# Prussian Blue: A Safe Pigment with Zeolitic-Like Activity
## 1 Introduction  
A zeolite is a crystalline, microporous, hydrated aluminosilicate mineral having an infinite, open, three-dimensional structure. They have a framework structure that encloses interconnected cavities occupied by large metal cations (Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), and others) and water molecules. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Zeolites are further able to lose and gain water reversibly without change of crystal structure. The large structural cavities and the entry channels leading into them contain water molecules, which form hydration spheres around exchangeable cations. On removal of water by heating at 350-400 \({}^{\circ}\)C, small molecules can pass through entry channels, but larger molecules are excluded--the so-called \"molecular sieve\" property of crystalline zeolites [1]. In the last 60 years, the study and characterization of zeolites has been an important factor for industrial progress, given their catalytic properties and their high adsorption and desiccant capabilities, permitting oil refining, industrial gasses treatments, industrial ovens, and industrial waste treatments. Another application is water remediation, consisting of the removal of cationic pollutants such as ammonium as adsorbents in pollution control or the handling and storage of nuclear wastes [2]. Prussian blue (PB) and their analogues (PBA) present important similarities with zeolites in relation its adsorption ability of cations. PB and PBA are widely recognized as thefirst coordination network materials in which transition metal ions are assembled through cyano-bridges, generating a 3D cubic structure. PB is a mixed-valence system exhibiting the general formula A\({}_{1-x}\)Fe\({}^{\text{III}}\)[Fe\({}^{\text{II}}\)(CN)\({}_{6}\)]\({}_{1-x}\)/4\(\square\)\({}_{x}\)/4, in which A is a ion (K\({}^{+}\), Na\({}^{+}\) or NH\({}_{4}\)\({}^{+}\)) and \(\square\) denotes the hexacyanoferrate vacancies [3]. PBA are coordination polymers with the general formula A\({}_{x}\)M\({}_{y}\)[M\({}^{\prime}\)(CN\({}_{6}\))\({}_{2}\), where A is an alkaline metal cation and M and M\({}^{\prime}\) cations in oxidation state +2 or +3 [4]. Depending on the exact stoichiometry and conditions of preparation, PB can be divided into insoluble PB and soluble PB [5]. The zeolitic-like properties of PB are inherent to insoluble form. Some defects exist in insoluble PB due to the interstitial water molecules, which could be divided into two types according to their coordination: the water molecules coordinated to Fe (II) sites (coordinative water) and the ones inside cavities which do not coordinate to metal sites (zeolitic or crystallized water) [6]. Both kinds of water can be distinguished by IR: the absorption peak at 3630 cm\({}^{-1}\) is characteristic of O-H group of coordinated water, whereas the peak at 3400 cm\({}^{-1}\) is derived of O-H group of crystallized water [3]. Throughout the decades, PB has been recognized by scientists to be an inorganic coordination complex with great application potential in many fields, such as electrochromic displays, electrocatalysts, secondary batteries, ion- and biosensors, photomagnets, and hydrogen storage [7]. PB are assembled into nano-sized architectures through cyanobridged ligands, the PB nanoparticles. Currently, the synthesis of PB at the nanoscale has a great interest due to several properties arising from their molecule-based nature: solubility, stability, flexible molecular structure, porosity, low density (\(\sim\)1.8 g cm\({}^{-3}\)), and adjustable physical and chemical properties [8]. The development of PB nanoparticles has involved an upturn of their use in a myriad of biomedical applications [9]. Such nanoparticles benefit from an excellent biocompatibility and biosafety since the insoluble form of PB was approved by the Food and Drug Administration (FDA) as material with good safety for human use.