Layer by Layer Optimization of Langmuir–Blodgett Films for Surface Acoustic Wave (SAW) Based Sensors for Volatile Organic Compounds (VOC) Detection

Abstract

Rayleigh surface acoustic wave (RSAW)-based resonant sensors, functionalized with single and multiple monomolecular layers of Langmuir–Blodgett (LB) films, were thickness and density optimized for the detection of volatile organic compounds (VOC), which could impose a serious threat on the environment and human health. Single layers of a phospholipid (SLP), hexane dissolved arachidic acid (HDAA), and chloroform dissolved arachidic acid (CDAA) were used for the LB film preparation. Several layers of these compounds were deposited on top of each other onto the active surface of high-Q 434 MHz two-port RSAW resonators in a LB trough to prepare a highly sensitive vapor detection quartz surface microbalance (QSM). Frequency shift was measured with a vector network analyzer (VNA). These devices were probed with saturated vapors of hexane, chloroform, methanol, acetone, ethanol, and water after each deposited layer to test the behavior of the QSM’s insertion loss, loaded Q, vapor sensitivity, and to find the optimum trade-off between these parameters for the best real-life sensor performance. With 2200 ppm and 3700 ppm sensitivity to chloroform, HDAA and CDAA coated QSM devices reached the optimum sensor performance at 15 and 11–15 monolayers, respectively. Surface pressure optimized single monolayers of phospholipid LB films were found to provide up to 530 ppm sensitivity to chloroform vapors with a negligible reduction in loss and loaded Q. This vapor sensitivity is higher than the mass of the sensing layer itself, making SLP films an excellent choice for QSM functionalization.

Langmuir-Blodgett Films of Arachidic and Stearic Acids as Sensitive Coatings for Chloroform HF SAW Sensors

“…We study the influence of the layer’s number on the morphology of the LB film of SA, and obtain evidence that the 17-layer films are optimum for sensing. This result was close to the optimum number of layers for LB film of AA reported earlier [ 17 ]. The concentration dependencies for chloroform are measured for two acoustic devices, namely, for a two-port Raleigh type SAW resonator (414 MHz) based on AT-quartz and for a SAW delay line (157.5 MHz) ST-X based on ST-quartz.…”

“…In this work, a two-port Rayleigh-type surface acoustic wave resonator (RSAW) implemented on AT, X-quartz (Eugler angles 0°, 125°, 0°) with gold electrodes and a central frequency of 414 MHz was used [ 17 ]. Figure 2 shows a schematic view (a) and an optical photo of the central part of the resonator (b) obtained by an optical confocal microscope Lext OLS5000 (Olympus Corp., Tokyo, Japan).…”

“…At present, few sensor configurations for detecting volatile organic compounds (VOCs), including chloroform, have already been developed. They are based on the bulk (BAW) and surface (SAW) acoustic wave resonators implemented on AT and ST quartz substrates, operating at 9 MHz to 435 MHz frequency range [ 8 , 9 , 10 , 11 , 12 ], the SAW delay lines (f = 38 MHz) implemented on lithium niobate crystal [ 13 ], the two-port SAW resonators (f = 434 MHz) [ 14 , 15 , 16 , 17 ], the film bulk acoustic resonators (FBAR) [ 18 ], the Love SAW devices [ 19 ], etc. The main problem for any sensor configuration and any type of sensor is a sorbent film whose physical properties should be changed remarkably and selectively in presence of chloroform vapor.…”

“…In our previous work, Langmuir-Blodgett (LB) monolayer films fabricated from dipalmytoyl phosphatidyl ethanolamine head labeled with nitrbenzoxadiazole (DPPE-NBD) on a two-port SAW resonator (f = 411 MHz) [ 17 , 23 ] have demonstrated extremely high sensitivity to chloroform. Actually, the absorbed chloroform mass was bigger than the mass of the sensing monolayer mass, which is very rare [ 17 ]. However, it is known that phospholipids are difficult to deposit as a multilayer, which was no exception for the DPPE-NBD molecules [ 24 ].…”

One-Step Hydrothermal Synthesis of 3D Interconnected rGO/In2O3 Heterojunction Structures for Enhanced Acetone Detection

“…Recently, many kinds of sensors based on optical, electric, gravimetric transduction techniques, etc., have been developed for detecting VOCs [5,6]. Avramov et al presented an optimization-mass-sensitive sensor using a surface acoustic wave (SAW)-based twoport resonator as a highly sensitive quartz crystal microbalance (QCM), which exhibited excellent VOC-sensing properties [7]. Kanawade et al reported a new negative-axicon fiberoptic sensor platform, which exhibits many desirable characteristics for sensing various VOCs at room temperature [8].…”