Mini-review: fluorescence imaging in cancer cells using dye-doped nanoparticles

Fluorescence imaging has gained increased attention over the past two decades as a viable means to detect a variety of cancers. Fluorescence imaging has the potential to provide physicians with high resolution images with enhanced contrast, which will allow them to be able to better diagnose and treat patients with cancer. Early detection and treatment are key to eradicating cancer in a patient, and fluorescence imaging has the ability to identify non-advanced, even pre-cancerous, tumors where imaging based on white light or radiation overlooked them. Several fluorescent dyes have been identified as possible fluorophores for enhanced fluorescence imaging, such as cyanine, squaraine, porphyrin, phthalocyanine, and borondipyrromethane dyes. These dyes have high fluorescence quantum yields, which provides a high target to background ratio; however, these dyes are often plagued by low water solubility. This low solubility can be ameliorated by conjugating or covalently attaching these dyes to polymeric crosslinked micelles, polymersomes, or polymer-core nanoparticles. These particle & dye systems then can become platforms on which secondary components can be attached to enhance the systems functionality. For example, dyes attached to these nanocarriers can target tumors through passive targeting; however, active targeting can be achieved by further modifying these nanocarriers with ligands that have a binding affinity for receptors overexpressed in tumor cells, cell surface receptors located on the tumor cell membrane, or endothelium. Fluorescence activation of the probes is another promising technology for the early detection of cancer. Activation requires that there be a change in fluorescence, whether it be an emission wavelength change or a fluorescence "on/off" signal when in the presence of some external stimuli. Activation increases the target to background ratio and enhances the contrast of the obtained image. This review serves to highlight the recent developments of (1) improved fluorescent dyes for the detection of cancer, with a specific focus on dyes that are being coupled to nanocarriers; (2) dye & nanocarrier systems that target, both actively and passively, tumors, and (3) fluorescence activation of these fluorophore systems for better image quality.