Nanoarchitectonics of tunable aminosalicylate sodium encapsulated gold nanoparticles enabling multi-faceted role as capping, reducing, stabilizing and colorimetric detection of metal ions

Despite all the advancements in aqueous synthesis of gold nanoparticles, certain features like one-pot/one-step method with minimal reactants using greener solvents are still demanding. The challenge in the aqueous phase synthesis is to balance the nucleation and precise growth of nanoparticles avoiding aggregation. In this work, we report a unique versatile unexplored molecule aminosalicylate sodium (Na-4-ASA) which functions as a capping, reducing, stabilizing and more interestingly as an encapsulating agent for gold nanoparticles. This multi-faceted molecule showed excellent control in synthesizing monodisperse tunable encapsulated nanoparticles of sizes (60 nm, 53 nm and 12 nm) exhibiting absorbance bands at 560 nm, 540 nm and 520 nm respectively. X-ray diffraction and Fourier Transmission Infra-Red validated crystalline structure and binding of Na-4-ASA onto gold nanoparticles surface respectively. Furthermore, the AuNPs were investigated for their ability to detect metal ions through colorimetric change where purification via centrifugation turned out to be a key parameter in enabling the detection. Selectivity towards Al³⁺was observed with the 12 nm sized nanoparticles at 0.5 ppm metal ion concentration. The AuNPs of sizes 60 nm and 53 nm detected Al³⁺/Cr³⁺/Fe³⁺and Al³⁺/Fe³⁺respectively indicating the impact of size in heavy metal ions detection. The greater the size of AuNPs, lower is the selectivity where detection of three metal ions were observed and vice versa i.e. smaller-sized AuNPs showed high selectivity by detecting single metal ion. Also, the time duration for detection increased with decreasing size of the AuNPs. Finally, LOD for the heavy metal ions Al³⁺, Cr³⁺, and Fe³⁺were calculated as 67 ppb, 78 ppb, 76 ppb respectively.