Starting my PhD in Raman Spectroscopy, I often encounter the challenging dilemma of explaining not only the importance of my project but also its fundamental nature. How, indeed, can one convey the value of something without a basic understanding of it? To illustrate the significance and utility of an idea or tool, I find it useful to draw comparisons with aspects of our own biology and culture.
In our biology and our culture, what fundamentally separates humans from animals? Is it our capacity for communication, our knack for negotiation, or our ability to forge peace and relationships? Or, perhaps more fundamentally, is it rooted in the capabilities of our vision? By examining these aspects, we can better appreciate the unique insights that Raman Spectroscopy offers, much like how our vision allows us to perceive the world.
Homo sapiens and birds of the feather
Homo sapiens are a rare species amongst mammals because of our unique vision. A vision which is comparable to that of insects, fish and birds. Human beings operate their vision from 400nm to 700nm. Within this spectral region, we perceive different colours at various levels of sensitivity.
There are of course other animals which can see different ranges of the visible spectrum at different intensity levels. Butterflies and bees can see colours which are well into the UV range of below 300nm. This capability enhances the UV-reflective structures found in both male and female butterfly wings, as well as illuminating the UV-reflective properties of flowering petals -- allowing flying insects to see each other and food.
This range in vision is not exclusive to insects and humans, but is also native to birds, octopi, crabs, and fish. Modern butterflies and mantis shrimp exhibit what could be described as hyper-spectral sensitivity to colour, although their range remains limited to approximately 300–700 nm. Unfortunately, their nervous systems do not process the individual pigments separately, which is a drawback.
Here are several other animals, yet we observe that the majority fall within the same visible range as human vision.
The general human population has 3 colour perceptual channels: Red, Green and Blue. The intensity to which each of these channels are experienced varies over a Gaussian distribution. While other mammals, such as dogs, are commonly thought to be color-blind, they actually possess only two cones, enabling them to distinguish between yellow and blue hues. Capitalizing on their heightened sense of smell, mammals adeptly navigate their surroundings, rather than using their sight.
Cold blooded animals such as snakes and pythons use bolometric sensing organs: an organ sensitive to infrared radiation. As oppose to cones coiled up in the retinal preceptors, bolometric sensing organs expand and warm in the presence of an IR emitter.
Sea creatures like dolphins and whales, along with bats, utilize echolocation to discern features in their environment. They emit high-frequency sound waves; the echoes that bounce back enable them to determine the distances and shapes surrounding them. This ability extends to analysing how these echoes are altered by objects, allowing these creatures to visualize the structures of prey and the sizes of predators effectively.
The general population of human beings have 3 cones: we are trichromats. However, a 12% of women in the human population have an addition colour receptor: they are tetrachromatic. This is an additional cone which is due to an X chromosome inactivation and it has indeed been demonstrated that they are indeed tetrachromatic and have an additional colour perceptual channel. [1]
As I've demonstrated, there's an entire invisible world around us—a natural world that employs unique visual and spectroscopic abilities to enhance its search for mates and food.
The Rise of Value
Our vision, our human vision, is what gave rise to the our evolutionary development. While snakes use warm objects to distinguish prey, dolphins emit sound waves to track terrestrial structures, human beings see the world with the speed of light: the fastest way of recognising immediate objects around us. Combining our ability of vision with our upright posture on two legs-- leading to our remaining limbs to utilise tools and weaponry--further combined with our ability to tackle as many calories with as little energy expenditure as possible, led to our increase in brain size and our cognitive abilities. This combination ultimately culminated into excess thought which we applied to thinking about the future, thinking about our families, about art and culture, about stories and about metaphors. Our ability to see and our ability of vision led to the rise of value.
We can distinguish the world around us through our vision. We can classify structure, materials an texture purely based on light. We need not feel the texture of brick to know that it is brick, nor touch the surface of glass to know that it is glass.
Like human vision, identifying structures, materials, and textures based solely on light—Raman spectroscopy offers a similar revelatory power. Just as we can recognize a brick's texture or identify glass by sight without the need for physical contact, Raman spectroscopy uses light to reveal the molecular composition of materials. By analysing the light scattered off objects, it can determine their chemical structures, making the chemical details of the world accessible to us.
SERS spectra reveal the differences among cultivars of Flos Chrysanthemum. a Spectrum and image of Golden Queen cultivar. b Spectrum and image of Mini Red Peach cultivar. c The differences between these two spectra. d The contrast between their distribution patterns of normalized characteristic peaks. [2]
Real value comes with a cost
In reality, Raman spectrscopy has has a lot of problems. The technology can not begin to be compared with the human eye. It is inconsistent and uncertain. It fluctuates with cosmic rays and fluorescence interference. It requires whole protocols of data processing, all of which, when ultimately applied to different materials are cumbersome and unreliable.
If Raman spectroscopy is to give value to the world, just as human spectroscopy has given value to human beings, application scientists must focus on enhancing the speed, acquisition and data processing capabilities of spectroscopy.
References
[1] JORDAN, G., DEEB, S. S., BOSTEN, J. M. & MOLLON, J. D. 2010. The dimensionality of color vision in carriers of anomalous trichromacy. Journal of Vision, 10, 12-12.
[2] Zhang, H., Chen, Z., Li, T. et al. Surface-enhanced Raman scattering spectra revealing the inter-cultivar differences for Chinese ornamental Flos Chrysanthemum: a new promising method for plant taxonomy. Plant Methods 13, 92 (2017).
