A sonochemical synthesis of SrTiO₃ supported N-doped graphene oxide as a highly efficient electrocatalyst for electrochemical reduction of a chemotherapeutic drug

Graphical abstract

Highlights

• •A sonochemical synthesis of SrTiO₃/NGO electrode materials without any toxic reducing agents.
• •The morphological, chemical and electrochemical characterizations of the SrTiO₃/NGO composite was carried out.
• •SrTiO₃/NGO/LIGE exhibits excellent electrochemical sensing ability towards anti-cancer drug.
• •SrTiO₃/NGO/LIGE platform is successfully applied to practicality in biological fluids.

Abstract

A sonochemical based green synthesis method playa powerful role in nanomaterials and composite development. In this work, we developed a perovskite type of strontium titanate via sonochemical process. SrTiO₃ particles were incorporated with nitrogen doped graphene oxide through simple ultrasonic irradiation method. The SrTiO₃/NGO was characterized by various analytical methods. The nanocomposite of SrTiO₃/NGO was modified with laser-induced graphene electrode (LIGE). The SrTiO₃/NGO/LIGE was applied for electrochemical sensor towards chemotherapeutic drug detection (nilutamide). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques have been used to examine the electrochemical performance of nilutamide (anti-cancer drug). DPV was found to be more sensitive and found to exhibit a sensitivity 8.627 µA µM⁻¹ cm⁻² for SrTiO₃/NGO/LIGE with a wide linear range (0.02–892 µM) and low Limit of detection (LOD: 1.16 µM). SrTiO₃/NGO/LIGE has been examined for the detection of nilutamide in blood serum and urine samples and obtained a good recovery in the range of 97.2–99.72 %. The enhanced stability and selectivity and practical application results indicates the suitability of SrTiO₃/NGO/LIGE towards the detection of nilutamide drug in pharmaceutical industries.

1. Introduction

The second most common type of cancer found in men is prostate cancer, considered as the top 6^th cause for deaths from cancer [1]. The percentage of prostate cancer is kept on increasing in many of the cities across the globe and may reach 1.8 million cases by the end of 2030 [2]. So, there is need of medicinal treatment for this prostate cancer and it is in practice. Nilutamide (IUPAC:5,5-dimethyl-3-(4-nitro-3-(trifluoromethyl) phenyl) imidazolidine-2,4 dione) is a type of non-steroidal drug mainly focuses on blockage of androgen response element transcription through ligand binding [3]. Nilutamide is used as a chemotherapeutic drug to treat the metastatic prostate cancer. Nilutamide blocks the action of androgens and limits the spreading of cancerous cells [4]. The high dosage of nilutamide causes severe issues to the patients. The major side effects of nilutamide are hemeralopia, visual disturbance, nausea, sexual dysfunction, blood in urine, nocturnal amblyopia, pneumonitis, disturbed sleeping, vomiting and etc [5], [6]. Hence, there is a need to monitor the pharmaceutical content in the human body. Many methods are available in the literature for the determination on nilutamide liquid chromatography, spectrophotometry, and electroanalytical methods [7], [8], [9], [10], [11], [12]. Assay of drugs using electrochemical sensors takes the lead compared to many other techniques. The basis of this relies with its low cost, easy to handle, wide linear range, high sensitivity and good selectivity [13].

Unmodified electrodes are not well suited for the detection of drugs due to low mobility of electrons and over peak potentials. Also, electrodes will be fouling towards drug analysis. Hence the electrodes are preferred to modify using catalysts. The materials used to modify the electrode surface should have the properties like conductivity, and easy mobility of electrons [14], [15]. Perovskites are the compounds exhibit the general formula ABO₃, where A and B are the metals having cations and O helps in binding A and B. SrTiO₃ is one such perovskite demonstrate numerous characteristics like high stability, enhanced surface area, intrinsic defects in the structure, tunable bandgap and appropriate band edge potential for water splitting reactions [16], [17]. Due to these features, SrTiO₃ were used in photo-catalysis, solar cells, supercapacitors and water splitting applications [18], [19]. SrTiO₃ particles has been explored for the detection of arsenic, tryptophan, drugs, simultaneous detection of dopamine and ascorbic acid, and nitro aromatics [20], [21]. Many methods were used for the synthesis of SrTiO₃ include hydrothermal, sol–gel, micro-emulsion, co-precipitation, solid-state synthesis approaches [16], [22]. These methods look advantageous still show signs of one or the other limitations like agglomeration, low surface to volume ration, uneven structure, use of hazardous solvents, and tedious process during synthesis [23].

Green approach using ultrasonication method for the synthesis of SrTiO₃ is highly pleasing among the researchers. The sonochemical synthesis method is very fast and may lead to get versatile structures due to the supply of shock waves and produce cavities during ultrasonication [24], [25]. In addition, during the sonochemical synthesis, acoustic cavitation, turbulent mixing, inter-particle collision, high temperature due to collision, and acceleration of suspended solid particles in liquid medium results in great morphology, structure, particle size, homogenous distribution of different metals will occur in the perovskites based nanomaterials [26], [27]. The electrochemical behavior of SrTiO₃ could be enhanced by decorating it with the graphene oxide (GO). The GO is a layered material with Sp² hybridization demonstrates good conductivity, flexibility and stability [28], [29], [30]. In addition, a potential candidate acts as a good matrix material for the nanocomposite fabrication and exhibit superior the electrochemical, photochemical performances in many applications. Doping heteroatoms like N, S, B and O to GO additionally increases the electrochemical behavior of the nanocomposites [29], [31]. Doping of N to the GO could create the defects and helps in the easy mobility of electrons during the redox reactions occurs at the surface of the modified electrodes. The electrode used for detection of analyte is also an important feature need to concentrate during fabrication of sensors. Many electrodes were used like glassy carbon electrode (GCE), screen printed electrode (SPCE), and 3D printing electrodes [32], [33], [34]. Laser induced graphene electrode (LIGE) is a promising electrode for sensor applications since the LIGE shape is well controlled by computer design, and its surface area and resistance is comparable with typical graphene [35], [36].

The properties of SrTiO₃, NGO and SrTiO₃/NGO were analyzed by various methods. Sonochemical synthesis is taken up and fabricated a SrTiO₃/N-doped graphene oxide (SrTiO₃/NGO composites) on LIGE. Enhanced electrochemical performance of is observed in SrTiO₃/NGO/LIGE than SrTiO3/LIGE and NGO/LIGE. SrTiO₃/NGO/LIGE has been examined for its electrochemical behavior for the detection of nilutamide in human urine and serum samples. The fabricated electrode exhibited wide linear range, sensitivity, and selectivity towards the detection of nilutamide and hence could serve as a choice of material in pharmaceutical Industries for its assay.

2. Experimental part

2.1. Synthesis of SrTiO₃/NGO

SrTiO₃ particles were decorated on NGO sheets using a simple sonochemical method. First, 0.3 mol of Ti(C₄H₉O)₄ was mixed with 30 mL of ethylene glycol under the magnetic stir. 0.3 mol of Sr(OH)₂·8H₂O was added into the reaction solution under the magnetic stirrer. Then, the reaction solution was processed for sonochemical reactions by (3 s on and 3 s off, total 6 s cycle;) based on a high-intensity Ti-horn probe at 50 kHz for 2 h. Finally, 20 mg of NGO was added under constant stirring. Preparation of NGO is given in the supporting information file [37], [38]. Finally, a perovskite type SrTiO₃/NGO was centrifuged, washed several times with water and ethanol and dried under dryer. The Scheme 1 depicts the sonochemical synthesis of SrTiO₃/NGO composite.

Scheme 1.

Green approach and synthesis of SrTiO₃/NGO and the composite modified laser-induced graphene electrode.

2.2. Preparation of SrTiO₃/NGO modified electrode

SrTiO₃/NGO (8 µL) was drop-casted on carbon surface of LIGE and dried at room temperature and labeled as SrTiO₃/NGO/LIGE. Same procedure is followed to prepare NGO/LIGE and SrTiO₃/LIGE for control experiments. More details of LIGE preparation are given in the addition information file (SI).

2.3. Details of the real sample and preparation

The real samples of human urine and serum samples were collected from CGMU, Taiwan (Chang-Gung Medical University) and used as such without any purification and placed in refrigerator and were subjected to electrochemical evaluation of nilutamide (after adding appropriate amount).

3. Results and discussions

3.1. XRD analysis of SrTiO₃/NGO and SrTiO₃

Fig. 1A depicts the XRD patterns of SrTiO₃ and SrTiO₃/NGO. The XRD pattern of the SrTiO₃ exhibit diffraction peaks at 2θ 23.0, (1 0 1) 33.64 (1 2 1), 37.2, (1 0 2), 39.2, (0 3 1), 40.9 (2 2 0), 47.4_, (2 0 2), 59.5_, (1 2 3), 69.7^, (2 4 2), and 79.8 (1 6 1) degree ((JCPDS No.#35–0734) [39], [40] and suggests the orthorhombic structure of SrTiO₃ nanomaterials. The XRD pattern of SrTiO₃/NGO exhibits all the peaks of SrTiO₃ nanomaterial in addition to a small broad peak in between 10 and 15^° assigned to (0 0 2) plane indicates the formation of existence of SrTiO₃/NGO without any other impurities. The synthesized compounds average crystal size was calculated using Debye-Scherrer formula and equation (1) [36].

$$d=\frac{K\lambda }{\beta Cos\theta }$$ (1)

where K is Scherrer’s constant, $\lambda$ is the wavelength of X-rays, $\theta$ is the Bragg diffraction angle, and $\beta$ is the full width at half-maximum (FWHM) of the diffraction peaks. The average particle size for SrTiO₃ was found to be 255 nm according to the full width at half maximum (FWHM) of the (1 1 0) crystal plane. The comparison of crystal sizes suggest that the crystallinity of SrTiO₃/NGO is higher than that of the SrTiO₃. In addition, cubic (space group: Pm-3 m) crystal structure of the SrTiO₃ particles is given in polyhedral model (Fig. 1B), space-filling crystal model (Fig. 1C), and last one is ball-stick crystal model of the sonochemically synthesized SrTiO₃ (Fig. 1D). based on the XRD analysis and cubic crystal structure, the unit-cell volume is calculated as 59.27 Å³. Therefore, the formation of SrTiO₃ particles and the crystal of the particle were evaluated clearly in the Fig. 1A-D.

Fig. 1.

(A) XRD analysis of SrTiO₃/NGO. Cubic crystal structure of SrTiO₃ particles with different models (B-D).

3.2. Structural analysis of SrTiO₃/NGO catalyst

The scanning electron microscope (SEM) produces enormous signals at the solid surfaces using high-beam electrons. The SEM monographs of the samples makes us to understand the crystalline structure, components, texture, and orientations. Morphological analysis of the SrTiO₃ material was examined by SEM and TEM method (Fig. 2A-C). Sonochemically synthesized SrTiO₃ shows a sphere like nanoparticles in Fig. 2C. SEM and TEM images of SrTiO₃/NGO, SrTiO₃ nanoparticles are covered with NGO sheets in Fig. 2D-F and NGO is layered nanosheets. Energy Dispersive Spectrometers can perform qualitative analysis (identifying all components in a specimen spectrum). In elemental mapping result of SrTiO₃/NGO is given in Fig. 2G indicates the existence of Sr, Ti, N, C and O the % wt. composition of individual element is depicted in Fig. 2H. All these results indicate the proper decoration of SrTiO₃ to NGO sheets.

Fig. 2.

SEM (A, B) and TEM images (C) of SrTiO₃ nanoparticles. SEM (D, E) and TEM (F) images of SrTiO₃/NGO. EDS analysis of SrTiO₃/NGO nanocomposite (G) and quantitative analysis of composite (H).

To identify the chemical structure and composition of the obtained SrTiO₃/NGO material, X-ray photoelectron spectroscopy (XPS) measurement was analyzed. Fig. 3A present the overall survey spectrum of the SrTiO₃/NGO composite. The obtained survey spectrum is confirmed the presence of Sr, Ti, O, N and C elements in sonochemically prepared SrTiO₃/NGO composite [41]. Moreover, the spectrum of Ti 2p is presented at 458 to 465 eV, and it suggesting the existence of Ti as Ti⁴⁺ state in the SrTiO₃/NGO composite [41]. Mainly, the characteristic peak for Sr 3d is presented at around 134 eV, it shows that strontium existed in the form of Sr²⁺ in the SrTiO₃/NGO composite [41]. Finally, the XPS spectrum for N 1 s, O 1 s and C 1 s reveals that the characteristic peaks of the SrTiO₃/NGO composite [42], [43].

Fig. 3.

(A), XPS analysis of SrTiO₃/NGO composite. (B) EIS analysis of bare electrode, NGO/LICE, SrTiO₃/LIGE and SrTiO₃/NGO/LIGE in 5 mM [Fe(CN)₆)]^3−/4− having 0.1 M KCl (frequency 1.0 Hz to 100 kHz at 0 V).

3.3. Electrochemical characterization of the modified SrTiO₃/NGO/LIGE

Electrochemical performance of the electrodes and modified electrodes are primarily associated with the reaction mechanism involving electron and charge mobility [44], [45]. EIS analysis was carried out for better understanding the behavior at the interface of electrode and electrolyte [46], [47], [48]. Fig. 3B indicates the Nyquist plot of unmodified LIGE, NGO/LIGE, SrTiO₃/LIGE and SrTiO₃/NGO/LIGE in 5 mM [Fe(CN)₆)]^3−/4− having 0.1 M KCl. The modification of electrode is further confirmed due to the different R_ct values. The charge low transfer resistance (97.5 Ω) and semi-circular nature of SrTiO₃/NGO/LIGE indicates its ability towards sensing different materials.

3.4. Electrochemical reduction of nilutamide at SrTiO₃/NGO/LIGE by voltammetry

The CV curves of 200 µM nilutamide in 0.05 M PBS at the scan rate at 50 mV/s of bare LIGE, NGO/LIGE andSrTiO₃/NGO/LIGE are given in Fig. 4A. The bare LIGE did not responding to the nilutamide shows its inefficiency to sense the model drug. A small current is observed, smaller oxidation and reduction peaks are observed for nilutamide in presence of NGO/LIGE electrode. In presence of SrTiO₃/NGO/LIGE a sharp irreversible cathodic peak is obtained at −0.18 V, indicates an efficient reduction of nilutamide to amine hydroxyl with 4H⁺/4e^- system. A peak towards the oxidation of nilutamide is observed at the potential 0.3 V and a small peak towards reduction of nilutamide is observed at 0.33 V follows 2H⁺/2e^-quasi-irreversible reaction attributed to the conversion of amine hydroxyl to nitroso group [3], [49], [50], [51]. The reduction peak at −0.18 V is considered for since it is sharp, and the peak current is high compared to others for further evaluation.

Fig. 4.

CV analysis of (a) LIGE, (b) NGO/LIGE and (c) SrTiO₃/NGO/LIGE (c) containing 200 µM of nilutamide under 0.05 M PBS at 50 mV/s (A). Different pH analysis with nilutamide (pH 3.0 to pH 11.0) in 0.05 M PBS at SrTiO₃/NGO/LIGE (B).

The pH of the medium where the reaction of the analyte occurs is one of the prominent features in the electrochemical detection. Hence the electrochemical evaluation of nilutamide was examined at different pH in presence of SrTiO₃/NGO/LIGE. Fig. 4B depicts the CVs of nilutamide at pH from 3 to 11. The observed reduction current is linear from pH 3 to 7 and found to decline at pH 9 and 11. The current response at the basic pH is less and shows shift towards negative potential. This could attribute to the availability of more hydroxyl ions and less H⁺ ions at basic pH and speaks about involvement of protons in the reduction of nilutamide. Hence, pH 7 considered as optimum which is a preferential pH since it is a physiological pH of humans.

Different concentration of nilutamide (25–300 μM) was used to evaluate the effect of [nilutamide] on its reduction in presence of SrTiO₃/NGO/LIGE. Fig. 5A indicates the increase in the current response with the increase in the concentration and attains a maximum current response of around 25 μA at 300 μM nilutamide due to the formation of larger reduction product and enhanced ionic stability in the electrolyte. The corresponding calibration curve is given in Fig. 5B shows good linearity with 0.9961 R² value. Scan rate has been varied and evaluated the CV of nilutamide in presence of SrTiO₃/NGO/LIGE. Fig. 6A depicts the CVs of nilutamide at different scan rate (0.02–0.30 V/s). The curves look linear with increase in the scan rate with R² = 0.9905 (Fig. 6B), It indicates the diffusion-controlled reduction of nilutamide based on SrTiO₃/NGO/LIGE [6], [52].

Fig. 5.

(A) SrTiO₃/NGO/LIGE was utilized for the different concentrations from 25 to 300 μM of nilutamide in pH 7 at a scan rate of 50 mV/s, and (B) the corresponding linear calibration plot between cathodic current vs [nilutamide]/μM.

Fig. 6.

(A) CV responses for different scan rate in nilutamide drug at SrTiO₃/NGO/LIGE. (B) The linear plot of square root of scan rate versus peak current/µA.

3.5. Electrocatalytic reduction of nilutamide at SrTiO₃/NGO/LIGE by DPV method

A more sensitive technique compared to CV, DPV is used to further examine the nilutamide. Fig. 7A shows the DPV curves of nilutamide at different concentration ranging from 10 nM − 892 µM under optimized conditions. A good linearity is observed with respect to current upon increasing the [nilutamide]. Fig. 7B depicts the corresponding calibration curve, shows the wide linear range from 0.01 to 892 µM follows regression equation y = -0.1561x −1.12 with R² = 0.9912. The sensitivity of the modified sensor is calculated as 8.67 µA µM⁻¹ cm⁻². LOD has been calculated using formula 1 and found to be 1.16 nM [14], [44].

$$\mathit{LOD}=3S/b$$ (2)

Fig. 7.

(A) DPV analysis of nilutamide under 50 mM PB (in pH 7.0) using SrTiO₃/NGO/LIGE and (B) calibration plot between cathodic current vs [nilutamide]/μM.

Where, S-slope and b-intercept of the plot. The low LOD, wide dynamic linear range, good sensitivity of SrTiO₃/NGO/LIGE towards electrochemical detection of nilutamide is superior and comparable to many other reported methods as shown in Table 1. In the modified electrode of SrTiO₃/NGO/LIGE is shows excellent electrochemical performance and ability of reduction towards detection anti-cancer drug in pH 7.0.

Table 1.

Nilutamide sensor performance at different modified electrode and the SrTiO₃/NGO/LIGE.

Electrode	Limit of detection (nM)	Sensitivity (µA µM−1 cm−2)	Linear range (µM)	Ref.
WS2/N, B-rGO	3.0	2.693	0.1–250	[53]
ZnO NPs/CPE	3.21	–	–	[11]
Samarium tungstate	2.6	0.918	0.05–318	[54]
GdV/HNT/GCE	1.8	1.33	0.5–478	[55]
ZnFe2O4	6.0	0.794	0.001–166	[56]
FeMnO3@f-CNF	5.3	–	0.001–244.5	[57]
SrTiO3/NGO/LIGE	1.16	8.627	0.02–892	This work

3.6. Selectivity, stability, and reproducibility of SrTiO₃/NGO/LIGE

Selectivity in sensing the target material is an important feature of any modified electrode. The common interfering species like drugs and other biological analytes were used for selectivity studies using SrTiO₃/NGO/LIGE through DPV technique. 100 μM nilutamide was taken in N₂ purged phosphate buffer of pH 7.0 and examined using DPV. The selectivity study, 10-folds high volume of drugs (dopamine, uric acid, folic acid, epinephrine, tanshinol, Adriamycin) and biological chemicals (acetylcholine, adenine, calciferol, cysteine, estradiol, guanine) were added and studied. No considerable change is observed in detecting nilutamide (<7.5 % deviation) even in presence of drugs and biological analytes shows the good selectivity of good selectivity SrTiO₃/NGO/LIGE towards the detection of nilutamide. Stability of the modified electrodes is another important feature in electrochemical sensors. 200 μM nilutamide was used for DPV analysis to evaluate the stability of SrTiO₃/NGO/LIGE for 30 days. The SrTiO₃/NGO/LIGE managed to retain 97.4 % of its original current even after 30 days indicates the good stability of the modified electrode. Reproducibility of SrTiO₃/NGO/LIGE was examined by fabricating six different electrodes and evaluated the detection of 200 μM nilutamide in N₂ purged PB of pH 7.0 using DPV. Moreover, the RSD of DPV peaks is less and found to be 1.86 % speaks about the very good reproducibility of the SrTiO₃/NGO/LIGE. Repeatability of SrTiO₃/NGO was evaluated by examining the nilutamide (200 μM) using DPV for 20 times. The lower RSD of 1.49 % indicates the superior repeatability of the SrTiO₃/NGO/LIGE.

3.7. Real same analysis of nilutamide in biological fluids using SrTiO₃/NGO/LIGE

Practical applicability of the modified electrodes relies on its efficiency to detect the analyte in biological samples. The electrochemical performance of SrTiO₃/NGO/LIGE towards detection of nilutamide has been evaluated in the sample of serum and urine using DPV method. Based on our previous experiment knowledge, a known concentration of nilutamide drug was dissolved in fluids. After that, we decided to analysis four different concentrations of nilutamide like 50, 100, 200 and 500 µM of added volume for the DPV analysis in phosphate buffer using SrTiO₃/NGO/LIGE by following standard addition real sample method. After the analysis, the recovery of the analysis was calculated as 97.2–99.72 % (Table 2). Those results indicate the outstanding performance in real sample analysis.

Table 2.

Nilutamide analysis in various biological fluids based on SrTiO₃/NGO/LIGE by DPV method.

Real samples	Added (nM)	Obtained (nM)	Recovery (%)	RSD (%)
Serum	0	–	–	–
	50	49.4	98.8	1.74
	100	98.5	98.5	1.86
	200	197.9	98.95	1.03
	500	498.6	99.72	1.09
Urine	0	–	–	–
	50	48.6	97.2	2.61
	100	98.9	98.9	1.34
	200	198.4	99.2	1.21
	500	497.1	99.42	0.89

4. Conclusion

In this report, NGO sheets covered SrTiO₃ NPs is developed via green sonochemical approach. Moreover, the different characterization results indicate the successful decoration of SrTiO₃ with NGO sheets. A non-enzymatic sensor has been fabricated for drug determination in biological fluids. The anti-prostate cancer drug of nilutamide is analyzed based on the modified LIGE. The enhanced electrochemical performance is observed in SrTiO₃/NGO/LIGE that could be attributed to the combined properties of SrTiO₃ (surface activity) and NGO sheets (conductivity and surface area). A wide linear range of 0.01–892 µM nilutamide and nano-molar detection (1.16 nM) was able to detect using SrTiO₃/NGO/LIGE in DPV analysis. The recovery results of real sample analysis speak about the practical applicability in pharmaceutical industries. Enhanced sensitivity, low LOD, good repeatability and reproducibility were indicated the efficiency of SrTiO₃/NGO/LIGE towards the non-enzymatic detection of nilutamide.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Data availability

No data was used for the research described in the article.