Size |
Tunable (2-10 nm) |
Typically <0.5 nm (protein or small molecule) |
Absorption Spectra |
Broad, discrete bands |
Narrow |
Emission Spectra |
Narrow (symmetric, size-tunable) |
Broad (asymmetric with red-shift tail) |
Molar Absorptivity (ε) |
Large (typically on the order of 0.5 to 5 × 106 cm−1M−1) |
Small (typically on the order of 0.5 to 2.5 × 105 cm−1M−1) |
Emission Spectra |
Narrow (symmetric, size-tunable) |
Broad (asymmetric with red-shift tail) |
Quantum Yield |
High (0.1-0.8 in visible range and 0.2-0.7 in NIR) |
Low (0.5-1.0 in visible range and 0.05-0.25 in NIR) |
Fluorescence Excited State Lifetime |
Long (10 to 100 nanoseconds with multi-exponential decay) |
Short (< 10 nanoseconds with mono-exponential decay) |
Two-Photon Cross Section |
Large (typically on the order of 0.2 to 5 × 10−46 cm4 sec photon−1) |
Small (typically on the order of 1 × 10−49 cm4 sec photon−1) |
Detection Sensitivity |
High |
Low |
Resistance to Photo-bleaching |
High |
Low |
Aqueous Solubility |
Low (need for surface coating or functionalization) |
High (control via chemistry) |
Tissue Penetration Depth |
Medium (size dependent) |
High |
Spectral Multiplexing Capability |
High |
Low (spectral overlap) |
Quantification Capability |
High |
Low |
Toxicity |
Potential for accumulation and heavy metal leaching, typically slow metabolism |
Chemistry dependent, typically rapid metabolism |
Key Strengths |
Ease of spectra tunability |
Typically stable, water-soluble materials |
Quantum yield (up to 100 times brighter) |
Ease of bioconjugation (many available commercially) |
High photostability |
Deep tissue penetration |
Key Challenges |
Potential toxicity in vivo
|
Lack of tunability |
Poor aqueous solubility and stability |
Prone to photo-bleaching |
Difficult to alter biodistribution |
More difficult to enable in vivo multiplex imaging |