USC researchers say they've made a big improvement in a new breed of electronic detectors for viruses and other biological materials — one that may be a valuable addition to the battle against epidemics.
It consists of a piece of synthetic antibody attached to a nanowire that's attached to an electrical base, sitting in liquid.
If the protein the antibody binds to is present in the liquid, it will bind to these antibodies, immediately creating a sharply measurable jump in current through the nanowire. (see diagram).
The basic principle of nanotube and nanowire biosensors for protein detection was first demonstrated in 2001, but the new design by a team headed by Chongwu Zhou of the USC Viterbi School of Engineering and chemist Mark Thompson of the USC College uses two new elements.
First, it takes advantage of bioengineered synthetic antibodies, much, much smaller versions of the natural substances that are designed to bind with a specific protein and only that protein.
Second, it uses indium oxide (In2O3) nanowires instead of silicon and other materials previously tried. Metal oxides, according to the article just published in ACS Nano do not, unlike silicon, develop "an insulating native oxide layer that can reduce sensitivity."
The result, according to the paper, "Label-Free, Electrical Detection of the SARS Virus N-Protein with Nanowire Biosensors Utilizing Antibody Mimics as Capture Probes," is a device that can detect its target molecules with a sensitivity as great as the best alternative modes, do so more rapidly and without use of chemical reagents.
It is also potentially considerably cheaper than alternatives.
"We believe," the authors write, "that nanowire bisensor devices functionalized with engineered proteins … can have important applications ranging from disease diagnosis to homeland security."
Additionally, the system can be useful in basis research, in helping to establish certain important parameters for two-part biological systems like the antibody/target protein pair.
The protein the prototype system detects is the SARS (severe acute respiratory syndrome) virus n-protein, which infected more than 8,000 people in 2002-2003, killing nearly 10 percent of them. Commercial systems using enzyme-linked immunosorbent assay (ELISA) now exist to test for SARS, but the new system has advantages in time, cost and portability.
The first step was the creation, by Richard Roberts and Mark Thompson, chemists, and their team of the synthetic antibody, including both the active area, design to interact with the protein and, at the other end, a chemical "hook" that would bind it to nanowire at this point and only this point.
"This … strategy allows every bound [detector molecule] to retain full activity, a clear advantage over antibodies, which [in earlier biosensor designs] are often bound to nanowire surface via amine containing residues randomly distributed over the antibody surface," the authors say in the paper.
The Zhou lab, which has specialized in nanowire and nanotube technology for years, performed the complex set of procedures to synthesize the wires, attaching
No reaction to blank detectors: The system without the antibody mimic protein (red in the diagram above) on the nanowires does not respond.
In tests, the group performed if anything better than predictions, showing a standard and low level of activity when no SARS protein was present, leaping quickly to a higher level when the protein was introduced, in response patterns that varied consistently according to concentration of the SARS protein. Devices complete except for the detector molecule showed no response at all. (see chart)
The response was complete in less than ten minutes, compared to hours needed for results from ELISA tests - which are basically present/not present tests with relatively little quantitative elements.
Next steps are to enable detection in more complex environments, such as Serum and whole blood, by integrating the nanobiosensor with micro systems such as microfluidics chips and micro filters.
Here are descriptions of other existing nanobiosensor systems:
The USC team believes their new system has potential to be cheaper and more portable than either.
In addition to Zhou (from the Viterbi School's Ming Hsieh Department of Electrical Engineering), Thompson (of the USC College Department of Chemistry) and Roberts (who has a joint appointment in both engineering and the college), the team also included Fumiaki Ishikawa, Hsaio-Kang Chang, Po-Ching Chen from Electrical Engineering; Marco Curreli, Rui Zhang, and C. Anders Olson from Chemistry, Richard J. Cote of the Keck School of Medicine at USC Department of Pathology, and Hsiang-I Liao and Ren Sun of the UCLA Department of Medical Pharmacology.
The Whittier Foundation and the National Institutes of Health funded the research.