A função mysql_escape_string() está obsoleta use mysql_real_escape_string() Codeigniter

/**
  * Escape String
  *
  * @param string
  * @param bool whether or not the string will be used in a LIKE condition
  * @return string
  */
 public function escape_str($str, $like = FALSE)
 {
  if (is_array($str))
  {
   foreach ($str as $key => $val)
      {
    $str[$key] = $this->escape_str($val, $like);
      }

      return $str;
     }

  $str = is_resource($this->conn_id) ? mysql_real_escape_string($str, $this->conn_id) : addslashes($str);

  // escape LIKE condition wildcards
  if ($like === TRUE)
  {
   return str_replace(array($this->_like_escape_chr, '%', '_'),
      array($this->_like_escape_chr.$this->_like_escape_chr, $this->_like_escape_chr.'%', $this->_like_escape_chr.'_'),
      $str);
  }

  return $str;
 }