Research in Focus

This months research in focus presents some of the work done by Daniel White fibre optic SERS sensors.

The following is as abstract from the paper 'Fibre-Optic SERS Sensors with Well-Defined Nanoscale Structures' (D.J. White and P.R. Stoddart) which is to be presented at the ICORS 2004 conference in August.

Abstract: A new approach to fabricating reproducible and stable SERS-active coatings on the tip of fibre optic probes is described. A standard 350 mm diameter coherent optical fibre imaging bundle was drawn down to a diameter of 30-40 mm, which reduces the diameter of the individual picture elements to about 150 nm. After etching the cleaved tip of the drawn fibre, the picture elements provide a regular nanostructured array. We demonstrate that this array is well suited to SERS.

(a)	(b)	(c)
Fig 1. (a)(b) Imaging fibre tip after 1 minute etch. (c) SEM image taken at an angle of 30 degrees showing nano-scale structures between wells

The sensitivity of surface enhanced Raman scattering (SERS) has led to notable recent applications in single-molecule spectroscopy [1], gene detection [2] and glucose sensing [3]. At the same time, progress continues to be made in the development of compact, field-portable Raman spectrometers for new on-line and in-vivo applications. Compact instrumentation is typically fibre coupled in order to avoid alignment difficulties and allow in-situ monitoring [4]. In their recent paper on glucose biosensing, the Van Duyne group identified the development of a fibre optic probe with a SERS-active tip as one of four remaining milestones before a minimally invasive, in-vivo glucose sensor is achieved [3].

To this end, there have been a number of previous attempts to produce SERS surfaces on the tips of optical fibres. These efforts have made use of a variety of techniques, including deposition of colloidal metal particles [5], vapour deposited metal islands, metal films over nanoparticles or metal films over abrasively roughened surfaces [6]. In general, these methods fail to achieve well-defined and uniform features and therefore suffer from poor reproducibility. The immobilization of gold or silver colloid appears to have delivered the best results thus far, with a reported reproducibility of about 10% [5]. This may still be too high for many analytical requirements.

In the present work, an optical fibre imaging bundle (Fujikura FIGH-10-350S), consisting of approximately 10,000 individual fibres fused together, has been used as the fibre platform. The individual fibres were reduced to sub-micron scales by heating the image fibre in an oxyacetylene torch and drawing it to a smaller diameter. The drawn fibre was then cleaved at a diameter of 30 - 40 mm and immersed in a selective etchant. It is known that standard imaging fibre can be etched to form micron-sized wells in a honeycomb-like pattern [7]. The nanoscale structures revealed in one of the drawn fibres are shown in Fig. 1. The individual fibres are not expected to guide light on this scale and so the drawn fibre can be regarded as a single homogeneous waveguide. After etching, the fibre tip was coated with a 100 nm layer of silver in a vapour deposition system.

Fig 2. (a) Thiophenol spectrum after 1 minute etch on a 30um fibre and (b) after a 2 minute etch on a 38um fibre.

The etched and silver coated fibre tips were soaked for 10 minutes in a 10 mM solution of thiophenol in ethanol, which provides a convenient and stable reference spectrum. As can be seen in Fig. 2, strong thiophenol SERS spectra were obtained from the free surface of two different drawn fibres. The uncontrolled fibre drawing process does not allow spectral measurements to be performed through the fibre in these samples. The measurements were performed on a Renishaw System RM2000 with a 20x objective and approximately 1 mW of 633 nm excitation at the sample. The broad background apparent in Fig. 2(b) is not yet understood, but is believed to result from variations in size and shape (aspect ratio) of the etched structures. The reproducibility of the sensors and the influence of the nanostructure geometry will be examined in detail in future studies. These results indicate that high-quality SERS substrates can readily be produced from optical fibre imaging bundles. The method has the potential for mass production of inexpensive devices for use in a number of different chemical and biological sensing applications.

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