Rajdip Bandyopadhyaya

Improved electrochemical performance of SnO2-mesoporous carbon hybrid as a negative electrode for lithium ion battery applications.

To utilize the high specific capacity of SnO2 as an anode material in lithium-ion batteries, one has to overcome its poor cycling performance and rate capability, which result from large volume expansion (∼300%) of SnO2 during charging-discharging cycles. Hence, to accommodate the volume change during cycling, SnO2 nanoparticles of 6 nm diameter were synthesized specifically only on the outer surface of the mesopores, present within mesoporous carbon (CMK-5) particles, resulting in an effective buffering layer. To that end, the synthesis process first involves the formation of 3.5 nm SnO2 nanoparticles inside the mesopores of mesoporous silica (SBA-15), the latter being used as a template subsequently to obtain SnO2-CMK-5 hybrid particles. SnO2-CMK-5 exhibits superior rate capabilities, e.g. after 30 cycles, a specific discharge capacity of 598 mA h g(-1), at a current density of 178 mA g(-1). Electrochemical impedance spectroscopy reveals that the SnO2-CMK-5 electrode undergoes a significant reduction in solid-electrolyte interfacial and charge transfer resistances, with a simultaneous increase in the diffusion coefficient of lithium ions, all these in comparison to an electrode made of only SnO2 nanoparticles. This enhances the potential of using the SnO2-CMK-5 hybrid as a negative electrode, in terms of improved discharge capacity and cycling stability, compared to other electrodes, such as only SnO2 or only CMK-5.

State of Dispersion of Magnetic Nanoparticles in an Aqueous Medium: Experiments and Monte Carlo Simulation

Monte Carlo simulation results predicting the state of dispersion (single, dimer, trimer, and so on) of coated superparamagnetic iron oxide (Fe(3)O(4)) nanoparticles in an aqueous medium are compared with our experimental data for the same. Measured values of the volume percentage of particles in the dispersion, core particle diameter, coating-shell thickness, grafting density of the coating agent, saturation magnetization, and zeta potential for the citric acid-coated and poly(acrylic acid) [PAA]-coated particles have been used in our simulation. The simulation was performed by calculating the total interaction potential between two nanoparticles as a function of their interparticle distance and applying a criterion for the two particles to aggregate, with the criterion being that the minimum depth of the secondary minima in the total interaction potential must be at least equal to k(B)T. Simulation results successfully predicted both experimental trends-aggregates for citric acid-coated particles and an individual isolated state for PAA-coated particles. We have also investigated how this state changes for both kind of coating agents by varying the particle volume percentage from 0.01 to 25%, the particle diameter from 2 to 19 nm, the shell thickness from 1 to 14 nm, and grafting density from 10(15) to 10(22) molecules/m(2). We find that the use of a lower shell thickness and a higher particle volume percentage leads to the formation of larger aggregates. The possible range of values of these four variables, which can be used experimentally to prepare a stable aqueous dispersion of isolated particles, is recommended on the basis of predictions from our simulation.

Molecular dynamics study of nanocomposite polymer electrolyte based on poly(ethylene oxide)/LiBF4

Interactions of Li + and BF − ions with TiO2 clusters were investigated using ab initio quantum chemistry methods. Classical force fields have been developed for poly(ethylene oxide)/LiBF4/TiO2, and molecular dynamics simulations have been performed on poly(ethylene oxide)/LiBF4 polymer electrolyte with and without embedded TiO2 nanoparticles using the developed force field. Addition of a TiO2 nanoparticle to PEO/LiBF4 solid polymer electrolyte resulted in the formation of a highly structured layer with a thickness of 5–6 A that had more than an order of magnitude slower mobility than that of bulk PEO/LiBF4. The PEO and ions in the layers extending from 6 to 15 A from the TiO2 nanoparticle also revealed some structuring and reduced dynamics, whereas the PEO/LiBF4 located further than 15 A was basically unaffected by the presence of the TiO2 nanoparticle. Both cations and anions tended to form a region with an increased concentration in the interfacial layers extending from 5 to 15 A. No ions were dissolved by the first interfacial layer of PEO. Addition of a nanoparticle with soft-repulsion interactions with PEO resulted in the formation of a PEO interfacial layer with reduced PEO density but increased ion concentration. The PEO and ion mobility in the interfacial layer next to the soft-repulsive nanoparticle were higher than those of bulk PEO/LiBF4 by 20–50%, whereas the conductivity of the nanocomposite electrolyte with the soft-repulsive particle increased only by 10%.

Polyacrylic acid-coated iron oxide nanoparticles for targeting drug resistance in mycobacteria.

The emergence of drug resistance is a major problem faced in current tuberculosis (TB) therapy, representing a global health concern. Mycobacterium is naturally resistant to most drugs due to export of the latter outside bacterial cells by active efflux pumps, resulting in a low intracellular drug concentration. Thus, development of agents that can enhance the effectiveness of drugs used in TB treatment and bypass the efflux mechanism is crucial. In this study, we present a new nanoparticle-based strategy for enhancing the efficacy of existing drugs. To that end, we have developed poly(acrylic acid) (PAA)-coated iron oxide (magnetite) nanoparticles (PAA-MNPs) as efflux inhibitors and used it together with rifampicin (a first line anti-TB drug) on Mycobacterium smegmatis. PAA-MNPs of mean diameter 9 nm interact with bacterial cells via surface attachment and are then internalized by cells. Although PAA-MNP alone does not inhibit cell growth, treatment of cells with a combination of PAA-MNP and rifampicin exhibits a synergistic 4-fold-higher growth inhibition compared to rifampicin alone. This is because the combination of PAA-MNP and rifampicin results in up to a 3-fold-increased accumulation of rifampicin inside the cells. This enhanced intracellular drug concentration has been explained by real-time transport studies on a common efflux pump substrate, ethidium bromide (EtBr). It is seen that PAA-MNP increases the accumulation of EtBr significantly and also minimizes the EtBr efflux in direct proportion to the PAA-MNP concentration. Our results thus illustrate that the addition of PAA-MNP with rifampicin may bypass the innate drug resistance mechanism of M. smegmatis. This generic strategy is also found to be successful for other anti-TB drugs, such as isoniazid and fluoroquinolones (e.g., norfloxacin), only when stabilized, coated nanoparticles (such as PAA-MNP) are used, not PAA or MNP alone. We hence establish coated nanoparticles as a new class of efflux inhibitors for potential therapeutic use.

Janus silica film with hydrophobic and hydrophilic surfaces grown at an oil-water interface

We report a new methyltrimethoxysilane (MTMS) based route to growing a Janus silica film at the oil–water interface, which upon drying shows anisotropic wetting by water on its two surfaces. The contact angle of water on the surface grown in contact with the oil-side is found to be ∼150°, but it is much smaller, ∼65°, on the side which grew in contact with the aqueous phase. This large difference in the contact angle is found to be primarily because of two reasons: (i) orientation of hydrophobic methyl groups towards the oil-side of the film as confirmed by micro-Raman spectroscopy, and (ii) microstructural differences in the oil and water-side surfaces of the film. The inherently hydrophobic silica cluster network on the oil-side surface also exhibits larger pores that provide an air cushion for the water droplet and engenders a large contact angle. Effects of oil–water interfacial tension on the film growth and on its wetting and microstructural properties are also investigated by addition of cationic and anionic surfactants in the aqueous subphase. Static and dynamic wetting properties of the oil-side surface indicate that these do not change significantly due to variations in either the microstructure or chemical nature of the surface alone, but is a combined effect of both. Interestingly, the Janus films showing asymmetric surface properties can also be grown directly and thus integrated with a variety of porous surfaces like cotton, paper, hydrogel and ceramic substrates by having these surfaces straddle an oil–water interface.

Biofouling prevention using silver nanoparticle impregnated polyethersulfone (PES) membrane: E. coli cell-killing in a continuous cross-flow membrane module

Biofouling significantly decreases membrane performance. So silver nanoparticle (Ag-NP) was impregnated selectively on a sulfonated polyethersulfone (SPES) membrane and its efficacy was tested in a continuous, cross-flow membrane module. The main challenges are: (i) to prevent biofouling on the membrane surface, (ii) achieve zero bacterial cell (E. coli) count in the permeate water, (iii) maintain Ag concentration in the permeate stream within the permissible limit of drinking water and (iv) maintain a high tensile strength of the membrane to prevent mechanical failure. Addressing these factors would ensure a long and productive service-life of the membrane. To this end, 104CFU/ml of E. coli cell-suspension was passed through the Ag-SPES membrane of 150μm total thickness, which has a narrow (1.74μm thickness), upper surface of Ag-NPs. We achieved zero E. coli cell-count and a minimum (10μg/L) Ag concentration in the permeate stream; simultaneously increasing the tensile strength from 2.78MPa to 3.92MPa due to Ag-NP impregnation. Thus, for a continuous inlet flow of E. coli contaminated water, the membrane module could deliver an almost constant permeate flow rate of 3.45L per hour, due to complete E. coli cell-killing. Simultaneously, Ag concentration in permeate stream is well-below the WHOs recommended limit of 100μg/L, for potable quality water. Therefore, the Ag-SPES membrane can be used as an anti-biofouling membrane in a continuous operational mode.

Role of coating agent in iron oxide nanoparticle formation in an aqueous dispersion: Experiments and simulation.

Iron oxide (Fe3O4) nanoparticle was synthesized by coprecipitation and was modeled and solved using a hybrid (discrete-continuous) model, based on a kinetic Monte Carlo (kMC) simulation scheme. The latter was combined with the constant number MC method, to improve both speed and accuracy of the simulation. Complete particle size distribution (PSD) from simulation matches very well with PSD of both uncoated and coated (with either polyacrylic acid or dextran) Fe3O4 nanoparticles, obtained from our experiments. The model is general, as the time scales of various processes (nucleation, diffusion-growth and coagulation-growth) are incorporated in rate equations, while, input simulation parameters are experimentally measured quantities. With the help of the validated model, effect of coating agent on coagulation-growth was estimated by a single, fitted, coagulation-efficiency parameter. Our simulation shows that, logarithm of coagulation-efficiency scales linearly with logarithm of inverse of the molecular weight of the coating agent. With this scaling law, our model is able to a priori predict the experimental PSD of Fe3O4 nanoparticles, synthesized with an even higher molecular weight of dextran.

Impact of density of coating agent on antibacterial activity of silver nanoparticle impregnated plasma treated activated carbon

To use stabilized nanoparticles (NPs) in water as disinfectants over a very long period, the amount of coating agent (for NP stabilization) needs to be optimized. To this end, silver nanoparticles (Ag-NPs) with two different coating densities of tri-sodium citrate (12.05 and 46.17molecules/nm2, respectively), yet of very similar particle size (29 and 27 nm, respectively) were synthesized. Both sets of citrate capped NPs were then separately impregnated on plasma treated activated carbon (AC), with similar Ag loading of 0.8 and 0.82wt.%, respectively. On passing contaminated water (containing 104 CFU Escherichia coli/mL of water) through a continuous flow-column packed with Ag/AC, zero cell concentration was achieved in 22 and 39 min, with Ag-NPs (impregnated on AC, named as Ag/AC) having lower and higher coating density, respectively. Therefore, even on ensuring similar Ag-NP size and loading, there is a significant difference in antibacterial performance based on citrate coating density in Ag/AC. This is observed in lower coating density case, due to both: (i) higher Ag+ ion release from Ag-NP and (ii) stronger binding of individual Ag-NPs on AC. The latter ensures that, Ag-NP does not detach from the AC surface for a long duration. TGA-DSC shows that Ag-NPs with a low coating density bind to AC with 4.55 times higher adsorption energy, compared to Ag/AC with a high coating density, implying stronger binding. Therefore, coating density is an important parameter for achieving higher antibacterial efficacy, translating into a faster decontamination rate in experiments, over a long period of flow-column operation.

Synergistic antibacterial activity of a combination of silver and copper nanoparticle impregnated activated carbon for water disinfection

Silver and copper nanoparticles impregnated separately on plasma-treated activated carbon (AC) (named Ag-AC and Cu-AC, respectively), when mixed in an optimized 50 : 50 weight ratio, show synergistic water disinfection compared to only Ag-AC or only Cu-AC. Time-dependent cryo-SEM imaging and reactive oxygen species (ROS) measurement of E. coli prove that (in both batch mode and continuous flow column) Cu-AC forms pits faster on the cell membrane compared to Ag-AC and together with the latter facilitates faster cell death. The composite of Ag-AC and Cu-AC in batch mode showed a 4 log reduction of live E. coli cells in the treated water in 8 minutes, as confirmed by both plate count and fluorescence spectroscopy. Scaling up this result in a continuous flow column (of 8 cm diameter and 25 cm height, packed with the composite), one could achieve zero live cell concentration in only 7 minutes of contact time, translating into a very high flow rate of 5.32 L h−1 of treated water. Therefore, plate count, fluorescence spectroscopy, time-dependent ROS measurement and cryo-SEM imaging establish the mechanism of synergy and contact time required for cell death. The disinfection performance to zero live E. coli cells could be maintained with an even higher inlet E. coli concentration of 106 CFU ml−1 (albeit at a lower throughput from the column, as expected) and also non-stop for at least 5 days, demonstrating a potential long-term performance, with more than adequate Ag and Cu left in the column. Finally, the steady state concentrations of Ag and Cu in the treated water were 23 and 56.4 μg L−1, respectively, which are well within the permissible limits of 100 and 1000 μg L−1 for Ag and Cu, respectively. Therefore, the synergistic antibacterial activity of the composite resulted in production of decontaminated water at the highest throughput over a long time period.

Design of Ultrasensitive Protein Biosensor Strips for Selective Detection of Aromatic Contaminants in Environmental Wastewater

Phenol and its derivatives constitute a class of highly toxic xenobiotics that pollute both river and groundwater. Here, we use a highly stable enzyme-based in vitro biosensing scaffold to develop a chip-based environmental diagnostic for in situ accurate, direct detection of phenol with selectively down to 10 ppb. Mesoporous silica nanoparticles (MCM41) having a pore diameter of 6.5 nm was screened and found to be the optimal solid support for creation of a robust immobilized protein based sensor, which retains stability, enzyme activity, sensitivity, and selectivity at par with solution format. The sensor strip exhibits minimal cross reactivity in simulated wastewater, crowded with several common pollutants. Moreover, this design is competent towards detection of phenol content with 95% accuracy in real-time environmental samples collected from local surroundings, making it a viable candidate for commercialization. The enzyme has been further modified via evolution driven mutagenesis to generate an exclusive 2,3-dimethylphenol sensor with equivalent selectivity and sensitivity as the native phenol sensor. Thus, this approach can be extended to generate a battery of sensors for other priority aromatic pollutants, highlighting the versatility of the biosensor unit. This novel biosensor design presents promising potential for direct detection and can be integrated in a device format for on-site pollutant monitoring.