Detecting nitric oxide (NO) is crucial for tracking air quality since NO emissions from burning fossil fuels lead to issues like acid rain and smog. In the medical field, NO acts as a significant signaling molecule and is utilized as a biomarker for asthma. A research team has introduced a new material capable of reversibly detecting NO. This material operates on low power and has high sensitivity and specificity: it is a two-dimensional metal-organic framework (MOF) infused with copper and electrically conductive.
Metal-organic frameworks (MOFs) are structured like a lattice, featuring metal “nodes” interconnected by organic links (ligands). A novel group of MOFs known as electrically conducting frameworks consists of layered formations. These two-dimensional conducting MOFs (2D-cMOFs) have shown great promise as chemiresistive sensors, which alter their electrical resistance upon interacting with specific molecules. This trait could lead to highly sensitive and low-power detection of harmful gases. However, these systems have faced issues like cross-reactivity with various gases and limited reusability due to the permanent binding of analytes.
Katherine A. Mirica, Christopher H. Hendon, and their research team from Dartmouth College (Hanover, NH, USA), the University of Oregon (Eugene, OR, USA), and Ulsan National Institute of Science and Technology (South Korea), have created a reusable 2D-cMOF designed for the precise detection of NO. They based their framework on copper and hexaiminobenzene, identified as Cu3(HIB)2. Using a different synthetic method (where the linker was combined as an undissolved powder with a solution of Cu2+ ions and potassium acetate), the team achieved a material with notably higher crystallinity, resulting in rod-shaped crystals that are approximately 500 nm long.
The crystalline structure consists of several layers forming a web-like arrangement of six-membered rings, interconnected by copper ions that are chemically bonded to nitrogen atoms. Analytical and computational studies identified that the NO binding sites are in the Cu-bis(iminobenzosemiquinone) units of the copper-based 2D-cMOFs. Conversely, a similar compound composed of nickel did not show significant NO absorption. It appears that the copper ions carrying a single positive charge, present in limited quantities alongside doubly charged ions, play a critical role in NO binding. Computational analysis indicates that when NO is absorbed, it causes notable distortion in the structure, destabilizing the bonded state, which can be linked to the advantageous reversibility of the NO adsorption.
This innovative sensor detects NO effectively at room temperature and a low voltage of 0.1 V, boasting a high sensitivity with a detection limit around 1.8 ppb. It can be reused at least seven times without requiring regeneration. Additionally, successful quantitative measurements of NO have been recorded even in moist conditions, showing a marked increase in sensor response to NO compared to other gases, such as nitrogen dioxide, hydrogen sulfide, sulfur dioxide, ammonia, and carbon oxides.
