Raman surface plasmons

In order to make meaningful physical measurements on monolayer and multilayer L-B films, it is essential that the films be characterized to determine the molecular structure, unformity of the film and layer-to-layer structural correlation. To probe the molecular structure, our research group has utilized a variety of spectroscopic techniques such as u.v.-visible spectroscopy, ATR-IR, wave-guide Raman, surface-plasmon wave Raman spectroscopy and inelastic electron tunneling. The spectroscopic techniques... 

In these sensors, the intrinsic absorption of the analyte is measured directly. No indicator chemistry is involved. Thus, it is more a kind of remote spectroscopy, except that the instrument comes to the sample (rather than the sample to the instrument or cuvette). Numerous geometries have been designed for plain fiber chemical sensors, all kinds of spectroscopies (from IR to mid-IR and visible to the UV from Raman to light scatter, and from fluorescence and phosphorescence intensity to the respective decay times) have been exploited, and more sophisticated methods including evanescent wave spectroscopy and surface plasmon resonance have been applied. 

Pettinger B., Wenning U., Wetzel H., Surface-plasmon enhanced Raman-scattering frequency and angular resonance of Raman scattered-light from pyridine on Au, Ag and Cu electrodes, Surf. Sci. 1980 101 409-416. 

Surface-enhanced Raman scattering (SERS) has emerged as a powerful technique for studying species adsorbed on metal films, colloidal dispersions, and working electrodes. SERS occurs when molecules are adsorbed on certain metal surfaces, where Raman intensity enhancements of ca. 105-106 may be observed. The enhancement is primarily due to plasmon excitation at the metal surface, thus the effect is limited to Cu, Ag, and Au, and a few other metals for which surface plasmons are excited by visible radiation. 

Although chemisorption is not essential, when it does occur there may be further enhancement of the Raman signal, since the formation of new chemical bonds and the consequent perturbation of adsorbate electronic energy levels can lead to a surface-induced RR effect. The combination of surface and resonance enhancement (SERRS) can occur when adsorbates have intense electronic absorption bands in the same spectral region as the metal surface plasmon resonance, yielding an overall enhancement as large as 10lo-1012. 

A number of methods are available for the characterization and examination of SAMs as well as for the observation of the reactions with the immobilized biomolecules. Only some of these methods are mentioned briefly here. These include surface plasmon resonance (SPR) [46], quartz crystal microbalance (QCM) [47,48], ellipsometry [12,49], contact angle measurement [50], infrared spectroscopy (FT-IR) [51,52], Raman spectroscopy [53], scanning tunneling microscopy (STM) [54], atomic force microscopy (AFM) [55,56], sum frequency spectroscopy. X-ray photoelectron spectroscopy (XPS) [57, 58], surface acoustic wave and acoustic plate mode devices, confocal imaging and optical microscopy, low-angle X-ray reflectometry, electrochemical methods [59] and Raster electron microscopy [60]. 

Optical sensors have the advantage of an easily measured signal that can be seen by the naked eye in some cases. Optical detection methods include fluorescence, surface plasmon resonance spectroscopy, Raman, IR, and chemiluminescence (Fabbrizzi and Poggi 1995 deSilva et al. 1997). However, the fabrication and development of optical MIP sensors requires that a colored, emissive, or fluorescent monomer... 

Kelvin-probe) AFM, and STM] to analyze the local phenomena and structure after top contact constmction. Development of new techniques such as surface plasmon resonance Raman, for example, could be applied to advantage for junctions fabricated with plasmonically active metals such as silver [125] or gold [126]. 

In principle, optical chemosensors make use of optical techniques to provide analytical information. The most extensively exploited techniques in this regard are optical absorption and photoluminescence. Moreover, sensors based on surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) have recently been devised. 

One simple explanation for these results was as follows The electric field at a metal vacuum interface can be >10 times larger than in free space when the conditions required for a surface plasma resonance are met (47). Since the Raman cross-section is proportional to the square of the field, surface plasmons could produce enhancements of >10. This enhancement is probably not large enough to explain the tunneling junction results by itself, but an enhancement in signal of a factor of 100 by the excitation of surface plasmons would increase the Raman intensity from near the limits of detectibility. 

Figure 16. Raman scattering signal from A1-A lOx-4-pyridine-COOH-A g tunneling junctions with three different thicknesses of CaFs evaporated on the substrates before making the samples ("451. Thicker CaFs films have larger roughness amplitudes. The rougher Ag films couple better to surface plasmons and give larger Raman scattering enhancements.

Figure 17. Reflectivity (O) and Raman scattering intensities (9) for an Al-AlOx-4-pyridine-COOH-Ag tunneling junction prepared on a diffraction grating substrate, as a function of the angle between the incident laser beam and the normal to the grating surface (45). At the same angles that absorption by surface plasmons causes reflectivity dips, the Raman signal shows peaks.

Metal labels have been proposed to resolve problems connected with enzymes. Metal ions [13-16], metal-containing organic compounds [17,18], metal complexes [19-21], metalloproteins or colloidal metal particles [22-28] have served as labels. Spectrophotometric [22,25], acoustic [25], surface plasmon resonance, infrared [24] and Raman spectroscopic [28] methods, etc. were used. A few papers have been dealing with electrochemical detection. However, electrochemical methods of metal label detection may be viewed as very promising taking into account their high sensitivity, low detection limit, selectivity, simplicity, low cost and the availability of portable instruments. 

The transducers most commonly employed in biosensors are (a) Electrochemical amperometric, potentiometric and impedimetric (b) Optical vibrational (IR, Raman), luminescence (fluorescence, chemiluminescence) (c) Integrated optics (surface plasmon resonance (SPR), interferometery) and (d) Mechanical surface acoustic wave (SAW) and quartz crystal microbalance (QCM) [4,12]. 

---