A universal tunable nanofluidic diode via photoresponsive host–guest interactions

Abstract

Fluidics: controlling direction of flow
A nanoscale device that allows optical control of the flow of electrically charged liquids has been developed by Chinese researchers. Nanofluidics, the control of fluid flow in nanometer-wide channels, is useful for processing and analyzing small liquid samples such as blood. Many of the components required in nanofluidic circuits echo the functionality of electronic components. Diodes, for example, integral to every electronic circuit, allow flow in only one direction. Pei Liu from the Technical Institute of Physics and Chemistry, Beijing, and colleagues created a nanofluidic diode comprising a conical channel made from a polymer resin in which flow can be activated by light. Flow control is achieved by harnessing the optically induced reactions between different organic molecules embedded in the channel walls. The diode direction can be switched by altering the pH level in the device.

Full text

Liu et al. NPG Asia Materials (2018) 10: 849–857
DOI 10.1038/s41427-018-0079-5 NPG Asia Materials
ARTICLE Open Access
A universal tunable nanofluidic diode via
photoresponsive host–guest interactions
Pei Liu
1,2
,GanhuaXie
1
,PeiLi
3
, Zhen Zhang
1
,LinsenYang
1
,YuanyuanZhao
1
, Congcong Zhu
1,2
, Xiang-Yu Kong
1
,
Lei Jiang
1,2,3
and Liping Wen
1,2,3
Abstract
Inspired by biological ion channels, scientists have fabricated various artificial nanosystems. However, nanofluidic
diode systems with replaceable functional groups are rarely reported. In this work, we demonstrated a universal
tunable nanofluidic diode based on a conical polyimide nanochannel. Using host–guest interactions between β-
cyclodextrin and azobenzene, bidirectional nanofluidic diodes were prepared, and the degree of rectification could be
adjusted with high precision by tuning the pH conditions. This work provides a novel approach to create a nanofluidic
platform with replaceable surface functional groups, which has great potential in fields such as photosensitive
nanofluidic devices, drug transport and release, and nanofluidic logic devices.
Introduction
Biological ion channels
1–3
embedded within cell mem-
branes play a significant role in exchanging material,
transporting ions, and transforming energy with the
extracellular world.
4–6
Inspired by their genius designs and
outstanding properties, scientists have developed various
artificial nanosystems
7–10
that are capable of sensing or
separating a diverse variety of ions in aqueous solu-
tions.
11,12
Recent advances in smart nanofluidic devices in
chemistry, materials science, and nanotechnology have
gained increasing attention, and nanofluidic diodes, in
particular, have evolved quickly.
13–17
One of the main
characteristics of ion transport in these diode-like nano-
systems is the ion-rectifying effect.
13,14,17,18
Like diodes in
electronic devices, nanofluidic diodes exhibit ion uni-
directional transport behavior owing to several factors, the
main of which is the asymmetric distribution of surface
charges on the inner surface of nanochannels.
17,18
The
transport direction of ions can be changed by altering the
effective surface charge.
14,19–21
However, existing studies
on controlling the ion transport direction in a nanofluidic
diode mainly focus on regulating the functional groups,
which are normally fixed and non-replaceable, through
pH,
22–24
voltage,
15
light,
25,26
ions,
14,21,27
or other sti-
muli,
28,29
resulting in restrictions in changing the type of
nanofluidic system as needed. Therefore, fabricating a
universal tunable nanofluidic diode
6,30,31
is an interesting
and challenging task.
In this work, we demonstrated a pH-regulated bidirec-
tional nanofluidic diode via photoresponsive host–guest
interactions based on a conical polyimide (PI) nano-
channel. Among addressable nanodevices, light-
responsive nanochannels stand out owing to their
reversibility, short response times, and remote control and
can potentially be applied in various fields, such as optical
information storage and drug release.
32–34
In our case,
light is used as the stimulus to realize the functionaliza-
tion of the system. As one type of widely used photo-
responsive molecule, azobenzene (Azo) groups, change
into the trans conformation under irradiation with visible
light (vis light, 430 nm) and interact with β-cyclodextrins
driven by hydrophobic and van der Waals interactions,
© The Author(s) 2018
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Correspondence: Xiang-Yu Kong (kongxiangyu@mail.ipc.ac.cn)or
Liping Wen (wen@mail.ipc.ac.cn)
1
CAS Key Laboratory of Bioinspired Materials and Interfacial Science, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing
100190, China
2
School of Future Technology, University of Chinese Academy of Science,
Beijing 100049, China
Full list of author information is available at the end of the article.
These authors contributed equally: Pei Liu, Ganhua Xie
1234567890():,;
1234567890():,;
1234567890():,;
1234567890():,;
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whereas Azo groups return to the cis conformation under
irradiation with ultraviolet light (UV light, 365 nm),
35,36
resulting in a mismatch between the host and
guest.
30,31,37–40
Here, by immobilizing β-cyclodextrin (β-
CD) as the “host”and preparing various functionalized
Azo-end groups as “guests”, a universal tunable system
that returns to the original state under different light
stimuli can be realized (Scheme 1).
38
In addition, the pH-
responsive functional groups make it possible to regulate
the rectification property and perform bidirectional
rectification, which can lead to enhanced functionalities of
the artificial nanofluidic diode. In addition, the powerful
host–guest chemistry has been employed to regulate the
wetting behavior and conducting states of the nanopore
reversibly with light and electric fields by grafting the Azo
group onto the surface of the nanopore.
41
Based on the
reversible and adjustable host–guest interaction, nano-
fluidic diodes with different surface properties and states
can be achieved; thus, various functions can be integrated
into a single nanochannel.
Scheme 1 The universal tunable nanofluidic diode controlled via photoresponsive host–guest interactions. Under visible light irradiation,
azobenzene with different functional groups bind to the β-cyclodextrins on the channel surface and dissociate from the β-cyclodextrins under
ultraviolet light, which allows control of the functional groups and their locations in the nanochannel
Liu et al. NPG Asia Materials (2018) 10: 849–857 850
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Materials and methods
Materials
PI (GSI, Darmatadt, Germany, 12 µm thick) film,
potassium chloride (KCl), N-hydroxysulfosuccinimide
(NHSS), 1-ethyl−3-(3-dimethyllaminopropyl) carbodii-
mide, (EDC), potassium iodide (KI), sodium hypochlorite
(NaClO, 13%), 4-(phenylazo)benzoic acid (Azo-COOH),
and Azo were purchased from Sinopharm Chemical
Reagent Beijing Co., Ltd. (SCRC, China). Mono-6-amino-β-
cyclodextrin (EDA-β-CD) and ((phenyldiazenyl)phenoxy)
propan-1-amine (Azo-NH
2
) were synthesized according to
the reported protocol (Supporting Note 1 and Figure S1).
42
All solutions were prepared in MilliQ water (18.2 M Ω).
Fabrication of nanochannel-based nanofluidic diodes
The single conical PI nanochannel was prepared via an
asymmetric track-etched technique with a single-ion track
(Supporting Note 2 for details). In brief, the ion track PI
membrane was mounted between two chambers of a
homemade conductivity cell. The membrane was etched
using etching solution (NaClO, 13%) in one chamber and
stopping solution (1 M KI) in the other chamber at 60 ℃.
After the etching process, EDA-β-CDs were successively
immobilized on the nanochannel surface by a conven-
tional EDC/NHSS coupling reaction with the carboxyl
groups on the nanochannel surface. Prior to the reaction,
the carboxyl groups on the PI film were activated in 1 mL
of an aqueous solution containing 15 mg EDC and 3 mg
NHSS for 1 h at room temperature. Then, the PI film was
washed and immersed in 10 mM EDA-β-CD aqueous
solution overnight. Finally, the modified film was washed
several times with distilled water.
Characterization of the nanofluidic diode system
The asymmetric ion transport properties of the
nanofluidic system were evaluated by the current-
voltage (I–V) curves with a Keithley 6487 picoammeter
(Keithley Instruments, Cleveland, OH) in 0.1 M KCl. A
single channel in the center of the PI membrane was
mounted between the two chambers of the etching cell.
Ag/AgCl electrodes were used to apply a transmem-
brane potential. The main transmembrane potential
used in this work was a scanning voltage that varied
from –2V to +2 V. The anode faced the large opening
(base) of the nanochannel, and the cathode faced the
small opening (tip) of the nanochannel. All measure-
ments were carried out at room temperature. The
nanofluidic diodes were characterized by SEM, XPS, and
UV-vis spectroscopy measurements (Supporting Note 2,
Supporting Note 5, and Supporting Note 6 for details).
Asymmetric modification
The asymmetric modification setup is shown in Fig-
ure S9. By changing the types and the position of the Azo
solution, two asymmetric states, “T(+)-B(−)”and “T
(−)-B(+)”, were formed. The intensity of the light in the
center of the setup was 22 mW.
Theoretical simulation of ion transport
The ion rectification phenomenon was theoretically
investigated with the commercial finite-element software
package COMSOL (version 5.0) Multiphysics using the
“electrostatics (Poisson equation)”and “Nernst-Planck
without Electroneutrality”modules (Supporting Note 7
for details). Four states (positively charged nanochannel,
negatively charged nanochannel, T(+)-B(–) and T(−)-B
(+) were theoretically investigated using the above
method.
Results and discussion
The single conical nanochannel in this work was fab-
ricated through UV treatment and chemical etching on an
ion-tracked PI membrane.
43,44
The diameter of the
large opening (base side) of the conical nanochannel was
~ 400 nm, whereas the diameter of the narrow opening
(tip side) was calculated to be ~ 20 nm (Supporting Note 2
and Figure S2). Resulting from the asymmetric structure
of the nanochannel and the deprotonation of the -COOH
groups on the etched surface, the rectification of the
conical nanochannel was obtained by characterizing the
asymmetric I–Vcurve with a positive rectification degree
(pR, pR =log [|I
−2V
/I
+2V
|], where the base number is 10
and I
−2V
and I
+2V
represent the current at −2 V and +
2 V, respectively). Here, the rectifying direction with
positive pR was defined as the forward rectifying direc-
tion, and vice versa.
EDA-β-CDs, as intermediates, were grafted into the
nanochannel using a well-studied EDC-NHSS coupling
reaction. The ion transport property of the nanochannel
before and after modification with β-CDs was character-
ized by transmembrane ion current measurements in 0.1
M KCl solutions of different pH values. After modification
with β-CDs, owing to the reduction of effective negative
charges on the surface, the ion current across the channel
was reduced from −25.3 nA to −5.3 nA (pH 7), and the
corresponding pR was reduced from 1.18 to 0.89 (pH 7)
(Figure S3). The results of the current measurements
along with the X-ray photoelectron spectroscopy (XPS)
analysis revealed that β-CDs were modified onto the
surface of the nanochannel (Figure S4 and Figure S5).
Based on the β-CD-immobilized nanochannel,
(Azo-NH
2
, 10 mM) was induced through host–guest
interactions under visible light irradiation. A bidirectional
rectification behavior was observed by measuring the
transmembrane ion current in 0.1 M KCl solution of
different pH values. In acidic electrolytes (pH 2–3), a
backward rectification was observed, where the ion cur-
rents at positive voltages were higher than those at
Liu et al. NPG Asia Materials (2018) 10: 849–857 851
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negative voltages (Fig. 1a, left), whereas the system
changed into a forward rectification state and showed the
opposite phenomenon in alkaline solution (pH 7–11)
(Fig. 1a, right). This could be ascribed to changes in the
effective pore size, wettability, and surface charge induced
by the plugged Azo-NH
2.17,45,46
Owing to the protonation
of Azo-NH
2
and unreacted -COOH, pR was < 0 in solu-
tions with pH < 4. However, when the environmental pH
was above 7, pR was > 0 because of the deprotonation of
groups on the surface (Fig. 1b).
To further investigate how the pH affects the charge
status of the nanochannel as well as ion transport, I–V
curves were measured in 0.1 M KCl solution at a series of
asymmetric pH values, and the corresponding pR values
were calculated, which are summarized in Fig. 1c. The
results show that the pH of the electrolyte solution has a
significant effect on the rectification of the nanochannel.
The pR value can be divided into the following three
conditions: (a) approaching 0 (weak area: green in Fig. 1c),
showing almost no rectification; (b) > 0 (strong area with
forward rectification: red and orange in Fig. 1c), which
means |I
−2V
/I
+2V
| > 1; and (c) < 0 (strong area with
backward rectification: blue in Fig. 1c), which means |I
−2
V
/I
+2V
| < 1. When pH
Tip
(pH near the tip side of the
nanosystem) is in the range of 7–11 and pH
Base
(pH near
the base side of the nanosystem) is below 4, pR is located
inside the strong area with values above 2, and the cor-
responding rectification ratios are higher than 100. Under
such conditions, the functional groups on the tip side
(pH 7–11) are deprotonated, and the functional groups on
the base side (pH < 4) are protonated. The asymmetric
geometry along with the asymmetric charge distribution
of the channel worked simultaneously and thus achieved
very high rectification ratios. On the other hand, the pR
value did not vary much over a long and narrow area
(pH
Tip
< 3 and pH
Base
5~11) with values below −0.6,
which showed that the states on the basic side had a
relatively small impact on the whole system.
In addition, the nanofluidic diode shows good reversi-
bility owing to the photocontrolled host–guest interaction
between β-CD and Azo-NH
2
. After treating β-CD-
immobilized nanochannels with Azo-NH
2
under irradia-
tion with visible light, Azo groups change into a trans
conformation and bind with β-CDs, causing an increase in
the positive charges; thus, the ion current at positive
voltages ( +2 V) increases from 0.065 nA to 5.65 nA at pH
3. Then, when irradiated by UV light, the Azo groups
changed into a cis conformation and peeled off from
the pore surface, which was confirmed by UV-vis spec-
trophotometry (Figure S6), and the corresponding ion
current returned to ~ 0.065 nA (Fig. 2a). Moreover, the
light-gated nanofluidic diode exhibits good stability and
reversibility. Based on the β-CD molecules immobilized
on the channel surface, reversible light-responsive
switching between the two different ion conduction
states of the nanofluidic diode was achieved by irradiating
Fig. 1 Nanofluidic diodes functionalized with Azo-NH
2
exhibit bidirectional rectification behavior. a I–Vcurves of the single asymmetrical
nanochannel modified with Azo-NH
2
under different pH conditions, exhibiting bidirectional rectification depending on the pH: a backward
rectification is observed below pH 4, whereas a forward rectification is observed above pH 7. bSchematic illustration of the bidirectional rectification
behavior (blue circle: -COOH, red circle: Azo-NH
2
). The -COOH groups and modified -NH
2
groups on the side of the channel surface act as pH-
responsive functional groups, and their protonation or deprotonation results in a sensitive and reversible pH-responsive device. cThe contour plot of
the pR value at asymmetric pH values (pH
Tip
=2, 3, 5, 7, 9, 11; pH
Base
=2, 3, 5, 7, 9, 11)
Liu et al. NPG Asia Materials (2018) 10: 849–857 852
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visible and UV light (Fig. 2b), which can be expressed as
the pR switching between ~ −0.7 and ~ 0.1 for at least
five cycles at pH 3 (Fig. 2c). Evidently, the nanofluidic
diode possesses dual light and pH response and bidirec-
tional adjustment, and these processes are reversible and
recyclable owing to the light-responsive properties of Azo
and the protonation and deprotonation of the functional
groups on the surface of the channel under certain pH
conditions.
Furthermore, our universal tunable nanofluidic diode
system can be easily functionalized by other functional
molecules. Here, Azo-COOH (10 mM) was chosen to
adjust the rectification of the system. Similar to Azo-NH
2
,
Azo-COOH could also be attached to the pore surface
through interactions with β-CDs, and the corresponding
system exhibited dual light and pH response. After
introducing Azo-COOH, the ion current decreased from
−0.091 nA to −8.99 nA at −2 V, and the pR value
increased from −0.07 to 0.88 owing to the increase in
-COOH groups on the surface (Fig. 3a). After exposure to
UV light, the ion current at negative voltages was reduced
because the Azo groups returned to the cis conformation
and then dissociated from the β-CDs. In addition, the pR
value of this system switched between ~ 0.85 and ~ –0.15
for at least five cycles, and no obvious change was
observed in each state at pH 3 (Figure S7). This demon-
strates good reversibility and cycling performance of the
nanofluidic diode. Furthermore, the pH-responsive func-
tional groups could precisely regulate the rectification
property. With an increase in the pH of the electrolyte,
the pR value of the system increased steady from −0.86
(at pH 2) to 1.45 (at pH 3) to 1.79 (at pH 4) and finally
remained at ~ 1.8 (at pH 5, 7, 9) (Fig. 3b). The degree of
protonation on the surface increased with increasing pH,
resulting in the gradual addition of negative charges on
the channel surface and eventually to a saturated state. In
addition to Azo-COOH and Azo-NH
2
, Azo was employed
to functionalize the system as a control (Figure S8).
By controlling the distributions and density of available
charge carriers inside the nanochannels, a system with
flexible and tunable surface charges can be realized. The
locations of the functional groups (Azo) in the nano-
channel could be fine-tuned, and two asymmetric mod-
ified systems with light- and pH-responsive properties
were built (Fig. 4). To effectively construct a single
nanochannel with controlled effective charges on its
surface, Azo-NH
2
and Azo-COOH, two types of Azo
molecules with opposite charges in certain circumstances,
were separately grafted onto the tip side (or base side) and
the base side (or tip side) of the conical nanochannel
bridged by β-CDs (Fig. 4a, S9).
After Azo-NH
2
was bound to the tip side and Azo-
COOH to the base side through host–guest interactions
with β-CDs, the system presented in the “T(+)-B(−)”
state (left, Fig. 4a). In this state, the ion currents at positive
voltages were much higher than those at negative voltages
in 0.1 M KCl solution at pH 2 owing to protonation of the
-NH
2
groups on the tip side (Figure S10a). To investigate
the influence of pH on the above system, I–Vcurves in
0.1 M KCl solution at asymmetric pH values of the two
states were explored. It is worth noting that there are
three sections in the contour plot: (a) approaching 0
(weak area: green in Fig. 4b), showing almost no rectifi-
cation; (b) > 0 (strong area with forward rectification: red
Fig. 2 Reversible functionalization of the nanofluidic diode controlled by supramolecular photochemistry. a I–Vcurves of the nanochannels
with Azo-NH
2
at pH 3 under irradiation with visible (■) and UV (●) light. bSchematic illustration of the reversible light-responsive nanochannel.
Using β-CDs as intermediates, Azo-NH
2
could be bound to the surface of the channel under visible light and then released under UV light.
cReversible and recyclable switching of the pR value of the nanochannel at pH 3 between two different rectification states by the inclusion (shaded
column) and exclusion (unshaded column) of β-CD with Azo-NH
2
Liu et al. NPG Asia Materials (2018) 10: 849–857 853
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in Fig. 4b), which means |I
−2V
|>|I
+2V
|; and (c) < 0
(strong area with backward rectification: blue in Fig. 4b),
which means |I
−2V
|<|I
+2V
|. When pH
Tip
5 ~ 11 and
pH
Base
3 ~ 11, the system was located in a large strong
area with pR > 1.6, which means the relevant rectification
ratios were > 44. By introducing negative charges on the
base side, the system with strong forward rectification
extended over a long range compared with the system
functionalized with Azo-NH
2
(Fig. 1c). When pH
Tip
<3
and pH
Base
< 3, the system showed strong backward rec-
tification with pR < −0.43 and a minimum pR of −1.93.
In the “T(−)-B(+)”state (right, Fig. 4a), the ion current
at negative voltage decreased much after modification in
0.1 M KCl solution at pH 11 (Figure S10b), and there are
Fig. 3 Dual light and pH response of the Azo-COOH-modified nanochannel. a I–Vcurves of the nanochannel with Azo-COOH after irradiation
with visible light (■) and UV light (●)atpH3.bThe pR values of the Azo-COOH modified channel at different pH values (from 2 to 9) undergo three
main states; the value first increases in negative charge and then reaches a saturated state (blue circle: -COOH, black circle: Azo-COOH)
Fig. 4 Nanofluidic diodes with an asymmetric structure and chemistry. a Schematic illustration of the two states of the asymmetric modification:
in the “T(+)-B(−)”state, Azo-NH
2
was introduced on the tip side and Azo-COOH on the base side, and in the “T(−)-B(+)”state, Azo-COOH was
introduced on the tip side and Azo-NH
2
on the base side. bContour plot of the pR values of the “T(+)-B(−)”state at asymmetric pH values (pH
Tip
=2,
3, 5, 7, 9, 11; pH
Base
=2, 3, 5, 7, 9, 11). cContour plot of the pR values of the “T(−)-B(+)”state under asymmetric pH values (pH
Tip
=2, 3, 5, 7, 9, 11;
pH
Base
=2, 3, 5, 7, 9, 11)
Liu et al. NPG Asia Materials (2018) 10: 849–857 854
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two long and narrow zones with opposite rectification
directions in the contour plot in Fig. 4c. The forward (pR
> 0.85) and backward (pR < −0.17) rectification zones
were located in the range of (pH
Tip
5 ~ 11, pH
Base
< 3) and
(pH
Tip
<2, pH
Base
2 ~ 11), respectively. When pH
Tip
was
decreased from 11 to 2 and pH
Base
was increased from 2
to 11, the system gradually stepped from the forward
rectification area to the backward rectification area, which
means the pR transforms from positive to negative
(Fig. 4c). As shown in Fig. 4c, the distribution of pR is
unlike the situation in the “T(+)-B(−)”state (Fig. 4b),
indicating the capability of tuning the ion transport by
replacing the surface functional groups in the proposed
system. In addition, both nanofluidic systems in the “T
(+)-B(−)”and “T(−)-B(+)”states show good reversibility
and cycling performance (Figure S11).
To better understand the asymmetric geometry- and
charge-dependent rectification behavior, finite-element
simulations were performed to qualitatively describe the
mechanism based on coupled PNP (Poisson-Nernst-
Planck) equations.
47–50
The total length of all simulated
models was uniformly set to 2000 nm (Supporting
Note 7 and Figure S12). In Fig. 5a, the concentration
profile shows that ions, mainly anions (Cl
−
), accumulate
in the positively charged nanochannel under a positive
voltage bias. However, reversing the direction of the
applied electric field drives the anions (Cl
−
)outward
from the channel, causing the formation of a depletion
zone with a low concentration profile. In contrast, the
ions, mainly cations (K
+
), accumulate strongly at
negative voltage and deplete at positive voltage when the
entire nanochannel is negatively charged (Fig. 5b). By
introducing an asymmetric charge distribution, the
concentration profiles change intensively. For the posi-
tively charged tip and negatively charged base (Fig. 5c)
nanochannel, which is the “T(+)-B(−)”state in Fig. 4a,
the concentration profiles at +2V and −2V are
strengthened compared with those in Fig. 5a. The rea-
sons for the strengthened ion accumulation and deple-
tion in this asymmetric geometry- and charge-
dependent nanochannel can be divided into three
parts: (1) no external voltages were applied—the cations
(K
+
) were mainly in the negatively charged base side,
whereas the anions (Cl
−
)wereenrichedinthepositively
charged tip side; (2) positive voltages were applied (from
thebasesidetothetipside)—both anions and cations
were driven toward the junction, and after the system
reached a steady state, the ions in the channel were
enriched, causing a high ion current; and (3) negative
voltages were applied (from the tip side to the base side)
—both anions and cations migrate outward from the
border area under opposite electrical fields, leading to a
depletion of ions in the channel and causing a large
decrease in the ion current.
7,23
In contrast, a significant
increase in the concentration profile was observed at −2
V(Fig.5d) when the tip side was negatively charged and
thebasesidewaspositivelycharged(“T(−)-B(+)”state).
The simulated results agreed well with our experimental
measurements and provided visual ion concentration
distributions for analysis. The simulation shows that our
designed replaceable functional group mechanism for
rationally controlling the charge distribution can finely
tune the ion transport property.
Conclusion
In summary, we successfully fabricated a pH-regulated
bidirectional nanofluidic diode by employing photo-
responsive host–guest interactions based on a PI mem-
brane. Through host–guest interactions between β-CD and
Azo, nanofluidic diodes with different functional groups
Fig. 5 Numerical simulation of the nanofluidic diode with varied surface charge and the corresponding concentration profile of the ion
distribution in the nanochannel at +2 V and −2V.aPositively charged nanochannels result in ion (mainly anions) accumulation at +2 V and
depletion at −2V. bNegatively charged nanochannels result in ion (mainly cations) accumulation at +2 V and depletion at −2V. cIons deplete
strongly at −2 V and accumulate strongly at +2 V when the tip side is positively charged and the base side is negatively charged. dIons accumulate
strongly at −2 V and deplete strongly at +2 V when the tip side is negatively charged and the base side is positively charged
Liu et al. NPG Asia Materials (2018) 10: 849–857 855
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(Azo-COOH and Azo-NH
2
) could be prepared as needed,
and the degree of rectification could be adjusted with high
precision by tuning the pH. Furthermore, this work pre-
sents great potential in smart device establishment by
introducing host–guest chemistry into nanofluidics. The
dual light and pH response of the PI single nanofluidic
systems may allow potential applications in fields such as
photosensitive nanofluidic devices, light-controlled drug
transport,
51–53
pH-activated drug release and devices for
optical information storage. In addition, the reversible and
bidirectional switching characteristics allow further devel-
opment in nanofluidic logic devices.
54–57
Acknowledgements
We thank the Material Science Group of GSI (Darmstadt, Germany) for
providing the ion-irradiated samples. This work was supported by the National
Key R&D Program of China (2017YFA0206904, 2017YFA0206900), the National
Natural Science Foundation of China (21625303, 51673206, 21434003), and the
Key Research Program of the Chinese Academy of Sciences (QYZDY-SSW-
SLH014).
Author details
1
CAS Key Laboratory of Bioinspired Materials and Interfacial Science, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing
100190, China.
2
School of Future Technology, University of Chinese Academy
of Science, Beijing 100049, China.
3
Key Laboratory of Bio-Inspired Smart
Interfacial Science and Technology of Ministry of Education School of
Chemistry and Environment, Beihang University, Beijing 100190, China
Conflict of interest
The authors declare that they have no conflict of interest.
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
Supplementary information is available for this paper at https://doi.org/
10.1038/s41427-018-0079-5.
Received: 1 April 2018 Revised: 2 July 2018 Accepted: 3 July 2018.
Published online: 31 August 2018
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