Table 1.

Advantages and weaknesses of major classes of functional probes that can be conjugated to QDots for biological labeling and imaging.

Targeting Probe	Advantages	Disadvantages
Organic Ligand	Uses known small-molecule drugs/agonists/inhibitors; a much more diversified base of ligand candidates compared to peptides/antibodies. Readily applied to binding/affinity assays and structure-function relationship studies. Potential in pharmacological or physiological applications.	Lack of commercially available sources. Fairly sophisticated organic chemistry is required for ligand synthesis and purification. Requires rigorous spectroscopic analyses. Requires additional pharmacological analyses. Potential issues with multivalency of the final conjugates.
Peptide	Readily available custom peptide synthesis services and well-established solid-phase synthesizers on the market. Applicable for intracellular targets when combined with cell penetrating peptides. Good biocomparibility Lower cellular toxicity in general compared to organic ligand. Particularly useful for certain neuropeptide receptors	Fewer reported peptides applicable for membrane protein targeting. Relatively short half-life in general compared with organic ligands; prone to degradation mechanisms and proteolytic digestion. May require additional pharmacological analyses. Relatively poor receptor specificity compared to the monoclonal antibody.
Antibody	Rich commercially accessible and feasible resources. Routinely used in biochemical and cell-based assays; gold standard in protein detection/labeling. Excellent biocompatibility; Low cellular toxicity concern in most cases.	Bulky, significantly increase the hydrodiameter after conjugation. Concerned with batch to batch variability, particularly with the use of polyclonal antibodies. Prone to degradation mechanisms and proteolytic digestion.