Optoelectronic detection of DNA molecules using an amorphous silicon photodetector
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Optoelectronic detection of DNA molecules using an amorphous silicon photodetector F. Fixe1,2, D.M.F. Prazeres2, V. Chu1 and J.P. Conde1,3 1 INESC Microsistemas e Nanotecnologias, Lisbon, Portugal 2 Center for Biological & Chemical Engineering, Instituto Superior Técnico, Lisbon, Portugal 3 Department of Materials Engineering, Instituto Superior Técnico, Lisbon, Portugal ABSTRACT This work demonstrates the use of an amorphous silicon (a-Si:H) photodetector to measure the density of covalently-bound DNA molecules tagged with a fluorescent molecule. This device is based on the photoconductivity of a-Si:H in a coplanar electrode configuration. Excitation of a fluorescently-tagged biomolecule with near UV/blue light results in the emission of visible light. The emitted light is then converted into an electrical signal in the photodetector, thus allowing the detection of the presence of the tagged DNA molecules. The design, fabrication and characterization of this integrated a-Si:H-based bio-detector is described. The detection limit of the present device is of the order of 20 pmol/cm2. A surface density of ≤ 30 pmol/cm2 for DNA covalently-bound to an active silica layer was measured with the a-Si:H-based bio-detector. INTRODUCTION DNA microarrays have the potential to revolutionize the acquisition and analysis of genetic information [1,2]. Microarray technology bridges fields such as materials science, microelectronics, biochemistry and physics. In these massively parallel microarrays, DNA hybridization can be tested with different capture single-strand DNA “probes” immobilized at specific sites in a matrix. Current DNA-chip data acquisition is based on the use of fluorescence microscope image capture of the emission from a fluorescent tag bound to DNA “target” molecules that hybridize with immobilized DNA “probe” molecules in the array [38]. Although these optical systems show high sensitivity, they require the use of complex image acquisition and processing systems. On-chip electronic data acquisition would improve both the speed and the reliability of DNA chip hybridization pattern analysis. This would be particularly important for applications such as clinical point of care diagnostics. In this paper, a detection approach in which the presence of fluorescently-tagged DNA molecules is optoelectronically detected by an integrated thin-film a-Si:H-based photodetector is demonstrated. EXPERIMENTAL Device fabrication The device (Figure 1) is fabricated on a glass substrate. The length of the aluminum parallel contacts ranges between 30 µm and 3 mm and their separation is between 5 µm and 500 µm. The Al metal lines are defined by photolithography in a clean room environment. The a-Si:H, silicon nitride (SiNx), amorphous silicon-carbon (a-SiC:H) and silicon dioxide (SiO2) thin-films are deposited by RF-PECVD. The SiO2 film is functionalized by a silanization and cross-linking process (see below) and becomes an active layer upon which the probe DNA molecule can be immobilized.
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Spectral response measurement
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