Onsite detection of cadmium and nickel using gold nanospheres and nanotriangles capped with Cyclea peltata phytochemicals

Small-sized gold nanospheres and nanotriangles (CPAuNPs) were synthesized to visually identify low concentrations of cadmium and nickel (20 ppm) in polluted water samples. Three factors i.e Cyclea peltata extract concentration (X1), temperature (X2), pH (X3) and agitation speed (X4) were optimized for maximum CPAuNPs synthesis by central composite design. The optimized conditions were found to be CP extract concentration of 24.20%, pH of 7.68 and agitation speed of 43 RPM at which maximum yield of 0.412 g/L was obtained with a fold increase of 1.42 as compared to unoptimized conditions. Gold nanospheres were in the size range of 25–36 nm and nano triangles of 100–140 nm with zetapotential of −31.8 mV. The synthesized CPAuNPs demonstrated good selectivity for visual detection of cadmium and nickel at ppm levels, where the colour of CPAuNPs changed from pink to purple. Upon interaction of CPAuNPs with Cd⁺³ and Ni⁺² in the varied concentration ranges, a maximum red shift was seen at 20 ppm, which confirmed agglomeration. The variation in absorbance ratio (A_640nm/A_523nm) for Cd⁺³ and (A_610nm,/A_523nm) for Ni⁺² showed linearity upto 50 ppm with respect to increased concentration in both the cases with R² = 0.926 in case of Cd⁺³ and R² = 0.950 for Ni⁺². Hence, a portable low-cost detection kit for specific heavy metals could be constructed, employing this plant reduced nanospheres, for on-site detection sensing of cadmium and nickel in industrial effluents upto 20 ppm limits.