Debjit Chattopadhyay

Peroxidase activity of enzymes bound to the ends of single-wall carbon nanotube forest electrodes

This communication reports the first example, to our knowledge, of enzymes covalently attached onto the ends of vertically oriented single-wall carbon nanotube (SWNT) forest arrays used as electrodes. Quasi-reversible FeIII/FeII voltammetry was observed for the iron heme enzymes myoglobin and horseradish peroxidase coupled to carboxylated ends of the nanotube forests by amide linkages. Results suggest that the “trees” in the nanotube forest behaved electrically similar to a metal, conducting electrons from the external circuit to the redox sites of the enzymes. Electrochemically manifested peroxidase activity of myoglobin and horseradish peroxidase attached to the SWNT forests was demonstrated, with detection limits for hydrogen peroxide in buffer solutions of ∼100 nM. These prototype SWNT-forest biosensors are easy to prepare, and enzyme layers were stable for weeks.

Complete elimination of metal catalysts from single wall carbon nanotubes

The complete removal of entrapped metallic impurities (i.e. Ni and Co) incorporated within single wall carbon nanotubes (SWNTs) has been a long-standing issue. A sonication-mediated treatment of as-obtained SWNT soot in a 1:1 mixture of aqueous hydrofluoric and nitric acids resulted in the complete elimination of these impurities as shown by energy dispersive X-ray analysis (EDAX). Contact angle measurements indicated that the wetting of SWNTs is enhanced in the presence of HF. The presence of HNO3 and surfactant was found essential in removing the catalyst due to SWNT etching of end-caps/defects and providing better dispersion, respectively. Moreover, Raman spectroscopy indicated that the structural purity of the SWNTs is not compromised by the HF/HNO3 purification treatment.

Electrochemical Properties of Single-Wall Carbon Nanotube Electrodes

The electrochemical properties of single-wall carbon nanotube (CNT) electrodes in the form of sheets or papers have been examined. Thermal annealing has produced significant changes in a range of properties of the material including increased hydrophobicity and elimination of electroactive surface functional groups and other impurities. As a result of these changes. the treated electrodes exhibit lower double-layer capacitance, absence of faradaic responses and associated pseudocapacitance, and a better frequency response. The basic electrochemical behavior of the CNT paper electrodes is not markedly affected by relatively large differences in electrolyte ion size, consistent with an average pore size of 9 nm. Increases in both CNT sheet thickness and surface area induce a slower electrode response in agreement with the porous nature of the electrode matrix.

APPLICATION OF POLYANION/Fe3+ MULTILAYERED MEMBRANES IN PREVENTION OF BIOSENSOR MINERALIZATION

ABSTRACT Controlled analyte permeability, calcification resistance and biocompatibility are among the many prerequisites necessary for the fabrication of outer membranes for implantable biosensors. In this study, multilayered films of polyelectrolytes (i.e., NafionTM, a perfluorinated ionomer, and Humic Acids (HAs), naturally occurring biopolymers) were investigated as potential semi-permeable membranes for implantable glucose sensors. These films were fabricated using the layer-by-layer self-assembly of polyanions (either Nafion or HAs) with oppositely charged ferric ions. Spectroscopic and quartz crystal microbalance (QCM) studies point towards a stepwise film growth pattern, with growth rates as high as 47 and 24.3 nm per layer for Nafion and HAs, respectively. These assemblies were characterized by growth rates that are strongly dependent on the pH and ionic strength of the polyanion solution. Nafion/Fe3+ assembled films exhibit an order of magnitude lower calcification level as compared to dip-coated and annealed Nafion films. Additionally, these self-assembled films do not require annealing to impart insolubility. Sig nificantly, after four-week immersion in DMEM cell culture media, HAs/Fe3+ assembled films were devoid of calcium phosphate. Moreover, their hydrolytic stability was found to be dependent on film growth condi tions and optimum stability was obtained when the films were assembled at pH and ionic strengths comparable to those of the in vitro testing media.

Fabrication, morphology and actuation from novel single-wall carbon nanotube/nafion composites

Mechanical actuators that simultaneously provide high power densities and large force generation capacities are of great scientific and technological interest. Recently single wall carbon nanotube (SWNT) sheets (bucky papers) were shown to possess significant promise as electrochemical actuators. Embedding polyelectrolytes, like Nafion, within the nanotube matrix has the potential to address the limitations of SWNT bundling and tube slippage thus increasing force generation. In this paper two types of Nafion/SWNT composite actuators have been investigated depending on the method of fabrication. In the first case, infiltration of Nafion within SWNT sheet matrix was followed by annealing at 150 degree(s)C to invert Nafions micellar structure and render it insoluble. This has resulted in a substantially exfoliated layer morphology that causes a reduction in both conductivity and actuation strain (c.a. 0.03%). In the second case, slow casting of a methanolic suspension of Nafion and SWNT soot, followed by annealing at 150 degree(s)C, resulted in a more homogeneous structure. This composite, upon electrochemical cycling between -1 and +1 V in aqueous electrolytes, exhibited actuation strains (as high as 0.43%). However, these higher strains are accompanied by an order of magnitude reduction in modulus largely due to Nafion swelling.

Carbon Nanotube/Polyelectrolyte Composites as Novel Actuator Materials

Mechanical actuators that simultaneously provide high power densities and large force generation capacities are of great scientific and technological interest. Recently single wall carbon nanotube (SWNT) papers (“bucky” papers) were shown to possess significant promise as electrochemical actuators. Embedding polyelectrolytes, like Nafion™, within the nanotube matrix has the potential to address the limitations of SWNT bundling and tube slippage thus increasing force generation. In this paper two types of Nafion/SWNT composite actuators have been investigated depending on the method of fabrication. In the first case, infiltration of Nafion within SWNT paper matrix was followed by annealing at 150°C to invert Nafions micellar structure and render it insoluble. This has resulted in a substantially exfoliated layer morphology that causes a reduction in both conductivity and actuation strain (c.a. 0.03%). In the second case, slow casting of a methanolic suspension of Nafion and SWNT soot, followed by annealing at 150°C, resulted in a more homogeneous structure. This composite, upon electrochemical cycling between -1 and +1 V in aqueous electrolytes, exhibited actuation strains (as high as 0.43%). However, these higher strains are accompanied by an order of magnitude reduction in modulus largely due to Nafion swelling.