Electronic transistors have revolutionized the fields of microelectronics, computers, and mobile devices. Their ability to digitize electronic signals allows high fidelity data transfer as well as formation of logic gates. Inspired by electronic transistors, transistor-like organic materials have been under intensive investigation to amplify biological signals in a broad range of applications such as biosensing, diagnostic imaging, and therapeutic delivery. This Account highlights the inception and implementation of a "proton transistor" nanoparticle that can digitize acidotic pH signals in biological systems. Similar to electronic transistors, the ultra-pH-sensitive (UPS) nanoparticles derive their binary threshold response from phase separation phenomena. Hydrophobic micellization drives nanophase separation from unimers to aggregated polymeric micelles, which is responsible for the all-or-nothing proton distribution between the micelle and unimer states. Depending on the assembly status, conjugated fluorophores are quenched (micelle state) or freely fluoresce (solution unimer state) allowing robust detection of the phase transition behavior across a narrow pH range. Based on this mechanistic insight, we created a UPS nanoparticle library encompassing a broad physiological pH range from 4.0 to 7.4. For biological applications, we engineered a barcode-like nanosensor capable of digitizing multiple pH signals at a single organelle resolution in live cells. The barcode system allowed easy identification of mutant Kirsten rat sarcoma viral oncogene (KRAS), a common mutation involved in tumorigenesis, which leads to rapid cellular proliferation, as the protein driver for accelerated organelle acidification and lysosome catabolism in a broad set of isogenic as well as heterogeneous cancer cell lines. Adoption of the technology to an ON-OFF/Always-ON design allowed the quantification of proton flux across the membranes of endocytic organelles. For medical applications, we demonstrate the ability to achieve binary detection of solid cancers with clear tumor margin delineation by near-infrared fluorescence imaging. Image-guided resection of head/neck and breast tumors resulted in significantly improved long-term survival over white light or tumor debulking surgeries in tumor-bearing mice, catapulting the clinical evaluation of the UPS nanosensor in cancer patients. This Account serves as the first comprehensive summary of the molecular mechanism and biological applications of the digital pH threshold sensors. Building on the concept of cooperative phase transition behavior, we hope this Account will promote the rational design and development of additional transistor-like chemical sensors to digitize analog biological signals.