Atul N. Parikh

Bio: Atul N. Parikh is an academic researcher from University of California, Davis. The author has contributed to research in topics: Bilayer & Lipid bilayer. The author has an hindex of 45, co-authored 177 publications receiving 10402 citations. Previous affiliations of Atul N. Parikh include Stony Brook University & University of New Mexico.

Comparison of the Structures and Wetting Properties of Self-Assembled Monolayers of n- Alkanethiols on the Coinage Metal Surfaces, Cu, Ag, Au'

Abstract: Long-chain alkanethiols, HS(CH2),CH3, adsorb from solution onto.the surfaces of gold, silver, and copper and form monolayers. Reflection infrared spectroscopy indicates that monolayers on silver and on copper (when carefully prepared) have the chains in well-defined molecular orientations and in crystalline-like phase states, as has been observed on gold. Monolayers on silver are structurally related to those formed by adsorption on gold, but different in details of orientation. The monolayers formed on copper are structurally more complex and show a pronounced sensitivity to the details of the sample preparation. Quantitative analysis of the IR data using numerical simulations based on an average single chain model suggests that the alkyl chains in monolayers on silver are all-trans zig-zag and canted by - 12' from the normal to the surface. The analysis also suggests a twist of the plane containing the carbon backbone of -45' from the plane defined by the tilt and surface normal vectors. For comparison, the monolayers that form on adsorption of alkanethiols on gold surfaces, as judged by their vibrational spectra, are also trans zig-zag extended but, when interpreted in the context of the same single chain model, have a cant angle of -27O and a twist of the plane of the carbon backbone of -53'. The monolayers formed on copper (when they are obtained in high quality) exhibit infrared spectra effectively indistinguishable from those on silver and thus appear to have the same structure. Films on copper are also commonly obtained that are structurally ill-defined and appear to contain significant densities of gauche conformations. These spectroscopically based interpretations are compatible with inferences from wetting and XPS measurements. The structure of the substrate-sulfur interface appears to control molecular orientations of the alkyl groups in these films. An improved structural model, incorporating a two-chain unit cell and allowing for the temperature-dependent population of gauche conformations, is presented and applied to the specific case of the structures formed on gold.

The targeted delivery of multicomponent cargos to cancer cells by nanoporous particle-supported lipid bilayers

Abstract: Encapsulation of drugs within nanocarriers that selectively target malignant cells promises to mitigate side effects of conventional chemotherapy and to enable delivery of the unique drug combinations needed for personalized medicine. To realize this potential, however, targeted nanocarriers must simultaneously overcome multiple challenges, including specificity, stability and a high capacity for disparate cargos. Here we report porous nanoparticle-supported lipid bilayers (protocells) that synergistically combine properties of liposomes and nanoporous particles. Protocells modified with a targeting peptide that binds to human hepatocellular carcinoma exhibit a 10,000-fold greater affinity for human hepatocellular carcinoma than for hepatocytes, endothelial cells or immune cells. Furthermore, protocells can be loaded with combinations of therapeutic (drugs, small interfering RNA and toxins) and diagnostic (quantum dots) agents and modified to promote endosomal escape and nuclear accumulation of selected cargos. The enormous capacity of the high-surface-area nanoporous core combined with the enhanced targeting efficacy enabled by the fluid supported lipid bilayer enable a single protocell loaded with a drug cocktail to kill a drug-resistant human hepatocellular carcinoma cell, representing a 10(6)-fold improvement over comparable liposomes.

Self-assembled monolayers and multilayers of conjugated thiols, α,ω-dithiols, and thioacetyl-containing adsorbates. Understanding attachments between potential molecular wires and gold surfaces

Abstract: This paper describes tudies of the formation of self-assembled monolayers (SAMs) and multilayers on gold surfaces of rigid-rod conjugated oligomers that have thiol, cr,a.r-dithiol, thioacetyl, or cqor{'ithioacetyl end groups' The SAMs were analyzed usingillipsom-etry, X-ray photoelectron spectroscopy Q(PS), and infrared extemal reflectance spec6oscopy. The thiol moieties usually dominate adsorption on the gold sites; interactions with the conjugated z-systems are weaker. Rigid rod cr,al-dithiols form *r"*bli"r in which one thiol gouP binds to the surface while the second thiol moiety projects upward at the exposed surface of the SAlvl. ln sinr deprotection of the thiol moieties by deacylation of thioacetyl groups using NFlaOFtpermits formation of SAMs without having to isolate the oxidatively unstable free thiols. Moreover, directidsorption, without exogenous base, of the thioacetyl-terminated oligomers can be accomplished to generate gold surface-bound thiolates. However, in the non-base-promoted adsorptions, higher concentrations of the thioaietyl groups, relative to that of thiol groups, are required to achieve monolayer coverage in a given interval. A thiol-teinninated phenylene-ethynylene system was shown to have a tilt angle of the long molecular axis of <20o from the normal to the substrate surface. These aromatic o-ro-dithiol-derived monolayers provide the basis for studies leading to the design of molecular wires capable of bridging proximate

An Intrinsic Relationship between Molecular Structure in Self-Assembled n-Alkylsiloxane Monolayers and Deposition Temperature

Abstract: We have studied the effect of preparation temperature, in the range 5-65 OC, on the structures of n-octadecylsiloxane monolayers prepared by self-assembly from dilute solution of n-octadecyltrichlorosilane onto the surface of freshly hydrated, oxidized silicon substrates Structural features of the films were characterized using a combination of liquid drop contact angle measurements, null ellipsometry, and infrared transmission spectroscopy The contact angle data confirm a previously reported observation of a critical temperature, Tc - 28 f 5 OC, below which the surface energy is constant at a near-limiting value of a pure CH3 surface and above which the surface energy monotonically increases with increasing temperature Coverages and chain organization, as measured by ellipsometry and vibrational spectroscopic features (peak positions and integrated intensities of methylene C-H stretching modes), respectively, show changes in the same temperature region as the wetting behavior We conclude that when prepared below T,, the films exhibit a heterogeneous structure with closely spaced islands of densely packed, nearly all-trans alkyl chains arranged nearly vertical to the surface In contrast, when prepared above T,, the films exhibit monotonically diminishing coverage with increasing preparation temperature and the alkyl chains increasingly assume higher contents of conformational disorder Further, the infrared data indicate that these higher temperature films are heterogeneous with coexisting domains of high and low chain conformational ordering All the data, taken together, are in good conformity with a film formation mechanism which involves, prior to siloxy group cross-linking, the intervention of intermediate structural phases of mobile alkylsiloxy species adsorbed on a water layer adjacent to the solid substrate surface In support of this mechanism, a strong parallel is apparent between Tc and the triple point temperature at which gas (G), liquid-expanded (LE), and liquid-condensed (LC) phases coexist for chain Langmuir monolayers at the air/bulk water interface Below Tc the self-assembled film structure is similar to that of the nearly pure LC Langmuir phase while above Tc the film structure is similar to that of coexisting LE and LC phases Deviations of the self-assembled film structures for the analogous equilibrium Langmuir phase structures occur at higher preparation temperatures and are rationalized in terms of both the known occurrence of nonequilibrium phases in Langmuir films above the triple point temperature and the relative acceleration of the Si-O-Si cross-linking reaction in the self-assembled film to form structures with frozen-in defects

In vivo lipidomics using single-cell Raman spectroscopy

Abstract: We describe a method for direct, quantitative, in vivo lipid profiling of oil-producing microalgae using single-cell laser-trapping Raman spectroscopy. This approach is demonstrated in the quantitative determination of the degree of unsaturation and transition temperatures of constituent lipids within microalgae. These properties are important markers for determining engine compatibility and performance metrics of algal biodiesel. We show that these factors can be directly measured from a single living microalgal cell held in place with an optical trap while simultaneously collecting Raman data. Cellular response to different growth conditions is monitored in real time. Our approach circumvents the need for lipid extraction and analysis that is both slow and invasive. Furthermore, this technique yields real-time chemical information in a label-free manner, thus eliminating the limitations of impermeability, toxicity, and specificity of the fluorescent probes common in currently used protocols. Although the single-cell Raman spectroscopy demonstrated here is focused on the study of the microalgal lipids with biofuel applications, the analytical capability and quantitation algorithms demonstrated are applicable to many different organisms and should prove useful for a diverse range of applications in lipidomics.

Mussel-Inspired Surface Chemistry for Multifunctional Coatings

Abstract: We report a method to form multifunctional polymer coatings through simple dip-coating of objects in an aqueous solution of dopamine. Inspired by the composition of adhesive proteins in mussels, we used dopamine self-polymerization to form thin, surface-adherent polydopamine films onto a wide range of inorganic and organic materials, including noble metals, oxides, polymers, semiconductors, and ceramics. Secondary reactions can be used to create a variety of ad-layers, including self-assembled monolayers through deposition of long-chain molecular building blocks, metal films by electroless metallization, and bioinert and bioactive surfaces via grafting of macromolecules.

Formation and Structure of Self-Assembled Monolayers.

Abstract: The field of self-assembled monolayers (SAMs) has witnessed tremendous growth in synthetic sophistication and depth of characterization over the past 15 years.1 However, it is interesting to comment on the modest beginning and on important milestones. The field really began much earlier than is now recognized. In 1946 Zisman published the preparation of a monomolecular layer by adsorption (self-assembly) of a surfactant onto a clean metal surface.2 At that time, the potential of self-assembly was not recognized, and this publication initiated only a limited level of interest. Early work initiated in Kuhn’s laboratory at Gottingen, applying many years of experience in using chlorosilane derivative to hydrophobize glass, was followed by the more recent discovery, when Nuzzo and Allara showed that SAMs of alkanethiolates on gold can be prepared by adsorption of di-n-alkyl disulfides from dilute solutions.3 Getting away from the moisture-sensitive alkyl trichlorosilanes, as well as working with crystalline gold surfaces, were two important reasons for the success of these SAMs. Many self-assembly systems have since been investigated, but monolayers of alkanethiolates on gold are probably the most studied SAMs to date. The formation of monolayers by self-assembly of surfactant molecules at surfaces is one example of the general phenomena of self-assembly. In nature, self-assembly results in supermolecular hierarchical organizations of interlocking components that provides very complex systems.4 SAMs offer unique opportunities to increase fundamental understanding of self-organization, structure-property relationships, and interfacial phenomena. The ability to tailor both head and tail groups of the constituent molecules makes SAMs excellent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular-substrates and molecule-solvent interactions like ordering and growth, wetting, adhesion, lubrication, and corrosion. That SAMs are well-defined and accessible makes them good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. SAMs provide the needed design flexibility, both at the individual molecular and at the material levels, and offer a vehicle for investigation of specific interactions at interfaces, and of the effect of increasing molecular complexity on the structure and stability of two-dimensional assemblies. These studies may eventually produce the design capabilities needed for assemblies of three-dimensional structures.5 However, this will require studies of more complex systems and the combination of what has been learned from SAMs with macromolecular science. The exponential growth in SAM research is a demonstration of the changes chemistry as a disciAbraham Ulman was born in Haifa, Israel, in 1946. He studied chemistry in the Bar-Ilan University in Ramat-Gan, Israel, and received his B.Sc. in 1969. He received his M.Sc. in phosphorus chemistry from Bar-Ilan University in 1971. After a brief period in industry, he moved to the Weizmann Institute in Rehovot, Israel, and received his Ph.D. in 1978 for work on heterosubstituted porphyrins. He then spent two years at Northwestern University in Evanston, IL, where his main interest was onedimensional organic conductors. In 1985 he joined the Corporate Research Laboratories of Eastman Kodak Company, in Rochester, NY, where his research interests were molecular design of materials for nonlinear optics and self-assembled monolayers. In 1994 he moved to Polytechnic University where he is the Alstadt-Lord-Mark Professor of Chemistry. His interests encompass self-assembled monolayers, surface engineering, polymers at interface, and surfaces phenomena. 1533 Chem. Rev. 1996, 96, 1533−1554

A reactive potential for hydrocarbons with intermolecular interactions

Abstract: A potential function is presented that can be used to model both chemical reactions and intermolecular interactions in condensed-phase hydrocarbon systems such as liquids, graphite, and polymers. This potential is derived from a well-known dissociable hydrocarbon force field, the reactive empirical bond-order potential. The extensions include an adaptive treatment of the nonbonded and dihedral-angle interactions, which still allows for covalent bonding interactions. Torsional potentials are introduced via a novel interaction potential that does not require a fixed hybridization state. The resulting model is intended as a first step towards a transferable, empirical potential capable of simulating chemical reactions in a variety of environments. The current implementation has been validated against structural and energetic properties of both gaseous and liquid hydrocarbons, and is expected to prove useful in simulations of hydrocarbon liquids, thin films, and other saturated hydrocarbon systems.

Conductance of a Molecular Junction

Abstract: Molecules of benzene-1,4-dithiol were self-assembled onto the two facing gold electrodes of a mechanically controllable break junction to form a statically stable gold-sulfur-aryl-sulfur-gold system, allowing for direct observation of charge transport through the molecules. Current-voltage measurements at room temperature demonstrated a highly reproducible apparent gap at about 0.7 volt, and the conductance-voltage curve showed two steps in both bias directions. This study provides a quantative measure of the conductance of a junction containing a single molecule, which is a fundamental step in the emerging area of molecular-scale electronics.