Sustainable synthesis of a PtNPs@rGO nanohybrid for detection of toxic fluoride ions using hand-made screen-printed electrodes in aqueous medium

Abstract

High fluoride (F⁻) concentrations in groundwater affect over 200 million people across 25 countries, making accurate detection and quantification of fluoride in water essential for safety assessment. There is a growing demand for advanced water quality testing systems that provide real-time, location-specific data without requiring specialized expertise. This study presents the development of a simple, eco-friendly, and cost-effective nanosensor for electrochemical F⁻ detection in environmental water samples. To our knowledge, this is the first report on the green synthesis of platinum nanoparticles (PtNPs) using Ficus religiosa (sacred fig) leaf extract via a co-precipitation method. Additionally, PtNPs were synthesized ex situ and decorated on reduced graphene oxide (rGO) to form a nanohybrid using ultrasonication. The PtNPs@rGO nanohybrid was then deposited on a disposable screen-printed carbon electrode (SPCE) to fabricate the PtNPs@rGO/SPCE nanosensor using a drop-casting technique. This approach enhances the specificity and sensitivity of the sensor, addressing current analytical challenges. The PtNPs@rGO nanohybrid was characterized by Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis, contact angle (CA) measurement, and electrochemical techniques such as differential pulse voltammetry (DPV) and cyclic voltammetry (CV). The PtNPs@rGO/SPCE nanosensor exhibited a wide linear range from 0.001 to 160 μM for F⁻ concentrations, with a limit of detection of 10 nM and a limit of quantification of 0.036 μM. The sensitivity was 4.126 μA μM⁻¹ cm⁻². The sensor demonstrated excellent reproducibility and strong anti-interference properties. It was successfully applied for F⁻ detection in tap, drain, and tube well water samples, yielding satisfactory recoveries, and its performance surpasses those of previously reported sensors for aqueous F⁻ sensing.

---

1. Introduction

Recently, the determination of electroactive anions has gained paramount importance in research predominantly in aqueous environments. Among them, fluoride (F⁻) has gained considerable attention owing to its significance in biological systems. It is a naturally occurring anion found in groundwater globally, characterized by its small anionic radius, high charge density, and hard Lewis base nature. It plays a crucial role in biological and medical applications, being essential for the development and maintenance of teeth, hair, nails, and bones and for the treatment of osteoporosis. However, excessive exposure to F⁻ can lead to dental and skeletal fluorosis, as well as kidney and acute gastric issues, due to its ability to be readily absorbed and slowly excreted by the body. Major sources of fluoride exposure include drinking water, food, dental products, and pesticides. The World Health Organization (WHO) and the Environmental Protection Agency (USEPA) recommend fluoride concentrations of 1.0–1.5 mg L⁻¹ (ppm) and 2–4 mg L⁻¹ in drinking water, respectively.¹ In the USA, water is artificially fluoridated to levels between 0.7 and 1.2 mg L⁻¹ to meet dietary needs, whereas populations in areas with contaminated groundwater may be exposed to levels as high as 6–12 mg L⁻¹.² Globally, F⁻ poisoning contributes significantly to dental caries, with 2.4 billion people affected by caries in permanent teeth and 486 million children affected by caries in primary teeth. Furthermore, over-intake of F⁻ leads to nerve damage and might potentially result in fatality.³ Therefore, it is crucial to obtain information regarding the F⁻ concentration in the groundwater source prior to its consumption as drinking water. Thus, there arises a need to investigate a meticulous and sensitive method for detecting F⁻ in environmental samples from various locations to ensure effective water quality monitoring.

Many analytical F⁻ detection techniques, such as surface-enhanced Raman scattering,⁴⁸F nuclear magnetic resonance spectroscopy,⁵ gas chromatography-mass spectrometry,⁶ ion chromatography,⁷ colorimetry,⁸ mass spectrometry,⁷ potentiometry, electrochemical methods,⁹ and fluorescence methods,^10–12 have been used to identify F⁻ over the years. However, many of these methods are unsuitable for on-site, real-time water quality monitoring due to their complex procedures and reliance on manual operation. In contrast, point-of-care (POC) sensing technologies have gained significant attention for the management of F⁻ due to their potential to provide rapid, portable, and user-friendly detection systems. These devices are particularly valuable for real-time monitoring in resource-limited settings and areas with endemic fluorosis. Recent developments in POC sensors, including electrochemical platforms and portable optical devices, have demonstrated enhanced sensitivity and specificity for F⁻ detection.¹³ However, challenges such as interference from coexisting ions and the need for cost-effective materials remain key areas of focus in the advancement of these technologies.¹⁴

The screen-printed carbon electrode, i.e., SPCE, is a kind of electrochemical nanosensor that has garnered significant interest in recent decades due to its ability to be customized and adapted to meet the user's specific needs, thereby presenting great potential for commercial use.¹⁵ Nevertheless, an unaltered SPCE exhibits inadequate sensitivity and elevated overpotential, resulting in unavoidable surface fouling over a while. Enhancing the SPCE analytical responses remains a persistent challenge. Modification of the surface electrodes is a crucial aspect for the electrochemical F⁻ detection.¹⁶ Therefore, the development of selective and sensitive electrochemical sensor nanoplatforms can be accomplished by effective surface alterations and fabrication procedures.¹⁵

Sensor molecules and F⁻ interact through various forces such as hydrogen bonding, electrostatic interactions, Lewis acid coordination, and chemical reactions. Among these, molecular detection based on specific chemical reactions tends to offer higher selectivity and stability compared to noncovalent interactions. Recently, numerous nanomaterials have been employed for the removal of aqueous F⁻ and for the design of nanosensors. These materials include gold nanoparticles, CeO₂ NPs, semiconductor quantum dots (QDs), carbon QDs, metal–organic frameworks, micellar nanoparticles, SiO₂ nanoparticles (SiO₂ NPs), and graphene oxide (GO). Their popularity is due to their large surface areas, well-defined pores, improved solubility, ease of fabrication, cost-effectiveness, high sensitivity, and good biocompatibility.^17–19 Significantly, reduced graphene oxide, i.e., rGO, which is a graphene derivative, has got ample attention owing to its unique characteristics.^20,21 It is generally regarded as the incomplete GO reduced product, which is the transitional state of GO and graphene. As a result of the partial reduction, rGO possesses various oxygen-containing functional groups as well as defects.²² These characteristics enable rGO to provide chemically active sites for catalytic reactions and facilitate interactions with metal nanoparticles.²³ Thus, rGO has shown excellent potential for applications in biosensors,²⁴ optical device fabrication,²⁵ plastic electronics,²⁶ photocatalysts,²⁷ and solar cells. Currently, rGO is extensively employed as a substrate for active transition metals due to its vast surface area and exceptional conductive properties.²⁸ Nevertheless, the folding of the rGO sheet remains a formidable obstacle for an electrode that has been modified. The process of decorating metals or non-precious metals onto rGO has been extensively used to enhance the rGO conductivity.²⁹

Amongst the metals, noble metal nanoparticles are utilized as sensory elements for electrochemical biosensors due to their outstanding biocompatibility, remarkable thermal and electrical conductivity, reliable chemical stability, and considerable surface area.³⁰ Platinum nanoparticles (PtNPs) that belong to the class of noble metal nanoparticles possess distinct characteristics, including the macroscopic quantum tunneling effect, quantum size impact, volume effect, and surface effect.³¹ These nanoparticles have been extensively used to fabricate sensors to detect bioactive molecules like hormones, glutamic acid, and glucose, and they have been synthesized employing various approaches, including chemical and physical methods.³² However, the use of harmful ingredients and rigorous synthesis circumstances eventually result in issues related to health and the environment. Hence, synthesizing environmentally benign materials necessitates the implementation of “green chemistry”.³³ Therefore, the use of plant-assisted biosynthesis has been integrated into the PtNP preparation, as the utilization of plant extracts to synthesize metallic nanoparticles has experienced a significant increase in popularity in recent times.³⁴ Various plant extracts have been utilized for the eco-friendly synthesis of PtNPs such as Pinus resinosa,³⁵Diospyros kaki,³⁶Anogeissus latifolia,³⁷Musa paradisiaca,³⁸Ocimum sanctum,³⁹Gardenia jasminoides,⁴⁰Pulicaria glutinosa,⁴¹Cinnamomum camphora,⁴² and Curcuma longa.⁴³ These natural sources provide bioactive compounds that facilitate the green production of PtNPs,³⁷ as highlighted in prior studies. Ficus religiosa, commonly known as the sacred fig or peepal, holds significant importance among herbal flora. Practically every component of this tree, including its fruits, seeds, bark, and leaves, finds application in the formulation of herbal remedies. It contains abundant bioactive compounds such as phenols, saponins, tannins, alkaloids, and flavonoids, among others, which contribute to its therapeutic efficacy in treating various ailments.⁴⁴ Consequently, the leaf extract of Ficus religiosa was utilized as an ideal alternative to precious metals to enhance the electrocatalytic performance of electrochemical sensors through reduction processes.

Considering this fact, we introduce for the first time the synthesis of a green nanohybrid nano-sensing platform consisting of PtNPs loaded onto rGO (PtNPs@rGO) for electrochemical label-free F⁻ detection. The PtNPs-adorned rGO nanohybrid was synthesized using a simple and environmentally friendly method. To date, no study has investigated the green synthesis of PtNPs using Ficus religiosa leaf extract for electrochemical determination of F⁻ on a SPCE. Using the drop-casting technique, the nanohybrid was then deposited to modify the SPCE surface and develop PtNPs@rGO/SPCE. The different developed electrodes were then characterized using electrochemical techniques like differential pulse voltammetry (DPV), cyclic voltammetry (CV), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX) analysis, and high-resolution transmission electron microscopy (HRTEM). It was shown that the sensor displayed better sensitivity with a low detection limit of 10 nM and a good linearity for F⁻ between 0.001 and 160 μM using DPV. The modified PtNPs@rGO/SPCE electrode exhibited good interference capacity against F⁻ with acceptable reproducibility. The validation of the technique is directed at real environmental water samples.

2. Experimental section

2.1. Materials and reagents

Sodium fluoride (≥99%), absolute alcohol (99.8%), and chloroplatinic acid (H₂PtCl₆, 99.8%) were ordered from Sigma Aldrich, India. Potassium ferricyanide (K₃[Fe(CN)₆]) (99%), sodium chloride (NaCl; ≥99.0%), sodium phosphate monobasic anhydrous (NaH₂PO₄; ≥98%), potassium ferrocyanide (K₄[Fe(CN)₆]; ≥99.95%), sodium phosphate dibasic dihydrate (Na₂HPO₄·H₂O; ≥99.0%), and graphite powder were acquired from Fisher Scientific, India. The dried leaves of Ficus religiosa (sacred fig) were collected from the local campus of Jawaharlal Nehru University, New Delhi. The SPCE was prepared in our laboratory using a commercialized graphite ink. The polyvinyl chloride (PVC) substrate for the SPCE was brought from local vendors in New Delhi, India. The silver chloride paint and commercialized graphite ink are procured from the Nanochemazone company. For the whole work, Milli-Q (18.2 MΩ cm⁻¹) was used. Deionized (DI) water was obtained from Milli Q, a Millipore unit (Elix, USA) of 18.2 MΩ resistance. The standard solution of PBS with various pH values was prepared utilizing Na₂HPO₄ as well as NaH₂PO₄ along with the addition of NaCl (0.9%) having 5 mM [Fe(CN₆)]^3−/4− as the redox probe. The solutions of anions were prepared from salts of NaF, KCl, KBr, and Zn(NO₃)₂, respectively. Each of the working solutions was formulated using DI sourced from Millipore. The chemicals used in this investigation were of analytical grade and did not need any further processing or modifications. For the voltammetric assessment, we used an Autolab Galvanostat/Potentiostat (EcoChemie). All electrochemical tests were conducted at ambient temperature.

2.2. Instrumentation

The PtNPs@rGO nanohybrid's crystalline characteristics were analyzed employing a Rigaku Miniflex 600 X-ray diffractometer (XRD) from Japan, operated at a 1.5406 Å wavelength. The analysis was conducted at a step size of 0.02 in the 5° to 90° range and at a 5° per minute scanning rate. Scanning electron microscopy-elemental diffraction X-ray (SEM-EDX) analysis was conducted using the JSM-6700F electron microscope to analyze the PtNPs@rGO nanohybrid and PtNPs surface morphology with an accelerating voltage of 20 kV. In addition, a high-resolution transmission electron microscope (HRTEM) was utilized to verify the shape of the PtNPs in the PtNPs@rGO nanohybrid through the HR-TEM instrument, JEOL JEM-2200 FS, from Japan, functioning at a voltage of 200 kV. The functional group analysis on PtNPs and the nanohybrid was done using a PerkinElmer FTIR instrument from 400 to 4000 cm⁻¹. All the electrochemical analysis were done on an Autolab Galvanostat/Potentiostat (EcoChemie, The Netherlands) electrochemical workstation under ambient conditions. The three-electrode system consisted of a carbon working electrode (area: 0.071 cm²), carbon as a counter, and a reference electrodes coated with a silver chloride paint. The whole electrochemical study was performed on a PtNPs@rGO/SPCE electrode in PBS, comprising a redox species, by utilizing CV as well as DPV. The SPCE was fabricated using a Grafica Flextronica Nano-Print 1015 printing system.

2.3. Synthesis of PtNPs and the PtNPs@rGO nanohybrid

2.3.1. Preparation of Ficus religiosa leaf extract. The PtNPs were synthesized using the dried leaves of Ficus religiosa gathered from the JNU university, New Delhi. Initially, 10 g of dried leaves was weighed, thoroughly cleansed to eliminate any debris or impurities, and allowed to dry in the atmosphere. Subsequently, the leaves were cut down into tiny pieces in order to enhance their overall surface to volume ratio. These fragments were then transferred to a beaker containing 100 mL of DI. The solution was subjected to constant stirring at 400 rpm while being heated at 80 °C for a duration of 1 h. The solution was subjected to cooling, filtration, and subsequent storage in a refrigerator for next experimentation. Every step was executed meticulously within a sterile environment to ensure the accuracy and efficacy of the outcomes.

2.4. Green synthesis of platinum nanoparticles (PtNPs)

Green synthesis, also referred to as eco-friendly or sustainable synthesis, relates to the advancement of chemical procedures and techniques that decrease or eradicate the utilization of dangerous components, diminish waste production, and support environmental sustainability. It has numerous advantages in comparison to conventional synthesis processes.⁴⁵ The green technique was chosen to synthesize PtNPs by utilizing the leaf extract of Ficus religiosa, which served both as the reducing as well as stabilizing agent for the NPs. The tree's anticonvulsant, antibacterial, wound healing, analgesic, anti-diabetic, and anti-inflammatory characteristics have contributed to its popularity in the herbal market.⁴⁶ Its many sections are widely used as essential components in the current pharmaceutical industry.

The PtNPs were synthesized using a 10 mM solution of H₂PtCl₆ as the precursor. Initially, the precursor was introduced into 10 mL of DI and agitated for 15 min at a speed of 300 rpm. Both the H₂PtCl₆ solution and leaf extract were heated to a temperature of 80 °C. Once the temperature reached 80 °C, the leaf extract was gradually added to the H₂PtCl₆ solution while continuously stirring at a speed of 300 rpm. During the reaction, a dark reddish-brown color developed, indicating the bio-reduction process and the synthesis of PtNPs within 1 h. Ultimately, the PtNPs solution was subjected to sonication for a duration of 30 min to disperse the tiny particles in liquids and disintegrate agglomerated NPs.

Furthermore, the reduced NPs were cleaned using several cycles of centrifugation at a speed of 10 000 rpm for 15 min using DI and ethanol to eliminate any contaminants. The PtNPs were believed to undergo reduction and show stability through the involvement of phytochemicals such as flavonoids, quinones, and proteins. Most significantly, the reaction was simple and convenient to handle.

2.5. Graphene oxide (GO) synthesis

The entire protocol of synthesizing GO from graphite was executed employing the modified Hummer's process as reported in our previous work.⁴⁷

2.6. Ex situ synthesis of the PtNPs@rGO nanohybrid

The nanohybrid comprising PtNPs embedded on rGO was synthesized utilizing the ultrasonication technique. To prepare the PtNPs@rGO nanohybrid, two solutions were prepared simultaneously. The initial step involved in the preparation of the aqueous PtNPs solution was combining 10 mg of PtNPs with 10 mL of DI and stirring the mixture for 30 min at a speed of 350 rpm. Furthermore, the GO solution was formed by dissolving 10 mg of GO in 10 mL of DI and subjecting it to ultrasonication for a time span of 3 h for exfoliation of the graphene sheets. Subsequently, the GO solution was gradually introduced into the PtNPs solution, and the resulting mixture was stirred for an additional duration of 15 min. Next, this solution was subjected to ultrasonication for a duration of 24 h for reducing it and to form a PtNPs@rGO nanohybrid. Then, the nanohybrid was rinsed with DI using centrifugation at a speed of 8000 rpm for a duration of 10 min. Finally, the nanohybrid was air-dried in the oven overnight, and the product was crushed and utilized for subsequent characterization.

2.7. Fabrication of the PtNPs@rGO/SPCE nanosensor

2.7.1. Manufacture of SPCE strips. The SPCE for electrochemical measurement was fabricated using a screen-printing process according to our previous work.⁴⁸ The three-electrode strips were fabricated using a screen-printing machine. The printing process utilized two squeegees of varying sizes and polyester mesh. The stencil screen was subjected to moderate pressure, resulting in 500 mm s⁻¹ printing speed. For this process, we applied a commercially available graphite ink (10 mL) onto a PVC sheet via screen printing. The ink was then dried at a temperature of 60 °C for 30 min. This resulted in the development of a working electrode with a diameter of 3 mm, as well as a counter electrode. The configuration consists of three electrodes that are printed on the same strip. Subsequently, the direct application of AgCl paste was done on the PVC-based SPCE to develop a reference electrode, which was then subjected to drying at a temperature of 60 °C for a period of one night. Besides, a layer of insulating ink was applied to the three electrodes to inhibit reverse flow. The SPCE was dried at ambient temperature and then stored in a desiccator.

2.7.2. Sensor fabrication. The proposed technique to manufacture the electrochemical sensor consisted of depositing the synthesized nanohybrid onto the SPCE working area using the drop-casting technique. To achieve this goal, 1 mg of the synthesized nanohybrid (PtNPs@rGO) was combined with 1 mL of DI and evenly dispersed using an ultrasonic chamber for the duration of 3 h. Next, a precise amount of 4 μL of the suspension solution was carefully dropped onto the SPCE working area after determining the optimal volume of stock concentrations. The resulting solution was subsequently dried in air to remove the solvent through evaporation. Scheme 1 depicts the process of synthesizing the PtNPs@rGO nanohybrid using an extract from Ficus religiosa leaves. Additionally, it shows the development of a sensing nanoplatform labeled PtNPs@rGO/SPCE for detecting F⁻.

Scheme 1 The ex situ PtNPs@rGO nanohybrid synthesis using Ficus religiosa leaf extract and PtNPs@rGO/SPCE development employing a drop-casting approach for F⁻ determination.

2.8. Electrochemical measurement

A solution containing 1 mM F⁻ was made and utilized as a primary stock solution that was diluted with PBS (1×, pH 7.0) solution for obtaining F⁻ analyte solutions ranging from 0.001 μM to 160 μM. In the investigation of F⁻ determination, an electrochemical measurement was conducted using 40 μL of 0.2 M PBS (pH 7.0) electrolyte comprising 5 mM redox species, which was poured onto the PtNPs@rGO/SPCE surface. The electrode's performance was evaluated using CV by subjecting it to a 50 mV s⁻¹ scan rate in a potential window of −0.8 to +8.0 V. Furthermore, DPV was performed within the −0.4 V to +0.8 V voltage range at a 50 mV s⁻¹ scan rate. Subsequently, an analyte volume of 5 μL of the F⁻ dilutions was added to the nanosensor surface in a PBS solution of 0.2 M with pH 7.0 to determine F⁻.

2.9. Preparation of the PtNPs@rGO nanohybrid dispersion and diverse concentrations of fluoride ions

A stock solution of the PtNPs@rGO nanohybrid at a concentration of 1 mg mL⁻¹ was made by dispersing it in DI. It was this stock solution that formed the foundation for the electrode modification procedure. For electrode modification, 4 μL of the PtNPs@rGO nanohybrid dispersion was meticulously pipetted onto the working surface of the developed SPCE. In order to delineate the region for alteration and avoid overflow, adhesive tape was affixed to the SPCE before applying the nanohybrid dispersion. Following the application of the dispersion, the electrodes were positioned in an oven adjusted to a suitable temperature to promote the drying process. This procedure guaranteed the formation of a stable and consistent layer of the PtNPs@rGO nanohybrid on the surface of the SPCE for following electrochemical operations.

Preparation of diverse concentrations of F⁻ (i.e., 0.001 μM to 160 μM) was carried out using the phosphate buffered saline (PBS) having pH 7.0 and 0.2 M from a stock solution of 1 mM concentration. From this primary stock solution, further F⁻ concentrations were prepared through serial dilutions.

3. Result and discussion

3.1. XRD

The crystalline structures of the synthesized PtNPs and PtNPs@rGO nanohybrid were investigated using XRD. The powered XRD pattern [Fig. 1A(a)] of PtNPs shows four diffraction peaks at 2θ = 80.8°, 67.1°, 47.0°, and 39.1°, which are ascribed to (311), (220), (200), and (111), planes, respectively, of the FCC Pt crystal as referred from JCPDS file no. 03-065-2868.⁴⁹ Furthermore, the green synthesized PtNPs showed maximum orientation towards the (111) plane. The sharpening of the diffraction peaks confirms the crystalline nature of Pt. However, an additional peak was seen in the XRD pattern of PtNPs at 2θ = 28.9°, which might be due to the presence of plant extract.¹⁹ In contrast, it is absent in the case of the XRD curve of the nanohybrid [Fig. 1A(b)], which exhibited a broad peak at 25.06° attributed to the diffraction plane (002) resulting from the amorphous rGO present in the nanohybrid along with the appearance of other peaks characteristic of PtNPs as shown in [Fig. 1A(b)]. Also, a peak that appeared at 2θ = 10.4° indicates the partial reduction of GO. Furthermore, the reduction in peak intensity, along with the slight shifting of peaks in the case of the nanohybrid, symbolizes the incorporation of PtNPs into the rGO nanosheet. Moreover, as illustrated by eqn (1), the Scherrer equation was utilized for estimating the PtNPs crystallite size, which is calculated to be 1.75 nm along the most intense plane (111) in the nanohybrid.

d = kλ/β cos θ_max (1)

Here, β is the FWHM of the highest intensified peak (in radians), θ symbolizes the diffraction angle of the nanohybrid, λ is the copper Kα X-ray radiation wavelength, i.e., 0.154 nm, k is the constant of the crystallite shape (0.9), and d is the crystallite size of the intense peak (nm).

Fig. 1 (A) XRD patterns and (B) FTIR spectra of (a) PtNPs and (b) the PtNPs@rGO nanohybrid.

3.1.1. Growth mechanism of the PtNPs@rGO nanohybrid. The green synthesis of PtNPs@rGO using Ficus religiosa leaf extract is an eco-friendly approach that utilizes natural phytochemicals as reducing and stabilizing agents. Graphene oxide (GO) is initially reduced to reduced graphene oxide (rGO), providing a conductive surface with active sites for nanoparticle nucleation. Platinum ions from a precursor like H₂PtCl₆ are reduced to metallic platinum (Pt⁰) by bioactive compounds present in the Ficus religiosa extract, such as phenolics, flavonoids, and terpenoids.⁵⁰ These compounds not only facilitate the reduction process but also act as capping agents, stabilizing the Pt nanoparticles and preventing their aggregation. The rGO matrix anchors the PtNPs, ensuring uniform dispersion through electrostatic and π–π interactions.⁵¹ This synthesis route avoids the use of toxic chemicals and enhances the catalytic performance of the PtNPs@rGO nanohybrid making it suitable for biosensor applications.

3.2. FTIR

To further confirm the functional groups present, the FTIR spectroscopy of the PtNPs and PtNPs@rGO nanohybrid was performed as depicted in Fig. 1B(a) and (b), respectively. In the FTIR spectrum of PtNPs shown in Fig. 1B(a), the probable biomolecules accountable for reducing and stabilizing Pt ions are confirmed by the appearance of prominent peaks showing the vibrating modes particular to the presence of the functional groups, i.e., at 1013 cm⁻¹, 1422 cm⁻¹, 1561 cm⁻¹, 3275 cm⁻¹ ascribed to C–O, C–C (from aromatic rings), NH(CO) (that develops inside the cage of cyclic peptides), and O–H stretching, respectively.⁵² Furthermore, the FTIR patterns of the nanohybrid show the successful reduction of GO after ultra-sonication and its conversion to rGO. The intensity of peaks of the nanohybrid is reduced and shifted to a higher wavenumber at 1589 cm⁻¹ when PtNPs are incorporated into rGO sheets, Fig. 1B(b), thus indicating the presence of functional groups.⁵³ Therefore, the FTIR analysis of the PtNPs@rGO nanohybrid indicates the incorporation of the PtNPs onto rGO sheets showing successful preparation and capping of biomolecules on PtNPs present in the leaf extract of Ficus religiousa due to the green synthesis.

3.3. Contact angle

The investigation of contact angle (CA) was performed by the sessile drop method for checking the hydrophilic as well as the hydrophobic behavior of the SPCE, PtNPs/SPCE and PtNPs@rGO/SPCE (Fig. S1, ESI†). The CA of SPCE is presented in Fig. S1(a) (ESI†), showing a value of 45.6°. This shows that the in-home SPCE is hydrophilic in nature. While in the case of PtNPs/SPCE (Fig. S1(b), ESI†) and PtNPs@rGO/SPCE (Fig. S1(c), ESI†), CAs were estimated to be 64.1° and 122.1° indicating hydrophobic behavior. The hydrophobicity of PtNPs@rGO/SPCE is needed for the SPCE to prevent the contact of the nanohybrid during the drop-casting procedure.

3.4. SEM-EDX

The morphological structures of PtNPs and the PtNPs@rGO nanohybrid were determined using SEM-EDX analysis. For this, a thin layer of the respective material was deposited on the SPCE surface [Fig. 2(a)–(e)]. Fig. 2(a) and (b) displays the agglomeration of spherical-shaped PtNPs synthesized using the leaf extract of Ficus religiosa at low (a) and high scales (b). The PtNps exhibit significant aggregation, potentially resulting from the formation of reactive and mobile nanoparticles during the synthesis process, leading to collisions and the induction of van der Waals forces between the nanoparticles. Furthermore, in Fig. 2, low (c) and high scale (d) small spherical PtNPs could be seen decorated on the thin surface of rGO by weak van der Waals forces of attraction or electrostatic interaction,¹⁹ leading to the PtNPs@rGO nanohybrid formation using the green method. Furthermore, EDX analysis was executed to estimate the elemental composition of individual elements present in the nanohybrid (Fig. 2(e)), i.e., Pt (22.97), O (30.61), and C (46.42), where the values in the brackets display the %wt of the elements. Fig. 2(f) shows the nanohybrid's elemental mapping spectrum, thus confirming elements that exist in the nanohybrid with different colors.

Fig. 2 SEM images of PtNPs: (a) low resolution and (b) high resolution; the PtNPs@rGO nanohybrid: (c) low resolution and (d) high resolution; and (e) EDX and (f) elemental mapping of the PtNPs@rGO nanohybrid.

3.5. HR-TEM

High resolution transmission electron microscopy (HR-TEM) was employed to study the microstructures of the PtNPs@rGO nanohybrid, as depicted in Fig. 3(a)–(d). The image in Fig. 3(a) displays the PtNPs@rGO nanohybrid, revealing two distinct stages. The rGO sheets are represented by a bright and broad phase, while the metallic PtNPs are observed as black dots, decorating the rGO sheets. The image verified the effective distribution and adornment of PtNPs on the rGO nano-surface (Fig. 3(b)). In addition, the presence of PtNPs clusters exhibiting aggregation and random distribution on the rGO surface has also been observed. The HR-TEM analysis was conducted to determine the d-spacing value of PtNPs, as shown in Fig. 3(c). The calculated value was found to be 0.22 nm, indicating that it corresponds to the (111) plane of PtNPs embedded in the rGO matrix. The selected area electron diffraction (SAED) pattern (Fig. 3(d)) of the PtNPs@rGO nanohybrid demonstrates multiple diffractions, indicating the high crystallinity of the PtNPs present in the nanohybrid, demonstrating the FCC structure of PtNPs. The particle size distribution of the synthesized PtNPs was analyzed from the TEM images. As shown in Fig. S2 (ESI†), the particles exhibit a narrow size distribution with an average diameter of 1.18 ± 0.29 nm. The size distribution confirms the uniformity of the nanoparticles, further validating the synthesis process.

Fig. 3 HRTEM images of the PtNPs@rGO nanohybrid at (a) low resolution and (b) high resolution, (c) HRTEM, and (d) SAED pattern.

4. Electrochemical analysis

4.1. CV responses of the PtNPs@rGO nanohybrid in [Fe(CN)₆]^3−/4− solution

To evaluate the effectiveness of the novel nanohybrid material for electroanalytical applications, all materials were characterized using electrochemical redox probes, specifically a ferro/ferricyanide mixture. The SPCEs were nano-engineered through a drop casting method, and electrochemical experiments were conducted by comparing the bare SPCE made with a carbon-based conductive ink and the modified electrodes, namely, PtNPs/SPCE and PtNPs@rGO/SPCE. CV experiments were performed with a 5 mM [Fe(CN)₆]^3−/4− solution in 0.2 M PBS as the supporting electrolyte, as shown in Fig. 4(c). The CV results showed two similar peaks in shape and intensity for each modified electrode, indicating the electrochemical reversibility of the redox probes used. The electrodes were compared by measuring the peak heights and the potential difference (ΔE) between the cathodic (I_pc) and anodic (I_pa) peaks. The performance of PtNPs/SPCE (I_pa = 52.64 μA; ΔE = 0.53 V) and PtNPs@rGO/SPCE (I_pa = 67.04 μA; ΔE = 0.37 V) was found to be better than the bare SPCE (I_pa = 45.39 μA; ΔE = 0.61 V). The higher currents and lower potential difference (ΔE) relative to the bare SPCE confirmed the enhancement of electrochemical properties due to the presence of PtNPs and rGO modifiers.

Fig. 4 (a) pH study, (b) volume optimization of PtNPs@rGO/SPCE stock solution (in μL), and (c) CV and (d) DPV spectra of (i) SPCE, (ii) PtNPs/SPCE, and (iii) PtNPs@rGO/SPCE.

Notably, the PtNPs@rGO nanohybrid exhibited the highest current peaks, demonstrating a synergistic effect between PtNPs and rGO. This is likely due to the incorporation of Pt atoms into the rGO layer, which increased interstitial space and promoted the rearrangement of the rGO nanohybrid, as evidenced by XRD and SEM images. PtNPs@rGO/SPCE showed the highest peak currents and the lowest potential difference (I_pa = 67.04 μA; ΔE = 0.377 V), indicating improved electrochemical processes at this modified interface, making it highly suitable for electrochemical sensing. Among the tested electrodes, PtNPs@rGO/SPCE exhibited excellent conductivity and higher current values, suggesting superior electrochemical active sites and better response in the presence of target species.

4.2. pH effect

The pH study aimed to analyze the electrochemical behavior of the PtNPs@rGO/SPCE nanosensor in PBS using the DPV approach and a redox probe, [Fe(CN)₆]^3−/4−. The −0.4 to +0.8 V potential window and the scan rate of 50 mV s⁻¹ were set. The investigation involved preparing PBS with varying pH levels, specifically from 6.0 to 8.0, i.e., 6.0, 6.6, 7.0, 7.4, and 8.0. Fig. 4(a) illustrates the correlation between peak currents and the pH of the solution. An observation was made that as the pH rises, the peak current increases until the pH reaches 7.0 (53.55 μA) but then reduces when the pH is further increased to 8.0.⁵⁴ Thus, the pH 7.0 of the PBS solution was chosen as the optimal pH for investigating the electrochemical reaction of the PtNPs@rGO/SPCE electrode, as it exhibited the highest observed current at this pH.

4.3. Volume optimization of PtNPs@rGO nanohybrid stock solution

The optimization of stock solution (1 mg mL⁻¹) of PtNPs@rGO samples was performed by drop casting volumes ranging from 1 μL to 5 μL onto the SPCE working area. The DPV peak current for each SPCE is depicted in Fig. 4(b). The highest DPV peak current was attained at 4 μL volume of stock solution using 0.2 M PBS, pH = 7.0, comprising a redox coupler. But further increasing the coating volume to 5 μL resulted in the formation of broader barrier layers, which hindered the flow of electrons between the electrode surface and electrolyte. On the other hand, smaller amounts of coating volumes (1 to 3 μL) resulted in fewer layers with less binding, but a faster binding rate was found.⁵⁵ However, the most optimal conditions are attained with a quicker electron flow at 4 μL volume. Therefore, a solution of 4 μL of 1 mg mL⁻¹ PtNPs@rGO nanohybrid stock was used to modify the SPCE for subsequent electrochemical evaluations.

4.4. Electrode studies

The electrochemical performance of several electrodes was examined employing CV as well as DPV, as shown in Fig. 4(c) and (d). The CV profiles of different electrodes, namely SPCE, PtNPs/SPCE, and PtNPs@rGO/SPCE, are displayed in curves from (i) to (iii), respectively, as depicted in Fig. 4(c). These measurements were conducted in PBS (pH 7.0), and the potential range was set from −0.8 to +0.8 V. The figure clearly demonstrates that the response of the SPCE electrode [curve (i)] is the lowest, measuring 45.39 μA, due to its limited effective surface area. Consequently, the electron transfer rate is sluggish in the SPCE. However, when using the PtNPs/SPCE electrode, the anodic current is enhanced by 52.64 μA, compared to the SPCE electrode, as demonstrated in the curve (ii). This phenomenon may be attributed to an enhanced electron transfer occurring at the interface between PtNPs and the SPCE, which is facilitated by the larger surface area of PtNPs. The anodic peak current is the highest for the PtNPs@rGO/SPCE nanosensor [67.04 μA, curve (iii)] compared to the SPCE and PtNPs/SPCE electrodes. This is because the PtNPs are incorporated onto the rGO surface, which increases the conductivity as well as surface volume ratio of the nanohybrid.⁵⁵ As a result, the electrochemical characteristics of the nanohybrid are improved. The nanohybrid's high conductance is attributed to the synergistic action of the nanomaterials, namely PtNPs and rGO.¹⁹ In a similar manner, the DPV graphs exhibit a comparable pattern of increasing current values for the SPCE (curve (i): 5.44 μA), PtNPs/SPCE (curve (ii): 5.77 μA), and PtNPs@rGO/SPCE (curve (iii): 12.11 μA), as observed in Fig. 4(d) with that of CV plots.

The anodic and cathodic peak potentials (E_pa and E_pc), oxidation and reduction peak current densities (I_pa and I_pc), and the potential difference (ΔE_p) are summarized in Table 1. PtNPs@rGO/SPCE exhibited lower ΔE_p and higher redox peak currents compared to the other modified and unmodified electrodes, likely due to the favorable electrostatic interactions between the PtNPs@rGO surface and the [Fe(CN)₆]^3−/4− ions. We evaluated each of the electrodes by quantifying the heights of the peaks (Fig. S3, ESI†) and their potential change between the anodic peaks. Observations indicated that every electrode arrangement had responses in our CV examination. The untreated SPCE (bare SPCE) had a maximum current (I_pa) of 45.39 μA and a maximum potential difference (ΔE) of 0.61 V. The aforementioned values function as a reference point for comparing with the altered electrodes. The addition of PtNPs to the thin-layer bare SPCE electrodes led to a rise in the maximum current to 52.64 μA, suggesting enhanced electrochemical performance. Moreover, there was a reduction in the margin between peak potentials to 0.53 V, indicating a displacement in the redox potentials. These findings suggest that the inclusion of PtNPs influences the behaviour by facilitating the transfer of charge and modifying the electrical properties of the electrode. A significant enhancement was observed in the PtNPs@rGO nanohybrid/SPCE, characterised by a maximum current of 67.04 μA and a wider range of peak potentials, reaching 0.37 V. The combination of PtNPs with rGO leads to enhanced electrochemical performance characterized by increased peak current and charge transfer, facilitated by the presence of a larger surface area with enhanced electron mobility. Therefore, based on the analysis of peak electrical currents and the difference in peak potentials, we have selected the PtNPs@rGO nanohybrid as the material for investigating F⁻.

Table 1 CV data of the different nanomodified electrodes in 0.2 M PBS containing [Fe(CN)₆]^3−/4− species

S. no.	Modified electrodes	E pa (V)	E pc (V)	I pa (μA cm^−2)	I pc (μA cm^−2)	ΔEp (V)
1	Bare SPCE	0.347	−0.270	45.39	−43.670	0.61
2	PtNPs/SPCE	0.415	−0.121	52.64	−47.637	0.53
3	PtNPs@rGO/SPCE	0.327	−0.050	67.04	−64.147	0.37

---

4.5. Scan rate studies

The PtNPs@rGO/SPCE nanosensor response was measured by varying the scan rate from 10 to 100 mV s⁻¹ in the −0.8 to +0.8 V voltage range, as depicted in Fig. 5(a). The CV approach was employed to investigate the redox characteristics as well as interfacial kinetics. The electrode exhibited a positive as well as a negative drift for oxidation and reduction peaks, respectively, as the scan rate increased. This means the cathodic (I_pc) and anodic (I_pa) peak currents both displayed a linear correlation with the square root of the scan rate, as depicted in Fig. 5(b).⁵⁶ In addition, when the scan rate rose, the distance between the peak-to-peak separation (ΔE_p) also increased and switched along the higher potential side, as depicted in Fig. 5(c). This indicates that the process is diffusion-controlled, i.e., quasi-reversible in nature.⁵⁷ The linear fitting plots displays the following equations (eqn (2)–(4)).

I_{pa(PtNPs@rGO/SPCE)} = [21.83 μA (s mV⁻¹) × (scan rate [mV s⁻¹]^1/2)] + 41.29 μA, R² = 0.988 (2)

I_{pc(PtNPs@rGO/SPCE)} = −[20.34 μA (s mV⁻¹) × (scan rate [mV s⁻¹]^1/2)] + 34.50 μA, R² = 0.995 (3)

ΔE_{p(PtNPs@rGO/SPCE)} = [0.072 V (s mV⁻¹) × (scan rate [mV s⁻¹]^1/2)] − 0.007 V, R² = 0.994 (4)

Fig. 5 (a) Scan rate study of the PtNPs@rGO/SPCE nanosensor. (b) and (c) The linear plots of the PtNPs@rGO/SPCE nanoplatform illustrating the correlation of anodic and cathodic peak currents and ΔE_p with the square root of the scan rate, respectively.

The value of the diffusion coefficient “D” is 2.58 × 10⁻¹¹ cm² s⁻¹ estimated using the Randles–Sevcik formula as given by eqn (5).

I_p = (2.69 × 10⁵)Cn^3/2D^1/2v^1/2A, (5)

where I_p is the electrode's I_pa (108.3 μA), n represents the number of transferred electrons, A is the electrode surface area (0.071 cm²), D signifies the diffusion coefficient, C denotes the redox coupler concentration, which is 5 mM, and v represents the scan rate, i.e., 50 mV s⁻¹.

Moreover, the Brown–Anson equation⁵⁸ was employed for estimating the average surface concentration of electroactive absorbed ions (I*), as represented by eqn (6)

I* = 4RTI_pa/n²F²Aν (6)

Here, F stands for the Faraday constant having a value of 96 485 C mol⁻¹, R is the gas constant, i.e., 8.314 mol⁻¹ K⁻¹, and T denotes the room temperature, i.e., 300 K. The estimated I* value for the PtNPs@rGO/SPCE nanosensor was 3.26 × 10⁻⁸ mol cm⁻².

Also, the two significant factors responsible for electron transfer reversible kinetics are K_s, i.e., the heterogeneous electron transfer rate constant, and the scan rate constant. Hence, the K_s value estimated using the Laviron equation,⁵⁹ shown in eqn (7), is 0.94 s⁻¹.

K_s = mnFv/RT (7)

Here, the difference in peak potential (V) is given by m. Thus, the K_s of the PtNPs@rGO/SPCE nanosensor illustrates the fast transfer of electrons between the redox species of the electrolyte and the electrode's surface. The different kinetic interface factors for the PtNPs@rGO/SPCE nanosensor are depicted in Table 2.

Table 2 The different kinetic interface parameters for the nanosensor

Electrode	A (cm^2)	K s (s^−1)	I* (mol cm^−2)	D (cm^2 s^−1)
PtNPs@rGO/SPCE	0.071	0.94	3.26 × 10^−8	2.58 × 10^−11

---

4.6. Electrochemical response studies

The electrochemical sensing performance of the PtNPs@rGO/SPCE nanoplatform was examined by varying the amounts of F⁻ ions, which range from 0.001 μM to 160 μM, i.e., 0.001, 0.01, 0.1, 0.5, 1.0, 10 20, 40, 80 and 160 μM. The DPV approach was employed for this study in a PBS solution with a pH of 7.0. The sensing procedure was conducted using a redox couple, with a 50 mV s⁻¹ scan rate, throughout the voltage window of −0.4 to +0.8 V. Fig. 6(a) demonstrates a progressive drop in DPV peak current as the F⁻ concentrations rise. Fig. 6(b) displays a magnified view of the electrochemical response of F⁻ concentrations. The gradual reduction in maximum current can be ascribed to the occurrence of F⁻ aggregation on the electrode's surface. The accumulation of F⁻ ions hinders the activity of the electrode's surface, leading to a reduction in current.⁶⁰

Fig. 6 (a) Electrochemical sensing plot depicting the DPV response study of the PtNPs@rGO/SPCE nanosensor against various concentrations of F⁻, (b) zoomed version of the sensing graph of PtNPs@rGO/SPCE, and (c) calibration curve of the PtNPs@rGO/SPCE nanoplatform versus various fluoride concentrations.

Furthermore, the PtNPs@rGO nanohybrid surface provides a greater surface-to-volume ratio and facilitates the quick movement of electrons due to the combined effect, resulting in improved electron transfer across the interfacial area. The existence of tiny PtNPs enhances the connection between the graphene layers of rGO, creating a continuous pathway for charge transfer and facilitating the diffusion of electrolyte ions into the planar film surface. The nanohybrid planar designs increase the electrochemical zones of activity for desorbing or absorbing electrolyte ions and create additional interfaces at the nanohybrid interlayer regions to facilitate charge movement during the electrochemical analysis.¹⁸Fig. 6(c) displays a linear relationship between the PtNPs@rGO/SPCE peak currents and the concentrations of F⁻. This correlation may be described by the linear regression equation (eqn (8)). An analysis revealed that the PtNPs@rGO nanohybrid outperformed both the SPCE and SPCE modified with PtNPs and rGO with respect to sensitivity and accuracy. The regression equations, slope, correlation coefficient (R²), and standard error for this sensor are given in eqn (8).

I_pa = [−0.293 (μA μM) × F⁻ (μM⁻¹) + 0.009 (μA)], R² = 0.991 (8)

The linear plot reveals a linearity range from 0.001 to 160 μM, having a regression coefficient (R²) of 0.991. Additionally, it has a low detection limit (LOD) of 10 nM, a limit of quantification (LOQ) of 0.036 μM as well as a sensitivity of 4.126 μA μM cm⁻². The given LOD and LOQ were determined utilizing the standard equations (eqn (9) and (10)).⁶¹

Detection limit = 3σ/sensitivity (9)

LOQ = 10σ/sensitivity (10)

where σ is the intercept's standard error of the linear plot for the PtNPs@rGO/SPCE nanosensor having a value of 0.015. Moreover, sensitivity was estimated using the slope of the calibration curve/area of the nanosensor as illustrated in eqn (11).⁶²

Sensitivity = slope of the calibration curve/working area of the nanosensor (0.071 cm²) (11)

Analysis of the regression equations and R² values indicates that the PtNPs@rGO nanohybrid has potential as a material for electrochemical applications in sensing. This technology provides enhanced sensitivity and precision, making it a notable breakthrough in the area of electroanalytical chemistry.

Moreover, Table 3 shows the comparison of earlier reported electrochemical F⁻ sensors' parameters, depicting their sensitivity, LOD, techniques, and linear range, with those reported in the current study. The sensor reported in the current work shows high sensitivity and a lower detection limit in comparison to those reported in other documented research studies. Furthermore, it displayed a wide linear range owing to the synergistic effect of PtNPs and rGO of the nanohybrid.

Table 3 Comparative performances of the PtNPs@rGO/SPCE nanoplatform and earlier reported electrochemical F⁻ sensors

Electrode	Techniques	Linear range (M)	Sensitivity	LOD (M)	Ref.
PAPBA/GR	Potentiometric	5 × 10^−4 to 0.05	—	—	63
MPBA/Au	SWV	1 × 10^−8 to 0.01	—	—	64
SPOSi/SWCNTs/GCE	CV	0.5 × 10^−6 to 0.001 × 10^−2	—	0.083 × 10^−6	65
[FeF6]3^−/BDDE	SWV	5 × 10^−6–2.5 × 10^−5	—	0.6 × 10^−6	66
FITC-OSI-p-MWCNT-GCE	CV	0 × 10^−6 to 100 × 10^−6	—	0.26 × 10^−6	67
PtNPs@rGO/SPCE	DPV	1 × 10^−9 to 160 × 10^−6	4.126 μA μM cm^−2	0.010 × 10^−6	This work

---

4.6.1. Possible mechanisms of detection of fluoride ions. The electrode modified with the PtNPs@rGO nanohybrid demonstrated a significantly higher redox activity compared to the bare SPCE and electrode modified with PtNPs alone. As illustrated in Fig. 5(c), the peak current in PtNPs@rGO/SPCE was more than that of the electrodes with only PtNPs or SPCE. In this nanohybrid, rGO provides a conductive framework for electron transfer, while PtNPs offer electrocatalytic activity for F⁻ oxidation. The combination of rGO and PtNPs creates a synergistic effect that enhances the electrocatalytic performance of PtNPs in the oxidation of F⁻, resulting in a stable, sensitive, and cost-effective sensing tool. This property was leveraged to develop the proposed sensor platform. The probable mechanism is based on the interaction of F⁻ with the nanohybrid, which is capable of hydrogen bonding (through carboxyl or hydroxyl groups present on the surface of the PtNPs@rGO/SPCE sensing electrode, as they form strong hydrogen bonds).⁶⁸ This is because fluoride is highly electronegative and tends to form very strong hydrogen bonds compared to other halides. Furthermore, excessive fluoride ions can form strong hydrogen bonds or other interactions with molecules in the electrolytic solution, which can alter the local environment around the electrode. These interactions can impede the efficient transfer of electrons between the fluoride ions and the electrode surface, resulting in a decreased current response. As a result of the reduced adsorption and hindered electron transfer, the overall current generated in the electrochemical detection process decreases, leading to decrement in the overall electrochemical activity.⁶⁹ Moreover, at high fluoride concentrations, there is a possibility of forming stable fluoride complexes with the PtNPs.⁷⁰ These complexes can reduce the number of free active sites available for the electrochemical reaction, thus further lowering the current. Thus, formation of such complexes can also alter the catalytic properties of the PtNPs, making them less effective in facilitating the redox reaction. The probable interactions taking place during sensing are shown in Scheme 2.

Scheme 2 The possible mechanism of PtNPs@rGO/SPCE for electrochemical detection of F⁻. Created with https://BioRender.com.

4.7. Control, interference, and reproducibility analyses

The control analysis investigated the interaction behavior between PtNPs/SPCE and various concentrations of F⁻ ranging from 0.001 μM to 160 μM, utilizing the DPV approach. This is shown in Fig. 7(a). The results indicate that adding F⁻ did not significantly change the current. This suggests that PtNPs/SPCE did not effectively bind with F⁻ alone for an extensive sensing range with a relative standard deviation (%RSD) of 3.71%. Furthermore, the reproducible behavior of six distinct PtNPs@rGO/SPCE electrodes manufactured under similar conditions was assessed using the same technique illustrated in Fig. 7(b). The sensor electrode exhibited satisfactory, consistent performance, as evidenced by an %RSD value of 8.83%.

Fig. 7 (a) Control analysis with the PtNPs/SPCE electrode and (b) reproducibility and (c) interference analysis of the PtNPs@rGO/SPCE nanosensor in PBS containing a redox couple.

Next, in order to assess the PtNPs@rGO/SPCE nanosensor's selectivity towards F⁻, the impact of different interfering substances on the electrode's potential was investigated utilizing the DPV method in a PBS solution comprising redox species. In this study, the quantity of ethanol was increased by a factor of 30, the content of fructose was raised by a factor of 200, and the proportion of numerous inorganic ions, including Zn²⁺, C1⁻, K⁺, Cu²⁺, NO₃⁻, Na⁺, Fe²⁺, SO₄²⁻, and Ca²⁺, was enhanced by a factor of 100, all with reference to the concentration of F⁻.⁷¹ The ions selected for testing the selectivity of the present sensor were chosen based on their potential to interfere with the target ion under similar environmental or physiological conditions. Specifically, we have considered the chemical similarity, co-existence in real samples and interference potential criteria to test the selectivity of the sensor. No discernible alteration in the DPV peak current was seen following the introduction of each interferent (5 μL). Nevertheless, the introduction of a particular quantity of F⁻ (80 μM) results in a reduction of peak current, indicating that the PtNPs@rGO/SPCE nanosensor exhibits exceptional selectivity for F⁻. The %RSD for different interfering agents was determined to be in close conformity, which ranges from 1.61% to 6.03%. This is illustrated in Fig. 7(c). These findings indicate that the nanohybrid exhibits a high degree of selectivity and sensitivity in detecting F⁻. The reliability of the sensor and technique makes the approach rather appropriate for applications that need selective detection of F⁻.

4.8. Analytical applications of the PtNPs@rGO/SPCE nanosensor

In this work, we conducted a thorough validation of our analytical techniques to evaluate sensitivity, reliability, precision, and bias (Table 4). To assess the actual efficacy of the PtNPs@rGO/SPE sensor, the presence of fluoride ions in real samples was measured using the DPV technique. For this experiment, three real samples were collected, including drain water, tube well water, and tap water. Also, the drain water and tube well water were gathered from the JNU campus, New Delhi, whereas the tap water was obtained from our laboratory. To conduct the investigation, 5 μL of each real sample was introduced into the sensor that contained PBS. For all the samples, it was seen that no F⁻ ions were identified in any of the real samples, as well as the peak current was determined to be comparable to that of the nanosensor.

Table 4 Electrochemical detection using the PtNPs@rGO/SPCE electrode in the spiked real sample demonstrating peak currents during standard electrochemical response as well as after spiking with different environmental samples for F ⁻ detection

Real samples	Spiked concentrations	DPV current in the standard sample (μA)	DPV current in spiked real samples (μA)	(%) RSD	(%) Recovery
Drain water	0	10.00	11.28	8.51	112.8
	0.001	9.55	9.05	3.8	94.76
	1	8.60	8.06	4.58	93.72
	160	8.00	7.88	1.07	98.50
Tube well water	0	10.00	9.09	6.74	90.9
	0.001	9.55	8.10	11.62	84.81
	1	8.60	7.47	9.94	86.86
	160	8.00	7.23	7.15	90.37
Tap water	0	10.00	11.83	11.86	118.3
	0.001	9.55	10.58	7.24	110.78
	1	8.60	9.51	7.11	110.58
	160	8.00	9.13	9.33	114.12

---

Furthermore, the samples were analyzed through a spike/recovery test. To determine the percentage of recovery, we examined the standard measured current values with spiked concentrations of fluoride ion samples. Table 4 provides the % recovery for spiked samples with specific fluoride concentrations, i.e., 0.001 μM, 1 μM, and 160 μM, along with their corresponding %RSD values. It shows that the %recoveries ranged from 84.81% to 118.30%, as well as the average %RSD values ranged from 1.07% to 11.86%. The PtNPs@rGO/SPCE nanosensor that has been constructed can effectively detect and measure the concentration of F⁻ in water samples. Thus, using PtNPs@rGO/SPCE as an electrochemical sensing platform offers an efficient and highly sensitive approach for accurately measuring F⁻ in real samples.

5. Conclusion

In summary, an eco-friendly and greener PtNPs@rGO/SPCE nanosensing platform has been successfully developed for sensitive, selective, rapid, simple, and cost-effective F⁻ detection in real water samples. Additionally, this PtNPs@rGO/SPCE nanohybrid is the first sensing platform designed using an environmentally benign sustainable approach to detect F⁻ ions. Also, the PtNPs@rGO nanohybrid was made ex situ via a co-precipitation technique, where spherical PtNPs were synthesized from Ficus religiosa leaf extract for the first time followed by ultrasonicating GO solution to form the PtNPs@rGO nanohybrid. The drop casting procedure was utilized to fabricate the nanoplatform by depositing PtNPs@rGO on the SPCE. The PtNPs@rGO/SPCE nanosensor exhibits good conductivity, excellent electrochemical performance, and a high specific surface area due to the synergistic properties of PtNPs and rGO. As described, the PtNPs@rGO/SPCE sensor demonstrates a broad linear range spanning from 0.001 to 160 μM. It has a detection limit of 10 nM, LOQ of 0.036 μM, and sensitivity of 4.126 μA μM cm⁻². The nanosensor's detection limit was 10 nM, which falls within the 0.001–160 μM range, much lower than the acceptable levels of drinking water proposed by the WHO and EPA. The %RSD values for interferents and reproducibility demonstrated satisfactory outcomes, indicating high selectivity towards F⁻. A further assessment of the PtNPs@rGO/SPCE nanohybrid's suitability has been successfully applied to analyze actual environmental samples including drain water, tube well water, and tap water directly, yielding satisfactory % recovery rates. The findings from the examination of these actual samples illustrate the pragmatic suitability of the sensor. In contrast to previous detection techniques, the innovative electrochemical detection assay provides an affordable and user-friendly platform for the detection of F⁻. The proposed method is expected to replace intricate and labour-intensive investigations, demonstrating considerable promise for future progress. This study further demonstrated that PtNPs@rGO/SPCE may be employed as a single-use SPCE technology for the direct detection of F⁻ in water, offering a novel approach for the measurement and quantification of fluoride using this nanohybrid.

Author contributions

Damini Verma: methodology, investigation, validation, visualization, formal analysis, conceptualization, writing – original draft, writing – reviewing and editing; Amit K. Yadav: investigation, validation, visualization, formal analysis, writing – original draft, writing – reviewing and editing, supervision; Kunal Kumar Gupta: methodology, formal analysis; and Pratima R. Solanki: writing – reviewing and editing, resources, project administration, supervision, funding acquisition.

Data availability

Data shall be made available upon genuine request to the authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the Advanced Instrumentation Research Facility (AIRF), JNU, New Delhi, for providing the characterization facilities, and the Biomedical Device and Technology Development (BDTD) Project, DST, New Delhi, India, for providing financial support to carry out this research work. Amit K. Yadav expresses gratitude for the financial support provided by the Prime Minister Research Fellowship, funded by the Ministry of Education, Government of India.

References

1. M. Cametti and K. Rissanen, Chem. Commun., 2009, 2809–2829.
2. A. Maikap, K. Mukherjee, N. Mandal, B. Mondal and A. K. Meikap, Electrochim. Acta, 2018, 264, 150–156.
3. Y. Li, C. Liang, C. W. Slemenda, R. Ji, S. Sun, J. Cao, C. L. Emsley, F. Ma, Y. Wu and P. O. Ying, J. Bone Miner. Res., 2001, 16, 932–939.
4. X. Yue, Y. Su, X. Wang, L. Li, W. Ji and Y. Ozaki, ACS Sens., 2019, 4, 2336–2342.
5. M. Malet-Martino, V. Gilard, F. Desmoulin and R. Martino, Clin. Chim. Acta, 2006, 366, 61–73.
6. E. Pagliano, J. Meija, J. Ding, R. E. Sturgeon, A. D’Ulivo and Z. Mester, Anal. Chem., 2013, 85, 877–881.
7. H. N. Kim, Z. Guo, W. Zhu, J. Yoon and H. Tian, Chem. Soc. Rev., 2011, 40, 79–93.
8. X. Wang, Q. Zhang, C. Nam, M. Hickner, M. Mahoney and M. E. Meyerhoff, Angew. Chem., Int. Ed., 2017, 56, 11826–11830.
9. A. J. Bandodkar and J. Wang, Trends Biotechnol., 2014, 32, 363–371.
10. Y. Zhou, J. F. Zhang and J. Yoon, Chem. Rev., 2014, 114, 5511–5571.
11. N. Kaur and G. Jindal, Spectrochim. Acta, Part A, 2019, 223, 117361.
12. Y. Shen, X. Zhang, Y. Zhang, H. Li and Y. Chen, Sens. Actuators, B, 2018, 258, 544–549.
13. S. Mukherjee, M. Shah, K. Chaudhari, A. Jana, C. Sudhakar, P. Srikrishnarka, M. R. Islam, L. Philip and T. Pradeep, ACS Omega, 2020, 5, 25253–25263.
14. V. Mani, W.-Y. Li, J.-A. Gu, C.-M. Lin and S.-T. Huang, Talanta, 2015, 131, 121–126.
15. A. D. Ambaye, M. Muchindu, A. Jijana, S. Mishra and E. Nxumalo, Mater. Today Commun., 2023, 35, 105567.
16. D. Singh, S. Shaktawat, S. K. Yadav, R. Verma, K. R. B. Singh and J. Singh, Int. J. Biol. Macromol., 2024, 265, 130867.
17. D. Verma, D. Chauhan, M. Das Mukherjee, K. R. Ranjan, A. K. Yadav and P. R. Solanki, J. Appl. Electrochem., 2021, 51, 447–462.
18. D. Verma, A. K. Yadav, M. Das Mukherjee and P. R. Solanki, J. Environ. Chem. Eng., 2021, 9, 105504.
19. D. Verma, T. K. Dhiman, M. Das Mukherjee and P. R. Solanki, J. Electrochem. Soc., 2021, 168(9), 097504.
20. D. Verma, K. R. Ranjan, M. Das Mukherjee and P. R. Solanki, Biosens. Bioelectron.:X, 2022, 100217.
21. R. Verma, S. K. Yadav, K. R. B. Singh, R. Verma, D. Kumar and J. Singh, ACS Appl. Bio Mater., 2023, 6, 5842–5853.
22. R. Verma, K. R. B. Singh, R. Verma and J. Singh, Surf. Interfaces, 2024, 49, 104406.
23. Y. Long, C. Zhang, X. Wang, J. Gao, W. Wang and Y. Liu, J. Mater. Chem., 2011, 21, 13934–13941.
24. D. Verma, S. K. Yadav, A. Kalkal, R. Pradhan and G. Packirisamy, IEEE Sens. Lett., 2023, 7(12), 1–4.
25. N. Chaudhary, D. Verma, A. K. Yadav, J. G. Sharma and P. R. Solanki, Talanta Open, 2023, 8, 100259.
26. G. Eda and M. Chhowalla, Adv. Mater., 2010, 22, 2392–2415.
27. Q. Xiang, J. Yu and M. Jaroniec, Chem. Soc. Rev., 2012, 41, 782–796.
28. G. Xiang, J. He, T. Li, J. Zhuang and X. Wang, Nanoscale, 2011, 3, 3737–3742.
29. M. M. Shahid, P. Rameshkumar, A. Numan, S. Shahabuddin, M. Alizadeh, P. S. Khiew and W. S. Chiu, Mater. Sci. Eng., C, 2019, 100, 388–395.
30. A. Paisanpisuttisin, P. Poonwattanapong, P. Rakthabut, P. Ariyasantichai, C. Prasittichai and W. Siriwatcharapiboon, RSC Adv., 2022, 12, 29491–29502.
31. C. Chen, M. Long, H. Wu and W. Cai, Sci. China: Chem., 2013, 56, 354–361.
32. B. Zheng, T. Kong, X. Jing, T. Odoom-Wubah, X. Li, D. Sun, F. Lu, Y. Zheng, J. Huang and Q. Li, J. Colloid Interface Sci., 2013, 396, 138–145.
33. H. Duan, D. Wang and Y. Li, Chem. Soc. Rev., 2015, 44, 5778–5792.
34. M. S. Jameel, A. A. Aziz and M. A. Dheyab, Green Process. Synth., 2020, 9, 386–398.
35. P. V. Kumar and K. SMJ, Int. J. Recent Res. Asp., 2018, 608–612.
36. S. A. Fahmy, E. Preis, U. Bakowsky and H. M. E.-S. Azzazy, Molecules, 2020, 25, 4981.
37. S. Jadoun, R. Arif, N. K. Jangid and R. K. Meena, Environ. Chem. Lett., 2021, 19, 355–374.
38. N. Ishak, S. K. Kamarudin, S. N. Timmiati, N. A. Karim and S. Basri, J. Adv. Res., 2021, 28, 63–75.
39. P. Hanumanthaiah, H. Panari, A. Chebte, A. Haile and G. Belachew, Arab. J. Med. Aromat. Plants, 2020, 6, 105–127.
40. F. E. Ettadili, S. Aghris, F. Laghrib, A. Farahi, S. Saqrane, M. Bakasse, S. Lahrich and M. A. El Mhammedi, J. Mol. Struct., 2022, 1248, 131538.
41. A. Ghosh, R. V. Hegde, S. S. Gholap, S. A. Patil and R. B. Dateer, Funct. Nanomater. Catal. Appl., 2021, 303–328.
42. R. B. Wani and D. K. Dwivedi, J. Pharm. Negat. Results, 2023, 7260–7269.
43. S. Krishnamurthy, S. Muthuswamy and Y.-S. Yun, J. Biosci. Bioeng., 2009, 108, S91–S92.
44. S. F. Adil, M. E. Assal, M. Khan, A. Al-Warthan, M. R. H. Siddiqui and L. M. Liz-Marzán, Dalton Trans., 2015, 44, 9709–9717.
45. M. Patyal, D. Verma, N. Gupta and A. K. Malik, Environ. Pollut., 2023, 336, 122420.
46. K. Saware and A. Venkataraman, J. Cluster Sci., 2014, 25, 1157–1171.
47. A. K. Yadav, D. Verma and P. R. Solanki, ACS Appl. Bio Mater., 2023, 6(10), 4250–4268.
48. D. Verma, N. Dubey, A. K. Yadav, A. Saraya, R. Sharma and P. R. Solanki, Mater. Adv., 2024, 5(5), 2153–2168.
49. A. Bolhan, N. A. Ludin, M. S. Suait, M. A. Mat-Teridi, K. Sopian, N. M. Mohamed and H. Arakawa, Malay. J. Catal., 2018, 3(1) DOI:10.11113/mjcat.v3n1.88.
50. P. Pang, H. Li, Y. Liu, Y. Zhang, L. Feng, H. Wang, Z. Wu and W. Yang, Anal. Methods, 2015, 7, 3581–3586.
51. S. Pushparajah, S. Hasegawa, T. S. H. Pham, M. Shafiei and A. Yu, Materials, 2023, 16, 7622.
52. N. Prabhu and T. Gajendran, IOSR J. Biotechnol. Biochem., 2017, 3, 107–112.
53. V. H. Le, T. H. Nguyen, H. H. Nguyen, L. T. N. Huynh, A. Le Vo, T. K. T. Nguyen, D. T. Nguyen and V. Q. Lam, Int. J. Photoenergy, 2020, 2020, 1–10.
54. D. Verma, S. Z. H. Hashmi, G. Lakshmi, R. K. Sajwan, A. Kumar and P. R. Solanki, Microchem. J., 2023, 108964.
55. S. Kasturi, S. R. Torati, Y. J. Eom, S. Ahmad, B.-J. Lee, J.-S. Yu and C. Kim, RSC Adv., 2020, 10, 13722–13731.
56. N. Chaudhary, A. K. Yadav, D. Verma, J. G. Sharma and P. R. Solanki, Mater. Adv., 2024, 5(4), 1597–1613.
57. A. K. Yadav, D. Verma, A. Kumar, A. N. Bhatt and P. R. Solanki, Int. J. Biol. Macromol., 2023, 124325.
58. N. Chaudhary, A. K. Yadav, J. G. Sharma and P. R. Solanki, J. Environ. Chem. Eng., 2021, 9, 106771.
59. D. Verma, R. K. Sajwan, G. Lakshmi, A. Kumar and P. R. Solanki, Environ. Sci.: Nano, 2022, 9, 3992–4006.
60. F. Yan, X. Ma, Q. Jin, Y. Tong, H. Tang, X. Lin and J. Liu, Microchim. Acta, 2020, 187, 1–8.
61. A. K. Yadav, D. Verma, G. Lakshmi, S. Eremin and P. R. Solanki, Food Chem., 2021, 363, 130245.
62. A. K. Yadav, D. Verma and P. R. Solanki, Mater. Today Chem., 2021, 22, 100567.
63. H. Çiftçi, Y. Oztekin, U. Tamer, A. Ramanavicine and A. Ramanavicius, Talanta, 2014, 126, 202–207.
64. P. Ćwik, U. E. Wawrzyniak, M. Jańczyk and W. Wróblewski, Talanta, 2014, 119, 5–10.
65. J. Tao, P. Zhao, Y. Li, W. Zhao, Y. Xiao and R. Yang, Anal. Chim. Acta, 2016, 918, 97–102.
66. E. Culková, P. Tomčík, Ľ. Švorc, K. Cinková, Z. Chomisteková, J. Durdiak, M. Rievaj and D. Bustin, Electrochim. Acta, 2014, 148, 317–324.
67. R. Appiah-Ntiamoah, B. T. Gadisa and H. Kim, New J. Chem., 2018, 42, 12263.
68. C. Ding, X. Meng, X. Meng, S. Ma, J. Huo, Z. Chen, F. Guo and P. Xie, J. Fluoresc., 2023, 1–16.
69. I.-H. Cho, D. H. Kim and S. Park, Biomater. Res., 2020, 24, 6.
70. A. N. Wagassa, T. A. Shifa, A. Bansiwal and E. A. Zereffa, Environ. Sci. Pollut. Res., 2023, 30, 119084–119094.
71. M. Talbi, A. Al-Hamry, P. R. Teixeira, L. G. Paterno, M. Ben Ali and O. Kanoun, Chemosensors, 2022, 10, 40.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tb02115k

‡ These authors contributed equally.

---