High resolution portable platform for pH sensing via colorimetric fluorescence of carbon dots

• Carbon Dots synthesized from resorcinol show pH-responsive polyphenol molecules. • Dual (blue and green) band emission exhibits intensity variations with pH. • The optical responsivity is engaged to fabricate a nitrocellulose platform for smartphone-based pH detection. • Colorimetric method enables high resolution pH measurements. Portable sensing platforms based on nanoparticles are becoming increasingly significant across diverse fields, as they enable real-time measurements in various environments without complex lab protocols. Carbon dots (CDs), known for low toxicity and intense fluorescence, can exhibit pH-dependent optical emission, generally related to proton exchange-capable surface groups. In addition, CDs’ hydrophilicity, biocompatibility, and stability make them advantageous for pH sensing and, despite limited application in portable sensing devices, their full potential can be realized with precise control of chemistry and optical properties. In this study, we fabricate a portable nitrocellulose-based platform for pH sensing, expressly designed to take advantage of the fluorescence properties of resorcinol-based CDs. First, responsivity of CDs fluorescence features on pH is carefully spectroscopically characterized and linked to acid-base equilibria of polyphenols (hydroxylated polyaromatic rings) present within the CDs carbonaceous structure. Notably, this also offers a titrimetric method to estimate CD concentration. Next, nitrocellulose-based wax-printed circular pads are functionalized with CDs and integrated with a smartphone-based fluorescence colorimetric optical detection system. The developed portable device exhibits a measurement resolution of ± 0.2 pH units (in the range between 2.5 and 6.7) and of ± 0.3 pH units (in the 6.7 – 7.0 range), compatible with most practical pH measurement needs in biological, food and drink, wastewater sample analysis and others. Moreover, although some portable pH sensing devices are already widespread and commercially available, this platform is easy to use, it necessitates only very small sample volumes and is potentially adaptable to various environments and automated analysis processes that does not need prior-use calibrations.