Securing your notes in OS X is pretty easy, just select a note, click the lock icon in the toolbar, and click on «Block this note», the option is also available by Ctrl + Click (Right Click) to select it from the context menu. On the other hand, it is possible that at that moment the system will ask you to create a password if you have not already done so.
By default, said locked note will still be visible and will only have an open padlock icon next to it. To completely lock all notes, from the padlock icon top right in the same window, we will select the option «Close all locked notes». In this way we will ensure that all the notes that we have marked as blocked, stop being shown in the preview and no content will be displayed.
Obviously, to reverse the process, it will be enough for us to select the specific note that we have created, unlock it with the password to show the content and again in the padlock menu select "Remove Padlock" to remove the password, if on the other hand we only want to remove it , we will select with the context menu (Ctrl + Click) the option to delete, this will move the note to "Recently Deleted" for a period of 30 days in case we want to recover it.
The process in IOS is a bit more complicated, if only because it is not so obvious. To lock a note on your iPhone or iPad, you must enter the note in question and then click on the share icon at the top right (shaped like a square and an arrow), at that moment a menu will appear where we will click on lock note and we will be asked for a password as in OS X, once entered it is it will activate right next to the previous icon a specific one with an open padlock that if we press it, it will automatically lock the note and it will appear closed, leaving the preview as it appears in the image below.
From now on we can unlock them with Touch ID, taking into account that Apple has focused the Touch ID partnership with a single password since if we enter different passwords for each of the notes we can only unlock the first with Touch ID and for the others we will have to enter them manually. This indicates that Apple has only focused on taking a single password approach to notes.
Remember that this option is only available for the latest versions of the respective operating systems, that is, both for OS X 10.11.4 as for iOS 9.3 onwards