Selective generation of gold nanostructures mediated by flavo-enzymes to develop optical biosensors

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) (λexc = 320 nm, λem = 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.

In situ enzymatic generation of Au/Pt nanoparticles as an analytical photometric system: proof of concept determination of tyramine

In situ enzymatic generation of bimetallic nanoparticles, mainly Au/Pt, overcomes the drawbacks (continuous absorbance drift, modest LOQ, and long-time reaction) observed when AuNP alone are produced. In this study, Au/Pt nanoparticles have been characterized by EDS, XPS, and HRTEM images using the enzymatic determination of tyramine with tyramine oxidase (TAO) as a model. Under experimental conditions, the Au/Pt NPs show an absorption maximum at 580 nm which can be related to the concentration of tyramine in the range 1.0 × 10^-6M to 2.5 × 10^-4M with a RSD of 3.4% (n = 5, using 5 × 10^-6M tyramine). The Au/Pt system enables low LOQ (1.0 × 10^-6 M), high reduction of the absorbance drift, and a significant shortening of the reaction time (i.e., from 30 to 2 min for a [tyramine] = 1 × 10^-4M); additionally, a better selectivity is also obtained. The method has been applied to tyramine determination in cured cheese and no significant differences were obtained compared to a reference method (HRP:TMB). The effect of Pt(II) seems to involve the previous reduction of Au(III) to Au(I) and NP generation from this oxidation state. Finally, a three-step (nucleation-growth-aggregation) kinetic model for the generation of NPs is proposed; this has enabled us to obtain a mathematical equation which explains the experimentally observed variation of the absorbance with time.