Table 1.

Comparison between the optical properties of traditional organic dyes and QDs for biological application

Properties	Organic dyes	QDs	Advantages of QD
Excitation spectrum	Narrow	Broad	Organic dyes can only be excited by light of a specific wavelength due to the narrow excitation spectrum vs QDs that may be excited by light of a range of wavelengths, allowing multicolour QDs to be excited by a single wavelength of light.
Emission spectra	Broad and asymmetrical	Narrow and symmetrical	The broad emission spectra of conventional dyes may overlap and this limits the number of fluorescent probes that can be tagged to biomolecules for simultaneous imaging in a single experiment. QDs have narrow emission spectra that can be controlled by altering the size, composition and surface coatings of the dots. Hence, multiple QDs emitting different colours can be excited by a single wavelength of light, making them ideal for multiplexed imaging.
Photobleaching threshold	Low	High	Organic dyes bleach within a few minutes on exposure to light whereas QDs are extremely photostable due to their inorganic core, which is resistant to metabolic degradation and can maintain high brightness even after undergoing repeated cycles of excitation and fluorescence for hours. Hence, they can be used for long-term monitoring and cell-tracking studies.
Decay lifetime	Fast (<5 ns)	Slow (30–100 ns)	The fluorescence lifetime of QDs is considerably longer than typical organic dyes that decay within a few nanoseconds. This is valuable in overcoming the autofluorescence of background tissues, hence improving signal to noise ratio.
Quantum yield	Low	High	QDs have higher quantum yields, a larger absorbance cross section and a larger saturation intensity than organic fluorophores in aqueous environments, making them much brighter probes for in vivo studies and continuous tracking experiments over extended periods of time.
Absorbance cross section	Low	High
Saturation intensity	Low	High