Silica nanoparticles-walled microcapsules

Abstract

Colloidosomes are microcapsules with shells consisting of coagulated or partially fused colloid particles [1]. Since Dinsmore and collaborators [2] proposed the first concept of colloidosomes, many types of colloidosome structures have been proposed and discussed, and the research related to colloidosomes had become an attractive subject of many research groups. For example, Paunov et al. [3] reported the fabrication of ‘‘hairy’’ colloidosomes with shells of polymeric microrods. Wang et al. [4] produced the magnetic colloidosomes using a nanoparticle interfacial selfassembly method. Using the principles of colloidosomes, Binks and collaborators [5] investigated the temperatureinduced inversion of nanoparticle-stabilized emulsions. Due to its novel microstructures, colloidosomes are expected to have potential applications in many areas of science and technologies [6]. It has been recognized that the colloidosome membranes offer a great potential in controlling the release rate of entrapped species. Their major advantage is that the membrane pores size can be varied by varying the size of the particles and by controlling their degree of fusion. The fabrication of colloidosomes not only provides a way to construct novel microstructures with nanosized building-blocks, but also gives a new way to generate microcapsules with controllable permeability. Currently, the fabrication processes of colloidosomes are mainly based on the assembly of nanoparticles at the oil–water interfaces. In those processes, solid fine particles are first prepared and then these solid particles are assembled at the oil–water interface to produce the colloidosome structures. In this letter, we demonstrate the direct fabrication of silica colloidosome via a simple sol–gel process. The resultant silica colloidosomes are hollow, silica nanoparticles-walled microcapsules. Microcapsules are novel structures which are designed for the protection, transmission and controlled releasing of active ingredients such drugs, proteins, vitamins, flavors, gas bubbles, or even living cells. Versatile core materials are encapsulated for several reasons, such as improvement of long-time efficiency, stabilization against environmental degradation, easy handling through solidification of liquid core, and maintenance of non-toxicity of degradation products [7]. In a microcapsule system, the controlling behavior of the microcapsule is greatly determined by the structure and properties of the shell material used. Currently, the commonly used wall materials are urea formaldehyde and melamine formaldehyde (MF) resins. With the expanding applications of microcapsules, in some case, these microcapsules will be used in more rigorous conditions, e.g., at high temperatures. The commonly used wall materials cited above cannot meet these challenges. In addition, how to well construct the microstructures of the walls is still a challenge to researchers. Herein, we demonstrate the fabrication of a new type of silica nanoparticles-walled microcapsule. The schematic microstructure of the silica nanoparticleswalled microcapsule is shown in Fig. 1. This silica nanoparticles-walled microcapsule can be well constructed by using a two-step sol–gel process shown in Fig. 2. The principle for the formation of uniform silica nanoparticles is based on a two-step sol-gel process which is found in a literature method [8]. Using this two-step sol–gel method, uniform silica nanoparticles can be prepared. In this two-step sol–gel method, silica precursors were firstly hydrolyzed in aqueous media under the catalysis of diluted acid, and then ammonia was introduced for the Y. Liu (&) AE X. Chen AE J. H. Xin Nanotechnology Center, Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, P. R. China e-mail: tcliuyy@inet.polyu.edu.hk J Mater Sci (2006) 41:5399–5401 DOI 10.1007/s10853-006-0248-8