Inorganic nanoparticles as enzyme mimics

The ambitious goal of biomimetic chemistry to mimic the structural and functional aspects of natural enzymes is at the center of interest by contemporary scientists. In the past decades, intensive efforts have been made to synthesize inorganic nanomaterials capable of mimicking natural enzymes termed as “artificial enzymes,” that are more stable and cost efficient compared to their natural counterpart (Wei and Wang, 2013; Breslow, 2006; Breslow and Overman, 1970). Based on previous studies of catalytically active model compounds, including metal complexes (Kirby and Hollfelder, 2009), 532polymers (Kirkorian et al., 2012; Klotz, 1984; Kofoed and Reymond, 2005; Wang et al., 2014; Wulff, 2001), supramolecules (Dong et al., 2011; Raynal et al., 2014) and biomolecules (Aiba et al., 2011; Breaker and Joyce, 1994; Mader and Bartlett, 1997; Pollack et al., 1986; Tramontano et al., 1986), new materials have been identified to imitate the biological functions of natural enzymes. Still, mimicking enzymatic reactions inside living organisms-especially in the presence of other competing reactions-remains a great challenge. Recently, several biocompatible inorganic nanomaterials were found to exhibit enzyme-like activities. Therefore, applications inside living cells or organisms could be possible in the future (Fan et al., 2012; Kim et al., 2012; Ragg et al., 2014). However, it is still an open question whether and in which form inorganic nanoparticles (NPs) can mimic the high efficiency and exceptional specificity of their natural counterparts (André et al., 2013). At the same time, enzyme-catalyzed reactions are greatly dependent on specific reaction conditions like temperature, pH or chemical structure of the substrates. For example, enzymes generally suffer from low stability in body fluids or organic solvents, short shelf life and high production costs. In contrast, inorganic NPs provide significant advantages compared to their natural counterpart, e.g., cost-efficient synthesis up to industrial scale and tolerance of major changes in reaction conditions, like temperature, pH or solvent. Additionally, NPs constitute the essential feature of enhanced chemical activity due to their large surface area leading to an increase in catalytic activity. Furthermore, the surface of NPs can be modified by post-synthetic steps. Suitable stabilizing ligands are based on chelating agents that bind tightly to the NP surface, which makes them versatile tools in various applications. Due to the functionalization process, NPs offer the possibility of specific cellular targeting in combination with drug delivery, enhancement of the solubility in different media or increase of their physiological compatibility as well as active site-substrate interactions (Breslow and Overman, 1970; André et al., 2013; D’Souza et al., 1987). Recently, great efforts have been made in identifying new materials with enzyme-like properties that are equally or even more efficient than their natural counterparts. An exceeding number of peroxidase mimics are among the reported materials, whereas 533only a few other enzymatic systems (e.g., superoxide dismutases, catalases, oxidases, haloperoxidases) have been explored so far.