Silver Nanoparticle-Embedded Conductive Hydrogels for Electrochemical Sensing of Hydroquinone

Abstract

In this work, a conductive hydrogel was successfully synthesized, taking advantage of the high number density of active amino and hydroxyl groups in carboxymethyl chitosan and sodium carboxymethyl cellulose. These biopolymers were effectively coupled via hydrogen bonding with the nitrogen atoms of the heterocyclic rings of conductive polypyrrole. The inclusion of another biobased polymer, sodium lignosulfonate (LS), was effective to achieve highly efficient adsorption and in-situ reduction of silver ions, leading to silver nanoparticles that were embedded in the hydrogel network and used to further improve the electro-catalytic efficiency of the system. Doping of the system in the pre-gelled state led to hydrogels that could be easily attached to the electrodes. The as-prepared silver nanoparticle-embedded conductive hydrogel electrode exhibited excellent electro-catalytic activity towards hydroquinone (HQ) present in a buffer solution. At the optimum conditions, the oxidation current density peak of HQ was linear over the 0.1-100 μM concentration range, with a detection limit as low as 0.12 μM (signal-to-noise of 3). The relative standard deviation of the anodic peak current intensity was 1.37% for eight different electrodes. After one week of storage in a 0.1 M Tris-HCl buffer solution at 4 °C, the anodic peak current intensity was 93.4% of the initial current intensity. In addition, this sensor showed no interference activity, while the addition of 30 μM CC, RS, or 1 mM of different inorganic ions does not have a significant impact on the test results, enabling HQ quantification in actual water samples.

Keywords: conductive hydrogel; electrochemical sensor; hydroquinone; silver nanoparticle-embedded.

Figure 1

Fabrication of silver nanoparticle-embedded conductive hydrogel (Ag NP-CH) on a glassy carbon electrode (GCE).

Figure 2

(A) SEM image and corresponding element mapping and energy spectrum of the surface of Ag NP-CH/GCE; (B) FT-IR; and (C) XRD spectra of Ag NP-CH.

Figure 3

(A) CVs of 1.0 × 10⁻⁴ mol/L HQ at bare GCE (red line), CH (black line), and Ag NP-CH/GCE (blue line) in 0.10 M Tris-HCl buffer, pH 7, scan rate: 0.1 V/s. (B) EIS of 1.0 × 10⁻⁴ mol/L HQ at bare GCE (red line), CH (black line), and Ag NP-CH/GCE (blue line) in 0.10 M Tris-HCl buffer, containing 5 mM Fe(CN)₆³⁻/Fe(CN)₆⁴⁻, pH 7. (C) Anodic peak current intensities for different buffer solutions (pH = 7, 1.0 × 10⁻⁴ mol/L HQ). (D) Anodic peak current intensities at different pH values (0.10 M Tris-HCl buffer, 1.0 × 10⁻⁴ mol/L HQ). (E) CV spectra of 1.0 × 10⁻⁴ M in 0.10 M Tris-HCl (pH = 7.0) at different scan rates. The signals shown, going from the inner to the outer spectra, corresponded to 0.01, 0.02, 0.03, 0.06, 0.1, 0.2, 0.3, and 0.5 V/s. (F) Linear fit to the peak current intensities as a function of the square root of scan rate of HQ.

Figure 4

(A) DPVs of Ag NP-CH/GCE at various HQ concentrations = (0, 0.1, 0.7, 1, 3, 7, 10, 30, 70, and 100 μM). (B) Calibration plot of HQ (pulse amplitude: 0.05 V; pulse width: 0.05 s; scan rate: 0.02 V/s; electrolyte solution: Tris-HCl solution at pH = 7; each point was measured three times).

Figure 5

(A) DPVs of Ag NP-CH/GCE with 30 μM HQ, 30 μM HQ and 30 μM CC, and 30 μM HQ and 30 μM RS, respectively. (B) DPVs of Ag NP-CH/GCE with 30 μM HQ with 1 mM of different kinds of inorganic ions. (C) Current intensity response for eight different electrodes with 30 μM HQ and (D) current response obtained on one electrode that was kept in Tris-HCl buffer solution at 4 °C for one week and detected every other day with 30 μM HQ.