Surface-directed assembly

Surfaces can do more than stimulate the growth of amyloid fibrils the presence of a surface during assembly can also influence fibril morphology. Fibril diameter has been observed to be smaller on surfaces than in the bulk for some polypeptide systems (Zhu et al., 2002). This may be due to the constraints imposed by the surface or could result from a different mechanism of fibril growth. The shape of fibrils formed on surfaces can also differ to the shape of fibrils formed in the bulk solution. 

Surface assembly can also be used to orientate fibrils relative to the surface, generating fibrils that are horizontal or perpendicular to a surface. Zhang observed this behavior for the artificial GAV9 peptide which contains the consensus sequences from a-synuclein, A[3, and the prion protein (Zhang et al., 2006). This peptide assembles horizontally on HOPG surfaces and assembles perpendicular to the surface of mica. 

External microscopy tools can be used to further control the surface-based assembly of fibrils on a surface. Kowalewski used an AFM tip to move the initial aggregates that first form during fibril assembly and change their position on a surface (Kowalewski and Holtzman, 1999). An AFM tip has also been used to induce fibril disassembly in restricted areas (Kellermayer et al., 2008), and both methods offer a way to create patterns of fibrils on a surface. 

One possible drawback of surface-directed assembly is that this phenomenon has been observed to decrease for some fibrils, such as A (125-35, as the fibrils reach the later stages of assembly (Karsai et al., 2007). This temporal change could possibly be avoided by using higher concentrations of A (125-35 peptide during assembly. Chemical cross-linking could also be used to capture the fibrils on the surface and stabilize the fibril network. 

Information on the stability of fibrils will be critical for assessing the potential of fibril-coated surfaces for biotechnology applications. Fibrils such as insulin remain strongly bound to the surface after formation and remain on the surface after rinsing with water (Zhu et al., 2002). This is a desirable feature for the construction of new materials, although there may be some instances where surface transfer is required. 

---

Islands occur particularly with adsorbates that aggregate into two-dimensional assemblies on a substrate, leaving bare substrate patches exposed between these islands. Diffraction spots, especially fractional-order spots if the adsorbate fonns a superlattice within these islands, acquire a width that depends inversely on tire average island diameter. If the islands are systematically anisotropic in size, with a long dimension primarily in one surface direction, the diffraction spots are also anisotropic, with a small width in that direction. Knowing the island size and shape gives valuable infonnation regarding the mechanisms of phase transitions, which in turn pemiit one to leam about the adsorbate-adsorbate interactions. 

Knowledge about protein folding and conformation in biological systems can be used to mimic the design of a desired nanostructure conformation from a particular MBB and to predict the ultimate conformation of the nanostructure [152]. Such biomimetic nano-assembly is generally performed step by step. This wiU allow observation of the effect of each new MBB on the nanostructure. As a result, it is possible to control accurate formation of the desired nanostmcture. Biomimetic controlled and directed assembly can be utilized to investigate molecular interactions, molecular modeling, and study of relationships between the composition of MBBs and the final conformation of the nanostmctures. Immobilization of molecules on a surface could facilitate such studies [153]. 

More recently, a new method of assembling multilayers of PB on surfaces has been described.110 In contrast to the familiar process of self-assembly, which is spontaneous and leads to single monolayers, directed assembly is driven by the experimenter and leads to extended multilayers. In a proof-of-concept experiment, the generation of multilayers of Prussian blue (and the mixed Fein/Run analog ruthenium purple) on gold surfaces by exposing them alternately to positively charged ferric cations and [Fe(CN)6]4- or [Ru(CN)f,]4 anions has been demonstrated.110... 

Figure 11.4. Fluidic flow-directed assembly of NWs. (a,b) Schematic (a) and SEM image (b) of regular NW arrays obtained by flowing NW solution over a chemically patterned surface. (c,d) Parallel and crossed NW device arrays obtained with fluidic flow assembly. [Adapted from Refs. 44 and 49.]...

A novel mesoporous intercalate belonging to the class of mesostructured solid acids known as porous clay heterostructures (PCH) has been synthesized through the surfactant - directed assembly of silica in the two - dimensional galleries of saponite. The new saponite PCH, denoted SAP-PCH, exhibits a basal spacing of 32.9 A, a BET surface area of 850 m2/g and pore volume of 0.46 cm3/g. SAP-PCH is an effective catalyst for the condensed phase Friedel-Crafts alkylation of bulky 2,4-di-tert-butylphenol (DBP) with cinnamyl alcohol to produce a large flavan, namely, 6,8-di-tert-butyl-2,3-dihydro[4H]benzopyran. 

The assembly of monolayers of alkylbromide-functionalized Co nanoparticles onto amino-terminated silicon surfaces through direct nucleophilic substitution was reported by Kim et al.17 The nanoparticle density on the surface can be controlled by changing the immersion time of the silicon surface in the nanoparticle solution. Directed assembly of nanoparticles was observed on a chemically patterned surface. 

Zhi ZL, Powell AK, Turnbull JE (2006) Fabrication of carbohydrate microarrays on gold surfaces direct attachment of noderivatized oligosaccharides to hydrazide-modified self-assembled monolayers. Anal Chem 78 4786-4793... 

Merzlyakov M, Li E, Hristova K. Directed assembly of surface-supported bilayers with transmembrane helices. Langmuir 2006 22 1247-1253. 

It has been demonstrated that DPN can be used to create nanostructures on a surface which can then be used for directed assembly of nanoparticles [90]. In this application also the basic patterning by DPN was done with MHA as ink on an Au substrate. Subsequently, alkylamine-modified DNA was coupled to the MHA nanopatterns so created. By creating an ordered pattern of two distinct oligonucleotide sequences it was possible to create an ordered assembly of two different nanoparticles from a randomly dispersed system. 

Directed Assembly of Programmable Matter Surfaces with Hydro dynamically Switchable Affinities... 

Keywords Interface science Metal surfaces Metal-directed assembly ... 

The control and fabrication of single compounds, and the organization of metal-organic units in ID coordination polymers or 2D coordination arrays is of interest in metallosuparmolecular engineering on surfaces. The key features and their role in the potential functionality of the particularly versatile nanoporous networks achieved by metal-directed assembly in vacuum are illustrated in Scheme 1. The work on the formation of supramolecular architectures on surfaces containing metal centers will here be divided into the two approaches illustrated in Scheme 2. 

---