Jan M. Schnorr

Selective detection of ethylene gas using carbon nanotube-based devices: utility in determination of fruit ripeness.

Ethylene, the smallest plant hormone, plays a role in many developmental processes in plants. For example, it initiates the ripening of fruit, promotes seed germination and flowering, and is responsible for the senescence of leaves and flowers. The rate-limiting step in the biosynthetic pathway to ethylene, elucidated by Yang et al., is catalyzed by 1aminocyclopropane-1-carboxylic acid (ACC) synthase. Ethylene production in plants is induced during several developmental stages as well as by external factors. The ripening process is the result of ethylene binding to the receptor ETR1, which leads to the translation of ripening genes and eventually the production of enzymes that induce the visible effects of ripening. The monitoring of the ethylene concentration is of utmost importance in the horticultural industries. The internal ethylene concentration in fruit can serve as an indicator for determining the time of harvest, while the monitoring of the atmospheric ethylene level in storage facilities and during transportation is crucial for avoiding overripening of fruit. We herein present a reversible chemoresistive sensor that is able to detect sub-ppm concentrations of ethylene. Our detection method has high selectivity towards ethylene and is simply prepared in few steps from commercially available materials. The sensing mechanism relies on the high sensitivity in resistance of single-walled carbon nanotubes (SWNTs) to changes in their electronic surroundings. These principles have been employed in a variety of sensing applications. For the selective recognition of ethylene we employ a copper(I) complex, inspired by nature, where Cu has been found to be an essential cofactor of the receptor ETR1. As a result of its small size and lack of polar chemical functionality, ethylene is generally hard to detect. Traditionally, ethylene concentrations are monitored through gas chromatography or laser acoustic spectroscopy, which both require expensive instrumentation and are not suitable for in-field measurements. Other techniques suggested are based on amperometric or electrochemical methods or rely on changes in luminescence properties. Furthermore, gas-sampling tubes based on a colorimetric reaction are available. The carbon nanotube based sensing concept we have developed is shown in Scheme 1.

Rapid prototyping of carbon-based chemiresistive gas sensors on paper

Significance This paper describes a rapid, solvent-free, two-step procedure for the fabrication of selective gas and vapor sensors from carbon nanotubes and graphite on the surface of paper that overcomes challenges associated with solvent-assisted chemical functionalization and integration of these materials into devices. The first step generates solid composites from carbon nanotubes (or graphite) and small molecules (chosen to interact with specific types of gases and vapors) by mechanical mixing and subsequent compression into a form similar to a conventional pencil “lead”. The second step uses mechanical abrasion (“drawing”) of these solid composites on the surface of paper to generate functional devices. The use of diverse composites yields sensing arrays capable of detecting and differentiating gases and vapors and part-per-million concentrations. 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 solvent-free 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 part-per-million and part-per-thousand concentrations.

Cavitand-functionalized SWCNTs for N-methylammonium detection.

Single-walled carbon nanotubes (SWCNTs) have been functionalized with highly selective tetraphosphonate cavitand receptors. The binding of charged N-methylammonium species to the functionalized SWCNTs was analyzed by X-ray photoelectron spectroscopy and confirmed by (31)P MAS NMR spectroscopy. The cavitand-functionalized SWCNTs were shown to function as chemiresistive sensory materials for the detection of sarcosine and its ethyl ester hydrochloride in water with high selectivity at concentrations as low as 0.02 mM. Exposure to sarcosine and its derivative resulted in an increased conductance, in contrast to a decreased conductance response observed for potential interferents such as the structurally related glycine ethyl ester hydrochloride.

Wiring-up catalytically active metals in solution with sulfonated carbon nanotubes

Highly water soluble sulfonate MWCNTs were synthesized and could be used to facilitate the electron transfer between Pd and Cu in a Wacker-type oxidation in solution.