Near-Infrared Fluorophores Based on Heptamethine Cyanine Dyes: From Their Synthesis and Photophysical Properties to Recent Optical Sensing and Bioimaging Applications

This review summarizes diverse synthetic methodologies to prepare a wide range of heptamethine cyanine dyes, including symmetrical, unsymmetrical, and meso-substituted derivatives, and their photophysical properties and uses. Their sensing applications are also discussed, considering the nature of their electronic structures and photophysical properties. The optical sensing applications of these compounds can be potentially expanded via structural modification, broadening their absorption amplitude and spectral emission range from the ultraviolet to near-infrared (NIR) region. In this review, several recent practical applications of the heptamethine cyanine dyes for ion and small molecule sensing, as well as bioimaging, are covered and discussed.