Rational Design of Xanthene-based Photoacoustic Probes

Photoacoustic imaging (PAI) is an emerging biomedical imaging modality with promise as a point-of-care diagnostic. This imaging modality relies on optical excitation of an absorber followed by production of ultrasound through the photoacoustic effect, resulting in high spatial resolution with imaging depths in the cm range. The classical xanthene dye scaffold has found numerous applications in fluorescence imaging, however, xanthenes are rarely utilized for PAI since they do not typically display NIR absorbance.
This thesis starts with a brief overview of the current landscape of xanthene-based photoacoustic probes (Chapter 1). Next, we describe our first efforts to use Nebraska Red dyes for photoacoustic applications, resulting in a photoacoustic turn-on probe for hypochlorous acid (HOCl) (Chapter 2). While fluorescence brightness has guided the design and optimization of improved fluorophores for fluorescence imaging, no such benchmarking parameter exists for PAI. Consequently, there is no consensus on rational design strategies to improve photoacoustic signal from small molecule dyes. In Chapter 3, we disclose the discovery of the first benchmarking parameter for small molecule dye performance in PAI, which we term the acoustic loudness factor (ALF). ALF inspired the synthesis of louder dye scaffolds with improved in vitro and in vivo loudness. Building off this work, we describe attempts to increase the sensitivity of turn-on photoacoustic probes for HOCl (Chapter 4). In the concluding chapter (Chapter 5) an outlook for future design strategies and applications of photoacoustic probes based on Nebraska Red dyes is provided