Claims:I claim:
1. A method of fabricating gas sensor device comprising of steps:
dispersing silver nanowires in a suitable medium;
dispersing carbon nanotubes in a suitable medium;
having a substrate and forming a first layer of plurality of metal nanowires
thereon, and drying at an optimum temperature;
forming a second layer of atleast one or plurality of carbon nanotubes adjacent to
the said first layer and drying at an optimum temperature obtaining a
heterostructure therebetween; and
forming a pair of conductive electrodes on top surface of the heterostructure along
two opposite edges providing a means to connect to an external circuit.
2. The method of fabricating gas sensor device as claimed in claim 1, wherein the first layer consists of atleast one or plurality of layers of silver nanowires, copper nanowires, nickel nanowires, gold nanowires or combination thereof, preferably silver nanowires.
3. The method of fabricating gas sensor device as claimed in claim 2, wherein the plurality of layers is obtained through filtration, preferably vacuum filtration.
4. The method of fabricating gas sensor device as claimed in claim 1, wherein the medium is ethanol.
5. The method of fabricating gas sensor device as claimed in claim 1, wherein the medium is N-methyl pyrrolidone.
6. The method of fabricating gas sensor device as claimed in claim 1, wherein the optimum temperature for drying room temperature in the range of 22 – 33oC.
7. The method of fabricating gas sensor device as claimed in claim 1, wherein the second layer consisting of carbon nanotubes layers is selected from either single-walled carbon nanotubes or double-walled carbon nanotubes or multi-walled carbon nanotubes, preferably single walled carbon nanotubes.
8. The method of fabricating gas sensor device as claimed in claim 1, wherein the contact electrodes is selected from a group of silver, gold, copper, platinum, aluminium, preferably silver.
9. The method of fabricating gas sensor device as claimed in claim 2, wherein the first layer is of uniform thickness having an amount of silver nanowire ranging from 0.08 mg/cm2 to 0.24 mg/cm2, preferably 0.16 mg/cm2.
10. The method of fabricating gas sensor device as claimed in claim 1, wherein the second layer is of uniform thickness having a constant amount of 0.032 mg/cm2 carbon nanotubes.
11. The method of fabricating gas sensor device as claimed in claim 1, wherein the supporting substrate is flexible selected from a group consisting of filter paper, embossed paper, cellulose sheet, microfiber sheet, fabric sheet, polymer reinforced sheet, preferably filter paper.
12. The method of fabricating gas sensor device as claimed in claim 1, wherein the supporting substrate is non-flexible selected from a group consisting of glass, quartz, silica.
13. A gas sensor device to detect gas in sensor ambience comprising of:
a supporting substrate;
a first layer deposited adjacent to the said substrate consisting of atleast one or
plurality of metal nanowires;
a second layer deposited adjacent to the first layer consisting of atleast one or
plurality of carbon nanotubes;
said first and second layer of metal nanowires and carbon nanotubes forming a
heterostructure therebetween;
said heterostructure having a pair of conducting electrodes thereon, spaced apart
and providing a means of contact to an external circuit thereof; and
a chamber enclosing the said substrate, said heterostructure and the said
electrodes with one or more openings to facilitate the said connection to the
external circuit.
14. The gas sensor device as claimed in claim 13, wherein the conducting electrodes are deposited on top surface of the heterostructure, preferably coated along any of two opposite edges.
15. The gas sensor device as claimed in claim 13, wherein the gas detected is selected from a group consisting of vapours of ammonia, methanol, ethanol, isopropyl alcohol and acetone, preferably ammonia.
16. The gas sensor device as claimed in claim 13, wherein the gas detection limit is in the range of 62.5 – 1000 ppm.
17. A gas sensor characterized in that:
heterostructure showing a decrease in sheet resistance from 11.98 k?/? to
5.02 k?/?;
heterostructure showing increase in response;
exhibiting shorter response and recovery times;
showing reduced sensor noise of 0.0119;
exhibiting stability during bending and unbending cycles; and
capable of being disposed through ignition in 5 seconds.
18. The gas sensor as claimed in claim 17, wherein the heterostructure consists of silver nanowire in the range 0.08 mg/cm2 to 0.16 mg/cm2 and 0.032 mg/cm2 single-walled carbon nanotube.
19. The gas sensor as claimed in claim 17, wherein the gas detected is preferably ammonia.
20. The gas sensor as claimed in claim 17, wherein the increase in response (S) with 0.16 mg/cm2 of silver nanowire is 31 for 62.5 ppm ammonia.
21. The gas sensor as claimed in claim 17, wherein the heterostructure consisting of silver nanowires in the range of 0.08 mg/cm2 to 0.16 mg/cm2 show response time in the range of 93 s to 15 s, preferably 15 s for 0.16 mg/cm2 silver nanowires.
22. The gas sensor as claimed in claim 17, wherein the heterostructure consisting of silver nanowires in the range of 0.08 mg/cm2 to 0.16 mg/cm2 show recovery time in the range of 70 s to 68 s, preferably 68 s for 0.16 mg/cm2 silver nanowires.
23. The gas sensor as claimed in claim 17, wherein the sensor noise value is for 0.16 mg/cm2 silver nanowires.
24. The gas sensor as claimed in claim 17, wherein the bending angle is 60o and number of bending and unbending cycles is 25.
25. The gas sensor as claimed in claim 17, wherein the disposability is characteristic of cellulose based substrate.
, Description:Field of Invention
The present invention relates to Silver Nanowires-Carbon Nanotubes based heterostructure gas sensor. Particularly the invention relates to heterostructure gas sensor having high sensitivity at room temperature and method of fabrication thereof.
Background
Many types of nanomaterials based gas sensors have been experimented and reported. Carbon nanotubes (CNT), especially single walled carbon nanotubes (SWCNT) have recently gained more attention as a gas sensing material that can be applied to various industrial fields due to their high chemical reactivity, high surface to volume ratio and high conductivity. These sensing applications utilize metal oxides, carbon nanotubes (CNT), nanowires and graphene and have its own disadvantages such as high cost, shorter life time, and less sensitivity or longer response time.
Chemiresistors based on CNT are realized on variety of substrates like silicon, glass, plastics and cellulose. Recently, paper-based chemiresistors gained more interest because of its unique functionalities including light weight, physical flexibility, bio-degradability and disposability, which are not possible with the conventional sensors. Fabrication of chemiresistors on paper with fast response and recovery is a challenging task because the increased amount of deposition of single walled carbon nanotubes (SWCNT) on paper produced longer response and recovery times (Shobin and Manivannan, Sens. Actuators, B 2015, 220, 1178–1185). Generally, to increase the sensitivity, selectivity, reduction of the response and recovery times, metal decoration (Star et al., J. Phys. Chem. B 2006, 110 (42), 21014–21020) or metal CNT hybrid formation is considered as the preferred route. Despite wide research using CNT, hybrid structures realized using metal nanostructures and CNT focuses on decorating metal nanoparticles or nanowires on CNT for selective and sensitive gas sensing.
To achieve better sensor performance in terms of sensitivity and response the baseline resistance has to be low. The low baseline resistance is generally achieved by increasing the amount of SWCNT on the sensor substrate. However, increased SWCNT content will decrease the sensing performance due to domination of metallic SWCNT in the network and thickness effect. To address this issue, interdigitated electrodes are often used, which requires clean room facilities and lithography process for fabrication and the process are messy and expensive. Albeit wide research on these nanomaterials, enhanced gas sensing ability is attained through metal doping, metal coding, functionalization of CNT. Hence, the researchers are trying to reduce the baseline resistance without disturbing the SWCNT network.
US 8152991 B2 discusses an electrochemical based nanotube sensor for analyzing the ammonia gas in the sensor ambience. CN103076370A describes a flexible sensor capable of detecting ammonia at normal temperature employing photographic paper as a substrate and SWNT-PABS functional layer.
US20100089772A1 elaborates on nanomaterial-based gas sensors with an electrochemically functionalized semiconductive nanomaterial using nanoparticle elemental metal, doped polymer, and metal oxide to detect many gases including ammonia.
The above mentioned sensors have their limitations of unstablility, high fabrication cost and longer response and recovery times, lower reduction in resistance. Notwithstanding these limits, the substrate of the sensor plays a significant role depending on the sensor application. Though a transparent and hard substrate may be preferred for a stable sensor, need based demand of flexible sensors exist in many arenas and one such is gas detection. Detection of gases, specifically ammonia, is experimented extensively by coating SWCNT and silver nanowires (AgNW) individually on a variety of substrates. Nevertheless, heterostructure of SWCNT and AgNW as gas sensor has not been realized yet. Hence flexible sensors using the heterostructure for the detection of gas at room temperature with enhanced response compared to the individual materials is presented in this disclosure.
Advantages
In the present invention, AgNW are introduced below the SWCNT to reduce the resistance of the sensor. As a result, the sensor response is increased due to the fast exchange of charge carriers. The method of coating SWCNT on AgNW reduces the baseline resistance, supports the improved sensor performance of the individual materials used. The disclosed method is simple to fabricate at room temperature in normal atmosphere, compatible with the solution process and capable of producing control over the deposition of nanomaterials. It does not require any metal doping, lithography and clean room facilities. Another important advantage of the paper-based devices is its flexibility. SWCNT are found to be good sensing material with high strength under strain. Another component in the hybrid is AgNW, which also has high elastic modulus (2.64 GPa), superior mechanical properties. The fabricated sensor can effectively be used in safety and homeland security. Disposability is another crucial property of the sensors and is essential in a variety of personal safety and diagnostic applications in the medical field where the paper based sensors are mostly preferred. The sensor disclosed in the present invention is easily disposable in 5 seconds by just igniting with fire.
Summary
A gas sensor device fabricated based on heterostructure of nanowires and carbon nanotubes on a supporting substrate that can detect vapors in the ambience upto 1000 ppm at room temperature. The heterostructure consists of nanowire layer deposited on the substrate on top of which carbon nanotubes are layered. The gas sensor exhibited a reduced resistance of 5.02 k?/? and shorter response and recovery times compared to pristine carbon nanotubes sensor. Dynamic response of heterostructure sensors with 0.16 mg/cm2 nanowires for 62.5 ppm ammonia is 7.25 times greater than the pure carbon nanotube sensor. The sensor realized on flexible substrate such as filter paper has the advantage of being stable upto 25 bending and unbending cycles as well as disposability on ignition in 5 s. The sensor thus has applications in many fields such as invisible switches, touch screen fabrication and transparent conducting films.
Brief Description of Figures
Figure 1: Schematic of fabrication process of AgNW-SWCNT heterostructure sensor on paper.
Figure 2: Sheet resistance of AgNW and AgNW-SWCNT network as a function of amount of
AgNW deposited on filter paper. Inset shows the expanded view of the marked portion.

Figure 3: FE-SEM images of (a) SWCNT deposited on paper (0.032 mg/cm2), (b) AgNW
deposited on paper (0.08 mg/cm2), (c) AgNW-SWCNT heterostructure (HS1) and (d)
schematic diagram of the AgNW-SWCNT heterostructure.

Figure 4: FE-SEM images of (a) HS0, (b) HS1, (c) HS2 and (d) HS3
Figure 5: Room temperature dynamic response of the AgNW, SWCNT (HS0) and AgNW-
SWCNT (HS1) sensor towards 62.5 ppm ammonia vapor.

Figure 6: Temporal responses of (a) HS0 (b) HS1 (c) HS2 and (d) HS3 towards ammonia vapor.
Figure 7: Comparison of ammonia sensing response of HS0, HS1, HS2 and HS3.
Figure 8: Comparison of response against various vapors at 1000 ppm measured at room
temperature.

Figure 9: Response of the sensor HS2 towards 62.5 ppm of ammonia at different bending and
unbending cycles.

Figure 10: Photographs showing the disposal property of the sensor. (a) before the ignition of
sensor using a matchstick, (b), (c) burning of sensor and (d) residue from the burnt
sensor.

Detailed description
The present disclosure, in one aspect of the invention, focuses on nanomaterial based heterostructure gas sensor device for detecting gases in the sensor ambience, specifically ammonia. Any person skilled in the art will appreciate that there are other methods available to carry out commonly known processes such as dispersion of nanoparticles, layering of nanoparticles and heating. Therefore the foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the description and claims.
The flexible substrate used in this invention, filter paper (Whatman, thickness-390 µm, pore size -11 µm), acts as an insulator due to the presence of cellulose fibers possessing hydroxyl groups that promote the uniform distribution of SWCNT over it.
The main embodiment of the invention is a gas sensor fabricated by forming a heterostructure of AgNW-CNT. In one embodiment the nanomaterial may be selected from either SWCNT or double-walled carbon nanotubes (DWCNT) or multi-walled carbon nanotubes (MWCNT). However SWCNT are selected for further study in the present invention due to the advantages of maximum surface area for contact and their transport properties. At ambient temperature, SWCNT act as a p-type semiconductor with holes as majority carriers. Hence it is to be construed by any person of ordinary skill in the art that the disclosed embodiments are not limited to the use of SWCNT. The AgNW used in this invention is synthesized through polyol process as published by the applicant (Shobin and Manivannan, Electron. Mater. Lett. 2014, 10 (6), 1027–1031). The SWCNT used in this invention are purchased commercially (Arc discharged, 60–70 wt% purity, average diameter ~1.3 nm, Iljin Nanotechnology Inc., Korea).
In a preferred embodiment of the invention the gas sensor based on heterostructure of AgNW –CNT consists of a first layer of AgNW (003) adjacent to the flexible substrate surface (001). A second layer of CNT (004) is coated over the first layer of AgNW in such a manner the CNT fill in the gaps within the AgNW mesh. Layered structure of AgNW-SWCNT hybrid is prepared by successively filtering using vacuum filtration method (002) (Figure 1). Careful deposition of these conducting materials is essential to attain the heterostructure. Vacuum suction is helpful to achieve a good contact between these two nanomaterials due to the capillary forces.
The gas sensing mechanism is based on good electrical conductivity of SWCNT and the charge interaction between the analyte and SWCNT reducing the sheet resistance. The presence of AgNW beneath the SWCNT layer acts as backbone as well as a conductor. This characteristic of AgNW used in this invention is attributed to absence of chemically responsive interparticle boundaries (CRIBs) due to which electron can flow along the axis of AgNW freely. This heterostructure provides a good percolation behavior due to the contact between SWCNT and AgNW. SWCNT network being closely entwined over the AgNW matrix act as a bridge between the nanowires and performing conduction pathways. Both the layers together provide a heterostructure thus acting coordinated in reducing sheet resistance.
The observed sensor response can be explained based on the charge transfer from analyte to CNT. The vapour molecules donate electrons to the nanotube network and reduce the hole carrier concentration which cause an increase in the resistance of the network. The change in resistance is measured with respect to change in current flowing through it or the change in voltage. This change in magnitude of the resistance is calibrated with known concentration of the analyte to record a stable baseline value. The concentration of the analyte is obtained by comparing with the baseline value.
In the present invention, the synthesized AgNW standalone is insensitive towards gases, specifically ammonia, due to absence of CRIBs. This suggests that AgNW used in this invention acts as a perfect conductor and not as a resistive type sensor. AgNW easily forms a mesh structure and facilitate the current conduction which is a highly desired feature in the fabrication of transparent conducting film. The AgNW beneath the SWCNT reduced the resistance of the sensor, which promoted the sensor response to higher values without affecting the sensing surface of CNT.
In a preferred embodiment the purified AgNW are dispersed in any suitable organic solvent known in the art such as ethanol, methanol, water, IPA, mixer of ethanol and water, mixer of water and IPA using one of the dispersing methods available such as bath sonication, probe sonication and magnetic stirring, shaken well and filtered through a vacuum filter and dried at room temperature of 22 – 33ºC thus forming a layer of AgNW network. Plurality of AgNW layers are obtained by filtering the known amount of AgNW successively as many times required through the filter paper.
In an exemplary embodiment of the present invention, the AgNW (003) is filtered 5, 10, and 15 times to obtain plurality of layers. The amount of AgNW in the layer is varied by varying the amount of AgNW filtered on to the substrate. In an exemplary embodiment of the present invention the amount of AgNW filtered onto the substrate is 0.08 mg/cm2, 0.16 mg/cm2, 0.24 mg/cm2 (Figure.3). In the present invention, the role of AgNW in gas sensing is characterized by increasing the network density of AgNW deposited beneath a constant amount of SWCNT. Figure 4 shows the FE-SEM image of the SWCNT and heterostructure sensors. In the SWCNT sensor (HS0), SWCNT were forming a mat like structure over the filter paper (Figure 4a). In the heterostructure sensors, when lower amount of AgNW were used (HS1), AgNW were forming a well dispersed net like structure. The gap between the AgNW (~7µm) was covered by the SWCNT network (Figure 4b). When the concentration of AgNW was increased the gap between the AgNW was decreased (<5µm) and uniform SWCNT network on the AgNW (HS2) was observed (Figure 4c). In the sensor HS3, SWCNT network over the AgNW was not uniform because of the protrusions and aggregations produced by the AgNW, due to the increased amount (Figure 4d).
In an embodiment a fixed quantity of the SWCNT is dispersed in any suitable solvent known in the art and filtered over the AgNW network deposited on the substrate and dried at room temperature in the range of 22 – 33oC forming a AgNW-SWCNT layered heterostructure.
In an exemplary embodiment of the present invention the solvent used is N-methyl pyrrolidone and dried at a temperature of 30oC.
In another embodiment the sensor is prepared by cutting the middle portion of the heterostructure layered substrate which is uniformly coated and the edges coated with metal as contact electrodes (005) selected from a group consisting of silver, gold, copper, platinum, aluminium.
In yet another embodiment the sensor is placed inside a chamber equipped to hold the analyte gases and having connectivity with the electrical circuit (006). Gas sensing setup consists of a vapor generating system, delivery system to send ammonia to sensing chamber and a resistance measurement unit attached to a computer. The sensor disclosed in the present invention is placed in the sealed test chamber. The capacity of the gas sensing chamber is one liter. Gas flow rate through the test chamber is maintained at 1000 mL/min when exposed to all test vapors.
In varied embodiments the heterostructure sensor is prepared by varying the amount of AgNW deposited beneath a constant amount of SWCNT layer. The optimum amount of AgNW that exhibits high sensitivity to a particular analyte selected from a group consisting of Ammonia, Methanol, Ethanol, Isopropyl alcohol (IPA) and Acetone. Sensor is also prepared using pristine SWCNT alone.
In an embodiment the method of sensing the analyte involves measurement of change in resistance of the sensor on exposure to the analyte. The sensor response is stabilized in air atmosphere prior to the measurements. Stable baseline resistance is reached by purging the air at room temperature in the range of 22 - 33ºC. Different concentrations of vapor to be analyzed such as Ammonia, Methanol, Ethanol, IPA and Acetone are prepared by the serial dilution factor method and the evolved vapor is allowed to pass through the test chamber at 30oC with the help of synthetic air which is used as a carrier gas using a mass flow controller. Each testing cycle comprises of exposure and release of vapor from the test chamber. Resistance of the sensor is measured (Model 6517A, Keithley Instruments, Inc., USA) and recorded as a function of operating time in data processing unit.
In an embodiment the analyte concentration in the range from 60 to 1000 ppm is measured by measuring the change in resistance of the sensor.
In another embodiment the sheet resistance of the AgNW-SWCNT heterostructure is measured using the four point probe technique (Figure 2). The average value is obtained from several measurements at different points for each sample. A sheet resistance of 11.98 k?/? is measured when the substrate is coated with 0.08 mg/cm2 of AgNW alone (007), whereas the the resistance reduced to 5.02 k?/? on coating SWCNT on this network (0.032 mg/cm2) (008). This is attributed to the property of SWCNT to effectively connect the AgNW network.
The heterostructure sensor showed 3.25 times increased response (009) when compared to the SWCNT pristine sensor realized on the filter paper (Figure 5). The prime reason for increase in response is the decrease in sheet resistance (Rs). The AgNW buried under the SWCNT reduced the resistance of the sensor which promoted the sensor response to higher values without affecting the sensing surface of SWCNT.
Response and recovery times of the heterostructure sensor decreased compared to the pristine SWCNT sensor (Figure 5) thus confirming the role of AgNW as charge transport layer too. The charge conduction in the SWCNT network is rapidly transferred to AgNW which facilitates it to reach the contact electrodes. Hence, the response and recovery are fast. In an exemplary embodiment, 0.16 mg/cm2 AgNW (HS2) reflected a reduced response and recovery time of 15 s and 68 s (011) respectively whereas for the pristine SWCNT sensor (HS0), it is 289 s and 86 s (010) respectively.
Dynamic response of heterostructure sensors with different volume of AgNW is shown in Figure 7. Response of the sensor is increased logarithmically while increasing the gas concentration. The response of the sensor is increased 2.2 fold when the AgNW amount is increased from 0.08 mg/cm2 (HS1) (012) to 0.16 mg/cm2 (HS2) (013) for 62.5 ppm ammonia exposure. HS2 response (013) is 7.25 times greater than the pure SWCNT sensor (014).
Examples
Example 1
Purification and dispersion of SWCNT
The commercial SWCNT (Arc discharged, 60–70 wt% purity, average diameter ~1.3 nm, Iljin Nanotechnology Inc., Korea) is purified by dry oxidation followed by acid treatment to enhance the purity. Purified SWCNT are sonicated using a bath type sonicator with N-methyl pyrrolidone (NMP) at a concentration of 2 mg/100 mL for 3 h at 30ºC.
Example 2
Fabrication of heterostructure sensors
Purified AgNW are dispersed in ethanol at a concentration of 1 mg/10 mL using a bath sonicator for 2 min and shaken well before the filtration. AgNW network is formed on the commercial filter paper (Whatman, thickness-390 µm, pore size -11 µm) by using a vacuum filter set up and dried at 30ºC for 10 min. Increasing the amount of AgNW filtered through the filter paper leads to increase in density of the AgNW network on paper. 20 mL of SWCNT dispersed in NMP is vacuum filtered over the AgNW network deposited paper and dried at 30ºC for 30 min. Thus, the AgNW-SWCNT layered structure is formed on the filter paper. Middle portion of heterostructure deposited filter paper of area 2 cm × 2 cm is cut with a scissor and silver electrodes are coated over the two edges. The prepared structure is used as a sensor (Figure.1). Heterostructure sensors prepared with different amount of AgNW are named as HS1 (0.032 mg/cm2 SWCNT on 0.08 mg/cm2 AgNW), HS2 (0.032 mg/cm2 SWCNT on 0.16 mg/cm2 AgNW) and HS3 (0.032 mg/cm2 SWCNT on 0.24 mg/cm2 AgNW). Sensor prepared from SWCNT (0.032 mg/cm2 SWCNT) alone is named as HS0.

Example 3
Sheet resistance of sensors
The sheet resistances of AgNW, SWCNT, AgNW-SWCNT coated papers are measured by using the four point probe technique (S-302-4, Lucas Labs, USA). The average value is obtained from seven measurements at different points for each sample (Figure.2). When the amount of AgNW is 0.08 mg/cm2, it showed the sheet resistance as 11.98 k?/? (007). After coating the SWCNT on this network (0.032 mg/cm2), the resistance reduced to 5.02 k?/? (008) which can be attributed to the fact that the SWCNT effectively connected the AgNW network. 20 mL SWCNT dispersed NMP deposited on filter paper without AgNW showed sheet resistance value of 5.64 M?/?. AgNW acts as backbone as well as a conductor whereas SWCNT acts as connecters with good electrical conductivity and gas sensing property in the heterostructure. When the amount of AgNW is increased, the sheet resistance reduced drastically.
Example 4
Room temperature dynamic response of sensors
Response of AgNW, SWCNT and AgNW-SWCNT heterostructure sensors exposed to 62.5 ppm ammonia vapor is shown in Figure 5. Ammonia vapor alone with different vapor concentrations is introduced into the sensing chamber along with air using a mass flow controller. Resistance of the sensor is measured and stored in data processing unit while varying the ammonia concentration in the range from 62.5 to 1000 ppm.
Response of the sensor is defined as
S = [(RA-RS)/RS] x 100
where RA and RS are the resistance values of the sensor with and without ammonia exposure respectively. The positive S values denote the resistance of the sensor, which increase upon exposure to ammonia. Unfortunately, AgNW network did not respond to ammonia test vapor. Rather SWCNT network (HS0) response towards the ammonia vapor (S=4) is confirmed by the increase in resistance of the network. The HS1 show increased response (S=13) when compared to sensors realized from the individual SWCNT. The heterostructure sensor show 3.25 times increased response when compared to SWCNT pristine sensor realized on the filter paper.
Example 5
Dynamic response of heterostructure sensors
Dynamic response of heterostructure sensors with different volume of AgNW as seen in Figure 6 shows a logarithmic increase with increase in gas concentration. Figure 7 shows the response of all the sensors to ammonia vapor. Increase in AgNW amount from 0.08 mg/cm2 (HS1) to 0.16 mg/cm2 (HS2) for 62.5 ppm ammonia exposure triggered a 2.2 fold increase in response. HS2 response was 7.25 times greater than the pure SWCNT sensor. The response decreased with higher amount (0.24 mg/cm2) of AgNW (HS3). These characteristics implies the causal factor to aggregation of AgNW and metallic nature of the sensor.
Example 6
Response and recovery times of sensor
Table 1: Comparison of response and recovery times of sensors with varying amount of AgNW-SWCNT

Sl.
No. Materials Response
(S) Response/
recovery time (s) Experimental/
theoretical detection limit
1 0.032 mg/cm2 SWCNT (HS0) 4 for 62.5 ppm 289/86
62.5 ppm/
20 ppb
2 0.032 mg/cm2 SWCNT on 0.08 mg/cm2 AgNW (HS1) 13 for 62.5 ppm 93/70
62.5 ppm/
5 ppb
3 0.032 mg/cm2 SWCNT on 0.16 mg/cm2 AgNW (HS2) 31 for 62.5 ppm 15/68
62.5 ppm/
1 ppb
4 0.032 mg/cm2 SWCNT on 0.24 mg/cm2 AgNW (HS3) 11.5 for 62.5 ppm 213/150
62.5 ppm/
6 ppb

Reduction in response and recovery times (Table 1) in the presence of AgNW compared to pristine SWCNT sensor prove that AgNW act as charge transport layer too. This suggests that charge transfer which occurs in the SWCNT network is rapidly transferred to AgNW and reaches the contact electrodes providing a faster response and recovery. HS2 reflected a shorter response and recovery times to 15 and 68 s respectively. Contrarily HS3 exhibited an increase in response and recovery times to 213 and 150 s respectively with increase in AgNW.
Example 7
Comparison of sensor sensitivity to various vapors
Sensor’s ability to discriminate ammonia from common reagents, such as acetone, ethanol, methanol and IPA was investigated by individually sending 1000 ppm of these vapors. Response of the best sensor (HS2) towards the vapors of these common reagents is depicted in Figure 8. The sensor responded to all the vapors. Nevertheless, the magnitude of the response is low for alcohol and acetone vapors when compared to ammonia. It was found that the response of ammonia is 12 times greater than the response of IPA, which clearly denotes that the sensors are more selective to ammonia. The selective sensing performance towards the ammonia is due to the high electron donating nature of ammonia at vapor state compared to alcohol and acetone vapors. This also confirms that the charge transfer from ammonia to SWCNT is the key factor for gas sensing in this study.
Example 8
Sensor noise is calculated by measuring the variation in the baseline of sensor using root mean square (RMS) deviation. The RMSnoise is calculated as
RMSnoise= v((?¦(y_i-y)²)/N) -------------------------------------------------- (1)
where yi is the measured data points, y is the corresponding value calculated from sixth-order polynomial fit of the data points and N is the number of data points used in the curve fitting. In our case, 100 points of the sensor baseline are used to calculate the RMSnoise. The sensor noise is calculated as 0.0322, 0.0242, 0.0119 and 0.0089 for the sensors HS0, HS1, HS2 and HS3 respectively. The sensor noise is decreased while increasing the amount of AgNW in the sensor. Reduction of noise can be accounted to the perfect conduction path formed between the SWCNT and AgNW.
Example 9
The performance of sensors on flexible substrates suffer under substrate bending, folding and rolling, leading to reduced response. It depends on the bending stability. In order to examine the stability, bending test is performed manually, by bending and unbending the sensors in a convex fashion. Response of the HS2 is measured after every 25 bending and unbending cycles. In the bent state (60°) the response of the sensor is similar to the unbent state (Figure 9), however the response (226 s) and recovery (337 s) times are increased when compared to the flat state. After 25 bending and unbending cycles, response of the HS2 is decreased by 6% when compared to the response of the same sensor before the bending and unbending cycles and the response and recovery times are found to be 221 s and 441 s respectively. After 50 bending and unbending cycles, 53% decrease in response is observed with response time 253 s and recovery time 473 s.
AgNW-SWCNT hybrid network on the paper remains connected until 50 bending and unbending cycle, and hence producing a reasonable response. After 70 bending and unbending cycles, the sensor was ruptured out and lost the sensing performance due to discontinuity in the network caused by the mechanical bending.
Example 10
The flexible sensors fabricated in the present invention are light weight, can be creased into desired size and disposable after usage. Suitability of disposal property is tested by igniting the sensor with fire and found that the sensors are disposable within 5 s (Figure 10). Disposal property of the sensors is essential in a variety of personal safety and diagnostic applications in the medical field where the paper based sensors are mostly preferred.