Hybrid nanocomposites of semiconductor nanoparticles and conjugated polyelectrolytes and their application as fluorescence biosensors

Abstract

Weakly emissive silicon nanoparticles with an average diameter of about 5 nm are prepared via pulsed laser ablation of silicon wafers in water. Electrostatic assembly of water-soluble conjugated polyelectrolytes on the surface of the silicon nanoparticles steadily enhances the photoluminescence of these nanocomposites, indicating the possibility of energy transfer between the semiconductor nanoparticles and the conjugated polymer, or silicon nanoparticle-induced elimination of chain aggregates of the conjugated polyelectrolyte. Fluorescence emission of the hybrid silicon-conjugated polymer nanocomposites is steeply quenched by cytochrome c, and the minimum detection concentration for the redox-active protein is found to be 50 nM. The sensitization is realized by ultrafast photoinduced electron transfer between the electron-deficient protein and the conjugated polyelectrolyte binding on the silicon nanoparticle surfaces. The results offer guidelines to explore novel sensors for detecting nanoparticles, and also help develop high-efficiency sensory materials based on electrostatic complexes of conjugated polyelectrolytes and inorganic semiconductor nanoparticles.

Introduction

Semiconductor nanomaterials offer unique opportunities to study materials in the regime between molecular and bulk states due to the quantum confinement effect imposed upon their charge carriers [1]. Silicon plays a key role in mesoscopic semiconductor electronics. As an indirect band material, silicon is an inefficient emitter but shows dramatic changes in optical properties when its size is reduced to less than 10 nm. Recent investigations on silicon nanocrystals and one-dimensional silicon quantum wires show dramatic changes in their light-emission behavior [2], [3]. The potential applications of silicon nanomaterials in optoelectronic devices, fluorescent labels for biological cell imaging, highly sensitive biological and chemical sensors, and non-volatile memory devices have triggered broad interests [4], [5], [6], [7], [8], [9]. Silicon nanomaterials have been fabricated by electrochemical etching of silicon wafers, laser pyrolysis, high-temperature aerosol reactions, plasma deposition, and chemical reduction techniques in colloids or micro-emulsions [10], [11], [12], [13], [14]. To meet the requirements of biosensor applications, silicon nanoparticles should be water-dispersible, highly luminescent, and have functional groups for conjugation with biological molecules in aqueous environments.

The photoluminescence in the visible region renders the silicon nanoparticles potential fluorescent probes in biological systems. However, post-synthesis procedures for semiconductive nanomaterials are generally required to afford a substantial photoluminescence quantum yield and hydrophilicity to prevent aggregation and precipitation in a biological environment. For example, the silicon nanoparticles have been chemically modified with allylamine by a platinum-catalyzed addition reaction to hydrogen-terminated surfaces [15]. In comparison with these chemical processes, we describe a rather simple method by electrostatic adsorption of oppositely charged conjugated polyelectrolytes on the silicon nanoparticle surfaces. The chemical structure of the polyelectrolyte, poly[(2-methoxy-5-propyloxy sulfonate-1,4-phenylene vinylene)-alt-(1,4-phenylene vinylene)], is shown in Fig. 1. The water-soluble polyelectrolyte has a π-conjugated backbone, which can be regarded as an array of charged chromophores (or fluorophores) that are fully conjugated [16], [17]. Thus it can absorb/emit light that is consistent with its band gap, and most importantly, effectively transport electrons or energy along the “molecular wire” of chromophores [18], [19], [20], [21], [22], [23]. Electrostatic assembly thereby affords dual-fluorescent composite nanoparticles with inorganic semiconductor cores and shells of ultrathin organic semiconductor layers. This hybrid nanocomposites can serve as a high-sensitivity fluorescence sensor for the detection of the redox-active metalloprotein cytochrome c.