Modifying the surfaces of metal nanoparticles with self-assembled monolayers of functionalized thiols provides a simple and direct method to alter their surface properties. Mixed self-assembled monolayers can extend this approach since, in principle, the surfaces can be tuned by altering the proportion of each modifier that is adsorbed. However, this works best if the composition and microstructure of the monolayers can be controlled. Here, we have modified preprepared silver colloids with binary mixtures of thiols at varying concentrations and modifier ratios. Surface-enhanced Raman spectroscopy was then used to determine the effect of altering these parameters on the composition of the resulting mixed monolayers. The data could be explained using a new model based on a modified competitive Langmuir approach. It was found that the composition of the mixed monolayer only reflected the ratio of modifiers in the feedstock when the total amount of modifier was sufficient for approximately one monolayer coverage. At higher modifier concentrations the thermodynamically favored modifier dominated, but working at near monolayer concentrations allowed the surface composition to be controlled by changing the ratios of modifiers. Finally, a positively charged porphyrin probe molecule was used to investigate the microstructure of the mixed monolayers, i.e., homogeneous versus domains. In this case the modifier domains were found to be <2 nm.

Modification of citrate and hydroxylamine reduced Ag colloids with thiocholine bromide, a thiol functionalized quaternary ammonium salt, creates particles where the zeta potential is switched from the normal values of ca.-50 mV to ca. +50 mV. These colloids are stable but can be aggregated with metal salts in much the same way as the parent colloids. They are excellent SERS substrates for detection of anionic targets since their positive zeta potentials promote adsorption of negatively charged ions. This is important because the vast majority of published SERS studies involve cationic or neutral targets. Moreover, the fact that the modifier is a quaternary ammonium ion means that the negative surface charge is maintained even at alkaline pH. The modified colloids can be used to detect compounds which cannot be detected using conventional negatively-charged citrate or hydroxylamine reduced metal nanoparticles, for example the detection limit was 5.0 × 10(-5) M for perchlorate and <8.7 × 10(-7) M for tetraphenylporphine tetrasulfonic acid (TPPS). More importantly, picric acid (an explosive) and diclofenac (a non-steroidal anti-inflammatory) could also be analysed quantitatively at low concentrations, 2.5 × 10(-5) M and 1.9 × 10(-5) M, respectively. Interestingly, the correct choice of aggregating agent is important for achieving high sensitivity since the anion in the aggregating salt may compete with anionic targets for surface binding sites. Finally, since the modification procedure simply involves reaction of nanoparticles with a small alkyl thiol derivative, it can easily be adapted to other particle morphologies or metals.

The potential of IR absorption and Raman spectroscopy for rapid identification of novel psychoactive substances (NPS) has been tested using a set of 221 unsorted seized samples suspected of containing NPS. Both IR and Raman spectra showed large variation between the different sub-classifications of NPS and smaller, but still distinguishable, differences between closely related compounds within the same class. In initial tests, screening the samples using spectral searching against a limited reference library allowed only 41% of the samples to be fully identified. The limiting factor in the identification was the large number of active compounds in the seized samples for which no reference vibrational data were available in the libraries rather than poor spectral quality. Therefore, when 33 of these compounds were independently identified by NMR and mass spectrometry and their spectra used to extend the libraries, the percentage of samples identified by IR and Raman screening alone increased to 76%, with only 7% of samples having no identifiable constituents. This study, which is the largest of its type ever carried out, therefore demonstrates that this approach of detecting non-matching samples and then identifying them using standard analytical methods has considerable potential in NPS screening since it allows rapid identification of the constituents of the majority of street quality samples. Only one complete feedback cycle was carried out in this study but there is clearly the potential to carry out continuous identification/updating when this system is used in operational settings.

Here we report an example of a mixed thiol monolayer on the surface of Ag nanoparticles which promotes adsorption and quantitative SERS detection of 3,4-methylenedioxymethamphetamine (MDMA, "Ecstasy"); the thiols in the mixed monolayers act synergistically since MDMA does not adsorb onto colloids modified with either of the thiols separately.

Surface‐enhanced Raman spectroscopy (SERS) is now widely used as a rapid and inexpensive tool for chemical/biochemical analysis. The method can give enormous increases in the intensities of the Raman signals of low‐concentration molecular targets if they are adsorbed on suitable enhancing substrates, which are typically composed of nanostructured Ag or Au. However, the features of SERS that allow it to be used as a chemical sensor also mean that it can be used as a powerful probe of the surface chemistry of any nanostructured material that can provide SERS enhancement. This is important because it is the surface chemistry that controls how these materials interact with their local environment and, in real applications, this interaction can be more important than more commonly measured properties such as morphology or plasmonic absorption. Here, the opportunity that this approach to SERS provides is illustrated with examples where the surface chemistry is both characterized and controlled in order to create functional nanomaterials.

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The goal of transforming SERS from an interesting novelty into a viable quantitative analytical technique has been pursued for many years, but it is only recently that the first indications that quantitative SERS could be generally achievable have appeared. Up to this point, the challenges of preparing sensitive and reproducible enhancing media and the need to develop a deep understanding of the fundamental mechanisms of the effect (or at least to have a working knowledge of the relationship between the microstructure of the enhancing materials, their surface chemistry and the enhancements that they provide; see also Chapter 1) have been sufficient to occupy the attention of researchers who were interested in using SERS for quantitative measurements. However, many of these issues have now either been fully resolved or at least brought to the stage where they are no longer impediments. For example, new generations of enhancing media have been developed; the links between microstructure, plasmon resonances and enhancements have been established; and the role of the media's surface chemistry is being addressed. This chapter discusses these new developments and also considers the extent to which the separate strands can be combined to create standard, generally applicable methods for quantitative SERS. In addition, it highlights the need to begin the process of judging the success of new SERS methods against competing technologies, rather than against previous SERS results. SERS MediaThe choice of enhancing media for SERS studies has grown to an extraordinary extent, although they can still be broadly divided into solution-phase suspensions of nanoparticles and micro-textured solid substrates. This division dates back to the earliest work in the area, which used either colloidal suspensions of silver or gold nanoparticles [1] or the surfaces of roughened noble-metal electrodes [2]. Since this

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