The Link Between Electrolytes and Metals Photoelectron Spectroscopy Maps a Gradual Transition for Solvated Electrons in Ammonia

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CHEMICAL PHYSICS The link between electrolytes and metals Photoelectron spectroscopy maps a gradual transition for solvated electrons in ammonia

By Christine M. Isborn reveal the energetics of metal-ammonia so- These questions have motivated many lutions across a large concentration range. studies of hydrated and ammoniated elec- olutions of alkali metals in liquid am- These studies provide the missing energetic trons over the years (6, 7). One of the most monia have fascinated scientists for link to characterize the journey of solvated effective techniques for studying electronic more than 200 years. The “fine blue electrons from electrolyte to metal. energy levels is photoelectron spectroscopy, colour” of a dilute solution indicative Electrons are usually either relatively lo- which measures the kinetic energy of elec- of solvated electrons was first noted calized in atomic or molecular orbitals or trons emitted after a substance is irradiated by Sir Humphry Davy in 1808 [(1), p. can be delocalized in the energy bands of with light (based on the principles of the pho- S63] and independently published by W. Weyl extended solids. For the much less common toelectric effect), revealing electronic binding in 1864 (2). A bronze sheen as the solution case of solvated electrons—which exist in a energies. This technique has been used to becomes highly concentrated (see the figure) number of solvents, including water, but are characterize hydrated electrons and ammo- develops as these solvated electrons coalesce most stable in ammonia—questions remain niated electrons, albeit at low concentrations, into a metallic continuum. Charles Kraus, an about the extent of localization and the de- well below the electrolyte-to-metal transition. Downloaded from early pioneer of the study of these solutions gree of association with the parent ions and The development of a new apparatus en- (3), noted that “these solutions…constitute a surrounding solvent. The solvent environ- abled the application of x-ray photoelectron link between electrolytes, on the one hand, ment may change to support these species, spectroscopy to a liquid ammonia microjet and metals, on the other” [(4), p. 83]. On page and as the concentration increases, the elec- (8), allowing Buttersack et al. to collect the 1086 of this issue, Buttersack et al. (5) com- trons may interact to become spin-paired photoelectrons from the volatile, polar re- bine low-temperature x-ray photoelectron dielectrons and ultimately coalesce into a frigerated metal-ammonia solution across http://science.sciencemag.org/ spectroscopy with high-level simulations to metallic state. the wide concentration range of 0.012 to 9.7 mol % metal. Over this range, the solution color changes from a lighter to a deeper blue A fascinating color change and finally develops a bronze metallic sheen. Alkali metal-ammonia solutions change from blue to bronze as the concentration of the metal is increased. The photoelectron spectra reveal the onset This color change marks the shift of the solution from electrolyte to metallic. The photoelectron and growth of a peak beginning at a concen- spectroscopy experiments by Buttersack et al. across the concentration range show how the electron tration of 0.08 mol % metal (see the figure). gradually changes from localized to metallic. The energy of this peak is independent of the identity of the alkali metal in the solu-

tion, which suggests that the peak arises only Solvated electron Plasmon peaks Conduction on July 30, 2020 band from the ammoniated electron and that the metal parent ion does not play a direct role Fermi edge in this transition. As the concentration of the alkali metal increases, this peak gradually grows into a metallic conduction band with a sharp Fermi edge, and an additional plasmon peak appears. This plasmon peak is responsible for the characteristic bronze color of the metallic solution. This gradual transition is in contrast Coalescence of to the sharp transition proposed in recent solvated electrons work on metal-ammonia nanodroplets (9), which shows that the bulk liquid and small solvent clusters support different solvated Increase in electron structures at higher concentrations metal concentration and therefore different metallic onsets. Electrolyte Metallic Modeling by Buttersack et al. comple- solution solution ments the photoelectron spectroscopy data. They apply a metallic free-electron gas model to fit the growth of the conduction band and the sharp Fermi edge in the photoelectron spectra as the solution undergoes the electrolyte-to-metal transition. These SCIENCE Solvated electrons Metallic electrons A localized peak arises from the At higher concentrations, metallic features Chemistry and Chemical Biology, University of solvated electron. A similar appear in the spectrum at concentrations California Merced, Merced, CA 95343, USA.

feature appears for the dielectron. well below the visual color change to bronze. Email: [email protected] BICKEL/ C. GRAPHIC:

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Published by AAAS metallic spectral features begin to reveal NEUROSCIENCE themselves at concentrations lower than when the solution begins to visually appear metallic bronze. Unblinding with infrared On the dilute side of the concentration range (only one solvated electron or one solvated dielectron), ab initio molecular dy- nanosensors namics provide structures of the electron and dielectron solvated by ammonia molecules. Gene therapy and nanotechnology come together These structures are then used for high-level vertical dissociation energy computations. to fight degenerative blindness Performing ab initio molecular dynamics of an excess electron or dielectron in bulk am- By Katrin Franke and Anna Vlasits ment in patients’ visual function (5). Other monia is a substantial computational feat therapeutic strategies include electrical im- and advances the field beyond previous static any cases of blindness result from plants and optogenetic gene therapies, which cluster calculations to reveal a diffuse ammo- progressive loss of photoreceptors, aim at restoring vision (4, 6). nia solvation shell that is similar for both the which are the light-sensing cells in The optogenetic approach induces the electron and the dielectron. The spin densi- the eye. For individuals with such expression of light-sensitive ion channels ties suggest that the ammoniated electron progressive blindness, potential through gene therapy to restore light sensi- resides within a cavity that is less structured therapies aim at restoring vision by tivity to retinal neurons. Several molecular than that of the hydrated electron. The high- Mmaking the retina light-sensitive again while candidates can restore light sensitivity in ani- Downloaded from level vertical dissociation energy calculations minimally interfering with any healthy pho- mals and in postmortem human retinas (7), show that the energies to ionize the electron toreceptors—goals that are usually contradic- and some are now in clinical trials. But cur- and the dielectron both fall within the mea- tory. Many current therapeutic strategies in- rently these molecules require much more sured photoelectron signal. terfere with remaining vision, making them light than normal photoreceptors to become With greatly increased computational re- primarily suitable for patients who have lost activated, and most therapies would require sources, ab initio molecular dynamics simu- all light sensitivity. On page 1108 of this issue, video goggles to boost the effective bright- http://science.sciencemag.org/ lations could be expanded to a larger scale Nelidova et al. (1) present a potential solution to ness of incoming images. For patients with to include metal ions dissolved in ammonia this conundrum: making the retina sensitive some remaining vision, this strategy would at a variety of concentrations. Studies such to infrared light, which is largely undetect- saturate or possibly even damage their re- as these may be necessary to resolve some able by human photoreceptors. They use en- maining functional photoreceptors. of the remaining controversy of the localized gineered nanoparticle sensors and gene ther- Nelidova et al. circumvent these problems versus delocalized nature of the hydrated apy to induce infrared light sensitivity in mice by making the retina sensitive to infrared and ammoniated electron (10–13). These re- with inherited degenerative blindness and in light, which is light beyond the visible spec- sults would provide additional atomistic and postmortem human retinas. This approach trum and emitted by, for example, warm ob- electronic details of the electrolyte-to-metal might avoid damage to functional photore- jects. Their approach combines gene therapy

transition. For now, the spectroscopic stud- ceptors by preventing saturation or hyper- with the use of gold nanorods, an emerging on July 30, 2020 ies by Buttersack et al. provide the missing activation while inducing light sensitivity in nanotechnology for activating molecules energetic and spectroscopic link and reveal patients with partial retinal degeneration. in the human body (8). Nelidova et al. use the gradual transition to metallic behavior At the level of the retina, a multilayered ar- gold nanorods as antennae for infrared light, before our eyes can see it. j ray of more than 100 types of neurons sorts transforming the light into heat through a complex visual features, such as motion and process called surface plasmon resonance. REFERENCES AND NOTES color, into separate channels to send to the Genetic constructs injected into the eye then 1. S. J. M. Thomas, P. P. Edwards, V. L. Kuznetsov, ChemPhysChem 9, 59 (2008). brain (2). When photoreceptors fail, the en- cause the expression of temperature-sensitive 2. W. Weyl, Ann. Phys. 197, 601 (1864). tire downstream network is affected, and transient receptor potential (TRP) channels 3. C. A. Kraus, J. Am. Chem. Soc. 43, 749 (1921). restoring the visual system’s physiological in photoreceptors. Such TRP channels are 4. C. A. Kraus, J. Chem. Educ. 30, 83 (1953). 5. T. Buttersack et al., Science 368, 1086 (2020). function becomes challenging. In addition, normally found in mammalian heat-sensing 6. M. T. J. H. Lodge et al., J. Phys. Chem. B 117, 13322 (2013). mammalian rod and cone photoreceptors, nerves in the skin, as well as in the infrared- 7. E. Zurek, P. P. Edwards, R. Hoffmann, Angew. Chem. Int. unlike those of some species in the animal sensing organs of some snakes and vampire Ed. 48, 8198 (2009). 8. T. Buttersack et al., J. Am. Chem. Soc. 141, 1838 (2019). kingdom, cannot regenerate. For patients bats, and are able to transform heat into elec- 9. S. Hartweg, A. H. C. West, B. L. Yoder, R. Signorell, Angew. with degenerative blindness, the aim of ther- trical changes in the membranes of cells (9). Chem. Int. Ed. 55, 12347 (2016). apy is therefore twofold: to slow degenera- The authors use antibodies to link the heat- 10. I. A. Shkrob, J. Phys. Chem. A 110, 3967 (2006). tion while preserving remaining vision, and emitting gold nanorods to heat-sensitive TRP 11. R. E. Larsen, W. J. Glover, B. J. Schwartz, Science 329, 65 (2010). to restore some vision once degeneration is channels. Thus, infrared light can activate 12. L. D. Jacobson, J. M. Herbert, Science 331, 1387 (2011). complete. Several types of therapies show photoreceptors (see the figure). 13. C.-C. Zho, E. P. Farr, W. J. Glover, B. J. Schwartz, J. Chem. promise in slowing degenerative blindness, To test whether this strategy can restore Phys. 147, 074503 (2017). including targeted gene therapies and stem visual function, Nelidova et al. express TRP ACKNOWLEDGMENTS cell–based approaches (3, 4). One example channels in cone photoreceptors of a mouse The author is supported by grants from the U.S. Department is Leber congenital amaurosis, a disease for model of degenerative blindness. They find of Energy, Basic Energy Sciences, Computational and which a gene therapy has led to an improve- that neural activity measured in the retina Theoretical Chemistry and Condensed Phase and Interfacial Molecular Science programs (DE-SC0019053 and and visual cortex correlated with infrared DE-SC0020203). Institute for Ophthalmic Research, Bernstein Center light stimuli. In addition, they show that for Computational Neuroscience, Center for Integrative Neuroscience, Tübingen University, 72076 Tübingen, treated blind mice can use their infrared light 10.1126/science.abb9717 Germany. Email: [email protected] sensitivity to learn a simple visually guided

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