Biomedical Engineering Reference
In-Depth Information
14.1.3 DNA
DNA is the substance that makes up chromosomes and carries the genetic code in human beings
and almost all other organisms. The information in DNA is stored as a code made up of four
chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). These DNA bases pair
up with each other in the rule of A with T and C with G to form the basic units of DNA called base
pairs. Each base is attached together to a sugar molecule and a phosphate molecule to construct the
unit of nucleotide. Nucleotides are arranged in two long strands that form a spiral called double
helix by the interaction of A-T and C-G [129].
DNA can also be used as molecular recognition element in biosensors. Based on the types of DNA
used in biosensors and the mechanisms of action, there are in total two kinds of DNA biosensors,
namely, single-strand DNA (ssDNA) biosensor and double-strand DNA (dsDNA) biosensor. The
ssDNA biosensor uses an immobilized ssDNA as a probe to detect its own complementary ssDNA
series by the interaction of DNA bases. The dsDNA biosensor is used to detect some molecules or
ions that can interact with immobilized dsDNA based on the signals produced in the interaction.
Furthermore, based on the application of markers, DNA biosensors can be classifi ed as marked
DNA biosensor and unmarked DNA biosensor. In a marked biosensor, probe or target DNA is
marked by fl uorescent agents or electroactive agents, and the detection is fulfi lled by the changes of
fl uorescent signals or redox signals. In contrast, no marks are used in an unmarked DNA biosensor,
and the detection is realized by the changes of some physical or chemical information such as mass
and the refractive indexes before and after the hybridization process of DNAs directly.
Similar to enzyme and microorganism biosensors, DNA biosensors can also be classifi ed into
electrochemical DNA biosensor, optical DNA biosensor, surface plasma DNA biosensor, etc. based
on the difference of signal transducer.
Examples of DNA biosensors are listed in Table 14.8.
14.1.4 A
NTIGENS
-A
NTIBODIES
Various substances act as antigens such as toxins, bacteria, foreign blood cells, and the cells of
transplanted organs, which stimulate the production of antibodies when they are introduced into
other bodies. Antibodies are protein substances produced in the blood or tissues in response to
specifi c antigens. Antibodies can destroy or weaken bacteria and neutralize organic poisons, thus
forming the basis of immunity. The binding of an antibody with its specifi c antigen that stimu-
lated the formation of the antibody will result in agglutination, precipitation, complement fi xation,
greater susceptibility to ingestion, and destruction by phagocytes, or neutralization of an exotoxin.
Based on the special reaction of antigen with antibody, antigens (antibodies) can be used to pre-
pare immunobiosensors for the qualitative or quantitative determination of corresponding antibod-
ies (antigens) or even their accelerators and inhibitors by the changes of some physical or chemical
signals such as optical, electrochemical, quality, and thermal signals during the reaction. Till now,
lots of antigens (antibodies) have been used to construct immunobiosensors. Some of them are listed
in Table 14.9.
14.2 INTERMEDIAMATERIALS
Intermedia materials are important components of biosensors. Up to now, various kinds of biosen-
sors have been fabricated and their development depends on the application of new intermedia
materials to a great extent. Because of their special characters, the intermedia materials are able
to immobilize biorecognition molecules effectively or can function as excellent signal transducers.
Usually more than one intermedia materials are needed in a biosensor to achieve the best stability
and sensitivity. In this part, we review intermedia biomaterials widely used in biosensors. Some of
them have been investigated for many years, while the others have been incorporated in biosensors
only for a couple of years.
