Introduction

The synthesis of novel materials has received significant interest across many disciplines including analytical chemistry. In particular, the synthesis of transition metal chalcogenides has been the subject of considerable attention and derived materials have been widely utilized in supercapacitors1, solar cells2, batteries3, and sensors4 due to their high energy density, long cycling stability, and excellent electrochemical and charge transfer properties5,6,7. Accordingly, transition metal chalcogenides such as metal sulfide, selenide, telluride, nitride, boride, and phosphide have also been widely prepared and employed in energy and sensor applications8,9,10,11. Given their superior electronic characteristics, selenide based chalcogenides are recognized as superior to other metal chalcogenides. Recently, many chalcogenides selenides such as Ni, Co, Fe and Mo selenides have been reported to date. Metal selenides are typically prepared by using chemical bath deposition12, chemical vapor deposition13, electrodeposition14, simple chemical synthesis15, and hydrothermal techniques16. Among these, hydrothermal techniques are shown to generate metal selenides of varying structure and high crystallinity17. In the present work, we have synthesized Cr2Se3 hexagon (h-Cr2Se3) using a simple hydrothermal method for the first time.

4-nitrophenol (4-NP) is well-known phenolic compound that has been widely used in the industrial manufacture of products from pesticides and fungicides to paracetamol and dyes18. However, 4-NP is also considered as a major water pollutant with serious health implications for both humans and animals19. Reliable and sensitive detection of 4-NP in water samples is therefore essential to the treatment and provision of safe water supplies. To date, various analytical methods such as mass spectrometry20, high performance liquid chromatography21, spectrophotometry22, flow injection analysis23, and electrochemical methods24,25,26,27,28,29 have all been applied to the sensitive determination of 4-NP concentrations in water samples. However, the determination of 4-NP levels by electrochemical techniques is demonstrated to be less complex and less expensive than other reported methods25,26. Owing to their increased surface area, high conductivity, and unique physical and chemical properties, chemically modified electrodes have been widely used for electrochemical determination of 4-NP in recent years. Accordingly, electrodes modified with boron doped diamond film27, Hg (mercury hanging drop)24, and its amalgam (Ag, Cu, Au, Bi, Sn, or Zn with liquid mercury)28,29 have been widely applied in the determination of 4-NP. In addition, the composites of carbon micro/nano materials30,31, metal oxides32 and conducting polymers33 have also been routinely used in the sensitive detection of 4-NP. The main goal of the present work is to fabricate a sensitive and selective sensor for detection of 4-NP by using h-Cr2Se3 as a model active electrode material. It is noted that the application of metal selenides in electrochemical determination of 4-NP is limited. However, given its high conductivity and high relative surface area, the application of h-Cr2Se3 modified electrodes in the determination of 4-NP may demonstrate significant advantages in terms of high sensitivity, a wide linear range, and reduced detection limit (LOD).

Results and Discussion

Characterizations of h-Cr2Se3

The surface morphology of as-synthesized h-Cr2Se3 was examined by SEM. The SEM image of as prepared h-Cr2Se3 is shown in Fig. 1A, and clearly demonstrates the hexagonal structure of Cr2Se3. Figure 1B shows the EDX of as-synthesized h-Cr2Se3 and confirms the presence of Cr and Se in h-Cr2Se3. In addition, the elemental mapping of h-Cr2Se3 (Fig. 1C,D) reveals the uniform distribution of Cr and Se in h-Cr2Se3. The results confirmed the formation of h-Cr2Se3. We have used XRD to confirm the crystalline characteristics of h-Cr2O3. Figure 2A shows the XRD pattern of h-Cr2Se3, and all major diffraction peaks obtained concur with the standard data for Cr2Se3 (JCPDS NO. 98-010-6520) with hexagonal structure (space group R-3, NO. 148). The cell parameters of Cr2Se3 hexagon are a = 6.25 Å, b = 6.25 Å, c = 17.28 Å. In this XRD pattern, the diffraction peaks of h-Cr2Se3 are located at 30.48°, 32.58°, 42.83°, 45.33°, 51.73°, 56.06°, 61.63°, and 68.31° for corresponding lattice plane of (001), (011), (002), (310), (102), (100), (211), and (022). The sharp peaks highlight the high crystalline purity of h-Cr2Se3. Accordingly, the reported method is suited to the preparation of a metal chalcogenide with high purity.

Figure 1
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(A) SEM image of h-Cr2Se3, and EDX (B) and elemental mapping (C–E) of h-Cr2Se3.

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Figure 2
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(A) XRD pattern of as-synthesized h-Cr2Se3. (B) EIS Nyquist curve of (a) h-Cr2Se3/SPCE and (b) bare SPCE in 5 mM of [Fe(CN)6]3−/4− containing 0.1 M of KCl in the frequency range from 0.1 Hz to 100 kHz.

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EIS is a powerful tool to investigate electron charge transfer processes at the interface between electrode and electrolyte related to double layer capacitance (cdl), solution resistance (Rs), Warburg impendence (ZW), and charge transfer resistance (Rct)34. In general, the semicircle region at higher frequency region and its diameter are ascribed to charge transfer resistance (Rct). Figure 2B shows the EIS profile of bare SPCE and h-Cr2Se3/SPCE in 5 mM [Fe(CN)6]3−/4− containing 0.1 M of KCl in the frequency range from 0.1 Hz to 100 kHz. The Rct values for bare SPCE and h-Cr2Se3/SPCE were calculated as 136.8 and 28.45 Ω, respectively. This confirms h-Cr2Se3/SPCE has a higher electron transfer ability than the bare SPCE.

Cyclic voltammetry (CV) was used to investigate the electron transfer ability of bare SPCE and h-Cr2Se3/SPCE, with electrochemical experiments employing an electrolyte of 5 mM of [Fe(CN)6]3−/4− contain 0.1 M of KCl, and at a scan rate of 50 mV/s. The obtained voltammetry data are shown in Figure. S1A. When compared to the un-modified SPCE, the h-Cr2Se3/SPCE displays clearly enhanced oxidation and reduction peak current, and a peak-to-peak separation of 0.18 V (Figure. S1B) 90 mV lower than observed for the un-modified SPCE. This result demonstrates the enhanced electron transfer capacity and reversibility because of h-Cr2Se3/SPCE modification.

The surface chemical state of Cr and Se elements in h-Cr2Se3 was probed by using XPS analysis as shown in Fig. 3. The survey spectrum of h-Cr2Se3 (Fig. 3A) clearly exhibits the major peaks of Cr, Se, O, and C. This reveals that the sample h-Cr2Se3 mainly contains the Cr and Se elements at near to surface range while the presence of carbon (as the reference) and oxygen is assigned to the surface adsorption of hydrocarbon contaminants and moisture. Figure 3B shows the XPS spectra of Cr 2p with two energy band at 586.8 and 576.9 eV for corresponding Cr 2p1/2 and Cr 2p3/2 states respectively, where the peak binding energy separation was determined as approximately 9.9 eV. The Se 3d spectrum in Fig. 3C shows the band for Se 3d5/2 at 54.6 eV, which confirms the presence of the metal selenium bond. The XPS analysis evidently proves the chemical states of both Cr and Se in the h-Cr2Se3, in agreement with the published literature35,36.

Figure 3
figure 3

(A) Wide scan XPS spectra of h-Cr2Se3. (B) XPS spectra of Cr 2p, and (C) Se 3d.

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Electrochemical behavior of 4-NP

The schematic representation for the electrochemical reduction of 4-NP at h-Cr2Se3 modified SPCE is shown in Fig. 4. The electrocatalytic activity of h-Cr2Se3/SPCE and bare SPCE towards the detection of 4-NP was studied by CV. Figure 5A shows the CV response of h-Cr2Se3/SPCE and bare SPCE in presence and absence of 4-NP at pH 7. The h-Cr2Se3/SPCE exhibits a sharp reduction peak at −0.75 V in the presence of 476 µM 4-NP (curve c), which is due to the direct reduction of 4-NP into hydroxylaminophenol37 as in Fig. 4. In addition, a quasi-reversible anodic peak was observed at 0.14 V due to oxidation of hydroxylaminophenol into 4-nitrosophenol. It should be noted that no such peaks were obseved at h-Cr2Se3/SPCE in the absence of 4-NP (curve a). It is notable that the reduction peak potential of 4-NP was 70 mV lower at h-Cr2Se3 modified SPCE when compared to the response observed for bare SPCE (curve b). In addition, the observed reduction peak current of 4-NP at h-Cr2Se3/SPCE was 2-fold higher than bare SPCE. The unique properties of h-Cr2Se3 on SPCE results in enhanced sensitivity and low potential detection for 4-NP. Hence, h-Cr2Se3 modified SPCE can be used for sensitive and lower potential detection of 4-NP.

Figure 4
figure 4

Schematic representation for the eelectrochemical reduction of 4-NP at h-Cr2Se3 modified SPCE.

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Figure 5
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(A) CV response of h-Cr2Se3/SPCE in the absence (a) and presence (c) of 476 µM 4-NP at pH 7 at scan rate of 50 mVs−1. At same conditions, CV response of bare SPCE in the presence of 476 µM 4-NP at pH 7. (B) CVs obtained for h-Cr2Se3 modified SPCE in 476 µM of 4-NP at pH 7 against an increasing scan rate from 10 to 100 mVs−1. The inset figure shows the relationship between the square root of scan rate and the resulting current response. (C) CV response of h-Cr2Se3 modified SPCE in 476 µM 4-NP for pH values from pH 3 to pH 11 at a scan rate of 50 mVs−1. (D) Corresponding plot of pH vs. Epc and pH vs. Ipc.

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Generally, the electrochemical behaviour of the modified electrodes is greatly controlled by the effect of applied scan rate. Therefore, the effect of scan rate on the surface of h- Cr2Se3/SPCE towards detection of 476 µM 4-NP was studied by CV. Figure 5B shows the CV response of h-Cr2Se3/SPCE at pH 7 containing 476 µM 4-NP at increasing scan rates. The reduction peak current of 4-NP increases with the scan rate from 10 to 100 mVs−1. In addition, the oxidation peak current of 4-nitrosophenol also increases with increasing the scan rates. As shown in Fig. 5B inset, the reduction current of 4-NP has a linear relationship with the squre root of scan rates. The correlation coeffecient (R2) was found to be 0.995. The result implies that the overall electrochemical reduction reaction of 4-NP at h-Cr2Se3/SPCE is a typical diffusion controlled process38.

The electrocatalytic ability of the modified electrode towards 4-NP was studied at different pH, since electrochemical activity of the modified electrode can be easily affected by pH. Figure 5C shows the CV response of h-Cr2Se3/SPCE in 476 µM 4-NP across a range of pH values from pH 3 to pH1 at a scan rate of 50 mV/s. The h-Cr2Se3 modified SPCE shows a sharp reduction peak current response in the presence of 4-NP at each pH, with the maximum reduction peak current response was observed at pH 7 (Fig. 5D). This may be due to the high activity of h-Cr2Se3 modified SPCE in pH 7 than other pHs. Accordingly, pH 7 was selected as optimal for further electrochemical studies. Figure 5D also illustrates the linear relationship between pH and current peak potential. The linear regression was calculated as E (V) = −0.507 − 0.399 pH with an R2 value of 0.9977. The negative sign indicates the proton to be directly involved in the electrochemical reduction of 4-NP, and such findings are consistent with previously reported results38.

Determination of 4-NP

The electroctalytic ability of h-Cr2Se3 modified SPCE towards the detection of different concetration of 4-NP was investigated by CV. Figure. S2 shows the CV response of h-Cr2Se3 modified SPCE in the absence (a) and presence of 100 µM (b), 380 µM (c), 650 µM (d) and 909 µM 4-NP at pH 7, and at scan rate of 50 mV/s. In the absence of 4-NP, the h-Cr2Se3 modified SPCE did not show any apparent electrochemical response at pH 7, while a significant reduction peak current was observed at h-Cr2Se3 modified SPCE in the presence of 100 µM of 4-NP. The reduction peak current increases with increasing concentrations of 4-NP at pH 7 (c-e) and indicates a high electro-reduction capacity of h-Cr2Se3 modified SPCE towards 4-NP.

Amperometric i-t method was used to determine the 4-NP using h-Cr2Se3 modified SPCE. Under optimized conditions, the h-Cr2Se3 modified SPCE is used for the detection of 4-NP in constantly stirred pH 7 with the working potential of −0.73 V. As shown in Fig. 6A, a sharp amperometric response was observed for each addition of different concetration of 4-NP (0.05–958.0 µM) at pH 7. The sensor also shows a stable response for addition of 0.05 µM (a), 0.1 µM (b), 0.2 µM (c), 0.7 µM (d), 1.5 µM (e), 2.5 µM (f), 4 µM (g) and 5.7 µM (h) 4-NP into the constantly stirred PBS (Fig. 6A lower inset). The response time of the sensor was calculated as 4 s and reveals the fast electrocatalytic reduction of 4-NP by h-Cr2Se3 modified SPCE. In addition, the amperometric response current of 4-NP was linear over concentrations ranging from 0.05 µM to 908.0 µM (Fig. 6A upper inset). The linear equation for the calibration plot is I (μA) = 0.15 + 4.71 C (µM) and the R2 is 0.9967. The sensitivity of the sensor is 1.24 µAµM−1 cm−2 as calculated from the slope of the calibration plot/electrochemically active surface area (0.12 cm2) of the h-Cr2Se3 modified SPCE. The detection limit (LOD) of the sensor was estimated as 0.01 µM based on 3 ∗ standard deviation of the blank response/slope of the calibration plot (0.15), where the blank response currents are 0.0124, 0.0131 and 0.0121 µA (Fig. 6A lower inset). To further verfiry the advantages of the Cr2Se3 modified SPCE for 4-NP sensor applications, the LOD, senstivity and linear response range of the Cr2Se3 modified SPCE sensor was compared with previously reported modified electrodes. The comparative results are shown in Table 1, and clearly show the h-Cr2Se3 modified SPCE has lower LOD, wider linear response range and higher senstivity for the detection of 4-NP than previously reported modified electrodes. For instance, the LOD of h-Cr2Se3 modified SPCE (0.01 µM) was lower than nano- Au25 (8 µM), graphene-chitosan26 (0.08 µM), chitosan/ZnO nano needles30 (0.23 µM), cyclodextrin-reduced graphene oxide33 (0.05 µM), activated carbon31 (0.16 µM), nano-Cu2O32 (0.5 µM), Ag nanoparticles38 (0.015 µM), carbon dot39 (0.028 µM), ZnO40 (0.029 µM) and hydroxyapatite nano powder41 (0.6 µM) modified electrodes. In addition, the senstivity and linear response of the sensor was more comparable with the previously reported sensors25,26,30,31,32,33,38,39,40,41,42.

Figure 6
figure 6

(A) Amperometric i-t response of h-Cr2Se3 modified SPCE for addition of different concentration of 4-NP into the constantly stirred pH 7. Inset is calibration plot for current response vs. [4-NP], and an enlarged view of amperometric response of h-Cr2Se3 modified SPCE for addition of 0.05 µM (a), 0.1 µM (b), 0.2 µM (c), 0.7 µM (d), 1.5 µM (e), 2.5 µM (f), 4 µM (g) and 5.7 µM (h) 4-NP at the working potential of −0.73 V. (B) Amperometric i-t response of h-Cr2Se3 modified SPCE for addition of 10 µM 4-NP and 500 µM additions of dihydroxybenzene isomers and metal ions at pH 7 with the working potential of −0.73 V. (C) At similar conditions, amperometric i-t response of h-Cr2Se3 modified SPCE for addition of 10 µM of 4-NP and 500 µM additions of phenolic and nitro compounds.

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Table 1 Comparison of analytical performance of h-Cr2Se3 modified SPCE with previously reported modified electrodes for determination of 4-NP. Abbreviations LOD – limit of detection; GCE – glassy carbon electrode; CV – cyclic voltammetry; Gr – graphene; CHI – chitosan; ABPE – acetylene black paste electrode; LSV – linear sweep voltammetry; NDs – nano needles; DPV – differential pulse voltammetry; CD – cyclodextrin; RGO – reduced graphene oxide; AC –activated carbon; PDPA – poly(diphenylamine); HA-NP – hydroxyapatite nano powder.
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Selectivity studies

The selectivity of the h-Cr2Se3 modified SPCE towards detection of 4-NP was investigated by amperometric method. Figure 6B shows the amperometric response of h-Cr2Se3 modified SPCE for the addition of 10 µM of 4-NP and 500 µM additions of catechol, resorcinol, hydroquinone, Mn2+, Cu2+, Ni2+, Pb2+, Cl−, Br−, SO42− and Zn2+ at pH 7 with an applied potential of –0.74 V. A well-defined and sharp amperometric response was observed for the addition of 4-NP. The previously discussed electroactive interferences did not show any response on h-Cr2Se3 modified SPCE. Accordingly, the results clearly reveal that h-Cr2Se3 modified SPCE is highly selective for detection of 4-NP in the presence of phenolic compounds and metal ions. We have also tested the selectivity of the sensor in the presence of electrochemically reducible compounds such as 4-acetamidophenol, 4-aminophenol, 4-nitrobenzoic acid, 4-nitrobenzyl alcohol and H2O2. The selectivity studies were performed in the presence of 50-fold additions of the aforementioned electrochemically reducible compounds by amperometry and the results are shown in Fig. 6C. It can be seen that the Cr2Se3 modified SPCE shows a stable amperometric response for the addition of 10 µM of 4-NP and 500 µM additions of electrochemically reducible compounds did not result in any measureable response. Hence, the modified sensor can be used for the selective detection of 4-NP in the presence of electrochemically reducible compounds and metal ions.

Stability, repeatability, and reproducibility of the sensor

Stability, repeatability, and reproducibility are critical to utilization of the sensor in real time applications. Fig. S3 shows the operational stability of h-Cr2Se3 modified SPCE for the addition of 10 µM of 4-NP into the constantly stirred electrolyte at pH 7, and the background current response up to 1200 s. The amperometric profile of h-Cr2Se3 modified SPCE clearly shows that the sensor retains 97.3% of its initial current response after 1200 s. This result demonstrates high operational stability of the h-Cr2Se3 modified SPCE in the detection of 4-NP. To evaluate the repeatability of the sensor, a single h-Cr2Se3 modified SPCE was used in five set of pH 7 containing 476 µM of 4-NP by CV. In the same manner, five independently prepared h-Cr2Se3 modified SPCEs were used for the detection of single sample containing 476 µM of 4-NP. Other experimental condtions are similar to Fig. 5A. The h-Cr2Se3 modified SPCEs demonstrate a relative standard deviation (RSD) of approximately 2.2%, and show appropriate repeatability in the detection of 4-NP. In addition, the RSD about 2.1% was observed for five independently prepared h-Cr2Se3 modified SPCEs towards detection of 4-NP. The result also demonstrates good reproducibility of the sensor matrix.

Real sample analysis

The practicality of amperometric analysis employing the h-Cr2Se3 modified SPCE was verified using 4-NP spiked tap water and river water samples. Contaminant-free water samples were used as collected, with no treatment prior to analysis, and known concentrations of 4-NP added at pH7. Recovery was calculated using the standard addition method. The 4-NP concentrations determined for spiked tap and river water samples are tabulated in Table 2. The obtained recovery values range from 97.9 to 98.8%, with an average relative standard deviation of 2.9%. These results confirm the practical application of h-Cr2Se3 modified SPCE for the determination of 4-NP in water samples.

Table 2 Determination of 4-NP in different water samples using h-Cr2Se3 modified SPCE. RSD is related to 3 measurements.
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Conclusions

In summary, we have synthesized h-Cr2Se3 using a simple hydrothermal method and employed it as an electrode material for the first time in the sensitive detection of 4-NP. The physicochemical characterizations confirm the presence of pure h-Cr2Se3. The h-Cr2Se3 modified SPCE demonstrated significant advantages over previously reported 4-NP sensors, such as low LOD, wider linear response range, and high detection sensitivity. In addition, the h-Cr2Se3 modified electrode exhibited a superior capacity for the selective detection of 4-NP. The stability of the sensor is shown to be appropriate for the precise detection of 4-NP in real samples. The sensor also demonstrates highly selective detection of 4-NP in the presence of electrochemically reducible compounds, phenolic compounds and metal ions.

Experimental

Materials

Chromium (II) acetate (Cr2(CH3CO2)4(H2O)2), selenium powder (Se), and 4-nitrophenol were purchased from sigma Aldrich. Hydrazine hydrate was purchased from Acros Oroganics. The supporting electrolyte was prepared using 0.05 M Na2HPO4 and NaH2PO4 in double distilled water, and adjusted to pH7 through addition of NaOH or H2SO4. Analytical grade reagents were used without further purification. Screen-printed carbon electrodes were purchased from Zensor R&D Co., Ltd., Taipei, Taiwan.

Hydrothermal synthesis of h- Cr2Se3 and electrode modifications

A simple hydrothermal method was used for the synthesis of h- Cr2Se3. In brief, a suspension of 20 mM chromium acetate and 80 mM selenium was prepared in 25 mL of double distilled water. 6 mL of hydrazine hydrate was gradually added to this suspension under continuous stirring. After 30 min, the mixed solution was transferred to a Teflon sealed auto clave and heated to 180 °C for 16 h. The obtained product (h- Cr2Se3) was cooled at room temperature, washed with double distilled water and ethanol, and dried in an air oven for 12 h.

The h-Cr2Se3 dispersion was prepared by the addition of 1 mg of h-Cr2Se3 to 1 mL of ethanol, and mixing by sonication for 15 minutes. Electrode modification consisted of 6 µL h-Cr2Se3 suspension drop-coated onto the SPCE surface, and oven drying of the electrode at 45 °C. This h-Cr2Se3 modified SPCE was used for all reported electrochemical detection of 4-NP.

Characterization techniques

Scanning Electron Microscopy (SEM) was performed using Hitachi S-3000 H electron microscope. Energy dispersive X-ray (EDX) spectrum was recorded using HORIBA EMAX X-ACT attached with Hitachi S-3000 H scanning electron microscope. XRD characterization was carried out using XPERT-3 diffractometer with Cu Kα radiation (K = 1.54 Å). Electrochemical impedance spectroscopy (EIS) was performed using IM6ex ZAHNER impedance measurement unit. CV and amperometric studies were performed using CHI611A electrochemical analyzer. Conventional three-electrode system was used for electrochemical studies. The modified SPCE was used as a working electrode, and saturated Ag/AgCl and platinum electrodes were used as the reference and auxiliary electrodes respectively. CV experiments were performed in a potential range from 0.4 to −0.7 V at a scan rate of 50 mV s−1. All electrochemical measurements were carried out at a room temperature in N2 saturated electrolyte solution at pH 7.