A Mechanochemically Triggered “Click” Catalyst

Mechanochemical generation of aryne

Mechanical force is unique in promoting unusual reaction pathways and especially for the generation of reactive intermediates sometimes inaccessible to other forms of activation. The mechanochemical generation of reactive species could find application in synthetic and materials chemistry alike. However, the nature of these reactive intermediates has been mostly limited to radicals or carbenes. Here, we present a new mechanophore that generates a reactive aryne intermediate upon dissociation of a benzocyclobutene (BCB) core via a force-promoted retro [2 + 2] cycloaddition.

Mechanoresponsive Protein Crystals for NADH Recycling in Multicycle Enzyme Reactions

NAD(H)-dependent enzymes play a crucial role in the biosynthesis of pharmaceuticals and fine chemicals, but the limited recyclability of the NAD(H) cofactor hinders its more general application. Here, we report the generation of mechano-responsive PEI-modified Cry3Aa protein crystals and their use for NADH recycling over multiple reaction cycles. For demonstration of its practical utility, a complementary Cry3Aa protein particle containing genetically encoded and co-immobilized formate dehydrogenase for NADH regeneration and leucine dehydrogenase for catalyzing the NADH-dependent l-tert-leucine (l-tert-Leu) biosynthesis has been produced. When combined with the PEI-modified Cry3Aa crystal, the resultant reaction system could be used for the efficient biosynthesis of l-tert-Leu for up to 21 days with a 10.5-fold improvement in the NADH turnover number.

Polymer mechanochemistry: from single molecule to bulk material

The field of polymer mechanochemistry has experienced a renaissance over the past decades, primarily propelled by the rapid development of force-sensitive molecular units (i.e., mechanophores) and principles governing the reactivity of polymer networks for mechanochemical transduction or material strengthening. In addition to fundamental guidelines for converting mechanical energy input into chemical output, there has also been increasing focus on engineering applications of polymer mechanochemistry for specific functions, mechanically adaptive material systems, and smart devices. These endeavors are made possible by multidisciplinary approaches involving the development of multifunctional mechanophores for mechanoresponsive polymer systems, mechanochemical catalysis and synthesis, three-dimensional (3D) printed mechanochromic materials, reasonable design of polymer network topology, and computational modeling. The aim of this minireview is to provide a summary of recent advancements in covalent polymer mechanochemistry. We specifically focus on productive mechanophores, mechanical remodeling of polymeric materials, and the development of theoretical concepts.

Triggered Crosslinking of Main-Chain Enediyne Polyurethanes via Bergman Cyclization

Crosslinking chemistries occupy an important position in polymer modification with a particular importance when triggered in response to external stimuli. Enediyne (EDY) moieties are used as functional entities in this work, known to undergo a pericyclic Bergman cyclization (BC) to induce a triggered crosslinking of polyurethanes (PU) via the intermediately formed diradicals. Diamino-EDYs, where the distance between the enyne-moieties is known to be critical to induce a BC, are placed repetitively as main-chain structural elements in isophorone-based PUs to induce reinforcement upon heating, compression, or stretching. A 7-day compression under room temperature results in a ≈69% activation of the BC, together with the observation of an increase in tensile strength by 62% after 25 stretching cycles. The occurrence of BC is further proven by the decreased exothermic values in differential scanning calorimetry, together with characteristic peaks of the formed benzene moieties via IR spectroscopy. Purely heat-induced crosslinking contributes to 191% of the maximum tensile strength in comparison to the virgin PU. The BC herein forms an excellent crosslinking strategy, triggered by heat or force in PU materials.

Naphthopyran molecular switches and their emergent mechanochemical reactivity

Naphthopyran molecular switches undergo a ring-opening reaction upon external stimulation to generate intensely colored merocyanine dyes. Their unique modularity and synthetic accessibility afford exceptional control over their properties and stimuli-responsive behavior. Commercial applications of naphthopyrans as photoswitches in photochromic ophthalmic lenses have spurred an extensive body of work exploring naphthopyran-merocyanine structure-property relationships. The recently discovered mechanochromic behavior of naphthopyrans has led to their emergent application in the field of polymer mechanochemistry, enabling advances in the design of force-responsive materials as well as fundamental insights into mechanochemical reactivity. The structure-property relationships established in the photochemical literature serve as a convenient blueprint for the design of naphthopyran molecular force probes with precisely tuned properties. On the other hand, the mechanochemical reactivity of naphthopyran diverges in many cases from the conventional photochemical pathways, resulting in unexpected properties and opportunities for deeper understanding and innovation in polymer mechanochemistry. Here, we highlight the features of the naphthopyran scaffold that render it a powerful platform for the design of mechanochromic materials and review recent advances in naphthopyran mechanochemistry.

Validation of an Accurate and Expedient Initial Rates Method for Characterizing Mechanophore Reactivity

Understanding structure-mechanochemical reactivity relationships is important for informing the rational design of new stimuli-responsive polymers. To this end, establishing accurate reaction kinetics for mechanophore activation is a key objective. Here, we validate an initial rates method that enables the accurate and rapid determination of rate constants for ultrasound-induced mechanochemical transformations. Experimental reaction profiles are well-aligned with theoretical models, which support that the initial rates method effectively deconvolutes the kinetics of specific mechanophore activation from the competitive process of nonspecific chain scission.

Covalent Mechanochemistry and Contemporary Polymer Network Chemistry: A Marriage in the Making

Over the past 20 years, the field of polymer mechanochemistry has amassed a toolbox of mechanophores that translate mechanical energy into a variety of functional responses ranging from color change to small-molecule release. These productive chemical changes typically occur at the length scale of a few covalent bonds (Å) but require large energy inputs and strains on the micro-to-macro scale in order to achieve even low levels of mechanophore activation. The minimal activation hinders the translation of the available chemical responses into materials and device applications. The mechanophore activation challenge inspires core questions at yet another length scale of chemical control, namely: What are the molecular-scale features of a polymeric material that determine the extent of mechanophore activation? Further, how do we marry advances in the chemistry of polymer networks with the chemistry of mechanophores to create stress-responsive materials that are well suited for an intended application? In this Perspective, we speculate as to the potential match between covalent polymer mechanochemistry and recent advances in polymer network chemistry, specifically, topologically controlled networks and the hierarchical material responses enabled by multi-network architectures and mechanically interlocked polymers. Both fundamental and applied opportunities unique to the union of these two fields are discussed.

Sonopharmacology: controlling pharmacotherapy and diagnosis by ultrasound-induced polymer mechanochemistry

Active pharmaceutical ingredients are the most consequential and widely employed treatment in medicine although they suffer from many systematic limitations, particularly off-target activity and toxicity. To mitigate these effects, stimuli-responsive controlled delivery and release strategies for drugs are being developed. Fueled by the field of polymer mechanochemistry, recently new molecular technologies enabled the emergence of force as an unprecedented stimulus for this purpose by using ultrasound. In this research area, termed sonopharmacology, mechanophores bearing drug molecules are incorporated within biocompatible macromolecular scaffolds as preprogrammed, latent moieties. This review presents the novelties in controlling drug activation, monitoring, and release by ultrasound, while discussing the limitations and challenges for future developments.

Competitive Activation Experiments Reveal Significantly Different Mechanochemical Reactivity of Furan–Maleimide and Anthracene–Maleimide Mechanophores

During the past two decades, our understanding of mechanochemical reactivity has advanced considerably. Nevertheless, an incomplete knowledge of structure-activity relationships and the principles that govern mechanochemical transformations limits molecular design. The experimental development of mechanophores has thus benefited from simple computational tools like CoGEF, from which quantitative metrics like rupture force can be extracted to estimate reactivity. Furan-maleimide (FM) and anthracene-maleimide (AM) Diels-Alder adducts are widely studied mechanophores that undergo retro-Diels-Alder reactions upon mechanical activation in polymers. Despite possessing significantly different thermal stability, similar rupture forces predicted by CoGEF calculations suggest that these compounds exhibit similar mechanochemical reactivity. Here, we directly probe the relative mechanochemical reactivity of FM and AM adducts through competitive activation experiments. Ultrasound-induced mechanochemical activation of bis-adduct mechanophores comprising covalently tethered FM and AM subunits reveals pronounced selectivity-as high as ∼13:1-for reaction of the FM adduct compared to the AM adduct. Computational models provide insight into the greater reactivity of the FM mechanophore, indicating a more efficient mechanochemical coupling for the FM adduct compared to the AM adduct. The methodology employed here to directly interrogate the relative reactivity of two different mechanophores using a tethered bis-adduct configuration may be useful for other systems where more common sonication-based approaches are limited by poor sensitivity.

Mechanoresponsive Carbamoyloximes for the Activation of Secondary Amines in Polymers

Mechanophores are molecular moieties that are incorporated into polymers and respond to force with constitutional, configurational, or conformational bond rearrangements to enable functionality. Up to today, several chemically latent motifs have been activated by polymer mechanochemical methods, but the generation of secondary amines remains elusive. Here we report carbamoyloximes as mechanochemical protecting groups for secondary amines. We show that carbamoyloximes undergo force-induced homolytic bond scission at the N-O oxime bond in polymers thus producing the free amine, as the reaction proceeds via the carbamoyloxyl and aminyl radicals, analogously to its photochemical counterpart. Eventually, we apply the carbamoyloxime motif in a force-activated organocatalytic Knoevenagel reaction. We believe that this protecting strategy can be universally applied for many other secondary and primary amines in polymer materials.

Allosteric Binding-Induced Intramolecular Mechanical-Strain Engineering

Recently, polymer mechanochemistry has attracted much scientific interest due to its potential to develop degradable polymers. When the two ends of a polymer chain experience a linear pulling stress, molecular strain builds up, at sufficiently strong force, a bond scission of the weakest covalent bond results. In contrast, bond-breaking events triggered by conformational stress are much less explored. Here, we discovered that a Zn salen complex would undergo conformational switching upon allosteric complexation with alkanediammonium guests. By controlling the guest chain length, the torsional strain experienced by Zn complex can be modulated to induce bond cleavage with chemical stimulus, and reactivity trend is predicted by conformational analysis derived by DFT calculation. Such strain-release reactivity by a Zn(salen) complex initiated by guest binding is reminiscent of conformation-induced reactivity of enzymes to enable chemical events that are otherwise inhibited.

Polymer mechanochemistry for the release of small cargoes

The field of force-induced release of small cargoes within polymeric materials has experienced rapid growth over the past decade, not only including achieving diversified functional materials that report force, trigger degradation, activate drugs and release catalysts, but also involving investigations on the interesting force-coupled reactivity of mechanophores, such as ferrocenes. In this highlight article, we review the recent progress on polymer mechanochemistry that releases small cargoes, including small molecules and metal ions. Since mechanophores play a key role in force-responsive materials, we introduce the progress by discussing different types of mechanophores and their mechanochemical reactions for the release of acids, gases, fluorophores, drugs, iron ions, and so on. At the end, we provide our perspectives on the remaining challenges and future targets in this growing field.

Ultrasound controlled mechanophore activation in hydrogels for cancer therapy

Mechanophores are molecular motifs that respond to mechanical perturbance with targeted chemical reactions toward desirable changes in material properties. A large variety of mechanophores have been investigated, with applications focusing on functional materials, such as strain/stress sensors, nanolithography, and self-healing polymers, among others. The responses of engineered mechanophores, such as light emittance, change in fluorescence, and generation of free radicals (FRs), have potential for bioimaging and therapy. However, the biomedical applications of mechanophores are not well explored. Herein, we report an in vitro demonstration of an FR-generating mechanophore embedded in biocompatible hydrogels for noninvasive cancer therapy. Controlled by high-intensity focused ultrasound (HIFU), a clinically proven therapeutic technique, mechanophores were activated with spatiotemporal precision to generate FRs that converted to reactive oxygen species (ROS) to effectively kill tumor cells. The mechanophore hydrogels exhibited no cytotoxicity under physiological conditions. Upon activation with HIFU sonication, the therapeutic efficacies in killing in vitro murine melanoma and breast cancer tumor cells were comparable with lethal doses of H2O2 This process demonstrated the potential for mechanophore-integrated HIFU combination as a noninvasive cancer treatment platform, named "mechanochemical dynamic therapy" (MDT). MDT has two distinct advantages over other noninvasive cancer treatments, such as photodynamic therapy (PDT) and sonodynamic therapy (SDT). 1) MDT is ultrasound based, with larger penetration depth than PDT. 2) MDT does not rely on sonosensitizers or the acoustic cavitation effect, both of which are necessary for SDT. Taking advantage of the strengths of mechanophores and HIFU, MDT can provide noninvasive treatments for diverse cancer types.

Mechanochromic polymers with a multimodal chromic transition: mechanophore design and transduction mechanism

This review presents the recent progress in multi-chromic polymers embedded with mechanophores concentrating on transduction mechanisms and design concepts.

Harnessing the Power of Force: Development of Mechanophores for Molecular Release

Polymers that release small molecules in response to mechanical force are promising materials for a variety of applications ranging from sensing and catalysis to targeted drug delivery. Within the rapidly growing field of polymer mechanochemistry, stress-sensitive molecules known as mechanophores are particularly attractive for enabling the release of covalently bound payloads with excellent selectivity and control. Here, we review recent progress in the development of mechanophore-based molecular release platforms and provide an optimistic, yet critical perspective on the fundamental and technological advancements that are still required for this promising research area to achieve significant impact.

Mechanochemistry for the synthesis of non-classical N-metalated palladium(II) pincer complexes

The peculiarities of cyclopalladation of a series of non-classical pincer-type ligands based on monothiooxalyl amides bearing ancillary N- or S-donor groups in the amide units have been scrutinized both under conditions of conventional solution-based synthesis and in the absence of a solvent according to a solid-phase methodology including mechanochemical activation. Grinding the functionalized monothiooxamides with PdCl₂(NCPh)₂ in a mortar or vibration ball mill is shown to serve as an efficient and green alternative to the synthesis of these complex metal-organic systems in solution that can offer such advantages as the absence of any auxiliary and significant rate and yield enhancement, especially for the challenging ligands. The realization of S,N,N- or S,N,S-monoanionic tridentate coordination in the resulting pincer complexes has been confirmed by multinuclear NMR (including 2D NMR) and IR spectroscopy and, in some cases, X-ray diffraction. The course and outcome of the solid-phase reactions have been studied by a combination of different spectroscopic methods as well as SEM/EDS analysis. The preliminary evaluation of cytotoxic activity against several human cancer cell lines has revealed the high potency of some of the cyclopalladated derivatives obtained, rendering further development of solvent-free synthetic routes to this type of complexes very urgent.

Force-Induced Near-Infrared Chromism of Mechanophore-Linked Polymers

A near-infrared (NIR) mechanophore was developed and incorporated into a poly(methyl acrylate) chain to showcase the first force-induced NIR chromism in polymeric materials. This mechanophore, based on benzo[1,3]oxazine (OX) fused with a heptamethine cyanine moiety, exhibited NIR mechanochromism in solution, thin-film, and bulk states. The mechanochemical activity was validated using UV-vis-NIR absorption/fluorescence spectroscopies, gel permeation chromatography (GPC), NMR, and DFT simulations. Our work demonstrates that NIR mechanochromic polymers have considerable potential in mechanical force sensing, damage detection, bioimaging, and biomechanics.

A modular approach to mechanically gated photoswitching with color-tunable molecular force probes

Molecular force probes conveniently report on mechanical stress and/or strain in polymers through straightforward visual cues. Unlike conventional mechanochromic mechanophores, the mechanically gated photoswitching strategy decouples mechanochemical activation from the ultimate chromogenic response, enabling the mechanical history of a material to be recorded and read on-demand using light. Here we report a completely redesigned, highly modular mechanophore platform for mechanically gated photoswitching that offers a robust, accessible synthesis and late stage diversification through Pd-catalyzed cross-coupling reactions to precisely tune the photophysical properties of the masked diarylethene (DAE) photoswitch. Using solution-phase ultrasonication, the reactivity of a small library of functionally diverse mechanophores is demonstrated to be exceptionally selective, producing a chromogenic response under UV irradiation only after mechanochemical activation, revealing colored DAE isomers with absorption spectra that span the visible region of the electromagnetic spectrum. Notably, mechanically gated photoswitching is successfully translated to solid polymeric materials for the first time, demonstrating the potential of the masked diarylethene mechanophore for a variety of applications in force-responsive polymeric materials.

Reversible regulation of metallo-base-pair interactions for DNA dehybridization by ultrasound

Mechanical force applied by ultrasound in solution leads to the dissociation of DNA metallo-base-pair interactions when these motifs are functionalized with oligodeoxynucleotide sequences of sufficient length. The annealing and force-induced denaturing process is followed by the attachment of distance-sensitive fluorescent probes and is found to be reversible.

Force-modulated reductive elimination from platinum(ii) diaryl complexes

Coupled mechanical forces are known to drive a range of covalent chemical reactions, but the effect of mechanical force applied to a spectator ligand on transition metal reactivity is relatively unexplored. Here we quantify the rate of C(sp2)-C(sp2) reductive elimination from platinum(ii) diaryl complexes containing macrocyclic bis(phosphine) ligands as a function of mechanical force applied to these ligands. DFT computations reveal complex dependence of mechanochemical kinetics on the structure of the force-transducing ligand. We validated experimentally the computational finding for the most sensitive of the ligand designs, based on MeOBiphep, by coupling it to a macrocyclic force probe ligand. Consistent with the computations, compressive forces decreased the rate of reductive elimination whereas extension forces increased the rate relative to the strain-free MeOBiphep complex with a 3.4-fold change in rate over a ∼290 pN range of restoring forces. The calculated natural bite angle of the free macrocyclic ligand changes with force, but 31P NMR analysis and calculations strongly suggest no significant force-induced perturbation of ground state geometry within the first coordination sphere of the (P-P)PtAr2 complexes. Rather, the force/rate behavior observed across this range of forces is attributed to the coupling of force to the elongation of the O⋯O distance in the transition state for reductive elimination. The results suggest opportunities to experimentally map geometry changes associated with reactions in transition metal complexes and potential strategies for force-modulated catalysis.

Mechanochemical Reactivity of Bottlebrush and Dendronized Polymers: Solid vs. Solution States

We explored the mechanochemical degradation of bottlebrush and dendronized polymers in solution (with ultrasonication, US) and solid states (with ball-mill grinding, BMG). Over 50 polymers were prepared with varying backbone length and arm architecture, composition, and size. With US, we found that bottlebrush and dendronized polymers exhibited consistent backbone scission behavior, which was related to their elongated conformations in solution. Considerably different behavior was observed with BMG, as arm architecture and composition had a significant impact on backbone scission rates. Arm scission was also observed for bottlebrush polymers in both solution and solid states, but only in the solid state for dendronized polymers. Motivated by these results, multi-mechanophore polymers with bottlebrush and dendronized polymer architectures were prepared and their reactivity was compared. Although dendronized polymers showed slower arm-scission, the selectivity for mechanophore activation was much higher. Overall, these results have important implications to the development of new mechanoresponsive materials.

The influence of polymer architecture in polymer mechanochemistry

Polymer architecture is an important factor in polymer mechanochemistry. In this Feature Article, we summarize recent developments in utilizing polymer architecture to modulate mechanochemical reactions within polymers, or more specifically, the location and rates of bond scission events that lead to polymer fragmentation or mechanophore activation. Various well-defined architectures have been explored, including those of cyclic, intramolecularly cross-linked, dendritic, star, bottlebrush, and dendronized polymers. We primarily focus on describing the enhancement or suppression of mechanochemical reactivity, with respect to analogous linear polymers, as well as differences in solution- and solid-state behavior.

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

Activation of a Copper Biscarbene Mechano-Catalyst Using Single-Molecule Force Spectroscopy Supported by Quantum Chemical Calculations

Single-molecule force spectroscopy allows investigation of the effect of mechanical force on individual bonds. By determining the forces necessary to sufficiently activate bonds to trigger dissociation, it is possible to predict the behavior of mechanophores. The force necessary to activate a copper biscarbene mechano-catalyst intended for self-healing materials was measured. By using a safety line bypassing the mechanophore, it was possible to pinpoint the dissociation of the investigated bond and determine rupture forces to range from 1.6 to 2.6 nN at room temperature in dimethyl sulfoxide. The average length-increase upon rupture of the Cu-C bond, due to the stretching of the safety line, agrees with quantum chemical calculations, but the values exhibit an unusual scattering. This scattering was assigned to the conformational flexibility of the mechanophore, which includes formation of a threaded structure and recoiling of the safety line.

Mechanochemical Release of N-Heterocyclic Carbenes from Flex-Activated Mechanophores

We have discovered a new flex-activated mechanophore that releases an N-heterocyclic carbene (NHC) under mechanical load. The mechanophore design is based upon NHC-carbodiimide (NHC-CDI) adducts and demonstrates an important first step toward flex-activated designs capable of further downstream reactivities. Since the flex-activation is non-destructive to the main polymer chains, the material can be subjected to multiple compression cycles to achieve iterative increases in the activation percentage of mechanophores. Two different NHC structures were demonstrated, signifying the potential modularity of the mechanophore design.

Mechanochemical Release of Non-Covalently Bound Guests from a Polymer-Decorated Supramolecular Cage

Supramolecular coordination cages show a wide range of useful properties including, but not limited to, complex molecular machine-like operations, confined space catalysis, and rich host-guest chemistries. Here we report the uptake and release of non-covalently encapsulated, pharmaceutically-active cargo from an octahedral Pd cage bearing polymer chains on each vertex. Six poly(ethylene glycol)-decorated bipyridine ligands are used to assemble an octahedral PdII6 (TPT)4 cage. The supramolecular container encapsulates progesterone and ibuprofen within its hydrophobic nanocavity and is activated by shear force produced by ultrasonication in aqueous solution entailing complete cargo release upon rupture, as shown by NMR and GPC analyses.

Self-Diagnostic Polymers-Inline Detection of Thermal Degradation of Unsaturated Poly(ester imide)s

Monitoring polymer degradation is an important quest, particularly relevant for industry. Although many indirect methodologies for assessing polymer degradation exist, only few are applicable for an inline-monitoring via optic detection-systems. An inline-monitoring system is introduced for the thermal degradation of crosslinked poly(ester imide)s (PEIs) by embedding trifluoroacetyl functionalized stilbene molecules, serving as chemosensors to track the release of generated alcoholic byproducts. Nucleophilic addition of an alcohol to the sensors trifluoroacetyl functionality triggers hemiacetal formation which is accompanied by significant changes in optical properties, in turn allowing monitoring of sensor activation by direct spectroscopy. Fluorescence spectroscopy offers an easy detection tool for the inline thermal monitoring of PEI-degradation.

Bioinspired Artificial "Clickase" for the Catalytic Click Immunoassay of Foodborne Pathogens

The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click reaction has drawn increasing attention in the field of analytical science. However, the poor stability of Cu(I) usually hinders not only the simplicity of the click reaction but also its applications in precise analyses. Therefore, the development of a nanocatalyst containing stable Cu(I) is of great significance for broadening the application of CuAAC-based assays. Herein, inspired by the active center structure of natural multicopper oxidases (MCOs), we successfully prepared a novel nanocatalyst containing abundant stable Cu(I) as an artificial "clickase" (namely, CCN) by using glutathione to stabilize Cu(I). The stability and enzyme-like catalytic activity in the CuAAC reaction of the prepared CCN clickase were studied, and the catalytic mechanism of the CCN clickase-mediated CuAAC reaction between 3-azide-7-hydroxycoumarin (Azide 1) and propargyl alcohol (Alkyne 2) was also revealed. Compared with the existing solid CuO nanocatalysts used in CuAAC-based assays, CCN clickases exhibited plenty of superior properties (including high stability, excellent catalytic activity, no requirements of dissolution and reducing agents/radical initiator during the detection, well-defined porosities benefiting the substrate diffusion, and good biocompatibility), which can greatly increase the reaction efficiency and shorten the detection time. Encouraged by these remarkable performances, CCN clickases were used as labels to establish a new catalytic click fluorescence immunoassay for foodborne pathogens. Notably, the proposed CCN clickase-based immunoassay exhibited high analytical performances for the quantification of Salmonella enteritidis in the linear range of 10²-10⁶ CFU/mL with a limit of detection as low as 11 CFU/mL. The developed method has also been used in the determination of S. enteritidis in food samples, showing its great potential in the detection of foodborne pathogens.

The many flavours of mechanochemistry and its plausible conceptual underpinnings

Mechanochemistry describes diverse phenomena in which mechanical load affects chemical reactivity. The fuzziness of this definition means that it includes processes as seemingly disparate as motor protein function, organic synthesis in a ball mill, reactions at a propagating crack, chemical actuation, and polymer fragmentation in fast solvent flows and in mastication. In chemistry, the rate of a reaction in a flask does not depend on how fast the flask moves in space. In mechanochemistry, the rate at which a material is deformed affects which and how many bonds break. In other words, in some manifestations of mechanochemistry, macroscopic motion powers otherwise endergonic reactions. In others, spontaneous chemical reactions drive mechanical motion. Neither requires thermal or electrostatic gradients. Distinct manifestations of mechanochemistry are conventionally treated as being conceptually independent, which slows the field in its transformation from being a collection of observations to a rigorous discipline. In this Review, we highlight observations suggesting that the unifying feature of mechanochemical phenomena may be the coupling between inertial motion at the microscale to macroscale and changes in chemical bonding enabled by transient build-up and relaxation of strains, from macroscopic to molecular. This dynamic coupling across multiple length scales and timescales also greatly complicates the conceptual understanding of mechanochemistry.

Mechanochemical bond scission for the activation of drugs

Pharmaceutical drug therapy is often hindered by issues caused by poor drug selectivity, including unwanted side effects and drug resistance. Spatial and temporal control over drug activation in response to stimuli is a promising strategy to attenuate and circumvent these problems. Here we use ultrasound to activate drugs from inactive macromolecules or nano-assemblies through the controlled scission of mechanochemically labile covalent bonds and weak non-covalent bonds. We show that a polymer with a disulfide motif at the centre of the main chain releases an alkaloid-based anticancer drug from its β-carbonate linker by a force-induced intramolecular 5-exo-trig cyclization. Second, aminoglycoside antibiotics complexed by a multi-aptamer RNA structure are activated by the mechanochemical opening and scission of the nucleic acid backbone. Lastly, nanoparticle-polymer and nanoparticle-nanoparticle assemblies held together by hydrogen bonds between the peptide antibiotic vancomycin and its complementary peptide target are activated by force-induced scission of hydrogen bonds. This work demonstrates the potential of ultrasound to activate mechanoresponsive prodrug systems.

Single-Molecule Activation and Quantification of Mechanically Triggered Palladium-Carbene Bond Dissociation

Metal-complexed N-heterocyclic carbene (NHC) mechanophores are latent reactants and catalysts for a range of mechanically driven chemical responses, but mechanochemical scission of the metal-NHC bond has not been experimentally characterized. Here we report the single-molecule force spectroscopy of ligand dissociation from a pincer NHC-pyridine-NHC Pd(II) complex. The force-coupled rate constant for ligand dissociation reaches 50 s^-1 at forces of approximately 930 pN. Experimental and computational observations support a dissociative, rather than associative, mechanism of ligand displacement, with rate-limiting scission of the Pd-NHC bond followed by rapid dissociation of the pyridine moiety from Pd.

Controlling Optical and Catalytic Activity of Genetically Engineered Proteins by Ultrasound

Ultrasound (US) produces cavitation-induced mechanical forces stretching and breaking polymer chains in solution. This type of polymer mechanochemistry is widely used for synthetic polymers, but not biomacromolecules, even though US is biocompatible and commonly used for medical therapy as well as in vivo imaging. The ability to control protein activity by US would thus be a major stepping-stone for these disciplines. Here, we provide the first examples of selective protein activation and deactivation by means of US. Using GFP as a model system, we engineer US sensitivity into proteins by design. The incorporation of long and highly charged domains enables the efficient transfer of force to the protein structure. We then use this principle to activate the catalytic activity of trypsin by inducing the release of its inhibitor. We expect that this concept to switch "on" and "off" protein activity by US will serve as a blueprint to remotely control other bioactive molecules.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

The Mechanism of Flex-Activation in Mechanophores Revealed By Quantum Chemistry

Flex-activated mechanophores can be used for small-molecule release in polymers under tension by rupture of covalent bonds that are orthogonal to the polymer main chain. Using static and dynamic quantum chemical methods, we here juxtapose three different mechanical deformation modes in flex-activated mechanophores (end-to-end stretching, direct pulling of the scissile bonds, bond angle bendings) with the aim of proposing ways to optimize the efficiency of flex-activation in experiments. It is found that end-to-end stretching, which is a traditional approach to activate mechanophores in polymers, does not trigger flex-activation, whereas direct pulling of the scissile bonds or displacement of adjacent bond angles are efficient methods to achieve this goal. Based on the structural, energetic and electronic effects responsible for these observations, we propose ways of weakening the scissile bonds experimentally to increase the efficiency of flex-activation.