Smart Sensor for Mercury Detection in Novel Food ^†

Abstract

In this work, we present a simple, fast, and cheap procedure for the determination of mercury [Hg(II)] by square wave anodic stripping voltammetry (SWASV) at nanocomposite screen-printed graphite electrodes. The nanocomposite surfaces were obtained by the electrodeposition of poly(L-aspartic acid) on graphite screen-printed working electrodes using cyclic voltammetry (CV) and, then, of gold nanoparticles using chronoamperometry. A calibration curve by SWASV was obtained with a wide dynamic range (1–60 μg/L) and a limit of detection of 0.25 μg/L. The developed sensor was applied for the analysis of novel food samples.

1. Introduction

Mercury [Hg(II)] is one of the most well-known toxic contaminants of natural and anthropogenic origin in aquatic ecosystems. Low-level Hg(II) exposure is considered to be a significant human health hazard because the accumulation of Hg(II) in the human body through the food chain can lead to serious problems to the central nervous system, including damages to the brain, kidney, lungs, and the developing fetus. Hence, analytical techniques capable of detecting trace levels of Hg (II) in food and environment are highly required.

In this work, we propose a smart Hg(II) detection by square wave anodic stripping voltammetry (SWASV) at nanocomposite graphite screen-printed electrodes as an analytical tool for food applications. The nanocomposite surfaces were obtained by modifying screen-printed graphite electrodes with poly(L-aspartic acid) and gold nanoparticles, and they were characterized using electrochemical techniques. An exhaustive study of the experimental conditions involved in the voltammetric stripping measurements was addressed to develop a reliable method capable of measuring Hg(II) concentration in the low μg/L range. The sensor was integrated in a smart setup, comprising a pocket instrument connected to a smartphone, proving its applicability for disposable and cost-effective on-site analysis. The applicability of the developed platform was assessed by determining Hg(II) in novel food samples.

2. Materials and Methods

2.1. Chemicals

Tetrachloroauric acid (HAuCl4), sulfuric acid 95–97% (H2SO4), potassium ferrocyanide [K4[Fe(CN)6], potassium ferricyanide [K3[Fe(CN)6], potassium chloride (KCl), nitric acid (HNO3), mercury chloride (HgCl2), sodium chloride (NaCl), sodium di-hydrogen phosphate (NaH2PO4), di-sodium hydrogen phosphate (Na2HPO4) and L-aspartic were purchased from Merck (Darmstadt, Germany). All solutions were prepared using MilliQ water (obtained from Milli-Q Water Purification System, Millipore, UK).

2.2. Instrumentation

Screen-printed electrochemical cells (SPCs) based on a graphite working electrode, a graphite counter electrode, and a silver pseudo-reference electrode (EcoBioServices srl, Sesto Fiorentino, Italy) were used for the electrochemical experiments. The voltammetric measurements were carried out at room temperature using Sensit Smart portable potentiostat/galvanostat (PalmSens BV, Houten, The Netherlands) connected to an Android© smartphone equipped with the PSTouch app for data acquisition and elaboration.

3. Discussion

Firstly, the electrodeposition of poly(L-aspartic acid) on graphite screen-printed working electrodes was performed using cyclic voltammetry (CV). Then, gold nanoparticles were electrodeposited on modified graphite screen-printed electrodes. Unmodified GSPE and AuNPs@p(L-Asp)/GSPEs were characterized by cyclic voltammetry and electrochemical impedance spectroscopy (EIS) using the reversible redox couple [Fe(CN)6]−4/−3 to assess for an enhancement of the performance of the graphite—working electrode due to the modification.

Looking for the best analytical response of the AuNPs@p(L-Asp)/GSPEs for mercury determination, key experimental parameters, such as time and deposition potential, were initially optimized. Using optimized experimental conditions, a calibration curve by SWASV was obtained both in beaker and in drop configuration with a wide dynamic range (1–100 μg/L and 1–60 μg/L, respectively) and with a limit of detection of 0.28 μg/L and 0.25 μg/L, respectively. The developed sensor was applied for the analysis of cricket flour and seaweed samples. Firstly, the real samples were mineralized with a microwave oven following a standard procedure for mercury analysis and then were analyzed by the standard addition method. The obtained results were compared with a reference method.