## 2 Experimental Section  
### Synthesis and Surface Functionalization of Gd\({}_{3}\)N@C\({}_{80}\)  
Gadolinium containing fullerenes, Gd\({}_{3}\)N@C\({}_{80}\), were prepared by vaporization of graphite rods packed with a mixture of Gd\({}_{2}\)O\({}_{3}\), graphite powder and metallic copper with a weight ratio of 2.0/1.0/2.1 under a dynamic flow of He and N\({}_{2}\) (flow rate ratio of N\({}_{2}\):He = 3:100) in a Kratschmer-Huffman generator as previously reported [43]. Fullerene products were extracted from the soot with xylene in a Soxlet extractor and the soluble empty-caged fullerenes and endohedral metallofullerenes are dissolved in the solvent. The Gd\({}_{3}\)N@C\({}_{80}\) product was purified by loading the solution containing fullerenes and metallofullerenes onto a cyclopentadiene functionalized Merrifield resin column eluted with toluene, which selectively removes empty-caged fullerenes [44]. Removal of the empty cage fullerenes was verified by HPLC (column: Cosmosil 5-PBB; flow-rate: 1 mL/min; mobile phase: toluene). As can be seen in Figure 1 (top trace), the empty cage fullerenes are a predominant product in the direct soot extraction with the endohedral Gd\({}_{3}\)N@C\({}_{80}\) species having a retention time around 28 min. After elution through the Merrifield resin column [Figure 1 (bottom trace)], the empty cages are greatly reduced and further endohedral products become evident beyond the Gd\({}_{3}\)N@C\({}_{80}\) peak. In order to obtain only the Gd\({}_{3}\)N@C\({}_{80}\) product, further purification was performed by fraction collecting with multi-stage HPLC (column: Cosmosil 5-PYE; flow-rate: 2 mL/min; mobile phase: toluene), which separates different species of metallofullerenes. Verification of the pure Gd\({}_{3}\)N@C\({}_{80}\) product was performed by HPLC and laser desorption time-of-flight mass spectrometry (LD-TOF-MS). Figure 2 (top) shows only one peak for the fraction-collection purified product. The experimental isotopic distribution obtained for this product by LD-TOF-MS matched the theoretical isotopic distribution calculated for Gd\({}_{3}\)NC\({}_{80}\) as shown in Figure 2 (bottom) giving verification of the product being pure Gd\({}_{3}\)N@C\({}_{80}\).  
Figure 1: HPLC traces of the fullerene and metallofullerene solution before (**top**) and after (**bottom**) selective removal of empty-caged fullerenes with cyclopentadiene functionalized Merrifield resin column.  
Figure 2: **(top)** HPLC trace of purified Gd\({}_{3}\)N_@_C\({}_{80}\) and (**bottom**) laser desorption time-of-flight (LD-TOF) mass spectrometry of the purified Gd\({}_{3}\)N_@_C\({}_{80}\). Inset: expansion of the spectrum showing theoretical and experimental isotopic distribution.  
Toluene was then removed by rotovap to yield the dry purified Gd\({}_{3}\)N@C\({}_{80}\) product, and functionalization of the Gd\({}_{3}\)N@C\({}_{80}\) with carboxyl and hydroxyl groups was carried out by methods detailed previously [34]. Briefly, succinic acid acyl peroxide (3.2 mg) was added to Gd\({}_{3}\)N@C\({}_{80}\) (4 mg) dissolved in 5 mL of o-dichlorobenzene. The solution was de-oxygenated with flowing argon and heated at 84 \({}^{\circ}\)C for 48 h. At 12 h intervals, additional succinic acid acyl peroxide (3.2 mg) was added to ensure adequate carboxyl functionalization for aqueous solubility. After 48 h, a brown precipitant was formed from which the water soluble product was extracted and endowed with hydroxyl groups by addition of 8 mL of 0.2 M NaOH. The aqueous solution was concentrated and purified using a Sephadex G-25 (Parmacia) size-exclusion gel column. The product was eluted using deionized water as a narrow brown band with pH = 7-7.5. Following the purification steps, the Gd\({}^{3+}\) concentration was determined by inductively coupled plasma mass spectrometry (ICP-MS). This was performed by digesting triplicate samples in Optima grade nitric acid and measuring for Gd against a standard curve.