Size-dependent differential interaction of allylamine-capped silicon quantum dots with surfactant assemblies studied using photoluminescence spectroscopy and imaging technique

Abstract

We have demonstrated the size-dependent charge specific surfactant induced aggregation and subsequent deaggregation of allylamine-capped silicon quantum dots (Si-QDs) by means of photoluminescence (PL) spectroscopy, dynamic light scattering (DLS), and imaging techniques. The heterogeneous size distribution of allylamine-capped Si-QDs results in differential interaction with negatively charged sodium dodecyl sulfate (SDS) surfactant. Initial aggregation and subsequent deaggregation have been observed for both smaller and bigger size Si-QDs in the presence of SDS. These aggregates dissociated slowly beyond 1 mM SDS concentration due to formation of spherical micelles. It has been observed that the smaller size QDs get incorporated inside the micellar Stern layer while the larger size QDs remain in aqueous phase. No such effect has been observed for cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) surfactant. The results at pH 4.5 and pH 7.1 indicate the involvement of specific electrostatic as well as hydrogen-bonding interaction between the amine groups and negatively charged sulfate headgroups. The extent of aggregation is more at pH 4.5 than pH 7.1 due to favorable electrostatic interaction between cationic Si-QDs and anionic SDS surfactant. The effect of local surface pH around the SDS micelles is considered to interpret the PL behavior at pH 7.1. We have performed PL microscopy of individual allylamine-capped Si-QDs to get further information about their inherent PL properties at single dot resolution. © 2013 American Chemical Society.