FIELD OF INVENTION:
This invention relates to the development of a method to use composites of metal nanoparticles and single walled carbon nanotubes (SWNTs) to sense, detect and measure gases using the visible emission from the composite. More specifically, several gases of industrial importance like hydrogen, helium and methane can be detected using this composite and its light emission. The properties of the gases such as their pressure can be measured using this composite.
PRIOR ART:
Carbon nanotubes are an allotropic form of carbon. These consist of cylindrical structures, made entirely of carbon atoms. They can be imagined to be formed by the rolling of graphene sheets, along a given direction and curvature, thereby forming what is known as single walled carbon nanotubes (SWNTs). These have been exciting materials due to diverse properties, one being their large surface area, allowing adsorption and storage of various gases. Surface areas up to 1600 m2/g for SWNTs and other carbon materials have been reported (Lafi, L., Cossement, D., Chahine, R., Carbon, 2005, 43, 1247 - 1357). The gas-storage capability of these materials is thus one of the widely researched areas of science. Storing gases such as hydrogen and methane find practical applications in fuel cell industry and related applications. The interest in the development of such products is to store maximum amount of gas for minimum mass of the material. Thus, SWNTs being light and porous are an attractive proposition. Equally important to storage is the ability to detect gases, often at trace levels. Several prior art exists in this direction, some of them being:
(a) United States patent 6528020 - provides a method to fabricate aligned arrays of carbon nanotubes for molecular sensing.
(b) United States patent 6919730 - provide a method to sense liquids and gases by measuring the current-voltage (l-V) characteristics across the ends of the tube.
These, along with a few other carbon nanotube based gas sensors exist (United States patent 7052468, United States patent 7081385). In general, the stress has been to monitor the electrical characteristics of carbon nanotubes, upon adsorption of various gases and liquids. This generally requires lithographic tools to fabricate an

array of aligned nanotubes. The measurement of l-V characteristics often requires special instruments. These factors make this type of sensors economically unviable to produce on large scale. On the other hand, very limited art is available on monitoring the changes in the vibrational and optical characteristics of nanotubes upon exposure to gases. Herein, we conceive and demonstrate a method to fabricate gas sensors, relying on the visible fluorescence from metal nanoparticle-SWNT composites. Leakage of gases like methane, hydrogen and helium are often dangerous, from the perspective of their flammability, particularly in industrial scenarios. In this disclosed method, SWNTs which act as storage materials, are made as sensors for the stored gases. This makes it possible to have the same device for both storing and sensing applications.
DESCRIPTION OF INVENTION:
This invention describes one such gas sensor, fabricated from metal nanoparticle-single walled carbon nanotube composites. This is based on the visible fluorescence in such composites that has been reported and documented earlier (Indian Patent Application No 421 CHE/2006 dated 9th March 2006; Indian Patent No 943/CHE/2007 dated 03th May, 2007). The present patent describes a method to use the composite material as a gas sensor using the fluorescence. The fluorescence from the composite has been shown to be sensitive to the quantity and type of gas to which it is exposed to.
The method herein proposes to fabricate a composite film of gold or silver nanoparticles and carbon nanotubes at a liquid-liquid interface and use it to sense gases.
The nanoparticles of gold, stabilized by citrate groups (Au@citrate) were synthesized by a procedure reported in the literature (Turkevich, J.; Stevenson, P.L.; Hiller, J. Discuss. Faraday Sod 951, 11, 55). The typical size of the particles synthesized by this method is 12-15 nm in diameter. Silver nanoparticles (Ag@citrate) of mean diameter 60 - 70 nm, are also prepared from a similar protocol by reduction with trisodium citrate. Gold nanorods (AuNRs), of aspect ratio 2.8 and diameter 11 nm, are prepared from a seed-mediated template assisted method (Sau, T. K., Murphy, C. J., Langmuir, 2004, 20, 6414). Single walled carbon nanotubes were purchased

from Sigma Aldrich. This was sonicated in N, N dimethylformamide and centrifuged at 20000 rpm for 30 minutes. The resulting mother liquor was collected and subjected to several sonication-centrifugation cycles to remove all metal and carbon impurities. The brown colored supernatant solution was found to be stable for several weeks.
The method according to this invention comprises the steps of fabricating a composite film having nanoparticles and single walled carbon nanotubes as the components. This composite film was formed at the liquid-liquid interface. The as-prepared gold and silver nanoparticles, using the citrate reduction route (Turkevich, J.; Stevenson, P.L.; Hiller, J. Discuss. Faraday Soc.1951, 11, 55), was the aqueous phase (liquid phase 1) while diethyl ether formed the organic phase (liquid phase 2). To this biphasic system, the dispersion of SWNTs in tetrahydrofuran (THF) was added. The film formed at the liquid-liquid interface was transferred to a glass substrate and allowed to dry in ambience. Various other sources of SWNTs were used. Gold nanoparticles, nanorods and silver nanoparticles have been used in the aqueous phase to form the composite termed as Au-SWNT, AuNR-SWNT and Ag-SWNT, respectively. The vibrational and fluorescent properties of such composites were studied by Raman spectroscopy, the details of the instrumental setup is given below.
For Raman characterization, the film was irradiated with a 514.5 nm Argon ion laser, 40 mW maximum power, through a 20X microscope objective and the light obtained from the sample were collected by the same objective and sent to a spectrometer through a multimode fiber. A super-notch filter placed in the path of the signal effectively cuts off the excitation radiation. The signal was then dispersed using a 150 grooves/mm grating and the dispersed light was collected by a Peltier cooled charge coupled device (CCD).
DESCRIPTION WITH REFERENCE TO DRAWINGS:
The sample compartment consists of an Oxford MicrostatN cryostat suitably modified to conduct the gas experiments. The experimental set-up (see Figure 1) consists of a gas line connected to the sample stage of the confocal Raman microscope. The gas line is connected to a mercury manometer through valve 1 using which the pressure

is monitored and controlled. Commercially available high purity (>99.5%) gas cylinders are used as gas sources throughout the experiments. The desired gas cyljnder(s) is connected to the gas line through valves 2 or 3 and a known amount of gas can be admitted inside the sample stage through valve 4. The stage is a part of the confocal Raman microscope. Yet another valve (5) is connected to the rotary pump (Ri) so that the gases can be removed. The sample stage is also connected to a separate rotary pump (R2) in order to get the composite to a vacuum of 10-2 torr.
Figure 1. Schematic representation of the setup used to study the properties of the composite upon exposure to various gases.
The composite film, formed at liquid-liquid interface was transferred on a freshly cleaved mica surface and dried in vacuum. Samples for point-contact current-image atomic force microscopy (PCI-AFM) were prepared by masking one half of the sample surface using a cover glass slip and fabricating a gold electrode (thickness 30 nm) on the other half by thermal vacuum deposition of pure Au metal. The PCI-AFM measurement is carried out on a JEOL JSPM-4210 instrument equipped with 2 function generators (WF 1946, NF Corporation). The l-V characteristics along the long axis of the nanotube composite were measured using Ti-Pt coated conductive cantilevers. The bias voltage was applied on the gold electrode and the cantilever was grounded.
Upon irradiating the sample with the excitation laser source, a red visible emission between 600 - 700 nm was obtained. Typical signatures of single walled carbon nanotubes, namely the radial breathing mode (RBM) and the D and G bands were found to be superimposed on the background of this emission, confirming that the emission was an inherent property of the carbon nanotubes. This emission has been documented earlier in our patent (Indian Patent application No 421 CHE/2006 dated 9th March 2006).
All measurements mentioned here are done at the ambient temperature.
Specific example of the working of the device
Variation of Au-SWNT fluorescence upon exposure to hydrogen

As-synthesized citrate reduced gold nanoparticles of 12-15 nm mean diameter in aqueous medium and diethyl ether were taken in equal volumes to form the aqueous-organic liquid-liquid interface. To this biphasic system, a dispersion of SWNTs in THF was added. This was immediately followed by the formation of a composite film at the interface. This film was transferred onto a 0.2 mm cover-glass substrate and dried in ambience.
This Au-SWNT composite sample is mounted inside the sample compartment and evacuated using R2. Simultaneously, the gas line is also evacuated using R1. The Raman spectrum from the composite is recorded after evacuation. The laser intensities are kept constant throughout the experiment. A shutter is used to shine the laser on the sample only during the analysis, to avoid laser-induced damage to the sample. The valve 5 and the tri-junction valve connecting the gas line to the sample stage are closed. The desired gas is then leaked into the gas line by opening valve 2. The amount of gas leaked into the gas line is monitored using the Hg manometer. After leaking, valve 2 is closed. The tri-junction valve is now opened carefully with simultaneous monitoring of the pressure inside the gas line using the Hg manometer. This way a controlled amount of gas is leaked into the sample compartment, where it interacts with the composite sample. The Raman spectrum from the sample is measured after allowing the gas to interact with the composite for 5 minutes so that the response of the system equilibrates. The gas pressure inside the sample compartment is varied systematically from 10 to 500 torr with its fluorescence being monitored simultaneously. The resulting series of spectra till 500 torr are shown in Figure 2A. The experimental geometry in the present set-up does not allow us to measure gas-pressures greater than their atmospheric pressure.
We find that the fluorescence of the composite is dependent on the partial pressure of the gas inside the sample compartment. In case of hydrogen, the fluorescence is seen to decrease with systematic variation of the partial pressure of the gas. Since the G' band of SWNT is located close to the fluorescence maxima, the intensity of this peak is taken to be the measure of fluorescence from the sample. A plot of the intensity of the G' band with respect to the partial pressure of the gas is shown in Figure 2B.

Figure 2. (A) Raman spectra of Au-SWNT composite acquired at various pressures of H2. (B) Plot of G' band intensity as a function of H2 gas pressure. Inset shows the semi-log plot of (B). The red ovals indicate regions of saturation, which are discussed in the text.
Similar experiments have been carried out with gases and vapours like helium, methane, hexane, cyclohexane and nitrogen. It is seen that the response of the composite depends on the size of the gas molecule and its nature of interaction with the SWNTs in the composite. The adsorption of gases on to SWNTs can occur at four distinct sites:
(a) Endohedral sites - referring to sites inside the nanotubes.
(b) Groove sites - referring to the troughs formed in between adjacent SWNTs constituting the lattice.
(c) Interstitial sites - referring to sites available in-between SWNT constituting the bundle.
(d) External sites - referring to sites present on the outer surface of the bundle. The exact site of adsorption of a gas molecule on a SWNT depends on its size, its interaction energy and the stability of such an interaction contributed to the system. These aspects have been documented theoretically (Stan, G., et al., Phys. Rev. B., 2000, 62, 2173 - 2180) and experimentally (Kondraytuk, P., Yates Jr., J. T., Ace. Chem. Res., 2007. 40, 995 - 1004). The results show that the effect of the interaction of a gas can be used to understand the type of adsorption site.
Separate set of experiments are conducted by exposing Au-SWNT to gases like nitrogen and cyclohexane and monitoring the vibrational spectrum in a method similar to that carried out for hydrogen. No significant change in the emission intensity is observed for these gases, as shown in Figure 3.
Figure 3. The response of Au-SWNT composite upon exposure to (A) nitrogen and (B) cyclohexane.
Thus sensitivity of the fluorescence to the presence of certain types of gases is established. This sensitivity arises due to the size and nature of interaction of a given gas with a SWNT bundle. Theoretical studies by Stan et al {Phys. Rev. R,

2000, 62, 2173 - 2180) indicates that the molecules like hydrogen and helium prefer to adsorb in both the interstitial and endohedral channels of SWNT bundles. In a similar treatment, it has been shown that nitrogen adsorbs preferentially in the endohedral spaces and not the interstitial channels. Thus, the decreases in fluorescence intensity from the composite with specific adsorption of hydrogen can be linked to its occupation of the interstitial sites. Cyclohexane being a globular molecule, in hindered from entering into interior pores of the SWNT present in the composite. The filling up of the endohedral, interstitial and groove sites of the SWNT bundle by the gas present is important for affecting the visible fluorescence. The adsorption of gases on the endohedral sites is thought to open up radiative decay pathways, thereby leading to decrease in fluorescence intensity. On the other hand, adsorption of gases on the interstitial and groove sites loosens up the SWNT bundle and spatially separates the nanotubes present in them. This causes the metallicity of the bundle to increase, thereby resulting in quenching of fluorescence. Of these two possible mechanisms, the size of cyclohexane may be hindering its adsorption and subsequent interaction with the composite whereas the non-specific nature of interaction of nitrogen with the composite does not influence the latter's electronic response.
Variation of Au-mSWNT fluorescence upon exposure to hydrogen
Metallic SWNT (mSWNTs) were purified according to an established procedure (J. Am. Chem. Soc, 2005, 127, 10287) and their purity was estimated using confocal microRaman and PCI-AFM measurements. Composites of Au nanoparticles were prepared with mSWNTs using a similar protocol, at the liquid-liquid interface. Such a composite is henceforth designated as Au-mSWNT. The vibrational and electrical transport properties of such a composite is studied in the presence of nitrogen and hydrogen using confocal microRaman and PCI-AFM techniques. The variation of the G-band is monitored in presence of different gases. G-band is sensitive to the chirality of SWNTs and its lineshape provides important pointers. As seen from Figure 4A, the G-band lineshape exhibits marked variations in the presence of nitrogen and hydrogen. It shows a lorentzian lineshape (full width at half maximum, FWHM = 21cm''') for Au-mSWNT composite indicating its semiconducting nature. This is seen in presence of nitrogen also. However, in presence of hydrogen, the G-band assumes a broad, asymmetric lineshape (FWHM-54 cm""*) signifying metallic

behavior. To verify this observation, PCI-AFM measurements were conducted on Au-mSWNT in nitrogen and hydrogen ambiences. Using the AFM probe, it was possible to topographically map the same area in presence of nitrogen and hydrogen, simultaneously collecting its l-V characteristics along several bundles. A plot of conductance (dl/dV) versus applied bias voltage is made to assess the variation in the electrical transport properties of the Au-mSWNT composite. The value of conductance at zero bias voltage can be related to the densty of states at the Fermi level. This value is zero for Au-mSWNT in presence of nitrogen while it shifts to a non-zero value in presence of hydrogen. These observations confirm that the semiconducting bundles in Au-mSWNT composite revert back to metallic state, upon exposure of hydrogen. This explains the drop in fluorescence for the composite upon hydrogen exposure. Thus nanotube composite acts like a switch upon the adsorption of gases.
Figure 4. (A) Raman spectra of (a) purified mSWNTs, (b) Au-mSWNT composite, (c) Au-mSWNT upon exposure to 500 torr H2 and (d) Au-mSWNT composite after pumping out H2 exposed in (c). Spectra (a) to (d) are recorded at the same point on the composite sample. (B) Plot of conductance versus bias voltage constructed at various point of the Au-mSWNT composite under an atmosphere of nitrogen (red traces) and hydrogen (black traces).
Other composites like Ag-SWNT and AuNR-SWNT have also been tested and observed to give similar responses. This phenomenon has also been demonstrated with liquids like n-hexane and cyclohexane. Due to its molecular shape and size, n-hexane is able to influence the fluorescence from the Au-mSWNT composite, while cyclohexane (which does not adsorb in interstitial channels) does not affect it.
In all cases, the response is reversible and the fluorescence intensities come back to the original value upon evacuation. The response has been reproduced at east fifty times in all the cases.
The objective of making a nanoparticle-nanotube composite is to cover the nanotube structure with nanoparticles such that there is electronic interaction between the two. This nanoparticle cover can be achieved by various means such as spraying a

nanoparticle dispersion over nanotubes, evaporating a nanoparticle dispersion over a nanotube covered substrate, mixing nanoparticles and nanotubes intimately by a physical method such as grinding, etc. Any method which covers the nanoparticles on the nanotubes may be used to meet the desired objective.

WE CLAIM :
1. A sensor having alterable optical and electrical characteristics for detecting a
gas or a liquid connprising :
a. a composite film formed with selective metal nanopartides and single walled
carbon nanotubes (SWNT), the said composite having a visible fluorescence
sensitive to a particular gas or liquid, and
b. a fluorescence sensor in the form of a composite film for producing a variation
in its own fluorescence thereby determining the presence of the gas or the
liquid in the proximity of the composite film.
2. A sensor with altering optical and electrical characteristics for detecting a gas
or a liquid comprising :
a. a composite film formed with selective metal nanopartides and SWNT, the
said composite having a visible fluorescence sensitive to a particular gas or a
liquid with alterable fluorescence, and
b. a fluorescence sensor in the form of a composite film for sensing the
fluorescence thereby determining the presence of the gas or the liquid in the
proximity of the composite film.
3. A method of using a sensor for sensing the presence of a gas or a liquid by detecting the alteration in optical characteristics of a sensitive composite film formed with selective metal nanopartides and SWNTs having an alterable optical characteristic, with a first fluorescence in the absence of the gas or the liquid and a second fluorescence in the presence of the gas or the liquid , and where the second fluorescence is different from the first fluorescence.
4. A method of using a sensor for sensing the presence of a gas or a liquid by detecting the alteration in optical characteristics of a sensitive composite film formed with selective metal nanopartides and SWNTs with altering optical characteristic, with a first fluorescence in the absence of the gas or the liquid and altering to a second fluorescence in the presence of the gas or the liquid, and where the second fluorescence is different from the first fluorescence.

5. A method of using a sensor for sensing the presence of a gas or a liquid with
a gas or a liquid sensor having alterable optical characteristic as claimed in claims 3
and 4 comprising the steps of:
a. using a sensitive composite film formed with selective metal nanoparticles and
SWNTs for trapping the gas or the liquid exposed within SWNTs in the
composite, which is adapted to absorb the exposed gas or liquid, and alter
thereafter its own fluorescence which is representative of the absorbed gas or
the liquid,
b. sensing the fluorescence and the alteration of the fluorescence in the
composite, and
c. determining the presence of the exposed gas or the liquid using the
fluorescence response, where the fluorescence of the composite after
exposure to the gas or the liquid is different from the fluorescence of the
composite before its exposure to the gas or the liquid.
6. The sensor as claimed in claims 1 to 5 is a gas sensor.
7. The sensor as claimed in claims 1 to 5 is a liquid sensor.
8. The nanoparticles claimed in claims 1 to 5 wherein the shape of the nano particles is rod, sphere, triangle or any other shape.
9. The said metal nanoparticles as claimed in claims 1 to 5 are selected from the group consisting of gold, silver, copper, platinum, rhodium, palladium, any alloys thereof and any compounds thereof.
10. The said film as claimed in claims 1 to 5 is formed on a substrate, the substrate selected from the group consisting of glass, silica, silicon, mica, quartz, highly ordered pyrolytic graphite or any such similar material.
11. The composite as claimed in claims 1 to 5 is formed with a method selected from the group consisting of dispersion, vapour condensation, thermal evaporation, gas phase deposition and spraying.

12. The composite as claimed in claims 1 to 5 is irradiated with laser for detecting its optical characteristics.
13. The composite as claimed in claims 1 to 5 is irradiated with white light or laser of 514.5 nm or any other wavelength for detecting its optical characteristics.
14. The composite as claimed in claims 1-5 having alterable fluorescence intensity indicative of the pressure of the gas or the vapour pressure of the liquid present.
15. The composite as claimed in claims 1-5 having alterable fluorescence intensity indicative of the quantity of the gas or the liquid present.
16. The SWNT forming the composite in claims 1-5 may be metallic or semi conducting.
17. The composite as claimed in claims 1-5 is a reversible metal-semi conductor-metal composite and wherein the reversibility occurs with gas or the liquid exposure.
18. The composite as claimed in claims 1-5 is formed with metallic type SWNTs (mSWNTs) and wherein the composite is capable of transforming into semiconducting type upon composite formation and reversing back to metallic type upon exposure to a specific gas or liquid.
19. The composite as claimed in claims 1-5 is such that it exhibits variable electrical characteristics upon exposure of specific gases or liquids,
20. The gas or the liquid detectable as claimed in claims 1-5 shall be capable of interacting with nanotubes and adsorbable in interstitial sites.
21. The gas detectable as claimed in Claim 1-5 & 6, is hydrogen.
22. The gas detectable as claimed in Claim 1-5 & 6, is helium.

23. The liquid detectable as claimed in Claim 1-5 & 7, is n-hexane.
24. The liquid detectable as claimed in Claim 1-5 & 7 is chloroform.
25. The composite as claimed in claims 1-5, is such that adsorption of gases or
liquids results in a variation of fluorescence.