Current Densities at Chemically Modified Pt Electrodes
No redox peaks could be found for bare Pt electrode. An increase in redox peak current was observed only for Pt/ZnO/AChE/Chitosanbioelectrode, which confirmed the ZnO nanoparticles facilitated direct electron transfer. When the cyclic voltammograms of Pt/ZnO/Chitosan and Pt/CeO2/Chitosan were recorded in the absence of AChE enzyme, the peak current density was clearly decreased. On the other hand, the cyclic voltammograms of Pt/ZnO/AChE/Chitosan and Pt/CeO2/AChE/Chitosan showed a drastic increase in peak current densities, implying that immobilized AChE enzyme was effectively immobilized on the Pt/ZnO and Pt/CeO2 modified bioelectrodes. This might be also due to the high surface area and abundant electron transfer sites of ZnO and CeO2 nanoparticles, which facilitated high AChE enzyme loading. Comparing with other modified Pt electrodes, the ratio of cathodic to anodic peak current density for Pt/CeO2/Chitosanbioelectrode was greater than 2 with a value of 2.16. It indicated that CeO2 nanoparticles transferred a large number of electrons in the cathodic process than in the anodic process.
Formal Potential of Various Modified Pt Electrodes
The formal potentials (E0 = (Epa+ Epc)/2) of Pt/CeO2/Chitosan, Pt/ZnO/Chitosan, Pt/AChE/Chitosan, Pt/CeO2/AChE/Chitosan and Pt/ZnO/AChE/Chitosanbioelectrodes were estimated to be 243.5, 243, 241, 241.5 and 241.5 mV vs Ag/AgClrespectively. These values were similar to the values of formal potential reported for Au/SiO2/CaCO3/Nano-Au electrode (E0 = 233.5 mV vs. Ag/AgCl) in the literature [1]. All these results demonstrated that the immobilized AChE enzyme retained its original activity even after immobilization on ZnO and CeO2nanoparticles. It also indicated that the redox peaks of all the modified Pt electrodes should be ascribed to electrochemical reaction of the thiocholine (red)/thiocholine (ox) couple.
Change in Peak Potentials of Chemically Modified Pt Electrodes
The peak to peak separations (ΔEp) observed at the cyclic voltammograms of various modified Pt bioelectrodes were greater than 60 mV, which revealed a quasi-reversible electron transfer process. These large peak to peak separations observed in Pt/CeO2/AChE/Chitosan and Pt/ZnO/AChE/Chitosanbioelectrodes were ascribed to the various orientations of immobilized AChE enzyme.
AChE Enzyme Mimicking Nature of CeO2 and ZnO Nanoparticles
In the absence of ATChCl, no peaks were observed in Pt/ZnO/Chitosan, Pt/CeO2/Chitosan, Pt/ZnO/AChE/Chitosan and Pt/CeO2/AChE/Chitosanbioelectrodes. However, in the presence of ATChCl, a pair of distinct and well defined peak was observed. Moreover, as reported earlier, the measured formal potentials of the thiocholine (red)/thiocholine (ox) couple were close to the formal potential values reported for Au/SiO2/CaCO3/Nano-Au electrode in the literature [1]. All these results indicated that the AChE enzyme mimicking nature of CeO2 and ZnO nanoparticles.
Electron Transfer Rate Constant
The values of electron transfer rate constant (Ks = Ip/Q, where, Ip is the peak current and Q is the amount of charge consumed in the electrochemical process) of Pt/CeO2/Chitosan, Pt/ZnO/Chitosan, Pt/AChE/Chitosan, Pt/CeO2/AChE/Chitosanbioelectrodes were estimated as 0.040, 0.076, 0.108 and 0.139 s-1 in the cathodic process and 0.038, 0.072, 0.104 and 0.136 s-1 in the anodic process respectively, which were smaller than that of the Pt/ZnO/AChE/Chitosanbioelectrode (Ks = 0.167 in cathodic process and Ks = 0.164 s-1 in the anodic process). All the above results depicted that the presence of ZnO nanoparticles can enhance the electron transfer rate of AChE enzyme. ZnO nanoparticles provided increased surface area and good conductivity, which improved the electron transport efficiency and enhanced the electron transfer rate of AChE enzyme.
Surface Coverage of Adsorbed ElectroactiveThiocholine
The surface coverage (Γ) of the adsorbed electroactivethiocholine biomolecules on the modified Pt electrode was estimated according to the Eq. (1),
(1)
where is the Faraday’s constant, is the transfer coefficient, is the room temperature, is the number of electrons involved in the charge transfer step, is the surface density of reactant species, is the irreversible peak current, is the gas constant, is the scan rate, is the number of electrons in the redox reaction and is the area of Pt working electrode. As given in Table S1, the surface coverage value adsorbed electroactive thiocholine on the Pt/ZnO/AChE/Chitosan bioelectrode (11.903 mol cm-2 in the cathodic process and 8.548 mol cm-2 in the anodic process) was higher than that of adsorbed thiocholine on other modified Pt electrodes. The larger surface coverage of adsorbed thiocholine may be due to the larger surface area of the Pt/ZnO/AChE/Chitosanbioelectrode.
Compared withPt/CeO2/AChE/Chitosanbioelectrode, the Pt/ZnO/AChE/Chitosanbioelectrode showed larger percentage relative signal (change in the signal observed in the absence and presence of ATChCl) (Fig. S1). This might be resulted from the following reasons (i) the values of Γ and Ks at Pt/ZnO/AChE/Chitosanbioelectrode were larger than that of values measured at other bioelectrodes and (ii) ZnO nanoparticles had the ability to hydrolyze ATChCl [1]. Owing to the high percentage relative signal, Γ and Ks, Pt/ZnO/AChE/Chitosanbioelectrode was chosen to conduct the subsequent experiments.
Optimization of Experimental Parameters
The influence of pH on Pt/ZnO/AChE/Chitosanbioelectrode response was examined in the range 5 - 10. As shown in Fig. S2, maximum peak current density was obtained at pH 8.0. The responses of Pt/ZnO/AChE/Chitosanbioelectrode increased rapidly at pH lower than 8.0, whereas the current density decreased with an increase of pH from 8 to 10. Therefore pH 8.0 was preferred for subsequent experiments.
The effect of the concentration of AChE enzyme loading ranging from 0.10 to 0.40 U mL-1 on the Pt/ZnO/AChE/Chitosanbioelectrode response towards 0.5 mM ATChCl in 0.1 M PBS was investigated (Fig. S3). With the increasing concentration of AChE, the current density increased quickly at first (0.10 - 0.30 U mL-1), then slowly increased (0.30 - 0.35 U mL-1) and finally inclined to a stable value at 0.35 - 0.40 U mL-1. The stable current density response at higher concentration was might be due to the higher AChE enzyme loading, which increased the film thickness and hence the electron transfer to the Pt/ZnO/AChE/Chitosanbioelectrode surface was hindered. Thus AChE concentration of 0.35 U mL-1 was preferred for the modification of Pt/ZnO/AChE/Chitosanbioelectrode.
Simultaneous optimization of pH and AChE enzyme amount plays an important role in registering maximum current density response of Pt/ZnO/AChE/Chitosanbioelectrode to a given concentration of binary mixture of TCDD and PCP. The effect of pH and concentrations of AChE on the Pt/ZnO/AChE/Chitosanbioelectrode response was investigated in 0.1 M PBS (pH 8.0) containing 0.5 mM ATChCl (Fig. S4). As can be seen, the maximum current density could be observed with 0.35 U mL-1 of AChE at a pH of 8.0. Thus the pH of 8.0 and AChE amount of 0.35 U mL-1 were selected for subsequent experiments.
Influence of Scan Rate on the Electrochemical Behaviour of Pt/ZnO/AChE/ChitosanBioelectrode
Fig.S5 shows the cyclic voltammograms of Pt/ZnO/AChE/Chitosanbioelectrode recorded at various scan rates ranging from 0.01 to 0.09 Vs-1. As can be seen, both current density (Fig. S6) and peak potential (Fig.S7) are dependent on the scan rate. With the increasing scan rate from 0.03 to 0.09 Vs-1, both the anodic (Jpa (µA cm-2) = -99397.18 [ν] (µA cm-2/Vs-1) - 2967.92, R2 = 0.99) and cathodic peak current density (Jpc (µA cm-2) = 81278.43 [ν] (µA cm-2/Vs-1) + 5298, R2 = 0.99) increased linearly, indicating that the nature of redox process was controlled by surface confined electroactivethiocholine biomolecule adsorbed on to the surface of Pt/ZnO/AChE/Chitosanbioelectrode. Shift in peak potentials (ΔEp (V) = 0.292 [ν] (V/Vs-1) + 0.080, R2 = 0.99) was noticed with increasing scan rate (0.03 - 0.09 Vs-1) indicated that the electrochemical reaction at the surface of Pt/ZnO/AChE/Chitosanbioelectrode had electroactivethiocholine adsorption complications. According to Laviron theory, using variations of peak potentials with logarithm of scan rate, the electron transfer rate constant of surface confined electroactivethiocholine biomolecule can be calculated.
(2)
where, R = 8.314 mol-1 K-1, F = 96, 485 C mol-1 and T = 298 K. As shown in Fig. S7, when the scan rate was greater than 0.03 Vs-1, both the cathodic (Epc (V) = -0.019 log (ν) (V/Vs-1) + 0.171, R2 = 0.97) and anodic potentials (Epa (V) = 0.018 log (ν) (V/Vs-1) + 0.316, R2 = 0.97) were proportional to the logarithm of scan rate. The calculated value for the electron transfer coefficient was 0.49, which indicated that the energy barrier of redox reaction was symmetric. Using the equations, and cathodic , the number of electron transferred in the cathodic and anodic process was estimated to be one.
References
[1]ChauhanN, NarangJ,PundirCS (2011)Immobilization of rat brain acetylcholinesterase on porous gold-nanoparticle–CaCO3 hybrid material modified Au electrode for detection of organophosphorous insecticides. IntJ Biol Macromol 49:923-929
Figure captions
Fig.S1Percentage relative signal change in the cyclic voltammograms of different modified Pt bioelectrodes.
Fig.S2The effect of pH on the cyclic voltammetric current response of Pt/ZnO/AChE/Chitosanbioelectrode in the presence of 0.5 mM ATChCl in PBS performed at a scan rate of 0.055 Vs-1.
Fig.S3The effect of amount of AChE enzyme on Pt/ZnO/AChE/Chitosanbioelectrode surface performed in the presence of 0.5 mM ATChCl in PBS (pH 8.0) at a scan rate of 0.055 Vs-1.
Fig.S4Simultaneousoptimization of pH and AChE enzyme loading on Pt/ZnO/AChE/Chitosanbioelectrode surface performed in the presence of 0.5 mM ATChCl in PBS (pH 8.0) at a scan rate of 0.055 Vs-1.
Fig.S5Cyclic voltammograms of Pt/ZnO/AChE/Chitosanbioelectrode immobilized with 0.35 mL-1 of AChE at different scan rates in the presence of 0.5 mM ATChCl in PBS (0.1 M, pH 8.0).
Fig.S6The plot of peak current density versus scan rate.
Fig.S7The plot of peak potential versus the logarithm of scan rate.
Fig.S8Effect of incubation time (10, 15, 20, 25, 30 and 35 min) on Pt/ZnO/AChE/Chitosan bio-electrode response.
Table captions
Table S1Comparison of electrochemical parameters for various Pt working bioelectrodes.
Table S2Permeabilities of Pt/ZnO/AChE/Chitosanbioelectrode to TCDD, PCP, TCDD and PCP.
Table S3Electrochemical parameters estimated from the cathodic process for various concentrations of PCP and TCDD.
TableS4 Electrochemical parameters estimated from the anodic process for various concentrations of PCP and TCDD.
Table S5Added and predicted concentrations of PCP for different linear response models in the cathodic and anodic process.
Table S6Added and predicted concentrations of TCDD for different linear response models in the cathodic and anodic process.
Table S7Comparison of analytical parameters of the developed electrochemical biosensor with the previously reported electrochemical biosensors for PCP and TCDD determination.
Fig.S1
Fig.S2
Fig.S3
Fig.S4
Fig.S5
Fig.S6
Fig.S7
Fig.S8
Electrode / Ip(µA) / Ep
(mV) / ΔEp
(mV) / FWHM
(mV) / Ks
(s-1) / Γ
(M cm-2)
Cathode / Anode / Cathode / Anode / Cathode / Anode / Cathode / Anode / Cathode / Anode
Pt/CeO2/Chitosan / 13.040 / 6.0355 / 189 / 298 / 109 / 262 / 221 / 0.040 / 0.038 / 6.008 / 2.780
Pt/ZnO/Chitosan / 17.715 / 9.4299 / 181 / 305 / 124 / 259 / 224 / 0.076 / 0.072 / 8.162 / 4.344
Pt/AChE/Chitosan / 21.141 / 11.266 / 173 / 309 / 136 / 255 / 223 / 0.108 / 0.104 / 9.740 / 5.190
Pt/CeO2/AChE/Chitosan / 23.700 / 15.671 / 168 / 315 / 147 / 254 / 227 / 0.139 / 0.136 / 10.919 / 7.220
Pt/ZnO/AChE/Chitosan / 25.835 / 18.554 / 163 / 320 / 157 / 252 / 229 / 0.167 / 0.164 / 11.903 / 8.548
Table S1
Table S2
Chitosan / 10 nM PCP / 10 nM TCDDPt/ZnO/AChE/Chitosan / Pt/ZnO/AChE / Permeability / Pt/ZnO/AChE/Chitosan / Pt/ZnO/AChE / Permeability
wt% / I (%) / I (%) / (%) / I (%) / I (%) / (%)
0.50 / 17 / 35 / 48.57 / 15 / 40 / 37.50
0.45 / 19 / 35 / 54.28 / 21 / 40 / 52.50
0.40 / 23 / 35 / 65.71 / 28 / 40 / 70
0.35 / 27 / 35 / 77.14 / 35 / 40 / 87.50
0.30 / 33 / 35 / 94.28 / 40 / 40 / 100
0.25 / 35 / 35 / 100 / 40 / 40 / 100
0.20 / 35 / 35 / 100 / 40 / 40 / 100
Chitosan / 10 nM PCP + 10 nM TCDD
Pt/ZnO/AChE/Chitosan / Pt/ZnO/AChE / Permeability
wt% / I (%) / I (%) / (%)
0.50 / 31 / 70.7 / 43.84
0.45 / 40 / 70.7 / 56.57
0.40 / 54 / 70.7 / 76.37
0.35 / 58 / 70.7 / 82.03
0.30 / 68 / 70.7 / 96.18
0.25 / 70.7 / 70.7 / 100
0.20 / 70.7 / 70.7 / 100
Table S3
PCP / TCDD / Inhibition / Jpc / Q / Ks / Γ / REA(nM) / (nM) / (%) / (µA cm-2) / (µC) / (s-1) / (1012 M cm-2) / (%)
1 / 20 / 52.498 / 7950.336 / 288.812 / 0.864 / 0.064 / 47.501
3 / 17 / 57.046 / 7189.191 / 279.321 / 0.808 / 0.058 / 42.953
5 / 15 / 61.606 / 6425.889 / 273.111 / 0.738 / 0.052 / 38.393
7 / 13 / 66.154 / 5664.744 / 265.912 / 0.668 / 0.046 / 33.845
10 / 10 / 70.702 / 4903.599 / 259.866 / 0.592 / 0.039 / 29.297
13 / 7 / 75.249 / 4142.454 / 253.949 / 0.512 / 0.033 / 24.750
15 / 5 / 79.797 / 3381.309 / 250.339 / 0.424 / 0.027 / 20.202
17 / 3 / 84.358 / 2618.008 / 245.333 / 0.335 / 0.021 / 15.642
20 / 1 / 88.738 / 1884.894 / 237.424 / 0.249 / 0.015 / 11.261
Table S4
PCP / TCDD / Inhibition / Jpa / Q / Ks / Γ / REA(nM) / (nM) / (%) / (µA cm-2) / (µC) / (s-1) / (1012 M cm-2) / (%)
1 / 20 / 37.703 / 7488.151 / 128.300 / 1.832 / 0.060 / 62.296
3 / 17 / 40.969 / 7095.573 / 130.531 / 1.706 / 0.057 / 59.030
5 / 15 / 44.244 / 6701.884 / 134.300 / 1.566 / 0.054 / 55.755
7 / 13 / 47.510 / 6309.306 / 136.000 / 1.456 / 0.051 / 52.489
10 / 10 / 50.776 / 5916.728 / 141.162 / 1.316 / 0.048 / 49.223
13 / 7 / 54.042 / 5524.151 / 144.321 / 1.201 / 0.044 / 45.957
15 / 5 / 57.308 / 5131.573 / 147.805 / 1.090 / 0.041 / 42.691
17 / 3 / 60.583 / 4737.883 / 149.623 / 0.994 / 0.038 / 39.416
20 / 1 / 63.729 / 4359.763 / 155.512 / 0.880 / 0.035 / 36.270
Table S5
Model / Predicted PCP1 / 3 / 5 / 7 / 10 / 13 / 15 / 17 / 20
(nM) / (nM) / (nM) / (nM) / (nM) / (nM) / (nM) / (nM) / (nM)
Cathode / PCP vs I (%) / 0.497 / 2.902 / 5.312 / 7.716 / 10.12 / 12.524 / 14.929 / 17.340 / 19.655
PCP vs Jpc / 0.497 / 2.901 / 5.312 / 7.716 / 10.120 / 12.525 / 14.929 / 17.339 / 19.655
PCP vs Q / -0.509 / 3.189 / 5.609 / 8.415 / 10.772 / 13.078 / 14.485 / 16.437 / 19.519
PCP vs Ks / 1.259 / 2.984 / 5.140 / 7.297 / 9.638 / 12.103 / 14.814 / 17.556 / 20.205
PCP vs Γ / 0.551 / 2.887 / 5.223 / 7.559 / 10.284 / 12.619 / 14.955 / 17.291 / 19.627
PCP vs REA / 0.497 / 2.902 / 5.312 / 7.716 / 10.121 / 12.524 / 14.929 / 17.339 / 19.655
Anode / PCP vs I (%) / 0.497 / 2.901 / 5.312 / 7.716 / 10.120 / 12.525 / 14.929 / 17.339 / 19.655
PCP vs Jpa / 0.497 / 2.901 / 5.312 / 7.716 / 10.120 / 12.525 / 14.929 / 17.339 / 19.655
PCP vs Q / 1.192 / 2.778 / 5.459 / 6.668 / 10.340 / 12.587 / 15.065 / 16.358 / 20.547
PCP vs Ks / 0.188 / 2.718 / 5.530 / 7.739 / 10.550 / 12.860 / 15.089 / 17.017 / 19.306
PCP vs Γ / 0.676 / 2.951 / 5.225 / 7.499 / 9.774 / 12.806 / 15.081 / 17.355 / 19.629
PCP vs REA / 0.497 / 2.901 / 5.312 / 7.716 / 10.120 / 12.525 / 14.929 / 17.339 / 19.655
Table S6
Model / Predicted TCDD1 / 3 / 5 / 7 / 10 / 13 / 15 / 17 / 20
(nM) / (nM) / (nM) / (nM) / (nM) / (nM) / (nM) / (nM) / (nM)
Cathode / TCDD vs I (%) / 0.564 / 2.880 / 5.292 / 7.696 / 10.101 / 12.505 / 14.910 / 17.321 / 19.726
TCDD vs Jpc / 0.564 / 2.880 / 5.292 / 7.696 / 10.101 / 12.505 / 14.910 / 17.321 / 19.726
TCDD vs Q / 0.668 / 3.762 / 5.720 / 7.132 / 9.447 / 11.812 / 14.628 / 17.057 / 20.769
TCDD vs Ks / 0.053 / 2.692 / 5.424 / 8.126 / 10.581 / 12.914 / 15.063 / 17.212 / 18.931
TCDD vs Γ / 0.593 / 2.929 / 5.266 / 7.602 / 9.938 / 12.663 / 14.999 / 17.335 / 19.671
TCDD vs REA / 0.564 / 2.881 / 5.292 / 7.696 / 10.101 / 12.505 / 14.910 / 17.321 / 19.726
Anode / TCDD vs I (%) / 0.564 / 2.880 / 5.292 / 7.696 / 10.101 / 12.505 / 14.910 / 17.321 / 19.726
TCDD vs Jpa / 0.564 / 2.880 / 5.292 / 7.696 / 10.101 / 12.505 / 14.910 / 17.321 / 19.726
TCDD vs Q / -0.279 / 3.890 / 5.178 / 7.645 / 9.882 / 13.538 / 14.742 / 17.411 / 18.990
TCDD vs Ks / 0.896 / 3.190 / 5.122 / 7.356 / 9.670 / 12.488 / 14.701 / 17.519 / 20.055
TCDD vs Γ / 0.594 / 2.868 / 5.142 / 7.416 / 10.447 / 12.721 / 14.995 / 17.269 / 19.543
TCDD vs REA / 0.564 / 2.880 / 5.292 / 7.696 / 10.101 / 12.505 / 14.910 / 17.321 / 19.726
Table S7
TCDDTechnique / LOD / Linear range / Recovery / Repeatability / References
(nM) / (nM) / (%) / (% RSD)
Surface plasmon resonance / 1.55310-3 / --- / --- / --- / [1]
Gas chromatography-Mass spectrometry / 155.3 / --- / --- / --- / [2]
Surface plasmon resonance / 3.10510-5 / 31.058 - 310.587 / --- / --- / [3]
Gas chromatography-Mass spectrometry / 1.86310-4 / --- / 86.4 / 11.9 / [4]
Gas chromatography-Mass spectrometry / 0.5 / --- / --- / --- / [5]
Quartz crystal microbalance / 3.105 / 3.105 - 15.534 / --- / --- / [6]
Gas chromatography-Matrix isolation-Fourier transform infra red spectrometry / --- / --- / 52 / 30 / [7]
Cyclic voltammetry / 0.768 / 1 - 20 / 100.285 -100.909 / < 0.9 / Present work
PCP
Materials / Methods / Linear range / LOD / Reproducibility / Stability / Recovery / References
(nM) / (nM) / (%RSD) / (%) / (%)
CuS nanocomposites / Cyclic volammetry / 1880 - 7500 / 625 / 3.45 / --- / 88.5 / [8]
Chitosan nanoparticles / Cyclic voltammetry / 100 - 5000 / 40 / 1.8 / 88 (28 days) / 97.1 - 102.8 / [9]
Mg-Al nanosheets / Differential pulse voltammetry / 1 - 200 / 0.4 / 1.5 / 92 (30 days) / 95 - 102.8 / [10]
TiO2 nanparticles / Squarwave voltammetry / 50 - 7500 / 10 / 3.5 / 21 days / 101.1 - 108.7 / [11]
ZnO nanoparticles / Cyclic voltammetry / 1 - 20 / 0.534 / < 1.7 / 95.2 (21 days) / 100.129 - 100.666 / Present
work
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