Biosensing Based on Luminescent Semiconductor Quantum Dots and Rare Earth Up-Conversion Nanoparticles

Abstract

A biosensor is a device incorporating a biological sensing element either intimately connected to or integrated within a transducer, which is mainly determined by specific molecular recognition such as enzyme-substrate, antibody-antigen and so on. Currently, with the development of nanoscience and nanotechnology, more and more interest has been focused on using nanoparticles to fabricate biosensors (Alivisatos, 2004; Katz, 2004; Rosi 2005). Several kinds of biological sensors based on semiconductor quantum dots (QDs), gold nanoparticles (GNPs), carbon nanotubes (CNTs), fullerene, dendrimer nanoparticles have been presented. Semiconductor QDs are a new class of fluorescent materials for biosensor. In comparison with conventional organic dyes and fluorescent proteins, they have unique optoelectronic properties with size-tunable light emission, superior signal brightness, resistance to photobleaching and broad absorption spectra for simultaneous excitation of multiple fluorescence colors (Alivisatos, 1996). However, the use of semiconductor QDs for biosensor application still has some limitations (Jaiswal, 2004), for instance, the potential toxicity of QDs may pose risks to human health and the environment under certain conditions (Derfus, 2004). In addition, the absorption of UV and visible light by biological samples often induces autofluorescence, which interferes with fluorescent signals obtained from exogenous biomarkers. Moreover, if biological samples are prolonged exposure to UV radiation, it would cause the samples photo-damage and mutation. The drawbacks of QDs in biosensing application have prompted the development of upconverting nanoparticles (UCNs) emerged as another class of new biosensing materials. Usually, UCNs exhibit intense visible or near-infrared light excited by near-infrared light according to the anti-stokes law. The UCNs also show a sharp emission bandwidth, long lifetime, tunable emission, high photostability, low bio-toxicity and good biocompatibility, which are less harmful to biological samples and have greater penetration depth through biological samples than conventional ultraviolet excitation. Moreover, UCNs can be easily coupled to proteins or other biological macromolecular systems and used in a variety of