Biosensing with NV-centers in optically trapped nanodiamonds

Nanodiamonds with color centers are to an increasing degree investigated as intracellular biosensors for magnetic fields, electrical fields, or temperature changes, as well as for abundance of free radicals or pH inside live cells. A common color-center is the nitrogen-vacancy (NV) center, where a substitutional nitrogen atom is positioned next to a vacancy within the diamond host crystal. The nanodiamond with negatively charged NV^- center is particularly versatile due to its biocompatibility and its purely optical addressability. Our work aims to use NV-center nanodiamonds both as intracellular biosensors and as probe particles within an optical trap to determine the viscoelastic properties of the intracellular environment in single-cell studies. For this to be successful, several prior steps are needed: 1) The uptake of nanodiamonds within the cells should be characterized, including studies of subcellular localization, and a controllable protocol developed; 2) Any effect of the trapping laser on the NV-center sensing should be characterized and understood, and a protocol for stable trapping along with accurate biosensing should be developed. In this work we summarize the preliminary findings of our ongoing investigations to address these points. We show results of T1-relaxometry with and without CW NIR laser irradiation in a suspension cell model, analyze optical trapping of nanodiamonds with CW light in an adhesion cell model, and investigate implications of the presence of the optical trapping laser on T1-relaxometry measurements.