Tridib K. Sarma

Biogenic Growth of Alloys and Core-Shell Nanostructures Using Urease as a Nanoreactor at Ambient Conditions

Biomineralization is an extremely efficient biologically guided process towards the advancement of nano-bio integrated materials. As a prime module of the natural world, enzymes are expected to play a major role in biogenic growth of inorganic nanostructures. Although there have been developments in designing enzyme-responsive nanoparticle systems or generation of inorganic nanostructures in an enzyme-stimulated environment, reports regarding action of enzymes as reducing agents themselves for the growth of inorganic nanoparticles still remains elusive. Here we present a mechanistic investigation towards the synthesis of metal and metallic alloy nanoparticles using a commonly investigated enzyme, Jack bean urease (JBU), as a reducing as well as stabilizing agent under physiological conditions. The catalytic functionality of urease was taken advantage of towards the development of metal-ZnO core-shell nanocomposites, making urease an ideal bionanoreactor for synthesizing higher order nanostructures such as alloys and core- shell under ambient conditions.

Self-assembled monolayer coated gold-nanoparticle catalyzed aerobic oxidation of α-hydroxy ketones in water: an efficient one-pot synthesis of quinoxaline derivatives

For the first time, 4-aminothiophenol self-assembled monolayer-coated gold-nanoparticles (Au-NPs) which catalyze the aerobic oxidation of aryl substituted α-hydroxy ketones to aryl 1,2-diketones are reported. In addition, a one-pot synthesis of quinoxalines has been successfully achieved via in situ oxidation of α-hydroxy ketones and subsequent condensation with aryl 1,2-diamines in water. This method offers the potential for simple self-assembled monolayer-coated Au-NPs to exhibit catalytic activity for the aerobic oxidation reaction in a green and efficient manner.

Ultrasound mediated synthesis of α-aminophosphonates and 3,4-dihydropyrimidin-2-ones using graphene oxide as a recyclable catalyst under solvent-free conditions

Graphene oxide is utilized as an environmentally friendly and efficient catalyst for an ultrasound mediated multi-component coupling reaction that involves aldehyde, amine/ethylacetoacetate and diethyl phosphite/urea, under solvent-free conditions, leading to the bioactive α-aminophosphonates and 3,4-dihydropyrimidin-2-ones with excellent yield.

Enzymes as bionanoreactors: glucose oxidase for the synthesis of catalytic Au nanoparticles and Au nanoparticle–polyaniline nanocomposites

The use of biomaterials such as enzymes for the synthesis of functional materials is important because such biologically guided processes can significantly reduce energy consumption in manufacturing processes. Glucose oxidase (GOx) has been exploited as a reducing as well as a stabilising agent for the green chemical synthesis of Au nanoparticles at pH 7.0 under ambient conditions. The synthesized Au nanoparticle-GOx composite was found to act as a highly-effective catalyst towards the reduction of p-nitrophenol to p-aminophenol in the presence of NaBH4. The catalytic activity of GOx was largely inhibited after its participation in the reduction of metal salt to form nanoparticles. A detailed mechanistic investigation was carried out using fluorescence spectroscopy, Fourier transform infra-red (FTIR) spectroscopy and circular dichroism (CD) to gain insights into the conformational changes in the enzyme structure. The catalytic activity of GOx towards the oxidation of glucose was taken advantage of to form a Au nanoparticle-polyaniline (Au NP-PANI) composite at room temperature. The production of the green emeraldine salt form of polyaniline (PANI) was extremely low in case of Au nanoparticle-GOx composite, where GOx was involved as a reducing agent. However, H2O2 generated during the catalytic reaction of GOx acted as a simultaneous reducing and oxidizing agent, leading to the formation of an Au NP-PANI core-shell composite in a controlled fashion.

Gold Nanoparticle–Polydopamine–Reduced Graphene Oxide Ternary Nanocomposite as an Efficient Catalyst for Selective Oxidation of Benzylic C(sp3)−H Bonds Under Mild Conditions

A ternary nanocomposite comprising of Au nanoparticles (NPs), polydopamine, and reduced graphene oxide exhibits excellent activity and selectivity towards the oxidation of C−H bonds in benzylic hydrocarbons under mild conditions in the presence of N‐hydroxyphthalimide. All the components in the nanocomposite play an important role in the effectiveness of the catalyst. Sufficient electron transfer from polydopamine to the Au NPs afforded a more negatively charged nanoparticle surface and was favorable for molecular oxygen activation, leading to C−H bond oxidation. The reaction followed a free radical pathway as shown by detailed mechanistic studies. Further, easy separation and excellent reusability without significant loss in activity over several iterations make the ternary nanocomposite an excellent heterogeneous catalyst for C−H oxidation reactions.

Coordination polymer hydrogels through Ag(I)-mediated spontaneous self-assembly of unsubstituted nucleobases and their antimicrobial activity

Spontaneous formation of metallogels through the self-assembly of unsubstituted nucleobases with Ag(I) ions is reported. Whereas adenine, cytosine, thymine and uracil form metallogels under deprotonated conditions with nanofibrillar morphology, gelation of guanine with Ag(I) occurs only under acidic conditions through the self-organization of nanoscale metal–organic particles. In situ reduction of Ag salts occurs in Ag–pyrimidine gels to yield Ag nanoparticles decorated on the gel nanofibers. The Ag–nucleobase hydrogels showed excellent antimicrobial properties against both Gram positive as well as Gram negative bacteria.

Chirality control of multi-stimuli responsive and self-healing supramolecular metallo-hydrogels

A metallo-hydrogel based on histidine and Zn2+ ions is designed using coordination driven self-assembly. The hydrogel formation is instantaneous and it exhibits stimuli-responsive behaviour with respect to pH, heat and external chemicals. The gelation ability depends on the optical purity and enantiomeric excess of the amino acids. The supramolecular chirality mediated by the Zn–histidine complex and subsequent fiber formation showed sign inversion of the CD signal with respect to that of native amino acids (L- or D-) and can be tuned by changing the ratio of the two chiral species, L-, D-amino acids. The Jobs plot from the circular dichroism probing supramolecular chirality confirms the 1 : 1 complex of Zn and histidine. The viscoelastic properties and kinetics of the hydrogel can conveniently be tuned by changing its enantiomeric excess (ee%), concentration and the ratio of Zn and histidine. Moreover, the resulting metallo-hydrogel is thixotropic and exhibits instantaneous, intrinsic self-healing behaviour.

Catalytic activity of various pepsin reduced Au nanostructures towards reduction of nitroarenes and resazurin

Pepsin, a digestive protease enzyme, could function as a reducing as well as stabilizing agent for the synthesis of Au nanostructures of various size and shape under different reaction conditions. The simple tuning of the pH of the reaction medium led to the formation of spherical Au nanoparticles, anisotropic Au nanostructures such as triangles, hexagons, etc., as well as ultra small fluorescent Au nanoclusters. The activity of the enzyme was significantly inhibited after its participation in the formation of Au nanoparticles due to conformational changes in the native structure of the enzyme which was studied by fluorescence, circular dichroism (CD), and infra red spectroscopy. However, the Au nanoparticle-enzyme composites served as excellent catalyst for the reduction of p-nitrophenol and resazurin, with the catalytic activity varying with size and shape of the nanoparticles. The presence of pepsin as the surface stabilizer played a crucial role in the activity of the Au nanoparticles as reduction catalysts, as the approach of the reacting molecules to the nanoparticle surface was actively controlled by the stabilizing enzyme.

White light emission by controlled mixing of carbon dots and rhodamine B for applications in optical thermometry and selective Fe3+ detection

A simple mixing of rhodamine B with fluorescent carbon dots in water led to aggregation of the dye molecules on the carbon dot surface. Controlling the emission of free rhodamine B dye with that of the resultant carbon dot-aggregated rhodamine B composite resulted in efficient white light emission with the CIE coordinate (0.33, 0.32). The white light emitting system can be incorporated into a gel or polymer matrix for solid-state processibility. Further, selective sensing of Fe3+ ions as well as reversible and thermo-responsive emission in the temperature range of 25–80 °C in water shows the versatility in application potential of the nanocomposite.

Zn(II)-nucleobase metal-organic nanofibers and nanoflowers: synthesis and photocatalytic application

The formation of hybrid metal–organic nanoscale materials using basic biological building blocks is highly appealing due to the rich functional group chemistry as well as green and benign environment offered by biomolecules which can significantly reduce the energy consumption in manufacturing processes. In this contribution, the spontaneous formation of crystalline hybrid organic–inorganic nanoscale materials such as nanofibers and nanoflowers through the supramolecular interaction of Zn2+ and unsubstituted nucleobases is reported. The interaction of cytosine and guanine with Zn2+ ions in alkaline medium led to the formation of metallogels with nanofibrous morphology. Surprisingly, the addition of Zn2+ ions to a solution containing a mixture of cytosine and guanine led to the formation of flower shaped crystalline coordination polymer particles. The catalytic activity of the three materials has been exploited for the photocatalytic degradation of environmental pollutant organic dyes such as methylene blue and methyl orange.