Studies on the binding of CO to low-spin [Fe(II)(Por)L2] complexes: an aid to understanding the binding of CO to haemoglobin and myoglobin.

The visible and Mössbauer spectra of [Fe(II)(Por)L2] and [Fe(II)(Por)L(CO)] complexes (where Por = protoporphyrin IX (PPIX) or tetra(p-sulfophenyl)porphyrin (TPPS) and L = an aliphatic or aromatic nitrogenous base) are reported and discussed. The results are compared to those of previously reported [Fe(II)(Por)L(CO)] complexes (where Por = PPIX, TPPS, PMXPP, TPP, OMTBP and OEP; L = a nitrogenous aromatic ligand) and HbCO (where Hb = haemoglobin) and MyCO (where My = myoglobin). A new approach, to extracting information from the Mössbauer parameters has been developed by plotting those of the [Fe(II)(Por)L2] complexes against those of [Fe(II)(Por)L(CO)] complexes for the same ligands, has yielded a series of trend lines that show a significant dependence on both the nature of the porphyrin and also of the nitrogenous ligand. Different trend lines were found for aromatic nitrogenous ligands to aliphatic nitrogenous ligands showing that the porphyrins could donate different amounts of charge to the Fe(II) cations as the L ligand changed, and hence, they display electron sink properties. From the plots, it was shown that haemoglobin and myoglobin both bind CO very strongly compared to the model complexes studied herein. Using the reported structural and Mössbauer data for the [Fe(II)(Por)L2] and [Fe(II)(Por)L(CO)] complexes, it proved possible and instructive to plot the Mössbauer parameters against a number of the bond lengths around the Fe(II) cations. The interpretation of the resulting trend lines both supported and facilitated the extension of our findings enabling further understanding of the geometry of the bonding in CO haemoglobin and CO myoglobin.

Studies on the binding of nitrogenous bases to protoporphyrin IX iron(II) in aqueous solution at high pH values.

Studies are reported on the formation of low-spin six-coordinate [Fe(PPIX)L2] complexes from iron(II) protoporphyrin where L is one of a series of nitrogenous ligands (aliphatic, aromatic or heterocyclic). The bonding constants have been determined by titration of the metal complex with these ligands and are compared in relation to previous studies. The adduct formation was monitored utilising optical spectroscopy. In addition, MÓ§ssbauer spectroscopic experiments were conducted to monitor the electronic environment around the central iron atom in these complexes. The two complementary spectroscopic methods indicated that all nitrogen ligands formed low-spin octahedral complexes. The magnitude of the overall binding constants (ß2 values) are discussed and related to (a) the pKa values of the free ligands and (b) the Mössbauer parameter ΔEQ, which represents the quadrupole splitting of the haem iron. The ß2 and ΔEQ values are also discussed in terms of the structure of the ligand. Cooperative binding was observed for nearly all the ligands with Hill coefficients close to 2 for iron(II) protoporphyrin; one of these ligands displayed a much greater affinity than any we previously studied, and this was a direct consequence of the structure of the ligand. Overall conclusions on these and previous studies are drawn in terms of aliphatic ligands versus aromatic ring structures and the absence or presence of sterically hindered nitrogen atoms. The implications of the work for the greater understanding of haem proteins in general and in particular how the nitrogenous ligand binding results are relevant to and aid the understanding of the binding of inhibitor molecules to the cytochrome P450 mono-oxygenases (for therapeutic purposes) are also discussed. Changes in the electronic absorption spectra of five-coordinate [Fe(II)(PPIX)(2-MeIm)] that occurred as the temperature was lowered from room temperature to 78° K.

New Developments in Cathodoluminescence Spectroscopy for the Study of Luminescent Materials.

Herein, we describe three advanced techniques for cathodoluminescence (CL) spectroscopy that have recently been developed in our laboratories. The first is a new method to accurately determine the CL-efficiency of thin layers of phosphor powders. When a wide band phosphor with a band gap (Eg > 5 eV) is bombarded with electrons, charging of the phosphor particles will occur, which eventually leads to erroneous results in the determination of the luminous efficacy. To overcome this problem of charging, a comparison method has been developed, which enables accurate measurement of the current density of the electron beam. The study of CL from phosphor specimens in a scanning electron microscope (SEM) is the second subject to be treated. A detailed description of a measuring method to determine the overall decay time of single phosphor crystals in a SEM without beam blanking is presented. The third technique is based on the unique combination of microscopy and spectrometry in the transmission electron microscope (TEM) of Brunel University London (UK). This combination enables the recording of CL-spectra of nanometre-sized specimens and determining spatial variations in CL emission across individual particles by superimposing the scanning TEM and CL-images.

Structure and luminescence analyses of simultaneously synthesised (Lu1-xGdx)2O2S:Tb3+ and (Lu1-xGdx)2O3:Tb3.

Herein we describe the synthesis and luminescence of nanosized (Lu1-y-xGdx)2O2S:Tby and (Lu1-y-xGdx)2O3:Tby phosphors with y = 0.1 mol% Tb3+ and y = 2 mol% Tb3+ and x ranging between 0 and 1. The concentration of Gd3+ (x) was varied in steps of 0.1 (molar ratio Gd3+). The samples at 0.1 < x < 0.7 contained a mixture of (Lu1-xGdx)2O3:Tb3+ and (Lu1-xGdx)2O2S:Tb3+, while the samples at x = 0 contained only Lu2O3:Tb3+. At 0.1 < x < 0.7 Lu2O2S:Tb3+ and Gd2O2S:Tb3+ did not form a solid solution, but rather crystallised into two slightly different hexagonal structures. This behaviour has been explained in terms of segregation of Lu and Gd between the oxide and oxysulfide phases: the oxide phase is more Lu-rich whereas the second oxysulfide phase is more Gd-rich. The photoluminescence spectra of the phosphors with 0.1 mol% Tb3+ showed a modest colour change of the fluorescence light from cyan to green when x was increased from 0 to 1, whereas the samples of the series with 2 mol% Tb3+ yielded essentially green light. From this analysis it was concluded that the colour change of (Lu1-xGdx)2O2S:0.1%Tb3+ is caused by increasing energy transfer of the 5D3-level of Tb3+ to the charge transfer band of (Lu1-xGdx)2O2S:Tb3+ upon increasing x. Since the samples with 100% Lu consisted of pure cubic Lu2O3:Tb3+, we had the opportunity to also study the symmetry-related PL of this compound. From this study we concluded that the C2-C3i doublet of the Tb3+ 5D4 → 7F5 transition behaves in the same way as the corresponding doublet in cubic Y2O3:Tb3+.

Contrast and decay of cathodoluminescence from phosphor particles in a scanning electron microscope.

Cathodoluminescence (CL) studies are reported on phosphors in a field emission scanning electron microscope (FESEM). ZnO: Zn and other luminescent powders manifest a bright ring around the periphery of the particles: this ring enhances the contrast. Additionally, particles resting on top of others are substantially brighter than underlying ones. These phenomena are explained in terms of the combined effects of electrons backscattered out of the particles, together with light absorption by the substrate. The contrast is found to be a function of the particle size and the energy of the primary electrons. Some phosphor materials exhibit a pronounced comet-like structure at high scan rates in a CL-image, because the particle continues to emit light after the electron beam has moved to a position without phosphor material. Image analysis has been used to study the loss of brightness along the tail and hence to determine the decay time of the materials. The effect of phosphor saturation on the determination of decay times by CL-microscopy was also investigated.

Effects of the host lattice and doping concentration on the colour of Tb3+ cation emission in Y2O2S:Tb3+ and Gd2O2S:Tb3+ nanometer sized phosphor particles.

Y2O2S and Gd2O2S phosphor lattices activated with a range of Tb(3+) concentrations have been successfully prepared as nanoparticles and their emission properties have been characterized using SEM, XRPD, photoluminescence spectroscopy and cathodoluminescence. (5)D3-(5)D4 cross relaxation processes between Tb(3+) cations were observed in both Y2O2S and Gd2O2S as a function of Tb(3+) concentration. In the Y2O2S host lattice, the predominant emission colour shifts from blue to green with increased Tb(3+) concentration. In contrast, green emission is always predominant in Gd2O2S at Tb(3+) concentrations from 0.1 mol% to 5 mol%. This finding is explained in accordance with previous reports on the bulk materials that found the Gd2O2S lattice has a lower charge transfer state than the Y2O2S host lattice.

Contrasting behaviour of the co-activators in the luminescence spectra of Y2O2S:Tb3+,Er3+ nanometre sized particles under UV and red light excitation.

Nanometre sized particles of terbium and erbium co-doped yttrium oxysulfide up-converting phosphors were prepared by a urea homogeneous-precipitation method. Results from X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) and photoluminescence spectroscopy studies on the microstructure and luminescent properties of the materials are reported. Upconversion emission was observed from the Er(3+) cations when particles were excited with laser light of 632.8 nm wavelength. Under these conditions no interactions between the Er(3+) cations and the Tb(3+) cations were observed. In contrast there was evidence from the Stokes emission of the Er(3+) cations under 254 nm excitation for an interaction between the Er(3+) and Tb(3+) cations reducing intensity of the latter's blue and green emission bands by cross relaxation processes.

Surface plasmon resonance imaging detection of silver nanoparticle-tagged immunoglobulin.

The detection sensitivity of silver nanoparticle (AgNP)-tagged goat immunoglobulin G (gIgG) microarrays was investigated by studying surface plasmon resonance (SPR) images captured in the visible wavelength range with the help of a Kretchmann-configured optical coupling set-up. The functionalization of anti-gIgG molecules on the AgNP surface was studied using transmission electron microscopy, photon correlation measurements and UV-visible absorption spectroscopy. A value of 1.3×10(7) M(-1) was obtained for the antibody-antigen binding constant by monitoring the binding events at a particular resonance wavelength. The detection limit of this SPR imaging instrument is 6.66 nM of gIgG achieved through signal enhancement by a factor of larger than 4 owing to nanoparticle tagging with the antibody.