Applications briefs: Localization of Human Hair Structural Lipids Using Nanoscale Infrared Spectroscopy and Imaging

Key points


  • AFM-IR capable of producing IR spectra and absorption images at a sub-micrometer spatial resolution.
  • Enables human hair to be spectroscopically characterized at levels not previously possible.
  • In particular, it was possible to determine the location of structural lipids in the cuticle and cortex of hair.

Key words

AFM-IR | nano IR spectroscopy | Hair Analysis | nanoFTIR

Authors

Curtis Marcott, Michael Lob, Kevin Kjoller, Francoise Fiat, Nawel Baghdadli, Guive Balooch, and Gustavo S. Luengo

Abstract

Samples of human hair were embedded, cross-sectioned, and mounted on ZnSe prisms. A tunable IR laser generating pulses of the order of 10 ns was used to excite sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever.

Differences are observed in the IR absorbance intensity of long-chain methylene-containing functional groups between the outer cuticle, middle cortex, and inner medulla of the hair. An accumulation of structural lipids is clearly observed at the individual cuticle layer boundaries. This method should prove useful in the future for understanding the penetration mechanism of substances into hair as well as elucidating the chemical nature of alteration or possible damage according to depth and hair morphology.

Contact mode AFM height image (top right) and 12 AFM-IR spectra of a human hair cross-section (top left) near the interface between the outer cuticle and cortex layers. The IR band labeled with an asterisk at 2924cm-1 is due to the absorbance of the long-chain CH2 antisymmetric stretching vibration of a lipid component.

AFM-IR single-wavenumber intensity ratio images (2930cm-1/1525cm-1) collected from cross-sections of three different human hair samples from the same source (bottom). The IR band located at 2930cm-1 is due to the absorbance of the long-chain CH2 antisymmetric stretching vibration of the lipid component, while the band at 1525cm-1 is due to the absorbance of the amide II vibration of the hair keratin protein. The white-colored areas are locations of higher lipid concentration.

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