Flexible Nanowire Cluster as a Wearable Colorimetric Humidity Sensor

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

Eye-Readable and Wearable Colorimetric Sensor Arrays for In Situ Monitoring of Volatile Organic Compounds

Wearable sensors utilize changes in color as a response to physiological stimuli, making them easily recognizable by the naked eye. These colorimetric wearable sensors offer benefits such as easy readability, rapid responsiveness, cost-effectiveness, and straightforward manufacturing techniques. However, their applications in detecting volatile organic compounds (VOCs) in situ have been limited due to the low concentration of complex VOCs and complicated external interferences. Aiming to address these challenges, we introduced readable and wearable colorimetric sensing arrays with a microchannel structure and highly gas-sensitive materials for in situ detection of complex VOCs. The highly gas-sensitive materials were designed by loading gas-sensitive dyes into the porous metal-organic frameworks and further depositing the composites on the electrospun nanofiber membrane. The colorimetric sensor arrays were fabricated using various gas-sensitive composites, including eight dye/MOF composites that respond to various VOCs and two Pd2+/dye/MOF composites that respond to ethylene. This enables the specific recognition of multiple characteristic VOCs. A microfluidic channel made of polydimethylsiloxane (PDMS) was integrated with different colorimetric elements to create a wearable sensor array. It was attached to the surface of fruits to collect and monitor VOCs using the DenseNet classification method. As a proof of concept, we demonstrated the feasibility of the wearable sensing system in monitoring the ripening process of fruits by continuously measuring the VOC emissions from the skin of the fruit.

Plasmon-Enhanced Refractive Index Sensing of Biomolecules Based on Metal-Dielectric-Metal Metasurface in the Infrared Regime

Infrared plasmonic sensors offer enhanced biomolecule detection potential over visible sensors due to unique spectral fingerprints, enhanced sensitivity, lower interference, and label-free, nondestructive analysis capabilities. Moreover, multimode plasmonic sensors are highly advantageous for their ability to outperform single-mode counterparts through long-wavelength tuning, enhanced information retrieval, and reduced false results through multimode data cross-referencing. In this study, to achieve a high quality factor and enhanced sensitivity simultaneously, we employed silver square block arrays (SSBs) in a metal-dielectric-metal configuration. The proposed design supports three modes resulting from gap plasmons and propagating surface plasmon resonances, enabling the detection of a broad spectrum of biomolecules. Designed sensors demonstrate notable sensitivities in different modes: Mode I achieves 525 nm/RIU, Mode II reaches 1287 nm/RIU, and Mode III records 812 nm/RIU, while maintaining the quality factor of Mode I-17, Mode II-356, and Mode III-107. The figure of merit for Mode I is 7 RIU-1, for Mode II it is 375 RIU-1, and for Mode III it is 98 RIU-1. Different concentrations of glucose and hemoglobin are efficiently detected with the proposed sensor, showing great potential for its biosensing application and real-time monitoring of biomolecule dynamics. Taken together, the proposed sensor exhibits the capability to identify diverse types of biomolecules and holds the potential to serve as a preliminary screening tool for various biomolecules.

MoO2 Pump-Enhanced Flexible TiO2 Nanojungle-Based Chemiresistors for Rapid Room-Temperature Detection of H2S at Parts-per-Billion Levels

In this study, we developed a gas sensing platform that can sensitively and specifically detect trace H2S in a high-humidity atmosphere at RT. Upon integrating a carbon nitride (C3N4) nanofilm and molybdenum dioxide (MoO2) nanosheets onto nanojungle-like TiO2 nanotube arrays (TiNTs), the fabricated chemiresistor showed rapid response (38 s)/recovery (58 s) abilities and remarkable detection sensitivity for H2S at concentrations down to 2 ppb, with an estimated detection limit of 1.13 ppb at RT and room-environmental light (REL). Importantly, the gas sensor exhibited satisfactory H2S sensing performance even in dark conditions with a response of 1.9 at 200 ppb. In this design, apart from the architectural advantages of the nanojungle-like TiNTs for accelerating the gas flow efficiency and the abundant sensing sites provided by the C3N4 film, the MoO2 nanosheets act as the essential electron pump not only for the H2S response but also for the subsequent recovery process in air. After employing the MoO2 pump onto C3N4/TiNTs, the response time and recovery time of the system are shortened to ∼35 and ∼11%, respectively. Moreover, we demonstrated the good performance of the flexible gas sensor in detecting trace H2S in human exhaled breath with good humidity resistance. These results highlight the possibility of designing chemiresistors operating in RT and REL conditions and to use these environmentally friendly TiO2-based sensors in real applications.

Materials with Tunable Optical Properties for Wearable Epidermal Sensing in Health Monitoring

Advances in wearable epidermal sensors have revolutionized the way that physiological signals are captured and measured for health monitoring. One major challenge is to convert physiological signals to easily readable signals in a convenient way. One possibility for wearable epidermal sensors is based on visible readouts. There are a range of materials whose optical properties can be tuned by parameters such as temperature, pH, light, and electric fields. Herein, this review covers and highlights a set of materials with tunable optical properties and their integration into wearable epidermal sensors for health monitoring. Specifically, the recent progress, fabrication, and applications of these materials for wearable epidermal sensors are summarized and discussed. Finally, the challenges and perspectives for the next generation wearable devices are proposed.

Au@Ag Nanorods-PDMS Wearable Mouthguard as a Visualized Detection Platform for Screening Dental Caries and Periodontal Diseases

The development of easy-to-use, low-cost, and visualized detection platforms for screening human dental caries and periodontal diseases is in urgent demand. In this work, a Au@Ag nanorods-poly(dimethylsiloxane) (Au@Ag NRs-PDMS) wearable mouthguard, which can visualize the tooth lesion sites through the color change of it at the corresponding locations, is presented. The Au@Ag NRs-PDMS composite exhibits a distinct color response to hydrogen sulfide (H₂ S) gas generated by bacterial decay at the lesion sites. Moreover, the Au@Ag NRs-PDMS mouthguard is demonstrated to own desired mechanical properties, excellent chemical stability, as well as good biocompatibility, and can accurately locate the lesion sites in human oral cavity. These findings suggest that the mouthguard has the potential to be utilized on a large scale to help people self-monitor their oral health in daily life, and treat oral diseases locally.

Biomarkers and Detection Platforms for Human Health and Performance Monitoring: A Review

Human health and performance monitoring (HHPM) is imperative to provide information necessary for protecting, sustaining, evaluating, and improving personnel in various occupational sectors, such as industry, academy, sports, recreation, and military. While various commercially wearable sensors are on the market with their capability of "quantitative assessments" on human health, physical, and psychological states, their sensing is mostly based on physical traits, and thus lacks precision in HHPM. Minimally or noninvasive biomarkers detectable from the human body, such as body fluid (e.g., sweat, tear, urine, and interstitial fluid), exhaled breath, and skin surface, can provide abundant additional information to the HHPM. Detecting these biomarkers with novel or existing sensor technologies is emerging as critical human monitoring research. This review provides a broad perspective on the state of the art biosensor technologies for HHPM, including the list of biomarkers and their physiochemical/physical characteristics, fundamental sensing principles, and high-performance sensing transducers. Further, this paper expands to the additional scope on the key technical challenges in applying the current HHPM system to the real field.

A Camel Nose-Inspired Highly Durable Neuromorphic Humidity Sensor with Water Source Locating Capability

Numerous emerging applications in modern society require humidity sensors that are not only sensitive and specific but also durable and intelligent. However, conventional humidity sensors do not have all of these simultaneously because they require very different or even contradictory design principles. Here, inspired by camel noses, we develop a porous zwitterionic capacitive humidity sensor. Relying on the synergistic effect of a porous structure and good chemical and thermal stabilities of hygroscopic zwitterions, this sensor simultaneously exhibits high sensitivity, discriminability, excellent durability, and, in particular, the highest respond speed among reported capacitive humidity sensors, with demonstrated applications in the fast discrimination between fresh, stale, and dry leaves, high-resolution touchless human-machine interactive input devices, and the real-time monitoring humidity level of a hot industrial exhaust. More importantly, this sensor exhibits typical synapse behaviors such as paired-pulse facilitation due to the strong binding interactions between water and zwitterions. This leads to learning and forgetting features with a tunable memory, thus giving the sensor artificial intelligence and enabling the location of water sources. This work offers a general design principle expected to be applied to develop other high-performance biochemical sensors and the next-generation intelligent sensors with much broader applications.

Electrospun Nanofiber Membranes from 1,8-Naphthimide-Based Polymer/Poly(vinyl alcohol) for pH Fluorescence Sensing

Accurately and sensitively sensing and monitoring the pH in the environment is a key fundamental issue for human health. Nanomaterial and nanotechnology combined with fluorescent materials can be emerged as excellent possible methods to develop high-performance sensing membranes and help monitor pH. Herein, a series of fluorescent nanofiber membranes (NFMs) containing poly-1,8-naphthimide derivative-3-[dimethyl-[2-(2-methylprop-2-enoyloxy)ethyl]azaniumyl]propane-1-sulfonate (PNI-SBMA) are fabricated by electrospinning the solution of PNI-SBMA blended with poly(vinyl alcohol) (PVA). The surfactant-like functionalities in side chains of PNI-SBMA endow the NFMs with outstanding hydrophilicity, and the naphthimide derivatives are sensitive to pH by photoinduced electron transfer effect, which contribute to highly efficient pH fluorescence sensing applications of NFMs. Specifically, the PNI-SBMA/PVA NFM with a ratio of 1:9 (NFM2) shows high sensitivity and good cyclability to pH. This work demonstrates an effective strategy to realize a fluorescent sensor NFM that has a fast and sensitive response to pH, which will benefit its application of pH sensor monitoring in the water treatment process.

Evaluation Humidity Sensing Properties of Graphene/HS-TiO₂ Nanocomposites

This paper reports on a nanocomposite synthesized by sol-gel procedure comprising graphene sheets with hollow spheres of titanium dioxide (G/HS-TiO₂) with varying weight percentages of graphene for the purpose of humidity sensors. The surface morphology of the nanocomposite was characterized using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The structural properties were examined using X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The response to 12-80% RH at room temperature exhibited sensitivity (S = 135). However, the relative humidity range of 12-90% at room temperature exhibited higher sensitivity (S = 557). Sensors fabricated using the proposed nanocomposite exhibited high sensitivity to humidity, high stability, rapid response times, and rapid recovery times with hysteresis error of less than 1.79%. These results demonstrate the outstanding potential of his material for the monitoring of atmospheric humidity. This study also sought to elucidate the mechanisms underlying humidity sensing performance.

Single-sided and integrated polyaniline/ poly(vinylidene fluoride) flexible membrane with micro/nanostructures as breathable, nontoxic and fast response wearable humidity sensor

Harmless and breathable flexible humidity sensor has important applications in continuous and real-time detection of human physiological activities. In this work, with hydrophobic poly (vinylidene fluoride) (PVDF) membrane as both the template and substrate and cetyltrimethylammonium bromide as a structure regulator, polyaniline (PANI) was unilaterally deposited on a PVDF microporous membrane to facilely fabricate a single-sided integrated flexible humidity sensor (IFHS). Such IFHS is featured with unique micro/nano structure and good air permeability. Moreover, it exhibits good humidity sensing properties at room temperature including fast response, small hysteresis and stable response even under bending deformation. The flexible sensor could realize non-contact monitoring of human respiration and speaking activities. Unilateral deposition of PANI and good breathability of IFHS avoids direct contact between PANI and human skin, thus averting harms to human and minimizing the deterioration of humidity sensing properties of PANI layer. The simple method is universal to the preparation of single-sided, integrated, breathable, nontoxic and fast response wearable humidity sensors based on PANI and hydrophobic microporous polymer membranes, offering useful references for the construction of advanced flexible sensors.

An ultrafast-response and flexible humidity sensor for human respiration monitoring and noncontact safety warning

The humidity sensor is an essential sensing node in medical diagnosis and industrial processing control. To date, most of the reported relative humidity sensors have a long response time of several seconds or even hundreds of seconds, which would limit their real application for certain critical areas with fast-varying signals. In this paper, we propose a flexible and low-cost humidity sensor using vertically aligned carbon nanotubes (VACNTs) as electrodes, a PDMS-Parylene C double layer as the flexible substrate, and graphene oxide as the sensing material. The humidity sensor has an ultrafast response of ~20 ms, which is more than two orders faster than most of the previously reported flexible humidity sensors. Moreover, the sensor has a high sensitivity (16.7 pF/% RH), low hysteresis (<0.44%), high repeatability (2.7%), good long-term stability, and outstanding flexibility. Benefiting from these advantages, especially the fast response, the device has been demonstrated in precise human respiration monitoring (fast breathing, normal breathing, deep breathing, asthma, choking, and apnea), noncontact electrical safety warning for bare hand and wet gloves, and noncontact pipe leakage detection. In addition, the facile fabrication of the flexible platform with the PDMS-Parylene C double layer can be easily integrated with multisensing functions such as pH sensing, ammonium ion sensing, and temperature sensing, all of which are useful for more pattern recognition of human activity.

Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability

Wearable electronics have revolutionized the way physiological parameters are sensed, detected, and monitored. In recent years, advances in flexible and stretchable hybrid electronics have created emergent properties that enhance the compliance of devices to our skin. With their unobtrusive attributes, skin conformable sensors enable applications toward real-time disease diagnosis and continuous healthcare monitoring. Herein, critical perspectives of flexible hybrid electronics toward the future of digital health monitoring are provided, emphasizing its role in physiological sensing. In particular, the strategies within the sensor composition to render flexibility and stretchability while maintaining excellent sensing performance are considered. Next, novel approaches to the functionalization of the sensor for physical or biochemical stimuli are extensively covered. Subsequently, wearable sensors measuring physical parameters such as strain, pressure, temperature, as well as biological changes in metabolites and electrolytes are reported. Finally, their implications toward early disease detection and monitoring are discussed, concluding with a future perspective into the challenges and opportunities in emerging wearable sensor designs for the next few years.

Magnetic-Assisted Transparent and Flexible Percolative Composite for Highly Sensitive Piezoresistive Sensor via Hot Embossing Technology

A highly transparent and flexible percolative composite with magnetic reduced graphene oxide@nickel nanowire (mGN) fillers in EcoFlex matrix is proposed as a sensing layer to fabricate high-performance flexible piezoresistive sensors. Large excluded volume and alignment of mGN fillers contribute to low percolation threshold (0.27 vol %) of mGN-EcoFlex composites, leading to high electrical conductivity of 0.003 S m^-1, optical transmittance of 71.8%, and low Young's modulus of 122.8 kPa. Large-scale microdome templates for sensors are prepared by hot embossing technology cost-effectively and COMSOL Multiphysics is utilized to optimize the sensor performances. Piezoresistive sensors fabricated experimentally show superior average sensitivity of 1302.1 kPa^-1 with a low device-to-device variation of 3.74%, which provides a new way to achieve transparent, highly sensitive, and large-scale electronic skin.

A novel quartz-crystal microbalance humidity sensor based on solution-processible indium oxide quantum dots

Having a large surface area, like the quantum confinement effect also caused by the nano-level size of quantum dots (QDs), creates fantastic potential for humidity sensing. A high concentration of surface adsorption sites initiates an increased response. Porosity between QDs allows fast water vapor penetration and outflow. Here, a quartz-crystal microbalance (QCM) humidity sensor was prepared using indium oxide (In₂O₃) QDs, synthesized via a solvothermal method. After the In₂O₃ QDs were directly spin-coated onto the QCM, an annealing process removed organic long chains and exposed more moisture adsorption sites on the surfaces of the QDs. The annealed QCM humidity sensor exhibited high sensitivity (56.3 Hz per %RH at 86.3% RH), with a fast response/recovery time (14 s/16 s). Long carbon chains were broken down, and hydrogen-bonded hydroxyl groups were chemisorbed to the QDs. The chemical reaction was reduced by these chemisorbed hydrogen-bonded hydroxyl groups. Mass change was mostly caused by fast multilayer physisorption. Thus, the transducer can effectively and precisely monitor the moisture from a person's breath. In₂O₃ QD-modified QCM sensors demonstrate promising humidity-sensing applications in daily life.

A high-performance QCM humidity sensor was prepared based on In₂O₃ QDs with a high specific surface area.

Cs4PbX6 (X = Cl, Br, I) Nanocrystals: Preparation, Water-Triggered Transformation Behavior, and Anti-Counterfeiting Application

As a promising material, Cs₄PbX₆ (X = Cl, Br, I) nanocrystals (NCs) have attracted much attention. However, their luminescent property is still under debate. In this work, we first systematically studied the colloidal preparation of Cs₄PbX₆ NCs. It is found that the critical parameter for the formation of Cs₄PbX₆ NCs is the ratio between Cs and Pb. Pure Cs₄PbX₆ NCs are nonluminescent. The luminescence property of previous reported Cs₄PbX₆ NCs may come from the impurity of luminescent CsPbX₃ NCs. No coexistence of both Cs₄PbX₆ and CsPbX₃ phase has been found in one single nanoparticle. The water-triggered transformation from nonluminescent Cs₄PbX₆ NCs to luminescent CsPbX₃ NCs has been quantitatively studied. The potential application of Cs₄PbX₆ NCs in humidity sensor and anticounterfeiting have been demonstrated. This work is important because it not only confirmed the nonluminescent nature of Cs₄PbX₆ NCs but also demonstrated the potential application of such NCs.

Plasmonic molecular assays: Recent advances and applications for mobile health

Plasmonics-based biosensing assays have been extensively employed for biomedical applications. Significant advancements in use of plasmonic assays for the construction of point-of-care (POC) diagnostic methods have been made to provide effective and urgent health care of patients, especially in resourcelimited settings. This rapidly progressive research area, centered on the unique surface plasmon resonance (SPR) properties of metallic nanostructures with exceptional absorption and scattering abilities, has greatly facilitated the development of cost-effective, sensitive, and rapid strategies for disease diagnostics and improving patient healthcare in both developed and developing worlds. This review highlights the recent advances and applications of plasmonic technologies for highly sensitive protein and nucleic acid biomarker detection. In particular, we focus on the implementation and penetration of various plasmonic technologies in conventional molecular diagnostic assays, and discuss how such modification has resulted in simpler, faster, and more sensitive alternatives that are suited for point-of-use. Finally, integration of plasmonic molecular assays with various portable POC platforms for mobile health applications are highlighted.

Soft Material-Enabled, Flexible Hybrid Electronics for Medicine, Healthcare, and Human-Machine Interfaces

Flexible hybrid electronics (FHE), designed in wearable and implantable configurations, have enormous applications in advanced healthcare, rapid disease diagnostics, and persistent human-machine interfaces. Soft, contoured geometries and time-dynamic deformation of the targeted tissues require high flexibility and stretchability of the integrated bioelectronics. Recent progress in developing and engineering soft materials has provided a unique opportunity to design various types of mechanically compliant and deformable systems. Here, we summarize the required properties of soft materials and their characteristics for configuring sensing and substrate components in wearable and implantable devices and systems. Details of functionality and sensitivity of the recently developed FHE are discussed with the application areas in medicine, healthcare, and machine interactions. This review concludes with a discussion on limitations of current materials, key requirements for next generation materials, and new application areas.