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Gold nanoparticles can also serve as simple colorimetric test to detect mercury in water. Developed in 2007 in Northwestern University by Chad Mirkin and his team, the principle is based on the different colours of 15 nm diameter gold nanoparticles in solution- red when separated and blue when agglomerated. 6 Complementary oligonucleotides with a single nucleotide (thymidine-thymidine, T-T) mismatch are attached to the surface of the gold nanoparticles. These allow the nanoparticles to agglomerate, however they can be forced apart by heating the solution. In the presence of mercury (which binds strongly to T-T mismatches) the temperature required to break the interaction between nanoparticles is raised, and directly quantifiable with the amount of mercury present. The system is extremely sensitive (detecting mercury at levels as low as 100 nM). The team wants to extend research on other metals to provide simple, portable detection devices. Researchers of University of North Dakota have developed fluorescent dye-doped silica nanoparticles functionalized with oligonucleotides as labels for chip-based sandwich DNA assays. The nanoparticles are composed of a silica matrix that encapsulates large numbers of fluorophores and can detect DNA target down to 1fM. They have also used similar particles to detect single bacterium cells by modification of the fluorescent nanoparticles with specific monoclonal antibodies. (Tan et al., 2003). Recent work at UC Davis has seen the creation of a new type of nanoparticle, between 100 and 200 nm in size, which possesses magnetic and luminescent properties. These nanoparticles comprise a magnetic core of iron oxide doped with cobalt and neodymium (Nd:Co:Fe2O3) encapsulated in a luminescent shell of europium and gadolinium oxide (Eu:Gd2O3). When stimulated with a laser, europium emits red light. The nanoparticles can also be manipulated with magnets and detected by fluorescence or coated with short pieces of DNA and used for bio analysis (e.g. ricin, botulinum toxin) (D. Dosev et al., 2007). 6 http://www.sciencedaily.com/releases/2007/04/070427072203.htm 13
Quantum dots Quantum dots, with their broad excitation spectra, sharp emission spectra, and easily tuneable emission properties, are strong candidates for replacing conventional fluorescent markers in biodetection assays (N.L. Rosi and C.A. Mirkin, 2005). Studies have looked at functionalising quantum dots directly with biomolecules (such as oligonucleotides) or incorporating quantum dots in microbeads, that are subsequently functionalised. By preparing a panel of quantum dots or microbeads functionalised with different oligonucleotides, for example, it is possible to simultaneously detect multiple target DNA sequences (multiplexing) by virtue of the different emission spectra of the quantum dots or microbeads (Nie et al., 2001, Medintz et al., 2005). See Figure 2.14 for an illustration. Figure 2.14 Quantum dots (incorporated in beads) can be employed for detecting multiple targets in a single assay. Specifically, varying the numbers and ratios of different quantum dots per target results in a unique fluorescent signal for each individual target. Source: http://faculty.washington.edu/xgao/Images/QD-beads.pdf 14
Page 1 and 2: Tenth Nanoforum Report: Nanotechnol
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Page 5 and 6: About Nanoforum This European Union
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