Minimum Detection Limit Research Articles

A Non-Source Optical Fiber Sensor for Multi-Point Methane Detection.

Fast, accurate, real-time measurement of gas concentration is an important task for preventing coal mining disasters. In order to develop an accurate monitoring method for methane gas concentration at different locations in a mine environment, a non-source optical fiber sensor for multi-point methane detection has been developed in this paper. A 16-channel fiber splitter and a multi-channel time-sharing acquisition module are employed within the sensor, enabling simultaneous detection of methane gas at 16 points by a host. Furthermore, the methane sensors are connected to the monitoring host via an all-optical method, achieving non-source and long-range detection of methane. To assess the performance of the methane gas sensor, experiments were conducted to evaluate its detection range, response time, and stability. The results indicated that the average detection error was approximately 1.84% across the full range, and the response time did not exceed 10 s. The minimum detection limit of the sensor, as determined by the 1σ criteria, was obtained as 58.42 ppm. Additionally, the concentrations of methane gas measured at varying distances (1 km, 2 km, 5 km) were found to be essentially consistent over an extended period. These results suggest that the development of this non-source optical fiber sensor holds significant potential for providing a method for mine environment, multi-point online methane gas detection.

Hydrothermal synthesis of NdFeO3 nanoparticles and their high gas-sensitive properties

N-butanol, a volatile organic compound prevalent in numerous industrial processes, poses significant risks to human health and the environment if not properly monitored. Consequently, the development of high-sensitivity n-butanol sensors is imperative. For this reason, NdFeO3 nanoparticles were successfully synthesized by hydrothermal method and used for the detection of n-butanol. They were fully analyzed using characterization techniques such as XRD, BET, EDX, SEM, TEM and XPS. The test results revealed that the prepared samples exhibited a granular structure with an average diameter of approximately 65 nm. The specific surface area measured 15.415 m2/g, and the pore size was determined to be 16.638 nm. The NdFeO3 sensor responds up to 441 for 100 ppm n-butanol at 260 °C, response and recovery times were 33 s and 7 s respectively. The response value of the NdFeO3 sensor is still 4.3 for 1 ppm n-butanol with a minimum detection limit of 0.024 ppm. Simultaneously, the NdFeO3 sensor has excellent selectivity, stability and moisture resistance. These good sensing properties indicate that this work provides a possible strategy for developing an economical and repeatable high performance gas sensor for the detection of n-butanol.

Flexible piezo-resistive strain sensors based on silver nanowires and graphene nanoplatelets reinforced polydimethylsiloxane for human motion detection

In this work, flexible sensors consisting of silver nanowires (Ag NWs), graphene nanoplatelets (GNP), and polydimethylsiloxane (PDMS) are fabricated using a straightforward, affordable, and simple solution processing technique. The performance of the flexible devices with structure Ag NWs/GNP/PDMS has been studied, in which Ag NWs/GNP-based ternary conductive hybrid nanocomposites are used as acting sensing layers. The diameter of Ag NWs and flake size of GNPs were determined to be 80–160 nm and 30–190 nm, respectively. The Raman peaks of Ag NWs within the composite are appeared at 392.08, 482.12, 543.7, 679.42 and 854.87 cm−1. The peaks existing at 1317.68, 1565.36 and 2639.75 cm−1 represent the D, G and 2D bands of GNP, respectively. The gauge factor is determined to be 865 within the strain range of 0–71 %. The response and recovery time of the fabricated sensor are calculated to be 101 and 110 ms, respectively. The sensor exhibits a minimum detectable limit of 0.8 %. The sensor shows a highly reproducible response for more than 1150 bending and stretching cycles suggesting long-term durability which may be attributed to the Ag NWs/GNPs conductive networks for significant strengthening the hybridisation with PDMS. Different human activities, viz. finger bending, stretching, mouse/mobile screen scrolling, swallowing through the throat and drinking are monitored by the fabricated Ag NWs/GNP/PDMS-based flexible strain sensor, indicating promising applications in wearable electronics.

A novel quantitative real-time PCR with the GAPDH reference gene for peste des petits ruminants.

Peste des petits ruminants (PPR) isaserious acute, highly contagious disease caused bythe peste des petits ruminants virus (PPRV). This study aims toestablish aqRT-PCR assay with aninternal amplification control for the rapid and accurate detection ofPPRV. The primers and probes for PPRV Nwere based onthe national standard ofthe diagnostic techniques for PPR ofChina, and apair ofprimers and TaqMan probes for the internalreference gene ofglyceraldehyde-3-phosphate dehydrogenase (GAPDH) was designed. Optimisation ofthe reaction conditions, specificity, sensitivity and reproducibility tests, and clinical sample detection were conducted. The results showed that the optimal primers and probe concentrations ofPPRV were 0.4μmol/l and 0.4μmol/l, respectively, and were 0.4μmol/l and 0.2μmol/l for the reference gene GAPDH, respectively. The established method has nocross-reaction with other viruses. The minimum detection limit was 6.8copies/μl for PPRV and 190copies/μl for GAPDH. The coefficients ofvariation (CV%) ofPPRV and GAPDH were both lower than 2%. The results suggest that the PPRV qRT-PCR method containing internal reference genes has strong specificity, high sensitivity, and good reproducibility. The addition ofinternal reference genes for the sample quality control improves the accuracy ofthe detection.

Regulated Inductively Coupled Plasma–Optical Emission Spectrometry Detectible Elements in Utah Lake: Characterization and Discussion

During the 2021 (n = 15) and 2022 (n = 13) summers, we measured the total and dissolved (<0.45 μm) concentration of 25 elements in Utah Lake using Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES) with detection limits in the order of a few parts-per-billion (ppb). This resulted in 1400 measurements, which is a unique dataset in terms of sensitivity and temporal resolution. Regulated elements are not commonly measured at the ppb-level; thus, these data provide insight into both the behavior and existence of these elements in an aquatic environment and have implications for both the management and regulation of the lake. Utah regulates twelve of these elements. While ICP-OES has ppb-level sensitivity, it is not the approved regulatory analysis method for these elements. All regulations are for dissolved concentrations, except aluminum (Al) and phosphorus (P), which are for total recovery. We found total Al above the allowable concentration, but dissolved concentrations were well below allowable concentrations. We attribute high total concentrations to suspended clays. This suggests that regulatory methods should be reviewed for lakes with a high suspended-solid content. Dissolved copper (Cu) concentrations were below regulatory levels in 2021, but some samples were above regulatory levels in 2022. This could be related to the use of Cu-based algaecide treatments, or from other sources. Lead (Pb) data were inconclusive; dissolved Pb concentrations were well below the acute (1 h average) limit, but the chronic concentration limit (4 h average) was below the ICP-OES minimal detection limit. Arsenic (As) concentrations exhibited a seasonal trend that we attribute to groundwater inflows—they were below regulatory levels for aquatic environments but around the levels for drinking water. This ppb-level study with high temporal resolution provides insight into regulated elements in Utah Lake previously not available due to the high sensitivity of the method and measurements of both total and dissolved concentrations.

A fluorescent probe for the detection of Al (III) based on a novel unsymmetrical trisubstituted 1,3,5-triazine

A highly selective and sensitive assay for Al (III) has been reported. A series of unsymmetrical trisubstituted 1,3,5-triazine compounds were synthesized using a one-pot method, in which probe 8-((4,6-diphenyl-1,3,5-triazin-2-yl)oxy)quinoline (2a) utilized for the detection of aluminum ions. The fluorescence characteristics of the 2a probe were then thoroughly studied, including its selectivity towards various metal ions, solvent effects, ion interference testing, response time, maximum detection limit, Job’s curve analysis, and research on the reversibility of EDTA. The linearity of the fluorescence concentration titration was found to be good within the range of 100–300 μM with R2 = 0.981 for Al3+. The minimum detection limit was determined to be 0.29 μM and the response time was only 10 s. These results indicate that probe 2a exhibits a specific response to Al3+ and holds great potential for application in aluminum ion detection.

An enhancement filtering method based on CEEMDAN-EP-WTD for TDLAS gas sensing

In tunable diode laser absorption spectroscopy (TDLAS) gas detection, the accuracy of the detection system is reduced due to the impact of nonlinear noise on the second harmonic. In this paper, a noise reduction method CEEMDAN-EP-WTD was proposed. Combining wavelet threshold method and complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) via evaluation parameter (EP) and sample entropy (SE). The CEEMDAN-EP-WTD can evaluate the noise level of the (intrinsic mode function) IMF component. It calculates the unknown noise standard deviation of the IMF component in the actual signal, which is related to the wavelet threshold. Based on simulation and experimental analysis, the signal-to-noise ratio of the harmonic signal is improved from 4.41 to 28.8596. The minimum detection limit of the experimental system is 4.72 ppm and the detection accuracy is 4.4 %. The results confirm that the algorithm successfully suppresses system noise and enhances the performance of the detection system.

In Situ High-Precision Measurement of Deep-Sea Dissolved Methane by Quartz-Enhanced Photoacoustic and Light-Induced Thermoelastic Spectroscopy.

Rapid and accurate realization of in situ analysis of deep-sea dissolved gases imperative to the study of ecological geology, oil and gas resource exploration, and global climate change. Herein, we report for the first time the deep-sea dissolved methane (CH4) in situ sensor based on quartz-enhanced photoacoustic and light-induced thermoelastic spectroscopy. The developed sensor system has a volume of φ120 mm × 430 mm and a power consumption of 7.6 W. The sensor, in the manner of frequency division multiplexing, is able to simultaneously measure the photoacoustic signals and light-induced thermoelastic signals, which can accurately correct laser-intensity induced influence on concentration. The spectral response of CH4 concentration varying from 0.01 to 5% is calibrated in detail based on the pressure and temperature in the application environment. The trend of the photoacoustic signal of CH4 at different water molecule (H2O) concentrations is investigated. An Allan variance analysis of several hours demonstrates a minimum detection limit of 0.21 ppm for the CH4 spectrometer. The sensor combined with the gas-liquid separation and enrichment unit is integrated into a compact marine standalone system. Since the specifically designed photoacoustic cell has a volume of only 1.2 mL, the time response for dissolved CH4 detection is reduced to 4 min. Furthermore, the sensor is successfully deployed in the vicinity of the "HaiMa" cold seeps at 1380 m underwater in the South China Sea, completing three consecutive days of measurements of dissolved CH4.

Polyaniline/ZnO heterostructure-based ammonia sensor self-powered by electrospinning of PTFE-PVDF/MXene piezo-tribo hybrid nanogenerator

Self-powered sensors are receiving increasing attention for their environmental friendliness and energy efficiency. In this work, self-powered polyaniline/zinc oxide (PANI/ZnO) heterostructure-based ammonia (NH3) sensor was driven by polytetrafluoroethylene (PTFE)-polyvinylidene fluoride/Ti3C2Tx (PVDF/MXene) piezo-tribo hybrid nanogenerator (PTNG) at room temperature. PVDF/MXene nanofiber membranes were prepared using electrospinning technology to construct a PTFE-PVDF/MXene PTNG. This enhances the effective contact area and dielectric properties of the composite film, thereby increasing the surface charge density. Additionally, the synergy between the triboelectric and piezoelectric effects significantly improves the output performance of the PTNG, effectively enabling energy harvesting for wearable devices. In addition, ZnO/PANI composite materials with a large specific surface area and high active sites were prepared, and a self-powered ammonia sensor was constructed based on the impedance matching effect. Furthermore, by constructing a PANI/ZnO p-n heterojunction, the number of active sites has been increased, significantly enhancing the sensing performance. The sensor has a minimum detection limit of 0.1 ppm, and at an NH3 concentration of 10 ppm, the response rate reaches 96.1 %. This study provides insights for the development of self-powered gas detection systems and holds significant research and practical value in enhancing the performance of NH3 sensors.

Highly sensitive CH4, C2H2 and CO simultaneous measurement LITES sensor based on multi-pass cell with overlapped spots pattern and QTFs with low resonant frequency

This paper presents a simultaneous measurement light-induced thermoelectric spectroscopy (LITES) sensor with high sensitivity for detecting methane (CH4), carbon monoxide (CO) and acetylene (C2H2). It employs a multi-pass cell (MPC) with an overlapped spots pattern and low resonant frequency circle-head quartz tuning forks (QTFs) for the first time. The fiber-coupled MPC with an optical length (OPL) of 40 m was combined with a thin-film filter (TFF) to improve the laser absorption and enable light spots multiplexing on mirrors. Three self-designed circle-head QTFs with low resonant frequencies of less than 10 kHz and a quality factor of ∼ 11500 were adopted to improve the detection ability. The LITES sensor detected multi-gas signals based on their unique absorption spectrum, enabling real-simultaneous measurement of CH4, CO, and C2H2. After optimization, the minimum detection limits (MDLs) of 0.5 ppm, 126.9 ppm and 0.4 ppm for these gases were obtained, respectively. With integration times of 300 s for CH4 and 200 s each for CO and C2H2, the MDLs could be further reduced to 0.09 ppm, 57.1 ppm, and 0.07 ppm, correspondingly. The paper concludes with a discussion of potential strategies for further improving the performance of such LITES sensors.

Pollen-Modified Flat Silk Cocoon Pressure Sensors for Wearable Applications.

Microstructures have been proved as crucial factors for the sensing performance of flexible pressure sensors. In this study, polypyrrole (PPy)/sunflower pollen (SFP) (P/SFP) was prepared via the in situ growth of PPy on the surface of degreased SFP with a sea urchin-like microstructure; then, these P/SFP microspheres were sprayed onto a flat silk cocoon (FSC) to prepare a sensing layer P/SFP-FSC. PPy-FSC (P-FSC) was prepared as an electrode layer through the in situ polymerization of PPy on the FSC surface. The sensing layer P/SFP-FSC was placed between two P-FSC electrode layers to assemble a P/SFP-FSC pressure sensor together with a fork finger electrode. With 6 mg/cm2 of optimized sprayed P/SFP microspheres, the prepared flexible pressure sensor has a sensitivity of up to 0.128 KPa-1 in the range of 0-13.18 KPa and up to 0.13 KPa-1 in the range of 13.18-30.65 KPa, a fast response/recovery time (90 ms/80 ms), and a minimum detection limit as low as 40 Pa. This fabricated flexible P/SFP-FSC sensor can monitor human motion and can also be used for the encrypted transmission of important information via Morse code. In conclusion, the developed flexible P/SFP-FSC pressure sensor based on microstructure modification in this study shows good application prospects in the field of human-computer interaction and wearable electronic devices.

Photoacoustic spectroscopy based on vertical cruciform multi-stepped photoacoustic cell achieving ppb-level acetylene detection

A photoacoustic spectroscopy (PAS) based on vertical cruciform multi-stepped photoacoustic cell (CMS-PAC) was developed to achieve highly sensitive detection of trace acetylene (C2H2) gas. Compared with the T-type PAC, the CMS-PAC has improved sensitivity through optimization of both the structural design and the direction of laser incidence. Through finite element simulation analysis, when the incidence direction of modulated laser was placed horizontally, the acoustic intensity of the CMS-PAC was ∼2.45 times higher than that of the T-type PAC. When the modulated laser was applied vertically, the acoustic intensity achieved by the CMS-PAC was ∼11 times higher than that along the horizontal incidence direction. The sensitivities of the CMS-PAC-based PAS system have reached 15.06 and 1.83 pm/ppm, along the vertical and horizontal directions, respectively. Combined with assessment using Allan deviation, the CMS-PAC-based PAS system achieved a minimum detection limit of 3.04 ppb@100 s in the vertical direction, and 23.06 ppb@100 s in the horizontal direction. The proposed CMS-PAC not only enhances acoustic intensity through optimized resonant cavity structures but also improves detection sensitivity with the optimized direction of laser incidence, providing a novel perspective for designing highly sensitive and compact PACs.

MXene (Ti3C2Tx)/Rh-doped SnO2 composites for improved acetone sensing properties

Rh-doped SnO2 nanofibers have demonstrated high selectivity and response in detecting acetone. However, these nanofibers operated at high temperature and exhibited a long recovery time. In this study, a two-dimensional material MXene was introduced into Rh-doped SnO2 nanofibers, and MXene (Ti3C2Tx)/Rh–SnO2 composites were synthesized via hydrothermal method. The morphology, structure and composition of MXene (Ti3C2Tx)/Rh–SnO2 were systematically characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller techniques (BET). The results indicated that MXene (Ti3C2Tx)/Rh–SnO2 composites formed compact heterostructures. The effects of adding different volumes (0.5, 1, 1.5, and 2 ml) of 5 mg/ml MXene on the sensing properties of the composites were investigated. The sensing results showed that the composite of (1 ml) MXene (Ti3C2Tx)/Rh–SnO2 achieved a response as high as 82.46 % to 100 ppm acetone gas at 100 °C and exhibited rapid response and recovery times of 3/8 s, compared to the optimal operating temperature (190 °C) and recovery time (43 s) observed with the pristine Rh-doped SnO2 nanofibers, there is a significant improvement. It also demonstrated outstanding humidity resistance and a minimum detection limit of 0.6 ppm. Moreover, the addition of MXene not only significantly affects the sensing performance of the pristine Rh-doped SnO2 nanofibers but also preserves its sensing advantages. The sensing mechanism of the MXene (Ti3C2Tx)/Rh–SnO2 composites is also discussed. Herein, MXene (Ti3C2Tx)/Rh–SnO2 composites presents a feasible strategy for enhancing the sensing properties of gas sensors.

Adaptation of Conductometric Monoenzyme Biosensor for Rapid Quantitative Analysis of L-arginine in Dietary Supplements.

The present study reports on the development, adaptation, and optimization of a novel monoenzyme conductometric biosensor based on a recombinant arginine deiminase (ADI) for the determination of arginine in dietary supplements with a high accuracy of results. Aiming for the highly sensitive determination of arginine in real samples, we studied the effect of parameters of the working buffer solution (its pH, buffer capacity, ionic strength, temperature, and protein concentration) on the sensitivity of the biosensor to arginine. Thus, it was determined that the optimal buffer is a 5 mM phosphate buffer solution with pH 6.2, and the optimal temperature is 39.5 °C. The linear functioning range is 2.5-750 µM of L-arginine with a minimal limit of detection of 2 µM. The concentration of arginine in food additive samples was determined using the developed ADI-based biosensor. Based on the obtained results, the most effective method of biosensor analysis using the method of standard additions was chosen. It was also checked how the reproducibility of the biosensor changes during the analysis of pharmaceutical samples. The results of the determination of arginine in real samples using a conductometric biosensor based on ADI clearly correlated with the data obtained using the method of ion-exchange chromatography and enzymatic spectrophotometric analysis. We concluded that the developed biosensor would be effective for the accurate and selective determination of arginine in dietary supplements intended for the prevention and/or elimination of arginine deficiency.

Multi-gas sensing system based on miniaturized cruciform photoacoustic cell

Photoacoustic spectroscopy has the advantage of multi-gas sensing with only one sound detector because of its non-selectivity. A photoacoustic cell (PAC) is an important part of the system as it amplifies the intensity of sound pressure. In this paper, we report on the development of a multi-gas sensing system based on a specially designed cruciform photoacoustic cell (CF-PAC), whose structure contains two beam paths and is suitable for multiple laser beams incident from different directions independently. It is especially suitable where beams are not easy to combine due to large wavelength differences. The structure of the cell is carefully designed using the finite-element method (FEM) and the resonant frequency of it is about 5050 Hz and the external dimension is only 67×60×40 mm with the inner volume of about 24 cm3. We also present a contrast experiment between the CF-PAC and the conventional “H-type PAC” with similar parameters and the result shows that the former sacrifices only a small amount of performance, but adds beam path effectively. The sensor is designed to detect methane (CH4), nitrogen dioxide (NO2), and carbon dioxide (CO2), and the minimum detection limit (MDL) with a 5 s integration time are 113 ppb, 2.1 ppm, and 1.6 ppm, respectively. The 1σ normalized noise equivalent absorption coefficients (NNEA) of the three gases are 2.54×10−9 cm−1 W Hz−1/2, 5.70×10−8 cm−1 W Hz−1/2, and 2.31×10−6 cm−1 W Hz−1/2, respectively. Modified formulas based on nonlinear regression will be given to make the detection results more accurate in the multi-component gas environment.

Signal enhancement of the gas detection based on quartz-enhanced photothermal spectroscopy technology

This paper presents an improved gas sensor based on the dual-excitation of quartz-enhanced photothermal spectroscopy (QEPTS) using a single quartz tuning fork (QTF) for signal detection. The silver coating on one side of the QTF was chemically etched to increase the laser power interacted with QTF for QEPTS signal excitation. By etching the silver coating on one side of QTF, the reflection structure between the silver coating of the other side of QTF and the external flat mirror was established. The device uses an absorption gas cell with an optical range length of 3 m, making the laser beam interact with the gas more completely and posing more gas concentration information. Acetylene was selected as the target gas to verify the performance of the sensor. The experimental results show that the signal amplitude with a flat mirror was 1.41 times that without a flat mirror, and 2.47 times that of traditional QEPTS sensor. The system has a minimum detection limit (MDL) of 1.10 ppmv, corresponding to a normalized noise equivalent absorption coefficient (NNEA) of 7.14 × 10−9 cm−1·W·Hz−1/2. Allan variance analysis results show that when the integration time is 700 s, the MDL of the system is 0.21 ppmv. The proposed gas sensor can play an important role on detecting trace gas in many fields.

High-sensitivity trace gas detection based on differential Helmholtz photoacoustic cell with dense spot pattern

A high-sensitivity photoacoustic spectroscopy (PAS) sensor based on differential Helmholtz photoacoustic cell (DHPAC) with dense spot pattern is reported in this paper for the first time. A multi-pass cell based on two concave mirrors was designed to achieve a dense spot pattern, which realized 212 times excitation of incident laser. A finite element analysis was utilized to simulate the sound field distribution and frequency response of the designed DHPAC. An erbium-doped fiber amplifier (EDFA) was employed to amplify the output optical power of the laser to achieve strong excitation. In order to assess the designed sensor's performance, an acetylene (C2H2) detection system was established using a near infrared diode laser with a central wavelength 1530.3 nm. According to experimental results, the differential characteristics of DHPAC was verified. Compared to the sensor without dense spot pattern, the photoacoustic signal with dense spot pattern had a 44.73 times improvement. The minimum detection limit (MDL) of the designed C2H2-PAS sensor can be improved to 5 ppb when the average time of the sensor system is 200 s.

CRISPR/Cas13a-based genome editing for establishing the detection method of H9N2 subtype avian influenza virus

Avian influenza virus (AIV) subtype H9N2 has significantly threatened the poultry business in recent years by having become the predominant subtype in flocks of chickens, ducks, and pigeons. In addition, the public health aspects of H9N2 AIV pose a significant threat to humans. Early and rapid diagnosis of H9N2 AIV is therefore of great importance. In this study, a new method for the detection of H9N2 AIV based on fluorescence intensity was successfully established using CRISPR/Cas13a technology. The Cas13a protein was first expressed in a prokaryotic system and purified using nickel ion affinity chromatography, resulting in a high-purity Cas13a protein. The best RPA (recombinase polymerase amplification) primer pairs and crRNA were designed and screened, successfully constructing the detection of H9N2 AIV based on CRISPR/Cas13a technology. Optimal concentration of Cas13a and crRNA was determined to optimize the constructed assay. The sensitivity of the optimized detection system is excellent, with a minimum detection limit of 10° copies/μL and didn't react with other avian susceptible viruses, with excellent specificity. The detection method provides the basis for the field detection of the H9N2 AIV.

PMA-ddPCR method for detecting high ethanol-producing klebsiella pneumoniae in viable but non-culturable state

Objective: To establish an absolute quantitative method for high ethanol-producing klebsiella pneumoniae in a viable non-culturable (VBNC) state. Methods: High ethanol-producing Klebsiella pneumonia was induced to enter the VBNC state and then the ethanol production was evaluated. A PMA-ddPCR method was established to count the copies of live cell genes in the VBNC state of high ethanol-producing Klebsiella pneumoniae using single-copy genes. Further, the sensitivity and adaptability of ddPCR for detecting low-concentration samples were evaluated in VBNC fecal simulation. Results: The lower detection limit of ddPCR for quantitative analysis of high ethanol-producing Klebsiella pneumoniae gradient diluent was 10 times that of qPCR. At low temperature and low nutritional state, high ethanol-producing Klebsiella pneumoniae entered the VBNC state on the 45th day. The quantitative results of PMA-ddPCR on VBNC state cells were (5.46±0.05) log10 DNA copies/ml. The ethanol production in the VBNC state was<2.2 mmol/L and the ability to produce ethanol was restored after recovery. The minimum detection limit for ddPCR in fecal simulated samples with VBNC state was 3.2 log10 DNA copies/ml. Conclusion: The ddPCR detection method for high ethanol-producing Klebsiella pneumoniae with VBNC state has good sensitivity and adaptability, and can be used for the detection of VBNC state cells in clinical samples.

Ultra-low concentration detection of dissolved C2H2 in oil using fixed-frequency RF-assisted calibration-free wavelength modulation spectroscopy

The monitoring sensitivity of dissolved C2H2 in transformer oil is crucial to ensuring the safe operation of electric power systems. A highly sensitive sensor system based on fixed-frequency radio frequency assisted calibration-free wavelength modulation spectroscopy (FF-RF-WMS-2f/1f) for dissolved C2H2 in transformer oil was developed and evaluated. Lowering the headspace sampling pressure to 0.1 atm effectively increased the outgassing ratio and detection signal of dissolved C2H2, while also reducing background interference. By employing a novel fixed-frequency RF-assisted modulation technique, we effectively suppressed the amplitude and influence of optical fringes and improved the system’s linear fitting R-squared value for gas-phase concentration from 0.9997 to 0.9999. Furthermore, the minimum detection limit (MDL), as calculated by Allan deviation, was reduced from 8.58 ppb to 4.36 ppb, while the optimal integration time increased slightly from 62 s to 79 s. The equivalent MDL for dissolved C2H2 reached 0.9 ppb. The comparative experiments showed that the error was less than 0.2 ppm or 8 %, the relative standard deviations (RSDs) of continuous measurement were less than 3 %, and the RSD at ultra-low concentration was less than 8 %. The experiments verified the high sensitivity and accuracy of the sensor system, making it suitable for ultra-low concentration dissolved C2H2 detection in transformer insulating oil.