The aim of this work was the development of disposable electrochemical PADs using a dual fabrication approach which combines programmable pen-drawing and screen-printing. For this purpose, hydrophobic barriers were delimited by x-y plotting with a hydrophobic commercial marker pen. Electrodes were formed by depositing carbon electrodes on the PADs via screen-printing. Applicability of the PADs was demonstrated for electrochemical detection of organic compounds by differential pulse voltammetry and of heavy metals by stripping voltammetry. This research is co-financed by Greece and the European Union (European Social Fund-ESF) through the Operational Programme \"Human Resources Development, Education and Lifelong Learning 2014-2020\" in the context of the project \"Development of paper-based integrated fluidic electrochemical biosensors using nanomaterial labels\" (MIS 5047930). ## References  
* Fu and Wang (2018) Fu, L.M. ; Wang, Y.N. Detection methods and applications of microfluidic paper-based analytical devices. _TrAC_**2018**, _107_, 196-211. * Ozer et al. (2020) Ozer, T.; McMahon, C.; Henry, C.S. Advances in paper-based analytical devices. _Annu. Rev. Anal. Chem._**2020**, _13_, 85-109.  
different morphologies by taking advantage of the characteristics of the flexible sensor, and it is expected that such a sensor could be applied to various attachment sites on fish. Study of the Influence of MWCNTs Addition to the Ion-Selective Membrane on the Analytical Parameters of Electrodes Sensitive to Nitrates  
**K. Pietrzak * and C. Wardak**  
Department of Analytical Chemistry, Faculty of Chemistry, Institute of Chemical Sciences-Maria Curie-Sklodowska University, Maria Curie-Sklodowska Sq. 3, 20031 Lublin, Poland * Correspondence: karolina.pietrzak@poczeta.umcs.lublin.pl  
Potentionetry is one of the simplest electrochemical techniques that allows us to determine the concentration of inorganic and organic ions in liquid samples. The most popular potentiometric sensors are ion-selective electrodes, the potential of which depends on the activity of ions in the sample. In combination with a reference electrode (with a constant potential), they form a cell whose measurement of the electromotive force enables the determination of the concentration of ions in the solution [1]. In order to improve the analytical parameters of sensors, new chemicals are constantly searched for, which act as an active substance in the membrane, enabling the achievement of good selectivity of the sensors. Various materials are also developed that can provide good stability and reversibility of the potential (ion-electron transducers) in ion-selective electrodes with solid contact which do not contain an internal solution. Conductive polymers were the first substances used as solid contact, but nowadays, due to the great popularity of all nano-sized materials, nanomaterials and nanoparticles are widely used for this purpose. Nanomaterials are characterized by a high surface to volume ratio, very good electrocatalytic activity, as well as high electrical conductivity and mechanical strength [2]. Their use as an intermediate layer or a component of the ion-selective membrane improves the stability of the electrode capacitance and reduces the membrane resistance [3]. So far, carbon nanomaterials, such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), graphene, fullerenes, carbon nanofibers, and carbon black, have been used as a solid contact. Initial studies of ion-selective electrodes for the determination of nitrate (V) ions were performed, for the construction of which various types of MWCNTs were used. The effect of modifying the composition of the ion-selective membrane of the obtained potentiometric sensors was tested by determining the basic analytical parameters, which include slope of calibration curve, detection limit, measuring range, pH range, response time, and potential stability.