Department of Chemistry

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    Revealing the role of electrostatics in gold-nanoparticle-catalyzed reduction of charged substrates
    (ACS, 2017-09) Rao, Anish
    The potency of electrostatic effects arising from nanoparticle (NP) surface in Au-NP-catalyzed reduction of charged substrates are presented. The electrostatic potential around Au NPs is controlled by varying the nature of ligands and ionic strength of the medium. Favorable interactions arising from the attraction between oppositely charged Au NP and substrates results in the channeling of substrates to the NP surface, which in turn enhances the catalytic reduction. The positively charged ([+]) Au NP outperformed other NP systems despite having comparable or even lower surface area for adsorption, proving the exclusivity of electrostatics in catalysis. At least an order of magnitude higher concentration of negatively charged ([−]) Au NP is required to compete with the catalytic activity of [+] Au NP.
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    Emergence of selectivity in inherently nonselective gold nanoparticles through preferential breaking of interparticle interactions
    (2018-10) Rao, Anish
    We demonstrate a fundamentally unique identification strategy to impart selectivity to a traditionally and inherently nonselective carboxylate-functionalized gold-nanoparticles ([-] AuNPs), without the aid of any analyte specific ligands. The common practice is to use the ability of divalent ions to trigger the aggregation process in a kinetically trapped dispersed solution of [-] AuNPs. Aggregation of NPs being a thermodynamically favourable process will result in a uniform and nonselective turn-off response from most of the strongly binding divalent ions. Our approach is to use the abilities of various divalent ions to break a thermodynamically stable inter-nanoparticle precipitates containing [+] and [-] AuNPs (nanoionic precipitates), as the means of identification. Importantly both [+] and [-] AuNPs, independently, were ‘blind’ in terms of selectivity towards divalent ions. Remarkably, a hybrid-system composed of such nonselective nanoparticles was able to discriminate between the hard-to-distinguish pair of Pb2+ and Cd2+ ions. The rationale is that only the strongest of strongly binding ions will be able to break the interactions between the NP precipitates (thermodynamically stable state) and re-disperse them back in solution (kinetically trapped state). This is in stark contrast with the conventional idea of forming an interaction between NPs and divalent ions, with the help of analyte-specific ligands.
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    Fabrication of Au decorated MoS2 Langmuir Blodgett film as SERS sensing scaffold for detection of thiram in aqueous solution and in carrot peels
    (Elsevier, 2024-10) Pande, Surojit
    This paper is focused on simple fabrication of robust and reproducible SERS active substrate through self-assembly of gold nanoparticles (AuNps) entrapped in the Langmuir Blodgett (LB) film of MoS2 flakes. The as prepared Au-MoS2 substrate can detect SERS signals of 4-Mercapto Pyridine (4-Mpy) molecules at trace concentrations down to 10-12 M. The substrate has been further employed in detecting thiram (TH) both in aqueous solution and in carrot peels (CPls). The limit of detection (LOD) of TH in aqueous medium and in CPls have been estimated to be ∼ 5.18 and 10.16 pM respectively. We believe that the applicability of this SERS active substrate may be extended in the trace detections of explosives, drugs and in diagnostic applications.
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    Kinetics of Nanoparticle–Membrane Adhesion Mediated by Multivalent Interactions
    (ACS, 2019-01) Jana, Pritam Kumar
    Multivalent adhesive interactions mediated by a large number of ligands and receptors underpin many biological processes, including cell adhesion and the uptake of particles, viruses, parasites, and nanomedical vectors. In materials science, multivalent interactions between colloidal particles have enabled unprecedented control over the phase behavior of self-assembled materials. Theoretical and experimental studies have pinpointed the relationship between equilibrium states and microscopic system parameters such as the ligand–receptor binding strength and their density. In regimes of strong interactions, however, kinetic factors are expected to slow down equilibration and lead to the emergence of long-lived out-of-equilibrium states that may significantly influence the outcome of self-assembly experiments and the adhesion of particles to biological membranes. Here we experimentally investigate the kinetics of adhesion of nanoparticles to biomimetic lipid membranes. Multivalent interactions are reproduced by strongly interacting DNA constructs, playing the role of both ligands and receptors. The rate of nanoparticle adhesion is investigated as a function of the surface density of membrane-anchored receptors and the bulk concentration of nanoparticles and is observed to decrease substantially in regimes where the number of available receptors is limited compared to the overall number of ligands. We attribute such peculiar behavior to the rapid sequestration of available receptors after initial nanoparticle adsorption. The experimental trends and the proposed interpretation are supported by numerical simulations.
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    Gram Level Synthesis of Lead-Free Solder in the Nanometer Length Scale Obtained from Tin and Silver Compounds Using Silicone Oil
    (ACS, 2008-07-19) Pande, Surojit
    A straightforward route to gram level synthesis of a pure phase of the Sn−Ag nanoalloy in an eutectic composition (Sn/Ag 96.5:3.5) in silicone oil is reported. The composition, morphology, and microstructure of the alloy were fully characterized. In a mixture of ethylene glycol and silicone oil, direct reduction of Sn(II) acetate and Ag(I) nitrate gave the Sn−Ag nanoalloy. The nanoalloy disintegrates by sonication and reforms by heating, leading to smaller particles with a melting point as low as 128 °C.
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    Dendrimer-encapsulated nanoparticles: New synthetic and characterization methods and catalytic applications
    (RSC, 2011) Pande, Surojit
    In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized using a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently reduced chemically to yield nearly size-monodisperse particles having diameters in the 1–2 nm range. Monometallic, bimetallic (alloy and core@shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica, but also as a stabilizer for the nanoparticle. In this perspective, we report on progress in the synthesis, characterization, and applications of these materials since our last review in 2005. Significant advances in the synthesis of core@shell DENs, characterization, and applications to homogeneous and heterogeneous catalysis (including electrocatalysis) are emphasized.
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    Polymer with Competing Depolymerization Pathways: Chain Unzipping versus Chain Scission
    (ACS, 2020) Addy, Partha Sarathi
    Interest in triggered depolymerization is growing, driven by needs in sustainable plastics, self-healing materials, controlled release, and sensory amplification. For many triggered depolymerization reactions, the rate-limiting step does not directly involve the stimulus, and therefore, depolymerization kinetics exhibit only weak or no correlation to the concentration and reactivity of the stimulus. However, for many applications, a direct relationship between the stimulus and the depolymerization kinetics is desired. Here we designed, synthesized, and studied a polymer in which a nucleophile-induced chain scission (NICS) mechanism competes with the chain unzipping pathway. We find that the choice of the chain end functionality and the character of the nucleophile determines which of these is the predominant pathway. The NICS pathway was found to be dependent on the stimulus concentration, in contrast to the chain unzipping mechanism. We demonstrate transferability of these molecular-scale, structure–property relationships to nanoscale materials by formulating the polymers into host nanoparticles.
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    Gram Level Synthesis of Lead-Free Solder in the Nanometer Length Scale Obtained from Tin and Silver Compounds Using Silicone Oil
    (ACS, 2008) Basu, Mrinmoyee
    A straightforward route to gram level synthesis of a pure phase of the Sn−Ag nanoalloy in an eutectic composition (Sn/Ag 96.5:3.5) in silicone oil is reported. The composition, morphology, and microstructure of the alloy were fully characterized. In a mixture of ethylene glycol and silicone oil, direct reduction of Sn(II) acetate and Ag(I) nitrate gave the Sn−Ag nanoalloy. The nanoalloy disintegrates by sonication and reforms by heating, leading to smaller particles with a melting point as low as 128 °C.
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    A Green Chemistry Approach for the Synthesis of Flower-like Ag-Doped MnO2 Nanostructures Probed by Surface-Enhanced Raman Spectroscopy
    (ACS, 2009) Basu, Mrinmoyee
    Novel hierarchical flower-like nanostructures of Ag-doped MnO2 have been obtained by facile wet chemical and photochemical routes. UV−visible absorption spectroscopy measurement reveals that doping of Ag nanoparticles in MnO2 nanostructures leads to a red shift of the absorption edge and reduces the optical band gap energy from 2.68 to 2.51 eV while compared with undoped MnO2. Raman study reveals that the band broadens and shifts toward higher wavenumbers as the MnO6 octahedron is contorted by Ag doping and thus the loss of translational symmetry activates otherwise Raman-forbidden oxygen vibrations. Finally, SERS activity upsurges from Ag-doped MnO2 with Rhodamine 6G and 2-aminothiophenol as probe molecules.
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    Room Temperature Ferromagnetic Ni Nanocrystals: An Efficient Transition Metal Platform for Manifestation of Surface-Enhanced Raman Scattering
    (ACS, 2009) Basu, Mrinmoyee
    A simple solid-phase synthetic approach has been deliberately exploited for the synthesis of room temperature ferromagnetic, phase pure, fcc Ni nanocrystals on resin matrix. Self-assembly directed chainlike hierarchical nanostructures on the matrix could be engendered from magnetic dipole−dipole interaction between the nanocrystallites. Then, a practical virtue of the transition metal nanoparticle, Ni, was expressed from the rich and high-quality vibrational information of a chelating ligand, 1,10-phenanthroline (phen), onto the magnetically separated metal particles. Thus, surface-enhanced Raman scattering (SERS) has emerged exclusively from the time-dependent surface complexation of the chemically adhered probe molecule. Finally, kinetic effect has bestowed Ni(II)-phen chelate which later on demonstrates unique SERS activity on fcc Ni nanocrystals. The results provide a benchmark illustration of the value of transition metal for aiding interpretation of the vibrational signature of the adsorbate attainable from SERS studies.