[Show abstract][Hide abstract] ABSTRACT: SANS study shows that in presence of different valent ions the effect of interactions among protein molecules become prominent with lowering the solution pD towards the isoelectric point.
Chemical Physics Letters 08/2014; 610-611:405-410. · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Layer-by-layer structures of dodecanethiol-encapsulated Au nanoparticles have been formed on Si(001) and quartz substrates at different surface pressures by Langmuir Blodgett method. Optical absorption spectra and out-of-plane structural information have been obtained from UV-Vis spectroscopy and X-ray reflectivity measurements, respectively. With time the thickness of the film decreases keeping the layered structure unchanged but finally monolayer like structure forms. Localized surface plasmon resonance peaks obtained from the UV-Vis spectra show that coupling between Au nanoparticles occurs at the initial stage of the reorganization process as the interparticle distance decreases and as a result, a redshift in the plasmon peak wavelength takes place. Maximum redshift occurs for the monolayer and the peak shift linearly decreases for the multilayer structures. After prolonged reorganization when all layered structures transform into monolayer like structure again redshift occurs but in this process the redshift is reverse with respect to the previous one. In the later process, redshift is minimum for the monolayer structure and increases nearly linearly for the multilayer structures. Two different mechanisms responsible for these two processes are proposed.
Journal of Applied Physics 07/2014; 116(2):024316. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cold gelation behavior of BSA protein has been studied mixing with mono-, di- and tri-valent ions at pD ≈ 7.0. Small angle neutron scattering (SANS) studies show that for the heat treated BSA fractal structure grows with the addition of salts. Heat treated BSA without salt shows intermediate range electrostatic repulsion and long range attraction. However, in presence of salts, cold gelation occurs but the fractal dimension depends upon the salt type and salt concentration. Variation of fractal dimension with salt concentration is nearly similar for Cu2+ and Fe3+ ions however similarity exists between Na+ and Ca2+ ions.
Chemical Physics Letters 01/2014; 593:140–144. · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Prolonged reorganization behaviour of mono-, di-, tri- and multi-layer films of Au nanoparticles prepared by Langmuir-Blodgett method on hydrophobic Si(001) substrates have been studied by using X-ray scattering techniques. Out-of-plane study shows that although at the initial stage the reorganization occurs through the compaction of the films keeping the layered structure unchanged but finally all layered structures modify to monolayer structure. Due to this reorganization the Au density increases within the nanometer thick films. In-plane study shows that inside the reorganized films Au nanoparticles are distributed randomly and the particle size modifies as the metallic core of Au nanoparticles coalesces.
[Show abstract][Hide abstract] ABSTRACT: Langmuir-Blodgett films of standard amphiphilic molecules like nickel arachidate and cadmium arachidate are grown on wet chemically passivated hydrophilic (OH-Si), hydrophobic (H-Si), and hydrophilic plus hydrophobic (Br-Si) Si(001) surfaces. Top surface morphologies and height-difference correlation functions g(r) with in-plane separation (r) are obtained from the atomic force microscopy studies. Our studies show that deposited bilayer and trilayer films have self-affine correlation behavior irrespective of different passivations and different types of amphiphilic molecules, however, liquid like correlation coexists only for a small part of r, which is located near the cutoff length (1/κ) or little below the correlation length ξ obtained from the liquid like and self-affine fitting, respectively. Thus, length scale dependent surface correlation behavior is observed for both types of Langmuir-Blodgett films. Metal ion specific interactions (ionic, covalent, etc.,) in the headgroup and the nature of the terminated bond (polar, nonpolar, etc.,) of Si surface are mainly responsible for having different correlation parameters.
Journal of Applied Physics 03/2013; 113(11). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Hydrophilic/hydrophobic nature of the Cl-passivated Ge(001) surface is investigated directly by contact angle (CA) measurement and indirectly by growing nickel arachidate Langmuir-Blodgett (LB) films on the Cl-passivated Ge(001) surface. Passivation of Ge(001) surface by Cl atoms is confirmed by X-ray photoelectron spectroscopy measurement. CA measurements show that the Cl-passivated Ge(001) surface has intermediate wettability, i.e., the surface has intermediate hydrophilic/hydrophobic behavior. Structural information obtained from the deposited LB films by using X-ray reflectivity and atomic force microscopy analysis shows that the surface is homogeneous and hydrophilic (~ 85%), although very few effectively hydrophobic (~ 15%) regions are present. Structural study in molecular level thus helps to identify the surface nature in nanometer level, which is not possible by simple macroscopic CA measurement. Specific electrostatic and dispersive effects of Cl atoms are possibly responsible for such hydrophilic-like nature of the Cl-passivated Ge(001) surface.
Chemical Physics 10/2012; 406:72. · 1.96 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Structure and stability of cadmium arachidate (CdA) Langmuir-Blodgett (LB) films on homogeneous (i.e., OH-, H-passivated Si(001) substrates) and heterogeneous (i.e., Br-passivated Si(001) substrates) surfaces were studied using x-ray reflectivity and atomic force microscopy techniques and compared with those of nickel arachidate (NiA) LB films. While on OH-passivated Si, asymmetric monolayer (AML) structure starts to grow, on H-passivated Si, symmetric monolayer (SML) of CdA forms, although for both the films, pinhole type defects are present as usual. However, on heterogeneous Br-passivated Si substrates, combination of AML, SML, shifted SML and SML on top of AML (i.e., AML/SML), all types of structures are found to grow in such a way that, due to the variation of heights in the out-of-plane direction, ring-shaped in-plane nanopatterns of CdA molecules are generated. Probably due to stronger head-head interactions and higher metal ioncarboxylic ligand bond-strength for CdA molecules compared to NiA, easy flipping of SML on top of another preformed SML, i.e., SML/SML structure formation was not possible and as a result a wave-like modulation is observed for CdA film on such heterogeneous substrate. Presence of hydrophilic/hydrophobic interfacial stress on heterogeneous substrate thus modifies the deposited molecular structure so that the top surface morphology for CdA film is similar with monolayer buckling while that for NiA film it is similar with monolayer collapse.
The European Physical Journal E 08/2012; 35:79. · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dodecanethiol-encapsulated Au nanoparticles have been deposited on Si(001) and quartz substrates by Langmuir-Blodgett method at different surface pressures for producing layer-by-layer structure. Out-of-plane structural reorganization and optical absorption spectra have been obtained from x-ray reflectivity and UV-Vis spectroscopy measurements, respectively. With time, positions of the nanoparticles reorganize so that films become more compact and thickness decreases. Localized surface plasmon resonance peaks show that coupling between Au nanoparticles occur as the interparticle distance decreases after reorganization and as a result, a redshift in the plasmon peak wavelength takes place. Variation in redshift depends upon the deposition pressure or layer number.
Journal of Applied Physics 07/2012; 112(1). · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hydrophobic to hydrophilic transition of HF-treated Si surface strongly depends upon the metal ions, which are present in the headgroups of the deposited Langmuir-Blodgett (LB) film. Structure of LB films studied by X-ray reflectivity technique and chemical analysis of LB film-substrate interfaces studied by X-ray photoelectron spectroscopy combinedly suggest that the partial transition or partial oxidation of the HF-treated Si surface takes place under the subphase water but further transition or oxidation is possible only in the presence of metal ions. Electrovalent and covalent natures of the metal ions tune this transition or oxidation. Ni ions, for which bonding with headgroups are electrovalent in nature, are favorable for such transition/oxidation and as a result, complete transition/oxidation takes place when nickel arachidate LB film is deposited. On the other hand, Cd ions, for which bonding with headgroups show covalent nature, is not favorable for such transition and can not oxidize the underlying H-passivated Si substrate totally when cadmium arachidate LB film is deposited on such HF-treated Si surface. This ion-specific hydrophobic to hydrophilic transition is visualized by X-ray reflectivity, contact angle and X-ray photoelectron spectroscopy measurements.
Materials Chemistry and Physics 05/2012; 134:549. · 2.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Complexation of zwitterionic lipid, dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and protein, bovine serum albumin (BSA) at the air-water interface has been studied by surface pressure (π) - mean molecular area (A) isotherms and X-ray reflectivity. Although BSA has isoelectric point nearly at pH≈4.8, possibility of complex formation with lipid molecules has been investigated from low (≈4.0) to high (≈9.0) pH range in presence of divalent cation, Ca(2+) in the water subphase. Both the isotherm and reflectivity analysis show that the interaction of BSA with lipid monolayer takes place from that low to high subphase pH range, i.e., complexation occurs both below and above of the isoelectric point. Only one layer of BSA forms below the lipid monolayer and the probable reasons for such complex formation have been proposed.
[Show abstract][Hide abstract] ABSTRACT: Growth of dodecanethiol-encapsulated Au nanoparticles on differently terminated (OH-, H-, or
Br-terminated) Si(001) substrates by Langmuir-Blodgett method at a constant monolayer surface
pressure and their out-of-plane structural modification with time have been investigated. As the
substrates have different gradations in the hydrophilic/hydrophobic nature, three different
out-of-plane structures have been formed. On H-terminated Si (hydrophobic surface), a fluctuating
monolayer of Au nanoparticles has been formed, whereas on Br- (coexistence of hydrophilic and
hydrophobic surfaces) and OH-terminated Si (hydrophilic surfaces), trilayer of Au nanoparicles
have been formed, but the top layer coverage is more for the OH-terminated Si. The growth of Au
nanoparticles on H-terminated Si is similar to the Frank-van der Marwe mode, whereas on Br- and
OH-terminated Si, the growth is similar to the Stranski-Krastanov mode. These three different
structures modify with time and finally become a thicker monolayer of high density, and positions
of naoparticles within the monolayer become random. AFM images of the films also show that
positions of the Au nanoparticles are random. Density of the final layer becomes maximum on
OH-terminated Si and minimum on H-terminated Si, whereas it becomes intermediate on
Br-terminated Si. Reorganization thus helps to obtain nanostructures of tunable nanoparticle
Journal of Applied Physics 12/2011; 110(82):114302-222512. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A monolayer of dodecanethiol-encapsulated Au nanoparticles when compressed laterally transforms into layer-by-layer assemblies on water surface. These layer-by-layer assemblies of Au nanoparticles have been deposited on H-terminated Si(001) substrates by using one down-up cycle (two strokes) in the Langmuir-Blodgett (LB) method. The transformation from monolayer to layer-by-layer assembly on a water surface is irreversible; i.e., if the compressed film is decompressed the layer-by-layer structure cannot regenerate the monolayer structure. Unlike layer-by-layer growth, only odd numbers of layers grow from the monolayer on the H-terminated Si(001) substrates by using different numbers of down-up cycles. Z-type LB deposition occurs only in the first down-up cycle of the hydrophobic substrate, whereas Y-type LB deposition takes place in the successive cycles. Such layer-by-layer assemblies of Au nanoparticles, which are made on bare silicon surfaces and where thickness can be controlled at the nanoscale level, are very promising for their novel applications in the field of nanoscience.
[Show abstract][Hide abstract] ABSTRACT: A Langmuir monolayer of octadecylamine (ODA) in the presence of aqueous HAuCl4 subphase produces nanoparticles/nanocrystals of Au at the air–water interface. Au-attached monolayer films were deposited at a constant surface pressure (πT) of the ODA monolayer but at different times after reaching the πT. X-ray reflectivity analysis gives the out-of-plane structures of the deposited films and shows time dependent growth behavior of Au adsorption below the ODA monolayer. Atomic force microscopy, scanning electron microscopy and X-ray diffraction analysis mainly shows that the worm-like Au nanoparticles become globular and with time gets crystal structures, which are effectively disc-like.
Chemical Physics Letters 01/2011; 508(1):80-85. · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ferric stearate (FeSt) Langmuir–Blodgett (LB) films have been reacted chemically with H2S gas for making iron sulphide within the organic matrix. Films, before and after the reaction with H2S, have been analyzed with the X-ray reflectivity (XRR), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) studies. After sulphidation, more ‘pinhole’ defects form which changes the film morphology and the number of layers increases due to the rearrangement of the molecules. Formation of less ordered iron sulphide within the stearic acid multilayers after sulphidation increases the interfacial roughness that decreases the reflectivity. XPS analysis shows that polysulphide forms within the microenvironment of the FeSt LB films after reaction with H2S whereas both mono and polysulphide are produced when the reaction occurs with FeSt in bulk.
[Show abstract][Hide abstract] ABSTRACT: HF-treated Si surface, which is hydrophobic in nature and quite stable in air and inside pure water, can become completely hydrophilic during nickel arachidate Langmuir-Blodgett film growth. Such transition is clearly evident from the structures of films deposited by different number of strokes and can be understood by considering partial oxidation of Si surface inside subphase water and further or complete oxidation in air, in presence of Ni head-groups. Attached Ni head-groups weaken the nearby Si-Si covalent bonding and easily oxidize those Si atoms. Depending upon the amount of those head-groups and its distribution, oxidation or transition can even complete.
Chemical Physics Letters 11/2010; 500:90. · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nickel arachidate (NiA) Langmuir-Blodgett (LB) films have been deposited on hydrophilic Si(0 0 1) substrates by three (up-down-up) and five (up-down-up-down-up) strokes. During deposition, substrates were kept inside the water subphase for different times after each down stroke. Structural information of all the LB films have been obtained from X-ray reflectivity (XRR) studies. One and two symmetric monolayer (SML) was deposited on top of the asymmetric monolayer (AML) in three and five stokes respectively. All the preformed LB films were then used to go through the air-water interface with the same speed that was used at the time of film deposition. Structural information obtained from the XRR studies show that mainly the top layer density decreases after passing through the air-water interface but the layered structure remains the same. Information obtained from both the XRR and atomic force microscopy (AFM) studies suggest that molecules peeled from the top SML layer do not reincorporate with the LB film through tail-tail hydrophobic interaction. Our study shows that NiA LB film has better stability compared with cadmium arachidate LB film inside the water subphase without forming any out-of-plane molecular reorganization.
Journal of Colloid and Interface Science 10/2010; 353(1):316-21. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Polyelectrolyte-surfactant complexes have been deposited on hydrophilic silicon substrates by using a horizontal deposition technique. DNA and carboxymethyl cellulose (carboxyMC) were used as short and long polyelectrolyte and dodecyltrimethylammonium bromide (DTAB) was used as water-soluble surfactant. Varying the surfactant concentration, the structural and morphological information have been obtained for these polyelectrolyte-surfactant complexes. Morphology and out-of-plane structures have been obtained by atomic force microscopy and X-ray reflectivity studies. Electron density profiles obtained from the reflectivity study show that DNA-DTAB complexes form lamellar like multilayered structure but carboxyMC-DTAB complexes form coil-like structure. At lower DTAB concentration, these DNA-DTAB and carboxyMC-DTAB complexes form Gibbs layer, whereas at higher surfactant concentration, DTAB molecules themselves form lamellar like multilayered structure that coexists with the structure formed by the complexes.
Journal of Colloid and Interface Science 04/2010; 344(2):547-55. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Growth of Langmuir-Blodgett (LB) films of nickel arachidate (NiA) on differently terminated (OH-, H-, or Br-terminated) Si(001) substrates and their structural evolution with time have been investigated by x-ray reflectivity technique and complemented by atomic force microscopy. Stable and strongly attached asymmetric monolayer (AML) of NiA is found to grow on freshly prepared oxide-covered Si substrate while unstable and weakly attached symmetric monolayer (SML) of NiA grows on H-terminated Si substrate, corresponding to stable hydrophilic and unstable hydrophobic natures of the substrates, respectively. The structure of LB film on Br-terminated Si substrate, however, shows intermediate behavior, namely, both AML and SML are present on the substrate, indicative of coexisting (hydrophilic and hydrophobic) nature of this terminated surface. Such coexisting nature of the substrate shows unusual growth behavior of LB films: (i) hydrophilic and hydrophobic attachments of NiA molecules in single up stroke of deposition and (ii) growth of few ring-shaped large-heights islands in subsequent deposition. These probably occur due to the presence of substrate-induced perturbation in the Langmuir monolayer and release of initially accumulated strain in the film structures near hydrophilic/hydrophobic interface, respectively, and provide the possibility to grow desired structures (AML or SML) of LB films by passivation-selective surface engineering.