The impedance data of a device exposed only to dry N2 provides a 'baseline' for the device electrical properties without exposure to other chemical environments as plotted as a Bode-Nyquist diagram as shown in figure (1B). The native device exhibits a single arc at frequencies between approximately 100 Hz and 100 kHz which is qualitatively characteristic of these devices exhibiting behavior as an effective circuit with capacitive and resistive behaviors acting in parallel. Furthermore, it is seen that different arc geometries are observed at different applied voltages- indicating that the particular effective elements possess some degree of non-linearity in their response behavior. This behavior is commensurate to the general expectations of the architecture of this device which is essentially constructed as an electrical channel constructed from low-density semi-conductor/semi-metal with a capacitive contact at each end of the channel.
Figure (1C) shows the shift in electrical impedance response that results from exposure of the architecture to water vapor. It may be seen that the response retains the characteristic qualitative single arc of values between approximately 100 Hz and 100 kHz. However, the quantitative values of the impedance are significantly altered and the qualitative characteristics of the arc have shifted somewhat. In particular, the magnitude of both the real and imaginary components of the impedance are substantially reduced for all frequency test values. Additionally, most of the response values for the impedance are more tightly clustered across different applied test voltage values from 0.01V–1V whereas the values obtained at a test voltage of 2V which provides a significantly differentiated response curve.
It is worth recognizing that the VANTA architecture is inherently very low density and the carbon nanotubes constitute a volume fraction typically below 15% and commonly below 1% of the physical space in the spatial region which the structure occupies. As such, applied electrical fields do not terminate at the 'surface' of the VANTA as is the case for traditional metals but penetrates for tens of microns into the array which is similar to the Debye length for undoped silicon.
To visualize the impact of the analyte on the system, Nyquist plots have been constructed that shows the shift in device response from a baseline impedance value curve in which the device was purged with N2 that was taken immediately prior to the analyte test. As such figure (1D) represents the Nyquist plot that results from the figure (1C) after subtracting the data of figure (1B). Thus these figures illustrate the difference of the impedance signal of the device as a result of its exposure to an analyte. It should be noted that this procedure of subtracting the non-exposed baseline data from the device electrical behavior in the presence of chemical analyte simply provides visualization of the difference in measured electrical parameters of the device resulting from chemical exposure. This baseline subtraction is not intended, of itself, to provide direct data regarding the operation of the electrical circuit per se. Further raw data figures are provided in the supplementary information.
Upon exposure to various analytes, the sensor shows both a strong shift in electrical properties in response to all four chemical analytes tested as well as significant differentiating features for each chemical species as shown in figures (1D), (1E), (1F) and (1G).
Water vapor acts to simultaneously reduce the magnitude of the real and imaginary components of the electrical impedance of the device for almost all tested parameter values as shown in figure (1D) while still retaining the basic qualitative characteristics of the circuit behavior. The qualitative shifts in the Nyquist plot suggest that in both real- and imaginary-valued components of the impedance are altered by the presence of water vapor.
In contrast, exposure to organic vapors- ethanol (EtOH), ethyl acetate (EtOAc) and toluene- all led to larger average values for the real component of the impedance as well as larger magnitude negative values for the imaginary impedance component. This again suggests that resistance has been reduced and capacitance has been increased for the VANTA in response to these species. However, the details of the response are different for each species.
Specifically, EtOH, shown in figure (1E), produces a device response wherein the semi-circular arc of the difference signal is roughly maintained and wherein the apparent resistance and capacitance are modified by a similar factor.
Similarly, toluene, shown in figure (1F), the device produces a response wherein the semi-circular arc of the difference signal is roughly maintained and wherein the apparent resistance and capacitance are modified by a similar factor though the details of the response are different for toluene as compared to ethanol.
For EtOAc, shown in figure (1G), the device also produces a response possessing mostly larger average values for the real component of the impedance as well as larger magnitude negative values for the imaginary impedance component. However, for EtOAc, a clear inflection point is seen in the differential curves which is most pronounced for applied test voltage of 0.1V but is seen for all voltages though with decreasing strength for voltages above 0.1V. This inflection point behavior is unique to ethyl acetate out of the analytes tested here.