Full Paper Amperometric Detection of Morphine at Preheated Glassy Carbon Electrode Modified with Multiwall Carbon Nanotubes Abdollah Salimi,* Rahman Hallaj, Gholam-Reza Khayatian Department of Chemistry, Faculty of Science, Kurdistan University, P.O. Box 416, Sanandaj, Iran *e-mail: absalimi@yahoo.com Received: July 12, 2004 Final version: September 13, 2004 Abstract A highly sensitive and fast responding sensor for the determination of morphine is described. The multiwall carbon nanotubes immobilize on preheated glassy carbon electrode (5 min at 50 8C) by gently rubbing of electrode surface on a filter paper supporting the carbon nanotubes.The results indicated that carbon nanotubes(CNTs) modified glassy carbon electrode exhibited efficiently electrocatalytic oxidation for morphine with relatively high sensitivity, stability and long life. Under conditions of cyclic voltammetry, the potential for oxidation of morphine is lowered by approximately 100 mV and the current is enhanced significantly (10 times) in comparison to the bare glassy carbon electrode at wide pH range (2 – 9). The electrocatalytic behavior is further exploited as a sensitive detection scheme for morphine determination by hydrodynamic amperometry. Under the optimized conditions the calibration plots are linear in the concentration range 0.5 – 150 mM with the calculated detection limit (S/N ¼ 3) of 0.2 mM and sensitivity of 10 nA/mM and a relative standard deviation (RSD) of 2.5% (n ¼ 10). The amperometric response is extremely stable, with no loss in sensitivity over a continual 30 min operation. Such attractive ability of multiwall carbon nanotubes (MWCNTs) modified GC electrode, suggests great promise for a morphine amperometric sensor. Finally the ability of the modified electrode was evaluated for simultaneous determination of morphine and codeine. Keywords: Glassy carbon, Modified electrode, Multiwall carbon nanotubes, Morphine, Amperometry, Codeine 1. Introduction Morphine (M) is a highly effective and preferred drug for the treatment of moderate to severe pain. The purposes of morphine in biological samples are mainly to monitor therapeutic levels in patients, drug concentrations in human and animal pharmacokinetic studies, investigation of opiate abuse for epidemiological purposes of drug abuse control as well as in forensic cases as an indicator of heroin usage and to identify causes of intoxication or death in cases of clinical and pathological interest [1, 2]. Several methods have been developed for the detection and determination of morphine and its major metabolites. The most common analytical techniques currently used are gas chromatography – mass spectrometry [3, 4], solid phase extraction – high perform- ance liquid chromatography [5, 7], liquid chromatography with UV [8], fluorescence [9] and mass spectrometry detection [10, 11], capillary electrophoresis with mass spectrometry [12], laser induced fluorescence detection [13] and surface plasmon resonance based immunosensors [14, 15]. The use of HPLC with electrochemical detection was recommended for the determination of morphine in biological fluids for its high sensitivity [16 – 23].The use of bare unmodified electrodes as an electrochemical sensors for morphine and its derivatives detection has a number of limitations such as low sensitivity and reproducibility, slow electron transfer reactions, low stability over a wide range of solution compositions and high overpotential at which the electron transfer process occurs and some of them are not sensitive enough for real sample analysis. The chemical modification of inert substrate electrodes with redox active thin films offers significant advantages in the design and development of electrochemical sensors. Different modi- fied electrodes have been used for electrooxidation of morphine and its derivatives [24, 26]. Unfortunately these modified electrodes have certain disadvantages, such as considerable leaching of electron transfer mediator and poor long-term stability, furthermore the methods of preparations are more expensive and difficult. Hence it is pertinent to explore and develop a simple and reliable method to fabricate modified electrodes with new electron transfer mediators. Recent studied demonstrated the ex- cellent electrocatalytic ability and antifouling properties of carbon nanotubes on the electrochemical devices. The excellent electrocatalytic properties of electrodes modified with single and multiwall carbon nanotubes have been reported for different molecules and biomolcules such as; insulin [27], NADH [28], carbohydrates [29], hydrogen peroxide [30], trinitrotoluene [31], nucleic acids [32], dopamine, ascorbic and uric acids [33], norepinephrine [34], aminophenol [35], 6-mercuptopurine [36], nitric-oxide [37], cytochrome c [38], myoglobine [39], thymine [40] and glucose [41]. Usually the CNTs were immobilized on to the electrode surfaces by using a CNTs solution as the dispersing medium. As carbon nanotubes are insoluble in most solvents and dissolution processes need a long period time, 873 Electroanalysis 2005, 17, No. 10  2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200403166