Abstract:
Volatile organic compounds or oxidizing gases are detected when the work function of a metal-organic framework is measured to produce a sensor signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and hereby claims priority to German Application No. 10 2011 075 396.6 filed on May 6, 2011, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The detection or measurement of volatile organic compounds (VOC) is of great interest for a number of applications. Examples are assessment of the air quality in rooms, the energy-efficient control of air conditioning plants, medical diagnostics, safety technology such as the detection of explosives or the monitoring of purified air. A problem with volatile organic compounds is that they come in a very great molecular variety. Important representatives are, for example, aldehydes, alcohols, amines, carboxylic acids, esters, ketones, hydrocarbons, terpenes, thiols, etc. This variety makes it difficult to realize a sensor which gives a sufficiently high sensor signal for a major part of the volatile organic compounds. Odors are often made up of a plurality of compounds which are also frequently present in very small amounts. Volatile organic compounds generally have such a low odor threshold that concentrations which are clearly perceptible to human beings are at the same time very difficult to detect technically. 
     SUMMARY 
     Described below is an apparatus for the detection of at least one volatile organic compound or at least one oxidizing gas, by which even small amounts of the volatile organic compound or of the oxidizing gas can be measured. 
     The apparatus described below uses a measuring material whose electronic work function is changed on interaction with a volatile organic compound or an oxidizing gas. The measuring material includes a material selected from the group consisting of metal-organic frameworks (MOFs). The apparatus measures changes in the electronic work function of the measuring material. 
     The inventors recognized that the particular characteristic of metal-organic frameworks is a large hollow space in the middle of a unit cell of the metal-organic framework. This hollow space makes the use of metal-organic frameworks in sensors for gases possible. Here, the large internal surface area (up to more than 4000 m 2 /g) is important and a great advantage. This large internal surface area is available for interaction with gases. Furthermore, a high selectivity for particular substances can advantageously be achieved by fixing the size of the pores or of the hollow space. This can in turn be controlled via the size of the organic ligands. Matching to a gas to be detected can be carried out in this way. 
     It has surprisingly been found to be possible to use metal-organic frameworks in gas sensors. The position of the hollow spaces in the middle of the unit cells of the metal-organic framework is at a large distance, on the scale of electronic interaction, from measuring a change in the work function. In other words, the interaction with the gas at the interior surfaces of the metal-organic framework is at a comparatively large distance from the surface of the measuring material. A measurable change in the work function as a result of influences of gas was therefore not to be expected. Nevertheless, experiments have shown that gas sensors using metal-organic frameworks give good results for volatile organic compounds. 
     In an advantageous embodiment, the apparatus includes a heater device, for example a heating meander structure. This can serve firstly to bring the metal-organic framework to a required operating temperature. Secondly, it is also possible, in the case of a measurement at room temperature, to reset the sensor by heating if some of the chemical reactions are irreversible or reversible only with difficulty at room temperature. 
     In the process of producing an apparatus for measuring at least one volatile organic compound or at least one oxidizing gas, a measuring material whose electronic work function is changed on interaction with the volatile organic compound or the oxidizing gas is applied to a support, in particular to a silicon substrate. The measuring material may include a material from the group consisting of metal-organic frameworks. Furthermore, changes in the electronic work function of the measuring material are measured. Here, the measuring material is applied as a layer using an organic binder which is not gas-sensitive, in particular ethylcellulose. The organic binder remains in the layer and ensures satisfactory adhesion of the layer to the support. The layer can, for example, be applied by known screen printing or spin coating processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic cross section of a gas sensor having a sensitive layer with a metal-organic framework and 
         FIG. 2  is a graph of a measurement result of the gas sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  shows, in highly schematic form, an illustrative structure of a sensor. A substrate  1 , for example a silicon substrate, bears a field effect transistor structure which has a drain electrode and a source electrode and is not shown in more detail. Both are covered by a passivation layer  2 . Above the passivation layer  2  and separated by an air gap  6 , there is a support  3  having a gas-sensitive layer  4 . The gas-sensitive layer  4  has a metal-organic framework, in this example a porous Cu 3 BTC 2  metal-organic framework. 
     Sensors can also be produced using, for example, the metal-organic frameworks Basolite F300 iron benzene-1,3,5-tricarboxylate and C300 copper benzene-1,3,5-tricarboxylate available from BASF. Basolite A100 aluminum terephthalate and Basolite Z1200 2-methylimidazolezinc can also be used. 
     Gas molecules  5  permeate into the air gap  6 . Here, they become incorporated in the gas-sensitive layer  4 , i.e. the metal-organic framework, and change the work function of the latter. This change is measured by the field effect transistor structure. For example, the flow of current through the field effect transistor structure is monitored and changes in the current are recorded as sensor signal. A further possibility, which is indicated in  FIG. 1 , is electrical connection of the gas-sensitive layer  4  to the substrate  1 . A voltage source located in-between regulates the shift current which is generated in the case of changes in the work function to 0 and the voltage necessary for this is used as sensor signal. 
       FIG. 2  shows two measurement curves of the sensor described by way of example. Here, the change in the work function is plotted as a function of time. In addition, the concentration in which the gases toluene and pentanal were fed to the sensor is shown in the lower part of the graph. Both measurement curves show no appreciable deflection in the region of the addition of toluene which was used in a maximum concentration of 10 ppm. However, the addition of likewise not more than 10 ppm of pentanal leads to a very distinct sensor deflection and sensor signal. 
     A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide  v.  DIRECTV,  358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).