Quo vadis surface-enhanced Raman scattering?

The title of the perspective article in this themed issue by Richard Van Duyne’s group (Surface-enhanced Raman spectroscopy of dyes: from single molecules to the artists’ canvas), summarizes in a single sentence, the mind-boggling variety of topics that can be included today under the umbrella of SERS. Indeed, the last few years have seen the technique moving fast towards new and yet-unexplored areas of biology, forensic science, analytical chemistry, art and conservation science (to mention just a few). It can be argued—not without a certain risk of controversy—that Raman is one of the most universal types of spectroscopies one can hope for. For example, light does not even need to be absorbed by the sample (as is it necessary in fluorescence spectroscopy) and its non-invasive nature, together with possible combinations with different types of microscopies (confocal, etc), and resonance conditions, makes it truly universal and versatile. SERS, in addition, adds a completely new dimension to the technique by boosting Raman signals to a degree that allows one (under the right conditions) to observe single molecules.

The ultra-sensitive character of SERS as a spectroscopic technique has, for many years, appealed to a wide community for both purely scientific and applied reasons. If one manages to tame the conditions under which one can observe (reliably and reproducibly) single molecules of certain types, one has (effectively) the ultimate analytical tool at hand. From there, it can only get worse! Still, this goal has remained in the horizon of possibilities and (like the horizon itself) it seems to move forward every time the technique experiences some progress in its direction. A long discourse could follow from here, but I would like to summarise it by the statement that: we do not seem to be able (yet) to fully control the “nano-world” completely at will; at least to the degree of precision that would be needed for the task. Remarkable steps forward have been taken in that direction though, with the development of better and more reproducible SERS substrates, and the notable development of TERS (tip-enhanced Raman spectroscopy) in the last few years. Yet, the detection of single molecules (which is a big driving force in the field as a whole) still suffers from what Natan has described informally as the “SERS uncertainty principle”, i.e. the fact that the more we want to control the nano-world (responsible for the huge enhancements needed to observe single molecules), the less the fine control over the geometrical aspects of the substrate we seem to have. Whether we will be able to control the cause of the effect (the SERS enhancement) all the way down to the required precision (∼1 nm) for fast and reproducible detection of single molecules in easy-to-use experimental layouts remains to be seen. The implementation of SERS in its ultra-sensitive capabilities for standard analytical work of single molecules continues to be a task (and a question mark) for the future. Nevertheless, this is only a fictitious limitation, for SERS outperforms (in sensitivity) other techniques even in the limit where we do not aim for single molecule detection. Some examples of direct applications of the technique for analytical work in this “intermediate range” (where we are not exploiting the ultra-sensitive capabilities of SERS to an extreme) can be seen in some of the articles in this issue.

This special themed issue of PCCP has enough different material to give the interested reader a flavour of current topics and directions in SERS. I will not duplicate the information on the content of the papers here, but only give an overview comment on the general trends that can be perceived from them. While some papers concentrate directly on applications (detection of pesticides, or development of pH-sensors, for example), others attack the very many (still) unresolved fundamental aspects of the technique (chemical enhancements, single molecule detection, vibrational pumping, etc.). The ever-present (and always important) interface with electrochemistry and biology are also well represented, as well as theoretical aspects of the electromagnetic properties of nano-particles and density functional theory (DFT) of molecules in close proximity to metallic surfaces. The latter seems to be a growing trend in the field at the moment; i.e. to try to understand the microscopic interaction of molecules with surfaces at a theoretical level with DFT. Last, but not least, the endless variety of combinations leading to the development of new SERS substrates for specific applications (nano-biosensors, for example) is also well represented in the collection of articles.

In the last few years, SERS has received considerable attention in the literature. The Royal Society of Chemistry (RSC) has been particularly proactive in this, with the publication of a special issue in the prestigious Faraday Discussions in 2006, and the more recent special issue in the high impact articles of the Chemical Society Reviews in 2008. This themed issue in PCCP, therefore, continues a trend of engagement of the RSC which is greatly valued and appreciated by the practitioners of the technique at the boundary between chemistry and physics. It is my personal hope that the collection of articles in this issue will attract more interest from existing and future practitioners, and that the mixture of activities will blend into a new layer of understanding that will surely contribute to the furtherance of the technique.

I would like to thank the RSC for inviting me to be the guest Editor of this issue. In particular, special thanks are due to Anna Roffey at the RSC, who was in charge of finalising the project and showed a great deal of patience with all my broken promises to deliver things on time. Thanks are also due to my personal collaborators at Victoria University of Wellington in New Zealand, with whom I have shared the excitement (and sometimes the frustration) of a very active, and at times controversial, field of research like SERS. In particular: Dr Eric Le Ru who has been an endless source of inspiration and the driving force behind many of the results in my group, and the (long-suffering) students: Matthias Meyer (who has provided the table of contents figure of this introduction; symbolising the very many areas that span from the SERS effect), Chris Galloway, Evan Blackie and Stefan Meyer. The contributions from our group to this issue (and beyond) are more their merit than mine.

Pablo G. Etchegoin (Guest Editor), Victoria University of Wellington, Wellington, New Zealand.

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