Conductivity for Soot Sensing: Possibilities and Limitations

In this study we summarize the possibilities and limitations of a conductometric measurement principle for soot sensing. The electrical conductivity of different carbon blacks (FW 200, lamp black 101, Printex 30, Printex U, Printex XE2, special black 4, and special black 6), spark discharge soot (GfG), and graphite powder was measured by a van der Pauw arrangement. Additionally the influence of inorganic admixtures on the conductivity of carbonaceous materials was proven to follow the percolation theory. Structural and oxidation characteristics obtained with Raman microspectroscopy and temperature programmed oxidation, respectively, were correlated with the electrical conductivity data. Moreover, a thermophoretic precipitator has been applied to deposit soot particles from the exhaust stream between interdigital electrodes. This combines a controlled and size independent particle collection method with the conductivity measurement principle. A test vehicle was equipped with the AVL Micro Soot Sensor (photoacoustic soot sensor) to prove the conductometric sensor principle with an independent and reliable technique. Our results demonstrate promising potential of the conductometric sensor for on-board particle diagnostic. Furthermore this sensor can be applied as a simple, rapid, and cheap analytical tool for characterization of soot structure

Sorption of Silver Nanoparticles to Environmental and Model Surfaces

The fate of engineered nanoparticles in environmental systems is controlled by changes in colloidal stability and their interaction with different environmental surfaces. Little is known about nanoparticle–surface interactions on the basis of sorption isotherms under quasi-equilibrium conditions, although sorption isotherms are a valuable means of studying sorbate-sorbent interactions. We tested the extent to which the sorption of engineered silver nanoparticles (<i>n</i>Ag) from stable and unstable suspensions to model (sorbents with specific chemical functional groups) and environmental (plant leaves and sand) surfaces can be described by classical sorption isotherms. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) qualitative and quantitative analyses were also used to assess the morphology and nanomechanical parameters of the covered surfaces. The sorption of <i>n</i>Ag from stable suspensions was nonlinear and best described by the Langmuir isotherm. Langmuir coefficients varied with sorbent surface chemistry. For <i>n</i>Ag sorption from an unstable suspension, the sorption isotherms did not follow any classical sorption models, suggesting interplay between aggregation and sorption. The validity of the Langmuir isotherm suggests monolayer sorption, which can be explained by the blocking effect due to electrostatic repulsion of individual nanoparticles. In unstable suspensions, aggregates are instead formed in suspension and then sorbed, formed on the surface itself, or formed in both ways

Exploring the Potential of Stable Isotope (Resonance) Raman Microspectroscopy and Surface-Enhanced Raman Scattering for the Analysis of Microorganisms at Single Cell Level

Raman microspectroscopy is a prime tool to characterize the molecular and isotopic composition of microbial cells. However, low sensitivity and long acquisition times limit a broad applicability of the method in environmental analysis. In this study, we explore the potential, the applicability, and the limitations of stable isotope Raman microspectroscopy (SIRM), resonance SIRM, and SIRM in combination with surface-enhanced Raman scattering (SERS) for the characterization of single bacterial cells. The latter two techniques have the potential to significantly increase sensitivity and decrease measurement times in SIRM, but to date, there are no (SERS-SIRM) or only a limited number (resonance SIRM) of studies in environmental microbiology. The analyzed microorganisms were grown with substrates fully labeled with the stable isotopes ¹³C or ²H and compounds with natural abundance of atomic isotopes (¹²C 98.89% or ¹H 99.9844%, designated as ¹²C or ¹H, respectively). Raman bands of bacterial cell compounds in stable isotope-labeled microorganisms exhibited a characteristic red-shift in the spectra. In particular, the sharp phenylalanine band was found to be an applicable marker band for SIRM analysis of the <i>Deltaproteobacterium</i> strain N47 growing anaerobically on ¹³C-naphthalene. The study of <i>G. metallireducens</i> grown with ¹³C- and ²H-acetate showed that the information on the chromophore cytochrome <i>c</i> obtained by resonance SIRM at 532 nm excitation wavelength can be successfully complemented by whole-organism fingerprints of bacteria cells achieved by regular SIRM after photobleaching. Furthermore, we present here for the first time the reproducible SERS analysis of microbial cells labeled with stable isotopes. <i>Escherichia coli</i> strain DSM 1116 cultivated with ¹²C- or ¹³C-glucose was used as a model organism. Silver nanoparticles synthesized <i>in situ</i> were applied as SERS media. We observed a reproducible red-shift of an adenine-related marker band from 733 to 720 cm^–1 in SERS spectra for ¹³C-labeled cells. Additionally, Raman measurements of ¹²C/¹³C-glucose and -phenylalanine mixtures were performed to elucidate the feasibility of SIRM for nondestructive quantitative and spatially resolved analysis. The performed analysis of isotopically labeled microbial cells with SERS-SIRM and resonance SIRM paves the way toward novel approaches to apply Raman microspectroscopy in environmental process studies

Label-Free in Situ Discrimination of Live and Dead Bacteria by Surface-Enhanced Raman Scattering

Techniques to distinguish between live and dead bacteria in a quantitative manner are in high demand in numerous fields including medical care, food safety, and public security as well as basic science research. This work demonstrates new nanostructures (silver nanoparticles coating bacteria structure, Bacteria@AgNPs) and their utility for rapid counting of live and dead bacteria by surface-enhanced Raman scattering (SERS). We found that suspensions containing Gram-negative organisms as well as AgNPs give strong SERS signals of live bacteria when generated selectively on the particle surface. However, almost no SERS signals can be detected from Bacteria@AgNPs suspensions containing dead bacteria. We demonstrate successful quantification of different percentages of dead bacteria both in bulk liquid and on glass surfaces by using SERS mapping on a single cell basis. Furthermore, different chemicals have been used to elucidate the mechanism involved in this observation. Finally, we used the Bacteria@AgNPs method to detect antibiotic resistance of <i>E. coli</i> strains against several antibiotics used in human medicine

Photoinduced C–C Reactions on Insulators toward Photolithography of Graphene Nanoarchitectures

On-surface chemistry for atomically precise sp² macromolecules requires top-down lithographic methods on insulating surfaces in order to pattern the long-range complex architectures needed by the semiconductor industry. Here, we fabricate sp²-carbon nanometer-thin films on insulators and under ultrahigh vacuum (UHV) conditions from photocoupled brominated precursors. We reveal that covalent coupling is initiated by C–Br bond cleavage through photon energies exceeding 4.4 eV, as monitored by laser desorption ionization (LDI) mass spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) gives insight into the mechanisms of C–Br scission and C–C coupling processes. Further, unreacted material can be sublimed and the coupled sp²-carbon precursors can be graphitized by e-beam treatment at 500 °C, demonstrating promising applications in photolithography of graphene nanoarchitectures. Our results present UV-induced reactions on insulators for the formation of all sp²-carbon architectures, thereby converging top-down lithography and bottom-up on-surface chemistry into technology