Green Synthesis of Gold Nanoparticles with Pectinase: a Highly Selective and Ultra-Sensitive Colorimetric Assay for Mg2+

A highly selective and ultra-sensitive colorimetric assay has been developed for detecting the magnesium ion (Mg 2+ ) based on pectinase (PE)-protected Au nanoparticles (PE-AuNPs). This is the first study reporting the use of pectinase for green synthesizing gold nanoparticles. Aggregation of PE-AuNPs was induced immediately following the addition of Mg 2+ ion under Tris-HCl buffer at pH 4.0, yielding a color change from red to blue, and the characteristic surface plasmon resonance (SPR) peak of PE-AuNPs was red-shifted to 665 nm. The effects of parameters such as pH, the amount of PE-AuNPs, and incubation time on the sensitivity of colorimetric assay were investigated in detail. The Mg 2+ -induced aggregation of PE-AuNPs could be monitored by both the naked eye and UV–vis spectrophotometry. The lowest detection concentration with the naked eye is 0.52 μM. A linear relationship between the absorbance ratio ( A 665 / A 523 ) and Mg 2+ concentration was observed in two ranges of Mg 2+ concentration by UV–vis spectrophotometry, including 5.34 × 10 −7  μM to 51.4 μM and 5.12 × 10 2  μM to 3.19 × 10 3  μM. The Mg 2+ detection limit was determined to be 4.0 × 10 −9  μM with UV–vis spectrophotometer. The proposed colorimetric assay possesses a highly selective response for Mg 2+ over other metal ions. This method has been successfully applied to determine the Mg 2+ ion in some water samples.