Numerous advancements in today’s technology had impact on research of (bio)chemical sensors. Focus has been shifted towards realization of wearable analytical devices. Wearable (bio)chemical sensors are analytical tools that provide the opportunity for continuous and non-invasive analysis of body fluid content (such as sweat, tears, saliva, interstitial fluid or breath), as well as the ability to detect substances in the environment of their user. Optical (bio)chemical sensors represent the subclass of (bio)chemical sensors where interaction with electromagnetic radiation is employed in order to extract useful information. They are most commonly named “optodes” in the literature, due to great similarity with their electrochemical equivalents-electrodes. Certain problems such as choosing appropriate materials, external power sources and credibility of harvested data still represent the challenge that needs to be resolved in order to start commercial production of wearable (bio)chemical sensors. A growing interest in using paper and textile as substrate materials has been observed. Availability and affordability of low-cost paper are main reasons to choose this material. On the other hand, textile-based sensors would be most practical due to simple integration with existing clothing material. The ultimate goal of this thesis was to investigate the mechanism of ion-selective optodes on different substrates. Potassium-selective optodes based on valinomycin carrier and lipophilized Nile Blue indicator dye were fabricated with potential application in analyzing the sweat content. Fabrication of sensors was followed with their analytical characterization. Experimental results have shown that chromatography grade paper seems to be the most appropriate substrate material, considering the investigated mechanism. Such paper-based optodes were characterized in greater detail, exhibiting dynamic range that covers over three orders of magnitude (10^-1 M - 10^-4 M K+), making them suitable for predicted application. Average potassium concentration in human sweat lies within that range. It should be emphasized that the usability of these sensors is limited owing to the well-known leaching problem, where sensing components are being washed out of the system. Characterization parameters such as repeatability, reproducibility and stability have clarified that apparent problem. Paper-based potassium-selective optodes investigated in this research could therefore be used for predicted application as single-use, disposable paper strips.