Fabrication of Solid-State Gas Sensors by Drawing: An Undergraduate

Laboratory Experiment pubs.acs.org/jchemeduc

Fabrication of Solid-State Gas Sensors by Drawing: An Undergraduate and High School Introduction to Functional Nanomaterials and Chemical Detection Merry K. Smith, Daphnie G. Martin-Peralta, Polina A. Pivak, and Katherine A. Mirica* Burke Laboratory, Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States S Supporting Information *

ABSTRACT: Carbon nanomaterials have promising utility in chemical sensing including applications in preserving occupational safety, monitoring of environmental pollution, and human health. While recent advances in device fabrication and molecular design of functional materials have enabled rapid fabrication of chemical sensors from carbon nanomaterials, limited efforts have focused on translating these discoveries into undergraduate curriculum. This paper describes a safe and engaging laboratory exercise that introduces undergraduates and younger students to modern nanoscience while illustrating fundamental principles of general chemistry that include concepts of orbital hybridization, allotropes, and intermolecular interactions. Solid-state devices are prepared on shrinkable polymer film substrates equipped with hand-drawn graphite electrodes. Chemiresistive sensing materials, carbon nanotubes (CNTs) and graphite powder, are compressed into pellets and drawn directly into this device architecture to produce functional sensors capable of detecting and differentiating gases and vapors based on differences in intermolecular interactions. KEYWORDS: First-Year Undergraduate/General, Upper-Division Undergraduate, Laboratory Instruction, High School/Introductory Chemistry, Hands-On Learning/Manipulatives, Organic Chemistry, Materials Science, Conductivity, Gases, Nanotechnology aseous chemical sensing is a well-developed field critical to protecting the environment,1 as well as human health and safety.2,3 Many effective gaseous chemical sensing protocols require complex or expensive methods, such as large mass spectrometers4 or trained sniffer canines.5 The need for portable, low-cost chemical detection has motivated the development of a range of simple technologies including an emerging class of chemiresistors:6,7 simple electrical resistors with integrated conductive materials capable of changing their resistance upon chemical exposure. Carbon nanotubes (CNTs) are well-known chemiresistive materials.6,7 Although the chemical sensing industry is projected to reach commercial values of $1.9 billion per year in 2017,8 there is a limited number of recently reported (since 2000) laboratory exercises aimed toward undergraduates that deal with gaseous chemical sensors.9,10 Furthermore, laboratory protocols that involve the chemistry of CNTs are primarily computational11 or deal with composites,12,13 despite the high visibility of CNTs in both the scientific and popular literature. With nanoscience at the forefront of modern chemistry, the importance of applied nanomaterials provides an excellent opportunity for their introduction within undergraduate and high school laboratories. When a chemiresistive material is placed between two electrodes and an input potential bias (V) is applied, the resulting output current (I) changes based on the resistance

G

© XXXX American Chemical Society and Division of Chemical Education, Inc.

(R) of chemiresistor before and after analyte exposure, according to Ohm’s Law (eq 1)14 and as shown in Figure 1:

R=

V I

(1)

Low-dimensional nanomaterials, such as CNTs, are particularly effective chemiresistive materials.15−17 CNTs share many similarities with the more common allotrope of carbon, graphite: both are fully conjugated, conductive, and display high surface area-to-volume ratios. However, graphite and CNTs also have distinct differences related to the morphological structural disparity between the layered 2D graphite sheets and the cylindrical nanotubes: for example, the π-cloud distortion from uniform in graphite to asymmetric in nanotubes leads to differences in the properties of the materials and in their performance in chemiresistors.18 For the stacked 2D sheets of graphite, analyte binding can occur anywhere on the outer surface, allowing electrical current to circumvent binding sites and limiting the magnitude of the current change. Alternatively, in nanotubes, as a quasi-1D system, electrical charge transport is directional along the axis of the tube, so Received: December 20, 2016 Revised: May 18, 2017

A

DOI: 10.1021/acs.jchemed.6b00997 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Cartoon illustrating function of a simple chemiresistive device. (A) Electrical bias is applied between two electrodes. Without a chemiresistive material joining the two electrodes, no current will be observed. (B) When a conductive sensing material is incorporated, the circuit is closed, and a resistance (designated R1) is measured. (C) When exposed to a gaseous analyte, the resistance of the chemiresistive material changes and is measured (designated R2). (D) After analyte dosing is removed, the device returns to its original state, and the measured resistance recovers. (Note: this cartoon indicates a case of full device recovery. In some cases, chemiresistors exhibit incomplete recovery due to irreversible doping effects.)

Figure 2. Stepwise procedure for the fabrication of chemiresistive devices. (A) Preparation of the conductive pellets used to prepare four chemiresistive (CR) devices. Pure nanocarbon (G-CR and CNT-CR) or nanocarbon + selector (G/S-CR and CNT/S-CR) powders are pressed into pellets in step 1. (B) In step 2, the electrode is drawn onto a shrinkable film substrate using a standard HB pencil. In step 3, the devices are thermally miniaturized (163 °C) and tested for short circuits. In step 4, by using the DRAFT method, sensing material is deposited though abrasion (by drawing) using the pellets of chemiresistor material made in panel A until the closed circuit has a resistance between 100 and 300 kΩ. (C) After step 4, the electrode pads of devices are clipped to a multimeter with alligator clips and suspended in the headspace of an analyte-containing beaker to sense, as shown in the photos of the actual laboratory experiment.

ically rich, with the potential to introduce students to both fundamental and modern chemical principles and data analysis while still remaining a safe, hands-on, engaging experiment for students of multiple levels. We provide four different laboratory protocols: one for high school students, one for early undergraduate students, and two versions for upper-level undergraduates, which contain corresponding levels of sensor and analyte complexity and are detailed in the Supporting Information (see Section SIX). Students at the high school level gained a working understanding of general chemistry principles including molecular allotropes, intermolecular interactions, and Ohm’s Law. Students at the undergraduate level built on this understanding by also developing their mastery of materials and organic chemistry principles including the chemical and electrical implications of orbital hybridization of sensor and analyte, intermolecular forces, chemiresistive sensing, the structure/property relationship of nanomaterials morphology with respect to sensor performance, and data processing.

analyte binding anywhere on the surface of the tube will produce electrical perturbation.19 Moreover, the introduction of surface heterogeneity from both doping effects and nanotube junctions20 contributes to the chemiresistance of CNT sensors. Other doping effects, either from CNT defects or adsorbed atmospheric dopants (i.e., H2O or O2), contribute strongly to the sensitivity of the sensor due to increased charge carrier density.21 For all these reasons, the charge transport changes are magnified in the nanotubes compared to the graphite sheets.18 While conductive low-dimensional nanomaterials are sensitive chemiresistors capable of detecting certain analytes in parts-per-million concentrations, their selectivity is often limited.6 Indeed, both pure graphite (metallic) and CNTs (ptype semiconductor) can differentiate between oxidizing and reducing gases, due the concurrent changes in charge carrier density, but have restricted selectivity within these regimes.6,22 Consequently, the response of a CNT sensor to one analyte may be similar to its response to a different analyte. In these cases, small-molecule “selectors” can be introduced to enhance the response of the chemiresistor to specific analytes.23a Herein, we describe a laboratory exercise in which students prepare simple, fully drawn CNT and graphite-based chemiresistive sensors and then quantitatively test the performance of the sensors with an assortment of biologically and warfare-relevant gaseous analytes. This protocol is pedagog-

■

EXPERIMENTAL OVERVIEW Broadly, and for all levels of protocol, this laboratory exercise can be divided into two sections: (1) device fabrication and (2) chemical sensing. Lower level laboratory procedures include fewer devices and analytes and are discussed independently (see Supporting Information Section SIX) from the advanced undergraduate procedure described herein. The entire exercise B




Figure 3. Mode for enhanced CNT chemiresistor selectivity when molecular selectors are used. (A) When CNT nanomaterials are combined with the HHPFN molecular selector (red), the resulting CNT/selector system is composed of cofacially π-bonded selector coating the nanotube. (B) When HHFPN selector is exposed to a ketone analyte (C6H10O), the selector will hydrogen bond with the ketone as shown (blue).

can be accomplished from start to finish in a typical undergraduate laboratory timeline (4−5 h) assuming approximately one instructor for every six students working in pairs.

specificity. The selector used herein, 2-(2-hydroxy-1,1,1,3,3,3hexafluoropropyl)-1-naphthol (HHFPN), is a hydrogen bond donor that interacts selectively with ketone-containing groups, as shown in Figure 3B. The selector is capable of both hydrogen-bonding with analyte molecules and cofacial π−π interaction with the nanotube surface (Figure 3A), providing a pedagogical opportunity to discuss intermolecular interactions. Graphite, on the other hand, when blended with selector, can only be coated by selector on the exposed 2D surfaces of the nanomaterial, preventing signal enhancement for graphite. For these reasons, the CNT/selector chemiresistor (CNT/S-CR) will provide a larger response to certain analytes than the graphite/selector device (G/S-CR).

Device Fabrication

Device Fabrication through Thermal Miniaturization. In Part (1) of the procedure, students prepare functional devices. Electrodes are drawn onto a shrinkable polystyrenebased polymer substrate (similar to “Shrinky-Dinks”) using a standard HB pencil and then thermally miniaturized (see Supporting Information Section SIV) using a method developed by Smith et al. (Figure 2).23b Each student prepares three devices equipped with interdigitated electrodes using a tracer (see Section SIII) for consistency. The miniaturization of the substrate is not only engaging for students, but also serves several practical purposes: (i) the resulting devices are too small to be drawn easily by hand, and (ii) the miniaturization provides 3D character to the graphite wires that improves the available surface area for sensor/wire contact.24 Deposition of Carbon-Based Sensing Materials. The sensing materials, CNT and graphite powders, are pressed into pellets (Figure 2A) and drawn directly into the solid-state device using the DRAFT (deposition of resistors with abrasion fabrication technique) developed by Mirica et al.,23a,c who used the method to prepare CNT-chemiresistive sensors on a variety of substrates. We found that by differentiating electrode drawing from resistor DRAFTing, students were less likely to deposit the sensor material at the wrong time or location on the devices, and they developed an understanding for the chemiresistor architecture with respect to electrodes versus chemiresistive material. Students monitored the change in resistance from infinite resistance of no electrical contact between two interdigitated electrodes (R = infinity) to a functional device (R = 100−300 kΩ) using a multimeter as they abrade the sensing material over the interdigitated portion of their drawn electrodes (Figure 2: Step 4). For the basic procedure, each student prepared two functional sensors, one CNT chemiresistor (CNT-CR) and one graphite device (GCR). Deposition of Selective Sensing Materials. To illustrate the differences in properties between graphite and CNTs with respect to sensing performance, two additional sensors are prepared using pellets of carbon-based chemiresistor blended with a “selector.” Chemical selectors are small molecules that enhance the response of CNTs to specific analytes by (i) dispersing nanotube bundles and coating the tubes and (ii) interacting chemically with the analyte with a high degree of

Chemical Sensing

Part 2 of the basic procedure involves using the fully drawn, functional devices to detect gaseous analytes in the saturated headspace vapor over analyte-containing liquid solutions. Students select their best (closest to the R = 100−300 kΩ range) CNT-CR and G-CR device and use these to detect two analytes: (i) ammonia (NH3) and (ii) nitric oxides (NOx). Both NH3 and NOx are common toxic pollutants:1,2,4 sensing of NH3 is particularly relevant to commercial refrigeration, where toxic leaks can cause worker asphyxiation in facilities that utilize large cooling systems. Atmospheric pollution responsible for smog and acid rain contains assorted compounds, including NOx gases generated from automobiles. Consequently, these analytes provide a pedagogical opportunity for the instructor to discuss environmental industrial pollutants and permissible workplace exposure limits. In both systems, device exposure to analytes is accomplished through headspace sampling (Figure 2C). Sensing chambers in the hoods consist of a beaker containing a small volume of liquid analyte (ammonium hydroxide or nitric acid for NH3 and NOx, respectively) sealed with parafilm and fully saturated with analyte vapors.25 Students use multimeters equipped with alligator clips to monitor the device resistance through a (i) baseline, (ii) exposure, and (iii) recovery cycle. The CNT-CR and G-CR devices are (i) exposed to the atmosphere, (ii) dosed with analyte by suspending the device in the headspace of the sensing chamber through a slit in the parafilm, and then (iii) recover in the atmosphere (Figure 4). The third analyte, cyclohexanone (C6H10O), a target analyte in explosives detection, is also detected using all four devices CNT-CR, G-CR, CNT/S-CR, and G/S-CR.3 The inclusion of a warfare-relevant analyte provides the instructor with an opportunity to discuss a major focus in commercial chemical C




inhalation should be avoided. Nitric acid gives off toxic fumes, and C6H10O is considered an irritant and should not be inhaled. Nitric acid is highly corrosive and strongly oxidizing, and students should be educated about proper disposal, accidental spillage or exposure before the laboratory begins. The safety data sheets for all chemicals used should be available to students before the laboratory exercise is commenced.

■

RESULTS The basic procedure reported herein was conducted at Dartmouth College with 17 “Upward Bound” high-school students from different high schools in Vermont and New York. The upper-level procedure was conducted at Dartmouth College in Hanover, New Hampshire with 24 Honors general chemistry undergraduates (see Supporting Information Sections SVIII and SIX). All students at both levels fabricated fully drawn working devices and observed changes in device resistance concurrent with analyte dosing. Representative sensing results obtained by an undergraduate student are shown in Figure 4. Students were able to observe and quantify the sensor response changes by normalizing their worksheet resistance data (R) to percentages(ΔR/Ro) according to the formula in eq 2, where Rf = final and Ro = initial resistance value:

Figure 4. Representative student-obtained sensing traces for all analytes through a baseline (atmosphere), exposure (analyte dosing), and recovery (atmosphere) cycle. All traces are normalized in terms of resistance and vertically offset for clarity. Two devices for each sensor are depicted to show reproducibility. (A) Response of CNT-CR (blue) and G-CR (green) devices to exposure (gray bar) to NH3. The partial device recovery after dosing suggests a partially reversible analyte binding event. (B) Response of CNT-CR and G-CR devices to exposure to NOx. The irreversible recovery suggests irreversible analyte binding. (C) Response of CNT-CR (blue), G-CR (green), CNT/S-CR (purple), and G/S-CR (orange) devices to exposure to C6H10O highlights the enhanced response of CNT/S-CR devices to ketone-containing analytes.

(R f − R o) ΔR (%) = × 100 Ro Ro

(2)

The normalized data were then plotted and compared for each analyte and sensor by students (Figure 4) in their assessment or laboratory writeup. The percent change in resistance versus time provided a visual representation of sensor performance, easily quantified and directly comparable between devices and analytes. The main analytes in this study, NH3 and NOx, were selected, in part, because their differences in chemical properties lead to opposite sensing responses (Figure 4A,B). In all cases, students observed and noted this difference and used the opportunity to think deeply about the chemical implications of the corresponding increase or decrease in conductance with respect to orbital hybridization and electron donating (NH3) or accepting (NOx) capability of analytes. The molecular-level interaction of the analyte with the sensor is a complicated problem with many variables including adsorbed layers of atmospheric gases and water in the sensing devices, but the consistent, opposite response for these two analytes provides an excellent “real-world” tangible example of observable differences in electron donating/accepting properties of small molecules. Representative normalized percent response changes are reported for three devices with standard deviations calculated using device/device variability (see Section SVII for values). For NH3, the normalized response changes for G-CR = 14 ± 7% and for CNT-CR = 15 ± 4%. For NOx, G-CR = −62 ± 9% and CNT-CR = −66 ± 3%.27 The sensing of C6H10O with both allotrope devices and allotrope/selector devices demonstrated the structure/property relationship between the sensing materials and device performance. For the sensing of C6H10O, G-CR, CNT-CR, and G/SCR are not effective sensors, with responses of 0 ± 0%, 1 ± 1%, and −3 ± 15%, respectively. Devices made with CNT/S-CR sensors, on the other hand, demonstrated a response of 198 ± 8%, which highlighted the enhanced selectivity of selectorcoated CNTs (Figure 4C).

sensing. Each student will likely have experience with chemical sensors in airports or have seen bomb sniffing dogs at work. These types of sensors are needed for portable detection of dangerous materials, the discussion of which was particularly engaging for students. This exercise serves to sense C6H10O and compare the sensor performance between carbon allotropes with and without additives. Data points of resistance measurements (collected every 30 s) are recorded into a provided worksheet (see Section SIXE) to be normalized, plotted, and analyzed for each sensor and analyte to quantify relative performance of chemiresistive sensors. Step-by-step directions for this analysis are detailed in the Supporting Information (Section SIXG).

■

HAZARDS This laboratory exercise is particularly safe, as students never need to handle or transfer analyte chemicals. The fabrication procedure should be conducted in a well-ventilated space, and the sensing stations should be placed in fume hoods. Proper personal protective equipment (safety goggles, lab coat, and gloves) should be worn at all times, even when fabricating devices. Devices are thermally miniaturized in a 163 °C oven, and thermal burns can be avoided by using heat resistant gloves to handle pans. The toxicity of CNTs is debated;26 therefore, care should be taken to avoid ingestion, inhalation of powders, and skin contact with the CNT pellets. Similar considerations should be in place for graphite. Students do not need to directly handle the analyte chemicals, as they should be enclosed in secondary containment in sensing hoods. Ammonium hydroxide emits irritating NH3 fumes, and any D



■


(3) Yamazoe, N. Toward Innovations of Gas Sensor Technology. Sens. Actuators, B 2005, 108 (1−2), 2−14. (4) Arshak, K.; Moore, E.; Lyons, G. M.; Harris, J.; Clifford, S. A Review of Gas Sensors Employed in Electronic Nose Applications. Sens. Rev. 2004, 24 (2), 181−198. (5) Stitzel, S. E.; Stein, D. R.; Walt, D. R. Enhancing Vapor Sensor Discrimination by Mimicking a Canine Nasal Cavity Flow Environment. J. Am. Chem. Soc. 2003, 125 (13), 3684−3685. (6) Fennell, J. F.; Liu, S. F.; Azzarelli, J. M.; Weis, J. G.; Rochat, S.; Mirica, K. A.; Ravnsbæk, J. B.; Swager, T. M. Nanowire Chemical/ Biological Sensors: Status and a Roadmap for the Future. Angew. Chem., Int. Ed. 2016, 55 (4), 1266−1281. (7) Aswal, D. K.; Gupta, S. K. Science and Technology of Chemiresistor Gas Sensors; Nova Science Publishers, Inc.: New York, 2007; pp 33− 94. (8) Chemical Sensors Industry Study 3058; Freedonia Group, 2017. http://www.freedoniagroup.com/industry-study/3058/chemicalsensors.html (accessed April 2017). (9) Riechel, T. L. A Gas-Sensor-Based Urea Enzyme Electrode, Its Construction and Use in the Undergraduate Laboratory. J. Chem. Educ. 1984, 61 (7), 640−642. (10) Aristov, N.; Habekost, G.; Habekost, A. Kundt’s Tube: An Acoustic Gas Analyzer. J. Chem. Educ. 2011, 88 (6), 811−815. (11) Simpson, S.; Lonie, D. C.; Chen, J.; Zurek, E. A Computational Experiment on Single-Walled Carbon Nanotubes. J. Chem. Educ. 2013, 90 (5), 651−655. (12) de Dios, M.; Salgueirino, V.; Pérez-Lorenzo, M.; Correa-Duarte, M. A. Synthesis of Carbon Nanotube-Inorganic Hybrid Nanocomposites: An Instructional Experiment in Nanomaterials Chemistry. J. Chem. Educ. 2012, 89 (2), 280−283. (13) Hobbs, J. M.; Patel, N. N.; Kim, D. W.; Rugutt, J. K.; Wanekaya, A. K. Glucose Determination in Beverages Using Carbon Nanotube Modified Biosensor: An Experiment for the Undergraduate Laboratory. J. Chem. Educ. 2013, 90 (9), 1222−1226. (14) Ohm, G. S; Taylor, R. Die Galvanische Kette, Mathematisch Bearbeitet: Scientific Memoirs; Wikisource: London, 1841; Vol. 2. https://en.wikisource.org/wiki/Scientific_Memoirs/2/The_ Galvanic_Circuit_investigated_Mathematically (accessed April 2017). (15) Baptista, F. R.; Belhout, S. A.; Giordani, S.; Quinn, S. J. Recent Developments in Carbon Nanomaterial Sensors. Chem. Soc. Rev. 2015, 44 (13), 4433−4453. (16) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Carbon Nanotubes − The Route Toward Applications. Science 2002, 297 (5582), 787−792. (17) Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Carbon Nanomaterials for Electronics, Optoelectronics, Photovoltaics, and Sensing. Chem. Soc. Rev. 2013, 42 (7), 2824−286. (18) Kauffman, D. R.; Star, A. Carbon Nanotube Gas and Vapor Sensors. Angew. Chem., Int. Ed. 2008, 47 (35), 6550−6570. (19) Zhang, T.; Mubeen, S.; Myung, N. V.; Deshusses, M. A. Recent Progress in Carbon Nanotube-Based Gas Sensors. Nanotechnology 2008, 19 (33), 332001−332015. (20) Salehi-Khojin, A.; Khalili-Araghi, F.; Kuroda, M. A.; Lin, K. Y.; Leburton, J.-P.; Masel, R. I. On the Sensing Mechanism in Carbon Nanotube Chemiresistors. ACS Nano 2011, 5 (1), 153−158. (21) Goldoni, A.; Petaccia, L.; Lizzit, S.; Larciprete, R. Sensing Gases with Carbon Nanotubes: A Review of the Actual Situation. J. Phys.: Condens. Matter 2010, 22 (1), 013001−013009. (22) Latif, U.; Dickert, F. L. Graphene Hybrid Materials in Gas Sensing Applications. Sensors 2015, 15 (12), 30504−30524. (23) This laboratory experiment was inspired by and adapted from: (a) Mirica, K. A.; Azzarelli, J. M.; Weis, J. G.; Schnorr, J. M.; Swager, T. M. Rapid Prototyping of Carbon-Based Chemiresistive Gas Sensors on Paper. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, E3265−E3270. (b) Smith, M. K.; Jensen, K. E.; Pivak, P. A.; Mirica, K. A. Direct SelfAssembly of Conductive Nanorods of Metal-Organic Frameworks into Chemiresistive Devices on Shrinkable Polymer Films. Chem. Mater. 2016, 28 (15), 5264−5268. (c) Mirica, K. A.; Weis, J. G.; Schnorr, J.

SUMMARY The introduction of chemiresistive sensing using CNTs and graphite to undergraduates and high school students was found to be an engaging, straightforward, and pedagogically rich laboratory experiment. The simple fabrication of electrodes by drawing is a consistent, reproducible method that introduces students to hands-on basic electronic architectures. Subsequent deposition of sensing material and chemical detection bring the vast field of chemical sensing directly into the student laboratory. Finally, the topic of CNTs adds relevance to nanoscience and offers the opportunity for students to think deeply about molecular properties from small-molecule to nanoscale. The undergraduates who performed this laboratory exercise used analytical data-processing techniques to great effect in their laboratory assessments, including data normalization and plotting in graphing software. The practical exercise was highly straightforward, with easily manageable potential pitfalls (detailed in Section SX) including maintaining the electrical contact between the electrodes and the leads, and short-circuiting of the devices. This exercise offers pedagogically tunable opportunities for the laboratory instructor all the way from basic electronics and data processing to advanced organic chemistry and molecular structure/property relationships, and it introduces students to a fully applied demonstration of modern nanochemistry.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00997. Detailed laboratory handouts for high-school students, beginner, and upper-level undergraduates (including preand postlab questions, worksheets, data processing appendices, and Instructor Notes); data related to protocol optimization; related content (PDF, DOCX)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Katherine A. Mirica: 0000-0002-1779-7568 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS K.A.M. gratefully acknowledges support from startup funds provided by Dartmouth College (DC) and from Walter and Constance Burke Research Initiation Award. D.M.P. acknowledges support through the Women in Science Project and Sophomore Science Scholarship (DC), and P.A.P. is grateful for support provided by Project SEED through the American Chemical Society.

■

REFERENCES

(1) Lee, D.-D.; Lee, D.-S. Environmental Gas Sensors. IEEE Sens. J. 2001, 1 (3), 214−224. (2) Choi, N.-J.; Kwak, J.-H.; Lim, Y.-T.; Bahn, T.-H.; Yun, K.-Y.; Kim, J.-C.; Huh, J.-S.; Lee, D.-D. Classification of Chemical Warfare Agents Using Thick Film Gas Sensor Array. Sens. Actuators, B 2005, 108 (1− 2), 298−304. E




M.; Esser, B.; Swager, T. M. Mechanical Drawing of Gas Sensors on Paper. Angew. Chem., Int. Ed. 2012, 51 (43), 10740−10745. (24) The miniaturized devices were shown to be quantitatively superior in chemiresistive performance to devices drawn at the same scale as the shrunken devices (see Supporting Information Section SV). (25) If hoods are not available, and safety is a concern, high-proof ethanol may be used in place of toxic analytes, but the normalized percent response change will be smaller. For CNTs without selector, approximately −5.2 ± 0.6%. (26) Liu, Y.; Zhao, Y.; Sun, B.; Chen, C. Understanding the Toxicity of Carbon Nanotubes. Acc. Chem. Res. 2013, 46 (3), 702−713. (27) We found that, although the devices have an irreversible binding event for NH3 and NOx gases, they can be reused for exposure to other analytes without a significant loss of performance. Multiple exposures to the same analyte, however, limit their performance significantly, as shown in the Supporting Information (Section VI).

F


Fabrication of Solid-State Gas Sensors by Drawing: An Undergraduate

Recommend Documents