Biological materials naturally display an astonishing variety of sophisticated nanostructures that are difficult to obtain even with the most technologically advanced synthetic methodologies. As the needs for nanoengineered materials with improved performance characteristics are becoming increasingly important, the potential of biological scaffolds for the fabrication of novel types of nanostructures is being actively explored. This review presents an overview of “biotemplating” as an emerging, unique approach for the synthesis and organization of inorganic nanostructures into well-defined architectures. The technological significance of these architectures is emphasized. We review examples of biological templates explored to date (in terms of their origin and structure) and the success of a variety of methodologies in providing control over the size, crystallinity, and surface chemistry of the nanomaterials.

We report here a multistep route for the immobilization of DNA and proteins on chemically modified gold substrates using fourth-generation NH(2)-terminated poly(amidoamine) dendrimers supported by an underlying amino undecanethiol (AUT) self-assembled monolayer (SAM). Bioactive ultrathin organic films were prepared via layer-by-layer self-assembly methods and characterized by fluorescence microscopy, variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). The thickness of the AUT SAM base layer on the gold substrates was determined to be 1.3 nm from ellipsometry. Fluorescence microscopy and AFM measurements, in combination with analyses of the XPS/ATR-FTIR spectra, confirmed the presence of the dendrimer/biopolymer molecules on the multilayer sensor surfaces. Model proteins, including streptavidin and rabbit immunoglobulin proteins, were covalently attached to the dendrimer layer using linear cross-linking reagents. Through surface plasmon resonance measurements, we found that sensor surfaces containing a dendrimer layer displayed an increased protein immobilization capacity, compared to AUT SAM sensor surfaces without dendrimer molecules. Other SPR studies also revealed that the dendrimer-based surfaces are useful for the sensitive and specific detection of DNA-DNA interactions. Significantly, the multicomponent films displayed a high level of stability during repeated regeneration and hybridization cycles, and the kinetics of the DNA-DNA hybridization process did not appear to be influenced by surface mass transport limiting effects.

Two-dimensional (2-D) surface layer (S-layer) protein lattices isolated from the gram-positive bacterium Deinococcus radiodurans and the acidothermophilic archaeon Sulfolobus acidocaldarius were investigated and compared for their ability to biotemplate the formation of self-assembled, ordered arrays of inorganic nanoparticles (NPs). The NPs employed for these studies included citrate-capped gold NPs and various species of CdSe/ZnS core/shell quantum dots (QDs). The QD nanocrystals were functionalized with different types of thiol ligands (negative- or positive-charged/short- or long-chain length) in order to render them hydrophilic and thus water-soluble. Transmission electron microscopy, Fourier transform analyses, and pair correlation function calculations revealed that ordered nanostructured arrays with a range of spacings (approximately 7-22 nm) and different geometrical arrangements could be fabricated through the use of the two types of S-layers. These results demonstrate that it is possible to exploit the physicochemical/structural diversity of prokaryotic S-layer scaffolds to vary the morphological patterning of nanoscale metallic and semiconductor NP arrays.

Arrays of Au nanoparticles were created using the inherent repeating patterns of bacterial S-layer proteins. Bacterial self-assembling S-layer protein lattices display a highly repetitive surface structure that makes them particularly suitable as biotemplates to fabricate metallic/semiconducting nanostructures and arrays. One interesting S-layer for nanoparticle templating is the hexagonally packed intermediate (HPI) layer of Deinococcus radiodurans. This S-layer, displaying hexagonal (p6) symmetry, is comprised of a hexameric protein core unit with a central pore, surrounded by six relatively large openings (“vertex points”). In this work, the influences of particle properties and adsorption conditions on the formation of ordered arrays of 5-nm Au nanoparticles using HPI S-layers were investigated. Using transmission electron microscopy (TEM), it was found that the templating of citrate-capped Au nanoparticles on HPI layers under low ionic strength conditions resulted in hexagonal-packed ordered arrays with ∼18-nm interparticle spacings that corresponded with the pore-to-pore distance of the S-layer. Interestingly, nanoparticle binding occurs at the vertex points on the HPI layer and, due to repulsion forces, adsorption tends to be favored at every second vertex point. Upon increasing the ionic strength, ordered packing is still observed. However, because interparticle repulsions are less prominent, adsorption of nanoparticles occurs in virtually every available vertex point, resulting in the formation of a honeycomb-like pattern of nanoparticles extending throughout the HPI monolayer sheet. Combined with the results of additional investigations using either uncharged hydroxy-terminated particles or positively charged ferritin molecules, the experimental data suggest that the creation of ordered arrays through biotemplating of Au nanoparticles onto HPI S-layers depends on the electrostatic interactions between individual nanoparticles as well as the interaction with the HPI layer.