• Nanostructure-Based High Sensitivity Bioassays

An ideal assay is one that is cheap, can be performed quickly and by anyone, can be carried out at remote lo-cations and point-of-care settings, is accurate and general for any target biomolecule, can be used to detect multiple targets in a single test, and, finally, is able to detect the target molecule at the lowest levels when disease activity is at a minimum and the correct intervention would be most beneficial.

In our lab, nanostructure-based bioassays that address these points are being developed. One of the most promising is the bio-bar-code amplification assay. Briefly, the bio-bar-code as-say utilizes two different particles that are each designed to bind a target molecule (Figure 3).

One of the particles is magnetic, and the other is a target capture particle. Both particles have molecules on their surface that bind specifically to the target molecule. A target capture particle (barcode probe) carries with it hundreds to thousands of barcode DNA molecules which serve as surrogates for the target. The bio-barcode concept allows us to design a bar code for any number of target molecules. The number of barcodes is almost limitless (for a 30 base DNA sequence, there are 4^30 or ~1.15 x 10^18 choices!).

In a typical assay, we mix the particles and target (Figure 3). The magnetic and bar-code probes bind to the target molecule to form a complex. If the target is not present, there is no complex formation. The complex is separated using a magnet that pulls the complex (due to the presence of the magnetic particle) to the side of the container. Finally, the bar-codes are released from the probes via addition of water. The barcode DNA is responsible for the final signal output. Due to the large num-ber of bar-code DNA strands per particle, there is a significant increase in the effective signal. As there are numerous highly efficient ways to detect the bar-code DNA, one can accurately and sensitively detect its presence using a variety of techniques.

Thus, one can accurately detect the presence of a surrogate protein. The target molecules, in addition to proteins, can be DNA, RNA, and metal ions. This technology is useful to drug development and disease diagnostics because this method allows for looking for biomarkers for the use of surrogates in early and later-stage clinical trials. This signal amplification strategy could potentially replace the enzyme-linked immunosorbent assay (ELISA), but it should be focused on assays requiring PCR-like ultrasensitivity.