Define a string with single quotation marks? This is kind of a short question. All I want to know is, if I can define a string using single quotation marks ('), instead of the regular ones("). I want to use it in a function like this: 
print('Hello world'); 

I unfortunately do not have an Arduino nearby so I cannot test it myself.
No, you cannot. This rules comes from the basic C++ syntax on which the Arduino platform is built.