Cancer is a leading cause of death worldwide, with metastasis responsible for a majority of these deaths. Circulating tumour cells (CTCs) present in blood samples can be a precursor to metastasis and if detected in low numbers can indicate early stage cancer. In this work, fluorescent silica nanoparticles (NPs) were developed to detect these CTCs. NPs approximately 40 nm in diameter were doped with two different concentrations of Cyanine5 dye molecules, using the reverse microemulsion method. The NPs were over 100 times brighter than Cyanine5 free dye and the measured fluorescence intensity matched a homo-Förster Resonance Energy Transfer model. NPs were conjugated with anti-epithelial cellular adhesion molecule (EpCAM) antibody to the NP surface for immunospecific targeting. The NP labels were shown to selectively bind to model CTC lines, MCF-7 and LNCaP, a metastatic breast and prostate cancer cell line respectively, by targeting EpCAM present on the cell’s membrane. In flow cytometry experiments, the NPs were twice as bright as commercial anti-EpCAM red fluorophore conjugates, APC and AlexaFluor®647 on the MCF-7 cell line. This increase was achieved while keeping non-specific binding low with a non-metastatic cancer control cell line (HeLa). The NPs were also immunospecific in fluorescence microscopy experiments. Confocal microscopy was used to confirm that the NPs were located on the cell’s surface, matching with the location of the EpCAM marker. The NPs were tested in a mixed cell environment by spiking CTCs into lysed blood samples where the NPs successfully identified CTCs at different ratios to white blood cells. Some experiments were also conducted to investigate the use of NPs for secondary antibody staining of EpCAM on CTCs. In addition, gold grating plasmonic enhanced fluorescence imaging of CTCs was investigated briefly. In summary, the NP labels have excellent potential in biomedical applications, particularly in cell labelling and diagnostics.