Simple, rapid, portable and quantitative sensing of Fe3+ ions via analyte-triggered redox reactions mediating Tyndall effect enhancement of Au nanoparticles

Abstract

This study initially describes a new nanosensing method for colorimetric detection of iron (III) (Fe3+) ions with high sensitivity and selectivity using one of the most basic optical properties of colloids (namely the Tyndall effect, TE). It adopts colloidal Au nanoparticles (AuNPs) prepared by sodium citrate reduction as the light scattering signaling probes, and 3,3′,5,5′ -tetramethylbenzidine (TMB) as recognition reagent. In the presence of Fe3+ ions, the TMB can be oxidized to its positively-charged product (TMBox) to induce the aggregation of the negatively-charged AuNP via electrostatic adsorption, leading to a significant enhancement in their TE which positively relies on the analyte concentration in the sample. The results demonstrate that this nanosensor is simple, rapid, low-cost, and portable, because it takes just 15 min to complete one assay run where only a laser pointer pen is utilized as the hand-held light source to stimulate the TE creation and a smartphone as the mobile quantitative TE reader. In particular, it can linearly detect the Fe3+ in a level range from 0.5 to 64 µM with a detection limit of ∼ 370 nM, offering a ∼ 37-fold improvement in sensitivity but with ∼ 9 times reduction in nanoprobe consumption per assay in comparison with the most common AuNP-based naked-eye method with surface plasmon resonance signaling strategy. Its accuracy and practicability are additionally validated by determining Fe3+ ions in real complex samples including tap water, commercially-available drinking water and human serum with acceptable recovery results between 85.96 and 115.52%. To the best of our knowledge, this is the first report of designing an equipment-free quantitative visual assay for the Fe3+ analysis by taking the advantage of the TE of colloids like AuNP tested herein.