# Thermometric Characterization of Fluorescent Nanodiamonds Suitable for Biomedical Applications  
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1 Physics PhD Program, Department of Physics Research, University of Sonora, Rosales and Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
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2 Department of Physics, University of Sonora, Rosales y Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
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3 Department of Physics Research, University of Sonora, Rosales and Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
[PERSON]  
3 Department of Physics Research, University of Sonora, Rosales y Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
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4 Department of Geology, University of Sonora, Rosales and Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
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3 Department of Physics Research, University of Sonora, Rosales y Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
[PERSON]  
2 Department of Physics Research, University of Sonora, Rosales y Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
[PERSON]  
3 Department of Physics Research, University of Sonora, Rosales and Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
[PERSON]  
3 Department of Physics Research, University of Sonora, Rosales and Encinas, Hermosillo 83000, Sonora, Mexico; [EMAIL_ADDRESS]  
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5 Nanotechnology PhD Program, Department of Physics, University of Sonora, Rosales y Encinas, Hermosillo 83000, Sonora, Mexico; 583  
ratio [11] and charged surface [12], nanodiamonds exhibit remarkable loading/binding capacities for a wide variety of therapeutic compounds [13; 14; 15; 16; 17]. With highly ordered aspect ratios close to unity, nanodiamonds have shown to be biologically stable [18], enabling them to prevent stress-inflammatory and apoptotic cellular reactions [19]. Some studies have supported nanodiamonds inherently susceptible biological performance in water suspension once incubated with cells [11; 20]. Diamonds may contain luminescent defects or colour centres due to different impurities or defects embedded within the crystal lattice [21; 22]. Amongst these diamond colour centres, the nitrogen-vacancy (NV) colour centres are the most notable as they possess a stable fluorescence spectrum spanning from the visible and near-infrared regions of the electromagnetic spectrum [22], which also broadens and quenches as the nanodiamond particle size decreases [23; 24]. Characteristic emission lines are at 575 nm (NV\({}^{0}\)) and 637 nm (NV\({}^{-}\)) corresponding to the neutral or negatively charged centre, respectively. These colour centres are known to allocate 5 (NV\({}^{0}\)) or 6 (NV\({}^{-}\)) electrons and give rise to two possible electronic structures [25; 26; 27]. Each charge state population can be tuned to match a maximum of luminescent intensity given an excitation's wavelength and power [28]. Additionally, an excitation wavelength within 450-575 nm origins that the NV centres continuously switch between the two charge states [29]. Further studies show that under the typical 532 nm excitation wavelength, the probabilities of finding the NV centre in the NV\({}^{-}\) and NV\({}^{0}\) configurations are 70% and 30%, respectively [28; 30]. These optical properties make fluorescent nanodiamonds a suitable probe for bioimaging and thermic sensing applications in cellular environments [31; 32]. Moreover, the NV\({}^{-}\) centre is highly photostable, with no photobleaching signs, even under high power excitation at the single-molecule level [7; 8]. Many cellular events are temperature dependent [33; 34; 35] and several studies have demonstrated how cancer cells exhibit a higher temperature than healthy cells due to increased metabolism [35; 36]. At this point, accurate data on the local temperature in biological systems is necessary to understand their dynamical behaviour and general state [37]. In these systems, it is desirable to have a non-invasive method to perform local temperature measurements at micron and sub-micron scales.