Full-printing paper-based room temperature gas sensor and preparation method thereof

Abstract

The invention relates to a full-printing paper-based room temperature gas sensor and a preparation method thereof, the sensor uses an extensible digital manufacturing technology, and the limitation of a traditional sensing material and a manufacturing method is solved. The sensor adopts a novel nano composite material, the novel nano composite material is prepared into printable ink, the printable ink is formed by compounding two or more nano structures of metal oxide, graphene, precious metal and the like, and the ultralow detection limit and high response value of NO2, H2S and other gases at room temperature are achieved. The nano material is accurately deposited on the paper flexible substrate by adopting an ink-jet printing technology, and by optimizing the viscosity, surface tension and particle size of the gas-sensitive material and finely adjusting the printing speed and height of a printer, rapid, extensible, uniform and high-precision sensor manufacturing is realized, and meanwhile, the compatibility with flexible and wearable base materials is kept.

Description

Technical Field

[0001] The present invention belongs to the field of gas sensing technology, and specifically relates to a fully printed paper-based room-temperature gas sensor and a preparation method thereof, which is applicable to fields such as environmental monitoring, medical diagnosis and industrial safety. Background Art

[0002] With the rapid advancement of industrialization and urbanization, the emission of toxic and harmful gases (such as volatile organic compounds (VOCs), carbon monoxide (CO), hydrogen sulfide (H2S), and nitrogen oxides (NOx)) has become increasingly severe, posing a serious threat to environmental safety and human health. Even at extremely low concentrations, these gases can cause physical illness and sensory discomfort. For example, the World Health Organization (WHO) Global Air Quality Guidelines recommend an annual safe exposure limit of 5 ppb for nitrogen dioxide (NO2), while my country's Emission Standard for Odor Pollutants (GB 14554-93) stipulates an industrial perimeter concentration limit of approximately 20 ppb for hydrogen sulfide. Traditional gas sensor technology faces many challenges in detecting this type of gas: (1) Commonly used metal oxide semiconductor (MOS) sensors need to operate at high temperatures of 200-400°C, resulting in high energy consumption and difficulty in integration into portable devices; (2) Existing gas-sensitive film deposition technologies such as sputtering and spin coating have difficulty in taking into account both the porous structure and surface uniformity of the film layer, limiting the controllable preparation of sensor elements with high sensitivity, high stability and high consistency; (3) Current micro-nano manufacturing processes such as photolithography are complex, costly and have long cycles, making it difficult to meet the needs of rapid mass production of low-cost gas sensors.

[0003] Digital manufacturing technologies, such as inkjet printing, offer new avenues for the rapid and controllable fabrication of gas sensors. However, their application remains constrained by the limited availability of readily available materials. While existing printed sensors based on materials such as graphene, TiO2, and CdS exhibit some response to gases like ethanol, their sensitivity for detecting toxic and hazardous gases at low concentrations, even at the ppb level, remains insufficient. Furthermore, key technical challenges, such as the dispersion stability of nanomaterials in printed inks and the precise control of interfacial contact resistance within devices, remain a significant bottleneck hindering the large-scale production of highly sensitive sensors.

[0004] Therefore, there is an urgent need to develop a new type of composite gas sensor that has excellent gas-sensing performance at room temperature, can be quickly and efficiently mass-produced, and has an easy-to-control preparation process, so as to meet the urgent needs of future environmental monitoring and health and safety protection. Summary of the Invention

[0005] The present invention provides a fully printed paper-based room-temperature gas sensor and a preparation method thereof. The core of the invention lies in solving the deficiencies of traditional sensors in sensitivity, selectivity, stability and manufacturing process by combining new material design with inkjet printing technology.

[0006] Furthermore, the sensor consists of a paper-based substrate, inkjet-printed interdigitated electrodes, and a nanocomposite sensitive layer. The nanocomposite includes composite materials composed of reduced graphene oxide nanoparticles and metal oxide nanoparticles, such as gold-doped reduced graphene oxide / copper cobalt oxide and reduced graphene oxide-loaded tin oxide / copper oxide. Its operating principle is based on the chemical adsorption and charge transfer process between gas molecules and the surface of the sensitive layer. Molecules adsorb on the surface of the sensitive layer and chemically react with surface oxygen vacancies and nanometal particles to form adsorbed species. The adsorbed gas molecules capture electrons from the sensitive layer, resulting in a decrease in the resistance of the p-type semiconductor material. This resistance change is positively correlated with the concentration of the adsorbed gas. The resistance change is measured by the interdigitated electrodes and converted into an electrical signal for output, enabling quantitative detection of gas concentration.

[0007] Furthermore, to achieve ultra-low detection limits and high response values, the present invention adopts a new material design strategy: by regulating the charge redistribution around the metal atoms in the composite material, the oxygen vacancy concentration on the material surface is increased, and the gas adsorption capacity is improved; the incorporated noble metal nanoparticles act as electron sensitizers to promote the chemical adsorption and electron transfer of the gas to be measured, thereby reducing the reaction activation energy; the treated reduced graphene oxide provides a highly conductive scaffold, accelerating charge transfer, and its large specific surface area characteristics can increase gas adsorption sites, thereby optimizing the gas-sensing properties of the material.

[0008] Furthermore, inkjet printing technology is used in the present invention to precisely deposit interdigital electrodes and a sensitive layer. The specific process includes: dispersing the nanocomposite material to be used in an organic solvent (such as propanol, oleylamine, oleic acid, etc.), adjusting the ink viscosity and surface tension by adding thickeners and surfactants to maintain the viscosity between 10-50 mPa·s and the surface tension between 25-40 mN / m, and ensuring uniform dispersion of the nanoparticles by adjusting the grinding time, solvent concentration, and ultrasonic treatment power; using silver nano-ink to print interdigital electrodes on a paper substrate with an electrode spacing of 50-200 μm and a thickness of 1-5 μm to ensure high conductivity and uniformity; printing the nanocomposite ink on the interdigital electrodes, adjusting the thickness of the sensitive layer (5-30 μm) by controlling the number of prints, and curing at 60°C to form a uniform sensitive layer; and adjusting the printing speed, nozzle height, and substrate temperature to avoid the coffee ring effect, improve the uniformity of the film, and improve the consistency of sensor performance.

[0009] Furthermore, the sensor of the present invention offers the following performance advantages: high response values ​​to various toxic and hazardous gases, particularly NH3, CO, H2S, and volatile organic compounds (VOCs). Furthermore, the sensor of the present invention offers excellent environmental resilience and long-term stability, with minimal response fluctuations under varying temperatures, minimal response attenuation under high humidity conditions, and high response retention after long periods of continuous operation.

[0010] Furthermore, the sensor of the present invention has the following performance advantages: it adopts an inkjet printing process, has a fast production speed, is compatible with flexible substrates, and achieves scalable and uniform sensor manufacturing while maintaining compatibility with flexible and wearable substrates, making it suitable for large-scale production and wearable device integration.

[0011] Furthermore, the sensor of the present invention can be widely used in the following fields: integrated into air quality monitoring stations or portable devices to detect the concentration of toxic and harmful gases in the atmosphere in real time; used for early warning of toxic and harmful gas leaks in environments such as chemical plants and mines, with the detection threshold set at ppb to ppm level; and assisted in the diagnosis of respiratory diseases such as asthma and chronic obstructive pulmonary disease (COPD) by analyzing the content of specific components in human exhaled breath.

[0012] Furthermore, the innovations of the present invention lie in: achieving ultra-low detection limits and high response values ​​at room temperature through the synergistic effect of reduced graphene oxide / metal oxide composite materials; using inkjet printing technology to achieve high-precision, high-efficiency, and scalable sensor manufacturing; maintaining compatibility with flexible and wearable substrates to meet the needs of applications in some special fields such as wearable devices; and maintaining long-term stability in a wide temperature range and high humidity environment to meet practical application needs. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 :A finished sensor fabricated using Au-rGO / Cu2Co1O4 as an example and its structural diagram (including paper substrate, interdigitated electrodes, and sensitive layer)

[0014] Figure 2 :Dynamic response curve of sensor made of Au-rGO / Cu2Co1O4 nanocomposite material to 2.5ppb NO2

[0015] Figure 3 : Response performance of sensors made with different printing times when printing Au-rGO / Cu2Co1O4 sensitive layers to 2.5-100ppbNO2

[0016] Figure 4 :Sensing performance of Au-rGO / Cu2Co1O4 nanocomposite sensors for 50ppbNO2 at different operating temperatures

[0017] Figure 5 :The effect of humidity on the sensing performance of sensors made of Au-rGO / Cu2Co1O4 nanocomposites under 100ppb NO2 conditions

[0018] Figure 6 :Dynamic response curve of the sensor made of rGO-SnO2 / CuO nanocomposite to 100ppb-10ppm H2S at 120℃

[0019] Figure 7 :Dynamic response curve of the sensor made of rGO-SnO2 / CuO nanocomposite to 1ppm-5ppm H2S at room temperature DETAILED DESCRIPTION

[0020] Example 1: Preparation of Au-rGO / Cu2Co1O4 nanocomposite material and sensor assembly, including a paper-based substrate, interdigitated electrodes and an inkjet-printed Au-rGO / Cu2Co1O4 nanocomposite sensitive layer; the sensitive layer is composed of Au-doped reduced graphene oxide (rGO) and Cu2Co1O4 spinel nanoparticles, wherein the Au doping amount is 5% of the total molar amount of Cu and Co.

[0021] This embodiment describes in detail the preparation process of the sensor of the present invention:

[0022] Step A: Copper nitrate (Cu(NO3)2·3H2O, 0.16 mmol) and cobalt nitrate (Co(NO3)2·6H2O, 0.08 mmol) were dissolved in 20 mL of deionized water, 0.5 mmol of NaNO3 was added as a mineralizer, and magnetic stirring was performed for 30 minutes; then 0.012 mmol of HAuCl4 and 20 mg of rGO powder were added, and further ultrasonication was performed for 30 minutes; the mixed solution was then transferred to a 50 mL polytetrafluoroethylene autoclave and reacted at 180°C for 12 hours; after the reaction was completed, it was naturally cooled, the precipitate was collected by centrifugation, washed alternately with deionized water and ethanol three times, and vacuum dried at 60°C for 6 hours to obtain Au-rGO / Cu2Co1O4 nanoparticles.

[0023] Step B: Disperse 50 mg of the nanoparticle composite in 10 mL of isopropyl alcohol and add 0.1 wt% polyethylene glycol (PEG-400) as a dispersant. Ultrasonic treatment is then performed for 30 minutes to break up aggregated particles. Undispersed particles are then removed by centrifugation to obtain a supernatant. Next, thickeners and surfactants are added to maintain a viscosity between 10 and 50 mPa·s and a surface tension between 25 and 40 mN / m, respectively, to achieve an inkjet ink with acceptable viscosity and surface tension, as required for inkjet printing.

[0024] Step C: Sensor assembly and interdigital electrode printing: Use a flexible electronic printer to print interdigital electrodes on photo paper using silver nano-ink with a 250 μm dispensing tip, maintained at 60°C. After printing the interdigital electrodes, heat the paper at 90°C for one hour.

[0025] Step D: Print the sensitive layer, and print Au-rGO / Cu2Co1O4 ink layer by layer in the interdigital electrode area. Use a 90μm dispensing head and change the number of dispensing times to optimize the deposition thickness. The final sensor product is shown in the figure. Figure 1 .

[0026] Sensitivity tests were conducted. Before testing, the chamber was flushed with dry synthetic air for 20 minutes to stabilize the baseline, and then introduced to measure the sensor response. The sensor was placed in an airtight test chamber. During the gas sensing test, the mixture of dry air and target gas was adjusted to control the air concentration via a digital mass flow control system, with the gas flow rate set at 1200 mL / min. Resistance changes were measured using a two-probe method at a constant voltage of 5 V using an electrochemical workstation (CHI 760E). The response value was calculated as: S (%) = |Ra - Rg | / Rg × 100%, where Ra is the sensor resistance in air and Rg is the sensor resistance in a NO2 environment. After the test, the sensor was refreshed by introducing synthetic air to restore its baseline state. Unless otherwise stated, all gas sensing experiments were conducted at room temperature. The target gas for sensitivity testing was NO2 at concentrations ranging from 2.5 to 100 ppb.

[0027] Test results: The optimized sensor has ultra-high sensitivity, with response values ​​of 241.5% and 7.68% for 100ppb and 2.5ppb NO2 respectively. The theoretical detection limit is as low as 0.0545ppb. The response curve for 2.5ppb NO2 is shown in the following figure. Figure 2 .

[0028] Sensitivity tests of sensors with different sensitive layer thicknesses were performed. Au-rGO / Cu2Co1O4 nanocomposite ink was printed into three sensors with different film thicknesses using a dispensing technique. The sensitive layer of the three sensors was printed 2 times, 4 times, and 6 times, respectively. To ensure repeatability, three sensors were prepared for each thickness, and the films were inspected using SEM to evaluate their thickness and uniformity. The sensors of three different thicknesses were exposed to 2.5ppb, 4ppb, 6ppb, 10ppb, 20ppb, 50ppb, and 100ppb of NO2 (the sensors were operated at room temperature) to evaluate their dynamic reversible performance. All sensors exhibited a sudden change in resistance when exposed to NO2, and the resistance varied with the gas concentration. Figure 3The responses of three sensors with different numbers of sensitive layer printing times to various NO2 concentrations were compared, and normalized resistance response curves were presented. The Au-rGO / Cu2Co1O4 sensor printed twice (sensitive layer thickness of approximately 22μm) showed the best response, with response values ​​of 7.68%, 15.79%, 25.43%, 45.26%, 72.02%, 129.18%, and 241.48% corresponding to NO2 concentrations of 2.5, 4, 6, 10, 20, 50, and 100 ppb, respectively.

[0029] Temperature and humidity tests were conducted. The response effect of the sensor prepared by Au-rGO / Cu2Co1O4 nanocomposite materials was evaluated at 15℃, 30℃, 45℃, 60℃, 75℃, and 90℃. Figure 4 The response effect of the sensor prepared by Au-rGO / Cu2Co1O4 nanocomposite material was evaluated under the conditions of humidity of 0, 20, 40, 60, and 80. Figure 5 .

[0030] Test results: The optimized sensor performs extremely well in a wide temperature range and high humidity environment, meeting different practical application requirements.

[0031] Example 2: Preparation of rGO-SnO2 / CuO nanocomposite material and sensor assembly, characterized in that it includes a paper-based substrate, inkjet-printed interdigitated electrodes and a rGO-SnO2 / CuO nanocomposite material sensitive layer; the sensitive layer is composed of reduced graphene oxide (rGO), SnO2 nanoparticles and CuO nanoparticles.

[0032] The preparation process of the sensor of the present invention is as follows:

[0033] Step A: Dissolve tin tetrachloride (SnCl4·5H2O, 1.2 mmol) and cupric chloride (CuCl2, 0.6 mmol) in a mixture of 20 mL of oleic acid and 2.5 mL of oleylamine, and ultrasonicate for 20 minutes to obtain a dispersion.

[0034] Step B: Prepare reduced graphene oxide (rGO): Disperse 30 mg of graphene oxide (GO) in 20 mL of ethanol and sonicate for 1 hour. Add 200 mg of citric acid (CA) and reduce by stirring in an 80°C water bath for 24 hours. Remove undispersed particles by centrifugation, collect the product, wash three times with ethanol, and dry at 60°C to obtain partially reduced rGO powder. Add the rGO powder to deionized water at a concentration of 1 mg / mL and disperse it ultrasonically to obtain an rGO dispersion.

[0035] Step C: Synthesis of rGO-SnO2 / CuO Nanocomposite: To the dispersion prepared in Step A, rGO (5 mL) dispersion was added and vigorously stirred at 60°C to form a transparent solution. Ethanol (10 mL) was then added and stirred until transparent. The solution was transferred to an autoclave and maintained at 180°C for 12 hours. After cooling, it was washed several times with ethanol and hexane and then dried at 75°C for 12 hours. The resulting powder was then calcined in a muffle furnace at 400°C for 2 hours at a heating rate of 10°C per minute. After cooling, the powder was collected to obtain the rGO-SnO2 / CuO composite.

[0036] Step D: Under magnetic stirring, ethylene carbonate (EC) was uniformly dissolved in 5% 1-propanol (1:2) to obtain an EC binder. rGO-SnO2 / CuO powder, EC binder, and 1-propanol were then mixed in a 9:1:20 mass ratio. Ultrasonic treatment and stirring were performed for 20 minutes to break up particle aggregates. A thickener and surfactant were then added to maintain a viscosity between 10 and 50 mPa·s and a surface tension between 25 and 40 mN / m, as required for inkjet printing. This yielded an inkjet ink with acceptable viscosity and surface tension.

[0037] Step E: Assemble the sensor and print the interdigitated electrodes using a flexible electronic printer. Print the interdigitated electrodes on photo paper with silver nano-ink. The dispensing tip has a specification of 250 μm and is kept at 60°C. After printing the interdigitated electrodes, heat the paper at 90°C for one hour.

[0038] Step F: Print the sensitive layer. Print the inkjet ink prepared in step D layer by layer in the interdigitated electrode area. Use a dispensing head with a specification of 90 μm. Optimize the deposition thickness of the sensitive layer by changing the number of dispensing times. The thickness of the sensitive layer is about 22 μm.

[0039] Sensitivity testing: Before testing, the chamber was flushed with dry synthetic air for 20 minutes to stabilize the baseline, and then introduced to measure the sensor response. The sensor was placed in an airtight test chamber. During the gas sensing test, the mixture of dry air and target gas was adjusted to control the air concentration via a digital mass flow control system, with the gas flow rate set at 1200 mL / min. Resistance changes were measured using a two-probe method at a constant voltage of 5 V using an electrochemical workstation (CHI 760E). The response value was calculated as: S (%) = |Ra - Rg| / Rg × 100%, where Ra is the sensor resistance in air and Rg is the sensor resistance in H2S. After the test, the sensor was refreshed by introducing synthetic air to restore its baseline state. Unless otherwise noted, all gas sensing experiments were conducted at room temperature. The target gas for sensitivity testing ranged from 100 ppb to 10 ppm H2S.

[0040] Test results: The optimized sensor has ultra-high sensitivity, with a response value of up to 190 to 10ppm H2S at a low temperature of 120℃, and a reproducible and stable response at 100ppb. Figure 6 At the same time, it also has a high sensitivity response at room temperature and has a significant response to 1ppm H2S. Figure 7 .

[0041] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A fully printed paper-based room temperature gas sensor, characterized in that: It includes a paper-based substrate, inkjet-printed interdigitated electrodes and a nanocomposite sensitive layer. The nanocomposite includes a material composed of two or more nanostructures of metal oxide, graphene and precious metal Au.

2. The fully printed paper-based room temperature gas sensor according to claim 1, characterized in that: The nanocomposite ink required for inkjet printing is prepared by the following steps: Step A: Preparation of metal oxides, synthesis of redox graphene (rGO), noble metal doping, and composite of multiple nanomaterials: mixing a metal salt solution of appropriate concentration and an rGO dispersion at a certain temperature in an appropriate molar ratio, then adding an appropriate molar amount of noble metal salt to the mixture to dope the material with noble metals, mixing again to obtain a uniform mixture, and subjecting the mixture to a hydrothermal reaction for several hours to form a noble metal-doped redox graphene / metal oxide nanocomposite material; Step B: Precisely prepare and control the composite ink. First, the composite material prepared in Step A is ground and dispersed in an organic solvent. Stirring and ultrasonic treatment are used to ensure uniform dispersion of the particles. The grinding process uses appropriate solvents such as propanol and oleylamine, and stirring is carried out at 30-60°C for 10-30 minutes to promote full dispersion of the particles. Subsequently, ultrasonic treatment is used to further break up the particle aggregates, avoiding excessive temperatures to prevent particle damage or solvent volatilization. After ultrasonic treatment, incompletely dispersed particles are removed by centrifugation to obtain a supernatant. Next, the viscosity and surface tension of the ink are adjusted according to the requirements of inkjet printing, usually by adding thickeners and surfactants. The viscosity is maintained between 10-50 mPa·s and the surface tension is controlled between 25-40 mN / m to ensure smooth jetting and adhesion to the substrate. In addition, particle size control is also critical. The target particle size is usually 30-100 nm, which is achieved by adjusting the grinding time, solvent concentration, and ultrasonic treatment power. Finally, the performance of the ink is verified through consistency and printing tests to ensure its stability and uniformity in inkjet printing.

3. The fully printed paper-based room temperature gas sensor according to claim 1, characterized in that: The sensor substrate is a paper substrate, including photographic paper, dust-free paper, and nitrocellulose membrane, and has the advantages of being lightweight, flexible, environmentally friendly, replaceable, and wearable.

4. The fully printed paper-based room temperature gas sensor according to claim 1, characterized in that: The interdigitated electrodes are prepared by inkjet printing silver nano-ink, with an electrode spacing of 50-200 μm and a thickness of 1-5 μm.

5. The fully printed paper-based room temperature gas sensor according to claim 1, characterized in that: The sensitive layer is prepared by inkjet printing of nanocomposite ink, and the thickness of the sensitive layer is 10-30 μm. The thickness of the sensitive layer can be finely controlled by the number of printing times, thereby achieving a better gas-sensitive response effect.

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