Sonam Mandani

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.

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.

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.

Probing Carbocatalytic Activity of Carbon Nanodots for the Synthesis of Biologically Active Dihydro/Spiro/Glyco Quinazolinones and Aza-Michael Adducts

Herein, we report the fluorescent carbon dots as an effective and recyclable carbocatalyst for the generation of carbon-heteroatom bond leading to quinazolinone derivatives and aza-Michael adducts under mild reaction conditions. The results establish this nanoscale form of carbon as an alternative carbocatalyst for important acid catalyzed organic transformations. The mild surface acidity of carbon dots imparted by -COOH functionality could effectively catalyze the formation of synthetically challenging spiro/glycoquinazolinones under the present reaction conditions.

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.

Carbon Dots as Nanodispersants for Multiwalled Carbon Nanotubes: Reduced Cytotoxicity and Metal Nanoparticle Functionalization

The colloidal stabilization of multiwalled carbon nanotubes (MWCNTs) in an aqueous medium through noncovalent interactions has potential benefits toward the practical use of this one-dimensional carbonaceous material for biomedical applications. Here, we report that fluorescent carbon nanodots can efficiently function as dispersing agents in the preparation of stable aqueous suspensions of CNTs at significant concentrations (0.5 mg/mL). The amphiphilic nature of carbon dots with a hydrophobic graphitic core could effectively interact with the CNT surface, whereas hydrophilic oxygenated functionalization on the C-dot surface provided excellent water dispersibility. The resultant CNT-C-dot composite showed significantly reduced cytotoxicity compared to that of unmodified or protein-coated CNTs, as demonstrated by cell viability and proliferation assays. Furthermore, the reducing capability of C-dots could be envisaged toward the formation of a catalytically active metal nanoparticle-CNT-C-dot composite without the addition of any external reducing or stabilizing agents that showed excellent catalytic activity toward the reduction of p-nitrophenol in the presence of NaBH4. Overall, the present work establishes C-dots as an efficient stabilizer for aqueous dispersions of CNTs, leading to an all-carbon nanocomposite that can be useful for different practical applications.

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.