Inner filter effect between upconversion nanoparticles and Fe(ii)-1,10-phenanthroline complex for the detection of Sn(ii) and ascorbic acid (AA)

Development of an Fe2+ sensing system based on the inner filter effect between upconverting nanoparticles and ferrozine

The ferrozine (FZ) assay is a vital oxidation state-specific colorimetric assay for the quantification of Fe2+ ions in environmental samples due to its sharp increase in absorbance at 562 nm upon addition of Fe2+. However, it has yet to be applied to corresponding fluoresence assays which typically offer higher sensitivites and lower detection limits. In this article we present for the first time its pairing with upconverting luminescent nanomaterials to enable detection of Fe2+via the inner filter effect using a low-power continuous wave diode laser (45 mW). Upon near infra-red excitation at 980 nm, the overlap of the upconversion emission of Er3+ at approximately 545 nm and the absorbance of the FZ:Fe2+ complex at 562 nm enabled measurement in the change of UCNP emission response as a function of Fe2+ concentration in a ratiometric manner. We first applied large, ultra-bright poly(acrylic acid) (PAA)-capped Gd2O2S:Yb3+,Er3+ UCNPs upconverting nanoparticles (UCNPs) for the detection of Fe2+ using FZ as the acceptor. The probe displayed good selectivity and sensitivity for Fe2+, with a low limit of detection (LoD) of 2.74 μM. Analogous results employing smaller (31 nm) PAA-capped hexagonal-phase NaYF4:Yb3+,Er3+ UCNPs synthesised in our lab were achieved, with a lower LoD towards Fe2+ of 1.43 μM. These results illustrate how the ratiometric nature of the system means it is applicable over a range of particle sizes, brightnesses and nanoparticle host matrices. Preliminary investigations also found the probes capable of detecting micromolar concentrations of Fe2+ in turbid solutions.

Ascorbic acid detector based on fluorescent molybdenum disulfide quantum dots

A rapid and facile method is reported for the detection of ascorbic acid using molybdenum disulfide quantum dots (MoS₂ QDs) as a fluorescence sensor. Water-soluble and biocompatible MoS₂ QDs with the maximum fluorescence emission at 506 nm have been successfully synthesized by hydrothermal method and specific detection for ascorbic acid (AA) was constructed to utilize the modulation of metal ion on the fluorescence of MoS₂ QDs and the affinity and specificity between the ligand and the metal ion. The fluorescence of MoS₂ QDs was quenched by the irreversible static quenching of Fe³⁺ through the formation of a MoS₂ QDs/Fe³⁺ complex, while the pre-existence of AA can retain the fluorescence of MoS₂ QDs through the redox reaction between AA and Fe³⁺. Based on this principle, a good linear relationship was obtained in the AA concentration range 1 to 150 μM with a detection limit of 50 nM. The proposed fluorescent sensing strategy was proven to be highly selective, quite simple, and rapid with a requirement of only 5 min at room temperature (RT), which is particularly useful for rapid and easy analysis. Satisfactory results were obtained when applied to AA determination in fruits, beverages, and serum samples as well as AA imaging in living cells, suggesting its great potential in constructing other fluorescence detection and imaging platforms.