The use of nanoparticles for sensing, cell imaging, and therapy has seen an extraordinary increase, in recent years. But, one of the issues commonly encountered when using nanoparticles in bioapplications is their toxicity. Cetyltrimethylammonium bromide (CTAB) is a common surfactant used in nanoparticle synthesis. One of CTAB’s uses is to direct the growth and stabilize the shape of certain types of colloidal metallic nanoparticles, for example gold nanorods. The main drawback is that these metallic nanoparticles cannot be used in applications with cells because of their cytotoxicity. Despite many conjectures, it is not yet clear what the origin of this cytotoxicity is. Here we explore some possible reasons and mechanisms that could be accounted for to explain this cytotoxicity. Due to its molecular structure, CTAB is difficult to detect in solution using conventional methods. In this thesis work, electrospray ionization mass spectrometry (ESI-MS) was used as a means to detect CTAB in solution and in vitro and to study the source of its toxicity. From the data collected, we have evidence that there are two active mechanisms: 1) CTAB’s interaction with the phospholipid bilayer that destabilizes the cell membrane leading to cell death, and 2) the catalytic action of one of CTAB’s dissociation products, the CTA+ cation, which might cause the quenching of the enzyme ATPsynthase and thus lead to energy deprivation and death of the cell. Our results also indicate that it is this second mechanism, which depends on the concentration of CTA+, to be the most lethal. We have found that nanorods can be successfully formed with high concentrations of Br-, which is non-toxic, with limited amounts of CTAB, because CTA+ might be playing a more important role only for stability purposes. This will reduce the amount of CTAB in solution to below cytotoxic values. We have also found out that once the nanorods are formed using CTAB as the stabilizing surfactant, low concentrations of an iodine/iodide mixture are able to modify, post-fabrication, the aspect ratio of the nanorods and thus their optical properties. This discovery might lead to the development of a new category of gold nanorods, with decreased toxicity and tunable morphology that would be ideal for applications in conjunction to cells.