Natural flavonols as probes for direct determination of borax: From conventional fluorescence analysis to paper-based smartphone sensing.

Borax is strictly regulated in the food processing and pharmaceutical industry due to its physiological toxicity, and the development of a direct analytical method is essential for effectively monitoring the borax abuse. In this work, the fluorescence properties of flavonoids, including flavones, isoflavones and flavonols, were systematically investigated from aqueous to borax solutions, and it was found that the weak intrinsic fluorescence of flavonols could be pervasively sensitized by borax. A natural flavonol, morin, was subsequently chosen as a representative probe to develop a turn-on fluorescence sensing method for borax analysis, which achieved a linear response spanning four orders of magnitude with a detection limit of 1.07 µM (0.22 µg mL-1 in terms of Na2B4O7 content). Furthermore, a smartphone-assisted paper-based test device was designed and constructed by 3D printing technology. Using morin-impregnated test strips as the carrier, the borax could be visually detected by the RGB signals of the captured images, with a detection limit of 0.13 mM (27.05 µg mL-1 for Na2B4O7). Combining ion exchange treatment for food samples and sodium periodate oxidation for drug samples, the developed methods were successfully applied for the direct analysis of borax in various products with the recoveries of 86.9-106.3% for traditional fluorescence analysis and 82.7-108.8% for smartphone-assisted fluorescence sensing. The fluorescence property of the morin-borax system was studied using time-dependent density functional theory, and the sensing mechanism was discussed in conjunction with experimental research.

Non-destructive distinction of single seed for Medicago sativa and Melilotus officinalis by capillary electrophoresis with laser-induced fluorescence detection.

Flavonoids are a class of natural polyphenolic compounds with great health benefits, and the development of methods for their analysis is of continuing interest. In this work, apigenin, kaempferol and formononetin were selected as the typical representatives of flavone, flavonol and isoflavone, three subclasses of flavonoids. Fluorescence studies revealed that tetraborate complexation could significantly sensitize the weak intrinsic fluorescence of flavonoids in solution, with a maximum of 137-fold for kaempferol. Subsequently, an integrated strategy of derivatization and separation was proposed for the universal analysis of flavonoids by capillary electrophoresis (CE) with 405 nm laser-induced fluorescence (LIF) detection. Using a running buffer consisting of 20 mM sodium tetraborate, 10 mM SDS and 10% methanol (pH 8.5), the dynamic derivatization was realized in the capillary, and the baseline separation was achieved within 10 min, with the detection limits of 0.92-35.46 nM (S/N=3) for the total of 9 flavonoids. The developed CE-LIF method was employed to the quantitative analysis of some flavonoids in Medicago sativa (alfalfa) plants and granulated alfalfa with the recoveries of 80.55-94.25%. Combined with the principal component analysis, the developed method was successfully applied to the non-destructive distinction of single seed for alfalfa and Melilotus officinalis (sweet clover), two forage grass seeds with very similar apparent morphology. Furthermore, this method was used to continuously monitor the substance metabolism during the soaking process at the level of single seed.

Integrated strategy of derivatization and separation for sensitive analysis of salvianolic acids using capillary electrophoresis with laser-induced fluorescence detection.

Salvianolic acids (SAs) are a class of natural active substances that have been widely used in clinical treatment and food adjuvant therapy. In this work, we found that SAs could form the ternary complex with borax and 2-hydroxypropyl-ß-cyclodextrin (HP-ß-CD), thereby sensitizing their weak intrinsic fluorescence to maximized 92-fold for salvianolic acid B (SAB). The formation of ternary complex was dynamic and could complete once mixed, and the fluorescence intensity remained stable within 3 h. On this basis, an integrated strategy of derivatization and separation was proposed for the sensitive analysis of SAs using capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection. After the systematic investigation, a solution consisting of 20 mM borax and 20 mM HP-ß-CD (pH 8.5) was selected as the running buffer. By the direct injection of SAs, the dynamic derivatization was realized in the capillary, and the baseline separation was achieved within 6 min, with the detection limits of 1.2-21.2 nM for four SAs (S/N=3). Then, the developed CE-LIF method was successfully applied to the quantitative analysis of SAs in four traditional ginsengs, including Salvia miltiorrhiza, Codonopsis pilosula, Panax quinquefolius and Panax ginseng with the recoveries ranging from 95.2% to 110.7%. Except for four target SAs, a large number of unknown electrophoretic peaks had also been observed in four ginsengs, and then were utilized for the identification of ginseng species via principal component analysis. Furthermore, a hypoxia/reoxygenation model was constructed using Rat cardiomyocyte H9c2 cells, and subsequently, the developed method was applied to continuously monitor the consumption of SAB in cell culture medium after its intervention.

After enjoying curry: urine metabolism analysis of curcuminoids by microemulsion electrokinetic chromatography with laser-induced native fluorescence detection.

Considering pH-dependent fluorescence of curcuminoids, a microemulsion electrokinetic chromatographic (MEEKC) method was developed under acidic conditions for their separation and detection using laser-induced native fluorescence (LINF), so as to solve the analysis of urine metabolism for curcuminoids. The microemulsion composition was optimized by response surface methodology (RSM), and the effects of buffer pH and organic modifiers were systematically investigated. The optimal buffer for the separation of curcuminoids was chosen as follows: 2.8% (v/v) ethyl acetate, 80 mM SDS and 2.8% (v/v) n-butanol to form microemulsion, 28% (v/v) ethanol as organic modifier, and 20 mM phosphoric acid as electrolyte at pH 3.0. Under these conditions, four curcuminoids including curcumin, demethoxy curcumin (DMC), bisdemethoxy curcumin (BDMC) and demethyl curcumin (DEC) could be well separated within 18 min, and the detection limits (LOD, based on S/N=3) were calculated to be 71, 60, 22, and 147 pg mL-1, respectively. Combined with solid-phase extraction (SPE), the developed MEEKC-LINF method has been successfully applied to continuously monitor the curcuminoids and related metabolites in human urine collected from a healthy volunteer after oral administration of curry, testifying that this method has potential for evaluating the pharmacological activity of curcuminoids.