A novel molecularly imprinted thin film applied to a Love wave gas sensor

A novel molecularly imprinted thin film applied to a Love wave gas sensor

Sensors and Actuators 76 Ž1999. 93–97 www.elsevier.nlrlocatersna A novel molecularly imprinted thin film applied to a Love wave gas sensor B. Jakoby ...

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Sensors and Actuators 76 Ž1999. 93–97 www.elsevier.nlrlocatersna

A novel molecularly imprinted thin film applied to a Love wave gas sensor B. Jakoby a

a,)

, G.M. Ismail b, M.P. Byfield b, M.J. Vellekoop

a

Electronic Instrumentation Laboratoryr DIMES, Delft UniÕersity of Technology, Mekelweg 4, P.O. Box 5031, 2600 GA Delft, Netherlands b EEV, Waterhouse Lane, Chelmsford, Essex CM1 2QU, UK Received 6 November 1998; received in revised form 16 November 1998; accepted 27 November 1998

Abstract Novel molecularly imprinted materials ŽMIMs. have been deposited as thin films on acoustic Love wave devices. MIMs offer high selectivity towards target molecules while Love wave devices offer one of the highest sensitivities with respect to surface mass changes that can be achieved with microacoustic devices. Hence this specific sensor configuration promises high sensitivity and specificity at the same time. In this contribution we outline the device configuration and present experimental results obtained with this sensor. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Love wave device; Thin film; Sensor; Molecular imprinting

1. Introduction Commonly used biochemical interface films in biochemical sensors often show serious drawbacks in terms of ageing and physical instability. The application of molecularly imprinted materials ŽMIMs. as sensing films helps to overcome these disadvantages since they essentially consist of polymeric materials, which show significantly higher stability than comparable biosensing interface materials. The process of molecular imprinting is based on the fabrication of a polymer which comprises a certain amount of template molecules, i.e., the molecules to be detected. After polymerisation the template molecules can be washed out such that empty sites being characteristic for the template molecule Ž‘fingerprints’. remain in the polymer. Upon re-exposure to, for instance, a gas containing the template molecule, template molecules are preferably absorbed into the empty sites imprinted in the polymer. In that manner a selective film has been realized on the basis of polymer material. Detailed information about molecular imprinting can be found, e.g., in Ref. w1x. Whilst the

) Corresponding author. Tel.: q31-15-2785026; Fax: q31-152785755; E-mail: [email protected]

technique of molecular imprinting is relatively well-established for bulk materials, there are, as yet, no proven techniques for fabricating films of molecularly imprinted materials in the range of 0.1–5 mm required for most chemical sensor technologies. We deposited MIM films on microacoustic Love wave delay lines, which are incorporated as the frequency determining element in an oscillator Žsee Fig. 1.. Love waves are acoustic shear modes propagating in a waveguiding layer which is deposited on a substrate. A major advantage of Love wave devices is the high sensitivity that can be achieved w2x in liquid but also in gas sensing applications. This high sensitivity is essentially caused by the effective concentration of the acoustic energy close to the sensing surface. Table 1 compares typical values of the relative mass sensitivity 1 Sm for common acoustic sensors w3,4x. The given sensitivity ranges are typical for realized devices. It can be seen that only Lamb waves offer higher sensitivities than Love wave devices but in contrast to

1 For delay line-based sensors, the relative mass sensitivity can be defined as the relative change in wave velocity DÕr Õ due to surface mass loading D r s : Sm s Ž1r Õ .Ž DÕrD r s .. For resonating devices Sm is defined in terms of the relative change in resonance frequency D f r f : Sm s Ž1r f .Ž D f rD r s ..

0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 3 5 7 - 4

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B. Jakoby et al.r Sensors and Actuators 76 (1999) 93–97

Fig. 1. Ža. Cross-section of a Love wave delay line. Žb. Oscillator setup.

Love wave devices they require complicated technology and feature a fragile thin membrane.

2. Device description For our Love wave devices, the substrate is ST-cut quartz and the guiding layer is SiO 2 , which is deposited by Plasma Enhanced Chemical Vapor Deposition ŽPECVD.. The propagation direction of the wave is orthogonal to the crystalline X direction of the substrate, which is also orthogonal to the commonly used direction for Rayleigh waves on ST-cut quartz. The Love wave is excited and detected by means of interdigital transducers ŽIDTs. deposited between substrate and guiding layer. To improve the adhesion of the MIM film on the device a chromiumrgold layer has been deposited on top of the guiding layer Žthe chromium layer improves the adhesion of the gold layer on the SiO 2 .. The impact of such a metalization on the electric parameters of the delay line is discussed in Ref. w5x. For our devices the thickness of the guiding layer is 5.7 mm, the thickness of the chromium and gold layer are about 40 nm each. The utilized acoustic wavelength l is 52 mm where the transducers are split-finger IDTs with 50% metalization ration and 50 electric periods. The acoustic aperture was chosen as 50 l and the center to center distance of transmitting and receiving IDT is 125 l. The resulting oscillator frequency amounts to 86 MHz. To prepare the gold film surfaces of the devices for the MIM layer deposition, they were modified with a suitable Table 1 Typical sensitivities of microacoustic wave sensors Žsee also Refs. w3,4x. Sensorrwave type Quartz crystal microbalance Žbulk wave resonator. Rayleigh wave Acoustic plate mode device Lamb wave Žflexural plate mode. Love wave

Typical < Sm < in cm2 rg 10–20 100–200 10–40 200–1000 150–500

organosulphur reagent. Then pre-polymerisation liquid films were deposited on the modified gold surfaces by spraying. The pre-polymerisation liquid mixtures contained the following essential components: the imprint molecule Ž2-methoxy 3-methyl pyrazine, MMP, a substance used in perfume industry., the functional monomer Žmethacrylic acid., cross-linker Žethylene glycol dimethacrylate., photoinitiator Že.g., 2,2X-azobisŽ2-methylpropionitrile.., and solvent Žtoluene.. As controls, non-imprinted films have also been produced where the liquid mixture was the same as that for the imprint film but without the print molecules. After spraying, polymerisation was initiated by UV exposure. Finally, the surfaces were made active for sensing MMP vapor by washing with methanol. The resulting MIM film thicknesses are in the order of than 1 mm.

3. Physical impact of the MIM film Depositing the film changes the physical device characteristics. Firstly, due to the mass-loading represented by the film, the wave is slowed down yielding an associated shift in center frequency. Note that this is caused by the fact that we also covered the IDT regions of the device which yields increased sensitivity compared to the case where only the waveguide region between the IDTs is covered with the sensitive layer. In the latter case no shift in IDT center frequency would be observed but only a change in the delay time of the device. We remark that this again yields a gain in sensitivity compared to, e.g., Rayleigh wave devices, where no MIM supporting gold coating can be placed on the IDT regions in order to avoid a shorting of the transducers. Secondly, the viscoelastic properties of the film introduce acoustic losses leading to the an associated additional damping of the wave. Fig. 2 shows the effect on the device response characteristics, i.e., the forward scattering parameter S21 of the delay line. In the magnitude we see the typical sinŽ x .rx response being characteristic for a microacoustic delay line using unweighted IDTs Žsee, e.g., Ref. w6x.. For the coated device, the response appears slightly shifted to the left which

B. Jakoby et al.r Sensors and Actuators 76 (1999) 93–97

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Fig. 2. Transmission function Žforward scattering parameter S21 . of the delay line with and without coating. Electromagnetic cross-talk has been eliminated using a time domain gate.

Fig. 3. Experimental setup Ždimensions not to scale.. Bypassing the Dreschel bottle yields pure nitrogen flow.

corresponds to the mass-loading induced change of the wave velocity. The center frequency of the device is given by f c s Õrl where Õ denotes the phase velocity of the Love wave 2 and l denotes the wavelength at the center frequency which is essentially prescribed by the electrical period of the IDT patterns. Apart from this frequency shift D f we observe a shift downwards corresponding to the

2

Strictly speaking Õ is a frequency-dependent quantity itself because the Love wave is a dispersive mode.

increased insertion loss DIL induced by the viscoelastic effects in the MIM layer. The shift in phase also corresponds to the reduced phase velocity as the device phase shift is given by f s 2p flrÕ Ž l denotes the electrical length of the delay line.. Using these measurements, a relative measure of the film thickness can be obtained: If the film thickness is doubled, the obtained frequency shift Žand also change in phase shift. doubles as well. Note that this holds only if the film thickness is much smaller than the thickness of the guiding layer. Otherwise the MIM film itself becomes a major part of the waveguide which also

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Fig. 4. Ža. Used Love wave device. Žb. Device mounted in gas flow cell.

strongly increases the unwanted damping effect. For a theoretical analysis of this issue, see Ref. w7x. The frequency shift obtained due to coating will be called ‘coating frequency shift’ D f c below and will be used as a measure for the film thickness.

4. Chemical sensing results To test the response of the devices to MMP exposure, we mounted the devices in a gas flow cell as shown in Fig. 3. Saturated MMP vapor at a constant flow rate of 150 mlrmin has been generated using a bubbler and passed on to the flow cell. By means of an O-ring seal, only a part of top surface containing the sensitive surface has been exposed to the gas. To control possible unwanted influence of temperature changes, a temperature sensor has been mounted on the device. 3 Fig. 4 shows photographs of the Love wave chip mounted on a printed circuit board ŽPCB. carrier and the device mounted in a brass flow cell with the connected amplifier electronics. Fig. 5 shows the response of the device to saturated MMP vapor. Prior to MMP vapor, pure nitrogen was passed through the cell. As soon as the MMP vapor exposure started Žat about 4 min on the time scale shown in Fig. 5., the frequency dropped. After about 20 min the gas flow was changed back to pure nitrogen. The imprinted device was then exposed for a second time. It

3 A possible way to reduce unwanted temperature sensitivity is to use a dual setup with one coated and one uncoated device, where the temperature effect can be canceled to first order by taking the difference signal of the two sensors. For the sake of simplicity, we stick to single devices for our first experiments.

can be seen that the MIM response is roughly 180% of the control response. Furthermore the film can be regenerated to a large extent by cleaning it in a nitrogen flow. For the imprinted device also a second cycle has been measured. The frequency change is given in relative units to account for thickness differences of the MIM film between the two compared devices. While accurate thickness measurements were not available, the ratio between the thicknesses can be determined by measuring electrical characteristics of the delay line before and after coating. One of the characteristic parameters is the shift of the device center frequency due to the coating, D f c Žsee also Section 3.. The ordinate in Fig. 5 gives the measured frequency shift in the experiment, D f, scaled by the coating frequency shift D f c . This scaled response is characteristic for the film and eliminates the sensitivity of the sensing device as well as

Fig. 5. Relative frequency change D f rD fc due to MMP exposure for imprinted and non-imprinted Žcontrol. device.

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the influence of increased sensitivity due to increased film thickness. Correspondingly, the normalized responses should be equal regardless from used sensing device and film thickness. Indeed the obtained scaled response resembles that of those obtained for bulk wave devices with the same film material w8x. In terms of absolute frequency values, the maximum frequency shift in Fig. 5 for the MIM coated device corresponds to about 18 kHz. Oscillator stabilities in the order of a few Hertz can be achieved, which, together with the shown high sensitivity, yields very low detection limit. Experiments with MIM-coated bulk acoustic wave ŽBAW. devices yielded absolute frequency shifts of typically 200 Hz Žfor thicker films., which results in a worse detection limit. This illustrates the high sensitivity of Love wave devices compared to BAW devices.

5. Summary and conclusions Microacoustic Love wave devices have been covered with novel molecularly imprinted polymer films. In the experimental setup the response of the MIM-coated and a control device Žcoated with a non-imprinted but otherwise identical polymer. have been investigated. The results indicate the suitability of molecularly imprinted thin films and confirm the high sensitivity of the Love wave sensor.

Acknowledgements This work was partly supported by Brite-Euram project BE-95-1745: MIMICS. We thank the ICP group of the DIMES technology center for the fabrication of the devices. The help of J. Bastemeijer, A. Berthold, J. Groeneweg, and W. van der Vlist in preparing and conducting the experiments is gratefully appreciated. The used flow cell has been designed in cooperation with Biosensores S.L., Moncofar, Spain.

References w1x K. Mosbach, O. Ramstrom, The emerging technique of molecular imprinting and its future impact on biotechnology, Biotechnology 14 Ž2. Ž1996. 163–170. w2x G. Kovacs, A. Venema, Theoretical comparison of sensitivities of acoustic shear wave modes for Žbio.chemical sensing in liquids, Appl. Phys. Lett. 61 Ž1992. 639–641. w3x D.S. Ballantine, R.M. White, S.J. Martin, A.J. Ricco, E.T. Zellers,

w4x w5x

w6x w7x

w8x

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G.C. Frye, H. Wohltjen, Acoustic Wave Sensors, Academic Press, San Diego, 1997. M.J. Vellekoop, Acoustic wave sensors and their technology, Ultrasonics 36 Ž1998. 7–14. B. Jakoby, M.J. Vellekoop, Analysis and optimization of Love wave liquid sensors, IEEE Trans. on Ultrason., Ferroelec., Freq. Contr. 45 Ž1998. 1293–1302. D.P. Morgan, Surface-Wave Devices in Signal Processing, Elsevier, Amsterdam, 1985. B. Jakoby, M.J. Vellekoop, Viscous losses of shear waves in layered structures used for biosensing, in: Proc. IEEE Ultrason. Symp., 1998, in press. G.M. Ismail, B. Jakoby, R.H.J. Marshall, C.J. Bowden, M.P. Byfield, M.J. Vellekoop, Acoustic wave chemical sensors based on molecularly imprinted materials, in: 194th Meeting of the Electrochemical Society, 1998, in press.

Bernhard Jakoby was born in Neuß, Germany, in 1966. He obtained the Dipl.-Ing. ŽMSc. degree in Communication Engineering and the PhD degree in Electrical Engineering from the Vienna University of Technology, Austria, in 1991 and 1994, respectively. Currently he is assistant professor at the Electronic Instrumentation LaboratoryrDIMES at the Delft University of Technology, The Netherlands, working on microacoustic sensors. His research interests are focused on numerical and analytical methods in field theory, complex media in electromagnetics and acoustics, and microacoustics and its applications in general. Gulam Ismail graduated in Chemistry from Leicester University in 1983 and carried on to take his PhD in Chemistry from The City University, London in 1986 after working on the synthesis of novel fluorescent optical brighteners. He was then employed for two years at Nottingham University as a BTG postdoctoral fellow to work on the design and synthesis of carrier molecules for anti-cancer agents. Between 1988 and 1990 he worked as a senior scientist for Serono Diagnostics after which he joined General Electric ŽGEC. where he initially worked on biosensors until transferring to gas sensor R&D in 1996. He is now a Principal Engineer at EEV and works in the Chemical Sensor Systems group. Mark Byfield graduated in Chemistry in 1985 with a BA Hons. from The Queen’s College, Oxford. He then spent a year at Beecham Products Research before taking a PhD in Metalloprotein Chemistry from the University of Surrey in Guildford, UK. He joined the General Electric ŽGEC. in 1989 and has worked since then on the research and development of biosensors and chemical sensor array technologies. He is now Technology Manager for EEV Chemical Sensor Systems part of the Marconi Electronic Systems group within GEC. Michael J. Vellekoop was born in Amsterdam in 1960. He received the BSc degree in Physics from the HTS Dordrecht, The Netherlands, and the PhD degree in Electrical Engineering from the Delft University of Technology, the Netherlands, in 1982 and 1994, respectively. Currently he is an associate professor at the Delft University of Technology, Information Technology and Systems Faculty where he leads the Physical Chemosensors and Microacoustic Devices Group of the Electronic Instrumentation Laboratory, DIMES. His recent research activities have been in the areas of microacoustic sensor systems for gas and liquid sensing applications, solid state sensor technology, and physical chemosensors.