# Sensing of Transition Metals by Top-Down Carbon Dots
## 1 Introduction  
Carbon quantum dots (CQDs) are nanoparticles composed of a rich carbon core, with dimensions of less than \(\approx\)10 nm. Their most important characteristic is powerful luminescence, which is combined with many other advantages, such as low cost and ease of synthesis, high solubility in water, biocompatibility, high sensitivity to the external environment, and a marked ability to donate and accept electrons [1, 2, 3, 4]. The combination of all these features ensures that CQDs can be used in a very wide range of applications, including many different fields such as imaging, optoelectronics, and sensing [5, 6, 7, 8, 9, 10]. The CQDs optical absorption covers the UV-visible range, and this allows their photoexcitation in a very wide spectral range, causing emission tuning, that is, the position of the emission peak in the visible range changes with the excitation wavelength [2, 11, 12]. This gives a multi-coloured emission from blue to green, and sometimes to red, although absorption and emission efficiencies usually decrease at longer wavelengths [13, 14, 15]. This emission is often combined with a high quantum yield which, however, strongly depends on the synthesis conditions [16, 17, 18]. The origin of the optical properties of CQDs are still much debated in the literature, where at least two types of emission mechanisms are presumed, one originating exclusively from core-surface interactions, and another depending on the chromophores present on the surface [2, 5, 14, 18]. As far as the synthesis of CQDs is concerned, there are mainly two approaches: bottom-up, and top-down [19; 20; 21; 22]. Bottom-up synthesis makes it possible to use small molecular precursors, and through controlled pyrolysis it is possible to synthesise various types of carbon dots with very different structural properties, from crystalline to amorphous [22; 23; 24]. Regarding top-down synthesis, the precursors are larger and more extensive carbon allotropes [25; 26], such as graphite, carbon nanotubes, and fullerene. Top-down synthesis can be carried out by chemical oxidation [14; 15; 25] or laser ablation [26; 27; 28]. This plethora of synthesis methods makes it possible to design and construct different types of CQDs, with surfaces rich in heteroatoms, such as oxygen and nitrogen, enabling the exploitation of the presence of different surface chromophores, and to change the functional and emissive properties of the nanoparticles in a controlled manner [23; 28; 29; 30]. Due to the powerful sensitivity of the fluorescence to the external environment, and the ability of CQDs to behave as efficient electron donors [1; 24; 31; 32; 33], CQDs are often proposed as detectors of various harmful substances, e.g., heavy metals such as copper [34; 35], cobalt [36; 37], and iron [38; 39; 40]. Therefore, new CQDs design methodologies consistently aim to increase the sensitivity and selectivity of CQDs to one or more of these analytes. However, the interaction pathways between metal ions and CQDs are still barely understood, due to the lack of dedicated studies on the fundamental interaction mechanisms. A better understanding of these phenomena would greatly help not only to achieve better sensing, but also as a way for probing the fluorescent groups of CQDs, and learning or optimizing the emission mechanisms. In this paper, we present a study of sensing application of two different families of CQDs synthesised using a top-down approach. After characterising the structure and the optical features of the two types of prepared CQDs, we tested their interaction with heavy metal ions such as nickel, copper, cobalt, iron, and zinc through the combined use of steady-state and time-resolved optical spectroscopy. We found that the interactions of these metals with the two CQDs are different, resulting in different detection, sensitivity, and selectivity features.