Cysteine-Stabilised Silver Nanoparticles as a Colorimetric Probe and Adsorbent for Cu(II) and Ni(II) in Water

Cysteine-stabilised silver nanoparticles (cys-AgNPs) were prepared in a one-pot aqueous synthesis in which AgNO₃ was reduced with NaBH₄ and the particles were capped with cysteine at room temperature. Formation of the particles was indicated by a colour change from colourless to yellow and then reddish-brown, with a localised surface plasmon resonance (LSPR) band at 401 nm that changed little over 100 min. UV-vis, FTIR, TGA, CHNS microanalysis, powder XRD, SEM and TEM were used for characterisation. These techniques indicated crystalline face-centred cubic silver of about 5–48 nm with a bimodal size distribution. They also showed a substantial organic fraction (about 40–53 wt%), with cysteine probably bound to the silver surface through the thiol group. Addition of Cu(II), Ni(II) and Au(III) (2–20 ppm) caused a rapid colour change and a marked decrease and shift of the LSPR band within 5 min. Batch experiments with Cu(II) and Ni(II) examined the effects of initial concentration, contact time, adsorbent dose, pH and temperature. Uptake reached about 10 mg g⁻¹ at 20 ppm, was fast (Cu(II) near-complete within 5 min), followed pseudo-second-order kinetics and was best described by the Henry isotherm. Ni(II) uptake increased with temperature and with pH. These results show that cys-AgNPs can bind Cu(II) and Ni(II) rapidly from dilute solution and are a candidate material for combined colorimetric sensing and removal of heavy metals. Selectivity, working capacity and performance in real wastewater still need to be established.