Adjusting the PLN formulation to incorporate a different lipophilic pH indicator, Chromoionophore II (ChII), allowed us to measure across the biological pH range with the persistent luminescence format (Figure , 500 µs delay). The inverse relationship between pH and ChII absorbance at 700nm (Figure ) results in the increasing ZGO:Cr 3+ signal seen in Figure . Importantly, the pH PLN response remains the same with multiple delay times (Figure ) and when saline concentrations of ions are spiked into the buffered standards, indicating no significant response to ionic strength (Figure , pink). The pH PLN signal was also tested against Na + and K + (10µM -1M) -no change in signal occurred (Figure ). Additionally, the calibrations of the pH PLNs are consistent for 8 days (Figure ). Next, the pH PLN response was measured in pH-adjusted FBS from pH 3-9. An inner filter effect (IFE) impacted the sensor signal but can be adjusted for using established methods (see equation S2) . Using a delay-time, we can omit FBS autofluorescence and calibrate the pH PLNs in the serum while achieving the same fit parameters as the response in buffered standards (Figure ). Without a delay time, these measurements are not possible in FBS as the background signal is at least 5 times that of the sensor response (Figure ).