Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
  • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
  • G01N21/65—Raman scattering
  • G01N21/658—Raman scattering enhancement Raman, e.g. surface plasmons
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y15/00—Nanotechnology for interacting, sensing or actuating, e.g. quantum dots as markers in protein assays or molecular motors
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
  • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
  • G01N21/64—Fluorescence; Phosphorescence
  • G01N21/6428—Measuring fluorescence of fluorescent products of reactions or of fluorochrome labelled reactive substances, e.g. measuring quenching effects, using measuring "optrodes"
  • G01N21/643—Measuring fluorescence of fluorescent products of reactions or of fluorochrome labelled reactive substances, e.g. measuring quenching effects, using measuring "optrodes" non-biological material
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
  • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
  • G01N21/64—Fluorescence; Phosphorescence
  • G01N21/6489—Photoluminescence of semiconductors
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
  • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
  • G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
  • G01N27/128—Microapparatus

Definitions

• This invention relates to the development of a method to use composites of metal nanoparticles and single walled carbon nanotubes (SWNTs) to sense, detect, analyse and measure gases using the visible emission and the electrical conductivity of the composite. More specifically, several gases of industrial importance like hydrogen, helium and methane can be detected using this composite and its light emission and/or electrical conductivity. The properties of the gases such as their pressure can be measured using this composite.
• SWNTs single walled carbon nanotubes
• 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 their diverse properties, one being their large surface area, allowing adsorption and storage of various gases. Surface areas up to 1600 m 2 /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.
• SWNTs single walled carbon nanotubes
• United States patent 6528020 provides a method to fabricate aligned arrays of carbon nanotubes for molecular sensing.
• United States patent 6919730 provides a method to sense liquids and gases by measuring the current-voltage (I-V) characteristics across the ends of the tube.
• 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. Further, the device has also been demonstrated to exhibit significant changes in the current carrying capabilities upon adsorption of specific gases.
• the patent application no. 421/CHE/2006 dated 9/03/2006 relates to the discovery of visible, unblinking fluorescence from single walled carbon nanotubes (SWNTs) and the use of such emission in imaging nanotube structures.
• SWNTs single walled carbon nanotubes
• 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.
• Single walled carbon nanotubes obtained from a commercial source, were purified by repeated sonication and centrifugation in N,N-dimethyl formamide (DMF).
• the as- prepared silver nanoparticles using the citrate reduction route (Turkevich, J.; Stevenson, P. L.; Hiller, J. Discuss. Faraday Soc.1951 , 11, 55) constituted the aqueous phase (liquid phase 1) while diethyl ether formed the organic phase (liquid phase 2).
• the dispersion of SWNTs in DMF was added.
• the film formed at the liquid-liquid interface was transferred to a glass substrate and allowed to dry in ambience.
• SWNTs single- walled carbon nanotubes
• This invention provides a method of an arrangement and the use of this arrangement for transforming metallic carbon nanotubes into semiconducting carbon nanotubes.
• This invention specifically pertains to the modification of electrical transport properties of SWNTs. More specifically, it involves the transformation of a metallic SWNT to a semiconducting SWNT.
• the method herein proposes to fabricate a composite film of gold or silver nanoparticles and carbon nanotubes at a liquid-liquid interface and nanostructures based on this film.
• Such a film shows semiconducting properties throughout as all the metallic nanotubes were transformed into semiconducting ones.
• This technology is believed to have far-reaching applications in nanoelectronics, sensors and device- fabrication at the molecular level. It is possible to observe this transformation from single walled carbon nanotubes using a variety of other nanoparticles of gold, silver and other metals of various shapes and sizes.
• a method for transforming a metallic, conducting mSWNT (mSWNT refers to metallic SWNT) into a semiconducting SWNT comprising the steps of:-
• a preparing an organic liquid phase with diethyl ether; b. preparing , an aqueous liquid phase with citrate stabilized metal nanoparticles; c. mixing the two liquid phases to form a biphasic composition having a liquid-liquid interface; d. dispersing mSWNT (prepared by the procedure reported in Y. Maeda et a/, J. Am. Chem. Soc. 2005, 127, 10287) in a tetrahydrofuran (THF) solution; e. introducing the dispersed mSWNT to the biphasic composition; f.
• mSWNT prepared by the procedure reported in Y. Maeda et a/, J. Am. Chem. Soc. 2005, 127, 10287
• this invention is another attempt to create a sensor from SWNT composites.
• the present 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 9 th March 2006; Indian Patent No 943/CHE/2007 dated 03 rd 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 were synthesized by a procedure reported in the literature (Turkevich, J.; Stevenson, P. L.; Hiller, J. Discuss. Faraday Soc.1951 , 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.
• 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).
• the dispersion of SWNTs in DMF 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 fluorescence properties of such composites were studied by Raman spectroscopy, the details of the instrumental set-up is given below.
• the film, kept on a glass coverslip was irradiated with a 514.5 nm Argon ion laser, 40 mW maximum power, through a 2OX 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).
• CCD Peltier cooled charge coupled device
• a microelectronic switching device with interdigitated electrodes having a spacing of 10 ⁇ m, is fabricated on a silicon wafer by mask-assisted chemical vapor deposition of gold. The composite is deposited on this device and allowed to dry, for complete removal of any liquids. Electrical leads are made onto gold pads using silver paste. This is suspended in a cylindrical glass column sealed at both ends with a provision for flowing the desired gas. The I-V characteristics of the device were monitored using Keithley 2700 digital data acquisition system, interfaced to a computer. A schematic representation of the electrode along with the picture of the device is shown in the figure 5.
• 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 cylinder(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 (R 2 ) 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.
• PCI-AFM point-contact current-image atomic force microscopy
• a dispersion of SWNTs in DMF 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 mercury 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 manometer.
• Cyclohexane being a globular molecule, is hindered from entering into interstitial 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.
• adsorption of gases on the interstitial and groove sites loosens up the SWNT bundle which spatially separates the nanotubes present in them. This causes the metallicity of the bundle to increase, thereby resulting in quenching of fluorescence.
• 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.
• mSWNTs Metallic SWNT
• mSWNTs 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.
• a plot of differential conductance (dl/dV) versus applied bias voltage (V) 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 density 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.
• the observations from confocal Raman and PCI-AFM 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.
• nanotube composite acts like a switch upon the adsorption of gases.
• FIG. 4 Raman spectra of (a) purified mSWNTs, (b) Au-mSWNT composite, (c) Au-mSWNT upon exposure to 500 torr H 2 and (d) Au-mSWNT composite after pumping out H 2 exposed in (c). Spectra (a) to (d) are recorded at the same point on the composite sample.
• 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.
• FIG. 5 Photograph of the device setup with a cartoon representation of the microelectrode.
• the shaded circle in the cartoon is used to represent the sample with the yellow regions representing the gold electrode.
• Changes in electrical conductivity of Ag-mSWNT composite was studied in the device shown in Figure 5A.
• the glass column was first evacuated using a rotary pump after which the desired gas (H 2 or N 2 ) was introduced into the compartment.
• the current response before and after the introduction of the gas at a constant voltage was monitored using a Keithley 2700 digital data acquisition system, interfaced to a computer.
• One cycle of exposure consists of evacuation of the chamber, introduction of the desired gas at the desired pressure, followed by pumping out of the gas. Several such cycles were carried out, with the current being monitored continuously.
• Figure 5B is a plot of variation of current for a bias voltage of 5 V for Au- mSWNT composite in presence of H 2 (500 torr, black line) and N 2 (500 torr, red line).
• the ON and OFF states pertain to the presence and absence of gases, respectively. While the current for the ON state is constant, that due to the OFF state increases slowly with increase in cycles as hydrogen exposed during the previous cycle is not removed completely. There is an increase in the current response in the case of H 2 (black trace), which is not the case with N 2 (red trace).
• a sensor having alterable optical and/or electrical characteristics for detecting a gas or a liquid shall comprise of a composite film formed with selective metal nanoparticles and single walled carbon nanotubes (SWNT), the said composite having a visible fluorescence sensitive to a particular gas or liquid, and 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.
• SWNT single walled carbon nanotubes
• a sensor having alterable optical and electrical characteristics for detecting a gas or a liquid shall comprise of a composite film formed with selective metal nanoparticles and single walled carbon nanotubes (SWNT), the said composite having a visible fluorescence sensitive to a particular gas or liquid, 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, and an electrical characteristic sensor in the form of a composite film exhibiting distinct, traceable changes in the I-V characteristics upon exposure of certain gases and liquids.
• SWNT selective metal nanoparticles and single walled carbon nanotubes
• a sensor with altering optical and/or electrical characteristics for detecting a gas or a liquid shall comprise of a composite film formed with selective metal nanoparticles and SWNT, the said composite having a visible fluorescence sensitive to a particular gas or a liquid with alterable fluorescence, and 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.
• a sensor with altering optical and electrical characteristics for detecting a gas or a liquid shall comprise of a composite film formed with selective metal nanoparticles and SWNT, the said composite having a visible fluorescence sensitive to a particular gas or a liquid with alterable fluorescence, 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, and an electrical characteristic sensor in the form of a composite film exhibiting distinct, traceable changes in the I-V characteristics upon exposure of certain gases and liquids.
• 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 nanoparticles 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.
• the method of using a sensor as per the invention 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 nanoparticles 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.
• the method of using a sensor as per the invention for sensing the presence of a gas or a liquid by detecting the alteration in electrical characteristics of a sensitive composite film formed with selective metal nanoparticles and SWNTs shall have an alterable electrical characteristic, with a first electrical conductivity in the absence of the gas or the liquid and a second electrical conductivity in the presence of the gas or the liquid, and where the second electrical conductivity is different from the first electrical conductivity.
• the method of using a sensor as per the invention for sensing the presence of a gas or a liquid by detecting the alteration in electrical characteristics of a sensitive composite film formed with selective metal nanoparticles and SWNTs with altering electrical characteristic, with a first electrical conductivity in the absence of the gas or the liquid and altering to a second electrical conductivity in the presence of the gas or the liquid, and where the second electrical conductivity is different from the first electrical conductivity.
• the method of using a sensor as per the invention for sensing the presence of a gas or a liquid with a gas or a liquid sensor shall have alterable optical and/or electrical characteristic which comprising the steps of firstly 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 and/or its electrical conductivity which is representative of the absorbed gas or the liquid.
• the electrical characteristic sensor as per the invention is a microelectronic device which is capable of monitoring the I-V characteristics of the metal nanoparticle- carbon nanotube composite for sensing and measuring gases.
• microelectronic device as per the invention consists of gold electrodes and pads or of any other metal.
• microelectronic device as per the invention is made on silicon or any other equivalent susbtrate.
• the sensor as per the invention shall be a gas sensor and a liquid sensor.
• the shape of the nanoparticles as per the invention is rod, sphere, triangle or any other shape and further wherein the particles can be classified in the nanometer scale based on their size.
• the said metal nanoparticles as per the invention are selected from the group consisting of gold, silver, copper, iron, platinum, rhodium, palladium, any alloys thereof and any compounds thereof.
• the said composite film and the device as per the invention shall be 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.
• the composite film as per the invention shall be formed with a method selected from the group consisting of dispersion, vapor condensation, thermal evaporation, gas phase deposition and spraying.
• the composite film as per the invention shall be irradiated with white light or laser of 514.5 nm or any other wavelength for detecting its optical characteristics.
• the composite film as per the invention shall have alterable fluorescence intensity indicative of the amount of gas or vapour present.
• the composite film as per the invention shall have alterable electrical conductivity indicative of the quantity of the gas or the liquid present.
• the SWNT forming the composite film as per the invention may be metallic or semiconducting.
• the composite film as per the invention shall be a reversible metal-semiconductor- metal composite and wherein the reversibility occurs with gas or the liquid exposure.
• the composite film as per the invention shall be formed with metallic type SWNTs (mSWNTs) and wherein the composite film is capable of transforming into semiconducting type upon composite film formation and reversing back to metallic type upon exposure to a specific gas or liquid.
• mSWNTs metallic type SWNTs
• the composite film as per the invention shall exhibit variable electrical characteristics upon exposure of specific gases or liquids.
• the gas or the liquid detectable as per the invention is capable of interacting with nanotubes and adsorbable in interstitial sites.
• gases detectable as per the invention are hydrogen, helium, methane or propane or butanes or pentanes,
• the liquid/vapor detectable as per the invention are n-hexane and chloroform.
• the composite as per the invention is such that adsorption of gases or liquids results in a variation of its fluorescence and/or its electrical response.
• 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.
• the nanoparticles used can also be of differing sizes and shapes, as has been demonstrated in this case with Au nanoparticles, Ag nanoparticles and Au nanorods.

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