Talanta 81 (2010) 248–251 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Humidity adsorption kinetics of calix[4]arene derivatives measured using QCM technique Salih Okur a,∗ , Mahmut Kus ¸ b,∗∗ , Faruk Özel b , Vildan Aybek b , Mustafa Yılmaz c a Izmir Institute of Technology, Department of Physics, Urla, Izmir, Turkey b Selc ¸ uk University, Department of Chemical Engineering, Selc ¸ uklu, Konya, Turkey c Selc ¸ uk University, Department of Chemistry, Selc ¸ uklu, Konya, Turkey article info Article history: Received 29 May 2009 Received in revised form 22 November 2009 Accepted 29 November 2009 Available online 5 December 2009 Keywords: Calix[4]arene Humidity sensor Quartz crystal microbalance abstract This study focuses on the characterization of sulphonated calix[4]arene derivative films coated on a quartz substrate with a thickness of 40 nm by spin coating method for humidity detection. The humidity adsorption kinetics of the sulphonated calix[4]arene films was investigated by quartz crystal microbal- ance (QCM) technique. The Langmuir model was used to determine the adsorption rates and Gibbs free energy for various relative humidities between 11% and 97%. Our reproducible experimental results show that suphonated calix[4]arene films have a great potential for humidity sensing applications at room temperature operations. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Calixarenes are macrocyclic molecules consisting of several phe- nol units connected via methylene bridges and they can be easily functionalized. Many studies with these molecules show that the cylindrical-shaped calixarenes of varying cavity sizes can form a variety of host–guest types of inclusion complexes similar to cyclodextrins. However, – interaction is observed in calixarenes due to the benzene groups [1]. Water-soluble calixarenes have been also synthesized and reported by many authors in the literature. Shinkai et al. synthesized water-soluble calixarenes carrying sul- fonate groups in 1984 [2]. Thin films of calixarene derivatives have been widely used as chemical sensors, due to their zeolite-like capacity and selectiv- ity. Calixarenes became promising materials for sensor applications due to functional groups at the upper and lower rims, that determine the selectivity in host–guest interactions and physical properties [3–4]. They have been also used as gas or organic solvent vapour sensors [5–7]. Quartz crystal microbalance (QCM) [8–10] and ion-sensitive field-effect transistors (ISFETs) [11] technique have been reported as sensor applications. QCM have been widely employed for the ∗ Corresponding author. Tel.: +90 2327507706. ∗∗ Corresponding author. Tel.: +90 3322232131. E-mail addresses: salihokur@iyte.edu.tr (S. Okur), mahmut kus@yahoo.com (M. Kus ¸ ). determination of metals deposited onto the crystal and inves- tigations of the kinetics of adsorption of adsorbate molecules. QCM technique is a powerful technique for determining the sens- ing properties of materials before a sensor device design during development stages. Although it is known that some of calixarene derivatives adsorb water molecules [12], we have not reached any report on humidity sensing properties, adsorption kinetics of cal- ixarene based on QCM techniques. In this work, we have used QCM technique to investigate sensing properties of calix[4]arene films with carboxylic and sulphonic acid groups which are sensitive to water molecules. QCM is based on frequency shift of coated quartz crystal with sensing element due to adsorption of humidity atoms on the surface of the sensing material [13]. The mass change (m) on surface of the quartz crystal is calculated by using Sauerbrey equation [13] from the frequency change (f) m =- A √  2f 2 0 × f =-C × f (1) where f 0 is the resonant frequency of the fundamental mode of the QCM crystal, A is the area of the gold disk coated onto the crystal,  is the density of the crystal, and  is the shear modulus of quartz. Hence the frequency shift is directly proportional to the adsorbed mass on the calix[4]arene modified gold QCM electrodes. Calix[4]arene films were prepared on gold coated QCM elec- trodes using spin coating method at 2000 rpm. The change of resonance frequency of QCM was monitored with increasing rel- ative humidity (RH) up to 97%. The adsorption kinetic parameters 0039-9140/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2009.11.065