Squaraine dyes for biophotonics: sensing, imaging and theranostics

Squaraines, a celebrated class of organic dyes known for their remarkable photophysical properties, particularly fluorescence behaviour in the near infrared (NIR) region, have emerged as versatile molecular tools in the domain of biophotonics. They offer unparalleled opportunities for molecular sensing, bioimaging and theranostics. This review delves into the unique capabilities of these dyes, spotlighting advances in design and functionalization that enable their application across diverse biological systems. The sensitivity of squaraine dyes in biomolecular recognition is highlighted for detecting proteins such as bovine serum albumin (BSA) and enzymes like chymotrypsin. The fluorescence changes in squaraine dyes that signal binding events facilitate quantitative biomarker detection with superior detection limits. These signal responses enhance the utility of squaraines in DNA labeling, viscosity sensing, real time optical probing and cellular bioimaging. Beyond proteins and nucleic acids, the reactivity of squaraine dyes towards nucleophiles enables their use as both colorimetric and fluorimetric probes for the detection of thiol-containing biomolecules, lipids and metabolites. The dynamic optical responses of squaraine derivatives towards molecular interactions, such as reversible metal-ion binding, thiol-mediated interactions leading to fluorescence turn-on, etc., provide key insights into biochemical pathways. Innovative functionalization strategies and engineering of supramolecular assembly formation in squaraine derivatives further enhance their selectivity and signal modulations in complex biological systems. This review also highlights the emerging theranostic potential of squaraine dyes specifically for targeted therapeutic interventions and their chemical robustness for long-term bioimaging and diagnostics. The growing applications of squaraine-based functional materials in molecular sensing, bioimaging and theranostics pave the way for innovative advancements in diagnostics, personalized medicine and biomedical research.