Spectroscopic Properties of Inorganic and Organometallic Compounds: Techniques, Materials and Applications, Volume 41 (Specialist Periodical Reports - ... of Inorganic and Organometallic Compounds) - Hardcover

Hardcover

ISBN 10: 1847550479 ISBN 13: 9781847550477

Publisher: Royal Society of Chemistry, 2010

Synopsis

Reflecting the growing volume of published work in this field, researchers will find this book an invaluable source of information on current methods and applications.

About this title

About the Author

Professor Jack Yarwood is an emeritus professor at Sheffield Hallam University. Professor Simon Duckett is a research group leader at the University of York, UK. His group is mainly involved in the design, development and implementation of NMR methods, supported by the synthesis of inorganic and organometallic complexes. Dr Richard Douthwaite is at the University of York, UK. His main research interests include molecular and materials chemistry and photocatalysis. Both an EPSRC college member and fellow of the Royal Society of Chemistry, Dr DOuthwaite is also on the SCI National Materials Committee.

From the Back Cover

Spectroscopic Properties of Inorganic and Organometallic Compounds: Techniques, Materials and Applications provides a unique source of information in an important area of chemistry. Since Volume 40 the nature and ethos of this series have been altered to reflect a change of emphasis towards 'Techniques, Materials and Applications'. Researchers will now find up-to-date critical reviews which provide in-depth analyses of the leading papers in the field, with authors commenting of the quality and value of the work in a wider context. Focus areas will include structure-function relationships, photochemistry and spectroscopy of inorganic complexes, and catalysis; materials such as ceramics, cements, pigments, glasses and corrosion products; techniques such as advanced laser spectroscopy and theoretical methods.

Excerpt. © Reprinted by permission. All rights reserved.

Spectroscopic Properties of Inorganic and Organometallic Compounds

Techniques, Materials and Applications Volume 41

By J. Yarwood, R. Douthwaite, S. B. Duckett

The Royal Society of Chemistry

Copyright © 2010 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84755-047-7

Contents

Preface J. Yarwood, R. Douthwaite and S. B. Duckett, v, 
The inorganic chemistry of surface enhanced Raman scattering (SERS) Karen Faulds, Aaron Hernandez-Santana and W. Ewen Smith, 1, 
IR spectroscopy of clay minerals and clay nanocomposites Jana Madejová, Helena Pálková and Peter Komadel, 22, 
Applications of XPS to the study of inorganic compounds Karen Wilson and Adam Lee, 72, 
Applications of QM/MM in inorganic chemistry Tell Tuttle, 87, 
Bioinorganic electrochemistry Carola Schulzke, 111, 
In-situ Fourier transform infra red spectroelectrochemistry as a probe of electrocatalysis P. A. Christensen, 125, 
Femtosecond mid-infrared spectroscopy of liquid water and aqueous solutions H. J. Bakker, 166, 
Time resolved spectroscopy of inorganic complexes Tia E. Keyes, Robert J. Forster and Charles Blackledge, 211, 
In situ photochemistry with NMR detection of organometallic complexes Graham E. Ball, 262, 
Mass spectrometry in organometallic chemistry J. Scott McIndoe, 288, 
Nuclear quadrupole resonance spectroscopy K.B. Dillon, 310, 

CHAPTER 1

The inorganic chemistry of surface enhanced Raman scattering (SERS)

Karen Faulds, Aaron Hernandez-Santana and W. Ewen Smith

DOI: 10.1039/9781849730853-00001

1. Introduction

Surface enhanced Raman scattering (SERS) has enormous potential as a detection technique. Used correctly, it can combine many of the advantages of Raman spectroscopy such as stand off detection and selective identification of a species in situ, with the additional advantages of high sensitivity and even greater specificity.

The measurement of SERS involves adding an analyte to the roughened surface of a suitable metal and interrogating the surface using a Raman spectrometer. The metals most commonly used for practical SERS measurements are gold or silver. The reasons for this are that the electronic properties of gold and silver are suitable for SERS in that they have surface plasmons which lie in the visible region of the electromagnetic spectrum which coincides with the commonly used Raman excitation wavelengths, they have good resistance to corrosion allowing the surfaces produced to be reasonably long lived, and a large number of analytes adsorb effectively on them.

Raman spectroscopy is a selective technique, both because of the molecularly specific nature of the pattern of peaks obtained and because of the wide variation in the Raman cross section of different analytes. For example, water gives very weak Raman scattering and organic molecules usually have much larger scattering cross sections thereby enabling Raman scattering to be recorded from organic molecules in aqueous solution and allowing analytes to be identified in situ. However, the detection limits are usually high since Raman is an inherently weak process and surface enhancement provides much greater sensitivity. Surface enhancement was originally discovered by Fleischman when unusually intense Raman scattering was observed from pyridine adsorbed on a roughened silver electrode. The enhancement observed was calculated to be a factor of 106 over that expected for normal Raman scattering. It was subsequently discovered that the magnitude of the enhancement obtained with SERS can be increased further if the analyte used is a coloured molecule. In this technique, surface enhanced resonance Raman scattering (SERRS), the enhancement is due to a combination of surface enhancement and resonant enhancement obtained from molecular resonance from the coloured analyte when the wavelength of an electronic transition coincides with the laser excitation wavelength. Only Raman bands originating from the chromophore of the analyte are enhanced, which can afford selective detection of a resonant analyte in the presence of non-resonant contaminants. The enhancement factors claimed can be extremely high and have been calculated for a suitable analyte to be between 1013 and 1015. This form of spectroscopy has been shown in practice to rival or surpass fluorescence in sensitivity. However, the major advantage of SERS over fluorescence is the fact that SERS is a molecularly specific technique producing spectra which have sharp peaks whereas fluorescence spectra are broad and overlapping and less specific for a particular molecule. This enables much higher numbers of analytes to be discriminated in the one vessel by SERS/SERRS than by fluorescence where it is difficult to discriminate between more than 3 or 4 analytes in a mixture.

Thus, SERS is a label-less technology that has the advantage that many substances can be identified in situ and at low concentration but, if even higher sensitivity is required, this can be achieved by adding SERRS active labels. It should be noted that the labels need not be fluorophores or dyes, but can be any molecule which gives a strong SERS response and consequently a wider range of labels are available for SERS than for fluorescence techniques. A good example of this is that many haem compounds give very effective SERRS. These include chromophores which are native to proteins such as cytochrome C and cytochrome P-450.

Modern Raman spectroscopy is a simple and effective technique and this ease of use permits the development of SERS/SERRS methods where the advantages of selectivity, sensitivity and multiple analyte determination in a single cuvette or micro-titreplate well are required. Two difficulties have inhibited the growth of the technique and both are now better understood and both can be overcome. Firstly, SERS was originally discovered experimentally and the effect was poorly understood. Following much debate and many theoretical and practical studies the theory is now much better understood. The second difficulty concerns the need to obtain reliable and reproducible enhancement. Most roughened silver and gold surfaces which contain nanoscale features will give SERS and some will give very large enhancements. However, if the roughness features cannot be reproduced, it is not possible to obtain the same degree of enhancement for each surface making reliable analysis methods difficult to develop. A greater understanding of the nature of the effect has led to the design of reproducible and reliable substrates and the development of methods for obtaining reproducible results, overcoming some of the early problems with the technique. This article will initially discuss the basic theory required for practical use and then the main types of substrate that can be used to obtain reliable and reproducible results.

2. Theory

Many papers on SERS refer to two different effects which contribute to the SERS enhancement mechanism. The first, and dominant, effect is electromagnetic enhancement and the second is chemical or charge transfer enhancement.

Electromagnetic enhancement is observed when the analyte is on, or very close to, a metal surface. The analyte interacts with the surface plasmon, which is essentially a wave of electrons on the metal surface created by the interaction of the laser excitation with electrons bound to the surface. A surface plasmon created on a smooth surface does not scatter light since it is confined to the metal surface. To create scattering, the surface requires to be roughened to create a component of the plasmon with a vector perpendicular to the surface. The frequency of the plasmon is dependent on the properties of the metal and on the roughness of the metal surface. There are many experiments that show that plasmon enhancement is essential for effective SERS.

Although electromagnetic enhancement can explain important features of the SERS effect it does not take into account changes to the analyte as a result of bonding to the surface. Chemical enhancement models take into account the analyte surface interaction and are thought to operate independently from electromagnetic enhancement. For systems where both enhancements occur simultaneously, it is thought that the enhancement effects are additive. Chemical enhancement can result from charge-transfer between the metal and the analyte which can result in an increase in the polarisability, α, of the molecule. Therefore, a surface complex between the analyte and the metal must form before chemical enhancement can occur.

The simplest explanation for chemical enhancement is that new electronic states arise from the chemi-sorption of the analyte to the metal, which serve as new resonant intermediate states in Raman scattering. It is thought that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the adsorbate are symmetrical in energy with respect to the Fermi level of the metal. If this occurs, charge transfer excitation from the metal to the molecule and vice versa can occur at a lower energy than the intrinsic intramolecular excitations of the adsorbate.

It is very difficult to separate the two enhancement effects but this has been attempted by measuring the chemical enhancement from smooth metal surfaces where it was found that the chemical enhancement effect was small. This enhancement process is totally adsorbate dependant and requires an analyte with functionality capable of chemi- or physi-sorption to the metal surface. For instance, in a study involving rhodamine 6G molecules adsorbed on single Ag nanoparticles, the chemical enhancement factors were estimated to be only about 102–103, compared to the electromagnetic enhancement factors of up to 1011 quoted by others.

Rather than postulating two mechanisms, SERS is used here as a single effect which essentially follows the steps of the electromagnetic approach. The way in which charge transfer enhancement is implicit in the single mechanism description will be discussed after describing the effect itself. Basically, the process can be described in five stages:

1. The creation of the surface plasmon by the incoming laser light.

2. The interaction of the plasmon with the molecule adsorbed on the surface.

3. The Raman process in the molecule.

4. The transfer of energy back into the plasmon.

5. The scattering of the frequency shifted radiation from the metal.

These events occur on different length scales. The plasmon is excited using radiation which is often of the order of 500 nm, however the roughness required to obtain an effective plasmon for SERS with visible excitation is usually of the order of about 50 nm. It is probable that in practice many roughened surfaces have a smaller scale roughness within them but the features usually discriminated by electron microscopy and calculated in designs to shape plasmons for SERS, are in the order of 50 nm for visible excitation. However, the Raman process is a molecular event and the size of small molecules, such as pyridine, are closer to 1 nm in size. This means that light of 500 nm excites a plasmon of about 50 nm which in turn creates Raman scattering from a molecule on the 1 nm scale. Thus when the plasmon is excited (step 1), the exciting radiation can cover the whole of the roughened area, however it is shaped on the 50 nm scale by the metal, and the nature of the roughness features. The actual transfer of energy from the plasmon to the molecule (step 2) then occurs on the 50 to 1 nm scale and the actual Raman event (step 3) on the 1 nm scale.

The magnitude of the SERS enhancement drops off quickly with the distance of the molecule from the surface 16 in fact the field falls off as 1/r3 from the surface and although SERS has been recorded up to about 20 nm from the surface, more than 90% of the scattering occurs from the first layer. Within that layer, the most effective enhancement will occur where the adsorbed molecule is at a site where there are high field gradients. For example, if the surface is created by bringing two nano particles together, the highest field gradients are obtained close to where the particles touch or nearly touch and it is believed that this is where the maximum SERS enhancement occurs (Fig 1). This means, that specific molecules will give different enhancements depending on their position on the surface.

There are many good studies modelling the nature of the plasmons and the effect of the interstices which give confidence in the above explanation which lie outside the scope of this article. However, the exact way in which the energy is transferred between the plasmon and the molecule, and the precise nature of the effective SERS site, still requires further definition.

The above explanation emphasises the importance of the plasmon in causing the surface enhancement. However, calculations suggest that in many cases the plasmon enhancement that would be expected is lower than that obtained in practice leading to the belief that there may also be some interaction with the surface (charge transfer or chemical enhancement) involved. It should also be pointed out that no surface chemistry has been considered in the electromagnetic enhancement based approach described above. The surface layer can affect the plasmon, so it is essential that the chemistry involved between the analyte and the surface is considered even for the electromagnetic/plasmon mechanism, especially for substances such as silver where a metal oxide layer is expected to form on the surface if the substrate is left in air or water. Further, the way in which the molecule is attached to the surface will affect the electronic structure, and hence the polarisability, of the analyte. For example, if a molecule such as pyridine bonds to an Ag+ ion present on the silver surface, this will then essentially become an Ag+ complex in which the polarisability of the molecule is affected by the bonding in the complex. If however, a molecule such as pyridine is adsorbed onto clean metal surface in high vacuum, the dative bond formed in an Ag+ complex will not be present and the bonding will involve surface metal atoms i.e. Ag0. Note that the orbitals on the silver atoms used to form the bond and those used to describe the plasmon are essentially the same, therefore the properties of the metal plasmon are affected by the surface interaction between the analyte and the metal. Gold surfaces tend to be more inert than silver surfaces, however the reduced colloidal metal nanoparticles often used have significantly negative zeta potentials indicating that a physi-sorbed, if not a chemi-sorbed, surface layer is present and this may still affect the plasmon and the polarisation of the adsorbate. Thus, if SERS is to be understood, the surface chemistry has to be considered, both because of the direct effect it has on the plasmon frequency and because of the effect surface bonding can have on the polarisability of the adsorbed analyte.

The role of surface charge, in particular when using colloidal nano-particles, should not be neglected when carrying out SERS measurements. One of the basic requirements for SERS is close proximity of the analyte to the metal surface, a condition which may be easily disrupted when relying on electrostatic interactions. For instance, the SERS intensity of the amino acid glycine on gold can vary sharply with pH. This may be attributed to a shift in equilibrium between the positive, neutral and negative charged forms, which in turn affect the affinity of the amino acid for the metal surface (Fig. 2). In this example, however, it is likely that pH also has an effect on nanoparticle aggregation. Surface charge can direct nano-particle aggregation, so these parameters need to be addressed in order to carry out successful SERS measurements. Hence, by careful consideration of these parameters, it is possible to predict SERS activity of different analytes. For instance, spermine (a cationic polyamine) may be used as an effective aggregating agent in SERS analysis of DNA using colloidal silver. Spermine plays a dual role by neutralizing the negatively charged DNA backbone and the negatively charged silver particles, inducing the adsorption of DNA onto the metal surface.

3. Plasmons

SERS has been obtained from a large number of metals including lithium, palladium, cadmium, nickel, silver, gold and copper, and the size of the enhancement varies enormously with the metal. However, there is a limitation on the number of metals that can be used since we require a stable, reproducible and reliable practical substrate for SERS and metals such as lithium and sodium are essentially unsuitable since they will rapidly oxidise. For instance, copper is difficult to use in air because the surface is rapidly corroded and other metals such as palladium have low enhancement factors. The largest enhancements occur from surfaces that have been roughened at the nanoscale level. Some examples include chemically roughened electrodes, metal colloid assemblies produced via lithography, islands films, metal coated silica and magnetic beads, metal deposition over polystyrene nanospheres to produce ordered arrays of metal particles and silver coated AFM tips. The most commonly used substrates are silver and gold colloids since they are time stable, easily prepared, and provide a large surface area. They both have reasonably good chemical resistance and have surface plasmons which lie in the visible region, enabling good enhancement factors to be obtained when excited with the visible and near infrared lasers commonly used by Raman spectroscopists.

(Continues...)

Excerpted from Spectroscopic Properties of Inorganic and Organometallic Compounds by J. Yarwood, R. Douthwaite, S. B. Duckett. Copyright © 2010 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2010
• Language: English
• ISBN 10: 1847550479
• ISBN 13: 9781847550477
• Binding: Hardcover
• Number of pages: 334
• Editor: Jack Yarwood