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This term is about biosensors which are analytical tools for the analysis of bio-material samples to gain an understanding of their bio-composition, structure and function by converting a biological response into an electrical signal. Usually it is characterized into three parts the sensitive biological element, the transducer or the detector element, associated electronic or signal processors.
From chemistry point of view biosensors are used for blood glucose monitoring in which a blood glucose test is performed by piercing the skin (typically, on the finger) to draw blood, then applying the blood to a chemically active disposable 'test-strip' which interfaces with a digital meter. Within several seconds, the level of blood glucose will be shown on the digital display.
Further biosensors are classified as Electrochemical biosensors. Electrochemical biosensors are normally based on enzymatic catalysis of a reaction that produces or consumes electrons (such enzymes are rightly called redox enzymes). The sensor substrate usually contains three electrodes; a reference electrode, a working electrode and a sink electrode. The target analyte is involved in the reaction that takes place on the active electrode surface, and the ions produced create a potential which is subtracted from that of the reference electrode to give a signal.
A biosensor is an analytical device for the detection of an analyte that combines a biological component
With a physicochemical detector component. Biosensors: are analytical tools for the analysis of bio-material samples to gain an understanding of their bio-composition, structure and function by converting a biological response into an electrical signal. The analytical devices composed of a biological recognition element directly interfaced to a signal transducer which together relates the concentration of an analyte (or group of related analytes) to a measurable response.
It consists of 3 parts:
the sensitive biological element (biological material (eg. tissue, microorganisms, organelles, cell receptors, enzymes, antibodies, nucleic acids, etc), a biologically derived material or biomimic) The sensitive elements can be created by biological engineering.
the transducer or the detector element (works in a physicochemical way; optical, piezoelectric, electrochemical, etc.) that transforms the signal resulting from the interaction of the analyte with the biological element into another signal (i.e., transducers) that can be more easily measured and quantified;
associated electronics or signal processors that are primarily responsible for the display of the results in a user-friendly way.. This sometimes accounts for the most expensive part of the sensor device, however it is possible to generate a user friendly display that includes transducer and sensitive element(see Holographic Sensor).
A common example of a commercial biosensor is the blood glucose biosensor, which uses the enzyme glucose oxidase to break blood glucose down. In doing so it first oxidizes glucose and uses two electrons to reduce the FAD (a component of the enzyme) to FADH2. This in turn is oxidized by the electrode (accepting two electrons from the electrode) in a number of steps. The resulting current is a measure of the concentration of glucose. In this case, the electrode is the transducer and the enzyme is the biologically active component.
Recently, arrays of many different detector molecules have been applied in so called electronic nose devices, where the pattern of response from the detectors is used to fingerprint a substance. Current commercial electronic noses, however, do not use biological elements.
Basic Characteristics of a Biosensor
1. LINEARITY: Maximum linear value of the sensorcalibration curve. Linearity of the sensor must be high forthe detection of high substrate concentration.
2. SENSITIVITY: The value of the electrode response persubstrate concentration.
3. SELECTIVITY: Interference of chemicals must be minimized for obtaining the correct result.
4. RESPONSE TIME: The necessary time for having 95% ofthe response.
Biosensors from Chemistry point of view
Used for Blood glucose monitoring
Blood glucose testing, showing the size of blood drop required by modern meters.
Blood glucose monitoring is a way of testing the concentration of glucose in the blood (glycemia). Particularly important in the care of diabetes mellitus, a blood glucose test is performed by piercing the skin (typically, on the finger) to draw blood, then applying the blood to a chemically active disposable 'test-strip'. Different manufacturers use different technology, but most systems measure an electrical characteristic, and use this to determine the glucose level in the blood.
Healthcare professionals advise patients with diabetes on the appropriate monitoring regime for their condition. Most people with Type 2 diabetes test at least once per day. Diabetics who use insulin (all Type 1 diabetes and many Type 2s) usually test their blood sugar more often (3 to 10 times per day), both to assess the effectiveness of their prior insulin dose and to help determine their next insulin dose.
Blood glucose monitoring reveals individual patterns of blood glucose changes, and helps in the planning of meals, activities, and at what time of day to take medications.
Also, testing allows for quick response to high blood sugar (hyperglycemia) or low blood sugar (hypoglycemia). This might include diet adjustments, exercise, and insulin (as instructed by the health care provider).
Blood glucose meters
Four generations of blood glucose meter, c. 1993-2005. Sample sizes vary from 30 to 0.3 Î¼l. Test times vary from 5 seconds to 2 minutes (modern meters are typically below 15 seconds).
A blood glucose meter is an electronic device for measuring the blood glucose level. A relatively small drop of blood is placed on a disposable test strip which interfaces with a digital meter. Within several seconds, the level of blood glucose will be shown on the digital display.
Needing only a small drop of blood for the meter means that the pain associated with testing is reduced and the compliance of diabetic people to their testing regimens is improved. Although the cost of using blood glucose meters seems high, it is believed to be a cost benefit relative to the avoided medical costs of the complications of diabetes.
Recent and welcome advances include:
'alternate site testing', the use of blood drops for from other places than the finger, usually the palm or forearm. This alternate site testing uses the same test strips and meter, is practically pain free, and gives the real estate on the finger tips a needed break if they become sore. The disadvantage of this technique is that there is usually less blood flow to alternate sites, which prevents the reading from being accurate when the blood sugar level is changing.
'no coding' systems. Older systems required 'coding' of the strips to the meter. This carried a risk of 'miscoding', which can lead to inaccurate results. Two approaches have resulted systems that no longer require coding. Some systems are 'auto coded', where technology is used to code each strip to the meter. And some are manufactured to a 'single code', thereby avoiding the risk of miscoding.
'multi-test' systems. Some systems use a cartridge or a disc containing multiple test strips. This has the advantage that the user doesn't have to load individual strips each time, which is convenient and can enable quicker testing.
'Downloadable' meters. Most newer systems come with software that allows the user to download meter results to a computer. This information can then be used, together with health care professional guidance, to enhance and improve diabetes management. The meters usually require a connection cable, unless they are designed to work wirelessly with an insulin pump, or are designed to plug directly into the computer.
Continuous blood glucose monitoring
A continuous blood glucose monitor (CGM) determines blood glucose levels on a continuous basis (every few minutes). A typical system consists of:
a disposable glucose sensor placed just under the skin, which is worn for a few days until replacement
a link from the sensor to a non-implanted transmitter which communicates to a radio receiver
an electronic receiver worn like a pager (or insulin pump) that displays blood glucose levels with nearly continuous updates, as well as monitors rising and falling trends.
Continuous blood glucose monitors measure the glucose level of interstitial fluid. Shortcomings of CGM systems due to this fact are:
continuous systems must be calibrated with a traditional blood glucose measurement (using current technology) and therefore require both the CGM system and occasional "fingerstick"
glucose levels in interstitial fluid lag temporally behind blood glucose values
Patients therefore require traditional fingerstick measurements for calibration (typically twice per day) and are often advised to use fingerstick measurements to confirm hypo- or hyperglycemia before taking corrective action.
The lag time discussed above has been reported to be about 5 minutes. Anecdotally, some users of the various systems report lag times of up to 10-15 minutes. This lag time is insignificant when blood sugar levels are relatively consistent. However, blood sugar levels, when changing rapidly, may read in the normal range on a CGM system while in reality the patient is already experiencing symptoms of an out-of-range blood glucose value and may require treatment. Patients using CGM are therefore advised to consider both the absolute value of the blood glucose level given by the system as well as any trend in the blood glucose levels. For example, a patient using CGM with a blood glucose of 100 mg/dl on their CGM system might take no action if their blood glucose has been consistent for several readings, while a patient with the same blood glucose level but whose blood glucose has been dropping steeply in a short period of time might be advised to perform a fingerstick test to check for hypoglycemia.
Continuous monitoring allows examination of how the blood glucose level reacts to insulin, exercise, food, and other factors. The additional data can be useful for setting correct insulin dosing ratios for food intake and correction of hyperglycemia. Monitoring during periods when blood glucose levels are not typically checked (e.g. overnight) can help to identify problems in insulin dosing (such as basal levels for insulin pump users or long-acting insulin levels for patients taking injections). Monitors may also be equipped with alarms to alert patients of hyperglycemia or hypoglycemia so that a patient can take corrective action(s) (after fingerstick testing, if necessary) even in cases where they do not feel symptoms of either condition. While the technology has its limitations, studies have demonstrated that patients with continuous sensors experience less hyperglycemia and also reduce their glycosylated hemoglobin levels.
This technology is an important component in the effort to develop a closed-loop system connecting real-time automatic control of an insulin pump based on immediate blood glucose data from the sensor. One important goal is to develop an algorithm for automatic control, by which the system would function as an artificial pancreas. However, this is a long-term goal at this point for companies that manufacture such systems, as such an algorithm would need to be very complex in order to accurately control blood sugar levels without any user input.
Electrochemical Glucose Biosensor
Glucose + O2 à Gluconic Acid + H2O2
H2O2 à 2H+O2 +2 e-
0.6 V vs. SHE
The first and the most widespreadly used commercial biosensor: the blood glucose biosensor - developed by Leland C. Clark in 1962
Electrochemical biosensors are normally based on enzymatic catalysis of a reaction that produces or consumes electrons (such enzymes are rightly called redox enzymes). The sensor substrate usually contains three electrodes; a reference electrode, a working electrode and a sink electrode. An auxiliary electrode (also known as a counter electrode) may also be present as an ion source. The target analyte is involved in the reaction that takes place on the active electrode surface, and the ions produced create a potential which is subtracted from that of the reference electrode to give a signal. We can either measure the current (rate of flow of electrons is now proportional to the analyte concentration) at a fixed potential or the potential can be measured at zero current (this gives a logarithmic response). Note that potential of the working or active electrode is space charge sensitive and this is often used. Further, the label-free and direct electrical detection of small peptides and proteins is possible by their intrinsic charges using biofunctionalized ion-sensitive field-effect transistors.
Potentiometric biosensors are based on ion-selective electrodes (ISE) and ion-sensitive field effect transistors (ISFET). The primary outputting signal is possibly due to ions accumulated at the ion-selective membrane interface. Current flowing through the electrode is equal to or near zero. The electrode follows the presence of the monitored ion resulting from the enzyme reaction. For example, glucose oxidase can be immobilized on a surface of the pH electrode. Glucose has only minimal influence on pH in the working medium; however, the enzymatically formed gluconate causes acidification. A bio recognition element is immobilized on the outer surface or captured inside
the membrane. In the past the pH glass electrode was used as a physicochemical transducer. The Nernst potential of the pH glass electrode is described by the Nicolsky-Eisenman equation, of which the generalized form for ISE : (E potential, R the universal gas constant, T temperature, F Faraday constant, za followed and zi interfering ion valence, aa activity of measured and ai activity of interfering ion and Ka,i represents the selectivity coefficient).
Amperometric biosensors are quite sensitive and more suited for mass production than the
Potentiometric ones . The working electrode of the amperometric biosensor is usually either a noble metal or a screen-printed layer covered by the biorecognition component . Carbon paste with an embedded enzyme is another economic option . At the applied potential, conversion of electro active species generated in the enzyme layer occurs at the electrode and the resulting current (typically nA to Î¼A range) is measured. The principle of the previously mentioned YSI 23A can serve as an example:
Glucose + GOD(FAD) à gluconolactone + GOD(FADH2) (1)
GOD (FADH2) + O2 à GOD(FAD) + H2O2 (2)
H2O2 à O2 + 2H+ + 2e- (3)
The reactions (1) and (2) are catalyzed by glucose
The reactions (1) and (2) are catalyzed by glucose oxidase (GOD) containing FAD as a cofactor. The last reaction is the electrochemical oxidation of hydrogen peroxide at the potential of around +600 mV. Amperometric biosensors can work in two- or three-electrode configurations. The former case consists of reference and working (containing immobilized biorecognition component) electrodes.
The main disadvantage of the two-electrode configuration is limited control of the potential on
the working electrode surface with higher currents, and because of this, the linear range could be
Shortened. To solve this problem, a third auxiliary electrode is employed. Now voltage is applied
between the reference and the working electrodes, and current flows between the working and the auxiliary electrodes.
The amperometric biosensors are often used on a large scale for analytes such as glucose, lactate
and sialic acid Biological agents such as model Bacillus cereus and Mycobacterium smegmatis, the serological diagnosis of Francisella tularensis, a pharmacology study and the detection of pesticides and nerve agents have also been described.
There are many potential applications of biosensors of various types. The main requirements for a biosensor approach to be valuable in terms of research and commercial applications are the identification of a target molecule, availability of a suitable biological recognition element, and the potential for disposable portable detection systems to be preferred to sensitive laboratory-based techniques in some situations. Some examples are given below:
Glucose monitoring in diabetes patients â†historical market driver
Other medical health related targets
Environmental applications e.g. the detection of pesticides and river water contaminants
Remote sensing of airborne bacteria e.g. in counter-bioterrorist activities
Detection of pathogens
Determining levels of toxic substances before and after bioremediation
Detection and determining of organophosphate
Routine analytical measurement of folic acid, biotin, vitamin B12 and pantothenic acid as an alternative to microbiological assay
Determination of drug residues in food, such as antibiotics and growth promoters, particularly meat and honey.
Drug discovery and evaluation of biological activity of new compounds.
Protein engineering in biosensors.
Detection of toxic metabolites such as mycotoxins .
Fluorescent glucose biosensors
Fluorescent glucose biosensors are devices which measure the concentration of glucose in diabetic patients by means of sensitive protein which relay the concentration by means of fluorescence, an alternative to amperometric sension of glucose. No device has yet entered the medical market but due to the prevalence of diabetes, it is the prime drive in the construction of fluorescent biosensors.