Raman Spectroscopy of Carotenoid Compounds for Clinical Applications-A Review

Abstract

Carotenoid compounds are ubiquitous in nature, providing the characteristic colouring of many algae, bacteria, fruits and vegetables. They are a critical component of the human diet and play a key role in human nutrition, health and disease. Therefore, the clinical importance of qualitative and quantitative carotene content analysis is increasingly recognised. In this review, the structural and optical properties of carotenoid compounds are reviewed, differentiating between those of carotenes and xanthophylls. The strong non-resonant and resonant Raman spectroscopic signatures of carotenoids are described, and advances in the use of Raman spectroscopy to identify carotenoids in biological environments are reviewed. Focus is drawn to applications in nutritional analysis, optometry and serology, based on in vitro and ex vivo measurements in skin, retina and blood, and progress towards establishing the technique in a clinical environment, as well as challenges and future perspectives, are explored.

Keywords: Raman spectroscopy; beta carotene; blood; carotenoids; lutein; lycopene; optometry; skin; zeaxanthin.

Figure 1

Examples of carotenoid compounds of relevance to human health: (a) beta carotene, (b) lutein, (c) zeaxanthin, (d) meso-zeaxanthin and (e) lycopene (public domain images reproduced from Wikimedia Commons).

Figure 2

Absorption spectra of beta carotene aggregates in 1:1 aqueous ethanol (dot) and in pure ethanol (dash dot). Dash line represents the negative second derivative of the experimental results. Coloured lines are the Voigt fitting components and their sum (green) (Reprinted with permission from Ref. [54]. 2022, Elsevier).

Figure 3

Schematic depiction of non-resonant and resonant Raman scattering processes. Energy levels are represented by horizontal lines, and the vertical arrows show the transition from one state to another (Reprinted with permission from [85]. 2022, Elsevier).

Figure 4

(a) Beta carotene spectrum at multiple wavelengths and (b) 532 nm spectrum of beta carotene, lutein and zeaxanthin.

Figure 5

(a) Raman spectroscopic detector showing the argon ion laser, spectrograph, light delivery/collection module and excitation laser spot on the palm of a subject (Reprinted under the Creative Commons Licence from Ref. [119]. 2022, MDPI). (b) Diagrammatic description of the Raman spectroscopic detector on a skin surface [131].

Figure 6

Raman intensity mapping showing the distribution of total macula carotenoids and individual macula carotenoids, zeaxanthin and lutein in a human retinal section (Reprinted with permission from Ref. [91]).