## 3 Results and Discussion  
Figure 1A,B show typical TEM images of dried diluted suspensions of the Cd-CNPs prepared by MWS and THS, respectively. The MWS sample consists of nearly spherical NPs with size distribution, which can be fitted by a lognormal function with a mean size of 5 nm (right inset in Figure 1A). A HR-TEM image of a selected CNP reveals atomic planes with a lattice spacing of 0.22 \(\pm\) 0.01 nm (left inset in Figure 1A). This lattice parameter is close to the interatomic distance for the (01\(\overline{1}\)0) crystallographic plane of graphite [25] and it is corresponding to the in-plane periodicity in graphene-like nanosheets [26]. The same lattice spacing was observed early for CDs and CNPs synthesized by MCW [16,18], HTS [20] or similar methods at about 180 \({}^{\circ}\)C and slightly higher temperatures [26]. It is interesting to note that the graphene-like interlayer distances have been preserved in Gd-doped CNPs similar to the results for catalytically active CD-based nanocomposites, which were highly doped with different impurities such as aluminum and tellurium [26,27]. It means that the incorporation of both light and heavy elements does not necessarily destroy the atomic order in CDs and CNPs. An analysis of the TEM data of HTS samples (Figure 1B) demonstrates NPs with size distribution covered by a lognormal function centered at 4 nm (right inset in Figure 1B). In contrast to the MWS samples, the HTS ones do not exhibit a prominent lattice structure as it is checked by the HR-TEM analysis (left inset in Figure 1B). This fact can be explained by the relatively low temperature of 150 \({}^{\circ}\)C during the HTS treatment which is not sufficient to form the crystalline order in CNP. Note that the similar HTS procedure at 200 \({}^{\circ}\)C resulted in the formation of small graphite NPs, which were identified by the characteristic interatomic spacing of about 0.334 nm [19]. The graphite-like CNPs seem to be not optimal for bioimaging applications because of the nonradiative losses of light in the electrically conductive cores. According to the EDX analysis (see Figures S2 and S4 and Tables S1 and S2 in the Supplementary Materials) the Cd-content was about 0.04 and 0.5 at. % for the samples prepared by MWS and HTS methods, respectively. The higher Gd-doping level of the latter was achieved despite the lower Gd-content in the precursor solution in comparison with the MCW samples. This fact indicates the better incorporation of Gd-ions in amorphous CNPs than in the nanocrystalline ones. Note, the relatively high concentration of oxygen (14 at. %) and nitrogen (13 at. %) correlate with the higher Gd content in the HTS sample. In contrast, the MCW sample exhibits a rather low concentration of oxygen (7.8 at. %) and nitrogen (0.16 at. %) atoms. This fact can be interpreted as a predominant surface location of these atoms and Gd-impurities in the MWS Gd-CDs, while the HTS-prepared mostly amorphous samples can contain the impurities in the whole NPs' volume. According to the FTIR spectroscopy data (Figure 2A), the chemical bonds in the prepared Gd-CNPs are mostly related to the well-known vibration modes of the carbonyl, carboxyl, and other functional groups, which contain carbon, oxygen, hydrogen, and nitrogen (Table 1). In particular, the spectrum of the HTS-prepared sample exhibits a significantly higher signal at 1557 cm\({}^{-1}\), which is assigned to the vibration and deformation bands of N-H groups [28]. Additionally, there is a strong signal at 1391 cm\({}^{-1}\) associated with the C=C bond vibrations [28]. This line is also close to the position of C-O bond vibrations (1388 cm\({}^{-1}\)) detected early in Gd-CDs [19]. In contrast, the spectrum of the MWS sample exhibits a strong asymmetric peak at 1438 cm\({}^{-1}\), which can be related to the C-N bonds from aromatic amine groups and C-C vibrations of the carbon mesh frame [8; 10].