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Insights into the Role of Biomineralizing Peptide Surfactants on Making Nanoemulsion-Templated Silica Nanocapsules
Abstract
We recently developed a novel approach for making oil-core silica-shell nanocapsules using designed bifunctional peptides (also called biomineralizing peptide surfactants) having both surface activity and biomineralization activity. Using the bifunctional peptides, oil-in-water nanoemulsion templates can be readily prepared, followed by the silicification directed exclusively onto the oil droplet surfaces thus the formation of silica shell. To explore their roles in the synthesis of silica nanocapsules, two bifunctional peptides AM1 and SurSi were systematically studied and compared. The peptide AM1, which was designed as a stimuli-responsive surfactant, demonstrated a quick adsorption kinetics with a rapid decrease of the oil-water interfacial tension, thus resulting in the formation of nanoemulsions with a droplet size as small as 38 nm. Additionally, the nanoemulsions showed good stability over four weeks because of the formation of a histidine-Zn2+ interfacial network. In comparison, the peptide SurSi which was designed by modularizing an AM1-like surface-active module with a highly cationic biosilicification-active module was unable to effectively reduce the oil-water interfacial tension due to its high molecular charge at neutral pH. The slow adsorption resulted in the formation of less stable nanoemulsions with a larger size (60 nm) than that of AM1. Besides, both AM1 and SurSi were found able to induce biomimetic silica formation. SurSi produced well dispersed and uniform silica nanospheres in the bulk solution, while AM1 only generated irregular silica aggregates. Consequently, well-defined silica nanocapsules were synthesized using SurSi nanoemulsion templates, whereas silica aggregates instead of nanocapsules predominated when templating AM1 nanoemulsions. This finding indicated that the capability of peptide surfactants to form isolated silica nanospheres might play a role in the successful fabrication of silica nanocapsules. This fundamental study provides insights into the design of bifunctional peptides for making silica nanocapsules.
... Therefore, surfactants are required to increase their kinetic stability. Typically, there are two types of emulsions used for making silica nanocapules, that are, oil-in-water (O/W) [49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64] and water-in-oil (W/O) [65][66][67][68][69][70][71] emulsions ( Table 3). The formation of these types of emulsions is generally controlled by the volume fraction of the oil and water phases, the type and amount of surfactant(s) added to stabilize the emulsions as well as the conditions during emulsification such as temperature. ...
... In addition, the volume ratio of ethanol-to-water and the concentration of surfactant have also been demonstrated to play key roles in stabilizing TEOS droplets enabling the formation of hollow silica nanoparticles instead of solid silica nanoparticles ( Fig. 5B). Recently, biomolecules have attracted attention to facilitate silica nanocapsules formation by templating O/W nanoemulsions [60][61][62][63][64]. We developed an emulsion and biomimetic dual templating approach for making oil-core silicashell nanocapsules by using either rationally designed peptides ( Fig. 5C) [60] or proteins ( Fig. 5D) [63] to both stabilize emulsion droplets as well as catalyze silica formation preferentially at oil-water interfaces. ...
... Sol-gel reactions occur at the gas-liquid interface where gas bubbles Table 3 Representative examples of oil-in-water (O/W) and water-in-oil (W/O) emulsion droplet as a template for the synthesis of silica nanocapsules. Dispersed phase Continuous phase Surfactant Silica precursor Cargo Nanocapsule diameter [nm] Shell thickness [nm] Ref. [51] TEOS and n-heptane Water CTAB TEOS Gold nanoparticles 280 1 [52] Hexadecane Water Brij®97 OTMS and TMOS n/a 98 33 [53] Ethyl butyrate Water Tween®80 and lecithin OTMS and TMOS n/a 77 21 [54] Benzene Ionic liquid (BmimBF 4 ) Triton™X-100 TEOS n/a 95 n/a [55] Perfluoro-decalin Water Pluronic®F68 Prehydro-lyzed TMOS Perfluorode-calin 230-250 † n/a [56] Labrafac® WL 1349 Water Kolliphor™ELP APTES n/a 120 n/a [57] Miglyol®812 Water SurSi peptide TEOS Fipronil insecticide 140-288 † 8-44 ‡ [60][61][62] Miglyol®812 Water D4S2 protein TEOS n/a 249 26 [63] Soybean oil Water Lecithin and poly(lysine) TEOS Nile red and CdSe quantum dots ~200 † 5-20 ‡ [64] W/O emulsions Water Cyclohexane Triton™X-100 and n-hexanol (co-surfactant) ...
Silica nanocapsules have attracted significant interest due to their core-shell hierarchical structure. The core domain allows the encapsulation of various functional components such as drugs, fluorescent and magnetic nanoparticles for applications in drug delivery, imaging and sensing, and the silica shell with its unique properties including biocompatibility, chemical and physical stability, and surface-chemistry tailorability provides a protection layer for the encapsulated cargo. Therefore, significant effort has been directed to synthesize silica nanocapsules with engineered properties, including size, composition and surface functionality, for various applications. This review provides a comprehensive overview of emerging methods for the manufacture of silica nanocapsules, with a special emphasis on different interfacial engineering strategies. The review starts with an introduction of various manufacturing approaches of silica nanocapsules highlighting surface engineering of the core template nanomaterials (solid nanoparticles, liquid droplets, and gas bubbles) using chemicals or biomolecules which are able to direct nucleation and growth of silica at the boundary of two-phase interfaces (solid-liquid, liquid-liquid, and gas-liquid). Next, surface functionalization of silica nanocapsules is presented. Furthermore, strategies and challenges of encapsulating active molecules (pre-loading and post-loading approaches) in these capsular systems are critically discussed. Finally, applications of silica nanocapsules in controlled release, imaging, and theranostics are reviewed.
... The high positive charge of nanoemulsions is probably resulted from the positive charge of both AM-S and C 8 -AM at neutral pH, together with the association of Zn 2+ in the intermolecular networks at the interface. 23,24,46 As a rule of thumb, nanoemulsions with absolute ζ potentials above +30 mV are electrically stabilized. 47 Therefore, nanoemulsions prepared using a high concentration of surfactant peptides are considered to have strong electrical repulsion among droplets to enhance stability against coalescence. ...
... When the concentration is high enough (200 μM or above), the nanoemulsions prepared remain stable at both 4 and 25°C. 24,51 To further explore the histidine−metal ion interaction at the interface and thus the formation of an interfacial network, we added a small amount of a chelating agent (EDTA) to the peptide-stabilized nanoemulsions to disrupt metal−histidine binding. Figure 9 shows the stability of the nanoemulsions with and without EDTA over time. ...
Designed peptide surfactants offer a number of advanced properties over conventional petrochemical surfactants, including biocompatibility, sustainability, and tailorability of the chemical and physical properties through peptide design. Their biocompatibility and degradability make them attractive for various applications, particularly for food and pharmaceutical applications. In this work, two new peptide surfactants derived from an amphiphilic peptide surfactant (AM1) were designed (AM-S and C8-AM) to better understand links between structure, interfacial activity and emulsification. Based on AM1, which has an interfacial α-helical structure, AM-S and C8-AM were designed to have two modules, that is, the α-helical AM1 module and an additional hydrophobic moiety to provide for better anchoring at the oil–water interface. Both AM-S and C8-AM at low bulk concentration of 20 µM were able to adsorb rapidly at oil–water interface, and reduced interfacial tension to equilibrium values of 17.0 mN/m and 8.4 mN/m within 400 s, respectively. Their relatively quick adsorption kinetics allowed the formation of nanoemulsions with smaller droplet sizes and narrower size distribution. AM-S and C8-AM at 800 µM bulk concentration could make nanoemulsions of average diameter 180 nm and 147 nm, respectively, by simple sonication. With respect to the long-term stability, a minimum peptide concentration of 400 µM for AM-S and a lower concentration of 100 µM for C8-AM were demonstrated to effectively stabilize nanoemulsions over three weeks. Compared to AM1, the AM-S nanoemulsion retained its stimuli-responsive function triggered by metal ions, whereas the C8-AM nanoemulsions did not respond to the stimuli as efficiently as AM-S because of the strong anchoring ability of the hydrophobic C8 module. The two-module design of AM-S and C8-AM represents a new strategy in tuning the surface activity of peptide surfactants, offering useful information and guidance of future designs.
... We developed a novel and green biomimetic technology to form core-shell materials using dual-functional peptides [23][24][25][26][27][28][29]. These biomolecules have a unique two-module design, one module for surface activity to make stable droplets, and the other one for biosilisification activity. ...
Biomimetic nanomaterials have attracted tremendous research interest in the past decade. We recently developed biomimetic core-shell nanoparticles – silica nanocapsules, using a designer dual-functional peptide SurSi under room temperature, neutral pH and without use of any toxic reagents or chemicals. The SurSi peptide is designed capable of not only stabilizing nanoemulsions because of its excellent surface activity, but also inducing the formation of silica through biosilicification at an oil-water interface. However, it remains challenging to precisely control the peptide-induced nucleation and biosilicification specifically at the oil-water interface, thus forming oil-core silica-shell nanocapsules with uniform size and monodispersity. In this study, the fundamental mechanism of silica formation through a peptide catalyzed biosilicification was systematically investigated, so that the formation of oil-core silica-shell nanocapsules can be precisely controlled. The SurSi peptide induced hydrolysis and nucleation of biomineralized silica particles were monitored to study the biosilicification kinetics. Effects of pH, SurSi peptide concentration and pre-hydrolysis of silica precursors were also studied to optimize the formation of biomimetic silica nanocapsules. The fundamental understanding achieved through these systematic studies provides valuable insights for making core-shell nanoparticles via controlling nucleation and reaction at interfaces.
... Peptides as emulsifiers have attracted much attention due to their superior functionalities and chemical diversities, 9−12 which could be used to design emulsifiers with different applications. For example, Wibowo and co-workers 13,14 reported that AM1 (Ac-MKQLADS LHQLARQ VSRLEHA-CONH2) was designed to stabilize emulsion with high stability. Mondal et al. 15 demonstrated that SHR-FLLF (H 2 N-Phe-Aib-Leu-Ala-Aib-Leu-Phe-OH) with a rigid backbone could afford long-term emulsion stability, forming highly stable emulsions compared with traditional emulsifiers such as Tween 20 and SDS. ...
A lipo-dipeptide (C13-lysine-arginine, C13-KR) was designed as a potential emulsifier with good emulsifying properties under acidic condition. Compared with two traditional emulsifiers (whey protein isolate and Tween 80), C13-KR emulsion had the minimum mean size but the highest zeta potential (around +100 mV). Moreover, C13-KR emulsion showed better stability against environmental stresses, such as high salt concentrations and high temperature. The C13-KR particles had the fastest move rate around 400 Hz when it attained an equilibrium state. Furthermore, C13-KR emulsifier could sharply reduce the interfacial tension and had the lowest tension value at the oil/water interface. The interfacial tension of C13-KR emulsifier was only 3.6 mN/m (0.5% w/v). In conclusion, the lipo-dipeptide C13-KR could be considered as an emulsifier to produce emulsion under acidic condition.
... This strategy opens new perspectives for the design of targeting agents for example for tumor or cancer detection. An original method, based on the design of bifunctional peptides, to prepare the silica capsules has been proposed by Wibowo et al. [39,40]. First the authors have modified part of peptide surfactant for surface activity with a sequence for biosilicification. ...
Nano-emulsions known also as mini-emulsions, ultrafine emulsions or submicron emulsions are a specific kind of emulsions that have a sub-micrometer droplet size and a low polydispersity. Nano-emulsions, being kinetically stable systems, require energy input in order to be formed, either from mechanical devices or from the intrinsic physicochemical potential of the components. The properties of the nano-emulsions make them suitable for applications in various domains such as drug delivery, cosmetics, pesticides and in particular for the preparation of inorganic or hybrid nanostructured materials. This review discusses the recent progresses made in this latest field. We focus on inorganic or hybrid nanoparticles, nanocapsules, hollow spheres or composites prepared by combining the nano-emulsion technique and the sol–gel process. In that case nano-emulsions act as template. We also outline the most recent development, which consists in using the nano-emulsions as imprints to get hierarchical porous silica materials.
There has recently been a growing attention towards peptide and protein molecules as potential bioemulsifiers for the stabilization of foams and emulsions, thanks to their innate tendency towards interfacial adsorption. Additionally, peptides and proteins are biodegradable and biocompatible, making them less toxic if compared to traditional emulsifiers. This chapter provides a comprehensive overview of the different classes of peptide, protein and mixed protein–polysaccharide emulsifiers and discusses the emulsification mechanisms of these systems. In essence, peptide-mediated emulsification can occur either via traditional surfactant-like mechanism, where amphiphilic molecular peptide chains adsorb at the biphasic interface forming ‘spherical micelles’, or through peptide self-assembly into higher secondary structure (α-helices or β-sheets) with the formation of amphiphilic nanofibrous structures adsorbing at the interface. Moreover, peptides can self-assemble in the continuous aqueous phase forming nanofibrous network of viscous hydrogels that enhance system stability. On the other hand, emulsion stabilization by proteins is mainly achieved through either electrostatic repulsion or steric stabilization. The various characterization techniques for emulsification and interfacial stabilization will be visited throughout this chapter, focusing on structural, mesoscopic and macroscopic characterization of these systems.KeywordsEmulsionEmulgelEmulsifierSelf-assembled peptideProteinInterfacial self-assembly
Stimuli‐responsive peptides and proteins are an exciting class of smart biomaterials for various applications and have received significant attention over the past decades. A wide variety of stimuli such as temperature, pH, ions, enzymes, magnetic field, redox, etc., are explored. This article provides a review of five intensively studied types of stimuli‐responsive peptides and proteins, their design principles and applications, including temperature‐, pH‐, light‐, metal ion‐, and enzyme‐responsive with an emphasis on the key design concepts and switch function. Moreover, typical examples of their applications are discussed to provide a better understanding of the design concept and underlying methodology. This review will facilitate and inspire future innovation toward new peptide‐ and protein‐based materials and their diverse applications.
A protein corona forms around nanoparticles when they are intravenously injected into the bloodstream. The composition of the protein corona dictates the interactions between nanoparticles and the biological systems thus their immune evasion, blood circulation, and biodistribution. Here, we report for the first time the impact of nanoparticle stiffness on protein corona formation using a unique emulsion core silica shell nanocapsules library with a wide range of mechanical properties over four magnitudes (700 kPa to 10 GPa). The nanocapsules with different stiffness showed distinct proteomic fingerprints. The protein corona of the stiffest nanocapsules contained the highest amount of complement protein (Complement C3) and immunoglobulin proteins, which contributed to their high macrophage uptake, confirming the important role of nanocapsules stiffness in controlling the protein corona formation thus their in vitro and in vivo behaviors.
Emulsions are liquid-liquid dispersions with one liquid phase dispersed in the other liquid phase as small droplets. Nanoemulsions are nano-sized emulsions with sizes ranging from tens to hundreds of nanometers, and have great potential applications in pharmaceutics, foods and cosmetics due to their attractive properties, such as small sizes, high surface area per unit volume, improved dispersion of active hydrophobic components and enhanced absorption. The article provides an overview of nanoemulsions for drug delivery, starting with an introduction of emulsion types, nanoemulsion preparation and nanoemulsion stability. Surfactants play critical roles in producing and stabilizing nanoemulsions. Different types of surfactants are summarized including small molecule surfactants, particle surfactants, phospholipids, peptide and protein surfactants. Then the applications of nanoemulsions as nanomedicine in drug delivery are presented. Finally, clinical applications of nanoemulsions are discussed.
Iron oxide nanoparticles have been extensively studied for a wide variety of applications. However, there remains a challenge in developing hierarchical magnetic iron oxide nanoparticles as existing synthetic techniques require harsh, toxic chemical conditions and high temperatures or give poorly defined product with weak magnetic properties. In addition, drug loading is limited to post-loading methods such as chemical conjugation or surface adsorption that have poor loading efficiency and are prone to premature drug release. We report a facile biomimetic method for making iron oxide nanoparticle-loaded silica nanocapsules based on a bimodal catalytic peptide surfactant stabilized nanoemulsion template. Iron oxide nanoparticles can be preloaded into the oil phase of the nanoemulsion at tunable concentrations, and the excellent surface activity of the designed bimodal peptide in combination with sufficient electrostatic repulsion promotes the stability of the nanoemulsions. Biosilicification induced by the catalytic peptide module leads to the formation of silica shell nanocapsules containing a magnetic oil core. The bioinspired silica nanocapsules encapsulating iron oxide nanoparticles demonstrate the next-generation of magnetic nanostructures for drug delivery applications.
Silica materials are attractive protein supports for applications in catalysis, sensing, water purification, and protein therapy. However, current approaches for loading proteins in silica are limited by inefficient protein immobilization, unintended leaching, the requirement for large quantities of silica precipitants, and associated protein inactivation. Here, we report a highly efficient and robust strategy that directly synthesizes protein-embedded silica via a vault protein nanocapsule-templated biomimetic silicification route. The vaults serve as nucleation sites for the deposition and condensation of silica precursors to form vault/silica composites. Using vaults encapsulated with proteins as the templating agent enables direct embedding of proteins in silica composites with nearly 100% immobilization efficiency. The vaults also act as protective shelters to preserve high protein activity during silica condensation. With the dual advantages of vault templating and protection, the proteins immobilized via this strategy retained over 90% activity after 30 reuse cycles and exhibited high activity, minimal leaching, and improved stability, which would be valuable in a broad variety of medical, industrial, and environmental applications.
Background
The interest in peptide hydrogels of natural origin has dramatically increased given the potential applications in several fields -e.g. biomedicine and nanotechnology. Interestingly, despite the current knowledge on protein hydrolysates from food sources, which self-assemble and form gels, the extraction of single peptides that can form hydrogels from food products and/or their application in food and other areas remains poorly explored.
Scope and approach
This review provides a prospective analysis of the literature on the mechanism, production, toxicity and potential applications of food derived peptide hydrogels.
Key findings and conclusions
Food products can be an important source of single peptides that form hydrogels, and these can find important applications not only in food science, and other areas. However, research in this area is in its infancy and its progress is limited due in part to the lack of 1) tools that will allow one to predict peptide fragments within a food protein that can self-assemble and form gels and 2) efficient peptide purification protocols. Therefore, more research will have to be directed on these areas in conjunction with optimization of recombinant, and enzymatic/fermentation production protocols.
Among various drug‐delivery systems, core‐shell nanoparticles have many advantages. Inspired by nature, biomimetic synthesis has emerged as a new strategy for making core‐shell nanoparticles in recent years. Biomimetic mineralization is the process by which living organisms produce minerals based on biomolecule templating that leads to the formation of hierarchically structured organic–inorganic materials. In this minireview, we mainly focus on the synthesis of core‐shell nanoparticle drug‐delivery systems by biomimetic mineralization. We review various biomimetic mineralization methods for fabricating core‐shell nanoparticles including silica‐based, calcium‐based and other nanoparticles, and their applications in drug delivery. We also summarize strategies for drug loading in the biomolecule‐mineralized core‐shell NPs. Current challenges and future directions are also discussed.
Exploration of the bioinspired silicification of artificial scaffolds is crucial to understanding and engineering the hierarchically complex and elaborate three-dimensional (3D) frustules of diatoms, which have high porosity and mechanical stability with related gas diffusion and storage properties. Herein, we report on the bioinspired silicification of the nanostructured surfaces of hexagonally close-packed silica bead (hc-SB) arrays using a liquid-phase deposition (LPD) method. This process, governed by the kinetics of silicification, was controlled using the concentration of the reactants and the reaction temperature, and monitored in real time using a quartz-crystal microbalance, which allowed the investigation of the silicification on the surface during the LPD reaction. These heterogeneous LPD reactions on hc-SB arrays were optimized to mimic natural 3D hierarchical structures. Anisotropic silicification of the nanostructures occurred owing to differences in the energy and local concentration of silicic acid on the nanostructured surface. A 3D hierarchical pore network was realized via a heterogeneous LPD reaction by controlling the size, location, and arrangement of the SBs. We believe that our silicification process on nanostructured surfaces can lead to great improvements in the bioinspired morphogenesis-based engineering of 3D hierarchical structures.
A peptidic system is described containing linear and branched peptides based on the RRIL motif that is found in silaffin proteins in diatoms. These peptides form structured assemblies, in which the resulting morphology is dictated by single‐residue changes in their primary sequences as well as by changes in stereochemistry. The resulting supramolecular assemblies serve as templates that can precipitate nanostructured silica in a biomimetic manner under mild reaction conditions. Small changes with big impacts. Linear and branched peptides with the RRIL form structured assemblies, with nano‐ and microscale morphology dictated by single‐residue changes in their primary structures. The peptide nanomaterials are used to produce templated silica in a biomimetic manner under mild reaction conditions.
Inspired by biosilicification of glass sponges, we designed a catalytic peptide, which formed silica structures in the imidazole-buffered solution. The peptide was adsorbed selectively onto the surface of yeast cells, and the bioinspired silicification led to the formation of a cytoprotective silica shell on individual yeast cells.
A novel, bio-inspired templating platform technology is reported for the synthesis of biocompatible oil-core silica-shell nanocapsules with tunable shell thickness by utilizing a designed bifunctional peptide. Furthermore, facile encapsulation of an active molecule and its sustained release are demonstrated.
The intricate, hierarchical, highly reproducible, and exquisite biosilica structures formed by diatoms have generated great interest to understand biosilicification processes in nature. This curiosity is driven by the quest of researchers to understand nature's complexity, which might enable reproducing these elegant natural diatomaceous structures in our laboratories via biomimetics, which is currently beyond the capabilities of material scientists. To this end, significant understanding of the biomolecules involved in biosilicification has been gained, wherein cationic peptides and proteins are found to play a key role in the formation of these exquisite structures. Although biochemical factors responsible for silica formation in diatoms have been studied for decades, the challenge to mimic biosilica structures similar to those synthesized by diatoms in their natural habitats has not hitherto been successful. This has led to an increasingly interesting debate that physico-chemical environment surrounding diatoms might play an additional critical role towards the control of diatom morphologies. The current study demonstrates this proof of concept by using cationic amino acids as catalyst/template/scaffold towards attaining diatom-like silica morphologies under biomimetic conditions in ionic liquids.
A series of low-charge peptides were designed to explore the functionality of different amino acids in precipitating precursor titanium(IV) bis(ammonium lactato)-dihydroxide (TiBALDH), and to obtain insight into the mechanism of TiO2biomineralization.
Nanoscale control of the polymerization of silicon and oxygen determines the structures and properties of a wide range of
siloxane-based materials, including glasses, ceramics, mesoporous molecular sieves and catalysts, elastomers, resins, insulators,
optical coatings, and photoluminescent polymers. In contrast to anthropogenic and geological syntheses of these materials
that require extremes of temperature, pressure, or pH, living systems produce a remarkable diversity of nanostructured silicates
at ambient temperatures and pressures and at near-neutral pH. We show here that the protein filaments and their constituent
subunits comprising the axial cores of silica spicules in a marine sponge chemically and spatially direct the polymerization
of silica and silicone polymer networks from the corresponding alkoxide substrates in vitro, under conditions in which such syntheses otherwise require either an acid or base catalyst. Homology of the principal protein
to the well known enzyme cathepsin L points to a possible reaction mechanism that is supported by recent site-directed mutagenesis
experiments. The catalytic activity of the “silicatein” (silica protein) molecule suggests new routes to the synthesis of silicon-based materials.
An amendment to this article, which had been requested by the author, was inadvertently omitted.
On page R635, the first sentence of the abstract should read: We summarize procedures for producing `nanoemulsions' comprised of nanoscale droplets, methods for controlling the droplet size distribution and composition, and interesting physical properties of nanoemulsions.
The online article has also been corrected.
The stunning silica structures formed by diatoms are among the most remarkable examples of biological nanofabrication. In recent years, insight into the molecules and mechanism that allow diatoms to perform silica morphogenesis under ambient conditions has been gained.
Histidine is an amino acid present in proteins involved in biosilica formation and often found in peptides identified during phage display studies but its role(s) and the extent of its involvement in the silica precipitation process is not fully understood. In this contribution we describe results from an in vitro silicification study conducted using poly-histidine (P-His) and a series of different molecular weight synthetic polymers containing the imidazole functionality (polyvinylimidazole, PVI) for comparison. We show that the presence of imidazole from PVI or P-His is able to catalyze silicic acid condensation; the effect being greater for P-His. The catalytic mechanism is proposed to involve the dual features of the imidazole group—its ability to form hydrogen bonds with silicic acid and electrostatic attraction toward oligomeric silicic acid species.
In order to understand the role that proteins play in the generation of well regulated biosilica structures we need to understand the contribution of the components, singly and in combination. To this end we have performed a systematic study of the effect of amino acids and small peptide oligomers on silica formation from aqueous solution. Silicas produced from a potassium silicon catecholate salt at ca. pH 7 in the presence of the amino acids (Gly, Arg, Asn, Gln, Glx, Ser, Thr, Tyr, Pro, Ala, Lys) at a 2 Si : 1 amino acid molar ratio have shown that these amino acids affect the kinetics of small oligomer formation, the growth of aggregate structures and the morphology and surface properties of the silicas produced. The effects seen during the early stages of oligomer formation carry through to the properties of the particles and aggregates produced after extended periods of reaction. The behaviour of the amino acids relates to the pI and hydrophobicity of the individual amino acids. The presence of the nitrogen containing amino acids generates larger particles and the presence of amino acids containing hydroxyl and hydrophobic groups generates silicas with smaller particles than are produced for silicas produced in the absence of amino acids. An extensive study of the effect of the number of lysine and glycine units per peptide was also performed (for lysine, 1–5 and ca. 150 and for glycine, 1,4,5). Increasing the number of glycine units per additive molecule had little effect on the kinetics, aggregation, sample morphology, surface area and porosity of the silicas produced. A distinct relationship between the number of lysine units per additive molecule and an increased rate of oligomer formation, aggregate growth and a reduction in silica surface area and broadening of pore sizes was observed. A distinct change over in behaviour, particularly in regard to the porosity characteristics of the silica produced was noted for between (lys) 3 and (lys) 4 as well as this being the smallest size of peptide that was incorporated into the siliceous material formed. Aggregation was observed to accelerate exponentially over the full range of lysine oligomers used. Consecutive sequences of the same amino acid residues were shown to produce effects much larger than the sum of the effects of the individual residues, and at extremes mediate macroscopic morphological changes. The consequences of these findings for biosilicification are discussed. It is clear that all amino acid functional groups in proteins that are accessible to silica during the stages of formation from orthosilicic acid through to the final material have a role to play in determining the physical nature and structure of the material that forms.
This tutorial review provides an outlook on nanomaterials that are currently being used for theranostic purposes, with a special focus on mesoporous silica nanoparticle (MSNP) based materials. MSNPs with large surface area and pore volume can serve as efficient carriers for various therapeutic agents. The functionalization of MSNPs with molecular, supramolecular or polymer moieties, provides the material with great versatility while performing drug delivery tasks, which makes the delivery process highly controllable. This emerging area at the interface of chemistry and the life sciences offers a broad palette of opportunities for researchers with interests ranging from sol-gel science, the fabrication of nanomaterials, supramolecular chemistry, controllable drug delivery and targeted theranostics in biology and medicine.
We report the structure and Young's modulus of switchable films formed by peptide self-assembly at the air–water interface. Peptide surfactant AM1 forms an interfacial film that can be switched, reversibly, from a high- to low-elasticity state, with rapid loss of emulsion and foam stability. Using neutron reflectometry, we find that the AM1 film comprises a thin (approx. 15 Å) layer of ordered peptide in both states, confirming that it is possible to drastically alter the mechanical properties of an interfacial ensemble without significantly altering its concentration or macromolecular organization. We also report the first experimentally determined Young's modulus of a peptide film self-assembled at the air–water interface (E=80 MPa for AM1, switching to E<20 MPa). These findings suggest a fundamental link between E and the macroscopic stability of peptide-containing foam. Finally, we report studies of a designed peptide surfactant, Lac21E, which we find forms a stronger switchable film than AM1 (E=335 MPa switching to E<4 MPa). In contrast to AM1, Lac21E switching is caused by peptide dissociation from the interface (i.e. by self-disassembly). This research confirms that small changes in molecular design can lead to similar macroscopic behaviour via surprisingly different mechanisms.
Earth's biota produces vast quantities of polymerized silica at ambient temperatures and pressures by mechanisms that are not understood. Silica spicules constitute 75% of the dry weight of the sponge Tethya aurantia, making this organism uniquely tractable for analyses of the proteins intimately associated with the biosilica. Each spicule contains a central protein filament, shown by x-ray diffraction to exhibit a highly regular, repeating structure. The protein filaments can be dissociated to yield three similar subunits, named silicatein alpha, beta, and gamma. The molecular weights and amino acid compositions of the three silicateins are similar, suggesting that they are members of a single protein family. The cDNA sequence of silicatein alpha, the most abundant of these subunits, reveals that this protein is highly similar to members of the cathepsin L and papain family of proteases. The cysteine at the active site in the proteases is replaced by serine in silicatein alpha, although the six cysteines that form disulfide bridges in the proteases are conserved. Silicatein alpha also contains unique tandem arrays of multiple hydroxyls. These structural features may help explain the mechanism of biosilicification and the recently discovered activity of the silicateins in promoting the condensation of silica and organically modified siloxane polymers (silicones) from the corresponding silicon alkoxides. They suggest the possibility of a dynamic role of the silicateins in silicification of the sponge spicule and offer the prospect of a new synthetic route to silica and siloxane polymers at low temperature and pressure and neutral pH.
Diatom cell walls are regarded as a paradigm for controlled production of nanostructured silica, but the mechanisms allowing
biosilicification to proceed at ambient temperature at high rates have remained enigmatic. A set of polycationic peptides
(called silaffins) isolated from diatom cell walls were shown to generate networks of silica nanospheres within seconds when
added to a solution of silicic acid. Silaffins contain covalently modified lysine-lysine elements. The first lysine bears
a polyamine consisting of 6 to 11 repeats of the N-methyl-propylamine unit. The second lysine was identified as ɛ-N,N-dimethyl- lysine. These modifications drastically influence the silica-precipitating activity of silaffins.
Diatoms are of interest to the materials research community because of their ability to create highly complex and intricate silica structures under physiological conditions: what these single-cell organisms accomplish so elegantly in nature requires extreme laboratory conditions to duplicate-this is true for even the simplest of structures. Following the identification of polycationic peptides from the diatom Cylindrotheca fusiformis, simple silica nanospheres can now be synthesized in vitro from silanes at nearly neutral pH and at ambient temperatures and pressures. Here we describe a method for creating a hybrid organic/inorganic ordered nanostructure of silica spheres through the incorporation of a polycationic peptide (derived from the C. fusiformis silaffin-1 protein) into a polymer hologram created by two-photon-induced photopolymerization. When these peptide nanopatterned holographic structures are exposed to a silicic acid, an ordered array of silica nanospheres is deposited onto the clear polymer substrate. These structures exhibit a nearly fifty-fold increase in diffraction efficiency over a comparable polymer hologram without silica. This approach, combining the ease of processability of an organic polymer with the improved mechanical and optical properties of an inorganic material, could be of practical use for the fabrication of photonic devices.
Designer peptides have recently been developed as building blocks for novel self-assembled materials with stimuli-responsive properties. To date, such materials have been based on self-assembly in bulk aqueous solution or at solid-fluid interfaces. We have designed a 21-residue peptide, AM1, as a stimuli-responsive surfactant that switches molecular architectures at a fluid-fluid interface in response to changes in bulk aqueous solution composition. In the presence of divalent zinc at neutral pH, the peptide forms a mechanically strong 'film state'. In the absence of metal ions or at acid pH, the peptide adsorbs to form a mobile 'detergent state'. The two interfacial states can be actively and reversibly switched. Switching between the two states by a change in pH or the addition of a chelating agent leads to rapid emulsion coalescence or foam collapse. This work introduces a new class of surfactants that offer an environmentally friendly approach to control the stability of interfaces in foams, emulsions and fluid-fluid interfaces more generally.
Synergistically combining the merits of silica (e.g., mechanical robustness, biocompatibility and great versatility in surface functionalization) and capsular configurations (e.g., a large inner cavity, low density and favourable colloidal properties), silica-based nanocapsules (SNCs) with a size cutoff of 100 nm have gained growing interest in encapsulating bioactive molecules for bioimaging and controlled delivery applications. Within this context, this review provides a comprehensive overview of the synthetic strategies, structural control and biomedical applications of SNCs. Special emphasis is placed on size control at the nanoscale and material composition manipulation of each strategy and the newly emerging synthetic strategies. The applications of SNCs in bioimaging/diagnosis and drug delivery/therapy and the structure engineering that is critically important for the bio-performance of SNCs are also addressed in this review.
Nanometre-sized CdS semiconductor particles were synthesized in the presence of sodium citrate, and subsequently surrounded by a homogeneous silica shell. The coating procedure makes use of 3-(mercaptopropyl) trimethoxy silane (MPS) as a surface primer to deposit a thin silica shell in water. The dispersion is then transferred into ethanol, where thicker shells can be grown. The citrate-stabilized particles are slowly degraded through photochemical oxidation in the presence of dissolved oxygen. This destabilizing process is suppressed when a homogeneous, microporous silica shell is built up around the particles, through a limited access of O2 molecules to the CdS surface.
Magnetic nanocapsules were synthesized for controlled drug release, magnetically assisted delivery, and MRI imaging. These magnetic nanocapsules, consisting of a stable iron nanocore and a mesoporous silica shell, were synthesized by controlled encapsulation of ellipsoidal hematite in silica, partial etching of the hematite core in acid, and reduction of the core by hydrogen. The iron core provided a high saturation magnetization and was stable against oxidation for at least 6 months in air and 1 month in aqueous solution. The hollow space between the iron core and mesoporous silica shell was used to load anticancer drug and a T1-weighted MRI contrast agent (Gd-DTPA). These multifunctional monodispersed magnetic "nanoeyes" were coated by multiple polyelectrolyte layers of biocompatible poly-l-lysine and sodium alginate to control the drug release as a function of pH. We studied pH-controlled release, magnetic hysteresis curves, and T1/T2 MRI contrast of the magnetic nanoeyes. They also served as MRI contrast agents with relaxivities of 8.6 mM(-1) s(-1) (r 1) and 285 mM(-1) s(-1) (r 2).
Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.
The remarkable progress of nanotechnology and its application in biomedicine have greatly expanded the ranges and types of biomaterials from traditional organic material-based nanoparticles (NPs) to inorganic biomaterials or organic/inorganic hybrid nanocomposites due to the unprecedented advantages of the engineered inorganic material-based NPs. Colloidal mesoporous silica NPs (MSNs), one of the most representative and well-established inorganic materials, have been promoted into biology and medicine, and shifted from extensive in vitro research towards preliminary in vivo assays in small-animal disease models. In this comprehensive review, the recent progresses in chemical design and engineering of MSNs-based biomaterials for in vivo biomedical applications has been detailed and overviewed. Due to the intrinsic structural characteristics of elaborately designed MSNs such as large surface area, high pore volume and easy chemical functionalization, they have been extensively investigated for therapeutic, diagnostic and theranostic (concurrent diagnosis and therapy) purposes, especially in oncology. Systematic in vivo bio-safety evaluations of MSNs have revealed the evidences that the in vivo bio-behaviors of MSNs are strongly related to their preparation prodecures, particle sizes, geometries, surface chemistries, dosing parameters and even administration routes. In vivo pharmacokinetics and pharmacodynamics further demonstrated the effectiveness of MSNs as the passively and/or actively targeted drug delivery systems (DDSs) for cancer chemotherapy. Especially, the advance of nano-synthetic chemistry enables the production of composite MSNs for advanced in vivo therapeutic purposes such as gene delivery, stimuli-responsive drug release, photothermal therapy, photodynamic therapy, ultrasound therapy, or anti-bacteria in tissue engineering, or as the contrast agents for biological and diagnostic imaging. Additionally, the critical issues and potential challenges related to the chemical design/synthesis of MSNs-based "magic bullet" by advanced nano-synthetic chemistry and in vivo evaluation have been discussed to highlight the issues scientists face in promoting the translation of MSNs-based DDSs into clinical trials.
The relaxation in surface tension due to the adsorption of bulk-soluble, unbranched, long chain surfactants with small polar groups at the air-water interface is often characterized by an initial induction period in which the surface tension relaxes very slowly. In this study, the origin of this induction in the surface tension relaxation is attributed to intermolecular cohesive forces among the adsorbed surfactant molecules which develop as the surface coverage increases. Surfactant molecules with long, slender hydrocarbon chains and small polar groups are subject to strong, attractive van der Waals forces when surface crowding causes interchain contact. Two models are constructed to account for this cohesion. In the first, intermolecular attraction leads to the formation of a liquid phase from a gaseous state. The induction period arises as the liquid state is forming, and addition of further molecules by diffusion is not accompanied by a change in the surface pressure. In the second model, the intermolecular attraction causes a cooperative adsorption as the activation energy for desorption increases faster with surface coverage than for adsorption. The induction period arises as the presence of cohesion lowers the surface pressure, offsetting the effect of the large increase in surface concentration due to the cooperative adsorption. Equations of state and adsorption isotherms necessary to describe this cooperative adsorption/phase transition behavior are developed, and theoretical solutions of the diffusion limited mass transfer to a fresh surface coupled with these isotherms are presented. Experimental verification of these ideas is obtained by studying the adsorption of aqueous solutions of 1-decanol at the air-water interface. Surface tension relaxation profiles for 1-decanol are obtained by using pendant bubble tensiometry enhanced by video digitization, and these profiles compare favorably with the numerical solutions obtained by using the developed models.
The formation process of silica nanoparticles by the addition of tetraethylorthosilicate (TEOS) to an buffered, aqueous solution of lysine, is discussed. Optically clear mixtures can be prepared from lysine, TEOS, and water to give a specific molar composition. Clear sols were prepared by initially mixing lysine with distilled water, followed by addition of the prescribed amounts of TEOS. Small-sngle X-ray scattering (SAXS) patterns were collected to characterize the lysine-silica mixtures and to quantatively detect nanoparticles. It was observed that the formation of nanoparticles is limited by the rate of TEOS hydrolysis. This formation phenomena opens possibilities for application of the nanoparticles as high purity reagents for synthesis of biocompatible silicates, nanocomposites, films, and gels.
A protein isolated from a biosilica (shown schematically) catalyzes alkoxysilane polycondensation at neutral pH values and low temperatures. Replacement of either of two specific side chain functionalities (Ser-26 and His-165) significantly diminishes catalysis, supporting a reaction mechanism analogous to that of a well-known enzyme that is highly homologous to the silica protein. These results may be useful in the development of synthetic catalysts for environmentally benign synthesis of polysiloxanes.
In this Review, recent achievements in the multilevel interior-structured hollow 0D and 1D micro/nanomaterials are presented and categorized. The 0D multilevel interior-structured micro/nanomaterials are classified into four main interior structural categories that include a macroporous structure, a core-in-hollow-shell structure, a multishell structure, and a multichamber structure. Correspondingly, 1D tubular micro/nanomaterials are of four analogous structures, which are a segmented structure, a wire-in-tube structure, a multiwalled structure, and a multichannel structure. Because of the small sizes and complex interior structures, some special synthetic strategies that are different from routine hollowing methods, are proposed to produce these interior structures. Compared with the same-sized solid or common hollow counterparts, these fantastic multilevel hollow-structured micro/nanomaterials show a good wealth of outstanding properties that enable them broad applications in catalysis, sensors, Li-ion batteries, microreactors, biomedicines, and many others.
Inorganic hollow spheres have attracted considerable interest due to their singular properties and wide range of potential applications. In this critical review, we provide a comprehensive overview of the preparation and applications of inorganic hollow spheres. We first discuss the syntheses of inorganic hollow spheres by use of polymers, inorganic nonmetals, metal-based hard templates, small-molecule emulsion, surfactant micelle-based soft-templates, and the template-free approach. For each method, a critical comment is given based on our knowledge and related research experience. We go on to discuss some important applications of inorganic hollow spheres in 0D, 2D, and 3D arrays. We conclude this review with some perspectives on the future research and development of inorganic hollow spheres (235 references).
Silica and silver nanoparticles are relevant materials for new applications in optics, medicine, and analytical chemistry. We have previously reported the synthesis of pH responsive, peptide-templated, chiral silver nanoparticles. The current report shows that peptide-stabilized nanoparticles can easily be coated with a silica shell by exploiting the ability of the peptide coating to hydrolyze silica precursors such as TEOS or TMOS. The resulting silica layer protects the nanoparticles from chemical etching, allows their inclusion in other materials, and renders them biocompatible. Using electron and atomic force microscopy, we show that the silica shell thickness and the particle aggregation can be controlled simply by the reaction time. Small-angle X ray scattering confirms the Ag/peptide@silica core-shell structure. UV-vis and circular dichroism spectroscopy prove the conservation of the silver nanoparticle chirality upon silicification. Biological tests show that the biocompatibility in simple bacterial systems is significantly improved once a silica layer is deposited on the silver particles.
Nanocapsules containing intentionally trapped magnetic nanoparticles and defined anticancer drugs have been prepared to provide a powerful magnetic vector under moderate gradient magnetic fields. These nanocapsules can penetrate into the interior of tumors and allow a controlled on-off switchable release of the drug cargo via remote RF field. This smart drug delivery system is compact as all the components can be self-contained in 80-150 nm capsules. In vitro as well as in vivo results indicate that these nanocapsules can be enriched near the mouse breast tumor and are effective in reducing tumor cell growth.
By modifying a well-studied peptide sequence, we have designed two biosurfactants with the ability to reversibly and precisely control the stability of foams. Foam stabilization occurs when the peptide forms a cohesive interfacial film cross-linked by metal ions, while foam destabilization occurs when peptide-metal binding is disrupted. The parent sequence is an amphipathic peptide that adsorbs at fluid interfaces, but forms neither cohesive interfacial films nor stable foams at the concentrations tested. Two modified peptide sequences were designed in which internal sites were substituted with metal-binding histidine residues. The first derivative, AM1, contains two histidines and can undergo intermolecular cross-linking by metal at the air-water interface. AM1 forms cohesive interfacial films and stable foams in the presence of Zn(II), Co(II), or Ni(II), but not in the absence of metal ions. The second derivative, AFD4, has four histidine substitutions, and can undergo both intra- and intermolecular cross-linking by metal ions. AFD4 forms stronger interfacial films and more stable foams than AM1 in the presence of the same metal ions, and also undergoes helical structuring in solution in the presence of added metal ions. For both peptides, film formation and foam stabilization can be reversed by acidification of the bulk solution, or addition of a metal chelator.
A direct and efficient synthesis route to form vesicles of mesoporous silica by a sonochemical method in a very short preparation time is outlined. This approach does not require any specialized structure-directing reagents other than the normally used cetyltrimethylammonium bromide (CTAB) as the surfactant similar to the original work to prepare mesoporous silicates.
Hollow silica and silica-polymer spheres with diameters between 720 and 1000 nanometers were fabricated by consecutively assembling
silica nanoparticles and polymer onto colloids and subsequently removing the templated colloid either by calcination or decomposition
upon exposure to solvents. Scanning and transmission electron microscopy images demonstrate that the wall thickness of the
hollow spheres can be readily controlled by varying the number of nanoparticle-polymer deposition cycles, and the size and
shape are determined by the morphology of the templating colloid. The hollow spheres produced are envisioned to have applications
in areas ranging from medicine to pharmaceutics to materials science.
A family of mesoporous molecular sieves (denoted MSU-G) with vesiclelike hierarchical structures and unprecedented thermal (1000 degreesC) and hydrothermal stabilities (more than 150 hours at 100 degreesC) associated with high SiO4 cross-linking was prepared through a supramolecular assembly pathway that relies on hydrogen bonding between electrically neutral gemini surfactants of the type CnH2n+1NH(CH2)2NH2 and silica precursors derived from tetraethylorthosilicate. The vesicle shells are constructed of one or more undulated silica sheets that are about 3 nanometers thick with mesopores (average diameters from 2.7 to 4.0 nanometers) running both parallel and orthogonal to the silica sheets, which makes the framework structure bicontinuous and highly accessible. Catalytic metal ion centers [for example, Ti(IV) and Al(III)] have been incorporated into the framework with the retention of hierarchical structure.
In biological systems such as diatoms and sponges, the formation of solid silica structures with precisely controlled morphologies is directed by proteins and polysaccharides and occurs in water at neutral pH and ambient temperature. Laboratory methods, in contrast, have to rely on extreme pH conditions and/or surfactants to induce the condensation of silica precursors into specific morphologies or patterned structures. This contrast in processing conditions and the growing demand for benign synthesis methods that minimize adverse environmental effects have spurred much interest in biomimetic approaches in materials science. The recent demonstration that silicatein-a protein found in the silica spicules of the sponge Tethya aurantia--can hydrolyse and condense the precursor molecule tetraethoxysilane to form silica structures with controlled shapes at ambient conditions seems particularly promising in this context. Here we describe synthetic cysteine-lysine block copolypeptides that mimic the properties of silicatein: the copolypeptides self-assemble into structured aggregates that hydrolyse tetraethoxysilane while simultaneously directing the formation of ordered silica morphologies. We find that oxidation of the cysteine sulphydryl groups, which is known to affect the assembly of the block copolypeptide, allows us to produce different structures: hard silica spheres and well-defined columns of amorphous silica are produced using the fully reduced and the oxidized forms of the copolymer, respectively.
A systematic experimental study of the effect of several factors on the mean drop diameter, d32, during emulsification, is performed with soybean oil-in-water emulsions. These factors are (1) type of used emulsifier; (2) emulsifier concentration, CS; and (3) ionic strength of the aqueous solution. Three different types of emulsifier, anionic (sodium dodecyl sulfate, SDS), nonionic (polyoxyethylene-20 cetyl ether, Brij 58), and protein (whey protein concentrate), are studied. For all of the studied systems, two well-defined regions are observed in the dependence of d32 on CS: at low surfactant concentration, d32 increases significantly with the decrease of CS (region 1), whereas d32 does not depend on CS at high surfactant concentration (region 2). The model, proposed by Tcholakova et al. (Langmuir 2003, 19, 5640), is found to describe well the dependence of d32 on CS in region 1 for the nonionic surfactant and for the protein emulsifier at high electrolyte concentration, 150 mM NaCl. According to this model, a well defined minimal surfactant adsorption (close to that of the dense adsorption monolayer) is needed for obtaining an emulsion. On the other hand, this model is found inapplicable to emulsions stabilized by the ionic surfactant, SDS, and by the nonionic surfactant, Brij 58, at low electrolyte concentration. The performed theoretical analysis of drop-drop interactions, in the emulsification equipment, shows that a strong electrostatic repulsion between the colliding drops impedes the drop-drop coalescence in the latter systems, so that smaller emulsion drops are obtained in comparison with the theoretically predicted ones. The results for SDS-stabilized emulsions in region 1 are explained by a quantitative consideration of this electrostatic repulsion. The drop size in region 2 (surfactant-rich regime) is described very well by the Kolmogorov-Hinze theory of turbulent emulsification.
Micellar nanoparticles made of surfactants and polymers have attracted wide attention in the materials and biomedical community for controlled drug delivery, molecular imaging, and sensing; however, their long-term stability remains a topic of intense study. Here we report a new class of robust, ultrafine silica core-shell nanoparticles formed from silica cross-linked, individual block copolymer micelles. Compared with pure polymeric micelles, the main advantage of the new core-shell nanoparticles is that they have significantly improved stability and do not break down during dilution. We also studied the drug loading and release properties of the silica cross-linked micellar particles, and we found that the new core-shell nanoparticles have a slower release rate which allows the entrapped molecules to be slowly released over a much longer period of time under the same experimental conditions. A range of functional groups can be easily incorporated through co-condensation with the silica matrix. The potential to deliver hydrophobic agents into cancer cells has been demonstrated. Because of their unique structures and properties, these novel core-shell nanoparticles could potentially provide a new nanomedicine platform for imaging, detection, and treatment, as well as novel colloidal particles and building blocks for mutlifunctional materials.
We present a theory for the kinetics of surfactant adsorption at the interface between an aqueous solution and another fluid (air, oil) phase. The model relies on a free-energy formulation. It describes both the diffusive transport of surfactant molecules from the bulk solution to the interface, and the kinetics taking place at the interface itself. When applied to non-ionic surfactant systems, the theory recovers results of previous models, justify their assumptions and predicts a diffusion-limited adsorption, in accord with experiments. For salt-free ionic surfactant solutions, electrostatic interactions are shown to drastically affect the kinetics. The adsorption in this case is predicted to be kinetically limited, and the theory accounts for unusual experimental results obtained recently for the dynamic surface tension of such systems. Addition of salt to an ionic surfactant solution leads to screening of the electrostatic interactions and to a diffusion-limited adsorption. In addition, the free-energy formulation offers a general method for relating the dynamic surface tension to surface coverage without relying on equilibrium relations.