Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee Research paper Single-walled carbon nanotubes-calixarene hybrid for sub-ppm detection of NO 2 ☆ Tapan Sarkar a,b, ⁎ , Sira Srinives a,c a Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521, USA b University School of Chemical Technology, Guru Gobind Singh Indraprastha University, Dwarka, Sector 16C, New Delhi 110078, India c Chemical Engineering Department, Mahidol University, 25/25 Puttamonthon 4 Road, Nakorn Pathom 73170, Thailand ARTICLE INFO Keywords: Carbon nanotubes Calixarene NO 2 Chemiresistor Gas sensor ABSTRACT Calixarenes are exciting class of organic macromolecules and proved to be an excellent sensing material for optical and mass transaction based sensors. The limited conductivity of calixarenes is a major obstacle for the development of calixarene-only chemiresistive sensors. Here we report on a single-walled carbon nanotube (SWNT)-calixarene based chemiresistive senor for room temperature detection of NO 2 that is based on a hybrid material obtained by non-covalent functionalization of SWNTs with calixarene. This has two beneficial effects: (i) the use of SWNTs eliminates the conductivity issue, and this enables low-power chemiresistive sensing; (ii) the excellent affinity of calixarenes for certain analytes improves sensitivity. The hybrid was fabricated by solvent casting technique and its formation was confirmed through structural (SEM and TEM) and electrical (I D - V D and I D -V G ) characterizations. The hybrid exhibited higher sensitivity to NO 2 gas at concentrations as low as 0.25 ppm when compared to pristine SWNTs. The sensitivity and the limit of detection (LOD) of the hybrid was found to be ~283/ppm of NO 2 and ~25 ppb of NO 2 respectively. An improvement of the response time of the sensor was observed and the average response time of the hybrids was found to be < ~1 min for all con- centration of NO 2 exposure more than 1 ppm. 1. Introduction Single-walled carbon nanotubes (SWNTs) have gained considerable attention as materials for chemical sensor due to their excellent elec- tronic properties and chemical stability at ambient condition [1,2]. SWNTs are often functionalized through noncovalent [3–5] or covalent [6] functionalization with suitable recognition molecules to impart sensitivity or selectivity towards specific analyte [7]. In particular, noncovalent functionalization can be achieved easily through π-π in- teractions almost without disrupting the electronic properties of the SWNTs [4]. However, covalent functionalization alters the electronic properties of the SWNTs significantly [6]. Calixarenes are organic macro-cyclic compounds and attractive choices as functional material for noncovalent functionalization of SWNTs with highly polarizable aromatic ring structure that interacts strongly with π-conjugated graphenic sidewalls. Calixarenes have shown promises for use in high-sensitivity chemical sensors, typically with optical- [8,9] or mass-based [10] transduction. However, despite their promise in sensing applications, the application of calixarene for the development of chemiresistive sensor remains relatively unexplored due to their limited electrical conductivity [11]. In this regard, the use of calixarene functionalized SWNT hybrids has two beneficial effects: (i) the use of SWNTs eliminates the conductivity issue, and this enables low-power chemiresistive sensing; and (ii) the affinity of calixarene for certain analytes improves sensitivity. It is important to mention that the sensitivity of the hybrid may be tuned by manipulating the calixarene- analyte interaction through the selection of the right kind of calixarene having different cavity size and/or different end-terminal functional group while making the hybrid. Sensor selectivity is one of the major issues for the development of reliable sensor. The sensitivity issue can be resolved by choosing the proper recognition molecule that will bind only with the analyte. Alternatively, the selectivity issue can be addressed by building a sensor array consisting of multiple numbers of electrodes that uses different kinds of recognition molecule along with suitable electronic hardware and chemometric/pattern recognition software. The resulting sensor array generates a response pattern on interacting with analytes that may be used/analyzed by chemometrics to identify the analyte [12]. It is noteworthy to mention that the availability of wide variety of calixarene generates the possibilities of development of only calixarene https://doi.org/10.1016/j.mee.2018.05.004 Received 30 November 2017; Received in revised form 9 May 2018; Accepted 23 May 2018 ☆ The Chemical and Environmental Engineering, University of California, Riverside, CA 92507, USA. ⁎ Corresponding author at: University School of Chemical Technology, Guru Gobind Singh Indraprastha University, Dwarka, Sector 16C, New Delhi 110078, India. E-mail address: tapan@ipu.ac.in (T. Sarkar). Microelectronic Engineering 197 (2018) 28–32 Available online 25 May 2018 0167-9317/ Published by Elsevier B.V. T