# Carbon Dots with Up-Conversion Luminescence as pH Nanosensor
## 1 Introduction  
In recent years, there has been a rapidly growing interest in the nonlinear optical properties of carbon nanoparticles [1; 2; 3]. This interest has been caused, among other things, by the wide prospects for the use of carbon dots (CDs) and nanodiamonds as medical nanoagent photoluminescence markers for the rapid and sensitive detection of analyte, drug carriers, adsorbents, etc. [4; 5; 6; 7; 8]. Together with the photostability and a high quantum yield of luminescence, solubility in water, biocompatibility, and the possibility of surface modification, the recently discovered property of various carbon dots to luminsee in the anti-Stokes region significantly expands the prospects for the use of such CDs in biomedicine. Upon excitation of the up-conversion luminescence (UCL) of nanoparticles, the autofluorescence of biological tissue is not excited, while UCL itself falls into the spectral region of the biological \"transparency window\", which ensures deep penetration of radiation in tissue. The first report on the observation of UCL of carbon dots, upon the excitation of CDs obtained by laser ablation by pulsed laser radiation with a wavelength of 800 nm, was made by [PERSON] et al. [9] in 2007. The CDs' UCL spectrum was a broad band with a maximum near 470 nm. The authors found a quadratic dependence of the UCL intensity of the CDs on the excitation power and explained the observed UCL as a result of two-photon absorption. From 2007 to 2013, numerous publications appeared, including those in high-ranking journals, in which the authors reported on the observation of CDs' UCL when excited by a conventional xenon lamp of a spectrofluorimeter [10; 11; 12]. However, the authors of [13; 14] showed that the supposed CD UCL signals observed on the spectrofluorimeter are artifacts such as interference from leaking second-order diffraction of the excitation light when broad-band light sources and diffraction gratings are used for excitation wavelengthselection. According to [9; 14; 15], the main criterion for a \"real\" CD UCL is the quadratic dependence of the UCL intensity on the excitation radiation intensity. The results of active studies on CDs' UCL over the past 5 years have shown that the properties of UCL of CDs of different synthesis differ significantly [9; 14; 16; 17; 18]. This is explained by the existence of various mechanisms for the formation of UCL [19], including UCL of CDs: two-photon absorption through real and virtual levels [9; 14; 15; 20], multiphoton absorption [21], thermal activation [22], and the state of surface traps [20; 23]. In the vast majority of publications, two-photon absorption is considered to be the main one. However, the dependences of the CDs' UCL intensity on the power of the exciting radiation are not always quadratic. For example, for CDs doped with nitrogen by the photochemical method, these dependences resembled those for lanthanoid-based up-conversion nanoparticles and was linear in the double-log coordinates with the slope 0.69 [18], while for thermally-activated CDs, this dependence was found to be linear in standard linear coordinates (as well as in the double-log coordinates with the slope 1) [22]. The observed dependences of the CDs' UCL intensity on the excitation wavelength also differ. For CDs synthesized by different methods or doped with different atoms, these dependences have one [21], two [18; 24], or three [25] maxima with different positions. For other CDs, such as CDs simultaneously doped with S and Se [16] and for CDs synthesized by the Hummers method [17], excitation wavelength-independent NIR emission was observed. Thus, the accumulated results of UCL studies of various CDs at this stage are characterized by many contradictions and uncertainty. The next step is to search for correlations and systematize all the data obtained.