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DNAzyme-Directed Assembly of Au Nanoparticles: A Sensitive Biosensor for Colorimetric Metal Detection
[Reported by Alfred A. Zinn]

Recent progress in DNA-directed assembly of inorganic nanoparticles has led to programmable nanomaterials with a wide range of promising applications, such as using oligonucleotide-functionalized gold nanoparticles for selective colorimetric detection of complementary DNA. However, work on colorimetric metal sensors is still scarce. Realizing the latter would minimize or even eliminate most of the costs associated with fluorescence detection, which has recently seen rapid development , and could make on-site real-time detection of metal specieseasier.

Juewen Liu and Yi Lu (University of Illinois, Urbana-Champaign) have developed a method using DNAzymes (catalytic DNA) for directed assembly of gold nanoparticles, demonstrating their use for sensitive and selective colorimetric detection and quantification of metal ions, specifically lead. They reported their results on a lead sensor based on DNAzyme-directed assembly of gold nanoparticles consisting of 5'-thio-modified 12-mer DNA attached to gold nanoparticles (DNAAu), a DNAzyme called 17E, and its substrate (SubAu), in a recent issue of the Journal of the American Chemical Society1. They chose the "8-17" DNAzyme system (with a 17E enzyme strand and 17DS substrate strand) because it shows high sensitivity and selectivity toward Pb(II), known to cause adverse health effects.

The SubAu sequence design is such that it can hybridize specifically to a DNAAu nanoparticle on each end while maintaining the 17E recognition portion. These hybridizations cause aggregation of the gold nanoparticles, resulting in a purple color. However, in the presence of lead, the 17E catalyzes hydrolytic cleavage of SubAu, preventing individual nanoparticles from aggregating and thus resulting in a color change to red. The scientists confirmed experimentally that DNAzyme cleavage of SubAu by Pb(II) causes the color change and used the ratio of extinction at 522 nm and 700 nm to quantify the lead concentration. They found that this unoptimized system allows lead detection from 100 nM to 4 mM (480 nM is considered to be the toxic level for humans). Liu and Lu were able to tune the sensor detection range over several orders of magnitude by varying the relative concentrations of the 17E DNAzyme with a slightly modified DNAzyme. This is a unique capability of the developed sensor system, allowing accurate analyte quantification suitable for desirable applications involving different concentrations of analytes. Further tests showed that the sensor is highly selective for Pb(II) in the presence of other divalent cations such as Ca, Mn, Co, Ni, Cu, Zn, or Cd. The methodology described by the authors is applicable to the design of colorimetric DNAzyme-nanoparticle sensors for any desired analyte, since the well-established in vitro selection process, which is a combinatorial biology approach, allows identification of DNAzymes that can recognize a given analyte.

Juewen Liu and Yi Lu, "A calorimetric lead biosensor using DNAzyme-directed assembly of gold nanoparticles," Journal of the American Chemical Society 125 (2003) p. 6642.


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