Ractopamine as a novel reagent for the fabrication of gold nanoparticles: Colorimetric sensing of cysteine and Hg2+ ion with different spectral characteristics

•Ractopamine hydrochloride was used as a novel reagent for fabrication of AuNPs.•RA-AuNPs acted as a visual reader for colorimetric assaying of Cys and Hg2+ ion.•RA-AuNPs showed different spectral changes for Cys (red-shift) and Hg2+ ion (blue-shift).•RA-AuNPs successfully applied to detect Cys and Hg2+ in real samples. We have developed a facile and cost-effective chemical reduction procedure for the preparation of gold nanoparticles (AuNPs) using ractopamine hydrochloride (RA) as a novel reagent. The RA assembled AuNPs display high affinity towards cysteine (Cys) and Hg2+ ion, causing aggregation of RA-AuNPs, which accompanies a remarkable a red-shift (534 to 606 nm) for Cys and a blue-shift (534 to 506 nm) for Hg2+ ion in the surface plasmon resonance (SPR) band of RA-AuNPs. The UV–visible absorption spectral data revealed that linear calibration curves were obtained between absorption ratios (A606/A534 for Cys and A506 for Hg2+ ion) and the concentration range from 0.6 to 600 µM and from 1 to 400 µM with linear coefficients of 0.9910 and 0.9925 for Cys and Hg2+ ion. The RA-AuNPs-based colorimetric assay represents the detection limits of 0.35 and 0.30 µM for Cys and Hg2+ ion (3σ). This colorimetry assay is completed within 60 min, and successfully applied to detect Cys in urine and Hg2+ ion in water samples.