pH Sensing using UCNPs

It was shown in chapter 4 that the luminescence intensity of the UCNPs is independent of pH in the range from 3 to 10. Moreover, it is possible to quench the UCNP emission via the inner filter effect of dyes absorbing in the range of the particle emission. Therefore, these particles can be used for the development of a pH sensing system using suitable pH indicators to create a pH-dependent inner filter effect. This concept has already been shown with upconversion nanorods along with

bromothymol blue as the pH probe.¹ In this work, three different pH-indicators were chosen all of which have a pH dependent absorbance overlapping with the green peak of the UCNPs only. The red emission of the particles in turn remains unaffected by any pH changes. This allows for ratiometric measurements using the red peak as an internal standard.

The first pH probe investigated in this work (phenol red, PR) undergoes a large spectral shift (from 435 nm to 560 nm) and a distinct color change (from yellow to pink) with increasing pH. Absorption spectra at three typical pH values, and the overlap with the green emission of the nanoparticles are shown in figure 5.5, left. The pKa value of PR in water at 25 °C has been reported² to be 7.9. PR is not fluorescent by itself. Depending on whether PR is present in its (pink) base form or in its (yellow) acidic form, the dye was expected to either exert or not to exert an inner filter effect on the green emission of the nanoparticles. Experiments were performed with a mixture of PR (40 µM) and UCNPs (50 µg/mL) in buffered solution. This dye to particle ratio empirically was found to give the best results.

Figure 5.5 (Left) Absorption spectra of phenol red in aqueous solutions of pH 5, 7 and 9, respectively. Luminescence emission of NP-3. (Right) Luminescence spectra of NP-3 in a solution of phenol red at different pH values, spectra are normalized to the peak at 655 nm.

On changing their pH, the solutions undergo the color changes shown in figure 5.6. The intensity of the green emissions of the nanoparticles varies with pH even though the nanoparticles by themselves have an emission that is independent of pH. The red emission of the UCNPs, in contrast, is not affected by the filter effect of the pH indicator. Therefore, this peak can serve as internal standard to allow for ratiometric measurements. By determining the ratio of the green to the red peak

Figure 5.6 Ratio of the green to the red emission of NP-3 in a PR solution at different pH values after 980 nm excitation. Inset: Photograph of the PR and NP-3 system in solution at pH values between 4 and 10.

The ratio of the intensities (I541/I655) of the green and red UCNP emissions changes from 2.5 (at acidic pH) to 1.3 (at basic pH) as can be seen from figure 5.6. With this system, the neutral range is well covered.

Other pH ranges can be covered by choosing different pH indicators with lower or higher pKa values, respectively. For the weakly acidic range, methyl red (MR) is an adequate pH indicator with a pKa value of 5.1. MR changes its color from pink to yellow with increasing pH, with the absorbance maximum shifting from 525 nm to 430 nm. Absorption spectra at three typical pH values, and the overlap with the green emission of the nanoparticles are shown in figure 5.7, left. The dye to particle was the same as for the phenol red system.

Figure 5.7 (Left) Absorption spectra of methyl red in aqueous solutions of pH 3, 5 and 7, respectively. Luminescence emission of NP-3. (Right) Luminescence spectra of NP-3 in a solution of methyl red at different pH values, spectra are normalized to the peak at 655 nm.

In a solution of MR, the green emission of the UCNPs is screened away at pH values below 5 and remains unaffected at high pH values (see figure 5.7, right). The intensity of the red emission remains unchanged and again serves as internal standard. The solutions undergo a color change with increasing pH shown in figure 5.8. The ratio of the intensities of the green and red UCNP emissions (I541/I655) changes from 1.4 (at acidic pH) to 2.7 (at basic pH) as can be observed from figure 5.8. With this system, the weakly acidic range is well covered.

Figure 5.8 Ratio of the green to the red emission of NP-3 in a MR solution at different pH values after 980 nm excitation. Inset: Photograph of the MR and NP-3 system in solution at pH values between 3 and 8.

absorbance is peaking at 560 nm in the basic range. Absorption spectra at three typical pH values, and the overlap with the green emission of the nanoparticles are shown in figure 5.9, left. In a solution of PT, the green emission of the UCNPs is unaffected at pH values below 8 and filtered at high pH values (see figure 5.9, right).

The intensity of the red emission remains unchanged and again serves as internal standard. The dye to particle ratio was chosen as 66.7 µM of PT to 50 µg/mL of the UCNPs.

Figure 5.9 (Left) Absorption spectra of phenolphthalein in aqueous solutions of pH 7, 9 and 11, respectively. Luminescence emission of NP-3. (Right) Luminescence spectra of NP-3 in a solution of phenolphthalein at different pH values, spectra are normalized to the peak at 655 nm.

The solutions undergo a color change with increasing pH shown in figure 5.10. The ratio of the intensities of the green and the red emission of the UCNPs (I541/I655) changes from 2.5 (in the acidic and neutral range) to 0.7 (in the basic range) (figure 5.10). The inner filter effect of PT is very strong, the intensity of the green peak is almost completely screened away.

Figure 5.10 Ratio of the green to the red emission of NP-3 in a PT solution at different pH values after 980 nm excitation. Inset: Photograph of the PT and NP-3 system in solution at pH values between 6 and 11.

This concept for determining the pH via the inner filter effect on pH sensitive probes on the UCNP emission can be expanded to the detection of acidic or basic gases.

This is demonstrated in the following for the basic gas ammonia.