Selective thiourea optical probe based on thiourea-induced removal of chloroacetyl group from chloroacetylamine

A novel chromogenic probe for thiourea, N-chloroacetyl parafuchsin (CAP), was designed and synthesized. The method was based on the reaction of CAP with thiourea, resulting in the initial formation of the S-substituted thioformamidine hydrochloride, which would then undergo the intramolecular amidinolysis to liberate the corresponding amine, and thus leading to an absorbance increase at visible spectral range. Based on this mechanism, a highly selective optical probe for thiourea was developed. Under optimal conditions, absorbance increase (ΔA) at 562 nm is proportional to thiourea concentration up to 0.2 mmol L−1 with a detection limit of 2.1 μmol L−1 (3σ). Because of the selective removal of chloroacetyl group from CAP by thiourea, there is little interference by other molecules.