Phosphonic Acid Modified ZnO Nanowire Sensors: Directing Reaction Pathway of Volatile Carbonyl Compounds

Abstract

Surface molecular transformations on nanoscale metal oxides are inherently complex, and directing those reaction pathways is still challenging but important for designing their various applications, including molecular sensing, catalysts, and others. Here, a rational strategy to direct a reaction pathway of volatile carbonyl compounds (nonanal: biomarker) on single-crystalline ZnO nanowire surfaces via molecular modification is demonstrated. The introduction of a methylphosphonic acid modification on the ZnO nanowire surface significantly alters the surface reaction pathway of nonanal via suppressing the detrimental aldol condensation reaction. This is directed by intentionally decreasing the probability of two neighboring molecular activations on the nanowire surface. Spectrometric measurements reveal the correlation between the suppression of the aldol condensation surface reaction and the improvement in the sensor performance. This tailored surface reaction pathway effectively reduces the operating temperature from 200 to 100 °C while maintaining the sensitivity. This is because the aldol condensation product ((E)-2-heptyl-2-undecenal) requires a higher temperature to desorb from the surface. Thus, the proposed facile strategy offers an interesting approach not only for the rational design of metal oxide sensors for numerous volatile carbonyl compounds but also for tailoring various surface reaction pathways on complex nanoscale metal oxides.

Controlling Heterogeneous Catalysis with Organic Monolayers on Metal Oxides

“…Other groups have identified alternative reactions for which organic ligands can influence catalysis on oxides; these have particularly focused on selective site poisoning. For example, PA SAMs have been used to suppress undesired reactions on ZnO nanowires for sensor applications . In that case, the modifiers diminish the availability of adjacent sites needed for bimolecular aldol condensation reactions of aldehydes while still permitting aldehyde adsorption at significant coverages.…”

Organic Modifiers Promote Furfuryl Alcohol Ring Hydrogenation via Surface Hydrogen-Bonding Interactions

“…For example, self-assembled monolayers (SAMs) deposited from various precursors, such as thiols or amines, have been used to crowd the surface of metal catalyst particles in order to improve reactivity through steric and/or electronic effects. ,, Additionally, it has been shown that modifiers can be specifically designed to take advantage of π–π interactions between an aromatic reactant and modifier, leading to significantly higher yield in the selective hydrogenation of cinnamaldehyde . More recent work has explored the use of phosphonic acids (PAs) as surface modifiers, which typically deposit onto metal oxide catalysts and supports. − These studies have found that PA modifiers can enhance catalyst behavior by providing steric effects, improving catalyst stability, introducing additional active sites, or promoting the adsorption and subsequent reaction of an adsorbate. ,− Moreover, organic PA modifiers have been shown to be surprisingly stable (to >300 °C) under high-temperature hydrogenation conditions . They are also stable under hydrothermal conditions, though oxidizing conditions are known to degrade the organic ligands at temperatures near 200 °C. , …”

Moderate molecular recognitions on ZnO m-plane and their selective capture/release of bio-related phosphoric acids

“…[26][27][28][29][30] For example, a ZnO NW semiconductor-based electrical sensor was fabricated to detect nonanal, a biomarker for lung cancer. 31 In addition, biomolecular analysis systems based on specic interactions with biomolecules have been constructed using ZnO NWs. Examples include a highly sensitive and selective ZnO NW sensor for immunoglobulin G 32 and a ZnO NW microuidic device to detect cancer-related miRNA biomarkers from urinary extracellular vesicles.…”