CiteULike: kopelman's library [228 articles]

Photodynamic nanoplatforms offer alternative to standard cancer treatments

R Kopelman — 2007-10-09T17:18:30-00:00

SPIE Newsroom (2007)

New photodynamic cancer therapies combine the tumor detection and imaging capabilities of nanoparticles with the increased penetration depth of infrared two-photon excitation.

Space-and time-resolved diffusion-limited binary reaction kinetics in capillaries: experimental observation of segregation, anomalous exponents, and depletion zone

Yong Koo — 2007-08-16T19:27:43-00:00

Journal of Stat. Phys., Vol. 65 (1991), pp. 893-918.

An experimental investigation of one-dimensional, diffusion-limited A+B→C chemical reactions is reported. The persistence of reactant segregation and the formation of a depletion zone is observed and expressed in terms of the universal time exponents: α (motion of the boundary zone), β (width of instantaneous product formation zone), γ (rate of instantaneous local formation of product), δ (rate of instantaneous global formation of product), etc. There is good agreement with the recently predicted and/or simulated values: α =1/2, β =1/6, γ =2/3, δ =1/2, in contrast to classical predictions ( α =0, β =1/2, γ =0, δ =−1/2). Furthermore, classically the segregation would not be preserved and there would be no formation of a depletion zone and no motion (just dissipation) of the reaction zone. We also discuss the relations to electrode oxidation-reduction reactions, i.e., A+C→C where C is a catalyst, electrode, or “trap.”

Scaling properties of diffusion-limited reactions on fractal and euclidean geometries

Raoul Kopelman — 2007-08-16T19:24:55-00:00

Journal of Stat. Phys., Vol. 65 (1991), pp. 1969-1283.

We review our scaling results for the diffusion-limited reactions A + A → 0 and A+B→0 on Euclidean and fractal geometries. These scaling results embody the anomalies that are observed in these reactions in low dimensions; we collect these observations under a single phenomenological umbrella. Although we are not able to fix all the exponents in our scaling expressions from first principles, we establish bounds that bracket the observed numerical results.

Diffusive motion in a fractal medium in the presence of a trap

Shlomo Havlin — 2007-08-16T19:18:58-00:00

Physical Review A, Vol. 43, No. 10. (15 May 1991), 5228.

There have been many studies of transport in an unbounded fractal medium. Here we discuss a number of quantities related to the concentration of reactants diffusing in fractal media in the presence of a trapping point. This investigation is suggested by an extension of the Smoluchowski model to calculate reaction rates in such media [M. von Smoluchowski; Phys. Z. 16 ; 321 (1915); 17 ; 557 (1917); 17 ; 585 (1917)]. Results; some analytic and some based on a scaling argument; are given for the flux into the trap; which is the analog of the reaction rate; in addition to the concentration profile in the neighborhood of the trap and the time dependence of the distance between the trap and the nearest untrapped particle. The results of our theory are found to be in good agreement with Monte Carlo and exact enumeration calculations for the concentration profile on a Sierpinski gasket and on an infinite percolation cluster at criticality. Some scaling and numerical results are reported for the situation in which the traps move in the presence of fixed particles.

The diffusion-limited reaction A+B→0 on a fractal substrate

LM Sander — 2007-08-16T19:17:42-00:00

Journal of Stat. Phys., Vol. 65 (December 1991), pp. 919-924.

We show in this paper how the segregation of reactants in the diffusion-limited reaction A+B→0 on a fractal substrate arises. For spectral dimensions d s ⩽ 2 we obtain segregation controlled by the source and/or the intrinsic lifetime of the particles.

Visualizing chemical kinetics in fractal domains

Neal Hurlburt — 2007-08-16T18:46:40-00:00

(1991), pp. 364-367.

Scaling properties of diffusion-limited reactions: Fractals

Katja Lindenberg — 2007-08-16T18:42:44-00:00

Physical Review A, Vol. 43, No. 12. (15 June 1991), 7070.

We have generalized our previous scaling arguments [Sheu; Lindenberg; and Kopelman; Phys. Rev. A 42 ; 2279 (1990)] for diffusion-limited A + B →0 reactions to encompass various possible connectivity properties and reaction conditions on fractal structures. The theory now allows for a more complete range of possible reaction surface configurations. While our original result (which is a special case) places a bound that is consistent with very recent simulations on critical percolation clusters; the generalization is needed to account for the behavior on finitely ramified structures such as Sierpinski gaskets and Peano curve fractal constructions. Our results yield upper and lower bounds for the oscillations of the reactant decay exponent which are typical for hierarchical structures. Our approach unites under a single framework situations such as the A + B →0 reaction and the A + A →0 reaction that were previously treated separately.

Exotic behavior of the reaction front in the A+B-->C reaction-diffusion system

Haim Taitelbaum — 2007-08-16T18:37:34-00:00

Physical Review A, Vol. 46, No. 4. (1992), 2151.

A dynamic reaction zone is produced in the A + B → C reaction-diffusion system with initially separated components. Our perturbation analysis results predict a rich behavior for the kinetics of the reaction front in the short-time limit; with variety of universality classes. In particular; we show that the center of the front can change its direction of motion. Our experimental data support this prediction and demonstrate that this behavior is measurable over a time scale of hours in bimolecular reactions at room temperature.

Influence of External Steady Source Structure on Particle Distribution and Kinetics of Diffusion-Limited Reaction. I: A + A --> 0 Simulations

L Li — 2007-08-16T18:08:39-00:00

Journal of Physical Chemistry, Vol. 96 (1992), pp. 8079-8084.

Steady-state diffusion-controlled A+A-->0 reaction in Euclidean and fractal dimensions: Rate laws and particle self-ordering

Eric Clément — 2007-08-16T18:04:22-00:00

Physical Review A, Vol. 39, No. 12. (15 June 1989), 6472.

For diffusion-limited reactions of the type A+A→0; we present a theory for the pair correlation function and the macroscopic reaction law for the Euclidean dimensions d =1; 2; 3 and for self-similar fractal structures with spectral dimension such that 1≤ d s ≤2. In any dimension we define a length Λ ; which is the typical size of the depletion zone around each particle. Λ is expressed as a function of a fundamental size ξ= √ρD/R ; where ρ is the steady-state density; R the external source rate; and D the microscopic diffusion constant. For d s ≤2; we obtain the ‘‘fractal’’ reaction order X=1+2/ d s and a mesoscopic; density-dependent depletion zone around each particle. We give an interpretation of the particle self-ordering and anomalous rate laws in terms of the random-walk topological properties.

Submicrometer intracellular chemical optical fiber sensors

W Tan — 2007-08-16T17:55:24-00:00

Science, Vol. 258, No. 5083. (30 October 1992), pp. 778-781.

A thousandfold miniaturization of immobilized optical fiber sensors, a millionfold or more sample reduction, and at least a hundredfold shorter response time, all simultaneously, were achieved by combining nanofabricated optical fiber tips with near-field photopolymerization. Specifically, pH optical fiber sensors were prepared with internal calibration, making use of the differences in both fluorescence and absorption of the acidic and basic dye species. The submicrometer sensors have excellent detection limits, as well as photostability, reversibility, and millisecond response times. Successful applications include intracellular and intraembryonic measurements. Potential applications include spatially and temporally resolved chemical analysis and kinetics inside single biological cells and their substructures. 10.1126/science.1439785

Influence of external steady source structure on particle distributions and kinetics of diffusion-limited reactions. II. A + B --> 0 simulations

L Li — 2007-08-15T20:04:16-00:00

Chemical Physics, Vol. 174, No. 3. (15 August 1993), pp. 367-375.

Monte Carlo simulations were performed to study the effect of a steady external source structure (i.e., particle correlation and vertical reactions) on diffusion-limited A + B --> 0 reactions at steady state. Several methods were developed to describe the spatial organization of the system. They are the distributions of aggregates and inter-particle distance ("gap"), and a parameter based on the number of boundaries between A-rich and B-rich domains. The correlation of the particles in the steady external source reduces the local fluctuation in the particle landing process. The vertical reaction restrains the organization of particles, and does not allow it to reach total segregation. The degree of segregation affects the steady state kinetic behavior. The simulation results are consistent with existing theoretical predictions for the reaction order, the correlation length, and the segregation size.

Nonclassical A+B-->0 batch reactions: Effect of mobility on rate, order, aggregation and segregation

Panos Argyrakis — 2007-08-15T20:02:03-00:00

Physical Review E, Vol. 47, No. 5. (May 1993), 3757.

Monte Carlo simulations are performed for the elementary batch reaction A + B →0 on a one-dimensional lattice. The initial densities of the A and B species are always identical but the relative mobility is varied. We investigate the rates; rate laws; and particle distribution functions. The rate power law is conserved; i.e.; the density always decays in time algebraically with exponent 1/4. The rate coefficient is proportional to the relative mobility; as expected. The interparticle distribution functions (‘‘gaps’’ and nearest-neighbor distances) show that the aggregation does depend on the relative mobility but the segregation does not. However; this subtle difference has no effect on the asymptotic reaction order; which is close to 5.

Tip/sample interactions, contrast, and near-field microscopy of biological and solid-state samples

Steven Smith — 2007-08-15T19:58:27-00:00

Scanning Probe Microscopies II, Vol. 1855, No. 1. (1993), pp. 81-92.

Biological samples, molecular solids and solid state devices have been investigated by Near- Field Scanning Optical Microscopy (NSOM), Near-Field Optical Spectroscopy, and Near- Field Chemical Sensing. We report here on our progress in applying the NSOM technology to various biological and physical systems. Results demonstrating both spatial and spectral resolution as well as image contrast unique to the near-field technique are presented.

The diffusion-limited reaction A + A -->0 in the steady state: influence of correlations in the source

E Clement — 2007-08-15T19:53:55-00:00

Chemical Physics, Vol. 180, No. 2-3. (1 March 1994), pp. 337-341.

We study theoretically the kinetics of the diffusion-limited reaction + -->0 in the steady state. We consider the effect of an external source which adds correlated pairs of particles. We show that in dimensions below 2 and in the low-density limit spatial self-organization of reactants occurs and is controlled by the correlation in the source term. At higher densities there is a crossover to a regime similar to the random landing case, exhibiting a non-classical order of reaction.

100 femtosecond/100 nanometer near-field probe

S Smith — 2007-08-15T19:51:00-00:00

Ultramicroscopy, Vol. 57, No. 2-3. (February 1995), pp. 173-175.

Cross-correlation by upconversion has been used to measure pulse widths below 100 fs emitted from an aluminum-coated fiber near-field tip of the variety reported by Betzig et al. [Science 251 (1991) 1468]. The pulses are 0.8 [mu]m center-wavelength with 76 MHz repetition rate generated by a Ti: sapphire mode-locked laser. The size of the probe is estimated at 100 nm by SEM micrographs and previous experience with similar probes used in our continuous wave NSOM systems. We measure average tip output powers of ~ 1 nW for average input powers of ~ 500 [mu]W.

Implementation of an NSOM system for fluorescence microscopy

E Monson — 2007-08-15T19:48:29-00:00

Ultramicroscopy, Vol. 57, No. 2-3. (February 1995), pp. 257-262.

We describe our progress toward an NSOM system intended for fluorescence imaging of biological samples. This process included integration of shear-force feedback into an existing NSOM system. Topographic images acquired using uncoated tips are presented. We also present our initial effort at simulataneous acquisition of topographic and fluorescence data using an aluminum coated tip.

Is smaller better?--Scaling of characteristics with size of fiber-optic chemical and biochemical sensors

Raoul Kopelman — 2007-08-15T19:22:34-00:00

Chemical, Biochemical, and Environmental Fiber Sensors VIII, Vol. 2836, No. 1. (1996), pp. 2-11.

Micron and submicron fiber-optic chemical and biochemical sensors have been developed recently in our laboratory. Here we give a theoretical discussion, based on pragmatic modes of operation, of the advantages and disadvantages of optode miniaturization. In most instances we present explicit functional dependences of specific optode characteristics on the optode radius (r). For instance, the absolute detection limit decreases with r3 (good the response time reduces with r2 (good noise ratio decreases with r (bad our standard working conditions. Other features that improve with down-sizing include sample volume, sensitivity, invasiveness, spatial resolution, enzyme activity, heating of sensor and/or sample, toxicity and materials cost. Features that may worsen include fluorophore leaching and photodamage to sensor and/or sample. Methods for overcoming these disadvantages make use of the shorter response times of small optodes and forward optical signal collection with small samples (utilizing standard lab microscopes). We refer to the ultimate goal of non-destructive single molecule detection and imaging.

Fluorescent Fiber-Optic Calcium Sensor for Physiological Measurements

M Shortreed — 2007-08-15T19:14:11-00:00

Anal. Chem., Vol. 68, No. 8. (15 April 1996), pp. 1414-1418.

Abstract: A new optical sensor based on covalent immobilization of a newly synthesized calcium-selective, long-wavelength, fluorescent indicator has been constructed, with a response dynamic range optimal for physiological measurements. Immobilization occurs via photoinitiated copolymerization of the indicator with acrylamide on the distal end of a silanized 125 m diameter multimode optical fiber. The working lifetime of this sensor is limited only by photobleaching of the indicator. Due to the inherent hydrophilic nature of the acrylamide polymer, the response time of this new sensor is governed by simple aqueous diffusion of the ionic calcium. This results in sensor response times fast enough to monitor some concentration fluctuations at physiological rates. The ability to monitor calcium concentration fluctuations in a high background level of magnesium is also demonstrated with a calculated selectivity of 10-4.5.

Early-time scaling for the reaction front of a ternary, A+2B[over -->]C, reaction-diffusion system

Andrew Yen — 2007-08-15T18:39:24-00:00

Physical Review E, Vol. 57, No. 2. (February 1998), 2438.

We study the ternary A +2 B → C reaction-diffusion process with initially separated reactants. A perturbation approach gives a scaling of the global reaction rate with t 1/2 at early time; followed by a crossover to t -1/2 in the asymptotic time regime; a behavior which is the same as for the binary A + B → C reaction under initial segregation. Monte Carlo simulations are in good agreement with the predicted scaling.

Gel-free experiments of reaction-diffusion front kinetics

Sung Park — 2007-08-15T17:26:22-00:00

Physical Review E, Vol. 64, No. 5. (30 October 2001), 055102.

We present a gel-free experimental system to study the kinetics of the reaction front in the A + B → C reaction-diffusion system with initially-separated reactants. The experimental setup consists of a CCD camera monitoring the kinetics of the front formed in the reaction-diffusion process Cu 2+ + tetra [disodium ethyl bis(5-tetrazolylazo) acetate trihydrate] →1:1 complex; in aqueous; gel-free solution; taking place inside a 150 μm gap between two flat microscope slides. The experimental results agree with the theoretical predictions for the time dependence of the front’s width; height; and location; as well as the global reaction rate.

COOKING WITH NANOPARTICLES: A SIMPLE METHOD OF FORMING ROLL, PANCAKE, AND BREADED POLYSTYRENE MICROPARTICLES

JN Anker — 2007-07-23T16:08:07-00:00

European Cells and Materials, Vol. 3, No. 2. (2002), pp. 95-97.

Photodynamic Characterization and In Vitro Application of Methylene Blue-containing Nanoparticle Platforms

Wei Tang — 2007-08-14T19:36:39-00:00

Biomedical Photonics Handbook (2003), pp. 1-14.

This article presents the development and characterization of nanoparticles loaded with methylene blue (MB), which are designed to be administered to tumor cells externally and deliver singlet oxygen (1O2) for photodynamic therapy (PDT), i.e. cell kill via oxidative stress to the membrane. We demonstrated the encapsulation of MB, a photosensitizer (PS), in three types of sub-200 nm nanoparticles, composed of polyacrylamide, sol–gel silica and organically modified silicate (ORMOSIL), respectively. Induced by light irradiation, the entrapped MB generated 1O2, and the produced 1O2 was measured quantitatively with anthracene-9,10-dipropionic acid, disodium salt, to compare the effects of different matrices on 1O2 delivery. Among these three different kinds of nanoparticles, the polyacrylamide nanoparticles showed the most efficient delivery of 1O2, but its loading of MB was low. In contrast, the sol–gel nanoparticles had the best MB loading but the least efficient 1O2 delivery. In addition to investigating the matrix effects, a preliminary in vitro PDT study using the MB-loaded polyacrylamide nanoparticles was conducted on rat C6 glioma tumor cells with positive photodynamic results. The encapsulation of MB in nanoparticles should diminish the interaction of this PS with the biological milieu, thus facilitating its systemic administration. Furthermore, the concept of the drug-delivering nanoparticles has been extended to a new type of dynamic nanoplatform (DNP) that only delivers 1O2. This DNP could also be used as a targeted multifunctional platform for combined diagnostics and therapy of cancer.

Anomalous growth of the depletion zone in the photobleaching trapping reaction

SH Park — 2007-08-14T19:20:19-00:00

Phys. Rev. E, Vol. 67, No. 6. (June 2003), 060103.

We study the anomalous growth of the depletion zone at a single trap, as observed in a photobleaching trapping reaction in confined geometry. We provide experimental evidence for a nonuniversal growth of this depletion. We also find an early-time behavior of the depletion zone, owing to the finite size of the trap. Various laser powers are used in order to study the effects of trapping strength, interpreted theoretically in terms of an imperfect trap. The results are supported by numerical calculations. Comparison with other trapping reactions provides insight into finite-size traps.

Poly(decyl methacrylate)-based fluorescent PEBBLE swarm nanosensors for measuring dissolved oxygen in biosamples.

Y Cao — 2007-08-14T19:10:40-00:00

Analyst, Vol. 129, No. 8. (August 2004), pp. 745-750.

150-250 nm Poly(decyl methacrylate)(PDMA) fluorescent ratiometric nanosensors for dissolved oxygen have been developed. Platinum octaethylporphine ketone (PtOEPK), the oxygen-sensitive dye, and octaethylporphyrin (OEP), the oxygen-insensitive dye, have been incorporated into PDMA nanoparticles to make the sensors ratiometric. Based on the corresponding Stern-Volmer plot, these nanosensors exhibit almost complete linearity over the whole range of dissolved molecular oxygen from 0 to 42.5 ppm (deoxygenated to pure oxygen-bubbled water). The overall quenching response is up to 97.5%, the best so far for all dissolved oxygen optical sensors. These PEBBLE nanosensors also show very good reversibility and stability to leaching and photobleaching, as well as very short response times and no perturbation by proteins. In human plasma they demonstrate a robust oxygen sensing capability, little affected by light scattering and autofluorescence. Potential applications include intracellular oxygen imaging and microresolved pressure profiles in biological and other heterogenous environments.

Fabrication of uniform half-shell magnetic nanoparticles and microspheres with applications as magnetically modulated optical nanoprobes

BH Mcnaughton — 2007-08-14T18:36:22-00:00

ArXiv Condensed Matter e-prints (June 2005)

Magnetically Modulated Optical Nanoprobes (MagMOONs) are magnetic particles that indicate their angular orientation by emitting varying intensities of light for different orientations and have shown promise for a variety of applications. In this letter we describe a new method to fabricate uniform magnetic half-shell particles that can be used as MagMOONs. Cobalt was deposited onto commercially made polystyrene nanospheres and microspheres, using ultrahigh vacuum vapor deposition, producing particles with uniform size, shape and magnetic content. Additionally, the coercivity of the cobalt deposited on the nanospheres was enhanced compared to its bulk value.

Magnetically-modulated optical nanoprobes (MagMOONs) and systems

Jeffrey Anker — 2007-08-14T18:29:53-00:00

Journal of Magnetism and Magnetic Materials, Vol. 293, No. 1. (May 2005), pp. 655-662.

Magnetically-modulated optical nanoprobes (MagMOONs) are micro and nano particles that emit different fluxes of light in different orientations, and that rotate in changing magnetic fields. The particles blink as they rotate, allowing in situ background subtraction and chemical imaging with individual particles. The probes also function as torque sensors to measure the drag on the particle and local rheological properties.

Cu+- and Cu2+-sensitive PEBBLE fluorescent nanosensors using DsRed as the recognition element

James Sumner — 2007-08-14T18:26:14-00:00

Sensors and Actuators B: Chemical, Vol. 113, No. 2. (27 February 2006), pp. 760-767.

The red fluorescent protein (DsRed) has typically been used as either a fluorescent tag or FRET acceptor, but here, we detail its use as a recognition element for Cu2+ and Cu+ in a fluorescent photonic explorer for bioanalysis with biologically localized embedding (PEBBLE) nanosensor. DsRed and the reference dye Alexa Fluor 488 have been encapsulated within a polyacrylamide matrix by a microemulsion polymerization process, to produce spherical, ratiometric, 80 nm-sized sensors, which allow for spatially resolved real-time measurements. These nanobiosensors have excellent selectivity and sensitivity towards copper ions, with a detectable range in the nanomolar (ppb) regime, even in the presence of other divalent and heavy metal ions. The nanosensors are both photostable and reversible, which allows for continuous monitoring.

DsRed as a highly sensitive, selective, and reversible fluorescence-based biosensor for both Cu(+) and Cu(2+) ions.

JP Sumner — 2007-08-14T18:21:46-00:00

Biosens Bioelectron, Vol. 21, No. 7. (15 January 2006), pp. 1302-1308.

The wild type form of Red fluorescent protein (DsRed), an intrinsically fluorescent protein found in tropical corals, is found to be highly selective, reversible and sensitive for both Cu(+) and Cu(2+), with a nanomolar detection limit. The selectivity towards these ions is retained even in the presence of other heavy metal ions. The K(d) values for monovalent and divalent copper, based on single binding isotherms, are 450 and 540 nM, respectively. The wild type DsRed sensitivity to Cu(2+) (below 1 ppb) is seven orders of magnitude better than that of the related wild type Green Fluorescent protein (GFP), and it is even 40 times more sensitive than engineered mutants of GFP. Potential binding sites have been proposed, based on amino acid sequences for copper binding and the distance from the chromophore, with the aid of computer modeling.

Optical Nanoprobes for Biosensing and Therapy

Martin Philbert — 2007-08-14T18:00:46-00:00

Lasers & Electro-Optics Society, IEEE (2006), pp. 380-381.

The sol-gel based oxygen nanoprobe that provides for quantitative intracellular measurement of dissolved oxygen in living cells provides a compelling example of the use of these biologically-compatible nanoplatforms for noninvasive monitoring function in living cells. Photonic Explorers for Bioanalysis with Biologically Localized Embedding (PEBBLEs) are flexible polymer nanoplatforms that encapsulate sensing elements in a biocompatible format for quantitative measurement of intracellular ions or small molecules. These biosensors capitalize on micelle or condensation/hydrolysis techniques that allow for nanoscale fabrication of polyvinyl chloride, polydecyl methacrylate, polyacrylamide hydrogels, sol-gels or organically-modified silicates with controlled diameters in the range 20-250 nm

Photonic explorers based on multifunctional nanoplatforms for biosensing and photodynamic therapy

Yong-Eun Koo — 2007-08-14T17:48:17-00:00

Applied Optics, Vol. 46, No. 10. (13 March 2007), pp. 1924-1930.

Nanoparticle-based photonic explorers have been developed for intracellular sensing and photodynamic therapy (PDT). The design employs nanoparticles made of various matrices as multifunctional nanoplatforms, loading active components by encapsulation or covalent attachment. The nanoplatform for biosensing has been successfully applied to intracellular measurements of important ionic and molecular species. The nanoplatform for PDT has shown high therapeutic efficacy in a rat 9L gliosarcoma model. Specifically, a multifunctional nanoplatform that encompasses magnetic resonance imaging (MRI) and PDT agents inside, as well as targeting ligands on the surface, has been developed and applied in vivo, resulting in much improved MRI contrast enhancement and PDT efficacy.

"Nanosized Voltmeter" Enables Cellular-Wide Electric Field Mapping

Katherine Tyner — 2007-08-14T17:43:43-00:00

Biophys. J., Vol. 93, No. 4. (15 August 2007), pp. 1163-1174.

Previously, all biological measurements of intracellular electric fields (E fields), using voltage dyes or patch/voltage clamps, were confined to cellular membranes, which account for <0.1% of the total cellular volume. These membrane-dependent techniques also frequently require lengthy calibration steps for each cell or cell type measured. A new 30-nm "photonic voltmeter", 1000-fold smaller than existing voltmeters, enables, to our knowledge, the first complete three-dimensional E field profiling throughout the entire volume of living cells. These nanodevices are calibrated externally and then applied for E field determinations inside any live cell or cellular compartment, with no further calibration steps. The results indicate that the E fields from the mitochondrial membranes penetrate much deeper into the cytosol than previously estimated, indicating that, electrically, the cytoplasm cannot be described as a simple homogeneous solution, as often approximated, but should rather be thought of as a complex, heterogeneous hydrogel, with distinct microdomains. 10.1529/biophysj.106.092452

Variable range cluster model of exciton migration: Dimensionality and critical exponents for naphthalene

R Kopelman — 2007-08-14T17:38:15-00:00

Journal of Luminescence, Vol. 18-19, No. Part 1. (January 1979), pp. 41-46.

Relative luminescence intensities for randomly substituted ternary systems with two major components and a minor one (sensor), for four triplet and one singlet exciton systems, identify the maximal effective exciton interaction distance for each system. The critical exponents [beta] and [gamma] show an effective 2-dimensional exciton topology and are consistent with dynamic exciton percolation.

Exciton percolation III. Stochastic and coherent migration in binary and ternary random lattices

P Argyrakis — 2007-08-14T17:33:23-00:00

Journal of Theoretical Biology, Vol. 73, No. 2. (20 July 1978), pp. 205-236.

We develop a method for calculating energy migration in random heterogeneous aggregates, with potential application to the primary process in photosynthetic units. A Monte Carlo technique is employed to study several types of random walk motion in a random binary lattice. Our computations include 2 and 3 dimensional lattices of different topology and employ correlated steps with a Gaussian distribution of directional memory. The effects of the characteristics of the motion and its parameters are displayed and discussed. The lower threshold for efficient visitation by the walker is given by the critical percolation concentration. However, a higher threshold is found in the case of coherent motion. This new "turning point" appears to play an important role in the process of exciton transport. The exciton percolation formalism is utilized, giving results for ternary random lattices where the third component is very dilute and acts as a sensor. The results are applied to a system representing the 11B2u, naphthalene exciton dynamics in an isotopic and chemically mixed crystal, which by itself is supposed to mimic the exciton transport in the photosynthetic units of green plants. Physically reasonable parameters, trends and limits are discussed. Also, an analytical solution is derived and tested for a physically reasonable limit of semicoherent motion in a perfect lattice. The ramification of this work on bioexciton transfer is discussed, especially concerning the light harvesting units in green plants. It leads to a simple minded model that rationalized the ratio of antenna to active-center molecules. Our most important result is that incoherent exciton transfer, i.e. simple random walk, is the most efficient process for significantly heterogeneous aggregates.

Energy trapping and funnels in mixed naphthalene crystals

P Argyrakis — 2007-08-14T17:24:52-00:00

Journal of Physical Chemistry, Vol. 87, No. 9. (1983), pp. 1467-1469.

From singlet energy transport studies in several mixed molecular crystals, at liquid helium temperatures, we deduce relative trapping efficiencies for several ?-methylnaphthalenes and anthracene in naphthalene, and compare these to the efficiency of naphthalene in perdeuterionaphthalene (C10H8/C10D8). We find that all these species are more efficient than naphthalene, but only by a factor of 2-8, with small energy funnels (order of 4 molecules) being generated around these trap molecules. We also discuss the possible contributions to trapping from phonon (and localized phonon) effects. © 1983 American Chemical Society.

Fractal-like exciton transport and fusion in disordered naphthalene

Laurel Harmon — 2007-08-14T17:23:39-00:00

Journal of Luminescence, Vol. 31-32, No. Part 2. (December 1984), pp. 660-662.

Steady-state fluorescence and phosphorescence spectra od semi-crystalline naphthalene are presented. Singlet and triplet exciton transport are strongly dependent on temperature and degree of annealing. Analysis of delayed fluorescence decays reveals fractal-like transport in the more disordered samples.

Luminescent fractal reactions: Exciton fusion on percolation clusters

Lola Anacker — 2007-08-14T17:22:06-00:00

Journal of Luminescence, Vol. 31-32, No. Part 2. (December 1984), pp. 648-650.

Experimental measurements of exciton annihilation in isotopic mixed crystals of naphthalene-h8 in naphthalene-d8 exhibit fractal reaction kinetics on samples well below and slightly above the percolation threshold. The effective spectral dimensions are in very good agreement with the superuniversality conjecture and 2-dim. random walk simulations. Crystals well above the percolation threshold exhibit the expected classical behavior.

Fractal exciton fusion: Simulations on long-range percolation clusters

James Newhouse — 2007-08-14T17:20:12-00:00

Journal of Luminescence, Vol. 31-32, No. Part 2. (December 1984), pp. 657-659.

Computer simulations of exciton fusion on percolating clusters were performed for interaction distances N=2, 3, and 4. The data yield slopes less than those expected from superuniversality (f=2/3) at interaction distances above 2, when the probability of sucessively longer hops falls by a factor of 1/10.

Fractal-like energy transport and fusion in a naphthalene-doped polymeric glass

Irene Newhouse — 2007-08-13T20:13:50-00:00

Journal of Luminescence, Vol. 31-32, No. Part 2. (December 1984), pp. 651-653.

Singlet energy transport in naphthalene-doped poly (methylmethacrylate) (PMMA) samples has been found, by doping with anthracene, to follow Stern-Volmer kinetics with a transfer rate constant ca. 2x109. Delayed fluorescence is easily observed; triplet energy transport seems to have fractal-like properties.

Dynamics of anisotropic exciton hopping in molecular crystals

P Argyrakis — 2007-08-13T20:12:27-00:00

Journal of Physical Chemistry, Vol. 88, No. 10. (1984), pp. 1973-1976.

We investigate excitonic energy transfer in molecular crystals including the effect of anisotropic interactions and we apply our approach to the first singlet excited state of naphthalene. This is done through a random-walk formalism similar to our previous work, but incorporating here the weaker out-of-plane interactions explicitly. The problem is also treated via numerical simulations, which turn out to be consistent with the analytical formalism. Comparing the results with the experimental time decay curves, we find good agreement. The derived stepping time is of the order of 1 ps. © 1984 American Chemical Society.

Fractal energy transport: Random walk simulations on long-range percolation clusters

Panos Argyrakis — 2007-08-13T20:11:37-00:00

Journal of Luminescence, Vol. 31-32, No. Part 2. (December 1984), pp. 654-656.

Luminescene from naphthalene alloys is quenched by long-range exciton hops. These are modeled by long-range random walks on long-range percolation clusters with a range-dependent hopping time. Both linear and exponential range dependencies are simulated, over to neighbor hops. At critical percolation thresholds the random walk properties obey the super-universality hypothesis (spectral or fraction dimension of about 4/3). However this asymptonic limit is approached at different rates for different functional forms of the hopping time (constant, r, er, 10r).

One-dimensional luminescence kinetics; a submicron probe

Jagdish Prasad — 2007-08-13T20:09:20-00:00

Journal of Luminescence, Vol. 40-41 (February 1988), pp. 643-644.

Cylindrical naphthalene wires (10-1200 nanometer diameter) exhibit one-dimensional exciton annihilation kinetics for 10-30 nm wires, three-dimensional behavior for 100-1200 nm wires and a crossover diameter increasing from 4 to 77 K. Nuclear channel pore membranes (polycarbonate) serve as one-dimensional templates. Vycor glass pores are found to be effectively one-dimensional. The triplet exciton migration (multiple hopping) length is 25-50 nm. The recombination involves free and bound excitons.

Simulations of one-dimensional and fractal luminescence kinetics

Li Li — 2007-08-13T20:08:11-00:00

Journal of Luminescence, Vol. 40-41 (February 1988), pp. 688-689.

Exciton and electron-hole recombination reactions were simulated on one-dimensional and fractal networks. Particle distributions depend on dimensionality and electric fields. Geminate vs. non-geminate and pulsed vs. steady-state generation give very different reaction orders and population distributions.

Recombination kinetics in low dimensions

Panos Argyrakis — 2007-08-13T20:06:52-00:00

Journal of Luminescence, Vol. 40-41 (February 1988), pp. 690-691.

Computer simulations predict very anomalous delayed luminescence delayed luminescence decays for low-dimensional materials. Classical kinetics theory fails because excitons, electrons and holes have no convection. The mode of stirring plays a very important role in the rate of reactions.

LOW-DIMENSIONAL EXCITON REACTIONS.

Raoul Kopelman — 2007-08-13T20:05:36-00:00

Philos Mag B, Vol. 56, No. 6., pp. 717-723.

A formalism for low-dimensional reaction kinetics is presented. Triplet-exciton annihilation experiments demonstrate the utility of this approach for low-dimensional and ultra-small samples: (1) naphthalene wires ranging down to a radius of a few nanometers; (2) percolation clusters at criticality (and above) where a site is a single naphthalene molecule but the system is a single crystal (ideal isotopic alloy). Exciton hopping and fusion provide a new, sensitive tool for the topological mapping of disordered films, surface aggregates, porous materials and fine powders. It also suggests a new approach to soliton and electron-hole recombination kinetics and to diffusion-limits reactions on surfaces, in pores and in other locally heterogeneous media.

Self-stirred vs. well-stirred reaction kinetics

P Argyrakis — 2007-08-13T20:03:46-00:00

Journal of Physical Chemistry, Vol. 91, No. 11. (1987), pp. 2699-2701.

For low-dimensional reactions (on surfaces or in pores) and solid-state reactions the stirring mechanism is usually limited to self-stirring, i.e., self-diffusion. Simulations of A + A ? A2? reactions show classical behavior for well-stirred reactions but drastic nonclassical behavior for self-stirred reactions in low-dimensional systems: (1) one-dimensional system, (2) square lattice, (3) two-dimensional percolating cluster, (4) three-dimensional percolating cluster. In contrast, a three-dimensional isotropic system (cubic lattice) shows a near-classical behavior even for self-stirred reactions. Different universalities in kinetic behavior are shown for the well-stirred and for the self-stirred reactions. © 1987 American Chemical Society.

Fractal-like molecular reaction kinetics: Solute photochemistry in porous membranes

J Prasad — 2007-08-13T20:02:22-00:00

Journal of Physical Chemistry, Vol. 91, No. 2. (1987), pp. 265-266.

A new, fractal-like, reaction kinetics is documented for the diffusion-controlled photodimerization of naphthalene molecules in a liquid solvent that is trapped inside porous polymeric membranes. The anomalous molecular diffusion of the solute molecules leads to anomalous reaction kinetics for the elementary, binary reaction N + N ? N2. The nonclassical kinetics can be expressed via a long-time-limit time-dependent rate coefficient k = k0t-h where h ? 1/3. The effective (spectral) dimension for the low-dimensional reaction environment is about 4/3. © 1987 American Chemical Society.

Phonon sidebands of ordered and disordered media: Naphthalene crystals and molecularly doped polymers

YE Koo — 2007-08-13T20:00:33-00:00

Journal of Physical Chemistry, Vol. 93, No. 5. (1989), pp. 1677-1680.

We present the first Raman phonon sideband study of naphthalene crystals (77-300 K). The results are compared with neutron studies, density-of-states calculations, and previous ultraviolet and infrared phonon sideband studies. We also present the first study of phonon sidebands in disordered materials - a molecularly doped polymer: naphthalene in poly(methyl methacrylate). The doped polymer studies are consistent with exciton kinetics studies regarding the polymer morphology (segregation of crystallites). The promise of this technique is in differentiating normal from anomalous phonons in disordered media. © 1989 American Chemical Society.

Stirring in chemical reactions

P Argyrakis — 2007-08-13T19:59:08-00:00

Journal of Physical Chemistry, Vol. 93, No. 1. (1989), pp. 225-229.

Monte Carlo simulations of diffusion- and/or convection-controlled homoreactant (A + A) and heteroreactant (A + B) binary kinetics are given for one-, two-, and three-dimensional lattices and for disordered fractal lattices (percolation clusters in two and three dimensions). The rate laws are compared for different degrees and methods of convective stirring, including global versus local stirring in space and continuous versus periodic stirring in time. The results are parametrized in terms of the fractional time exponent of the reciprocal reactant density. © 1989 American Chemical Society.

Nanometer light source and molecular exciton microscopy

R Kopelman — 2007-08-13T19:56:34-00:00

Journal of Luminescence, Vol. 45, No. 1-6. ( 1990), pp. 298-299.

We describe the development of a new nanometer light source based on exciton transmitted optical radiation as well as a new biologically non-invasive ultraresolution light microscopy, based on combining the energy transfer "spectral ruler" method with the micro-movement technology employed in STM. We use near-field scanning optical microscopy, with micropipettes containing crystals of energy packaging donor molecules in the tips that can have apertures below 5 nm. The excitation of these tips extends near field microscopy well beyond the 50 nm limit. The theoretical resolution limit for this spectrally sensitive light microscope is well below 1 nm. The Z-distance feedback control is illustrated by the exciton-plasmon transfer from an anthracene crystal tip to a thin gold film (transparent to light).