Electrochemical Nanoparticle Sizing Via Nano-Impacts: How Large a Nanoparticle Can be Measured?

The field of nanoparticle (NP) sizing encompasses a wide array of techniques, with electron microscopy and dynamic light scattering (DLS) having become the established methods for NP quantification; however, these techniques are not always applicable. A new and rapidly developing method that addresses the limitations of these techniques is the electrochemical detection of NPs in solution. The ‘nano-impacts’ technique is an excellent and qualitative in situ method for nanoparticle characterization. Two complementary studies on silver and silver bromide nanoparticles (NPs) were used to assess the large radius limit of the nano-impact method for NP sizing. Noting that by definition a NP cannot be larger than 100 nm in diameter, we have shown that the method quantitatively sizes at the largest limit, the lower limit having been previously reported as ∼6 nm.¹     

Hydration energy or hydration force? Origin of ion-specificity in ion selective electrodes

The origin of ion-specificity (also known as Hofmeister effect) in potentiometric ion selective electrodes (ISE) with polymeric membranes has been traditionally assigned to the differences in lipophilicities of ions, with hydration energies described in the framework of Born theory as being of purely electrostatic nature. This is in clear contrast to the current understanding of the Hofmeister effect in colloid and interface science, where it is viewed as resulting from an interplay between the electrostatic and non-electrostatic interactions, the latter often referred to as “hydration” forces. The two approaches to ion-specificity in ISE, simplistically termed “hydration energy” (ion partitioning between an aqueous phase and the ISE membrane) and “hydration force” (ion adsorption from an aqueous phase to the electrically charged ISE membrane) are described and compared. Two major conclusions are drawn: i) ion-specificity can be included in both approaches, although it is more natural within the “hydration force” approach with ion-surface interactions; ii) both ion partitioning into, and ion adsorption onto the ISE membrane should be considered in order to fully understand the origin of ion-specificity in ISE.     

Phosphonium-phosphates/thiophosphates: Ionic liquids or liquid ion pairs? NMR spectroscopic classification

Structurally unique ionic liquids phosphonium-phosphate and phosphonium-thiophosphate, having both phosphorus based counter ions, in which the anionic part is represented by di-aryl phosphate or di-aryl thiophosphate and cations been tetraalkylphosphonium groups, behave differently in terms of their NMR behaviour. While phosphonium-phosphates show significant changes in its ¹H, ¹³C and ³¹P NMR chemical shifts vis. á vis. corresponding chemical shifts for a physical mixture of tetraalkylphosphonium bromide and di-aryl phosphate, phosphonium-thiophosphates behave almost similarly in terms of NMR with their synthetic precursors, hence indicating phosphate-phosphonium interaction has a significant covalent component resembling more to a liquid ion pair while thiophosphate-phosphonium interaction is principally ionic in nature. Translational diffusion behavior studied by PFGSE-NMR experiments and ionic conductivities of these ionic liquids in chloroform solution corroborated the hypothesis. The effect of variable alkyl chain length in phosphonium cation is effectively observed in the extent of ion association. Results of this study may provide insight into the solution state behavior of these ionic liquids, would help to classify those in terms of their strength of ion association and thus potential application thereof.     

What determines MALDI ion yields? A molecular dynamics study of ion loss mechanisms

Ion recombination in matrix-assisted laser desorption/ionization (MALDI) is as important as any ion formation process in determining the quantity of ions observed but has received comparatively little attention. Molecular dynamics simulations are used here to investigate some models for recombination, including a Langevin-type model, a soft threshold model and a tunneling model. The latter was found to be superior due to its foundations in a widespread physical phenomenon, and its lack of excessive sensitivity to parameter choice. Tunneling recombination in the Marcus inverted region may be a major reason why MALDI is a viable analytical method, by allowing ion formation to exceed ion loss on the time scale of the plume expansion. Ion velocities, photoacoustic transients and pump-probe measurements might be used to investigate the role of recombination in different MALDI matrices, and to select new matrices.     

How does a thin volatile film move?

When a water film evaporates from a mica substrate, an interface similar to a solidification front develops, separating two films of different thicknesses. We show experimentally that the evolution dynamics is controlled mainly by material diffusion through the vapor phase rather than by hydrodynamic flow through the film. Our results illustrate the role of different contributions to pattern formation of volatile liquid films.     

Bridge-bonded formate: active intermediate or spectator species in formic acid oxidation on a Pt film electrode?

We present and discuss the results of an in situ IR study on the mechanism and kinetics of formic acid oxidation on a Pt film/Si electrode, performed in an attenuated total reflection (ATR) flow cell configuration under controlled mass transport conditions, which specifically aimed at elucidating the role of the adsorbed bridge-bonded formates in this reaction. Potentiodynamic measurements show a complex interplay between formation and desorption/oxidation of COad and formate species and the total Faradaic current. The notably faster increase of the Faradaic current compared to the coverage of bridge-bonded formate in transient measurements at constant potential, but with different formic acid concentrations, reveals that adsorbed formate decomposition is not rate-limiting in the dominant reaction pathway. If being reactive intermediate at all, the contribution of formate adsorption/decomposition to the reaction current decreases with increasing formic acid concentration, accounting for at most 15% for 0.2 M DCOOH at 0.7 VRHE. The rapid build-up/removal of the formate adlayer and its similarity with acetate or (bi-)sulfate adsorption/desorption indicate that the formate adlayer coverage is dominated by a fast dynamic adsorption-desorption equilibrium with the electrolyte, and that formate desorption is much faster than its decomposition. The results corroborate the proposal of a triple pathway reaction mechanism including an indirect pathway, a formate pathway, and a dominant direct pathway, as presented previously (Chen, Y. X.; et al. Angew. Chem. Int. Ed. 2006, 45, 981), in which adsorbed formates act as a site-blocking spectator in the dominant pathway rather than as an active intermediate.     

What ion is generated when ionizing acetonitrile?

It has long been assumed that ionizing neutral acetonitrile produces ions with the same atomic connectivity, CH(3)CN(+*). Recent calculations on the C(2)H(3)N(+*) potential energy surface have suggested that it may be difficult to generate pure CH(3)CN(+*) when ionizing acetonitrile. We have probed the interconversion of CH(3)CN(+*) and its lower energy isomer CH(2)CNH(+*) by calculation, collision-induced dissociation mass spectrometry and ion-molecule reaction. The latter ion, ionized ketenimine, is co-generated upon electron or chemical ionization of neutral acetonitrile in the ion source of a mass spectrometer. An estimate of the ratio of the two isomers can be obtained from their respective ion-molecule reactions with CO(2) or COS. CH(3)CN(+*) reacts by proton-transfer with CO(2) and charge transfer with COS, whereas CH(2)CNH(+*) is unreactive.     

Quantitative Analyse durch beschr?nkte (limited) Oxydation

Ohne Zusammenfassung     

Contact isotopic- and contact ion exchange between two adsorbents—II Kinetics of film diffusion controlled processes

The kinetics of contact ion exchange between two adjacent ion exchanger beads in water was investigated. Since in this case diffusion of the counter ions in the intermingling electric double layers between the beads was the rate determining step, the rates of these processes were described theoretically with help of the Nernst-Planck equations. Model calculations revealed that the half times of contact ion exchange are the longer the stronger the ion exchanger particles absorb one of the two counter ions (separation factor α₂¹ ⪢ 1 or α₂¹ ⪡ 1). Contact isotopic exchange between labelled and unlabelled ions 1 will be faster than contact ion exchange between ions 1 and ions 2 if D₁/D₂ > 1 (D_i = diffusion coefficients), regardless of the value of the separation factor. Experimentally, the rates of the contact exchange processes Cs⁺ −Cs⁺, Cs⁺ −Na⁺ and Cs⁺ −Ag⁺ were determined, using ¹³⁴Cs as a radiotracer. The half times of these reactions were found to be between 20 and 30 hr, if the upper of the two 0.8 mm Φ beads rested with its own weight on the other one. If the upper bead was weighted with a mass of 2g, the half times were reduced by a factor of about 20. The experiments are in quantitative agreement with the theory.     

Utilization of a spiropyran derivative in a polymeric film optode for selective fluorescent sensing of zinc ion

Spiropyrans are the most studied families of func- tional materials due to their reversible structural con- version in response to external optical, chemical, and thermal stimulation[1]. Irradiation with ultraviolet light causes formation of an extended π-conjugation open form (merocyanine form) by heterolytic cleavage of the C (spiro)-O bond, which generates an intense ab- sorption in the visible region. Under the irradiating of visible light, the opened form will come back to the closed spi…     

Layer by layer assembly of DNA film via Zr(Ⅳ) ion and its intercalation property

In order to study the molecular recognition ability of DNA and different behavior of dyes incorporated into the base pairs, DNA molecule was assembled layer by layer via a Zr(Ⅳ) ion. The UV absorption spectra showed the uniform layer assembly of the DNA film. The fabricated DNA film was water-insoluble and maintained the native B-form structure. UV and CD measurements showed that the DNA film could interealate ethidium bromide (EtBr).     

What is the mechanism of soap film entrainment?

Classical Frankel's law describes the formation of soap films and their evolution upon pulling, a model situation of film dynamics in foams (formation, rheology, and destabilization). With the purpose of relating film pulling to foam dynamics, we have built a new setup able to give an instantaneous measurement of film thickness, thus allowing us to determine film thickness profile during pulling. We found that only the lower part of the film is of uniform thickness and follows Frankel's law, provided the entrainment velocity is small. We show that this is due to confinement effects: there is not enough surfactant in the bulk to fully cover the newly created surfaces which results in immobile film surfaces. At large velocities, surfaces become mobile and then Frankel's law breaks down, leading to a faster drainage and thus to a nonstationary thickness at the bottom of the film. These findings should help in understanding the large dispersion of previous experimental data reported during the last 40 years and clarifying the pulling phenomenon of thin liquid films.     

Specialist or generalist?

Analytical and Bioanalytical Chemistry -     

Field asymmetric waveform ion mobility spectrometry studies of proteins: Dipole alignment in ion mobility spectrometry?

Approaches to separation and characterization of ions based on their mobilities in gases date back to the 1960s. Conventional ion mobility spectrometry (IMS) measures the absolute mobility, and field asymmetric waveform IMS (FAIMS) exploits the difference between mobilities at high and low electric fields. However, in all previous IMS and FAIMS experiments ions experienced an essentially free rotation; thus the separation was based on the orientationally averaged cross-sections Omega(avg) between ions and buffer gas molecules. Virtually all large ions are permanent electric dipoles that will be oriented by a sufficiently strong electric field. Under typical FAIMS conditions this will occur for dipole moments >400 D, found for many macroions including most proteins above approximately 30 kDa. Mobilities of aligned dipoles depend on directional cross-sections Omega(dir) (rather than Omega(avg)), which should have a major effect on FAIMS separation parameters. Here we report the FAIMS behavior of electrospray-ionization-generated ions for 10 proteins up to approximately 70 kDa. Those above 29 kDa exhibit a strong increase of mobility at high field, which is consistent with predicted ion dipole alignment. This effect expands the useful FAIMS separation power by an order of magnitude, allowing separation of up to approximately 10(2) distinct protein conformers and potentially revealing information about Omega(dir) and ion dipole moment that is of utility for structural characterization. Possible approaches to extending dipole alignment to smaller ions are discussed.     

Infrared multiphoton dissociation spectroscopy study of protonated p-aminobenzoic acid: does electrospray ionization afford the amino- or carboxy-protonated ion?

Infrared multiphoton dissociation spectra of protonated p-aminobenzoic acid generated by electrospray ionization (ESI) from aqueous methanol and acetonitrile solutions were recorded in the gas phase from 2800-4000 cm(-1). The O-protonated ion is more stable than the N-protonated structure in the gas phase, whereas the opposite is true in both solutions. When CH(3)OH/H(2)O was used as the ESI solvent, only the O-protonated ion was observed. In contrast, a 70:30 mixture of the O- and N-protonated species were produced from CH(3)CN/H(2)O. These structural assignments are based on an assortment of experimental data (action spectra, photofragments, photofragmentation kinetics, and H/D exchange) and are fully supported by extensive computations. This work shows that ESI can lead to isomerization and that the ionization site may be varied by changing the solvent from which the substrate is analyzed.