Introduction

Tin oxide nanostructures have been the most important n-type semiconductor with a lot of properties and characteristics that may justify their potential applications within various fields. A large band gap of 36 eV [1], catalytic activity, good compatibility and biocompatibility, non-toxic, inexpensive, green material, good chemical stability and medium conductivity [2, 3] are some of these properties. Different types of tin oxides structures have been introduced for application in electrochemical sensor and biosensor such as nanotubes [1], nanospheres [2], core shell [4], nanowires [5], hollow spheres [6] and nanosheets [7]. Additionally, the composite of SnO2 with some materials such as Ni [8], Cu [9], graphene [10], reduced graphene oxide [11] and Co3O4 [12] have been reported in literature. These structures are used in optical materials [13], gas sensor [14], electrochemical sensors and biosensor [15, 16] and secondary batteries [17].

This study describes synthesis and characterization of some novel nanocomposites of tin oxide such as SnO2/CuS, SnO2/SnS and Cu@SnS/SnO2. These new nanoparticles are used as modifier for preparation of carbon paste modified electrodes and their electrocatalytic behaviors have been investigated by cyclic voltammetry in the presence of some important chemical compounds. The carbon paste modified electrode with SnO2/SnS has been utilized for determination PAT and HQ in real samples.

Experimental

Chemicals and apparatus

Tin(II) chloride dihydrate (SnCl2 2H2O), tin(IV) chloride (SnCl4), thiourea (NH2CSNH2), thioacetamide (CH3CSNH2) and ammonium acetate (NH4CH3COO) were bought from Scharlau (http://www.scharlab.com). Sodium hydrogen phosphate salt and all other solvents and reagents were obtained from Merck (https://www.merck.com). All the chemicals were ultrapure or reagent grade and were used without further purification.

Cyclic voltammetry measurements were performed by PalmSens EmStat3+ and PSTrace software for Windows. Electrochemical impedance spectroscopy experiments were performed using μAutolab type III/FRA2 potentiostat/galvanostat with NOVA software. A three electrode system: Ag|AgCl|KCl (3 M) as reference electrode a platinum wire as auxiliary electrode and homemade carbon paste modified electrode as working electrode with a diameter of 2 mm was used. All experiments were carried out at room temperature (25 ± 1 °C).

Preparation of SnO2/SnS nanocomposite

First, 8.0 mL of 0.5 M SnCl2 aqueous solution was mixed under stirring with 8.0 mL of 1.0 M NH4CH3COO solution. In the next step, 10.0 mL of SnCl4 8% (V/V) aqueous solution and 10.0 mL of 0.4 mol L−1 CH3CSNH2 solution was added to the initial mixed solution and stirred for 30 min. Then, de-ionized water was added to the prepared mixed solution to make a total volume of 50 mL. Next, the prepared reaction solution was transferred into an autoclave and heated with temperature of 80 °C for 24 h. After that, the mixture was centrifuged at 8000 rmp and isolated nanoparticles were washed several times with de-ionized water. Next the nanoparticles were filtered through filter paper (0.45 μm porosity) and dried at 50 °C for 24 h.

Preparation of CuS@SnO2 nanoparticles

The procedure was same as in the previous section except that the dried precipitate dispersed in 150 mL of de-ionized water in an Erlenmeyer flask to form a suspension. Then, 1.0 mL of copper acetate solution 0.002 M was added to 25 mL of the above suspension drop by drop to obtain the CuS@SnO2 nanoparticles. The homogenous precipitation of CuS@SnO2 nanoparticles was filtered through filter paper (0.45 μm porosity) washed several times by de-ionized water and dried at 50 °C for 24 h.

Preparation of cu@SnS/SnO2 nanoparticles

The Cu@SnS/SnO2 nanoparticles were synthesized according to the procedure explained in the literature with some modification [18]. The procedure is presented in the Electronic Supporting Material in detail.

Electrochemical analysis procedure

The experiments were carried out in 0.1 M phosphate buffer (pH 4.0). Differential pulse voltammograms from 0.2 to 0.8 V were recorded by adjusting the following parameters: amplitude of 0.08 V, pulse width of 0.05 s, pulse period of 0.5 s and time of equilibrating for 4 s. The impedance analysis was performed under the oscillation potential 0.005 V from 200,000 Hz to 0.1 Hz at open circuit potential.

Sample preparation and procedures

The urine sample of a healthy man was stored 2 h in refrigerator (at 4 °C). Then, it was centrifuged for 30 min at 5000 rpm and filtered using filter paper and diluted 100 times with phosphate buffer pH = 4. PAT tablet from Aria Iranian Company was examined for estimation of its acetaminophen content. Five tablets of PAT (500 mg of PAT in each tablet) were accurately weighed and powdered in a mortar. An adequate amount of the powders was weighed and transferred to a 100 mL volumetric flask and then dissolved in distilled water. The standard addition method was used for analyzing PAT and HQ-spiked samples for validation of the CPE modified with SnO2/SnS.

Characterization methods

The structure and surface morphology of nanomaterials was investigated by scanning electron microscopy (FE-SEM-EDX MIRA3 TESCAN). The crystallization degree and the size of nanoparticles were specified by X-ray powder diffractometer (XRD PHILIPS - PW1730).

Preparation of working electrodes

To investigate the electrochemical properties of nanomaterials, the carbon paste electrodes were prepared by hand-mixing of 0.9 g graphite fine powder 30 mg mineral oil and 70 mg from each of the synthesized nanomaterials with a mortar and pestle in 30 min for total uniformity. The resulting carbon paste was packed at the bottom of a polypropylene tube (internal diameter of 2.1 mm). The electrical connection was performed by a copper wire.

Results and discussion

Choice of materials

Electrodes modified with carbon nanomaterials [19,20,21], nanostructured metal oxides [22, 23], conductive polymers [24,25,26], various metal nanoparticles [27,28,29] and other nanostructured [30,31,32,33] have been demonstrated to enhance the electroanalysis efficiency of PAT and HQ. Composite or hybrid materials containing two or more constituents are of great interest. They can exhibit combination of individual properties of the components or unique new physical and chemical properties for various applications. Among these, composite of SnO2 nanoparticles attract considerable attention due to their fascinating physical and chemical properties. Metal sulfide nanocrystals have been studied the most due to their great number of applications in different technological areas including biological labeling and diagnostics, photovoltaic devices, sensors and electroluminescent devices. CuS was found to show interesting properties including metal-like electrical conductivity which may have potential application in electrochemical sensors [34, 35]. SnS is a p-type semiconductor with a band gap of 1.3 eV. The most significant aspect of SnS is being cheap and nontoxic [36]. Additionally, the preparation of their hybrid materials is convenient. Based on the above-mentioned notes, the investigation of sensing properties of new composites of SnO2 and CuS, SnS and their utilization in constructing electrochemical sensors are worthy of studying.

Microstructural analysis of the electrocatalysts

Characterizations of the SnO2/CuS, SnO2/SnS and Cu@SnS/SnO2 nanoparticles were investigated by SEM and XRD techniques. The microstructural characterization of modified electrode surface have significant role in the response of electrochemical sensor. Figure 1 shows FE-SEM micrographs of surface morphologies of SnO2/CuS, SnO2/SnS and Cu@SnS/SnO2 nanoparticles respectively. As seen, the homogenous dispensation of the very fine spherical shape particles with a diameter of approximately 10–30 nm is observed. Figure 2d shows the EDX analysis of SnO2/SnS nanoparticles which indicates the existence of Sn, O, S, and Cu elements in the chemical composition.

Fig. 1
figure 1

SEM images of SnO2/CuS (a), SnO2/SnS (b) and Cu@SnS/SnO2 (c). (a′-c′) are EDX analysis of SnO2/CuS, SnO2/SnS and Cu@SnS/SnO2 respectively

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Fig. 2
figure 2

Powder XRD patterns of (a) SnO2/CuS (b) SnO2/SnS and (c) Cu@SnS/SnO2 composites

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Powder X-ray diffraction (XRD) is carried out to investigate the crystal structure analysis of the prepared nanocomposites (Fig. 2). As seen, XRD pattern of SnO2 in Fig. 2a–c shows the peaks at 266°, 340° and 518° which are assigned to (110), (101), and (211) indicating the rutile structure of SnO2 ((JCPDS card: 41–1445).

Figure 2a shows the peaks at 273°, 281°, 342°, 541° and 588°. These can be attributed to the hexagonal covellite crystalline phase of CuS with characteristic (101), (102), (006), (110) and (116) planes. This is in good agreement with the standard data for CuS (JCPDS Card No 06–0464).” Fig. 2b shows the XRD pattern of SnO2/SnS composite. As seen, in addition to the peaks observed for SnO2, the peaks around 220°, 271°, 315°, 519.5° and 658° are observed. These are related to lattice planes of (110), (120), (111), (151) and (251), SnS (JCPDS Card No 39–0354). Concerning Cu@SnS/SnO2 nanocomposite, the XRD pattern (Fig. 2c) contains the peaks around 430° and 501°. These are assigned to (110) and (200) planes of copper and agree with the standard diffraction card of JCPDS No 04–0836.

The crystallite size of the nanocomposites powders are calculated according to Scherrer equation. The value of 17.7 nm for SnO2/CuS, 15.0 nm for SnO2/SnS and 16.6 nm for Cu@SnS/SnO2 are obtained respectively.

Electrochemical characterization

Electrochemical properties of carbon paste modified electrodes are evaluated by cyclic voltammetry and electrochemical impedance spectroscopy. Figure 3a, b show the cyclic voltammograms and the Nyquist plots for unmodified carbon paste electrode (a) SnO2/CuS (b) SnO2/SnS (c) and Cu@SnS/SnO2 (d) in phosphate buffer containing 1 mmol L−1 [Fe(CN)6]3− and 1 mmol L−1 [Fe(CN)6]4−. As can be seen in Fig. 3a, a well-defined redox peaks is observed for all modified electrodes. However, partial differences are observed for peak to peak potential separation (ΔEp). The ΔEp value for SnO2/CuS, SnO2/SnS, Cu@SnS/SnO2 modified CPEs and bare CPE are calculated as 145, 130, 140 and 260 mV versus Ag/AgCl respectively. These show increasing electron transfer rate for the modified electrodes.

Fig. 3
figure 3

a Cyclic voltammograms (b) Nyquist plots for 1 mM [Fe(CN)6]3−/4– in a solution of 0.1 M KH2PO4 at the surface of modified CPEs. Scan rate: 50 mV s−1, EIS condition: frequency range: 200 KHz – 0.1 Hz, perturbation amplitude: 5 mV

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The effective surface area of the electrodes was examined by recording CVs of 1 mmol L−1 K3[Fe(CN)6] at various scan rates for a reversible process by the Randles–Sevcik equation:

$$ \mathrm{Ip}=269\times 105{\mathrm{AD}}^{1/2}{\mathrm{n}}^{3/3}\mathrm{C}{\upupsilon}^{1/2} $$
(1)

where Ip refers to the peak current (A), n is the electron transfer number, A is the effective surface area (cm2), D is the diffusion coefficient of K3[Fe(CN)6] in the solution (7.66 × 10−6 cm2 s−1 at 0.1 mol L−1 KCl [37], C is the concentration of K3[Fe(CN)6] (mol cm−3) and υ is the scan rate (V/s). The calculated effective surface areas (A) for bare CPE, SnO2/CuS, SnO2/SnS and Cu@SnS/SnO2 modified CPEs are calculated as 0.257, 0.597, 0.773 and 0.457 cm2 respectively.. As a result, current responses for the redox couple of [Fe(CN)6]3−/4− increase with rise in the effective surface area of the modified electrodes. Accordingly, the highest current is observed for CPE modified with SnO2/SnS (Fig. 3a).

The EIS can provide information about the electron transfer rate of reaction. The Fig. 3b represents the EIS curve obtained from the modified and unmodified electrodes. The Nyquist-shaped EIS curves contain a semicircular part and a linear part. The semicircle in the Nyquist plot is related to the charge-transfer limiting processes at high frequencies. Moreover, the semicircle’s diameter corresponds to the charge transfer resistance (Rct) of the redox probe at the interfacial surface while the linear portion represents the diffusion process. Howsoever Rct is small, the system is kinetically facile. As can be seen in Fig. 3b, the Rct values are much lower for the modified electrodes compared with the Rct of the bare carbon paste electrodes which indicate increased electron transfer at their surface.

Electrocatalytic oxidation of some important species

The electrocatalytic properties of the carbon paste modified electrodes are studied toward some important drugs, biological and environmental species such as ascorbic acid (AC), uric acid (UA), salicylic acid (SA), paracetamol (PAT), carbidopa (CA), methyl paraben (MP), hydroquinone (HQ) and hydrazine (HZ). The CVs responses are shown in Fig. 4. As seen, good responses are observed at the carbon paste modified electrodes for all those redox systems. For example, the overpotential for oxidation of HQ and hydrazine are significantly decreased to 335 and 680 mV at the surface of CPE modified with SnO2/CuS compared to 455 and 880 mV at bare carbon paste electrode respectively. On the other hand, the onset potentials for all tested compounds are dramatically decreased as shown in Fig. 4. Moreover, considerable increases in peak currents are also observed due to the enhancement in reversibility of the electron transfer processes. These suggest an efficient electrocatalytic reaction for HQ and hydrazine at the carbon paste modified electrodes.

Fig. 4
figure 4

Cyclic voltammograms of 0.5 mmol L−1 (a) ascorbic acid (b) uric acid (c) Salicylic acid (d) PAT (e) carbidopa (f) HQ (g) methyl paraben and (h) hydrazine, recorded at the surface of bare and modified CPEs. Scan rate: 50 mV s−1, pH = 4.0. Graphite (bare electrode) (......), SnO2/CuS (— — —), Cu@SnS/SnO2 (———) and SnO2/SnS (— • — •)

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In spite of observed electrocatalytic behavior, it should be noted that oxidation potentials are still high for some tested compounds. This may cause fouling of the electrode surface and selectivity reduction in determining the analyte in the real samples.

Differential pulse voltammetric determinations

Case study evaluation of linear concentration range, sensitivity and detection limit using the differential pulse voltammetry (DPV) method has been used for PAT and HQ at surface of modified electrodes. The results of these initial studies are shown in Fig. S1. Figure 5 shows differential pulse voltammetry responses of PAT and HQ in different concentrations at surface of CPE modified with SnS/SnO2. The experiments were performed under the optimal conditions by keeping concentration of one component constant at 20.0 μmol L−1. A working voltage of 0.390 V (vs. Ag/AgCl) for PAT and 0.550 V (vs. Ag/AgCl) for HQ were used for obtaining current data. Detection limits were calculated using 3σ/s definition, where σ is the standard deviation of the blank signals (n = 5) and s is the slope of the calibration curve. As seen, the peak currents increase linearly with the increase in the concentration of the target molecules. In both, two linear dynamic ranges are observed. The relationship between peak currents PAT and concentration is in the range of 1–36 μmol L−1, and the linear equations of PAT are Ipa (μA) = 0.290 C (μmol L−1) + 0.190 (1–8.5 μmol L−1, R = 0.9973) and Ipa (μA) = 0.154 C (μmol L−1) + 1.758 (8.5–36 μmol L−1, R = 0.9912). The limit of detection (LOD) was calculated to be 0.06 μmol L−1. Similarly, the anodic peak current for HQ is proportional to its concentration from 1.0 to 85 μmol L−1. The linear equations are Ipa (μA) = 0.0756 C (μmol L−1) + 0.060 (1.0–10 μmol L−1, R = 0.9919) and Ipa (μA) = 0.038 C (μmol L−1) + 0.340 (10–85 μmol L−1, R = 0.9968) with a LOD of 0.2 μmol L−1. The sensitivities are 7.07 μA μM−1 cm−2 for PAT and 1.8 μA μM−1 cm−2 for HQ, respectively. The sensitivity and detection limit are comparable with or sometimes better than previously reported electrochemical methods for PAT and HQ in Table S1.

Fig. 5
figure 5

a Differential pulse voltammograms at surface CPME with SnS/SnO2 in phosphate buffer (pH 4.0) a for a binary mixture of 20.0 μmol L−1 HQ and different concentrations of PAT (1–36 μmol L−1). b For a binary mixture of 20.0 μmol L−1 PAT and different concentrations of HQ (1–85 μmol L−1). The inserts are the relationships between the anodic peak currents and concentrations

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The relative standard deviations for the determination of 10 μmol L−1 of PAT and HQ were obtained 3.9 and 2.5% by successive 5 measurements, respectively. These indicate that the CPMEs have excellent repeatability. The stability of all modified electrodes is also evaluated by measuring the anodic peak currents responses at a fixed 15 μM of the some substances under study over a period of 5 months. The peak potentials are unchanged and the current signals are decreased by less than 2%. These results show that the modified electrodes have a long-term stability.

Interference study

The potential interference of some inorganic and organic species was investigated accurately to detect PAT and HQ in the real sample. The results indicate that the 300 fold Na+, Ca2+, Mg+2 and K+, 300 fold of NO2−, NO3−, H2PO4−, HPO42−, SO42−, Cl−, 50 fold of glucose, ascorbic acid, quercetin, allura red, 80 fold of codeine sulphate and ibuprofen (as PAT ingredients) and 100 fold of benserazide and bisoprolol (as PAT similar structure) have no notable interference with PAT and HQ determination (the peak current response change <5%). This suggests that CPE modified with SnO2/SnS has a great selectivity for PAT and HQ determination without sensible problem in presence of usual interfering species.

Real sample analysis

To evaluate the efficiency of the CPME with SnS/SnO2, the standard addition method was used. For this purpose, known quantities of PAT and HQ within the linear range of PAT and HQ were added to tablet, urine and tap water samples. The analyses of their concentrations were performed by DPV method in the optimal conditions. The results have been shown in Table 1. As seen, the PAT and HQ concentrations determined using this sensor is in good agreement with the spiked value.

Table 1 Determination of PAT and HQ with modified carbon past electrode in Tablet, urine and tap water
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Conclusions

Electrocatalytic properties of the novel SnO2/CuS, SnO2/SnS and Cu@SnS/SnO2 nanocomposites toward several important molecules were investigated. The results indicate that the novel tin oxide nanocomposites can be used in construction of electrochemical sensors and biosensors as an effective electrocatalyst. Differential pulse voltammetric determinations of PAT and HQ were accomplished at CPE modified with SnO2/SnS. Additionally, the novel nanocomposites can also offer a high electrocatalytic activity towards other electroactive species such as ascorbic acid, uric acid, salicylic acid, carbidopa and methyl paraben. The oxidation potentials of the tested species are relatively high and this can limit selectivity. However, this new modified electrodes show good sensitivity, repeatability and reproducibility.