Chelator-Free Conjugation of ^99mTc and Gd³⁺ to PEGylated Nanographene Oxide for Dual-Modality SPECT/MR Imaging of Lymph Nodes

PEGylated ultrasmall nanographene oxide (usNGO-PEG) has exhibited a great potential in nanotheranostics due to its newly discovered physicochemical properties derived from the rich functional groups and bonds. Herein, we developed a general, simple, and kitlike preparation approach for ^99mTc‑ and Gd-anchored NGO-PEG using a chelator-free strategy. In this strategy, [^99mTc^I(CO)₃(OH₂)₃]⁺ (abbreviated to ^99mTc^I) and GdCl₃ were mixed with usNGO-PEG to yield ^99mTc‑ and Gd-usNGO-PEG via the synergistic coordination of N and O atoms from NGO and PEG with ^99mTc^I and Gd³⁺ without additional exogenous chelators. Under optimized conditions, the nanoprobes ^99mTc‑ and Gd-usNGO-PEG were reliably prepared with high yields and good stability. Serial comparative experiments of the labeling yield, the measurements of −NH₂ density and ζ-potentials, and various characterizations including energy-dispersive X-ray analysis spectroscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy demonstrated that both usNGO and PEG synergistically provide the electron-donating atoms O and N to coordinate with ^99mTc^I and Gd to form stable nanocomplexes. Furthermore, both ^99mTc- and Gd-usNGO-PEG exhibited excellent in vivo imaging of lymph nodes using single-photon emission computed tomography/computed tomography (SPECT/CT) and magnetic resonance (MR) imaging after local injection. Therefore, these results showed the successful establishment of ^99mTc‑ and Gd-anchored usNGO-PEG using a chelator-free strategy and the potential of multimodality SPECT/CT and MR imaging of lymph nodes.