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ANALYTICAL CURRENTS Quantum-dot-tag with style Want to screen nucleic acids and proteins in high-throughput fashion? Tag them with some fancy colored quantum dots (QDs). Shuming Nie and co-workers at Indiana University take advantage of the optical properties of semiconductor QDs for massive parallel coding of polymeric microbeads, which can then be used for high-throughput analysis of biological molecules in gene expression studies and medical diagnostics. In this work, the tags were constructed by embedding precisely controlled ratios of different-sized QDs consisting of ZnS-capped CdSe nanocrystals into 1.2-µm microbeads. Molecular probes were arrayed on the microbead surface for studies of biological events such as
DNA–DNA hybridization and antibody– antigen/ligand–receptor interactions. Imaging and spectroscopic measurements showed that the QD-tagged beads were very uniform and reproducible, yielding bead identification accuracies as high as 99.99%. A demonstration of the approach using triple-color encoded beads for a DNA hybridization assay shows that the coding and target signals can be simultaneously read at the singlebead level. The number of codes increases exponentially with multiple wavelengths and multiple intensities. According to the researchers, a realistic scheme using five or six colors of QDs and six intensity levels would generate as many as
Fluorescence micrograph of a mixture of CdSe/ZnS QD-tagged beads emitting singlecolor signals at 484, 508, 547, 575, and 611 nm. (Adapted with permission. Copyright 2001 Nature Publishing.)
40,000 recognizable codes. (Nat. Biotechnol. 2001, 19, 631–635)
Three Ms These Ms represent microsphere sensors
by comparing the increased fluorescence
other assays, but this problem is limited
for multicomponent monosaccharide solu-
of the analyte solution with a calibration
by the recessed position of the receptor
tions—a versatile, aqueous assay to identi-
plot. Cross-talk—the diffusion of fluores-
microspheres. Deepening the cavities,
fy sugars and small ions. The sensor was
cent reporter molecules—may decrease
leaving at least one cavity vacant between
developed by Jason Shear and his col-
sensitivity or interfere with the results of
microspheres, and erecting barriers be-
leagues at the University of Texas–Austin,
tween rows and columns inhibit cross-
who derivatized agarose microspheres
talk even further.
with monosaccharide-specific dehydro-
Microspheres washed between as-
genases and then trapped them in ad-
says with a nonionic detergent enjoy an
dressable cavities on a silicon chip where
expanded lifetime of >30 trials without a
they await their “prey”.
significant loss in activity and with
