Dynamic Light Scattering Data Research Articles

Interaction of Jatrorrhizine with Human Gamma Globulin in membrane mimetic environments: Probing of the binding mechanism and binding site by spectroscopic and molecular modeling methods

The interaction between Jatrorrhizine and Human Gamma Globulin (HGG) in AOT/isooctane/water microemulsions was studied by using fluorescence quenching, UV absorption spectroscopy, circular dischroism (CD) spectroscopy and dynamic light scattering (DLS). Fluorescence data in water-surfactant molar ratio ( ω 0) 25 microemulsions revealed the presence of the binding site of Jatrorrhizine on HGG and its binding constants at four temperatures were obtained. The affinities in microemulsions were similar to that in buffer solution. The alterations of ΗGG secondary structure in the microemulsions in the absence and presence of Jatrorrhizine compared with the free form of HGG in buffer were analyzed by CD spectroscopy. In addition, the DLS data suggested that HGG may locate inside the microemulsion and Jatrorrhizine could interact with them. Furthermore, the thermodynamic functions, i.e. standard enthalpy ( Δ H 0 ) and standard entropy ( Δ S 0 ) for the reaction were also calculated, according to Van’t Hoff equation. These data showed that hydrophobic and electrostatic interaction played the main role in the binding of Jatrorrhizine to HGG.

Physicochemical and pharmacological characterization of α-tocopherol-loaded nano-emulsion system

The main purpose of the present study is to develop a novel nano-emulsion (NE) formulation of α-tocopherol (α-TC) with enhanced oral bioavailability and pharmacological effects. Three NE formulations of α-TC at different loading amounts (10%, 30% and 50%) were prepared by a mechanochemical method. Physicochemical properties of NE formulations were characterized with a focus on the morphology by transmission electron microscopy (TEM), droplet size distribution and zeta-potential by dynamic light scattering (DLS), and long-term stability. According to the TEM images and DLS data, mean diameters of NE droplets ranged from 80 to 400 nm, in proportion to the amount of loaded α-TC. Although all NE formulations of α-TC were found to be negatively charged with the zeta-potential of ca −40 mV, NE formulations at α-TC content of 30% or higher exhibited severe aggregation of droplets in NE formulations during long-term storage. After oral administration of 10% α-TC-loaded NE formulation (30 mg α-TC/kg) in rats, higher α-TC exposure was observed with a 2.6-fold increase of bioavailability as compared to the control mixture of oil and α-TC. In streptozotocin-induced diabetic rats, oral administration of the α-TC-loaded NE formulation (30 mg α-TC/kg) exhibited a significant reduction of lipoperoxidant in several organs, especially the liver; however, the control mixture was less effective. With these findings, the NE approach might be efficacious to improve the oral bioavailability and anti-oxidative activities of α-TC.

Effect of an amphiphilic diol (Surfynol ®) on the micellar characteristics of PEO–PPO–PEO block copolymers in aqueous solutions

The aggregation and clouding behaviors of a series of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymers, PEO–PPO–PEO (Pluronic ®) containing ethylene oxide (EO) content 30% (P103 and P123), 40% (P84), 50% (P85) and 70% (F87 and F127) in the presence of an amphiphilic diol (C 14diol or Surfynol ®104) were investigated using cloud point, viscometry, dynamic light scattering (DLS) and small angle neutron scattering (SANS). DLS and SANS data reveal micellar growth/transition in the presence of diol; such changes strongly dependant on the EO content and hence on the hydrophilic lipophilic balance (HLB) value of the copolymer. Copolymer with low HLB value showed noticeable growth in the micellar size as compared to one with high HLB. P123 and P103, the least hydrophilic (HLB ∼ 8/9) among the copolymers studied here, show a series of structural changes with the addition of Surfynol. Highly temperature sensitive microstructures and hence the rheological behavior were also observed in P103. These structural changes in micelles are explained in terms of the hydrophobic interaction between the copolymer and diol. The observed results correlate well with the reported HLB values of the block copolymer.

Acoustic properties of metal oxides aqueous suspensions

The feasibility of using aqueous suspensions of several metal oxide nanoparticles as contrast agents for ultrasonic cardiovascular visualization has been investigated. Nanopowders of Al, Fe, and Zr oxides were prepared by the electrical explosion of metallic wires, and characteristics such as particle size, specific area, and particle size distribution, were obtained, as were TEM photographs. Dynamic light-scattering data demonstrated that the aggregation of nanoparticles took place in their aqueous suspensions in the function of mean particle size. Acoustic properties of stable suspensions of nanopowders were studied in an experimental model of circulation with simultaneous measurements performed by a commercially available standard ultrasonic apparatus used in medical institutions. The results indicate that the intensity of the reflected echo signal depends on the chemical nature of the metal oxide and the structure of nanoparticles and aggregates that are formed in an aqueous medium. The presented data show the high ultrasonic echo activity of several metal oxides and, at the same time, form a scientific background for a better understanding of the interactions between nanoparticles and the ultrasonic wave.

A New Approach for the Characterization of Reverse Micellar Systems by Dynamic Light Scattering

This paper reports the use of dynamic light scattering (DLS) to study reverse micelles formed by the water/Igepal CO-520/cyclohexane system over a large range of global compositions. A novel approach for data analysis is presented, based on the realization that micelles of a given size must be in equilibrium with free surfactant of a fixed concentration. Compilation of the DLS data into sets of fixed micelle size but differing global compositions therefore allows for the determination of parameters such as free surfactant concentration, micellar molar composition, surfactant interfacial area, and aggregation numbers. Importantly, this method gives access to the variation of each of these parameters with micelle size, as is essential for the characterization of reverse micelles formed by nonionic surfactants. This approach constitutes a significant complement to other available characterization methods. The analysis also provides insight into the primary factors controlling the equilibrium distribution of surfactant within the system and the relative stability of the micelles.

Fraction estimation of small, dense LDL using autocorrelation function of dynamic light scattering

Small, dense low-density lipoprotein (sdLDL) in total LDL is strongly related with the cardiovascular risk level. An optical technique using dynamic light scattering (DLS) measurement is useful for point-of-care testing of sdLDL. However, the sdLDL fraction estimated from the particle size distribution in DLS data is sensitive to noise and artifacts. Therefore, we derived analytical solutions in a closed form to estimate the fraction of scatterers using the autocorrelation function of scattered light from a polydisperse solution. The effect of the undesired large particles can be eliminated by the pre-processing of the autocorrelation function. The proposed technique was verified using latex standard particles and LDL solutions. Results suggest the feasibility of this technique to estimate the sdLDL fraction using optical scattering measurements.

Effect of nanoparticle concentration on zeta-potential measurement results and reproducibility

The effect of nanoparticle concentration on zeta-potential measurement results at dilute concentrations was evaluated. The values of the zeta-potential for four different types of nanoparticles, Ludox (silica), multi-walled carbon nanotubes (bamboo-shaped and hollow nanotubes) and gold, at various concentrations, were obtained using a laser Doppler electrophoresis instrument. The size of the nanoparticles on dilution was measured using dynamic light scattering (DLS). The results show that there is a concentration range within which the zeta-potential, and particle size, are not affected by nanoparticle concentration. The lower concentration limit for the system to produce consistent results was dependent on the nature of the sample under study and ranged between 10 −2 and 10 −4 wt%. Below this concentration, there was an apparent shift in zeta-potential values to less negative values, which was accompanied by an increase in the particle size. The shift in zeta-potential was attributed to an increase in contribution of the signal from extraneous particulate matter. The increase in particle size was attributed to the nature of the homodyne optical configuration of the instrument. The aim of this study was to elucidate the range in nanoparticle concentration that allows for accurate and reliable measurement of the zeta-potential and DLS data.

Spectroscopic determination of the size of cadmium sulfide nanoparticles formed under environmentally relevant conditions

A multi-technique approach was adopted using UV/visible spectroscopy, fluorescence spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM) in the characterization of cadmium sulfide (CdS) nanoparticles. CdS nanoparticle sizes could be determined by the Brus equation, which relates the UV/vis spectroscopic properties of CdS nanoparticles to size based on the quantum confinement effect. The diameters calculated from the wavelength of absorbance in UV/vis spectra were within 10% of the mean nanoparticle diameter measured in TEM. UV/vis spectroscopy provided an aqueous phase measurement of the CdS core size of the nanoparticles (<2 nm to approximately 10 nm diameter) that is based on a physical property rather than light scattering. CdS nanoparticles readily formed upon addition of sulfide to a Cd(ii)-thiolate complex with the thiolate molecules acting as capping agents that passivate the nanoparticle surface, thereby preventing bulk mineral precipitation. Using UV/vis as a method of nanoparticle sizing, we were able to demonstrate how aqueous conditions dictate the resulting nanoparticle size. High pH, capping thiolate concentration and Cd : S ratio all resulted in smaller nanoparticles. Ionic strength did not influence nanoparticle size, but DLS data indicate the formation of aggregates above ionic strengths of 0.1 M.

Synthesis, micelle formation, and bulk properties of poly(ethylene glycol)‐b‐poly(pentafluorostyrene)‐g‐polyhedral oligomeric silsesquioxane amphiphilic hybrid copolymers

AbstractThe synthesis, micelle formation, and bulk properties of semifluorinated amphiphilic poly(ethylene glycol)‐b‐poly(pentafluorostyrene)‐g‐cubic polyhedral oligomeric silsesquioxane (PEG‐b‐PPFS‐g‐POSS) hybrid copolymers is reported. The synthesis of amphiphilic PEG‐b‐PPFS block copolymers are achieved using atom transfer radical polymerization (ATRP) at 100 °C in trifluorotoluene using modified poly(ethylene glycol) as a macroinitiator. Subsequently, a proportion of the reactive para‐F functionality on the pentafluorostyrene units was replaced with aminopropylisobutyl POSS through aromatic nucleophilic substitution reactions. The products were fully characterized by 1H‐NMR and GPC. The products, PEG‐b‐PPFS and PEG‐b‐PPFS‐g‐POSS, were subsequently self‐assembled in aqueous solutions to form micellar structures. The critical micelle concentrations (cmc) were estimated using two different techniques: fluorescence spectroscopy and dynamic light scattering (DLS). The cmc was found to decrease concomitantly with the number of POSS particles grafted per copolymer chain. The hydrodynamic particle sizes (Rh) of the micelles, calculated from DLS data, increase as the number of POSS molecules grafted per copolymer chain increases. For example, Rh increased from ∼60 nm for PEG‐b‐PPFS to ∼80 nm for PEG‐b‐PPFS‐g‐POSS25 (25 is the average number of POSS particles grafted copolymer chain). Static light scattering (SLS) data confirm that the formation of larger micelles by higher POSS containing copolymers results from higher aggregation numbers (Nagg), caused by increased hydrophobicity. The Rg/Rh values, where Rg is the radius of gyration calculated from SLS data, are consistent with a spherical particle model having a core‐shell structure. Thermal characterization by differential scanning calorimetry (DSC) reveals that the grafted POSS acts as a plasticizer; the glass transition temperature (Tg) of the PPFS block in the copolymer decreases significantly with increasing POSS content. Finally, the rhombohedral crystal structure of POSS in PEG‐b‐PPFS‐g‐POSS was verified by wide angle X‐ray diffraction measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 152–163, 2010

Unusual behavior in light scattering experiments of poly(N-vinylimidazole) prepared by precipitation polymerization

Poly(N-vinylimidazole) (PVI) was synthesized by the precipitation polymerization using 2,2’-azobis(isobutyronitrile) (AIBN; initiator) and benzene (solvent) at two different monomer/initiator ratios. The solution polymerization was also performed with the following initiator/solvent systems: AIBN/water–methanol mixture (1:1 by volume) and 4,4’-azobis(4-cyanovaleric acid)/aqueous HCl solution (pH 0.8). All the four preparations of PVI in ethanol and in 0.2 M NaCl (pH 3 with HCl) were examined by dynamic light scattering (DLS). The CONTIN analysis of DLS data for each preparation from the solution polymerization showed a unimodal distribution in both ethanol and aqueous solvents. A good agreement was obtained between the molar masses in these different solvents by static light scattering (SLS). However, the polymers from the precipitation polymerization exhibited a heterogeneous bimodal distribution in DLS under the same conditions as above, indicating that the SLS data as in reference [6] (Savin et al. Macromolecules 37:6565) cause a serious error in the understanding of solution behavior of PVI.

Fluctuation dynamics of spherical vesicles: Frustration of regular bulk dissipation into subdiffusive relaxation

Spherical lipid vesicles obtained by the extrusion method are nonequilibrium membrane structures more curved than the zero spontaneous curvature equilibrium state of the bilayer. Furthermore, these structures are quite rigid as compared to spontaneous vesicles or microemulsion droplets made of soluble surfactants. The dynamical description of the shape fluctuations derived by Milner and Safran (MS) [Phys. Rev. A 36, 4371 (1987)], which is based on the elastic Helfrich energy referred to the equilibrium state, could be misleading in these cases. In the present contribution, shape fluctuations of unilamellar palmitoyl-oleyl-phosphocholine (POPC) vesicles (radius Rh<or=100 nm ) prepared by extrusion are studied by means of neutron spin echo (NSE) in combination with dynamic light scattering (DLS). The relaxation of the fluctuation modes is inferred from the DLS autocorrelation functions and from the intermediate NSE scattering functions measured for several different values of the wave vector. The observed relaxations are compatible with a stretched-exponential decay rather than the single-exponential behavior. Dynamical frustration of the bulk dissipation mechanism in the way described by Zilman and Granek (ZG) for weak amplitude fluctuations [Phys. Rev. Lett. 77, 4788 (1996)] is invoked as a plausible scenario for explaining the subdiffusive nonexponential relaxations experimentally observed. The combined analysis of NSE and DLS data allows a calculation of the bending elastic constant of POPC vesicles kappa=19+/-2 kBT , in excellent agreement with literature data. The ZG approach is revealed as the adequate extension of the MS theory to describe fluctuation dynamics of rigid membranes.

Design and Synthesis of a Stable Supramolecular Trigonal Prism Formed by the Self-Assembly of a Linear Tetrakis(Zn2+−cyclen) Complex and Trianionic Trithiocyanuric Acid in Aqueous Solution and Its Complexation with DNA (Cyclen = 1,4,7,10-Tetraazacyclododecane)

A new supramolecular complex, {(Zn(4)L(4))(3)-(TCA(3-))(4)}(12+), was designed and synthesized by the 3:4 self-assembly of a linear tetrakis(Zn(2+)-cyclen) complex (Zn(4)L(4))(8+) and trianionic trithiocyanurate (TCA(3-)) in aqueous solution (cyclen = 1,4,7,10-tetraazacyclododecane). The {(Zn(4)L(4))(3)-(TCA(3-))(4)}(12+) complex, which should have a trigonal prism configuration, was found to be very stable in aqueous solution at neutral pH and 25 degrees C, as evidenced by (1)H NMR titration, potentiometric pH and UV titrations, and MS measurements. The complex does not dissociate into the starting building blocks in the presence of Zn(2+)-binding anions such as phosphates and double-stranded DNA. The results of the competitive binding assays with ethidium bromide and calf-thymus DNA, thermal melting experiments, gel mobility shift assays, and dynamic light-scattering data strongly indicated that the trigonal prism functions as a polycationic template to induce the aggregation of double-stranded DNA.

EARLY CATARACT DETECTION BY DYNAMIC LIGHT SCATTERING WITH SPARSE BAYESIAN LEARNING

Dynamic light scattering (DLS) is a promising technique for early cataract detection and for studying cataractogenesis. A novel probabilistic analysis tool, the sparse Bayesian learning (SBL) algorithm, is described for reconstructing the most-probable size distribution of α-crystallin and their aggregates in an ocular lens from the DLS data. The performance of the algorithm is evaluated by analyzing simulated correlation data from known distributions and DLS data from the ocular lenses of a fetal calf, a Rhesus monkey, and a man, so as to establish the required efficiency of the SBL algorithm for clinical studies.

Quantification of initial steps of nucleation and growth of silica nanoparticles: An in-situ SAXS and DLS study

The initial steps of silica polymerization and silica nanoparticle formation have been studied in-situ and in real-time. The experiments were carried out in near neutral pH (7–8) solutions with initial silica concentrations of 640 and 1600 ppm ([SiO 2]) and ionic strengths (IS) of 0.02, 0.05, 0.11 and 0.22 M. The polymerization reactions were induced by neutralizing a high pH silica solution (from pH 12 to 7) and monitored by the time-dependent depletion in monosilicic acid concentration over time. The accompanied nucleation and growth of silica nanoparticles (i.e., change in particle size over time) was followed in-situ using time-resolved synchrotron-based Small Angle X-ray Scattering (SAXS) and conventional Dynamic Light Scattering (DLS) combined with scanning and (cryo)-transmission electron microscopy (SEM/cryo-TEM). The critical nucleus diameter was quantified (1.4–2 nm) and results from SAXS and DLS showed that over 3 h the particle diameter increased to a final size of ∼8 nm. SEM and TEM photomicrographs verified the SAXS and DLS data and confirmed the spherical and hydrous structure of the forming silica nanoparticles. Furthermore, fractal analysis (i.e., fractal dimension, D m ∼ 2.2) indicated that the formed particles consisted of open, polymeric, low-density structures. For the nucleation and growth of silica nanoparticles a 3-stage growth process is proposed: (1) homogeneous and instantaneous nucleation of silica nanoparticles, (2) 3-D, surface-controlled particle growth following 1st order reaction kinetics and (3) Ostwald ripening and particle aggregation.

Sodium Glycodeoxycholate and Glycocholate Mixed Aggregates in Gas and Solution Phases

This paper deals with electrospray ionization mass spectrometry (ESIMS), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) measurements in order to provide information on the existence, aggregation, composition, and structure of the two-component aggregates of sodium glycocholate (NaGC) and sodium glycodeoxycholate (NaGDC) in the gas and solution phases. Five samples, containing 100% NaGC and 100% NaGDC, and NaGDC/NaGC molar ratios of 3 (75D), 1 (50D), and 1/3 (25D), have been analyzed by ESIMS in positive-ion detection mode starting from 10(-3) and 10(-2) M total bile salt concentration in aqueous solutions. Generally, dimers or trimers prevail in the 100% NaGC or NaGDC samples, respectively, as observed in the preceding one-component ESIMS measurements and in agreement with the proposed micellar aggregate structures in aqueous solution. Moreover, it is observed that the composition of multimers in the samples 75D, 50D, and 25D deviates from the one expected on the basis of a random association of the monomers, the NaGDC contribution generally prevailing on the NaGC one. It happens also under the same percentage condition (50D sample), in agreement with a greater aggregation ability of NaGDC with respect to NaGC. SAXS and DLS data were recorded on six samples containing a NaGC+NaGDC 40 mM total concentration, one bile salt having 40, 32, 24, 16, 8, and 0 mM concentration and the other the complementary one, keeping constant the NaCl concentration (0.6 M). The NaGDC 40 mM sample presents SAXS curves in agreement with a cylindrical shape of the aggregates as shown in a previous paper. For the bile salt mixtures, the progressive decrease of the sizes and change of the aggregate morphology, toward a globular-like geometry, are observed by increasing the NaGC fraction, thus confirming the hypothesis about the ability of trihydroxy salts to inhibit the growth of dihydroxy salt aggregates. Fits on the basis of cylindrical model can be accomplished for all the SAXS spectra, however, when the extracted cylinder parameters are used to estimate theoretical hydrodynamic radii a reasonable agreement is obtained only for the samples at high fraction of NaGDC (NaGDC>or=24 mM).

Complex Formation between a Fluorescently-Labeled Polyelectrolyte and a Triblock Copolymer

The association between a randomly pyrene labeled PAA polymer (PAAMePy55) and a PEO-PPO-PEO triblock copolymer (P123) in aqueous solutions of different NaCl concentrations and pHs has been studied by means of dynamic light scattering (DLS) and steady-state fluorescence spectroscopy at 40 degrees C. At acidic pH values, in the low P123/PAAMePy55 molar ratio regime (i.e., at low P123 concentrations), the relaxation time distributions retrieved from the DLS data analysis were monomodal and very similar to those obtained for the pure PAAMePy55 solution. The apparent hydrodynamic radius of PAAMePy55 at low pH is 18 nm. At higher molar ratios (i.e., at high P123 concentrations), still in the acidic pH regime, bimodal relaxation time distributions were obtained, where the fast relaxation mode is connected to the translational diffusion of free P123 micelles with a hydrodynamic radius obtained at infinite P123 dilution (R(H,P123=0)) of 10-11 nm. This value coincides perfectly with the hydrodynamic radius of the pure P123 micelles at 40 degrees C, which was found to be ca. 10 nm at all pH values. The second mode corresponds to a complex consisting of one PAAMePy55 polymer chain and about 42 P123 micelles and with a R(H,P123=0) between 35 and 36 nm depending on pH. At pH 9, the mixed system also presented bimodal relaxation time distributions. At this high pH, the intermolecular association between PAAMePy55 and P123 is less strong than at acidic pH according to the steady-state fluorescence measurements. The fast mode is also in this case attributed to free P123 micelles whereas the second mode is related to the so-called "slow mode" commonly observed for polyelectrolyte solutions. In this system, it is related to the formation of multichain domains, that is, large domains formed by several PAAMePy55 chains that move in a common electrostatic field (i.e., a structure factor effect). The presence of P123 micelles does not lead to the total disruption of these domains. They may either contain entrapped P123 micelles or hydrophilic diblock impurities (originating from the P123 sample) that associate with the PAAMePy55 chains.

Formation and stability of nanoemulsions with mixed ionic–nonionic surfactants

A simple, low-energy two-step dilution process has been applied with binary mixtures of ionic-nonionic surfactants to prepare nanoemulsions. The systems consist of water/DDAB-C(12)E(5)/decane. Nanoemulsions were obtained by dilution of concentrates located in bicontinuous microemulsion or lamellar liquid crystal phase regions. The nanoemulsions generated were investigated both by contrast-variation small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The SANS profiles show that C(12)E(5) nanodroplets suffer essentially no structural change on incorporation of the cationic DDAB surfactant, except for increased electrostatic repulsive interactions. Interestingly, SANS indicated that the preferred droplet sizes were hardly affected by the surfactant mixture composition (up to a DDAB molar ratio (m(DDAB)/(m(DDAB) + m(C(12)E(5))) of 0.40) and droplet volume fraction, phi, between 0.006 and 0.120. No notable changes in the structure or radius of nanoemulsion droplets were observed by SANS over the test period of 1 d, although the droplet number intensity decreased significantly in systems stabilized by C(12)E(5) only. However, the DLS sizing shows a marked increase with time, with higher droplet volume fractions giving rise to the largest changes. The discrepancy between apparent nanoemulsion droplet size determined by DLS and SANS data can be attributed to long-range droplet interactions occurring outside of the SANS sensitivity range. The combined SANS and DLS results suggest flocculation is the main mechanism of instability for these nanoemulsions. The flocculation rate is shown to be significantly retarded by addition of the charged DDAB, which may be due to enhanced electrostatic repulsive forces between droplets, leading to improved stability of the nanoemulsions.

Dual thermo-responsive and ion-recognizable monodisperse microspheres

A novel type of dual stimuli-responsive microspheres that simultaneously exhibit ion-recognition property based on the supramolecular host–guest complexation of crown ether receptors (benzo-18-crown-6-acrylamide) (BCAm) with specific ions and thermo-sensitivity based on the phase transition of poly(N-isopropylacrylamide) (PNIPAM) is fabricated in this study. The prepared poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) (P(NIPAM-co-BCAm)) microspheres are characterized by FT-IR spectroscopy, UV–vis absorption spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS). SEM images and DLS data show that the synthesized microspheres exhibit nearly perfect spherical shape and high monodispersity. Moreover, according to the DLS results, P(NIPAM-co-BCAm) microspheres exhibit satisfactory thermo-responsive behavior and ion-recognition property. In K+ solutions, due to the formation of crown ether/K+ complexes, the LCST of P(NIPAM-co-BCAm) microspheres shifts to a higher temperature and the colloidal stability is increased. The P(NIPAM-co-BCAm) microspheres undergo a volume change from shrunken state to swollen network isothermally at a certain temperature by the addition of metal ions. Due to dual thermo-responsive and ion-recognition behaviors, this kind of microspheres would serve as promising candidates for sensors, controlled drug delivery systems and possibly new biomaterials.

Synthesis and Colloidal Properties of Polyether−Magnetite Complexes in Water and Phosphate-Buffered Saline

Biocompatible magnetic nanoparticles show great promise for many biotechnological applications. This paper addresses the synthesis and characterization of magnetite nanoparticles coated with poly(ethylene oxide) (PEO) homopolymers and amphiphilic poly(propylene oxide-b-ethylene oxide) (PPO-b-PEO) copolymers that were anchored through ammonium ions. Predictions and experimental measurements of the colloidal properties of these nanoparticles in water and phosphate-buffered saline (PBS) as functions of the polymer block lengths and polymer loading are reported. The complexes were found to exist as primary particles at high polymer compositions, and most formed small clusters with equilibrium sizes as the polymer loading was reduced. Through implementation of a polymer brush model, the size distributions from dynamic light scattering (DLS) were compared to those from the model. For complexes that did not cluster, the experimental sizes matched the model well. For complexes that clustered, equilibrium diameters were predicted accurately through an empirical fit derived from DLS data and the half-life for doublet formation calculated using the modified Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Deviation from this empirical fit provided insight into possible additional interparticle hydrophobic interactions for select complexes for which the DLVO theory could not account. While the polymers remained bound to the nanoparticles in water, most of them desorbed slowly in PBS. Desorption was slowed significantly at high polymer chain densities and with hydrophobic PPO anchor blocks. By tailoring the PPO block length and the number of polymer chains on the surface, flocculation of the magnetite complexes in PBS was avoided. This allows for in vitro experiments where appreciable flocculation or sedimentation will not take place within the specified time scale requirements of an experiment.

A Femtosecond Study of Solvation Dynamics and Anisotropy Decay in a Catanionic Vesicle: Excitation‐Wavelength Dependence

The structure and dynamics of a catanionic vesicle are studied by means of femtosecond up-conversion and dynamic light scattering (DLS). The catanionic vesicle is composed of dodecyl-trimethyl-ammonium bromide (DTAB) and sodium dodecyl sulphate (SDS). The DLS data suggest that 90 % of the vesicles have a diameter of about 400 nm, whereas the diameter of the other 10 % is about 50 nm. The dynamics in the catanionic vesicle are compared with those in pure SDS and DTAB micelles. We also study the dynamics in different regions of the micelle/vesicle by varying the excitation wavelength (lambda(ex)) from 375 to 435 nm. The catanionic vesicle is found to be more heterogeneous than the SDS or DTAB micelles, and hence, the lambda(ex)-dependent variation of the solvation dynamics is more prominent in the first case. The solvation dynamics in the vesicle and the micelles display an ultraslow component (2 and 300 ps, respectively), which arises from the quasibound, confined water inside the micelle, and an ultrafast component (<0.3 ps), which is due to quasifree water at the surface/exposed region. With an increase in lambda(ex), the solvation dynamics become faster. This is manifested in a decrease in the total dynamic solvent shift and an increase in the contribution of the ultrafast component (<0.3 ps). At a long lambda(ex) (435 nm), the surface (exposed region) of a micelle/vesicle is probed, where the solvation dynamics of the water molecules are faster than those in a buried location of the vesicle and the micelles. The time constant of anisotropy decay becomes longer with increasing lambda(ex), in both the catanionic vesicle and the ordinary micelles (SDS and DTAB). The slow rotational dynamics (anisotropy decay) in the polar region (at long lambda(ex)) may be due to the presence of ionic head groups and counter ions.