In photoacoustic spectroscopy (PAS), wavelength dependent optical attenuation of biological tissue presents a challenge to measure the absolute oxygen saturation of hemoglobin (sO 2 ). Here, we employ the combination of photoacoustics and acousto-optics (AO) at two optical wavelengths to achieve quantification, where AO serves as a sensor for the relative local fluence. We demonstrate that our method enables compensation of spatial as well as wavelength dependent fluence variations in PAS without a priori knowledge about the optical properties of the medium. The fluence compensated photoacoustic images at two excitation wavelengths are used to estimate the absolute oxygen saturation of blood in a spatially and spectroscopically heterogeneous phantom.
A method that uses an integrating sphere as a platform for photoacoustic measurement of the Grüneisen parameter Γ of absorbing liquids is developed. Derivation of a simple equation for determining Γ is presented. This equation only requires the voltage peak-to-peak value of the photoacoustic signal detected by a flat transducer and the relative energy of the incident light measured by a photodetector. Absolute detector sensitivities are not required. However, a calibration procedure is necessary. An experimental setup is constructed in order to implement and verify the method. Aqueous ink solutions are used as absorbing liquids to determine the calibration (instrument) constants. Validation of the equation is done by determining Γ of ethanol at room temperature. The obtained value of Γ(ethanol) = 0.72 ± 0.06 has a 7% relative difference to the calculated value from known thermal properties reported in literature.
The optical properties of PEGylated gold nanorods (PEG-GNR) in interaction with cells have been investigated with Raman and fluorescence microspectroscopic imaging. The emission spectra were compared with those from dispersions of GNR, which can be characterized by a broad emission bandwidth of approximately 60 nm with a band maximum around 675 nm. These properties are in good agreement with observations from various other gold substrates and (nano)particles. The emission spectra from cells incubated with PEG-GNR were dominated by Raman scattering from locations where no GNR were present. Intense fluorescence spectral lines, with peak amplitude comparable with the Raman scattering from cells, were observed from locations containing GNR. The frequency range of the fluorescence emission spectra coincided mainly with the Raman fingerprint region from 500 cm−1 to 1800 cm−1, excited by the laser emission line at 647.1 nm. No surface-enhanced Raman spectra were observed. It was furthermore observed from cluster analysis of the Raman and fluorescence hyperspectral datasets that the GNR-related integrated fluorescence emission band from an individual cell could be sub-divided in multiple bands with slightly varying band maxima. Raman difference spectra of cells with GNR minus control cells showed that the amplitude of lipid signal in cells incubated with PEG-GNR was increased. An excellent correlation was found between the increased lipid signals and locations of the nanorods. This positive correlation between Raman signals from lipids and fluorescence signals from gold nanorods supports that gold nanorods are locally accumulating in lipid vesicles within the cells.
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