BHARATHIDASAN
UNIVERSITY

RESEARCH INTEREST

GaN nanostructures:

III-Nitride semiconductors (GaN, InN and AlN) are technologically important wide bandgap semiconductors widely used for the commercial production of light emitting diodes in the visible region, particularly high brightness Blue and Ultra-violet for the primary applications in cell phone back lighting, traffic signals, outdoor displays, projection television, white light source and other high-end consumer products. The unique intrinsic properties of nitride system with polarisation charges make them suitable for the high temperature and high power electronic devices, which are widely used in wireless communication and high speed defence radar communications and also there is a widespread interest in automobiles and power transmissions. The nitride NWs are non-toxic to biomolecules and offers unique biocompatibility which creates a new avenue for the biosensing application. The large surface to volume ratio could provide higher detection surface, more binding sites for biomolecules and direct path, might increase the sensitivity limit to even detect single molecule. Nitride NWs are better than Silicon and Carbon nanotubes in several aspects. We can acquire the technology to fabricate the well ordered defect free nitride nanowires and basic understanding of driving mechanism for self-catalytic approach. As these are the technology important materials, nitride NWs can be explored for varieties of above described applications ranging from nanoelectronics to life sciences.  

The Semiconductor nanowire based FET BIO sensors transduce chemical and biological binding events into electrical signals suggest the potential for a highly sophisticated interface between nano and biological information processing system. The interfacing of nitride semiconductor NWs with biological system will lead to the detection and quantification of biological and chemical species which are critical to many areas of health care and life sciences, from diagnosing to the discovery and screening of new drug molecules. The most commonly used 1D nanostructures in biosensing have been silicon nanowires (SiNWs) and carbon nanotubes (CNTs). The success of SiNWs is due to the semicoducting properties with controlled charge carriers (electrons and holes). CNTs have also been successful for biosensing applications, however, unlike Silicon, CNT exhibits semiconducting or metallic conductivities depending on their chirality make them less attractive for detection of biomolcules and also toxic to the living system at certain extend. The Gallium Nitride (GaN) based NW FETs are highly suitable for binding of biological molecules due to the non-toxic to the living system, biocompatibility, chemically very stable and does not require complicated surface modifications as compared to CNT and SiNWs. GaN NW FET can be fabricated and functionalized for powerful detection of DNA, real time analyses of low concentration proteins and viruses. We anticipate the demonstration of direct and label-free biosensor utilizing GaN NWs for specific DNA sequence identification and other biological molecules with ultra high sensitivity that is exceptionally attractive for many applications in medicine and life sciences.    

Interfacing Nanowire with biosystem:

Life on earth has a fundamental dependence on organic chemistry. The laws of quantum physics allow the molecules and compounds to be built up into the very complex chemical structures of living organism. In general, there are four organic compounds, Amino acids, Carbohydrates, Lipids and Nucleic acids which include DNA and RNA. DNA is, in fact, the fundamental information processing material that directs the development and functioning of all living organism. DNA provides the long term storage for the information needed to construct the building blocks of an organism, it is also passes the genetic information to the next generation.

The identification and characterisation of DNA is one of the greatest scientific breakthroughs of twentieth century and its implications are overwhelming [1]. As a result, we know that DNA is a long polymer chain with repeating units called nucleotides A, T, G and C. In nature, DNA typically exists not as a single strand but as a double helix because of their nucleotides have a very specific attraction for A with T (A-T) and G with C (G-C), known as base pairing. Hence, two single strands of DNA will join together to form double helix structure. Genes are long sequence of DNA base pairs that encode information to evolve like our activity, chromosome levels, blood cells and likelihood to develop infections and etc. The HUMAN GENOME, for example, contains about 25,000 Genes and six billion individual nucleotides. In order to detect and classify, we need a way to read the exact base pair pattern present in DNA structure.

Currently employed optical detection methods are slow and expensive; sequencing a human genome costs approximately 20 million USD and takes several years [2]. There is no much benefit of human genome sequence for humankind because of high cost and time period. In order to circumvent bottleneck on Genome research and expedite, we need to sequence genomes very quickly and cheaply at affordable to all. As the leading researchers in this filed have shown dissatisfaction to the rate of progress of genome sequence, to stimulate the technology X-Prize Foundation has announced a 10 million USD award for producing sequence of 100 human genomes in 10 days or less with the amount of 10,000 USD per Genome [3]. The currently employed methods are based on the optical detection of DNA- hybridisation using DNA micro-arrays in which fluorescent tagging or markers are used. In fact, detection of single fluorescence molecule is challenging. However, DNA-micro arrays technology provides the multiplex detection of DNA by binding the known single stranded DNA with unknown single strand molecule and thus reading the matching of nucleotides base pair with optical light. Despite the vastly multiplex reactions and speedy sequences, the labelling or reagents are time consuming, environmentally-harmful and complex to implement.   

Current sensing DNA-Hybridization and biomolecule detection using semiconductor nanowires and quantum dots/nanoparticles provide ideal platform for medicine and life sciences; in particular, genetics, drug discovery and targeted drug delivery [4]. IN recent times, the one -dimensional nanostructures such as nanowires and nanotubes have become the focus of intense research due to their unique properties and building blocks for future nanoscale devices including sensor, nanoelectronics and optoelectronics [5]. The 1 D system is the smallest dimension structure that can be used to efficiently transport the electrons and optical excitation. Because of high-aspect ratio and large-surface to volume-ratio of NWs and tuneable electron transport properties due to quantum confinement, their electrical conduction in nanowires are strongly influenced by the minor perturbation on the surface. The biomoleulces are naturally charged, binding with the semiconductor nanowire surface leads to the depletion or accumulation of electrons in the bulk region [6] (in contrast to the surface depletion or accumulation of carriers only to the gate area of the conventional FET) depending on the nature of carrier types (electrons or holes). Thus giving rise to large change in conductivities of the nanowire which enables the detection of even single biomolecules in ultrahigh sensitivity [4-6]. There is a natural connection between the nanostructures and life sciences in which the diameter of the nanowire and quantum dots are comparable to that of biological and chemical species (protein, virus, cells and etc) being sensed. Hence, the nanowire based biosensor devices have many key features, including direct, label-free and real-time electrical signal transduction, ultra-high sensitivity, exquisite selection of bio molecules, potential for multiplex detection and ability to reuse [5-7]. Here we proposed to use GaN based nanowires for the detection of biological molecules because they have a number of unique properties over silicon and carbon nanotubes that make them ideal candidates for BIOSENSOR:

Ø        Chemically very stable

Ø        Non-toxic to living system

Ø        Transparent to visible electromagnetic radiation

Ø        Stable characteristics at high temperature and etc.

Multiplex detection of biomolecules can be possible with the functionlisation of different chemical species to specifically bind the particular biomolcules such as DNA, Protein and virus. The proposed work involves interfacing of biosystem with inorganic nanowires of Gallium Nitride in which two major part of the work will be carried out. First one is the fabrication of self-assembled Gallium Nitride Nanowires by catalytic and self-catalytic assisted vapour liquid solid (VLS) approach using chemical vapour deposition and characterisation of nanowirs. The second part is to interface the nanowires with biosystem. Here it involves the functionlisation of nanowires and fabrication of two terminal Field Effect Transistor (FET) for the detection of biomolecule. The charged bio molecules act as a gate for the FET to control the flow of electrons between the source and drain. As due to large surface to volume ratio of nanowire, the binding of molecules to the receptor group functionalised in the NWs (n-type) lead to depletion of electrons in the bulk region, thus the no electrons flow to the drain and hence the no conductivity in the wires. On the other hand if there is no binding, the electrons in the NWs and thus the conductivity remain as in the control FET. In contrast, the situation reverses in the case of p-type semiconductor, binding of molecule would accumulate the electrons and resulting in increase in conductivity. From the strength of the signal level, the sensitivity of detection limit can be measured, it is expected to detect below the atto Molar (aM) concentration.

References:

1.      Purves W. K, Sadawa D, Orians G.H and Heller H.C, Life: The science of Biology, Sixth Edition, Sunderland – Sinauer Associates 2001)

2.      Church George M, Genome for ALL, Scientific American (Jan 206) 47-54.

3.      Cohen Jon, Sequencing in a Flash, Technological Review (June 2007) 72-77.

4.      Semiconductor Physics, S. M. Sze

5.      Patolsky F, Zheng G  and Leber C.M, Nanowire based Biosensors, Analytical Chemistry (2006) 4261.

6.      Patolsky F, Zheng G and Leber C.M, Nanowire sensor for medicine and the life sciences, Nanomedicine (2006) Vol., 51.

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