Chemiresistive sensors are becoming increasingly important as they offer an inexpensive option to conventional analytical instrumentation, they can be readily integrated into electronic devices, and they have low power requirements. Nanowires (NWs) are a major theme in chemosensor development. High surface area, interwire junctions, and restricted conduction pathways give intrinsically high sensitivity and new mechanisms to transduce the binding or action of analytes. This Review details the status of NW chemosensors with selected examples from the literature. We begin by proposing a principle for understanding electrical transport and transduction mechanisms in NW sensors. Next, we offer the reader a review of device performance parameters. Then, we consider the different NW types followed by a summary of NW assembly and different device platform architectures. Subsequently, we discuss NW functionalization strategies. Finally, we propose future developments in NW sensing to address selectivity, sensor drift, sensitivity, response analysis, and emerging applications.

Chemically functionalized carbon nanotubes (CNTs) are promising materials for sensing of gases and volatile organic compounds. However, the poor solubility of carbon nanotubes hinders their chemical functionalization and the subsequent integration of these materials into devices. This manuscript describes a solventfree procedure for rapid prototyping of selective chemiresistors from CNTs and graphite on the surface of paper. This procedure enables fabrication of functional gas sensors from commercially available starting materials in less than 15 min. The first step of this procedure involves the generation of solid composites of CNTs or graphite with small molecule selectors-designed to interact with specific classes of gaseous analytes-by solvent-free mechanical mixing in a ball mill and subsequent compression. The second step involves deposition of chemiresistive sensors by mechanical abrasion of these solid composites onto the surface of paper. Parallel fabrication of multiple chemiresistors from diverse composites rapidly generates cross-reactive arrays capable of sensing and differentiating gases and volatile organic compounds at partper-million and part-per-thousand concentrations. mechanochemistry | gas sensor arrays | pencil | nanocarbon | electronic nose D evelopment of simple and low-cost technologies for detecting and identifying gases and volatile organic compounds (VOCs) is critically important for improving human health, safety, and quality of life (1-3). Carbon nanotubes (CNTs) are promising materials for sensing gases and VOCs because they can be integrated into portable, sensitive, cost-effective, and lowpower devices (4-6). The molecular structure of CNTs renders the electrical conductance of these materials extremely sensitive to changes in their local chemical environment (7), and the compatibility of these materials with covalent (8-11) and noncovalent (11, 12) chemical modification enables fabrication of selective sensors (4).Multiple research groups have exploited several electronic architectures for CNT-based gas and vapor sensors (e.g., chemiresistors, field-effect transistors) with the goal of optimizing various characteristics, such as sensitivity, selectivity, response time and recovery, reproducibility in performance, power requirements, ease of fabrication, and cost (4, 6, 13-15). As a result, various methods have been developed for integrating CNTs into these electronic architectures [e.g., chemical vapor deposition (7, 16), drop casting (17), spin coating (18, 19), spray coating (20, 21), inkjet printing (22, 23), transfer printing (24, 25), and mechanical abrasion (26)]. These methods provide a number of options for integrating CNTs into devices either as individual CNTs, highly aligned arrays of CNTs, or randomly oriented networks of CNTs (4,6,13,15).Chemiresistors based on randomly oriented networks of CNTs offer significant advantages over other types of architectures for CNT-based sensors in terms of simplicity of design, ease of fabrication, compatibility with chemical func...

Chemical sensing is of critical importance to human health, safety, and security, yet it is not broadly implemented because existing sensors often require trained personnel, expensive and bulky equipment, and have large power requirements. This study reports the development of a smartphone-based sensing strategy that employs chemiresponsive nanomaterials integrated into the circuitry of commercial near-field communication tags to achieve non-line-of-sight, portable, and inexpensive detection and discrimination of gas-phase chemicals (e.g., ammonia, hydrogen peroxide, cyclohexanone, and water) at part-per-thousand and part-per-million concentrations.RFID | NFC | sensor | nanomaterials | wireless P ortable chemical sensors are needed to manage and protect the environment (1), human health (2, 3), and quality of life (4). Examples include sensors for point-of-care diagnosis of disease (5), detection of explosives and chemical warfare agents (6), indication of food ripening and spoilage (7), and monitoring of environmental pollution (1). Connecting sensors with information technology through wireless rf communication is a promising approach to enable cost-effective onsite chemical detection and analysis (8). Although rf technology has been recently applied toward wireless chemical sensing, current approaches have several limitations, including lack of specificity to selected chemical analytes, requirements for expensive, bulky, fragile, and operationally complex impedance and network analyzers, and reliance on extensive data processing and analysis (8-13). We report herein the adaptation of a nascent technology embedded in modern smartphones-near-field communication (NFC)-for wireless electronic, portable, non-line-of-sight selective detection of gasphase chemicals ( Fig. 1 and Fig. S1). We demonstrate this concept by (i) incorporating carbon-based chemiresponsive materials into the electronic circuitry of commercial NFC tags by mechanical drawing and (ii) using an NFC-enabled smartphone to relay information regarding the chemical environment (e.g., presence or absence of a chemical) surrounding the NFC tag. This paper illustrates the ability to detect and differentiate partper-million (ppm) concentrations of ammonia, cyclohexanone, and hydrogen peroxide. We demonstrate the ability to couple wireless acquisition and transduction of chemical information with existing smartphone functions [e.g., Global Positioning System (GPS)] (Movie S1).Many commercial smartphones and mobile devices are equipped with NFC hardware configured to communicate wirelessly with NFC "tags"-simple electrical resonant circuits comprising inductive (L), capacitive (C), and resistive (R) elements on a plastic substrate (Fig. 1). The smartphone, such as the Samsung Galaxy S4 (SGS4) used in this study, communicates with the battery-free tag by powering its integrated circuit (IC) via inductive coupling at 13.56 MHz (14). Power transferred from the smartphone to the IC is, among other variables, a function of the transmission frequency (f), the re...

Human exposure to hazardous chemicals can have adverse short- and long-term health effects. In this Communication, we have developed a single-use wearable hazard badge that dosimetrically detects diethylchlorophosphate (DCP), a model organophosphorous cholinesterase inhibitor simulant. Improved chemically actuated resonant devices (CARDs) are fabricated in a single step and unambiguously relate changes in chemiresistance to a wireless readout. To provide selective and readily manufacturable sensor elements for this platform, we developed an ionic-liquid-mediated single walled carbon nanotube based chemidosimetric scheme with DCP limits of detection of 28 ppb. As a practical demonstration, an 8 h workday time weighted average equivalent exposure of 10 ppb DCP effects an irreversible change in smartphone readout.