Before you pass your sting to Uri.UnescapeDataString()funcation, handel that character or replace them with standard characters.
some thing like...
..........
string strTest = "‘testing string’";
strTest = strTest.Replace("’","'");
............
or you can also create funcation which will handel this type of special characters in your string like this...
public string ReplaceCharacter(string str){
str= str.Replace("’","'").Replace("`", "'");
}
string strTest = "‘testing string’";
strTest = ReplaceCharacter(strTest);