Micrometer-scale ion current rectification at polyelectrolyte brush-modified micropipets

Article abstract of DOI:10.1021/jacs.6b11696

Here we report for the first time that ion current rectification (ICR) can be observed at the micrometer scale in symmetric electrolyte solution with polyimidazolium brush (PimB)-modified micropipets, which we call micrometer-scale ion current rectification (MICR). To qualitatively understand MICR, a three-layer model including a charged layer, an electrical double layer, and a bulk layer is proposed, which could also be extended to understanding ICR at the nanoscale. Based on this model, we propose that when charges in the charged layer are comparable with those in the bulk layer, ICR would occur regardless of whether the electrical double layers are overlapped. Finite element simulations based on the solution of Poisson and Nernst-Planck equations and in situ confocal laser scanning microscopy results qualitatively validate the experimental observations and the proposed three-layer model. Moreover, possible factors influencing MICR, including the length of PimB, electrolyte concentration, and the radius of the pipet, are investigated and discussed. This study successfully extends ICR to the micrometer scale and thus opens a new door to the development of ICR-based devices by taking advantage of ease-in-manipulation and designable surface chemistry of micropipets.

Full text of DOI:10.1021/jacs.6b11696

Communication
pubs.acs.org/JACS
Micrometer-Scale Ion Current Rectification at Polyelectrolyte Brush-
Modified Micropipets
Xiulan He, Kailin Zhang, Ting Li, Yanan Jiang, Ping Yu,* and Lanqun Mao†,‡
†
,‡
†
†,‡
†
,†,‡
†Beijing National Laboratory for Molecular Science, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of
Chemistry, the Chinese Academy of Sciences, Beijing 100190, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
*
S Supporting Information
21
flow. Shao et al. and Zhu et al. observed ICR at a micrometer
22,23
ABSTRACT: Here we report for the first time that ion
current rectification (ICR) can be observed at the
micrometer scale in symmetric electrolyte solution with
polyimidazolium brush (PimB)-modified micropipets,
which we call micrometer-scale ion current rectification
scale by using PEI-coated or biconical pipets.
To the best of
our knowledge, no efforts have been made on polyelectrolyte
brush-modified micropores to realize ICR, although polyelec-
trolyte brushes have been used to tailor the inner surface of
24−28
nanopores in literature.
(
MICR). To qualitatively understand MICR, a three-layer
Herein, we find that micrometer-scale ICR (MICR) can be
easily obtained at polyimidazolium brush (PimB)-modified
micropipets. PimB-modified micropipets were controllably
fabricated by surface-initiated atom transfer radical polymer-
ization (SI-ATRP, Supporting Information S1) at their inner
surfaces. As shown in Figure 1A, a standard linear current−
model including a charged layer, an electrical double layer,
and a bulk layer is proposed, which could also be extended
to understanding ICR at the nanoscale. Based on this
model, we propose that when charges in the charged layer
are comparable with those in the bulk layer, ICR would
occur regardless of whether the electrical double layers are
overlapped. Finite element simulations based on the
solution of Poisson and Nernst−Planck equations and in
situ confocal laser scanning microscopy results qualitatively
validate the experimental observations and the proposed
three-layer model. Moreover, possible factors influencing
MICR, including the length of PimB, electrolyte
concentration, and the radius of the pipet, are investigated
and discussed. This study successfully extends ICR to the
micrometer scale and thus opens a new door to the
development of ICR-based devices by taking advantage of
ease-in-manipulation and designable surface chemistry of
micropipets.
Figure 1. (A) I−V curves obtained at a bare micropipet (black curve)
and PimB-modified pipet (red curve) in 10 mM KCl solution. The
voltage sweep was in the direction from −1 V to +1 V. Scan rate, 50
mV/s. (B) SEM image of a PimB-modified micropipet. Scale bar, 5
μm.
on current rectification (ICR) is a physical phenomenon by
I
which ion current in one direction is greater than that in the
other, which can be attributed to uneven transport of anions
voltage curve consistent with previous reports was obtained at
bare pipets (i.e., without PimB modification) of 5 μm in radius
(black curve) in 10 mM KCl solution, because the pipet radius
1
−3
and cations across a biological or nanostructured channel.
ICR by solid-state nanopores and nanochannels has recently
attracted much attention because of its potential applications in
fluidic logic circuits (i.e., iontronics),
biosensors.
nanosystems,
pores, conical glass pores,
protein channels. However, ICR has so far been observed
mostly at nanometer scales, and it is difficult to achieve when
the pore diameter is 10 times larger than the Debye length.
was much larger than the thickness of the electrical double layer
4
−6
7
11,29
nanoionics, and
(ca. 3 nm).
Interestingly, a typical nonlinear current−
8
−10
Hence, ICR has been investigated in various
voltage curve (that is, the current in positive potential window
was far greater than that in negative potential window) was
observed in 10 mM KCl solution after the micropipet was
modified with PimB (red curve). The scanning electron
microscopy (SEM) image shows the orifice of a PimB-modified
pipet with a radius of ca. 5 μm (Figure 1B), essentially
demonstrating that inner modification with PimB did not
change the pipet diameter. In other words, MICR occurred at
the PimB-modified micropipet even though the thickness of the
1
−19
11,12
including nanopipets,
conical polymer
3
,4
13−16
17
SiN nanochannels, and
1
,8
20
There are quite limited reports on observing ICR at a
micrometer scale mainly by introducing more asymmetric
factors.
2
1−23
For example, Mayer et al. demonstrated a method
to generate ICR at micropipets with diameters ranging from 10
nm to 2.2 μm by using asymmetric electrolyte solution, and
assigned the main driving force for ICR to the electroosmotic
Received: November 11, 2016
Published: January 18, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b11696
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
electric double layer (i.e., κ− ≈ 3 nm) was much smaller than
the radius (i.e., α ≈ 5 μm) of the pipet (i.e., κα ≈ 1670).
To investigate the influence of polyelectrolyte swelling in salt
solution on MICR, we prepared PimB-modified silica nano-
particles (SiNPs) and characterized the size of as-prepared
PimB-modified particles by SEM in a dried state and dynamic
light scattering (DLS) in 10 mM KCl. The difference in the size
of PimB-modified SiNPs obtained from SEM imaging (i.e., 186
1
Therefore, we proposed to use the total amounts of ions (i.e.,
quantity of electrical charges) to predict the occurrence of ICR.
Governed by the rule of electric neutrality, the total amount of
ions in CL and DL when no potential was applied across the
pipet could be estimated on the basis of the surface charge of
PimB obtained from the following equation:
Q = ρNzefπL(2α + L tan θ)
s
(1)
±
4 nm) and DLS (i.e., 216 ± 4.84 nm) indicates a ca. 30 nm
where ρ is the inner surface grafting density, N is the degree of
polymerization, z is the charge number of the monomer, e is the
elementary charge, f is the dissociation of PimB, L is the
effective length of the pipet, α is the orifice radius, and θ is the
swelling of the polyelectrolyte in 10 mM KCl (Supporting
30
Information S2, Figure S1A,B), which was negligible as
compared to the pipet radius, further excluding an overlap of
electrical double layers in Figure 1A (red curve). Although
previous studies have pointed out that a complete overlap of
half cone angle of the pipet. The surface charge (Q ) was
s
−7
calculated to be 1.65 × 10 C under the present conditions
2
0,31,32
the double layers is not necessary for ICR,
the
(
Supporting Information S5).
occurrence of ICR at such a large κα value has never been
observed for a polyelectrolyte brush-modified channel. To
confirm this phenomenon, we fabricated the micropipets in
different radii150 nm, 1 μm, and 10 μmand found that all
pipets showed nonlinear current−voltage (I−V) curves (i.e.,
MICR) (Supporting Information S3, Figure S2).
The occurrence of MICR was considered to originate from
the charge of the polyelectrolyte brush, which was then
confirmed by the reversed rectification when the negatively
charged polyelectrolyte brush was used to modify the pipet
For the BL presumably under no influence of electrostatic
forces, the total amount of ions could be estimated from the
electrolyte concentration by using the following equation:
Q = 2V C N e
b b A
(2)
b
where V is the volume of the BL, C is the electrolyte
b
b
concentration, NA is the Avogadro constant, and e is the
elementary charge. The total quantity of charges carried by ions
−7
was calculated to be 1.38 × 10 C when the electrolyte
concentration of KCl was 0.01 M (Supporting Information S6).
When a potential was applied across the pipet, ions in the CL
and the DL were controlled by both electrostatic force and bias
electric field force, while ions in the BL were controlled only by
the bias electric field. The ion concentration in the BL would be
(
Supporting Information S4, Figure S3). The main difference
between nanoscale ICR and MICR appeared to be associated
with the difference in the charge density on the inner surface of
the pipets: for nanoscale ICR with glasses or polymers, the
surface charge density varied from 1 mC/m to several tens of
mC/m , while for MICR in our case, the space charge density
in the polyelectrolyte layer was estimated to be 4.3 × 10 C/m .
To qualitatively understand MICR, a three-layer model was
proposed, as shown in Figure 2. As we all know, the ionic
2
the same in both positive and negative potentials, considering
2
the same mobility of K+ and Cl−. Therefore, the current
7
3
difference at positive and negative potentials was dominated by
free counterions in the CL and the DL (i.e., Q ). When Q ≪
s
s
Q , no ICR occurred, and a linear I−V curve was obtained
b
(
Figure 1, black curve). After the micropipet was modified with
PimB, Q and Q determined from the equations above became
s
b
comparable, and MICR was observed in terms of the nonlinear
I−V curve shown in Figure 1 (red curve). Taking these
together, we proposed that when Q is larger than or
s
comparable with Q , ICR would take place.
b
Based on this three-layer model (Figure 2), the current at a
specific potential can be reduced to calculate the resistance in
each part at steady state. Note that, in the present system, we
only considered the CL and the BL to simplify the calculation,
since we believe that the DL bears the same variation trend of
resistance with the CL. By dividing the resistance of each zone
into massive infinitesimal parallel-plate resistance, the current
was given by the following equation (for derivation see
Supporting Information S7):
Figure 2. Schematic illustration of the proposed three-layer model
including a charged layer (CL), a double layer (DL), and a bulk layer
(
BL).
current through a nanochannel or nanopore is dominated by
α+L tan θ
αn+1 − αn
the ionic concentration near the orifice of the pore at a specific
1
5,21
I = 2U/ ∑
+ 2U
2
potential.
There are three kinds of ions contributing to the
C αn
α
b,n
total ionic current in the present system: the counterions in
PimB (defined as charged layer (CL), green zone in Figure 2,
right panel), the ions in the electrolyte diffusion layer (defined
as the electrical double layer (DL), pink zone in Figure 2, right
panel), and the ions in the middle of the pipet, which are
unaffected by the surface electrostatic field (defined as the bulk
layer (BL), blue zone in Figure 2, right panel). For a nanopore,
the cross-sectional area at the opening can be used to estimate
α+L tan θ
αn+1 − αn
/
∑
−1
C α (l + κ )
α
s,n
n
(3)
where Cs,n and Cb,n are the concentration of counterions in the
CL and the concentration of the bulk solution in a specific
infinitesimal unit (n), respectively.
When the positive potential was applied, C increased due to
s
11
−
the possibility of ICR, as demonstrated by Bard et al.
However, both simulation and experiments show that the
the accumulation of ions (i.e., Cl ) at the CL and DL by ion
movement from the high charge density area to the low charge
density area, resulting in the high conductive state (Figure 3A,
31−33
distribution of ions at the pore opening is nonuniform.
B
DOI: 10.1021/jacs.6b11696
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
micropipets with different polymerization times. For ATRP
reaction, the length of the polymer brush was mainly controlled
by polymerization time when other conditions were kept the
34,35
same according to previous reports.
As shown in Figure 4A,
Figure 4. (A) Dependence of the rectification ratio (RR) on
polymerization time of PimB. (B) Dependence of RR on KCl
concentration for the PimB-modified micropipets.
with increasing polymerization time, the length of the polymer
brush (i.e., the total surface charge) increased, resulting in an
increase of RR (i.e., the current ratio at the opposite voltage) at
pipets with the same radius. When the polymerization time was
long enough so that the monomer concentration in solution
became the rate-limiting factor, the length of the polymer brush
stopped increasing as time prolonged, and RR leveled off to
yield a plateau in Figure 4A, which was consistent with the
results on the SiNPs surface (Figure S1C) and previously
Figure 3. (A) Schematic illustration of the ion distribution at negative
and positive potentials based on the three-layer model proposed in
Figure 2. (B) Simulated I−V curve of a 5-μm-radius pore with a 300-
nm-length polyelectrolyte brush layer. The space charge density was 4
6
3
×
10 C/m . Inset, the total ion distribution at the junction of the
36
polyelectrolyte brush layer and the bulk layer.
reported results. This result experimentally demonstrated the
dominance of MICR by the total surface charge (Q ), further
s
validating the proposed three-layer model.
inset in positive potential). Conversely, C decreased due to the
The dependence of RR on the electrolyte concentration was
also investigated at the PimB-modified micropipet. As shown in
Figure 4B, a peak-shaped curve was drawn between RR and the
logarithmic concentration of KCl, and the largest RR was
obtained at 10 mM KCl solution for the 5-μm-radius
micropipet. This is a result of a synergistic combination of
various factors. One is the thickness of the DL, which is also the
s
depletion of ions in both the CL and DL when the negative
potential was applied, and the resulting low conductive state
was observed (Figure 3A, inset in negative potential; for
derivation see Supporting Information S8).
To validate the proposed model, finite element simulation
was carried out based on solution of the Poisson−Nernst−
Planck equation for symmetric KCl solution (10 mM) within a
positively charged polyelectrolyte-modified micropipet (Sup-
porting Information S9). As shown in Figure 3B, the
numerically simulated I−V response for the micropipet with a
radius of 5 μm mainly supports our experimental observation
that rectification occurred upon inner surface modification by
polyelectrolytes. Moreover, the ion concentration in the middle
part was almost the same as that in the bulk solution (Figure
S5). Close inspection of the vicinity of the inner wall suggests
three kinds of layers, corresponding to the CL, the DL, and the
3
dominant factor in nanoscale ICR, and another is the length of
the polymer brush that shows a complex change with the
36
electrolyte concentration. Moreover, the dissociation degree
of the polymer brush at different salt concentrations would also
37
affect RR. Different from ICR at a nanopore, MICR could be
clearly observed at relatively high electrolyte concentrations up
to 1 M KCl (i.e., κα ≈ 16 700) (Figure 4B), suggesting that
microdevices based on MICR may be used in high-salt systems
(e.g., serum or brain microdialysate). Moreover, the MICR at
the PimB-modified micropipets was almost pH-independent
(Supporting Information S11, Figure S9).
BL, as proposed in our model. To validate the role of Q , the
s
space charge density of the polyelectrolyte layer (ρ ) and the
In summary, we have observed ICR at the polyelectrolyte
brush-modified micropipet, essentially extending the knowledge
of ICR from the nanoscale to the microscale (MICR).
Compared with ICR at the nanoscale, MICR bears the
following unique properties: (1) the thickness of the electric
double layer is much smaller than the orifice radius; (2) MICR
can be obtained in high-salt solutions; (3) MICR can be
modulated by tuning the polyelectrolyte length and, as such,
the type of polyelectrolytes; (4) the diversity of polyelectrolyte
essentially endows MICR with more designability; and (5) the
relatively large size of the micropipets makes MICR-based
devices easier to handle, which is particularly useful for practical
c
length of the polyelectrolyte brush (L ) were investigated
c
(
Figures S6 and S7). With increasing ρ and L (i.e., Q ), the
c c s
rectification ratio (RR) was increased. When ρ was very small,
c
that is, Q ≪ Q , no obvious rectification occurred (Figure S6).
s
b
When Q was comparable with or larger than Q , rectification
s
b
occurred. Furthermore, in situ confocal laser scanning
microscopy results experimentally highlighted the role of the
polyelectrolyte brush in anionic enrichment (Supporting
Information S10, Figure S8, and movies 1−6).
To experimentally explore the effect of the total surface
charge on MICR, we prepared a series of PimB-modified
C
DOI: 10.1021/jacs.6b11696
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
applications, such as in vivo sensing, and will thus pave a new
avenue for nanoionics and iontronics.
(10) Umehara, S.; Karhanek, M.; Davis, R. W.; Pourmand, N. Proc.
Natl. Acad. Sci. U. S. A. 2009, 106, 4611−4616.
(
11) Wei, C.; Bard, A. J.; Feldberg, S. W. Anal. Chem. 1997, 69,
4
(
1
(
1
(
1
(
627−4633.
12) Sa, N.; Lan, W.; Shi, W.; Baker, L. A. ACS Nano 2013, 7,
1272−11282.
13) Momotenko, D.; Girault, H. H. J. Am. Chem. Soc. 2011, 133,
4496−14499.
14) Guerrette, J. P.; Zhang, B. J. Am. Chem. Soc. 2010, 132, 17088−
7091.
15) Lan, W.; Holden, D. A.; White, H. S. J. Am. Chem. Soc. 2011,
33, 13300−13303.
16) Zhang, B.; Galusha, J.; Shiozawa, P. G.; Wang, G.; Bergren, A. J.;
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b11696.
■
*
S
Experimental details, Scheme S1, and Figures S1−S9
(
PDF)
Movie 1, real-time concentration change of PimB-
modified micropipet in 10 mM KCl containing 1 μM
1
(
5
-carboxyfluorescein at an applied potential of −3 V
Jones, R. M.; White, R. J.; Ervin, E. N.; Cauley, C. C.; White, H. S.
Anal. Chem. 2007, 79, 4778−4787.
(
G.; Branton, D. Nano Lett. 2004, 4, 1333−1337.
(18) Jin, P.; Mukaibo, H.; Horne, L. P.; Bishop, G. W.; Martin, C. R.
J. Am. Chem. Soc. 2010, 132, 2118−2119.
(
2
(
1
(
(
(
AVI)
Movie 2, real-time concentration change of PimB-
modified micropipet in 10 mM KCl containing 1 μM
17) Chen, P.; Mitsui, T.; Farmer, D. B.; Golovchenko, J.; Gordon, R.
5
-carboxyfluorescein at an applied potential of +3 V
(
AVI)
19) Macrae, M. X.; Blake, S.; Mayer, M.; Yang, J. J. Am. Chem. Soc.
010, 132, 1766−1767.
20) Kovarik, M. L.; Zhou, K.; Jacobson, S. C. J. Phys. Chem. B 2009,
Movie 3, real-time concentration change of bare
micropipet in 10 mM KCl containing 1 μM 5-
carboxyfluorescein at an applied potential of −3 V
13, 15960−15966.
(
AVI)
21) Yusko, E. C.; An, R.; Mayer, M. ACS Nano 2010, 4, 477−487.
Movie 4, real-time concentration change of bare
micropipet in 10 mM KCl containing 1 μM 5-
carboxyfluorescein at an applied potential of +3 V
22) Liu, S.; Dong, Y.; Zhao, W.; Xie, X.; Ji, T.; Yin, X.; Liu, Y.; Liang,
Z.; Momotenko, D.; Liang, D.; Girault, H. H.; Shao, Y. Anal. Chem.
2
012, 84, 5565−5573.
(
AVI)
(23) Chang, F.; Chen, C.; Xie, X.; Chen, L.; Li, M.; Zhu, Z. Chem.
Commun. 2015, 51, 15316−15319.
Movie 5, real-time concentration change of PimB-
modified micropipet in 10 mM KCl containing 1 μM
Rhodamine B at an applied potential of −3 V (AVI)
Movie 6, real-time concentration change of PimB-
modified micropipet in 10 mM KCl containing 1 μM
Rhodamine B at an applied potential of +3 V (AVI)
(24) Yameen, B.; Ali, M.; Neumann, R.; Ensinger, W.; Knoll, W.;
Azzaroni, O. J. Am. Chem. Soc. 2009, 131, 2070−2071.
(
25) Tagliazucchi, M.; Rabin, Y.; Szleifer, I. J. Am. Chem. Soc. 2011,
1
(
2
(
33, 17753−17763.
26) Zeng, Z.; Ai, Y.; Qian, S. Phys. Chem. Chem. Phys. 2014, 16,
465−2474.
27) Buchsbaum, S. F.; Nguyen, G.; Howorka, S.; Siwy, Z. S. J. Am.
Chem. Soc. 2014, 136, 9902−9905.
28) de Groot, G. W.; Santonicola, M. G.; Sugihara, K.; Zambelli, T.;
Reimhult, E.; Voros, J.; Vancso, G. J. ACS Appl. Mater. Interfaces 2013,
, 1400−1407.
29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals
AUTHOR INFORMATION
Corresponding Author
yuping@iccas.ac.cn
■
*
(
̈
̈
5
(
ORCID
Lanqun Mao: 0000-0001-8286-9321
and Applications; John Wiley & Sons: New York, 2001.
Notes
(30) Guo, X.; Ballauff, M. Langmuir 2000, 16, 8719−8726.
(
(
(
31) Kubeil, C.; Bund, A. J. Phys. Chem. C 2011, 115, 7866−7873.
The authors declare no competing financial interest.
32) White, H. S.; Bund, A. Langmuir 2008, 24, 2212−2218.
33) Momotenko, D.; Cortes-Salazar, F.; Josserand, J.; Liu, S.; Shao,
́
ACKNOWLEDGMENTS
This work is financially supported by NSF of China (Grant
Nos. 21475138 and 21322503 for P.Y. and 21621062,
■
Y.; Girault, H. H. Phys. Chem. Chem. Phys. 2011, 13, 5430−5440.
34) Ma, H.; Hyun, J.; Stiller, P.; Chilkoti, A. Adv. Mater. 2004, 16,
38−341.
35) He, X.; Yang, W.; Pei, X. Macromolecules 2008, 41, 4615−4621.
(36) Fan, X.; Lin, L.; Dalsin, J. L.; Messersmith, P. B. J. Am. Chem.
(
3
(
2
1435007, and 21210007 for L.M.), the National Basic
Research Program of China (2013CB933704 and
016YFA0200104), and the Chinese Academy of Sciences.
2
Soc. 2005, 127, 15843−15847.
The authors thank Dr. Long Luo at The University of Texas at
Austin for helpful discussions about the simulation.
(37) Biesalski, M.; Johannsmann, D.; Ru
̈
he, J. J. Chem. Phys. 2002,
1
17, 4988−4994.
REFERENCES
■
(
1) Caterina, M. J.; Schumacher, M. A.; Tominaga, M.; Rosen;
Levine, J. D.; Julius, D. Nature 1997, 389, 816−824.
2) Rollings, R. C.; Kuan, A. T.; Golovchenko, J. A. Nat. Commun.
(
2
016, 7, 11408.
(
3) Siwy, Z. S. Adv. Funct. Mater. 2006, 16, 735−746.
(
4) Swiy, Z. S.; Howorka, S. Chem. Soc. Rev. 2010, 39, 1115−1132.
(
5) Ali, M.; Mafe, S.; Ramirez, P.; Neumann, R.; Ensinger, W.
Langmuir 2009, 25, 11993−11997.
6) Chun, H.; Chung, T. D. Annu. Rev. Anal. Chem. 2015, 8, 441−
62.
(
4
(
(
(
7) Cheng, L.; Guo, L. J. Chem. Soc. Rev. 2010, 39, 923−938.
8) Clapham, D. E. Nature 2003, 426, 517−524.
9) Howorka, S.; Siwy, Z. Chem. Soc. Rev. 2009, 38, 2360−2384.
D
DOI: 10.1021/jacs.6b11696
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX