A competitive strategy based on cucurbit[7]uril supramolecular interaction for simple and sensitive detection of dibucaine

Coptisine acts as a nucleolus fluorescent probe in vitro

Coptisine (COP) is a natural protoberberine isoquinoline alkaloid that is isolated from Coptis chinensis and exhibits a variety of pharmacological activities, such as the inhibition of tumor growth, bacterial infection, inflammation and oxidative stress. In this study, COP penetrated and produced fluorescent signals in living tumor cell lines, primary MEF cells and polyformaldehyde-fixed cells. The fluorescent signal was detected at a wavelength of 488 nm. The fluorescent signal of COP was observed predominantly in the nucleoli and colocalized with nucleolus fibrillarin and B23. The fluorescence intensity of COP was associated with tumor malignancy. Compared with cells with high fluorescent signals, cells with low fluorescent signals were highly malignant. Taken together, these data suggest that COP can function as a nucleolus probe and a probe candidate for distinguishing tumor cell malignancy.

A supramolecular electrochemical probe based on a tetrazole derivative pillar[5]arene/methylene blue system

For the first time, an original synthetic approach has been developed that enables the introduce ten tetrazole fragments into the pillar[5]arene structure. A supramolecular electrochemical probe was assembled for the first time from the obtained macrocycles and an electrochemically active signal converter: methylene blue (MB) dye. The ability of pillar[5]arene containing tetrazole fragments to selectively bind MB was confirmed by UV-vis and 2D 1H-1H NOESY spectroscopy. The stoichiometry of the resulting pillar[5]arene/MB complex = 1 : 2. This new supramolecular probe pillar[5]arene/MB allowed the detection of changes in the electrochemical signals of MB implemented in the supramolecular complex depending on the presence or absence of some metal ions (Zn2+ and Co2+) that do not exert their own redox activity. This will find further applications for the enhancement of the range of analytes detected by their influence on host-guest complexation and for the design of biosensors based on specific DNA-MB interactions.

Selective detection of paraquat by a cucurbit[7]uril-based fluorescent probe

A simple fluorescent "on-off" system that can be utilized for the selective identification and determination of paraquat (PQ) is presented herein. 1H NMR spectroscopic data indicated that in aqueous solution the alkaloid palmatine can be partially encapsulated within the cucurbit[7]uril (Q[7]) cavity, whereby a stable 1 : 1 host-guest inclusion complex is formed. Other characterization techniques including mass spectrometry, UV-Vis and fluorescence spectroscopy also provided further evidence, and the host-guest inclusion complex was found to exhibit reasonable fluorescence intensity. It is noteworthy that the addition of PQ resulted in quenching the fluorescence of the host-guest inclusion complex, whereas the presence of 12 other pesticides did not significantly affect the fluorescence intensity. Given the linear relationship between the intensity of the fluorescence and the PQ concentration, the PQ concentration in aqueous solution was easily detected. Thus, a new method for identifying and determining the fluorescence quenching of PQ has been developed in this work.

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.

Benzimidazole-Piperazine-Coumarin/Cucurbit[7]uril Supramolecular Photoinduced Electron Transfer Fluorochromes for Detection of Carnosol by Stimuli-Responsive Dye Displacement and pK a Tuning

A new fluorescent dye (4PBZC) comprising coumarin (C), piperazine (P), and benzimidazole (BZ) was designed, prepared, and complexed to cucurbit[7]uril (CB7) to detect carnosol (CAR), an anti-breast cancer drug, in sub-nanomolar concentrations utilizing the supramolecular indicator displacement assay strategy, the CB7-assisted pK _a shift, and the CB7-retarded photoinduced electron transfer process. The host-guest complexation was confirmed by UV-visible absorption, fluorescence, and ¹H NMR spectroscopy, which established the binding of 4PBZC to CB7. CB7 preferentially binds the indicator dye (4PBZC) via the protonated BZ residue compared to the neutral BZ one, demonstrated by a higher binding constant of the complex in its di-protonated form, which led to an increase in the pK _a of the BZ moiety by ca. 3.0 units after the addition of CB7. In aqueous solution (pH 6), switching the emission signals between 4PBZH⁺C/CB7 (ON state) and 4PBZC (OFF state) was achieved by displacement of the protonated dye from the cavity of CB7 by the CAR analyte. An efficient sensor was obtained for the sensitive detection of CAR in aqueous solution with a low detection limit of 0.148 ng/mL (0.45 nM) and a linear range from 20 to 627 ng/mL.

Supramolecular complexation between cucurbit[7]uril and folate and analytical applications

Folate (FA) plays a key role in the biosynthesis of amino acids, purines, and pyrimidines in the human body, and intracellular folate metabolism has become an attractive target of tumor chemotherapy. In this work, an inclusion interaction was found between FA and cucurbit[7]uril (CB[7]), and the formation of a CB[7]-FA 2:1 supramolecular inclusion complex was confirmed by fluorescence spectra, UV-Vis absorption spectroscopy, 1H NMR, and molecular modeling calculations. In addition, FA is generally determined through the indirect fluorescent method because it shows weak fluorescence in aqueous solution. Therefore, a simple, direct fluorescence probe method for rapidly measuring FA was investigated, and the linear equation of FA was ΔF = 14.691C + 37.366 within the concentration ranges of 0.82 ~ 18.31 µg mL–1. The proposed direct fluorescence method was applied to the determination of spiked plasma. We demonstrated that this method could provide an experimental basis for the targeted administration of the CB[7]-FA complex, and it could be extended as a promising fluorescence detection method for drugs in vivo.

A cucurbit[7]uril-based supramolecular fluorescent probe for the detection of metronidazole with high sensitivity and strong anti-interference capacity

We propose a new method for the selective detection of the antibiotic metronidazole (MNZ) using CB[7]-JAT (cucurbit[7]uril = CB[7] and JAT = jatrorrhizine) as a fluorescent probe, which is based on the competitive reaction between MNZ and JAT for the occupancy of the CB[7] cavity. The proposed method gives a good calibration curve in the concentration range of 0.38–60 μM, and the limit of detection for MNZ is 65 ng mL−1 with those obtained by the standard curve method. Moreover, the proposed method was successfully applied for the determination of MNZ in liquid milk. Most importantly, due to the high binding affinity between CB[7] and MNZ, the proposed method shows great anti-interference capacity to accurately detect MNZ in the presence of other antibiotics.

An "on-off" fluorescent probe based on cucurbit[7]uril for highly sensitive determination of ammonia nitrogen in aquaculture water

A novel "on-off" fluorescent probe was synthesized for highly sensitive and ultra-trace determination of ammonia nitrogen in aquaculture water. Ammonium can react with formaldehyde and sodium hydroxide to form a ring substance (urotropine), which shows no fluorescence signal. Palmatine hydrochloride (PAL) can enter the hydrophobic cavity of cucurbit[7]uril (CB[7]), eventually forming a 1 : 1 host guest complex called PAL@CB[7] under neutral or acidic conditions, which has strong green fluorescence with the maximum excitation (λ_ex) wavelength at 343 nm, and the maximum emission (λ_em) wavelength at 500 nm, while urotropine has a fluorescence quenching effect on the fluorescence enhancement system of PAL@CB[7]. Therefore, a fluorescent chemosensor based on PAL@CB[7] and the reaction of ammonia nitrogen with formaldehyde was developed. The results indicate that the linearity range and the limit of detection of the proposed method are 1-300 μg L^-1 with a good correlation coefficient (r² = 0.9966) and 1.8 × 10^-2 μg L^-1, respectively. Under the optimal conditions, the method was employed for the detection of ammonia nitrogen in real aquaculture water samples, revealing high selectivity and sensitivity. In the future, the combination of the "on-off" fluorescence method, a portable hardware system and intelligent algorithms will provide technology support for the design of on-line sensors for measuring ammonia nitrogen in aquaculture water.

A supramolecular indicator displacement assay for acetyl amantadine, a proxy biomarker for spermidine/spermine N1-acetyltransferase (SSAT) activity

Acetyl amantadine (AcAm) is produced from amantadine (Am) in vivo upon catalysis by spermidine/spermine N1-acetyl transferase (SSAT). SSAT is a biomarker for multiple aggressive cancers, and the analysis of AcAm in urine has been promoted as a proxy measure for the early detection of cancer. We report here the development and optimization of cucurbit[7]uril–dye pair based indicator displacement assay (IDA) for the detection of AcAm in solution. In deionized water, using Rhodamine B as the dye, the limit of detection of AcAm was 0.087 μM with a linear response range from 0 to 1 μM. Using berberine as the dye, the limit of detection was 0.077 μM with the same range of linear response. Our efforts and difficulties in translating this assay to function in human urine are also described. We achieve a partial response of the berberine IDA to the presence of AcAm in urine that has undergone a simple PD-10 desalting step.

Determination of phenformin hydrochloride employing a sensitive fluorescent probe

A complexation of non-fluorescent phenformin hydrochloride (PFH) with cucurbit [7]uril (CB [7]) in aqueous solution was investigated using the fluorescent probe of palmatine (PAL) coupled with CB [7]. The fluorescent probe of CB [7]-PAL exhibited strong fluorescence in aqueous solution, which was quenched gradually with the increase of PFH. This effect is observed because when PFH was added to the host-guest system of CB [7]-PAL, PFH and PAL competed to occupy the CB [7] cavity. Portions of the PAL molecule were expelled from the CB [7] cavity owing to the introduction of PFH. Based on the significant quenching of the supramolecular complex fluorescence intensity, a fluorescence method of high sensitivity and selectivity was developed to determine PFH with good precision and accuracy for the first time. The linear range of the method was 0.005-1.9 μg mL(-1) with a detection limit of 0.003 μg mL(-1). In this work, association constants (K) of PFH with CB [7] were also determined. KCB [7]-PFH=(2.52±0.05)×10(5) L mol(-1). The ability of PFH to bind with CB [7] is stronger than that of PAL. The results of a density functional theory calculation authenticated that the moiety of PFH was embedded in the hydrophobic cavity of CB [7] tightly, and the nitrogen atom is located in the vicinity of a carbonyl-laced portal in the energy-minimized structure. The molecular modelling of the interaction between PFH and CB [7] was also confirmed by (1)H NMR spectra (Bruker 600 MHz).