Switchable Conductive MOFâNanocarbon ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43782−43789

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Switchable Conductive MOF−Nanocarbon Composite Coatings as Threshold Sensing Architectures Pascal Freund, Irena Senkovska, and Stefan Kaskel* Inorganic Chemistry I, Technische Universität Dresden, Bergstraße 66, 01062 Dresden, Germany S Supporting Information *

ABSTRACT: Switchable metal−organic frameworks (MOFs) showing pronounced and stepwise volume changes as a response toward external stimuli such as partial pressure changes were integrated into electron conductive composites to generate novel threshold sensors with pronounced resistivity changes when approaching a critical partial pressure. Two “gate pressure” MOFs (DUT-8(Ni), DUT = Dresden University of Technology, and ELM-11, ELM = Elastic Layer-structured MOF) and one “breathing” MOF (MIL-53(Al), MIL = Material Institute Lavoisier) are shown to cover a wide range of detectable gas concentrations (∼20−80%) using this concept. The highest resistance change is observed for composites containing a percolating carbon nanoparticle network (slightly above the percolation threshold concentration). The volume change of the MOF particles disrupts the percolating network, resulting in a colossal resistance change up to 7500%. Repeated threshold detection is particularly feasible using MIL-53(Al) due to its high mechanical and chemical stability, even enabling application of the composite sensor concept in ambient environment for the detection of volatile organic compounds at high concentration levels. KEYWORDS: metal−organic frameworks, composites, switchable materials, gas sensing, threshold sensors

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INTRODUCTION Modern life and workspaces force mankind to spend an ever increasing time in closed buildingsin some cases up to 90% of their lifetime.1,2 Low-energy architecture and energy-efficient buildings are the key to a sustainable development for an ever increasing population. In such buildings accumulation of gaseous toxicants is a critical issue, as it is provoked by enhanced internal heat and air regeneration systems. Monitoring gaseous species and their feedback toward active control-regulation systems is therefore crucial for the development of safe and environmentally friendly buildings. Common less-toxic but fatiguing gases frequently surpassing critical threshold values in closed buildings are CO2 and humidity, especially in congested office rooms or lounges for only short peak times. On the other hand, accumulation of formaldehyde, or VOCs in general, caused by extensive use of plywood elements, flooring material, furniture, and textiles may cause severe health problems. Ozone is a common indoor toxicant reaching peak concentrations at office workplaces.2 A wide variety of toxic gases but only at trace levels and their impact on human health are yet to be analyzed. For advanced indoor air control threshold chemical sensors suitable for electronic integration are required, ideally offering a high degree of selectivity. They indicate via signaling whenever the maximum allowable concentration is surpassed and activate special filter systems or vents.3 For CO2 and humidity regulation at higher concentration levels is relevant. Another important application of threshold chemical sensors is the chemical industry and their processes, in which also higher critical concentrations in the range of 10−70% such as CO2 © 2017 American Chemical Society

(CH4/CO2 streams), CO in synthesis gas (CO/H2), humidity, O2 (explosion limits), or organic product concentrations in a reactor shall not be exceeded. Moreover, mining industries suffer from high levels of CO2 and methane gas present in hard coal mines. Threshold chemical sensors are also important for safety control in industry whenever a critical TLV (threshold limit value) or explosion limits (for example, for n-butane the lower and upper flammability limits are 1.8 and 8.4%, respectively) should not be surpassed.4 Metal−organic frameworks (MOFs) stand out in porosity and are highly selective in their interaction with gases and gas mixtures.5 MOFs have been proposed as gas sensing materials due to their high surface area and a number of physical response functions.6−9 A unique class of porous materials are switchable MOFs, in which gas uptake triggers a stepwise structural transition, causing a distinct volume change of the crystallites and in some cases changes in optical and magnetic properties.10−15 Switchability (stepwise structural changes) in porous solids gives rise to a number of unprecedented adsorption phenomena, such as gating,16 breathing,17 and negative gas adsorption (NGA).18 The first example of gating was discovered in a framework now termed ELM-11 (ELM = Elastic Layer-structured MOF).19 Despite the fact that quite a number of such switchable MOFs have been reported,15 so far the rational design of such solids is still under development.20 Received: September 13, 2017 Accepted: November 28, 2017 Published: November 28, 2017 43782

DOI: 10.1021/acsami.7b13924 ACS Appl. Mater. Interfaces 2017, 9, 43782−43789

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Illustration of the Stimulus-Induced MOF-Crystal Expansion Disrupting the Percolating Carbon−Nanoparticle Network Causing an Abrupt Loss of Electrical Conductivity within the Composite Coating

DMF), 2,6-naphthalenedicarboxylic acid (2,6-H2ndc) (1.51 g (7 mmol) in 75 mL of DMF), and 1,4-diazabicyclo[2.2.2]octane (dabco) (0.46 g (3.5 mmol) in 45 mL of methanol) were prepared separately, mixed together in a 250 mL Teflon inlet, placed in a stainless steel autoclave, and heated at 393 K for 48 h. After cooling, the mother liquor was decanted, and the green product washed with DMF (6 times), ethanol (3 times), and DCM (9 times) in a period of 6 days before activation under vacuum at 393 K overnight. After this activation step a color change from green to yellow occurs, indicating the formation of the closed pore form of DUT-8(Ni). Estimation of Percolation Thresholds. To estimate the conductivity percolation threshold, defined amounts of carbon black (starting at 0.1 wt % followed by 0.1 wt % stepwise increase) were added to the MOF (more details on preparation procedure are given below), and the conductivity of the resulting materials was measured. The conductivity percolation threshold was assigned to the critical concentration of the conductive carbon material in a mixture with MOF, at which the conductivity showed a stepwise change from the conductivity of the dielectric component (MOF) to the conductivity of the conductive component (CB). Synthesis of MOF/Carbon Composites. Three different approaches were tested for the preparation of the MOF/carbon composites. Approach 1: ball milling. The MOF/carbon mixture was suspended in ethanol and then treated in a ball mill for 30 s. Approach 2: stirring in ethanol. The MOF/carbon mixture was suspended in ethanol and stirred with a stirring bar at 200 rpm for 30 min. Approach 3: mixing of dry powders. The MOF/carbon composite was produced by adding a defined amount of carbon black to the MOF-powder followed by homogenization with a spatula for approximately 3 min until the mixture appeared uniform. To prepare the composites for sensing measurements, the amount of added carbon was chosen to be slightly above the estimated percolation thresholds: 6.8 wt % for ELM-11 and 5.9 wt % for DUT8(Ni) as well as for MIL-53(Al) (dry mixing). As the transformation from the activated, flexible form ELM-11 (Cu(bpy) 2 (BF 4 ) 2 ) to the rigid, hydrated pre-ELM-11 (Cu(bpy)2(BF4)2(H2O)2) takes place under ambient conditions, the resulting ELM-11-based composites were activated again under vacuum at 403 K for 3 h and subsequently handled under inert gas atmosphere. Synthesis of MIL-53(Al)/Carbon/PTFE Composites for Cyclization. Approximately 10 mg of a PTFE in ethanol suspension (60 wt %) was added to a mixture of MIL-53(Al) (90 mg) and carbon black (8 mg). Subsequently, 6 mL of ethanol was added, and the mixture was stirred with a spatula until the ethanol was evaporated. Characterization. Powder X-ray diffraction patterns were measured in transmission geometry using a STOE STADI P diffractometer operated with monochromatic Cu Kα1 (λ = 0.154 05 nm, 40 kV, 30 mA) radiation, equipped with a 0D gas-filled detector and with a scan speed of 15 s step−1 and a step size of 0.3°. Nitrogen physisorption measurements were performed at 77 K on a Quadrasorb SI apparatus from Quantachrome. The n-butane adsorption isotherms (n-butane purity: 99.95%) at 298 K up to 1 bar were measured

While the stepwise response of gating MOFs is ideal for the integration into threshold sensors, so far no conductive switchable MOF has been reported, and a platform of switchable and conductive MOFs showing well-defined transitions is yet to be established. Thus, in order to integrate nonconductive switchable MOFs into electronically addressable sensors, we have developed now nanostructured composite materials with specific response toward gases at a certain threshold value coupled to a pronounced change in conductivity (resistivity) conceptually related to swelling polymer sensor composites.21 Polymers in general show a more or less pronounced gradual swelling in the presence of gases or vapors, while gating MOFs show stepwise volume changes up to 250%.22 In the following we present the conceptual integration of such switchable MOFs, showing pronounced stepwise volume expansion, into threshold concentration sensitive devices for repeated use even under ambient humid conditions.

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EXPERIMENTAL SECTION

Materials. Copper(II) tetrafluoroborate and 4,4′-bipyridine were purchased from Sigma-Aldrich, and methanol was from VWR. Acetylene Carbon Black (CB) was purchased from ABCR (density 0.2 g/cm3, average particle diameter of 42 nm, surface area 64 m2/g). HKUST-1 was received from Materials Center Dresden. All the chemicals were used as received without further purification. Before the mixing processes, the carbon was sieved using a 250 μm sieve to suppress agglomeration of the particles. MOFs Synthesis. ELM-11. The hydrated form of ELM-11, preELM-11 (Cu(bpy)2(BF4)2(H2O)2), was synthesized based on the modified procedure reported by Cheng et al.23 Therefore, a solution (375 mL) containing copper(II) tetrafluoroborate in deionized water (0.04 mol L−1) was heated to 343 K for 2 h. Subsequently, the solution of 4,4′-bipyridine in methanol (375 mL, 0.08 mol L−1) was slowly added, and the resulting mixture was held again at 343 K for another 2 h under reflux. After settling for 15 h, the obtained mixture containing pre-ELM-11 was filtered and according to the activation procedure by Bon et al.24 soaked in ethanol at room temperature for 2 days and dried under vacuum at 393 K for 16 h to produce ELM-11 (Cu(bpy)2(BF4)2). MIL-53(Al). Following a procedure published by Loiseau et al.,17 Al(NO3)3·9H2O, 1,4-benzenedicarboxylic acid (H2bdc), and deionized water were mixed in a ratio of 1(Al) : 0.5(H2bdc) : 80(H2O) in a Teflon-inlet stainless steel autoclave and then heated up to 493 K for 72 h. After filtration, washing with deionized water, and drying, the assynthesized white product MIL-53(Al)_as (Al(OH)(bdc)(H2bdc)0.70) was heated again for 72 h at 603 K to remove the encapsulated H2bdc molecules from the pores and to obtain the MIL-53(Al)_ht. DUT-8(Ni). In a scaled-up synthesis described by Klein et al.,25 three solutions containing Ni(NO3)2·6H2O (2.03 g (7 mmol) in 30 mL of 43783


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ACS Applied Materials & Interfaces volumetrically using a BELSORP-max instrument (Microtrac BEL). Scanning electron microscopy (SEM) was performed on a HITACHI SU 8020 instrument. Resistance Measurements. To measure the resistance change in the material, 45 mg of the composite was handled in argon atmosphere and pressed into a pellet using a die set with a diameter of 8 mm and an applied force of 1.5 kg. The average thickness of the pressed composite pellet was 2 mm. Typically, the sensor was prepared under an argon atmosphere where the pellet was fixed between a glass slide and an interdigitated Au electrode (six electrode fingers with a width of 1.5 mm and a distance of 0.7 mm) before it was incorporated in the setup and exposed to a 50 mL min−1 nitrogen flow containing defined amounts of n-butane under ambient pressure. A schematic depiction of the setup can be seen in Figure S2.

coefficients are also representative for the actual expansion behavior of MOF powders under the chosen conditions in the laboratory, in a preliminary evaluation we performed macroscopic powder expansion tests for all materials at 298 K using nbutane as probe molecule (Figure 1).

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Figure 1. n-Butane triggered gate opening and accompanying volume change of the powder inside a syringe at 298 K for switchable MOFs: (a) ELM-11, (b) DUT-8(Ni), and (c) MIL-53(Al).

RESULTS AND DISCUSSION Conceptual Sensor Design and MOF Composite Preparation. The concept for such MOF-based threshold sensors is schematically depicted in Scheme 1. In this conceptual implementation design, the pristine material consists of a homogeneous composite of MOF crystallites and carbon nanoparticles in which the carbon particles form a percolating network giving rise to continuous electron conducting pathways forming a composite with an overall low specific resistivity (Scheme 1a). Switchable MOFs show a distinct guest-induced structural transformation associated with a crystal volume change at a characteristic, threshold gas concentration (partial pressure).15 Gas adsorption above this critical threshold induces a significant and stepwise volume change of the MOF crystals in the composite and subsequently the disruption of percolating carbon nanoparticles network. This causes an abrupt increase in resistivity due to the MOF expansion and loss of electron conducting pathways (Scheme 1b). This concept requires a composite containing nonconductive gating MOF with high volume change in the gating process, which volume change is able to disrupt the percolation network. We have selected three suitable switchable metal− organic frameworks varying in adsorption behavior (transition steps and gate pressures) but with well-characterized crystal structures showing structural transitions at ambient conditions, namely ELM-11, MIL-53(Al), and DUT-8(Ni).24 ELM-11 contains a Cu(4,4′-bipyridine)2+ (4,4)-net capped by two BF4− anions per Cu2+, but no covalent bonds between the layers exist. DUT-8(Ni) also consists of neutral (4,4) square nets ((Ni2+)2(2,6-ndc2−)4/2)n (2,6-ndc = 2,6-naphthalene dicarboxylate) pillared by dabco (dabco = 1,4-diazabicyclo[2.2.2]octane) via strong bonds to the N-donor ligand. ELM-11 and DUT-8(Ni) are typical gate pressure MOFs and show one transition step from the closed pore form (cp) to the large pore form (lp) after surpassing a certain pressure.16,25 The MIL-53 structure resembles a typical wine-rack motif and is known for its breathing behavior. Thus, two transition steps occur during the adsorption of e.g. n-butane: from the large pore to a narrow pore (np) and then again to the large pore form.17 Both transition processes, gate pressure and breathing, involve at least one unit cell volume expansion step while adsorbing the gas. The pore sizes calculated for lp forms using Zeo++ software (probe radius 1.2 Å) amount to 5 Å for ELM-11, 7.0 Å for MIL-53(Al), and 10.2 Å for DUT-8(Ni) (Figure S17, Supporting Information). The resulting cell volume expansion coefficients calculated from the crystal structure of the cp (or np for MIL-53) and lp phases for all three materials were reported by Bon et al. and amount to 1.5 for ELM-11 and MIL53(Al), and 2.4 for DUT-8(Ni).24 To analyze if these calculated

At ambient pressure and 100% n-butane atmosphere (298 K, p/p0 = 0.42) these tests show somewhat different expansion characteristics for powders as compared to the crystallographic expansion coefficients (cexp = Vop/Vcp): for ELM-11 cexp is larger (approximately 2.0) while DUT-8 and MIL-53(Al) show smaller volume changes (cexp(DUT-8) = 1.8 and cexp(MIL-53) = 1.3). This discrepancy is attributed to packing effects. The powders were packed using a pressure of 3 kg cm−2. Thus, the observed initial density is closer to a “tapped density”. However, the powder can freely expand without load when exposed to n-butane, resulting in a value closer to a “powder bulk density”. The differences for the three materials reflect the differences in particle size distribution (DUT-8(Ni) has up to 50 μm sized crystals while for the other materials particles are much smaller). The visual tests were crucial in the beginning to evaluate basic processing parameters as the conductive composites are prepared in an analogous approach. In order to realize the new sensor concept, it is essential to manufacture composites with a concentration of the conductive carbon material above the conductive percolation concentration (cpt) to achieve a percolating network of carbon particles inside the composite. This network provides the paths for electrons enfolding the insulating MOF particles, resulting in an overall electrically conductive composite as required for the detection principle. Table 1 shows the specific carbon percolation threshold concentrations for the composites obtained using different preparation approaches. We have explored several scenarios for preparing homogeneous carbon nanoparticle distributions and suitable composites using ELM-11, as this is the most fragile and sensitive among the three MOFs investigated with respect to processing conditions. An established method to achieve homogeneous Table 1. Percolation Threshold of the Composites Obtained by Different Mixing Methods material ELM-11/CB

MIL-53(Al)/CB DUT-8(Ni)/CB MIL-53(Al)/CB/ PTFE 43784

mixing method dry mixing EtOH stirring ball milling dry mixing dry mixing EtOH stirring

percolation threshold concn (cpt) [wt % CB] 6.5 4.0 2.0 5.2 3.6 5.4


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More importantly, the isotherms of ELM-11 for n-butane (Figure 4) demonstrate only minor changes in the adsorption

composites is ball milling which affords the lowest percolation threshold concentration. Also a slurry, produced by dispersing the MOF and carbon nanoparticles for 30 min in ethanol, leads to a stable and homogeneous mixture, but with a slightly higher cpt. However, as MOFs are often sensitive toward mechanical stress, ELM-11 is highly damaged after these processes according to SEM, XRD, and nitrogen physisorption measurements (see also Figure S1). The latter can be avoided using a solvent-free dry mixing approach. According to XRD powder patterns (Figure 2c), the original framework ELM-11 is still intact, and due to the quite gentle attrition most of the crystals remain nearly undamaged (Figure 3).26

Figure 4. n-Butane physisorption isotherms at 298 K for ELM-11 (black squares) and an ELM-11/CB composite after mixing (red triangles) and pelletizing (blue circles). Closed symbols refer to adsorption and open symbols to desorption branch.

behavior for the dry mixed sample. However, pressing the composite into a thin pellet shows a minor influence on the sample, causing a shift of the gate-opening toward higher pressure. The latter should be considered in the interpretation of the resistance measurements described in the following. As the dry mixing method was found ideal for further processing, all other composites were produced in the same way based on DUT-8(Ni) and MIL-53(Al) as active materials, taking the respective differences in cpt into account. However, the DUT-8(Ni) and MIL-53(Al) show better mechanical stability and may be in principle also formulated using other techniques. Resistance Measurements Using a Threshold Sensing Architecture. The composites are easily integrated into prototype sensor architectures using interdigitated electrodes on glass substrates. A thin pellet (maximum thickness 2 mm) of the composites was contacted to the electrodes, and the resistivity was measured vs increasing gas concentrations in a controlled gas atmosphere (see also Figure S2). As an initial proof of concept we used an ELM-11/carbon composite (6.8 wt % CB) and studied the response to n-butane by stepwise increasing the concentration from 0 to 70%. Figure 5 shows a typical response of a composite and its relative resistance change with increasing concentration of nbutane in a mixture with nitrogen. At the gate opening pressure, when the threshold concentration is surpassed, the composite shows a colossal resistivity change of up to 5000% due to the expansion of MOF crystals, with a short response time of only few seconds. In the dynamic sensor test, the threshold is reached at 30 vol % n-butane concentration, which is slightly higher as compared to the gate opening pressure observed in the isotherm for pure ELM-11 and the ELM-11/CB composite, but it is in reasonable agreement with the isotherm of a pressed pellet (Figure 4). Comparison of the flexible with a rigid MOF (HKUST-1) composite shows the significance of the measured resistance evolution; i.e., nearly no resistance change over the whole range of n-butane concentration is detected for the rigid MOF. These positive results motivated us to probe the general applicability of our concept implementing other switchable MOFs, namely DUT-8(Ni) and MIL-53(Al) (Figure 6).

Figure 2. XRD pattern of the ELM-11/carbon composites prepared using (a) ball milling, (b) stirring in EtOH, and (c) dry mixing approach compared to (d) the theoretical pattern of activated ELM11.

Figure 3. SEM images of ELM-11 (a) and ELM-11/CB composite after dry mixing (b), EtOH stirring (c), and ball milling (d). Agglomerates of carbon black are encircled.

However, dry mixing also results in the highest percolation threshold concentration requiring around 6.5% carbon black. An explanation may be gained from the SEM data in Figure 3 since we found larger CB aggregates near areas in which the CB particles are homogeneously distributed over the composite. In general in the case of ELM-11, composite formation is accompanied by partial fragmentation of the initially large (20− 40 μm) sized ELM-11 crystallites into smaller domains of only a few micrometers which may also affect the gating pressure in such materials.24 Further composite optimization is feasible for implementation and miniaturization but beyond the scope of this initial proof of concept study. 43785


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indicated by the expansion test, this has not necessarily its origin in the colossal volume change of the compound but may also originate from intrinsic textural features of the composite. However, after the measurement the pressed pellet starts to disintegrate more seriously than it was the case for ELM-11 material. DUT-8(Ni) crystals tend to fracture after adsorbing nbutane, which causes a gate pressure drift and also leads to an increased contact resistance to the electrode due to crumbling (see also Figure S4).24 To overcome this challenge, it is sufficient to increase the fraction of carbon additive in the composite beyond the percolation threshold but the resistance changes should remain in a reasonable measuring range. MIL-53(Al) discovered in the pioneering work of Férey17 is probably the most widely studied flexible MOF with a characteristic breathing transition, a two-step transformation (lp → np → lp) starting form a large pore (lp) form. After traversing through an intermediate narrow pore (np) form, the adsorption branch reaches a second step to form again lp form. MIL-53 stands out due to its high chemical and mechanical stability during preparation (Figures S5 and S6) but also represents here the material of smallest particle size. In the following we use the np form, which was obtained by subsequent purging the activated powder with n-butane and nitrogen. The MIL-53-based composite shows a characteristic threshold at 40 vol % n-butane (Figure 6b) which is clearly detected as a sharp increase of the resistivity of the composite. Our results undoubtedly demonstrate the general applicability of the new MOF-threshold sensor concept using either gating or breathing MOFs showing stepwise transitions at welldefined partial pressure. By choosing the appropriate MOF, the threshold of the sensor can be adjusted in a wide range of concentrations (30, 40, and 70% n-butane for ELM-11, MIL53(Al), and DUT-8(Ni), respectively). It is not the aim of this contribution to establish a trace gas sensor but to establish the general concept for the integration of switchable MOFs into threshold sensors at potentially low cost. Cycling Characteristics of MOF-Composite Threshold Sensors. Repeated usability of threshold sensors is not necessarily a prerequisite for filter systems, as contaminated toxic gas filters are typically disposed after exposure. However, a stable cycling is beneficial for systems that are regenerated and reused several times. In the following, as a proof of concept we demonstrate the repeated detection for at least ten concentration gradient cycles resulting in comparable resistance change for each cycle. Two aspects are essential to achieve repeated threshold detection: (i) The formation of macroscopic deformations and cracks in the pellet-shaped composite should be suppressed. This was managed using a suitable polymeric binder additive, flexible enough to enable the expansion but strong enough to preserve the composite shape. PTFE (polytetrafluoroethylene, ethanoic suspension) turned out to be a suitable binder as it does not block the MOF pores. (ii) The MOF itself needs to be intrinsically stable toward adsorption cycling even after formulation with the binder. Among the MOFs tested, only MIL-53(Al) meets both criteria. For ELM-11, the PTFE additive changes the adsorption characteristics to an impractical performance, while DUT-8(Ni) shows a history-dependent gate opening pressure and a drift toward higher n-butane concentrations with repeated cycling.24 On the other hand, MIL-53(Al) can be cycled up to 100 times without pronounced changes in breathing characteristics.

Figure 5. Relative resistance change against concentration of n-butane in nitrogen for ELM-11 (squares) and HKUST-1 (triangles) based composites.

Figure 6. Relative resistance of composites based on DUT-8 (Ni) (a) and MIL-53(Al) (b) covering different threshold concentrations of nbutane.

The DUT-8(Ni) material consists of significantly larger crystallites (about 50 μm in size) as compared to ELM-11. It also proved to be more robust against the preparation procedures than the quite fragile 2D-layered ELM-11. DUT8(Ni)-based composites show nearly no alteration of the gateopening pressure and only a negligible decrease in the overall uptake compared with pristine DUT-8(Ni) (Figure S3). Sensor measurements show even higher relative resistance changes (7500%) compared to ELM-11 composites (Figure 6a). As 43786


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raising concerns and relevant to air filtration, we were also curious if the sensor concept is applicable in an open air system. In particular, the presence of humidity may hamper the operability of the system and reduce the cycling stability. Intriguingly, operating the above-described MIL-53/CB/ PTFE composite under ambient conditions demonstrates stable cycling (Figure 8), and clear threshold detection with only minor indications of aging is observed.

Figure 7 shows the cycling behavior of a pelletized composite prepared from a mixture of MIL-53(Al), carbon additive, and

Figure 7. Resistance measurement of the MIL-53/CB/PTFE composite in a closed system and dry atmosphere during ten cycles (a) and relative resistance change and corresponding response time for each cycle (b).

PTFE. For each cycle the sample was flushed alternately with nitrogen (100%, 40 min) and n-butane (50 vol %, 20 min). The overall resistance change for all ten cycles is lower for the PTFE containing composites due to the partial expansion suppression by the binder additive. Figure 7b illustrates the relative resistance changes as well as the response times (tr) for each cycle, determined for either a relative resistance change of 5% or 10%. We hereby observed retardation from the first to the following cycles. Whereas tr in the first cycle is in the range of 30−40 s, for the following cycles tr is always significantly longer. In parallel, the relative resistance change drops after the first cycle. Analyzing the raw data (Figure 7a), we observed the maximum in the absolute resistance retaining a constant level, but the “background” resistance value after desorption and shortly before each measurement increases after the first cycle. This phenomenon may originate from an increased contact resistance to the electrode, incomplete composite contraction after the nitrogen purge, or a small fraction of MIL-53 particles retaining in the lp form. However, for all cycles the resistance change is quite pronounced and threshold detection in this binary gas mixture system is clearly demonstrated. As n-butane also represents a highly volatile organic compound (VOC), and thus a class of compounds frequently

Figure 8. Resistance measurement for the MIL-53/CB/PTFE composite in an open system under atmospheric conditions for ten cycles (a) and corresponding relative resistance change and response time for each cycle (b).

Again, the resistance change for the first cycle is higher than for the following cycles, and the overall resistance change is again slightly diminished. However, threshold indication is evident even for the 10th cycle. The latter is a remarkable finding and supports the robustness of the new sensor concept.

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CONCLUSION In a “proof of concept” study we have demonstrated for the first time implementation of switchable MOFs into threshold sensors using an efficient and low-cost transducer principle suitable for electronic integration. So far we have only explored the applicability of such systems toward higher concentrations as it is relevant for threshold sensing in monitoring industrial feed streams, VOC filter architectures, or high-pressure storage cylinders containing organics, CO2, or other gases. However, within the several thousand MOF compounds known, the family of switchable MOFs is an emerging one. Eventually, rational prediction or serendipity may even lead to MOFs 43787


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(5) Sabetghadam, A.; Seoane, B.; Keskin, D.; Duim, N.; Rodenas, T.; Shahid, S.; Sorribas, S.; Le Guillouzer, C.; Clet, G.; Tellez, C.; Daturi, M.; Coronas, J.; Kapteijn, F.; Gascon, J. Metal Organic Framework Crystals in Mixed-Matrix Membranes: Impact of the Filler Morphology on the Gas Separation Performance. Adv. Funct. Mater. 2016, 26 (18), 3154−3163. (6) Stassen, I.; Burtch, N.; Talin, A.; Falcaro, P.; Allendorf, M.; Ameloot, R. An Updated Roadmap for the Integration of Metal− Organic Frameworks with Electronic Devices and Chemical Sensors. Chem. Soc. Rev. 2017, 46 (11), 3185−3241. (7) Campbell, M.; Dincă, M. Metal−Organic Frameworks as Active Materials in Electronic Sensor Devices. Sensors 2017, 17 (5), 1108. (8) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal-Organic Framework Materials as Chemical Sensors. Chem. Rev. 2012, 112 (2), 1105−1125. (9) Hendon, C. H.; Rieth, A. J.; Korzynski, M. D.; Dinca, M. Grand Challenges and Future Opportunities for Metal-Organic Frameworks. ACS Cent. Sci. 2017, 3 (6), 554−563. (10) Horike, S.; Shimomura, S.; Kitagawa, S. Soft Porous Crystals. Nat. Chem. 2009, 1 (9), 695−704. (11) Deng, H.; Olson, M. A.; Stoddart, J. F.; Yaghi, O. M. Robust Dynamics. Nat. Chem. 2010, 2 (6), 439−443. (12) Coudert, F.-X. Responsive Metal−Organic Frameworks and Framework Materials: Under Pressure, Taking the Heat, in the Spotlight, with Friends. Chem. Mater. 2015, 27 (6), 1905−1916. (13) Sakata, Y.; Furukawa, S.; Kondo, M.; Hirai, K.; Horike, N.; Takashima, Y.; Uehara, H.; Louvain, N.; Meilikhov, M.; Tsuruoka, T.; Isoda, S.; Kosaka, W.; Sakata, O.; Kitagawa, S. Shape-Memory Nanopores Induced in Coordination Frameworks by Crystal Downsizing. Science 2013, 339 (6116), 193−196. (14) Bennett, T. D.; Cheetham, A. K.; Fuchs, A. H.; Coudert, F.-X. Interplay between Defects, Disorder and Flexibility in Metal-Organic Frameworks. Nat. Chem. 2016, 9 (1), 11−16. (15) Schneemann, A.; Bon, V.; Schwedler, I.; Senkovska, I.; Kaskel, S.; Fischer, R. A. Flexible Metal−Organic Frameworks. Chem. Soc. Rev. 2014, 43 (16), 6062−6096. (16) Li, D.; Kaneko, K. Hydrogen Bond-Regulated Microporous Nature of Copper Complex-Assembled Microcrystals. Chem. Phys. Lett. 2001, 335 (1), 50−56. (17) Loiseau, T.; Serre, C.; Huguenard, C.; Fink, G.; Taulelle, F.; Henry, M.; Bataille, T.; Férey, G. A Rationale for the Large Breathing of the Porous Aluminum Terephthalate (MIL-53) Upon Hydration. Chem. - Eur. J. 2004, 10 (6), 1373−1382. (18) Krause, S.; Bon, V.; Senkovska, I.; Stoeck, U.; Wallacher, D.; Többens, D. M.; Zander, S.; Pillai, R. S.; Maurin, G.; Coudert, F.-X.; Kaskel, S. A Pressure-Amplifying Framework Material with Negative Gas Adsorption Transitions. Nature 2016, 532 (7599), 348−352. (19) Kanoh, H.; Kondo, A.; Noguchi, H.; Kajiro, H.; Tohdoh, A.; Hattori, Y.; Xu, W.-C.; Inoue, M.; Sugiura, T.; Morita, K.; Tanaka, H.; Ohba, T.; Kaneko, K. Elastic Layer-Structured Metal Organic Frameworks (ELMs). J. Colloid Interface Sci. 2009, 334 (1), 1−7. (20) Kitagawa, S. Future Porous Materials. Acc. Chem. Res. 2017, 50 (3), 514−516. (21) Wisser, F. M.; Grothe, J.; Kaskel, S. Nanoporous Polymers as Highly Sensitive Functional Material in Chemiresistive Gas Sensors. Sens. Actuators, B 2016, 223, 166−171. (22) Bon, V.; Klein, N.; Senkovska, I.; Heerwig, A.; Getzschmann, J.; Wallacher, D.; Zizak, I.; Brzhezinskaya, M.; Mueller, U.; Kaskel, S. Exceptional Adsorption-Induced Cluster and Network Deformation in the Flexible Metal-Organic Framework DUT-8(Ni) Observed by in Situ X-Ray Diffraction and EXAFS. Phys. Chem. Chem. Phys. 2015, 17 (26), 17471−17479. (23) Cheng, Y.; Kajiro, H.; Noguchi, H.; Kondo, A.; Ohba, T.; Hattori, Y.; Kaneko, K.; Kanoh, H. Tuning of Gate Opening of an Elastic Layered Structure MOF in CO2 Sorption with a Trace of Alcohol Molecules. Langmuir 2011, 27 (11), 6905−6909. (24) Bon, V.; Kavoosi, N.; Senkovska, I.; Kaskel, S. Tolerance of Flexible MOFs toward Repeated Adsorption Stress. ACS Appl. Mater. Interfaces 2015, 7 (40), 22292−22300.

showing structural changes as a response to significantly lower partial pressures. Recent discoveries indicate gating pressures to be adjustable to a wide extent by manipulating crystal size13,15,27 or defects,28 and some switchable MOFs such as Co(BDP), which is prominent due to five steps in the nitrogen isotherm, show a first step already at very low concentration (0.1%).29 The latter is promising for the application of gating MOFs at lower concentration in threshold sensors using our concept. The cycling stability was shown to critically depend on mechanical stability of the MOF and composite, but even ambient sensing can be achieved repeatedly. The systems are valuable for in situ monitoring gas concentrations in chemical industry, for example, pressure swing systems, in which a certain concentration must not be surpassed due to safety concerns (explosion limits) or for the protection of subsequent catalytic converters from overloading or poisoning. Given the high degree of selectivity currently explored for switchable MOFs30,29,31,32 we will explore this concept further in future for various vapors and toxic gases by making use of the modular construction of switchable MOFs and their gating behavior which may be adjusted by tailoring linkers, nodes, mesostructure, and outer surface functionalization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13924. N 2 physisorption isotherms at 77 K, schematic illustration of sensing setup, n-butane physisorption isotherms at 298 K, images of degradation of pellets, XRD pattern of differently prepared composites, raw data of resistance measurements, SEM images of parent materials and composites, pore size distributions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.K.). ORCID

Irena Senkovska: 0000-0001-7052-1029 Stefan Kaskel: 0000-0003-4572-0303 Funding

The authors gratefully acknowledge support by the DFG (FOR 2433: KA 1698/29-1). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Sc. Sebastian Ehrling and M.Sc. Kai Eckhardt are acknowledged for REM measurements and electrode printing, respectively.

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REFERENCES

(1) Danzon, M.; Van Leeuwen, R.; Krzyzanowski, M. Air Quality Guidelines for Europe; 2000. (2) Weschler, C. J. Changes in Indoor Pollutants since the 1950s. Atmos. Environ. 2009, 43 (1), 153−169. (3) Fraden, J. Handbook of Modern Sensors: Physics, Designs, and Applications; Springer: 2016. (4) Yaws, C. L.; Braker, W. Matheson Gas Data Book; McGraw-Hill Chemical Engineering Series: 2001. 43788


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