Abstract:
Optical-fluorescence imaging provides molecular specificity and spatio-temporal resolution necessary for noninvasive imaging of cellular interactions and bio-electrical activity. However, these techniques suffer from limited imaging depth constrained by optical absorption and scattering in tissue. This research investigates the feasibility of photoacoustic (PA) sensing of biopotentials, which relies on absorption of light by voltage- sensitive probes and subsequent generation/detection of ultrasound. PA-based voltage sensing could noninvasively provide voltage maps with spatial and temporal resolutions that are adequate for monitoring changes in neuronal cell-membrane potential in intact functional circuits. We have demonstrated the detectability and sensing of bio-electrical activity using PA-based voltage sensing. We have achieved this in two ways: (a) by characterizing the optical-absorption properties of voltage-sensitive dyes as a function of membrane potential change using a custom absorption spectrophotometer, and (b) by using in vitro experiments involving cell cultures to demonstrate that the photoacoustic signal from cells labelled with voltage-reporting probes track the change in cell potential. Pheochromocytoma (PC12) cells were tagged with voltage-sensitive probes such as the commercially available voltage-sensitive absorption dye RH155 and a novel voltage- reporting nano-construct consisting of a CdSe-CdS/ZnS core-shell quantum dot (QD) conjugated to a peptide-fullerene bioconjugate. Cells were depolarized by administering potassium chloride (KCl), which was verified using whole-cell patch-clamp. A spectrophotometer was used to characterize the corresponding change in optical absorption. PA-signal amplitude exhibited a monotonic change with increasing cell- membrane potential and the dynamics of the PA-signal change was consistent with the theoretically modelled change in membrane potential. In summary, we have shown that PA-based voltage sensing can provide signal-to-noise-ratio and temporal response that are comparable to fluorescence sensing. When biopotentials were chemically altered in cell cultures, the photoacoustically measured voltage exhibited the same temporal dynamics as those observed by fluorescence. The methodologies developed in this research effort can potentially alleviate challenges encountered in current fluorescence-based techniques, and facilitate the study of neurological activity in healthy, diseased, and injured brain in intact biological models.
