Inkjet printing and electrical characterisation of DNA-templated cadmium sulphide nanowires.

Cadmium sulphide can be templated on λ-DNA molecules to form an aqueous dispersion of CdS/λ-DNA nanowires. Subsequent addition of ethylene glycol to 50% v/v is sufficient to formulate an ink suitable for printing using piezoelectric drop-on-demand technology. Printed droplet arrays show a coffee-ring morphology of individual deposits by fluorescence and Raman microscopy, but upon increasing the number of layers of printed material by repeated printing over each droplet, the dry deposit approaches closer to a disc shape. It is also possible to print parallel tracks by reducing the droplet separation in the array until neighbouring droplets overlap before they dry. The droplets coalesce to form a strip of width roughly equal to the diameter of the droplets. Evaporation-driven capillary flow sends the nanowires to the edges of the strip and when dry they form parallel tracks of CdS/λ-DNA nanowire bundles. Both droplets and tracks were printed onto Pt-on-glass interdigitated microelectrodes (10 µm width, 10 µm gap). The current-voltage characteristics of these two-terminal devices were approximately ohmic, but with some hysteresis. The conductance increased with temperature as a simple activated process with activation energies of 0.57 ± 0.02 eV (tracks) and 0.39 ± 0.02 eV (droplets). The impedance spectra of the printed films were consistent with hopping between CdS grains.

Core and valence exciton formation in x-ray absorption, x-ray emission and x-ray excited optical luminescence from passivated Si nanocrystals at the Si L(2,3) edge.

Resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy and x-ray excited optical luminescence (XEOL) have been used to measure element specific filled and empty electronic states over the Si L(2,3) edge of passivated Si nanocrystals of narrow size distribution (diameter 2.2 ± 0.4 nm). These techniques have been employed to directly measure absorption and luminescence specific to the local Si nanocrystal core. Profound changes occur in the absorption spectrum of the nanocrystals compared with bulk Si, and new features are observed in the nanocrystal RIXS. Clear signatures of core and valence band exciton formation, promoted by the spatial confinement of electrons and holes within the nanocrystals, are observed, together with band narrowing due to quantum confinement. XEOL at 12 K shows an extremely sharp feature at the threshold of orange luminescence (i.e., at ∼1.56 eV (792 nm)) which we attribute to recombination of valence excitons, providing a lower limit to the nanocrystal band gap.

Dispersions of alkyl-capped silicon nanocrystals in aqueous media: photoluminescence and ageing.

Alkyl-capped silicon nanocrystals can be dispersed in aqueous media by shaking or stirring their solutions in organic solvents (DMSO, ether, THF) with excess water. THF is the most straightforward choice with which to prepare stable aqueous dispersions, because the nanocrystals are very soluble in THF and it is also miscible with water. As little as 0.01% v/v tetrahydrofuran is sufficient. DMSO and ether were the preferred choices for subsequent staining of live cells because THF shows some acute toxicity even when very dilute. The luminescence intensity of the aqueous dispersions is linear in particle concentration and independent of pH over the range 5-9. The sols retain their photoluminescence and are stable against flocculation for at least 6 months.

A kinetic model of the formation of organic monolayers on hydrogen-terminated silicon by hydrosilation of alkenes.

We have analyzed a kinetic model for the formation of organic monolayers based on a previously suggested free radical chain mechanism for the reaction of unsaturated molecules with hydrogen-terminated silicon surfaces (Linford, M. R.; Fenter, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc 1995, 117, 3145). A direct consequence of this mechanism is the nonexponential growth of the monolayer, and this has been observed spectroscopically. In the model, the initiation of silyl radicals on the surface is pseudo first order with rate constant, ki, and the rate of propagation is determined by the concentration of radicals and unreacted Si-H nearest neighbor sites with a rate constant, kp. This propagation step determines the rate at which the monolayer forms by addition of alkene molecules to form a track of molecules that constitute a self-avoiding random walk on the surface. The initiation step describes how frequently new random walks commence. A termination step by which the radicals are destroyed is also included. The solution of the kinetic equations yields the fraction of alkylated surface sites and the mean length of the random walks as a function of time. In mean-field approximation we show that (1) the average length of the random walk is proportional to (kp/ki)1/2, (2) the monolayer surface coverage grows exponentially only after an induction period, (3) the effective first-order rate constant describing the growth of the monolayer and the induction period (kt) is k = (2ki kp)1/2, (4) at long times the effective first-order rate constant drops to ki, and (5) the overall activation energy for the growth kinetics is the mean of the activation energies for the initiation and propagation steps. Monte Carlo simulations of the mechanism produce qualitatively similar kinetic plots, but the mean random walk length (and effective rate constant) is overestimated by the mean field approximation and when kp >> ki, we find k approximately ki0.7kp0.3 and Ea = (0.7Ei+ 0.3Ep). However the most striking prediction of the Monte Carlo simulations is that at long times, t >> 1/k, the effective first-order rate constant decreases to ki even in the absence of a chemical termination step. Experimental kinetic data for the reaction of undec-1-ene with hydrogen-terminated porous silicon under thermal reflux in toluene and ethylbenzene gave a value of k = 0.06 min(-1) and an activation energy of 107 kJ mol(-1). The activation energy is in reasonable agreement with density functional calculations of the transition state energies for the initiation and propagation steps.

The effect of reduced myocardial cyclic AMP content on the response to milrinone in the isolated guinea pig heart.

BACKGROUND: Cardiac beta receptor down-regulation is associated with a reduction of tissue cyclic adenosine monophosphate (AMP) content. Milrinone exerts its effects by inhibiting the metabolism of existing cyclic AMP. The purpose of this study was to evaluate the effect of reduced myocardial cyclic AMP content on the pharmacologic action of milrinone. METHODS: A reduction of myocardial cyclic AMP content was produced by creating catecholamine depletion in the hearts of adult guinea pigs with intraperitoneal reserpine. Control animals received the reserpine vehicle. Isolated heart perfusion was maintained with modified Krebs buffer, and hearts were paced at 270 beats/min. A latex balloon and transducer-tipped catheter were inserted into the left ventricle. Isovolemic work was maintained at a constant balloon volume. Hearts from control and reserpine treated animals were perfused for 20 minutes with buffer containing either no milrinone, 1.7 x 10(-6), or 1.0 x 10(-4) mol/L milrinone (n = 12 for each dose). Maximal positive and negative dP/dt were assessed. The hearts were then frozen and cyclic AMP was measured. RESULTS: Cyclic AMP content was significantly lower in the reserpine-treated hearts at each milrinone concentration (0.33 +/- 0.01 vs 0.46 +/- 0.01; 0.33 +/- 0.01 vs 0.53 +/- 0.01; 0.30 +/- 0.01 vs 0.61 +/- 0.02 pmol/mg wet weight, p < 0.05). Milrinone significantly increased positive and negative dP/dtmax (p < 0.05), but no difference was observed between control and reserpine-treated hearts. CONCLUSIONS: Endogenous catecholamine depletion reduces myocardial cyclic AMP content but does not attenuate the response to milrinone in the isolated heart.

Isolated four-chamber working swine heart model.

BACKGROUND: Isolated heart models separate cardiac characteristics from systemic characteristics with subsequent findings used in cardiac research, including responses to pharmacologic, mechanical, and electrical components. The model objective was to develop the ability to represent in situ physiologic cardiac function ex vivo. METHODS: Swine hearts were chosen over rat or guinea pig models due to their notably greater anatomical and physiologic similarities to humans. An in vitro apparatus was designed to work all four chambers under simulated in situ physiologic conditions. Using standard cardiac surgical techniques, 12 porcine hearts (mean weight 331 +/- 18 g) were explanted into the apparatus. Preload and afterload resistances simulated in situ input and output physiologic conditions. Hemodynamic characterizations, including cardiac output, max +/- dP/dt, and heart rate, were used to determine in situ function leading to explantation (prethoracic operation, postmedial sternotomy, and postperidectomy) and during in vitro function (t = 0, 60, 120, and 240 minutes). RESULTS: In vitro performance decayed with time, with statistical differences from base line (t = 0) function at t = 240 minutes (p > 0.05). CONCLUSIONS: An isolation and in vitro explantation protocol has been improved to aid in the study of isolated cardiac responses, and to determine cardiac hemodynamic function during open chest operation, transplantation, and in vitro reanimation with a crystalloid perfusate. The resulting model offers similar working physiologic function, with real-time imaging capabilities. The resulting model is advantageous in representing human cardiac function with regard to anatomic and physiologic functions, and can account for atrial and ventricular interactions.

Measurement of the rate of uptake and subcellular localization of porphyrins in cells using fluorescence digital imaging microscopy.

A fluorescence imaging system incorporating a cooled slow-scan charge-coupled device camera was used to study the rate of uptake and subcellular localization of prophyrins in living cells. Measurements were carried out on human dermal fibroblasts (D532) using two different prophyrins meso-tetra(4-N-methylpyridyl)porphine (TMPP) and meso-tetra(4-N-hexylpyridyl)porphine (THPP). It was observed that TMPP was rapidly taken up by cells and principally located in the nucleus. The THPP, on the other hand, internalized more slowly and exhibited a particulate distribution in the cytoplasm.

The potential for increased risk of infection due to the reuse of convective air-warming/cooling coverlets.

BACKGROUND: The use of convective air warming and/or cooling for the prevention of hypothermia or to induce hypothermia is growing rapidly. To date, there is no information available as to the potential risks for infection associated with either the post-surgical reuse or the repositioning of coverlets closer to the wound. We hypothesized that use of coverlets either intra- or postoperatively leads to increased contamination. METHODS: The bacterial contamination of commercially available coverlets before (control group, n = 10) and after patient application (n = 18) was investigated. From 3 predetermined sites, 1 cm x 2 cm pieces of coverlet were removed and analyzed for bacterial contamination. RESULTS: Even prior to use, coverlet samplings provided identifiable contamination (3 out of 30 sites, 10%), but this could be within our study's sampling error. Nevertheless, following clinical use the frequency of contamination was considerably increased; 17 out of 57 sampled sites (31.5%) elicited contamination (P < 0.05, Fisher's exact test). CONCLUSION: This study demonstrates that the use of the coverlets, intra- or postoperatively, can lead to significant bacterial contamination. It is concluded that it is not advisable to reuse coverlets for multiple clinical applications.

Voltammetric measurements of the interaction of metal complexes with nucleic acids.

Cyclic voltammetry and differential-pulse voltammetry at mm-sized electrodes were used to measure the decrease in the rate of diffusion of metal complexes upon binding to DNA and to extract the binding constants and effective binding site sizes. A linear correlation was observed between the site size determined electrochemically and the diameter of the complexes [site size: Cu(phen)2(2+) > Fe(phen)3(2+) > Co(bipy)3(3+) approximately Fe(bipy)3(2+) > Ru(NH3)6(3+)]. The binding constants were found to be influenced by the charge of the metal complex, the nature of ligand and the geometry about the metal centre. Competition experiments, in which differential pulse voltammetry was used to observe the release of bound metal complex on addition of a second DNA-binding molecule to the solution, were sensitive to the nature and location of the binding sites for the two species. Steady-state voltammetric experiments at microelectrodes are shown to have a number of advantages over cyclic voltammetry and differential pulse voltammetry at mm-sized electrodes for determination of binding constants. In particular, the steady-state diffusion limited current is directly proportional to the diffusion coefficient, rather than its square root, which improves the discrimination between DNA-bound and freely diffusing metal complex. Further, the kinetics of the binding process do not affect the steady state measurement, whereas for transient techniques, e.g., cyclic voltammetry, only a range of values can be extracted corresponding to the limits of fast and slow binding kinetics compared to the experimental timescale.

An examination of potential matrices for the affinity chromatography of NADP+-linked dehydrogenases with special reference to 6-phosphogluconate dehydrogenase.

1. 6-phosphogluconate dehydrogenase from sheep liver has been purified 350-fold by affinity chromatography with a final specific activity of 18 micronmol of NADP+/reduced min per mg of protein and an overall yield of greater than 40%. 2. A systematic investigation of potential ligands has been carried out: these included 6-phosphogluconate and NADP+, pyridoxal phosphate and several immobilized nucleotides. The results indicate that NADP+ is the most suitable ligand for the purification of 6-phosphogluconate dehydrogenase. 3. The effects of pH and alternative eluents have been examined in relation to the parameters known to affect the desorption phase of affinity chromatography; careful manipulation of the elution conditions permitted the separation of glucose 6-phosphate dehydrogenase, glutathione reductase and 6-phosphogluconate dehydrogenase from sheep liver on NADP+-Sepharose 4B. 4. A large-scale purification scheme for 6-phosphogluconate dehydrogenase is presented that uses the competitive inhibitors inorganic pyrophosphate and citrate as specific eluents.

Monomer, dimer, tetramer, polymer: structural diversity in zinc and cadmium complexes of chelate-tethered nucleobases.

A series of ZnII and CdII complexes of adenine and guanine derivatives containing a diamine tether have been isolated from aqueous solutions and characterised by single crystal X-ray analysis. These studies reveal a wide range of structural types including monomeric, dimeric, tetrameric and polymeric architectures. The extended structures arise from the ability of the ligands to bridge metal ions using the chelating tether in conjunction with N7 of the nucleobase. Additional metal-nucleobase co-ordination is generally observed at the N3-site of the adenine derivatives. With CdII, ethylenediamine-N9-ethylguanine forms an inverted G-tetrad type structure.

Macrochelation, cyclometallation and G-quartet formation: N3- and C8-bound PdII complexes of adenine and guanine.

The reactions of Pd(II) ions with a series of chelate-tethered derivatives of adenine and guanine have been studied and reveal a difference in the reactivity of the purine bases. Reactions of [PdCl2(MeCN)2] and A-alkyl-enH x Cl (alkyl = propyl or ethyl, A adenine, en = ethylenediamine) yield the monocationic species [PdCl(A-N3-Et-en)]+ (1) and [PdCl(A-N3-Pr-en)]+ (2). Both involve co-ordination at the minor groove site N3 of the nucleobase as confirmed by single-crystal X-ray analysis. Reactions with the analogous G-alkyl-enH x Cl derivatives (G=guanine, alkyl = ethyl or propyl) were more complex with a mixture of species being observed. For G-Et-en HCI a product was isolated which was identified as [PdCl(G-C8-Et-en)]+ (3). This compound contains a biomolecular metal-carbon bond involving C8 of the purine base. Crystallography of a product obtained from reaction of G-Pr-enH x Cl and [Pd(MeCN)4][NO3]2 reveals an octacationic tetrameric complex (4), in which each ligand acts to bridge two metal ions through a combination of a tridentate binding mode involving the diamine and N3 and monodentate coordination at N7.

Immobilisation and synthesis of DNA on Si(111), nanocrystalline porous silicon and silicon nanoparticles.

Oligonucleotides have been synthesized on hydrogen-terminated Si(111) and porous silicon using surface hydrosilation of difunctional molecules (1,(omega)-dimethoxytritylundecenol) to produce a monolayer bearing suitable reactive groups to allow automated solid-phase DNA synthesis. The absence of an intervening oxide enables electrochemical characterisation of the surface-bound oligonucleotides. Complementary sequences to the DNA synthesized on Si(111) undergo hybridisation at the surface and a straightforward electrochemical quantitation of the amount of synthesized DNA and its hybridisation efficiency (47%) is possible using Ru(NH3)6(3+) as a redox label. In the case of DNA synthesized in porous silicon, electron transfer (ET) between DNA and the underlying bulk semiconductor can be studied by cyclic voltammetry, however the anisotropic diffusion inside the porous layer and the large resistance of the porous silicon results in voltammograms for which thin-layer behaviour is not observed and the peak currents increase with the square root of scan rate. We interpret these voltammograms in terms of charge transport limitations in the layer of metal centres bound to the DNA inside the pores. Further evidence for this interpretation has been obtained using scanning electrochemical microscopy (SECM) to study the charge transport between redox species in films of DNA synthesized on Si(111) surfaces that are in contact with an aqueous phase. As the bulk concentration of Ru(NH3)6(3+) is reduced below about 250 microM the SECM feedback indicates that the rate of charge transport between surface-bound Ru(NH3)6(3+) exceeds that due to diffusion in the liquid phase. Electrochemical quantitation of the DNA is not possible in this situation, however we have been able to obtain independent determinations using radioassay based on 32P or UV/VIS spectrophotometry of dimethoxytrityl cation cleaved from the porous layer. In the case of the former, use of labelled complementary sequences shows an inverse relationship between the current density used to prepare the porous silicon and the amount of hybridisation. This can be interpreted in terms of the specific surface area of the porous silicon layers since the hybridisation efficiencies (ca. 40%) obtained by comparing DMT+ cleaved from sequences synthesized on the surface and then from complementary sequences after hybridisation were relatively insensitive to the current density used to prepare the layers. Our recent work has also been concerned with individual Si nanocrystals generated by breaking up porous silicon during thermal hydrosilation reactions. FTIR spectroscopy shows these particles are also coated with an organic Si-C-bonded monolayer and they form stable, non-turbid and strongly luminescent (lambdamax = 600-650 nm) dispersions in apolar solvents (L. H. Lie, M. S. Duerdin, E. M. Tuite, A. Houlton and B. R. Horrocks, J. Electroanal. Chem., 2002, 538/539, 183). The effect of carrying out synthetic reactions on the porous silicon prior to breaking up the layer is to produce instead larger, micron-scale assemblies with a nanometre scale internal structure. Micron-sized particles of porous silicon produced by breaking up the layer can be probed by confocal Raman spectroscopy using the electric field of a focused laser to trap such particles. Although these particles are also luminescent, the use of relatively long wavelength laser excitation (lambda = 785 nm) allows acquisition of Raman spectra from individual particles in the optical trap. The bulk optical phonon mode at ca. 520 cm(-1) characteristic of crystalline silicon is red-shifted and broadened providing evidence for an internal nanometre scale substructure in these micron-sized particles and we also see evidence for this mode in the colloidal suspensions of the Si nanoparticles. We propose a model for the formation of these two types of particles and briefly discuss the prospects to extend our solid-phase synthesis on porous silicon to allow the facile synthesis of luminescent Si nanocrystals bearing DNA or other biomolecules.