Atomic force microscope microcantilevers used as sensors for monitoring humidity C. Steffens a,b,⇑ , A. Manzoli b , F.L. Leite a,c , O. Fatibello a,d , P.S.P. Herrmann a,b a Department of Biotechnology, Federal University de São Carlos (UFSCar), SP 13565-905, Brazil b National Nanotechnology Laboratory for Agribusiness, Embrapa Instrumentation, São Carlos, SP 13560-970, Brazil c Department of Physics Mathematics and Chemistry (DFMQ), Federal University of São Carlos (UFSCar), Sorocaba, SP 18052-780, Brazil d Department of Chemistry, Federal University of São Carlos (UFSCar), São Carlos, SP 13565-905, Brazil article info Article history: Received 20 January 2013 Received in revised form 8 July 2013 Accepted 23 July 2013 Available online 2 August 2013 Keywords: Microcantilever sensor Humidity Polyaniline Sensitivity abstract A microcantilever sensor is presented. Functionalization of the cantilever with a polyaniline (PANI) sen- sitive layer and its use as a humidity sensor were investigated. Polyaniline was produced by interfacial synthesis and the sensitive layer was deposited on the microcantilever surface by the spin-coating method. The microcantilever deflection at various levels of relative humidity (RH) was read by means of the optical lever of an atomic force microscope (AFM Veeco Dimension V). A range of RH from 20% to 70% was introduced into the AFM chamber by mixing streams of dry and wet nitrogen. The sensitivity and reversibility of the sensors were assessed at various RH and temperatures (10, 20 and 30 °C). A large deflection was observed in the coated microcantilever sensors, with faster response time at 10 °C and bet- ter sensitivity and reversibility at 30 °C. These results demonstrate that the spin-coated microcantilever can be used as a sensor to detect relative humidity at various different temperatures. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The development of microelectromechanical systems (MEMs) has motivated the construction of new miniature sensing systems of high selectivity and sensitivity [1]. In this context, the cantilever sensors used in atomic force microscopy (AFM) have emerged as very sensitive and selective miniature devices [2–4]. The working principle of microcantilever sensors is based on the adsorption of analytes on a sensitive surface, which usually induces a surface stress and an increase in microcantilever mass [5–7]. These sensors have several advantages, compared to conventional analytical techniques, such as small analyte volume (lL), high sensitivity, low cost, simple and non-hazardous procedures and fast response [3,8]. The sensitivity of these microcantilever sensors depends on the sensitive layer, which reacts with the target molecules [9–11]. Microcantilever sensors used to measure humidity need to show good sensitivity over a wide humidity range, low hysteresis, good reproducibility and durability. A way of potentially satisfying these requirements is to use a thin layer of conductive polymer as the sensitive layer [12,13]. Conducting polymers suffer many changes when exposed to an analyte, such as modifications of the backbone conformation, solvation effects on the polymer chain and the attraction of dopant counter ions or transfer of electrons. Thus, changing the electron mobility of charge carriers and on swelling of the polymer matrix are converted into electrical and/ or mechanical signals [14]. The conducting polymer that is most promising as a sensitive layer is polyaniline (PANI) [15–17]. The great interest in PANI is due to its low cost, ease of synthesis and doping in aqueous solution, environmental stability, electronic properties and moderately high conductivity relative to other con- ducting polymers. PANI is the only conducting polymer whose con- ductivity is controlled by the doping level [15]. In the undoped reduced state, the conductivity is very low but can be increase by ten orders of magnitude or more by exposure to doping acids. This doping process can be reversed during dedoping by exposure to bases [18]. PANI exhibits the fastest adsorption/desorption of va- pors among the tested polymeric materials [19,20], and is a suit- able material to be investigated as a sensitive layer to detect water molecules [21]. Several techniques have been reported for the functionalization of the surface of a microcantilever with polymers. Lahav et al. [12] used an electrochemical method to deposit PANI on the cantilever and detected a deflection on electrochemical oxidation/reduction of the PANI film. Fig. 1 schematically represents the mechanical deflection of the cantilever and the vertical displacement (Dz) of the tip during the redox reactions occurring when exposed to var- ious stimuli. However, this method of deposition is not as simple as the spin-coating technique, in which the conducting polymer 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.07.015 ⇑ Corresponding author at: Department of Biotechnology, Federal University de São Carlos (UFSCar), SP 13565-905, Brazil. Tel.: +55 5435209000; fax: +55 5435209090. E-mail addresses: clarices@uricer.edu.br, claristeffens@yahoo.com.br (C. Stef- fens). Microelectronic Engineering 113 (2014) 80–85 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee