Somnath Bhattacharjee

Nanoparticle-Containing Membranes for the Catalytic Reduction of Nitroaromatic Compounds

Layer-by-layer deposition of polyelectrolyte/metal nanoparticle films in porous alumina, track-etched polycarbonate, and nylon substrates yields catalytic membranes. With all three substrates, scanning electron microcopy images demonstrate a high density of well-separated nanoparticles in the membrane pores. These nanoparticles catalyze the reduction of nitroaromatic compounds by sodium borohydride with rate constants that are the same as those for nanoparticles immobilized on alumina powder. Moreover, the membranes selectively catalyze the reduction of nitro groups in compounds containing other reducible functionalities such as cyano, chloro, and styrenyl moieties. With nitrophenols and nitroanilines, the only reduction product is the corresponding amine. In contrast, nitrobenzene, nitrotoluenes, nitrobenzonitriles, chloronitrobenzenes, and m-nitrostyrene also form a nitroso product. Membrane catalysts are particularly attractive for controlling product distributions through variation of solution fluxes, as demonstrated by the formation of increased levels of nitroso compounds at high flux.

Selectivity as a Function of Nanoparticle Size in the Catalytic Hydrogenation of Unsaturated Alcohols

Layer-by-layer adsorption of poly(acrylic acid)-Pd(II) complexes and poly(ethylenimine) on alumina powder followed by reduction of Pd(II) with NaBH(4) yields Pd-nanoparticle catalysts embedded in multilayer polyelectrolyte films. The use of different ratios of poly(acrylic acid) to Pd(II) in deposition solutions gives a series of films with Pd nanoparticles whose average diameters range from 2.2 to 3.4 nm, and the catalytic selectivities of these nanoparticles vary dramatically with their size. Turnover frequencies (TOFs) for the hydrogenation of monosubstituted unsaturated alcohols increase with decreasing average nanoparticle size, whereas multisubstituted unsaturated alcohols show the opposite trend. Hence, the ratio of TOFs for the hydrogenation of allyl alcohol and crotyl alcohol is 39 with average particle diameters of 2.2 nm and only 1.3 with average particle diameters of 3.4 nm. Ratios of TOFs for hydrogenation of allyl alcohol and beta-methallyl alcohol are as high as 240 with the smallest nanoparticles, but substantial isomerization of beta-methallyl alcohol complicates this comparison. Increasing selectivity with decreasing average particle size occurs with both films deposited on alumina powder and nanoparticles stabilized by polyelectrolytes in solution. Presumably, high selectivities occur on the smallest nanoparticles because the active sites on the smallest Pd nanoparticles are less available for binding and hydrogenation of multisubstituted double bonds than are active sites on larger particles.

Ring-Opening Polymerization of Prodrugs: A Versatile Approach to Prepare Well-Defined Drug-Loaded Nanoparticles†

The synthesis of polymer-drug conjugates from prodrug monomers consisting of a cyclic polymerizable group that is appended to a drug through a cleavable linker is achieved by organocatalyzed ring-opening polymerization. The monomers polymerize into well-defined polymer prodrugs that are designed to self-assemble into nanoparticles and release the drug in response to a physiologically relevant stimulus. This method is compatible with structurally diverse drugs and allows different drugs to be copolymerized with quantitative conversion of the monomers. The drug loading can be controlled by adjusting the monomer(s)/initiator feed ratio and drug release can be encoded into the polymer by the choice of linker. Initiating these monomers from a poly(ethylene glycol) macroinitiator results in amphiphilic diblock copolymers that spontaneously self-assemble into micelles with a long plasma circulation, which is useful for systemic therapy.

Selective hydrogenation of monosubstituted alkenes by Pd nanoparticles embedded in polyelectrolyte films.

Pd nanoparticles embedded in multilayer polyelectrolyte films can be easily prepared through layer-by-layer adsorption of poly(acrylic acid) (PAA) and poly(ethyleneimine)-Pd2+ (PEI-Pd(II)) complexes followed by reduction of Pd(II) with NaBH4. Transmission electron microscopy confirms the formation of Pd particles with diameters of 1-3 nm. Remarkably, [PAA/PEI-Pd(0)]3PAA films catalyze the hydrogenation of monosubstituted alkenes with turnover frequencies that are as much as 100-fold higher than turnover frequencies for hydrogenation of multiply substituted double bonds. Selectivities in the hydrogenation of monosubstituted over multisubstituted double bonds are higher than those of Wilkinsons catalyst. Moreover, the turnover frequency for the hydrogenation of allyl alcohol did not change when the catalyst was recycled three times. Intramolecular selectivity for the hydrogenation of monosubstituted alkenes also occurs when substrate molecules contain both mono and multiply substituted double bonds. The combination of the encapsulating polyelectrolyte film and small nanoparticles apparently results in hindered access of multiply substituted double bonds to catalytic sites.

Formation of high-capacity protein-adsorbing membranes through simple adsorption of poly(acrylic acid)-containing films at low pH.

Layer-by-layer polyelectrolyte adsorption is a simple, convenient method for introducing ion-exchange sites in porous membranes. This study demonstrates that adsorption of poly(acrylic acid) (PAA)-containing films at pH 3 rather than pH 5 increases the protein-binding capacity of such polyelectrolyte-modified membranes 3-6-fold. The low adsorption pH generates a high density of -COOH groups that function as either ion-exchange sites or points for covalent immobilization of metal-ion complexes that selectively bind tagged proteins. When functionalized with nitrilotriacetate (NTA)-Ni(2+) complexes, membranes containing PAA/polyethylenimine (PEI)/PAA films bind 93 mg of histidine(6)-tagged (His-tagged) ubiquitin per cm(3) of membrane. Additionally these membranes isolate His-tagged COP9 signalosome complex subunit 8 from cell extracts and show >90% recovery of His-tagged ubiquitin. Although modification with polyelectrolyte films occurs by simply passing polyelectrolyte solutions through the membrane for as little as 5 min, with low-pH deposition the protein binding capacities of such membranes are as high as for membranes modified with polymer brushes and 2-3-fold higher than for commercially available immobilized metal affinity chromatography (IMAC) resins. Moreover, the buffer permeabilities of polyelectrolyte-modified membranes that bind His-tagged protein are ~30% of the corresponding permeabilities of unmodified membranes, so protein capture can occur rapidly with low-pressure drops. Even at a solution linear velocity of 570 cm/h, membranes modified with PAA/PEI/PAA exhibit a lysozyme dynamic binding capacity (capacity at 10% breakthrough) of ~40 mg/cm(3). Preliminary studies suggest that these membranes are stable under depyrogenation conditions (1 M NaOH).

Increased protein sorption in poly(acrylic acid)-containing films through incorporation of comb-like polymers and film adsorption at low pH and high ionic strength.

In principle, incorporation of comb-like block copolymers in multilayer polyelectrolyte films can both increase film thickness relative to coatings containing linear polymers and provide more swollen films for increased sorption of proteins. In the absence of added salt, alternating adsorption of 5 bilayers of protonated poly(allylamine) (PAH) and comb-like poly(2-hydroxyethyl methacrylate)-graft-poly(acrylic acid) (PHEMA-g-PAA) leads to ∼2-fold thicker coatings than adsorption of PAH and linear PAA, and the difference in the thicknesses of the two coatings increases with the number of bilayers. Moreover, the (PAH/PHEMA-g-PAA)n films sorb 2- to 4-fold more protein than corresponding films prepared with linear PAA, and coatings deposited at pH 3.0 sorb more protein than coatings adsorbed at pH 5.0, 7.0, or 9.0. In fact changes in deposition pH and addition of 0.5 M NaCl to polyelectrolyte adsorption solutions alter protein sorption more dramatically than variations in the constituent polymer architecture. When deposited from 0.5 M NaCl at pH 3.0, both (PAH/PHEMA-g-PAA)5 and (PAH/PAA)5 films increase in thickness more than 400% upon adsorption of lysozyme. These films contain a high concentration of free -COOH groups, and subsequent deprotonation of these groups at neutral pH likely contributes to increased protein binding. Lysozyme sorption stabilizes these films, as without lysozyme films deposited at pH 3.0 from 0.5 M NaCl desorb at neutral pH. Films deposited at pH 9.0 from 0.5 M NaCl are more stable and also bind large amounts of lysozyme. The high binding capacities of these films make them attractive for potential applications in protein isolation or immobilization of enzymes.

Site‐Specific Zwitterionic Polymer Conjugates of a Protein Have Long Plasma Circulation

Many proteins suffer from suboptimal pharmacokinetics (PK) that limit their utility as drugs. The efficient synthesis of polymer conjugates of protein drugs with tunable PK to optimize their in vivo efficacy is hence critical. We report here the first study of the in vivo behavior of a site‐specific conjugate of a zwitterionic polymer and a protein. To synthesize the conjugate, we first installed an initiator for atom‐transfer radical polymerization (ATRP) at the N terminus of myoglobin (Mb‐N‐Br). Subsequently, in situ ATRP was carried out in aqueous buffer to grow an amine‐functionalized polymer from Mb‐N‐Br. The cationic polymer was further derivatized to two zwitterionic polymers by treating the amine groups of the cationic polymer with iodoacetic acid to obtain poly(carboxybetaine methacrylate) with a one‐carbon spacer (PCBMA; C1), and sequentially with 3‐iodopropionic acid and iodoacetic acid to obtain PCBMA(mix) with a mixture of C1 and C2 spacers. The Mb‐N‐PCBMA polymer conjugates had a longer in vivo plasma half‐life than a PEG‐like comb polymer conjugate of similar molecular weights (MW). The structure of the zwitterion plays a role in controlling the in vivo behavior of the conjugate, as the PCBMA conjugate with a C1 spacer had significantly longer plasma circulation than the conjugate with a mixture of C1 and C2 spacers.

Dendrimer antibody conjugate to target and image HER-2 overexpressing cancer cells

Although many breast and lung cancers overexpress human epidermal growth factor receptor-2 (HER-2), no methods currently exist for effective and early detection of HER-2-positive cancers. To address this issue, we designed and synthesized dendrimer-based novel nano-imaging agents that contain gold nanoparticles (AuNPs) and gadolinium (Gd), conjugated with the humanized anti-HER-2 antibody (Herceptin). Generation 5 (G5) polyamidoamine (PAMAM) dendrimers were selected as the backbone for the nano-imaging agents due to their unique size, high ratio of surface functional groups and bio-functionality. We modified G5 PAMAM dendrimer surface with PEG and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelators to encapsulate AuNPs and complex Gd. These dendrimer entrapped AuNPs were further conjugated with Herceptin through copper-catalyzed azide- alkyne click reaction to construct the nano-imaging agent Au-G5-Gd-Herceptin. The targeted nano-imaging agent bound selectively to HER-2 overexpressing cell lines, with subsequent internalization into the cells. More importantly, non-targeted nano-imaging agent neither bound nor internalized into cells overexpressing HER-2. These results suggest that our approach could provide a platform to develop nano-diagnostic agents or nano-therapeutic agents for early detection and treatment of HER-2-positive cancers.

Atherosclerosis and Nanotechnology: Diagnostic and Therapeutic Applications.

Over the past several decades, tremendous advances have been made in the understanding, diagnosis, and treatment of coronary artery disease (CAD). However, with shifting demographics and evolving risk factors we now face new challenges that must be met in order to further advance are management of patients with CAD. In parallel with advances in our mechanistic appreciation of CAD and atherosclerosis, nanotechnology approaches have greatly expanded, offering the potential for significant improvements in our diagnostic and therapeutic management of CAD. To realize this potential we must go beyond to recognize new frontiers including knowledge gaps between understanding atherosclerosis to the translation of targeted molecular tools. This review highlights nanotechnology applications for imaging and therapeutic advancements in CAD.

Nanoparticle-macrophage interactions: A balance between clearance and cell-specific targeting

The surface properties of nanoparticles (NPs) are a major factor that influences how these nanomaterials interact with biological systems. Interactions between NPs and macrophages of the reticuloendothelial system (RES) can reduce the efficacy of NP diagnostics and therapeutics. Traditionally, to limit NP clearance by the RES system, the NP surface is neutralized with molecules like poly(ethylene glycol) (PEG) which are known to resist protein adsorption and RES clearance. Unfortunately, PEG modification is not without drawbacks including difficulties with the synthesis and associations with immune reactions. To overcome some of these obstacles, we neutralized the NP surface by acetylation and compared this modification to PEGylation for RES clearance and tumor-specific targeting. We found that acetylation was comparable to PEGylation in reducing RES clearance. Additionally, we found that dendrimer acetylation did not impact folic acid (FA)-mediated targeting of tumor cells whereas PEG surface modification reduced the targeting ability of the NP. These results clarify the impact of different NP surface modifications on RES clearance and cell-specific targeting and provide insights into the design of more effective NPs.