Comparison of the characteristics and applications between traditional organic fluorophores and QDs.
| Property | Traditional organic fluorophores[32–35] | Quantum dots[29, 36–42] |
|---|---|---|
| Chemical properties | Chemical resistance is often poor | Resistant to chemical degradation; sensitivity to pH determined by coatings |
| Size scale | Molecular, <0.5 nm | Colloidal, 1.5 nm to 10 nm diameter |
| Hydrodynamic radius | Small, <0.6 nma | Variable, 1.4 nm to 40 nmb |
| Absorption spectra | Discrete bands, FWHMc, 35 nmd to 80 nm to 100 nme | Strong and broad |
| Emission spectra | Broad, red-tailed, and asymmetric, FWHM, 35 nm to 70 nm to100 nm | Narrow, symmetric, FWHM, 30 nm to 90 nm |
| Two-photon cross-section | 10 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 yield | Variable, 0.05 to 1.0 | High, >20%g |
| Fluorescence lifetime | Short, <5 ns, mono-exponential decay | Long, >10 ns, typically multi-exponential decay |
| Solubility or dispersibility | Control by substitution pattern | Control via surface chemistry (ligands) |
| Thermal stability | Dependent on dye class; can be critical for NIR-wavelength dyes | High; depends on shell or ligands |
| Photostability | Usually poor | Excellent resistance to photobleaching; observation time of minutes to hours |
| Bioconjugation labels | Monovalent to multivalent labeling possible | Scaffolds; Monovalent conjugation can be challenging; distribution of multivalences often encountered |
| Applicability to single molecule analysis | Moderate; limited by photobleaching | Good; limited by blinking |
| Spectral Multiplexing | Possible | Ideal for multi-color experiments; up to five colors demonstrated |
| Multifunctionality | Difficult and few | Great potential |
| Toxicity | Variable, based on dye | Related 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