Journal of Alumni Association of Biotechnology (2021) 3(2):1-7
Enzyme Based Biosensor for
Onsite Detection of Chromium
Rasleen Kaur, S. Keshavkant*
School of Studies in Biotechnology, Pt. Ravishankar
Shukla University, Raipur 492 010, India
Received in revised form
Environment has own permissible limit of heavy
metals (HMs) in its every component (soil, water and air). Excess use of HMs
overcomes this limit hence, leading to toxicity affecting the life on Earth.
Due to the oxidizing property of the chromium (Cr), one of the HMs, it is
widely used for a number of purposes such as manufacturing of stainless
steel, in tannery industry, as cleaning agent for glassware, etc.
Chromium has two main stable oxidation states; hexavalent (VI) and trivalent
(III), out of which earlier is comparatively more toxic than the later form.
Solubility of Cr (VI) in ground water had led to the need of designing a
sensitive device which can be quite efficient in monitoring of its presence
in the environment. In this review, an attempt has been made to collate
information, so far available, on the use of microbial system for intake and
detoxification of Cr following use of chromate reductase (CR) enzyme.
Keywords: Biosensor; Cromium; Chromate
Metals are constituent of the Earth’s
crust with varying content at different places based on the environmental
factors. Metallic elements having atomic weight higher than 40.04 (atomic mass of
Ca) (Ming-Ho 2005) are considered as heavy metals (HMs). Heavy metals possess
range of atomic weights which varies between 63.5 to 200.6 g mol−1.
Specific gravity defines HMs with value greater than 5 g cm−3.
Cobalt, copper, iron, manganese, molybdenum, vanadium, strontium and zinc are
some of the essential HMs required by living organisms in trace amounts.
Non-essential HMs such as cadmium, arsenic, chromium (Cr), lead, mercury and
antimony affects surface water system (Oehme and Wolfbeis 1997). Heavy metals
are categorized as pollutants adversely affecting the environment. Accumulation
of HM in the environment is a serious problem globally. Compared to organic
contaminants which are easily degraded in nature, HM contaminants possess
non-biodegradable and persistent properties. Heavy metals tend to concentrate
in living tissues thus becoming part of food chain causing various health
issues in organisms (Kouba et al. 2010; Vinod et al. 2010). Among many
environmental problems, HM play negative role and causing soil and water
pollution thereby affecting 235 million hectares of arable land worldwide (Bermudez et al. 2012). Enrichment of
soil with ions of HMs not only disturbs the biodiversity and productivity but
also affects the structure and function of ecosystem (Mayor et al. 2013). Industries and mining sites
indiscriminately discharge hazardous and harmful waste containing HMs at
elevated concentration thus causing environmental pollution, which is one of
the growing concerns all around the world (Ruggaber and Talley 2006).
Heavy metals are easily available on the Earth surface due to their natural
sources such as natural weathering of the Earth’s crust, mining, soil erosion, etc.
(Ming-Ho 2005). Presence of HMs
beyond the threshold limits of soil is due to anthropogenic sources such as
urban runoff, sewage effluents, industrial discharge, disease control agents
applied to plants and a number of other reasons contribute to their abundance
leading to toxicity. Heavy metal contamination follows a loop of industry,
atmosphere, soil, water, food and human being. Toxicity of HM is a function of
concentration (Agency for Toxic Substance and Disease Registry [ATSDR] (2008).
High concentration of these leads to carcinogenicity, allergy, inhibition of
sensitive enzyme function, etc. (Koropatrick and Leibbrandt 1995).
Among various elements found in Earth
crust, Cr exists as 21st most abundantly found metal (Gallios and
Vaclavikova 2008). Chromium owns its name from a Greek word “chroma” meaning
color. This trace metal gives Ruby and Emerald its red and green color
respectively. Chromium holds 24th position in periodic table as member
of group VIB d-block element, intermediate to molybdenum and tungsten. It has
four naturally occurring non-radioactive isotopes; 50Cr (4.35%), 52Cr (83.80%),
53Cr (9.50%) and 54Cr (2.35%). Atomic mass of Cr is 51.996 g mol-1
and, it has a range of oxidation states from -2 to +6, and is useful for
manufacturing of varied chemicals (Shekawat et al. 2015). It exhibits
ubiquitous nature and found naturally in water, soil, rocks and air. Chromium
concentration in air is <0.01 μg m-3 (ATSDR
2008), in water is 0.2-114.4 μg l-1 (ATSDR
2008) and in soil is 1-1,000 mg kg-1 (Richard
and Bourg 1991). Geochemistry of any region is responsible for availability of
Cr in the Earth crust. Chromium exists as grey colored solid and melts at high
temperature. Its ore has many forms; mineral chromite is the economically
extractable form. The annual world production of chromite shows that the South
Africa is the leading producer of Cr with India contributing 14% of total
output. About 98% of Cr produced from India is contributed by the Sukinda
Valley Mines of Odisha alone. The total mean concentration of Cr ranges from 7
to 150 ppm (Jankiewicz and Ptaszynski 2005). However, its content and
distribution in the soil mainly depend on the type of mother rock of soil. The
presence of additional amount of Cr in the soil is caused by anthropogenic
activity. Among various oxidation states; trivalent Cr (III) and hexavalent Cr (VI)
species are most prevalent, which differ in their individual physiochemical and
toxicological properties (Ishibashi et al. 1990). The Cr(VI) is relatively more
water soluble, bioavailable, oxidizing, reactive and toxic than the Cr(III).
These properties allows Cr(VI) to easily cross the cell membrane, hence cause
oxidative damage to biomolecules (Cervantes
et al. 2001).
Chromium compounds are considered as mutagenic (Venitt and Levy 1974) and
teratogenic (Bauthio 1992) in nature to living systems. Elevated concentration
of Cr due to the anthropogenic activities in the environment is major concern.
Due to the toxicity of Cr(VI), the U.S.
Environmental Protection Agency (EPA) has classified Cr as a Group ‘A’ human
carcinogen and is one of the main pollutants. The permissible limit of Cr in the
sandy soil up to 15 cm is 2-30 ppm and, loamy soil has 14.8-81.0 ppm (Dudka
1992). The directive of the Ministry of the Environment states that the
concentration of Cr in the urban areas, up to depth of 0.3-15 m should be
150-190 ppm. The toxicity and mutagenicity of Cr(VI) are approximately 100 (Beleza
et al. 2001) and
1,000 (Czako-Ver et al. 1999) times more than the Cr(III)
respectively (Pal et al. 2005).
The Earth’s crust contains 0.1-0.3 mg
kg-1 of Cr in bound form (Molokwane et al. 2008). Large quantity of Cr is dumped
into the environment due to improper handling of Cr containing waste, leakage,
accidental release, improper storage conditions and untreated industrial
effluent (Zazo et al. 2008). About 35% of Cr(VI)
is contributed to the environment by anthropogenic sources. Different oxidation
state of Cr has many applications. Elemental Cr(0) is rarely found state which
limiting its use. The Cr(II) readily oxidizes into Cr(III) due to unstable
nature, hence less useful for industry. The Cr(III) easily adsorbed on soil
surface thus has high potential for contaminating aquifers and surface water.
The Cr(VI) is most widely used form of Cr in industries. Its compounds are used
in the form of chromate and dichromate due to the oxidizing property. Under
acidic condition dichromate is used to soak glassware in laboratories (Dhala et
al. 2013). Due to the corrosion resistant property, they are popularly used in
the steel industry to manufacture stainless steel. In electroplating, they are
used as protective coating, construction of alloy products, reactor vessels in
nuclear and aircraft, in tannery industry used to soften the animal hides, dye
and ceramics syntheses, paint, pigments and wood preservative make use of Cr in
large quantity. For leather tanning, 80-90% Cr chemicals are used (Papp 2004).
In addition to these, spill and dumping of Cr compounds cause entry into soil
thereby environmental problems (Rani
and Upadhaya 2013). As a result of excessive use of Cr in manufacturing and
industrial sector, about 1,70,000 tons of Cr is discarded as waste, annually,
into the environment (Kamaludeen 2003). Combustion of coal and oil releases
1700 metric tons Cr year-1. About 35% of Cr released through human
activities is Cr(VI) only.
1. Use of chromium in different industrial sectors.
The Cr(VI) is categorized under group A
human carcinogen because of its mutagenicity (Petrilli and Flora 1977), carcinogenicity (Gruber and
Jennette 1978) and teratogenicity (Gale 1978). Chromium possess high toxicity
to plants, animals, microbes, humans and fishes (
ATSDR 2008). Toxicity of Cr(VI) can results in skin ulcer also known as chrome
holes, nasal ulcer, lung dysfunction, respiratory problems, altered kidney
functions, altered liver functioning and
immune system (Costa and Klein 2006). The
EPA has categorized Cr(III) under Group D. Trivalent Cr being insoluble cannot
cross cell membrane (Gonzalez et al. 2003). Moreover, in humans, Cr(III) is an
essential trace nutrient that plays an essential role in carbohydrate, lipid,
protein and mineral metabolism, and acts as a regulator of insulin activity. Its deficiency can lead to Type II diabetes, heart
diseases and nervous system disorders. Therefore, according to
the U.S. National Research Council, daily dietary intake of Cr(III) should be
50-200 μg for adults. The daily average intake of Cr(III)
from air is less than 0.2 to 0.4 μg, water
is 2.0 μg, and food is 60 μg (ATSDR
1998). Trivalent Cr is required by body in trace
amount. Long exposure to Cr(III) may cause adverse effects to humans, animals
and plants (Guertin 2004).
Reproductive and Developmental Effects
Chromium (VI) affects the
growth of child during pregnancy, while Cr(III) does not possess any effect
on reproductive development.
Chromium Toxicity in Animals
Studies have shown that the
exposure of Cr(VI) can lead to severe
developmental effects in mice such as gross abnormalities and loss of reproductive health like
reduction in clutch size, low sperm count, and altered functioning of seminiferous tubules.
Chromium Toxicity in Plants
The concentration range and regulatory
limit of Cr in soil is 0.005-3950
mg kg-1 and 100 mg kg-1 respectively (Salt 1995). Chromium(III)
has greater affinity of complex formation with the cell biomolecules leading to
their altered functions and affecting central dogma (Nickens et al. 2010).
Chromium(VI) affects the plant growth; both morphologically and genetically
such as inhibition of electron transport, chromosomal DNA damage, altered
transpiration and enzyme activity, decreased nutrient uptake, reduced
photosynthesis, seed germination, etc. (Cervantes et al. 2001).
A number of physical and chemical
treatments such as reduction, precipitation, adsorption, ion exchange, reverse
osmosis and electro-dialysis, or disposal through landfill are used to remove
Cr from industrial effluent and soil respectively. These methods has
disadvantage of incomplete metal removal, high economic cost, high energy and
reagent consumption, while also generating secondary waste and polluting
environment. Some of the chemical treatments done for Cr removal are feasible
only at concentration 1 to 100 mg l-1, and are not effective for
lower concentration (Cossich et al. 2002). To overcome the limitation of
conventional methods, metal- microbe interaction studies are focused as an
alternative solution to clean environment.
Reduction in Microbes
Bioremediation is an effective and
attractive approach for cleaning Cr polluted environment. Microorganisms such
as bacteria and fungi play crucial role in bioremediation process. The Cr(VI)
reducing microorganisms can be easily isolated from natural and contaminated
ecosystems, such group of microbes are named Cr resistant bacteria (CRB)
(Schmieman et al. 1998). Gram negative bacteria are more sensitive for Cr(VI)
reduction as compared to the Gram positive ones (Coleman 1988). Chromate
reduction and, resistance are two independent properties of bacteria. It is not
necessary that CRB also participate in Cr reduction. During Cr cycling in soil,
oxidation and reduction occurs simultaneously. Detoxification of Cr by
reduction from higher oxidation state to lower is easier than the oxidation of
Cr(III) to Cr(VI) (Cervantes et al. 2001). The pH of the medium influences the
oxidation and reduction of Cr in soil. Oxidative power of Cr is directly
proportional to high pH value, while reduction takes place at low pH. Chromium
reduction capacity of microorganisms varies strain to strain because of
difference in biogeochemical activities and nutrient utilization ability.
Chromium(VI) detoxification in CRB is related to plasmid genes. Microbes
exhibit several protective systems against metal stress at higher
concentration. In bacteria, Cr reduction can either be directly by enzymatic
activity or indirectly by producing compound that can reduce Cr(VI). Chromium(VI)
has soluble nature due to which it easily diffuses from cell membrane. Within
cell, ROS are produced leading to toxicity. Chromium reduction is possible
under both aerobic and anaerobic conditions. Some bacteria are capable of both
aerobic and anaerobic reduction of Cr.
Abundant release of Cr contaminated
industrial waste into the environment causes unavoidable problems. The Cr(VI)
released from industries, and high amount of Cr(III) produced due to the
natural and induced degradation needs preventive measures. Detection of Cr
comes prior to its removal from the environment to avoid its ill effects on
life forms. Many different methods such as standard methods, lab based methods,
sensors and biosensors are used for detection of Cr in potable water. Till date, methods used for Cr determination
includes absorption and emission spectroscopy, inductively coupled plasma mass
spectroscopy and chromatographic techniques. These techniques offers high
sensitivity, selectivity, reliability and accuracy but has limitations of
laboratory testing requiring skilled persons, complicated sample collection,
pre-treatment of samples and time taking long protocols (Thompson et al. 1998; Han et al. 2001). Spectroscopic method with the use of diphenylcarbazide chemical
is time demanding, complex and not economical. Lab techniques are not used
much. Biosensors, the electrochemical methods such as ion-selective electrodes,
polarography and other voltametric techniques are more preferred due to the
selectivity, sensitivity, simple instrument and shorter response (Han et al.
2001). Biosensors are more precise, rapid and accurate. The biggest property of
biosensor is portability i.e. on site monitoring of pollutants. In this
review our focus is on detection of Cr with the help of biosensors. International
Union of Pure and Applied Chemistry (IUPAC) (namely Physical Chemistry and
Analytical Chemistry Divisions) defines biosensor as “a self-contained
integrated device that is capable of providing specific quantitative or
semi-quantitative analytical information using a biological recognition element
(biochemical receptor) which is in direct spatial contact with a transduction
element” (Pearson et al. 2000). Biosensors are designed with purpose to detect
electronic signal directly proportional to the analyte present in the sample.
Leland C. Clark Jr. (1962) was the pioneer person in biosensor development. He
worked with electrode biosensor to detect glucose.
Design of Biosensor
as the name suggests consist of a biological substance that acts as recognition
part, which can be enzyme, antibody, DNA, etc. A transducer acts as an
interface between biological material and sensing element. The third important
component is the amplifying unit to process the data generated through
biological interaction and display in some form. Biosensors are categorized
based on their bioelement and transducer.
Fig 3.Classification of biosensor based on bioelement used
and the sensing property.
Fig4. Working principal of a biosensor.
Cross talk between bioelement and transducer play very
important role in the functioning of biosensor. Bioelement is attached on the
surface of transducer without loosing its biological functioning and structure
loss. Methods such as adsorption of bioelement on transducer surface,
crosslinking with the help of functional groups, strong covalent binding
immoblization and sol gel entrapments are used to establish connection between
bioelement and transducer. Whatsoever
immoblization technique is used, it must fulfill certain criteria like;
Biological activity of biomolecules to be used in sensing
should not get altered during their immoblization over the transducer.
The senstivity of biosensor depends upon how closely the
bioelement is bound to the biosensing element. Higher the proximity more is the
sensitivity of the biosensor.
The immoblization technique must be durable with biological
activity for long run.
must not interfere with specificity of biosensor for analyte detection (Collings 1997).
The technique should support reproducablity and reuse of same
biomaterial (D'Souza 2001).
In biosensor, transducer functions to
convert physical, chemical and biological reactions occurring between the
bioelement and sensing element into the electrical readable signals (Lowe
2007). On the basis of output signals from transducer different biosensors are
designed. Electrochemical biosensor based on electrochemical properties are the
most studied since 1960s for glucose oxidase enzyme activity in glucose
biosensor (Clark 1962). Various mechanisms involved in the transduction are;
electrochemical, electrical, optical, thermal, polarimetric, piezoelectric and surface acoustic
wave, optical and field effect transducers, thermal
and piezoelectric (Pearson et al. 2000). Response of transducer may vary due to
1. Change in hydrogen ion
2. Reaction involving emission of gases
such as ammonia.
3. Electrons being utilized or evolved
4. Emission of light.
5. Emission of heat.
6. Absorption or reflectance.
7. Change in mass.
Different electrochemical biosensors
are constructed such as impedimetric, potentiometric or amperometric, each with
its advantages and disadvantages (Pohanka 2008). Signals generated through
interaction between biomolecule and transducer directly relates to the presence
and amount of analyte. Of all the electrochemical biosensors, most studied is
the amperometric biosensor for Cr sensing in the environment due to easy
miniaturization and activity with less volume. From biorecognition side, this
review highlights the enzyme based sensing of Cr from contaminated site. Enzyme
based biosensors utilize the enzymes active in CRB capable to reduce toxic
oxidation state of Cr(VI) to nontoxic state Cr(III). Microbial degradation of
environmental HM pollutants is slow as compared to particular enzymes isolated
from microbial species (Sutherland et al. 2004). Enzymatic biosensors are more
specific in its functioning and devoid of toxic by-products thus not posing any
negative impact on the environment. Designing of enzyme based biosensor can use
either free enzyme or immobilized enzyme as bioelement. Immoblized enzymes have
several advantages over free enzymes, which are listed below (Berezhetskyy et
of enzymes required to be immobilized on the surface is thousand times lower
than the free enzymes for any reaction.
enzymes avoid unnecessary pre-incubation.
analysis of analyte.
of enzyme activity is not necessary as in case of reversible inhibition.
Enzymatic biosensors have two modes of
actions for sensing of HM ions such as Cr either by inhibition or activation. Sometimes HM ions become integral
part of the enzyme structure and acts as their cofactor to enhance the overall
functions of enzymes, for example metalloproteins and thus activate the enzyme.
Functioning of enzyme biosensor based on inhibition of enzyme activity is due
to the interference of metal ions with the thiol or methylthiol groups of amino
acids present at the active site of enzymes. Many enzymes such as oxidases and
dehydrogenase work on enzyme inhibition (Krawczyk et al. 2000). For Cr
determination inhibition based biosensors are developed using L-lactate
dehydrogenase, hexokinase and pyruvate kinase. Presence of Cr inhibits the
action of these enzymes. Heavy metal ions present in the sample
are measured in terms of % Inhibition and it is given like;
% Inhibition = [(Ao – A)/Ao x 100 %]
absorbance of substrate obtained before incubation in HM
Ao: absorbance of urea obtained after
incubation in HM
The amperometric biosensor has two
electrodes; working and reference. On applying constant potential,
electroactive material is either oxidized or reduced which results in electron
flow between two electrodes. The change in current determines the analyte
concentration which is related to mass transfer to the electrodes. Caroline et
al. (2003) developed first amperometric biosensor using CR enzyme and Cytochrome
c3 from anaerobic sulphate reducing bacteria. This biosensor was designed using
different immobilization methods. When reduction potential is applied immoblized
cytochrome get oxidized and resulting in electron release which is used in Cr
reduction. Chromium amperometric biosensor has rapid quantification with
sensitivity range 1.8 nA µM-1 and detection limit; 0.20-6.84 mg l-1.
With higher Cr concentration, efficiency of sensing was reduced. Aiken et al. (2003) used
enzyme inhibition by nitrate reductase for HM detection. Another sensor was
designed based on oxidizing ability of Acidothiobacillus
fero oxidans using oxygen probe transducer. Bacterial biomass was
immobilized on cellulose filter. Bacterial biosensor has very high sensitivity
816 µM l-1 and response time of 51 sec for 2×10-5
concentration Cr2O7. Caroline et al. (2006) quantified Cr
in ground water using Cytochrome c3 of D. norvegicum
(DSM 1741). Biosensor was assessed
under different pH, temperature and ionic strength. Cytochrome c3 was
immobilized on glassy carbon electrode with the help of dialysis membrane. The
lowest detection limit of Cr(VI) of Cytochrome c3 biosensor was 0.2 mg l-1.
Nepomuscene et al. (2007) worked with urease isolated from the Dolichos uniforms. He studied sol gel
immobilization method to detect Cr and determined the difference in working of
free and immobilized enzyme. Immobilized urease has maximum velocity (Vm) of
3.28 × 10-3 mM min-1 which is very less as compared to
free urease 10.62 mM min-1. For restoration of immobilized urease
activity, the sensor was soaked in buffer solution containing ethylene
diaminetetraacetic acid (EDTA). Maria et al. (2008) developed amperometric
biosensor using glucose oxidase enzyme. Electro synthesized poly-o-phenylene diamine
was used for immobilizing glucose oxidase. Glucose oxidase amperometric
biosensor could easily detect toxic Cr(VI). This study used hydrogen peroxide
decomposition by metal ions to estimate enzyme inhibition interference. Ana et
al. (2014) used chronoamperometric assay to quantify Cr(III) and Cr(VI) in
samples. For Cr(III) detection, tyrosinase isolated from mushroom was
immobilized on screen printed carbon electrodes by cross linking with mediator
tetrathiafulvalene (TTF). Inhibitory activity of tyrosinase to pyrocatechol was
measured. Limit of detection was 2.0±0.2 mM and reproducibility was 5.5%.
Quantification of Cr(VI) was done through glucose oxidase isolated from Aspergillus
niger, which was
immobilized on modified screen printed electrodes using ferricyanide as
mediator. In the presence of Cr, chronoamperometric response was reduced for
glucose with limit of detection 90.5±7.6 nM and reproducibility of 6.2%. Aisha
et al. (2004) worked with horseradish peroxidase enzyme for Cr detection with
the help of hydrogen peroxide. This was the first study utilizing horseradish
peroxidase for detection of Cr(III) and Cr(VI) through enzyme inhibition.
Enzyme was immobilized directly on carbon film electrode or electropolymerized
using gulutaraldehyde with bovine serum albumin. The detection limit was 0.4
and 2.5 μM respectively for Cr(III) and Cr(VI). Amperometric biosensor has two
approaches for Cr quantification; the direct approach which utilizes anaerobic Cytochrome
c3 showed a linear relation between increase in Cr concentration and current flow.
The second approach was indirect quantification following enzyme inhibition
activity in the presence and absence of substrate. Biosensor based on indirect
approach showed exponential decay of current with Cr concentration. The best
amperometric biosensor for Cr detection was urease with the highest detection
limit 10×103 to 50×103 μmol l-1.
In enzymatic reaction, charged species
are either consumed or released which leads to change in ionic strength of
sample. Conductometric biosensors are designed with two electrodes dipped in
solution. Enzymatic reactions are used to charge neutral species and the change
in conductance of medium was measured. Compared to other biosensors,
conductometric biosensor offers size miniaturization, cost effectiveness with
less power consumption. The Cr(VI) contamination was accessed using
thiosulphate oxidizing bacteria (TOB) and, based on thiosulphate oxidation pH,
production of sulphate and electrical conductivity were determined. Using
thiosulphate as electron donor, biosensor was developed with Cr(VI) detection
limit of 100 µg l-1. Many researchers have utilized inhibition
activity of sulphur reducing bacteria (SOB) to estimate Cr in sample. Under
oxygen rich condition, SOB uses SO as electron donor and oxygen as electron
acceptor; and by oxidation reaction produces sulphate and protons which
increases the electrical conductivity by increase in protons. Presence of HMs
affects oxidation of sulphur to sulphate by SRB. Qambrani et al. (2016) worked for
Cr quantification using inhibition activity of SOB, best at 2.0 mg l-1 Cr(VI)
with 98% inhibition. Fourou et al. (2016) developed Cr(VI) biosensor based on
β-galactosidase inhibition, isolated from Aspergillus
oryzoe. The β-galactosidase was immobilized by cross linking with
glutaraldehyde on electrochemical transducer. The β-galactosidase was inhibited
by the presence of Cr(VI). The limit of detection for this biosensor was
Potentiometric biosensor was based on oxidation
or reduction potential of an electrochemical reaction. It consists of ion
selective electrodes and transistors that convert biological reactions into
electrical signals. Nernst equation was used to study relationship between
potential difference and analyte in the electrochemical cell.
Ecell =E0cell –
Cell potential at zero current
Universal gas constant
Absolute temperature in kelvin
Number of charge transferred during electrode reaction
Ratio of anode ion concentration/ Cathode ion concentration
Microbial fuel cell (MFC) as biosensor
for real time Cr monitoring was designed by Bingchuan et al. (2001), based on
potentiometric principal. It is least utilized type of biosensor.
Resonant biosensor is based on change
in mass of the membrane when analyte molecule is attached to bioelement coupled
with acoustic wave transducer. The bioelement in this biosensor is an antibody
molecule having affinity for antigen molecule. The change in mass directly
changes the resonant frequency which is measured and displayed (Higson et al.
These biosensors measures the light as
electrical signal generated in any reaction. These can be based on optical
diffraction or electrochemiluminescence. Optical diffraction biosensors are
more efficient to response against minor change in refractive index or
thickness when cells get bound to immobilize receptors on transducer. Change in
light can be directly correlated to changes in mass number and molecules (Syam et al. 2012).
Thermal biosensor is based on principal
that biological reactions involve liberation or involvement of heat. Such types
of biosensors are used in the detection of pesticides and pathogenic bacteria
(Syam et al. 2012).
By the integration of nanotechnology
with biosensors, nanobiosensors are developed. Nanoparticles such as silver,
gold, carbon, etc., are utilized in the sensing of biomolecules with
more sensitivity. Such types of biosensors are particularly used in drug
delivery system (Rai et al. 2012).
Piezoelectric (PZ) biosensor has
advantage of real-time output, simple operation and economic cost. When the
surface of the PZ sensor bounded with a selectively binding substance binds to
its counter parts, the crystal mass increases with proportional decrease in
resonance frequency of oscillation (Lazcka et al. 2007).
Bioluminescence biosensors are based on
enzymes ability to emit photon in the reactions. These sensors are highly
useful for bacterial detection. The lux
gene in bacteria codes for luciferase enzyme under the control of constitutive
or inducible promoter. To detect the concentration of interest of compound the
gene is fused with the lux gene resulting in fusion product. By
measuring the bioluminensance intensity the concentration of compound can be
known (D’Souza 2001).
Microorganisms provide the
environmental condition for reaction to be feasible. They adapt to new
conditions by bringing modifications in their genome. Whole cell can be used as
bioelement in biosensor either in viable or non-viable form. Metabolism of organic
compounds releases carbon dioxide, ammonia and acids as end products which are
measured by the transducers. Most of the microbial biosensors are developed to
access environmental problems, such as biological oxygen demand estimation
based on oxygen depletion rate (Rogers et al. 1999).
In this review, we have
summarized various biosensors capable of detecting Cr using different
transducer elements. Most widely used electrochemical method for Cr sensing is
amperometric biosensor. The CRB has potential to detoxify Cr from toxic higher
oxidation state +6 to lower +3 with the help of CR
enzyme. Chromium metal exhibit different colors at different oxidation state
which can be one of the possible ways for Cr sensing. Chromium detection
involving CR as a bio-recognition element in biosensor will possibly be a
sensitive technique and will be efficient in detection of Cr, even present in
trace amounts, in soil and water samples.
The use of microbes as a
remedy to solve environmental problems is a promising technique. Dumping site
of tannery effluent and mining ore site are the mostly Cr polluted sites. Different
sophisticated techniques involving complex instrumentation are being used by
the researchers to detect Cr. Use of enzyme based biosensors will not only
helpful in Cr detection but also detoxify it. Biosensors developed so far has
very low limit of detection, therefore; improvements can be made to enhance
their detection limits for finer detection. Another improvement in Cr detection
following the use of enzyme biosensors is to overcome the detection of
background noise which often can lead to false positive detection. Developing
such Cr biosensor with very simple operation and quick detection will help us
to tackle Cr hazards and its finer sensing.
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