Abstract
Chemsensors have been playing a crucial role in various aspects of biomedical science, analytical and environmental chemistry. The toxic metal ions like Zn, Cd, Cu, Pb and Hg have increased gradually but now have reached an alarming situation, crossing the threshold value. Due to high toxicity of these heavy metals there is an obvious need for a sensor system to detect their presence. Chemsensors including surface acoustic wave sensors, enzymes, carbon nanotubes, nanoparticles, and chromophore-based sensors have attracted increasing attention over the last few years. Chemsensors prove very promising as the system is rapid, selective, sensible, low-cost, easy-to-use, and has the ability to provide real-time signals. However, recently, considerable effort has been devoted to the synthesis of sterically encumbered selenium containing species reported to display strong affinities with Hg2+ or Ag2+. This chapter reviews the basic principles involved in the design of chemsensors, their variety and applications in various established and emerging fields.
TopIntroduction
Till today a great variety of sensors has been synthesized by man, few of which are those that allows us to detect chemical species, this particular type is called a chemical sensor, and is defined by IUPAC as, a device that transforms chemical information, ranging from the concentration of a specific sample component to total composition analysis, into an analytically useful signal (Seyma, Fueki, Shiokawa et al., 1983). This device can be either macroscopic (e.g. a pH measuring electrode) or microscopic, it refers to those molecule (or an assembled supra-molecular unit) that could selectively bind the target analyte and furnish information about this binding, therefore acting as a microscopic chemical sensor.
Molecular recognition and signal transduction are the two different processes that take place in chemical sensing through analyte detection. A chemsensor usually constitute three parts: A receptor (which is responsible for the selective analyte binding), an active unit (whose properties should change upon the aforementioned binding) and, in few cases, a spacer that is able to modify the geometry of the system and tune the electronic interaction between the two other components (White, 1996).
Figure 1. Source: White, 1996 Actually, chemsensors are sensory receptors that transduce a chemical signal into an action and have vast potential as detecting units and thus, are continuously emerging as a tool for industrial, research, scientific, and pharmaceutical markets. In this series coordination compounds are also emerging as a potent tool in which chalcogen (O, S, Se and Te) containing compounds plays a significant role. Selenium is one of the elements from chalcogen series which is a naturally occurring element considered a link between metals and non-metals. It is found in nature in small concentration in rocks, plants, coal and fossil fuels (Shapira, 1973). One of such sterically encumbered selenium compound, tetrakis(iso-propyl seleno methyl benzene) on complexation with Hg2+, Ag2+, Pb2+& Cd2+ showed selectivity towards Hg2+ ions though coordination to selenium is not an exclusive feature for mercury. As confirmed by physicochemical data the presence of multiple soft selenium donor, the flexibility of the arms, steric bulk and open exterior geometry make these molecules potent tool for trapping Hg (II) selectively. Thus, there is a great potential in designing of sterically hindered organoselenium molecule to bind environmentally toxic metal ion selectively. Hence such tailored ligand and complexes could serve a great potential for trapping environmentally toxic metal ion and thus has application as sensors.
TopBackground
Large number of solid-state sensor devices based on various principles and materials are known which detect gaseous components for example semiconductor gas sensors which uses metal oxides, which detect the presence of inflammable gases in air such as CH4, LPG and H2 are therefore are used in large scale as gas leakage alarms domestically (Yamazoe & Miura, 1992). Sensors have also become increasingly important in food industries for the detection of various volatile gases or smells generated from food or food materials. However, the most burning issue today is of energy and environment are increasing the necessity of those sensors which can detect air pollutants in environment like SOX, NOX, etc., as well as can be applied for control systems of combustion exhaust from stationary facilities and automobiles.
Key Terms in this Chapter
Physicochemical Data: Relating to both physical and chemical properties of any sample or substance.
Inductively Coupled Plasma Mass Spectrometry or ICP-MS: Is an analytical technique used for elemental determinations. It combines a high-temperature ICP (Inductively Coupled Plasma) source with a mass spectrometer.
Biomacromolecules: Are the biomolecules which have molecular weights higher than 1000 daltons (Da) approximately and found in the acid soluble pool in the chemical analysis of the living tissue. For e.g. proteins, nucleic acids etc.
Glucose Biosensor: Are devices that measure the concentration of glucose in diabetic patients by means of sensitive protein that relays the concentration by means of fluorescence, an alternative to amperometric sension of glucose.
Heteroatom: This term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure. Typical heteroatoms are nitrogen, oxygen, sulphur, phosphorus, chlorine, bromine and iodine.
Coumarin: It is a fragrant organic chemical compound in benzopyrone chemical class may also be seen as subclass of lactone.
Polarography: A method of analysis in which a sample is subjected to electrolysis using a special electrode and a range of applied voltages, a plot of current against voltage showing steps corresponding to particular chemical species and proportional to their concentration.
Atomic Absorption Spectroscopy (AAS): It is a spectroanalytical procedure for the quantitative determination of chemical elements using the absorption of optical radiation (light) by free atoms in the gaseous state.
Fluoroscent Probes: These are molecules that absorb light of a specific wavelength and emit light of a different, typically longer, wavelength (a process known as fluorescence).
Fluoroscence Quantum Yield: Ratio of number of photons emitted by the fluorophore to the number absorbed. The possible values are between 0 and 1.
Spectrofluorometer: An instrument which takes advantage of fluorescent properties of compounds in order to provide information regarding their concentration and chemical environment in a sample. A certain excitation wavelength is selected, and the emission is observed either at a single wavelength, or a scan is performed to record the intensity versus wavelength, also called an emission spectra.
Phosphorescence: It is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for “glow-in-the-dark” materials which are “charged” by exposure to light. In phosphorescence, the electron which absorb the photon (energy) undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity usually a triplet state from singlet state, which then returns to the ground state.
Organochalcogen Compound: Is a compound containing at least one carbon-chalcogen bond.
Cold Vapor Atomic Absorption Spectroscopy or CVAAS: Is one of the primary techniques for mercury analysis. It has a peristaltic pump that transports sample and stannous chloride into a Gas Liquid Separator (GLS) where a stream of pure, dry gas (typically argon) is introduced to the liquid mixture to release mercury vapor.