Device: Researchers at Tel-Aviv University have developed a device—based entirely on nanowires—that can collect and separate specific proteins for analysis from blood or urine. The strategy relies on two sets of nanowires integrated into one chip. First, antibodies attached to a “forest” of upright silicon nanowires (or nanoposts) collect and concentrate proteins from a drop of patient sample, while unwanted cells and proteins are washed away. Then, the concentrated proteins are washed from the nanowire forest to a set of nanowire sensors, also covered in antibodies targeted to the protein of interest, which convert protein binding to an electrical signal.
Within about 10 minutes, the chip can detect proteins at a high concentration—about 0.4 micromolar. The device can also be rigged with two sets of antibodies, allowing the scientists to detect two proteins in one sample. In work published last month (August 2) in Nano Letters, the device’s creators demonstrate its abilities to detect both green fluorescent protein (GFP) and hemoglobin in GFP-spiked blood.
What’s New: “The major advance here is using silicon wires as both the collection and detection piece,” said Eric Lagally, recently of the University of British Columbia, who did not participate in the research. Previous work had done these steps separately. The current study ties together work done on nanoposts and nanowires and “shows you can do some pretty elegant experiments,” said Lagally.
In addition to enabling the collection and detection of proteins of interest in one device, the nanowire chip also incorporates the key step of desalting. High salt concentrations will interfere with the nanosensor’s ability to transform protein binding into an electrical signal, explained Amy Herr, who researches microfluidic-based biosensors at the Unviersity of California, Berkeley, but was not involved in the project. The nanowire collection forest avoids this problem by desalting the sample as it concentrates the proteins, providing a “really nice compatibility between the different functions—between the upstream preparation and the downstream detection modality,” Herr said.
Importance: The use of antibodies to target specific proteins makes the strategy easily applicable to a range of possible proteins. Ideally, using such devices to measure protein levels in blood and urine would make diagnosing illness, from heart attacks (by detecting levels of the protein Troponin T) to prostate cancer (by detecting PSA), quick and easy.
“The fact that it’s an electronic sensing is also very nice,” Herr noted, because this reduces the equipment, like imaging screens, needed for reading the protein concentration of a biological sample. This simplifies the sensor’s application in a clinical setting, and, theoretically, makes it cheaper and easier to use.
Needs Improvement: Although the strategy is promising, the researchers “have a long way to go to figure out performance,” said Herr. The nanowire forest-based chip “is not yet ready to diagnose anything.” It will take more work to show that the chip can detect proteins in widely varying human samples, and consistently detect a specific disease state, Herr explained.
Lagally agreed that several hurdles need to be overcome before the chips are ready for diagnostic use. The antibodies are currently attached to the nanowires by a weak silane bond. It’s easy to get the antibodies bound using silane bonds, said Lagally, but these bonds “also have the tendency to oxidize. The bonds fall apart over time, and the antibodies come back off the nanoposts over course of day.” This can easily be remedied, however, by using different chemical steps to covalently bind the antibodies, he said.
Furthermore, the current strategy for applying a sample, such as blood, to the chip may limit its ability to detect rare proteins in small volumes, Lagally pointed out. Rather than flowing a sample over the chip, the researchers drop the sample on, which means that if a protein is rare enough, there may not be enough in one droplet to meet the chip’s limit of detection, said Lagally. Although the chip is capable of giving a readout from high concentration proteins within 10 minutes, it will take much longer for low-concentration proteins to adhere to the antibodies in amounts sufficient to be detected. Flowing larger sample volumes over the chip could solve this problem, he noted.
V. Krivitsky et al., “Si nanowires forest-based on-chip biomolecular filtering, separation and preconcentration devices: nanowires do it all,” Nano Letters, doi: 10.1021/nl3021889, 2012.