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A system composed of upconversion nanoparticles (UCNPs) and N,N-diethyl-p-phenylenediamine (EPA) is shown to be a useful probe for highly sensitive and selective fluorometric determination of ferric ion. The fluorescence of the UCNPs (under the 980 nm excitation) has peaks at 546, 657, 758 and 812 nm. EPA is readily oxidized by Fe(III) to generate a dye with a peak at 552 nm. This causes an inner filter effect on the fluorescence peaks at 546 nm, whereas the emissions at 657, 758 and 812 nm remained unchanged. Therefore, the iron concentration can be quantified by measurement of the ratio of fluorescence at 546 and 758. Under optimal condition, the ratio drops linearly in the 0.25 to 50 μM. Fe(III) concentration ranges, with a detection limit of 0.25 μM. The method is highly selective and was applied to the analysis of spiked samples (wastewater) where it gave recoveries of between 100.9 and 107.3%; and RSD values between 0.8 and 1.4%. Results are approximately the same as those obtained by AAS. Graphical abstract A method is presented for fluorimetric determination of Fe(III). Fe(III) reacts with N,N-diethyl-p-phenylenediamine (EPA) to generate EPA oxide. The fluorescence peaking at 546 nm is reduced in presence of oxidized EPA via an inner filter.

Cocoa beans were quality graded innovatively using a near-infrared chemo-dyes system as aroma sensor to capture and detect their volatiles.