Au@ZnO functionalized three–dimensional macroporous WO3: A application of selective H2S gas sensor for exhaled breath biomarker detection

Hydrogel-based radio frequency H2S sensor for in situ periodontitis monitoring and antibacterial treatment

Periodontitis, a chronic disease, can result in irreversible tooth loss and diminished quality of life, highlighting the significance of timely periodontitis monitoring and treatment. Meanwhile, hydrogen sulfide (H2S) in saliva, produced by pathogenic bacteria of periodontitis, is an important marker for periodontitis monitoring. However, the easy volatility and chemical instability of the molecule pose challenges to oral H2S sensing. Here, we report a wearable hydrogel-based radio frequency (RF) sensor capable of in situ H2S detection and antibacterial treatment. The RF sensor comprises an agarose hydrogel containing conjugated silver nanoparticles-chlorhexidine (AG-AgNPs-CHL hydrogel) integrated with split-ring resonators. Adhered to a tooth, the hydrogel-based RF sensor enables wireless transmission of sensing signals to a mobile terminal and a concurrent release of the broad-spectrum antibacterial agent chlorhexidine without complex circuits. With the selective binding of the AgNPs to the sulfidion, the RF sensor demonstrates good sensitivity, a wide detection range (2-30 μM), and a low limit of detection (1.2 μM). Compared with standard H2S measurement, the wireless H2S sensor can distinguish periodontitis patients from healthy individuals in saliva sample tests. The hydrogel-based wearable sensor will benefit patients with periodontitis by detecting disease-related biomarkers for practical oral health management.

Multi-metallic MOF based composites for environmental applications: synergizing metal centers and interactions

The escalating threat of environmental issues to both nature and humanity over the past two decades underscores the urgency of addressing environmental pollutants. Metal-organic frameworks (MOFs) have emerged as highly promising materials for tackling these challenges. Since their rise in popularity, extensive research has been conducted on MOFs, spanning from design and synthesis to a wide array of applications, such as environmental remediation, gas storage and separation, catalysis, sensors, biomedical and drug delivery systems, energy storage and conversion, and optoelectronic devices, etc. MOFs possess a multitude of advantageous properties such as large specific surface area, tunable porosity, diverse pore structures, multi-channel design, and molecular sieve capabilities, etc., making them particularly attractive for environmental applications. MOF-based composites inherit the excellent properties of MOFs and also exhibit unique physicochemical properties and structures. The tailoring of central coordinated metal ions in MOFs is critical for their adaptability in environmental applications. Although many reviews on monometallic, bimetallic, and polymetallic MOFs have been published, few reviews focusing on MOF-based composites with monometallic, bimetallic, and multi-metallic centers in the context of environmental pollutant treatment have been reported. This review addresses this gap by providing an in-depth overview of the recent progress in MOF-based composites, emphasizing their applications in hazardous gas sensing, electromagnetic wave absorption (EMWA), and pollutant degradation in both aqueous and atmospheric environments and highlighting the importance of the number and type of metal centers present. Additionally, the various categories of MOFs are summarized. MOF-based composites demonstrate significant promise in addressing environmental challenges, and this review provides a clear and valuable perspective on their potential in environmental applications.

SnTe/SnSe Heterojunction Based Ammonia Sensors with Excellent Withstand to Ambient Humidities

Non-invasive breath testing has gained increasing importance for early disease screening, spurring research into cheap sensors for detecting trace biomarkers such as ammonia. However, real-life deployment of ammonia sensors remains hindered by susceptibility to humidity-induced interference. The SnTe/SnSe heterojunction-based chemiresistive-type sensor demonstrates an excellent response/recovery to different concentrations of ammonia from 0.1 to 100 ppm at room temperature. The improved sensing properties of the heterojunctions-based sensors compared to single-phased SnTe or SnSe can be attributed to the stronger NH3 adsorptions, more Te vacancies, and hydrophobic surface induced by the formed SnTe/SnSe heterojunctions. The sensing mechanisms are investigated in detail by using in situ techniques such as diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), Kelvin probe, and a.c. impedance spectroscopy together with the Density-Function-Theory calculations. The formed heterojunctions boost the overall charge transfer efficiency between the ammonia and the sensing materials, thus leading to the desirable sensing features as well, with excellent resistance to ambient humidities.

SnO2QDs sensitized ZnFe2O4spheres with enhanced acetone sensing performance

Herein, SnO2QDs (<10 nm) with small size instead of conventional nanoparticles was employed to modify ZnFe2O4to synthesize porous and heterogeneous SnO2/ZnFe2O4(ZFSQ) composites for gas sensing. By an immersion process combined with calcination treatment, the resultant porous ZFSQ composites with different contents of SnO2QDs were obtained, and their sensing properties were investigated. Compared with bare ZnFe2O4and SnO2QDs, porous ZFSQ composites based-sensors showed much improved sensor response to acetone. For contrast, the sensor performance of ZFSQ composites was also compared with that of ZnFe2O4sphere modified by SnO2nanoparticles with different size. The porous ZFSQ composite with 5 wt% SnO2QDs (ZFSQ-5) showed a better acetone sensing response than that of other ZFSQ composites, and it exhibited a high response value of 110-100 ppm of acetone and a low detection limit of 0.3 ppm at 240 °C. In addition to the rich heterojunctions and porous structure, the size effect of SnO2QDs was other indispensable reasons for the improved sensor performance. Finally, the ZFSQ-5 composite sensor was attempted to be applied for acetone sensing in exhaled breath, suggesting its great potential in monitoring acetone.

WO3 Nanoplates Decorated with Au and SnO2 Nanoparticles for Real-Time Detection of Foodborne Pathogens

Nowadays, metal oxide semiconductor gas sensors have diverse applications ranging from human health to smart agriculture with the development of Internet of Things (IoT) technologies. However, high operating temperatures and an unsatisfactory detection capability (high sensitivity, fast response/recovery speed, etc.) hinder their integration into the IoT. Herein, a ternary heterostructure was prepared by decorating WO3 nanoplates with Au and SnO2 nanoparticles through a facial photochemical deposition method. This was employed as a sensing material for 3-hydroxy-2-butanone (3H-2B), a biomarker of Listeria monocytogenes. These Au/SnO2-WO3 nanoplate-based sensors exhibited an excellent response (Ra/Rg = 662) to 25 ppm 3H-2B, which was 24 times higher than that of pure WO3 nanoplates at 140 °C. Moreover, the 3H-2B sensor showed an ultrafast response and recovery speed to 25 ppm 3H-2B as well as high selectivity. These excellent sensing performances could be attributed to the rich Au/SnO2-WO3 active interfaces and the excellent transport of carriers in nanoplates. Furthermore, a wireless portable gas sensor equipped with the Au/SnO2-WO3 nanoplates was assembled, which was tested using 3H-2B with known concentrations to study the possibilities of real-time gas monitoring in food quality and safety.

In-MIL-68 derived In2O3/Fe2O3 shuttle-like structures with n-n heterojunctions to improve ethanol sensing performance

Metal-organic frameworks (MOFs) have a variety of structures and unique properties that make them suitable for use in gas sensors. Herein, In2O3/Fe2O3 was successfully synthesized using simple solvothermal and impregnation methods. The response to 100 ppm of ethanol gas reached 67.5 at an optimum working temperature of 200 °C, and the response/recovery time was 9 s/236 s. The composite also exhibited excellent selectivity, repeatability, and long-term stability. SEM, TEM, XRD, and XPS were used for the characterization of materials. The excellent sensing performance of the sensors is attributed to the construction of n-n heterojunctions, an increase in oxygen vacancies, and the unique structural characteristics of MOFs. The above experimental results indicate that In-MIL-68-derived In2O3/Fe2O3 is a promising ethanol sensing material.

Achieving One Part Per Billion Hydrogen Sulfide (H2S) Level Detection through Optimizing Composition and Crystallinity of Gold-Decorated Tungsten Trioxide (Au-WO3) Nanofibers

As a common environmental pollutant and an important breath biomarker for several diseases, it is essential to develop a hydrogen sulfide gas sensor with a low-ppb level detection limit to prevent harmful gas exposure and allow early diagnoses of diseases in low-resource settings. Gold doped/decorated tungsten trioxide (Au-WO3) nanofibers with various compositions and crystallinities were synthesized to optimize H2S-sensing performance. Systematically experimental results demonstrated the ability to detect 1 ppb H2S with a response value (Rair/Rgas) of 2.01 using a 5 at % Au-WO3 nanofibers with average grain sizes of around 15 nm. Additionally, energy barrier difference of sensing materials in air and nitrogen (ΔEb) and power law exponent (n) were determined to be 0.36 eV and 0.7, respectively, at 450 °C indicating that O- is predominately ionic oxygen species and adsorption of O- significantly altered the Schottky barrier between the grain. Such quantitative analysis provides a comprehensive understanding of H2S detection mechanism.

A flexible and wearable PET-based chemiresistive H2S gas sensor modified with MoS2-AgCl@AgNPs nanocomposite for the dynamic monitoring of egg spoilage

In this study, a flexible and wearable chemiresistive hydrogen sulfide (H2S) sensor is developed by modifying the MoS2-AgCl@AgNPs (MAAN) nanocomposite on a flexible PET-based Au interdigital electrode (FPAIDE) (MAAN/FPAIDE) to monitor egg spoilage at room temperature inexpensively. A new method is developed for the low-cost batch fabrication of MAAN/FPAIDEs by laser direct writing. The morphology and composition of the synthesized MAAN nanocomposite are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and transmission electron microscopy (TEM). Based on the oxygen adsorption model, a new H2S sensing mechanism is discussed, which is related to the formation of p-n junctions between MoS2 and AgCl and the specific adsorption of H2S by AgNPs on the MAAN sensing layer, causing a decrease in resistance. X-ray photoelectron spectroscopy (XPS) is used to characterize the charge transfer between gas molecules and the MAAN sensing layer and sulfide generation during the response process. The concentration of H2S can be detected down to 27 ppb at 25 °C. Finally, the prepared sensor has been successfully utilized in the real-time monitoring of egg spoilage with satisfactory results, indicating its great potential for the application of fresh food quality and safety supervision and the smart packaging of poultry eggs.

PtPd NPs-functionalized metal-organic framework-derived α-Fe2O3 porous spindles for efficient low-temperature detection of triethylamine

In recent years, metal-organic framework (MOF) derivatives have gradually become ideal materials for gas sensors due to their controllable composition, diverse structures and open metal sites. In this research, a simplified hydrothermal method was applied to successfully prepare MOF-derived α-Fe2O3 spindles, and an in situ reduction method was then utilized to deposit Pt, Pd and PtPd bimetallic nanoparticles (NPs) on the α-Fe2O3 spindles. The effects of noble metals Pt, Pd and PtPd on the gas-sensing properties of Fe2O3 were systematically examined. The PtPd/α-Fe2O3 sensor has enhanced gas-sensing performance for triethylamine (TEA), especially at PtPd content of 1.5 wt% and mass ratio of Pt : Pd = 90 : 10, where the response of the sensor to 100 ppm TEA at a lower temperature of 150 °C is 442, which is 34 times higher than that of the original α-Fe2O3 (response of 13). Additionally, the sensor demonstrated improved response/recovery properties and very respectable selectivity, repeatability, long-term stability within 30 days and lower detection limit (500 ppb) at 150 °C. Combining the results of XPS and O2-TPD, the enhanced gas-sensing properties of PtPd bimetallic-modified α-Fe2O3 over monometallic (Pt or Pd) modified α-Fe2O3 were analyzed, which can be attributed to the chemical and electronic sensitization of noble metals and the synergistic effect of the PtPd bimetallic NPs, resulting in more surface defects and enhanced oxygen adsorption capacity of the sensing material. This work provided an effective gas-sensing material for the low-temperature detection and analysis of triethylamine gas.

A Gas Sensor with Enhanced Sensing Properties towards Butyl Acetate: Vascular Bundle Structure Zn2 SnO4 Derived from Maize Straw

The development of butyl acetate sensors with high sensitivity and selectivity has been highly desirable for its harmful effects on human health. In this work, we developed a high-performance butyl acetate sensor based on vascular bundle structure Zn2 SnO4 nanomaterial derived from maize straw. The vascular bundle structure Zn2 SnO4 with higher specific surface area obtained by calcination to remove the maize straw template, plays the dual role of accelerating the diffusion of gas molecules and providing more active sites. Our research showed that the sensor had a response of 18 to 100 ppm butyl acetate at a working temperature of 250 °C, with a fast response recovery rate (18 s/25 s), which showed significant improvement compared to the Zn2 SnO4 sensor prepared without templates. The improved performance can be attributed to the cross-linked nanoparticle chains and gas collision mechanism of the sensor. These findings highlight the potential of our sensor for the detection of butyl acetate gas.

High-performance metal-oxide gas sensors based on hierarchical core-shell ZnFe2O4 microspheres for detecting 2-chloroethyl ethyl sulfide

Mustard gas, an erosive chemical agent, is primarily used as a chemical weapon, which seriously threatens human life and health. Therefore, detecting mustard gas and its simulant, 2-chloroethyl ethyl sulfide (2-CEES), is a very important task. As a binary metal oxide with a spinel structure, ZnFe₂O₄ is widely used for fabricating gas sensors because of its stable chemical structure and abundant oxygen vacancies. In this study, gas-sensing ZnFe₂O₄ microspheres with a hierarchical core-shell nanosheet structure were prepared via a simple one-step solvothermal method. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and N₂ adsorption analyses were performed to characterize the morphology, structure, and chemical composition of these microspheres. A gas sensor was fabricated from the as-synthesized material, and its gas sensing performance was evaluated, using 2-CEES as a target gas. The obtained ZnFe₂O₄-based sensor exhibited a high sensitivity of 9.07 to 1 ppm 2-CEES at the optimal working temperature of 250 °C. The sensor response and recovery times were 18 and 546 s, respectively, and its detection sensitivity of 2.87 achieved at a 2-CEES concentration of 0.01 ppm was within an acceptable range. Additionally, the sensor demonstrated sufficiently high 2-CEES selectivity, repeatability, and long-term stability.

Facile Synthesis of ZnO/WO3 Nanocomposite Porous Films for High-Performance Gas Sensing of Multiple VOCs

Volatile organic compounds (VOCs) in indoor environments have typical features of multiple components, high concentration, and long duration. The development of gas sensors with high sensitivity to multiple VOCs is of great significance to protect human health. Herein, we proposed a sensitive ZnO/WO₃ composite chemi-resistive sensor facilely fabricated via a sacrificial template approach. Based on the transferable properties of self-assembled monolayer colloidal crystal (MCC) templates, two-dimensional honeycomb-like ordered porous ZnO/WO₃ sensing matrixes were constructed in situ on commercial ceramic tube substrates with curved and rough surfaces. The nanocomposite thin films are about 250 nm in thickness with large-scale structural consistency and integrity, which facilitates characteristic responses with highly sensitivity and reliability. Furthermore, the nanocomposite sensor shows simultaneous responses to multiple VOCs that commonly exist in daily life with an obvious suppression sensing for traditional flammable gases. Particularly, a detection limit of 0.1 ppm with a second-level response/recovery time can be achieved, which is beneficial for real-time air quality assessments. We proposed a heterojunction-induced sensing enhancement mechanism for the ZnO/WO₃ nanocomposite film in which the formation of abundant heterojunctions between ZnO and WO₃ NPs significantly increases the thickness of the electron depletion layer in the bulk film and improves the formation of active oxygen species on the surface, which is conducive to enhanced responses for reducing VOC gases. This work not only provides a simple approach for the fabrication of high-performance gas sensors but also opens an achievable avenue for air quality assessment based on VOC concentration detection.

Heteronanostructural metal oxide-based gas microsensors

The development of high-performance, portable and miniaturized gas sensors has aroused increasing interest in the fields of environmental monitoring, security, medical diagnosis, and agriculture. Among different detection tools, metal oxide semiconductor (MOS)-based chemiresistive gas sensors are the most popular choice in commercial applications and have the advantages of high stability, low cost, and high sensitivity. One of the most important ways to further enhance the sensor performance is to construct MOS-based nanoscale heterojunctions (heteronanostructural MOSs) from MOS nanomaterials. However, the sensing mechanism of heteronanostructural MOS-based sensors is different from that of single MOS-based gas sensors in that it is fairly complex. The performance of the sensors is influenced by various parameters, including the physical and chemical properties of the sensing materials (e.g., grain size, density of defects, and oxygen vacancies of materials), working temperatures, and device structures. This review introduces several concepts in the design of high-performance gas sensors by analyzing the sensing mechanism of heteronanostructural MOS-based sensors. In addition, the influence of the geometric device structure determined by the interconnection between the sensing materials and the working electrodes is discussed. To systematically investigate the sensing behavior of the sensor, the general sensing mechanism of three typical types of geometric device structures based on different heteronanostructural materials are introduced and discussed in this review. This review will provide guidelines for readers studying the sensing mechanism of gas sensors and designing high-performance gas sensors in the future.

Nanometer-Thick ZnO/SnO2 Heterostructures Grown on Alumina for H2S Sensing

Designing heterostructure materials at the nanoscale is a well-known method to enhance gas sensing performance. In this study, a mixed solution of zinc chloride and tin (II) chloride dihydrate, dissolved in ethanol solvent, was used as the initial precursor for depositing the sensing layer on alumina substrates using the ultrasonic spray pyrolysis (USP) method. Several ZnO/SnO₂ heterostructures were grown by applying different ratios in the initial precursors. These heterostructures were used as active materials for the sensing of H₂S gas molecules. The results revealed that an increase in the zinc chloride in the USP precursor alters the H₂S sensitivity of the sensor. The optimal working temperature was found to be 450 °C. The sensor, containing 5:1 (ZnCl₂: SnCl₂·2H₂O) ratio in the USP precursor, demonstrates a higher response than the pure SnO₂ (∼95 times) sample and other heterostructures. Later, the selectivity of the ZnO/SnO₂ heterostructures toward 5 ppm NO₂, 200 ppm methanol, and 100 ppm of CH₄, acetone, and ethanol was also examined. The gas sensing mechanism of the ZnO/SnO₂ was analyzed and the remarkably enhanced gas-sensing performance was mainly attributed to the heterostructure formation between ZnO and SnO₂. The synthesized materials were also analyzed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray, transmission electron microscopy, and X-ray photoelectron spectra to investigate the material distribution, grain size, and material quality of ZnO/SnO₂ heterostructures.

A multi-platform sensor for selective and sensitive H2S monitoring: Three-dimensional macroporous ZnO encapsulated by MOFs with small Pt nanoparticles

The high-selectivity and high-sensitivity determination of trace concentrations of toxic gases is a major challenge when using semiconductor metal oxide (SMO) gas sensors in complicated real-world environments. In this study, by strategically combining a three-dimensional inverse opal (3DIO) macroporous ZnO substrate and a ZIF-8 outer filter membrane, two series of sensors with Pt NPs loaded at different locations are developed. In the optimal 3DIO ZnO@ZIF-8/Pt sensor, the existence of small Pt NPs in ZIF-8 cavities can effectively accelerate the absorption of H₂S, capture electrons from the N site of ZIF-8, and donate the electron to the S site of H₂S, as indicated by density functional theory simulations, leading to a significantly increased response to H₂S. Together with the molecular-sieving effect that ZIF-8 exerts on gas molecules with larger kinetic diameters, the 3DIO ZnO@ZIF-8/Pt sensor exhibits a high response to H₂S (118-5.5 ppm), a detection limit of 40 ppb, and importantly, a 59-fold higher selectivity to H₂S against typical interference gases. In addition, the 3DIO ZnO@ZIF-8/Pt sensor is developed as a multi-platform sensor to evaluate trace concentrations of H₂S in meat quality assessment, halitosis diagnosis, and automobile exhaust assessment.

Efficient detection of hazardous H2S gas using multifaceted Co3O4/ZnO hollow nanostructures

The rapid increases in environmental hazardous gases have laid dangerous effects on human health. The detection of such pollutants gases is mandatory using various optimal techniques. In this paper, porous multifaceted Co₃O₄/ZnO nanostructures are synthesized by pyrolyzing sacrificial template of core-shell double zeolitic imidazolate frameworks (ZIFs) for gas sensing applications. The fabricated exhibit superior gas sensor response, high selectivity, fast response/recovery times, and remarkable stability and sensitivity to H₂S gas. In particular, the multifaceted Co₃O₄/ZnO nanostructures show a maximum response of 147 at 100 ppm of H₂S under optimum conditions. The remarkable gas sensing performances are mainly ascribed to high porosity, wide surface area multifaceted nanostructures, presence of heterojunctions and catalytic activity of ZnO and Co₃O₄, which are beneficial for H₂S gas sensors industry.

Ultra-sensitive H2S sensor based on sunflower-like In-doped ZnO with enriched oxygen vacancies

In–ZnO with oxygen vacancies exhibits a higher sensing response and a shorter recovery time for H2S compared to ZnO.

Selective Determination of Hydrogen Sulfide Using SnO2–Ag Sensor Working in Non-Stationary Temperature Regime

The application of a non-stationary regime of temperature modulation in metal oxide semiconductor sensor based on SnO2–Ag leads not only to a strongly increased sensor response, but also to a considerably improved sensor selectivity toward hydrogen sulfide. Selectivity with respect to other reducing gases (CO, NH3, H2) is about five orders of magnitude, enabling a correct selective determination of H2S in the presence of interfering gas components.

Ethanol Sensing Properties and First Principles Study of Au Supported on Mesoporous ZnO Derived from Metal Organic Framework ZIF-8

It is of great significance to develop ethanol sensors with high sensitivity and low detection temperature. Hence, we prepared Au-supported material on mesoporous ZnO composites derived from a metal-organic framework ZIF-8 for the detection of ethanol gas. The obtained Au/ZnO materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (SEM), field emission transmission electron microscopy (TEM) and nitrogen adsorption and desorption isotherms. The results showed that the Au/ZnO-1.0 sample maintains a three-dimensional (3D) dodecahedron structure with a larger specific surface area (22.79 m² g⁻¹) and has more oxygen vacancies. Because of the unique ZIF structure, abundant surface defects and the formation of Au-ZnO Schottky junctions, an Au/ZnO-1.0 sensor has a response factor of 37.74 for 100 ppm ethanol at 250 °C, which is about 6 times that of pure ZnO material. In addition, the Au/ZnO-1.0 sensor has good selectivity for ethanol. According to density functional theory (DFT) calculations, the adsorption energy of Au/ZnO for ethanol (−1.813 eV) is significantly greater than that of pure ZnO (−0.217 eV). Furthermore, the adsorption energy for ethanol is greater than that of other gases.

CuxO Nanostructure-Based Gas Sensors for H2S Detection: An Overview

H2S gas is a toxic and hazardous byproduct of the oil and gas industries. It paralyzes the olfactory nerves, with concentrations above 100 ppm, resulting in loss of smell; prolonged inhalation may even cause death. One of the most important semiconducting metal oxides for the detection of H2S is CuxO (x = 1, 2), which is converted to CuxS upon exposure to H2S, leading to a remarkable modulation in the resistance and appearance of an electrical sensing signal. In this review, various morphologies of CuxO in the pristine form, composites of CuxO with other materials, and decoration/doping of noble metals on CuxO nanostructures for the reliable detection of H2S gas are thoroughly discussed. With an emphasis to the detection mechanism of CuxO-based gas sensors, this review presents findings that are of considerable value as a reference.

A review of low-temperature H2S gas sensors: fabrication and mechanism

Reduced detection temperature of hazardous gases such as H2S can lower power consumption and increase the long-term stability. The decreased operating temperature can be achieved via physical and chemical modification of the sensing layer.

Microscopic Nature of Gas Adsorption on WO3 Surfaces: Electron Interaction and Localization

Even though WO₃ has been used in gas-sensors for many years, little is known about the gas-sensing mechanism. Using first-principles density functional theory and experimental methods, we study the adsorption of CO, H₂, NH₃, and NO₂ on the surface of WO₃ (001). The results indicate that the surface undergoes reconstruction after gas adsorption, which inevitably causes the localization of surface electrons and a change of the electric (sensing) signal. Through the analysis of atomic orbital and molecular orbital, the adsorption mechanism can be effectively predicted. The above analysis confirms that the resistance change is only related to the electronic behavior of the surface and the gas. We corrected problems associated with adsorption energy to characterize the adsorption strength of a gas on a surface, and we investigated the effect of the test temperature and test environment on both electronic interaction and the final electric sensing signal.