Self-assembly of Gold Nanoparticles on Porphyrin Monolayers Anchored on Inorganic Substrates.

Gold nanoparticles (Au NPs) have attracted great attention due to their tuneable and distinctive chemical, optical, magnetic, and electronic properties, employed in a wide variety of areas, such as electronics, photonics, catalysis, and biomedicine.1 Depending on their size, Au nanoparticles can alternately show surface plasmons or luminescence. In fact, the increase in the size of the Au NPs, and the appearance of the surface plasmons may result in the disappearance of luminescence. In this context, the design of hybrid plasmonic and luminescent nanomaterials using a bottom-up approach is suitable for the manufacture of functional nanocomposites that display structural control and novel properties, e.g. optical, electronic or catalytic properties, in the perspective of their applications in different fields of nanotechnology.2 In addition, the combination of different optical properties belonging to the different molecular components represents an advanced method to manufacture hybrid assemblies, useful for improved optical applications. Porphyrins and metalloporphyrins exhibit luminescence properties, sensing capabilities, non-linear optical behaviours, biological roles, magnetic properties, and more.3 Furthermore, porphyrin molecules, covalently grafted to electroactive metal oxide substrates, can also be used for molecular-based information storage materials, whereby information can be stored in the discrete redox states of the molecules. In this context, the aim of this work was the synthesis of a new composite assembly consisting of porphyrin monolayer covalently anchored to inorganic functionalized substrates, conjugated with an additional Au NP monolayer, thanks to the strong binding affinity of Au NPs to pyridyl moieties present in the structure of porphyrin molecules. This nanocomposite 3D architecture exhibits both a strong surface plasmon, due to the gold nanoparticles, and an intense luminescence signal from the porphyrin monolayer molecules. Therefore, we fabricated an optically active archetypal setup that shows distinct optical outputs depending on the excitation wavelength (input), which is of interest for advanced signaling and communication systems. Moreover, the present plasmonic material may be also of interest for photocurrent generation in solar energy conversion systems. Finally, the appropriate choice of this porphyrin as a molecular cross-linker resulted in the structural control of the gold surface structures that are parallel to each other, and evidence a long-range linear order.