Are you simply asking how to load a specific variable? E.g.,
variable_to_load = 'X';
do some work on the variable
Another formulation you might consider is the following:
y = load(['nano',num2str(n),'.mat'],variable_to_load);
z = y.(variable_to_load);
all downstream code then uses variable named z
This latter formulation has the advantage that your downstream code can use a variable name (in this case z) that is known to you up front, regardless of the actual variable name you picked to load.