CA2105869A1 - Sensors based on nano-structured composite films - Google Patents

Abstract

An electrically reactive composite article (20) comprising a random or regular array of microstructures (16) partially encapsulated within an encapsulating layer (12), microstructures (16) each comprising a whisker-like structure (14), optionally having a conformal coating (13) enveloping the whisker-like structure (14) is described. The composite article (20) is useful as an electrically conducting component of a circuit, antenna, microelectrode, reactive heater, and multimode sensor to detect the presence of vapors, gases, or liquid analytes.

Description

W0 92/17774 ~ 3 ~ PCT/US92/01372 XEN30~ ~A~ED ON NA~0-8~UC~URED CO~PO~ITB ~I~M~

3~o~r~una o th~ gr~nt Sn~nt~ OD

Th~ in~entlon rel~te~ to Q Gomp~3ite ~rtlclo compri~ g r~ndomly or ragu~arly array~d osi~nted micro~tructur~s partially encap~u~ated within ~ l~yer, in particular to the method o~ msking the e~me and to ~o use Or the comp~lte erticle ~6 ~n ~l~ctric~lly conduct~ng polym~r, thin ~i~ r~onant c~rcuit, ~ntenn~ microelect~od~ or r~sist~ve heater, ~nd ~6 multimode 3ensor t~ detect the presenc~ of v~pors, g~sc~, or li~uld ~n~lyt~D.

2. ~ 5Y{~L~o ll _OD- D~e-CDmpo~ite article~ contalnlnq or exhtbiting ~ :
layere~ s~ru~ture haYe been prepared by m~ny d1f~r~nt : .
typeo o~ chumlc~l ~nd phy~icnl d~po~itt on proc~ 3.
For 8x~mplu~ U.S Patent No. 4,812,35a ~l~c10808 an art~olc comprl~inq ~ 6ubstr~te h~ving ~ ~:
~nlcrol4yer ~icrontruGtured-l~yer) th~ comprl~e~ ~ :
un~rmly o~$~nt~dl crystalllno, ~olld, orgAnlc micro6truoturH~ Yeral ten~ o~ nanometer~ ~n croos~
~ction and ~ method o~ mAk~ng th~ same. ~urther, ~352 ~e~che~ optlonally conrorm~l ~o~t~ng th~ m~cr~yer And 0naapsul~ltlng th~ ~onform~ ontod m~ c:rc~lay~r .
Dlrk~ ot al. in "co~um~ar Microstructureo in :u ~apor-Dapo~ d ~nln ~llm~ hln Sol ~ Eilm~, vol. 47, .-~
~0 ~1~77), pg~ 219-3~ revi~w r3e~eral methods ~own in th~
~rt that c~n yi~ld co~umnar micro~tructur~, how~vc~, -. . .
a~ ~irks ~t ~l. po~nt out t~e ~tructuro~ are no~
d~rablo o~ a ~ought-a~ter ou~coma o~ v~por- :
depos~t~on.
U.~:. Pat~nt- No. 3/96!~rS65 ds2:crib6~ ~ vacu~m deposition te~hni~ue that can produce or~an~ or ~norg~nlc mlcrostr~c~ure~.

~lJ13STITUTE SffEET
' ' W092/~7774 ~ t~ 5 3 ~ PCT/US92/01372 Floro et al. in "Ion-Bombardment-Induced Whisker Formation of Graphite," J._Vac._Sci. Technol.
A, vol. 1, no. 3, July/September (1983) pgs 1398-1402 describe graphite whisker-liXe s~tructures produced by an ion-bombardment process.
Flexible conducting media known in the art, typically having a layered structure, exist in a variety of distinct formats. For example, U.S. Patent No. 4,674,320 discloses a conducting powder-like material, such as carbon, dispersed throughout a polymeric binder at concentrations sufficient to enable conduction by charge transfer from particle to particle. Such an arrangement results in an isotropicly conducting sheet, that is, resistivity perpendicular to the plane of the sheet is the same as the in-plane resistivity.
Bartlett et al., Sensors and Actuators, vol.
20, pg 287, 1989, disclose a conductive polymer film made by electrochemical polymerization. Resistivities of these polymer films are three-dimensionally isotropic and tend to be relatively high.
Other examples known in the art teach an -article comprising a conducting layer applied to a flexible polymer sheet by vacuum coating processes, electrochemical or electroless plating processes, printing, particle embedding and the like. However, in these cases, the conductive coating, for example, a solid metallic layer, will have a low resistivity and is not easily controllable. Additionally, since the conductive layer is on the surface of a polymer substrate, adhesion of the conductive layer to the polymer substrate is often a problem. The a~hesion problem is particularly apparent when the conducting layer is carrying current. If a very thin 0r discontinuous conductive layer is applied to the polymer substrate to increase the surface resistivity, the power carrying capability of the conductive layer :- ~ ,. . , - - - .. . ,: : . :-W~92/17774 2 ~ 3 PCT/~S92/01372 tends to be compromised and the problem of adhesion tends to be exacerbated.
Electrical properties are useful as sensors, however, most prior art gas and vapor sensors are based on many of the prior art layered structures. The sensor media can be thin or thick film devices utilizing either surface acoustic wave (SAW) technology or chemiresistors incorporating solid electrolytes, polymers with bulk gas sensitivity, metal or semiconductor (inorganic or organic) thin films, or homogeneous dispersions of conducting particles in insulating matrices. -Generally, sensors based on SAW technology are costly to manufacture and tend to be used only for re~ersible sensing. They are generally not used for nonreversible sensors, such as dosimetry monitoring, see Snow et al., "Synthesis and Evaluation of Hexafluorodimethyl carbinol Functionalized Polymers as SAW Microsensor Coatings," Polymer Reprints, 30(2), 213 (1989); Katritzky et al., "The Development of New Microsensor Coatings and a Short Survey of ~icrosensor Technology," Analytical Chemistry 2l(2), 83 (1989).
On the other hand, chemiresistor based sensors tend to be reversible or nonreversible depending on the chemical and physical composition of the sensing medium, see Katritzky et al., "New Sensor Coatings for the Detection of Atmospheric Contaminants and Water, 1I Review of Heteroatom Chemistry, 3, 160 (l990). Generally, the prior art sensing media exhibit isotropic or homogeneous gas sensing properties. Media having an isotropic sensing property display the same resistivity in all directions of the media. Such media are typically capable of only a single mode of detection. In contrast, media having an ani-sotropic 35 impedance sensing property disp1ay different in-plane -~
and out-of-plane gas sensing impedances. Thus, anisotropic media permit multi-mode operation.

W092/17774 2 1 3 ~ ~ ~ 9 PCT/US~2/01372 Generally, conduction through chemiresistor devices occurs between conducting particles dispersed throughout the media. For example, U.S Patent No.

4,674,320 teaches a chemiresistive gas sensor comprising a layer of organic semiconductor disposed between two electrodes, wherein dispersed within the layer of organic semiconductor is a high conductivity material in the form of very small particles, or islands. Adsorption of a gaseous contaminant onto the layer of organic semiconductor modulates the tunneling current.
U.S. Pat2nt No. 4,631,952 discloses an apparatus and a method for sensing organic liquids, vapors, and gases that includes a resistivity sensor means comprising an admixture of conductive particles and a material capable of swelling in the presence of the liquid, gas, or vapor contaminant.
Ruschau et al., "0-3 Ceramic/Polymer Composite Chemical Sensors," Sensors and Actuators, vol. 20, pgs 269-75, (1989) discloses a composite article consisting of carbon black and vanadium oxide conductive fillers in polyethylene, a polyurethane, and polyvinyl alcohol for use as chemical sensors. The polymer matrices swell reversibly in the presence of li~uid and gaseous solvents, disrupting the conductive pathway and proportionally increasing the resistance. -U.S. Patent No. 4,224,595 discloses an adsorbing type sensor having electrically conductive particles embedded in a surface, forming an electrically conductive path through the sensor.
U.S. Patent No. 4,313,338 discloses a gas sensing device comprising a gas sensing element comprising a gas-sensitive resistive film formed of an aggregate of ultrafine particles of a suitable material deposited on the surface of a substrate of an electrical insulator formed with electrodes.
U.S. Patent No. 3~820,958 discloses an -- . - -- ~ : ~ . , .~, , . . .

W092/17774 2 ~ PCT/US92tO1372 apparatus and a method for determining the presence of hydrogen sulfide in a gas mixture. Silver is deposited on a thin dielectric film. Electrical resistance across the film before and after exposure of the film to hydrogen sulfide containing gas mixture is utilized to determine the amount of hydrogen sulfide present.
U.S. Patent No. 4,906,440 discloses a sensor for a gas detector comprising a metallic/metallic oxide gas sensitive discontinuous film. The gas changes the conductivity of the film and causes the RC network to react.
U.S. Patent No. 3,045,198 discloses a detection device comprising an electrical element sensitive to exposure to liquids, vapors or gases. The 15 detection element includes a broad and long base having ~ -an electrically non-conductive, relatively resilient surface on which is anchored a stratum of exposed electrically conductive discrete adsorbent particles. ~ `
Sadaoka et al., Effects of Morphology on N0 Detection in Air at Room Temperature with Phthalocyanine Thin Films," J. of Mat'l Sci. 25, 5257 (l990) disclose that crystal size in films is affected by the nature of the substrate, ambient atmosphere, and annealing time. The variations of the crystals can effect the detection of NO2 in air.

Summary of the Pre~ent Invention :`
Briefly, this invention provides a composite article with an electrically conductive surface comprising a layer having a dense array of discrete, oriented microstructures partially encapsulated and optionally having 2 conformal coating wherein one end of the microstructures is exposed and coincident with the conductive surface. The conformal coating, preferably is a conducting material. The encapsulant, is preferably a dielectric. Advantageously, the ~

".,- ,..

W0~2/17774 ~ PCT/US92/01372 anisotropic structure of the composite article provides anisotropic impedance, that is, the impedance parallel to the surface plane of the composite article is resistive, while the impedance perpendicular to the surface plane of the article is predominantly capacitive.
In another aspect, a resonant circuit is described wherein the composite article provides the resistive (R) and capacitive (C) component of the circuit. Advantageously, the resonant circuit can be constructed as a low-pass filter, a high-pass filter, a band-pass ~ilter and the like. Furthermore, the composite article can be fabricated such that the conducting layer is formed in patterns suitable for building electronic circuits. This is achieved, by depositing the crystalline microstructures in patterns, or conformally coating th~ microstructures through a mask, or by encapsulating the coated microstructures through a mask, or by any combination of the above.
In yet another aspect of the present invention, a multimode sensor is described. The unique construction of the composite article enables selection of the conformal coating and the encapsulant for their responses to a particular analyte molecule of interest.
The effect of gas/vapor/liquid molecules on the multimode sensor is detected by monitoring the changes in the composite article's electrical properties, that is the resistance and the capacitance.
In this application:
"whisker-like structure" refers to individual repeating units such as, for example, material structures, whiskers, rods, cones, cylinders, laths, pyramids and other regular or irregular geometric shaped structures;
"microst~ucture" refers to the whisker-like structure that has been conformally coated;
"microstructured-layer" refers to a layer ,: . . .
~- ..

W092/t7774 2 i ~ PCT/US92/01372 formed by all the microstructures taken together; ~ : -"conformal-coated" means a material is deposited onto the sides and an end of each whisker-like structure element to envelope the element such that the deposited material conforms to the shape of the whisker-like structure element; - ~ .
"uniformly oriented" means the microstructures are approximate:Ly perpendicular to the surface of the substrate;
"solidified" means the encapsulant undergoes ::~
a change in state, typically from a liquid or liquid- ::~
like phase to a more rigid, solid, or solid-like phase, : ~
such as may occur as a result of drying, chemical .
setting, cooling, freezing, gelling, polymerization, lS etc.; ~: :
"continuous" means coverage of a surface without interruption of the coating;
"discontinuous" means coverage of a surface . wherein there is periodic or non-periodic interruption 20 of the coating; :`~
"uniform" with respect to size, means that the major dimension of the cross-section of the individual microstructures varies no more than about +
25~ from the mean value of the major dimension and the minor dimension of the cross-section of the individual microstructures varies no more than about ~25% from the . mean value of the minor dimension;
"areal number density" means the number of microstructures per unit area; :.
"gas" means a state of matter existing in the :--gaseous state at standard temperature and pressure, but can be liquified by pressure; and :;
"vapor" means an air dispersion of molecules .
of a substance that is liquid or solid in its normal 35 state, that is at standard temperature and pressure, ~ .
sometimes called fumes.

~,..

213~ .3:~ , g Detailed De~criptio~ of the P:referred Embodiment The present invention discloses a composite article having an electrically co:nducting surface, the S~BSTlTt,lTF S~IE1 ... . .

.

21~3~6~

-- 10 -- .
process for making such a film with variable surface resistivity, and use of the invention as a flexible -~
electric circult element havin~ both capacitance and resistancP properties are described. Specific examples 5 are given demonstrating the suitability of the media for use directly as passive RC filter networ~s with significant power dissipation potential. Additional examples demonstrate the suitability of the media for use as gas, vapor and liquid analyte sensors that derive sensing properties from the properties of the nanostructured composite film surface. The sensor functions uniquely in two distinct ways, first in terms of the dual mechanisms by which vapor/gas/liquid molecules affect sensor properties, and secondly with respect to the independent resistance and capacitance impedance properties that can be measured as a function of exposure to the vapor, gas, or liquid.
A composite article comprises encapsulant layer, for example, a polymer that has been encapsulated 20 in encapsulant layer arrayed microstructures, which may --also be composites preferably initially oriented normal to the substrate. Each microstructure comprises a whisker-like structure and optionally, a conformal coating enveloping the whisker-like structure. The chemical composition of the microstructures is determined by the starting material deposited on the substrate to form the whisker-like structures and the conformal -coating subsequently applied to the whisker-like structures. Microstructures may be randomly or regularly arrayed in the encapsulating layer.
The composite article is partially delaminated from the substrate and delamination of the composite article is occurring at interface between the encapsulating layer and the substrate. Delamination of the composite article from the substrate takes the microstructures along, embedded precisely in the surface of encapsulant layer ~ 5~E~~

.: . . , , , . :
~ ` . ` ` , . . ` . .

2 ~ u ~;l and exposes one cross-sectional end of each microstructure, wherein a surface of the encapsulating material of the encapsulant layer and the exposed cross-sectional ends of microstructures are coincident on a common side. The topography of the delaminated surface, or exposed surface of the composits article is the inverse of the topography of the surface of the substrate from which it is delaminated. Furthermore, the exposed surface of the composite article is electrically reactive, that is, exhibits surface electronic phenomena, such as resis.ance and capacitance. If the surtace of the substrate is perfectly smooth, the exposed cross-sectional ends of the microstructures and the delaminated surface of the encapsulating layer will be on a common plane.
The unique fracture and adhesion properties of the whisker-like structures at the substrate interface allow the whisker-like structures to withstand the coating and encapsulating processes, yet be easily and cleanly delaminated from the substrate.
The thickness of the conformal coating applied to the whisker-like structures, and intrinsic resistivity of the conformal coating, are the primary parameters controlling the surface electronic conductivity of the composite article.
It should be noted that the composite article can have the conducting area formed into patterns suitable for building electronic clrcuits by several means. For example, starting material, for example, perylene pigment, can be deposited through a mask, or the conducting conformal coating can be applied to the whisker-like structures through a mask, or the encapsulant can be photolithographically applied to encapsulate the coated whisker-like structures image-wise. The small volume and flexibility of the mediumof the present invention allows it to be used in a wide S~ S~

W092/~7774 2 ~ ~ ~ 8 ~ ~ - 12 - PCT/US92/01372 variety of resonant circuit constructions.
Additionally, the exposed surface of the composite article, that is the reactive ~;urface, can be coated in a patterned manner with an insulator or other dielectrics.
Materials useful as a substrate for the present invention include those which maintain their integrity at the temperatures and pressures imposed upon them during any deposition and annealing steps of lo subsequent materials applied to the substrate. The substrate may be flexible or rigid, planar or non-planar, convex, concave, aspheric or any combination thereof.
Preferred substrate materials include organic or inorganic materials, such as, polymers, metals, ceramics, glasses, semiconductors. Preferred organic substrates include polyimide film, commercially ~-available under the trade designation KAPTON~ from DuPont Corp., Wilmington, DE. Additional examples of substrate materials appropriate for the present invention can be found in U.S. Patent No. 4,812,352 and is incorporated herein by reference.
Starting materials useful in preparing the whisker-like structures include organic and inorganîc compounds. The whisker-like structures are essentially a non-reactive or passive matrix ~or the subsequent conformal coating and encapsulating material. In addition to starting materials that produce whisker-like structures, several techniques or methods are -~
useful for producing the whisker-like configuration of the particles.
For example, methods for making organic -microstructured layers are disclosed in J. Sci. Technol. A, vol. 5, no. ~, July/August (1987), pgs 1914-16; J. ScL. Technolol. ~., vol. 6, no. 3, May/June (1988), pgs 1907-11; Thin Solid Films, vol.
186, (1990), pgs 327-47; U.S. Pa'. No. 3,969,545; Rapid WO92/17774 - 13 2 ~ PCT/US92/01372 Quenched Metals, (Proc. of the Fifth Int'l Conf. on Rapidly Quenched Metals), Wurzburg, Germany, Sept. 3-7 (1984); s. Steeb et al. Eds. Elsevier Science Publishers B.V., New York (1985), pgs 1117-24; U.S.
Pat. ~o. 4,568,598; Photo. sci. and Enq., vol. 24, no.
4, July/August, (1980), pgs 211-16; and U.s. Pat. No.
4,340,276, the disclosures of which are incorporated herein by reference.
Methods for making ino:rganic-, metallic-, or semiconductor-based microstructu:red-layers or whisker-like structures are disclosed in U.S. Pat. No.
4,969,545; J. Vac. Sci. Tech. A , vol. 1, no. 3, July/Sept. (1983), pgs 1398-1402; U.S. Pat. No.
4,2S2,864; U.S. Pat. No. 4,396,643; U.S. Pat. No.
4,148,294; U.S. Pat. No. 4,155,781; and U.S. Pat No.
4,209,008, the disclosures of which are incorporated herein by reference.
The organic compounds include planar molecules comprising chains or rings over which 20 ~-electron density is extensively delocalized. These ~`
organic materials generally crystallize in a herringbone configuration. Preferred organic materials can be broadly classified as polynuclear aromatic hydrocarbons and heterocyclic aromatic compounds.
Polynuclear aromatic hydrocarbons are described in Morrison and Boyd, Orqanic Chemistry, 3rd ed., Allyn -~
and Bacon, Inc. (Boston, 1974), Chap. 30. Heterocyclic aromatic compounds are described in Chap. 31 of the same reference.
Preferred polynuclear aromatic hydrocarbons include, for example, naphthalenes, phenanthrenes, perylenes, anthracenes, coronenes, and pyrenes. A
preferred polynuclear aromatic hydrocarbon is N,N'-di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide), commercially available under the trade desi~nation of C. I. Pigment Red 149 (American Hoechst Corp., Sommerset, NJ) [hereinafter referred to as perylene . , - . - - ~ . .
t : . :

,. ...
- . . .. . :: . . .. .,. :: :

',J ~ h3 red].
Preferred heterocyclic aromatic compounds include, for example, phthalocyanines, porphyrins, carbazoles, purines, and pterins More preferred heterocyclic aromatic compounds include, for example, porphyrin , and phthalocyanine, and their metal complexes, for example copper phthalocyanine. Such a ~
compound is available, from Eastman Kodak, Rochester, NY.
The organic material for whisker-like structures may be coated onto a substrate using well- -known techniques in the art for applying a layer of an organic material onto a substrate including but not limited to vacuum evaporation, sputter coating, ~;
chemical vapor deposition, spray coating, Langmuir-Blodgett, or blade coating. Preferably, the organic layer is applied by physical vacuum vapor deposition (i.e., sublimation of the organic material under an applied vacuum). The preferred temperature of the substrate during deposition is dependent on the organic material selected. For perylene red, a substrate temperature near room temperature (i.e., about 25C) is satisfactory.
In the preferred method for generating organic whisker-like structures, the thic~ness of the organic layer deposited will determine the major dimension of the microstructures which form during an ., annealing step. Whisker-like structures are grown on a substrate with the characteristics and process described in U.S. Patent Application Serial No.
07/271,930, filed Nov. 14, 1988 and incorporated herein ~-by reference. The process for obtaining the whisker-like structures islalso descri~ed in Example 1 hereinbelow. Preferably, when the or~anic material is perylene red the thic~ness of the layer, prior to annealing is in the range from about 0.05 to about 0.25 micrometer, more preferably in the range of 0.05 to 2 ~

0.15 micrometer. The organic materials are annealed and produce a whisXer~ e structure. Preferably, the whisker-like structures are monocrystalline or polycrystalline rather than amorphous. The properties, S both chemical and physical, of the layer of whisker-like structures are anisotropic due to the crystalline nature and uniform orientation of the microstructures.~
Typically, the orientation of the whisXer-like structures is uniformly related to the substrate surface. The structures are preferably oriented normal to the substrate surface, that is, perpendicular to the substrate surface. Preferably, the major axes of the whis~er-like structures are parallel to one another.
The whisker-like structures are typically uniform in size and shape, and have uniform cross-sectional dimensions along their major axes. The preferred length of each structure is in the range of 0.1 to 2.5 micrometers, more preferably in the range of 0.5 to 1.5 micrometers. The diameter of each structure is preferably less than 0.1 micrometer.
Preferably, the whisker-like structures, are substantially uniaxially oriented. Microstructures, submicrometer in width and a few micrometers in length, are composites comprising an organic pigment core whisker conformally coated with a conducting material.
The whisXer-like structures preferably have a high aspect ratio, (i.e., a length to diameter ratio in the range from about 3:1 to about 100:1). The major dimension of each whisker-like structure is directly proportional to the thic~ness of the initially deposited organic layer. The areal number densities of the conformally coa~ed microstructures are preferably in the range of 40 to 50 per square micrometers.
The conformal coating material will generally S~ U~ S~

,~ .

. ~ . . ,: . , ~. .. .

W092/17774 h 1 ~ ~ 8 ~ ~ PCT~US92/01372 strengthen the microstructures comprising the microstructured-layer. Preferably, the conformal coating material has electrically conductive properties and is selected from the group consisting of an organic material, such as electrical conducting organic materials, for example see "the Organic Solid state"
Cowen et al., Chem & En~. News, July 21 (1986) pgs 78-45, a metallic material, or a semiconductor inorganic material, such as silicon or ga:Llium arsenide. More preferably, the conformal coating material is a metal or metal alloy. Preferably, the metallic conformal coating material is selected from the group consisting of aluminum, cobalt, nickel chromium, cobalt chramium, copper, platinum, silver, gold, iron, and nickel.
Preferably, the organic conformal coating material is selected from the group consisting of hetrocyclic polynuclear aromatics. The preferred inorganic conformal coating material is a semiconductor.
Pre~erably, the wall thickness of the conformal coating surrounding the whisker-like structure is in the range from about 0.5 nanometers to about 30 nanometers.
The conformal coating may be deposited onto 25 the microstructured-layer using conventional ~-techniques, including, for example, those described in U.S. Patent Application No. 07/271,930, su~ra.
Preferably, the conformal coating is deposited by a method that avoids the disturbance of the -microstructured-layer by mechanical or mechanical-like forces. More preferably, the conformal coating is ~
deposited by ~acuum deposition methods, such as, vacuum -subiimation, sputtering, vapor transport, and chemical vapor deposition.
Preferably, the encapsulating material is such that it can be applied to the exposed surface of ;;
the conformal-coated microstructured-layer in a liquid WO92/17774 21~ ~rj~ 3 ~ ~ PCT/US92/01372 or liquid-like state, which can be solidified. The encapsulating material may be in a vapor or vapor-like state that can be applied to the exposed surface of the conformal-coated microstructured-layer. Alternatively, the encapsulating material is a solid or solid-like material, preferably powder or powder-like, which can be applied to the exposed surface o~ the conformal-coated microstructured-layer, transformed (e.g., by heating) to a liquid or liquid-like state (without adversely affecting the confcrmal-coated microstructured-layer composite), and then resolidified.
More preferably, the encapsulating material is an organic or inorganic material. The encapsulating material may exhibit sensitivity to gas or vapor contaminants to be detected. Additionally, it is preferable, although not required, that the encapsulant be permeable to gas or vapor contaminants.
Preferred organic encapsulating materials are molecular solids held together by van der Waals' forces, such as organic pigments, including perylene red, phthalocyanine and porphyrins and thermoplastic polymers and co-polymers and include, for example, polymers derived from olefins and other vinyl monomers, condensation polymers, such as polyesters, polyimides, polyamides, polyethers, polyurethanes, polyureas, and natural polymers and their derivatives such as, cellulose, cellulose nitrate, gelation, proteins, and rubber. Inorganic encapsulating materials that would be suitable, include ror example, gels, sols, or semiconductor, or metal oxides applied by, for example, vacuum processes.
Preferably, the thickness of the coated encapsulating material is in the range from about l micrometer to about lO0 micrometers, and more preferably in the range from about 6 micrometers to about S0 micrometers.

: . .

The encapsulating material may be applied to the conformal-coated microstructured-layer by means appropriate for the particular encapsulating material.
For example, an encapsulating mat:erial in a liquid or liquid-like state may be applied to the exposed surface of the conformal-coated microstructured-layer by dip coating, vapor condensation, spray coating, roll coating, knife coating, or blade coating or any other coating method known to those skilled in the art. An encapsulating material may be applied in a vapor or vapor-like state by using conventional vapor deposition techniques including, for example, vacuum vapor deposition, chemical vapor deposition, or plasma vapor deposition.
An encapsulating material which is solid or solid-like may be applied to the exposed surface of the conformal-coated microstructured-layer liquified by applying a sufficient amount of energy, for example, by conduction or radiation heating to transform the solid or solid-like material to a liquid or liquid-like material, and then solidifying the liquid or liquid-like material.
The applied encapsulating material may ~e solidified by means appropriate to the particular material used. Such solidification means include, for example, curing or polymerizing techniques known in the art, including, for example, radiation, free radical, anionic, cationic, step growth process, or combinations thereof. Other solidification means include, for 30 example, freezing and gelling. ;
After the polymer is cured, the resulting composite article comprising a conformal-coated microstructured-la~er and an encapsulating layer is delaminated from the substrate at the original substrate interface by mechanical means such as, for example, pulling the composite layer from the substrate, pulling the substrate from the . . .. . . . .

2 ~

composite layer, or both. In solne instances, the composite layer may self-delaminate during solidification of the encapsulating material.
Capacitive properties of the composite article are determined by the dielectric constants of the encapsulating material, film thickness and planar area used. Intimate contact of the conductive particles wit~ the surrounding encapsulant permits the full dielectric response of the encapsulant to be realized with only physical contact of a circuit lead to the conductlng side of the composite, that is, without evaporation or sputter coating of a metal overlayer on the polymer surface as is usually necessary to bring a conductor into full electrical contact with a dielectric surface.
It is also a unique property of this medium's structural anisotropy that the complex impedance is anisotropic. That is, the impedance parallel to the surface of the composite film is predominantly resistive, while the impedance in the direction perpendicular to the surface is predominantly capacitive, being determined by the very large reactance of the much thicker encapsulant layer.
In all cases the resistance and capacitance character is spatially distributed over the entire area o~ the composite article. A conductive metal foil tape is applied to the side of the encapsulating polymer opposite the conducting nanostructured side.
Electrical contact can then be made at three points x, y, and z. This is equivalent in a first order approximation to the three terminal networX. Depending on which pairs of terminals are used as input and output for an alternating current S~ ~

2 ~ ~ ~ 3 ~ 3 (AC) voltage signal, the composite strip can function as a low-pass or high-pass filter circuit. For example, applying the input signal across terminals x and z (or y and z) and taking the output across terminals y and z (or x and z) i.s equivalent to a low-pass filter.~ On the other hand, applying the input - across terminals z and y (or z and x) and the output acrosc terminals x and y (or y and x) produces a high-pass filter. Applying the output and input signals to the third combination or termlnal pairs, for example, an input signal applied across terminals x and -~
y (or y and x) and output measured across terminals z and y (or z and x), gives a simple capacitively coupled voltage divider. The metal foil tape need not be -~-~
applied in a single piece and thus could produce multiple terminals. As used herein, "low pass filter"
means a filter network that passes all frequencies below a specified frequency with little or no loss.
The term "high-pass filter" means a wave filter having a single transmission band extending from some critical frequency up to infinite frequency. The term "voltage divider" means a resistor or reactor connected across a voltage and tapped to make a fixed or variable fraction of the applied voltage available.
The composite film can be configured to form a four terminal network with two pieces of the nanostructured composite film arranged in "parallel"
such that the conductive sides of the composite film are facing outward and the corresponding equivalent electric circuit. Electrical contact is made at the ends of each side. This can similarly be utilized to have different filter characteristics depending on the various combinatio~s of terminals used for input and output, or to form various two and three terminal networks.

~3~B~T

The composite film can also be configured to form a four terminal network with two pieces of the composite film arranged in "series" and a simplified e~uivalent electrical circuit. This arrangement is approximately equivalent to a band pass filter~
Electrical contact is made at the ends of each side.
The term "band pass filter" means a wave filter with a single transmission band, wherein the filter attenuates frequencies on either side of this band. A metal foil tape can be replaced with other composite strips.
As a sensor, it is found that the in-plane surface resistivity of the nanostructured side of the composite article, the impedance to current flow in the plane of the whisker-like structures, is a simple yet sensitive pro~e of gas, vapor, or liquid analyte effects. The electrical conductance mechanism may involve both electron 'Ipercolation'' from point-to-point where adjacent whiskers touch, and tunneling through or charge injection into the thin intermediate encapsulating material interstitially located between the conductive conformal coated microstructures.
Therefore, if the conductivity of this conformal coating applied to the whisker-like structures, or the relative separation o~ the microstructures, or the charge transport properties of the intermediate encapsulating material are affected by the analyte, the surface impedance of the composite article is altPred.
The initial surface resistivity is easily varied over a wide range by controlling the thickness of the conductive conformal coating applied to the whisker-like structure prior to encapsulation.
- Sensor medium is produced in a convenient flexible polymer fdrm which may be cut into arbitrary sizes and shapes. Electrical connections are simply made by contact with the conducting, chemically active ~U~ 1TIJ~E S~E~
.. . .. .. , . , .. . .,.,,,.~ ., . .. . . , ~ .. .

2 ~

surface.
Again the physical structure of the composite article, utilized as a gas, liquid or vapor sensor, comprises a polymer film, optionally, sPnsitive to the vapor or gas of interest, having encapsulated in its surface a dense, random array of discrete whisker-like structures. The whisker-like structures are typically~
a~out one to a few micrometers in length and submicrometer in width. Microstructures comprise organic pigment core whisker-iike structures with a conformal coating, typically a conducting material, and optionally, sensitive to the vapor or gas to be sensed.
Preferably, the encapsulating material and the "
conformal coating may be selected for sensitivlty to lS the gas/vapor/liquid analyte see Katritzky et al., "New Sensor-Coatings for the Detection of Atmospheric ~ontamination and Water," supra. Gases, vapors or liquids typically sensed include but are not limited to acetone, methyl ethyl ketone, toluene, isopropyl alcohol, hydrogen sulfide, ammonia, carbon dioxide, carbon monoxide, nitrous oxide, sulfur dioxide, organophosphorus compounds in general, dimethyl methylphosphonate, chloroethyl ethyl sulfide, xylene, benzene, l,l,1-trichloroethane, styrene, hexane, ethyl acetate, perchloro-ethylene, cyclohexane, VMP naphtha, cellosolves, chloroform, methylene chloride, Freon~
113, ethanol, ethylene oxide, hydrogen fluoride, chlorine, hydrogen chloride, hydrogen cyanide, toluene `
diisocyanate, methylene di-p-phenylene isocyanate, and formaldehyde. The preferred sensing property of the sensor is the electrical impedance.
The sensing composite article of the present in~ention is a dual mode sensor since the conductive conformal coating and the polymer encapsulant may each S.UBST~ E St~E~

,, - , : ; - . : . ', , . ........ . . ..... .. .. i ., ~ . . .. .
.... . . . . - . .. i.: . . . . .. ... - . .. . .. ..

2~

be selected for their individual response to a particular analyte molecule of interest.
The sensing composite article is a dual sensor in a second aspect, as we:Ll. By constructing a sensor the effect of vapor/gas molecules absorbed by the encapsulant on its dielectric properties can be sensed by changes in the capacitance being measured.
Since this impedance in the perpendicular direction is predominantly determined by capacitance, and is unaf~ected by the in-plane resistivity of the whisker surface layer, the perpendicular-capacitance and in-plane resistance values are independent.
Since the microstructure's conformal coating and the encapsulant may independently be chosen to have varying degrees of sensitivity to an arbitrary specific gas, vapor or liquid analyte, it is possible to combine a variety of such individually compris~d sensors into a multiplexed array, whereby the integrated response of the array as a whole to an unknown gas, vapor or liquid composition, could be used to determine the composition of the unknown gas, vapor or liquid, the relative fractions of the components making up the later, or for a single analyte, the absolute concentration. .

JTE~

2 ~ f~ - 24 -Exam~les Objects and advanta~es of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention. Unless otherwise stated or apparent all materials used in the following examples are 10 commercially available. -Examples 1 and 2 illustrate the basic procedure for preparing the composite articles o~ the present invention.
, Example 1 Organic pigment C.I. Pigment Red 149, (N,N'-di(3,5-xylyl)perylene-3,4:9,10-bis(dicarboximide)) [hereinafter referred to as perylene red], available from American Hoechst-Celenese, (Somerset, NJ) was vacuum vapor deposited onto a stretched, 0.0125 mm thic~ sheet of copper coated polyimide, formed into a disc 8.3 cm in diameter. The resulting copper coated polyimide, having a 1000 Angstrom thick film of perylene red, was then annealed in vacuum, heating the entire continuous perylene red film coating the polyimide, by thermal conduction through the polyimide substrate. The perylene red film was heated at approximately 280C
over a period of 90 minutes. After vacuum annealing, the disc had a nanostructured layer of discrete, oriented crystalline whiskers 1 to 2 ~m in length.
CoCr (86%/14~) was then sputter coated conformally onto the whiskers, using a conventional radio frequency (rf) glow discharge for 3 minutes at 13.7 MHz, with a 20 cm diameter target, 10 cm substrate-to-target distance, 24 mTorr of Argon (Ar), 500 watts of forward power and 1200 volts target bias.
Five milliliters (mL) of DUC0~ Cement "Household Cement" (Devcon Corporation), a solution of thermoplastic resin in toluene and other solvents, were applied to the center of the sample disc while spinning at 200 rpm. The disc was stopped when the cement flowed out to the perimeter of the sample disc. After-air drying at room temperature for approximately 5 hours, the resulting nanostructured composite easily delaminated from the original polyimide substrate, producing a smooth surface where the now solidified cement had interfaced with the polyimide. The resulting dried thickness of the composite film was approximately 0.12 mm.
Two rectangular pièces of the composite were cut from different sections of the sample disc to give a sample with an area of 1.55 cm2. The end-to-end resistance of one strip was measured to be 12,060 ohms and the second strip measured 2910 ohms. The strips were pressed together between glass microscope slides with the electrically conducting nanostructured surfaces facing outward. The sinewave output from a signal generator over the frequency range of 1 kHz to 10 MHz was applied across the conductive surfaces on one end of the composite strip. The output signal developed across the conductive surface were monitored with an oscilloscope having a 1 megohm, 20 picofarad (pF) input impedance using a lX probe, or with a lOX
probe having a 10 megohm, 13 pF impedance.
The composite strips have an electronic transfer function similar to a low pass RC network, with a fall-off of approximately 6 dB/decade.
,, :~

SUB~ UTE S~

-. . . - ~ , . . . ' ' - 1 , ' - ' . ' ' ' - ' ' ' - ' 2~ ~a~3 ~

Exam~le 2 .
A second 8 cm diameter sample disc was prepared as described in Example 1, except CoCr was sputtered onto the perylene whis~ers for 4 minutes at the conditions of E~ample 1, followed by encapsulation with 3 ml of DUCO~ cement. Two pairs of rectangular -strips werP cut from the sample and pressed together between glass microscope slides to form two composite strips. The same AC signal transfer function was me sured as a function of frequency for each of these dual strips. The resistances of both sides of the dual composite strip (area of 2.7 cm2) were approximately 4500 ohms. The resistances were approximately 2200 ohms and area approximately 6.5 cm~. The dual composite strip thicknesses were approximately 0.05 mm.
The frequency "cut-off" values shift in response to the capacitance and resistance of the strips.

-- Exam~les 3-10 The following examples illustrate a range of nanostructured composite sample types, varying with respect to the coating on the perylene whiskers and the polymer encapsulant used, to generate a series of equivalent RC network circuits with low-pass and high-pass cut-off frequencies that vary over several orders of magnitude. All samples were identically prepared up to and including the growth of the perylene whiskers.
In each example, the sample type was identified according to the composition listed in Table 1. In each case, Scotch~'brand aluminum foil backed adhesive tape (3M Co., St. Paul) was applied to the polymer 35 encapsulant sicle of each sample plece. Electrical -. SuBs~lT~lTE 5~1E~

21~ 5 3 ~ ~9 - contact was then made to the ends of the sample strip on the nanostructured side, and the metal foil tape on the opposing side, to form three terminal networ~s. The resistance across the conducting side of each sample was measured with a Keithley model 617 digital electrometer. The capac:itance of each sample was measured as described below. A conventional 1.025-megohm resistor was placed in series with terminal x of 3 a sample capacitor and a squarewave signal in the 100 Hz to several kHz range was applied to the 1.025 megohm resistor and terminal z across the two circuit elements. The voltage signal decay across the sample terminals y and z was monitored with an oscilloscope llOX probe) and the RC time constant read directly from the sample's capacitance waveform, allowing C
(capacitance~ to be calculated from Equation I

t = RC (I) where R is the resistance in ohms, C is the capacitance in farads and t is the decay time in seconds for capacitance to discharge to l/e of the initial charge. ~-- - ~

S~3e~ T~ S~El ~ ~ ' ; i ; . ' 'r . ' .. ~ ' ,', 2 ~

....~
-- Table 1 .
Sample Conformal Type Coating ~ncapsulant A Cu DUCO~ Cement1 B Cu Urethane/Vinyl2 C CuFluorenone polyester2 .
D AgDUCO~ Cement 10 E AgFluorenone polyester E AuDUCO~ Cement G CoCrDUCO~ Cement H CoCrFluorenone polyester I AuFluorenone polyester 1~ _ , _ 1 Devcon Corp., Danvers, MA
2 3M Co., St. Paul, MN

Examples 3-S illustrate the passive network :-response of small flexible strips of type D samples. A
type D oriented nanostructure was made by first evaporating 750 Angstroms (A) mass equivalent of Ag onto perylene whiskers in a conventional diffusion pumped bell jar vacuum system operating at approximately 10-6 Torr pressure range, and then encapsulating the nanostructure as described in Example 1. The sample parameters are shown in Table 2.
Examples 6-~ illustrate the passive RC
networ~ response of small flexible strips of type H
samples. The type H oriented nanostructure was made by sputtering CoCr (5 minutes under the conditions o~
Example 1) onto per~lene whiskers and encapsulating the ~;U~35TI~'JTE S~E~ . .

3 ~' ~

nanostructure in fluorenone polyester (FPE) by spin coating 7 ml of a 5% solution in cyclohexanone at a revolution rate sufficient to just cover the entire 8 cm diameter sample disc, followed by air drying for 16 hours at room temperature and 4.5 hours at approximately 70OC. The sample parameters are shown in Table 2.
Example g illustrates the passive RC networ~ j response of a small flexible strip of type G sample.
The oriented nanostructure was made by sputtering 750 Angstroms mass equivalent of CoCr onto the perylene whiskers and encapsulating them in 5 ml of DUCQ~ cement as in Example 2. The sample parameters are shown in Table 2.
Example 10 illustrates the passive RC network response of a small flexible strip of type A sample.
The oriented nanostructure was prepared by sputtering Cu to a mass equivalent of approximately 600 A onto the perylene whiskers and encapsulating in DUCO~ cement.
The results are shown in Tables 2a and 2b.
The response of the samples appear to be a band pass frequency response rather than high-pass frequency response. This is due to oscilloscope input impedance, which in combination with the sample strips' half resistances, act as a low pass filter following the high pass circuit configuration.
: ~ . .:. .

S~ S~E~

- 2 ~

_ ~
_ Table 2a Example ¦ Sample ¦ Area ¦ Thi.cknPss ¦ Resistance No. Type (cm2) (mm) (ohms) 3 D 12.1 .04 26 x 106 4 D 1.54 .04 1.86 x 106 D 3.52 .04 .20 x 1o6 6 H 2.37 .025 .023 x 106 ~ .

7 H 13 .02 ~1180-1290 8 H 3.9 .02 560 9 G 2.5 .07 1760 A 2.1 064 1005 : .-.
Table 2b Ex. ¦ Capacitance ~ -:
(pF) ::
.: ' 3 1~6 :
4 61.5 6 63 .:

9 40 .

U~ E~

WO 92/17774 h ~ 3 ~l PCT/US92/01372 Examples 11-19 In Examples 11-19, the power dissipation capability of the nanostructured composite article, used in purely a resistive mode, is demonstrated and compared to a conventional carbon resistor and a thin metal film coated polymer.
In Examples 11-18, thin strips of varying surface resistance, formed with various metal/polymer lo combinations as described in Table 1, were heated by passing current through the strip until the test strip failed. The strips were laid against a glass slide with the nanostructured side against the glass and a temperature probe pressed against the opposite polymer side of the strip to monitor the temperature rise as a function of current level. The glass slide was not cooled. The plots of temperature rise versus electrical power dissipated in the composite strips was observed to be linear. Table 3 summarizes the results of eight sample strips, made from five sample types, as described in Table 1. Table 3 summarizes ~T/~P, the slope of the linear temperature versus power plot, the -test strip resistance, area, thickness, volume, and the maximum current dsnsity at the time of failure. The current density is calculated assuming the current carrying layer of the strip is approximately 2 ~m thick, which is the known thickness of the nanostructured region of the composite article.
The last entry o~ Table 3 identified as example 19, shows similar measurements from a standard 12 ohms, l/4 Watt carbon resistor, having cylindrical geometry. The current density of the carbon resistor is calculated using the inner carbon volume diameter.
It is seen that the nanostructured composite films can support current densities 50 to 70 times larger than standard resistors of equivalent resistance and volume, for a similar temperature rise. This is due in large WO92/17774 2 1 ~ ~ 8 ~ ~ - 32 - P~T/US92/0137t part to the larger surface area for heat dissipation.
For Examples 11-19, it can be shown that the thermal conductivity of the polymer forming the bulk of the strip is the limiting thermal dissipation factor.

W O 92/17774 2 ~ P ~ /US92/01372 N ~D ~D cn ~ CO 1~1 t~ 11'1 0 ~ ~ rl ô
~, N N r I o r` Ir) O N ,~ N

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~o ~ G~ CO ~ O ,~

C- . ,`~'' ':' ~' ~ s r ,~ o N ~ N
O . . ' _ . ~''''" . ,, ~ , Q o C~ N N ~ o o o o o . ~ .

h = . . ~ :

Q)~ ~ r~ CO CO 1~ ~, ~Y o o o o o o o O :z: : ..
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a)a~ ~ ' `' ", a ~ c~ a ~ 3 u~ _ , ~ ' .

X .-1 N ~ Ç1 r co al _ ~ .

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, WO92/17774 2 ~ 34 - PCT/US92/01372 ExamPles 20-23 In Examples 20-22, sample strips similar to those described in Examples 11-18 were resistively heated while heat-sinked to maximize the total power dissipation, and compared to Example ~3, a cobalt film sputter-deposited on 0.05 mm thick polyimide. The sample strips were pressed tightly against a water cooled copper block with a thin ~ilm of heat transfer grease applied between the block and the polymer side of the nanostructured composite strips. Nextel~ (3M
Co., St. Paul) insulating material was pressed against the nanostructured side of the strip, and a 0.025 mm diameter chromel-alumel (Type X) thermocouple measured the temperature at the midpoint of the conducting side of the strip through a small hole in the Nextel~ sheet.
In this configuration, the surface temperature of the strip's conducting side was measured as a function of the input power, with thermal conductivity determined by the composite strip's polymer and its thickness, or in the case of the comparative Example 23, the polyimide substrate. The heat transfer grease, appiied extremely thin, was observed to have a significant effect. The bulk thermal conductivity, k, across the ~
2~ thickness, d, of the polymer strip is simply related to .--the temperature drop across the strip, ~T, the planar area of the strip, A, the electrical power dissipated in the strip, P, as shown in Equation II.

k = Pd (II) A~T
The thermal conductivity was found typically to be on the order of 2 mWatts/cm2DC, indicative of a solid, polymer material.
For Examplle 20, composite article was formed by evaporating gold to a mass equivalent thickness of 1500 Angstroms onto an 8 cm diameter disc of perylene 21~ ~ 3 ~ ~ ~

whisker coated polyimide, and encapsulating the latter with 6 ml of 4% solids FPE in cyclohexanone to form the nanostructured surface composite (type I) as described in Table 1. A test strip with an area of 4.0 cm2, thic~ness of o.005 mm and an end~-to-end resistance of 10.8 ohms was placed on the cu b:Lock assembly described above. The temperature difference was measured across~
the strip versus power until failure of the strip ~ ~
occurred. --For Example 21, a surface composite of type E
(see Table l) was formed by evaporating 1500 Angstroms of Ag onto the whiskers and encapsulating with 10 ml of 4% FPE. A strip with an area of 4.0 cm2, O.OS mm thickness and 2.9 ohms resistance was mounted on the Cu block assembly. The temperature difference was measured across the test strip versus the power dissipated in the strip.
For Example 22, a surface composite of type E
was formed by evaporating 1950 Angstroms of Ag onto the whiskers and encapsulating with 10 ml of 4% FPE. A
test strip with an area of 4.9 cm2, thickness 0.014 mm and resistance of 2.2 ohms was placed on the Cu block assembly. The temperature difference was measured across the strip versus the power dissipated in the strip.
For comparative Example 23, approximately 1250 Angstroms of cobalt was sputter deposited onto a --0.05 mm thick polyimide web, using the conditions of Example l. A strip was cut with an area of 4.4 cm2, and an end-to-end resistance of 2.8 ohms. The temperature difference across the polyimide strip versus the power input until failure, measured in same way as for Examples 20-22.

SU~STlTUTE S3 1EE~

2 1 ~ 36 -Example 24 This example shows the use of the nanostructured composite film, with a metal coating and a polymer encapsulant, as a sensor capable of reversibly responding to a saturated vapor of acetone with a rapid response time and a sensitivity, according to Equation III

S = (R--Ro) /Ro = 10% (III) The organic pigment C.I. Pigment Red 149, -(N,N'-di(3,5-xylyl) perylene-3,4:9,10 bis(dicarboximide), (available from American Hoechst-Celenese), was vacuum vapor deposited onto a stretched, 0.05mm thick sheet of copper coated polyimide, formed into a disc with a diameter of 8.3 cm. The disc was vacuum annealed to form a nanostructured layer of discrete, oriented crystalline 20 whiskers approximately 1.5 micrometers tall, as , described in Example l. CoCr (86%/14%) was then sputter coated conformally onto the whiskers, using a conventional rf glow discharge at 13.7 MXz for 8 minutes with 20 cm diameter targets, 10 cm substrate-to-target distance, 24 mTorr of Ar, 500 Watts forward power, 1200 volts target bias and water cooling of the target and substrate.
Three milliliters of uncured photopolymer, (cyclohexyl methacrylate, hexamethylene diisocyanate trimethylolpropane 5 (CHMA, XMDI-TA5)), as prepared in U.S. Patent No. 4,785,064 was applied to the center of the polyimide disc and hand tilted to cause the solution to uniformly flow over and encapsulate the CoCr coated perylene whiskers. The photopolymer was then cured by exposing it to the appropriate W lamps, under N2, for one-half hour.
The resulting nanostructured composite easily : ~

2 ~ ~ J~ ~ ~
~v092/l7~74 PCT/~IS92/013~2 _ ~7 _ :
delamin~ted ~rom thR orlglnal polyl~ido 3ubætrate, produclng a ~mooth, re~l~ctlve surface where the now aolldl~l~d polymer ellcnpsulant hsd lnter~Dced wlth the polylmlde. An ~rreg~llarly ~haped pl~ce of the brittle compo~ite, approxlmately S c~ long, 1.25 om wlde at the center and 0.5 c~ wlde at each end, ~aS
'oroken from the orlglnal dlsl-. Electrlcal leads were ~ttachad to th~ end~ by crlmlplng on t~nned ~older lugs and coating them with conduct~va palnt. The total lo res~stanoe of the ~ampl~ plece n~ descr~bed, wa5 s43 ohms.
Wlth laads from a K~lthley modal ~16 electro~ter ~ttached to ~a~ur~ th~ re~l~tance, and the latter drivlng a tlm~ basad chart r~corder, the sample was placed inslde a covsred 400 ~l po~yethylene bea~er. ~ith only alr in tha bea~r, the reslstance remalned con~tant at 840 ohm~ ~or approxlmately 40 mlnutes. Acetone was then added to the covered beaker to a depth of 3 ~m, 80 ~S to e~po~a the ~en~or to a saturated vapor. The resl~tance ~R) ~gan to lncrease and rose to 855 ohms over a two mlnute lnterval. For approxl~ately 15 mlnuteo, th~ ~ r~ma~nQd at 855 ohms and then R lncreased ~harply agaln to approxlm~tely 875 ohms ovQr a period of 30 ~ocond~ and re~ained constant ~ `
25 for 12 mln~te~. R then ~ump~ to 900 oh~ in a p~r~od ;~
o~ two mlnutes, thereafter r~a~ning in the rang~ o~
9oO to 890 ohms for 70 m~nut~s. ~t this polnt, the ~ensor ~s~embly Wa5 r~moved from tha beaker ~nd lald on the laboratory bench, whereupon R began dropplng wlthln ~0 eeconds, r~achlng 790 ohm~ ln 7 mlnut~ and st~ylng '~
constant ~or 12 ~lnute~ untll put back lnto ~he acetone vapor. R ~mmed~tely began lncreas~ng aga~n, reachlng 9oO ohms ln 9 minute~ wher~ lt r~alned oon~tant.
-In ~mmary, thls ~xamplx o~ ~ n~n~tructured composite ~nsor with coCr con~ormal coating ~nd c~, ~DI-TAS encapsul~nt has d~monstratad th~ eapabl lity to r~pidly and reverslbly ~on90 a room tempcr~t~re ~0 9~/1777~ f ~ J~ YCT/US92/01372 - 38 ~
saturated vapor of acetone with ~ sensitivity of approximately 10%.
The following example classes demonstrate the utility of the nanostructured ~_omposite film as 5 gas/vapor sensors for H2S, Hg vapor, H2O and organic vapors of methyl ethyl ketone (MEK), acetone, toluene, isopropyl and ethyl alcohol, and as a liquid a~alyte sensor for aqueous Br. In all cases basic whisker structured perylene films deposited on copper coated polyimide sheets such as described in Example 24 were used as the starting point, and various combinations of metal conformal coatings and polymer encapsulants used to form thirteen different types of nanostructured composite films. Table 4 lists these different samples according to metal coating and polymer encapsulant as types A-M, which are referenced for brevity in the following Examples.
Photopolymer A was prepared as described in U.S. Patent No. 4,262,072, Examples l and 2 and is incorporated herein by reference.
Photopolymer B is a radiation-curable composition prepared as described in U.S. Patent No.
4,9~6,496, Example 4 and is incorporated herein by reference.
Components Parts urethane acrylate oligomer 68 (XP51-85~, Cargile, Inc.) tetraethylene glycol diacrylate l9 (SR-268~, Sartomer, Co.) diethoxyacetonphenone 5 (DE~P~, Upjohn Co.) fluorochemical surfactant z.5 (FC-431~, 3M Co.) n-vinyl pyrrolidone 5 (GAF, Inc.) W light stabilizer 0.5 (TIN W IN 770~, Ciba Geigy, Inc.) - -- . -- - - - - . ......... . . .

- , . . : .~ ~ - , , .: . , - ., :- : ;~ . . :' i : : ~ . ' .

-. .

WO92/1777~ 2 1 ~ PCT/US92/01372 - 39 - ~
: . ' Table 4 , Sample Conformal Type Coating Encapsulant ._ = , _ ., A Ag Photopolymer B Ag DUCO~ Cement2 C Ag Photopolymer B3 :~
D Ag Vinol polyvinyl alcohol E Cu Fluorenone polyester4 F Au W optical adhesive5 G . Au DUCO~ Cement . H CoCr Photopolymer CHMA, HMDI-TA56 I CoCr Photopolymer A
lS J coCr Fluorenone polyester K Fe DUCO~ Cement . --.
L Cu DUCO~ Cement M CoCr DUCO~ Cement .. _..................... . ~ . .
20 l U.S. Patent No. 4,262,072 .
2 Devcon Corp., Danvers, MA
3 U.S. Patent No. 4,986,496 4 3M Co., St. Paul, MN :.
Norland Products, Inc., New Brunswick, NJ
25 6 U.S. Patent No. 4,785,064 (cyclohexyl :
methacrylate, hexamethylene diisocyanate -trimethylolpropane 5) ExamDles 25-33 These examples demonstrate the utility of .
samples of types A, B, C, D, and E as irreversible 3S sensors or dosimeters for H2S gas under conditions of high humidity, and illustrate the dependence of sensor sensitivity on the initial resistivity for quantit~tive analyses. .

, '' '' .' . ~ . . .. .... .. .. .. .. - ....... . . . . .. ,. -......... . . - . .

WO92/l7774 ~ PCT/US9Z/01372 Example 25 illustrates that type B samples produce a significant response to H2S/N2 concentrations as low as 30 ppm in times as short as 30 seconds under conditions of 50% relative humidity (R.H.) and 30 1/min flow rates.
A strip of type B sample, made by evaporating 900 Angstroms mass equivalent of Ag and encapsulating to a thickness of 0.045mm, was cut 6 mm wide and 4.5 cm long. Electrical contact to the strip was made by simply clipping smooth-jawed miniature alligator clips to the ends of the strip. The initial resistance was 870 ohms. The strip was supported within a sealed 9 oz. glass jar and the resistance continuously monitored while gas mixtures of known composition and flow rate were admitted and allowed to vent through tubes penetrating the jar cover. Two sources of gases were mixed in a preliminary 9 oz. jar that supplied the final mixed gas to the sample containing jar. The first gas source was wet N~, produced by flowing N2 over a humidistat (General Eastern) controlled water ~apor bath through a glass flow meter tube (LabCrest No.
4S0-688).
The second gas source was either pure N2 or 108 ppm H2S/N2 tUnion Carbide Industrial Gases Inc.) supplied to the mixing jar via a flow meter (Ace Glass Inc., tube #35). While flowing only humidified N2 into the sample jar, the resistance remained constant at 869 ohms over a period of 35 minutes during which time the relative humidity was increased from 50% to 78% in the first gas source, flowing at 16 l/min~, and the second gas source of dry N2 flowed at approximately 14 l/min., to produce a total flow of approximately 30 l/min. of humidified N2 at approximately 25% to 39% RH. This demonstrates that a type B sensor is unaffected by water vapor, such las would exist in the vicinity of human breath. The valving of the second gas source was quickly switched to admit the 108 ppm H2S at 2i~6~ ~
92/171'4 - 41 PCT/~S92/013~2 approxlmately 5 l/min. ln place of thQ dry N2 and i~medlatoly th~ reslstanc~ began r~pldly dropplng at a rat~ ~xceedlng 100 ohms/mln.
finally approachlng ~ stable and nonreverslble r~sl~tance of 490 oh~s a dQcr~a~ o~ 44~. Thls represent~ an average relnt~ve reslstance change of 11%/mln. over th~ flrst two ~l~ut~s. The relat've flow rates and mixture value~ ply that the rapld and large response of ~he strlp g re~lstancQ was produced ~y spprox'~ately 30 ppm H25/~ t approxlmat~ly 40~ RH.
For E~ample 26, a second -trlp of type 9 ~ampla 4.5 cm x 5 mm having ~ lower re~i~tlvlty than ~x~mple 25 was mounted ln the oame t~t apparatu~ as Example 25. The inltlal reslstanc~ o~ the strlp was ~onstant at 130.7 oh~s while ~xpo~ed to the wet N2 gas mixture flowlng into the m'x'ng ~ar at approx'mately 23 l/min. and S~% ~H. Upon ~wltching to a mlxture ~ ;
contalnlng 35 ppm H2S gas the reslstance began dropplng withln seconds r~aching approxlmately 105 ohms 'n 2 ~lnutns and Qventually 82.2 ohms a~ter 20 mlnutes. This represents an average relatlve oh~nge of resi~tance of approxlmately lo~ n. over the fir~t two minute~ slm'lar to Exampla 25 desplte the dlfference in inltl~l resistlvity. It should be noted that the very staole reslstance to nearly 1 part per thousand 'mplies thot even ~u~t a 25 ohms change ~rom 130 ohmq is still a s'gnal to nol~ ratlo o~ 20%10.1% or ~00/1.
~or Example 27 a third strlp o~ type sample uQed in Exa~ples 25 end 26 had an inltlal r~lstance of 3900 ohms. Unlike the previous examples some sensltiv~ty to water vapor w~Y noted. ~he ~trlp was ~ounted in the ~a~e te~t apparatus but ~ slmpler gas adnls~lo~ ~ystem was used in whlch elther dry N2 or the 108 pp~ H2S/~2 gas mixture oould ~e ad~ltted dlrectly to the ~ar and vented through a ~oond tube ln the ~ar ~over. ~wo ~1 o~ dlstilled water wa~ added to the bottom o~ the test apparatus. Th~ ~low rates were . :.. -, 2 . ' ~ ~J ~
WO92/17~7~ PC~/US92/01372 not quantified, but produced a fast bubble out the 3mm diameter (O.D.) vent tube when its outer end was placed in water. Upon switching from 1:he pure N2 to the H2S/N2 gas, the sample resistance increased briefly to 4150 ohms over 30 seconds, then plummeted to lO00 ohms in 90 seconds, a sensitivity of 50%/min, and reached 260 ohms after 6 minutes, In comparison to the previous examples, this example indicates that the nanostructured composite sensitivity to H2S may be correlated to the initial resist:ivity.
For Example 28, a similar strip of type B
sample, with lO00 Angstroms mass equivalent Ag, and a very low initial resistance of 15.4 ohms, was exposed to the same gas flow conditions as used in Examples 25 and 26. No response to the H2S gas was noted after switching from wet N2. This comparative example to Examples 25 to 27 indicates that too low an initial resistivity is ~ot desirable. Sensitivity may be correlated to initial resistivity, probably because a different conduction mechanism may be dominating the current flow which is less sensitive to initial small degrees of reaction with the H2S.
For Example 29, a strip of type D sample, made by annealing the perylene whiskers at 240C for 80 2, minutes, vacuum evaporating lO00 Angstroms mass equivalent of Ag onto the whiskers, and solution coating with a 5~ solution of vinol-polyvinyl alcohol in water with 0.1% Triton X-lO0, (Rohm & Haas, Philadelphia, PA) was cut 5 cm lon~ and 6 mm wide. The strip's initial resistance under flowing dry N2 in the simpler gas flow arranyement of Example 27 was 34 ohms.
Switching to the 108 ppm H2S/N2 gas source, in the absence of any water vapor in the apparatus produced no change in resistance. Three milliliters of water were added to the appara,tus and the gas flow sequence repeated. With water vapor present, the resistance began dropping 90 seconds after switching to the H2S

2i~3~

and exhibited a relative resistance drop of 10% over a period-of approximately 11 minutes. This degree of change is also qualitatively consistent for such a low initial resistivity and the observations of the previous examples. It also indicates with this polymer encapsulant the need for finite relative humidity.
For Example 30, a strip of type C sample, made by annealing the perylene at 280C for 90 minutes, vacuum evaporating 1035 Angstroms mass equivalent of Ag and spin coating 3 ml of photopolymer B onto an 8 cm diameter disc and W curing, was cut 3.7 cm long and 6 mm wide. ~he strip was exposed to N2 and a 108 ppm H2S/N2 gas mixture in the simpler gas flow arrangement of Example 27. Distilied water was present in the bottom of the apparatus. The initial resistance of the strip was 1.36K ohms and remained constant in a pure N2 flow. Switching to the 108 ppm H2S/N2 flow, the resistance began dropping after l.0 minute, and decreased logarithmically.
For Example 31, a strip of type B sampl~ with an initial resistance of 898 ohms was placed in the simpler gas flow arrangement of Example 27. DistilIed water was present in the bottom of the apparatus. With only pure N2 flowing into the apparatus, adequate to produce a fast bubble from the outlet tube, the resistance was constant. Upon switching to the 108 ppm H2S~N2 flow, the resistance versus time curve broke at -t=12 seconds after the start of the H2S flow dropping -at an initial rate of 36%/min, reaching less than 400 ohms within two minutes.
Example 32 shows that the resistance can also increase due to H2S exposure with a different metal coating on the whisXers and polymer encapsulant.
An example of type E was prepared by annealing the perylene whiskers for 160 minutes and sputter coating Cu for two minutes under the same rf : . . -. .. .... ; . . . .. '.. . , .. . - , . ... ...

f~ Ji' ~ ~ ~

and Ar pressure as cited in Example 24. It was then encapsulated in fluorenone polyester (FPE) by spin coating 5 ml of a 5% solutlon of F~E in cyclohexanone at a rate of 170 rpm onto an 8 cm diameter disc sample.
After air drying, the FPE encapsulated nanostructured composite was cleanly delaminated from the original copper coated polyimide substrate. A piece with an initial resistance of 104 ohms was cut and mounted in the simpler gas flo-~ arrangement described in Example 27. There was no water vapor present. Upon switching from pure N2 to the 108 ppm H2S/N2 flow the resistance began increasing very slowly. After several hours the resistance had increased to only 113.7 ohms, at which point the flow was stopped and the remnant vapors left in the sealed apparatus. The resistance began changins much more rapidly during the static conditions, until the pure N2 flow was restarted.
In contrast to Examples 25 to 31, Example 33 illustrates that the resistance can increase upon H2S
exposure even if Ag is used to coat the whiskers but a different encapsulant is used.
A type A sample was prepared by vapor coating a whis~ered perylene sample with approximately 800 Angstroms of Ag, spin coating 5 ml of the ORP
photopolymer at 450 rpm onto an 8 cm diameter sample disc, and W curing. A piece with an initial resistance of 40.8 ohms was mounted in the simpler gas flow arrangement of Example 27. Distilled water was present in the bottom of the apparatus. With pure N2 flowing, the resistance was constant to within 0.1 ohm.
Upon switching to the 10~ ppm/N2 flow, the resistance began changing within 30 seconds, dipped briefly to 40.0 ohms over 2 minutes, and then began increasing monotonically, ultimately reaching 54.5 ohms after approximately 90 minutes. As in Example 32, it was observed that under static conditions, the rate of SO~;TY~UTE S~tEI~l .

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resistance change was faster than with positive flow.
This effect is interpreted as due to nonequilibrium gas mixing in the simple single jar flow arrangement descrihed in Example 27, and lowerinq of the relative humidity in the ~ar when the dry gas mixture is admitted.
In summary, Examples 25 to 33 show that the ~
polymer encapsulant and metal coating both contribute to the response of the sensor to H2S, even causing the 10 resistance to change in o~posite directions for ;
different com~inations of metal and encapsulant, providing evidence that the conduction mechanisms and gas sensitivity invol~e both constituents of the-nanostructure, unlike the prior art. These examples also show that a particularly useful combination for a fast, sensitive H2S sensor,`which is not affected by humid air alone is Ag and DUC0~ cement.
Specifically for this combination and 108 ppm H2S/N2, Examples 27, 28 and 31 illustrate a logarithmic dependence of the initial rate of relative resistance change, l/Ro(dRo/dt), (or dSo/dt in %/min), on initial resistance, Ro~ This result is potentially very important since it indicates a simple means to quantify the analyte concentration with these sensors. It is logical to assume that the slope of the straight line will vary with the relative concentration of H2S, and in fact this is supported by Examples 25 and 26 where effective concentrations of 30 and 35 ppm respectively produced initial sensitivity rates of ~0 approximately 10%/min for initial resistances of 870 and 131 ohms respectively. Gi~en then two or more sensors, of different initial resistance, responding to the same gas concen'tration, it is only necessary to measure the initial rate of resistance Sl) BSTITUTE 5^rl11 2 ~ ~3 ~

change for each sensor, over one to two minutes, calculate the slope of a plot of the initial resistance rate of change versus resistance and compare to a calibration table to determine the analyte concentration. All of this could be done via integrated circuitry, and a multiplexed array of several of these sensors of regularly varying - -resistance could perhaps glve good accuracy as well.

Examples 34 and 35 Examples 34 and 35 show that using gold as the conformal coating on the whiskers produces a mercury vapor sensor with a reaction mechanism dominated by solid state diffusion.
For Example 34, a type F sample was prepared by vapor coating 1500 Angstroms mass equivalent of gold onto a whisker coated substrate disc, spin coating with NOA 81 optical adhesive (Norland Products, Inc.) at approximately 250 rpm producing a W cured film approximately 0.3 mm thick. The nanostructured composite was cleanly delaminated from the polyimide substrate. Strips of the composite, approximately 5 mm x 35 mm, were cut and individually mounted inside a test apparatus comprising a sealed 9 oz. glass jar having electrical leads penetrating the jar cover. The initial resistances were in the range of 120 to 600 ohms. The Hg vapor was generated by adding a few milliliters of pure Hg to the apparatus and placing the apparatus assembly into an air oven at controlled temperatures. After a resistance versus time run was complete at one temperature, a new sample strip was used to obtain another run at a new temperature, and hence vapor pressure. The Hg vapor pressure, hence the concentration was taken as the equilibrium vapor pressure of Hg at the given temperature. Four sample strips were exposed in this manner to Hg vapor between SlJ~STlTUTE SHE1 room temperature and 91C. The sensitivity, defined by Equation III demonstrates a strong temperature dependence and approximately a square root of time dependence, an indication that solid state diffusion is the rate limiting step.
Assuming a model for the gold coated whisker composite as a distribution of gold "posts" which are ~
being converted to a AuHg alloy by solid state diffusion of Hg through the alloy to the alloy/Au interface, which is propagating down the length of the "post" as tl/2, the sensitivity can be expressed in terms or the resistivities of the Au and alloy, and a diffusion coefficient of Hg through the alloy. The temperature dependence of the diffusion coefficient can be extracted from a plot of sensitivity versus reciprocal temperature. - -For Example 35, a type G sample was prepared by vapor coating a mass equivalent of 2500 Angstroms of Au onto a whisker coated substrate, followed by encapsulation in DUC0~ cement by spin coating 4 ml of the adhesive at ~40 rpm onto an 8 cm diameter polyimide disc and allowing it to air dry. As in Example 34, strips were cut from the delaminated composite and exposed to mercury vapor at various temperatures. The 25 resistances of the strips were initially in the range ~- -of 5 to 20 ohms. For each new temperature, the sample was first monitored in a Hg free apparatus at the designated temperature, establishing that at any temperature the resistance was constant in air. When switched to the Hg containing apparatus, the resistance - increased as shown ~y a sensitivity plot, wherein a significantly larger response was recorded than with the type F sample i'n Example 34.
Examples 34 and 35 show that the sensitivity of the sensor for Hg, using Au as the reactive metal, is strongly dependent on the initial sample resistance, assuming the type of polymer is not as important in SU~3STlTUTE S'~E!El . . .
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this case, and that a reaction mechanism can be extracted suggesting solid state diffusion and amalgamation. This implies that even more sensitive Hg vapor sensors could be created with different metal coatings, such as Al, although the Au may be beneficial where no reaction to water vapor or high temperature is desired.

ExamDle 36 Example 36 shows that using copper as the conformal coating on the whiskers produces an irreversible sensitive indicator of total water vapor exposure.
For Example 36, a type E sample was prepared as described in Example 32. Several strips were cut, 2-3 cm long and 6-8 mm wide, having initial resistances in the 50 to a few hundred ohms range. The strips, attached to the leads of an electrometer, were suspended directly over warmed water contained in an insulated Dewar. The resistance change versus time was then recorded for different average water temperatures, the latter generally remaining constant to within 1 degree durin~ the exposure. The resistance change can vary over several orders of magnitude as the Cu coating on the whiskers is oxidized. There appear to be at least two regimes of behavior or kinetics, and the mid-range can be modelled assuming an ArrhPnius relationship between S ((R-Ro) /Ro) and exposure time, S=S0exp(mt). Using the slope at the inflection points of the log(S) vs. time curves to get m as a function of temperature, from which an apparent heat of enthalpy of only 11 kcal/mole r'esults. This is considerably lower ..5~ 1TlJTlE S; lE~l .... - - . . ~ ... , , ~ . .. . .. ... . .... . .

WO92/17774 2 ~ ~ ~ i3 ~ ~ PCT/US92/01372 than-reported heats of formation of most metal oxides, in the 30 - 200 kcal/mole range, with cu approximately 60 kcal/mole. This high reactivity is probably a consequence of the high surface area of the whiskers and small size, which leads to faster response and greater sensitivity than solid thin film based sensors would exhibit.

Example 37 . `,, Example 37 demonstrates the potential for selecting the polymer encapsulant to specifically sense organic solvent vapors. The gas concentrations were ~-not varied or controlled in these experiments, rather the room temperature vapor pressures were used to simply demonstrate for different metal/polymer combinations that the response can be reversible and that the sensitivity is very dependent on the permeability of the gas or vapor into the encapsulant.
In Example 37, nanostructured film samples were made using four different combinations of metal coating and polymer encapsulant, in addition to that of -Example 24. Strips similar to those described in previous Examples were cut from sample discs and electrical contact made to the ends by various means, including crimped indium foil and conductive silver paint. The resistance of each strip was then recorded against time, first with each strip suspended within a dry polyethylene beaker, in air, and then continuing after solvent was added to the bottom of the beaker.
Table 5 summarizes the observed average rate of resistance change for various solvents and~indicates whether the response was reversible, when tested over one or two cycles. The total change, expressed as sensitivity S is ~lso given where appropriate. The rate of sensitivity change increases with initial resistivity, since all pieces were about the same size W092/17774 h~ ~ 3 ~ ~ Pcr/us92/0l372 ~ 50 -and shape. Also, the affinity of the polymer for the solvent is presumably the primary reason for the large disparity of response of a single sensor type to various solvents.

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W092/17774 2 ~ 52 - PCT/US92/01372 The resistance changes, which in all cases were increases, occur most probably due to ~oth polymer swelling and the attendant increase in inter-whisker spacing, and changes in the intrinsic electronic transport properties of the interwhisker polymer material after sorbing solvent molecules. Similar resistance changes in three dimensional dispersions of carbon black particles in polymers is well Xnown and often described in terms of percolation theory and the Hildebrand solubility parameter of the polymer and solvent. It is also conceivable that the polymer/metal interface of the coated whiskers is a controlling factor in charge injection or tunneling as well. The potential to tailor the polymer and metal combination for desired responses from specific gases or vapors would appear feasible. The results suggest, as with H2S, that the rate of sensitivity change may be correlatable with vapor concentration.

Example 3~

Example 38 demonstrates the use of the composite medium as a liquid analyte sensor.
A piece of type B sample, 6mm x 40mm, was held in the form of a semicircle by the electrical leads from an electrometer and immersed in a 180 ml volume of distilled water, contained in a 250 ml glass jar, so that the electrical clip leads were just above the water surface. The initial air resistance, 149.3 ohms, did not change when immersed in the water, until after several minutes, at which time the resistance began to slowly decrease at a constant rate of 0.26 ohms/min. over a period of 45 minutes. At this point, l milliliter of an equilibrium solution of ~r2 in distilled water was injected by syringe'into the center of the 180 ml sample ~olume and brie~ly stirred, giving a 68 ppm aqueous Br solution. The resistance remained stable for h ~ 3 approximately 2 minutes, then began rapidly decreasing at a rate,,of approximately 4 ohms/min., ~inally -~' equilibrating a~ter fifteen minut:es for a total sensitivity change of 26%.
'-~
Exam~le ^lg Example 39 demonstrates that the capacitance measured perpendicular to the film plane can be used as the sensor property.
Small area pieces, on the order of 2 cm2, were cut from B, E, L, and M type samples. Aluminum foil backed adhesive tape was applied to the polymer encapsulant side of each piece so as to form a simple capacitor with the nanostructured surface of the composite forming one conductive side of the capacitor and the aluminum foil the other. The thickness of the adhesive layer on the tape was much thinner than the composite film, the latter being in the 0.05 to 0.125 mm thickness range. Electrical contact to the formed capacitor was made by smooth jawed alligator clips. To measure the capacitance, a 1.025 megohm resistor was placed in series with the sample capacitor, a squarewave ' signal in the 100 H7 to several kHz range was applied across the two circuit elements, and the voltage signal across the sample capacitor was monitored with an oscilloscope. The RC time constant was read directly from the sample capacitor waveform, and the sample's capacitance calculated from Equation I. The sample test 30 assembly was then placed in a covered beaker, directly '` ' over acetone liquid in the bottom, to expose the,sample capacitor to a nominally saturated vapor. The RC time - constant was then periodically read off the oscilloscope, the total circuit capacitance calculated and the scope probe capacitance subtracted to give the sample capacitance.

S~.J113~iTlTI ITE S~lEFt By combining the capacitance property of the Exampl-e with the demonstrated surface resistance properties of Example 37, it is clear that the sensor media could be made into a resonant, tuned circ~it element with a fast frequency shift type response.
Various modifications and alterations of this invention will become apparent to those skilled in the~
art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

.

50E~5TlTUTE S~lE~f

Claims (15)

MINNESOTA MINING & MANUFACTURING CO.
Cur re??: D 555 PCT

New Main Claim

1. A composite article with an electrically conductive surface, comprising an encapsulating layer having a regular or random array of discrete microstructures, each microstructure comprising a whisper-like structure and an optional conformal coating enveloping said whisker-like structure, wherein said array of discrete microstructures is encapsulated in said encapsulating layer, such that one end of each microstructure is embedded within said encapsulating layer and the other end of each microstructure is exposed and coincident with the surface of said encapsulating layer.

2. The composite article according to Claim 1, wherein said whisker-like structure comprises an organic or inorganic material.

3. The composite article according to Claim 2, wherein said organic material is selected from the group consisting of polynuclear aromatic hydrocarbons and heterocyclic aromatic compounds.

4. The composite article according to Claim 3, wherein said organic material is selected from the group consisting of perylenes, phthalocyanines, and porphyrins.

5. The composite article according to Claim 2, wherein said inorganic material is a metal oxide.

6. The composite article according to Claim 4, wherein said organic material is N,N'-di(3,5-xylyl)perylene-3,4:9,10 bis(dicarboximide).

7. The composite article according to Claim 1, wherein said conformal coating is a conducting 53a material selected from a group consisting of metals, inorganic compounds, and conducting polymers.

8. The composite article according to Claim 7, wherein said metal is selected from the group consisting of aluminum, cobalt, cobalt chromium, nickel, nickel chromium, platinum, silver, gold, iron, copper and other transition metals.

9. The composite article according to Claim 1, wherein said encapsulating material is organic or an inorganic material.

10. The composite article according to Claim 9, wherein said organic material is selected from the group consisting of molecular solids held together by van der Waals' forces, such as organic pigments, including perylene red, phthalocyanine and porphyrins and thermoplastic polymers and co-polymers such polymers derived from olefins and other vinyl monomers, condensation and addition polymers, such as polyesters, polyimides, polyamides, polyethers, polyurethanes, polyureas, and natural polymers and their derivatives such as, cellulose, cellulose nitrate, gelation, proteins, and rubber.

11. The composite article according to Claim 9, wherein said inorganic material is selected from the group consisting of gels, sols, or semiconductor, or metal oxides.

12. The composite article according to Claim 1, wherein capacitance impedance is measured perpendicular to the surface plane of said article and resistance impedance is measured parallel to the surface plane of said article.

13. A sensor comprising the composite article in accordance with Claim 1, wherein said conformal coating, said encapsulating material and said whisker-like structures are independently selected for their responsiveness to gas, vapor, or liquid analytes.

14. A flexible thin film according to Claim 13.

15. A method for preparing a composite article having an electrically conductive surface comprising the following steps:
(a) providing whisker-like structures on a substrate;
(b) optionally coating said whisker-like structures with a conformal coating, wherein each microstructure comprises a whisker-like structure conformally coated is produced;
(c) encapsulating said microstructures with an encapsulating material, wherein a layer comprising encapsulated microstructures is produced; and (d) delaminating said layer from said substrate, such that at the surface of said layer delaminated from said substrate, one end of said microstructures is exposed, such that said exposed ends and surface of said layer are coincident on a common side of said layer.