Heterogeneous reactions of tropospheric trace-gases on solid model aerosol surface:a laboratory study

The first part of this laboratory study deals with heterogeneous reactions of bromine- and chlorine-containing species relevant to the atmosphere with solid model alkali salt substrates. The second part of this work treats the heterogeneous interaction of acidic compounds with various mineral dust substrates. The aim of this study is to investigate the role of adsorbed water, H2O(a) and the influence of defect surface sites on the reactions. Furthermore, reaction mechanisms are proposed for the different interactions. Steady state and pulsed valve uptake experiments have been performed in a low-pressure flow reactor. In order to characterise the solid samples images have been taken using a scanning electron microscope (SEM). Furthermore, a few scans using a photoacoustic cell coupled to a FTIR spectrometer have been performed. First, the interaction of gaseous Cl2, HCl, BrCl, Br2, Cl2O and HOCl with solid KBr and furthermore, the reaction of Cl2O and HOCl on solid NaCl have been investigated. The reaction products of the reactions of Cl2, Cl2O and HOCl with KBr have been Br2 and BrCl. The reaction of Cl2O with KBr leads to slow additional formation of HOCl, BrOCl and Cl2, and the reaction of HOCl with KBr leads to slow formation of HOBr, Cl2O, Br2O and BrOCl. The reaction of Cl2O on solid NaCl leads to formation of Cl2 but also a slow formation of HOCl has been observed which is attributed to the reaction with adsorbed H2O(a). The interaction of HOCl with NaCl was immeasurably slow under our conditions. In order to measure the amount of adsorbed H2O(a) desorption experiments have been carried out and a calibration curve has been obtained. While performing desorption experiments on ground KBr grains and thin KBr films sprayed on a gold-coated substrate desorption of HOBr has been observed which we attributed to the presence of surface adsorbed molecular bromine. Using the SEM we found that pumping and heating the substrate lead to a better-ordered surface. The exposure of the salt sample to an excess of chlorine or bromine leads also to a recrystallisation of the surface. Furthermore, all reactions on alkali salt samples have shown that adsorbed halogen species are retained on the surface. These adsorbed species play a crucial role for the reactions of halogens with alkali salt. In the case of HOCl on KBr we explain the autocatalytic behaviour of the reaction by the presence of adsorbed halogen species, particularly Br2. In the Cl2/KBr system a regeneration of the surface has been observed which we explain with desorption of adsorbed intermediates, but also by crystallisation of KCl that may lead to a recycling of reactive sites. In contrast to Cl2/KBr no regeneration of the substrate towards formation of gaseous bromine-containing species has been observed in the HOCl/KBr system. We explain this with the fact that KOH thermodynamically does not tend to crystallise in the presence of H2O as does the alkali halide. In the second part the reactions of CO2, SO2, HCl and HNO3 on various CaCO3 substrates such as roughened and polished marble and low- and high-ordered precipitated CaCO3 have been carried out. Additionally, a few HNO3 uptake experiments have been performed on dust substrates such as Saharan Dust, Kaolinite and Arizona Road Dust. On these substrates a fast uptake of HNO3 but no reaction products have been observed. In the case of SO2 and HCl interacting with precipitated CaCO3 the observed reaction product has been CO2. Whereas HNO3 uptake experiments on precipitated CaCO3 additionally lead to a slow formation of H2O. HCl uptake experiments on polished marble plates have not led to formation of any reaction product. However, HNO3 uptake experiments on polished and roughened marble led to formation of H2O but not of CO2. We explain these observations with the dependence of the rate of surface chemistry on the morphology of the sample. The HCl and HNO3 uptake experiments on precipitated CaCO3 have shown a delay in the formation of CO2 whereas it is immediately released after the exposure of the CaCO3 sample to SO2. Furthermore, an uptake of CO2 has been observed on precipitated CaCO3. All uptake experiments on CaCO3 have shown saturation due to a limited number of reactive sites. In order to explain these results we have speculated the existence of an intermediate species Ca(OH)(HCO3). This intermediate may be formed by exposure of the CaCO3 sample to ambient H2O and CO2. Acidic compounds such as HCl and HNO3 react rapidly with the basic OH part of the intermediate and subsequently more slowly with the bicarbonate part of the intermediate. This explains the delay of formation of CO2 in the reaction of HCl and HNO3 with CaCO3. In contrast to acidic species, SO2 attacks with preference the bicarbonate part of the intermediate. This leads to an immediate release of CO2 after exposure of the CaCO3 sample to SO2. The reaction of SO2 with the OH part of the intermediate leads to loss of SO2. The mass balance between loss of SO2 and yield of CO2 has shown a deficiency of CO2, which may be explained with a slow reaction with the OH part of the intermediate

Absorbance enhancement in microplate wells for improved-sensitivity biosensors

A generic optical biosensing strategy was developed that relies on the absorbance enhancement phenomenon occurring in a multiple scattering matrix. Experimentally, inserts made of glass fiber membrane were placed into microplate wells in order to significantly lengthen the trajectory of the incident light through the sample and therefore increase the corresponding absorbance. Enhancement factor was calculated by comparing the absorbance values measured for a given amount of dye with and without the absorbance-enhancing inserts in the wells. Moreover, the dilution of dye in solutions with different refractive indices (RI) clearly revealed that the enhancement factor increased with the ΔRI between the membrane and the surrounding medium, reaching a maximum value (EF>25) when the membranes were dried. On this basis, two H2O2-biosensing systems were developed based on the biofunctionalization of the glass fiber inserts either with cytochrome c or horseradish peroxidase (HRP) and the analytical performances were systematically compared with the corresponding bioassay in solution. The efficiency of the absorbance-enhancement approach was particularly clear in the case of the cytochrome c-based biosensor with a sensitivity gain of 40 folds and wider dynamic range. Therefore, the developed strategy represents a promising way to convert standard colorimetric bioassays into optical biosensors with improved sensitivity

Strong Improvement of Long-Term Chemical and Thermal Stability of Plasmonic Silver Nanoantennas and Films

Silver (Ag) nanostructures and thin films are advantageous plasmonic materials as they have significantly lower losses than gold (Au). Unfortunately, Ag nanostructures suffer from poor chemical and thermal stability, which limit their applications. Here, the mechanisms leading to the deterioration of Ag nanostructures are clarified. It is first shown that oxygen alone cannot oxidize Ag nanostructures. Then, experiments using X-ray photoelectron spectroscopy reveal that the amount of sulfur in ambient air is too low for efficient tarnishing of the Ag surface. Finally, water is found to be the most critical factor for the degradation of Ag nanostructures and thin films. At high relative humidity, adsorbed water forms a thin film enabling the migration of Agions at the Ag/air interface, which deteriorates the Ag nanostructures. A dehydration treatment is developed which alters the morphology of the deposited silver, leading to an improved chemical and thermal stability of the Ag nanostructures and films, which then remain stable for more than 14 weeks under ambient laboratory conditions. In addition, dehydration also improves significantly the root-mean-square roughness for Ag thin films deposited on a glass substrate

Surface-to-volume ratio controls the radiation of stratified plasmonic antennas

Surface plasmons are excited at a metal/dielectric interface, through the coupling between conduction electrons and incident photons. The surface plasmon generation is therefore strongly determined by the accessibility of the surface to the incoming electromagnetic field. We demonstrate the role of this surface for plasmonic nanoantennas with identical volumes and resonant lengths. An antenna is stratified parallel to the plane of its main dipolar resonance axis and the influence of the number of layers and the spacing between them on the optical properties of the antenna are investigated experimentally. We show that increasing the number of layers and, hence, increasing the total accessible surface of the antenna, results in an enhanced scattering cross section and a redshift which indicates that lower energy photons are required to couple to the metal electrons. In particular, the far-field enhancement observed for double-layer nanostructures suggests that standard single-layer metal deposition can be easily and advantageously substituted with metal/dielectric/metal deposition to boost light scattered by a plasmonic antenn

Trapping and Sensing 10 nm Metal Nanoparticles Using Plasmonic Dipole Antennas

The Optical trapping of Au nanoparticles with dimensions as small as 10 nm in the gap of plasmonic dipole antennas is demonstrated. Single nanoparticle trapping events are recorded in real time by monitoring the Rayleigh scattering spectra of individual plasmonic antennas. Numerical simulations are also performed to interpret the experimental results, indicating the possibility to trap nanoparticles only a few nanometers in size. This work unveils the potential associated with the integration of plasmonic trapping with localized surface plasmon resonance based sensing techniques, in order to deliver analyte to specific, highly sensitive regions ("hot spots")

Coupling Strength Can Control the Polarization Twist of a Plasmonic Antenna

The far-field polarization of the optical response of a plasmonic antenna can be tuned by subtly engineering of its geometry. In this paper, we develop design rules for nano antennas which enable the generation of circular polarized light via the excitation of circular plasmonic modes in the structure. Two initially orthogonal plasmonic modes are coupled in such a way that a rotational current is excited in the structure. Modifying this coupling strength from a weak to a strong regime controls the helicity of the scattered field. Finally, we introduce an original sensing approach that relies on the rotation of the incident polarization and demonstrates a sensitivity of 0.23 deg·nm -1 or 33 deg·RIU-1, related to changes of mechanical dimensions and the refractive index, respectively. © 2013 American Chemical Society