Green solid-state based curcumin mediated rhamnolipids stabilized silver nanoparticles: Interaction of silver nanoparticles with cystine and albumins towards fluorescence sensing.

Curcumin mediated rhamnolipids stabilized Ag Nanoparticles (NPs) have been successfully synthesized in a green solid-state procedure and in a solvent free environment. Depending on growth time, the synthesis process offers Ag NPs of different sizes and spherical shapes as reflected in the scanning electron microscope (SEM) images. The absorbance of rhamnolipids stabilized Ag NPs consistently increases with increase in growth time in rhamnolipids medium reconfirming growth in size of the particles. Even after 7 days of growth time in the rhamnolipids medium, the size slightly increases but the shapes remain spherical unlike the case observed in ethanolic environment. During reduction process, curcumin is conjugated at the surfaces of Ag NPs making Ag NPs fluorescent active. The fluorescence/RRS spectral measurements show an enhancement in fluorescence/SFS intensity without changing the spectral position suggesting more curcumin molecules are bound in larger particles. The fluorescence of rhamnolipids stabilized Ag NPs increases while interacting with cystine and albumins. Fluorescence of Ag NPs can estimate cystine and albumins in the 1 μM-100 μM concentration ranges. While binding with cystine is 1:1 with curcumin molecules, for Human Serum Albumin (HSA) and Bovine Serum Albumin (BSA) it is more. The association constant of cystine with rhamnolipids stabilized Ag NPs is high whereas for HSA it is moderate and for BSA is weak, thus, these Ag NPs have better selectivity for cystine sensing.