<unk> s (SiQDs) possess size-tunable photoluminescence that is similar to that observed for conventional<unk> <unk> s. The luminescence is routinely tuned throughout the visible and into the near-infrared region by defining particle size. In general, there are two distinct luminescence bands that dominate<unk>  properties. Long-lived luminescence excited states (S-band, slow decay rate) are typically associated with size-dependent photoluminescence ranging from yellow/orange to the near-infrared. Short-lived luminescent excited states (F-band, fast decay rate) are typically associated with size-independent blue photoluminescence and in some cases nitrogen impurities have been implicated in these processes. The S-band is typically attributed to the size-dependent band gap of the<unk> s. This emission can be tuned from yellow (600 nm) into the infrared (1000 to 1100 nm) by changing the diameter of the<unk> s from about 2 to 8 nm. Some reports also describe the preparation of green-emitting<unk> s prepared by decreasing the size, however, these materials are challenging to isolate and require further development.<unk>  luminescence may also be tuned by defining their surface chemistry. Attaching different surface species allows tuning of<unk>  luminescence throughout the visible spectrum while the<unk>  dimensions remain unchanged. It is interesting to note that this surface tuning is typically accompanied by the appearance of nanosecond lifetimes like those seen for F-band luminescence.<unk>  photoluminescence<unk>  yields are typically in the range of 10 to 40%, with a handful of synthetic protocols providing values in excess of 70%. The long-lived excited state of<unk>  S-band luminescence that starkly contrasts photoemission from conventional<unk> <unk> s is often attributed to the inherent indirect band gap of<unk>  and lends itself to unique material applications. Combining long-lived excited states with the biological compatibility of<unk> s enables time-gated biological imaging. The large Stokes shift allows them to convert photons from the ultraviolet range into the visible or infrared range and is particularly beneficial in the design and implementation of luminescent solar concentrators because it limits self-absorption while down converting the light. Importantly, SiQDs are biologically compatible and do not contain heavy metals (e.g., cadmium, indium, lead). The biological compatibility of these materials has been carefully studied both in vitro and in vivo. During in vitro studies, SiQDs have been found to exhibit limited toxicity in concentrations up to 72 µg/mL in HeLa cells and 30 µg/mL in epithelial-like cells (MDA-MB-231). In vivo studies assessing biological compatibility of SiQDs have been undertaken in mice and monkeys (rhesus macaques), that found “no signs of toxicity clearly attributable to SiQDs.” In bacteria, SiQDs have been shown to be less toxic than both CdSe and CdSe/ZnS<unk> <unk> s.
Silicon quantum dot
Hemichrome
Silver nanoparticle
Respiratory pigment
Vapours 
Tetrachlorodecaoxide
Hematoporphyrin
WAIFW matrix