**3. Results and discussion**

### **3.1. Electrochemical behaviors of Bi-Ag bimetallic modified electrode**

The preliminary investigation of the electroactivity of the bismuth-silver bimetallic nanofilm electrode (Bi-AgFE) was done by using cyclic voltammetry (CV) and differential pulse adsorptive stripping voltammetry (DPAdSV) measurements. To obtain optimal conditions, it is very important to study the influence of supporting electrolyte, dimethylglyoxime concentration, deposition potential, deposition time, and stability test in DPAdSV mode. In this study, different electrolytes such as 0.1 M hydrochloric acid, 0.2 M sodium acetate (pH = 4.7), 0.1 M phosphate (pH = 7.0), and 0.1 M phosphate (pH = 9.0) buffers were tested as supporting electrolytes using the bismuth-silver bimetallic nanofilm electrode (Bi-AgFE). The cyclic voltammograms (CVs) of the resulting electrode obtained in the four different buffer solutions (not shown) showed that the redox response peak height was improved in the presence of 0.2 M sodium acetate buffer solution. Thus, for voltammetric measurements, a solution of acetic acid and sodium acetate was used as the optimal buffer solutions. The results obtained showed anodic peaks at −0.2 and −0.6 V (vs. Ag/AgCl) and cathodic peaks at +0.1 and +0.4 V (vs. Ag/AgCl). On closer inspection the CV results for Bi-AgFE sensor at a scan rate of 50 mV s−1, it is seen that redox couples for Bi3+/Bi2+and Ag<sup>+</sup> /Ag are present.

#### **3.2. Effect of reagent concentration**

In differential pulse adsorptive stripping voltammetric (DPAdSV) analysis, the ligand concentration in solution has a profound effect on the voltammetric peak height. Palladium has a definite adsorption voltammetric peak in acidic medium if dimethylglyoxime (DMG) is used as complexing agent. Dimethylglyoxime is suggested by Georgieva and Pihlar [37] as the complexing agent if sodium acetate is used as supporting electrolyte. In this investigation, the effect of DMG concentrations on the PGMs (Pd, Pt, Rh) peak currents was examined in the range from 5 × 10−6 to 5 × 10−5 M (**Figure 1**). The effect of DMG concentration on Pd(II), Pt(II), and Rh(III) peak currents in 0.2 M sodium acetate buffer (pH = 4.7) solution has shown that concentration of 1 × 10−5 M DMG gave the best results for the SPCE/Bi-AgF sensor, and it was decided to conduct all other stripping experiments using this DMG concentration. **Figure 1** presents the results for the current responses of PGMs complexes ((e.g., Pd(HDMG)<sup>2</sup> , Pt(HDMG)<sup>2</sup> , Rh(HDMG)<sup>3</sup> ) and the evaluated potentials in different concentrations of DMG.

#### **3.3. Deposition potential and time studies**

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

126 Recent Progress in Organometallic Chemistry

temperature [35, 36].

**3. Results and discussion**

it is seen that redox couples for Bi3+/Bi2+and Ag<sup>+</sup>

**3.2. Effect of reagent concentration**

tive differential pulse stripping voltammetry measurements.

**3.1. Electrochemical behaviors of Bi-Ag bimetallic modified electrode**

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 adsorp-

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

The preliminary investigation of the electroactivity of the bismuth-silver bimetallic nanofilm electrode (Bi-AgFE) was done by using cyclic voltammetry (CV) and differential pulse adsorptive stripping voltammetry (DPAdSV) measurements. To obtain optimal conditions, it is very important to study the influence of supporting electrolyte, dimethylglyoxime concentration, deposition potential, deposition time, and stability test in DPAdSV mode. In this study, different electrolytes such as 0.1 M hydrochloric acid, 0.2 M sodium acetate (pH = 4.7), 0.1 M phosphate (pH = 7.0), and 0.1 M phosphate (pH = 9.0) buffers were tested as supporting electrolytes using the bismuth-silver bimetallic nanofilm electrode (Bi-AgFE). The cyclic voltammograms (CVs) of the resulting electrode obtained in the four different buffer solutions (not shown) showed that the redox response peak height was improved in the presence of 0.2 M sodium acetate buffer solution. Thus, for voltammetric measurements, a solution of acetic acid and sodium acetate was used as the optimal buffer solutions. The results obtained showed anodic peaks at −0.2 and −0.6 V (vs. Ag/AgCl) and cathodic peaks at +0.1 and +0.4 V (vs. Ag/AgCl). On closer inspection the CV results for Bi-AgFE sensor at a scan rate of 50 mV s−1,

/Ag are present.

In differential pulse adsorptive stripping voltammetric (DPAdSV) analysis, the ligand concentration in solution has a profound effect on the voltammetric peak height. Palladium has a definite adsorption voltammetric peak in acidic medium if dimethylglyoxime (DMG) is In electroanalytical chemistry, differential pulse voltammetry (DPV) is used as an effective and common technique when the content of analyte is very low due to its sensitivity [38]. The influence of deposition potential (*E*d) and time (*t*d) is always important factors on the

**Figure 1.** Effect of varying dimethylglyoxime (DMG) concentrations on the peak current results for PGMs at a SPCE/ Bi-AgFE sensor. The solutions consisted of 0.2 M acetate buffer, (pH = 4.7) containing: (A) 1 ng L−1 Pd(II) with deposition time of 30 s V; (B) 1 ng L−1 Pt(II) with deposition time of 30 s; and (C) 1 ng L−1 Rh(III) with deposition time of 30 s. Three different concentrations of DMG were employed as demonstrated in the graphs.

sensitivity and detection limit in DPV methods. To enhance the electroanalytical performance of the Bi-Ag bimetallic sensor, the deposition potential and time were optimized. The dependence of deposition potential on the variation of stripping peak current for 1 ng L−1 Pd(II), Pt(II), and Rh(III) at the bismuth-silver nanosensor surface (**Figure 2A**). The optimization of deposition potential was done by varying the potential from −0.4 to +1.0 V (vs. Ag/ AgCl). In the optimization results for deposition potentials have shown that for Pd(II) and Pt(II), a steady increase in the peak current responses was observed up to a *E*d value of −0.7 and −0.9 V (vs. Ag/AgCl). In the case for Rh(III), a sharp increase in peak current was observed at −0.9 V (vs. Ag/AgCl). Optimum deposition potential for Pd(II), Pt(II) and Rh(III) determination in 0.2 M acetate buffer (pH = 4.7) solution is in the range from −0.7, −0.9 and −0.8 V (vs. Ag/AgCl), respectively.

The dependence of deposition time on the stripping peak current of Pd(II), Pt(II) and Rh(III) was investigated using the bismuth-silver bimetallic nanosensor (**Figure 2B**). In adsorptive stripping voltammetry (ASV), complexing agent in the electrolyte solution, after reaction, forms complex ions in the solution, and the complex is accumulated onto the sensor surface in the amount proportional to the deposition time [39]. The dependence of deposition time on the stripping peak current for Pd(II) and Rh(III) decreases almost linearly with longer deposition times. A deposition time at 30 s was chosen as the optimum deposition time in this investigation. In the case for Pt(II), stripping peak current increases with the increasing in the deposition time between 30 and 90 s and became nearly constant above 90 s due to the surface saturation of the bismuthsilver bimetallic nanosensor. In this study for all subsequent Pt(II) measurements, deposition time of 90 s was employed due to surface saturation of the bimetallic sensor.

**Table 1** illustrates a summary of the optimized working conditions for the adsorptive differential pulse stripping voltammetric (AdDPSV) determination of a series of standard (or model) solutions of Pd(II), Pt(II), and Rh(III) metal ions. It was observed that Pd(II) and Rh(III) have the same deposition time with different deposition potentials. Other optimized working conditions such as DMG concentration, supporting electrolyte and potential window was the same for of Pd(II), Pt(II), and Rh(III) throughout the study.

**Figure 2.** Results obtained for the effect of: (A) varying deposition potential (*E*d) upon adsorptive stripping voltammetric responses for 1 ng L−1 Pd(II), Pt(II) and Rh(III); and (B) varying deposition times (*t*d) upon adsorptive stripping voltammetric responses for 1 ng L−1 Pd(II), Pt(II) and Rh(III) at the SPCE/Bi-AgFE sensor. The solutions used consisted of 0.2 M acetate buffer (pH = 4.7) containing 1 × 10−5 M DMG concentration.


**Table 1.** Summary of optimum stripping voltammetry conditions for the determination of Pd(II), Pt(II) and Rh(III) with the constructed GC/Bi-AgFE bimetallic nanosensor [40].

#### **3.4. Analytical features of the adsorptive stripping procedure**

sensitivity and detection limit in DPV methods. To enhance the electroanalytical performance of the Bi-Ag bimetallic sensor, the deposition potential and time were optimized. The dependence of deposition potential on the variation of stripping peak current for 1 ng L−1 Pd(II), Pt(II), and Rh(III) at the bismuth-silver nanosensor surface (**Figure 2A**). The optimization of deposition potential was done by varying the potential from −0.4 to +1.0 V (vs. Ag/ AgCl). In the optimization results for deposition potentials have shown that for Pd(II) and Pt(II), a steady increase in the peak current responses was observed up to a *E*d value of −0.7 and −0.9 V (vs. Ag/AgCl). In the case for Rh(III), a sharp increase in peak current was observed at −0.9 V (vs. Ag/AgCl). Optimum deposition potential for Pd(II), Pt(II) and Rh(III) determination in 0.2 M acetate buffer (pH = 4.7) solution is in the range from −0.7, −0.9 and −0.8 V (vs.

The dependence of deposition time on the stripping peak current of Pd(II), Pt(II) and Rh(III) was investigated using the bismuth-silver bimetallic nanosensor (**Figure 2B**). In adsorptive stripping voltammetry (ASV), complexing agent in the electrolyte solution, after reaction, forms complex ions in the solution, and the complex is accumulated onto the sensor surface in the amount proportional to the deposition time [39]. The dependence of deposition time on the stripping peak current for Pd(II) and Rh(III) decreases almost linearly with longer deposition times. A deposition time at 30 s was chosen as the optimum deposition time in this investigation. In the case for Pt(II), stripping peak current increases with the increasing in the deposition time between 30 and 90 s and became nearly constant above 90 s due to the surface saturation of the bismuthsilver bimetallic nanosensor. In this study for all subsequent Pt(II) measurements, deposition

**Table 1** illustrates a summary of the optimized working conditions for the adsorptive differential pulse stripping voltammetric (AdDPSV) determination of a series of standard (or model) solutions of Pd(II), Pt(II), and Rh(III) metal ions. It was observed that Pd(II) and Rh(III) have the same deposition time with different deposition potentials. Other optimized working conditions such as DMG concentration, supporting electrolyte and potential window was the

**Figure 2.** Results obtained for the effect of: (A) varying deposition potential (*E*d) upon adsorptive stripping voltammetric responses for 1 ng L−1 Pd(II), Pt(II) and Rh(III); and (B) varying deposition times (*t*d) upon adsorptive stripping voltammetric responses for 1 ng L−1 Pd(II), Pt(II) and Rh(III) at the SPCE/Bi-AgFE sensor. The solutions used consisted

time of 90 s was employed due to surface saturation of the bimetallic sensor.

same for of Pd(II), Pt(II), and Rh(III) throughout the study.

of 0.2 M acetate buffer (pH = 4.7) containing 1 × 10−5 M DMG concentration.

Ag/AgCl), respectively.

128 Recent Progress in Organometallic Chemistry

According to the literature, the determination of palladium by DPAdSV at the surface of the hanging mercury drop electrode (HMDE) was first described by Wang and Varughese [41]. Dimethylglyoxime was used as the complexing ligand in slightly acidic media (pH = 5.15) for the deposition of palladium-dimethylglyoxime complex (Pd-(HDMG)<sup>2</sup> ). In the present study, the determination of Pd-(HDMG)<sup>2</sup> was done in 0.2 M acetate buffer (pH = 4.7) solution at the surface of a bismuth-silver bimetallic nanosensor. The DPAdSV current of the Pd-(HDMG)<sup>2</sup> complex at optimal conditions yielded well-defined peaks, in the concentration range 0.4–1.0 ng L−1 shown in **Figure 3A**. The five concentrations used yielded a linear response and the equation of the linear calibration curve is *y* = 0.773*x* + 0.6151 with a correlation coefficient of 0.9911.

The differential pulse adsorptive stripping voltammetric (DPAdSV) current for the Pt-(HDMG)<sup>2</sup> complex was measured at optimal conditions using a bismuth-silver bimetallic nanosensor in **Figure 3B**. In these measurements, a series of Pt-(HDMG)<sup>2</sup> complex concentrations ranging from 0.2 to 0.8 ng L−1 in 0.2 M acetate buffer (pH = 4.7) solution with 30 s deposition time was used. The peaks observed in the differential pulse voltammograms are well defined and the five concentrations used yielded a linear response, and the equation of the linear calibration curve is *y* = 0.690*x* + 0.718 with a correlation coefficient of 0.9881.

The determination of the Rh-(HDMG)<sup>3</sup> complex in 0.2 M acetate buffer (pH = 4.7) solution was performed by DPAdSV analysis under optimized working conditions, and the voltammograms are shown in **Figure 3C**. Well-defined stripping peaks were observed at

**Figure 3.** Differential pulse adsorptive stripping voltammetry results for increasing concentrations of (A) 0.4–1.0 ng L−1 Pd(II) with *E*d = −0.7 V (vs. Ag/AgCl), (B) 0.2–0.8 ng L−1 Pt(II) with *E*d = −0.9 V (vs. Ag/AgCl) and *t* <sup>d</sup> = 120 s, (C) 0.2–0.8 ng L−1 Rh(III) with *E*d = −0.7 V (vs. Ag/AgCl) and *t* <sup>d</sup> 30 s at a SPCE/Bi-AgFE sensor, (D) corresponding calibration curves for the obtained DPAdSV curves. The electrolyte used consisted of 0.2 M acetate buffer (pH = 4.7) containing 1 × 10–5 M DMG concentration.

the bismuth-silver bimetallic nanosensor in the concentration ranging from 0.2 to 0.8 ng L−1. The results indicated that dimethyglyoxime (DMG) can greatly promote the deposition of the Rh-(HDMG)<sup>3</sup> complex at the bismuth-silver bimetallic nanosensor and significantly increase the sensitivity of the determination of the Rh-(HDMG)<sup>3</sup> complex. The inset in **Figure 3C** showed that the DPAdSV peak currents have a linear response for the five concentrations evaluated, and the equation of the linear calibration curve is *y* = 3.9527*x* + 0.5798, with a correlation coefficient of 0.9703.

#### **3.5. Interference and stability studies**

In differential pulse adsorptive stripping voltammetry (DPAdSV), several trace metals can interfere with the determination of platinum group metals (PGMs) absorbing competitively onto the bismuth-silver bimetallic film electrode (Bi-AgFE) surface. They also complexing competitively with DMG producing signals close to that of the different PGMs or completely suppress the peaks. A number of metal ions that could potentially interfere with these PGMs were investigated such as Ni(II), Co(II), Fe(III), and Na+. The sulfates and phosphates were also investigated, and 1 ng L−1 of these interfering ions was added to the model solutions. These ions were chosen because they might reasonably be expected to exhibit redox activity at the SPCE/Bi-AgF sensor and exist in real samples. The behavior of Pd(II), Pt(II), and Rh(III) at concentrations of 0.5–1.5 ng L−1 in the presence of these cations and anions was investigated. This study showed that these ions have not interferes on the determination of Pd(II), Pt(II) and Rh(III).

The stability of the fabricated bismuth-silver bimetallic nanosensor was investigated for the peak current after every 7 h over a period of 28 h. The electrode was kept in deionized water after each measurement. Using the above optimized conditions, the bismuth-silver bimetallic nanosensor was utilized for the determination of 1 ng L−1 concentration of the PGMs evaluated. It was found that the peak current intensities decreased only slightly for the bismuth-silver bimetallic nanosensor, indicating that the nanosensor has good stability and repeatability (data not shown).

### **3.6. Analysis of environmental samples**

the bismuth-silver bimetallic nanosensor in the concentration ranging from 0.2 to 0.8 ng L−1. The results indicated that dimethyglyoxime (DMG) can greatly promote the deposition of

**Figure 3.** Differential pulse adsorptive stripping voltammetry results for increasing concentrations of (A) 0.4–1.0 ng L−1

for the obtained DPAdSV curves. The electrolyte used consisted of 0.2 M acetate buffer (pH = 4.7) containing 1 × 10–5 M

Pd(II) with *E*d = −0.7 V (vs. Ag/AgCl), (B) 0.2–0.8 ng L−1 Pt(II) with *E*d = −0.9 V (vs. Ag/AgCl) and *t*

**Figure 3C** showed that the DPAdSV peak currents have a linear response for the five concentrations evaluated, and the equation of the linear calibration curve is *y* = 3.9527*x* + 0.5798, with

In differential pulse adsorptive stripping voltammetry (DPAdSV), several trace metals can interfere with the determination of platinum group metals (PGMs) absorbing competitively onto the bismuth-silver bimetallic film electrode (Bi-AgFE) surface. They also complexing

increase the sensitivity of the determination of the Rh-(HDMG)<sup>3</sup>

complex at the bismuth-silver bimetallic nanosensor and significantly

<sup>d</sup> 30 s at a SPCE/Bi-AgFE sensor, (D) corresponding calibration curves

complex. The inset in

<sup>d</sup> = 120 s, (C) 0.2–0.8

the Rh-(HDMG)<sup>3</sup>

DMG concentration.

a correlation coefficient of 0.9703.

**3.5. Interference and stability studies**

ng L−1 Rh(III) with *E*d = −0.7 V (vs. Ag/AgCl) and *t*

130 Recent Progress in Organometallic Chemistry

The determination of PGMs was conducted in road dust and roadside soil samples collected in the Western Cape Province at Bottelary Road close to Stellenbosch and Old Paarl Road close to Klapmuts, outside Stellenbosch using the SPC/Bi-AgFE nanosensor. The bioavailability of the PGMs in the road dust and roadside soil samples was determined by subjecting the samples to a three-step sequential extraction procedure [42, 43].

The two sets of results obtained for road dust and roadside soil samples are shown in **Table 2**. The results for the dust and soil samples have shown that the method was successfully applied using the SPC/Bi-AgFE nanosensor. Relatively good results were obtained for



BOT, Bottelary Road; OP, Old Paarl Road.
