Green synthesis of gold nanoparticles and their deposition on ITO surfaces

The use of plasmonic nanoparticles for biomedical applications has been widely explored, resulting in significant advances in the construction of optical biosensors. The shape and size of AuNPs determines the spectral signature of their Localized Surface Plasmon Resonance (LSPR) and, therefore, the features of their plasmonic band can be used to monitor surface changes such as those related to protein binding or nanoparticle aggregation. In this work, gold nanoparticles (AuNPs) were produced based on a green and sustainable methodology using tea leaves. The phytochemicals present in tea act as reducing and stabilizing agents. To optimize the AuNPs deposition (nanomaterial proximity, homogenization and substrate coverage), ITO surfaces were modified with different materials, namely sol-gel matrices (e.g. (3-aminopropyl) triethoxysilane (APTES)), cross-linking agents (e.g., glutaraldehyde) and biopolymers (e.g., Bovine Serum Albumin (BSA)). The produced AuNPs were deposited directly onto ITO surfaces functionalized with APTES or in a mixture of BSA and glutaraldehyde; these matrices are transparent and thus suitable for optical applications. The functionalization procedure of ITO surfaces with the referred materials was performed by two methodologies: i) direct deposition of the matrix solution using a micropipette and ii) ultrasound irradiation. The resulting functionalized ITO surfaces were compared and characterized by light transmission spectroscopy. Accordingly, the tea-AuNPs deposited in the presence of BSA and glutaraldehyde provided the best plasmonic response, being the most promising ones for the development of an optical immunosensor.