## 4 Conclusions  
Finally, the comparative study of Gd-doped carbon NPs synthesized through microwave and hydrothermal methods reveals their specific structural and physical properties for potential applications in bimodal fluorescence/MRI bioimaging. The results of the structure analysis and optical spectroscopy show that the MWS samples are more crystalline ordered and possess stronger absorption in the visible spectral range which results in the high photoluminescence efficiency of the red-NIR under green light excitation. This fact suggests that microwave synthesis could be preferable when targeting luminescence-based imaging techniques, enabling sensitive cellular or subcellular visualization. Based on the relaxometry measurements, the HTS sample exhibited superior MRI relaxivity properties. The relatively long duration and spatially homogeneous temperature profile during the hydrothermal synthesis allowed us to realize the efficient incorporation of Gd-ions on the surface of CNPs, leading to enhanced MRI contrast. Thus, the hydrothermal method could be advantageous for producing Gd-doped carbon NPs with excellent MRI contrasting, enabling accurate anatomical imaging in a clinical setting. The longitudinal and transverse relaxivities for the HTS Gd-CNPs are significantly larger than that for commercially  
Figure 4: (**A**) Dependences of the reciprocal time of the longitudinal proton relaxation in aqueous suspensions of Gd-CNPs prepared by MWS (red circles) and HTS (blue squares) on Gd-concentration. (**B**) Concentration dependencies of the reciprocal time of the transverse proton relaxation for the same samples. The indicated longitudinal relaxivity, r\({}_{1}\), and transverse one, r\({}_{2}\), are obtained from linear fits of the corresponding concentration dependencies. The different titrations in the concentration dependencies for the MCW and HTS samples are related to their different initial Gd-content. available Gd-based CAs. The divergent properties observed between the two synthesis methods highlight the importance of selecting an appropriate synthesis approach based on the desired imaging modality. If MRI contrast enhancement is the primary goal, the hydrothermal method appears to be more suitable. Conversely, for biomedical applications where luminescence bioimaging is of greater importance, e.g., at the cell level and intraoperative visualization, the microwave method may be preferable. Future research should focus on optimizing the synthesis conditions of both methods to further improve the desired imaging properties. Additionally, exploring hybrid approaches that combine the strengths of both methods could potentially yield optimized Gd-doped CNPs and CDs with enhanced bimodal imaging capabilities. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13169322/s1](https://www.mdpi.com/article/10.3390/app13169322/s1)]([https://www.mdpi.com/article/10.3390/app13169322/s1](https://www.mdpi.com/article/10.3390/app13169322/s1)), Figure S1: TEM images and EDX elemental maps for HTS Gd-CDs; Figure S2: EDX spectrum of HTS Gd-CDs; Figure S3: EDS-TEM images and composition of an individual Gd-CD; Figure S4: EDX-TEM images and composition of MWS Gd-CD; Table S1: Elemental content of HTS Gd-CNPs; Table S2: Elemental content of MWS Gd-CNPs. Investigation and writing--original draft preparation, D.U.M. ; methodology and investigation, A.N.K. ; visualization and formal analysis, A.V.S. ; resources, methodology, and investigation, V.S.V. ; investigation, N.D.M. ; investigation, O.S.P. ; methodology and data curation, Y.A.P. ; methodology and writing--review and editing, A.N.B. ; supervision and writing--review and editing, V.Y.T. All authors have read and agreed to the published version of the manuscript. A.V.S. and V.S.V. acknowledge the financial support from the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2021-606). Not applicable. The data presented in this study are available on request from the corresponding authors.