Raman spectroscopy. Raman spectra were obtained using a custom-built Raman spectroscopy system (Figure ) consisting of a multimodal Raman spectroscopy probe with a central excitation fiber surrounded by seven collection fibers (EmVision) connected to a 785 nm diode laser with maximum output of 500 mW (B&W TEK Inc.) for Raman excitation and collected using a fibercoupled QEPro spectrograph with a 1200 grooves mm -1 grating and a 50 μm slit (OceanOptics Inc., Dundin, FL). The Raman probe has a spot size of around 500 µm and, for comparison, 785 nm light penetration through tissues (depending on the tissue) is on the order of 1 mm. Raman spectra were acquired continuously with a 1 second integration time and 100 mW output power using custom, inhouse developed scripts (MATLAB 2017a, Mathworks, USA). Spectral processing was also performed in MATLAB and consisted of wavelength calibration using a NeAr lamp, system spectral response correction for CCD dark noise and fiber-optic probe background, fluorescence background subtraction (Whittaker filter, λ = 100,000), normalization to the area under the curve, and smoothing using a first order Savitzky-Golay filter with a frame length of 7.