Solution studies showed the strong interaction of [5-(dimethylamino) naphthalene-1-sulfonyl 4-phenylsemicarbazide] (NSP) with Er(III) ions. NSP was used as a sensing material during construction of carbon paste Er(III) sensors. The electrodes were modified with 1-n-butyl-3-methylimidazolium tetrafluoroborate, [bmim]BF4, as room temperature ionic liquid (RTIL) and multi-walled carbon nanotube (MWCNT). Potentiometric sensors constructed with [bmim]BF4 and MWCNTs show better sensitivity, selectivity, response time, and response stability compared to Er(III) carbon paste sensors. The best performance for the modified sensor was obtained with an electrode composition of 20% [bmim]BF4, 20% NSP, 45% graphite powder and 15% MWCNT. This particular sensor formulation exhibits a Nernstian response (19.8 +/- 0.3 mV decade(-1)) toward Er(III) ions in the range of 1.0 x 10(-7) to 1.0 X 10(-1) mol L-1 with a detection limit of 5.0 x 10(-8) mol L-1. The proposed modified Er(III) sensor can be used over the pH range from 3.5 to 9.0. (C) 2009 Elsevier Ltd. All rights reserved.