Citation: Raub, A.A.M.; Hamidah, I.; Nandiyanto, A.B.D.; Ridwan, J.; Mohamed, M.A.; Buyong, M.R.; Yunas, J. ZnO NRs/rGO Photocatalyst in a Polymer-Based Microfluidic Platform. Polymers 2023, 15, 1749. https://doi.org/10.3390/ polym15071749 Academic Editor: Shi-Yong Liu Received: 10 January 2023 Revised: 21 February 2023 Accepted: 26 February 2023 Published: 31 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article ZnO NRs/rGO Photocatalyst in a Polymer-Based Microfluidic Platform Aini Ayunni Mohd Raub 1 , Ida Hamidah 2 , Asep Bayu Dani Nandiyanto 2 , Jaenudin Ridwan 1 , Mohd Ambri Mohamed 1 , Muhamad Ramdzan Buyong 1 and Jumril Yunas 1, * 1 Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia (UKM), Bangi 43600, Malaysia 2 Faculty of Engineering Education, Universitas Pendidikan Indonesia, Jl. Dr. Setiabudhi 207, Bandung 40154, Indonesia * Correspondence: jumrilyunas@ukm.edu.my Abstract: This paper reports the development of ZnO NRs/rGO-based photocatalysts integrated into a tree-branched polymer-based microfluidic reactor for efficient photodegradation of water con- taminants. The reactor system includes a photocatalytic reactor, tree-branched microfluidic channels, and ZnO nanorods (NRs) coated with reduced graphene oxide (rGO) on a glass substrate within an area of 0.6 × 0.6 cm 2 . The ZnO NRs/rGO acts as a photocatalyst layer grown hydrothermally and then spray-coated with rGO. The microfluidic system is made of PDMS and fabricated using soft lithography (micro molding using SU-8 master mold patterned on a silicon wafer). The device geometry is designed using AutoCAD software and the flow properties of the microfluidics are simu- lated using COMSOL Multiphysics. The microfluidic platform’s photocatalytic process aims to bring the nanostructured photocatalyst into very close proximity to the water flow channel, reducing the interaction time and providing effective purification performance. Our functionality test showed that a degradation efficiency of 23.12 %, within the effective residence time of less than 3 s was obtained. Keywords: microfluidic reactor; SU-8 master mold; reduced graphene oxide/zinc oxide nanorods; methylene blue; water treatment 1. Introduction Water treatment is an essential part of the water cycle that must be managed through- out the water management cycle: from freshwater intake, treatment, distribution, use, collection, and post-treatment to reuse and eventually return to the environment from which the water source was recovered. Compared to water supply concerns, water man- agement receives less societal and political attention, especially in relation to water scarcity. According to the United World Water Development Report 2017, better wastewater man- agement leads to social, environmental, and economic advantages critical for long-term development and achieving the 2030 Agenda for Sustainable Development [1]. Moreover, due to the volume and cost of storing drinkable water on the international space station (ISS), recycling used water is the best option for obtaining clean water. The Water Pro- cessor Assembly (WPA) is the technology currently in use on the ISS, and it uses high temperature and pressure to complete an oxidation catalytic reaction for the elimination of volatile organic compounds [2]. The problem with this technology is that its high oper- ating temperature and pressure necessitate frequent replacement of its weak parts [3]. A novel photocatalytic water purification microreactor that operates at standard pressure and temperature must be designed to overcome these difficulties. Numerous metal oxides have been used as photocatalysts in water treatment. By implementing binary or ternary semiconductors, band gap narrowing can be achieved for a wide range of optoelectronics applications [4,5]. A ZnO and GO-based nanocomposite has successfully degraded different dyes such as MB (methylene blue) [6,7], MO (methyl Polymers 2023, 15, 1749. https://doi.org/10.3390/polym15071749 https://www.mdpi.com/journal/polymers