Highly sensitive mercury detection using electroactive gold-decorated polymer nanofibers

Highlights

• A novel electroactive 3D polymeric nanofiber network with increased binding sites for Hg detection is reported.

• Poly (aniline-co-o-aminphenol) fibers decorated with uniformly distributed gold nanoparticles.

• Identifying the optimum conditions to grow the nanofibers through controlled electrodeposition process.

• A detection limit of 0.23 nM was obtained; the Hg²⁺ detection in water and fish samples is demonstrated.

Abstract

Easy to use devices for the rapid detection of toxic metal ions in water or food samples are essential for identifying, monitoring and effectively managing the effects of heavy metal pollution. We report a novel conductive nanofibrillar structure with a high number of nitrogen binding sites as a general strategy for enhancing detection sensitivity of electrodes for mercury ions (Hg²⁺) quantification by anodic stripping voltammetry (ASV). The nanofibers are made of a conductive copolymer, poly(aniline-co-o-aminophenol) – PANOA – decorated with gold nanoparticles (Au NPs) homogenously distributed throughout the fibrillar structure grown directly onto the surface of low cost disposable working electrodes. The synergistic effect arising from the high affinity provided by the large number of nitrogen functional groups (imine, amino, amido) in the PANOA and the Au NPs enabled the development of a very sensitive sensor for Hg²⁺ with a detection limit of 0.23 nM and a linear dynamic range between 0.8 and 12.0 nM, using a 180 s pre-concentration step. The sensor was selective for Hg²⁺ in the presence of As, Pb, Cu, Zn and Cd ions. An example application for detection of Hg²⁺ in river water as well as fish samples was demonstrated. The method provides a broadly applicable strategy to increase sensitivity of electrochemical sensors for field monitoring of heavy metal ion pollution.

Introduction

While essential for living organisms, the presence of high levels of metal ions in the environment (soil, water) and food supply results in accumulation in humans and animals, disturbing the normal physiological processes of the brain, kidney and the nervous system. Mercury (Hg, Hg²⁺) is among the most hazardous pollutants that can cause neurotoxic effects at very low concentrations. The maximum allowable concentration for Hg²⁺ in drinking water imposed by the World Health Organization (WHO) and European Union (EU) is 1 ppb (4.98 nM), and 2 ppb (9.97 nM) by the US-Environmental Protection Agency (EPA) [1]. Despite efforts to remove Hg from consumer products over the years, Hg contamination remains a major environmental and health problem. A number of analytical methods and instrumental techniques are available for Hg determination including spectroscopic methods, UV–vis, fluorescence, Raman, atomic absorption or emission (AAS and AES), and inductively coupled plasma mass spectrometry (ICP-MS) [[2], [3], [4], [5]] with limit of detection (LOD) ranging from 0.016 pg/mL (0.08 pM) to 0.8 μg/mL (4 μM) [5]. Conventional analytical methods such as AAS, AES and ICP-MS, while sensitive, involve the use of expensive instrumentation that cannot be easily deployed and skilled personnel, which limits the availability and increases the analysis costs.

Due to the simple operation, relatively low cost, portability, high sensitivity and selectivity, electrochemical methods with anodic stripping voltammetric (ASV) detection [3] provide an alternative low cost analysis tool for the detection of heavy metals including Hg²⁺ [1,6]. The selectivity of these methods is due to the oxidation potential, which is specific for each metallic ion, and their sensitivity varies with the electrode type, material, and its modification procedure [7]. Initially, mercury-films were the materials of choice for the working electrodes in ASV. However, metallic mercury and mercury salts are toxic, and therefore the use of mercury-free electrodes is desired [8]. Electrodes like glassy carbon (GCE) or carbon paste (CPE) modified with enzymes, DNA, metallic nanoparticles (Au, Ag, Pt), or organic compounds with high affinity for Hg²⁺ have been reported [1,4], some of which have achieved low limits of detection [9]. For example, using a solvent exfoliated molybdenum disulphide (MoS₂), the LOD was lowered to 1 fM due to the high affinity between the Hg²⁺ and S²⁻ groups [10]. Many of these electrodes involve multistep fabrication and electrodes such as GCE are expensive, require polishing, and are not manufactured in large quantities.

Due to their mass production capability in a reproducible manner, screen-printed carbon electrodes (SPCE) are a more appropriate choice for on-site analysis of heavy metals by voltammetric techniques [11]. These electrodes are low cost and field deployable but, they need surface modification with materials with high affinity for mercury in order to achieve detection limits required by the EPA. Examples of surface modification include coatings with metallic NPs (Au, Ag,) or films (Au, Sb, Bi), polymers, thiol compounds, DNA, or enzymes [1,8]. Wang and Tian [12] were the first to report trace analysis of Hg using a metallic gold film modified SPCE achieving a LOD of 0.5 μg/L (2.49 μM). Gold film SPCE have shown good selectivity for Hg²⁺ in presence of other ions, and their good accuracy was verified by ICP-MS [13]. Other materials include thiol-modified magnetic particles [14], AuNPs [15] and carbon black-gold NP composite [16]. SPCE have also been modified with polymeric materials deposited by drop casting and chemical or electro-polymerization, allowing the accumulation of heavy metals on the electrode surface. This process generates an increase of the oxidation current resulting in detection of lower metal ion concentrations [7]. To minimize the resistivity, electroactive conductive polymers and composites [17] are used to enhance the electron transfer between electrode and the metal ions [18]. It was demonstrated that the nitrogen atoms on polyaniline (PANI) and its derivatives enhance Hg²⁺ adsorption [19], and using SPCEs modified with polypyrrole/carbonaceous nanospheres (PPy–CNSs) [20], poly(3-methylthiophene) [21], or polyaniline nanoparticles [22] enabled Hg detection with a LOD ranging between 0.04 and 2.5 μg/L (0.2 and 12.5 nM). However, for a pH higher than 4, the PANI has low conductivity, which significantly limits its use for sensing purposes at neutral pH, the common value for the natural water systems [23]. To overcome this inconvenience, the aniline was co-polymerized with aniline derivatives such as 2-aminophenol, creating poly(aniline-co-o-aminophenol) (PANOA), that can retains the PANI structure and the electrical conductivity of PANI at extended pH values [23,24]. However, these polymers are deposited as a film-like structure that provides limited binding sites for metal ion detection. Designing morphologies with three-dimensional (3D) structures made of these materials would provide a high abundance of binding sites for metal ion adsorption and enhance metal ion accumulation on the electrode surface, that would enhance the sensitivity of sensing devices.

In this work, we report a synthetic modification procedure to generate a conductive nanofibrillar PANOA structure with a high number of nitrogen binding sites for Hg²⁺. Using electrochemical deposition, the PANOA fibers were decorated with AuNPs, known for high affinity for Hg²⁺ that provide high detection sensitivity for Hg²⁺ measurements by square wave anodic stripping voltammetry (SWASV). The nanofibrillar Au/PANOA structure is grown directly onto the surface of low-cost, disposable SPCE sensors, imparting these electrodes with a strong recognition capability for Hg²⁺ detection. The synergistic effect arising from the large number of nitrogen functional groups (imine, amino, amido) in PANOA and the high AuNPs adsorption capacity for Hg²⁺, enhance the Hg²⁺ detection capability. The co-polymer was electrochemically synthesized from purified aniline and 2-aminophenol using a cyclic voltammetry procedure [25]. The 3D nanofibrilar structure of the newly created Au/PANOA network provides a large surface area for binding and pre-concentration of Hg²⁺, which resulted in a low LOD for Hg²⁺ detection. This work provides a systematic study of the deposition parameters of the Au/PANOA nanofibers and describes examples of Hg²⁺ measurements in river water and fish samples.