Surface enhanced Raman spectroscopy (SERS) [1] was discovered in 1974 and is now a powerful analysis technique which is becoming more and more widespread due to the development of cheaper lasers and spectrometer systems. SERS derives information about the type of chemical bonds in trace amounts of analyte molecules when these are adsorbed onto, or placed adjacent to, a metal surface or structure: a so-called SERS substrate. As it is vibrations in the chemical bonds of the analyte that cause a Raman shift, most chemical species can in theory be identified. For this reason SERS is used as a versatile analytical tool for both chemical and biochemical sensing in liquid and gas phases. [2][3][4][5][6] It is widely recognized that the SERS phenomena is based on laser excitation inducing an electromagnetic field at the surface of noble metals. Areas with particularly large electromagnetic fields, also called "hot spots", are found in between adjacent metal nanostructures if these are located sufficiently close to each other, i.e., on the order of a few nanometers. If an analyte molecule is located inside this hot spot, it will result in a comparably large Raman signal from the analyte. Recent studies have shown that hot spots account for a disproportionally large contribution to the total Raman signal from a SERS substrate. [7,8] There is generally an emphasis on reproducibility and uniformity to ensure consistent chemical detection sensitivity across the surface of a SERS substrate. A trade-off between reproducibility and enhancement is often mentioned. The current dilemma is that the more uniform the enhancement is across a substrate, i.e. the better the reproducibility, which is often achieved using a top down lithographic approach, the lower the enhancement tends to be and vice versa.[9] Currently, the inability to mass produce large areas of cost effective nanostructured SERS substrates with large numbers of hot spots and hence suitable Raman enhancement is impeding the use of SERS sensors in both laboratories and mobile applications.Ideally, a SERS substrate should enhance the Raman effect sufficiently to enable suitable chemical detection levels while at the same time being practical to use in a sensor system. Broadly speaking, SERS substrates can be classified into two categories, i) metallic nanoparticles in colloidal solution and ii) roughened metallic surfaces. Colloidal suspensions of metallic nanoparticles have so far been reported to have the largest enhancement factors and even single molecule detection has been demonstrated. [10][11][12] However, with colloidal solutions it is a challenge to bring the metal nanoparticles sufficiently close to form hot spots around the analyte, while at the same time preventing the formation of large conglomerations of nanoparticles before the analyte is introduced.Rough silver and gold surfaces can be fabricated by a number of methods including chemical etching, mechanical deformation, electroplating and oblique angle deposition. [13][14][15] Processes where well define...
The fabrication and performance of an electrophoretic separation chip with integrated optical waveguides for absorption detection is presented. The device was fabricated on a silicon substrate by standard microfabrication techniques with the use of two photolithographic mask steps. The waveguides on the device were connected to optical fibers, which enabled alignment free operation due to the absence of free-space optics. A 750 microm long U-shaped detection cell was used to facilitate longitudinal absorption detection. To minimize geometrically induced band broadening at the turn in the U-cell, tapering of the separation channel from a width of 120 down to 30 microm was employed. Electrical insulation was achieved by a 13 microm thermally grown silicon dioxide between the silicon substrate and the channels. The breakdown voltage during operation of the chip was measured to 10.6 kV. A separation of 3.2 microM rhodamine 110, 8 microM 2,7-dichlorofluorescein, 10 microM fluorescein and 18 microM 5-carboxyfluorescein was demonstrated on the device using the detection cell for absorption measurements at 488 nm.
Sealing of the flow channel is an important aspect during integration of microfluidic channels and optical waveguides. The uneven topography of many waveguide-fabrication techniques will lead to leakage of the fluid channels. Planarization methods such as chemical mechanical polishing or the etch-back technique are possible, but troublesome. We present a simple but efficient alternative: By means of changing the waveguide layout, bonding pads are formed along the microfluidic channels. With the same height as the waveguide, they effectively prevent leakage and hermetically seal the channels during bonding. Negligible influence on light propagation is found when 10-mum-wide bonding pads are used. Fabricated microsystems with application in absorbance measurements and flow cytometry are presented.
This paper presents the results of an investigation of the influence of soft baking temperature on the lithographic performance of the negative photoresist SU-8. The work was initiated in order to obtain a lithographic resolution suitable for integration of diffractive optical components for near-infrared wavelengths. The study was carried out on 40 µm SU-8 layers on thermally oxidized silicon wafers, a widespread platform for integration of microfluidic systems and waveguides. A series of experiments covering soft bake temperatures in the range 65–115 °C were performed under otherwise identical processing conditions. The influence of the soft bake temperature on polymerization temperature as well as cracking, lithographic resolution and hardness of the resist was investigated. The kinetics of the polymerization process were observed to change with soft bake temperature, leading to changes in sensitivity and contrast of the resist, as well as changes in the material strength of the developed structures. Soft baking at 65 °C proved superior with respect to all the inspected properties, providing a sample showing full resolution of 3.8 µm wide trenches and no stress-related cracking.
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