Figure 3.

(a) pH-dependent upconversion emission spectra of bromothymal blue conjugated NaYF₄:Er³⁺/Yb³⁺ film upon 980 nm excitation (Reproduced from Ref. [78] with permission from The Royal Society of Chemistry). (b) The upconversion spectrum of Y₂O₂S:Er₃₊/Yb₃₊ UCNPs (red line, left y-axis) and the extinction spectra (right y-axis) of bromocresol green-doped silica films in pH 3.0 (orange) and buffer 7.0 (green). Inset shows the red luminescence with higher resolution. (c) The calibration curves of bromocresol green conjugated Y₂O₂S:Er³⁺/Yb³⁺ films, without (black dots) and with (red points) passing through porcine tissue. Left inset shows the photograph of the sensor film sandwiched between two porcine tissues for pH calibration. The inset in the right side shows the schematic representation of the pH sensor. (d) pH determination in real-time with the sensor film passing through porcine tissue. The red dots exhibit the variation of pH due to the bacterial development at the interface of the TSA plate and pH-sensor; the black dots show the variation of pH of the controlled sample with same volume of phosphate buffer solution. The corresponding pH values, calculated according to the calibration curve, are shown in the right y-axis. (e) The photograph at the left side shows the pH sensor film by the end of the real-time experiment with S. epidermidis and at the right side shows the film of the control sample by the end of acquisition (Figure 3b–d: Reproduced from Ref. [34] with permission of John Wiley & Sons Ltd.).