In addition to conventional down-conversion fluorescence emissions, reports have also shown that certain CQDs have up-conversion fluorescence emission properties. Up-conversion fluorescence emission is an optical phenomenon wherein the fluorescence emission wavelength is shorter than the used excitation wavelength, which is particularly attractive for in vivo bioimaging since bioimaging at longer wavelengths especially in the NIR region is usually preferred owing to the improved photon tissue penetration and reduced background auto-fluorescence.