## 1 Introduction
### Carbon Nanostructures Properties and Uses  
Carbon nanostructures represent a large family of materials based on carbon characterized by a diversity of morphologies and structures (Figure 2) [27]. Typically, they present very interesting electronic and conductive properties that arise from the extended conjugation of sp\({}^{2}\) atoms, although exceptions exist. For instance, nanodiamonds contain mainly sp\({}^{3}\)-hybridized carbon atoms, as the name suggests [28]. Nanocarbons have been engineered to feature sp-hybridized carbon atoms as well, especially for energy conversion and storage applications [29]. In particular, sp-hybridized one-dimensional \"synthetic carbon allotropes\" are emerging as attractive molecular wires for advanced applications in electronics and opto-electronics [30].  
Among the most studied carbon nano-allotropes are zero-dimensional fullerenes [33], which can be considered as soccer-ball shaped structures, one-dimensional carbon nanotubes (CNTs), which have a tubular morphology [34], and two-dimensional graphene-based materials [35], which generally consist of nanosheets. Other well-known examples include nano-onions (CNOs) [36], which consist of concentric fullerenes, and nanohorns (CNHs), which arise from clusters of nanocones [37]. In recent years, the class of carbon dots has attracted attention thanks to their ultrasmall structure (<10 nm), which provides them with characteristic luminescent properties [38].  
Carbon nanostructures have found a wide variety of applications over the years thanks to their electronic and thermal conductivity, low density, and high mechanical strength, as well as the ability to undergo chemical functionalization to further tune their properties as needed for the intended use [39]. They are being studied especially for energy [40] and catalysis [41; 42; 43], including electro-catalysis [44; 45] and nanozymes [46], as well as for the development of advanced electronic applications [47], including supercapacitors [48; 49] and batteries [50], wearable electronics [51], electro-catalytic water-splitting [52], electromagnetic interference (EMI) shielding materials [53], molecular magnets [54], thermal-energy harvesting [55], photo-detectors [56], and electrochemical sensors [57]. In particular, in the area of sensing [58], recent developments have been made in the areas of nano-mass and nano-force sensors [59], gas sensors [60], biosensors [61], temperature sensors [62], and the growing field of touch or motion-driven sensors, or \"haptics\" [63]. Another area of growing interest regards environmental remediation [64], including water purification [65] and the detection of various pollutants, such as pesticides [66] and pharmaceuticals [67].  
Figure 2: Different types of well-known carbon nanostructures (not to scale). Reproduced from [31]. The nano-onion is reproduced with permission from [32], copyright © 1996, Elsevier.  
In materials science, they are well-known as nano-fillers [68; 69], but also used as flame-retardants [70].  
Finally, carbon nanomaterials can be applied for biomedical use [71; 72; 73; 74], especially in oncology [75; 76], theranostics [77], drug delivery [78], antimicrobials [79; 80], and DNA analysis [81]. A biomedical area that is progressing at a fast pace is tissue engineering [82], especially for nerve [83], cardiac [84], and bone [85] tissues. In bioelectronics, synaptic transistors and neuromorphic computing [86] are progressing at a fast pace.