Imaging of epithelial tissue in vivo based on excitation of multiple endogenous nonlinear optical signals

We demonstrate an integrated optical microscope to image the living epithelial tissue by simultaneously exciting multiple endogenous nonlinear optical signals. By employing the spectral lifetime detection capability, this technology provides a unique approach to sensing the fine structure, the protein distribution, and the cellular metabolism of epithelial tissue in vivo. In particular, we investigated the two-photon excitation fluorescence (TPEF) of tryptophan, an essential amino acid serving as the building block of protein. Our findings show that the TPEF of cellular tryptophan produces a good contrast to reveal the morphology of the epithelial cell layer, and the contrast can be further enhanced by applying low-concentration acetic acid.