Teodora A. Doneva

Atomic force microscopy studies of nanofiltration membranes: surface morphology, pore size distribution and adhesion

Abstract Atomic force microscopy has been used to quantify surface morphology, pore size distributions and particle adhesion for three nanofiltration membranes. It was found that the mean pore diameters were ∼ 1 nm and that the relative sizes corresponded to measured separation performance. Adhesion of polystyrene particles was greater than that of silica particles, due partly to greater electrostatic double-layer repulsion between the negatively charged membranes and silica, and partly to short-range repulsive interactions associated with the silica surface. It is likely that short-range repulsive interactions are also responsible for the low adhesion of the negatively charged silica particles even at the membrane which was positively charged.

Separation of humic acid from a model surface water with PSU/SPEEK blend UF/NF membranes

Abstract Membranes made from blends of polysulphone and sulphonated poly(ether ether ketone) (PSU/SPEEK) have high porosity, high charge and a pore size at the boundary between nanofiltration (NF) and ultrafiltration (UF). Their application to the treatment of surface water for drinking purposes has been investigated through study of their ability to remove humic acid (HA) from a model water, in comparison with two commercial membranes. The PSU/SPEEK membranes, in planar or tubular form, showed excellent retention of HA and very high flux. Their high charges gave rise to critical fluxes for deposition. As an additional benefit, HA deposits forming on the charged membranes above the critical fluxes had a loose structure, as visualised by atomic force microscopy (AFM), and were consequently efficiently removed by rinsing. Three multivalent ions, Fe 3+ , Al 3+ and Mn 2+ , at their concentrations in a natural water did not affect the membrane process significantly.

The use of atomic force microscopy to quantify membrane surface electrical properties

Abstract An atomic force microscope has been used to quantify the electrical double layer interactions between a silica particle and a Desal nanofiltration membrane in electrolyte solutions of various ionic strengths. The experimental data has been compared to DLVO predictions incorporating a numerical solution of the non-linear Poisson–Boltzmann equation. At the lowest ionic strength studied (10 −3 M) there was good agreement between the experimental data and the theoretical calculations. The membrane surface potential calculated from the best-fit membrane charge under these conditions was in good agreement with the zeta potential obtained from streaming potential data. The discrepancy between DLVO calculations and experimental data increased as the ionic strength increased and in particular as the membrane surface roughness became greater in magnitude than the solution Debye length. Under such conditions, colloid–membrane electrical double layer interactions may at present only be quantified by direct experimental measurement. Atomic force microscopy measurement of pull-off (adhesion) forces provides a further direct evaluation of fouling propensity.

The effect of sulfonated poly(ether ether ketone) additives on membrane formation and performance

Sulfonated poly(ether ether ketone) (SPEEK), a strong polyelectrolyte, was investigated as an additive in polysulfone (PSU)/SPEEK/N-methyl-2-pyrrolidinone (NMP) systems. Membrane characterisation was carried out using filtration studies and by atomic force microscopy (AFM). The effect of SPEEK in the charged membrane formation was investigated through thermodynamic and viscosity studies and correlated with the structure and properties of the resulting membranes. A “Polyelectrolyte behaviour” of SPEEK was observed for the first time in the membrane forming systems. Charged ultrafiltration/nanofiltration membranes prepared from 20 wt.% PSU solutions with the ratio of SPEEK/PSU in the range of 0.005–0.05, have substantially increased permeability, salt rejection, porosity, and a greatly reduced fouling tendency compared to the base PSU membrane. The charged membranes offer the possibility of effective removal of humic substances from surface waters in a highly efficient manner.

Protein deposition during cross-flow membrane filtration: AFM studies and flux loss

An atomic force microscope (AFM) has been used to study the deposition of bovine serum albumin (BSA) onto the surface of two polymeric membranes of similar molecular weight cut-off but different surface roughness and surface chemistry (ES404 and EM006 PCI membranes (UK)). BSA coated probes were used to quantify the interaction of BSA with both clean and BSA fouled membranes. Clean water flux (CWF) loss suffered by both the membranes under different BSA feed concentrations (0.1, 1 and 10 g l−1) and two pHs (pH 5 and 9) was measured in cross-flow filtration experiments. Subsequent comparison was made between the CWF loss, membrane morphology and surface forces for clean and fouled membranes. Protein coverage of membrane surfaces has been determined using profiles of force–distance retraction curves. Preferential adsorption takes place at specific locations on the membrane surface and is a function of pH, surface roughness and surface chemistry.

Atomic force microscopy studies of membrane—solute interactions (fouling)☆

Abstract AFM has been proved to be a rapid method of assessing membrane—solute interactions (fouling) of membranes under process conditions. The interactions of developmental membranes (SPEEK/PSU and PSU) with different solutes have been directly measured and linked to process performance. Correlation of the performance of two commercial PCI membranes (ES404 and EM006) with AFM measurements has been carried out for two types of separation of industrial importance: ultrafiltration in the pulp and paper industry and protein concentration/ fractionation. AFM expertise has been further exploited in a study of the efficiency of membrane cleaning after a process of whey concentration.

Atomic force microscopy characterization of ultrafiltration membranes : correspondence between surface pore dimensions and molecular weight cut-off

Atomic force microscopy (AFM) has been used to quantify the mean surface pore diameters of a family of five ultrafiltration membranes with molecular weight cut-off (MWCO) in the range 1000–10 000 aspect ratio tips were used and the images were treated with fast Fourier transform filtering to remove distortions. The mean pore diameters were found to be in the range 1.93–3.14 nm, increasing systematically with the MWCO. Mean pore diameters have also been evaluated from the MWCO values using three published expressions relating MWCO to solute radius. Depending on the expression used, the ratio of the mean pore diameters obtained from MWCO data and AFM measurements (Rr) was in the range 1.04–2.42. Agreement between the diameters obtained in the two ways was best for the membranes with the lowest MWCO values. The overall agreement was substantially better than has been reported previously. The good correspondence between mean surface pore dimensions and MWCO demonstrates the usefulness of AFM in quantifying such a key membrane property. Copyright

Recruitment of heterogeneous nuclear ribonucleoprotein A1 in vivo to the LMP/TAP region of the major histocompatibility complex.

Sequences containing the matrix recognition signature were identified adjacent to theLMP/TAP gene cluster in the human and mouse major histocompatibility complex class II region. These sequences were shown to function as nuclear matrix attachment regions (MARs). Three of the five human MARs and the single mouse MAR recruit heterogeneous nuclear ribonucleoprotein A1 (hnRNP-A1) in vivo during transcriptional up-regulation of the major histocompatibility complex class II genes. The timing of this recruitment correlates with a rise in mature TAP1 mRNA. Two of the human MARs bind hnRNP-A1 in vitro directly within a 35-bp sequence that shows over 90% similarity to certain Alu repeat sequences. This study shows that MARs recruit and bind hnRNP-A1 upon transcriptional up-regulation.

HnRNP-A1 binds directly to double-stranded DNA in vitro within a 36 bp sequence

The heterogeneous nuclear ribonucleoprotein A1 (hnRNP-A1) is known as an RNA- and single-stranded DNA-binding protein involved in alternative splicing of mRNA, RNA transport and maintenance of chromosome telomere length. In this study we tested whether this protein could bind directly to double-stranded DNA (dsDNA). Using PCR amplification of target DNA-sequences from human chromosome 11q13 followed by their incubation with hnRNP-A1 and atomic force microscopy (AFM) of the DNA/protein complexes, we found that this protein bound to DNA within a 36 bp sequence. These results were confirmed by electrophoretic mobility shift assay (EMSA). This sequence was found widely dispersed throughout the genome. There is no overlap between the 36 bp sequence and known target sequences in RNA for binding hnRNP-A1.

Development and AFM study of porous scaffolds for wound healing applications

An engineering approach to the development of biomaterials for promotion of wound healing emphasises the im- portance of a well-controlled architecture and concentrates on optimisation of morphology and surface chemistry to stimulate guidance of the cells within the wound environment. A series of three-dimensional porous scaffolds with 80-90% bulk porosity and fully interconnected macropores were prepared from two biodegradable materials - cellulose acetate (CA) and poly (lactic- co-glycolic acid) (PLGA) through the phase inversion mechanism of formation. Surface morphology of obtained scaffolds was determined using atomic force microscopy (AFM) in conjunction with optical microscopy. Scanning Electron Microscopy (SEM) was applied to characterise scaffolds bulk morphology. Biocompatibility and biofunctionality of the prepared materials were assessed through a systematic study of cell/material interactions using atomic force microscopy (AFM) methodologies to- gether with in vitro cellular assays. Preliminary data with human fibroblasts demonstrated a positive influence of both scaffolds on cellular attachment and growth. The adhesion of cells on both biomaterials were quantified by AFM force measurements in conjunction with a cell probe technique since, for the first time, a fibroblast probe has been successfully developed and optimal conditions of immobilisation of the cells on the AFM cantilever have been experimentally determined.