Colorimetric-Electrochemical Combined Method for the Identification of Drugs of Abuse in Blotter Papers: A Powerful Screening Technique Using Three Analytical Responses

Lysergic acid diethylamide (LSD) and phenylethylamine derivatives (NBOHs and NBOMes) are commonly found on seized blotter papers, posing public health risks. Efficient screening methods for identifying these substances are currently limited. To address this, a novel protocol combining colorimetric and electrochemical techniques was developed as a screening method for drugs of abuse in blotter papers. The method uses Emerson's colorimetric reagent (CR) for NBOH identification combined with voltammetric detection via differential pulse stripping adsorptive voltammetry (AdSDPV) using graphite screen-printed electrodes (SPE-Gr). This approach offers, for the first time, an unambiguous identification of NBOHs through three analytical responses: (1) a color change following the addition of the CR; (2) an electrochemical signal indicating the NBOHs' redox process; and (3) a selective electrochemical signal of the colorimetric reaction product (CR-NBOH) on SPE-Gr. It also differentiates NBOH, NBOMes, 2Cs, and LSD, enabling rapid identification of drugs commonly found in blotter papers. Compared to previous sensors, this method provides selective detection of these drugs at the same pH, offering simplicity for forensic applications. The proposed method showed strong electrochemical stability with low variability (<2.3% RSD) and a low detection limit (0.3 μg mL^-1) over a wide linear range (10-1000 μg mL^-1), offering a simple and fast quantitative analysis of illicit drugs in these materials. The combined method was successfully applied to 33 real seized samples, with results confirmed by definitive methods.