Synthesis of Citrate-Capped Gold Nanoparticles

All glassware used in the synthesis of gold nanosphere particles was washed thoroughly with deionized water followed by ultrapure water before use. Citrate-capped spherical gold nanoparticles (AuNPs) were synthesized following a method previously described (27 (link)). Briefly, 50 mL of 0.02% gold (III) chloride solution (Sigma, USA) was boiled in distilled water and 1.2 mL of 1% sodium citrate solution (Merck, USA) was added immediately with constant stirring. A color change from grey to blue and purple-violet occurred within 50 - 60 seconds. After the color change, the heat was turned off, the solution was stirred for 2 - 3 minutes, and allowed to cool at room temperature. The final colloidal solution suspension was characterized by ultraviolet-visible spectroscopy (UV-Vis) by scanning between 400 and 800 nm. The batch having λ-max between 525 and 535 nm was used to prepare antibody conjugate. Enhanced colloidal stability was measured using NaCl solution.The gold particles were stable in normal water solution but started aggregating in different concentration of NaCl (125mM, 250mM, 500mM and 1M). (Supplementary Material Figure S1).