A colorimetric assay for ultrasensitive detection of copper (II) ions based on pH-dependent formation of heavily doped molybdenum oxide nanosheets.

Here we report a simple and low-cost colorimetric assay for ultrasensitive detection of copper (II) ions (Cu2+) based on pH-dependent formation of plasmonic MoO3-x nanosheets. The reaction between ascorbic acid (AA) and Cu2+ gives birth to hydrogen ions, which remarkably promotes the reduction of MoO3 nanosheets by AA to form oxygen vacancies-rich MoO3-x nanosheets that increase the free carrier concentration, generating a strong local surface plasmon resonance (LSPR) absorption. The Cu2+-triggered LSPR is utilized for development of the colorimetric assay. Under the optimal experimental conditions, this colorimetric assay can be used for selective detection of Cu2+, and the limit of detection is 0.8 nM, which is lower than that of most of the reported methods. Additionally, the present colorimetric assay has also been successfully employed for Cu2+ determination in real serum and human hair samples with satisfactory recoveries ranging from 96% to 108%.