Towards circular analytical chemistry: Optimization of colorimetric assay for sustainable and selective dopamine detection in pharmaceutical formulations using green solvents

The drive towards sustainability in analytical chemistry has prompted the replacement of hazardous solvents with greener alternatives, in line with circular chemistry principles. We here report a rapid, low-cost, and sustainable colorimetric assay for dopamine (DA) detection in pharmaceutical formulations. Building on our previously developed dimethyl sulfoxide (DMSO)-based melanochrome (MN) assay, three bio-based solvents—Cyrene™, γ-valerolactone (GVL), and N-butylpyrrolidone (NBP)—were evaluated as DMSO substitutes. Screening indicated that DMSO and NBP supported effective MN formation, whereas GVL and Cyrene were unsuitable. NBP was selected for optimization using a Design of Experiments (DoE) strategy to assess pH, solvent fraction, and temperature effects. Temporal contributions and interactions of parameters were analyzed through dynamic DoE modeling, with curvature effects evaluated via triplicate center-point measurements. Model predictivity was verified at the center and at the predicted maximum absorbance region, achieving the highest signal at 30 min. The optimized assay (NBP:buffer 1:4 v/v, pH 10.0, 45 °C) exhibited excellent linearity (R² = 0.9998) across 11–80 μg/mL, a low detection limit (LOD 1.35 μg/mL), and quantification limit (LOQ 4.59 μg/mL). Compared to the reference DMSO system, the NBP assay showed improved sensitivity and reduced solvent usage, minimizing environmental impact. Selectivity was confirmed against adrenergic drugs (adrenaline, noradrenaline, ephedrine, etilefrine, dobutamine), none of which produced MN chromophore. Thes results demostrated method robustness in clinically relevant contexts. Overall, the proposed NBP-based assay integrates eco-friendly solvent selection with DoE-guided optimization, delivering sensitivity, selectivity, and practicality for pharmaceutical quality control. Beyond simple solvent substitution, this study presents a systematic green re-evaluation of a validated analytical protocol, effectively balancing sustainability, functionality, and analytical performance.