Metabolic imaging of tumor tissue samples using planar fluorescence lifetime-based pH sensors and a SPAD array imager

Extracellular pH (pHe) is an important indicator of the homeostatic condition of mammalian tissue and its pathophysiological state such as tumor development and tissue acidosis. Optochemical sensors with internal referencing capabilities can perform dynamic quantitative imaging of pHe in live tissue samples. Here we have used planar solid-state pH sensors based on a fluorescent octaethylporphin-ketone (OEPK) dye in plasticized PVC matrix, which produce robust fluorescence lifetime response in the physiological pH range and stable lifetime calibration. A custom-built fluorescence lifetime imager, with a 405 nm picosecond diode laser excitation and a photon counting SPAD array detector (120 x128 pixels) with wide-field Fluorescence Lifetime Imaging (FLIM) capability, was used to image the planar pH sensor brought in contact with freshly excised normal and tumor tissue. The FLIM based sensor system has demonstrated good analytical performance and suitability for in vivo use. Producing detailed maps of tissue pHe, such sensors can facilitate a deeper understanding of tissue metabolism and physiology, and can find use in fluorescence guided surgery of solid tumors.