Siddhartha Das | University of Dhaka, Bangladesh (original) (raw)

Papers by Siddhartha Das

Inorganic Chemistry, 2005

A series of complexes with the formula [Mn(III/IV)2(mu-O)2(L)2(X)2]3+ have been prepared in situ ... more A series of complexes with the formula [Mn(III/IV)2(mu-O)2(L)2(X)2]3+ have been prepared in situ from Mn(II)LCl2 precursors by a general preparative method (L = terpy, Cl-terpy, Br-terpy, Ph-terpy, tolyl-terpy, mesityl-terpy, t Bu3-terpy, EtO-terpy, py-phen, dpya, Me2N-terpy, or HO-terpy, and X = a labile ligand such as water, chloride, or sulfate). The parent complex, where L = terpy and X = water, is a functional model for the oxygen-evolving complex of photosystem II (Limburg, et al. J. Am. Chem. Soc. 2001, 123, 423-430). Crystals of Mn(II)(dpya)Cl2, Mn(II)(Ph-terpy)Cl2, Mn(II)(mesityl-terpy)Cl2, and an organic-soluble di-mu-oxo di-aqua dimanganese complex, [Mn(III/)(IV)2(mu-O)2(mesityl-terpy)2(OH2)2](NO3)3, were obtained and characterized by X-ray crystallography. Solutions of the in situ-formed di-mu-oxo dimanganese complexes were characterized by electrospray mass spectrometry, EPR spectroscopy, and UV-visible spectroscopy, and the rates of catalytic oxygen-evolving activity were assayed. The use of Mn(II)LCl2 precursors leads to higher product purity of the Mn dimers while achieving the 1:1 ligand to Mn stoichiometry appropriate for catalytic activity assay. These methods can be used to screen the catalytic activity of other di-mu-oxo dimanganese complexes generated by using a ligand library.

Science, 2006

Although enzymes often incorporate molecular recognition elements to orient substrates selectivel... more Although enzymes often incorporate molecular recognition elements to orient substrates selectively, such strategies are rarely achieved by synthetic catalysts. We combined molecular recognition through hydrogen bonding with C-H activation to obtain high-turnover catalytic regioselective functionalization of sp3 C-H bonds remote from the -COOH recognition group. The catalyst contains a Mn(mu-O)2Mn reactive center and a ligand based on Kemp&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s triacid that directs a -COOH group to anchor the carboxylic acid group of the substrate and thus modify the usual selectivity for oxidation. Control experiments supported the role of hydrogen bonding in orienting the substrate to achieve high selectivity.

Colloids and Surfaces B-biointerfaces, 2007

In this paper, a generalized surface-kinetics based model is developed to analytically investigat... more In this paper, a generalized surface-kinetics based model is developed to analytically investigate the influences of the substrate types and the buffer compositions on the macromolecular transport and hybridization in microfluidic channels, under electrokinetic influences. For specific illustration, three typical microchannel substrates, namely silanized glass, polycarbonate and PDMS, are considered, in order to obtain analytical expressions for their zeta potentials as a function of the buffer pH and the substrate compositions. The expressions for the zeta potential are subsequently employed to derive the respective velocity distributions, under the application of electric fields of identical strengths in all cases. It is also taken into consideration that the charged macromolecules introduced into these channels are subjected to electrophoretic influences on account of the applied electric fields. Closed form expressions are derived to predict the transport behaviour of the macromolecules and their subsequent hybridization characteristics. From the analysis presented, it is shown that the modification of the channel surface with silane-treatment becomes useful for enhancing the macromolecular transport and surface hybridization, only if the buffer pH permits a large surface charge density. The analytical solutions are also compared with full-scale numerical solutions of the coupled problem of fluid dynamic and macromolecular transport in presence of the pertinent surface reactions, in order to justify the effectiveness of closed-form expressions derived in this study. .in (S. Chakraborty). determining ions are the protons and the hydroxyl groups, and the ionic concentration is described by the pH (pH = −log[H + ]). With variations in the concentration of the charge-determining ions, the surface charge density may change from positive to negative or vise versa. The concentration of charge determining ions corresponding to a neutral or zero-charged surface is known as the point of zero charge (p.z.c.). For oxide surfaces, for instance, the surface is covered with negative charges (hydroxyl groups), in case pH > p.z.c. On the other hand, for pH < p.z.c., the surface gets positively charged. In the solution phase, both co-ions and counter-ions do exist, the distribution of which is determined by a combination of Columbic or electrostatic forces, entropic forces and the Brownian motion. As a result, the concentration of counter-ions is a maximum near the solid substrate and decreases progressively as one moves further away from the same, giving rise to the formation of an electric double layer (EDL) . In effect, a potential difference establishes across the EDL between the surface layer of immobile counter-ions and the electrolyte in the far-field, which is known as the zeta potential.

Sensors and Actuators B-chemical, 2006

The present study develops closed-form expressions depicting the rate of DNA hybridization in the... more The present study develops closed-form expressions depicting the rate of DNA hybridization in the presence of electroosmotic and pressure-driven flows in a microchannel. The model assumes a diffusion-reaction mechanism of DNA hybridization using second-order hybridization kinetics. Analytical solutions of the pertinent partial differential equation of species conservation are established under time dependant reactive boundary conditions at the channel walls, describing the concentration variation in the presence of both advection and diffusion fluxes. From the analytical solutions, it is revealed that a saturation state of the hybridization reaction is obtained faster in a pure electroosmotic flow, as compared to the case of a pressure-driven flow. A further assessment of the present solutions also reveals very good agreements with full-scale numerical solutions.

Analytica Chimica Acta, 2006

In this paper, analytical solutions are derived, describing the transport characteristics of a no... more In this paper, analytical solutions are derived, describing the transport characteristics of a non-Newtonian fluid flow in a rectangular microchannel, under the sole influence of electrokinetic forces. Apart from estimating the fully-developed velocity and temperature distributions, ...

Microfluidics and Nanofluidics, 2006

An integrated thermofluidic analysis of DNA hybridization, in the presence of combined electrokin... more An integrated thermofluidic analysis of DNA hybridization, in the presence of combined electrokinetically and/or pressure-driven microchannel flows, is presented in this work. A comprehensive model is developed that combines bulk and surface transport of momentum, heat and solute with the pertinent hybridization kinetics, in a detailed manner. Results confirm that electrokinetic accumulation of DNA occurs within a few seconds or minutes, as compared to passive hybridization that could sometimes take several hours. Further, it is observed that by increasing the accumulation time, significantly higher concentration of DNA can be achieved at the capture probes. However, this eventually tends to attain a saturation state, due to a lesser probability of successful hybridization on account of a prior accumulation of target DNA molecules on the capture probe strands. While favorable pressure gradients augment DNA hybridization rates that are otherwise established by the electro-osmotic transport, adverse pressure gradients of comparable magnitude may turn out to be much less consequential in retarding the same. Such effects can be of potential significance in the designing of a microfluidic arrangement to achieve the fastest rate of DNA hybridization.

Aiche Journal, 2007

The present study examines the modality, in which localized transverse electric fields can be suc... more The present study examines the modality, in which localized transverse electric fields can be successfully employed, to augment the rate of DNA hybridization at the capturing probes that are located further downstream relative to the inlet section of a rectangular microchannel. This is in accordance with an enhanced strength of convective transport that can be achieved, on account of increments in the wall zeta potential at the transverse electrode locations. In the present model, the overall convective transport, which is an implicit function of the magnitude and the location of the transverse electrical field being employed, is essentially coupled with the surface kinetics of the bare silica wall and also the kinetics that are involved in the dual mechanisms of DNA hybridization. Parameters that govern the overall transport phenomena, such as the pH of the inlet buffer, the length of the transverse electrodes, and the voltages at which these electrodes are maintained are critically examined, in an effort to obtain an optimized wall pH distribution, which in turn can ensure favorable DNA hybridization rates at the capturing probe locations. Practical constraints associated with the upper limits of the strength of the transverse electrical fields that can be employed are also critically analyzed, so as to ensure that an optimized rate of DNA hybridization can be achieved from the bio-microfluidic arrangement, without incurring any adverse effects associated with the overheating of the DNA molecules leading to their thermal denaturation. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Physical Review E, 2008

We pinpoint the limitations in traditional electroviscous analysis for narrow fluidic confinement... more We pinpoint the limitations in traditional electroviscous analysis for narrow fluidic confinements. We show that because of neglecting the convective transport of ions originated out of the established streaming field itself, the traditional approach may result in physically inconsistent flow rate predictions. We show that the larger the value of an ionic Peclet number and narrower the confinement, the more conspicuous are these erroneous predictions, within threshold limits of the nondimensional potential. We come up with an improved mathematical model to overcome such discrepancies.