(Digital Presentation) Rapid Electrical Impedance Analysis to Identify Liquid Isomers Using Nanoparticle Thin Film
Authors: Kano Shinya , Mekaru Harutaka 
Published: November 2022

Chemical liquids are transparent and indistinguishable by eyes. For rapid and noncontact analysis of chemical liquids, detecting a headspace vapor from the liquids is a promising technique. Semiconductor sensor1,2, mass spectrometry with inhomogeneous electrostatic deflection3, and quartz crystal microbalance4 have been studied for vapor sensing. In these techniques, a target vapor concentration is typically lower than 100 ppm and it takes >1 min to measure the vapor electronically. On the contrary, a headspace vapor from the chemical liquids in the vessel is much concentrated. An alternative approach to measure the highly concentrated vapor is required for noncontact liquid analysis.
In this study, we propose a rapid noncontact analysis to identify liquid isomers by using the concentrated headspace vapor at room temperature. The key of the analysis is to use an insulating nanoparticle thin film as a sensing layer to capture the vapor5. Figure 1(a) shows the schematic of the sensing mechanism. When a nanoparticle thin film is exposed to a chemical vapor, the vapor starts to diffuse into a void space between the nanoparticles. The driving force of the diffusion is the vapor pressure of each chemical. Due to the Kelvin effect6, nanoscale voids effectively condense a chemical vapor into liquids under the normal pressure. When the chemicals are polar molecules, the condensation is detected by monitoring ion current through the condensed liquids between insulating nanoparticles. Four butanol liquid isomers (1-butanol, 2-butanol, 2-methyl-1-propanol, and tert-butyl alcohol) are detected within 30 s for the proof-of-concept study.
The microsensor chip with an insulating nanoparticle thin film was fabricated as follows. Gold interdigitated electrodes with 20μm line and spacing were prepared on a thermally oxidized silicon wafer by standard photolithography and dry-etching processes. The surface of the electrodes was cleaned by reactive-ion etching with oxygen plasma. Then, a silica nanoparticle thin film was formed by coating a colloidal solution of the nanoparticles. The average diameter of the silica nanoparticle in the solution was 50 nm. The concentration of the nanoparticle was roughly 1% (w/v) in ethanol and a 20μl of the solution was spin-coated. Figure 1(b) shows a scanning electron microscope (SEM) image of a coated nanoparticle film on the electrodes. The deposited nanoparticle film was dried at ≈50°C.
Time response of the film impedance modulated by chemical vapors was evaluated under 26°C and 60 % of relative humidity. The electrical impedance was measured by introducing the sensor into a glass vessel filled with a headspace vapor. Figure 1(c) shows the picture of the measurement configuration. The vessels contained 2 mL of pure chemical liquids and the sensor was placed at ≈3 cm above the liquid surface. It should be noted that the sensor was not immersed into the liquid. The exposure time under the vapor was 30 s.
Figure 1(d) shows the response to the vapor from four pure butanol isomers. Colored region indicates the exposure time. Response times (t
90) of the curves are 10.6, 4.7, 7.5, and 2.4 s, in 1-butanol, 2-butanol, 2-methyl-1-propanol, and tert-butyl alcohol, respectively. On the contrary, recovery times (t
10) of the curves are less than 2 s (indicated by the arrow), which is not obviously dependent on the liquids. The quickness for saturation of the impedance is largely attributed to a high vapor pressure of the chemicals. The vapor pressure of the chemicals at 20°C are 0.9, 2.3, 1.5, and 5.4 kPa, in 1-butanol, 2-butanol, 2-methyl-1-propanol, and tert-butyl alcohol, respectively. The saturated minimum impedance of the response is related to the conductivity of the polar liquids. Figure 1(e) classifies the minimum impedance versus the response time in each curve. The advantage of the proposed noncontact method is a quick recovery without a heating element.
Reference:
1. X. Y. Bai et al., Cryst. Growth Des., 17, 423–427 (2017).
2. N. J. Kybert, M. B. Lerner, J. S. Yodh, G. Preti, and A. T. C. Johnson, ACS Nano, 7, 2800–2807 (2013).
3. F. Filsinger et al., Angew. Chemie - Int. Ed., 48, 6900–6902 (2009).
4. H. Shiigi et al., Electrochem. Solid-State Lett., 6, 3–6 (2003).
5. S. Kano and H. Mekaru, Nanotechnology, 33, 105702 (2022).
6. L. R. Fisher, R. A. Gamble, and J. Middlehurst, Nature, 290, 575–576 (1981).
Figure Caption: Figure 1 (a) Schematic of sensing mechanism. (b) Cross-section of nanoparticle film. (c) Sensor exposed to headspace vapor. (d) Impedance response under exposure to isomer vapor. (e) Two-dimensional plot of minimum impedance and response time of impedance response. A group of ethanol is shown as a reference.

Figure 1