1) Make sure you are using MYSQL 5.5, only then you can change the sorting to utf8mb4_something
2) Make sure the table columns that are going to receive emoji have their own utf8mb4_something settings
3) Edit the database.php configuration file
$db['default']['char_set'] = 'utf8mb4';
$db['default']['dbcollat'] = 'utf8mb4_unicode_ci';