Reduction-Driven Modulation of Relaxivity in Graphene Oxide Derivatives

Graphene oxide (GO)-based nanomaterials have emerged as promising platforms in biomedicine, particularly for their potential use as carriers of magnetic resonance imaging (MRI) contrast agents (CAs). In this study, we investigate the MRI CA performance of reduced graphene oxide and its dependence on the degree of reduction. To ensure full control, GO was synthesized in-house via a modified Hummers' method and subsequently reduced by two distinct approaches: mild thermal reduction (rGO1) and selective chemical reduction with sodium borohydride (rGO2). Comprehensive physicochemical characterization, including Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, differential scanning calorimetry, and thermogravimetric analysis (TGA), was performed to evaluate the structural changes induced by each reduction process. The MRI performance of GO, rGO1, and rGO2 was assessed at 1.0 T alongside electron paramagnetic resonance (EPR) measurements, while inductively coupled plasma-mass spectrometry (ICP-MS) quantified residual manganese and iron impurities. EPR and ICP-MS demonstrated that these impurities made a negligible contribution to the relaxivity. Our findings reveal a significant enhancement in MRI relaxivities with an increasing degree of reduction. Chemically reduced rGO2 exhibits the highest longitudinal relaxivity (r ₁), approximately 3.9-fold greater than that of pristine GO, and transverse relaxivity (r ₂), approximately 4.6-fold greater. Strong linear correlations between relaxivities and XPS-derived structural parameters, including the C/O ratio and C-C/CC content, indicate that carbon structural defects govern MRI contrast performance. Overall, these results establish a framework for the rational design of rGO-based CAs through controlled reduction of two-dimensional carbon materials, enabling optimized imaging performance and supporting future biomedical translation.