Naked eye and optical biosensing of cysteine over the other amino acids using β-cyclodextrin decorated silver nanoparticles as a nanoprobe

Abstract

A simple, visual colorimetric sensor for rapid, sensitive and selective determination of cysteine (Cys) using the naked eye has the potential for impact in the biological and environmental sciences. Herein, we report the sensitive and highly selective sensing of Cys amino acid over the other natural amino acids using β-cyclodextrin functionalised silver nanoparticles (β-CD AgNPs) as a colorimetric assay. Upon the addition of Cys to β-CD AgNPs, the surface plasmon resonance (SPR) band of β-CD AgNPs at 400 nm decreased and a new peak appeared at 535 nm, while the colour also changed from yellow to wine red. The observed spectral and colour changes are caused by aggregation of β-CD AgNPs, and this Cys induced aggregation was further corroborated by high resolution transmission electron microscopy (HR-TEM), dynamic light scattering (DLS) and zeta potential measurements. The highly specific detection of Cys in these experiments were due to the unmodified β-CD molecules favouring Ag–S bond formation, whereas they prevented the interaction of amino acids containing other functional groups on the surface of the AgNPs. Cys contains the thiol group so it could interact with the β-CD AgNP surface, as evidenced using FT-IR spectral techniques. Furthermore, this sensing approach exhibits a good linear range with Cys concentrations from 3.00 × 10⁻⁷ mol dm⁻³ to 45.00 × 10⁻⁷ mol dm⁻³, and the lowest limit of detection was 4.65 × 10⁻⁹ mol dm⁻³. The proposed colorimetric assay was effectively utilized for the quantification of Cys in human blood serum and urine samples.