Fluorescent nanoparticles are more stable and modifiable than organic fluorescent dyes. For two decades, quantum dots (QDs) have been the leading fluorescent nanoparticle in biomedical applications. Recently, we have developed a group of novel fluorescent nanoparticles with superior and unique properties that is now poised to challenge and even supplant QDs in many applications.

These novel nanoparticles are known as upconverting nanoparticles (UCNs), so named because of their ability to absorb low-energy light and emit high-energy radiation. This property is advantageous for biomedical applications because filters that allow reverse fluorescence block out all natural auto-fluorescence, which results in very high signal-to-background ratios. The particles are chemically stable, resistant to photobleaching and their fluorescence is independent of environmental effects. Furthermore, they can be excited using safe near-infrared rays – a big advantage for all applications dealing with live cells/tissues/animals/humans.

Progress in the field

The development of a single-step hydrothermal synthetic procedure pioneered in the Cellular and Molecular Bioengineering Laboratory (CMBL) at NUS resulted in the production of relatively uniform sub-100 nm water-soluble UCNs, and opened up the field to a host of other applications. The resulting nanoparticle had a polar polymer coat, which not only made the structure water soluble, but enabled the attachment of other molecules to enhance and add functions. For example, adding a targeting molecule to the surface enabled these UCNs to be specifically identified and taken up by cancer cells, as demonstrated in a subsequent paper. Following this, we reported a series of enhancements, driving down the size of the particles and improving the uniformity and solubility. Our most recent paper describes the development of silica-coated highly uniform sub-50 nm UCNs, unthinkable less than a decade ago.

The demonstration of multicoloured emission was another important development, which opened up the field of in vitro multiplexed assays. Here, differently coloured nanoparticles are attached to different cells so that the effect of a single chemical on these cells can be investigated in parallel, which saves time.

The unique long-wavelength excitation and emission characteristics of these particles theoretically allows sensitive detection in tissues and from within tissues. The first report on imaging of UCNs in live cells and animals was presented by us. We demonstrated that fluorescence from UCNs, injected under the skin of small animals like mice and rats, could be seen from the surface if excited with an infrared laser. This potentially can be used to detect the accumulation of tagged cells and drugs from under the skin. We are now in the process of submitting results, which demonstrate that live cells injected below the skin, tagged with UCNs, can be detected by laser confocal microscopy and followed for several hours to determine their activity.

To enable these in vivo applications, we have performed several tests to demonstrate the biocompatibility of these nanoparticles with a variety of cell types. Also, we have determined the bio-distribution of these particles in rats and shown that there is little residual accumulation of these particles after less than a week post-injection, meaning that no long-lasting adverse effects can be expected from their use.

Apart from imaging and diagnostics, we have also harnessed the optical properties of UCNs for therapeutics. UCNs have been demonstrated to act as nano-transducers for photodynamic therapy of diseased cells. By this method, infrared light is used to trigger the activation of anti-cancer drugs called photosensitizers deep within the body in specific areas of interest only. This limits the toxicity of the drugs and increases the efficiency of the photodynamic therapy. Recently, we reported an enhancement of this system, replacing the polymer coat with porous silica to hold the photosensitizer drugs.