Polyethyleneimine-Starch Functionalization of Single-Walled Carbon Nanotubes for Carbon Dioxide Sensing at Room Temperature

Abstract

There is an ever-growing interest in the detection of carbon dioxide (CO₂) due to health risks associated with CO₂ emissions. Hence, there is a need for low-power and low-cost CO₂ sensors for efficient monitoring and sensing of CO₂ analyte molecules in the environment. This study reports on the synthesis of single-walled carbon nanotubes (SWCNTs) that are functionalized using polyethyleneimine and starch (PEI-starch) in order to fabricate a PEI-starch functionalized SWCNT sensor for reversible CO₂ detection under ambient room conditions (T = 25 °C; RH = 53%). Field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy are used to analyze the physiochemical properties of the as-synthesized gas sensor. Due to the large specific surface area of SWCNTs and the efficient CO₂ capturing capabilities of the amine-rich PEI layer, the sensor possesses a high CO₂ adsorption capacity. When exposed to varying CO₂ concentrations between 50 and 500 ppm, the sensor response exhibits a linear relationship with an increase in analyte concentration, allowing it to operate reliably throughout a broad range of CO₂ concentrations. The sensing mechanism of the PEI-starch-functionalized SWCNT sensor is based on the reversible acid-base equilibrium chemical reactions between amino groups of PEI and adsorbed CO₂ molecules, which produce carbamates and bicarbonates. Due to the presence of hygroscopic starch that attracts more water molecules to the surface of SWCNTs, the adsorption capacity of CO₂ gas molecules is enhanced. After multiple cycles of analyte exposure, the sensor recovers to its initial resistance level via a UV-assisted recovery approach. In addition, the sensor exhibits great stability and reliability in multiple analyte gas exposures as well as excellent selectivity to carbon dioxide over other interfering gases such as carbon monoxide, oxygen, and ammonia, thereby showing the potential to monitor CO₂ levels in various infrastructure.

Figure 1

(a) Schematic diagram for the fabrication of a PEI-starch-functionalized SWCNTs gas sensor device; (b) Experimental setup for sensor testing.

Figure 2

(a–b) SEM and TEM micrographs of the as-grown pristine SWCNTs, respectively (inset show the SAED pattern); (c–d) SEM and TEM micrographs for the PEI-starch-sfunctionalized SWCNTs.

Figure 3

(a) Raman spectra of as-grown pristine SWCNTs, and PEI-starch-functionalized SWCNTs (the inset shows the comparison of RBMs before and after functionalization); (b) FTIR spectra of as-grown pristine SWCNTs, and PEI-starch-functionalized SWCNTs.

Figure 4

I–V characteristics for as-grown pristine SWCNTs and PEI-starch-functionalized SWCNTs under ambient room conditions.

Figure 5

(a) Dynamic sensing responses for the PEI-starch-functionalized SWCNT sensor for a range of CO₂ concentrations (50–500 ppm) at T = 25 °C for V_bias = 12 V; (b) Variation of sensor response (%) with CO₂ concentration and its linear fitted curve.

Figure 6

(a) Real-time sensor response and recovery for 500 ppm CO₂ in the background of N₂ flow under ambient room conditions; (b) and (c) Response and recovery times of the sensor device as a function of CO₂ gas concentration, respectively.

Figure 7

(a) Repeatability of the PEI-starch-functionalized sensor to successive cycles of 500 ppm CO₂ exposure under ambient conditions; (b) Cross-sensitivity histogram of the sensor to different analyte gases at room temperature.

Figure 8

Schematic representation of the proposed CO₂ sensing mechanism under dry conditions (a); and in the presence of hygroscopic starch (b).