L-Cysteine Functionalized Gold Nanoparticles as a Colorimetric Sensor for Ultrasensitive Detection of Toxic Metal Ion Cadmium

Abstract

The toxic metal ion cadmium is known to be a widespread environmental contaminant and adversely affect human health. Exposure is largely via the respiratory or gastrointestinal tracts. Important non industrial sources of exposure are cigarette smoke and food (from contaminated soil and water). The kidney is the main organ affected by chronic Cd exposure and toxicity this has made us much more aware of the consequences of chronic environmental (or nonindustrial) exposure to heavy metal, and their impact on human health. Milk and dairy products can be contaminated in different ways. One of the most important and most challenging contaminations is due to heavy metals cadmium ions.. There are various ways to measure the amount of contamination in milk and dairy products. Therefore, reliable detection technique of metal ion levels in foods is essential for improving the public safety issues. In current investigation, we have developed l-cysteine modified gold nanoparticle (AuNPs) based colorimetric assays technique for detecting the toxic metal ion in food. It is a simple and low-cost alternative of spectroscopy based methods. Synthesized colloidal AuNPs possess intrinsically strong surface plasmon resonance (SPR) absorptions, with extremely high extinction coefficients, in the visible wavelength range. The extinction coefficient of the Au NPs and the wavelengths at which NPs absorb and scatter light depend on their shapes and sizes, the dielectric constant of their surrounding aqueous media, and their interactions with neighboring particles. We have synthesized small size L- cysteine AuNPs with dark red colour which has potent ability to aggregates with toxic metal ions and there is change in color. Red Au NPs change into deep blue. The aggregation of Au NPs was controlled by employing surface ligands that bind to target heavy metals ions in foods. When metal ions are introduced, it binds through the ligands of multiple Au NPs, which was caused due to uncontrolled aggregation and subsequent color change of the solution. The 24 nm spherical Au NPs were prepared by reduction of 1.0 mM HAuCl4.3H2O with sodium tri-citrate (38.8 mM). The average size of monodisperse AuNPs sizes (DLS and TEM) was 31 nm, which have given significant results