An Ultrasensitive, Carbon-Based Probe For Biomolecules

In the drive to detect cancer at its earliest stages, researchers are searching for probes that can detect tumor biomarkers at vanishingly low concentrations. Photoelectrochemical (PEC) probes, made from materials that generate current when hit with light, are sensitive and simple to use but often damage biomolecules. Now, researchers in China have demonstrated a water-soluble PEC probe based on fullerenes—part of a two-component system—for detecting cancer biomarkers at zeptomole concentrations (Anal. Chem. 2013, DOI: 10.1021/ac4028005).

So far researchers have constructed most PEC sensors from semiconductors and metal oxides, materials that generate a large current when excited with ultraviolet light. However, these materials aren’t useful for building solution biosensors because they corrode easily and oxidize biomolecules readily.

Chengguo Hu of Wuhan University, in China, and his colleagues decided to construct the probe, the first part of the PEC sensing system, out of fullerenes. Fullerenes are a better choice for a biological sensing system because they are less reactive than metals. They also respond to visible light instead of UV and have good conductivity, Hu says. To make the probe, the researchers simply ground together a mixture of C₆₀, carbon nanotubes, and Congo red dye in a mortar to create a nanohybrid material. The Congo red allowed this nanohybrid to disperse in solution, and the fullerenes generate electrons when excited with green laser light. The researchers then attached an antibody for carcinoembryonic antigen (CEA), a biomarker for human colorectal cancer, to the nanotubes.

To complete the two-part PEC sensing system, the researchers also created a sensing electrode that carried a different antibody for CEA. The electrode consisted of indium tin oxide modified with a layer of carbon nanotubes and a second antibody on the surface. In solution, the biomarker can sandwich itself between the antibodies on both the electrode and the nanohybrid probe. When researchers shine green light from an inexpensive laser pen on this complex, the fullerenes reach an excited state, which causes a current to flow to the electrode. Researchers can then measure the current, allowing them to calculate the concentration of the CEA biomarker.

The group tested their system on serum samples from healthy people and cancer patients and found that the sensitivity (0.1 pg/mL) was comparable to the best existing PEC biosensors (Anal. Chem. 2012, DOI: 10.1021/ac302853y). Hu expects that they could boost the sensitivity up to tenfold if they could lengthen the nanotubes from 100 nm up to several micrometers. He and his colleagues are also testing different types of fullerenes and fullerene complexes to improve nanohybrid performance.

The most interesting aspect of the work is that the nanohybrid allows detection over a wide range of sample concentrations, says Dario Bassani, a photochemist at the University of Bordeaux, in France. He is also intrigued by the fact that simply crushing nanotubes and fullerenes together can functionalize the material. “That could be interesting for further research—to find out how that works,” he says.