Electronic Supplementary Material
Surface molecularly imprinted polydopamine films for recognition of
immunoglobulin G
Aleksei Tretjakov, Vitali Syritski, Jekaterina Reut, Roman Boroznjak, Olga Volobujeva, and
Andres Öpik
Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn
ESTONIA, [email protected]
1. Experimental part
1.1 Cyclic voltammetry
CV measurements were performed in the solution containing 4 mM of K3[Fe(CN)6]/K4[Fe(CN)6]
redox probe in 1M KCl. The potential was cycled between 0 and 0.5 V at a scan rate of 50 mV•s -1.
For each electrode modification stage 3 potential scans were applied.
1.2 EIS
EIS measurements were performed in the solution containing 4 mM of K3[Fe(CN)6]/K4[Fe(CN)6]
redox couple in 0.1 M KCl as the supporting electrolyte at the formal potential of the system (E =
270 mV) and an alternating potential with amplitude of 10 mV at the frequency range from 0.1 Hz
to 100 kHz. The experiments were repeated three times. The impedance spectra were fitted to an
equivalent electrical circuit by using Gamry Echem Analyst software from Gamry Instruments, Inc.
2. Results and discussion
2.1 IgG immobilization on the gold electrode surface. Electrochemical impedance
spectroscopy studies
The EIS spectra represented as experimental and simulated complex plane plots (Nyquist plots) for
the bare and the modified Au electrodes are shown in Fig. S1. The plots are characterized by the
depressed semicircle located at the high frequencies corresponding to the electron-transfer kinetics
of the redox probe at the electrode interface, and by the linear region at the low frequencies where
the impedance response is dominated by the mass transfer of the redox species to and from the
electrode surface.
The equivalent circuits used for fitting the impedance data are presented in Fig. S2. The equivalent
circuit in Fig. S2a consisting of a solution resistance (RU), a charge transfer resistance (RP),
Warburg impedance resulting from the diffusion of ions from the bulk electrolyte to the electrode
interface (Wd) and constant phase element (CPE) was used to describe the bare and 4-ATP modified
Au electrodes. The use of CPE in the equivalent circuits instead of ideal capacitors was determined
by the appearance of the depressed semicircles in the Nyquist plots. CPE reflects roughness and
inhomogeneity of the surface, and was shown to be important for the modeling of primary protein
layers on an electrode surface [1]. To fit the EIS data of the 4-ATP/DTSSP and 4-ATP/DTSSP/IgG
modified electrodes the additional components were added to the circuit in order to account for the
blocking effect due to the insulating multilayered structure formation on the electrode surface (Fig.
S2b): Rp1 and corresponding CPE1 describe the resistance and capacitance associated with the 4ATP monolayer on gold, while Rp2 and corresponding CPE2 describe the resistance and capacitance
associated with the 4-ATP/DTSSP or DTSSP/IgG layers. The analogous circuit has been employed
elsewhere for the similar interfaces [2]. The parameters obtained from EIS data fitting are presented
in Table 1.
The modification of the Au surface by 4-ATP is clearly seen on the EIS spectra (Fig. S1). The
semicircle measured at the bare Au electrode is poorly defined due to the fast electrode reaction.
With the 4-ATP monolayer coated electrode Rp is clearly greater than Rp of the unmodified Au
electrode due to inhibition of the electron transfer rate. After subsequent electrode modification by
DTSSP Rp continues to increase indicating the more efficient electrode blocking. A significant
increase of Rp after incubation of the 4-ATP/DTSSP modified electrode in IgG containing solution
indicates that the electron transfer process is considerably hindered in this case, suggesting
successful antibody immobilization.
Fig. S1. Impedance spectra of the bare Au (a), 4-ATP (b), 4-ATP/DTSSP (c), and
4-ATP/DTSSP/IgG (d) modified electrodes. The data were recorded in 4mM Fe(CN)63−/Fe(CN)64−
containing 1 M KCl.
Fig. S2. The equivalent circuit used to fit the impedance spectra for the bare and 4-ATP modified
Au electrodes (a), 4-ATP/DTSSP and 4-ATP/DTSSP/IgG modified Au electrodes (b).
Table S1. Model parameters extracted from EIS data (Fig. S1) by fitting to the equivalent circuits
in Fig. S2. α is a CPE exponent reflecting the extent of system inhomogeneity.
Applied
model
Ru (Ohm)
CPE1 (S·sα)
α1
Rp1 (Ohm)
CPE2 (S·sα)
α2
Rp2 (Ohm)
Wd (S·s1/2)
Goodness of
Fit
Au
CPE with
diffusion (Fig.
S2a)
16.25
2.58 ×10-5
94.41 ×10-2
3.44
80.90 ×10-4
51.96 ×10-6
Au/4-ATP
CPE
with
diffusion
(Fig.
S2a)
21.81
2.25 ×10-6
82.06 ×10-2
94.32
59.85 ×10-5
57.29 ×10-5
Au/4-ATP/DTSSP
two CPE elements with
diffusion (Fig. S2b)
23.42
8.14 ×10-7
85.37 ×10-2
15.04 ×102
77.73 ×10-7
68.84 ×10-2
13.12 ×103
51.82 ×10-5
16.03 ×10-5
Au/4-ATP/DTSSP/IgG
two CPE elements with
diffusion (Fig. S2b)
21.25
8.44 ×10-7
84.81 ×10-2
50.79 ×10 2
55.34 ×10-7
66.71 ×10-2
64.13 ×10 3
89.92 ×10-5
30.35 ×10-6
2.2 Effect of mercaptoethanolic treatment on PDA thin films stability
In the presented strategy the procedure of template protein removal from the PDA matrix includes
mercaptoethanolic treatment at approx. 100 ºC to disrupt the disulfide bonds of the DTSSP linker.
Thus, there is a potential risk of the polymer degradation under such harsh washing conditions
leading to its detachment from the electrode surface. In order to evaluate the influence of the
mercaptoethanolic treatment on PDA stability the cyclic voltammograms of the PDA coated Au
electrode before and after the treatment were recorded and compared with the cyclic volatmmogram
of the bare Au electrode. As it clearly seen redox peaks of Fe(CN)63−/Fe(CN)64− pair completely
disappear when the bare Au electrode is coated by the PDA film (Fig. S3a and b) yielding a
featureless cyclic voltammogram, which remains very similar to that after the mercaptoethanolic
treatment (Fig. S3b and c). This supports that the PDA film is stable enough to withstand this
treatment and keep its original structure, what is additionally proven by the SEM micrographs (Fig.
6b and c)
Fig.S3. Cyclic voltammograms of the bare Au electrode (a), PDA coated Au electrode before (b)
and after (c) the mercaptoethanolic treatment. The data were recorded in 1M KCl containing 4mM
Fe(CN)63−/Fe(CN)64− at scan rate 50 mV•s-1.
2.3 SEM microscopy
The SEM micrographs in Fig. S4 provide additional evidence for the PDA film deposition on the 4ATP/DTSSP/IgG modified electrode, where this polymer with morphology of uniformly sized
agglomerates of fine particles can be seen.
Fig. S4. SEM images of the bare Au electrode surface of the QCM sensor (a), the PDA film
electrodeposited on the Au/4-ATP/DTSSP/IgG modified Au electrode (b) and the PDA film after
mercaptoethanolic treatment (c).
2.4 Evaluation of IgG-SIPs films rebinding capability
Fig. S5. The log plot of the IgG adsorption isotherms for the IgG-SIP (black circle) and NIP
(hollow circle) of the PDA various thicknesses. Curves represent fits of the data to FreundlichLangmuir (FL) isotherm model.
Fig. S6. The IgA and IgG adsorption isotherms on the IgA-SIP, IgG-SIP and corresponding NIP
films. The curves represent fits of the data to Freundlich-Langmuir (FL) isotherm model.
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