A sensitive electrochemical Hg²⁺ ions sensor based on polypyrrole coated nanospherical platinum

Abstract

We report the synthesis and characterization of polypyrrole coated on nanospherical platinum (Pt/PPy NSs) composites for the detection of mercury ions. The synthesis was completed via the direct reduction of potassium tetrachloro platinate(II) aqueous solution in the presence of pyrrole monomers in NaOH. X-ray diffraction and field emission scanning electron microscopy results indicated that the Pt cations were completely reduced to Pt with the concurrent formation of Pt/PPy NSs nanospherical morphology, respectively. The electrochemical properties of Pt/PPy NSs electrode were studied by cyclic voltammetry, differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy. From the DPV results, a linear working range for the concentration of mercury ions between 5 to 500 nM with LOD 0.27 nM (S/N = 3) was obtained. The sensitivity of this linear segment is 1.239 μA nM⁻¹ cm⁻². The interference from Ag⁺, Fe²⁺, Mn²⁺, K⁺, Pd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, Sn²⁺ and Zn²⁺ was negligible, thus is a bright prospect for the electrochemical detection of Hg(II).

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1 Introduction

The detection of mercury cations is of paramount importance, due to its toxicity in binding with sulfur-containing proteins and enzymes.¹ Due to these effects, the development of a rapid, simple and economical sensor for the detection of mercury cations is in high demand in clinical diagnosis, industrial, food and environmental applications.^2,3 Several techniques have been developed for the detection of mercury cations such as atomic absorption spectrophotometry (AAS),⁴ inductively coupled plasma-atomic emission spectroscopy (ICPAES),⁵ inductively coupled plasma-mass spectrometry (ICP-MS),⁵ X-ray fluorescence (XRF)⁶ and neutron activation analysis (NAA),⁷ but such applications are limited by their expensive cost. Electrochemical techniques have been widely used for the detection of Hg(II), due to the high sensitivity, low cost and ease of miniaturization. E. Wang et al.⁸ reported a three-dimensional fibril-like carbon fiber mat electrode (CFME) decorated with Au nanoparticles (AuNPs). They showed that the highly porous feature of CFME and the high affinity of AuNPs could increase the sensitivity of mercury detection. The use of conducting polymers such as polypyrrole (PPy), polythiophene and polyaniline is another method of electrode fabrication for the detection of Hg²⁺. It is notable that small perturbations at the surface or in the bulk of conducting polymers could trigger strong changes in the electroactivity, which can be probed by amperometry or potentiometry.

PPy is a widely used conducting polymer due to its good electrical conductivity and environmental stability.^9,10 The composite of metal or metal oxide/PPy has better electronic properties compared to the pure material.¹¹ Recent reports demonstrated that the composites of electroactive polymers with noble metal or metal oxide nanoparticles are good candidates for the detection of different types of analytes.^12–14 Since the size of the composites may influence the sensitivity of detection of the analytes,^15,16 and due to the unique advantages of nanosize materials, many researchers are developing facile synthetic methods for the production of nanocomposites of electroactive polymers with noble metals or metal oxides nanoparticles. One of the advantages of the combination of electroactive polymers and noble metals nanoparticles is the increase in the electrical conductivity, compared to the pure materials.^17,18 Moreover, the synthesis of electroactive polymers in the presence of metal nanoparticles could increase the available surface area, as the nanoparticles and electroactive polymers could function as the core and the shell, respectively. With the increase of the surface area, high surface reaction activity, high catalytic efficiency and strong adsorption ability are useful for improving the sensor stability and sensitivity.^19–21

In this work, we have developed a facile method for the preparation of polypyrrole coated platinum nanospherical (Pt/PPy NSs) composites via the direct reduction of platinum cations in the presence of pyrrole monomers. The application and performance of these new nanocomposites toward Hg²⁺ sensing are also investigated.

2 Experimental methods

2.1 Synthesis of polypyrrole coated palatinate nanospherical (Pt/PPy NSs) composites

All chemicals such as potassium tetrachloro-platinate(II), 99.9% metals basis, dimethyl formamide for molecular biology ≥99% (DMF), potassium ferricyanide (K₃Fe(CN)₆, 99.2%), ferric chloride, Na₂HPO₄ and NaH₂PO₄ were procured from Sigma-Aldrich (St. Louis, Mo, USA), while potassium ferrocyanide K₄Fe(CN)₆·3H₂O was procured from Fischer Scientific (Shah Alam, Selangor, Malaysia). The synthesis of Pt/PPy NSs can be divided into three steps. First, 1 mL of 0.1 M potassium tetrachloro-platinate(II) solution was added into 30 mL 7 M NaOH solution in a reaction vessel; the mixture was kept at room temperature with continuous stirring at 500 rpm and a homogenous mixture was observed after 30 minutes. Second, 0.5 mL pyrrole monomer was added into the mixture and turned brown in color. The mixture was stirred for another 30 min to complete the redox reaction between the pyrrole monomers and Pt cations. Third, 0.01 mL hydrazine monohydrate was added into the reaction mixture and the temperature was increased to 60 °C at 1.5 °C min⁻¹ and this process was sustained for another 60 minutes. Upon completion of the reaction, the mixture was centrifuged at 4000 rpm for 10 minutes to separate the Pt/PPy NSs from the mixture, followed by drying in a vacuum oven at 50 °C for 12 h. Moreover, pure PPy was synthesized as control material via chemical polymerization, by dissolving 0.1 mol ferric chloride in 150 mL distilled water in a reaction vessel and continuously stirred at 800 rpm with a mechanical stirrer. A solution of 0.05 mol distilled pyrrole monomer dissolved in 50 mL water was added drop-wise into the ferric chloride solution. The stirring was continued for 2 h at room temperature to complete the polymerization. The PPy was filtered and washed with distilled water repeatedly and dried in a vacuum oven at 40 °C for 24 h.

2.2 Electrode preparation

The synthesized Pt/PPy NSs (1 mg) was added in DMF (1 mL) and sonicated for 10 minutes to obtain a brown homogenous suspension. Then, 5 μL of the homogenous mixture was drop-casted onto the surface of a polished glassy carbon electrode (GCE) (polished with 0.05 μm alumina slurry using Buehler polishing kit), cleaned and dried at room temperature. The current density was calculated from the active area of GCE (0.07 cm²) which was covered by the synthesized Pt/PPy NSs.

2.3 Instrumentation and characterizations

Field emission scanning electron microscopy (FESEM, Quanta 200F) and energy dispersive X-ray (EDX) spectroscopy were used to investigate the morphology and weight percentage of the Pt/PPy NSs. The structure and phase composition of the prepared Pt/PPy NSs were characterized by X-ray diffraction (Siemens D5000) using CuKα radiation. The infrared spectrum was recorded at room temperature on a Spectrum 400 (FT-IR/FT-FIR spectrometer). Electrochemical impedance spectroscopy (EIS) measurements were performed by a potentiostat/galvanostat from Autolab, PGSTAT-302N (Ecochemie, Netherlands), controlled by a USB IF030 (MetrohmAutolab) interface card with the FRA.EXE software (version: 409.007, distributor: Metrohm Malaysia) installed in a PC. The quality of the fitting of the equivalent circuit was judged, first, by the x²-value (i.e. the sum of the square of the difference between the theoretical and experiment points) and second, by limiting the relative error in the value of each element in the equivalent circuit to 5%. The electrochemical cell was a three-electrode system where the modified GCE was the working electrode. A platinum foil and a saturated calomel electrode (SCE) were the counter and reference electrodes, respectively. The electrolyte for EIS measurement was 1 mM Fe(CN)₆^3−/4− (1 : 1) solution with 0.1 M KCl supporting electrolyte (K₃Fe(CN)₆, 99.2% and K₄Fe(CN)₆·3H₂O).

3 Results and discussions

3.1 Materials characterizations

The nanospherical morphology of the Pt/PPy NSs can be confirmed by the FESEM images (Fig. 1(a) and (b)) which show that the nanospheres are bundled by the PPy. The nanospherical structure is clearly confirmed from a higher magnification of the FESEM image in Fig. 1(b). The size of each nanospheres shows the existence of a large surface area of the Pt/PPy NSs, which can be utilized as a catalyst for the Hg²⁺ detection. The weight percentage of Pt/PPy NSs was determined by EDX and is shown in Fig. 1(c). The EDX results show the existence of C (from PPy) and Pt (from reduced Pt(II)). The weight percentage of each element is given in Table 1. The existence of carbon in the EDX spectrum of Pt/PPy NSs confirms the presence of PPy as a coating in the Pt/PPy NSs in Fig. 1(c).

Fig. 1 (a) FESEM images of the Pt/PPy NSs (b) a higher magnification image of (a). (c) EDX of Pt/PPy NSs.

Table 1 EDX data of the Pt/PPy NSs

Elements	C	Pt
W%	2.806 ± 0.321	97.193 ± 0.321

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The XRD pattern of Pt/PPy NSs and PPy are shown in Fig. 2(a) and (b) respectively. The intensity of the (111), (200) and (220) peaks in the diffractogram are related to the Pt/PPy NSs (Ref. code: 00-001-1194). The XRD result of Pt/PPy NSs shows a broad amorphous diffraction peak between 2θ = 5–20°, which is related to the scattering of the bare polymer chains at the interplanar spacing.^22,23 The XRD of PPy shows a broad amorphous diffraction peak at 2θ = 10–40° (Fig. 2(b)). The broad diffraction peak of PPy in the Pt/PPy NSs is replaced with the peak (111) of Pt, indicating that the crystallinity of PPy is much lower than the Pt/PPy NSs. Based on the XRD result, we can conclude that the Pt²⁺ was converted to Pt metal in the presence of pyrrole monomers during the synthesis of Pt/PPy NSs. Moreover, from the XRD and FESEM results, it can be concluded that the polymerized PPy encapsulates the surface of Pt nanoparticles, forming a nucleus for further growth. These results can be explained by the Ostwald ripening process,^24–26 which acts as a barrier against the corrosive NaOH environment.²⁷

Fig. 2 XRD patterns of: (a) the Pt/PPy NSs, (b) synthesized PPy in the present of FeCl₃.

Fig. 3(a) and (b) is the FTIR spectrum of the Pt/PPy NSs and PPy (the control spectrum, synthesized in the presence of FeCl₃ solution), respectively. The polymerization of pyrrole monomers in the presence of Pt²⁺ can be confirmed by the presence of the following peaks in Fig. 3(a). First, the peak at 3350 cm⁻¹ is related to the N–H bond. Second, the peak at 1670 cm⁻¹ is related to the C–N–C bond or the C=O group, which suggests that an over-oxidation of the pyrrole monomers had occurred during the PPy polymerization. The over-oxidation of PPy takes place through the apparition of the C–OH and C=O functional groups in the polymer backbone, as well as the formation of CO₂ at sufficiently positive potentials or in the presence of a strong oxidizer.²⁸ Third, the peak at 1389 cm⁻¹ is attributed to a typical PPy ring vibration. Moreover, the peaks at 1180 and 1080 cm⁻¹ are due to the C–N and C–H groups in the composite spectrum, respectively. All of the described peaks are observed in the FTIR of PPy as the control spectrum, therefore it clearly confirms the polymerization of the pyrrole monomers and the over-oxidation of PPy in the presence of Pt²⁺ cations. On the other hand, the Pt²⁺ acts as an oxidizing agent and is reduced to metallic Pt during the polymerization of pyrrole.

Fig. 3 FTIR of: (a) Pt/PPy NSs, (b) synthesized PPy in the present of FeCl₃.

3.2 EIS results

The interfacial properties of the GCE electrodes modified with Pt/PPy NSs were studied by EIS. The Nyquist plots of the Pt/PPy NSs/GCE (1), PPy/GCE (2), bare (3) and their equivalents circuits in 1 mM Fe(CN)₆^3−/4− (1 : 1) with 0.1 M KCl supporting electrolyte are shown in Fig. 4. A comparison between the Nyquist plots of the Pt/PPy NSs GCE (1), PPy/GCE (2) and bare GCE (3) clearly confirms the effect of Pt/PPy NSs and PPy on the charge transfer resistance. The decrease in the charge transfer resistance from GCE to Pt/PPy NSs can be attributed to the conductivity of PPy and Pt. The presence of a peak related to the over-oxidation of polymerized PPy in the presence of Pt²⁺ in the FTIR spectrum, confirms that this phenomenon could decrease the conductivity of PPy, but the existence of Pt NPs could decrease the over-oxidation effect on the PPy conductivity. This effect can be confirmed from the semicircle diameter in the Nyquist plots of PPy/GCE and Pt/PPy NSs; together with the R_ct (charge transfer resistance) obtained from the simulation of the EIS results of Pt/PPy NSs/GCE, PPy/GCE and the bare GCE (Table 2). The R_ct is related to the primary resistance against electron transfer. The Nyquist plots of Pt/PPy NSs/GCE, PPy/GCE and bare GCE show a “depressed semi-circle” with the center of the semi-circle below the Z_re axis, which is due to the deviation from the double-layer capacitance. The value of “n” is related to the slope of the log Z vs. log f, in the Bode diagram. The decrease of “n” from unity is due to the porous and inhomogeneous surface. The increase of CPE of the Pt/PPy NSs/GCE compared to PPy/GCE (Table 2) confirms the existence of pores in the composite but the decrease of “n” is due to the increase of the surface roughness. Based on our results, it can be concluded that the polymerization of pyrrole monomers in the presence of Pt²⁺ has some advantages and disadvantages for the detection of Hg²⁺. The FESEM results show that Pt can play a role as the core and the pyrrole monomers (as the shell) are polymerized on the surface of the Pt nanoparticles, and this phenomenon increases the available surface area of PPy for the reaction with Hg²⁺. The second advantage is that the conductivity of the nanocomposite is greater than the pure PPy. Third, from this core–shell structure, numerous types of pores can be present in the nanocomposite. The main disadvantage is that the decrease in the surface roughness can decrease the available surface area for the reaction with the analyte.

Fig. 4 Nyquist plots of the Pt/PPy NSs/GCE (1), PPy/GCE (2), bare (3) and their equivalents circuits in 1 mM Fe(CN)₆^3−/4− (1 : 1) with 0.1 M KCl supporting electrolyte.

Table 2 Electrochemical parameters obtained from the simulation of the EIS results of the Pt/PPy NSs/GCE in 1 mM Fe(CN)₆^3−/4− (1 : 1) with 0.1 M KCl supporting electrolyte

Electrode	R1 (Rs) ohm cm^2	R2 (Rct) ohm cm^2	Q1Y° (mohm^−1 s^n cm^−2)	n1	WY° (mS s^1/2 cm^−2)
GCE	2.27	3950.10	0.0013	0.9902	0.1086
Pt NSs-PPy	2.81	177.12	0.1460	0.9262	2.8370
PPy/GCE	2.93	898.21	0.0548	0.7884	—

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3.3 Electrochemical behavior of Hg²⁺

The cyclic voltammograms (CVs) of Pt/PPy NSs/GCE (1), PPy/GCE (2) and bare GCE (4) in the presence of 10 μM Hg²⁺ in a mixture of 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ as supporting electrolyte, at pH 6.9 are shown in Fig. 5(a). The same conditions were employed for the comparison of the CV of Pt/PPy NSs/GCE (2) in the absence of Hg²⁺ from −0.8 V to 1.2 V (vs. SCE) at 50 mV s⁻¹. Taking into account the fact that the redox potential of Hg²⁺ (E⁰ (2Hg²⁺/Hg₂²⁺) = +0.67 V (vs. SCE)) is considerably higher than E_redox of PPy, therefore the polymer is supposed to be partially oxidized by means of the Hg²⁺ ions (trapped in the polymer matrix after the adsorption step) when the latter is reduced to Hg₂²⁺, to form a complex between the Hg₂²⁺ and PPy at the surface of the electrode.^29–31 The anodic peak located at 0.57 V in the CV of Pt/PPy NSs/GCE (1) in the presence of 10 mM Hg²⁺ is attributed to the electro-oxidation of the as-formed Hg₂²⁺ (in the complex) to the Hg²⁺ ions. The high sensitivity of the nanocomposite in the detection of Hg²⁺ is due to the high conductivity of the Pt nanoparticles which facilitates faster electron transfer in the polymer backbone, thus promoting the favorable reduction to Hg₂²⁺. Moreover, the cathodic peak at around +0.15 V is related to the reduction of Hg²⁺ to Hg₂²⁺ and the reduction of oxidized PPy from the reaction with Hg²⁺. This interpretation can be confirmed form the comparison of CVs of Pt/PPy NSs in the presence (1) and absence (2) of Hg²⁺. The results show a small shift and the increase of the size of the reduction peak in the presence of Hg²⁺. This peak can be seen in the CV of PPy/GCE (2) and is related to the reduction of the oxidized PPy. Theoretically, mercury like other 3d metal, such as copper, cadmium and zinc have poorly defined reduction waves, but show sharp and intense re-oxidation stripping waves.³² The cathodic peak at 0.7 V is related to the reduction of the over-oxidized PPy in the CVs of Pt/PPy NSs, in the absence and presence of Hg²⁺. In summary, after the adsorption step, due to the chelating complexation (a common feature of M²⁺ ions), the selective electrochemical detection of Hg²⁺ can occur owing to its characteristic value of E⁰ (Hg²⁺/Hg₂²⁺) which triggers the reduction of Hg²⁺ → Hg₂²⁺, driven chemically (with the help of PPy) or electrochemically (under more negative potential bias). Fig. 5(b) shows the reproducibility of the synthesized Pt/PPy in the detection of Hg²⁺. We synthesized Pt/PPy three times and checked the detection of Hg²⁺ separately. The result shows an acceptable reproducibility of the synthesized Pt/PPy.

Fig. 5 (a) Cyclic voltammograms (CVs) of Pt/PPy NSs/GCE (1) and PPy/GCE (2), bare GCE (4) in the presence of 10 μM Hg²⁺ in 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ as supporting electrolyte, at pH 6.9. The CV of Pt/PPy NSs/GCE (2) in the absence of Hg²⁺ in 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ as supporting electrolyte at pH 6.9 (scan rate: 50 mV s⁻¹). (b) Reproducibility of synthesized Pt/PPy to detection of Hg²⁺.

3.4 Effects of the pH of the solution

The effect of pH on the electrochemical response of the Pt/PPy NSs/GCE upon the addition of 10 μM Hg²⁺ was investigated using CV. The change in the peak current with pH (pH range of 6.3–7.2) is shown in Fig. 6. Since this electrode is utilized in the detection of Hg²⁺ in ground water and pipe water, where the pH is close to 7, we only considered a narrow range of pH around 7. It can be observed that the anodic peak current increases with pH until pH 6.9. The reasons can be explained as follows. With the decrease of pH and increase in the concentration of H⁺, the competition between Hg²⁺ and H⁺ for the reactive sites in PPy is intensified. This is notable where the first step in the detection of Hg²⁺ is the adsorption and trapping of the Hg²⁺ in the PPy matrix. On the other hand, the current density decreases with the increase of pH > 7, due to the interaction of Hg²⁺ with OH⁻. Therefore, the buffering at pH 6.9 which is close to the pH of pure water is used in the rest of the work.

Fig. 6 The change in peak current with pH (pH range of 6.5–7.4).

3.5 Effects of scan rate

Fig. 7(a) shows the CVs of Pt/PPy NSs/GCE at different scan rates in the presence of 10 μM Hg²⁺ in 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ at pH 6.9. The increase of the oxidation and reduction peak currents with the scan rate shows that the electrode reaction of the immobilized Pt/PPy NSs redox couple is a surface-confined electrochemical process. The linear regression equation can be expressed as I_pa = 0.03υ (mV s⁻¹) + 0.40 (R² = 0.996) and I_pc = −0.03 (mV s⁻¹) + 0.50 (R² = 0.993) (Fig. 7(b)).

Fig. 7 (a) CVs of Pt/PPy NSs/GCE at different scan rates (10, 20, 30, 40, 50, 60, 70, 80, 90, 100 and 200 mV s⁻¹) in the presence of 10 μM Hg²⁺ in 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ as supporting electrolyte, at pH 6.9. (b) Linear plots of the oxidation and reduction currents versus scan rate.

3.6 Determination of Hg²⁺ using differential pulse voltammetry

A typical amperometric response of the Pt/PPy NSs/GCE to Hg²⁺ concentration changes was studied with differential pulse voltammetry (DPV) 10 μM Hg²⁺ in 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ at pH 6.9. Fig. 8(a) shows the dependence of the anodic peak current (I_pa) on the Hg²⁺ concentration. The sensor was calibrated three times and the standard deviations were calculated. The calibration curve for the Pt/PPy NSs/GCE shows a linear segment: from 5 to 500 nM with a linear regression equation of I_pa = 0.001 (mA μM⁻¹ cm⁻²) + 0.024 (R² = 0.996) (Fig. 8(b)). The limit of detection (LOD) (S/N = 3) and the limit of quantification (LOQ) (S/N = 10) of the Pt/PPy NSs/GCE were calculated from the following equations:³³

LOD = 3S_B/b (4)

LOQ = 10S_B/b (5)

where S_B is the standard deviation of the blank solution and b is the slope of the analytical curve, as shown in Fig. 8(b). The estimated LOD (S/N = 3), LOQ (S/N = 10) for the linear segment is 0.277 nM, and 0.924 nM, respectively. In addition, the sensitivity of the linear segment is 1.239 μA nM⁻¹ cm⁻². The increase of the response of Pt/PPy NCs/GCE toward the Hg²⁺ concentration can be explained by the following reasons: first, the role of the Pt nanoparticles is to increase the conductivity of the composites and to decrease of over-oxidation effect due to the polymer. Second, the Pt nanoparticles are the core and therefore decrease the size of the PPy shell. Third, the size decrease of the PPy increases the total surface area of the polymer; this phenomenon promotes the interaction of PPy with the Hg²⁺ ions resulting in more adsorption and entrapment of Hg²⁺ into the polymer matrix.

Fig. 8 (a) The DPV curves of different concentration of Hg²⁺ in 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ as supporting electrolyte (5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400 and 500 nM) at pH 6.9. (b) The calibration curve. Error bars represent ± standard deviation.

3.7 Interference effects and recovery experiment

The selectivity of the sensor was determined by investigating it with the presence of alkaline earth metal ion (K⁺) and transition metal ions (Ag⁺, Cu²⁺, Cu²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Pd²⁺, Sn²⁺, Zn²⁺). Fig. 9 shows the electrochemical response of Hg²⁺ in the presence of a 100-fold individual species in the solution with respect to Hg²⁺. Among the various metal ions studied, Hg²⁺ shows the highest signal gain change compared to the other metal ions. Likewise, the sensor's response to Hg²⁺ is unaffected by the presence of the mixture of metal ions.

Fig. 9 The selectivity of the sensor by investigating it with alkaline earth metal ion (K⁺) and transition metal ions (Ag⁺, Cu²⁺, Cu²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Pd²⁺, Sn²⁺, Zn²⁺). The electrochemical response of Hg²⁺ in the presence of 100-fold individual species in the solution with respect to Hg²⁺. The illustrated error bars represent the standard deviations of measurements taken from at least three independent experiments.

For analytical applications, a recovery experiment was also carried out to examine the prospective use of the Pt/PPy NSs/GCE. Three different concentrations of Hg²⁺ (50, 200 and 400 nM) were added in a mixture of 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ at pH 6.9. The Hg²⁺ was detected by the renewed electrode with acceptable relative standard deviation (RSD), which was estimated from 6 parallel measurements. This result shows that the prepared Pt/PPy NSs/GCE sensor could be applied repeatedly without a significant decrease in its accumulation capacity for Hg²⁺, with high reproducibility and regeneration ability (Table 3).

Table 3 The detection of Hg²⁺ concentration in test samples (results based on six replicate determinations per sample)

Sample	Added (nM)	Found (nM)	RSD%	Recovery%
1	50	49.423	3.305	98.84
2	200	199.571	2.338	99.78
3	500	497.521	1.667	99.50

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3.8 Analysis of real samples

The analytical application of the Pt/PPy NSs/GCE electrode was tested with ground water and pipe water in University of Malaya. Linear correlations (n = 3) between the peak current and the corresponding concentrations of Hg²⁺ were attained by using samples spiked at different concentration levels. The LODs for Hg²⁺ in the water samples, calculated from the analyte concentration, where the peak current is three times (S/N = 3), were 1.825 and 1.129 nM, for the ground water and pipe water, respectively (Table 4).

Table 4 Analysis of real samples

Sample	Linear range (nM)	R%	LOD (nM)
Pipe water	10–100	99.1	1.125
Ground water	10–100	98.7	1.825

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Table 5 shows that the LOD of Hg²⁺ for Pt/PPy NSs/GCE is lower compared to thiol-functionalized AgNPs,³⁴ quantum dot/DNA/AuNPs ensemble,³⁵ fluorescently labeled single-stranded DNA probe³⁶ and the present result is comparable to AuNPs and thymine-rich hairpin DNA probes.³⁷

Table 5 A summary and comparison of the estimated LOD values from the present work with previous reports

Type of electrode	Detection method	Sensitivity	Performance LOD/nM	Linear range/nM	Reference
Thiol-functionalized AgNPs	Surface-enhanced Raman scattering	—	2.4	10–2000	34
Quantum dot/DNA/AuNPs ensemble	Fluorometry	Fluorescence quenching efficiency as a function of the Hg^2+ concentration: 0.007 F nM^−1	2	2–60	35
Fluorescently labeled single-stranded DNA probe and carbon nanoparticles	Fluorometry	1.85 μA nM^−1	10	0–250	36
AuNPs and thymine-rich hairpin DNA probes	Colorimetry	—	0.1	0.1–100	37
Coordination of Hg^2+ to AuNPs associated nitrotriazole	Colorimetry	—	7	10–500	38
Thiolated ferrocene (Fc)-tagged DNA oligomer	Fluorometry	—	100	10–2000	39
Au-MPA-HFHAHFAF peptide modified electrode	Electrochemistry	0.5 μA μM^−1	9.5	0.25–0.81	40
Pt/PPy NSs (nanospherical)	Electrochemistry	1.239 μA nM^−1	0.27	5–500	This work

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4 Conclusions

Polypyrrole coated platinum nanospherical (Pt/PPy NSs) composites were synthesized by a direct reduction of Pt²⁺ in the presence of pyrrole monomers in aqueous solution. The results demonstrated that the surface modification of the glassy carbon electrode with Pt/PPy NSs resulted in a superior electrocatalytic activity toward the electro-oxidation of 10 μM Hg²⁺ in a mixture of 0.33 M phosphate buffer solution, 0.5 M NaCl and 0.1 M LiClO₄ at pH 6.9. The higher electroactivity of the Pt/PPy NSs was due to the following reasons. First, the FESEM results show that Pt can play the role of a core and pyrrole monomers (as the shell) can polymerize on the surface of the Pt nanoparticles. Due to this phenomenon, the size of the polymer decreased and the available surface area of the PPy increased. Second, the composite has larger conductivity than the PPy and third, the appearance of numerous types of pores in the composites. The higher sensitivity of the composite for the detection of Hg²⁺ is due to the higher conductivity of the Pt nanoparticles, which promotes faster electron transfer process along the polymer backbone, thus induced more favorable reduction to Hg₂²⁺. The estimated LOD (S/N = 3) and LOQ (S/N = 10) for the linear segment was 0.277 nM and 0.924 nM, respectively. In addition, the sensitivity of this linear segment was 1.239 μA nM⁻¹ cm⁻².

Acknowledgements

The authors wish to thank Mojdeh Yeganeh for valuable discussion. This research is supported by High Impact Research MoE Grant UM.C/625/1/HIR/MoE/SC/04 from the Ministry of Education Malaysia, PRGS grant PR002-2014A, GC001C-14SBS, RP038C-15HTM and University Malaya Centre for Ionic Liquids (UMCiL).

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