# Nanoscale electrochemical sensing of tramadol with a polyoxometalate-modified electrode

Donald Eric Njiajo Talla, Astree Lottie Djuffo Yemene, Sherman Lesly

Zambou Jiokeng, Konstantin M Schüttler, Israel Mbomekalle, Boris Mizaikoff,

Ignas Kenfack Tonlé

► To cite this version:

Donald Eric Njiajo Talla, Astree Lottie Djuffo Yemene, Sherman Lesly Zambou Jiokeng, Konstantin M Schüttler, Israel Mbomekalle, et al.. Nanoscale electrochemical sensing of tramadol with a polyoxometalate-modified electrode. Journal of Electroanalytical Chemistry, 2025, 998, pp.119522. 10.1016/j.jelechem.2025.119522. hal-05450981

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# Nanoscale electrochemical sensing of tramadol with a polyoxometalate-modified electrode

Donald Eric Njiajo Talla<sup>a</sup>, Astree Lottie Djuffo Yemene<sup>a</sup>, Sherman Lesly Zambou Jiokeng<sup>b,*</sup>, Konstantin M. Schüttler<sup>c</sup>, Israel Mbomekalle<sup>d</sup>, Boris Mizaikoff<sup>b,e</sup>, Ignas Kenfack Tonlé<sup>a,*</sup>

## ARTICLE INFO

Keywords:

Tramadol detection
electrochemical sensor
polyoxometalate-modified electrode
square wave voltammetry
pharmaceutical analysis

## ABSTRACT

Ensuring the accuracy and reliability of analytical methods is essential to ensure the efficacy and safety of pharmaceutical treatments. This study describes a rapid and cost-effective electroanalytical nanosensing platform for tramadol detection, an opioid analgesic widely used in clinical settings but prone to misuse. The sensing material is based on a polyoxometalate, $[H_4PW_{17V}(IV)O_{62}]^{9-}$ (POM), which was successfully synthesized, then characterized by Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), powder X-ray diffraction (PXRD), and X-ray photoelectron spectroscopy (XPS) which together allow to confirm its morphology, composition, crystalline structure, and surface chemical states. A glassy carbon electrode (GCE) modified with this POM (POM/GCE) was evaluated by cyclic voltammetry using $[Fe(CN)_6]^{3-}$ and $[Ru(NH_3)_6]^{3+}$ as redox probes, and square wave voltammetry was employed for tramadol detection in aqueous solution. Key experimental parameters including pH, suspension volume, frequency, and accumulation time were optimized to enhance the sensor sensitivity. Under optimal conditions (pH 3, 3 µL modifier, 25 Hz, no accumulation time), the sensor exhibited a linear response from 0.06 to 0.78 µM ($R^2 = 0.998$), with a detection limit of 4.03 nM and quantification limit of 13.4 nM. The method showed a good reproducibility (RSD 2.2 %) and stability, and was successfully applied to the analysis of pharmaceutical formulations, yielding recovery rates between 97 and 101 %. In comparison to UV-Visible Spectroscopy, the proposed method demonstrated higher sensitivity, a lower detection limit, and improved selectivity for tramadol determination in pharmaceutical formulations. These advantages highlight its potential as a reliable alternative for routine analysis and drug quality control.

## 1. Introduction

Pain, an unpleasant sensation primarily caused by tissue damage, is influenced by psychological, behavioral, and social factors [1]. It can significantly impair quality of life by affecting the ability to work, sleep, and interact with others, while also causing emotional disturbances such as anxiety and depression [2]. Many drugs have been used to either reduce the impact of pain or eliminate it altogether, among which opioids are particularly notable. However, the long-term use and misuse of opioids lead to severe health issues. Opioids function by binding to opioid receptors located in the brain, spinal cord, and other organs, modulating sensations of pain and pleasure [2]. Among them, tramadol

(TRA) 2-(dimethylamino)methyl-1-(3-methoxyphenyl)cyclohexanol hydrochloride is widely employed to relieve moderate to severe pain [3]. TRA is also misused by individuals with substance use disorders as an anti-addictive agent; however, due to its action on opioid receptors in the central nervous system, cases of severe dependence have been reported [4,5]. In combination with opioids, antipyretics and analgesics, TRA can be used to treat advanced cancer [6]. Its consumption beyond 400 mg per day for adults can lead to severe adverse effects such as seizures, respiratory depression, and potentially fatal serotonin syndrome [7]. These risks underline the necessity of strict quality control for pharmaceutical products containing TRA. High-performance liquid chromatography (HPLC) [8,9] and UV-visible spectroscopy [10,11] are

<footnote><sup>a</sup> Department of Chemistry, Faculty of Science, University of Dschang, P.O. Box. 67, Dschang, Cameroon</footnote>

<footnote><sup>b</sup> Institute of Analytical and Bioanalytical Chemistry, University of Ulm, 89081 Ulm, Germany</footnote>

<footnote><sup>c</sup> Core Facility Elemental, Molecular, and Materials Analysis, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany</footnote>

<footnote><sup>d</sup> Institut de Chimie Physique – ICP Université Paris-Saclay – CNRS UMR 800, Paris, France</footnote>

<footnote><sup>e</sup> Hahn-Schickard, 89077 Ulm, Germany</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: sherman.jiokeng-zambou@uni-ulm.de (S.L.Z. Jiokeng), ignas.tonle@univ-dschang.org (I.K. Tonlé).</footnote>
among the most commonly used and reliable analytical techniques for
quality control of TRA. These methods are well established and offer
high accuracy and reproducibility. However, their application may be
limited in some contexts due to relatively high operational costs, long
analysis times and the requirement for sophisticated instrumentation
[12]. In addition, although they are generally robust, they do not always
offer optimal selectivity and sensitivity for specific analytical challenges
[13]. The ideal analytical tool for the applications should provide a fast
and reliable response, preferably at the point of sample collection, and
should be reasonably affordable. In this context, the development of
alternative, faster and more cost-effective analysis methods is of
particular interest. Electrochemical analysis appears to be a promising
approach, offering several advantages over conventional techniques.
These include rapid detection, low reagent and energy consumption,
ease of implementation and cost-effectiveness [14,15]. In addition,
electrochemical sensors can distinguish specific target species in com-
plex mixtures, with high sensitivity, good selectivity and miniaturiza-
tion potential, making them particularly suitable for in situ analysis
[16]. Various researchers have explored the use of electrochemical
methods for tramadol analysis with different modified sensors. How-
ever, the performance of electrochemical sensors is highly dependent on
the nature of the electrode-modifying materials and can be significantly
affected by matrix interferences. The composition of sensing layer plays
a crucial in determining the sensitivity and selectivity of electrochemical
sensor [15]. Chemically modified electrodes (CMEs) have been widely
considered as efficient tools for the determination of trace amounts of
important species. Notably, in 2019, Rokhsefid et al. developed a sensor
based on a carbon paste electrode modified with a nanocomposite
comprising gold nanoparticles, graphene nanosheets, and 4-hydroxy-2-
(triphenylphosphonio)phenolate [17]. Similarly, Saichanapan and col-
laborators designed a sensor using a glassy carbon electrode (GCE)
modified with graphene oxide nanoplatelets [18]. Given the constant
evolution of science, the emergence of new synthesized materials that
push current boundaries in terms of sensitivity, selectivity, and cost, and
the necessity of having diverse approaches to address a problem, we aim
to develop a new innovative electrochemical analysis method for tra-
madol detection using polyoxometalate (POM) as electrode material.
POMs are anionic metal clusters made of transition metals like W, Mo,
and V bonded to oxygen atoms, forming structures such as Keggin,
Dawson, or Anderson [19]. Their rapid and reversible redox transitions
make them ideal for sensors, energy storage, catalysis, and biotech-
nology applications [20,21]. In electrochemistry, POMs have been
shown effective to detect analytes such as phenols, amines, and phar-
maceuticals [22]. Similar approaches using advanced electrode mate-
rials have also been successfully applied for phenolic compound
detection, for example, the controllable selenization strategy for V<sub>2</sub>CT<sub>x</sub>
MXene-based multiphase electrodes enabling simultaneous detection of
hydroquinone and catechol in drinking water and beverages [23]. They
have notable attributes including thermal stability, high negative
charges, remarkable redox capabilities, unique ligand properties, po-
tential for organic grafting, and well-defined molecular structures. They
also exhibit catalytic properties and the ability to be modified and
fabricated into different composite materials [24]. These useful and
applicable properties of POMs make them excellent candidates for ap-
plications in various fields, especially electrochemistry [25–27]. In this
study, an electrochemical sensor was developed based on a Keggin-type
polyoxometalate [H<sub>4</sub>PW<sub>17</sub>V<sup>(IV)O<sub>62</sub></sup><sup>9-</sup>, for the sensitive and selective
detection of TRA. [H<sub>4</sub>PW<sub>17</sub>V<sup>(IV)O<sub>62</sub></sup><sup>9-</sup> was selected for its unique redox-
active properties, chemical stability, and the presence of a vanadium(IV)
center, which enhances its electrocatalytic activity [28]. This type of
polyoxometalate has also been employed for hydrogen production and
for the facile synthesis of metal nanoparticles [29,30]. The lacunary
Keggin structure provides multiple active sites and a high capacity for
electron transfer, facilitating effective interactions with electroactive
analytes such as tramadol. These features make [H<sub>4</sub>PW<sub>17</sub>V<sup>(IV)O<sub>62</sub></sup><sup>9-</sup>
especially suitable for electrochemical sensing applications. The central

objective of this work is to demonstrate that the deposition of this POM
onto a GCE significantly improves the sensor performance in terms of
sensitivity, selectivity, and operational stability; meanwhile leading to a
simple and low-cost analytical tool. Comparative studies between un-
modified and POM-modified GCE were conducted, along with system-
atic optimization of key parameters, including scan rate, pH of the
supporting electrolyte, film thickness, frequency, accumulation time,
and TRA concentration. Furthermore, the repeatability, detection
sensitivity, and long-term stability of the sensor were evaluated to
validate the reliability of the developed platform. Finally, the practical
applicability of the sensor was demonstrated via the quantification of
TRA in commercial pharmaceutical formulations.

## 2. Materials and methods

### 2.1. Materials and chemicals

All chemicals used were of analytical grade and employed without further purification. Tramadol hydrochloride (TRA), alumina (Al₂O₃), potassium hexacyanoferrate(III) (K₃[Fe(CN)₆], > 99 %), and hexaammineruthenium(III) chloride ([Ru(NH₃)₆]Cl₃, > 99 %) were obtained from Sigma-Aldrich. Methanol (CH₃OH, 99 %), acetic acid (CH₃COOH, 99.5 %), sodium acetate (CH₃COONa, 100.0 %), boric acid (H₃BO₃, 99 %), phosphoric acid (H₃PO₄, 99 %), potassium chloride (KCl, 37 %), dipotassium hydrogen phosphate (K₂HPO₄, 99 %), potassium dihydrogen phosphate (KH₂PO₄, 99 %), and sodium hydroxide (NaOH, 98 %) were sourced from Prolabo. Phosphate buffer (PB), Britton Robinson buffer (BRB) and acetate buffer (AB) solution were prepared and used for voltammetry analyses. The POM, [H₄PW₁₇V<sup>(IV)O<sub>62</sub></sup><sup>9-</sup> was prepared following a synthetic procedure previously reported [29,31]. Its electrochemical behavior was extensively characterized in the cited work, which showed no significant redox peaks in the potential window relevant to the electrooxidation of TRA. Consequently, these data were not repeated here, as they are not relevant to the scope of this study. Ultrapure water was used for solution preparation, and pH measurements were conducted using a VWR pH meter (pHenomenal) with a glass electrode.

### 2.2. Preparation of working electrode and electrochemical studies

The sensor was obtained using the “drop coating” method. The GCE was first polished with alumina paste and rinsed with a water-ethanol mixture to remove impurities. A dispersion of POM was prepared by mixing 1 mg of POM with 1 mL of distilled water, followed by 5 min sonication. Then, 3 µL (unless specified) of the dispersion was deposited onto the surface of the GCE and dried in an oven at 50 °C for 5 min. The resulting electrode was labeled POM/GCE, while the bare GCE was referred as GCE. Voltammetric measurements were performed at ambient temperature using an Autolab potentiostat connected to a computer for recording the voltammograms, in a standard three-electrode electrochemical cell configuration. The working electrodes were either the GCE or POM/GCE; a Silver/Silver chloride (Ag/AgCl) electrode served as the reference electrode while a stainless-steel auxiliary electrode was used. The electrochemical characterization provided insights into the ionic accumulation and exchange properties of the material. For this purpose, redox probe electroanalysis was conducted using the [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ and [Ru(NH₃)₆]³⁺/[Ru(NH₃)₆]²⁺ redox couples, selected for their ideal electrochemical behavior. The experiments were performed in 0.1 M KCl solution (supporting electrolyte) over a potential range of -0.6 V to 0.7 V for [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ and of -0.45 V to 0.1 V for [Ru(NH₃)₆]³⁺/[Ru(NH₃)₆]²⁺, with a scan rate of 50 mV/s. A potentiostat (Autolab) connected to a computer was used to record the voltammograms. SWV measurements were conducted in a 0.1 M buffer solution containing 1 mM TRA, with a potential range of +1 V to +1.8 V. All experiments were carried out in triplicate, and the data points represent the average of
three repeated experiments.

## 2.3. Material characterization

The synthesized POM was characterized by FT-IR spectroscopy in the mid-infrared region, 4000–400 cm⁻¹ using a commercial FT-IR spectrometer (Alpha Platinum, Bruker, Germany) equipped with a single-bounce diamond internal reflectance element. For morphological characterization, micrographs were obtained using a Helios Nanolab 600 (Thermo Fisher Scientific, USA). SEM images were recorded at 3.0 kV and 86 pA using a through-the-lens detector. The Powder X-ray diffraction (PXRD) result was obtained with a PANalyticalX'Pert Pro Diffractometer handled at 40 kV and 40 mA with Cu Kα radiation in the 2θ range of 10°-90°. XPS analysis was conducted using a Physical Electronics photoelectron spectrometer (PHI 5800 ESCA System) equipped with a hemispherical electron analyzer, a monochromatic Al Kα X-ray source (1486.6 eV), and a sample neutralizer to prevent sample charging. XPS measurements were performed using monochromatic Al Kα (1486.6 eV) radiation at an emission angle of 45°. For narrow-scans, 58.7 eV pass energy and 0.125 eV step width were used; for survey scans, 93.9 eV and 0.4 eV were used. The binding energy scale was calibrated using the C 1 s peak attributed to adventitious carbon (combined C—C and C—H signal) as a reference at a binding energy of 284.8 eV. Peak fitting and data analysis were performed using the CasaXPS software (version 2.3.26PR1.0). For XPS measurements, the POM powder sample was prepared on a conductive carbon pad. Sample neutralization was used to compensate for charging of the sample.

# 3. Results and discussion

## 3.1. Structure characterization of POM

The FT-IR spectrum (Fig. 1a) of the POM shows characteristic bands of the Dawson-type framework. Broad absorptions at 3540–3460 cm⁻¹ and 1615 cm⁻¹ correspond to O—H stretching and H—O—H bending, confirming hydration [32]. In the fingerprint region, bands at 1045, 931, 864, and 742 cm⁻¹ are assigned to P—O, W=O, W—O—W, and V—O vibrations, evidencing vanadium incorporation into the structure [33,34]. The SEM micrograph of [H₄PW₁₇V(IV)O₆₂]⁹⁻ (insert, Fig. 1a) shows a well-defined lamellar morphology, characterized by sharp-edged micrometric polygonal crystals, which is typical of Keggin-type polyoxometalates and their lacunar derivatives. These highly ordered and regular arrangements reflect the high crystallinity and structural integrity commonly reported for POM materials. The cracks observed between the platelets most likely result from mechanical stresses experienced during drying and sample preparation, a phenomenon frequently mentioned in the literature for thin-film and bulk POM samples. The XRD pattern of [H₄PW₁₇V(IV)O₆₂]⁹⁻ (Fig. 1b) displays sharp reflections at 2θ ≈ 8.8°, 10.1°, 18.4°, 25.8°, and 30.2°, consistent with Dawson-type POMs [20,35]. The presence of well-defined peaks

and absence of broad halos confirm its crystalline nature and successful V(IV) incorporation without disruption of the framework.

XPS, with its capability to study the elemental and chemical composition of the surface and near-surface region of a solid sample, was used to characterize the POM material (Fig. S1, Supporting Information). Table 1 provides an overview of the elemental composition of the POM sample. Besides the components of the POM (P, W, V, and O), only C could be detected by XPS, as shown by the survey spectrum of POM in Fig. S1 (Supporting Information). XPS narrow scans of the W(4f), V(2p), and O(1 s) regions were acquired and evaluated to derive further information on the oxidation state and chemical nature of the respective components. Both W(4f) and V(2p) exhibit strong chemical shifts, which allows for distinguishing different oxidation states based on their peak position [36,37]. Fig. 2a shows an XPS narrow scan of the W(4f) region. The spectra exhibit a dominant peak doublet of W(4f) at 36.1 eV binding energy as well as the corresponding W(5p₃/₂) feature at 42.0 eV, clearly indicative of W⁶⁺ [36]. The remaining XPS intensity at slightly lower BE, between 33.0 and 34.5 eV, might be indicative of W species in a slightly lower oxidation state. Note that these species might also originate from W⁶⁺ species reduced by X-ray irradiation during the measurement. Moreover, clear W⁴⁺ signals are expected to appear much sharper and show a clear doublet structure, which is not apparent from the measured data. For data evaluation in the V(2p) region, the peak fit model presented by Biesinger et al. [37] was used. In the O(1 s) region (Fig. 2b), three peaks ascribed to lattice oxygen (530.6 eV), hydroxyls (531.8 eV), and adsorbed water (533.1 eV), as well as an O(1 s) satellite (537.0 eV), are distinguished based on their binding energy [37]. These peaks correspond to 36.7 at.% lattice oxygen, 12.1 at.% OH and 5.98 at.% water. The inset in Fig. 2b shows the XPS narrow scan of the V(2p) region. A distinct V⁴⁺ signal is observed at binding energy of 516.4 eV, corresponds to 0.66 at.% V⁴⁺. On the high binding energy side of the V⁴⁺ signal, the fit model also suggests the presence of a V⁵⁺ component at 518.0 eV. Including this in the fitting, as shown in the inset of Fig. 2b, yields a theoretical V⁵⁺ concentration of 0.06 at.%. However, this value does not allow for a clear confirmation or exclusion of V⁵⁺ species.

## 3.2. Electrochemical characterization of the POM-modified GCE

The electrochemical characterization of modified electrodes provides a valuable method to understand the properties and behavior of materials. By analyzing electrochemical responses, particularly through voltammetric curves, the accumulation and ion exchange properties of the materials can be investigated. In this study, the complex ions [Fe

<table><caption>**Table 1**

Elemental composition of the POM sample determined by XPS, values given in at.%.</caption><thead><tr><th>Element</th><th>C</th><th>O</th><th>P</th><th>K</th><th>V</th><th>W</th></tr></thead><tbody><tr><td>XPS analysis</td><td>24.1</td><td>57.0</td><td>0.86</td><td>5.93</td><td>0.71</td><td>11.4</td></tr></tbody></table>

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** (a) FT-IR spectra, (b) Powder X-ray diffractogram of the POM. The inset in (a) corresponds to SEM image of the POM.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** XPS high-resolution spectra of the (a) W(4f) region, (b) O(1s) and V(2p) regions. The inset shows the V(2p) region in more detail.</figcaption></figure>

([CN]₆]³⁻ and [Ru(NH₃)₆]³⁺ were used as redox markers due to their well-defined electrochemical behavior, enabling verification of the ionic exchange properties of the developed sensor. The analysis of [Fe(CN)₆]³⁻ was conducted in the potential range of -0.6 V to 0.7 V in a 0.1 M KCl solution containing 1 mM [Fe(CN)₆]³⁻. The results are shown in Fig. 3a. The signals recorded on the bare GCE exhibited higher reversible intensities (Ipa = 14.65 µA, Ipc = 14.11 µA, Ipa/Ipc ≈ 1) compared to those on the modified GCE/POM electrode (Ipa = 7.10 µA, Ipc = 7.44 µA). This reduction in current intensity on GCE/POM suggests electrostatic repulsion between the negatively charged [Fe(CN)₆]³⁻ ions and the negatively charged surface of the POM-modified electrode, which limits electrochemical reactivity. Despite this repulsion, the oxidation and reduction signals of [Fe(CN)₆]³⁻ remain visible, likely due to the porous nature of the POM material. These pores may facilitate the accommodation of [Fe(CN)₆]³⁻ ions, allowing oxidation-reduction processes to occur. For [Ru(NH₃)₆]³⁺ (Fig. 3b), the voltammograms revealed higher peak current intensities on GCE/POM (Ipa = 21.23 µA,

Ipc = 21.30 µA) compared to GCE (Ipa = 13.01 µA, Ipc = 13.11 µA). This increase in signal intensity can be attributed to the electrostatic attraction between the positively charged [Ru(NH₃)₆]³⁺ ions and the negatively charged oxo groups of the POM. Additionally, the porous structure of the POM promotes the adsorption of [Ru(NH₃)₆]³⁺, leading to a progressive accumulation on the electrode surface. These findings are consistent with the observations of Gu and collaborators, who demonstrated the influence of ionic charge interactions and accumulation on the electrochemical properties of polyoxometalate cluster multilayers [38]. The results highlight the strong affinity for positively charged species of GCE/POM, while its interaction with negatively charged species is limited by electrostatic repulsion. This dual behavior underscores the potential of the modified electrode for selective electrochemical sensing applications.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Electrochemical response of GCE (i) and POM/GCE (ii): (a, b) Multicyclic voltammograms in 0.1 M KCl of 1 mM [Fe(CN)₆]³⁻ and [Ru(NH₃)₆]³⁺ (v = 50 mV/s, 50 cyclic scans); (c) Cyclic voltammograms (v = 25 mV/s), and (d) Square wave voltammograms in 0.1 M Pb solution (pH 3.3) in 1 mM TRA. Blank solution (Dark yellow), GCE (red) and POM/GCE (black). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
### 3.3. Electrochemical behavior of tramadol

To evaluate the effect of modification and the electrochemical behavior of TRA, its redox behavior was investigated by CV and SWV on GCE and POM/GCE (Fig. 3c,d). CV presented in Fig. 3c shows exclusively a single oxidation peak obtained on each electrode. The absence of a reduction signal shows that the TRA describes an irreversible system on these electrodes. Furthermore, the peak current gradually increases from 0.844 µA to 2.12 µA when going from GCE to POM/GCE, this with a decrease in potential from 1.549 V to 1.44 V. In those obtained in Fig. 3d, the peak current gradually increases from 4.124 µA to 11.3 µA when going from GCE to POM/GCE, and the potential decreases from 1.569 V to 1.473 V. This improvement represents an increase in current of the order of 2.51-fold in CV and 2.34-fold in SWV, followed by a decrease in potential characteristic of the electro-catalytic character of POM. These results obtained can be explained by the fact that, the POM by its structure porous promotes better adsorption of TRA molecules on the surface of the electrode [20]. This porous structure provides a larger specific surface area, thus allowing increased contact between the TRA molecules in solution and the catalytic material. In addition, the electrostatic attraction existing between the highly negatively charged POM [39], and TRA of opposite charge, strengthens their affinity. Overall, the synergistic effect of the material porosity and the electrostatic affinity promote a very good accumulation of tramadol, demonstrating the effectiveness of POM as a good mediator, as it facilitates the electronic transfer between the chemical species in solution and the electrode surface.

### 3.4. Effect of potential scan rate

The scan rate is a key parameter in CV, as it provides insights into the mechanism governing analyte transport to the electrode surface. To

achieve this, current responses were recorded at scan rates from 10 to 100 mV/s. The results are presented in Fig. 4(a-c), as illustrated in Fig. 4a, increasing the scan rates leads to higher oxidation current intensities and a shift of the peak potentials toward more positive (anodic) values. Furthermore, plotting the anodic peak current (Ipa) versus the square root of the scan rate (v¹/²) yields a linear relationship: Ipa = 4.51 × 10⁻⁷v¹/² - 6.88 × 10⁻⁷, with correlation coefficient R² = 0.998. This strong linearity (R² value obtained being close to 1) suggests that the analyte is transported to the electrode surface primarily by diffusion. To further confirm the diffusion-controlled process, log(Ipa) was plotted against log(v) (Fig. 4c), resulting in the equation: log(Ipa) = 0.64 log(v) - 4.77 (R² = 0.996) with a slope value of 0.64. This slope value indicated that diffusion is the predominant transport mechanism, but some adsorption may also be involved. Typically, a slope of 0.5 indicates pure diffusion, a value between 0.5 and 1 suggests mixed diffusion – adsorption, and a slope of 1 corresponds to pure adsorption [40].

### 3.5. Influence of detection medium and modifier loading

The supporting electrolyte critically influences electrochemical processes by facilitating ion conduction and modulating species stability/reactivity. To identify the optimal medium for TRA detection, three decimolar electrolyte solutions, AB, BRB, and PB all at pH 4.5 were evaluated using SWV on POM/GCE with 0.1 mM TRA (Fig. S2, Supporting Information). PB yielded the highest current response (11.5 µA), surpassing AB (4.23 µA) and BRB (8.52 µA), alongside a cathodic peak potential shift (PB: 1.483 V vs. AB: 1.493 V). This improvement reflects the superior compatibility of PB with PB, likely due to its favorable diffusion properties and ion-exchange characteristics. These results are in line with the literature demonstrating the efficacy of PB for the electrooxidation of TRA [41,42]. The amount of POM deposited on the electrode (modifier loading) significantly impacted sensor performance.

<figure><img src="image_6.png" /><figcaption>Fig. 4. (a) Multicyclic voltammograms obtained in 0.1 M PB solution (pH 3.3) containing 0.1 mM TRA on POM/GCE at scan rates of 10 to 100 mV/s and, (b) peak current as a function of v¹/²; (c) log(Ipa) as a function of log(v); (d) SWV peak current as a function of the amount of POM deposited on the GCE, recorded in 0.1 M PB solution (pH 4.5) containing 0.1 mM TRA. Experiments were performed in triplicate.</figcaption></figure>
Excessive film thickness (higher mass) can impede analyte diffusion, while insufficient loading limits the number of active sites. Optimization studies (Fig. 4d) revealed that the peak current was maximized when 3 µg of POM was deposited (corresponding to 3 µL of a 1 mg/mL suspension), with a current of 12.3 µA at 1.488 V. Lower (1–2 µg) or higher (4–5 µg) amounts resulted in decreased current responses. This trend confirms the critical balance between accessible catalytic sites and mass transport efficiency.

### 3.6. Influence of pH on the peak potential and current of TRA

The pH of the supporting electrolyte significantly affects the electrochemical response of analytes by altering their speciation and interaction with the electrode surface. In this study, the pH of PB solutions was varied (2, 3, 4, 5, 6) and the results obtained are presented in Fig. 5. As illustrated in Fig. 5b(i), the peak current for TRA increases from pH 2 to 3, reaching a maximum of 14.3 µA at pH 3, and then decreases gradually as the pH increases to 7. This trend can be attributed to electrostatic interaction between the TRA and the POM film. At pH 3, TRA is predominantly in its cationic form due to protonation of its amine and phenol groups (the pKa of amine and phenol are around 9.41 and 13.08 respectively), which enhances its attraction to the negatively charged POM/GCE [43]. Additionally, the porous structure of the POM may facilitate the adsorption and immobilization of TRA molecules on the electrode surface. As the pH increases beyond 3, TRA gradually loses its positive charge, reducing its affinity for the POM and resulting in lower peak currents. These observations are consistent with previous reports [44–46]. Besides, the anodic peak potential (Epa) followed a linear dependence over pH (Fig. 5b(ii)), with a slope of –0.00121 V/pH (R² = 0.994). The very low slope observed for the potential–pH curve indicates that the underlying electrochemical mechanism is largely independent of the solution pH. This behavior can be attributed to the role of the POM, which acts as a redox mediator and facilitates a direct electron transfer between TRA and the electrode, without significant involvement of hydrogen ions in the rate-determining step [47]. The reaction can be represented as follows for POM-based systems, where the redox potential remains stable across a broad pH range due to the intrinsic properties of the POM framework and its electron transfer mechanism. Tomita et al. demonstrated that the redox potential of mono-transition-metal-substituted POMs is essentially pH-independent, as their redox processes do not involve proton transfer [48].

Similarly, Iwase et al. showed that molybdenum-substituted Keggin-type POMs function as stable redox mediators with negligible pH sensitivity in catalytic applications [49]. Thus, the very low pH sensitivity of the oxidation potential in our system confirms that the process follows a pure electron transfer mechanism, characteristic of POM-based catalytic systems, and enables robust detection of tramadol over a wide pH range. To conclude this section, a PB solution at pH 3 was chosen as the supporting electrolyte for further experiments.

### 3.7. Influence of frequency and accumulation time on the TRA signal

In SWV electrochemical analysis, frequency variation plays a critical role in influencing the kinetics of electrochemical reactions. By adjusting the frequency, one can control the interaction rate between reactive species and the electrode, directly affecting both the speed and efficiency of the electrochemical process. In this work, the frequency was studied from 10 to 90 Hz, and the results obtained are shown in Fig. S3a (Supporting Information). The peak current increase progressively with the frequency. This trend can be attributed to the fact that higher frequencies reduce the time available for diffusion and interfacial reactions at the electrode surface. While a moderate increase in frequency can increase current by accelerating reaction rates, frequencies that are too high can limit diffusion of the analyte to the electrode and reduce current due to the lack of time for effective charge transfer or possible desorption of the analyte. As Kissinger and Heineman point out, higher frequencies can also improve the signal-to-noise ratio by minimizing capacitive (non-faradic) contributions, thus improving the sensitivity of faradic current detection [50]. Accumulation time refers to the duration for which TRA is allowed to absorb onto the electrode surface prior to measurement, potentially increasing sensitivity and selectivity by enriching the analyte concentration at the interface. In this work, accumulation time was varied from 0 to 240 s (Fig. S3b, Supporting information). The optimal current response was observed at zero accumulation time. This time is very short compared to those observed in the literature, such as 300 s used by Afkhami et al. for a NiFe₂O₄/graphene nanocomposite modified electrode [51]. However, the absence of an accumulation time allows for instantaneous acquisition of electrochemical signals, eliminating unwanted side reactions or electrode degradation that could occur during a longer accumulation period. This approach not only provides more accurate and reproducible results, but also significantly reduces the overall analysis time [52].

### 3.8. Calibration plot

Under optimized conditions, the sensitivity of the developed POM/GCE sensor for TRA detection was evaluated in the concentration range of 0.06–0.78 µM (Fig. 6). The results show that an increasing TRA concentrations lead to a proportional increase in peak current, with a strong linear dependence expressed by the equation: $I_{pA} = (5.52 \pm 0.08) [TRA] (\mu M) + (6.89 \times 10^{-7} \pm 0.08 \times 10^{-7})$ (inset in Fig. 6). The correlation coefficient ($R^2 = 0.998$, $n = 8$) confirms excellent linearity between TRA concentration and peak current. Based on the calibration curve, the limit of detection (LoD) and limit of quantification (LoQ) of 4.03 nM and 13.4 nM, respectively, were calculated using a 3σ/slope ratio and 10σ/slope ratio, where σ is the standard deviation of the blank, determined according to IUPAC recommendations [53]. Compared to these reported methods (Table 2), the proposed sensor exhibits a particularly low detection limit (4.03 nM) and a linear detection range

<figure><img src="image_7.png" /><figcaption>Fig. 5. (a) Effect of pH of the supporting electrolyte on the electrochemical response of 0.1 mM TRA in 0.1 M PB solution on the POM/GCE and (b) $I_{pA}$ as a function of pH (i) and linear relationship between Ep and pH (ii). Experiments were performed in triplicate.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** SWV responses recorded on POM/GCE in 0.1 M PB solution (pH 3) containing TRA at different concentrations: (a-h): 0.06–0.78 µM. Inset shows the corresponding calibration graph. Experiments were performed in triplicate.</figcaption></figure>

<caption>**Table 2**</caption>

Comparison of the efficiency of some electroanalytical sensors in the determination of TRA.

<table><thead><tr><th>Working electrodes</th><th>Linear range (µM)</th><th>LoDs (µM)</th><th>References</th></tr></thead><tbody><tr><td>Poly(Nile blue) modified GCE<sup>a</sup></td><td>1–310</td><td>0.5</td><td>[40]</td></tr><tr><td>Graphite/Co<sub>3</sub>O<sub>4</sub> modified SPE<sup>b</sup></td><td>0.1–500</td><td>0.03</td><td>[42]</td></tr><tr><td>CuO-NPs and polypyrrole modified CPE<sup>c</sup></td><td>0.005–380</td><td>0.001</td><td>[54]</td></tr><tr><td>MgO/SWCNTs nanocomposite +1,3-dipropylimidazolium bromide modified CPE</td><td>0.05–280</td><td>0.008</td><td>[55]</td></tr><tr><td>Au Nanoparticles/Cysteic acid modified GCE</td><td>0.5–63.6</td><td>0.17</td><td>[56]</td></tr><tr><td>Nanographene/tramadol-imprinted polymer nanoparticles/ionic liquid modified CPE</td><td>0.003–1000</td><td>0.002</td><td>[57]</td></tr><tr><td>LDH/Fe<sub>3</sub>O<sub>4</sub> modified GCE</td><td>1–200</td><td>0.3</td><td>[58]</td></tr><tr><td>Carbon nanoparticles modified GCE</td><td>10–1000</td><td>5.00</td><td>[59]</td></tr><tr><td>UiO-66-NH<sub>2</sub> MOF-PAMAM<sup>d</sup>/GCE</td><td>0.5–500</td><td>0.2</td><td>[60]</td></tr><tr><td>POM modified GCE</td><td>0.08–0.78</td><td>0.00403</td><td>This work</td></tr></tbody></table>

(0.06–0.78 µM) suitable for trace-level quantification. While other sensors, such as the CPE modified with CuO-NPs/polypyrrole and the CPE modified with nanographene-printed polymer nanoparticles/tramadol, have wider linear ranges and similar or slightly lower detection limits, our method offers competitive sensitivity with excellent reproducibility and simplicity in electrode preparation. To the best of our knowledge, there are no prior reports of polyoxometalate-based electrodes for the electrochemical detection of tramadol, highlighting the novelty of the present work. The existing tramadol sensors primarily rely on carbon nanomaterials, metal oxides, and metal-organic frameworks (MOFs) [40,42,54–60].

### 3.9. Reproducibility, repeatability, long-term stability, and interference study

To assess the accuracy of the sensor, reproducibility was evaluated using five POM-modified GCEs prepared independently under identical experimental conditions. As shown in Fig. 7a, a relative standard deviation (RSD) of 2.2% was obtained, corresponding to a repeatability of 97.8%. RSD values between 2 and 4% are generally considered to be indicative of good repeatability. For comparison, Arabali and collaborators reported an RSD of 3.8% using a pencil graphite electrode modified with CuO nanoparticles, which was described as satisfactory [54]. Repeatability was evaluated by performing five consecutive measurements with the same electrode, resulting in an RSD of 9.04% (Fig. 7b). The stability of the POM-modified GCE was assessed by subjecting them to ambient storage at room temperature for a duration of several days, with evaluations conducted every day utilizing SWV. After 5 days of ambient storage, the current signals of POM/GCE were 72.5 ± 5.5%, demonstrating good long-term stability. These results confirm that the POM/GCE sensor exhibits high reproducibility, repeatability, and durability, which are essential for reliable electrochemical detection of tramadol in various samples.

To evaluate the selectivity of the developed sensor, its electrochemical behavior was investigated in the presence of potential interfering substances, including glucose (GLU), uric acid (UAC), acetaminophen (ACE), amoxicillin (AMO), caffeine (CAF) and clonacillin (CLO). The interference of these interferents on the TRA signal was assessed at three concentration ratio (TRA:interferent = 1:0.5; 1:1; 1:2), using a fixed TRA concentration of 0.1 mM. The measurements were performed by SWV under identical experimental conditions. The results are shown in Fig. 8. The data indicate that the presence of interfering species alters the electrochemical response of TRA. Specially, CAF, AMOX, GLU, and UA tend to decrease the peak current of TRA, suggesting possible signal suppression and reduced recovery. In contrast, ACE and CLO enhance the TRA current response, with the effect varying depending on their concentrations. These observations highlight the

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Reproducibility (a), and repeatability (b) study of POM-modified GCE (detection of 0.1 mM TRA in 0.1 M PB solution (pH 4.5). For (b), a step of cleaning the surface of the modified electrode between each successive measurement has been applied under stirring for 15 s in 0.1 M PB solution (to remove the adsorbed TRA oxidation products).</figcaption></figure>

<footnote><sup>a</sup> GCE: Glassy carbon electrode, <sup>b</sup>SPE: Screen-printed electrode, <sup>c</sup>CPE: Carbon paste electrode, <sup>d</sup>MOF/PAMAM: Metal-organic framework/poly(amidoamine).</footnote>
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Effect of potentially interfering species on the peak currents of 10 µM TRA in 0.1 M PB solution (pH 3) on POM/GCE. Experiments were performed in triplicate.</figcaption></figure>

selective response of the sensor toward TRA, despite the presence of structurally or electrochemically actives species, confirming its suitability for real sample analysis.

### 3.10. Analytical application

To demonstrate the practical applicability of the modified electrode, TRA contents was determined in pharmaceutical formulations using the standard addition method. Three commercial products were analyzed: TRAMAGEN® tablets (50 mg TRA hydrochloride), TRAMADIS® am-poules (100 mg/2 mL TRA injectable) and Tramadol Denk 50 mg. In this method, a TRA solution of known concentration was incrementally added to the diluted sample solution, and the peak current was recorded after each addition. The analyte concentration was then calculated from the linear regression of current versus added concentration (Fig. S4, Supporting information). The calibration equations obtained were: TRAMAGEN®; $I_{pa}$ (A) = 0.14[TRA](M) + 2.99 × 10⁻⁶ (R² = 0.998, n = 6), TRAMADIS®; $I_{pa}$ (A) = 0.14[TRA](M) + 4.24 × 10⁻⁶ (R² = 0.998, n = 6), and Tramadol Denk; $I_{pa}$ (A) = 0.31[TRA](M) + 4.43 × 10⁻⁶ (R² = 0.995 n = 6). For comparison, the TRA content of the same samples was also quantified by UV-Visible spectroscopy at a wavelength of 277 nm. This method involves plotting absorbance as a function of added TRA concentration to determine the original content in the matrix, based on the y-intercept of the calibration line obtained (Fig. S5, Supporting information). The calibration equations obtained were: TRAMAGEN®; Abs = 0.03[TRA] (µM) + 0.27 (R² = 0.997, n = 14), TRAMADIS®; Abs = 0.03[TRA] (µM) + 0.23 (R² = 0.993, n = 14), and Tramadol denk; Abs = 0.05 [TRA] (µM) + 0.01 (R² = 0.xx, n = 14). under the tested conditions, the electrochemical method yielded recovery rates ranging from 92.9 and 101 %, demonstrating high accuracy (see Table 3). In contrast, UV-Visible method showed lower recovery rates between 73.55 % and 97.75 %. These differences can be attributed to the greater selectivity and sensitivity of the electrochemical approach, which is less affected by interference from the sample matrix.

### 4. Conclusions

A simple, sensitive, and reliable voltammetric sensor based on [H₄PW₁₇V(IV)O₆₂]⁹⁻ polyoxometalate was developed for quality control of tramadol-containing pharmaceuticals. A polyoxometalate was successfully characterized by XPS, PXRD, FT-IR and SEM. The sensor, prepared by modifying a GCE, exhibited a 2.51-fold current increase in cyclic voltammetry and 2.34-fold in square wave voltammetry. Optimal

<table><caption>**Table 3**

Results of analysis of drug samples containing TRA by SWV and UV-Visible Spectrophotometry.</caption><thead><tr><th>Sample</th><th></th><th>Theoretical mass</th><th>Recovery mass</th><th>Recovery (%)</th></tr></thead><tbody><tr><td rowspan="2">TRAMAGEN®</td><td>SWV</td><td>50 mg</td><td>50.12 mg</td><td>100.25 %</td></tr><tr><td>UV-Vis</td><td>50 mg</td><td>43.75 mg</td><td>87.5 %</td></tr><tr><td rowspan="2">TRAMADIS®</td><td>SWV</td><td>100 mg / 2 mL</td><td>97.56 mg / 2 mL</td><td>97.56 %</td></tr><tr><td>UV-Vis</td><td>100 mg / 2 mL</td><td>73.55 mg / 2 mL</td><td>73.55 %</td></tr><tr><td rowspan="2">Tramadol Denk 50 mg</td><td>SWV</td><td>50 mg</td><td>46.45 mg</td><td>97.95 %</td></tr><tr><td>UV-Vis</td><td>50 mg</td><td>48.975 mg</td><td>92.90 %</td></tr></tbody></table>

conditions were established using a phosphate buffer pH 3, 3 µL polyoxometalate, a frequency of 90 Hz, and no accumulation time. The method showed high sensitivity, with a detection limit of 4.03 nM and a quantification limit of 13.4 nM, outperforming previously reported electrochemical approaches for tramadol determination. The sensor was successfully applied to the analysis of commercial formulations, yielding recovery rates between 97 % and 101 %, thus demonstrating the reliability of the method for routine pharmaceutical quality control. Although the present study focused on commercial formulations, future work will investigate the applicability of this sensor to more complex matrices, such as environmental and biological samples, in order to assess its potential for broader applications, including the detection of misuse and environmental contamination.

### CRediT authorship contribution statement

**Donald Eric Njiajo Talla:** Writing – original draft, Methodology, Data curation. **Astree Lottie Djuffo Yemene:** Writing – original draft, Methodology, Investigation, Data curation. **Sherman Lesly Zambou Jiokeng:** Writing – review & editing, Data curation, Conceptualization. **Konstantin M. Schüttler:** Investigation, Writing – original draft. **Israel Mbomekalle:** Writing – review & editing, Validation, Investigation. **Boris Mizaikoff:** Writing – review & editing, Funding acquisition. **Ignas Kenfack Tonlé:** Writing – review & editing, Validation, Project administration, Conceptualization.

### Declaration of competing interest

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

### Acknowledgments

The authors gratefully acknowledge the support of the Alexander von Humboldt Foundation (Germany). We thank the Open Access Publishing Fund of the University of Ulm for funding. Special thanks are extended to Dr. Gregor Neusser at the Focused Ion Beam Center UULm (Institute of Analytical and Bioanalytical Chemistry, Ulm University, Germany) for assistance with the SEM studies and to Cyrille Ghislain Fotsop (Institute of Chemistry, Faculty of Process and Systems Engineering, Otto-von-Guericke-University Magdeburg, Germany) for the assistance with the powder X-ray diffraction measurements.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jelechem.2025.119522.

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