Efficient Preparation of a Nonenzymatic Nanoassembly Based on Cobalt-Substituted Polyoxometalate and Polyethylene Imine-Capped Silver Nanoparticles for the Electrochemical Sensing of Carbofuran

Abstract

The ever-growing exploitation of pesticides and their lethal effects on living beings have made it a dire need of the day to develop an accurate and reliable approach for their monitoring at trace levels. The designing of an enzyme-free electrocatalyst to electrochemically detect the pesticide residues is currently gaining much importance. In this study, a novel redox-sensing film was constructed successfully based on cobalt-substituted Dawson-type polyoxometalate [P₂W₁₇O₆₁ (Co²⁺·OH₂)]^7- (Co-POM) and polyethylene imine (PEI)-capped silver nanoparticles (AgNPs). A nanohybrid assembly was fabricated on a glassy carbon electrode's surface by alternately depositing Co-POM and PEI-AgNPs using the layer-by-layer self-assembly method. The surface morphology of the immobilized CoPOM/AgNP multilayer nanoassembly was analyzed through scanning electron microscopy along with energy-dispersive spectroscopy for elemental analysis. The redox properties and surface morphologies of fabricated assemblies were evaluated by cyclic voltammetry and electrochemical impedance spectroscopy. The practicability and feasibility of the proposed sensing layer was tested for the detection of a highly toxic insecticide, that is, carbofuran. The fabricated sensor exhibited a limit of detection of 0.1 mM with a sensitivity of 13.11 μA mM^-1 for carbofuran. The results depicted that the fabricated nonenzymatic hybrid film showed excellent electrocatalytic efficiency for the carbofuran oxidation. Furthermore, the obtained value of "apparent Km", that is, 0.4 mM, illustrates a good electro-oxidation activity of the sensor for the detection of carbofuran. The exceptionally stable redox activity of Co-POM, high surface area and greater conductivity of AgNPs, and the synergistic effect of all components of the film resulted in an excellent analytical performance of the proposed sensing assembly. This work provides a new direction to the progress and designing of nonenzymatic electrochemical sensors for pesticide determination in real samples.

Scheme 1. Schematic Illustration of Construction of an Enzyme-Free Electrochemical Sensor by LBL Immobilization of Polyanionic Co-POM and Cationic PEI-Wrapped AgNPs on Substrate GCE for the Electro-Oxidation of Carbofuran Pesticide

Figure 1

FT-IR spectra (A) and X-ray diffraction pattern (B) of K₈ [CoP₂W₁₇O₆₁·H₂O]·16H₂O.

Figure 2

SEM micrographs of the CoPOM/AgNP nanoassembly comprising eight bilayers having outer a cationic AgNP layer (A) and outer anionic POM layer (B). Energy-dispersive spectrum of Co-POM/AgNP-based multilayer nanoassembly: inset shows percentage composition of each element present in the material (C).

Figure 3

(A) Cyclic voltammograms taken after the stepwise accumulation of the multilayer film comprising 16 monolayers (outermost layer of Co-POM) on GCE using pH 3.5 buffer as the electrolyte (0.1 M Na₂SO₄/20 mM CH₃COOH) at 100 mV s^–1 scan rate with negative scan direction, (B) relationship between the number of layers deposited on the surface of GCE and the corresponding charge accumulated for the second tungsten-oxo (W–O) redox process.

Figure 4

Redox behavior of Co-POM/AgNP-based multilayer assembly with a surface coverage 0.1 nmol·cm² of in pH 3.5 buffer (0.1 M Na₂SO₄/20 mM CH₃COOH) on GCE (at scan speed 100 mV s^–1, negative scan direction).

Figure 5

(A) Overlay of cyclic voltammograms recorded for the multilayer nanoassembly in pH 3.5 buffer at various scan rates [sweep rates: 10 (innermost), 20, 30, 40, 50, 60, 70, 80, 90, 100 125, 150, and 175 mV s^–1]. (B) Plot of oxidation and reduction peak currents (I_pa and I_pc) against scan rate for multilayer assembly for the second W–O electrochemical process.

Figure 6

(A) Cyclic voltammograms obtained from the multilayer assembly comprising 16 layers with the outermost layer of POM contacting aqueous buffer solutions of pH varying from 2 to 5. Scan rate was 100 mV s^–1. (B) Graph of pH vs E_1/2 for the second W–O electrochemical reaction.

Figure 7

Nyquist plots of the Co-POM/AgNP multilayer assembly with the increase in number of layers (a) cleaned GCE, (b) GCE/PDDA, (c) GCE/PDDA/Co-POM, (d) GCE/PDDA/POM/AgNP, and (e) GCE/PDDA/POM/AgNP/POM (10^–1 to 10² KHz; signal amplitude = 5 mV; E = +230 mV; electrolyte: K₄[Fe(CN)₆]/K₃[Fe(CN)₆]/0.1 M KCl).

Figure 8

(A) Cyclic voltammograms of GCE coated with Co-POM/AgNP multilayer nanoassembly comprising 16 layers in the absence and presence of carbofuran up to 2 mM in Britton–Robinson buffer pH 4.0 (a) bare GCE, (b) 0.0, (c) 0.2, (d) 0.4, (e) 0.6, and (f) 0.8 mM carbofuran at a scan rate of 10 mV s^–1; inset represents the calibration curve between the catalytic current (I_cat) and concentration of carbofuran. (B) Lineweaver Burk plot between inverse of the concentration of carbofuran (1/CBF) and steady-state current response (1/I_cat) of Co-POM/AgNP/GCE.

Figure 9

Amperometric response of Co-POM/AgNP/GCE in 0.1 M PBS (pH 7.0) with the addition of (a) Mg²⁺, (b) Na⁺, (c) Ca²⁺, (d) SO^3–, (e) NO^3–, and (f) catechol.