Microcontact Printing of Thiol-Functionalized Ionic Liquid Microarrays for "Membrane-less" and "Spill-less" Gas Sensors

Materials for Chemical Sensing: A Comprehensive Review on the Recent Advances and Outlook Using Ionic Liquids, Metal–Organic Frameworks (MOFs), and MOF-Based Composites

The ability to measure and monitor the concentration of specific chemical and/or gaseous species (i.e., “analytes”) is the main requirement in many fields, including industrial processes, medical applications, and workplace safety management. As a consequence, several kinds of sensors have been developed in the modern era according to some practical guidelines that regard the characteristics of the active (sensing) materials on which the sensor devices are based. These characteristics include the cost-effectiveness of the materials’ manufacturing, the sensitivity to analytes, the material stability, and the possibility of exploiting them for low-cost and portable devices. Consequently, many gas sensors employ well-defined transduction methods, the most popular being the oxidation (or reduction) of the analyte in an electrochemical reactor, optical techniques, and chemiresistive responses to gas adsorption. In recent years, many of the efforts devoted to improving these methods have been directed towards the use of certain classes of specific materials. In particular, ionic liquids have been employed as electrolytes of exceptional properties for the preparation of amperometric gas sensors, while metal–organic frameworks (MOFs) are used as highly porous and reactive materials which can be employed, in pure form or as a component of MOF-based functional composites, as active materials of chemiresistive or optical sensors. Here, we report on the most recent developments relative to the use of these classes of materials in chemical sensing. We discuss the main features of these materials and the reasons why they are considered interesting in the field of chemical sensors. Subsequently, we review some of the technological and scientific results published in the span of the last six years that we consider among the most interesting and useful ones for expanding the awareness on future trends in chemical sensing. Finally, we discuss the prospects for the use of these materials and the factors involved in their possible use for new generations of sensor devices.

Finely Tunable Surface Wettability by Two-Dimensional Molecular Manipulation

Local molecular environment governs material interface properties, especially the substrate's exposing behavior and overall functionality expression. Although current techniques can provide efficient surface property modification, challenges in molecule spatial distribution and composition controls limited the generation of homogeneous and finely tunable molecular environment. In this study, Au-thiolate rupturing operation in chemical lift-off lithography (CLL) is used to manipulate the substrate interface molecular environment. The creation of randomly distributed artificial self-assembled monolayer defects generates vacancies for substrate property modification through back-insertion of molecules with opposite functionalities. Surface wettability adjustment is utilized as an example, where well-controllable molecule distribution provides finely tunable substrate affinity toward liquids with different physical properties. The distinct property difference between two surface regions assists microdroplet formation when liquids flow through, not only water solution but also low-surface-tension organic liquids. These microdroplet arrays become a template to guide material assembly in its formation process and act as pH-sensitive platforms for high-throughput detection. Furthermore, the tunability of the molecular pattern in this approach helps minimize the coffee-ring effect and the sweet-spot issue in matrix-assisted laser desorption/ionization mass spectrometry. Two-dimensional molecular manipulation in the CLL operation, therefore, holds the capability toward controlling homogeneous material surface property and toward exhibiting behavior adjustments.

Covalently Assembled Monolayers of Homo- and Heteroleptic FeII -Terpyridyl Complexes on SiOx and ITO-Coated Glass Substrates: An Experimental and Theoretical Study

Well-defined Fe^II -terpyridyl monolayers were fabricated on SiO_x and conductive ITO-coated glass substrates through covalent-bond formation between the metallo-organic complexes and a preassembled coupling layer. Three different homo- and heteroleptic complexes with terminal pyridyl, amine, and phenyl groups were tested. All the films were found to be densely packed and homogeneous, and consist of molecules standing upright. They exhibited high thermal (up to ≈220 °C) and temporal (up to 5 h at 100 °C) stability. The UV/Vis spectra of the monolayers showed pronounced metal-to-ligand charge-transfer bands with a significant redshift compared with the solution spectra of the metallo-ligands with a pendant pyridyl group quaternized with the coupling layer, whereas the shift was significantly smaller when the coupling layer was bonded to the primary amine (-NH₂ ) group of the complex. Cyclic voltammograms of the monolayers showed reversible, one-electron redox behavior and suggested strong electronic coupling between the confined molecules and the underlying substrate. Analysis of the electrochemistry data allowed us to estimate the charge-transfer rate constant between the metal center and the substrate. Additionally, detailed quantum-chemical calculations were performed to support and rationalize the experimentally observed photophysical properties of the Fe^II -terpyridyl complexes both in the solution state and when bound to a SiO_x -based substrate.

Electropolymerized Pyrrole-Based Conductive Polymeric Ionic Liquids and Their Application for Solid-Phase Microextraction

Pyrrole was covalently bonded to 1-methyl and 1-benzylimidazolium ionic liquids (ILs) via an N-substituted alkyl linkage to prepare electropolymerizable IL monomers with excellent thermal stability. The methylimidazolium IL, [pyrrole-C₆MIm]⁺, was then electropolymerized on macro- and microelectrode materials to form conductive polymeric IL (CPIL)-modified surfaces. Electrochemical characterization of a 1.6 mm diameter Pt disk electrode modified with poly[pyrrole-C₆MIm]⁺ demonstrated a selective uptake for an anionic redox probe while rejecting a cationic redox probe. Furthermore, electropolymerization of [pyrrole-C₆MIm]⁺ doped with single-walled carbon nanotubes (SWNT) on 125 μm platinum wires produced 42 μm thick poly[pyrrole-C₆MIm]⁺/SWNT films compared to 17 μm in the absence of SWNT and 5 μm for the previously reported poly[thiophene-C₆MIm]⁺ coatings. The poly[pyrrole-C₆MIm]⁺/SWNT films were prepared with reproducible thicknesses as well as thermal properties sufficient for high-temperature applications, such as solid-phase microextraction (SPME) with gas chromatographic analysis. The utilization of the CPIL sorbent materials in SPME experiments provided excellent extraction efficiencies and selectivity toward organic aromatic analytes. The CPIL sorbent coatings also yielded outstanding fiber-to-fiber reproducibility on the basis of extraction efficiencies and improved response for a range of analytes relative to commercial 100 μm poly(dimethylsiloxane) fibers when normalized for differences in film thickness. Poly[pyrrole-C₆MIm]⁺ CPIL coatings doped with SWNT are therefore promising new sorbent materials for SPME analyses.