H2S-Sensing Studies Using Interdigitated Electrode with Spin-Coated Carbon Aerogel-Polyaniline Composites

Trends on Aerogel-Based Biosensors for Medical Applications: An Overview

Aerogels are unique solid-state materials composed of interconnected 3D solid networks and a large number of air-filled pores. This structure leads to extended structural characteristics as well as physicochemical properties of the nanoscale building blocks to macroscale, and integrated typical features of aerogels, such as high porosity, large surface area, and low density, with specific properties of the various constituents. Due to their combination of excellent properties, aerogels attract much interest in various applications, ranging from medicine to construction. In recent decades, their potential was exploited in many aerogels' materials, either organic, inorganic or hybrid. Considerable research efforts in recent years have been devoted to the development of aerogel-based biosensors and encouraging accomplishments have been achieved. In this work, recent (2018-2023) and ground-breaking advances in the preparation, classification, and physicochemical properties of aerogels and their sensing applications are presented. Different types of biosensors in which aerogels play a fundamental role are being explored and are collected in this manuscript. Moreover, the current challenges and some perspectives for the development of high-performance aerogel-based biosensors are summarized.

Valorization of Agricultural Waste as a Chemiresistor H2S-Gas Sensor: A Composite of Biodegradable-Electroactive Polyurethane-Urea and Activated-Carbon Composite Derived from Coconut-Shell Waste

In this study, a high-performance H2S sensor that operates at RT was successfully fabricated using biodegradable electroactive polymer-polyurethane-urea (PUU) and PUU-activated-carbon (AC) composites as sensitive material. The PUU was synthesized through the copolymerization of biodegradable polycaprolactone diol and an electroactive amine-capped aniline trimer. AC, with a large surface area of 1620 m2/g and a pore diameter of 2 nm, was derived from coconut-shell waste. The composites, labeled PUU-AC1 and PUU-AC3, were prepared using a physical mixing method. The H2S-gas-sensing performance of PUU-AC0, PUU-AC1, and PUU-AC3 was evaluated. It was found that the PUU sensor demonstrated good H2S-sensing performance, with a sensitivity of 0.1269 ppm-1 H2S. The H2S-gas-sensing results indicated that the PUU-AC composites showed a higher response, compared with PUU-AC0. The enhanced H2S-response of the PUU-AC composites was speculated to be due to the high surface-area and abounding reaction-sites, which accelerated gas diffusion and adsorption and electron transfer. When detecting trace levels of H2S gas at 20 ppm, the sensitivity of the sensors based on PUU-AC1 and PUU-AC3 increased significantly. An observed 1.66 and 2.42 times' enhancement, respectively, in the sensors' sensitivity was evident, compared with PUU-AC0 alone. Moreover, the as-prepared sensors exhibited significantly high selectivity toward H2S, with minimal to almost negligible responses toward other gases, such as SO2, NO2, NH3, CO, and CO2.

Polyaniline/Graphene-Functionalized Flexible Waste Mask Sensors for Ammonia and Volatile Sulfur Compound Monitoring

A flexible resistive-type polyaniline-based gas sensor was fabricated by simple dip-coating of graphene combined with in situ polymerization of aniline on a flexible waste mask substrate. The prepared polypropylene/graphene/polyaniline (PP/G/PANI) hybrid sensor demonstrated a fast response (114 s) and recovery time (23 s), ppb-level detection limit (100 ppb), high response value (250% toward 50 ppm NH₃, which is over four times greater than that of the pristine PANI sensor), acceptable flexibility, excellent selectivity, and long-term stability at room temperature. The morphological and structural properties of the composite sensor materials were characterized by scanning electron microscopy and energy-dispersive spectroscopy characterization, and the surface chemistry of the hybrid sensors was analyzed by Fourier transform infrared spectroscopy. The excellent sensing performance was mainly ascribed to the larger specific surface area and efficient conducting paths of the porous PP/G/PANI network. Moreover, the PP/G/PANI hybrid gas sensor exhibited excellent sensing capability on volatile sulfur compounds contained in human breath, indicating that the hybrid sensor can be applied to breath analysis and kidney disease diagnosis.

Micro and Nano Interdigitated Electrode Array (IDEA)-Based MEMS/NEMS as Electrochemical Transducers: A Review

Micro and nano interdigitated electrode array (µ/n-IDEA) configurations are prominent working electrodes in the fabrication of electrochemical sensors/biosensors, as their design benefits sensor achievement. This paper reviews µ/n-IDEA as working electrodes in four-electrode electrochemical sensors in terms of two-dimensional (2D) planar IDEA and three-dimensional (3D) IDEA configurations using carbon or metal as the starting materials. In this regard, the enhancement of IDEAs-based biosensors focuses on controlling the width and gap measurements between the adjacent fingers and increases the IDEA's height. Several distinctive methods used to expand the surface area of 3D IDEAs, such as a unique 3D IDEA design, integration of mesh, microchannel, vertically aligned carbon nanotubes (VACNT), and nanoparticles, are demonstrated and discussed. More notably, the conventional four-electrode system, consisting of reference and counter electrodes will be compared to the highly novel two-electrode system that adopts IDEA's shape. Compared to the 2D planar IDEA, the expansion of the surface area in 3D IDEAs demonstrated significant changes in the performance of electrochemical sensors. Furthermore, the challenges faced by current IDEAs-based electrochemical biosensors and their potential solutions for future directions are presented herein.

Designing Cu2+ as a Partial Substitution of Protons in Polyaniline Emeraldine Salt: Room-Temperature-Recoverable H2S Sensing Properties and Mechanism Study

Hydrogen sulfide (H2S) sensors are in urgent demand in the field of hermetic environment detection and metabolic disease diagnosis. However, most of the reported room-temperature (RT) H2S sensors based on transition metal oxides/salts unavoidably suffer from the poisoning effect, resulting in the unrecoverable behavior to restrain their application. Herein, copper(II) chloride-doped polyaniline emeraldine salt (PANI-CuCl2) was devised for RT-recoverable H2S detection, where the copper ion (Cu2+) was designed as a partial substitution of protons (H+) in PANI. The prepared gas sensor exhibited full recovery capability toward 0.25-10 ppm H2S, good repeatability, and long-term stability under 80% RH. Meanwhile, the changes of the PANI-CuCl2 during the H2S sensing period were analyzed via multiple analytical methods to reveal the reversible sensing behavior. Results showed that doping of Cu2+ not only promoted the PANI's response through the formation of conductive copper sulfide (CuS) and following H+ redoping in the PANI but also facilitated the sensor's recovery behavior because of the Cu2+ regeneration under the H+/oxygen environment. This work not only proves the changes of the interaction between the PANI and Cu2+ during the H2S sensing period but also sheds light on designing recoverable H2S sensors based on transition metal salts.

Aerogels for Biomedical, Energy and Sensing Applications

The term aerogel is used for unique solid-state structures composed of three-dimensional (3D) interconnected networks filled with a huge amount of air. These air-filled pores enhance the physicochemical properties and the structural characteristics in macroscale as well as integrate typical characteristics of aerogels, e.g., low density, high porosity and some specific properties of their constituents. These characteristics equip aerogels for highly sensitive and highly selective sensing and energy materials, e.g., biosensors, gas sensors, pressure and strain sensors, supercapacitors, catalysts and ion batteries, etc. In recent years, considerable research efforts are devoted towards the applications of aerogels and promising results have been achieved and reported. In this thematic issue, ground-breaking and recent advances in the field of biomedical, energy and sensing are presented and discussed in detail. In addition, some other perspectives and recent challenges for the synthesis of high performance and low-cost aerogels and their applications are also summarized.

The Room Temperature Highly Sensitive Ammonia Gas Sensor Based on Polyaniline and Nitrogen-Doped Graphene Quantum Dot-Coated Hollow Indium Oxide Nanofiber Composite

Hollow indium trioxide (In₂O₃) nanofibers fabricated via an effectively combined method of electrospinning and high-temperature calcination were coated with nitrogen-doped graphene quantum dots (N-GQDs) prepared by a hydrothermal process through electrostatic interaction. The N-GQD-coated hollow In₂O₃ nanofibers served as a core for the synthesis of polyaniline (PANI)/N-GQD/hollow In₂O₃ nanofiber ternary composites using in situ chemical oxidative polymerization. The chemical structure and morphology of the fabricated ternary composites were characterized using Fourier transform infrared, field-emission scanning electron microscopy, and transmission electron microscopy. The gas-sensing performances of the ternary composites were estimated by a homemade dynamic test system which was supplied with a real-time resistance acquisition platform at room temperature. The response value of the PANI/N-GQD/hollow In₂O₃ nanofiber sensor with a loading of 20 wt% N-GQD-coated hollow In₂O₃ nanofiber and an exposure of 1 ppm NH₃ was 15.2, which was approximately more than 4.4 times higher than that of the PANI sensor. This ternary composite sensor was proved to be very sensitive in the detection of NH₃ at a range of concentration between 0.6 ppm and 2.0 ppm at room temperature, which is crucial in the detection of hepatic or kidney disease in human breath. The PANI/N-GQD/hollow In₂O₃ nanofiber sensor also revealed higher selectivity and repeatability when exposed to 1.0 and 2.0 ppm NH₃ at room temperature. Because of the excellent selectivity and repeatability in the detection of 1.0 and 2.0 ppm NH₃ at room temperature achieved in this study, it is considered that the PANI/N-GQD/hollow In₂O₃ nanofiber composite sensor will be a favored gas-sensing material applied on human breath for the detection of hepatic or kidney disease.

Polyaniline/Biopolymer Composite Systems for Humidity Sensor Applications: A Review

The development of polyaniline (PANI)/biomaterial composites as humidity sensor materials represents an emerging area of advanced materials with promising applications. The increasing attention to biopolymer materials as desiccants for humidity sensor components can be explained by their sustainability and propensity to absorb water. This review represents a literature survey, covering the last decade, which is focused on the interrelationship between the core properties and moisture responsiveness of multicomponent polymer/biomaterial composites. This contribution provides an overview of humidity-sensing materials and the corresponding sensors that emphasize the resistive (impedance) type of PANI devices. The key physicochemical properties that affect moisture sensitivity include the following: swelling, water vapor adsorption capacity, porosity, electrical conductivity, and enthalpies of adsorption and vaporization. Some key features of humidity-sensing materials involve the response time, recovery time, and hysteresis error. This work presents a discussion on various types of humidity-responsive composite materials that contain PANI and biopolymers, such as cellulose, chitosan and structurally related systems, along with a brief overview of carbonaceous and ceramic materials. The effect of additive components, such as polyvinyl alcohol (PVA), for film fabrication and their adsorption properties are also discussed. The mechanisms of hydration and proton transfer, as well as the relationship with conductivity is discussed. The literature survey on hydration reveals that the textural properties (surface area and pore structure) of a material, along with the hydrophile-lipophile balance (HLB) play a crucial role. The role of HLB is important in PANI/biopolymer materials for understanding hydration phenomena and hydrophobic effects. Fundamental aspects of hydration studies that are relevant to humidity sensor materials are reviewed. The experimental design of humidity sensor materials is described, and their relevant physicochemical characterization methods are covered, along with some perspectives on future directions in research on PANI-based humidity sensors.