A sensitive colorimetric probe for detection of the phosphate ion

In the present article, we report a novel colorimetric probe (TNT@MB) for the detection of the phosphate ion, which is based on the strong binding affinity between the phosphate ion and titanium dioxide nanotubes (TNTs). TNTs were synthesized from TiO₂ nanoparticles by hydrothermal treatment. The obtained TNTs had an average length of 200 ± 50 nm and an average width of 12 ± 5 nm. TNT@MB was prepared by adsorbing methyl blue onto TNTs in acidic condition. The optimal synthesis conditions for TNT@MB consisted in having 0.05 g of TNTs react with 1 μmole of methyl blue at pH 2 for 90 min. TNTs and TNT@MB were characterized by UV-vis diffuse reflection spectroscopy, TEM, FTIR, and XPS. The phosphate-ion sensing behavior of TNT@MB was investigated by UV-visible spectroscopy. The phosphate-ion concentration linear range and detection limit of this method based on TNT@MB were 1-40 μM and 0.59 μM, respectively. A sample of lake water was used as a real sample, and analyte recovery rates were measured in the 102.5-103.6% range, with relative standard deviations below 5.6% (n = 3). We also found that this probe could be reused after regeneration in alkaline solution. These results indicate that as a colorimetric probe, TNT@MB has the advantages of being environmentally friendly, inexpensive, and simple to use, as well as giving rise to an easily observable color change.