Towards new fluorometric methodologies based on the in-situ generation of gold nanoclusters

In this manuscript a method for the fluorometric determination of tyramine is described. It is based on the direct reaction between Au(III) and tyramine in a phosphate buffer which produces fluorescent gold nanoclusters (AuNC) (λ_exc = 320 nm, λ_em = 410 nm) with a diameter of 1.50 ± 0.06 nm. The Au(III) and buffer solutions are mixed and after 140 s, tyramine solution is added; which produces a fast and stable fluorescence signal. The formation of AuNC is demonstrated by STEM and, more importantly, this reaction could be followed by Atomic Fluorescence Microscopy (AFM). The method allows the determination of tyramine in the range from 6.0x10^-7 M (limit of quantification) up to 1.2x10^-4 M; with a relative standard deviation (RSD) ranges from 1.8% to 4.4% depending on the tyramine concentration. The mechanism of AuNC formation involves the Au(III) reduction via the phenol group and the complexation with the amine group. Putrescine and cadaverine do not produce interference, meanwhile histamine causes a proportional decrease in the signal which can be overcome by the standard addition method. The method was applied to the determination of tyramine in a tuna and cheese samples and the results obtained are in statistical agreement with these obtained using a validated or standard method.