Sensors and Actuators B 134 (2008) 324–331 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Voltammetric behavior of multi-walled carbon nanotubes modified electrode-hexacyanoferrate(II) electrocatalyst system as a sensor for determination of captopril B. Rezaei ∗ , S. Damiri Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran article info Article history: Received 15 January 2008 Received in revised form 3 May 2008 Accepted 7 May 2008 Available online 14 May 2008 Keywords: Multi-walled carbon nanotube Iron oxide nanoparticle Captopril Hexacyanoferrate Cyclic voltammetry abstract The study of electrochemical behavior and determination of captopril, as an angiontensin-converting enzyme inhibitor, is reported on multi-walled carbon nanotube (MCNT) modified glassy carbon electrode (GCE) and hexacyanoferrate(II) (HCF) electrocatalyst. The cyclic voltammetric results indicate that MCNTs and HCF system can remarkably enhance electrocatalytic activity toward the oxidation of captopril in acidic solutions. It is leading to a considerable improvement of the anodic peak current for captopril, and allows the development of a highly sensitive voltammetric sensor for detection of captopril in pharmaceu- tical and clinical samples. The investigation of captopril oxidation on the iron oxide nanoparticles modified carbon paste electrode does not show any electrocatalytic effect on the oxidation of captopril, suggesting that iron oxide impurities in the MCNTs are not the active sites in captopril sensing. Under optimized conditions on MCNTs-HCF system, the proposed method with respect to other reported electrochemical methods shows wider dynamic range (0.5–600 M) with suitable selectivity, practical detection limit of 0.2 M and good precision (R.S.D. <3%). © 2008 Elsevier B.V. All rights reserved. 1. Introduction S-Captopril, 1-(3-mercapto-2-(S)-methyl-1-oxopropyl)- S(L)proline, is a synthetic dipeptide serving as an orally active inhibitor of the angiontensin-converting enzyme (ACE) and has been widely used as antihypertensive drug [1] and to moderate heart failure [2]. It is the only inhibitor of ACE bearing a thiol group and can take up free radicals in living systems and exhibits antioxidant properties [3–5]. Captopril contains two asymmetric centers, one associated with the proline moiety and another associated with the 3-mercapto-2-methylpropionic acid side chain. Accordingly there are three other possible stereoisomers. One of these stereoisomers is R-captopril, 1-(3-mercapto-2(S)- methyl-1-oxopropyl)-R(D)-proline. It has been reported that the biological activity resides mainly in S-captopril while R-captopril possesses non-ACE inhibiting activity. R-Captopril is a by-product and impurity formed in the synthesis of S-captopril. It is essential for pharmaceutical treatment that tablets must be very pure containing only the S-enantiomer [6–8]. Other uses for captopril ∗ Corresponding author. Tel.: +98 311 3912351; fax: +98 311 3912350. E-mail address: rezaei@cc.iut.ac.ir (B. Rezaei). have also been reported including decreasing high blood pressure caused by blood vessels in the kidneys, decreasing symptoms of cystinuria, reducing rheumatoid arthritis symptoms, treating Raynaud’s phenomena and progression of kidney disease in peo- ples with diabetes [9]. Captopril is generally well tolerated and some side effects are reported for it. A dry, persistent cough has been reported with the use of captopril and other ACE inhibitors. Coughing resolves after discontinuing the medication. Captopril has some other side effects such as abdominal pain, constipation, diarrhea, dizziness, fatigue, headache, loss of taste, loss of appetite, nausea and vomiting, easy bruising or bleeding, chest pain, chills, difficulty breathing, severe dizziness or fainting, fever, numbness or tingling in the hands or feet, rash, and a sore or swollen throat. In rare instances, liver dysfunction and skin yellowing (jaundice) have been reported with captopril [9]. Captopril is metabolized in liver (it is oxidized into the corre- sponding disulfide) and is mainly excreted with the urine (40–60% of the excreted drug remain unchanged in the urine) [10]. Therefore, the determination of captopril is important from a physiologi- cal point of view as well as for the purposes of quality control. In some real sample analysis, it is important also to discrimi- nate between enantiomers and the enantiopurity tests must follow the general methods for determination of base molecule. Sev- eral methods have already been reported for the determination 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.05.004