Venumadhav Korampally

Nanomaterial processing using self-assembly-bottom-up chemical and biological approaches

Nanotechnology is touted as the next logical sequence in technological evolution. This has led to a substantial surge in research activities pertaining to the development and fundamental understanding of processes and assembly at the nanoscale. Both top-down and bottom-up fabrication approaches may be used to realize a range of well-defined nanostructured materials with desirable physical and chemical attributes. Among these, the bottom-up self-assembly process offers the most realistic solution toward the fabrication of next-generation functional materials and devices. Here, we present a comprehensive review on the physical basis behind self-assembly and the processes reported in recent years to direct the assembly of nanoscale functional blocks into hierarchically ordered structures. This paper emphasizes assembly in the synthetic domain as well in the biological domain, underscoring the importance of biomimetic approaches toward novel materials. In particular, two important classes of directed self-assembly, namely, (i) self-assembly among nanoparticle-polymer systems and (ii) external field-guided assembly are highlighted. The spontaneous self-assembling behavior observed in nature that leads to complex, multifunctional, hierarchical structures within biological systems is also discussed in this review. Recent research undertaken to synthesize hierarchically assembled functional materials have underscored the need as well as the benefits harvested in synergistically combining top-down fabrication methods with bottom-up self-assembly.

Fluorescence enhancement from nano-gap embedded plasmonic gratings by a novel fabrication technique with HD-DVD

We demonstrate strong electromagnetic field enhancement from nano-gaps embedded in silver gratings for visible wavelengths. These structures fabricated using a store-bought HD-DVD worth

Entropy driven spontaneous formation of highly porous films from polymer–nanoparticle composites

10 and conventional micro-contact printing techniques have shown maximum fluorescence enhancement factors of up to 118 times when compared to a glass substrate under epi-fluorescent conditions. The novel fabrication procedure provides for the development of a cost-effective and facile plasmonic substrate for low-level chemical and biological detection. Electromagnetic field simulations were also performed that reveal the strong field confinement in the nano-gap region embedded in the silver grating, which is attributed to the combined effect of localized as well as propagating surface plasmons.

Femtogram-level detection of Clostridium botulinum neurotoxin type A by sandwich immunoassay using nanoporous substrate and ultra-bright fluorescent suprananoparticles

Nanoporous materials have become indispensable in many fields ranging from photonics, catalysis and semiconductor processing to biosensor infrastructure. Rapid and energy efficient process fabrication of these materials is, however, nontrivial. In this communication, we describe a simple method for the rapid fabrication of these materials from colloidal dispersions of Polymethyl Silsesquioxane nanoparticles. Nanoparticle-polymer composites above the decomposition temperature of the polymer are examined and the entropic gain experienced by the nanoparticles in this rubric is harnessed to fabricate novel highly porous films composed of nanoparticles. Optically smooth, hydrophobic films with low refractive indices (as low as 1.048) and high surface areas (as high as 1325 m(2) g(-1)) have been achieved with this approach. In this communication we address the behavior of such systems that are both temperature and substrate surface energy dependent. The method is applicable, in principle, to a variety of nanoparticle-polymer systems to fabricate custom nanoporous materials.

Development of a Miniaturized Liquid Core Waveguide System With Nanoporous Dielectric Cladding—A Potential Biosensing Platform

We report a simple, robust fluorescence biosensor for the ultra-sensitive detection of Clostridium botulinum Neurotoxin Type A (BoNT/A) in complex, real-world media. High intrinsic signal amplification was achieved through the combined use of ultra-bright, photostable dye-doped nanoparticle (DOSNP) tags and high surface area nanoporous organosilicate (NPO) thin films. DOSNP with 22 nm diameter were synthesized with more than 200 times equivalent free dye fluorescence and conjugated to antibodies with average degree of substitution of 90 dyes per antibody, representing an order of magnitude increase compared with conventional dye-labeled antibodies. The NPO films were engineered to form constructive interference at the surface where fluorophores were located. In addition, DOSNP-labeled antibodies with NPO films increased surface roughness causing diffuse scattering resulting in 24% more scattering intensity than dye-labeled antibody with NPO films. These substrates were used for immobilization of capture antibodies against BoNT/A, which was further quantified by DOSNP-labeled signal antibodies. The combination of optical effects enhanced the fluorescence and, therefore, the signal-to-noise ratio significantly. BoNT/A was detected in PBS buffer down to 21.3 fg mL(-1) in 4 h. The assay was then extended to several complex media and the four-hour detection limit was found to be 145.8 fg mL(-1) in orange juice and 164.2 fg mL(-1) in tap water, respectively, demonstrating at least two orders of magnitude improvement comparing to the reported detection limit of other enzyme-linked immunosorbent assays (ELISA). This assay, therefore, demonstrates a novel method for rapid, ultra-low level detection of not only BoNT/A, but other analytes as well.

Large sensitivity enhancement in semiconducting organic field effect transistor sensors through incorporation of ultra-fine platinum nanoparticles

We present a high-throughput optofluidic light waveguide system consisting of etched microchannels in silicon using water as the core and an ultra low refractive index nanoporous dielectric (ND) as the cladding organosilicate nanoparticulate films with refractive index of 1.16 have been used as the cladding layer. Although NDs offers many advantages over Teflon AF for use as the cladding layer, integration of these coatings to the waveguide design is not trivial. In this paper, we address the various integration issues of the NDs to the liquid core waveguide architecture followed by testing of these waveguides for their light guiding capability. Compared to uncoated channels, ND clad channels offer a high light guiding efficiency. In addition, the high surface areas associated with them could be potentially used to immobilize higher density of sensor probes implying a great potential for biosensor applications in an integrated system.

Plasma modification of polymer surfaces and their utility in building biomedical microdevices

We report remarkable improvement in sensitivity of pentacene-based field effect transistor devices towards trace nitro-aromatic explosive vapors through the incorporation of high density, sub-2 nm platinum nanoparticles (NPs) within these structures. Exploiting the unique electronic properties of these NPs, we have demonstrated a detection limit of 56.6 parts per billion of 2,4-dinitrotoluene (DNT) vapor while control samples without any embedded NPs showed no observable sensitivity to DNT vapor. We attribute this remarkable enhancement in sensitivity to the ability of these NPs to function as discrete nodes, participating in the charge transfer with adsorbed nitro-aromatic molecules.

Optimization of Design and Fabrication Processes for Realization of a PDMS-SOG-Silicon DNA Amplification Chip

Polymers are widely used in micro-systems for biological detection and sensing and to provide easier alternatives for fabrication of Biomedical Micro-devices (BMMDs). The most widely used polymeric system amenable to micro-fabrication is silicone rubber, particularly poly(dimethylsiloxane) (PDMS). The principal advantage that silicone rubber offers is its ability to get replicated with high aspect ratios by micro-molding. In addition to PDMS, other polymer systems, like resists or epoxies, find extensive use in micro-fabrication providing many aspects such as good interlayer bonding, selective patterning, modified physical properties like variable electrical or optical properties, etc. Most polymer systems are amenable to rapid changes in their surface energies as they are exposed to gas plasmas or UV radiation. Such changes can sometimes be reversible and the exposed surfaces can regain their original configuration with time called hydrophobic recovery. In general, polymer surfaces after such external stimuli become constitutionally highly dynamic and this makes them well suited to prominent applications in fabrication of BMMDs. Our group has extensively worked in the area of polymer surface modification by external stimuli and its characterization and in this paper we have attempted to review some of the groups work.

A comparative evaluation of microarray slides as substrates for the development of protease assay biosensors.

A novel on-chip inexpensive platform to perform DNA amplification has been fabricated by optimizing the design and microfabrication processes using polydimethyl siloxane (PDMS) and silicon. The silicon base contains a set of microfabricated platinum heater structures on the bottom with a 140-nm-thick spin-on-glass (SOG) layer on the top and a 3 mul replica molded PDMS chamber with feed channels and inlet-outlet ports bonded to this film. The plasma exposed SOG surface is found to undergo recovery of hydrophobicity with time as indicated by an increase in advancing contact angle and bonds very well to another plasma exposed PDMS piece. The bonding protocol developed can be used for a diverse range of substrates, which may form a basis for integration of fluidic assays with microelectronics. The hydrophobic recovery of the microchamber and channels also eliminate the need for various surface passivation techniques for polymerase chain reaction (PCR) chips. A thermal cycler with flexible PCR cycle control is designed and implemented by using a sensing thermocouple. The amplification has been tested using picogram-level template DNA concentration. We have further been able to show negligible nonspecific binding of the template DNA to the hydrophobic interiors of our device by fluorescence measurements and have been able to successfully demonstrate the possibility of multiple usage of this chip without cross-contamination from the previous run

Novel nanostructured platform and nanoparticles for sensitive detection of biological materials

The application of commercially available microarray slides as substrates for fluorogenic protease assays has been explored in terms of binding efficiency, stability, and activity. A fluorescent, biotinylated substrate for botulinum neurotoxin A (BoNTA) was attached via self-assembled monolayer of Streptavidin to amine-reactive aldehyde, epoxy, hydrogel, and polymer slides. Nexterion Slide P® was found to have optimal protein binding efficiency and stability of the slides examined. Addition of glycerol to the printing buffer improved spot morphology significantly and polyvinylpyrrolidone provided long-term stability, allowing chips to be stored for up to 1 month with good viability. Detection of a recombinant BoNTA light chain was then carried out at 37°C and a sub-lethal dose was detected in 2 hours.