PCCP Blog

Despite 35 years of research and over 10 000 publications on surface-enhanced Raman spectroscopy (SERS), a universal agreement regarding the physical mechanism responsible for the effect still appears to be elusive. Is SERS in actual fact better understood than the impression given by a number of tenacious misconceptions? Professor Martin Moskovits believes so.

In his recent PCCP Perspective, Moskovits discusses several important topics in SERS, which are often subject to dispute or contradictions in the current literature, including its physical origin. He explains plasmonic theory, the mechanism that underlies the current understanding, and reasons that alternatively proposed mechanisms, such as chemical enhancement, have not only impeded the establishment of a universally accepted understanding, but have hindered the use of SERS in applications such ultrasensitive chemical and biological sensors.

SERS is a surface-sensitive technique that enhances Raman signals from Raman-active analyte molecules absorbed onto specially prepared metal surfaces.  SERS is sufficiently sensitive to allow the detection of single molecules.

SERS theory, as it is commonly applied, is a classical theory largely based on electrostatics. To obtain a more complete understanding of SERS, a theory based on quantum mechanics, and approached as a dynamical theory, needs developing, Moskovits explains in his review. He suggests that in the meantime, researchers should deal with the surface chemistry that arises in a given experiment on a molecule-by-molecule basis.

Read the details in this HOT PCCP Perspective today:

Persistent misconceptions regarding SERS
Martin Moskovits
DOI: 10.1039/C2CP44030J

If you enjoyed this perspective, you may also be interested in our web collection on SERS.

Rapid passage effects have been demonstrated in a Lamb dip experiment for the first time, by researchers from Oxford, UK.

Grant Ritchie’s team conducted Lamb dip spectroscopy on a low pressure sample of NO using a narrow-linewidth, high-power, single-mode quantum cascade laser. They were able to determine the laser linewidth, and that the power was sufficient to induce significant population transfer of up to 35%, from the widths of the Lamb dips. The population transfer efficiencies could be controlled by the laser chirp rate.

The lamb dip signals became increasingly asymmetric as the chirp rate was increased. At sufficiently high chirp rates rapid passage oscillations on the Lamb dips were observed due to the narrow laser linewidth, and were seen to be affected by the hyperfine splitting in the transition.

Furthermore, the team showed that it was possible to separate the asymmetry effect from the rapid passage oscillations by pumping a single velocity group using a second laser at a fixed frequency.

Read this HOT article today:

Coherent transient spectroscopy with continuous wave quantum cascade lasers
James M. R. Kirkbride, Sarah K. Causier, Elin A. McCormack, Damien Weidmann and Grant A. D. Ritchie
DOI: 10.1039/C2CP44116K

The commercial ATTO 465 dye, based on 9-alkylated proflavine, appears ever so simple: a 2,7-diamino-9-alkyl-acridinium fluorophore with a relatively remote carboxylate group. Even so Arden-Jacob and co-workers find that the photophysical behaviour is far from straightforward. This despite the fact that the cousin: ATTO 495 with the 2,7-bis(dialkylamino)-9-alkyl-acridinium dye, is perfectly simple.

ATTO 465 is interesting. As dye it is not spectacular: it absorbs light at roughly a fifth of the best possible and emits around half of the number of photons it absorbs. Nothing unusual here, but the shape of the spectra, the way the dye changes depending on which solvent that surround it, and mechanisms it uses to dissipate energy; that is unusual. And apparently the explanation lies with the hydrogen atoms on the peripheral amino groups?

A dye can be reduced to a group of electrons moving in a box of nanometer dimensions: a small antenna. The macroscopic counter-piece is a radio antenna, which contains a number of electrons that move in the volume of the antenna. If the dye is changes, the interaction between the nano-antenna and light changes. It can be a variation in colour or a difference in the amount of light emitted. ATTO 465 is a box where both the shape and the number of electrons can change. By bonding hydrogen to the amino groups electrons are removed, the bonding also twists the amino groups resulting in a differently shaped box. Unfortunately the ‘twisting’ is not discreet and many different shapes will be present at all times, as long as there are protons (hydrogen atoms) in the solution that surrounds ATTO 465. The result is the complex photophysical behaviour observed and discussed by Arden-Jacob and co-workers.

Much more detail can be found in the full PCCP article:

Ultrafast photoinduced dynamics of the 3,6-diaminoacridinium derivative ATTO 465 in solution
Jutta Arden-Jacob, Karl-Heinz Drexhage, Sergey I. Druzhinin, Maria Ekimova, Oliver Flender, Thomas Lenzer, Kawon Oum and Mirko Scholz
DOI: 10.1039/C2CP43493H

by Dr Thomas Just Sørensen

Chiral structures interact with circularly polarised light. Gennaro Pescitelli and co-workers have investigated the effects of the close contact between handed molecules in micro-crystals in their interaction with light. The well-rounded study explains—as promised–the specific case, but does it take us closer to a general structure-property relationship regarding the preferential absorption of one handedness of light? And is such as a correlation even possible?

In ”Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study” by Gennaro Pescitelli, Daniele Padula and Fabrizio Santoro, the finer details of the interaction between circularly polarised light and chiral matter is discussed. In this specific case, homochiral microcrystals are investigated experimentally and theoretically. The results presented are impressive, as the theoretical model fits and explains the experimental observation, but why is this study so important? What is this chirality? And which answers are we looking for?

Chirality and the word chiral derive from the Greek word cheir or hand: chiral can be read as handed and chirality as handedness. The definition of a chiral object is that it cannot be superimposed on its mirror image, just as our hands. Chirality is hugely important as every biological building block that makes up our hands, and indeed the rest of us, is handed. It comes in a left-hand and a right-hand form.

As it is the entire biological machinery on earth is almost exclusively left-handed. How this came about, we do not know, but it is suggested that we, in our part of space, have an abundance of circularly polarised light emitters of a certain handedness, and that this has caused the handedness of our biology. Chirality and circularly polarised light are linked. Each can be described of having a specific handedness and one handedness of light interacts with one handedness of chiral matter. An abundance of right-handed light in our part of the universe may have induced the single handedness of our biology.

In order to figure out how the homo-chirality of the biosphere on earth arose, we need to understand how circularly polarised light and chiral material interacts; exactly what Gennaro Pescitelli and co-workers are investigating. Studies like this, where experiment and highly advanced theory is used to correlate observations and the chiral structure of the material, are needed if we are to understand how chirality in a molecular framework absorbs one specific handedness of light.

We understand how regular material (not handed) and normal light (linearly polarised) interact, and how to design materials that interact with light in a desired fashion. We cannot yet design materials that absorb a specific handedness of light. The paper  ”Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study” by Gennaro Pescitelli, Daniele Padula and Fabrizio Santoro takes us one step closer, it was published in of Physical Chemistry Chemical Physics 2013, 15, 795-802.

by Dr Thomas Just Sørensen

Read this fascinating study today:

Intermolecular exciton coupling and vibronic effects in solid-state circular dichroism: a case study
Gennaro Pescitelli, Daniele Padula and Fabrizio Santoro
DOI: 10.1039/C2CP43660D

We know much about the energetics in solutions and of species in solution. We can directly measure most parameters, when we measure energy. When it comes to the structure of solution or in solutions, we know next to nothing. Indirect measurements are our main source of information, as we cannot see the fluctuating solution structures. Miyuki Tanaka and co-workers have performed one of the most informative studies using indirect methods to probe solvent structures that I have come across.

In a recent paper in PCCP by Miyuki Tanaka, Tomoaki Yago and Masanobu Wakasa the microscopic structure of ionic liquid solvents is investigated. The diffusion of single particles is compared to the macroscopic measure of viscosity. They find that the ionic liquids are ‘more sticky’, when looking at the diffusion of single molecules, than more commonly used solvents; although the viscosities of the two solvents are identical.

In “Local structure of ionic liquids probed by self-quenching of thiobenzophenone” the movement of single microscopic particles in solution (molecular diffusion), are compared to measurement determining how difficult a macroscopic object moves in the same solution (the viscosity). A single particle is excited using light and gains energy. The energy can be released the by collision with another particle in the solution.  By following the evolution of the population of excited particles the molecular movements can be followed.

The result presented in this paper is another piece of the puzzle we have to assemble in order to understand the microstructure of solvents, and the conundrum of solvation. Solvents are everywhere, most ubiquitous is water. We have a limited understanding of the structure of pure solvents, and know even less about the structure of complex solutions. The collection of small molecules that constitute most liquids can have an astoundingly complex structure. A structure that we have to know and understand, if we are to comprehend the complex condensed phases such as the cells that make up most living things.

At the most advanced facility for structural investigation of matter, the SLAC national accelerator laboratory, an entire team of elite scientists has been assembled ‘just’ to elucidate the structure of water. The method applied by Tanaka and co-workers is a much simpler route to understanding molecular movements in solution. When a large library of different solvents has been investigated, we will be able to deduce effects of specific solvation. Following this, we may be able to explain exactly why the ionic liquids are ‘more sticky’ than traditional solvents.

“Local structure of ionic liquids probed by self-quenching of thiobenzophenone” by Miyuki Tanaka, Tomoaki Yago and Masanobu Wakasa was published in Physical Chemistry Chemical Physics (PCCP) at the beginning of 2013: M. Tanaka, T. Yago, M. Wasaka Phys. Chem. Chem. Phys.2013, 15, 787-794

By Dr Thomas Just Sørensen