Introduction Heavy metals are a huge threat to the environment and human health due to the fact that they are not biodegradable and very high levels of pollution

Heavy metals are a huge threat to the environment and human health due to the fact that they are not biodegradable and very high levels of pollution. Hence, monitoring of trace heavy metals is vital due to the potential health and ecological hazard they present. Such monitoring systems have to be accurate and have to be able to detect heavy metals in low concentrations 1-3. There are numerous methods to detect heavy metal ions, such as atomic absorption spectroscopy, inductively coupled plasma atomic emission spectrometry, and inductively coupled plasma mass spectrometry 4. However, these spectrometric methods are expensive and not suitable for in situ analysis due to the tedious and complicated instruments. On the contrary, the electrochemical method, as an alternative to these spectroscopic techniques, has been accepted as an efficient method to detect heavy metal ions due to their excellent sensitivity, short analysis time, portability, and low cost 5.
The emerging trends towards miniaturization, portability and requirements of high sensitivity and selectivity in electrochemical sensors have motivated an increased demand for metal ion sensors that are low cost and environmentally friendly. Apart from economic and environmental considerations, two of the factors that determine the success of a metal ion sensor are high sensitivity and selectivity. The use of the chemically modified electrodes tremendously improves the efficiency of accumulating target analytes and has been developed as a fascinating and effective way for the anodic stripping voltammertry (ASV) determination of heavy metal ions 6. The nanostructured materials are extremely attractive to modify electrodes for electrochemically detecting heavy metal ions due to their unique electronic, chemical, thermal, and mechanical properties in comparison with conventional materials 7.
Mercury-coated film modified electrodes have been used recently for trace heavy metal detection in connection with voltammetric stripping analysis. The usage of mercury-based electrodes is critical due to its toxicity. For this reason the development of carbon-based material 2,3, boron-doped diamond electrodes 8, bismuth-modified electrodes 9, polymer modified electrodes 10,11 and other environmental friendly materials is being studied. However, mercury film electrodes are still being used. This kind of electrode in comparison to conventional ones used so far (i.e., hang drop mercury electrode, HDME) reduces the amount of mercury used in the analysis. Apparently, more sensitive and selective mercury based modified electrodes are still needed to be developed for the determination of heavy metals, due to its environmental and biological significance 12.
Novel materials with their better electrochemical performances are being investigated and will improve the sensitivity of electrochemical devises, decrease the operational costs, and widen the range of applications. Quantum dots (QDs) are semiconductor nanoparticles with inherent electronic properties, enormous surface area-to-volume ratio, and specific surface area. Among various NPs, QDs have shown great potential because of their unique advantages: nanoscale size similar to proteins, broad excitation, spectra for multicolor imaging, robust, narrow band emission, enhancing the signal and versatility in surface modifications 13-16.
In this work, Hg(II) was electrochemically immobilized on cadmium selenide quantum dot modified graphite electrode. The modified electrode was characterized by FESEM, EDS, CV, and electrochemical impedance spectroscopy. The modified electrode exhibited excellent stripping performance for trace analysis of lead (II) ion. Finally, a highly sensitive method for the detection of lead was proposed, which shows the satisfactory results for the detection of the lead in water samples. As a result, the introduction of QDs modified electrode can extend the analytical application of various metal ion sensors. To date, however, no reports have appeared demonstrating the formation of stable mercury immobilized in the CdSe QDs modified electrodes of known size for use in routine electroanalytical applications in trace metal analysis.
Materials and Methods
Chemicals and Instruments
Spectroscopic grade graphite rod (3 mm), and cadmium chloride hemi penta hydrate were purchased from Aldrich, Germany. L-cysteine ethyl ester hydrochloride was purchased from Hi-media. Lead nitrate was purchased from Qualigens, India. All other chemicals and reagents were of analytical grade and used without further purification. All aqueous solutions were prepared with doubly distilled water.
The pH of the solutions was measured using a digital pH meter (Elico India, Model LI 120) at 27°C. The electrochemical experiments were carried out using a CHI 660B electrochemical workstation (CH Instruments, USA). A conventional three-electrode system was used that consisted of the CdSe QDs/GO nanocomposite modi?ed as the working electrode, a platinum wire auxiliary electrode and a saturated calomel reference electrode (SCE). Field emission scanning electron microscopic analyses were performed using FESEM (Model SU6600, HITACHI, JAPAN) attached with EDS. Transmission electron microscopic images were obtained using TEM (Model H7650, HITACHI, JAPAN). All experiments were carried out at ambient temperature.
Synthesis of CdSe QDs
L-Cysteine capped CdSe QDs were synthesized using an already reported procedure 17 with a slight modification. In a three-necked round bottom flask, CdCl2.2.5H2O (1.46 mg) and L-Cysteine ethyl ester hydrochloride (0.012 g) were mixed with 20 mL aqueous solution and the pH was adjusted to 11 using 1 M NaOH. This was followed by the addition of 20 mL of NaHSe which was prepared by adding selenium metal (0.506 mg) and sodium borohydride (0.612 mg) in an inert atmosphere. The mixture was stirred for 30 minutes and then refluxed for 4 h under nitrogen atmosphere. After 4 h an orange red colloidal CdSe QDs was formed.
Fabrication of CdSe QDs-Hg modified electrode
PIGE was prepared as reported 18 and used for electrode modification. One end of the electrode was carefully polished with emery paper and then with 0.05µm alumina slurry, washed with distilled water and dried in air. An optimised volume of as prepared CdSe QDs was drop casted on the polished surface of PIGE and allowed to dry at ambient conditions. The electrode was washed with double distilled water to remove weakly adsorbed CdSe QDs and this electrode was used for further modification. CdSe QDs modified PIGE was held at a negative potential of -1.2 V in solution containing 1× 10-4 M of HgCl2 in 0.1 M KCl for 180 s for the reduction of Hg which will henceforth called as CdSe QDs – Hg modified electrode and used for further studies.
Results and discussion
Characterization of CdSe QDs
In order to investigate the surface morphology of CdSe QDs, it was examined by TEM. Fig. 1a shows the TEM micrograph of CdSe QDs dispersed uniformly with spherical shape and the size of the particles ranges between 2-8 nm. To identify the elements present in the CdSe QDs, EDS measurement was carried out. The corresponding EDS is shown in Fig. 1b. The appearance of Cd, Se, S, C, N and O peaks confirm the formation of CdSe QDs.
Characterization of CdSe QDs – Hg modified electrode
FESEM studies can give useful insights about the formation of deposited mercury on the CdSe QDs surface. Fig. 2a shows the typical FESEM image of CdSe QDs – Hg modified electrode. As can be seen from the figure, Mercury spheres was homogeneously deposited and distributed on the CdSe QDs surface with some agglomeration. The formed mercury spheres increase the surface area and this could be obviously reflected on the sensitivity in the determination of target metal ions. The elements present in the CdSe QDs – Hg modified electrode was confirmed with EDS measurements and the results are shown in Fig. 2b. The presence of Cd, Se, N, O, C and Hg peaks confirms the deposition of Mercury on the surface of CdSe QDs.
Optimization of parameters influencing CdSe QDs – Hg modified electrode
Some key parameters can influence the electrochemical behaviour of the CdSe QDs – Hg modified electrode drastically. The three main parameters that can be considered in this particular electrode modification are, volume of CdSe QDs loaded on the PIGE, deposition time and deposition potential for Hg deposition.
The optimization of CdSe QDs amount is an important parameter in the electrode preparation. The amount of CdSe QDs loaded on the PIGE was controlled for the deposition Hg over CdSe QDs surface. The effect of stripping response of Hg (II) on the amount of CdSe QDs loaded on the PIGE was studied in the range from 5 µL to 20 µL and is illustrated in Fig. 3a. The stripping peak current for Hg (II) reached a maximum value for 10 µL of CdSe QDs but at higher volume the stripping peak current decreased drastically. This is explained by the fact that loading higher volume leads to formation of cracks on the surface and this can decrease the efficiency of the deposition of Hg.
The effect of deposition potential on the stripping signals of Hg (II) was checked in the potential range of -0.8 V to -1.2 V. The stripping peak current for Hg (II) increased considerably on increasing the potential from -0.8 V and reaches a maximum at a potential of -1.2 V. Further on increasing the deposition potential, the stripping peak current for Hg (II) decreased due to the occurrence hydrogen evaluation. Therefore, an optimized potential of -1.2 V was chosen for the deposition of Hg.
The effect of deposition time on the stripping peak current of Hg (II) was studied in the range of 60 – 300 s. Fig. 3b shows the plot of DPASV response of Hg (II) vs deposition time. As shown in the figure, the stripping peak current increased linearly with increase in deposition time upto 180 s. When the deposition time was increased beyond 180 s the stripping peak current remains constant, which is due to the surface saturation of Hg deposition over CdSe QDs.
Electrochemical characterization of CdSe QDs – Hg modified electrode
The Hg deposited over the surfaces of CdSe QDs modified and bare electrode were characterized using cyclic voltammetry and the results are shown in Fig. 4. The curves ‘a’ and ‘b’ correspond to the CV response of bare and CdSe QDs – Hg modified electrode respectively in 0.1 M KCl at a scan rate of 50 mV/s. As can be seen from the figure, the anodic and cathodic peak current for the oxidation and reduction of Hg in the CdSe QDs modified electrode was found to be higher when compared to bare electrode. From the results, it could be understood that the CdSe QDs provides larger surface area and acts as a suitable anchoring medium for the deposition of increased no. of Hg spheres.
Electrochemical impedance spectroscopy (EIS) is an effective method, which could be used to study the surface features of the modified electrodes. Fig. 5 compares the Nyquist plots for the bare (a) and CdSe QDs – Hg modified (b) electrodes recorded in 0.1 M KCl containing 5 mM Fe(CN)63-/4- as an electrochemical redox probe. In EIS, the semicircle segment observed at higher frequencies corresponds to the electron-transfer limited process and its diameter is equal to charge transfer resistance (Rct). The linear segment appeared at lower frequencies attributed to the diffusion limited electron transfer process. An Rct value of 455 ? and 25 ? were observed for bare, and CdSe QDs – Hg modified electrodes, respectively. Compared with the bare electrode, the CdSe QDs – Hg modified electrode showed a low Rct value. This observation implies that the deposition of Hg over CdSe QDs surface increases the electron transfer rate. Thus, it can concluded that the CdSe QDs – Hg modified electrode can efficiently accelerate the electron transfer and enhances the sensitivity of the target metal ions.

Differential pulse anodic stripping voltammetric determination of Pb (II) ion
Fig. 6 shows the DPASV response of bare electrode (a) and CdSe QDs – Hg modified electrode (b) in 0.1 M HClO4 solution containing 1.9 µg/L of Pb (II). The sharper and higher peak current for Pb (II) was obtained for CdSe QDs – Hg modified electrode when compared to the bare electrode. The enhancement of the voltammetric response is probably due to the presence of CdSe QDs, which can provide more active surface area for the deposition of increased amount of Hg. The Hg spheres anchored into the CdSe QDs can form amalgam with the metal ion and thereby improves the performance of the modified electrode.
In stripping analysis, the application of adequate deposition potential is very important to achieve the best sensitivity. Thus, the e?ect of the deposition potential on the peak current of 0.1 µg/L M Pb (II) was studied in the potential range from -0.8 to -1.4 V in 0.1 M HClO4. When the deposition potential shifts from -0.8 to -1.2 V, the stripping peak currents for Pb (II) increased and reached a maximum at a potential of -1.2 V. When a deposition potential more negative than -1.2 V was employed, a decrease in the current response was observed, this is probably due to the generation of hydrogen at the electrode surface. Thus, an optimum deposition potential of -1.2 V was chosen for subsequent experiments.
The effect of deposition time on the stripping peak current of 0.1 µg/L Pb(II) was studied in the range of 60 – 300 s. The stripping peak current increased linearly with increasing the deposition time upto 180 s. When the deposition time was increased beyond 180 s, the stripping peak currents remains almost constant, which is due to the surface saturation at high metal ion concentration.
In addition the modified electrode was applied for the successive determination of Pb (II). The DPASV response of different concentrations of Pb (II) and the calibration plot are illustrated in Fig. 7 a and b. On increasing the concentration of Pb (II) the stripping peak currents were found to be increase in the concentration range of 5.2 ng/L – 4.2 µg/L with the correlation coefficient of 0.997 and the detection limit was found to be 1.7 ng/L (S/N = 3). For comparison, various methods and electrode materials reported for the detection of Pb(II) ion are summarized and listed in Table 1. In particular, the CdSe QDs were proved to be an effective immobilization of Hg in the electrode surface and overall the modified electrode showed excellent performance for the detection of Pb(II) ion.

Abbreviations: PXOFME, poly xylenol orange film modified electrode; LOD, Limit of detection; BDD, Boron doped diamond; DPASV, Differential pulse anodic stripping voltammetry; SWASV, Square wave anodic stripping voltammetry; SPCE, Screen printed carbon electrode

Interference studies
Under optimal conditions, the selectivity of the proposed sensor was tested by evaluating the possible interferences in the determination of Pb(II). The selectivity was studied by spiking various foreign substances in a standard solution containing 1.6 µg/L. It was observed that a large no. of anions (Cl-, NO3- and SO42-) and cations (Mg2+, Ca2+, Ba2+, and Al3+) did not have any influence on the stripping currents of Pb(II). The observation implies that the proposed modified electrode could be used for the selective determination of Pb(II).

Stability of CdSe QDs – Hg modified electrode towards Pb(II) ion
To evaluate the stability of the modified electrode, a series of 10 repetitive measurements of DPASV response for 5 × 10-6 M of Pb (II) in 0.1 M HClO4 was performed. The current response of Pb(II) ion remained constant at 97.5% of initial value showing excellent working stability towards Pb(II) ion determination. The long-term stability of the modified electrode was also studied for a period of 60 days. The modified electrode was used to determine the same concentration of Pb(II) ion which showed 96% of initial current response, indicating that the modified electrode exhibits excellent long-term stability. The above results reveals that the CdSe QDs acts as a suitable anchoring medium for Hg on the electrode surface and has not leached out.

Determination of Pb(II) in different water samples

To access the applicability of the sensor, the proposed modified electrode was applied for the determination of Pb(II) ion in different water samples. The water samples were spiked with known amount Pb(II) and were analyzed. The results are listed in Table. 2. The recoveries obtained are varied from 99.2% to 100.5%. The results indicate that the proposed electrode can be used for the analysis of Pb(II) in real water samples with good recovery.

In this work, a high performance and sensitive platform for the stripping analysis of Pb(II) ion detection using Hg/CdSe QD modified electrode was reported for the first time. Such a high surface area CdSe QDs modified electrode was used to immobilize Hg in the electrode surface robustly and it greatly facilitates the electron-transfer process and the sensing behavior of metal ion detection, leading to a remarkable improved sensitivity and selectivity. The resulting proposed sensor exhibits fine applicability for the detection of Pb(II) ion in water samples. The modified electrode will be attractive to detect toxic heavy metal ions.
The authors acknowledge the financial assistance from University of Madras through National Centre for Nanoscience and Nanotechnology.