Thermometry – the measurement of temperature – is critical to a wide range of applications, including many industrial processes, biomedical monitoring, and environmental regulatory systems. However, measuring temperature in the presence of high RF (radio frequency) or other electromagnetic fields – such as are found in aerospace, automotive and medical systems – cannot be accomplished using electrical thermometric probes.
In these cases, optical sensors that allow scientists to perform thermometry based on thermally-driven changes in ﬂuorescence (brightness) are the instrument of choice. While typical ﬂuorescence thermometers use millimeter-scale optical probes, smaller devices are needed to measure temperatures in intracellular and other nanoscale environments. As a result, the field has witnessed the development of nanoscale ﬂuorescence thermometers based on quantum dots, rare-earth ions and nanogels.
Recently, scientists at the University of California, Santa Barbara, Ames Laboratory, and the University of Chicago have demonstrated extremely sensitive ﬂuorescence thermometry techniques based on nitrogen vacancy (NV) centers in diamond – paramagnetic defects in which electron spins can be manipulated at room temperature by applying a combination of microwave radiation and light.
This results in sharp resonances in the defects’ photoluminescence intensity as a function of the frequency of the microwave radiation, with the frequency dependent on external variables such as magnetic fields, electric fields, and, in this work, temperature. The researchers also showed that these techniques can be applied over a broad temperature range and in both finite and near-zero magnetic field environments, suggesting that single spin quantum coherence could be leveraged for sensitive thermometry in a broad range of biological and microscale systems.
Nitrogen vacancy center in diamond. (A) Depiction of a nitrogen vacancy (NV) center in the diamond lattice. The wavy green arrow represents the 532-nm laser used for optical excitation and the wavy red arrow represents the phonon-broadened fluorescence used to measure the spin state. (B) Fine structure of the NV center ground state as a function of axial magnetic field. The light bulbs represent the relative fluorescence difference for the ms = 0 and ms = ±1 states. Temperature changes shift the crystal field splitting (D), whereas magnetic fields (Bz) split the ms = ±1 sublevels. This difference enables dynamical decoupling pulse sequences that move the spin between all three states to selectively measure temperature shifts and mitigate magnetic noise. (C) Ramsey measurement performed on the ms = 0 to ms = −1 transition (Bz = 40 G). Inset illustrates the pulse sequence. The short inhomogeneous spin lifetime (T2*) limits the sensitivity of conventional NV center thermometry. The uncertainties in IPL, estimated from the photon shot noise, are ∼0.003. The microwave carrier frequency was detuned from the mI = 0 hyperfine resonance by ∼3.5MHz to induce oscillations in IPL. The fluorescence signal exhibits a beating caused by the three hyperfine resonances and weak coupling to a nearby 13C spin. Gray dashed lines show the fluorescence intensity of ms = 0 and ms = −1 as determined by independent measurements. Copyright © PNAS, doi:10.1073/pnas.1306825110