**2. Materials and methods**

#### **2.1. Reagents**

result in an ever-increasing demand [2, 3]. According to El Mhammedi et al. [4], heavy metal analysis has been carried out in laboratories with time-consuming sampling, transportation and storage steps. Spectroscopy instrumentation was used for the simultaneous analysis of metal ions in water and, sediment biota samples. For the detection of trace metals, electro-

In electroanalysis, much attention has been dedicated to the development of mercury-free sensors the last decade [8, 9]. Various materials have been used as working electrodes for heavy metal analysis [10, 11]. One of these working electrodes is bismuth film electrodes (BiFEs) and has been widely explored as a replacement for the toxic hanging drop mercury electrodes and films [12]. In differential pulse adsorptive stripping voltammetry (DPAdSV), nanoparticles play also a very important role. Due to their unique electronic, chemical, mechanical and thermal properties of nanoparticles make them extremely attractive for heavy metal electrochemical sensors in comparison with conventional materials [13]. In comparison with traditional macro-electrodes, nanoparticle electrodes show a decreased influence of the solution resistance, an increased mass-transport rate, and a higher signal-

Rahman et al. [15] use gold-silver bimetallic nanoparticles for the sensing of an environmental toxin, pyrene. These bimetallic nanoparticles were prepared by chemical reduction

zine as reducing agent, cetryl trimethyl ammonium bromide as the capping agent, deionized water as the solvent and iso-octane as a co-precipitator. In another study, Mailu et al. [16] used overoxidized-polypyrrole/Ag-Au bimetallic nanoparticles for the determination of anthracene, using square wave voltammetry as technique. Bimetallic nanoparticles have been mainly used for the construction of biosensors and electrochemical sensors for determination of PGMs [17], arsenic [18], glucose and ascorbic acid [19], nitrite [20], carcinogenic organic

This study focuses on the development of a new working electrode, such as bismuth-silver bimetallic nanoparticles for heavy metal analysis. The work done by Van der Horst et al. [23] describes the development of a DPAdSV procedure for the determination of platinum in environmental samples. In this procedure, a glassy carbon electrode modified with a bismuth film (GCE/Bi-AgF) was constructed. The optimization of several stripping voltammetric parameters such as dimethylglyoxime (DMG) concentration, composition of supporting electrolyte, pH, deposition potential and deposition time was performed. In this study, the results obtained showed low detection limits of 0.2 ng L−1 for Pt(II). These bismuth-silver bimetallic nanoparticles have been utilized for the determination of Pd(II), Pt(III), and Rh(III) [17] and Pt-Rh and Pd-Rh complexes [24] and the improved detection

The main aim of the work described in this chapter was to investigate the construction and application of a disposable screen-printed carbon electrode (SPCE) that utilizes bismuthsilver bimetallic nanomaterials for PGMs determination. The electrochemical behavior of PGMs in road dust and roadside soil samples was investigated by voltammetry, and the optimum instrumental conditions were defined by DPAdSV measurements. The bismuth-silver

and HAuCl4

in aqueous solution of 2% hydra-

chemical analysis has always been recognized as a powerful tool [5–7].

to-noise ratio [14].

124 Recent Progress in Organometallic Chemistry

of ascorbic acid [25].

method involving the reduction of AgNO<sup>3</sup>

compounds [21] and antidepressant drug [22].

Sodium acetate (NaOAc), ammonia (NH<sup>3</sup> ) (25%), ammonium chloride (NH4 Cl), hydrochloric acid and nitric acid were supplied by Merck (South Africa). All precious and heavy metal standards (1000 mg L−1 AAS), dimethylglyoxime (DMG) were purchased from Sigma-Aldrich (South Africa). Glacial acetic acid (95%), ethanol (95%), hexamethylenetetramine (HMTA), dichloromethane, hydrazine sulfate, and formaldehyde solution were supplied by Kimix (South Africa). Poly(vinyl) alcohol (PVA) was also obtained from Sigma-Aldrich (South Africa). Next, 0.5–20 µg L−1 solutions of platinum group elements were prepared by diluting the corresponding standard stock solutions. A 0.01 M ammonia buffer solution (pH = 9.0) was prepared by mixing ammonium chloride with concentration ammonia and served as the supporting electrolyte. A 0.2 M sodium acetate buffer (pH = 4.7) was prepared by mixing sodium acetate with acetic acid and deionized water. The 0.01 M DMG solution was prepared in 95% ethanol and served as the chelating agent. All solutions were prepared by Milli-Q (Millipore 18 M Ohm cm) water.

#### **2.2. Instrumentation**

Differential pulse adsorptive stripping voltammetric measurements were performed using PalmSens® portable potentiostat/galvanostat, with the PS Trace program and accessories (PalmSens® Instruments BV, 3992 BZ Houten, The Netherlands). The portable potentiostat was connected to a microcomputer controlled by PS 2.1 software for data acquisition and experimental control. All the DPAdSV measurements were performed in a conventional electrochemical cell of 20.0 mL, employing the screen-printed carbon electrode modified with bismuth-silver nanoparticles (SCPE/Bi-Ag) with 4 mm diameter provided by Dropsens (Oviedo, Spain) as working electrodes [26–28]. All experiments were performed at ambient temperatures [29].

#### **2.3. Preparation of the bismuth-silver bimetallic film**

The Bi-Ag bimetallic nanoparticles were dispersed through ultrasonic vibration in a solution of *N,N*-dimethylformamide (DMF) to form a suspension. A defined quantity of the suspension was applied to a clean surface of SPCE and dried at room temperature to get a thin film on the SPCE surface [30–32]. After each voltammetric cycle, the cleaning of the Bi-Ag bimetallic nanofilm was performed by holding the potential of the electrode at +1.0 V. Traces of the remaining DMG complexes on the electrode surface were reduced and quickly desorbed at this potential. A short cleaning period of 30 s was required to refresh the electrode surface completely [33].

## **2.4. Procedure for the determination of PGMs**

A 10 mL of 0.2 M acetate buffer (pH = 4.7) solution containing 1 × 10−5 M DMG was used as electrolyte in the cyclic and stripping voltammetric procedures. The SPCE/Bi-Ag nanosensor was immersed into the solution and an accumulation potential of −0.7 V (vs. Ag/AgCl) for Pd(II) and −0.6 V (vs. Ag/AgCl) for Pt(II), and −0.7 V (vs. Ag/AgCl) for Rh(III) was applied, while the solution was stirred. A 30 s quiet time was used, and the voltammogram was scanned from +0.8 to −1.4 V (vs. Ag/AgCl) at a scan rate of 60 mV s−1 for cyclic voltammetry measurements, while scanning was performed from −0.8 to −0.1 V (vs. Ag/AgCl) for adsorptive differential pulse stripping voltammetry measurements.

For dust or soil extracted solution, 1 mL aliquot of both extracted solutions was added to 9 mL of 0.2 M sodium acetate buffer (pH = 4.7) solution, containing 1 × 10−5 M DMG and 0.5 µg L−1 PGM standard, respectively, to give a final volume of 10 mL. The determination of Pd(II), Pt(II), and Rh(III) was performed using both adsorptive differential pulse stripping voltammetry (AdDPSV) [34]. The PGMs were introduced into the solution after the background voltammogram was recorded. All the experiments were performed in the presence of oxygen and at room temperature [35, 36].
