Comparison between Linear and Branched Polyethylenimine and Reduced Graphene Oxide Coatings as a Capture Layer for Micro Resonant CO2 Gas Concentration Sensors

Abstract

The comparison between potential coatings for the measurement of CO2 concentration through the frequency shift in micro-resonators is presented. The polymers evaluated are linear polyethylenimine, branched polyethylenimine and reduced graphene oxide (rGO) by microwave reduction with polyethylenimine. The characterization of the coatings was made by using 6 MHz gold-plated quartz crystals, and a proof-of-concept sensor is shown with a diaphragm electrostatic microelectromechanical systems (MEMS) resonator. The methods of producing the solutions of the polymers deposited onto the quartz crystals are presented. A CO2 concentration range from 0.05% to 1% was dissolved in air and humidity level were controlled and evaluated. Linear polyethylenimine showed superior performance with a reaction time obtained for stabilization after the concentration increase of 345 s, while the time for recovery was of 126 s, with a maximum frequency deviation of 33.6 Hz for an in-air CO2 concentration of 0.1%.

Capacitive Based Micromachined Resonators for Low Level Mass Detection

“…The wheel CCM, as shown in Figure 22 , was used as a gas sensor for low-level mass detection of carbon dioxide. An external opening in the clamped membrane of this structure is provided to reduce damping due to the air flow passing through the plates of the sensor [ 76 ]. Due to the large openings in the wheel CCM, the overall mass of the sensor was reduced, which led to a higher resonant frequency for the structure, as per Equation (18) and Equation (21).…”

“…However, this also led to a lower quality factor for the structure, which can be traded off by an array configuration of the wheel CCMs and averaging the measurements. This structure is coated with linear polyethylenimine (PEI) solution, which is used for carbon dioxide detection due to its high selectivity, low sensitivity to humidity, and linear reversibility [ 76 ].…”

“…This wheel CCM [ 76 ] is fabricated by employing a sacrificial technique, PolyMUMPS [ 77 ]. The mass detection area is at the center of the wheel shaped structure.…”

“…Any added mass like the conventional CCM was detected through a decrease in the resonant frequency of the wheel CCM. The nominal resonant frequency of the fabricated wheel CCM was 1.15 MHz with a quality factor of 8 at atmospheric pressure and 300 at an ambient pressure of 133.32 Pa [ 76 ]. As the PEI coating was applied to the wheel CCM, the structure’s resonant frequency decreased to 1.139 MHz.…”

Branched versus Linear Structure: Lowering the CO2 Desorption Temperature of Polyethylenimine-Functionalized Silica Adsorbents

“…PEI has another structure, which is linear PEI (LPEI). The utilization of LPEI for CO 2 capture was also extensively studied [19][20][21][22][23][24][25][26][27]. The adsorption capacity of LPEI is slightly low compared to BPEI, but LPEI has much better tolerance against oxidative conditions [24].…”