Nucleic Acid Foldamers for Sensors, Medicine and Nano-electronics
The availability of many nucleic acid aptamers, riboswitches, ribozymes and DNAzymes has opened a new horizon for a wealth of applications such as novel sensors for protein, RNA, DNA, metabolite and metal-ion detection, drug discovery and nanotechnology. Navani and Li have recently reviewed the fast growing developments in this field . Moreover, several nucleic acid aptamers are presently in clinical trials, illustrating the real potential of these functional nucleic acid foldamers as drugs and therapeutics [7, 206]. Therapeutic agents such as small interfering siRNAs, ribozymes, and antisense RNAs show significant potential in new molecular approaches to down regulate specific gene expression in cancerous or virus-infected cells. The development of safe, efficient, specific and nonpathogenic nanoparticles for the packaging and delivery of multifunctional therapeutic RNA is thus highly desirable. Recent investigations suggest that the use of antigen-free 20-40 nm programmable self-assembled RNA particles presenting multiple therapeutic functionalities [184, 207] might hold promise as delivery and therapeutic systems for the repeated long-term treatment of chronic diseases . Although RNA is chemically more fragile than DNA, such "instability" may actually permit a higher degree of flexibility in the design of activatable structures or triggered assembly or degradation of the engineered target in a timely fashion.
Several strategies have been used to generate nanowires taking advantage of nucleic acids as templates for nano-electronic applications (Fig. 10.15). One of
Fig. 10.15 Nucleic acid foldamers for nano-electronics. (a) Metallic nano-wires can be fabricated from a DNA template by silver deposition and glutaraldehyde reduction  or silver photo-induced metallization ; (b, c) Metallic particles coated with (b) DNA  or (c) RNA  can be used to direct the specific bottom-up assembly of nanowires between two electrodes [212, 213]; (d) Positively charged nanoparticles can be aligned on DNA linear structures by electrostatics ; (e) Cationic metallic particles can also be positioned with exquisite regularity on RNA 1-D ladder
Fig. 10.15 Nucleic acid foldamers for nano-electronics. (a) Metallic nano-wires can be fabricated from a DNA template by silver deposition and glutaraldehyde reduction  or silver photo-induced metallization ; (b, c) Metallic particles coated with (b) DNA  or (c) RNA  can be used to direct the specific bottom-up assembly of nanowires between two electrodes [212, 213]; (d) Positively charged nanoparticles can be aligned on DNA linear structures by electrostatics ; (e) Cationic metallic particles can also be positioned with exquisite regularity on RNA 1-D ladder scaffolds by electrostatics, size and shape recognition ; (f) Periodic programmable 2-D arrays can be used as template for building regular metallic nanoparticle arrays either by noncovalently binding DNA coated particles or directly incorporating gold particle into the tile design by gold-DNA conjugation [187, 198, 199, 201]; (g) It is possible to use fully programmable and addressable nucleic acid nano-arrays of finite size as templates for exquisite positioning and ordering of different nanoparticles or other components on a surface to create exotic composite materials.
them is to direct metalization of DNA nanostructure by using glutaraldehyde as reducing agent for silver deposition , another is to use UV light for silver photo-induced metalization . The former method was used for the fabrication of highly conductive silver nanowires by using DNA self-assembling 1-D architectures as templates [166, 193]. Interestingly, the specific incorporation in nucleic acid of modified triphosphates that bear functions that can be further de-rivatized with aldehyde groups via the use of click chemistry can offer an alternative route to the selective metalization of DNA or RNA molecules . This method can potentially lead to the development of more complex metallic nucleic acid templated nanowires. Another strategy involves the use of metallic particles coated with DNA to direct their specific assemblies  between electrodes  (Fig. 10.15). Recently, conductive self-assembling nanowires were constructed by assembling gold-derivatized DNA particles with magnesium dependent loop-receptor tectoRNAs . One RNA-based metallic wire located between lithographically fabricated nano-electrodes was shown to exhibits activated conduction by electron hopping at temperatures in the range 150-300 K . Another interesting potential of well-defined nucleic acid architectures, like tectosquare 1-D ladders [35, 214], is that they can act as scaffolds to control the positioning of cat-ionic nanoparticles not only based on electrostatics  but also size and shape recognition  (Fig. 10.15). The applicability of such type of assemblies in nano-electronics remains to be demonstrated, however. A very powerful way to precisely organize metallic particles takes advantage of periodic programmable 2 D arrays (Fig. 10.15). By noncovalently binding DNA-coated particles or directly incorporating gold particles into the DNA tile design, it is possible to build very regular metallic particle arrays of different sizes [187, 198-201]. For instance, the use of fully programmable and addressable nanogrids open the way to precise positioning of nanoparticles of different composition and sizes  (Fig. 10.15). These nucleic acid based technologies could be particularly promising for the development of several applications including the fabrication of microelectronic architectures, hybrid electronic, optoelectronic devices and sensing.
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