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. 2020 Mar 25;20(7):1824.
doi: 10.3390/s20071824.

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

Alberto Prud'homme  1 , Frederic Nabki  1
Affiliations

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

Alberto Prud'homme et al. Sensors (Basel). .

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%.

Keywords: CO2 sensor; coatings; gas sensor; humidity sensor; mass sensor; micro-resonator; polyethylenimine; reduced graphene oxide.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
(a) Branched polyethylenimine molecule [33], and (b) lineal polyethylenimine molecule [34].
Figure 2
Figure 2
Coating deposition technique, (a) spin coater for the quartz crystal used for initial coating tests, (b) micro-drop deposition by piezo probe used for the micro-resonator sensor.
Figure 3
Figure 3
Reduced graphene oxide by microwave reduction.
Figure 4
Figure 4
SEM image of the quartz crystal coated with the (a) branched PEI layer, (b) linear PEI layer and (c) rGO with linear PEI layer.
Figure 5
Figure 5
Infrared (IR) absorption spectroscopy result for the three deposited coatings on the quartz crystals.
Figure 6
Figure 6
Diagram of the characterization setup.
Figure 7
Figure 7
(a) Setup for the quartz crystals and the resonator characterization, (b) close-up of the micro-resonator during the characerizacion with the laser from the vibrometer visible on the micro-resonator die.
Figure 8
Figure 8
Cross-section of the micro-resonator with oxide as sacrificial layer.
Figure 9
Figure 9
(a) Three-dimensional (3D) view of the real micro-resonator obtained by the Olympus (Shinjuku, Tokyo, Japón) LEXT OLS4000-Confocal microscope, (b) Eigenfrequency FEM COMSOL simulation of the first mode of resonance of the microelectromechanical systems (MEMS) resonator.
Figure 10
Figure 10
Quartz crystal frequency shift in response to different CO2 concentration cycles for (a) branched PEI coated crystal, and (b) linear PEI coated crystal.
Figure 11
Figure 11
Frequency shift of branched PEI and linear PEI coated crystals at different humidity levels while in absorption and recovery.
Figure 12
Figure 12
Frequency shift of the quartz crystal with linear PEI with rGO in response to different CO2 concentration.
Figure 13
Figure 13
Response of the three different coatings at CO2 concentrations of (a) 0.1%, (b) 0.5%, and (c) 1%.
Figure 14
Figure 14
Frequency shift of each coating from 0.05% to 1% of CO2.
Figure 15
Figure 15
(a) Adsorption time for each coating at different CO2 concentrations, (b) recovery time for each coating at different CO2 level. The recovery time for the linear PEI with rGO reflects only the time to stabilize the desorption to the captured CO2 value.
Figure 16
Figure 16
Frequency shift of the coatings at different range of air humidity from 15 to 75 %RH.
Figure 17
Figure 17
Response of the coated micro-resonator to a 0.8% CO2 concentration variation.
Figure 18
Figure 18
Behavior of the micro-resonator over time at different concentrations of CO2.
See this image and copyright information in PMC

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