Surface thiolation was recently found to affect two important properties of gold nanoparticles: their size and the presence of visible luminescence under UV stimulation. We explored these phenomena by systematically analysing alkanethiolate coatings with different carbon chain lengths, from 3-mercaptopropionic acid to 16-mercaptohexadecanoic acid. In all cases, the coating was achieved by rapid reduction of metal ions stimulated by intense x-ray irradiation. For small surfactant-to-metal ratios in the solution, the nanoparticle size monotonically decreases with the length. Photoluminescence is present for the smallest nanoparticles, but its intensity becomes more intense as the carbon chain length increases. With 16-mercaptohexadecanoic acid coating, the nanoparticles achieved a 28% quantum efficiency which make them very bright fluorophores with a large Stokes shift, both strongly desired performances for biomedical imaging. These particles also yield strong luminescence with multiphoton excitation which could find imaging application with standard experimental setups. Besides their fundamental interest in clarifying the mechanism for luminescence, these results could find interesting practical applications in nanoparticle production and in the use of fluorescence for imaging in biology and other areas.