ORCID Profile
0000-0002-6296-314X
Current Organisations
University of Manchester
,
China University of Geosciences
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Publisher: American Chemical Society (ACS)
Date: 13-01-2009
DOI: 10.1021/LA8028325
Abstract: We have exploited the spatial and kinetic resolution of ellipsometry to monitor the lateral movement of inhomogeneous patches of material in mixed adsorption layers of poly(ethylene imine) and sodium dodecyl sulfate at the air/liquid interface. We show that the choice of s le preparation methods can have a profound effect on the state of the interface for chemically equivalent s les. The extent of aggregation in the bulk solution on relevant time scales is affected by specific details of the polymer/surfactant mixing process, which produces varying numbers of aggregates that can become trapped in the interfacial layer, resulting in an enhanced and fluctuating ellipsometry signal. It can be beneficial to apply the surface-cleaning method of aspiration prior to physical measurements to remove trapped aggregates through the creation of a fresh interface. At low pH, the ellipsometry signal of s les prepared with surface cleaning is remarkably constant over a factor of >500 in the bulk composition below charge equivalence, which is discussed in terms of possible adsorption mechanisms. At high pH, through observing temporal fluctuations in the ellipsometry signal of s les prepared with surface cleaning, we reveal two important processes: there is the spontaneous adsorption of aggregates > 0.2 microm in diameter into the interfacial layer, and with time there is the fusion of smaller aggregates to generate new large surface aggregates. We attribute the favorability of the adsorption and fusion processes at high pH to reduced electrostatic barriers resulting from the low surface charge density of the aggregates. It is inappropriate in this case to consider the interface to comprise a homogeneous adsorption layer that is in dynamic equilibrium with the bulk solution. Our work shows that it can be helpful to consider whether there are macroscopic particles embedded in molecular layers at the air/liquid interface for systems where there is prior knowledge of aggregation in the bulk phase.
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D0NR06621D
Abstract: Self-assembly of insoluble surfactants imposes curvature restrictions on the air–water interface which leads to 3D nanopatterns that can be deposited onto solid surfaces.
Publisher: American Chemical Society (ACS)
Date: 29-04-2010
DOI: 10.1021/LA9047177
Abstract: This study increases the understanding at a molecular level of the interactions between DNA and poly(amido amine) (PAMAM) dendrimers on solid surfaces, which is a subject of potential interest in applications such as gene therapy. We have used in situ null ellipsometry and neutron reflectometry to study the structure of multilayer arrangements formed by PAMAM dendrimers of generation 2 (G2), 4 (G4), and 6 (G6) and DNA on silica surfaces. Specifically, we adsorbed cationic dendrimer layers, then we condensed DNA to form dendrimer-DNA bilayers, and last we exposed further dendrimer molecules to the interface to encapsulate DNA in dendrimer-DNA-dendrimer trilayers. The dendrimer monolayers formed initially result in the deformation of the cationic adsorbates as a result of their strong electrostatic attraction to the hydrophilic silica surface. The highest surface excess and most pronounced deformation occurs for the G6 molecules due to their relatively large size and high surface charge density. G6-functionalized surfaces give rise to the highest surface excess of DNA during the bilayer formation process. This result is explained in terms of the high number of charged binding sites in the G6 monolayer and the low electrostatic repulsion between DNA and exposed patches of silica surface due to the relatively thick G6 monolayer. The binding strengths of the silica-dendrimer and dendrimer-DNA interactions are demonstrated by the high stability of the interfacial bilayers during rinsing. For the formation of trilayers of dendrimers, DNA, and dendrimers, G2 adsorbs as a smooth layer while G4 and G6 induce the formation of less well-defined structures due to more complex DNA layer morphologies.
Publisher: American Chemical Society (ACS)
Date: 04-10-2019
DOI: 10.26434/CHEMRXIV.9929684.V1
Abstract: A simple, insoluble monolayer of fatty acid is shown to induce 3D nanotexturing of the water–air interface. This advance has been achieved through the study of monolayers of a methyl-branched long chain fatty acid, analogous to those found on the surface of hair and wool, directly at the water–air interface. Specular neutron reflectometry combined with AFM probing of deposited monolayers shows pronounced 3D surface domains, which are absent for unbranched analogues and which are attributed to hydrocarbon packing constraints. The resulting surface topographies of the water far exceed the height perturbation that can be explained by the presence of capillary waves of a free liquid surface. These have hitherto been considered the only source of perturbation of the flatness of a planar water interface under gravity in the absence of topographical features from the presence of extended, globular or particulate matter. This amounts to a paradigm shift in the study of interfacial films and opens the possibility of 3D texturing of the water–air interface.
Publisher: American Chemical Society (ACS)
Date: 24-09-2014
DOI: 10.1021/JP507230M
Abstract: We have determined how the bulk behavior of mixtures of small cationic poly(amidoamine) dendrimers (generation 2, PAMAM-G2) and sodium dodecyl sulfate (SDS) affects the structure and composition of the adsorbed layers at the air-water interface. The aim is to reveal how the size of a well-defined hyperbranched polyelectrolyte affects the interfacial and bulk solution behavior of mixtures with oppositely charged surfactants, when the size of the polyelectrolyte approaches that of the surfactant. A combination of electrophoretic mobility, UV-vis spectroscopy, dynamic light scattering, and small-angle X-ray scattering measurements have been employed to characterize the interactions in the bulk solution. PAMAM-G2 associates strongly with SDS in the bulk, forming large aggregates where the size and the charge depend on the bulk composition. We show that kinetically trapped aggregates can be formed at compositions outside the equilibrium two-phase region, and the positively charged aggregates are larger than the negative ones. Surface tensiometry, neutron reflectometry, and ellipsometry have been used to reveal the properties of the interfacial layers. The interfacial structures formed depend strongly on the bulk composition: structured layers are present for s les inside the two-phase region, whereas intact nanostructured aggregates adsorb for s les just outside the two-phase region. The interfacial behavior of PAMAM-G2/SDS mixtures is compared with that of small amines or multivalent ions and oppositely charged surfactants. The implications of aggregate adsorption, dissociation, and spreading processes are discussed as well as the potential of small dendrimers for applications involving the delivery of functional molecules to interfaces.
Publisher: American Chemical Society (ACS)
Date: 29-09-2010
DOI: 10.1021/JZ101179F
Publisher: American Chemical Society (ACS)
Date: 07-12-2011
DOI: 10.1021/JP2088803
Abstract: We show for the oppositely charged system poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate that the cliff edge peak in its surface tension isotherm results from the comprehensive precipitation of bulk complexes into sediment, leaving a supernatant that is virtually transparent and a depleted adsorption layer at the air/water interface. The aggregation and settling processes take about 3 days to reach completion and occur at bulk compositions around charge neutrality of the complexes which lack long-term colloidal stability. We demonstrate excellent quantitative agreement between the measured surface tension values and a peak calculated from the surface excess of surfactant in the precipitation region measured by neutron reflectometry, using the approximation that there is no polymer left in the liquid phase. The nonequilibrium nature of the system is emphasized by the production of very different interfacial properties from equivalent aged s les that are handled differently. We go on to outline our perspective on the "true equilibrium" state of this intriguing system and conclude with a comment on its practical relevance given that the interfacial properties can be so readily influenced by the handling of kinetically trapped bulk aggregates.
Publisher: American Chemical Society (ACS)
Date: 16-07-2014
DOI: 10.1021/LA500621T
Abstract: We discuss different nonequilibrium mechanisms by which bulk aggregates directly modify, and can even control, the interfacial structure and morphology of an oppositely charged polyelectrolyte/surfactant (P/S) mixture. S les are categorized at the air/water interface with respect to the dynamic changes in the bulk phase behavior, the bulk composition, and the s le history using complementary surface-sensitive techniques. First, we show that bulk aggregates can spontaneously interact with the adsorption layer and are retained in it and that this process occurs most readily for positively charged aggregates with an expanded structure. In this case, key nonequilibrium issues of aggregate dissociation and spreading of surface-active material at the interface have a marked influence on the macroscopic interfacial properties. In a second distinct mechanism, aggregates inherently become trapped at the interface during its creation and lateral flocculation occurs. This irreversible process is most pronounced for aggregates with the lowest charge. A third mechanism involves the deposition of aggregates at interfaces due to their transport under gravity. The specificity of this process at an interface depends on its location and is mediated by density effects in the bulk. The prevalence of each mechanism critically depends on a number of different factors, which are outlined systematically here for the first time. This study highlights the sheer complexity by which aggregates can directly impact the interfacial properties of a P/S mixture. Our findings offer scope for understanding seemingly mysterious irreproducible effects which can compromise the performance of formulations in wide-ranging applications from foams to emulsions and lubricants.
Publisher: American Vacuum Society
Date: 06-2008
DOI: 10.1116/1.2976448
Abstract: The application of scattering methods in the study of biological and biomedical problems is a field of research that is currently experiencing fast growth. In particular, neutron reflectometry (NR) is a technique that is becoming progressively more widespread, as indicated by the current commissioning of several new reflectometers worldwide. NR is valuable for the characterization of biomolecules at interfaces due to its capability to provide quantitative structural and compositional information on relevant molecular length scales. Recent years have seen an increasing number of applications of NR to problems related to drug and gene delivery. We start our review by summarizing the experimental methodology of the technique with reference to the description of biological liquid interfaces. Various methods for the interpretation of data are then discussed, including a new approach based on the lattice mean-field theory to help characterize stimulus-responsive surfaces relevant to drug delivery function. Recent progress in the subject area is reviewed in terms of NR studies relevant to the delivery of lipids and DNA to surfaces. Lastly, we discuss two case studies to exemplify practical features of NR that are exploited in combination with complementary techniques. The first case concerns the interactions of lipid-based cubic phase nanoparticles with model membranes (a drug delivery application), and the second case concerns DNA compaction at surfaces and in the bulk solution (a gene delivery application).
Publisher: Royal Society of Chemistry (RSC)
Date: 2015
DOI: 10.1039/C4SM02712D
Abstract: Molecular recognition of nucleic acids by dendrimers with non-covalently attached nucleolipids can be tuned by nucleolipid type and order of addition.
Publisher: Elsevier BV
Date: 10-2009
Publisher: American Chemical Society (ACS)
Date: 02-07-2012
DOI: 10.1021/JP304564X
Abstract: We show conclusively that multilayers at interfaces of an oppositely charged polyelectrolyte/surfactant system can result from the transport under gravity of bulk aggregates with internal molecular structure. This process was demonstrated by measurements of poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate solutions at the air/liquid and solid/liquid interfaces using neutron reflectometry. In the latter case a novel approach involving the comparison of reflection up versus down measurements provided key evidence. Interfacial multilayers indicated by a strong Bragg peak and clear off-specular scattering are exhibited under three conditions: (1) only for s les in the phase separation region, (2) only for fresh s les where a suspension of bulk aggregates remains in solution, and (3) only when the creaming or sedimentation process occurs in the direction of the interface under examination. This bulk transport mechanism is an alternative route of formation of interfacial multilayers to surface induced self-assembly. The two processes evidently give rise to interfaces with very different structural and rheological properties. Such directionality effects in the formation of nanostructured liquid interfaces may have implications for a broad range of soft matter and biophysical systems containing macromolecules such as synthetic polymers, proteins, or DNA.
Publisher: Wiley
Date: 09-2021
DOI: 10.1002/LDR.3730
Abstract: Although it is generally believed the Grain for Green programme (GFG) implemented in China has attenuated soil erosion, the extent to which it is effective still needs verification. Taking Yan'an in the Loess Plateau as the study area, we analysed both total effect and efficiency differences during GFG implementation. Results showed that, while soil erosion on average decreased from 4,884.49 to 4,087.57 t km −2 yr −1 , counties with higher GFG implementation intensity achieved a lower soil conservation effect. For ex le, Wuqi ranks third in the GFG implementation intensity among all counties in Yan'an, but its actual soil erosion reduction is the lowest, only 54.1% of Yan'an's average level. To analyse the reason for the efficiency difference, the concept of soil conservation potential was proposed. It is concluded that the soil conservation effect is controlled by the soil conservation potential. Ideally, regions with high soil conservation potential should get priority in the GFG application, yet there is a significant spatial mismatch between the GFG implementation intensity and the soil conservation potential because the correlation coefficient is only −0.05, which weakened the soil control effect. A dynamic implementation mechanism was put forward for the formulation and optimization of ecological programmes and projects in future: first, using the soil conservation potential to determine the implementation intensity in each region second, adjusting the intensity to the changes of the soil conservation potential in the following implementation third, repeating above steps to ensure high efficiency of soil erosion control, and achieving the sustainability and effectiveness of the ecological projects.
Publisher: Royal Society of Chemistry (RSC)
Date: 2009
DOI: 10.1039/B905270D
Publisher: Elsevier BV
Date: 05-2018
DOI: 10.1016/J.CHEMPHYSLIP.2018.01.006
Abstract: Deuteration of biomolecules has a major impact on both quality and scope of neutron scattering experiments. Cholesterol is a major component of mammalian cells, where it plays a critical role in membrane permeability, rigidity and dynamics, and contributes to specific membrane structures such as lipid rafts. Cholesterol is the main cargo in low and high-density lipoprotein complexes (i.e. LDL, HDL) and is directly implicated in several pathogenic conditions such as coronary artery disease which leads to 17 million deaths annually. Neutron scattering studies on membranes or lipid-protein complexes exploiting contrast variation have been limited by the lack of availability of fully deuterated biomolecules and especially perdeuterated cholesterol. The availability of perdeuterated cholesterol provides a unique way of probing the structural and dynamical properties of the lipoprotein complexes that underly many of these disease conditions. Here we describe a procedure for in vivo production of perdeuterated recombinant cholesterol in lipid-engineered Pichia pastoris using flask and fed-batch fermenter cultures in deuterated minimal medium. Perdeuteration of the purified cholesterol was verified by mass spectrometry and its use in a neutron scattering study was demonstrated by neutron reflectometry measurements using the FIGARO instrument at the ILL.
Publisher: Royal Society of Chemistry (RSC)
Date: 2020
DOI: 10.1039/C9NH00722A
Abstract: The air–water interface can be textured according to the same self-assembly packing criteria that determine the wealth of 3D structures found in micellar and other hiphilic meso-phases.
Publisher: Royal Society of Chemistry (RSC)
Date: 2008
DOI: 10.1039/B801630E
Publisher: Elsevier BV
Date: 12-2015
Publisher: American Chemical Society (ACS)
Date: 11-05-2010
DOI: 10.1021/JP9119809
Abstract: Poly(amido amine) (PAMAM) dendrimers have previously been shown, as cationic condensing agents of DNA, to have high potential for nonviral gene delivery. This study addresses two key issues for gene delivery: the interaction of the biomembrane with (i) the condensing agent (the cationic PAMAM dendrimer) and (ii) the corresponding dendrimer/DNA aggregate. Using in situ null ellipsometry and neutron reflection, parallel experiments were carried out involving dendrimers of generations 2 (G2), 4 (G4), and 6 (G6). The study demonstrates that free dendrimers of all three generations were able to traverse supported palmitoyloleoylphosphatidylcholine (POPC) bilayers deposited on silica surfaces. The model biomembranes were elevated from the solid surfaces upon dendrimer penetration, which offers a promising new way to generate more realistic model biomembranes where the contact with the supporting surface is reduced and where aqueous cavities are present beneath the bilayer. The largest dendrimer (G6) induced partial bilayer destruction directly upon penetration, whereas the smaller dendrimers (G2 and G4) leave the bilayer intact, so we propose that lower generation dendrimers have greater potential as transfection mediators. In addition to the experimental observations, coarse-grained simulations on the interaction between generation 3 (G3) dendrimers and POPC bilayers were performed in the absence and presence of a bilayer-supporting negatively charged surface that emulates the support. The simulations demonstrate that G3 is transported across free-standing POPC bilayers by direct penetration and not by endocytosis. The penetrability was, however, reduced in the presence of a surface, indicating that the membrane transport observed experimentally was not driven solely by the surface. The experimental reflection techniques were also applied to dendrimer/DNA aggregates of charge ratio = 0.5, and while G2/DNA and G4/DNA aggregates interact with POPC bilayers, G6/DNA displays no such interaction. These results indicate that, in contrast to free dendrimer molecules, dendrimer/DNA aggregates of low charge ratios are not able to traverse a membrane by direct penetration.
Publisher: American Chemical Society (ACS)
Date: 12-05-2014
DOI: 10.1021/LA500473R
Abstract: We relate the adsorption from mixtures of well-defined poly(amidoamine) (PAMAM) dendrimers of generations 4 and 8 with sodium dodecyl sulfate (SDS) at the air-water interface to the bulk solution properties. The anionic surfactant shows strong attractive interactions with the cationic dendrimers at pH 7, and electrophoretic mobility measurements indicate that the association is primarily driven by electrostatic interactions. Optical density measurements highlight the lack of colloidal stability of the formed bulk aggregates at compositions close to charge neutrality, the time scale of which is dependent on the dendrimer generation. Adsorption at the air-water interface was followed from s les immediately after mixing using a combination of surface tension, neutron reflectometry, and ellipsometry measurements. In the phase separation region for dendrimers of generation 4, we observed high surface tension corresponding to a depleted surfactant solution but only when the aggregates carried an excess of surfactant. Interestingly, these depleted adsorption layers contained spontaneously adsorbed macroscopic aggregates, and these embedded particles do not rearrange to spread monomeric material at the interface. These findings are discussed in relation to the interfacial properties of mixtures involving dendrimers of generation 8 as well as polydisperse linear and hyperbranched polyelectrolytes where there is polyelectrolyte bound to a surfactant monolayer. The results presented here demonstrate the capability of dendrimers to sequester anionic surfactants in a controllable manner, with potential applications as demulsification and antifoaming agents.
Publisher: American Chemical Society (ACS)
Date: 06-2008
DOI: 10.1021/JP710170D
Abstract: The interfacial behavior of aqueous solutions of four different neutral polymers in the presence of sodium dodecyl sulfate (SDS) has been investigated by surface tension measurements and ellipsometry. The polymers comprised linear poly(ethylene oxide) with low and high molecular masses (10(3) and 10(6) Dalton (Da), respectively), and two high molecular mass methacrylate-based comb polymers containing poly(ethylene oxide) side chains. The adsorption isotherms of SDS, determined by Gibbs analysis of surface tension data, are nearly the same in the presence of the high molecular mass linear polymer and the comb polymers. Analysis of the ellipsometric data reveals that while a single surface layer model is appropriate for films of polymer alone, a more sophisticated interfacial layer model is necessary for films of SDS alone. For the polymer/surfactant mixtures, a novel semiempirical approach is proposed to determine the surface excess of polymer, and hence quantify the interfacial composition, through analysis of data from the two techniques. The replacement of the polymer due to surfactant adsorption is much less pronounced for the high molecular mass linear polymer and for the comb polymers than for the low molecular mass linear polymer. This finding is rationalized by the significantly higher adsorption driving force of the larger polymer molecules as well as by their more hiphilic structure in the case of the comb polymers.
Publisher: American Chemical Society (ACS)
Date: 06-05-2013
DOI: 10.1021/LA400774P
Abstract: This work addresses structural and nonequilibrium effects of the interactions between well-defined cationic poly(amidoamine) PAMAM dendrimers of generations 4 and 8 and the anionic surfactant sodium dodecyl sulfate (SDS) at the hydrophilic silica-water interface. Neutron reflectometry and quartz crystal microbalance with dissipation monitoring were used to reveal the adsorption from premixed dendrimer/surfactant solutions as well as sequential addition of the surfactant to preadsorbed layers of dendrimers. PAMAM dendrimers of both generations adsorb to hydrophilic silica as a compact monolayer, and the adsorption is irreversible upon rinsing with salt solution. SDS adsorbs on the dendrimer layer and at low bulk concentrations causes the expansion of the dendrimer layers on the surface. When the bulk concentration of SDS is increased, the surfactant layer consists of aggregates or bilayer-like structures. The adsorption of surfactant is reversible upon rinsing, but slight changes of the structure of the preadsorbed PAMAM monolayer were observed. The adsorption from premixed solutions close to charge neutrality results in thick multilayers, but the surface excess is lower when the bulk complexes have a net negative charge. A critical examination of the pathway of adsorption for the interactions of SDS with preadsorbed PAMAM monolayers and premixed PAMAM/SDS solutions with hydrophilic silica revealed that nonequilibrium effects are important only in the latter case, and the application of a thermodynamic model to such experimental data would be inappropriate.
Publisher: American Chemical Society (ACS)
Date: 23-02-2009
DOI: 10.1021/JP807314S
Publisher: Royal Society of Chemistry (RSC)
Date: 2014
DOI: 10.1039/C4SM01733A
Abstract: Functionalization of dendrimer layers with nucleolipids allows selective molecular recognition of nucleic acids.
Publisher: American Chemical Society (ACS)
Date: 29-12-2008
DOI: 10.1021/LA802766N
Abstract: Liquid-crystalline cubic-phase nanoparticles (CPNPs) (known as Cubosome particles), based on the lipid glycerol monooleate and stabilized by the nonionic block copolymer Pluronic F-127, interact with supported model membranes consisting of dioleoylphosphatidylcholine (DOPC) in a complex and dynamic fashion. Neutron reflectivity measurements on the interaction of CPNPs with bilayers of different coverage have increased our understanding of an interfacial exchange mechanism that is relevant to delivery applications. To access the composition of the adsorption layer, the method of isotopic contrast between the components was exploited by using DOPC with perdeuterated acyl chains, which are distinguishable (high scattering contrast) from the hydrogenous components of the CPNPs. The exchange of material between CPNPs and the bilayer takes place regardless of the initial bilayer coverage. However, this parameter has a strong influence on the physical nature of the layer formed upon interaction. For a bilayer of "high coverage" (80%), extensive exchange takes place between the CPNP components and the bilayer, and at steady state the surface layer comprises 72% glycerol monooleate and 8% DOPC, with no change in the solvent content. An analogous experiment involving pure glycerol monooleate liquid crystals shows that lipid exchange occurs even in the absence of the stabilizing polymer. For bilayers of "low coverage" (55%), the exchange mechanism involves an initial adsorption of material from the CPNPs to fill in the bilayer defects. However, most of the bilayer breaks up and only 15% coverage remains after 30 h. The evolution of a Bragg diffraction peak was monitored in this case to show that the bound nanoparticles occupy >7% surface coverage and have a periodicity in the density of the internal lipid structure that decreases with time. This progression is attributed to the incorporation of d-DOPC molecules within the internal cubic structure of the nanoparticles. The broadening of the diffraction peak with time, together with a final mean position that is closely related to the periodicity of the lamellar phase organization of GMO, shows that the lipid-exchange process results in either a contraction of the unit cell of the cubic-phase nanoparticles or a progression of the lipid arrangement to the lamellar phase.
Publisher: Royal Society of Chemistry (RSC)
Date: 2008
DOI: 10.1039/B714911E
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Richard Campbell.