## 3 Optical Sensing with NDs
### Other Implementations of NV-NDs  
Local, environmental, mechanical stress acts as a strain field on the fluorescent PLA NDs that can be detected via ODMR as an additional source of the ODMR fluorescence dip splitting. We demonstrated in [64] that such a field is well explained by taking into account the spin-Hamiltonian of the NV center with the added of a potential term, Vgs, which accounts for the interaction of the NV center with the local strain field **e**. Strain fields down to \(10^{-5}\) were detected and good compatibility with Raman peak shifts was proven. Figure 17 reports the wide-field ODMR where micrometric-sized regions-of-interest (ROIs) can be extrapolated in ODMR curves [64].  
NDs are able to determine the structure of single proteins like ferritin. [PERSON] et al. [111] where able to detect the presence of ferritin molecules on the NDs' surface via the increased magnetic relaxation experienced in such a situation compared to uncoated NDs. The strongly fluctuating spin bath created by the ferritin coating at room temperature made the spin relaxation time (T1) of the NV center relax 10 times faster than that of the uncoated NDs, see Figure 18. It is worth mentioning here that single Gd ions have been detected on the surface of NDs with this technique [112].  
To conclude this section on sensing by NV-containing NDs, we want to underline that PLA NDs permit \"doping\" of the NDs with, for example, silicon, germanium, or other dopants, by using doped graphite targets. This will contribute to the exploration of other emission spectral regions (see Figure 19), which could open a wealth of new sensing opportunities.  
Figure 17: Spatially resolved ODMR wide-field imaging and strain-field dependence. Panel (**a**): Schematics of the inverted wide-field ODMR microscope. The green laser is steered onto the sample and focused through the 40\(\times\) objective through a dichroic mirror transmitting the red photoluminescence. The RF irradiation is obtained with an Au-coated copper loop, here represented by the microwave antenna wire located under the sample’s surface. Panel (**b**): Temporal diagram of the continuous wave electron spin resonance experiment. The camera detects the PL image synchronously with the RF irradiation, and the laser is kept ON all along the measurement time. Panel (**c**) Photoluminescence images as the RF frequency sweeps in the 2.85-2.90 GHz range. An ROI (red square) containing a bright spot was selected and the average intensity inside that ROI was plotted as a function of the RF frequency to generate an ODMR spectrum. Panel (**d**) reports three typical examples of signals on diamond nano-agglomerates showing different strain effects (curves d1, d2, and d3). Reproduced with permission from [64], Royal Society of Chemistry, 2018.  
Figure 19: Fluorescence spectra of color centers in diamonds under 532 nm laser excitation. Lead-related (Pb-rel), germanium-vacancy (GeV), tin-vacancy (SnV), and silicon-vacancy (SiV). Reproduced with permission from [113], Springer, 2013.  
Figure 18: (**a**) High resolution Transmission Electron Microscopy (TEM) of ferritin molecules (right) showing the iron-containing core and a single ND covered with ferritin (left). (**b**) The spin relaxation time T1 of naked NV (left) and in ferritin-coated NDs (left, inset). Statistical distribution of the T1 for bare NDs (right, blue bars) and for ferritin-coated NDs (right, red bars). Reprinted with permission from [111], American Chemical Society, 2013.