Bare Au-gated and thioglycolic acid functionalized Au-gated AlGaN/GaN high electron mobility transistors were used to detect mercury (II) and copper (II) ions. Fast detection of < 5 s was achieved for thioglycolic acid functionalized sensors. The thioglycolic acid functionalization increased the sensitivity for detection of mercury by 2.5 times over the bare Au-gated surface. Both surfaces had a selectivity of approximately 100-fold over other contaminating ions of sodium, magnesium, and lead and can be easily recycled. Our results show that portable, selective, and fast Cu2+ and Hg2+ sensors can be realized by combining bare Au-gated and thioglycolic acid-functionalized surface in one sensor. (c) 2007 The Electrochemical Society. All rights reserved.