A Highly Sensitive Electrochemical Sensor for Cd²⁺ Detection Based on Prussian Blue-PEDOT-Loaded Laser-Scribed Graphene-Modified Glassy Carbon Electrode

Abstract

Heavy metal ion pollution has had a serious influence on human health and the environment. Therefore, the monitoring of heavy metal ions is of great practical significance. In this work, we describe the development of an electrochemical sensor to detect cadmium (Cd²⁺) using a Prussian blue (PB), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT)-loaded laser-scribed graphene (LSG) nanocomposite-modified glassy carbon electrode (GCE). In this nanocomposite material, we successfully brought together the advantages of an extraordinarily large surface area. The accumulation of PB nanoparticles results in an efficient electrochemical sensor with high sensitivity and selectivity and fast detection ability, developed for the trace-level detection of Cd²⁺. Electrochemical features were explored via cyclic voltammetry (CV), whereas the stripping voltammetry behavior of modified electrodes was analyzed by utilizing differential pulse voltammetry. Compared with bare GCE, the LSG/PB-PEDOT/GCE modified electrode greatly increased the anodic stripping peak currents of Cd²⁺. Under the optimized conditions, the direct and facile detection of Cd²⁺ was achieved with a wide linear range (1 nM–10 µM) and a low LOD (0.85 nM).

1. Introduction

As estimated, one billion people produce billions of tons of wastewater, sludge, and solid waste every year. Approximately 80% of this wastewater is discharged untreated (composed of highly toxic industrial discharges, animal and human waste, organic matter, pharmaceuticals, salinity, and plastics) into the natural environment [1]. The amount of human waste entering the water each day is approximately 2 million tons. The economics of a country or region can be adversely affected by contaminated water.

Heavy metals have severely polluted the environment and its components. Despite the fact that heavy metals are naturally occurring compounds, anthropogenic activities have been observed to introduce large amounts of them into different areas of the world’s environment [2]. This affects not only the health of people, animals, and plants, but also the environment’s ability to support life. Due to the fact that heavy metals are not biodegradable, they accumulate in food chains. A heavy metal concentration above the permissible limit can be toxic and harmful to human health, according to the World Health Organization (WHO) [3]. In view of this, heavy metal detection is of the utmost importance and urgency. Cadmium is a heavy metal with a serious and chronic influence on the gastrointestinal, immune, and nervous systems. It is categorized as a toxic environmental contaminant by the United States Environmental Protection Agency (USEPA) [4]. The long half-life and deleterious effects of this heavy metal make it one of the most toxic industrial and environmental pollutants. A trace level of Cd²⁺ can sometimes be found in the groundwater. In humans, the presence of Cd²⁺ in the environment causes a number of disorders, including kidney dysfunction and bone degeneration, lung insufficiency, liver damage, as well as hypertension [4,5]. According to the WHO, the permissible concentration level for Cd in drinking water is approximately 3 parts per billion (ppb). Hence, it is extremely important to develop simple and reliable methods for detecting and monitoring the presence of these toxic metal ions in water.

At present, a series of techniques and methods are utilized to detect metal ions, including solid-phase spectrometry (SPS), electrothermal atomic absorption spectrometry (EAS), ICP-MS, ultraviolet–visible spectroscopy (UV–vis) and colorimetric and ion chromatography [3,5]. It should be noted that these methods have some limitations, i.e., the need for highly qualified technicians for their operation, the difficulty of analyzing large samples with the instrument and the time-consuming process and difficulty in field analysis.

Among recently developed techniques for the detection of Cd²⁺, electrochemical sensors are considered to be economical, since the equipment and operating procedures tend to be simpler and more affordable than those involved in other techniques [6]. Moreover, electrochemical sensors have many advantages, including being low-cost, portable (in-situ and real-time detection) and simple, and they offer higher sensitivity and specificity than the detection techniques mentioned in the above section [7,8]. It is evident that the electrochemical device is considered to be environmentally friendly, since electrochemical processes and technologies operate with the electron as their reagent, which is non-polluting [9]. It can be found that nanocomposite materials are core components and play an important role in the development of electrochemical-based cadmium ion sensors.

In recent Cd²⁺ electrochemical detection studies, Pu et al. determined Cd²⁺ using a Fe₃O₄/Bi₂O₃/C₃N₄ modified glassy carbon electrode (GCE) with high adsorption capacities. The linear response sensor was observed in the range of 0.01 to 3 μmol/L and exhibited high levels of sensitivity, where the minimal detectable concentration of Cd²⁺ was less than 3 × 10⁻⁹ mol/L [10]. Sreekanth et al. developed an electrochemical sensor based on DNA to detect Cd²⁺ using ethyl green (EG) and multiwalled carbon nanotubes (MWCNT). In this method, the linear detection range is characterized by a sensitivity of around 5 nA/nM and the limit of detection is 2 nM [11]. Lee et al. utilized a structure switching aptamer duplex immobilized on an electrochemically reduced graphene (ERGO) electrode and measured Cd²⁺ concentrations between 1 fM and 1 nM, and the LOD could be 0.65 fM. Qin et al. reported an electrochemical sensor for detecting Cd²⁺ using gold nanoparticle (AuNP)-functionalized ß-cyclodextrin (ß-CD). The nanocomposites presented in this work exhibit excellent electrical properties due to the presence of AuNPs and GS and show high capture capability due to the strong Cd²⁺ adsorption properties of the ß-CD. Under optimal stripping conditions, the peak currents for Cd²⁺ in the interface are linearly proportional to the concentrations of these elements over the range of 40–1200 μg/L [12]. The preparation of these modified electrodes, however, is complex and time-consuming. Thus, the development of simple, fast, and reliable protocols for monitoring and detecting Cd²⁺ is crucial.

Graphene (Gr) and graphene-based carbon materials have attracted wide interest due to their large surface area, high electrical conductivity, rapid heterogeneous electron transfer and excellent mechanical properties [13,14,15,16]. In order to manufacture graphene-based materials, chemical vapor deposition (CVD), self-reduction of graphene oxide or other approaches are often employed, which are expensive and labor-intensive, thus significantly limiting their commercial applications [17,18]. Therefore, it is imperative to develop a low-cost method for fabricating graphene and graphene-based nanomaterials on a large scale. Interestingly, laser-scribed graphene has been successfully demonstrated as an efficient method of producing 3D porous conductive graphene using thermoplastic resins, including polyimide (PI) [19,20,21]. Recent developments in electrochemical sensors based on LSG have successfully detected small molecules, potential biomarkers and neurotransmitters [22,23,24]. To date, no laser-scribed graphene-based electrochemical sensor for the detection of heavy metals ions has been reported.

Moreover, composite materials made from transition metals and conducting polymers have been used to prepare a wide range of biosensors based on electrochemical transducers [25,26,27,28]. Prussian blue (PB, ferric ferrocyanide Fe₄^III [Fe^II (CN)₆]₃) belongs to the group of inorganic–organic compounds [29]. It is a nitrogen-containing, mixed-valence compound, having a large surface area, high-density Fe-N₄ coordination structure and it is relatively inexpensive for use in biosensors due to its excellent redox property and superior catalytic properties [30,31,32,33]. The soluble nature of PB facilitates easy diffusion from the electrode surface into the electrolyte solution, resulting in poor performance [26]. Therefore, finding suitable strategies for PB immobilization without sacrificing its activity has remained a challenge for researchers. In recent decades, conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh) and their derivatives have been broadly applied in the area of biosensing [27,34,35,36]. Among them, poly(3,4-ethylenedioxythiophene) (PEDOT) has been regarded one of the most promising conducting polymers because its synthesis is extremely straightforward, it possesses excellent redox properties and it has superior conductivity along with greater environmental stability [37,38,39]. Ernst et al. developed an enzyme-based biosensor for the detection of H₂O₂ by fabricating a layer-by-layer structured PB-PEDOT thin film [40]. Huang et al. described the synthesis of pyrolyzed PB-PEDOT nanocomposites as bifunctional catalysts for oxygen reduction reactions (ORR) [30]. According to our knowledge, no study has reported the detection of Cd²⁺ using PB-PEDOT-coated LSGs.

Hence, in the present study, we have developed a simple and effective synthesis method to combine a conductive polymer—poly(3,4-Ethylenedioxythiophene) polystyrene sulfonate (PEDOT)—with Prussian blue (PB) and laser-scribed graphene (LSG) to design an electrochemical platform for the detection of Cd²⁺ in tap water and wastewater samples. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM) measurements of the PB/PEDOT/LSG nanocomposite were used to explore its physicochemical properties. Additionally, the as-prepared nanocomposite was used to modify GCE by drop casting, and the electrocatalytic performance of such a modified nanocomposite was systematically studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Subsequently, the modified electrode was utilized toward the detection of Cd²⁺ ions in acetate buffer solution (ABS; pH 5.0) using the differential pulse voltammetry (DPV) method. By modifying GCE with PB/PEDOT/LSG, exceptional electrochemical performance was achieved for Cd²⁺ detection, and the reason for this may be the synergistic effect of PB, PEDOT and LSG on the electronic microstructure. On the other hand, the presence of the -COOH group on LSG chelates with Cd²⁺ to form COO-Cd-OOC, thereby enriching Cd²⁺ on the electrode surface. It is shown that the PB/PEDOT/LSG/GCE sensor exhibits high sensitivity and selectivity and fast detection capabilities, with an LOD in the nM range, making it a practical and cost-effective heavy metal sensor for monitoring drinking water quality.

2. Materials and Methods

2.1. Reagents and Materials

There was no further purification of the chemicals used in this study; all of them were reagent-grade chemicals. Polyimide film (PI) was obtained from American Taiwan Professional Plastics Co., Ltd., Zhubei City, Taiwan. Hydrochloric acid (HCl) was obtained from Cheng Yi Chemicals and Materials Co., Ltd., Kaohsiung City, Taiwan. Potassium hexacyanoferrate (II) trihydrate (K₄Fe (CN)₆ · 3H₂O) was obtained from Strem Chemicals, Inc., Taiwan. Ethanol was obtained from Chung Yuan Chemicals, Inc., Taiwan. Glacial acetic acid (CH₃CO₂H) was obtained from Emperor Chemical Co., Ltd., Taipei, Taiwan. Potassium hexacyanoferrate (III) (K₃Fe (CN)₆), iron (III) chloride hexahydrate (FeCl₃ · 6H₂O), 3,4-ethylenedioxythiophene (EDOT) (C₂H₄O2C₄H₂S), potassium chloride (KCl) and sodium acetate trihydrate (CH₃COONa · 3H₂O) solutions were all purchased from Sigma Aldrich, Taiwan. All aqueous solutions were prepared in ultra-pure Milli-Q water with a resistivity of 18.2 MΩ cm (at 25 °C).

2.2. Instrumentation

The morphology analysis was carried out by using a emission scanning electron microscope (JEOL JSM-7610F SEM, Tokyo, Japan). Raman spectroscopy was performed by using the micro-Raman spectrum (ACRON, UniNanoTech Co., Ltd., Yongin-si, Korea). Powder diffraction analysis was performed using low-angle XRD (EMPYREAN PANalytical, The Netherlands). The above-mentioned physicochemical characterization was performed for confirmation of the material properties and electrode modification. This characterization was carried out on a conventional three-electrode system with a PalmSens4 electrochemical workstation equipped with the PS Trace 5.6 software (Palm Sens BV, Houten, The Netherlands). A GCE modified with the synthesized samples served as a working electrode, while a Pt wire and an Ag/AgCl were used as counter and reference electrodes, respectively. Prior to experiments, nitrogen gas was used to de-aerate the solutions. In order to conduct CV measurements, we used three-electrode systems in the presence of a [Fe (CN)₆]^3−/4− redox couple dissolved at 0.1 M KCl. The CV measurements were conducted in a potential range of −0.2 V to 0.6 V at a scan rate of 50 mVs⁻¹. The DPV measurements were carried out in the potential range of −0.2 V to 0.6 V in PBS (pH 5), at 50 mVs⁻¹ and an amplitude of 0.05 V, as well as a step potential of 0.07 V.

2.3. Fabrication of LSG

A schematic representation of the fabrication of LSG and the synthesis of PB-PEDOT nanoparticles for the sensor is shown in Figure 1. The Snapmaker Original 3-in-1 3D printer was used to engrave the graphene directly on the PI sheets. Prior to laser etching, the PI sheets were cleaned by rinsing them with absolute ethanol followed by ultra-pure water and then drying them in an oven at 60 °C. An LSG or carbonized layer was produced by engraving pre-designed patterns directly onto the PI films using a portable laser scribing system with a power between 150 and 400 mW under air to produce a black layer of carbonization. Two distinct areas of the film could be clearly distinguished: the black laser-induced LSG film (Figure 1) and the orange PI film that was not exposed to the laser. We removed the PI sheets after laser etching and scraped them with a spoon, after which we ground them to a powder using a mortar and pestle. The resultant material was defined as LSG.

2.4. Synthesis of PB-PEDOT Nanoparticles

In order to synthesize PB-PEDOT nanoparticles, the synthesis process was performed in a 50 mL round-bottom flask equipped with a magnetic stirrer. Solution A was prepared by sonicating 20 mL of a 0.1 M solution of HCl containing 4 mM FeCl₃ and 4 mM K₃Fe (CN)₆ for 10 min. Furthermore, 10 mL of ethanol containing 50 µL of PEDOT was added to 20 mL of 0.1 M HCl solution to produce solution B. The slow addition of solution A to solution B was performed, stirring vigorously while the mixture was added. An overnight stir at room temperature was performed on the resulting suspension. Later, the mixture gradually changed color from green to dark blue, indicating the emergence of the PB-PEDOT nanocomposite. Lastly, after centrifugation and repeated washing with ethanol and water, the mixture was dried in an oven at 40 °C for several hours.

2.5. Synthesis of PB-PEDOT-LSG Nanocomposite

For the GCE electrode modification, 0.5 µm of alumina powder was used to clean the electrode, and then it was washed with distilled water and dried. After this, 1 mg of LSG and 1 mg of synthetic PB-PEDOT were mixed together in a 1:1 ratio (optimized) of water and ethanol solution with ultrasonication for 30 min in order to obtain a homogeneous suspension. On the surface of GCE, 10 µL of the suspension was dropped and dried. The electrode developed during these experiments was denoted PB-PEDOT/LSG/GCE.

2.6. Fabrication of Nanocomposite-Modified Electrodes

Before the surface modification, a GCE with a diameter of 3 mm was polished using 0.05 m alumina powder on a polishing cloth, rinsed thoroughly with double-distilled water and allowed to dry at room temperature. In order to prepare the electrochemical Cd²⁺ sensor, five microliters of a 1 mg ml⁻¹ PB-PEDOT-LSG composite dispersion were drop-cast onto the pretreated GCE electrode and allowed to dry at room temperature. The same procedure was used to prepare PB-PEDOT and LSG-modified GCE for comparison.

3. Results and Discussion

3.1. Physiochemical Characterization

SEM analyses were performed to examine the morphology of LSG. The SEM image of the LSG is presented in Figure 2a. It is evident that the morphology is porous and there are numerous edge plane areas, as demonstrated by the SEM images [22]. There has been evidence that focused laser beams could create high local temperatures (more than 2500 °C), resulting in the carbonization and graphitization of polymer surfaces [41]. In this case, the laser scribing was conducted in air, and oxygen and moisture that were readily available during the graphitization process may have burned some carbon, resulting in a porous structure. The LSG patterns with edge plane electron transfer sites provide a highly accessible electrochemical surface area, while the 3D morphology will enhance electron transfer behavior. The SEM image of the PB-PEDOT nanoparticles is presented in Figure 2b. PB-PEDOT nanoparticles possess a coral-like structure with many separate tentacles. We observe significant clustering as a result of aggregation and agglomeration upon drying [42]. The SEM image of the LSG/PB-PEDOT nanocomposite (Figure 2c) reveals that large quantities of PB-PEDOT are attached to the surface of the LSG. No isolated PB-PEDOT is observed, indicating the strong interaction between PB-PEDOT and LSG.

The EDS spectra of LSG, PB-PEDOT and LSG/PB-PEDOT nanocomposites are presented in Figure S1a–c, respectively. EDS analysis was used to study the atomic composition of each sample, such as its elements or chemicals. Carbon, oxygen, iron and sulfur elements were utilized in the synthesis of LSG, PB-PEDOT and LSG/PB-PEDOT, as indicated by the EDS spectra. This confirms the successful synthesis of the PB-PEDOT/LSG nanocomposite [43].

XRD was used to investigate the structural characteristics of LSG, PB-PEDOT and LSG/PB-PEDOT electrodes. Based on the XRD pattern in Figure S2, LSG exhibits a graphitic structure. The peak positions at 2θ = 15.00°, 23.42° and 27.07° are correlated with the layer-to-layer structure of LSG, with distance (d-spacing) of approximately 0.86 nm [13]. The broadening is further accompanied by a lack of periodicity on the c-axis. The XRD spectra of PB-PEDOT show major peaks at 2θ = 16°, 25°, 35° and 40° and are well correlated with JCPDS, PDF#521907 [30]. For LSG/PB-PEDOT, the XRD data show major peaks at 2θ = 15.55°, 24.32°, 35° and 40°, which confirms the nanocomposite formation.

3.2. Electrochemical Characterization of Modified Electrodes

To investigate the electrochemical characteristics of bare GCE, LSG, PB-PEDOT and LSG-PB-PEDOT nanocomposite-based modified GCE, cyclic voltammetry (CV) measurements in the presence of 1 mM [Fe(CN)₆]^3−/4− in 0.1 M KCl at a scan rate of 50 mV s⁻¹ were performed. As shown in Figure 3a, the peak shape of the CV shows a typical reversible electrochemical reaction in which the diffusion of the electroactive species governs the rate of reaction to the surface of a planar electrode [44]. However, from the PB-PEDOT/LSG-modified GCE, the redox peak current is enhanced (Ipa = 56.0 μA, Ipc = −57.24 μA; ΔEp = 0.235 mV) as PB-PEDOT has high electrical conductivity and the multilayer stacked graphene nanosheets of LSG have a large surface area. The cathodic peak at −0.168 V vs. Ag/AgCl observed after coating with Prussian blue (PB) is due to the reduction of PB to Prussian white (PW) (colorless) [29]. The reduction process can be described by the following Equation (1).

$${\mathrm{KFe}}^{\mathrm{III}}\left[{\mathrm{Fe}}^{\mathrm{II}}\right(\mathrm{CN}{)}_6]+{\mathrm{e}}^{-}+{\mathrm{K}}^{+}\to {\mathrm{K}}_2{\mathrm{Fe}}^{\mathrm{II}}[{\mathrm{Fe}}^{\mathrm{II}}\left(\mathrm{CN}{)}_6\right]$$

(1)

The nanocomposite was found to provide more active sites, increasing their electrochemical efficiency and current response. Electrochemical characteristics of other modified electrodes are described in Table S1. Figure 3b shows a comparative DPV analysis of all the modified electrodes, among which the LSG/PB-PEDOT/GCE electrode exhibits the highest current peak.

3.3. Electrochemical Detection of Cadmium Ions

Various regulatory agencies are involved in the process of drafting policies and directives to set maximum allowable concentration limits in drinking water in order to monitor and reduce the risks of heavy metals to human health. According to the US EPA and WHO, the concentration of Cd²⁺ should not exceed 3–5 ppb. The European Union sets a Cd²⁺ standard of 0.45 ppb in their water policy frameworks [45,46]. In response to these standards, researchers are developing materials for sensitive electrochemical sensors that can detect these heavy metals at an ultra-trace level. We used PB-PEDOT/LSG-modified glassy carbon electrodes to analyze target metal Cd²⁺. Figure 4a,b show the DPV curves of Cd²⁺ concentrations from 1 nM to 10 µM measured at the PB-PEDOT/LSG/GCE electrode in 0.1 M acetate buffer at pH 5. A clearly defined anodic peak is observed at 0.95 V that increases with Cd²⁺ concentration. However, the peak potential response to the Cd²⁺ concentration shifts slightly. Calibration curves for Cd²⁺ concentrations ranging from 1 nM to 1000 nM and 1000 nM to 10 µM versus oxidation peak currents are shown in Figure 4c,d. In Figure 4a, the circle denotes the lower concentrations. The calibration curves both show linearity, with a correlation coefficient of 0.993 and 0.983, respectively. From the lower concentration calibration curve (Figure 4d), the linear regression equation can be expressed as Ip (μA) = 4.316 C (nM) + 15.11 (R² = 0.983). The limit of detection (LOD) was calculated to be 0.85 nM (S/N = 3), which is much lower than the respective concentration of 3 µg L⁻¹ specified by the WHO [47]. It is believed that the enhanced electrocatalytic performance is a result of the unique composite nanostructures, the reduced electrical resistance and the synergistic effect of the different components. In addition, well-dispersed PEDOT nanoparticles on the LSG layer surface increased the accessible surface area by preventing aggregation and facilitating the fast diffusion of Cd²⁺ ions at the surface and in the bulk. Based on these results, it is concluded that the PB-PEDOT-LSG/GCE electrode as built would be well suited for the electrochemical detection of Cd²⁺ levels in water.

Table 1. Comparison of Cd²⁺ sensing using proposed method with some previous reports.

Electrode Materials	Method	Linear Range (nM)	LOD (nM)	Reference
a GDY/GCE	g SWASV	10–1000	0.46	[48]
b PA/PPy/GO/GCE	h DPV	44–1330	19	[49]
c Nano-PPCPE	DPV	100–3000	78	[50]
d Fe3O4/RGO/GCE	DPV	0–800	56	[3]
e GCE/SBA-15-NH2-Nafion	i SWV	360–1680	710	[51]
f CFP/CoMOF/AuNPs/GSH	SWV	1–20,000	1	[52]
PB/PEDOT/LSG/GCE	DPV	1–10,000	0.85	This work

^a Graphdiyne-modified glassy carbon electrode; ^b Phytic acid-functionalized polypyrrole–graphene oxide functionalized glassy carbon electrode; ^c Nanoporous pseudo carbon paste electrode; ^d Magnetite-reduced graphene oxide-modified glassy carbon electrode; ^e Amino-functionalized mesoporous silica powder coated with Nafion-modified glassy carbon electrode; ^f Carbon fiber paper–cobalt metal oxide framework–gold nanoparticles–glutathione; ^g Square wave anodic stripping voltammetry; ^h Differential pulse voltammetry; ⁱ Square wave voltammetry.

presents a comparison of the analytical performance of the newly developed electrode with previous studies [3,48,49,50,51,52]. The electro-catalytic performance of our sensor was superior or comparable to that of other modified electrodes. The low detection limit and wide linear response to Cd²⁺ for the proposed sensor are satisfactory. Moreover, this modified electrode possesses high selectivity, simple synthesis and inexpensive materials.

3.4. Performance Assessment

In order to evaluate the reproducibility of the PB-PEDOT/LSG/GCE electrodes, five equally electroactive material-coated electrodes were fabricated and interrogated using the DPV technique in terms of the response to 500 µM Cd²⁺ in 0.1 M acetate buffer (pH 5.0). The oxidation peak potential of Cd²⁺ species was observed at −0.95 V, irrespective of the electrodes, with negligible current variation, as shown in Figure 5a. The results showed a relative standard deviation (RSD) of 1.97%, which reveals the excellent reproducibility of the proposed electrode.

For evaluating the interferences in the electrochemical detection of Cd²⁺ by PB-PEDOT/LSG/GCE, the influence of 10 folds of Mn²⁺, Pb²⁺, Ca²⁺ and Zn²⁺ with 500 μM Cd²⁺ was tested using chronoamperometry. Figure 5b shows that 500 μM Cd²⁺ can be determined even in 10-fold potential interference. It is indicated that PB-PEDOT/LSG/GCE is suitable for the analytical application and experiences no considerable interference by Mn²⁺, Pb²⁺, Ca²⁺ and Zn²⁺.

To investigate the operational stability of the fabricated sensors, we have evaluated the DPV current response of PB-PEDOT/LSG/GCE towards 500 µM Cd²⁺ in 0.1 M acetate buffer (pH 5.0) and 25 continuous cycles were carried out at a scan rate of 50 mVs⁻¹, as shown in Figure S3. After running 25 continuous cycles, a negligible loss of approximately 2.3% in the current signal was noticed, retaining 98.7% of the initial current response, thereby indicating the efficient operational stability and antifouling properties of the PB-PEDOT/LSG/GCE electrode or Cd²⁺ sensor.

Moreover, this electrochemical detection system was used to detect the concentration of Cd²⁺ in tap water and drinking water samples to evaluate its detection performance in a practical application. The results are summarized in Figure 6a,b; they show that the electrochemical detection system using the PB-PEDOT/LSG/GCE produced satisfactory detection results. The tap water sample was spiked with 100 μM, 500 μM, 700 μM, 1 mM and 10 mM, respectively. The packaged drinking water sample was spiked with 100 μM, 300 μM, 500 μM, 700 μM and 1 mM, respectively. These results demonstrate that the electrochemical detection system could act as an effective method for the on-site detection of Cd²⁺ at low (μM) levels in packaged drinking and tap water samples. In

Table 2. Analysis of Cd²⁺ in tap water and packaged drinking water samples.

Real Samples	Added Concentration (μM)	Measured Concentration (μM)	Recovery (%)
Packaged drinking water	1000	940.00	93.8
	700	672.00	96.0
	500	478.90	95.8
	300	292.86	97.6
	100	99.55	99.6
Tap water	10,000	9320	93.2
	1000	980	97.7
	700	709.52	101.4
	500	516.10	103.2
	100	94.69	94.7

, the real samples are presented, for which the recovery rates were found to be in the range of 93.8–103.2%.

4. Conclusions

A highly sensitive electrochemical sensor for the detection of Cd²⁺ is described in this paper. Combining the advantages of PB-PEDOT and LSG nanocomposites, good electrical conductivity and high specific surface area were obtained. The nanocomposites for the electrode were characterized by various spectroscopy and analytical methods. The sensor can achieve an LOD of 0.85 nM and has good reproducibility. After running 25 continuous cycles, a negligible loss of approximately 2.3% in the current signal was noticed, retaining 98.7% of the initial current response; this electrochemical sensor demonstrated better long-term stability. In addition, it was found that this analytical method proved effective even at interferences that were 10-fold greater than the measured value. The feasibility of the electrochemical detection system for the detection of Cd²⁺ was further examined by the analysis of tap water and packaged drinking water samples. The electrochemical detection system using PB-PEDOT/LSG/GCE produced satisfactory Cd²⁺ detection results for water quality monitoring.