Enlightening LEDs
The 2014 Nobel Prize in Physics, awarded “for the invention of efficient blue light-emitting diodes,” has put LEDs under the spotlight, particularly as a stable, robust and efficient light source in analytical sciences.
Since the emergence of white LEDs for lighting applications in general, it was clear that such breakthrough technology would not only change many aspects of everyday life, it would also have a big impact on science. Harnessing their power for analytical science has been a continuous process for more than three decades (among the first breakthrough examples was a miniature LED-based oximeter introduced in the early 1970s).
Mainstream analytical use of LEDs emerged in the early 1990s and over the years they have shown many advantages over traditional incandescent and arc lamp lighting. Importantly for the analytical scientist, the robustness of solid-state emitter technology, small size, low cost (for well-established, mass-produced LEDs), and excellent stability (resulting in low noise in optical detection) make LEDs an ideal light source. In addition, LEDs can readily provide pulsed light (up to GHz frequencies), allowing larger light output in the “on” period, and enabling optical techniques requiring pulsed light source, such as wavelength multiplexing and time-resolved fluorescence.
Optical detection and imaging are the main areas for the analytical use of LEDs, and they are proving useful for analytical photochemistry, including photolithography in microfabrication and photopolymerization when using polymer monolithic stationary phases. LEDs also have many amazing applications in the broader area of life sciences, such as aiding tissue healing with near infrared (NIR) therapy, utilizing the pulsed capability of LEDs for fluorescence imaging of living cells, and reducing induced oxidative stress in cells... and the list goes on.
When assessing the potential of LEDs, we should really ask, “What can’t LEDs do for the analytical scientist?” Clearly, LEDs have different properties to laser diodes. Apart from light coherence and directionality of laser diodes, their higher optical output compared to LEDs has often been to their advantage. Nevertheless, this is changing, as many new LEDs now achieve laser-like light output levels. And, what about the strengths and weaknesses of LEDs when compared with classical light sources?
LEDs produce quasi-monochromatic light, which can be advantageous, but in my view, what we really need is an LED that can provide a broadband light source ranging from deep-UV (~200 nm) through the visible region to NIR. Such technology could replace bulky, fragile and expensive deuterium and similar lamps. A convincing proof of concept has been demonstrated for spectrophotometric detection; the technology exhibits lower noise and offers limits of detection that are several times higher than a standard D2 lamp (1). Will such a LED-functional equivalent of a D2 lamp become available in the future? Well, it will depend on two factors: namely, demand and the availability of deliverable technology. The first factor appears fulfilled. The second factor is trickier for me to judge as an analytical chemist, because it has everything to do with solid-state physics.
Clearly, it is possible to make LEDs with wavelengths at or below 200 nm, even when they are not yet commercially available for wavelengths below circa 230 nm. So, creating a broadband LED light source for deep UV might be a worthwhile challenge; however, this is where my judgment reaches its limits, so I’ll stick to following future developments in IT, consumer electronics and other areas (including those used in greeting cards).
I believe we analytical scientists must follow LED developments in other areas and be ready to utilize them in our own research. For example, deep-UV LEDs with a wavelength below 250 nm have their uses for sterilization and water purification. Such use of light in ‘non-analytical’ applications is good for getting commercially available LEDs with enough optical power for analytical devices (especially those for fluorescence analysis) to market.
So, what is the future of LEDs and their use in analytical science? Bearing in mind their strengths and limitations, and their immense potential in many other areas, I think it is very bright indeed.
- T. Piasecki, M. C. Breadmore and M. Macka, “White LEDs as Broad Spectrum Light Sources for Spectrophotometry: Demonstration in the Visible Spectrum Range in a Diode-Array Spectrophotometric Detector”, Electrophoresis, 31: 3737–3744 (2010). DOI: 10.1002/elps.201000341
Mirek Macka graduated in general and analytical chemistry from the Masaryk University, Brno, Czechoslovakia, and obtained his PhD degree at the University of Tasmania. He has extensive experience from industrial pharmaceutical research in Europe as well as a productive academic research record. His main interests are separation science including capillary electrophoresis, electrochromatography, liquid chromatography and related methods, microfluidic chip-based separations, as well as in areas of molecular spectrophotometry and instrumental design. His research philosophy dares to cross boundaries between disciplines and fundamental and applied research. He is also a pioneer in utilizing modern high-tech-low-cost smart gadgets and gives Short Courses at Pittcon on solid-state light sources used in chemical analysis.
