mysql_real_escape_string转换为mysqli

Im trying to convert mysql_real_escape_string() into mysqli and have been trying with:

$mysqli->real_escape_string($this->email);

However this brings up an error and doesn't seem to work. The error:

Undefined variable: mysqli in..

... Which refers to this line: $mysqli->real_escape_string($this->name),

Any suggestions on how I get this to work in my setup?

class registerForm {

    var $name;
    var $email;

    function registerContactFormular(){

    }

    function init() {
        //some different init stuff

        return true;
    }

    function save() {
        if(isset($_REQUEST['field'])) {
            $sql = sprintf("
            INSERT INTO contact (
            name,
            email)
            VALUES ('%s','%s','%s)",
            mysqli_real_escape_string($this->name),
            mysqli_real_escape_string($this->email));

            $res = mysql_query($sql) or die( mysql_error());
        }

        if($res) {
            return true;
        }
    }

}