Hybrid nanocomposites of semiconductor nanoparticles and conjugated polyelectrolytes and their application as fluorescence biosensors
Graphical abstract
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.
Section snippets
Materials
The conjugated polyelectrolyte was obtained by the Wittig condensation reaction [24], [25], and the number- and weight-average molecular weights of it are 12,100 Da and 19,800 Da, respectively. Horse heart cytochrome c (molecular mass 12,384 Da) from Sigma and cetyltrimethylammonium bromide (CTAB) from Shanghai Chemical Reagent Company were used as received. Silicon wafers were obtained from Huajing Microelectronics (Wuxi, China). The water used in making the stock solution of the conjugated
Photophysical properties of hybrid nanocomposites
Herein, the silicon nanoparticles are prepared by pulsed laser ablation (PLA) [26], [27] of silicon wafer in aqueous solution containing cationic surfactant of CTAB (2.3 mM). The as-prepared nanoparticles have an average diameter of about 5 nm observed by a transmission electron microscope (Fig. 2a). The result of inductively coupled plasma-atomic emission spectroscopy indicates that the silicon nanoparticles are at a concentration of 2.0 μg/mL. The photoluminescence spectrum (Fig. 2b) shows
Conclusion
Water-dispersible silicon nanoparticles can readily take up conjugated polyelectrolytes by electrostatic assembly to afford dual-fluorescent composite nanoparticles with inorganic semiconductor cores and shells of ultrathin organic semiconductor layer. Such inorganic-organic hybrid semiconductors have high surface density of ionic moieties and simultaneous high fluorescence yield and can be exploited to develop sensors for trace analysis of biological molecules. The nanocomposites are highly
Acknowledgement
This work is supported by the National Natural Science Foundation of China under Contract No. 20774040.
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