Kirigami-Structured, Low-Impedance, and Skin-Conformal Electronics for Long-Term Biopotential Monitoring and Human-Machine Interfaces.

Epidermal dry electrodes with high skin-compliant stretchability, low bioelectric interfacial impedance, and long-term reliability are crucial for biopotential signal recording and human-machine interaction. However, incorporating these essential characteristics into dry electrodes remains a challenge. Here, a skin-conformal dry electrode is developed by encapsulating kirigami-structured poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/polyvinyl alcohol (PVA)/silver nanowires (Ag NWs) film with ultrathin polyurethane (PU) tape. This Kirigami-structured PEDOT:PSS/PVA/Ag NWs/PU epidermal electrode exhibits a low sheet resistance (≈3.9 Ω sq-1 ), large skin-compliant stretchability (>100%), low interfacial impedance (≈27.41 kΩ at 100 Hz and ≈59.76 kΩ at 10 Hz), and sufficient mechanoelectrical stability. This enhanced performance is attributed to the synergistic effects of ionic/electronic current from PEDOT:PSS/Ag NWs dual conductive network, Kirigami structure, and unique encapsulation. Compared with the existing dry electrodes or standard gel electrodes, the as-prepared electrodes possess lower interfacial impedance and noise in various conditions (e.g., sweat, wet, and movement), indicating superior water/motion-interference resistance. Moreover, they can acquire high-quality biopotential signals even after water rinsing and ultrasonic cleaning. These outstanding advantages enable the Kirigami-structured PEDOT:PSS/PVA/Ag NWs/PU electrodes to effectively monitor human motions in real-time and record epidermal biopotential signals, such as electrocardiogram, electromyogram, and electrooculogram under various conditions, and control external electronics, thereby facilitating human-machine interactions.

Electrospun VO2/carbon fibers for aqueous zinc-ion batteries.

Aqueous zinc-ion batteries (AZIBs) have become one of the most potential energy storage devices due to their high safety and low cost. Vanadium oxide is an ideal cathode material for AZIBs because of its unique tunnel structure and multivalent nature. In this work, electrospun VO2/carbon fibers (VO2@CPAN) with a three-dimensional (3D) network are obtained by an electrospinning strategy combining with a controlled heat treatment. As cathode for AZIBs, the 3D network of the carbon fiber significantly improves the conductivity of VO2, avoids the agglomeration of VO2, and increases the stability of VO2. Therefore, VO2@CPAN delivers a specific capacity of 323.2 mA h g-1 at 0.2 A g-1, which is higher than pure VO2. At the same time, excellent capacity retention of 76.6% is obtained at high current density of 10 A g-1 after 3000 cycles.

Construction of a Chiral Fluorescent Probe for Tryptophan Enantiomers/Ascorbic Acid Identification.

Chiral recognition of amino acid enantiomers is critical in enhancing drug efficacy, detecting disease markers, and understanding physiological processes. Enantioselective fluorescent identification has gained attention among researchers due to its nontoxicity, easy synthesis, and biocompatibility. In this work, chiral fluorescent carbon dots (CCDs) were produced through a hydrothermal reaction followed by chiral modification. The fluorescent probe, Fe3+-CCDs (F-CCDs), was constructed by complexing Fe3+ with CCDs to differentiate between the enantiomers of tryptophan (Trp) and determine ascorbic acid (AA) through an "on-off-on" response. It is worth noting that l-Trp can greatly enhance the fluorescence of F-CCDs with a blue shift, whereas d-Trp does not have any effect on the fluorescence of F-CCDs. F-CCDs showed a low limit of detection (LOD) for l-Trp and l-AA, with an LOD of 3.98 and 6.28 µM, respectively. The chiral recognition mechanism of tryptophan enantiomers using F-CCDs was proposed based on the interaction force between the enantiomers and F-CCDs, as confirmed by UV-vis absorption spectroscopy and density functional theory calculations. The determination of l-AA by F-CCDs was also confirmed through the binding of l-AA to Fe3+ to release CCDs, as seen in UV-vis absorption spectra and time-resolved fluorescence decays. In addition, AND and OR gates were constructed based on the different responses of CCDs to Fe3+ and Fe3+-CCDs to l-Trp/d-Trp, demonstrating the significance of molecular-level logic gates in drug detection and clinical diagnosis.

A two-photon iridium(III) complex probe for sensitive detection of SO2 derivatives in living cell mitochondria.

The derivatives of sulfur dioxide (HSO3-) formed in the biological environment play a vital role in the circulation system. Excessive SO2 derivatives will cause serious damage to the living system. Herein, a two-photon phosphorescent probe based on Ir(III) complex (named as Ir-CN) was designed and synthesized. Ir-CN is extremely selective and sensitive to SO2 derivatives with significant phosphorescent enhancement and increased phosphorescent lifetime. The detection limit of Ir-CN for SO2 derivatives reaches 0.17 µM. More importantly, Ir-CN preferentially accumulates in mitochondria, so bisulfite derivatives can be detected at subcellular level, which enriching the application of metal complex probe in biological detection. In addition, both single-photon and two-photon images can clearly show that Ir-CN is targeted to mitochondria. Benefits from its good biocompatibility, Ir-CN may be used as a reliable tool to detect SO2 derivatives in mitochondrion of living cells.

Biomimetic supramolecular polyurethane with sliding polyrotaxane and disulfide bonds for strain sensors with wide sensing range and self-healing capability.

To prolong the service life of flexible electronic materials, polymeric matrixes with excellent self-healing capability and integrated mechanical properties are highly desirable, but the balance between the self-healing capability and mechanical properties is a grand challenge. Here, polyrotaxanes as sliding crosslinkers and dynamic disulfide bonds are incorporated into the main chains of polyurethane (PU) via one-pot synthesis, which endows the PU with polydisperse hard/soft segments, high density of self-healing points and energy dissipation. Based on this judicious molecular design, the PU elastomers exhibit exceptional mechanical properties, such as high stretchability (1167 % with a tensile strength of 3.49 MPa), high fracture energy (20,775 J m-2) and high puncture energy (200.70 mJ). Moreover, due to the presence of dynamic reversible hydrogen and disulfide bonds, the elastomer could achieve stress and strain repair efficiencies of 93.98 % and 99.21 % at 100 ℃ within 1 h, respectively. The above-mentioned superiorities enable the bioinspired strain sensors to possess a large sensing range (∼596 %), high sensitivity (∼79.98), short response time (∼128 ms), along with excellent reliability and self-healing ability. Besides, the strain sensor exhibits remarkable recyclability and prominent reprocessability, which nicely solves the pollution by discarded electronics.

Sensing for hydrazine of a pyrene chalcone derivative with acryloyl terminal group.

Hydrazine, as a toxic substance, seriously endangers human health and the environment. Based on the excellent luminescent properties and low biological toxicity of pyrene derivatives, combing with chalcone derivatives easily attacked by nucleophilic group, a pyrene derivative PCA decorated by acryloyl terminal group as fluorescent probe for hydrazine was developed. The compound shows fluorescent peak red shift and intensity enhancement with increasing solvent polarity from hexane (459 nm) to methanol (561 nm). Based on strong fluorescence emission in methanol, methanol-HEPES mixed solution was used as the solvent in the spectral recognition experiments. The probe exhibits fluorescent change from yellow fluorescence (576 nm) to blue fluorescence (393 nm) with 800-fold ratiometric fluorescence enhancement (I393nm/I576nm) after the reaction with hydrazine. The probe can recognize hydrazine in fast response rate with kinetic constant calculated being 2.7 × 10-3 s-1 and 15 min as response time. The probe also can monitor hydrazine in real water samples and various soils.

2D Materials for Skin-Mountable Electronic Devices.

Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

A selective "turn-on" sensor for recognizing In3+ and Zn2+ in respective systems based on imidazo[2,1-b]thiazole.

A new simple and easily synthesized multitarget sensor, (E)-N'-(4-(diethylamino)-2-hydroxybenzylidene)imidazo[2,1-b]thiazole-6-carbohydrazide (X), was designed and synthesized using imidazo[2,1-b]thiazole-6-carboxylic acid and 4-(diethylamino)-2-hydroxybenzaldehyde. X could be used as a sensor to detect In3+ in DMF-H2O buffer solution and detect Zn2+ in EtOH-H2O buffer solution through fluorescence enhancement with detection limits of 1.02 × 10-9 M and 5.5 × 10-9 M, respectively. X exhibited an efficient "off-on-off" fluorescence behavior by cyclic addition of metal ions (In3+ and Zn2+) and EDTA. The stoichiometry between X and metal ions (In3+ and Zn2+) was 1 : 1. The binding mode and sensing mechanism of X with metal ions (In3+ and Zn2+) was verified by theoretical calculations using Gaussian 09 based on B3LYP/6-31G(d) and B3LYP/LANL2DZ basis, respectively. Moreover, X could be applied as a potential sensor for the quantitative detection of In3+ and Zn2+ with a satisfactory recovery and relative standard deviation (RSD) in real water samples.

Structurally regular arrangement induced fluorescence enhancement and specific recognition for glutathione of a pyrene chalcone derivative.

Glutathione (GSH) is an important antioxygen and free radical scavenger in the organism. Level of GSH in vivo is associated with many diseases and specific recognition for GSH is very important. Here, a pyrene chalcone derivative 1 1-(2-hydroxyphenyl)-3-(1-pyrenyl)-2-propen-1-one as specific probe for GSH was developed. The probe can give rise to rapid blue fluorescence enhancement for GSH based on Michael addition reaction in pure PBS solution with high sensitivity, fast response rate and high specificity. The compound also can be applied for GSH detection in HeLa cell. Simultaneously, the compound exhibits blue fluorescence emission enhancement in methanol-water (1:1, v/v) solution with fluorescence quantum yield being 0.45 due to the competition of water molecules for hydrogen bonds between hydroxyl and carbonyl and the formation of structurally regular rodlike crystals, which allows regulating fluorescence emission by different solvent condition.

An Omni-Healable and Highly Sensitive Capacitive Pressure Sensor with Microarray Structure.

Capacitive pressure sensors with high flexibility, sensitivity, and excellent healable properties are desirable for a wide variety of applications, such as e-skin. However, implementing these characteristics onto a device presently remains a great challenge. In this work, a flexible pressure sensor with high sensitivity and strong healable properties has been developed based on healable polyurethane (HPU), silver nanowires and graphene. The HPU-based microstructured capacitive pressure sensor exhibited a high sensitivity of 1.9 kPa-1 (<3 kPa), a fast response time (<100 ms), low detection limit (10 Pa), and long-term durability (1000 cycles). Touch-finger and vocal-cord vibration detection have been demonstrated and exhibit a high sensitivity to both static and dynamic pressure. More notably, the entire pressure sensor including the dielectric layer and electrodes is omni-healable after complete separation. The prototype has experimentally shown tremendous potential for wearable, healable applications, such as healthcare monitoring and human-machine interfaces.

Cyanide and biothiols recognition properties of a coumarin chalcone compound as red fluorescent probe.

A novel coumarin chalcone derivative 1 was designed, synthesized and characterized by nuclear magnetic resonance spectra and high resolution mass spectrum. The photophysical and recognition properties of the compound as red fluorescent probe for cyanide and biothiols including cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) have been discussed systematically. Red fluorescence probe 1 was able to achieve rapid and selective identification for cyanide anion and biothiols in aqueous solutions with red fluorescence quench. In addition, the recognition mechanism of 1 was demonstrated by in situ 1H NMR. The compound has two potential nucleophilic sensing sites including carbon-carbon double bond and 4-position of coumarin. The results indicate that cyanide anions can be bonded to these two sites to afford 2:1 bonding product. But biothiols only are bonded to carbon-carbon double bond by Michael addition reaction. The bonding of both cyanide and biothiols to the probe disrupts intramolecular charge transfer and leads to fluorescence quench.

Advancements in Copper Nanowires: Synthesis, Purification, Assemblies, Surface Modification, and Applications.

Copper nanowires (CuNWs) are attracting a myriad of attention due to their preponderant electric conductivity, optoelectronic and mechanical properties, high electrocatalytic efficiency, and large abundance. Recently, great endeavors are undertaken to develop controllable and facile approaches to synthesize CuNWs with high dispersibility, oxidation resistance, and zero defects for future large-scale nano-enabled materials. Herein, this work provides a comprehensive review of current remarkable advancements in CuNWs. The Review starts with a thorough overview of recently developed synthetic strategies and growth mechanisms to achieve single-crystalline CuNWs and fivefold twinned CuNWs by the reduction of Cu(I) and Cu(II) ions, respectively. Following is a discussion of CuNW purification and multidimensional assemblies comprising films, aerogels, and arrays. Next, several effective approaches to protect CuNWs from oxidation are highlighted. The emerging applications of CuNWs in diverse fields are then focused on, with particular emphasis on optoelectronics, energy storage/conversion, catalysis, wearable electronics, and thermal management, followed by a brief comment on the current challenges and future research directions. The central theme of the Review is to provide an intimate correlation among the synthesis, structure, properties, and applications of CuNWs.

Two 3-hydroxyflavone derivatives as two-photon fluorescence turn-on chemosensors for cysteine and homocysteine in living cells.

Two 3-hydroxyflavone derivatives as one- and two-photon fluorescent chemosensors for cysteine (Cys) and homocysteine (Hcy) were synthesized. The recognition properties and mechanism of the chemosensors for Cys and Hcy were investigated systematically. The experiment results indicate that 3-hydroxyflavone compound 1 (6-bromo-2-(9-ethyl-9H-carbazol-3-yl)-3-hydroxy-chromen-4-one) after the addition of nickel ions exhibits good recognition properties for Cys and Hcy with fluorescence enhancement and 65nm absorption peak blue shift based on nickel displacement reaction mechanism. The detection limits (DL) with fluorescence as detected signal are 4.06 × 10-3µM (Cys, linear range of 10-80µM) and 5.8 × 10-3µM (Hcy, linear range of 10-100µM), respectively. But acrylate substituted 3-hydroxyflavone compound 2 (4-oxo-2-(4-diethylamino-phenyl)-4H-chromen-3-yl acrylate) can specially identify Cys with fluorescence turn-on (DL = 1.87 × 10-3µM, linear range of 4-22µM) based on Cys leading to acrylate hydrolysis mechanism and succedent excited-state intramolecular proton transfer process of 3-hydroxyflavone compound. Then Cys and Hcy biological thiols can be recognized at one time by these two 3-hydroxyflavone derivatives. The bioimaging experiment indicates that both the compounds can be successfully applied to the detection of Cys/Hcy in living cells and compound 2 also can be applied to bioimaging Cys in zebrafish by one- and two-photon fluorescence mode. Then these two compounds have a potential in the application of biological sample analysis.

Three-Dimensional Graphene Structure for Healable Flexible Electronics Based on Diels-Alder Chemistry.

Wearable electronics with excellent stretchability and sensitivity have emerged as a very promising field with wide applications such as e-skin and human motion detection. Although three-dimensional (3D) graphene structures (GS) have been reported for high-performance strain sensors, challenges still remain such as the high cost of GS preparation, low stretchability, and the lack of ability to heal itself. In this paper, we reported a novel self-healing flexible electronics with 3D GS based on Diels-Alder (DA) chemistry. Furfurylamine (FA) was employed as a reducing as well as a modifying agent, forming GS by FA (FAGS)/DA bonds contained polyurethane with the "infiltrate-gel-dry" process. The as-prepared composite exhibited excellent stretchability (200%) and intrinsic conductivity with low incorporation of graphene (about 2 wt %), which could be directly employed for flexible electronics to detect human motions. Besides, the FAGS/DAPU composite exhibited lower temperature retro-DA response for the continuous graphene networks. Highly effective healing of the composites by heat and microwave has been demonstrated successfully.

Binary Synergistic Sensitivity Strengthening of Bioinspired Hierarchical Architectures based on Fragmentized Reduced Graphene Oxide Sponge and Silver Nanoparticles for Strain Sensors and Beyond.

Recently, stretchable electronics have been highly desirable in the Internet of Things and electronic skins. Herein, an innovative and cost-efficient strategy is demonstrated to fabricate highly sensitive, stretchable, and conductive strain-sensing platforms inspired by the geometries of a spiders slit organ and a lobsters shell. The electrically conductive composites are fabricated via embedding the 3D percolation networks of fragmentized graphene sponges (FGS) in poly(styrene-block-butadiene-block-styrene) (SBS) matrix, followed by an iterative process of silver precursor absorption and reduction. The slit- and scale-like structures and hybrid conductive blocks of FGS and Ag nanoparticles (NPs) provide the obtained FGS-Ag-NP-embedded composites with superior electrical conductivity of 1521 S cm-1 , high break elongation of 680%, a wide sensing range of up to 120% strain, high sensitivity of ≈107 at a strain of 120%, fast response time of ≈20 ms, as well as excellent reliability and stability of 2000 cycles. This huge stretchability and sensitivity is attributed to the combination of high stretchability of SBS and the binary synergistic effects of designed FGS architectures and Ag NPs. Moreover, the FGS/SBS/Ag composites can be employed as wearable sensors to detect the modes of finger motions successfully, and patterned conductive interconnects for flexible arrays of light-emitting diodes.

A novel silicon-oxygen aurone derivative assisted by graphene oxide as fluorescence chemosensor for fluoride anions.

A novel silicon-oxygen aurone derivative TBDPSA was synthesized and used for the detection of fluoride anions in aqueous solution based on a specifically F--triggered silicon-oxygen cleavage. Even though the compound has shown high selectivity, obvious absorption and fluorescence response for fluoride anions in aqueous solution, but it also is suffered from many limits, such as low detection sensitivity and long response time. Here the compound was successfully assembled on the graphene oxide (GO) surface by π-π stacking. GO improves recognition sensitivity and shortens response time of TBDPSA for fluoride anions by taking advantage of the nanocarrier GO. Compared with TBDPSA, the response time of GO/TBDPSA is shortened greatly from 1h to <5s and the detection limit is lowered about four times with fluorescence as detected signal. Generally speaking, GO is an excellent promoter for accelerate recognition.

Recent Advancements in Flexible and Stretchable Electrodes for Electromechanical Sensors: Strategies, Materials, and Features.

Stretchable and flexible sensors attached onto the surface of the human body can perceive external stimuli, thus attracting extensive attention due to their lightweight, low modulus, low cost, high flexibility, and stretchability. Recently, a myriad of efforts have been devoted to improving the performance and functionality of wearable sensors. Herein, this review focuses on recent remarkable advancements in the development of flexible and stretchable sensors. Multifunction of these wearable sensors is realized by incorporating some desired features (e.g., self-healing, self-powering, linearity, and printing). Next, focusing on the characteristics of carbon nanomaterials, nanostructured metal, conductive polymer, or their hybrid composites, two major strategies (e.g., materials that stretch and structures that stretch) and diverse design approaches have been developed to achieve highly flexible and stretchable electrodes. Strain sensing performances of recently reported sensors indicate that the appropriate choice of geometric engineering as well as intrinsically stretchable materials is essential for high-performance strain sensing. Finally, some important directions and challenges of a fully sensor-integrated wearable platform are proposed to realize their potential applications for human motion monitoring and human-machine interfaces.

Highly Stretchable and Sensitive Strain Sensor Based on Facilely Prepared Three-Dimensional Graphene Foam Composite.

Wearable strain sensors with excellent stretchability and sensitivity have emerged as a very promising field which could be used for human motion detection and biomechanical systems, etc. Three-dimensional (3D) graphene foam (GF) has been reported before for high-performance strain sensors, however, some problems such as high cost preparation, low sensitivity, and stretchability still remain. In this paper, we report a highly stretchable and sensitive strain sensor based on 3D GF and polydimethylsiloxane (PDMS) composite. The GF is prepared by assembly process from graphene oxide via a facile and scalable method and possesses excellent mechanical property which facilitates the infiltration of PDMS prepolymer into the graphene framework. The as-prepared strain sensor can be stretched as high as 30% of its original length and the gauge factor of this sensor is as high as 98.66 under 5% of applied strain. Moreover, the strain sensor shows long-term stability in 200 cycles of stretching-relaxing. Implementation of the device for monitoring the bending of elbow and finger results in reproducibility and various responses in the form of resistance change. Thus, the developed strain sensors exhibit great application potential in fields of biomechanical systems and human-interactive applications.

Layer-by-layer assembly of multifunctional porous N-doped carbon nanotube hybrid architectures for flexible conductors and beyond.

Coassemble diverse functional nanomaterials with carbon nanotubes (CNTs) to form three-dimensional (3D) porous CNTs hybrid architectures (CHAs) are potentially desirable for applications in energy storage, flexible conductors, and catalysis, because of diverse functionalities and synergistic effects in the CHAs. Herein, we report a scalable strategy to incorporate various functional nanomaterials with N-doped CNTs (N-CNTs) into such 3D porous CHAs on the polyurethane (PU) sponge skeletons via layer-by-layer (LbL) assembly. To investigate their properties and applications, the specific CHAs based on N-CNTs and Ag nanoparticles (NPs), denoted as PU-(N-CNTs/Ag NPs)n, are developed. The unique binary structure enables these specific CHAs conductors to possess reliable mechanical and electrical performance under various elastic deformations as well as excellent hydrophilicity. Moreover, they are employed as strain-gauge sensor and heterogeneous catalyst, respectively. The sensor could detect continuous signal, static signal, and pulse signal with superior sustainability and reversibility, indicating an important branch of electromechanical devices. Furthermore, the synergistic effects among N-CNTs, Ag NPs, and porous structure endow the CHAs with excellent performance in catalysis. We have a great expectation that LbL assembly can afford a universal route for incorporating diverse functional materials into one structure.

Strain-driven and ultrasensitive resistive sensor/switch based on conductive alginate/nitrogen-doped carbon-nanotube-supported Ag hybrid aerogels with pyramid design.

Flexible strain-driven sensor is an essential component in the flexible electronics. Especially, high durability and sensitivity to strain are required. Here, we present an efficient and low-cost fabrication strategy to construct a highly sensitive and flexible pressure sensor based on a conductive, elastic aerogel with pyramid design. When pressure is loaded, the contact area between the interfaces of the conductive aerogel and the copper electrode as well as among the building blocks of the nitrogen-doped carbon-nanotube-supported Ag (N-CNTs/Ag) aerogel monoliths, changes in reversible and directional manners. This contact resistance mechanism enables the hybrid aerogels to act as strain-driven sensors with high sensitivity and excellent on/off swithching behavior, and the gauge factor (GF) is ∼15 under strain of 3%, which is superior to those reported for other aerogels. In addition, robust, elastomeric and conductive nanocomposites can be fabricated by injecting polydimethylsiloxane (PDMS) into alginate/N-CNTs/Ag aerogels. Importantly, the building blocks forming the aerogels retain their initial contact and percolation after undergoing large-strain deformation, PDMS infiltration, and cross-linking of PDMS, suggesting their potential applications as strain sensors.