The big advantage or may be a curse for Biosensing is that great strides are being made on every component of a Biosensor. Be it ‘Bio’ where new methods of making and capturing biorecognition element are being invented everyday or ‘Sensing’ where nanotechnology is bringing in a wide array of sensitive transducers. I said curse because the task of integrating and validating ‘New Bio’ with ‘New Sensor’ is perhaps the most challenging job. But this discussion belongs to some other forum.
The advantage I am talking about is the report in recent PNAS issue about the use of ‘relativey new’ nanoparticle luminescent probes for cell imaging applications. The luminescent probes are lanthanide doped nanocrystals that adsorb low evergy near infra red (NIR) radiations and emit higher energy visible light-hence the name ‘Upconverting’ (Good review). Absorbing low energy radiation and emitting high energy light is opposite of the organic and protein fluorescent dyes that absorb higher energy light and emit red shifted low energy radiation.
The advantages that make UCNPs particularly attractive for cell imaging applications are
- Biological materials do not “Upconvert” hence no background: Better signal to background ratio
- Excitation wavength for UCNPs is low energy continuous wave laser of ~980nm. At 980nm, there is no problem of autofluorescence from biological material: Better signal to background ratio
- No “Blinking” as is typically seen in Quantum Dots
- Photostable: No loss of intensity was seen even after 1.0hr of continuous excitation
- UCNPs can be coated with polymers to make them water soluble and functionalize them with biomolecules or ligands
- Single molecule imaging
- Multicolor UCNPs possible by “mix-and-match” of nanocrystal structure and lanthanide dopant concentration and composition
Higher efficiency (105) of upconversion compared to 2-photon excitation of fluorescent organic dyes
All the features that make UCNPs attractive for cell imaging also make them useful for biosensing. UCNPs have already been used for immunoassyas, lateral-flow assays, DNA microarrays, immuno-histochemistry. A lateral-flow immunoassay device using UCNPs was commercialized by Orasure Technologies but its current status is not clear. Since the emission spectrum of UCNPs are extremely sharp they are also ideal for FRET type studies (Fluorescent Resonance Energy Transfer). Biological material are transparent at 980nm (the excitation wavength of UCNPs) and the large anti-stokes shift of UCNPs allows several organic dues to be used as acceptor molecules for FRET applications.
Considering the advantages UCNPs provide over conventional organic dues or even Quantum Dots, the number of reports using UCNPs for biosensing/bioassays have been rather limited. Some of the reasons are
- Methods for synthesis of monodisperse UCNPs in repordcucible fashion arre still eveolving
- Emission intensities of UCNPs are lower than the bulk material
- Coating chemistries of UCNPs are still being devloped
The unique advantages of UCNPs for various applications will inevitably attract the attention of wider scientific community and help solve some of the problems and widen their application for Biosensing.