# Carbon Dots with Up-Conversion Luminescence as pH Nanosensor
## 4 Conclusions  
In this work, the UCL for aqueous solutions of carbon dots with a polyfunctional surface and a surface functionalized with carboxyl groups was studied. It was found that UCL is absent in CDs functionalized with hydroxyl surface groups. The obtained quadratic dependence of the CDs' UCL intensity on the power of the exciting radiation proves the fact that the UCL for the studied CDs is caused by two-photon absorption processes. The differences in the UCL of CD-poly and CD-COOH aqueous solutions and its absence for CD-OH indicates that the mechanisms of nonlinear optical properties of CDs \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Measurement Method** & **Water** & **Egg White 1** & **Egg White 2** & **Egg White 3** & **Egg White 4** \\ \hline Electrode & 6.73 \(\pm\) 0.05 & 8.33 \(\pm\) 0.05 & 8.36 \(\pm\) 0.05 & 8.23 \(\pm\) 0.05 & 8.44 \(\pm\) 0.05 \\ CD-poly & 6.74 \(\pm\) 0.12 & 8.34 \(\pm\) 0.16 & 8.32 \(\pm\) 0.18 & 8.16 \(\pm\) 0.19 & 8.48 \(\pm\) 0.16 \\ CD-COOH & 6.73 \(\pm\) 0.22 & 8.35 \(\pm\) 0.31 & 8.33 \(\pm\) 0.33 & 8.17 \(\pm\) 0.368 & 8.55 \(\pm\) 0.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The pH values of the water and chicken egg white obtained using a pH electrode and from the spectra of CDs’ UCL. Figure 9: UCL of CDs in chicken egg white (lower half of the image) upon excitation at a wavelength of 720 nm. (photoluminescence in the result of two-photon absorption), such as linear ones (classic photoluminescence in the Stokes region), depend on the type and composition of surface functional groups. It was found that the UCL for aqueous solutions of CD-poly and CD-COOH in water depends on the wavelength of the excitation radiation and on the pH value of the solution. The analysis of the obtained results showed that CDs' UCL is most effectively excited at a wavelength of 720 nm. It has been shown that the studied CD-COOH and CD-poly with up-conversion luminescence can be used as pH nanosensors in the linear response pH ranges from 2 to 9 and from 5 to 9, respectively. In these ranges, the estimated accuracy in the determination of the pH of the environment by UCL spectra for aqueous solutions of CDs was 0.13 for CD-poly and 0.21 for CD-COOH. Approbation of pH nanosensors in biological tissue--chicken egg white--was carried out, which demonstrated the prospects of using the proposed method of using CDs' UCL to determine the pH of biological media. Obviously, CD-COOH are more promising for biomedical problems as pH nanosensors, since they have high biocompatibility and can measure pH in a larger range of its linear changes in biological tissue. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app122312006/s1](https://www.mdpi.com/article/10.3390/app122312006/s1)]([https://www.mdpi.com/article/10.3390/app122312006/s1](https://www.mdpi.com/article/10.3390/app122312006/s1)), Figure S1: Reproducibility of up-conversion luminescence of CD aqueous suspension obtained in different synthesis batches under the same synthesis conditions, Figure S2: Demonstration of the up-conversion luminescence stability of aqueous suspension of CDs for 10 days. Conceptualization, T.D. and K.L. ; methodology, S.B., M.K., and K.L. ; software, A.V. ; validation, M.K. and A.V. ; formal analysis, A.V. ; investigation, M.K. and K.L. ; resources, T.D. ; data curation, T.D. and K.L. ; writing--original draft preparation, T.D. and K.L. ; writing--review and editing, T.D. ; visualization, M.K. ; supervision, T.D. ; project administration, T.D. ; funding acquisition, T.D. All authors have read and agreed to the published version of the manuscript. The study was carried out at the expense of a grant from the Russian Science Foundation No. 22-12-00138, [[https://rscf.ru/project/22-12-00138/](https://rscf.ru/project/22-12-00138/)]([https://rscf.ru/project/22-12-00138/](https://rscf.ru/project/22-12-00138/)) (accessed on 2 September 2022). Not applicable.