Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38062-38067
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Metallic versus Semiconducting SWCNT Chemiresistors: A Case for Separated SWCNTs Wrapped by a Metallosupramolecular Polymer Shinsuke Ishihara,*,† Curtis J. O’Kelly,†,‡ Takeshi Tanaka,§ Hiromichi Kataura,§ Jan Labuta,† Yoshitaka Shingaya,† Tomonobu Nakayama,† Takeo Ohsawa,∥ Takashi Nakanishi,† and Timothy M. Swager⊥ †
World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), ‡International Center for Young Scientists (ICYS), and ∥Research Center for Functional Materials, National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan § Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ⊥ Department of Chemistry, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: As-synthesized single-walled carbon nanotubes (SWCNTs) are a mixture of metallic and semiconducting tubes, and separation is essential to improve the performances of SWCNT-based electric devices. Our chemical sensor monitors the conductivity of an SWCNT network, wherein each tube is wrapped by an insulating metallosupramolecular polymer (MSP). Vapors of strong electrophiles such as diethyl chlorophosphate (DECP), a nerve agent simulant, can trigger the disassembly of MSPs, resulting in conductive SWCNT pathways. Herein, we report that separated SWCNTs have a large impact on the sensitivity and selectivity of chemical sensors. Semiconducting SWCNT (S-SWCNT) sensors are the most sensitive to DECP (up to 10000% increase in conductivity). By contrast, the responses of metallic SWCNT (M-SWCNT) sensors were smaller but less susceptible to interfering signals. For saturated water vapor, increasing and decreasing conductivities were observed for S- and M-SWCNT sensors, respectively. Mixtures of M- and S-SWCNTs revealed reduced responses to saturated water vapor as a result of canceling effects. Our results reveal that S- and M-SWCNTs compensate sensitivity and selectivity, and the combined use of separated SWCNTs, either in arrays or in single sensors, offers advantages in sensing systems. KEYWORDS: gas sensors, chemiresistors, chemical warfare agents, carbon nanotubes, semiconductors, supramolecular polymers, metal ligand complexes
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INTRODUCTION Low-cost, portable, low-energy, and internet-connected chemical sensors can be differentiated from conventional disposable chemical sensors (e.g., colorimetric indicator paper and detector tube), in that big data readily collected from numerous sensors can be monitored, stored, shared, and analyzed at real time.1,2 Such advanced chemical sensor networks3 will offer opportunities to improve personal health, public safety, and security.4,5 Chemiresistive materials,6 which display changes in electric resistance in response to chemical stimuli, are ideally suited to meet this goal because resistance can be monitored without complicated signal transduction. Single-walled carbon nanotubes (SWCNTs) are promising chemiresistive materials that operate at ambient temperatures,7−11 which is in contrast to metal oxide semiconductors that generally require heating over 200 °C.6 The resistance of the SWCNT network is © 2017 American Chemical Society
influenced by factors including (i) the doping state of individual tubes, (ii) the charge transfer across tube−tube junction, and (iii) the Schottky barrier at tube−electrode junctions. Each of these processes can be influenced by molecular events (e.g., physisorption, chemisorption, reaction, and swelling) on or near the SWCNT surfaces, thereby creating resistive responses for specific chemical analytes.11 It is well-known that the chirality and diameter of a SWCNT significantly affect its optoelectrical properties, and assynthesized SWCNTs are a mixture of metallic and semiconducting tubes.12 Separation of these different species has proven to be essential to improve the performances of Received: August 28, 2017 Accepted: October 12, 2017 Published: October 12, 2017 38062
DOI: 10.1021/acsami.7b12992 ACS Appl. Mater. Interfaces 2017, 9, 38062−38067
Research Article
ACS Applied Materials & Interfaces
perturbations, would be expected to provide a superior sensor. It is also important to understand the sensitivity of metallic and semiconducting behavior of the SWCNTs on confounding factors, such as humidity, oxygen, and temperature. In this paper, we report a study using separated M- and SSWCNTs, which reveals their role in determining the sensitivity and selectivity in our MSP-based chemical sensors. S-SWCNTs displayed the highest sensitivity to the target analytes, whereas M-SWCNTs had lower responses to confounding stimuli. Mixtures of M- and S-SWCNTs reveal a merged character of each and offer some advantages such as a reduced response to saturated water vapor. Our results imply that chemiresistors using separated SWCNTs can make for more reliable and advanced sensing systems.
SWCNT-based electric devices. For example, separated semiconducting SWCNTs (S-SWCNTs) improve the on/off switching ratio of field-effect transistors,13−15 and separated metallic SWCNTs (M-SWCNTs) perform optimally in transparent conductive films.16,17 There are various approaches to separate S- and M-SWCNTs from the as-synthesized bulk materials.12 However, the separated SWCNTs have rarely been utilized for chemical sensors,7−11,18−20 presumably as a result of high cost of materials as well as difficulty of removing the dispersing reagent used in the purification process. For example, wrapping the SWCNTs with π-conjugate polymers offer facile and large-scale separation of SWCNTs;21−23 however, this method suffers from subsequent removal of polymers from the SWCNT except for a few examples that utilize supramolecular polymers.24,25 Recently, some of us reported highly sensitive and selective chemiresistive gas sensors prepared by wrapping SWCNTs with a Cu2+-based MSP (Figure 1a).26 The MSP had integrated
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RESULTS AND DISCUSSION The separation of SWCNTs was carefully designed and performed to minimize the influences of dispersing reagents. First, the as-synthesized SWCNTs (HiPco, NanoIntegris, USA) were sonicated and dispersed in water using the mixed surfactant (sodium dodecyl sulfate and sodium cholate). Next, the surfactant-wrapped SWCNTs were separated by a Sephacryl gel (GE Healthcare) column, as reported previously.27 The Sephacryl gel strongly adsorbs the S-SWCNTs on top of the column, and the fraction containing M-SWCNTs was collected as an eluant. After the complete collection of MSWCNTs, S-SWCNTs are extracted by using a sodium deoxycholate surfactant by aqueous elution. This method enables continuous and large-scale separation of SWCNTs with excellent purity (ca. 95% for S-SWCNTs and ca. 90% for MSWCNTs). In contrast to polymer-based dispersing reagents, the surfactant is easily removed from the SWCNTs by washing with solvents because the surfactant molecules are incapable of maintaining an assembled structure below critical micelle concentrations. To determine if the separation processes (e.g., sonication and surfactant) had irreversibly altered the M- and S-SWCNTs, we have reconstituted the mixtures to give compositions similar to as-synthesized SWCNTs (i.e., MSWCNTs/S-SWCNTs = 1:2, denoted as M/S-SWCNTs). Our MSP readily dispersed the separated SWCNTs in toluene/o-dichlorobenzene (DCB) mixtures under 30 min of sonication. The color of M-SWCNT/MSP dispersions has a red−green tint and S-SWCNT/MSP dispersions were green tinted, reflecting the original colors of separated SWCNTs.28 As shown in Figure 2a, the UV−vis−NIR absorption spectra of SWCNT/MSP dispersions displayed the characteristic transition bands associated with M-SWCNTs (M11) and SSWCNTs (S11), confirming a successful separation. The S22 transition bands associated with S-SWCNTs are overlapped with the absorption of MSPs. The UV−vis−NIR absorption spectrum of M- and S-SWCNT/MSP compositions represents a summation of SWCNTs and MSP contributions. Compared with those of S-SWCNTs, the S11 transitions of the SSWCNTs/MSPs are characteristically bathochromically shifted by 10−30 nm. By contrast, the M11 transitions of M-SWCNTs and M-SWCNTs/MSPs were almost identical. This result reflects that the electronic structure of S-SWCNTs is influenced by the MSP. In addition, Raman spectra of S-SWCNTs demonstrated a split in the G′ band upon interaction with the MSP, which also implies the electronic interaction between SSWCNTs and the MSP (Figure S2).29 Changes in G′ band by MSPs were not observed for M-SWCNTs. X-ray photoelectron spectroscopy peaks of S-SWCNTs/MSPs corresponding to
Figure 1. (a) Schematic illustration of chemiresistive sensors composed of SWCNTs and metallosupramolecular polymers (MSPs). (b) Chemical structure of an MSP. (c) Sensing device constituted by SWCNTs/MSPs bridging two gold electrodes. Monitoring of electric current (I(t)) under 0.1 V.
polarizable anthracene groups, solubilizing alkyl chains, binary salicylaldimine ligands, and square-planar Cu2+ ions (Figure 1b). The square-planar configuration ensures that the anthracene units are aligned in the same plane, effectively interacting with SWCNTs through π−π interactions. Thus, the MSP is very effective at wrapping and debundling SWCNTs. The macromolecular structure is critical, and in the absence of Cu2+, the ligand monomer is incapable of dispersing SWCNTs. With this condition established, the triggered disassembly of MSPs by strong electrophiles, such as diethyl chlorophosphate (DECP, a nerve agent simulant) and/or thionyl chloride (a choking agent simulant), causes unwrapping of the MSP from the SWCNT surface. Unwrapping of the MSP by electrophiles leads to a dosimetric increase in conductivity (i.e., response is proportional to the concentration of the analyte and exposure time), and thus our sensors (Figure 1c) can detect 0.1 ppm DECP. In our previous work, as-synthesized SWCNTs were used to prepare sensors. To develop these concepts further, it is important to understand if the chemical transduction relies only on electrical connectivity between carbon nanotubes and/or if the disassembly process also results in changes of the conductance of the nanotubes. In the former, purely MSWCNTs having a robust conductance would be preferred. In the latter, or a confluence of the two mechanisms, S-SWCNTs, with limited numbers of carriers that are sensitive to electronic 38063
DOI: 10.1021/acsami.7b12992 ACS Appl. Mater. Interfaces 2017, 9, 38062−38067
Research Article
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sensor was fixed (Figure S5). The sensor response was determined by monitoring the electric current (I(t)) of several sensors simultaneously using a multiplexer, under a constant voltage (0.1 V). Sensing responses were evaluated as the normalized change in electric current described by (I(t) − I0)/ I0 × 100%, where I0 is the baseline electric current and I(t) is the electric current at time t. This normalized procedure takes into account the small differences in the initial resistance resulting from our manual fabrication process and allows for clear device to device comparisons. Figure 3a details the magnitude of response of sensors to ppm-level vapors from DECP30 scales as S-SWCNTs/MSPs >
Figure 2. (a) Ultraviolet−visible−near-infrared (UV−vis−NIR) spectra of separated SWCNTs, MSP, and SWCNTs/MSP in odichlorobenzene (DCB)/toluene mixtures (2:1 in volume). [SWCNT] = 0.033 mg/mL, [MSP] = 0.17 mg/mL, and optical length = 1 mm. Spectra were shifted vertically for clarity. (b) Dynamic force microscopy (DFM) image of M/S-SWCNTs/MSP drop-casted on SiO2/Si substrate. [SWCNT] = 0.08 mg/mL, [MSP] = 0.40 mg/ mL, and DCB/toluene = 1:4 (in volume). (c) DFM image of M/SSWCNTs/MSP spin-coated on SiO2/Si substrate from diluted dispersed solution ([SWCNT] = 0.008 mg/mL and [MSP] = 0.040 mg/mL). Concentration of SWCNTs and MSPs were estimated based on the degree of dilution from the stock solution.
carbon (C 1s) and nitrogen atoms (N 1s) are shifted from those of S-SWCNTs and MSPs, suggesting that anthracene and imine moieties in the MSP are interacting with the SWCNT surface (Figure S3). Figure 2b shows a DFM image of a dropcast film of M/S-SWCNTs/MSPs with similar order of density of the SWCNT networks used in our sensors. The SWCNTs/ MSPs appeared more aggregated/bundled than expected, and this is likely to occur during solvent evaporation. Spin coating at low concentration appears to avoid this behavior to give a lower density coverage of well-isolated SWCNTs (Figure 2c). The height of the SWCNTs/MSPs determined by DFM was approximately 1.1−1.2 nm (Figure S4), which is consistent with the expected values for debundled HiPco SWCNTs. Chemiresistive sensors were prepared by drop-casting a homogeneously dispersed solution of SWCNTs/MSPs on gold electrodes (Figure 1c). Depositing the same quantities of SWCNTs/MSPs revealed that the resistances of sensors prepared from S-SWCNTs/MSPs were an order of magnitude higher than that of M-SWCNTs/MSPs, reflecting the higher conductivity of the latter. For testing, we adjusted the amounts for all as-produced sensors to be approximately 1 MΩ. The adjustment of initial resistance causes the difference in the loading amount of SWCNTs/MSPs on each sensor; however, we have confirmed that similar sensing responses can be obtained when the loading amount of SWCNTs/MSPs on each
Figure 3. (a) Normalized electric current trace of SWCNT/MSP sensors upon exposure to ppm-level vapors from DECP in a N2 flow (300 mL/min). Initial resistances of sensors were 1.0 MΩ for M/SSWCNTs, 1.4 MΩ for S-SWCNTs, and 1.2 MΩ for M-SWCNTs. (b) Normalized electric current trace of S-SWCNT/MSP sensors during a field test. Whole measurement was performed in static air. Sensors were physically transferred between room (23 ± 1 °C, 30% RH) and outside (8.3 ± 1 °C, 30% RH) temperature and then exposed to saturated DECP for 3 s. Initial resistance of the sensors was 2.2 MΩ. DECP vapor was directly taken from a freshly opened reagent bottle using a plastic syringe.
M/S-SWCNTs/MSPs > M-SWCNTs/MSPs, thereby indicating a higher sensitivity from S-SWCNTs. If the threshold for detection is conservatively set at 100% increase in conductivity, successful analyte detection occurs in 5 s with an S-SWCNT/ MSP sensor. The superior response of the S-SWCNTs indicates that the transduction involves increasing the conductance of individual S-SWCNTs and enhanced inter38064
DOI: 10.1021/acsami.7b12992 ACS Appl. Mater. Interfaces 2017, 9, 38062−38067
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Figure 4. (a) Chemiresistive responses of SWCNT/MSP sensors upon 100 s exposure to various vapors in N2 (300 mL/min). Responses to these analytes were nearly saturated within several tens of seconds. Response to temperature change in dry N2 is also shown in the right. (b) Electric current trace of SWCNT/MSP sensors under dry N2 flow (300 mL/min) and static air. (c) Electric current trace of SWCNT/MSP sensors under dry N2 and dry O2 flow (1 L/min).
junctions is a major portion of the response resulting from the removal of the insulating MSP wrapper. The viability of our sensors to function under real-world condition was confirmed through a field test (Figure 3b). The S-SWCNTs/MSP sensors experience background responses (±20%) when moved between a warm room (23 ± 1 °C, 30% RH) and the cold outside (8.3 ± 1 °C, 30% RH) environment. However, this background response, which is largely attributed to temperature, was found to be reversible and is negligible in comparison with much greater response to saturated DECP vapor (up to ca. 10000%). As we reported previously,26 SWCNT/MSP sensors display excellent sensitivity to electrophiles such as DECP, thionyl chloride, acetyl chloride, and trifluoroacetic acid. The sensors display much weaker responses to water as well as common volatile organic compounds (Figures 4a and S6−S14). When compared with M-SWCNTs/MSP, S-SWCNT/MSP sensors exhibit higher sensitivity to polar chemicals (e.g., water, alcohols, and amines) as well as temperature. In the case of nonpolar chemicals (e.g., toluene), responses of M-SWCNTs/ MSPs and S-SWCNTs/MSPs are similar because this effect is governed by swelling of the network. In any case, the use of SSWCNTs is effective in enhancing the selectivity for the target analyte (e.g., DECP) over nonpolar chemicals. Interestingly, opposite responses to saturated water vapor were observed for M-SWCNTs/MSPs and S-SWCNTs/MSPs, with M/S-SWCNTs/MSPs demonstrating a reduced response to saturated water vapor by an apparent cancelation of signals (Figure S6). It is unclear why M- and S-SWCNTs show opposite responses to saturated water vapor. However, M/S-
SWCNTs and SWCNT−electrode junction electrical transport. The role of the electrode interfaces should not be overlooked, and the resistance at the metal−semiconducting junction (i.e., Schottky barrier) can also be highly sensitive to the SWCNT electronic states.31,32 The response of the M/S-SWCNT/MSP sensor was decidedly lower than that of S-SWCNTs/MSPs. Although smaller, the response of the M-SWCNT/MSP sensor gives a very significant response (ca. 200%) to ppm-level vapors from DECP. The conductivity of individual M-SWCNTs is understood to be largely independent from external stimuli as a result of the absence of a band gap.7 Therefore, we assume that the observed response in M-SWCNT/MSP sensors is the result of better electric inter-M-SWCNT contact created by DECPtriggered disassembly of MSPs. After the sensor responses to DECP reached saturation, they were exposed to air, revealing gradual decrease in electric current for S-SWCNTs/MSPs and M/S-SWCNTs/MSPs (Figure 3a). The decrease of response was further accelerated when these sensors were exposed to wet air (>90% RH), indicating that the enhanced device conductance was quenched in part by water. However, after exposure to wet air, the SSWCNT/MSP response is maintained at ca. 1500%, a value that is still very high relative to typical SWCNT chemiresistor sensory responses.6−11 If it is assumed that the water reverses a majority of the analyte-induced intra-SWCNT conductance, then the irreversible part of the response can be inferred to be the result of improved electric transport across the interSWCNTs and SWCNT-electrode junctions. Clearly, it is not possible to confirm such an assertion with the data at hand; however, it is very likely that improved conductance through 38065
DOI: 10.1021/acsami.7b12992 ACS Appl. Mater. Interfaces 2017, 9, 38062−38067
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ACS Applied Materials & Interfaces SWCNT/MSP sensors may offer superior performance under conditions with large humidity variations. Baseline drifts are often observed in SWCNT sensors with changes in carrier gases, and this is often the result of doping by surface absorption of oxygen. Hence, the direction of baseline shift is opposite under N2 and air, and the magnitude of the shift between the compositions scales as S-SWCNTs/MSPs > M/S-SWCNTs/MSPs > M-SWCNTs/MSPs (Figure 4b). Thus, M-SWCNTs/MSPs are superior in terms of a steady signal. A lack of stability in SWCNT sensors is often ascribed to reorganization of the aggregation of SWCNTs.33 However, aggregation is not an issue because (i) SWCNT/MSP sensors are stable over months when preserved under dry N2 and (ii) the baseline shifts are reversible. The baseline shift derived from O2 could be the result of hole−carrier injection for SSWCNTs.34,35 However, all our SWCNT/MSP sensors show only tiny responses when the carrier gas was replaced from dry N2 to pure dry O2 (Figure 4c). Other studies have reported that SWCNTs are intrinsically sensitive to ppb levels of NOx.7 As air contains such levels of NOx,36 it is possible that baseline shift comes from these pollutants contained in air. NOx molecules cumulatively bind on the SWCNT surfaces, and UV irradiation is capable of promoting desorption of these molecules (Figure S15).37
Timothy M. Swager: 0000-0002-3577-0510 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by World Premier International (WPI) Research Center Initiative on Materials Nanoarchitectonics (MANA) from Japan Society for the Promotion of Science (JSPS) and National Science Foundation (DMR1410718). This work was partly supported by JSPS KAKENHI, grant no. 25220602. Kumiko Hara is acknowledged for assisting the research.
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CONCLUSIONS In conclusion, separated S- and M-SWCNTs, respectively, demonstrated different sensitivities and selectivities when wrapped with MSPs. S-SWCNTs displayed the largest sensitivity to the target analytes, and ppm-level vapors from DECP could be discriminated in 5 s. S-SWCNTs have advantages for commercialization because larger responses are obtained. By contrast, the responses of M-SWCNT sensors were smaller but less prone to confounding signals. Thus, MSWCNT sensors could have advantages when small variations of conductivity are monitored by precise instrumentation. M/SSWCNT sensors have intermediate signals that incorporate attributes of both S- and M-SWCNT sensors and give reduced response to saturated water vapor relative to pure M- and SSWCNT/MSP sensors, as a result of the merged character of Sand M-SWCNTs. We expect that this “cancellation” effect can be generalized by mixing two types of chemiresistive materials (with opposite response) in a single sensor network to cancel out unwanted signals.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12992. Materials, general experimental procedures, sensing experimental procedures, and detailed sensing data (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shinsuke Ishihara: 0000-0001-6854-6032 Takeshi Tanaka: 0000-0001-7547-7928 Hiromichi Kataura: 0000-0002-4777-0622 Takashi Nakanishi: 0000-0002-8744-782X 38066
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