In this instance, you should clean the PTFE stirrer bar and the cuvette (if you are not using disposable cells) very well before continuing transport experiments to remove any possible contamination of other compounds. If the spectrofluorometer response is still significant after running a second blank, the vesicles are likely leaking. Therefore, the results obtained with that batch of leaky vesicles are not reliable. The experiment should be stopped, and a new batch of vesicles prepared. Leaking vesicles is not common and should not occur if the vesicle preparation procedure is followed closely.