Robert S. Urquhart

Photoelectrochemical properties of ‘Q-state’ CdS particles in arachidic acid Langmuir–Blodgett films

The photoelectrochemical (PEC) characteristics of thin films of ‘Q-state’ CdS semiconductor particles in arachidic acid Langmuir–Blodgett (LB) layers have been examined. Initially, ‘Q-state’ size CdS particles of ca. 2 nm were formed in the film and then progressively grown to ca. 10 nm by a cyclic sequence, which involves exposing the film to H2S(g) following its immersion in a CdCl2(aq) solution. A PEC cell was constructed using a layered glass/ITO–CdS/LB plate as the photoanode and a standard calomel electrode (SCE) as the reference electrode. The photoelectrical response of electrodes coated with LB films containing CdS particles of various sizes was investigated using white light. The reproducibility of current–voltage polarisation curves indicated the relatively good stability of the film to PEC degradation processes over several scanning cycles in the range 0 to –1000 mV vs. SCE. Values for the open-circuit (Voc) and the short-circuit current for the PEC cells were obtained and fill factors in the range 20 to 70–75% were calculated. Voc decreased from ca. –200 to ca. –800 mV vs. SCE with increasing particle size. This effect was attributed to the dielectric properties of the arachidic acid LB film matrix.

Immobilisation of IgG onto gold surfaces and its interaction with anti-IgG studied by surface plasmon resonance

The optical excitation of surface plasmon resonance (SPR) at a metal dielectric interface has been used to study the binding of immunoglobulin G (IgG) to gold and anti-IgG to immobilised IgG layers. In these studies both a monoclonal mouse and polyclonal sheep IgG were used as receptor layers for anti-IgG. The kinetics of binding were investigated by monitoring the reflectivity of light at an angle close to plasmon resonance. Both the initial rate of change and final reflectivity were measured during and after protein binding. The amount of protein bound to the surface was found to be less for the monoclonal mouse IgG compared to the polyclonal sheep IgG, these two IgG nominally being of the same dimensions and molecular weight. Further, anti-IgG binding produced greater changes in reflectivity than the initial IgG layers. By fitting the full angle-dependent reflectivity data to the Fresnel equation the effective protein layer thicknesses of IgG and anti-IgG as a function of concentration were determined. Differences in the effective thickness of the bound layer for the two IgG was observed, the mouse IgG having a thinner effective thickness compared with the sheep IgG. The limitations of direct binding of protein to metal surfaces in SPR biosensor applications are discussed.

Size-quantised cadmium sulfide particles in Langmuir–Blodgett films: film thermal stability

The fabrication of particles in Langmuir–Blodgett films of cadmium icosanoate and nonacosa-10,12-diynoate is followed using a quartz crystal microbalance. The mass changes accompanying the exposure of the film to hydrogen sulfide are consistent with a chemical reaction in the film which also produces the acid form of the surfactant. Film thermal stability is greatly reduced after the particles are formed, a result also consistent with formation of the more volatile acid form. The extent of film removal by heating/outgassing suggests that particles are capped by adsorbed icosano or diynoic acid. It is likely that this capping plays a role in particle growth control.

Photoelectrochemical behaviour of Q-state CdSxSe(1 –x) particles in arachidic acid Langmuir–Blodgett films

CdS Q-state particles, with mean diameters varying from 2 to 10 nm, grown in arachidic acid Langmuir–Blodgett (LB) films, have been exposed to H2Se(g) to form the corresponding Q-state CdSxSe(1 –x) particles. These particles are considered to be made up of a core of CdS coated with about a monolayer of CdSe. Q-state CdSxSe(1 –x) particle formation was verified by X-ray photoelectron spectroscopy (XPS) and by monitoring a red shift in the UV–VIS absorbance spectra relative to that of CdS. XPS results on 6 nm diameter CdS particles that had been grown in an LB film and then extensively exposed to H2Se(g), revealed a stable average composition of CdS0.4Se0.6. A study of the photoelectrochemical behaviour of these systems was conducted through current–voltage polarisation curves in the range 0 to –1000 mV vs. SCE. An average drop of 100 mV in the open-circuit voltage and a marked increase in the short-circuit current was observed when LB films with Q-state CdS particles were exposed to H2Se(g). Evidence is also presented which shows that, as the mean diameter of the core-shell particles increases from 2 to 10 nm, better connection between the particles in the film is achieved.

Investigation of immuno-reactions in a flow-injection system using surface plasmon resonance

Abstract The effect of flow rate on the binding of polyclonal sheep immunoglobulin G (IgG) and anti-sheep IgG to gold, and to immobilized anti-IgG and IgG receptor layers, respectively, has been studied using the optical technique of surface plasmon resonance at a gold-solution interface. The kinetics of protein binding were investigated by monitoring the reflectivity of light at a fixed angle close to the plasmon resonance. Fixed-angle reflectivity data are presented in terms of protein surface coverage. IgG and anti-IgG effective thicknesses were determined by fitting the full angle-dependent reflectivity data to Fresnel theory. The effective thicknesses of IgG and anti-IgG bound to the gold surface and to each other (as pre-adsorbed layers) were found to be independent of the flow rate of protein solution. Gold-adsorbed IgG and anti-IgG layers were found to be 63 ± 15 A and 68 ± 10 A thick, respectively, and 91 ± 11 A (anti-IgG) and 20 ± 7 A (IgG) thick when subsequently bound to their biospecific partner. The differences in the thickness values between the gold-adsorbed and protein-adsorbed layers are explained in terms of the orientation of the immobilized proteins and their deformation upon binding to the gold surface. Non-specific binding between immobilized sheep IgG layers and anti-mouse IgG was shown to be negligible.

Ultra-fast photoresponses of CdS nanoparticles in Nafion films

The ultra-fast dynamics of transient absorbance bleaching and recovery of cadmium sulphide nanoparticles with sizes ranging from 2.7 to 4.9 nm in Nafion polymer films was studied upon excitation with a femtosecond laser at 400 nm. All samples, with the exception of the sample containing the smallest particles, showed maximum bleaching within the time resolution of the experiment. The bleaching then recovered with multi-exponential kinetics. Bi-exponential fits to the recovery of absorbance for samples which exhibited transient photobleaching showed that the lifetime of the short component increased as the size of the particles became larger. This behaviour was attributed to greater average distances between separated electrons and holes in the larger particles. Transient spectral studies showed that there was a change in the wavelength of maximum photobleaching with time in the cases of 4.3 and 4.9 nm CdS particles. In these cases, the peak of transient photobleaching showed a progressive red shift as the time between the pump and probe pulses was increased. There was no change in the wavelength of maximum bleaching with time for Nafion films containing 3.5 nm particles. The differences in transient spectral behaviour could be due either to their being a more polydisperse size distribution of CdS nanoparticles in films containing larger particles or to films containing larger particles having different types of trapping sites on the particle surface. It is possible that both factors are contributing to the observed transient photobleaching behaviour.

Pressure-area isotherms and fluorescence behaviour of 12-(1-pyrenyl)dodecanoic acid at the air-aqueous solution interface

Abstract Surface pressure-area (π- A ) isotherms of 12-(1-pyrenyl)dodecanoic acid (PDA), alone, and in mixed films with eicosanol, on pure water and basic subphases, were measured concomitantly with the emission spectra of the film at various stages of compression. On a pure water subphase certain portions of the π- A curves could be clearly correlated with the formation of aggregates, primarily composed by PDA molecules. The main driving force for aggregate formation we attribute to hydrogen bonding between the carboxylic moieties of the PDA. Adjustment of the subphase pH such that the carboxylic groups are ionised leads to a large reduction in the degree of PDA aggregation which is seen by a distinct change in the profile of the π- A curves and more clearly by a dramatic reduction in the amount of pyrene excimer emission from the film.

Reactions of complex ions of platinum and palladium in Langmuir-Blodgett films of dimethyldioctadecylammonium chlorometallates

Abstract Langmuir-Blodgett films containing complex ions of platinum(II), platinum(IV) and palladium(II) were prepared from compressed monolayers of dimethyldioctadecylammonium bromide on subphases of these ions. The films were characterized by measuring the surface pressure vs. area per molecule ( π A ) isotherms and by UV/visible spectroscopy, grazing angle FTIR spectroscopy and X-ray photoelectron spectroscopy. The reactions of these films with H2S and hydrazine were investigated by the above spectroscopic techniques and by atomic force microscopy (AFM). The absorbance onsets of the H2S-exposed films are dramatically blue shifted relative to the bulk bandgaps of PtS2, PtS and PdS. This is consistent with the formation of particles in the “Q state” size regime. AFM images of hydrazine-treated (DDA)2(PdCl4) films on mica revealed metal particles with an average diameter of 3.6 nm.

Ultrafast dynamics of transient bleaching of surface modified cadmium sulphide nano-particles in Nafion films

Abstract Ultrafast dynamics of transient photobleaching and recovery of cadmium sulphide (CdS) nanoparticles in Nafion films before and after treatment with excess cadmium ions have been studied upon excitation at 400 nm with a femtosecond laser system. Both samples gave transient bleaching in less than 1 ps and a multi-exponential recovery in the wavelength region between ca. 410 and 500 nm. Cd2+ treatment of the nanoparticles resulted in several dramatic changes to the fluorescence and transient bleaching spectra from the films: (1) A new fluorescence band at higher energy appeared in addition to an increase in the ‘defect’ fluorescence band at longer wavelengths. (2) A remarkable increase in the extent of transient bleaching was observed below about 440 nm, which recovered faster than that at longer wavelengths. (3) The recovery rate of the transient bleaching became faster, relative to the untreated nanoparticles, over the whole wavelength region of the bleaching. These results have been interpreted in terms of the recombination of photogenerated electrons and holes from traps or defects in/on the nanoparticles.

The fabrication of titania Q-state particles in Langmuir–Blodgett films

Optical absorption and XPS analyses have shown that titania Q-state particles (<10 nm diameter) can be fabricated within arachidic (icosanoic) acid and cadmium arachidate Langmuir–Blodgett (LB) films by sequential exposure to gaseous titanium tetrachloride and water vapour; when a cadmium arachidate film was subsequently gassed with H2S, sulfur was not incorporated into the film; CdS was not formed.