Selective fluorescence quenching of papain-Au nanoclusters by self-polymerization of dopamine.

In this paper, we synthesized a papain-stabilized fluorescent Au nanocluster (NC) probe and studied its interaction with dopamine. As fluorescence of papain-Au NCs is quenched in the presence of dopamine under alkaline conditions, we were able to establish a simple, selective analytical method for dopamine determination. By studying the fluorescence lifetime and dynamic light scattering of the NCs before and after interaction with dopamine, we found that this fluorescence quenching mechanism was possibly due to dopamine self-polymerization that produced polydopamine that cross-linked papain-Au NCs. Based on this new phenomenon, we proposed a highly selective analytical method for dopamine detection. Other small organic molecules, such as amino acids, ascorbic acid and uric acid did not interfere with dopamine detection. Dopamine in the range 20-100 μM can be linearly detected by the fluorescence quenching ratio of gold nanoclusters. Dopamine detection could be visually realized by watching color changes of papain-Au NCs under UV light or daylight, as both fluorescence and absorption of the papain-Au NCs changed during dopamine polymerization.