Ag-coated submicron particles of polystyrene formed by dewetting process and their application in multi-functional biosensor-chips

Abstract

The formation of submicron polystyrene (PS) particles based on dewetting process during spin-coating was investigated. The surface morphologies on glass substrates were analyzed. The formation of particles was achieved using PS with relatively high molecular weight (Mw ≥ 3250). In addition, it was observed that the particle formation proceeded through sequential morphological changes in the order of initial wet thin film, secondary spinodal-decomposed structures, and eventual dewetted particles. One of the fabricated samples exhibited obvious optical resonance in the visible region based on the relationship between particle dimeters and optical wavelengths in the visible region. The optical resonance strongly depended on the ambient refractive index, which can be used for optical sensor-chips. Finally, a multi-functional biosensor-chip was demonstrated to detect glucose in water solutions.

Introduction

Polymer materials exhibit various morphologies based in their wetting/dewetting characteristics on substrates [[1], [2], [3], [4]], similar to the wettability of aqueous solutions on various surfaces. The wettability-based phenomena are usually observed in polymer materials under thermal treatment [[5], [6], [7], [8]] and in polymer material solutions during solvent evaporation [[9], [10], [11]]. Various morphologies can be observed, including spherical particles [7,8,[11], [12], [13], [14], [15]], ring-shaped structures [[16], [17], [18]], ridges (or fibrils) [6,9,11,15,19], porous membranes [6,12,17,19], and networks (or mosaic patterns) [6,7,9,14,16]. The formation of these morphologies is associated closely with following phenomena of wetting, dewetting, spinodal decomposition, phase separation, solvent evaporation, and thermal transformation. These phenomena that start from initial thin films often occur either intermittently, simultaneously, sequentially, and/or synchronously [2,12,16,20]. As a result, such phenomena make wetting/dewetting morphologies complex and attractive. Similar phenomena can be observed in cases of metallic materials [[21], [22], [23]]. The produced morphologies can be used in various applications, such as polymeric material patterning [10,24,25], hydrophobic surface [11,26], and lens arrays [14,15].

Metal nanoparticles (spheres, tringles, and rings) [[27], [28], [29], [30], [31], [32]] and metal-coated polymer particles [[33], [34], [35], [36], [37], [38]] are important candidates in the field of optical sensor-chips, including plasmonic sensor-chips. Polymer patterning based on wetting/dewetting can be employed as polymer templates for constructing metal-coated polymer particles. A typical configuration of the sensor-chip is formed by a monolayer of polystyrene (PS) colloidal spheres coated by thin layer of noble-metal [33,34,38,39]. The degree of order in the PS colloidal sphere monolayer is directly associated with the characteristic of the optical resonance. When PS sphere arrangement is highly regulated (i.e., closed-packed hexagonal array), sharp peak and valley can be observed in the extinction and reflection (scattered) spectra, respectively, as distinct optical resonance. Because the peak shift of the resonant wavelength usually depends on the change in ambient refractive index, metal-coated PS particle monolayer can be used as a sensor-chip to detect adsorption of substances, which is available for chemical sensors and biosensors. On the other hand, when the order of arrangement in the monolayer is relatively low, the extinction and reflection (scattered) spectra are broadly distributed, which differs from the case of highly-regulated monolayer [33]. Therefore, if the randomly distributed PS particles covered by noble metals are prepared, the sample is expected to exhibit a broad resonance and negligible changes with the variations in the ambient refractive index in the reflection (scattered) spectrum. It is considered that these characteristics are not suitable for sensing devices. However, a multi-functional sensor-chip could be obtained based on Ag-coated PS particles having a broad optical resonance in the scattered spectrum, contrary to the initial expectations. Indeed, Zhang and coworkers reported on efficient surface-enhanced Raman scattering (SERS) substrates which were fabricated using dewetting process of thin polymer blend films [40]. In this paper, we report on the formation of PS particles and the dependence on preparation conditions. We also investigated the sensing ability of the Ag-coated PS particles using glucose solutions.