Photoacoustic (PA) imaging is an emerging imaging modality that combines the advantages of optical imaging and ultrasound imaging. In particular, activatable PA probes, which visualize the presence or the activity of target molecules in terms of a change of the PA signal, are useful tools for functional imaging. In this chapter, we describe the development of small-molecule-based activatable PA probes, focusing on the design and synthesis of PA-MMSiNQ, our recently developed activatable PA probe for HOCl. We also describe the protocols used for evaluation of PA-MMSiNQ with a UV-vis spectrometer and a PA imaging microscope.

An increasing number of intracellular and extracellular proteins are shown to interact with membrane lipids under physiological conditions. For rapid and robust quantitative measurement of lipid-protein interaction, we developed a sensitive fluorescence quenching-based assay that is universally applicable to all proteins and lipids. The assay employs fluorescence protein (FP)-tagged proteins whose fluorescence emission intensity is decreased when they bind vesicles containing quenching lipids. This simple assay can be performed with a fluorescence plate reader or a spectrofluorometer and optimized for different proteins with various combinations of FPs and quenching lipids. The assay allows a rapid, sensitive, and accurate determination of lipid specificity and affinity for various lipid-binding proteins, and high-throughput screening of molecules that modulate their membrane binding.