Polyethyleneimine-Enabled Tunable Electrostatic Nanoparticle Assemblies on Ultrathin Protein Nanofibers for Plasmonics-Based Solar Energy Harvesting

Controlled assembly of nanoparticles (NPs) onto ultrathin, protein-based biotemplates was previously achieved mainly through the chemical or genetic engineering of the surface chemistry of the templates. However, not only is this approach tedious and complicated, but also such surface engineering is case-specific and lacks generality. Biotemplates with one type of surface chemistry are only suitable to assemble one or a few types of NPs, and different engineering methods are demanded for different types of nanomaterials. Here, instead of engineering the biotemplates, we developed a simple, universal plug-and-play approach through the engineering of the surface chemistry of NPs. We discovered that coating NPs with short-chain polyethyleneimine (PEI) can lead to the highly efficient and controllable electrostatic assembly of NPs onto unmodified protein-based bionanofibers. The PEI molecular weight played a key role in the NP–biotemplate interaction. Specifically, we found that only low-molecular-weight PEI-coated NPs could be loaded densely onto the bionanofibers, while the high-molecular-weight PEI, which individually carried more charges, only led to low-density NP assembly. Our method is facile and universal in several ways. It can assemble NPs of various compositions, sizes, and surface ligand structures onto different wild-type bionanofibers, including flagella, bacteriophages, and pili, without the hassle of the case-specific surface engineering of the biotemplates. Such 1D nanoparticle assembly has its unique advantage as starting materials in fabricating 3D structured linear plasmonic nanochains with broad optical bands for solar harvesting applications.