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Development of Micro/Nano Electro Mechanical System based Sensors for Societal Applications

M. S. VINCHURKAR1, V. SEENA1,3, D. AGARWAL1, NEHUL GULLAIYA1, S. MUKHERJI2 and V. RAMGOPAL RAO1

1 Centre of Excellence in Nanoelectronics, Department of Electrical Engineering

2 Department of Biosceiences and Bio-engineering Indian Institute of Technology, Bombay, India – 400 076

3 Indian Institute of Space Science and Technology, Thiruvananthapuram

*Email: rrao@ee.iitb.ac.in

Abstract

Using surface functionalized piezoresistive polymer composite cantilevers, prototypes of systems have recently been demonstrated at IIT Bombay for point of care cardiac diagnostics and for vapour phase explosive detection. Prototypes of these systems are currently being tested in the field conditions with the help of NanoSniff Technologies Pvt. Ltd., a company incubated at IIT Bombay based on this technology. Keywords : biosensor, microsensor, biofunctionalization SU-8-CB, cantilever.

1. Introduction

Microsensors based on Micro/nano electro mechanical systems are among the first examples when the investigation of microelectronic application of microfabrication technology was started. Electronic sensor applications require fast, comprehensible, economical and ultra sensitive methods for

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recognition of target analytes. MEMS applications with ‘‘Microcantilever’’ transducers have emerged from the increasing demand for these miniaturized high throughput sensors supported by advancement in microfabrication. MEMS devices based on micromechanical cantilevers offer an enhanced sensitivity and an exceptional dynamic range which lead to their diverse sensing applications. Many physical1, 2, chemical3, and biological4–6 microcantilever based sensors have been reported till date. MEMS sensors with microcantilever as a transducer work on the basis of conversion of physical parameters such as temperature changes around them or chemical or biomolecular interactions occurring directly on their surface into either static displacements (static mode) or a change in resonance frequency (dynamic mode) of the microcantilever beam6–8. Depending on the sensor application, the microcantilever design needs to be optimized differently with respect to the transduction modes used. Either optical (external) or electrical (integrated) transduction techniques are applied to monitor the nanomechanical motion. An external optical readout mechanism like a laser beam needs extra device alignment step which can be eliminated by incorporation of a transduction mechanism within the mechanical element. The practical limitations of external transduction method are overcome by employing self-sensing microcantilevers with integrated electrical transduction mechanisms. In a microcantilever with an integrated piezoresistor, electrical transduction of strain is performed by measuring the corresponding resistance change. When a surface functionalized microcantilever with molecules or thin film coatings that are specific to the target molecules is exposed to the ambient containing analyte, the selective molecular adsorption creates a differential surface stress between the top and bottom surfaces of the microcantilever. This differential surface stress leads to a change in resistance of the piezoresistive layer. The surface stress sensitivity is proportional to the the ratio of the gauge factor (K) of

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the piezoresistive film to the Young’s modulus (E) of the cantilever structural material. Stable polymers such as SU-8 (glycidylether of bis-phenol A) with low E values are favored as structural layers for microcantilevers over the conventional materials like silicon. SU- 8 microcantilevers with piezoresistive layers such as gold and polysilicon have been reported earlier9,10,11. Performance improve-ment of the piezoresistive SU-8 microcantilever sensorshas been achieved by integrating SU-8-carbon black (CB) nanocomposite as the strain sensitive layer12-16.

2. Device Design and Fabrication

The dimensions of the microcantilevers are decided based on their stiffness which reflects in their sensitivity in static mode and the resonance frequency of the microcantilever, which determines their stability under external vibrations.14. The piezoresistive SU-8 microcantilever device has a set of measurement and reference microcantilever beams as depicted in the schematics of the polymer device die (Fig. 1)14.

Fig. 1 : Device design of the SU-8-CB peizoresistive micro-cantilever; planar schematic (A); Cross sectional schematic (B)14

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The fabrication of these microcantilevers with SU-8/CB nanocomposite piezoresistive layers sandwiched between two SU-8 layers involves five levels of lithography15. The process sequence for fabricating this composite microcantilever is shown in Fig. 2 13,11,16.

Fig. 2: Process sequence for SU-8-CB microcantilevers (a) First layer of SU-8 (b) Cr/Au for contacts (c) strain sensitive composite layer (d) encapsulating SU-8 (e) Thick SU-8 (f) Release of cantilever die from the substrate15

Silcon wafer used as the dummy susbstrate has been RCA cleaned and silicon dioxide which acts as a sacrificial layer has been deposited. The piezoresistive layer is a polymer nanocomposite of Carbon Black (CB) Conductex 7067 Ultra in SU-8. For this, optimum carbon black filler loading in the range of 8–9 vol% was used13,14. By adjusting the lithographic parameters, this nanocomposite was successfully patterned just like any other negative UV resist. Wet etching of the silicon dioxide layer releases the polymer device chips

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from silicon substrate. The photograph of the unreleased and released devices along with an SEM image of one of the fabricated devices are shown in Fig. 3.

Fig. 3: Photograph of strips of different polymer nanocomposite micro-cantilever device chips after the release process (A); SEM image of one type of SU-8/CB nanocomposite microcantilevers (B); Optical micrographs of SU-8/CB nanocomposite microcantilevers devices on the wafer (C and D);13,14,16

3. Device Characterization

Electromechanical characterization of the fabricated microcantilevers was performed for verifying the piezoresistive transduction and extracting the sensor sensitivity. These devices showed deflection sensitivity ( R/R for unit deflection) of 1.1

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ppm nm−1. Using the ‘Polytek Doppler’ vibrometer the resonance frequency of these microcantilevers was extracted (Fig. 4). The resonance frequency was found to be well above the lower limit needed for external vibrational noise immunity.

Fig. 4: Resonance frequency plot of SU-8-CB microcantilever.23

4. Sensor Applications

In recent years, microcantilevers have been identified to be suitable for developing chemical and biological sensors because of their advantages such as small size, high sensitivity, label free detection, low power consumption and versatility for incorporating sensors arrays in a single miniature package.17-24. Among the transduction schemes for these sensors, optical readout schemes are known to be sensitive and simple. However, this class of sensors suffer from practical limitations such as incompatibility with opaque solutions, time consuming laser alignment, difficulty in integration with sensor electronics etc. Hence, for the biomolecular sensing applications, microcantilever sensors with sensitive electrical transduction schemes would be a better

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candidate as compared to their optical counterpart. Application of the peizoresistive polymeric (SU-8-CB) microcantilevers as cardiac biosensor and explosive detector are described in the following sections.

5. Cardiac Biosensor

According to the World Health Organization (WHO) survey, by 2030, Cardiovascular Diseases (CVDs) will cause 23.6 million deaths around the world25.There is an urgent need for a sensor which can detect the cardiac markers at an early stage of Acute Myocardial Infarction (AMI). Authors have combined the advantages of polymeric microcantilevers and piezoresistive readout into an inexpensive and highly sensitive platform for point of care applications. The integrated system called "infarcSens" or "iSens", is a peizoresistive microcantilever based affinity biosensor array for sensing myocardial infarction and subsequent cardiac status prognosis, using a suite of molecular markers. It is particularly useful in detecting heart attacks that go unnoticed before a major or fatal attack occurs. The target cardiac biomarkers are Human fatty acid binding protein (hFABP), Troponin, Myoglobin and CK-MB.

6. Bio-functionalization

In order to use the peizoresistive SU-8-CB microcantilevers for biosensor applications, the cantilever should be bio-functionalized with biomolecules specific to the target analyte such as an antibody. The protocol for the bio-functionalization has been developed earlier26,27.

Briefly, functional group modification of either top or bottom surface of the microcantilever is done as the first step. A dry method involving the pyrolytic (filament temperature of 1450°C) dissociation of Ammonia gas inside a hot wire Chemical Vapour Deposition (CVD) chamber is employed to get surface amine

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groups on the SU-8-CB microcantilevers26. After the CVD treatment the microcantilevers are mounted in an in-house built liquid flow cell and the rest of the biofunctionalization steps were carried out inside the flow cell. The microcantilever surface amine groups are then reacted with the homo-bifunctional linker glutaraldehyde for subsequent covalent binding of the antibodies on the surface. The unsaturated aldehyde sites and nonspecific adsorption sites on the antibody immobilized surface are blocked using bovine serum albumin (BSA). The efficiency of the surface biofunctionalization is monitored by a control experiment using labeled antibodies and fluorescence microscopy. A typical fluorescence microscopic image of the biofunctionalized SU-8-CB microcantilevers is shown in Fig. 5.

Fig. 5: Fluorescence image of biofunctionalized SU-8-CB microcantilever die

7. Read Out Mechanism and Measurement Set-up

A differential microcantilever surface stress is created because of the binding of the marker protein to the immobilized antibody causing a bending of the microcantilever, which is in the order of a few tens of nanometers9. The corresponding change in resistance of the piezoresistive nanomechanical cantilevers is in the range of

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a few parts per million. An instrumentation setup28 that generates a voltage signal analogous to the resistance changes down to 14 parts per million (ppm) with a maximum sensitivity of 2.4 V/ppm integrated with the peizoresistive microcantilever-liquid flow cell unit comprises the table-top sensor for cardiac diagnostics. All the measurements on the biofunctionalized microcantilevers are done with respect to the nonfunctionalized reference microcantilevers in the liquid flow cell. A representative example of the sensor signal in the case of cardiac marker protein h-FABP24 is given in Fig. 6 (A) along with a photograph of the cardiac biosensor prototype “iSens” Fig. 6 (B).

Fig. 6: Signal recorded as a change in voltage for the detection of cardiac marker protein FABP. (Baseline was adjusted to zero before analyte introduction) (A); iSens: A point of care system for cardiac diagnostics (B)24.

It is largely due to the high economical and social value of modern medical devices that new materials and processes are incorporated at a very early stage into new products. The point of care cardiac diagnosis system “iSens” developed at IIT-Bombay is one such example. As micro machined micro/nano size cantilevers are batch-fabricated they offer the advantage of parallelization into sensor arrays which can be used to detect various markers

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simultaneously. Our ultimate aim is to develop a lab on a chip piezoresistive microcantilever based over-the-counter, field portable, high-throughput, protein detection systems as a common platform for characterizing a wide spectrum of diseases. Such a "protein-chip" would detect multiple markers in a single step in contrast to the currently available assays which require a separate reaction for each analyte. The sensing electronics and the associated data management software for tracking the markers, with the time required for creating an epidemiological database is also being developed. This will reduce both the time and cost of diagnosis.

8. Explosive Detector

Ion mobility spectrometry (IMS) and mass spectrometry are the conventional explosive detection techniques but their use is limited due to longer response times besides being bulky and expensive. As explosives have very low vapour pressures, detectors with ultra high sensitivity are required for their vapour phase detection. Use of microcantilever based MEMS devices in explosive detection has been reported in the literature29,30. In order to use microcantilevers for explosive detection, functionalization of the cantilever surface with a chemically selective layer or receptor for the target molecule is needed. The functionalization procedure is based on the receptor and microcantilever surface chemistry. Specific receptors like 4-MBA (4-mercaptobenzoic acid), SXFA, etc.30,31 have been reported for the most popular explosives such as TNT, RDX and PETN. In our case, for the explosive vapour detection, the peizoresistive SU-8-CB microcantilevers were functionalized using 4-MBA as a receptor layer29. The sulfhydryl group 4-MBA forms a stable monolayer on a gold surface. Thus, for a selective functionalization with 4-MBA, one of the microcantilever surfaces was coated with 30 nm of gold using titanium as the adhesion layer. The peizoresistive SU-8-CB microcantilevers with the monolayer of 4-MBA were used in the explosive sensor. The functionalized and non-functionalized (as control) microcantilever devices were stored

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in a humidity controlled chamber. The devices were then connected to a signal conditioning circuit and kept inside a gas flow cell for explosive vapour detection. Prolonged dry nitrogen purging of the device chamber to reduce the humidity levels was done before starting the measurement. The vapour generator containing the TNT source was maintained at a constant temperature using a calibrated temperature controller. The carrier gas for the TNT was dry nitrogen and the gas flow of 30 sccm was maintained using a mass flow controller (MFC). The sensor was regenerated with dry nitrogen purging. The calibrated vapour generator is capable of delivering TNT vapours with concentrations of less than 10 ppb. A photograph of the experimental setup along with microcantilever response recorded during a cycle of exposure to TNT and nitrogen are given in Fig. 7.

Fig. 7: (a) Experimental set up for explosive vapour experiments (b)Response of 4-MBA coated Microcantilever to TNT vapours (c) A prototype of the explosive

detector system.23

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These devices could detect TNT vapour concentrations down to a few ppb. Thus an effective chemical sensing application of the peizoresistive SU-8-CB microcantilevers for the detection of explosive vapours has been successfully demonstrated.

Acknowledgement

The authors wish to acknowledge the funding received from the Department of Information Technology, Government of India, through the Centre of Excellence Nanoelectronics. The cardiac diagnostics work is supported by the National Programme on Micro & Smart Systems (NPMASS) while the explosive detector work is supported by the Principal Scientific Adviser’s office, Govt. of India.

References

1. Berger R, Lang H P, Gerber Ch, Gimzewski J K, Fabian J H, Scandella L, Meyer E and G¨untherodt H-J (1998) Micromechanical thermogravimetry. Chem. Phys. Lett. 294, 363–9.

2. Chen L T, Lee C Y and Cheng W H (2008) MEMS-based humidity sensor with integrated temperature compensation mechanism. Sensors Actuators A 147, 522–8.

3. Lim S H, Raorane D, Satyanarayana S and Majumdar A (2006) Nano-chemo-mechanical sensor array platform for high-throughput chemical analysis. Sensors Actuators B 466–74

4. Carrascosa L G, Moreno M, A´ lvarez M and Lechuga L M (2006) Nanomechanical biosensors: a new sensing tool. Trends Anal. Chem. 25, 196–206.

5. Arntz Y, Seelig J D, Lang H P, Zhang J, Hunziker P, Ramseyer J P, Meyer E, Hegner M and Gerber C (2003) Label-free protein assay based on a nanomechanical cantilever array. Nanotechnology 14, 86–90.

6. Ziegler C (2004) Cantilever-based biosensors. Anal. Bioanal.Chem. 379, 946–59.

7. Fritz J, Baller M K, Lang H P, Rothuizen H, Vettiger P, Meyer E, G¨untherodt H, Gerber C and Gimzewski J K (2000) Translating biomolecular recognition into nanomechanics. Science 288 316–8.

8. Hosaka S, Chiyoma T, Ikeuchi A, Okano H, Sone H and Izumi T (2006) Possibility of a femtogram mass biosensor using a self-sensing cantilever.Curr. Appl. Phys. 6, 384–8.

85

9. Thaysen J, Yalcinkaya A D, Vettiger P and Menon A (2002) Polymer-based stress sensor with integrated readout. J. Phys.D: Appl. Phys. 35, 2698–703.

10. Kale N S, Nag S, Pinto R and Rao V R (2009) Fabrication and characterization of a polymeric microcantilever with an encapsulated hotwire CVD polysilicon piezoresistor. J. Microelectromech. Syst. 18, 79–87.

11. Seena V, Kale N, Nag S, Joshi M, Mukherji S, Rao V R (2009) Developing a polymeric microcantilever platform technology for biosensing applications. International Journal of Micro and Nano Systems 1 (1), 65-70.

12. Gammelgaard L, Rasmussen P A, Calleja M, Vettiger P and Boisen A (2006) Microfabricated photoplastic cantilever with integrated photoplastic/carbon based piezoresistive strain sensor. Appl. Phys. Lett. 88, 1508.

13. Seena V, Rajorya A, Pant P, Mukherji S and Rao V R (2009) Poly-mer microcantilever biochemical sensors with integrated polymer composites for electrical detection. Solid State Sci. 11, 1606–11.

14. Seena V, Fernandes A, Pant P, Mukherji S, and Rao V R (2011) Polymer nanocomposite nanomechanical cantilever sensors: Material characterization, device development and application in explosive vapour detection. Nanotechnology 22, 1–11.

15. Seena V, Rajorya A, Fernandes A, Dhale K, Pant P,Mukherji S and Rao V R (2010) Fabrication and characterization of novel polymer composite microcantilever sensors for explosive detection. IEEE 23rd Int. Conf. MEMS (IEEE MEMS 2010) Tech. Dig. 851–854.

16. Seena V, Fernandes A, Mukherji S, Rao V R (2011) Photoplastic Microcantilever Sensor Platform for Explosive Vapour Detection. International Journal of Nanoscience 10, 739-43.

17. Fritz J, (2008) Cantilever biosensors Analyst 3 855-63. 18. Prashanthi K, Miriyala N, Gaikwad R D, Moussa W, Rao V R and

Thundat T (2013) Vibrational energy harvesting using photo-patternable piezoelectric nanocomposite cantilevers. Nano Energy (Elsevier) Accepted for publication.

19. Raval H N, Sutar D S and Rao V R (2013) Copper(II) phthalo-cyanine based organic electronic devices for ionizing radiation dosimetry applications. Organic Electronics (Elsevier) Accepted for publication.

20. Raval H N, Sutar D S, Nair P R and Rao V R (2013) Investigation of effect of ionizing radiation exposure on material properties of

86

organic semiconducting oligomer – Pentacene. Organic Electronics (Elsevier) Accepted for publication.

21. Ray P, Rao V R (2013) Al-Doped ZnO thin-film transistor embedded micro-cantilever as a piezoresistive sensor. Applied Physics Letters (APL) Accepted for publication.

22. Reddy C V B, Khaderbad M A, Gandhi S, Kandpal M, Patil S, Chetty K N, Rajulu K G, Chary P C K, Ravikanth M and Rao V R (2012) Piezoresistive SU-8 Cantilever with Fe(III)porphyrin coating for CO Sensing. IEEE Transactions on Nanotechnology 11, 701-6.

23. Seena V, Dudhe R S, Raval H N, Patil S, Kumar A, Mukherji S, Rao V R (2011) Organic sensor platforms for environmental and security applications. Electro-Chemical-Society (ECS) Transactions 35, 67-77.

24. Agarwal D K, Gandhi S I, Narayan Y, Aithal S, Chatterji G, Surya S G, Seena V, Mukherji S, Rao V R (2012) Detection of heart-type human Fatty Acid Binding Protein(h-FABP) using piezoresistive polymer micro-cantilevers. Nano-Mechanical sensors conference (NMC).

25. http://www.who.int/mediacentre/factsheets/fs317/en/ index.html. 26. Joshi M, Kale N, Lal R, Rao V R, Mukherji S (2007) A novel dry

method for surface modification of SU-8 for immobilization of biomolecules Bio-MEMS. Biosensors and Bioelectronics 22, 2429–35.

27. Joshi M, Pinto R, Rao V R, Mukherji S (2007) Silanization and antibody immobilization on SU-8. Applied Surface Science 253, 3127–32.

28. Surya S, Nag S, Fernandes A, Gandhi S, Agarwal D, Chatterjee G, Rao V R (2011). IEEE International Symposium on Electronic System Design 34.

29. Pinnaduwage L A, Hawk J E, Boiadjiev V, Yi D and Thundat T (2003) Use of microcantilevers for the monitoring of molecular binding to self-assembled monolayers. Langmuir 19, 7841–4.

30. Senesac L and Thundat T (2008) Nanosensors for trace explosive detection. Mater. Today 11, 28–33.

31. Chen Y, Xu P and Li X (2010) Self-assembling siloxane bilayer directly on SiO2 surface of micro-cantilevers for long-term highly repeatable sensing to trace explosives. Nanotechnology 21, 265501.