Ho3+ carbon paste sensor based on multi-walled carbon nanotubes: Applied for determination of holmium content in biological and environmental samples

For the first time a novel multi-walled carbon nanotubes (MWCNTs) modified Ho3+ carbon paste sensor is introduced. The electrode with a composition containing 20% paraffin oil, 60% graphite powder, 15% N-(1-thia-2-ylmethylene)-1,3-benzothiazole-2-amine (TBA) as an ionophore, and 5% MWCNTs, exhibits a stable potential response to Ho3+ ions with a nice Nernstian behavior (19.3 ± 0.3 mV decade− 1) in a wide dynamic linear concentration range of Ho3+ ions (1 × 10− 8–1.0 × 10− 2 M). In the absence of MWCNTs, sensitivity of the Ho3+ sensor was relatively poor. The proposed modified Ho3+ sensor shows very low detection limit (7.0 × 10− 9 M) and a fast response time (13 s). It has a long life time (more than 2 months) and its response is independent of pH in the range of 3.8–7.5. In term of selectivity, Ho3+ sensor has a good selectivity over all lanthanide members and common alkali and alkaline earth metal ions. The Ho3+ sensor was applied for the determination of Ho3+ ion concentration in water, holmium alloys and synthetic human serum.