A green solvent-free method for methylene blue quantification in water by dSPE-DRIFTS using MCM-41 mesoporous silica

A solvent-free analytical method for the determination of methylene blue (MB) in water was developed by coupling dispersive solid-phase extraction (dSPE) with diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), using mesoporous silica MCM-41 as the solid phase. The sorbent material exhibited high surface area, uniform mesoporosity and a negatively charged surface at neutral pH, enabling efficient retention of cationic MB. Key experimental variables affecting the extraction process, including pH, sorbent amount and contact time, were optimized to maximize the MB/MCM-41 loading ratio and ensure reproducible spectroscopic responses. Under the optimized conditions, the method showed good linearity (R² = 0.997) over the concentration range 0.5-3.20 mg/L, with limits of detection (LOD) and quantification (LOQ) of 0.2 and 0.5 mg/L, respectively. Satisfactory intra- and inter-day precision was obtained, and common inorganic ions typically present in natural waters did not significantly affect MB quantification. To extend the applicability toward lower concentrations levels, a sequential loading strategy was implemented, enabling reliable quantification down to 0.07 mg/L. The method was successfully applied to a real river water sample, confirming its suitability for environmental monitoring. Overall, the proposed dSPE-DRIFTS approach provides a simple, cost-effective, and environmentally sustainable analytical alternative for routine MB determination in aqueous matrices.