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CAPACITANCE-TYPE CHEMICAL SENSORS 255 · · and outdoor air quality monitoring, climate monitoring, and agricultural monitoring (e.g., in viniculture). In addition, capacitance-type sensors are the most likely candidates to be adopted in RFID-based sensing, where a sensor could be integrated with RFIDs to allow for remote power- ing and readout of chemical sensors. Sensor arrays: Several devices may easily be fabricated on the same chip, but more important, their sensitivity and selectivity may easily be modified among the different sensors in the array by simply changing the chemically sensitive material or the geometry of the devices. Sensor performance: The performance of capacitance-type chemical sensors is expected to be im- proved, for example, by changing the geometry of interdigital electrode sensors (Kummer and Hierlemann 2006). Use of novel materials, on the other hand (e.g. carbon nanotubes or silicon nanowires) will also improve selectivity and sensitivity. In addition, new operating principles in capacitive chemical sensing based on ultrathin membranes promise to open new horizons in the limit of detection. REFERENCES Amírola J., Rodríguez A., Castañer L., Santos J.P., Gutiérrez J., and Horrillo M.C. (2005) Micromachined silicon microcantilevers for gas sensing applications with capacitive read-out. Sens. Actuators B 111­112, 247­253. Boltshauser T. and Baltes H. (1991a) Capacitive humidity sensors in SACMOS technology with moisture absor- bing photosensitive polyimide. Sens. Actuators A 26, 509­512. Boltshauser T., Chandran L., Baltes H., Bose F., and Steiner D. (1991b) Humidity sensing properties and electrical permittivity of new photosensitive polyimides. Sens. Actuators B 5, 161­164. Boutopoulos C., Tsouti V., Goustouridis D., Chatzandroulis S., and Zergioti I. (2008) Liquid phase direct laser printing of polymers for chemical sensing applications. Appl. Phys. Lett. 93, 91109. Brahim S., Colbern S., Gump R., and Grigorian L. (2008) Tailoring gas sensing properties of carbon nanotubes. J. Appl. Phys. 104, 024502. Britton C.L. Jr., Jones R.L., Oden P.I., Hu Z., Warmack R.J., Smith S.F., Bryan W.L., and Rochelle J.M. (2000) Multiple-input microcantilever sensors. Ultramicroscopy 82, 17­21. Buchold R., Nakladal A., Gerlach G., and Neumann P. (1998) Design studies on piezoresistive humidity sensors. Sens. Actuators B 53(1­2), 1­7. Burg T.P., Mirza A.R., Milovic N., Tsau C.H., Popescu G.A., Foster J.S., and Manalis S.R. (2006) Vacuum- packaged suspended microchannel resonant mass sensor for biomolecular detection. J. Microelectromech. Syst. 15, 1466­1476. Casalini R., Kilitziraki M., Wood D., and Petty M.C. (1999) Sensitivity of the electrical admittance of a poly- siloxane film to organic vapors. Sens. Actuators B 56, 37­44. Cha M., Shin J., Kim J.-H., Kim I., Choi J., Lee N., Kim B.-G., and Lee J. (2008) Biomolecular detection with a thin membrane transducer. Lab Chip 8, 932­937. Chandran L., Baltes H., and Korvink J. (1991) Three-dimensional modeling of capacitive humidity sensors. Sens. Actuators A 25, 243­247. Chang S.-M., Iwasaki Y., Suzuki M., Tamiya E., Karube I., and Muramatsu H. (1991) Detection of odorants using an array of piezoelectric crystals and neural-network pattern recognition. Anal. Chim. Acta 249, 323­329. Chatzandroulis S., Tegou E., Goustouridis D., Polymenakos S., and Tsoukalas D. (2004) Capacitive-type chemical sensors using thin silicon-polymer bimorph membranes. Sens. Actuators B 103, 392­396.