Assembly of carboxylated zinc phthalocyanine with gold nanoparticle for colorimetric detection of calcium ion

A series of water-soluble carboxylated zinc phthalocyanine (ZnPc-COOH) were obtained from a facile hydrolyzation of terminating nitriles groups of zinc phthalocyanine synthesized via bisphthalonitrile based precursor. After the gold nanoparticles (Au NPs) with positively charged surfactant CTAB was added to as-prepared ZnPc-COOH solution, the electronic interaction between them would contribute to the tunable conjugate of Au NPs/ZnPc-COOH and lead to a red-shifted absorption peak in UV–Vis spectrum. Specially, both of the amount of phthalocyanine rings and concentrations of ZnPc-COOH would make a big difference in the interaction with Au NPs. In the presence of different metal ions, the ZnPc-COOH/Au NPs aqueous solution revealed selective response to Ca2+, leading to an increased aggregation extent and the naked eye visualized color change. The further experiment revealed that the red-shift was accessible in a wide concentration range of Ca2+, and the red-shift degree was proportional to the concentration of Ca2+ in the range of 2–8 µM with a limit of detection defined as 1 µM. Combing the photosensitivity of ZnPc-COOH and localized surface resonance plasmon of Au NPs, this label-free probe would provide a potential application in colorimetric detection and photosensitization under physiological environment.