Abstract
Deuterium labelling is increasingly used in coherent Raman imaging of complex systems, such as biological cells and tissues, to improve chemical specificity. Nevertheless, quantitative coherent Raman susceptibility spectra for deuterated compounds have not been previously reported. Interestingly, it is expected theoretically that -D stretch vibrations have a Raman susceptibility lower than -H stretch vibrations, with the area of their imaginary part scaling with their wavenumber, which is shifted from around 2900 cm-1 for C-H into the silent region around 2100 cm-1 for C-D. Here, we report quantitative measurements of the nonlinear susceptibility of water, succinic acid, oleic acid, linoleic acid and deuterated isoforms. We show that the -D stretch vibration has indeed a lower area, consistent with the frequency reduction due to the doubling of atomic mass from hydrogen to deuterium. This finding elucidates an important trade-off between chemical specificity and signal strength in the adoption of deuterium labelling as an imaging strategy for coherent Raman microscopy.
Highlights
Raman micro-spectroscopy is increasingly used as a label-free, chemically specific imaging modality in cell biology.[1]
We have shown that the Raman scattering coefficients and the coherent Raman susceptibility χ of the –H stretch vibrations approximately scale with the vibrational frequency under isotopic substitution of hydrogen to deuterium
This is consistent with theoretical expectations of a sum rule for the Raman polarizability
Summary
Raman micro-spectroscopy is increasingly used as a label-free, chemically specific imaging modality in cell biology.[1]. Deuterated compounds have been used extensively to provide specific chemical contrast when examining lipid bilayers,[5,6] drug delivery,[7] protein synthesis,[8] metabolic dynamics,[9] glucose metabolism,[10] and neurodegenerative disorders.[11] Triggered by these developments, a basic question arises, namely, how is the magnitude of the Raman scattering cross-section changing upon deuterium substitution, and in turn, how is the CRS susceptibility amplitude scaling. In Martín and Montero,[17] ethane was measured in the gas phase and fitted with a bond-polarizability model, followed by ethene[21] and benzene.[22] The reported scattering cross-sections for C–H and C–D stretch vibrations are used here to evaluate the sum rule prediction. The deviation from 1 (i.e., a constant s independent of the isotopes) is close to the reported measurement error; we can conclude that the scaling of the scattering coefficients with the vibrational frequency for deuteration is approximately valid for these examples. We are not aware of a quantitative comparison of the CRS susceptibility of deuterated versus nondeuterated compounds
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