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An ultra-sensitive photoelectrochemical (PEC) sensor based on perovskite composite was developed for the determination of alkaline phosphatase (ALP) in human serum. In contrast to CsPbBr3 or Y6 that generated anodic current, the heterojunction of CsPbBr3/Y6 promoted photocarriers to separate and generated cathodic photocurrent. Ascorbic acid (AA) was produced by ALP hydrolyzing L-ascorbic acid 2-phosphate trisodium salt (AAP), which can combine with the holes on the photoelectrode surface, accelerating the transmission of photogenerated carriers, leading to enhanced photocurrent intensity. Thus, the enhancement of PEC current was linked to ALP activity. The PEC sensor exhibits good sensitivity for detection of ALP owing to the unique photoelectric properties of the CsPbBr3/Y6 heterojunction. The detection limit of the sensor was 0.012 U·L−1 with a linear dynamic range of 0.02–2000 U·L−1. Therefore, this PEC sensing platform shows great potential for the development of different PEC sensors. [ABSTRACT FROM AUTHOR]

The detection of botulinum neurotoxin A (BoNT/A) endopeptidase activity by pregnancy test paper based on human chorionic gonadotropin (hCG)-functionalized peptide-modified magnetic nanoparticles (MNs) is described for the first time. HCG-functionalized SNAP-25 peptide substrate with hydrolysis recognition sites was optimally designed. HCG can be recognized by pregnancy test strips. BoNT/A light chain (BoNT-LcA) is the central part of the endopeptidase function in holotoxin, which can specifically hydrolyze SNAP-25 peptide to release the hCG-peptide probe, and the hCG-peptide probe released can be quantitatively detected by pregnancy test strips, achieving indirect determination of BoNT/A. By quantifying the T-line color intensity of test strips, the visual detection limit for BoNT-LcA is 12.5 pg/mL, and the linear range of detection for BoNT-LcA and BoNT/A holotoxin was 100 pg/mL to 1 ng/mL and 25 to 250 ng/mL. The ability of the method to quantify BoNT/A was validated in human serum samples. This method shows the potential for sensitive detecting BoNT/A and has prospects for the diagnosis and prognosis of clinical botulism. [ABSTRACT FROM AUTHOR]

Graphene@MXene composite nanomaterials were utilized to construct an electrochemical sensor for alanine aminotransferase (ALT) detection. The combination of graphene nanosheets with MXene avoids the self-stacking of MXene and graphene, and broadens the charge transfer channel. In addition, the composite nanomaterial provides increased loading sites for pyruvate oxidase. The principle of ALT detection is a two-step enzymatic reaction. l-Alanine was initially transferred to pyruvate catalyzed by ALT. The formed pyruvate was then oxidized by pyruvate oxidase, generating H2O2. Through the detection of the generated H2O2, ALT activity was measured. The linear range of the sensor to ALT was from 5 to 400 U·L−1 with a detection limit of 0.16 U·L−1 (S/N = 3). For real sample analysis, the spiked recovery test results of ALT in serum samples were between 96.89 and 103.93% with RSD < 5%, confirming the reliability of the sensor testing results and potential clinical application of the sensor. [ABSTRACT FROM AUTHOR]

An electrochemical sensor for the detection of uric acid was constructed using cobalt oxide-modified porous carbon@multi-walled carbon nanotube (MWCNTs) composite material for the modification of the electrode. Firstly, ZIF-67 is generated on carbon nanotubes using the surfactant cetylammonium bromide (CTAB) as template. The vesicles generated by CTAB act as nucleation sites for the in situ growth of ZIF-67. Then, cobalt oxide-modified porous carbon was obtained after high-temperature carbonization of ZIF-67, leading to the formation of cobalt oxide-modified porous carbon@MWCNT composite materials. Co-N and Co-O active sites on the composite material can improve the oxidation of uric acid on the electrode surface, leading to enhanced sensitivity and selectivity for uric acid detection. The sensor has a good linear range from 1 to 40 μM for uric acid detection with a detection limit of 0.09 μM. The sensor was utilized for determination of uric acid in actual serum samples. [ABSTRACT FROM AUTHOR]

The weak fluorescence efficiency of copper nanoclusters (Cu NCs) limits their wide applications in biosensing and bioimaging areas, while the aggregation-induced emission (AIE) effect is anticipated to increase their luminescence intensity. Herein, the weak red emission of Cu NCs is increased considerably by the addition of lanthanide Tb3+, ascribed to the AIE effect. Monitoring of spores contamination can be carried out by determining the level of 2, 6-dipicolinic acid (DPA), which is a marker of spores. Due to the stronger synergy between DPA and Tb3+ for its clamped configuration of adjacent pyridine nitrogen group with the carboxylic acid group, the addition of DPA leads Tb3+ to be taken away from Cu NCs through a stronger coordination effect, causing Cu NCs to return to the dispersed state and weakened fluorescence. Based on this, an "off–on-off" fluorescent probe for DPA sensing was built, in which Tb3+ was used as a bridge to achieve AIE enhanced fluorescence effect on Cu NCs as well as a specific recognizer of DPA. The detection range for DPA was 0.1–60 μM and the detection limit was 0.06 μM, which was much lower than the infectious dose of anthrax spores. Since DPA is a unique biomarker for bacterial spores, the method was applied to the detection of actual bacterial spores and satisfactory results were obtained with a detection limit of 4.9*103 CFU mL−1. [ABSTRACT FROM AUTHOR]

A silica ring-based ratiometric fluorescence nanosensing and imaging platform was established for the quantification of miRNA-21 with carbon dots and nucleic-functionalized gold nanoparticles inside and outside the ring, respectively. In the presence of miRNA-21, on one hand, the fluorescence emission signal of the originally quenched 6-carboxyfluorescein (FAM), which was labeled on the nucleic acid molecules that modified on the gold nanoparticles, was re-emitted. On the other hand, the fluorescence intensity of the carbon dots inside the ring remained essentially unchanged, which served to eliminate background interference. The ring rather than the shell well maintained the fluorescence of the carbon dots. The ratio of the recovered fluorescence intensity of FAM to the fluorescence intensity of the carbon dots was linearly correlated with the logarithm of miRNA-21 concentration. The linear range of the probe for miRNA detection was more than 3 orders of magnitude, and the detection limit was as low as 2.5 aM. The feasibility of the method for clinical application was verified by the quantitative analysis of intracellular miRNA. [ABSTRACT FROM AUTHOR]

B-doped core–shell Fe@BC nanozyme was synthesized. The peroxidase (POD) like activity of Fe@BC nanozyme was studied and utilized for detecting the activity of alanine aminotransferase (ALT). In the presence of ALT as well as ALT co-substrates l-alanine and α-ketoglutarate, l-glutamate is generated. The following catalytic oxidation of l-glutamate by glutamate oxidase leads to the generation of H2O2. The POD-like activity of Fe@BC can oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to oxTMB in the presence of H2O2, generating a blue-colored compound. Through the detection of the amount of H2O2 generated, ALT activity can be determined through measuring the absorbance intensity variation at 450 nm. The limit of detection of the assay is 4 U/L, with a linear range from 10 to 1000 U/L. For human serum samples, the ALT levels determined by our assay are comparable to those determined by the hospital with a correlation coefficient of 0.991, demonstrating the reliability of our assay results. [ABSTRACT FROM AUTHOR]

Nanozyme based on Prussian blue nanocubes (PB NCs) loaded with copper nanoparticles (Cu@PB NCs) was synthesized. The peroxidase (POD)-like activity of Cu@PB NCs was studied and utilized for detecting the activity of alkaline phosphatase (ALP). The Cu@PB NCs possess higher POD-like activity compared with PB NCs and natural horseradish peroxidase (HRP) due to the loading of copper nanoparticles. 3,3′,5,5′-Tetramethylbenzidine (TMB) can be oxidized to oxTMB in the presence of Cu@PB NCs and H2O2, generating blue-colored compound, while introduction of pyrophosphate (PPi) leads to the POD-like activity of Cu@PB NCs decreased obviously. In the presence of ALP, PPi was hydrolyzed and then the POD-like activity of Cu@PB NCs was restored. So, according to the change of the POD-like activity of Cu@PB NCs, a sensitive colorimetric assay for ALP activity was reported. The limit of detection of the assay is 0.08 mU/mL, with linear range from 0.1 to 50 mU/mL. In addition, the assay was also applied for screening the inhibitors of ALP. [ABSTRACT FROM AUTHOR]

Gold nanoparticle (AuNP)–anchored BP nanosheets were synthesized through in situ growth of AuNPs onto BP. Due to the strong chelating ability of P or phosphorus oxides with AuNPs, the stability of BP is improved. As proof-of-concept demonstration of the functionalized BP, electrochemical detection of circulating tumor cells (CTCs) based on BP@AuNPs@aptamer as a probe combined with immunomagnetic separation is reported. The aptamer can specifically bind with CTCs, while the phosphorus oxides including phosphite ion and phosphate ion (PxOy species) on BP and aptamer can react with molybdate to generate an electrochemical current, leading to dual signal amplification. The biosensor is applied to MCF-7 cell detection and displays good analytical performance with a detection limit of 2 cell mL−1. Furthermore, the practicality of this biosensor was validated through sensitive determination of MCF-7 cells in human blood. Therefore, the reported biosensor could be applied to detect other biomarkers, offering an ultrasensitive strategy for clinical diagnostics. [ABSTRACT FROM AUTHOR]

A fluorescence off-on sensing platform was developed based on thioglycolic acid-stabilized cadmium sulfide quantum dots (CdS QDs) as fluorescence probe for the sensitive and selective detection of 2,6-pyridinedicarboxylic acid (DPA) in spores. The fluorescence emission intensity of the quantum dots at 650 nm when excited at 460 nm was first quenched by mixing with europium ions (Eu3+) and then recovered after the addition of DPA. The interaction of DPA with Eu3+ relieved the quenching effect of Eu3+ toward CdS QDs. As the DPA concentration increases, the color of the probe changes from colorless to red. The method exhibits a wide linear range from 1 to 120 μM for DPA determination, with a detection limit of 0.2 μM. The CdS QDs based nanoprobe was successfully applied for sensitive determination of DPA released from bacteria spores. In this case, the detection limit is 3.5 × 104 CFU·mL−1. [ABSTRACT FROM AUTHOR]

In this work, an aptamer-based photoelectrochemical (PEC) assay is reported for the determination of MCF-7 breast cancer cells using hexagonal carbon nitride tubes (HCNTs) as photoactive material. The aptamer immobilized on the HCNT surface can specifically bind with mucin 1 protein (MUC1) that is overexpressed on the surface of MCF-7 cell. Thus, the PEC assay has high specificity for the determination of MCF-7. The determination of MCF-7 is due to the binding of MCF-7 onto HCNT that suppressed the photocurrent intensity. The PEC assay displays good performances for MCF-7 determination with a linear range from 1 × 102 to 1 × 105 cell mL−1 and limit of detection down to 17 cells mL−1. Meanwhile, the PEC assay can distinguish MCF-7 from normal cells in blood samples, which may have potential applications in cancer diagnostics and therapeutics. [ABSTRACT FROM AUTHOR]

• Biosensor based on Pt nanoparticles modified MXene is developed for analysis of pyruvate. • Pyruvate oxidase oxidizes pyruvate to produce H 2 O 2. • H 2 O 2 was detected by PtNPs-modified MXene to generate electrochemical signals. • The sensor was applied for determination of pyruvate in human serum. In this study, we developed a biosensor based on Pt nanoparticles (PtNPs) modified 2D MXene for sensitive analysis of pyruvate in human serum. The pyruvate oxidase oxidizes pyruvate to produce H 2 O 2 , which is catalyzed by PtNPs-modified MXene, generating electrochemical signals for pyruvate detection, with a linear range of 10–1000 μM and a detection limit of 0.7 μM. The practical application of the sensor was verified by the determination of pyruvate in human serum. The utilization of MXene as biosensing materials may find wide application in the design of different biosensors. [ABSTRACT FROM AUTHOR]

Prussian Blue nanoparticles were deposited on g-C3N4 nanosheets. The resulting nanocomposite possesses peroxidase-like (POx) activity and can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine at room temperature in the presence of H2O2. This leads to formation of a blue product with an absorption maximum at 650 nm. The formation of the Prussian Blue nanoparticles on the g-C3N4 nanosheets, and the POx-like activity of the composite were characterized in detail. The POx mimic was used for determination of L-lactic acid via detection of H2O2 that is produced by the enzyme lactate oxidase (LOx). The assay has a linear range that extends from 5 to 100 μM, and the detection limit is 2.2 μM. The method was successfully applied to the determination of L-lactic acid in spiked human serum. [ABSTRACT FROM AUTHOR]

Electrochemical sensing of L-ascorbic acid (AA) is reported based on the use of a redox-active molybdophosphate film on a glassy carbon electrode (GCE). Molybdophosphate is formed by reacting hydroxyapatite nanoparticles with sodium molybdate. The modified GCE can be utilized for detection of AA, typically at a working potential of 0.4 V (vs. Ag/AgCl). The GCE has a decreased overpotential and enhanced sensitivity (219 μA·mM−1·cm−2). Response is linear in the 1 μM to 1.5 mM AA concentration range, and the limit of detection is 4 nM. The selectivity of this sensor makes it a useful tool for accurate determination of AA in practical samples as shown for a vitamin C tablet and for spiked beverages. [ABSTRACT FROM AUTHOR]

The authors report a sensitive electrochemical aptamer-based assay for the cancer biomarker human epidermal growth factor receptor-2 (HER2). It is based on the use of MnO2 nanosheets that were functionalized with phosphate and a HER2 binding aptamer and serve as the electrochemical probe. The assay follows a sandwich protocol. A peptide that can specifically recognize HER2 was immobilized on a gold electrode for the capture of HER2. Then, the functionalized MnO2 nanosheets were linked to the electrode via the binding between HER2 and HER2 aptamer on the MnO2 nanosheets. The reaction of phosphate and aptamer on the MnO2 nanosheets with molybdate leads to the formation of redox-active molybdophosphate. This results in dual signal amplification. The generated electrochemical current was measured at 0.22 V (vs. Ag/AgCl). The assay allows HER2 to be determined in the 0.1 to 500 pg·mL−1 concentration range, and the detection limit is as low as 0.05 pg·mL−1. The assay was successfully applied for the detection of HER2 in spiked human serum samples. [ABSTRACT FROM AUTHOR]

Hierarchical Co3O4@NiMoO4 core-shell nanowires (NWs) were synthesized utilizing a two-step hydrothermal method. The NWs show a high chemiresistive response (at a temperature of 255 °C) to xylene, with an Rgas/Rair ratio of 24.6 at 100 ppm xylene, while the response towards toluene, benzene, ethanol, and acetone, CO, H2S and NO2 is much weaker. In contrast, pure Co3O4 nanowires exhibit weak responses to all the vapors/gases and poor selectivity. The new NW sensor displays an almost linear response (1–100 ppm) to xylene and a lower detection limit of 424 ppb. The remarkable gas sensing characteristics are attributed to the synergistic catalytic effect and the formation of a heterostructure between Co3O4 and NiMoO4. Graphical abstract Schematic presentation of a xylene vapor chemiresistive sensor based on Co3O4@NiMoO4 core-shell nanowires. The Co3O4@NiMoO4 core-shell nanowires-based sensor exhibits a high response (24.6) to 100 ppm xylene at 255 °C and high response/recovery speed (13–15 and 25–29 s). [ABSTRACT FROM AUTHOR]

Nanoparticles are usually used as carrier to load more antibody and enzyme to improve the sensitivity of enzyme-linked immunosorbent assay (ELISA). However, limited by high density and complicated modification procedure, the traditional nanoparticles such as Au nanoparticles (AuNPs) usually induce large background signal and poor reproducibility in ELISA. In this work, functional lightweigh nanoparticle polystyrene@polydopamine (PS@PDA) was prepared and induced as the carrier of detection antibody and horseradish peroxidase (HRP) to form PS@PDA@Ab 2 /HRP biojungates. The appropriate density (close to water) and good hydropilicity ensure the good dispersion of PS@PDA@Ab 2 /HRP in solution, preventing the physical sedimention and decreasing the background signal even though the bioconjugate's size is close to 200 nm. The large surface area and abundant active group from PDA facilitate the loading of detection antibody and HRP, improving the loading efficiency and stability of biojungates. Based on it, taking interleukin-17A (IL-17A, a biomarker of psoriasis) as the detection target, we developed a PS@PDA-based sandwich ELISA, achieving a sensitive dynamic range from 0.3 to 80 pg/mL and a detection limit of 0.2 pg/mL. Furthermore, the contents of IL-17A were assayed successfully in 10-fold diluted serum samples from psoriasis patients. Compared with those commercial or AuNP-based ELISA, our PS@PDA-based ELISA method exhibits higher sensitivity, lower background interference, and higher stability, which will significantly improve the application of ELISA in the low-abundant biomolecule assays. [Display omitted] • Lightweight polystyrene@polydopamine (PS@PDA) nanoparticles are easy to synthesize, stable and easy to store. • Bioconjugates are less susceptible to environmental influences and have less nonspecific adsorption. • PS@PDA-ELISA has high sensitivity and wide dynamic range. [ABSTRACT FROM AUTHOR]

Ratiometric sensors with background correction are expected to get around the interference from complicated systems. However, there is currently a scarcity of viable carriers for quickly and easily assembling fluorophores with varying fluorescence emission. In this work, we developed a novel aptamer-based ratiometric fluorescence sensor for analysis of adenosine triphosphate (ATP) based on the properties of aptamer that enable conformational transformations upon binding to the target. The carbon dots (CDs) and thioflavin T (ThT) are employed as the reference and detection signal for the ratio-dependent sensor, which are both integrated with the aptamer. The green fluorescence of ThT was initially turn-on after binding with aptamer and then quenched in the presence of ATP, whereas the red fluorescence of CDs remained practically steady. This approach exhibits remarkable selectivity and can detect ATP in the range of 0.5–60 μM with a detection limit of 0.167 μM. Moreover, the method was successfully applied to the determination of ATP in human serum as well as in A549 cells, demonstrating the potential for medical diagnostic applications. • We developed ratiometric fluorescence sensor for adenosine triphosphate. • Carbon dots and thioflavin T are employed as the reference and detection signal. • The green fluorescence of ThT changed in the presence of adenosine triphosphate. • The red fluorescence of carbon dots remained practically steady. • The sensor was applied for determination of adenosine triphosphate in biological samples. [ABSTRACT FROM AUTHOR]

By combining microbial cell lysis and DNA induced current detection, we develop a new genosensor for culture independent total bacteria determination. • Amperometric genosensor could be used to quantitate bacteria via reaction between molybdate and phsphate ions on the DNA. • Bacterial DNA is corelated to the number of bacteria and bacterial viability levels, reflecting in the variations of current generated • The genosensor shows a low detection limit of broad-spectrum bacteria and can achieve bacteria count. Bacterial plate count for general assessment of water quality requires lengthy bacterial culturing. We report here a new DNA induced current genosensor for culture independent total bacteria determination. Since the amount of bacterial DNA is correlated to the number of bacteria, the genosensor measures the amount of bacterial DNA to determine bacterial count. The approach relies on bacteria lysis to release DNA which can react with molybdate to form redox molybdophosphate and measured electrochemically. Analysis of E. coli and S. aureus demonstrated that the DNA generated current is highly correlated with the level of bacteria lysis which was confirmed by spectrometric measurement. Culture independent measurement of S. aureus bacterial load suggests limit of detection is 21.9 CFU/mL, with linear range from 3 × 102 to 3 × 107 CFU/mL and correlation coefficient of 0.992. For E. coli analysis, the detection limit is 25.1 CFU/mL with the same linear range. The use of electrochemical microbial DNA quantitation for culture independent bacterial count is a new approach, the genosensor measurement is rapid (within 1 h) and has potential use for analysis of broad-spectrum bacteria for various applications. [ABSTRACT FROM AUTHOR]

Optical immunoassay for the detection of protein biomarker tumor necrosis factor-α (TNF-α) is reported based on core–shell Fe@BC structure nanozyme as label. The nanozyme is composed of Fe nanoparticles as core and carbon layer as shell with element B doped into the carbon shell. The nanozyme displays good peroxidase-like activity that can oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) to oxTMB in the presence of H 2 O 2 , generating blue colored compound. The detection of TNF-α was following the ELISA protocol by replacing traditional enzyme horseradish peroxidase (HRP) with nanozyme Fe@BC. The assay can be finished by about 2 h and has wide linear range for TNF-α from 5 pg/mL to 1 ng/mL with limit of detection of 2 pg/mL. The assay was applied for the detection of TNF-α in human serum samples with detection results comparable to commercial ELISA. [ABSTRACT FROM AUTHOR]

• A new sensor for the fluorescence detection of AA was designed. • The assay is highly selective for the detection of AA. • The sensor was successfully applied to determine AA in beverage. Water dispersible silicon quantum dots (SiQDs) showing blue fluorescence were synthesized with 3-aminopropyltriethoxysilane (APTES) as silicon source. Based on the synthesized SiQDs as the photoluminescence unit, MnO 2 nanosheets (NS) as the quencher, a "switch-on" fluorescence assay for the determination of ascorbic acid (AA) was designed. The fluorescence of SiQDs can be effectively quenched by MnO 2 NS because of the internal filtration effect. In the presence of AA, MnO 2 is reduced to Mn2+, so that the fluorescence of SiQDs is partially recovered. The recovered fluorescence intensity was related to the concentration of AA. Under the optimal experimental conditions, the linear response range of the assay to AA is 1–80 µM, and the detection limit is 0.48 µM. The method for the determination of AA has the advantages of simple, low cost, good selectivity and sensitivity. The assay has been successfully applied to the quantification of AA in beverage (mizone) samples, which proves the practicability of the assay. [ABSTRACT FROM AUTHOR]

Molecularly imprinted electrochemical sensor for ascorbic acid detection was prepared based on MXene modified glassy carbon electrode. • Molecularly imprinted electrochemical sensor detection was prepared. • MXene was utilized to modify glassy carbon electrode. • Mxene with layered structure was prepared by chemical etching method. • O-phenylenediamine was electrochemically polymerized onto MXene modified electrode. • The imprinted polymer provides binding sites for target analytes. Molecularly imprinted electrochemical sensor for ascorbic acid (AA) detection was prepared based on MXene modified glassy carbon electrode (MIPs/MXene/GCE). MXene with layered structure was synthesized by chemical etching method and the o-phenylenediamine (o-PD) polymer was electrochemically polymerized onto MXene modified electrode surface. With the binding of AA molecules onto electrode, the current response of the sensor was decreased. Under the optimized experimental conditions, the current response change of the sensor was in linear with the concentration of AA in the range from 0.5 μM to 10 μM with detection limit of 0.27 μM. The sensor has good selectivity, which was applied to the determination of AA in vitamin C tablets. [ABSTRACT FROM AUTHOR]

In this work, WO 3 nanosheets are synthesized successfully through the hydrothermal method. Ru is thereafter efficiently loaded on the surface of WO 3 using the ethylene glycol-assisted solvothermal method. The gas sensing test shows that 0.5 wt% Ru-WO 3 exhibits the best performance towards xylene detection. Compared with the pure WO 3 sensor, the gas response of the sensor based on 0.5 wt% Ru-WO 3 in 100 ppm xylene increases from 11 to 73, and the operating temperature decreases from 375 °C to 280 °C. And an ultra-low detection limit of 25 ppb is obtained. In addition, the gas sensing performance of the Ru-loaded WO 3 sensor is much better than that of Au, Pt and Pd-loaded WO 3 sensors. The enhanced gas sensing performance can be attributed to the catalytic overflow of Ru and the formation of heterojunction between RuO 2 and WO 3. It is expected that this finding gives rise to further efforts on efficient and low-cost xylene sensors that are based on Ru-loaded WO 3. Image 1 • WO 3 nanosheets were prepared by a simple hydrothermal method and loaded Ru on WO 3 by ethylene glycol hydrothermal method. • Hydrothermal method could effectively loading Ru on WO 3. • Ru-WO 3 sensor has an ultra-low detection limit of 23 ppb to xylene. [ABSTRACT FROM AUTHOR]

• Nb-doped NiO nanosheets with different dopant concentration were prepared through a facile hydrothermal process. • Dopant concentration variation influenced morphology, carrier concentration and sensing property. • Nb-NiO sensor exhibits excellent sensitivity and good sensitivity to xylene. It's demonstrated that doping of aliovalent atom can greatly influence the sensing performance of metal oxides-based gas sensors. In this work, Nb-doped nickel oxides with Nb contents in the range of 6.2–29.1 at% have been synthesized by a one-step hydrothermal method. The gas sensing test results indicates that the 20.2 at% Nb doped NiO possesses an ultrahigh response (335.1–100 ppm), excellent selectivity and theoretical ppb-level detection limit (2 ppb) to xylene at 370 °C, which is much better than that of pure NiO sensor. The higher specific surface area and the enhanced catalytic activity caused by higher ratio Ni3+/Ni2+ are considered as the main reasons for the enhanced gas sensor performance. [ABSTRACT FROM AUTHOR]