Antioxidant properties of gold nanoparticles studied by ESR spectroscopy

The Au-containing nanocomposites were synthesized by UV irradiation followed by the thermal treatment of chitosan oligomer solutions doped by HAuCl₄. The size of the formed gold nanoparticles depends on the concentration of the dopant, which is proved by UV—Vis absorption spectroscopy and small-angle X-ray scattering (SAXS). The antioxidant activity of the gold nanoparticles with respect to hydroxy radicals significantly depends on the specific surface of the particles, which was found using the secondary radical spin-trapping technique. The change in the ^·OH radical concentration was monitored by the intensity of the ESR signal of the adduct of the spin trap (α-phenyl-N-tert-butylnitrone) with the Me^·radicals formed in the reaction of ^·OH with DMSO. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 509–512, March, 2008.     

Mechanical properties of alkanethiol monolayers studied by force spectroscopy

The mechanical properties of alkanethiol monolayers on Au(111) in KOH solution have been studied by force spectroscopy. The analysis of the vertical force versus penetration curves showed that monolayer penetration is a stepped process that combines elastic regions with sudden penetration events. The structural meaning of these events can be explained both by the creation of gauche defects on the hydrocarbon chains and by a cooperative molecular tilting model proposed by Barrena et al. [J. Chem. Phys. 113, 2413 (2000)]. The validity of these models for alkanethiol monolayers of different compactness and chain length has been discussed. The Young's modulus (E) of the monolayers has been calculated by using a recently developed model which considers the thickness of the monolayer as a parameter, thus allowing a decoupling of the mechanical properties of the thiol layer from those of the Au(111) substrate. As a result, the calculated E values are in the range of 50-150 Pa, which are remarkably lower than those previously reported in the literature.     

Early stages of ZnS nanoparticle growth studied by in-situ stopped-flow UV absorption spectroscopy.

The early stages of ZnS nanoparticle growth from supersaturated solution are investigated in situ by stopped-flow UV absorption spectroscopy with a time resolution of 1.28 ms. A model for data analysis is suggested which makes it possible to study both the average particle radius and the concentration. The average radii lie in the sub-nanometer range. During the first 40 ms, growth is predominantly governed by ripening.     

Intercalation interactions between dsDNA and acridine studied by single molecule force spectroscopy

In this letter, we report on the direct measurement of the intercalation interactions between acridine and double-stranded DNA (dsDNA) using single molecule force spectroscopy. The interaction between acridine and dsDNA is broken by force of 36 pN at a loading rate of 5.0 nN/s. The most probable rupture force between acridine and dsDNA is dependent on the loading rate, indicating that the binding of acridine and dsDNA is a dynamic process. The combination of SMFS experimental data with the theoretical model clearly suggests the presence of two energy barriers along with an unbinding trajectory of acridine-dsDNA.     

Optical properties of single semiconductor nanocrystals

We present an overview of the current progress in the understanding of the (steady state) optical properties of individual II-VI semiconductor nanocrystals. We begin with a presentation of the conceptual development of the theory required to model the electronic structure of these systems. This is followed by an overview of the current experimental results obtained from the spectroscopy of individual semiconductor nanocrystals, and in particular, we focus on the study of photoluminescence intermittency (blinking) and spectral diffusion. Where possible, we link the experimental observations to the predictions of current theories. We conclude that the surface of small semiconductor crystals plays an important role in determining their optical properties.     

Adsorption of As (V) on iron oxide nanoparticle films studied by in situ ATR-FTIR spectroscopy

Stabilization of arsenic contaminated soils by iron oxides has been proposed as a remediation technique to prevent leaching of arsenate into the environment. Fundamental studies are needed to establish under which conditions the complexes formed are stable. In the present work, a powerful technique, viz. ATR-FTIR spectroscopy, is adapted to the studies of adsorption of arsenate species on iron oxides. This technique facilitates acquisition of both quantitative and qualitative in situ adsorption data.In the present work, about 800 nm thick films of 6-lineferrihydrite were deposited on ZnSe ATR crystals. Arsenate adsorption on the ferrihydrite film was studied at pD values ranging from 4 to 12 and at an arsenate concentration of 0.03 mM in D₂O solution. The amount of adsorbed arsenate decreased with increasing pD as a result of the more negatively charged iron oxide surface at higher pD values. The adsorption and desorption kinetics were also studied. Arsenate showed a higher adsorption rate within the first 70 min and a much lower adsorption rate from 70 to 300 min. The low adsorption rate at longer reaction times was partly due to a low desorption rate of already adsorbed carbonate species adsorbed at the surface. The desorption of carbonate species was evidenced by the appearance of negative absorption bands. The desorption of adsorbed arsenate complexes was examined by flushing with D₂O at pD 4 and 8.5 and it was found that the complexes were very stable at pD 4 suggesting formation of mostly inner-sphere complexes whereas a fraction of the complexes at pD 8.5 were less stable than at pD 4, possibly due to the formation of outer-sphere complexes.In summary, the ATR technique was shown to provide in situ information about the adsorption rate, desorption rate and the speciation of the complexes formed within a single experiment, which is very difficult to obtain using other techniques.     

Optical and electrical properties of three-dimensional interlinked gold nanoparticle assemblies

The optical and electrical properties of 11-20 nm thick films composed of approximately 4 nm gold nanoparticles (Au-NPs) interlinked by six organic dithiol or bis-dithiocarbamate derivatives were compared to investigate how these properties depend on the core of the linker molecule (benzene or cyclohexane) and its metal-binding substituents (thiol or dithiocarbamate). Films prepared with the thiol-terminated linker molecules, (1,4-bis(mercaptomethyl)benzene, 1,4-bis(mercaptomethyl)cyclohexane, 1,4-bis(mercaptoacetamido)benzene, and 1,4-bis(mercaptoacetamido)cyclohexane), exhibit thermally activated charge transport. The activation energies lie between 59 and 71 meV. These films show distinct plasmon absorption bands with maxima between 554 and 589 nm. In contrast, the film prepared with 1,4-cyclohexane-bis(dithiocarbamate) has a significantly red-shifted plasmon band ( approximately 626 nm) and a pronounced absorbance in the near infrared. The activation energy for charge transport is only 14 meV. These differences are explained in terms of the formation of a resonant state at the interface due to overlap of the molecular orbital and metal wave function, leading to an apparent increase in NP diameter. The film prepared with 1,4-phenylene-bis(dithiocarbamate) exhibits metallic properties, indicating the full extension of the electron wave function between interlinked NPs. In all cases, the replacement of the benzene ring with a cyclohexane ring in the center of the linker molecule leads to a 1 order of magnitude decrease in conductivity. A linear relationship is obtained when the logarithm of conductivity is plotted as a function of the number of nonconjugated bonds in the linker molecules. This suggests that nonresonant tunneling along the nonconjugated parts of the molecule governs the electron tunneling decay constant (beta(N)(-)(CON)), while the contribution from the conjugated parts of the molecule is weak (corresponding to resonant tunneling). The obtained value for beta(N)(-)(CON) is approximately 1.0 (per non-conjugated bond) and independent of the nanoparticle-binding group. Hence, the molecules can be viewed as consisting of serial connections of electrically insulating (nonconjugated) and conductive (conjugated) parts.     

The electronic properties of potassium doped copper-phthalocyanine studied by electron energy-loss spectroscopy

The authors have studied the electronic structure of potassium doped copper-phthalocyanine using electron energy-loss spectroscopy. The evolution of the loss function indicates the formation of distinct KxCuPc phases. Taking into account the C1s and K2p core level excitations and recent results by Giovanelli et al. [J. Chem. Phys. 126, 044709 (2007)], they conclude that these are K2CuPc and K4CuPc. They discuss the changes in the electronic excitations upon doping on the basis of the molecular electronic levels and the presence of electronic correlations.     

Magnetic properties of ultrafine ferrihydrite clusters studied by Mossbauer spectroscopy and by thermodynamical analysis

Magnetic properties of ultrafine clusters of Fe₅HO₈·₄H₂O (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a secondorder magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jumplike first order magnetic phase transition (JMT). The critical radius R _cr below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R _cr = 2 α β η/(1 - T _cc/T ₀), where α, β and η are constants derived from surface energy, magnetostriction, compressibility and T _cc = 3/2 Nκ _B T _o ² ηβ ² (N is the number of iron atoms, κ _B is the Boltzmann constant, T _o is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be R _cr ～ 1.5–2.0 nm and T _JMT ～ 4.2–6 K, respectively. Satisfactory agreement between the value R _cr, estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.     

The CdS nanoparticle with capped surface and its nonlinear optical properties studied by hyper-Rayleigh scattering

The CdS nanoparticles synthesized in AOT (bis(2-ethylhexyl) sulfosuccinate, disodium salt) reverse micelle in addition of the electric neutral surface-capping agent of pyridine are characterized by XPS, TEM and the absorption spectra. The values of the first-order hyperpolarizability β for two kinds of nanoparticles, CdS/AOT^- and CdS/pyridine/AOT^-, have been experimentally measured in solution by the newly developed hyper-Rayleigh scattering technique. The values of β are larger than 10^-27 esu, which are among the largest values reported for solution species.     

Protolytic properties of polyamine wasp toxin analogues studied by 13C NMR spectroscopy

Acid-base properties of the natural polyamine wasp toxin PhTX-433 (1) and seven synthetic analogues [PhTX-343 (2), PhTX-334 (3), PhTX-443 (4), PhTX-434 (5), PhTX-344 (6), PhTX-444 (7), and PhTX-333 (8)], each having four protolytic sites, were characterized by 13C NMR spectroscopy. Nonlinear, multiparameter, simultaneous fit of all chemical shift data obtained from the NMR titration curves yielded macroscopic pKa values as well as intrinsic chemical shift data of all differently protonated macrospecies. Analyses of the chemical shift data demonstrated strong interactions between all four sites and provided information about complex relationships between chemical shift values and protonation state. Deprotonation of fully protonated forms starts at the central amino group of the polyamine moiety, and the extent of this trend depends on the distance to the flanking, protonated amino groups. The pKa1 values of 1-8 are in the range 8.2-9.4. Hence, some of the toxins are incompletely protonated at the pH and ionic strength conditions used for assessment of their interactions with ionotropic glutamate and nicotinic acetylcholine receptors, and the degree of protonation is expected to have pharmacological importance in the ion-channel binding event.     

Dielectric properties of a single nanochannel investigated by high-frequency impedance spectroscopy

High-frequency electric impedance spectroscopy is used to characterize the dielectric properties of a single nanochannel in a micro/nanofluidic device. The simulated electric impedance results lead to the determination of two conductance regimes, a non-ideal capacitance and its surface charge.     

Furanized rubber studied by NMR spectroscopy

The double bonds of natural rubber latex (stabilized by a nonionic surfactant) were reacted with an approximately equimolar amount of performic acid at room temperature with a limited amount of formic acid present. Product analysis by ¹H-NMR during the course of the reaction showed that 69–90% epoxidation occurred before the advent of ring opening and ring expansion to produce furanized rubber; hence the rate of epoxidation was greater than the rate of furanization. Indeed, at lower concentrations of formic acid and rubber latex, epoxidation occurred to 90% and furanization was prevented; it was subsequently brought about by the addition of a catalytic amount of orthophosphoric acid. Increased formic acid concentration caused early coagulation of the modified rubber latex. By ¹H- and ¹³C-NMR, it was found that the furanized rubber probably consisted of tetrahydrofuran rings linked together by C? C bonds at positions adjacent to the hetero atom and contained a terminal hydroxy group. The number average sequence length was 2–9, but only the sample with an average sequence length of 9 was effective as a cation binder.     

Isonitrile ligand properties as studied by He I/He II photoelectron spectroscopy

A new series of isonitrile-substituted cobalt tricarbonyl nitrosyl (Co(CO)₂(NO)CNR, R = Me, Et, ⁿPr, ⁱPr, ⁿBu, ⁿPe, CH₂Si(CH₃)₃) has been synthesized, and their He I ultraviolet photoelectron spectra are reported. The assignment of the bands in the low energy part of the spectra was performed with the aid of DFT calculations. The first vertical ionization energies of the complexes were found to be 7.73 (CNMe), 7.58 (CNEt), 7.59 (CNⁿPr), 7.70 (CNⁿBu), 7.67 (CNⁿPe), 7.77 (CNⁱPr), and 7.54 ± 0.03 eV (CNCH₂Si(CH₃)₃). In the case of ⁿPr- and CH₂Si(CH₃)₃- substitutions, He II photoelectron spectra were also recorded. The relative importance of electronic and steric effects of the isonitrile ligands, as a function of the size of group –R, is discussed.     

Optical limiting of gold nanoparticle aggregates induced by electrolytes

The electronic absorption spectra and optical-limiting (OL) properties of gold nanoparticle (AuNP) aggregates induced by KCl and NaCl have been investigated using 4.1-ns laser pulses at 532 nm. Although the individual AuNP colloid shows no optical-limiting effect, the AuNP aggregates exhibit significant optical-limiting characteristics. With an increased concentration of KCl and NaCl, the surface plasmon resonance (SPR) band shifts to a longer wavelength, and the optical-limiting performance is enhanced. Both the electronic absorption and optical limiting are influenced by the particle size. The larger the individual nanoparticle, the further red-shifted the SPR band and the stronger the optical limiting. Optical limiting of aggregates induced by KCl is stronger than that of aggregates induced by NaCl. Mechanistic studies reveal that free-carrier absorption is the dominant contributor to the optical limiting, with negligible contribution from nonlinear scattering.     

Early stages of ZnS growth studied by stopped-flow UV absorption spectroscopy: effects of educt concentrations on the nanoparticle formation

The growth of ZnS nanoparticles by precipitation from supersaturated aqueous solution is studied by stopped-flow UV absorption spectroscopy. The average size, size distribution, and concentration of the particles are monitored within the sub-second time regime with a resolution of 1.28 ms. Particle growth at these early stages is governed by pronounced ripening. The UV absorption data strongly suggest that growth occurs by preferential adsorption of HS- anions relative to Zn(2+) or ZnOH(+) cations. Correspondingly, the initial sulfide concentration has a much more pronounced influence on the growth kinetics than the initial zinc concentration. These findings are verified by zeta-potential measurements which confirm that the particle surfaces are negatively charged under near-neutral pH conditions.     

Wide-field single metal nanoparticle spectroscopy for high throughput localized surface plasmon resonance sensing

Noble metal nanoparticles (mNPs) have a distinct extinction spectrum arising from their ability to support Localized Surface Plasmon Resonance (LSPR). Single-particle biosensing with LSPR is label free and offers a number of advantages, including single molecular sensitivity, multiplex detection, and in vivo quantification of chemical species etc. In this article, we introduce Single-particle LSPR Imaging (SLI), a wide-field spectral imaging method for high throughput LSPR biosensing. The SLI utilizes a transmission grating to generate the diffraction spectra from multiple mNPs, which are captured using a Charge Coupled Device (CCD). With the SLI, we are able to simultaneously image and track the spectral changes of up to 50 mNPs in a single (～1 s) exposure and yet still retain a reasonable spectral resolution for biosensing. Using the SLI, we could observe spectral shift under different local refractive index environments and demonstrate biosensing using biotin-streptavidin as a model system. To the best of our knowledge, this is the first time a transmission grating based spectral imaging approach has been used for mNPs LSPR sensing. The higher throughput LSPR sensing, offered by SLI, opens up a new possibility of performing label-free, single-molecule experiments in a high-throughput manner.     

States of molecular assembly and physical properties of crystalline long-chain compounds studied by vibrational spectroscopy

Various types of molecular assembly of long-chain compounds in solid states were investigated by means of infrared absorption, Raman and Brillouin spectroscopic methods. As for the polymorphism in even-numbered n-fatty acids, three monoclinic modifications, B, C, and E, all consisting of the orthorhombic polyethylene sublattice, give rise to their characteristic infrared and Raman spectra. A dynamical equilibrium between cis and trans conformations of the hydrogen-bonded carboxyl groups in modification C, which is related to the high-temperature stable character of this phase, is reflected to a dramatic change with temperature in the low-frequency Raman spectra. A new type of reversible solid state phase transition was found between two A-type (triclinic) modifications of myristic, palmitic, and stearic acids. The γ→α phase transition of oleic acid was found to be caused by a conformational disordering of polymethylene chains at the lamellar interfacial region. Two basic polytype structures, Mon and Orth II, of stearic acid B were investigated, and it was found that the low-frequency phonon frequencies (below 50 cm⁻¹) were strongly influenced by the polytype structure. Based on the spectroscopic considerations, Orth II was predicted as the thermodynamically stable phase around room temperature compared with Mon, and the stability is responsible for the vibrational free energy term. Some experimental findings which support this prediction were obtained. The values of the stiffness tensor elements of Mon and Orth II, measured by Brillouin scattering, indicate that the mechanical behavior of bulk crystals is very dependent on the polytype structure. The relationship between the mobility of chain molecules and the width of the spectral bands was investigated in a quantitative manner for the case of n-alkane molecules entrapped in the urea inclusion adducts. The changes in the half-width for the polarization components of various Raman bands on the transition from the orthorhombic to the hexagonal phase are interpreted in terms of the correlation functions of the Raman tensor related to the rotational motion of the alkane molecules around the chain axes.     

Methyl-radical association studied by time-resolved mass spectroscopy

Time-resolved mass spectroscopy has been utilized to study the kinetics of methylradical association. The rate coefficient for the association process was determined by following the time dependence of ethane formation after flash photolysis of CH₃I in the presence of N₂. The net rate coefficient for the process 2CH₃ (+M) → C₂H₆(+M) has a high-pressure limit of (4.0 ± 0.3) × 10^?11 cm³ molecule^?1 sec^?1 at 313°K. This rate coefficient has found to be insensitive to the third-body number densities for pressures ranging from 13 Torr down to below 0.5 Torr, indicating that the lifetime for dissociation of the intermediate C₂H₆* species is greater than 2 × 10^?7 sec at 313°K for a gas pressure of 0.5 Torr.