Heavy enzyme sensors

Nabok A., Haron S., and Ray A., Registration of heavy metal ions and pesticides with ATR planar waveguide enzyme sensors, Appl. Surf. Sci., 238(1-4), 423-428, 2004. 

Determination of traces of pollutants has become very important. The environment and living organisms are especially vulnerable to the toxic heavy metals. In many cases, a simple and fast method for the determination of heavy metals is desirable. Enzymes which are inhibited by the heavy metals offer an interesting possibility in this context, and several applications of this idea have been made earlier such as for the determination of small concentrations of mercury and copper. We have shown that heavy metals (mercury) can also be detected, e.g., with the use of urease in combination with an IrTMOS ammonia sensor. Although several parameters can still be optimized, our results suggest that simple equipment for field use may be constructed around this type of sensor. The choice of enzyme (urease) was made out of convenience. There may be other enzymes which perform better. Furthermore the choice of enzyme will also determine the specificity of the enzyme-heavy metal system. It may be valuable to have both general heavy metal sensors as well as specific ones. 

Biosensors application for heavy and toxic metals determination is extensively reviewed in some recent works [7, 22-24]. Relevant examples include inhibition-based enzyme sensors, DNA-, antibody-, and whole-cell-based sensors. 

Enzyme/indicator optrodes for registration of enzyme reactions and their inhibitors, such as heavy metal ions and pesticides, can be produced by PESA technique. Composite films containing the enzyme urease and cyclo-tetra-chromotropylene as indicator molecules show some characteristic spectral transformations caused by urea decomposition. The reaction of inhibition of urease by heavy metal ions can be also registered with this method. Further development of the enzyme sensors and sensor arrays lies in finding suitable pairs of enzyme/indicator, their deposition by PESA method and studying the enzyme reactions (including inhibition) with UV-vis spectroscopy. 

For these reasons, microbial sensors are less suitable for the determination of individual analytes. However, some practical apphcations for biosensors based on enzymes or antibodies for the specific determination of environmentally relevant compounds can be expected soon [11]. Furthermore, in some cases defined specific metabolic pathways in microorganisms are used, leading to microbial sensors for more selective analysis for those environmental pollutants which cannot be measured by the use of simple enzyme reactions, e.g., aromatic compounds and heavy metals. In this context it is also important to mention the aspect of bio availability, a parameter which is included by the measuring procedure of microbial sensors as an integral effect. 

Enzyme inhibition sensors are the most commonly reported enzyme-based biosensors for the detection of toxic compounds and heavy metal ions. The sensors are based on the selective inhibition of specific enzymes by classes of compounds or by the more general inhibition of enzyme activity. Most of the research carried out has been directed toward the detection of organophosphorus and carbamate insecticides and the triazine herbicides and metal ions analysis [72,73]. Several enzymes have been used in inhibition sensors for pesticides and heavy metal analysis using water, soil, and food samples including choline esterase, horseradish peroxidase, polyphenol oxidase, urease, and aldehyde dehydrogenase. 

Biosensors, known to monitor various analytes both selectively and sensitively, have also been reported for heavy metal detection. Several electrode configurations using whole cells, enzymes or apoenzymes have been designed 4-6). The main advantage of such biosensors is that samples often require little pretreatment and the bioavailable concentration of the toxic heavy metal is measured, rather than the total concentration. However, a limited selectivity and quite low sensitivity characterize these sensors described in the literature. 

Enzyme inhibition sensors are of interest in the environmental context, the most used being those involving acetylcholinesterase inhibited by pesticides e.g. these can involve rather complex architectures, and characterisation of such systems by EIS is becoming more widespread. Complex sensor architectures have been used for endocrine disrupters, with similar characterisation by electrochemical impedance. Recently, EIS was used for the first time to characterise the response of glucose biosensors in the presence of heavy metal ion inhibition. ... 

Ghica ME, Carvalho RC, Amine A, Brett CMA (2013) Glucose oxidase enzyme inhibition sensors for heavy metals at carbon film electrodes modified with cobalt or copper hexacyanofeirate. Sensor Actuator B 178 270-278... 

In addition to being used for glucose detection, electrical sensors have been developed for pH, ions, heavy metals, small molecules, nucleotides, and enzymes/proteins. Proof of concept has also been demonstrated utilizing antibodies and whole cells as recognition elements. We discuss some characteristic examples below, by order of application as opposed to chronologically. 

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