Methylated Poly(ethylene)imine Modified Capacitive Micromachined Ultrasonic Transducer for Measurements of CO2 and SO2 in Their Mixtures

Abstract

A gravimetric gas detection device based on surface functionalized Capacitive Micromachined Ultrasound Transducers (CMUTs) was designed, fabricated and tested for detection of carbon dioxide (CO2) and sulfur dioxide (SO2) mixtures in nitrogen. The created measurement setup of continuous data collection, integrated with an in-situ Fourier Transform Infrared (FT-IR) spectroscopy, allows for better understanding of the mechanisms and molecular interactions with the sensing layer (methylated poly(ethylene)imine) and its need of surface functionalization for multiple gas detection. During experimentation with CO2 gases, weak molecular interactions were observed in spectroscopy data. Linear sensor response to frequency shift was observed with CO2 concentrations ranging from 0.16 vol % to 1 vol %. Moreover, the Raman and FT-IR spectroscopy data showed much stronger SO2 and the polymer interactions, molecules were bound by stronger forces and irreversibly changed the polymer film properties. However, the sensor change in resonance frequency in the tested region of 1 vol % to 5 vol % SO2 showed a linear response. This effect changed not only the device resonance frequency but also affected the magnitude of electroacoustic impedance which was used for differentiating the gas mixture of CO2, SO2, in dry N2.

Mass Sensors Based on Capacitive and Piezoelectric Micromachined Ultrasonic Transducers—CMUT and PMUT

“…Employing these technologies provides batch fabrication, high yield and uniformity, ability for array fabrication, and an ease of circuit integration due to the material properties of silicon as the substrate [11,16]. The two most common CMUT fabrication techniques are the sacrificial release process [19], and the wafer bonding process [4], both of which are investigated and discussed in this paper. Utilizing advanced microfabrication technology, individual CMUT cells can be fabricated to form an array of sensors with various arrangements including circular, square, O-ring, as well as hexagonal shapes [20][21][22].…”

“…In the spin coating technique, the polymer is diluted into a proper solvent, then it is spin coated within a specific time to achieve a uniform and desirable polymer thickness. This technique is used in functionalizing the CMUT structure with different polymers such as methylated (polyethylene-imine) (mPEI), which acts as a detector of CO 2 and SO 2 [4].…”

“…Surface functionalization using polymers produces gas-sensitive films which alter the resonant frequency of the device due to their imposed mass on the device when applied to the surface of a sensor [4]. Further mass changes are expected during sensing upon interaction of the analytes with the polymer, which ultimately leads to adsorption [17].…”

“…MEMS-based mass resonant sensors benefit from a low power consumption, a high selectivity and a low limit of detection (LOD) whilst being integrable in a wide range of applications [1]. Amongst them, micromachined ultrasonic transducers (MUT) are shown as potential candidates in sensing applications [4][5][6]. In an unconventional approach, these devices can be employed in gas sensing technology due to their unique structures that provide design flexibility and contribute to their potential high-performance capabilities.…”

“…MUT-sensors operate based on a change in the mass of their sensing components when used for gas sensing and volatile organic compound (VOC) detection. In addition, MUT devices benefit from advanced micro fabrication technologies, and therefore, these micromachined-sensors can be fabricated in an array structure using various sensing materials in order to improve their selectivity in complex environments [4,5,7]. A common…”