A diode laser based clinical diagnostic system using shifted excitation resonance Raman difference spectroscopy for the in vivo detection of ss-carotene in human skin

Non-invasive in vivo monitoring of the antioxidant level of human skin, e.g. by measuring carotenoid marker substances, can deliver significant data on human health status and can provide diagnostic data during medical treatments. Among others, resonance Raman spectroscopy is a promising contactless tool to detect carotenoids in human skin within the necessary nanomolar concentration range. Unfortunately, laser induced fluorescence and ambient light can obscure the Raman signals even when using the resonance Raman effect and exciting the carotenoids at wavelengths around 500 nm. Here, shifted excitation resonance Raman difference spectroscopy (SERRDS) is a powerful and easy-to-use tool separating the wanted from the unwanted signals. For this purpose, a portable clinical diagnostic system including a compact Raman handheld probe and a miniaturized wavelength-tunable frequency-doubled diode laser-based 488 nm light source was developed. The diode laser can be tuned over 2 nm providing the two excitation lines with a flexible spectral distance for SERRDS, resulting fluorescence-free skin Raman spectra. For reliable and representative measurements of human skin, an excitation spot diameter of 3 mm was selected. An excitation power of 9 mW at the sample provides a power density of 1.3 mW/mm(2) meeting the laser safety regulations with a maximum permissible exposure of 20 mW/mm(2). A calibration procedure was performed using skin phantoms containing ss-carotene at selected concentrations and a limit of detection of 0.05 nmol g(-1) of ss-carotene is achieved using the 3-sigma criterion.