AlGaN/GaN-sensors for Monitoring of Enzyme Activity by pH-Measurements
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AlGaN/GaN-sensors for Monitoring of Enzyme Activity by pH-Measurements Gabriel Kittler1, Armin Spitznas1, Benedikt Luebbers1, Vadim Lebedev1, Dennis Wegener2, Michael Gebinoga3, Frank Weise3, Andreas Schober3, and Oliver Ambacher1 1 Institute of Micro- and Nanotechnologies, Technical University Ilmenau, Gustav-Kirchhoff-Str. 7, Ilmenau, 98963, Germany 2 Institute for Physical High Technology Jena, P.O. Box 100239, Jena, 07702, Germany 3 Center for Innovation Competence MacroNano®, Microfluidics and Biosensors, GustavKirchhoff-Str. 7, Ilmenau, 98693, Germany
ABSTRACT GaN based pH-sensors were used to measure the reaction and the kinetics of a lipase assay producing caprylic acid. Sensor structures consist of an AlGaN/GaN-heterostructure grown by plasma-induced molecular beam epitaxy with a thin GaN cap layer. The carrier concentration of the two-dimensional electron gas is manipulated by changing the surface potential due to changes in pH value. INTRODUCTION Group-III nitrides are known to be a suitable material system for microelectronic devices and sensors. Their outstanding properties like high chemical stability, optical transparency in the visible range, and high spontaneous and piezoelectric polarization offer a huge variety of applications like surface sensitive sensors, high electron mobility transistors (HEMT), and light emitting diodes (LED) or lasers (LD) [1]. The difference of spontaneous and piezoelectric polarization within an AlGaN/GaNheterostructure leads to a two-dimensional accumulation of electrons close to the interface. In our experiments we used the possibility to manipulate the carrier density within the twodimensional electron gas (2DEG) by changing the surface potential caused by different concentrations of ionic solutions. The principle of ion-sensitive field effect transistors (ISFET) was described by Bergveld [2]. First pH measurements with GaN based structures were reported by Steinhoff [3] and described theoretically by Bayer [4]. According to the site dissociation theory [5] amphoteric hydroxyl groups are responsible for the ion-sensitive response of sensor structures. These groups can be either protonated or deprotonated leading to a change in surface potential. In recent years GaN based sensors were used for measuring cell action potential [6] or to detect proteins [7]. We describe the use of such AlGaN/GaN heterostructures for monitoring a lipase based enzymatic reaction by measuring the pH value. Lipases are versatile biocatalysts used for many industrial biotransformation reactions such as alcoholysis, acidolysis, and hydrolysis. [8]
EXPERIMENT The AlGaN/GaN heterostructures were grown by plasma-induced molecular beam epitaxy on a sapphire substrate with an AlN nucleation layer. A 16 nm thick Al0.25Ga0.75N barrier layer was grown on a GaN bulk layer. These heterostructures were capped by 2 nm GaN. The carrier concentration determined by CV measurements was about 6 x 1012 cm-2. Metal contacts consist of 20 nm Ti, 80 nm Al, 30 nm Ti, and 100 nm Au. Contacts were passi
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