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A highly sensitive and selective NH3 gas sensor was developed based on single-layer pristine graphene doped with copper(II) oxide (CuO) nanoparticles of a specific size. High-quality single-layer graphene was grown using chemical vapor deposition. Approximately 15 nm-sized CuO colloidal nanoparticles were fabricated by a microwave-assisted thermal method using copper acetate as the precursor, and dimethylformamide as the reducing and stabilizing agent. Pristine graphene was doped with an aqueous suspension of CuO nanoparticles at a coating speed of 1500 rpm using a simple spin coater. CuO nanoparticle doping induces changes in the electronic properties of graphene; in particular, p-type doping significantly altered graphene resistivity in the presence of NH3 gas. Upon exposure of the pristine graphene surface to NH3 gas, NH3 reacted with O2−/ O−/ O2− species on the graphene surface and released electrons into graphene. This caused a change in the concentration of charge carriers in the valence channel of graphene and an increase in graphene resistivity, facilitating real-time NH3 monitoring with quick response and rapid recovery at 25 ℃ and ~ 55% relative humidity. Our results indicated that graphene doped with ~ 15 nm-sized CuO nanoparticles can sense NH3 gas selectively with a resistivity response of ~ 83%. Moreover, the sensor exhibited good reusability, fast response (~ 19 s), and rapid recovery (~ 277 s) with a detection limit of 0.041 ppm and a relative standard deviation of 0.76%. [ABSTRACT FROM AUTHOR]

Gold nanoparticles (AuNPs) of various sizes were synthesized by modifying the citrate concentration; 45 nm AuNPs were found to respond to Cr 6+ ions selectively. Label-free AuNPs, 45 nm, showed localized surface plasmon resonance bands at 530 nm, which decreased linearly upon addition of Cr 6+ . The addition of Cr 6+ also resulted in the appearance of a new band at 750 nm, along with a visible color change in the solution from wine red to violet. The decrease in absorbance and color change were due to AuNP aggregation upon coordination of Cr 6+ . The detection limit for Cr 6+ was 0.4 nM, and excellent selectivity was observed in the presence of other metal ions and anions. The binding site and sensing mechanism for Cr 6+ and label-free AuNPs were characterized by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry. This method was applied to tap, pond, and waste water samples, and validated using inductively coupled plasma-optical emission spectrometry, illustrating the utility of the sensitive, selective, and simple AuNP sensor in the detection of Cr 6+ . [ABSTRACT FROM AUTHOR]

Incurvate gold nanohexahedrons (AuNHs) are feasible colorimetric probes for the monitoring of chlorite (ClO 2 −) ions. In this study, incurvate AuNHs were found to respond to ClO 2 − ions selectively, and the addition of ClO 2 − ions to water samples induced selectively the etching of incurvate AuNHs, eroding the sharp apexes to rounded edges, which brought in hypsochromic shift absorption attended by a color alternation from cobalt blue to violet pink. The process of etching of incurvate AuNH vertices was characterized using X-ray photoelectron spectroscopy and atom probe tomography. ClO 2 − ions were successfully detected using the incurvate AuNH-based probe at pH 2.0 and 25 °C with 20 mM NaCl. The A 563 nm /A 631 nm were linear in proportion to the ClO 2 − concentrations with two different slopes in the ranges 0.00–0.06 μM and 0.075–0.15 μM, and detection limits of 3.03 and 3.27 nM in tap water and waste water, respectively. The hypersensitive and selective optical assay reported herein has been excellently implemented to determine ClO 2 − concentrations in spiked tap water, waste water, Clorox cleaning solution, and disinfecting wipes. [Display omitted] • Incurvate AuNHs were synthesized as viable optical nanoprobe of ClO 2 − ions. • ClO 2 − ions induced selectively the etching for the sharp vertices of AuNHs. • The etching of AuNHs resulted in a color change from cobalt blue to violet pink. • The AuNH probe of ClO 2 − was optimized at pH 2, 25 °C, and 20 mM NaCl concentration. • The AuNH probe showed a LOD of 3.03 nM ClO 2 − in tap water. [ABSTRACT FROM AUTHOR]