## 3 Results and Discussion
Both samples exhibit similar absorption peaks in the region of 2500-3500 cm\({}^{-1}\), which corresponds to the superposition of the stretching vibrations of N-H, O-H, and C-H groups [29; 30]. Note, the total absorption by the chemical bonds in the MWS sample is lower than that for the HTS one (see Figure 2A). For example, the absorption at 1078 cm\({}^{-1}\), which corresponds to the C-O and C-N groups, is five times stronger for the HTS Gd-CNPs than that for the MWS ones. This fact correlates with the higher contents of oxygen and nitrogen in the formers (see Tables S1 and S2 in the Supplementary Materials). Figure 1: **(A)** TEM image of MWS Gd-CNPs; the left-hand and right-hand insets show an HR-TEM image of an individual NP and size distribution obtained from the TEM image analysis, respectively. **(B)** TEM image of HTS Gd-CNPs; the left-hand and right-hand insets show an HR-TEM image of an individual NP and size distribution obtained from the TEM image analysis, respectively. The size distributions were obtained by counting N = 189 and 188 particles for the TEM images of MWS and HTS samples, respectively. Figure 2B shows UV-visible-NIR absorption spectra of the aqueous suspensions of both types of Gd-CNPs of the same total concentration. The spectra consist of three main bands at about 280, 370, and 490 nm. While the UV absorption bands are similar for both types of Gd-CNPs, the visible absorption band is significantly stronger for the MWS sample than for the HTS one. Because the former exhibits the higher absorption via the C-N bonds from aromatic amine groups it can be supposed that the visible absorption band also originates from those amine groups [8,29]. Figure 3A shows PL spectra of the prepared aqueous suspensions of Gd-CNPs under UV excitation. The samples of both types are fluorescent mainly in the blue-green spectral range with tails in the red-NIR region. The PL intensity of the MWS sample is slightly larger than that of the HTS one. Since the UV absorption spectra of both samples are almost the same value (see Figure 2B), the difference in the PL intensity indicates the different ratio between the radiative and non-radiative processes in the MWS and HTS samples. The non-radiative losses in the latter can be related to the larger Gd-content since Gd-ions can act as quenchers of the fluorescence of CNPs as it was found for different metal cations [7,29,30]. The stronger PL QY of the MWS sample can be related to the better passivation of the non-radiative recombination sites due to the higher temperature of the synthesis. This fact correlates with the better crystallinity of the MWS Gd-CNPs (see Figure 1A), which represent mostly nanocrystalline CDs covered by thin shells of the functional groups listed \begin{table}
\begin{tabular}{c c c} \hline
**Wavenumber (cm\({}^{-1}\))** & **Functional Groups and Bands** & **References** \\ \hline
3375–3468 & N–H & [8,20,30] \\
3060–3431 & O–H & [8,20,22] \\
3050 & N–H & [30] \\
2854 & C–H & [22] \\
1652 & C=C isolated and conjugated & [29] \\
1651 & C=O stretching & [28] \\
1557 & N–H deformation & [29] \\
1438 & C–N in aromatic amine and C–C of the carbon mesh frame & [8,10] \\
1391 & C=C stretching and bending & [28] \\
1388 & C–O & [19] \\
1254 & C–O–C & [18] \\
1078–1122 & C–O, C–N & [9,28,29] \\ \hline \end{tabular}
\end{table} 
Table 1: Assignment of the infrared absorption bands for Gd-CNPs. Figure 2: **(A)** FTIR absorption spectra of dried Gd-CNPs prepared by MWS (red curve) and HTS (blue curve); **(B)** UV–visible absorption spectra of aqueous suspensions of Gd-CNPs prepared by MWS (red curve) and HTS (blue curve). in Table 1. Note, the PL QY of the latter under the UV excitation is estimated to be about 80 \(\pm\) 10% which indicates the dominant role of the radiative recombination in the MWS Gd-NPs. Being excited with the green light at 532 nm, the MWS-prepared Gd-CNPs exhibit significantly stronger PL emission compared to the HTS samples, as shown in Figure 3B.