## 1 Introduction  
With the fast development of nanotechnology in the last decades, the realm of nanostructures has unveiled all its beauty, giving unimaginable quantum systems, spanning all fields of physics, chemistry, and biology. The autonomous ordering and self-assembly of atoms and molecules on atomically well-defined surfaces [1], two dimensional (2D) elemental materials such as graphene [2], silicene [3] and all those 2D beyond graphene [4; 5; 6], including van der Waals 2D heterostructures [7], transition metal dichalcogenides [8], perovskite systems [9; 10] and bio-polydopamine composite materials [11] are only a small part of this, inaccessible to naked eye, extraordinary world. The close conjugation between the atomic and nanoscopic scale of materials and their physical properties, mainly derived from the 3D, 2D, 1D, or 0D quantum size confinement [12; 13; 14], directly takes us into the mechanics applied to nanoscience and its complexity [15; 16]. So far, the world of chemistry and biology has been able to strongly embrace that of physics, revealing unexpected structural, electronic, and catalytic properties of nanostructured materials that have ubiquitously flowed into the daily life, through devices and sensors. To date, cupric oxide (CuO) has been diffusively studied thanks to its great potential applications in various fields, such as catalysis [17], solar cells [18], optical and photoconductive response [19]. It could be used, also, for its field emission properties [20], being a p-type semiconductor with a narrow band gap of 1.2 eV. One of the CuO fascinating features is its ability to form nanostructures that include many shapes, such as flower-like structures composed of hierarchical 2D nanosheets and spherical architectures. They are constituted of ultrathin nanowalls, of about 10 nm in thickness [21; 22], or large honeycombs assembly on copper foils [23]. In the context of gas sensing, these CuO flower-like nanostructures are applied as electrode sensors capable of having a high non-enzymatic electrocatalytic activity for hydrogen peroxide detection [24; 25]. To produce these extraordinary flower-like CuO nanostructures, chemical methods were typically used [21; 22; 23; 24; 25] with copper mainly treated in alkaline conditions by persulfate [21; 22; 23; 24; 25]. In these studies, the (-111) X-ray diffraction (XRD) reflection at 20 = 35.744\({}^{\circ}\)[21; 24; 25] and the Raman peak at 295 cm\({}^{-1}\)[26] were utilized to identify the run products. CuS nanoparticle clusters and sponge-like structures of nanoflakes were also synthesized by the chemical method by using copper chloride and thioacetamide as reagents [27; 28]. These CuS were studied by scanning electron microscopy (SEM), XRD, and Raman spectroscopy to characterize their nanostructure morphology. In particular, XRD peaks specific to the hexagonal CuS structure [27] and the sharp Raman peak at 473 cm\({}^{-1}\) assigned to the S-S stretching mode of S\({}_{2}\) ions at the 4e sites [27; 28; 29; 30] were used for featuring the obtained compound. Peculiar copper sulphide nanostructures, such as spheres and nanotubes, were successfully synthesized, by using a microwave-assisted solvothermal method on Cu complexes [29]. Indeed, pure CuS (hcp) with flower-like, spherical hollows and tubular structures were obtained by means of CuCl\({}_{2}\bullet\)2H\({}_{2}\)O and CH\({}_{3}\)CSNH\({}_{2}\) dissolved in ethylene glycol, by varying the pH by the addition of NaOH [30]. An interesting and promising application of CuS metal sulphide involves its use as anode material in lithium ion batteries [31]. Interestingly, hybrid organic-inorganic nanoflowers were accidently discovered by adding copper sulphate (CuSO\({}_{4}\)) to phosphate-buffered saline solutions, containing 0.1 mg/mL bovine serum albumin at pH 7.4 and 25 \({}^{\circ}\)C [32]. A few days after, a precipitate appeared containing porous flower-like structures.