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Field-effect calcium sensor for the determination of the risk of urinary stone formation
Sensors and Actuators B 144 (2010) 374–379
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Field-effect calcium sensor for the determination of the risk of urinary stone formation Stefan Beging a , Daniela Mlynek a , Sudkanung Hataihimakul a , Arshak Poghossian a,b , Gerhard Baldsiefen c , Heinz Busch c , Norbert Laube d , Lisa Kleinen e , Michael J. Schöning a,b,∗ a
Institute of Nano- and Biotechnologies (INB), Aachen University of Applied Sciences, Campus Jülich, 52428 Jülich, Germany Institute of Bio- and Nanosystems (IBN-2), Research Centre Jülich GmbH, 52425 Jülich, Germany NTTF Coatings GmbH, 53619 Rheinbreitbach, Germany d Division of Experimental Urology, Department of Urology, University of Bonn, 53105 Bonn, Germany e Institute of Thin Film Technology (IDST), Kaiserslautern University of Technology, 53619 Rheinbreitbach, Germany b c
a r t i c l e
i n f o
Article history: Available online 14 December 2008 Keywords: Field-effect Calcium sensor Urolithiasis Bonn-Risk-Index EMIS sensor
a b s t r a c t Urinary stone formation has been evolved to a widespread disease during the last years. The reason for the formation of urinary stones are little crystals, mostly composed of calcium oxalate, which are formed in human kidneys. The early diagnosis of the risk for urinary stone formation of patients can be determined by the “Bonn-Risk-Index” method based on the potentiometric detection of the Ca2+ -ion concentration and an optical determination of the triggered crystallisation of calcium oxalate in unprocessed urine. In this work, miniaturised capacitive field-effect EMIS (electrolyte-membrane-insulator-semiconductor) sensors have been developed for the determination of the Ca2+ -ion concentration in human native urine. The Ca2+ -sensitive EMIS sensors have been systematically characterised by impedance spectroscopy, capacitance–voltage and constant–capacitance method in terms of sensitivity, signal stability and response time in both CaCl2 solutions and in native urine. The obtained results demonstrate the suitability of EMIS sensors for the measurement of the Ca2+ -ion concentration in native urine of patients. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the past 20 years in the developed countries, urolithiasis (urinary stone formation) has been evolved to a widespread disease, like diabetes or gout. For instance, in Europe, North America and Japan, 5–15% of the population are struck on urinary stones every year [1–3]. The prevalence in Germany currently ranges around 4.7% and the incidence amounts to approximately 1.5%; this refers to annually 1.2 million patients with a symptomatic first-time or recurrent stone episode [4]. This causes more than 10 million days of disability. Furthermore, most of the first-struck patients become manifest in the age class of 25–50 years, that means in the so-called “rush-hour of life” (family, job) [5,6]. Thus, the socio-economic costs (e.g. in Germany with approximately 175 million D /year) for the health-care systems are enormous. Urolithiasis is just the symptom of underlying (mostly) multifactorial metabolic disturbances. Alimentary factors, like diets rich in fat and protein, lacking fibre intake combined with inactivity, resembling the so-called modern industrialised life style and
∗ Corresponding author. Tel.: +49 241 6009 53215; fax: +49 241 6009 53235. E-mail address: [email protected] (M.J. Schöning). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.12.012
genetic predisposition enhance developing urolithiasis. Typical symptoms of an acute stone colic are, inter alia, agony, sickness and hematuria. Urinary calculus formation is caused by disturbed urinary compositions with altered urinary pH, increased concentrations of lithogenic components as, e.g. calcium, oxalate, phosphate and a lack in inhibitoric substances as, e.g. citrate and magnesium. Calcium oxalate represents the most frequent mineral phase found in uroliths with a frequency of approximately 70–75% [7]. Owing to the high recurrence rate of calcium oxalate stone formation in case of inadequate treatment, evaluation of the individual causes for calcium oxalate urolithiasis is of utmost clinical importance [8]. To improve the early diagnosis and thus, to prevent urinary stone formation at persons with increased risk factors, the Bonn-RiskIndex (BRI) constitutes a trusty method [8–11]. The BRI method is based on the potentiometric detection of the free Ca2+ -ion concentration (activity) by means of an ion-selective electrode (ISE) together with an optical determination of the induced crystallisation of calcium oxalate in native urine. The BRI is determined as ratio:
BRI =
[Ca2+ ] (Ox2− )
(1)
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Fig. 1. Diagram for the determination of the Bonn-Risk-Index (BRI); “low-risk persons” and stone patients under successful therapy are differentiated from high-risk persons still “healthy” but prone to calcium oxalate formation and patients with inadequate metaphylaxis at a BRI = 1/l.
where [Ca2+ ] is the native concentration of free urinary calcium ions, and (Ox2− ) is the amount of ammonium oxalate required for crystal formation [8–11]. Although just defined by only two parameters, high significance is reached by BRI in discrimination between persons “without risk” and persons “at risk” (see Fig. 1). Based on this method, the risk for urinary stone formation can be automatically determined by using the commercially available device “Urolizer® [12]. However, the big sizes and high costs of ISEs used, the necessity of their careful cleaning and frequent calibration after the measurement in urine with cost-intensive solutions as well as low life-time in urine, limit a broad implementation of these devices in routine medical practices. In the present work, miniaturised field-effect capacitive Ca2+ sensitive EMIS (electrolyte-membrane-insulator-semiconductor) sensors have been developed for the determination of the free Ca2+ -ion concentration in native urine of high-risk patients. The advantages of capacitive EMIS sensors are robustness, small sizes, simple layout, and easy and cost-effective fabrication by using silicon technology (usually, no photolithographic or complicated encapsulation processes are needed) [13–15].
Fig. 2. Measurement set-up (a) and cross-section (b) of the layer structure for the capacitive EMIS sensor with a polymeric membrane.
onto the sensor surface. For evaporating the solvent of the PVC membrane, the EMIS sensors have been stored under dry conditions for 48 h. Fig. 3 depicts a scanning electron microscopy picture of the Ca2+ -sensitive PVC membrane onto the Ta2 O5 layer. Before testing, the sensors have been preconditioned in a 0.1 M CaCl2 standard solution (Fluka) for at least 48 h. The modified EMIS sensors have been characterised in both CaCl2 solutions with different concentrations from 0.01 to 100 mM
2. Experimental Fig. 2 shows the measuring set-up (a) and a cross-section of the layer structure (b) of the developed EMIS sensor. The EMIS sensor consists of an Al-p-Si-SiO2 -Ta2 O5 structure (300 nm Al film as rear-side contact layer, p-Si with = 5–10 cm, ∼30 nm thermally grown SiO2 and 50–60 nm Ta2 O5 , chip size: 10 mm × 10 mm) modified with an ion-selective polyvinyl chloride (PVC) membrane containing the Ca2+ ionophore ETH 1001. The Ta2 O5 layer has been prepared by means of thermal oxidation of sputtered Ta in an oxygen atmosphere at 520 ◦ C for about 2 h. To improve the adhesion of the Ca2+ -sensitive PVC membrane onto the Ta2 O5 layer, prior to the membrane deposition, the sensor surface has been cleaned by helmanex solution, iso-propanol, ethanol and DI-water for 5 min in an ultrasonic bath. Then, the sensors have been dried in N2 flow following of dehydrating on a hotplate at 160 ◦ C for 10 min. After the cleaning process, the Ta2 O5 surface has been silanised by means of pipetting hexamethyldisilazane (HMDS) and treated on a hotplate at 100 ◦ C. The silanised sensor surface has been characterised by contact angle method (the contact angle was ∼60 ◦ ). The Ca2+ -ionophore-based PVC membrane has been deposited onto the silanised Ta2 O5 layer by dropping a commercial membrane cocktail (C-Cit, Wädenswil)
Fig. 3. Scanning electron microscopy picture of the Ca2+ -sensitive PVC membrane on the Ta2 O5 layer.
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(background electrolyte of 0.5 M KCl was used for adjusting constant ionic strength) and native urine by means of impedance spectroscopy, capacitance–voltage (C–V) and constant–capacitance (ConCap) methods using an impedance analyzer (Zahner Elektrik, Kronach). For the measurements, the EMIS sensor was mounted into a home-made measuring cell (see measurement set-up in Fig. 2(a)), sealed by an O-ring and contacted on its front side by the electrolyte and a reference electrode, and on its rear side by a gold-plated pin. The side walls and backside contact of the EMIS sensor chip were protected from the electrolyte solution by means of an O-ring, thus, avoiding the complicated encapsulation process. The contact area of the EMIS sensor with the solution was about 0.5 cm2 . For operation, a DC polarisation voltage is applied via the reference electrode (conventional double junction Ag/AgCl electrode (Metrohm, Filderstadt) with a salt bridge of 3 M KCl) to set the working point of the EMIS sensor, and a small AC voltage (20 mV) is applied to the system in order to measure the capacitance of the sensor. All potential values are referred to a Ag/AgCl electrode. The impedance spectroscopy measurements were carried out in a frequency range varying from 1 Hz up to 1 MHz. The C–V measurements have been performed in a bias potential range from −2.5 to 1.0 V. The C–V and ConCap measurements have been performed at a frequency of 30 Hz. All measurements have been performed in a Faraday cage at room temperature.
3. Results and discussion For the EMIS structures, the bulk ion-selective polymeric membrane can be described as a parallel circuit of the membrane geometric capacitance CM and resistance RM in series with the bare EIS (electrolyte-insulator-semiconductor) structure [13–17]. The experimentally measured capacitance of the EMIS sensor, C, can be expressed as [16,17]: C = CEIS
2 C 2 ω2 1 + RM M 2 (C 2 2 1 + RM EIS CM + CM )ω
(2)
where CEIS is the capacitance of the original EIS structure without membrane, ω = 2f and f is the measuring frequency. Thus, the measured capacitance is affected by a membrane resistance. As it has been demonstrated in [15,18–20], generally, any series resistance (e.g. membrane resistance, bulk resistance of low-ionic strength electrolyte, resistance of Si substrate, backside contact resistance as well as impedance of the reference electrode) can lead to a deformation of the frequency-dependent C–V curves of the capacitive field-effect structures. The impedance spectra for the developed Ca2+ -sensitive capacitive EMIS sensors have been recorded in the accumulation, depletion and inversion range of the C–V curves. As an example, Fig. 4(a) shows the impedance spectra (Bode plot) in the depletion range (at the polarisation potential of −650 mV) measured in 1 mM CaCl2 solution and native urine. For comparison, the impedance spectrum of an EIS structure without ion-sensitive membrane is presented, too. As it could be expected, in comparison with the impedance spectrum for the bare EIS structure, the addition of the ion-sensitive membrane shifts the impedance curve to a lowfrequency range. Such a behaviour of the impedance curve can be assigned to the bulk resistance and capacitance of the ion-sensitive membrane in parallel [13–17]. At lower frequencies (<50 Hz) the membrane impedance is lower, thus making the impedance of the p-Si-SiO2 -Ta2 O5 structure more significant. Therefore, the further C–V and ConCap characterisation of the developed EMIS sensors in terms of sensitivity, stability, linear concentration range and detection limit have been performed at a chosen frequency of 30 Hz.
Fig. 4. Impedance spectra in the depletion range (at a polarisation potential of −650 mV) measured in 1 mM CaCl2 solution and native urine: (a) Bode plot; (b) Nyquist plot. For comparison, the impedance spectrum of an EIS structure without ion-sensitive membrane is presented, too.
Fig. 4(b) shows the corresponding Nyquist plot for the same EMIS sensor, from which the characteristic frequency of the ionsensitive membrane (fM ), the membrane resistance and capacitance have been calculated by using equation (3) [17]: 2fM CM RM = 1.
(3)
The evaluated values for the PVC membrane capacitance and resistance for four tested EMIS sensors at a characteristic frequency of fM = 10–15 kHz were in the range of 0.3–0.4 nF and 25–30 k, respectively. The calculated membrane thickness, taking into account a value of the dielectric constant of the PVC membrane of εr = 4.8 [21] and a contact area of about 0.5 cm2 , amounts to ∼10 m, that is in a good agreement with results measured by ellipsometry (9.4 m). Fig. 5 depicts a typical set of C–V curves measured in CaCl2 solutions of different concentrations from 0.1 to 10 mM CaCl2 . As expected, with increasing the Ca2+ concentration in the solution, the C–V curves are shifted along the voltage axis in the direction of a more negative flat-band voltage due to the change of the additional potential drop at the electrolyte/membrane interface. These shifts can clearly be recognised from the inset graph in Fig. 5. Fig. 6 shows a typical dynamic ConCap response of the Ca2+ -sensitive capacitive EMIS sensor in CaCl2 solutions. In this experiment, the capacitance of the EMIS sensor has been kept at a fixed value within the depletion region of the C–V curve (nearly the inflection point) using a feedback-control circuit, and the Ca2+
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Fig. 5. Typical set of C–V curves of a Ca2+ -sensitive capacitive EMIS sensor measured in CaCl2 solutions of different concentrations from 0.1 to 10 mM CaCl2 .
concentration-dependent signal changes were directly recorded. As can be seen, the sensor signal shows a clear dependence on the Ca2+ concentration with a Nernst-like slope of 25 mV/pCa. The long-term stability of the developed sensors has been tested during six months by periodically checking the Ca2+ -sensitivity values and drift behaviour. For four tested sensors, an average sensitivity of (27 ± 2) mV/pCa in the range of 0.1–10 mM Ca2+ has been achieved. The calcium sensitivity values were comparable to those that have been previously reported for ion-sensitive field-effect transistors (28 mV/pCa) [22] and light-addressable potentiometric sensors (25–27 mV/pCa) [23,24]. The drift of the sensor output signal was ∼0.2–0.3 mV/min, dependent on the particular CaCl2 concentration. The detection limit was ∼0.01 mM Ca2+ , which is below the lowest concentration of 0.1–0.2 mM Ca2+ in native urine. This is important for estimating very low Ca2+ -ion concentrations in urine samples on urinary stone strucked patients. Fig. 7 demonstrates examples of the ConCap response of the realised Ca2+ -sensitive EMIS sensor recorded for five times in the same native urine sample to check the reproducibility of the sensor signal (a) and in two different urine samples from two persons (b). For accurate experiments, prior to measurements in native urine samples, the EMIS sensor has been calibrated in CaCl2 solutions of different concentrations. The concentrations of calibration solutions in Fig. 7(a) have been chosen to be comparable with the
Fig. 7. ConCap response of a Ca2+ -sensitive EMIS sensor recorded for five times in the same native urine sample to check the reproducibility of the sensor signal (a), and in two different urine samples from two persons (b). The sensor response in calibration solutions before and after the measurements in urine was recorded, too.
free Ca2+ -ion concentration in human native urine (0.1–5.0 mM). After the relatively large signal drift during the first contact with native urine, the EMIS sensor shows a reproducible sensor signal by repeated measurements in the same urine sample. The response time (t90% ) in native urine was higher (∼50 s) than in CaCl2 solutions (∼30 s). The evaluated Ca2+ -ion concentration in urine by using a sensitivity value of 26 mV/pCa for the tested sensor was ∼0.5 mM. This is in good agreement with the Ca2+ -ion concentration value measured by a commercial Ca2+ ion-selective macro-electrode presently used for estimating the risk of the formation of calcium oxalate stones. In order to prove, whether the sensitivity of the EMIS sensor changes after the experiments in different urine samples, the ConCap response in calibration solutions has been recorded before and after the measurements in urine (Fig. 7(b)). Although due to the drift effects the sensor response in the same calibration solutions as well as in the same urine samples were slightly shifted along the voltage axis, the sensitivity (26.5 ± 0.5 mV/pCa) does not change after repeated measurements in two different urine samples. The evaluated values of Ca2+ -ion concentration in the two different native urine samples from two persons were 4.8 mM (sample 1) and 0.9 mM (sample 2), respectively. 4. Conclusions
Fig. 6. Typical dynamic ConCap response of the developed Ca2+ -sensitive EMIS sensor in CaCl2 solutions.
Miniaturised Ca2+ -sensitive capacitive field-effect EMIS sensors with an ETH 1001-containing PVC-based membrane have been realised with the aim of future application for determining the risk of urinary stone formation by patients. The developed EMIS
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sensors have been systematically characterised by impedance spectroscopy, capacitance–voltage and constant–capacitance methods in both CaCl2 solution of different concentrations and human native urine. The geometric capacitance and resistance of the deposited Ca2+ -sensitive PVC membrane has been determined from the Nyquist plot. The developed Ca2+ EMIS sensors show a nearly Nernstian calcium sensitivity of ∼25–29 mV/pCa in the concentration range from 0.01 to 10 mM Ca2+ . The lower detection limit was ∼0.01 mM Ca2+ . The response time (t90% ) in CaCl2 solutions and in native urine samples was around 30 and ∼50 s, respectively. The sensors’ stability in both calibration solutions and real urine samples were comparable. In addition, the measured values of Ca2+ concentration in native urine by the EMIS sensors were in good agreement with those that have been measured with a commercial macro-ISE. The obtained results demonstrate the suitability of the developed miniaturised capacitive field-effect EMIS sensors for the application in native urine. Future experiments will be focussed on the characterisation of the developed calcium sensors in terms of long-term stability and life-time under clinical conditions as well as the sensor’s behaviour in native urine samples with extremely high concentrations of lithogenic substances and disturbed urinary compositions with respect to macromolecular fractions (e.g. macro-proteinuria). Acknowledgements The authors gratefully thank H.P. Bochem for technical support. This work was financially supported by the Federal Ministry of Education and Research (Germany), project “UroSens”. References [1] A. Trinchieri, Epidemiological trends in urolithiasis: impact on our health care systems, Urol. Res. 34 (2006) 151–156. [2] K.K. Stamatelou, M.E. Francis, C.A. Jones, L.M. Nyberg, G.C. Curhan, Time trends in reported prevalence of kidney stones in the United States: 1976–1994, Kidney Int. 63 (2005) 1817–1823. [3] O.W. Moe, Kidney stones: pathophysiology and medical management, Lancet 367 (2006) 333–344. [4] A. Hesse, E. Brändle, E. Wilbert, K.U. Köhrmann, P. Alken, Study on the prevalence and incidence of urolithiasis in Germany comparing the years 1979 vs. 2000, Eur. Urol. 44 (2003) 709–713. [5] W.L. Strohmaier, Ökonomie des Harnsteinleidens: rechnet sich die HarnsteinMetaphylaxe? URO-News 6 (2001) 32–37. [6] A. Hesse, E. Brändle, E. Wilbert, K.U. Köhrmann, P. Alken, Prevalence and incidence of urolithiasis in Germany—an epidemiological update, Urol. Res. 31 (2003) 106. [7] G. Schubert, Stone analysis, Urol. Res. 34 (2006) 146–150. [8] N. Laube, S. Hergarten, B. Hoppe, M. Schmidt, A. Hesse, Determination of the calcium oxalate crystallization from urine samples: the BONN risk index in comparison to other risk formulas, J. Urol. 172 (2004) 355–359. [9] J. Kavanagh, In vitro calcium oxalate crystallisation methods, J. Urol. 34 (2006) 139–145. [10] N. Laube, A. Schneider, A. Hesse, A new approach to calculate the risk of calcium oxalate crystallization from unprepared native urine, Urol. Res. 28 (2000) 274–280. [11] N. Laube, V. Labedzke, S. Hergarten, A. Hesse, The expression of the urinary calcium oxalate formation risk in three generation of family, J. Chem. Inf. Comp. Sci. 42 (2002) 633–639. [12] N. Laube, H. Busch, U. Grabowy, in: M.D.I. Gohel, D.W. Au (Eds.), Kidney Stones: Inside & Out, 10th International Symposium on Urolithiasis, Hong Kong China, 25–28 May, 2004, pp. 269–270. [13] Y. Mourzina, D.-T. Mai, A. Poghossian, Y. Ermolenko, T. Yoshinobu, Y. Vlasov, H. Iwasaki, M.J. Schöning, K+ -selective field-effect sensors as transducers for bioelectronic applications, Electrochim. Acta 48 (2003) 3333– 3339. [14] A. Poghossian, D.-T. Mai, Y. Mourzina, M.J. Schöning, Impedance effect of an ion-sensitive membrane: characterisation of an EMIS sensor by impedance spectroscopy, capacitance–voltage and constant–capacitance method, Sens. Actuators B 103 (2004) 423–428. [15] A. Poghossian, M.J. Schöning, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 9, American Scientific Publisher, Stevenson Ranch, 2006, pp. 463–533. [16] A. Demoz, E.M.J. Verpoorte, D.J. Harrsion, An equivalent circuit model of ionselective membrane/insulator/semiconductor interfaces for chemical sensors,
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Biographies Stefan Beging received his diploma degree in biomedical engineering at Aachen University of Applied Sciences, Campus Jülich, in the Laboratory of Chemical Sensors and Biosensors in 2006. Following, he is working as a member of the research staff on ion-selective microelectrodes and field-effect (bio-)chemical sensors for medical applications fabricated by silicon and thin-film technologies. Daniela Mlynek finished her diploma degree in biomedical engineering at Aachen University of Applied Sciences, Campus Jülich, in the Laboratory of Chemical Sensors and Biosensors in 2008. Her research activities during her thesis have been focussed on miniaturised chemical sensors for medical applications. Sudkanung Hataihimakul received her bachelor degree in chemical engineering at the University of Kasetsart, Bangkok (Thailand). After working as a technical engineer at Rayong Olefins Co., Ltd. in Rayong, she studied biomedical engineering at the Aachen University of Applied Sciences, Campus Jülich, where she received her master degree in 2008. During her thesis, she has been working on miniaturised chemical sensors for medical applications in the Laboratory of Chemical Sensors and Biosensors. Arshak Poghossian received his PhD degree in solid-state physics from Leningrad Electrotechnic Institute (Russia) in 1978 and the Dr. Sci. (Engineering) degree in solid-state electronics and microelectronics from the State University of Yerevan (Armenia) in 1995. After being an associate professor at State Engineering University of Armenia and director of Microsensor Ltd. (Yerevan) from 1991 to 1996, he has been a professor at the University of Management and Information (Yerevan). Since 1998, he has been in the Institute of Thin Films and Interfaces (now, Institute of Bio- and Nanosystems) at the Research Centre Jülich, and since 2004, he is with the Aachen University of Applied Sciences, Campus Jülich, Germany. In 2008, he has been appointed as honorary professor. His research interests are solid-state chemical sensors and biosensors, sensor materials, nano-devices, microsystem technology, nano- and biotechnology. Gerhard Baldsiefen received his PhD degree in physics at the University of Bonn. He was working in the sector of information and telecommunication before he started to work at NTTF GmbH in the field of research and development for medical technology. Heinz Busch received his PhD degree in nuclear and solid-state physics at University of Bonn. In the year 2000, he was co-founder of the company NTTF (New Technologies in Thin Films) GmbH which is called today NTTF Coatings GmbH. His core business is the development of biocompatible and biofunctional layer systems as well as the development and fabrication of devices for diagnosis and therapy in medicine. Norbert Laube received his diploma degree in mineralogy and PhD degree in geophysics and soil sciences at University of Bonn. After working as member of the research staff at the chair of geodynamics (University of Bonn), he changes as postdoctoral research fellow to the German Research Centre for Geosciences (GFZ) in Potsdam. In 1998, he became head of laboratory at the Division of Experimental Urology at the University Hospital of Bonn, and in 2003, head of the department. In 2006, he received his habilitation in the field of “Clinical Biomineralisation”. Since 2008, he is also scientific head of the German Urinary Stone Centre in Bonn. Lisa Kleinen received her diploma degree in physics at University of Bonn (Institute of Nuclear Physics). After a triannual occupation in industry, she starts to work as member of the research staff at the Institute for Thin Film Technology of Kaiserslautern University of Technology in the year 2000. Her main research activities are plasma-deposited multi-functionalised layer systems for medical applications.
S. Beging et al. / Sensors and Actuators B 144 (2010) 374–379 Michael J. Schöning received his diploma degree in electrical engineering (1989) and his PhD in the field of semiconductor-based microsensors for the detection of ions in liquids (1993), both from the Karlsruhe University of Technology. In 1989, he jointed the Institute of Radiochemistry at the Research Centre Karlsruhe. Since 1993, he has been with the Institute of Thin Films and Interfaces (now, Institute of Bioand Nanosystems) at the Research Centre Jülich, and since 1999 he was appointed as
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full professor at Aachen University of Applied Sciences, Campus Jülich. Since 2006, he serves as a Director of the Institute of Nano- and Biotechnologies (INB) at the Aachen University of Applied Sciences. His main research subjects concern siliconbased chemical and biological sensors, thin-film technologies, solid-state physics, microsystem and nano(bio-)technology.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Field-effect calcium sensor for the determination of the risk of urinary stone formation Stefan Beging a , Daniela Mlynek a , Sudkanung Hataihimakul a , Arshak Poghossian a,b , Gerhard Baldsiefen c , Heinz Busch c , Norbert Laube d , Lisa Kleinen e , Michael J. Schöning a,b,∗ a
Institute of Nano- and Biotechnologies (INB), Aachen University of Applied Sciences, Campus Jülich, 52428 Jülich, Germany Institute of Bio- and Nanosystems (IBN-2), Research Centre Jülich GmbH, 52425 Jülich, Germany NTTF Coatings GmbH, 53619 Rheinbreitbach, Germany d Division of Experimental Urology, Department of Urology, University of Bonn, 53105 Bonn, Germany e Institute of Thin Film Technology (IDST), Kaiserslautern University of Technology, 53619 Rheinbreitbach, Germany b c
a r t i c l e
i n f o
Article history: Available online 14 December 2008 Keywords: Field-effect Calcium sensor Urolithiasis Bonn-Risk-Index EMIS sensor
a b s t r a c t Urinary stone formation has been evolved to a widespread disease during the last years. The reason for the formation of urinary stones are little crystals, mostly composed of calcium oxalate, which are formed in human kidneys. The early diagnosis of the risk for urinary stone formation of patients can be determined by the “Bonn-Risk-Index” method based on the potentiometric detection of the Ca2+ -ion concentration and an optical determination of the triggered crystallisation of calcium oxalate in unprocessed urine. In this work, miniaturised capacitive field-effect EMIS (electrolyte-membrane-insulator-semiconductor) sensors have been developed for the determination of the Ca2+ -ion concentration in human native urine. The Ca2+ -sensitive EMIS sensors have been systematically characterised by impedance spectroscopy, capacitance–voltage and constant–capacitance method in terms of sensitivity, signal stability and response time in both CaCl2 solutions and in native urine. The obtained results demonstrate the suitability of EMIS sensors for the measurement of the Ca2+ -ion concentration in native urine of patients. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the past 20 years in the developed countries, urolithiasis (urinary stone formation) has been evolved to a widespread disease, like diabetes or gout. For instance, in Europe, North America and Japan, 5–15% of the population are struck on urinary stones every year [1–3]. The prevalence in Germany currently ranges around 4.7% and the incidence amounts to approximately 1.5%; this refers to annually 1.2 million patients with a symptomatic first-time or recurrent stone episode [4]. This causes more than 10 million days of disability. Furthermore, most of the first-struck patients become manifest in the age class of 25–50 years, that means in the so-called “rush-hour of life” (family, job) [5,6]. Thus, the socio-economic costs (e.g. in Germany with approximately 175 million D /year) for the health-care systems are enormous. Urolithiasis is just the symptom of underlying (mostly) multifactorial metabolic disturbances. Alimentary factors, like diets rich in fat and protein, lacking fibre intake combined with inactivity, resembling the so-called modern industrialised life style and
∗ Corresponding author. Tel.: +49 241 6009 53215; fax: +49 241 6009 53235. E-mail address: [email protected] (M.J. Schöning). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.12.012
genetic predisposition enhance developing urolithiasis. Typical symptoms of an acute stone colic are, inter alia, agony, sickness and hematuria. Urinary calculus formation is caused by disturbed urinary compositions with altered urinary pH, increased concentrations of lithogenic components as, e.g. calcium, oxalate, phosphate and a lack in inhibitoric substances as, e.g. citrate and magnesium. Calcium oxalate represents the most frequent mineral phase found in uroliths with a frequency of approximately 70–75% [7]. Owing to the high recurrence rate of calcium oxalate stone formation in case of inadequate treatment, evaluation of the individual causes for calcium oxalate urolithiasis is of utmost clinical importance [8]. To improve the early diagnosis and thus, to prevent urinary stone formation at persons with increased risk factors, the Bonn-RiskIndex (BRI) constitutes a trusty method [8–11]. The BRI method is based on the potentiometric detection of the free Ca2+ -ion concentration (activity) by means of an ion-selective electrode (ISE) together with an optical determination of the induced crystallisation of calcium oxalate in native urine. The BRI is determined as ratio:
BRI =
[Ca2+ ] (Ox2− )
(1)
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Fig. 1. Diagram for the determination of the Bonn-Risk-Index (BRI); “low-risk persons” and stone patients under successful therapy are differentiated from high-risk persons still “healthy” but prone to calcium oxalate formation and patients with inadequate metaphylaxis at a BRI = 1/l.
where [Ca2+ ] is the native concentration of free urinary calcium ions, and (Ox2− ) is the amount of ammonium oxalate required for crystal formation [8–11]. Although just defined by only two parameters, high significance is reached by BRI in discrimination between persons “without risk” and persons “at risk” (see Fig. 1). Based on this method, the risk for urinary stone formation can be automatically determined by using the commercially available device “Urolizer® [12]. However, the big sizes and high costs of ISEs used, the necessity of their careful cleaning and frequent calibration after the measurement in urine with cost-intensive solutions as well as low life-time in urine, limit a broad implementation of these devices in routine medical practices. In the present work, miniaturised field-effect capacitive Ca2+ sensitive EMIS (electrolyte-membrane-insulator-semiconductor) sensors have been developed for the determination of the free Ca2+ -ion concentration in native urine of high-risk patients. The advantages of capacitive EMIS sensors are robustness, small sizes, simple layout, and easy and cost-effective fabrication by using silicon technology (usually, no photolithographic or complicated encapsulation processes are needed) [13–15].
Fig. 2. Measurement set-up (a) and cross-section (b) of the layer structure for the capacitive EMIS sensor with a polymeric membrane.
onto the sensor surface. For evaporating the solvent of the PVC membrane, the EMIS sensors have been stored under dry conditions for 48 h. Fig. 3 depicts a scanning electron microscopy picture of the Ca2+ -sensitive PVC membrane onto the Ta2 O5 layer. Before testing, the sensors have been preconditioned in a 0.1 M CaCl2 standard solution (Fluka) for at least 48 h. The modified EMIS sensors have been characterised in both CaCl2 solutions with different concentrations from 0.01 to 100 mM
2. Experimental Fig. 2 shows the measuring set-up (a) and a cross-section of the layer structure (b) of the developed EMIS sensor. The EMIS sensor consists of an Al-p-Si-SiO2 -Ta2 O5 structure (300 nm Al film as rear-side contact layer, p-Si with = 5–10 cm, ∼30 nm thermally grown SiO2 and 50–60 nm Ta2 O5 , chip size: 10 mm × 10 mm) modified with an ion-selective polyvinyl chloride (PVC) membrane containing the Ca2+ ionophore ETH 1001. The Ta2 O5 layer has been prepared by means of thermal oxidation of sputtered Ta in an oxygen atmosphere at 520 ◦ C for about 2 h. To improve the adhesion of the Ca2+ -sensitive PVC membrane onto the Ta2 O5 layer, prior to the membrane deposition, the sensor surface has been cleaned by helmanex solution, iso-propanol, ethanol and DI-water for 5 min in an ultrasonic bath. Then, the sensors have been dried in N2 flow following of dehydrating on a hotplate at 160 ◦ C for 10 min. After the cleaning process, the Ta2 O5 surface has been silanised by means of pipetting hexamethyldisilazane (HMDS) and treated on a hotplate at 100 ◦ C. The silanised sensor surface has been characterised by contact angle method (the contact angle was ∼60 ◦ ). The Ca2+ -ionophore-based PVC membrane has been deposited onto the silanised Ta2 O5 layer by dropping a commercial membrane cocktail (C-Cit, Wädenswil)
Fig. 3. Scanning electron microscopy picture of the Ca2+ -sensitive PVC membrane on the Ta2 O5 layer.
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(background electrolyte of 0.5 M KCl was used for adjusting constant ionic strength) and native urine by means of impedance spectroscopy, capacitance–voltage (C–V) and constant–capacitance (ConCap) methods using an impedance analyzer (Zahner Elektrik, Kronach). For the measurements, the EMIS sensor was mounted into a home-made measuring cell (see measurement set-up in Fig. 2(a)), sealed by an O-ring and contacted on its front side by the electrolyte and a reference electrode, and on its rear side by a gold-plated pin. The side walls and backside contact of the EMIS sensor chip were protected from the electrolyte solution by means of an O-ring, thus, avoiding the complicated encapsulation process. The contact area of the EMIS sensor with the solution was about 0.5 cm2 . For operation, a DC polarisation voltage is applied via the reference electrode (conventional double junction Ag/AgCl electrode (Metrohm, Filderstadt) with a salt bridge of 3 M KCl) to set the working point of the EMIS sensor, and a small AC voltage (20 mV) is applied to the system in order to measure the capacitance of the sensor. All potential values are referred to a Ag/AgCl electrode. The impedance spectroscopy measurements were carried out in a frequency range varying from 1 Hz up to 1 MHz. The C–V measurements have been performed in a bias potential range from −2.5 to 1.0 V. The C–V and ConCap measurements have been performed at a frequency of 30 Hz. All measurements have been performed in a Faraday cage at room temperature.
3. Results and discussion For the EMIS structures, the bulk ion-selective polymeric membrane can be described as a parallel circuit of the membrane geometric capacitance CM and resistance RM in series with the bare EIS (electrolyte-insulator-semiconductor) structure [13–17]. The experimentally measured capacitance of the EMIS sensor, C, can be expressed as [16,17]: C = CEIS
2 C 2 ω2 1 + RM M 2 (C 2 2 1 + RM EIS CM + CM )ω
(2)
where CEIS is the capacitance of the original EIS structure without membrane, ω = 2f and f is the measuring frequency. Thus, the measured capacitance is affected by a membrane resistance. As it has been demonstrated in [15,18–20], generally, any series resistance (e.g. membrane resistance, bulk resistance of low-ionic strength electrolyte, resistance of Si substrate, backside contact resistance as well as impedance of the reference electrode) can lead to a deformation of the frequency-dependent C–V curves of the capacitive field-effect structures. The impedance spectra for the developed Ca2+ -sensitive capacitive EMIS sensors have been recorded in the accumulation, depletion and inversion range of the C–V curves. As an example, Fig. 4(a) shows the impedance spectra (Bode plot) in the depletion range (at the polarisation potential of −650 mV) measured in 1 mM CaCl2 solution and native urine. For comparison, the impedance spectrum of an EIS structure without ion-sensitive membrane is presented, too. As it could be expected, in comparison with the impedance spectrum for the bare EIS structure, the addition of the ion-sensitive membrane shifts the impedance curve to a lowfrequency range. Such a behaviour of the impedance curve can be assigned to the bulk resistance and capacitance of the ion-sensitive membrane in parallel [13–17]. At lower frequencies (<50 Hz) the membrane impedance is lower, thus making the impedance of the p-Si-SiO2 -Ta2 O5 structure more significant. Therefore, the further C–V and ConCap characterisation of the developed EMIS sensors in terms of sensitivity, stability, linear concentration range and detection limit have been performed at a chosen frequency of 30 Hz.
Fig. 4. Impedance spectra in the depletion range (at a polarisation potential of −650 mV) measured in 1 mM CaCl2 solution and native urine: (a) Bode plot; (b) Nyquist plot. For comparison, the impedance spectrum of an EIS structure without ion-sensitive membrane is presented, too.
Fig. 4(b) shows the corresponding Nyquist plot for the same EMIS sensor, from which the characteristic frequency of the ionsensitive membrane (fM ), the membrane resistance and capacitance have been calculated by using equation (3) [17]: 2fM CM RM = 1.
(3)
The evaluated values for the PVC membrane capacitance and resistance for four tested EMIS sensors at a characteristic frequency of fM = 10–15 kHz were in the range of 0.3–0.4 nF and 25–30 k, respectively. The calculated membrane thickness, taking into account a value of the dielectric constant of the PVC membrane of εr = 4.8 [21] and a contact area of about 0.5 cm2 , amounts to ∼10 m, that is in a good agreement with results measured by ellipsometry (9.4 m). Fig. 5 depicts a typical set of C–V curves measured in CaCl2 solutions of different concentrations from 0.1 to 10 mM CaCl2 . As expected, with increasing the Ca2+ concentration in the solution, the C–V curves are shifted along the voltage axis in the direction of a more negative flat-band voltage due to the change of the additional potential drop at the electrolyte/membrane interface. These shifts can clearly be recognised from the inset graph in Fig. 5. Fig. 6 shows a typical dynamic ConCap response of the Ca2+ -sensitive capacitive EMIS sensor in CaCl2 solutions. In this experiment, the capacitance of the EMIS sensor has been kept at a fixed value within the depletion region of the C–V curve (nearly the inflection point) using a feedback-control circuit, and the Ca2+
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Fig. 5. Typical set of C–V curves of a Ca2+ -sensitive capacitive EMIS sensor measured in CaCl2 solutions of different concentrations from 0.1 to 10 mM CaCl2 .
concentration-dependent signal changes were directly recorded. As can be seen, the sensor signal shows a clear dependence on the Ca2+ concentration with a Nernst-like slope of 25 mV/pCa. The long-term stability of the developed sensors has been tested during six months by periodically checking the Ca2+ -sensitivity values and drift behaviour. For four tested sensors, an average sensitivity of (27 ± 2) mV/pCa in the range of 0.1–10 mM Ca2+ has been achieved. The calcium sensitivity values were comparable to those that have been previously reported for ion-sensitive field-effect transistors (28 mV/pCa) [22] and light-addressable potentiometric sensors (25–27 mV/pCa) [23,24]. The drift of the sensor output signal was ∼0.2–0.3 mV/min, dependent on the particular CaCl2 concentration. The detection limit was ∼0.01 mM Ca2+ , which is below the lowest concentration of 0.1–0.2 mM Ca2+ in native urine. This is important for estimating very low Ca2+ -ion concentrations in urine samples on urinary stone strucked patients. Fig. 7 demonstrates examples of the ConCap response of the realised Ca2+ -sensitive EMIS sensor recorded for five times in the same native urine sample to check the reproducibility of the sensor signal (a) and in two different urine samples from two persons (b). For accurate experiments, prior to measurements in native urine samples, the EMIS sensor has been calibrated in CaCl2 solutions of different concentrations. The concentrations of calibration solutions in Fig. 7(a) have been chosen to be comparable with the
Fig. 7. ConCap response of a Ca2+ -sensitive EMIS sensor recorded for five times in the same native urine sample to check the reproducibility of the sensor signal (a), and in two different urine samples from two persons (b). The sensor response in calibration solutions before and after the measurements in urine was recorded, too.
free Ca2+ -ion concentration in human native urine (0.1–5.0 mM). After the relatively large signal drift during the first contact with native urine, the EMIS sensor shows a reproducible sensor signal by repeated measurements in the same urine sample. The response time (t90% ) in native urine was higher (∼50 s) than in CaCl2 solutions (∼30 s). The evaluated Ca2+ -ion concentration in urine by using a sensitivity value of 26 mV/pCa for the tested sensor was ∼0.5 mM. This is in good agreement with the Ca2+ -ion concentration value measured by a commercial Ca2+ ion-selective macro-electrode presently used for estimating the risk of the formation of calcium oxalate stones. In order to prove, whether the sensitivity of the EMIS sensor changes after the experiments in different urine samples, the ConCap response in calibration solutions has been recorded before and after the measurements in urine (Fig. 7(b)). Although due to the drift effects the sensor response in the same calibration solutions as well as in the same urine samples were slightly shifted along the voltage axis, the sensitivity (26.5 ± 0.5 mV/pCa) does not change after repeated measurements in two different urine samples. The evaluated values of Ca2+ -ion concentration in the two different native urine samples from two persons were 4.8 mM (sample 1) and 0.9 mM (sample 2), respectively. 4. Conclusions
Fig. 6. Typical dynamic ConCap response of the developed Ca2+ -sensitive EMIS sensor in CaCl2 solutions.
Miniaturised Ca2+ -sensitive capacitive field-effect EMIS sensors with an ETH 1001-containing PVC-based membrane have been realised with the aim of future application for determining the risk of urinary stone formation by patients. The developed EMIS
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sensors have been systematically characterised by impedance spectroscopy, capacitance–voltage and constant–capacitance methods in both CaCl2 solution of different concentrations and human native urine. The geometric capacitance and resistance of the deposited Ca2+ -sensitive PVC membrane has been determined from the Nyquist plot. The developed Ca2+ EMIS sensors show a nearly Nernstian calcium sensitivity of ∼25–29 mV/pCa in the concentration range from 0.01 to 10 mM Ca2+ . The lower detection limit was ∼0.01 mM Ca2+ . The response time (t90% ) in CaCl2 solutions and in native urine samples was around 30 and ∼50 s, respectively. The sensors’ stability in both calibration solutions and real urine samples were comparable. In addition, the measured values of Ca2+ concentration in native urine by the EMIS sensors were in good agreement with those that have been measured with a commercial macro-ISE. The obtained results demonstrate the suitability of the developed miniaturised capacitive field-effect EMIS sensors for the application in native urine. Future experiments will be focussed on the characterisation of the developed calcium sensors in terms of long-term stability and life-time under clinical conditions as well as the sensor’s behaviour in native urine samples with extremely high concentrations of lithogenic substances and disturbed urinary compositions with respect to macromolecular fractions (e.g. macro-proteinuria). Acknowledgements The authors gratefully thank H.P. Bochem for technical support. This work was financially supported by the Federal Ministry of Education and Research (Germany), project “UroSens”. References [1] A. Trinchieri, Epidemiological trends in urolithiasis: impact on our health care systems, Urol. Res. 34 (2006) 151–156. [2] K.K. Stamatelou, M.E. Francis, C.A. Jones, L.M. Nyberg, G.C. Curhan, Time trends in reported prevalence of kidney stones in the United States: 1976–1994, Kidney Int. 63 (2005) 1817–1823. [3] O.W. Moe, Kidney stones: pathophysiology and medical management, Lancet 367 (2006) 333–344. [4] A. Hesse, E. Brändle, E. Wilbert, K.U. Köhrmann, P. Alken, Study on the prevalence and incidence of urolithiasis in Germany comparing the years 1979 vs. 2000, Eur. Urol. 44 (2003) 709–713. [5] W.L. Strohmaier, Ökonomie des Harnsteinleidens: rechnet sich die HarnsteinMetaphylaxe? URO-News 6 (2001) 32–37. [6] A. Hesse, E. Brändle, E. Wilbert, K.U. Köhrmann, P. Alken, Prevalence and incidence of urolithiasis in Germany—an epidemiological update, Urol. Res. 31 (2003) 106. [7] G. Schubert, Stone analysis, Urol. Res. 34 (2006) 146–150. [8] N. Laube, S. Hergarten, B. Hoppe, M. Schmidt, A. Hesse, Determination of the calcium oxalate crystallization from urine samples: the BONN risk index in comparison to other risk formulas, J. Urol. 172 (2004) 355–359. [9] J. Kavanagh, In vitro calcium oxalate crystallisation methods, J. Urol. 34 (2006) 139–145. [10] N. Laube, A. Schneider, A. Hesse, A new approach to calculate the risk of calcium oxalate crystallization from unprepared native urine, Urol. Res. 28 (2000) 274–280. [11] N. Laube, V. Labedzke, S. Hergarten, A. Hesse, The expression of the urinary calcium oxalate formation risk in three generation of family, J. Chem. Inf. Comp. Sci. 42 (2002) 633–639. [12] N. Laube, H. Busch, U. Grabowy, in: M.D.I. Gohel, D.W. Au (Eds.), Kidney Stones: Inside & Out, 10th International Symposium on Urolithiasis, Hong Kong China, 25–28 May, 2004, pp. 269–270. [13] Y. Mourzina, D.-T. Mai, A. Poghossian, Y. Ermolenko, T. Yoshinobu, Y. Vlasov, H. Iwasaki, M.J. Schöning, K+ -selective field-effect sensors as transducers for bioelectronic applications, Electrochim. Acta 48 (2003) 3333– 3339. [14] A. Poghossian, D.-T. Mai, Y. Mourzina, M.J. Schöning, Impedance effect of an ion-sensitive membrane: characterisation of an EMIS sensor by impedance spectroscopy, capacitance–voltage and constant–capacitance method, Sens. Actuators B 103 (2004) 423–428. [15] A. Poghossian, M.J. Schöning, in: C.A. Grimes, E.C. Dickey, M.V. Pishko (Eds.), Encyclopedia of Sensors, vol. 9, American Scientific Publisher, Stevenson Ranch, 2006, pp. 463–533. [16] A. Demoz, E.M.J. Verpoorte, D.J. Harrsion, An equivalent circuit model of ionselective membrane/insulator/semiconductor interfaces for chemical sensors,
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Biographies Stefan Beging received his diploma degree in biomedical engineering at Aachen University of Applied Sciences, Campus Jülich, in the Laboratory of Chemical Sensors and Biosensors in 2006. Following, he is working as a member of the research staff on ion-selective microelectrodes and field-effect (bio-)chemical sensors for medical applications fabricated by silicon and thin-film technologies. Daniela Mlynek finished her diploma degree in biomedical engineering at Aachen University of Applied Sciences, Campus Jülich, in the Laboratory of Chemical Sensors and Biosensors in 2008. Her research activities during her thesis have been focussed on miniaturised chemical sensors for medical applications. Sudkanung Hataihimakul received her bachelor degree in chemical engineering at the University of Kasetsart, Bangkok (Thailand). After working as a technical engineer at Rayong Olefins Co., Ltd. in Rayong, she studied biomedical engineering at the Aachen University of Applied Sciences, Campus Jülich, where she received her master degree in 2008. During her thesis, she has been working on miniaturised chemical sensors for medical applications in the Laboratory of Chemical Sensors and Biosensors. Arshak Poghossian received his PhD degree in solid-state physics from Leningrad Electrotechnic Institute (Russia) in 1978 and the Dr. Sci. (Engineering) degree in solid-state electronics and microelectronics from the State University of Yerevan (Armenia) in 1995. After being an associate professor at State Engineering University of Armenia and director of Microsensor Ltd. (Yerevan) from 1991 to 1996, he has been a professor at the University of Management and Information (Yerevan). Since 1998, he has been in the Institute of Thin Films and Interfaces (now, Institute of Bio- and Nanosystems) at the Research Centre Jülich, and since 2004, he is with the Aachen University of Applied Sciences, Campus Jülich, Germany. In 2008, he has been appointed as honorary professor. His research interests are solid-state chemical sensors and biosensors, sensor materials, nano-devices, microsystem technology, nano- and biotechnology. Gerhard Baldsiefen received his PhD degree in physics at the University of Bonn. He was working in the sector of information and telecommunication before he started to work at NTTF GmbH in the field of research and development for medical technology. Heinz Busch received his PhD degree in nuclear and solid-state physics at University of Bonn. In the year 2000, he was co-founder of the company NTTF (New Technologies in Thin Films) GmbH which is called today NTTF Coatings GmbH. His core business is the development of biocompatible and biofunctional layer systems as well as the development and fabrication of devices for diagnosis and therapy in medicine. Norbert Laube received his diploma degree in mineralogy and PhD degree in geophysics and soil sciences at University of Bonn. After working as member of the research staff at the chair of geodynamics (University of Bonn), he changes as postdoctoral research fellow to the German Research Centre for Geosciences (GFZ) in Potsdam. In 1998, he became head of laboratory at the Division of Experimental Urology at the University Hospital of Bonn, and in 2003, head of the department. In 2006, he received his habilitation in the field of “Clinical Biomineralisation”. Since 2008, he is also scientific head of the German Urinary Stone Centre in Bonn. Lisa Kleinen received her diploma degree in physics at University of Bonn (Institute of Nuclear Physics). After a triannual occupation in industry, she starts to work as member of the research staff at the Institute for Thin Film Technology of Kaiserslautern University of Technology in the year 2000. Her main research activities are plasma-deposited multi-functionalised layer systems for medical applications.
S. Beging et al. / Sensors and Actuators B 144 (2010) 374–379 Michael J. Schöning received his diploma degree in electrical engineering (1989) and his PhD in the field of semiconductor-based microsensors for the detection of ions in liquids (1993), both from the Karlsruhe University of Technology. In 1989, he jointed the Institute of Radiochemistry at the Research Centre Karlsruhe. Since 1993, he has been with the Institute of Thin Films and Interfaces (now, Institute of Bioand Nanosystems) at the Research Centre Jülich, and since 1999 he was appointed as
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full professor at Aachen University of Applied Sciences, Campus Jülich. Since 2006, he serves as a Director of the Institute of Nano- and Biotechnologies (INB) at the Aachen University of Applied Sciences. His main research subjects concern siliconbased chemical and biological sensors, thin-film technologies, solid-state physics, microsystem and nano(bio-)technology.