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Search Results: 1 - 10 of 301485 matches for " Wolfgang J. Parak "
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Characterization of protein adsorption onto FePt nanoparticles using dual-focus fluorescence correlation spectroscopy
Pauline Maffre,Karin Nienhaus,Faheem Amin,Wolfgang J. Parak
Beilstein Journal of Nanotechnology , 2011, DOI: 10.3762/bjnano.2.43
Abstract: Using dual-focus fluorescence correlation spectroscopy, we have analyzed the adsorption of three human blood serum proteins, namely serum albumin, apolipoprotein A-I and apolipoprotein E4, onto polymer-coated, fluorescently labeled FePt nanoparticles (~12 nm diameter) carrying negatively charged carboxyl groups on their surface. For all three proteins, a step-wise increase in hydrodynamic radius with protein concentration was observed, strongly suggesting the formation of protein monolayers that enclose the nanoparticles. Consistent with this interpretation, the absolute increase in hydrodynamic radius can be correlated with the molecular shapes of the proteins known from X-ray crystallography and solution experiments, indicating that the proteins bind on the nanoparticles in specific orientations. The equilibrium dissociation coefficients, measuring the affinity of the proteins to the nanoparticles, were observed to differ by almost four orders of magnitude. These variations can be understood in terms of the electrostatic properties of the proteins. From structure-based calculations of the surface potentials, positively charged patches of different extents can be revealed, through which the proteins interact electrostatically with the negatively charged nanoparticle surfaces.
Time-Resolved Fluorescence Immunoassay for C-Reactive Protein Using Colloidal Semiconducting Nanoparticles
Harri H?rm?,Juha Toivonen,Juhani T. Soini,Pekka H?nninen,Wolfgang J. Parak
Sensors , 2011, DOI: 10.3390/s111211335
Abstract: Besides the typical short-lived fluorescence with decay times in the nanosecond range, colloidal II/VI semiconductor nanoparticles dispersed in buffer also possess a long-lived fluorescence component with decay times in the microsecond range. Here, the signal intensity of the long-lived luminescence at microsecond range is shown to increase 1,000-fold for CdTe nanoparticles in PBS buffer. This long-lived fluorescence can be conveniently employed for time-gated fluorescence detection, which allows for improved signal-to-noise ratio and thus the use of low concentrations of nanoparticles. The detection principle is demonstrated with a time-resolved fluorescence immunoassay for the detection of C-reactive protein (CRP) using CdSe-ZnS nanoparticles and green light excitation.
Quantification of the internalization patterns of superparamagnetic iron oxide nanoparticles with opposite charge
Christoph Schweiger, Raimo Hartmann, Feng Zhang, Wolfgang J. Parak, Thomas H. Kissel, Pilar Rivera_Gil
Journal of Nanobiotechnology , 2012, DOI: 10.1186/1477-3155-10-28
Abstract: The physicochemical characterization of the nanoparticles showed particles of approximately the same size and shape as well as similar magnetic properties, only differing in charge due to different surface coatings. Incubation of the cells with both nanoparticles resulted in strong differences in the internalization rate and in the intracellular localization depending on the charge. Quantitative and qualitative analysis of nanoparticles-organelle colocalization experiments revealed that positively charged particles were found to enter the cells faster using different endocytotic pathways than their negative counterparts. Nevertheless, both nanoparticles species were finally enriched inside lysosomal structures and their efficiency in agarose phantom relaxometry experiments was very similar.This quantitative analysis demonstrates that charge is a key factor influencing the nanoparticle-cell interactions, specially their intracellular accumulation. Despite differences in their physicochemical properties and intracellular distribution, the efficiencies of both nanoparticles as MRI agents were not significantly different.
Getting Across the Plasma Membrane and Beyond: Intracellular Uses of Colloidal Semiconductor Nanocrystals
Camilla Luccardini,Aleksey Yakovlev,Stéphane Gaillard,Marcel van ‘t Hoff,Alicia Piera Alberola,Jean-Maurice Mallet,Wolfgang J. Parak,Anne Feltz,Martin Oheim
Journal of Biomedicine and Biotechnology , 2007, DOI: 10.1155/2007/68963
Abstract: Semiconductor nanocrystals (NCs) are increasingly being used as photoluminescen markers in biological imaging. Their brightness, large Stokes shift, and high photostability compared to organic fluorophores permit the exploration of biological phenomena at the single-molecule scale with superior temporal resolution and spatial precision. NCs have predominantly been used as extracellular markers for tagging and tracking membrane proteins. Successful internalization and intracellular labelling with NCs have been demonstrated for both fixed immunolabelled and live cells. However, the precise localization and subcellular compartment labelled are less clear. Generally, live cell studies are limited by the requirement of fairly invasive protocols for loading NCs and the relatively large size of NCs compared to the cellular machinery, along with the subsequent sequestration of NCs in endosomal/lysosomal compartments. For long-period observation the potential cytotoxicity of cytoplasmically loaded NCs must be evaluated. This review focuses on the challenges of intracellular uses of NCs.
Light triggered detection of aminophenyl phosphate with a quantum dot based enzyme electrode
Waqas Khalid, Gero G?bel, Dominik Hühn, Jose-Maria Montenegro, Pilar Rivera-Gil, Fred Lisdat, Wolfgang J Parak
Journal of Nanobiotechnology , 2011, DOI: 10.1186/1477-3155-9-46
Abstract: Colloidal quantum dots (QDs), which are fluorescent semiconductor nanoparticles, have recently brought impact to various disciplines, as has been highlighted in various review articles [1-5]. QDs have been recently discussed also as new building blocks for the construction of electrochemical sensors [6-12]. Upon optical illumination (below the wavelength of the first exciton peak QDs have a a continuous absorption spectrum, with a local maximum at the exciton peak [13]) electron hole pairs are generated inside QDs. Due to these charge carriers electrons can be transferred to or from the QDs. QDs thus can be oxidized/reduced and can serve as light-controlled redox active element and can be integrated in electrochemical signal chains [9,14-16]. The key advantage hereby is that the redox reaction of the QD surface can be virtually switched on and off by light. QD have been also used as elements of signal transduction of enzymatic reactions [17,18].In the present work we wanted to apply QDs as light-controlled redox active element for the enzymatic detection of p-aminophenyl phosphate (pAPP) with alkaline phosphatase (ALP). ALP is a widely used enzyme in bioanalysis as it has a high turnover rate and broad substrate specificity [19]. The enzyme is particularly interesting as label for immunoassays [20,21]. Very sensitive substrate recycling schemes have been also reported [22,23]. Four different groups of substrates are known for ALP: i) ?-glycerophosphate and hexose phosphate [24-26], ii) phenyl phosphate [27,28] and ?-naphthyl phosphate [29], iii) p-nitrophenyl phosphate [30] and phenolphthalein diphosphate [31,32], 4-methyl-umbellipheryl phosphate [33] and p-aminophenyl phosphate (pAPP) [34], and iv) phosphoenol pyruvate [35]. Electrochemical detection has been reported for a number of ALP substrates [36,37], in particular for phenyl phosphate. However, pAPP is claimed to be a better substrate for ALP than phenyl phosphate, as its product 4-aminophenol (4AP) is more
Magnetic capsules for NMR imaging: Effect of magnetic nanoparticles spatial distribution and aggregation
Azhar Zahoor Abbasi,Lucía Gutiérrez,Loretta L. del Mercato,Fernando Herranz,Oksana Chubykalo-Fesenko,Sabino Veintemillas-Verdaguer,Wolfgang J. Parak,M Puerto Morales,Jesús M González,Antonio Hernandoe,Patricia de la Presa
Physics , 2011, DOI: 10.1021/jp1118234
Abstract: Magnetic and NMR relaxivity properties of {\gamma}-Fe2O3 nanoparticles embedded into the walls of polyelectrolyte multilayer capsules and freely dispersed in a sodium borate buffer solution have been investigated. The different geometric distribution of both configurations provides the opportunity to study the relationship of water accessibility and magnetic properties of particles on the NMR relaxivity. Changes in their blocking temperature and average dipolar field were modeled as a function of packing fraction in the ensemble of free and entrapped nanoparticles. For free nanoparticles with relatively low concentration, relaxivity values increase with packing fraction according to an increase in the dipolar field and larger water accessibility. However for embedded NPs in the capsule wall, packing fractions should be limited to optimise the efficiency of this system as magnetic resonance imaging (MRI) contrast agent.
Gel Electrophoresis of Gold-DNA Nanoconjugates
T. Pellegrino,R. A. Sperling,A. P. Alivisatos,W. J. Parak
Journal of Biomedicine and Biotechnology , 2007, DOI: 10.1155/2007/26796
Abstract: Gold-DNA conjugates were investigated in detail by a comprehensive gel electrophoresis study based on 1200 gels. A controlled number of single-stranded DNA of different length was attached specifically via thiol-Au bonds to phosphine-stabilized colloidal gold nanoparticles. Alternatively, the surface of the gold particles was saturated with single stranded DNA of different length either specifically via thiol-Au bonds or by nonspecific adsorption. From the experimentally determined electrophoretic mobilities, estimates for the effective diameters of the gold-DNA conjugates were derived by applying two different data treatment approaches. The first method is based on making a calibration curve for the relation between effective diameters and mobilities with gold nanoparticles of known diameter. The second method is based on Ferguson analysis which uses gold nanoparticles of known diameter as reference database. Our study shows that effective diameters derived from gel electrophoresis measurements are affected with a high error bar as the determined values strongly depend on the method of evaluation, though relative changes in size upon binding of molecules can be detected with high precision. Furthermore, in this study, the specific attachment of DNA via gold-thiol bonds to Au nanoparticles is compared to nonspecific adsorption of DNA. Also, the maximum number of DNA molecules that can be bound per particle was determined.
The rational formula from the runhydrograph
M Parak, GGS Pegram
Water SA , 2006,
Abstract: The rational formula is possibly the simplest flood estimation technique available using rainfall-runoff relationships. In spite of the many criticisms regarding its over-simplification of the processes of rainfall conversion into runoff, it remains possibly the most widely used method for estimating peak flood flows for urban drainage systems and small (<100 km2) rural catchments. However, as a result of the criticisms, the formula carries with it many cautions. One such caution regards the determination of the formula\'s runoff coefficient c, which is seen as the main difficulty in the design application of the formula. Mindful of this, it was decided to investigate the calibration of this coefficient, on past flood peak and flood volume pairs for a number of catchments in South Africa. To this end the “data set” of runhydrographs, which describe the characteristic peak and volume discharges of a catchment for a given recurrence interval, was used to calibrate the coefficients for selected catchments and to explore the assumptions underpinning this simple model. This article describes the methods employed in achieving this as well as a discussion of the results. Water SA Vol.32 (2) 2006: pp.163-180
A review of the regional maximum flood and rational formula using geomorphological information and observed floods
Geogg Pegram, Mohamed Parak
Water SA , 2004,
Abstract: Flood estimation methods in South Africa are based on three general approaches: empirical, deterministic and probabilistic. The \"quick\" methods often used as checks are the regional maximum flood (RMF) and the rational formula (RF), which form part of the empirical and deterministic methods respectively. A database of annual flood peaks was used in a probabilistic approach to review these methods and to provide preliminary insight into their estimates of flood peaks. This paper examines the following: the relationship between floods and landscape; the estimation of the return period of the RMF; the use of ratios in scaling RMF flood peak estimates to flow rates of shorter return periods; the applicability of the modified rational formula (MRF); the examination of the relationship between scaling parameters and regional parameters. It turns out that the RMF is the best of all methods examined in this preliminary study (other than statistical) in estimating the 200-year flood peak at an ungauged location. WaterSA Vol.30 (3) 2004: 377-392
Radiation damage in biological material: electronic properties and electron impact ionization in urea
C. Caleman,C. Ortiz,E. Marklund,F. Bultmark,M. Gabrysch,F. G. Parak,J. Hajdu,M. Klintenberg,N. Timneanu
Physics , 2008, DOI: 10.1209/0295-5075/85/18005
Abstract: Radiation damage is an unavoidable process when performing structural investigations of biological macromolecules with X-ray sources. In crystallography this process can be limited through damage distribution in a crystal, while for single molecular imaging it can be outrun by employing short intense pulses. Secondary electron generation is crucial during damage formation and we present a study in urea, as model for biomaterial. From first principles we calculate the band structure and energy loss function, and subsequently the inelastic electron cross section in urea. Using Molecular Dynamics simulations, we quantify the damage and study the magnitude and spatial extent of the electron cloud coming from an incident electron, as well as the dependence with initial energy.
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