A colorimetric probe based on 4-mercaptophenol and thioglycolic acid-functionalized gold nanoparticles for determination of phytic acid and Fe(III) ions

As a first of its kind, we developed a highly sensitive colorimetric nanoprobe for phytic acid (PA) and Fe(III) ion detection based on 4-mercaptophenol (4MP) and thioglycolic acid (TGA)-functionalized gold nanoparticles {AuNPs@(4MP-TGA)}. AuNPs were easily derivatized by 4MP and TGA through –SH binding to gold. Fe(III) ions possibly are bound first to the phenolate groups of 4MP-AuNPs, and further coordinated several nanoparticles via the carboxylate groups of TGA-AuNPs to cause aggregation, resulting in a red-to-purple color change and a bathochromic shift in the SPR absorption band of the nanoprobe. With the addition of PA to the AuNPs@(4MP-TGA)-Fe(III) system, the aggregated particles were released due to strong complex formation between Fe(III) and PA, resulting in a restoration of the color (purple-to-red) and of the SPR band to the original 520 nm wavelength maximum. Thus, the 650-nm absorption is attenuated and the 520-nm band is enhanced upon PA-Fe(III) chelation. This means that the absorption ratio A650/A520 is an indication of Fe(III) whereas the reverse ratio A520/A650 of the PA content of complex samples. The limits of detection (LOD) of the AuNPs@(4MP-TGA) were 1.0 μM for Fe(III) ions and 0.15 μM for PA. Phytic acid extracted from bean grains was determined with the proposed probe, yielding good recoveries. In addition, common metal ions, anions, and several biomolecules did not show an adverse effect on the nanoprobe performance for ferric ions and phytate. The developed method was statistically validated against a LC–MS/MS literature method.