Miodrag Micic

Surface chemistry of Alzheimer's disease: A Langmuir monolayer approach

Amyloid beta (1-40) and (1-42) peptides are the major constituents of hallmark senile plaques found in Alzheimers disease (AD) patients. Study of aggregation of Abeta (1-40) and (1-42) peptides and the truncated Abeta fragments could lead towards the mechanism of AD. Langmuir monolayer approach is one of the excellent methods to investigate the mechanism and origin of AD. Particularly, to study the steps involved in the formation and assembly of beta-sheet structures leading to formation of amyloid fibrils. Surface pressure- and surface potential-area isotherms provide information regarding the nature of short-range and long-range interactions between the molecules especially the lipids and the Abeta peptides. Spectroscopic methods like IRRAS, PM-IRRAS, FTIR-ATR, and GIXD at the air-water interface provide insight into the structural characterization, and orientation of the molecules in the Langmuir monolayer.

Strong and Selective Adsorption of Lysozyme on Graphene Oxide

Biosensing methods and devices using graphene oxide (GO) have recently been explored for detection and quantification of specific biomolecules from body fluid samples, such as saliva, milk, urine, and serum. For a practical diagnostics application, any sensing system must show an absence of nonselective detection of abundant proteins in the fluid matrix. Because lysozyme is an abundant protein in these body fluids (e.g., around 21.4 and 7 μg/mL of lysozyme is found in human milk and saliva from healthy individuals, and more than 15 or even 100 μg/mL in patients suffering from leukemia, renal disease, and sarcoidosis), it may interfere with detections and quantification if it has strong interaction with GO. Therefore, one fundamental question that needs to be addressed before any development of GO based diagnostics method is how GO interacts with lysozyme. In this study, GO has demonstrated a strong interaction with lysozyme. This interaction is so strong that we are able to subsequently eliminate and separate lysozyme from aqueous solution onto the surface of GO. Furthermore, the strong electrostatic interaction also renders the selective adsorption of lysozyme on GO from a mixture of binary and ternary proteins. This selectivity is confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), fluorescence spectroscopy, and UV–vis absorption spectroscopy.

Reciprocating flow-based centrifugal microfluidics mixer

Proper mixing of reagents is of paramount importance for an efficient chemical reaction. While on a large scale there are many good solutions for quantitative mixing of reagents, as of today, efficient and inexpensive fluid mixing in the nanoliter and microliter volume range is still a challenge. Complete, i.e., quantitative mixing is of special importance in any small-scale analytical application because the scarcity of analytes and the low volume of the reagents demand efficient utilization of all available reaction components. In this paper we demonstrate the design and fabrication of a novel centrifugal force-based unit for fast mixing of fluids in the nanoliter to microliter volume range. The device consists of a number of chambers (including two loading chambers, one pressure chamber, and one mixing chamber) that are connected through a network of microchannels, and is made by bonding a slab of polydimethylsiloxane (PDMS) to a glass slide. The PDMS slab was cast using a SU-8 master mold fabricated by a two-level photolithography process. This microfluidic mixer exploits centrifugal force and pneumatic pressure to reciprocate the flow of fluid samples in order to minimize the amount of sample and the time of mixing. The process of mixing was monitored by utilizing the planar laser induced fluorescence (PLIF) technique. A time series of high resolution images of the mixing chamber were analyzed for the spatial distribution of light intensities as the two fluids (suspension of red fluorescent particles and water) mixed. Histograms of the fluorescent emissions within the mixing chamber during different stages of the mixing process were created to quantify the level of mixing of the mixing fluids. The results suggest that quantitative mixing was achieved in less than 3 min. This device can be employed as a stand alone mixing unit or may be integrated into a disk-based microfluidic system where, in addition to mixing, several other sample preparation steps may be included.

A Novel Photoscissile Poly(ethylene glycol)‐Based Hydrogel

drawbacks, such as slow gelation rate, need for potentially toxic initiators, and thermal or storage instability. In addition, a limited number of these photoinduced systems demonstrate photoreversibility. [2,3,9,10] Andreopoulos et al. [3] have synthesized a partially reversible hydrogel via photopolymerization of cinnamylidene-terminated PEG. The physical properties of the hydrogel membrane such as pore size and swellability were controlled in a predictive way by alternating the wavelength (>300/254 nm) and sequence of irradiation. The photoreversibility efficiency of the PEG-cinnamylidene hydrogel, however, was compromised by photoscission light inefficiency, cinnamylidene photodegradation, and side polymerization reactions. [10] In the work reported here, we designed a new photocrosslinked and photoscissile hydrogel based on an eight-branched PEG with nitrocinnamate as pendant groups. Cinnamate is known to undergo trans‐cis isomerization and [2+2] cycloaddition upon UV irradiation at wavelengths longer than 290 nm, and the formed cyclobutane ring can be cleaved to regenerate the starting cinnamate groups at wavelengths below 260 nm. [11] The photocrosslinking property of cinnamates has been broadly utilized in the field of photolithography and the semiconductor industry. [12] However, the application of nitrocinnamate photoreactivity in PEG chemistry, especially in PEG-based hydrogel formation, has rarely been reported. [6,13] On the other hand, cinnamate derivatives possess excellent thermal and storage stability superior to cinnamylidene systems. [14] PEG-cinnamylidene polymers have been shown to be unstable and undergo gelation when they are kept at room temperature for a period of a few weeks. Nitrocinnamate demonstrates the stability characteristics of the unsubstituted cinnamate and at the same time is 350 times more photoreactive. [11,15]

A multiplexed immunoassay system based upon reciprocating centrifugal microfluidics

A novel, centrifugal disk-based micro-total analysis system (μTAS) for low cost and high throughput semi-automated immunoassay processing was developed. A key innovation in the disposable immunoassay disk design is in a fluidic structure that enables very efficient micro-mixing based on a reciprocating mechanism in which centrifugal acceleration acting upon a liquid element first generates and stores pneumatic energy that is then released by a reduction of the centrifugal acceleration, resulting in a reversal of direction of flow of the liquid. Through an alternating sequence of high and low centrifugal acceleration, the system reciprocates the flow of liquid within the disk to maximize incubation/hybridization efficiency between antibodies and antigen macromolecules during the incubation/hybridization stage of the assay. The described reciprocating mechanism results in a reduction in processing time and reagent consumption by one order of magnitude.

Probing the lignin nanomechanical properties and lignin–lignin interactions using the atomic force microscopy

By combining atomic force microscopy (AFM) force and environmental scanning electron microscopies (ESEMs), herein we present an evidence for the existence of strong intermolecular forces, which are responsible for holding lignin globules together in higher ordered structures. Based on this observation, we provide a support for the hypothesis that lignin globules consist of at least two individual spherical layers, with space in between filled with solvent or gas.

Correlated topographic and spectroscopic imaging beyond diffraction limit by atomic force microscopy metallic tip-enhanced near-field fluorescence lifetime microscopy

A near-field optical imaging approach is demonstrated for simultaneous topographic and spectroscopic imaging with spatial resolution beyond the optical diffraction limit. The method combines metallic-tip-based tapping-mode atomic force microscopy (AFM) with fluorescence lifetime imaging microscopy (FLIM). The AFM metallic tip was formed by sputter coating a Si tapping mode tip with Au, in a way that forms a globular tip apex. Such tip apex generates high local electric field enhancement under laser illumination, which provides a strong electric-field interaction between the AFM tip and the fluorescent molecules under the tip. The tip perturbation of fluorescence gives the fluorescence lifetime changes that provide the AFM–FLIM imaging contrast. A finite element method simulation was used to further evaluate the electric near-field enhancement and electric field distribution originating from the metallic Au-coated AFM tapping-mode tip. We have demonstrated that spatially mapping the change in fluorescence ...

Studies of a novel polymerizable amphiphilic dendrimer

Abstract In the present study, we have synthesized a polymerizable amphiphilic dendrimer by attaching 10,12-pentacosadiynoic acid (PDA) to a third generation poly(amidoamine) (PAMAM) dendrimer core (PDA-PAMAM). This dendrimer was characterized by 1H NMR and MALDI-TOF mass spectroscopy. The amphiphilic property of this dendrimer has been studied. Surface pressure and surface dipole moment–area isotherm measurements have shown that PDA-PAMAM forms a stable monolayer at the air–water interface with a limiting molecular area of 460 A2 per molecule. The compressed monolayer can be readily polymerized upon UV irradiation, as like other PDA derivatives. The topography of the monolayer was observed by Brewster angle microscopy (BAM) as well as by the environmental scanning electron microscopy (ESEM). We found a good correlation between the size and shape of monolayer domains observed with the BAM and ESEM techniques. To our knowledge, this is the first time wet mode ESEM has been used to characterize L–B film. The wet mode ensures that L–B films remain intact without being damaged by the high vacuum system of traditional SEM. As a preliminary study, we have found that this new dendrimer forms colloidal particles in chloroform solution and can be readily polymerized by UV irradiation.

Surface chemistry of lipid raft and amyloid Aβ (1-40) Langmuir monolayer

Lipid rafts being rich in cholesterol and sphingolipids are considered to provide ordered lipid environment in the neuronal membranes, where it is hypothesized that the cleavage of amyloid precursor protein (APP) to Aβ (1-40) and Aβ (1-42) takes place. It is highly likely that the interaction of lipid raft components like cholesterol, sphingomylein or GM1 leads to nucleation of Aβ and results in aggregation or accumulation of amyloid plaques. One has investigated surface pressure-area isotherms of the lipid raft and Aβ (1-40) Langmuir monolayer. The compression-decompression cycles and the stability of the lipid raft Langmuir monolayer are crucial parameters for the investigation of interaction of Aβ (1-40) with the lipid raft Langmuir monolayer. It was revealed that GM1 provides instability to the lipid raft Langmuir monolayer. Adsorption of Aβ (1-40) onto the lipid raft Langmuir monolayer containing neutral (POPC) or negatively charged phospholipid (DPPG) was examined. The adsorption isotherms revealed that the concentration of cholesterol was important for adsorption of Aβ (1-40) onto the lipid raft Langmuir monolayer containing POPC whereas for the lipid raft Langmuir monolayer containing DPPG:cholesterol or GM1 did not play any role. In situ UV-vis absorption spectroscopy supported the interpretation of results for the adsorption isotherms.

New Insights into the Structural Organization of the Plant Polymer Lignin

Abstract: The organizational features of lignin structure and the mechanism of its synthesis have significant implications for the response of the plant to stress. It was unknown whether the enzymic formation of lignin in the cell wall is an uncontrolled process or finely regulated in time and space. In vitro scanning tunneling microscopy (STM), atomic force microscopies (AFM), near‐field scanning optical microscopy (NSOM). and the novel environmental scanning electron microscopy (ESEM) imaging studies of the lignin model compounds have directly shown its highly ordered structure and elucidated its modular and fractal organization. Direct evidence was presented for the existence of strong intermolecular forces responsible for holding lignin globules together in highly ordered structures. Fractal analysis was applied as a theoretical approach, to show regularity and modular organization of lignin. Surface chemistry studies of the lignin monolayer reveal intrinsic properties that may be a key to osmotic pressure and cell size control mechanism in the higher plant cells. The obtained data contribute to the explanation of the mechanisms of cell wall synthesis in vivo.