Arjan J. G. Mank

Catalytic processes monitored at the nanoscale with tip-enhanced Raman spectroscopy

Heterogeneous catalysts play a pivotal role in the chemical industry, but acquiring molecular insights into functioning catalysts remains a significant challenge. Recent advances in micro-spectroscopic approaches have allowed spatiotemporal information to be obtained on the dynamics of single active sites and the diffusion of single molecules. However, these methods lack nanometre-scale spatial resolution and/or require the use of fluorescent labels. Here, we show that time-resolved tip-enhanced Raman spectroscopy can monitor photocatalytic reactions at the nanoscale. We use a silver-coated atomic force microscope tip to both enhance the Raman signal and to act as the catalyst. The tip is placed in contact with a self-assembled monolayer of p-nitrothiophenol molecules adsorbed on gold nanoplates. A photocatalytic reduction process is induced at the apex of the tip with green laser light, while red laser light is used to monitor the transformation process during the reaction. This dual-wavelength approach can also be used to observe other molecular effects such as monolayer diffusion.

A critical assessment of laser ablation ICP-MS as an analytical tool for depth analysis in silica-based glass samples

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a versatile technique for trace element analysis with respect to depth in solid samples. The high sensitivity of ICP-MS makes it possible to determine most elements in the periodic table at trace levels (<1 µg g –1 ). Recent trends in the development of instrumentation have led to the possibility of analysing craters of smaller diameter with variable depth. However, a major limitation to this approach for depth profiling, preventing accurate and precise analysis, is element-selective, non-reproducible ablation. The ability for representative sampling during depth analysis is tested in this study by ablating into homogeneous silica-based glass materials. Elemental relative response deviations of up to 300% are observed for selected elements during progressive ablation into the glass target. The geometry of the ablation crater controls the accuracy of sampling of the material at depth. Elemental fractionation becomes significant for some elements (e.g., Zn, Pb) when the depth/diameter ratio of the ablation crater is >6, corresponding to a 50% reduction in analyte response. Large diameter craters, if ablated with sufficient laser power density, reduce elemental fractionation and give a larger signal for a longer period of time, providing more suitable conditions for representative analysis. LA-ICP-MS can be a powerful technique for depth profiling provided that optimum analytical conditions are selected.

Separation of time-resolved phenomena in surface-enhanced Raman scattering of the photocatalytic reduction of p-nitrothiophenol.

Abstract Straightforward analysis of chemical processes on the nanoscale is difficult, as the measurement volume is linked to a discrete number of molecules, ruling out any ensemble averaging over rotation and diffusion processes. Raman spectroscopy is sufficiently selective for monitoring chemical changes, but is not sufficiently sensitive to be applied directly. Surface‐enhanced Raman spectroscopy (SERS) can be applied for studying reaction kinetics, but adds additional variability in the signal as the enhancement factor is not the same for every location. A novel chemometric method described here separates reaction kinetics from short‐term variability, based on the lack of fit in a principal‐component analysis. We show that it is possible to study effects that occur on different time scales independently without data reduction using the photocatalytic reduction of p‐nitrothiophenol as a showcase system. Using this approach a better description of the nanoscale reaction kinetics becomes available, while the short‐term variations can be examined separately to examine reorientation and/or diffusion effects. It may even be possible to identify reaction intermediates through this approach. With only a limited number of reactive molecules in the studied volume, an intermediate on a SERS hot spot may temporarily dominate the spectrum. Now such events can be easily separated from the bulk conversion process by making use of this chemometric method.

Investigation of the Kinetics of a Surface Photocatalytic Reaction in Two Dimensions with Surface‐enhanced Raman Scattering

Heterogeneous catalysis is a surface phenomenon. Yet, though the catalysis itself takes place on surfaces, the reactants and products rapidly take the form of another physical state, as either a liquid or a gas. Catalytic reactions within a self‐assembled monolayer are confined within two dimensions, as the molecules involved do not leave the surface. Surface‐enhanced Raman spectroscopy is an ideal technique to probe these self‐assembled monolayers as it gives molecular information in a measured volume limited to the surface. We show how surface‐enhanced Raman spectroscopy can be used to determine the reaction kinetics of a two‐dimensional reaction. As a proof of principle, we study the photocatalytic reduction of p‐nitrothiophenol. A study of the reaction rate and dilution effects leads to the conclusion that a dimerization must take place as one of the reaction steps.

Laser ablation inductively coupled plasma mass spectrometry as a tool for studying heterogeneity within polymers

The distribution (heterogeneous or homogeneous) of inorganic additives within an organic polymer phase is important because it directly influences the physical and chemical properties of the polymer. Heterogeneity must also be assessed when producing reference materials. When producing a polymer reference material certified for the concentrations of its inorganic additives, assessment of heterogeneity is crucial. In this work the use of LA-ICP-MS to demonstrate the occurrence of heterogeneity in polymers, and polyethylene in particular, is described. The polymers studied were candidate reference materials. The conclusions reached correspond well with those obtained by means of solid sampling-Zeeman atomic absorption spectrometry (SS-ZAAS). As LA-ICP-MS is a multi-elemental technique the data was obtained in a fraction of the time required by SS-ZAAS. Hence, LA-ICP-MS can be used as a screening tool to quickly reject materials on the basis of unacceptable heterogeneity, thereby avoiding unnecessary time-consuming SS-ZAAS analyses.

Selective and sensitive in vitro detection method for aluminum phthalocyanine photosensitizers, using liquid chromatography and diode-laser induced fluorescence

Abstract A combination of conventional-size liquid chromatography (LC) and visible diode-laser induced fluorescence (DIO-LIF) detection has been applied for the detection of photosensitizers used in photodynamic therapy. Of these photosensitizers, a mixture of disulfonated aluminum phthalocyanines (AlPcS 2 ) has been chosen as a model. First, the influence of pH and solvent on the spectroscopic properties of di- and tetrasulfonated aluminum phthalocyanine has been examined, indicating the presence of multi-step ionization equilibria and the formation of aggregates. Direct fluorescence detection of AlPcS 2 is found to be unsatisfactory. For biological samples, the introduction of LC showed clear advantages, especially for faeces extracts. A detection limit of 7 × 10 −12 M was found for AlPcS 2 ( S/N = 3 for the major component) in both rat urine and faeces extracts using the DIO-LIF system, which is 20-fold more sensitive than a conventional fluorescence detection system, in combination with chromatography.

Column liquid chromatography with diode laser-induced fluorescence detection of carboxylic acids after pre-column derivatisation

Abstract Several pre-column approaches to fluorescence derivatisation of carboxylic acids, compatible with diode laser-induced fluorescence (DIO-LIF) detection, were compared. For this purpose red-absorbing labels containing hydrazide, bromoacetamide and iodoacetamide functionalities were studied. The sensitivity of DIO-LIF detection is illustrated by the (3–10) × 10−12 M (75–250 amol) detection limit for various derivatised carboxylic acids. The real-life detection limit for the carboxylic acids is 3 × 10 −8 M , because derivatisation is not quantitative at lower concentrations. Derivatised n-alkylcarboxylic acids and non-steroidal anti-inflammatory drugs are easily separated by column liquid chromatography, despite the attachment of the bulky red-absorbing labels. The determination of the anti-inflammatory drug naproxen in saliva is shown as an example.

In Situ Raman Analysis of Gas Formation in NiMH Batteries

In this paper Raman spectrometry is introduced in the field of sealed battery research for in situ gas-phase analysis and for long-term measurements. For this purpose, a new method was successfully applied in order to model battery behavior without interfering with operation. It is shown that oxygen, hydrogen, and nitrogen are responsible for the pressure increase that occurs during overcharging. The relative contribution of the different gases depends on the current imposed on the battery as well as the operating temperature. Reproducible and stable signals could be obtained even under severe conditions such as high pressure and elevated temperature. Oxygen and hydrogen are produced in side reactions taking place during battery operation. However, as nitrogen is unlikely to be a reacting gas inside the battery, the change in its partial pressure can be attributed to electrode expansion and a change in the electrolyte volume.

Determination of the anticancer drug metabolite WR1065 using pre-column derivatization and diode laser induced fluorescence detection

A liquid chromatographic (LC) procedure using alumina as stationary phase in both the pre- and the analytical column, is reported for the determination of WR1065, the active metabolite of the amino- and thiol-containing anticancer drug WR2721. After pre-column derivatization of the thiol group, the analyte is determined by LC with diode laser induced fluorescence detection in the near-infrared. Selective removal of excess label is achieved by means of column switching; it allows the detection of 5 x 10(-9) M WR1065 in water and 10-fold diluted, deproteinated plasma samples. The detection limit is determined by the derivatization reaction and not by the fluorescence detection of the labelled analyte. Endogeneous thiols do not interfere.

Opto-mechanical probe for combining atomic force microscopy and optical near-field surface analysis

We have developed a new easy-to-use probe that can be used to combine atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). We show that, using this device, the evanescent field, obtained by total internal reflection conditions in a prism, can be visualized by approaching the surface with the scanning tip. Furthermore, we were able to obtain simultaneous AFM and SNOM images of a standard test grating in air and in liquid. The lateral resolution in AFM and SNOM mode was estimated to be 45 and 160 nm, respectively. This new probe overcomes a number of limitations that commercial probes have, while yielding the same resolution.