Morteza Hosseini

A novel sensitive method for detection of DNA methylation was developed with thioglycollic acid (TGA)-capped CdTe quantum dots (QDs) as fluorescence probes. Recognition of methylated DNA sites would be useful strategy due to the important roles of methylation in disease occurrence and developmental processes. DNA methylation occurs most often at cytosine-guanine sites (CpG dinucleotides) of gene promoters. The QDs significantly interacted with hybridized unmethylated and methylated DNA. The interaction of CpG rich methylated and unmethylated DNA hybrid with quantum dots as an optical probe has been investigated by fluorescence spectroscopy and electrophoresis assay. The fluorescence intensity of QDs was highly dependent to unmethylated and methylated DNA. Specific site of CpG islands of Adenomatous polyposis coli (APC), a well-studied tumor suppressor gene, was used as the detection target. Under optimum conditions, upon the addition of unmethylated dsDNA, the fluorescence intensity increased in linear range from 1.0 × 10(- 10) to 1.0 × 10(- 6)M with detection limit of 6.2 × 10(- 11) M and on the other hand, the intensity of QDs showed no changes with addition of methylated dsDNA. We also demonstrated that the unmethylated and methylated DNA and QDs complexes showed different mobility in electrophoresis assay. This easy and reliable method could distinguish between methylated and unmethylated DNA sequences.

A modified non-cross-linking gold-nanoparticles (Au-NPs) aggregation strategy has been developed for the label free colorimetric detection of DNAs/RNAs based on self-assembling target species in the presence of thiolated probes. Two complementary thiol-modified probes, each of which specifically binds at one half of the target introduced SH groups at both ends of dsDNA. Continuous disulfide bond formation at 3′ and 5′ terminals of targets leads to the self-assembly of dsDNAs into the sulfur-rich and flexible products with different lengths. These products have a high affinity for the surface of Au-NPs and efficiently protect the surface from salt induced aggregation. To evaluate the assay efficacy, a small part of the citrus tristeza virus (CTV) genome was targeted, leading to a detection limit of about 5 × 10 −9 mol.L −1 over a linear ranged from 20 × 10 −9 to 10 × 10 −7 mol.L −1. This approach also exhibits good reproducibility and recovery levels in the presence of plant total RNA or human plasma total circulating RNA extracts. Self-assembled targets can be then sensitively distinguished from non-assembled or mismatched targets after gel electrophoresis. The disulfide reaction method and integrating self-assembled DNAs/RNAs targets with bare AuNPs as a sensitive indicator provide us a powerful and simple visual detection tool for a wide range of applications. Colorimetric assays have attracted a great deal of attention because of the simple, speedy and a direct visual detection methods they offer without the need for any complicated equipment 1,2. Gold nanoparticles (GNPs) have been used as the sensing material for colorimetric detections due to their unique optical properties and ease of surface modifications 3,4. The optical properties of AuNPs correspond to a phenomenon known as localized surface plasmon resonance (LSPR), which originates from the resonance collective oscillation of valence electrons in the conduction band of metallic nanoparticles, upon interacting with an electromagnetic field 5. AuNPs' LSPRs usually occur between 520 and 800 nm and the spectral position of the LSPR band is strongly dependent on the particle size and shape. The bright colors of AuNPs, which is the basis for their application as markers, is due to the presence of a plasmon absorption band, which is formed when the frequency of the incident photon resonates with the collective excitation of the conductive electrons of the particle 6. The surface plasmon absorption peak for GNPs is very sensitive to the environment, shifting towards longer wavelengths (red shift) and broadening significantly upon GNP aggregation 7. This is a direct result of electric dipole–dipole interactions and couplings between the plasmons of the neighboring particles in the aggregates formed 6. The interparticle