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This work describes investigations into the synthesis of new potential multimodal agents for medical imaging of cancer cells. For this purpose, two approaches were used: (i) based on novel fluorescent nanohybrids based on iron oxide nanoparticles, and (ii) based on a symmetric tripodal core that can be functionalised, giving rise to bifunctional chelating molecular and nanocomposite agents. Each Chapter of this thesis describes the synthesis andcharacterisation of the different components of the desired probes. The cellular viability of the as-prepared imaging agents was evaluated using different cell lines (e. g., PC-3, EMT6 and FEK-4) and assays, such as MTT and crystal violet. Laser scanning confocal microscopy investigations were carried out on the new fluorescent conjugates synthesised hereby to study their localisation within cancer cells. Chapter 1 describes the context of this work, starting with a background of cancer and tumour hypoxia, and including overviews of different imaging modalities particularly applied to diagnosis, staging and follow-up of prostate cancer (PET, SPECT, MRI, confocal and epifluorescence microscopy, FLIM and TCSPC). The synthesis and development of some radioisotopes for use in PET/SPECT imaging is detailed. The use of imaging probes based on thiosemicarbazide derivatives is described together with their advantages, due to the importance of this type of compounds in tumour hypoxia detection and multimodality imaging potential. Moreover, the use of iron oxide nanoparticles as scaffolds for biomedical and nanomedicine applications, with special importance for magnetic resonance imaging, is reviewed. Finally, the use of bombesin, a GRP-targeting peptide, as a vector for the recognition of cancer cells and its incorporation into imaging probes is discussed. Chapter 2 describes synthetic approaches towards the functionalisation of a tripodal symmetric organic core containing -CH2Br units used as starting material. Novel fluorophores based on BODIPY were synthesised and characterised hereby, together with other compounds including different linkers. The most promising fluorescent molecules synthesised and characterised hereby were tested in PC-3 cells using single and two-photon laser scanning confocal microscopy, and their cell viability was evaluated by means of MTT assays. Furthermore, two deferoxamine-based molecules were synthesised and radiolabelled with zirconium-89. Chapter 3 describes the synthesis of a peptide sequence (the [7-13] bombesin fragment, of interest for cancer cells targeting) using solid-phase peptide synthesis, and the attempts to incorporate this into the fluorescent tripodal molecule synthesised in the previous chapter. Moreover, the synthesis of a deferoxamine-bombesin conjugate as a new chelator for main group and transition metals is reported herein. Chapter 4 describes the synthesis of novel thiosemicarbazide-based ligands, some structural investigations using NMR spectroscopy and the single crystal X-ray diffraction data collected to fully characterise the main new molecules emerging from this study. The complexation tests to incorporate several metals from main group and transition metals (e. g., Zr(IV), Ga(III) and Cu(II)) and the results obtained are reported. The behaviour of the ligands in three different cell lines was assessed via crystal violet assays, and their IC50 values calculated after 24, 48 and 72 hours of incubation in cells. Chapter 5 describes the synthesis and characterisation of novel imaging probes based on benign iron oxide nanoparticles coated with a silica shell. These were functionalised with two types of fluorophores: quantum dots (Cd0.1Zn0.9Se) and a potential hypoxia targeting unit, and three radiolabelling methods using gallium-68 were developed. An alternative system based on citric acid-coated iron oxide nanoparticles and functionalised with a BODIPY derivative was synthesised. The most promising nanocomposites in terms of kinetic stability were tested in PC-3 cells, using single and two-photon laser scanning confocal microscopy to study their fluorescent properties. Their cell viability was tested in PC-3 cells using MTT assays over a range of conditions and timescales. Chapter 6 constitutes a summary of the work carried out and results found during this thesis and includes some proposals for future work based on the research findings described here. Chapter 7 contains all the experimental details and characterisation data for the compounds described in this thesis. The Appendices provide supporting spectroscopic evidence and X-ray diffraction data for the new compounds synthesised in this work.