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Surface technology

Label-free chemical imaging

Using spectroscopic imaging to characterise samples

We use a range of techniques to characterise surfaces.

Raman and non-linear optical spectroscopy

Raman scattering can be used to measure and map chemical distribution within samples based on molecular vibrations. The energy required to excite molecular vibrations within molecules depends upon the masses of atoms, the types of bonds and the chemical microenvironment. Therefore, the scattering of incident monochromatic radiation generates frequencies related to the functional groups present.

Confocal Raman and stimulated Raman scattering (SRS) microscopy are available at NPL and are suitable for non-destructive analysis of a range of sample types with minimal requirements for sample preparation. Both techniques generate signals which are linear with concentration, enabling some degree of quantitative analysis.

Confocal Raman microscopy

  • Renishaw InVia Qontor Raman microscope
  • Can reach sub-micron spatial resolution
  • Visible and NIR laser excitation sources
  • Temperature controlled stage and incubator suitable for cell imaging.

Example applications include:

  • Surface chemistry of coatings
  • Identification of crystalline phases and defects
  • Characterisation of pharmaceutical and consumer health products
  • Drug and chemical distribution in cells and tissues
  • Identification of oxidation and corrosion products

Stimulated Raman scattering (SRS) microscopy

In SRS, two lasers are used, where the frequency difference is tuned to match the vibrational frequency of a molecule of interest (frequency can be identified in advance using confocal Raman). This stimulates the specific vibrational mode targeted and allows rapid imaging. The lasers can be raster scanned across the sample for 3D imaging at speeds approaching video-rate. Due to the modulation transfer detection mechanism, unlike coherent anti-Stokes Raman scattering (CARS), SRS images are comparatively free from non-resonant background such as emitted fluorescence, and the signal is linear with concentration.

  • Spatial resolution: ~400nm laterally, ~800 nm axially
  • Label-free chemical imaging (although deuteration can be helpful to improve chemical specificity in complex samples)
  • Rapid high resolution imaging with single frequency Raman contrast per image. Imaging can then be repeated at different frequencies to reveal the distribution of multiple components (for example, the skin cream image below shows the aqueous phase in blue overlaid with the lipid phase in red).

Example applications include:

  • Visualisation of ingredients within formulated products such as creams
  • Fate of topically applied drugs/chemicals
  • Drug/chemical distribution in biological tissues
  • Controlled release studies

Contact us

Lead researcher

Natalie Belsey

Senior Research Scientist

Natalie Belsey is a Senior Research Scientist in the Surface Technology Group.

View Natalie Belsey

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