Sonochemical construction of hierarchical strontium doped lanthanum trisulfide electrocatalyst: An efficient electrode for highly sensitive detection of ecological pollutant in food and water

Graphical abstract

Highlights

• •Innovative, non-toxic and sonochemical synthesis of Sr@La₂S₃ electrode materials without any toxic reducing agents.
• •The morphological, spectral and electrochemical characterization of the Sr@La₂S₃ electrode platform was carried out.
• •Sr@La₂S₃/GCE and Sr@La₂S₃/RDE platforms are exhibits excellent electrochemical properties for MTO detection.
• •Sonochemically fabricated modified electrode platform is successfully applied to practicality test.

Abstract

Herbicides are used constantly in agriculture to enhance productivity across the globe. This herbicide monitoring requires utmost importance since its high dose leads to ecological imbalance and a negative impact on the environment. Moreover, a quantification of toxic herbicide is one of the important problems in the food analysis. In this work, deals with the development of a simple, and facile one-pot sonochemical synthesis of strontium doped La₂S₃ (Sr@La₂S₃). Morphological and structural characterization confirms the doping of Sr@La₂S₃ to generate a hierarchical layered structure. The electrochemical performance of modified with rotating disk electrode (RDE) using Sr@La₂S₃ composite is high, compared to La₂S₃ and bare electrodes towards the quantitative detection of mesotrione (MTO) in phosphate buffer. Sr@La₂S₃/RDE showed good sensitivity for MTO detection and it exhibit a range of 0.01–307.01 μM and limit of detection of 2.4 nM. Besides, the selectivity of fabricated electrode is high as it can electrochemically reduce MTO particularly, even in the presence of other chemicals, biological molecules and inorganic ions. The repeatability of MTO detection is high even after 30 days with a lower RSD values. Hence, simple fabrication of Sr@La₂S₃/RDE could be a novel electrode for the sensitive, selective, and reproducible determination of herbicides in real-time applications.

1. Introduction

Interestingly, the sonochemical approach has been proved to be a non-toxic method to obtain novel nanomaterials. The chemical effects of ultrasonic irradiation arise from acoustic cavitation and activation, in other words, the formation, growth and implosive collapse of bubbles in a chemical reaction medium, which results in a spontaneously high temperature and pressure pulse [1], [2], [3]. Based on this sonochemical reaction conditions of high pressure, temperature, and intense micromixing process attained during acoustic cavitation abilities lead to different and remarkable properties of nanomaterials in the irradiated reaction mixture [1], [4]. Based on an environmental perspective, the sonochemical irradiation is one of the advantageous, because it could be utilized to facilely synthesize doped nanomaterials without using toxic reagents and chemicals [5]. To prepare an economical and eco-friendly nanomaterial, sonochemical approach may be among the best way as a synthesis direction.

Lanthanum sulfides are the class of rare-earth chalcogenides materials in various forms; LaS, La₂S₃, and La₃S₄. Among these, dilanthanum trisulfides (La₂S₃) is the stable one consists of both positive (La⁺³) and negatively charged ions (S^2-). Lanthanum metal having a unique band structure due to the existence of f-orbital electrons, which can generate d-π coordination complexes with aromatic ring carbons [6]. Besides, the sulfide groups could easily form an electrostatic bond with the sensing analytes and along with this, it enhances the porous and hydrophilic nature. Compared to other metal-based sulfides, La₂S₃ seems less toxic and stable. La₂S₃ exhibits properties like good conductivity, variable oxidation state, enhanced surface area and ability to undergo redox reaction, it has been used as electrode materials in supercapacitors and water purification through adsorption and photocatalytic approach [7]. In additional, enhance the electrocatalytic performance of La₂S₃, it is more preferred to dope with other rare earth metals. Mainly, strontium (Sr) is one of the rare earth metals used as dopant to enhance electrocatalytic properties [8]. When Sr is doped to La₂S₃, there will be a generation of two cations with one sulfur anions. This doping of metal-to-metal sulfide will enhance the electrochemical performance by enhancing the electron mobility upon structural changes.

The properties of La₂S₃ depend on the structure, phase, crystallinity, shape etc. These properties are attributed to the synthetic approach used. La₂S₃ has been synthesized by thermal decomposition, spray pyrolysis, and sulphurization of oxide in presence of H₂S, successive ion layer adsorption and reactions methods [9], [10]. All these methods involve one or the other ambiguities like the use of hazardous solvents, time-consuming, expensive, high-energy input etc. So there is a need of a simple, cost-effective method for the generation of Sr@La₂S₃ with good structural properties. Authors in the present work describe a simple, facile, and economical one-pot synthesis of Sr doped La₂S₃ by hydrothermal approach. At present, the population across the globe is increasing and is expected to reach 9 billion by the year 2050 [11]. Once there is an increase in the population the need for food production is to be raised by 75 to 90 % [11]. There are several issues related to crop production like weeds, animal pests, pathogens, crop management, and socioeconomic constraints. Weeds are the unwanted plants that show competition with crops for space, nutrients, water, sunlight and additionally act as hosts for the growth of several insects that harm the growth of crops [12], [13]. Herbicides are the solution for the removal of unwanted weeds in the field due to its ease of application and economical since it saves labour cost. One of the triketone family herbicides used as selective weed control in maize, sugarcane, and rice is mesotrione (IUPAC name: 2-(4-methylsulphonyl-2-nitrobenzyl) cyclohexane-1,3-dione). Syngenta marketed mesotrione (MTO) in the year 2001 [14]. Due to the outstanding weed control property of MTO it is used to control the growth of broad leaves and weeds during their pre and its post-emergence [14]. Triple action formula is observed in MTO i.e. on roots, shoots as well as leaves. Main crops like Paddy, cane and maize are highly tolerant to MTO since they can metabolize the herbicide into inactive products. Like other herbicides, MTO could also be washed from leaves to soil and aquatic compartments [15]. Later accomplish in both surface and groundwater. Accumulation of MTO exhibits toxicity on non-target organisms and adversely disturbs the aquatic ecosystem by affecting the growth of microalgae. MTO and its metabolites are active to disturb the prokaryotic structures in water and lead to algal blooms as observed in the blue-green algae disaster that happened in 2007 which damaged the ecological unbalance [15]. Human beings are also affected even when a trace amount of MTO is consumed either in the form of food or water. Monitoring the MTO concentration in soil and water is of high importance as it falls under environmental remediation. Efficient, sensitive, selective, cost-effective, accurate and precise technique is needed for the detection of MTO.

Several analytical methods have been credited for the determination of MTO. The methods available for the detection of mesotrione are high-performance liquid chromatography (HPLC), gas-chromatography (GC), liquid chormatography-mass spectroscopy (LC-MS), colorimetric and diode array detectors [15], [16], [17], [18], [19], [20], [21]. These methods are sensitive but involve the use of hazardous reagents/solvents; time-consuming some may not be selective in detection. Hence, there is a need for selective as well as a sensitive method for the determination of MTO. Among several available new techniques for analysis, sensor-based techniques are of high priority. Among the different types of sensors available, florescent probe sensors, optical sensors, biosensors and etc [22], [23], electrochemical sensors are well used for the sensitive determination of herbicides [18]. The main advantage of electrochemical sensors is simple, limited use of hazardous materials, quick, and low cost. Different electrodes used in electrochemical sensors are glassy carbon electrode, gold electrode, screen-printed carbon electrode, carbon paste electrode and etc [24], [25], [26]. When these electrodes are used as such, the high efficiency of the electrode will not be reached due to high overpotential, slow kinetics during the electrochemical process. Hence, modification of the electrodes is much needed for the sensitive detection of analytes. The material used to modify electrodes should be having superior electrical conductivity, rich redox centers on the surface, swift electrochemical reactions, exceptional electron/electrode interface, specific structural defects and etc [27], [28], [29]. Therefore, materials like metal oxides, metal sulfide, carbon materials, metal vanadate’s, nanocomposites are used to modify electrodes could suit best for the electrochemical redox reactions for sensitive and selective analytical detection of analytes.

The characterization of Sr@La₂S₃ showed a hierarchical layered structure which is advantageous in electrochemical applications due to the easy mobility of electrons. The Sr@La₂S₃ modifier coated GCE showed an enhanced electrochemical sensing performance for the detection of mesotrione herbicide. The developed method is having good sensitivity, selectivity, stability along with practical applicability. The Sr@La₂S₃ could be developed as an alternative composite for the detection of herbicides, pharmaceuticals or other organic molecules that readily undergo redox reactions (Scheme 1).

Scheme 1.

Schematic representation for the electrochemical sensors and its electrocatalytic application.

2. Experimental

2.1. Materials and methods of the work

Mesotrione, lanthanum (III) nitrate hexahydrate, thioacetamide, strontium nitrate, and all other chemicals and reagents were collected from Sigma-Aldrich, Taiwan and used without further purification and washing process. The sonochemical synthesis was used by the probe of JY92-IIN Scientz (Xinzhi Co). Further, details of methods and instrumentation details were given in the supporting file.

2.2. Green based sonochemical synthesis of Sr@La₂S₃

In a synthesis of Sr@La₂S₃, the precursors 10 mM of strontium nitrate (ACS reagent, ≥99.0 %), 20 mM of lanthanum(III) nitrate hexahydrate (≥99.0 %), and 100 mM of sodium sulfide nonahydrate (ACS reagent, ≥98.0 %) were added into the 60 mL of deionized water. And then, the mixture was stirred homogeneous using magnetic stirrer at room temperature. After 10 min, the reaction solution was sonicated for 4 h. The ultrasonication process and reaction is conducted under optimum frequency at 50 kHz and optimum power at 200 W. After the reaction time, the obtained solution was centrifuged at 8000 RPM to collect the product and supernatant excess solution was discarded. At last, the resultant precipitate was filtered and washed with pure water and dried in a vacuum oven. Finally, the product was got after consecutively cleaned and dried process to obtain Sr@La₂S₃ material (Scheme 2).

Scheme 2.

Schematic representation for the synthesis of Sr@La₂S₃ catalyst.

2.3. Fabrication of Sr@La₂S₂ modified GCE/RDE

The working electrode; GCE/RDE has been modified using Sr@La₂S₃. First, the GCE/RDE was subjected to polishing using 0.05 μm alumina powder and sonicated in water. Two mg mL⁻¹ of suspension was dispersed in deionized water through sonication, 8 μL of the suspension was drop casted on GCE/RDE and used after drying at 75 °C.

3. Results and discussion

3.1. XRD and XPS analysis of Sr@La₂S₃ electrode material

The XRD spectrum of the Sr@La₂S₃ material is chemical and structural analysis in the 2θ range of 20° to 85° (Fig. 1A), which shows peaks with corresponding planes of (2 1 1), (3 2 1), (3 3 0), (2 1 2), (4 1 2), (6 1 0), (6 2 0), (2 1 3), and (2 0 9). The observed 2θ with corresponding XRD planes are well-matched with the ICSD card of 01–089–4035 and it’s indicate successful crystallographic of tetragonal La₂S₃ [30]. Moreover, the peaks are appearing at 34° to 42° corresponds to the Sr (Xiaobin et al. 2021). Based on this data, the XRD analysis suggest that Sr has been doped with La₂S₃ particles at the prescribed conditions, and it act as a Sr@La₂S₃ nanomaterials [31], [32].

Fig. 1.

(A) XRD pattern of La₂S₃and Sr@La₂S₃. (B) Crystal analysis of La₂S₃ particle.

XPS analysis was used to investigate the detailed surface elemental composition as well as the exact ionic states of the as-synthesised sample, and the resulting full survey spectrum is shown in Fig. 2A. The total survey spectrum clearly demonstrates the presence of La, Sr, S and O in the composites, and the results are consistent with the elemental results in Fig. 3I. The high resolution XPS spectrum of Sr 3d states is shown in Fig. 2B. Two resolved peaks at 133.3 eV and 135.1 eV belonging to Sr²⁺ 3d_5/2 and 3d_3/2, respectively are detected [33]. Fig. 2C depicts the XPS spectra of La, which mainly consist of the spin orbit splitting of La 3d_5/2 and La 3d3/2. La 3d_5/2 spectra, which contain two electronic transitions, the first of which is situated at about 837.6 eV and the second at 834.5 eV [34]. The peak at 835 eV is attributed to a satellite generated by a valence-band electron transitioning to an empty 4f orbital at the same time as a 3d electron transitioning to a continuum state. The binding of lanthanum to sulphur in La₂S₃ is responsible for these two peaks in the La 3d_5/2 spectrum. In trivalent lanthanum compounds including halide, oxide, and sulphide, this sort of satellite structure has been seen frequently. The peaks of La 3d_3/2 were located at 852.5 eV and 856.3 eV, clearly suggesting that La is in its La³⁺ oxidation state [6], [7]. The same sample's S 2p core level XPS spectra revealed two significant peaks at 161.8 eV and 163.0 eV [35], corresponding to the S 2p_3/2 and S 2p_1/2 spin orbit splits which is in good agreement with the literature (Fig. 2D).

Fig. 2.

XPS analysis of Sr@La₂S₃ (A), Sr 3d (B), La3d (C), S 2p (D).

Fig. 3.

SEM of Sr@La₂S₃ (A1-A3) and TEM analysis of Sr@La₂S₃ (B1-B3 and C). SAED pattern of Sr@La₂S₃ (D). Elemental mapping analysis of composite (E-G) and elemental analysis of Sr@La₂S₃ (H).

3.2. Morphological analysis of Sr@La₂S₃

Fig. 3A1, A2, and A3 represents the SEM images of Sr@La₂S₃ composite. In Fig. 3A2 shows the circular district strontium particles were accumulating on the flake surface of the La₂S₃. Fig. 3A3 reveals the exact structure of the composite at higher magnification. The TEM images of the composite were observed in Fig. 3B1, B2, and B3 at different magnifications. These TEM images shows the dark and light spots on the composite is due to different angle of orientation of the composite crystals. The HR-TEM image Fig. 3C show the different direction orientation of the fringes of the Sr@La₂S₃ composite and sample has a polycrystalline in nature. The fringes have distance of 321 nm, which is corresponds to the d spacing of the Sr and La₂S₃ in the XRD analysis. The SAED patterns of the Sr@La₂S₃ composite shown in Fig. 3D, the hkl values of the Sr@La₂S₃ composite were matched with the XRD values of corresponding d spacing of Fig. 1A the data gives the supporting evidence to the poly crystals of Sr@La₂S₃ composite. The elemental mapping of the Sr@La₂S₃ composite analysed in Fig. 3E, F and G, indicates the presence of Sr, La, and S respectively. The EDS spectra was showing the all the elements of the Sr@La₂S₃ composite were observed in the Fig. 3H.

3.3. Electrocatalytic activity of Sr@La₂S₃/GCE

Electrochemical performance of the synthesized materials is mainly related to the reaction mechanism that is allied with electron and charge mobility. Electrochemical impedance spectroscopy (EIS) analysis has been used to understand the electrode-solution interface [36], [37] and data is given in Fig. 4A. Nyquist plot of GCE, La₂S₂/GCE, and Sr@La₂S₃/GCE in 5 mM [Fe(CN)₆)]^3−/4− having 0.1 M KCl shows different R_ct values due to the surface modification. The inset (Randle's circuit) of exhibits lower charge transfer resistance in Sr@La₂S₃/GCE (32.6 Ω) along with semi-circular nature when compared to La₂S₂/GCE. The electrochemical surface area of the Sr@La₂S₃/GCE was calculated using a Randles-Sevcik Eq. (1) [38], [39], [40]. The electrochemical active-surface area was calculated by using of the unmodified and modified electrodes (Fig. 4B and C).,

$$\mathit{Ip}=2.69\times {10}^{5}A{D}^{1/2}{n}^{3/2}{v}^{1/2}C$$ (1)

Fig. 4.

(A) EIS and (B), CV analysis of unmodified GCE, La₂S₃/GCE, and Sr@La₂S₃/GCE in 5 mM [Fe(CN)6)]^3−/4− containing 0.1 M of KCl as the electrolyte (inset: Randles circuit). (C), corresponding calibration plot.

Herein, A, D, I_p, n, v, and C are representing the active surface area (cm²), diffusion coefficient (7.6 × 10^{− 6}), response peak current, number of electrons in the electrochemical reactions, scan rate of the reaction in CV analysis (Vs^{− 1}) and the concentration of the reaction molecules, respectively. Based on the Randles-Sevcik equation and obtained electrochemical parameters, the electrochemical active surface area (ECSA) was calculated. Moreover, more significant properties of ECSA for Sr@La₂S₃/GCE is 0.126 cm². To compare with other electrodes of La₂S₃/GCE (0.103 cm²) and bare GCE (0.019 cm²), the Sr doped La₂S₃ modified electrode has higher ECSA.

To find out the difference in the electrocatalytic performance of bare and modified GCE, cyclic voltammetry has been carried out. The CV of 100 µM MTO at bare GCE, La₂S₃/GCE and Sr@La₂S₃/GCE in 0.05 M PB (pH = 7) at a 50 mVs⁻¹ scan rate is depicted in Fig. 5A. A negligible reduction peak (−0.74 V) was observed with 4.1 µA current response for bare GCE which indicates the unsuitability of GCE towards the detection of MTO. Upon modification of GCE with La₂S₂ reduction peak was observes at −0.76 V with current 13.2 µA shows its electrochemical behavior towards the detection of MTO. When Sr@La₂S₃/GCE is used as the electrode, a considerable decrease in the potential with enhanced current response was observed. The potential of Sr@La₂S₃/GCE for the detection of MTO was found to be −0.63 V with 23.6 µA current. This enhanced current and lowered peak potential of Sr@La₂S₃/GCE for the detection of MTO are due to the synergistic effect of Sr metal on La₂S₃ and its layered structure. The Sr@La₂S₃/GCE's enhanced surface area, conductivity and layered structure accompany the easy and swift mobility of electrons during the redox reaction of the MTO [14].

Fig. 5.

(A) CV response for 100 µM MTO at bare GCE, La₂S₃/GCE and Sr@La₂S₃/GCE in 0.05 M PB (pH = 7) at a 50 mVs⁻¹scan rate. (B-C) pH analysis in Sr@La₂S₃/GCE with MTO.

The electrochemical performance of the material is mainly dependent on the pH of the electrolyte. Fig. 5B, describes the effect of pH on the electrochemical behavior of Sr@La₂S₃/GCE in presence of MTO. The pH was varied from 1 to 11, the current density response of Sr@La₂S₃/GCE was linear from pH 1 to 7 and a decrease in the current response was observed after pH 7. Hence, pH 7 is set as optimum to carry out further studies. The corresponding linear plot is given in Fig. 5C which indicates the dependence of peak current with the potential with regression coefficient R² = 0.995. The mechanism of electro-reduction of MTO at 0.05 M PB in presence of Sr@La₂S₃/GCE is given in Scheme 3. The reduction of MTO takes place in two steps. Step A is a chemical reaction that involves the keto-enol tautomerism and forms the protonated enol form of MTO. Step B is an electrochemical reaction, where MTO undergoes irreversible reduction by gaining 4 electrons and 4H⁺ in 0.5 M PB solution [14].

Scheme 3.

Electrochemical reduction of MTO at Sr@La₂S₃ modified electrode in 0.05 M of PB (pH 7.0).

To know the range of MTO sensing by Sr@La₂S₃/GCE, CV has been carried out in 0.05 M PB by varying the concentration of MTO. Fig. 6A, indicates the peak potential linear with the concentration i.e. upon rising the concentration the reduction peaks current increases with a slight shift in the peak towards more negative potential. A corresponding calibration graph is given Fig. 6B which follows the equation as I = −0.1022 µA µM⁻¹ X-4.0779 (R² = 0.9938). Fig. 6C, indicates the plot of log concentration of MTO against log current, which is found to be linear with R² = 0.9983. CV of 100 μM MTO in presence of Sr@La₂S₃/GCE was recorded by varying the scan rate from 0.02 to 0.4 Vs⁻¹ in 0.05 M PB solution (Fig. 6D). It is found that the current response was linear with the scan rate indicates the superior electrochemical behaviour of Sr@La₂S₃ towards the detection of MTO. The inset is a plot of the square root of the scan rate versus current shows the good linearity with the linear regression equation −29.468x-4.8805 with regression co-efficient R² = 0.9915 (inset). This indicates the diffusion-controlled phenomena that occur during the electrochemical detection of MTO in presence of Sr@La₂S₃/GCE [41], [42]. Fig. 6E is the calibration graph of log scan rate against potential, which is found to be linear with regression co-efficient value 0.9916. Another calibration curve (Fig. 6F) indicates the dependence of current on the applied potential. The current is linear with the applied potential with the R² value 0.9966.

Fig. 6.

(A) CV of Sr@La₂S₂/GCE at different quantities of MTO (10 to 200 μM) in 0.05 M PB (pH = 7.0) and (B-C) corresponding calibration plot. (D) CV platform of Sr@La₂S₃/GCE containing 100 μM MTO at different sweep rates 0.02–0.4 Vs⁻¹ in 0.05 M of PB (pH = 7) and (E-F) corresponding linear calibration plots.

3.4. Amperometric detection

Sr@La₂S₃/RDE has been further subjected to amperometric detection of different concentrations of MTO to find out the analytical parameters like sensitivity and limit of detection (LOD). Fig. 7A depicts the amperometric profile of MTO in 0.05 M of PB (pH = 7.0). With the increase in the addition of MTO has observed to increase the peak potential. A wide range of detection is observed with LOD of 2.4 nM [43], [44], [45]. Its corresponding calibration curve of different concentrations of MTO against the current is given in Fig. 7B. The reduction of MTO is linear with the increase in the concentration of MTO, still, a slight variation is observed in lower and higher concentrations of MTO. The linear regression equation at lower concentration (up to around 100 μM) equation Ip_c (μA) = 0.7322 μM – 1.9366 with 0.9948 R² value. At higher concentration of MTO (above 100 μM) the equation follows I_pa (μA) = 0.6976 μM – 2.1696 with 0.9995 R² value. This indicates, the amperometric approach is best suited in both higher and lower concentrations of MTO, which is a much-needed sensing characteristic of a material. The sensitivity of Sr@La₂S₂/RDE is very higher (40.677 & 38.755 µA⋅µM⁻¹⋅cm⁻²) with a wide linear range (0.01–307.01 μM) and low LOD values indicate its superiority over different methods available in the literature by using various techniques towards the electrochemical detection of MTO (Table 1). Moreover, limit of quantification is calculated as 10 nM in the modified sensor.

Fig. 7.

(A) amperometric responses of Sr@La₂S₃/RDE at different quantities of MTO (0.01–307.01 μM) in 0.05 M of PB (RPM = 1200). (B) linear dependence for the concentration of MTO (μM) vs current (μA).

Table 1.

Comparison of electrochemical parameters for the detection of MTO toxic chemical at Sr@La₂S₃ modified RDE with other research articles.

Electrode	Methods	Sensitivity (µA⋅µM−1⋅cm−2)	Linear range (µM)	LOD (nM)	Reference
Carbon/GCE	DPV	2.58	0.25–8.5	70	[20]
carbon quantum dots	MIFP	–	0.015 to 3.0	4.7	[16]
Organoclay/GCE	SWV	6.54	0.25–2.5	25	[46]
silver/amalgam electrode	DPV	–	0.10–10	50	[47]
carbon black/GCE	SWV	2.34	0.040–7.2	26	[48]
NfrA2@MgAl-NPs	AMP	18.4	5–60	300	[18]
Fe(III) complex	UV–vis	0.379	0.2–10.0	53	[19]
HPPD	CV	–	1–237	65	[49]
Sr@La2S3/RDE	AMP	40.677 & 38.755	0.01–307.1	2.4	This work

3.5. Interference studies

A stable sensor can detect the analyte even in the presence of other interferants. To find out the selectivity of Sr@La₂S₃/RDE towards the detection of MTO, interference studies have been carried out and are given in Fig. 8A. The current response Sr@La₂S₃/RDE in amperometric detection MTO (50 µM) in presence of various pharmaceutical drugs, biological analytes and metals (10-fold excess) are given in Fig. 8A-C, respectively. A clear response is observed only for MTO, the response for other analytes is very negligible and does not affect MTO response. The results indicate, Sr@La₂S₃/RDE is a selective electrode for sensing MTO in 0.05 M PB and can be detected even in the presence of organic and inorganic species along with analyte. Mainly, the possible reason for the selective detection of MTO is each interferents have specific potentials with adsorption sites on the surface of Sr@La₂S₃/RDE, while the adsorption of MTO could be remarkably higher compare to other analytes. The applied potential is drastically lowered which is one of the important reasons for selectivity.

Fig. 8.

(A-C) Interference analysis based on Sr@La₂S₂/RDE in 100 μM MTO and different interferences. (D) Stability analysis at Sr@La₂S₃/RDE in MTO.

The stability of the Sr@La₂S₃/RDE for the detection of MTO have been evaluated by carrying out the amperometric studies for 30 days (Fig. 8D). After the modification of Sr@La₂S₃/RDE was placed at 4 °C, when not in use. The Sr@La₂S₃/RDE is found to exhibit high stability with a constant current response even up to 30 days (more than 95 %). Further, MTO has been detected in different water samples and grape extract samples using Sr@La₂S₂/RDE by optimized amperometric method.

3.6. Real sample demonstration using Sr@La₂S₃ catalyst

In the real sample studies, we used the amperometric method at Sr@La₂S₃/RDE for analysis of MTO in industrial, river water and fruit extract samples. Based on the previous literature, MTO is contaminated in water resources and fruits. Real sample preparation is given in the supporting information. For the experiment, the obtained real samples were directly analyzed based on the optimized conditions. Furthermore, an MTO was spiked and prepared known concentration of real samples. The spiked real samples were tested and current response was observed using Sr@La₂S₃/RDE. Results for the recovery of MTO in the industrial, river water and fruit extract samples are shown in Table 2. As a result of the testing, Sr@La₂S₃ modified electrode appeared to have excellent capabilities for MTO detection in various real samples. The RSD values are less than 2 %, which indicates a very good selective detection of MTO not only with other species but also at the different environments of the analyte.

Table 2.

Detection of MTO in water and fruit samples at Sr@La₂S₃ catalyst modified electrode.

sample	Added (nM)	Found (nM)	Recovery (%)	RSD/±% (n = 3)
Industrial water	0	0	–	–
	200	198.5	99.25	0.94
	500	496.2	99.24	0.59
River water	0	0	–	–
	200	197.9	98.95	1.02
	500	497.2	99.44	0.91
Fruit extract (grape)	0	0	–	–
	200	198.8	99.4	0.84
	500	498.1	99.62	0.69

4. Conclusion

The work involves the simple one-pot hydrothermal synthetic method for the generation of layered strontium doped dilanthanum trisulfide. The electrocatalytic activities of the La₂S₃ and Sr@La₂S₃ have been used as modifiers on RDE. The electrocatalytic results indicate Sr@La₂S₃/RDE exhibit enhanced activity than La₂S₃/GCE for the detection of MTO chemical in 0.05 phosphate buffer. Results show that doping of Sr to La₂S₃ positively enhanced the electrochemical performance due to increased conductivity and active sites on the surface. The Sr@La₂S₃/GCE showed very good stability, sensitivity, and selectivity for MTO detection with a wide range and lower LOD values. This is the first report on metal-doped lanthanum sulfide for the detection of an herbicide. This report opens up the gate for the research on metal-doped lanthanum sulfides towards the detection of organic molecules in the chemical and pharmaceutical industries.

CRediT authorship contribution statement

Umamaheswari Rajaji: Conceptualization, Formal analysis, Investigation, Writing – original draft. K. Yogesh Kumar: Validation, Writing – review & editing. Rameshkumar Arumugam: Data curation, Methodology. Asma A. Alothman: Writing – review & editing, Conceptualization. Mohamed Ouladsmane: Funding acquisition, Writing – review & editing. Ren-Jei Chung: Investigation, Supervision, Conceptualization. Ting-Yu Liu: Project administration, Supervision, Funding acquisition, Writing – original draft, Writing – review & editing.

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: