Cucurbit[n]uril-based fluorescent indicator-displacement assays for sensing organic compounds

The widespread conversion of synthetic receptors into luminescent sensors has been achieved via the use of fluorescent-indicator displacement assays (F-IDAs). Due to their rigid structures and efficient binding affinities, cucurbit[n]urils, combined with a variety of fluorescent guests, have gained extensive utilization in fluorescent-indicator displacement assays for sensing non-fluorescent or weakly fluorescent organic compounds (analytes) in a selective and specific manner. This mini-review summarizes recent advances in the design of cucurbit[n]uril-based fluorescent-indicator displacement assays and discusses the current challenges and future prospects in this area.

Gram-scale synthesis of nitrogen-doped carbon dots from locusts for selective determination of sunset yellow in food samples

Locust powder was converted into water-soluble fluorescent nitrogen-doped carbon dots (N-CDs) with gram-scale yield through a self-exothermic reaction between nitric acid and diethylenetriamine (DETA) within 10 min. The morphology, elemental information, and optical properties of the N-CDs were characterized using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared, ultraviolet-visible and fluorescence spectroscopy. Spectroscopic investigation indicated that the fluorescence emission behaviour of N-CDs is excitation wavelength dependent, with the strongest emission peak at 470 nm using a 390 nm excitation wavelength. The strong absorption peak of sunset yellow (SY) at 482 nm overlaps substantially with the blue emission peak (470 nm) of N-CDs. This enables the fluorescence emission of N-CDs to be obviously quenched by SY through the inner filter effect. There was a good linear relationship between the fluorescence quenching degree and the concentrations of SY within the range 0.5-40 μM. The detection limit of developed fluorescence assay for SY is 28 nM, and the relative standard deviation is 2.3% (c = 10 μM). The N-CDs derived from locusts by the self-exothermic reaction are highly selective and sensitive fluorescent probes for SY, which were applied to the fluorescence sensing of SY in different food samples with satisfactory results.

Indicator displacement assays: from concept to recent developments

Overcoming the synthetic burden related to covalently connected receptors with appropriate indicators for sensing various analytes via an indicator spacer receptor (ISR) approach, the indicator displacement assay (IDA) seems to be a very sophisticated and versatile supramolecular sensing paradigm, and it has taken the phenomenon of molecular recognition to the next level in the realm of host-guest chemistry. Due to the unavailability of a comprehensive report on what has been done in the last decade in relation to IDAs, we decided to set down this account illustrating diverse indicator displacement assays (IDAs) in detail from the concept stage to recent developments relating to the detection of cationic, anionic, and neutral analytes. The authors conclude this account with future perspectives and highlight the limitations and challenges relating to IDAs which need to be overcome in order to realize the full potential of this popular sensing phenomenon. While we were finalizing our account for publication, a tutorial review by the research groups of Anslyn, Sessler, and Sun was published, which focuses mainly on diverse aspects of the chemistry related to IDAs. As can be seen, our review, besides discussing various basic IDA concepts, has a vast collection of information published in the past decade and hence, hopefully, will be very informative for the supramolecular community. We believe that this work will offer new insights for the construction of novel sensors operating through the IDA approach.