Versatile hollow fluorescent metal-silica nanohybrids through a modified microemulsion synthesis route

Abstract

Silica-metal nanohybrids are common materials for applications in biomedicine, catalysis or sensing. Also, hollow structures are of interest as they provide additional useful features. However, in these materials the control of the size and accessibility to the inner regions of the structure usually requires complex synthesis procedures. Here we report a simple colloidal procedure for synthesizing hollow silica-metal nanohybrids, driven by the diffusion of metal precursors through the porous silica shell and subsequent reduction in aqueous solutions. The formation of hollow nanoparticles is controlled by the colloidal conditions during synthesis, which affect the ripening of hollow nanoparticles in presence of organosilanes. The modification of the conditions during synthesis affected the growth of silica precursors in presence of fluorescein isothiocyanate (FITC). The limited access to water molecules during the hydrolysis of silica precursors is attributed to the hydrophobicity of organic fluorescent molecules linked to the condensing silica clusters at the initial stages of nanoparticle formation and to the limitation of water content in the microemulsion method used. Finally, the growth of metal nanoseeds at the core of hollow nanoparticles can be easily achieved though a simple method in aqueous environment. The pH and thermal conditions during the reduction process affect the formation of metal-silica nanohybrids and their structural features.

Introduction

The potential of nanotechnology for transforming the scientific and industrial world along with our everyday life is already a matter of fact [1], [2]. New fabrication procedures are being investigated for the development of new nanomaterials with numerous applications [3], [4]. Of especial interest are nanocomposites and nanohybrids [5], [6], which show unique properties with applications in catalysis [7], [8], medicine [9], sensing [10], energy storage [11] or photonics [12], to cite a few. Furthermore, using specific fluorescent markers, these hybrid nanomaterials have proven their utility as tracers for risk assessment in nanosafety applications [13], [14], [15].

The silica-metal nanohybrids combine the chemical activity of metal clusters with a versatile nanoscale silica shell [16], [17], [18], [19], [20]. These have been loaded with drug molecules to obtain efficient smart-responsive drug delivery systems [21] or with luminescent moieties to produce light-reactive nanoparticles [22] as well as with superparamagnetic iron oxides to enable magnetic response [16], [23]. The extraordinary thermal and chemical stability, high hydrophilicity and biocompatibility of the silica matrix have allowed further applications in other areas [24].

The synthesis of nanohybrids is considered a laborious procedure, difficult to be scaled up and prone to reproducibility problems [25], [26]. Conventional synthesis methods involve the formation of voids within solid nanoparticles and further incorporation of small metallic clusters into cavities [27] or by means of sacrificial cores enclosed in a solid shell [28], [29]. Other procedures based in the Ostwald ripening have been also reported to produce core-shell structures in a variety of compositions [30]. For the synthesis of silica nanohybrids, the Stöber method is usually the reference procedure [31], [32]. Microemulsion methods have been proposed to produce complete nanoscale shells, which show excellent control in the surface hydroxylation [33]. Yet, the synthesis of functional silica-metal nanohybrids with narrow particle size distributions remains a great challenge [34].

Here we report a novel procedure to synthesize fluorescent hollow silica-metal nanohybrids. To this end, monodisperse fluorescent silica nanoparticles were synthesized in a water-in-oil microemulsion and further stabilized in anhydrous ethanol. To shed light on the process of formation of hollow nanostructures, we have tested the influence of several parameters, namely ionic strength, pH, temperature, and reagent concentration. Under the appropriate conditions, fluorescent silica-metal nanohybrids have been obtained by secondary processing of the hollow nanostructures in aqueous solutions of the suitable metal precursors.