ReviewSquaraine dyes: The hierarchical synthesis and its application in optical detection

- Summary of frequently-used synthetic strategies to SQ dyes.
- SQ-based colorimetric and fluorescence probe for metal ions, anions, pH, thiol-based compounds and biomolecules.
- Current challenges and perspectives in the field of synthesis methods and optical applications of SQ dyes.

Squaraine dyes, a four-membered ring system with structural rigidity, possess unique photoelectrical properties and are marked by their exceptionally sharp and intense absorption associated with a strong fluorescent emission in solution. These favorable characteristics have prompted their exploitation in a number of state of the art applications including photoconductivity, data storage, light-emitting field-effect transistors, solar cells and fluorescent histological probes. In this review, we first summarize the recently proposed novel methods in the synthesis of these versatile derivatives. Subsequently, their extensive applications in the prevalent optical detection of the surrounding medium such as ions, pH, thiol-based compounds, biomolecules and cell over the past decades are covered and discussed. In addition, different categories for the synthesis and sensing mechanisms for various squaric acid-based chemo-/bio- sensors are illustrated. Finally, the challenges and opportunities in the synthesis and application of these derivatives are also briefly discussed.

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