Bio-Inspired Carbon Monoxide Sensors with Voltage-Activated Sensitivity

Abstract

Carbon monoxide (CO) outcompetes oxygen when binding to the iron center of hemeproteins, leading to a reduction in blood oxygen level and acute poisoning. Harvesting the strong specific interaction between CO and the iron porphyrin provides a highly selective and customizable sensor. We report the development of chemiresistive sensors with voltage-activated sensitivity for the detection of CO comprising iron porphyrin and functionalized single-walled carbon nanotubes (F-SWCNTs). Modulation of the gate voltage offers a predicted extra dimension for sensing. Specifically, the sensors show a significant increase in sensitivity toward CO when negative gate voltage is applied. The dosimetric sensors are selective to ppm levels of CO and functional in air. UV/Vis spectroscopy, differential pulse voltammetry, and density functional theory reveal that the in situ reduction of Fe^III to Fe^II enhances the interaction between the F-SWCNTs and CO. Our results illustrate a new mode of sensors wherein redox active recognition units are voltage-activated to give enhanced and highly specific responses.

Keywords: Carbon monoxide; carbon nanotubes; iron porphyrin; sensors; voltage-activated.

Figure 1

Carbon monoxide detection by a bio-inspired sensors. Schematic of a field-effect transistor (FET) substrate with Au source-drain electrodes, and Ti adhesion layer deposited on SiO₂ dielectric layer and Si gate electrode. Chemical structures of pyridyl-functionalized single-walled carbon nanotubes (F-SWCNTs) and iron porphyrin (Fe-(tpp)ClO₄), depicting the coordination chemistry of the pyridyl group to the iron center of the porphyrin.

Figure 2

Sensing responses at no gate voltages. a) Average changes in the conductance and standard deviations (N ≥ 6 sensors) in response to 2 min exposures to 200 ppm of CO for F-SWCNTs without Fe(tpp)ClO₄ (black), pristine SWCNTs with Fe(tpp)ClO₄ (green), and three densities of functionalization (red, blue, violet). b) Conductance changes of F-SWCNT-1 with Fe(tpp)ClO₄ in response to various concentrations of CO gas diluted in N₂.

Figure 3

Robustness and selectivity of the CO sensors. a) Conductance curves of F-SWCNT-1 with Fe(tpp)ClO₄ sensors in response to 2 min of 200 ppm of CO gas in air (42% relative humidity) and dry N₂. b) Comparison between the response to CO in both N₂ and air to the responses to CO₂ and O₂.

Figure 4

UV/Vis investigation of reactivity of Fe(tpp)ClO₄ to CO in solution of THF. a) Fe(tpp)ClO₄ before and at various times after addition of Na metal. b) Photograph of the color change with the addition of sodium metal and subsequent addition of carbon monoxide. c) Non-reduced Fe(tpp)ClO₄ before and after addition of CO. d) Blue shift in the spectra of fully reduced porphyrin upon addition of CO.

Figure 5

Enhancement in sensitivity via application of the gate voltage. a) Conductance curves of F-SWCNT-1 with Fe(tpp)ClO₄ sensors in response to 2 min of 200 ppm of CO at +3, 0, and −3 V gate voltage. b) Change in conductance towards an exposure of 2 min at 200 ppm of CO as a function of the gate voltage. Dashed line to guide the eye. c) Schematic of band diagram of SWCNT and Fe^IIIpy(tpp)ClO₄ (py = pyridine) and interactions between the two upon application of gate voltage.

Figure 6

Computed change in the Fermi energy (ΔE_F) upon addition of Fe^II porphyrin and subsequent addition of CO or O₂ relative to the Fermi energy of the pristine SWCNT with inserts of the ground-state geometries. For these molecules the Fermi level is defined as the level of the HOMO.