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Detection of Fake Alcoholic Beverages Using Electrolyte-Free Nanogap Electrochemical Cells Tse-Hsien Ou, Yifei Wang, Dan Fang, Sri R Narayanan, and Wei Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18729 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Applied Materials & Interfaces

Detection of Fake Alcoholic Beverages Using Electrolyte-Free Nanogap Electrochemical Cells Tse-Hsien Ou a,‡, Yifei Wang a,‡, Dan Fang b, S. R. Narayanan b, Wei Wu * ,a a. Ming Hsieh Department of Electrical Engineering, University of Southern California b. Department of Chemistry, University of Southern California

KEYWORDS alcohol electrolysis, electrolyte-free electrolysis, nanogap electrochemical cell, bifunctional mechanism, fake alcoholic beverage

ABSTRACT

Due to the similarity of odor, appearance, and chemical structure of methanol and ethanol, measuring the low concentration of methanol in an alcoholic beverage is difficult to perform in a quick, quantitative, and repeatable fashion. However, it is important for people to monitor the content of methanol in a liquor since a high amount of methanol absorbed will result in blindness, coma, and death. In response to this need, we have developed electrolyte-free methanol electrolysis and ethanol electrolysis based on the nanogap electrochemical cells for the methanol and ethanol sensing. Upon applying a voltage, a high electric field across the nanogap cell enhances the solution ionization and the ions transport rate. Moreover, the nanoscale distance

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between the electrodes provides a shorter path for electrolysis to easily occur. The nanogap electrochemical cells not only make the direct electrolyte-free organic solvent electrolysis possible but also enhance the sensitivity of the chemical of interest in low concentration solutions without the influence of added electrolyte. The nanogap electrochemical cells have been demonstrated having high sensitivity to detect 0.15% methanol volume concentration in deionized water solutions without adding any electrolyte, and its ability for the fake alcoholic beverages’ detection has successfully demonstrated.

INTRODUCTION As drinking alcoholic beverages become an important part of people’s lives, the worldwide consumption of alcohol increases annually.1-3 A quarter of the consumption, however, is unrecorded alcoholic drinks (even worst in certain regions like Islamic states of the Eastern Mediterranean and in the South-East Asian)4 coming from homemade alcohol, illegally produced, or sold outside normal government controls. In addition, alcohol is recognized for its high revenue-generating potential, and its market is estimated to reach $1,594 billion by 2022.5 Due to the increasing demand and the great profit of alcoholic beverages, some illegal alcohol manufacturers used denatured alcohol rather than edible alcohol as a substitute to produce alcoholic beverages, which normally called fake alcoholic drink, for seeking a higher profit.6-8 The denatured alcohol, however, contains 5% or more volume concentration of methanol, which is highly toxic to people when digested or absorbed.9 The high content of methanol in a human body will form formaldehyde, which is poisonous to human’s central nervous system and leads to blindness, coma, and death.10-15 The human tolerable content of methanol for a 60% ethanol concentration of alcohol is around 0.05% to 0.15%16, but this


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tolerable content varies with the person’s physical condition and it is very difficult to define a safe, accurate value for the individual to avoid digesting a high methanol content. Therefore, it is necessary to monitor the content of methanol in alcoholic drinks to prevent digesting a too high content of methanol. Conventionally, the methanol content in alcoholic beverages is measured by liquid chromatography, gas chromatography, and spectroscopy.17-21 Although these methods can provide a precise quantity of methanol, they require bulky equipment and cumbersome preparations, so that it is hard to apply them to daily life. The fast and convenient method to relative measure methanol content is the test strips method, whereas the test strips method can only qualitatively test the chemical and cannot provide an actual methanol content for people.11, 22

To conveniently and quantitatively monitor the methanol content, there is a need to develop a

method that can precisely, quickly, repeatably, and conveniently measure the low methanol content. According to our previous study23, when applied a voltage, a strong electric field exists between the two electrodes of the nanogap cell that not only enhanced the solution ionization and ions transport rate but also provided a shorter path to easily trigger electrolysis. Note that with enhanced ions transport rate, the rate determining steps of electrolysis when performed by NECs is primarily dominated by the charges transfer rate at the electrodes and is no longer controlled by the ions transport rate as traditional electrochemical cells do. In this way, quite different from well-established theories, the presence of electrolytes in the solution has limited effects on the electrolysis, and even non-conductive liquid, like deionized water, could be efficiently electrolyzed. This fundamental discovery provides us with a new approach for chemical sensing based on the direct electrolyte-free solvent electrolysis. Moreover, since the electrolysis is performed under an electrolyte-free condition, we can study purely the interested chemical in a 3 Environment ACS Paragon Plus


low-concentration solution without the influence of the electrolyte ions. Besides, the current in the current-voltage curve can help us calculate the numbers of electrons and therefore the concentration of the chemicals. Accordingly, the nanogap electrochemical cells (NECs) can be applied to the field of chemical sensing and can be a solution to fit all our needs for lowconcentration methanol sensing.

RESULTS AND DISCUSSION In this study, the NECs have been applied to the electrolyte-free methanol and ethanol electrolysis to study each alcohol’s electrolysis behavior. The comparison of each alcohol’s electrolysis behavior and the direct measurement of methanol in an ethanol deionized water solution were also studied. The schematic of the fabrication processes and cell structures of the NECs are shown in Figure 1a. The structure of NECs was designed as a sandwiched-like structure with a layer of 60 nm pure Pt as a top cathode, a layer of 60 nm Ru-Pt alloy (composition ratio of Ru to Pt, 1:1) as a bottom anode, and a middle layer of 72 nm silicon nitride as a dielectric layer as shown in Figure 1b. The schematic of the testing setup is presented in Figure 1c, and the testing liquid is only dropped on the top of the grating region for studying the electrolysis. At the cathode, platinum was chosen for the efficient hydrogen evolution.24-30 The purpose of using Ru-Pt alloy at the anode is to prevent the poisoning effect and is commonly-used for the efficient direct alcohol electrolysis. For efficient methanol electrolysis, the optimal composition ratio of Ru to Pt has been found as 1:1.31-33


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Figure 1. a) The schematic fabrication processes and cell structures of nanogap electrochemical cell. b) The cross-sectional SEM image of the NEC. c) The schematic testing setup of NEC.

The difference of electrolysis behavior in these two alcohols can be recognized by the overall electrolysis reaction of the saturated alcohol, considering the complete oxidation of the alcohol into CO2, is as following: 𝐶" 𝐻$"%& 𝑂𝐻 + (2𝑛 − 1)𝐻$ 𝑂 → 𝑛𝐶𝑂$ + 3𝑛𝐻$

(1)

Two half-reactions occurring at each electrode are as following: Cathode: 6𝑛𝐻% + 6𝑛𝑒 3 → 3𝑛𝐻$

(2)

Anode: 𝐶" 𝐻$"%& 𝑂𝐻 + (2𝑛 − 1)𝐻$ 𝑂 → 𝑛𝐶𝑂$ + 6𝑛𝐻% + 6𝑛𝑒 3

(3)



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Compared to the methanol (as n=1) and ethanol (as n=2) in Equation 1, a higher electrode potential is required for ethanol electrolysis than methanol electrolysis to have a complete oxidation since the carbon-carbon bond that is very strong has to be broken to form CO2.34-35 This electrode potential difference can be seen in the threshold voltages in the current-voltage curve that helps to distinguish methanol and ethanol electrolysis. As the kinetics of alcohol electrolysis is relatively slow the threshold voltages are increased to 0.45-0.50 V compared to the thermodynamic (or reversible) electrode potentials for both alcohol electrolysis even using the best catalyst.24 Since the cathode is where the hydrogen evolution reaction (Equation 2) occurred, the electrode potential at this electrode is almost zero vs. normal hydrogen electrode (NHE). Also, the hydrogen evolution reaction is facile and thus the overpotential losses are negligible. Under these conditions, the measured current-voltage curve for the cell is largely governed by the reaction at the anode given by Equation 3. In addition to the reactions, the adsorption of dissociated components of the alcohol molecule on electrodes and the electron transfer from the adsorbed component to the electrode are also crucial. The adsorption and the dehydrogenation reaction of dissociated methanol components on platinum are provided as Equation S1-S624,

26, 36

shown in the Supporting

Information. To apply methanol electrolysis to the methanol sensing, the most desired adsorption on the anode is Pt-Had, since it will release the electrons to the external circuit. Nevertheless, other adsorbed components will also occupy the active site of Pt, and among them, the Pt-(CO)ad is the most strongly adsorbed and long-lived component that should be avoided. The oxidation reactions of CO to CO2 by adsorbed -OH groups at a Ru-Pt alloy electrode are provided in the Supporting Information as Equation S7-S1031,

36-38

shown. Therefore, the site of Pt can be

regenerated and continued to adsorb protons after the CO2 gases released. Therefore, the electrolysis behavior of each alcohol can be differentiated by monitoring the current at a certain


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threshold voltage based on NECs measurement. A range of voltages, 0-1.1 V was applied on NECs to analyze the electrolysis behavior of each alcohol aqueous solution. Various volume concentrations of methanol deionized water solutions were used for the methanol electrolysis measurement. The water electrolysis was also performed by using the deionized water and was regarded as a background current when discussing the methanol and ethanol electrolysis in the later. In here, deionized water was chosen not only to make the experiments more controllable as well as repeatable, but also to study the behaviors of each alcohol’s electrolysis. The current of deionized water electrolysis was comparable to that of the drinking water (a commercial bottle water, ARROWHEAD® 100% MOUNTAIN SPRING WATER), which contains a few of minerals39 that can be treated as electrolytes, as shown in Figure 2a, indicating that the conductivities under NECs setup of these two water sources were comparable. The comparable electrical property is because the rate determining step of the electrolysis in the NECs is primarily dominated by the charges transfer rate at the electrodes, and the presence of electrolytes in the solution has much smaller effects on the electrolysis in NECs than in traditional electrochemical cells.23 Besides, the enhanced conductivity of deionized water can be explained as the presence of a high electric field in the bulk solution and the dissolution of carbon dioxide from the ambience.7,

23, 40

Since the

experiments we performed without further purifying the deionized water, the water already contains a few of electrolytes in the water and its electrolysis’ behavior is comparable to the normal drinking water. The results in Figure 2b demonstrated that the NECs had the sensitivity to detect methanol concentration as low as to 0.15%. Besides, the currents increased when the methanol concentrations increased, whereas the threshold voltages decreased with the increasing methanol concentrations. The increasing concentration of methanol in an deionized water solution 7 Environment ACS Paragon Plus


provided more dissociated adsorbed components, CH2OH- and H-, making the methanol electrolysis occur easier and resulted in a higher current but a smaller threshold voltage on the IV curve. However, in Figure 2c, when the methanol concentration increased to higher than 7.4%, the currents decreased with the increasing methanol concentrations, and the threshold voltages increased with the increasing methanol concentrations. The inverse trend observed before and after 7.4% could be explained by the bifunctional mechanism. At the lower methanol concentrations, the added Ru in Ru-Pt alloy anode facilitated the re-oxidation of -COads, but once the methanol concentration increased to a higher concentration, a saturation of all the sites for the formation of Pt-COad was reached, resulting in a poisoned anode due to the inadequacy of water adsorption to produce the needed level of -OHads species. To further demonstrate the poisoning effect that caused the reduced currents when the methanol concentrations increased, the currents of 4%, 7.4%, 16.7%, and 40.5% of methanol electrolysis were subtracted by the current of water electrolysis to purely study the effect of the adsorption of dissociated methanol components on the anode. As shown in Figure 2d, in the region I, 0 V-0.3 V, the methanol dissociated and formed CH2OHads and Hads in the water solution producing increasing current when the applied voltage increased. In the region II, 0.30.6 V, the currents, however, decreased sharply as methanol concentration increased. The slope of each current-voltage curves, varied with the methanol concentration as observed from the steepness of each current in the region II, suggested that the poisoning effect became aggravated when the methanol concentration increased. In the region III, after 0.6 V the currents started to increase due to the adsorbed OH- from water24 helped reduce the adsorbed CO and regenerated the Pt, which became active again and continued the methanol electrolysis.


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Figure 2. a) The I-V curves of deionized water and drinking water electrolysis b) The I-V curves of deionized water electrolysis and 0.15%, 0.4%, 4%, and 7.4% methanol deionized water solution electrolysis. c) The I-V curves of 7.4%, 16.7%, and 40.5% methanol deionized water solution electrolysis. d) The logarithm-scale of 4%, 7.4%, 16.7%, and 40.5% methanol electrolysis subtracted by deionized water electrolysis.

To investigate fake alcoholic beverages detection using NECs, 40% and 60% ethanol deionized water solutions were first studied as Figure 3a shown. However, only 60% ethanol deionized water solution was used as a reference to represent the commercial alcoholic beverages since that the fake alcoholic beverages are more prevailing in Asian, and the alcohol



concentration for the popular, valuable fake alcoholic beverages is usually around 60%. In Figure 3a, the results of electrolysis show that a higher electrode potential is required for ethanol electrolysis for complete oxidation to form CO2: a higher temperature environment (normally higher than 80 °C) is usually required for the efficient ethanol electrolysis.26, 32 Since all the measurements we performed were at room temperature, the reduced current at a higher ethanol concentration was due to a lower degree of adsorption of the dissociated components of ethanol. To further study the adsorption that occurred on the anode in the ethanol electrolysis, the current observed in water electrolysis was also subtracted from the ethanol electrolysis data as shown in Figure 3b. Compared to the methanol electrolysis, for 40% ethanol concentration, there was no reduced current observed at the range of 0.3-0.6 V as we observed in the methanol electrolysis, and even at a very high ethanol concentration, 60%, only a slight reduction of current was observed indicating that under a room temperature environment, the ethanol electrolysis was much less prone to poisoning than methanol electrolysis. This decreased effect of poisoning could be because ethanol does not breakdown as easily as methanol and form adsorbed species unless the temperature is raised. Though ethanol dissolved in deionized water was used in most experiments for better controllability, we did compare the ethanol dissolved in deionized water and the ethanol dissolved in drinking water with the real alcohol at the same concentration to study the difference of each electrolysis’ behavior. Here, two 40% ethanol aqueous solutions mixed by deionized water and drinking water and a commercial 40% alcoholic beverage (ABSOLUT® VODKA) were studied and presented in Figure 3c. The curves of two 40% ethanol aqueous solutions were nearly identical and both were only slightly higher than the current of the 40% alcoholic beverage. Besides, the electrolysis’ behavior of ethanol aqueous solution was very similar to the electrolysis’ behavior of real alcoholic beverage. This similarity allowed us to


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approximate the electrolysis’ behavior of ethanol deionized water solutions to the electrolysis’ behavior of ethanol drinking water solutions and real alcoholic drinks, and it again demonstrated that the presence of electrolytes in the solution had limited effects on the electrolysis’ behavior under NECs setup.



Figure 3. a) The I-V curves of deionized water electrolysis, 40% ethanol deionized water solution electrolysis, and 60% ethanol deionized water solution electrolysis. b) The logarithmscale of 40% and 60% ethanol electrolysis subtracted by deionized water electrolysis. c) The I-V


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curves of real alcoholic beverage (ABSOLUT® VODKA), 40% ethanol deionized water solution electrolysis, and 40% ethanol drinking water solution electrolysis.

In Figure 4a, the threshold voltage of each alcohol could be easily differentiated. For the methanol electrolysis, the threshold voltage was around 0.7 V, whereas the threshold voltage for the ethanol electrolysis was around 0.8 V. Compared these values to the literature value 0.450.50 V as mentioned above, the higher value of threshold voltage via NECs measurement might due to using the thin film structure of catalyst rather than using the nanostructured catalyst that is likely to be more active with a larger reaction area for electrolysis. For the methanol sensing ability (Figure 4b), when the applied cell voltage to the NECs was 1.0 V, the current for 4%, 7.4%, and 16.7% methanol deionized water solutions obviously were higher than 60% ethanol deionized water solutions indicating the NECs were capable to distinguish these three methanol concentrations when compared to the 60% ethanol deionized water solutions. Besides, a range of methanol sensing ability to the 60% ethanol deionized water solutions from 1% to 40.5% methanol concentration can be extracted from this data. The direct measurement of different methanol concentrations in a 60% ethanol deionized water solution as Figure 4c clearly demonstrated that the NECs could directly distinguish the 4% and 7.4% methanol concentration in a background of 60% ethanol deionized water solution.




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Figure 4. a) The comparison of the current of 4%, 7.4%, and 16.7% methanol deionized water solutions electrolysis to the 60% ethanol deionized water solutions electrolysis. b) The comparison of the currents of different methanol deionized water solution electrolysis to the 60% ethanol deionized water solution electrolysis at 1.0 V vs. NHE (the grey region indicates the error bar region of the 60% ethanol deionized water solution electrolysis). c) The direct measurement of 4% and 7.4% methanol concentrations in a 60% ethanol deionized water solution.

The NECs currently have more than enough sensitivity to detect the denatured alcohol to help alcohol manufacturers differentiate the denatured alcohol and edible alcohol. Similarly, alcohol users can differentiate high methanol-contained alcoholic drinks at a concentration between 1% to 40.5%. Although the current of 0.15% methanol deionized water solution, compared to a 60% ethanol deionized water solution, is not enough to prove the drink is safe to drink, there are many improvements possible to enhance the current of 0.15% methanol deionized water solution like finding the optimum composition of the Ru-Pt alloy for the NECs as well as fabricating nanostructured electrodes for a higher reaction area. Therefore, the detection sensitivity can be greatly improved.

CONCLUSIONS The electrolyte-free alcohol electrolysis based on nanogap electrochemical cells has been studied and been applied to the detection of fake alcohol (methanol) in alcoholic beverages. The observed behavior of the electrolysis of methanol and ethanol can be attributed to the differences



in the adsorption of the dissociated methanol and the ethanol components as well as the bifunctional mechanism of catalysis that operates on platinum-ruthenium catalysts. A clear difference of threshold cell voltages of the methanol and ethanol electrolysis could be observed. By using the NECs for methanol detection, currently, the measurable range for the methanol in a 60% ethanol deionized water solution at 1.0 V is around 1% to 40.5%. Further improvements to extend the detection to 0.15% methanol deionized water solution to a 60% ethanol deionized water solution will require optimizing the composition of platinum-ruthenium alloy catalysts at the anode and using nanostructured electrodes for the higher reaction area for the direct application of fake alcoholic drinks detection.

EXPERIMENTAL SECTIONS Device fabrication A layer of 3 nm Ti layer was deposited on a 3-inch silicon wafer (100) with a 100 nm thermally grown silicon dioxide layer by e-beam evaporator (Temescal BJD-1800). A 60 nm binary Pt-Ru film was co-sputtered by using a custom-built multisource sputter deposition system with platinum (Plasmaterials, 99.99%) and ruthenium (Plasmaterials, 99.9%). A 72 nm silicon nitride layer was deposited at 350 °C using PECVD (Oxford PlasmaPro System 100). Two resist layers were spin-coated on the substrate for the following photolithography process. A 200 nm liftoff layer (Microposit LOL 2000) was firstly coated as an adhesive layer of photoresist and for the later easier metal-liftoff process. A 1 µm photoresist layer (AZ MIR 701) was subsequently coated to define the patterns. Photolithography was performed with a custom-designed photomask with 3.5 mm by 3.5 mm contact pads and 1 cm by 1 mm 1-D gratings with a 10 µm pitch. Afterward, the patterns were developed by AZ 300 MIF Developer to develop both the 16 Environment ACS Paragon Plus

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liftoff layer and photoresist layer. A 2 nm Ti and a 60 nm Pt layer as a cathode electrode were deposited by the e-beam evaporator. A liftoff process was performed by acetone and AZ 300 MIF Developer to remove all the residual resist layers. Finally, the silicon nitride was etched by the low DC-bias etching technique41, which we developed to avoid the short-circuited devices due to the re-deposition of metal atoms on the sidewall of devices during the etching process.

Chemicals The methanol aqueous solution and ethanol aqueous solution were prepared by ≥99.9% methanol and ≥99.5% ethanol, respectively, both from Sigma-Aldrich.

Measurements The I-V curves measurements were performed by Agilent 4156B Semiconductor Parameter Analyzer. The setting parameters: the voltage scanning range from 0 to 1.1 V; each scanning step is 15mV; 1 second for both the hold time and delay time for extracting data.

Supporting Information The Supporting Information is available free of charge. The absorption and dehydrogenation reaction of dissociated methanol components on platinum electrode, the reduction reaction of carbon monoxide on binary platinum-ruthenium electrode. (PDF)



Corresponding Author *E-mail: [email protected]

ORCID Tse-Hsien Ou: 0000-0002-0892-908X Yifei Wang: 0000-0002-6670-7995 S. R. Narayanan: 0000-0002-7259-3728 Wei Wu: 0000-0001-6404-0317

Author Contributions ‡T.-H.O and Y.W. contributed equally to this project. D.F. contributed to deposit the cathode electrode materials used in this research. Funding Sources This research is supported by Prof. Wei Wu’s start-up fund from the University of Southern California. ACKNOWLEDGMENT T.-H.O thanks Prof. S. R. Narayanan for the helpful discussions in electrochemistry. T.-H.O also thanks Prof. Chongwu Zhou for use of the measuring equipment (probe station and the semiconductor parameter analyzer).


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