<unk> s have been used in prototype applications owing to their biocompatibility and the ubiquitous nature of<unk> , compared to other types of<unk> <unk> s. In addition to these fundamental properties, the unique optical properties of<unk> s (i.e., long-lived excited states, large Stokes shift and tunable luminescence) can be advantageous for certain applications. Owing to these (and other) properties, the potential applications of SiQDs are diverse, spanning medical, sensing, defense, and energy related fields. Biological imaging The biocompatibility of<unk> s along with their long luminescent lifetimes and near-infrared emission makes them well-suited for fluorescence imaging in biological systems. Due to this promise,<unk> s have been applied for both in vitro and in vivo imaging. While steady-state imaging is traditionally used, the keen advantage of<unk>  comes into play for time-gated imaging. Time-gated imaging employs a delay between the excitation and the luminescence detection, this allows fluorophores with short lifetimes to relax, thus highlighting those with long lifetimes. This type of fluorescence imaging is useful for biological imaging as many tissues exhibit autofluorescence that can interfere with imaging. By using this technique, the signal to background ratio for imaging SiQDs can be increased up to 3x over conventional steady-state imaging techniques. Other modes of imaging have also been explored for<unk>  nanomaterials. For example, the<unk>  core of large<unk>  nanoparticles has been used for 29Si MRI in mice models. By modifying the surface with a ligand that can coordinate 64Cu, PET imaging is also accessible. Further, doping with paramagnetic centers show promise for T1 and T2 weighted 1H MRI. A photograph of glass slides with a<unk>  coating under UV light.<unk>  coated slides such as these are useful for luminescent solar concentrators. Courtesy of Applied<unk>  Materials Inc. Luminescent solar concentrators Luminescent solar concentrators take advantage of the large Stokes shift of the<unk> s to convert light into electricity. The large Stokes shift allows the SiQDs to convert UV light into red/near infrared light that is effectively absorbed by<unk>  solar cells, while having limited self absorption. The LSCs are designed to collect light and use the glass to waveguide the re-emitted light towards the edges of the glass, where the solar cells collect the light and convert it to electricity. By designing the LSC carefully, the<unk> s can be prepared as a transparent film over the glass limiting losses due to scattering, while making them suitable as replacements for windows in buildings. To do this effectively, the surface of the<unk> s can be modified with various ligands to improve polymer compatibility. It is also desirable to push the absorbance of the SiQDs into the visible to correspond better with the solar spectrum, which can be accomplished by adding a dye. Light-emitting diodes<unk> <unk>  displays utilize<unk> <unk> s to produce pure monochromatic light. Most of the work designing LEDs based on<unk> s have focused on electroluminescence of the<unk> s. By changing the size of the SiQDs, the LED emission can be tuned from deep red (680 nm) to orange/yellow (625 nm). Despite promising initial results and advances towards improving the external<unk>  efficiency of the resulting LEDs, future work is required to overcome the broad luminescence emission. A photograph of an array of SiQDs on a paper filter illuminated by UV light that is used for sensing applications. Courtesy of Applied<unk>  Materials Inc. Sensing Photochemical sensors take advantage of the<unk>  photoluminescence by quenching photon emission in the presence of the analyte. Photochemical sensors based on<unk> s have been used to sense a wide variety of analytes, including pesticides, antibiotics, nerve agents, heavy metals, ethanol, and pH, often employing either electron transfer or fluorescence resonance energy transfer (FRET) as the method of quenching. Hazardous high energy materials, such as nitroaromatic compounds (i.e., TNT and DNT), can be detected at nanogram levels via electron transfer. In the electron transfer method, the energy level of LUMO of the molecule is between the valence and conduction bands of the<unk> s, enabling the transfer of an excited state electron to the LUMO, and, therefore, preventing radiative recombination of the electron hole pair. This also works when the HOMO of the analyte is just above the conduction band of the SiQD, enabling the electron to transfer from the analyte to the SiQD. Alternative methods of detection via quenching of the SiQD core have also been explored. By functionalizing the<unk> <unk> s with enzymes, various biologically relevant materials can be sensed due to the formation of metabolites. Using this method, glucose can be detected via the formation hydrogen peroxide that quenches luminescence. Another method uses ratiometric sensing, where a fluorescent molecule is used as a control and the relative intensities of the two fluorescent labels are compared. This method was used to detect organophosphate nerve agents visually at a lower concentration than can be observed for SiQD quenching alone.
Silicon quantum dot
Hemichrome
Silver nanoparticle
Respiratory pigment
Vapours 
Tetrachlorodecaoxide
Hematoporphyrin
WAIFW matrix