Chemical Sensors and Imaging: Molecular, Materials, and Biological Platforms

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

Carbon dots (CDs), as a new type of carbon-based nanomaterial, have attracted broad research interest for years, because of their diverse physicochemical properties and favorable attributes like good biocompatibility, unique optical properties, low cost, ecofriendliness, abundant functional groups (e.g., amino, hydroxyl, carboxyl), high stability, and electron mobility. In this Outlook, we comprehensively summarize the classification of CDs based on the analysis of their formation mechanism, micro-/nanostructure and property features, and describe their synthetic methods and optical properties including strong absorption, photoluminescence, and phosphorescence. Furthermore, the recent significant advances in diverse applications, including optical (sensor, anticounterfeiting), energy (light-emitting diodes, catalysis, photovoltaics, supercapacitors), and promising biomedicine, are systematically highlighted. Finally, we envisage the key issues to be challenged, future research directions, and perspectives to show a full picture of CDs-based materials.

Single-molecule localization microscopy is used to construct super-resolution images, but generally requires prior intense laser irradiation and in some cases additives, such as thiols, to induce on-off switching of fluorophores. These requirements limit the potential applications of this methodology. Here, we report a first-in-class spontaneously blinking fluorophore based on an intramolecular spirocyclization reaction. Optimization of the intramolecular nucleophile and rhodamine-based fluorophore (electrophile) provide a suitable lifetime for the fluorescent open form, and equilibrium between the open form and the non-fluorescent closed form. We show that this spontaneously blinking fluorophore is suitable for single-molecule localization microscopy imaging deep inside cells and for tracking the motion of structures in living cells. We further demonstrate the advantages of this fluorophore over existing methodologies by applying it to nuclear pore structures located far above the coverslip with a spinning-disk confocal microscope and for repetitive time-lapse super-resolution imaging of microtubules in live cells for up to 1 h.