Squaraine-based NIR dyes for phototheranostics

Phototheranostics, a promising approach combining optical imaging and phototherapy, holds great promise for early cancer diagnosis and precise treatment. It is vital to develop multi-functional phototheranostic agents with easy processability and high biocompatibility. Squaraine dyes, known for their zwitterionic structure with an electron-deficient central four-membered ring, are particularly attractive for phototheranostics. These dyes are easily synthesized via condensation reactions and offer remarkable optical properties, such as strong absorption and emission in the visible to near-infrared (NIR) regions, high fluorescence quantum yields, and exceptional photostability, making them ideal for phototheranostics. This review outlines the main synthetic strategies and photophysical properties of squaraine dyes, followed by a discussion on their recent applications in bioimaging, photodynamic therapy (PDT), and photothermal therapy (PTT). Additionally, future prospects for advancing squaraine dyes in phototheranostic applications are explored.