but also by the ligands coordinated to them, as reported in recent studies. [2] In this sense, the coordination of metal-NCs with biomolecules is an emerging area of research since this biomolecular capping can endow multiple capabilities to the NCs, including biocompatibility, stability in biological media, and biological functionality (e.g., binding and inhibitory functions) resulting in hybrid bionanomaterials with a plethora of potential applications. [3] n this context, it has been shown

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... at the capping and stabilization of metal NCs with peptides and proteins affect the optical properties of the NCs and the interaction with living matter by means of specific biological interplay. [4] Taking into account the role of the stabilizing agents on the NCs properties, an extra level of tailoring can be achieved by exploiting protein engineering. Metal coordinating engineered proteins are strongly inspired by natural metalloproteins, whose metalbinding capabilities allow to perform multiple functions, such as storage and transport, [5, 6] sensing, [7, 8] or catalysis. [9, 10] In this framework, designed repeat proteins emerged as an interesting option to display custom metal coordination sites. [13] [14] The possibility of adding tailored chemical Gold nanoclusters (AuNCs) are nanomaterials with interesting photoluminescent properties that can be endowed with biomolecular recognition and biocompatibility when stabilized with proteins. The interplay between the optical features of AuNCs and the function added by the protein makes them perfect candidates for generating hybrid protein-inorganic nanomaterials. Focusing on protein stabilized-AuNCs, hitherto most of the work has covered the use of natural proteins for in situ growth of AuNCs. However, the exploitation of design proteins for such endeavors enables fine-tuning of the photoluminescent assets of AuNCs. In this work, rational protein engineering of modular protein scaffolds is applied for capping of non-emissive, non-passivated naked AuNCs, resulting in a fast and easy method for the synthesis of customizable and emissive protein-AuNC nanomaterials. Tuning of the photoluminescent properties of the final hybrid module is obtained by appropriate choice of the coordination residues grafted on the same protein scaffold. The effects of ligands and coordination bonds are studied using time-resolved photo luminescence and X-ray absorbance spectroscopies, shedding light on the mechanisms behind the emerging properties of these hybrid materials. Moreover, the described versatile strategy opens new avenues for the synthesis of on-demand photoluminescent hybrids for a wide spectrum of optical applications.