Black Phosphorus-New Nanostructured Material for Humidity Sensors: Achievements and Limitations

Paper-Based Humidity Sensors as Promising Flexible Devices, State of the Art, Part 2: Humidity-Sensor Performances

This review article covers all types of paper-based humidity sensor, such as capacitive, resistive, impedance, fiber-optic, mass-sensitive, microwave, and RFID (radio-frequency identification) humidity sensors. The parameters of these sensors and the materials involved in their research and development, such as carbon nanotubes, graphene, semiconductors, and polymers, are comprehensively detailed, with a special focus on the advantages/disadvantages from an application perspective. Numerous technological/design approaches to the optimization of the performances of the sensors are considered, along with some non-conventional approaches. The review ends with a detailed analysis of the current problems encountered in the development of paper-based humidity sensors, supported by some solutions.

One-Step Electrochemical Synthesis of AlO x -Passivated Twisted-Phosphorene Nanosheets for Potentially Stable Energy Storage Devices

Black phosphorus (BP), a promising 2D material for electronics, energy storage, catalysis, and sensing, has sparked a research boom. However, exfoliated thin-layered BP is unstable and can easily be degraded under environmental conditions, severely limiting its practical applications. In this context, a simple and cost-effective method has been proposed that involves electrochemically exfoliating BP and simultaneously electrochemically depositing aluminum oxide (AlO _x ) for passivation of the exfoliated BP. The ambient stability of the exfoliated BP is studied using a time-dependent atomic force microscope (AFM). The AlO _x capping layer significantly improves the environmental stability of BP compared to uncapped BP. The thermal stability of the resulting BP is evaluated using power-dependent Raman spectroscopy. The results show that the AlO _x -passivated BP has increased thermal stability, with only a slight shift in peak position toward higher Raman power intensity. These properties can make the material suitable for stable energy storage devices. Interestingly, the electrochemical exfoliation and passivation processes resulted in the BP with a twist angle (9.86°), which is expected to exhibit unique electronic properties similar to those of graphene with a twist angle.

Black Phosphorous Aptamer-based Platform for Biomarker Detection

Black phosphorus nanostructures (nano-BPs) mainly include BP nanosheets (BP NSs), BP quantum dots (BPQDs), and other nano-BPs-based particles at nanoscale. Firstly discovered in 2014, nano-BPs are one of the most popular nanomaterials. Different synthesis methods are discussed in short to understand the basic concepts and developments in synthesis. Exfoliated nano-BPs, i.e. nano-BPs possess high surface area, high photothermal conversion efficacy, excellent biocompatibility, high charge carrier mobility (~1000 cm^-2V^-1s^-1), thermal conductivity of 86 Wm^-1K^-1; and these properties make it a highly potential candidate for fabrication of biosensing platform. These properties enable nano-BPs to be promising photothermal/drug delivery agents as well as in electrochemical data storage devices and sensing devices; and in super capacitors, photodetectors, photovoltaics and solar cells, LEDs, super-conductors, etc. Early diagnosis is very critical in the health sector scenarios. This review attempts to highlight the attempts made towards attaining stable BP, BP-aptamer conjugates for successful biosensing applications. BP-aptamer- based platforms are reviewed to highlight the significance of BP in detecting biological and physiological markers of cardiovascular diseases and cancer; to be useful in disease diagnosis and management.

Two-Dimensional Non-Carbon Materials-Based Electrochemical Printed Sensors: An Updated Review

Recently, there has been increasing interest in electrochemical printed sensors for a wide range of applications such as biomedical, pharmaceutical, food safety, and environmental fields. A major challenge is to obtain selective, sensitive, and reliable sensing platforms that can meet the stringent performance requirements of these application areas. Two-dimensional (2D) nanomaterials advances have accelerated the performance of electrochemical sensors towards more practical approaches. This review discusses the recent development of electrochemical printed sensors, with emphasis on the integration of non-carbon 2D materials as sensing platforms. A brief introduction to printed electrochemical sensors and electrochemical technique analysis are presented in the first section of this review. Subsequently, sensor surface functionalization and modification techniques including drop-casting, electrodeposition, and printing of functional ink are discussed. In the next section, we review recent insights into novel fabrication methodologies, electrochemical techniques, and sensors' performances of the most used transition metal dichalcogenides materials (such as MoS₂, MoSe₂, and WS₂), MXenes, and hexagonal boron-nitride (hBN). Finally, the challenges that are faced by electrochemical printed sensors are highlighted in the conclusion. This review is not only useful to provide insights for researchers that are currently working in the related area, but also instructive to the ones new to this field.

SnS2 Nanosheets as a Template for 2D SnO2 Sensitive Material: Nanostructure and Surface Composition Effects

Two-dimensional nanosheets of semiconductor metal oxides are considered as promising for use in gas sensors, because of the combination of a large surface-area, high thermal stability and high sensitivity, due to the chemisorption mechanism of gas detection. In this work, 2D SnO₂ nanosheets were synthesized via the oxidation of template SnS₂ nanosheets obtained by surfactant-assisted one-pot solution synthesis. The 2D SnO₂ was characterized using transmission and scanning electron microscopy (TEM, SEM), X-ray diffraction (XRD), low-temperature nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and IR spectroscopy. The sensor characteristics were studied when detecting model gases CO and NH₃ in dry (RH₂₅ = 0%) and humid (RH₂₅ = 30%) air. The combination of high specific-surface-area and increased surface acidity caused by the presence of residual sulfate anions provides a high 2D SnO₂ sensor's signal towards NH₃ at a low temperature of 200 °C in dry air, but at the same time causes an inversion of the sensor response when detecting NH₃ in a humid atmosphere. To reveal the processes responsible for sensor-response inversion, the interaction of 2D SnO₂ with ammonia was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in dry and humid air at temperatures corresponding to the maximum "positive" and maximum "negative" sensor response.

Recent Development of Gas Sensing Platforms Based on 2D Atomic Crystals

Sensors, capable of detecting trace amounts of gas molecules or volatile organic compounds (VOCs), are in great demand for environmental monitoring, food safety, health diagnostics, and national defense. In the era of the Internet of Things (IoT) and big data, the requirements on gas sensors, in addition to sensitivity and selectivity, have been increasingly placed on sensor simplicity, room temperature operation, ease for integration, and flexibility. The key to meet these requirements is the development of high-performance gas sensing materials. Two-dimensional (2D) atomic crystals, emerged after graphene, have demonstrated a number of attractive properties that are beneficial to gas sensing, such as the versatile and tunable electronic/optoelectronic properties of metal chalcogenides (MCs), the rich surface chemistry and good conductivity of MXenes, and the anisotropic structural and electronic properties of black phosphorus (BP). While most gas sensors based on 2D atomic crystals have been incorporated in the setup of a chemiresistor, field-effect transistor (FET), quartz crystal microbalance (QCM), or optical fiber, their working principles that involve gas adsorption, charge transfer, surface reaction, mass loading, and/or change of the refractive index vary from material to material. Understanding the gas-solid interaction and the subsequent signal transduction pathways is essential not only for improving the performance of existing sensing materials but also for searching new and advanced ones. In this review, we aim to provide an overview of the recent development of gas sensors based on various 2D atomic crystals from both the experimental and theoretical investigations. We will particularly focus on the sensing mechanisms and working principles of the related sensors, as well as approaches to enhance their sensing performances. Finally, we summarize the whole article and provide future perspectives for the development of gas sensors with 2D materials.

Current Advances in Black Phosphorus-Based Drug Delivery Systems for Cancer Therapy

Cancer has been one of the major threats to the lives of human beings for centuries. Traditional therapy is more or less faced with certain defects, such as poor targeting, easy degradation, high side effects, etc. Therefore, in order to improve the treatment efficiency of drugs, an intelligent drug delivery system (DDS) is considered as a promising solution strategy. Due to their special structure and large specific surface area, 2D materials are considered to be a good platform for drug delivery. Black phosphorus (BP), as a new star of the 2D family, is recommended to have the potential to construct DDS by virtue of its outstanding photothermal therapy (PTT), photodynamic therapy (PDT), and biodegradable properties. This tutorial review is intended to provide an introduction of the current advances in BP-based DDSs for cancer therapy, which covers topics from its construction, classified by the types of platforms, to the stimuli-responsive controlled drug release. Moreover, their cancer therapy applications including mono-, bi-, and multi-modal synergistic cancer therapy as well as the research of biocompatibility are also discussed. Finally, the current status and future prospects of BP-based DDSs for cancer therapy are summarized.

Current Trends in Nanomaterials for Metal Oxide-Based Conductometric Gas Sensors: Advantages and Limitations. Part 1: 1D and 2D Nanostructures

This article discusses the main uses of 1D and 2D nanomaterials in the development of conductometric gas sensors based on metal oxides. It is shown that, along with the advantages of these materials, which can improve the parameters of gas sensors, there are a number of disadvantages that significantly limit their use in the development of devices designed for the sensor market.

Anisotropic protein diffusion on nanosurface

The unique puckered structure of α-phase phosphorene carbide (α-PC) results in anisotropic electronic and thermal transporting properties. In the present work, the interactions between a model protein, villin headpiece sub-domain (HP35), and the surface of α-PC and monolayer black phosphorus (MBP, another puckered nanostructure) were explored by molecular dynamic (MD) simulations. It is found that HP35 diffuses quickly only along the zigzag direction of the α-PC surface. On the MBP surface, HP35 migrates mainly along the zigzag direction but can also easily stride over the ridges and grooves along the armchair direction. Moreover, the mild binding strength between α-PC and HP35 does not cause distortion in the protein structure. The intrinsic biocompatibility of α-PC, which is distinct from several other widely studied nanomaterials, such as carbon nanotubes, graphene and MoS2, makes it a promising candidate in functional biomedical applications.