Short communicationImmobilisation of DNA to polymerised SU-8 photoresist
Introduction
A future goal of the lab-on-a-chip is to integrate every step needed for detecting an analyte in the sample. The analysis of a sample is often based on size separations or interaction with a probe molecule either in solution or attached to a solid support. Using high throughput solid phase reactions like microarray, tens of thousands of analytes can be detected in one batch process. An efficient procedure for immobilising DNA or proteins to a solid support is essential for solid phase biochemical reactions. DNA has been successfully attached to glass (Zammatteo et al., 2000), oxidized silicon (Chrisey et al., 1996), silicon wafers (Strother et al., 2000), gold surfaces (Steel et al., 2000) and PMMA (Fixe et al., 2004a, Fixe et al., 2004b) which all are relevant surfaces that can be micro-fabricated.
Because of its attractive mechanical properties, the epoxy-based photoresist SU-8 has become widely used for the fabrication of mechanical structures such as probes for scanning probe microscopy (Genolet et al., 1999, Genolet et al., 2001). Furthermore, SU-8 can be fabricated into optical waveguides (Lee et al., 2002) and into microfluidic structures (Chuang et al., 2003, Heuschkel et al., 1998, Jackman et al., 2001, Seidemann et al., 2002a, Seidemann et al., 2002b). Those functionalities can be combined and integrated to build complete biosensors (Calleja et al., 2003, Mogensen et al., 2003). Since SU-8 is an attractive material for MEMS and micro fluidic devices, it is desirable to develop procedures for immobilising DNA directly on its surface. Here we describe a one-step procedure for attaching DNA directly onto cured micropatterned SU-8.
Section snippets
Processing of photoresist
Commercially available SU-8 5 (glycidyl ether of bisphenol A) (Microresist Technology, Berlin, Germany) and resist thinner (gamma butyrolactone) were mixed in a 2:1 ratio. An SU-8 thickness of 0.8 μm was obtained when spin-coating the resist onto a glass wafer at a spinning speed of 3000 rpm. After spinning, the excess solvent was evaporated in a soft bake for 3 min at 65 °C and 2 min at 95 °C. The SU-8 was exposed to UV-light through a mask for 30 s at 9 mW/cm2 and baked for 1 h at 90 °C. The wafers
Auto-fluorescence of SU-8
SU-8 was deposited onto glass wafers at different thicknesses and scanned in the Cy3 channel to evaluate the optical properties of SU-8 in a Cy3-based fluorescent assay. An SU-8 layer of 0.8 μm thickness had approximately 20% higher fluorescence signal than the glass wafer alone, while 1.7, 12 and 120 μm thick layers of SU-8 had 50, 400 and 6600% more signal than glass, respectively, indicating that the thinnest SU-8-layer as possible should be used in order to minimise the background signal.
Conclusions
SU-8 could be functionalised with DNA capture probes with functional probe densities of 6–13 fmol/mm2. The immobilised DNA has excellent specificity down to single base pair level indicating that probes immobilised on SU-8 could be used for genotyping. The stable bond between SU-8 and DNA makes regeneration of sensor surfaces possible. The immobilising of DNA directly to SU-8 avoids several process steps otherwise needed to link DNA to glass and simplifies integration of DNA in biosensors like
Acknowledgements
We thank Dorthe Thybo Ganzhorn for spending time in the cleanroom fabricating the photoresist substrate. This project was funded by the Danish research council (Grant #2014-00-0003, DABIC and Grant #26-02-0280, Polymeric cantilevers)
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2012, European Polymer JournalCitation Excerpt :Wang and coauthors showed UV-illumination mediated covalent linking of various monomers onto the SU-8 without any pre-treatment of the surface [18]. Single stranded DNA has been immobilized onto untreated SU-8 cantilevers with either adsorption or with a reaction of epoxy and amine groups of DNA [19]. Successful antibody immobilization on large area SU-8 surfaces was also achieved, with an initial sulfuric acid-assisted epoxy ring opening and a subsequent silanization and glutaraldehyde treatment [20].