Construct std::function from function with one known input argument and one unknown input argument

I have the function

int testFunction(double a,std::string b)
{
    return 0;
}

of which I want to make a std::function<int(std::string)> in which a is known and b is unknown. So something like

std::function<int(std::string)> testFunction=testFunction(2.3,std::string b);

but this syntax does not work.

What is the correct syntax?