Photon upconverting nanoparticles convert near-infrared into visible light (anti-Stokes emission), which strongly reduces the background of autofluorescence and light scattering in biological materials. Hexagonal NaYF(4) nanocrystals doped with Yb(3+) as the sensitizer and Er(3+)/Ho(3+)/Tm(3+) as the activator display at least two emission lines that respond differently to temperature changes. The ratio of the main emission line intensities enables a self-referenced optical readout of the temperature in the physiologically relevant range from 20 to 45 °C. Upconverting nanoparticles of the type NaYF(4):Yb, Er covered by an inactive shell of NaYF(4) are bright and allow for resolving temperature differences of less than 0.5 °C in the physiological range. The optical readout of this nanoparticle-based thermometer offers many options for imaging the two-dimensional distribution of temperature.
The pH sensor exploits the phenomenon of upconversion luminescence and is based on a hydrogel matrix containing (a) nanorods of the NaYF(4):Er,Yb type that can be excited with 980-nm laser light to give a green and red (dual) emission, and (b) a longwave absorbing pH probe that causes a pH-dependent inner filter effect.
Turned off by oxygen: Luminescent upconverting nanoparticles (UCNPs) of the type NaYF4:Yb,Tm are employed in an entirely new type of optical sensor for oxygen (see picture). Upon laser excitation at 980 nm, these UCNPs act as nanolamps, the blue emission of which is used to photoexcite an iridium complex dissolved in ethyl cellulose. Its green emission, in turn, is dynamically and fully reversibly quenched by molecular oxygen.
Upconverting nanoparticles (UCNPs) display the unique property of converting near-infrared light (with wavelengths of typically 800-1,000 nm) into visible luminescence. Following a short introduction into the mechanisms leading to the effect, the main classes of materials used are discussed. We then review the state of the art of using UCNPs: (1) to label biomolecules such as antibodies and (synthetic) oligomers for use in affinity assay and flow assays; (2) to act as nanolamps whose emission intensity is modulated by chemical indicators, thus leading to a novel kind of chemical sensors; and (3), to act as donors in luminescence resonance energy transfer in chemical sensors and biosensors.
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