Processes at the solid-liquid interface are important in many applications across a variety of fields from solar energy to corrosive protection. These critical processes include physisorption and chemisorption, electrochemical reactions, various interactions involving (dis)charging of colloids, and photocharging. These processes all involve the evolution of surface charge, making surface charge desirable to monitor. In addition to the monitoring of surface charge, concurrent monitoring of the pertinent observables is equally beneficial to understand the chemical nature of these processes with the possibility of enhanced spatial and time resolution. Much of the existing approaches in previous literature solve this problem through optical tweezers-based methods. Regardless, these techniques have a drawback in that the obtained valuable and sensitive measurements of the overall surface charge do not provide insight into the chemical nature of the surface and surface species interactions. To this note, there is need to develop an experimental platform that will enable sensitive measurement of time-resolved surface charge at solid-liquid interfaces at microscopic scales combined with spectroscopic capabilities.
Researchers led by professor Oksana Ostroverkhova at Oregon State University developed an experimental platform which utilized a highly-sensitive method to measure surface charge on microscopic particles suspended in various environments combined with spectroscopic capabilities on the same particles. They hoped their technique would enable time-resolved measurements of surface charge evolution and energy transfer processes while under photoexcitation. Their work is published in the research journal Optics Express.
Their research technique combined measuring surface charge of microspheres with optical tweezers while simultaneously monitoring the fluorescence of the same spheres under photoexcitation via spectroscopy. To demonstrate their technique, micron-sized silica spheres were coated with physisorbed fluorescent organic semiconductor molecules and their blends. These spheres were suspended in different environments including water and toluene. As a control, uncoated silica microspheres were studied as well.
The authors observed that uncoated silica spheres in water had a higher surface charge density compared to coated silica spheres in both water and toluene. The fluorescence spectra for trapped coated spheres was also observed under photoexcitation. For example, the fluorescence spectra of one coated sphere indicated the interplay between monomeric and aggregate behavior of the molecules of the coating. Other spheres coated with donor-acceptor blends demonstrated exciplex formation, a behavior similar to that in spin-cast thin films of the same blends. Significantly, while the researchers monitored fluorescence from adsorbed molecules, they noted that other light-induced processes could also be studied with this technique including photoinduced charge and energy transfer between the molecules in the coating and in the environment, charge dependent Raman shifts, and (dis)charging under photoexcitation. The potential for time-resolved charge measurements was also evaluated.
Oksana Ostroverkhova and her team demonstrated a novel empirical platform by combining spectroscopic capabilities with time-resolved measurements of effective surface charge at solid-liquid interfaces. Altogether, this versatile platform has enhanced possibilities to study a variety of photo-induced processes simultaneously with measurements of surface charge and can also be incorporated in devices such as micro-reactors and microfluidics.
Figure: Silica microsphere coated with organic semiconductor layer, suspended in liquid, is placed in the AC electric field and optically trapped using a 800 nm laser (left). Upon excitation with 532 nm, the organic semiconductor emits fluorescence (right) which is detected and analyzed. The method enables simultaneous measurement of surface charge, with an elementary charge resolution, and fluorescence from the coated microsphere.
R. Grollman, G. Founds, R. Wallace, O. Ostroverkhova. Simultaneous fluorescence and surface charge measurements on organic semiconductor-coated silica microspheres in (non)polar liquids. Volume 25, Number. 23 | 13 Nov 2017 | Optics Express 29161.
Go To Optics Express