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Article

Self-Powered Photoelectrochemical Assay for Hg2+ Detection Based on g-C3N4-CdS-CuO Composites and Redox Cycle Signal Amplification Strategy

by
Yonghuan Su
1,
Lixia Su
1,
Bingqian Liu
1,*,
Youxiu Lin
2 and
Dianping Tang
3,*
1
Guizhou Engineering Laboratory for Synthetic Drugs (Ministry of Education of Guizhou Province), College of Pharmacy, Guizhou University, Guiyang 550025, China
2
Key Laboratory of Modern Analytical Science and Separation Technology of Fujian Province, College of Chemistry, Chemical Engineering and Environment, Minnan Normal University, Zhangzhou 363000, China
3
Key Laboratory of Analytical Science for Food Safety and Biology (MOE & Fujian Province), Department of Chemistry, Fuzhou University, Fuzhou 350108, China
*
Authors to whom correspondence should be addressed.
Chemosensors 2022, 10(7), 286; https://doi.org/10.3390/chemosensors10070286
Submission received: 26 May 2022 / Revised: 12 July 2022 / Accepted: 17 July 2022 / Published: 18 July 2022
(This article belongs to the Special Issue Photoelectrochemical (Bio)sensors for Biological, Food, and Environmental Analysis)
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Abstract

:
A highly sensitive self-powered photoelectrochemical (spPEC) sensing platform was constructed for Hg2+ determination based on the g-C3N4-CdS-CuO co-sensitized photoelectrode and a visible light-induced redox cycle for signal amplification. Through successively coating the single-layer g-C3N4, CdS, and CuO onto the surface of an electrode, the modified electrode exhibited significantly enhanced PEC activity. The microstructure of the material was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). However, the boost in photocurrent could be noticeably suppressed due to the consumption of hole-scavenging agents (reduced glutathione) by the added Hg2+. Under optimal conditions, we discovered that the photocurrent was linearly related to the Hg2+ concentration in the range of 5 pM–100 nM. The detection limit for Hg2+ was 0.84 pM. Moreover, the spPEC sensor demonstrated good performance for the detection of mercury ions in human urine and artificial saliva.

1. Introduction

It is well known that heavy metals are very harmful to the human body. Among such metals, mercury is the most widely used heavy metal, and it is used in the manufacture of compact fluorescent-lamp light bulbs, jewelry, thermometers, blood pressure monitors, and silver dental fillings [1]. Mercury ions (Hg2+) are the general form of mercury and these can cause various diseases, such as hypertension, enteritis, bronchitis, and pulmonary edema [2]. At present, several methods have been utilized for the detection of mercury ions, including fluorescent and colorimetric sensors [3,4,5], surface-enhanced Raman scattering [6], and electrochemistry [7,8]. Although these methods have the advantages of high sensitivity and accuracy [9], they are limited by complex sample pretreatment and high operation cost [10]. Therefore, developing a mercury-ion detection method with a facile pretreatment process, and low cost, is imperative.
Photoelectrochemical (PEC) sensors demonstrate significant potential for analysis owing to advantages such as weak background noise, simple operation, high sensitivity, and quick response [11]. The self-powered photoelectrochemical (spPEC) sensor is an emerging photoelectrochemical detection method owing to its portability, lack of external power supply, and sustainability [12]. The spPEC sensor is primarily based on a three-electrode system, including the type of anode or cathode. The three-electrode scheme adopts a single working electrode as the signal source and a specific recognition platform [13]. However, the interaction between the optical electrode and the reducing agent or the target molecule inevitably leads to deterioration of the performance of the PEC sensor [14]. Cao et al. [15] constructed an ingenious visible light-induced membraneless self-powered PEC biosensing platform by integrating a signal amplification strategy for PEC bioanalysis. Owing to the simple enzyme-induced chemical redox cycle process, the signal could be effectively and repeatedly regenerated through coupling reduction and oxidation reactions, which were used for ultrasensitive PEC analysis.
Therein, the selection of photoactive materials is the key to improving the detection of photoelectrochemical sensors. Currently, g-C3N4 is considered as the photoactive material with the highest potential owing to its advantages such as suitable band gap, excellent stability, and low toxicity [16]. Nevertheless, the photocatalytic efficiency of single-phase g-C3N4 is low owing to the lack of surface redox active sites and the high recombination rate of electron–hole (e−-h+) pairs [17]. To effectively improve the photocatalytic activity of g-C3N4, it is generally combined with other photoactive materials with lower band gaps. As n-type narrow-band gap semiconductors, CdS quantum dots (QDs) are often used for the study of visible light-active materials [18]. However, CdS QDs are not only photocorrosive [19] but also exhibit a high electron–hole recombination rate [20], which can be resolved by combining them with other p-type photoactive materials. CuO nanoparticles (NPs) are p-type semiconductors [21] that have been widely employed as photocatalysts for different reactions owing to their high conductivity and low band gap [22]. Therefore, combining n-type CdS and p-type CuO to form composites [23] may serve as an effective strategy for enhancing stability and inhibiting electron–hole recombination.
Herein, we propose a spPEC assay for Hg2+ detection based on g-C3N4-CdS-CuO composites and a redox cycle signal amplification strategy. As presented in Scheme 1A, the g-C3N4-CdS-CuO material is prepared via layer-by-layer self-assembly as a self-powered system. A fluorine-doped tin oxide (FTO) electrode is modified using the composite material as a photoanode. As illustrated in Scheme 1B, under irradiation, glutathione (GSH) is oxidized by CuO to form oxidized glutathione (GSSG) for generating a strong photocurrent owing to the GSH-oxidase and peroxidase-like activities of CuO NPs [24,25]. In the presence of GSH reductase (GR), GSSG is reduced to GSH using a reductive coenzyme II (NADPH) as a substrate, resulting in a redox cycle system [26], signal amplification is achieved through redox cycle. However, in the presence of Hg2+, GSH can bind to Hg2+ based on the hard–soft acid–base (HSAB) theory [27], which influences the above redox cycle system and significantly reduces the photocurrent. Based on these results, we can conclude that the g-C3N4-CdS-CuO composite was successfully used to develop a spPEC senor for the detection of Hg2+ in human urine and artificial saliva.

2. Materials and Methods

2.1. Materials and Reagents

All chemicals (Table 1) were of analytical reagent grade, ultrapure water used in all solutions was from tap water purified by a Water Milli-Q system (Merck KGaA, Darmstadt, Germany). All instrumentations used in this work were listed in Table 2.

2.2. Photoelectrochemical Measurement

Prior to the photoelectrochemical measurement, g-C3N4 and CdS were prepared. The detailed preparation processes of these composites are illustrated in Support Information. Firstly, the bared FTO was cleaned ultrasonically in water, ethanol and acetone in sequence and dried at 60 °C. Then, 20 μL g-C3N4 suspension (1.0 mg mL−1) was dropped on the FTO electrode with an active area of 0.25 cm2 and dried at room temperature. After that, 20 μL CdS (5 mg mL−1) and 20 μL CuO (1.0 mg mL−1) were dropped on the FTO in the same operation, successively, to form layer upon layer of electrode materials. A three-electrode system was used to measure the photocurrent in CHI660E Electrochemical Workstation. The modified FTO was used as the working electrode, Pt wire as the counter electrode, and the saturated Ag/AgCl electrode as the reference electrode. GSH (15 mM, 1 mL), GR (170 u mg−1, 2 μL) and NADPH (10 mg mL−1, 250 μL) were added into the 0.1 M phosphate buffer solution (PBS) (5 mL, pH 7.4) as a redox cycle to realize signal amplification. The excitation light was turned on every 10 s in the photoelectric measurement process, and the voltage was constant at 0.0 V. The change in photocurrent was observed by adding different concentrations of mercury ions into the electrolyte.

2.3. Real Sample Processing

Urine samples [28]: 1 mL 50% hydrochloric acid and 0.8 mL 0.1 mol L−1 KBrO3/0.084 mol L−1 KBr were added to 10 mL urine samples; after reaction for 15 min, the appropriate amount of hydroxylamine (120 g L−1) hydrochloride/sodium chloride (120 g L−1) solution was added into the above mixture until the yellow color disappeared; samples were diluted to 50 mL with deionized water and diluted 10 times with PBS buffer before use.
Artificial saliva [29]: 0.6 mg mL−1 disodium hydrogen phosphate, 0.6 mg mL−1 anhydrous calcium chloride, 0.4 mg mL−1 potassium chloride, 0.4 mg mL−1 sodium chloride, 4 mg mL−1 mucin and 4 mg mL−1 urea was dissolved in 1 mL of the deionized water. This was set to pH 7.2, stored in the refrigerator and diluted 10 times with PBS buffer before use.

3. Results and Discussion

3.1. Characterizations of Composite Material

Scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) were used to characterize the prepared composites and to analyze the electrode preparation process.
SEM images reveal that the bare FTO electrode is a uniform spongy layer (Figure 1A) [30]. After g-C3N4 is loaded onto FTO, the surface of the g-C3N4/FTO electrode presents a typical plate-like structure (Figure 1B), which results from aggregation of the g-C3N4 sheets [31]. A large number of CdS QD nanoparticles are distributed on the surface of g-C3N4 (Figure 1C), which is consistent with the literature [32]. The large surface area of g-C3N4 serves as an effective anchor for loading CdS NPs. When the CdS QD-g-C3N4/FTO electrode is covered with CuO nanoparticles (Figure 1D), the CuO nanoparticles appear to be nearly spherical [33]. SEM (Figure 1E,F) and elemental mapping by energy dispersive X-ray spectroscopy (Figure 1I) reveal that C, N, Cd, S, Cu, and O exist in the g-C3N4-CdS-CuO electrode, indicating that the composite material is successfully loaded onto the FTO electrode.
The XRD analysis results of the crystal phase of the synthesized material are presented in Figure 1G. The XRD spectrum of g-C3N4 exhibits two evident diffraction peaks at 24.3° and 27.0°. These peaks represent the (100) and (002) planes of the graphite material, respectively, corresponding to the in-plane structure of the triazine ring and layered superposition of the conjugated aromatic groups [34]. The diffraction peaks of CdS at 26.5°, 37.6°, and 43.7° correspond to the (002), (102), and (103) crystal planes, respectively, which correlate with the wurtzite phase standard cards (JCPDS 80-0006) [35]. The diffraction peaks of the CuO nanoparticles at 35.6°, 38.7°, and 48.8°, in accordance with their monoclinic and crystalline nature, correspond to the (110), (111), and (200) crystal planes, respectively [33].
In the FT-IR spectra (Figure 1H) with the direct detection (Resolution: 4, Number of scans: 8, Detector: DTGS), g-C3N4 (curve a) exhibits strong absorption peaks in the range of 1200–1600 cm−1, which correspond to C-N heterocycles. The absorption band at 806 cm−1 corresponds to the typical breathing mode of the tri-s-triazine ring, and the intensity of the absorption peak near 2900–3400 cm−1 corresponds to the N–H stretching vibration [36]. The Cd-S band of CdS (curve b) demonstrates stretching and bending vibration absorption peaks at 631 cm−1, whereas the peak at 1630 cm−1 corresponds to the O–H stretching vibrations [37]. For the CuO NPs (curve c), the peaks appearing at 701 cm−1 belong to the CuO NPs and confirm the formation of NPs [38]. Moreover, peaks corresponding to OH− and C–O stretching vibrations can be observed from 2800 to 3600 cm−1 [39]. These results confirm that the composite and modified electrodes were successfully synthesized.

3.2. Electrochemical Activity of g-C3N4-CdS-CuO Electrode

The electrochemical properties of the bare and material electrodes were studied, including the diffusion coefficient and heterogeneous electron-transfer rate constant. First, the electrochemical performances of the bare and modified electrodes were determined using cyclic voltammetry, in a 5 mM [Fe(CN)6]3−/4− solution containing 0.1 M KCl, by employing platinum as the counter electrode and saturated Ag/AgCl as the reference electrode (See supporting literature for details). As illustrated in Figure 2A,D, the current response increases gradually with an increase in the scanning rate and demonstrates a good linear relationship with the square root of the potential scanning rate, indicating that the redox reaction on the surface of the bare and modified electrodes is a diffusion control process. The electrically active area of the bare and g-C3N4-CdS-CuO electrodes was calculated according to the Randles–Sevcik equation [40]: 𝐼p = 2.69 × 105 A × 𝑛3⁄2 × D1⁄2 × C × υ1⁄2; where Ip denotes the maximum current (A), D denotes the diffusion coefficient (cm2/s), C = [Fe (CN)6]3−/4− (mol/cm3), N represents the number of transferred electrons, υ denotes the scanning rate (V/s), and A indicates the effective working area (cm2) of the modified electrode. The effective working area (A) can be calculated by a linear fitting of Ip to υ1/2. The active area of the bare electrode was 53.72 cm2 (Figure 2B) and that of the g-C3N4-CdS-CuO electrode was 72.40 cm2 (Figure 2E). Thus, the larger the specific surface area of the g-C3N4-CdS-CuO electrode, the more numerous the active sites are, and this is conducive to the amplification of electrical signals. In addition, the electron transfer rate constants of the bare and g-C3N4-CdS-CuO electrodes can be calculated according to Nicholson’s equation [41]: φ = 𝑘0[πD𝑛FV⁄(RT)]−1⁄2; where φ denotes the peak-to-peak separation (δ Ep), k0 indicates the heterogeneous electron transfer rate constant, D represents the electroactive material diffusion coefficient, F denotes the Faraday constant, R represents the molar gas constant, and T denotes the temperature. The slope of the φ pair [πDnF/(RT)]−1/2 υ−1/2 curve corresponds to the heterogeneous electron transfer rate constant 𝑘0, which is 3.35 × 10−4 cm s−1 (Figure 2C) for the bare electrode and 2.72 × 10−4 cm s−1 (Figure 2F) for the g-C3N4-CdS-CuO electrode. This indicates that the electrochemical equilibrium time required by the g-C3N4-CdS-CuO-modified electrode was not significantly shortened.

3.3. Feasibility of the Designed Strategy

To evaluate the spPEC properties of the fabricated CuO-CdS-g-C3N4/FTO electrode, photocurrent responses of bare FTO, g-C3N4/FTO, CdS-g-C3N4/FTO, and CuO-CdS-g-C3N4/FTO were monitored (Figure 3A). The photocurrent of CuO-CdS-g-C3N4/FTO (column c) is better than that of g-C3N4/FTO (column a) and CuO-CdS-g-C3N4/FTO (column b), which is further verified by electronic impedance spectroscopy (EIS), recorded by the EIS different changes of preparing the electrodes in ferricyanide/ferrocyanide mixed solution ([Fe(CN)6]3−/4−) (See supporting literature for details). As illustrated in Figure 3B, the resistance of g-C3N4/FTO (curve b) is larger than that of FTO (curve a), which may be attributed to the poor conductivity of g-C3N4/FTO. The resistance of CdS-g-C3N4/FTO (curve c) is lower than that of FTO, indicating that CdS promotes g-C3N4 electron transfer. Moreover, the resistance of CuO-CdS-g-C3N4/FTO (curve d) is less than that of CdS-g-C3N4/FTO, which indicates that CuO increases the transfer of electrode current and further increases the photocurrent. The feasibility analysis further verifies this principle.
To verify the amplification caused by the redox cycle strategy, the developed spPEC sensor was studied using the photocurrent output. For comparison, we also carried out the comparison experiment of redox single signal amplification and double signal amplification; that is, under the same conditions, different concentrations of Hg2+ (0.5 nM, 10 nM) were added to GSH (Figure 3C) and GSH, NADPH, and GR (Figure 3D) solutions respectively. As presented in Figure 3C, GSH is oxidized by CuO to GSSG to generate a strong photocurrent (column b vs. a) and single signal amplification was achieved. When Hg2+ was added from 0.5 nM to 10 nM, the photocurrent △I increased from 0.97 μA to 2.27 μA (column c and d). These results demonstrate that the GSH could specifically recognize Hg2+, and the photocurrent was inversely proportional to the concentration of Hg2+. Evidently, when NADPH and GR are added (Figure 3D), the photocurrent reaches a maximum (column b vs. a) and double signal amplification. In the presence of GR, GSSG is reduced to GSH using NADPH as a substrate, thereby resulting in a self-powered redox cycle system. When Hg2+ was added from 0.5 nM to 10 nM, the photocurrent △I increased from 2.49 μA to 3.59 μA (columns c and d), which was the same as the photocurrent trend in Figure 3C, indicating that both methods were beneficial to PEC detection, and the detection effect of double signal amplification was better. Therefore, the GSH-NADPH-GR redox cycle can be used as a signal amplification strategy to improve the detection sensitivity for Hg2+.
Ultraviolet–visible diffuse reflection spectroscopy (DRS) was used to study the electronic orientation of the composite materials. The band gap of the three semiconductors was calculated according to the equation (αhν)2 = C (hν—Eg) (where α, hν, C, and Eg are the absorption coefficient, photoenergy, the constant, and band gap energy, respectively) [42] and extrapolated the intercept of the linear part of (αhν)2 vs. hν plot to the x-axis shown in Figure S1A. The band gaps (Eg) of g-C3N4, CdS, and CuO are 2.84 eV, 2.10 eV, and 1.38 eV, respectively. The conduction bands (ECB) and valence bands (EVB) were estimated using the following formula: ECB = X − Ec − Eg/2 (Ec = 4.5 eV) and EVB = ECB + Eg, where X and Eg denote the geometric average value of the absolute electronegativity of each atom in the semiconductor and the band gap width of the semiconductor, respectively. The ECB values of g-C3N4, CdS, and CuO were 0.92 eV, −0.36 eV, and 0.62 eV, respectively, and the EVB values of g-C3N4, CdS, and CuO were 3.76 eV, 1.74 eV, and 2.0 eV, respectively. The band gaps of g-C3N4, CdS, and CuO reported in the literature are presented in Scheme 2A [43]. When g-C3N4, CdS, and CuO are in contact, the energy level differences between CdS and g-C3N4 and between CdS and CuO (Fermi–Dirac statistics) cause electrons to flow from CdS (high energy level) to g-C3N4 and CuO (low energy level). If the material is described in terms of a Fermi–Dirac distribution, this electron transfer is referred to as a Fermi level alignment. The redistribution of electrons between CdS and g-C3N4 and between CdS and CuO is assumed to trigger the downward and upward movement of CdS, g-C3N4 and CuO band edges, respectively. Therefore, we can infer that the electron orientation of the g-C3N4-CdS-CuO electrode demonstrates a stepped structure, as illustrated in Scheme 2B. The edges of the conduction and valence bands of these three materials increased in the following order: g-C3N4 < CdS < CuO; that is, the CdS layer was inserted between g-C3N4 and CuO to increase the conduction band edge of CuO, providing a higher driving force for the injection of excited-state electrons from outside the CuO layer. When g-C3N4-CdS-CuO was photoexcited under irradiation, the stepped structure at the edge of the band proved not only beneficial for electron injection in g-C3N4-CdS-CuO, but also for hole recovery. Under irradiation, the g-C3N4-CdS-CuO material generated a photocurrent, and the GSH-NADPH-GR redox cycle acted as a self-powered system to further increase the photocurrent and achieve signal amplification (Figure S1B).

3.4. Optimization of Experimental Conditions

To improve the sensitivity of the sensor, we optimized the experimental parameters by using the photocurrent of PEC. We primarily optimized the concentration of NADPH (Figure S2A), and a maximum change was noted when the amount of NADPH was 0.40 μg/μL (250 μL). The amount of GR was also optimized, which is one of the most important factors in the signal detection process. As illustrated in Figure S2B, when different concentrations of GR are added to the bath solution, a maximum photocurrent signal can be obtained at 2.08 ng/μL (2 μL). Next, we determined the optimal GSH concentration (Figure S2C). The maximum signal was obtained at a concentration of 15 mM, after which the signal slowly diminished. Thus, 15 mM was selected as the optimal concentration for GSH. Finally, we optimized the mass ratio of g-C3N4 to CuO. As presented in Figure S2D, the photocurrent signal of the material reaches a maximum when the ratio of g-C3N4 and CuO is 1:1. As shown in Figure S2E, when using a Na2CO3-NaHCO3 (0.1 M, pH 7.4) and tris-HNO3 (0.1 M, pH 7.4) buffer, the signal response of the photocurrent decreases. In contrast, no corresponding interference can be observed in PBS (0.1 M, pH 7.4). Therefore, we think that using PBS (0.1 M, pH 7.4) to prepare the bath solution will result in a better signal response.

3.5. Analytical Performance and Selectivity and Stability

To verify the analytical performance of the proposed spPEC sensing system, different concentrations of Hg2+ were added and measured under the optimal conditions. As illustrated in Figure 4A, the photocurrent signal gradually weakens with increasing Hg2+ concentration. Hg2+ is linearly and logarithmically correlated in the range between 5 pM and 100 nM. The regression equation was Y = 0.90LogCHg2+ − 2.40 (R2 = 0.993), and the calculation formula for detection limit is S/N = 3 [44]. The detection limit was 0.84 pM (Figure 4B). Compared with other Hg2+ detection methods, this sensor demonstrated a picomolar detection sensitivity for Hg2+ without requiring aptamer fixing and pretreatment processes (Table S1). Therefore, we can conclude that the detection efficiency of the sensor is significantly improved.
The experimental results indicated that the PEC response did not change significantly within 200 s (Figure 4C), and the relative standard deviation (RSD) of the PEC response change was less than 0.77%. To assess the selectivity of the constructed spPEC sensor, we investigated its specificity for Hg2+ (0.5 nM). Among several analogs, including Mg2+, Mn2+, Cd2+, Fe3+, Zn2+, K+, and Na+ (50 nM), a much lower current value was recorded with Hg2+ than with other interfering species (Figure 4D). This result demonstrates the excellent selectivity of the spPEC method for Hg2+. The stability of the sensor was also evaluated for the detection of 0.5 nM Hg2+. Moreover, to evaluate the repeatability of the proposed sensor, six replicate measurements were performed with the sensor using 0.5 nM Hg2+ (Figure S3). The results revealed an RSD of 0.53%, indicating reasonable repeatability of the spPEC sensor for Hg2+ detection.

3.6. Analysis of Real Samples

To investigate whether the proposed method could be used to detect Hg2+ in complex samples, the experiment was conducted for two different substances (the urine of healthy people and artificial saliva). The standard addition method was used to evaluate the feasibility of the method for actual samples, as presented in Table 3. The concentration of Hg2+ was in the range 0.1–10 nM, and the recovery rate was between 93.56% and 103.02%, indicating possible applicability of the spPEC sensor for detecting Hg2+ in real-sample application.

4. Conclusions

A new spPEC sensor for Hg2+ detection based on g-C3N4-CdS-CuO composites, as well as a redox-cycle signal-amplification strategy were developed. The novel g-C3N4-CdS-CuO co-sensitized modified electrode, which exhibits large surface area and improved photoelectric response due to a cascade structure, was successfully used to detect target ions in complex samples. In addition, the self-powered sensor demonstrated satisfactory sensitivity owing to double signal amplification (GSH and GSH-NADPH-GR) and a limit of detection of 0.84 pM for Hg2+. More importantly, in vitro application of the GSH-NADPH-GR redox cycle demonstrated its potential as a photoelectrochemical sensor and, therefore, presented a new method for constructing novel sensors. The sensor also demonstrated significant potential for detecting Hg2+ in human urine and artificial saliva samples. More significantly, the developed approach can also be used to detect other heavy metal ions according to the HSAB theory and may serve as a new analytical tool for specific, sensitive, and reliable analysis of heavy metal ions in clinical toxicology, food, and the environment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemosensors10070286/s1, Figure S1: The band gaps of g-C3N4, CdS, and CuO were measured by the UV-vis diffuse reflection spectra (DRS) (A), the picture shows the direction of electrons in the photoelectric sensor under light conditions (B); Figure S2: (A) Concentration of NADPH; (B) Concentration of GR; (C) Concentration of GSH; (D) Mass ratio of g-C3N4 to CuO; (E) Different types of buffer solutions (0.1M); Figure S3: Reproducibility of g-C3N4-CdS-CuO at 0.5 nM Hg2+; Table S1: Comparison of other analysis strategies for Hg2+ detection. References [10,25,34,45,46,47,48,49,50] are cited in the supplementary materials.

Author Contributions

Conceptualization, B.L. and Y.L.; methodology, Y.S.; validation, Y.S.; formal analysis, L.S.; investigation, L.S.; data curation, Y.S.; writing—original draft preparation, Y.S.; writing—review and editing, D.T. and B.L.; supervision, D.T. and B.L.; funding acquisition, B.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (21864007, 21904056, 21605029), and Guizhou Provincial Natural Science Foundation (Qian Ke He Ji Chu [2020]1Y042, [2017]5788 Qian Ke He Platform for Talents, [2018]5781 Qian Ke He Platform for Talents).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemosensors 10 00286 sch001 550
Scheme 1. Schematic diagram of electrode preparation process (A) and schematic diagram of redox cycle on photoanode of self-powered device and test in actual sample (B).
Scheme 1. Schematic diagram of electrode preparation process (A) and schematic diagram of redox cycle on photoanode of self-powered device and test in actual sample (B).
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Chemosensors 10 00286 g001 550
Figure 1. SEM images of naked FTO (A), g-C3N4 (B), CdS (C) and CuO (D); (E,F) SEM mapping image; XRD (G) includes a: FTO, b: g-C3N4/FTO, c: CdS-g-C3N4/FTO, d: CuO-CdS-g-C3N4/FTO; FT-IR (H) of a: g-C3N4, b: CdS and c: CuO; and (I) EDX spectrum of g-C3N4-CdS-CuO.
Figure 1. SEM images of naked FTO (A), g-C3N4 (B), CdS (C) and CuO (D); (E,F) SEM mapping image; XRD (G) includes a: FTO, b: g-C3N4/FTO, c: CdS-g-C3N4/FTO, d: CuO-CdS-g-C3N4/FTO; FT-IR (H) of a: g-C3N4, b: CdS and c: CuO; and (I) EDX spectrum of g-C3N4-CdS-CuO.
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Figure 2. The cyclic voltammetry (CV) curves of the bare electrode (A) and the g-C3N4-CdS-CuO modified electrodes (D) at different scan rates (0.005~ 0.02 V/s), and bare electrode (B) and the g-C3N4-CdS-CuO modified electrodes (E) of linear relationship between the anodic and cathodic peak currents versus the square of the root of the scan rate, the plots of φ values vs. [πDnF/(RT)]−1/2 υ−1/2: (C) the bare electrode and (F) the g-C3N4-CdS-CuO modified electrodes.
Figure 2. The cyclic voltammetry (CV) curves of the bare electrode (A) and the g-C3N4-CdS-CuO modified electrodes (D) at different scan rates (0.005~ 0.02 V/s), and bare electrode (B) and the g-C3N4-CdS-CuO modified electrodes (E) of linear relationship between the anodic and cathodic peak currents versus the square of the root of the scan rate, the plots of φ values vs. [πDnF/(RT)]−1/2 υ−1/2: (C) the bare electrode and (F) the g-C3N4-CdS-CuO modified electrodes.
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Chemosensors 10 00286 g003 550
Figure 3. PEC responses of the FTO electrode in PBS buffer solutions containing: g-C3N4/FTO; CdS-g-C3N4/FTO; CuO-CdS-g-C3N4/FTO (A); and EIS of the electrode, a, bare FTO; b, g-C3N4/FTO; c, CdS-g-C3N4/FTO; d, CuO-CdS-g-C3N4/FTO (B). The photocurrent response of a, CuO-CdS-g-C3N4/FTO; b, CuO-CdS-g-C3N4/FTO +GSH; c, CuO-CdS-g-C3N4/FTO + GSH + 0.5 nM Hg2+; d, g-C3N4-CdS-CuO + GSH + 10 nM Hg2+ (C); and the photocurrent response of a, CuO-CdS-g-C3N4/FTO + GSH; b, CuO-CdS-g-C3N4/FTO + GSH + NADPH + GR; c, CuO-CdS-g-C3N4/FTO + GSH + NADPH + GR + 0.5 nM Hg2+; d, CuO-CdS-g-C3N4/FTO + GSH + NADPH + GR + 10 nM Hg2+ (D).
Figure 3. PEC responses of the FTO electrode in PBS buffer solutions containing: g-C3N4/FTO; CdS-g-C3N4/FTO; CuO-CdS-g-C3N4/FTO (A); and EIS of the electrode, a, bare FTO; b, g-C3N4/FTO; c, CdS-g-C3N4/FTO; d, CuO-CdS-g-C3N4/FTO (B). The photocurrent response of a, CuO-CdS-g-C3N4/FTO; b, CuO-CdS-g-C3N4/FTO +GSH; c, CuO-CdS-g-C3N4/FTO + GSH + 0.5 nM Hg2+; d, g-C3N4-CdS-CuO + GSH + 10 nM Hg2+ (C); and the photocurrent response of a, CuO-CdS-g-C3N4/FTO + GSH; b, CuO-CdS-g-C3N4/FTO + GSH + NADPH + GR; c, CuO-CdS-g-C3N4/FTO + GSH + NADPH + GR + 0.5 nM Hg2+; d, CuO-CdS-g-C3N4/FTO + GSH + NADPH + GR + 10 nM Hg2+ (D).
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Chemosensors 10 00286 sch002 550
Scheme 2. Band gap of g-C3N4, CdS, and CuO nanomaterial (A), Band structure of electrode after electron redistribution between g-C3N4, CdS, and CuO interface (Fermi-level arrangement) (B).
Scheme 2. Band gap of g-C3N4, CdS, and CuO nanomaterial (A), Band structure of electrode after electron redistribution between g-C3N4, CdS, and CuO interface (Fermi-level arrangement) (B).
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Figure 4. (A) PEC reaction of Hg2+ at different concentrations; (B) Linear relationship between photocurrent and logarithm value of Hg2+ concentration; (C) Sensor stability within 200 s; (D) Selectivity of PEC system.
Figure 4. (A) PEC reaction of Hg2+ at different concentrations; (B) Linear relationship between photocurrent and logarithm value of Hg2+ concentration; (C) Sensor stability within 200 s; (D) Selectivity of PEC system.
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Table
Table 1. Chemicals.
Table 1. Chemicals.
ChemicalsManufacturerAddress
Melamine (C3H6N6, 99.0%)Macklin Co., Ltd.Shanghai, China
Cadmium chloride hemi (pentahydrate) (CdCl2·2.5H2O, 98.0%)Macklin Co., Ltd.Shanghai, China
Cupric oxide (CuO NPs, 99.5%)Macklin Co., Ltd.Shanghai, China
Sodium sulfide nonahydrate (Na2S·9H2O, 98.0%)Xilong Scientific Co., Ltd.Guangdong, China
Hydroxylammonium chloride (AR)Cameo Chemical Reagent Co., Ltd.Tianjin, China
Hydrochloric acid (HCl, AR)Chuandong Chemical Engineering Co., Ltd.Chongqing, China
Sodium chloride (NaCl, 99.5%)Jinshan Chemical Reagent Co., Ltd.Chendu, China
Calcium chloride anhydrous (CaCl, 96.0%)Sinopharm Chemical Reagent Co., Ltd.Shanghai, China
MucinSangong Bioengineering Co., Ltd.Shanghai, China
UreaAladdin Industrial Co., Ltd.Shanghai, China
Potassium bromate (KBrO3, 99.9%)Macklin Co., Ltd.Shanghai, China
Potassium bromide (KBr, 99%)Macklin Co., Ltd.Shanghai, China
Sodium carbonate anhydrous (Na2CO3, 99.8%)Yongda Chemical Reagent Co., Ltd.Tianjin, China
Sodium hydrogen carbonate (NaHCO3, 99.8%)Yongda Chemical Reagent Co., Ltd.Tianjin, China
TrisCameo chemical reagent Co., Ltd.Tianjin, China
Nitric acid (HNO3, 65.0–68.0%)Chuandong Chemical Engineering Co., Ltd.Chongqing, China
Potassium chloride (KCL, 99.5%)Jinshan Chemical Reagent Co., Ltd.Chendu, China
Reduced glutathione (GSH, 98.0%)Solarbio Science & Technology Co., Ltd.Beijing, China
Glutathione reductase (GR, AR)Solarbio Science & Technology Co., Ltd.Beijing, China
Dihydronicotinamide-adenine dinu-cleotide phosphate, tetrasodium salt (NADPH, >98.0%)Solarbio Science & Technology Co., Ltd.Beijing, China
Sodium phosphate dibasic (Na2HPO4, 99.0%)Cameo Chemical Reagent Co., Ltd.Tianjin, China
Sodium dihydrogen phosphate (NaH2PO4, >99.0%)Cameo Chemical Reagent Co., Ltd.Tianjin, China
Potassium ferricyanide (K3[Fe(CN6)], >99.5%)Jinshan Chemical Reagent Co., Ltd.Chendu, China
Potassium ferrocyanide (K4[Fe(CN6)], >99.5%)Jinshan Chemical Reagent Co., Ltd.Chendu, China
Mercury chloride (HgCl2, >99.5%)Silver Lake Chemical Co., Ltd.Guizhou, China
Table
Table 2. Apparatus.
Table 2. Apparatus.
ApparatusModelAddress
Electrochemical workstationCHI660EShanghai Chenhua
Scanning electron microscopy (SEM)Zeiss Sigma 300Germany
Powder X-ray diffraction (XRD)Bruker D8 AdvanceGermany
UV-visible diffuse reflectance spectrum (UV-vis DRS)Shimadzu UV-3600 Plus Japan
Energy dispersive X-ray Spectroscopy (EDS)Oxford SpectroscopyBritain
Fourier transform infrared spectroscopy (FT-IR)Perkin Elmer Spectrum Two FTIR-DTGBritain
Table
Table 3. Comparison of Hg2+ determination in human urine samples and artificial saliva.
Table 3. Comparison of Hg2+ determination in human urine samples and artificial saliva.
SampleApplied (nM)Found (nM)Recovery (%)RSD (%)
Urine0--1.76
0.10.0997.142.03
10.9898.812.74
1010.3103.022.19
Saliva0--2.71
0.10.0993.562.88
10.9797.972.33
109.9299.243.07
“-” Indicates that it is below detection limit.
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Su, Y.; Su, L.; Liu, B.; Lin, Y.; Tang, D. Self-Powered Photoelectrochemical Assay for Hg2+ Detection Based on g-C3N4-CdS-CuO Composites and Redox Cycle Signal Amplification Strategy. Chemosensors 2022, 10, 286. https://doi.org/10.3390/chemosensors10070286

AMA Style

Su Y, Su L, Liu B, Lin Y, Tang D. Self-Powered Photoelectrochemical Assay for Hg2+ Detection Based on g-C3N4-CdS-CuO Composites and Redox Cycle Signal Amplification Strategy. Chemosensors. 2022; 10(7):286. https://doi.org/10.3390/chemosensors10070286

Chicago/Turabian Style

Su, Yonghuan, Lixia Su, Bingqian Liu, Youxiu Lin, and Dianping Tang. 2022. "Self-Powered Photoelectrochemical Assay for Hg2+ Detection Based on g-C3N4-CdS-CuO Composites and Redox Cycle Signal Amplification Strategy" Chemosensors 10, no. 7: 286. https://doi.org/10.3390/chemosensors10070286

APA Style

Su, Y., Su, L., Liu, B., Lin, Y., & Tang, D. (2022). Self-Powered Photoelectrochemical Assay for Hg2+ Detection Based on g-C3N4-CdS-CuO Composites and Redox Cycle Signal Amplification Strategy. Chemosensors, 10(7), 286. https://doi.org/10.3390/chemosensors10070286

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