INTERROGATION OF A SENSOR Field of_the invention
This invention relates to a method for changing an optical characteristic of s holographic sensor. Background to trie Invention
Holographic sensors may be used for the detection of a variety of analytes W095/2649B discloses a holographic sensor, based on a volume hologram. This sensor comprises an analyte-sensitive matrix having an optical transducing structure disposed throughout its volume. Because of this physical arrangement of the transducer, an optical signal generated by the sensor is very sensitive to volume changes or structural rearrangements taking place in the anaiyte-sensitive matrix as a result of interaction or reaction with the analyte.
WQ03/DS7899 describes a method of continuous sensing using a holographic sensor. A fluid comprising the analyte is passed over the sensor, the analyte reacting reversibly with the holographic support medium. Summary of the invention
The present invention is based on the realisation that a holographic sensor can usefully be sensitive to physical interaction, e.g. in the presence of a change of temperature, magnetic, electric or-other radiation field, light and/or pressure. The sensor, e.g. a volume hologram in which a medium supporting a hologram is affected by the physical interaction, undergoes a change that can be ooserved. This change in an optical characteristic is preferably reversible.
According to the invention, a sensing method comprises subjecting a holographic sensor to an external physical influence to which the sensor is sensitive, and observing a change in the image. The physical influence may be understood as remote or direct interrogation that induces a variation in a physical property of a medium in which a holographic element is supported.
The holographic sensor may be sensitive to a physical influence such as temperature, magnetism, light and/or pressure. The invention has particular relevance to security/authentication, The observed change, i.e. a change in the optical characteristic of the sensor, may.be detectable either directly by eye, or using any suitable apparatus, for example a spectrometer.
Brie" Description pf the Drawings
Figure " is a graph of peak wavelength (nm) against temperature f°C): anc
Figure 2 is a graph of intensity (counts) against time (minsj. Description of the Invention
A holographic, sensor for use in the invention typically comprises a holographic support medium and, disposed throughout the volume of the medium & hologram. A method of the invention preferably involves the use of a sensor wherein the support medium is sensitive to a physical property, e.g. temperature. By subjecting the sensor to interrogation, the (local) physical property may be changed, causing a change in a property of the support medium. This results in a change in an optical characteristic of the element.
The sensor may oe interrogated by any suitable means. As an example of remote interrogation, a laser of suitable power., when directed at the sensor, may cause the support medium to expand. A sensor of the invention may thus be light-addressable.
Other-forms of interrogation are not remote, and include direct application of pressure., e.g. manually or by the application of a weight., in compression or stretcnmg The hologram may be sensitive to a physical effect such as temperature, whether increased or reduced.
The property of the.support medium that varies may be its charge density, volume, shape, density, viscosity, strength, hardness, charge, hydrophobicity, sweliability, integrity, cross-link density or any other physical property. Variation of such a property causes a variation of an optical characteristic, such as polarisability, reflectance, refractance or absorbance of the holographic element. If any change occurs whilst the hologram is being replayed by incident broad band, non-ionising electromagnetic radiation, then an optical property varies and a colour or intensity change, for example, may be observed.
The property that varies is preferably the size or volume of the support medium. This may be achieved by incorporating into the support matrix., groups which cause an expansion or contraction of the support medium. The support medium preferably comprises a native or modified matrix with viscoelastic properties.
In a preferrec embodiment, the support medium comprises £ spiropyran grout, or a derivative thereof Spiropyrans, upon exposure to UV light, undergo a reversible conversion to merocyanine. This transtormation is accompanied by an increase in charge, and may cause the suopori medium to expand. The conversion can be reversed by interrogating the sensor with visible light or heat.
In order to achieve a magnetic transition in the support medium, the medium may comprise e ferromagnetic polymer, or one or more ''molecular magnets' Molecular magnets are known and can be synthesised as poiymensable monomers.
Tne holographic sensor may comprise a plurality of holographic elements, each element being sensitive to & different property. The holographic elements may be ir the form of an array
The hologram may be/for example, a transmission'or reflection hologram. In a reflection hologram, the fringes are parallel to s surface of the support medium; this causes rays to leave by the same surface at which the incident rays enter This fringe geometry provides s hologram that may be particularly sensitive to changes in temperature,
The sensitivity of the sensor to external physical interaction may be the consequence of the bulk properties of the sensor or support medium. Alternatively i; may be the consequence of chemical or other modification of the holographic element; this may be particularly appropriate for more sensitive reaction and/or to remote interrogation.
As illustrated in Example 2, alight-sensitive hologram can be fabricated by subjecting the sensor to interrogation by light, the properties of the hologram are altered in such a way that a change in an optical characteristic of the holographic element occurs. In Example 2, a particular bleach is used, but it will be understood that others are suitable. Modification of the bleaching protocol and/or the polymer backbone can -be used to modulate photosensitivity. Similarly, other physical influences can be used to modulate sensitivity and thus the response of the sensor.
Alternative light-sensitive holographic sensors may be fabricated using an azobenzene moiety which undergoes a trans to cis photoisomerization under UV interrogation This changes the free energy of mixing of the medium supporting
the Holographic image and therefore results in a change in the optical properties of tne associated hologram. Ye1, another approach involves the use of nphenyimethane leuco ayes incorporated into the holograms. Such dyes photo-dissociate in tne presence of UV light resulting in a charged species which causes the holographic matrix to swell, altering the holographic image.
Particularly for a laser-sensitive sensor the medium is preferably b polymer comprised of eiastomenc monomers. Alternatively, the Holographic medium may comprise a polymer such as poly(N-isopropyiacrylamicJe) ("NiPAMM") wnich has a conformation that is highly sensitive to changes in temperature As the temperature rises, the rigid polymer structure collapses, producing & significant shift in the wavelength of reflection
The invention is particularly relevant to security. For example, an authentication tag may comprise a holographic sensor having a built-in "message'. When the sensor is interrogated, for example, say, with a magnetic field, the sensor displays the "message". The "message" is preferably viewable directly by eye
Magnetic fields have the potential for use as security features. For example simply by passing a magnet over a hologram, tne resulting magnetic field would change the colour or image of the hologram, showing that it was genuine Furthermore, they could be used to create integrated magnetic strips (like tnose on credit cards) with holograms on them.
Pressure sensors could be used for food quality monitoring where a vacuum or a pressurised atmosphere is used to sea! a product and keep it fresh. Loss of the vacuum or pressured atmosphere would change the hologram. Also, tnis could be an authenticity feature where pressure from touch changes the colour or image to show that the hologram is genuine.
Other temperature-sensitive embodiments of the invention are a device that changes colour with the ambient temperature of, say, a fermentation system, or a replacement, power-free thermometer which can be interrogated from a distance using light. A security device which changes colour when it comes close to body temperature, e.g. when pressing a finger onto it, or part of an array of sensors m bacteria! or human diagnostics monitoring, are further embodiments
Trie sensor ma\ be sensitive tc an anaiyte whicr. is a chemical, biochemical or biological species Tne present invention relates to a method of detection oi anv such anaiyte in a sample, which comprises contacting the sample with the- sensor, and detecting any change of its optical characteristic.
Tne presen: invention also relates to an article comprising e sensor according to tne invention where the article ie a device such as a transaction card panKriote passport, identification care, smart card driving licence, share certificate bond, cheque, cheque card, tax Panderoie, gift voucher, postage stamp, raii or air ticket., telephone card, lottery card, event ticket, credit or debit care business card, or an item used in consumer, brand or product protection for the purpose of distinguishing genuine products from counterfeit products or identifying stolen products. The article can also be an item of intelligent packaging wnicn is a system that comprises a container, wrapper or enclosure to monitor, test or indicate product information on quality or environmental conditions that will affect product quality, shelf life or safety. Typical applications include indicators showing time-temperature, freshness, moisture, alcohol, gas, physical damage and the like.
The article can be an industrial or handicraft item comprising b decorative element, selected from items of jewellery, items of clothing (including footwear), fabric, furniture, toys, gifts, household items (including crockery and glassware), architecture (including glass, tile, paint, .metals, bricks, ceramics, wood, plastics and other internal and external installations), art (including pictures, sculpture, pottery and iignt installations), stationery (including greetings cards, letterheads and promotional material) and sporting goods. The article can be a product or device for use in agricultural studies, environmental studies, human or veterinary prognostics, theranostics, diagnostics, therapy or chemical analysis which can be a test strip, chip, cartridge, swab, tube, pipette, contact lens, sub-conjunctivai implant, sub-dermal implant, breathalyser, catheter or a fluid sampling or analysis device.
The invention also relates to a transferable holographic film comprising a sensor according to the invention. The film can be present on a hot stamping tape or can be used tc enhance the security of an article, by transferring onto the article the sensor from the film.
Trie preseni invention furtner relates to a product comprising a sensor of the invention which is capable of generating data and a system which uses such aata for date reading, processing, storage, control transmission, distributing, reporting and/or modelling. Such systems include mobile telephones personal digita; assistants and other portable electronic devices.
The following Examples illustrate the invention. Example "
/v-lsopropyiacrylamide ana /vyV-methylenebisacrylamide (MBA; in a molar ratio of 24 1 were dissolved in dimethyl sulphoxide (DMSO), to form a 40 wt% (of total monomer) solution. An appropriate amount of 5% 2,2-dimethoxy-2-phenyiacetophenone (DMPA) in methanol was then added to the monomer solution to give a final concentration of 0.24% DMPA.
80 ul aliquots of the monomer solution were polymerised for 60 minutes using UV light under siianised glass slides. Holograms were recorded in these polymers using the silver diffusion method.
The hologram was incubated in 3.7 ml 20 mM sodium phosphate-buffered saline (pH 6.5. 3Q°C; ionic strength = 5D mM) in a 4 ml-cuvette. The temperature of the cuvette was modulated by an external water bath and temperature was measured in the bulk solution.
Peak diffraction wavelengths were recorded from spectral date acquired at an acute angle. Ail measurements reflect the final peak signals at equilibrium. Results are shown, in Figure 1 of the accompanying drawings. Example_2
A light-sensitive holographic sensor was fabricated from a gelatin-based hologram. The resultant hologram was bleached using a Fe(ilt)-based formulation with 'KBr, to create photosensitive holographic fringes. The light-sensitive holographic sensor was then exposed to white light for 1000 min and the intensity of the light diffracted by the'hologram was monitored and recorded for the duration of the experiment, as shown in Figure 2 of the accompanying drawings. The intensity of the diffracted signal (reflectivity) was observed to decrease by 15% during the first 200 mm of exposure to white light. Furthermore, this effect was visible to the naked eye. Example 3
An acryiamide co-polymer with s 2:1 ratio of acrylarnide:methacryiamide and 5% o{ the cross-linker MBA was prepared. 200 pi of the monomer solution was polymerised onto & treated glass 'shoe to create a thick polymer layer. A noiogram was recorded within this polymer while it was soaking in a water bath using £ frequency doubled Nd:YAG laser, resulting in a green diffraction signal (replay < of 52£ nm in deionisec water The hologram was then covered with a second glass slide and clamped with a pair of G-clamps, spaced about 15 mm apan The diffraction signal (replays of the hologram in between the G-clamps changed to 526 nm once the clamps had been tightened, taking care not to crack the glass shoes. The pressure of the clamps on the hologram resulted in a contraction in the volume of the hologram, thus causing the diffraction signal to blue-shifi by a total of 3 nm. Example 4
A co-polymer of PANiCNQ, produced from polyaniline (PANi) and an acceptor molecule, tetracyanoquinodimethane (TCNQ) can be synthesized as described in W003/062305 and inZaidi-et al, Polymer 45 (2004) 5583-89, the contents of which are incorporated herein by reference. This gives rise to b polymer material that exhibits magnetic properties at room temperature. The polymer can be coated onto ,a glass surface and used as is or cross-linked by-diffusion of co-monomers containing cross-linkers into the PANiCNQ and polymerizing them. The PANiCNQ polymer can then be treated as a recording material using silver diffusion, as described in W095/26499, to produce a hologram. Alternatively, the monomer solution can be cross-linked using a UV laser, tc produce silver-free holograms, as described in W02004/081676.
As the entire polymer is magnetic, on interrogation with a magnetic field, the polymer can be made to expand or contract, thereby red-shifting or blue shifting respectively, the diffraction wavelength (replay) of the hologram when it is illuminated with white light. This could be used to quantify the magnetic field or to cause a change in the colour/image of the hologram in the presence of a suitable magnetic, field.

We claim:
1. A sensing method, which comprises:
subjecting a holographic sensor to an external physical interaction, wherein the sensor is a volume hologram recorded in a support medium that is directly sensitive to the external physical interaction, wherein the volume hologram produces a holographic image; and
observing a change in the holographic image;
wherein the external physical interaction is temperature, and wherein the change in the holographic image is a change of color or a change of image to show that the volume hologram is genuine;
wherein the support medium is a polymer that is highly sensitive to changes in temperature such that the polymer has a rigid structure which collapses upon an increase in temperature, thereby producing a significant shift in a wavelength of reflection of the polymer; and
wherein the volume hologram comprises a reflection hologram.
2. The method as claimed in claim 1, wherein the support medium comprises a polymer.
3. The method as claimed in claim 1, wherein the change in the holographic image is reversible.
4. The method as claimed in claim 3, wherein the change in the holographic image is a change of color or a change of image to show that the volume hologram is genuine for the purpose of authentication.
5. The method as claimed in claim 1, wherein the volume hologram is viewable under white light, UV light or infra-red radiation.
6. The method as claimed in claim 1, wherein the change in the holographic image is a change of color or a change of image to show that the volume hologram is genuine for the purpose of authentication.
7. The method as claimed in claim 1, wherein the polymer is poly(N-isopropylacrylamide).
8. A sensing method, which comprises:
subjecting a holographic sensor to an external physical interaction, wherein the sensor is a volume hologram recorded in a support medium that is directly sensitive to the external physical interaction, wherein the volume hologram produces a holographic image; and
observing a change in the holographic image;
wherein the external physical interaction is magnetism, and wherein the change in the holographic image is a change of color or a change of image to show that the volume hologram is genuine;
wherein the support medium is magnetic; and
wherein the volume hologram comprises a reflection hologram.

9. The method as claimed in claim 8, wherein the support medium comprises a polymer.
10. The method as claimed in claim 8, wherein the change in the holographic image is reversible.
11. The method as claimed in claim 8, wherein the volume hologram is viewable under white light, UV light or infra-red radiation.
12. The method as claimed in claim 8, wherein the change in the holographic image is a change of color or a change of image to show that the volume hologram is genuine for the purpose of authentication.
13. The method as claimed in claim 10, wherein the change in the holographic image is a change of color or a change of image to show that the volume hologram is genuine for the purpose of authentication.
14. The method as claimed in claim 8, wherein the support medium comprises a molecular magnet.
15. The method as claimed in claim 8, wherein the support medium comprises a ferro-magnetic polymer.
16. The method as claimed in claim 8, wherein the support medium comprises a ferri-magnetic polymer.
17. The method as claimed in claim 8, wherein the support medium is a copolymer of aniline and tetracyanoquinoline.