A General Approach to Convert Hemicyanine Dyes into Highly Optimized Photoacoustic Scaffolds for Analyte Sensing**

Most photoacoustic (PA) imaging agents are based on the repurposing of existing fluorescent dye platforms that exhibit non-optimal properties for PA applications. Herein, we introduce PA-HD, a new dye scaffold optimized for PA probe development that features a 4.8-fold increase in sensitivity and a red-shift of the λ_abs from 690 nm to 745 nm to enable ratiometric imaging. Computational modeling was used to elucidate the origin of these enhanced properties. To demonstrate the generalizability of our remodeling efforts, we developed three probes for β-galactosidase activity (PA-HD-Gal), nitroreductase activity (PA-HD-NTR), and H₂O₂ (PA-HD-H₂O₂). We generated two cancer models to evaluate PA-HD-Gal and PA-HD-NTR. We employed a murine model of Alzheimer's disease to test PA-HD-H₂O₂. There, we observed a PA signal increase at 735 nm of 1.79±0.20-fold relative to background, indicating the presence of oxidative stress. These results were confirmed via ratiometric calibration, which was not possible using the parent HD platform.