Principle of working of a potentiometric sensor for the analysis of amino acids, sugars and esters. The enzymes are immobilized at the pH-electrode surface and the change in pH caused due to enzymatic conversion of the substrate is recorded, which is proportional to analyte concentration.
... the past two decades chemically modified electrodes (CME) have attracted considerable interest of researchers to exert a direct control over the chemical nature of electrodes. CMEs have found a large number of useful applications in different fields viz. selective electro- organic synthesis, biomedical analysis, electro analyses etc. The ability to manipulate the molecular architecture of bulk matrix of the electrode, particularly its surface has led to a wide range of analytical applications of CMEs and created opportunities for electroanalysts to fabricate useful biosensors The CMEs have shown remarkable specificity for biological recognition processes which has led to the development of highly selective biosensing devices. The electrochemical biosensors hold a leading position among the bioprobes currently available and hold great promise for the tasks of study of in-vivo mechanism of action of large number of drugs. These electrochemical biosensors consist of two components (1) a biological entity that recognises the target analyte and (2) the electrode transducer that translates the biorecognition event in to a useful electrical signal. Modern methods of analysis specially designed for drug discovery are mostly high-through put systems. The target samples are either obtained by natural origin generated by combinational chemistry or produced by biochemical methods. Hundreds and thousands of synthetic compounds are available in modern substance libraries, which have to be tested individually for their use as useful drugs. In addition to this, some natural resources including tropical rain forests and marine environments are of great interest for the development of potential new drugs. In the past, a large number of plants and samples obtained from biological sources of these habitats have been used as traditional medicine of native population. Scientific knowledge about these traditional medicines has attracted the attention of scientists working in the field of development of nature derived drugs. In the present times most of the screening systems used are enzyme or whole cell based and these biological substances have also been used as biological recognition elements of biosensors. We may consider a biosensor as a device consisting of a biological part and a physical transducer (Figure-1). The first scientifically proposed as well as successfully commercialized biosensors were those based on electrochemical sensors for multiple analysis. More than fifty percent sensors reported in the literature are electrochemical and can be classified as amperometric, potentiometric or conductometric sensors (Meadows,1996). Electrochemical biosensors have been studied for a long time. They have been the subject of basic as well as applied research for nearly fifty years. Leland C Clark introduced the principle of the first enzyme electrode with immobilized glucose oxidase (Clark et al., 1962). The first commercially produced biosensor was introduced in the market in 1975. This biosensor was used for the fast glucose assay in blood samples from diabetics. Today there is a large number of proposed and already commercialized devises based on the principle of biosensor including those for the analysis of pathogens and toxins. In amperometric sensors, an enzyme is typically immobilized at the surface of an amperometric electrode; this immobilized enzyme reacts with the substrate (e.g. phenolic compounds/sugar) and produces current that depends on the concentration of the analyte (Figure-2). This type of biosensor is based on the use of ion selective electrodes and ion-sensitive field effect transistors. Possibly the primary out put signal is due to the ions accumulated at the ion-selective membrane interface. The presence of the monitored ion due to reaction at the electrode surface is indicated by change in some physical parameters like pH etc. For example, the enzyme glucose oxidase can be immobilized at the pH electrode surface. The compound glucose has minimal influence on pH in working medium but the formation of gluconate due to its interaction at the enzyme immobilized pH electrode the solution becomes acidic, which can be easily detected. In general a potentiometric biosensor can be represented as under Figure 3. Some semiconductor based physico-chemical transducers are commonly used for the construction of biosensors. The ion selective field effect transistors (ISFET) and light addressable potentiometric sensors (LAPS) are convenient biosensor materials. The working principle of ISFET is based on the generation of potential by surface ions in a solution (Yuging et al., 2003, 2005). The generated potential modulates the current flow across silicon semiconductors. A selective membrane fabricated from compounds viz. Si 3 N 4 (silicon nitrite), Al 2 O 3 (Alumina), ZrO 2 (Zirconium oxide), Ta 2 O 5 (Tantalum oxide) is used to cover the transistor gate surface to enable pH measurement. However, the LAPS working are based on semiconductor activation by light emitting diode (LED). Both types of biosensors have proved their applicability for bioassay. These types of biosensors measure either impedance or its components resistance/conductance and capacitance. These biosensors have been used for the assay of urea, using urease as biorecognition component. Though, the use of impedimetric biosensor is less frequent as compared to potentiometric and amperometric biosensors but their use in the study of hybridization of DNA fragments previously amplified by a polymerase chain reaction and also in monitoring the microorganism growth due to the production of conductive metabolites (Silley et al., 1996) and some other studies has produced promising results. With increasing demand for the development of low cost analytical techniques for selective and accurate analysis of drugs and other analytes and also for suggesting the mechanism of action of drugs, scientists working in the field of electroanalytical chemistry have designed electrochemical biosensors. The developed biosensors have been successfully used for pharmaceutical analysis (Gil et al., 2010) and also for biomedical purpose. Since a large variety of biological systems can be used as recognizing agents, as such, it allows the fabrication of specific biosensors for a large variety of analytes and the electrochemical transducers impart high sensitivity to these devices (Lojou et al., 2006). The analyte-bio-recognizing agent interaction is monitored by the transducer and in the case of an electrochemical biosensor the signal detection occurs at the electrode-solution interface, which may be dynamic or static. In case of former methods i.e. voltammetry, amperometric biosensors are used, the interaction involves redox process followed by transfer of electrons. Whereas, in static methods, potentiometric biosensors are used to monitor the concentration of charged species as a function of electrochemical potential (Ravishanker, et al 2001) . The correct choice of a biosensor for the study of a particular analyte depends on the selection of the recognizing agent and transducer, both suited to the target molecule. Thus, the selection of the above biosensors depends on the characteristics of the analyte i.e. while using amperometric biosensors the organic/inorganic species should undergo a redox process at working potentials. Whereas, the static methods (using potentiometric biosensors) involve charged species. In pharmaceutical analysis, the commonly used recognizing agents are enzymes, antibodies, DNA, drug receptors etc. For the fabrication of electrochemical biosensors the main transducing elements in use are some noble metals viz. Pt and Au and also some carbon based electrode materials viz. glassy carbon, carbon paste. These electrodes, after proper modification are being widely used in the field of pharmaceutical and biomedical analysis. In addition to the above materials the advancement made in the field of materials science and nanotechnology, has led to significant development of many electrochemical transducers like conducting polymers with suitable characteristic for electrochemical sensors, carbon nanotubes, nanomaterials with molecular dimensions (porous and monodispersive particles of clay with high superficial area etc). Nevertheless, the efficient immobilization of recognizing agents for transducer is still a challenging task in electrochemical biosensor technology. Some immobilization techniques have been reported in the literature (Nakamura et al., 2003). The field of research and development of biomedical and pharmaceutical analysis embraces comprehensive procedures in a bid to fulfill the requirements of the analysis i.e. accuracy, selectivity, precision, simplicity and low cost. Since, biosensors fulfill the above requirements, their possible use in the field of biomedical, pharmaceutical, food and environment etc analytes is widely being proposed. Chemotherapy is an important weapon for the treatment of cancer. A large number of compounds have been developed as potential candidates for anticancer drugs, but only handful of them have become effective in clinical protocols. As such, the need to develop drugs which can effectively treat various forms ...
Ad you may be interested in