Tunable Electrochemical Sensors Based on Carbon Nanocomposite Materials towards Enhanced Determination of Cadmium, Lead and Copper in Water †

: Many carbon materials are well-known conductive materials, widely used in the fabrication of composite electrodes. In this work, diverse allotropic forms of carbon such as graphite, MWCNTs and rGO were tested. Furthermore, these materials allow the construction of cheaper, smaller, portable, reliable and easy-to-use devices, which can be easily modiﬁed. The above-mentioned composite electrodes were developed for metal analysis in water such as Cu, Cd and Pb that, at a high concentration, can have consequences on human health. SWASV is the selected technique. It would be ideal to exploit the potential properties of mercury for metal detection by tuning the electrode’s surface. Due to mercury’s hazardous properties and to reduce the amount of this substance used in polarography, the use of nanoparticles is a good option due to their properties. Mercury nanoparticles were used to modify the surface of the composite electrodes to improve electroanalytical sensor response. For this reason, using these modiﬁed composite electrodes can lower detection limits and widen the linear range that can be achieved for Cd (0.05–1 mg · L − 1 ) and Pb (0.045–1 mg · L − 1 ). However, for Cu (0.114–1.14 mg · L − 1 ), meaningful variations were not observed compared to the bare electrode.


Introduction
Water is fundamental for all Earth's living forms, and a key issue for social and economic development.Currently, water analysis is a vital topic, for because monitoring some parameters is important to prevent some health problems.One of the parameters that has become important involves determining the concentration of heavy metals in water.To do this, several analysis techniques are used, such as atomic absorption spectroscopy (AAS) [1], inductively coupled plasma (ICP) [2], high-performance liquid chromatography (HPLC) [3], etc.Some of the metals that can be found in water are Cu, Cd and Pb and, at high concentrations, can have consequences on human health [4][5][6].
In this work, a voltametric technique has been chosen, known as square-wave anodic stripping voltammetry (SWASV) [7,8].SWASV consists of two steps: first, applying a potential to preconcentrate the analyte on the surface of the electrode; second, taking a measurement by applying staircase potential to record the current generated.To use this technique, composite electrodes were construct using different carbon materials and a non-conductor epoxy.The behavior of graphite, reduced graphene oxide (rGO) and carbon nanotubes (CNTs) were tested in the detection of Cd, Pb and Cu.However, we work with the bare electrode; the modification of their surface with mercury nanoparticles (Hg-NPs) was also tested [9].Mercury was used, a long time ago, in polarography, and it is well-known for its ability to form amalgams with some metals, reducing the potential where they appear [10,11].Hence, taking advantage of these properties, the aim of this work is to reduce the amount of mercury used in polarography for the determination of Cd, Pb, and Cu.

Composite Electrode Construction
Composites were constructed using three different carbon materials: graphite, CNTs and rGO.The first step is to weld a copper sheet to a commercial connector; after that, it is placed in a PVC tube.A mixture of one of the carbon materials and Epotek H77 is prepared, and the PVC tube (2.1 cm, ∅6 mm) is filled with this mixture.Then, it is cured for 2 days at 80 • C.Then, the surface must be polished.
The percentages tested of carbon materials are shown in Table 1.These percentages were optimized previously, and they are related to their respective improvement in the electroanalytical properties of developed sensors, in terms of detection limit and sensitivity [12].

Composite Electrode Chacaterization
Electrodes were characterized using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) using a computer-controlled Multi-AUTOLAB M101 (Eco Chemie, Utrecht, The Netherlands) with a three-electrode cell: a platinum-based electrode 53-671 (Crison Instruments, Alella, Barcelona, Spain) as a counter electrode, an Ag/AgCl handmade electrode as a reference electrode and the constructed composite electrodes as working electrodes.The characterization was performed in solution composed of 0.01 M K 4 Fe(CN 6 ), 0.01 M K 3 Fe(CN 6 ) and 0.1 M KCl.For CV, the scan rate was 10 mV•s −1 and the rate of frequencies used in EIS was 0.01 to 10 4 Hz.
The behavior of the 15% rGO electrode was unusual, possibly related to the orientation of the layers in the Epotek H77 matrix, and its characterization using CV and EIS was not successful.In Figure 1, the characterization of the rest of the carbon electrodes, with graphite or CNTs, can be observed.The most notable difference is showed in EIS, where the 20% graphite presents the lower charge transference resistance.Thus, a highly conductive surface is then available for the preconcentration of cationic metals.

Composite Electrode Modification with Hg-Nps
After electrode characterization, the surface of the electrode is modified with mercury nanoparticles (Hg-NPs) following the synthesis from [9].In the synthesis, 78 mg Hg2(NO3)2 • 2 H2O is used, 1 mL 1M HNO3 is added and then 0.5 mL of a solution of 3.5 g of PVA (Polyvinyl Alcohol) added to 16 mL of Milli-Q water.All the steps of the synthesis were performed at 25 °C and under stirring conditions.
A total of 20 µL of the nanoparticle solution is drop casted on the electrode surface and dried in the oven at 80 °C for 2h.The modified electrodes were characterized using scanning electron microscopy (SEM) (MerlinFe-SEM, Carl Zeiss, Germany) and the Hg-NPs were characterized using transmission electron microscopy (TEM) (JEM-2011 200 kV, Jeol, Peabody, MA, USA) (see Figure 2).

Composite Electrode Modification with Hg-Nps
After electrode characterization, the surface of the electrode is modified with mercury nanoparticles (Hg-NPs) following the synthesis from [9].In the synthesis, 78 mg Hg 2 (NO 3 ) 2 •2H 2 O is used, 1 mL 1 M HNO 3 is added and then 0.5 mL of a solution of 3.5 g of PVA (Polyvinyl Alcohol) added to 16 mL of Milli-Q water.All the steps of the synthesis were performed at 25 • C and under stirring conditions.
A total of 20 µL of the nanoparticle solution is drop casted on the electrode surface and dried in the oven at 80 • C for 2 h.The modified electrodes were characterized using scanning electron microscopy (SEM) (MerlinFe-SEM, Carl Zeiss, Germany) and the Hg-NPs were characterized using transmission electron microscopy (TEM) (JEM-2011 200 kV, Jeol, Peabody, MA, USA) (see Figure 2).

Composite Electrode Modification with Hg-Nps
After electrode characterization, the surface of the electrode is modified with mercury nanoparticles (Hg-NPs) following the synthesis from [9].In the synthesis, 78 mg Hg2(NO3)2 • 2 H2O is used, 1 mL 1M HNO3 is added and then 0.5 mL of a solution of 3.5 g of PVA (Polyvinyl Alcohol) added to 16 mL of Milli-Q water.All the steps of the synthesis were performed at 25 °C and under stirring conditions.
A total of 20 µL of the nanoparticle solution is drop casted on the electrode surface and dried in the oven at 80 °C for 2h.The modified electrodes were characterized using scanning electron microscopy (SEM) (MerlinFe-SEM, Carl Zeiss, Germany) and the Hg-NPs were characterized using transmission electron microscopy (TEM) (JEM-2011 200 kV, Jeol, Peabody, MA, USA) (see Figure 2).

Bare Composite Electrodes
For metal determination, the technique chosen was SWASV.This consists of applying a potential (−1.4 V) for 7 min that reduces the metal ions on the electrode surface; then, staircase potential is applied and the current generated is recorded.This process is performed under N 2 bubbling.Moreover, a modification in the electrochemical cell is used.Instead of using a handmade reference electrode, the one used for the measurements is Orion 900 electrode (Thermo Scientific, Beverly, MA, USA).
Firstly, the bare electrodes were used for the electrochemical detection of Cd, Pb and Cu.The results for all electrodes studied are shown in Figure 3.

Bare Composite Electrodes
For metal determination, the technique chosen was SWASV.This consists of applying a potential (−1.4 V) for 7 min that reduces the ions on the electrode surface; then, staircase potential is applied and the current generated is recorded.This process is performed under N2 bubbling.Moreover, a modification in the electrochemical cell is used.Instead of using a handmade reference electrode, the one used for the measurements is Orion 900 electrode (Thermo Scientific, Beverly, MA, USA).
Firstly, the bare electrodes were used for the electrochemical detection of Cd, Pb and Cu.The results for all electrodes studied are shown in Figure 3.As can be seen, 20% graphite electrodes showed the best response, as it has a better sensitivity compared with 15% graphite and 10% CNTs composite electrodes for three metal cations analyzed.

Hg-NPs Drop Casted Electrodes
The next step is to modify the surface of the 20% graphite electrode with Hg-NPs, as mentioned above.Once the surface is modified, the electrode is tested for Cd, Pb and Cu determination using SWASV.The corresponding results are shown in Figure 4.As can be seen, 20% graphite electrodes showed the best response, as it has a better sensitivity compared with 15% graphite and 10% CNTs composite electrodes for three metal cations analyzed.

Hg-NPs Drop Casted Electrodes
The next step is to modify the surface of the 20% graphite electrode with Hg-NPs, as mentioned above.Once the surface is modified, the electrode is tested for Cd, Pb and Cu determination using SWASV.The corresponding results are shown in Figure 4.

Figure 1 .
Figure 1.CV (a) and EIS (b) characterization of the different electrodes.

Figure 1 .
Figure 1.CV (a) and EIS (b) characterization of the different electrodes.

Figure 1 .
Figure 1.CV (a) and EIS (b) characterization of the different electrodes.

Figure 3 .
Figure 3. Calibration curves for Cd (a), Pb (b) and Cu (c) for each raw material.

Figure 3 .
Figure 3. Calibration curves for Cd (a), Pb (b) and Cu (c) for each raw material.

Table 1 .
Percentages used in the construction of each electrode.