Spectrophotometric determination of Fe(III) by using casein-functionalized gold nanoparticles

The preparation and application of casein-capped gold nanoparticles (AuNPs) as a specific probe for ferric ions Fe(III) is reported. The functionalized AuNPs exhibit narrow size distribution and form stable dispersions in water of different ionic strengths and basicity. The presence of diverse functional groups from the side chain of peptides warrants colloidal stability of AuNPs and also assists recognition of Fe(III) in versatile conditions. Fe(III) ion reportedly causes the aggregation of AuNPs and a red-shift in absorbance toward longer wavelength (660 nm). A spectrophotometric method is appropriate for selective detection of Fe(III) and the spectral shift is also accompanied by a color change from red to blue. The aggregation of AuNPs is not suppressed after the addition of NaOH or at moderate ionic strength. The resulting spectrophotometric method works for Fe(III) in the concentration range of 0.1 to 0.9 μM and has a detection limit of 450 nM. The AuNP probe can also detect Fe(III) ion content in real samples at the same detection limit, which is much lower than the maximum contaminant level allowed for Fe(III) in drinking water (5.37 μM) by the U.S. Environmental Protection Agency. [Figure not available: see fulltext.].