De Cuyper M., Bulte J.W.M. - Physics and chemistry basis of biotechnology (Vol. 7) (2002)(en)

Aleksey Nedoluzhko and Trevor Douglas

Current – voltage curves of the resulting wire were quite surprising, and two interesting features were revealed. First, at low voltage zero current, and, consequently, an extremely high resistance was observed. However, at a higher bias the wire showed usual ohmic behaviour. The value of threshold bias was found to depend on the amount of silver metal deposited on DNA. Second, electric current was dependent on the direction of the voltage scan. Possible explanations for these phenomena are based on the concept that the metal wire is not really continuous, but contains a number of silver grains that can require simultaneous charging to provide the wire conductivity. These effects may also appear as a result of chemical processes on grain boundaries.

A different kind of interaction was employed by Cassell et al [125] for the synthesis of DNA – fullerene composites. C60 fullerenes modified with N,N- dimethylpyrrolidinium iodide reacted with DNA through the interaction with phosphate groups. The resulting composite was observed by TEM without any additional staining, Fullerene-DNA complexes tended to agglomerate, but this could be prevented by the addition of anionic or zwitterionic surfactants.

It must be noted that although all the above works describe the use of nucleotide chains as templates for the material synthesis, the synthetic procedures does not employ specific nucleotide-nucleotide interactions to form inorganic materials. The synthesis is based on the interactions between specific nucleotide groups and the metal cation (or pyrrole ring). In principle, similar results could be achieved using other polymer molecules containing appropriate active groups. However, DNA has shown to be a suitable template for the crystal growth, and recent progress in alteration of the DNA geometry [126, 127] opens new perspectives for the material synthesis involving the use of nucleotide chains with tuneable and complex architectures.

3.7.2. Synthesis involving nucleotide-nucleotide interactions

The first studies describing the use of specific nucleotide interactions for the creation of ordered structures were devoted to the organisation of gold nanoparticles modified with oligonucleotide ligands through thiol bridges.

Alivisatos et al [128] demonstrated the procedure for the organisation of gold particles into dimers and trimers. In this study, gold particles were first modified with 18-base nucleotide chains. They reacted with a single-stranded DNA molecule that contained two or three sequences complementary to the oligonucleotides attached to Au particles. The reaction with 37-base DNA template yielded particle dimers. Depending on the orientation of the complementary sequences one against the other inside the DNA strain, either “parallel” or “anti-parallel” dimers may be formed. The formation of “parallel” trimers in reaction of the oligonucleotide-modified gold nanoparticles with the 56-base DNA template was also reported.

In later work by Loweth et al [129] this synthetic strategy was developed further to obtain various relative spatial arrangements of gold nanoparticles (5 and 10 nm size) heterodimers and heterotrimers (Figure 17). TEM analysis of these assemblies showed that they are not rigid, even in the case where the DNA does not have single-stranded pieces (“nicks”) in it.

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Figure 17. Organisation of gold nanoparticles into dimers and trimers using oligonucleotides. Reprinted by permission from Angew. Chem. Int. Ed [I29] Copyright 1999 Viley-VCH Verlag.

Another approach was employed in the work of Mirkin et al [130]. In this study, 13 nm Au particles were modified with 8-base oligonucleotides of two kinds, which were not complementary. Addition of single-stranded DNA, containing two 8-base sequences, each complementary to either of the oligonucleotides attached to the gold particles, resulted in precipitation of the colloid. TEM analysis revealed the occurrence of a wellordered network of the gold particles separated uniformly by 60 Å The Au particles were linked together with DNA duplexes (Figure 18). To confirm this, the aggregate was heated to the temperature of the DNA duplex denaturation. As expected, at the elevated temperature DNA duplex dissociated into single-stranded oligonucleotides yielding the initial reactants.

The particle self-assembly was accompanied by a decrease of the Au surface plasmon band and its shifting to a longer wavelength, changing the initial red colour of the colloid to blue. This feature of the assembly reaction may be used for the colorimetric determination of DNA nucleotide sequence [131]. Using gold nanoparticles with attached appropriate oligonucleotide sequences it is possible to determine whether a DNA strand is exactly complementary to these sequences or not through the colour changes of the gold colloid.

Later, the strategy developed for the nanoparticles assembly was employed for the assembly of structures containing gold particles of two different sizes [132]. Using nucleotide-modified gold particles (8 nm and 31 nm in diameter) in an appropriate ratio, the assembly of structures containing larger Au particle surrounded by many smaller Au particles was demonstrated.

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Figure 18. Strategy to create network of gold particles linked with DNA. Reprinted by permissionfrom J. Cluster Sei [135] Copyright 1997.

Using nucleotide-nucleotide recognition for nanoparticle assemblies into ordered structures was extended to include semiconductor particles [ 133]. Specifically, highly luminescent CdSe – core ZnS – shell particles were used. These semiconductor nanoparticles were considered to be of a particular interest since they had been reported to possess a stable fluorescence with a relatively narrow (33 nm at half-maximum) band originating from band-to-band transitions with a quantum yield of 0.5 at room temperature [134]. The semiconductor particles were first synthesised in trioctylphosphine oxide to produce particles with strongly hydrophobic surface. Then, the particle surface was modified with 3-mercaptopropionic acid to provide the solubility in water. This procedure as well as the subsequent attachment of thiolmodified 22-base single strand DNA did not affect the fluorescence of the semiconductor. However, assembly of particles into ordered structure in the presence of complementary DNA template caused some decrease (by 26 %) of the fluorescence intensity. Interestingly, ordered networks of CdSe/ZnS particles linked by DNA duplexes remained in the solution, although they could be separated by centrifugation at relatively low speeds compared to the single semiconductor particles modified with nucleotides and mercaptopropionic acid.

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Since both Au and CdSe/ZnS particles may be organised into the DNA-linked assemblies in a similar way, the creation of binary metal-semiconductor networks is possible, and has been demonstrated [ 133].

For reviews on the use of nucleotide chains in material synthesis see [135, 136]. 3.8. BIOLOGICAL SYNTHESIS OF NOVEL MATERIALS

In the pursuit of novel materials the yeasts Candida glabratu and Schizosaccharoyces pombe have been induced to produce quantum sized crystallites of the semiconductor cadmium sulphide (CdS). The yeasts, cultured in the presence of cadmium salts, produced crystallites (~20Å diameter), coated by short chelating peptides of the general formula (y-Glu-Cys)n -Gly which were suggested to control the crystal nucleation and growth as well as stabilising the particles from aggregation [137, 138]. These crystallites were more monodisperse than those produced by conventional chemical means and also were shown to be an unusual polymorph of CdS. Some other microorganisms were found to produce CdS nanoparticles too. Bacteria Klebsiella pneumonae were shown to synthesise CdS particles (up to 200 Å size) on the cell surface [139]. The formation of metal sulphide by these bacteria showed cadmium specificity [140]. Biosynthesised CdS nanoparticles could protect the cells against UV radiation [14 1] and possessed photocatalytic properties similar to CdS nanocrystals obtained by chemical methods [142].

Clearly, this detoxification by the organism has some similarities to the iron sequestration by ferritin but the phytochelatin polypeptide retains significant flexibility to accommodate the chelation of both molecular and nano-materials.

Another interesting example of the synthesis of novel materials by living organisms was demonstrated in the work of Fritz et al. [143]. A highly organised composite material - a “flat pearl” was grown on disks of glass, mica, and MoS2 inserted between the mantle and shell of Huliotis rufescens (red abalone). The biosynthesis of this material proceeded through a developmental sequence closely resembling that occurring at the growth front of the natural shell.

3.9. ORGANIZATION OF NANOPARTICLES INTO ORDERED STRUCTURES

Some examples of the organisation of nanocrystals into the structures ordered at a larger scale have been already mentioned in the previous sections. Langmuir-Blodgett multilayers as well as liquid crystal templates are two examples of techniques used in the synthesis of three-dimensional nanoparticle arrays. Another well-developed procedure for the creation of “nanocrystal superlattices” includes dispersion of highly monodisperse spherical nanoparticles into organic solvent with subsequent solvent evaporation. This method was employed in the work of Murray et al. [144] for the synthesis of assemblies of CdSe nanoparticles of a narrow (+ 3%) size distribution. The formation of three-dimensional ordered structure was confirmed by TEM, HRSEM and ED. The interparticle distance was shown to depend entirely on the capping substance used in the preparation of CdSe particles. Samples prepared with hexadecyl phosphate,

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trioctylphosphine oxide and tributylphosphine oxide had interparticle distances of 17, 1 1, and 7 Å respectively.

Figure 19 Aggregation of biotinylated ferritin containing iron oxide particles in the presence of streptavidin Reprinted by permission from Chem Mater [I47] Copyright 1999 ACS Publications

Certain organic molecules may be employed to form organised nanoparticle arrays through covalent linking of the particles. This technique was demonstrated, for example, for the synthesis of two-dimensional arrays of 3.7 nm gold particles linked by an aryl di-isonitrile (1,4-di(4-isocyanophenylethynyl)-2-ethylbenzene) [145],

More biological approach to the nanoparticle assembly uses highly specific interactions between certain biomolecules. Besides nucleotide-nucleotide interactions, two other biological recognition systems were used to assemble inorganic nanoparticles. In the study of Shenton et al. [146] 12 nm Au and Ag nanoparticles with attached either IgE or IgG antibodies formed arrays in the presence of bivalent antigens with two appropriate functionalities. Filaments consisting of closely packed metal particles were observed. The other strategy uses streptavidin – biotin interaction. This is of particular interest because of the large association energy of the complex. In the presence of streptavidin the assembly of biotinylated ferritin shells with inorganic cores was demonstrated [147] (Figure 19). Streptavidin was also shown to promote aggregation of gold nanoparticles modified with a disulfide biotin analogue [ 148].

Most of the above-mentioned methods employ specific organic-organic interactions. A more direct approach to nanoparticle assembly involves specific

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interactions between the nanoparticle itself and the capping ligands. An important step in this direction has been made in a recent study aimed at the selection of peptides that may bind specifically to a certain semiconductor surface [149]. In this study phagedisplay libraries, based on a combinatorial library of random peptides, containing 12 amino acids each, were used to probe selective adsorption on specific crystallographic planes of GaAs, InP and Si. This strategy opens the perspectives for nanoparticle assembly with the peptide chains capable to recognise specific solid surfaces.

Biomimetic materials chemistry is at a stage when scientists are able to use the knowledge and tools of a number of disciplines in a pursuit of novel materials. Thus liquid crystal chemistry, polymer chemistry, inorganic materials chemistry, nucleic acid chemistry, protein chemistry and molecular biology all continue to make contributions to this hybrid field.

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