Raman Diffusion-Ordered Spectroscopy

30 June 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


The Stokes-Einstein relation, which relates the diffusion coefficient of a molecule to its hydrodynamic radius, is commonly used to determine molecular sizes in chemical analysis methods. Here, we combine the size-sensitivity of such diffusion-based methods with the structure-sensitivity of Raman spectroscopy by performing Raman diffusion-ordered spectroscopy (Raman-DOSY). The core of the Raman-DOSY setup is a flow cell with a Y-shaped channel containing two inlets, one for the sample solution and one for the pure solvent. The two liquids are injected at the same flow rate, giving rise to two parallel laminar flows in the channel. After the flow stops, the solute molecules diffuse from the solution-filled half of the channel into the solvent-filled half, at a rate determined by their hydrodynamic radius. The arrival of the solute molecules in the solvent-filled half of the channel is recorded in a spectrally resolved manner by Raman micro-spectroscopy. From the time series of Raman spectra, a two-dimensional Raman-DOSY spectrum is obtained, which has Raman frequency on one axis and diffusion coefficient (or equivalently, hydrodynamic radius) on the other. In this way, Raman-DOSY spectrally resolves overlapping Raman peaks arising from molecules with different sizes. We demonstrate Raman-DOSY on samples containing up to three compounds and derive the diffusion coefficients of small molecules, proteins, and supramolecules (micelles), illustrating the versatility of Raman-DOSY. Raman-DOSY is label-free and does not require deuterated solvents, and can thus be applied to samples and matrices that might be difficult to investigate with other diffusion-based spectroscopy methods.



Supplementary materials

Raman Diffusion-Ordered Spectroscopy: Supplementary Information
Conventional Raman spectra of the component solutions and Singular-Value Decomposition of the raw data.


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