New Applications of Zirconium Phosphate Nanomaterials

ConspectusThe 2-D layers of the inorganic ion exchanger α-zirconium phosphate (Zr(HPO4)2·H2O, α-ZrP) make this compound particularly stable to low pH, high temperature, and ionizing radiation. Initially studied for its ion exchange properties, once the conditions for its synthesis in crystalline form was accomplished by James Stynes and Abraham Clearfield in 1964, numerous other types of studies and applications followed. Extensive studies in the 1960s and 1970s on the thermodynamics of ion exchange led to insights into the intercalation mechanism of this material. The Clearfield group solved the crystal structure in 1968 and refined it in 1977. Powder methods were pioneered by the Clearfield group to solve the structure of this type of materials. In 1968 Giulio Alberti reported means to prepare zirconium phosphonates expanding the chemistry of these layered compounds. New phases of ZrP were also discovered (e.g., γ, θ, λ, τ) and the applications ranged from heterogeneous catalysis to intercalation chemistry and solid-state proton conductors. Methods to exfoliate the layers of ZrP were developed in the 1990s as interest grew in new applications of these types of materials. For example, protein and enzyme intercalation was accomplished starting in the 1990s by the McLendon, Mallouk, and Kumar groups. In the early 2000s, the Colón group pioneered the use of the θ phase of ZrP for the direct intercalation of large inorganic metal complexes that could not be directly intercalated into the α phase. Initial studies in the Colón group ranged from applications of these directly intercalated ZrP derivatives in photophysics and photochemistry, amperometric biosensors, vapochromism, and vapoluminescence. Over the past decade, new applications of these materials have been developed in anticancer drug delivery and electrocatalysis of the oxygen evolution reaction (OER). ZrP has now proven to be a promising drug nanocarrier and its unique chemical microenvironment provided by the α-type layers and the interlayer space enhances catalytic activity for numerous types of reactions. Further elucidation of the catalytic active species under operando conditions as well as the chemical structure of drug-intercalated derivatives should provide new insights that will advance the design and development of new compounds with desired properties. The initial pioneering efforts of Clearfield and Alberti are being continued by numerous research groups providing new exciting areas of development on the chemistry of layered M(IV) phosphate and phosphonate compounds. In this Account we present the efforts of the Colón group during the past decade on studies of the applications of ZrP for anticancer drug delivery and electrocatalysis of the OER.