# Carbon Dots with Up-Conversion Luminescence as pH Nanosensor
## 3 Results and Discussion  
Dependence of the UCL Intensities of Aqueous Solutions of CDs on the Wavelength of the Exciting Radiation  
To study the UCL of CDs, the UCL spectra of CDs' aqueous solutions at pH 11.7 (initial pH for studied aqueous solutions) were obtained under excitation by laser radiation with different wavelengths in the range from 670 to 840 nm with excitation radiation power of 50 mW (Figure 5). From them, the dependences of the integral intensities of these spectra on the wavelength of the exciting radiation were calculated (Figure 6). For CD-OH, no photoluminescence signal was observed in the specified spectral range, what corresponds to the absence of the intense absorption bands in the optical absorption spectra of this sample in the range from 250 to 500 nm. Additionally, it is worth noting that the synthesized CDs were characterized by stable UCL, both in terms of reproducibility of the up-conversion  
Figure 4: Scheme of the experimental setup for the study of CDs’ UCL: 1—pulsed nanosecond Nd:YAG laser; 2—parametric light generator; 3—light filter KS-11; 4—rotatory mirror; 5—power meter; 6—mechanical shutter; 7—rotatory mirror and lens; 8—cuvette with the solution of nanoparticles; 9—optical system of two lenses; 10—light filters SZS-22 and SZS-23; 11—registration system consisting of a monochromator and a CCD camera.  
luminescence of CD aqueous suspension obtained in different synthesis batches under the same synthesis conditions (see Figure S1) and in terms of the UCL stability of aqueous suspensions of CDs for 10 days (see Figure S2).  
The processing of the spectra consisted in averaging 4 consecutively recorded spectra and normalizing their intensity to the power of the exciting radiation and to the time of accumulation of the spectra. The spectra were smoothed by a third-order polynomial using the Savitsky-Golay method.  
It can be seen from the obtained results that the maxima of the dependences of the UCL integral intensity on the wavelength of the exciting radiation for the two types of CDs are close and are in the region of 720 nm (Figure 6). Therefore, this excitation wavelength can be considered optimal for both types of CDs, and it was used for the excitation of the CDs' UCL in following measurements. The proximity of the excitation wavelengths that provide the maximum integrated intensity of UCL of CD-COOH and CD-OH is explained by the same adsorption features of these CDs' solutions related to their functional groups (Figure 3). Nevertheless, it should be noted that, upon excitation of UCL by radiation at a wavelength of 720 nm, the UCL intensity of an aqueous solution of CD-COOH is approximately 1.7 times higher than that of an aqueous solution of CD-poly at pH = 11.7. Thus, the presence/absence of CDs' UCL and its characteristics depend on the functionalization of the nanoparticle surface.  
Figure 5: UCL spectra of aqueous solutions of CD-poly and CD-COOH upon excitation by laser radiation at wavelengths of 680, 720, and 800 nm (pH = 11.7).  
Figure 6: Dependences of the integrated UCL intensities of aqueous solutions of CD-poly and CD-COOH on the wavelength of the exciting radiation (pH = 11.7).