Recent Advances in DNA Biosensor - International Frequency ...

Sensors & Transducers Journal, Vol. 92, Issue 5, May 2008, pp. 122-133
7. DNA Biochip
Microarrays, DNA arrays, gene chips or biochips are same terminology often being intermixed to
describe analysis of complex DNA samples and expression of genes [59, 60, 77, 78, 79, 80]. The most
attractive features of these devices are the miniaturization, speed and accuracy. Accordingly, this DNA
microchip technology offers an enormous potential for rapid multiplex analysis of nucleic acid
samples, including the diagnosis of genetic diseases, detection of infectious agents, measurements of
differential gene expression, drug screening or forensic analysis. Such use of DNA microarrays is thus
revolutionizing many aspects of genetic analysis.
The biochips are fabricated from glass, silicon or plastic supports, and comprise thousands of
10-100 µm reaction zones onto which individual oligonucleotides have been deposited. This results in
high densities (up to 10 6 sites/cm 2 ) in connection with typical 1-2 cm 2 -size chips. The exact number of
probes varies in accordance with the application. The actual construction of gene chips involves the
immobilization or synthesis of an array of DNA probes on a solid support. High-density DNA arrays
often require the use of physical delivery (e.g. microjet deposition technology), involving the
dispension of picoliter volumes onto discrete locations on the chip. It is essential to activate the surface
for a covalent attachment of the oligonucleotide probes.
Successful implementation of DNA chip technology requires development of methods for fabricating
the probe arrays, detecting the target hybridization, algorithms for analyzing the data, and
reconstructing the target sequence. Such array technology thus integrates molecular biology, advanced
microfabrication / micromachining technologies, surface chemistry, analytical chemistry, software,
robotics and automation. The automation of gene chip systems greatly facilitates their production and
accelerates their operation, while eliminating human errors. The detection of the DNA hybridization
(at the individual spots) relies on the signal generated by the binding event. The most common
application of DNA/oligonucleotide microarray is gene expression analysis. In this technique, RNA
isolated from two samples are labeled with two different fluorochromes (generally the green cyanine 3
and the red cyanine 5 (Cy3, Cy5)) before being hybridised to a microarray consisting of large numbers
of cDNAs / oligonucleotides orderly arranged onto a glass microscope slide. After hybridization under
stringent conditions, a scanner records, after excitation of the two fluorochromes at given wavelengths,
the intensity of the fluorescence emission signals that is proportional to transcript levels in the
biological samples. The microarray data are analyzed using specific software that enables clustering of
genes with similar expression patterns, assuming that they share common biological functions.
8. Conclusions and Future Prospects
From the first discovery of electrochemistry of nucleic acids by Palecek at the end of the 1950’s [81],
huge progress can be observed, particularly at the development of electrochemical DNA biosensors
based on the nucleic acid as biorecognition element. Different types of electrodes immobilized with
specific probes can be used to detect the presence of complementary target sequence by hybridization
technique. Besides the different immobilization methods, electroactive hybridization indicators (metal
complexes, daunomycin, methylene blue, etc.) and different conducting polymer based nanocomposites
are also used for development of electrochemical biosensors.
SPR, Quantum-Dot and piezoelectric biosensors are the emerging area of molecular diagnosis. The
Intelligent Opto sensors interfacing based on universal frequency-to-digital converter has opened new
opportunities for development of DNA biosensors [82]. Some success has been achieved in the
commercialization of optical fiber sensors. However, they still suffer from competition with other
mature sensor technologies and new ideas are being continuously developed and tested not only for the
traditional measurands but also for new applications [83-84].
129