Introduction

In recent decades, obesity has emerged as a global epidemic, with a rapidly increasing prevalence worldwide and strong associations with chronic diseases such as diabetes, cardiovascular disorders, and metabolic syndrome. Consequently, weight management has become a major health concern, leading to the widespread consumption of weight-loss products, including herbal-based supplements and functional foods. These products are often marketed as “natural” and safe; however, numerous studies have reported the adulteration of such formulations with undeclared synthetic pharmaceutical compounds, posing serious risks to public health [1,2]. Among the commonly detected adulterants, sibutramine (SIB), phenolphthalein, caffeine, and fluoxetine are frequently identified [3]. In particular, SIB, a serotonin-norepinephrine reuptake inhibitor (SNRI), was originally developed as an antidepressant and was later approved by the U.S. Food and Drug Administration (FDA) in 1997 for the treatment of obesity [4,5]. However, due to its severe adverse effects, including increased blood pressure, elevated heart rate, and the risk of serotonin syndrome, sibutramine was withdrawn from the market in 2010 [6]. In Vietnam, SIB has been officially banned from health-protective food products under Circular No. 10/2021/TT-BYT [7]. Despite strict regulations, the illegal addition of sibutramine into herbal weight-loss products remains prevalent, primarily driven by the desire to enhance rapid weight-loss efficacy. To address this issue, various analytical techniques have been developed for the detection and quantification of SIB, including reversed-phase high-performance liquid chromatography (RP-HPLC) [8], HPLC [9], high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) [10], gas chromatography-MS (GC-MS) [1,11] and electrochemical analysis [12]. While chromategraphic and mass spectrometric methods provide high sensitivity and accuracy, they typically require expensive instrumentation, sophisticated operation, and time-consuming sample preparation, thereby limiting their applicability to rapid, on-site screening. Simpler techniques, such as spectrophotometry or capillary electrophoresis, are lower-cost but often have limited sensitivity and selectivity, particularly in complex matrices such as herbal formulations. In recent years, electrochemical methods have gained considerable attention as promising alternatives for pharmaceutical analysis due to their inherent advantages, including simplicity, low cost, rapid response, high sensitivity, and potential for miniaturization and portable applications. Importantly, the analytical performance of electrochemical sensors can be significantly enhanced by modifying the electrode surface with nanostructured materials. Graphene-based materials, especially reduced graphene oxide (rGO), have been widely employed for their excellent electrical conductivity, large specific surface area, and strong adsorption capacity [13]. However, pristine rGO still suffers from certain limitations, such as restacking of graphene layers and a limited number of active catalytic sites, which restrict its electrocatalytic performance. On the other hand, metal nanoparticles, particularly silver nanoparticles (AgNPs), exhibit outstanding catalytic activity and can effectively accelerate electron-transfer kinetics [14]. Nevertheless, AgNPs alone tend to aggregate and lack structural stability when directly applied as electrode modifiers. To overcome these drawbacks, the integration of AgNPs with rGO into a hybrid nanocomposite provides a synergistic effect, in which rGO serves as a conductive and high-surface-area support to disperse AgNPs, while AgNPs act as active catalytic centres that enhance the electrochemical oxidation of target analytes. Therefore, in this study, a novel electrochemical sensor based on a glassy carbon electrode (GCE) modified with silver nanoparticles-electrochemically reduced graphene oxide (AgNPs-ErGO) was developed for the determination of sibutramine in herbal-based weight-loss products. The proposed sensor exploits the synergistic interaction between AgNPs and ErGO to enhance electrocatalytic activity and analytical performance. The method was systematically optimized and validated, and its applicability was demonstrated by analysing real samples collected from the market. The obtained results were further compared with those from a conventional HPLC-DAD method to confirm the reliability and accuracy of the proposed approach.