pH-dependent Valence Electronic Properties of Aqueous Aerosols

We probed the valence electronic structures of Cys aqueous aerosols via the aerosol VUV photoelectron spectroscopy apparatus. The photoelectron spectra of Cys show distinct band shapes at varying pH conditions, reflecting the altered molecular orbital characters when its dominating form changes. The ionization energy of Cys is determined to be 8.98 ± 0.05 eV at low pH. A new feature at a binding energy of 6.97 ± 0.05 eV is observed at high pH, suggesting that the negative charge on the thiolate group becomes the first electron to be removed upon ionization. This work implies that when Cys is involved in redox processes, the charge transfer mechanism may be entirely altered under different pH conditions.

The significance of aerosols has been increasingly recognized, encompassing numerous important research fields including the environmental science, atmospheric chemistry, public health, and biomedical science. However, due to the complex nature of aerosols which are often composed of multicomponents with various size, compositional ratio, and architecture, it has been challenging to probe the intrinsic properties of aerosols at the atomic and molecular level in a composition- and size-controlled way. The valence electronic structure is a particularly important property for aerosols, as it determines the chemical activity of aerosols and thus, their coupling with the surrounding. From this perspective, the newly developed aerosol VUV photoelectron spectroscopy technique not only exhibits the capability to probe the solvated species but also sheds new light for one to investigate the valence electronic structure of aerosols and complex molecular assemblies in a size-selective and composition-controlled way, as well as their evolution into the bulk. By utilizing the aerosol VUV photoelectron spectroscopy technique, it is promising to address numerous fundamental but critical issues regarding aerosols in their related fields, including the environmental science, the atmospheric chemistry, and the biomedical science.