Biosensors are chemical sensors that take advantage of the high selectivity and sensitivity of a biologically active material. It is well known that the resonant frequency of an oscillating piezoelectric crystal can be affected by a change in mass at the crystal surface. Piezoelectric immunosensors are able to measure a small change in mass.
A biosensor is an analytical tool consisting of biologically active material used in close conjunction with a device that will convert a biochemical signal into a quantifiable electrical signal. Biosensors have many advantages, such as simple and low-cost instrumentation, fast response times, minimum sample pretreatment, and high sample throughput. Although biosensors are beginning to move toward field testing and commercialization in the United States, Europe, and Japan, relatively few have been commercialized. Increased research in this area demands the development of novel materials, new and better analytical techniques, and new and improved biosensors. Some potential applications of biosensors are agricultural, horticultural and veterinary analysis; pollution, water and microbial contamination analysis; clinical diagnosis and biomedical applications; fermentation analysis and control; industrial gases and liquids; mining and toxic gases; explosives and military arena; and flavors, essences and pheromones.
A biosensor has two components: a receptor and a detector. The receptor is responsible for the selectivity of the sensor. Examples include enzymes, antibodies, and lipid layers. The detector, which plays the role of the transducer, translates the physical or chemical change by recognizing the analyte and relaying it through an electrical signal. The detector is not selective. For example, it can be a pH-electrode, an oxygen electrode or a piezoelectric crystal. Figure 1 describes a typical biosensor configuration that allows measurement of the target analyte without using reagents.
The device incorporates a biological-sensing element with a traditional transducer. The biological-sensing element selectively recognizes a particular biological molecule through a reaction, specific adsorption, or other physical or chemical process, and the transducer converts the result of this recognition into a usable signal, which can be quantified. Common transduction systems are optical, electro-optical, or electrochemical; this variety offers many opportunities to tailor biosensors for specific applications. For example, the glucose concentration in a blood sample can be measured directly by a biosensor (which is made specifically for glucose measurement) by simply dipping the sensor into the sample.
Two classes of bio-recognition processes-bio-affinity recognition-and bio-metabolic recognition, offer different methods of detection. Both processes involve the binding of a chemical species with another, which has a complementary structure. This is referred to as shape-specific binding. In bio-affinity recognition, the binding is very strong, and the transducer detects the presence of the bound receptor-analyte pair. The most common types of processes are receptor-ligand and antibody-antigen binding.
In bio-metabolic recognition, the analyte and other co-reactants are chemically altered to form the product molecules. The biomaterials that can be recognized by the bio-recognition elements are as varied as the different reactions that occur in biological systems. Table 1 lists a number of common analytes that could prove attractive for developing biosensors of appropriate specificity and sensitivity.
Almost all types of biological reactions, (chemical or affinity), can be exploited for biosensors. The concept of shape-specific recognition is commonly used to explain the high sensitivity and selectivity of biological molecules, especially antigen-antibody systems. The analyte molecule has a complementary structure to the antibody, and the bound pair is in a lower energy state than the two separate molecules. This binding is very difficult to break. Table 2 summarizes a variety of biosystem-transducer combinations in terms of transducer, measurement mode and potential application.
The interaction of antibodies with their corresponding antigens is an attractive reason for attempting to develop antibody-based chemical biosensors, i.e. immunosensors. Theoretically, if an antibody can be raised against a particular analyte, an immunosensor could be developed to recognize it. Despite the high specificity and affinity of antibodies towards complementary ligand molecules, most antibody-antigen interactions do not cause an electronically measurable change. However, the remarkable selectivity of antibodies has fueled much research to overcome this intrinsic problem. The piezoelectric effect in various crystalline substances is a useful property that leads to the detection of analytes. Figure 2 shows a schematic diagram of an immunosensor device.
The piezoelectric immunosensor is thought to be one of the most sensitive analytical instruments developed to date, being capable of detecting antigens in the picogram range. Moreover, this type of device is believed to have the potential to detect antigens in the gas phase as well as in the liquid phase.
A piezoelectric sensor that could reliably detect the mycobacterial antigen in biological fluids would be of enormous use. For instance, detection of the antigen in saliva could constitute a noninvasive method of screening high-risk populations. One tested piezoelectric crystal sensor gives results within a couple of hours after exposing the electrode to a liquid containing the antigen. The apparatus would be quite portable, so the immunological tests could be performed virtually anywhere, and the results could be obtained very quickly.
A piezoelectric sensor that could reliably detect the mycobacterial antigen in biological fluids would be of enormous use. For instance, detection of the antigen in saliva could constitute a noninvasive method of screening high-risk populations. One tested piezoelectric crystal sensor gives results within a couple of hours after exposing the electrode to a liquid containing the antigen. The apparatus would be quite portable, so the immunological tests could be performed virtually anywhere, and the results could be obtained very quickly.
Theoretical Principals
The basic equations describing the relationship between the resonant frequency of an oscillating piezoelectric crystal and the mass deposited on the crystal surface have been derived by Sauerbrey, Stockridge, and Lostis. Each followed a different path, but their final equations are similar, the Sauerbrey equation being the most widely accepted. In 1959, Sauerbrey developed an empirical equation for AT-cut quartz crystals vibrating in the thickness shear mode that describes the relationship between the mass of thin metal films deposited on quartz crystals and the corresponding change in resonant frequency of the crystal:
where, DF = frequency change in oscillating crystal in Hz, F = frequency of piezoelectric quartz crystal in MHz, DM = mass of deposited film in g, and A = area of electrode surface in cm2.
These relationships not only apply to film deposition but also to particulate deposition. When vibrating in the thickness-shear mode, the oscillating frequency of an AT-cut quartz crystal is given by:
 | (1) |
where, DF = frequency change in oscillating crystal in Hz, F = frequency of piezoelectric quartz crystal in MHz, DM = mass of deposited film in g, and A = area of electrode surface in cm2.
These relationships not only apply to film deposition but also to particulate deposition. When vibrating in the thickness-shear mode, the oscillating frequency of an AT-cut quartz crystal is given by:
where F is the frequency of the crystal, N is the material constant (N = 1.66 MHz-mm for AT-cut quartz crystal), and a is the thickness of the crystal plate. Thus,
for finite amount of change, we may write
Dividing Equation 4 by Equation 2 gives:
If the thickness is defined as
where M is the mass of the electrically driven portion of the crystal, A is the area of the electrically driven portion of the crystal, and r is the density of the crystal.
Assuming constant density, we have:
If the changes are finite,
Using Equation 6 yields
Thus,
Since F and M are constants, we may say:
The oscillating frequency of the crystal changes linearly with the change in mass on the crystal. The mass change occurs due to deposition of materials on the surface of the crystal. This relationship is only valid for small mass changes. For larger mass changes, it would be invalid since the density would change.
Equation 10 may also be stated as:
This shows that the term
would increase if the base oscillating frequency of the crystal is increased. The term itself is the sensitivity of the crystal sensor, and Equation 12 shows that sensitivity is directly proportional to the base frequency of the crystal.
If a gas stream is sent flowing over the surface of the piezoelectric crystal and if it contains an analyte with concentration C, then
where C is the concentration of the analyte in the gas stream, DM is the mass of analyte in the gas stream, and V is the volume of the gas in the stream. The volume of the gas stream is related to the sampling time by the following relationship:
where q is the flow rate of the gas stream, and t is the sampling time. Equation 13 can be rewritten as:
where E is the collection efficiency of the coating material. If E is assumed to be equal to one and substituting Equation 15 into Equation 12, we may say
or
Thus, the change in the oscillating frequency of the crystal is related to both the sampling time and the concentration of the particles in the carrier gas. If we keep the sampling time, career gas flow rate and the mass of the analyte in the gas stream constant, we may restate Equation 17 as
Equation 18 shows that the change in frequency of the crystal is directly proportional to the concentration of the analyte in the gas stream flowing over it.
The piezoelectric crystal detector can be a very powerful analytical tool because of the relationship shown for the change in frequency to the analyte concentration with high sensitivity. Conversely, the above explanation shows that the crystal detector indiscriminately changes frequency due to the deposition of mass of any material on its surface. Thus, it is the task of the researcher to choose a coating that will undergo a highly selective chemical or physical binding with the substance to be detected. Only then can a highly selective sensor be constructed that will be sensitive to the subject to be detected.
 | (3) |
 | (4) |
 | (5) |
 | (6) |
Assuming constant density, we have:
 | (7) |
 | (8) |
 | (9) |
 | (10) |
DF = -k DM | (11) |
Equation 10 may also be stated as:
 | (12) |
DF DM |
If a gas stream is sent flowing over the surface of the piezoelectric crystal and if it contains an analyte with concentration C, then
 | (13) |
V = q · t | (14) |
 | (15) |
 | (16) |
 | (17) |
DF = K C | (18) |
The piezoelectric crystal detector can be a very powerful analytical tool because of the relationship shown for the change in frequency to the analyte concentration with high sensitivity. Conversely, the above explanation shows that the crystal detector indiscriminately changes frequency due to the deposition of mass of any material on its surface. Thus, it is the task of the researcher to choose a coating that will undergo a highly selective chemical or physical binding with the substance to be detected. Only then can a highly selective sensor be constructed that will be sensitive to the subject to be detected.
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1 comments:
cool.... The piezoelectric crystal detector can be a very powerful analytical tool because of the relationship shown for the change in frequency to the analyte concentration with high sensitivity?
February 24, 2011 at 12:23 AMPost a Comment