16 research outputs found
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 20ā150
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>āAminothiophenol to <i>p</i>,<i>p</i>ā²āDimercaptoazobenzene on Gold Nanoparticle: Surface-Enhanced Raman Scattering Investigation and Potential for Nitrite Detection
The stunning large enhancement factor
(ā¼10<sup>8</sup>)
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 āb<sub>2</sub>ā bands (ca. 1140,1390, 1432 cm<sup>ā1</sup>) 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>ā²-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 NO<sub>2</sub><sup>ā</sup> 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 NO<sub>2</sub><sup>ā</sup> (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 (H<sub>2</sub>S) 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 H<sub>2</sub>S-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 H<sub>2</sub>S. Upon target
introduction, Au@TPt-NCs were deactivated to different degrees dependent
on H<sub>2</sub>S 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 H<sub>2</sub>S 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 (Īø<sub>Pt</sub> = 2) were superior for H<sub>2</sub>S-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 H<sub>2</sub>S with a wide linear range
of 10ā100 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 H<sub>2</sub>S-spiked real samples, and the recovery
in all cases was 91.6ā106.7%. 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<sup>ā1</sup>. 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
B<sub>1</sub>, AFB<sub>1</sub>, 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 AFB<sub>1</sub>ābovine serum
albumin (AFB<sub>1</sub>āBSA) conjugates as the tags. Then,
the carried AgNPs with AFB<sub>1</sub>āBSA 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 AFB<sub>1</sub> in a dynamic working range
from 10 pg mL<sup>ā1</sup> to 15 ng mL<sup>ā1</sup> at
a low limit of detection (LOD) of 3.0 pg mL<sup>ā1</sup> 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 N<sub>2</sub>O in the Off-Gas of Wastewater Treatment Plants
Different
configurations of photoacoustic (PA) setups for the online-measurement
of gaseous N<sub>2</sub>O, employing semiconductor lasers at 2.9 and
4.5 Ī¼m, were developed and tested. Their performance was assessed
with respect to the analysis of N<sub>2</sub>O emissions from wastewater
treatment plants. For this purpose, the local N<sub>2</sub>O emissions
of a wastewater treatment bioreactor was sampled by a dedicated mobile
sampling device, and the total N<sub>2</sub>O 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 Ī¼m, 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>ā<i>T</i> relaxation. This observation in combination with the limited
dynamic range (max conc. ā¼ 3000 ppmv) led us to the use of
the less-sensitive but spectroscopically more robust 2.9 Ī¼m
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 N<sub>2</sub>O 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 colloidābacterial suspension.
The total assay time required is only 10 min and the total reactantsā
volume needed to analyze bacteria in a real environment is as low
as 1 mL. Particularly, only one droplet of 3 Ī¼L 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 Ć 10<sup>2</sup> 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 A<sub>10</sub>[Ge<sub>9</sub>]<sub>2</sub>[WO<sub>4</sub>] As Well As A<sub>10+<i>x</i></sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>1ā<i>x</i></sub>Nb<sub><i>x</i></sub>O<sub>4</sub>] with A = K and Rb Containing [Ge<sub>9</sub>]<sup>4ā</sup> Polyanions
Semitransparent dark-red or ruby-red moisture- and air-sensitive
single crystals of A<sub>10+<i>x</i></sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>1ā<i>x</i></sub>Nb<sub><i>x</i></sub>O<sub>4</sub>] (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>2<sub>1</sub><i>/c</i>, <i>Z</i> = 4) with <i>a</i> = 13.908(1) Ć
, <i>b</i> = 15.909(1) Ć
, <i>c</i> = 17.383(1) Ć
,
and Ī² = 90.050(6)Ā° for K<sub>10.35(1)</sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>0.65(1)</sub>Nb<sub>0.35(1)</sub>O<sub>4</sub>]; <i>a</i> = 14.361(3) Ć
, <i>b</i> = 16.356(3)
Ć
, <i>c</i> = 17.839(4) Ć
, and Ī² = 90.01(3)Ā°
for Rb<sub>10.35(1)</sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>0.65(1)</sub>Nb<sub>0.35(1)</sub>O<sub>4</sub>]; <i>a</i> = 13.8979(2)
Ć
, <i>b</i> = 15.5390(3) Ć
, <i>c</i> = 17.4007(3) Ć
, and Ī² = 90.188(1)Ā° for K<sub>10</sub>[Ge<sub>9</sub>]<sub>2</sub>WO<sub>4</sub>; and <i>a</i> = 14.3230(7) Ć
, <i>b</i> = 15.9060(9) Ć
, <i>c</i> = 17.8634(9) Ć
, and Ī² = 90.078(4)Ā° for
Rb<sub>10</sub>[Ge<sub>9</sub>]<sub>2</sub>WO<sub>4</sub>. The compounds
contain discrete Ge<sub>9</sub><sup>4ā</sup> Wadeās <i>nido</i> clusters and WO<sub>4</sub><sup>2ā</sup> (or
NbO<sub>4</sub><sup>3ā</sup>) anions, which are packed according
to a hierarchical atom-to-cluster replacement of the Al<sub>2</sub>Cu prototype and are separated by K and Rb cations, respectively.
The alkali metal atoms occupy the corresponding tetrahedral sites
of the Al<sub>2</sub>Cu 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 [Ge<sub>9</sub>]<sup>4ā</sup> 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 A<sub>10</sub>[Ge<sub>9</sub>]<sub>2</sub>[WO<sub>4</sub>] As Well As A<sub>10+<i>x</i></sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>1ā<i>x</i></sub>Nb<sub><i>x</i></sub>O<sub>4</sub>] with A = K and Rb Containing [Ge<sub>9</sub>]<sup>4ā</sup> Polyanions
Semitransparent dark-red or ruby-red moisture- and air-sensitive
single crystals of A<sub>10+<i>x</i></sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>1ā<i>x</i></sub>Nb<sub><i>x</i></sub>O<sub>4</sub>] (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>2<sub>1</sub><i>/c</i>, <i>Z</i> = 4) with <i>a</i> = 13.908(1) Ć
, <i>b</i> = 15.909(1) Ć
, <i>c</i> = 17.383(1) Ć
,
and Ī² = 90.050(6)Ā° for K<sub>10.35(1)</sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>0.65(1)</sub>Nb<sub>0.35(1)</sub>O<sub>4</sub>]; <i>a</i> = 14.361(3) Ć
, <i>b</i> = 16.356(3)
Ć
, <i>c</i> = 17.839(4) Ć
, and Ī² = 90.01(3)Ā°
for Rb<sub>10.35(1)</sub>[Ge<sub>9</sub>]<sub>2</sub>[W<sub>0.65(1)</sub>Nb<sub>0.35(1)</sub>O<sub>4</sub>]; <i>a</i> = 13.8979(2)
Ć
, <i>b</i> = 15.5390(3) Ć
, <i>c</i> = 17.4007(3) Ć
, and Ī² = 90.188(1)Ā° for K<sub>10</sub>[Ge<sub>9</sub>]<sub>2</sub>WO<sub>4</sub>; and <i>a</i> = 14.3230(7) Ć
, <i>b</i> = 15.9060(9) Ć
, <i>c</i> = 17.8634(9) Ć
, and Ī² = 90.078(4)Ā° for
Rb<sub>10</sub>[Ge<sub>9</sub>]<sub>2</sub>WO<sub>4</sub>. The compounds
contain discrete Ge<sub>9</sub><sup>4ā</sup> Wadeās <i>nido</i> clusters and WO<sub>4</sub><sup>2ā</sup> (or
NbO<sub>4</sub><sup>3ā</sup>) anions, which are packed according
to a hierarchical atom-to-cluster replacement of the Al<sub>2</sub>Cu prototype and are separated by K and Rb cations, respectively.
The alkali metal atoms occupy the corresponding tetrahedral sites
of the Al<sub>2</sub>Cu 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 [Ge<sub>9</sub>]<sup>4ā</sup> 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