Employing Halogen Bonding Interactions in Chemiresistive Gas Sensors

Anion Sensing through Redox-Modulated Fluorescent Halogen Bonding and Hydrogen Bonding Hosts

Anion sensing via either optical or electrochemical readouts has separately received enormous attention, however, a judicious combination of the advantages of both modalities remains unexplored. Toward this goal, we herein disclose a series of novel, redox-active, fluorescent, halogen bonding (XB) and hydrogen bonding (HB) BODIPY-based anion sensors, wherein the introduction of a ferrocene motif induces remarkable changes in the fluorescence response. Extensive fluorescence anion titration, lifetime and electrochemical studies reveal anion binding-induced emission modulation through intramolecular photoinduced electron transfer (PET), the magnitude of which is dependent on the nature of both the XB/HB donor and anion. Impressively, the XB sensor outperformed its HB congener in terms of anion binding strength and fluorescence switching magnitude, displaying significant fluorescence turn-OFF upon anion binding. In contrast, redox-inactive control receptors display a turn-ON response, highlighting the pronounced impact of the introduction of the redox-active ferrocene on the optical sensing performance. Additionally, the redox-active ferrocene motif also serves as an electrochemical reporter group, enabling voltammetric anion sensing in competitive solvents. The combined advantages of both sensing modalities were further exploited in a novel, proof-of-principle, fluorescence spectroelectrochemical anion sensing approach, enabling simultaneous and sensitive read out of optical and electrochemical responses in multiple oxidation states and at very low receptor concentration.

ZnSnO3 based gas sensors for pyridine volatile marker detection in rice aging during storage

This study reports the development of ZnSnO₃ based gas sensors for pyridine detection in rice aging. Pyridine is one of heterocyclic markers formed via Maillard reaction and lipid oxidation. Herein, graphitic carbon nitride (g-C₃N₄) decorated ZnSnO₃ microstructures were obtained through a template-free approach. And the sensing results reveal that 5 wt%g-C₃N₄ decorated ZnSnO₃ exhibited a high sensitivity (47.9), a short response/recovery time (14/120 s) and a low detection limit (0.45 ppm), which is due to the catalysis of g-C₃N₄ nanosheets, the decorated microstructure and the formation of heterojunctions. Meanwhile, the practical experiment demonstrates that the sensitivity towards volatiles generated from Japonica rice aging is 48.7, which is around 4 and 2.5 times higher than those of Indica rice and Polished Glutinous rice, indicating that the sensor has anticipated application in the development of a high-performance E-nose for the quality inspection of rice and other products.

Halogen bonding and chalcogen bonding mediated sensing

Sigma-hole interactions, in particular halogen bonding (XB) and chalcogen bonding (ChB), have become indispensable tools in supramolecular chemistry, with wide-ranging applications in crystal engineering, catalysis and materials chemistry as well as anion recognition, transport and sensing. The latter has very rapidly developed in recent years and is becoming a mature research area in its own right. This can be attributed to the numerous advantages sigma-hole interactions imbue in sensor design, in particular high degrees of selectivity, sensitivity and the capability for sensing in aqueous media. Herein, we provide the first detailed overview of all developments in the field of XB and ChB mediated sensing, in particular the detection of anions but also neutral (gaseous) Lewis bases. This includes a wide range of optical colorimetric and luminescent sensors as well as an array of electrochemical sensors, most notably redox-active host systems. In addition, we discuss a range of other sensor designs, including capacitive sensors and chemiresistors, and provide a detailed overview and outlook for future fundamental developments in the field. Importantly the sensing concepts and methodologies described herein for the XB and ChB mediated sensing of anions, are generically applicable for the development of supramolecular receptors and sensors in general, including those for cations and neutral molecules employing a wide array of non-covalent interactions. As such we believe this review to be a useful guide to both the supramolecular and general chemistry community with interests in the fields of host-guest recognition and small molecule sensing. Moreover, we also highlight the need for a broader integration of supramolecular chemistry, analytical chemistry, synthetic chemistry and materials science in the development of the next generation of potent sensors.

Monolayer-Protected Gold Nanoparticles Functionalized with Halogen Bonding Capability─An Avenue for Molecular Detection Schemes

The use of functionalized nanoparticles (NPs) and their aggregation in the presence of a targeted analyte is a well-established molecular detection strategy predicated on harnessing specific molecular interactions to the NP periphery. Molecules able to specifically interact with the functionalized NPs alter the unique optical and electrochemical properties of the NPs as a function of interparticle spacing. While many intermolecular interactions have been successfully exploited in this manner in conjunction with aqueous NP systems, the use of non-aqueous NPs in the same capacity is significantly less explored. A fundamental interaction that has not been previously investigated in NP schemes is halogen bonding (XB). XB is an orthogonal, electrostatic interaction between a region of positive electrostatic potential (δ+) on a halogen atom (i.e., XB donor) and a negative (δ-) Lewis base (XB acceptor) molecule. To couple XB with NP systems, ligands featuring a molecular structure that promotes XB interactions need to be identified, optimized, and synthesized for subsequent attachment to NPs. Herein, density functional theory (DFT) and NMR techniques are used to identify a strong XB-donor moiety (-C₆F₄I) and a synthetic scheme for a thiolate ligand featuring that functionality is devised and executed with high purity/yield (78%). Ligand-exchange reactions allow functionalization of non-aqueous alkanethiolate-protected gold NPs or monolayer-protected clusters (MPCs) with the XB-donor ligands. Functionalized MPCs (f-MPCs), within both assembled films and in solution, are shown to engage in XB interactions with target XB-acceptor molecules. Molecular recognition events, including induced aggregation of the f-MPCs, are characterized with UV-vis spectroscopy, cyclic voltammetry, TEM imaging, and diffusion-ordered spectroscopy NMR with limits of detection of 50-100 nM for strong XB acceptors. While fundamental exploration of XB interactions is ongoing, this study represents a step toward utilizing XB within molecular detection schemes, an application with implications for supramolecular chemistry, forensic, and environmental chemical sensing.

C60Br24/SWCNT: A Highly Sensitive Medium to Detect H2S via Inhomogeneous Carrier Doping

H₂S is a toxic and corrosive gas, whose accurate detection at sub-ppm concentrations is of high practical importance in environmental, industrial, and health safety applications. Herein, we propose a chemiresistive sensor device that applies a composite of single-walled carbon nanotubes (SWCNTs) and brominated fullerene (C₆₀Br₂₄) as a sensing component, which is capable of detecting 50 ppb H₂S even at room temperature with an excellent response of 1.75% in a selective manner. In contrast, a poor gas response of pristine C₆₀-based composites was found in control measurements. The experimental results are complemented by density functional theory calculations showing that C₆₀Br₂₄ in contact with SWCNTs induces localized hole doping in the nanotubes, which is increased further when H₂S adsorbs on C₆₀Br₂₄ but decreases in the regions, where direct adsorption of H₂S on the nanotubes takes place due to electron doping from the analyte. Accordingly, the heterogeneous chemical environment in the composite results in spatial fluctuations of hole density upon gas adsorption, hence influencing carrier transport and thus giving rise to chemiresistive sensing.

Evaluating Halogen-Bond Strength as a Function of Molecular Structure Using Nuclear Magnetic Resonance Spectroscopy and Computational Analysis

Halogen bonding (XB) is a highly directional, non-covalent intermolecular interaction between a molecule (XB donor) presenting a halogen with an electron-deficient region or sigma hole (σ-hole) and an electron-rich or Lewis-base molecule (XB acceptor). A systematic, experimental, and theoretical study of solution-phase XB strength as a function of the molecular structure for both XB donor and acceptor molecules is presented. The impact of specific structural features is assessed using ¹⁹F and ¹H nuclear magnetic resonance (NMR) titrations to determine association constants, density functional theory calculations for interaction energies and bond lengths, as well as ¹⁹F-¹H HOESY NMR measurements of intermolecular cross-relaxation between the interacting XB donor-acceptor adducts. For XB donor molecules (perfluoro-halogenated benzenes), results indicate the critical importance of iodine coupled with electron-withdrawing entities. Prominent structural components of XB acceptor molecules include a central atom working in conjunction with a Lewis-base atom to present high electron density directed at the σ-hole (e.g., tributylphosphine oxide). Additionally, larger surrounding aliphatic R groups (e.g., butyl and octyl) were found to significantly stabilize strong XB, particularly in solvents that promote the interaction. With a more thorough understanding of structure-optimized XB, one can envision harnessing XB interactions more strategically for specific design of optimal materials and chemical applications.

Zn(II) Heteroleptic Halide Complexes with 2-Halopyridines: Features of Halogen Bonding in Solid State

Reactions between Zn(II) dihalides and 2-halogen-substituted pyridines 2-XPy result in a series of heteroleptic molecular complexes [(2-XPy)₂ZnY₂] (Y = Cl, X = Cl (1), Br (2), I (3); Y = Br, X = Cl (4), Br (5), I (6), Y = I, X = Cl (7), Br (8), and I (9)). Moreover, 1-7 are isostructural (triclinic), while 8 and 9 are monoclinic. In all cases, halogen bonding plays an important role in formation of crystal packing. Moreover, 1-9 demonstrate luminescence in asolid state; for the best emitting complexes, quantum yield (QY) exceeds 21%.

Size Discrimination of Carbohydrates via Conductive Carbon Nanotube@Metal Organic Framework Composites

Traditional chemical sensing methodologies have typically relied on the specific chemistry of the analyte for detection. Modifications to the local environment surrounding the sensor represent an alternative pathway to impart selective differentiation. Here, we present the hybridization of a 2-D metal organic framework (Cu₃(HHTP)₂) with single-walled carbon nanotubes (SWCNTs) as a methodology for size discrimination of carbohydrates. Synthesis and the resulting conductive performance are modulated by both mass loading of SWCNTs and their relative oxidation. Liquid gated field-effect transistor (FET) devices demonstrate improved on/off characteristics and differentiation of carbohydrates based on molecular size. Glucose molecule detection is limited to the single micromolar concentration range. Molecular Dynamics (MD) calculations on model systems revealed decreases in ion diffusivity in the presence of different sugars as well as packing differences based on the size of a given carbohydrate molecule. The proposed sensing mechanism is a reduction in gate capacitance initiated by the filling of the pores with carbohydrate molecules. Restricting diffusion around a sensor in combination with FET measurements represents a new type of sensing mechanism for chemically similar analytes.

Pentiptycene Polymer/Single-Walled Carbon Nanotube Complexes: Applications in Benzene, Toluene, and o-Xylene Detection

We report the dispersion of single-walled carbon nanotubes (SWCNTs) using pentiptycene polymers and their use in chemiresistance-based and QCM-D sensors. Poly(p-phenylene ethynylene)s (PPEs) incorporating pentiptycene moieties present a concave surface that promotes π-π interactions and van der Waals interactions with SWCNTs. In contrast to more common polymer-dispersing mechanisms that involve the wrapping of polymers around the SWCNTs, we conclude that the H-shape of pentiptycene groups and the linear rigid-rod structure creates a slot for nanotube binding. UV-vis-NIR, Raman, and fluorescence spectra and TEM images of polymer/SWCNTs support this dispersion model, which shows size selectivity to SWCNTs with diameters of 0.8-0.9 nm. Steric bulk on the channels is problematic, and tert-butylated pentiptycenes do not form stable dispersions with SWCNTs. This result, along with the diameter preference, supports the model in which the SWCNTs are bound to the concave clefts of the pentiptycenes. The binding model suggests that the polymer/SWCNTs complex creates galleries, and we have demonstrated the binding of benzene, toluene, and o-xylene (BTX) vapors as the basis for a robust, sensitive, and selective sensing platform for BTX detection. The utility of our sensors is demonstrated by the detection of benzene at the OSHA short-term exposure limit of 5 ppm in air.

Halogen bonding for molecular recognition: new developments in materials and biological sciences

This review highlights recent developments of halogen bonding in materials and biological sciences with a short discussion on the nature of the interaction.

Chlorotellurate(iv) supramolecular associates with “trapped” Br2: features of non-covalent halogen⋯halogen interactions in crystalline phases

Reactions of chlorotellurates(iv) and Br₂ afford formation of supramolecular complexes Cat₂{[TeCl₆](Br₂)} (Cat = Me₃N⁺ (1), PyH⁺ (2), 4-MePyH⁺ (3) and 1-MePy⁺ (4)) where dibromine fragments are “trapped” by [TeCl₆]³⁻via Br⋯Cl halogen bonding.

Chemiresistive Carbon Nanotube Sensors for N-Nitrosodialkylamines

N-Nitrosamines are environmental genotoxicants that are widely encountered in air, water, and food. Contamination of indoor and outdoor air with N-nitrosamines has been reported on many occasions. Conventional detection of airborne N-nitrosamines requires sophisticated instrumentation, field sampling, and laboratory analysis. Herein, we report ultrasensitive carbon nanotube based chemiresistive sensors utilizing a cobalt(III) tetraphenylporphyrin selector element for the detection of N-nitrosamines. Concentrations as low as 1 ppb N-nitrosodimethylamine, N-nitrosodiethylamine, and N-nitrosodibutylamine were detected. We also demonstrate the integration of these sensors with a field deployable sensing node wherein the sensor response can be read online remotely.

Halogen Bonding Interactions for Aromatic and Nonaromatic Explosive Detection

Improved sensing strategies are needed for facile, accurate, and rapid detection of aromatic and nonaromatic explosives. Density functional theory was used to evaluate the relative binding interaction energies between halogen-containing sensor model molecules and nitro-containing explosives. Interaction energies ranged from -18 to -14 kJ/mol and highly directional halogen bonding interactions were observed with bond distances ranging between 3.0 and 3.4 Å. In all geometry optimized structures, the sigma-hole of electropositive potential on the halogen aligned with a lone pair of electrons on the nitro-moiety of the explosive. The computational results predict that the strongest interactions will occur with iodine-based sensors as, of all the halogens studied, iodine is the largest, most polarizable halogen with the smallest electronegativity. Based on these promising proof-of-concept results, synthetically accessible sensors were designed using 1,4-dihalobenzene (X = Cl, Br, and I) with and without tetra-fluoro electron withdrawing groups attached to the benzene ring. These sensing molecules were embedded onto single walled carbon nanotubes that were mechanically abraded onto interdigitated array electrodes, and these were used to measure the responses to explosive model compounds cyclohexanone and dimethyl-dinitro-benzene in nitrogen gas. Amperometric current-time curves for selectors and control molecules, including concentration correlated signal enhancement, as well as response and recovery times, indicate selector responsiveness to these model compounds, with the largest response observed for iodo-substituted sensors.

Carbon Nanotube Chemical Sensors

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Halogen bonding in the structures of pentaiodobenzoic acid and its salts

Structural characterization of pentaiodobenzoic acid (PIBA) and its salts was performed for the first time.

Ionic Liquid-Carbon Nanotube Sensor Arrays for Human Breath Related Volatile Organic Compounds

High sensitivity, selectivity, and stability are key requirements for carbon nanotube (CNT)-based sensors to realize their full potential in applications ranging from chemical warfare agent detection to disease diagnostics. Herein we demonstrate the sensing of volatile organic compounds (VOCs) relevant to human diseases using an array of chemiresistive carbon nanotube (CNT)-based sensors functionalized with ionic liquids (ILs). The ILs are fluid at ambient temperature and were selected to produce a discriminating sensor array capable of the gas-phase detection of human disease-related VOCs. We find that sensor arrays consisting of imidazolium-based ILs with different substituents and counterions provide selective responses for known biomarkers of infectious diseases of the lungs. Specifically, the sensors discriminate the various volatile biomarkers for tuberculosis based on their polarity, solubility, and chemical affinities. In addition to selectivity, the sensors also show a high level of reversibility and promising long-term stability, which renders them to be suitable candidates for practical applications in breath analysis.

Chemiresistor Devices for Chemical Warfare Agent Detection Based on Polymer Wrapped Single-Walled Carbon Nanotubes

Chemical warfare agents (CWA) continue to present a threat to civilian populations and military personnel in operational areas all over the world. Reliable measurements of CWAs are critical to contamination detection, avoidance, and remediation. The current deployed systems in United States and foreign militaries, as well as those in the private sector offer accurate detection of CWAs, but are still limited by size, portability and fabrication cost. Herein, we report a chemiresistive CWA sensor using single-walled carbon nanotubes (SWCNTs) wrapped with poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives. We demonstrate that a pendant hexafluoroisopropanol group on the polymer that enhances sensitivity to a nerve agent mimic, dimethyl methylphosphonate, in both nitrogen and air environments to concentrations as low as 5 ppm and 11 ppm, respectively. Additionally, these PEDOT/SWCNT derivative sensor systems experience negligible device performance over the course of two weeks under ambient conditions.

CNTs based improved chlorine sensor from non-covalently anchored multi-walled carbon nanotubes with hexa-decafluorinated cobalt phthalocyanines

To study the effect of synergetic interactions between metal-phthalocyanine and carbon nanotubes for gas sensing characteristics of carbon nanotubes, we have synthesized F₁₆CoPc/MWCNTs–COOH hybrid.

The nature of intermolecular interactions in pyridinium–anion–β-hexachlorocyclohexane molecular crystals

The nature of intermolecular interactions in different molecular crystal configurations formed by pyridinium cations, chloride or bromide anions as well as β-hexachlorocyclohexane (β-HCH) molecules has been investigated using high level ab initio quantum chemistry methods.

Bimodal supramolecular functionalization of carbon nanotubes triggered by covalent bond formation

Many applications of carbon nanotubes require their chemical functionalization. Both covalent and supramolecular approaches have been extensively investigated. A less trodden path is the combination of both covalent and noncovalent chemistries, where the formation of covalent bonds triggers a particularly stable noncovalent interaction with the nanotubes. We describe a series of naphthalene diimide (NDI) bisalkene molecules that, upon mixing with single-walled carbon nanotubes (SWNTs) and Grubbs' catalyst, undergo two different reaction pathways. On one hand, they ring-close around the SWNTs to form rotaxane-like mechanically interlocked derivatives of SWNTs (MINTs). Alternatively, they oligomerize and then wrap around the SWNTs. The balance of MINTs to oligomer-wrapped SWNTs depends on the affinity of the NDI molecules for the SWNTs and the kinetics of the metathesis reactions, which can be controlled by varying the solvent. Thorough characterization of the products (TGA, TEM, AFM, Raman, UV-vis-NIR, PLE, XPS and UPS) confirms their structure and shows that each type of functionalization affects the electronic properties of the SWNTs differently.