a holographic sensor generally comprises a holographic support medium and , disposed throughout the volume of the medium , a hologram . the support medium interacts with an analyte resulting in a variation of a physical property of the medium . this variation induces a change in an optical characteristic of the holographic element , such as its polarisability , reflectance , refractance or absorbance . if any change occurs whilst the hologram is being replayed by incident broad band , non - ionising electromagnetic radiation , then a colour or intensity change , for example , may be observed . there are a number of basic ways to change a physical property , and thus vary an optical characteristic . the physical property that varies is preferably the size of the holographic element . this may be achieved by incorporating specific groups into the support matrix , wherein these groups undergo a conformational change upon interaction with the analyte , and cause an expansion or contraction of the support medium . a group is preferably the specific binding conjugate of an analyte species . another method would be to change the active water content of the support medium . a holographic sensor may be used for detection of a variety of analytes , simply by modifying the composition of the support medium . the medium preferably comprises a polymer matrix the composition of which must be optimised to obtain a high quality film , i . e . a film having a uniform matrix in which holographic fringes can be formed . the matrix is preferably formed from the copolymerisation of ( meth ) acrylamide and / or ( meth ) acrylate - derived monomers , and may be cross - linked . in particular , the monomer hema ( hydroxyethyl methacrylate ) is readily polymerisable and cross - linkable . polyhema is a versatile support material since it is swellable , hydrophilic and widely biocompatible . other examples of holographic support media are gelatin , k - carageenan , agar , agarose , polyvinyl alcohol ( pva ), sol - gels ( as broadly classified ), hydro - gels ( as broadly classified ), and acrylates . further materials are polysaccharides , proteins and proteinaceous materials , oligonucleotides , rna , dna , cellulose , cellulose acetate , siloxanes , polyamides , polyimides and polyacrylamides . gelatin is a standard matrix material for supporting photosensitive species , such as silver halide grains . gelatin can also be photo - cross - linked by chromium iii ions , between carboxyl groups on gel strands . when the analyte is relatively large in relation to the dimensions of the pores in the polymer matrix and / or the polymer has little or no associated porosity , diffusion of the analyte into and throughout the matrix is inhibited . thus the sensor may become slower to respond to changes in analyte concentration . a sensor of the invention comprises a holographic support medium , which may be formed by the polymerisation of monomers or comonomers in the presence of an agent which produces a porous polymer matrix . the agent may be optimally selected for producing pores of a specific dimension . this is particularly relevant when the analyte is sterically bulky , e . g . large biological molecules such as haemoglobin . the pore - forming agent or porogen may be a liquid , gas or solid , e . g . of particles such as bicarbonate , carbonate or pvc . when solid particles are used , they are preferably insoluble in the polymerisation mixture , such that post - polymerisation they are still present in the matrix , from which they can subsequently be removed by reaction ( e . g . acid ), dissolution or rinsing . when the agent is a gas , this may be bubbled through the polymerisation mixture . an example of a pore - producing agent is water . by incorporating water into a monomer mixture , such as hema monomers , small pockets may be produced during the polymerisation process , resulting in a more porous polymer matrix . for example , the agent may be a non - solvent for the polymer . another example is a salt which can be present in high concentration during polymerisation . a metal alginate could be used , and removed by washing with edta / acid ( to take out the metal ) followed by dissolution . a protein or liquid may be removed enzymatically . the agent may also be removed by physical methods , e . g . laser irradiation or ablation . a uv absorber could be heated locally , using local differences in temperature to cause pore formation . the agent may be a gas , which could be generated in situ . electrolysis or physical movement may stimulate gas formation in a suitable system . if the matrix incorporates a solvent saturated with gas , removal of the solvent will generate bubbles . bubble formation may be stabilised by the presence of a surfactant such as pluronic . a polymer matrix was formed by polymerisation of hema monomers in water and 4 % methanol ( w / v ). for reference , a polymer was made by polymerisation of hema monomers in isopropanol . upon formation , each polymer was soaked for 2 hours in 50 mg / ml haemoglobin , and the respective absorption spectra were determined , as shown in fig1 . the absorption spectrum of a control solution of 0 . 25 mg / ml of haemoglobin is also shown for comparison . the presence of water in the polymerisation mixture resulted in a more porous polymer matrix . as shown in the absorption spectra , the increased porosity of the matrix allows greater diffusion of the relatively large haemoglobin molecules , producing an absorption correlating more closely to that of the haemoglobin solution . a pair of holographic polymer matrices were produced , each having a monomer composition of 70 % hema , 20 % ethylene dimethacrylate ( edma ) and 10 % methacrylic acid ( maa ). one of the polymers was produced by polymerisation of the monomers in propanol ; the other in water and 8 % methanol ( w / v ). holographic recording material was then disposed on each support , and the holograms recorded . the developed holograms were immersed in an analyte sample . the response times to increasing analyte concentration are shown in fig2 . the presence of water in the polymerisation mixture produced a microporous polymer structure , resulting in a more responsive holographic sensor .