Quantification signalling via transition of solution inhomogeneity: determination of iron content in human serum by the naked eye

Abstract

Visual detection triggered by sensing events has been one of the central themes in contemporary chemistry. Demonstrated here is that phase transition can successfully signal the concentration levels of analytes. The proof-of-concept example is a rapid screening scheme which allows for the visual determination of whether the amount of iron(III) in a sample falls under a diagnostic concentration range by the naked eye. The screening scheme is achieved by utilizing an iron–ligands polymerisation followed by the formation of flocculates that cause phase segregation, whose rate is dependent on the concentration of iron ions. The assay solution appears homogeneous until the introduced iron(III) reaches a concentration threshold, triggering the formation of heterogeneous flocculates. The two-phase assay is demonstrated enabling the determination of the diagnostic range of iron(III) concentration for human serum samples (10–35 μM, equivalent to 0.56–1.95 ppm). With a handy laser pointer, semi-quantification of sub-ppm (μM-level) iron(III) can be performed conveniently by the naked eye through the Tyndall effect, in which the light is scattered by a colloidal suspension. By carefully selecting the experimental conditions, the visual sensing method can be applied to iron(III) detection in serum in 1 min. For example, for a 5.4-fold diluted serum sample, an iron(III) concentration of as low as 4.7 μM (0.26 ppm) can be determined visually. The sensing mechanism of flocculation is elucidated via a series of characterisations.