Polyethylene glycol (PEG) is a polyether compound commonly used in biological research and medicine because it is biologically inert. *
This simple polymer exists in variable chain lengths (and molecular weights). As they are devoid of any contiguous π-system, PEGs are expected to lack fluorescence properties. However, recent studies suggested the occurrence of fluorescence properties in non-traditional fluorophores like PEGs. *
In the article “Polyethylene Glycol 20k. Does It Fluoresce?” Bethany F. Laatsch, Michael Brandt, Brianna Finke, Carl J. Fossum, Miles J. Wackett, Harrison R. Lowater, Alex Narkiewicz-Jodko, Christine N. Le, Thao Yang, Elizabeth M. Glogowski, Scott C. Bailey-Hartsel, Sudeep Bhattacharyya and Sanchita Hati report a comprehensive investigation of the molecular basis of the fluorescent properties of PEG 20k molecules using combined experimental and computational methods. Fluorescence measurements of PEG 20k were conducted in aqueous solutions as well as in the presence of metal ions. *
Also, atomic force microscopy (AFM), dynamic light scattering (DLS), fluorescence anisotropy, and proton nuclear magnetic resonance (1H NMR) were used to analyze PEG 20k aggregates, assess their sizes, and investigate the presence of delocalization. Atomistic molecular dynamics (MD) simulations were performed to explore the structural properties of PEG 20k. Quantum chemical calculations were performed at the level of density functional theory with improved functionals to probe if the “through-space” delocalization of the PEG 20k aggregates gives rise to the fluorescence properties. *
For the investigation by atomic force microscopy samples were prepared by applying 100 μg/mL, 20 mg/mL, 200 mg/mL, and 300 mg/mL PEG 20k solutions to freshly cleaved pieces of highly oriented pyrolytic graphite (HOPG) as to cover the whole surface of the HOPG. Samples on HOPG were incubated for 30 min. After 30 min, excess PEG 20k solution was removed from the HOPG substrate with a dry Kimwipe by capillary action. *
The sample was then dried in vacuo for 30 min and scanned using a commercial atomic force microscope. *
Scans were conducted in a 5 μm area using the NANOSENSORS uniqprobe qp-BioAC AFM probe and a force constant of 0.15–0.55 N/m. The scan rate was maintained at 0.4 Hz, and the scans were done with 512 scan points and lines. All samples were run in duplicate. *
Results of this combined experimental and computational study suggested that although PEG 20k could exhibit “through-space” delocalization of lone pairs of electrons in aggregates/clusters, formed via intermolecular and intramolecular interactions, the actual contributor of fluorescence between 300 and 400 nm is the stabilizer molecule, i.e., 3-tert-butyl-4-hydroxyanisole present in the commercially available PEG 20k. *
Therefore, the reported fluorescence properties of PEG should be taken with a grain of salt, warranting further investigation. *
Figure 4 from « Polyethylene Glycol 20k. Does It Fluoresce?» by Bethany F. Laatsch, et al:
AFM images of the PEG 20 k on the HOPG substrate. (a) Height-retraced and (b) amplitude-retraced images of PEG 20k on the HOPG substrate. Each AFM image size is 5 μm × 5 μm. The key in each image indicates a length of 1 μm.
*Bethany F. Laatsch, Michael Brandt, Brianna Finke, Carl J. Fossum, Miles J. Wackett, Harrison R. Lowater, Alex Narkiewicz-Jodko, Christine N. Le, Thao Yang, Elizabeth M. Glogowski, Scott C. Bailey-Hartsel, Sudeep Bhattacharyya and Sanchita Hati
Polyethylene Glycol 20k. Does It Fluoresce?
ACS Omega 2023, 8, 15, 14208–14218
DOI: https://doi.org/10.1021/acsomega.3c01124
Open Access: The article “Polyethylene Glycol 20k. Does It Fluoresce?” by Bethany F. Laatsch, Michael Brandt, Brianna Finke, Carl J. Fossum, Miles J. Wackett, Harrison R. Lowater, Alex Narkiewicz-Jodko, Christine N. Le, Thao Yang, Elizabeth M. Glogowski, Scott C. Bailey-Hartsel, Sudeep Bhattacharyya and Sanchita Hati is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/.