Ultra-low Concentration Research Articles

Vanadium oxide nanostructured thin films prepared by Aerosol Spray Pyrolysis for gas sensing and thermochromic applications

Vanadium oxide (VxOy) nanostructured films were prepared by Aerosol Spray Pyrolysis technique with different concentrations of precursor solution (0.005 M, 0.01 M, 0.02 M), using ammonium metavanadate (NH4VO3) as precursor. The deposition was carried out onto glass substrates (Corning glass) at two different substrate temperatures (300 °C and 400 °C). Furthermore, oxalic acid was added to the precursor solution in order to grow the thermochromic VO2(M) monoclinic phase via reduction of the Vanadium oxides in which Vanadium is in a higher oxidizing state. All films were characterized by X-Ray Diffraction technique and UV/Vis/NIR spectroscopy to identify their structural and optical properties, respectively. V2O5 films were tested against ozone, having a sensitivity of 29% in ultra-low concentration of 5ppb ozone, at room temperature. Moreover, nanostructured VO2 films appeared to be thermochromic, having an IR-switching of about 5% at λ = 2500 nm, upon heating.

Dehydrogenation characteristics of lean methane in a thermal reverse-flow reactor

In order to study the dehydrogenation reaction mechanism of ultra-low concentration methane in a thermal reverse-flow reactor, the effects of the cyclic period (120s–240s), the lean methane volume flow (90 Nm3/h to 180 Nm3/h), and the methane concentration (0.2 vol% to 0.8 vol%) on the dehydrogenation performance were studied systematically by using a thermal reverse-flow experimental system. When the methane concentration is 0.2 vol%, the reactor can achieve self-heat maintaining operation. With the increase in the methane concentration, the width of the high-temperature zone, the exhaust gas temperature, the methane conversion rate, and the maximum temperature of the heat-accumulator bed increase. With the increase in the lean methane volume flow, the width of the high-temperature zone, the distance between the center of the high-temperature zone and the center of the reactor, the maximum temperature, the exhaust gas temperature, and the methane conversion rate increase. With the increase in the cyclic period, the exhaust gas temperature and the deviation of the high-temperature zone increase, but the methane conversion rate and the maximum temperature decrease slightly.

Micro-fabricated packed metal gas preconcentrator for enhanced monitoring of ultralow concentration of isoprene

A novel non-silicon-based micro-preconcentration device, as a pretreatment component in a portable gas chromatography system, was developed for the preconcentration one of the trace volatile organic compounds (VOCs) in the exhaled gases, which is one typical biomarker for the chronic liver disease (CLD). The device was designed as an array of manifold-shaped rectangular metal micro-channels with flat dimensions of 16 mm × 12.6 mm and the internal empty volume is 14.4 μL on the copper substrate. Instead of the non-silicon fabrication process, the traditional laser etching technology (LET) was optimized to etch micro-channels, and vacuum diffusion welding (VDW) was applied to form internal channels. The fabricated chip was filled with Carbopack X adsorbent. In the testing, the metal gas preconcentrator (MGP) was installed in a commercial GC (gas chromatography) and nitrogen was used as carrier gas and desorbed gas. With the MPG, up to 352 of concentration factor can be achieved for 10 ppb isoprene. The developed MGP, which has advantages of high strength, low cost, good thermal conductivities, can potentially be used for non-invasive screening of advanced liver fibrosis by monitoring isoprene concentrations in exhaled breath.

Heterogeneous and cross-distributed metal structure hybridized with MoS2 as high-performance flexible SERS substrate

The heterogeneous metal nanostructures have attracted great interest in various applications due to the synergistic effects between two noble metals, especially in surface enhanced Raman scattering (SERS) region. Herein, we prepared a 3D SERS active substrate based on heterogeneous and cross-distributed metal structure hybridized with MoS2by in situ synthesizing gold nanoparticles (AuNPs) on MoS2 membrane. The AuNPs-AgNPs/MoS2/P-Si hybrid SERS substrate were characterized by a scanning electron microscope (SEM), a transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) to investigate the character and the content of elements. In virtue of the heterogeneous and cross-distributed structure and ultra-narrow interparticle gap generating strong electric fields enhancement, the ultra-low concentration of probe molecule were detected (the LOD of 10-12 M for R6G and CV, 10-11 M for MG), serving the optimal SERS performance. The excellent uniformity and reproducibility were achieved by the proposed substrate. Moreover, the flexible MoS2/AuNPs-AgNPs/PMMA pyramidal SERS substrate was applied to detect melamine molecule in liquid milk (the LOD reached 10-9 M), which revealed great potential to be an outstanding SERS substrate for biological and chemical detection.

Effect of salt concentration in spinning solution on fiber diameter and mechanical property of electrospun styrene-butadiene-styrene tri-block copolymer membrane

Electrospun nanofiber is regarded as a promising candidate for many high-end applications due to its advantageous unique structure. Ultrafine fiber and good mechanical property are essential to ensure its performance. In this work, tunable ultrafine diameter and significantly enhanced mechanical strength of electrospun Styrene-butadiene-styrene tri-block copolymer (SBS) membrane are attained by means of the addition of LiBr into SBS solution dissolved in tetrahydrofuran/dimethylformamide (THF/DMF). Solution conductivity is found to grow in two steps with the increasing lithium bromide (LiBr) concentration and a critical point is observed at the ultralow concentration of 0.005 wt%, sharp increase below it but modest increase above it. Interestingly, the diameter of electrospun fiber shows a significant reduction with increase of LiBr concentration below this critical point but minor fluctuation above this. Welded structure is observed between adjacent fibers and the degree of welding goes deeper with increase of salt concentration. Accordingly, the mechanical strength of electrospun membrane is gradually enhanced and reaches to the highest value of 5.7 MPa at the critical concentration, with enhancement of 134% in comparison with pure electrospun SBS membrane, and then declines when further adding LiBr. TEM observation confirms more oriented and distinct self-assembly domains in the finer fibers, which contribute greatly to the property enhancement for electrospun SBS membrane obtained below critical concentration. The finding of critical concentration is important for the preparation of high performance electrospun membrane by adding salt in the spinning solution.

Detection of Ultralow Concentration NO2 in Complex Environment Using Epitaxial Graphene Sensors.

We demonstrate proof-of-concept graphene sensors for environmental monitoring of ultralow concentration NO2 in complex environments. Robust detection in a wide range of NO2 concentrations, 10-154 ppb, was achieved, highlighting the great potential for graphene-based NO2 sensors, with applications in environmental pollution monitoring, portable monitors, automotive and mobile sensors for a global real-time monitoring network. The measurements were performed in a complex environment, combining NO2/synthetic air/water vapor, traces of other contaminants, and variable temperature in an attempt to fully replicate the environmental conditions of a working sensor. It is shown that the performance of the graphene-based sensor can be affected by coadsorption of NO2 and water on the surface at low temperatures (≤70 °C). However, the sensitivity to NO2 increases significantly when the sensor operates at 150 °C and the cross-selectivity to water, sulfur dioxide, and carbon monoxide is minimized. Additionally, it is demonstrated that single-layer graphene exhibits two times higher carrier concentration response upon exposure to NO2 than bilayer graphene.

Rapid and Highly Efficient Detection of Ultra-low Concentration of Penicillin G by Gold Nanoparticles/Porous Silicon SERS Active Substrate

A new method for the detection of ultra-low concentration of penicillin G (PG) antibiotic was innovated using gold nanoparticles/porous silicon (AuNPs/PSi) SERS active substrate. PSi was employed as a template and a reducing agent to deposited aggregated AuNPs with a high density of hot spot regions. A highly enhancement performance and reproducible AuNPs/PSi SERS substrate was fabricated. The AuNPs/PSi SERS substrate was investigated for detection PG drug at different concentrations ranging from 10−3 to 10−9 M. The results showed that the proposed AuNPs/PSi SERS substrate provide an efficient way for detection the lowest concentration of PG. The adsorption process of the PG drug on the surface of the AuNPs/PSi was investigated, it was found that PG was close to the surface of AuNPs/PSi through the carboxylate group. The enhancement factor (EF) of 5 × 107 and a 4.5% relative standard deviation of reproducibility were obtained at an ultra-low PG concentration of 10−9 M. The influence of the pH value on the EF for PG antibiotic under acidic and alkaline conditions (EFpH) was studied, it was shown that the highest EFpH with efficient activity toward PG was obtained at pH = 5.

Large-Scale Fabrication of Ultrasensitive and Uniform Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Pesticides.

Technology transfer from laboratory into practical application needs to meet the demands of economic viability and operational simplicity. This paper reports a simple and convenient strategy to fabricate large-scale and ultrasensitive surface-enhanced Raman scattering (SERS) substrates. In this strategy, no toxic chemicals or sophisticated instruments are required to fabricate the SERS substrates. On one hand, Ag nanoparticles (NPs) with relatively uniform size were synthesized using the modified Tollens method, which employs an ultra-low concentration of Ag+ and excessive amounts of glucose as a reducing agent. On the other hand, when a drop of the colloidal Ag NPs dries on a horizontal solid surface, the droplet becomes ropy, turns into a layered structure under gravity, and hardens. During evaporation, capillary flow was burdened by viscidity resistance from the ropy glucose solution. Thus, the coffee-ring effect is eliminated, leading to a uniform deposition of Ag NPs. With this method, flat Ag NPs-based SERS active films were formed in array-well plates defined by hole-shaped polydimethylsiloxane (PDMS) structures bonded on glass substrates, which were made for convenient detection. The strong SERS activity of these substrates allowed us to reach detection limits down to 10−14 M of Rhodamine 6 G and 10−10 M of thiram (pesticide).

Specific Two-Photon Imaging of Live Cellular and Deep-Tissue Lipid Droplets by Lipophilic AIEgens at Ultralow Concentration

Lipid droplets are highly associated with obesity, diabetes, inflammatory disorders, and cancer. A reliable two-photon dye for specific lipid droplets imaging in live cells and live tissues at ultralow concentration has rarely been reported. In this work, four new aggregation-induced emission luminogens (AIEgens) based on the naphthalene core were designed and synthesized for specific two-photon lipid droplet staining. The new molecules, namely, NAP AIEgens, exhibit large Stokes shift (>110 nm), high solid-state fluorescence quantum yield (up to 30%), good two-photon absorption cross section (45–100 GM at 860 nm), high biocompatibility, and good photostability. They could specifically stain lipid droplets at ultralow concentration (50 nM) in a short time of 15 min. Such ultralow concentration is the lowest value for lipid droplets staining in live cells reported so far. In vitro and ex vivo two-photon imaging of lipid droplets in live cells and live mice liver tissues were successfully demonstrated. In ad...

Tetracycline@silver ions-functionalized mesoporous silica for high bactericidal activity at ultra-low concentration.

Aim: Polyether pores were designed and tetracycline-loaded mesoporous silica materials, with their surface decorated by silver ions, were prepared, with the aim of reaching high antibacterial activity. Meanwhile, mammalian cell cytotoxicity and hemolytic effects were not observed using material concentrations tenfold the ones optimized for the bactericidal tests. Methods: Pore size was tuned by changing the polyether content and the surface was covalently decorated with silver thiolate groups. Results: We showed that the biological activity was enhanced by modulating silver ions and tetracycline releases by tuning silver thiolate group concentration on the silica surface and/or by modulating the pH of the environment. Conclusion: The combined use of tetracycline and silver ions with the mesoporous drug-delivery carrier was a very effective strategy against susceptible and tetracycline-resistant Escherichia coli bacteria.

Superhydrophobic Ag–Cu Composite Metal Film as Surface-Enhanced Raman Scattering Substrate

In this paper, a superhydrophobic Ag–Cu composite metal film was fabricated as surface-enhanced Raman scattering substrate. Ag–Cu composite metal film was prepared from galvanic displacement on commercial copper foil. The prepared silver films were modified by PFDT to create a stable superhydrophobic SERS platform. Superhydrophobic substrates were characterized by a variety of characterization methods, including scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-Vis, XPS and Raman spectra. The superhydrophobic substrate could enrich probing molecules with the volatilization of solvent. Mathematical model was presented and applied to investigate concentration factor of hydrophobic substrate. The measurements were in good agreement with the calculation results. The superhydrophobic Ag–Cu composite metal film can reduce the detection limits and provide a new way to prepare efficient SERS substrates for ultra-low concentration detection.

1T' Transition Metal Telluride Atomic Layers for Plasmon-Free SERS at Femtomolar Levels.

Plasmon-free surface enhanced Raman scattering (SERS) based on the chemical mechanism (CM) is drawing great attention due to its capability for controllable molecular detection. However, in comparison to the conventional noble-metal-based SERS technique driven by plasmonic electromagnetic mechanism (EM), the low sensitivity in the CM-based SERS is the dominant barrier toward its practical applications. Herein, we demonstrate the 1T' transition metal telluride atomic layers (WTe2 and MoTe2) as ultrasensitive platforms for CM-based SERS. The SERS sensitivities of analyte dyes on 1T'-W(Mo)Te2 reach EM-comparable ones and become even greater when it is integrated with a Bragg reflector. In addition, the dye fluorescence signals are efficiently quenched, making the SERS spectra more distinguishable. As a proof of concept, the SERS signals of analyte Rhodamine 6G (R6G) are detectable even with an ultralow concentration of 40 (400) fM on pristine 1T'-W(Mo)Te2, and the corresponding Raman enhancement factor (EF) reaches 1.8 × 109 (1.6 × 108). The limit concentration of detection and the EF of R6G can be further enhanced into 4 (40) fM and 4.4 × 1010 (6.2 × 109), respectively, when 1T'-W(Mo)Te2 is integrated on the Bragg reflector. The strong interaction between the analyte and 1T'-W(Mo)Te2 and the abundant density of states near the Fermi level of the semimetal 1T'-W(Mo)Te2 in combination gives rise to the promising SERS effects by promoting the charge transfer resonance in the analyte-telluride complex.

DNA-Conjugated Amphiphilic Aggregation-Induced Emission Probe for Cancer Tissue Imaging and Prognosis Analysis.

Detection of an ultralow concentration of mRNA is important in the prognosis of gene-related diseases. In this study, a DNA-conjugated amphiphilic aggregation-induced emission probe (TPE-R-DNA) was synthesized for cancer tissue imaging and prognosis analysis based on an exonuclease III-aided target recycling technique. TPE-R-DNA comprise two components: a hydrophobic component that serves as the "turn-on" long wavelength fluorescence imaging agent (TPE-R-N3); and a hydrophilic single DNA strand (Alk-DNA) which acts as specific recognition part for target mRNA. In the absence of target mRNA, TPE-R-DNA had almost no fluorescence because of its high water solubility. Conversely, the TPE-R-DNA was digested by exonuclease III (Exo III) in the presence of MnSOD mRNA to release the hydrophobic fluorogens (TPE-R-AT). Subsequently, TPE-R-AT formed aggregates, and therefore, fluorescence signal was distinctly observed. For the first time, the structure of the hydrolysis product (TPE-R-AT), containing two bases A and T, was proved by the mass spectrum (MS) and high-performance liquid chromatography (HPLC). Moreover, the detection limit toward mRNA could be achieved in as low as 0.6 pM. Furthermore, the fluorescent signal can be used to confirm the MnSOD mRNA expression level in cancer tissue. The MnSOD mRNA expression in renal cancer was lower than in renal cancer adjacent tissue. In particular, the expression level was analyzed to predict prognosis of cancer patients. Our results demonstrate that a shorter survival time was evident among patients in lower MnSOD mRNA expression. Thereby, it indicates great potential for the development of an ultrasensitive biosensing platform for the application in disease prognosis.

Emulsion-Assisted Polymerization-Induced Hierarchical Self-Assembly of Giant Sea Urchin-like Aggregates on a Large Scale.

Hierarchical solution self-assembly has become an important biomimetic method to prepare highly complex and multifunctional supramolecular structures. However, despite great progress, it is still highly challenging to prepare hierarchical self-assemblies on a large scale because the self-assembly processes are generally performed at high dilution. Now, an emulsion-assisted polymerization-induced self-assembly (EAPISA) method with the advantages of in situ self-assembly, scalable preparation, and facile functionalization was used to prepare hierarchical multiscale sea urchin-like aggregates (SUAs). The obtained SUAs from amphiphilic alternating copolymers have a micrometer-sized rattan ball-like capsule (RBC) acting as the hollow core body and radiating nanotubes tens of micrometers in length as the hollow spines. They can capture model proteins effectively at an ultra-low concentration (ca. 10 nm) after functionalization with amino groups through click copolymerization.

Silver island deposited titanium oxide composite substrate for surface-enhanced Raman spectroscopy with high enhance factor and ultra low detection concentration

Silver island deposited titanium oxide composite substrates for surface-enhanced Raman spectroscopy (SERS) have been synthesized by successively growth of titanium oxide on glass substrate and in situ deposition of silver islands. The underlying titanium oxide film is porous in structure with an uniform thickness of about 100 nm, and the silver particles are deposited both as flower-like islands and small nanoparticles with a size of 2–3 nm on the TiO2. The silver island deposited titanium oxide composite substrates show excellent SERS performance. The detection concentration for 4-aminothiophenol (PATP) is measured to be as low as 10−10 M, while the enhance factor reaches to 108∼9, which demonstrate the good SERS properties of the silver island deposited titanium oxide composite substrates and their great potential for molecular detection.

Development and characterization of a highly sensitive fluorometric transducer for ultra low aqueous ammonia nitrogen measurements in aquaculture

The accurate measurement of ammonia nitrogen concentration is of great significance for the study of aquatic ecological processes. Unfortunately, the commonly used colorimetric and ammonia sensitive electrode methods suffer from poor consistency, especially when the concentration of ammonia nitrogen is low. This paper presents the development and characterization of a highly sensitive fluorometric transducer, intended for measuring ultra-low ammonia nitrogen concentration in natural waters. The design of the transducer is based on molecular fluorescence analysis and focused on the ammonia-orthophthaldialdehyde-sulfite complexation reaction in alkaline aqueous solutions. LED excitation and photodiode detection are integrated in a compact pressure hull, with a small dichroic beam-splitter installed on the optical path. The beam-splitter reflects a small percentage of ultraviolet light into the reference photodiode to evaluate variations of the excitation energy and simultaneously guide fluorescence light into the sample photodiode by total internal refection. As a major innovation, pseudo-random sequence modulation and self-calibration are introduced into the amplitude measurement of the fluorescence for the first time. Specifications include a limit of detection (LOD) of 0.16 μg·L−1, a measurement range of 0–419.31 μg·L−1, and the R-square of 0.99. Two sets of algorithms with inputs of pH, temperature, salinity, and atmospheric pressure are embedded into the device for the high-precision measurement.

Carrier concentration-dependent piezotronic and piezo-phototronic effects in ZnO thin-film transistor

The piezotronic and piezo-phototronic effects have been confirmed as promising methodologies to optimize the performances of electronic/optoelectronic devices. Here, the carrier concentration dependence of both the piezotronic and piezo-phototronic effects are systematically investigated in ZnO thin-film transistors (TFTs) that fabricated by high-concentration solution hydrothermal method (ZnO NW TFT) and radio frequency magnetron sputtering (ZnO seed TFT) method. Significant performance improvement is achieved in the ZnO NW TFT with moderate carrier concentration by the piezotronic effect, whereas the photoresponse performance of the ZnO NW TFT to 365 nm UV illumination shows no modulation by the piezo-phototronic effect. In contrast, the performance of the ZnO seed TFT with ultra-low carrier concentration almost keeps unchanged when introducing the piezotronic effect, while significant photoresponse improvement of the ZnO seed TFT to 365 nm UV illumination is obtained by the piezo-phototronic effect. After careful analysis and comparation, the different influences of piezotronic and piezo-phototronic effects in the ZnO NW TFT and the ZnO seed TFT are resulted from the different carrier concentrations in ZnO NW film and ZnO seed film with or without UV illumination. This study not only presents in-depth understanding about carrier concentration dependence of the piezotronic and piezo-phototronic effects in ZnO TFTs, but also provides feasible, compatible and adjustable methodologies to enhance/optimize the performances of electronic/optoelectronic devices.

Performance tuning of gas sensors based on epitaxial graphene on silicon carbide

In this study, we investigated means of performance enhancement in sensors based on epitaxial graphene on silicon carbide (SiC). Epitaxially grown graphene on SiC substrates were successfully decorated with metal oxide nanoparticles such as TiO2 and Fe3O4 using hollow cathode pulsed plasma sputtering. Atomic Force Microscopy and Raman data verified that no damage was added to the graphene surface. It could be shown that it was easily possible to detect benzene, which is one of the most dangerous volatile organic compounds, with the Fe3O4 decorated graphene sensor down to an ultra-low concentration of 5 ppb with a signal to noise ratio of 35 dB. Moreover, upon illumination with a UV light LED (265 nm) of the TiO2 decorated graphene sensor, the sensitivity towards a change of oxygen could be enhanced such that a clear sensor response could be seen which is a significant improvement over dark conditions, where almost no response occurred. As the last enhancement, the time derivative sensor signal was introduced for the sensor data evaluation, testing the response towards a change of oxygen. This sensor signal evaluation approach can be used to decrease the response time of the sensor by at least one order of magnitude.

Micelles with ultralow critical micelle concentration as carriers for drug delivery

Conventional micellar carriers disassemble into free surfactants when diluted at concentrations below the critical micelle concentration (CMC). This limits the bioavailability in vivo of injected hydrophobic drugs encapsulated in micellar systems. Here, we show that a micelle comprising a superhydrophilic zwitterionic polymer domain and a superhydrophobic lipid domain has an undetectable CMC below 10-6 mM-a value that is orders of magnitude lower than the CMCs (>10-3 mM) of typical micellar systems. We also show that zwitterionic moieties or zwitterionic polymers added to a micelle solution stabilize the micelles at concentrations below their inherent CMC. In a mouse model of melanoma, ultralow-CMC micelles encapsulating docetaxel led to the complete eradication of tumours, whereas conventional docetaxel micellar formulations did not reverse tumour growth. Ultralow-CMC micelles might become next-generation carriers for drug delivery.

32.1 MECHANISMS OF ABNORMAL POSTTRANSLATIONAL PROTEIN PROCESSING IN SCHIZOPHRENIA BRAIN

32.1 MECHANISMS OF ABNORMAL POSTTRANSLATIONAL PROTEIN PROCESSING IN SCHIZOPHRENIA BRAIN