Multi-pass Differential Photoacoustic Sensor for Real-Time Measurement of SF6 Decomposition Component H2S at the ppb Level.

We designed and implemented a photoacoustic (PA) sensor for H2S detection in SF6 background gas based on a multi-pass differential photoacoustic cell (MDPC) and a near-infrared distributed feedback (DFB) laser. In the MDPC apparatus, two resonators with identical geometric parameters were vertically and symmetrically embedded. The differential processing algorithm of two phase-reversed signals realized the effective enhancement of the PA signal and suppressed the flow noise in the dynamic sampling process. In addition, the λ/4 buffer chamber in the MDPC was utilized as a muffler to further reduce the flow noise and realize the dynamic detection of H2S. The collimated excitation light was reflected 30 times in a multi-pass structure constituted of two gold-plated concave mirrors, and an absorption path length of 4.92 m was achieved. Due to the high gas density of SF6, the relationship between the signal-to-noise ratio (SNR) and the gas flow was different between SF6 and N2 background gases. The maximum flow rate of the characteristic gas components detected in the SF6 background is 150 standard cubic centimeters per minute (SCCM), which is lower than 350 SCCM in N2. The linearity property was analyzed, and the results show that the sensitivity of the sensor to H2S in the SF6 background was 27.3 µV/ppm. With the structure, parameters, temperature, gas flow, and natural frequency of the MDPC been optimized, a minimum detection limit (MDL) of 11 ppb was reached with an averaging time of 1000 s, which furnished an effective preventive implement for the safe operation of gas insulation equipment.

Fiber-Optic Photoacoustic Gas Sensor with Multiplexed Fabry-Pérot Interferometric Cantilevers.

A fiber-optic photoacoustic (PA) gas sensor with multiplexed Fabry-Pérot (F-P) interferometric cantilevers is demonstrated. A compact cylindrical nonresonant PA tube with a volume of only 0.45 mL is designed. The PA signal is measured by two symmetrically installed fiber-optic interferometric cantilever microphones (FOICMs) to improve the signal-to-noise ratio (SNR). For multiplexing the two cantilevers by a single demodulation system, a dual cavity length synchronous measurement method based on total-phase demodulation algorithm with ultrahigh resolution is developed. The PA signal detection is realized by the second-harmonic wavelength modulation spectroscopy (2f-WMS) technique. The sensor performance is verified by conducting the detection of trace acetylene (C2H2). The normalized noise equivalent absorption (NNEA) coefficient is 2.5 × 10-9 cm-1·W·Hz-1/2, and the minimum detection limit (MDL) downs to about 0.2 ppm with an averaging time of 1 s. The fiber-optic PA gas sensor has characteristics of high resolution and immunity to electromagnetic and vibration interference. Furthermore, the technical scheme of the multiplexed cantilever demodulation shows great potential for remote multipoint monitoring of gases in harsh environments.

Dynamic detection of ppb-level SO2 based on a differential photoacoustic cell coupled with UV-LED.

We design a photoacoustic (PA) SO2 sensor based on the coupling of a differential photoacoustic cell (DPAC) and cost-effective UV-LED, which realized the dynamic monitoring of SO2 gas at the ppb level. Aiming at the limitation of UV-LED divergence, a light source combination module with high condensing efficiency was devised based on a lens through theoretical derivation and experimental analysis. The PA signal with the optimum matching of the lens was 20-times larger than the direct coupling of the UV-LED. Due to the excellent beam collimation effect of the lens assembly, the background interference was only 1 ppm. In addition, the DPAC gathered the merits of doubling the PA signal and reducing the flow noise interference. The analysis of Allan-Werle deviation showed that the detection limit of SO2 was 1.3 ppb with the averaging time of 100 s.

Chemiluminescence assay for kanamycin based on target recycling strategy.

A chemiluminescence (CL) sensing strategy for kanamycin residue detection in fish samples was established based on luminol-functionalized gold nanoparticles as CL nanoprobe materials combined with DNA hairpin structure and carboxyl-modified magnetic beads. Relying on nucleic acid amplification technology, the system can successfully realize the recycling of kanamycin, so that the biosensor can release a large number of luminol-functionalized gold nanoparticles with excellent CL performance even at a low residual levels of kanamycin. The biosensor strategy showed a good linear relationship with kanamycin in the range 0.09-130 nM, the detection limit was as low as 0.04 nM. This method proves the excellent performance of the sensing strategy and provides a low-cost and high-sensitivity CL analysis strategy for the detection of kanamycin and even other antibiotics.

Self-enhancement photoelectrochemical strategy for kanamycin determination with amino functionalized MOFs.

Based on the self-enhanced photoelectrochemical quality of Cu-MOF-NH2, a photoelectrochemical biosensor for kanamycin detection with a Cu-MOF-NH2 modified electrode was constructed. This biosensor took full advantage of the low electron hole recombination rate due to the ligand-to-metal charge-transfer mechanism of excited electron transfer in Cu-MOF-NH2. The kanamycin aptamer was modified onto Cu-MOF-NH2 by Schiff base reaction. In the presence of kanamycin, the holes on the amino group in Cu-MOF-NH2 oxidize kanamycin, making kanamycin itself as a signal enhancement substance, and achieving the effect of self-enhancement. The dynamic monitoring range for kanamycin is 0.5 to 650 nM, and the detection limit is 0.1 nM. The sensor has been successfully applied to the determination of kanamycin in fish with recoveries of 95.7-105.0% and RSD of 1.5-4.0%. This work provides a broad path for the development of self-enhanced photoelectrochemical sensors.

Photoelectrochemical aptasensor with low background noise.

In photoelectrochemical (PEC) detection, enhancing the PEC signal and depressing the blank signal are conducive to improve the sensitivity. Because the carbon nanotube (CNT) effectively transfers photogenerated electrons from SnSe to the electrode, the composite nanomaterial CNTs/SnSe generates a strong PEC signal. Methionine (Met), AuNPs, and probe DNA are woven together forming a nanoprobe which is used as a quencher to quench the PEC signal of CNTs/SnSe. When the nanoprobe and CNTs/SnSe are modified onto the electrode, there is a low blank signal. In the presence of metastatic breast cancer cells, the cells interact with the aptamer of dsDNA; concomitantly, cDNA is released to trigger catalytic hairpin assembly (CHA). As a result, a new dsDNA which has an overhang is formed. The nanoprobe on the surface of the electrode hybridizes with the newly formed dsDNA. Subsequently, the nanoprobe is released from the surface of the electrode and the quenching effect between the nanoprobe and the CNTs/SnSe disappears. The PEC aptasensor is linear in the concentration range of 300-5,000 cells/mL, and the detection limit is 180 cells/mL under optimized conditions. The relative standard deviation (RSD) is 3.6% at 10,000 cells/mL. This work demonstrates a promising strategy using CNTs/SnSe as the photoactive material and Met-AuNPs as the quencher to establish a PEC aptasensor with a high PEC response and low blank signal. It can be used to detect bioactive substances at ultralow levels prospectively. Graphical abstract.

NiFe-coordinated zeolitic imidazolate framework derived trifunctional electrocatalyst for overall water-splitting and zinc-air batteries.

Developing high-efficient non-noble metal electrocatalysts toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), water-splitting, and the zinc-air battery is essential but challenging. Zeolitic imidazole frameworks (ZIFs) are generally employed as ideal platforms for the design and fabrication of energy-related catalysts by exploiting their porous structure with high surface area and flexibility. This work presents the preparation of NiFe-bimetallic species decorated N-doped porous carbon composite (NiFe@NPC) through pyrolyzing the NiFe-coordinated ZIF precursor. The obtained NiFe@NPC shows a larger surface area and porous nanostructure comprising the active bimetallic species evenly distributed in the conductive carbon matrix. The nanocomposite demonstrates excellent trifunctional catalytic activity toward ORR, OER, and HER. For ORR, NiFe@NPC offers a half-wave potential value of 0.87 V, which is positively shifted by 30 mV relative to that of Pt/C in 1 M KOH. NiFe@NPC exhibits OER activity with superior overpotential, reaction kinetics, and durability to those of IrO2. It also demonstrates the desirable HER activity with a low overpotential of 150 mV at 10 mA/cm2 and excellent durability in an acidic electrolyte. Additionally, the water-splitting configuration and zinc-air battery assembled with NiFe@NPC catalyst reveal superior performance to noble-metal catalysts. Such excellent electrocatalytic performance can be attributed to the distinct chemical composition of evenly distributed bimetallic active sites on highly conductive carbon sheets, and the porous nanostructure with large surface area and fast mass transfer.

3D hierarchical porous nitrogen-doped carbon/Ni@NiO nanocomposites self-templated by cross-linked polyacrylamide gel for high performance supercapacitor electrode.

Three-dimensional nitrogen-doped carbon network incorporated with nickel@nickel oxide core-shell nanoparticles composite (3D NC/Ni@NiO) has been facilely prepared, self-templated by the cross-linked polyacrylamide aerogel precursor containing NiCl2. Characterizations reveal that the Ni@NiO nanoparticles distribute homogeneously in the 3D nitrogen-doped carbon matrix and the composite is of hierarchical porous structure. When used as supercapacitor electrode in a three-electrode system, the 3D NC/Ni@NiO exhibits enhanced electrical conductivity and excellent electrochemical performance, presenting a high specific capacitance (389F g-1 at 5 mV s-1), good rate capability (276 F g-1 at 100 mV s-1) and outstanding cycling performance (with the capacitance retention of 70.2% after 5000 charge-discharge cycles). This is due to the synergistic effects of conductive metallic nickel, pseudocapacitive nickel oxide as well as in situ nitrogen doping of carbon network. Moreover, an asymmetric supercapacitor (ASC) was fabricated with NC/Ni@NiO as positive electrode and active carbon as negative electrode. The ASC device exhibits a maximum energy density of 19.4 W h kg-1 at a power density of 700 W kg-1 and shows good cycling stability (73.8% capacity retention after 3000 cycles), indicating that it has great promise for practical energy storage and conversion application.

A cascade autocatalytic strand displacement amplification and hybridization chain reaction event for label-free and ultrasensitive electrochemical nucleic acid biosensing.

Herein, an autocatalytic strand displacement amplification (ASDA) strategy was proposed for the first time, which was further ingeniously coupled with hybridization chain reaction (HCR) event for the isothermal, label-free and multiple amplification toward nucleic acid detection. During the ASDA module, the target recognition opens the immobilized hairpin probe (IP) and initiates the annealing of the auxiliary DNA strand (AS) with the opened IP for the successive polymerization and nicking reaction in the presence of DNA polymerase and nicking endonuclease. This induces the target recycling and generation of a large amount of intermediate DNA sequences, which can be used as target analogy to execute the autocatalytic strand displacement amplification. Simultaneously, the introduced AS strand can propagate the HCR between two hairpins (H1 and H2) to form a linear DNA concatamer with cytosine (C)-rich loop region, which can facilitate the in-situ synthesis of silver nanoclusters (AgNCs) as electrochemical tags for further amplification toward target responses. With current cascade ASDA and HCR strategy, the detection of target DNA could be achieved with a low detection limit of about 0.16 fM and a good selectivity. The developed biosensor also exhibits the distinct advantages of flexibility and simplicity in probe design and biosensor fabrication, and label-free electrochemical detection, thus opens a promising avenue for the detection of nucleic acid with low abundance in bioanalysis and clinical biomedicine.

Photoelectrochemical platform for cancer cell glutathione detection based on polyaniline and nanoMoS2 composites modified gold electrode.

Herein, a visible light photoelectrochemical (PEC) platform based on polyaniline (PANI) and nanoMoS2 composites as optoelectronic material for glutathione detection without any auxiliary of biomolecules or labeled materials was developed. Firstly, the nanoMoS2 was prepared via a simple ultrasound exfoliation method. The PANI was synthesized by chemical oxidative polymerization method. Then composite of PANI and nanoMoS2 was used to modify gold electrode. It was found that the composite membrane showed excellent PEC properties. And glutathione enhanced the PEC signal greatly. Based on this finding a method for glutathione detection was fabricated. Under the optimum conditions, the linear response of glutathione concentrations ranged from 1.0 × 10-10 to 1.0 × 10-4 mol L-1 was obtained with a detection limit of 3.1 × 10-11 mol L-1. The relative standard deviation was 2.9% at 2.0 × 10-9 M (n = 10). This method showed high sensitivity and simpleness which opened up a new promising signal-on PEC platform for future bioassay.

Surfactant-free microemulsion composed of oleic acid, n-propanol, and H2O.

Generally, a microemulsion consists of oil, water, surfactant, and sometimes cosurfactant. Herein, we report a surfactant-free microemulsion (denoted as SFME), consisting of oleic acid (oil phase), water, and n-propanol without the amphiphilic molecular structure of a traditional surfactant. The phase behavior of the ternary system was investigated, showing that there were a single-phase microemulsion region and a multiphase region in the ternary phase diagram. The electrical conductivity measurement was employed to investigate the microregions of the single-phase microemulsion region, and three different microregions, that is, water-in-oleic acid (W/O), a bicontinuous (B.C.) region, and oleic acid-in-water (O/W), were identified, which were further confirmed by freeze-fracture and cryogenic transmission electron microscopy (FF-TEM and Cryo-TEM) observations. The polarity and the salt solubility of water domains in the W/O SFME were investigated by UV-visible spectroscopy using methyl orange and potassium ferricyanide as probes, respectively. Experimental results showed that the water domains in the W/O microemulsion had a lower polarity than bulk water and a normal solubility for salt species, indicating that the SFMEs have much significance in the preparation of various nanomaterials.

A hierarchical Co-Fe LDH rope-like nanostructure: facile preparation from hexagonal lyotropic liquid crystals and intrinsic oxidase-like catalytic activity.

Hierarchical rope-like structures based on Co-Fe layered double hydroxide (LDH) nanosheets were synthesized by the coprecipitation method from a hexagonal lyotropic liquid crystal (LLC) nanoreactor, and were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric (TG) and inductively coupled plasma (ICP) analyses. It was found that the rope-like LDH structures were composed of LDH nanosheets with a lateral size of about 200-400 nm and an average thickness of 4.47 nm in the form of face-to-edge interactions. The length and the diameter of the rope-like assemblies were about 3-6 µm and 150-300 nm, respectively, and their aspect ratio was as high as 20. Interestingly, the LDH rope-like assemblies were ordered to form an array with the oriented directions parallel to each other. A formation mechanism for the hierarchical LDH structures in the LLC media was proposed. In addition, the catalytic activity of the hierarchical rope-like LDH assemblies for the oxidation reaction of the typical horseradish peroxidase (HRP) substrate, 3,3',5,5'-tetramethylbenzidine (TMB), was examined, and results revealed that they had a higher oxidase-like catalytic activity towards the oxidization of TMB by dissolved oxygen. We expect that the hierarchical rope-like LDHs can offer the potential applications in aqueous redox catalysts, biosensors, medical diagnostics and so on.