Diffusion of metal complexes inside of silica-surfactant nanochannels within a porous alumina membrane

The present paper describes diffusivities of a series of metal complexes inside of silica-surfactant nanochannels (channel diameter = 3.4 nm), which were formed within a porous alumina membrane by a surfactant-templated method using cetyltrimethylammonium bromide (CTAB) as a template surfactant. The metal complexes used in this study were Fe(CN)6(3-), Ru(NH3)6(3-), ferrocenecarboxylic acid (Fc-COO-), (ferrocenylmethyl)-trimethylammonium (Fc-NMe3+), N,N-(dimethylamminomethyl)ferrocene (Fc-NMe2), and ferrocene methanol (Fc-OH). Apparent diffusion coefficients of these metal complexes were estimated by measuring their mass transports through the silica-surfactant nanochannels. The estimated apparent diffusion coefficients were on the order of 10(-11) cm2 s(-1) for Fe(CN)6(3-) and Ru(NH3)6(3-), and these values were five orders of magnitude smaller than those in a bulk aqueous solution. For the ferrocene derivatives, the apparent diffusion coefficients of charged ferrocene derivatives are almost the same (5.3 x 10(-11) cm2 s(-1) for Fc-COO- and 5.4 x 10(-11) cm2 s(-1) for Fc-NMe3+), whereas neutral ferrocene derivatives (Fc-NMe2 and Fc-OH) show faster diffusion than the charged species. In addition, the apparent diffusion coefficient of Fc-NMe2 (27 x 10(-11) cm2 s(-1)) was about three times larger than that of Fc-OH (10 x 10-11 cm2 s(-1)). The difference in these diffusion coefficients is discussed by considering the mesostructure of the silica-surfactant nanochannels, that is, an ionic interface with cationic head groups of CTA and their counteranions, a hydrophobic interior of the micellar phase, and a silica framework. As a result, it is inferred that the slow diffusivities of the charged metal complexes are due to the electrostatic interaction between the charged species and the ionic interface, whereas less interaction between neutral ferrocenes and the ionic interface causes distribution of metal complexes into the hydrophobic micellar phase, which is a less viscous medium compared to the ionic interface, resulting in the faster diffusivities of the neutral species.     

Voltammetric detection of organohalides by redox catalysis: improved sensitivity by immobilisation within a cubic phase liquid crystal

Two combined strategies are reported for improving the sensitivity of organohalide detection by redox catalysis. These are, improvement of the second order rate constant (k) for catalytic reduction of the organohalide, and improvement of the rate of substrate diffusion. Values of k are calculated for both alkyl and aryl halides, from slow scan rate cyclic voltammograms in homogeneous solution. It is shown that a Zn(ii) porphyrin exhibits higher catalytic rates than the previously used Co(ii) porphyrin or Co(i) salen. Amperometric and rotating disk electrode studies of electropolymerised films of the Zn(ii) porphyrin, reveal that at optimum thickness, mediator-substrate reaction and substrate diffusion are the rate limiting steps. Hence, immobilisation of the Zn(ii) porphyrin within the more open structure of a cubic phase liquid crystal produces an increase in sensitivity of approx. 10 times, and lowers the limit of detection by one order of magnitude. The optimised sensor responds linearly to seven organohalides in the range 0.1 microM to 1.0 microM with a sensitivity of 6.95 A M(-1) cm(-2). Chronoamperometric experiments with a microdisk electrode show that the rate of charge transport in the cubic phase films (apparent diffusion coefficient, D(E)= 5.65 x 10(-10)+/- 0.11 x 10(-10) cm(2) s(-1)) is faster than in the electropolymerised films (D(E)= 3.64 x 10(-11)+/- 0.02 x 10(-11) cm(2) s(-1)). The variation of D(E) with the concentration of Zn(ii) in the cubic phase suggests that diffusion of charge is predominantly by electron self-exchange, rather than by physical movement.     

Two-focus fluorescence correlation spectroscopy: a new tool for accurate and absolute diffusion measurements.

We present a new method to measure absolute diffusion coefficients at nanomolar concentrations with high precision. Based on a modified fluorescence correlation spectroscopy (FCS)-setup, this method is improved by introducing an external ruler for measuring the diffusion time by generating two laterally shifted and overlapping laser foci at a fixed and known distance. Data fitting is facilitated by a new two-parameter model to describe the molecule detection function (MDF). We present a recorded MDF and show the excellent agreement with the fitting model. We measure the diffusion coefficient of the red fluorescent dye Atto655 under various conditions and compare these values with a value achieved by gradient pulsed field NMR (GPF NMR). From these measurements we conclude, that the new measurement scheme is robust against optical and photophysical artefacts which are inherent to standard FCS. With two-focus-FCS, the diffusion coefficient of 4.26 x 10(-6) cm2s(-1) for Atto655 in water at 25 degrees C compares well with the GPF NMR value of 4.28 x 10(-6) cm2s(-1).     

Immobilizing single lipid and channel molecules in artificial lipid bilayers with annexin A5

The effects of annexin A5 on the lateral diffusion of single-molecule lipids and single-molecule proteins were studied in an artificial lipid bilayer membrane. Annexin A5 is a member of the annexin superfamily, which binds preferentially to anionic phospholipids in a Ca2+-dependent manner. In this report, we were able to directly monitor single BODIPY 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (DHPE) and ryanodine receptor type 2 (RyR2) labeled with Cy5 molecules in lipid bilayers containing phosphatidylserine (PS) by using fluorescence microscopy. The diffusion coefficients were calculated at various annexin A5 concentrations. The diffusion coefficients of BODIPY-DHPE and Cy5-RyR2 in the absence of annexin A5 were 4.81 x 10(-8) cm(2)/s and 2.13 x 10(-8) cm(2)/s, respectively. In the presence of 1 microM annexin A5, the diffusion coefficients of BODIPY-DHPE and Cy5-RyR2 were 2.2 x 10(-10) cm(2)/s and 9.5 x 10(-11) cm(2)/s, respectively. Overall, 1 microM of annexin A5 was sufficient to induce a 200-fold decrease in the lateral diffusion coefficient. Additionally, we performed electrophysiological examinations and determined that annexin A5 has little effect on the function of RyR2. This means that annexin A5 can be used to immobilize RyR2 in a lipid bilayer when imaging and analyzing RyR2.     

NMR diffusion spectroscopy as a measure of host-guest complex association constants and as a probe of complex size

The complexes of cyclohexylacetic acid and cholic acid with beta-cyclodextrin were studied by NMR diffusion coefficient measurements. The diffusion coefficient of the 1:1 cyclohexylacetic acid/beta-cyclodextrin complex, K(a) = 1800 +/- 100 M(-1), is slightly slower (3.23 +/- 0.07 x 10(-6) cm(2) s(-1)) than that of beta-cyclodextrin (3.29 +/- 0.07 x 10(-6) cm(2) s(-1)). The diffusion coefficient of the 1:1 cholic acid/beta-cyclodextrin complex, K(a) = 5900 +/- 800 M(-1), is significantly slower (2.93 +/- 0.07 x 10(-6) cm(2) s(-1)) than that of beta-cyclodextrin. The results indicate that caution should be exercised when studying host-guest complexation by the so-called 'single point' technique. A novel data treatment is introduced which takes into account the diffusion behavior of all of the species when determining K(a). Experimentally determined diffusion coefficients of complexes are also a useful probe of the size of host-guest complexes.     

Proton diffusion across membranes of vesicles of poly(styrene-b-acrylic acid) diblock copolymers

A study of proton diffusion across membranes of block copolymer vesicles in dilute solution is described. The vesicles were formed by the self-assembly of a diblock copolymer of poly(styrene-b-acrylic acid) (PS(310)-b-PAA(36), where the numbers represent the degree of polymerization for individual blocks). A pH gradient was created across the vesicle membrane with the interior pH (pH(in)) of ca. 2.9 and the exterior pH (pH(out)) of ca. 8.5. The permeability of the polystyrene (PS) membrane was tuned by the addition of different amounts of dioxane (0-40 wt %) to the external aqueous solution. Proton concentrations in the solution outside of the vesicles were followed by monitoring the spectrum of a pH-sensitive fluorescent dye, namely 8-hydroxypyrene-1,3,6-trisulfonate. After the start of the experiment, the proton concentrations increase linearly with the square root of time, while the slopes of the lines increase with dioxane content. To calculate the diffusion coefficients of the protons across the vesicular membrane, the concentration data were fitted using a model, which describes the diffusion of species across the membrane of a reservoir. The apparent diffusion coefficient (D*, which equals the true diffusion coefficient multiplied by the partition coefficient of protons between PS and water) increases from 1.1 x 10(-18) cm(2)/s at 7 wt % dioxane in the external solution to 1.2 x 10(-14) cm(2)/s at 40 wt %. The increase of D* with dioxane content is related to its plasticization of the PS membrane, which can be used as a gating mechanism.     

A highly sensitive colour reaction for chromium(VI) with the iodide-basic xanthene dye-pva system

A highly sensitive spectrophotometric method has been developed for determination of chromium(VI), based on oxidation of I(-) to I(-)(3) in acid medium, then formation of a 1:1 ion-association compound of I(-)(3) with a basic xanthene dye in the presence of poly(vinyl alcohol). The molar absorptivity is 1.79 x 10(5) 1.mole(-1).cm(-1) at 600 nm for the Rhodamine B system, 1.56 x 10(5) 1.mole(-1).cm(-1) at 600 nm with Ethylrhodamine B, 1.56 x 10(5) 1.mole(-).cm(-1) at 570 nm with Rhodamine 6G and 2.01 x 10(5) 1.mole(-1).cm(-1) at 590 nm with Butylrhodamine B, respectively. Preconcentration of chromium(VI) by an improved trioctylamine extraction procedure greatly increases the selectivity of the method and can be applied in the spectrophotometric determination of trace amounts of chromium(VI) in tap water, hot-spring water and river water.     

Sweeping on a microchip: concentration profiles of the focused zone in micellar electrokinetic chromatography.

On-line sample concentration by sweeping was investigated in microchip micellar electrokinetic chromatography (MEKC), By changing the distance between the injection cross and the detection points, the profile of the concentration process and the diffusion process in sweeping was elucidated. Rhodamine B injected for 4 s was best concentrated by sweeping at 9.4 mm from the injection cross and the enhancement factor was 450. At the longer distance from this point the peak of Rhodamine B was broadened and diluted by diffusion. The diffusion constant of Rhodamine B calculated from the experiment was 5.7 x 10(-6) cm2s(-1). The mixture of rhodamine B, sulforhodamine B, and cresyl fast violet was concentrated by sweeping and separated by MEKC at the same time.     

Lateral diffusion coefficients of an eicosanyl-based bisglycerophosphocholine determined by PFG-NMR and FRAP

We report the lateral diffusion properties of 2,2'-di-O-decyl-3,3'-di-O-(eicosanyl)-bis-(rac-glycero)-1,1'-diphosphocholine (C20BAS) using pulsed-field gradient NMR (PFG-NMR) and fluorescence recovery after photobleaching (FRAP). C20BAS membranes display a melting transition at Tm = 15.7 degrees C, as determined by differential scanning calorimetry and 31P NMR chemical shift anisotropy. The lateral diffusion coefficient of C20BAS, as determined by PFG-NMR and FRAP, at 25 degrees C, were DPFG-NMR = 1.9 +/- 0.6 x 10(-8) cm2/s and DFRAP C20BAS = 1.2 +/- 0.1 x 10(-8) cm2/s, respectively. In comparison, the lateral diffusion coefficient of the monopolar phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), was 1.8 +/- 0.9 x 10(-8) and 2.5 +/- 0.9 x 10(-8) cm2/s using PFG-NMR and FRAP, respectively.     

Nonlinear studies of Pararosanilin dye in liquid and solid media

Solid-state dye-doped polymers are attractive alternative to the conventional liquid dye solutions. In this paper, nonlinear properties of the dye Pararosanilin has been studied. The third-order nonlinear optical properties of Pararosanilin dye in 1-butanol and dye-doped polymer film were measured by the Z-scan technique using 532 nm diode pumped Nd:Yag laser. This material exhibits negative optical nonlinearity. The dye at 0.4 mM concentration exhibited nonlinear refractive coefficient (n(2) = -6.8 x 10(-8) and -7.11 x 10(-8) (cm(2)/W) in liquid and solid media, respectively), nonlinear absorption coefficient (beta = -7.7 x 10(-4) and -7.93 x 10(-4)cm/W in liquid and solid media, respectively) and susceptibility (chi((3))=3.38 x 10(-6) and 3.53 x 10(-5)esu in liquid and solid media, respectively). These results show that Pararosanilin dye has potential applications in nonlinear optics.     

Ion-exchange voltammetry of dopamine at Nafion-coated glassy carbon electrodes: quantitative features of ion-exchange partition and reassessment on the oxidation mechanism of dopamine in the presence of excess ascorbic acid

The incorporation/exclusion features of dopamine (DA), ascorbic acid (AA) and uric acid (UA) are evaluated for Nafion (NA)-coated glassy carbon electrodes (GCE) of different thicknesses. The ion-exchange partition of DA(+) between the NA film and the sodium phosphate electrolyte is evaluated by determining the partition coefficient (k(D)) and the apparent diffusion coefficient (D(app)) in thick NA films which were 401 and 1.5 x 10(-9) cm(2) s(-1), respectively. The solution diffusion coefficient was found to be 6.0 x 10(-6) cm(2) s(-1). Also, the effect of NA loading and of the voltammetric timescale on DA voltammetry in the presence of excess AA is assessed, at physiologic like conditions. It is demonstrated that, although AA is excluded at the NA coating, a catalytic regeneration of DA, induced by AA, occurs at the interface NA film/electrolyte resulting from the diffusion of the o-quinone product of DA oxidation from the electrode surface to that interface. The interference of AA in the voltammetric signal of DA is eliminated using 18 microg mm(-2) NA films and v> or =0.5 V s(-1). Therefore, fast, selective and sensitive voltammetric analysis of DA at concentrations<100 microM in the presence of excess AA, e.g., 1 mM is achieved.     

Nonlinear studies of acid fuchsin dye in liquid and solid media

Solid-state dye-doped polymers are attractive alternative to the conventional liquid dye solutions. In this paper, nonlinear properties of the dye Acid Fuchsin has been studied. The third-order nonlinear optical properties of Acid Fuchsin dye in 1-butanol and dye-doped polymer film were measured by the Z-scan technique using 532 nm diode pumped Nd:Yag laser. This material exhibits negative optical nonlinearity. The dye at 0.4 mM concentration exhibited nonlinear refractive coefficient (n(2)=-8.72 x 10(-8) and -10.308 x 10(-8) (cm(2)/W) in liquid and solid media, respectively), nonlinear absorption coefficient (beta=-7.69 x 10(-4) and -8.294 x 10(-4)cm/W in liquid and solid media, respectively) and susceptibility (chi(3)=4.33 x 10(-6) and 5.13 x 10(-5)esu in liquid and solid media, respectively). These results show that Acid Fuchsin dye has potential applications in nonlinear optics.     

Molecular basis of carbon dioxide transport in polycarbonate membranes

Gas transport of carbon dioxide in poly[bisphenol A carbonate-co-4,4'-(3,3,5-trimethylcyclohexylidene)diphenol carbonate] films over a wide range of pressure is described. The interpretation of the experimental results in terms of the dual mode model allowed the evaluation of the parameters of the model that govern the gas permeation process. The value of the diffusion coefficient obtained for carbon dioxide at zero concentration was 2.4 x 10(-8) cm(2) s(-1), at 303 K. This parameter was also measured by using pulsed field gradient NMR finding that its value reaches a nearly constant value of (2.7 +/- 0.9) x 10(-8) cm(2) s(-1), at 298 K, for diffusion times greater than 20 ms. Both the diffusion and solubility coefficients were also computed by using simulation methods based on the transition states theory and the Widom method, respectively. The value obtained for the diffusion coefficient was 1.8 x 10(-8) cm(2) s(-1), at 303 K, which compares very favorably with the experimental measurements. The drop of the simulated solubility coefficient with increasing pressure is sharper than that of the experimental one, at low pressures, and similar, at high pressures.     

Diffusion coefficient of 36Cl in silver chloride

Diffusion coefficient of 36Cl in silver chloride was revealed to be about 1 x 10(-10)-4 x 10(-11) cm2 . s-1 in the range of 400 degrees C-200 degrees C by means of radioactive tracer measurements. Activation energy for diffusion was calculated about 0.13 eV. These results were different from those obtained by Maurer and Compton.     

Diffusivities of tris(2,2'-bipyridyl)ruthenium inside silica-nanochannels modified with alkylsilanes.

The apparent diffusion coefficients of tris(2,2'-bipyridyl)ruthenium ([Ru(bpy(3))](2+)) are estimated in silica-nanochannels which are assembled inside columnar alumina pores in an anodic alumina membrane, and are modified with alkylsilanes such as trimethylchlorosilane (C1), butyldimethylchlorosilane (C4), and dodecyldimethylchlorosilane (C12). The estimation is performed by observing the lag-time, which is defined as the time required for [Ru(bpy)(3)](2+) to diffuse through alkylsilane-modified silica-nanochannels in the alumina membrane. When ethanol is used as a solvent, the apparent diffusion coefficients of [Ru(bpy)(3)](2+) are estimated as 2.1 x 10(-10) and 3.2 x 10(-10) cm(2) s(-1) in the C1- and C4-modified silica-nanochannels, respectively. These values are about 10(4) times smaller than that obtained in bulk ethanol. Based on the experimental results on the solvent dependency of the lag-time, the hydrogen-bonding interaction between ethanol molecules is considered to be stronger in the C1- and C4-modified silica-nanochannels than in bulk ethanol, and the hydrogen-bonding interaction plays a critical role for the slow diffusivity in those nanochannels. In contrast, the apparent diffusion coefficient in the C12-modified silica-nanochannel is at least two orders of magnitude larger than those in the C1- and C4-modified silica-nanochannels. This relatively fast diffusion is most likely explained by the presence of a long alkyl chain of C12, which reduces a hindrance effect that is originates in the hydrogen-bonding interaction.     

Examination of cucurbit[7]uril and its host-guest complexes by diffusion nuclear magnetic resonance

The self-diffusion of cucurbit[7]uril (CB[7]) and its host-guest complexes in D2O has been examined using pulsed gradient spin-echo nuclear magnetic resonance spectroscopy. CB[7] diffuses freely at a concentration of 2 mM with a diffusion coefficient (D) of 3.07 x 10(-10) m(2) s(-1). At saturation (3.7 mM), CB[7] diffuses more slowly (D = 2.82 x 10(-10) m(2) s(-1)) indicating that it partially self-associates. At concentrations between 2 and 200 mM, CsCl has no effect on the diffusion coefficient of CB[7] (1 mM). Conversely, CB[7] (2 mM) significantly affects the diffusion of 133Cs+ (1 mM), decreasing its diffusion coefficient from 1.86 to 0.83 x 10(-9) m(2) s(-1). Similar changes in the rate of diffusion of other alkali earth metal cations are observed upon the addition of CB[7]. The diffusion coefficient of 23Na+ changes from 1.26 to 0.90 x 10(-9) m(2) s(-1) and 7Li+ changes from 3.40 to 3.07 x 10(-9) m(2) s(-1). In most cases, encapsulation of a variety of inorganic and organic guests within CB[7] decreases their rates of diffusion in D2O. For instance, the diffusion coefficient of the dinuclear platinum complex trans-[[PtCl(NH3)2}2mu-dpzm](2+) (where dpzm is 4,4'-dipyrazolylmethane) decreases from 4.88 to 2.95 x 10(-10) m(2) s(-1) upon encapsulation with an equimolar concentration of CB[7].     

Temperature dependence of exciton diffusion in conjugated polymers

The temperature dependence of the exciton dynamics in a conjugated polymer is studied using time-resolved spectroscopy. Photoluminescence decays were measured in heterostructured samples containing a sharp polymer-fullerene interface, which acts as an exciton quenching wall. Using a 1D diffusion model, the exciton diffusion length and diffusion coefficient were extracted in the temperature range of 4-293 K. The exciton dynamics reveal two temperature regimes: in the range of 4-150 K, the exciton diffusion length (coefficient) of approximately 3 nm (approximately 1.5 x 10 (-4) cm2/s) is nearly temperature independent. Increasing the temperature up to 293 K leads to a gradual growth up to 4.5 nm (approximately 3.2 x 10 (-4) cm2/ s). This demonstrates that exciton diffusion in conjugated polymers is governed by two processes: an initial downhill migration toward lower energy states in the inhomogenously broadened density of states, followed by temperature activated hopping. The latter process is switched off below 150 K.     

End-to-end diffusion on the microsecond timescale measured with resonance energy transfer from a long-lifetime rhenium metal-ligand complex

We measured the end-to-end diffusion coefficient of an alkyl chain-linked donor-acceptor pair using the time-resolved frequency-domain decay of the donor. The donor was a rhenium metal-ligand complex with a mean decay time ranging from 2.1 to 7.9 microseconds in the absence of the Texas red acceptor. The decay time was used to measure the donor-to-acceptor distance distribution and the mutual diffusion coefficient. Using this long lifetime donor, it was easily possible to determine a diffusion coefficient near 2 x 10(-8) cm2/s and diffusion coefficients as low as 1.3 x 10(-9) cm2/s were measurable. Such long lifetime donors should be valuable for measuring the flexing of peptides on the microsecond timescale, domain motions of proteins and lateral diffusion in membranes. The availability of microsecond decay time luminophores now allows luminescence spectroscopy to be useful generally for studies of microsecond dynamics of biological macromolecules.     

Proton dynamics in lithium-ammonia solutions and expanded metals

Quasielastic neutron scattering has been used to study proton dynamics in the system lithium-ammonia at concentrations of 0, 4, 12, and 20 mole percent metal (MPM) in both the liquid and solid (expanded metal) phases. At 230 K, in the homogenous liquid state, we find that the proton self-diffusion coefficient first increases with metal concentration, from 5.6x10(-5) cm2 s(-1) in pure ammonia to 7.8x10(-5) cm2 s(-1) at 12 MPM. At higher concentrations we note a small decrease to a value of 7.0x10(-5) cm2 s(-1) at 20 MPM (saturation). These results are consistent with NMR data, and can be explained in terms of the competing influences of the electron and ion solvation. At saturation, the solution freezes to form a series of expanded metal compounds of composition Li(NH3)4. Above the melting point, at 100 K, we are able to fit our data to a jump-diffusion model, with a mean jump length (l) of 2.1 A and residence time (tau) of 3.1 ps. This model gives a diffusion coefficient of 2.3x10(-5) cm2 s(-1). In solid phase I (cubic, stable from 88.8 to 82.2 K) we find that the protons are still undergoing this jump diffusion, with l=2.0 A and tau=3.9 ps giving a diffusion coefficient of 1.8x10(-5) cm2 s(-1). Such motion gives way to purely localized rotation in solid phases IIa (from 82.2 to 69 K) and IIb (stable from 69 to 25 K). We find rotational correlation times (tau(rot)) of the order of 2.0 and 7.3 ps in phases IIa and IIb, respectively. These values can be compared with a rotational mode in solid ammonia with tau(rot) approximately 2.4 ps at 150 K.     

Vertical diffusion of water molecules near the surface of ice

We studied diffusion of water molecules in the direction perpendicular to the surface of an ice film. Amorphous ice films of H(2)O were deposited on Ru(0001) at temperature of 100-140 K for thickness of 1-5 bilayer (BL) in vacuum, and a fractional coverage of D(2)O was added onto the surface. Vertical migration of surface D(2)O molecules to the underlying H(2)O multilayer and the reverse migration of H(2)O resulted in change of their surface concentrations. Temporal variation of the H(2)O and D(2)O surface concentrations was monitored by the technique of Cs(+) reactive ion scattering to reveal kinetics of the vertical diffusion in depth resolution of 1 BL. The first-order rate coefficient for the migration of surface water molecules ranged from k(1)=5.7(+/-0.6) x 10(-4) s(-1) at T=100 K to k(1)=6.7(+/-2.0) x 10(-2) s(-1) at 140 K, with an activation energy of 13.7+/-1.7 kJ mol(-1). The equivalent surface diffusion coefficients were D(s)=7 x 10(-19) cm(2) s(-1) at 100 K and D(s)=8 x 10(-17) cm(2) s(-1) at 140 K. The measured activation energy was close to interstitial migration energy (15 kJ mol(-1)) and was much lower than diffusion activation energy in bulk ice (52-70 kJ mol(-1)). The result suggested that water molecules diffused via the interstitial mechanism near the surface where defect concentrations were very high.