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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.

Current Research

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.

(a)Fig. 1a
(b) Fig. 1b

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).

Fig. 2

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.

(a)Fig. 3
(b) Fig. 3

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.

Fig. 4

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.


1. Synthesis and formation of one-dimensional Au nanoparticle chains, C. Smyth, R. Reilly, Y. Rakovich and E. McCabe, J.Surf.Sci.Nanotech. 7, 327-329, 2009
2.Photoluminescent lifetime of polyelectrolytes in thin films formed via layer by layer self assembly, R. Reilly, C. Smyth, Y. Rakovich and E. McCabe, Nanotechnology 20, 095707, 2009
3.Alignment and FLIM imaging of Ag nanowires with CdTe quantum dots, C.Smyth, Y. Rakovich, E. McCabe, 11th International Conference on Transparent Optical Networks Ponta Delgada University, Azores, Portugal, 2009
4. Investigation and characterisation of 1-dimensional plasmonic structures, C.Smyth, Y. Rakovich, E. McCabe, Photonics Ireland, Kinsale, Co. Cork , 2009
5. Synthesis and formation of one-dimensional Au nanoparticle chains, C. Smyth, R. Reilly, Y. Rakovich and E. McCabe, 4th International Conference on Solid Films and Surfaces,, 25, 228, 2008
6. Synthesis and formation of one-dimensional Au nanoparticle chains, C. Smyth, R. Reilly, Y. Rakovich and E. McCabe, 2nd International Conference on Advanced Nano Materials, Aveiro, Portugal, 29, 159, 2008
7.Fluorescent lifetime imaging as a new tool to investigate surface plasmon enhanced spontaneous emission, R Reilly, Y. Rakovich and E. McCabe, Photonics Ireland Conference, Galway, 33,, 2007
8. Imaging of Enhanced Fluorescence by Nanostructured Gold Particles, A. MacRaighine, Y. Rakovich, Y. Gunko, E. McCabe, Conference Quantum Electronics and Photonics (QEP-17), 2006
9. Emerging Light Fields from Liquid Crystal Microlenses, A. MacRaighne, E. McCabe and T. Scharf, Rev.Sci.Instrum. 77, 055103, 2006
10.Confocal microscopy using variable-focal-length microlenses and an optical fibre bundle, L. Yang, A. MacRaighne, E. McCabe, L. Dunbar and T. Scharf, Appl. Opt. 44, 5928-5936, 2005
11.Variable focal-length microlens arrays in confocal microscopy, A. MacRaighne, J. Wang, E. McCabe and T. Scharf, Proc.SPIE 5701, 93-100, 2005, Three-dimensional and Multi-dimensional Microscopy: Image Acquisition and Processing
12.Variable Focal Length Lenses for Confocal Microscopy, A. MacRaighne, J. Wang, T. Scharf and E. McCabe, European Conference on Biomedical Optics, Proc.SPIE 5860, 58600L, 2005
13. Three-dimensional confocal microscopy of fluorescent microspheres: imaging size and determination, A. MacRaighne, L. Yang and E. McCabe, Proc.SPIE 5827, 299, 2005
14.Confocal Microscopy and Variable-Focal-Length Microlenses, A. MacRaighne, L. Yang, L. Dunbar, E. McCabe and T. Scharf, Proc.SPIE 5324, 55-64, 2004
15. Three-dimensional imaging of microspheres with confocal and conventional polarization microscopes, L. Yang, C. Taylor, Y. Rakovich and E. McCabe, Appl.Opt. 42, 5693-700, 2003
16.Three-dimensional imaging of microspheres with confocal and conventional polarization microscopes, L. Yang, C. Taylor, Y. Rakovich and E. McCabe, Appl.Opt. 42, 5693-700, 2003
17. The three-dimensional imaging of microspheres in confocal and conventional polarization microspheres, L. Yang, C. Taylor, Y. Rakovich and E. McCabe, Proc.SPIE 4964, 66-72, 2003
18. Effects of source coherence and aperture array geometry on optical sectioning strength in direct-view microscopy, C. Taylor and E. McCabe, J.Opt.Soc.Am.A 19,7,1406-1416, 2002
19.Switchable fibre coupling using variable-focal-length microlenses, C. Taylor, P. Smith, E. McCabe, D. Selviah, S. Day and L. Commander, Rev.Sci.Instrum. 72, 7, 2001
20. Programmable array microscopy using a ferroelectric liquid crystal SLM, P. Smith, C. Taylor, A. Shaw and E. McCabe, Appl.Optics 39, 16, 2664-2669, 2000