Zn2+ and EDTA Cooperative Switchable Nanofluidic Diode Based on Asymmetric Modification of Single Nanochannel

Article abstract of DOI:10.1002/chem.201504616

Design and fabrication of smart switchable nanofluidic diodes remains a challenge in the life and materials sciences. Here, we present the first example of a novel Zn²⁺/EDTA switchable nanofluidic diode system based on the control of one-side o

Full text of DOI:10.1002/chem.201504616

DOI: 10.1002/chem.201504616
Communication
&
Biomimetic Synthesis
Zn2+ and EDTA Cooperative Switchable Nanofluidic Diode Based
on Asymmetric Modification of Single Nanochannel
Yao Sun+, Fan Zhang+, Zhongyue Sun, Miaomiao Song, Demei Tian, and Haibing Li*[a]
asymmetric multiresponsive nanochannel system could be
Abstract: Design and fabrication of smart switchable
easily realized by asymmetric chemical modification ap-
nanofluidic diodes remains a challenge in the life and ma-
proaches to functionalize diverse specific local areas with dif-
terials sciences. Here, we present the first example of
ferent functional molecules. Hence, the combination of the
a novel Zn2+/EDTA switchable nanofluidic diode system
symmetrically shaped nanochannel with asymmetric modifica-
based on the control of one-side of the modified hour-
tion could be realize to build a more complex system, which
glass-shaped nanochannel with salicylaldehyde Schiff base
moves one step farther toward the development of smart
(SASB). The nanofluidic diode can be turned on in the re-
nanochannel systems for real-world applications.[7] However,
sponse of Zn2+ and turned off in response to EDTA solu-
the present research towards asymmetric modification ap-
tion with good reversibility and recyclability.
proaches for building responsive nanochannel systems is still
in its early stages.
Zn2+, the second most abundant transition metal ion in the
The coherent regulation of ion channels in controlling ion flow
across the cell membrane is attached with great importance to
the various significant physiological functions in life process-
es.[1] Recently, by mimicking biological ion channels, synthetic
nanochannels have triggered interest because of their robust
mechanical and chemical properties.[2] Ion current rectification
(ICR), as the most promising feature of these artificial ion chan-
nels, is characterized by a nonlinear diode-like current–voltage
response.[3] By various chemical modifications, these nanoflu-
idic diodes can respond to external stimuli, such as specific
ions, pH, temperature and molecules, which result in distinct
changes in ICR.[4] However, functionalities of these artificial
nanochannels are mainly realized by modification the whole
inner surface of nanochannels, and how to endow them with
complex and multiple functions is a remaining challenging
task. The components of most biological nanochannels are
asymmetrically distributed between membrane surfaces to im-
plement complex biological functions.[5] Inspired by this natu-
ral asymmetrical design, Hou et al. utilized a symmetric hour-
glass shaped nanochannel (H-shaped) with asymmetric plasma
modification approaches to develop a responsive rectifier
nanochannel system.[6] Indeed, the symmetrically shaped nano-
channel system has several advantages. For example, the sym-
metric hourglass-shaped pore provides the nonhomogeneity
of the distribution of a chemical introduced from one side of
the pore along the pore centerline, once the system has
achieved steady-state. Furthermore, the development of an
human body after iron, is known to actively participate in bio-
logical processes such as regulation of enzymes, neural signal
transmission and modulation of ion channels.[8] For example,
the Cys loop ligand gated ion channel can be activated by zinc
binding induced conformational change of the channel pro-
tein.[9] Considering the roles of Zn2+ in living systems, building
biomimetic Zn2+-regulated ion channels in vitro will have
great applications in the field of biotechnology, like biosensors,
controlled nanofluids and drug delivery systems.[10] Salicylalde-
hyde Schiff bases (SASB) with N and O as donor atoms are
known to form strong complexes with Zn2+ and are widely
used as ionophores in sensors for Zn2+ [11]
Moreover, decom-
.
plexation of Zn2+/Schiff base complexes could be realized by
washing with EDTA solution.[12] Inspired by these results,
herein, we present the first example of a novel Zn2+/EDTA co-
operative switchable nanofluidic diode system based on an
asymmetric SASB-modified H-shaped nanochannel. By asym-
metric modification with SASB, we can realize the shift from
symmetric shape to the broken symmetry of the nanochannel,
and obtain an obvious ion rectification change. Furthermore,
the state of rectification of the SASB functional channel can be
controlled over the external stimulus (Zn2+/EDTA). The results
show that the rectification of nanofluidic diode can be turned
on in response to Zn2+ and turned off in response to EDTA so-
lution (Figure 1 and Scheme 1). After several cycles there was
little decay in the rectification gating behavior, and this switch-
able nanofluidic diode shows excellent reproducibility and re-
versibility. Such a device could potentially spark further experi-
ment and theoretical efforts to explore the rectification of bio-
inspired intelligent nanomachines. To our knowledge, studies
on the rectification in a symmetric nanochannel to fabricate an
artificial switchable nanofluidic diode, which is the focus of our
current study, are rarely reported.
[a] Y. Sun,+ F. Zhang,+ Z. Sun, M. Song, D. Tian, Prof. H. Li
Department Key Laboratory of Pesticide and Chemical Biology Ministry of
Education, College of Chemistry, Central China Normal University
Wuhan, 430079 (P.R. China)
E-mail: lhbing@mail.ccnu.edu.cn
[+] These authors contributed equally to this work.
To produce the H-shaped nanochannel, etching of the poly-
ethylene terephthalate (PET, 12 mm thick) membrane contain-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504616.
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Communication
Figure 2. A) SASB was immobilized on one side of the H-shaped nanochan-
nel. B) The corresponding contact angles of the PET membrane before and
after modification. C) LSCM observation before and after modification on
one side of the nanochannel. D) Current–voltage (I–V) properties of the
single nanochannel before SASB (black) and after SASB (red) modification on
one side of the H-shaped nanochannel. Each data point is the average of
four measurements. Error bars are not shown in the I–V curves because they
are approximately equivalent to the width of the lines in the plot. These re-
sults showed that SASB was successfully immobilized on one side of the
inner surface of the H-shaped nanochannel.
Figure 1. By mimicking natural asymmetry nanochannels, an artificial asym-
metric nanochannel was fabricated by asymmetric modification.
I–V curves of symmetric modification with SASB on both sides
of the nanochannel exhibit a distinct behavior, which further
confirms the successful of one-side modification procedure
(Figure 2D vs. Figure S4 in the Supporting Information). More-
over, we examined the wettability (Figure 2B) before and after
modification. As expected, modification of the PET membrane
with SASB molecules leads to a remarkable change on the
wettability of the surface (from 72.18Æ3.08 to 51.18Æ1.28) cor-
responding to a change of the chemical composition. The suc-
cessful nanochannel surface modification with SASB was also
verified by laser scanning confocal microscopy (LSCM). A
strong fluorescent signal appeared after the modification and
the fluorescent signal showed the membrane thickness to be
about 6.0(Æ0.5) mm which agrees with its half of actual thick-
ness (Figure 2C). Also, X-ray photoelectron analysis shows
a new N peak at 406 eV (Table S2 in the Supporting Informa-
tion), which further confirmed that we had successfully immo-
bilized SASB on the inner surface of the H-shaped nanochan-
nel.
Scheme 1. A switchable nanofludic diode based on Zn2+/EDTA over a one-
side modified H-shaped nanochannel with SASB.
ing a single ion track in the center was performed from both
sides at room temperature. The large opening (base) was
about 420 nm wide, as observed by scanning electron micros-
copy (SEM, Figure S2 in the Supporting Information), and the
tip was calculated to be 12 nm by an electrochemical meth-
ods.[13] The interior surface of one side of the H-shaped nano-
channel was modified with SASB by a two-step coupling reac-
tion. During the modification process, the PET film was mount-
ed between the two halves of the cell. One half of the cell was
firstly tested with classical EDC/NHSS coupling reactions to
produce amine-reactive NHSS ester molecules. The other half
of the cell was filled with phosphate buffer of pH 10 as stop-
ping medium during the activation process (Figure S3 in the
Supporting Information).[14] Subsequently the succinimidyl in-
termediate was covalently coupled with SASB through a stable
amide linkage (Figure 2A). After modification, the PET film was
washed and treated with deionized water, overnight, before
further experiments.
After successful asymmetrical incorporation SASB as metal-
binding site, we investigated whether this modified H-shaped
nanochannel could respond to external metal ions. The experi-
ment was carried out with 1 mm solutions of different metal
ions including Mg2+, Ca2+, Cd2+, Ba2+ and Zn2+ in electrolyte
KCl/Tris-HCl (pH 6.8). When the bare nanochannel was exposed
to an electrolyte (pH 6.8) containing 1 mm of ions tested, the
ionic current stayed nearly unchanged (Figure S5 in the Sup-
porting Information). However, it is worth mentioning that
there was an obvious asymmetric ionic current change when
the functional H-shaped nanochannel was exposed to 1 mm
Zn2+ (Figure 3A). The diode-like behavior of this functional
nanochannel can be quantified by the rectification ratio (R).
The values were calculated as the ratio of the currents record-
ed at +2 V to the absolute values of the currents recorded at
À2 V; high rectification ratio (>2) was defined as the “on”
state of rectifier switch and low rectification ratio (around 1) as
The success of the surface one-side modification was evi-
denced by the corresponding current–voltage (I–V) curves
measured using 0.5m KCl/Tris-HCl (pH 6.8) in both half-cells. As
shown in Figure 2D, significantly different I–V characteristics of
the H-shaped nanochannel before and after SASB immobiliza-
tion were observed. This is attributed to the neutral SASB di-
minishing the negative charge of the nanochannel surface,
which resulted in the loss of current behavior. In contrast, the
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the “off” state. Compared with other metal ions, only Zn2+
turned the rectification switch “on” (Figure 3B). Hence, it is evi-
dent that the asymmetrically modified H-shaped nanochannel
displayed good responsiveness to Zn2+ ions. In general, the
ionic transport properties of the single polymeric nanochan-
nels are determined by three major factors: surface charge, ef-
fective pore size and wettability.[15] Any asymmetric distribution
of these three factors may bring about asymmetric ionic trans-
port properties. Herein, the “on” state of rectifier switch was re-
alized by Zn2+ specifically binding with SASB immobilized on
the inner walls of H-shaped nanochannel, which can both
change surface charge and wettability. These two factors con-
tribute greatly to the ionic conductance.[16] To provide further
proof of the success of Zn2+ binding characteristics of SASB,
XPS analysis was carried out. After soaking the SASB-modified
membrane in 1 mm zinc chloride solution for 1 h, the XPS
spectrum showed a new Zn(2p3/2) peak at 1034 eV (Table S3
in the Supporting Information). Previous studies have shown
that SASB analogues act as good Zn2+ binding ligands.[11]
Having achieved open rectifying behavior of the nanofluidic
diode, we further evaluate the effect of Zn2+ concentration on
rectification (from 0.01 to 5 mm) in an electrolyte (pH 6.8). Fig-
ure S6 in the Supporting Information shows that a remarkable
increase in the rectification could be observed with rising Zn2+
concentrations (ratio from 0.8 to 2.1). The maximum rectifica-
tion ratio was seen in the present of 1 mm (ratio=2.1). This
can be explained in terms of the fact that by increasing Zn2+
concentration the interaction between SASB and Zn2+ leads to
a continuous decrease in the negative charge on the single
side of the channel surface. In addition, we exploited the recti-
fication of symmetric modification in response to Zn2+, which
is another evidence for the success of modification on one
side (Figure S7 in the Supporting Information).
Figure 3. A) I–V curves of the SASB modification on a single side of the H-
shaped nanochannel in 0.5m KCl/Tris-HCl (pH 6.8) with the addition of 1 mm
Mg2+, Ca2+, Cd2+, Ba2+ or Zn2+. Each data point is the average of four
measurements. Error bars are not shown in the I–V curves because they are
approximately equivalent to the width of the lines in the plot. B) The rectifi-
cation change ratios are in the presence of different metal ions. Herein, the
rectification change ratio was defined as (RmetalÀRBlank)/RBlank. The rectification
of this functional material can be “turned on” in response to Zn2+
.
Encouraged by the above results, we set out to explore clo-
sure of the rectifying switch of this SASB functional nanofluidic
diode. We chose EDTA as the second external stimulus. Fig-
ure 4A shows the fabrication and operating principle of this
nanofluidic diode system. It is interesting that the rectification
behavior vanishes after the Zn2+-binding nanofluidic diode
was exposed to an EDTA solution for 20 min (Figure 4B). There-
fore, the “on” state of this rectification switch was successfully
realized by EDTA. When designing a rectifier switch, it is very
important to demonstrate its reproducible and reversible char-
acters. Therefore, the switchable behavior was investigated
with repeated measurements of rectification ratio between
Zn2+ and EDTA. As shown in Figure 4C, good switchable ability
of the rectification property of this functional nanochannel was
observed. Hence, the measurements confirmed that this nano-
fluidic diode system can be switched between the “on” state
and “off” state in response to Zn2+ and EDTA.
Since the wettability plays a crucial role in the rectifying be-
havior, we further studied the wettability switching behavior of
the SASB-modified H-shaped nanochannel upon alternate
treatment with Zn2+ and EDTA. Figure S8 in the Supporting In-
formation indicates a remarkable change of the wettability of
the surface (from 72.18Æ3.08 to 51.18Æ1.28), every cycle in the
experiments was completed within 20 min. The relationship
Figure 4. A) The fabrication and operating principle of the rectifier switch
single nanochannel system. B) I–V curves of the SASB modification on
a single side of the nanochannel in the presence of Zn2+/EDTA. C) Cycling
experiments of the rectification ratio behavior of the single nanochannel.
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ble molecular recognition mechanism among SASB, Zn2+ and
EDTA by analyzing the fluorescence spectra. Figure S9A in the
Supporting Information shows that there is an obvious change
after the addition of Zn2+, which is attributed to the formation
of the Zn2+/SASB complex. After washing with EDTA, the fluo-
rescence of SASB recovered to the former state, which sug-
gests the decomplexation of Zn2+/SASB. A cycling experiment
again indicates that in the present of EDTA, Zn2+ could revers-
ibly interact with SASB (Figure S9B).
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21372092, 21102051), Natural
Science Foundation of Hubei Province (2013CFA112) and Self-
determined research funds of CCNU from the colleges’ basic
research and operation of MOE (CCNU13F005).
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Keywords: biomimetic synthesis · nanochannels · nanofluidic
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Received: November 17, 2015
Published online on February 16, 2016
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