Calcium Sensor for Photoacoustic Imaging

Abstract

We introduce a selective and cell-permeable calcium sensor for photoacoustics (CaSPA), a versatile imaging technique that allows for fast volumetric mapping of photoabsorbing molecules with deep tissue penetration. To optimize for Ca²⁺-dependent photoacoustic signal changes, we synthesized a selective metallochromic sensor with high extinction coefficient, low quantum yield, and high photobleaching resistance. Micromolar concentrations of Ca²⁺ lead to a robust blueshift of the absorbance of CaSPA, which translated into an accompanying decrease of the peak photoacoustic signal. The acetoxymethyl esterified sensor variant was readily taken up by cells without toxic effects and thus allowed us for the first time to perform live imaging of Ca²⁺ fluxes in genetically unmodified cells and heart organoids as well as in zebrafish larval brain via combined fluorescence and photoacoustic imaging.