Due to its remarkable properties, graphene has become a basis of many novel microelectronic devices. However its functional derivatives including graphene oxide (GO) can be mass-produced at lower costs and retain many properties of graphene in addition being water soluble. GO also exhibits fluorescence in the visible/near-IR suitable for applications in optoelectronics and biomedicine. The most novel biomedical applications of GO include, sensing, molecular imaging in visible/near-IR and drug delivery. In all these applications GO was mostly proposed as an agent for in-vitro or ex-vivo imaging due to its emission in visible or near-IR. In this work we combine GO with superparamagnetic iron oxide nanoparticles (Fe₃O₄ NPs) to provide the unique capacity of multimodal imaging and magnetic targeted drug delivery.

Fe₃O₄ NPs are biocompatible and have been used as biosensing, magnetic assisted drug delivery system and also a magnetic resonance contrast agent for MRI. Additionally, these NPs can be attracted by a magnet allowing for targeting of the attached therapeutics via the magnetic field to specific organ and/or cancer tumor.

In this work, we fabricate nanoconjugates of GO- Fe₃O₄ for combining GO fluorescence, sensing and drug delivery properties with magnetic targeting and a possibility of MRI imaging provided by Fe₃O_4. GO flakes were processed to have diameter mean size of 260 nm optimal for cellular internalization and conjugated to Fe₃O₄ NPs.

For the specific application of magnetic resonance imaging, the quality of an MRI contrast agent is often evaluated by the relaxivity parameters r1 or r2, which describe the ability of a contrast agent to shorten the T1 or T2 relaxation time of water. GO-Fe₃O₄ showed suitable for MRI imaging relaxivity values (longitudinal r₁ and transvers r₂ in water at 37°C), of r₁=6.6 mM^-1s^-1 and r₂=71.1 mM^-1s^-1. In fact, these values are better than those for single Fe₃O₄ NPs: the conjugated samples showed relaxivity ratio r₂/r₁>2, placing them in the category of negative contrast agent. The cytotoxicity of GO-Fe₃O₄ established via MTT assay in HeLa cells showed over 80% viability at the imaging concentrations mimicking that of GO. Cellular uptake and imaging studies show stable internalization and localized emission inside the cells. Additionally this system provides an efficient route to pH sensing of cellular environment as spectra of in GO-Fe₃O₄ are highly dependent on the pH of the media. This allows to assed the presence of acidic cancerous environments for cancer therapy and detection.

As a result, we propose that this new multifunctional GO-Fe₃O₄ composite can be used as a magnetic targeted drug delivery system, as well as biosensing and bimodal (MRI and fluorescence) imaging agent.