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

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.