Green approaches for the synthesis of layered double hydroxides and evaluation of their biological properties

Layered double hydroxides (LDH) consist of brucite-like layers containing di- and trivalent metal ions with interchangeable counter anions and solvation molecules that are present between the layers [1]. LDH find several applications in the pharmaceutical field due to their chemical stability, biocompatibility and high mechanical strength, and they are well known to support cellular processes, such as cell differentiation and viability [2]. LDH have very high potentialities for industrial applications, but their synthesis often requires a high number of steps, which could also lead to the production of great amounts of waste [3]. Given these premises, the goal of the work is the design of sustainable strategies for the synthesis of LDH in view of a continuous manufacturing process. LDH were produced through two well-known methods using new green synthesis conditions and their properties were compared to investigate the differences given by the LDH compositions. In particular, their biological properties were considered for a future application in the biomedical field, especially in tissue engineering. Nitrate salts of Mg2+, Zn2+, Cu2+, Al3+, and Fe3+, chosen as bio-enabling cations, were used. First, a co-precipitation method through a fast addition of salts and reduced aging time was developed, using ammonia as an alkaline agent. Few studies are reported using this chemical for the synthesis of LDH. Being a volatile molecule, the excess of ammonia could be easily removed and trapped, together with some of the byproducts that might be formed during the process. Second, the urea hydrolysis through microwave and hydrothermal treatment was considered. The synthesis parameters were adjusted to minimize the waste production. In all cases, the LDH were characterized both as prepared and upon centrifugation/washing, in view of a single step synthesis without water and components waste. The XRD patterns showed that LDH phases represent the major part in the unwashed samples