Keywords: calixarenes, nanoparicles, applications, catalysis
Summary:Nanoparticles are of extreme interest because of the new properties (such as chemical reactivity and optical behavior) they exhibit compared with larger particles. These new properties are again taken to a better level when they are bound to structures which promote their strength and tune them to find applications in various fields. Such structures can be the mesmerizing calixarenes, which are nothing but large bulkier macrocycles with an inherent hollow cavity. When calixarenes and their various derivatives merge with the concept of entities in nanometer scale, it gives birth to a new horizon of research. Calixarenes encapsulates the nanoparticles by modifying, templating, capping or stabilizing ways. Thus these functionalized calixarenes which possess flexible preformed nature, surface charge, easy charge transfer interactions and inherent hollow cavity helps anchoring and encapsulation of metal nanoparticles. In this context, we have been interested in the design of Calix-protected metal nanoparticles, such as calix-nanogold, calix-nanosilver, calix-nanopalladium, calix-nanoalloy of gold and silver, calix-platinum using different calix hydrazide systems. Moreover, exciting and fascinating applications of the prepared calix-nano hybrids as selective and sensitive sensors for various important analytes (metal ions and amino acids), as catalysts for organic chemical reactions: Suzuki, Heck, Sonogashira and Stille reactions have been witnessed. The biological applications are also explored using them as a better antimicrobial agent and can form a future basis for their potential use in new pharmaceutical formulations. We have synthesized the nanoparticles in a not much uncomplicated manner. A one-pot synthesis is carried out which involves a container to which the calix system acting as both capping and reducing agent is added with the concerned metal salt. The convergent nature of calixarene will beautifully encapsulate the nano-metallic particles. The properties of calix based metal nanoparticles relies on the reducing ability of the groups attached on calixarene moiety and stabilizing power of the web-like structure of parent calix platform. To prepare small size, mono-disperse and stable nanoparticles, the reductant and the stabilizing agent should effectively cap and passivate the surface of the metal atoms. Reduction of the metallic salt, for example, HAuCl4 by calix hydrazide system occurs via oxidation of the amino group i.e., transfer of electrons from the amine of calix-hydrazide to the Au3+ ions. Calix hydrazide system can passivate the surface of gold nanoparticles and stabilize nanoparticles owing to the coordination of nitrogen atoms of hydrazide (–NH–NH2) group with Au atoms at the surface of gold nanoparticles. The resulting metallic gold nucleates to form gold nanoparticles. Understanding the non-covalent interactions between the calixarene structures and nanoparticles is essential for predicting the development of novel applications. The chemistry behind how the calix system surrounds the metal nanoparticles in its cavity and how brilliantly these calix moieties stabilize the metal nanoparticles for long period of time is yet to be further explored. The non-covalent forces working inside the cavity of calix system or the binding sites available at either lower or upper rims can stabilize the nanoparticles.