In a recent study published in the journal Angewandte Chemie, Professors Guoqing Zhang, Shiyong Liu, Xiaoguo Zhou, and Xuepeng Zhang from the University of Science and Technology of China devised a method for detecting water-ice microstructures by using organic phosphorescent probes and phosphorescence spectroscopy.
Schematic illustration of the hydration state and the energy levels of the ADI aqueous system. Image Credit: Professor Guoqing Zhang’s team.
It is thought that ice was essential to the development of life. One explanation is that carefully ordered water molecules can exclude organic molecules into the spaces between the crystal lattice, which causes an accumulation of organic compounds.
Nonetheless, existing approaches to investigate organic compounds in ice, such as infrared and Raman spectroscopy, are primarily restricted to spectroscopic techniques based on absorption, which limits the sensitivity of the measurements.
The group suggested using emissions to investigate organic compounds in water ice. Acridine iodide (ADI), a phosphorescent probe, was used to determine the hydration state and reveal the microstructural variations in water ice (i.e., crystalline vs. glassy).
A trace number of organic compounds soluble in water can dramatically influence the microstructures of water ice. In particular, the ADI probe's AD+ cation and I- anion will be separated by bound water molecules if water ice remains amorphous at low temperatures.
This will result in long-lasting phosphorescence and a discernible greenish-yellow afterglow. When ADI probe molecules cluster to crystalline ice, the heavy atom action of iodine causes a brief red phosphorescence.
When ethylene glycol (EG) short molecules and monodispersed EG polymers (PDI = 1) were added to the aqueous solution of ADI, the emission spectra showed clear spectroscopic changes in the solution.
When EG (0.1%) is added in trace amounts, a fluorescence band appears at 480 nm. This is followed by a more powerful phosphorescence band with distinct vibronic progressions at 555, 598, and 648 nm. The spectrum data showed that ADI molecules in water ice changed from undissolved aggregates to dissolved ion states as a result of the addition of EG.
Low-temperature scanning electron microscopy (Cryo-SEM) images confirmed the findings of phosphorescence spectroscopy by demonstrating that the addition of trace EG to the water ice containing ADI produced localized areas with porous microstructures.
The addition of trace EG was sufficient to trigger a shift in the O-H vibration of water ice from a low-frequency crystalline state to a high-frequency glassy state, as confirmed by low-temperature Raman (LT-Raman) spectra.
Using more practical and sensitive phosphorescence spectroscopy, this study found that adding minor amounts of small or large molecule organics to water can considerably hinder the crystalline order of water ice.
When trace organics with different structures and the same concentration are added to water, phosphorescence spectroscopy can reveal morphological differences in the water-ice microstructures. This is consistent with scanning electron microscopy and Raman spectroscopy and offers a new technical means of studying the interactions between water, ice, and organics at lower concentrations and a wider temperature range.
Journal Reference:
Liu, H., et al. (2024) Water‐Ice Microstructures and Hydration States of Acridinium Iodide Studied by Phosphorescence Spectroscopy. Angewandte Chemie. doi.org/10.1002/anie.202405314.