Diplexer based microwave sensor for noninvasive detection of sucrose and sorbitol in pharmaceutical syrups

In this study, a novel low-pass/band-pass diplexer-based microwave sensor is introduced for the first time as a high-sensitivity platform for the detection and quantification of sucrose and sorbitol concentrations in pharmaceutical syrups. The proposed sensor features a compact footprint of 13.1 mm × 15.84 mm and integrates two distinct filtering paths, each optimized for specific spectral ranges. The band-pass and low-pass filters are constructed using a combination of M-notch-shaped and semicircular-shaped resonators, with the latter enhanced by integrated spiral lines to reduce both physical dimensions and resonant frequency. This geometric optimization enables stronger field confinement and improved sensitivity. An equivalent lumped-element LC circuit model is also presented to accurately predict the resonators' behavior and validate the electromagnetic performance. The sensor's functionality was experimentally verified using three commercially available pharmaceutical syrups-Diphenhydramine, Guaifenesin, and Salbutamol-each with varying concentrations of sweetening agents. The sensor achieved a maximum sensitivity of 9312.5 MHz/(g mL^-1) and a minimum detection limit (DL) of 0.00037 (dB g)/(MHz mL), demonstrating exceptional accuracy in resolving small dielectric variations. The observed nonlinear correlation between sweetener concentration and resonant frequency shift enables precise quantification of both sucrose and sorbitol across diverse formulations. The combination of high sensitivity, excellent repeatability, and miniaturized design makes the proposed sensor highly suitable for real-time, non-invasive quality control of pharmaceutical syrups. This work represents a significant advancement in dielectric-based sensing and offers promising potential for implementation in both laboratory testing and point-of-care diagnostics within the pharmaceutical industry.