Irresistible rise of the Raman Empire

Raman Spectroscopy has been described as like picking out the light emitted from a single firefly flying across the face of the Sun. Poor detection of the signal restricted RS as useful method for analysis. Then a group of chemists in Southampton decided to rough things up a little and Surface Enhanced Raman Spectroscopy was born In science, some discoveries come crashing down out of the nowhere like lightning bolts. The discoveries of the electron, the splitting of the atom, nuclear fission, the structure of DNA, are a handful of the more obvious examples. Other discoveries take time to sink in to the human psyche. In some cases they may never gain the full appreciation they deserve. Raman Spectroscopy (RS) falls into this category. It may not be splitting the atom but it is a tool for exploring molecules which is helping making the world a better and fairer place. More than 80 years after its discovery, RS has improved beyond measure our ability to identify molecules quickly and efficiently. Raman Spectroscopy and its spin-offs affect all our lives in so many ways, from security to health, from the economy to commerce. It is a shame that the heroes of its discovery remain unsung outside the narrow confines of the scientific community. Like most good scientific discovery stories there is a touch of romance about the Indian physicist CV Raman’s work on light scattering, first published in 1928. Raman told how it was while on a voyage to Calcutta via the Mediterranean, that he began musing on why the sea was so blue. He was unconvinced that the colour was a simple reflection of the blue sky. He dedicated the following years on investigating why it is that when light traverses a medium it experiences a change in wavelength. Raman received a Nobel Prize and a knighthood for showing that monochromatic light is scattered to produce light of both higher and lower wavelength as well as light of the same wavelength. His work was to demonstrate that the energy shift of the light is a function of the mass of the atoms involved and the strength and configuration of their bonds so that every individual chemical species shows its own unique spectral fingerprint. What makes a discovery such as the Raman Effect fascinating is that it was a case where theory predicted the experimental discovery. Einstein’s predictions about the nature of gravity and Niels Bohr’s on atomic structure, and the discovery of heavier elements before they were physically isolated were well known examples of theory pointing to the existence of some yet to be observed effect. In 1923, the Raman Effect was predicted by Adolf Smekal who carried out the mathematical groundwork to show the possible existence of what we describe as inelastic scattering of light. For years after it was discovered, Raman Spectroscopy was to remain in the side-lines with the development of more heavy-duty analytical techniques such as HPLC, Mass Spectrometry and Nuclear and Electron Spin Resonance. The Raman Effect could only be demonstrated using sunlight, a narrow band photographic filter to create monochromatic light, and a crossed filter to block this monochromatic light. The resulting signals were so weak they hampered early development of the technique into anything really useful. During the 30s, the mercury arc became the principal light source for Raman studies, first with photographic detection and then with spectro-photometric detection. Serious Raman spectroscopy only became possible when the laser was developed in the 1960s but even then the signals that Raman scattering produced were so weak and difficult to detect, it made meaningful work super challenging. The key breakthrough which thrust Raman Spectroscopy into the limelight came in 1973 in Southampton. Southampton University at that time was graced with some of the liveliest minds in Chemistry. It was the home of Richard Cookson the synthetic chemist, the photochemists Neville Jonathan and Alan Morris, Raymond Baker and a young David Phillips, who later went on to be president of the Royal Society of Chemistry. The department’s reputation in the field of electrochemistry was particularly well founded. One staff member was an enigmatic professor of electrochemistry from Czechoslovakia with a maverick approach to what he called “doing dirty chemistry” which was the same as saying “experimental chemistry”. Martin Fleischmann went on in the late-80s to become embroiled in the Cold Fusion controversy but in the early-70s he was experimenting with electrodes in his Southampton lab, coating them with various metals like silver and gold and trying to understand surface effects when placed in an electrolytic solution. It was Fleischmann who picked up the Raman baton in collaboration with the gifted research chemist Pat Hendra and a young post-doctoral research fellow Jim McQuillan. McQuillan, who today is Professor of Chemistry at the University of Otega in New Zealand, recalls with fondness the hard work and delight of those days in the lab at Southampton. McQuillan’s own account of the birth of SERS, written in 2009 in the notes and records of the Royal Society, is fascinating. He recalls: “Martin’s suggestion that Raman Spectroscopy might be used to probe the nature of adsorbed electrode species and electrode reaction intermediates was probably influenced by the ultraviolet-visible reflectance spectroelectrochemistry which Alan Bewick, a former PhD student of Fleischmann’s, had already begun. “My first project was to see whether we could detect the Raman signal from the anodic formation of insoluble mercury salts on mercury. “A large cylindrical glass cell was made with part of its wall flattened to facilitate collection of the signal. “We found that we could record Raman spectra from multiple layers of Hg₂Cl₂, Hg₂Br₂ and HgO on small mercury droplets formed by nucleation onto a platinum disc electrode.” This work was reported in early 1973 in Chemical Communications and this led to a note in New Scientist. Hg₂Cl₂ has a very strong Hg–Hg stretch Raman signal, but detection limits for Hg₂Cl₂ multilayers were still above the more interesting monolayer level. The variable sensitivity of the SERS signals in the beginning was not promising for its use as an analytical technique. But the team at Southampton expended much energy into the preparation and stabilisation of SERS substrates for analytical purposes. The work then turned to seeking ways to enhance the Raman signal. The possibilities of using phase-sensitive detection, resonance Raman, and large-surface-area electrodes were considered. Pat Hendra had been pioneering the use of Raman Effect to study species adsorbed on large-area oxide catalysts, so large surface area became the favoured direction. McQuillan wrote: “The question was: would metal-coated alumina particles or platinum black provide a detectable signal? By the March of 1973 our quest led to pyridine as a strong Raman scatterer and silver as an electrode material amenable to electrochemical surface roughening. “On August 30, following an afternoon and evening on the Cary 82 spectrometer, which I had to share with other completely unrelated research groups, I noted ‘quite a startling success’. “I had used the roughed up silver electrode in aqueous 0.05 mol dm K3 pyridine containing 0.1 mol dmK3 KCl using less than 100 mW of the 5145 A laser line. “The next page of my notes is pretty bare of details, probably reflecting the excitement of the results, but I did make this observation: ‘On electrode with no applied potential, bands at 1008 (2.4) broad, 1026 (3.0), and 1037 cmK1 (4.0), getting about 1000 counts sK1 on 1037 cmK1 band at 2 cmK1 slits—very sensitive.’ “A subsequent afternoon and evening run of experiments on September 4 with the same system led to the results that were in due course reported the following year in Chemical Physics Letters.” Jim, Pat and Martin Fleischmann could not have known the full implications of their discovery of a method for increasing the Raman signal strength. Their work had effectively unlocked the door to a fabulous new tool for analytical chemistry, one so powerful it could detect single molecules. It led to Martin Fleischmann being awarded the US Palladium Medal for Electrochemistry. Fleischmann said: “It is certainly remarkable how significant SERS has become. None of us made vast quantities of money from the discovery, but I always think that I did the chemistry for the fun of it, like most scientists. The outcome has given me a relatively secure existence with a good income.” Last summer, in recognition of that pioneering work, a Royal Society of Chemistry plaque was unveiled at Southampton University to commemorate the discovery. Fittingly Dr Hendra and Professor McQullion were back at their old department enjoying the moment and David Phillips returned to make tribute to the work of his erstwhile colleagues. Sadly Fleischmann wasn’t present. He died the year before aged 85 a year before the fortieth anniversary of his most incisive research work.  Whatever the wider view of the Cold Fusion brouhaha which came years later, nothing can detract from the results of his dogged determination to understand surface electrode effects which directly resulted in the coming of age of Raman Spectroscopy. Pat Hendra says: “SERS is arguably the most sensitive method of analysis on surfaces that anyone has ever come up with. However, at the time we had no idea how important it would become beyond the academic world, or the vast range of applications that would be developed." He has his own personal memories of the 70s working with Professor Fleischmann. Sometimes he would burst in on Hendra’s own lectures, with a beaming smile and unruly mop of thinning fly-away hair spilling coffee everywhere pronouncing in his is in a lyrical mid-European accent. “I have had an idea!” It was the kind of infectious enthusiasm that inspired his research students and it is a fair bet that CV Raman would have appreciated Fleischmann dogged determination wrestling with his “dirty chemistry”. …And all of this from one man’s quest to understand why the Mediterranean Sea is blue. The power of Raman spectroscopy and SERS shows no sign of diminishing. There has been an almost exponential increase in the applications and studies in the four decades since Pat Hendra, Jim McQuillan and Martin Fleischmann strode the stage. SERS has inspired thousands of academic papers and new methods and fresh applications are being developed on a regular basis. While the technology has found a welcome home in forensics and industry, Dr Sumeet Mahajan, senior lecturer in life sciences at Southampton is using the technique to advance stem cell therapy. Dr Mahajan says: “We already know that stem cells could hold the key to tackling many diseases. They develop into all the various kinds of cells needed in the body – blood, nerves and organs, but it is almost impossible to tell them apart, during initial development, even with the most advanced microscopes, without complex techniques. “Up to now, scientists have used intrusive fluorescent ‘markers’ to track each cell but this can alter or damage the cells and render them useless for therapeutic use. SERS technology is allowing the use of very tiny particles of gold, less than 1,000^th of the width of a human hair, as ‘nanoprobes’ to enter cells. “It is allowing the observation of the natural vibrations of molecules within the cell and making an almost invisible motion, detectable. This enables us to judge whether drugs are reaching cells correctly, and to detect abnormalities within cells on a molecular level.” The Engineering and Physical Sciences Research Council (EPSRC) fund Dr Mahajan’s work and some results have been published in the journal Nano Letters. He and his team are collaborating with major pharmaceutical companies to further develop the work for better drugs. The anthrax attacks in the US in 2001 underscored the necessity of a quick response to biological terrorism agents. Optimal organisation and capacity are critical to saving lives at minimal cost. In situations like that rapid biological agent identification aided by a field-ready analytical protocol is needed. Raman spectroscopy has proved uniquely capable of helping fill this need given its capacity for unique molecular identification, Indirect SERS detection methods are also available, such as labelling the analyte with a Raman-active dye and bringing it to the SERS substrate for readout. Nanometre-scale SERS-active tags conjugated with specific biological recognition moieties, thereby effecting direct binding to the analyte, is another possibility. An indirect biological detection system is now compatible with the Raman-based StreetLab Mobile. It employs SERS tags as unique labels for each target of interest in a so-called sandwich immunoassay format. Unique spectroscopic signatures are generated with SERS tags consisting of individual glass-encapsulated gold nanoparticles and surface-bound Raman active reporter molecules. The SERS tags are bound to a specific antibody and provide a strong, spectroscopically consistent label. Superparamagnetic particles conjugated to the antibodies capture and concentrate the SERS-labelled complex at the focal point of the Raman laser using a magnetic field. The simple SERS read out confirms the presence or absence of the analyte. In another recent development, scientists at ETH Zurich and the Lawrence Livermore National Laboratory (LLNL) in California have developed an innovative sensor for SERS. Thanks to its unique surface properties at nanoscale, the method can be used to perform analyses that are more reliable, sensitive and cost-effective. In experiments with the new sensor, the researchers were able to detect a certain organic species (1,2bis(4-pyridyl)ethylene, or BPE) in a concentration of a few hundred femtomoles per litre. A 100 femtomolar (10^-15 moles per litre) solution contains around 60 million molecules per litre. Hyung Gyu Park and Tiziana Bond two researchers on this project said recently that they now want bring their new concept to market, but they are looking for an industry partner for financial support. Their sensor differs from other comparable ultra-sensitive SERS sensors not only in terms of its structure, but also because of its relatively inexpensive and simple production process and the very large surface area of the 3D structures producing an intense, uniform signal. Park and Bond are still improving the sensitivity of the sensor, and they are also looking for potential areas of application. Park believes installation of the technology in portable devices, for example to facilitate on-site analysis of chemical impurities such as environmental pollutants or pharmaceutical residues in water. He stresses that invention of a new device is not necessary; it is simple to install the sensor in a suitable way. Author Dermot Martin Dermot Martin is a science writer, former editor of Analysis Europa and currently media advisor to Bournemouth and Poole College of Further Education