Niraj Sinha

Carbon nanotubes for biomedical applications

Carbon nanotubes (CNTs) have many unique physical, mechanical, and electronic properties. These distinct properties may be exploited such that they can be used for numerous applications ranging from sensors and actuators to composites. As a result, in a very short duration, CNTs appear to have drawn the attention of both the industry and the academia. However, there are certain challenges that need proper attention before the CNT-based devices can be realized on a large scale in the commercial market. In this paper, we report the use of CNTs for biomedical applications. The paper describes the distinct physical, electronic, and mechanical properties of nanotubes. The basics of synthesis and purification of CNTs are also reviewed. The challenges associated with CNTs, which remain to be fully addressed for their maximum utilization for biomedical applications, are discussed.

Electromechanical interactions in a carbon nanotube based thin film field emitting diode

Carbon nanotubes (CNTs) have emerged as promising candidates for biomedical x-ray devices and other applications of field emission. CNTs grown/deposited in a thin film are used as cathodes for field emission. In spite of the good performance of such cathodes, the procedure to estimate the device current is not straightforward and the required insight towards design optimization is not well developed. In this paper, we report an analysis aided by a computational model and experiments by which the process of evolution and self-assembly (reorientation) of CNTs is characterized and the device current is estimated. The modeling approach involves two steps: (i) a phenomenological description of the degradation and fragmentation of CNTs and (ii) a mechanics based modeling of electromechanical interaction among CNTs during field emission. A computational scheme is developed by which the states of CNTs are updated in a time incremental manner. Finally, the device current is obtained by using the Fowler-Nordheim equation for field emission and by integrating the current density over computational cells. A detailed analysis of the results reveals the deflected shapes of the CNTs in an ensemble and the extent to which the initial state of geometry and orientation angles affect the device current. Experimental results confirm these effects.

Tuning the surface enhanced Raman scattering and catalytic activities of gold nanorods by controlled coating of platinum

Galvanic replacement of silver (Ag) by platinum (Pt) on bi-metallic nanorods (NRs) having gold (Au) core and silver shell (Au@Ag) resulted in discontinuous coating of Pt over Au (Au@Pt-DC) NRs. However, a novel method has been developed in this work for the preparation of Au NRs having smooth and continuous coating of Pt (Au@Pt-C NRs) using galvanic replacement reaction of Au@Ag NRs in presence of sulphuric acid. Selective blocking by the bisulfate ions that are adsorbed on Pt surface, preventing Pt on Pt deposition seems to be the mechanism of formation of Au@Pt-C NRs. Effect of the nature of Pt shell (i.e. whether continuous or discontinuous) on SERS activity of the NRs was investigated with methylene blue (MB) as a reporter molecule. The specific enhancement of the Raman signals were in the order Au@ Pt-C NRs<Au NR<Au@Pt-DC NRs. Catalytic reduction of MB by NaBH4 using the NRs also followed a similar trend with Au@Pt-DC NRs showing almost three times better activity than Au and Au@Pt-C NRs.

Characterization of Self-Assembly and Evolution in Carbon Nanotube Thin Film Field Emitter

Carbon nanotubes (CNTs) are found to be good sources of cold cathode electron for a variety of technological applications. In this paper, we analyze the evolution and self-assembly of randomly oriented CNTs in a thin film during field emission under diode configuration. A model of the evolution of CNT thin film is proposed, where the CNTs are assumed to decay by fragmentation and formation of plasma consisting of carbon atoms and impurities. The random orientation of the CNTs and the electrodynamic interaction among themselves are modeled to explain the self-assembly caused by dynamic reorientation of the CNTs. Finally, the nucleation coupled degradation model and the electrodynamic forcing model are employed to estimate the current-voltage characteristics based on the modified Fowler-Nordheim equation for field emission. The simulated results are in close agreement with the experimental results.

Perfusion kinetics in human brain tumor with DCE-MRI derived model and CFD analysis

Cancer is one of the leading causes of death all over the world. Among the strategies that are used for cancer treatment, the effectiveness of chemotherapy is often hindered by factors such as irregular and non-uniform uptake of drugs inside tumor. Thus, accurate prediction of drug transport and deposition inside tumor is crucial for increasing the effectiveness of chemotherapeutic treatment. In this study, a computational model of human brain tumor is developed that incorporates dynamic contrast enhanced-magnetic resonance imaging (DCE-MRI) data into a voxelized porous media model. The model takes into account realistic transport and perfusion kinetics parameters together with realistic heterogeneous tumor vasculature and accurate arterial input function (AIF), which makes it patient specific. The computational results for interstitial fluid pressure (IFP), interstitial fluid velocity (IFV) and tracer concentration show good agreement with the experimental results. The computational model can be extended further for predicting the deposition of chemotherapeutic drugs in tumor environment as well as selection of the best chemotherapeutic drug for a specific patient.

Multi-mode phonon controlled field emission from carbon nanotubes: Modeling and experiments

The main idea proposed in this paper is that in a vertically aligned array of short carbon nanotubes (CNTs) grown on a metal substrate, we consider a frequency dependent electric field, so that the mode-specific propagation of phonons, in correspondence with the strained band structure and the dispersion curves, take place. We perform theoretical calculations to validate this idea with a view of optimizing the field emission behavior of the CNT array. This is the first approach of its kind, and is in contrast to the the conventional approach where a DC bias voltage is applied in order to observe field emission. A first set of experimental results presented in this paper gives a clear indication that phonon-assisted control of field emission current in CNT based thin film diode is possible.

Enhancing field emission from a carbon nanotube array by lateral control of electrodynamic force field

Fluctuation of field emission current from carbon nanotubes (CNTs) poses certain difficulties for their use in nano-biomedical X-ray devices and imaging probes. This problem arises due to deformation of the CNTs due to electrodynamic force field and electron–phonon interaction. It is of great importance to have precise control of emitted electron beams very near the CNT tips. In this paper, a new array configuration with stacked array of CNTs is analysed and it is shown that the current density distribution is greatly localised at the middle of the array, that the scatter due to electrodynamic force field is minimised and that the temperature transients are much smaller compared to those in an array with random height distribution.

Field Emission Properties of Carbon Nanotube Thin Films Grown on Different Substrate Materials

The electron field emission characteristics from multiwalled carbon nanotube (MWNT) films grown on two different substrate materials have been studied in this paper. The MWNT films were grown on a flat conductor surface (stainless steel) and a flat insulator surface (quartz). The field emission experiments were carried out under a diode configuration. It was found that the film on the quartz surface exhibits better emission capability, while the film on the stainless steel surface shows better emission stability. Energy-dispersive X-ray spectroscopy analysis revealed that carbon atoms evaporated and deposited on the surface of anode during field emission, which is in agreement to studies previously reported in literature. It is estimated that the degradation was due to local heating of tips of CNTs, leading to evaporation and deposition of carbon atoms during the field emission process.

Computational Implementation of a New Multiphysics Model for Field Emission from CNT Thin Films

Carbon nanotubes (CNTs) grown in a thin film have shown great potential as cathodes for the development several field emission devices. However, in modeling these important devices we face substantial challenges since the CNTs in a thin film undergo complex dynamics during field emission, which includes processes such as (1) evolution, (2) electromechanical interaction, (3) thermoelectric heating and (4) ballistic transport. These processes are coupled, nonlinear, and multiphysics in their nature. Therefore, they must be analyzed accurately from the stability and long-term performance view-point of the device. Fairly detailed physics-based models of CNTs considering some of these aspects have recently been reported by us. In this paper, we extend these models and focus on their computational implementation. All components of models are integrated at the computational level in a systematic manner in order to accurately calculate main characteristics such as the device current, which are particularly important for stable performance of CNT thin film cathodes in x-ray devices for precision biomedical instrumentation. The numerical simulations reported in this paper are able to reproduce several experimentally observed phenomena, which include fluctuating field emission current, deflected CNT tips and the heating process.

Experimental investigation of the crosstalk phenomenon and current stability in a carbon nanotube array

For biomedical applications, tomographic imaging of objects in rapid motion requires high speed and high temporal resolution. It has been found that a multipixel carbon nanotube (CNT) based field emission X-ray source can produce spatially and temporally modulated radiations. When a multi-pixel configuration is used, crosstalk (leaking of current from one pixel to the neighboring pixel) comes into picture. In order to achieve good image quality, the crosstalk should be negligible. We report a study on the crosstalk phenomenon in a multi-pixel CNT array. The study will help in improving the imaging capability of computed tomography (CT) scanner.