Thermo-/Mechano-Chromic Chiral Coordination Dimer: Formation of Switchable and Metastable Discrete Structure through Chiral Self-Sorting.

Although strong chiral self-sorting often emerges in extended covalent or supramolecular polymers, the phenomenon is generally weak in discrete assemblies (e.g., dimers and oligomers) of small molecules due to the lack of a cooperative growth mechanism. Consequently, chiral self-sorting has been overlooked in the design of switchable and metastable discrete supramolecular structures. Here, we report a butyl-benzo[h]quinoline-based iridium(III) complex (Bu-Ir) with helical chirality at its metal center, which forms preferentially a homochiral dimer and exhibits thermo-/mechano-chromism based on a monomer-dimer transformation. While a five-coordinate monomer is formed in a racemic or an enantiopure Bu-Ir solution at 25 °C, a six-coordinate homochiral dimer complex is formed almost exclusively at low temperatures, with a higher degree of dimerization in enantiopure Bu-Ir solution. Estimation of apparent dimerization binding constants (K) and thermodynamic parameters (ΔH and ΔS) based on variable temperature ultraviolet-visible (UV-vis) and 1H NMR spectra reveals a strong preference for homochiral dimerization (largest known value for the coordination complex, Khomo/Khetero > 50). Notably, crystals of the homochiral dimer are metastable, undergoing a distinct color change upon grinding (from yellow to red) due to mechanical cleavage of coordination bonds (i.e., a dimer to monomer transformation). A comparison with control compounds having different substituents (proton, methyl, isopropyl, and phenyl groups) reveals that Bu-Ir dimerization involves both strong homochiral self-sorting preference and connected thermo-/mechano-chromic behavior, which is based on matched propeller-shaped chirality and subtle steric repulsion between alkyl substituents that render the homochiral dimer switchable and metastable. These findings provide substantial insights into the emergence of dynamic functionality based on the rational design of discrete chiral assemblies.

Nonionic omnisoluble photosensitizer reference material for the estimation of singlet oxygen quantum yield.

Meso-Tetrakis-(3,4,5-tris{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}phenyl)porphyrin TEG12PH2 is reported as an 'omnisoluble' reference for singlet oxygen (1O2) generation quantum yield (ΦSO) estimation. TEG12PH2 is a highly soluble, nonionic compound possessing excellent 1O2 QY in a wide variety of common solvents, including water. TEG12PH2 was prepared on multigram scale by the 12-way O-alkylation of tetrakis(3,4,5-trihydroxyphenyl)porphyrin using 2-(2-(2-methoxyethoxy)ethoxy)ethyl 4-toluenesulfonate as a reaction solvent. The corresponding Zn(II) complex TEG12PZn was also prepared and studied. The 1O2 QYs of TEG12PH2 in the different solvents studied were found to be 0.86 (acetone), 0.59 (acetonitrile), 0.66 (chloroform), 0.85 (methanol), 0.45 (toluene) and 0.51 (water). TEG12PH2 can be considered a reliable and easy to implement omnisoluble reference compound for the estimation of the 1O2 generating activities of new materials, especially new porphyrinic compounds.

Dynamics of Meso-Chiral Interconversion in a Butterfly-Shape Overcrowded Alkene Rotor Tunable by Solvent Properties.

Elucidation of dynamics of molecular rotational motion is an essential part and challenging area of research. We demonstrate reversible diastereomeric interconversion of a molecular rotor composed of overcrowded butterfly-shape alkene (FDF). Its inherent dual rotatory motion (two rotors, one stator) with interconversion between two diastereomers, chiral trans-FDF and meso cis-FDF forms, has been examined in detail upon varying temperatures and solvents. The free energy profile of 180° revolution of one rotor part has a bimodal shape with unevenly positioned maxima (transition states). FDF in aromatic solvents adopts preferentially meso cis-conformation, while in non-aromatic solvents a chiral trans-conformation is more abundant owing to the solvent interactions with peripheral hexyl chains (solvophobic effect). Moderate correlations between the trans-FDF/cis-FDF ratio and solvent parameters, such as refractive index, polarizability, and viscosity were found.

Selective Phase Transfer Reagents (OxP-crowns) for Chromogenic Detection of Nitrates Especially Ammonium Nitrate.

Nitrogen and phosphorus-containing ions such as ammonium, nitrates and phosphates are anthropogenic pollutants while ammonium nitrate may be diverted for nefarious purposes in improvised explosive devices. Crown ether-oxoporphyrinogen conjugates (OxP-crowns) are used to selectively detect nitrates, especially their ion pairs with K+ and NH4 + , based on ion pair complexation of OxP-crowns under phase transfer conditions. The presence of phosphate and carbonate lead to deprotonation of OxP-crowns. OxP-1N18C6 is capable of extracting ion pairs with nitrate from aqueous phase leading to a selective chromogenic response. Deprotonation of the OxP moiety leads to [OxP- ]-1N18C6[K+ ] and is promoted by crown ether selective cation binding coupled with hydration of basic oxoanions, which are constrained to remain in the aqueous phase. This work illustrates the utility of molecular design to exploit partitioning and ion hydration effects establishing the selectivity of the chromogenic response.

Amphiprotism-Coupled Near-Infrared Emission in Extended Pyrazinacenes Containing Seven Linearly Fused Pyrazine Units.

Peripherally substituted tetradecaazaheptacene (N14Hp) compounds, exhibiting amphiprotism-coupled emission, have been synthesized. X-ray crystallography reveals a planar acene-like chromophore, and electronic absorption and emission occur in the near-infrared biological transparency window (650-900 nm). The compounds exhibit long-wavelength emission with photoluminescence quantum yields ΦPL up to ∼0.61 at 686 nm, with the monodeprotonated state ΦPL ≈ 0.58 at 712 nm. This unprecedented highly nitrogenous chromophore illustrates the stability and utility of the pyrazinacenes for different applications based on their photophysical properties and chemical structures.

Chiral sensing by nonchiral tetrapyrroles.

Enantiomeric excess (ee) is a measure of the purity of an enantiomer of a chiral compound with respect to the presence of the complementary enantiomer. It is an important aspect of chemistry, especially in the fields of pharmaceuticals and asymmetric catalysis. Existing methods for determination of enantiomeric excesses using nuclear magnetic resonance (NMR) spectroscopy mostly rely on special chiral reagents (auxiliaries) that form two or more diastereomeric complexes with a chiral compound. As a result of this, the NMR spectrum of each enantiomer is different, allowing the determination of enantiomeric excess. In this Account, we describe a molecular design process that has allowed us to prepare prochiral solvating agents for NMR determination of ee of a wide variety of analyte types. At the outset of this work, we initially encountered the phenomenon of NMR peak splitting in the oxoporphyrinogen (OxP) host component of a supramolecular host-guest complex, where the extent of the splitting is apparently proportional to the guests' ee. Upon closer examination of the mechanism of action, it was found that several complicating factors, including prototropic tautomerism, macrocyclic inversion (ring-flipping), and 1:2 host-guest stoichiometry, obstruct potential applications of OxP as a chiral solvating agent. By considering the molecular conformation of the OxP host, a saddle-shaped calix[4]pyrrole, we moved to study the tetraphenylporphyrin (TPP) dication since it has a similar form, and it was found that it could also be used to probe ee. However, although TPP does not suffer from disadvantageous tautomeric processes, it is still subject to macrocyclic inversion and has the additional serious disadvantage of operating for ee sensing only at depressed temperatures. The intrinsic disadvantages of the OxP and TPP systems were finally overcome by covalently modifying the OxP chromophore by regioselective N-alkylation at one face of the molecule. This procedure yields a host Bz2OxP that undergoes 1:1 host-guest interactions, cannot be protonated (and so does not suffer drawbacks due to tautomeric processes), and can interact solely through hydrogen bonding with a much wider range of analyte types, including acids, esters, amines (including amino acid derivatives), and ketones, for the determination of their ee at room temperature. Chiral sensing, in this case, can be understood by considering the breakdown of the host's symmetry when it interacts with a chiral guest under fast exchange. Furthermore, chirality discrimination (i.e., which is the major enantiomer in a sample) can be performed by addition of a small amount of one of the known enantiomers. Adaptation of a symmetrical molecule for ee sensing presents certain intrinsic advantages, including identical binding constants of each enantiomer. Our results indicate that other symmetrical molecules might also be useful as NMR probes of enantiopurity. These systems could provide insights into important chirality principles such as majority rule, intermolecular chirality transfer, and asymmetric reactions. The Bz2OxP system is also of note from the point of view that it does not rely on the formation of diastereomers.

Chiral guest binding as a probe of macrocycle dynamics and tautomerism in a conjugated tetrapyrrole.

We report chiral guest binding as a probe of prototropic tautomerism and macrocyclic inversion in a highly conjugated tetrapyrrole studied using (1)H NMR spectroscopy in conjunction with mandelic acid as the chiral guest. Both tautomerism and macrocycle inversion can be influenced in a non-trivial way depending on temperature and the respective concentrations of tetrapyrrole host, chiral guest or water. Chirality of the interacting guest is the key feature since it permits separation and detailed observation of macrocyclic inversion and tautomerism. Based on this, a methodology was developed to identify and characterize the dynamic processes. Our observations suggest that yields of products (e.g., of asymmetric reactions) can be affected by reactivity of functional groups (in molecules undergoing tautomerism or inversion) by varying solution properties including reagent concentrations and impurities such as water. This work establishes a connection between the important chemical concepts of chirality, tautomerism, and macrocyclic dynamics.

Aligned 1-D nanorods of a π-gelator exhibit molecular orientation and excitation energy transport different from entangled fiber networks.

Linear π-gelators self-assemble into entangled fibers in which the molecules are arranged perpendicular to the fiber long axis. However, orientation of gelator molecules in a direction parallel to the long axes of the one-dimensional (1-D) structures remains challenging. Herein we demonstrate that, at the air-water interface, an oligo(p-phenylenevinylene)-derived π-gelator forms aligned nanorods of 340 ± 120 nm length and 34 ± 5 nm width, in which the gelator molecules are reoriented parallel to the long axis of the rods. The orientation change of the molecules results in distinct excited-state properties upon local photoexcitation, as evidenced by near-field scanning optical microscopy. A detailed understanding of the mechanism by which excitation energy migrates through these 1-D molecular assemblies might help in the design of supramolecular structures with improved charge-transport properties.

Porphyrin-based sensor nanoarchitectonics in diverse physical detection modes.

Porphyrins and related families of molecules are important organic modules as has been reflected in the award of the Nobel Prizes in Chemistry in 1915, 1930, 1961, 1962, 1965, and 1988 for work on porphyrin-related biological functionalities. The porphyrin core can be synthetically modified by introduction of various functional groups and other elements, allowing creation of numerous types of porphyrin derivatives. This feature makes porphyrins extremely useful molecules especially in combination with their other interesting photonic, electronic and magnetic properties, which in turn is reflected in their diverse signal input-output functionalities based on interactions with other molecules and external stimuli. Therefore, porphyrins and related macrocycles play a preeminent role in sensing applications involving chromophores. In this review, we discuss recent developments in porphyrin-based sensing applications in conjunction with the new advanced concept of nanoarchitectonics, which creates functional nanostructures based on a profound understanding of mutual interactions between the individual nanostructures and their arbitrary arrangements. Following a brief explanation of the basics of porphyrin chemistry and physics, recent examples in the corresponding fields are discussed according to a classification based on physical modes of detection including optical detection (absorption/photoluminescence spectroscopy and energy and electron transfer processes), other spectral modes (circular dichroism, plasmon and nuclear magnetic resonance), electronic and electrochemical modes, and other sensing modes.

Subphthalocyanines as fluorescence sensors for metal cations.

Subphthalocyanines (SubPcs) and their aza-analogues (SubTPyzPzs) are fluorophores with strong orange fluorescence emission; however, their sensing ability towards metal cations remains uncharted. To fill this gap, we have developed an efficient method for introducing aza-crown moieties at the axial position of SubPcs and SubTPyzPzs to investigate the structure-activity relationship for sensing alkali (Li+, Na+, K+) and alkaline earth metal (Ca2+, Mg2+, Ba2+) cations. SubPcs showed better photostability than SubTPyzPzs and even a commonly utilized dye, 6-carboxyfluorescein. Selectivity toward metal cations was driven by the size of the aza-crown, irrespective of the counter anion. The stoichiometry of binding was found to be 1 : 1 in all cases, and the interaction between SubPcs and cations was characterized by the corresponding apparent binding constants (Ka). Notably, an unusually strong interaction of all sensoric SubPcs with Ba2+ compared to other studied cations was demonstrated. The role of the surrounding environment, i.e. the addition of water or methanol, in sensing cations is shown in detail as well. Selectivity towards K+ over Na+ was demonstrated in aqueous media with SubPcs bearing the 1-aza-6-crown-18 moiety in Tween 80 micelles. In this case, a 5-fold increase of the fluorescence quantum yield was observed upon binding K+ ions. The high brightness, photostability, and sensing activity in aqueous media make SubPc macrocycles promising fluorophores for metal cation sensing.

Dynamic breathing of CO2 by hydrotalcite.

The carbon cycle of carbonate solids (e.g., limestone) involves weathering and metamorphic events, which usually occur over millions of years. Here we show that carbonate anion intercalated layered double hydroxide (LDH), a class of hydrotalcite, undergoes an ultrarapid carbon cycle with uptake of atmospheric CO2 under ambient conditions. The use of (13)C-labeling enabled monitoring by IR spectroscopy of the dynamic exchange between initially intercalated (13)C-labeled carbonate anions and carbonate anions derived from atmospheric CO2. Exchange is promoted by conditions of low humidity with a half-life of exchange of ~24 h. Since hydrotalcite-like clay minerals exist in Nature, our finding implies that the global carbon cycle involving exchange between lithosphere and atmosphere is much more dynamic than previously thought.

Kinetic study of NADPH activation using ubiquinone-rhodol fluorescent probe and an IrIII-complex promoter at the cell interior.

Nicotine adenine dinucleotide derivatives NADH and NADPH are intimately involved in energy and electron transport within cells. The fluorescent ubiquinone-rhodol (Q-Rh) probe is used for NADPH activation monitoring. Q-Rh reacts with NADPH yielding its quenched hydroquinone-rhodol (H2Q-Rh) form with concurrent NADPH activation (i.e. NADP+ formation). NADPH activation can be enhanced by the addition of an IrIII-complex (i.e. [(η5-C5Me5)Ir(phen)(H2O)]2+) as a promoter. The rate of the Q-Rh fluorescence quenching process is proportional to the NADPH activation rate, which can be used to monitor NADPH. Experiments were performed in phosphate-buffered saline (PBS) solution and on HeLa cell cultures to analyze the kinetics of Q-Rh reduction and the influence of the IrIII-complex promoter on the activation of NADPH (in PBS) and of other intracellular reducing agents (in HeLa cells). There is a substantial increase in Q-Rh reduction rate inside HeLa cells especially after the addition of IrIII-complex promoter. This increase is partly due to a leakage process (caused by IrIII-complex-induced downstream processes which result in cell membrane disintegration) but also involves the nonspecific activation of other intracellular reducing agents, including NADH, FADH2, FMNH2 or GSH. In the presence only of Q-Rh, the activation rate of intracellular reducing agents is 2 to 8 times faster in HeLa cells than in PBS solution. When both Q-Rh and IrIII-complex are present, the rate of the IrIII-complex catalyzed reduction reaction is 7 to 23 times more rapid in HeLa cells. Concentration- and time-dependent fluorescence attenuation of Q-Rh with third-order reaction kinetics (reasonably approximated as pseudo-first-order in Q-Rh) has been observed and modelled. This reaction and its kinetics present an example of "bioparallel chemistry", where the activation of a molecule can trigger a unique chemical process. This approach stands in contrast to the conventional concept of "bioorthogonal chemistry", which refers to chemical reactions that occur without disrupting native biological processes.

Selective Detection of Toxic C1 Chemicals Using a Hydroxylamine-Based Chemiresistive Sensor Array.

Formaldehyde (FA) is a deleterious C1 pollutant commonly found in the interiors of modern buildings. C1 chemicals are generally more toxic than the corresponding C2 chemicals, but the selective discrimination of C1 and C2 chemicals using simple sensory systems is usually challenging. Here, we report the selective detection of FA vapor using a chemiresistive sensor array composed of modified hydroxylamine salts (MHAs, ArCH2ONH2·HCl) and single-walled carbon nanotubes (SWCNT). By screening 32 types of MHAs, we have identified an ideal sensor array that exhibits a characteristic response pattern for FA. Thus, trace FA (0.02-0.05 ppm in air) can be clearly discriminated from the corresponding C2 chemical, acetaldehyde (AA). This system has been extended to discriminate methanol (C1) from ethanol (C2) in combination with the catalytic conversion of these alcohols to their corresponding aldehydes. Our system offers portable and reliable chemical sensors that discriminate the subtle differences between C1 and C2 chemicals, enabling advanced environmental monitoring and healthcare applications.

NMR lineshape analysis using analytical solutions of multi-state chemical exchange with applications to kinetics of host-guest systems.

Nuclear magnetic resonance (NMR) lineshape analysis is a powerful tool for the study of chemical kinetics. Here we provide techniques for analysis of the relationship between experimentally observed spin kinetics (transitions between different environments [Formula: see text]) and corresponding chemical kinetics (transitions between distinct chemical species; e.g., free host and complexed host molecule). The advantages of using analytical solutions for two-, three- or generally N-state exchange lineshapes (without J-coupling) over the widely used numerical calculation for NMR spectral fitting are presented. Several aspects of exchange kinetics including the generalization of coalescence conditions in two-state exchange, the possibility of multiple processes between two states, and differences between equilibrium and steady-state modes are discussed. 'Reduced equivalent schemes' are introduced for spin kinetics containing fast-exchanging states, effectively reducing the number of exchanging states. The theoretical results have been used to analyze a host-guest system containing an oxoporphyrinogen complexed with camphorsulfonic acid and several other literature examples, including isomerization, protein kinetics, or enzymatic reactions. The theoretical treatment and experimental examples present an expansion of the systematic approach to rigorous analyses of systems with rich chemical kinetics through NMR lineshape analysis.

Reversible photoredox switching of porphyrin-bridged bis-2,6-di-tert-butylphenols.

Porphyrin derivatives bearing 2,6-di-tert-butylphenol substituents at their 5,15-positions undergo reversible photoredox switching between porphyrin and porphodimethene states as revealed by UV-vis spectroscopy, fluorescence spectroscopy, and X-ray single-crystal analyses. Photoredox interconversion is accompanied by substantial variations in electronic absorption and fluorescence emission spectra and a change of conformation of the tetrapyrrole macrocycle from planar to roof-shaped. Oxidation proceeds only under photoillumination of a dianionic state prepared through deprotonation using fluoride anions. Conversely, photoreduction occurs in the presence of a sacrificial electron donor. Transient absorption spectroscopy and electron spin resonance spectroscopy were applied to investigate the processes in photochemical reaction, and radical intermediates were characterized. That is, photooxidation initially results in a phenol-substituent-centered radical, while the reduction process occurs via a delocalized radical state involving both the macrocycle and 5,15-substituents. Forward and reverse photochemical processes are governed by different chemical mechanisms, giving the important benefit that conversion reactions are completely isolated, leading to better separation of the end states. Furthermore, energy diagrams based on electrochemical analyses (cyclic voltammetry) were used to account for the processes occurring during the photochemical reactions. Our molecular design indicates a simple and versatile method for producing photoredox macrocyclic compounds, which should lead to a new class of advanced functional materials suitable for application in molecular devices and machines.

Self-assembled pyrazinacene nanotubes.

Nanotubes of a pentacene derivative, 6,13-bis(1-n-dodecyl)-[a,c,l,n]-tetrabenzo-5,6,7,12,13,14-hexaazapentacene, have been prepared by a hierarchical self-assembly mechanism. The oligoazaacenes 1-3, referred to as pyrazinacenes due to their structures of linearly fused pyrazine heterocycles, can also be considered as two azatriphenylenes fused through a reduced pyrazine ring. Dissolution of 6,13-bis(1-n-dodecyl)-[a,c,l,n]-tetrabenzo-5,6,7,12,13,14-hexaaza pentacene in nearly boiling toluene followed by standing of the solution at room temperature yields self-assembled (sa) pyrazinacene (Pa) nanotubes (NT's), or sa-PaNTs. Self-assembled-PaNTs are formed after initial aggregation of the pyrazinacene giving a 130-nm-wide 2-dimensional tape followed by helical twisting of this tape into a hollow cylindrical form of 150-200 nm diameter which can exceed 10 µm in length. The morphologies of the tape and nanotube structures were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and electron absorption spectroscopy (UV/Vis). The latter indicates that the tubes may be formed by chromophore J-aggregation. Also, high resolution TEM of the tubes reveals that they can be composed of several tapes while powder X-ray diffraction revealed the lamellar structure of the tapes composing the tubes.

Pyrazinacenes exhibit on-surface oxidation-state-dependent conformational and self-assembly behaviours.

Acenes and azaacenes lie at the core of molecular materials' applications due to their important optical and electronic features. A critical aspect is provided by their heteroatom multiplicity, which can strongly affect their properties. Here we report pyrazinacenes containing the dihydro-decaazapentacene and dihydro-octaazatetracene chromophores and compare their properties/functions as a model case at an oxidizing metal substrate. We find a distinguished, oxidation-state-dependent conformational adaptation and self-assembly behaviour and discuss the analogies and differences of planar benzo-substituted decaazapentacene and octaazatetracene forms. Our broad experimental and theoretical study reveals that decaazapentacene is stable against oxidation but unstable against reduction, which is in contrast to pentacene, its C-H only analogue. Decaazapentacenes studied here combine a planar molecular backbone with conformationally flexible substituents. They provide a rich model case to understand the properties of a redox-switchable π-electronic system in solution and at interfaces. Pyrazinacenes represent an unusual class of redox-active chromophores.

Probing the micro-phase separation of thermo-responsive amphiphilic polymer in water/ethanol solution.

The temperature-induced coil-globule transition (micro-phase separation) in water/ethanol solutions of poly(vinyl methyl ether) was studied using NMR spectroscopy, differential scanning calorimetry, optical microscopy and dynamic light scattering. NMR sequence based on spin-echo was introduced in order to determine lower critical solution temperature with high accuracy. Variation in DSC profiles and enthalpy increments depending on ethanol concentration in a water/ethanol mixture was found. Evolution of morphology pattern during heating-cooling cycle was observed using optical microscopy. At lower PVME concentration the globule size distribution was determined using digital image processing. Relative number and relative mass size distributions of globules in a dilute sample were measured by differential scanning calorimetry and subsequently compared with those obtained by optical microscopy.

Poly(N,N'-Diethylacrylamide)-Based Thermoresponsive Hydrogels with Double Network Structure.

Temperature response of double network (DN) hydrogels composed of thermoresponsive poly(N,N'-diethylacrylamide) (PDEAAm) and hydrophilic polyacrylamide (PAAm) or poly(N,N`-dimethylacrylamide) (PDMAAm) was studied by a combination of swelling measurements, differential scanning calorimetry (DSC) and 1H NMR and UV-Vis spectroscopies. Presence of the second hydrophilic network in DN hydrogels influenced their thermal sensitivity significantly. DN hydrogels show less intensive changes in deswelling, smaller enthalpy, and entropy changes connected with phase transition and broader temperature interval of the transition than the single network (SN) hydrogels. Above the transition, the DN hydrogels contain significantly more permanently bound water in comparison with SN hydrogels due to interaction of water with the hydrophilic component. Unlike swelling and DSC experiments, a rather abrupt transition was revealed from temperature-dependent NMR spectra. Release study showed that model methylene blue molecules are released from SN and DN hydrogels within different time scale. New thermodynamical model of deswelling behaviour based on the approach of the van't Hoff analysis was developed. The model allows to determine thermodynamic parameters connected with temperature-induced volume transition, such as the standard change of enthalpy and entropy and critical temperatures and characterize the structurally different states of water.

Cascade Reaction-Based Chemiresistive Array for Ethylene Sensing.

Chemiresistive sensors, which are based on semiconducting materials, offer real-time monitoring of environment. However, detection of nonpolar chemical substances is often challenging because of the weakness of the doping effect. Herein, we report a concept of combining a cascade reaction (CR) and a chemiresistive sensor array for sensitive and selective detection of a target analyte (herein, ethylene in air). Ethylene was converted to acetaldehyde through a Pd-catalyzed heterogeneous Wacker reaction at 40 °C, followed by condensation with hydroxylamine hydrochloride to emit HCl vapor. HCl works as a strong dopant for single-walled carbon nanotubes (SWCNTs), enabling the main sensor to detect ethylene with excellent sensitivity (10.9% ppm-1) and limit of detection (0.2 ppm) in 5 min. False responses that occur in the main sensor are easily discriminated by reference sensors that partially employ CR. Moreover, though the sensor monitors the variation of normalized electric resistance (ΔR/R0) in the SWCNT network, temporary deactivation of CR yields a sensor system that does not require analyte-free air for a baseline correction (i.e., estimation of R0) and recovery of response. The concept presented here is generally applicable and offers a solution for several issues that are inherently present in chemiresistive sensing systems.