Current research in Zharov group is divided between three main areas: (1) functional membrane materials for energy and separations, (2) functional nanoparticles for biomedical applications and catalysis, and (3) nanoconfinement effects on chemical reactivity and on physical properties of hydrocarbons. Within these areas, the following projects are ongoing: (1) self-assembly of polymer brush nanoparticles into porous supercrystals, (2) ion-conducting membranes from self-assembly of polymer brush nanoparticles, (3) tailoring the nanoenvironment of diamond-supported noble metal nanoparticles for control of catalysis, (4) degradable silica nanoparticles, (5) investigation of nanoconfinement effect on reactivity of aryl cyanate esters, and (6) fluid-solid interactions inside nanopores.
1. SELF-ASSEMBLY OF POLYMER BRUSH NANOPARTICLES INTO POROUS SUPERCRYSTALS
Our earlier work on ionic and molecular transport in silica colloidal nanopores was followed by the preparation of free-standing colloidal membranes, with size selectivity towards macromolecules and pores filled with responsive polymer brushes. However, more recently we decided to shift our attention to colloidal membranes prepared by self-assembly of silica nanoparticles carrying polymer brushes on their surface (“hairy” nanoparticles, HNPs). This approach avoids the limitations related to the preparation of silica colloidal membranes, provides great flexibility in terms of surface chemistry and leads to many fundamental questions of great importance in the areas of self-assembly and polymer-polymer interactions. Our proof-of-concept work showed that HNPs can indeed reversibly assemble into robust mesoporous materials with tunable pore size. This led to our current NFS-funded project which combines experimental work with computation work performed in collaboration with Prof. Michael Grünwald of the Chemistry Department. The project is focused on preparing a series of HNPs with varying architecture and investigating their self-assembly as a function of HNP structural parameters (i.e. size, grafting densities, polymer chemistry, and chain length) and assembly conditions. The structure, mechanical properties and porosity of HNP assemblies are characterized by a range of experimental methods, including transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and BET analysis. These experimental studies will shed light on correlations between parameters of the polymer brush and properties of the resulting assembled superstructures. Molecular dynamics computer simulations, using coarse-grained pair potentials, provide information about the microscopic structure of the polymer chains that dictate nanoparticle assembly and are used to rationalize experimental findings and to guide the exploration of the parameter space associated with polymer brushes.
In the past year, we finished the preparation and characterization, and started modeling novel nanoporous membranes prepared by self-assembly of HNPs carrying polyelectrolyte copolymer brushes. We showed that self-assembly can be controlled by the polymer length and charge, which affect the repulsion between the nanoparticles during the membrane formation. We found that the properties of the membranes result from the nanoscale interactions between the polymer chains on the nanoparticle surface, and from the behavior of the polymer chains inside the nanoconfined spaces. The polymer plays three roles in the assembly and behavior of the membranes. First, it acts as a “molecular glue” such that polymer chains align themselves at points of contact between the particles to form a compact arrangement maximizing the polymer-polymer interactions and providing mechanical stability to the nanoparticle films. Secondly, the polymer chains extend into the interstitial spaces between the particles defining the nanopore size. Finally, polymer chains introduce charged groups into the nanopores, which, combined with the high surface area and tortuosity of the nanopores, leads to highly selective ionic transport through the nanopores despite their relatively large size. We also found that the polymer conformation responds to the ionic strength of the contacting solution in a unique way: long polymer brushes fill the nanopores and do not undergo significant swelling due to nanoconfinement, while short polymer brushes are able to swell 10 times more. This leads to an unusual response of the salt rejection to salt concentrations due to the interplay between the charge screening and polymer swelling.
Our current and future directions in this area are studying the effects of polymer brush conformation, degree of polymerization and charge density, polymer-polymer interactions, effect of solvent, compressibility and concentration polarization on the behavior of HNP membranes, as well as designing functional HNP membranes with nanopores responsive to various external stimuli.
Zharov, I.; Khabibullin, A. Acc. Chem. Res. 2014, 47, 440-449.
Khabibullin, A.; Zharov, I. ACS Appl. Mater. Interfaces 2014, 6, 7712-7718.
Bohaty, A. K.; Smith, J. J.; Zharov, I. Langmuir 2009, 25, 3096-3101.
Ignacio-de Leon, P. A.; Zharov. I. Chem. Commun. 2011, 47, 553-555.
Schepelina, O.; Poth, N.; Zharov, I. Adv. Funct. Mater. 2010, 20, 1962-1969.
Ignacio-de Leon, P. A.; Zharov. I. Langmuir 2013, 29, 3749-3756.
Khabibullin, A.; Fullwood, E.; Kolbay, P.; Zharov, I. ACS Appl. Mater. Interfaces 2014, 6, 17306-17312.
Eygeris, Y.; White, E. V.; Wang, Q.; Carpenter, J. E.; Grünwald, M.; Zharov, I. Submitted to ACS Nano.
2. ION-CONDUCTING MEMBRANES FROM SELF-ASSEMBLY OF POLYMER BRUSH NANOPARTICLES
As a part of the above NSF-funded project, we are working on novel designs of ion-conducting materials for fuel cells and lithium batteries. Our earlier work on sulfonated silica colloidal materials and proton conductivity in these materials led to a novel design for ion-conducting membranes, based on pore-filled colloidal crystals. These membranes possess a number of attractive properties, including high proton conductivity, mechanical stability, high water retention and non-swelling. In addition, this system allows for systematic studies of proton conductivity and fuel cell performance as a function of polymer composition. Our current and future work focuses on developing a new class of lithium-conducting HNP membrane materials whose lithium conductivity results from low molecular weight polymer brushes suitable for lithium ion transport and immobilized on nanoparticles. We found that when such low MW polymer brushes are used, their side chains can contain as few as two oxygen atoms, e.g. poly(ethoxyethyl methacrylate), pEEMA and still provide high lithium ion conductivity. In addition, we recently discovered that nanoporous membranes with relatively high proton conductivity and low swelling, which makes them particularly suitable for redox batteries, can be prepared by self-assembly of HNPs of two types, those carrying sulfonated polymer brushes and those carrying polymer brushes with hydrophobic side chains.
Smith, J. J.; Zharov, I. Chem. Mater. 2009, 21, 2013-2019.
Khabibullin, A.; Minteer, S. D.; Zharov, I. J. Mater. Chem. A. 2014, 2, 12761-12769.
Green, E.; Lifshitz, M.; Golodnitsky, D.; Zharov.; I. Manuscript in preparation.
3. TAILORING THE NANOENVIRONMENT OF DIAMOND-SUPPORTED NOBLE METAL NANOPARTICLES FOR CONTROL OF CATALYSIS
This is an NFS-funded collaborative project with Prof. Jennifer Shumaker-Parry of the Chemistry Department. The goal of the project is to create a controlled nanoenvironment for noble metal nanoparticles supported on synthetic diamond (ND) and silica nanoparticles (SNPs) using polymer brushes and to investigate the impact of this environment on catalysis. Our hypothesis is that the catalytic properties of these materials as well as their stability can be controlled by the structure and properties of the polymer brushes, prepared by polymerization initiated from the surface of the nanoparticles.
While working on this project we developed a method to prepare novel thiol-ene polymer-coated ND particles with reactive surfaces that function as a support by immobilizing Au, Pt and Pd nanoparticles that retain their catalytic activity. The polymer/ND particles offer a versatile surface with an inert core that will allow probing reaction environments that were previously inaccessible. The successful use of this support for Au, Pt and Pd nanoparticles suggests that it may be suitable for other transition metal and alloy nanoclusters. In an effort to further increase the catalytic efficiency of polymer/ND-supported catalysts we are working towards developing higher surface area ND supports and on catalytic properties of bare ND particles.
More recently, when attempting to synthesize SNPs with a catalytically active Pd2+ complex immobilized on the silica surface by treating SNPs carrying covalently bound dmp ligands with Pd(OAc)2 in acetone, we observed instead the formation of small PdNPs uniformly decorating the silica surface. After probing different solvents and temperatures we determined that an alcohol impurity in the reagent-grade acetone was likely responsible for the reduction of Pd(OAc)2 to Pd0 which then formed nanoparticles. In addition to being important in terms of catalytic mechanisms, this result provides a simple, reproducible, one-pot, room temperature preparation method for formation of monodisperse PdNPs supported on a silica surface.
Our future work will focus on varying the length, grafting density, polarity, structure and chemical composition of polymer brushes to create a tunable nanoenvironment for supported noble metal nanoparticles. We will also work on elucidating the mechanism of PdNP formation on silica surface in the presence of surface ligands.
Quast, A.; Bornstein, M.; Zharov, I.; Shumaker-Parry, J. S. ACS Catalysis 2016, 6, 4729–4738.
Quast, A.; Luke, R. C.; Zharov, I.; Shumaker-Parry, J. S. Submitted to ACS Appl. Nano Mater.
Bornstein, M.; Quast, A.; Park, R.; Parker, D. M.; Shumaker-Parry, J. S.; Zharov, I. Submitted to Angew. Chemie.
4. DEGRADABLE SILICA NANOPARTICLES
This work is an NIH-funded collaboration with Prof. Ghandehari of Pharmaceutics and Pharmaceutical Chemistry and Bioengineering at the University of Utah. It focuses on the preparation of internally functionalized biodegradable silica nanoparticles with applications in drug delivery, cancer treatment and MRI imaging. Silica nanoparticles (SNPs) are attractive for applications in delivery of drugs and imaging agents due to their ease of synthesis and scale up, robust structure, and controllable size and composition. Degradability is one important factor that limits biomedical applications of SNPs. Our interest in this area stems from the desire to develop novel theranostic materials and our approach to this problem is based on our ability to alter the internal composition of silica nanoparticle with various functional groups.
Most recently, we prepared unique hydrolysable silica nanoparticles (ICPTES-Sorbitol SNPs) by the incorporation of carbamate linkages into the silica matrix. For this purpose, a silsesquioxane ICPTES-Sorbitol containing carbamate groups was synthesized and used to produce the novel nanoparticles by co-condensation with tetraethoxysilane. ICPTES-Sorbitol SNPs completely degraded in water at neutral and acidic pH as the result of the carbamate linkage hydrolysis with monosilicic acid as the main degradation product. We predict that the novel ICPTES-Sorbitol SNPs are suitable to become superior building block for future theranostic agents. In addition, these SNPs became porous and formed primary amines on the surface in the process of their degradation and showed different hydrolysis rates at pH 2 and 4, which could be of interest for future applications of as multifunctional nanoparticles for drug delivery and imaging and may be of interest in oral-based drug delivery. In the future, we will study their drug loading and release and will work on incorporating targeting and imaging modalities into these nanoparticles.
Dubey, R.; Kushal, S.; Levin, M. D.; Mollard, A.; Oh, P.; Schnitzer, J. E.; Zharov, I.; Olenyuk, B. Z. Bioconj. Chem. 2015, 26, 78-89.
Brozek, E., Zharov, I. Chem. Mater. 2009, 21, 1451-1456.
Gao, Z.; Zharov, I. Chem. Mater. 2014, 26, 2030-2037.
Gao, Z.; Moghaddam, S. P. H.; Ghandehari, H.; Zharov, I. RSC Adv. 2018, 8, 4914-4920.
5. INVESTIGATION OF NANOCONFINEMENT EFFECT ON REACTIVITY OF ARYL CYANATE ESTERS
This is a Russian Science Foundation (RFS)-funded project which I lead at the A. M. Butlerov Chemistry Institute, Kazan Federal University, Russia. The main objective of this project is to investigate the effect of nanoconfinement on chemical reactions in terms of the mechanisms of this influence via surface-related (strength of substrate-surface interaction, catalysis by surface-grafted functional groups) and size-related (pore geometry) effects. The model reaction used in this work is the thermal polymerization of aryl cyanate esters. Our aim is to gain new understanding of the mechanism of the nanoconfinement effect and to investigate the role of substrate ordering induced by nanoconfinement on its reactivity. To study nanoconfinement effect we use silica colloidal crystals. Thermal polymerization of aryl cyanate esters provides a convenient model reaction because it is exothermic and thus can be easily followed by differential scanning calorimetry. The aryl cyanate substrates selected for this work provide the structural variations that will allow discerning both the substituent effects on polymerization kinetics and the effects of specific and non-specific interactions with the nanopore surface. This study will also use EPR and ENDOR spectroscopy to reveal the behavior of nanoconfined substrates. This will be achieved by studying the interactions between radiation-induced paramagnetic centers on the surface of the nanopores with nuclear spin momentum of 15N, 13С and 2H in substrates.
6. FLUID-SOLID INTERACTIONS INSIDE NANOPORES
This project is a critical part of a $10.75M DOE EFRC Center Multi-Scale Fluid-Solid Interactions in Architected and Natural Materials (MUSE). The EFRC is organized into four tightly integrated interdisciplinary thrusts: Materials Architecture, Dynamic Characterization, Multiscale Properties and Physical Properties. The overarching goal of the center is to develop fundamental understanding of confinement and surface interactions in mesoscale media with nanometer-sized pores on the phase behavior, thermodynamic and multiphase flow properties of multicomponent fluid mixtures. I am the Thrust Leader for Materials Architecture. As such, I will coordinate efforts between materials preparation, characterization, modeling and testing. Our own work will include the preparation of mesoporous silica and carbon materials with different pore sizes and geometries that will serve as model porous media. We will also build model materials of different compositions and with distinct nanopore surface chemistries. Finally, we will participate in studies of nanoconfinement effects on interfacial, thermodynamic, flow, and reactive properties of confined hydrocarbons.