Highly Selective Chemical Vapor Sensing by Molecular Recognition: Specific Detection of C1-C4 Alcohols with a Fluorescent Phosphonate Cavitand

Highly selective and discriminative detection of small alcohols based on a dual-emission macrocyclic samarium complex

Lower alcohols (C1-C7) have a close relationship with our lives and some of them are harmful to our body's health. For example, liquor mixed with a tiny amount of methanol is harmful to our health. Much of this study is about identifying one or two low-level alcohols. How to detect low-level alcohol and high-throughput and distinguish between analogues of alcohol remains a tremendous challenge. In this study, a new large ring Schiff base Sm(III) complex (Sm-2r) is synthesized with a double emission matrix using the template method. Its dynamic imine bond (CN) and organic ligands (H₂L2_r) with molecular rotor properties can respond to changes in viscosity and polarity in external environments. The PCA method is used to turn the data matrix into a fingerprint spectrum to distinguish different alcohols (C1-C7). Sm-2r enables the quantization of cyclopropyl and glycerol. Linear ranges of cyclopropanol and glycerol are 0-9.0% and 0-3.0% (v/v), respectively. In addition, Sm-2r has an excellent ability to distinguish the mixtures of n-PrOH and i-PrOH, C₅H₉OH and C₆H₁₁OH, n-PeOH and n-HeOH, 1,3-PDO and 1,2-PDO, MeOH and EtOH, 1,2-EG and 1,2-PDO at different volume ratios. We have provided a way to distinguish alcohol species based on their molecular polarity and viscosity.

Supramolecular fluorescence sensing of l-proline and l-pipecolic acid

Fluorescent mono-phosphonate calix[4]pyrrole cavitands display selectivity in amino acids' sensing.

Development of a smartphone-based real time cost-effective VOC sensor

Air pollution by various volatile organic compounds (VOC) is a matter of concern for us. So in this regard, designing real-time VOC responsive materials is gaining attention across the scientific community. In this present work, we have developed an inexpensive VOC sensor based on a Meisenheimer complex derived from picric acid and N,N'-dicyclohexylcarbodiimide. The sensor coated TLC plate was used as a sensor of potentially harmful VOCs. The sensor coated TLC plate looks deep red colored and does not show any fluorescence emission under 366 nm UV light. But in the presence of harmful volatile organic compounds like benzene, toluene, xylene, etc the sensor coated TLC plate becomes orange colored and it also shows strong yellow emission under 366 nm UV light. This property was utilized to detect the VOCs by fluorescence spectroscopy. The detection limit for various VOCs was found to be in the range of 0.7-9 ppm. To make the sensor user friendly, we have demonstrated a method where VOCs can be detected using a smartphone in real-time and also the setup is portable.

Vapochromic crystals: understanding vapochromism from the perspective of crystal engineering

Vapochromic materials, which undergo colour and/or emission changes upon exposure to certain vapours or gases, have received increasing attention recently because of their wide range of applications in, e.g., chemical sensors, light-emitting diodes, and environmental monitors. Vapochromic crystals, as a specific kind of vapochromic materials, can be investigated from the perspective of crystal engineering to understand the mechanism of vapochromism. Moreover, understanding the vapochromism mechanism will be beneficial to design and prepare task-specific vapochromic crystals as one kind of low-cost 'electronic nose' to detect toxic gases or volatile organic compounds. This review provides important information in a broad scientific context to develop new vapochromic materials, which covers organometallic or coordination complexes and organic crystals, as well as the different mechanisms of the related vapochromic behaviour. In addition, recent examples of supramolecular vapochromic crystals and metal-organic-framework (MOFs) vapochromic crystals are introduced.

Probing the Structural Determinants of Amino Acid Recognition: X-Ray Studies of Crystalline Ditopic Host-Guest Complexes of the Positively Charged Amino Acids, Arg, Lys, and His with a Cavitand Molecule

Crystallization of tetraphosphonate cavitand Tiiii[H, CH₃, CH₃] in the presence of positively charged amino acids, namely arginine, lysine, or histidine, afforded host-guest complex structures. The X-ray structure determination revealed that in all three structures, the fully protonated form of the amino acid is ditopically complexed by two tetraphosphonate cavitand molecules. Guanidinium, ammonium, and imidazolium cationic groups of the amino acid side chain are hosted in the cavity of a phosphonate receptor, and are held in place by specific hydrogen bonding interactions with the P=O groups of the cavitand molecule. In all three structures, the positively charged α-ammonium groups form H-bonds with the P=O groups, and with a water molecule hosted in the cavity of a second tetraphosphonate molecule. Furthermore, water-assisted dimerization was observed for the cavitand/histidine ditopic complex. In this 4:2 supramolecular complex, a bridged water molecule is held by two carboxylic acid groups of the dimerized amino acid. The structural information obtained on the geometrical constrains necessary for the possible encapsulation of the amino acids are important for the rational design of devices for analytical and medical applications.

Environmental Gas Sensing with Cavitands

Environmental gas sensing needs stringent sensor requirements in terms of sensitivity, selectivity and ruggedness. One of the major issues to be addressed is combining in a single device the conflicting requirements of molecular-level selectivity and low-ppb sensitivity. The exploitation of synthetic molecular receptors as sensing materials is particularly attractive to address the selectivity issue, to single out the desired analytes in the presence of overwhelming amounts of interferents. This minireview summarizes the strategies in environmental gas and vapor sensing using molecular receptors as selective hosts for specific analytes, with the main focus on cavitands. In particular, we highlight the use of these macrocycles as selective preconcentrator units to be integrated into portable devices for environmental monitoring. Depending on the class of analytes to be detected, the molecular recognition properties of cavitands can be manipulated through the proper choice of the bridging groups at the upper rim, and their transducer integration can be implemented through the manifold functionalization options at the lower rim.

Advances in developing rapid, reliable and portable detection systems for alcohol

Development of portable, reliable, sensitive, simple, and inexpensive detection system for alcohol has been an instinctive demand not only in traditional brewing, pharmaceutical, food and clinical industries but also in rapidly growing alcohol based fuel industries. Highly sensitive, selective, and reliable alcohol detections are currently amenable typically through the sophisticated instrument based analyses confined mostly to the state-of-art analytical laboratory facilities. With the growing demand of rapid and reliable alcohol detection systems, an all-round attempt has been made over the past decade encompassing various disciplines from basic and engineering sciences. Of late, the research for developing small-scale portable alcohol detection system has been accelerated with the advent of emerging miniaturization techniques, advanced materials and sensing platforms such as lab-on-chip, lab-on-CD, lab-on-paper etc. With these new inter-disciplinary approaches along with the support from the parallel knowledge growth on rapid detection systems being pursued for various targets, the progress on translating the proof-of-concepts to commercially viable and environment friendly portable alcohol detection systems is gaining pace. Here, we summarize the progress made over the years on the alcohol detection systems, with a focus on recent advancement towards developing portable, simple and efficient alcohol sensors.

The Origin of Selectivity in the Complexation of N-Methyl Amino Acids by Tetraphosphonate Cavitands

We report on the eligibility of tetraphosphonate resorcinarene cavitands for the molecular recognition of amino acids. We determined the crystal structure of 13 complexes of the tetraphosphonate cavitand Tiiii[H, CH3, CH3] with amino acids. (1)H NMR and (31)P NMR experiments and ITC analysis were performed to probe the binding between cavitand Tiiii[C3H7, CH3, C2H5] or the water-soluble counterpart Tiiii[C3H6Py(+)Cl(-), CH3, C2H5] and a selection of representative amino acids. The reported studies and results allowed us (i) to highlight the noncovalent interactions involved in the binding event in each case; (ii) to investigate the ability of tetraphosphonate cavitand receptors to discriminate between the different amino acids; (iii) to calculate the Ka values of the different complexes formed and evaluate the thermodynamic parameters of the complexation process, dissecting the entropic and enthalpic contributions; and (iv) to determine the solvent influence on the complexation selectivity. By moving from methanol to water, the complexation changed from entropy driven to entropy opposed, leading to a drop of almost three orders in the magnitude of the Ka. However, this reduction in binding affinity is associated with a dramatic increase in selectivity, since in aqueous solutions only N-methylated amino acids are effectively recognized. The thermodynamic profile of the binding does not change in PBS solution. The pivotal role played by cation-π interactions is demonstrated by the linear correlation found between the log Ka in methanol solution and the depth of (+)N-CH3 cavity inclusion in the molecular structures. These findings are relevant for the potential use of phosphonate cavitands as synthetic receptors for the detection of epigenetic modifications of histones in physiological media.

Covalent or supramolecular combinations of resorcinarenes and porphyrinoids

Cavitands and porphyrinoids are two major groups of molecules. If cyclodextrins are maybe the most famous type of cavitands, the properties of resorcinarenes are now well known and rise rapidly increasing attention. This mini-review aims at detailing all the combinations of resorcinarenes with porphyrinoids reported so far, evidencing the bright future of such combos. In addition, two newly synthesized porphyrins-resorcinarene hybrids are reported.

Ionic liquid-based fluorescence sensing paper: rapid, ultrasensitive, and in-site detection of methamphetamine in human urine

A low cost sensing paper chip was developed to perform a rapid and ultrasensitive in-site methamphetamine test in human urine.

A radical approach for fluorescent turn ‘on’ detection, differentiation and bioimaging of methanol

A Schiff base (RC) is presented herein as a smart fluorescent material for the selective detection and bioimaging of methanol. The key step behind same involves methanol induced opening of the cyclic control unit ofRCresulting in the formation of a highly fluorescent moiety,RO.

Chemoselective recognition with phosphonate cavitands: the ephedrine over pseudoephedrine case

The molecular origin of the selective recognition of ephedrine over pseudoephedrine by an achiral phosphonate cavitand receptor was revealed by the crystal structure of the respective complexes.

An ultrafast molecular rotor based ternary complex in a nanocavity: a potential “turn on” fluorescence sensor for the hydrocarbon chain

Formation of a ternary complex by an ultrafast molecular rotor (UMR) with a macrocyclic cavitand has been investigated for the sensitive detection of the alkyl chain of a surfactant.

Simple synthetic receptors for aspirin

Shallow methylene-bridged cavitands appended with simple H-bond donor/acceptor groups are shown to bind aspirin. The structural features needed in a synthetic receptor for aspirin binding are defined.

Cavitand-grafted silicon microcantilevers as a universal probe for illicit and designer drugs in water

The direct, clean, and unbiased transduction of molecular recognition into a readable and reproducible response is the biggest challenge associated to the use of synthetic receptors in sensing. All possible solutions demand the mastering of molecular recognition at the solid-liquid interface as prerequisite. The socially relevant issue of screening amine-based illicit and designer drugs is addressed by nanomechanical recognition at the silicon-water interface. The methylamino moieties of different drugs are all first recognized by a single cavitand receptor through a synergistic set of weak interactions. The peculiar recognition ability of the cavitand is then transferred with high fidelity and robustness on silicon microcantilevers and harnessed to realize a nanomechanical device for label-free detection of these drugs in water.

Selectivity assessment in host–guest complexes from single-crystal X-ray diffraction data: the cavitand–alcohol case

The solid-state selectivity of a cavitand receptor towards short alkyl chain alcohols was evaluated by analysis of X-ray diffraction data of isomorphous single crystals grown in competition binding experiments.

A BODIPY dye as a reactive chromophoric/fluorogenic probe for selective and quick detection of vapors of secondary amines

A new reaction based fluorescence turn-off strategy for detection of secondary amines was developed. The probe shows fast response and high selectivity to secondary amines in solution/film at sub-ppm levels through chromogenic and fluorescent dual-mode signal changes.

Supramolecular sensing with phosphonate cavitands

Molecular recognition is a recurrent theme in chemical sensing because of the importance of selectivity for sensor performances. The popularity of molecular recognition in chemical sensing has resulted from the progress made in mastering weak interactions, which has enabled the design of synthetic receptors according to the analyte to be detected. However, the availability of a large pool of modular synthetic receptors so far has not had a significant impact on sensors used in the real world. This technological gap has emerged because of the difficulties in transferring the intrinsic molecular recognition properties of a given receptor from solution to interfaces and in finding high fidelity transduction modes for the recognition event. This Account focuses on the ways to overcome these two bottlenecks, and we recount our recent efforts to produce highly selective supramolecular sensors using phosphonate cavitands as receptors. Through two examples, we present an overview of the different operating strategies that are implemented depending on whether the interface is vapor-solid or liquid-solid. First we describe the selective detection of short chain aliphatic alcohols in the vapor phase. In this example, we solved a key issue common to all sensors for organic vapors: the dissection of the specific interaction (between cavitand and the alcohol) from ubiquitous nonspecific dispersion interactions (between the analytes and interferents in the solid layer). We removed responses resulting from the nonspecific interactions of the analytes with interferents by directly connecting the recognition event at the interface to the transduction mechanism (photoinduced charge transfer). The second example addresses the specific detection of sarcosine in urine. Recent research has suggested that sarcosine can serve as reliable biomarker of the aggressive forms of prostate cancer. Tetraphosphonate cavitands can complex N-methyl ammonium salts with impressive selectivity in solution, and we used this property as a starting point. The sensor implementation requires that we first graft the cavitand onto silicon and gold surfaces as monolayers. The exclusive recognition of sarcosine by these supramolecular sensors originates from their operation in aqueous environments, where synergistic multiple interactions with the phosphonate cavitand are possible only for N-methyl ammonium derivatives. We couple that selectivity with detection modes that probe the strength of the complexation either directly (microcantilever) or via exchange with molecules that have comparable affinity for the cavity (fluorescence dye displacement).

Solid state deposition of chiral amphiphilic porphyrin derivatives on glass surface

Herein, we present a straightforward method to achieve optically active films based on porphyrin derivatives. The introduction of an aminoacid functionality on the porphyrin platform confers to the macrocycle both the amphiphilic and chiral character exploited for its solvent-promoted self-aggregation leading to the formation of chiral supramolecular architectures. These ordered suprastructures have the propensity to spontaneously layer as solid films on glass surfaces. The deposited material has been characterized by means of UV-visible, fluorescence emission, circular dichroism spectroscopy and AFM. The reported studies show once more how the stereochemical information stored on a single porphyrin framework can induce the formation of supramolecular chiral architectures, in solution, as well as in solid state. Furthermore, slight modifications on the porphyrin skeleton can influence the aggregation process and the structural features of the final assemblies, leading to solid surfaces featuring different morphologies. These combined aspects can be of great importance for the achievement of solid state chemical sensors with stereoselective properties.

Protein recognition by a self-assembled deep cavitand monolayer on a gold substrate

This paper details the first use of a self-folding deep cavitand on a gold surface. A sulfide-footed deep, self-folding cavitand has been synthesized, and its attachment to a cleaned gold surface studied by electrochemical and SPR methods. Complete monolayer formation is possible if the cavitand folding is templated by noncovalent binding of choline or by addition of space-filling thiols to cover any gaps in the cavitand adsorption layer. The cavitand is capable of binding trimethylammonium-tagged guests from an aqueous medium and can be deposited in 2 × 2 microarrays on the surface for characterization by SPR imaging techniques. When biotin-labeled guests are used, the cavitand:guest construct can recognize and immobilize streptavidin proteins from aqueous solution, acting as an effective supramolecular biosensor for monitoring protein recognition.

A deep cavitand with a fluorescent wall functions as an ion sensor

The synthesis and characterization of a deep cavitand bearing a fluorescent benzoquinoxaline wall is reported. Noncovalent host-guest recognition events are exploited to sense small charged molecules including acetylcholine. The cavitand also exhibits an anion dependent change in fluorescence that is used to differentiate halide ions in solution.