Chaotropic Monovalent Anion-Induced Rectification Inversion at Nanopipettes Modified by Polyimidazolium Brushes

Article abstract of DOI:10.1002/anie.201800335

A nonintuitive observation of monovalent anion-induced ion current rectification inversion at polyimidazolium brush (PimB)-modified nanopipettes is presented. The rectification inversion degree is strongly dependent on the concentration and species of monovalent anions. For chaotropic anions (for example, ClO₄^?), the rectification inversion is easily observed at a low concentration (5 mm), while there is no rectification inversion observed for kosmotropic anions (Cl^?) even at a high concentration (1 m). Moreover, at the specific concentration (for example, 10 mm), the variation of rectification ratio on the type of anions is ranged by Hofmeister series (Cl^?≥NO₃^?>BF₄^?>ClO₄^?>PF₆^?>Tf₂N^?). Estimation of the electrokinetic charge density (σ^ek) demonstrates that rectification inversion originates from the charge inversion owing to the over-adsorption of chaotropic monovalent anion. To qualitatively understand this phenomenon, a concentration-dependent adsorption mechanism is proposed.

Full text of DOI:10.1002/anie.201800335

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Accepted Article
Title: Chaotropic Monovalent Anion Induced Rectification Inversion at
Polyimidazolium Brushes Modified Nanopipettes
Authors: Ping Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201800335
Angew. Chem. 10.1002/ange.201800335
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Chaotropic Monovalent Anion Induced Rectification Inversion at
Polyimidazolium Brushes Modified Nanopipettes
Xiulan He, [a,b] Kailin Zhang, [a,b] Yang Liu, [a] Fei Wu, [a] Ping Yu,* [a,b] Lanqun Mao[a,b]
Abstract: Here we report a nonintuitive observation of monovalent
anion-induced ion current rectification inversion at polyimidazolium
brush (PimB) modified nanopipettes. The rectification inversion
shows great potential for constructing various ionic devices.5c
However, the influence of Hofmeister monovalent anion series
on man-made nanopores has not been investigated so far
although several papers have reported that it would strongly
influence the behavior of biochannel (e.g., α-hemolysin,
gramicidin) in lipid layer.6
degree is found to be strongly dependent on the concentration and
-
species of monovalent anions. For chaotropic anions (e.g., ClO
4
),
the rectification inversion is easily observed at a low concentration (5
mM), while there is no rectification inversion observed for
-
kosmotropic anions (e.g., Cl ) even at a high concentration (1 M).
0
.2
0.1
.0
1
.0
.5
KClO4
0.1 mM
1 mM
Moreover, at the specific concentration (e.g., 10 mM), the variation
of rectification ratio on the type of anions is ranged by Hofmeister
KCl
0.1 mM
A
BE / V
0
E / V
0.0
-0.5
8
0
-
-
-
-
-
-
series (Cl ≥ NO
3
> BF
4
> ClO
4
> PF
6
> Tf
2
N ). Estimation of the
-1
0
1
-
1
0
0
1
1
ek
-0.1
.4
electrokinetic charge density (σ ) demonstrates that rectification
inversion is originated from the charge inversion due to the over-
adsorption of chaotropic monovalent anion. To qualitatively
understand this phenomenon, the concentration-dependent
adsorption mechanism is proposed and discussed. This study shows
the first example on rectification inversion induced by monovalent
anions, which provides a new idea to construct ionic devices or
sensors by easily modulating the surface charge.
0
1
5
mM
mM
0.2
0.0
-0.2
5
4
E / V
E / V
0
-1
0
0
1
1
-
1
-
16
4
E / V
0
8
-
1
5
mM
E / V
-5
0
-1
0
0
1
1
-
-
10
5
2
4
6
8
0
E / V
10 mM
1
0
-
1
0
1
-
5
Nanoionics, dealing with ion transport in spatially confined
solutions, has attracted increasing scientific and technological
interest due to its broad applications in ionic and molecular
control, energy conversion, filtration and desalination, ionic
diodes and sensors.1 In this case, ion current measurements
across synthetic nanopores and/or nanochannels with surface
charge have revealed several counterintuitive ion transfer
phenomena, such as ion current rectification (ICR), negative
differential resistance, oscillation and hysteresis.2 Among them,
ICR behavior is the earliest reported and mostly studied one.
Rectification inversion is an unusual rectification behavior, by
which the direction of rectification is inverted with changing the
E / V
10 mM
10
-1
Figure 1. I-V curves recorded at the PvbimB modified nanopipette in KCl (A)
and KClO (B) solutions with different concentrations: 0.1 mM (black curve), 1
4
mM (red curve), 5 mM (blue curve), and 10 mM (magenta curve). The
diameter of tip was ca. 300 nm. Scan rate was 50 mV/s.
Herein, we study the influence of monovalent anions of the
Hofmeister series
on the rectification behavior at
polyimidazolium brush (PimB)-modified nanopipettes, and
interestingly find that rectification inversion could be induced by
3
concentration of counterions. So far, rectification behavior has
chaotropic monovalent anions (e.g., ClO -
). PimB-modified
4
been observed at various channel materials (e.g., nanopipette,
silicon, polymer), driving forces (e.g., electroosmotic force,
voltage, pressure), electrolytes (e.g., symmetric, asymmetric)
and solvents (e.g., aqueous solution, ionic liquid, organic
solution).4 Several studies have been conducted on the
relationship between the species of ions and rectification
nanopipettes were prepared by surface-initiated atom transfer
radical polymerization (Supporting Information S1) at their inner
surfaces, and the diameter of nanopipette used was ca. 300 nm.
We first recorded current-voltage curves of the poly(1-vinyl-3-
butylimidazolium) brush (PvbimB) modified nanopipettes in KCl
solutions with various concentrations. As shown in Figure 1A,
changing the KCl concentration does not reverse the current-
voltage curves recorded with the PvbimB-modified nanopipette,
even in a highly concentrated solution of KCl (e.g., 1 M) (See
supporting information S2, Figure S1), suggesting the
nanopipette remains anion-selective (i.e., positive rectification).7
Figure 1B shows current-voltage curves of the same PvbimB-
5
behavior. For example, Siwy et al. have reported that the type
of monovalent cations would influence the rectification degree.
More interestingly, they found that some of the multivalent
2
+
3+
cations (e.g., Ca and CoSep ) would induce the rectification
inversion at a negatively charged polymer pore channel, which
modified nanopipette as used in Figure 1A in KClO
4
solution with
[
a]
b]
Dr. X. He, K. Zhang, Y. Liu, Dr. F. Wu, Prof. P. Yu, Prof. L. Mao
Beijing National Laboratory for Molecular Sciences, Key Laboratory
of Analytical Chemistry for Living Biosystems
Institute of Chemistry, the Chinese Academy of Sciences (CAS),
Beijing 100190, China.
E-mail: yuping@iccas.ac.cn
Dr. X. He, K. Zhang, Prof. P. Yu, Prof. L. Mao
University of Chinese Academy of Sciences, Beijing 100049, China
various concentrations. In KClO solutions with concentrations of
4
0.1 mM and 1 mM (black and red curves), we observed the
same rectification behavior as those recorded in KCl solutions.
-
Interestingly, when the ClO
4
concentration was increased to 5
mM and 10 mM, we observed significant rectification inversion
(blue and magenta curves) as compared with the I - V curves
recorded in KCl solutions (Figure 1A), indicating the nanopipette
changed from anion selective (i.e., positive rectification) to
[
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Angewandte Chemie International Edition
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COMMUNICATION
cation selective (i.e., negative rectification). This result reveals
that rectification behavior shows strong concentration
dependence and that rectification inversion occurs in
only few papers observed the charge inversion phenomenon in
monovalent salt solution, in which the hydrophobic effect was
considered to play an important role for monovalent ion-induced
charge inversion in colloidal system.9
monovalent salt solution (e.g., KClO
reported so far.
4
), which have not been
We then studied the driving force for the charge inversion
observed by considering both the hydrophobicity of the anions
and the interface. First, we investigated the hydration enthalpies
by empirical equation (See supporting information S4). The
1
5
PvimB
5
0 nA
0 nA
-
B
A10
Cl
-
-
4
hydration enthalpies of Cl and ClO were estimated to be -387
PvbimB
PvoimB
and -263 KJ/mol, respectively. Moreover, the water contact
5
5
0
angle (WCA) of the PvbimB-modified borosilicate glass slices
-
-,
were determined to be 37 ± 1° and 75 ± 1° for Cl and ClO
4
50 nA
respectively (See supporting information S5, Figure S3). These
-
results demonstrate that ClO
4
bears stronger hydrophobicity
-
5
-
2
nA
than Cl . Second, we prepared nanopipettes modified with
polyimidazolium brush with different carbon chains (i.e., PvimB,
PvbimB and PvoimB, structures see Scheme S1), that is,
designing the interface with different hydrophilic/hydrophobic
properties. As shown in Figure 2B, there is no rectification
inversion for all the polyimidazolium brushes with different
carbon chains investigated here in 10 mM KCl solution (solid
-
PvimB
ClO4
-
-
-
-
10
15
20
25
1
0 nA
PvbimB
10 nA
PvoimB
1
10
100
1000
-1
0
1
C / mM
E / V
curves). For ClO -
, no rectification inversion occurred in the
4
Figure 2. (A) Plots of σek of the PvbimB modified SiNPs calculated based on
poly(vinylimidazolium) brush (PvimB) modified nanopipette
(black dashed). While, a significant rectification inversion was
observed in the poly(1-vinyl-3-octylimidazolium) brush (PvoimB)
modified nanopipettes (blue dashed). Moreover, the inversion
concentration at the PvoimB-modified nanopipette (i.e., 2.72 ±
-
-
eqn. 1 versus the concentration of Cl (black squares) and ClO
(
(
4
(red circles).
B) I-V curves recorded at the PvimB- (black), PvbimB- (red), and PvoimB-
blue) modified nanopipettes in 10 mM KCl (solid curves) and KClO (dashed
4
curves). The diameter of tip was ca. 300 nm. Scan rate was 50 mV/s.
1.81 mM) is lower than that at the PvbimB-modified nanopipette
(
i.e., 4.64 ± 3.15 mM). These results show that rectification
To investigate the origin of this observation, we estimated the
electrokinetic charge density (σ , the density of "active"
electrostatic charge) of PvbimB-modified silica nanoparticles
SiNPs) based on Zeta potential by the Grahame equation to
-
inversion in ClO
4
solution was more easily to be observed at
ek
more hydrophobic interface. These investigations indicated that
both the chaotropic/kosmotropic character of ions and the
hydrophilic/hydrophobic character of surfaces play important role
in the rectification inversion observed here.
(
indirectly demonstrate the surface charges of PvbimB-modified
8
a
nanopipettes:
푒푘
휎
= √8휀 휀 푐 푅푇 sinh(푧 퐹휁⁄2푅푇)
(1)
0
푟 0
where, c
0
is the bulk concentration of electrolyte. ε
r
and ε
0
are
the permittivity of vacuum and dielectric constant of water,
respectively. 휁 is the zeta potentials of the PvbimB-modified
SiNPs, which was obtained from Figure S2 The other symbols
have their standard meanings (for the values used for
ek
calculation, supporting information S3). σ accounts for both the
charges from the interface and any other sources such as
adsorbed ions. If there is no ionic adsorption on the interface,
the electrokinetic charge obtained is independent of electrolyte
concentration. Figure 2A shows the electrokinetic charge
changing with the electrolyte concentration. As shown, with
-
increasing the Cl concentration in solution (black squares), the
σek increases, demonstrating that the “active” charge on the
interface was increased, which was consistent with the previous
report.8a Moreover, the value of σ was always positive.
Figure 3. Schematic illustration of the concentration-dependent adsorption
mechanism.
ek
ek
-
Differently, the
concentration of ClO
concentration of ClO
interestingly, the value of σek changed from positive to negative,
σ
for ClO
4
initially increases when the
-
4
was low and then decreases when the
was further increased (red circles). More
Based on all the results described above and the
-
10
4
polyelectrolyte theory developed previously, we proposed a
concentration-dependent adsorption mechanism to understand
the occurrence of rectification inversion (Figure 3). For “strong”
polyelectrolyte (i.e., PimB), the concentration of counter anions
in the PimB brushes is low in the low concentration of electrolyte
and the polyelectrolyte chain is in the stretch state because of
indicating the occurrence of the charge inversion. With
increasing the concentration of ClO -
, the over-adsorption occurs,
4
resulting in the charge inversion. Although previous attempts
8
have reported the charge inversion induced by multivalent ions ,
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Angewandte Chemie International Edition
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COMMUNICATION
-
the electrostatic repulsion between the polyelectrolyte chain. As
a result, the charge of the polyelectrolyte brush interface
remains positive and the nanopipette thus exhibits the anion
2
Tf N ) was obtained, ruling out the effect of ion mobility on the
rectification inversion caused by ions in Hofmeister series
(Supporting Information S4). Although the mechanism
underlying Hofmeister series is still controversial, recent
investigation suggested that the interaction of monovalent ions
with interface was dominated by the chaotropic/kosmotropic
character of ions and the hydrophilic/hydrophobic character of
surfaces.8a The more chaotropic anion would induce the
stronger adsorption at the interface and further result in
decrease of positive charge density and consequently lead to
the decrease of the rectification ratio.
-
-
selective (i.e., positive rectification) for both Cl and ClO
4
(Figure
3). With increasing the concentration of electrolyte, the
concentration of counter anions in the PimB brushes increases,
and the high salt concentration leads to a strong screening of
electrostatic interactions between polyelectrolyte brush and thus
results in the shrink of PimB.10 Moreover, this high counter anion
concentration in polyelectrolyte brush would lead to the different
concentration distribution for the anions with different
-
hydrophobicity as shown in Figure 3. For Cl , it tends to dissolve
into the aqueous solution due to its relatively hydrophilic
A
B
-
-
-
20
Cl
property, resulting in the low and/or no adsorption of Cl on the
NO
3
kosmotropes
10
-
E / V
0.8
0.4
0.0
0.4
-
Cl
PimB interface. As a result, the ion distribution of Cl was in
0
-
Cl
-
accordance with the classic Gouy-Chapman-Stern model, and
the polymer brush was still anion-selective (i.e., positive
rectification). Consequently, there was no rectification inversion
observed in KCl solution (Figure 3, left panel). In contrast, the
20
10
0
BF4
-
E / V
E / V
E / V
NO3
-
NO
3
4
2
0
-
BF4-
BF
4
1
chaotropic property of ClO -
the hydrophobic interface. When the nanopipette was put into
makes it more easily to adsorb onto
-2
4
-
5
0
5
ClO
4
-
-
ClO4
-
PF6-
the higher concentration of ClO -
polymer brush allows the adsorption of ClO
, the higher concentration in
-10
4
-
-
E / V
E / V
ClO4
4
onto the
0
-
-
PF
6
Tf N
-
2
PF6
-0.8
-1.2
hydrophobic interface, which was evidenced from the different
inversion concentration at different interface (Figure 2B). This
adsorption further elicits the formation of hydrophobic PimB-
-5
0
10
20
chaotropes
-
-
1
0
-1
Tf N
-
-
2
Tf N
2
-
-30
10 mM
ClO
4
interface (Supporting Information S5, Figure S3B and S4),
-
which would be prone to adsorb more ClO
the over-adsorption of ClO -
4
and further lead to
4
and consequently the charge
Figure 4. (A) I-V curves recorded at the PvbimB-modified nanopipette in
different solutions of KCl, KNO NaBF KClO KPF and LiTf (the
concentration of electrolyte in each solution was 10 mM). The diameter of tip
was ca. 300 nm. Scan rate was 50 mV/s. (B) The dependence of lgRR (RR,
rectification ratio) on the kind of monovalent anions at 10 mM. The RR values
were calculated with the bias potentials of ±1 V.
inversion of the interface from positive to negative net charge.
This negative charge would attract cation due to electrostatic
interaction leading to the occurrence of rectification inversion
3
,
4
,
4
,
6
2
N
(
Figure 3, right panel).
To further investigate the relationship between the species of
anions and the rectification behavior, six kinds of monovalent
-
-, BF -
, ClO -
, PF -
-
anions (Cl , NO
3
4
4
6
, and Tf
2
N ) were studied.
It is worth to note that although the zeta potential results show
that charge inversion also occurred at the PvbimB-modified
Figure 4A shows the typical I - V curves obtained at the same
PvbimB-modified nanopipette in different solutions of
-
SiNPs (i.e., ca. 160 mM for ClO , See supporting information
4
monovalent anions with the same concentration (i.e., 10 mM).
S3), the concentration for rectification reversion occurrence in
-
-, BF -
-
All the investigated kosmotropic anions (e.g., Cl , NO
3
4
)
nanopipette was much lower (4.64 ± 3.15 mM for ClO ). More
4
show the normal I - V curves, while the chaotropic anions (e.g.,
interestingly, the charge inversion-induced rectification inversion
could also occur at the PvbimB-modified micropipettes
(Supporting Information S9, see Figure S8), and the order of
-, PF
-, and Tf
N ) show the clearly inverted I - V curves.
-
ClO
4
6
2
Moreover, at all the investigated concentrations, there is no
rectification inversion observed for NO - and BF -
(Supporting
3
4
inversed concentration in ClO - was ca. 160 mM (PvbimB-
4
Information S6, Figure S5A and 5B), which was similar with that
modified SiNPs)
micropipettes)
>
8.57
±
4.36 mM (PvbimB-modified
3.15 mM (PvbimB-modified
of Cl- (Figure 1A). But for PF -
and Tf
N , they exhibit the
-
6
2
>
4.64 ±
rectification inversion behavior even at the lower concentration
as compared with ClO -
(Supporting Information S6, Figure S5C
and S5D). More interestingly, as shown in Figure 4B and Figure
S6 (Supporting Information S7), the values of rectification ratio
nanopipettes). We assumed that this difference possibly resulted
from the ion concentration polarization effect in the
4
1
1
polyelectrolyte brush modified nanopippette. Moreover, the
electroosmotic flow effect may also contribute this difference.
In conclusion, we have demonstrated that the rectification
inversion induced by chaotropic monovalent anions occurs at
the PimB modified nanopipettes. Moreover, we have observed
the Hofmeister anions series in solid nanochannel for the first
time. The chaotropic/kosmotropic character of ions and the
hydrophilic/hydrophobic character of surfaces play a dominating
role in the rectification inversion system. Our results provide
some insights into design rules for nanochannel-based ionic
-
-
(
RR) decrease from Cl to Tf
2
N , which shows a good correlation
-
with the anion position in the original Hofmeister series (Cl ≥
3 4 4 6 2
> BF > ClO > Tf N ). Since the recorded ion
> PF -
-
-
-
-
NO
current are also associated with ion mobility, we then invetigated
the I - V curves in a fixed background electrolyte (i.e., 100 mM)
-
-, BF -
containing 30 mM of monovalent anions (i.e., Cl , NO
3
4
,
-
ClO
4
-, PF
6
-, and Tf
2
N ) (Supporting Information S8). As shown in
Figure S7, a good correlation with the anion position in the
-
-
-
-
-
original Hofmeister series (Cl ≥ NO
3 4 4 6
> BF > ClO > PF >
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Angewandte Chemie International Edition
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COMMUNICATION
device and sensors: the surface charge would be greatly
influenced by the species and concentration electrolyte.
Moreover, the present results would offer new perspective to
understanding the ionic distribution near the charged interface in
aqueous electrolytes.
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2
1475138, 21775151, and 21790053 for P. Yu and 21790390,
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National Basic Research Program of China (2016YFA0200104),
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Keywords: Ionic transport • Rectification inversion • Hofmeister
series • Solid nanochannel • Polyelectrolyte brush
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1
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Angewandte Chemie International Edition
10.1002/anie.201800335
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Xiulan He, Kailin Zhang, Yang Liu, Fei
Wu, Ping Yu,* Lanqun Mao
ICR
Inversion
ICR
Page No. – Page No.
E / V
E / V
Chaotropic Monovalent Anion
Induced Rectification Inversion at
Polyimidazolium Brushes Modified
Nanopipettes
Hofmeister series
Cl-
≥
NO3-
>
BF4-
>
ClO4-
>
PF6- > Tf N
-
Kosmotropes
Chaotropes
2
The rectification inversion and the Hofmeister anions series were for the first time
observed in the polyimidazolium brush modified nanopipettes. The
chaotropic/kosmotropic character of ions and the hydrophilic/hydrophobic character
of surfaces are considered to play a dominating role in the rectification inversion
system.
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