use of bio molecular templates for the fabrication of metal nanowires

MINIREVIEW

Use of biomolecular templates for the fabricationof metal nanowiresEhud Gazit

Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel

Bionanotechnology – the use ofbiological tools for nanotechnology

Many functional biological assemblies represent genu-

ine nanotechnological systems and devices [1,2]. These

nano-objects are formed by the process of self-

assembly, facilitated by molecular recognition events

between building blocks, resulting in the formation of

functional devices. Even the simplest living organism

contains functional complex elements such as motors,

pumps, and cables, all functioning at the nano-scale

[3]. Much research is being devoted to the use of

nanotechnology tools for the advancement of biology

(nanobiotechnology) [4]. This is directly related to the

use of nanotechnology to address biological and med-

ical needs (Fig. 1). However, another very interesting

research direction involves the use of ordered biologi-

cal building blocks for the fabrication of various non-

biological nanostructures [5]. In recent years there has

been increasing interest in the utilization of biological

tools for nanotechnological applications that are not

related to biology such as micro-electronics and nano-

electronics, micro-fluidics and nano-fluidics, and

micro-electromechanical and nano-electromechanical

systems. This general field could be referred to as ‘bio-

nanotechnology’, the use of biology (or biological tools

and scaffolds) for nanotechnology. The present review

will focus on bionanotechnological applications for the

formation of metal and other inorganic wires. As will

be discussed next, biology may actually provide unique

tools for such fabrication at the nano-scale (Fig. 1).

The biological building blocks include proteins, pep-

tides, nucleic acids (DNA and RNA), bacteriophages

(viruses that infect bacteria), and plant viruses. These

biologically templated nanostructures may have appli-

cations in diverse fields that are very remote, such as

electronics, telecommunication, and materials engineer-

ing. In this minireview, I will limit the discussion to

the scheme in which the biological assemblies define

the 1D nature of the nanowire. However, it is worth

Keywords

bionanotechnology; electroless deposition;

fibrils; molecular recognition; self-assembly

Correspondence

E. Gazit, Department of Molecular,

Microbiology and Biotechnology,

George S. Wise Faculty of Life Sciences,

Tel Aviv University, Tel Aviv 69978, Israel

Fax: +972 3 640 5448

Tel: +972 3 640 9030

E-mail: [email protected]

(Received 6 October 2006, accepted

3 November 2006)

doi:10.1111/j.1742-4658.2006.05605.x

The nano-scale spatial organization of metallic and other inorganic materi-

als into 1D objects is a key task in nanotechnology. Nano-scale fibers and

tubes are very useful templates for such organization because of their

inherent 1D organization. Fibrillar biological molecules and biomolecular

assemblies are excellent physical supports on which to organize the inor-

ganic material. Furthermore, these biological assemblies can facilitate high-

order organization and specific orientation of inorganic structures by their

utilization of highly specific biological recognition properties. In this mini-

review, I will describe the use of biomolecules and biomolecular assemblies,

including DNA, proteins, peptides, and even viral particles, which are

excellent templates for 1D organization of inorganic materials into wires.

This ranges from simple attempts at electroless deposition on inert biologi-

cal templates to the advanced use of structural motifs and specific protein–

DNA interactions for nano-bio-lithography as well as the fabrication of

multilayer organic and inorganic composites. The potential technological

applications of these hybrid biological–inorganic assemblies will be dis-

cussed.

FEBS Journal 274 (2007) 317–322 ª 2006 The Author Journal compilation ª 2006 FEBS 317

mentioning that other research directions involve bio-

logical modifications of nonbiological 1D objects such

as carbon nanotubes [6,7].

Use of DNA as a template for nanowireformation

DNA molecules are very intriguing building blocks

for nanotechnological applications. Interestingly, more

than two decades ago, Seeman [8,9] showed that

specific recognition between complementary DNA

single-strands allowed them to be engineered to form

well-ordered structures at the nano-scale. The inherent

addressing capabilities, facilitated by specific inter-

actions between complementary single strands, are

manifested in specific recognition and self-assembly

processes. The formation of 2D arrays as well as 3D

nanocubes could be achieved by clever design of the

building blocks [8,9].

DNA is also a very interesting biomolecule for

nanotechnological applications from the material sci-

ence point of view. The diameter of ssDNA is less than

1 nm, and that of dsDNA is �2 nm (Fig. 2). Further-

more, DNA molecules are chemically very robust and

their frequent use in molecular biology applications

has significantly reduced the cost of large-scale chem-

ical DNA synthesis. Consequently, large amounts of

native and modified DNA molecules (for example, by

biotinylation or thiolation) can be rapidly synthesized

at a relatively low cost.

One of the early applications of DNA for the forma-

tion of nanowires, in 1998, involved the metallization

of dsDNA between two electrodes to form conductive

silver nanowire [10]. More specifically, the researchers

used complementary ssDNA to bridge a 12–lm gap

between two gold electrodes. The dsDNA formed was

then coated with silver by a deposition and enhance-

ment process to form 12–lm long, 100nm-wide con-

ductive silver wires. Other seminal work paved the way

to form a gold nanowire based on the use of a DNA

template [11]. This was achieved by the intercalation of

functionalized gold nanoparticles into dsDNA, fol-

lowed by covalent photochemical attachment of the

intercalator [11]. The use of metal-coated DNA mole-

cules was also demonstrated for DNA-assisted wiring

of gold electrodes on silicone wafers [12] and for the

specific metallization of a Y-shaped DNA that incor-

porated a central biotin moiety [13]. These patterned

and directed metallization schemes hold promise for

novel applications in the design and manufacture of

nanoelectronic devices in the future [12,13]. Although

lithography methods are constantly being improved,

template-assisted nanowire formation may be very use-

ful for making interconnections between lithographic-

ally defined elements [14].

Other research into much higher resolution pattern-

ing involves specific recognition between proteins and

defined DNA sequences by a process termed ‘mole-

cular lithography’ [15] (Fig. 3). In this case RecA, a

sequence-specific DNA-binding protein, was allowed

ygolonhcetonaNygoloiB

ygolonhcetonanoiBserutcurtsonandelbmessa-fleS

slairetamderipsni-oiBscinortceleralucelom-oiB

seilbmessa-oibfonoitazillateM

ygolonhcetoibonaNpihc-A-nO-lleC

scitsongaidyarra-naNygoloibnistodmutnauQ

-onannognireenigneeussiTsetalpmet

Fig. 1. Interplay between biology and nano-

technology. Nanobiotechnology involves the

use of nanotechnological tools for various

biological and medical applications.

Bionanotechnology is the use of biological

and bio-inspired molecules and assemblies

for technological uses.

Biological templates for nanowire fabrication E. Gazit

318 FEBS Journal 274 (2007) 317–322 ª 2006 The Author Journal compilation ª 2006 FEBS

to bind a specific region on a DNA template before

the metallization process, thus serving as the equival-

ent of a ‘resist’ (Fig. 3). As the metallization process

proceeded, only noncovered parts of the DNA mole-

cule were coated, thus achieving nano-scale patterned

metallization of the DNA molecule [15]. RecA–DNA

interaction was also used to attach a genetically engin-

eered RecA containing a surface-associated cysteine

which allowed specific metal–thiol interactions [16].

Other DNA–protein complexes used for the formation

of ordered metallic assemblies at the nano-scale have

involved the strepavidin protein array of a 2D array of

biotinylated DNA, followed by metallization of the

array [17].

Use of the naturally occurring amyloidfibrils for metal coating

Another use of DNA is to utilize protein and peptide fi-

bers [18–20]. Such nano-scale fibrils are formed by the

assembly of various building blocks and could be pro-

duced in large amounts by over-expression. Unlike

tnemaliFnitcA mn7

lirbiFdiolymA mn01-7

ebutonanTNDA mn02

AND mn2

egahpsuotnemaliF mn6

Fig. 2. Molecular dimensions of 1D biological molecules and biomolecular assemblies for nanotechnological use. The biological molecules

and assemblies are schematically presented to provide an approximate indication of their dimensions. The DNA structure is formed by bio-

molecular assembly of double helix. All other structures are formed by self-assembly of the large number of nano building blocks.

Photo Lithography Molecular Lithography

Photoresist

SiO2

Wafer

Mask

UV radiationPhotoresist removalEtching

DNA

Recognitionsequences

DNA-bindingproteins

MetallizationProtein removal

Fig. 3. Use of DNA-binding proteins for ‘molecular lithography’. In photolithography, a photoresist layer is deposited on the silicone oxide

surface. The use of a mask allows differential treatment of the photoresist and the etching of specific parts of the layer. In molecular litho-

graphy, the specific DNA sequence is the equivalent of a mask, and the DNA-binding protein serves as the resist.

E. Gazit Biological templates for nanowire fabrication


DNA structures, these are supramolecular assemblies

formed by the recognition and association of numerous

building blocks to create ordered structures. Compared

with DNA, protein allows much more chemical and bio-

logical flexibility as well as providing building blocks

with heterogeneity. As discussed above in the case of

DNA-based structures, genetically engineered DNA-

binding protein is used to achieve such variability.

The first reported attempt to use naturally occur-

ring fibers to make conductive nanowires involved the

use of amyloid fibrils as template [20]. Amyloid fibrils

are naturally occurring fibrillar assemblies with a

diameter of �7–10 nm and a length that can reach

several microns (Fig. 2). These assemblies are usually

associated with human disorders [18–20]. Yet the for-

mation of typical amyloid fibrils is observed in cases

involving bacterial biofilms and in yeast ‘prion-pro-

teins.’ In a pioneering study, yeast-derived amyloid

fibrils were found to be a useful protein template for

the formation of conductive metal wires [18]. Over-

expressed yeast amyloid proteins were genetically

engineered to contain a cysteine residue (as described

above for the RecA-mediated DNA metallization)

[18]. This additional thiol group served as a nuclea-

tion site for the metallization of the fibrils. The

researchers were able to demonstrate the formation of

conductive nanowires by directly measuring the cur-

rent carried by the modified fibrils across a nano-scale

gap between electrodes.

The novel concept of the use of amyloid fibrils for

nanowire formation may actually be utilized to make

wires by coating amyloid fibrils formed by simpler

building blocks. As it has been demonstrated that typ-

ical amyloid fibrils can be formed by peptides as short

as pentapeptides and tetrapeptides [21,22], and as the

molecular structure of amyloid assemblies has been

revealed by high-resolution methods [23–25], simpler

peptide building blocks could be used for future appli-

cations of amyloid fibrils for bionanotechnology.

Simpler building blocks could be synthesized in large

quantities by solid-phase techniques, as previously des-

cribed for DNA oligomers.

Use of cytoskeletal elements for theassembly of nanowires

Another interesting use of naturally occurring fibers

for metal deposition is the use of cytoskeletal elements.

Various nano-scale fibers comprise part of the eukary-

otic cell skeleton including actin and tubulin as well as

intermediate filaments. Such fibers are ubiquitous in

the biological world, and homologous proteins, such

as the FtsZ protein, can also be found in bacteria.

The first use of cytoskeletal proteins for nanotech-

nology was the utilization of actin filaments as tem-

plates for nanowire formation [26]. Briefly, 7-nm actin

filaments were formed by self-assembly of the actin

protein, providing mechanical support for the cell

(Fig. 2). Preformed actin fibrils were covalently modi-

fied by the attachment of gold nanoparticles using an

amine-reactive agent (N-hydroxysuccinimide). This was

followed by disassembly using dialysis, repolymeriza-

tion of fibers, and an enhancement process, resulting

in the formation of a continuous gold nanowire.

The use of cytoskeletal elements adds another

dimension to the biological template of nanowires, as

these elements can be translated at the nano-scale

using biological nanomotors. The myosin nanomotor

can bind actin fibers and use ATP hydrolysis to gener-

ate force and can ‘walk’ along the filament. Thus, fur-

ther study of cytoskeletal modification may lead to

various nano-electromechanical system applications in

which mechanics, in addition to electrical conductivity,

is provided by the biological–inorganic complex.

Use of peptide nanostructures to formconductive nanowires

Another key research direction for the fabrication of

biological fibrils involves the use of peptide and

hybrid–peptide building blocks for the assembly of

bio-inspired fibrillar assemblies. Such bio-inspired

assemblies were also used for the fabrication of metal-

lic nanowires. The simple peptide and peptide–hybrid

building blocks could be synthesized in large amounts

and readily modified.

Various classes of peptide nanotube had already

been used for the formation of 1D metal assemblies.

Glycylglycine bolaamphiphile peptide nanotubes are

examples of such bio-inspired peptide nanostructures

[27]. The functionalization of these peptide nanotubes

with histidine-rich peptide motifs enabled the forma-

tion of copper coating on the nanotube surface [27].

Other studies utilized aromatic dipeptide nanotubes

(Fig. 2). The preferential entrance of metal ions into

the lumen of aromatic dipeptide nanotubes allowed

the reduction of silver ions, with the formation of

silver-filled nanotubes [28]. After the peptide coat is

removed, silver nanowires 20 nm in diameter are

formed [28]. Another study used aromatic dipeptide

nanotubes to assemble platinum nanoparticles [29]. In

a follow-up study, silver-filled peptide nanotubes were

further coated with gold to achieve trilayer coaxial

nanocables [30].

Peptide–amphiphile nanofibers form part of another

class of peptide-based nanostructures. These fibers are



formed by the self-assembly of hydrophilic peptide

building blocks that are conjugated to a hydrophobic

aliphatic tail [31,32]. Amphiphile nanofibers were

shown to form 1D arrays of gold nanoparticles on the

surface of modified peptide fibers [31]. Such peptide–

amphiphile nanofibers were also modified using the

paramagnetic gadolinium(III) metal ion, forming inor-

ganically modified peptide fibers that could be used for

magnetic resonance imaging [32].

Use of bacteriophages and viruses fornanowire assembly

Earlier in this minireview, I discussed the use of DNA

molecules or peptide and protein assemblies. Another

research direction in this organic–inorganic template-

assisted fabrication process is the use of much more

complex assemblies such as bacteriophages and viruses.

These viruses are self-assembled structures at the nano-

scale (Fig. 2). Viral structures are also very attractive

assemblies for fabricating 1D metallic objects. Both

viruses and bacteriophages have been used for this

purpose.

One of the first studies in bionanotechnology was the

metallization of tobacco mosaic virus particles [33,34].

This nano-scale biological entity is very effective as a

seamless template for the fabrication of various inor-

ganic materials. In the last few years, several protocols

for the deposition of various metals on the tobacco

mosaic virus surface have been developed [33,34].

Filamentous bacteriophages can provide an even bet-

ter molecular system for the formation of well-ordered

1D inorganic assemblies [35–38]. This is based on the

ability of bacteriophages to express various protein

motifs, including single-chain antibodies, on their sur-

face, a technique known as ‘phage display’. These are

proteins and peptides expressed on 6-nm elongated

fibrillar structures (Fig. 2). This technique, which is

widely used for selecting various peptide-binding

motifs, was later used for selecting peptide motifs that

can bind various inorganic metallic and semiconductive

nanoparticles [35–38]. This property was later used for

fabricating various metal and semiconductive nano-

wires by utilizing the bacteriophages. The bacterio-

phages used are engineered to express motifs that

interact with specific metal and semiconductive parti-

cles. These phages can then be aligned in such a way

that macroscopic metal or semiconductive wires are

formed. The application of these wires was recently

demonstrated for the fabrication of electrodes for thin

lithium-ion batteries [38]. The binding of gold to the

viruses followed by reduction of the cobalt ions resul-

ted in composite wires that contained both cobalt oxide

and gold, which serve as superb electrodes for batteries.

These wires have very good specific capacity, allowing

the production of batteries with high-energy density.

A very recent study used phage display technology

to select for single-chain antibodies (scFv) that specific-

ally discriminate between crystalline facets of a gallium

arsenide semiconductor [39]. The use of these recogni-

tion properties, combined with the metallization proto-

cols for bacteriophages, may allow further integration

of phage-based assemblies into electronic devices.

Conclusions

Ordered structures of biological molecules and assem-

blies at the nano-scale serve as excellent templates for

fabricating inorganic nanostructures. The structures

used range from single-stranded or double-stranded

nucleic acids and proteins to peptide assemblies and

even viral particles.

Acknowledgements

I thank the Israel Science Foundation (ISF) for their

support for this research.

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