Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J1/00—Photometry, e.g. photographic exposure meter
  • G01J1/48—Photometry, e.g. photographic exposure meter using chemical effects
• • 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
  • G01N21/75—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated
  • G01N21/77—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator
  • G01N21/78—Systems in which material is subjected to a chemical reaction, the progress or the result of the reaction being investigated by observing the effect on a chemical indicator producing a change of colour

Definitions

• metal ions can be detected through a color change brought about by the reaction of the ions with a ligand or chelating agent.
• Metals can inhibit the activity of enzymes, an aspect that can be used to quantify their concentration. They can also stimulate the activity of enzymes through the release of substrates for the enzyme; this heightened activity can also be quantified.
• Such methods are known to be applicable to molecules and ions other than metals.
• Immunoassay methods are known to be sensitive in their response to target analytes.
• Biologically active organisms such as Helicobacter pylori and Streptococcus species can be detected.
• the antibody-antigen interaction is utilized to produce a detectable event upon exposure of the device to the target analyte.
• Fluorescent species can be released that can be detected instrumentally; electrochemical means can be used to amplify the signal.
• Interference patterns can be induced by such reactions through a change in film thickness, which can be quantified.
• Ferrous ions can be chelated by reagents within a multilayer element that provides a buffered environment for its migration to a detection layer.
• the chelating agent may be resident in this detection layer.
• Similar arrangements are known for the detection of Ca ++ in aqueous solutions. Interferences may be removed by the presence of additional reagents such as the addition of calcium chelating reagents to a multilayer analytical element designated for detection of magnesium.
• Multilayer analytical elements have been devised for the assay of complex fluids, such as blood. Such elements can be composed to detect glucose, alcohol, cholesterol, or proteins.
• separation layers impregnated with separation agents are used to isolate the target analyte from the other components of the blood.
• analyses of biological fluids often rely upon enzymes to produce the color change for detection of the target analyte.
• the analyte can interfere with a color-producing reaction by the enzyme, or it can induce the enzyme to produce a colored species.
• Immunoassay methods make use of antibody/antigen binding to effect specific sensitivity in multilayer analytical elements. Enzymes specific to an analyte are synthesized and coated within such a device to bind the target molecule and induce a measurable readout signal through the release of a molecule which can migrate to a registration layer. Usually this is a fluorescent assay that requires a photodetector for interpretation.
• a novel means of broad screen detection is described for the presence of target analytes in the vapor phase, in solution, or eluted from a solid.
• the novel application of competitive dye desorption from a solid adsorbent is employed as a method of quantifying the presence of the molecule or target analyte.
• dye or dye-precursor molecules adsorbed on the surface of an adsorbent are caused to desorb through the adsorption of the target analyte on the adsorbent.
• the desorbed dye or precursor is made detectable through sequestering of a radiation detectable species in the device of the invention. Such detection may occur, e.g., through absorption or emission of radiation in regions of the spectrum extending from the ultra-violet through the visible and into the infra-red regions. In one aspect of the invention, these processes occur within a multi-layer analytical element, in which the functions of the device may be executed by different layers.
• a multi-layer element will include an analyte acquisition layer, which contains the substrate for dye desorption from which dye is desorbed in response to the presence of the analyte within the layer, and an underlayer in which the desorbed dye is sequestered in a detectable manner.
• Fig. 1 is an illustration of the manner in which dye desorption off of an adsorbent can serve as a vapor detection mechanism
• FIGS. 2 and 3 show enlarged sectional views of analytical elements incorporating the invention for the purpose of vapor and liquid analysis respectively;
• Fig. 4 shows a desirable relationship between the strengths of adsorption of a given dye onto an adsorbent and a mordant material
• Fig. 5 shows an analyte detection badge incorporating the analytical detection element of the invention
• Fig. 6 shows an analyte detection container incorporating the analytical detection element of the invention.
• a relatively loosely adsorbable dye or dye precursor is adsorbed onto an adsorbent to serve as the analyte detector.
• the adsorbent can be coated on a substrate, e.g., as a film or a coating on any surface in a binder medium that will permit adsorbate adsorption, desorption, and diffusion and used, e.g., in a badge for contaminant exposure detection. Exposure to analyte, e.g., in the air, displaces the dye or its precursor and the dye or its precursor is detected.
• materials can be provided elsewhere in the analytical element to react with it and produce a radiation detectable species.
• the term "dye” should be taken to include the use in this invention of molecules that are not directly detectable but can be converted to detectable form within the analytical element by chemical or physical processes.
• the displaced or newly formed dye then diffuses to a receiving registration layer, for example a dye mordanting layer. This layer is coated below the adsorbent layer and on a transparent base, and dye will accumulate there.
• the term "dye" is intended to include an embodiment in which the displaceable material adsorbed on the adsorbent is a catalyst for the formation of a radiation detectable species, e.g., for a color-forming reaction in which the color-forming precursor reagents are incorporated into other layers, for example, directly into the mordant.
• Competitive displacement of the catalyst by analyte adsorption enables the catalyst to come into contact with, e.g., the color precursors to produce color in the mordant. Because the system is catalytic, the amount of dye formed can be many times the amount of analyte adsorbed, depending on the catalyst turnover number and the amount of precursor reagents present.
• Diffusion into the registration layer is facilitated by including a polar, high boiling plasticizer/solvent in at least one of the layers of the analytical element to create a liquid-like environment.
• Plasticizers are high boiling solvents that permit substances to migrate within the coatings.
• the dye on the adsorbent and in the mordant are at equilibrium with dye solvated by the plasticizer which permeates the entire structure.
• the conceptual basis of the invention is revealed through an examination of model isotherms such as a Langmuir-type isotherm, for dye adsorption in the adsorbent and mordant layers.
• the selection of the adsorbent and mordant composition is made so that the free energy of binding of the dye to the two differs by at least an order of magnitude. This difference results in two distinctly different isotherms, as depicted in Fig. 4.
• the sensitivity of the system is a strong function of the absolute amount of dye in the carbon overlayer.
• a milligram of activated carbon can adsorb 2xl0- 7 moles of dye, so there is a wide range of sensitivity control available through the carbon coating coverage.
• the kinetics of the system must be adequate as well.
• the dye must have a low but non-zero rate of desorption from adsorbent into the plasticizer as well as a reasonable rate of diffusion,
• molecular size, weight, and polarity and molecular polarizability are usually the most important.
• Molecular size and weight affect an adsorbate's volatility and surface area available for binding to an adsorbent.
• Polarity determines the coulombic aspects of adsorption.
• Molecular polarity is composed of several components, but it can be visualized as the extent to which electronic charge is non-uniformly distributed in a molecule.
• activated carbon or related adsorbents the most strongly bound analytes will generally be materials of low polarity, high molecular weight, or both.
• the strength of adsorption of normal alkanes is proportional to the number of carbons in the molecule.
• polar adsorbents such as silica
• the most strongly bound adsorbents will generally be materials of high polarity, and the effect of molecular weight will be minimal.
• the plasticizer/solvent also will play a role in the energetics of adsorbent/adsorbate interactions. Adsorption will be strongest when the solubility of the adsorbate is lowest. Therefore, the choice of plasticizer/solvent can also influence device response and selectivity. As a result of the wide range of adsorbents and adsorbates that can be combined, the invention has very wide application. Embodiments can be developed on the basis of analyte polarity, solubility in the plasticizer/solvent, or on specific interactions with the adsorbent. Selectivity for particular materials can be attained by altering dye molecular structures to control relative binding strength to the adsorbate.
• the dye, adsorbent, and plasticizer/solvent act as a group to establish the selectivity for different analytes.
• the choice of other embodiment components, particularly any binder used to hold the adsorbent, can also affect the selectivity.
• the invention can be implemented to provide sensitivity to either a broad range of analytes, or to very specific types of analytes, depending on the type of adsorbent-adsorbate interactions selected. Specific examples of adsorbent/adsorbate pairs are given in Table 1.
• the invention is capable of being implemented so that a variety of incident radiation-detectable changes can provide the read-out method for a device incorporating this invention.
• Dye desorption can provide either a direct optical read-out for visual or instrumental assessment; systems can be constituted so that either the appearance of color or a change in color occurs.
• Other read-outs can include the appearance or disappearance of fluorescence or signaling at wavelengths outside the visual range.
• a system based on the disappearance of color can be constructed around the release of a colorless adsorbate that can bleach a dye already present in the registration layer.
• the binder of the adsorbent can serve as the registration or sequestering region, and the binder/adsorbent complex can be coated directly on a surface to enable analyte detection over an area.
• the mordant and transparent base of the sensor are replaced by a transparent base on which is coated a conductive transparent oxide layer.
• This layer can be tin oxide or indium tin oxide (ITO) or any other transparent conductive material.
• ITO indium tin oxide
• the structure of the resulting device is base/ITO/titania (optional) /carbon.
• the dye desorbed from the adsorbent, e.g., carbon, by the incident analyte molecule can diffuse to adsorb onto the ITO. The presence of the adsorbed dye will make the ITO photoconductive at the wavelengths where the dye absorbs light.
• the resistance of the ITO layer will change under illumination, and if electrical contacts are made to the ITO, this change in resistance can be quantified electronically to provide an electronic readout correlated with the concentration of the target analyte.
• the sensor element would be packaged as a cartridge that is inserted into a small electronic device constructed to measure its photoconductivity and, therefore, the extent of exposure to the target analyte.
• Another useful embodiment of this invention is one in which varying levels of one or more dyes are coated in separate sections of the analytical element, so that color appears in different places on the viewing side as a function of the amount or types of analyte detected. It is also possible to adsorb two dyes with differing adsorption strengths onto the adsorbate, so that as the level of analyte exposure increases, the color observed both increases and shifts hue.
• an analytical device incorporating competitive desorption as a means of chemical detection can be assembled is to build the device as an integral, multilayered analytical element, which can be worn, e.g., as an analyte detection badge, as shown in Fig. 5.
• the construction of such analytical elements using other methods of analysis is well-documented in the prior art.
• Such elements are simple in structure, easily manufactured at reasonable cost, and adapted to carry out the analysis in a simple and effective manner.
• a device is used for measurement of molecules in vapor phase above liquids and solid samples.
• a device consists, e.g., of a vial, jar or other container into which is placed a liquid or solid sample to be analyzed for the presence of volatile compounds, and of a cap with which the container is sealed.
• the analytical, or sensor, element is placed inside the closed vessel, either on the side of the jar or in the cap, in such a way that the readout is visible through the transparent walls of the vessel or through the transparent cap.
• the volatile components emanating from the sample are detected by the sensor element in the manner described above to induce a color readout visible from the outside of the vessel.
• the analytical element can be in the form of sheets, short strips, continuous tapes or strips, or as small sections or chips that may or may not be mounted in aperture cards or other external mountings.
• the detection element can be provided in multiple small pieces, e.g., as confetti or in shredded form, that can be spread over an area to detect the presence of target analyte (s) within that area.
• the preferred form of the invention is a multilayered element, the invention is not limited to this form. Rather, it is directed to any element using competitive displacement of dyes or radiant energy-detectable materials or precursors from adsorbents in a solid but molecularly permeable medium as a means for the detection and analysis of chemical substances.
• Radiant energy-detectable refers to a species that, through the formation or loss of color, fluorescence, or other characteristics, alters the radiant energy reflection or transmission characteristics of the analytical element.
• the integral, multilayer analytical element is typically laminar in construction, so that when observed as a transverse cross-section, the layers are seen as superposed. This construction permits the desorbed species to diffuse from the outermost layers into one or more interior layers. In use, the element may simply be exposed to the air or other gases containing vapors to be analyzed. Alternatively, liquids may be applied directly to an appropriately designed element top surface, for example as described in U.S. Pat. 3,992,158 or many other designs known in the art .
• the element includes a support, a layer overcoated or otherwise integrally formed on one side of the support and including at least one indicator substance, a layer providing a background for viewing the signal produced by the indicator substance, and a layer at or near the top of the layer stack that contains the adsorbent and competitively desorbable species, which is accessible by the analyte (s) to be detected.
• a working embodiment of an integral, multilayer analytical element needs to incorporate several functions: sample capture, competitive desorption, concentration of the released radiant energy-detectable materials and, if the adsorbent is colored or significantly opaque, provision of a background against which the radiant energy-detectable materials can be detected. In certain embodiments, it is possible to combine certain of these functions into single layers. For detection of vapor analytes, it is preferred to construct the element as a three layer structure atop a transparent base, readable by reflectance. Referring to Figs. la-Id and to Fig.
• analytical element (10) in order of distance from the base: the clear plastic base (12) itself, the concentrating layer (14) such as a dye mordanting layer, a reflecting layer (16), e.g., a white pigment, that also is sufficiently opaque to appear white under a dark adsorbent, and the adsorbent/indicator layer (18), which for vapor detection serves also as the sample capture layer.
• the concentrating layer (14) such as a dye mordanting layer
• a reflecting layer (16) e.g., a white pigment, that also is sufficiently opaque to appear white under a dark adsorbent
• the adsorbent/indicator layer (18) which for vapor detection serves also as the sample capture layer.
• analytes in the vapor phase (v) diffuse into adsorbent/indicator layer (18), where, as shown in Fig. lb, the analytes to be detected competitively displace the displaceable dye or dye precursor adsorbed onto the adsorbent.
• an appropriately designed top surface (22), as shown in Fig. 3 may be used to receive the liquid directly.
• the sample fluid can itself serve to swell the layers and transport the desorbed radiant energy-detectable species or precursor to the concentrating layer for read-out. However, it may be desirable either to increase swell or transport rates or to minimize sample size requirements to incorporate a fluid into one or more of the layers. In the case of vapor phase samples, such fluid incorporation is mandatory.
• This fluid will in essence be a plasticizer for the polymer film-formers in the structure, as it must be compatible with the polymers, capable of swelling them but not of altering their mechanical integrity.
• plasticizer/solvent It should have a sufficiently high boiling temperature so that it does not evaporate significantly during the use life of the element.
• This plasticizer must also be a solvent for the mobile species that will be or will lead to the detectable species. Thus, it will be referred to henceforth as the plasticizer/solvent.
• all references to plasticizer/solvent can also include, when relevant, the case of the sample fluid itself serving as the plasticizer/solvent.
• film-forming binders and plasticizer/solvents are selected based on the relative polarities and hydrophilicities of the analyte (s), adsorbents, and indicating materials (dye or precursor) .
• the dye or dye precursor could be chosen to be more polar so that the analyte could displace it from the carbon surface.
• the dye or dye precursor could be chosen to be more polar so that the analyte could displace it from the carbon surface.
• a sufficiently polar plasticizer/solvent that the dye would be dissolved and be transported by it.
• polymer binders to those that could dissolve significant amounts of plasticizer/solvent and be plasticized by it.
• a hydrophilic, high boiling organic solvent that is not so polar as to exclude the analyte of interest might be selected.
• solvents can include alcohols, diols and higher polyols, sulfoxides, amides, esters, carbonates, ketones, and the like. Any solvent that will achieve the above goals and be stable under coating, storage, and use conditions can be a candidate.
• the polymer (s) used as binders for the different layers must be selected for compatibility with the selected plasticizer/solvent.
• water soluble or highly swellable polymers such as polyvinyl alcohol, gelatin, hydroxyalkylcellulose ethers, N-vinyl pyrrolidone polymers and copolymers, carrageenan and other natural polysaccharide gums, carboxymethylcellulose, polyacrylamide and its copolymers, and other film forming, hydrophilic polymers could be suitable.
• polar but non-hydrogen bond donor solvents such as amides or sulfoxide
• water-insoluble polymers such as polyvinyl butyral, aerylate copolymers, polyvinyl acetate and copolymers, cellulose esters and ethers, copolymers of styrene with polar monomers such as maleic anhydride or N-vinyl pyrrolidone, and other polymers with the right solubility parameters for the solvent in question would be appropriate.
• suitable solvents include dialkyl diesters such as those sold as the DBE series (DuPont) , diesters of aryl compounds including those commonly used as plasticizers for, e.g., polyvinyl chloride, polyvinyl acetate, cellulose nitrate, and the like, such as diethyl phthalate, long chain esters such as isopropyl palmitate, phosphate esters like tributyl phosphate, liquid chloroparaffins, sulfonamides like N-ethyl-o,p-toluenesulfonamide, amides such as diethyl lauramide, ethoxylated or propoxylated phenols such as Igepal OD-410 (Rhodia) , and many others.
• a specific example might be the combination of polystyrene with dimethyl sebacate.
• polymers that become insoluble on drying for instance polyvinyl acetate lattices, or ones that become insoluble on heating such as ammonium salts of base-soluble but neutral water-insoluble polymers like Carboset 525 (BFGoodrich) or Scripset 520
• Fig. 2 represents a three layer, wet coated film that demonstrates this concept. The analyte is adsorbed on the side opposite the transparent base, and the signal is read through the transparent base.
• any substrate that can receive the coated layer and that does not interfere with optical detection is suitable.
• Such substrates can include plastic films, paper, nonwoven sheets, metal foils and the like.
• the coating substrate must have some degree of transparency. This is most easily achieved by the use of plastic films.
• the thickness of these films can be any amount that is convenient and compatible with the coating apparatus used.
• These films can be composed of materials such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, polycarbonate, polyolefin, polystyrene, cellulosics such as cellulose triacetate, polyphenylene sulfide, polysulfone, polyacrylates and related copolymers. It may be desirable to surface treat the substrate film in order to permit coating with aqueous or other high surface tension fluids. This may be done using one or more treatments known in the art such as subbing, corona treatment, the use of coupling agents like organotitanates, and the like. Suitably pretreated, transparent films such as subbed polyethylene terephthalate are commercially available from several sources, and are preferred.
• the bottom layer of the coated stack is designed to collect and concentrate the chemical responsible for the radiant energy-detectable species, for example a dye. Effective concentration requires that this layer be highly permeable to dye, and must show some degree of thermodynamic preference for the dye over the other layers in the integral element.
• This dye affinity can be the result of electrostatic interactions (ionic or dipolar), covalent bonding, hydrophobicity matching, hydrogen bonding, or any other physical mechanism that encourages the accumulation of the detectable species in this layer.
• the affinity of the mordanting agent In the case of direct desorption of a dye from an adsorbent, the affinity of the mordanting agent must be balanced against that of the adsorbent so as to prevent spontaneous transfer of dye to the mordant in the absence of analyte.
• Mordanting layers are well-known in instant photography and prior art integral analytical elements.
• a mordant layer consists of an ionic polymer of opposite charge to the dye to be mordanted.
• the dye affinity of the mordant can be controlled by altering its charge density either by copolymerization or dilution with a compatible non-mordanting polymer.
• the mordant polymer can be crosslinked to reduce its degree of swell and hence the amount of binding sites actually available for dye binding.
• anionic dyes such as those commonly used in photographic applications, a wide range of cationic polymers, both commercial and custom-made, have been proposed.
• Cationic dyes may be mordanted with anionic polymers such as carboxymethyl cellulose, polystyrenesulfonic acid salts, polyacrylic acid salts, naturally occurring anionic polymers such as alginic acid and carrageenan, copolymers of maleic acid such as those in the Gantrez line (ISP) , and many others. These may be diluted with compatible nonionic polymers such as those mentioned earlier.
• the polymer counterions may be varied to adjust compatibilities, regulate crosslinking, and achieve differing degrees of dye affinity.
• this layer be transparent and uncolored. This is achieved by selecting components for compatibility in both the wet and dry states. This includes small molecules such as the plasticizer/solvent (s) , preservatives, and surfactants. The last can be of value in insuring optimal coating quality, thereby maximizing transparency. In general, it is desirable to avoid surfactants with charge opposite to the mordanting polymer.
• a cationic wetting or leveling agent may be used, for example Fluorad FC-135 (3M) or Aerosol C-61 (Cytec) .
• Anionic mordants can benefit from anionic wetting agents such as Aerosol OT (Cytec) or Rhodacal DSB (Rhodia) .
• anionic wetting agents such as Aerosol OT (Cytec) or Rhodacal DSB (Rhodia) .
• either type of mordant could be compatible with many of the typical nonionic and amphoteric surfactants.
• the mordant layer it is generally necessary for the mordant layer to be largely insoluble in the solvent being used to coat the next layer atop it. This may be achieved either by appropriate selection of component polymers, by design of the overcoat fluid, or by crosslinking the mordanting layer.
• a hydrophilic mordant layer such as one composed of sodium carboxymethylcellulose would tolerate being overcoated with a reflecting layer using an organic solvent such as isopropanol or 2-butanone as its solvent.
• the mordant layer can have its solubility and degree of swell reduced to a usable level by adding water-insoluble binders such as polyvinylbutyral (added as e.g.
• the coating process is simplified if the crosslinking agents are heat activated. Then the mordant layer can be coated without concern for fluid viscosity buildup during the coating process. Crosslinking is then triggered by the drying process.
• Another possible implementation is to coat multiple layers at once, as is done in the photographic industry using cascade coating techniques; by suitable adjustment of coating layer densities and viscosities it is often possible to coat several water-based layers atop each other at once with minimal mixing.
• the next layer to be coated is a reflecting layer.
• Opacity and reflectance is achieved by the use of reflecting pigments such as titanium dioxide, barium sulfate, aluminum powder, zinc oxide, or other scattering particles.
• Permeability is achieved by choosing polymer film formers that are well swollen by the plasticizer/solvent. For example, polyols such as glycerol are excellent plasticizers for most grades of polyvinyl alcohol, and dyes that are somewhat soluble in glycerol move easily through an appropriately formulated layer of titanium dioxide dispersed in polyvinyl alcohol.
• Factors that control the effectiveness of the reflecting layer include pigment type, pigment volume concentration, pigment dispersant type and degree of dispersion, and coating thickness (coverage) . It is necessary to optimize these parameters for each combination of materials making up the reflecting layer in order to get high opacity and reflectivity without impeding dye migration, layer mechanical integrity, and coating quality.
• Titanium dioxide is the preferred pigment because of its high reflectivity and covering power.
• rutile pigments are preferred to anatase titanium dioxide because of their lower photoactivity.
• Average primary particle size for optimum optical properties is in the range of from 0.1 to 1.0 micrometers and preferably of from 0.3 to 0.5 micrometers. It is further preferred to use a rutile pigment that is surface modified by precipitation of a shell of other oxide, typically one or more of silicon dioxide, aluminum oxide, or zirconium dioxide, over the pigment particles.
• These core-shell pigments display reduced dye photodegradation and easier dispersability which are both desirable.
• modified titanium dioxides can be found in most titanium dioxide pigment manufacturer's lines, as exemplified by the pigments of the Ti-Pure R-900 line (DuPont), Kemira's RDD and 650, Kronos 2310, 2102 and 2160, and similar pigments.
• the amount of pigment in the layer can vary over a wide range, depending on the particular pigment, how well dispersed it is, what other materials are in the pigment layer, and how thick this layer is coated. For titanium dioxide, coverages can range from 5 g/m 2 to 40 g/m 2 , preferably in the range of 10 to 25 g/m 2 .
• Layer thickness can be in the range of from 3 to 30 ⁇ m, preferably from 5 to about 20 ⁇ m.
• Pigment volume concentration can range from 25 percent to 75 percent, limited by the rheology of the coating fluid, the mechanical integrity of the dry layer, and the resistance to dye passage through this layer caused by the solid pigment particles.
• the required amount of titanium dioxide can be reduced be incorporation of low refractive index particles such as polytetraflouroethylene latices or Ropaque (Rohm & Hass) air encapsulates as is well known in the paint industry. In order to achieve sufficient opacity to hide the detection layer from view, the pigment particles need to be thoroughly dispersed.
• Standard pigment dispersion methods as used in the paint industry are suitable, for example, ball milling, homogenizing, sonicating, impinging high-pressure flows, Cowles or Hockmeyer blades, and the like. It is possible to prepare the titanium dioxide as a dispersion in coating solvent, and then add the film-forming binder in a second step, as is often done for paint manufacture, or it is possible in some cases to disperse the pigment directly into a solution of the binder resin in coating solvent and plasticizer/solvent.
• dispersant additives to insure complete dispersion and reduce flocculation and settling; this may however be unnecessary if the binder polymer has sufficient pigment dispersing properties itself, for example gelatin.
• Pigment dispersants can be surfactants or polymers, charged or uncharged, and may be used singly or in combination. It is necessary that the dispersants chosen be compatible with the binder and plasticizer/solvent. Otherwise the coating fluid may be unstable, or pigment flocculation and surface defects may occur during drying.
• Titanium dioxide is generally dispersed in aqueous media with the aid of low molecular weight, anionic polymers such as polyacrylates like Colloid 111 (Rhodia) , polymethacrylates such as Daxad 30 (Hampshire Chemical) , naphthalenesulfonate-formaldehyde condensates such as Tamol
• CT-131 Air Products
• Small molecules like pyrophosphate salts and anionic surfactants such as Geropon SS-O-75 (Rhodia) may also be suitable. It is desirable that the charge on the dispersant be the same as that of the dye or dye precursor, so as to avoid mordanting of the dye in the reflecting layer.
• cationic dispersants such as Aerosol C-61 (Cytec) , Witflow 953 (Witco) , and cationic polymers like Nalco TX 7991 (Nalco Chemical) may be useful.
• nonionic dispersants such as the Soprophor series (Rhodia) , Glucopon 425N (Henkel) , or Gafac P-904 (ISP) may be tried, but they are generally somewhat less effective. Mixtures of charged and uncharged dispersants are also useful. Similar more hydrophobic dispersants of all three charge types are available for use in organic solvent-based coating fluids.
• a pigment wetting or grind aid as part of the dispersion process.
• Representative materials include Surfynol 104, CT-121, and CT-136 (Air Products), Witco 960, Witflow 963 (Witco) , and Disperbyk 183 (Byk) .
• Surfynol CT-136 and Disperbyk 183 can also serve as dispersants.
• the weight ratio of pigment to binder can range of from 1:1 to 20:1, preferably in the range of from 5:1 to 9:1. The preferred ratio will depend to some degree on the choice of binder and the amount of plasticizer/solvent incorporated into it.
• Detecting Layer Over the reflecting layer is coated the layer containing the dye desorption detection system.
• exemplary adsorbents useful in the detection system include activated carbon, silica, alumina, ion exchange resins and molecular sieves. Also useful are polymers or polymer latices capable of binding dyes ion exchange resins like Amberlite IR 120 (Rohm & Hass) and molecular sieves such as 4A° sieves (W. R. Grace) . Also useful are polymers or polymer latices capable of binding dyes.
• polymers containing vinylprrolidone units are known to have an affinity for many classes of dyes, but as the interactions are weak, other molecules with comparable polarity and hydrogen- bonding characteristics could serve to displace these dyes and hence be detected.
• Particulate neutral polymeric adsorbents such as Amberlite XAD series (Rohm & Hass) are also applicable. If colorless dye precursors and a white adsorbent are used, it may be possible to omit the reflecting layer. However, in most cases it will be advantageous to separate the reflection and detection functions to achieve maximum reflectivity without limiting the selection of detection materials.
• the reflecting layers described above can be made sufficiently opaque to completely hide a layer of activated carbon and still be permeable to dyes.
• the combined material can be coated over the reflecting layer using the same or different binders.
• Requirements for the binder are that the combination of binder and coating fluid not desorb the indicating species and that the binder must not block release of the indicating species, either directly or due to insufficient permeability to the analytes of interest.
• Finding appropriate binders is a process that is sensitive to the choice of plasticizer/solvent as well as adsorbent and dye or dye precursor.
• plasticizer/solvent as well as adsorbent and dye or dye precursor.
• polyvinyl alcohol is a satisfactory binder for a titanium dioxide reflective layer, but not for the activated carbon detecting layer.
• cellulosic polymers such as Klucel (Hercules) or Methocel (Dow) do permit dye to leave the carbon surface and migrate to the mordant layer.
• wetting agents and other coating aids must be selected carefully to avoid interference with the detection system. Combinations of polymers may be particularly useful in this layer to maximize permeability to both analytes and dyes while providing the mechanical robustness necessary for an unprotected top layer.
• Example 1 A vapor detection system based on dye desorption was prepared as follows: a mordant layer comprising the commercially available polymer Gantrez AN-169 (ISP) at 1300 mg/m 2 , 2-methyl-l, 3-propanediol at 7786 mg/m 2 , and Rhodacal DSB (Rhodia) at 13.0 mg/m 2 was coated from water using a wound-wire rod onto a transparent, subbed polyester support from water and dried 5 minutes at 115 °C. Over this layer was coated a reflecting layer containing the following materials: MATERIAL COVERAGE (mg/m 2 )
• Airvol 107 Air Products
• 2673 ammonium titanium lactate (50% solution) (Aldrich) 406
• This layer was coated from water with a wire-wound rod, and air-dried.
• the completed detector was tested by attaching it to a clear glass plate that was then glued with Polectron 430 adhesive latex (ISP) onto a jar containing a small beaker of test solvent that served as a source of vapors to be detected.
• ISP Polectron 430 adhesive latex
• This detector prior to exposure, had an optical reflection density (OD) in the magenta (CIE status A) of 0.22 and a white visual appearance. After 160 min. of exposure to dichloromethane vapors, the viewing side of the detector became deep magenta with a magenta reflection density of 0.84. When exposed to toluene vapors at ambient temperature for 8 hours, the detector displayed a reflection density of 1.44.
• OD optical reflection density
• CIE status A magenta
• AN-169 in a manner analogous to Example 1, except that the amount and type of base was varied.
• Bases used included sodium and potassium hydroxides and lithium carbonate, as well as amines including ethanolamine, diethanolamine, and triethanolamine (all from Aldrich) . These mordants were used to prepare multi-layer structures comparable to Example 1. All gave significant color on exposure to toluene vapors. Maximum optical densities were as follows:
• Example 3 Various loadings of Pyronin Y on carbon were prepared and incorporated into the top layer of a multi-layer coating similar to Example 1. Pieces of these coatings were exposed to vapors of various solvents until the color on the mordant side did not increase further. The results are reported in units of percentage reflectance as measured on a home-built spectrophotometer.
• Airvol 125 Air Products
• ISP polyvinylpyrrolidone K-15
• Dur-O-Set SB-321 polyvinyl acetate latex (National Starch) 258.6
• Example 6 The maximum viewing surface dye densities resulting from exposure to various vapors of the coated structure of Example 1 were determined by reflection densitometry using an X-Rite 310 densitometer (X-Rite Systems, Grandville, MI) in Status A mode. Two coatings were compared, that of Example 1 and another identical except for the replacement of Pyronin Y by the blue-violet dye Thionin perchlorate. Pyronin Y 60 mg/g Thionin C104 on Darco G-60 120 mg/g on Darco G-60 toluene 0.71/0.30/1.72/0.99 1.69/1.09/2.11/0.63
• Dye densities are in units of reflectance optical density in the order visual/cyan/magenta/yellow.
• Example 7 Various liquids were applied to Example 1 coatings using a wire-wound rod to apply a 15 ⁇ m thick coating over the carbon layer.
• the untreated coatings had a D ⁇ of 0.21/0.22/0.24/0.23:
• Example 8 The effect of toluene vapor on three devices identical except for containing three different dyes was examined. The dye loadings were chosen to give comparable D m values:

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