Stretchable gold fiber-based wearable textile electrochemical biosensor for lactate monitoring in sweat.

Past several years have witnessed growing interest in developing wearable biosensors for non-invasive monitoring vital signs of chemical/biological markers such as lactate. In this context, textiles can be seen as a promising platform for the integration of wearable chemical sensors due to their inherent breathability, flexibility, softness and comfortableness. Gold is regarded as a preferred active sensing material due to its excellent biocompatibility, chemical inertness and wide electrochemical window. Here, a dry-spinning method was used to fabricate stretchable, strain-insensitive and highly conductive gold fibers. Such gold fibers could be used to fabricate lactate-sensing working electrodes, reference electrode, counter electrodes and further weaved into textiles in a standard three-electrode system with a planar layout. The textile lactate biosensors showed a high sensitivity of 19.13 µA/mM cm2 in phosphate-buffered solution (PBS) and 14.6 µA/mM cm2 in artificial sweat. This sensitivity could be maintained under high tensile strain up to 100% without external structural design. The results presented here indicate the potential application of wearable smart textile towards non-invasive lactate monitoring.

Stretchable gold fiber-based wearable electrochemical sensor toward pH monitoring.

Sweat pH is a key health indicator related to metabolism and homeostasis level through hydrogen ion concentration in biological bio-fluid. Therefore, increasing research efforts have been directed to develop wearable pH sensors towards continuous non-invasive monitoring of sweat pH values in the out-of-hospital environments. Herein, we report a stretchable gold fiber-based electrochemical pH sensor based on our recently developed elastomer-bonded gold nanowire coating technology. The densely packed gold film offers superior strain-insensitive conductivity, high stretchability and large electrochemical active surface area (EASA). By electrodepositing polyamine (PANI) and Ag/AgCl onto the gold fibers, we could selectively detect the pH based on open circuit potentials in an ion-selective electrode design. The obtained fiber-based pH sensors feature a great sensitivity (60.6 mV per pH), high selectivity against cationic interference and high stretchability (up to 100% strain). One of the attributes for the fiber-based sensors is that they can be weaved into textiles, holding great potential for integration into everyday clothing for "unfeelable" personal health monitoring.

Disruptive, Soft, Wearable Sensors.

The wearable industry is on the rise, with a myriad of technical applications ranging from real-time health monitoring, the Internet of Things, and robotics, to name but a few. However, there is a saying "wearable is not wearable" because the current market-available wearable sensors are largely bulky and rigid, leading to uncomfortable wearing experience, motion artefacts, and poor data accuracy. This has aroused a world-wide intensive research quest for novel materials, with the aim of fabricating next-generation ultra-lightweight and soft wearable devices. Such disruptive second-skin-like biosensing technologies may enable a paradigm shift from current wearable 1.0 to future wearable 2.0 products. Here, the state-of-the-art progress made in the key phases for future wearable technology, namely, wear → sense → communicate → analyze → interpret → decide, is summarized. Without a doubt, materials innovation is the key, which is the main focus of the discussion. In addition, emphasis is also given to wearable energy, multicomponent integration, and wireless communication.

Hierarchically Structured Vertical Gold Nanowire Array-Based Wearable Pressure Sensors for Wireless Health Monitoring.

We have recently demonstrated that vertically aligned gold nanowires (v-AuNWs) are outstanding material candidates for wearable biomedical sensors toward real-time and noninvasive health monitoring because of their excellent tunable electrical conductivity, biocompatibility, chemical inertness, and wide electrochemical window. Here, we show that v-AuNWs could also be used to design a high-performance wearable pressure sensor when combined with rational structural engineering such as pyramid microarray-based hierarchical structures. The as-fabricated pressure sensor featured a low operation voltage of 0.1 V, high sensitivity in a low-pressure regime, a fast response time of <10 ms, and high durability with stable signals for the 10 000 cycling test. In conjunction with printed electrode arrays, we could generate a multiaxial map for spatial pressure detection. Furthermore, our flexible pressure sensor could be seamlessly connected with a Bluetooth low-energy module to detect high-quality artery pulses in a wireless manner. Our solution-based gold coating strategy offers the benefit of conformal coating of nanowires onto three-dimensional microstructured elastomeric substrates under ambient conditions, indicating promising applications in next-generation wearable biodiagnostics.

Highly Stretchable and Strain-Insensitive Fiber-Based Wearable Electrochemical Biosensor to Monitor Glucose in the Sweat.

Development of high-performance fiber-shaped wearable sensors is of great significance for next-generation smart textiles for real-time and out-of-clinic health monitoring. The previous focus has been mainly on monitoring physical parameters such as pressure and strains associated with human activities. Development of an enzyme-based non-invasive wearable electrochemical sensor to monitor biochemical vital signs of health such as the glucose level in sweat has attracted increasing attention recently, due to the unmet clinical needs for the diabetic patients. To achieve this, the key challenge lies in the design of a highly stretchable fiber with high conductivity, facile enzyme immobilization, and strain-insensitive properties. Herein, we demonstrate an elastic gold fiber-based three-electrode electrochemical platform that can meet the aforementioned criteria toward wearable textile glucose biosensing. The gold fiber could be functionalized with Prussian blue and glucose oxidase to obtain the working electrode and modified by Ag/AgCl to serve as the reference electrode; and the nonmodified gold fiber could serve as the counter electrode. The as-fabricated textile glucose biosensors achieved a linear range of 0-500 µM and a sensitivity of 11.7 µA mM-1 cm-2. Importantly, such sensing performance could be maintained even under a large strain of 200%, indicating the potential applications in real-world wearable biochemical diagnostics from human sweat.

Highly Stretchable Fiber-Shaped Supercapacitors Based on Ultrathin Gold Nanowires with Double-Helix Winding Design.

The ability of developing highly durable fiber-shaped electronic devices is crucial for next-generation smart textile electronics. Past several years have witnessed encouraging progress made in stretchable fiber-shaped supercapacitors using carbon materials, transition metal oxides, and conducting polymers. Here, we report a dry-spun strategy to produce scalable ultrathin gold nanowire-based fibers, which can lead to highly stretchable fiber-based supercapacitors using a double-helix winding design. Hildebrand's and Hansen's solubility parameters of gold nanowire-binding oleylamine ligands match those of styrene-ethylene/butylene-styrene and tetrahydrofuran, enabling the formation of high-quality dry-spun fibers. In conjunction with conductivity enhancement by electroless plating and pseudocapacitance by polyaniline, we obtained fiber-shaped supercapacitors stretchable up to 360% with a capacitance of 16.80 mF cm-2. The capacitance retention is about 85% after 2000 cycles of 0-200-0% stretching/releasing. Our fiber capacitors can be woven into an everyday glove, with negligible capacitance changes for normal finger movements.

Self-assembled gold nanorime mesh conductors for invisible stretchable supercapacitors.

Thin, skin-conformal, transparent and stretchable energy devices are ideal for powering future wearable and implantable electronics. However, it is difficult to achieve such "unfeelable" and "invisible" devices with traditional materials and design methodologies because of the challenge of simultaneously achieving high optical transparency, high electrical conductivity and high mechanical stretchability. Here, we report a two-step nanowire growth approach for fabricating gold nanorime mesh conductors, enabling skin-thin, transparent and stretchable supercapacitors. Solution-state oleylamine-capped 2 nm-thin gold nanowires self-assemble into highly transparent nanomeshes, which then serve as templates for growing highly conductive vertically aligned nanowires. This two-step solution-plus-surface nanowire growth strategy leads to elastic gold nanorime mesh conductors with an optical transparency up to 90.3% at 550 nm, a low sheet resistance as low as 1.7 ± 0.8 Ω sq-1, and a stretchability of over 100% strain. Such elastic conductors are successfully used to construct symmetrical supercapacitors that can simultaneously achieve high areal capacitance and high stretchability, demonstrating the potential to power future bio-integratable electronics.

Photoelectrochemical Conversion from Graphitic C3N4 Quantum Dot Decorated Semiconductor Nanowires.

Despite the recent progress of developing graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst, the synthesis of nanostructured g-C3N4 has still remained a complicated and time-consuming approach from its bulk powder, which substantially limits its photoelectrochemical (PEC) applications as well as the potential to form composites with other semiconductors. Different from the labor-intensive methods used before, such as exfoliation or assistant templates, herein, we developed a facile method to synthesize graphitic C3N4 quantum dots (g-CNQDs) directly grown on TiO2 nanowire arrays via a one-step quasi-chemical vapor deposition (CVD) process in a homemade system. The as-synthesized g-CNQDs uniformly covered over the surface of TiO2 nanowires and exhibited attractive photoluminescence (PL) properties. In addition, compared to pristine TiO2, the heterojunction of g-CNQD-decorated TiO2 nanowires showed a substantially enhanced PEC photocurrent density of 3.40 mA/cm(2) at 0 V of applied potential vs Ag/AgCl under simulated solar light (300 mW/cm(2)) and excellent stability with ∼82% of the photocurrent retained after over 10 h of continuous testing, attributed to the quantum and sensitization effects of g-CNQDs. Density functional theory calculations were further carried out to illustrate the synergistic effect of TiO2 and g-CNQD. Our method suggests that a variety of g-CNQD-based composites with other semiconductor nanowires can be synthesized for energy applications.

Three-dimensional WS2 nanosheet networks for H2O2 produced for cell signaling.

Hydrogen peroxide (H2O2) is an important molecular messenger for cellular signal transduction. The capability of direct probing of H2O2 in complex biological systems can offer potential for elucidating its manifold roles in living systems. Here we report the fabrication of three-dimensional (3D) WS2 nanosheet networks with flower-like morphologies on a variety of conducting substrates. The semiconducting WS2 nanosheets with largely exposed edge sites on flexible carbon fibers enable abundant catalytically active sites, excellent charge transfer, and high permeability to chemicals and biomaterials. Thus, the 3D WS2-based nano-bio-interface exhibits a wide detection range, high sensitivity and rapid response time for H2O2, and is capable of visualizing endogenous H2O2 produced in living RAW 264.7 macrophage cells and neurons. First-principles calculations further demonstrate that the enhanced sensitivity of probing H2O2 is attributed to the efficient and spontaneous H2O2 adsorption on WS2 nanosheet edge sites. The combined features of 3D WS2 nanosheet networks suggest attractive new opportunities for exploring the physiological roles of reactive oxygen species like H2O2 in living systems.

A flexible ligand-based wavy layered metal-organic framework for lithium-ion storage.

A substantial challenge for direct utilization of metal-organic frameworks (MOFs) as lithium-ion battery anodes is to maintain the rigid MOF structure during lithiation/delithiation cycles. In this work, we developed a flexible, wavy layered nickel-based MOF (C20H24Cl2N8Ni, designated as Ni-Me4bpz) by a solvothermal approach of 3,3',5,5'-tetramethyl-4,4'-bipyrazole (H2Me4bpz) with nickel(II) chloride hexahydrate. The obtained MOF materials (Ni-Me4bpz) with metal azolate coordination mode provide 2-dimensional layered structure for Li(+) intercalation/extraction, and the H2Me4bpz ligands allow for flexible rotation feature and structural stability. Lithium-ion battery anodes made of the Ni-Me4bpz material demonstrate excellent specific capacity and cycling performance, and the crystal structure is well preserved after the electrochemical tests, suggesting the potential of developing flexible layered MOFs for efficient and stable electrochemical storage.