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Comparing selectivity of functionalized-graphenes used for chemiresistive hydrocarbon vapor detection Sanjay V Patel, Stephen Hobson, Sabina Cemalovic, and William K. Tolley ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00852 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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ACS Applied Nano Materials

Comparing selectivity of functionalized-graphenes used for chemiresistive hydrocarbon vapor detection Sanjay V. Patel*, Stephen T. Hobson†, Sabina Cemalovic, William K. Tolley Seacoast Science, Inc., 2151 Las Palmas Drive, Suite C, Carlsbad, California 92011 *Corresponding author: E-mail: [email protected]

ABSTRACT: Portable or wearable sensors for Volatile Organic Compounds (VOCs) such as benzene and naphthalene are important for occupational health monitoring of workers near refueling operations. Six commercially-available, plasma-processed, functionalized-graphene nanoplatelet (fGNP) materials were dispersed between electrodes as disordered films to form chemiresistors. Putative functional groups included amino, carboxyl, fluoroalkyl, and hydroxyl. Sensor response trends were determined upon exposure to both non-polar (fuel related alkanes and arenes) and polar compounds (alcohols, acetone, trichloroethylene, and water). The relative sensitivity to the fuel-related hydrocarbon compounds did not correlate with the functional group; but any functionalization significantly increased the sensor response compared to unfunctionalized graphene. Most notably, there is increased sensitivity to polar protic vapors.

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The results suggest that the dominant mechanism for sensor response is swelling caused by intercalation of vapor between particles, with sorption capacity enhanced by defects introduced during processing. N2-fGNP was the most sensitive to alcohol vapors, demonstrating limits of detection below 10ppm for ethanol and methanol vapors in dry air. Among the target fuelrelated compounds, some selectivity was demonstrated. Sensors prepared from the O2-fGNP showed limited (~20%) selectivity for isooctane over benzene; in contradistinction, the carboxylfGNP showed over 2x sensitivity for benzene over isooctane for identical exposure levels. The fluoroalkyl fGNP had an attenuated response to all analytes. The carboxyl-fGNP was particularly sensitive to the target refueling vapors with room temperature limits of detection below 50 ppbV for naphthalene and below 1 ppmV for benzene, in dry air.

Keywords: Graphene, functionalized graphene, naphthalene, benzene, chemical sensor, exposure monitor, chemiresistor, short term exposure limit.

INTRODUCTION Graphene-based materials have been studied as chemically-sensitive layers for many applications.

Graphene’s high electrical conductivity1 makes it suitable for a variety of

applications: chemical2,3 and biological sensors;4 electrode materials;5,6 and wearable sensors.7 Several groups8 have reported graphene-based sensors responding to various analytes in applications ranging from biological (both direct9 and indirect10) targets to gases and organic vapors.11,12 Whereas initial applications used pristine graphene for chemical sensing, recent


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work has shown that surface defects dominate graphene’s electrical characteristics and that the number of defects substantially enhance the sensor response during chemical exposure.13 In other words, tailoring graphene surfaces improves chemical sensitivity.

Many routes to

functionalize graphene surfaces have been reported5,14,15,16,17 and modified and unmodified graphene powders or suspensions18,19,20 are available from various commercial vendors. Despite this availability, there are few reports detailing functionalized-graphenes to detect targets other than inorganic gases (CO, CO2, H2), with the majority of work focuses on the electrical performance of single layers of graphene.21,22,23,24 Various grades of graphene have been demonstrated with many types of chemical transducers, with different effects on analytical performance.25 In a chemiresistor, where the chemosensitive layer is formed from overlapping conductive or semi-conductive particles alone (e.g. graphene), the layer’s conductivity can be disrupted by surface sorption of analytes, which affects the surface potential (Figure S1).2,3 In addition, sorption of the analyte can disrupt the interparticle electron hopping between overlapping, disordered, nanoparticle flakes by increasing interparticle spacing.26,27,28 Finally, graphene materials are likely not defect free, especially at the edges, where the defects can affect the local electron density and therefore the conductivity-based response to sorbed chemicals for graphene-based resistive sensors. For a graphene-based chemiresistor, the expectation is that the quantity of sorbed analyte depends on the strength of reversible chemical interactions (van der Waals, π-stacking, hydrogen-bonding

strength,

and

dipolar

interactions)

as

in

polymer-based

nanocomposites,29,30,31,32 functionalized carbon nanotubes,33 Si-nanowires,34,35 and metallic nanoparticles.36,37 Hence, the introduction of diverse chemical moieties should influence the


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strength of the graphene-analyte interaction, thus modifying the analyte’s ability to disrupt interparticle conductivity or the local surface potential resulting in a change in the sensor response. As chemical detection devices become smaller and less expensive, they become more appealing for wearable and mass-market applications, such as portable personal protection and community-wide, crowd-sourced monitoring.38 However, to be useful for personal protection, these devices must be sensitive to concentrations of target chemicals well below government mandated hazardous levels regulated by the Occupational Safety & Health Administration (OSHA) in the United State (US). Sensors for VOCs such as benzene and fuel-related chemicals found near refueling operations are of particular interest.

In light of this, the U.S. National Research Council identified

naphthalene as a serious health hazard for personnel working with jet fuels and oil-based sealants, with JP-8 jet fuel being the “most common and abundant potential chemical exposure of Department of Defense (DOD) and NATO military personnel.”39 Naphthalene and similar aromatic hydrocarbons (e.g. benzene) make up 22-67% by volume of JP-8. The US DOD and its NATO partners use 5 billion gallons of JP-8 annually; commercial aircraft world-wide use over 58 billion gallons annually of Jet-A, which poses an equivalent risk. Finally, isopentane and isooctane are key markers of fresh and weathered gasoline,40,41 respectively, and their detection is useful in environmental remediation applications. Chemiresistors directly deposited onto printed circuit boards (PCBs) are an established technology commercialized in the Cyranose® 320, in the late 1990’s - early 2000s.42 The Cyranose® 320 used an array of 32 chemiresistor sensors composed of polymer-carbon black composites deposited onto a proprietary PCB substrate. More recently, researchers have also reported the use of carbon nanotube composites deposited upon PCBs to detect VOCs.43 The


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cost savings from reducing part-counts, process steps, and simplifying manufacturing are important factors in bringing low-cost wearable sensors to the market. With the market pull and the recent technological advances, a question remains: can the same strategies used to augment the selectivity and sensitivity of polymer29 and other nanomaterialbased33,35 sensors be applied to sensors that use disordered functionalized graphene films as the sensing element? In this work, we report what effect, if any, surface-functionalization of graphene has on the selectivity and sensitivity of conductivity-based chemical sensors.

EXPERIMENTAL Conductive carbons: Unfunctionalized graphene powder (GR) and graphene oxide (GRO) in aqueous suspension were purchased from Angstron Materials19 (Dayton, OH); functionalized graphene nanoplatelets (fGNPs) were purchased from Graphene Supermarket18 (owned by Graphene Laboratories Inc., Calverton, NY); all carbon materials were used as received without further purification. The fGNPs were specified as being functionalized using the Haydale plasma process17 (HDPlas®) (Table S1). Solvents and target VOCs: Isopentane, isooctane, benzene, toluene, naphthalene, trichloroethylene (TCE), acetone, ethanol, methanol, were purchased from Sigma Aldrich Corporation (St. Louis, MO), and used as received. For benzene tests below 50 parts-per-million by volume (ppmV), vapor was delivered from a certified gas cylinder (100ppmV benzene in air, Matheson Tri-Gas, Inc., San Marcos, CA). Deionized water was sourced from an in-house system. Dried laboratory air from an air compressor was passed through Drierite as the carrier gas for all tests.


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Chemiresistor substrates: Printed circuit board (PCB) platforms were fabricated by Advanced Circuits (Aurora, CO) using proprietary designs. Compared to typical microfabricated electrode designs, electrode designs44,45,46 in this study were both larger and more widely spaced. Each 2.4cm x 1cm platform has nine electrode pairs, grouped in sets of three. Each set has three different electrode spacings: 1.0mm, 1.8mm, and 2.5mm. Each electrode pad consists of a goldplated, lead-free solder-mask that was 2.0mm by 1.3mm. Plated vias conduct traces from each electrode to the back of the platform. Platforms have conventional PCB thickness (1.6mm). Standard 0.1inch pitch pins are soldered to the back of each platform make electrical contact with a standard socket for testing. To remove residual flux, the PCB platforms are cleaned with ethanol and allowed to air dry prior to deposition of the chemiresistor media. Chemiresistor films: Duplicate sensor films were deposited on adjacent electrode pairs to provide statistical confidence in the data. GR (0.1wt%) and fGNP (0.25wt%) allotropes were suspended in toluene. GRO, received as an aqueous commercial stock-solution, was diluted to 1wt% with DI water. Suspensions were homogenized by ultrasonication for up to 3 hours prior to deposition on the electrode pairs.27,47 The suspensions were deposited directly on the PCB platform (Figure S2) by drop casting using a micropipette (0.3 - 20 µL of the stock solution per site). Following film deposition, all chemiresistors were dried at (laboratory) ambient temperature (~22˚C) in air. The coated platforms were stored at ambient conditions until tested. Individual film thickness was variable and contributed to variability in test results. Based on the amount of material deposited and the surface of the electrode pairs, the average film thickness was ~ 5 – 20µm. Coatings were deposited to obtain a target resistance ≤ 10kΩ; care was taken during deposition of the suspensions to maintain uniform dispersion of the particles to prevent


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electrical short between electrodes. The films form ohmic contact with the gold electrodes, as demonstrated by linear I-V curves (Figure S3). Sensors were tested in a temperature and humidity controlled test system capable of mixing and diluting vapors from a variety of vapor sources, including bubblers, gas cylinders, and permeation ovens.48,49 Target analytes were delivered by passing a known flow of dry air through a bubbler, held at 10°C, filled with the analyte, or, in the case of naphthalene, by passing air through loosely packed solid powder.50 Mass-flow controllers regulated the dilution with dry air to produce the desired analyte concentrations. For selectivity testing, the same % P/Psat was used to normalize test-system related effects that can arise from using differing flows and to ensure that the chemical was generated at an appreciable concentration to be well equilibrated at the sensor. Vapor concentrations were calculated via the Antione Equation and were verified gravimetrically.51 A computer-controlled datalogger switch unit (Agilent #34970A) was used to record the resistances (2-point DC) of the chemiresistors during testing. Typically, the resistance of 16 chemiresistors was measured sequentially, as the datalogger measured resistance of each sensor sequentially in 100MΩ), chemiresistors fabricated from GRO proved unsuccessful. As the baseline resistance (R0) of the different types of chemiresistors varies by up to an order of magnitude, the absolute peak-to-peak noise (ohms) depended on the capability of the data logger. However, the measurement noise can be represented by the RSD of the baseline: in this study, RSD ranged from 0.000013Ω to 0.0014Ω and had a median value of 0.00006Ω (N=43 sensors). This RSD (0.00006Ω) is used as a general value for the noise floor. This noise measurement is taken from a 2-minute (>10 points) time segment (dry air exposure without analyte present, T=25°C). Selectivity. To highlight selectivity trends, relatively high vapor concentrations were used to generate a larger response. Pristine, unfunctionalized graphene (GR) chemiresistors were the least responsive to all of the vapors tested.

The relative response (∆R/R0)fGNP of each

functionalized graphene was compared to the relative response of the GR chemiresistors, using the ratio of relative responses (∆R/R0)fGNP)/(∆R/R0)GR. The data show that the functionalization process enhances the relative sensitivity of the materials for the same vapor exposure of a given chemical (Figure 3).


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Figure 3. Relative response ratio of the neat allotrope chemiresistors to the unfunctionalized graphene chemiresistor response ((∆R/R0)fGNP)/(∆R/R0)GR). (ppmV) are given with the chemical name.

Exposed vapor concentrations

A response ratio >1 indicates that the sensor

prepared from the functionalized graphene is more sensitive than the sensor prepared from pristine graphene. Concentrations correspond to 1% of Psat at 10°C (except naphthalene at 25°C). Response to water is not shown because the GR sensors did not show a significant response to 1% water vapor.

As the chemiresistor films were dried under ambient (humid) conditions, some hydration of polar functional groups on the graphene surface and especially at edges and defects may occur. Exposure to water vapor is known to p-dope unfunctionalized graphene.26 Advantageous water can also form hydrogen bonds at nitrogen or oxygen substitutions on functionalized graphene.14,53,54,55,56 This may explain why the most noticeable differences between the pristine and functionalized graphenes, were observed in the relative responses to the polar (acetone) and polar-protic (methanol and ethanol) analytes as these compound have a favorable interaction energy with the hydrated functionalized graphene. Responses of the fGNP-based sensors to the polar compounds could be ranked from strongest to weakest: N2, Carboxyl > Argon (Ar), O2 >NH3 > fluorocarbon (FC).

This order is essentially unchanged for acetone, ethanol and

methanol. Under this model, the stronger sensitivity of fGNP-Carboxyl compared to fGNP-O2, to the polar compounds, may be attributed to stronger hydrogen-bonding interactions that aid analyte sorption and thus increase the relative response. Nitrogen and ammonia functionalization are known to increase conductivity (n-doping) and add defects to graphene.26,56 In fact we observed that fGNP-Carboxyl and fGNP-N2 behavior


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were nearly identical, with much stronger responses to the polar compounds relative to fGNPNH3. One expects little effect from argon-functionalization, (e.g. Ar-plasma cleaning), in terms of modifying polarity or hydrogen-bonding strength; however, the observed responses are similar to that of fGNP-O2, which should be capable of hydrogen-bonding. Both fGNP-O2 and fGNP-Ar have strong relative responses for the alkanes (isooctane and isopentane) and toluene. For these nonpolar analytes, subtle differences are observed in the selectivity patterns; for example, fGNPO2 and fGNP-Ar have a larger relative response to isooctane, than fGNP-N2 and fGNP-Carboxyl, but are less sensitive to benzene. Therefore, we conclude that the films resulting from Ar and O2 functionalization are the least polar of the fGNPs. The order of relative responses to benzene (N2, Carboxyl > Ar, O2, NH3 > FC) is similar to that of the polar compounds, but distinctly different than the order of sensitivity for isooctane: Ar, O2 > NH3 > N2, Carboxyl > FC. This observation and the strong response of the fGNP-Carboxyl films over fGNP-O2 to the polar compounds, are inconsistent with simple models of intermolecular interactions, e.g. polar/nonpolar or H-bonding. Compared to the other functionalized graphene materials tested in this study, fluorocarbon functionalization produced relatively smaller responses, but the general shape of the pattern is consistent compared to the other fGNPs. It has been reported that fluorine is effective for pdoping graphene, which would reduce the film conductivity, perhaps dampening the sensor response.57 Fluorine-doped graphene films have also been reported to be more hydrophobic than types of fGNPs.56,58 This may explain the attenuated response of the fluorocarbon films, relative to the other fGNPs, although the exact structure of the resultant functionalization needs to be studied by other methods (e.g. Raman or X-ray photoelectron spectroscopy).14


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These data suggest that the defects resulting from the functionalization process may have as much to do with the enhanced sensitivity as the putative structure on the graphene surface resulting from functionalization.13,27,59,60 In addition, these data are also consistent with the work of Myers et al.,28 who compared sorption of various aromatic compounds to cyclohexane in aqueous solutions with octadecylamine-functionalized graphene chemiresistors. They suggest that the primary detection mechanism relies more on sorption-based swelling, increasing interparticle distances, than on electron doping interactions with the functional group, or on π-π interactions with the basal plane of the graphene. To compare the similarity of the sensors, a table of Pearson correlation coefficients, from the fGNP and GR response (∆R/R0) data, was generated (Table S2). The correlation analysis confirms that the GR sensor data is the least similar to the other functionalized graphenes. Significant correlation between the fGNPs is observed, with fGNP-Carboxyl being the least correlated to the others. Also, fGNP-N2 is similar to fGNP-Carboxyl and fGNP-FC, even though the putative chemical functionality is expected to be significantly different for each of these fGNPs. This confirms the observations from Figure 3, that for all the studied fGNPs, the selectivity patterns are generally similar to each other. This similarity is further illustrated by comparing the individual exposure responses for selected fGNPs versus fGNP-Carboxyl (Figure 4).


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Figure 4. Relative responses (∆R/R0) for GR, fGNP-O2, -NH3, -AR, -N2, and -FC each plotted against fGNP Carboxyl. The 1:1 line (indicating similar response) is shown to guide the eye. The five fuel component analytes are circled to emphasize their relative locations.

In these plots, data from small exposures of water vapor (1% of Psat at 10°C, ~125ppmV) were included as an additional interferent vapor. All sensors tested had very small response to this level of water vapor. The general locations of most of the nonpolar chemicals with respect to each other is similar. However, note how the relative locations of toluene with respect to


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naphthalene, benzene with respect to isooctane, and methanol with respect to ethanol, which indicates some subtle differences in selectivity among the fGNPs. The location of these data above or below the 1:1 line helps to indicate the relative response differences in the fGNPs. Also, the GR vs. fGNP carboxyl plot shows the stronger relative sensitivity that the fGNP has toward the polar compounds (acetone, ethanol and methanol). Principal components analysis (PCA) was used to model the chemical exposure data with and without the GR sensor in the array (Figure 5).

Figure 5. PCA result from the six fGNP sensors (a) alone and (b) with the GR sensors. The percentage of total variance is given with each PC axis title.

Even though the response of the fGNP devices seemed similar, their differences allow distinction of several of the non-polar analytes, including the alkanes, toluene and naphthalene. Adding GR to the array improves discrimination of benzene and water from the cluster of other


15


chemicals. In both cases, the more polar chemicals (alcohols and acetone) clustered together, as do the alkanes. Sensitivity. Chemiresistors prepared from neat functionalized and unfunctionalized graphene all strongly responded to toluene and naphthalene; however, there is a significant difference in relative vapor pressures of the two compounds. The limits of detection (LOD) are estimated from the raw data, using the IUPAC simplified relation.61,62,63 Exposed concentrations, 1% of Psat of each chemical at 10°C, are estimated in ppm by volume. LODs for benzene and naphthalene with the GR devices were (average ± 1 SD) 164 ± 171 ppmV and 0.6 ± 0.6 ppmV, with the sensor-to-sensor variation (coating reproducibility) reflected in the standard deviation (SD) (Table 1).

Table 1. Average limits of detection (LODs) and standard deviation (SD), in ppmV, for pristine graphene (GR) and fGNP chemiresistors, and relevant short-term exposure limits (STEL) and time weighted average (TWA) guidelines. NIOSH64 TWA LOD (ppmV) Chemical, exposure (ppmV)

Fuel Components

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(ppmV) fGNPfGNPfGNP- fGNP- fGNP- fGNP- 15-min 8-hr Carbox FC O2 N2 AR NH3 STEL TWA yl

GR

Benzene, 622

164 171

i-Octane, 301

125 ± 48

i-Pentane, 5160

1500 935

Toluene, 164 35 ± 47

± 25 ± 8 3 ± 0.2 9 ± 5

3 ± 0.7 64 ± 26 13 ± 3

41 ± 23 4 ± 0.4 6 ± 4

6 ± 1.1 65 ± 48 15 ± 7

± 408 108

± 148 41

± 89 ± 63 135 83

± 527 378

4 ± 0.8 1 ± 0.3 1 ± 0.8 1 ± 0.8 7 ± 5

± 148 96 2±1

±

1

0.1

385

75

610

120

150

100


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Naphthalene, 0.1 0.6 ± 0.6 3.4 0.03 Acetone, 1470

246 ± 89

TCE65, 466

106 114

± 0.03 0.01

± 0.02 0.02

± 0.2 0.1

68 ± 32 32 ± 38 16 ± 8 9 ± 2

120 39

± 19 ± 9 5 ± 1

± 0.03 0.02

8±8

Ethanol, 304 168 ± 66 24 ± 10 12 ± 14 6 ± 3

Interferents

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Methanol, 731

295 152

Water, 125

NA

± 36 ± 13 6 ± 2 102 66

5±3

± 0.05 0.03

±

± 27 ± 9

48 ± 52 10 ± 8

15

10

1000

250

25

10

3 ± 0.6 46 ± 16 11 ± 3 1000

13 ± 9 6 ± 1

57 ± 32 35 ± 7

± 48 ± 56 27 ± 11 13 ± 4 99 ± 60 37 ± 13

1000

250

200

NA

NA

Significant sensitivity improvement, compared the GR films, was observed in chemiresistors prepared from fGNPs. For example, the fGNP-Carboxyl sensor produced the lowest LOD (3 ± 0.2ppmV) for benzene. While this is not as stringent as the NIOSH guideline level, this is well below the current U.S. OSHA short-term (15-minute) exposure limit of 5 ppmV66. Several other sensors tested in this study produced LODs that were below the U.S. NIOSH exposure levels for several of the target vapors. For example, four of the fGNP chemiresistors (fGNP-N2, -Carboxyl, -O2, and -NH3) demonstrated average LODs below 0.05 ppmV for naphthalene, and below 2 ppmV for toluene with lower variance. Note that the fGNP-Carboxyl is not the most sensitive to water as would be expected if the sensing mechanism is directly related to the chemical functionality. We speculate that the observed increase in sensitivity of the functionalized materials is from: 1) the addition of defects created by the functionalization process, and possibly, to a lesser extent, 2) the specific type of functional groups added to the graphene surface, which enhances sorption through improved hydrogen bonding, dipolar, or van der Waals interactions. Tests performed in this work cannot distinguish between the two factors. Complicating this analysis is the fact that the number of


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functional groups, on per graphene basis, is not known67 and unlikely to be consistent between fGNPs. Finally, aging effects were not studied in this work. Response isotherms for these devices were consistent with the observed trends above (Figure 6).

Figure 6. Relative response curves for benzene, ethanol, and isooctane vapor in dry air at 25°C from fGNP-O2, (upper) and fGNP-Carboxyl (lower) chemiresistors. The lines are meant to guide the eye, and the RSD is denoted by the horizontal dashed line near zero.

For example, the fGNP-O2 is only 20% more sensitive to isooctane than benzene at ~60ppmV. In contrast, the fGNP-carboxyl chemiresistor, generally more sensitive, is itself more sensitive to benzene than isooctane by a factor of 2.7 at ~60ppmV. These data tend to confirm that the type of functionalization plays some role in selectivity.


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Response trends remained consistent when the sensors were tested in the presence of a limited number of interference compounds. Addition of humidity or a background interferent compound (100ppmV or greater of acetone, TCE, or ethanol) shifted the relative response (∆R/R0) of the sensors. LODs were estimated from these tests (Table 2). Detection limits were consistently poorer when interferents were added to the test system due in part to the greater variability of the vapor concentrations in the test system resulting from the added variability in delivering the interferent.

Addition of the interfering agent generally increased the response variability

resulting in reduced sensitivity and a poorer average LOD. Thus, the presence of the additional interfering compounds in the background, especially high levels of water vapor, had the practical effect of reducing the sensitivity by as much as a factor of 10.

Table 2. LOD (± 1 s.d., N=3) from fGNP-Carboxyl for benzene exposure (30 - 300 ppmV) with interferents in the background. Background Condition

Interferent (ppmV)

LOD (ppmV)

Dry Air

none

3±0.2

40% RH

none

35±3

70% RH

none

29±12

Dry Air

acetone (100)

22±4

40% RH

acetone (100)

14±3

Dry Air

TCE (120)

14±6

40% RH

TCE (120)

16±3

Dry Air

ethanol (120)

28±14

40% RH

ethanol (120)

29±16


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For the fGNP-Carboxyl device, water, the most polar of the interferents, is a significant factor, decreasing the observed response to non-polar analytes by approximately 10-fold. The other fGNPs also showed similar reduction in benzene sensitivity with background humidity. A combination of interferences, water vapor plus a 2nd chemical, generally resulted in an additive effect on response, thus these materials will require humidity compensation to be effective sensor materials.

The observed sensitivity to benzene was most negatively impacted when the

interfering compounds were most polar protic (i.e. water and ethanol). The sensitivity towards benzene was less affected by a background of acetone or TCE. We hypothesize that the overwhelming presence of the polar compounds makes the surface (and bulk) less amenable towards sorption of benzene compared to TCE or acetone and may prevent interaction between the functional group and benzene.32 In the previous tests, the fGNP-Carboxyl devices were fabricated with ~ 50µg deposited from a from 2.5wt% dispersion in toluene. To explore the possibility of improving sensitivity, a new set of fGNP-Carboxyl sensors was fabricated, using a significantly more concentrated dispersion (30wt%) of functionalized graphene and increasing the absolute amount of deposited material (Figure 7).


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Figure 7. Effect of increasing fGNP quantity on the average response of fGNP-Carboxyl sensors to benzene vapor at 25°C in dry air. Sensors were tested simultaneously with films containing different quantities of deposited nanoplatelets. Figure legend indicates the estimated mass of 30wt% fGNP-solution deposited.

Use of more concentrated suspensions in sensor fabrication resulted in additional deposited mass and increased sensitivity, with the LOD (0.1 ± 0.03) for benzene being lowest from the 100µg films (0.4µL deposited from 30 wt%), well below the NIOSH STEL.

No further

improvement was observed from greater masses of graphene per sensor. The same process of using more concentrated solutions was tested with GR; however, the sensors quickly become very conductive (< 300 Ω) with poor reproducibility. We hypothesize that further optimization of the sensitivity may be achieved by modifying the type of nanoplatelet, graphene flake size, deposition processing variables such as adding dispersant to the suspension, tighter temperature control during deposition, and modified surface roughness of the finished chemiresistor are all factors that are likely to influence sensor response.

CONCLUSIONS Chemiresistive sensors were fabricated from pristine and functionalized graphene powders. Sensors prepared from commercially-sourced, plasma-functionalized graphenes were more sensitive than those prepared from unfunctionalized graphene films. Optimal chemiresistors prepared from functionalized graphenes showed exceptional sensitivity to benzene and naphthalene, analytes of concern due to their deleterious effects on human health. In fact, even with these multi-layer, macroscopically-disordered films, there are observable differences in


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selectivity and sensitivity depending upon type of functionalization. However, the sensor responses could not be correlated with the physical and chemical characteristics of the functionalizing group, suggesting as-yet-unrecognized interactions between the target vapors and the functionalized graphene. In the studied sensor architecture, for these films of disordered and overlapping nanoplatelets, the sensing mechanism is dominated by sorption-based swelling; the extent to which a particular target analyte is absorbed by the sensing film is modulated by surface and edge defects affecting the film conductivity and sensor performance.

The specific chemical character of the

functionalization plays a small role in conferring selectivity to the films, compared to the addition of defects, although there is a definite effect exerted by the more polar protic functional groups. Further sensor sensitivity / selectivity improvements are likely achievable by optimization of the deposition process of the materials (e.g. spray coating) and by reduction of the sensor size. Miniaturization can be accomplished using ink-jet deposition.68 The many commercial sources of graphenes, the degree of functionalization (number of functional groups/graphene), and preparation methods likely also influence performance when used in a chemiresistor. This is not claimed to be an exhaustive survey of all graphene materials, both functionalized and pristine, and that there are likely to be modified graphenes that produce significantly larger sensor responses than those reported herein. Future work will include the study of the long-term aging and stability of resistances of these sensors. Studies of thermal properties should help elucidate the specific sensing mechanism, and establish if these types of sensors can be useful for long-term sensing applications. ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acsanm.XXXXXXX. Conceptual diagram of chemiresistor; List of carbon allotropes used for chemiresistors; Photograph of printed circuit board platforms with graphene and two fGNP coatings deposited between gold electrodes (2.5mm spacing); Representative I-V curves from fGNP-FC and GR films in the operational range of the measurement electronics showing the sensors have ohmic contact; Pearson correlation results from sensor response data. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID *Sanjay V. Patel: 0000-0001-9540-9957 †Stephen T. Hobson: 0000-0001-6634-6264 Present Addresses † Department of Biology & Chemistry, Liberty University, 1971 University Blvd, Lynchburg, VA 24515. Author Contributions The manuscript was written through contributions of all authors. ACKNOWLEDGMENT


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This work is supported by the US Army Medical Research and Materiel Command under Contract No. W81XWH-15-C-0181. The views, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision unless so designated by other documentation. ABBREVIATIONS GR pristine unfunctionalized graphene; fGNP functionalized graphene nanoplatelet; TCE trichloroethylene; ppmV parts-per-million by volume.

REFERENCE

(1) Kim, H.; Abdala,

A.A.; Macosko, C.W. Graphene/Polymer Nanocomposites.

Macromolecules 2010, 43, 6515-6530. (2) Nallon, E. C.; Schnee, V. P.; Bright, C.; Polcha, M. P.; Li, Q. Chemical Discrimination with an Unmodified Graphene Chemical Sensor. ACS Sens. 2016, 1, 26-31. (3) Dan, Y.; Lu, Y.; Kybert, N.J.; Luo, Z.; Johnson, A.C. Intrinsic Response of Graphene Vapor Sensors. Nano Lett., 2009, 9, 1472-1475. (4) Shao, Y. Wang, J. Wu, H. Liu, J. Aksay, I.A. Lin, Y.; Graphene Based Electrochemical Sensors and Biosensors: A Review, Electroanalysis 2010, 22, 1027-1036. (5) Chang, H.; Wang, G.; Yang, A.; Tao, X.; Liu, X.; Shen, Y.; Zheng, Z. A Transparent, Flexible, Low-Temperature, and Solution-Processible Graphene Composite Electrode. Adv. Funct. Mater., 2010, 20, 2893–2902.


24

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Sriprachuabbwonga, C.; Karuwana, C.; Wisitsorrata, A.; Phokharatkula, D. Inkjet-Printed Graphene-PEDOT:PSS Modified on Screen Printed Carbon Electrode for Sensing Applications, Proceedings of 14th International Meeting on Chemical Sensors - IMCS 2012, 2012, 1336 – 1338. (7) Benz, M.; Patel, S.V. Freestanding Chemiresistive Polymer Composite Ribbons as HighFlux Sensors, J. Appl. Polym. Sci., 2012, 125, 3986-3995. (8) Liu, Y.; Dong, X.; Chen, P.

Biological and Chemical Sensors Based on Graphene

Materials, Chem. Soc. Rev., 2012, 41, 2283-2307. (9) Lu, Y.; Goldsmith, B. R.; Kybert, N. J.; Johnson, A. T. C. DNA-Decorated Graphene Chemical Sensors, Appl. Phys. Lett., 2010, 97, 083107. (10) Arjmandi-Tash, H.; Belyaeva, L.A.; Schneider, G.F. Single Molecule Detection with Graphene and Other Two-Dimensional Materials: Nanopores and Beyond, Chem. Soc. Rev. 2016, 45, 476-493. (11) Schedin, F.; Geim, A. K.; Morozov, S.V.; Hill, E.W.; Blake, P.; Katsnelson, M.I.; Novoselov, K.S. Detection of Individual Gas Molecules Adsorbed on Graphene, Nat. Mater., 2007, 6, 652-655. (12) Sun, J.; Muruganathan, M.; Mizuta, H. Room Temperature Detection of Individual Molecular Physisorption using Suspended Bilayer Graphene, Sci. Adv., 2016, 2, E1501518. (13) Kumar, B.; Min, K.; Bashirzadeh, M.; Farimani, A. B.; Bae, M.-H.; Estrada, D.; Kim, Y. D.; Yasaei, P.; Park, Y. D.; Pop, E.; Aluru, N.R.; Salehi-Khojin, A. The Role of External


25


Page 26 of 34

Defects in Chemical Sensing of Graphene Field-Effect Transistors. Nano Lett. 2013, 13, 1962– 1968. (14) Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications. Chem. Rev. 2012, 112, 6156–6214. (15) Fang, X.; Donahue, J.; Shashurin, A.; Keidar, M. Plasma-Based Graphene Functionalization in Glow Discharge, Graphene, 2015, 4, 1-6. (16) Feng W.; Long P.; Feng Y.; Li, Y. Two-Dimensional Fluorinated Graphene: Synthesis, Structures, Properties and Applications. Adv. Sci., 2016, 3:1500413. (17) Haydale Graphene Industries Plc. Ammanford, U.K.; http://www.haydale.com/ourtechnology/hdplas-process/ (18)

Graphene

Laboratories,

Inc.,

Calverton,

NY:

https://graphene-

supermarket.com/Functionalized-GNPs/ (19) Angstron Materials, Inc., Dayton, OH: http://www.angstronmaterials.com/ (20) Goodfellow Corporation, Coraopolis, PA: http://www.goodfellowusa.com/ (21) Yavari, F.; Koratkar, N. Graphene-Based Chemical Sensors, J. Phys. Chem. Lett., 2012, 3, 1746−1753. (22) Varghese, S. S.; Varghese, S. H.; Swaminathan, S.; Singh, K.K.; Mittal, V. TwoDimensional Materials for Sensing: Graphene and Beyond. Electronics, 2015, 4, 651-687.


26

Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Paul, R. K.; Badhulika, S.; Saucedo, N. M.; Mulchandani, A. Graphene Nanomesh as Highly Sensitive Chemiresistor Gas Sensor. Anal. Chem. 2012, 84, 8171–8178. (24) Fowler, J. D.; Allen, M. J.; Tung, V. C.; Yang, Y.; Kaner, R. B.; Weiller, B. H. Practical Chemical Sensors from Chemically Derived Graphene. ACS Nano 2009, 3, 301–306. (25) Wang, T.; Huang, D.; Yang, Z.; Xu, S.; He, G.; Li, X.; Hu, N.; Yin, G.; He, D.; Zhang, L. A Review on Graphene-Based Gas/Vapor Sensors with Unique Properties and Potential Applications. Nano-Micro Lett. 2016, 8, 95–119. (26) Novoselov, K. S. Geim, A.K. Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigonrieva, I.V.; Firsov, A.A. Electric Field Effect in Atomically Thin Carbon Films. Science, 2004, 306, 666–669. (27) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Novoselov, K.S.; Geim, A.K. Graphene-Based Liquid Crystal Device. Nano Lett. 2008, 8, 1704–1708. (28) Myers, M.; Cooper, J.; Pejcic, B.; Baker, M.; Raguse, B.; Wieczorek, L. Functionalized Graphene as an Aqueous Phase Chemiresistor Sensing Material. Sens. Actuators, B 2011, 155, 154–158. (29) Snow, A.W.; Sprague, L.G.; Soulen, R.L.; Grate, J.W.; Wohltjen, H. Synthesis and Evaluation of Hexafluorodimethylcarbinol Functionalized Polymers as Microsensor Coatings. J. Appl. Polym. Sci., 1991, 43, pp.1659-1671.


27


Page 28 of 34

(30) McGill, R.A.; Abraham, M.H.; Grate, J.W. Choosing Polymer Coatings for Chemical Sensors, Chemtech. 1994, 24, 27–37. (31) Hansen, C.M. Hansen Solubility Parameters, A User’s Handbook; CRC Press: Boca Raton, FL, 2000, Appx. A1. (32) Barton, A.F.M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; 2nd ed.; CRC Press: Boca Raton, FL, 1983. (33) Zhang, T.; Mubeen, S.; Myung, N. V.; Deshusses, M. A. Recent Progress in Carbon Nanotube-Based Gas Sensors. Nanotechnology 2008, 19, 332001. (34) Wang, B.; Cancilla, J. C.; Torrecilla, J. S.; Haick, H. Artificial Sensing Intelligence with Silicon Nanowires for Ultraselective Detection in the Gas Phase. Nano Lett. 2014, 14, 933–938. (35) Paska, Y.; Stelzner, T.; Christiansen, S.; Haick, H. Enhanced Sensing of Nonpolar Volatile Organic Compounds by Silicon Nanowire Field Effect Transistors. ACS Nano 2011, 5, 5620–5626. (36) Wohltjen, H.; Snow, A.W. Colloidal Metal− Insulator− Metal Ensemble Chemiresistor Sensor. Anal. Chem., 1998, 70, 2856-2859. (37) Grate, J. W.; Nelson, D. A.; Skaggs, R. Sorptive Behavior of Monolayer-Protected Gold Nanoparticle Films: Implications for Chemical Vapor Sensing. Anal. Chem. 2003, 75, 1868– 1879.


28

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Miskell, G.; Salmond, J.; Williams, D.E. Low-Cost Sensors and Crowd-Sourced Data: Observations of Siting Impacts on a Network of Air-Quality Instruments, Sci. Total Environ., 2017, 575, 1119-1129. (39) Toxicologic Assessment of Jet-Propulsion Fuel 8, Subcommittee on Jet-Propulsion Fuel 8, Committee on Toxicology, National Research Council, National Academies Press, D.C., 2003. (40) Douglas, G. S.; Emsbo-Mattingly, S. D.; Stout, S. A.; Uhler, A. D.; McCarthy, K. J. In Hydrocarbon Fingerprinting Methods, In Introduction to Environmental Forensics, (3rd Edition); Academic Press, San Diego, 2015; Chapter 8, p. 272. (41)

Organics

in

fresh

and

weathered

gasoline

are

listed

at:

http://bcn.boulder.co.us/basin/waterworks/gasolinecomp.pdf (accessed 4/25/2018) (42) www.sensigent.com/products/cyranose.html (43) Abraham, J. K.; Philip, B.; Witchurch, A.; Varadan, V. K.; Reddy, C. C. A Compact Wireless Gas Sensor Using a Carbon Nanotube/PMMA Thin Film Chemiresistor. Smart Mater. Struct. 2004, 13, 1045. (44) Ho, C. K.; Hughes, R. C. In-Situ Chemiresistor Sensor Package for Real-Time Detection of Volatile Organic Compounds in Soil and Groundwater, Sensors. 2002, 2, 23–34. (45) Lei, H.; Pitt, W. G.; McGrath, L. K.; Ho, C. K. Resistivity Measurements of Carbon– Polymer Composites in Chemical Sensors: Impact of Carbon Concentration and Geometry. Sens. Actuators B 2004, 101, 122–132.


29


Page 30 of 34

(46) Scholten, K.; Bohrer, F.I.; Dattoli, E.; Lu, W.; Zellers, E.T. Organic Vapor Discrimination with Chemiresistor Arrays of Temperature Modulated Tin-Oxide Nanowires and ThiolateMonolayer-Protected Gold Nanoparticles, Nanotechnology 2011, 22, 125501. (47) Polyakova Stolyarova, E. Y.; Rim, K. T.; Eom, D.; Douglass, K.; Opila, R. L.; Heinz, T. F.; Teplyakov, A. V; Flynn, G. W. Scanning Tunneling Microscopy and X-Ray Photoelectron Spectroscopy Studies of Graphene Films Prepared by Sonication-Assisted Dispersion. ACS Nano 2011, 5, 6102–6108. (48) Patel, S. V.; Hobson, S. T.; Cemalovic, S.; Mlsna, T. E. Detection of Methyl Salicylate Using Polymer-Filled Chemicapacitors. Talanta 2008, 76, 872–877. (49) Mlsna, T. E.; Cemalovic, S.; Warburton, M.; Hobson, S. T.; Mlsna, D. A.; Patel, S. V. Chemicapacitive Microsensors for Chemical Warfare Agent and Toxic Industrial Chemical Detection. Sens. Actuators B 2006, 116, 192–201. (50) Nelson, G.O. Gas Mixtures: Preparation and Control, CRC Press, Boca Raton, FL, 1992. (51) Yaws, C.L. Yaws' Handbook of Antoine Coefficients for Vapor Pressure; Gulf Publishing Company, 2007. (52) Woltornist, S. J.; Oyer, A. J.; Carrillo, J. M. Y.; Dobrynin, A. V.; Adamson, D. H. Conductive Thin Films of Pristine Graphene by Solvent Interface Trapping. ACS Nano 2013, 7, 7062–7066.


30

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(53) Wang, H.; Maiyalagan, T.; Wang, X. Review on Recent Progress in Nitrogen-Doped Graphene: Synthesis, Characterization, and Its Potential Applications. ACS Catal. 2012, 2, 781– 794. (54) Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H.; Guo, J.; Dai, H. NDoping of Graphene Through Electrothermal Reactions with Ammonia. Science 2009, 324, 768771. (55) Boukhvalov, D. W. Oxidation of a Graphite Surface: The Role of Water; J. Phys. Chem. C 2014, 118, 27594−27598. (56) Hu, Y.; Sun. X. in Advances in Graphene Science; Aliofkhazraei, M., Ed.; IntechOpen, 2013; Chapter 7; pp. 161-189. (57) Walter, A. L.; Jeon, K. J.; Bostwick, A.; Speck, F.; Ostler, M.; Seyller, T.; Moreschini, L.; Kim, Y. S.; Chang, Y. J.; Horn, K.; Rotenberg, E. Highly p-Doped Epitaxial Graphene Obtained by Fluorine Intercalation. Appl. Phys. Lett. 2011, 98, 184102. (58) Shen B.; Chen J.; Xue Q. Synthesis of Fluorine-Doped Multi-Layered Graphene Sheets by Arc-Discharge. RSC Advances 2012, 2, 6761-6764. (59) Mattson, E. C.; Pande, K.; Unger, M.; Cui, S.; Lu, G.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J.; Hirschmugl, C. J. Exploring Adsorption and Reactivity of NH3 on Reduced Graphene Oxide. J. Phys. Chem. C 2013, 117, 10698–10707.


31


Page 32 of 34

(60) Tripathi, K.M.; Kim, T.; Losic, D.; Tung, T.T. Recent Advances in Engineered Graphene and Composites for Detection of Volatile Organic Compounds (VOCs) and Non-Invasive Diseases Diagnosis. Carbon 2016, 110, 97-129. (61) Currie, L.A. Nomenclature in Evaluation of Analytical Methods Including Detection and Quantification Capabilities (IUPAC Recommendations 1995). Pure Appl. Chem. 1995, 67, 16991723. LOD = 3.29 * σ0 * C / ∆R (62) Currie, L.A. Detection and Quantification Limits: Origins and Historical Overview. Anal. Chim. Acta 1999, 391, 127-134. (63) Simplified relation for the limit of detection based on the IUPAC standard; assumes constant standard deviation and requires = 95% confidence of positive detection. (64) Reference exposure limits (REL) are from NIOSH Pocket Guide to Chemical Hazards, http://www.cdc.gov/niosh/npg/; TWA = time-weighted average; STEL = short-term exposure limit. (65) Limits in the table for TCE are: American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV): 10 ppmV TWA; 25 ppmV STEL; for comparison, the NIOSH RELs are 2ppmV for 10hr TWA. ACGIH values are used as they use the

same

time

frame

as

the

other

data

in

the

table.

www.osha.gov/dts/chemicalsampling/data/CH_273000.html (accessed 5/14/2018)


32

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(66) www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10042; accessed 9/13/2016. (67) 4/12/2017 Personal communication with: Graphene Laboratories, Inc. Customer Support Team, Calverton, NY. www.graphene-supermarket.com (68) Capasso, A.; Del Rio Castillo, A.E.; Sun, H.; Ansaldo, A.; Pellegrini, V.; Bonaccorso, F. Ink-Jet Printing of Graphene for Flexible Electronics: An Environmentally-Friendly Approach. Solid State Commun. 2015, 224, 53–63.


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