Multimodal Sensing Strategy Using pH Dependent Fluorescence Switchable System.

A. Muthurasu,V. Ganesh

doi:10.1038/srep39452

Abstract

Biomolecules assisted preparation of fluorescent gold nanoparticles (FL–Au NPs) has been reported in this work using glucose oxidase enzyme as both reducing and stabilizing agent and demonstrated their application through multimodal sensing strategy for selective detection of cysteine (Cys). Three different methods namely fluorescence turn OFF–ON strategy, naked eye detection and electrochemical methods are used for Cys detection by employing FL–Au NPs as a common probe. In case of fluorescence turn–OFF method a strong interaction between Au NPs and thiol results in quenching of fluorescence due to replacement of glucose oxidase by Cys at neutral pH. Second mode is based on fluorescence switch–ON strategy where initial fluorescence is significantly quenched by either excess acid or base and further addition of Cys results in appearance of rosy-red and green fluorescence respectively. Visual colour change and fluorescence emission arises due to etching of Au atoms on the surface by thiol leading to formation of Au nanoclusters. Finally, electrochemical sensing of Cys is also carried out using cyclic voltammetry in 0.1 M PBS solution. These findings provide a suitable platform for Cys detection over a wide range of pH and concentration levels and hence the sensitivity can also be tuned accordingly.

Highlights

Biomolecules assisted preparation of gold nanoparticles (Au NPs) and nanoclusters (NCs) have received a great deal of attention in the field of nanoscience and technology due to their small size and bio-compatibility along with their intrinsic and exciting physical, chemical and optical properties
Inset displays the photographs obtained for these solutions under UV illumination showing the appearance of yellowish green fluorescence behaviour upon neutralization. It can be visualized from the insets that as-prepared glucose oxidase (GOx) stabilized Au NPs that are in alkaline pH, do not show any fluorescence (Inset–ii) and in contrary the same solution upon reducing the pH to neutral value display a bright yellowish green fluorescence (Inset–i)
Further it can be noted from UV–Vis spectra that as-prepared GOx–Au NPs (a) and GOx–FL Au NPs (b) showed a prominent characteristic absorbance peak at 535 nm corresponding to the surface plasmon resonance (SPR) peak of Au NPs

Summary

Introduction

Biomolecules assisted preparation of gold nanoparticles (Au NPs) and nanoclusters (NCs) have received a great deal of attention in the field of nanoscience and technology due to their small size and bio-compatibility along with their intrinsic and exciting physical, chemical and optical properties. Fluorescent property can arise from the interaction of metal core of Au NPs with the specific surface ligand, in case where the size of these Au NPs is relatively larger than 2 nm They exhibit SPR and fluorescence emission can be enhanced by the plasmonic resonance. Cysteine (Cys; 2-amino-3-sulfhydrylpropanoic acid) is an important amino acid essentially consisting of a thiol (−SH) group apart from amine and carboxylic groups, which enables the bond formation between a metal surface and Cys containing proteins and peptide molecules It is involved in many significant biological functions such as protein synthesis, detoxification and metabolism. Fluorescence of these NPs can be drastically quenched with the aid of acid (tuning pH to acidic range of 5) and subsequent addition of Cys leads to a colour change from red to violet This mixed solution exhibits bright rosy-red colour fluorescence emission upon exposure to UV light.

Methods

Results

Conclusion