On the stability of carboxylic acid derivatized gold colloidal particles: the role of colloidal solution pH studied by optical absorption spectroscopy

Results of an investigation into the influence of hydrosol pH on the optical properties of gold colloidal particles capped with a novel aromatic bifunctional molecule, 4-carboxythiophenol (4-CTP), are presented. Changes in the optical properties of the carboxylic acid derivatized clusters have been interpreted as arising due to flocculation of the clusters and quantified using a "flocculation parameter" (Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763). It is observed that there is a large fall in the flocculation parameter above hydrosol pH = 4 which then is constant above pH = 7. This indicates that the cluster distribution is very stable at high pH due to complete charging of the clusters and maximization of the repulsive electrostatic interaction. Contact angle titration measurements on a selfassembled monolayer of 4-CTP on gold revealed an analogous trend with the contact angle falling above pH = 6 and then remaining constant above pH = 8. This indicates that monolayer formation of 4-CTP on planar and curved surfaces is similar. However, these results are at variance with earlier similar studies on carboxylic acid functionalized alkanethiols where a decrease in the flocculation parameter was observed for intermediate pH values (3 to 7) (Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763) and a possible explanation is presented.