Surface Plasmon Enhanced Imaging
Zero and one-dimensional nanostructures are desirable in modern technology for their potential applications as the smallest possible carriers for electrons or plasmonic signals. The ability to direct the signal from source to destination could have major implications in the fields of nanoelectronics and photonics. The interaction between biomolecules and these excited surface plasmons could also have applications in sensing. One of the major challenges facing the application of these 0D and 1D nanostructures lies in the preparation of working substrates, either in the dispersion or in the alignment of the nanomaterials.
Surface-enhanced Raman Spectroscopy (SERS) is a phenomenon that occurs due to the adsorption of target molecules to roughened metal surfaces or to metal nanostructures. Potentially, it is a diagnostic technique that offers a great amount of detail in terms of compound structure, and indeed is capable of differentiating between structural isomers. It is also a highly sensitive technique, offering enhancements up to the order of 108-fold. Reproducibility issues that plagued the early development of SERS have begun to be addressed, and effective substrates based on the nanoscale features required for the technique are being developed.
As the cross-section of conventional Raman scattering is small compared to that of radiation examined in other spectroscopic techniques, SERS offers a potentially important diagnostic technique that offers a great amount of detail in terms of sensitivity and compound structure. One of the primary causes of the SERS effect is through an enhancement of local electric fields at the metal-analyte interface. This is caused through the generation of surface plasmons when the laser source is incident on the metal surface. Understanding and exploiting how these interactions occur is of great importance in the optimization of SERS substrates and is a key element of this work. Application of SERS to biosensing is a primary focus of this work. Metal colloids are examined with respect to their SERS efficiency regarding a group of biologically-important cofactors in the pterin family. These molecules are important in pigment colouration and also in the mediation of many biochemical reactions such as in enzyme catalysis. In cancer patients it has been found that increased pterin concentrations occur in urine samples. Pterins are biological compounds, the basic unit of which is a pyrazine and a pyrimidine ring. Many derivations occur, based on differing functional groups. Certain techniques such as high-performance liquid chromatography (HPLC) and high-performance capilliary electrophoresis (HPCE) have proved successful as highly sensitive diagnosis tools, but they are limited in providing detailed structural information. SERS offers an alternative to these approaches. Reproducibility has proven to be a key issue in realizing the potential of SERS. It is clearly important to have control over all of the factors which influence the localized surface plasmon resonance in order to maximize signal strength. These factors include material size, shape, interparticle spacing and also dielectric environment. All of these factors must be carefully controlled to ensure that the incident laser light excites the localized surface plasma residence in a reproducible manner. This work will consider reproducibility issues in relation to a reference molecule.
Figure 1. Both silver and gold nanostructures have been synthesized, as shown in (a) and (b) respectively. These structures have been examined in various applications, particularly concerning Fluorescence Lifetime Imaging Microscopy (FLIM) and Surface-Enhanced Raman Spectroscopy (SERS).
Figure 2. Confocal microscopy can be used to examine the fluorescence properties of a specimen in order to form images. When an incident light source illuminates a single point on the focal plane of a specimen, fluorescent light is emitted, channelled back through the objective lens and through a dichroic mirror. It then impinges upon the confocal aperture, which is located in the primary image plane of the objective. The confocal aperture allows all of the in-focus light from the region of interest on the sample to pass through to the detector. The light rays coloured red and green in the figure represent the fluorescent emission from different depths in the sample, and upon contact with the confocal aperture these rays are not in focus. As such very little of this light gets through to the detector and so adds very little to the final image obtained.
Figure 3.Image (a) on the left shows a light microscope image of a group of nanowires and nanoparticles. The wires were deposited onto a substrate that consisted of 2 bilayers of polyelectrolytes and 1 monolayer of 580nm-emitting CdTe quantum dots on a glass microslide. This area was imaged under FLIM, the result of which is shown in image (b). The average lifetime measured shows bright spots according to where the nanostructures are located.
Figure 4. The image on the left is a light microscope image of isoxanthopterin dried onto the silicon substrate. The compound tends to aggregate, hence the clusters evident here. The image on the right is a scanning Raman image taken as an average count across the largest Raman peak for isoxanthopterin, at about 1305cm-1. It can be seen that the most concentrated area of the isoxanthopterin aggregation gives rise to the most intense area of the scanning Raman image.
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