Alamgir Karim

Interaction of gold nanoparticles with common human blood proteins

In order to better understand the physical basis of the biological activity of nanoparticles (NPs) in nanomedicine applications and under conditions of environmental exposure, we performed an array of photophysical measurements to quantify the interaction of model gold NPs having a wide range of NP diameters with common blood proteins. In particular, absorbance, fluorescence quenching, circular dichroism, dynamic light scattering, and electron microscopy measurements were performed on surface-functionalized water-soluble gold NPs having a diameter range from 5 to 100 nm in the presence of common human blood proteins: albumin, fibrinogen, gamma-globulin, histone, and insulin. We find that the gold NPs strongly associate with these essential blood proteins where the binding constant, K, as well as the degree of cooperativity of particle--protein binding (Hill constant, n), depends on particle size and the native protein structure. We also find tentative evidence that the model proteins undergo conformational change upon association with the NPs and that the thickness of the adsorbed protein layer (bare NP diameter <50 nm) progressively increases with NP size, effects that have potential general importance for understanding NP aggregation in biological media and the interaction of NP with biological materials broadly.

Sinusoidal phase grating created by a tunably buckled surface

We investigate a buckling instability by both small angle light scattering and atomic force microscopy, demonstrating that a tunable phase grating can be created with a mechanical instability. The instability is realized in a prestressed silicone sheet coated with a glassy polymer film. Compression of the sample results in a sinusoidally wrinkled surface where the amplitude is controlled by the degree of compression and the wavelength by film thickness. We model the system with Fourier optics, explaining the positions and relative intensities of the diffraction orders.

Polymer surfaces, interfaces and thin films

Mobility of polymers near surfaces, J. Baschnagel et al tailoring polymer interfaces through confinement, A.C. Balazs et al depth profiling of thin polymer films - extracting segmental interaction parameters of binary mixtures, J. Klein tailoring polymer interfacial properties by end group modification, J.T. Koberstein polymer adsorption at polymer/solid interfaces, R. Oslanec and R.J. Composto dielectric response of polymer films confined between mica surfaces, Y.-K. Cho et al the glass transition and relaxation dynamics in thin polymer films, J.A. Forrest and R.A.L. Jones.

A rapid prototyping technique for the fabrication of solvent-resistant structures

We demonstrate a rapid prototyping technique for the fabrication of solvent-resistant channels up to and exceeding one millimeter in height. The fabrication of channels with such dimensions by conventional lithography would be both challenging and time consuming. Furthermore, we show that this technology can be used to fabricate channels with a depth that varies linearly with distance. This technique requires only a long-wavelength ultraviolet source, a mask made by a desktop printer and a commercially available optical adhesive. We demonstrate two lithographic methods: one that fabricates channels sealed between glass plates (close-faced) and one that fabricates structures on a single plate (open-faced). The latter is fully compatible with silicon replication techniques to make fluid handling devices.

Dynamic Thermal Field-Induced Gradient Soft-Shear for Highly Oriented Block Copolymer Thin Films

As demand for smaller, more powerful, and energy-efficient devices continues, conventional patterning technologies are pushing up against fundamental limits. Block copolymers (BCPs) are considered prime candidates for a potential solution via directed self-assembly of nanostructures. We introduce here a facile directed self-assembly method to rapidly fabricate unidirectionally aligned BCP nanopatterns at large scale, on rigid or flexible template-free substrates via a thermally induced dynamic gradient soft-shear field. A localized differential thermal expansion at the interface between a BCP film and a confining polydimethylsiloxane (PDMS) layer due to a dynamic thermal field imposes the gradient soft-shear field. PDMS undergoes directional expansion (along the annealing direction) in the heating zone and contracts back in the cooling zone, thus setting up a single cycle of oscillatory shear (maximum lateral shear stress ∼12 × 10(4) Pa) in the system. We successfully apply this process to create unidirectional alignment of BCP thin films over a wide range of thicknesses (nm to μm) and processing speeds (μm/s to mm/s) using both a flat and patterned PDMS layer. Grazing incidence small-angle X-ray scattering measurements show absolutely no sign of isotropic population and reveal ≥99% aligned orientational order with an angular spread Δθ(fwhm) ≤ 5° (full width at half-maximum). This method may pave the way to practical industrial use of hierarchically patterned BCP nanostructures.

The Form of the Enriched Surface Layer in Polymer Blends

The concentration profile at the surface in blends of deuterated and protonated polystyrene (d-PS and PS) is inferred from measurements of neutron reflectivity and secondary-ion mass spectrometry, using constraints provided by forward recoil spectrometry and X-ray reflectometry results on the same samples. The surface is enriched in d-PS, the volume fraction and the decay length of which are in good agreement with the predictions of mean-field theory but the form of the profile shows small, but statistically significant, deviations from that predicted by the theory.

Multifunctional ToF-SIMS: combinatorial mapping of gradient energy substrates

Abstract We present a simple method for chemical modification of chlorosilane self-assembled monolayers (SAMs) on Si surfaces by exposure to a gradient of UV-ozone radiation to create stable substrates with a range of contact angles ( θ H 2 O ≈5–95°) and surface energies on a single substrate. These gradient energy substrates are developed to potentially generate libraries for combinatorial studies of thin film phenomenology, where a systematic variation of interfacial surface energy represents one of the significant parameters along one axis. The graded oxidation process presents a systematic variation of surface chemical composition. We have utilized contact angle measurements and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate this variation for a series of ions, among which are SiCH 3 + , SiOH + and COOH − . We show that the macroscopic measurements of surface free energy/contact angle correlate with the detailed analysis of surface chemistry (as assessed by ToF-SIMS) on these test substrates.

Ordering in asymmetric poly (ethylene–propylene)–poly (ethylethylene) diblock copolymer thin films

We have used neutron reflection and phase contrast microscopy to investigate the morphology and surface topology of thin films of nearly symmetric (f=0.55) and asymmetric (f=0.77), poly (ethylene–propylene)–poly (ethylethylene) (PEP–PEE) diblock copolymers (f being the PEP volume fraction) and have identified three important differences in their ordering properties. First, annealed films of the asymmetric diblocks do not form the lamellar microstructure found in symmetric diblocks; their structure can instead be modeled in terms of the hexagonal packing of PEE cylinders observed in bulk small‐angle neutron scattering measurements. However, the cylinders show in‐plane distortions, which we interpret in the context of nonintegral layering. These distortions are amplified at the surfaces where the PEE assumes lamellarlike form. Second, as‐cast films of the asymmetric diblock are characterized by a microstructure lacking long‐range order, pinned between strongly segregated PEE at both surfaces. These films ca...

Formation and dissolution of phase‐separated structures in ultrathin blend films

ABSTRACT: The phase separation of ultrathin polymer blend films of deuterated poly-(styrene)/poly(vinylmethylether) leads to a variety of film morphologies, dependingon polymer composition. Phase-separation measurements are made at a constant tem-perature difference from the critical temperature, leading to a bicontinuous spinodaldecomposition pattern for near-critical blend compositions and to ‘‘mounds’’ and ‘‘holes’’for PVME-rich and dPS-rich off-critical mixtures, respectively. Reverse temperaturejumps of the phase-separated blend films into the one-phase region result in dissolutionof the undulating surface patterns, confirming the phase-separation origin of the filmpatterns. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 191–200, 1998 Keywords: ultrathin films; polymer blends; phase separation; atomic force microscopy INTRODUCTION than1 mm. 1 Moreover,thiseffectissubstratedepen-dent, shifting to higher temperatures with decreas-Polymer blends have been the focus of decades of ing film thickness on gold and to lower tempera-research owing to their technological and scien- tures on glass.

Langmuir Adsorption Study of the Interaction of CdSe/ZnS Quantum Dots with Model Substrates: Influence of Substrate Surface Chemistry and pH

We investigate the utility of Langmuir adsorption measurements for characterizing nanoparticle-substrate interactions. Spherical CdSe/ZnS core-shell nanoparticles were chosen as representative particles because of their widespread use in biological labeling measurements and their relatively monodisperse dimensions. In particular, the quantum dots were functionalized with 11-mercaptoundecanoic acid, and we utilized an amine-terminated self-assembled monolayer (SAM) as a model substrate. SAMs with different end-groups (-CH(3) and -COOH) were also considered to contrast with the adsorption behavior on the amine-terminated SAM substrates. We followed the kinetics of nanoparticle adsorption on the aminosilane layer by quartz crystal microgravimetry (QCM) over a range of particle concentrations and determined the corresponding Langmuir adsorption isotherms. Analysis of both equilibrium adsorption and kinetic adsorption data allowed us to determine a consistent value of the Langmuir adsorption equilibrium constant for the amine-terminated SAM at room temperature (K(L) approximately 2.7 (micromol/L)(-1)), providing a useful characterization of the nanoparticle-substrate interaction. The effect of varying solution pH on Langmuir adsorption was also investigated in order to gain insight into the role of electrostatic interactions on nanoparticle adsorption. The equilibrium extent of adsorption was found to be maximum at about pH 7. These changes of nanoparticle adsorption were further quantified and validated by X-ray photoelectron spectroscopy (XPS) and confocal fluorescence microscopy measurements. We conclude that Langmuir adsorption measurements provide a promising approach for quantifying nanoparticle-substrate interactions.