Implant associated infections caused by biofilms are a major cause of implant rejection. One promising strategy to help combat biofilms are “smart” anti-biofilm release surfaces. These surfaces are often loaded with antibiotics or other antimicrobials and respond to the creation of a microenvironment characteristic of biofilms by releasing their cargo. One such characteristic biofilm microenvironment is low pH, whereas in healthy tissue the pH should lie close to 7.4 at the biofilm substrate interface the pH can decrease to 5 and below. These heterogeneous pH environments have been studied previously using electrodes and optical devices. However, here a novel ratiometric sensor which relies on the same processes for sensing as for antimicrobial release is presented in order to optically probe the interfacial biofilm surface pH. Calcium phosphate nanoparticles doped with europium were synthesised and directly deposited onto silicon wafer chips by flame spray pyrolysis, the deposited film was then further stabilised by in situ annealing. These phosphorescent nanoparticles exhibit pH dependent dissolution, with an associated decrease in luminescence intensity which provides a sensor response and known pH buffers were used to draw a calibration curve. A panel of clinically relevant biofilm forming bacteria of both the gram-negatives and gram-positives were selected to measure their interfacial pH. Both gram-negatives and gram-positives demonstrated low pH environments, however, differences between bacterial strains and species were observed. These pH responsive silicon chips can therefore be used to guide the development of anti-biofilm surfaces to target particular bacterial infections.