Fundamentals of Quantum Sensing Free Radicals
The Basics Behind T1 Relaxometry and Nanodiamonds
Figure 1: Energy diagram of an electron which gets excited by a green photo from the ground state to the excited state and emits red photons from the excited state
Quantum sensing of free radicals relies on the excitation of electrons. When photons with appropriate wavelengths interact with electrons, they excite the electrons from their ground state to an excited state. As these excited electrons decay back to their ground state, they release photons. These emitted photons exhibit a red shift compared to the initial excitation wavelengths, allowing us to observe them.
The rate at which photons are emitted is influenced by the polarization of the electrons. Magnetic fields play a crucial role in altering the behavior of electrons in the ms = ±1 energy states. The stronger the magnetic interference around an electron, the shorter the decay time to the ground state. This principle forms the foundation of quantum sensing.
Figure 2: Nanodiamond (non-realistic representation of shape) with an NV center
To measure the polarization of the electrons, sensors are required which are sensitive to magnetic interference. The quantum sensors used for T1 relaxometry are nanodiamonds (FND), which contain Nitrogen-Vacancy (NV) centers. In the diamond lattice, a carbon atom is removed using an electron beam and replaced with a nitrogen atom. The size difference between the nitrogen and carbon atoms creates a vacancy next to the nitrogen atom. This vacancy contains unbound electrons that can be excited by laser light. The vacancy electrons are excited by laser light in the range of 500-600 nm and emit light in the range of 650-800 nm.
Free Radical Detection by Nanodiamonds
Free radicals are molecules that have an unbound electron. An unbound electron, with its magnetic dipole moment, causes a magnetic interference in its direct vicinity. The electrons in an NV-center are sensitive to this magnetic moment and as such alter the emission of photons. Free radicals include the hyperactive superoxide, hydroxyl radicals and others. The oxygen free radicals cause oxidative stress which damages the DNA, RNA, Proteins, lipids and ultimately leads to cell death if the concentration gets too high.
Nanodiamonds can be added to cells and are absorbed into the cells via endocytosis. Via a modified confocal microscopy system, it is possible to get measurements of the free radical concentration to a maximum of 50nm around the nanodiamond. The nanodiamonds can be functionalized as well. The nanodiamonds can be functionalized with antibodies, tags such as biotin or PEG. By directing the nanodiamonds, it is possible to get an accurate readout of free radical production and neutralization e.g., in the mitochondria.
Figure 3: A representation of the Quantum Sensing method in single cells. FNDs are introduced into cells, after which a laser is pulsed at the FND. The emitted photons are detected and measured to obtain a T1 curve
To determine the free radical concentration, emission time is correlated with the time between excitations. Laser pulses with increasing intervals excite the electrons, and the initial re-excitation is monitored to determine the photon emission rate. The dark time and emitted photons are plotted to create a relaxation curve. By fitting a mathematical model to this curve, the T1 relaxation time can be determined.
This method enables a new avenue to explore free radicals in single cells. Explore the difference of free radicals in dysfunctional and normal cells and correlate it to disease phenomena. At QT Sense we enable the detection of free radicals in single cells with unprecedented sensitivity, precision and temporal resolution.
