Selective surface reactions for Janus ORMOSIL particles with multiple functional groups using an ordered monolayer film at liquid–liquid interface

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Abstract

Monodispersed, submicron-sized Janus ORMOSIL particles with multiple functional groups were prepared by the selective surface reaction of a monolayer film formed at a hexane–water interface. A well-ordered monolayer film was obtained by self-assembly of ORMOSIL particles with multiple functional groups at hexane–water interface. The photopolymerization of an ordered monolayer containing ORMOSIL particles yields a rigid film strong enough to maintain its integrity for transfer and further chemical reaction. The chemical reaction of this ordered film with organic and inorganic functional groups produced Janus ORMOSIL particles with multiple functional groups. The morphologies, structures, and chemical compositions of monolayer films and Janus ORMOSIL particles were characterized by FT-IR, solid state NMR, X-ray diffraction (XRD), optical microscopy (OM), electron microscopies (SEM and TEM), and confocal laser scanning microscopy.

Highlights

► Monodispersed Janus ORMOSIL particles with multiple functional groups were prepared. ► A monolayer film was obtained by self-assembly of ORMOSIL particles at interface. ► Photopolymerization of a monolayer yield a rigid film to perform further reaction. ► Selective surface reaction of a rigid film produced Janus ORMOSIL particles.

Introduction

Monodispersed colloidal particles with multiple functionalities and submicron sizes have potential for use in various types of industrial materials [1], [2], [3] and biological applications [4], [5], [6]. To be suitable for these applications, colloidal particles must possess specific chemical and physical properties such as controlled structures and chemical compositions. Several techniques have been developed for synthesizing a variety of submicron particles with proper structures and compositions, including core–shell synthesis, layer-by-layer techniques, multi-block polymer emulsifications, and surface modifications [7], [8], [9], [10], [11]. Colloidal silica particles have been extensively studied, as they can be easily synthesized with controlled sizes and shapes [12], [13], [14]. Organically modified silica (ORMOSIL) particles are hybrid materials that can be prepared by surface modification of silica particles from tetraethylorthosilicate (TEOS) or using an organo-silane monomer as a starting material [15], [16]. However, these ORMOSIL particles usually contain only a single type of functional group. Therefore, multiple sequential modification steps must be employed to attach multitasking components such as imaging components, or drugs and targeting moieties. For wider biological applications, the development of a simple and efficient preparation method for particles with various functionalities is of great importance.

Multiple functionalities of colloidal particles can also be provided by asymmetry in shape and/or chemical composition of the particles. Janus particles, which were first mentioned by de Gennes in 1991, are asymmetric and usually exhibit different physical/chemical properties on opposite sides [17]. Their asymmetric dual functionality makes them effective as solid surfactants in emulsion stabilization, imaging/targeting/delivery probes and sensors in biochemical applications, and as photonic or display materials in electronic devices, as well as making them good building blocks for complex structures [18], [19], [20], [21], [22], [23], [24]. Janus particles are generally prepared by direct syntheses or indirect chemical or physical modifications. Micro-fluidic methods, electrohydrodynamic jetting, and self-assembly of block copolymers are typical direct synthetic routes, while indirect chemical and physical modifications involve gel trapping methods, metal sputtering, and Pickering emulsion methods [25], [26], [27], [28], [29], [30], [31], [32]. Although excellent control of shape and anisotropic features can be achieved using these methods, the submicron-size region is not readily accessible, and an inability to load chemical and biological components such as dyes, drugs, and image contrasting agents limits their use in biochemical applications. Therefore, the multiple chemical functionalities of Janus particles provide a great opportunity but raise significant challenges in the precise control of the dimensions, morphologies, and physicochemical properties of these asymmetric particles.

In this study, we describe the synthesis and the formation of an ordered monolayer with ORMOSIL particles with multiple functional groups, the photo-polymerization of a monolayer to generate a rigid film, and finally, the preparation of Janus particles using an asymmetric synthesis method. The synthetic method described in this study is very simple and produces monodispersed Janus ORMOSIL particles with homogenously distributed multiple functional groups inside and outside of spherical particles. These Janus ORMOSIL particles are not only the first example of Janus particles with multiple functional groups, but also can be further modified by the simple surface reaction to provide versatile materials in nano- and bio-applications such as multicolor imaging, signal amplification, self-assembly to ordered structures, and multiplex tasking.

Section snippets

Materials

Phenyltrimethoxysilane [PTMS] (94%, Aldrich), 3-mercaptopropyltrimethoxysilane [MPTMS] (97%, Fluka), 3-aminopropyltrimethoxysilane [APTMS] (99%, Aldrich), silver nitrate (99.8%, Junsei), iron (II) sulfate heptahydrate (98%, Sam Chun Chemicals), and hydrazine monohydrate (80%, Sam Chun Chemicals) were used without further purification. Ammonium hydroxide solution (30 wt.% as NH3), nitric acid (60%), hexane (95%), and methyl-methacrylate [MMA] (99%) were obtained from Sam Chun Chemicals.

Preparation of ORMOSIL particles with tri-functional groups (phenyl, amine, and mercapto functionalities)

A 250-mL

Results and discussion

Monodispersed, submicron-sized ORMOSIL particles having three different chemical functional groups were synthesized by a simple one-pot method, and a well-ordered monolayer film made up of these ORMOSIL particles was prepared using a self-assembly process at the interface between water and hexane. To obtain a rigid monolayer film, photochemical polymerization of the methyl-methacrylate monomers onto a monolayer formed with ORMOSIL particles was carried out by injecting the monomers into the top

Conclusions

In conclusion, monodispersed, submicron-sized Janus ORMOSIL particles with multiple functional groups were prepared by the asymmetric reaction of a monolayer film formed at a hexane–water interface. ORMOSIL particles with three different chemical functional groups were synthesized by a simple one-pot synthesis method. A well-ordered monolayer film was obtained by self-assembly of ORMOSIL particles with multiple functional groups at a hexane–water interface. The photochemical polymerization of

Acknowledgments

This work is the outcome of a Manpower Development Program for Energy supported by the Ministry of Knowledge and Economy (MKE) and was also supported by the Converging Research Center Program through the Converging Research Headquarter for Nanotechnology-based Information Energy funded by the Ministry of Education, Science and Technology (2010K001385).

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