A large variability of the concentration of K\({}^{+}\) is indicative of a large number of diseases, for example cardiac arrhythmia or myasthenia [7], alcoholism, diabetes, cancer and others [8]. The determination of potassium concentration [K\({}^{+}\)], therefore, is carried out on a large scale in clinical labs. Inside cells, the [K\({}^{+}\)] typically is 150 mM, while it is only 4 mM in the extracellular space, for example in serum [9]. [K\({}^{+}\)] is routinely analyzed in the clinical lab via ion-selective electrodes [10] or optical sensor membranes [11]. These usually consist of the ionophore, valinomycin [12], incorporated into a (plasticized) solid polymer that permits K\({}^{+}\) ions to be transported into or through the membrane to reach the electrochemical or optical signal transducer, whilst any other ions are excluded. Such sensors suffer, however, from several drawbacks. Electrodes can undergo signal drift and fouling [13], while optical sensors can undergo photobleaching or give weak optical signals or signal changes [14, 15]. Chemical sensors based on the use of photonic crystals (PhCs) represent a fairly new class of sensing devices [16]. PhCs consist of periodically arranged structures [17, 18] of a dielectric material often placed in a polymer [19]. They reflect light of a certain wavelength that is dependent on: (i) the angle of the incident light; (ii) the distances of beads within the structure; and (iii) the refractive indices of the structure and the surrounding medium [20]. If the initial distance in the PhC is kept constant, for example by placing ordered micro- or nano-particles in a hydrogel matrix, changes in the reflected wavelength will occur if the matrix swells or shrinks [21]. This has been widely used in physical and chemical sensor technology, as this mechanism can be made chemically selective and PhCs possess several advantages, such as low-cost production, ease of up-scaling, ease of readout and being label-free [16]. Numerous PhC-based schemes have been described for use in chemical [19, 22] and biological [23, 24, 25] sensing. In the field of ion sensing, sensors are known that respond to ionic strength in general [21, 26], and, due to the pioneering work of the groups of [PERSON] [27, 28], [PERSON] [29] and [PERSON] [30], more specifically, to ions of cobalt, copper, nickel, zinc [28], lead [27], mercury [29] and potassium [30]. The sensors are based on the modification of the hydrogel matrix with either enzymes [29] or crown ethers [27, 28, 30]. Some ions can be discriminated only according to their physical properties, such as radius and charge, or by the inhibitory effect on enzymes displayed by certain heavy metal ions. In order to enable highly-specific sensing in combination with the advantages of PhC technology, such as simple (often visual) readout, the ion recognition must be significantly improved. The sensor for [K\({}^{+}\)] described in [30] uses a porous gel that undergoes color changes depending on [K\({}^{+}\)], but suffers from the limited selectivity of the crown ether used. Moreover, it was prepared via a templating technique that results in a material of limited temporal and chemical stability. An all-plastic sensor (like practically all other sensors for K\({}^{+}\)) is highly desirable, because such sensors can be re-used for quite some time or even operated continuously. Ionophores are indispensable tools in terms of sensing K\({}^{+}\), and valinomycin is superior to any crown ether [10, 31]. We are demonstrating here, by extending the work of [PERSON] _et al._ on PhC sensing technology, a combination of a 3D photonic crystal film with a plasticized PVC membrane containing valinomycin to form a two-layer system. This sensor arrangement is shown to be highly selective for K\({}^{+}\) and to respond to K\({}^{+}\) over a wide concentration range that also covers the level of K\({}^{+}\) encountered in whole blood. We also show that such sensor films can be imaged using either a microscope or a digital camera.