Citrate-reduced silver nanoparticles: Synthesis temperature dependent properties

• XPS analysis reveals increased surface Ag + following Ca 2+ addition. • Surface Ag + promotes chemisorption of anionic species. • Cationic analytes readily adsorb onto silver nanoparticles lacking surface Ag + . • SERS activation relies on analyte adsorption before nanoparticle aggregation. Citrate-reduced silver colloids synthesized at 90 °C (cit-AgNPs90) and under boiling conditions (cit-AgNPs100) exhibit distinct differences in analyte adsorption and surface-enhanced Raman scattering (SERS) sensitivity. X-ray photoelectron spectroscopy (XPS) confirmed the presence of surface Ag + on cit-AgNPs90, which confers a more positive surface potential and thereby promotes the spontaneous adsorption of the anionic citrate surfactant. In contrast, the cationic dye Nile Blue (NB) did not adsorb spontaneously at 10 −8 M and was only detected on the cit-AgNPs90 surface at concentrations above 5 × 10 −7 M, following the displacement of pre-adsorbed citrate. Conversely, NB at 10 −8 M readily adsorbed onto cit-AgNPs100, while neither citrate nor Cl − exhibited detectable adsorption at concentrations up to 10 −3 M. However, the supplementation of the cit-AgNPs100 with Ca 2+ determined the adsorption of both anionic species. Concurrently, XPS analysis confirmed the presence of surface Ag + on cit-AgNPs100 subsequent to Ca 2+ supplementation. Differences in citrate binding strength and geometry were observed between the two colloids. Halides (Br − and I − ) displaced citrate more readily (10 -6 M) from cit-AgNPs100 compared to cit-AgNPs90 (10 -4 M). This study highlights how synthesis conditions modulate the functional properties of citrate-stabilized AgNPs and emphasizes the critical role of analyte adsorption onto the metal surface for SERS activation.