![]() To study the host–guest complexation between WP6 and ST, 1H NMR spectroscopy was first performed. Results and Discussion Complexation of ST With WP6 All the experiments were conducted at room temperature (298 K). Displacement assay for theophylline and theobromine was performed at pH 7.2 with WP6 at varying concentrations of theophylline and theobromine, respectively. The measurement was repeated three times for each experiment. ![]() To prevent the dilution effect during titration, WP6 stock solutions were produced using the same ST solution. Fluorescence spectra were obtained by using the Agilent Cary Eclipse fluorescence spectrophotometer. Nuclear magnetic resonance (NMR) spectra were obtained using the Bruker Avance III HD 400 spectrometer with the deuterated solvent as the lock and the residual solvent as the internal reference. WP6 ( Yu et al., 2012) was synthesized by following the known procedures. The reagents used were marketable and applied directly without further purification. The assay seems selective for theophylline and theobromine. Thus, we realised novel host–guest recognition between water-soluble pillararene (WP6) and safranine T (ST) and revealed the operation of this host–guest recognition motif as an FID assay in caffeine detection ( Scheme 1). Therefore, caffeine detection remains inconvenient for public usage. Caffeine detection can be realised with costly and complex methods, such as HPLC-MS and immunoassay ( Wu, et al., 2000 Oberleitner, et al., 2014). However, excessive intake of caffeine may possibly cause a headache, high blood pressure, irregular small muscle movement, and allergy, especially in teenagers and pregnant women ( Nehlig, et al., 1992 Rapuri, et al., 2001 Smith, 2002 Lovallo et al., 2005). Appropriate caffeine intake may enhance alertness, attention, and nerve cell activity and decrease the possibility of type 2 diabetes. Caffeine is the most widely consumed psychostimulant drug worldwide. This study established an FID assay with a water-soluble pillararene for caffeine detection. The macrocyclic hosts, such as cyclodextrins ( Crini, 2014 Pal, et al., 2015), calixarenes ( Koh, et al., 1996 Hennig, et al., 2007 Guo and Liu, 2014 Zheng, et al., 2018), cucurbiturils ( Florea and Nau, 2011 Praetorius, et al., 2008 Barrow, et al., 2015 Sonzini, et al., 2017) and pillararenes ( Wang P, et al., 2014 Bojtár, et al., 2015 Bojtár, et al., 2016 Hua, et al., 2016 Bojtár, et al., 2017 Hua, et al., 2018 Xiao, et al., 2018, Xiao, et al., 2019a Xiao, et al., 2019b), combined with various dyes have been applied as receptors in F-IDAs for specific and selective sensing in drugs, biomolecules, or other organic compounds. Macrocyclic hosts typically provide ideal receptors for use because of their particular composition and excellent functions. In F-IDAs, the competitive binding principle is used: after binding a fluorescent indicator to the receptor, when a competing analyte is introduced into the indicator–receptor pair, the indicator is discharged from the receptor to induce a fluorescence change ( Wiskur, et al., 2001 Nguyen and Anslyn, 2006).
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