Synthesis of Cone-Shaped Colloids from Rod-Like Silica Colloids with a Gradient in the Etching Rate

Abstract

We present the synthesis of monodisperse cone-shaped silica colloids and their fluorescent labeling. Rod-like silica colloids prepared by ammonia-catalyzed hydrolysis and condensation of tetraethyl orthosilicate in water droplets containing polyvinylpyrrolidone cross-linked by citrate ions in pentanol were found to transform into cone-shaped particles upon mild etching by NaOH in water. The diameter and length of the resulting particles were determined by those of the initial rod-like silica colloids. The mechanism responsible for the cone-shape involves silica etching taking place with a varying rate along the length of the particle. Our experiments thus also lead to new insights into the variation of the local particle structure and composition. These are found to vary gradually along the length of the rod, as a result of the way the rod grows out of a water droplet that keeps itself attached to the flat end of the bullet-shaped particles. Subtle differences in composition and structure could also be resolved by high-resolution stimulated emission depletion confocal microscopy on fluorescently labeled particles. The incorporation of a fluorescent dye chemically attached to an amine-based silane coupling agent resulted in a distribution of fluorophores mainly on the outside of the rod-shaped particles. In contrast, incorporation of the silane coupling agent alone resulted in a homogeneous distribution. Additionally, we show that etching rods, where a silane coupling agent alone was incorporated and subsequently coupled to a fluorescent dye, resulted in fluorescent silica cones, the orientation of which can be discerned using super-resolution confocal microscopy.

Sculpting Silica Colloids by Etching Particles with Nonuniform Compositions

“…Thus, various growth conditions actually exist in the solution as a result of the rapid cooling down process. Furthermore, the slow gradient in composition, which was initially observed for silica rods grown at room temperature, 27 cannot be observed in these particles; the first segment of the rod does not transform into a concave shape upon etching with the mild base. This is probably caused by the higher temperature during the growth of the first segment that makes this segment more resistant against etching.…”

“…The thin shell again originates from the attachment of the precursor directly from the oil phase and is similar to the layer found in earlier work. 27 The fluorescence from the shell is lower than that from the tail segment. The fluorescent segment appears to be shorter in length than the segment grown, but as described in earlier work, the dye cannot infiltrate the particle far enough.…”

“…Nevertheless this confocal image gives a good indication that the functional group is distributed throughout the tail segment of the particles. 27 In the following we illustrate how differences in the rate of silica etching in both base and HF can be used to achieve unusual silica morphologies from spherical core–shell particles.…”

“…26 Close inspection of this fluorescence pattern has shown that the dye is mainly located in an outer thin silica shell. 27 However, upon incorporating APTES alone, followed by the fluorescent labeling with FITC (fluorescein isothiocyanate) afterward, a homogeneous distribution of fluorophores was found. This pattern of fluorescence indicates that the local chemical composition of the particle can be tuned by changing the properties of the reacting species.…”

“…This gradient was previously used for the transformation of rods into cone shaped particles, while, depending on the reaction conditions, biconcave rods could also be synthesized. 27 …”