Abstract

Cyanine dyes are excellent candidates as bioimaging and therapeutic agents. Their brightness, high quantum yields and highly conjugated and rigid structure with combination of several modification sites give them unique properties. They have absorbance in NIR I region (650-900nm) and NIR II (900- 1500nm), which are known as therapeutic window, promoting deeper penetration, less light scattering, and lower fluorescence signal from biomolecules. By increasing the conjugation between heterocycles their absorbance can be shifted to NIR II region to improve their imaging properties. Their structural malleability allows for a variety of different modifications. For example, by incorporating different moieties on the heterocycles to increase solubility and altering at the meso position to introduce pH sensitivity for activable targeting.

Presence of different functional groups provide versatile conjugation sites on the cyanine scaffold for various active groups and small molecules e.g., targeting ligands, antibodies, and drugs etc. While cyanine dyes can be used as small molecules for bioimaging, they can be linked to targeting moieties to be used as active targeting agents or they can be encapsulated with nanoparticles to enhance their solubility and circulation time in body. In addition to nanoparticles, they can be linked to nanorods, proteins, polymers, and micelles. Herein, the structural modifiability of cyanine scaffold will be discussed for different conjugation modalities including drug delivery, targeted therapy, and targeted imaging.