RESEARCH ARTICLE Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America SENSOR LETTERS Vol. 5, 1–5, 2007 Enhanced Sensitivity of Polyphenylacetylene and Poly[Phenylacetylene-(Co-2-Hydroxyethyl Methacrylate)] Nanobeads to Humidity Iole Venditti 1 , A. Bearzotti 2 ∗ , A. Macagnano 2 , M. V. Russo 1 1 Chemistry Department of Università of Rome La Sapienza, P.le A. Moro5, 00189 Rome, Italy 2 I.M.M.-C.N.R., Research Area Tor Vergata, Via del Fosso del Cavaliere 100, 00133 Rome, Italy (Received: 29 May 2007. Accepted: 25 June 2007) Humidity sensors based on nanobeads of conjugated polymers, i.e., polyphenylacetylene (PPA) and poly[phenylacetylene-(co-2-hydroxyethyl methacrylate] P(PA/HEMA), prepared by modified emulsion polymerization technique, have been developed using resistive type and quartz crystal microbalance (QCM) devices. The investigations on the electrical response of PPA and P(PA/HEMA) nanobeads were performed and resistive humidity sensors based on ordered arrays of these nanos- tructured polymeric materials showed fast and reproducible current intensity variations in the range of relative humidity RH 5–90%. Polymeric nanostructured films deposited on the quartz electrode of QCM showed a linear sensitivity of 0.024 Hz/ppmv and short response time. Keywords: Resistive Humidity Sensor, Quartz Crystal Microbalance (QCM), Nanostructured Polymers, Polyphenylacetylene. 1. INTRODUCTION In recent years, noticeable efforts have been dedicated to the research for the development of sensors that allow the detection of gases and in particular of humidity. 1–14 New strategies are focussing on the fabrication of devices that are fast, non destructive and low cost. In this perspective various types of sensors are reported in the literature and among them some attractive ones are resistive sensors and quartz crystal microbalance (QCM). The first types of sen- sors are based on the change of the electrical resistance of semi-conductive films when exposed to vapors; the second type shows high sensitivity and selectivity to little vari- ation of mass when particular coatings are realized. The application of polymers or copolymers as sensing mem- branes, compared to the more often used metal oxides, 4–7 is advantageous because these materials offer greater pos- sibilities of structural variation and process ability. 11–14 It is note worthy that the chance of preparing these materials in nanostructured fashion leads to the rise of new properties and functionalities that can be modulated by a selective control of morphology, dimensions and assem- bling of the nanoparticles. 15–19 ∗ Corresponding author; E-mail: andrea.bearzotti@imm.cnr In this context, polyphenylacetylene (PPA), a versatile material with luminescent and semiconducting properties, and its copolymer P(PA/HEMA), (PA = phenylacetylene, HEMA = 2-hydroxyetylmethacrylate), were recently pre- pared in the form of nanospheres 15 with a reduced dispersion; these polymeric nanoparticles were success- fully tested for applications in optoelectronics 20 and biotechnology. 21 Our first attempts to use PPA as active membrane for humidity sensors were based on the preparation of films of amorphous iodine-doped PPA. 22 These studies demon- strated that good reproducibility and stability of the conductivity variations in the range 20–90% could be achieved and the mechanism of interaction between PPA and H 2 O molecules was interpreted on the basis of XPS studies performed on PPA and substituted polyacetylene membranes. 23 In this research work we extended our investigations with the intent to improve the sensitivity and the selec- tivity of our materials for the development of resis- tive and QCM humidity sensors. The achievement of nanostructures represents a new way to enhance the superficial area, where the interaction between poly- mer and gas occurs; moreover, peculiar electric prop- erties, that descend from nano-morphology, can be Sensor Lett. 2007, Vol. 5, No. 3 1546-198X/2007/5/001/005 doi:10.1166/sl.2007.227 1