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Organophosphonate-Based PNA-Functionalization of Silicon Nanowires for Label-Free DNA Detection
Abstract
We investigated hydroxyalkylphosphonate monolayers as a novel platform for the biofunctionalization of silicon-based field effect sensor devices. This included a detailed study of the thin film properties of organophosphonate films on Si substrates using several surface analysis techniques, including AFM, ellipsometry, contact angle, X-ray photoelectron spectroscopy (XPS), X-ray reflectivity, and current-voltage characteristics in electrolyte solution. Our results indicate the formation of a dense monolayer on the native silicon oxide that has excellent passivation properties. The monolayer was biofunctionalized with 12 mer peptide nucleic acid (PNA) receptor molecules in a two-step procedure using the heterobifunctional linker, 3-maleimidopropionic-acid-N-hydroxysuccinimidester. Successful surface modification with the probe PNA was verified by XPS and contact angle measurements, and hybridization with DNA was determined by fluorescence measurements. Finally, the PNA functionalization protocol was translated to 2 microm long, 100 nm wide Si nanowire field effect devices, which were successfully used for label-free DNA/PNA hybridization detection.
... Organophosphonate chemistry presents a promising alternative to silane chemistry. Compared to silanes, phosphonate films can achieve greater monolayer density, surface coverage, and stability, and have a lower tendency to form multilayered structures [270,307]. Shang et al. [126] demonstrated covalent immobilization of amine-bearing glycan and glycoprotein bioreceptors on silicon MRRs using an organophosphonate surface coating and an amine-vinyl sulfone linker ( Figure 15). After treating the surface with piranha solution to increase the number of available surface hydroxyl groups for organophosphonate grafting, the sensor surface was coated with a monolayer of 11-hydroxyundecylphosphonic acid (UDPA). ...
... In this work, the MRRs demonstrated excellent stability and reproducibility across multiple cycles of chemical regeneration and longterm storage at ambient conditions. A similar strategy was used to functionalize the surface of silicon nanowires with cysteine-modified PNA oligonucleotides [270]. Here, 3-maleimidopropionic-acid-N-hydroxysuccinimidester was used instead of DVS as a heterobifunctional linker to attach the thiol-containing PNA oligonucleotides to the UDPAmodified nanowires. ...
Silicon photonic (SiP) sensors offer a promising platform for robust and low-cost decentralized diagnostics due to their high scalability, low limit of detection, and ability to integrate multiple sensors for multiplexed analyte detection. Their CMOS-compatible fabrication enables chip-scale miniaturization, high scalability, and low-cost mass production. Sensitive, specific detection with silicon photonic sensors is afforded through biofunctionalization of the sensor surface; consequently, this functionalization chemistry is inextricably linked to sensor performance. In this review, we first highlight the biofunctionalization needs for SiP biosensors, including sensitivity, specificity, cost, shelf-stability, and replicability and establish a set of performance criteria. We then benchmark biofunctionalization strategies for SiP biosensors against these criteria, organizing the review around three key aspects: bioreceptor selection, immobilization strategies, and patterning techniques. First, we evaluate bioreceptors, including antibodies, aptamers, nucleic acid probes, molecularly imprinted polymers, peptides, glycans, and lectins. We then compare adsorption, bioaffinity, and covalent chemistries for immobilizing bioreceptors on SiP surfaces. Finally, we compare biopatterning techniques for spatially controlling and multiplexing the biofunctionalization of SiP sensors, including microcontact printing, pin- and pipette-based spotting, microfluidic patterning in channels, inkjet printing, and microfluidic probes.
... During all studies, a 3 µl water droplet was used. All measurements were performed 3 times by SiO 2 (1 µm) on another study [149] for dense mono -layers of phosphonates with OH-terminates. As demonstrated in Figure 5.10c, after the incubation, no significant FITC signal was obtained from the film while the signal arising from the ss -DNA strands were in the same range. ...
... 149] The o -Acylisourea molecule, which is very unstable in the buffer[137], was obtained via CH 3 terminal of the THPMP molecule. In this case, the unstability of the o -Acylisourea form was quite beneficial for recovering the surface anti -fouling properties after activating the surface in the presence of the EDC molecules. ...
... Layer thicknesses were obtained by averaging five measurement points for each sample. In this study, a refractive index value of n = 1.46 [21,32,33] was used for the organic layer for the thickness calculation. ...
... Herein, a hypothesis was introduced to explain the change process of NDM SAMs formation on Ge surface. Firstly, when the [21,32,33] was used for NDM SAMs thickness calculation, the inset in (a) was the partly amplified thickness results. reaction temperature was low, 4 • C for example, the SAMs formation process was slow and the Ge substrate was partly coated, which might result in the disorder of NDM molecules [35]. ...
As a typical semiconductor material, germanium has the potential to replace silicon for future-generation microelectronics, due to its better electrical properties. However, the lack of stable surface state has limited its extensive use for several decades. In this work, we demonstrated highly stable self-Assembled monolayers (SAMs) on Ge surface to prevent oxidization for further applications. After the pretreatment in hydrochloric acid, the oxide-free and Cl-terminated Ge could be further coated with 1-dodecanethiol (NDM) SAMs. The influence factors including reaction time, solvent component and reaction temperature were optimized to obtain stable passivated monolayer for oxidation resistance. Contact angle analysis, atomic force microscopy, ellipsometer and X-ray photoelectron spectroscopy were performed to characterize the functionalized Ge surface respectively. Meanwhile, the reaction mechanism and stability of thiols SAMs on Ge (1 1 1) surface were investigated. Finally, highly stable passivated NDM SAMs on Ge surface could be formed through immersing oxide-free Ge in mixture solvent (water/ethanol, v/v = 1:1) at appropriately elevated temperature (∼80 °C) for 24 h. And the corresponding optimized passivated Ge surface was stable for more than 10 days even in water condition, which was much longer than the data reported and paved the way for the future practical applications of Ge.
... The upper bound on the dynamic range for analytes of ∼20 mer in length is typically in the pM range [51], [125]. Herein, we aim to provide a short list of recent advances in nucleic acids detection based on CMOS compatible (or mostly compatible) Si-NW FETs [51], [53], [56], [74], [79], [82], [104], [126]- [132]. The most common configuration for nucleic acid sensing is to immobilize one strand of nucleic acid on the surface of the Si-NW FET as receptor, and use that to detect its complementary strand in solution. ...
... Although they are well-suited as analytes due to their high negative charges, they make poor receptors due to both electrostatic repulsion with analytes and high background noise due to movement of bound probes in solution. An important advancement in the field was thus the replacement of the negatively charged ssDNA probe with the neutrally charged PNA, which significantly improved DNA binding efficiency by minimizing electrostatic repulsion between the DNA analyte and the probe [126]. Gao et al. analyzed various factors (gate voltage, buffer ionic strength, and probe concentration) of their triangular Si-NW sensors and optimized the sensor performance for DNA sensing [128]. ...
Silicon nanowire field-effect transistors (Si-NW FETs) have been demonstrated as a versatile class of potentiometric nanobiosensors for real time, label-free, and highly sensitive detection of a wide range of biomolecules. In this review, we summarize the principles of such devices and recent developments in device fabrication, fluid integration, surface functionalization, and biosensing applications. The main focus of this review is on CMOS compatible Si-NW FET nanobiosensors.
... In the year 2008, A.Cattani-Scholz et al. [31] investigated PNA functionalized organophosphonate-based SilNW sensor with self-assembled hydroxyalkylphosphonate monolayer interface system for label-free DNA sensing. Upon electrolyte 1μM DNA buffer solution injection, a decline in wire resistance value corresponding to variation in surface potential of approximately 1.5 mV was reported. ...
Due to the distinctive optical, mechanical and electrical characteristics, silicon nanowire (SilNW), one of the one dimensional nanostructures, has become a potential sensing nanomaterial. SilNWs have drawn interest in high-sensitive sensor fabrication, primarily because of their large surface-volume ratios, which significantly enhanced the detection limit to femtomolar concentrations and also provided high sensitivity. Due to its high charge sensitivity, SilNW FET-based sensors had been employed extensively for sensing various chemical as well as biological species. In this work, the sensing performance and applications of different SilNW biosensors, gas sensors, chemical and metal ion sensors were studied. In this study, we have also elaborated the most current developments as well as the sensing performance of various SilNW-based Covid-19 sensors.
... The thickness measurements for the AhexPA layer support the picture of a formed single (mono-)layer with the ~1 nm long AhexPA molecules tilted by ~53° with respect to the surface normal. [21] The corresponding measurement for the OTG-bR layer led to the conclusion that OTG-bR assembled into bilayers on the amine-terminated TiN surface, which can be attributed to vesicle collapse instead of fusion. The thickness of the protein patches is in line with previous reports concerning OTG-bR layers on amineterminated SiO2. ...
... This indicated that Pd-SiNWs is a selective hydrogen sensor with fast response time. Organophosphonate and thiol-terminated SiNWs served as platforms to attach peptide nucleotide acid (PNA) probes for DNA sensing [118,119]. In the last case, DNA detection was achieved by monitoring the conductance, via current-voltage measurements, of the functionalized SiNWs at various concentration of target DNA in solution. ...
Silicon nanowires are attractive materials from the point of view of their electrical properties or high surface-to-volume ratio, which makes them interesting for sensing applications. However, they can achieve a better performance by adjusting their surface properties with organic/inorganic compounds. This review gives an overview of the main techniques used to modify silicon nanowire surfaces as well as characterization techniques. A comparison was performed with the functionalization method developed, and some applications of modified silicon nanowires and their advantages on those non-modified are subsequently presented. In the final words, the future opportunities of functionalized silicon nanowires for chipless tag radio frequency identification (RFID) have been depicted.
... The DNA sensor is a molecular recognition and sensing technology for detection purposes, 18 namely, the various signal conversion and sensing techniques are often jointed with DNA for simple, rapid, high sensitive and high selective detection. 19 At present, DNA recognition to the small molecule has attracted a number of scientific attention. ...
In this paper, DNA/carboxyl MWCNTs was immobilized on a glassy carbon electrode (GCE) to develop an electrochemical sensor for simultaneous determination of liquiritigenin (LG) and liquiritin (LQ). The adsorptive voltammetric behaviors of LG and LQ at DNA/carboxyl MWCNTs composite film modified GCE were investigated by differential pulse voltammetry (DPV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The calibration curves for LG and LQ were obtained in the ranges of 0.08–1.60 μM and 5.0–100.0 μM with the detection limits of 0.02 and 0.8 μM (S/N = 3), respectively. The method was applied successfully for the detection of LG and LQ in human blood serum with satisfactory recoveries. It provides a new tool for the research of the pharmacological mechanism of LG and LQ in vivo.
... 19 Short (15 bases), single-stranded PNA molecules, modified with reactive SH sites at different points of the backbone, were coupled to SAMPs-terminated silicon native oxide either in a vertical ( ⊥ PNA) or horizontal ( ∥ PNA) surface conformation (Scheme 1). The applied PNA immobilization chemistry depends on standard maleimido cross-coupling 20,21 and exploits both the hydrolytic stability of organophosphonate anchor groups 22 and the protective properties of ethylene glycol (EG) units against the nonspecific surface binding of charged biomolecules 23−25 making them attractive bioreceptor platforms for DNA/RNA detection in a physiological environment. Scheme 1. Schematic Representation of the Peptide Nucleic Acid (PNA) Surface Immobilization Protocol a a Single-stranded peptide nucleic acid (PNA, red) is coupled to the self-assembled monolayer of phosphonic acids (SAMPs, blue) either in a vertical ( ⊥ PNA) (pathway A) or horizontal ( || PNA) (pathway B) fashion using standard maleimido-based cross-coupling (green) methods (not drawn to scale). ...
Label-free detection of charged biomolecules, such as DNA, has experienced an increase in research activity in recent years, mainly to obviate the need for elaborate and expensive pretreatments for labeling target biomolecules. A promising label-free approach is based on the detection of changes in the electrical surface potential on biofunctionalized silicon field-effect devices. These devices require a reliable and selective immobilization of charged biomolecules on the device surface. In this work, self-assembled monolayers of phosphonic acids are used to prepare organic interfaces with a high density of peptide nucleic acid (PNA) bioreceptors, which are a synthetic analogue to DNA, covalently bound either in a multidentate (∥PNA) or monodentate (⊥PNA) fashion to the underlying silicon native oxide surface. The impact of the PNA bioreceptor orientation on the sensing platform's surface properties is characterized in detail by water contact angle measurements, atomic force microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. Our results suggest that the multidentate binding of the bioreceptor via attachment groups at the γ-points along the PNA backbone leads to the formation of an extended, protruding, and netlike three-dimensional metastructure. Typical "mesh" sizes are on the order of 8 ± 2.5 nm in diameter, with no preferential spatial orientation relative to the underlying surface. Contrarily, the monodentate binding provides a spatially more oriented metastructure comprising cylindrical features, of a typical size of 62 ± 23 × 12 ± 2 nm2. Additional cyclic voltammetry measurements in a redox buffer solution containing a small and highly mobile Ru-based complex reveal strikingly different insulating properties (ion diffusion kinetics) of these two PNA systems. Investigation by electrochemical impedance spectroscopy confirms that the binding mode has a significant impact on the electrochemical properties of the functional PNA layers represented by detectable changes of the conductance and capacitance of the underlying silicon substrate in the range of 30-50% depending on the surface organization of the bioreceptors in different bias potential regimes.
... This lowering in contact angle indicates azido-PNA coupling to the surface, as the increased hydrophilicity is expected from the polar structural groups. 33 Thiol−yne chemistry was performed by exposing the 1,8-nonadiyne monolayer to a solution of thiol-PNA in phosphate-buffered saline (PBS) under illumination with a 365 nm light source. The thiol-PNA-functionalized surface changed the contact angle from 87.6°± 1.1 after 1,8-nonadiyne to 46.5°± 3.2 after thiol-PNA, again indicating a hydrophilic surface and thus proper functionalization. ...
Silicon nanowire chips can function as sensors for cancer DNA detection, whereby selective functionalization of the Si sensing areas over the surrounding silicon oxide would prevent loss of analyte and thus increase the sensitivity. The thermal hydrosilylation of unsaturated carbon-carbon bonds onto H-terminated Si has been studied here to selectively functionalize the Si nanowires with a monolayer of 1,8-nonadiyne. The silicon oxide areas, however, appeared to be functionalized as well. The selectivity towards the Si-H regions was increased by introducing an extra HF treatment after the 1,8-nonadiyne monolayer formation. This step (partly) removed the monolayer from the silicon oxide regions, whereas the Si-C bonds at the Si areas remained intact. The alkyne headgroups of immobilized 1,8-nonadiyne were functionalized with PNA probes by coupling azido-PNA and thiol-PNA by click chemistry and thiol-yne chemistry, respectively. Although both functionalization routes were successful, hybridization could only be detected on the samples with thiol-PNA. No fluorescence was observed when introducing dye-labelled non-complementary DNA, which indicates specific DNA hybridization. These results open up the possibilities for creating Si nanowire-based DNA sensors with improved selectivity and sensitivity.
... The formation of SAMs provides a common means to modify the surfaces of conductors and semiconductors [23]. The occurrence of oxidation and reduction reactions on the nanoparticle exterior can be limited using SAMs, [17] and the type of headgroup used in the production of the monolayer is vital to the stability of the surface [24]. Phosphonic acids are ideal for the production of SAMs on the surface of NiTi nanoparticles due to the low acid dissociation constant, making them safe for use within the human body [6]. ...
Nitinol (NiTi) nanoparticles are a valuable metal alloy due to many unique properties that allow for medical applications. NiTi nanoparticles have the potential to form nanofluids, which can advance the thermal conductivity of fluids by controlling the surface functionalization through chemical attachment of organic acids to the surface to form self-assembled alkylphosphonate films. In this study, phosphonic functional head groups such as 16-phosphonohexadecanoic acid, octadecylphosphonic acid, and 12-aminododecylphosphonic acid were used to form an ordered and strongly chemically bounded film on the NiTi nanopowder. The surface of the NiTi nanoparticles was modified in order to tailor the chemical and physical properties to the desired application. The modified NiTi nanoparticles were characterized using infrared spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, and ³¹P solid-state nuclear magnetic resonance. The interfacial bonding was identified by spectroscopic data suggesting the phosphonic head group adsorbs in a mixed bidentate/monodentate binding motif on the NiTi nanoparticles. Dynamic light scattering and scanning electron microscopy-energy dispersive X-ray spectroscopy revealed the particle sizes. Differential scanning calorimetry was used to examine the phase transitions. Zeta potential determination as a function of pH was examined to investigate the surface properties of charged nanoparticles. The influence of environmental stability of the surface modifications was also assessed.
... 27 Because of these high ratios and their unique quasi-one-dimensional electronic structures, SiNW-based devices have properties that can outperform those of their traditional counterparts in many potential applications in the design of sensors, 28 solar cells, 29,30 Li-ion batteries, 31 and superhydrophobic surfaces. 32 Most modification processes for oxidized SiNW are based on silanization, 33 esterification, 34 and phosphonate attachment reactions, 35 although hydrosilylation in particular has been used heavily for oxide-free H-terminated SiNWs. 36−38 Each of these functionalization approaches typically requires at least several hours (sometimes even overnight) and often also elevated temperatures to reach completion, which strongly limits scale-up and concomitant industrial applications. ...
Inspired by the homogeneous catalyst tris(pentafluorophenyl) borane [B(C6F5)3], which acts as a promotor of Si-H bond activation, we developed and studied a method of modifying silicon oxide surfaces using hydrosilanes with B(C6F5)3 as the catalyst. This dedihydrosiloxanation reaction yields complete surface coverage within 10 min at room temperature. Organic monolayers derived from hydrosilanes with varying carbon chain lengths (C8-C18) were prepared on oxidized Si(111) surfaces, and the thermal and hydrolytic stabilities of the obtained monolayers were investigated in acidic (pH 3) medium, basic (pH 11) medium, phosphate-buffered saline (PBS), and deionized water (neutral conditions) for up to 30 days. DFT calculations were carried out to gain insight into the mechanism, and the computational results support a mechanism involving silane activation with B(C6F5)3. This catalyzed reaction path proceeds through a low-barrier-height transition state compared to the noncatalyzed reaction path.
... In literature, further ZnO nanowire use for DNA sensory applications can be observed [23]. There have been several studies focusing on the development of Si-NW sensors during the last decade due to availability and ease of fabrication [24][25][26]. Si-NWs are CMOS compatible [27]. But ZnO-NWs are not CMOS compatible but it works quite well under ambient conditions and oxygen rich environment [28][29][30]. ...
A comparative study of various electrical properties of n-channel Si And ZnO Gate-all-around (GAA) nanowire Field-Effect Transistors has been assessed and presented. The simulation results shown here are found on the basis of self-consistent solution of energy balance equation. The temperature, channel diameter and gate length dependence of various electrical parameters like subthreshold swing (SS), threshold voltage and transconductance have been simulated. ZnO nanowire FET showed better threshold voltage but poor subthreshold swing and transconductance with respect to Si nanowire FET.
... 27 Because of these high ratios and their unique quasi-one-dimensional electronic structures, SiNW-based devices have properties that can outperform those of their traditional counterparts in many potential applications in the design of sensors, 28 solar cells, 29,30 Li-ion batteries, 31 and superhydrophobic surfaces. 32 Most modification processes for oxidized SiNW are based on silanization, 33 esterification, 34 and phosphonate attachment reactions, 35 although hydrosilylation in particular has been used heavily for oxide-free H-terminated SiNWs. 36−38 Each of these functionalization approaches typically requires at least several hours (sometimes even overnight) and often also elevated temperatures to reach completion, which strongly limits scale-up and concomitant industrial applications. ...
Inspired by the homogeneous catalyst tris(pentafluorophenyl) borane [B(C6F5)3], which acts as a promotor of Si-H bond activation, we developed and studied a method of modifying silicon oxide surfaces using hydrosilanes with B(C6F5)3 as the catalyst. This dedihydrosiloxanation reaction yields complete surface coverage within 10 min at room temperature. Organic monolayers derived from hydrosilanes with varying carbon chain lengths (C8-C18) were prepared on oxidized Si(111) surfaces, and the thermal and hydrolytic stabilities of the obtained monolayers were investigated in acidic (pH 3) medium, basic (pH 11) medium, phosphate-buffered saline (PBS), and deionized water (neutral conditions) for up to 30 days. DFT calculations were carried out to gain insight into the mechanism, and the computational results support a mechanism involving silane activation with B(C6F5)3. This catalyzed reaction path proceeds through a low-barrier-height transition state compared to the noncatalyzed reaction path.
... [14][15][16][17][18][19][20] Nevertheless, the design of effective sensing agents that retain and possibly enhance the molecular recognition properties at the solid-state interface continues to be a major challenge. [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] The development of device-quality monolayerbased sensors requires not only selectivity and sensitivity towards a specific analyte, but also a high degree of stability and a fast nondestructive read-out process. ...
We report on an uranyl complex monolayer that easily allows the optical detection of a nervine gas simulant, namely the dimethyl methylphosphonate. Both UV-vis and photoelectronic data confirm that the functional hybrid material coordinates this Lewis basis by means of the P=O group that interacts with the uranium equatorial site available for complexation.
... 17 For both processes, substrates were rinsed after each step to remove untethered molecules and the coating process was repeated to promote uniform layer formation. 28 3.2. Chemical Composition and Coverage. ...
Although drug-eluting stent technologies have significantly improved clinical outcomes over the past decade, substantial issues with postimplantation vessel reocclusion still remain. To combat these issues, bioactive amphiphilic macromolecules (AMs), comprised of a functional end group, a branched hydrophobic domain, and a hydrophilic poly(ethylene glycol) tail, were investigated as a therapeutic coating to reduce smooth muscle cell (SMC) proliferation and platelet adhesion. In this study, grafting-from and grafting-to approaches for AM surface functionalization were compared to determine the effects of fabrication method on bioactive delivery characteristics, including the AM loading, release, and biological activity. Grafted-from coatings were formed by stepwise synthesis of phosphonate AMs, 1pM, on the substrate, first by alkyl phosphonate coordination to stainless steel and subsequent carbodiimide coupling to conjugate the hydrophobic and hydrophilic domains. In contrast, grafted-to monolayers were assembled utilizing presynthesized 1pM in a tethering by aggregation and growth technique. Coatings formed using the grafting-from approach yielded high AM grafting density and a highly ordered layer, which corresponded to a slower release rate and sustained bioactivity over 28 days. In contrast, the grafted-to coatings yielded less dense, heterogeneous layers, which released faster and were therefore less efficacious in suppressing prolonged SMC proliferation. Both coatings significantly reduced platelet adhesion compared to an uncoated control, but similar platelet adhesion results between grafted-from and grafted-to coatings suggest that both surfaces maintained a molecular density favorable for antiplatelet activity. Overall, the grafting-from method produced uniform coatings with improved loading, release, and bioactive properties compared to the grafting-to approach, highlighting the potential of AM controlled release coatings for therapeutic delivery.
... In particular, chiral PNAs can be obtained by introducing substituents on the original N-(2aminoethyl)glycine backbone and are usually divided into three groups (α-PNA, β-PNA, and γ-PNA) based on the position of the substituents in the PNA backbone [2]. In this work we have focused on the optimization of a sensing platform for DNA detection based on the immobilization of a commercially available 15mer γ-PNA on Si/SiO2 through organophosphonate chemistry as a valid alternative to siloxane chemistry [5] (Fig. 1). Covalent attachment via the γ-end groups is supposed to immobilize the γ-PNA horizontally on the sensor surface, with potential advantages towards the counter-ion screening problem at the sensor interface [6]. ...
Silicon-based field effect devices have been widely investigated in recent years for the label-free detection of DNA hybridization. The devices rely on detecting changes in the electrical surface potential that occur as a result of adsorbing charged DNA. To provide surface-immobilized affinity receptors for DNA hybridization, a suitable organic interface is obligatory that has a high density of receptor binding sites and a short distance between surface and probe DNA or its analogue, peptidic nucleic acid (PNA), to minimize electrolyte screening effects. In this work we report on the bio-functionalization and characterization of silicon oxide-terminated surfaces with γ-PNA through organophosphonate interfacial chemistry. Functionalizing via attachment groups at the γ-points along the PNA backbone allows for multidentate binding of the PNA receptor in a lying configuration on the device surface, with potential application in label-free biosensing device optimization.
... PNA functionalised Au NWs were used to detect 100 fM of mRNA (Fang and Kelley, 2009), while a previous study reported PNA functionalised Si FET sensor to detect 10 fM of DNA (Gao et al., 2007). One method of covalent binding of PNA molecules to SiO2 NW has been described by Cattani-Scholz et al. (2008), which is an alternative way to physisorption. Zhang et al. (2010) recently reported a highly sensitive and rapid sensor based on PNA-DNA hybridisation to detect reversetranscription-polymerase chain reaction (RT-PCR) product of Dengue serotype 2 (DEN-2). ...
Nanodevices and biomolecules have incredibly strong correspondence in terms of size and physical properties. In this review, three major types of nanodevices, namely cantilevers, nanowires and carbon nanotubes, have been discussed and how they have resulted in new sensor designs or helped push the limits of detection in existing schemes. After brief overview of each type and the ways it could be used in biosensing, recent research efforts are presented to emphasise the challenges and achievements in that particular category.
... 12 −14 In comparison with common monolayers, such as silanes and thiols, OPA monolayers have a better surface coverage, no self-polymerization at ambient conditions and better hydrolytic stability in alkaline conditions. 15,16 As a result, OPA has raised expectation to give rise to robustly grafted uniform monolayers, 17 exhibit superior hydrolytic and thermal stability than siloxanes, and display better resistance to mechanical stress, which is important in determining device lifetime for biosensors, 18 molecular, 19 and organic electronics. 20,21 On top of these expected advantages, OPA monolayers can form stable, covalently bound self-assembled monolayers (SAMs) on various metal oxide surfaces used in organic electronics 20,21 and biosensor applications, 22 such as aluminum oxide (Al 2 O 3 ), 12,23 mica, 24 titanium dioxide (TiO 2 ), 16,25 indium tin oxide (ITO), 26−28 or zinc oxide (ZnO). ...
... 12 −14 In comparison with common monolayers, such as silanes and thiols, OPA monolayers have a better surface coverage, no self-polymerization at ambient conditions and better hydrolytic stability in alkaline conditions. 15,16 As a result, OPA has raised expectation to give rise to robustly grafted uniform monolayers, 17 exhibit superior hydrolytic and thermal stability than siloxanes, and display better resistance to mechanical stress, which is important in determining device lifetime for biosensors, 18 molecular, 19 and organic electronics. 20,21 On top of these expected advantages, OPA monolayers can form stable, covalently bound self-assembled monolayers (SAMs) on various metal oxide surfaces used in organic electronics 20,21 and biosensor applications, 22 such as aluminum oxide (Al 2 O 3 ), 12,23 mica, 24 titanium dioxide (TiO 2 ), 16,25 indium tin oxide (ITO), 26−28 or zinc oxide (ZnO). ...
Formation of dense monolayers with proven atmospheric stability using simple fabrication conditions remains a major challenge for potential applications such as (bio)sensors, solar cells, surfaces for growth of biological cells, and, also, molecular, organic and plastic electronics. Here, we demonstrate a single-step modification of organophosphonic acids (OPA) on 1D and 2D structures using supercritical carbon dioxide (SCCO2) as a processing medium, with high stability and significantly shorter processing times than those obtained by the conventional physisorption-chemisorption method (2.5 h vs. 48-60 h).The advantages of this approach in terms of stability and atmospheric resistivity are demonstrated on various 2D materials, such as Indium-Tin-Oxide (ITO) and 2D Si surfaces. The advantage of the reported approach on electronic and sensing devices is demonstrated by Si nanowire Field Effect Transistors (SiNW FETs), which have shown a few orders of magnitude higher electrical and sensing performances, compared with devices obtained by conventional approaches. The compatibility of the reported approach with various materials and its simple implementation with a single reactor makes it easily scalable for various applications.
... Furthermore, a concentration-dependent detection of let-7b by the SiNW biosensor was investigated ( Figure 7B); the more the target miRNA molecules are hybridized, the higher the resistance increased. The results indicate that the biosensor is capable of detecting target miRNA at concentrations as low as 1 fM, which is 1 order of magnitude higher than that reported for detection of DNA (Cattani-Scholz et al. 2008). The assay was further tested for the detection of miRNA in real samples by analyzing let-7b in total RNA extracted from HeLa cells. ...
Monitoring the molecular recognition, binding, and disassociation between probe and target is important in medical diagnostics and drug screening, because such a wealth of information can be used to identify the pathogenic species and new therapeutic candidates. Nanoelectronic biosensors based on silicon nanowire field-effect transistors (SiNW-FETs) have recently attracted tremendous attention as a promising tool in the investigation of biomolecular interactions due to their capability of ultra-sensitive, selective, real-time, and label-free detection. Herein, we summarize the recent advances in label-free analysis of molecule-molecule interactions using SiNW-FETs, with a discussion and emphasis on small molecule-biomolecule interaction, biomolecule-biomolecule interactions (including carbohydrate-protein interaction, protein-protein or antigen-antibody binding, and nucleic acid-nucleic acid hybridization), and protein-virus interaction. Such molecular recognitions offer a basis of biosensing and the dynamics assay of biomolecular association or dissociation. Compared to the conventional technologies, SiNW-FETs hold great promise to monitor molecule-molecule interactions with higher sensitivity and selectivity. Finally, several prospects concerning the future development of SiNW-FET biosensor are discussed.
Nanobiodevices, a technology characterized by engineered nanostructures, have attracted extensive interest as a new generation bioanalytical tool that allows accessing the information contained in nucleic acids. Distinct properties of nanobiodevice technologies have shown great promise in improvements in sensitivity, speed, and accuracy for nucleic acid analysis, providing a cutting-edge analytical tool to facilitate the analysis of nucleic acid in biomedical applications. In this chapter, an overview of nanobiodevices and how this emergent technology can be combined with other analytical approaches for addressing different challenges in biomedical applications will be provided. The recent advances in nanobiodevice for nucleic acid analysis will be highlighted in the following categories: (i) nanobiodevice for preparation of nucleic acid and (ii) nanobiodevice for detection of nucleic acid.
A new low-cost, top-down nanowire fabrication technology is presented not requiring nanolithography and suitable for any conventional microtechnology cleanroom facility. This novel wafer-scale process technology uses a combination of angled thin-film deposition and etching of a metal layer in a precisely defined cavity with a single micrometer-scale photolithography step. Electrically functional silicon and metallic nanowires with lengths up to several millimeters, lateral widths of ∼100 nm, and thicknesses ∼20 nm have been realized and tested. Device characterization includes a general description of device operation, electrochemical biasing, and sensitivity for sensor applications followed by electrical measurements showing linear i-v characteristics with specific contact resistivity G c ∼ 4 × 10-4 Ω cm 2 and electrochemical behavior of the oxidized silicon nanowires is described with the site-binding model.
Silicon nanowires (Si NWs) are one of the most important one-dimensional semiconductors, partly due to their ready implementation in modern technology. As sensing applications using Si NWs increase, it is necessary to have well-defined chemical attachment schemes that will provide the desired biomolecular recognition properties, chemical stability, and good interfacial electrical properties. Such surface functionalization will allow the controlled immobilization of biomolecules on Si NWs, which is an important step in designing highly selective and sensitive biosensors. Moreover, such surface modification of Si NWs is not only dedicated to sensing, but is also useful for many other applications (not described in this chapter) such as device integration, controlled cell micropatterning, Si NW internalization, and analyte confinement that is high amount of analytes concentrated in a restricted area to improve their detection by mass spectrometry. Finally, advantages of using photochemistry on Si NWs to either immobilize or release probes are discussed as well as hybrid Si NWs coated with various inorganic materials.
Advancement in novel materials for sensing application has revolutionized the field of biological and chemical sensors. Nanomaterials such as nanowires, nanorods, nanobelts, carbon nanotubes, graphene, and MXenes have served as the building blocks in the fabrication of a variety of sensor technologies using both bottom-up and top-down approaches. Nanomaterials continue to be attractive for the development of high-performance sensors due to their extraordinary electrical and physical properties deriving from their 1D and 2D structures. These properties can be tuned through experimentation to achieve the desired sensor characteristics and sensing performance. This review will discuss the advances in sensor fabrication technology that rely on nanomaterials, their synthesis, properties, and applications. The current limitations of these nanomaterials and their prospects as sensing materials are also discussed.
The architecture of electrically contacting the self-assembled monolayer (SAM) of an organophosphonate has a profound effect on a device where the SAM serves as an intermolecular conductive channel in the plane of the substrate. Nanotransfer printing (nTP) enabled the construction of top-contact and bottom-contact architectures; contacts were composed of 13 nm thin metal films that were separated by a ca. 20 nm gap. Top-contact devices were fabricated by assembling the SAM across the entire surface of an insulating substrate and then applying the patterned metallic electrodes by nTP; bottom-contact ones were fabricated by nTP of the electrode pattern onto the substrate before the SAM was grown in the patterned nanogaps. SAMs were prepared from (9,10-di(naphthalen-2-yl)anthracen-2-yl)phosphonate; here, the naphthyl groups extend laterally from the anthracenylphosphonate backbone. Significantly, top-contact devices supported current that was about 3 orders of magnitude greater than that for comparable bottom-contact devices and that was at least 100,000 times greater than for a control device devoid of a SAM (at 0.5 V bias). These large differences in conductance between top- and bottom-contact architectures are discussed in consideration of differential contact-to-SAM geometries and, hence, resistances.
Electrografting of gold and graphene surfaces by functional p-(N-maleimido)phenyl groups was performed by reduction of p-(N-maleimido)phenyldiazonium tetrafluoroborate. The reduction was carried out using cyclo voltammetry coupled with micro-gravimetric measurements by means of electrochemical quartz crystal microbalance (EQCM). The overall deposited mass on gold was higher than on graphene. However, the Faradaic efficiency was lower on Au (14%) compared to graphene (22%) after the first potential scan. Subsequently, the maleimide functional groups have been tested for immobilization of terminal thiols using 2-(4-nitrobenzene)-ethane-thiol for the functionalized graphene surface and a cysteine-modified peptide for the functionalized gold surface. The functionalization by p-(N-maleimido)phenyl groups and the following thiol coupling of the particular surface was proven by infrared spectroscopic ellipsometry (IRSE). In addition, the interaction of the tetrabutylammonium and tetrafluoroborate ions present in the electrolyte with the Au and graphene electrodes was investigated by EQCM and revealed less electrostatic interaction of graphene with these ions in solution compared to the metal (Au) surface.
Optimization of interfaces in inorganic-organic device systems depends strongly on understanding both the molecular processes that are involved in surface modification, and of the effects that such modification have on the electronic states of the material. In particular, the last several years have seen passivation and functionalization of semiconductor surfaces to be strategies to realize devices with superior function by controlling Fermi level energies, band gap magnitudes, and work functions of semiconducting substrates. Among all the synthetic routes and deposition methods available for optimization of functional interfaces in hybrid systems, organophosphonate chemistry has been found to be a powerful tool to control at molecular level the properties of materials in many different applications. In this review, we focus on the relevance of organophosphonate chemistry in nanotechnology, giving an overview about some recent advances in surface modification, interface-engineering, nanostructure optimization, and bio-integration.
Electrografting of gold and graphene surfaces by functional p‐(N‐maleimido)phenyl groups was performed by reduction of p‐(N‐maleimido)phenyldiazonium tetrafluoroborate. The reduction was carried out using cyclic voltammetry coupled with micro-gravimetric measurements by means of electrochemical quartz crystal microbalance (EQCM). The overall deposited mass on gold was higher than on graphene. However, the Faradaic efficiency was lower on Au (14%) compared to graphene (22%) after the first potential scan. Subsequently, the maleimide functional groups have been tested for immobilization of terminal thiols using (4-nitrobenzyl)mercaptan for the functionalized graphene surface and a cysteine-modified peptide for the functionalized gold surface. The functionalization by p‐(N‐maleimido)phenyl groups and the following thiol coupling of the particular surface was proven by infrared spectroscopic ellipsometry (IRSE). In addition, the interaction of the tetrabutylammonium and tetrafluoroborate ions present in the electrolyte with the Au and graphene electrodes was investigated by EQCM and revealed less electrostatic interaction of graphene with these ions in solution compared to the metal (Au) surface.
We report the impact of geometrical constraint on intramolecular interactions in self-assembled monolayers (SAMs) of alkylphosphonates grown on anodically oxidized aluminum (AAO). Molecular order in these films was determined by sum frequency generation (SFG) spectroscopy, a more sensitive measure of order than infrared absorption spectroscopy. Using SFG we show that films grown on AAO are, within detection limits, nearly perfectly ordered in an all-trans alkyl chain configuration. In marked contrast, films formed on planar, plasma-oxidized aluminum oxide or α-Al2O3 (0001) are replete with gauche defects. We attribute these differences to the nanocylindrical structure of AAO, which enforces molecular confinement.
Electrografting of gold and graphene surfaces by functional p-(N-maleimido)phenyl groups was performed by reduction of p-(N-maleimido)phenyldiazonium tetrafluoroborate. The reduction was carried out using cyclo voltammetry coupled with micro-gravimetric measurements by means of electrochemical quartz crystal microbalance (EQCM). The overall deposited mass on gold was higher than on graphene. However, the Faradaic efficiency was lower on Au (14%) compared to graphene (22%) after the first potential scan. Subsequently, the maleimide functional groups have been tested for immobilization of terminal thiols using 2-(4-nitrobenzene)-ethane-thiol for the functionalized graphene surface and a cysteine-modified peptide for the functionalized gold surface. The functionalization by p-(N-maleimido)phenyl groups and the following thiol coupling of the particular surface was proven by infrared spectroscopic ellipsometry (IRSE). In addition, the interaction of the tetrabutylammonium and tetrafluoroborate ions present in the electrolyte with the Au and graphene electrodes was investigated by EQCM and revealed less electrostatic interaction of graphene with these ions in solution compared to the metal (Au) surface.
Organophosphonate self-assembled monolayers (SAMPs) fabricated on SiO2 surfaces can influence crystallization of vapor-deposited pentacene and thus can affect device performance of pentacene-based organic thin film transistors. Polarized Raman spectroscopy is demonstrated to be an effective technique to determine the degree of anisotropy in pentacene thin films deposited on three structurally different, aromatic SAMPs grown on silicon oxide dielectrics. Vibrational characterization of pentacene molecules in these films reveals that the molecular orientation of adjacent crystalline grains is strongly correlated on the SAMP-modified dielectric surface, which results in enhanced interconnectivity between the crystallite domains, well beyond the size of a single grain. It is found that vibrational coupling interactions, relaxation energies, and grain size boundaries in pentacene thin films vary with the choice of SAMP. This information clearly shows that molecular assembly of pentacene thin films can be modulated by controlling the SAMP-modified dielectric surface, with potentially beneficial effects on the optimization of electron transfer rates. Copyright
In situ photoelastic-modulated Fourier transform infrared reflection absorption spectroscopy has been applied for the investigation of interfacial stability of organothiol and organosilane monolayer films on nanocrystalline zinc oxide thin films. It has been shown that for octadecyltriethoxysilane films, exposure to high water activities results in physisorption of water in the cross-linked film. This high water activity at the interface leads to a reversible wet de-adhesion of the interfacial silanol groups from the ZnO surface. However, the organothiol seems to form a denser monolayer and a stable by S-Zn bond that is resistant to the competition with adsorbed water. The reversible attachment for cross-linked organosilanol films has been demonstrated for the first time by means of an in situ spectroscopic method on model ZnO surfaces.
Development of high-performance biosensors vastly facilitates the analysis and detection of various biological species, including nucleic acids, protein, cell, etc. Functional nanomaterials (e.g., silver/gold nanoparticles, carbon nanotubes, graphene, silicon nanowires, etc) serve as new platform for design of nano-biosensors featuring high sensitivity and specificity. Taking advantage of the attractive merits of silicon nanowires (SiNWs) (e.g., unique electronic/optical properties, huge surface-to-volume rations, surface tailorability, fast response and good reproducibility, and compatibility with conventional silicon technology), SiNWs have been widely employed for constructing various kinds of electrochemical and optical biosensors, enabling ultrasensitive, specific, and reproducible detection of DNA and protein. We introduce a number of typical SiNWs-based biosensors (e.g., field-effect transistor (FET), amperometric-, surface-enhanced Raman scattering (SERS), and fluorescence-based biosensors) in this chapter, aiming to summarize the representative progresses of this research field in recent years. These kinds of high-quality silicon-based sensors show potentially great promise for myriad practical applications, such as medical diagnosis, food safety, drug security, environment monitoring, as well as anti-bioterrorism and so forth.
Monolayers of alkyl bisphosphonic acids (bisPAs) of various carbon chain lengths (C4, C8, C10, C12) were grown on aluminum oxide (AlO x ) surfaces from solution. The structural and electrical properties of these self-assembled monolayers (SAMs) were compared with those of alkyl monophosphonic acids (monoPAs). Through contact angle (CA) and Kelvin-probe (KP) measurements, ellipsometry, and infrared (IR) and x-ray photoelectron (XPS) spectroscopies, it was found that bisPAs form monolayers that are relatively disordered compared to their monoPA analogs. Current-voltage (J-V) measurements made with a hanging Hg drop top contact show tunneling to be the prevailing transport mechanism. However, while the monoPAs have an observed decay constant within the typical range for dense monolayers, β mono = 0.85 ± 0.03 per carbon atom, a surprisingly high value, β bis = 1.40 ± 0.05 per carbon atom, was measured for the bisPAs. We attribute this to a strong contribution of 'through-space' tunneling, which derives from conformational disorder in the monolayer due to strong interactions of the distal phosphonic acid groups; they likely form a hydrogen-bonding network that largely determines the molecular layer structure. Since bisPA SAMs attenuate tunnel currents more effectively than do the corresponding monoPA SAMs, they may find future application as gate dielectric modification in organic thin film devices.
The detection of cardiac biomarkers is playing an important role in diagnosis of cardiovascular disease. In addition to traditional laboratory-based methods, the development of new devices that enable sensitive and rapid analysis of the biomarkers could help clinicians improve their diagnostic ability as well as provide an early and accurate indication of cardiac cellular necrosis. Silicon nanowire (SiNW)-based device is emerging as a highly sensitive, label-free, electrical biosensor for the direct detection of cardiac biomarkers. Here, we discuss fabrication of the SiNW biosensor using complementary metal-oxide semiconductor field-effect transistor-compatible technology, and its real-time detection of troponin T (cTnT) and multiplexed detection of three biomarkers, cTnT, creatine kinase MM (CK-MM) and creatine kinase MB (CK-MB), in serum simultaneously. The SiNW array format was produced through conventional optical lithography, etching and oxidation. Antibody was covalently immobilized on the SiNW surface. The real-time detection of cTnT was successfully demonstrated in an assay buffer solution of concentration down to 1 fg/ml, as well as in an undiluted human serum environment of concentration as low as 30 fg/ml. For the multiplexed detection, three antibodies were spotted on the SiNW array surface. The multiplexed detection was accomplished by incubating the biomarkers to the various antibodies-spotted SiNW surface. The SiNW sensor arrays were demonstrated to have the required selectivity and sensitivity for multiplexed detection of 100 fg/ml cTnT, CK-MM, and CK-MB in untreated serum. The multiplexed detection method is independent of the ionic strength of the sample solution, allowing the SiNW sensor to directly analyze the biomarkers in serum simultaneously. The developed SiNW biosensor shows a promising potential of constructing a miniaturized device for point-of-care diagnostics.
MicroRNA, a small non-coding RNA molecule that regulates geneexpression, is involved in cancer initiation and progression. It isbelieved that miRNA gene alterations play a critical role in almost allkinds of human cancers. The small size of miRNA makes not only itsdetection challenging, but also its selective pairing difficult. Currently, major methods of detecting miRNA are dependent on hybridization, in which a target miRNA molecule is hybridized to a complementarylabeled probe molecule. Recently developed detection methodsintroduce nanomaterials to the hybridized duplex to greatly enhancethe sensitivity. However, all of them are indirect, involving labeling orthe conjugating process. On the other hand, most of the current assayssuffer from low sensitivity. To get sufficient miRNA for detection, enrichment and amplification in the process of isolation of miRNA fromthe cell are needed prior to detection. However, the additional stepsmay lead to loss of miRNA, increase the analysis time, and involve largesample volume. In this chapter, we propose a new approach that enableshighly sensitive, specific, label-free direct detection of miRNA by usingpeptide nucleic acid (PNA)-functionalized silicon nanowires (SiNWs) biosensor. The biosensor is capable of detecting target miRNA as low as1 fM, as well as identifying fully matched versus mismatched miRNAsequences, especially discriminating between signal base differences, asin single nucleotide polymorphism (SNP). More importantly, the SiNWbiosensor enables miRNA direct detection in total RNA extracted fromcancer cells, providing a promising tool for early cancer detection inwhich the species of miRNAs in the cancer cells are different from thoseof normal cells. The developed PNA-functionalized SiNW biosensorshows potential applications in label-free, early detection of miRNAas a biomarker in cancer diagnostics with very high sensitivity andgood specificity.
A "nanonet", acronym for "NANOstructured NETwork", is defined as a network of one-dimensional nanostructures with high aspect ratio and randomly oriented on a substrate. In this work, a comprehensive study of nanonets based on silicon nanowires is presented for integration into DNA sensors. First, a simple method for the network fabrication has been developed in order to obtain homogeneous and reproducible nanonets. Then, the nanowire surface has been functionalized, so that the DNA hybridization detection is possible by fluorescence. The elaborated sensors exhibit excellent selectivity and a better sensitivity limit than planar substrates. The electrical properties of the silicon nanonets have also been investigated which resulted in the description of the conduction mechanisms of these networks. It has been shown that the electrical behaviour of such structures is ruled by the numerous nanowire-nanowire junctions and follows the electrical percolation theory. Moreover, an optimization procedure of these junctions has allowed stabilizing the electrical properties of silicon nanonets.
Therefore, these networks have attractive characteristics which arise not only from the individual components, the nanowires with a high specific surface, but also from the structural properties of the network itself which can be simply manipulated, at a low cost, on macroscopic scales. This work paves the way for the integration of silicon nanonets into DNA sensors based on electrical detection.
Using Density Functional Theory (DFT) including van der Waals interactions, we examine work function (WF) tuning of H:Si(111) over a range of 1.73 eV through adsorption of alkyl monolayers with general formula -[Xhead-group]-(CnH2n)-[Xtail-group], X = O(H), S(H), NH(2). The WF is practically converged at 4 carbons (8 for oxygen), for head-group functionalization. For tail-group functionalization and with both head- and tail-groups, there is an odd-even effect in the behavior of the WF, with peak-to-peak amplitudes of up to 1.7 eV in the oscillations. This behavior is explained through the orientation of the terminal-group's dipole. The shift in the WF is largest for NH2-linked and smallest for SH-linked chains and is rationalized in terms of interface dipoles. Our study reveals that the choice of the head- and/or tail-groups effectively changes the impact of the alkyl chain length on the WF tuning using self-assembled monolayers. This is an important advance in utilizing hybrid functionalized Si surfaces.
Silicon nanowire field-effect transistors (SiNW-FETs) are emerging as powerful chemical and biological sensors with various attractive features including-high sensitivity and direct electrical readout. However, limited systematic studies have appeared on how the working voltage affects their sensitivity. Here we demonstrate that the current change rate of SiNW-FETs can be exponentially enhanced in the subthreshold regime by both analyses of FET's theory model and electrical characteristics. On that basis, the back-gate controlled sensors' detection sensitivity for DNA and pH value appears great improvement when working in the subthreshold regime, which shows that optimization of SiNW-FET operating conditions, can provide significant improvement for the limits of SiNW-FET nanosensors, making it possible for higher-accuracy chemical and biological molecules detection. (C) 2013 The Japan Society of Applied Physics
A protocol for the preparation of improved phosphonate monolayers on a titanium (Ti) substrate is presented. Zirconium ions were used to enhance the bonding between the phosphonate headgroup and the pretreated Ti surface. Contact angle and X-ray photoelectron spectroscopy were used to characterize self-assembled monolayers (SAMs) of alkylphosphonic acid that formed spontaneously on Zr-mediated Ti (Zr/Ti) surfaces. The surfaces that were treated with an aqueous solution of zirconium oxychloride showed significantly enhanced stability in buffer compared to those formed directly on the native oxidized Ti. A bifunctional molecule, 10-mercaptodecanyl phosphonic acid (MDPA), was also used to form SAMs on Zr/Ti surfaces using an identical method; which enabled us to regulate the surface functionality through the terminal functional group. Protein patterning on the surface was carried out using UV irradiation through a mask to selectively degrade regions of the MDPA molecules. The surface was then backfilled with a protein-resistant molecule in the exposed regions followed by selective immobilization of proteins to the unexposed areas using a heterobifunctional linker molecule. The presented strategy significantly improved the stability of the phosphonate SAMs on oxidized Ti surfaces, which provided an ideal approach foundation for biomolecular immobilization and patterning onto the Ti surfaces. Thus, this method provided a versatile platform to activate the surfaces of biomedical Ti implants.
Rapid progress in identifying disease biomarkers has increased the importance of creating high-performance detection technologies. Over the last decade, the design of many detection platforms has focused on either the nano or micro length scale. Here, we review recent strategies that combine nano- and microscale materials and devices to produce large improvements in detection sensitivity, speed and accuracy, allowing previously undetectable biomarkers to be identified in clinical samples. Microsensors that incorporate nanoscale features can now rapidly detect disease-related nucleic acids expressed in patient samples. New microdevices that separate large clinical samples into nanocompartments allow precise quantitation of analytes, and microfluidic systems that utilize nanoscale binding events can detect rare cancer cells in the bloodstream more accurately than before. These advances will lead to faster and more reliable clinical diagnostic devices.
A facile method to coat silica surfaces with THPMP is introduced, forming a simultaneously protein resistant and bioconjugable surface. The coating is experimentally identified and its anti-fouling and bioconjugable characteristics are demonstrated.
We report the first successful preparation of polyelectrolyte brushes using an ATRP initiator that was covalently grafted to silica and mica substrates via an organophosphonic acid. Covalent attachment of the initiator to silica and mica and the subsequent synthesis of polyacrylic acid (PAA) and poly(sulfopropyl methacrylic acid) brushes by water mediated-ATRP were confirmed by ATR-FTIR, ellipsometry, AFM, and contact angle measurements. The initiator–substrate bond was robust and could resist a large range of pH in the absence of salt. Interactions between PAA brushes anchored to mica via the organophosphonic acid initiator were investigated using a surface forces apparatus. The results confirmed the robustness of the initiator–mica bond as the brushes could resist shearing and compression under relatively high applied loads.
Ligation-mediated PCR method is widely applied for detecting short-length DNA target. The primary principle of this method is based on the linkage of two separated DNA probes as PCR template via simultaneous hybridization with DNA target by DNA ligase. Even before taking into account of low ligation efficiency, a 1:1 stoichiometric ratio between DNA target and the produced PCR template would put an intrinsic limitation on the detection sensitivity. In order to solve this problem, we have developed an improved ligation-mediated PCR method. It is designed such that, a transcription reaction by T7 RNA polymerase is integrated into the ligation reaction. In this way, the produced joint DNA strand composed by two DNA probes can be used as a template both in the transcription reaction and the following PCR process. Then a great number of RNA strands containing the same sequence as DNA target, are transcribed to act as target to initiate new cyclic reactions of ligation and transcription. The results indicate that our proposed method can improve the detection sensitivity by ~2 orders of magnitude compared with the conventional ligation-mediated PCR method.
Self-assembly is an interesting process both for its biological relevance and because it provides a novel approach to complex structures having nanometer-scale dimensions. These structures are difficult or impossible to prepare by traditional methods. In this article, a general review on the use of self-assembled monolayers for chemical patterning is provided. In the first part, functional group transformation of SAMs on flat gold surfaces by chemical reactions is discussed in detail. The use of monolayer-protected gold nanoparticles as model systems for flat gold surfaces is covered with special focus on the transformation of functional groups on the outer SAM layer. Furthermore, techniques that have been used to pattern SAMs are discussed. Among these techniques, microcontact printing and scanning probe lithography are discussed in detail for both their advantages and limitations.
An integrated array of silicon field-effect transistor structures is used for electronic detection of label-free DNA. Measurements of the dc current–voltage characteristics of the transistors gives us access to reproducible detection of single- and double-stranded DNA, locally adsorbed on the surface of the device. We combine this approach with allele-specific polymerase chain reaction, to test for the 35delG mutation, a frequent mutation related to prelingual nonsyndromic deafness. © 2004 American Institute of Physics.
A critical evaluation of the possibilities and limitations of the label-free detection of deoxyribonucleic acid (DNA) hybridization by means of field-effect-based devices is discussed. A new DNA-detection method is introduced, which utilizes an ion-sensitive field-effect device as transducer. The upon the DNA hybridization induced redistribution of the ion concentration within the intermolecular spaces and/or the alteration of the ion sensitivity of the device is proposed as detection mechanism. The theoretical calculations predict a substantial change in the average ion concentration within the intermolecular spaces induced upon hybridization that are enough to obtain a detectable sensor signal.
We report the selective and real-time detection of label-free DNA using an electronic readout. Microfabricated silicon field-effect sensors were used to directly monitor the increase in surface charge when DNA hybridizes on the sensor surface. The electrostatic immobilization of probe DNA on a positively charged poly-l-lysine layer allows hybridization at low ionic strength where field-effect sensing is most sensitive. Nanomolar DNA concentrations can be detected within minutes, and a single base mismatch within 12-mer oligonucleotides can be distinguished by using a differential detection technique with two sensors in parallel. The sensors were fabricated by standard silicon microtechnology and show promise for future electronic DNA arrays and rapid characterization of nucleic acid samples. This approach demonstrates the most direct and simple translation of genetic information to microelectronics.
A new method is described to prepare strongly bonded, compact monolayer films of alkyl- or arylphosphonates on the native oxide surface of Si (SiO(2)/Si). This method is illustrated for octadecyl- and alpha-quarterthiophene-2-phosphonates. For both cases, AFM shows comprehensive coverage of the SiO(2)/Si surface. The thickness of the continuous film of 4TP/SiO(2)/Si was measured both by AFM and by X-ray reflectivity to be ca. 18 A. Direct gravimetric analysis shows surface coverage by alpha-quarterthiophene-2-phosphonate to be about 0.66 nmol/cm(2), which corresponds to molecular packing in the film close to that of crystalline alpha-quarterthiophene. Coverage by octadecylphosphonate was ca. 0.90 nmol/cm(2), corresponding to a cross-sectional area of about 18.5 A(2)/molecule, consistent with close-packed alkyl chains.
A novel concept for the integration of liquid phase charge sensors into microfluidic devices based on silicon-on-insulator (SOI) technology is reported. Utilizing standard silicon processing we fabricated basic microfluidic cross geometries comprising of 5-10-mm-long and 55-μm-wide channels of 3 μm depth by wet sacrificial etching of the buried oxide of an SOI substrate. To demonstrate the feasibility of fluid manipulation along the channel we performed electroosmotic pumping of a dye-labeled buffer solution. At selected positions along the channel we patterned the 205-nm thin top silicon layer into freely suspended, 10-μm wide bars bridging the channel. We demonstrate how these monolithically integrated bars work as thin-film resistors that sensitively probe changes of the surface potential via the field effect. In this way, a combination of electrokinetic manipulation and separation of charged analytes together with an on-chip electronic detection can provide a new basis for the label-free analysis of, for example, biomolecular species as envisaged in the concept of micrototal analysis systems (μTAS) or Lab-on-Chip (LOC).
Analysis of the shape of the curve of reflected x-ray intensity vs glancing angle in the region of total reflection provides a new method of studying certain structural properties of the mirror surface about 10 to several hundred angstroms deep. Dispersion theory, extended to treat any (small) number of stratified homogeneous media, is used as a basis of interpretation.Curves for evaporated copper on glass at room temperature are studied as an example. These curves may be explained by assuming that the copper (exposed to atmospheric air at room temperature) has completely oxidized about 150A deep. If oxidation is less deep, there probably exists some general reduction of density (e.g., porosity) and an electron density minimum just below an internal oxide seal. This seal, about 25A below the nominal surface plane, arrests further oxidation of more deeply-lying loose-packed copper crystallites.All measurements to date have been carried out under laboratory atmospheric conditions which do not allow satisfactory separation or control of the physical and chemical variables involved in the surface peculiarities. The method, under more controlled conditions of preparation and treatment of the surface, promises to be useful.
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PNA (peptide nucleic acid) is a DNA mimic with a pseudopeptide backbone composed of N-(2-aminoethyl)glycine units with the nucleobases attached to the glycine nitrogen via carbonyl methylene linkers. PNA was first described in 1991 and has since then attracted broad attention within the fields of bioorganic chemistry, medicinal chemistry, physical chemistry and molecular biology due to its chemical and physical properties, in particular with regard to efficient and sequence specific binding to both single stranded RNA and DNA as well as to double stranded DNA. The present review discusses the structural features that provide the DNA mimicking properties of PNA and gives an overview of structural backbone modifications of PNA.
The development of electrically addressable, label-free detectors for DNA and other biological macromolecules has the potential to impact basic biological research as well as screening in medical and bioterrorism applications. Here we report two-terminal silicon nanowire electronic devices that function as ultrasensitive and selective detectors of DNA. The surfaces of the silicon nanowire devices were modified with peptide nucleic acid receptors designed to recognize wild type versus the ΔF508 mutation site in the cystic fibrosis transmembrane receptor gene. Conductance measurements made while sequentially introducing wild type or mutant DNA samples exhibit a time-dependent conductance increase consistent with the PNA−DNA hybridization and enabled identification of fully complementary versus mismatched DNA samples. Concentration-dependent measurements show that detection can be carried out to at least the tens of femtomolar range. This nanowire-based approach represents a step forward for direct, label-free DNA detection with extreme sensitivity and good selectivity, and could provide a pathway to integrated, high-throughput, multiplexed DNA detection for genetic screening and biothreat detection.
The surface coverage and subsequent arginine-glycine-aspartic acid (RGD) derivatization of the Ti-6Al-4V surface which can be readily effected by silanization in amounts far higher on a surface-bound Ti phosphate interface that can be accomplished on the native oxide by standard methods were shown. Nonetheless, the inherent hydrolytic instability under physiological conditions of surface siloxanes ultimately limits their utility. Thus, a successful surface chemistry approach for creating stable osteoconductive surfaces on Ti and Ti-6Al-4V was found.
We describe a way to form multiple asymmetric layers of organic molecules that may be applicable to the fabrication of electrooptical switching elements and other second-order nonlinear optical devices. The molecular layers are formed sequentially and are held together by strong zirconium phosphate and zirconium phosphonate bonds. Such multilayer films have been constructed on both gold and oxidized silicon surfaces. The asymmetric orientation of the molecules in each layer is ensured by a three-step process, which is a modification of the two-step process that Mallouk and co-workers recently developed to deposit symmetric multilayers (J. Phys. Chem. 1988, 92, 2597-2601). First, an hydroxy-terminated surface is phosphorylated to give surface phosphate groups. Second, zirconium(IV) is bound to this phosphate surface. Finally, an organic molecule containing a phosphonic acid group at one end and a hydroxy group at the other is allowed to self-assemble into a monolayer on the zirconium(IV) surface. This results in a new hydroxy-terminated surface, and the sequence can then be repeated. We demonstrate this methodology with (11-hydroxyundecyl)phosphonic acid, a simple difunctional organic molecule. The individual surface reactions are followed by X-ray photoelectron spectroscopy, and the layer-by-layer growth is followed by ellipsometry. The extension of this methodology to organic molecules with large nonlinear optical coefficients is currently being pursued.
Self-assembled monolayers (SAMs) formed from nitrile-functionalized alkanethiols (AT), NC(CH2)16SH (NC-C16), on (111) gold and silver substrates were characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and contact angle measurements. The average chain tilt angles in NC-C16/Ag and NC-C16/Au were estimated to be 29.5° ± 5° and 42.5° ± 5° from the surface normal, respectively, while the data suggest lower ordering for NC-C16/Au. The −CN bonds were found to be predominantly oriented in the surface plane with a tilt angle of 65° ± 7° for both NC-C16/Au and NC-C16/Ag. Comparison with previous data on CH3-terminated SAMs reveals that substitution of weakly interacting CH3 groups by the CN entities results in an increase in the average tilt angles of the alkyl chains by 7.5° and 17.5° in AT/Au and AT/Ag, respectively. A strong electrostatic interaction between the polar nitrile groups is assumed to underlie the structural behavior by controlling a balance between the headgroup−substrate and interchain interactions. The near-parallel orientation of the nitrile groups to the surface in both of these SAMs can be explained on the basis of minimization of the unfavorable CN−CN dipole−dipole interactions.
Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold surfaces and form ordered, oriented monolayer films. The properties of the interfaces between the films and liquids are largely independent of chain length when n > 10; in particular, wetting is not directly influenced by the proximity of the underlying gold substrate. The specific interaction of gold with sulfur and other "soft" nucleophiles and its low reactivity toward most "hard" acids and bases make it possible to vary the structure of the terminal group, X, widely and thus permit the introduction of a great range of functional groups into a surface. Studies of the wettability of these monolayers, and of their composition using X-ray photoelectron spectroscopy (XPS), indicate that the monolayers are oriented with the tail group, X, exposed at the monolayer-air or monolayer-liquid interface. The adsorption of simple n-alkanethiols generates hydrophobic surfaces whose free energy (19 mJ/m2) is the lowest of any hydrocarbon surface studied to date. In contrast, alcohol and carboxylic acid terminated thiols generate hydrophilic surfaces that are wet by water. Measurement of contact angles is a useful tool for studying the structure and chemistry of the outermost few angstroms of a surface. This work used contact angles and optical ellipsometry to study the kinetics of adsorption of monolayer films and to examine the experimental conditions necessary for the formation of high-quality films. Monolayers of thiols on gold appear to be stable indefinitely at room temperature but their constituents desorb when heated to 80°C in hexadecane. Long-chain thiols form films that are thermally more stable than films formed from short-chain thiols.
Self-assembled monolayers (SAMs) provide ideal molecularly defined platforms to study reactions in two dimensions. The surface chemistry of SAMs can easily be controlled by the head group of the surfactant molecules, which self-assemble into perfectly ordered, crystalline monolayers. A wide range of organic transformations can then be carried out to change the surface chemistry. These reactions have important applications in biological microarray fabrication, such as DNA and peptide chips. In this Microreview we will give an overview of all the classes of reactions that have been performed on SAMs. We will discuss the fundamental difficulties related to synthesis on monolayers, focussing on issues like steric hindrance, characterization, determination of yields and possibilities for multistep syntheses. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2003)
The covalent attachment of small organic molecules and larger functional biomolecules such as DNA, enzymes, or other proteins on semiconductors is a new field of basic research with a pronounced interdisciplinary flavour. The ultimate goal of this endeavour is the creation of novel organic/inorganic heterostructures which can provide a direct link between the complex worlds of biology and digital electronics on a nanometer scale. The purpose of the present survey is to provide an overview over basic physics aspects, preparation methods, as well as possible applications of biofunctionalized semiconductors based on different material systems. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Highly sensitive, sequence-specific and label-free DNA sensors were demonstrated by monitoring the electronic conductance of silicon nanowires (SiNWs) with chemically bonded single-stranded (ss) DNA or peptide nucleic acid (PNA) probe molecules. For a 12-mer oligonucleotide, tens of pM of target ss-DNA in solution was recognized when the complementary DNA oligonucleotide probe was attached to the SiNW surfaces. In contrast, ss-DNA samples of 1000 concentration with a single-base mismatch produce only a weak signal due to nonspecific binding. In order to gain a physical understanding of the change in conductance of the SiNWs with the attachment of the DNA targets and the probes, process and device simulations of the two-dimensional cross sections of the SiNWs were performed. The simulations explained the width dependence of the SiNW conductance and provided understanding to improve the sensor performance.
A physical model is presented which quantitatively describes the threshold voltage instability, commonly known as drift, in n-channel Si3N4-gate and as well as Al2O3-gate pH ISFETs. The origin of the so-called drift is postulated to be associated with the relatively slow chemical modification of the gate insulator surface as a result of exposure to the electrolyte. The chemical modification of the surface is assumed to result from a transport-limited reaction whose rate is modeled by a hopping and/or trap-limited transport mechanism known as dispersive transport. The change in the chemical composition of the insulator surface leads to a decrease in the overall insulator capacitance with time, which gives rise to a monotonic temporal increase in the threshold voltage.
Electronic DNA sensors based on field-effect transistor arrays operating in liquid environment, offer an alternative method for the detection of biomolecular binding events, without the requirement to label the target molecules. These semiconductor devices are sensitive to electrical charge variations that occur at the surface/electrolyte interface. Using such devices, the hybridization reaction of oligonucleotides with complementary single-stranded oligonucleotides, which are immobilized on the oxide surface of the transistor gate, can be detected. The detection principle is based on the intrinsic: charge of the nucleic acid molecules in liquid environment. In this article we present a summary of the 'DNA BioFET' project, which has been conducted in our group in recent years. We compare an electrostatic and a covalent immobilization protocol for the probe DNA. Most important for reliable signal readout is the use of a differential approach, for which a DNA microarray represents an ideal bioassay. With our FET setup we can either use a chip-to-chip reference with two electrically identical chips, which are coated with different probe sequences. A more sophisticated method, which makes use of the DNA microarray technology, was established with an aligned microspotting system enabling the coating of different channels out of the microarray with different probe sequences. So far our current chip design uses 16 channels enabling first proof-of-principle experiments. The detection method itself, however, offers the possibility to be up scaled to many thousands of sensor spots like currently used in fluorescence based DNA microarray chips. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
In this paper we present a concept for a microfluidic chamber optimized for x-ray reflectivity studies at solid-liquid interfaces. Experiments of this kind are usually considerably limited by strong beam attenuation due to interactions with the aqueous environment. First experiments at synchrotron sources using supported model membranes showed that the microfluidic setup yields a very effective solution for minimizing background scattering and beam absorption, which are often accompanied by radiation damage of biological samples. Additionally, the setup is also well suited for the application of fluorescence microscopy. The application of these two different techniques on the same sample offers unique possibilities for complementary studies.
Although the importance of the polyelectrolyte character of DNA has been recognized for some time (Felsenfeld & Miles 1967), few of the implications have been explored, primarily because of a lag in translating the breakthroughs in polyelectrolyte theory of the last decade into a form that is well adapted to the analysis of the specialized problems of biophysical chemistry. Perhaps an analogous situation existed in the field of protein chemistry during the period after the formulation and confirmation of the Debye—Hückel theory of ionic solutions but before Scatchard's incorporation of the theory into his analysis of the binding properties of proteins. An achievement for polynucleotide solutions parallel to Scatchard's was recently presented by Record, Lohman, & de Haseth (1976) and further developed and reviewed by Record, Anderson & Lohman (1978).
The binding of a mixed-sequence pentadecamer PNA (peptide nucleic acid) containing all four nucleobases to the fully complementary as well as various singly mismatched RNA and DNA oligonucleotides has been systematically investigated using thermal denaturation and BIAcore surface-interaction techniques. The rate constants for association (k(a)) and dissociation (k(d)) of the duplex formation as well as the thermal stability (melting temperature, T(m)) of the duplexes have been determined. Upon binding to PNA tethered via a biotin-linker to streptavidin at the dextran/gold surface, DNA and RNA sequences containing single mismatches at various positions in the center resulted in increased dissociation and decreased association rate constants. T(m) values for PNA x RNA duplexes are on average 4 degrees C higher than for PNA x DNA duplexes and follow quantitatively the same variation with mismatches as do the PNA x DNA duplexes. Also a faster k(a) and a slower k(d) are found for PNA x RNA duplexes compared to the PNA x DNA duplexes. An overall fair correlation between T(m), k(a), and k(d) is found for a series of PNA x DNA and PNA x RNA duplexes although the determination of k(a) seemed to be prone to artifacts of the method and was not considered capable of providing absolute values representing the association rate constant in bulk solution.
We describe the complementary use of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy to quantitatively characterize the immobilization of thiolated (dT)(25) single-stranded DNA (ssDNA) on gold. When electron attenuation effects are accurately accounted for in the XPS analysis, the relative coverage values obtained by the two methods are in excellent agreement, and the absolute coverage can be calculated on the basis of the XPS data. The evolution of chemically specific spectral signatures during immobilization indicates that at lower coverages much of the DNA lies flat on the surface, with a substantial fraction of the thymine bases chemisorbed. At higher immobilization densities, the (dT)(25) film consists of randomly coiled ssDNA molecules each anchored via the thiol group and at possibly one or two other bases. We use two examples to demonstrate how the quantitative analysis can be applied to practical problems: the effects of different buffer salts on the immobilization efficiency; the immobilization kinetics. Buffers with divalent salts dramatically increase the efficiency of immobilization and result in very high surface densities (>5 x 10(13)/ cm(2)), densities that may only be possible if the divalent counterions induce strong attractive intermolecular interactions. In contrast with previous reports of alkanethiol adsorption kinetics on gold, ssDNA immobilization in 1 M phosphate buffer does not occur with Langmuir kinetics, a result attributable to rearrangement within the film that follows the initial adsorption.
Low lysine levels: The authors present a device (see picture) based on silicon-on-insulator (SOI) substrates that enables the detection of poly-L-lysine at concentrations of only 1 nM (80 ng mL-1).
A direct comparison of surface loading, interface shear strength, and interface hydrolytic stability was made between a phosphonate and two siloxane monolayers formed on the native oxide surface of Ti-6Al-4V. Surface loading for the phosphonate was ca. four times greater (on a nanomole/area basis) than for the siloxanes; mechanical strengths per surface-bound molecule were comparable, but the hydrolytic stability (pH 7.5) of the siloxanes was poor. These results suggest that phosphonate monolayer interfaces are more desirable than comparable siloxane ones for applications where such interfaces contact even slightly alkaline water.
Four approaches have been explored for the preparation of maleimido-functionalized self-assembled monolayers (SAMs) on silicon. SAMs prepared by self-assembly of maleimido-functionalized alkyltrichlorosilanes (11-maleimido-undecyl-trichlorosilane) on oxide-covered silicon yield higher signals from maleimido functionalities in ATR-IR (attenuated total reflection IR) spectroscopy and XPS (X-ray photoelectron spectroscopy) than the other three methods. The surface composition of maleimido groups was tailored further by the formation of mixed monolayers with nonfunctionalized alkyltrichlorosilanes (decyltrichlorosilane). The order of the alkyl chains within the monolayers only slightly depends on the composition of the mixed monolayers. We utilized the maleimido-terminated SAMs to bind various nucleophilic compounds, alkylamines, alkylthiols, and thiol-tagged DNA oligonucleotides by means of conjugate addition.
Maleimido-terminated self-assembled monolayers were prepared via a one-step reaction of a maleimido-functionalized trichlorosilane with an oxide-covered surface. Through conjugate addition, thiol-tagged DNA was immobilized on these maleimido-functionalized self-assembled monolayers, and these immobilized oligonucleotides were further hybridized with complementary strands. The surface concentrations of oligonucleotides were quantified with radioactivity measurements. The surface concentrations of immobilized oligonucleotides increase in a nonlinear manner with increasing molar fraction of maleimido moieties on the surface and achieve high values of 1.6 (+/-0.6) x 10(13) oligonucleotides/cm2 if monolayers having full coverage of maleimido groups are used. High hybridization efficiencies of complementary strands (approximately 75%) are obtained even if the surface concentration of immobilized strands exceeds a value of 5.0 x 10(12) oligonucleotides/cm2. Furthermore, five cycles of denaturation and rehybridization result in only a 4% loss of the immobilized DNA after each run.
Supported lipid membranes constitute one of the most important model systems for cell membranes. The properties of lipid membranes supported by the hydrophobic solid polymer cyclic olefin copolymer (COC) were investigated. Lipid layers consisting of varying amounts of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP, cationic) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, neutral) prepared by vesicle fusion and solvent exchange were compared. All lipid mixtures coated the COC surface homogeneously forming a fluid membrane as verified by fluorescence microscopy and fluorescence recovery after photobleaching (FRAP). The exact structure of the supported membranes was determined by synchrotron reflectivity experiments using a microfluidic chamber. The X-ray data are in agreement with a compressed (head-to-head distance = 29 angstroms) and less densely packed bilayer.
We report the preparation of loosely packed hydroxyl-terminated self-assembled monolayers (SAMs) on gold by the adsorption of bis(11,11'-dithioundecyl)perfluoroheptanoate and base-mediated cleavage of the fluorocarbon terminal group. As shown through complementary characterization methods, the partially fluorinated SAM exhibits a structure in which the outer surface contains mostly -CF(3) groups, the fluorocarbon groups are slightly canted on average, and the hydrocarbon chains underneath are in a fluidlike state. Upon cleavage of the fluorocarbon group, the hydroxyl-terminated alkyl chains relax into an increasingly canted, fluidlike state. The resulting monolayer packing exposes both methylene and hydroxyl functionalities, yielding an intermediate surface energy (theta(a)(H(2)O) approximately 68 degrees ). As compared to a densely packed hydroxyl-terminated SAM prepared from bis(11-hydroxyundecyl)disulfide, the cleaved films are thinner because of the greater average chain cant and exhibit a approximately 50% higher capacitance and a factor of 5 lower charge-transfer resistance. The addition of THF to the electrolyte solution as a cosolvent intercalates into the loosely packed SAM to double the charge-transfer resistance and increase the capacitance by approximately 20% but does not affect the capacitance of the densely packed SAM. The loosely packed SAM is also more easily exchanged upon exposure to a solution of n-docosanethiol.
Here, we report the development of a peptide-nucleic acid (PNA)-modified ion-sensitive field-effect transistor (IS-FET)-based biosensor that takes advantage of the change in the surface potential upon hybridization of a negatively charged DNA. PNA was immobilized on a silicon nitride gate insulator by an addition reaction between a maleimide group introduced on the gate surface, the succinimide group of N-(6-maleimidocaproyloxy) succinimide, and the thiol group of the terminal cysteine in PNA. The surface was characterized after each step of the reaction by X-ray photoelectron spectroscopy analysis, and the kinetic analysis of the hybridization events was assessed by surface plasmon resonance. In addition, we measured the -potential before and after PNA-DNA hybridization in the presence of counterions to investigate the change in surface charge density at the surface-solution interface within the order of the Debye length. On the basis of the zeta-potential, the surface charge density, DeltaQ, calculated using the Grahame equation was approximately 4.0 x 10(-3) C/m2 and the estimated number of hybridized molecules was at least 1.7 x 10(11)/cm2. The I-V characteristics revealed that the PNA-DNA duplexes induce a positive shift in the threshold voltage, VT, and a decrease in the saturated drain current, ID. These results demonstrate that direct detection of DNA hybridization should be possible using a PNA-modified IS-FET-based biosensor. PNA is particularly advantageous for this system because it enables highly specific and selective binding at low ionic strength.
Arrays of highly ordered n-type silicon nanowires (SiNW) are fabricated using complementary metal-oxide semiconductor (CMOS) compatible technology, and their applications in biosensors are investigated. Peptide nucleic acid (PNA) capture probe-functionalized SiNW arrays show a concentration-dependent resistance change upon hybridization to complementary target DNA that is linear over a large dynamic range with a detection limit of 10 fM. As with other SiNW biosensing devices, the sensing mechanism can be understood in terms of the change in charge density at the SiNW surface after hybridization, the so-called "field effect". The SiNW array biosensor discriminates satisfactorily against mismatched target DNA. It is also able to monitor directly the DNA hybridization event in situ and in real time. The SiNW array biosensor described here is ultrasensitive, non-radioactive, and more importantly, label-free, and is of particular importance to the development of gene expression profiling tools and point-of-care applications.
Strobel for helpful discussions and experimental support. This work was financially supported by the DFG via SFB563 and the Nanosystems Initiative Munich, and by the Fujitsu Laborato-ries of Europe. M.T. gratefully acknowledges funding by the BMBF (Junior Research Group
- Acknowledgment Arinaga
- S Birner
- F Blobner
- D Dorfner
- A G Hansen
- E L Hanson
- U Rant
Acknowledgment. We are grateful to K. Buchholz and A. So-lovev for their earlier contributions and thank K. Arinaga, S. Birner, F. Blobner, D. Dorfner, A.G. Hansen, E.L. Hanson, U. Rant, and S. Strobel for helpful discussions and experimental support. This work was financially supported by the DFG via SFB563 and the Nanosystems Initiative Munich, and by the Fujitsu Laborato-ries of Europe. M.T. gratefully acknowledges funding by the BMBF (Junior Research Group " Nanotechnology ", grants 03N8713 and 03X5513). J.S. and M.D. thank the National Sci-ence Foundation for support of their research (CHE-0612572).
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