Web Date: August 10, 2012
Disease Proteins Spotted In Minutes
To identify disease-related proteins in a patient’s blood, doctors send samples to a lab, sometimes waiting days for results. Researchers want to find faster, simpler tests so doctors can diagnose patients more quickly. Israeli researchers have now made a device based on silicon nanowires that pulls out proteins from blood and identifies them in less than 10 minutes (Nano Lett., DOI: 10.1021/nl3021889).
For certain diseases, including heart disease and some cancers, researchers have found specific proteins that appear in patients’ blood at the early stages of the conditions. In the clinic, tests that look for these proteins could lead to early diagnoses. But it’s a challenge to rapidly detect tiny quantities of these signature proteins in a soup of cells, salts, and other proteins, says Fernando Patolsky, a professor of chemistry at Tel Aviv University.
Some research groups have developed microfluidic devices that can sense low levels of proteins in blood. However, the methods require researchers to prepare the sample—to remove the blood’s cells and salts and to extract its proteins—outside of the chip. These manual steps slow down analysis. Scientists want to develop an integrated device that does it all: extracts, purifies, and senses proteins from a tiny drop of blood.
Patolsky and his colleagues made such a device from silicon nanowires. The device, which is a few centimeters wide, consists of two chambers, one for separating proteins from blood and the other for sensing them. A narrow channel with a valve connects the two sections.
The separation chamber contains a forest of vertical silicon nanowires. To make the array, the researchers deposit a layer of polystyrene beads on a silicon substrate and then etch the substrate using a hydrogen fluoride solution. The beads protect the silicon below them from the acid. So when the researchers remove the beads, they leave behind silicon pillars with rough surfaces that are 5 to 10 µm long and 250 nm wide. Meanwhile, the sensing chamber contains transistors that consist of horizontal silicon nanowires bridging two electrodes. The team made these wires through a previously reported method. The researchers coat the nanowires in both chambers with antibodies that bind a protein that they want to detect.
After the researchers apply a 250 µL blood sample to the separation chamber, the antibodies on the nanowires capture any target protein in the blood. The forest of silicon pillars creates a large antibody-coated surface area to catch as many of the proteins as possible. A buffer washes through the chamber to remove cells and other blood components not attached to the nanowires. Then the researchers flush the chamber with a second buffer that releases the proteins from the nanowires. This wash carries the proteins through the connecting channel into the sensing chamber. There, the proteins bind to the nanowires, changing the wires’ conductance and producing an easily detectable electrical signal.
The researchers tested the device with human blood samples spiked with the protein troponin T. Elevated levels of this protein are an early indicator of heart attack. In less than 10 minutes, the chip could detect the protein at sub-picomolar concentrations; the protein’s level in blood ranges from 0.1 pM in a healthy person to micromolar levels during a heart attack. By using nanowires coated with antibodies for two proteins, the researchers could also detect two proteins with one chip.
Peter Ertl, a biosensor technology researcher at the Austrian Institute of Technology, says the device’s key novelty is its elimination of tedious sample preparation. Another strength is its ability to look at multiple disease proteins at once, he says. The device’s inherent speed, ease of operation, and ability to detect low amounts of biomarkers in complex backgrounds make it attractive for point-of-care diagnostics, he adds.
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