Controllable growth of substrate-scale 2D ReSe2 thin films and their application for molecular detection via the SERS technique.

Rhenium diselenide (ReSe2) has attracted great interest due to its unique anisotropic structure and unusual in-plane anisotropic electrical and optical properties. However, efficient fabrication of large-area and high-quality 2D ReSe2 continuous films has become an increasingly important challenge. In this work, centimeter-scale 2D ReSe2 continuous films with the layer number varying from monolayer to 12 layers were successfully grown on a mica substrate using our space-confined CVD system via changing the position of the substrate. The fluorescence quenching effect and surface enhanced Raman scattering (SERS) effect on 2D ReSe2 films with different layer numbers were investigated using rhodamine 6G (R6G) dye molecules as a Raman probe. The large-area ReSe2 films show the layer-number-dependent nature of the SERS effect and a robust suppression effect of fluorescence. Our work explores the practical application of 2D ReSe2 films for molecular detection via the SERS technique.

Wood Derived Composites for High Sensitivity and Wide Linear-Range Pressure Sensing.

Natural wood possesses a unique 3D microstructure containing hierarchical interconnected channels along its growth direction. This study reports a facile processing strategy to utilize such structure to fabricate carbon/silicone composite based flexible pressure sensors. The unique contribution of the multichannel structure on the sensor performance is analyzed by comparing the pressure response of the vertically cut and horizontally cut composite structures. The results show that the horizontally cut composite based sensors exhibit much higher sensitivity (10.74 kPa-1 ) and wider linear region (100 kPa, R2 = 99%), due to their rough surface and largely deformable microstructure. Besides, the sensors also show little hysteresis and good cycle stability. The overall outstanding sensing properties of the sensors allow for accurate continuous measurement of human pulse and respiration, benefiting the real-time health signal monitoring and disease diagnoses.

Terbium-doped gadolinium oxide nanoparticles prepared by laser ablation in liquid for use as a fluorescence and magnetic resonance imaging dual-modal contrast agent.

Dual-modal lanthanide-doped gadolinium nanoparticles (NPs), which exhibit an excellent magnetic resonance imaging (MRI) spatial resolution and high fluorescence imaging (FI) sensitivity, have attracted tremendous attention in biotechnology and nanomedicine applications. In this paper, terbium (Tb) ion doped gadolinium oxide (Gd2O3:Tb) NPs with varied Tb concentrations were synthesized by a laser ablation in liquid (LAL) method. The characterization of the structure, morphology, and composition shows that these NPs are spherical with excellent crystallinity. The effects of Tb ion concentration on the visible green fluorescence and longitudinal relaxivity were investigated, indicating that the fluorescence properties were significantly influenced by the Tb ion concentration, but all samples were still efficient T1-weighted contrast agents. Furthermore, the optimum Tb doping concentration was determined to be 1%. The cell viability, cellular fluorescence imaging and in vivo MRI of this dual-modal nano-probe were studied, with the results revealing that the Gd2O3:Tb NPs did not have a significant cytotoxic effect, making them good candidates for use as a dual-modal contrast agent for MRI and fluorescence imaging.

Sub-10 nm monoclinic Gd2O3:Eu3+ nanoparticles as dual-modal nanoprobes for magnetic resonance and fluorescence imaging.

Monoclinic Gd2O3:Eu(3+) nanoparticles (NPs) possess favorable magnetic and optical properties for biomedical application. However, how to obtain small enough NPs still remains a challenge. Here we combined the standard solid-state reaction with the laser ablation in liquids (LAL) technique to fabricate sub-10 nm monoclinic Gd2O3:Eu(3+) NPs and explained their formation mechanism. The obtained Gd2O3:Eu(3+) NPs exhibit bright red fluorescence emission and can be successfully used as fluorescence probe for cells imaging. In vitro and in vivo magnetic resonance imaging (MRI) studies show that the product can also serve as MRI good contrast agent. Then, we systematically investigated the nanotoxicity including cell viability, apoptosis in vitro, as well as the immunotoxicity and pharmacokinetics assays in vivo. This investigation provides a platform for the fabrication of ultrafine monoclinic Gd2O3:Eu(3+) NPs and evaluation of their efficiency and safety in preclinical application.

Ligand-free gadolinium oxide for in vivo T1-weighted magnetic resonance imaging.

Gadolinium oxide (Gd2O3), which can be used as a T1-weighted magnetic resonance imaging (MRI) contrast agent, has attracted intense attention in recent years. In this paper, ligand-free monoclinic Gd2O3 nanocrystals of 7.1 nm in diameter are synthesized by a simple and green approach, namely microsecond laser ablation of a gadolinium (Gd) target in deionized water. These nanocrystals obtain high r1 relaxivity of 5.53 s(-1) mM(-1), and their low toxicity was demonstrated by the cell viability of S18 cells and apoptosis in RAW264.7 cells. In vitro and in vivo MR images show these particles to be good T1-weighted MRI contrast agents. Base on the experimental results and theoretical analysis, we suggest that the purity of the Gd2O3 contributes to its high r1 relaxivity value, while the low toxicity is due to its good crystallinity. These findings show that laser ablation in liquid (LAL) is a promising strategy to synthesize ligand-free monoclinic Gd2O3 nanocrystals for use as high efficient T1-weighted MRI contrast agents.

Highly Sensitive Piezoresistive Pressure Sensor Based on Super-Elastic 3D Buckling Carbon Nanofibers for Human Physiological Signals' Monitoring.

The three-dimensional (3D) carbon nanostructures/foams are commonly used as active materials for the high-performance flexible piezoresistive sensors due to their superior properties. However, the intrinsic brittleness and poor sensing properties of monolithic carbon material still limits its application. Rational design of the microstructure is an attractive approach to achieve piezoresistive material with superior mechanical and sensing properties, simultaneously. Herein, we introduce novel three-dimensional buckling carbon nanofibers (3D BCNFs) that feature a unique serpentine-buckling microstructure. The obtained 3D BCNFs exhibit superior mechanical properties, including super-elasticity (recovery speed up to 950 mm s-1), excellent flexibility (multiple folds), high compressibility (compressed by 90%), and high fatigue resistance (10,000 bending cycles). The pressure sensor fabricated by the 3D BCNFs shows a high sensitivity of 714.4 kPa-1, a fast response time of 23 ms, and a broad measuring range of 120 kPa. The pressure sensor is further applied to monitor the physiological signals of humans, and is capable of detecting the characteristic pulse waves from the radial artery, fingertip artery, and human-breath, respectively.

Flexible pressure and temperature dual-mode sensor based on buckling carbon nanofibers for respiration pattern recognition.

Breathing condition is an essential physiological indicator closely related to human health. Wearable flexible breath sensors for respiration pattern recognition have attracted much attention as they can provide physiological signal details for personal medical diagnosis, health monitoring, etc. However, present smart mask based on flexible breath sensors using single-mode detection can only detect a relatively small number of respiration patterns, especially lacking the ability to accurately distinguish mouth breath from nasal one. Herein, a smart face mask incorporated with a dual-sensing mode breathing sensor that can recognize up to eight human respiration patterns is fabricated. The breathing sensor uses novel three dimensional (3D) buckling carbon nanofiber mats as active materials to realize the function of pressure and temperature sensing simultaneously. The pressure model of the sensors shows a high sensitivity that are able to precisely detect pressure generated by respiratory airflow, while the temperature model can realize non-contact temperature variation caused by breath. Benefit from the capacity of real-time recognition and accurate distinguishing between mouth breath and nasal breath, the face mask is further developed to monitor the development of mouth breathing syndrome. The dual-sensing mode sensor has great potential applications in health monitoring.

Ultrafast-Response/Recovery Flexible Piezoresistive Sensors with DNA-Like Double Helix Yarns for Epidermal Pulse Monitoring.

A key challenge in textile sensors is to adequately solve the hysteresis for more broad and exacting applications. Unlike the conventional strategy in integrating elastic polymers into the textile, the hysteretic issue is critically addressed here through the structural design of yarns to provide a twisting force. The underlying mechanism is fully discussed based on theory and modeling, which are in good agreement with experimental data. Impressively, the pressure sensor outperforms almost all reported textile-based sensors in terms of recovery index, which refers to the ability to overcome the lagged deformation reflected by the hysteresis (5.3%) and relaxation time (2 ms). Besides, the sensor superiority is also demonstrated by way of its ultrafast response time (2 ms). Thanks to these merits, this pressure sensor is demonstrated to be capable of monitoring epidermal pulses and meanwhile shows great potential to advance the standardization and modernization of pulse palpation in traditional Chinese medicine.

PCA-Based Multi-Wavelength Photoplethysmography Algorithm for Cuffless Blood Pressure Measurement on Elderly Subjects.

The prevalence of hypertension has made blood pressure (BP) measurement one of the most wanted functions in wearable devices for convenient and frequent self-assessment of health conditions. The widely adopted principle for cuffless BP monitoring is based on arterial pulse transit time (PTT), which is measured with electrocardiography and photoplethysmography (PPG). To achieve cuffless BP monitoring with more compact wearable electronics, we have previously conceived a multi-wavelength PPG (MWPPG) strategy to perform BP estimation from arteriolar PTT, requiring only a single sensing node. However, challenges remain in decoding the compounded MWPPG signals consisting of both heterogeneous physiological information and motion artifact (MA). In this work, we proposed an improved MWPPG algorithm based on principal component analysis (PCA) which matches the statistical decomposition results with the arterial pulse and capillary pulse. The arteriolar PTT is calculated accordingly as the phase shift based on the entire waveforms, instead of local peak lag time, to enhance the feature robustness. Meanwhile, the PCA-derived MA component is employed to identify and exclude the MA-contaminated segments. To evaluate the new algorithm, we performed a comparative experiment (N = 22) with a cuffless MWPPG measurement device and used double-tube auscultatory BP measurement as a reference. The results demonstrate the accuracy improvement enabled by the PCA-based operations on MWPPG signals, yielding errors of 1.44 ± 6.89 mmHg for systolic blood pressure and -1.00 ± 6.71 mm Hg for diastolic blood pressure. In conclusion, the proposed PCA-based method can improve the performance of MWPPG in wearable medical devices for cuffless BP measurement.

Flexible Organic/Inorganic Hybrid Near-Infrared Photoplethysmogram Sensor for Cardiovascular Monitoring.

Wearable photoplethysmogram (PPG) sensors offer convenient and informative measurements for evaluating daily physiological states of individuals. In this work, epidermal and flexible near-infrared (NIR) PPG sensors integrating a low-power, high-sensitivity organic phototransistor (OPT) with a high-efficiency inorganic light-emitting diode are developed. By exploiting an organic bulk heterojunction active layer and a bilayer gate dielectric design, a low voltage (<3 V) operated OPT with NIR responsivity as high as 3.5 × 105 A W-1 and noise equivalent power of 1.2 × 10-15 W Hz-1/2 is achieved, greatly surpassing commercial available silicon-based photodetectors. In addition, the ultrathin encapsulation structure renders the device highly flexible and allows transfer printing of the device directly onto human skin. It is demonstrated that the epidermal/flexible PPG sensors are capable of continuously monitoring heart rate variability and precisely tracking the changes of pulse pressure at different postures of human subjects with the aid of electrocardiogram monitoring, exhibiting more reliable performance than commercial PPG sensors while consuming less power. The study suggests that the hybrid PPG sensor design may provide a promising solution for low-power, real-time physiological monitoring.

Flexible Piezoelectric-Induced Pressure Sensors for Static Measurements Based on Nanowires/Graphene Heterostructures.

The piezoelectric effect is widely applied in pressure sensors for the detection of dynamic signals. However, these piezoelectric-induced pressure sensors have challenges in measuring static signals that are based on the transient flow of electrons in an external load as driven by the piezopotential arisen from dynamic stress. Here, we present a pressure sensor with nanowires/graphene heterostructures for static measurements based on the synergistic mechanisms between strain-induced polarization charges in piezoelectric nanowires and the caused change of carrier scattering in graphene. Compared to the conventional piezoelectric nanowire or graphene pressure sensors, this sensor is capable of measuring static pressures with a sensitivity of up to 9.4 × 10-3 kPa-1 and a fast response time down to 5-7 ms. This demonstration of pressure sensors shows great potential in the applications of electronic skin and wearable devices.

Hollow-Structured Graphene-Silicone-Composite-Based Piezoresistive Sensors: Decoupled Property Tuning and Bending Reliability.

A versatile flexible piezoresistive sensor should maintain high sensitivity in a wide linear range, and provide a stable and repeatable pressure reading under bending. These properties are often difficult to achieve simultaneously with conventional filler-matrix composite active materials, as tuning of one material component often results in change of multiple sensor properties. Here, a material strategy is developed to realize a 3D graphene-poly(dimethylsiloxane) hollow structure, where the electrical conductivity and mechanical elasticity of the composite can be tuned separately by varying the graphene layer number and the poly(dimethylsiloxane) composition ratio, respectively. As a result, the sensor sensitivity and linear range can be easily improved through a decoupled tuning process, reaching a sensitivity of 15.9 kPa-1 in a 60 kPa linear region, and the sensor also exhibits fast response (1.2 ms rising time) and high stability. Furthermore, by optimizing the density of the graphene percolation network and thickness of the composite, the stability and repeatability of the sensor output under bending are improved, achieving a measurement error below 6% under bending radius variations from -25 to +25 mm. Finally, the potential applications of these sensors in wearable medical devices and robotic vision are explored.

A general top-down approach to synthesize rare earth doped-Gd2O3 nanocrystals as dualmodal contrast agents.

Dualmodal contrast agents of rare earth doped gadolinium oxide (Gd2O3) nanoparticles with high spatial resolution for magnetic resonance imaging (MRI) and high sensitivity for fluorescence imaging have attracted intensive attention in biomedical imaging. However, the rare earth doped nanoparticles mentioned above have been so far synthesized by the hydrothermal method, which is a bottom-up method, requiring high purity chemical reagents and relying on the availability of the respective precursors and strict reaction conditions. Here, we propose a facile and environmentally friendly top-down technique to synthesize the rare earth doped-Gd2O3 nanocrystals at an ambient environment. Using this approach, we synthesize a series of Tm3+, Tb3+, and Eu3+ doped-Gd2O3 nanoparticle colloids and observe strong blue, green, and red visible fluorescence from the as-synthesized nanoparticle colloids. Cell confocal microscope images show that these synthesized nanoparticle colloids are good fluorescence imaging contrast agents. Taking Gd2O3:Eu3+ nanoparticles as an example, we evaluate their performance in MRI in vitro and in vivo. These results indicate that the synthesized rare earth doped-Gd2O3 nanocrystals can be used as MRI and fluorescence imaging dualmodal contrast agents. The developed technique is expected to be a general, facile and environmentally friendly strategy towards synthesizing rare earth doped nanoparticles for biomedical applications.