Sensors and Biosensors Based on AlGaN/GaN heterostructures N. Sofikiti , a G. Tsiakatouras ,A. Georgakilas , N. Chaniotakis b,c b,c a a Laboratory ofAnalytical Chemistry, Dept. of Chemistry, University of Crete, Voutes, P.O. Box 2208, Iraklion- Greece URL: www. analytical_chemistry.uoc.gr 71003 Crete, Microelectronics Research Group, IESL, FORTH, P.O. Box 1527, 71110 Iraklion-Crete, Greece Dept. of Physics, University of Crete, Iraklion-Crete, Greece b c nchan@chemistry.uoc.gr, Sensors and Biosensors Based on AlGaN/GaN heterostructures Introduction The so-called III-nitride based materials have attracted research interest mainly due to the improvements in their fabrication technology. Their unique physical and chemical properties allow for of applications. These materials are used for the development of high power FETs and microelectronic devices [1-2]. recently a variety In our previous work we have shown that Gallium Nitride (GaN) (0001) grown on sapphire can be used as sensor element, or transducer, for the development of electrochemical sensors. In particular we developed an all solid-state GaN-based sensor that shows anion sensitivity. Potentiometric and impedance spectroscopy studies proved that the observed electrochemical sensitivity originates from the direct interaction of the anions in the solution with the Lewis acidic gallium (III) on the surface of the GaN (0001) crystal [3-6]. Additionally GaN was used as transducer for the development of composite cation sensitive electrochemical sensors, by depositing a cation selective electrochemically active membrane, for monitoring of cations, such as potassium and ammonium [7]. According to the above the affinity of the GaN surface to anionic substances as well as its exceptional chemical and physical stability make it an ideal electrode surface for the immobilization of biomolecules such as enzymes, peptides, oligonucleotides, DNA, RNA etc. Based on this we present here our recent results in which GaN (c-plane and a-plane) is used as transducer for the development of enzymatic biosensor systems. GaN crystal GaAs substrates Construction of GaN/Urease Biosensor The crystals used for sensor development consisted of heteroepitaxial Ga-polarity GaN (0001) [c-plane], and . Films were grown on Al O (0001) and (1-102) substrates, respectively, with a total thickness of 2-3 m. Pieces of the wafer, approximately 5 by 5 mm, were used for the construction of the sensor. A0.1 mm diameter Pt wire was attached with indium at the edge of the GaN. All surfaces except the GaN sensing area were electrically insulated. GaN (11-20) [a-plane] GaAs substrates, one with n GaAs (111)B [As-face] and one with n GaAs (111)A [Ga-face] were also evaluated for comparison reasons. The Urease-biosensor was prepared by covering the GaN or GaAs electrode with 20-55 L of enzyme solution, depending on the concentration. Enzyme solutions were prepared in 0.01M MES buffer solution (pH 6.5). The Urease-modified electrode was left to dry, for approximately 30min, and then was rinsed with 0.01M MES buffer solution (pH 6.5) for several times. All the electrochemical experiments were performed in 0.01M MES buffer solution with a Lawson Labs, Inc. EMF16 Precision Electrochemistry EMF Interface. GaN/Urease biosensors were stored at +4 C. 2 3 m + + o m References [1]O.Ambacher, J. Phys. D:Appl. Phys. 31 (1998) 2653-2710 [2]M. Stutzmann, G. Steinhoff, M. Eickhoff, O. Ambacher, C.E. Nebel, J. Schalwig, R. Neuberger, G. Muller, Diamond Relat.Mater. 11(2002)886-891 [3] N.A. Chaniotakis, Y. Alifragis, G. Konstantinidis, A. Georgakilas, Anal. Chem. 2004, 76, 5552-5556 [4] N.A. Chaniotakis, Y. Alifragis, A. Georgakilas, G. Konstantinidis, Appl. Phys. Lett. 86, 164103 (2005) [5] Y. Alifragis, A. Georgakilas, G. Konstantinidis, E. Iliopoulos, A. Kostopoulos, N.A. Chaniotakis, Appl. Phys. Lett. 87, 253507 (2005) [6] Y. Alifragis, G. Konstantinidis, A. Georgakilas, N.A. Chaniotakis, Electroanalysis 2005, 17, No.5-6, 527-531 [7] Y. Alifragis, A. Volosirakis, N.A. Chaniotakis, G. Konstantinidis, A. Adikimenakis, A. Georgakilas, Biosensors and Bioelectronics 22 (2007) 2796-2801 Acknowledgments The work is carried out with financial support from the European Community and GSRT (Hellenic Ministry of Development) through the projects NMP4-CT-2003-505641 “GANANO”, LSHB-CT-2007-O36812 “NANOMYC” and PENED 03ED 436 (supported from MARAC ELECTRONICS). Ionic and pH response Enzyme Response Experimental Log[KCl] (M) Relative Response (mV) V (mV) pH Log[Urea] (M) Aim of the work c- and a-plane nzyme /Urease one with GaAs (111)B [As-face] and another with GaAs (111)A [Ga-face] substrate, na similar way for comparison reasons. GaN crystals are tested as transducers for the development of GaN/Urease potentiometric biosensors. The e is physically adsorbed on the crystal surface. Two GaAs biosensors, are also developed i As illustrated in Fig.1A, both c- and a-plane GaN sensors show very similar anionic response . GaAs (111)A [Ga-face] also shows anionic response as it was expected since its crystal structure is similar to Ga- polarity c-plane GaN. On the contrary, only a slight potentiometric response is observed for GaAs (111)B [As-face] sensor. Regarding the pH study, c- and a-plane GaN sensors show similar response, while GaAs (both Ga- and As- face) sensors show even greater sensitivity (Fig.1B). These results for GaAs [As-face] seem quite contradictable and thus need further investigation. The enzymatic reaction of urease is: As illustrated in Fig.2A, both (c- & a-plane) GaN/Urease biosensors exhibit similar sensitivity to urea. The decrease in the potentiometric signal reveals that both GaN crystals respond to the anions produced from the enzymatic reaction. Additionally, as it appears from a preliminary storage stability study (Fig.2B) GaN crystals are able to retain enzyme on their surface. On the other hand, the two GaAs/Urease biosensors [Ga- & As-face] show no response to urea, although GaAs [Ga-face] substrate have shown anionic response. Based on these results we conclude that GaN (c- & a- plane) crystals can serve both as enzyme immobilization matrices as well as signal transducers. However, sensitivity improvement and further storage stability studies are required. 4 6 8 10 12 -800 -700 -600 -500 -400 -300 GaN c-plane (DV = -117 mV) GaN a-plane (DV = -129 mV) GaAs As-face (DV = -372 mV) GaAs Ga-face (DV = -270 mV) -6 -5 -4 -3 -2 -480 -460 -440 -420 -400 -380 -360 -340 GaN c-plane (Slope = -23mV/decade) GaN a-plane (Slope = -28.2mV/decade) GaAs As-face (Slope = -7.5mV/decade) GaAs Ga-face (Slope = -45.2mV/decade) Results Figure 1. A) Cl and B) pH response in 0,01M MES buffer solution. _ Potentiometric response to of GaN & GaAs sensors A B Relative Response (mV) Figure 2. A) Potentiometric response to urea of GaN & GaAs/Urease biosensors and B) Storage stability study /Urease of GaN/Urease biosensors A 1 2 3 0 10 20 30 40 50 60 70 Urease/GaN c-plane Urease/GaN a-plane |ÄV| (mV) Days B -6 -5 -4 -3 -2 -400 -390 -380 -370 -360 -350 -340 -330 -320 -310 GaN c-plane (Slope = -27.9mV/decade) GaN a-plane (Slope = -18.5mV/decade) GaAs As-face (Slope = -1.7mV/decade) GaAs Ga-face (Slope = -5.2mV/decade) View publication stats View publication stats