Carbon nanomaterials (CNMs) are a class of materials that aroused great interest in the last decades in several research fields. In particular, CNMs showed great potentialities in biomedical applications because their advantageous nanometric size and biocompatibility. In addition, CNMs have been also investigated as materials for electrocatalysis due to high surface and outstanding electro-mechanical properties. In particular, doping with heteroatoms have shown to be efficiently improve their electroactivity due to the formation of active sites in the inherent graphitic skeleton. Carbon nano-onions (CNOs) are a member of the carbon family with a structure composed of concentric graphitic shells akin to that of an onion. CNOs were firstly discovered by Ugarte in 1992, but only in the last two decades aroused great interest in the scientific community due to peculiar features such as small size, large surface area and interesting physical and chemical properties. To date, CNOs have been successfully applied in several applicative area with remarkable outcomes in biological and electrochemical applications. The aim of the present thesis is to developed novel CNO derivatives for possible applications in biomedicine and electrocatalysis in order to expand the current knowledge in these research fields. Chapter 1 contains an introduction on the fascinating world of carbon nanomaterials with particular emphasis given to carbon nano-onions. The structural characteristics and the reported production methods are discussed in details in the first part, while the second part illustrates the remarkable physico-chemical properties, the functionalization strategies exploited to enhance their dispersing abilities, concluding with the description of current applications involving CNOs. A detailed description has been provided in two applicative fields, which are of interest for the present thesis. In the first part, the biological investigations performed on CNOs have been described with particular attention on the in vitro and in vivo bio-evaluation, showing that CNOs are a biocompatible material. In the second part, the operative principles of fuel cells and information about oxygen reduction reaction have been discussed in detailed. Later, a literature review on the doping strategies performed on carbon nanomaterials for electrocatalysis has been provided with particular emphasis given on the use of CNOs as electrocatalysts for the oxygen reduction reaction. Chapter 2 illustrates the synthesis and characterization of 5 nm-sized pristine CNOs (p-CNOs). Thermal annealing of detonation nanodiamonds was employed to fabricate CNOs and the successful formation was corroborated by several characterization techniques such as x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), Raman, electron energy loss (EEL) and Fourier-transform infra-red (FTIR) spectroscopies Chapter 3 reports the development of a novel CNO-based biological imaging agent. A novel far-red fluorescent BODIPY dyes, synthetized in our laboratory, was attached by esterification reaction to the CNO surface bearing carboxylic acid functionalities and tested on human breast cancer cells (MCF-7). Fluorescent CNOs exhibited bright fluorescence upon internalization and negligible effect on the cell viability at different concentrations. Low cytotoxicity, ease of internalization and remarkable emission properties confirmed the outstanding abilities of CNO derivatives in biological applications. Chapter 4 illustrates the development of an efficient and low cost CNO-based electrocatalyst for the oxygen reduction reaction (ORR). The introduction of boron and nitrogen into the CNO framework was accomplished by an in-situ co-doping strategy based on thermal annealing of a mixture of detonation nanodiamonds and boric acid. The electroactivity towards ORR of the CNO derivatives were compared with that of p-CNOs and standard carbon-supported platinum catalysts (Pt/C). The results showed that the co-doped materials exhibit enhanced ORR performance, higher long-term stability and negligible metal crossover effect as a consequence of the presence of large amount of active sites (i.e. pyridinic N and substitutional B species), thus confirming the abilities of the proposed co-doping approach to create a new material able to efficiently catalyze the ORR. Chapter 5 describes the materials and methods used for the experimental work of the proposed thesis. Finally, Chapter 6 contains the conclusions as well as the future perspectives of the presented doctoral activity.