Graphene oxide (GO) is a promising candidate for humidity sensing, and the uniformity and thickness of GO films are important for the reproducibility and test signal strength of humidity sensors. In this paper, uniform and thickness-controllable GO films are first formed by the surface tension of different concentrations of GO solution and then transferred to surface acoustic wave (SAW) humidity sensors. This GO film formation and transfer process has very good repeatability and stability, as evidenced by the humidity response of the sensors. With the help of the uniform and highly oxidized GO film, the humidity sensors show a significantly high sensitivity (absolute sensitivity of 25.3 kHz/%RH and relative sensitivity of 111.7 p.p.m./%RH) in a wide test range from 10%RH to 90%RH with very little hysteresis (<1%RH). The sensors achieve good reversibility, excellent short-term repeatability and stability. Moreover, the humidity sensors also show a fast response and recovery time of <10 s. A technique that creates uniform films of graphene oxide enables the production of humidity sensors with high accuracy and stability. Technological advancement is fueling a demand for humidity sensors, but reliably producing uniform, high accuracy sensors at a low cost is still a challenge. Jin Xie and his team from China’s Zhejiang University have now developed a surface tension method to create uniform-thickness films of graphene oxide (GO), a humidity sensing material, which can then be transferred to sensor platforms. In tests, the resulting sensors showed high sensitivity from 10% to 90% relative humidity and quick response times to changing conditions. Altering the concentration of GO solutions produces films of varying thickness. This technique may be further developed for use in applications ranging from medical, to agriculture, to consumer electronics. With the rapid development of the internet of things, the demand for humidity sensors is also growing. Humidity sensors not only are required to have high sensitivity, a wide test range, small hysteresis, and fast response and recovery but also need to have low cost, low energy consumption, and easy integrability3. Currently, research on humidity sensors is mainly based on optical4, resistive6, capacitive8, and acoustic resonant devices10. The capacitive and resistive devices are small, simple, and low cost, but they have the disadvantage of poor test accuracy. Although the optical devices have high test accuracy, they consume too much energy and are difficult to integrate.In recent years, applications of humidity sensors have expanded from traditional industry and agriculture to medical, consumer electronics, and smart homes12, simple processing, and high test accuracy due to its quasi-digital output signal13. Combined with aluminum nitride (AlN) as the piezoelectric material, the fabrication of SAW devices is compatible with the Complementary Metal Oxide Semiconductor (CMOS) process14. Moreover, the chemical stability15 and hydrophobicity16 of AlN are beneficial to the stability of the SAW device during humidity sensing.As one of the acoustic resonators, the surface acoustic wave sensor (SAW) has advantages of small size, high sensitivity17, semiconductors18, polymers19, and nanomaterials20. Recently, graphene oxide (GO) has been widely used by researchers on different humidity-sensing platforms23. In addition to their large surface-to-volume ratio as a carbon nanomaterial, GO films also have high hydrophilicity24 and electrical insulation25 due to its oxygen-containing functional groups on the surface. These features make GO films very suitable for covering the surface of SAW devices as a humidity-sensing material. Our previous work has demonstrated that an AlN SAW resonator combined with a GO film has good performance in humidity sensing26. For practical applications, the thickness and uniformity of the GO film should be controlled, because they will greatly affect the test signal strength27 of the SAW humidity sensors as well as sensor-to-sensor reproducibility28. Nevertheless, how to deposit a uniform and thickness-controllable GO film on the surface of a device is still a problem. Although the existing methods of drop casting30, spin coating22, and dip coating2 are simple, the uniformity and thickness of the GO films formed on devices by these methods are difficult to accurately control, especially when the piezoelectric material is hydrophobic and the surface of the piezoelectric layer is covered with electrodes. Using ink-jet printing23, spray coating31, and ultrasonic atomization32, the thickness of GO films can be controlled, but the obtained GO films still have poor uniformity. In summary, because of the limitation of the surface structure of the SAW device and the hydrophobicity of the piezoelectric material, it is difficult to directly deposit a uniform and thickness-controllable GO film on the surface of the AlN SAW device.In addition to the sensing platform, it is also important to choose appropriate humidity-sensing materials to meet the requirements of humidity sensors with high performance. Commonly used humidity-sensing materials can be divided into ceramics33, we propose a simple and convenient GO film-forming method based on the surface tension of GO solution by using copper rings in this paper. When the water in the GO solution evaporates, uniform GO films are gradually formed and attached to the copper rings. The thickness of the GO films can be controlled by the concentration of the GO solution. Good uniformity and thickness controllability of the GO films can be achieved by this method. After the GO films are transferred to the surface of the SAW devices, the performances of the SAW humidity sensors are investigated systematically, including their sensitivity, sensor-to-sensor reproducibility, sensing mechanism, hysteresis characteristic, stability, reversibility, repeatability, response, and recovery time.Inspired by a fabrication process of free-standing inorganic sheets34, the W/W0 electrode structure was used. W and W0 are acoustic apertures and total IDT apertures and the lengths of W and W0 are 62λ and 65λ, respectively. An AlN/Si layered structure was adopted to fabricate the SAW resonator to achieve good thermal stability15. After 1 µm AlN was deposited over the Si wafer by reactive sputtering, a 200 nm gold layer was patterned on the surface of the AlN layer to form the IDTs and reflectors through a lift-off process.Schematics of the SAW humidity sensor before and after being covered with a GO film are illustrated in Fig. 36 and the copper rings with a 4.5 mm diameter were made of 0.1 mm diameter copper wire. To achieve the formation of different thicknesses of GO films, some of the original solution was further diluted to 1.5 and 1 mg/ml with deionized water. When the copper rings in the solutions were slowly removed, the GO solutions became liquid films adhered to the copper rings due to the surface tension of the solutions  survey scan of GO in Fig. −1 in the Raman spectrum correspond to the well-known D and G bands, respectively, and the band intensity ratio, ID/IG, can be used as an indicator for evaluating the oxidation degree of GO41. The higher the ID/IG ratio is, the higher the degree of oxidation (the ID/IG ratio is 1.01 for the selected GO). In addition, the C/O ratio in GO can also be used to measure the degree of oxidation of GO42. Highly oxidized GO typically has a lower C/O ratio and the calculated C/O ratio of selected GO is 1.85.Increasing the oxidation degree of GO can improve its water absorption performance11 and S21) of the SAW resonator with a GO film. The resonant frequency of the sensor is ~226.3 MHz according to the peak position on the spectrum curve. During the test, we can track the peak position to reflect the resonant frequency change of the sensor. To study the stability of the GO film prepared by the surface-tension method and sensing mechanism of the humidity sensor, we prepared six sensor samples and the sensor characteristics are summarized in Table 43. During the humidity test, the sensors were fixed successively in a relatively sealed metallic chamber, as shown in Fig. 2 into the chamber, the humidity inside the chamber can be adjusted (5% RH to 95% RH), whereas the humidity and temperature inside the box were calibrated in real time through a commercial hygrothermograph (TASI-621) connected to the outlet of the chamber. A network analyzer (E5061B) together with a PC with the LabVIEW program was used to detect and record changes in the resonant frequencies of the sensors.Compared with the GO film deposited directly on the surface of the SAW device by the drop-casting method, the film obtained by the proposed surface-tension method has obvious advantages in uniformity and smoothness, as displayed in Fig. The humidity-sensing performance of sensor D1 was first tested repeatedly from 10%RH to 90%RH with an interval of 10%RH. Figure 44. GO films with different thicknesses have different adsorption capacities for water moisture, so the thickness of the GO films is one of the factors affecting the sensing performance of the sensors. Figure 45. For this reason, the GO film can be regarded as an acoustic thin film46 and the viscoelastic change in the GO film has little effect on sensor response. In addition, the conductivity of the GO film increases with increasing humidity, but as the frequency of the test signal increases, the magnitude of the change in the conductivity of the GO film gradually decreases47. SAW devices usually operate at frequencies of a few hundred megahertz and the conductivity of the GO film should be insensitive to humidity changes at such high frequencies. To further verify the effect of GO film conductivity changes on the sensor response, the sensing performance of sensor D6 with a metallized path was tested and compared with that of sensor D1. As shown in Fig. 48, the relation between the mass change of the GO film (Δm) and the resonant frequency shift of the sensor (Δf) can be described as:f0 is the resonant frequency, C is a constant, and A is the sensing area. There is a linear relationship between the mass change and the resonant frequency shift according to the above equation. However, as shown by the humidity response curves of all the sensors, the resonant frequency shifts increase nonlinearly with increasing relative humidity, which means that the mass increases of GO films are nonlinear during this process. This phenomenon is because at high humidity levels, the water molecules gradually penetrate into the interlayer of the GO film and enlarge the layer spacing of the film39, thereby increasing the water absorption capacity of the film, resulting in a greater increase in the mass change. The sensing performance of the sensors is quantified as absolute sensitivity (Sa) and relative sensitivity (Sr) for easy comparison , and the absolute sensitivity and relative sensitivity of our sensors reach up to 25.3 kHz/%RH and 111.7 p.p.m./%RH. Compared with other SAW and QCM humidity sensors based on different sensing materials summarized in Table During the humidity test, AlN and Si do not absorb water, and only the GO film absorbs water. The changes in the resonant frequency of the sensors are mainly caused by changes in the mass, viscoelasticity, and conductivity of the GO films after water moisture absorptionentclass1pt{minimaAs a humidity-sensing layer, the GO film can quickly absorb and desorb water molecules, thus allowing the humidity sensors to have low hysteresis. Among all the sensors, sensor D1 was used to study hysteresis, because it has the thickest GO film. The humidity hysteresis characteristic of sensor D1 was tested by changing the relative humidity from 10%RH to 90%RH and then quickly back to 10%RH. In Fig. The continuous response of sensor D1 to humidity changes was investigated by continuously adjusting the humidity from 15%RH to different humidity levels and then back to 15%RH. The resonant frequency changes of the sensor were measured and recorded with a time interval of 1 s and the sensor shows very good tracking performance as the humidity is continuously changed between different relative humidities. When the humidity is set back to 15%RH, the sensor can quickly recover to its initial state, as indicated in Fig. The response and recovery speed of sensors D1, D4, and D5 were investigated by rapidly changing the relative humidity between 15%RH and 80%RH, and detailed response and recovery processes of the sensors are shown in Fig. In summary, we proposed a surface-tension method to obtain uniform and thickness-controllable GO films for use as humidity-sensing layers to improve the performance of SAW humidity sensors. The thickness of the GO films can be controlled by adjusting the concentration of GO solution. This GO film formation and transfer process has good repeatability and stability. Compared with other GO-based SAW humidity sensors, our sensors have significantly high sensitivity because of the use of highly oxidized GO. Increasing the thickness of the GO film can further improve the sensing performance of the humidity sensor, as the water absorption of the GO film is the main cause of the resonant frequency change of the sensor. Furthermore, theoretical analysis and experimental tests were carried out to investigate the effects of GO film viscoelasticity and conductivity changes on sensor response during humidity sensing, illustrating that the mass loading effect is the main contributor to the sensing mechanism of the sensor. Furthermore, very little humidity hysteresis and good stability of the sensors are obtained. The humidity sensors also show excellent reversibility and short-term repeatability, a fast response and a short recovery time.


The detection mechanisms pertaining to various gases are also discussed. In conclusion part, some existing problems which may hinder the sensor applications are presented. Several possible methods to solve these problems are proposed, for example, conceived solutions, hybrid nanostructures, multiple sensor arrays, and new recognition algorithm.Graphene-based gas/vapor sensors have attracted much attention in recent years due to their variety of structures, unique sensing performances, room-temperature working conditions, and tremendous application prospects, etc. Herein, we summarize recent advantages in graphene preparation, sensor construction, and sensing properties of various graphene-based gas/vapor sensors, such as NH GasesAs one of the most fascinating materials, graphene has aroused scientists’ great enthusiasms in its synthesis, modification, and applications in many fields since 2004 , due to In principle, a sensor is a device, purpose of which is to sense  some characteristics of its environs. It detects events or changes in quantities and provides a corresponding output, generally as an electrical or optical signal. According to different forms of reaction with external atmospheres, gas/vapor sensors can be classified into chemiresistor, silicon-based field-effect transistor (FET), capacitance sensor (CS), surface work function (SWF) change transistor, surface acoustic wave (SAW) change transistor, optical fiber sensor (OFS), and so on . Among tThrough real-time monitoring and analyzing the response curves of sensing devices, the realistic realization of vapor detection can be achieved. Figure For evaluating the performance of gas/vapor sensors, there are a few critical parameters including component resistance, measure resistance, sensitivity, limit of detection, response time, recovery time, and selectivity. The definitions and formulas of these parameters are summarized in Table 2/Si substrate, and the high-purity, single-layered graphene was obtained 2/Si subsobtained , Ru 70]2/Si subsobtained . Copper obtained .2 wafer [2SO4, HNO3, or HClO4, is used to transform graphite to graphite oxide. (2) complete exfoliation of graphite will take place, and molecular-level dispersion of individual graphene oxide (GO) in water or other polar solvent via ultrasonication will be achieved. (3) through the reduction of GO suspended in water or organic solvents, reduced graphene oxide (RGO) can be prepared without changing its morphology. Conductivity of RGO would be partly recovered too. The RGO sheets have quite high specific surface areas, which can be considered as a promising candidate for gas detection.In 2006, Stankovich et al. created a bottom-up approach when they incorporated graphene sheets in a composite material and the far-reaching method, which called chemical reduction of graphene oxide, pave the way for graphene’s large-scale production, modification, and application . Figure 2 wafer . In geneBrodie method , StaudenIn a sense, the development of chemical reduction can provide equivalent routes for production and modification of graphene materials via wet chemical techniques. As such, the reductant is so important since it can affect the properties of vapor detecting devices to a large degree –111. Fan3 and showed higher sensitivity with concentrations at ppm levels compared to those of the RGO attached with acid-doped PANI and de-doped PANI [Recently, we found that PPD-reduced RGO exposed to DMMP exhibited much better response than that of the RGO reduced from hydrazine . At the ped PANI .3, NO2, H2, CO, SO2, H2S, and volatile organic compounds (VOCs). Subsequently, some information from related works was summarized and discussed. Efforts have been made to exploit these sensitivities in the development of new sensor technologies.Graphene has shown excellent sensing properties toward NH3) is a compound of nitrogen and hydrogen with the formula NH3, which is a colorless gas with a characteristic pungent smell. Ammonia not only contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers, but also is a building-block for the synthesis of many pharmaceuticals, and is used in many commercial products. Although widely used, this gas is both caustic and hazardous, and thus it is harmful to human and would pollute environment. Therefore, the detection of NH3 is a pressing requirement for the modern society.Ammonia  was lower than that of PANI (ca. 10 ppm). This indicated that the sensitivity of graphene/PANI sensors for NH3 detection was enhanced by introduction of graphene into PANI. A simple, low-cost, and practical inkjet-printing technique for fabricating an innovative flexible gas sensor based on graphene–poly :poly (styrene sulfonate) (PEDOT:PSS) composite films with high uniformity over a large area was created by Seekaew et al. [3 in a low concentration ranging from 25 to 1000 ppm at room temperature. This novel and convenient method would provide a new thought for the controllable and mass manufacture of gas detectors. Table 3 detection based on graphene.Recently, a great deal of efforts had presented a great leap forward in the development of graphene gas sensors for ammonia detection. Gautam and his team investigated ammonia gas-sensing behaviors of graphene synthesized by CVD, of which the sensitivity and the recovery time were enhanced by the deposition of gold nanoparticles on the surface of graphene films . Yavari etectors . They fooperties . The resw et al. . Figure 2. On one hand, this reddish-brown gas, as one kind of the important chemical feedstocks, is an intermediate in the industrial synthesis of nitric acid. On the other hand, the toxic gas has characteristic sharp, biting odor, and is a prominent air pollutant. The whole society has a strong demand for NO2 detection, in order to curb environmental pollution and keep the safety and health of human beings.Nitrogen dioxide is one of several nitrogen oxides with the formula NO2 sensing showing the lower detection limit, higher response, and more practical manufacturing techniques. Choi and his co-workers reported a highly sensing NO2 gas sensor based on multilayered graphene films synthesized by a CVD method on a microheater-embedded flexible substrate [2 at sub-ppm (<200 ppb) levels. It also presented high responses and a short response time, when it was exposed to 1 ppm NO2 at room temperature. Hoa et al. reported that they built a gas sensor with hybrid structures of 2D graphene and 2D NiO nanosheets, sensitivity of which was two orders higher than those of devices based on NiO nanosheets alone toward NO2 even at 1 ppm level [2O3-modified RGO nanosheets were prepared by Dong’s team [2 at room temperature, which was 65.5 times higher than that of pure graphene, and the detection limit could be decreased down to 0.18 ppm. Huang et al. fabricated a gravure-printed chemiresistor-type NO2 sensor based on sulfonated RGO decorated with Ag nanoparticles (RGO/S + Ag)  gas sensors composed of reduced graphene oxides using commercially available yarns and molecular glues through an electrostatic self-assembly method [2 gas at room temperature with selectivity to other gases such as acetone, ethanol, ethylene, and CO2. Herein, we summarized recent researches about graphene-based gas sensors for NO2 detection, as shown in Table Compared to the development of ammonia detection, there are several reports about NOubstrate . The mulpm level . As showg’s team . The 3D-y method . The res2) is not very reactive under standard conditions, it does form compounds with most elements. As one of the most important industrial chemicals and potential clean energy facing the future, hydrogen has aroused a great attention. Large-scale preparation, transportation, and application of this material have a strong demand for rapid detection and accurate analysis, which makes H2 detection become a research hotspot recent years.While hydrogen  were observed dependent on the background temperature: 0.832 eV for 30–60 °C, 0.396 eV for 60–100 °C, and 0.057 eV for 100–170 °C. Their results contribute to the theoretical research of gas/vapor detection. Meanwhile, Chu and his team studied the effect of thickness of the Pt metal layer on hydrogen-sensing sensitivity of Pt-coated and multilayered graphene, and they concluded that the Pt coating improved the response time of the graphene sensor, but decreased the sensitivity [2 molecules on graphene with mono-atom-vacancies by using density functional theory (DFT) calculations [−35 mol L−1 theoretically. The reaction pathway of H2 molecule dissociative adsorption on pristine graphene and treated graphene with a monoatom-vacancy was displayed in Fig. 2 detection based on graphene.Johnson and his co-workers reported a novel Pd-functionalized multilayered graphene nanoribbon networks with excellent sensitivity to Hperature . The relu et al. . Three Esitivity . When thsitivity . Jiang eulations . They de2), carbon monoxide (CO), and methane (CH4) are very familiar to our daily life, industrial manufacture, and environmental protection. CO2 is not only the primary source of carbon in life, but also of significant impact in air pollution, which can cause global warming. CO is toxic to humans when encountered in concentrations above about 35 ppm. Besides, this colorless, odorless, and tasteless gas is one kind of gaseous fuels, which is widely used as reducing agent in industry. CH4 is the simplest alkane and the main component of natural gases. On one hand, the relative abundance of methane makes it an attractive fuel. On the other hand, it is the chief culprit of a gas explosion. In a word, detection and early warning of these gases is a pressing need for modern society.Carbon dioxide (CO2 of graphene/Y2O3 quantum dots (QDs) [2O3 QDs [2O3 QDs [2, NH3, CO2, N2, H2O, and H2) on Li-decorated T graphene, using DFT [2, CO and CH4 detection, as shown in Table Nemade et al. have carried out a lot of work focusing on graphene-based carbon dioxide sensor over the recent years . They fats (QDs) , graphen[2O3 QDs , and gra[2O3 QDs , respectsing DFT . They fosing DFT . This hy2) and hydrogen sulfide (H2S) are very harmful to mankind and animals. In recent years, some researchers reported some novel gas sensors for the detection of SO2 and H2S based on graphene composites. Shen and his team demonstrated that GO nanosheets derived from chemically tailoring acted as a promising material for SO2 gas sensing [2, indicating its potential application in SO2 detection [2 molecular sensors [2S molecules on Ag-decorated defective graphene composite were investigated using first principles calculations based on DFT by Xian’s team [2S molecule. Zhou et al. fabricated a RGO/Cu2O nanocomposite-based sensor with a very low detection limit of 5 ppb at room temperature, which might be on account of high surface activity adsorption of H2S gas molecules due to the absence of any surfactant capping [2S within 500 μs, as well as a fast recovery time of less than 30 s [2O3/graphene nanosheets. The experimental results illustrated that structural orientation of nanosheets played an essential role in maximizing efficiency of the device. In a word, their remarkable jobs and significant results have greatly promoted the development of graphene-based gas sensors. Table 2 and H2S detection based on graphene.As main atmospheric pollutants, sulfur dioxide  are organic chemicals that have a high vapor pressure at room temperature. VOCs are numerous, varied, and ubiquitous. They refer to gases which containing organic compounds, including aromatic hydrocarbon, nitro hydrocarbon, halogenated hydrocarbon, long chain alkane, alcohol, ether, acetone, grease, hydrazine, and so on. Most of them are toxic, flammable, and explosive gases. At present, as the terrible activities are of high frequency, the detection of explosives and chemical warfare agents (CWAs) attracts an increasing attention in many fields and is becoming a hot topic for research.In general, the study of graphene-based vapor sensors for detection of VOCs, explosives, and CWAs is relatively immature. As such, many novel approaches have been developed to explore the terra incognita.2O3/graphene nanocomposites with the aid of supercritical CO2 derived from graphene oxide [2O3/graphene nanocomposites were firstly reported on the basis of catalytic chemiluminescence mechanisms. They boldly broke through the limitation of the traditional preparation and measurement methods, leading a new way to tackle relevant problems. In the same year, Zhang et al. reported an intrinsic polymer optical fiber (POF) sensor based on graphene, which was described for the purpose of acetone vapor sensing for the first time [Dua and co-works developed a rapid and one-step method for the conversion of exfoliated GO into RGO using aqueous vitamin C as a mild and green reducing agent . The RGOne oxide . The ethrst time . Gautam’rst time .−16 M), which was based on p-aminothiophenol functionalized graphene nanosheets decorated with silver nanoparticles [Tang et al. established a prominent analytical platform for electrochemical sensing determination of nitroaromatic explosive compounds, such as 2,4,6-trinitrotoluene (TNT), which was superior to other TNT-sensing platforms, using uniform and rich-wrinkled graphene films prepared by electrophoretic deposition techniques . The detarticles . GO modiarticles . Their earticles . Though We have given a brief introduction to the classification of gas/vapor sensors. Considering that the gas-sensing mechanisms of graphene is uncertain and related research is rare, herein, we just give a recognized point of view as a general introduction of the reference of other related literatures –203.Graphene is intrinsically inert and nonselective. Its great efficiency to conduct electricity and distinguishing features of ballistic transport of charges decide that this two-dimensional material is an ideal candidate to serve as a platform or a supporter, in which we can realize many specific functions by doping or compositing with other materials. Once combined with other materials physically or chemically, graphene can show the characteristics of the semiconductor in normal circumstances, of which conductivity is determined by carriers’ concentration. For chemiresistor-type sensors, sensing materials show response to externalities by the change of conductivity, that is the variation of concentration of hole or electron carriers. Bulk porous materials usually have a large specific surface area, hence gas molecules can be easily adsorbed, following by the interaction between gas molecules and specific groups in the graphene surface, and then the gas molecules capture or donate electrons from the sensing material, which changes concentration of the semiconductor’s carriers.3, it would get electrons from the gas molecules, leading to an increase of the electron concentration, i.e., a decrease of graphene’s resistance occurs. Likewise, when it is exposed to an oxidation atmosphere, such as NO2, it will deliver electrons to the gas molecules, leading an increase of hole concentration, leading to an increase of graphene’s resistance. Figure Different doping and reaction conditions may lead to different types of graphene-based semiconductors (p-type or n-type). As we all know, p-type semiconductors refer to those who have a larger hole concentration than electron concentration. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. As opposed to p-type semiconductors, n-type semiconductors have a larger electron concentration than hole concentration. In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. For example, one doped graphene shows characteristic of n-type semiconductors: when it is exposed to a reducing atmosphere, such as NHZhou et al. have found that the total flow rate had a significant effect on the initial electric resistance of the sensors and their sensing properties to target gases. In addition, an appropriate quantity of deposited RGO solution was critical for sensors’ sensing response and sensitivity. Finally, they raise a novel sensing mechanism for chemiresistors based on RGO at room temperature . Zhu andThe interests in the study of nanomaterials have escalated in the recent decades, while the application is still in its infancy. This so-called “game changing” technology has met, one after another, many impediments on its way to large-scale industrialization . Can graTheory can indicate a direction for practice. However, till now, the mechanism of gas sensing based on nanomaterials is not very clear, and quantitative calculation is almost impossible. There is little doubt that graphene thin film has great sensitivity; however, this may lead to another result that it is sensitive to many kinds of gases. Cross-sensitivity means sensor shows similar responses to the different types of gases, and this character may result in false detecting. For example, cross-sensitivity can be a problem in the detection of ethylene oxide, as ethylene oxide requires a very active working electrode catalyst and high operating potential for its oxidation. Therefore, gases which are more easily oxidized like alcohols and carbon monoxide will also give a response. Once a technique reaches the stage of mass production, it will be a completely different compared with the laboratory. One of dire challenges we confronted with is the nonrepeatability of device fabrication. From preparation of sensing materials to construction of gas/vapor sensors, from building of experimental platforms to characterization parameters, none of the uniform criteria is listed, and neither specification of laboratory equipment nor the unified presentation of technological process and synthesis method was reported.In order to overcome the problems mentioned above, we put forward several worthwhile schemes and directions. Sensing materials are the core of gas detection in a real-world application. The development of synthesizing novel materials with high sensitivity and selectivity is one of the mainstream trends of gas sensors. Multicomponent classification and hybrid nanostructures which have multifunctions and outstanding performances in practical tasks are at the forefront of current research. By the improved preparation techniques such as modification of graphene, 3D structure tailoring, and thermal treatments, we may make sensors to suit the ideal state.2, NO2, and humidity and those of pristine graphene. The comparison explicitly instructs that modification can change the sensing properties to a large extent. Yavari and his team manufactured a macro-graphene foam-like 3D network which had both the advantages of the nanostructured and conventional solid-state and conducting-polymer sensors. A surprising sensing property and ppb level detection of NH3 and NO2 in air at room temperature had been demonstrated for this robust, flexible, and novel material [Figure material . The mic2O3 nanowire-like network directly on the surface of coplanar sensors array by structure replication from sacrificial CNTs was obtained on the basis of screen-printing technology and calcination. The device showed that excellent gas-sensing properties benefited from fabrication of coplanar gas sensors arrays and materials, which had special porous nanowire-like network micromorphology. Figure 2O3 nanowire-like network and the related devices.Based on the technology of microelectromechanical systems, multiple sensor arrays, in which every unit has different heterostructure and shows different sensing characteristics, can be assembled and expected to have higher sensitivity and improved selectivity. Yi et al. presented a novel materials—sensor integration fabrication strategy, which involved the introduction of micro-injection to fabricate sensing devices. The In2-based gas sensor, and the quantitative analyses of the pure pesticide vapor and their mixture were performed by fast Fourier transformation [The employment of the new signal-processing technology and recognition algorithm based on single-chip system is an important direction for the development of gas-sensing devices. By the application of dynamic detection, signal processing, and recognition algorithm, gas/vapor sensors with low power consumption, portable volume, and intelligent operation could be achieved –215. Huaormation . The resThe future of graphene-based gas/vapor sensors looks bright. Continued progress in this field will overcome the current challenges, get through the close siege, and lead to a class of gas sensors with superior sensitivity, excellent selectivity, reduced size, and extended lifetimes for a wide range of environments and applications.


By testing the turbidity and atomic force microscopy (AFM) of OZN and the mechanical properties and water vapor permeability of the composite films, the comprehensive properties of composite films with different mass ratios were analyzed. It showed that the addition of OZN improved the mechanical and moisture barrier properties and extended the release time of OPO. When the ratio of OZN and CS is 5:5, the highest elongation at break and tensile strengths is achieved, at values of 30.91% ± 2.52% and 12.19 ± 1.97 MPa respectively. The relative release concentration of OPO was highest at a ratio of 5/5, and over time it would last longer to maintain a higher release concentration. Besides, the oxidation resistance of the composite film was good, especially when the ration of starch CS to OZN was 5/5, it had the highest DPPH radical scavenging activity (30.16% ± 1.69%). Thus, it can be used as a bio-active edible food packaging film to ensure the safety of food products and reduce environmental pressure to some extent.Zein, corn starch (CS), and orange-peel oil (OPO) extracted from orange peels were used to prepare novel corn starch/orange-peel oil/zein nanocapsules (OZN) bio-active food packaging materials. The results showed that the OZN were round, smooth and in compact morphology with an average diameter of 102.7 ± 10.5 nm from OPO and zein (3:10,  In recent years, the full use of plastic food packaging materials has brought about increasingly severe challenges to the global ecological environment. Meanwhile, growing awareness of food safety has made biodegradable materials more and more popular . Due to CS was added into the matrix of polycaprolactone (PCL)/pomegranate peel extracts to prepare antibacterial packaging materials and it was found through bacteriostatic experiments that PCL/CS/pomegranate (PR) composite film had an excellent inhibitory effect on Staphylococcus aureus . CS not In this experiment, OPO from orange peels was extracted and made into various concentrations under different process conditions. Then, the corn starch/orange-peel oil/zein nanocapsules (OZN) were prepared in order to find optimal mixing proportions. Finally, the composite film was prepared by the casting film formation method. The thickness, density, mechanical properties, relative release antibacterial activities of the composite films were compared, and optimum conditions were established and its application was analyzed.3) used in this study was supplied by Chengdu Kelong Chemical Reagent Factory. The Anhydrous ethanol (effective substance content up to 99%) was also supplied by it. The CS was ordered from Chengdong Food Co., LTD in Shuangliu County of Chengdu. Orange used in this study were from a spontaneous mutation of the cultivar produced in Fengjie County (Sichuan). All above was of edible grade, and all chemicals and solvents were of reagent grade or higher purity, which were purchased from Yan’an Wanke Reagent Co., China, unless otherwise indicated.The glycerin  Tween 20 at a ratio of 10% of the total weight was incorporated to the solution under a magnetic stirrer. OPO was added into this solution drop-wise gradually. Mixed emulsions were finished using an ultrasonic homogenizer  at applied power of 400 W, 20 kHz, 12 mm probe diameter) at 25 °C (temperature was controlled by a container of ice and water) and the power of 350 W for 10 min. OZN for testing was obtained by steps of rotary evaporation at 60 °C for 15 min and freeze-drying.OPO and zein were weighed in different radios of 1/10, 2/10, 3/10, 4/10, 5/10, and zein was dissolved in 50 mL of 80% aqueous ethanol and stirred at 350 rpm for 1 h at room temperature. 0.1%  by the transmission of light through a 2-mm path-length cuvette.1 is the mass of the OPO before forming nanocapsules; N2 is the mass of OPO in OZN.The content of OPO in OZN was determined and calculated as follows: 1 g of OPO–zein nanocapsules was dissolved in 20 mL N, N-dimethylformamide (DMF) and 20 mL deionized water was added, then mixed for 1 min. OPO was extracted with DMF by heating the sample in glass tubes at 45 °C in a water bath with intermittent mixing. The cells were cooled to room temperature and DMF was separated from the aqueous phase by centrifugation at 4000 rpm for 20 min [To prepare CS/OZN solutions, first, the CS was placed in a water bath at 80 °C for 1 h to complete gelatinization. After the film liquid was cooled down to room temperature, the OZN were mixed with CS solutions, which were weighed at mass ratios of 10/0, 1/9, 3/7, 5/5, and 7/3. Then 1 wt% glycerin was added to the solutions as a plasticizer ,22. At tt of the composite film. The thickness of the composite film is determined to provide a basis for the determination of other indicators [The thickness of each composite film was measured by a thickness gauge, and 10 points were uniformly taken at the center and the periphery of the sample to the nearest 0.01 mm, and the average value was taken as the thickness value dicators .The light transmission test performed in the visible light region and film color properties were evaluated by using the CIELAB color space by means a KONICA CM-3600d COLORFLEX-DIFF2, HunterLab, Hunter Associates Laboratory, Inc, . Color coordinates, L (lightness), a* (red–green) and b* (yellow–blue) were measured. The instrument was calibrated with a standard white tile. Measurements were carried out in quintuplicate at random positions over the film surface. Average values for these five tests were calculated. Total color difference (ΔE) was calculated as follows:Nitrogen was used as the oxygen carrier gas (relative humidity ∼50%) at a temperature of 23 °C; the oxygen and nitrogen flow rates were 20 and 10 mL/min, respectively. The water vapor transmission coefficient of the composite film was measured by a PERMETM W3/031 water vapor transmission rate tester and oxygen transmission rate test was measured by a PERME TM OX2/231 permeability tester .2.Tensile strength and elongation at break of composite films were tested according to ASTM D828-97 standard test method . Before To obtain more details, the surface morphology of the composite film was analyzed by SEM , and a 3 × 3 mm square was cut from the sample composite film for scanning.2 substrate and dried at room temperature for 30 min. Then the image analysis by AFM was performed [AFM  was used to verify the morphological characterization and particle size distribution of OZN. AFM images were recorded in tapping mode  using a WITec alpha 300 RSA + microscope . To prepare the optimum sample for AFM imaging, 1 µL droplets of unloaded OZN in ultrapure water were put on CaFerformed . −1, and the environment temperature was 25 °C, to measure the precision of wavenumber in 4000–500 cm−1 of the absorption spectrum. The collected spectra were analyzed by OMNIC8.0 software.A FTIR  instrument was used to collect the infrared spectra of the composite film surface. The scanning frequency was set at 32 times/s, the resolution was 4 cm2 is the mass of OPO in OZN; N3 is the release of CS/OZN composite films.OPO released content was quantified by measuring absorbance at 252 nm by a UV–visible spectrophotometer . Aqueous solutions containing 95% of ethanol were used as the liquid food simulant, and films of 8 × 8 cm were introduced into 11 mL vials, containing 2 mL of each food simulant, which were hermetically closed and slightly stirred during storage. Samples (100 μL of simulant solution) were taken with a syringe through a septum in each vial for testing. The relative release of OPO was calculated as follows:A DPPH free radical scavenging assay was carried out according to previous research with slight modification . Twenty-p > 0.05) were calculated using analysis of variance—Tukey’s.All preparations were carried out in triplicate, statistical differences , and CS/OZN composite films which were at the mass ratios of 10/0, 9/1, 7/3, 5/5, and 3/7 . It can −1 due to C–H bending, 1645 cm−1 due to C=C stretching, and 2826 and 2971 cm−1 due to C–H stretching [−1 was due to the O–H stretching vibration of the hydroxyl group. In the spectra of zein and CS/OZN composite films, the characteristic peaks of the hydroxyl group were at 3306 and 3316 cm−1, respectively, which was similar to the result of Li et al. [−1  and 1538 cm−1 . Besides, in the spectra of OZN, both the characteristic peaks of OPO and zein appeared. The characteristic peak of the C–O–C bond is 1014 cm−1, and the absorption peak is blue-shifted to 1019 cm−1. This suggests that the addition of OZN may hinder the formation of hydrogen bonds in the matrix. Also, comparing to OPO, a new peak was found in the spectrum of CS/OZN composite films at 1655 cm−1, corresponding to the characteristic peak of zein. Similar results were also observed in the spectrum of zein/rhamnolipid composite nanoparticles [Infrared spectra of OPO, zein, OZN, CS/OZN composite films were obtained, as shown in retching . The bani et al. . The firarticles .In this study, the amount of OPO released relative to the composite film within 300 h was recorded. The results in Compared with the control, the films with OZN incorporated had significantly higher DPPH radical scavenging activity with increasing OZN concentrations than the control. The higher antioxidant activities of films mixed with OZN were because of the fact that OPO is a monoterpene compound . These rThe main purpose of the study was to develop a modified corn film prepared by the casting film method after the OZN were finished by exploring the effects of the mass ratios of CS/OZN on the density, transmittance, mechanical properties, water vapor transmission rate, and oxygen permeability of composite films, combined with SEM analysis of composite films. The apparent performance of the composite film was found to be good. The light transmittance of composite films was decreased, the mechanical properties and barrier performance were improved greatly. The addition of OZN also extended the release time of OPO. As a result, a new idea was provided to control the sustained release of volatile oil, that is, it should be encapsulated first and then combined with the film-forming substance. Therefore, this experimental composite film is expected to be used as a new type of food packaging material, in particular for dry matter in the food industry to reduce the use of internal plastic packaging to alleviate ecological pressure and it has a broad prospect for development.


Autophagy, a process for degrading intracellular substances to maintain basal metabolic turnover, is known to be perturbed in gastric cancer. Programmed cell death-1 (PD-1) with its ligand (PD-L1) are important immune checkpoint proteins and their regulation by autophagy has been reported in mouse melanoma and human ovarian cancer. Here, we explored the interplay between autophagy and the PD1/PD-L1 axis in gastric cancer.The expression of PD-L1 in gastric cancer cells was detected by Western blot and flow cytometry analysis. The effect of autophagy inhibition on PD-L1 expression was examined in vitro and in vivo. The molecular mechanisms of the regulation of PD-L1 by autophagy were evaluated in gastric cancer cell lines. The clinical relevance of autophagy-related markers p62/SQSTM1 and LC3 with PD-L1 was evaluated in 137 patients with gastric cancer.p < 0.001; p62/SQSTM1, p < 0.05). The expression of PD-L1 was also positively correlated with tumor lymphocyte infiltration (p < 0.001).We found that inhibition of autophagy by pharmacological inhibitors or small interfering RNAs increased the levels of PD-L1 in cultured gastric cancer cells and in xenografts. Interferon (IFN)-γ also promoted PD-L1 gene transcription, whose action was enhanced by autophagy inhibition. Mechanistically, autophagy inhibition led to the accumulation of p62/SQSTM1 and activation of nuclear factor (NF)-κB, in which NF-κB inhibition or p62/SQSTM1 knockdown attenuated PD-L1 induction by autophagy inhibition. Immunohistochemical staining of primary tumor tissues of 137 patients with gastric cancer showed that LC3 and p62/SQSTM1 protein levels were positively correlated with PD-L1  contains supplementary material, which is available to authorized users. Gastric cancer is a prevalent malignant tumor of the digestive tract and remains the third leading cause of cancer-related death worldwide. An estimated 951,600 newly diagnosed cases and 723,100 deaths of gastric cancer occurred in 2012 [Autophagy is a highly conserved homeostatic process that involves the formation of a double-membrane structure, autophagosome, which subsequently fuses with the lysosome to generate autolysosome leading to the degradation of the cellular proteins and damaged organelles. This catabolic pathway plays a pivotal role in cell survival, cellular metabolism and immune responses. Growing evidence reveals that the role of autophagy in tumorigenesis is complex and context-dependent . On one PTEN and LKB1 as well as epithelial-mesenchymal transition-related molecules [L/L mice with pancreatitis [Programmed cell death-1 (PD-1) with its ligand (PD-L1) are important immune checkpoint proteins. Elevated expression of PD-L1 receptors on cancer cell membranes has been observed in many cancer types. PD-L1 can interact with PD-1 and CD80 on the surface of T-cells, thus protecting cancer cells from immune-mediated rejection by inhibition of T effector functions . PD-L1 eolecules , 13. Morolecules . Recent olecules . Defectireatitis . The linEight gastric cancer cell lines  and a human normal gastric epithelial cell line (GES-1) were used in this study. Cell lines were maintained in RPMI-1640 medium or DMEM medium with 10% fetal bovine serum.One hundred and thirty-seven primary gastric cancer samples were collected during surgical resection at Peking University Cancer Hospital in Beijing, China. None of these patients received preoperative chemotherapy or radiotherapy. The diagnoses of gastric cancer were all histologically confirmed and all subjects provided informed consent for obtaining the study specimens. The study protocol was approved by the Clinical Research Ethics Committee of Peking University Cancer Hospital and Institute.RPMI1640 medium (72400) and DMEM medium (10564) are products from Life Technologies. 3-methyladenine (M9281), bafilomycinA1 (B1793), chloroquine (C6628), rapamycin (R0395) and phytohemagglutinin-M  are from Sigma-Aldrich. BMS 345541 (S8044) is from Selleck. The following primary antibodies were used: microtubule-associated light chain 3 , LC3A/B , p62/SQSTM1 , PD-L1 , PD-L1 , PD-L1 , ATG5 , ATG7 , β-actin , CD45 , CD8a , CD4 , FITC Mouse IgG1, PD-L1 , APC Mouse IgG2b , and 7-AAD .The expression of ATG5, ATG7, PD-L1, SQSTM1 and p65 was lowered using target-specific small interfering RNA (siRNA) molecules purchased from Qiagen as follows: Control siRNA (SI03650318), ATG5 siRNA (SI02655310), ATG7 siRNA (SI02655373), PD-L1 siRNA , SQSTM1 siRNA (SI00057596), RELA siRNA . Two hundred picomoles of gene-specific or control siRNA was transfected into cells at 40–60% confluence using Lipofectamine™ 3000 reagent  according to the manufacturer’s instructions.7 cells in 0.1 ml phosphate-buffered saline) were injected subcutaneously into the dorsal left flank of 4-week-old male BALB/c nude mice (n = 5 per group). Tumor diameter was measured every 2 days for 3 weeks. Pharmacological modulation of autophagy was achieved by intraperitoneal administration of chloroquine (50 mg/kg) every other day for 3 weeks. Tumor volume (mm3) was estimated by measuring the longest and shortest diameter of the tumor and calculating as previously described. All experimental procedures were approved by the Animal Ethics Committee of Peking University Cancer Hospital and Institute.MKN45 cells  were isolated from heparinised peripheral blood samples obtained from gastric cancer patients by Ficoll-Paque  density gradient centrifugation. To induce production of PD-1, PBMC were resuspended in RPMI-1640 containing 5 mg/mL PHA, 5% heat inactivated human AB serum, 1% penicillin, streptomycin, and amphotericin (Gibco), and incubated for 48 h . This PBGastric cancer cells were plated on two sets in 12-well plates, and on the following day, treated either with DMSO, CQ (chloroquine), 3-MA (3-methyladenine), Baf (bafilomycin A1) or Rap (rapamycin). After another 24-h period, one set was treated with media and the other set with INF-γ. The final concentrations of the drugs were 16 μmol/L for CQ, 10 mmol/L for 3-MA, 10 nmol/L for Baf, 100 nmol/L for Rap, and 200 U/mL for INF-γ. In the case of cocultures, all the steps and conditions were the same, and on the next day, a suspension of lymphocytes (primed as described in the above) was added to each well. The final concentration of lymphocytes was 550,000 cells/mL. Each assay was repeated at least twice.Formalin-fixed and paraffin-embedded blocks were sectioned at 5 μm and stained with hematoxylin and eosin. Immunohistochemistry was performed on paraffin sections of gastric cancer tissues using anti-LC3B antibody (1:2000), anti-p62/SQSTM1 antibody (1:2000) or anti-PD-L1 antibody (1:100). The immunostaining score was estimated based on the positive cell and the staining intensity, as described previously . The perRNA was extracted using Trizol reagent  and reverse-transcribed using SuperScript® III Reverse Transcriptase . Real-time PCR  was performed using the Power SYBR® Green PCR Master Mix with gene-specific primers: PD-L1, 5′-CAATGTGACCAGCACACTGAGAA-3′ and 5′-GGCATAATAAGATGGCTCCCAGAA-3′.Gastric cancer cells were probed with phycoerythrin-conjugated anti-PD-L1antibody. In the coculture experiments to distinguish gastric cancer cells from the immune cells, samples were stained with both anti-PD-L1 and anti-CD45  of death associated with expression of autophagy markers and other predictor variables were estimated by univariate Cox proportional hazards regression model. The difference in tumor growth rate between the two groups of nude mice was determined by repeated-measures analysis of variance. P values < 0.05 were taken as statistically significant.The results are expressed as mean ± standard deviation (SD). Differences between two groups were compared by the Mann-Whitney U test or Student’s t test where appropriate. Multiple group comparisons were made by the Kruskal-Wallis test or one-way analysis of variance (ANOVA) where appropriate. The χThe protein levels of PD-L1 in 8 gastric cancer cell lines and a human normal gastric epithelial cell line (GES1) were first determined by Western blots and flow cytometry . Moreover, inhibition of NF-κB signaling by knocking down the expression of p65 decreased the levels of PD-L1 proteins in NCI-N87 and AGS cells treated by chloroquine or 3-MA . These results indicated that autophagy inhibitors upregulated the expression of PD-L1 by activation of NF-κB signaling.PD-L1 expression in tumor cells could be triggered by intrinsic cellular signaling molecules, including NF-κB, MAPK, PI3K, mTOR and JAK/STAT . The ada+ T-cell activation and suppressed cytokine secretion, including tumor necrosis factor (TNF)-α, IFN-γ and interleukin (IL)-10 following anti-CD3/anti-CD28 activation [p < 0.05).In patients, lymphocytes very often infiltrate tumors and therefore tumor cells are directly exposed to the secreted cytokines. Our results so far indicated that expression of PD-L1 was increased in gastric cancer cells by autophagy inhibition Figs.  and 3. Mtivation , which mWe further evaluated the clinicopathological and prognostic significance of autophagy markers, namely LC3B and p62/SQSTM1, in relation to PD-L1 in patients with gastric cancer. Marked expression of total LC3B and p62/SQSTM1 were found in cancer cells but not in surrounding stromal cells as revealed by immunohistochemistry. LC3 was mainly expressed in the cytoplasm of the cancer cells, while p62/SQSTM1 was expressed in both the nucleus and cytoplasm  was determined by immunofluorescence in AGS and NCI-n87 cells exposed to autophagy inhibitors, bafilomycin A1  and chloroquine  for 24 h. Scale bar, 50 μm. Figure S3. The effect of chloroquine on the expression of PDL1 in in vivo subcutaneous xenograft models. Figure S4. (A) The effect of pharmacological inhibitors of autopahgy on the protein levels of p65, p-p65, IκBα, p-IκBα, IKKα/β and p-IKKα/β was detected by Western blots in AGS cells. Figure S5. (A) The protein levels of PD-L1, STAT1, p65 and p-p65 were detected by Western blots in AGS and NCI-n87 cells treated with IFN-γ for 24 h. Figure S6. (A) Representative images of double staining for LC3 and PD-L1. (DOC 5030 kb)


Upon the simultaneous presence of both metals, sensor response is dominated by Pb2+ rather than Cd2+ ions. To explain the sensing mechanisms and difference in adsorption behavior of Pb2+ and Cd2+ ions on EG in water-based solutions, we performed van-der-Waals (vdW)-corrected density functional theory (DFT) calculations and non-covalent interaction (NCI) analysis, extended charge decomposition analysis (ECDA), and topological analysis. We demonstrated that Pb2+ and Cd2+ ions act as electron-acceptors, enhancing hole conductivity of EG, due to charge transfer from graphene to metal ions, and Pb2+ ions have preferential ability to binding with graphene over cadmium. Electrochemical measurements confirmed the conductometric results, which additionally indicate that EG is more sensitive to lead than to cadmium.In this work, we investigated the sensing performance of epitaxial graphene on Si-face 4H-SiC (EG/SiC) for liquid-phase detection of heavy metals , showing fast and stable response and low detection limit. The sensing platform proposed includes 3D-printed microfluidic devices, which incorporate all features required to connect and execute lab-on-chip (LOC) functions. The obtained results indicate that EG exhibits excellent sensing activity towards Pb and Cd ions. Several concentrations of Pb Nowadays, among water pollutants, heavy metals (HMs) are considered as the most serious source to pollute the biosphere, posing a significant threat to human health, because they are non-biodegradable and accumulate in soft tissues . Some HMThe main trend in modern heavy metal sensors concepts has been their gradual shift from traditional electrochemical quantification of analytes to real-time non-invasive optical sensing of toxic substances, including fluorescent ,37, surf2 was processed through two different sputtering steps in order to realize four circular (θ = 1 mm) electrical contacts on the corners, needed to bias the sensor and to collect the output signal. All contacts were fabricated through sputter deposition of 2 nm of titanium and 200 nm of gold sequentially [® CAD software and printed by a Form 1 + 3D printer (FormLabs) with a proprietary resin Clear Type 02 [The sensor system proposed in this work integrates the extraordinary features offered by an epitaxial graphene sensor with a 3D-printed microfluidic lab-on-chip . The senentially . Four-poentially . Both mi Type 02 . The res Type 02 ,46. A 26Electrochemical measurements were performed by using a computer-controlled potentiostat . The custom-built electrochemical cell of O-ring type was assembled with a three-electrode system: EG/SiC, Ag/AgCl, and platinum wire were used as the working electrode, reference electrode, and counter electrode, respectively. For more information about the design of the electrochemical cell, see our previous work . The str2+), ranging from 125 nM to 500 µM, were prepared by diluting a powder of lead chloride, PbCl2 (purchased from Sigma-Aldrich), in deionized water (dH2O). In order to compare the response of EG to other heavy metals, a concentration of cadmium ions (Cd2+) of 500 µM was prepared by diluting a powder of cadmium chloride, CdCl2 (purchased from Sigma-Aldrich), in dH2O. Moreover, for cross-sensitivity studies, both solutions, containing lead and cadmium ions with a concentration of 500 µM for each, were mixed and analyzed. For electrochemical measurements, aqueous solutions of Cd2+ and Pb2+ were prepared by dissolving the appropriate amounts of CdCl2 and Pb(NO3)2 salts in buffer solution (0.1 mol·L−1 HClO4 in Milli-Q-water) with pH = 4.5. Analyte solutions, with concentrations of lead ions  calculations, with consideration of the water as a solvent phase. The nature of the interaction between heavy metals (Cd and Pb in different charge states) and graphene was elucidated using the Gaussian 09 Rev. D.01 program package . As a mocoronene ) with edcoronene  with concoronene . In ordecoronene , was useanalysis  and the analysis  were appanalysis . The natanalysis  and exteanalysis ,58.2+ > Pb2+ for gas-phase to Pb2+ < Cd2+ for water, respectively. Such reordering is originating from the solvent-mediated interaction between metal cations and carbon rings, as was explained in the classical work by Kumpf and Dougherty [2+ and Pb2+ divalent ions to occupy the hollow site . The corresponding adsorption heights are estimated to be 3.42 Å and 2.45 Å, respectively. For each of the three cases studied, we noticed that the initial charge on ionic species  before interaction with graphene tends to decrease during the interaction. This suggests that divalent metal cations on graphene act as electron-accepting adsorbates/dopants. In particular, the Mulliken/Hirshfeld charge magnitudes on separately adsorbed cadmium and lead are about +1.98/+1.90 and +1.40/+1.16, while for cadmium simultaneously adsorbed with lead on the graphene these values were slightly different for lead case: +1.98/+1.90 and +1.37/+1.12, respectively. The electron localization function (ELF) and localized orbital locators (LOL) analyses give more evidence on charge redistribution in the interacting systems  adsorption of individual cadmium ions, (ii) adsorption of individual lead ions, and (iii) simultaneous adsorption of both metals. The current work is a continuation of our previous research efforts towards deep understanding of the adsorption behavior of metal ions on graphene in aqueous phase . In partougherty . Furthertems see d–i. From2+ and graphene was observed.Charge decomposition analysis (CDA), which is reported in the 2+ ions, adsorbed on graphene, behave as electron-accepting dopants, with a preferential charge transfer from graphene to divalent ions [2 diluted in dH2O, several concentrations of Pb2+ solutions, ranging from 125 nM to 500 µM, were measured. Since the water molecules on graphene act as electron-accepting (p-type) dopants [p-type conductivity of graphene. DFT calculations indicate that Pbent ions . This pr dopants , we obseThe sensor used in this study was used for more than one year in different operating conditions  without demonstrating any degradation in performance. 2+, more than three measurement cycles were measured  and the differential resistance (ΔR) and experimental error were calculated and are reported in 2+ and the recovery value obtained by the cleaning of the chip after each measurement cycle. 2+. In particular, the response recorded for the lowest concentration of lead ions exhibits a signal-to-noise ratio (SNR) of 7.1 dB, which portends the possibility to further reduce the measurable detection limit of the sensor system.For each concentration of Pb2+ concentration leads to an increase of the ΔR value  that is much higher than the sensitivity (SH = 0.10 Ω/µM) estimated for high concentrations (50–500 µM) of Pb2+. This result demonstrates how the system is more sensitive to low concentrations of the analyte. Moreover, a detection limit of 95 nM was extrapolated from the calibration curve , which is lower than the recommended safe limit (180 nM) provided by the World Health Organization (WHO) for lead levels in drinking water [Moreover, in accordance with Langmuir’s law which describes the adsorption of a monolayer of species onto simple surfaces, increasing the PbΔR value , since mng water . Still, ng water ,39), butMeasurements with lead dissolved in drinking water were also performed in order to evaluate the interaction between different species in complex matrixes that also includes other ions . In that2+) and is reported in r) and fall (tf) times are the times needed to switch from 10% to 90%, or vice versa, of the signal amplitude. Both the rise and fall times of the sensor after each injection of solution containing heavy metals were analyzed. The data can be fitted assuming a decreasing exponential equation for both, in the form of: The EG response to lead was compared to the one obtained by measuring the same molar concentration (500 µM) of cadmium ions  and negative during its emptying (cleaning-phase). τ is the time constant and it has different values during the filling and emptying processes. The filling time constant (τf) is 11.2 ± 0.4 s for lead and 47.9 ± 1.2 s for cadmium. Once the data was fitted, rise (recovery time) and fall time (response time) values could be calculated, each being about three times the corresponding characteristic time (rτ and fτ respectively), hence we obtained ft = 33.6 ± 1.2 s for lead and ft = ~143.7 ± 3.6 s for cadmium.In Equation (1), τr) is 61.0 ± 1.4 s and consequentially, rt is 183.0 ± 4.2 s.In the worst case (highest concentration of lead), the emptying time constant , and a little shoulder due to the presence of Cd b.The presence of the shoulder is due to the different interaction nature of cadmium and graphene compared to lead, as demonstrated by DFT calculations. Moreover, due to the lower affinity with graphene exhibited by cadmium compared to lead, more time is needed for the interaction to occur. 2+. The detailed description of the sensing mechanism of Pb species can be found in our previous work [2+ concentrations, we then changed measurement conditions. After reaching the equilibrium concentration of 1.2 µM for both metals in solution, we then continued to increase only the Cd2+ concentration, while maintaining the Pb concentrations at the same level. Under accumulation conditions, the Cd-related peak was detected only at concentrations of 100 µM or higher. We noticed the enhancement of this peak at higher concentrations of metals. Such difference in behavior between the two metals can be ascribed to the interaction nature and preference for Pb adsorption compared to Cd. It is likely that under conditions of the simultaneous presence of both metals, Pb predominantly tends to occupy the available electrochemical reactive sites at the graphene surface, while Cd species are not involved in the stripping process within the corresponding concentration range. It can be explained by the very small intrinsic adsorption energy of Cd on the surface of the graphene electrode, which is not enough to accumulate the metal species required for generating the stripping current. The presence of cadmium could only be detected at high concentrations (500 µM), since the EG response to lead is much stronger due to the higher affinity. This result was also confirmed by the electrochemical measurements performed and reported in ous work . In contIn most cases, at small concentrations of Cd and Pb, the corresponding stripping peaks at SWASV are well-separated. In such accumulation conditions, the intensity of the stripping peaks is mainly dependent on the binding order and adsorption preference of considered metal species on graphene. On the other hand, at high metal concentrations, the peak potential of Pb is shifted to more negative potentials as the concentration values increase, while the peak of Cd is still very weak. It means that the simultaneous quantification of Pb and Cd by graphene electrode can be attained in very limited ways and is feasible neither in a low-concentration regime due to preferential Pb adsorption, nor in the high-concentration regime due to the existence of a binary mixture of metals. To explain the experimental results, we simulate the solvent-mediated interaction between elemental heavy metals and graphene. Since the graphene response during the electrochemical process is mainly determined by the interaction between graphene and neutral Cd and Pb adatoms, we believe that a fundamental understanding of the adsorption of these heavy metals will help to address the selectivity phenomenon. In this regard, we adapted the approach, which we used to explore the adsorption of divalent ions, to investigate the behavior of elemental heavy metals in the same adsorption configurations a. Briefl2+ due to the extreme sensitivity of the material. Indeed, a detection limit of 95 nM was obtained, which is much lower than the recommended limit provided by WHO for Pb levels in drinking water, and it can be still improved by increasing the EG area exposed to the solution relative to the total device area. Moreover, a full set of different concentrations of Pb2+ solutions, ranging from 125 nM to 500 µM, were analyzed, showing a Langmuir correlation between the signal and the Pb2+ concentration, fast response, good stability, and reproducibility over time. It was observed that for low-concentrations (0.125–5 µM) of Pb2+ the system exhibits a sensitivity (SL = 13.90 Ω/µM) that is much higher than the one (SH = 0.10 Ω/µM) obtained for high-concentrations (50–500 µM) of Pb2+. Real measurements with lead dissolved in drinking water were also performed to evaluate the interaction with different species in complex matrixes that includes other ions [In this work we showed design and characterization of a new sensing platform based on an epitaxial graphene sensor coupled to 3D-printed microfluidic chips for the real-time detection of heavy metals. The use of an EG sensor and a 3D-printed microfluidic chip allowed for the detection of low traces of Pbher ions . Compari2+ and Pb2+ adsorption can be understood in terms of the charge-transfer reactions and subsequent solvent-mediated stabilization of the carbon–metal bonding. It was found that the adsorption of electron-accepting Pb ions is governed by an orbital interaction, while Cd2+ behavior on graphene is predominantly regulated by long-range dispersion forces. This is evidenced by NCI, ECDA, and topological analyses. The different interaction nature of cadmium and graphene compared to lead, as demonstrated by DFT calculations, introduces a small shoulder in the experimental response to Cd2+. Moreover, due to the lower affinity with graphene exhibited by cadmium compared to lead, more time is needed for the interaction to occur. The comparison between the EG response to Pb and Cd was discussed, and the higher affinity to Pb was demonstrated. Indeed, from the cross-sensitivity analysis, only at high-concentration levels of Cd was it possible to detect its presence when mixed with lead. DFT calculations allowed us to get deep insights into the nature of the interaction between lead (cadmium) species and graphene as well as to explain the exceptionally high sensitivity of the EG to Pb compared to Cd. It was found that the major difference between CdTo confirm the conductometric results, we have also performed electrochemical tests by the probing of simultaneously present Cd and Pb in aqueous solution using the SWASV method. Comprehensive DFT calculations enabled us to elucidate the nature of the non-bonding interaction between elemental heavy metals and graphene. It is proposed that the mechanism of preferential electrochemical detection of lead is driven by dispersion forces. All of the features described in this work show that this system can be used for accurate and sensitive identification of heavy metals in the environment.


The determination of the weights of decision makers (DMs) is an important problem in multi-attribute group decision making. Many approaches have been presented to determine DMs’ weights. However, the computed weight vectors of DMs are usually assumed to be constant in existing studies, and this may cause irrationalities in the decision results. Therefore, this article proposes a novel method to determine DMs’ weights based on variable weights theory in which the evaluation information is described as intuitionistic fuzzy sets (IFSs). First, DMs provide their assessment with IFSs, and the intuitionistic fuzzy weighted averaging (IFWA) operator is applied to obtain weighted decision matrix based on the prior given DMs’ and attributes’ weights. Second, the DMs’ weights are obtained based on variable weights theory, and an alternative decision can be computed. Finally, the converted value of the achieved IFS of each alternative is calculated, and the best appropriate alternative is acquired. Two illustrative examples and the comparisons with exsiting approaches are also used to reflect the effectiveness of the proposed approach. Multi-attribute group decision making (MAGDM) is usually used to choose the best alternative from a group of ones according to the multi-attributes  . The purAs an extension of fuzzy sets , the intUsually, most the resolution of MAGDM and FMAGDM problem consists of two stages: aggregation and exploration , 19. In It is easy to see that MAGDM problems with IFSs usually have three common characteristics, including multiple DMs, alternatives and multi-attributes with incomparable units, in which the DMs’ weights play a key role , 4, 20. Many approaches have been proposed to obtain DMs’ weights. For example, Bodily  presenteIn these studies of determining DMs’ weights in MAGDM problems, Yue did more in-depth research with objective and subjective evaluation information with real numbers, interval numbers, intuitionistic fuzzy sets (IFSs), interval-valued intuitionistic fuzzy sets (IVIFSs), and other. For instance, Yue  determinThe abovementioned studies have contributed substantially to obtaining DMs’ weights under MAGDM and can be divided into two categories , 23, 24.But, through reviewing the literature, we found that the previous researches on the determination of DMs in MAGDM share something in common, namely, the calculated weight vectors of DMs are generally constant. That is to say, once the weight vector of the DMs is obtained, its quantitative value will not change throughout the entire computation process. For example, although our previous studies , 23 usedTherefore, from the above analysis, we can see that neither the subjective or objective weighting method can handle the problem very well. Therefore, to solve the above situation, we propose a novel approach to obtain the DMs’ weights for intuitionistic fuzzy group decision making based on the variable weights theory , 43. In Our method is composed of the following main steps. First, DMs provide their assessment information based on each attribute with IFSs and compute the weighted decision matrix according to the intuitionistic fuzzy weighted averaging (IFWA) operator with the prior given attributes and DMs’ weights. Second, the determination of the DMs’ weights is based on the variable weighting method, and the conversion of individual decisions to alternative decisions. Third, the alternatives are ranked.The article is organized as follows. Section 2 presents the basic concepts of IFSs and variable weights theory. Based on an extended variable weight vector, Section 3 develops a MAGDM methodology with intuitionistic fuzzy numbers that can determine the DMs’ weights. Section 4 gives a comparison between our method and other studies. Section 5 presents two illustrative examples, and Section 6 concludes the article.In this segment, we will review the basic concepts related to IFSs and variable weights theory.A be an intuitionistic fuzzy (IF) set in the universe of discourse X, where A = {〈xi,μA(xi),vA(xi)〉|xi∈X}, and μA and vA are the membership function and the non-membership function of the membership degree and the non-membership degree of an element, respectively. xi belongs to the IF set A, where μA(xi)∈, vA(xi)∈, 0≤μA(xi)+vA(xi)≤1 and 1≤i≤m. The degree of indeterminacy πA(xi) of element xi belonging to the IF set A is equal to 1−μA(xi)+vA(xi), where πA(xi)∈ and 1≤i≤m. According to  refer to the weighted average or weighted mean, and λk} are constant, the formula represents a constant synthetic weight on the situation {xk} with respect to relevant factors. Let λ = . Afterwards, the λ is defined as the constant weight vector. The constants reflect the strength or relative importance of the correlative factors.A common multifactor function is the mapping (operator), that is applied in additional decision-making systems and is as follows:λ is always fixed, in spite of the essential structure or configuration of the objective function ϑ(f(u)) = (ϑ1(f1(u)),ϑ2(f2(u)),…,ϑt(ft(u))).Refer to the constant weight, the weight vector u and the vector ϑ(f(u))’s elements. The constant vector λ in Eq  with no changes when X =  varies. Hence, the application of constant weights has its limitations. Li t→tx1,x2,…,xt)↦λkthat meet the below axioms:(λ.1). NORMALITY.λ.2). CONTINUITY. λk is continuous in the t-dimensional space; andλ.3). PENALTY. λk is monotonically decreasing according to xk.λ(x) = (λ1(x),(λ2(x),…,λt(x)), and we define it with the variable weight vector with a penalty. If we change λ.3) to meet the condition that λk is monotonically increasing according to xk, we define it as the variable weight vector with a reward.Let To capture the varying rule of weights, the axiomatic definition of the state variable weight vector was modified in the literature as follows.Definition 4: A mappingSk:t→t and X→Sk(X) is defined as an  state variable weight vector with a penalty if S satisfies the following axioms:s.1). xa≥xb→Sa(X)≤Sb(X),s.2). Sa is continuous according to each variable xb, ands.3). it satisfies the variable weight vector acquired from Eq (λ.1) -λ.3) in Definition 3, where λ =  is a constant weight vector and λ*S(X) = (λ1S1(X),…,λt*St(X)). from Eq  asλ(x)=1tλkSk(X)Eq (5) ss.1) to meet the conditions that xa≥xb→Sa(X)≥Sb(X) and λk is monotonically increasing according to xk, we call it the state variable weight vector with a reward.If we change Definition 2. The proposed approach performs the below three steps: (1) Preparatory stage. In the preparatory stage, DMs present their assessment information based on each attribute using intuitionistic fuzzy numbers and establish the decision matrix. Then, we compute the weighted decision matrix based on the IFWA operator with the prior given attributes and DMs’ weights. (2) Computation stage. In the computation stage, a variable weighting approach is proposed to obtain the DMs’ weights. After that, we aggregate the decision matrix based on the IFWA operator and work out the composite fuzzy scores of individual alternatives. (3) Decision stage. In the end, in the decision stage, the most desired alternatives are then selected with the crisp values of composite scores, which are calculated by the accuracy degrees operator (ADO) of IFSs from M = {1,2,…,m}, N = {1,2,…,n} and T = {1,2,…,t}, where i∈M,j∈N,k∈T. Let A = {A1,A2,…,Am} (m≥2) be a discrete set of m feasible alternatives, U = {u1,u2,…,un} be a finite set of attributes, and D = {d1,d2,…,dt} be a group of DMs.Let Specifically, the following notations are defined:M: M = {1,2,…,m}, where i∈M;N: N = {1,2,…,n}, where j∈N;T: T = {1,2,…,t}, where k∈T;A: A = {A1,A2,…,Am} (m≥2) denotes a discrete set of m feasible alternatives;U: U = {u1,u2,…,un} denotes a finite set of attributes;D: D = {d1,d2,…,dt} denotes a group of DMs;λk: λk denotes the initial DMs’ weights;q: q =  denotes the accuracy degrees;λ(q): λ(q) = (λ1(q),λ2(q),…,λt(q)) denotes the variable weight vector;An MAGDM with IFSs can be described as follows.LetIn the field of MAGDM research, many scholars have realized that DMs’ weights play a key role in MAGDM problems , 38, 41.w1, w2,…, and wj are the given attribute weights, and they meet the conditions wj∈First, we need to compute the weighted decision matrix operator :IFWA0.,0.4 denote the prior DMs’ weights based on the opinions from interviewing the senior DM. We called it the initial DMs’ weights, which meet λk∈.First, let q be the accuracy degrees of Definition 2, where qk∈l. In addition, let λ(q) = (λ1(q),λ2(q),…,λt(q)) be the variable weight vector shown as follows:k = 1,2,…,t; qk = (μk+vk) and parameter γ satisfy:Then, let Theorem 1:λ(q) is a variable weight vector. When γ>0, λ(q) is the variable weight vector with a penalty; when γ<0, λ(q) is variable weight vector with a reward.Proof.λ(q) satisfies the continuity in Definition 3. Ifq∈t, when λ(q) satisfies the normality in Definition 3.Obviously, γλk(q)/γλk = γ(qk−1)<0, λk(q) is monotonically decreasing with variable qk. That is, λ(q) is a variable weight vector with a penalty.Because qb and thus meets Definition 4.In addition, LetDefinition 4, S(q) =(S1(q),S2(q),…St(q)) is the penalty variable weight vector of λ(q).Based on λ(q) is a reward variable weight vector, which is defined as Definition 3, and S(q) is also a state variable weights vector with a reward because its form is the same as Eq  is a variable weight vector with a penalty; when γ>0, λ(q) is a variable weight vector with a reward; and when γ = 0, λ(q) = λ is the initial given weight. This reflects that we can obtain diverse variable weight vectors when we regulate the parameters accordingly, which provides a great convenience in practical use.According to After the DMs’ weights are computed, then we can compute the comprehensive evaluation value based on the IFWA operator , using (Ai(i∈m) according to Si(i∈m) in descending order according to Definition 2.Now, we can sort the alternatives The primary steps of the proposed method are summarized as follows:Step 1. Construct the individual decision matrix ps by Eq ,Step 2. Compute the weighted normalized decision matrix λk,ik by Eq  with theStep 3. Determine the DMs’ weights by using Eqs  of the DMs, andon by Eq  with theStep 5. Rank all the alternatives according to the value of Si by using Eq , scholars (d2) and students (d3), who are the DMs  for the grading. There are three attributes u1, u2 and u3 that are used to evaluate the alternatives A1, A2, A3, and A4, as shown below:An annual report is needed to assess satisfaction in regard to leadership at Chinese universities. Let the alternatives u1=working experience,u2=academic performance, andu3=personality.X1, X2 and X3 are represented by IFs given by the decision makers d1, d2 and d3, respectively and are shown in Suppose that the decision matrices λ1, λ2 and λ3 of the DMs d1, d2 and d3 are 0.33, 0.34 and 0.33, respectively. Suppose that the attribute weights u1, u2 and u3 given by the DMs d1 are 0.4, 0.2 and 0.4, respectively, i.e., u1, u2 and u3 given by the DMs d2 are 0.3, 0.3 and 0.4, respectively, i.e., u1, u2 and u3 given by the DMs d3 are 0.4, 0.4 and 0.2, respectively, i.e., Suppose that the initial weights 【Step1】: Based on the decision matrices d1, d2 and d3 and the weights of the attributes given by the DMs, we can obtain the weighted (on attributes) score matrices ik by Eq , as show【Step2】: Based on Eq  is that we want to examine the extreme effect of the variable weight in the range of the permit. Then, we can obtain the respective weights of the DMs in different situations, as shown in ed on Eq  and Eq  when γ = −7.2. Therefore, the calculation result suggests that through the adjustment of the parameter, top management can adjust the preferences to DMs according to the parameter ∂, and this may affect the final decision result. Apparently, via the above discussion, we can draw a conclusion that the variable weight based weighting approach can to some extent amend the unreasonable sorting results with effect by adjusting the DMs weights, especially when the prior given DMs weights are unconscionable. By adjusting different parameters, the variable weight vector of the DMs also can efficient reflect the differences of each DM’s knowledge, personal preferences and so on, which will lead to more scientific decisions.As shown above, with the proposed approach, when 9.46, Xu ’s method9.46, Xu ’s method9.46, Xu ’s methodg and Su ’s methodExample 2: adapted from Chen et al :A1, A2 and A3 to be evaluated as follows:An investment corporation intends to invest a sum of money in the best firm. There are three alternatives firm A1: a car firm,A2: a TV firm, andA3: a food firm.u1, u2 and u3 are to be used to evaluate the three alternatives A1, A2 and A3 by the three DMs, i.e., the director (d1), the manager (d2) and the assistant manager (d3), as shown below:Three attributes u1: the risk criterion,u2: the growth criterion, andu3: the environmental impact criterion.X1, X2 and X3 given by the decision makers d1, d2 and d3, respectively, are shown in The decision matrices λ1, λ2 and λ3 of the DMs d1, d2 and d3 are 0.36, 0.32 and 0.32, respectively. Suppose that the weights of the attributes u1, u2 and u3 given by the DMs are 0.01, 0.49 and 0.50, respectively, i.e., w1 = 0.01, w2 = 0.49 and w3 = 0.50.Suppose that the weights 【Step1】: Based on the decision matrices d1, d2 and d3 and the weights of the attributes given by the DMs, we can get the weighted (on attributes) scores matrices ik by Eq , as show【Step2】: Based on Eq  is because example 1 has examined the effect of extreme values, and the purpose of example 2 is to examine a modest effect of the variable weight in the permitted range. Then, we can get the weights of the DMs in different situations, as shown in ed on Eq  and Eq  Unlike most existing studies that treat the weights of DMs as a constant vector , 40, 41,This study enriches the methodology and theory for determining DMs’ weights in MAGDM with IFSs, and also has below practical implications: (1) Our method takes full advantage of experts. By applying variable weights theory in FMAGDM, our study can effectively avoid irrationalities caused by DM’s knowledge and personal preferences bias. We also add adjustable parameters in decision process, which making the final chosen solution more reasonable and scientific; (2) Our study offers universality and practicability. The analysis of illustrative examples prove the universality and practicability of our approaches, and it can be integrated with other approaches for apply in a variety of decision-making problems, such as partner selection, facility location, etc.Although the available study results, there is work remaining to be done in the future. First, this study only focuses on the determination of DMs weights in MAGDM, and future work should pay more attention to the determination of attributes weights to make the proposed method more comprehensively. Second, future work should also focuses on develop new aggregation methods to make the evaluation information aggregation phase more efficient. Finally, the language set of this study is in the form of intuitionistic fuzzy numbers, and future work should be implement to support other language sets, such as interval-valued intuitionistic fuzzy numbers, rough sets or vague sets and so on.