In the current article it is described the preparation of a carbon paste electrode modified with biochar (BM-CPE) decorated with nanostructured copper hexacyanoferrate (nCuHCF) and its application as amperometric sensor for isoniazid (INZ). Decoration of biochar was carried out using copper ions previously adsorbed at activated biochar surface, followed by nCuHCF electrochemical synthesis. Morphological investigation revealed the presence of nanostructures with sizes of 305 +/- 42 nm randomly distributed on the electrode surface. All parameters that influenced the formation of nCuHCF structures on the electrode's surface were evaluated in order to obtain the best conditions for INZ determination. After optimization, the nCuHCF-BM-CPE promoted a significant improvement in the current signal for INZ oxidation at a potential peak of 140 mV, which is less positive compared to the unmodified electrode. A linear response between 1.0 x 10(-6) and 1.0 x 10(-5) mol L-1 INZ (R-2 = 0.992) and Limits of Detection (LOD) and Quantification (LOQ) of 6.3 x 10(-8) mol L-1 and 2.1 x 10(-7) mol L-1, respectively, were obtained. The effect of concomitant species on the amperometric response was investigated and the proposed methodology was applied for INZ determination using synthetic human urine samples spiked with INZ (three concentration levels). Recovery values varied from 98.0 to 99.4% demonstrating an excellent analytical performance for INZ quantification. (C) 2018 Elsevier Ltd. All rights reserved.