New Perimidine based Squaraine Dyes for Optoacoustic Imaging

Optoacoustic imaging is an emerging bioimaging technique that combines optical excitation and ultrasound detection to generate a signal. The signal is generated through the optoacoustic effect, and the signal generated can be tracked at multiple wavelengths by Multispectral optoacoustic tomography (MSOT). As sound waves are the detection method, they have lower light scattering in tissue compared to light, allowing for deeper tissue penetration and higher-resolution images. For optoacoustic imaging contrast agents, the two main criteria are optimal: strong absorbance and weak fluorescence. Current agents are designed for fluorescence imaging, resulting in poor signal intensity and spectral shape, making them not applicable for optoacoustic imaging. Squaraine dyes are a class of Near Infrared (NIR) region scaffolds that have shown promising biological applications. The drive to achieve NIR region optical properties is due to biomolecules having limited to no interference in this region. This allows for deeper penetration into the cell and a better signal-to-noise ratio. Current squaraine dyes primarily utilize the indolium heterocycles, which are designed for fluorescence imaging in the NIR region. In addition, indolium-based squaraine dye has an absorbance below 700 nm, which underutilizes the full NIR potential. When the perimidine heterocycle is introduced to squaraine dyes, the absorbance is redshifted to over 800 nm. Perimidine-based squaraine is designed to be an optoacoustic imaging contrast agent due to strong absorbance and weak fluorescence. Herein, we report the synthesis, optical properties, and initial optoacoustic imaging data for the new squaraine dyes. These perimidine-based squaraine dyes have shown high optoacoustic signal and unique spectral shape that range the NIR region.