Tethering of twisted-fiber artificial muscles.

Twisted-fiber artificial muscles, a new type of soft actuator, exhibit significant potential for use in applications related to lightweight smart devices and soft robotics. Fiber twisting generates internal torque and a spiral architecture, exhibiting rotation, contraction, or elongation as a result of fiber volume change. Untethering a twisted fiber often results in fiber untwisting and loss of stored torque energy. Preserving the torque in twisted fibers during actuation is necessary to realize a reversible and stable artificial muscle performance; this is a key issue that has not yet been systematically discussed and reviewed. This review summarizes the mechanisms for preserving the torque within twisted fibers and the potential applications of such systems. The potential challenges and future directions of research related to twisted-fiber artificial muscles are also discussed.

The Power of Fiber Twist.

Nature's evolution over billions of years has led to the development of different kinds of twisted structures in a variety of biological species. Twisted fibers from nanoscale- to micrometer-scale diameter have been prepared by mimicking natural twisted structures. Mechanically inserting twist in a yarn is an efficient and important method, which generates internal stress, changes the macromolecular orientation, and increases compactness. Recently, twist insertion has been found to produce interesting fiber properties, including chemical, mechanical, electrical, and thermal properties. This Account summarizes recent progress in how twist insertion affects the chemical and physical properties of fibers and describes their applications in artificial spider silk, artificial muscles, refrigeration, and electricity generation.Twist and associated chirality widely arise in nature from molecules to nano- and microscale materials to macroscopic objects such as DNA, RNA, peptides, and chromosomes. Such twisted architectures play an important role in improving the mechanical properties and enabling biological functions. Inspired by the beauty and interesting properties of twisted structures, a wide range of artificial chiral materials with twisted or coiled structures have been prepared, from organic and inorganic nanorods, nanotubes, and nanobelts to macroscopic architectures and buildings.An efficient way to prepare twisted materials is by inserting twist in fibers or yarns, which is an ancient technique used to make yarns or ropes (Wang, R., et al. Science 2019, 366, 216-221. Mu, J., et al. Science 2019, 365, 150-155). During the twisting process, torque is generated in fibers or yarns, the structure of the polymer chains becomes helically oriented, and the fibers in a yarn become more compact. Therefore, the twisting of fibers and yarns can produce novel chemical, mechanical, electrical, and thermal properties (Dou, Y., et al. Nat. Commun. 2019, 10, 1-10. Kim, S. H., et al. Science 2017, 357, 773-778). This Account focuses on the novel properties generated by twist insertion. The mechanical stress and strain can be optimized in a yarn by twist insertion, and different types of fibers exhibit rather different mechanisms.In the first section, we will focus on recent progress in improving the mechanical properties of twisted fibers, including carbon nanotube yarns, single-filament fibers, and hydrogel fibers. Torque was generated by twist insertion in a fiber or a yarn, and the balance of internal torsional stress can be changed by causing a change in yarn volume. This will result in twist release and torsional and tensile actuations of the yarn, which will be described in the second section. Twisting a yarn generally makes it more compact, which will result in a mechanically induced change in capacitance, supercapacitance, and other useful electrochemical properties when a conducting yarn is in an electrolyte. Such processes were used to develop novel devices for twist-based electricity generation, called twistrons, which will be discussed in the third section. Twist insertion or release also changes the polymer chain orientation or crystal structure, resulting in changes in entropy. This is called the twistocaloric effect, which was used to develop a new cooling method, and will be discussed in the last section.

DNAzyme-controlled plasmonic coupling for SERS-based determination of Salmonella typhimurium using hybridization chain reaction self-assembled G-quadruplex.

A facile and rapid SERS strategy for S. typhimurium detection based on hybridization chain reaction (HCR) self-assembled G-quadruplex DNAzyme (GQH DNAzyme)-controlled plasmonic coupling was developed. GQH DNAzyme is introduced as a biocatalyst to catalyze the oxidation of L-cysteines to cysteines (thiols to disulfides) to assist SERS signal transduction. This is the first time that the self-assembled split GQH DNAzyme-controlled plasmonic coupling is integrated with SERS sensing. The results reveal the proposed SERS strategy can quantify S. typhimurium with a wide linear range (5 to 105 cfu mL-1) and a low detection limit (4 cfu mL-1; n = 5, mean ± standard deviation) and RSD of 7%. The method exhibited preeminent detection performance in spiked samples with recoveries of 93.1-117%. The proposed strategy has great potential for being a versatile SERS platform for detecting a wide spectrum of analytes by replacing them with the corresponding recognition elements. Therefore, this study not only creates a practical platform for pathogenic bacteria identification and related food safety testing and environmental monitoring, but also provides a new paradigm for building SERS sensor. A facile and rapid SERS strategy for S. Typhimurium detection based on hybridization chain reaction (HCR) self-assembled G-quadruplex DNAzyme-controlled plasmonic coupling.

A facile signal-on electrochemical DNA sensing platform for ultrasensitive detection of pathogenic bacteria based on Exo III-assisted autonomous multiple-cycle amplification.

A facile signal-on electrochemical DNA biosensor has been developed for ultrasensitive detection of pathogenic bacteria using an Exo III-assisted autonomous multiple-cycle amplification strategy. The strategy relies on pathogens and aptamer binding-initiated release of a trigger, which combines with the 3'-protruding terminus of the hairpin probe 1, leading to the formation of double-stranded DNA with a blunt 3' terminus which starts the Exo III-assisted multiple signal amplification reaction. In addition, hairpin probe 2 labeled with an electroactive reporter at the middle of the loop region is ingeniously designed to contain a short hairpin-embedded segment, which can fold into a hairpin structure via an Exo III-assisted cleavage reaction, thus bringing the redox molecule in proximity to the electrode surface for "signal-on" sensing. Under optimal conditions, this biosensor exhibits a very low detection limit as low as 8 cfu mL-1 and a wide linear range from 1.0 × 101 to 1.0 × 107 cfu mL-1 of target pathogenic bacteria. As far as we know, this is the first time that the Exo III-assisted autonomous multiple-cycle amplification strategy has been used for signal-on electrochemical sensing of pathogenic bacteria. In addition, the proposed sensor can also be used for highly sensitive detection of other targets by changing the aptamer sequence, such as nucleic acids, proteins and small molecules. Therefore, the proposed signal-on electrochemical sensing strategy might provide a simple and practical new platform for detection of pathogenic bacteria and related biological analysis, food safety inspection and environmental monitoring.

Circular exponential amplification of photoinduced electron transfer using hairpin probes, G-quadruplex DNAzyme and silver nanocluster-labeled DNA for ultrasensitive fluorometric determination of pathogenic bacteria.

The authors describe a fluorometric strategy for the detection of pathogenic bacteria with ultrasensitivity and high specificity. This strategy relies on the combination of target-modulated photoinduced electron transfer (PET) between G-quadruplex DNAzyme and DNA (labeled with silver nanoclusters) along with hairpin probe-based circular exponential amplification. The reaction system involves three hairpin probes (H1, H2 and H3). Probe H1 contains an aptamer against S. Typhimurium and the recognition sequence for nicking endonuclease. It is used to recognize S. Typhimurium and participates in polymerase-catalyzed target recycle amplification and secondary-target recycle amplification. Probe H2 contains an aptamer against hemin and is used to form the G-quadruplex DNAzyme in the presence of hemin and potassium ion. It acts as the electron acceptor and quenches the fluorescence of the labeled DNA. Fluorescence is best measured at excitation/emission wavelengths of 567/650 nm. Probe H3 contains the template sequence for the synthesis of AgNCs and the H2-annealing sequence. Both H2 and H3 are utilized to perform a strand displacement reaction and to achieve PET between G-quadruplex DNAzyme and DNA/AgNCs. To the best of our knowledge, this is the first example of a PET between G-quadruplex DNAzyme and DNA/AgNCs coupled with circular exponential amplification. The assay has an ultra-low detection limit 8 cfu·mL-1 of S. Typhimurium. The assay is rapid, accurate, inexpensive and simple. Hence, the strategy may represent a useful platform for ultrasensitive and highly specific detection of pathogenic bacteria as encountered in food analysis and clinical diagnosis. Graphical abstract The reaction system includes three hairpin probes (H1, H2 and H3), primer probe (P), Phi 29 DNA ploymerase (Phi 29) and nicking endonuclease Nt.AlwI (Nt.AlwI). Phi 29 and Nt.AlwI -assisted signal amplification leads to the recycling of target and produces numerous single stranded-DNAs (S). Strand displacement amplification leads to photoinduced electron transfer (PET) between G-quadruplex DNAzyme and DNA/AgNCs. HAP-based circular exponential amplification of PET results in an ultrasensitive fluorometric assay.

Tuning the reversibility of hair artificial muscles by disulfide cross-linking for sensors, switches, and soft robotics.

Tensile and torsional artificial muscles from biocompatible and biodegradable materials are highly desired for soft robotics, sensors, and controllers in bio-related applications. Twisted fibers can be used to prepare tensile and torsional artificial muscles, while torsional tethering is always required to avoid release of the inserted twist, which adds complexity to the device design. Moreover, the tuning of the reversibility of twisted fiber artificial muscles has not been realized. Here disulfide cross-linking was used to prepare novel tether-free hygroresponsive tensile and torsional fiber artificial muscles in twisted hair fibers. Increasing the cross-linking level converted the fiber artificial muscle from irreversible to reversible actuation. Different types of actuations including rotation, contraction, and elongation were realized for the twisted, the homochirally coiled, and the heterochirally coiled hair fibers, respectively. A reversible torsional fiber artificial muscle showed 122.4° mm-1 rotation, homochiral and heterochiral fiber artificial muscles showed 94% contraction and 3000% elongation, respectively, and a maximum work capacity and energy density of 6.35 J kg-1 and 69.8 kJ m-3, respectively, were realized, on exposure to water fog. This work provides a new strategy for preserving the inserted twist in bio-fiber artificial muscles and for tuning of muscle reversibility, which show application perspectives in biocompatible smart materials, sensors, and robotics.

Low-background and visual detection of antibiotic based on target-activated colorimetric split peroxidase DNAzyme coupled with dual nicking enzyme signal amplification.

Herein, we have reported the development of a simple, rapid, and low cost colorimetric method for the detection of antibiotic based on target-activated split peroxidase DNAzyme coupled with dual nicking enzyme signal amplification (NESA). To lower background signal in G-quadruplex DNAzyme-based detection, the two split G-rich parts are caged into two different hairpin probes, respectively, preventing the two parts from assembling into the G-quadruplex structure. By the combination of restriction endonuclease-assisted cleavage reaction with the spilt G-quadruplex probes, target-modulated release of the two split G-rich parts is achieved, affording high specificity of antibiotic detection. Our strategy features with several aspects. First, the less background signal produced by the self-assembly of G-quadruplex in the absence of target is effectively eliminated owing to the pre-blocking of the two split G-rich parts. Second, dual NESA coupled G-quadruplex DNAzyme amplification strategy is integrated with colorimetric assay of antibiotic, which significantly improves the detection sensitivity. Third, peroxidase-mimicking DNAzyme is used as biocatalyst in our reaction system, which can catalyze the oxidation of 2,2' - azino - bis (3 - ethylbenzothiozoline - 6 - sulfonic acid) (ABTS2-) mediated by H2O2 to generate the colored radical anion (ABTS•-), allowing to low cost and visual detection of antibiotic by the naked eye. Under optimized conditions, the results revealed the proposed biosensor exhibits excellent specificity and sensitivity toward kanamycin with a detection limit as low as 14.7 pM. Hence, the target-activated split G-quadruplex DNAzyme and dual NESA-based strategy provides a useful and practical platform for antibiotic residues determination and other analytes detection in bio-analysis.

A novel sandwich-type electrochemical aptasensor based on GR-3D Au and aptamer-AuNPs-HRP for sensitive detection of oxytetracycline.

In this paper, a novel sandwich-type electrochemical aptasensor has been fabricated and applied for sensitive and selective detection of antibiotic oxytetracycline (OTC). This sensor was based on graphene-three dimensional nanostructure gold nanocomposite (GR-3D Au) and aptamer-AuNPs-horseradish peroxidase (aptamer-AuNPs-HRP) nanoprobes as signal amplification. Firstly, GR-3D Au film was modified on glassy carbon electrode only by one-step electrochemical coreduction with graphite oxide (GO) and HAuCl4 at cathodic potentials, which enhanced the electron transfer and loading capacity of biomolecules. Then the aptamer and HRP modified Au nanoparticles provide high affinity and ultrasensitive electrochemical probe with excellent specificity for OTC. Under the optimized conditions, the peak current was linearly proportional to the concentration of OTC in the range of 5×10-10-2×10-3gL-1, with a detection limit of 4.98×10-10gL-1. Additionally, this aptasensor had the advantages in high sensitivity, superb specificity and showed good recovery in synthetic samples. Hence, the developed sandwich-type electrochemical aptasensor might provide a useful and practical tool for OTC determination and related food safety analysis and clinical diagnosis.