Heterogeneous Growth of UiO-66-NH2 on Oxidized Single-Walled Carbon Nanotubes to Form "Beads-on-a-String" Composites.

In this work, we demonstrate a facile synthesis of UiO-66-NH2 metal-organic framework (MOF)/oxidized single-walled carbon nanotubes (ox-SWCNTs) composite at room temperature. Acetic acid (HAc) was used as a modulator to manipulate the morphology of the MOF in these composites. With a zirconium oxide cluster (Zr) to 2-aminoteraphthalate linker (ATA) 1:1.42 ratio and acetic acid modulator, we achieved predominately heterogeneous MOF growth on the sidewalls of CNTs. Understanding the growth mechanism of these composites was facilitated by conducting DFT calculations to investigate the interactions between ox-SWCNTs and the MOF precursors. The synthesized composites combine both microporosity of the MOF and electrical conductivity of the SWCNTs. Gas sensing tests demonstrated higher response for UiO-66-NH2/ox-SWCNT hybrid toward dry air saturated with dimethyl methylphosphonate (DMMP) vapor compared to oxidized single-walled carbon nanotubes (ox-SWCNTs) alone.

Breath Acetone Sensing Based on Single-Walled Carbon Nanotube-Titanium Dioxide Hybrids Enabled by a Custom-Built Dehumidifier.

Acetone is a metabolic byproduct found in the exhaled breath and can be measured to monitor the metabolic degree of ketosis. In this state, the body uses free fatty acids as its main source of fuel because there is limited access to glucose. Monitoring ketosis is important for type I diabetes patients to prevent ketoacidosis, a potentially fatal condition, and individuals adjusting to a low-carbohydrate diet. Here, we demonstrate that a chemiresistor fabricated from oxidized single-walled carbon nanotubes functionalized with titanium dioxide (SWCNT@TiO2) can be used to detect acetone in dried breath samples. Initially, due to the high cross sensitivity of the acetone sensor to water vapor, the acetone sensor was unable to detect acetone in humid gas samples. To resolve this cross-sensitivity issue, a dehumidifier was designed and fabricated to dehydrate the breath samples. Sensor response to the acetone in dried breath samples from three volunteers was shown to be linearly correlated with the two other ketone bodies, acetoacetic acid in urine and ß-hydroxybutyric acid in the blood. The breath sampling and analysis methodology had a calculated acetone detection limit of 1.6 ppm and capable of detecting up to at least 100 ppm of acetone, which is the dynamic range of breath acetone for someone with ketosis. Finally, the application of the sensor as a breath acetone detector was studied by incorporating the sensor into a handheld prototype breathalyzer.

Modification of Carbon Nitride/Reduced Graphene Oxide van der Waals Heterostructure with Copper Nanoparticles To Improve CO2 Sensitivity.

Carbon nitride/reduced graphene oxide (rGO) van der Waals heterostructures (vdWH) have previously shown exceptional oxygen sensitivity via a photoredox mechanism, making it a potential material candidate for various applications such as oxygen reduction reaction catalysis and oxygen sensing. In this work, the electronic structure of a carbon nitride/rGO composite is modified through the introduction of copper nanoparticles (NPs). When incorporated into a chemiresistor device, this vdWH displayed a newfound CO2 sensitivity. The effects of humidity and light were investigated and found to be crucial components for the CO2 sensitivity. Density functional theory calculations performed on a carbon nitride/copper NP@rGO model system found an enhanced stabilization of CO2 caused by H-bonds between the carbon nitride layer and chemisorbed CO2 on copper, pointing to the important role played by humidity. The synergetic effect between the carbon nitride layer interfaced with CuNP@rGO, in combination with humidity and light (395 nm) irradiation, is found to be responsible for the newfound sensitivity toward CO2.

Tetrahydrocannabinol Detection Using Semiconductor-Enriched Single-Walled Carbon Nanotube Chemiresistors.

Semiconductor-enriched single-walled carbon nanotubes (s-SWCNTs) have potential for application as a chemiresistor for the detection of breath compounds, including tetrahydrocannabinol (THC), the main psychoactive compound found in the marijuana plant. Herein we show that chemiresistor devices fabricated from s-SWCNT ink using dielectrophoresis can be incorporated into a hand-held breathalyzer with sensitivity toward THC generated from a bubbler containing analytical standard in ethanol and a heated sample evaporator that releases compounds from steel wool. The steel wool was used to capture THC from exhaled marijuana smoke. The generation of the THC from the bubbler and heated breath sample chamber was confirmed using ultraviolet-visible absorption spectroscopy and mass spectrometry, respectively. Enhanced selectivity toward THC over more volatile breath components such as CO2, water, ethanol, methanol, and acetone was achieved by delaying the sensor reading to allow for the desorption of these compounds from the chemiresistor surface. Additionally, machine learning algorithms were utilized to improve the selective detection of THC with better accuracy at increasing quantities of THC delivered to the chemiresistor.

Holey Graphene Metal Nanoparticle Composites via Crystalline Polymer Templated Etching.

While graphene has sparked enormous research interest since its isolation in 2004, there has also been an interest in developing graphene composite materials that leverage graphene's extraordinary physical properties toward new technologies. Oxidative analogues such as graphene oxide and reduced graphene oxide retain many of the same properties of graphene. While these materials contain many functional moieties, defect formation through current oxidation methods is random which, despite reductive treatments, can never fully recover the properties of the starting material. In the interest of bridging the divide between these two sets of materials for composite materials, here we show a methodology utilizing 2-D covalent organic frameworks as templates for hole formation in graphene through plasma etching. The holes formed act as edge-only chemical handles while retaining a contiguous sp2 structure. Holey graphene structures generated act as autoreduction sites for small noble metal nanoparticles which return many of graphene's original electrical properties that can be used for functional composites. Composite materials here show 103 enhancement of the Raman signal of the underlying holey graphene as well as excellent calculated limits of detection in gas sensing of H2S (3 ppb) and H2 (10 ppm).

Growth of ZIF-8 on molecularly ordered 2-methylimidazole/single-walled carbon nanotubes to form highly porous, electrically conductive composites.

The combination of porosity and electrical conductivity in a single nanomaterial is important for a variety of applications. In this work, we demonstrate the growth of ZIF-8 on the surface of single-walled carbon nanotubes (SWCNTs). The growth mechanism was investigated and a molecularly ordered imidazole solvation layer was found to disperse SWCNTs and promote crystal growth on the sidewalls. The resultant ZIF-8/SWCNT composite demonstrates high microporosity and electrical conductivity. The ZIF-8/SWCNT composite displayed semiconducting electrical behavior and an increase in sensor sensitivity toward ethanol vapors versus pristine SWCNTs.