Comparison of the characteristics and applications between traditional organic fluorophores and QDs.

PropertyTraditional organic fluorophores[3235]Quantum dots[29, 3642]
Chemical propertiesChemical resistance is often poorResistant to chemical degradation; sensitivity to pH determined by coatings
Size scaleMolecular, <0.5 nmColloidal, 1.5 nm to 10 nm diameter
Hydrodynamic radiusSmall, <0.6 nmaVariable, 1.4 nm to 40 nmb
Absorption spectraDiscrete bands, FWHMc, 35 nmd to 80 nm to 100 nmeStrong and broad
Emission spectraBroad, red-tailed, and asymmetric, FWHM, 35 nm to 70 nm to100 nmNarrow, symmetric, FWHM, 30 nm to 90 nm
Two-photon cross-section10 GM to 500 GM(2,000 to 47,700) GMf
Molar absorption coefficient(103 to 105) cm-1mol-1L(105 to 106) cm-1mol-1L
Quantum yieldVariable, 0.05 to 1.0High, >20%g
Fluorescence lifetimeShort, <5 ns, mono-exponential decayLong, >10 ns, typically multi-exponential decay
Solubility or dispersibilityControl by substitution patternControl via surface chemistry (ligands)
Thermal stabilityDependent on dye class; can be critical for NIR-wavelength dyesHigh; depends on shell or ligands
PhotostabilityUsually poorExcellent resistance to photobleaching; observation time of minutes to hours
Bioconjugation labelsMonovalent to multivalent labeling possibleScaffolds; Monovalent conjugation can be challenging; distribution of multivalences often encountered
Applicability to single molecule analysisModerate; limited by photobleachingGood; limited by blinking
Spectral MultiplexingPossibleIdeal for multi-color experiments; up to five colors demonstrated
MultifunctionalityDifficult and fewGreat potential
ToxicityVariable, based on dyeRelated to the heavy metal

a:Except for fluorescent proteins, GFP 4.6×2.4 nm cylindrical shape

b:Coating, ligand, and bioconjugate-dependent

c:FWHM, full width at half height of the maximum.

d:Dyes with resonant emission, such as fluoresceins, rhodamines and cyanines.

e:CT dyes.

f:Wavelength-dependent; GM: Goeppert–Mayer units

g:Ligand, coating and solvent-dependent