Patent Publication Number: US-2020292489-A1

Title: Sensor apparatus and associated methods

Description:
TECHNICAL FIELD 
     The present disclosure relates particularly to sensors, associated methods and apparatus. Certain embodiments specifically concern an apparatus comprising a chemical and/or biological sensor. Some embodiments may relate to portable electronic devices, in particular, so-called hand-portable electronic devices which may be hand-held in use (although they may be placed in a cradle in use). Such hand-portable electronic devices include so-called Personal Digital Assistants (PDAs) and tablet PCs. 
     The portable electronic devices/apparatus according to one or more disclosed example aspects/embodiments may provide one or more audio/text/video communication functions (e.g. tele-communication, video-communication, and/or text transmission, Short Message Service (SMS)/Multimedia Message Service (MMS)/emailing functions, interactive/non-interactive viewing functions (e.g. web-browsing, navigation, TV/program viewing functions), music recording/playing functions (e.g. MP3 or other format and/or (FM/AM) radio broadcast recording/playing), downloading/sending of data functions, image capture function (e.g. using a (e.g. in-built) digital camera), and gaming functions. 
     BACKGROUND 
     Research is currently being done to develop new sensor devices. 
     The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. 
     SUMMARY 
     According to a first aspect, there is provided an apparatus comprising a pyroelectric layer, a two dimensional conductive channel and a floating gate, the apparatus configured such that the pyroelectric layer is capacitively configured with respect to each of the two dimensional conductive channel and the floating gate so that the two dimensional conductive channel and the floating gate can each act as respective capacitive plates for each respective, electrically connected, first and second portions of the pyroelectric layer, the respective first and second portions of the pyroelectric layer themselves configured to act as corresponding capacitive plates;
         the floating gate comprising electrically connected first and second portions, the first portion of the floating gate being in thermal proximity to the first portion of the pyroelectric layer, the second portion of the floating gate configured to overlie and gate flow of electrical charge through the two dimensional conductive channel by charge in the second portion of the floating gate,   wherein at least the first portion of the floating gate is functionalised to detect one or more proximal specific species, the detection of which gives rise to heat flow to or from the thermally proximal pyroelectric layer to allow the pyroelectric layer to generate an electrical signal dependent upon one or more of the presence and amount of the specific detected species.       

     The second portion of the floating gate may be separated from the two dimensional conductive channel by a dielectric layer configured to prevent electrical contact therebetween. 
     The dielectric layer may be a layer of native oxide formed on one or more of the second portion of the floating gate and the two dimensional conductive channel. 
     At least the first portion of the floating gate is functionalised to detect one or more chemical and/or biological species, for example by anchoring/attaching/tethering/conjugating/immobilizing a detector species to the exposed floating gate surface for reaction with a corresponding sample species. 
     The at least first portion of the floating gate may be functionalised to detect a plurality of different specific species. 
     The pyroelectric layer may be supported by two supporting legs at opposite sides of the pyroelectric layer to thermally isolate the pyroelectric layer. 
     The apparatus may comprise source and drain electrodes in electrical contact with the two dimensional conductive channel, the source and drain electrodes each connected between respective conductive paths associated with each of the supporting legs and the two dimensional conductive channel. 
     The apparatus may further comprise a border element located at the periphery of the pyroelectric layer, the border element configured to contain a liquid sample deposited on the apparatus. The border element may comprise a physical wall/barrier, or a (super)hydrophobic layer, for example. 
     The two dimensional conductive channel may comprise one or more of: graphene; graphene related materials (GRM), reduced graphene oxide, MOS 2 , phosphorene, silicon nanowires, carbon nanotubes, and also hybrid structures containing a combination of materials. 
     The at least first portion of the floating gate may be functionalised by one or more of: an enzyme, cholesterol oxidase, chymotrypsin, glucose oxidase, catalase, penicillinase, trypsin, amylase, invertase, urease, and uricase. The first portion of the floating gate may be functionalised to react with a corresponding sample species comprising one or more of: a protein, cholesterol, an ester, glucose, hydrogen peroxide, penicillin, a peptide, starch, sucrose, urea, and uric acid. A highly sensitive calorimetric device can be provided, particularly in these example cases. 
     The first and second portions of the pyroelectric layer may be:
         first and second portions of a common pyroelectric layer; or   respective separate electrically connected first and second pyroelectric layer elements.       

     That is, in some examples a single common pyroelectric layer/slab of material may be present in the apparatus, and in other example, at least two pyroelectric layers/slabs may be electrically connected together and used in an apparatus. 
     The area of the first portion of the floating gate may be one or more of: two times, three times, four times, five times, ten times, 20 times, 30 times, 50 times, 100 times and more than 100 times the area of the second portion of the floating gate. 
     At least the first portion of the floating gate may be functionalised by a proximal detector layer, and the detector layer may be configured to allow a plurality of reactions to take place with corresponding sample species. In some examples the detector layer may be configured to detect the same species over several separate sensing experiments/measurements, for example by comprising multiple layers of sensing species. In some examples the detector layer may be configured to detect different species over one or several separate sensing experiments/measurements, for example by comprising different types of detector species. 
     The apparatus may be electrically connected to and thermally isolated from a further such apparatus, apart from the at least first portion of the floating gate of the further apparatus not being functionalized. The apparatus and further apparatus together may be configured to form a potential divider. 
     The apparatus may be configured to detect the presence of a specific species at the functionalised first portion of the floating gate by allowing for a determination of a change of one or more of: thermal mass of the apparatus; optical absorbance of the apparatus; and reflectance of the apparatus, by using a controlled photon source to illuminate the apparatus. The controlled photon source may be configured to provide photons of a wavelength corresponding to an expected absorption resonance of a specific detected species. 
     The apparatus may further comprise a filter coating configured to allow one or more specific wavelengths of light from the controlled photon source to reach the specific species (and thereby block the passage of one or more other specific wavelengths of light from reaching the specific species). 
     According to a further aspect, there is provided a method comprising:
         for an apparatus comprising a pyroelectric layer, a two dimensional conductive channel and a floating gate, the apparatus configured such that the pyroelectric layer is capacitively configured with respect to each of the two dimensional conductive channel and the floating gate so that the two dimensional conductive channel and the floating gate can each act as respective capacitive plates for each respective, electrically connected, first and second portions of the pyroelectric layer, the respective first and second portions of the pyroelectric layer themselves configured to act as corresponding capacitive plates, the floating gate comprising electrically connected first and second portions, the first portion of the floating gate being in thermal proximity to the first portion of the pyroelectric layer, the second portion of the floating gate configured to overlie and gate flow of electrical charge through the two dimensional conductive channel by charge in the second portion of the floating gate, wherein at least the first portion of the floating gate is functionalised to detect one or more proximal specific species, the detection of which gives rise to heat flow to or from the thermally proximal pyroelectric layer to allow the pyroelectric layer to generate an electrical signal dependent upon one or more of the presence and amount of the specific detected species;   detecting the presence of a specific species proximal to the apparatus by measuring the electrical signal from the apparatus.       

     The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated or understood by the skilled person. 
     Corresponding computer programs for implementing one or more steps of the methods disclosed herein are also within the present disclosure and are encompassed by one or more of the described example embodiments. 
     In a further example there is provided a computer readable medium comprising computer program code stored thereon, the computer readable medium and computer program code being configured to, when run on at least one processor, control the operation of an apparatus, the apparatus comprising:
         a pyroelectric layer, a two dimensional conductive channel and a floating gate, the apparatus configured such that the pyroelectric layer is capacitively configured with respect to each of the two dimensional conductive channel and the floating gate so that the two dimensional conductive channel and the floating gate can each act as respective capacitive plates for each respective, electrically connected, first and second portions of the pyroelectric layer, the respective first and second portions of the pyroelectric layer themselves configured to act as corresponding capacitive plates,   the floating gate comprising electrically connected first and second portions, the first portion of the floating gate being in thermal proximity to the first portion of the pyroelectric layer, the second portion of the floating gate configured to overlie and gate flow of electrical charge through the two dimensional conductive channel by charge in the second portion of the floating gate,   wherein at least the first portion of the floating gate is functionalised to detect one or more proximal specific species, the detection of which gives rise to heat flow to or from the thermally proximal pyroelectric layer to allow the pyroelectric layer to generate an electrical signal dependent upon one or more of the presence and amount of the specific detected species;   the control providing for:
           detection of the presence of a specific species proximal to the apparatus by measuring the electrical signal from the apparatus.   
               

     One or more of the computer programs may, when run on a computer, cause the computer to configure any apparatus or device disclosed herein or perform any method disclosed herein. One or more of the computer programs may be software implementations, and the computer may be considered as any appropriate hardware, including a digital signal processor, a microcontroller, and an implementation in read only memory (ROM), erasable programmable read only memory (EPROM) or electronically erasable programmable read only memory (EEPROM), as non-limiting examples. The software may be an assembly program. 
     One or more of the computer programs may be provided on a computer readable medium, which may be a physical computer readable medium such as a disc or a memory device, or may be embodied as a transient signal. Such a transient signal may be a network download, including an internet download. 
     The present disclosure includes one or more corresponding aspects, example embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. Corresponding means for performing one or more of the discussed functions are also within the present disclosure. 
     The above summary is intended to be merely exemplary and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A description is now given, by way of example only, with reference to the accompanying drawings, in which:— 
         FIG. 1 a    shows an apparatus according to examples described herein; 
         FIG. 1 b    shows a cross section through a portion of the apparatus of  FIG. 1   a;    
         FIG. 2  shows an equivalent schematic circuit diagram for the apparatus of  FIG. 1   a;    
         FIG. 3  shows an example graph of current versus time measured from an apparatus as shown in  FIG. 1   a;    
         FIG. 4  shows a schematic example of a functionalised first portion of a floating gate according to examples described herein; 
         FIG. 5  shows a further apparatus according to examples described herein; 
         FIG. 6  shows a method according to examples described herein; and 
         FIG. 7  shows a computer-readable medium comprising a computer program configured to perform, control or enable a method described herein. 
     
    
    
     DESCRIPTION OF SPECIFIC ASPECTS/EMBODIMENTS 
     In conventional biosensing technology, temperature changes may be determined using thermistors at the ends of packed bed columns containing immobilised enzymes at a constant temperature. Using such a system up to 80% of the heat generated in a reaction between a sample and the detector species in the packed bed columns may be registered as a temperature change in the sample stream. The temperature change can be calculated from the enthalpy change and the amount of sample reacted. For example, if a 1 mM reactant/sample is completely converted to product in a reaction generating 100 kJ mole −1  then each ml of sample solution generates 0.1 J of heat. At 80% efficiency, this will cause a change in temperature of around 0.02° C. This level of temperature change is typical of biological reactions, and requires a detector temperature resolution of 0.0001° C. for the biosensor to be generally useful. 
     The heat output (molar enthalpies) of some example enzyme catalysed reactions are as follows: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                   
                 Heat output- 
               
               
                   
                 Reactant 
                 Enzyme 
                 □H(kJ mole− 1 ) 
               
               
                   
                   
               
             
            
               
                   
                 Cholesterol 
                 Cholesterol oxidase 
                 53 
               
               
                   
                 Esters 
                 Chymotrypsin 
                  4-16 
               
               
                   
                 Glucose 
                 Glucose oxidase 
                 80 
               
               
                   
                 Hydrogen peroxide 
                 Catalase 
                 100  
               
               
                   
                 Penicillin G 
                 Penicillinase 
                 67 
               
               
                   
                 Peptides 
                 Trypsin 
                 10-30 
               
               
                   
                 Starch 
                 Amylase 
                  8 
               
               
                   
                 Sucrose 
                 Invertase 
                 20 
               
               
                   
                 Urea 
                 Urease 
                 61 
               
               
                   
                 Uric acid 
                 Uricase 
                 49 
               
               
                   
                   
               
            
           
         
       
     
     It can be seen that a particular reactant/sample and enzyme/detector pair reacts with a particular heat output. Thus, if the heat output is accurately determined, then the nature of the reaction can be determined. 
     Conventional calorimetric sensors often use hotplates or chemical reactions, where thermistors are used to detect the difference in temperature. The detection of the change in temperature can be limited by the thermistor sensitivities. By using a pyroelectric device, the detection sensitivity may be significantly improved. 
     calorimetric biosensor may also encounter difficulties in closely matching the characteristic temperature constants of the measurement and reference thermistors. An equal movement of only 1° C. in the background temperature of both thermistors can cause an apparent change in the relative resistances of the thermistors equivalent to 0.01° C., which in many cases is a temperature change as large as the temperature change trying to be detected due to a reaction. It is clearly of great importance that environmental temperature changes are avoided as far as possible. 
     Isothermal microcalorimetry (IMC) is a laboratory method for real-time monitoring and dynamic analysis of chemical, physical and biological processes. Over a period of hours or days, IMC can be used to determine the onset, rate, extent and energetics of processes for specimens in small ampoules (e.g. 3-20 ml) at a constant set temperature (around 15° C.-150° C.). However, this can be a cumbersome process and does not reliably provide high level accuracy and high resolution detection within a short period of time (e.g., a few seconds). 
     There will now be described an apparatus and associated methods that may address one or more of the abovementioned issues. 
     Apparatus disclosed herein may be considered to provide a way of performing chemical and/or biological analysis using calorimetric principles. A two-dimensional conductive channel (for example, a graphene channel) is used in an individual pyroelectric detector/apparatus. A “floating gate” structure is used, where a portion of the apparatus is functionalized to trigger a chemical or biological reaction and transduce the resulting release of heat with area-dependent gain. Also described in a method to actively probe the occurrence of a reaction by, for example, illuminating the system with a controlled source of photons. 
     The apparatus  100  uses a calorimetric transducer based on a field effect transistor fabricated on a pyroelectric material. To obtain a high temperature change sensitivity, in some examples the apparatus may be fabricated on a “suspended” apparatus comprising a thin pyroelectric layer mounted on two support members/legs to help ensure that, for a fixed amount of heat delivered to the apparatus (by a reaction taking place on the apparatus), the resulting temperature change predominantly occurs within the apparatus and is induced in the apparatus due to its small thermal mass and low thermal conductivity to the substrate. 
     The sensing mechanism relies on the production/absorption of heat (i.e. heating or cooling). The heat is delivered by a chemical or biological reaction which occurs when a particular analyte (sample species) is present in the sample under consideration. To efficiently deliver the heat to the pyroelectric layer of the apparatus, the reaction takes place on the apparatus itself. If a large proportion (e.g., 80% or more) of the surface of the pyroelectric layer is overlaid with a conductive floating layer/gate/pad, then the reaction should take place on the pad. In order to achieve this, the pad is functionalised or “activated” by the presence of detector species (e.g., enzymes in the case of a biological reaction) to trigger the reaction with the analyte/sample species sought. The reaction may be, for example, a chemical binding, a chemical dissociation, a redox reaction, or other reaction; the only requirement is that the reaction brings about a hear transfer (i.e. it is exothermal or endothermal) so that the heat can be transferred to the pyroelectric substrate via the pad and provide a temperature change. 
       FIG. 1  illustrates an example apparatus  100 . Such an apparatus may also be called a pixel in that it is a standalone sensing element. A plurality of pixels may be connected together and used as a sensing array in some examples (one or more (e.g. groups of) pixels in the array may or may not be configured to be functionalised with respect to the same or different proximal species). The apparatus of each pixel may in some examples have an uppermost surface area of around 20×20=400 μm 2 . The pyroelectric layer may in some examples have a thickness of around 0.5 μm. The apparatus may be supported off an underlying substrate at a distance of around 2.5 μm. The apparatus  100  comprises a pyroelectric layer  102 , a two dimensional conductive channel  114  and a floating gate  104 . The two dimensional conductive channel  114  may comprise one or more of graphene; graphene related materials (GRM); reduced graphene oxide, MOS 2 , phosphorene, silicon nanowires, carbon nanotubes, and also hybrid structures containing a combination of materials. In this example the floating gate  104  comprises two first portions  104   a , and a second portion  104   b , which are electrically connected in an “H” shape, with the second portion  104   b  forming the cross-bar of the “H”. Of course other geometries may be used. 
     In this embodiment the first portions  104   a  of the floating gate  104  are directly overlying (and in physical contact with) the first portions  102   a  of the pyroelectric layer  102 , i.e. they are in thermal proximity to the first portions  102   a  of the pyroelectric layer  102 . The physical contact/thermal proximity between the floating gate and pyroelectric layer here acts to facilitate efficient heat transfer from the functionalised floating gate  104  on which a reaction takes place with a sample species, and the pyroelectric layer  102  which changes its physical properties due to a change in temperature/heat transfer. The arrangement of the first portion  104   a  of the floating gate  104  directly overlying the first portion  102   a  of the pyroelectric layer  102  forms a capacitor (see  FIG. 2 ). 
     A charge build-up at the surface of the pyroelectric layer  102 ,  202 , for example due to a change in temperature of the pyroelectric layer  102 ,  202 , cannot flow from the pyroelectric layer  102 ,  202  to the first portion  104   a ,  204   a  of the floating gate  104 ,  204  because the built-up charges are bound to the pyroelectric layer  102 ,  202 . Thus the pyroelectric layer acts as a first plate of a capacitor C 3 , as well as a dielectric/insulator of the capacitor C 3  because the flow of (bound) charge from the pyroelectric layer  102 ,  202  is prevented due to the nature of the charges in the pyroelectric layer  102 ,  202 . The first portion  104   a ,  204   a  of the floating gate  104 ,  204  acts as a second capacitor plate C 3 . 
     A second portion  104   b ,  204   b  of the floating gate  104 ,  204  is configured to overlie the second portion  102   b ,  202   b  of the pyroelectric layer  102 ,  202 , and is configured to overlie and gate flow of electrical charge through the two dimensional conductive channel  114 ,  214  by charge in the second portion  104   b ,  204   b  of the floating gate  104 ,  204 . Between the second portion  104   b ,  204   b  of the floating gate  104 ,  204  and the second portion  102   b ,  202   b  of the pyroelectric layer  102 ,  202  are located a two dimensional conductive channel  114 ,  214  and a dielectric layer  112 ,  212 . 
       FIG. 1 b    shows a cross section through the centre of the apparatus  100  in the region of the second portion  104   b  of the floating gate  104 . It can be seen that the two dimensional conductive channel  114  directly overlies the second portion  102   b  of the pyroelectric layer  102 , and the dielectric layer  112  is located between the two dimensional conductive channel  114  and the overlying second portion  104   b  of the floating gate  104 . Also shown are source and drain contacts  108 ,  110  with the two dimensional conductive channel  114  located therebetween. 
     Again with reference to  FIG. 2 , the dielectric layer  112 ,  212  prevents direct electrical contact between the second portion  104   b ,  204   b  of the floating gate  104 ,  204  and the underlying two dimensional conductive channel  114 ,  214 , thereby forming a capacitor arrangement C 2  with the dielectric layer  112 ,  212  acting as a dielectric/insulator between the conductive second portion  104   b ,  204   b  of the floating gate  104 ,  204  (a first plate of the capacitor C 2 ) and the underlying two dimensional conductive channel  114 ,  214  (a second plate of the capacitor C 2 ). 
     The dielectric layer  112 ,  212  may be formed by depositing (e.g. using atomic layer deposition) aluminium oxide, hafnium oxide, Group III oxides, carbon nano-membrane, and any conventional oxide such as silicon dioxide. In some cases, however, one or more of the second portion  104   b ,  204   b  of the floating gate  104 ,  204  and the two dimensional conductive channel  114 ,  214  may be formed from a material (e.g. a 2D material such as hexagonal boron nitride) which forms a native oxide on exposure to air/oxygen in the surrounding environment. In this scenario, there is no need to deposit a separate dielectric layer  112 ,  212  provided the native oxide is sufficient to prevent electrical contact between the second portion  104   b ,  204   b  of the floating gate  104 ,  204  and the two dimensional conductive channel  114 ,  214 . 
     The arrangement of the two dimensional conductive channel  114 ,  214  directly overlying the second portion  102   b ,  202   b  of the pyroelectric layer  102 ,  202  also forms a capacitor C 1 . A charge build-up at the surface of the pyroelectric layer  102 ,  202  cannot flow from the pyroelectric layer  102 ,  202  to the two dimensional conductive channel  114 ,  214  because the built-up charges are bound to the pyroelectric layer  102 ,  202 . Thus the pyroelectric layer  102 ,  202  acts as a first plate of a capacitor C 1 , as well as a dielectric/insulator. The two dimensional conductive channel  114 ,  214  acts as a second capacitor plate of capacitor C 1 . 
     Therefore, it can be said that the apparatus  100  is configured such that the pyroelectric layer  102 ,  202  is capacitively configured with respect to each of the two dimensional conductive channel  114 ,  214  and the floating gate  104 ,  204  so that the two dimensional conductive channel  114 ,  214  and the floating gate  104 ,  204  can each act as respective capacitive plates for each respective, electrically connected, first  102   a ,  202   a  and second  102   b ,  202   b  portions of the pyroelectric layer  102 ,  202 . The respective first  102   a ,  202   a  and second  102   b ,  202   b  portions of the pyroelectric layer  102 ,  202  are themselves configured to act as corresponding capacitive plates. In this example, the pyroelectric layer  102 ,  202  is notionally divided into a first portion  102   a ,  202   a  which underlies the floating gate  104 ,  204  (in particular, directly underlies the first portion  104   a ,  204   a  of the floating gate  104 ,  204 ) and a second portion  102   b ,  202   b  which underlies the two dimensional conductive channel  114 ,  214  (in the central region of the apparatus  100  as shown in  FIG. 1 ). 
     At least the first portion  104   a  of the floating gate  104  is functionalised to detect one or more proximal specific species. The detection of specific species, such as species present in a liquid sample deposited on the apparatus  100  or gaseous species present in the atmosphere/environment around the apparatus  100 , involves the specific species interacting with the detector species used to functionalise the floating gate  104 . The reactions give rise to heat flow to or from the thermally proximal pyroelectric layer  102 . This heat flow ultimately allows the pyroelectric layer  102  to generate an electrical signal dependent upon one or more of the presence and amount of the specific detected species. 
     The functionalisation may be present as one or more detector species attached to at least the first portion  104   a  of the floating gate  104 . For example, one or more of: an enzyme, cholesterol oxidase, chymotrypsin, glucose oxidase, catalase, penicillinase, trypsin, amylase, invertase, urease, and uricase may be attached to the first portion  104   a  of the floating gate  104  for reaction with a corresponding sample species. Such sample species include, for example: a protein, cholesterol, an ester, glucose, hydrogen peroxide, penicillin, a peptide, starch, sucrose, urea, and uric acid. A highly sensitive calorimetric apparatus can be provided particularly in these use cases. 
     In the example shown in  FIG. 1  the functionalisation is illustrated schematically by detector species  106  bound to the upper surface of the floating gate  104  (for example, each functionalisation region  106  may be imagined to be a particular detector species grafted/anchored to the floating gate  104  surface). 
     In some example, one type of detector species  106  may be used for an apparatus  100  which is sensitive to one particular sample species. In other examples, different types of detector species  106  may be used for an apparatus  100  which is sensitive to sensing a plurality of different particular sample species. This is illustrated schematically in  FIG. 4 , in which three types of detector species  406   a ,  406   b  and  406   c  are arranged on the surface of the floating gate  404 , each configured to detect a particular sample species. Such an apparatus configured to detect more than one type of sample species may be termed a “multi-parametric (bio)sensor”. Each detector species  406   a ,  406   b  and  406   c  may represent a different detector species bound to a floating gate. 
     In some examples, there may be an array of interconnected apparatus  100  and  FIG. 4  may be taken to schematically represent different apparatus  100  each functionalized with a particular detector molecule/receptor. In some examples each apparatus  100  in the array may be functionalised in the same way. In other examples there may be different apparatuses functionalised in different ways to respond to different sample species. A response to detecting a reaction for such an array of apparatus  100  may be measured as the net temperature change over the whole array when a specific sample molecule interacts with the separate apparatus  100  (or a plurality of specific molecules interact with correspondingly functionalised apparatuses  100 ). 
     In some examples the at least a first portion  104   a  of the floating gate  104  may be functionalised by a proximal detector layer. Such a layer may in some examples comprise a multilayer thickness of detector molecules overlaying the floating gate  104  to allow for multiple reaction events to take place using the apparatus  100 . Such a detector layer may be configured to allow only one type of sample species to be detected (if only one type of detector molecule is present in the detector layer). In other examples the detector layer may be configured to allow for a plurality of different reactions to take place with corresponding sample species (for example if different regions of the detector layer comprise different detector species configured to react with different particular sample species). For an example comprising a detector layer of multilayer thickness, the apparatus  100  may be used a plurality of times by, for example, performing a sensing experiment which consumes an upper layer of the detector layer, then removing the used upper layer to reveal a fresh sensing layer underneath for a subsequent sensing experiment. 
     The apparatus  100  in  FIG. 1  is arranged such that the pyroelectric layer  102  is supported by two supporting legs  120  at opposite sides (in this example at diagonally opposite corners) of the pyroelectric layer  102 . By suspending/supporting the apparatus  100  away from an underlying substrate to thermally isolate the pyroelectric layer  102  (for example, without the supporting legs  120 ) the apparatus  100  would be resting on a surface with the bottom surface of the pyroelectric layer  102  in substantially full contact with the underlying surface. Such thermal isolation improves measurement accuracy by reducing any change in temperature being detected which is not due to detection of a specific sample species (for example, by environmental temperature changes heating/cooling the surface which would also change the temperature of the apparatus if it was in substantially full contact with the surface, or by heat transfer due to the detection of a specific sample species being further transferred away from the apparatus to the underlying surface). 
     In some examples, the apparatus  100  may further comprise a border element (not shown) located at the periphery of the apparatus  100  or at the periphery of the pyroelectric layer  102 . The border element may be configured to contain a liquid sample deposited on the apparatus  100  and prevent it running off the surface of the apparatus  100 . For example, if water-based solutions containing sample species were to be analysed using the apparatus, then a hydrophobic or superhydrophobic layer may be located as a border element around the outside of the upper surface of the pyroelectric layer  102  to prevent the water-based solution running off the sides of the apparatus  100 . As another example, a physical border element, such as a wall, texturing, or other physical barrier element/container may be present to contain a liquid sample on the sensing (upper) surface of the apparatus  100 . 
     In this example the first  102   a  and second  102   b  portions of the pyroelectric layer  102  are first and second portions  102   a ,  102   b  of a common pyroelectric layer  102 . In other examples the first  102   a  and second  102   b  portions of the pyroelectric layer  102  may be respective separate electrically connected first and second pyroelectric layer elements. 
     When a sample species reacts with a detector species  106  anchored/attached to a conductive first portion/region of the floating gate  104   a , a reaction takes place which results in a transfer of heat (an increase in heat energy transferred to the apparatus for an exothermic reaction, or a increase in heat energy transferred away from the apparatus for an endothermic reaction). For an exothermic reaction, the heat transfer is away from the reaction site into the attached first portion  104   a  of the floating gate  104 , and from there into the underlying pyroelectric layer  102 , thereby increasing the temperature of the pyroelectric layer  102 . For an endothermic reaction, the heat transfer is towards the reaction site from the attached first portion  104   a  of the floating gate  104 , in turn from the underlying pyroelectric layer  102 , thereby decreasing the temperature of the pyroelectric layer  102 . 
     When the pyroelectric layer  102  undergoes a change in temperature its crystal structure changes, giving rise to a spontaneous internal separation of change (charge polarisation) within the pyroelectric crystal  102 . This charge separation is bound, meaning that the charges which arise are not free to flow but are bound/fixed within the crystal structure of the pyroelectric layer  102 . The charge separation causes a surface charge density and a corresponding electric field to form at the surface of the pyroelectric layer  102  proximal to the overlying floating gate  104 , 
     Because the surface charge density at the surface of the pyroelectric layer  102  is bound, the overlying first portion  104   a  of the floating gate  104 , can be in direct physical contact with the (insulating) associated first portion  102   a  of the pyroelectric layer  102  and a current cannot flow in-between because the charged species are not free to flow (i.e. there is no electrical short if the first portion  104   a  of the floating gate  104  is directly in contact with the first portion  102   a  of the pyroelectric layer  102 ). The pyroelectric layer  102  and the overlying first portion  104   a  of the floating gate  104  act as a capacitor, which behaves as if there is an insulating layer between the pyroelectric layer (one plate of the capacitor) and the first portion  104   a  of the floating gate  104  (the other plate of the capacitor) even though the two layers  102 ;  104   a , are in direct contact, because the bound charge cannot flow between the pyroelectric layer  102  and the first portion  104   a  of the floating gate  104 . 
     The first portion  104   a  of the floating gate  104  acts to screen the surface charges present at the surface of the pyroelectric layer  102  (that is, the first portion  104   a  of the floating gate  104  acts to balance out the charge “imbalance” due to the surface charge density) by opposite charges moving towards the underlying surface of the first portion  104   a  of the floating gate  104  closest to the pyroelectric layer  102  (the “inside” surface of the capacitor). The first portion  104   a  of the floating gate  104  is electrically isolated other than an electrical connection to the second portion  104   b  of the floating gate  104 . Thus for charges to form at the first portion  104   a  of the floating gate  104  to balance out the surface charge density at the pyroelectric layer  102 , the first portion  104   a  of the floating gate  104  draws charge from the second portion  104   b  of the floating gate  104  as this is the only charge reservoir available. The second portion  104   b  of the floating gate  104  also acts effectively like a capacitor plate in this apparatus, the capacitor being formed from the second portion  104   b  of the floating gate  104  as a first plate, and the two dimensional conductive channel  114  as a second plate, with a dielectric layer  112  in-between to prevent direction electrical contact between the (conducting) second portion  104   b  of the floating gate  104  and the two dimensional conductive channel  114 . 
     Because charge has been drawn from the second portion  104   b  of the floating gate  104  by the first portion  104   a  of the floating gate  104 , the capacitor formed from the first portion  104   a  of the floating gate  104  and the two dimensional conductive channel  114  acts to balance the change in charge by drawing charge from the two dimensional conductive channel  114 . The two dimensional conductive channel  114  is in electrical contact with source and drain contacts  108 ,  110 , and thus an electrical current flows between the source and drain contacts  108 ,  110  as charge is drawn from the two dimensional conductive channel  114 . Thus, overall, the apparatus  100  is configured such that the reaction of a sample species with a detector species gates the channel  114  of the apparatus  100 . The resulting current flow can be measured using external contacts  118  connected to the source and drain electrodes  108 ,  110 . 
     If there was no two dimensional conductive channel  114  from which to draw charge, then the first portion  104   a  of the floating gate  104 , would draw charge from the second portion  104   b  of the floating gate  104  acting as a capacitive plate with the surrounding air (i.e. the second portion  104   b  of the floating gate  104  would act as a one-plate capacitor). In principle in the apparatus  100 , an electrical field forms at the second portion  104   b  of the floating gate  104  with the surrounding air. However, because of the two dimensional conductive channel  114  which forms a parallel plate capacitor with the second portion  104   b  of the floating gate  104 , the capacitance of the “one-plate capacitor” of the second portion  104   b  of the floating gate  104  with the air is negligible, and charge is drawn from the two dimensional conductive channel  114 . 
     Consider the circuit diagram in  FIG. 2  representing the apparatus of  FIG. 1 a   . The pyroelectric layer  202  provides two possible capacitor plates  202   a ,  202   b  for capacitor C 3  and capacitor C 1  respectively (and the associated intervening insulating layer of the capacitor due to charge building up at the pyroelectric layer surface being bound to the pyroelectric layer and thus unable to flow as free charge). The two dimensional conductive channel  214  is also illustrated and provides a capacitor plate  214  for capacitors C 1  and C 2 . The first portion  204   a  and the second portion  204   b  of the floating gate  204  each form a complementary capacitor plate  204   a ,  204   b . The first portion  204   a  of the floating gate  204  forms a capacitor plate  204   a  with the capacitor plate  202   a  provided by the pyroelectric layer  202  to form a capacitor C 3 . The second portion  204   b  of the floating gate  204  forms a capacitor plate  204   b  with the capacitor plate  214  provided by the two dimensional conductive channel  214  having a dielectric layer  212  in between to form a capacitor C 2 . 
     If there was no dielectric  212  present in the apparatus to electrically separate the second portion  204   b  of the floating gate  204  from the two dimensional conductive channel  214  then the apparatus would only have a capacitance due to the capacitor C 1  formed by the capacitor plate  202   b  provided by the pyroelectric layer  202  and the capacitor plate  214  provided by the two dimensional conductive channel  214 . 
     At a particular temperature, the pyroelectric layer  202  produces a fixed amount of charge per unit area, indicated as σ(T)  222 . The electrostatic potential V 3  generated at C 3  does not depend on the geometry of C 3  (V 3 =Q 3 /C 3  with Q 3 =Δ(T)×Area(C 3 ). If the area of C 3  doubles, both Q 3  and C 3  double and V 3  stays constant). However, the charge Q 3  needed at C 3  to screen/balance the pyroelectric charge comes from C 2 , because the second portion  204   b  of the floating gate  204  forming a plate of the capacitor C 2  is a “floating” gate with no access to an external charge reservoir. For capacitors in series, Q 2 =Q 3 , and therefore the gate potential applied to the two dimensional conductive channel  214  is V 2 =Q 2 /C 2 =Q 3 /C 2 =V 3 ×C 3 /C 2 . So, the apparatus acts to amplify the natural gate voltage V 1  at capacitor C 1  with an additional gate voltage V 2  that scales with the capacitance ratio C 3 /C 2 . If the area of the first portion  204   a  of the floating gate  204  forming the capacitor plate  204   a  in direct contact with the pyroelectric substrate  202  is much larger than the overlap of the second portion  204   b  of the floating gate  204  with the two dimensional conductive channel  214 , a large C 3 /C 2  ratio, and a corresponding high temperature change sensitivity, is achieved. 
     Therefore better sensitivity to temperature change is achieved by increasing the ratio of C 3  to C 2 ; that is by having a very small overlap of second portion  204   b  of the floating gate  204  with the two dimensional conductive channel  214  to form the capacitor C 2 , and a very large first portion  204   a  of the floating gate  204  forming the capacitor plate  204   a , covering as much of the surface of the pyroelectric layer  202  as possible, to form the capacitor C 3 . 
     Thus in some examples, the area of the first portion  104   a ,  204   a  of the floating gate  104 ,  204  may be one or more of: two times, three times, four times, five times, ten times, 20 times, 30 times, 50 times, 100 times and more than 100 times the area of the second portion  104   b ,  204   b  of the floating gate  104 ,  204 . Theoretically, the larger the ratio of the area of the first portion  104   a ,  204   a  to the area of the second portion  104   b ,  204   b  is, the greater the “voltage amplification” effect of the apparatus  100  and the more sensitive the apparatus  100  is to changes in temperature. 
     The apparatus  100  may be considered to be a sensing pixel. A great advantage of calorimetric sensors based on pyroelectric materials such as apparatus  100  described above is that the sensitivity scales with the area of the apparatus  100 . While the temperature change induced in the apparatus  100  does not differ if one doubles its area (there is twice as much energy delivered to heat twice as much mass), the charge collected by the floating gate pad  104 ,  204  doubles, and thus the resistance change in the two dimensional conductive channel  114 ,  214  also doubles. One can thus increase the sensitivity (almost) at will at the expense of device area. For a chemical sensor or biosensor as described above, a single apparatus the size of a liquid droplet is perfectly realistic. 
     The inventors have experimentally measured that for a 300 μm×300 μm apparatus on z-cut LiNbO 3  the thermal coefficient of resistance (TCR) can reach values up to 150%/K as shown in  FIG. 3  which illustrates a change in current with time during a temperature change of an apparatus between 20° and 21. With a noise floor of about 0.5%, with the apparatus a minimum ΔT of ˜0.005° C. could be detected. Therefore to reach a 0.0001° C. resolution required the area needs to be increased by 50 times, i.e., using an apparatus 2.1 mm×2.1 mm in size. Such a size is still perfectly compatible with biosensing and chemical sensing applications. 
     The floating gate  104 ,  204  may be electrically conductive (e.g., so that it can act as a capacitor plate) and at the same time offer a good anchor point for receptor/detector species functionalization. For some reactions/receptors, metals such as gold, platinum and AgCl or other conductive materials such as graphene are suitable. If the desired receptor/detector species does not bind, or does not bind well, directly to the floating gate, then the floating gate may be coated a suitable buffer layer in order to increase the adhesion of the detector species to the floating gate pad. The buffer layer should not increase the overall thermal mass of the apparatus too much and should also possess good thermal conductivity to allow heat transfer between the reacting detector and sample species and the pyroelectric layer  102 ,  202  of the apparatus  100 . For some reactions, a charge transfer may occur between the reactants (i.e. sample and detector species) and the floating gate. Charge transfer into or out from the floating gate should be avoided to maintain a system acting as capacitive elements as illustrated in  FIG. 2 . To prevent charge transfer to/from the floating gate  104 .  204 , a thin insulating barrier may be deposited on the floating gate  104 ,  204 . 
     The table below indicates some pyroelectric materials which may be suitable for use in apparatus as described here for the pyroelectric layer  102 ,  202 . The choice of the pyroelectric material  102 ,  202  may be important for good operation of the apparatus. The table below shows some properties of common pyroelectric materials. A high pyroelectric coefficient may be desirable for high sensitivity, but one must also take into account other important features such as low internal leakage, and possible compatibility with CMOS electronics (e.g., aluminium nitride AlN exhibits a poor pyroelectric coefficient, but it is a highly compatible material with Si processing). 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Substrate 
                 Pyroelectric 
                   
                 Internal 
               
               
                 material 
                 coefficient (μC m −2  K −1 ) 
                 Implementation and cost 
                 discharge 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 LiTaO3 
                 230 
                 Crystal film (costly) 
                 Low 
               
               
                 LiNbO3 
                 120 
                 Crystal film (costly) 
                 Low 
               
               
                 AlN 
                 8 
                 Crystal film (less costly) 
                 Low 
               
               
                 ZnO 
                 7 
                 Crystal film (less costly) 
                 High 
               
               
                 PVDF 
                 27 
                 Flexible thin sheet (cheap) 
                 Low 
               
               
                 HydroFluoro- 
                 Unknown, but theory 
                 2D materials system 
                 Unknown 
               
               
                 Graphene 
                 suggests &gt; 0 
                 (eventually cheap) 
               
               
                   
               
            
           
         
       
     
     Apparatus as described herein have been experimentally tested to operate reliably over a period of several minutes and so are able to operate in DC mode and are suitable for monitoring “slow” reactions, such as biological reactions which can occur over a minute or more. 
     In some examples, the apparatus may be considered to be configured such that the “amplification” portion of the apparatus (i.e. the two dimensional conductive channel region  114 ,  214  shown in cross section in  FIG. 1 b   ) is integrated directly with the pyroelectric layer, rather than being a separate but attached element. While for a single-apparatus device (i.e. one apparatus) the cost-effectiveness importance is moderate, the apparatus architecture described herein provides advantages for medium-sized arrays (between around 20 to around 100 apparatus in an array) and larger. As many (bio)sensing solutions aim at multi-parametric analysis, namely that several analytes/samples are tested at the same time, the apparatus described herein can perform such multi-sample species detection. As described, the apparatus can be tailored to undergo multiple reactions by changing the functionalization within an apparatus or an array of apparatus. Further, the apparatus described herein can combine a common highly-sensitive calorimetric transducer with integrated readout electronics which makes the fabrication and interrogation of an array of such apparatus cost-effective. 
     In certain examples the reaction between the sample species and the detector species may be assumed to be “ongoing”, for example by continuing to supply the sample species over time (for example, over the timescale of a few seconds). In such a case, upon initially introducing the sample, the temperature of the pyroelectric layer will rise (assuming an exothermic reaction; the reverse is true of an endothermic reaction). Provided the sample is continually provided and the detector species are not all used up the temperature will remain at the higher level until the sample is removed from the vicinity of the reactant species (for example, by stopping supply of the sample and flushing and remaining sample away). The increase in temperature (and consequent current flow detected) is related to the nature of the reaction taking place, and thus the presence of a particular sample species may be determined. 
     In certain examples, the reaction between the sample species and the detector species may be assumed to be self-exhausting; that is, it occurs so quickly that all the sample species are consumed in a short timescale (for example, within the 10s or 100s of millisecond time frame). In such a case, upon initially introducing the sample, there will be a corresponding spike in temperature (again assuming an exothermic reaction) which will gradually decrease to substantially the initial pre-reaction temperature when the sample is consumed. The resulting temperature variation provides information on the nature of the reaction taking place (i.e. what is the sample species which is undergoing a reaction) from the height of the temperature spike. Also, the amount, or concentration, of the sample species present can be determined based on the time taken for the temperature to fall back down to substantially the pre-reaction temperature. 
     In certain examples, once the sample has reacted with the detector then the apparatus has been used and cannot be refreshed, e.g. if a DNA sample species reacts with a complementary DNA detector species. In such a case the apparatus is a one-use, disposable apparatus. 
     In certain examples, the apparatus may be used multiple times. For example, if the detector species is present as a thin (e.g., 100 nm) layer on the first portion  104   a  of the floating gate  104 , and if a reaction of sample with the detector species consumes the uppermost 1 nm layer of detector species, then after a single use, there remain a further  99  uses to use up the remaining 99 nm thickness of detector species material. The number of uses depends on the thickness of detector species material, the nature of the reaction taking place, and how much of the detector material is used up in each test. 
     To use the apparatus to detect sample species in solution, the solution may be applied to the apparatus, and after detection of the sample, the apparatus may be flushed e.g., with pure water, to remove any remaining sample solution and stop any further reactions. For example, an apparatus may have several different types of detector species present on the first portion  104   a ,  204   a  of the floating gate  104 ,  204 , each type configured to react with a particular sample species. A first sample may be applied, measurements taken, and when the apparatus can be flushed/cleaned and dried ready for a subsequent application of a further sample solution. 
     There is no need for the use of complex microfluidics if the apparatus is used to detect sample species in solution (although in some examples such microfluidics may be used). 
     In some examples, the apparatus may be passivated (for example, by coating it in a thin (e.g., 10 nm) oxide coating) to isolate the metallic parts (such as the floating gate  104 ) from any sample solution applied to the apparatus. The thermal mass of e.g., oxide applied would be very small and would not substantially detrimentally affect the operation of the apparatus. 
       FIG. 5  illustrates a further example apparatus which may be considered to provide a self-compensating architecture. A single apparatus  504  may be considered to act simply as a resistor whose resistance changes in response to a chemical or biological reaction (and an accompanying change in temperature). Two apparatus  504 ,  554  can then be combined as shown in  FIG. 5  to form a self-compensating potential divider  500 . To achieve this, the two apparatus  504 ,  554  may be connected as shown in  FIG. 5 , each having an electrode connected to a common output terminal V out    536 , one 504 having a second electrode connected to an input terminal V d    530 , and the other 554 having a second electrode connected to ground  532 . One apparatus  554  does not have any functionalisation of the floating gate. 
     The system in  FIG. 5  may be described as an apparatus  504  which is electrically connected to and thermally isolated from a further apparatus  554  apart from the at least first portion of the floating gate of the further apparatus  554  not being functionalized; the apparatus  504  and further apparatus  554  together being configured to form a potential divider  500 . 
     Absent any stimulus/sample molecules/reaction, both apparatus  504 ,  554  offer the same resistance (within the fabrication tolerance), so the signal V out  is roughly V d /2. Should there be any uncontrolled source of heat from the environment (air or water convection, impinging photons, etc.), both apparatus  504 ,  554  increase their temperature by the same amount because they have the same absorption and the same thermal mass. Hence, since both apparatus&#39; resistance is changing by the same amount, the divider  500  is still symmetrical and no change in signal (V out ) is detected. However, if some heat is also coming from a reaction taking place on one apparatus  504  due to functionalisation of that apparatus&#39;  504  floating gate (but is not taking place of the other apparatus  554  as there is no functionalisation here) this heat located on one apparatus  504  only will cause a temperature imbalance between the two apparatus  504 ,  554 , and consequently an asymmetry in the resistance of the two apparatus  504 ,  554 , and thus a change in the output signal V out . In essence, this architecture gives a differential response which is only dependent on the reaction heat, and the contribution of any other heat source (background noise) is filtered out. Note that the two apparatus  504 ,  554  must be fabricated on independent and isolated apparatus, or the extra heat produced at the functionalized apparatus  504  may quickly spread to the other apparatus  554  and bring the whole divider  500  into thermal equilibrium, suppressing any output V out . For this architecture, the integration of a two dimensional conductive channel amplifier within individual apparatus is of notable benefit. This is because one can place one graphene channel per pixel, and you need 2 FETs to realise this self-compensating apparatus. If the FETs could not be integrated (no graphene), the external wiring would become too cumbersome for an array, and would also increase the parasitic capacitance limiting the gain. 
     The above described apparatus may be considered to act as a passive sensor for which the reaction or event to be detected taking place on the functionalised floating gate offers sufficient thermal energy to activate the transducer and allow a current to be detected, the current being associated with the change in temperature taking place due to the reaction. There may be cases where the reaction under scrutiny offers too little thermal energy (or no energy at all) to perturb the thermal state of the apparatus in a measurable way. In these cases, the reaction or binding event has taken place on the functionalized pad, but the signal V out  has not significantly varied to an amount which allows the temperature change of the reaction to be identified. However, because a reaction has taken place, the coverage of the functionalized floating gate pad has indeed changed (in case of a positive test, it is now covered with the analyte) and this can change the properties of the apparatus by, for example, changing the thermal mass of the apparatus. These changes can then be probed by actively reading the optical properties of the system. Namely, the reaction apparatus  504  and the control apparatus  554  can both be illuminated by a controlled source of photons, with the aim of delivering some heat to both apparatus  504 ,  554 . With the assumption that, before the binding event/reaction, the absorption of both apparatus  504 ,  554  was identical (or, in any case, known), if the presence of the analyte/sample species has somewhat changed the absorption of the functionalized apparatus  504  then a temperature imbalance will arise. Because there is control over the source of heat (the illuminating photons), it can be made arbitrarily intense to match the sensitivity of the apparatus  504 ,  554 . In the divider geometry of  FIG. 5 , any uniform heating is cancelled out, and only the differential reading carries the fingerprint of the asymmetry generated by the binding event/reaction. 
     Active optical reading may be based on reflectivity only. Assuming the floating gate is made out of a reflecting material such as gold, the presence of the analyte/sample species on top of the floating gate will make that floating gate a less efficient mirror. Hence, heat delivery to the floating gate is increased for a wide range of wavelengths. A more sophisticated implementation could exploit the specific absorption resonances of the analyte/sample molecules, so external illumination at selected wavelengths can be used to interrogate the system more accurately. That is, the controlled photon source may be configured to provide photons of a wavelength corresponding to an expected absorption resonance of a specific detected species. A change in thermal mass may also be detectable due to the presence of sample species on one of the apparatus  504 . 
     That is, the apparatus may be configured to detect the presence of a specific species at the functionalised first portion of the floating gate by allowing for a determination of a change of one or more of:
         thermal mass of the apparatus;   optical absorbance of the apparatus; and   reflectance of the apparatus,
 
by using a controlled photon source to illuminate the apparatus
       

     There are various filters which can used for such implementations. The apparatus may in some examples be coated with one or more specific filters for specific selectivity and coatings can be used for each series of devices arrangement for specific detection in order to introduce spectral selectivity. Such filters may also filter wavelengths that might not be spectrally sensitive to the analyte/sample of interest. Thus the apparatus may further comprise a filter coating configured to allow one or more specific wavelengths of light from the controlled photon source to reach the specific species. 
     Overall, apparatus described herein may allow for different levels of sensing and signal processing alongside relatively straightforward integration. If using graphene as the two dimensional conductive channel, the excellent electrical properties and well understood manipulation of graphene materials allow exploitation of various key materials (thin-film crystals, polymers, 2D materials) as pyroelectric layers which may be chosen for their strong polarization response. 
       FIG. 6  shows schematically the main step of a method of detecting the presence of a specific species proximal to an apparatus by measuring the electrical signal from the apparatus  604 , wherein the apparatus comprises a pyroelectric layer, a two dimensional conductive channel and a floating gate, the apparatus configured such that the pyroelectric layer is capacitively configured with respect to each of the two dimensional conductive channel and the floating gate so that the two dimensional conductive channel and the floating gate can each act as respective capacitive plates for each respective, electrically connected, portion of the pyroelectric layer, the respective portions of the pyroelectric layer themselves configured to act as corresponding capacitive plates, the floating gate comprising electrically connected first and second portions, the first portion of the floating gate being in thermal proximity to the pyroelectric layer, the second portion configured to overlie and gate flow of electrical charge through the two dimensional conductive channel by charge in the second portion, wherein at least the first portion is functionalised to detect one or more proximal specific species, the detection of which gives rise to heat flow to or from the thermally proximal pyroelectric layer to allow the pyroelectric layer to generate an electrical signal dependent upon one or more of the presence and amount of the specific detected species  602 . 
       FIG. 7  illustrates schematically a computer/processor readable medium  700  providing a computer program according to one embodiment. The computer program may comprise computer code configured to perform, control or enable at least the method step  602  of  FIG. 6 . In this example, the computer/processor readable medium  700  is a disc such as a digital versatile disc (DVD) or a compact disc (CD). In other embodiments, the computer/processor readable medium  700  may be any medium that has been programmed in such a way as to carry out an inventive function. The computer/processor readable medium  700  may be a removable memory device such as a memory stick or memory card (SD, mini SD, micro SD or nano SD). 
     Other embodiments depicted in the figures have been provided with reference numerals that correspond to similar features of earlier described embodiments. For example, feature number  1  can also correspond to numbers  101 ,  201 ,  301  etc. These numbered features may appear in the figures but may not have been directly referred to within the description of these particular embodiments. These have still been provided in the figures to aid understanding of the further embodiments, particularly in relation to the features of similar earlier described embodiments. 
     It will be appreciated to the skilled reader that any mentioned apparatus/device and/or other features of particular mentioned apparatus/device may be provided by apparatus arranged such that they become configured to carry out the desired operations only when enabled, e.g. switched on, or the like. In such cases, they may not necessarily have the appropriate software loaded into the active memory in the non-enabled (e.g. switched off state) and only load the appropriate software in the enabled (e.g. on state). The apparatus may comprise hardware circuitry and/or firmware. The apparatus may comprise software loaded onto memory. Such software/computer programs may be recorded on the same memory/processor/functional units and/or on one or more memories/processors/functional units. 
     In some embodiments, a particular mentioned apparatus/device may be pre-programmed with the appropriate software to carry out desired operations, and wherein the appropriate software can be enabled for use by a user downloading a “key”, for example, to unlock/enable the software and its associated functionality. Advantages associated with such embodiments can include a reduced requirement to download data when further functionality is required for a device, and this can be useful in examples where a device is perceived to have sufficient capacity to store such pre-programmed software for functionality that may not be enabled by a user. 
     It will be appreciated that any mentioned apparatus/circuitry/elements/processor may have other functions in addition to the mentioned functions, and that these functions may be performed by the same apparatus/circuitry/elements/processor. One or more disclosed aspects may encompass the electronic distribution of associated computer programs and computer programs (which may be source/transport encoded) recorded on an appropriate carrier (e.g. memory, signal). 
     It will be appreciated that any “computer” described herein can comprise a collection of one or more individual processors/processing elements that may or may not be located on the same circuit board, or the same region/position of a circuit board or even the same device. In some embodiments one or more of any mentioned processors may be distributed over a plurality of devices. The same or different processor/processing elements may perform one or more functions described herein. 
     With reference to any discussion of any mentioned computer and/or processor and memory (e.g. including ROM, CD-ROM etc), these may comprise a computer processor, Application Specific Integrated Circuit (ASIC), field-programmable gate array (FPGA), and/or other hardware components that have been programmed in such a way to carry out the inventive function. 
     The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects/embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure. 
     While there have been shown and described and pointed out fundamental novel features as applied to different embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices and methods described may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. Furthermore, in the claims means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. 
     The research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement number 604391 Graphene Flagship.