An Auto‐Inductive Cascade for the Optical Sensing of Thiols in Aqueous Media: Application in the Detection of a VX Nerve Agent Mimic

Sulfur in Dynamic Covalent Chemistry

Sulfur has been important in dynamic covalent chemistry (DCC) since the beginning of the field. Mainly as part of disulfides and thioesters, dynamic sulfur-based bonds (DSBs) have a leading role in several remarkable reactions. Part of this success is due to the almost ideal properties of DSBs for the preparation of dynamic covalent systems, including high reactivity and good reversibility under mild aqueous conditions, the possibility of exploiting supramolecular interactions, access to isolable structures, and easy experimental control to turn the reaction on/off. DCC is currently witnessing an increase in the importance of DSBs. The chemical flexibility offered by DSBs opens the door to multiple applications. This Review presents an overview of all the DSBs used in DCC, their applications, and remarks on the interesting properties that they confer on dynamic chemical systems, especially those containing several DSBs.

Chemically triggered soft material macroscopic degradation and fluorescence detection using self-propagating thiol-initiated cascades

In this article, we present a new approach for thiol detection through chemically triggered polymeric macroscopic degradation using self-propagating cascades, coupled with photoluminescence.

Chemically Triggered Click and Declick Reactions: Application in Synthesis and Degradation of Thermosetting Plastics

In this Letter, we report that two amines can be coupled together rapidly and quantitatively through amine-thiol scrambling using a bisvinylogous thioester conjugate acceptor under mild conditions. The resulting bisvinylogous amide conjugate acceptors can be decoupled via an ethylene diamine-induced cyclization. Four representative conjugate acceptors have been utilized in the couple-decouple reactions, which were monitored and characterized by nuclear magnetic resonance, high-resolution mass spectrometry, and UV-vis spectroscopy. Further, we applied these small-molecule-based "click-declick" reactions to polymer synthesis and degradation. Highly cross-linked polymers, i.e., plastics, were quantitatively synthesized by simple reactions between commercial tris(2-aminoethyl)amine and the conjugate acceptors without solvent and any initiator or catalyst through ball milling within 60 min. Significantly, these thermosetting plastics can be degraded within 3-24 h via addition of ethylene diamine. The multiple architectures, application to plastics synthesis, and chemically triggered clean degradation to the thermosets at mild conditions with little input of energy herald a new generation of "intelligent" materials.

Chemically Triggered Hydrogel Transformations through Covalent Adaptable Networks and Applications in Cell Culture

In this article, we report a "smart" hydrogel system, which can be remodeled into multiple architectures through dynamic covalent adaptable networks. The topological changes in hydrogel structures yield dynamically tunable properties through the reformation of covalent chemical linkages via amine-thiol scrambling, thiol-thiol exchange, decoupling reaction, and disulfide formation. The stiffness of the hydrogels can be regulated via dynamic covalent bonding, with some hydrogels displaying self-healing and shear thinning properties, as demonstrated by rheological measurements. Significantly, the dramatic structural transformations are achieved under neutral aqueous conditions at room temperature. These "smart" hydrogels show good biocompatibility, which can induce cell growth in two-dimensional cell culture and effectively serve as a scaffold for encapsulating and releasing human mesenchymal stem cells in three-dimensional cell culture. Thus, the developed "smart" hydrogel system holds great potential in biomedical applications such as tissue engineering and cell therapy.

Chemically Triggered Synthesis, Remodeling, and Degradation of Soft Materials

Polymer topology dictates dynamic and mechanical properties of materials. For most polymers, topology is a static characteristic. In this article, we present a strategy to chemically trigger dynamic topology changes in polymers in response to a specific chemical stimulus. Starting with a dimerized PEG and hydrophobic linear materials, a lightly cross-linked polymer, and a cross-linked hydrogel, transformations into an amphiphilic linear polymer, lightly cross-linked and linear random copolymers, a cross-linked polymer, and three different hydrogel matrices were achieved via two controllable cross-linking reactions: reversible conjugate additions and thiol-disulfide exchange. Significantly, all the polymers, before or after topological changes, can be triggered to degrade into thiol- or amine-terminated small molecules. The controllable transformations of polymeric morphologies and their degradation herald a new generation of smart materials.

Self-Propagating Amplification Reactions for Molecular Detection and Signal Amplification: Advantages, Pitfalls, and Challenges

Self-propagating cascade reactions are a recent development for chemo-sensing protocols. These cascade reactions, in principle, offer low limits of detection by virtue of exponential signal amplification, and are initiated by a specific, pre-planned molecular detection event. This combination of selectivity for a detection event followed by in situ signal amplification is achieved by exploitation of mechanistic organic chemistry, and thus has resulted in various chemo-sensing protocols that employ one or more reagents to achieve the desired selectivity and sensitivity for an assay. Species such as hydrogen peroxide, thiols, and fluoride, have been used as active reagents to initiate the first examples of self-propagating signal amplification reactions, although many other active reagents should be compatible with the approaches. A common feature of the reagents that support the self-propagating signal amplification reactions is the involvement of quinonemethide intermediates resulting from elimination of optical reporters and/or active reagents, where the latter propagates the signal amplification reaction. The early examples of these amplification sequences, however, are slow to reach full signal, thus leaving time for background reactions to generate non-specific signals. This issue of background has limited practical applications of these self-propagating signal amplification reactions, as has challenging synthetic routes to the reagents, as well as the potential for other chemical species to interfere with the detection and signal amplification processes. Thus, the goal of this review is to summarize the progress of self-propagating signal amplification technology, identify the pitfalls of current designs, and by doing so, to stimulate future studies in this growing and promising research area.

Photography Coupled with Self-Propagating Chemical Cascades: Differentiation and Quantitation of G- and V-Nerve Agent Mimics via Chromaticity

Photography was employed for the quantitation and differentiation of G- and V-series nerve agent mimics with the use of self-propagating cascades. Fluoride anion and thiols, released from a G-nerve agent mimic (i.e., diisopropyl fluorophosphate) and a V-nerve agent mimic (i.e., demeton-S-methyl), respectively, were used to initiate self-propagating cascades that amplify fluorescence signals exponentially in a ratiometric manner. A homemade LEGO dark-box, a cell phone, and 96-well plates were employed to collect photographs of the fluorescence response to the analytes. The photographic images were digitally processed in the 1931 xyY color space using a watershed and morphological erosion algorithm to generate chromaticity vs concentration calibration curves. We show that the two different amplification routines are selective for their analyte class and thus successfully discriminated the G- and V-series nerve agent mimics. Further, accurate concentrations of the analytes are determined using the chromaticity and LEGO approach given herein, thus demonstrating a simple and on-site constructible/portable device for use in the field.

Bifunctional Fluorescent Probe for Sequential Sensing of Thiols and Primary Aliphatic Amines in Distinct Fluorescence Channels

Thiols and primary aliphatic amines (PAA) are ubiquitous and extremely important species in biological systems. They perform significant interplaying roles in complex biological events. A single fluorescent probe differentiating both thiols and PAA can contribute to understanding the intrinsic inter-relationship of thiols and PAA in biological processes. Herein, we rationally constructed the first fluorescent probe that can respond to thiols and PAA in different fluorescence channels. The probe exhibited a high selectivity and sensitivity to thiols and PAA. In addition, it displayed sequential sensing ability when the thiols and PAA coexisted. The application experiments indicated that the probe can be used for sensing thiols and PAA in human blood serum. Moreover, the fluorescence imaging of endogenous thiols and PAA as well as antihypertensive drugs captopril and amlodipine in living cells were successfully conducted.