# Carbon Dots with Up-Conversion Luminescence as pH Nanosensor
## 2 Materials, Their Characterization, and Research Methods
### IR Absorption Spectroscopy of Carbon Dots  
Analysis of the structure and functional surface groups of CDs was carried out using FTIR spectroscopy on a Bruker Invenio R IR spectrometer equipped with an ATR attachment based on a diamond crystal. The spectral resolution was 4 cm\({}^{-1}\). The obtained IR absorption spectra for the studied CDs are shown in Figure 2.  
The IR absorption spectra of three CD samples contain a similar set of bands that differ in relative absorption intensity. In the region of 3000-3700 cm\({}^{-1}\), there is a wide band caused by stretching vibrations of the surface hydroxyl and amino groups and OH groups of water molecules adsorbed on the surface of the samples. The relative intensity of this band for CD-poly and CD-COOH is significantly lower than for CD-OH. In the region of 2800-3000 cm\({}^{-1}\) there are bands of stretching vibrations of CH\({}_{\rm x}\) groups. In the region of 2055 cm\({}^{-1}\), there is a band that presumably belongs to overtones/combinations of various vibrations in CDs [36].  
The band at 1745 cm\({}^{-1}\) in the spectrum of CD-COOH corresponds to the stretching vibrations of C=O bonds of surface carboxyl groups. The weak relative intensity of this band is due to the fact that the COOH groups belong exclusively to the surface of this CD, while most other vibrational bands are due to the vibrations of the groups that are also in the CD's core. This situation is typical for CDs with a size of about 100 nm and does not manifest itself, for example, for nanodiamonds with a carboxylated surface: in them, the diamond core is not active, and the relative intensity of the vibration bands of all surface groups is much higher [37].  
Figure 2: IR absorption spectra of water and powders of synthesized CDs.  
In the region of 1640 cm\({}^{-1}\), there is a band of bending vibrations of OH bonds. In the spectra of CD-COOH and CD-OH, it manifests itself as a \"shoulder\" of a wide intense band with a maximum in the region of 1570 cm\({}^{-1}\), which is due to stretching vibrations of C=C bonds [36]. In the region of 1580-1650 cm\({}^{-1}\), the band of bending vibrations of NH groups overlaps with it.  
In the IR spectrum of the CD-poly, intense narrow bands are observed with maxima in the regions of 1600 and 1505 cm\({}^{-1}\), which, according to the authors of [36,38], are due to skeletal vibrations of the polycyclic aromatic elements of CDs. The absence of such bands in the spectra of CD-COOH and CD-OH indicates that the functionalization of these CDs occurred primarily due to the rupture and subsequent addition of radicals to such aromatic rings.  
The band with a maximum in the region of 1390 cm\({}^{-1}\) corresponds to stretching vibrations of C-O bonds in -COOH groups [39]. The band with a maximum in the region of 1335 cm\({}^{-1}\) is also due to C-O vibrations, but particularly in the C-O-C groups [40]. The set of bands in the range of 950-1180 cm\({}^{-1}\) is mainly due to stretching vibrations of C-O and bending vibrations of C-H bonds in various environments [36].  
An analysis of the IR spectra showed that the CD-poly contains a large number of polycyclic aromatic elements and various functional groups, such as C-O-C, C=C, N-H, OH. As a result of the functionalization of the CD-poly, on the surfaces of CD-COOH and CD-OH during the destruction of aromatic rings, a large number of oxygen-containing groups were formed, and the relative intensity of the bands due to vibrations of the C-O-C, C-O, and OH groups bonds increased. Additionally, in the spectra of CD-COOH bands corresponding to vibrations of carboxyl groups appeared. Thus, the results of IR absorption spectroscopy allow us to conclude that the functionalization of CD-COOH and CD-OH was successful (Figure 2).