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November 2003 |
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Start next semester with new chemistry experiments…and we’ll do the work for you. See Be Prepared, below.
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Around the same time, Van Duyne had been studying resonance Raman scattering and knew how much of a signal enhancement would be needed to see a single monolayer on a surface and it was much larger than what can be achieved by roughening a surface. He and his graduate student David Jeanmaire tried less roughening and found larger signals. (David Jeanmaire and Richard Van Duyne, Surface Raman spectroscoelectrochemistry,. Part 1. Heterocyclic, aromatic, and aliphatic amines adsorbed to anodized silver electrodes, J. Electroanal Chem., 84, 1, 1977. This led to recognition that an anomalous effect was enhancing the Raman signal of pyridine at silver electrodes. It remained an anomalous effect for several years until SERS was finally identified as stemming from two sources. One is a chemical enhancement similar to resonance Raman scattering. Some molecules, when chemisorbed to a surface, develop strong charge transfer bands that lead to an electronic absorption within the surface complex. This in turn leads to an enhancement of the Raman scattering. The other source of the enhancement arises purely from the physics of small noble metal particles. These particles, all in the periodic chart’s Group 1B, have a characteristic that leads to the SERS effect. They have free-electrons due to their full d-shells and a well shielded lone s electron. This creates an interesting electromagnetic property; particles of these metals react oppositely to electric fields than most other particles. In other words, when placed in an electric field at the frequency of light, the particles polarize in the direction from the light’s incident field. This is best understood pictorially.
One can see in the illustration above that a dielectric particle will reduce the electric field around itself. A free-electron particle will polarize to produce an electric inside (Einside) that is additive to the incident electric field (Eincident). The result is a total electric field (Etotal) near surface that is larger than the incident field. The key to SERS is not this initial large field near the particle. The crucial step is that this large field near the particle induces an even larger field inside the particle. Now the larger external field produces an even larger internal field and so on. This is a great example of a resonance! It is a very small perturbation in a system ready to oscillate, and it goes ballistic. Think about how small the electric field of a light wave is and how the SERS effect suddenly produces enhancements of a million to a hundred million through this iterative enhancement of the light’s electric field. This edition of Delta News is dedicated to SERS experiments that undergraduates can perform with the Advantage 200A. |
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Raman Spectroscopy
SERS: NanotechnologyNanotechnology is in vogue. We did a recent survey of articles on Raman and nanoparticles and as you can see it is an exponential growth. SERS provides a powerful method for physical chemists to study adsorption phenomena at surfaces and provides analytical chemists sensitivities far beyond that achievable with normal Raman scattering. We will discuss applications of SERS for understanding monolayer structures, adsorption studies, and detection of trace materials.
Structure of MonolayersMonolayers present a difficult challenge to spectroscopists. The small amount of material on a surface is very difficult to detect. Often the methods used to study monolayers involve expensive ultrahigh vacuum techniques or difficult infrared methods. Normal Raman spectroscopy of adsorbed monolayers is extremely difficult due to the low intensity of Raman scattering. However, with the discovery of SERS and the low-cost Advantage 200A makes monolayer studies simple and affordable.
1-Decanethiol provides an excellent choice for a simple study of adsorbed alkanethiols or the homologous series of alkanethiols provides an experiment suitable for advanced physical chemistry labs. The spectra above show spectra of liquid 1-decanethiol, solid 1-decanethiol, and a SERS spectrum of 1-decanethiol on silver colloids. These spectra can be interpreted in many ways to teach students about monolayers and SERS. Note the loss of the S-H stretch around 2500 cm-1 when the decanethiol reacts with the silver surface. Jeanne Pemberton and Mark Bryant did a number of studies about SERS and alkanethiols. (Mark Bryant and Jeanne Pemberton, Surface Raman Scattering of Self-Assembled Monolayers formed from 1-Alkanethiols at Ag, J. Am. Chem. Soc., 1991, 113, 3629) One of the simplest experiments is to look at the C-S stretching region. Many students not familiar with vibrational spectroscopy will expect one band. As the Newman diagrams beside show, many bands depend on the rotational conformation about the C-S bond. To illustrate this spectroscopically, we have zoomed in the C-S stretching region.
The liquid 1-decanethiol is free to rotate, and as students learn in organic chemistry, the non-eclipsed, or gauche configuration will be present. The energy for the barrier to rotation is much less than the thermal energy and all states are present. If you examine the top spectrum you can see the gauche states (G) and the trans state (T). When the 1-decanethiol is cooled, the amount of thermal energy present to allow free rotation goes down, and the conformation locks into the lowest energy state. Sterically this will be the trans conformation. This is clear in the spectrum of solid 1-decanethiol. How should a surface change the structure around the C-S bond? It depends on the coverage. This would make a great SERS study. Examine the G/T ratio as a function of coverage. At low coverage it should be liquid like. As the coverage increases, it should become trans as the intermolecular forces favor a trans conformation. This is the concept of self-assembly. You can see this in our SERS spectrum of 1-decanethiol. We did not achieve perfect self-assembly, but do have predominantly the trans conformation. Pyridine on Silver
When Jeanmaire and Van Duyne performed the original SERS experiments, they used state-of-the-art Raman equipment. But in 1977 that was not sufficient to see 0.05 M pyridine with sufficient S/N to report frequencies. It is quite easy with the Advantage 200A and 10 to 20 second integration times to achieve a good signal to noise spectrum. It is noisy, but the ring-breathing mode is apparent.
For this newsletter we chose to zoom in on the ring breathing mode and examine how it changes. Pyridine is a very good “probe” to study adsorption. In an aprotic environment, such as pure pyridine, the nitrogen is not attached to anything and the ring breathing mode is at 991 cm-1. When the pyridine hydrogen bonds to something (such as water) it shifts to 1005 cm-1; when it coordinates with metals, it is indicative of their electronegativity. With silver, the binding leads to a shift to 1008 cm-1. Note the ability of the Advantage 200A to distinguish between these small shifts. SERS and Analytical ChemistryThe examples discussed above fit more in the realm of physical chemistry. SERS can also be used in analytical chemistry. One series of experiments with SERS and SAMs that teaches many important concepts in analytical chemistry is adsorption of an analyte onto SAMs. This stems from work we performed several years ago. Surfaces can be classified by their energy. A high energy surface is very polar; a low energy surface is very nonpolar. Alkanethiol SAMs will produce a very non-polar surface. Conceptually, this is can be used to introduce the concept of chromatographic coatings.
The figure, above, of the SERS spectrum of 1-decanethiol on silver colloids contains bands for the various C-C stretches, C-S stretching, and some C-H bending modes. When 0.5 mL of this solution was exposed to 0.5 mL of a saturated benzene solution the strong ring-breathing mode of benzene shows up in the SERS spectrum. This is a very short ~10 minute experiment. Where it can diverge into several concepts for analytical chemistry is the preparation of a calibration curve for several different aromatic compounds. See our paper "Octadecylthiol Modified SERS Substrates: A New Method for the Detection of Aromatic Organic Compounds", Keith Carron, Laura Pietersen, and Mary Lewis, Environmental Science and Technology, 26, 1950, 1992. Examination of the "calibration curves" can lead into some pretty advanced ideas about isotherms. See our paper "Detection of Chlorinated Ethylenes at Octadecylthiol Modified Surfaces", Ken Mullen and Keith Carron, Anal. Chem., 66, 478, 1994. Last of all, if the alkanethiol chain length is varied one can determine a distance dependence for the SERS effect. See our paper "Determination of the Distance Dependence and Experimental Effects for Modified SERS Substrates Based on Self-Assembled Monolayers Formed Using Alkanethiols", Brian Kennedy, Samantha Spaeth, Matthew Dickey, and Keith Carron, J. Phys. Chem., 1999, 103, 3640-3646. |
Software/HardwareOur last newsletter’s Software/Hardware section was devoted to importing our data into Excel. This issue discusses an xyz translation state that greatly amplifies the number of applications that can be performed on the Advantage 200A. The examples presented in this issue covered applications using colloidal silver. However, all could be performed on solid silver substrates just as easily. The difficulty of using Raman with a solid substrate is the need for precise focusing. Solutions, such as colloids, only require the students to focus into the solution. Once the focus is inside the glass vial, a few millimeters won’t affect the signal. However, on a surface it is important to have the focus of the laser beam precisely placed on the surface. Our xyz translation stage fits directly onto the Advantage 200A and allows the user to move a sample along three orthogonal axes. This is ideal for examining silver foils, thin films, or large solid samples.
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Be Prepared!DeltaNu Lab 3: Determination of a Langmuir Isotherm using SERS (PDF format) Visit our website for prepared laboratories. This newsletter features a complete SERS experiment. It is suited for physical chemistry or advanced analytical chemistry. Students will learn about citrate reduction of silver nitrate to make silver colloids, dilutions, Raman spectroscopy of pyridine, and Langmuir isotherms. It represents a classic SERS experiment. Write for more details if you want hints on how to teach SERS without complicated electrodynamic calculations, or if you want to add a linear-free-energy concept to the adsorption isotherms. Lab 3 (PDF format)
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Contact Us:Website: www.deltanu.com Comments and questions are welcome at sales@deltanu.com. |
The
classic SERS experiment is also fun and easy to perform. Start with neat pyridine and have
the students take polarization spectra. The strong A1 ring breathing mode at
991 cm-1 is very obvious and strongly polarized. Since this mode involves
movement of the nitrogen atom which is part of the aromatic ring, its vibration frequency
will be indicative of the environment around the nitrogen.
The
eye-opener is what happens when a small amount of this solution is added to a colloidal
silver solution. Suddenly the spectrum of pyridine becomes very intense. At this point,
students can try to estimate the coverage of pyridine and the amount of pyridine in the
laser beam to find an enhancement factor. Another interesting study is the relative
intensity of the Raman peaks. Creighton and co-workers predict that the electric field
from the SERS effect is more intensive perpendicular to the surface than parallel.
Depending on the adsorption geometry and the symmetry of the vibrations, one can predict
the orientation of the pyridine.
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