06168274

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 11, NO. 4, JULY 2012 701

Piezoresistive SU-8 Cantilever With Fe(III)PorphyrinCoating for CO Sensing

C. Vijaya Bhaskar Reddy, Mrunal A. Khaderbad, Sahir Gandhi, Manoj Kandpal, Sheetal Patil, K. Narasaiah Chetty,K. Govinda Rajulu, P. C. K. Chary, M. Ravikanth, and V. Ramgopal Rao, Senior Member, IEEE

Abstract—Carbon monoxide detection is required for var-ious healthcare, environmental, and engineering applications.In this paper, 5,10,15,20-tetra (4,5-dimethoxyphenyl)-21H,23H-porphyrin iron(III) chloride (Fe(III)porphyrin) coated on apiezoresistive SU-8 microcantilever has been used as a CO sensor.Rapid detection of CO down to 2 ppm has been observed with afore-mentioned sensors. Cantilevers without Fe(III)porphyrin have notresponded to CO exposure. Fe(III)porphyrin-coated cantilever se-lectivity toward CO has been analyzed by measuring the sensorresponse to various gases such as N2 , CO2 , O2 , ethanolamine,N2O, and moisture. The sensor has exhibited a fast response andrecovery times and is fully recoverable after repeated exposures.

Index Terms—Cantilevers, carbon monoxide, iron porphyrin,piezoresistance, sensor.

I. INTRODUCTION

CONSIDERING its omnipresence, carbon monoxide (CO)is one of the most harmful compounds for human beings.

CO can jeopardize oxygen transport in blood and it is one of themost important gases to be detected for gas sensor-based firedetection [1] applications. The efficiency of fuel combustion incombustion engines, power plants, fuel cells, and automobilescan be monitored by quantifying CO emission. This providesinformation not only for feedback control of combustion pro-cesses, but also to indicate various fire and health hazards [2].This demand has stimulated research to realize low-power sen-

Manuscript received January 6, 2012; accepted February 24, 2012. Dateof publication March 12, 2012; date of current version July 11, 2012. Thisproject is supported by the Department of Information Technology, Ministryof Communication and Information Technology, Government of India underthe Indian Nanoelectronics Users Program (INUP) at Indian Institute ofTechnology Bombay (IIT Bombay). The review of this paper was arranged byAssociate Editor J. Li.

C. V. B. Reddy is with the Department of Mechanical Engineering, Srikala-hasteeswara Institute of Technology, Srikalahasthi 517640, India (e-mail:[email protected]).

M. A. Khaderbad, S. Gandhi, M. Kandpal, and V. Ramgopal Rao are with theCentre of Excellence in Nanoelectronics, Indian Institute of TechnologyBombay, Mumbai 400076, India (e-mail: [email protected];[email protected]; [email protected]; [email protected]).

S. Patil is with the Nanosniff Technologies Pvt. Ltd., Indian Institute ofTechnology Bombay, Mumbai 400076, India (e-mail: [email protected]).

K. N. Chetty and K. G. Rajulu are with the Department of MechanicalEngineering, Jawaharlal Nehru Technological University, Anantapur 515002,India (e-mail: [email protected]; [email protected]).

P. C. K. Chary is with the Department of Mechanical Engineering,Sree Vidyanikethan Engineering College, Tirupathi 517501, India (e-mail:[email protected]).

M. Ravikanth is with the Department of Chemistry, Indian Institute of Tech-nology Bombay, Mumbai 400076, India (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TNANO.2012.2190619

sors, which are compact and work in real-world conditions [3].Various sensing elements comprising of nanomaterials to im-prove the selectivity and sensitivity of detection have been ex-amined for this purpose.

The usability of copper chloride (CuCl) as a selective COsensor in the presence of 50 vol% hydrogen has been demon-strated for proton-exchange membrane fuel cell applications.However, the CO sensing performance of CuCl films is verysensitive to the synthesis and fabrication methods [4]. CO gassensors based on gold (Au)-doped tin oxide (SnO2) have shownefficient sensing in the 80–210 ◦C temperature range [5], [6].Pthalocyanine nickel(II)-coated piezoelectric crystal has alsobeen investigated as a CO sensor [7], [8]. Other types of sensorsfor CO detection include electrochemical sensors, thermoelec-tric, colorimetric detectors, and infrared detectors [9]–[12].

It is well known that molecules with a central metal atomsurrounded by neutral or charged ligands have been employedfor active gas sensing, where vacant sites or weakly bound co-ligand interact with the metal ion. In this interaction, the gaseousmolecule to be detected becomes a co-ligand either by occupy-ing free coordination site or by displacing other ligands. Col-orimetric sensor based on the interaction between CO and por-phyrin films has been explored for the estimation of the biologi-cal damage due to CO exposure [13]. Radhakrishnan et al., haveused polypyrrole functionalized with 5,10,15,20-tetraphenyl-21H, 23H-porphyrin iron(III) chloride (PPy–FeTPPCl) as anactive sensing material for CO gas. It was observed that semi-conducting PPy–FeTPPCl’s resistance was drastically increasedupon CO exposure [14], [15].

In this study, we demonstrate highly sensitive CO de-tection using 5,10,15,20-tetra(4,5-dimethoxyphenyl)-21H,23H-porphyrin iron(III) chloride (Fe(III)porphyrin)-coated SU-8nanocomposite microcantilevers with integrated piezoresistivereadout. As compared to conventional Si-based cantilevers,polymer (SU-8 as a structural layer)-based cantilevers providehigher sensitivity and use low cost fabrication techniques suchas spin coating, evaporation and wet etching etc. [16], [17].A typical (Fe(III)[T(4,5(OCH3)2 P)P]Cl)-CO interaction modi-fies surface stress on the cantilever when exposed to the CO gas.The change in surface stress results in a resistance change in theintegrated piezoresistor, thus enabling electrical detection.

II. EXPERIMENTATION

The polymer cantilevers for the fabrication of CO sen-sor were fabricated using the following process [17], [18].Nanocomposite-based polymer microcantilevers of the dimen-sions, 200, 50, and ∼3.5 μm, length, width, and thickness,

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702 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 11, NO. 4, JULY 2012

Fig. 1. Process sequence for polymer composite microcantilevers. (a) Silicondioxide as a sacrificial layer. (b) First layer of SU-8. (c) Cr/Au for contacts. (d)SU-8/CB composite layer. (e) Encapsulating SU-8. (f) Thick SU-8 die base. (g)Release of cantilever die from the substrate.

respectively, are used in this study in order to improve thesurface stress sensitivity, mechanical stability, and packagingcompatibility of the sensors. Nanocomposite, based on a disper-sion of carbon black nanoparticles in SU-8, a nonconductive,negative tone photoresist, is used as a piezoresistive layer inthese polymer cantilevers. The SU-8/carbon black nanocom-posite is obtained by homogeneously mixing the carbon blackpowder in the photosensitive SU-8 resin. The lower Young’smodulus of SU-8 compared to Si and the higher strain sensitiv-ity of SU8/carbon black nanocomposite provides these devicesthe required sensitivity to detect CO down to the ppm sensitivity.

The nanocomposite polymer microcantilever fabrication pro-cess sequence is illustrated in Fig. 1. The fabrication starts withRCA cleaning of silicon substrate with 500-nm thermally grownsilicon dioxide as a sacrificial layer. SU-8 structural layer (SU-8 2000.5, Microchem, MI) was spin coated and pre-exposurebaked at 70 ◦C and 90 ◦C for optimized timings with a slowramp up and ramp down to room temperature. To transfer themicrocantilever pattern [see Layer-1, Fig. 1(b)], the sampleswere exposed to UV light using Karl Suss MJB3 mask alignerand subjected to a post exposure bake cycle, development andrinsed with Iso Propyl Alcohol (IPA). A thin layer of Cr/Au(10 nm/200 nm) was deposited by sputtering, and the contactpads were patterned using PPR photolithography with the cor-responding mask [see Fig. 1(c)]. The Cr–Au layer was wetetched in respective enchants. To obtain an electrically conduc-tive and a strain sensitive layer, SU-8/CB nanocomposite wasprepared by dispersing the carbon black of 8–9 Vol.% in SU-8.The nanocomposite was spin coated and subsequently patternedusing mask for layer 3 [see Fig. 1(d)] by UV lithography, fol-lowed by additional ultrasonic cleaning step in IPA. This strain

Fig. 2. (a) Schematic of the porphyrin molecule and SEM images of the re-leased nanocomposite microcantilever devices. (b) Change in resistance (ΔR/R)as a function of deflection.

sensitive resistive layer is then encapsulated by 1.6 μm of SU-8(2002) that is spin coated and photolithographically patternedusing mask for layer 4 [see Fig. 1(e)]. Finally to form an an-chor [see Fig. 1(f)] for the cantilevers, a 180-μm thick SU-8was defined by spin coating and patterning of SU-8 (2100). Thedevices were released by wet etching the silicon dioxide layerin the buffered hydrofluoric acid approximately for 30 min.

The arrays of released SU-8 nanocomposite microcantileverchips were rinsed in DI water, isopropyl alcohol and allowed todry. Fig. 2(a) shows the SEM images of the released microcan-tilever devices. The microcantilever chips were characterizedelectromechanically to demonstrate the piezoresistive behavior.The tip of the microcantilever was deflected with a calibratedmicromanipulator needle from Suss Microtech with simulta-neous measurement of resistance using Keithley 4200 sourcemeasuring unit. The change in resistance (ΔR/R) as a functionof deflection is given in Fig. 2(b), the calculated deflection sen-sitivity is 1.1 ppm/nm [18], which is higher compared to thepolymer microcantilevers with Au as the strain gauge.

The piezoresistance value of the fabricated cantilevers wasmeasured to be 500 kΩ. The (Fe(III)[T(4,5(OCH3)2 P)P]Cl) wasdissolved in IPA solution (1 mg in 20 mL of solution) and theselected cantilevers were drop coated using a microdispenser.Further, the backside of the cantilever is coated with gold usingthe Nordiko sputter machine at a base pressure of 1.0 × 10−5

mbar and sputter pressure of 2.6 × 10−3 mbar to avoid theinteraction of porphyrins with the CO vapors on the bottom sidethus enhancing the system’s electrical response.

The cantilever is then mounted onto a printed circuit board(PCB) and electrical connections were made between the can-tilever and the preexisting contact leads on the PCB using

REDDY et al.: PIEZORESISTIVE SU-8 CANTILEVER WITH FE(III)PORPHYRIN COATING FOR CO SENSING 703

Fig. 3. (a) Cantilever mounted on PCB and flow cell arrangement for gassensing. (b) Experimental setup.

conductive silver epoxy (1:1) which included a 80 ◦C heat treat-ment for curing purposes. The cantilever is then enclosed in aflow cell made of Teflon and sealed for further measurements.This standard flow cell has an inlet and an outlet for the gas flowas shown in Fig. 3(a).

The experimental setup for sensor calibration used is shownin Fig. 3(b). CO and the carrier gas (N2) were allowed to flowout of the cylinders through flow controllers. The flow cell wasconnected to a gas-mixing chamber as inlet and the flow celloutlet was connected to a pump (not shown). The cantileversensor is connected to a Wheatstone’s bridge and the dc voltagewas recorded using the ADS123X TI board (Texas InstrumentBoard). The wheatstone bridge consists of four arms of whichone arm is cantilever and the remaining three arms have po-tentiometers. Initially, the bridge is balanced by matching R4with cantilever and R1 with R2. As the change in the resistanceΔR due to strain in the cantilever is very small, in orders offew ohms in 100 kΩ, nonlinearity error of the bridge is negli-gible. The output of the bridge is fed to one of the three inputchannels of ADC. The strain on the piezoresistive layer of thenanoelectromechanical cantilever results in the deflection sensi-tivity, which is an important performance parameter. The relativechange in the resistance (ΔR) with respect to the fixed arm of thebridge is determined by the deflection sensitivity. By means ofthe Wheatstone bridge the change in the resistance is measuredin terms of voltage. Sensitivity calculation of the current systemis based on the change in output voltage for the correspondingresistance change in one of the arms of the bridge.

N2 purge was carried out before the start of the experiment.Fe(III)porphyrin-CO interaction causes the cantilever to deflect,which changes the resistance of cantilever [14]. The change inthe resistance was measured as a voltage change in the output[17]–[20].

III. RESULTS AND DISCUSSION

Fig. 4(a) shows the response of a bare SU-8/CB cantileverfor alternating cycles of CO and N2 gases at 500 sccm flow

Fig. 4. (a) Response of a bare SU-8/CB polymer composite microcantileverfor consecutive cycles of CO and N2 . (b) Response of a Fe(III)porphyrin-coatedmicrocantilever for consecutive cycles of CO and N2 .

rate. It can be clearly seen that the cantilevers did not respondto either of the gasses. Similarly, the cantilevers functional-ized with Fe(III)porphyrin were exposed to alternating cyclesof CO and N2 (500 SCCM) and the response is as shown inFig. 4(b). From the figure, it is clear that there is an abruptincrease in the sensor’s response due to CO adsorption. A volt-age output of ∼80 mV was observed on each alternating cycle,which shows the recovery and repeatability of the sensor. Thesensor response and recovery times have been measured to be1 and 2 s. This clearly demonstrates that only porphyrin func-tionalized microcantilevers respond to CO gas, while the barecantilevers do not respond to the CO exposure. The overall timeresponses demonstrate that Fe(III)porphyrin-coated films showa very good response to CO at room temperature and ambientconditions.

The Fe(III)porphyrin functionalized cantilevers were thenexposed to various concentrations of CO ranging from 7 to70 sccm and the response was plotted as shown in the Fig. 5.The porphyrin functionalized sensor was also tested for its se-lectivity by exposing it to various other gases such as CO2 ,O2 , N20, and ethanolamine (300 sccm). Fig. 6 shows the re-sponse indicating that the Fe(III)porphyrin functionalized can-tilevers did not respond to the other gases. Output response of thesame cantilever for 300 sccm CO exposure was around 12 mV,depicting the high selectivity of the sensor toward CO. Fur-ther, Fe(III)porphyrin and CO binding was analyzed by Fouriertransform infrared spectroscopy (FTIR). The FTIR spectra ofFe(III)porphyrin coated on Si before and after CO exposureis shown in Fig. 7. In the spectrum, an IR peak is observed


Fig. 5. Response of porphyrin functionalized SU-8/CB microcantilevers fordifferent CO flow rates.

Fig. 6. Response of porphyrin functionalized SU-8/CB microcantilevers fordifferent gases.

Fig. 7. FTIR spectroscopy of Fe(III)porphyrin on Si before and after COexposure.

around 1820 cm−1 after CO exposure, which is because of themetal–CO binding frequency. This indicates that CO interactswith the central metal ion of porphyrin, by forming a coordinatecovalent bond with Fe with its lone pair of electrons [14], [21].

Moreover, microcantilevers coated with free base tetraphenyl porphyrin (H2TPP) and zinc(II)tetra phenyl porphyrin(Zn(II)TPP) were tested for CO selectivity. These cantileversdid not show any sensing behavior toward CO. This is becauseof the lack of CO binding site (no metal) in freebase porphyrinand group 12 metals such as zinc. In Zn(II)TPP, Zn does notbind to CO as its d-orbitals are not really available. This con-

firms that the Fe(III)porphyrin reported in this paper is efficientin CO binding and the functionalized cantilevers are suitable forCO sensing applications. Moreover, porphyrin molecules areknown to be stable in solution as well as in solid-state form,for the long-term operation of these sensors [22], [23]. In addi-tion, moisture effect on SU8 devices can be overcome with dif-ferential measurements and through postfabrication processesinvolving hard baking. Such baking for longer times is knownto increase the polymer’s cross-linking density, which, in turn,decreases the sensitivity of the material to the environmentalhumidity [24], [25].

IV. CONCLUSION

In conclusion, microcantilever-based CO sensor has been fab-ricated based on Fe(III)porphyrin-coated SU-8/CB cantilever.Experimental results indicate that the Fe(III)porphyrin-coatedcantilevers have a very high sensitivity toward CO as comparedto bare SU-8 cantilevers. In addition, Fe(III)porphyrin-coatedcantilever’s selectivity toward CO was compared by measuringthe response with gases such as N2 , CO2 , O2 , ethanolamine,N2O, and moisture.

ACKNOWLEDGMENT

The authors would like to thank Prof. P. Mathur, Departmentof Chemistry, IIT Bombay, for CO facility.

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C. Vijaya Bhaskar Reddy received the B.E. degreefrom S. V. University, Tirupathi, and the Masters de-gree from J.N.T.University, Hyderabad, India. He iscurrently working towards the Ph.D. degree with theJ.N.T University, Anantapur, India.

He is presently working in SKIT Engineering col-lege Srikalahasthi, India as an Assistant Professor.His research interests are automotive sensors sys-tems, bioinstrumentation, nanofabrication, thin andthick film sensor systems.

Mrunal A. Khaderbad received the M.Tech. degreefrom VNIT, Nagpur and the M.Phil. from the Uni-versity of Cambridge, U.K. Currently, he is pursuingthe Ph.D. degree in Centre of Excellence in Nano-electronics, Electrical Engineering Department, IITBombay, India.

He was a Visiting Researcher at the Energy Re-search Institute at the NTU (Eri@N), Nanyang Tech-nological University, Singapore. His research areasare Cu/low-k interconnects, NEMS, Sensors, andgraphene electronics.

Sahir Gandhi is working toward the Ph.D. degree atImperial College London, U.K. advised by Dr. DannyO.Hare and Prof. Martyn Boutelle at the Bioengineer-ing department.

He is a postgraduate in biomedical engineeringfrom Imperial College London. He worked at IIT-Bas a Sr. Research Assistant on Lab-on-Chip devices.

Manoj Kandpal received the M.Tech degree in ma-terials science and engineering from the Indian Insti-tute of Technology (IIT) Kharagpur, India, in 2008.He is currently a Ph.D. student in the Department ofElectrical Engineering, IIT Bombay, India.

His research interests includes piezoelectricnanocomposite based devices and chemical sensors.

Sheetal Patil received the Ph.D. degree from the University of Pune, India, in2004.

She has worked as a Research Scientist and Research Faculty, at Departmentof Electrical Engineering, University of South Florida, Tampa and Bio-MEMSGroup, Department of Mechnical Engg., University of Maryland, College Park,respectively. She has published more than 25 peer reviewed journal papers in thefields of Microfabrication and Chem-Bio sensors. She is currently working as a‘Lead Manager (R&D)’ in ‘NanoSniff Tech. Pvt. Ltd.’, a company incubated atIIT-Bombay, Mumbai, India.

K. Narasaiah Chetty graduated in mechanical en-gineering from S.V. University Tirupathi, India andreceived the doctoral degree from Indian Institute oftechnology Madras.

After the Ph.D. degree he joined as an AssistantProfessor at J.N.T. University, Anantapur, India. He isa life member in Indian Society Technical Education,Solar Energy Society of India and Indian Society ofHeat and Mass Transfer.


K. Govinda Rajulu received the doctoral degreefrom the Indian Institute of Technology, Roorkee,India, in 1993.

After the Ph.D. degree he joined as an AssistantProfessor at J. N. T. University, Anantapur, India. Heis Fellow of Institutional of Engineers, INDIA, holdsCharted Engineer (INDIA) Certificate and Life mem-ber of Indian Society for Technical Education.

P. C. K. Chary received the B.Tech, M.E., Ph.D.,and MISTE degrees.

He is working as a Principal for SreeVidyanikethan Engineering College, Tirupati, In-dia. He did his Ph.D. in CAD/CAM from SriVenkateswara University, Tirupati, India. He haspublished 22 papers so far at National and Inter-national level. His research interests are IntelligentCAD/CAM Systems, MEMS, FlexibleManufactur-ing Systems and Applications of Computers in Man-ufacturing.

M. Ravikanth M. Ravikanth was born in AndhraPradesh, in 1966. He received the B.Sc. and M.Sc.degrees from Osmania University, Hyderabad, indiaand the Ph.D. degree from the Indian Institute ofTechnology, Kanpur, India, in 1994.

After his postdoctoral stay in USA and Japan, hejoined as a faculty at Indian Institute of Technology,Bombay, where he is currently a full Professor. Hiscurrent research interest includes porphyrin and re-lated macrocycles, and boron dipyrromethenes.

V. Ramgopal Rao (M’98–SM’02) is an InstituteChair Professor in the Department of Electrical En-gineering, IIT Bombay. He has over 300 publicationsin the area of Electron Devices & Nanoelectronics inrefereed international journals and conference pro-ceedings and has 16 patents issued or pending.

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