CdS and CdSe luminescent quantum dots (QDs) have been widely investigated for their luminescence properties. Luminescent QDs show a number of advantages compared to organic fluorophores commonly used in biological applications. QDs exhibit higher photostability than organic dyes. Their emission band is size dependent due to quantum confinement effects. For example, 3 nm CdSe QDs emit green light while 6 nm CdSe QDs emit red light. It is therefore possible to prepare a series of solutions showing different emission colours by using only one type of semiconductor material. The emission spectra of semiconductor QDs are symmetric and sharp with a full width at half maximum (FWHM) as narrow as 30 nm. On the other hand, emission spectra of organic dyes are asymmetric and broad. This precludes the simultaneous use of several organic dyes to analyze multi-analyte samples due to overlap between their broad emission peaks. Luminescent semiconductor QDs has a wide excitation spectrum which enables the excitation of QDs of different size with a single excitation wavelength. In contrast, multiple excitation wavelengths are needed to simultaneously excite several organic fluorophores. Recognizing their potential bioanalytical researchers have recently applied luminescent QDs as biological labels, selective ions probes and luminescent gas sensors.