The electrocatalytic determination of glucose is based on the in situ formation of Fe\({}_{2}\)O\({}_{3}\) by the reduction in and oxidation of Fe(III) during the preconcentration and stripping steps, respectively. The catalytic mechanism of the Fe\({}_{2}\)O\({}_{3}\) to glucose oxidation can be explained by the reaction: Fe\({}_{2}\)O\({}_{3}\) + Glucose \(\rightarrow\) 2 FeO + Gluconolactone + H\({}_{2}\)O. Under the optimum conditions (i.e., preconcentration for 240 s at \(-\)1.4 V) the sensor enables noninvasive glucose detection in sweat in the physiologically relevant concentration range of 50\(-\)500 umol L\({}^{-1}\) without interference from common electroactive metabolites. The 3D printing procedure adopted in this work overcomes the fabrication disadvantages of manufacturing technologies used in existing sensors (i.e., screen printed and sputtering), while the use of catalytic sensing of glucose on an Fe(III) cluster, as opposed to enzymatic sensing, endows the sensor with an attractive electroanalytical performance for sweat glucose monitoring, including high sensitivity, selectivity, reproducibility, and stability. This research has been co-financed by the European Regional Development Fund of the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH-CREATE-INNOVATE (project code:T2 EDK-00028/MIS 5067540). ## References  
* (1) Yokus, M.A. ; Songkakul, T.; Pozdin, V.A. ; Bozkurt, A.; Daniele, M.A. Wearable multiplexed biosensor system toward continuous monitoring of metabolites. _Biosens. Bioelectron._**2020**, _153_, 112038. * (2) Zhu, X.; Ju, Y.; Chen, J.; Liu, D.; Liu, H. Nonenzymatic wearable sensor for electrochemical analysis of perspiration glucose. _ACS Sens._**2018**, \(3\), 1135\(-\)1141. * (3) Katseli, V.; Economou, A.; Kokkinos, C. Smartphone-Addressable 3D-Printed Electrochemical Ring for Nonenzymatic Self-Monitoring of Glucose in Human Sweat. _Anal. Chem._**2021**, _93_, 3331\(-\)3336. * (4) Hamzah, H.H. ; Shafiee, S.A.; Abdalla, A.; Patel, B.A. 3D printable conductive materials for the fabrication of electrochemical sensors: A mini review. _Electrochem. Commun._**2018**, _96_, 27\(-\)31. * (5) _18.9. Electrochemical Paper-Based Devices Fabricated via Programmable Pen-Drawing and Screen-Printing_
* (6) D. Soulis **1**, V. Pagkali **1**, C. Kokkinos **1**, M. Prodromidis **2** and A. Economou **1,***
* (7) Department of Chemistry, National and Kapodistrian University of Athens, 15771 Athens, Greece
* (8) Department of Chemistry, University of Ioannina, 45100 Ioannina, Greece
* (9) Correspondence: aeconomo@chem.uoa.gr  
The field of paper-based analytical devices (PADs) experienced a rapid growth over the last decade. The key features of paper as a platform for the fabrication of PADs are: (a) flexibility; (b) low thickness and lightness; (c) absorbency; (d) high surface-to-volume ratio; \(\xi\) hydrophilicity and capillary action; (f) chemical and biological inertness; (g) disposability and biodegradability; and (h) low cost and wide worldwide availability. As a result of recent advances in this field, a host of paper-based devices has been developed for application in various fields such as clinical diagnostics and POC testing, environmental monitoring, and food quality control [1,2]. The patterning of the hydrophobic barriers and electrodes are the most critical step in the fabrication of electrochemical PADs, and several approaches have been proposed in the literature [3]. Pen-on-paper (PoP) strategies involve the use of a suitable pen or pencil to deposit functional materials on paper substrates with the view to create hydrophobic patterns on paper [4]. Compared to the traditional methods for fabricating PADs, the advantages of PoP approaches include simplicity, low cost, wide availability of materials, scope for rapid prototyping, and flexibility. On the other hand, screen-printing technology has facilitated enormously the development of low-cost and portable electrochemical paper-based devices [5].