BITS Faculty Publications
Permanent URI for this communityhttp://localhost:4000/handle/123456789/1867
Browse
785 results
Search Results
Item Electrostatically driven resonance energy transfer in “cationic” biocompatible indium phosphide quantum dots(RSC, 2017-03) Rao, AnishIndium Phosphide Quantum Dots (InP QDs) have emerged as an alternative to toxic metal ion based QDs in nanobiotechnology. The ability to generate cationic surface charge, without compromising stability and biocompatibility, is essential in realizing the full potential of InP QDs in biological applications. We have addressed this challenge by developing a place exchange protocol for the preparation of cationic InP/ZnS QDs. The quaternary ammonium group provides the much required permanent positive charge and stability to InP/ZnS QDs in biofluids. The two important properties of QDs, namely bioimaging and light induced resonance energy transfer, are successfully demonstrated in cationic InP/ZnS QDs. The low cytotoxicity and stable photoluminescence of cationic InP/ZnS QDs inside cells make them ideal candidates as optical probes for cellular imaging. An efficient resonance energy transfer (E ∼ 60%) is observed, under physiological conditions, between the cationic InP/ZnS QD donor and anionic dye acceptor. A large bimolecular quenching constant along with a linear Stern–Volmer plot confirms the formation of a strong ground state complex between the cationic InP/ZnS QDs and the anionic dye. Control experiments prove the role of electrostatic attraction in driving the light induced interactions, which can rightfully form the basis for future nano-bio studies between cationic InP/ZnS QDs and anionic biomolecules.Item Revealing the role of electrostatics in gold-nanoparticle-catalyzed reduction of charged substrates(ACS, 2017-09) Rao, AnishThe 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.Item Electrostatically regulated photoinduced electron transfer in “cationic” eco-friendly CuInS2/ZnS quantum dots in water(2018) Rao, AnishThe potency of eco-friendly copper indium sulfide/zinc sulfide core/shell quantum dots (CIS/ZnS QDs) as efficient light harvesters in water is presented. A place exchange protocol is developed to prepare the much demanded cationic ([+]) CIS/ZnS QDs carrying a permanent positive charge, with ∼60% retention of the QD photoluminescence (PL) in water. Both steady-state and time-resolved photophysical studies confirm efficient electron transfer from the photoexcited CIS/ZnS QDs to indocyanine green (ICG) dye. The electrostatic attraction between the oppositely charged [+] CIS/ZnS QDs and [−] ICG dye is responsible for the formation of a strong ground state complex, which is vital for achieving an efficient electron transfer process in water. The successful demonstration of the efficient light harvesting properties using [+] CIS/ZnS QDs will be decisive in the development of artificial photosynthetic systems based on eco-friendly quantum dots.Item Emergence of selectivity in inherently nonselective gold nanoparticles through preferential breaking of interparticle interactions(2018-10) Rao, AnishWe 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.Item Precise nanoparticle–reactant interaction outplays ligand poisoning in visible-light photocatalysis(ACS, 2018-11) Rao, AnishThe ability to move electrons under the influence of visible-light in an efficient manner is one of the most fundamental challenges in photocatalysis. (1−6) The “hot” charge carriers in metal nanoparticles (NPs) have been shown to participate effectively in various reductive and oxidative photocatalytic chemical transformations. (7−14) During this process, the electrons and holes, oftentimes, have to encounter the “insulating” organic ligands capped on the NPs. (15−19) Generally, the surface ligand plays a crucial role in stabilizing the NPs as well as dictating its physicochemical properties. (20−23) However, for applications in photocatalysis, where the stability as well as the surface accessibility of NPs is desirable, the role of surface ligands is conflicting. In principle, the surface ligands can “poison” a photocatalyst by hindering the (i) movement of electrons/holes (due to its insulating nature) (15,16) and (ii) accessibility of the NP surface to the reactants (due to steric effect). (17−19) The alternative is to deposit NPs onto a support or use “ligand free” NPs for catalysis. (24−27) However, the available surface area and stability of NPs are compromised during the course of catalysis. (24) Thus, metal NPs and surface ligands are two inseparable entities, and strategies have to be developed to accomplish photocatalysis by retaining and taking advantage of the ligands on the NP surface. We address this challenge by using ligands that can enhance the NP catalyst–reactant interactions, which in turn can facilitate the electron transfer process. Our hypothesis was tested in the model photocatalytic reaction of ferricyanide reduction by gold nanoparticles (AuNPs) in the presence of ethanol as the hole scavenger. (6,28,29) A favorable interaction between NP catalyst and ferricyanide reactant was created through precise surface engineering, which resulted in the enhancement of the photocatalytic activities (both in terms of hot electron transfer rate constant and conversion yield). Cationic ([+]) and anionic ([−]) organic ligands were functionalized on AuNP surface to generate favorable and unfavorable interactions with [−] ferricyanide, respectively. Our studies show that the favorable interaction, arising from the strong electrostatic attraction, increases the local concentration of [−] ferricyanide around the [+] AuNP catalyst. Consequently, the NP accessibility and probability of hot electron injection from [+] AuNP to [−] ferricyanide was enhanced. On the other hand, the local concentration of the reactants and catalytic activities were lower when standard [−] AuNP was used as the catalyst. For instance, the rate constant increased from ∼8 × 10–4 to ∼4 × 10–3 min–1 (∼5-fold increment in reaction rate) when the NP–reactant interaction was made favorable, along with an appreciable increase in the ferricyanide conversion yield (from ∼10% for [−] AuNP to ∼60% for [+] AuNP). The dependence of catalytic activities on the NP surface potential ascertained the potency of electrostatics in enhancing the visible-light photocatalysis. Thermodynamic analysis based on Marcus model of outer sphere electron transfer revealed a higher pre-exponential factor (Φ) for [+] AuNP catalyst, a parameter directly related to the local concentration of reactants. Thus, the introduction of favorable interaction improves the NP accessibility to the reactants and the probability of hot electron transfer, thereby suppressing the “poisoning” effect of the “insulating” organic ligands.Item InP/ZnS quantum dots as efficient visible-light photocatalysts for redox and carbon–carbon coupling reactions(ACS, 2019-03) Rao, AnishEnergy research is enormously inspired by one of the most fascinating and elegant phenomena known to mankind, called photosynthesis. (1,2) The efficient harvesting of visible light and movement of electrons through a number of molecules and redox active metal centers (leading to new chemical bonds) is the heart of photosynthesis. (3,4) Understanding and mimicking of such processes in artificial systems is the central idea of solar to chemical energy conversion research, especially photocatalysis. (5−13) A diverse pool of catalytic supplies ranging from organic to inorganic to polymeric materials has been explored for harvesting photons and driving various chemical reactions. (5−13) Among them, semiconductor nanoparticles or quantum dots (QDs) have emerged strongly due to their high absorption extinction coefficient (∼106 M–1 cm–1) and electron–hole mobility, size- and shape-tunable band gap, photostability, and flexible surface chemistry. (14−25) A thorough review of the literature reveals that the common practice in the area of QD photocatalysis is to use them in combination with other catalytic materials (like metal ions/complexes, semiconductors, 2D materials, etc.). (26−29) Strikingly, recent reports have shown the sole use of QDs as photocatalyst for various reactions, including C–C bond formation, without the aid of any cocatalysts or sacrificial reagents. (30−35) To hold this promise on a longer perspective, these exciting results with toxic metal-ion-based QDs should be tested and demonstrated with more environmentally friendly QDs. Even though extensive studies were performed on the fundamental properties of environmentally friendly QDs (synthesis, surface engineering, imaging and biotargeting, energy/charge transfer processes, etc.), (36−43) the photocatalytic aspects of them are still at its infancy. (27,35,44,45) For instance, recent reports have used CuAlS2/ZnS QD for carbon dioxide reduction (35) and InP/ZnS QD (as a sensitizer of nickel complex) for photocatalytic production of hydrogen. (27) To this end, a successful demonstration of environmentally friendly QDs photocatalyzing different classes of chemical reactions will strengthen their claim of potential “greener” alternatives for toxic metal-ion-based QDs. In this regard, we explored the potency of InP/ZnS QD as a visible-light photocatalyst for mimicking the two key classes of reactions in photosynthesis, namely metal-centered redox and carbon–carbon bond forming reactionsItem Förster resonance energy transfer regulated multicolor photopatterning from single quantum dot nanohybrid filmsR(ACS, 2019-05) Rao, AnishPrecise patterning and localization of functional nanomaterials is the key step for miniaturization and building of optoelectronic devices. Present study utilizes a robust methodology for the multicolor patterning of luminescent Indium Phosphide/Zinc Sulfide Quantum Dot (InP/ZnS QD) film, by taking the advantage of QD’s enhanced photostability over organic dyes. The photoirradiation regulates the composition of donor–acceptor pair, and thereby, the efficiency of Förster Resonance Energy Transfer (EFRET) in QD–dye nanohybrid film. The photopatterned films are reusable over multiple cycles without any compromise of the color clarity, owing to the reversible switching between FRET ON and OFF states. The highlight of the present work is the use of a single QD nanohybrid system to create multicolor luminescent patterns; as opposed to the common practice of using different-colored QDs. FRET assisted photopatterning of luminescent InP/ZnS QD films provides a fundamentally unique and cost-effective approach for the manufacturing of luminescent optoelectronic devices.Item Turn-on selectivity in inherently nonselective gold nanoparticles for Pb2+ detection by preferential breaking of interparticle interactions(ACS, 2019-08) Rao, AnishEstablishing a “precise” control over different interparticle interactions holds the promise of introducing inherently absent properties to nanosystems. In this direction, our aim is to introduce the notion of selectivity in inherently nonselective carboxylate-functionalized gold-nanoparticles ([−] AuNPs) toward strongly binding divalent metal ions (M2+). The common practice is to use the ability of M2+ ions to trigger the aggregation process in a kinetically trapped dispersed solution of [−] AuNPs. Aggregation of NPs being a thermodynamically favorable process will result in a uniform and nonselective turn-off response from most of the strongly binding divalent ions with [−] AuNPs. Our approach for identification is to use the preferential abilities of various M2+ ions to break a thermodynamically stable inter-nanoparticle precipitate containing [+] and [−] AuNPs (nanoionic precipitates). Importantly both [+] and [−] AuNPs, independently, are “blind” in terms of selectivity toward divalent ions. Remarkably, a hybrid system composed of such nonselective nanoparticles can discriminate between the hard-to-distinguish pair of Pb2+ and Cd2+ ions. Among different ions tested, Pb2+ can break the electrostatic interactions in [+]–[−] Au nanoionic precipitates and displace [+] AuNP to solution, thereby turning on the plasmonic wine-red color. The dominance of interaction energy for [−] AuNP–Pb2+ complexation over the inter-nanoparticle interactions is accountable for the selective discrimination of Pb2+ from other M2+ ions. A precise variation in strengths of different interparticle interactions helped in tuning both the selectivity and sensitivity of our identification protocol.Item Influence of Shear Core Curtailment on the Structural Response of Core-Wall Structures(Springer, 2018-08) Muthukumar, G.; Kumar, manojShear walls and shear cores are the major lateral load resisting elements in multi-storeyed framed structures because of their proven track record in mitigating the damage under severe earthquake ground motions including the recent ones. The contrasting deflected profiles of shear core and frame actually help in reducing the drift of the structure. The reduction is primarily attributed to the level of horizontal interaction between cores and frames. The present study aims to simulate the horizontal interaction between core wall and frames using the dimensionless parameter () capable of characterizing the individual behaviors of the components of the dual system using the finite element analysis with different levels of curtailment of shear core. The triangular loading has been considered, and the computations have been made considering the site located in zone 4 with medium soil characteristics. To this end, in order to identify the optimum level of curtailment, an analytical study has been done on some shear core buildings with different levels of curtailment of shear core. The structural performance of the buildings with different levels of curtailment has been assessed for different parameters, namely, storey drift, deflection, bending moment, and shear forces using finite element modeling and analysisItem Development of Copper Cobalt Sulfide with Cu : Co Ratio Variation on Carbon Cloth as an Efficient Electrode Material for the Oxygen Evolution Reaction(Wiley, 2019-10) Pande, SurojitWhen designing a device for energy conversion, development of a highly active catalyst for OER (oxygen evolution reaction) reaction in alkaline medium is of utmost importance. For energy conversion, herein, we developed copper cobalt sulfide on carbon cloth (CC) using a simple and facile hydrothermal method. A vertically grown 2D sheet-like structure of copper-cobalt sulfide is observed using L-cysteine as a sulfur source. An optimized ratio of copper(II) and cobalt(III) and the best sulfur source are found to boost the electrocatalytic activity of mixed metal thiospinel for OER. Several characterization techniques (XRD, Raman, FESEM, TEM, XPS, and ICP-AES) were used to confirm phase purity, morphology, and composition of copper cobalt sulfides. We have established that 2D nanosheets of CuCo3Sz/CC exhibited superior electrocatalytic performance over bare RuO2 in 1.0 M KOH solution. It is observed that CuCo3Sz/CC need only 105 mV overpotential to generate 5 mA/cm2 current density. CuCo3Sz/CC is very stable and able to produce unaltered current density under an applied potential of 1.615 V vs. RHE for 48 h. Electrochemically active surface area calculation is performed for all catalysts to confirm higher electrocatalytic activity. The catalyst, CuCo3Sz/CC is used in a broad pH region using 0.5 M KOH, saline water, and 0.5 M Na2SO4 electrolyte solution to widen the applicability of water splitting. This work shows an avenue to develop active and durable electrocatalyst for OER without using any noble metal electrocatalyst.