Biomedical Engineering Reference
In-Depth Information
barcodes using combinations of different emitting QDs. For application, they
developed QD barcode-based assay for multiplex analysis of nine different gene
fragments from pathogens such as hepatitis B (HBV), SK102 HIV-1, and HCV.
Nine barcodes were prepared by mixing a combination of two different emitting
QDs (500 and 600 nm) with the polymer poly(styrene-co-maleic anhydride) in
chloroform. Different emitting QD barcodes were conjugated with different cap-
ture strands, and a secondary oligonucleotide was conjugated with the dye Alexa
Fluor 647. A library of QD barcodes conjugated with capture strands are mixed
with the secondary oligonucleotide-Alexa Fluor 647 (denoted as SA). When the
target sequence was introduced, a sandwich structure of QD barcode-capture
strand/target sequence/SA was formed. By measuring the optical emission of this
assembled complex in a flow cytometer, a signal response to the target sequence
was observed. It showed the great potential of QD in rapid gene mapping and
infections disease detection [
23
].
3.2 QDs for RNA Detection
Ribonucleic acid is a ubiquitous family of large biological molecules that perform
multiple vital roles in the coding, decoding, regulation, and expression of genes,
including mRNA, tRNA, rRNA, snRNAs, and other noncoding RNAs. Together
with DNA, RNA comprises the nucleic acids, which, along with proteins, consti-
tute the three major macromolecules essential for all known forms of life. Like
DNA, RNA is assembled as a chain of nucleotides. One of the major differences
between DNA and RNA is the sugar, with 2-deoxyribose being replaced by the
alternative pentose sugar ribose in RNA. The four bases found in DNA are adenine
(abbreviated A), cytosine (C), guanine (G), and thymine (T). A fifth pyrimidine
base, called uracil (U), usually takes the place of thymine in RNA and differs from
thymine by lacking a methyl group on its ring. Table
3.1
shows the other differ-
ences between DNA and RNA.
Some noncoding RNAs such as siRNA, miRNA, and mRNA have attracted more
attention because these RNAs play important roles in regulating proteins and associ-
ated with various types of human cancers [
24
-
26
]. The sensitive and selective detec-
tion of RNAs is of great importance in the early clinical diagnosis of cancers, as
well as drug discovery. Northern blotting, quantitative, real-time PCR (qRT-PCR),
and microarray-based hybridization are the widely used standard methods for ana-
lyzing RNAs [
27
-
31
]. However, these methods have some limitations such as poor
reproducibility with interference from cross-hybridization, low selectivity, insuf-
ficient sensitivity, time-consuming, or large amounts of sample required. Thus, the
innovative new tools for rapid, specific, and sensitive detection of RNAs are an
important field of research. As we know, although some differences exist between
DNA and RNA, the chemical structure of RNA is very similar to that of DNA.
Therefore, we can detect RNA according to the methods of DNAs. QDs have been
successfully conjugated with DNA and used in many applications [
32
,
33
]. The
