Paper-based analytical devices (PADs) have gained widespread attention due to their potential as simple, low-cost, and sustainable analytical devices with the main scope of preliminary monitoring. A ready-to-use colorimetric paper-based sensor for nitrite detection in environmental and clinical samples was developed [10.1039/D4SD00308J]. The sensor is based on the colorimetric Griess reaction, a double-step reaction that includes a diazotization of sulfanilamide in acidic conditions, which forms a diazonium salt, and its coupling with N-(1-naphthyl)ethylenediamine to produce a pink azo dye. Paper has been used as support for reagent immobilization and an origami-based mechanism has been chosen to initiate the reaction without requiring additional handling of chemicals. Unlike conventional nitrite assays, which require multiple steps, specialized instrumentation, and qualified personnel, this sensor enables rapid, user-friendly detection through a smartphone camera, with the option for qualitative assessment via the naked eye. The paper-based sensor demonstrated satisfactory analytical performance when used to analyze drinking water, with recovery rates between 87% and 110%, and detection and quantification limits of 0.27 mg L^-1 and 1.11 mg L^-1, respectively. In addition, for nitrate determination, nitrate reductase was implemented prior to the Griess reaction. The system was stable for at least 1 month at +4°C, maintaining 98% of its performance. The selectivity was studied using ions and potential interferents like BSA, urea, or glucose as common compounds present in environmental or clinical matrices. Another important aspect of this system is its sustainability, which has been evaluated through the 12 principles of white analytical chemistry (WAC) using the 12 algorithms [10.1016/j.trac.2021.116223]. Its sustainability and ease of use make it a promising tool for environmental monitoring and point-of-care (POC) applications. The biosensor was used to detect nitrite and nitrate in water and clinical samples such as saliva and exhaled breath condensate to evaluate its potential as an indicator of inflammation or infection.