fulltext26 - Photon Upconversion in Homogeneous...

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Photon Upconversion in Homogeneous Fluorescence-based Bioanalytical Assays T ERO S OUKKA ,T ERHI R ANTANEN , AND K ATRI K UNINGAS University of Turku, Department of Biotechnology, Turku, Finland Upconverting phosphors (UCPs) are very attractive reporters for fluorescence resonance energy transfer (FRET)-based bioanalytical assays. The large anti-Stokes shift and capability to convert near-infrared to visible light via sequential absorption of multiple photons enable complete elim- ination of autofluorescence, which commonly impairs the performance of fluorescence-based as- says. UCPs are ideal donors for FRET, because their very narrow-banded emission allows mea- surement of the sensitized acceptor emission, in principle, without any crosstalk from the donor emission at a wavelength just tens of nanometers from the emission peak of the donor. In addition, acceptor dyes emitting at visible wavelengths are essentially not excited by near-infrared, which further emphasizes the unique potential of upconversion FRET (UC-FRET). These characteristics result in favorable assay performance using detection instrumentation based on epifluorometer configuration and laser diode excitation. Although UC-FRET is a recently emerged technology, it has already been applied in both immunoassays and nucleic acid hybridization assays. The technol- ogy is also compatible with optically difficult biological samples, such as whole blood. Significant advances in assay performance are expected using upconverting lanthanide-doped nanocrystals, which are currently under extensive research. UC-FRET, similarly to other fluorescence tech- niques based on resonance energy transfer, is strongly distance dependent and may have limited applicability, for example in sandwich-type assays for large biomolecules, such as viruses. In this article, we summarize the essentials of UC-FRET, describe its current applications, and outline the expectations for its future potential. Key words: lanthanides; photoluminescence; infrared-to-visible; upconverting phosphors; inor- ganic nanocrystals; fluorescence resonance energy transfer Introduction Photon upconversion is a unique type of pho- toluminescence, in which lower-energy excitation (near-infrared light) is converted into higher-energy emission (visible light). 1 , 2 In contrast to two-photon excitation, upconversion is a feature of inorganic rare earth–doped crystals (upconverting phosphors [UCPs]), whose long-lifetime metastable excited states enable sequential multiphoton excitation (in microsec- ond time scale) to emission state to produce in- tense anti-Stokes photoluminescence already under relatively low laser power (below 1 W/cm 2 ). 3–5 Dif- ferent rare earths can produce upconversion photo- luminescence at various emission wavelengths char- acteristic to the lanthanide ion using a single near- Address for correspondence: Tero Soukka, University of Turku, De- partment of Biotechnology, Tykist¨okatu 6 A 6th floor, FI-20520 Turku,
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This note was uploaded on 07/11/2010 for the course SPECTOGRAP 545 taught by Professor Gdf during the Spring '10 term at AIB College of Business.

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fulltext26 - Photon Upconversion in Homogeneous...

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