Please see Article 1, Section 9, Clause 8 of the Constitution: "No title of nobility shall be granted by the United States: and no person holding any office of profit or trust under them, shall, without the consent of the Congress, accept of any present, emolument, office, or title, of any kind whatever, from any king, prince, or foreign state."  The next section prohibits states from granting any title of nobility.

Of course, the framers of the Constitution might have been ill informed, just like me.