Biomolecular Templated NW Assembly

The Parkinson's-Reversing Breakthrough

Parkinson Disease Treaments

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The steady trend in the electronics industry toward components having ever-smaller dimensions has stimulated the development of alternative "bottom-up" fabrication technologies to compete with conventional micro- and nano-

lithography, which are expected to become extremely expensive as the feature sizes of future electronic circuits approach the limits of optical lithography (112). Bottom-up approaches, which rely on self-assembly (or self-organization) that often utilizes biological molecules, could provide viable solutions. Low fabrication costs and feature sizes below the current limit of optical lithography are two of the major advantages of bottom-up approaches. Biomolecules can also be employed as templates to deposit various solid materials to form nonhomogeneous NWs, which could possibly be used in electronics-based biosensors.

The chemical deposition of metals such as silver (42), gold (113), platinum (114,115), palladium (116,117), copper (118), and functionalized gold particles (119,120) on DNA has been investigated as a potential approach for creating conductive NWs. The molecular recognition properties of biomolecules are used for the defined buildup of a nanostructured circuit, and the electrical functionality is installed by the directed construction of a metallic wire on the biotemplate. Saxl (112) and Keren et al. (113) demonstrated that not only can conducting gold and silver NWs be constructed from DNA templates but also that specific regions of DNA molecules can be protected from metal deposition by associating proteins along sections of the DNA. The ability to control metallization spatially provides an important step toward the bottom-up assembly of functional nanocircuits. However, a nonconducting gap was sometimes observed at small bias voltages. Richter et al. (117) reported that palladium NWs chemically deposited on a DNA template showed highly conductive ohmic transport behavior. Figure 9 shows low-voltage (1 kV) SEM images of a single palladium metallized DNA strand with a length of approx 16 ^m corresponding to the length of a linear X-DNA molecule of 48,502 bp. The DNA molecule was positioned between macroscopic Au electrodes and metallized afterward with a two-step chemical deposition method, involving (1) activation of the template by treatment with Pd(II) complexes, which in part bind on the DNA strands, and (2) subsequent reduction of the complexes to form metallic clusters. Two-terminal I-V curves of such NWs showed a linear curve with a bias voltage down to 1 ^V (117). The specific conductivity for NWs with a diameter >50 nm was found only one order of magnitude below that of bulk palladium.

Recently, specific sequences of peptides were used to mineralize specific metals and semiconductors to produce highly crystalline nanocrystals that form nonhomogeneous NW assemblies. A good example is a new biological approach developed by Djalali et al. (121) to fabricate Au NWs using sequenced histidine-rich peptides as templates. Histidine-containing peptides are known to have high affinities to metal ions that damage central nervous systems by altering protein conformations into abnormal forms via histidine-

Fig. 9. SEM image of a single palladium metallized DNA strand with length of approx 16 |im corresponding to length of a ^-DNA molecule. Two gold electrodes are deposited on the strands to measure the electrical property across the strand. The inset shows a magnification of the middle part with a diameter of 50 nm. (Reprinted from ref. 117 with permission. Copyright 2001 American Institute of Physics.)

Fig. 9. SEM image of a single palladium metallized DNA strand with length of approx 16 |im corresponding to length of a ^-DNA molecule. Two gold electrodes are deposited on the strands to measure the electrical property across the strand. The inset shows a magnification of the middle part with a diameter of 50 nm. (Reprinted from ref. 117 with permission. Copyright 2001 American Institute of Physics.)

metal complexation, and this protein deformation may cause Parkinson disease and Alzheimer disease. Fabrication of the histidine-rich peptide involved four steps. First, fe(N-a-amido-glycylglycine)-1,7-heptane dicarboxylate molecules (10 mM) were self-assembled into NWs in a pH 5.5 citric acid/NaOH solution. Such NWs incorporate binding sites that have high affinity to biological molecules such as DNAs and proteins. Second, a histidine-rich peptide with the sequence A-H-H-A-H-H-A-A-D was immobilized on the heptane dicarboxylate NWs at the binding sites. Third, the histidine-rich peptide NWs were mixed with a ClAuPMe3 solution and incubated for 5 d to allow complete immobilization of Au ions. Finally, a reducing agent, NaBH4, was added to produce Au nanocrystals. By using this method, monodispersed Au nano-crystals were uniformly coated on the histidine peptide NWs with high-density coverage, and the crystalline phases of the Au nanocrystals were observed with TEM.

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