The enhanced humidity sensing performance of calixarene/PMMA hybrid layers: QCM sensing mechanism

Abstract

This paper describes the preparation of the calix-4/PMMA hybrid QCM sensor by electrospinning of calix[4]arene derivative having carboxylic acid moieties (Calix-4) and poly(methyl methacrylate) (PMMA) as the humidity sensing material. The characterization of calix-4/PMMA hybrid sensors was conducted by microscopic surface imaging and contact angle analysis. The proposed sensors were then tested towards different RH environments ranging from 11 to 98% in a closed testing chamber to get response/recovery cycles. Initial experiments revealed that the highest response was observed by calix-4/PMMA (5:10). The sensor sensitivity (S) of the calix-4/PMMA (5:10) coated QCM sensor was obtained as 0.974 Hz/%RH. The response and recovery times of the proposed sensor were found as 20 and 30 s. The sensor's repeatability and durability features were tested in five sequential adsorption/desorption cycles and during a month towards all relative humidity levels, respectively. The adsorption mechanism of humidity molecules was attributed to the hydrogen bonding due to -OH and -C=O groups in the structures of calix-4 and PMMA. Also, the adsorption phenomena were investigated by adapting the experimental results to the bimodal exponential adsorption model. The response and recovery rates were approached by using this model. Consequently, the QCM sensor combined with a suitable platform such as calix-4/PMMA may be a useful humidity detection agent.