α-Synuclein-Mediated Two-Dimensional Assembly of Gold Nanoparticles and Its Applications

Abstract

Nanoparticles exhibit exceptional chemical, optical, and electrical properties. To maximize those advantages in realistic devices, hierarchical assembly of nanoparticles into supra-structures is essential. Among various supra-structures, two-dimensional organization of nanoparticles is of great interest to utilize nanoparticles in diverse electrical and optoelectrical devices. To organize nanoparticles into two-dimensional structures, diverse macromolecules such as DNA or polymers have been employed. α-Synuclein (αS) is an amyloidogenic protein which produces amyloid fibrils via self-assembly and constitutes a major component of Lewy bodies observed in Parkinsons disease patients. The protein is a member of intrinsically disordered proteins (IDPs) characterized by multiple partner molecules due to their structural plasticity. Interestingly, we have found out that αS encapsulates gold nanoparticles (AuNPs). αS consists of 140 amino acids and can be fdivided as three regions of the basic N-terimus, the hydrophobic middle segement of non-amyloid-β component (NAC), and the acidic C-terminus. Due to the positive charge of N-termial region, the region was demonstrated to be main part of the interaction between αS and negatively charged citrate-capped AuNPs. Therefore, the hydrophobic NAC and the negatively charged C-terminal region of αS would be exposed. In this study, the adhesive property of αS-AuNP has been investigated in acidic solution. αS-AuNP showed omni-adhesive property at pH 4.5. Impressively, the αS-AuNP complex adsorbed onto all the surfaces that we have tested, including oxides, amorphous ceramic material, semiconductors, metal, polymers, polymeric organosilicon, carbon materials, and natural minerals. The most intriguing part was that αS-AuNP adsorbed as tightly packed single layer regardless of chemical and physical nature of the substrate. The adsorption was explained by two different interactions: (i) interactions between αS and substrate and (ii) repulsive interaction between C-terminus of αS molecules. Interaction between αS and diverse substrates was enabled by structural plasticity of αS and induced adsorption of αS-AuNP. Repulsive interaction between the C-terminus of αS molecules precluded additional adsorption. The omni-adhesive property of αS-AuNPs was found to have great feasibility in various systems. E-beam lithography enabled the patterning of αS-AuNP in 100-nm-resolution. The solution-based adsorption strategy also allowed conformal organization of AuNPs onto diverse 3-D objects from macro-glass crystals to nano-materials. We applied the αS-AuNP adsorption to memory development, fuel-cell, solar-cell, cell-culture platform, and photothermal cancer therapy. The outlying αS played versatile roles such as dielectric layer for charge retention, sacrificial layer to expose AuNPs for chemical catalysis, reaction center for silicification, and bio-interface for cell attachment, respectively. Here, we also introduce free-standing αS-AuNP film. There were a lot of struggles in making ultrathin films with high nanoparticle density over micro scale. We have employed omni-adhesive property of αS-AuNP film in the fabrication of free-standing film. First, we have made αS-AuNP complex and adsorbed αS-AuNP single layer onto polycarbonate (PC) substrate. After the adsorption, PC substrate was dissolved using chloroform. The organic solvent treatment induced direct self-assembly of αS which led to α-sheet bridges between nanoparticles.