Waveguide confined Raman spectroscopy for microfluidic interrogation† Praveen C. Ashok, * Gajendra P. Singh, Helen A. Rendall, Thomas F. Krauss and Kishan Dholakia Received 30th September 2010, Accepted 6th December 2010 DOI: 10.1039/c0lc00462f We report the first implementation of the fiber based microfluidic Raman spectroscopic detection scheme, which can be scaled down to micrometre dimensions, allowing it to be combined with other microfluidic functional devices. This novel Raman spectroscopic detection scheme, which we termed as Waveguide Confined Raman Spectroscopy (WCRS), is achieved through embedding fibers on-chip in a geometry that confines the Raman excitation and collection region which ensures maximum Raman signal collection. This results in a microfluidic chip with completely alignment-free Raman spectroscopic detection scheme, which does not give any background from the substrate of the chip. These features allow a WCRS based microfluidic chip to be fabricated in polydimethylsiloxane (PDMS) which is a relatively cheap material but has inherent Raman signatures in fingerprint region. The effects of length, collection angle, and fiber core size on the collection efficiency and fluorescence background of WCRS were investigated. The ability of the device to predict the concentration was studied using urea as a model analyte. A major advantage of WCRS is its scalability that allows it to be combined with many existing microfluidic functional devices. The applicability of WCRS is demonstrated through two microfluidic applications: reaction monitoring in a microreactor and detection of analyte in a microdroplet based microfluidic system. The WCRS approach may lead to wider use of Raman spectroscopy based detection in microfluidics, and the development of portable, alignment-free microfluidic devices. Introduction Optofluidic devices where optical detection techniques are incorporated into microfluidics and nanofluidics have played a crucial role in the advancement of Lab on a Chip technology. 1 In the field of bio-chemical analytics for qualitative and quanti- tative analyses of samples, microfluidics devices, combined with spectroscopic techniques have found a variety of applications. Raman spectroscopy is a powerful analytical tool which is receiving attention from the microfluidic community as a viable detection technique owing to its high chemical specificity and high information content. One of the main advantages of Raman spectroscopy over other spectroscopic methods is its ability to achieve multi-component detection in an analyte as each component would exhibit its own fingerprint bands in the acquired Raman spectrum. Hence it is possible to achieve simultaneous monitoring of multiple analyte components in a microfluidic channel. Microfluidic Raman spectroscopy (MRS), where Raman spectroscopy is used as a detection technique on a microfluidic platform, has been used for a variety of chemical and biological applications ranging from basic analyte detection to reaction monitoring in a microreactor. 2–8 Its applications are still limited, when compared to other methods such as fluorescence spec- troscopy or absorption spectroscopy. 9 The main hindrance has been the inherently low cross-section of the Raman scattering process. Also, in many cases, the weak Raman signal is over- whelmed by a strong spectral background arising from the substrate of the microfluidic chip. Hence, for the successful implementation of MRS, it is essential to ensure maximum collection efficiency and minimum interference of the back- ground from the substrate. In fact, the implementation of MRS has largely been restricted to free space, bulk optic geometries and, to the best of our knowledge, a genuine on-chip implementation has not been reported before. Bulk optic based Raman detection systems have two inherent limitations: background from the substrate and the lack of portability. A detailed discussion on the former issue can be found in some recent publications. 6,10 The other limitation of a bulk optic based system is its lack of portability. Whilst portable or bench top Raman microscopes are available for field applications of MRS, 11 the system still requires optical alignment expertise to collect the Raman signal. A true portable MRS system should be completely alignment-free and should be able to record Raman spectra in daylight. It is worth noting that other spectroscopic methods when implemented in microfluidics have moved from a free space, bulk optic geometry to embedded fiber based geometries to achieve miniaturization and portability of the system. 12 For example, in SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, Fife, KY16 9SS, UK. E-mail: pca7@st-andrews.ac.uk † Published as part of a LOC themed issue dedicated to UK Research: Guest Editors Professors Hywel Morgan and Andrew deMello. 1262 | Lab Chip, 2011, 11, 1262–1270 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Lab on a Chip Cite this: Lab Chip, 2011, 11, 1262 www.rsc.org/loc PAPER Downloaded by St Andrews University Library on 17 March 2011 Published on 11 January 2011 on http://pubs.rsc.org | doi:10.1039/C0LC00462F View Online