Chandrasekhara Venkata Raman was a child prodigy who obtained his graduate degree by the age of 19. He was partly encouraged by a case of mistaken identity in which the eminent British physicist Lord Rayleigh directed personal correspondence to him as professor Raman, responding to his already-published work. In those days, the colors of the sky and sea were still hot topics in heady dispute. Rayleigh had previously identified elastic scattering of light in Earth’s atmosphere that causes us to see the sky as blue; however, he had also claimed the azure sea for a subservient and reflective mirror of the sky. When Raman traveled across the Mediterranean, he was so taken by its color that he decided this could not be, and amended Rayleigh’s observations using a prism, spectroscope, and diffraction grating to determine that the sea was also scattering light on its own.
Raman’s career was founded upon a series of experiments in which monochromatic light acquired color and polarity when beamed through certain liquid or crystalline samples. When scattered photons move to lower or higher energy states than incident photons, the resulting Stokes or anti-Stokes shifts can be recorded as lines within Raman spectra. This inelastic light scattering is much weaker than Rayleigh scattering, and the change in frequency corresponds to minute changes in vibrational and rotational energy in chemical bonds within molecules. Raman scattering can thus be quantified and used as a molecular fingerprint, in principle with equipment not much more advanced from that which Raman himself used.
Although it is analogous to infrared (IR) spectroscopy, Raman spectroscopy (RS) can choose its monochromatic source from a much wider array of the electromagnetic spectrum. As a result, it lagged in utility behind IR for many years, with unsatisfactory available light sources that required painstaking subtraction of interfering ambient light and fluorescence. The advent of lasers in the 1960s, and the later addition of charge-coupled detectors improved its throughput, convenience, and accuracy substantially.
With these technical refinements, and its evolving interface with machine learning and informatics-based algorithms, RS is beginning to realize its full power as an analytical tool. Most significantly, it is a platform that uses a fundamental property of physics to accomplish the comparatively quotidian basic chemistry task of determining exactly what things are in a mixture of things. As a reagent-less and non-destructive procedure, it is exquisitely adaptable to measurement in the laboratory or in the field, and to biology in glass or in living tissue, in which repeated measurements over time hold the power to inform treatment and prognosticate outcome.
In the laboratory and clinic, RS can be a novel approach to define the molecular basis of disease, and use these principles to revolutionize diagnosis. The typical laboratory Raman apparatus is exemplified by the Renishaw InVia confocal Raman microscope, which has been used in several ex vivo studies of brain tissue to discriminate between normal white and grey matter, compared to invasive, tumor, and necrotic tissue, all of which bear intrinsic Raman spectral signatures.
In prostate, pancreatic, breast, and ovarian cancers in which early detection is both highly important and extremely difficult, surface-enhanced (SERS) and tip-enhanced (TERS) variations of Raman increase sensitivity several orders of magnitude by employing metallic nanoscale substrates, and are helping to identify new biomarkers of early malignancy. Diagnosis and monitoring in living tissue are still largely in proof-of-principle phases, in which miniaturization of Raman components can be tailored to each tissue under study, and employ custom fiber optic probes with integrated or external lasers and CCDs. These and other emerging RS-based procedures are helping to optimize screening, biopsy, tumor margin assessment, and continuous monitoring of treatment and its success or shortcomings.