Imaging Single\nPrussian Blue\nNanoparticles with Extraordinary Low-Spin Iron Capacity

Abstract

In attempts to obtain high-capacity Prussian blue nanomaterials,\ncurrent efforts are predominantly focused on the particle-ensemble-level\nunderstanding of their structure–activity relationships. Complementarily,\nit would be insightful to screen out extraordinary individuals from\nthe nanoparticle population. Using a simple and efficient technique\nof bright-field microscopy, this work enables, for the first time,\nquantitative characterization of the overall two-redox-center electrochemistry\nof single Prussian blue nanoparticles many at a time. Quantitative\noptical voltammograms with little interference from solvent breakdown\nand non-Faradaic electrode charging/discharging are extracted for\neach single nanoparticle, revealing clear heterogeneity among them.\nOn this basis, the microscopic method allows a detailed comparative\nanalysis between the two redox-active sites. It is found that while\nthe synthesized nanoparticles show a similar specific capacity of\nthe high-spin (HS-Fe) sites with STD/mean = 30%, most individual nanoparticles\nexhibit monodispersedly small capacities of the low-spin iron (LS-Fe)\nsites, only about 17±1 of the HS-Fe capacity. Most importantly,\nit is discovered that there is always a small fraction (∼8%)\nof the single nanoparticles showing an impressively tripled LS-Fe\ncapacity. Facilitated by optical imaging, the discovery of this easily\noverlooked extraordinary subpopulation confers alternative opportunities\nfor targeted efforts for material chemists to improve synthesis and\nmaterial design based on these unusual individuals, which in turn\nimplies the general significance of nanoparticle screening.