Coating Gold Nanoparticles Synthesized from Spent Coffee Ground Extract to Enhance Their Stability

Gold nanoparticles (AuNPs) were synthesized using the extract from cold-brew spent coffee grounds. The colloids were characterized by UV-vis spectrophotometry and transmission electron microscopy. When using 1.8 mM and 4.0 mM HAuCl4, the average diameter of AuNPs was 10.51 and 13.75 nm, corresponding to the peak at 533 and 539, respectively. The colloidal stability was studied using the results from spectrophotometry. The aggregation of as-synthesized AuNPs, reflected by the red shift of the spectrum, occurred after the centrifugation at 10000 rpm for 10 minutes but this was prevented after coating with 5%w/v PEG400, 5%w/v PEG6000, 0.1%w/v alginate, and 0.5%w/v SDS solutions. Electrostatic stabilization using alginate and SDS could withstand up to three times of centrifugation, while steric stabilization using PEG400 and PEG6000 could withstand the disturbance of Ca2+ more effectively. The smaller AuNPs were more affected by Ca2+ than the larger ones. In highly acidic solutions, all coated AuNPs showed the red shift and SDS-coated AuNPs were the most effective against acid disturbance, and in highly basic solutions, all coated AuNPs showed the blue shift. Besides, the coated AuNPs were still stable at 52oC. This study could be applied in situations where mechanical, chemical, thermal and acid disturbances are concerned.