Krishna V. Singh

Nano-patterned liquid metal electrode for the synthesis of novel Prussian blue nanotubes and nanowires

We report the fabrication of Prussian blue nanotubes and nanowires via a novel technique wherein a liquid metal surface is nano-patterned with a porous polycarbonate membrane used as a hard mask. Prussian blue as 1D nanowires and nanotubes is an excellent candidate for ultra-low level sensing, optical waveguides and model systems to test the one-dimensional Ising model. One of the most feasible techniques for nanowire fabrication is to use a porous template. However, Prussian blue dissolves in strong acids and decomposes in strong bases. Conventional procedures which use porous alumina blocked with a back side metal contact require strong acid and base treatment in the various steps of the nanowire fabrication. We report a novel technique which will pave a new route for the facile synthesis of nanowires and nanotubes of materials that are sensitive to strong acidic or basic treatment. Hence, nano-patterning of a liquid metal (Hg, which functions as a top-side metal contact) with polycarbonate membrane instead of porous alumina membrane excludes the necessity of both strong acid and base during the fabrication of nanowires. By attempting this innovative technique, we report the synthesis of novel organic Prussian blue nanotubes and nanowires.

Synthesis and characterization of peptide nucleic acid–platinum nanoclusters

Peptide nucleic acid (PNA) is an analogue of deoxyribonucleic acid (DNA) and possesses a neutral backbone that affords stronger hybridization, greater stability and higher specificity in base pairing. However, it has not been explored as much as DNA in self-assembling functional nanostructures or nanoelectronic devices. We report here for the first time the metallization of PNA with platinum (Pt) nanoparticles via chemical binding, reduction and deposition. Pt ions from a precursor salt solution are allowed to bind over the PNA fragments followed by a reduction and then growth into metal nanoparticles. PNA–Pt complexes form chains several hundred nanometres in length and by varying the duration of chemical reduction step, the dimension of the Pt nanoparticles can be controlled. The structural features and chemical composition of PNA–Pt nanoparticles have been characterized via scanning electron microscopy, transmission electron microscopy and Fourier transform-infrared spectroscopy. These results are also supported by modelling and analysis of the nature of high-lying molecular orbitals on PNA using density functional theory (DFT) method.

Temperature dependent transport in nanotube bioconjugates

Carbon nanotube (CNT) based bioconjugates are among the candidates for molecular level electronics. In this work, we have studied transport mechanism of single walled carbon nanotube (SWNT) - peptide nucleic acid (PNA) bioconjugates. Electrical conductivity of the bioconjugates was studied at temperatures ranging from 70 K to 350 K. Negative differential resistance (NDR) behavior was observed at almost all temperatures and conductivity was found to increase with increasing temperatures. NDR behavior can be attributed to alignment and misalignment of energy bands of SWNT and PNA under different voltages.

Fluorescent Tag Based Metrology for Self‐Assembled Molecular Devices

Directed self‐assembly is being researched as a promising alternative to manufacturing nanoscale‐devices. Properly designed self‐assembly methods may reduce the high cost of manufacturing nanoscale‐devices using lithography, at the same time pushing the device dimensions to the realm of molecular. In this paper, a method of molecular device self‐assembly is discussed. Because of the uniqueness of this method, new metrology challenges are likely to be encountered in manufacturing. To monitor the self‐assembly process, in‐line metrology systems will be needed to inspect nanotubes/nanowires, measure dimensions, test chemical properties, and provide fast feedback in real time. To meet this requirement, new techniques of micrography are needed to achieve molecular level resolution without complicated sample preparation. Also, because of water or other solvent based environments likely for self‐assembly, the future metrology systems may be required to operate in environments significantly different from those f...

Application of quantum-dots for analysis of nanosystems by either utilizing or preventing FRET

We have developed conjugates with quantum-dots (QDs) for the purpose of analysis of nanosystems that are organic or inorganic in nature such as DNA and carbon nanotubes. First, by employing Florescence Resonant Energy Transfer (FRET) principles, a hybrid molecular beacon conjugates are synthesized. For water- solubilization of QDs, we modified the surface of CdSe-ZnS core-shell QD by using mercaptoacetic acid ligand. This modification does not affect the size of QDs from that of unmodified QDs. After linking molecular beacons to the carboxyl groups of the modified QDs using 1-Ethyl-3- (3-dimethylaminopropyl) carbodiimide hydrochloride, hybrid molecular beacons are prepared as a DNA probe. After hybridization with specific target DNA and non-specific target DNA, the hybrid conjugates show high specificity to the target DNA with 5-fold increase in the intensity of fluorescence. By developing atomic model of the conjugates, we calculated with 8 numbers of molecular beacons on a single quantum dots, we could increase the efficiency of FRET up to 90%. In other hands, for application of quantum dots to the carbon nanotubes, FRET is a barrier. Thus, after employing 1 % sodium-dodecyl-sulfonate (SDS), single-walled carbon nanotubes are decorated with QDs at their outer surface. This enables fluorescent microscopy imaging of single-walled carbon nanotubes which is a more common technique than electron microscopy. In summary, QDs can be used for analysis or detection of both organic and inorganic based nanosystems.