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

Photoacoustic Signal Generation in Gold Nanospheres in Aqueous Solution: Signal Generation Enhancement and Particle Diameter Effects

Gold nanoparticles can be used as an exogenous contrast agent for biomedical photoacoustic (PA) imaging. The generation of PA signals in monodispersed gold nanosphere suspensions (diameters 20–150 nm) from pulsed-laser excitation (5 ns pulse width, wavelength 532 nm) was investigated experimentally and compared to signals measured in solutions of a homogeneous molecular absorber. The PA signal amplitude was found to increase linearly with excitation fluence for the homogeneous absorber and the nanospheres up to 80 nm in diameter. By contrast, the signal amplitude was found to increase quadratically with respect to fluence for larger nanospheres. In the linear regime, the PA signal amplitude in gold nanosphere suspensions was found to be on average 26% higher than that in the homogeneous absorber with identical absorption coefficient, which were measured using an integrating sphere. Furthermore, in suspensions with identical absorption coefficient, no dependence of the PA signal amplitude on nanosphere diameter was found in the linear regime, entailing that suspensions with identical extinction coefficient display a decreasing trend in PA signal amplitude with increasing nanosphere diameter due to increasing contribution of scattering. This study presents experimental evidence of some of the physical phenomena governing the photoacoustic signal generation in gold nanosphere suspensions, which may inform on approaches to molecular biomedical PA imaging

Nitrite-Triggered Surface Plasmon-Assisted Catalytic Conversion of <i>p</i>‑Aminothiophenol to <i>p</i>,<i>p</i>′‑Dimercaptoazobenzene on Gold Nanoparticle: Surface-Enhanced Raman Scattering Investigation and Potential for Nitrite Detection

The stunning large enhancement factor (∼10⁸) of the surface-enhanced Raman scattering (SERS) effect leads people to wonder about the underlying enhancement mechanisms of the effect. But, a strong evidence of the existence of one commonly accepted mechanism (chemical enhancement), the origin of the symbolic “b₂” bands (ca. 1140,1390, 1432 cm^–1) of <i>p</i>-aminothiophenol (PATP), was recently found to be a false explanation, which were actually arisen from the product of a surface plasmon-assisted coupling reaction of PATP, <i>p</i>,<i>p</i>′-dimercaptoazobenzene (DMAB). However, the debate is far from over, especially because the mechanism of the above reaction has not been fully understood yet. In this paper, we for the first time report a new surface plasmon-assisted catalytic conversion of PATP to DMAB that NO₂^– ions can trigger the formation of DMAB on gold nanoparticles (GNPs) suspension under light illumination. The mechanism of the conversion is also discussed. All relevant data suggest the nitrite-triggered conversion of PATP to DMAB on GNPs is a surface plasmon-assisted oxidation reaction, involving transfer of multiple electrons from PATP to NO₂^– (electron acceptors) and protons, leading to the formation of DMAB. The proposed mechanisms may also help to understand the unclear surface plasmon-assisted catalytic coupling of PATP on the SERS substrates. Furthermore, inspired by the high selectivity of the above nitrite-triggered catalysis reaction, a simple and fast nitrite screening method was also developed, exhibiting good sensitivity. Considering other advantages of the assay, such as rapidness, simplicity of the detection procedures, and requirement of no sample pretreatment, it is a promising method for on-site fast screening or point-of-care application

Target-Induced Nanocatalyst Deactivation Facilitated by Core@Shell Nanostructures for Signal-Amplified Headspace-Colorimetric Assay of Dissolved Hydrogen Sulfide

Colorimetric assay platforms for dissolved hydrogen sulfide (H₂S) have been developed for more than 100 years, but most still suffer from relatively low sensitivity. One promising route out of this predicament relies on the design of efficient signal amplification methods. Herein, we rationally designed an unprecedented H₂S-induced deactivation of (gold core)@(ultrathin platinum shell) nanocatalysts (Au@TPt-NCs) as a highly efficient signal amplification method for ultrasensitive headspace-colorimetric assay of dissolved H₂S. Upon target introduction, Au@TPt-NCs were deactivated to different degrees dependent on H₂S levels, and the degrees could be indicated by using a Au@TPt-NCs-triggered catalytic system as a signal amplifier, thus paving a way for H₂S sensing. The combination of experimental studies and density functional theory (DFT) studies revealed that the Au@TPt-NCs with only 2-monolayer equivalents of Pt (θ_Pt = 2) were superior for H₂S-induced nanocatalyst deactivation owing to their enhanced peroxidase-like catalytic activity and deactivation efficiency stemmed from the unique synergistic structural/electronic effects between Au nanocores and ultrathin Pt nanoshells. Importantly, our analytical results showed that the designed method was indeed highly sensitive for sensing H₂S with a wide linear range of 10–100 nM, a slope of 0.013 in the regression equation, and a low detection limit of 7.5 nM. Also the selectivity, reproducibility, and precision were excellent. Furthermore, the method was validated for the analysis of H₂S-spiked real samples, and the recovery in all cases was 91.6–106.7%. With the merits of high sensitivity and selectivity, simplification, low cost, and visual readout with the naked eye, the colorimetric method has the potential to be utilized as an effective detection kit for point-of-care testing

DNA-Based Hybridization Chain Reaction for Amplified Bioelectronic Signal and Ultrasensitive Detection of Proteins

This work reports a novel electrochemical immunoassay protocol with signal amplification for determination of proteins (human IgG here used as a model target analyte) at an ultralow concentration using DNA-based hybridization chain reaction (HCR). The immuno-HCR assay consists of magnetic immunosensing probes, nanogold-labeled signal probes conjugated with the DNA initiator strands, and two different hairpin DNA molecules. The signal is amplified by the labeled ferrocene on the hairpin probes. In the presence of target IgG, the sandwiched immunocomplex can be formed between the immobilized antibodies on the magnetic beads and the signal antibodies on the gold nanoparticles. The carried DNA initiator strands open the hairpin DNA structures in sequence and propagate a chain reaction of hybridization events between two alternating hairpins to form a nicked double-helix. Numerous ferrocene molecules are formed on the neighboring probe, each of which produces an electrochemical signal within the applied potentials. Under optimal conditions, the immuno-HCR assay presents good electrochemical responses for determination of target IgG at a concentration as low as 0.1 fg mL^–1. Importantly, the methodology can be further extended to the detection of other proteins or biomarkers

Silver Nanolabels-Assisted Ion-Exchange Reaction with CdTe Quantum Dots Mediated Exciton Trapping for Signal-On Photoelectrochemical Immunoassay of Mycotoxins

Mycotoxins, highly toxic secondary metabolites produced by many invading species of filamentous fungi, contaminate different agricultural commodities under favorable temperature and humidity conditions. Herein, we successfully devised a novel signal-on photoelectrochemical immunosensing platform for the quantitative monitoring of mycotoxins (aflatoxin B₁, AFB₁, used as a model) in foodstuffs on the basis of silver nanolabels-assisted ion-exchange reaction with CdTe quantum dots (QDs) mediated hole-trapping. Initially, a competitive-type immunoreaction was carried out on a high-binding microplate by using silver nanoparticle (AgNP)-labeled AFB₁–bovine serum albumin (AFB₁–BSA) conjugates as the tags. Then, the carried AgNPs with AFB₁–BSA were dissolved by acid to release numerous silver ions, which could induce ion-exchange reaction with the CdTe QDs immobilized on the electrode, thus resulting in formation of surface exciton trapping. Relative to pure CdTe QDs, the formed exciton trapping decreased the photocurrent of the modified electrode. In contrast, the detectable photocurrent increased with the increase of target AFB₁ in a dynamic working range from 10 pg mL^–1 to 15 ng mL^–1 at a low limit of detection (LOD) of 3.0 pg mL^–1 under optimal conditions. In addition, the as-prepared photoelectrochemical immunosensing platform also displayed high specificity, good reproducibility, and acceptable method accuracy for detecting naturally contaminated/spiked blank peanut samples with consistent results obtained from the referenced enzyme-linked immunosorbent assay (ELISA) method

Photoacoustic Spectroscopy for the Quantification of N₂O in the Off-Gas of Wastewater Treatment Plants

Different configurations of photoacoustic (PA) setups for the online-measurement of gaseous N₂O, employing semiconductor lasers at 2.9 and 4.5 μm, were developed and tested. Their performance was assessed with respect to the analysis of N₂O emissions from wastewater treatment plants. For this purpose, the local N₂O emissions of a wastewater treatment bioreactor was sampled by a dedicated mobile sampling device, and the total N₂O emissions were analyzed in the gastight headspace of the bioreactor. We found that the use of a quantum-cascade laser emitting at about 4.53 μm, operated in a wavelength modulation mode, in combination with a conventional longitudinal PA cell yielded the highest sensitivity (<100 ppbv). However, we also observed a strong cross-sensitivity to humidity, which can be explained by increased <i>V</i>–<i>T</i> relaxation. This observation in combination with the limited dynamic range (max conc. ∼ 3000 ppmv) led us to the use of the less-sensitive but spectroscopically more robust 2.9 μm laser. A detection limit below 1 ppmv, a dynamic range of more than 4 orders of magnitude, no influence of humidity or any other substance relevant to the off-gas analysis, as well as a comparable low price of the laser source made it the ideal tool for N₂O analyses of the off-gas of a wastewater treatment plant. Such a system was implemented successfully in a full-scale wastewater treatment plant. The results regarding the comparison of different PA setups can be transferred to other systems, and the optimum performance can be selected according to the specific demands

SERS Detection of Bacteria in Water by in Situ Coating with Ag Nanoparticles

The bio-sensing for the convenient detection of bacteria has been widely explored with the use of various sensing materials and techniques. It is still a challenge to achieve an ultrasensitive and selective, but simple, rapid, and inexpensive detection method for bacteria. We report on surface-enhanced Raman scattering (SERS) for the detection of living bacteria in drinking water by employing a synthesis of silver nanoparticles coating the cell wall of bacteria. We found that the Raman signals intensity of bacteria after AgNP synthesis mainly depends on the zeta potential of the cell wall. The enhancement of the Raman signal of bacteria using this strategy is about 30-fold higher than that in the case of a simply mixed colloid–bacterial suspension. The total assay time required is only 10 min and the total reactants’ volume needed to analyze bacteria in a real environment is as low as 1 mL. Particularly, only one droplet of 3 μL sample is necessary for each SERS measurement. Furthermore, we can use this novel strategy to discriminate three strains of Escherichia coli and one strain of Staphylococcus epidermidis by hierarchy cluster analysis. Finally, we can detect bacteria down to 2.5 × 10² cells/mL on a hydrophobic glass slide by SERS mapping. Thus, our detection method offers prominent advantages, such as reduced assay time, simple handling, low reactant volumes, small amount of sample, and higher sensitivity and selectivity compared to previously reported label free methods. This novel strategy may be extended to open an avenue for developing various SERS-based biosensors

Extreme Differences in Oxidation States: Synthesis and Structural Analysis of the Germanide Oxometallates A₁₀[Ge₉]₂[WO₄] As Well As A_10+<i>x</i>[Ge₉]₂[W_1–<i>x</i>Nb_<i>x</i>O₄] with A = K and Rb Containing [Ge₉]^4– Polyanions

Semitransparent dark-red or ruby-red moisture- and air-sensitive single crystals of A_10+<i>x</i>[Ge₉]₂[W_1–<i>x</i>Nb_<i>x</i>O₄] (A = K, Rb; <i>x</i> = 0, 0.35) were obtained by high-temperature solid-state reactions. The crystal structure of the compounds was determined by single-crystal X-ray diffraction experiments. They crystallize in a new structure type (<i>P</i>2₁<i>/c</i>, <i>Z</i> = 4) with <i>a</i> = 13.908(1) Å, <i>b</i> = 15.909(1) Å, <i>c</i> = 17.383(1) Å, and β = 90.050(6)° for K_10.35(1)[Ge₉]₂[W_0.65(1)Nb_0.35(1)O₄]; <i>a</i> = 14.361(3) Å, <i>b</i> = 16.356(3) Å, <i>c</i> = 17.839(4) Å, and β = 90.01(3)° for Rb_10.35(1)[Ge₉]₂[W_0.65(1)Nb_0.35(1)O₄]; <i>a</i> = 13.8979(2) Å, <i>b</i> = 15.5390(3) Å, <i>c</i> = 17.4007(3) Å, and β = 90.188(1)° for K₁₀[Ge₉]₂WO₄; and <i>a</i> = 14.3230(7) Å, <i>b</i> = 15.9060(9) Å, <i>c</i> = 17.8634(9) Å, and β = 90.078(4)° for Rb₁₀[Ge₉]₂WO₄. The compounds contain discrete Ge₉^4– Wade’s <i>nido</i> clusters and WO₄^2– (or NbO₄^3–) anions, which are packed according to a hierarchical atom-to-cluster replacement of the Al₂Cu prototype and are separated by K and Rb cations, respectively. The alkali metal atoms occupy the corresponding tetrahedral sites of the Al₂Cu prototype. The amount of the alkali metal atoms on these diamagnetic compounds corresponds directly to the amount of W substituted by Nb. Thus, the transition metals W and Nb appear with oxidation numbers +6 and +5, respectively, in the vicinity of a [Ge₉]^4– polyanion. The crystals of the mixed salts were further characterized by Raman spectroscopy. The Raman data are in good agreement with the results from the X-ray structural analyses

Extreme Differences in Oxidation States: Synthesis and Structural Analysis of the Germanide Oxometallates A₁₀[Ge₉]₂[WO₄] As Well As A_10+<i>x</i>[Ge₉]₂[W_1–<i>x</i>Nb_<i>x</i>O₄] with A = K and Rb Containing [Ge₉]^4– Polyanions

Semitransparent dark-red or ruby-red moisture- and air-sensitive single crystals of A_10+<i>x</i>[Ge₉]₂[W_1–<i>x</i>Nb_<i>x</i>O₄] (A = K, Rb; <i>x</i> = 0, 0.35) were obtained by high-temperature solid-state reactions. The crystal structure of the compounds was determined by single-crystal X-ray diffraction experiments. They crystallize in a new structure type (<i>P</i>2₁<i>/c</i>, <i>Z</i> = 4) with <i>a</i> = 13.908(1) Å, <i>b</i> = 15.909(1) Å, <i>c</i> = 17.383(1) Å, and β = 90.050(6)° for K_10.35(1)[Ge₉]₂[W_0.65(1)Nb_0.35(1)O₄]; <i>a</i> = 14.361(3) Å, <i>b</i> = 16.356(3) Å, <i>c</i> = 17.839(4) Å, and β = 90.01(3)° for Rb_10.35(1)[Ge₉]₂[W_0.65(1)Nb_0.35(1)O₄]; <i>a</i> = 13.8979(2) Å, <i>b</i> = 15.5390(3) Å, <i>c</i> = 17.4007(3) Å, and β = 90.188(1)° for K₁₀[Ge₉]₂WO₄; and <i>a</i> = 14.3230(7) Å, <i>b</i> = 15.9060(9) Å, <i>c</i> = 17.8634(9) Å, and β = 90.078(4)° for Rb₁₀[Ge₉]₂WO₄. The compounds contain discrete Ge₉^4– Wade’s <i>nido</i> clusters and WO₄^2– (or NbO₄^3–) anions, which are packed according to a hierarchical atom-to-cluster replacement of the Al₂Cu prototype and are separated by K and Rb cations, respectively. The alkali metal atoms occupy the corresponding tetrahedral sites of the Al₂Cu prototype. The amount of the alkali metal atoms on these diamagnetic compounds corresponds directly to the amount of W substituted by Nb. Thus, the transition metals W and Nb appear with oxidation numbers +6 and +5, respectively, in the vicinity of a [Ge₉]^4– polyanion. The crystals of the mixed salts were further characterized by Raman spectroscopy. The Raman data are in good agreement with the results from the X-ray structural analyses