You need to know first WHAT you are shooting - your question is a very common one by the way.
Take photo of what? people with the background? Then you should use a fill flash to illuminate them, plus a long enough exposure to capture the background.
Remember, a prime lens will give you a bigger aperture, but you then get a thin DOF. So you really need to know what it is you are trying to shoot, and use the right setup. "photo at night at Singapore river" is too vague - if you're talking about a night landscape, you can use any lens that gives you the field of view you want, then you would shoot at about f11-f16 anyway - definitely no flash.