Irshad Hussain

Photoreductive synthesis of water-soluble fluorescent metal nanoclusters

Water-soluble fluorescent copper, silver and gold nanoclusters with quantum yields of 2.2, 6.8 and 5.3%, respectively, are prepared by a robust photoreduction of their inorganic precursors in the presence of poly (methacrylic acid) functionalized with pentaerythritol tetrakis 3-mercaptopropionate.

Multifunctional microporous organic polymers

Functional microporous organic polymers (MOPs) are attractive in a wide range of applications including gas separation, catalysis, and energy storage. There is a lack of cost-effective processes to produce functional MOPs at industrial scale, which in fact limits their practical applications. Here, we propose a new low-cost strategy, based on the Scholl reaction, which can directly link rigid building blocks to obtain MOPs with high surface area and highly microporous structures. More importantly, this method is suitable for various building blocks and can be used as a general bottom-up approach to produce a variety of multifunctional MOPs. Multifaceted applications of these materials are also demonstrated by their large gas storage capacity, high catalytic activity, luminescence and semiconducting properties.

Controlled Step Growth of Molecularly Linked Gold Nanoparticles: From Metallic Monomers to Dimers to Polymeric Nanoparticle Chains.

The solution-phase assembly of 15 nm gold particles into relatively linear chains of fairly controllable length of up to 1 mum is achieved by molecularly linking nanoparticles with alkanedithiols. This step-growth process can be controlled to prepare dimers, oligomers, and polymer-like gold nanoparticle chains by varying the ratio of alkanedithiols to nanoparticles. These size-controlled, relatively linear aggregates remain suspended in ethanol solution without precipitation for several weeks to months depending on the chain length. The resulting soluble nanoparticle assemblies were characterized by a variety of techniques including cryogenic transmission electron microscopy. The surface plasmon coupling of regularly spaced gold nanoparticles in these chains could be of interest in the fabrication of optical waveguide and nanoelectronic systems.

Protein-mediated synthesis, pH-induced reversible agglomeration, toxicity and cellular interaction of silver nanoparticles

Casein, a milk protein, is used to produce biotolerable and highly stable silver nanoparticles with a fair control over their size without using any additional reducing agent. These silver nanoparticles undergo reversible agglomeration to form protein-silver nanoparticle composite agglomerates as pH approaches to the isoelectric point of casein protein (pI=4.6). These agglomerates can then easily be re-dispersed in alkaline aqueous media with no obvious change in their optical properties. The nanoparticles can withstand high salt concentration (~0.5M), and can also be freeze-dried, stored as dry powder and then dispersed in aqueous media whenever required. More interestingly, by controlling the concentration of casein protein and pH, it was also possible to control the self-assembly of silver nanoparticles to produce fairly uniform spherical agglomerates. The nanoparticles and their agglomerates were thoroughly characterized using UV-visible and FTIR spectroscopy, TEM, SEM and DLS, etc. Cytotoxicity of the hybrid materials was examined using a Resazurin based cytotoxicity assay. After determining the LD(50) using NIH/3T3 fibroblast cells, the cellular interaction of these hybrid nanoparticles was studied to examine the behavior of casein-coated nanoparticles for their potential bio-applications.

Highly water-soluble magnetic iron oxide (Fe3O4) nanoparticles for drug delivery: enhanced in vitro therapeutic efficacy of doxorubicin and MION conjugates

We report a simple one step protocol for the preparation of fairly monodisperse and highly water-soluble magnetic iron oxide nanoparticles (MIONs) through a co-precipitation method using a novel multifunctional, biocompatible and water-soluble polymer ligand dodecanethiol-polymethacrylic acid (DDT-PMAA). DDT-PMAA owing to its several intrinsic properties, not only efficiently controls the size of the MIONs but also gives them excellent water solubility, long time stability against aggregation and oxidation, biocompatibility and multifunctional surface rich in thioether and carboxylic acid groups. The molecular weight and concentration of the polymer ligand were optimized to produce ultrasmall (4.6 ± 0.7 nm) MIONs with high magnetization (50 emu g-1). The MIONs obtained with 1.5 mM DDT-PMAA (5330 g mol-1) are highly stable in solution as well as in dry powder form for an extended period of time. These MIONs show a high degree of monodispersity and are superparamagnetic at room temperature. The polymer ligand and MIONs@Polymer were characterized by GPC, 1H NMR, DLS, TEM, FTIR-Raman, XRD, TGA and VSM. In order to demonstrate the bio-applications of these magnetic nanoparticles (NPs), their toxicity was determined by MTT assay and they were found to be non-toxic and biocompatible. Finally, MIONs were conjugated with the anti-cancer drug doxorubicin (DOX) and its efficacy, as a model drug delivery system, was determined using HepG2 cells. The efficiency of the drug-NP conjugates i.e., covalently bound DOX-MIONs and electrostatically loaded DOX/MIONs, was found to be significantly higher than that of the free drug (DOX).

Freeze‐Align and Heat‐Fuse: Microwires and Networks from Nanoparticle Suspensions

The use of a simple freezing technique to produce microwires from a wide range of materials is demonstrated. Freezing has been used previously to prepare porous materials with new structures and useful properties. For example, Deville et al. have shown how to replicate the nacre structure of shells by using a simple freezing path. Also, highly porous fibers were synthesized by electrospinning into a cryogenic liquid. Hierarchical biohybrid materials were prepared by an icesegregation-induced self-assembly process. We have demonstrated the synthesis of three-dimensional aligned porous materials by directional freezing of aqueous/organic solutions and emulsions, and liquid CO2 solutions. [8] Microwires can be produced by electrospinning but this requires the use of soluble or fusible polymers in solution. Silicon and silica microwires were synthesized using a vapor– liquid–solid process. In addition, capillaries, fungi, and porous membranes/plates have all been used as templates to produce microwires. A self-seeding technique was adopted to prepare single-crystal polythiophene microwires on a silicon substrate, and a photolithographic route was applied to fabricate micro/nanowires of semiconductors. An external perpendicular magnetic field was applied to a CoPt3-nanocrystal sol to drive the formation of microwire structures. Electrically functional microwires were also assembled from metallic nanoparticles by dielectrophoresis. Herein, we show that directional freezing is a simple and flexible method that can be applied to a range of particulate materials (metal nanoparticles, metal-oxide nanoparticles, polymer colloids) with diameters ranging from 10 to 500 nm. The scope of the approach encompasses materials such as metals, metal oxides, and polymers. The method is also easy to scale up for bulk-quantity microwire synthesis, and the aggregated nanoparticles may in some cases be fused together by heat treatment. To produce gold microwires, a concentrated suspension of gold nanoparticles (GNPs; 15 nm in diameter) was injected dropwise into liquid nitrogen. The GNP sol droplets were observed to float for a few seconds at the liquid surface before freezing and sinking into the liquid nitrogen. Porous bead structures were produced after freeze-drying. Figure 1a shows the porous structure of a sectioned surface of a single gold-microwire bead. The bead consists of a randomly oriented porous shell surrounding radially aligned microwires, focusing at the center of the bead.

Novel POSS-based organic–inorganic hybrid porous materials by low cost strategies

Two kinds of POSS-based organic–inorganic hybrid porous materials have been synthesized via Friedel–Crafts and Scholl coupling reactions, for the first time, using low-cost building blocks i.e., octaphenylsilsesquioxanes and simple knitting approaches to obtain high Brunauer–Emmett–Teller (BET) surface area porous polyhedral oligomeric silsesquioxane (POSS)-based hybrid materials. N2 sorption isotherms of the polymers show that both these materials are predominantly microporous and mesoporous with BET surface areas of 795 m2 g−1 for the polymer of octaphenylsilsesquioxanes-1 (POPS-1) and 472 m2 g−1 for the polymer of octaphenylsilsesquioxanes-2 (POPS-2). Moreover, POPS-1 can reversibly adsorb 9.73 wt% CO2 (1 bar and 273 K) and 0.89 wt% H2 (1.13 bar and 77 K), and POPS-2 shows moderate gas uptake with 8.12 wt% CO2 (1 bar and 273 K) and 0.64 wt% H2 (1.13 bar and 77 K). In addition, the structural integrity of POSS based building blocks was completely preserved under relatively strong acidic conditions.

A scalable synthesis of highly stable and water dispersible Ag44(SR)30 nanoclusters

We report the synthesis of atomically monodisperse thiol-protected silver nanoclusters [Ag44(SR)30 ]m, (SR = 5-mercapto-2-nitrobenzoic acid) in which the product nanocluster is highly stable in contrast to previous preparation methods. The method is one-pot, scalable, and produces nanoclusters that are stable in aqueous solution for at least 9 months at room temperature under ambient conditions, with very little degradation to their unique UV-Vis optical absorption spectrum. The composition, size, and monodispersity were determined by electrospray ionization mass spectrometry and analytical ultracentrifugation. The produced nanoclusters are likely to be in a superatom charge-state of m = 4−, due to the fact that their optical absorption spectrum shares most of the unique features of the intense and broadly absorbing nanoparticles identified as [Ag44(SR)30]4− by Harkness et al. (Nanoscale, 2012, 4, 4269). A protocol to transfer the nanoclusters to organic solvents is also described. Using the disperse nanoclusters in organic media, we fabricated solid-state films of [Ag44(SR)30]m that retained all the distinct features of the optical absorption spectrum of the nanoclusters in solution. The films were studied by X-ray diffraction and photoelectron spectroscopy in order to investigate their crystallinity, atomic composition and valence band structure. The stability, scalability, and the film fabrication method demonstrated in this work pave the way towards the crystallization of [Ag44(SR)30]m and its full structural determination by single crystal X-ray diffraction. Moreover, due to their unique and attractive optical properties with multiple optical transitions, we anticipate these clusters to find practical applications in light-harvesting, such as photovoltaics and photocatalysis, which have been hindered so far by the instability of previous generations of the cluster.

Electron microscopy studies of the thermal stability of gold nanoparticle arrays

A series of monolayer protected gold nanoparticle colloidal solutions have been prepared with average sizes in the 2–15nm range. If a drop of such a colloidal suspension is deposited onto a Si3N4 substrate and the solvent allowed to evaporate, the particles have a tendency to self-assemble into monolayer rafts with varying degrees of structural order depending on the initial mono-dispersity of the particles. The thermal stability of these selfassembled gold nanoparticle rafts as a function of particle size, heating method, heating rate and ligand identity have been assessed in this study. In-situ TEM studies show that sub-8nm Au nanoparticles on Si3N4 have a tendency to coarsen upon slow heating, whereas those comprised of larger particles exhibit densification. Increasing the heating rate for the smaller particles promoted densification, forcing them to form highly interconnected string-like structures. Finally, rafts of sub-4nm alkanethiol protected Au nanoparticles are shown to sinter spontaneously under ambient conditions at room temperature on the timescale of several months. This unexpected effect may have important implications for the long term structural stability of any device constructed from sub-4nm gold nanoparticles.

Control of Surface Tension at Liquid−Liquid Interfaces Using Nanoparticles and Nanoparticle−Protein Complexes

Subtle changes in the monolayer structure of nanoparticles (NPs) influence the interfacial behavior of both NPs and NP-protein conjugates. In this study, we use a series of monolayer-protected gold NPs to explore the role of particle hydrophobicity on their dynamic behavior at the toluene-water interface. Using dynamic surface tension measurements, we observed a linear decrease in the meso-equilibrium surface tension (γ) and faster dynamics as the hydrophobicity of the ligands increases. Further modulation of γ is observed for the corresponding NP-protein complexes at the charge-neutralization point.