Sips adsorption model for DNA sensing with AlGaN/GaN high electron mobility transistors
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Sips adsorption model for DNA sensing with AlGaN/GaN high electron mobility transistors Espinosa Nayeli1, 2, Schwarz U. Stefan1,2, Cimalla Volker2 and Ambacher Oliver1,2 1
Department of Microsystem Engineering-IMTEK, University of Freiburg, Georges-KoehlerAllee 103, 79110 Freiburg, Germany 2 Fraunhofer Institute of Applied Solid State Physics, Tullastr. 72, 79108, Freiburg, Germany ABSTRACT This work presents an adsorption model based on the Sips isotherm for sensing different concentrations of DNA with open gate AlGaN/GaN high electron mobility field effect transistors (HEMTs). Probe-DNA was immobilized on the transistor gate before the application of targetDNA. Concentrations of 10-15 to 10-6 mol/L were tested. The sensor has a detection limit of 10-12 mol/L and saturates after the addition of 10-8 mol/L target-DNA. INTRODUCTION Identification of DNA sequences and other biomolecules is crucial for medical diagnosis, genomics, quality control in food industry and other biotechnological fields. To accomplish this task, conventional methods such as fluorescent labeling or gel electrophoresis are highly reliable. Nevertheless, these procedures are time consuming and require expensive equipment [1,2]. The development of portable and cheap semiconductor sensors have been shown to be an attractive alternative over the last 40 years [3]. For their realization, wide band gap devices, such as AlGaN/GaN transistors are biocompatible, highly sensitive and chemically stable in acidic and alkaline environments compared to silicon devices [4]. In AlGaN/GaN devices, the gradient of piezoelectric and spontaneous polarization at the heterojunction generates a two-dimensional electron gas (2DEG) that behaves as an electron conductive channel [5]. The sensing principle of AlGaN/GaN transistors is similar to the silicon ion sensitive FETs, on which the conductive channel is modulated by the presence of charge on the gate. To provide selectivity, the gate was bio-functionalized with probe-DNA before the addition of target-DNA. The DNA hybridization increases the gate boundary charge and shifts threshold voltage. In recent years, different concentrations of DNA and proteins have been successfully detected with AlGaN/GaN HEMTs [6-8]. Nonetheless, only a reduced number of papers propose a model for the charge adsorption due to the probe-target binding [9]. Since the biofunctionalization layer and the DNA binding are non-uniform, this work presents an adsorption model based on the Sips isotherm that considers a heterogeneous distribution of DNA hybridization energy.
SIPS ISOTHERM The DNA hybridization on substrates depends on the probe density, surface heterogeneity and non-specific adsorption. Assuming that the probe molecules have a quasi-Gaussian binding energy distribution at constant temperature, the total surface coverage is expressed by [10]: q = ((KC)a)/(1+(KC)a)
(1)
where q is the surface coverage, K is the DNA hybridization constant, C is the added target-DNA concentration and a is a parameter of the quasi-Gaussian distribut
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