## 2 Materials and Methods  
Microwave synthesis (MWS) was carried out by using a mixture of urea ((NH\({}_{2}\))\({}_{2}\)CO) 300 mg, citric acid (HOOC-CH\({}_{2}\)-C(OH)COOH-CH\({}_{2}\)COOH) 150 mg, and gadolinium chloride (GdCl\({}_{3}\)-6H\({}_{2}\)O) 30 mg as precursors, which were dissolved in 5 mL of deionized water (18 M\(\Omega\)-cm). Then, synthesis was carried out in a microwave reactor (Anton Paar Monowave 450, Strasnice, Czech Republic) at a temperature of 180 \({}^{\circ}\)C with stirring at 1200 rpm for 15 min. The precursors' ratios and synthesis conditions were chosen to obtain a high yield of small CNPs with efficient red PL [18; 19]. The MWS process resulted in the formation of yellow-colored solutions, which later precipitated as a white solid. Centrifugation was performed to separate the precipitate, and then 20 mL of isopropyl alcohol was added to the 5 mL solution in order to separate small CNPs and byproducts as larger amorphous graphite NPs because of a lower density and surface tension of the solvent. The supernatant of Gd-CNPs in isopropyl alcohol was selected and the solution was subsequently evaporated. Then, the dried Gd-CNPs were transferred back into water, resulting in a transparent solution with red color. For hydrothermal synthesis (HTS) we used aqueous solutions containing urea 50 mg/mL, citric acid 25 mg/mL, and gadolinium chloride 2 mg/mL as precursors. The synthesis was carried out in a 45 mL stainless steel hydrothermal reactor with a Teflon liner at 150 \({}^{\circ}\)C for 3 h followed by cooling down in a natural way for 2 h. The HTS process yielded transparent suspensions Gd-doped CNPs with red color. Prior to the structural studies and optical and MRI experiments, the prepared aqueous suspensions of Gd-CNPs were purified by dialysis in deionized water through MWCO 500 Da membranes for 6 h.  
To estimate the total mass concentration of Gd-CNPs in the solution, we used a GR-202 analytical scale (A&D Company, Ltd., Tokyo, Japan) with a measurement accuracy of 0.01 mg. Then, 100-500 mL of the aqueous solution of Gd-CNPs were taken and dried on a glass substate at 80 \({}^{\circ}\)C for 20 min. The Gd concentration in Gd-CNPs was estimated by means of the X-ray fluorescence analysis (XFA) with a Radian-02 X-ray diffractometer with a Cu-K\(\alpha\) radiation source for the samples dried on a molybdenum substrate at 80 \({}^{\circ}\)C for 20 min. A calibration of the XFA method was carried out by using dried solutions of GdCl\({}_{3}\) with known Gd concentration as the reference. The measurements were carried out at room temperature in the air. Sizes, structures, and compositions of the prepared NPs were studied by using a JEOL JEM-2100 transmission electron microscope (TEM) (JEOL, Inc., Peabody, MA, USA), which could also operate in a high-resolution mode (HR-TEM). Size distributions of Gd-CNPs were obtained from their TEM images by using free ImageJ software (version 1.53t). The elemental compositions of the prepared NPs were determined by means of the energy dispersive X-ray spectroscopy (EDX) with the same TEM apparatus equipped with an Aztech X-Max 100 EDX device (Oxford Instruments, Abingdon, UK). To analyze the chemical bonds in Gd-CNPs, measurements by means of the Fourier-transform infrared spectroscopy (FTIR) were carried out with a Bruker IFS-66v/s spectrometer (Bruker, Billerica, MA, USA) in the range of 500-4000 cm\({}^{-1}\) with steps of 4 cm\({}^{-1}\) in the geometry of attenuated total reflection (ATR) with a diamond ATR-crystal. Optical absorbance spectra of aqueous suspensions of Gd-CNPs in quartz cuvettes were measured by using a UV-VIS 752P spectrophotometer in the range of 250-900 nm with a resolution of 1 nm. The concentrations of the studied solutions were typically 0.1 mg/mL. PL spectra of the prepared aqueous suspensions of Cd-CNPs were measured in the spectral range from 350 to 900 nm by using a Mightex compact spectrometer (Mightex, Toronto, ON, Canada) with a CCD array.