Electronic Supporting Material
Highly sensitive and selective voltammetric determination of dopamine using a gold
electrode modified with a molecularly imprinted polymeric film immobilized on flaked
hollow nickel nanospheres
Yuan Liu1, Jie Liu2, Jiang Liu1, Wei Gan1, Bang-ce Ye3, 4, Yingchun Li1,2,3,*
1
School of Science, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China
2
Key Laboratory of Xinjiang Phytomedicine Resources for Ministry of Education, School of
Pharmacy, School of Pharmacy, Shihezi University, Shihezi 832000, China
3
Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan,
School of Chemistry and Chemical Engineering, Shihezi University, Shihezi 832003, China
4
State Key Laboratory of Bioreactor Engineering, East China University of Science and
Technology, Shanghai 200237, China
*Corresponding author: tel: +86-755-86239466, e-mail address: [email protected]
Optimization of the method
Optimization of electro-polymerization conditions including functional monomer, pH value
and molar ratio of template to monomer (T:M) was carried out by testing the differently
prepared sensor response towards DA at the same concentration. In all the experiments, the
concentration of DA (as template) during polymerization was 2 mM. The sensor responses
were displays in Table S1. In particular, the values in Table S1A, B and C refer to the
response (∆I, mA) of the sensor to DA (2×10−7 M) in the solution containing 0.1 M KNO3 and
0.1 M Fe(CN)63−/4− in the potential range from −0.2 to +0.6 V at a scan rate of 100 mV s−1.
(a) Optimization of monomer and pH value for preparing MIP
Monomer screening is essential in MIP preparation as it determines the functional groups in
polymer and their steric structure, thus directly affecting the interaction between template and
MIP.
The optimum functional monomer was selected among the generally used ones in
electro-polymerization of MIPs, including resorcinol, o-phenylenediamine, pyrrole,
methylene blue, and p-aminobenzoic acid. Since pH value has significant influence on
polymerization of monomers, the optimal pH values of these candidates were examined first
under their own suitable pH range. All polymerization occurred on the surface of bare GE
with a molar ratio of template to monomer (T:M) being 1:3. After polymerization and removal
of template, the electrodes modified with different MIP modifications were applied for
determination of DA at the same concentration and their reduction peak current shifts (∆I)
were calculated for evaluating the individual response level. Table S1A lists the ∆I values
obtained from different MIP/GE, and the optimal pH values for each monomer were selected
in accordance with the highest ∆I. As can be found, the optimum pH for resorcinol,
o-phenylenediamine, pyrrole, methylene blue and p-aminobenzoic acid are 7.0, 6.0, 2.0, 7.0
and 4.0, respectively.
(b) Optimization of the molar ratio of template to monomer
The following consideration taken into account was the ratio of T:M during
electropolymerization, which plays a significant role in sensor performance as monomer
concentration would directly influence the thickness of MIP film and the number of imprinted
sites in the polymer layer [34]. Here we tested electrodes modified by different MIPs with
molar ratio of T:M varying at 1:1, 1:2, 1:3, 1:4 and 1:5. Table S1B demonstrates that the ratio
of 1:3 brings out the highest response for all the MIP/GEs. This may be due to the following
reasons: when the ratio is over 1:3, the amount of monomers is not sufficient to maximize the
generation of template-monomer assemblies, hence leading to deficient sites in MIP matrix
for accomplishing template binding; on the other hand, excessive monomers can bring up
polymer chains lacking in cavities imprinted by template molecules, thereby blocking off the
passage of Fe3+/Fe4+ probe ions to electrode surface and lowering the efficiency of electron
transfer. The ratio also indirectly reflects the interaction between monomer and DA, which is
mainly non-covalent [43], including hydrogen bonding between polymer and amino group of
DA, electrostatic force from polymer towards two hydroxyls of DA, and van der Waals force.
Moreover, since it is found that utilization of resorcinol (pH=7.0, T:M= 1:3) and methylene
blue (pH=7.0, T:M=1:3) as monomer gave rise to the highest response to DA among all the
MIP/GEs, we further explored the potential synergistic function of the two optimal monomers.
They were tried as double monomers to fabricate MIP/GE (Table S1C). However, the results
are unexpected that the response from double-monomer MIP/GE is inferior to that from
single-monomer MIP/GE. This might be caused by interaction between these two monomers
due to their own acid-base properties (pKa value of resorcinol and methylene blue is 9.3 and
4.52, respectively). Thus, resorcinol with the optimized condition (pH=7.0, T:M= 1:3) was
selected as functional monomer in following work.
Table S1A Optimization of pH value during electro-polymerization at the T:M of 1:3 (n=3).
The values refer to the response (∆I, mA) of the sensor to DA (2×10−7 M) in the solution
containing 0.1 M KNO3 and 0.1 M Fe(CN)63−/4− in the potential range from −0.2 to +0.6 V (vs.
SCE) at a scan rate of 100 mV s−1.
2.0
3.0
4.0
4.5
5.0
pH
5.5
1
-
-
-
0.12
0.11
0.11
0.17
0.47
0.85
0.41
0.15
2
-
-
-
-
0.16
0.16
0.36
0.23
0.24
0.26
-
3
0.30
0.22
0.07
-
0.07
-
0.09
-
0.09
-
-
4
-
-
-
-
0.20
0.22
0.24
0.20
0.50
0.20
-
5
-
0.15
0.19
-
0.11
-
0.11
-
0.10
-
-
Monomer
a
6.0
6.5
7.0
7.5
8.0
1: resorcinol, 2: o-phenylenediamine, 3: pyrrole, 4: methylene blue, 5: p-aminobenzoic acid;
b
- refers that polymerization was unable to take place or was not implemented under this
condition.
Table S1B Optimization of molar ratio of template to monomer under optimum pH value
obtained from Table S1A (n=3). The values refer to the response (∆I, mA) of the sensor to DA
(2×10−7 M) in the solution containing 0.1 M KNO3 and 0.1 M Fe(CN)63−/4− in the potential
range from −0.2 to +0.6 V (vs. SCE) at a scan rate of 100 mV s−1.
Condition
1:1
1:2
1:3
1:4
1:5
resorcinol
pH=7.0
0. 18
0. 16
0.85
0.13
0.09
o-phenylenediamine
pH=6.0
0. 23
0. 27
0.36
0.25
0.22
pyrrole
pH=2.0
0.16
0.11
0.30
0.15
0.06
methylene blue
pH=2
pH=7.0
0.07
0.11
0.50
0.17
0.13
para aminobenzoic acid
pH=4.0
0. 15
0. 14
0.19
0.13
0.12
Table S1C Sensing response of MIP/GE with resorcinol and methylene as double monomer at
different molar ratios (n=3). The values refer to the response (∆I, mA) of the sensor to DA
(2×10−7 M) in the solution containing 0.1 M KNO3 and 0.1 M Fe(CN)63−/4− in the potential
range from −0.2 to +0.6 V (vs. SCE) at a scan rate of 100 mV s−1.
Resorcinol : methylene blue
(pH=7)
1:1
1:2
2:1
ΔI (mA)
0.35
0.22
0.32
Conditions of HPLC
The HPLC experiment was performed using an Essentia LC-15C system equipped with two
LC-15C Solvent Delivery Units, an LC Solution 15C workstation and an SPD-15C UV-Vis
Detector (Shimadzu, Japan). LC conditions were as follows: chromatographic separation was
performed on a Shimadzu WondaSil C18 column (150 mm×4.6 mm id, 5 μm), mobile phase
was PBS (pH=5.8)/methanol (98:2, v/v) with a flow rate of 1.0 mL/min, injection volume was
10 μL, and detection wavelength was set at 280 nm. The column temperature was ambient.
Dilution procedure for preparing real sample
The dilution process is as follow: mother liquor was prepared at the concentration around 10−6
M. Then the solution was diluted to 10−8 M (e.g. 1 mL→100 mL) and such 100-times dilution
was easy to handle and brought negligible error in real operation. Similarly, solution at 10 −10
M was obtained from 10−8 M. And then the similar step was repeated another 2 times and the
target concentration solution can be obtained. As there is no big jumping from high
concentration to low concentration, it is safe to say that accurate preparation of solution at
extremely low concentration can be ensured.
Differential pulse voltammetry (DPV) was performed with the DA-imprinted MIP sensor
before and after removal of DA template. As shown in Fig. S1, since the template molecule
DA is electroactive, the electrode exhibits an apparent peak at 0.236 V before extraction
(curve a in Fig. S1). After extracting the template by cyclic voltammetric scanning between
−0.5 and +0.5 V in 0.1 M NaOH, there was no current response observed in the DPV
measurement (curve b in Fig. S1), which verified the successful removal of the template.
Fig. S1. DPVs of dopamine imprinted film at MIP/hNiNS/GE before (a) and after dopamine
removal (b). DPV was performed in the solution of 0.1 M PBS (pH7.4), with the potential
range from 0.6 to −0.4 V (vs. SCE) the potential increment of 4 mV, the pulse amplitude of 50
mV, the pulse width of 50 ms and the pulse period of 0.2 s.
Energy disperse spectroscopy (EDS) was also applied to prove the removal of DA template.
As shown in Fig. S2, before removing DA, the peak of N element exists in EDS spectra,
which is unique to DA molecule in this work, while this peak disappears after removing DA.
It indicates that DA molecules have been removed completely.
Fig. S2. EDS spectra of DA imprinted film at MIP/hNiNS/GE before (A) and after dopamine
removal (B).
Fig. S3. Cyclic voltammegrams for electropolymerization of different monomers (A:
p-aminobenzoic acid, B: pyrrole, C: methylene blue, D: o-phenylenediamine) in the presence
of DA at a scan rate of 50 mV s−1. The electro-polymerization was carried out in ABS solution
containing 6 mM monomer and 2 mM DA. The numbers inside represent the number of
scanning cycles.
Fig. S4. Cyclic voltammograms of different electrodes for the responses to 5×10−11 M DA in
the solution containing 0.1 M KNO3 and 0.1 M Fe(CN)63−/4− in the potential range from −0.2
to +0.6 V (vs. SCE) at a scan rate of 100 mV s−1. The top-left inset is partially amplified
cyclic voltammograms of the responses from MIP/hNiNS/GE and MIP/GE towards DA.
Fig. S5. Calibration curves of different electrodes towards DA at increased concentration
Fig. S6. Chemical structures of DA, its structural analogues and co-existing substances.
Table S2 The responses (∆I, mA), α value and β value of MIP/hNiNS/GE and
NIP/hNiNS/GE to different substances at the same concentration of 6×10−13 M. The CV was
performed in the solution containing 0.1 M KNO3 and 0.1 M Fe(CN)63−/4− in the potential
range from −0.2 to +0.6 V (vs. SCE) at a scan rate of 100 mV s−1 (n=3).
ΔI of
Substance
MIP/hNiNS/GE
(mA)
ΔI of NIP/hNiNS/GE
(mA)
α of
α of
MIP/
NIP/ hNiNS
hNiNS/GE
/GE
β
DA
0.241
0.045
-
-
-
L-adrenaline brtartrate
0.035
0.024
6.89
1.88
3.66
α-phenylethylamine
0.012
0.034
20.08
1.32
15.2
uric acid
0.037
0.026
6.51
1.73
3.76
ascorbic acid
0.020
0.039
12.05
1.15
10.48