Analyst PAPER Cite this: Analyst, 2023, 148, 4072 Received 5th April 2023, Accepted 12th July 2023 DOI: 10.1039/d3an00533j rsc.li/analyst Fabrication of a paper-based facile and low-cost microfluidic device and digital imaging technique for point-of-need monitoring of hypochlorite† Snehasish Debnath, a,b Riya Ghosh, a,b Pragti, c Suman Mukhopadhyay, * c Kamesh Viswanathan Baskaran* d and Pabitra B. Chatterjee * a,b Lab-on-a-paper-based devices are promising alternatives to the existing arduous techniques for point- of-need monitoring. The present work reports an instant and facile method to produce a microfluidic paper-based analytical device (μPAD). The fabricated μPAD has been used to detect hypochlorite (OCl - ) by incorporating newly synthesized chromo-fluorogenic ratiometric probes 1 and 2 into the sample reception zone. The probes showed high selectivity and fast response (<10 s) toward OCl - with an excel- lent linear relationship in the concentration range of 0–100 μM. The concentration-dependent fluoro- metric change driven by the reaction of 1@μPAD with OCl - has been monitored using gel-doc imaging systems, which is unprecedented. Digitizing the intensity of the colour solution with different mathemat- ical models of colour has developed a straightforward method for monitoring OCl - without any inter- ference from its competitors. 1@μPAD can detect OCl - at ∼10 times lower than the WHO recommended limit. The detection limit of 1@μPAD via a digital camera-based fluorescence technique was found to be better over digital camera-based cuvette assays. Therefore, 1@μPAD has been successfully utilized to monitor OCl - in actual environmental water samples with portability, ease of use, and sensitivity. The analytical RSD was found to be ≤3% based on fluorimetric detection using 1@μPAD. The chemodosi- metric reaction between OCl - and the probe was evidenced by UV-vis and fluorescence spectroscopy, 1 H NMR, and ESI-MS. The rapid response time, biocompatibility, low cytotoxicity, 100% aqueous solubility, ratiometric feature, and exclusive OCl - selectivity over other competitive ROS/RNS successfully lead to the application of the probes for bioimaging of exogenous as well as endogenous OCl - in normal cells (HEK293) and cancerous cells (HeLa). Introduction As a strong oxidant, a specified amount of HOCl/OCl - (10 -2 – 10 -5 M) is used in water and sewage treatment, sterilization, household bleaching agents, and other disinfectant purposes. 1,2 However, when in excess, it produces many toxic by-products causing eye/nose irritations, stomach discomfort, etc, 3 thereby posing potential risks to the environment and humans. As per a World Health Organization (WHO) report, OCl - is one of the essential factors of residual chlorine, and the limit of residual chlorine has been set at 5 mg L -1 (∼100 μM). 4 During the Covid-19 pandemic, excessive use of OCl - raised concerns about environmental monitoring due to its easy solubility and mobility in water. 5 Therefore, it is imperative to develop a rapid, easily accessible, affordable, and off-laboratory simple analytical tool for point-of-need monitor- ing of OCl - at <100 μM as per the WHO guidelines. 4,6,7 In this context, low-cost μPADs (microfluidic paper-based analytical devices) have emerged as an attractive platform for developing a lab-on-a-paper-based convenient tool that can be used in environmental applications. 8–10 In contrast to silicon and glass-fabricated microfluidic devices, cellulose paper-based microfluidic devices have many advantages due to their high porosity, hydrophilicity, and the high surface-to-volume ratio of the cellulosic network. 11 Reduced analytical costs, high reproducibility, easy disposability, and usability in low resource settings help μPADs avoid the need for other tiring instrumental techniques. 11 Despite tremendous progress in developing μPADs after the pioneering effort by Whitesides † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ 10.1039/d3an00533j a Analytical & Environmental Science Division and Centralized Instrument Facility, CSIR-CSMCRI, G. B. Marg, Bhavnagar, India. E-mail: pbchatterjee@csmcri.res.in b Academy of Scientific and Innovative Research (AcSIR), Ghaziabad-201002, India c Department of Chemistry, Indian Institute of Technology Indore, Indore, India d Dr. K. C. Patel Research & Development Centre, Charotar University of Science and Technology, Changa, India 4072 | Analyst, 2023, 148, 4072–4083 This journal is © The Royal Society of Chemistry 2023 Published on 24 July 2023. Downloaded by Charotar University of Science and Technology on 1/17/2024 5:23:13 AM. View Article Online View Journal | View Issue