Most synthetic sensors are designed with covalent attachment between a receptor and a reporter moiety. In this report, we describe the current progress of our use of noncovalently attached indicators to signal binding of analytes. With these systems, analyte binding leads to indicator displacement from the binding cavity, which in turn yields an optical signal modulation. We include previous examples, the strategies involved in our development, and the advantages as well as disadvantages of this method. Finally, our latest research in this field is briefly presented.

The microporosity of covalent organic frameworks (COFs) is tailored using a facile synthetic approach that introduces alkyl functionalities into the pore and generates networks with pore diameters between 1-2 nm. The added substituents significantly alter the host-guest properties of the resulting materials.

Molecular recognition has evolved from a science designed to understand biological systems into a much more diverse area of research. While work continues to elucidate “nature's tricks” with respect to intermolecular interactions, much attention has turned to the perspective that molecular recognition, by design, can lead to new technologies. Applications ranging from molecular sensing to information storage and even working molecular machines have been envisioned. This review will highlight a few historical hallmarks of molecular recognition oriented at studying the basic science of intermolecular interactions, but then detail recent advances in molecular recognition aimed towards applications in the field of molecular sensing. Rational design can be used to create synthetic receptors with a good deal of predictability and selectivity, and many signal transduction mechanisms exist for converting these receptors into sensors. This is the first topic discussed. The concept of “differential” or “generalized” sensing is then presented, where one uses an array of sensors that do not necessarily conform to the “lock and key” principle. This approach to sensing is inspired by the mammalian senses of taste and smell, which we briefly describe. To mimic senses of taste and smell, one is naturally led to the use of combinatorial libraries, a direction of research that has seen continued growth over the past few years. We summarize the current state of the art in synthetic combinatorial receptors/sensors, and then predict a future direction that the field of molecular recognition will possibly take. The review is not meant for the specialist, but instead for a general audience. It does not present a highly detailed analysis of each individual topic: synthetic receptors, sensors, olfaction/gustation, and combinatorial receptors/sensors. Instead, this review shows how all these fields complement each other and fit together to create sensing devices. Our conclusion is that specific analyte sensing, differential sensing, and combinatorial chemistry can and will be combined to create sensor arrays, and give the subfield of molecular recognition that uses synthetic systems a bright future in this type of sensing scenario.

The stability and bulk properties of two-dimensional boronate ester-linked covalent organic frameworks (COFs) were investigated upon exposure to aqueous environments. Enhanced stability was observed for frameworks with alkylation in the pores of the COF compared to nonalkylated, bare-pore frameworks. COF-18Å and COF-5 were analyzed as "bare-pore" COFs, while COF-16Å (methyl), COF-14Å (ethyl), and COF-11Å (propyl) were evaluated as "alkylated-pore" materials. Upon submersion in aqueous media, the porosity of alkylated COFs decreased ∼25%, while the nonalkylated COFs were almost completely hydrolyzed, virtually losing all porosity. Similar trends were observed for the degree of crystallinity for these materials, with ∼40% decrease for alkylated COFs and 95% decrease for nonalkylated COFs. SEM was used to probe the particle size and morphology for these hydrolyzed materials. Stability tests, using absorbance spectroscopy and (1)H NMR, monitored the release of monomers as the COF degraded. While nonalkylated COFs were stable in organic solvent, hydrolysis was rapid in aqueous environments, more so in basic compared to neutral or acidic aqueous media (minutes to hours, respectively). Notably, alkylation in the pores of COFs slows hydrolysis, exhibiting up to a 50-fold enhancement in stability for COF-11Å over COF-18Å.

A covalent organic framework (COF-18Å) based on poly(boronate ester)s has been successfully synthesized through a facile dehydration process in 85−95% isolated yield. Spectroscopic characterization confirms formation of the intended ester linkages as the key structural motif forming infinite 2D hexagonally porous sheets. Powder X-ray diffraction studies were used to determine the stacking orientation between the ester-linked sheets, such that atoms in adjacent layers lie directly over each other, resulting in a hexagonal array of 1D, 18 Å pores. COF-18Å exhibits rigid, thermally stable (to 500 °C) pores with high surface area (1260 m2/g) and a micropore volume of 0.29 cm3/g.