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Sensors & Transducers Journal, Vol. 113, Issue 2, February 2010, pp. 1-17

The most interesting property manifested by these particles is, when tested on mouse, they self- destruct and are cleared from the body within a short period of time without producing any toxic effect. Their work presents a new type of multifunctional nanostructure for in vivo applications with low toxicity. Other uses of silicon nanoporous particles as effective carriers for in vivo simultaneous application for different nanotherapeutics have been reported recently [116].

4. Biochips or Lab-on-a-Chips

A biochip or a lab-on-a chip can be broadly classified as three categories: (a) microarray-based chips for genomic analysis, (b) microfluidic-based chips for executing biochemical assays, and (c) in vitro chips as drug reservoirs and/or monitoring purposes. A microarray contains large number of miniaturized test sites, which can perform thousands of biochemical reactions instantaneously, such as decoding genes, DNA/protein analysis, etc., in a few seconds. The microfluidic-based biochips are widely used for on-chip implementation of several biochemical laboratory assays, for sample preparation, dilution and mixing [119, 120]. These chips use only nanoliter volumes of fluids and thus offer the advantages of low sample and reagent consumption, high throughput and sensitivity, and minimal intervention. The fluidic operations can be performed on-chip either in a continuous fashion (continuous-flow microfluidic chips), or in a discrete fashion (digital microfluidic biochips). Their applications include clinical diagnostics, enzymatic analysis, e.g., glucose and lactate assays, DNA analysis, immunoasays, and environmental toxicity monitoring. The third type of biochips are those, which can be implanted inside the human body or administered orally, for drug release or for controlling/monitoring some biological functioning, in vivo. All these biochips need several types of sophisticated optical and electronic sensors as interface.

In microarray type of chips, the term “gene expression” is used to describe the transcription of the information contained within the DNA, into messenger RNA (mRNA) molecules that are then translated into the proteins that perform most of the critical functions of the cells. In our body, all genes are not “expressed” in the same cell, though almost all cells contain the same gene. Many genes represent unique features to a particular type of cell. For example, liver cells express genes for enzymes that detoxify poisons, while pancreas cells express genes for making insulin. Scientists are working in these areas to identify which genes are expressed by each type of cells.

In a microarray, mRNA molecules bind specifically to a complementary DNA, to hybridize and to form a double helix structure. By using an array containing many DNA samples, scientists can determine, in a single experiment, the expression levels of hundreds or thousands of genes within a cell by measuring the amount of mRNA bound to each site on the microarray. The amount of mRNA bound to the spots on the microarray is precisely measured by a microprocessor attached to it, generating a profile of gene expression in the cell.

In a microarray, nucleic acid sensing is done by immobilizing single stranded oligonucleotide (5 to 50 nucleotides long) probe onto transducer surface forming a recognition layer that binds its complementary (target) DNA sequence to form a hybrid for the purpose of expression profiling, monitoring expression levels for thousands of genes simultaneously or for comparative genomic hybridization. The hybridization reaction means coupling of any four different nucleotides, adenine (A), thymine (T), guanine (G), and cytosine (C) with its complementary one e.g. the complementary sequence of G-T-C-C-T-A is C-A-G-G-A-T. Fig. 4 [121] shows how a hybridization reaction takes place. This process of hybridization helps in identifying diseases, where fluorescently labeled nucleic acid molecules are used as mobile probes to identify the complementary molecular sequences that are able to base-pair with one another.

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