Nanodiamond Relaxometry-Based Detection of Free-Radical Species When Produced in Chemical Reactions in Biologically Relevant Conditions

Abstract

Diamond magnetometry is a quantum sensing method involving detection of magnetic resonances with nanoscale resolution. For instance, T1 relaxation measurements, inspired by equivalent concepts in magnetic resonance imaging (MRI), provide a signal that is equivalent to T1 in conventional MRI but in a nanoscale environment. We use nanodiamonds (between 40 and 120 nm) containing ensembles of specific defects called nitrogen vacancy (NV) centers. To perform a T1 relaxation measurement, we pump the NV center in the ground state (using a laser at 532 nm) and observe how long the NV center can remain in this state. Here, we use this method to provide real-time measurements of free radicals when they are generated in a chemical reaction. Specifically, we focus on the photolysis of H 2 O 2 as well as the so-called Haber–Weiss reaction. Both of these processes are important reactions in biological environments. Unlike other fluorescent probes, diamonds are able to determine spin noise from different species in real time. We also investigate different diamond probes and their ability to sense gadolinium spin labels. Although this study was performed in a clean environment, we take into account the effects of salts and proteins that are present in a biological environment. We conduct our experiments with nanodiamonds, which are compatible with intracellular measurements. We perform measurements between 0 and 10 8 nM, and we are able to reach detection limits down to the nanomolar range and typically find T1 times of a few 100 μs. This is an important step toward label-free nano-MRI signal quantification in biological environments.

Quantum monitoring of cellular metabolic activities in single mitochondria

“…This time, called relaxation time or T1 , is shortened by the presence of spin noise ( 46 ) from free radicals. In a previous work ( 45 ), we found that, under controlled conditions, the obtained plots could be fitted with the following equation …”

“…Gray lines show individual experiments on single cells. The left gray axis represents the approximated concentration determined from a calibration with *OH radicals in solution from the previous work ( 45 ). The experiments were repeated nine times.…”

“…This could make them more or less sensitive to magnetic dipole located on it. While both constants were found dependent on the surrounding free radical concentration, the longer one Tb was more susceptible to changes in magnetic noise and thus is more sensitive to changes in the surrounding radical concentration ( 45 ). For this reason, Tb was selected from these two relaxation constants for analysis and quantification, as it has proven to be more sensitive to changes from the surrounding …”

“…To estimate the radical concentrations from T1 values, we used a calibration in a controlled environment. This calibration was obtained from measurements of known concentrations of *OH radicals [for more information, see ( 45 )].…”

“…This sequence is especially convenient because it does not require microwaves. The technique has already been used for several different applications including detection of gadolinium ( 39 ), magnetic structures ( 40 ), nanoparticles ( 41 – 43 ), and, most recently, free radicals ( 44 , 45 ). We demonstrate their use for monitoring metabolic activity in single cells or organelles.…”