Ultrasensitive Determination of L-Cysteine with g-C3N4@CdS-Based Photoelectrochemical Platform

Abstract

L-cysteine, a component of the symmetric L-cystine, is essential in numerous biological activities. Thus, detecting cysteine rapidly, selectively, and sensitively is of tremendous interest. Herein, g-C₃N₄@CdS composites were employed as sensing elements in a photoelectrochemical platform for L-cysteine sensing. In this system, g-C₃N₄@CdS composites provided much better optoelectronic function than bare CdS materials owing to their high photon-to-current conversion efficiency and excellent anti-photocorrosion properties. The innovative photoelectrochemical sensor has a wide determination range of 5 to 190 µM, a very low detection limit of 1.56 µM, a fast response time, and good long-term stability (ca. 1 month). Without applying any separation procedures, a low concentration of CySH was successfully detected in human urine samples, which is compatible with the results of chemiluminescence.

1. Introduction

L-cysteine (CySH) is an important amino acid owing to its crucial role in biological systems [1,2]. It is known to be an active site in the catalytic function of certain enzymes known as cysteine proteases, as well as in many other peptides and proteins [3,4]. Furthermore, the couple L-cystine/CySH is commonly used as a model for the role of the disulfide bond and thiol group in proteins in a variety of biological media [5]. Given the wide range of physiological and pathophysiological effects [6,7], several strategies for quantifying this critical amino acid have been developed, including cyclic voltammetry [8,9,10,11,12], chemiluminescence [13,14], photoelectrochemistry [15,16,17], fluorimetry [18,19], capillary zone electrophoresis [20], and liquid chromatography [21]. Among these, photoelectrochemical analysis has received a great deal of attention for CySH sensing because of its inherent advantages like simplicity, easy miniaturization, high sensitivity, and low cost.

In the case of the photoelectrochemical detection process, light is used to excite photoactive materials on the electrode, and the photocurrent is used as the detection signal. Thus, the development of photoactive materials with good optoelectronic properties is critical to the creation of an excellent photoelectrochemical sensor. Generally, inorganic semiconductors nanoparticles (such as TiO₂, ZnO, Cu₂O, CdS, ZnS, and Bi₂MoO₆) have been regarded as attractive candidates for photoelectric applications due to their size-tunable optical and electronic properties, as well as their efficient multiple charge carrier generations [22,23,24,25,26,27,28,29]. Among these inorganic semiconductors, CdS, a non-centrosymmetric hexagonal zinc ore crystal structure, has been used to design the photoelectrochemical platform for CySH sensing because it can easily absorb CySH from solution via the formation of Cd-S bonds [30,31,32]. For instance, Long et al. designed an ultra-sensitive photoelectrochemical sensor with a low detection limit of 0.1 µM for CySH employing methyl viologen-coated CdS quantum dots as photoactive materials [33]. Even though ultra-detection has been achieved by using CdS as photoactive materials, the low photon-to-current conversion efficiency and photocorrosion of CdS severely limit future improvements in photoelectrochemical properties for photoelectrochemical sensors. Therefore, there is still a significant amount of interest in finding low-cost, highly efficient, stable, and sensitive photoactive materials for photoelectrochemical sensors.

Recently, a polymeric photocatalyst known as graphitic carbon nitride (g-C₃N₄) has received a lot of attention due to its abundance, high thermal and chemical stability, and visible light response [34,35,36,37]. On the basis of the excellent properties mentioned above, it has been reported that g-C₃N₄ materials improve performance for photocatalysis [38,39,40,41] and biosensing [42,43,44]. Herein, combining the excellent photoelectrochemical properties of g-C₃N₄ nanosheets and CdS, g-C₃N₄@CdS composites were synthesized in a simple way. To our knowledge, there are few reports on CySH sensing based on g-C₃N₄@CdS composites. Using these photoactive materials, a novel photoelectrochemical platform was developed and used to assay CySH in human urine. It was discovered that such a photoelectrochemical sensor has a fast response, high sensitivity, high stability, and excellent selectivity for CySH.

2. Materials and Methods

All chemicals were obtained from commercial sources and utilized without further purification. Thioacetamide, polyvinylpyrrolidone (PVP, K-30), melamine, and chemiluminescence reagent were obtained from Sigma-Aldrich. CdCl₂·2.5H₂O (99%) was purchased from Alfa. The PBS buffer was made from sodium phosphate (NaH₂PO₄/Na₂HPO₄, 81:19 (molar ratio)) and sodium chloride dissolved in deionized water at final concentrations of 10 mmol L-1 (pH: 7.4).

XRD patterns of the samples were carried out with a D/MAX 2500V/PC X-ray diffraction (Cu Kα radiation, λ = 0.15406 nm) at 40 kV and 30 mA. The scanning range of 2θ was 10–80°. XPS was performed with an ESCALAB-MK II 250 photoelectron spectrometer using Al as the X-ray source for excitation. TEM measurements were carried out with a TECNAI G2 high-resolution transmission electron microscope and a 200 kV accelerating voltage. Fluorescence emission spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer at an excitation wavelength of 325 nm. All electrochemical experiments were performed with a CHI660A Electrochemical Workstation (CHI). ITO or modified ITO was used as the working electrode, a platinum wire was used as the auxiliary electrode, and an Ag/AgCl electrode (3 M KCl) was used as the reference electrode. In this work, E(RHE) = E(Ag/AgCl) + 0.198 V + 0.059 pH. The PBS solution was used as a supporting electrolyte. Before the experiment, the PBS solution was bubbled with N₂ for 15 min. LED light with different wavelengths (365 nm, 425 nm, 470 nm, 545 nm, and 645 nm, Beijing Perfectlight Technology) was used as the source of the photoelectrochemical sensor. Electrochemical impedance spectroscopy (EIS) measurements were carried out using a Solartron 1255 B Frequency Response Analyzer (Solartron Inc.UK) in a 0.1 M PBS aqueous solution.

g-C₃N₄ was first synthesized with thermal polycondensation of melanin in argon at 550 °C for 2 h at a heating rate of 2.5 °C min⁻¹. The obtained bulk g-C₃N₄ was rinsed with water and ethanol three times and dried at 70 °C. Then, 50 mg of bulk g-C₃N₄ was dispersed in 50 mL of water and ultrasonicated for 2 h. The ultrathin g-C₃N₄ (utg-C₃N₄) was dispersed in the supernatant, which was centrifuged at 3500 r·min⁻¹ for 20 min, and the supernatant was collected and prepared for use. In order to prepare the g-C₃N₄@CdS nanocomposites, 1.3 mL of NH₂OH and 0.2 g of PVP were dispersed into a 25 mL g-C₃N₄ dispersion solution. After vigorous stirring, 0.057 g CdCl₂·2.5H₂O and 0.1 g thioacetamide were added. The mixed solution was reacted for 4 h at 80 °C under vigorous stirring. Finally, the obtained product was centrifuged at 6500 rpm and then dried at 60 °C in a vacuum drier. As a control experiment, the bare GR and CdS were also synthesized under the same process without adding g-C₃N₄ and CdCl₂·2.5H₂O, respectively.

The ITO electrode was washed with 1 M NaOH and then 30% H₂O₂. Then, the obtained electrode was further cleaned with acetone and twice-distilled water three times. Finally, the obtained electrode was dried at room temperature. To create a g-C₃N₄@CdS modified ITO electrode, 100 L of the g-C₃N₄@CdS suspension solution (5 mg mL⁻¹) was cast onto the ITO electrode and dried at room temperature. Similar steps were used to prepare the CdS-modified ITO electrode.

The working electrode was illuminated from the back (an illumination power on the working electrode of 73.89 mW/cm²), effectively preventing interference from the color sample. Each sample was detected three times, and the average value was calculated. The photoelectrocurrent was obtained using the following procedure: I = I_smpale − I_blank (I_smpale, the photoelectrochemical current of the sample; I_blank, the photoelectrochemical current without the sample).

3. Results and Discussion

We first characterized the XRD patterns of the as-prepared photoactive materials. As shown in Figure 1a, two diffraction peaks were observed on the XRD pattern of g-C₃N₄ nanosheets. The stacking of conjugated double bonds was represented by one prominent diffraction peak at 27.4°. This peak was indexed as the (002) peak for graphitic materials and closely matched the interlayer d-spacing of 0.336 nm for g-C₃N₄. The other faint diffraction peak at 13.0° corresponded to a 0.672 nm interplanar separation, which was indexed as (100) in JCPDS 87-1526. These two diffraction peaks correlate well with the g-C₃N₄ values given in the literature [37,40,44]. Four discernible diffraction peaks were observed on the XRD spectra of CdS nanoparticles, which were indexed as (100), (101), (110), and (112) in JCPDS 89-0440 [45]. The g-C₃N₄@CdS composite samples presented diffraction peaks for both g-C₃N₄ and CdS, which demonstrated that g-C₃N₄@CdS was successfully synthesized.

The morphology of the g-C₃N₄@CdS nanocomposites was investigated by TEM. As shown in Figure 1b and Figure S1, CdS nanoparticles with diameters of about 20 nm were found to be uniformly dispersed on the surface of g-C₃N₄ nanosheets. The efficient optoelectronic properties of the g-C₃N₄@CdS nanocomposites could be guaranteed by the good distribution and high coverage of CdS on the g-C₃N₄ nanosheets. XPS measurements were further carried out to investigate the chemical composition of g-C₃N₄@CdS nanocomposites. High-resolution spectra of C1s (Figure 1c) at 285.5 eV and N1s at 398.5 eV were assigned to the sp² C=N bond in the s-triazine ring [23,40]. Peaks in the C1s zone at 288.3 eV and 284.6 eV are attributed to electrons originating from an sp² C atom attached to a -NH₂ group and an aromatic carbon atom [35]. The contributions in the N1s zone at 399.5 and 401.2 eV are ascribed to N atoms that are bound to three C atoms; these N atoms are present in the heptazine ring and as a bridging atom, respectively [37]. After the dispersion of CdS nanoparticles onto the surface of g-C₃N₄ nanosheets, the peaks of Cd3d and S2p could be observed on the XPS spectra of g-C₃N₄@CdS. As shown in Figure 1c, two peaks at 405.2 eV and 411.5 eV were observed on the high-resolution spectrum of Cd3d, which could be assigned to the Cd²⁺ ions of the CdS. An s2p peak at 161.5 eV was also observed, as expected for the sulfide in CdS [45]. All the above results demonstrate that g-C₃N₄@CdS nanocomposites were successfully synthesized.

The as-prepared g-C₃N₄@CdS photoactive materials were further applied to design a photoelectrochemical platform for CySH sensing. As shown in Figure 2a, a photocurrent of 353 nA was observed on a bare CdS modified ITO electrode (CdS/ITO) in 0.1 M PBS under 545 nm light irradiation, whereas a photocurrent of 449 nA was observed on a g-C₃N₄@CdS modified ITO electrode (g-C₃N₄@CdS/ITO), indicating that the addition of g-C₃N₄ improves the photocurrent conversion efficiency of CdS. To explore the above reason, electrochemical impedance spectroscopy was conducted at 0.0 V under 545 nm light illumination. As shown in Figure 2b, the charge transfer resistance of g-C₃N₄@CdS/ITO was 189.7 kΩ, which is much smaller than that of bare materials (517.2 kΩ). This smaller arc radius implies a much higher charge transfer efficiency [23,25]. These results were supported by the data of photoluminescence spectra (Figure S2). Furthermore, after adding 50 µM CySH to the electrolyte solution, the photocurrent reached 994 nA on g-C₃N₄@CdS/ITO, nearly 2.1 times that of CdS/ITO. The increase in the photocurrent was attributed to the oxidization of CySH by the photogenerated holes, effectively avoiding electron–hole recombination. The photoelectrochemical process of g-C₃N₄@CdS/ITO for CySH oxidation is proposed in Scheme 1. Upon irradiation with light, CdS was excited and underwent charge separation to yield electrons (e-) and holes (h⁺). The photoelectrons can arrive at g-C₃N₄ through electron tunneling and subsequently to the ITO electrode [46,47,48]. Finally, the photocurrent was produced. When CySH was introduced, the h⁺ of the CdS could be refilled by electrons from the CySH, ensuring that the occupied holes were available for the following excitation. The photocurrent could be greatly improved by this process. It was noted that the intensity of the photocurrent was consistently enhanced by increasing concentrations of CySH, thus promising the quantitative determination of CySH.

The irradiation wavelength for the g-C₃N₄@CdS-based photoelectrochemical platform was optimized. As shown in Figure 2c, under 645 nm irradiation, a slight photocurrent of 59 nA was observed on the g-C₃N₄@CdS/ITO with an applied potential of 0.0 V in 50 μM CySH electrolyte solution. When the irradiation wavelength was decreased to 545 nm, the photocurrent significantly increased to 971 nA under the same conditions. Furthermore, we found that the photocurrent increased sharply as the exciting wavelength below 470 nm. Although the photocurrent responses exhibited a preferential sensitivity at short wavelengths, this causes additional issues of serious interference due to coexisting species during actual sample examinations (e.g., glucose and ascorbic acid), which should be avoidable at weaker wavelength irradiations [23]. Therefore, 545 nm was chosen for the detection of CySH, which could simultaneously ensure the excellent sensitivity and stability of this photoelectrochemical sensor.

We further investigated the photoelectrochemical properties of the as-prepared photoelectrochemical sensor under different working potentials. As shown in Figure 2c, the photocurrent increment increased significantly as the applied potential rose from −0.2 to 0.0 V and tended to reach a maximum at 0.0 V, while from 0.0 to 0.3 V, essentially no discernible change was observed under 545 nm illumination in 50 μM CySH solution. The photocurrent increased again as the applied voltage increased to 0.4 V. Nevertheless, the photocurrent at 0.0 V was 70.1% of the photocurrent at 0.5 V, showing adequate sensitivity for the photoelectrochemical detection of CySH. Additionally, a voltage of 0.0 V was selected for the photoelectrochemical sensing of CySH in the subsequent tests to make it convenient to develop the two-electrode system for a photoelectrochemical CySH instrument.

The photocurrent of the photoelectrochemical sensor was investigated with increasing concentrations of CySH to validate its performance in the determination of CySH. As shown in Figure 3a, the photocurrent increased as the concentration of CySH increased. Good linear ranges are from 5 to 190 μM (R² = 0.997) for the detection of CySH with a lower detection limit of 1.56 μM. The ultra-low detection limit was superior to some CySH sensors reported in the literature [49,50,51,52,53,54]. The photoelectrochemical sensor has outstanding features, such as a wide response range, a low detection limit, and a rapid response, which ensures that real samples are detected. The reproducibility of the g-C₃N₄@CdS/ITO was investigated. Six electrodes were tested at the same concentration of CySH (50 μM), and the results showed that g-C₃N₄@CdS/ITO had good reproducibility with a relative 2.9%. Additionally, since stability is crucial for sensors, the operation stability as well as the long-term stability of such a photochemical sensor were studied. The photocurrent response remained at 90.3% of the value of the initial reaction after four weeks. Additionally, even after more than 1400 s of scanning, the photocurrent of CySH maintained 96.6% of its starting value (Figure 3c), which meets the requirements for CySH sensing. Yet, the photocurrent could decrease further after more than 1400 s test due to photocorrosion, a similar phenomenon was observed in the literature [55]. All these results demonstrated that g-C₃N₄@CdS would perform exceptionally well in the quantitative analysis of CySH.

The selectivity of the photoelectrochemical sensor was confirmed by evaluating its photoresponse toward various probable interference species found in the detection solution containing 50 μmol L⁻¹ CySH. In total, 1000 times Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, Ni²⁺, Cl⁻, NO₃⁻, ClO₄⁻, CO₃²⁻, SO₄²⁻, and PO₄³⁻; 500 times L-proline, ethanol, L-histidine, L-glycine, methanol, L-threonine, fructose, and glucose; and 100 times uric acid were texted. As shown in Figure 4, the results revealed that these species did not cause significant interference with the detection of CySH on the as-prepared g-C₃N₄@CdS-based photoelectrochemical platform.

To confirm the feasibility, the proposed method was applied to determine CySH in human urine. Three human serum samples were detected with the g-C₃N₄@CdS-based photoelectrochemical platform. The corresponding CySH detection results are shown in Table 1. All the results were found to be consistent with the chemiluminescence results. The performance of practical investigations demonstrated that the current photoelectrochemical platform is excellent for detecting CySH.

4. Conclusions

In conclusion, a novel photoelectrochemical platform for the detection of CySH was developed by employing g-C₃N₄@CdS composites as photoactive materials. The photoelectrochemical sensor exhibits a linear response of 5–190 µM, a lower detection limit of 1.56 μM, a fast response time, and good long-term stability (ca. 1 month). CySH was effectively tested at low concentrations in human urine samples, which was consistent with the chemiluminescence results. Compared with previous reports, this photoelectrochemical sensor has significant advantages, such as good anti-interference qualities and a high level of stability and reproducibility, and it was also demonstrated to be a practical and cost-effective method for CySH detection.

Author Contributions

Conceptualization, W.M.; data curation, H.Z. and S.Q.; investigation, G.Z. and K.Z.; methodology, H.Z. and G.Z.; resources, H.W.; validation, S.Q. and K.Z.; supervision, W.M.; writing—original draft preparation, H.Z. and W.M.; writing—review and editing, W.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available upon request from the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures, Scheme and Table

Figure 1. (a) XRD patterns of g-C3N4, CdS, and g-C3N4@CdS nanocomposites. (b) TEM image of g-C3N4@CdS nanocomposites. (c) High-resolution XPS spectra of C 1s, N1s, Cd3d, and S2p regions of g-C3N4@CdS nanocomposites.

Scheme 1. Schematic illustration of the photoelectrochemical process for CySH sensing at g-C3N4@CdS modified ITO electrode.

Figure 2. (a) Photocurrent responses of CdS (a, a0) and g-C3N4@CdS/ITO electrode without (a,b) and with (a0, b0) 50 µM CySH. (b) EIS plots of CdS/ITO and g-C3N4@CdS/ITO at 0.0 V under 545 nm irradiation. (c) Photocurrent responses of g-C3N4@CdS/ITO at different potentials. (d) Photocurrent responses of g-C3N4@CdS/ITO at different wavelengths.

Figure 3. (a) Photocurrent responses of g-C3N4@CdS/ITO with different concentrations of CySH. (b) The linear calibration curve of the g-C3N4@CdS/ITO for CySH. (c) The stability of 50 μM CySH on g-C3N4@CdS/ITO.

Figure 4. Photocurrent response of g-C3N4@CdS/ITO upon addition of 1000 times Na+, K+, Ca2+, Mg2+, Zn2+, Ni2+, Cl−, NO3−, ClO4−, CO32−, SO42−, and PO43−; 500 times L-proline, L-glycine, L-histidine, ethanol, methanol, L-threonine, fructose, and glucose; and 100 times uric acid in 0.1 M PBS (pH = 7.4) containing 50 μM CySH at 0 V under 545 nm light excitation.

Table 1

Results of analysis of CySH in real samples (n = 3).

Sample	Photoelectrochemical Sensor (μM)	Chemiluminescence (μM)
Urine 1	5.6 ± 0.5	6.0 ± 0.3
Urine 2	12.7 ± 1.9	11.5 ± 0.6
Urine 3	7.3 ± 0.8	6.9 ± 0.2

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/sym15040896/s1, Figure S1: TEM image of bare g-C₃N₄; Figure S2: Photoluminescence (PL) spectra of pure gC₃N₄ and g-C₃N₄@ CdS samples.

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