Engineering a Gd₂(WO₄)₃–P@rGO heterostructure for enhanced electrochemical sensing and therapeutic drug monitoring of erdafitinib

Engineering a Gd₂(WO₄)₃–P@rGO heterostructure for enhanced electrochemical sensing and therapeutic drug monitoring of erdafitinib | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Engineering a Gd₂(WO₄)₃–P@rGO heterostructure for enhanced electrochemical sensing and therapeutic drug monitoring of erdafitinib Rasha M. K. Mohamed, Rania H Taha, Ibrahim Hotan Alsohaimi, Khulaif Alshammari, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8213846/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Feb, 2026 Read the published version in Microchimica Acta → Version 1 posted 12 You are reading this latest preprint version Abstract Erdafitinib (ERDF), a tyrosine-kinase inhibitor approved for metastatic urothelial carcinoma, possesses a narrow therapeutic index and risk of severe adverse effects, making its precise quantification essential for therapeutic drug monitoring, pharmacokinetic profiling, pharmaceutical quality control, and environmental surveillance. To address the lack of highly sensitive and accessible analytical platforms, a hybrid Gd₂(WO₄)₃–P@rGO nanocomposite was fabricated using a hydrothermal synthesis followed by ultrasonication-assisted integration and subsequently immobilized onto a glassy carbon electrode (GCE). Comprehensive structural and chemical characterization (SEM, TEM, XRD, FT-IR, and XPS) verified the successful formation of a well-distributed heterostructure with enhanced defect density, abundant active sites, and improved electronic coupling between Gd₂(WO₄)₃ and P-doped rGO. Electrochemical assessment using CV and DPV demonstrated a significantly amplified ERDF oxidation response, yielding an ultralow detection limit of 0.0024 nM (S/N = 3), high sensitivity (16.670 µA nM⁻¹ cm⁻²), and a broad linear range spanning 0.01–800 nM (R² = 0.9980). The modified electrode demonstrated excellent selectivity against common interferents, along with strong operational stability and reproducibility. Validation in spiked human serum and urine showed accurate recoveries with minimal matrix effects, meeting internationally recognized bioanalytical standards. Scalable fabrication without toxic reducing agents further underscores the platform’s environmental and practical advantages. Collectively, these attributes position the Gd₂(WO₄)₃–P@rGO sensor as a promising tool for therapeutic drug monitoring, personalized dosing, pharmacokinetics, and future point-of-care diagnostics. Erdafitinib Gd₂(WO₄)₃ P@rGO Synergism Electrochemical analysis Biological fluids Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Urothelial carcinoma is a common cancer where aberrant fibroblast growth factor receptor (FGFR) signaling is a key driver [1]. Normally, FGFRs control cell proliferation and survival, but in urothelial tumors, mutations, gene fusions, or overactivation lead to constitutive oncogenic signaling. The high frequency of these FGFR alterations has made them a prime target for precision therapeutics [2, 3]. The orally administered pan-FGFR inhibitor Erdafitinib (ERDF) is indicated for locally advanced or metastatic urothelial carcinoma characterized by specific FGFR2 or FGFR3 genetic alterations [4]. It works by blocking aberrant FGFR signaling, which is often implicated in tumor growth and progression. It represents a significant advancement in targeted therapy for bladder cancer, especially in patients who have progressed following platinum-based chemotherapy [5]. The most common methods in the literature found for determination of ERDF are UPLC-MS/MS [6, 7], LC-MS/MS [8], and HPLC-UV [9]. Current methods are limited by high cost, complex, time-consuming protocols, and the use of toxic organic solvents [10–12]. For example, LC–MS/MS is costly and not widely accessible, while HPLC–UV has long run times and lower sensitivity. Electrochemical techniques have emerged as powerful analytical platforms that overcome many of the limitations associated with conventional chromatographic. In addition to their low operational cost, fast response, and minimal reagent consumption, these techniques provide excellent sensitivity, broad linear ranges, and compatibility with real-time monitoring [13, 14]. Their analytical performance can be significantly improved through strategic surface engineering of the electrode interface. Incorporating nanostructured materials, conductive polymers, metal oxides, or carbon-based modifiers enhances electron-transfer kinetics, increases the active surface area, and strengthens analyte–electrode interactions. Such modifications not only boost sensitivity and selectivity but also enable the detection of trace-level analytes in complex biological and environmental matrices, reinforcing the role of electrochemical sensing as a versatile and high-performance alternative to traditional methods [15–17]. The working electrode—particularly when surface-engineered—is the functional center of any electrochemical sensing platform, as it governs electron-transfer kinetics and mediates the redox transformations of the target analyte [18]. Among emerging electrode modifiers, gadolinium tungstate (Gd₂WO₆) has attracted significant attention owing to its unique combination of ferroelectric behavior, chemical robustness, low toxicity, and structural versatility. These characteristics have enabled its integration into diverse technologies, including supercapacitors, photocatalysis, hydrogen/oxygen evolution, and electrocatalytic systems [19–23]. Recent advances in synthetic strategies have further expanded its utility in electroanalysis. Tailored nanostructures—such as hydrothermally derived 2D Gd₂WO₆ nanoflakes and uniformly dispersed nanoparticles—exhibit enhanced electrical conductivity, enlarged specific surface area, and accelerated charge-transport pathways, all of which are critical parameters for high-performance sensing [24, 25]. Such morphology-engineered Gd₂WO₆ architectures have demonstrated remarkable electrocatalytic activity and improved analytical sensitivity, underscoring their potential as next-generation modifiers for electrochemical detection platforms [26]. For instance, Chakavak Esmaeili et al. used Gd₂WO₆ nanoparticles on a carbon paste electrode to detect progesterone [26]. Hybridizing rare-earth tungsten oxides with conductive nanomaterials like carbon nanotubes and graphene improves their conductivity and electrocatalytic performance. These enhanced composites are now promising materials for next-generation electrochemical sensing and energy applications [27, 28]. Owing to its unique suite of properties—including exceptional electrical conductivity, an extraordinarily high specific surface area, and remarkable mechanical and chemical robustness—graphene has emerged as a premier conductive scaffold in electrochemical sensing platforms [29, 30]. Within the broader family of carbon-based nanomaterials employed for electrode modification [31, 32], heteroatom-doped graphene derivatives have shown particular promise. Incorporating heteroatoms such as phosphorus into reduced graphene oxide (P@rGO) induces electronic redistribution, generates abundant defect sites, and improves wettability—all of which collectively enhance electrocatalytic activity and facilitate rapid interfacial electron transfer. Motivated by these advantages, we engineered a composite system in which P@rGO is coupled with Gd₂WO₆ to generate a synergistic electrode material. The integration of P@rGO significantly boosts the electrical conductivity and active surface area of Gd₂WO₆, enabling superior charge-transport dynamics and positioning the hybrid as a high-performance platform for sensitive electrochemical detection. In this work, a Gd₂WO₆/P@rGO material was synthesized through a simple yet efficient hydrothermal approach and employed as a high-performance electrochemical platform for the detection of ERDF. The incorporation of P@rGO markedly improved the intrinsic catalytic behavior of Gd₂WO₆ by enhancing its electrical conductivity, electron-transfer rate, and accessible active sites. When integrated onto a glassy carbon electrode (GCE), the composite enabled sensitive and reliable quantification of ERDF in complex matrices, including human serum and urine. The development of a Gd₂WO₆/P@rGO composite, reported here for the first time, marks a significant advance in the design of rare-earth tungstate–based electrochemical sensors. 2. Experimental 2.1. Materials and reagents Erdafitinib (ERDF, 99.8%), gadolinium nitrate hexahydrate ((Gd (NO 3 ) 3 •6H 2 O, 98.5%)), sodium tungstate dihydrate ((Na 2 WO 4 •2H 2 O, 98.3%)), graphite (AR), ammonium phosphate ((NH 4 ) 3 PO 4 , AR)), glucose (98.6%), adenine (98.9%), glutathione (99.8%), guanine (97.8%), cysteine (98.8%), glycine (97.5%), ascorbic acid (98.8%), dopamine HCl (98.4%), uric acid (98.8%), tryptophan (98.7%), capmatinib (98.8%), cytarabine (98.6%), topotecan (98.9%), vismodegib (99.7%), doxorubicin (98.8%), idarubicin (99.5%), and methotrexate (98.7%) were procured from Sigma Aldrich. KCl, NaOH, HCl, K 2 HPO 4 , KH 2 PO 4 , KMnO 4 , H 2 O 2 , H 2 SO 4 , urea, K 3 [Fe(CN) 6 ], K 4 [Fe(CN) 6 ], dimethyformamide (DMF), acetonitrile, and ethanol were procured from Merck. 2.2. Preparation of Gd₂(WO₄)₃ Gd₂(WO₄)₃ was synthesized via a simple one-pot hydrothermal approach. In a typical procedure, equimolar quantities of Gd(NO₃)₃•6H₂O and Na₂WO₄•2H₂O were dissolved in 50 mL of ultrapure water under sonication for 15 min to ensure complete dissociation and homogeneous mixing of the metal precursors. Subsequently, 0.88 g of urea was added as a precipitating and pH-modulating agent; upon hydrolysis at elevated temperatures, urea gradually releases OH⁻ and CO₃²⁻ ions, which facilitates controlled nucleation and growth of the metal tungstate framework [33]. The mixture was transferred to a Teflon-lined stainless-steel autoclave and heated hydrothermally at 200°C for 10 hours, a process that promotes the growth of a well-defined crystalline phase under self-generated pressure. After natural cooling to room temperature, the formed precipitate was isolated by centrifugation (6000 rpm, 20 min), thoroughly washed with ethanol and ultrapure water to remove unreacted ions and residual organics, and subsequently dried at 60°C for 48 h. 2.3. Preparation of P@rGO P@rGO was synthesized using a facile one-pot hydrothermal strategy. In a typical procedure, 5 mg of graphene oxide (GO), previously prepared through a modified Hummers oxidation method [34], was dispersed in deionized distilled water containing 0.75 M ammonium phosphate ((NH₄)₃PO₄) as the phosphorus precursor. The mixture underwent sequential bath sonication and probe-sonication for 2 h to ensure complete exfoliation of GO sheets and uniform interaction with phosphate ions. The resulting homogeneous suspension was transferred into a Teflon-lined stainless-steel autoclave and heated at 160°C for 4 h, during which simultaneous reduction of GO and heteroatom (P) incorporation occurred through hydrothermal deoxygenation and phosphate decomposition pathways. Following hydrothermal treatment, the product was isolated by centrifugation at 6000 rpm for 15 min, and repeatedly washed with ultrapure water to remove unreacted species and loosely adsorbed phosphate residues, ensuring high purity of the final material. The purified dispersion was then freeze-dried at − 60°C for 48 h to maintain the structural integrity and prevent restacking of graphene layers. 2.4. Preparation of Gd₂(WO₄)₃-P@rGO The Gd₂(WO₄)₃–P@rGO composite was synthesized through a simple ultrasonic-assisted blending protocol designed to promote intimate interfacial coupling between the metal tungstate and doped graphene domains. In a typical procedure, 20.0 mg of Gd₂(WO₄)₃ and an equimass portion of P@rGO were dispersed in 5 mL of DMF. The suspension was sonicated for 20 min to ensure uniform dispersion, enhance adsorption of Gd₂(WO₄)₃ nanoparticles onto the rGO surface, and promote formation of strong electrostatic and coordination-driven interactions at the interface. The homogenized mixture was subsequently dried at 70°C for 10 h to evaporate the solvent and stabilize the hybrid architecture through gradual solvent removal. The dry composite was lightly ground using an agate mortar to obtain a fine, uniform powder suitable for structural, spectroscopic, and electrochemical characterization. 2.5. Fabrication of GCE with Gd₂(WO₄)₃-P@rGO Before surface modification, the GCE (3 mm diameter) was mechanically polished with 0.05 µm alumina slurry to obtain a mirror-like finish, followed by thorough rinsing with ultrapure water. The electrode was then ultrasonically cleaned in ethanol and water to remove residual particulates and ensure a contaminant-free surface. Subsequently, 5.0 µL of a 4.0 mg mL⁻¹ aqueous dispersion of the Gd₂(WO₄)₃–P@rGO composite was drop-cast onto the pretreated GCE and allowed to dry at 40°C. This procedure formed a uniform, adherent catalytic film on the electrode surface, producing the Gd₂(WO₄)₃–P@rGO/GCE configuration used for all subsequent electrochemical measurements. The modified electrode was equilibrated in the supporting electrolyte prior to analysis to stabilize the composite layer and ensure reproducible electron-transfer performance. 2.6. Preparation of samples Plasma and urine samples were obtained from healthy volunteers after informed consent, following the approved ethical guidelines of Assiut University. Serum (3.0 mL) was separated by centrifugation and subjected to protein precipitation using acetonitrile (1.5 mL), followed by a second centrifugation step to ensure complete removal of macromolecular interferents. The resulting clear supernatant was evaporated to dryness, and the residue was reconstituted in phosphate buffer (pH 5.5). The reconstituted solution was further diluted ten-fold to minimize matrix effects while maintaining analyte detectability. Urine samples were analyzed without prior treatment other than simple dilution, owing to their relatively low protein content. Both processed serum and untreated urine were examined for ERDF using the Gd₂(WO₄)₃–P@rGO/GCE. To ensure analytical reliability in complex biological environments, quantification was performed using the standard addition method. Recovery values were calculated by comparing the electrochemical responses of samples spiked with known ERDF concentrations to those obtained from an external calibration curve, thereby validating method accuracy, precision, and matrix tolerance. 3. Results and discussions 3.1. Morphological and structural characterization The morphology and microstructural features of the Gd₂(WO₄)₃–P@rGO hybrid were systematically examined using SEM and TEM (Fig. 1 ). As shown in Fig. 1 A and 1 D, the P@rGO framework exhibits a characteristic wrinkled and crumpled-sheet morphology, reflecting the successful reduction of GO and the preservation of its intrinsic flexibility. These undulated nanosheets, with variable lateral dimensions, provide a mechanically robust scaffold and abundant anchoring sites for metal–oxide nucleation. SEM and TEM images in Fig. 1 B and 1 E further confirm the formation of interconnected hexagonal Gd₂(WO₄)₃ nanosheets (≈ 55–76 nm), whose well-defined geometry and orientation suggest a uniform nucleation–growth process facilitated by controlled hydrothermal conditions and strong precursor coordination. Notably, Fig. 1 C and 1 F demonstrate that Gd₂(WO₄)₃ nanosheets are uniformly anchored along the P@rGO surfaces and edges, a distribution promoted by ultrasonication-assisted dispersion and localized pulverization that prevents particle agglomeration. This hierarchical architecture creates an enlarged contact interface, inhibits rGO restacking through an effective spacer mechanism, and promotes rapid ion/electron transport. Such structural synergy is expected to significantly enhance catalytic activity, mechanical integrity, and long-term electrochemical stability of the composite electrode. The elemental composition and chemical homogeneity of the Gd₂(WO₄)₃–P@rGO composite were confirmed by EDX spectroscopy ( Fig. S1 A ). The detected weight percentages of C (34.76%), O (14.34%), W (22.45%), and Gd (28.45%) closely match the theoretical stoichiometry of the hybrid, verifying the successful incorporation of P@rGO and Gd₂(WO₄)₃ without measurable deviation in composition. The absence of extraneous elemental signals further indicates high material purity and the effectiveness of the hydrothermal synthesis in preventing contamination or secondary phase formation. Elemental mapping images ( Fig. S1 B–F ) reveal a uniform, co-distributed presence of C, O, W, and Gd throughout the entire matrix, demonstrating intimate interfacial coupling between the tungstate nanosheets and the conductive P-doped rGO scaffold. Such homogeneous dispersion is particularly important for electrochemical applications, as it maximizes the number of accessible catalytic sites, enhances the percolation network of electron pathways, and minimizes localized charge-transfer barriers. The resulting strong electronic coupling and continuous conductive framework are expected to significantly accelerate electron transport and, consequently, improve the sensor’s analytical performance toward ERDF detection. FTIR spectroscopy was employed to elucidate the functional groups and interfacial interactions within the individual components and the hybrid composite ( Fig. S2A ). The FTIR spectrum of pristine Gd₂(WO₄)₃ exhibits prominent bands at 1070, 880, and 720 cm⁻¹, corresponds to the asymmetric stretching of W–O, the symmetric stretching of W–O, and the characteristic Gd–O lattice vibrations, respectively [35]. These bands represent the typical vibrational fingerprints of rare-earth tungstates, confirming the formation of the expected crystalline tungstate framework. For P@rGO, absorption peaks at 3420, 1690, 1455, and 1180 cm⁻¹ are assigned to O–H stretching, C = O stretching, O–H bending, and P = O stretching vibrations, respectively, reflecting the presence of residual oxygenated groups and successful phosphorus doping. In the spectrum of the Gd₂(WO₄)₃–P@rGO composite, all major tungstate-related bands remain present, indicating the structural integrity of Gd₂(WO₄)₃ upon hybridization. Notably, subtle shifts in band positions and observable variations in peak intensities are detected for both WO₄²⁻ and P@rGO-related vibrations. These spectral modifications signify strong interfacial coupling between the tungstate nanosheets and the P-doped rGO matrix, likely arising from electrostatic interactions, surface complexation, and partial charge transfer at the interface. The preservation of all characteristic vibrational modes, combined with these shifts, provides compelling evidence for successful composite formation and intimate chemical integration, which is expected to enhance electron transport and catalytic activity in subsequent electrochemical applications. The crystalline structures of Gd₂WO₆, P@rGO, and the Gd₂(WO₄)₃–P@rGO composite were examined by X-ray diffraction ( Fig. S2B ). The diffraction pattern of pristine Gd₂WO₆ displays well-defined peaks at 18.9°, 27.6°, 32.6°, 35.4°, 47.8°, 53.6°, and 58.4°, which correspond to the (011), (321), (040), (600), (-602), (-361), and (-921) planes, respectively. These reflections match the orthorhombic Gd₂(WO₄)₃ phase, in agreement with JCPDS card No. 00-023-1074 [36], confirming the successful synthesis of a highly crystalline tungstate lattice. The XRD profile of P@rGO exhibits a broadened (002) reflection at 24.8°, indicative of turbostratic stacking and partial restoration of the graphene sp² network after reduction, along with a weak (100) peak at 43.7°, characteristic of in-plane graphitic ordering [37]. In the XRD pattern of the Gd₂(WO₄)₃–P@rGO composite, all major reflections associated with the Gd₂WO₆ phase remain clearly observable, confirming that the intrinsic crystal structure of the tungstate nanodomains is preserved during hybridization. However, the peaks exhibit noticeable weakening and broadening relative to pure Gd₂WO₆. This attenuation can be attributed to several synergistic effects: (i) restricted crystallite growth due to confinement on the P@rGO surface, (ii) enhanced dispersion of smaller tungstate domains across the conductive matrix, and (iii) strong interfacial interactions that modify local crystallinity. The absence of any additional peaks or secondary phases further verifies the purity of the composite. These observations collectively demonstrate the successful integration of Gd₂WO₆ with the P@rGO scaffold, forming a structurally coherent hybrid in which nanoscale confinement and graphene coupling are expected to enhance electron mobility and electrochemical performance. XPS analysis was conducted to elucidate the surface composition and electronic states of the Gd₂(WO₄)₃–P@rGO hybrid ( Fig. S3 ). The survey spectrum ( Fig. S3A ) displays distinct signals corresponding to C 1s, O 1s, 2 P 2p, W 4f, and Gd 4d, confirming the coexistence of the expected elements and the successful formation of the composite without detectable extraneous species. High-resolution spectra further resolved the chemical environments of the constituent elements. The C 1s profile ( Fig. S3B ) was deconvoluted into contributions from sp²-hybridized C–C/C = C (≈ 286.8 eV), C–O (≈ 287.3 eV), and C = O/P = O (≈ 291.2 eV) functionalities [38], indicating partial retention of oxygenated groups on P-doped rGO, which may facilitate interfacial bonding with tungstate domains. The O 1s spectrum ( Fig. S3C ) exhibited two major components: a lower-binding-energy peak at ≈ 533.6 eV assigned to lattice oxygen in Gd–O and W–O bonds, and a higher-binding-energy contribution at ≈ 535.4 eV arising from oxygen in C–O, C = O, and O–C = O surface groups [39]. This dual-oxygen signature demonstrates the intimate interaction between metal-oxide structures and functionalized graphene frameworks. The P 2p spectrum ( Fig. S3D ) explores two prominent peaks at 124.6 eV and 124.8 eV, corresponding to PO 3 -C and PO 3 -O-C-, respectively [40]. The W 4f region ( Fig. S3E ) features a well-defined spin-orbit doublet at ≈ 42.8 eV (W 4f₇/₂) and ≈ 45.7 eV (W 4f₅/₂), characteristic of W⁶⁺ species in tungstate lattices [41], confirming that tungsten retains its expected oxidation state within the composite. Meanwhile, the Gd 4d spectrum ( Fig. S3F ) reveals peaks at ≈ 149.5 eV (4d₅/₂) and ≈ 153.8 eV (4d₃/₂), consistent with the presence of Gd³⁺ cations in Gd₂(WO₄)₃ [42]. The absence of any shifted or additional components suggests that no substoichiometric or mixed-valence states are formed during synthesis. Collectively, the XPS results validate the successful incorporation of Gd₂(WO₄)₃ nanostructures onto the P-doped rGO scaffold and confirm the preservation of the expected oxidation states. The coexistence of metal–oxygen lattice bonds and residual oxygenated functionalities on rGO indicates strong interfacial coupling, which is anticipated to enhance charge transport, increase active-site accessibility, and improve the electrochemical response toward ERDF detection. An analysis of surface area and porosity was conducted via nitrogen adsorption-desorption ( Fig. S4 ). Gd₂(WO₄)₃ displayed a non-porous morphology, evidenced by a flat isotherm and a low surface area of 7.435 m²/g. Conversely, P@rGO exhibited a Type IV isotherm with a distinct hysteresis loop, confirming its mesoporous nature and high surface area of 38.45 m²/g. The Gd₂(WO₄)₃–P@rGO composite preserved this mesoporous framework, suggesting that the integration of Gd₂(WO₄)₃ did not significantly occlude the pore structure. The composite's intermediate surface area of 12.78 m²/g implies enhanced reactant accessibility and promising electrochemical properties compared to the pure Gd₂(WO₄)₃. 3.2. Electrochemical characterization To rigorously assess the performance of the engineered sensing interfaces, their interfacial electron-transfer characteristics were examined via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV profiles recorded in the Fe(CN)₆³⁻/⁴⁻ redox probe showed a substantial enhancement in peak current for the Gd₂(WO₄)₃–P@rGO/GCE, reflecting the formation of a highly conductive and electroactive surface (Fig. 2 A). This improvement can be attributed to the synergistic interplay between the redox-active Gd₂(WO₄)₃ nanostructure—which provides abundant catalytic sites—and the phosphorus-doped rGO framework, known for its accelerated charge mobility, defect-rich surface, and improved electronic density around the Fermi level. EIS measurements further substantiated this behavior by directly quantifying the charge-transfer resistance (Rct) (Fig. 2 B). The Nyquist plots revealed a dramatic reduction in Rct for the Gd₂(WO₄)₃–P@rGO/GCE (276.8 Ω), compared with the significantly higher resistance of the unmodified GCE (987.8 Ω). This decline in interfacial resistance indicates fast electron shuttling between the redox probe and the electrode surface, a hallmark of efficient electrocatalyst–carbon composite systems. The drastic enhancement in conductivity and charge-transfer kinetics confirms that coupling Gd₂(WO₄)₃ with P-doped graphene not only improves electrical pathways but also maximizes the electroactive surface area, ultimately enabling superior analytical sensitivity in subsequent sensing applications. The effective active surface area (EASA) of the fabricated electrodes was estimated from the cyclic voltammetric responses using the Randles–Ševčík equation, enabling a quantitative assessment of how surface morphology and interfacial architecture influence their electrochemical performance. By correlating peak current with scan rate (20–200 mV/s), this analysis provides a reliable measure of the accessible electroactive sites, thereby allowing direct comparison of the catalytic efficiency imparted by each modification layer ( Fig.S5A-D ). The EASA results demonstrated a pronounced enhancement in active surface area upon electrode modification, establishing a consistent performance hierarchy among the tested interfaces. The Gd₂(WO₄)₃–P@rGO/GCE exhibited the largest EASA (0.185 cm²), reflecting the strong synergistic coupling between the catalytically active Gd₂(WO₄)₃ domains and the conductive, defect-enriched P@rGO scaffold. This was followed by the P@rGO/GCE electrode (0.156 cm²), whose expanded surface and high electron mobility contributed to its improved performance. Electrodes modified solely with Gd₂(WO₄)₃ displayed a more moderate increase (0.127 cm²), indicating that although Gd₂(WO₄)₃ introduces additional catalytic sites, its intrinsic conductivity is insufficient to maximize interfacial utilization without a graphene-based support. All modified electrodes surpassed the bare GCE (0.087 cm²), confirming that surface engineering and composite formation substantially elevate the density of electroactive sites available for charge transfer. Techniques such as CV and DPV were systematically performed to elucidate the redox behavior of ERDF at the modified interfaces (Fig. 3 ). Control scans recorded in analyte-free electrolyte exhibited no discernible faradaic peaks, confirming negligible background currents and ensuring that the subsequent responses originated solely from ERDF oxidation. Upon introducing ERDF, a pronounced and sharp anodic peak was observed, with the highest current intensity obtained at the Gd₂(WO₄)₃–P@rGO/GCE. This substantial enhancement reflects the strong synergistic interaction between Gd₂(WO₄)₃ and phosphorus-doped rGO, where Gd₂(WO₄)₃ contributes abundant redox-active sites while P-doping introduces additional defects, increases electrical conductivity, and promotes faster charge transfer at the electrode–solution interface. The resulting composite provides a larger electroactive surface area, improved electron mediation pathways, and stronger adsorption affinity toward ERDF, collectively leading to amplified oxidation signals. Compared with CV, DPV exhibited superior peak resolution and a markedly higher signal-to-noise ratio, consistent with its well-recognized sensitivity for trace-level electroanalysis; therefore, DPV was selected as the optimal technique for quantitative determination of ERDF [43]. 3.3. Optimization of conditions To achieve maximum electrocatalytic efficiency, the loading concentration and deposition volume of the Gd₂(WO₄)₃–P@rGO suspension were systematically optimized. A series of dispersions with increasing concentrations in DMF were evaluated ( Fig. S6A ). The anodic peak current increased progressively with concentration, reaching its maximum at 5.0 mg mL⁻¹ (equivalent to 2.5 mg mL⁻¹ of each component). This enhancement is attributed to the higher density of electroactive sites, improved electron-transport pathways introduced by P-doped rGO, and the greater probability of ERDF adsorption at the composite surface. Beyond this concentration, a noticeable decline in peak current occurred, most likely due to particle agglomeration, excessive film thickness, and partial blockage of active sites—phenomena known to impede mass transport and hinder charge transfer in densely loaded films. Similarly, the deposition volume was optimized by drop-casting different aliquots (2–8 µL) of the 5.0 mg mL⁻¹ dispersion onto the GCE surface ( Fig. S6B ). The peak current increased with volume up to 5 µL, reflecting the formation of a uniform catalytic layer with sufficient conductive pathways. Additional loading beyond this volume produced a decrease in signal intensity, which can be explained by the development of overly thick films that introduce diffusion barriers, slow down electron migration, and reduce the efficiency of analyte–electrode interactions. Based on these trends, a concentration of 5.0 mg mL⁻¹ and a deposition volume of 5 µL were identified as the optimal conditions for fabricating high-performance Gd₂(WO₄)₃–P@rGO/GCE sensors in subsequent analyses. The electrochemical oxidation behavior of ERDF was further examined using cyclic voltammetry over a wide range of scan rates (20–220 mV s⁻¹) (Fig. 4 A). A progressive shift of the anodic peak toward more positive potentials was observed as the scan rate increased, confirming the irreversible nature of the electron-transfer process. Concurrently, the anodic peak current (Ipa) increased proportionally with the square root of the scan rate (ν¹ᐟ²), following the linear regression equation Ipa = 259.44ν ½ + 0.7248 (R² = 0.9973). This strong linearity indicates that the oxidation of ERDF is predominantly governed by a diffusion-controlled mechanism rather than surface-confined kinetics [44]. The diffusion-limited nature of the process was further validated by the excellent correlation coefficient and the absence of saturation effects across the applied scan-rate window (Fig. 4 B). A continuous positive shift in the anodic peak potential (Epa) with increasing scan rate was observed, further supporting the irreversible nature of the electron-transfer step involved in ERDF oxidation [45]. This behavior is characteristic of systems in which the oxidation kinetics cannot keep pace with the applied potential sweep, resulting in a rate-dependent displacement of the peak position. To corroborate this interpretation, a log(Ipa)–log(ν) analysis was conducted (Fig. 4 C). The resulting slope of 0.5848 (R² = 0.9997) aligns closely with the theoretical value of 0.5 expected for diffusion-controlled processes, thereby confirming that mass transport—rather than adsorption or surface confinement—governs the electrochemical oxidation of ERDF under the examined conditions [46]. The excellent linearity additionally indicates consistent diffusional behavior across all tested scan rates, with no evidence of mixed or adsorption-controlled contributions. Application of Laviron’s kinetic theory to the linear dependence of Epa on ln ν (Fig. 4 D) enabled quantitative evaluation of the electron-transfer parameters. The calculated slope and intercept yielded an electron-transfer coefficient (α) consistent with an irreversible system, and the number of electrons involved in the rate-determining step was determined to be approximately one. Coupled with the pH-dependent peak potential shift (discussed earlier), these results confirm that ERDF undergoes a coupled one-electron/one-proton (1e⁻/1H⁺) oxidation pathway. The combination of scan-rate analysis, log–log correlation, and Laviron modeling collectively verifies that the overall process is irreversible and governed primarily by diffusion rather than surface adsorption. The superior performance of the Gd₂(WO₄)₃–P@rGO/GCE in facilitating this oxidation can be attributed to its engineered physicochemical features: the high surface area increases accessible electroactive sites, Gd₂(WO₄)₃ provides redox-active centers, and phosphorus-doped rGO enhances electrical conductivity while minimizing charge-transfer resistance. These synergistic properties accelerate electron-transport kinetics and promote efficient interaction between ERDF and the electrode surface, ultimately enabling sensitive and reliable electrochemical detection. To further optimize the electrochemical response, the influence of solution pH on ERDF oxidation was systematically investigated. The anodic peak current displayed a strong dependence on proton concentration, reaching its maximum intensity at pH 5.5, after which it gradually decreased at more alkaline conditions (Fig. 5 A). The decline in current at higher pH values can be attributed to reduced proton availability, slower proton-coupled electron transfer kinetics, and possible shifts in ERDF speciation that weaken its electrochemical activity. In parallel, the anodic peak potential (Epa) shifted linearly toward more negative values with increasing pH, yielding a slope of approximately − 59 mV per pH unit (Fig. 5 B). This slope closely matches the theoretical Nernstian value for electrochemical processes involving an equal number of protons and electrons, thereby confirming that ERDF oxidation proceeds via a 1H⁺/1e⁻ proton-coupled electron-transfer mechanism [47, 48]. The excellent linearity further indicates that proton participation is integral to the rate-determining step of the oxidation process. Based on the combined trends of maximum signal intensity and favorable redox kinetics, pH 5.5 was selected as the optimal medium for all subsequent electroanalytical measurements. 3.4. Quantitative data Differential pulse voltammetry (DPV) was employed to quantify ERDF over a broad concentration range (0–800 nM) using the Gd₂(WO₄)₃–P@rGO-modified GCE in 0.1 M Britton–Robinson buffer at pH 5.5 (Fig. 6 A). A well-defined anodic peak centered at approximately 0.85 V (vs. Ag/AgCl) reflects the irreversible oxidation of ERDF and confirms efficient electrocatalytic mediation by the hybrid interface. The anodic peak current increased proportionally with analyte concentration, demonstrating rapid charge-transfer kinetics and stable surface adsorption behavior. The resulting calibration plot (Fig. 6 B) exhibited excellent linearity, described by Ipa (µA) = 0.1302 [ERDF] + 7.9447 (R² = 0.9980), supporting the reliability of quantitative determination across ultra-trace to sub-micromolar levels. The limit of detection, estimated at S/N = 3, was as low as 0.0024 nM, positioning the sensor among the most sensitive electrochemical platforms reported for ERDF to date. This remarkable sensitivity can be attributed to the synergistic contributions of the composite: (i) phosphorus-doped rGO enhances electrical conductivity, defect density, and π–π interaction with the aromatic ERDF structure; (ii) Gd₂(WO₄)₃ provides abundant redox-active sites and facilitates fast electron shuttling; and (iii) the hierarchical architecture increases the electroactive surface area and promotes rapid mass transport. Collectively, these features minimize overpotential, accelerate electron transfer, and enable efficient pre-concentration of ERDF at the electrode–solution interface. Such performance confirms the suitability of the Gd₂(WO₄)₃–P@rGO/GCE for trace detection in complex matrices, providing an analytical window relevant to therapeutic monitoring, pharmacokinetic studies, and environmental surveillance of anticancer residues. Table 1 presents a comparative evaluation of the analytical performance of the Gd₂(WO₄)₃–P@rGO/GCE against previously reported methods for ERDF determination. The proposed platform demonstrates a remarkably broad linear dynamic range together with an ultralow detection limit, outperforming most electrochemical, chromatographic, and spectroscopic approaches cited in the literature. Such advancement highlights the analytical superiority of the hybrid system, particularly for monitoring ERDF at clinically and environmentally relevant trace levels. Table 1. Performance comparison of Gd₂(WO₄)₃–P@rGO/GCE with reported ERDF detection techniques. Technique Sensor Linear range (nM) LOD (nM) Reference UPLC-MS/MS ------- ------- 1-2240 2.23-1119.6 ------ ------ [6] [7] LC-MS/MS ------- 6-2240 ------ [8] HPLC/UV ------- 110–4470 ------ [9] Spectrofluorometry Kolliphor RH 40 Nitrogen-doped carbon dots 110–1790 44–600,000 30 13 [49] [50] Voltammetry NPCS/GCE Gd₂(WO₄)₃–P@rGO/GCE 100–7380 0.01–800 11.2 0.0024 [44] This work 3.5. Stability, reproducibility, and repeatability The long-term operational stability of the Gd₂(WO₄)₃–P@rGO/GCE was evaluated by monitoring its DPV response over a 60-day storage period under ambient conditions ( Fig. S7A ). The electrode retained 85.87% of its initial peak current, demonstrating excellent resistance to surface passivation and structural deterioration. This sustained performance is attributed to the chemical robustness of the Gd₂(WO₄)₃ matrix and the anchoring effect of P-doped rGO, which suppresses nanoparticle aggregation, preserves conductivity, and prevents oxidative degradation—issues commonly encountered in rare-earth–oxide-based sensors. Reproducibility was assessed using seven independently prepared electrodes, yielding a low relative standard deviation of 3.56% ( Fig. S7B ). Such narrow variability reflects high batch-to-batch uniformity and confirms the reliability of the synthesis and modification protocol. This level of reproducibility is critical for large-scale fabrication, ensuring sensor consistency for regulated analytical settings and potential commercialization. Repeatability was further examined through ten consecutive DPV measurements in 100 nM ERDF, resulting in an RSD of only 2.34% ( Fig. S7C ). The minimal signal fluctuation and absence of peak distortion indicate stable electron-transfer kinetics and negligible electrode fouling during repeated use. Efficient analyte desorption and the defect-rich P@rGO surface likely mitigates signal drift by preventing accumulation at the interface. Collectively, the excellent stability, reproducibility, and repeatability affirm the robustness of the Gd₂(WO₄)₃–P@rGO/GCE and support its suitability for routine analytical applications, particularly those requiring prolonged storage, frequent measurements, and dependable inter-electrode performance—such as therapeutic drug monitoring, pharmaceutical quality control, and environmental screening of anticancer residues. 3.6. Selectivity and anti-interference The selectivity of the Gd₂(WO₄)₃–P@rGO/GCE toward ERDF was investigated by conducting DPV measurements in the absence and presence of the target analyte in complex mixtures containing potential interferents (Fig. 7 ). The tested species included abundant inorganic ions, endogenous biomolecules, and structurally related anticancer agents at elevated concentrations (1.0 mM), representing a > 10,000-fold excess relative to ERDF (100 nM). Under these challenging conditions, only ERDF generated a well-defined oxidation peak, whereas all interferents produced negligible current changes, confirming the absence of overlapping electrochemical signatures. The pronounced specificity can be attributed to several factors: (i) strong π–π and electrostatic interactions between ERDF’s aromatic structure and the defect-rich P-doped rGO surface; (ii) enhanced electron-transfer mediation by Gd₂(WO₄)₃, which selectively promotes ERDF oxidation at a distinct potential; and (iii) suppressed nonspecific adsorption due to the stable and hydrophilic hybrid interface. Together, these features minimize signal perturbation from coexisting species and prevent competitive surface occupation. These findings demonstrate that the Gd₂(WO₄)₃–P@rGO/GCE possesses exceptional interference tolerance and molecular discrimination capability, supporting its applicability for accurate ERDF quantification in complex biological matrices such as serum and urine, where high ionic strength and biochemical diversity often compromise electrochemical sensing performance. 3.7. Applications The real-sample applicability of the Gd₂(WO₄)₃–P@rGO/GCE platform was rigorously assessed through the quantification of ERDF in spiked human urine and serum. As summarized in Table 2 , the sensor achieved excellent recoveries of 95.7–104.6% (urine) and 97.4–104.8% (serum), accompanied by low %RSD values of 2.65–3.65%, demonstrating outstanding analytical accuracy and repeatability under physiological conditions. Notably, the absence of significant signal attenuation confirms that neither protein-binding interactions nor endogenous electroactive species interfered with the ERDF oxidation response. These findings highlight the sensor’s strong anti-matrix tolerance, which can be attributed to the synergistic electrocatalytic behavior of P-doped rGO—enhancing electron transfer—and the abundant active sites provided by Gd₂(WO₄)₃. Moreover, the obtained recovery performance aligns with internationally recognized bioanalytical validation criteria (e.g., FDA and ICH guidelines, allowing 85–115% recovery and ≤ 15% precision), thereby supporting the method’s suitability for routine clinical monitoring and therapeutic drug assessment. Collectively, these results establish the Gd₂(WO₄)₃–P@rGO/GCE sensor as a robust, highly reliable platform for ERDF determination in complex biological matrices, paving the way for future applications in point-of-care diagnostics, pharmacokinetic studies, and real-world patient sample analysis. Table 2. Determination of ERDF in urine and serum samples (n=3). Spiked (nM) Gd₂(WO₄)₃–P@rGO/GCE UPLC-MS/MS [7] Found (nM) Recovery % RSD % Found (nM) Recovery % RSD % Urine 3.0 5.0 7.5 10.0 2.87 ± 0.56 5.23 ± 0.22 7.67 ± 0.55 9.76 ± 0.43 95.7 104.6 102.2 97.6 3.24 2.67 3.28 3.65 3.17 ± 0.25 4.88 ± 0.19 7.43 ± 0.76 9.53 ± 0.66 105.7 97.6 99.1 95.3 3.43 2.42 2.87 3.24 Serum 3.0 5.0 7.5 10.0 3.08 ± 0.18 4.87 ± 0.19 7.78 ± 0.45 10.48 ± 0.32 102.7 97.4 103.7 104.8 2.65 3.07 2.65 3.08 2.87 ± 0.24 5.08 ± 0.43 7.89 ± 0.83 9.45 ± 0.78 95.7 101.6 105.2 94.5 2.87 2.56 3.29 4.13 4. Conclusions A novel Gd₂(WO₄)₃ nanostructure was successfully fabricated through a controlled hydrothermal route and subsequently integrated with phosphorus-doped reduced graphene oxide (P@rGO) via ultrasonication to construct a hybrid electroactive interface for ERDF sensing. This architecture exploits the complementary functions of both components: the redox-active Gd₂(WO₄)₃ provides abundant catalytic sites, while P-doping enhances the intrinsic conductivity and defect density of rGO, facilitating rapid charge transfer and improved adsorption of ERDF molecules. The resulting Gd₂(WO₄)₃–P@rGO/GCE exhibited markedly enhanced electrochemical behavior compared to its individual counterparts. Electrochemical characterization by CV and DPV revealed a well-defined oxidation peak with a broad linear response range of 0.01–800 nM (R² = 0.9980) and an ultralow detection limit of 0.0024 µM (S/N = 3), reflecting the sensor’s high sensitivity and fast electron-transfer kinetics. In addition, the modified electrode maintained excellent operational stability and reproducibility across multiple measurements, with negligible performance drift over prolonged storage. The selectivity studies confirmed strong anti-interference capability, as common endogenous biomolecules and structurally related drugs produced no significant signal perturbation. The platform’s practical utility was further demonstrated through ERDF quantification in spiked human urine and serum samples, achieving accurate recoveries with minimal matrix-induced suppression, thereby fulfilling FDA/ICH bioanalytical criteria. Taken together, these findings position the Gd₂(WO₄)₃–P@rGO hybrid as a promising sensing material for clinical bioanalysis, therapeutic drug monitoring, and potential translation into scalable point-of-care diagnostic systems. Declarations Competing interests The authors declare no competing interests. Contributions Rasha M. K. Mohamed, Rania H Taha : Writing-original draft, Project administration, Funding acquisition, Conceptualization. Ibrahim Hotan Alsohaimi, Khulaif Alshammari : Investigation, Formal analysis, Data curation. Hassan M. A. 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13:37:56","extension":"html","order_by":62,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":144179,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/56758a0a1e6d6cad0a5f12a9.html"},{"id":97356113,"identity":"d7b65bbe-c0f2-4a34-9d45-75013023f801","added_by":"auto","created_at":"2025-12-03 13:37:55","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":670097,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of P@rGO (A), Gd₂(WO₄)₃ (B), and Gd₂(WO₄)₃–P@rGO (C). TEM images of P@rGO (D), Gd₂(WO₄)₃ (E), and Gd₂(WO₄)₃–P@rGO (F).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/0cb1e9b009606e72cb54e456.png"},{"id":97356114,"identity":"b9c1f4e2-90a6-4c58-8a28-4fc6cd14ec97","added_by":"auto","created_at":"2025-12-03 13:37:55","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":362571,"visible":true,"origin":"","legend":"\u003cp\u003eCV scans (A) and EIS (B) of bare GCE, Gd₂(WO₄)₃/GCE, P@rGO/GCE, and Gd₂(WO₄)₃–P@rGO/GCE. Concentration of Fe(CN)₆³⁻/⁴⁻ redox probe is 5.0 mM.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/280972b42e824a17c20bf2d2.png"},{"id":97370865,"identity":"83d59090-fa72-4fdf-8fc2-00628ed38438","added_by":"auto","created_at":"2025-12-03 16:28:04","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":287676,"visible":true,"origin":"","legend":"\u003cp\u003eCV (A) and DPV (B) scans of bare GCE, Gd₂(WO₄)₃/GCE, P@rGO/GCE, and Gd₂(WO₄)₃–P@rGO/GCE in phosphate buffer (pH 5.5) containing ERDF (2.5 µM for CV and 100.0 nM for DPV).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/1b4d51ed345ad350a2af5ed4.png"},{"id":97356121,"identity":"120d6778-63e8-478d-9737-533fc9d3731f","added_by":"auto","created_at":"2025-12-03 13:37:55","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":224156,"visible":true,"origin":"","legend":"\u003cp\u003e(A) CV scans of 5.0 µM ERDF at different scan rates (20-220 mV/s), (B) plot relationship between ν\u003csup\u003e½\u003c/sup\u003e and current (Ipa), (C) plot relationship between log ν and log Ipa, (D) plot relationship between Ln ν and potential (E, V).\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/2c3f73dd7d0e1119c9907b9d.png"},{"id":97371183,"identity":"69bf0453-8b34-4c0e-9408-c6dea43e6bb4","added_by":"auto","created_at":"2025-12-03 16:28:30","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":147127,"visible":true,"origin":"","legend":"\u003cp\u003e(A) DPV scans of 100.0 nM ERDF at Gd₂(WO₄)₃–P@rGO/GCE using different pH values (4.0-7.0) and (B) corresponding plot between Epa (V) and pH value.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/4325a6a182108f83c3e004b0.png"},{"id":97371202,"identity":"54f33d57-d55a-4783-b25e-af2eb63e9b3e","added_by":"auto","created_at":"2025-12-03 16:28:31","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":238493,"visible":true,"origin":"","legend":"\u003cp\u003e(A) DPV responses for varying concentrations of ERDF (0-800 nM) at the Gd₂(WO₄)₃–P@rGO/GCE, and (B) the corresponding calibration plot.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/ac92a83115e276934cd8287b.png"},{"id":97371005,"identity":"7656609a-c2f8-4308-a056-1a564ceba0d3","added_by":"auto","created_at":"2025-12-03 16:28:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":178472,"visible":true,"origin":"","legend":"\u003cp\u003eDifferential pulse voltammetry (DPV) responses recorded at the Gd₂(WO₄)₃–P@rGO/GCE in phosphate buffer (pH 5.5). Panel 1 corresponds to 100 nM ERDF while Panels 2–25 show the signals obtained in the presence of 1.0 mM potential interferents, including inorganic ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, SO₄²⁻, Cl⁻, PO₄³⁻), endogenous biomolecules (glucose, adenine, glutathione, guanine, cysteine, glycine, ascorbic acid, dopamine·HCl, uric acid, tryptophan), and structurally related anticancer drugs (capmatinib, cytarabine, topotecan, vismodegib, doxorubicin, idarubicin, methotrexate).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/87f47e36746d03e3d21063b4.png"},{"id":103251279,"identity":"0a203704-802d-4c4a-81f2-28f627768346","added_by":"auto","created_at":"2026-02-23 16:07:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3668312,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/2ef91da5-b7c8-408f-b083-3fc84ef6217a.pdf"},{"id":97371102,"identity":"4889421f-f302-4fa1-99bd-2820e998e5b3","added_by":"auto","created_at":"2025-12-03 16:28:24","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2026919,"visible":true,"origin":"","legend":"","description":"","filename":"ElectronicSupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/f8252eebcd59be5247247b08.docx"},{"id":97356117,"identity":"2f22a89c-922c-4c5c-aac1-dfb254d3940b","added_by":"auto","created_at":"2025-12-03 13:37:55","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":237289,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1\u003c/strong\u003e Steps for preparation of Gd₂(WO₄)₃–P@rGO and modification of GCE for electro-catalytic determination of ERDF.\u003c/p\u003e","description":"","filename":"schemre1.png","url":"https://assets-eu.researchsquare.com/files/rs-8213846/v1/9c6ba5ce0721dd6002ae4af2.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Engineering a Gd₂(WO₄)₃–P@rGO heterostructure for enhanced electrochemical sensing and therapeutic drug monitoring of erdafitinib","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eUrothelial carcinoma is a common cancer where aberrant fibroblast growth factor receptor (FGFR) signaling is a key driver [1]. Normally, FGFRs control cell proliferation and survival, but in urothelial tumors, mutations, gene fusions, or overactivation lead to constitutive oncogenic signaling. The high frequency of these FGFR alterations has made them a prime target for precision therapeutics [2, 3]. The orally administered pan-FGFR inhibitor Erdafitinib (ERDF) is indicated for locally advanced or metastatic urothelial carcinoma characterized by specific FGFR2 or FGFR3 genetic alterations [4]. It works by blocking aberrant FGFR signaling, which is often implicated in tumor growth and progression. It represents a significant advancement in targeted therapy for bladder cancer, especially in patients who have progressed following platinum-based chemotherapy [5]. The most common methods in the literature found for determination of ERDF are UPLC-MS/MS [6, 7], LC-MS/MS [8], and HPLC-UV [9]. Current methods are limited by high cost, complex, time-consuming protocols, and the use of toxic organic solvents [10\u0026ndash;12]. For example, LC\u0026ndash;MS/MS is costly and not widely accessible, while HPLC\u0026ndash;UV has long run times and lower sensitivity.\u003c/p\u003e\u003cp\u003eElectrochemical techniques have emerged as powerful analytical platforms that overcome many of the limitations associated with conventional chromatographic. In addition to their low operational cost, fast response, and minimal reagent consumption, these techniques provide excellent sensitivity, broad linear ranges, and compatibility with real-time monitoring [13, 14]. Their analytical performance can be significantly improved through strategic surface engineering of the electrode interface. Incorporating nanostructured materials, conductive polymers, metal oxides, or carbon-based modifiers enhances electron-transfer kinetics, increases the active surface area, and strengthens analyte\u0026ndash;electrode interactions. Such modifications not only boost sensitivity and selectivity but also enable the detection of trace-level analytes in complex biological and environmental matrices, reinforcing the role of electrochemical sensing as a versatile and high-performance alternative to traditional methods [15\u0026ndash;17].\u003c/p\u003e\u003cp\u003eThe working electrode\u0026mdash;particularly when surface-engineered\u0026mdash;is the functional center of any electrochemical sensing platform, as it governs electron-transfer kinetics and mediates the redox transformations of the target analyte [18]. Among emerging electrode modifiers, gadolinium tungstate (Gd₂WO₆) has attracted significant attention owing to its unique combination of ferroelectric behavior, chemical robustness, low toxicity, and structural versatility. These characteristics have enabled its integration into diverse technologies, including supercapacitors, photocatalysis, hydrogen/oxygen evolution, and electrocatalytic systems [19\u0026ndash;23]. Recent advances in synthetic strategies have further expanded its utility in electroanalysis. Tailored nanostructures\u0026mdash;such as hydrothermally derived 2D Gd₂WO₆ nanoflakes and uniformly dispersed nanoparticles\u0026mdash;exhibit enhanced electrical conductivity, enlarged specific surface area, and accelerated charge-transport pathways, all of which are critical parameters for high-performance sensing [24, 25]. Such morphology-engineered Gd₂WO₆ architectures have demonstrated remarkable electrocatalytic activity and improved analytical sensitivity, underscoring their potential as next-generation modifiers for electrochemical detection platforms [26]. For instance, Chakavak Esmaeili et al. used Gd₂WO₆ nanoparticles on a carbon paste electrode to detect progesterone [26].\u003c/p\u003e\u003cp\u003eHybridizing rare-earth tungsten oxides with conductive nanomaterials like carbon nanotubes and graphene improves their conductivity and electrocatalytic performance. These enhanced composites are now promising materials for next-generation electrochemical sensing and energy applications [27, 28].\u003c/p\u003e\u003cp\u003eOwing to its unique suite of properties\u0026mdash;including exceptional electrical conductivity, an extraordinarily high specific surface area, and remarkable mechanical and chemical robustness\u0026mdash;graphene has emerged as a premier conductive scaffold in electrochemical sensing platforms [29, 30]. Within the broader family of carbon-based nanomaterials employed for electrode modification [31, 32], heteroatom-doped graphene derivatives have shown particular promise. Incorporating heteroatoms such as phosphorus into reduced graphene oxide (P@rGO) induces electronic redistribution, generates abundant defect sites, and improves wettability\u0026mdash;all of which collectively enhance electrocatalytic activity and facilitate rapid interfacial electron transfer. Motivated by these advantages, we engineered a composite system in which P@rGO is coupled with Gd₂WO₆ to generate a synergistic electrode material. The integration of P@rGO significantly boosts the electrical conductivity and active surface area of Gd₂WO₆, enabling superior charge-transport dynamics and positioning the hybrid as a high-performance platform for sensitive electrochemical detection.\u003c/p\u003e\u003cp\u003eIn this work, a Gd₂WO₆/P@rGO material was synthesized through a simple yet efficient hydrothermal approach and employed as a high-performance electrochemical platform for the detection of ERDF. The incorporation of P@rGO markedly improved the intrinsic catalytic behavior of Gd₂WO₆ by enhancing its electrical conductivity, electron-transfer rate, and accessible active sites. When integrated onto a glassy carbon electrode (GCE), the composite enabled sensitive and reliable quantification of ERDF in complex matrices, including human serum and urine. The development of a Gd₂WO₆/P@rGO composite, reported here for the first time, marks a significant advance in the design of rare-earth tungstate\u0026ndash;based electrochemical sensors.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials and reagents\u003c/h2\u003e\u003cp\u003eErdafitinib (ERDF, 99.8%), gadolinium nitrate hexahydrate ((Gd (NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026bull;6H\u003csub\u003e2\u003c/sub\u003eO, 98.5%)), sodium tungstate dihydrate ((Na\u003csub\u003e2\u003c/sub\u003eWO\u003csub\u003e4\u003c/sub\u003e\u0026bull;2H\u003csub\u003e2\u003c/sub\u003eO, 98.3%)), graphite (AR), ammonium phosphate ((NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, AR)), glucose (98.6%), adenine (98.9%), glutathione (99.8%), guanine (97.8%), cysteine (98.8%), glycine (97.5%), ascorbic acid (98.8%), dopamine HCl (98.4%), uric acid (98.8%), tryptophan (98.7%), capmatinib (98.8%), cytarabine (98.6%), topotecan (98.9%), vismodegib (99.7%), doxorubicin (98.8%), idarubicin (99.5%), and methotrexate (98.7%) were procured from Sigma Aldrich. KCl, NaOH, HCl, K\u003csub\u003e2\u003c/sub\u003eHPO\u003csub\u003e4\u003c/sub\u003e, KH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, KMnO\u003csub\u003e4\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, urea, K\u003csub\u003e3\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e], K\u003csub\u003e4\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e], dimethyformamide (DMF), acetonitrile, and ethanol were procured from Merck.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Preparation of Gd₂(WO₄)₃\u003c/h2\u003e\u003cp\u003eGd₂(WO₄)₃ was synthesized via a simple one-pot hydrothermal approach. In a typical procedure, equimolar quantities of Gd(NO₃)₃\u0026bull;6H₂O and Na₂WO₄\u0026bull;2H₂O were dissolved in 50 mL of ultrapure water under sonication for 15 min to ensure complete dissociation and homogeneous mixing of the metal precursors. Subsequently, 0.88 g of urea was added as a precipitating and pH-modulating agent; upon hydrolysis at elevated temperatures, urea gradually releases OH⁻ and CO₃\u0026sup2;⁻ ions, which facilitates controlled nucleation and growth of the metal tungstate framework [33]. The mixture was transferred to a Teflon-lined stainless-steel autoclave and heated hydrothermally at 200\u0026deg;C for 10 hours, a process that promotes the growth of a well-defined crystalline phase under self-generated pressure. After natural cooling to room temperature, the formed precipitate was isolated by centrifugation (6000 rpm, 20 min), thoroughly washed with ethanol and ultrapure water to remove unreacted ions and residual organics, and subsequently dried at 60\u0026deg;C for 48 h.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Preparation of P@rGO\u003c/h2\u003e\u003cp\u003eP@rGO was synthesized using a facile one-pot hydrothermal strategy. In a typical procedure, 5 mg of graphene oxide (GO), previously prepared through a modified Hummers oxidation method [34], was dispersed in deionized distilled water containing 0.75 M ammonium phosphate ((NH₄)₃PO₄) as the phosphorus precursor. The mixture underwent sequential bath sonication and probe-sonication for 2 h to ensure complete exfoliation of GO sheets and uniform interaction with phosphate ions. The resulting homogeneous suspension was transferred into a Teflon-lined stainless-steel autoclave and heated at 160\u0026deg;C for 4 h, during which simultaneous reduction of GO and heteroatom (P) incorporation occurred through hydrothermal deoxygenation and phosphate decomposition pathways. Following hydrothermal treatment, the product was isolated by centrifugation at 6000 rpm for 15 min, and repeatedly washed with ultrapure water to remove unreacted species and loosely adsorbed phosphate residues, ensuring high purity of the final material. The purified dispersion was then freeze-dried at \u0026minus;\u0026thinsp;60\u0026deg;C for 48 h to maintain the structural integrity and prevent restacking of graphene layers.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Preparation of Gd₂(WO₄)₃-P@rGO\u003c/h2\u003e\u003cp\u003eThe Gd₂(WO₄)₃\u0026ndash;P@rGO composite was synthesized through a simple ultrasonic-assisted blending protocol designed to promote intimate interfacial coupling between the metal tungstate and doped graphene domains. In a typical procedure, 20.0 mg of Gd₂(WO₄)₃ and an equimass portion of P@rGO were dispersed in 5 mL of DMF. The suspension was sonicated for 20 min to ensure uniform dispersion, enhance adsorption of Gd₂(WO₄)₃ nanoparticles onto the rGO surface, and promote formation of strong electrostatic and coordination-driven interactions at the interface. The homogenized mixture was subsequently dried at 70\u0026deg;C for 10 h to evaporate the solvent and stabilize the hybrid architecture through gradual solvent removal. The dry composite was lightly ground using an agate mortar to obtain a fine, uniform powder suitable for structural, spectroscopic, and electrochemical characterization.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5. Fabrication of GCE with Gd₂(WO₄)₃-P@rGO\u003c/h2\u003e\u003cp\u003eBefore surface modification, the GCE (3 mm diameter) was mechanically polished with 0.05 \u0026micro;m alumina slurry to obtain a mirror-like finish, followed by thorough rinsing with ultrapure water. The electrode was then ultrasonically cleaned in ethanol and water to remove residual particulates and ensure a contaminant-free surface. Subsequently, 5.0 \u0026micro;L of a 4.0 mg mL⁻\u0026sup1; aqueous dispersion of the Gd₂(WO₄)₃\u0026ndash;P@rGO composite was drop-cast onto the pretreated GCE and allowed to dry at 40\u0026deg;C. This procedure formed a uniform, adherent catalytic film on the electrode surface, producing the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE configuration used for all subsequent electrochemical measurements. The modified electrode was equilibrated in the supporting electrolyte prior to analysis to stabilize the composite layer and ensure reproducible electron-transfer performance.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.6. Preparation of samples\u003c/h2\u003e\u003cp\u003ePlasma and urine samples were obtained from healthy volunteers after informed consent, following the approved ethical guidelines of Assiut University. Serum (3.0 mL) was separated by centrifugation and subjected to protein precipitation using acetonitrile (1.5 mL), followed by a second centrifugation step to ensure complete removal of macromolecular interferents. The resulting clear supernatant was evaporated to dryness, and the residue was reconstituted in phosphate buffer (pH 5.5). The reconstituted solution was further diluted ten-fold to minimize matrix effects while maintaining analyte detectability. Urine samples were analyzed without prior treatment other than simple dilution, owing to their relatively low protein content. Both processed serum and untreated urine were examined for ERDF using the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE. To ensure analytical reliability in complex biological environments, quantification was performed using the standard addition method. Recovery values were calculated by comparing the electrochemical responses of samples spiked with known ERDF concentrations to those obtained from an external calibration curve, thereby validating method accuracy, precision, and matrix tolerance.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussions","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Morphological and structural characterization\u003c/h2\u003e\n \u003cp\u003eThe morphology and microstructural features of the Gd₂(WO₄)₃\u0026ndash;P@rGO hybrid were systematically examined using SEM and TEM (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eA and \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eD, the P@rGO framework exhibits a characteristic wrinkled and crumpled-sheet morphology, reflecting the successful reduction of GO and the preservation of its intrinsic flexibility. These undulated nanosheets, with variable lateral dimensions, provide a mechanically robust scaffold and abundant anchoring sites for metal\u0026ndash;oxide nucleation. SEM and TEM images in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eB and \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eE further confirm the formation of interconnected hexagonal Gd₂(WO₄)₃ nanosheets (\u0026asymp;\u0026thinsp;55\u0026ndash;76 nm), whose well-defined geometry and orientation suggest a uniform nucleation\u0026ndash;growth process facilitated by controlled hydrothermal conditions and strong precursor coordination. Notably, Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eC and \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eF demonstrate that Gd₂(WO₄)₃ nanosheets are uniformly anchored along the P@rGO surfaces and edges, a distribution promoted by ultrasonication-assisted dispersion and localized pulverization that prevents particle agglomeration. This hierarchical architecture creates an enlarged contact interface, inhibits rGO restacking through an effective spacer mechanism, and promotes rapid ion/electron transport. Such structural synergy is expected to significantly enhance catalytic activity, mechanical integrity, and long-term electrochemical stability of the composite electrode.\u003c/p\u003e\n \u003cp\u003eThe elemental composition and chemical homogeneity of the Gd₂(WO₄)₃\u0026ndash;P@rGO composite were confirmed by EDX spectroscopy (\u003cstrong\u003eFig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003eA\u003c/strong\u003e). The detected weight percentages of C (34.76%), O (14.34%), W (22.45%), and Gd (28.45%) closely match the theoretical stoichiometry of the hybrid, verifying the successful incorporation of P@rGO and Gd₂(WO₄)₃ without measurable deviation in composition. The absence of extraneous elemental signals further indicates high material purity and the effectiveness of the hydrothermal synthesis in preventing contamination or secondary phase formation. Elemental mapping images (\u003cstrong\u003eFig. \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003eB\u0026ndash;F\u003c/strong\u003e) reveal a uniform, co-distributed presence of C, O, W, and Gd throughout the entire matrix, demonstrating intimate interfacial coupling between the tungstate nanosheets and the conductive P-doped rGO scaffold. Such homogeneous dispersion is particularly important for electrochemical applications, as it maximizes the number of accessible catalytic sites, enhances the percolation network of electron pathways, and minimizes localized charge-transfer barriers. The resulting strong electronic coupling and continuous conductive framework are expected to significantly accelerate electron transport and, consequently, improve the sensor\u0026rsquo;s analytical performance toward ERDF detection.\u003c/p\u003e\n \u003cp\u003eFTIR spectroscopy was employed to elucidate the functional groups and interfacial interactions within the individual components and the hybrid composite (\u003cstrong\u003eFig. S2A\u003c/strong\u003e). The FTIR spectrum of pristine Gd₂(WO₄)₃ exhibits prominent bands at 1070, 880, and 720 cm⁻\u0026sup1;, corresponds to the asymmetric stretching of W\u0026ndash;O, the symmetric stretching of W\u0026ndash;O, and the characteristic Gd\u0026ndash;O lattice vibrations, respectively [35]. These bands represent the typical vibrational fingerprints of rare-earth tungstates, confirming the formation of the expected crystalline tungstate framework. For P@rGO, absorption peaks at 3420, 1690, 1455, and 1180 cm⁻\u0026sup1; are assigned to O\u0026ndash;H stretching, C\u0026thinsp;=\u0026thinsp;O stretching, O\u0026ndash;H bending, and P\u0026thinsp;=\u0026thinsp;O stretching vibrations, respectively, reflecting the presence of residual oxygenated groups and successful phosphorus doping. In the spectrum of the Gd₂(WO₄)₃\u0026ndash;P@rGO composite, all major tungstate-related bands remain present, indicating the structural integrity of Gd₂(WO₄)₃ upon hybridization. Notably, subtle shifts in band positions and observable variations in peak intensities are detected for both WO₄\u0026sup2;⁻ and P@rGO-related vibrations. These spectral modifications signify strong interfacial coupling between the tungstate nanosheets and the P-doped rGO matrix, likely arising from electrostatic interactions, surface complexation, and partial charge transfer at the interface. The preservation of all characteristic vibrational modes, combined with these shifts, provides compelling evidence for successful composite formation and intimate chemical integration, which is expected to enhance electron transport and catalytic activity in subsequent electrochemical applications.\u003c/p\u003e\n \u003cp\u003eThe crystalline structures of Gd₂WO₆, P@rGO, and the Gd₂(WO₄)₃\u0026ndash;P@rGO composite were examined by X-ray diffraction (\u003cstrong\u003eFig. S2B\u003c/strong\u003e). The diffraction pattern of pristine Gd₂WO₆ displays well-defined peaks at 18.9\u0026deg;, 27.6\u0026deg;, 32.6\u0026deg;, 35.4\u0026deg;, 47.8\u0026deg;, 53.6\u0026deg;, and 58.4\u0026deg;, which correspond to the (011), (321), (040), (600), (-602), (-361), and (-921) planes, respectively. These reflections match the orthorhombic Gd₂(WO₄)₃ phase, in agreement with JCPDS card No. 00-023-1074 [36], confirming the successful synthesis of a highly crystalline tungstate lattice. The XRD profile of P@rGO exhibits a broadened (002) reflection at 24.8\u0026deg;, indicative of turbostratic stacking and partial restoration of the graphene sp\u0026sup2; network after reduction, along with a weak (100) peak at 43.7\u0026deg;, characteristic of in-plane graphitic ordering [37]. In the XRD pattern of the Gd₂(WO₄)₃\u0026ndash;P@rGO composite, all major reflections associated with the Gd₂WO₆ phase remain clearly observable, confirming that the intrinsic crystal structure of the tungstate nanodomains is preserved during hybridization. However, the peaks exhibit noticeable weakening and broadening relative to pure Gd₂WO₆. This attenuation can be attributed to several synergistic effects: (i) restricted crystallite growth due to confinement on the P@rGO surface, (ii) enhanced dispersion of smaller tungstate domains across the conductive matrix, and (iii) strong interfacial interactions that modify local crystallinity. The absence of any additional peaks or secondary phases further verifies the purity of the composite. These observations collectively demonstrate the successful integration of Gd₂WO₆ with the P@rGO scaffold, forming a structurally coherent hybrid in which nanoscale confinement and graphene coupling are expected to enhance electron mobility and electrochemical performance.\u003c/p\u003e\n \u003cp\u003eXPS analysis was conducted to elucidate the surface composition and electronic states of the Gd₂(WO₄)₃\u0026ndash;P@rGO hybrid (\u003cstrong\u003eFig. S3\u003c/strong\u003e). The survey spectrum (\u003cstrong\u003eFig. S3A\u003c/strong\u003e) displays distinct signals corresponding to C 1s, O 1s, 2 P 2p, W 4f, and Gd 4d, confirming the coexistence of the expected elements and the successful formation of the composite without detectable extraneous species. High-resolution spectra further resolved the chemical environments of the constituent elements. The C 1s profile (\u003cstrong\u003eFig. S3B\u003c/strong\u003e) was deconvoluted into contributions from sp\u0026sup2;-hybridized C\u0026ndash;C/C\u0026thinsp;=\u0026thinsp;C (\u0026asymp;\u0026thinsp;286.8 eV), C\u0026ndash;O (\u0026asymp;\u0026thinsp;287.3 eV), and C\u0026thinsp;=\u0026thinsp;O/P\u0026thinsp;=\u0026thinsp;O (\u0026asymp;\u0026thinsp;291.2 eV) functionalities [38], indicating partial retention of oxygenated groups on P-doped rGO, which may facilitate interfacial bonding with tungstate domains. The O 1s spectrum (\u003cstrong\u003eFig. S3C\u003c/strong\u003e) exhibited two major components: a lower-binding-energy peak at \u0026asymp;\u0026thinsp;533.6 eV assigned to lattice oxygen in Gd\u0026ndash;O and W\u0026ndash;O bonds, and a higher-binding-energy contribution at \u0026asymp;\u0026thinsp;535.4 eV arising from oxygen in C\u0026ndash;O, C\u0026thinsp;=\u0026thinsp;O, and O\u0026ndash;C\u0026thinsp;=\u0026thinsp;O surface groups [39]. This dual-oxygen signature demonstrates the intimate interaction between metal-oxide structures and functionalized graphene frameworks. The P 2p spectrum (\u003cstrong\u003eFig. S3D\u003c/strong\u003e) explores two prominent peaks at 124.6 eV and 124.8 eV, corresponding to PO\u003csub\u003e3\u003c/sub\u003e-C and PO\u003csub\u003e3\u003c/sub\u003e-O-C-, respectively [40]. The W 4f region (\u003cstrong\u003eFig. S3E\u003c/strong\u003e) features a well-defined spin-orbit doublet at \u0026asymp;\u0026thinsp;42.8 eV (W 4f₇/₂) and \u0026asymp;\u0026thinsp;45.7 eV (W 4f₅/₂), characteristic of W⁶⁺ species in tungstate lattices [41], confirming that tungsten retains its expected oxidation state within the composite. Meanwhile, the Gd 4d spectrum (\u003cstrong\u003eFig. S3F\u003c/strong\u003e) reveals peaks at \u0026asymp;\u0026thinsp;149.5 eV (4d₅/₂) and \u0026asymp;\u0026thinsp;153.8 eV (4d₃/₂), consistent with the presence of Gd\u0026sup3;⁺ cations in Gd₂(WO₄)₃ [42]. The absence of any shifted or additional components suggests that no substoichiometric or mixed-valence states are formed during synthesis. Collectively, the XPS results validate the successful incorporation of Gd₂(WO₄)₃ nanostructures onto the P-doped rGO scaffold and confirm the preservation of the expected oxidation states. The coexistence of metal\u0026ndash;oxygen lattice bonds and residual oxygenated functionalities on rGO indicates strong interfacial coupling, which is anticipated to enhance charge transport, increase active-site accessibility, and improve the electrochemical response toward ERDF detection.\u003c/p\u003e\n \u003cp\u003eAn analysis of surface area and porosity was conducted via nitrogen adsorption-desorption (\u003cstrong\u003eFig. S4\u003c/strong\u003e). Gd₂(WO₄)₃ displayed a non-porous morphology, evidenced by a flat isotherm and a low surface area of 7.435 m\u0026sup2;/g. Conversely, P@rGO exhibited a Type IV isotherm with a distinct hysteresis loop, confirming its mesoporous nature and high surface area of 38.45 m\u0026sup2;/g. The Gd₂(WO₄)₃\u0026ndash;P@rGO composite preserved this mesoporous framework, suggesting that the integration of Gd₂(WO₄)₃ did not significantly occlude the pore structure. The composite\u0026apos;s intermediate surface area of 12.78 m\u0026sup2;/g implies enhanced reactant accessibility and promising electrochemical properties compared to the pure Gd₂(WO₄)₃.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Electrochemical characterization\u003c/h2\u003e\n \u003cp\u003eTo rigorously assess the performance of the engineered sensing interfaces, their interfacial electron-transfer characteristics were examined via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV profiles recorded in the Fe(CN)₆\u0026sup3;⁻/⁴⁻ redox probe showed a substantial enhancement in peak current for the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE, reflecting the formation of a highly conductive and electroactive surface (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eA). This improvement can be attributed to the synergistic interplay between the redox-active Gd₂(WO₄)₃ nanostructure\u0026mdash;which provides abundant catalytic sites\u0026mdash;and the phosphorus-doped rGO framework, known for its accelerated charge mobility, defect-rich surface, and improved electronic density around the Fermi level. EIS measurements further substantiated this behavior by directly quantifying the charge-transfer resistance (Rct) (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eB). The Nyquist plots revealed a dramatic reduction in Rct for the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE (276.8 Ω), compared with the significantly higher resistance of the unmodified GCE (987.8 Ω). This decline in interfacial resistance indicates fast electron shuttling between the redox probe and the electrode surface, a hallmark of efficient electrocatalyst\u0026ndash;carbon composite systems. The drastic enhancement in conductivity and charge-transfer kinetics confirms that coupling Gd₂(WO₄)₃ with P-doped graphene not only improves electrical pathways but also maximizes the electroactive surface area, ultimately enabling superior analytical sensitivity in subsequent sensing applications.\u003c/p\u003e\n \u003cp\u003eThe effective active surface area (EASA) of the fabricated electrodes was estimated from the cyclic voltammetric responses using the Randles\u0026ndash;\u0026Scaron;evč\u0026iacute;k equation, enabling a quantitative assessment of how surface morphology and interfacial architecture influence their electrochemical performance. By correlating peak current with scan rate (20\u0026ndash;200 mV/s), this analysis provides a reliable measure of the accessible electroactive sites, thereby allowing direct comparison of the catalytic efficiency imparted by each modification layer (\u003cstrong\u003eFig.S5A-D\u003c/strong\u003e). The EASA results demonstrated a pronounced enhancement in active surface area upon electrode modification, establishing a consistent performance hierarchy among the tested interfaces. The Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE exhibited the largest EASA (0.185 cm\u0026sup2;), reflecting the strong synergistic coupling between the catalytically active Gd₂(WO₄)₃ domains and the conductive, defect-enriched P@rGO scaffold. This was followed by the P@rGO/GCE electrode (0.156 cm\u0026sup2;), whose expanded surface and high electron mobility contributed to its improved performance. Electrodes modified solely with Gd₂(WO₄)₃ displayed a more moderate increase (0.127 cm\u0026sup2;), indicating that although Gd₂(WO₄)₃ introduces additional catalytic sites, its intrinsic conductivity is insufficient to maximize interfacial utilization without a graphene-based support. All modified electrodes surpassed the bare GCE (0.087 cm\u0026sup2;), confirming that surface engineering and composite formation substantially elevate the density of electroactive sites available for charge transfer.\u003c/p\u003e\n \u003cp\u003eTechniques such as CV and DPV were systematically performed to elucidate the redox behavior of ERDF at the modified interfaces (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e). Control scans recorded in analyte-free electrolyte exhibited no discernible faradaic peaks, confirming negligible background currents and ensuring that the subsequent responses originated solely from ERDF oxidation. Upon introducing ERDF, a pronounced and sharp anodic peak was observed, with the highest current intensity obtained at the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE. This substantial enhancement reflects the strong synergistic interaction between Gd₂(WO₄)₃ and phosphorus-doped rGO, where Gd₂(WO₄)₃ contributes abundant redox-active sites while P-doping introduces additional defects, increases electrical conductivity, and promotes faster charge transfer at the electrode\u0026ndash;solution interface. The resulting composite provides a larger electroactive surface area, improved electron mediation pathways, and stronger adsorption affinity toward ERDF, collectively leading to amplified oxidation signals. Compared with CV, DPV exhibited superior peak resolution and a markedly higher signal-to-noise ratio, consistent with its well-recognized sensitivity for trace-level electroanalysis; therefore, DPV was selected as the optimal technique for quantitative determination of ERDF [43].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Optimization of conditions\u003c/h2\u003e\n \u003cp\u003eTo achieve maximum electrocatalytic efficiency, the loading concentration and deposition volume of the Gd₂(WO₄)₃\u0026ndash;P@rGO suspension were systematically optimized. A series of dispersions with increasing concentrations in DMF were evaluated (\u003cstrong\u003eFig. S6A\u003c/strong\u003e). The anodic peak current increased progressively with concentration, reaching its maximum at 5.0 mg mL⁻\u0026sup1; (equivalent to 2.5 mg mL⁻\u0026sup1; of each component). This enhancement is attributed to the higher density of electroactive sites, improved electron-transport pathways introduced by P-doped rGO, and the greater probability of ERDF adsorption at the composite surface. Beyond this concentration, a noticeable decline in peak current occurred, most likely due to particle agglomeration, excessive film thickness, and partial blockage of active sites\u0026mdash;phenomena known to impede mass transport and hinder charge transfer in densely loaded films. Similarly, the deposition volume was optimized by drop-casting different aliquots (2\u0026ndash;8 \u0026micro;L) of the 5.0 mg mL⁻\u0026sup1; dispersion onto the GCE surface (\u003cstrong\u003eFig. S6B\u003c/strong\u003e). The peak current increased with volume up to 5 \u0026micro;L, reflecting the formation of a uniform catalytic layer with sufficient conductive pathways. Additional loading beyond this volume produced a decrease in signal intensity, which can be explained by the development of overly thick films that introduce diffusion barriers, slow down electron migration, and reduce the efficiency of analyte\u0026ndash;electrode interactions. Based on these trends, a concentration of 5.0 mg mL⁻\u0026sup1; and a deposition volume of 5 \u0026micro;L were identified as the optimal conditions for fabricating high-performance Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE sensors in subsequent analyses.\u003c/p\u003e\n \u003cp\u003eThe electrochemical oxidation behavior of ERDF was further examined using cyclic voltammetry over a wide range of scan rates (20\u0026ndash;220 mV s⁻\u0026sup1;) (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eA). A progressive shift of the anodic peak toward more positive potentials was observed as the scan rate increased, confirming the irreversible nature of the electron-transfer process. Concurrently, the anodic peak current (Ipa) increased proportionally with the square root of the scan rate (\u0026nu;\u0026sup1;ᐟ\u0026sup2;), following the linear regression equation Ipa\u0026thinsp;=\u0026thinsp;259.44\u0026nu;\u003csup\u003e\u0026frac12;\u003c/sup\u003e\u0026thinsp;+\u0026thinsp;0.7248 (R\u0026sup2; = 0.9973). This strong linearity indicates that the oxidation of ERDF is predominantly governed by a diffusion-controlled mechanism rather than surface-confined kinetics [44]. The diffusion-limited nature of the process was further validated by the excellent correlation coefficient and the absence of saturation effects across the applied scan-rate window (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eB). A continuous positive shift in the anodic peak potential (Epa) with increasing scan rate was observed, further supporting the irreversible nature of the electron-transfer step involved in ERDF oxidation [45]. This behavior is characteristic of systems in which the oxidation kinetics cannot keep pace with the applied potential sweep, resulting in a rate-dependent displacement of the peak position. To corroborate this interpretation, a log(Ipa)\u0026ndash;log(\u0026nu;) analysis was conducted (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eC). The resulting slope of 0.5848 (R\u0026sup2; = 0.9997) aligns closely with the theoretical value of 0.5 expected for diffusion-controlled processes, thereby confirming that mass transport\u0026mdash;rather than adsorption or surface confinement\u0026mdash;governs the electrochemical oxidation of ERDF under the examined conditions [46]. The excellent linearity additionally indicates consistent diffusional behavior across all tested scan rates, with no evidence of mixed or adsorption-controlled contributions. Application of Laviron\u0026rsquo;s kinetic theory to the linear dependence of Epa on ln \u0026nu; (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eD) enabled quantitative evaluation of the electron-transfer parameters. The calculated slope and intercept yielded an electron-transfer coefficient (\u0026alpha;) consistent with an irreversible system, and the number of electrons involved in the rate-determining step was determined to be approximately one. Coupled with the pH-dependent peak potential shift (discussed earlier), these results confirm that ERDF undergoes a coupled one-electron/one-proton (1e⁻/1H⁺) oxidation pathway. The combination of scan-rate analysis, log\u0026ndash;log correlation, and Laviron modeling collectively verifies that the overall process is irreversible and governed primarily by diffusion rather than surface adsorption. The superior performance of the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE in facilitating this oxidation can be attributed to its engineered physicochemical features: the high surface area increases accessible electroactive sites, Gd₂(WO₄)₃ provides redox-active centers, and phosphorus-doped rGO enhances electrical conductivity while minimizing charge-transfer resistance. These synergistic properties accelerate electron-transport kinetics and promote efficient interaction between ERDF and the electrode surface, ultimately enabling sensitive and reliable electrochemical detection.\u003c/p\u003e\n \u003cp\u003eTo further optimize the electrochemical response, the influence of solution pH on ERDF oxidation was systematically investigated. The anodic peak current displayed a strong dependence on proton concentration, reaching its maximum intensity at pH 5.5, after which it gradually decreased at more alkaline conditions (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eA). The decline in current at higher pH values can be attributed to reduced proton availability, slower proton-coupled electron transfer kinetics, and possible shifts in ERDF speciation that weaken its electrochemical activity. In parallel, the anodic peak potential (Epa) shifted linearly toward more negative values with increasing pH, yielding a slope of approximately \u0026minus;\u0026thinsp;59 mV per pH unit (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eB). This slope closely matches the theoretical Nernstian value for electrochemical processes involving an equal number of protons and electrons, thereby confirming that ERDF oxidation proceeds via a 1H⁺/1e⁻ proton-coupled electron-transfer mechanism [47, 48]. The excellent linearity further indicates that proton participation is integral to the rate-determining step of the oxidation process. Based on the combined trends of maximum signal intensity and favorable redox kinetics, pH 5.5 was selected as the optimal medium for all subsequent electroanalytical measurements.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4. Quantitative data\u003c/h2\u003e\n \u003cp\u003eDifferential pulse voltammetry (DPV) was employed to quantify ERDF over a broad concentration range (0\u0026ndash;800 nM) using the Gd₂(WO₄)₃\u0026ndash;P@rGO-modified GCE in 0.1 M Britton\u0026ndash;Robinson buffer at pH 5.5 (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eA). A well-defined anodic peak centered at approximately 0.85 V (vs. Ag/AgCl) reflects the irreversible oxidation of ERDF and confirms efficient electrocatalytic mediation by the hybrid interface. The anodic peak current increased proportionally with analyte concentration, demonstrating rapid charge-transfer kinetics and stable surface adsorption behavior. The resulting calibration plot (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eB) exhibited excellent linearity, described by Ipa (\u0026micro;A)\u0026thinsp;=\u0026thinsp;0.1302 [ERDF]\u0026thinsp;+\u0026thinsp;7.9447 (R\u0026sup2; = 0.9980), supporting the reliability of quantitative determination across ultra-trace to sub-micromolar levels. The limit of detection, estimated at S/N\u0026thinsp;=\u0026thinsp;3, was as low as 0.0024 nM, positioning the sensor among the most sensitive electrochemical platforms reported for ERDF to date. This remarkable sensitivity can be attributed to the synergistic contributions of the composite: (i) phosphorus-doped rGO enhances electrical conductivity, defect density, and \u0026pi;\u0026ndash;\u0026pi; interaction with the aromatic ERDF structure; (ii) Gd₂(WO₄)₃ provides abundant redox-active sites and facilitates fast electron shuttling; and (iii) the hierarchical architecture increases the electroactive surface area and promotes rapid mass transport. Collectively, these features minimize overpotential, accelerate electron transfer, and enable efficient pre-concentration of ERDF at the electrode\u0026ndash;solution interface. Such performance confirms the suitability of the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE for trace detection in complex matrices, providing an analytical window relevant to therapeutic monitoring, pharmacokinetic studies, and environmental surveillance of anticancer residues.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eTable 1\u003c/strong\u003e presents a comparative evaluation of the analytical performance of the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE against previously reported methods for ERDF determination. The proposed platform demonstrates a remarkably broad linear dynamic range together with an ultralow detection limit, outperforming most electrochemical, chromatographic, and spectroscopic approaches cited in the literature. Such advancement highlights the analytical superiority of the hybrid system, particularly for monitoring ERDF at clinically and environmentally relevant trace levels.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eTable 1.\u0026nbsp;\u003c/strong\u003ePerformance comparison of Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE with reported ERDF detection techniques.\u003c/p\u003e\n \u003ctable id=\"Taba\" border=\"1\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTechnique\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSensor\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eLinear range (nM)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eLOD (nM)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eReference\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eUPLC-MS/MS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-------\u003c/p\u003e\n \u003cp\u003e-------\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1-2240\u003c/p\u003e\n \u003cp\u003e2.23-1119.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e------\u003c/p\u003e\n \u003cp\u003e------\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[6]\u003c/p\u003e\n \u003cp\u003e[7]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLC-MS/MS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-------\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6-2240\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e------\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[8]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eHPLC/UV\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-------\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e110\u0026ndash;4470\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e------\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[9]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSpectrofluorometry\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eKolliphor RH 40\u003c/p\u003e\n \u003cp\u003eNitrogen-doped carbon dots\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e110\u0026ndash;1790\u003c/p\u003e\n \u003cp\u003e44\u0026ndash;600,000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003cp\u003e13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[49]\u003c/p\u003e\n \u003cp\u003e[50]\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eVoltammetry\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eNPCS/GCE\u003c/p\u003e\n \u003cp\u003eGd₂(WO₄)₃\u0026ndash;P@rGO/GCE\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e100\u0026ndash;7380\u003c/p\u003e\n \u003cp\u003e0.01\u0026ndash;800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e11.2\u003c/p\u003e\n \u003cp\u003e0.0024\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[44]\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eThis work\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5. Stability, reproducibility, and repeatability\u003c/h2\u003e\n \u003cp\u003eThe long-term operational stability of the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE was evaluated by monitoring its DPV response over a 60-day storage period under ambient conditions (\u003cstrong\u003eFig. S7A\u003c/strong\u003e). The electrode retained 85.87% of its initial peak current, demonstrating excellent resistance to surface passivation and structural deterioration. This sustained performance is attributed to the chemical robustness of the Gd₂(WO₄)₃ matrix and the anchoring effect of P-doped rGO, which suppresses nanoparticle aggregation, preserves conductivity, and prevents oxidative degradation\u0026mdash;issues commonly encountered in rare-earth\u0026ndash;oxide-based sensors.\u003c/p\u003e\n \u003cp\u003eReproducibility was assessed using seven independently prepared electrodes, yielding a low relative standard deviation of 3.56% (\u003cstrong\u003eFig. S7B\u003c/strong\u003e). Such narrow variability reflects high batch-to-batch uniformity and confirms the reliability of the synthesis and modification protocol. This level of reproducibility is critical for large-scale fabrication, ensuring sensor consistency for regulated analytical settings and potential commercialization.\u003c/p\u003e\n \u003cp\u003eRepeatability was further examined through ten consecutive DPV measurements in 100 nM ERDF, resulting in an RSD of only 2.34% (\u003cstrong\u003eFig. S7C\u003c/strong\u003e). The minimal signal fluctuation and absence of peak distortion indicate stable electron-transfer kinetics and negligible electrode fouling during repeated use. Efficient analyte desorption and the defect-rich P@rGO surface likely mitigates signal drift by preventing accumulation at the interface.\u003c/p\u003e\n \u003cp\u003eCollectively, the excellent stability, reproducibility, and repeatability affirm the robustness of the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE and support its suitability for routine analytical applications, particularly those requiring prolonged storage, frequent measurements, and dependable inter-electrode performance\u0026mdash;such as therapeutic drug monitoring, pharmaceutical quality control, and environmental screening of anticancer residues.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e3.6. Selectivity and anti-interference\u003c/h2\u003e\n \u003cp\u003eThe selectivity of the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE toward ERDF was investigated by conducting DPV measurements in the absence and presence of the target analyte in complex mixtures containing potential interferents (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e). The tested species included abundant inorganic ions, endogenous biomolecules, and structurally related anticancer agents at elevated concentrations (1.0 mM), representing a\u0026thinsp;\u0026gt;\u0026thinsp;10,000-fold excess relative to ERDF (100 nM). Under these challenging conditions, only ERDF generated a well-defined oxidation peak, whereas all interferents produced negligible current changes, confirming the absence of overlapping electrochemical signatures. The pronounced specificity can be attributed to several factors: (i) strong \u0026pi;\u0026ndash;\u0026pi; and electrostatic interactions between ERDF\u0026rsquo;s aromatic structure and the defect-rich P-doped rGO surface; (ii) enhanced electron-transfer mediation by Gd₂(WO₄)₃, which selectively promotes ERDF oxidation at a distinct potential; and (iii) suppressed nonspecific adsorption due to the stable and hydrophilic hybrid interface. Together, these features minimize signal perturbation from coexisting species and prevent competitive surface occupation. These findings demonstrate that the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE possesses exceptional interference tolerance and molecular discrimination capability, supporting its applicability for accurate ERDF quantification in complex biological matrices such as serum and urine, where high ionic strength and biochemical diversity often compromise electrochemical sensing performance.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n \u003ch2\u003e3.7. Applications\u003c/h2\u003e\n \u003cp\u003eThe real-sample applicability of the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE platform was rigorously assessed through the quantification of ERDF in spiked human urine and serum. As summarized in \u003cstrong\u003eTable\u0026nbsp;2\u003c/strong\u003e, the sensor achieved excellent recoveries of 95.7\u0026ndash;104.6% (urine) and 97.4\u0026ndash;104.8% (serum), accompanied by low %RSD values of 2.65\u0026ndash;3.65%, demonstrating outstanding analytical accuracy and repeatability under physiological conditions. Notably, the absence of significant signal attenuation confirms that neither protein-binding interactions nor endogenous electroactive species interfered with the ERDF oxidation response. These findings highlight the sensor\u0026rsquo;s strong anti-matrix tolerance, which can be attributed to the synergistic electrocatalytic behavior of P-doped rGO\u0026mdash;enhancing electron transfer\u0026mdash;and the abundant active sites provided by Gd₂(WO₄)₃. Moreover, the obtained recovery performance aligns with internationally recognized bioanalytical validation criteria (e.g., FDA and ICH guidelines, allowing 85\u0026ndash;115% recovery and \u0026le;\u0026thinsp;15% precision), thereby supporting the method\u0026rsquo;s suitability for routine clinical monitoring and therapeutic drug assessment. Collectively, these results establish the Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE sensor as a robust, highly reliable platform for ERDF determination in complex biological matrices, paving the way for future applications in point-of-care diagnostics, pharmacokinetic studies, and real-world patient sample analysis.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eTable 2.\u0026nbsp;\u003c/strong\u003eDetermination of ERDF in urine and serum samples (n=3).\u003c/p\u003e\n \u003ctable id=\"Tabb\" border=\"1\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\u0026nbsp;\u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eSpiked (nM)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003eGd₂(WO₄)₃\u0026ndash;P@rGO/GCE\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"3\"\u003e\n \u003cp\u003eUPLC-MS/MS [7]\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFound (nM)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRecovery %\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRSD %\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFound (nM)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRecovery %\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRSD %\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eUrine\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.0\u003c/p\u003e\n \u003cp\u003e5.0\u003c/p\u003e\n \u003cp\u003e7.5\u003c/p\u003e\n \u003cp\u003e10.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.56\u003c/p\u003e\n \u003cp\u003e5.23\u0026thinsp;\u0026plusmn;\u0026thinsp;0.22\u003c/p\u003e\n \u003cp\u003e7.67\u0026thinsp;\u0026plusmn;\u0026thinsp;0.55\u003c/p\u003e\n \u003cp\u003e9.76\u0026thinsp;\u0026plusmn;\u0026thinsp;0.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e95.7\u003c/p\u003e\n \u003cp\u003e104.6\u003c/p\u003e\n \u003cp\u003e102.2\u003c/p\u003e\n \u003cp\u003e97.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.24\u003c/p\u003e\n \u003cp\u003e2.67\u003c/p\u003e\n \u003cp\u003e3.28\u003c/p\u003e\n \u003cp\u003e3.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.25\u003c/p\u003e\n \u003cp\u003e4.88\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19\u003c/p\u003e\n \u003cp\u003e7.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.76\u003c/p\u003e\n \u003cp\u003e9.53\u0026thinsp;\u0026plusmn;\u0026thinsp;0.66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e105.7\u003c/p\u003e\n \u003cp\u003e97.6\u003c/p\u003e\n \u003cp\u003e99.1\u003c/p\u003e\n \u003cp\u003e95.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.43\u003c/p\u003e\n \u003cp\u003e2.42\u003c/p\u003e\n \u003cp\u003e2.87\u003c/p\u003e\n \u003cp\u003e3.24\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSerum\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.0\u003c/p\u003e\n \u003cp\u003e5.0\u003c/p\u003e\n \u003cp\u003e7.5\u003c/p\u003e\n \u003cp\u003e10.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.18\u003c/p\u003e\n \u003cp\u003e4.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.19\u003c/p\u003e\n \u003cp\u003e7.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.45\u003c/p\u003e\n \u003cp\u003e10.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e102.7\u003c/p\u003e\n \u003cp\u003e97.4\u003c/p\u003e\n \u003cp\u003e103.7\u003c/p\u003e\n \u003cp\u003e104.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.65\u003c/p\u003e\n \u003cp\u003e3.07\u003c/p\u003e\n \u003cp\u003e2.65\u003c/p\u003e\n \u003cp\u003e3.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.87\u0026thinsp;\u0026plusmn;\u0026thinsp;0.24\u003c/p\u003e\n \u003cp\u003e5.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.43\u003c/p\u003e\n \u003cp\u003e7.89\u0026thinsp;\u0026plusmn;\u0026thinsp;0.83\u003c/p\u003e\n \u003cp\u003e9.45\u0026thinsp;\u0026plusmn;\u0026thinsp;0.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e95.7\u003c/p\u003e\n \u003cp\u003e101.6\u003c/p\u003e\n \u003cp\u003e105.2\u003c/p\u003e\n \u003cp\u003e94.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.87\u003c/p\u003e\n \u003cp\u003e2.56\u003c/p\u003e\n \u003cp\u003e3.29\u003c/p\u003e\n \u003cp\u003e4.13\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eA novel Gd₂(WO₄)₃ nanostructure was successfully fabricated through a controlled hydrothermal route and subsequently integrated with phosphorus-doped reduced graphene oxide (P@rGO) via ultrasonication to construct a hybrid electroactive interface for ERDF sensing. This architecture exploits the complementary functions of both components: the redox-active Gd₂(WO₄)₃ provides abundant catalytic sites, while P-doping enhances the intrinsic conductivity and defect density of rGO, facilitating rapid charge transfer and improved adsorption of ERDF molecules. The resulting Gd₂(WO₄)₃\u0026ndash;P@rGO/GCE exhibited markedly enhanced electrochemical behavior compared to its individual counterparts. Electrochemical characterization by CV and DPV revealed a well-defined oxidation peak with a broad linear response range of 0.01\u0026ndash;800 nM (R\u0026sup2; = 0.9980) and an ultralow detection limit of 0.0024 \u0026micro;M (S/N\u0026thinsp;=\u0026thinsp;3), reflecting the sensor\u0026rsquo;s high sensitivity and fast electron-transfer kinetics. In addition, the modified electrode maintained excellent operational stability and reproducibility across multiple measurements, with negligible performance drift over prolonged storage. The selectivity studies confirmed strong anti-interference capability, as common endogenous biomolecules and structurally related drugs produced no significant signal perturbation. The platform\u0026rsquo;s practical utility was further demonstrated through ERDF quantification in spiked human urine and serum samples, achieving accurate recoveries with minimal matrix-induced suppression, thereby fulfilling FDA/ICH bioanalytical criteria. Taken together, these findings position the Gd₂(WO₄)₃\u0026ndash;P@rGO hybrid as a promising sensing material for clinical bioanalysis, therapeutic drug monitoring, and potential translation into scalable point-of-care diagnostic systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eRasha M. K. Mohamed, Rania H Taha\u003c/strong\u003e: Writing-original draft, Project administration, Funding acquisition, Conceptualization. \u003cstrong\u003eIbrahim Hotan Alsohaimi, Khulaif Alshammari\u003c/strong\u003e: Investigation, Formal analysis, Data curation. \u003cstrong\u003eHassan M. A. Hassan, Hossieny Ibrahim\u003c/strong\u003e: Software, Investigation, Formal analysis. \u003cstrong\u003eMohamed M. El-Wekil\u003c/strong\u003e: Writing-review \u0026amp; editing, Supervision, Methodology.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was funded by the Deanship of Graduates Studies and Scientific research at Jouf University under grant No. (DGSSR-2025-02-01417).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eS. Alouini, Risk factors associated with urothelial bladder cancer. Int. J. Environ. Res. Public Health, 21 (2024), p. 954. \u003cstrong\u003e\u003cu\u003e10.3390/ijerph21070954\u003c/u\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003cli\u003eS. Antoni, J. Ferlay, I. Soerjomataram, A. Znaor, A. Jemal, F. Bray, Bladder cancer incidence and mortality: A global overview and recent trends. Eur. Urol., 71 (1) (2017), pp. 96-108. \u003cstrong\u003e\u003cu\u003e10.1016/j.eururo.2016.06.010\u003c/u\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003cli\u003eM. 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Spectros.154 (2020), Article 104555. \u003cstrong\u003e\u003cu\u003e10.1016/j.saa.2022.121327\u003c/u\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003cli\u003eM. N. Goda, L. S. Alqarni, H. Ibrahim, A. B. H. Ali, M. M. El-Wekil, Selective detection of tyrosine kinase inhibitor erdafitinib using nitrogen-doped carbon dots synthesized at room temperature. Anal. Methods, 17 (2025), pp. 5929-5938. \u003cstrong\u003e\u003cu\u003e10.1039/D5AY00833F\u003c/u\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Scheme","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"microchimica-acta","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"miac","sideBox":"Learn more about [Microchimica Acta](https://link.springer.com/journal/604)","snPcode":"604","submissionUrl":"https://submission.springernature.com/new-submission/604/3","title":"Microchimica Acta","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Erdafitinib, Gd₂(WO₄)₃, P@rGO, Synergism, Electrochemical analysis, Biological fluids","lastPublishedDoi":"10.21203/rs.3.rs-8213846/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8213846/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eErdafitinib (ERDF), a tyrosine-kinase inhibitor approved for metastatic urothelial carcinoma, possesses a narrow therapeutic index and risk of severe adverse effects, making its precise quantification essential for therapeutic drug monitoring, pharmacokinetic profiling, pharmaceutical quality control, and environmental surveillance. To address the lack of highly sensitive and accessible analytical platforms, a hybrid Gd₂(WO₄)₃–P@rGO nanocomposite was fabricated using a hydrothermal synthesis followed by ultrasonication-assisted integration and subsequently immobilized onto a glassy carbon electrode (GCE). Comprehensive structural and chemical characterization (SEM, TEM, XRD, FT-IR, and XPS) verified the successful formation of a well-distributed heterostructure with enhanced defect density, abundant active sites, and improved electronic coupling between Gd₂(WO₄)₃ and P-doped rGO. Electrochemical assessment using CV and DPV demonstrated a significantly amplified ERDF oxidation response, yielding an ultralow detection limit of 0.0024 nM (S/N = 3), high sensitivity (16.670 µA nM⁻¹ cm⁻²), and a broad linear range spanning 0.01–800 nM (R² = 0.9980). The modified electrode demonstrated excellent selectivity against common interferents, along with strong operational stability and reproducibility. Validation in spiked human serum and urine showed accurate recoveries with minimal matrix effects, meeting internationally recognized bioanalytical standards. Scalable fabrication without toxic reducing agents further underscores the platform’s environmental and practical advantages. Collectively, these attributes position the Gd₂(WO₄)₃–P@rGO sensor as a promising tool for therapeutic drug monitoring, personalized dosing, pharmacokinetics, and future point-of-care diagnostics.\u003c/p\u003e","manuscriptTitle":"Engineering a Gd₂(WO₄)₃–P@rGO heterostructure for enhanced electrochemical sensing and therapeutic drug monitoring of erdafitinib","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-03 13:37:50","doi":"10.21203/rs.3.rs-8213846/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-18T12:58:43+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-18T07:09:04+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-16T06:01:07+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"51757815899059630375699774347823104103","date":"2025-12-13T06:33:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-12T07:18:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"4888866783202786150187495491864914705","date":"2025-12-12T00:08:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"328547735143579401175046761137683147785","date":"2025-12-11T16:58:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"7975511862040603947053509981586166204","date":"2025-12-11T13:48:32+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-01T14:22:50+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-27T09:38:49+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-27T01:48:45+00:00","index":"","fulltext":""},{"type":"submitted","content":"Microchimica Acta","date":"2025-11-26T14:03:41+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"microchimica-acta","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"miac","sideBox":"Learn more about [Microchimica Acta](https://link.springer.com/journal/604)","snPcode":"604","submissionUrl":"https://submission.springernature.com/new-submission/604/3","title":"Microchimica Acta","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1f520f7d-7e7d-4446-964e-92bc374fc1b2","owner":[],"postedDate":"December 3rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-23T16:04:55+00:00","versionOfRecord":{"articleIdentity":"rs-8213846","link":"https://doi.org/10.1007/s00604-026-07856-4","journal":{"identity":"microchimica-acta","isVorOnly":false,"title":"Microchimica Acta"},"publishedOn":"2026-02-18 15:58:36","publishedOnDateReadable":"February 18th, 2026"},"versionCreatedAt":"2025-12-03 13:37:50","video":"","vorDoi":"10.1007/s00604-026-07856-4","vorDoiUrl":"https://doi.org/10.1007/s00604-026-07856-4","workflowStages":[]},"version":"v1","identity":"rs-8213846","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8213846","identity":"rs-8213846","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}