High-Frequency Electron Paramagnetic Resonance Spectroscopy of Nitroxide-Functionalized Nanodiamonds in Aqueous Solution

Abstract

Nanodiamond (ND) is an attractive class of nanomaterial for fluorescent labeling, magnetic sensing of biological molecules, and targeted drug delivery. Many of those applications require tethering of target biological molecules on the ND surface. Even though many approaches have been developed to attach macromolecules to the ND surface, it remains challenging to characterize dynamics of tethered molecule. Here, we show high-frequency electron paramagnetic resonance (HF EPR) spectroscopy of nitroxide-functionalized NDs. Nitroxide radical is a commonly used spin label to investigate dynamics of biological molecules. In the investigation, we developed a sample holder to overcome water absorption of HF microwave. Then, we demonstrated HF EPR spectroscopy of nitroxide-functionalized NDs in aqueous solution and showed clear spectral distinction of ND and nitroxide EPR signals. Moreover, through EPR spectral analysis, we investigate dynamics of nitroxide radicals on the ND surface. The demonstration sheds light on the use of HF EPR spectroscopy to investigate biological molecule-functionalized nanoparticles.

Nanoscale Dynamic Readout of a Chemical Redox Process Using Radicals Coupled with Nitrogen-Vacancy Centers in Nanodiamonds

“…The hydrophobicity of TEMPO-based nitroxides requires efficient colloidal stabilization of the nanoparticles intended for biologically relevant sensing . Previous studies simply modified the diamond surface using silica. ,, However, silica coating without additional polymeric coating does not sufficiently stabilize NDs in a biological environment, , especially in combination with the hydrophobic molecules such as TEMPO. Our approach is based on (i) increasing the solubility of TEMPO by attaching a hydrophilic linker and (ii) creating a branched, structurally dense protecting layer of alkynated hydrogel poly­(glycerol) , that can accommodate TEMPO on the ND surface (Scheme B).…”

Nanoscale spin detection of copper ions using double electron-electron resonance at room temperature

“…EPR based measurements in biophysics most commonly involve labeling with nitroxides [1]. Likewise, for single spin detection using NV centers in diamonds, nitroxide spin labels have been widely used [26,28,29]. Recently, the developments in spin-labeling techniques have led to alternative labels that use other organic radicals or paramagnetic metal ions [2].…”