Heme Cofactor‐Resembling Fe–N Single Site Embedded Graphene as Nanozymes to Selectively Detect H2O2 with High Sensitivity

Over the past decade, the catalytic activity of nanozymes has been greatly enhanced, but their selectivity is still low and considered a critical issue to overcome. Herein, Fe–N₄ single site embedded graphene (Fe–N‐rGO), which resembles the heme cofactor present in natural horseradish peroxidase, shows a marked enhancement in peroxidase‐like catalytic efficiency of up to ≈700‐fold higher than that of undoped rGO as well as excellent selectivity toward target H₂O₂ without any oxidizing activity. Importantly, when Fe or N is doped alone or when Fe is replaced with another transition metal in the Fe–N₄ site, the activity is negligibly enhanced, showing that mimicking the essential cofactor structure of natural enzyme might be essential to design the catalytic features of nanozymes. Density functional theory results for the change in Gibbs free energy during the peroxide decomposition reaction explain the high catalytic activity of Fe–N‐rGO. Based on the high and selective peroxidase‐like activity of Fe–N‐rGO, trace amounts of H₂O₂ produced from the enzymatic reactions from acetylcholine and cancerous cells are successfully quantified with high sensitivity and selectivity. This work is expected to encourage studies on the rational design of nanozymes pursuing the active site structure of natural enzymes.     

Surface Modification of a Titanium Carbide MXene Memristor to Enhance Memory Window and Low-Power Operation

With the demand for low-power-operating artificial intelligence systems, bio-inspired memristor devices exhibit potential in terms of high-density memory functions and the emulation of the synaptic dynamics of the human brain. The 2D material MXene attracts considerable interest for use in resistive-switching memory and artificial synapse devices owing to its excellent physicochemical properties in memristor devices. However, few memristive and synaptic MXene devices that display increased switching performances are reported, with no significant results. Herein, the conductivity of MXene (Ti₃C₂T_x) is engineered via etching and oxidation to enhance the switching performance of the device. The exceptional properties of partially oxidized MXene memristors include large memory windows and low threshold biases, and the complex spike-timing-dependent plasticity synaptic rules are also emulated. The low threshold potential distribution, reliable retention time (10⁴ s), and distinct resistance states with a high ON–OFF ratio (>10⁴) are the main memory-related features of this device. The experimentally determined switching potentials of the optimized device are also uniformly distributed, according to a statistical probability-based approach. This investigation may promote the essential material properties for use in high-density non-volatile memory storage and artificial synapse systems in the field of innovative nanoelectronic devices.     

Controllable gas selectivity at room temperature based on Ph5T2-modified CuPc nanowire field-effect transistors

A dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2)-modified copper phthalocyanine (CuPc) single crystal nanowire field-effect transistor (FET) with gas dielectric was fabricated as an organic gas sensor. This device exhibits the high response and the excellent controllable selectivity at room temperature. Its detection limit for NO₂, NO, and H₂S is down to sub-ppm level. Prior to surface modification, the CuPc nanowire FET shows the response as high as 1088% to 10 ppm H₂S, but only 97.5% to 10 ppm NO₂. After Ph5T2 modification, the response to 10 ppm H₂S is decreased by one order of magnitude, but is dramatically improved up to 460% to 10 ppm NO₂. The responses towards H₂S and NO₂ respectively for pristine and the modified sensor are higher than those of most reported organic sensors. The gas-sensing results reveal that Ph5T2 modification can transform the selectivity of the sensor from H₂S to NO₂. The controllable modulation of gas selectivity is related to the formed organic heterojunctions between CuPc and Ph5T2, where the hole carriers of CuPc nanowire are modulated by these heterojunctions, resulting in the changed adsorption behavior towards different gases.     

Controlling the Synaptic Plasticity of a Cu2S Gap‐Type Atomic Switch

It is demonstrated that a Cu₂S gap‐type atomic switch, referred to as a Cu₂S inorganic synapse, emulates the synaptic plasticity underlying the sensory, short‐term, and long‐term memory formations in the human brain. The change in conductance of the Cu₂S inorganic synapse is considered analogous to the change in strength of a biological synaptic connection known as the synaptic plasticity. The plasticity of the Cu₂S inorganic synapse is controlled depending on the interval, amplitude, and width of an input voltage pulse stimulation. Interestingly, the plasticity is influenced by the presence of air or moisture. Time‐dependent scanning tunneling microscopy images of the Cu‐protrusions grown in air and in vacuum provide clear evidence of the influence of air on their stability. Furthermore, the plasticity depends on temperature, such that a long‐term memory is achieved much faster at elevated temperatures with shorter or fewer number of input pulses, indicating a close analogy with a biological synapse where elevated temperature increases the degree of synaptic transmission. The ability to control the plasticity of the Cu₂S inorganic synapse justifies its potential as an advanced synthetic synapse with air/temperature sensibility for the development of artificial neural networks.     

1T‐Phase WS2 Protein‐Based Biosensor

Metallic 1T‐phase transition metal dichalcogenides have been recognized for their desirable properties like high surface‐to‐volume ratio, high conductivity, and capacitive behavior, making them outstanding for catalytic and sensing applications. Herein, a hydrogen peroxide (H₂O₂) biosensor is constructed by the immobilization of hemoglobin (Hb) on 1T‐phase WS₂ (1T‐WS₂) sheets, and entrapment by glutaraldehyde. 1T‐WS₂ not only displays electrocatalytic activity toward the reduction of H₂O₂ but also provides a high surface‐to‐volume ratio and conductive platform for the immobilization of Hb and facilitation of its electron transfer to the electrode surface. The advantageous role of 1T‐phase WS₂ is further demonstrated for the construction of a heme‐based H₂O₂ biosensor compared to its 1T‐phase MoS_2, MoSe₂, and WSe₂ counterparts. Synergistic interactions between 1T‐WS₂ and Hb result in a H₂O₂ biosensor with high analytical performance in terms of wide range, sensitivity, selectivity, reproducibility, repeatability, and stability. These findings have profound impact in the research fields of electrochemical sensing and biodiagnostics.     

On Using the Volatile Mem-Capacitive Effect of TiO<Subscript>2</Subscript> Resistive Random Access Memory to Mimic the Synaptic Forgetting Process

In this work, we report on mimicking the synaptic forgetting process using the volatile mem-capacitive effect of a resistive random access memory (RRAM). TiO₂ dielectric, which is known to show volatile memory operations due to migration of inherent oxygen vacancies, was used to achieve the volatile mem-capacitive effect. By placing the volatile RRAM candidate along with SiO₂ at the gate of a MOS capacitor, a volatile capacitance change resembling the forgetting nature of a human brain is demonstrated. Furthermore, the memory operation in the MOS capacitor does not require a current flow through the gate dielectric indicating the feasibility of obtaining low power memory operations. Thus, the mem-capacitive effect of volatile RRAM candidates can be attractive to the future neuromorphic systems for implementing the forgetting process of a human brain.     

Transparent and Flexible Inorganic Perovskite Photonic Artificial Synapses with Dual-Mode Operation

With the rapid development of artificial intelligence, the simulation of the human brain for neuromorphic computing has demonstrated unprecedented progress. Photonic artificial synapses are strongly desirable owing to their higher neuron selectivity, lower crosstalk, wavelength multiplexing capabilities, and low operating power compared to their electric counterparts. This study demonstrates a highly transparent and flexible artificial synapse with a two-terminal architecture that emulates photonic synaptic functionalities. This optically triggered artificial synapse exhibits clear synaptic characteristics such as paired-pulse facilitation, short/long-term memory, and synaptic behavior analogous to that of the iris in the human eye. Ultraviolet light illumination-induced neuromorphic characteristics exhibited by the synapse are attributed to carrier trapping and detrapping in the SnO₂ nanoparticles and CsPbCl₃ perovskite interface. Moreover, the ability to detect deep red light without changes in synaptic behavior indicates the potential for dual-mode operation. This study establishes a novel two-terminal architecture for highly transparent and flexible photonic artificial synapse that can help facilitate higher integration density of transparent 3D stacking memristors, and make it possible to approach optical learning, memory, computing, and visual recognition.     

Tuning the synaptic behaviors of biocompatible synaptic transistor through ion-doping

Biodegradable and environmentally friendly artificial synapse devices are essential for the future development of neuromorphic computing. The emergence of synaptic transistors based on biocompatible polymer materials provides an ideal approach to achieve green electronics. However, modulating the synaptic properties in a wide range in a fixed biocompatible synaptic transistor is still challengeable, while it is vitally important for improving the adaptability of the synaptic device to achieve neuro-prosthetics in the future. Here, we reported the regulation of the synaptic behavior of biocompatible synaptic transistor through ion-doping, which allows to adjusting the response of the synaptic device according to a specific function. The ions doped into the insulating layer strengthen the formation of electric double layers (EDLs), which enables a remarkable regulation effect on post-synaptic current. Moreover, basic synaptic properties, including excitatory/inhibitory post-synaptic current (EPCS/IPSC), paired-pulse facilitation/depression (PPF/PPD), short-term/long-term memory (STM/LTM) are successfully demonstrated. In addition, high-pass and low-pass filtering functions are also implemented in a single synaptic device, indicating that the synapse attenuation can be effectively transformed according to the needs of the function. More importantly, this is the first work to demonstrate that the accuracy of pattern recognition of synaptic transistors, an important indicator of neuromorphic calculations, can be significantly improved via ion doping (as high as 75.96% relative to undoped devices of 41.68%). Our research provides a feasible strategy for precisely controlling synaptic behaviors, which has a profound impact on improving the adaptability of artificial synaptic devices in the field of neuromorphic computing.     

Tuning Memristivity by Varying the Oxygen Content in a Mixed Ionic–Electronic Conductor

The rising interest shown for adaptable electronics and brain‐inspired neuromorphic hardware increases the need for new device architectures and functional materials to build such devices. The rational design of these memory components also benefits the comprehension and thus the control over the microscopic mechanisms at the origin of memristivity. In oxide‐based valence‐change memories, the control of the oxygen drift and diffusion kinetics is a key aspect in obtaining the gradual analog‐type change in resistance required for artificial synapse applications. However, only a few devices are designed with this in mind, as they are commonly built around ionic insulating active materials. This shortcoming is addressed by using a mixed ionic–electronic conductor as functional memristive material. This work demonstrates how the oxygen content in La₂NiO_4+δ (L2NO4), tuned through post‐annealing treatments, has a critical influence on the memory characteristics of L2NO4‐based memristive devices. The presence of interstitial oxygen point defects in L2NO4 affects both its structure and electrical properties. High oxygen stoichiometry in the pristine state leads to an increased electrical conductivity, ultimately resulting in an improved memory window with highly multilevel, analog‐type memory programing capabilities, desirable for analog computing and synaptic applications in particular.     

Artificial Visual Systems With Tunable Photoconductivity Based on Organic Molecule-Nanowire Heterojunctions

The visual system, one of the most crucial units of the human perception system, combines the functions of multi-wavelength signal detection and data processing. Herein, the large-scale artificial synaptic device arrays based on the organic molecule-nanowire heterojunctions with tunable photoconductivity are proposed and demonstrated. The organic thin films of p-type 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene (C8-BTBT) or n-type phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) are used to wrap the InGaAs nanowire parallel arrays to configure two different type-I heterojunctions, respectively. Due to the difference in carrier injection, persistent negative photoconductivity (NPC) or positive photoconductivity (PPC) are achieved in these heterojunctions. The irradiation with different wavelengths (solar-blind to visible ranges) can stimulate the heterojunction devices, effectively mimicking the synaptic behaviors with two different photoconductivities. The long-term and multi-state light memory are also realized through synergistic photoelectric modulation. Notably, the arrays with different photoconductivities are adopted to build the hardware kernel for the visual system. Due to the tunable photoconductivity and response to multiple wavelengths, the recognition rate of neural networks can reach 100% with lower complexity and power consumption. Evidently, these photosynaptic devices are illustrated with retina-like behaviors and capabilities for large-area integration, which reveals their promising potential for artificial visual systems.     

An Electrical Switch‐Driven Flexible Electromagnetic Absorber

The development of a thin, tunable, and high‐performance flexible electromagnetic (EM) absorbing device that aims to solve signal interference or EM pollution is highly desirable but remains a great challenge. Herein, demonstrated is a flexible electrical‐driven device constructed by an insulated organic‐polymer substrate, carrier transmission layer, and core–shell structured absorber, enabling a narrow and tunable effective absorption region (f_E < 2.0 GHz) by controlling the external voltage toward this challenge. As a key design element, the selected absorber consists of an Sn/SnS/SnO₂ core and C shell, which exhibits an exceptional dielectric‐response ability at a small voltage, which is attributed to desirable carrier mobility and excitable carriers. Multiple f_E‐tuning regions (maximum up to 7.0), covering 90% of C‐band can be achieved for Sn/SnS/SnO₂@C‐based flexible device by selecting a low voltage (2–12 V). The strategy developed here may open a new avenue toward the design of flexible intelligent EM device for practical applications.     

The electrical conductivity of polycrystalline SnO2(Cu) films and their sensitivity to hydrogen sulfide

The effect of doping with copper on the sensor properties and the electrical conductivity of polycrystalline SnO₂(Cu) films has been investigated. It has been found that at room temperature the residual conductivity is observed after the films are exposed to H₂S. This made it possible to determine the character of the low-temperature conductivity of the films for different degrees of saturation with hydrogen sulfide. A comparison of the obtained data with the results of layerwise elemental analysis suggested a model that explains the mechanism of the gas sensitivity of SnO₂(Cu) to hydrogen sulfide. In contrast to the mechanisms, which are associated with the work done by the surface and which are standard for gas sensors, in the present case the change in the conductivity is due to the chemical reaction of the electrically active copper with sulfur in the entire volume of the film. This reaction determines the selectivity and high sensitivity of SnO₂(Cu) to H₂S. Fiz. Tekh. Poluprovodn. 31, 400–404 (April 1997)     

Gas Sensors: Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading (Adv. Funct. Mater. 24/2010)

This work presents a new route to suppress grain growth and tune the sensitivity and selectivity of nanocrystalline SnO₂ fibers. Unloaded and Pd‐loaded SnO₂ nanofiber mats are synthesized by electrospinning followed by hot‐pressing at 80 °C and calcination at 450 or 600 °C. The chemical composition and microstructure evolution as a function of Pd‐loading and calcination temperature are examined using EDS, XPS, XRD, SEM, and HRTEM. Highly porous fibrillar morphology with nanocrystalline fibers comprising SnO₂ crystallites decorated with tiny PdO crystallites is observed. The grain size of the SnO₂ crystallites in the layers that are calcined at 600 °C decreases with increasing Pd concentration from about 15 nm in the unloaded specimen to about 7 nm in the 40 mol% Pd‐loaded specimen, indicating that Pd‐loading could effectively suppress the SnO₂ grain growth during the calcination step. The Pd‐loaded SnO₂ sensors have 4 orders of magnitude higher resistivity and exhibit significantly enhanced sensitivity to H₂ and lower sensitivity to NO₂ compared to their unloaded counterparts. These observations are attributed to enhanced electron depletion at the surface of the PdO‐decorated SnO₂ crystallites and catalytic effect of PdO in promoting the oxidation of H₂ into H₂O. These phenomena appear to have a much larger effect on the sensitivity of the Pd‐loaded sensors than the reduction in grain size.     

Hydroxide‐Membrane‐Coated Pt3Ni Nanowires as Highly Efficient Catalysts for Selective Hydrogenation Reaction

Rational design of effective catalysts with both high activity and selectivity is highly significant and desirable for hydrogenation reaction. In this paper, for the first time an efficient approach to controllably construct 1D metal nanowires (NWs) coated with hydroxide (Ni_xM(OH)₂ (M = Mn, Fe, Co, Cu, and Al)) membranes as highly active and selective hydrogenation catalysts is reported. The optimized Ni₃₂Cu(OH)₂ membrane coated Pt₃Ni nanowires show much enhanced selectivity of 87.9% for the hydrogenation of cinnamaldehyde to hydrocinnamaldehyde instead of hydrocinnamyl alcohol, in contrast with the pristine Pt₃Ni nanowires and Pt₃Ni nanowires on Ni(OH)₂ membranes. The enhanced selectivity of Pt₃Ni@Ni₃₂Cu(OH)₂‐2 NWs is ascribed to confinement/poisoning effects of the coated Ni₃₂Cu(OH)₂ membranes as well as the intimate interaction between the Pt₃Ni NWs and Ni₃₂Cu(OH)₂ membranes, as confirmed by X‐ray photoelectron spectroscopy. The coated structures also show good stability after five recycle runs. The present work highlights the importance of surface engineering for the design of multicomponent composites with desirable activity and selectivity toward hydrogenation reaction and beyond.     

Au/AgI Dimeric Nanoparticles for Highly Selective and Sensitive Colorimetric Detection of Hydrogen Sulfide

The development of Au/AgI dimeric nanoparticles (NPs) is reported for highly selective colorimetric detection of hydrogen sulfide (H₂S). The detection mechanism is designed by taking advantage of the chemical transformation of AgI to Ag₂S upon reacting with sulfide, which leads to a shift in the plasmonic band of the attached Au NPs. The plasmonic shift is accompanied by a color change of the solution from purplish red to blue and finally to light green depending on the concentration of sulfide, thus enables a naked‐eye readout and UV–vis quantitation of the sulfide exposure. The Au/AgI dimeric NPs are further immobilized in agarose gels to produce test strips, which can be used for both naked‐eye readout and quantitative detection of sulfide using UV–vis spectroscopy thanks to its transparency in the visible region. Compared to commercial Pb(Ac)₂ test papers, the agarose gel strip has superior performance for detecting sulfide in terms of sensitivity, selectivity, stability, and fidelity. The agarose gel is also capable of detecting gaseous H₂S at important concentration thresholds, suggesting its practicability in real life applications. The potential of agarose gels is further highlighted by its ability in the enrichment and colorimetric detection of gaseous H₂S released during cell cultivation.     

Composition and bandgap control in Cu(In,Ga)Se2‐based absorbers formed by reaction of metal precursors

The control of composition and bandgap in chalcopyrite thin‐film absorber layers formed by a metal precursor reaction is addressed. Two processes using reaction with either H₂Se or H₂S as the final step of a three‐step reaction process were compared as follows: a three‐step H₂Se/Ar/H₂S reaction and a three‐step H₂Se/Ar/H₂Se reaction. In both processes, significant Ga homogenization was obtained during the second‐step Ar anneal, but the third‐step selenization resulted in Ga depletion near the Cu(InGa)Se₂ surface, whereas the third‐step sulfization did not. Solar cells were fabricated using absorbers formed using each method, and the surface Ga depletion significantly affected device performances. The solar cell incorporating the sulfization yielded a better device performance, with an efficiency of 14.4% (without an anti‐reflection layer) and an open‐circuit voltage of 609 mV. The bandgap control in the metal precursor reaction is discussed in conjunction with the device behavior. Copyright © 2014 John Wiley & Sons, Ltd.     

A Flexible Mott Synaptic Transistor for Nociceptor Simulation and Neuromorphic Computing

Designing transparent flexible electronics with multi-biological neuronal functions and superior flexibility is a key step to establish wearable artificial intelligence equipment. Here, a flexible ionic gel-gated VO₂ Mott transistor is developed to simulate the functions of the biological synapse. Short-term and long-term plasticity of the synapse are realized by the volatile electrostatic carrier accumulation and nonvolatile proton-doping modulation, respectively. With the achievement of multi-essential synaptic functions, an important sensory neuron, nociceptor, is perfectly simulated in our synaptic transistors with all key characteristics of threshold, relaxation, and sensitization. More importantly, this synaptic transistor exhibits high tolerance to the bending deformation, and the cycle-to-cycle variations of multi-conductance states in potentiation and depression properties are maintained within 4%. This superior stability further indicates that our flexible device is suitable for neuromorphic computing. Simulation results demonstrate that high recognition accuracy of handwritten digits (>95%) can be achieved in a convolution neural network built from these synaptic transistors. The transparent and flexible Mott transistor based on electrically-controlled VO₂ metal-insulator transition is believed to open up alternative approaches to developing highly stable synapses for future flexible neuromorphic systems.     

High selective gas sensors based on surface modified polymer transistor

Gas sensors based on organic field effect transistors have attracted great attentions and achieved much progress because of their inherent merits such as low-cost, room operating temperature, good portability and flexibility. However, high sensitivity and good selectivity are still challenging issues blocking their further development. Herein, we demonstrate a promising strategy for fabricating high selective organic gas sensors through introducing surface modification. Modified substrate with different self-assembling monolayers (SAMs), the sensing properties of polymer OFET gas sensors are varied with terminal group attached on the SAM. The bare device shows significant sensitive to NH₃, while it becomes much more sensitive to NO₂ if a fluorine substituent SAM adopted. The NH₂-terminal SAM and CH₃-terminal SAM bring moderate sensitivity to gases like NH₃ and NO₂. Those various sensitivity and device parameter evolution are promising to generate a patterning recognition for different gases, which may provide a potential promising approach for improving selectivity of organic gas sensors.     

High Yield Electrosynthesis of Hydrogen Peroxide from Water Using Electrospun CaSnO3@Carbon Fiber Membrane Catalysts with Abundant Oxygen Vacancy

Hydrogen peroxide (H₂O₂) production by electrochemical two-electron water oxidation reaction (2e-WOR) is a promising approach, where high-performance electrocatalysts play critical roles. Here, the synthesis of nanostructured CaSnO₃ confined in conductive carbon fiber membrane with abundant oxygen vacancy (O_V) as a new generation of 2e-WOR electrocatalyst is reported. The CaSnO₃@carbon fiber membrane can be directly used as a self-standing electrode, exhibiting a record-high H₂O₂ production rate of 39.8 µmol cm⁻² min⁻¹ and a selectivity of ≈90% (at 2.9 V vs reversible hydrogen electrode). The CaSnO₃@carbon fiber membrane design improves not only the electrical conductivity and stability of catalysts but also the inherent activity of CaSnO₃. Density functional theory calculation further indicates the crucial role of O_V in increasing the adsorption free energy toward oxygen intermediates associated with the competitive four-electron water oxidation reaction pathway, thus enhancing the activity and selectivity of 2e-WOR. The findings pave a new avenue to the rational design of electrocatalysts for H₂O₂ production from water.     

Mimic Drug Dosage Modulation for Neuroplasticity Based on Charge-Trap Layered Electronics

The human brain is often likened to an incredibly complex and intricate computer, rather than electrical devices, consisting of billions of neuronal cells connected by synapses. Different brain circuits are responsible for coordinating and performing specific functions. The reward pathway of the synaptic plasticity in the brain is strongly related to the features of both drug addiction and relief. In the current study, a synaptic device based on layered hafnium disulfide (HfS₂) is developed for the first time, to emulate the behavioral mechanisms of drug dosage modulation for neuroplasticity. A strong gate-dependent persistent photocurrent is observed, arising from the modulation of substrate-trapping events. By controlling the polarity of gate voltage, the basic functions of biological synapses are realized under a range of light spiking conditions. Furthermore, under the control of detrapping/trapping events at the HfS₂/SiO₂ interface, positive/negative correlations of the A_n/A₁ index, which significantly reflected the weight change of synaptic plasticity, are realized under the same stimulation conditions for the emulation of the drug-related addition/relief behaviors in the brain. The findings provide a new advance for mimicking human brain plasticity.