PDMPO: a specific silicon or silica, pH sensitive fluorescent probe?

Area of Science:

• Environmental Chemistry
• Soil Science
• Biogeochemistry

Background:

• Understanding silicon (Si) behavior and bioavailability in soils and plants is crucial.
• PDMPO (2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole) was proposed as a Si-specific, pH-dependent fluorescence marker.

Purpose of the Study:

• To evaluate PDMPO as a Si-specific fluorescence marker in complex environmental matrices.
• To investigate the interactions of PDMPO with silicon, aluminum, and humic acids.

Main Methods:

• Fluorescence spectroscopy was used to study six different systems with varying concentrations of humic acids (HA), Si, and aluminum (Al) at pH 4, 7, and 9.
• Al-PDMPO complex characterization using infrared spectroscopy.
• Nano tracking analysis for particle size distribution in solution.

Main Results:

• PDMPO fluorescence was not pH-dependent or Si-specific in the absence of standard pH buffers.
• PDMPO fluorescence signals were significantly enhanced in the presence of Si and HA, indicating ternary HA-PDMPO-Si complex formation.
• Aluminum addition strongly quenched fluorescence, suggesting quaternary HA-PDMPO-Si-Al complex formation.

Conclusions:

• PDMPO fluorescence is highly sensitive to complexable metals and natural organic matter.
• The complex interactions make PDMPO unsuitable for reliable quantification of Si or pH in complex environmental media like soils.