nanomaterials Article Laser-Tunable Printed ZnO Nanoparticles for Paper-Based UV Sensors with Reduced Humidity Interference Georges Dubourg 1,* , Marko Radovi´c 1 and Borislav Vasi´c 2 1 Center for Sensor Technologies, BioSense Institute, University of Novi Sad, Zorana Ðindi´ca,21101¯ Novi Sad, Serbia; [email protected] 2 Institute of Physics Belgrade, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia; [email protected] * Correspondence: [email protected] Abstract: Development of paper-based sensors that do not suffer with humidity interference is desirable for practical environmental applications. In this work, a laser processing method was reported to effectively modulate the cross-sensitivity to humidity of ZnO-based UV (Ultraviolet) sensors printed on paper substrate. The results reveal that the laser induced zinc oxide (ZnO) surface morphology contributes to the super-hydrophobicity of the printed ZnO nanoparticles, reducing humidity interference while enhancing UV sensitivity. Herein, this conducted research highlights for the first time that laser processing is an attractive choice that reduces the cross-sensitivity to water vapor in the UV sensing response of ZnO-based devices printed on paper, paving the way to low-cost and sophisticated paper-based sensors. Keywords: paper-based device; UV sensors; ZnO nanoparticles; humidity resistance 1. Introduction For a long time, cellulose fiber paper has been used as the main support for storing, Citation: Dubourg, G.; Radovi´c,M.; displaying, transferring information and connecting people in the form of missives, flyers Vasi´c,B. Laser-Tunable Printed ZnO or books. In recent years, however, its function as a writing medium has been declin- Nanoparticles for Paper-Based UV ing with the evolution of information and communication technologies. This evolution Sensors with Reduced Humidity drastically changed how people work, communicate and learn, gradually replacing the Interference. Nanomaterials 2021, 11, 80. https://doi.org/10.3390/ paper substrate with electronic support, such as computer, TV, e-book and e-library. Never- nano11010080 theless, this has generated a considerable amount of electronic waste resulting in severe environmental issues. In order to address the environmental concerns and end-of-life Received: 10 December 2020 disposal challenges, paper was reinvented as a building component in flexible electronics Accepted: 27 December 2020 because it is disposable, recyclable, inexpensive and one of the most abundant organic Published: 2 January 2021 materials. Therefore, paper-based substrate has been explored with the aim of developing a variety of components, such as electronic and optoelectronic devices [1,2], electrochemical Publisher’s Note: MDPI stays neu- biosensors [3–5] and physical sensors [6,7]. Among them, physical sensors which transduce tral with regard to jurisdictional clai- physical parameters, such as mechanical and optical signals, into processable electrical ms in published maps and institutio- signals represent a major category of paper-based devices and might play an important nal affiliations. role in the upcoming era of wearable electronics. Various functional nanomaterials have been employed in order to enhance the paper- based sensor performance. Among them, the zinc oxide (ZnO) nanostructures are a popular choice due to their multiple sensing modalities [8–10]. For instance, its direct wide bandgap Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. and 60 meV high exciton energy make ZnO nanostructures the ideal materials for UV This article is an open access article sensing applications [11,12]. Due to the growing importance of detecting/protecting UV distributed under the terms and con- light application in various fields, various reports have been published on the fabrication of ditions of the Creative Commons At- low-cost ZnO-based UV sensors on paper substrate [13–16]. Although many of the reported tribution (CC BY) license (https:// paper-based UV sensors work very well in laboratory scale proof of principle experiments, creativecommons.org/licenses/by/ they are still limited by their cross-sensitivity to background conditions, such as relative 4.0/). humidity than can vary in outdoor conditions, impeding their practical use in applications Nanomaterials 2021, 11, 80. https://doi.org/10.3390/nano11010080 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 11 Nanomaterials 2021, 11, 80 proof of principle experiments, they are still limited by their cross-sensitivity to 2back- of 11 ground conditions, such as relative humidity than can vary in outdoor conditions, imped- ing their practical use in applications such as environmental monitoring. Indeed, it has been demonstrated that water vapor, present in the real environment for monitoring, such as environmental monitoring. Indeed, it has been demonstrated that water vapor, strongly interacts with the surface or interface of a ZnO nanostructure, which leads to a present in the real environment for monitoring, strongly interacts with the surface or significant fluctuation of its electrical and UV sensing properties [17–19]. Although the interface of a ZnO nanostructure, which leads to a significant fluctuation of its electrical and effect of humidity on UV sensing properties of ZnO nanostructure has been investigated UV sensing properties [17–19]. Although the effect of humidity on UV sensing properties [20–22], the elimination of cross-sensitivity of ZnO-based UV sensors to environmental of ZnO nanostructure has been investigated [20–22], the elimination of cross-sensitivity humidity is still a major challenge. This is even more critical when they are made on paper of ZnO-based UV sensors to environmental humidity is still a major challenge. This is substrate. As the paper is hygroscopic, the effect of moisture can cause additional signifi- even more critical when they are made on paper substrate. As the paper is hygroscopic, cant fluctuations in the conductivity of paper-based devices. So far, the most applicable the effect of moisture can cause additional significant fluctuations in the conductivity of paper-basedapproaches devices.to reduction So far, of the mostinfluence applicable of humidity approaches include to either reduction coating of the paper influence with a ofsuperhydrophobic humidity include layer either [23–27] coating or paperits chemical with a modification superhydrophobic [28,29]. layer Even [ 23though–27] or those its chemicalapproaches modification are effective, [28 they,29]. require Even though addition thoseal processing approaches steps, are making effective, the they overall require pro- additionalcess more processingtime-consuming steps, makingand costlier. the overall One in processteresting more route time-consuming to process paper and substrate costlier. Oneis by interesting use of laser route processing to process technique, papersubstrate which represents is by use a of simple, laser processingfast and scalable technique, alter- whichnative represents to coating a simple,and chemical fast and modification scalable alternative techniques to coating and offers and chemical many advantages modification be- techniquescause of its and capability offers manyof selective advantages treatment because and its of itsfine capability spatial resolution of selective [30–32]. treatment It has andbeen its used fine spatialto control resolution the wetting [30– 32properties]. It has been of paper used substrate to control for the microfluidic wetting properties applica- oftions paper [32–34]. substrate In this for work, microfluidic we introduce applications the laser [32 –irradiation34]. In this post-processing work, we introduce technique the laserto withstand irradiation the post-processing influence of humidity technique and to withstandto improve the the influence electrical of properties humidity andof ZnO to improvebased UV the detectors electrical and properties screen-printed of ZnO devices based UV printed detectors on paper and screen-printedsubstrate. Effects devices of the printedlaser irradiation on paper on substrate. the conductivity Effects of and the UV laser sensing irradiation performance on the conductivity of the screen-printed and UV sensingZnO nanoparticles performance (NPs) of the were screen-printed studied in ZnO detail. nanoparticles Afterwards, (NPs) both were dark studied current in and detail. UV Afterwards,photocurrent both of the dark laser current treated and ZnO UV photocurrentUV sensors were of the measured laser treated in air ZnO with UV varied sensors rela- weretive humidity measured (RH) in air to with investigate varied relative the effect humidity of water (RH) vapor to investigateon the ZnO thefilm effect devices. of water Basic vapormechanisms on the ZnO for filmthe observed devices. Basicbehavior mechanisms were discussed for the observed and correlated behavior with were results discussed from andstructural correlated analysis with of results the ZnO from film. structural The resu analysislts showed of the that ZnO the film. laser The treated results UV showed sensor thathad the excellent laser treated compatibility UV sensor betw hadeen excellent stability compatibility and sensitivity, between response stability and and recovery sensitivity, time, responseand reducing and recovery the impact time, of humidity. and reducing the impact of humidity. 2.2. Materials Materials and and Methods Methods TheThe concept concept of of the the paper-based paper-based