Metallic versus Semiconducting SWCNT Chemiresistors: A Case for

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Metallic vs. Semiconducting SWCNT Chemiresistors: A Case for Separated SWCNTs Wrapped by Metallo-Supramolecular 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12992 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 14, 2017

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ACS Applied Materials & Interfaces

Metallic vs. Semiconducting SWCNT Chemiresistors: A Case for Separated SWCNTs Wrapped by Metallo-Supramolecular 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), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan ‡ International Center for Young Scientists (ICYS), NIMS, Tsukuba, Ibaraki 305-0044, Japan § Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan # Research Center for Functional Materials, NIMS, Tsukuba, Ibaraki 305-0044, Japan ¶ Department of Chemistry, Massachusetts Institute of Technology (MIT), Cambridge, Massachusetts 02139, United States 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 SWCNTs-based electric devices. Our chemical sensor monitors the conductivity of a SWCNTs network, wherein each tube is wrapped by insulating metallo-supramolecular polymer (MSP). Vapors of strong electrophiles such as diethyl chlorophosphate (DECP), a nerve agent simulant, can trigger disassembly of MSP, resulting in conductive SWCNTs pathways. Herein, we report that separated SWCNTs have a large impact on the sensitivity and selectivity of chemical sensors. Semiconducting SWCNTs (SSWCNTs) sensors are the most sensitive to DECP (up to 10,000% increase in conductivity). In contrast, the responses of metallic SWCNTs (M-SWCNTs) sensors were smaller, but less susceptible to interfering signals. For saturated water vapor, increasing and decreasing conductivity were observed for S-SWCNTs and M-SWCNTs sensors, respectively. Mixtures of metallic and semiconducting SWCNTs revealed reduced responses to saturated water vapor as a result of cancelling effects. Our results reveal that semiconducting and metallic SWCNTs compensate sensitivity and selectivity, and the combined use of separated SWCNTs, either in arrays or in single sensors, offer advantages in sensing systems.

KEYWODS: Gas sensors, chemiresistors, chemical warfare agents, carbon nanotubes, semiconducors, supramolecular polymers, metal ligand complexes INTRODUCTION Low cost, portable, low-energy, and internet-connected chemical sensors can be differentiated from conventional disposable chemical sensors (e.g., colorimetric indicator paper, 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 sensors 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, since resistance can be monitored without complicated signal

Figure 1. (a) Schematic illustration of chemiresistive sensor composed of SWCNTs and MSP. (b) Chemical structure of MSP. (c) Sensing device constituted by SWCNTs/MSP bridging two gold electrodes. Electric current (I(t)) under applying 0.1 V was monitored. transduction. Single-walled carbon nanotubes (SWCNTs) are promising chemiresistive materials with operation at ambient temperature,7−11 which is in contrast to metal oxide semiconductors that generally require heating over 200 ºC.6 The resistance of SWCNTs network is influenced by factors including (i) the doping state of individual tubes, (ii) the charge transfer across tubetube junction, (iii) and the Schottky barrier at tube-electrode junctions. Each of these processes can be influenced by molecular events (e.g., physisorption, chemisorption, reaction, swelling) on or near the SWCNT surfaces thereby creating resistive responses for specific chemical analytes.11

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It is well known that chirality and diameter of a SWCNT significantly affect its optoelectrical properties, and as-synthesized 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 SWCNT-based electric devices. For example, separated semiconducting SWCNTs (S-SWCNTs) improve ON/OFF switching ratio of field-effect transistors,13−15 and separated metallic SWCNTs (M-SWCNTs) perform optimally in transparent conductive films16,17. There are various approaches to separate S-SWCNTs and M-SWCNTs from the as-synthesized bulk materials.12 However, 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 SWCNT with π-conjugate polymers offers facile and large scale separation of SWCNTs,21−23 however this method suffers from subsequent removal of polymers from SWCNT except a few examples that utilize supramolecular polymers.24,25 Recently, some of us reported highly sensitive and selective chemiresistive gas sensor prepared by wrapping SWCNTs with a Cu2+-based metallo-supramolecular polymer (MSP) (Figure 1a).26 The MSP had integrated 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 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 monomeric ligand monomer is incapable of dispersing SWCNTs. With this condition established, triggered disassembly of MSP by strong electrophiles such as diethyl chlorophosphate (DECP, a nerve agent simulant) and/or thionyl chloride (a choking agent simulant), causes unwrapping of MSP from SWCNT surface. Unwrapping of MSP by electrophiles leads to a dosimetric increase in conductivity (i.e., response is proportional to concentration of analyte and exposure time), and thus our sensors (Figure 1c) can detect 0.1 ppm DECP. In our previous work, assynthesized 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 M-SWCNTs 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 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, that reveals their role in determining sensitivity and selectivity in our MSP-based chemical sensors. S-SWCNTs displayed the highest sensitivity to the target analytes, while MSWCNTs 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 implies that chemiresistors using separated SWCNTs can make for more reliable and advanced sensing systems. RESULTS AND DISCUSSIONS The separation of SWCNTs was carefully designed and performed in order to minimize the influences of dispersing reagents. First, as-synthesized SWCNTs (HiPco®, NanoIntegris, USA) were sonicated and dispersed in water using the mixed surfactant

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Figure 2. (a) UV-Vis-NIR spectra of separated SWCNTs, MSP, and SWCNTs/MSP in DCB:toluene mixtures (2:1 in volume). [SWCNT] = 0.033 mg/mL, [MSP] = 0.17 mg/mL, optical length = 1 mm. Spectra were shifted vertically for clarity. (b) DFM image of M/S-SWCNTs/MSP drop-casted on SiO2/Si substrate. [SWCNT] = 0.08 mg/mL, [MSP] = 0.40 mg/mL, DCB:toluene = 1:4 (in volume). (c) DFM image of M/S-SWCNTs/MSP spincoated on SiO2/Si substrate from diluted dispersed solution ([SWCNT] = 0.008 mg/mL, [MSP] = 0.040 mg/mL). Concentration of SWCNTs and MSP were estimated based on degree of dilution from stock solution. (sodium dodecyl sulfate and sodium cholate). Next, the surfactant-wrapped SWCNTs were separated by Sephacryl®-gel (GE Healthcare) column as we reported previously.27 Sephacryl®-gel strongly adsorbs the S-SWCNTs on top of column, and the fraction containing M-SWCNTs was collected as an eluent. After complete collection of the M-SWCNTs, the 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 M-SWCNTs). In contrast to polymer-based dispersing reagents, the surfactant is easily removed from the SWCNTs by washing with solvents since 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 Mand S-SWCNTs, we have reconstituted mixtures to give compositions similar to as synthesized SWCNTs (i.e., M-SWCNTs:SSWCNTs = 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-SWCNTs/MSP dispersions have a redgreen tint and S-SWCNTs/MSP dispersions were green tinted, reflecting the original colors of separated SWCNTs.28 As shown


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in Figure 2a, the ultraviolet-visible-near-infrared (UV-Vis-NIR) absorption spectra of SWCNTs/MSP dispersions displayed the characteristic transition bands associated with M-SWCNTs (M11) and S-SWCNTs (S11), confirming a successful separation. The S22 transition bands associated with S-SWCNTs are overlapped with the absorption of MSP. The UV-Vis-NIR absorption spectrum of M- and S-SWCNTs/MSP compositions represents a summation of SWCNTs and MSP contributions. Compared with S-SWCNTs, the S11 transitions of the S-SWCNTs/MSP are characteristically bathochromically shifted by 10-30 nm. In contrast, the M11 transitions of M-SWCNTs and M-SWCNTs/MSP were almost identical. This result reflects that electronic structure of S-SWCNTs is influenced by the MSP. In addition, Raman spectra of S-SWCNTs demonstrated split in G’ band upon interaction with MSP, which also implies electronic interaction between S-SWCNTs and MSP (Figure S2).29 Changes in G’ band by MSP was not observed for M-SWCNTs. X-ray photoelectron spectroscopy peaks of SSWCNTs/MSP corresponding to carbon (C 1s) and nitrogen atoms (N 1s) are shifted from those of S-SWCNTs and MSP, suggesting that anthracene and imine moieties in MSP are interacting with SWCNTs surface (Figure S3). Figure 2b shows a dynamic force microscopy (DFM) image of drop-cast film of M/SSWCNTs/MSPs with similar order of density of the SWCNTs networks used in our sensors. The SWCNTs/MSPs appeared more aggregated/bundled than expected and this is likely occurring 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 expected values for debundled HiPco SWCNTs. Chemiresitive sensors were prepared by drop-casting a homogeneously dispersed solution of SWCNTs/MSP on gold electrodes (Figure 1c). Depositing the same quantities of SWCNTs/MSP revealed that the resistance of sensors prepared from SSWCNTs/MSP were an order of magnitude higher than that of MSWCNTs/MSP, 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 loading amount of SWCNTs/MSP on each sensor, however, we have confirmed that similar sensing responses can be obtained when the loading amount of SWCNTs/MSP on each 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 applying 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 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 ppmlevel vapors from DECP30 scales as S-SWCNTs/MSP > M/SSWCNTs/MSP > M-SWCNTs/MSP, thereby indicating a higher sensitivity from S-SWCNTs. If threshold for detection is conservatively set at 100% increase in conductivity, successful analyte detection occurs in 5 sec with a S-SWCNTs/MSP sensor. The superior response of the S-SWCNTs indicates that the transduction involves increasing the conductance of individual SSWCNTs, and enhanced inter-SWCNTs and SWCNT-electrode junction electrical transport. The role of the electrode interfaces should not be overlooked, and the resistance at metalsemiconducting junction (i.e., Schottky barrier) can also be highly sensitive to the SWCNT electronic states.31,32 The response of M/S-SWCNTs/MSP sensor was decidedly lower than that of S-

SWCNTs/MSP. Although smaller, the response of MSWCNTs/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-SWCNTs/MSP sensors is the result of better electric inter-M-SWCNTs contact created by DECP triggered disassembly of MSP. After the sensor responses to DECP reached saturation, they were exposed to air, revealing gradual decrease in electric current for S-SWCNTs/MSP and M/S-SWCNTs/MSP (Figure 3a). The decrease of response was further accelerated when these sensors were exposed to wet air (> 90% RH), indicating that the enhanced

Figure 3. (a) Normalized electric current trace of SWCNTs/MSP sensors upon exposure to ppm-level vapors from DECP in N2 flow (300 mL/min). Initial resistance of sensors were 1.0 MΩ for M/S-SWCNTs, 1.4 MΩ for S-SWCNTs, 1.2 MΩ for MSWCNTs, respectively. (b) Normalized electric current trace of S-SWCNTs/MSP sensor during field test. Whole measurement was performed in static air. Sensor was physically transferred between room (23±1 ºC, 30% RH) and outside (8.3±1 ºC, 30% RH), then exposed to saturated DECP for 3 sec. Initial resistance of sensors was 2.2 MΩ. DECP vapor was directly taken from freshly-opened reagent bottle by plastic syringe.

device conductance was quenched in part by water. However,



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Figure 4. (a) Chemiresistive response of SWCNTs/MSP sensors upon 100 sec exposure to various vapors in N2 (300 mL/min). Responses to these analytes were nearly saturated within several tens of sec. Response to temperature change in dry N2 is also shown in the right. (b) Electric current trace of SWCNTs/MSP sensors under dry N2 flow (300 mL/min) and static air. (c) Electric current trace of SWCNTs/MSP sensors under dry N2 and dry O2 flow (1 L/min).

after exposure to wet air, the S-SWCNTs/MSP response is maintained at ca. 1,500%, a value that is still very high relative to typical SWCNTs chemiresistor sensory responses.6−11 If it is assumed that the water reverses a majority of the analyte-induced intraSWCNT conductance, then the irreversible part of response can be inferred to be the result of improved electric transport across the inter-SWCNTs 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 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 SSWCNTs/MSP sensors experience background responses (±20%) when moved between a warm room (23±1 ºC, 30% RH) and the cold outside (8.3±1 ºC, 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. 10,000%). As we reported previously,26 SWCNTs/MSP sensor 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 or-

ganic compounds (VOCs) (Figure 4a, Figure S6-S14). When compared with M-SWCNTs/MSP, S-SWCNTs/MSP sensors exhibit higher sensitivity to polar chemicals (e.g., water, alcohols, amines) as well as temperature. In case of non-polar chemicals (e.g., toluene) responses of M-SWCNTs/MSP and SSWCNTs/MSP are similar, because this effect is governed by swelling of network. In any case, the use of S-SWCNTs is effective in enhancing selectivity for the target analyte (e.g., DECP) over non-polar chemicals. Interestingly, opposite responses to saturated water vapor were observed for M-SWCNTs/MSP and S-SWCNTs/MSP, with M/S-SWCNTs/MSP demonstrating a reduced response to saturated water vapor by an apparent cancelation of signals (Figure S6). It is unclear why M-SWCNTs and S-SWCNTs show opposite responses to saturated water vapor. However, M/SSWCNTs/MSP sensor may offer superior performance under conditions with large humidity variations. Baseline drifts are often observed in SWCNTs 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 magnitude of the shift between the compositions scales as S-SWCNTs/MSP > M/SSWCNTs/MSP > M-SWCNTs/MSP (Figure 4b). Thus, MSWCNTs/MSP is superior in terms of a steady signal. A lack of


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stability in SWCNTs sensors is often ascribed to reorganization of aggregation of SWCNTs.33 However, aggregation is not an issue since (i) SWCNTs/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 holecarrier injection for S-SWCNTs.34,35 However, all our SWCNTs/MSP sensors show only tiny response when 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 CONCLUSIONS In conclusion, separated S- and M-SWCNTs respectively demonstrated different sensitivities and selectivities when wrapped with MSP. S-SWCNTs displayed the largest sensitivity to the target analytes, and ppm-level vapors from DECP could be discriminated in 5 sec. S-SWCNTs have advantages for commercialization since larger responses are obtained. In contrast, responses of M-SWCNTs sensor were smaller, but less prone to confounding signals. Thus, M-SWCNTs sensors could have advantages when small variations of conductivity are monitored by precise instrumentation. M/S-SWCNTs sensor 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 S-SWCNTs/MSP sensors, as a result of merged character of S- and 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 in order to cancel out unwanted signals.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials, general experimental procedures, sensing experimental procedures, and detailed sensing data (PDF).

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT 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 (DMR-1410718). This work was partly supported by JSPS KAKENHI, grant no. 25220602. Ms. Kumiko Hara is acknowledged for assisting the research.

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