Hydrothermal synthesis of AgInS₂@biochar nanocomposites for the photocatalysis and electrochemical sensing of glufosinate herbicides

Firdous Ahmad Ganaie^a, Irfan Nazir^a, Aaliya Qureashi^a, Zia ul Haq^a, Kaniz Fatima^a, Arshid Bashir^b, Altaf Hussain Pandith*^a and Mohsin Ahmad Bhat*^a
aLaboratory of Nanoscience and Quantum Computations, Department of Chemistry, University of Kashmir, Hazratbal, Srinagar-190006, J&K, India. E-mail: altafpandit23@gmail.com; mohsin@kashmiruniversity.ac.in; Fax: +91-194-2414049; Tel: +91-194-2424900 Tel: +91-7006429021 Tel: +91 9419033125
^bDepartment of Chemistry, Degree College, Beerwah, Budgam, J&K, India

First published on 25th July 2025

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1. Introduction

Globally, we are facing a scarcity of drinking water. This is largely due to the presence of various contaminants that are directly or indirectly discharged into water bodies.^1–3 These contaminants include potentially hazardous dyes, herbicides, heavy metal ions, and microplastics.^4,5 Particularly, potentially hazardous dyes, such as malachite green, methylene blue, and organophosphate herbicides, are the major contributors to water contaminants.⁶ Therefore, it is essential to treat wastewater before its discharge into the environment.^7,8 There is an urgent need for cost-effective and highly efficient materials for water treatment applications. A variety of semiconductors, including ZnO, CuO, CdS, ZnS, TiO₂, SnO₂, Ag₃PO₄, silver nanoparticles (Ag-NPs), and titanium oxide nanoparticles (TiO₂-NPs), have been utilized as photocatalysts for effective water purification. Despite their widespread use, some of these materials exhibit limited activity under visible light and are susceptible to photo-corrosion.^9–12 Hence, there is a need for innovative approaches that can address both the detection and removal aspects of contamination. Researchers around the world are continuously working to develop and modify new materials that enhance photocatalysis and electrochemical sensing of various contaminants.^13,14 Hybrid heterostructures can be used in electrochemical sensing applications utilizing the complementary properties of various materials. In this context, ternary semiconductor materials, such as TbCrO₃, CuInS₂, CuInSe₂, and AgInS₂, have gained significant attention owing to their low toxicity.^15,16 Among these, AgInS₂ stands out due to its remarkable properties, including a narrow emission band, broad excitation band, long fluorescence lifetime, high fluorescence emission, and excellent photostability.^17–19 It is a non-toxic and highly promising material, particularly effective as a solar light absorber for degrading organic pollutants. The unequal bonds between AgS and InS are beneficial for the formation of an internal electric field. Therefore, charges can be separated rapidly under illumination, leading to an improvement in the photocatalytic activity.^20–22 Despite its various advantages, AgInS₂ faces limitations, such as photocorrosion, charge recombination, tendency to agglomerate during reactions, and limited adsorption capacity, due to its relatively small specific surface area. To address these challenges, AgInS₂ has been combined with carbon materials, such as biochar, to produce a heterostructure, which can expand the light absorption range and promote charge separation to improve its effectiveness in treating toxic contaminants.^23–26 These properties are considered highly effective in reducing charge recombination and enhancing photocatalytic activity.^27–29

Biochar has evolved as an efficient and functional support material for the decoration of nanocatalysts. Its ease of preparation, chemical stability, versatile functional groups, electrical conductivity, large surface area, cost-effectiveness and unique properties make it suitable for various applications.^30–32 We fabricated an AgInS₂@biochar nanocomposite via a novel hydrothermal route utilizing cattail (Typha plant) biomass as a green source of biochar. The nanocomposite offers heterogeneous interfaces, flexible band gaps, and superior charge separation ability. The resulting synergy enhances sensitivity, selectivity, versatility, and overall performance, making these structures pivotal for the advancement of photocatalytic and electrochemical sensing technology.^33–35 Various analytical techniques have been utilized for the detection of contaminants, including gas chromatography (GC),³⁶ capillary electrophoresis,³⁷ mass spectrometry,³⁸ UV-visible spectrometry,³⁹ and high-performance liquid chromatography (HPLC).⁴⁰ Among these, the electrochemical method stands out owing to its operational simplicity, high sensitivity, and selectivity, making it particularly effective for real-time monitoring of environmental and food samples.^41–43

In this study, we develop a novel strategy for the design and synthesis of AgInS₂ and its composite (AgInS₂@biochar) using a two-step hydrothermal method. The biomass collected from the cattail typha plant (Typha latifolia L.) was pyrolyzed to obtain a biochar material that served as a sustainable carbon source. Both AgInS₂ and AgInS₂@biochar materials were employed for catalytic and sensing applications. Although pristine AgInS₂ demonstrated limited photocatalytic activity and a weak electrochemical response (CV and DPV), the AgInS₂@biochar composite exhibited significantly enhanced electrochemical sensitivity toward the detection of the toxic herbicide in 0.1 M KCl with glufosinate concentrations ranging from 1 to 10 μM, achieving a detection limit of approximately 0.026 μM-well within the limits recommended by the WHO. The AgInS₂@biochar sensor also demonstrated good stability, reusability, and improved detection limits compared to previously reported materials. Furthermore, the effective photocatalysis of malachite green (MG) dye was carried out with high photocatalytic activity. To the best of our knowledge, this is the first report on the electrochemical sensing properties of AgInS₂@biochar for herbicide detection.

2. Materials and methods

2.1 Materials

Cattail (Typha) plants were collected from the banks of Dal Lake, Srinagar; their chemical name is Typha latifolia L. with 95% purity, while silver nitrate (AgNO₃·5H₂O, 97.5%) was purchased from High Purity Laboratory Chemicals Pvt. Ltd. Indium chloride (InCl₃), thioacetamide (98.0%), and malachite green dye were purchased from MERCK. Glufosinate herbicide was obtained from Cheminova Pvt. Ltd. Double-distilled water was used to prepare the solutions in this study.

2.2 Preparation of biochar

The locally sourced cattail was initially desiccated at a temperature of 30 °C using a vacuum oven. Subsequently, the air-dried cattail underwent crumpling and filtration. Following this, the dehydrated cattail material was placed in a crucible and subjected to slow pyrolysis in a tubular furnace at a heating rate of 10 °C per minute for 2 h. This process resulted in the production of a dark black biochar material, which was then collected and stored for future experimental use.

2.3 Preparation of AgInS₂ nanocomposite material

AgInS₂ nanospheres were synthesized using a simple hydrothermal method. Specifically, 0.3 mmol of AgNO₃, 0.5 mmol of InCl₃·4H₂O, and an excess of thioacetamide were dissolved in 20 mL of deionized water for approximately 30 min. This solution was then transferred to a 50 mL Teflon-lined stainless-steel autoclave and kept at 120 °C for 8 h in a hot air oven. The resulting brownish-black colored powder was collected, washed, and dried at 50 °C.

2.4 Preparation of the AgInS₂@biochar composite

The AgInS₂@biochar nanocomposite was prepared following a hydrothermal method. Initially, 0.20 g of Cattail biochar was dispersed in 20 mL of distilled water under ultra-sonication for 1 h. To this biochar dispersion, 0.3 mmol silver nitrate (AgNO₃), 0.5 mmol indium tri chloride (InCl₃), and 2 mmol thioacetamide were added. The whole mixture was then stirred for 1 h and transferred to a hydrothermal reaction vessel maintained at 120 °C for 8 h. The precipitate was filtered and repeatedly rinsed with distilled water. The precipitate was then dried at 50 °C for 2 h to produce a solid black powder of AgInS₂@biochar (Scheme 1).

2.5 Catalyst characterisation

An X-ray diffractometer (XRD), Ultima-IV, Rigaku Corporation, Tokyo, Japan, was used to probe crystallinity. The morphology was examined with an analytical scanning electron microscope (Zeiss Ultraplus-4095) and a transmission electron microscope (HR-TEM) using a JEOL JEM 2100F (USA). Diffuse reflectance spectra (DRS-UV) were recorded with a Shimadzu 2600 spectrometer (Japan). The surface functionalization was analyzed with FTIR spectroscopy using a PerkinElmer Spectrum-100 FTIR. Photoluminescence (PL) spectra were recorded at room temperature with a Hi-tech F-4600 fluorescence spectrophotometer (λ_max = 440 nm). The X-ray photoelectron spectra of the samples were analyzed using a PHI 5000 Versa Probe III XPS instrument with monochromatized Al Kα (E = 1486.7 eV).

2.6 Photocatalytic activity test

The photocatalytic properties of the AgInS₂@biochar composite were evaluated by monitoring the degradation of malachite green (MG) using a flow batch mode method. A 300-W UVB lamp (Philips) was utilized in a photoreactor setup. The experiment involved adding 1 mg mL⁻¹ of the nanocomposite to 100 mL of a 10 ppm MG solution. This mixture was stirred in the dark for 1 h to reach adsorption–desorption equilibrium before irradiation. During the photocatalytic reaction, 2.5 mL aliquots were taken every 10 min and centrifuged to remove photocatalyst particles. The absorbance at 614 nm was measured using a double-beam UV-visible spectrophotometer (model: LI-2802). The degradation efficiency of MG was calculated using the following equation:

	(1)

where the absorbances C₀ and C are the concentrations of malachite green (MG) before and after exposure to visible light, respectively. The experimental data were analyzed to determine the kinetics of MG adsorption and degradation using the pseudo-first-order kinetic (PFOK) model. This model is represented by the equation typically used to describe the rate of reaction in photocatalytic processes.

	(2)

where t is the given time (min), [MG₀] is the initial concentration of MG (mg L⁻¹), and [MG]_t is the concentration of MG at another given time (mg L⁻¹).

2.7 Electrochemical studies

Electrochemical studies were conducted using a bio-logic-sas potentiostat (SP-150) for cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in a three-electrode configuration. The working electrode was a 3-mm diameter glassy carbon electrode (GCE) that had been modified with the synthesised material. The counter and reference electrodes were platinum wire and Ag/AgCl (3 M KCl), respectively. The studies used 0.1 M KNO₃ solution as the supporting electrolyte, with a potential window ranging from −1 V to +1 V and a scan rate of 5 mV s⁻¹. The electrolyte solution was purged with high-purity argon/nitrogen gas at atmospheric pressure (∼1 atm) for 20 minutes to eliminate dissolved oxygen and ensure an inert atmosphere. In addition, the glassy carbon electrode (GCE) was polished with alumina slurry (0.05 μm) on a polishing cloth. It was then thoroughly rinsed with distilled water and ethanol before drying at an ambient temperature. To modify the electrode, a stable suspension of the AgInS₂@biochar nanocomposite was created by dispersing 2 mg of the material in 1 mL of ethanol and 10 μL of 0.5% glutaraldehyde solution, followed by sonication for 30 minutes to ensure uniform dispersion. To create a homogenous and adherent nanocomposite film, 5 μL of the well-dispersed solution was drop-cast onto a clean GCE surface and dried at room temperature. A modified electrode (AgInS₂@biochar/GCE) was employed for electrochemical sensing of the glufosinate herbicide, and DPV was employed to evaluate this sensitivity. The limit of detection (LOD) was calculated using the following standard equation:

	(3)

where sigma is the standard deviation in experimental current values and S represents the slope in the calibration plot.

2.8 Computational analysis

Density functional theory (DFT) was employed in the computational investigations along with the Gaussian 09 arrangement of codes. B3LYP and LanL2DZ were applied as the practical and basis sets, respectively. Geometry optimization was carried out using Becke's three-parameter exchange functional combined with the Lee–Yang–Parr correlation function. The LanLDZ/6-311++g(d,p) basis sets were used for AgInS₂ and non-metal atoms, such as carbon. Moreover, the HUMO, LUMO and HUMO–LUMO energy gaps were assessed for AgInS₂ and AgInS₂@biochar.

3. Results and discussion

3.1 Characterization of the material

X-ray diffraction studies were conducted to analyze the phase and crystallinity of the as-prepared materials. The PXRD diffraction patterns for biochar, AgInS₂, and AgInS₂@biochar are presented in (Fig. 1a). The biochar samples display sharp diffraction peaks at approximately 2θ = 22.3° and 28.5°, corresponding to the (100) and (002) planes of the carbon structure, respectively. In AgInS₂, diffraction peaks at 24.3°, 28.2°, 31.2°, 34.0°, 40.2°, 49.8°, and 59.7° and 71° were observed. These results correspond to an orthorhombic structure (JCPDS No. 25–1328).^44,45 The AgInS₂@biochar composite exhibits peaks corresponding to both pristine biochar and AgInS₂. Notably, the peaks significantly deviate from their standard positions, and the presence of higher-intensity peaks indicates an increased number of atoms occupying the same lattice planes. This deviation and intensity suggest the successful formation of the AgInS₂@biochar nanocomposite through biochar impregnation.

In addition, the average particle size in AgInS₂ and AgInS₂@biochar was calculated using the Debye–Scherrer equation:

d = Kλ/βcosθ,

where d = grain size, λ = 1.54 Å (wavelength of the X-ray radiation), K = 0.89, β = peak width (in radians) at half-maximum height, and 2θ = Bragg's angle. For orthorhombic phase AgInS₂, the average grain size in AgInS₂ and AgInS₂@biochar were calculated to be ca. 27.3 and 38.2 nm, respectively.

FTIR analysis of the crafted samples was carried out to determine the functional groups present over the surface of the functional groups in biochar, AgInS₂, and AgInS₂@biochar nanocomposite (Fig. 1b). In all three samples, the peaks located around 3480 cm⁻¹ were attributed to the strong stretching vibrations of O–H. The peak at 2840 cm⁻¹ corresponds to a weak C–H stretching vibration band. In biochar and AgInS₂@biochar, a peak at around 1630 cm⁻¹ is attributed to the presence of CO in the sample. The peaks at 1520 cm⁻¹ and 1649 cm⁻¹ are due to the bending vibrations of –CH₃, the characteristic of the carbon material. The stretching bands at approximately 564 cm⁻¹, 982 cm⁻¹, and 1205 cm⁻¹ could be due to In–S or Ag–S bonds.⁴⁴ Moreover, the bands of AgInS₂ are present in the composite material (AgInS₂@biochar), indicating that AgInS₂ has been successfully incorporated into the biochar Matrix.

TGA analysis was carried out to determine the thermal stability of the synthesized materials (Fig. 1c). The biochar sample undergoes a two-step weight loss, with a 15% reduction between 100 °C and 150 °C, and a further weight loss starting from 590 °C to 700 °C results in an overall 23% weight reduction due to the decomposition of cellulosic, hemicellulose, and lignin compounds, after which the weight stabilizes.⁴⁶ In contrast, AgInS₂ starts losing weight around 150 °C, continuing up to 500 °C, which results in an 80% weight loss primarily due to the evaporation of adsorbed moisture. In AgInS₂@biochar the removal of adsorbed water is observed upto 100 °C. Another weight loss is observed upto 260 °C due to loss of adsorbed water and decomposition of organic matter in the composite. Thereafter the weight loss continues upto 540 °C due to decomposition of organic matter. Compared to pristine biochar, the composite exhibit delayed and reduced weight loss. This indicate the superior thermal stability of the composite material. The AgInS₂@biochar composite starts losing weight around 100 °C, continuing up to 280 °C, resulting in an 27% weight loss and further, continuing occurs up to 540 °C results 38% weight loss exhibits delayed and reduced weight loss, indicating superior thermal stability compared to AgInS₂, attributed to the expulsion of surface-bound water and decomposition of components other than carbon.

The surface area of the biochar and AgInS₂@biochar composite was measured using the multipoint Brunauer–Emmett–Teller (BET) method based on nitrogen adsorption–desorption isotherms. The isotherms displayed in (Fig. 1d) show a type-IV pattern with a hysteresis loop at a relative pressure (P/P₀) from 0.0 to 1.2, indicating a mesoporous structure. The BET analysis revealed a specific surface area of pristine biochar is 17.3 m² g⁻¹ (ref. 47) and that of the composite is 28.1 m² g⁻¹, confirming its advanced porous structure. This enhanced surface area provides a larger contact region, which is advantageous for photocatalytic and electrochemical sensing applications.

Scanning electron microscopy (SEM) was used to examine the surface morphology of biochar, AgInS₂, and the AgInS₂@biochar composite, as shown in (Fig. 2). The biochar exhibits interconnected bar-like structures resembling hollow tubes with a reticular surface (Fig. 2a and b). This surface structure of biochar is desirable for AgInS₂ to reunite into a spherical-like morphology, quick mass transport, and the growth of AgInS₂ interlaced spherical nanospheres²⁵ (Fig. 2c). In the AgInS₂@biochar composite (Fig. 2d), AgInS₂ appears as nanosphere units and particulate deposits on the biochar surface, suggesting interface interactions between the components. The heterostructure shows significant aggregation akin to the pristine composite. Transmission electron microscopy (TEM) analysis (Fig. 2e and f) reveals that the AgInS₂ particles were decorated on the interface of the composite to design AgInS₂@biochar hybrids without any destruction of the biochar or AgInS₂ nanospherical structures. We observe the successful stacking of AgInS₂ on the highly reticular surface of biochar, confirming the efficacious production of the composite material.

The EDS analysis was performed to assess the presence and distribution of elements in the AgInS₂@biochar composite. The results confirm a uniform distribution of all elements, specifically carbon (C), oxygen (O), silver (Ag), indium (In), and sulfur (S). These significant findings are illustrated in (Fig. 3).

3.2 Optical studies

UV-Vis diffuse reflectance spectroscopy was used to analyse the solid samples using a Shimadzu 2600 spectrometer (Japan), equipped with a 240-52454-01 integration sphere accessory. The solid material was ground in a pan grinder to a 100-mesh size. Then, this material was analyzed after being compressed carefully into the spectrometer cell. The DRUV-VIS spectra were converted to the Kubelka–Munk remission function defined by f(KM) = (1 − R)²/2R = k/s, where R is the reflectance, k is the absorption coefficient, and s is the scattering coefficient. Moreover, UV-vis diffuse reflectance spectroscopy was employed to analyze the optical properties of the synthesized materials, confirming the semiconductor characteristics of both AgInS₂ and AgInS₂@biochar composite, as depicted in (Fig. 4). AgInS₂ exhibited extended absorption in the visible range around 540 nm, while the AgInS₂@biochar composite showed a redshift of the absorption edge to approximately 685 nm. Band gaps were determined from Tauc plots of (αhν)² versus hν (Fig. 4b), revealing a value of 1.89 eV for AgInS₂ and 1.49 eV for the AgInS₂@biochar composite. The incorporation of biochar results in a lower band gap energy compared to pristine AgInS₂, which is attributed to enhanced charge transfer.

Photoluminescence (PL) analysis assessed the recombination rate of solar light-generated electron–hole pairs. (Fig. 4c) shows the PL spectra for AgInS₂ and AgInS₂@biochar, both excited at a wavelength of 670 nm, with an emission peak around 685 nm due to edge-free excitation luminescence. The PL intensity of the AgInS₂@biochar composite is notably lower than that of AgInS₂ alone, indicating that forming the composite effectively reduces the recombination of photo-induced carriers and enhances electron–hole pair separation. Biochar acts as an electron trap, improving electron trapping and resulting in a smaller PL peak for the composite. This suggests that AgInS₂@biochar composites can extend the lifespan of holes (h⁺) and activate hydroxyl groups to form ˙OH radicals on the biochar surface, aiding in the degradation of water pollutants.

3.3 XPS studies

X-ray photoelectron spectroscopy (XPS) analysis was performed to examine the chemical state of the synthesized AgInS₂@biochar composite. The survey spectrum revealed the presence of C 1s, O 1s, Ag 3d, In 3d, and S 2p signals, with no impurities observed (Fig. 5a). The weak presence of oxygen over the surface, as reflected by the XPS spectra, can be attributed to the held oxygen-containing bundles on the surface of the biochar. The C 1s spectra of the biochar, depicting the two different peaks at 284.6 and 286.1 eV, are attributed to C–C and C–O–C, respectively (Fig. 5b). The binding energies of the O 1s spectrum in (Fig. 5c) showed that the deconvoluted peaks at 531.5 and 529.2 eV were attributed to the O atoms ascribed to C–O and CO bonds, respectively. High-resolution spectra were also taken for the Ag 3d region (Fig. 5d), and the Ag 3d core splits into 3d_5/2 at (368.1 eV) and 3d_3/2 at (372.5 eV) peaks correspondingly, with a spin–orbit splitting of 4.4 eV.²⁵ In the high resolution In 3d spectra, the binding energies of In 3d_5/2 and In 3d_3/2 were 444.5 eV and 452.1 eV, respectively (Fig. 5e). In addition, the S 2p spectra correspond to two peaks of S 2p_3/2 and S 2p_1/2 peaks at 161.3 and 162.2 eV, that are assigned to S coordinated with Ag and In in AgInS₂, respectively (Fig. 5f). The binding energies of Ag 3d, In 3d and S 2p in AgInS₂ are consistent with those reported in the literature.^25,26 The current XPS analysis confirmed the presence of all elements in the composite, demonstrating the fruitful spreading of AgInS₂ on the surface of the biochar.

4. Zeta potential and impedance studies

The zeta potential (Fig. 6a) of AgInS₂@biochar nanocomposite becomes more negative at basic pH due to the deprotonation of surface functional groups and the increased presence of negatively charged species. This results in a higher negative charge density on the nanocomposite surface, thereby increasing the negative zeta potential. The more negative zeta potential enhances the electrostatic repulsion between the nanocomposite particles, improving the stability of the colloidal suspension and influencing the sensitivity of the sensor and photocatalytic efficiency. For electrochemical sensing of glufosinate, this negative charge can facilitate better interaction with positively charged analytes.

Moreover, impedance studies reveal that the AgInS₂-coated biochar exhibits superior charge transport abilities compared to pristine AgInS₂. (Fig. 6b) illustrates sample Nyquist plots corresponding to the EIS data recorded over AgInS₂ and AgInS₂@biochar. The depicted Nyquist plots seem typical of those expected from a Rendell's circuit, wherein the radius of the semicircle noted in the high-frequency region is taken as a measure of the resistance to charge transfer. The Nyquist plot shows a remarkably smaller semicircle radius for the AgInS₂@biochar composite compared to AgInS₂, indicating superior kinetics of heterogeneous electron transfer in the composite. The data were fitted to an electrochemical circuit model to estimate charge transfer resistance (R_ct). For AgInS₂@biochar, R_ct is 45.2 ohms, while for AgInS₂ on a glassy carbon electrode (GCE), it is 87.4 ohms, reflecting higher resistance and slower electron movement.^48,49 The lower R_ct for AgInS₂@biochar suggests that biochar integration significantly enhances electron transfer, thereby improving the composite's photocatalytic and electrocatalytic performance.

5. Photocatalytic study of MG

AgInS₂@biochar was explored for the photocatalytic degradation of malachite green (MG). First, 10 mg of the AgInS₂@biochar nanocomposite was added to a 100 mL solution of 10 ppm of MG. Before irradiation, the catalyst and MG solution were stirred for 1 h to facilitate adsorption on h-AgInS₂@biochar. Aliquots were periodically taken to monitor degradation via UV-vis spectrophotometry. The malachite green has an absorption peak at 614 nm, and the decreasing intensity of absorbance corresponding to MG specific in the depicted UV-visible spectra suggests the photocatalytic degradation of MG (Fig. 7a). The catalyst was found to degrade more than 97.4% of the MG within 70 min (Fig. 7b). The enhanced performance is attributed to the synergistic interaction between AgInS₂ and biochar that boosts the visible-light-harvesting ability of the nanocomposite owing to the surface plasmon resonance phenomenon and the graphitic structure of the carbon that increases the interfacial charge separation, thus decreasing the recombination of electron–hole pairs, which in turn increases the generation of reactive oxygen species (ROS) in the photocatalytic degradation procedure. In (Table 1), we compared the photocatalytic results of AgInS₂@biochar and other photocatalyst materials available in the literature. The results dictate the superior performance of AgInS₂@biochar. Subsequently, it revealed great photocatalytic performance. The kinetics (Fig. 7c) follow a pseudo-first-order model (eqn (2)) with a rate constant of k = 0.0433 min⁻¹.

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5.1 Adsorption study

To endorse the influence of the adsorption of MG onto AgInS₂@biochar on the total removal of MG, we performed the adsorption studies in a batch mode system at room temperature. Concisely, the 10 mg L⁻¹ MG solution was agitated with AgInS₂@biochar in the dark for a 70 min contact time. At the end of the adsorption period, the solution was centrifuged for 10 min at 5000 rpm. After centrifugation, small amounts of the liquid were taken and analysed using UV-vis absorption spectroscopy (Fig. 8a). The results specify a considerable decline in peak intensity. This suggests the dynamic role played by the process of adsorption in the overall mineralisation of MG. We report 24.8% MG removal by adsorption alone, as represented in (Fig. 8b). Hence, the catalyst material bids a sufficient surface for the effective adsorption of pollutants.

5.2 Mechanism of photocatalytic degradation of MG

In the photocatalytic mechanism, the light excitation of the photocatalyst material triggers the photo generation of radical species, eventually resulting in the degradation of the organic contaminants. In the present study, the photo irradiation of the AgInS₂@biochar composite is expected to result in the generation of electron hole pairs and finally the radical species⁵⁹ following eqn (4)–(8). In the photodegradation process of malachite green (MG), the primary mechanism involves the generation of electron–hole pairs through the photoexcitation of valence band electrons in AgInS₂@biochar, which are then promoted to the conduction band. The energy levels of these bands facilitate the oxidation of water by the holes in the valence band, resulting in the production of hydroxyl radicals (˙OH). Concurrently, the photogenerated electrons in the conduction band possess sufficient reducing power to interact with surface oxygen, leading to the formation of superoxide radicals (O2˙⁻). These radicals, O2˙⁻ and ˙OH, collaboratively degrade MG into smaller fragments, eventually converting it into carbon dioxide (CO₂) and water (H₂O). (Fig. 7d) illustrates a plausible sequence of steps involved in the AgInS₂@biochar-mediated photodegradation of malachite green (MG).

AgInS2@biochar + hν → AgInS2@biochar h+(VB) + e−(CB)	(4)

AgInS2@biochar h+(VB) + H2O → AgInS2@biochar + H+ + OH−	(5)

AgInS2@biochar − OH−ad + h(VB)+ → AgInS2@biochar + OH˙ad	(6)

AgInS2@biochar (eCB−) + O2 → AgInS2@biochar O2˙−	(7)

MG + (OH˙, OH˙ad,O2˙−,orhVB+) → intermediates CO2/H2O)	(8)

To further identify the degradation products of MG dye catalyzed by AgInS₂@biochar, we propose a probable degradation mechanism of MG catalyzed by AgInS₂@biochar. As shown in Scheme 2, the oxidation of MG at the tertiary carbon atom produces two main products: (4-(dimethylamino)phenyl) (phenyl)methanone and 4-(dimethylamino) phenol as well as N,N-dimethyl-4-((4-(methylamino)cyclohexa-2,5-dien-1-ylidene) methyl) aniline, via cleavage of the benzene group. Besides, N-demethylation reaction occurred for MG, creating the product of N-methyl-N-(4-((4-(methylamino)phenyl) (phenyl)methylene)cyclohexa-2,5-dien-1-ylidene)methanaminium, which may also undergo oxidation, and the oxidation products further undergo N-demethylation as well. Note that both oxidation and N-demethylation reactions were reported in the literature for MG.^60,61

6. Electrochemical studies

To assess the efficacy of the synthesized materials as electrode components, detailed voltammetric analyses were performed using AgInS₂- and AgInS₂@biochar-modified glassy carbon electrodes (GCE) in an argon-saturated 0.1 M KNO₃ electrolyte solution. (Fig. 9a) illustrates cyclic voltammograms (CVs) recorded at a potential scan rate of 20 mV s⁻¹ at 298.15 K, comparing bare GCE with those modified by AgInS₂ and AgInS₂@biochar composites in 0.1 M KNO₃. The CVs for the bare GCE show no faradaic signals across the potential window in the absence of glufosinate, while significant faradaic signals are evident in the CVs for the modified electrodes. Additionally, (Fig. 9b) presents CVs for the AgInS₂@biochar-modified GCE in 0.1 M KNO₃ with glufosinate recorded at various potential scan rates. An increase in scan rate was observed to increase the glufosinate-specific peak currents, suggesting diffusional control over the mass transfer of the herbicide. Interestingly, the CVs recorded over AgInS₂@biochar in the presence of glufosinate exhibited a single cathodic peak in the forward scan and one anodic peak, with peak positions that precisely match the anodic peaks noted in the CV records for the toxic herbicide. This suggests the potential of AgInS₂@biochar to facilitate sensitive and selective electrochemical sensing of glufosinate, the toxic herbicide. This was further confirmed by our detailed DPV investigations carried out on the AgInS₂@biochar-modified GCE for differently concentrated solutions of glufosinate in 0.1 M KNO₃. We conducted differential pulse voltammetric studies of AgInS₂@biochar in various concentrations of glufosinate (1–10 μM). The oxidation current increases with the concentration of glufosinate in the solution (Fig. 9c). The calibration plot reveals a linear relationship between the current and glufosinate concentration, with a correlation coefficient (R²) of 0.995, indicating high sensitivity towards glufosinate herbicide (Fig. 9d). The limit of detection was determined to be 0.026 μM, demonstrating that AgInS₂@biochar is effective for detecting glufosinate ions in solution. Both cyclic voltammetry and DPV confirm that AgInS₂@biochar nanoparticles are suitable for sensing glufosinate. The detection capability of the AgInS₂@biochar-modified glassy carbon electrode (GCE) for glufosinate is superior to that of other modified GCEs, as shown in Table 2.

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6.1 Possible interactions of hlufosinate with AgInS₂@biochar

The interaction between glufosinate and AgInS₂@biochar likely involves multiple mechanisms. Electrostatic attractions may play a significant role, as the charged groups on glufosinate can interact with the oppositely charged surface of AgInS₂@biochar. Hydrogen bonding is also probable due to the presence of hydrogen bond donors and acceptors in both glufosinate and the biochar matrix. The porous structure of biochar facilitates diffusion, allowing glufosinate molecules to penetrate and interact with internal surfaces. Electrocatalysis could be involved, where the AgInS₂ component of the biochar promotes electron transfer reactions, which enhances the interaction, as evidenced by the increase in the redox peak current, thereby facilitating the sensing of glufosinate. Additionally, complexation may occur, with glufosinate forming stable complexes with metal ions present in AgInS₂@biochar. These combined mechanisms contribute to the effective binding and interaction of glufosinate with AgInS₂@biochar (Scheme 3).

6.2 Density functional theory analysis

DFT calculations were conducted utilising the Lee–Yang–Parr (B3LYP) functional, the LanL2DZ basis set for the AgInS₂ atom, and the 6-311G(d,p) basis set for non-metal atoms to analyse the electronic characteristics of the materials. Utilising the Gaussian 03 arrangement of projects, we used the DFT method to determine their band hole energy values to focus on the effect of AgInS₂ on biochar. The HUMO–LUMO band hole energies were determined to be 2.54 eV and 1.58 eV for AgInS₂ and AgInS₂@biochar, respectively. This significant reduction in the band gap during the formation of the composite designates increased electron mobility, as a lower HOMO–LUMO gap correlates to a reduced activation overpotential for electron transfer between the electrode and analytes. The computed band gap values and orbital distributions are shown in (Fig. 10). The band hole energy values were in close agreement with the exploratory qualities, as displayed in Table 3. These results show that the association of AgInS₂ on the outer layer of biochar improved the optical response of the material. These outcomes indicate that the incorporation of AgInS₂ on the surface of biochar enhanced photo absorption towards the visible region, which in turn enhances the charge separation of photo-excited electrons.

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7. Selectivity and interference study

To assess the selectivity of the developed sensor towards glyphosate, cyclic voltammetry (CV) responses were recorded in the presence of glyphosate and other common interfering pesticides, including mancozeb, carbendazim, and tebuconazole (Fig. 11a). As shown in the CV plot, the sensor displayed a significantly distinct and well-defined redox response for glyphosate compared to the other analytes. The current response and peak shape for glyphosate were markedly different, indicating minimal interference from coexisting species. This suggests a high degree of selectivity of the sensor for glyphosate likely due to favorable binding interactions or specific affinity towards functional groups present in glyphosate. These results validate the applicability of the sensor in real sample analysis, even in complex matrices containing multiple agrochemical residues.

The repeatability and operational stability of the fabricated electrode were evaluated by measuring the current response for seven consecutive days. As shown in (Fig. 11b) in the bar graph, the current density remained nearly constant with only minor fluctuations, and all values were within a narrow error range (±0.002 mA cm⁻²). This consistent performance indicates the excellent repeatability and stability of the electrode under ambient storage conditions. The negligible variation in electrochemical response over time confirms that the sensor material does not degrade or lose activity, demonstrating its reliability for long-term applications in environmental sensing.

8. Conclusion

The novel bio-based hydrothermal synthesis of the AgInS₂@biochar nanocomposite has demonstrated significant potential in both photocatalysis and electrochemical sensing applications. The composite's low energy band gap of 1.89 eV and distinct morphological features of nano sphere-shaped AgInS₂ nanoparticles and rod-like structure of biochar provide a highly exposed surface area conducive to dual performance. The nanocomposite exhibited remarkable efficiency in degrading malachite green (MG), achieving 97.4% degradation in just 70 minutes, and showed excellent electrocatalytic activity in the oxidation of glufosinate herbicide, with a detection limit of 0.026 μM. Moreover, DFT studies were carried out on a simulated model material for the calculation of band gap energy values, and the consequences were in full accord with the experimental data. These results highlight the stability of the composite and efficient electroactive properties driven by the synergy between biochar and transition metals. This study opens new avenues for the development of advanced materials for environmental remediation and pollutant detection, showcasing the AgInS₂@biochar nanocomposite as a versatile and effective solution for addressing various environmental challenges.

Author contributions

Firdous Ahmad Ganaie: data curation, experimentation, investigation, methodology, and original draft. Irfan Nazir: review, editing, and writing. Aaliya Qureashi: experimentation, writing, and review. Zia-ul-Haq: investigation and methodology. Arshid Bashir: review, writing, and editing. Mohsin Ahmad Bhat: supervision, review, and writing. Altaf Hussain Pandith: conceptualization, project administration, supervision, resources, review, and editing.

Conflicts of interest

Authors declare no conflicts of interest.

Data availability

The data supporting the findings of this study are available within the article (Hydrothermal synthesis of AgInS₂@biochar nanocomposites for the photocatalysis and electrochemical sensing of glufosinate herbicides). Additional data are available from the corresponding author, and the data have not been submitted to any public repository.

Acknowledgements

We acknowledge the Department of Science & Technology, government of India, New Delhi, for providing facilities under DST-PURSE Programme (TPN-56945) to the Department of Chemistry, University of Kashmir, and for providing fellowship to Aaliya Qureashi under Women Scientist Scheme-A (WOS-A) [DST/WOS-A/CS-34/2021]. We are also thankful to NIT Srinagar for providing FE-SEM facility, IIT Delhi for providing TEM facility, and IISER Pune faculty for providing XPS facility for this work.

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