DNA condensation and formation of ultrathin nanosheets via DNA assisted self-assembly of an amphiphilic fullerene derivative

DNA nanotechnology propose various assembly strategies to develop novel functional nanostructures utilizing unique interactions of DNA with small molecules, nanoparticles, polymers, and other biomolecules. Although, well defined nanostructures of DNA and amphiphilic small molecules were achieved through hybridization of covalently modified DNA, attaining precise organization of functional moieties through non-covalent interactions remain as a challenging task. Herein, we report mutually assisted assembly of an amphiphilic fullerene derivative and various DNA structures through non-covalent interactions, which leads to initial DNA condensation and subsequent assembly yielding ordered fullerene-DNA nanosheets. The molecular design of the cationic, amphiphilic fullerene derivative (FPy) ensures molecular solubility in the 10% DMSO-PBS buffer system and facile interactions with DNA through groove binding and electrostatic interactions of fullerene moiety and positively charged pyridinium moiety, respectively. The formation of FPy/DNA nanostructures were thoroughly investigated in the presence of λ-DNA, pBR322 plasmid DNA, and single and double stranded 20-mer oligonucleotides using UV-visible spectroscopy, AFM and TEM analysis. λ-DNA and pBR322 plasmid DNA readily condense in presence of FPy leading to micrometer sized few layer nanosheets with significant crystallinity due to ordered arrangement of fullerenes. Similarly, single and double stranded 20-mer oligonucleotides also interact efficiently with FPy and form highly crystalline nanosheets, signifying the role of electrostatic interaction and subsequent charge neutralization in the condensation triggered assembly. However, there is significant differences in the crystallinity and ordered arrangements of fullerenes between these two cases, where longer DNA form condensed structures and less ordered nanosheets while short oligonucleotides lead to more ordered and highly crystalline nanosheets, which could be attributed to the differential DNA condensation. Finally, we have demonstrated the addressability of the assembly using a cyanine modified single strand DNA, which also forms highly crystalline nanosheets and exhibit efficient quenching of the cyanine fluorescence upon self-assembly. These results open up new prospects in the development of functional DNA nanostructures through non-covalent interactions and hence have potential applications in the context of DNA nanotechnology.

A Cross-Linkable Electron-Transport Layer Based on a Fullerene-Benzoxazine Derivative for Inverted Polymer Solar Cells

The synthesis, optoelectronic characterization and device properties of a cross-linkable fullerene derivative, [6,6]-phenyl-C₆₁ -butyric benzoxazine ester (PCBB) is reported. PCBB shows all the basic photophysical and electrochemical properties of the parent compound [6,6]-phenyl-C₆₁ -butyric methyl ester (PCBM). Thermal cross-linking of the benzoxazine moiety in PCBB resulted in the formation of cross-linked, solvent resistive adhesive films (C-PCBB). Atomic force microscopy (AFM) and optical microscopic studies showed dramatic reduction in the roughness and aggregation behaviour of P3HT-PCBM polymer blend film upon incorporation of C-PCBB interlayer. An inverted bulk heterojunction solar cell based on the configuration ITO/ZnO/C-PCBB/P3HT-PCBM/V₂ O₅ /Ag achieved 4.27 % power conversion efficiency (PCE) compared to the reference device ITO/ZnO/P3HT-PCBM/V₂ O₅ /Ag (PCE=3.28 %). This 25 % increase in the efficiency is due to the positive effects of C-PCBB on P3HT/C-PCBB and PCBM/C-PCBB heterojunctions.

Nanosheets and 2D-nanonetworks by mutually assisted self-assembly of fullerene clusters and DNA three-way junctions

Programmable construction of two dimensional (2D) nanoarchitectures using short DNA strands is of utmost interest in the context of DNA nanotechnology. Previously, we have demonstrated fullerene-cluster assisted self-assembly of short oligonucleotide duplexes into micrometer long, semiconducting nanowires. This report demonstrates the construction of micrometer-sized nanosheets and 2D-nanonetworks from the mutual self-assembly of fullerene nanoclusters with three way junction DNA (3WJ-DNA) and 3WJ-DNA with a 12-mer overhang (3WJ-OH), respectively. The interaction of unique sized fullerene clusters prepared from an aniline appended fullerene derivative, F-An, with two 3WJ-DNAs, namely, 3WJ-20 and 3WJ-30, having 20 and 30 nucleobases, respectively at each strand was characterized using UV-visible absorption, circular dichroism and fluorescence techniques. The morphological characterization of nanosheets embedded with F-An clusters was performed via AFM, TEM and DLS analyses. The programmability and structural tunability of the resultant nanostructures were further demonstrated using 3WJ-OH containing a cytosine rich, single stranded DNA 12-mer overhang, which forms entangled 2D-nanonetwork structures instead of nanosheets due to the differential interaction of F-An nanoclusters with single and duplex strands of 3WJ-OH. Moreover, the selective modification of the cytosine rich sequence present in 3WJ-OH with silver nanoclusters (AgNCs) resulted in significant enhancement in silver nanocluster fluorescence (∼40%) compared to 3WJ-OH/AgNCs owing to the additional stability of AgNCs embedded in 2D nanostructures. This unique strategy of constructing DNA based 2D nanomaterials and their utilization in the integration of functional motifs could find application in the area of DNA nanotechnology and bio-molecular sensing.

Chiral self-assembly of fullerene clusters on CT-DNA templates

Herein we discuss the differential interaction of three monosubstituted fullerene derivatives possessing pyridinium, aniline or phenothiazine end groups (F-Py, F-An and F-PTz, respectively) with calf thymus DNA (CT-DNA), probed via spectroscopic and imaging techniques. The pyridinium derivative, F-Py becomes molecularly dissolved in 10% DMSO-PBS and interacts with CT-DNA via groove binding and electrostatic interactions, leading to the initial condensation of CT-DNA into micrometer sized aggregates and subsequent precipitation. On the other hand, the aniline derivative F-An, which is reported to form nanoclusters of 3-5 nm size, interacts with DNA through ordered, chiral assemblies on the CT-DNA template, thus perturbing the highly networked structure of CT-DNA to form nanonetworks, which eventually transform into condensed aggregates. The binding interactions between CT-DNA and F-An nanoclusters were established via UV-Vis, AFM and TEM analysis, and the chiral nature of the fullerene nanocluster assemblies on CT-DNA was confirmed by the presence of induced circular dichroism that was exhibited around the 250-370 nm region, corresponding to F-An nanocluster absorption. In contrast, the phenothiazine derivative F-PTz, which forms larger nanoclusters of ∼70 nm size in 10% DMSO-PBS, exhibited only weak interactions with CT-DNA without affecting its network structure. These results demonstrate the role of the hydrophobic-hydrophilic balance in the design of DNA interacting fullerene derivatives by controlling their cluster size and interactions with CT-DNA, and are significant in applications such as DNA condensation, gene delivery and dimension controlled nanomaterial fabrication.

Fullerene Cluster Assisted Self-Assembly of Short DNA Strands into Semiconducting Nanowires

Programmable, hierarchical assembly of DNA nanostructures with precise organisation of functional components have been demonstrated previously with tiled assembly and DNA origami. However, building organised nanostructures with random oligonucleotide strands remains as an elusive problem. Herein, a simple and general strategy, in which nanoclusters of a fullerene derivative act as stapler motifs in bringing ordered nanoscale assembly of short oligonucleotide duplexes into micrometre-sized nanowires, is described. In this approach, the fullerene derivative, by virtue of its amphiphilic structure and unique hydrophobic-hydrophilic balance, pre-assembles to form 3-5 nm sized clusters in a mixture of DMSO-phosphate buffer, which further assists the assembly of DNA strands. The optimum cluster size, availability of DNA anchoring motifs and the nature of the DNA strands controls the structure of these nanomaterials. Furthermore, horizontal conductivity measurements through conductive AFM confirmed the charge transport properties of these nanowires. The current strategy could be employed to organise random DNA duplexes and tiles into functional nanostructures, and hence, open up new avenues in DNA nanotechnology.