Surface Plasmon Resonance Sensing Detection of Mercury and Lead Ions Based on Conducting Polymer Composite

et al. (2011) Surface Plasmon Resonance Sensing Detection of Mercury and Lead Ions Based on Conducting Polymer Composite. PLoS ONE 6(9): e24578. doi:10.1371/journal.pone.0024578 Surface Plasmon Resonance Sensing Detection of Mercury and Lead Ions Based on Conducting Polymer Composite Mahnaz M. Abdi 0 Luqman Chuah Abdullah 0 Amir R. Sadrolhosseini 0 Wan Mahmood Mat Yunus 0 Mohd Maarof Moksin 0 Paridah Md. Tahir 0 Matteo Rini, Joint Research Centre - European Commission, Germany 0 1 Laboratory of Biopolymer and Derivatives, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia , Serdang, Selangor , Malaysia , 2 Department of Physics, Faculty of Science, Universiti Putra Malaysia , Serdang, Selangor , Malaysia A new sensing area for a sensor based on surface plasmon resonance (SPR) was fabricated to detect trace amounts of mercury and lead ions. The gold surface used for SPR measurements were modified with polypyrrole-chitosan (PPy-CHI) conducting polymer composite. The polymer layer was deposited on the gold surface by electrodeposition. This optical sensor was used for monitoring toxic metal ions with and without sensitivity enhancement by chitosan in water samples. The higher amounts of resonance angle unit (DRU) were obtained for PPy-CHI film due to a specific binding of chitosan with Pb2+ and Hg2+ ions. The Pb2+ ion bind to the polymer films most strongly, and the sensor was more sensitive to Pb2+ compared to Hg2+. The concentrations of ions in the parts per million range produced the changes in the SPR angle minimum in the region of 0.03 to 0.07. Data analysis was done by Matlab software using Fresnel formula for multilayer system. - Funding: This work was supported by Ministry of Higher Education of Malaysia (GRF) and University Putra Malaysia. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. In recent years, surface plasmon resonance (SPR) has been widely demonstrated to be an effective optical technique for in situ investigation of the optical and electrical properties of conducting polymers, such as polypyrrole films [1]. Surface plasmon resonance (SPR) is a quantum optical-electrical phenomenon arising from the interaction of light with a metal surface [2]. The charge density oscillations that exist at a metal-dielectric (or metal/vacuum) interface are called surface plasmons that propagate in a direction parallel to the metal-dielectric interface and are on the boundary of the metal and the external medium (air or water for example) [3]. So that, these oscillations are very sensitive to any change of this boundary, such as adsorption of molecules to the metal surface. Heavy metals such as mercury and lead ions have long been recognized as a harmful environmental pollutant. Lower levels of lead can affect on the central nervous system, kidney, and blood cells [4] and in severe cases can cause convulsions, coma, and even death. Lead interferes with the development of the nervous system and is therefore particularly toxic to children and unborn babies, causing potentially permanent learning and behaviour disorders [5]. There are many techniques for analysis trace metal e.g., atomic absorption, atomic emission, and fluorescence spectrometry, inductively coupled plasma-mass spectrometry (ICP-MS), and electrochemical techniques (such as ion-selective potentiometry and anodic stripping voltammetry) [6]. In contrast to their attractive analytical figures of merit, each of these techniques suffers from some certain disadvantages. Applications of all of these methods require a knowledge of chemistry and instrumentation and need exactitude apparatuses [7]. A major disadvantage of ICP-MS is the high capital cost of the instrumentation. In addition, this instrument is bulky and not selective to different charge states of an element. The non linearity of the calibration curves is a disadvantage of the atomic absorption spectroscopy (AAS) technique (in the absorbance range higher than 0.5 to 1). Running and investment costs of inductively coupled plasma atomic emission spectroscopy (ICP-AES) are high. Another disadvantage of ICP-AES is spectral interferences (due to many emission lines). Voltammetric methods are simple, inexpensive, and portable but they suffer of interferences inherent in complex sample matrix. Anodic stripping voltammetry can only measure amalgamforming metal species [8]. Thereby, the complementary methods have developed to overcome some of these shortcomings. SPR is a method that provides an effective optical technique to detect monolayer thicknesses of the material on the conducting surface. This technique exhibits a good sensitivity, stability, and reproducibility; sometimes even changes in refractive index of approximately 1025 can be detected by SPR technique [9]. Conducting sensors based on PPy film with different dopant were developed for detecting volatile aromatic hydrocarbons [10] and volatile organic solvent sensors have also been fabricated using PPy films on conducting glass substrates [11]. Selective SPR sensors based on PPy have been reported for the detection of Cu2+ [12], and also for biosensor such as pseudomonas aeruginosa cells [13]. In previous work [14], we obtained the refractive indexes of PPy and PPy-CHI thin films (in contact with air) by SPR technique. Since they produced a sharp peak of resonance angle, it was shown these polymers are capable for using in sensitive optical sensors.The aim of the current study is to prepare optical sensors based on PPy and PPy-CHI composite films to detect trace amount of Hg2+ and Pb2+. The optical sensors were used for monitoring of toxic metal ions with and without sensitivity enhancement by chitosan. To our knowledge, this is the first report of a SPR sensor that uses PPy-CHI biocomposite for heavy metal detection. Materials and Methods Pyrrol monomer (Acros Organic) was distilled and stored below 4uC. All the other reagents including chitosan (a local company with 88% degree of deacetylation) and p-toluene sulfonate (Fluka) were of analytical grade,and used without further purification. All of the electropolymerization of pyrrole was done via a typical three-electrode electrochemical cell arrangement. A saturated calomel electrode (SCE) and a carbon rod were used as reference and counter electrode, respectively. The thin layer of gold covered onto the glass slide was used as working electrode. The glass microscope slides (ZF52, n = 1.8395) were cleaned using HCl/H2O at 60uC for 15 min following by rinsing the surface with deionized water and drying at 50uC. Each glass slide was coated with a thin layer of gold, using a sputtering coating (Model SC7940, Polaron). The thin layer of gold coated onto glass slide was used as working electrode in electrochemical deposition of polymer. The sam (...truncated)