Dual-Modal Photoelectrochemical and Visualized Detection of Copper Ions.

Copper is one of the extensively utilized heavy metals in modern industry and can be easily released into the environment due to high solubility of copper ions (Cu2+). Its percolation in water and accumulation along the food chain pose a serious threat to human health. Hence, it is of great significance to explore a novel, facile, and sensitive detection method for Cu2+. Based on the intriguing photo-to-electricity conversion process of CdS QDs, as well as desirable electrochromic property of WO3 NFs, a dual-modal photoelectrochemical (PEC) and visualized detection platform for Cu2+ is fabricated. The electrochromic WO3 NFs act as a display for the Cu2+ concentration, of which the color change could be observed directly by the naked eye, while the PEC signal provides accurate data for further analysis. In this work, a sensitive detection of Cu2+ in the range of 1 × 10-5 to 5 × 10-4 M is achieved, with a detection limit of 3.2 × 10-6 M. The dual-modal analysis gives more choices for signal readouts with enhanced quantification reliability, which is adaptive for diverse application scenarios, especially for on-site investigation. This protocol offers a prototype for quick and reliable detection of the Cu2+ concentrations, and is promising for other environmental pollutants.

Introduction

Copper ions (Cu2+) are one of the major heavy metal pollutants extensively found in waste water, which can spread through water circulation or accumulate along the food chain.[1] Although they are the third most abundant transition metal elements and essential for the catalytic process in human metabolism, an excess amount of Cu2+ is hazardous for normal oxidative and reductive processes, resulting in damage of liver and kidney as well as degenerative neurological disease.[2] Among the diverse analytical technologies for Cu2+,[3−9] photoelectrochemical (PEC) analysis has stood out and thrived due to its merits of high sensitivity, low background, low cost, and simple instrumentation.[10−15] However, most of the reported works relied on single signal outputs, which may show false positive or negative results. Besides, the results cannot be observed directly, making it inconvenient for on-site screening. Therefore, with acquisition of both intuitive visualized results and specific photoelectrochemical data, the dual-modal PEC and colorimetric analysis has garnered considerable attention.[16−18]

Electrochromism is a distinguishable color change phenomenon between two or more different redox states of a chemical species, induced by electrical current or potential stimulation.[19] This appealing technique has been implemented in the field of smart building materials,[20] batteries,[21,22] supercapacitors,[23] and biosensors.[24] Thanks to the relatively small driving force for color changes, the electrical current or potential required can be provided by the PEC process, instead of an external power supply, making self-powered photochromic devices. The PEC signal and the visualized signal can be obtained simultaneously in such devices, making it appropriate for field work. Compared to the traditional dual signal routes based on enzyme or enzymatic mimic catalytic protocols, utilization of electrochromic materials leads to higher stability and reproductivity. The key elements for such devices are a photoactive working electrode and an electrochromic counter electrode, where a rational choice is required for electrode materials to achieve a sufficient photovoltaic output and the corresponding high-contrast color change. In the past few years, efficient electrode material pairs have been reported including Bi2S3-Prussian blue (PB), Ni/FeOOH/BiVO4-PB, g-C3N4/Au/TiO2-PB, N749/TiO2–Ni/WO3, and so forth, and various disease-related biomarkers including carcinoembryonic antigens, prostate-specific antigens,[25] and pyrophosphate ions[26] were successfully detected. Nonetheless, with the ever-growing emergency for environmental protection, more effective material pairs are still needed to meet the requirements for diverse analytical targets, especially appropriate for environmental pollutants.

In this work, a simultaneous PEC and visualized assay was reported for sensitive detection of Cu2+, harnessing the electrochromic WO3 nanoflakes (NFs) as a display component for photocurrent changes of a CdS QD electrode. As illustrated in Scheme , high and stable photocurrent generated by the CdS QDs is strong enough to facilitate distinct color evocation of WO3 NFs from white to dark blue at zero bias voltage, leading to a self-powered photochromic process. Besides, based on the direct and fast interaction between Cu2+ and CdS QDs, the photocurrent of CdS QDs was quenched nearly completely, and the color of WO3 NFs is accordingly kept as original white. In this process, WO3 NFs act as a naked eye readable coulometer for photoelectrons generated by the photoactive anode, quickly giving information on the amount of Cu2+. This visualized signal is suitable for a semiquantitative assay for large-scale screening, and the exact data could be obtained using the photocurrent signal for further analysis. This self-powered and dual-modal detection method represented a novel signal readout with enhanced quantification reliability, which is adaptive for diverse application scenarios, especially outdoor investigation. This assay protocol offers a prototype for quick and reliable detection of the Cu2+ concentration, and is adaptive for other environmental pollutants.

Scheme 1

Schematic Illustration of the Photoelectrochemical and Visualized Detection of Cu2+

Results and Discussion

Characterization of the CdS QD Electrode

First, the microstructure and PEC performance of the synthesized CdS QD electrode were investigated. As shown in Figure A, CdS QDs showed a quasispherical morphology and good dispersity with a diameter of about 4 nm. The size distribution histogram demonstrated a nearly Gaussian distribution of its diameter (inset of Figure A). Figure B demonstrates the typical UV–vis absorption spectrum of CdS QDs, and its broad visible range before 480 nm indicates its potential as a photoactive material. After immobilized onto FTO through layer by layer adsorption, slight aggregation occurred due to the electrostatic interaction with a viscous polymer, polydimethyl diallyl ammonium chloride (PDDA) (Figure C). The fabricated CdS QD electrode possessed an irregular spherical morphology with a high surface area, which contributes to quick photoelectron transportation. As demonstrated in Figure D, a transient photocurrent response of about 138 μA over manifold cycles upon intermittent light irradiation was observed, confirming the high stability and intense photo-induced response of the CdS QD electrode.

Figure 1

(A) TEM image (inset: the corresponding size distribution histogram) and (B) UV–vis absorption spectrum of CdS QDs; (C) SEM image and (D) photocurrent stability of the CdS QD electrode.

Characterization of the WO3 NF Electrode

As the first discovered and most popular electrochromic material, WO3 was rationally selected as the counter electrode because of its fast color switching and high coloration efficiency.[27]Figure A depicts the SEM image of the hydrothermally prepared WO3 NFs. As is shown, the WO3 possessed a feather-like nanoflake morphology, with a length of about 1 μM. The X-ray diffraction patterns indicate the successful formation of WO3 (JCPDS no. 33-1387) on the surface of FTO (Figure S1). Its electrochromic property was subsequently investigated through cyclic voltammetry with observation of its color change simultaneously. As illustrated in Figure B, with the voltage changing negatively from +0.5 to −1.0 V, the WO3 NF electrode transformed from the bleached state (white) to the colored state (dark blue), and the corresponding current promptly increased when the voltage was below −0.4 V, indicating a relatively low energy requirement for WO3 NF coloration. In this process, the electrons and anions in the solution were injected into the lattice defects of WO3 NFs, leading to the reduction of W6+ to W5+ and a color change (eq ).[28] Reversely, when the voltage changed positively, the color was bleached to white. The WO3 NF electrode was photographed, and a distinct color change was observed (inset of Figure B) with the transmittance changing up to 75% in the visible range (Figure S2). The remarkable color change and relatively low energy requirements of the WO3 NF electrode suggested its potential as a naked eye readable coulometer for photoelectrons generated by CdS QDs.

Figure 2

(A) SEM image and the [inset of (A)] HRSEM image of the WO3 electrode; (B) cyclic voltammetry curve and [inset of (B)] the corresponding color change of the WO3 electrode.

Construction of the Dual-Modal Detection Platform

A dual signal photoelectrochemical and visualized detection platform was built through a three-electrode system utilizing CdS QDs as the working electrode and WO3 NFs as the counter electrode. In this protocol, a high photocurrent intensity was required to facilitate thorough coloration of WO3 NFs, thus ascorbic acid (AA) was selected as the electron donor to promote charge separation and transfer of the electron–hole pairs. As depicted in Figure A, compared to PBS solution, the photocurrent of the platform significantly increased from 3.8 to 137.2 μA in AA. Upon applying incident light irradiation, CdS QDs were excited, and the photogenerated electrons transferred from their conduction band to the surface states of WO3 through an external circuit, while the photogenerated holes were eliminated efficiently by AA,[29] leading to a high photocurrent that is enough for a complete color change of cathode WO3 NFs from white to dark blue without an external bias voltage. While without light irradiation, zero photocurrent was generated and the WO3 NFs still appeared original white (Figure B). These results confirmed that the color change of WO3 NFs was dependent only on the photocurrent intensity, and WO3 NFs were successfully demonstrated to be a display component for photocurrent changes of CdS QDs.

Figure 3

(A) Photocurrent of the CdS QD electrode in (a) 0.1 M PBS and (b) 0.1 M AA electrolyte; (B) photoresponse of the dual-modal platform (a) with and (b) without light irradiation, and [inset of (B)] the corresponding color of WO3 electrodes. The bias voltage was 0 V vs Ag/AgCl.

Photoelectrochemical and Visualized Detection of Cu2+

The fabricated self-powered dual signal detection platform is promising for outdoor detection of environmental pollutants, and Cu2+ was chosen as a model target. As depicted in Figure A, after immobilizing 1 × 10–3 M Cu2+ onto the CdS QD electrode, the photocurrent sharply decreased from 136.8 to 3.5 μA, with a quenching rate of 97.4%. In this process, Cu2+ was bound on the surface of CdS and partially reduced to Cu+ under illumination. Because the solubility product constant of CdS (pKsp = 26.2) is larger than that of CuS (x = 1, 2; pKsp(CuS) = 35.2, and pKsp(Cu = 47.6), CuS was formed through precipitation transformation. CuS generated a new dopant energy level between the energy gaps of CdS QDs, accelerating the recombination of the electron–hole pairs, leading to a decrement of photocurrent.[30] This could be validated by the red shift in the UV–vis absorption spectra of CdS QDs after Cu2+ addition (Figure S3). As the concentration of Cu2+ varied, the photocurrent intensity changed reversely (Figure B). With the concentration of Cu2+ increased from 1 × 10–5 to 5 × 10–4 M, the photocurrent decreased from 132.9 to 5.6 μA. Figure D shows the corresponding calibration curve, demonstrating the linear relationship between the photocurrent intensity and the logarithm of the Cu2+ concentration in the range of 1 × 10–5 to 5 × 10–4 M. The linear equation was I = 74.2 × lg C (Cu2+) – 235.0, with an R2 value of 0.994 and a detection limit of 3.2 × 10–6 M. The detection range meets the requirements of the maximum permissible limit of Cu2+ ions in drinking water (2 mg L–1, i.e., 3.12 × 10–5 M) established by the World Health Organization (WHO).[31] On the other hand, as the Cu2+ concentration increased, due to the decreased number of photoelectrons injected to the counter electrode WO3 NFs, its coloration process was retarded, and a lighter blue color was observed. The corresponding color change could be easily discriminated by the naked eye (Figure C) and quantified through its photograph with the help of Adobe Photoshop software. As shown in Figure E, the color intensity in the cyan channel showed a linear relationship with the concentration of Cu2+ from 2.5 × 10–5 to 5 × 10–4 M. Hence, this protocol realized photoelectrochemical and visualized analyses of the Cu2+ concentrations.

Figure 4

(A) Photocurrent change of the dual-modal platform (a) before and (b) after incubation with 1 × 10–3 M Cu2+; (B) photocurrent after incubation with different concentrations of Cu2+; (C) visualized signal after incubation with different concentrations of Cu2+; the linear calibration curve of (D) photocurrent and (E) color value in the cyan channel, respectively. The concentrations of Cu2+ were 1 × 10–5, 2.5 × 10–5, 5 × 10–5, 1 × 10–4, 2.5 × 10–4, and 5 × 10–4 M.

Selectivity of the Method

Figure shows the selectivity of the detection platform toward other heavy metals such as Zn2+, Cd2+, Mn2+, Fe3+, Pb2+, and Ag+, common cations such as K+, Mg2+, and Al3+, and anions such as Cl–, NO3–, SO42–, and S2–. As demonstrated, neither photocurrent nor color change was influenced by those ions, expect for Ag+. It is because Ag+ could also react with CdS, promoting the recombination of electron–hole pairs. Therefore, Br– was added as a mask agent to avoid the influence of Ag+, which had no influence on the signal generated by Cu2+.

Figure 5

Selectivity of the detection platform toward different ions. The concentrations of the ions were 5 × 10–4 M.

Real Sample Analysis

The feasibility of the dual-modal detection platform in a real sample matrix was evaluated in paddy water. The visualized signal was first used as a screening tool to obtain semiquantitative results. As shown in Figure , the second electrode possessed the lightest color, indicating the highest Cu2+ concentration in the sample. Compared with Figure C, the Cu2+ concentrations of the three samples were estimated to be 0, 1 × 10–4, and 2.5 × 10–5 M. Accurate data were further provided by the photoelectrochemical signal, and are shown in Table . When different concentrations of Cu2+ were spiked into paddy water, the average recoveries were between 91.2 and 105%, with the relative standard deviation between 2.15 and 6.90%. These results indicated acceptable accuracy and reproducibility of the protocol for real sample analysis, with a visualized signal for quick screening and a photoelectrochemical signal for further accurate data.

Figure 6

Visualized signal of the dual-modal detection platform in the presence of 0, 10–4, and 2.5 × 10–5 M Cu2+. The color values were obtained with Adobe Photoshop Software.

Table 1

Cu2+ Detection in Paddy Water

added concentration (M)	average photocurrent (μA)	average concentration detected (M)	average recovery (%)	RSD (%)
0	136.6			2.15
10–4	60.2	1.05 × 10–4	105	6.90
2.5 × 10–5	109.5	2.28 × 10–5	91.2	5.47

Conclusions

Based on the intense and stable photoresponse of CdS QDs, as well as the efficient photo-induced electrochromic phenomenon of WO3 NFs, a self-powered and dual-modal PEC visualized detection platform was built for sensitive detection of Cu2+. The color change of WO3 NFs was facilitated by the photocurrent of CdS QDs at zero bias voltage, and varied with the concentrations of Cu2+, thus establishing a color chart of Cu2+ for quick and on-site screening, and the photocurrent signal provided specific data of higher accuracy for further analysis. The dual modal detection platform detected Cu2+ with satisfactory selectivity, and the feasibility in real samples was verified in the matrix of paddy water. This work represented a prototype for fast, reliable, and on-site analysis of Cu2+ concentrations, and provided a new option for the detection of other environmental pollutants.

Materials and Methods

Materials and Apparatus

FTO electrodes (KV-FTO-R15-100100) were purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd. Cadmium chloride and thioglycolic acid (TGA) were purchased from Sigma-Aldrich (Shanghai, China). Sodium hydroxide, sodium sulfide, ethylene glycol, hydrochloric acid, and copper sulfate pentahydrate were purchased from Nanjing Chemical Reagent Co. LTD (Nanjing, China). Hydrogen peroxide (30%), tungstic acid, and ascorbic acid (AA) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). Polydimethyl diallyl ammonium chloride (PDDA, 20%, w/w in water, MW = 200 000–350 000) was purchased from Aldrich Chemical Reagents Co., LTD. Paddy water was collected from Pailou Teaching and Research Base of Nanjing Agricultural University. Other chemicals were of analytical reagent grade and used as received. All aqueous solutions were prepared with ultrapure water.

Scanning electron microscopy (SEM) images were recorded using a Hitachi S4800 scanning electron microscope (Hitachi Co., Japan). Transmission electron microscopy (TEM) was performed with a JEOL 2100F instrument operating at a 200 kV accelerating voltage. The UV–Vis absorption spectra were obtained on a Shimadzu UV-3600 UV–Vis–NIR spectrophotometer (Shimadzu Co., Japan). PEC measurements were performed with a homemade PEC system, and a 5 W LED lamp with monochromatic emission at 410 nm was used as the irradiation source. Photocurrent was measured on a CHI660E electrochemical workstation (Shanghai, China). The electrochromic performance of the WO3 NF electrode was tested on a CHI660E electrochemical workstation (Shanghai, China) with a three-electrode system: a prepared WO3 NFs electrode as the working electrode, a Pt wire as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode.

Experimental Methods

Fabrication of the CdS QDs/FTO Electrode

The utilized CdS QDs were synthesized according to the previous report.[32] In brief, 250 μL of TGA was added to 50 mL of 0.01 M CdCl2 aqueous solution with N2 bubbling throughout the solution for 30 min. During the period, 1.0 M NaOH was added to adjust pH to 11. Then, 5.0 mL of 0.1 M Na2S aqueous solution was injected into the solution, followed by heating to 110 °C and refluxing under a N2 atmosphere for 4 h. Finally, the as-obtained CdS QDs were diluted with the same volume of water and stored at 4 °C for further use. The CdS QDs/FTO electrode was fabricated by layer-by-layer assembly.[32] Briefly, the freshly cleaned FTO electrodes were then alternately dipped into a solution of 2% PDDA (containing 0.5 M NaCl) and the as-obtained CdS QD solution for 10 min, respectively, and this process was repeated four times to obtain desired photocurrent intensity. The electrodes were carefully washed with ultrapure water after each dipping step.

Fabrication of the WO3 NFs/FTO Electrode

WO3 NFs/FTO electrodes were synthesized using a hydrothermal method according to the previous report.[33] 5 g H2WO4 was added into 60 mL 30% H2O2 under refluxing at 95 °C, and the reaction solution was then naturally cooled down to room temperature and diluted to 200 mL to obtain the precursor solution (0.1 M). Then, 10.5 mL of the precursor solution, 3.5 mL of 3 M HCl, 10.5 mL of ultrapure water, and 31.5 mL of ethylene glycol were mixed together under constant stirring at room temperature to obtain a mixed solution. The cleaned FTO was tilted to the bottom of a 100 mL Teflon-lined stainless steel autoclave with the side facing down and immersed in the above solution, then sealed and thermally treated at 120 °C for 2 h. Afterward, the as-obtained WO3 film was rinsed three times with ethyl alcohol and dried at 60 °C.

Photoelectrochemical and Visualized Measurement

A dual modal detection platform was constructed with a three-electrode system: a prepared CdS QD electrode (the exposed active area was a geometrical circle with a diameter of 0.5 cm) as the working electrode, a WO3 NF electrode as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. The determination of Cu2+ was performed by adding 25 μL of Cu2+ solution of different concentrations onto the CdS QD electrode and incubating for 30 min at 25 °C. After rinsing three times with ultrapure water, the photocurrent of the resulted electrodes was measured at 0 V with an irradiation duration of 50 s. During this process, the colors of the resulted WO3 NF electrodes were photographed using a digital camera, and the color value in the cyan channel was obtained with the help of Adobe Photoshop software.

Anti-Interference Evaluation

25 μL of 5 × 10–4 M K+, Mg2+, Al3+, Zn2+, Cd2+, Mn2+, Fe3+, Pb2+, Ag+, Cl–, NO3–, SO42–, and S2– was dropped onto the electrodes, respectively, and incubated for 30 min at 25 °C. The photocurrent and color values were measured as stated.

Real Sample Analysis

The performance of the detection platform was tested in the matrix of paddy water. The collected paddy water was centrifuged and filtered through a 0.22 μm membrane before use. Different concentrations of Cu2+ were then added, and the resulting solutions were dropped onto the CdS QD electrode and incubated for 30 min at 25 °C. The photocurrent and color values were then measured as stated.