Chris Hodges - Academia.edu (original) (raw)

Papers by Chris Hodges

The adsorption of colloidal laponite at the solid/liquid interface on various substrates and over... more The adsorption of colloidal laponite at the solid/liquid interface on various substrates and over a range of laponite concentrations (10-1000 ppm) has been investigated. Although a wide range of surfaces were studied, only on a positively charged poly(diallyldimethylammonium chloride) (PDADMAC) surface was any adsorption of the laponite observed. This shows that when fully wetted, laponite adsorption depends primarily on the surface charge rather than the degree of hydrophobicity of the surface. The adsorption of spherical Ludox silica nanoparticles on PDADMAC surfaces was also examined for comparison with the disklike laponite. The QCM data for both laponite and Ludox show strong adsorption on PDADMAC surfaces; however, larger frequency shifts were seen for Ludox than laponite at all concentrations tested. Within the concentration range examined in this work, the dissipation data from the QCM suggested a simple monolayer formation for Ludox but a monolayer to multilayer transition for laponite as the concentration increases.

X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine ... more X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethylene-b-oxybutylene (E 23 B 8) copolymer film at the air-water interface. The XR data were fitted using both a one-and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.

The scientific community needs a rapid and reliable way of accurately determining the stiffness o... more The scientific community needs a rapid and reliable way of accurately determining the stiffness of atomic-force microscopy cantilevers. We have compared the experimentally determined values of stiffness for ten cantilever probes using four different methods. For rectangular silicon cantilever beams of well defined geometry, the approaches all yield values within 17% of the manufacturer's nominal stiffness. One of the methods is new, based on the acquisition and analysis of thermal distribution functions of the oscillator's amplitude fluctuations. We evaluate this method in comparison to the three others and recommend it for its ease of use and broad applicability.

Advances in Colloid and Interface Science, 2002

This review links together for the first time both the practicalities of force measurement and th... more This review links together for the first time both the practicalities of force measurement and the work carried out to date on force detection between polymeric surfaces in liquids Ž . using the atomic force microscope AFM . Also included is some of the recent work that has been carried out between surfactant surfaces and biologically coated surfaces with the AFM. The emphasis in this review is on the practical issues involved with force measurement between these types of surfaces, and the similarities and irregularities between the observed types of forces measured. Comparison is made between AFM and surface force apparatus Ž . SFA measurements, as there is a much longer history of work with the latter. Results indicate that forces between the surfaces reviewed here are a complicated mixture of steric-type repulsion, conformational behaviour on separation and long-range attraction, which is often ascribed to 'hydrophobic' forces. The origin of this latter force remains uncertain, despite its almost ubiquitous appearance in force measurements with these types of surfaces. ᮊ

Sensors and Actuators B: Chemical, 2012

Immunosensors depend on the specificity of the molecular recognition towards analytes of interest... more Immunosensors depend on the specificity of the molecular recognition towards analytes of interest provided by the antibodies. In this study, electrochemical impedance spectroscopy (EIS) was used to monitor the sensor surface assembly and recognition of two different analytes of cardiac pathology, cardiac troponin I (cTnI) and soluble lectin-like oxidized LDL receptor-1 (sLOX-1) on surfaces based on mixed self-assembled monolayers onto gold electrodes. The specificity and longer persistence of cTnI serves as a definitive biomarker in the diagnosis of acute myocardial infarction (AMI). Circulating levels of sLOX-1 serve as a marker for plaque instability or rupture before an AMI becomes apparent and facilitates early diagnosis of acute coronary syndrome (ACS). The immunosensors were highly specific, since a clinically relevant range of analyte concentrations was successfully detected in phosphate buffered saline and in serum with virtually no alteration to the slope of the calibration curve. Detection limits for cTnI and sLOX-1 immunosensors were at least 10 −13 M for each analyte. Atomic force microscopy studies were carried out to study each step of fabrication to elucidate the binding characteristics and surface nanotopography of the mSAM based immunosensors.

Nanotechnology, 2003

Langmuir, 2002

Using the atomic force microscope (AFM), the pull-off forces between flat glass or silicon surfac... more Using the atomic force microscope (AFM), the pull-off forces between flat glass or silicon surfaces, and silicon AFM tips or glass microspheres of different sizes, have been extensively studied as a function of relative humidity (RH) in the range 5-90%, as model systems for the behaviour of cohesive powders. The glass and silicon substrates were treated to render them either hydrophobic or hydrophilic. All the hydrophilic surfaces gave simple force curves and pull-off forces increasing uniformly with RH. Small contacts (R~20nm) gave pull-off forces close to values predicted by simple Laplace-Kelvin theory (~20 nN) but the values with microspheres (R~20µm) fell well below predictions for sphere-flat or sphere-sphere geometry, due to roughness and asperity contacts. The hydrophobic silicon surfaces also exhibited simple behaviour, with no significant RH dependence. The pull-off force again fell well below predicted values (JKRS contact mechanics theory) for the larger contacts. Hydrophobic glass gave similar adhesion to silicon over most of the RH range, but against both silicon tips and glass microspheres, there was an anomalously large adhesion in the RH range 20-40%, accompanied by a long range non-contact force. The adhesion on fully hydrophilic surfaces and its RH dependence can be mostly explained by current theories of capillary bridges, but the interpretation is complicated by the sensitivity of theoretical predictions to contact geometry (and hence to roughness effects) and by uncertainties in the thickness of adsorbed water layers. The anomalous behaviour on hydrophobic glass surfaces at intermediate values of RH is not fully understood, but possible causes are (1) dipole layers in the partially-formed water film, giving rise to patch charges and long range forces, or (2) fixed charges at a reactive glass surface, involving specific bonding reactions. The results may be useful in explaining the behaviour of cohesive powders with different coatings, or which show a large humidity dependence (e.g. zeolites) or show electrostatic charging effects (e.g. silica aerogels).

Langmuir, 2007

The adsorption of rodlike polymer-micelle aggregates of cetyltrimethylammonium 4-vinylbenzoate (p... more The adsorption of rodlike polymer-micelle aggregates of cetyltrimethylammonium 4-vinylbenzoate (p-C 16 TVB) at the silica-water interface has been characterized using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) studies. Adsorption isotherm data, recorded by QCM-D, indicate a two-stage mechanism: an adsorbed film of free CTA + ions is initially produced at low concentrations until the surface is charge reversed, whereupon the weakly anionic aggregates can adsorb and the adsorbed mass is seen to increase dramatically. The adsorbed rodlike micelle aggregates are seen to form a close-packed monolayer from AFM images with a high degree of order over micrometer length scales. AFM force-distance data indicate that the adsorbed aggregates retain their cylindrical structure and little or no flattening is seen. Rinsing of the film did not result in removal of the adsorbed layer, and the persistence of these nanoscale ordered films at the solid-liquid interface suggests many possible applications.

Langmuir, 2010

Langmuir, 2008

Surface films of two copolymers of ethylene oxide (E) and butylene oxide (B), namely E 23 B 8 and... more Surface films of two copolymers of ethylene oxide (E) and butylene oxide (B), namely E 23 B 8 and E 87 B 18 , have been examined by Brewster angle microscopy (BAM) and atomic force microscopy (AFM). Isotherms taken on unsupported films of these copolymers at the air-water interface showed a clear gas to liquid phase transition for E 87 B 18 and a barely discernible phase transition for E 23 B 8 . The BAM studies showed a gradual brightening of the films as the surface pressure was increased, which was associated with a film thickening and/or a film densification. Several bright spots were also observed within the films, with the number of spots increasing gradually as the film surface pressure was increased. AFM studies of these films did not show any localized ordering, which fits in with the results from our previous X-ray study of these copolymers [Hodges, C. S.; Neville, F.; Konovalov, O.; Gidalevitz, D.; Hamley, I. W.; Langmuir 2006, 22 (21), 8821-8825], where no long-range ordering was observed. AFM imaging showed two sizes of particulates that were irregularly spaced across the film. The larger particulates were associated with silica contaminants from the copolymer synthesis, whereas the smaller particulates were assumed to be aggregated copolymer. An analysis of the semidilute region of the isotherm showed that while both copolymers had intermixed ethylene oxide and butylene oxide units, the lower molecular weight E 23 B 8 copolymer manifested significantly more intermixing than E 87 B 18 .

Langmuir, 2002

Adhesion between polystyrene particles in high purity water has been measured for a wide range of... more Adhesion between polystyrene particles in high purity water has been measured for a wide range of particle sizes. The results are compared with predictions based on the JKR model using literature values of the solid-vapor surface energy. When the surface roughness of the particles is accounted for, the JKR adhesion is still too large. An optimum value of the surface energy is calculated for one tip particle size and used for predicting the pull-off force for all of the substrate particle sizes, and it was found that this produces much closer agreement between theory and experiment although the scatter in the data is large. The mean pull-off force for all of the particle-particle measurements studied here is of the order of a few nanonewtons. Because of the large scatter in the pull-off force data, no clear trend in the pull-off force for different particle sizes has been observed. It is suggested that the effect of surface roughness on the pull-off force is significant.

Langmuir, 2009

A quartz crystal microbalance (QCM) and an optical reflectometer have been used to quantify the l... more A quartz crystal microbalance (QCM) and an optical reflectometer have been used to quantify the long-term adsorption behavior of polyelectrolyte-surfactant aggregates of alkyltrimethylammonium and poly(4-vinylbenzoate) or pC n TVB at the silica-water interface. In solution, these polyelectrolyte-surfactant aggregates exist as weakly anionic semiflexible rodlike structures of several nanometers in radius and hundreds of nanometers in length. The optical reflectivity (OR) data confirmed our earlier proposed model of a two-stage adsorption process (Biggs, S.; Kline, S. R.; Walker, L. M. Langmuir, 2004, 20 where free CTA + ions initially adsorb and charge reverse the silica surface, thus allowing the weakly anionic aggregates to adsorb. Combining data from the two techniques allows a distinction to be made between contributions to the measured signal from the bulk and the interface. The isotherm determined by OR showed a clear plateau at higher concentrations, whereas the isotherm obtained by QCM continues to increase across all concentrations tested. This indicates a significant influence of the bulk fluid on the measured signals from the QCM as the concentration is increased. Slow changes in the apparent adsorbed mass observed with the QCM were not reproduced in the OR data, suggesting that these effects were also caused by the bulk and were not a densification of the adsorbed layer. The combination of techniques clarifies the adsorption kinetics and mechanism of adsorption in polyelectrolyte-surfactant aggregate systems.

Langmuir, 2009

A quartz crystal microbalance (QCM) and an optical reflectometer (OR) have been used to investiga... more A quartz crystal microbalance (QCM) and an optical reflectometer (OR) have been used to investigate the adsorption behavior of two different variants of the surfactant-hydrotropic counterion system, alkane trimethylammonium vinylbenzoate (C n TVB), onto silica surfaces. The C 18 TVB variant, with a longer hydrocarbon tail, produced a threestage adsorption isotherm in the OR, whereas the C 16 TVB surfactant showed a two-stage adsorption isotherm. This was explained in terms of the greater degree of hydrophobicity of the C 18 carbon chain requiring a significantly higher concentration of surfactant to be present on the surface before any further adsorption can occur. A concentration dependent adsorption rate was observed for both surfactants, with the faster adsorption rate being detected for C 18 TVB. The OR data showed that each surfactant could be completely rinsed off with the flow of water into the OR cell. This was not observed with the QCM data, where only a partial rinse off was seen. The difference between the two techniques was hypothesized to be due to the ability of the QCM to detect both interfacial and bulk behavior thus complicating the interpretation of the adsorption data.

Langmuir, 2006

X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine ... more X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethyleneb-oxybutylene (E 23 B 8 ) copolymer film at the air-water interface. The XR data were fitted using both a one-and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.

Langmuir, 2004

A simple method for using the JKR model to determine interfacial adhesion between two ideal rough... more A simple method for using the JKR model to determine interfacial adhesion between two ideal rough surfaces is demonstrated for individual asperity-asperity and asperity-flat contacts both in air and in water. The model takes into account the effect of a modified contact area at separation due to viscoelastic effects. The equilibrium version of the model significantly underestimates the measured adhesion, whereas the viscoelastic version of the model is much closer to the measured data. The asperity-flat geometry used with the viscoelastic version of the model seems to fit the experimental results best. This was thought to be due to the unlikely occurrence of direct asperity-asperity contacts. Instead, it would seem that the asperities have a far higher chance of fitting between each other on opposing surfaces, leading to correspondingly higher pull-off forces measured on separation. Many possible extensions to the roughness model described here may be made, allowing a much-improved understanding of the contact mechanics between two rough surfaces.

Journal of Nanoparticle Research, 2012

Nanoparticle suspensions (also called nanofluids) are often polydisperse and tend to settle with ... more Nanoparticle suspensions (also called nanofluids) are often polydisperse and tend to settle with time. Settling kinetics in these systems are known to be complex and hence challenging to understand. In this work, polydisperse spherical alumina (Al 2 O 3 ) nanoparticles in the size range of~10-100nm were dispersed in water and examined for aggregation and settling behaviour near its isoelectric point (IEP). A series of settling experiments were conducted and the results were analysed by photography and by Small Angle X-ray Scattering (SAXS). The settling curve obtained from standard bed height measurement experiments indicated two different types of behaviour, both of which were also seen in the SAXS data. But the SAXS data were remarkably able to pick out the rapid settling regime as a result of the high temporal resolution (10s) used. By monitoring the SAXS intensity, it was further possible to record the particle aggregation process for the first time. Optical microscopy images were produced on drying and dried droplets extracted from the suspension at various times. Dried deposits showed the rapid decrease in the number of very large particles with time which qualitatively validates the SAXS prediction, and therefore its suitability as a tool to study unstable polydisperse colloids.

Journal of Colloid and Interface Science, 2010

Dried deposits of spherical Ludox silica and disk-like laponite clay nanoparticles have been exam... more Dried deposits of spherical Ludox silica and disk-like laponite clay nanoparticles have been examined by dark-field optical microscopy and atomic force microscopy (AFM) to investigate the effects of nanoparticle shape on the deposit structure. Dark-field optical images indicated that a higher concentration of Ludox nanoparticles was required, compared to laponite, for an optically visible deposit to be formed. Compared with the relatively simple ring-like features observed at the edges of Ludox deposits, the laponite deposits were more complex, with dendritic features appearing below 10 ppm that disappeared at higher laponite concentrations. AFM images revealed that whilst the Ludox rim deposit structure gradually increased in height and width with increasing nanoparticle concentration, the laponite rim deposits increased steadily in height up to 1 ppm, above which the rim height suddenly decreased and the deposit structure became smoother. The widths of the rim deposits were observed to increase in a similar manner for both nanoparticle types. Nanoparticle shape is suggested as the main reason for differences in the structural features of the rim for each nanoparticle type. The disk-like laponite forms tall thin rim profiles at low concentrations, before creating a more uniform rim profile at higher concentrations. We suggest that a critical laponite rim height is reached before partial collapse of the nanoparticle stack at the rim takes place as the particle concentration is further increased. This produces much thinner and smoother films of laponite at high particle concentrations than is found for similar concentrations of Ludox. Our work suggests that both the shape and the concentration of the nanoparticles themselves are crucial in determining the structure of the final dried nanoparticle deposit.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2013

Biochimica et Biophysica Acta (BBA) - Biomembranes, 2006

Lipid A structure at the air-aqueous interface has been studied using pressure-area isotherm meth... more Lipid A structure at the air-aqueous interface has been studied using pressure-area isotherm methods coupled with the surface X-ray scattering techniques of X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD). Lipid A monolayers were formed at the air-aqueous interface to represent the lipid moiety of the outer membrane of Gram-negative bacteria. Lipid A structure was characterized at surface pressures between 10 and 35 mN/m. Interactions of α-helical antimicrobial peptides LL-37, SMAP-29 and D2A22 with lipid A monolayers were subsequently studied. Although insertion into the lipid A monolayers was observed with the α-helical peptides, little change was seen from the Xray data, suggesting that the lipid A hydrocarbon chains are involved in reorientation during insertion and that the hydrocarbon chains have a relatively rigid structure.

Advances in Colloid and Interface Science, 2002

Sensors and Actuators B: Chemical, 2012

Nanotechnology, 2003

Langmuir, 2002

Langmuir, 2007

Langmuir, 2010

Langmuir, 2008

Langmuir, 2002

Langmuir, 2009

Langmuir, 2006

X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine ... more X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) have been used to examine an oxyethyleneb-oxybutylene (E 23 B 8 ) copolymer film at the air-water interface. The XR data were fitted using both a one-and a two-layer model that outputted the film thickness, roughness, and electron density. The best fit to the experimental data was obtained using a two-layer model (representing the oxyethylene and oxybutylene blocks, respectively), which showed a rapid thickening of the copolymer film at pressures above 7 mN/m. The large roughness values found indicate a significant degree of intermixing between the blocks and back up the GIXD data, which showed no long range lateral ordering within the layer. It was found from the electron density model results that there is a large film densification at 7 mN/m, possibly suggesting conformational changes within the film, even though no such change occurs on the pressure-area isotherm at the same surface pressure.

Langmuir, 2004

Journal of Nanoparticle Research, 2012

Journal of Colloid and Interface Science, 2010

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2013

Biochimica et Biophysica Acta (BBA) - Biomembranes, 2006