Photosynthesis is inherently inefficient. In particular, the photosystem I protein (PSI) – a critical protein complex necessary for photosynthesis – loses energy when it transfers electrons to NADPH, a coenzyme necessary for CO₂ fixation. Researchers have sought to harness this lost energy by hybridizing PSI with synthetic catalysts. When illuminated, these biohybrid catalysts can perform the hydrogen revolution reaction to produce hydrogen gas from water, a “solar fuel” that could be used as a renewable energy source. 

Studies have shown that PSI photochemistry can drive assembly of Pt catalysts on the PSI surface, producing Pt nanoparticles hundreds of atoms in size. Although scanning electron and atomic force microscopies have shed some light on the structures of these nanoparticle catalysts, these approaches lack the atomic-level resolution necessary to understand the mechanisms behind catalyst formation. 

In a new study, a team of researchers from the U.S. Department of Energy’s (DOE) Argonne National Laboratory and Brookhaven National Laboratory, along with the University of Chicago, used several analytical methods to characterize PSI-Pt nanoparticle biohybrid catalysts. Their findings provide new insights into the mechanisms for photoreductive assembly of Pt nanoparticles on PSI and opportunities to harness this process for the creation of photosynthetic biohybrid catalysts for solar fuels production.  

Researchers used the resources of beamlines 11-ID-B and 12-ID-B at the Advanced Photon Source (APS), a DOE Office of Science user facility at Argonne. 

The researchers began by imaging platinum (Pt) clusters formed on Photosystem I (PSI) derived from cyanobacteria using scanning transmission electron microscopy (STEM) and high-angle annular dark-field (HAADF) imaging. These techniques revealed well-dispersed, crystalline Pt nanoparticles with an average diameter of approximately 1.7 nanometers. X-ray energy-dispersive spectroscopy confirmed the particles were associated with PSI, indicating their formation through a PSI-driven photochemical reaction.

To study the PSI–Pt assemblies in their native aqueous environment, the team used small-angle X-ray scattering (SAXS) and high-energy X-ray scattering (HEXS) at the APS. Unlike imaging techniques that require dried samples, these methods enabled analysis in the same solution used for nanoparticle formation. The results indicated that Pt clusters formed individually on PSI monomers and adopted a triangular arrangement on PSI trimers, with inter-cluster distances of approximately 144 Ångströms. Integrating these data with computational modeling, the researchers proposed that the nanoparticles formed on hydrophobic, solvent-exposed chlorophyll cofactors within PSI.

To follow nanoparticle formation from precursor salts, the team combined HEXS with pair distribution function (PDF) analysis. Prior to illumination, no signal characteristic of Pt nanoparticles was detected. However, after four hours of light exposure, a growing signal indicated the formation of nanoparticles reaching about 1.8 nanometers—consistent with microscopy data. Further investigation with SAXS and TEM suggested these particles were more likely two-dimensional disks than spherical structures.

In a final set of experiments, the researchers compared the growth of the Pt nanoclusters on PSI to similar Pt nanoclusters formed by atomic layer deposition (ALD) on porous anodic aluminum oxide (AAO) supports. ALD involves applying precursor materials to a substrate in repeated cycles. Analysis showed that the Pt clusters resulting from ALD had a similar physical appearance to those formed by photochemical synthesis on PSI, and both structures appeared to be formed through nucleation. However, the growth mechanism appears to be different for the two types of nanoclusters, with those grown on AAO relying on the formation of platinum oxides that were absent in those grown on PSI and ultimately forming a hemispherical shape.

Taken together, the findings point to new strategies for optimized synthesis of PSI–Pt nanoparticle biohybrids, enhancing their performance as catalysts for solar fuel production.  – Christy Brownlee

See: N.S. Ponomarenko¹, N.J. Zaluzec², X. Zuo¹, O.J. Borkiewicz¹, J.M. Hoffman¹, G. Kwon³, A.B.F. Martinson¹, L.M. Utschig¹, D.M. Tiede¹, “Structural characterization of the platinum nanoparticle hydrogen-evolving catalyst assembled on photosystem I by light-driven chemistry,” ACS Nano 2025, 19, 4, 4170-4185 (2025)

Author affiliations: ¹Argonne National Laboratory; ²University of Chicago; ³Brookhaven National Laboratory.

The authors acknowledge support for this work from the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Divisions of Chemical, Biological and Geological Science through Argonne National Laboratory under contract No. DE-AC02-06CH11357. This research used support and resources of the Advanced Photon Source, beamlines 12-IDB (SAXS/WAXS) and 11-IDB (HEXS-PDF), a U.S. Department of Energy (DOE) Office of Science user facility at Argonne National Laboratory and is based on research supported by the U.S. DOE Office of Science-Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Electron Microscopy was carried out using the Analytical Picoprobe Electron Microscope (Thermo Fisher Scientific Spectra Ultra X Iliad), which was developed as part of a CRADA #01300701 between ANL and Thermo Fisher Scientific Instruments and was supported in part by DOE Office of Science at Argonne National Laboratory under Contract No. DE-AC02-06CH11357, as well as in part by the National Science Foundation Major Research Instrumentation (MRI) Program (NSF DMR-2117896) at the University of Chicago.

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