Augmented nodal matrices and normal trees

Augmented nodal matrices play an important role in the analysis of different features of electrical circuit models. Their study can be addressed in an abstract setting involving two- and three-colour weighted digraphs. By means of a detailed characterization of the structure of proper and normal trees, we provide a unifying framework for the rank analysis of augmented matrices. This covers in particular Maxwell’s tree-based determinantal expansions of (non-augmented) nodal matrices, which can be considered as a one-colour version of our results. Via different colour assignments to circuit devices, we tackle the DC-solvability problem and the index characterization of certain differential-algebraic models which arise in the nodal analysis of electrical circuits, extending several known results of passive circuits to the non-passive context.     

Polarization behavior of polystyrene particles under direct current and low‐frequency (<1 kHz) electric fields in dielectrophoretic systems

The relative polarization behavior of micron and submicron polystyrene particles was investigated under direct current and very low frequency (<1 kHz) alternating current electric fields. Relative polarization of particles with respect to the suspending medium is expressed in terms of the Clausius–Mossotti factor, a parameter of crucial importance in dielectrophoretic‐based operations. Particle relative polarization was studied by employing insulator‐based dielectrophoretic (iDEP) devices. The effects of particle size, medium conductivity, and frequency (10–1000 Hz) of the applied electric potential on particle response were assessed through experiments and mathematical modeling with COMSOL Multiphysics^®. Particles of different sizes (100–1000 nm diameters) were introduced into iDEP devices fabricated from polydimethylsiloxane (PDMS) and their dielectrophoretic responses under direct and alternating current electric fields were recorded and analyzed in the form of images and videos. The results illustrated that particle polarizability and dielectrophoretic response depend greatly on particle size and the frequency of the electric field. Small particles tend to exhibit positive DEP at higher frequencies (200–1000 Hz), while large particles exhibit negative DEP at lower frequencies (20–200 Hz). These differences in relative polarization can be used for the design of iDEP‐based separations and analysis of particle mixtures.     

p-Nitroacetanilide in the presence of triethylamine as a photoinitiator system for vinyl polymerization

The polymerization rate of methyl methacrylate photosensitized by p-nitroacetanilide in the presence of triethylamine was measured as a function of the amine and monomer concentrations. The polymerization proceeds readily in high-polarity media (acetonitrile/monomer) but is negligible in nonpolar media. The polymerization rate increases with the amine concentration up to 0.15M. Further increase in amine concentration does not change the polymerization rate. A similar behavior was observed for the photobleaching of the nitro compound. ESR studies show the formation of a nitro and an amine free radical, the latter free radical being the active species that adds to the monomer. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3095–3100, 1997     

Comment on ‘A technique for image encryption using digital signature’

The security of a recently proposed technique for encryption images by Sinha and Singh [A. Sinha, K. Singh, Opt. Commun. 218 (2003) 229], based on the use of digital signatures and error correcting codes, is analyzed. The proposed cryptosystem is shown to have some weakness. In fact, the secret key and the original image can be recovered efficiently by a brute force attack.     

INTRAMICELLAR QUENCHING OF 2,3-DIMETHYLNAPHTHALENE FLUORESCENCE BY PEROXIDES and HYDROPEROXIDES

Hydrogen peroxide, t -butyl hydroperoxide, di- t -butyl peroxide and dibenzoyl peroxide efficiently quench the fluorescence of 2,3-dimethylnaphthalene incorporated into sodium dodecyl sulfate and cetyltrimethylammonium chloride micelles. Potassium persulfate only is effective when the probe is incorporated into a cationic micelle. The binding constant of the peroxides and the intramiccllar quenching rate constants have been evaluated for all the systems considered and the data are compared to that obtained in homogeneous solution. It is concluded that, even for processes which are not diffusion controlled, the quenching process is slower in the micelles.     

INCLUSION AND FLUORESCENE QUENCHING OF 2, 3-DIMETHYLNAPHTHALENE IN β-CYCLODEXTRIN CAVITIES

Abstract— The fluorescence quenching of 2,3-dimethylnaphthalene (DMN) incorporated to β-cyclodextrin (β-CD) cavities by different olefins (fumaronitrile, acrylonitrile, acrylamide and 2-hydroxyethylmethacrylate) has been measured as a function of the β-CD concentration. The quenching efficiency decreases when the β-CD concentration increases, but extrapolation of the data to infinite cavities concentration does not indicate complete protection. These results are interpreted in terms of two quenching processes, one of them taking place between 2,3-dimethylnaphthalene associated to a β-CD cavity and free quencher, and the other between the DMN and the quencher molecule, both associated with a different cavity. The rate constants of both quenching processes and the β-CD quencher association constant are obtained from the dependence of the quenching efficiency with β-CD concentration.     

Intramolecular interactions in the triplet excited states of benzophenone-thymine dyads

Time-resolved and product studies on the synthesized dyads 1 and 2 have provided evidence that the benzophenone-to-thymine orientation strongly influences intramolecular photophysical and photochemical processes. The prevailing reaction mechanism has been established as a Paterno-Büchi cycloaddition to give oxetanes 3-6; however, the ability of benzophenone to achieve a formal hydrogen abstraction from the methyl group of thymidine has also been evidenced by the formation of photoproducts 7 and 8. These processes have been observed only in the case of the cisoid dyad 1. Adiabatic photochemical cycloreversion of the oxetane ring is achieved upon direct photolysis to give the starting dyad 1 in its excited triplet state. The photobiological implications of the above results are discussed with respect to benzophenone-photosensitized damage of thymidine.     

Joule heating effects in optimized insulator‐based dielectrophoretic devices: An interplay between post geometry and temperature rise

Insulator‐based dielectrophoresis (iDEP) is the electrokinetic migration of polarized particles when subjected to a non‐uniform electric field generated by the inclusion of insulating structures between two remote electrodes. Electrode spacing is considerable in iDEP systems when compared to electrode‐based DEP systems, therefore, iDEP systems require high voltages to achieve efficient particle manipulation. A consequence of this is the temperature increase within the channel due to Joule heating effects, which, in some cases, can be detrimental when manipulating biological samples. This work presents an experimental and modeling study on the increase in temperature inside iDEP devices. For this, we studied seven distinct channel designs that mainly differ from each other in their post array characteristics: post shape, post size and spacing between posts. Experimental results obtained using a custom‐built copper Resistance Temperature Detector, based on resistance changes, show that the influence of the insulators produces a difference in temperature rise of approximately 4°C between the designs studied. Furthermore, a 3D COMSOL model is also introduced to evaluate heat generation and dissipation, which is in good agreement with the experiments. The model allowed relating the difference in average temperature for the geometries under study to the electric resistance posed by the post array in each design.     

The excited-state interaction of resazurin and resorufin with amines in aqueous solutions. Photophysics and photochemical reactions

The photophysics and photochemical behavior of the phenoxazin-3-one dyes, resazurin and resorufin, have been studied in aqueous solutions. The irradiation of resazurin in the presence of amines leads to deoxygenation of the N-oxide group, giving resorufin. This photoreaction is highly dependent on the amine structure and is efficient only in the presence of tertiary aliphatic amines. The absorption and fluorescence properties of these dyes are dependent on pH. At pH above 7.5 both dyes are in their anionic form. For resorufin this form is highly fluorescent (phiF = 0.75). At lower pH the fluorescence is strongly reduced. The N-oxide dye presents a very weak fluorescence quantum yield (0.11), which also is reduced at low pH. Flash photolysis experiments allowed characterization of the triplet state and the transients formed after irradiation of these dyes in the absence and presence of amines. The triplet quantum yields are 0.08 for resazurin and 0.04 for resorufin. The photodeoxygenation of N-oxide in the presence of amines occurs from the triplet state.     

Modification of Guanine with Photolabile N‐Hydroxypyridine‐2(1H)‐thione: Monomer Synthesis,Oligonucleotide Elaboration,and Photochemical Studies

The photochemistry of N‐hydroxypyridine‐2(1H)‐thione (NHPT), inserted as a photolabile modifier at the 6‐position of 2′‐deoxyguanosine or guanosine, has been evaluated. In particular, 6‐[(1‐oxidopyridin‐2‐yl)sulfanyl]‐ ( 1a ) and 6‐[(pyridin‐2‐yl)sulfanyl]‐2′,6‐dideoxyguanosine ( 2a ), novel photolabile derivatives of the natural nucleosides, were synthesized and characterized. The observed photolysis products of 1a in organic solvents could only be rationalized by assuming a rapid equilibrium with the corresponding 6‐[(2‐thioxopyridin‐1(2H)‐yl)oxy] analogue 3a (Scheme 5). Transient spectroscopy of 1a indicated a strong triplet‐excited state suitable for triplet → triplet energy transfer or singlet‐oxygen generation. The NHPT function was stable enough for (slightly modified) automated solid‐phase oligonucleotide synthesis. The utility of the above compounds is discussed, as well as their potential use in photosensitization of reactive oxygen species in DNA.