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.

We measure silane density and Sulfo-EMCS cross-linker coupling efficiency on aminosilane films by high-resolution X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. We then characterize DNA immobilization and hybridization on these films by (32)P-radiometry. We find that the silane film structure controls the efficiency of the subsequent steps toward DNA hybridization. A self-limited silane monolayer produced from 3-aminopropyldimethylethoxysilane (APDMES) provides a silane surface density of ~3 nm(-2). Thin (1 h deposition) and thick (19 h deposition) multilayer films are generated from 3-aminopropyltriethoxysilane (APTES), resulting in surfaces with increased roughness compared to the APDMES monolayer. Increased silane surface density is estimated for the 19 h APTES film, due to a ∼32% increase in surface area compared to the APDMES monolayer. High cross-linker coupling efficiencies are measured for all three silane films. DNA immobilization densities are similar for the APDMES monolayer and 1 h APTES. However, the DNA immobilization density is double for the 19 h APTES, suggesting that increased surface area allows for a higher probe attachment. The APDMES monolayer has the lowest DNA target density and hybridization efficiency. This is attributed to the steric hindrance as the random packing limit is approached for DNA double helices (dsDNA, diameter ≥ 2 nm) on a plane. The heterogeneity and roughness of the APTES films reduce this steric hindrance and allow for tighter packing of DNA double helices, resulting in higher hybridization densities and efficiencies. The low steric hindrance of the thin, one to two layer APTES film provides the highest hybridization efficiency of nearly 88%, with 0.21 dsDNA/nm(2). The XPS data also reveal water on the cross-linker-treated surface that is implicated in device aging.

A method for the preparation of smooth deposits of Pt on Au nanorods is described, involving sequential deposition steps with selective blocking of surface sites that reduces Pt-on-Pt deposition. The Au–Pt nanorods prepared by this method have higher long-term stability than those prepared by standard Pt deposition. Electrochemical data show that the resulting structure has more extended regions of Pt surface and enhanced activity toward the carbon monoxide oxidation and oxygen reduction reactions.

The amine density of 3-aminopropyldimethylethoxysilane (APDMES) films on silica is controlled to determine its effect on DNA probe density and subsequent DNA hybridization. The amine density is tailored by controlling the surface reaction time of (1) APDMES, or (2) n-propyldimethylchlorosilane (PDMCS, which is not amine terminated) and then reacting it with APDMES to form a mixed monolayer. High-resolution X-ray photoelectron spectroscopy (XPS) is used to quantify silane surface coverage of both pure and mixed monolayers on silica; the XPS data demonstrate control of amine density in both pure APDMES and PDMCS/APDMES mixed monolayers. A linear correlation between the atomic concentration of N atoms from the amine and Si atoms from the APDMES in pure APDMES films allows us to calculate the PDMCS/APDMES ratio in the mixed monolayers. Fluorescence from attached DNA probes and from hybridized DNA decreases as the percentage of APDMES in the mixed monolayer is decreased by dilution with PDMCS.

Chemical warfare agents (CWA) continue to present a threat to civilian populations and military personnel in operational areas all over the world. Reliable measurements of CWAs are critical to contamination detection, avoidance, and remediation. The current deployed systems in United States and foreign militaries, as well as those in the private sector offer accurate detection of CWAs, but are still limited by size, portability and fabrication cost. Herein, we report a chemiresistive CWA sensor using single-walled carbon nanotubes (SWCNTs) wrapped with poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives. We demonstrate that a pendant hexafluoroisopropanol group on the polymer that enhances sensitivity to a nerve agent mimic, dimethyl methylphosphonate, in both nitrogen and air environments to concentrations as low as 5 ppm and 11 ppm, respectively. Additionally, these PEDOT/SWCNT derivative sensor systems experience negligible device performance over the course of two weeks under ambient conditions.

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Alternatively the portal of CB[8] appears to be large enough such that it doesn't have sufficiently large dipole-dipole interactions with the PPyV chain to promote a strong threading equilibrium. However we find that the portal of CB [7] is optimal for the threading of PPyV. The PPyV-CB[n] system was further exploited to demonstrate a dual-action sensor platform, combining the PL-responsive behavior demonstrated by PPyV towards electron-rich analytes with the size-exclusion properties imparted by volume of the respective CB[n] cavities. Thin films of PPyV-CB [7] were found to display reversible photoluminescence quenching when exposed to vapors of the biologically relevant molecule indole which is recovered under ambient conditions, suggesting prospects for new size-exclusion based selective sensory schemes for volatile electron-rich analytes.

The effect of residual oxygen species in as-prepared Pt nanoparticle on partially reduced graphene oxide (Pt/PRGO) and partially reduced carboxylated-GO (Pt/PR­(GO–COOH)) supports was investigated using electrochemical CO stripping and density functional theory (DFT) analysis. Pt/PRGO and Pt/PR­(GO–COOH) revealed a clear negative shift in CO-stripping onset potential compared to commercial Pt/carbon black. DFT analysis confirmed that the presence of a −COOH group provides the most resistance for CO adsorption. This CO-Pt binding energy is significantly lower than that observed in the presence of an −OH group, which is the most abundant oxygen group in carbon supports. The Pt-CO dissociation energies (on a 42-atom graphene sheet) in the presence of various oxygen groups, in descending order, were OH > CO ≈ C–O–C > COOH. Although single-bonded carbon–oxygen groups (−OH and C–O–C) are more abundant on the GO basal plane and play an important role in Pt nanoparticle nucleation and distribution on graphene sheets, the double-bonded carbon–oxygen (CO and COOH) groups are more abundant residual species post Pt nanoparticle growth and play a vital role in enhancing CO tolerance.