UV light makes better biosensors

Ashley N. Paddock, ashley.paddock@photonics.com

Ultraviolet light can improve an established antibody-based molecule-detection process by increasing the number of antibodies available to do the sensing.

Quartz crystal microbalance (QCM) sensors detect pathogens in blood samples and help in protein synthesis studies. The sensors consist of antibodies anchored to gold electrodes on a piece of quartz crystal; the antibodies function like the hooks on a piece of Velcro, snagging molecules as they pass.

The more antibody “hooks” there are on the sensor’s surface, the more sensitive the QCM device becomes. But the converse also is true: If the antibodies adhere to the electrodes hook-side-down, they become useless for sensing and even put a damper on the QCM’s sensitivity as a whole.

To combat this problem, researchers from the University of Naples Federico II and the Second University of Naples irradiated antibodies with ultrashort pulses of ultraviolet light. They reported their findings in the open-access journal Biomedical Optics Express http://dx.doi.org/10.1364/BOE.2.003223.

“In a meeting with researchers at a small company involved in the realization of biosensors based on QCM, it was proposed to break the disulfide bridges by means of ultrashort UV pulses from a femtosecond laser system and to try to immobilize antibodies on gold surfaces with no previous chemical treatment,” said Raffaele Velotta, a professor at the University of Naples Federico II. “After a trial-and-error approach, we found the optimal conditions for the average power of the UV light and the irradiation time of the antibody solution.”

As tryptophan absorbs a UV photon, the disulfide bridges holding the antibody together are broken, creating a thiol group, which is exposed at the tail end of the antibody. The gold electrode attracts the thiol group, so that the upside-down position is hampered, and the antigen binding is more effective. Courtesy of Biomedical Optics Express.

The UV light is absorbed by the amino acid tryptophan, which breaks the disulfide bridges holding parts of the antibody together. This causes a particular part of the amino acid cysteine, called a thiol group, to become exposed at the tail end of the antibody. Thiol groups are more strongly attracted to gold electrodes than other parts of the antibody, so the bottom sides of the irradiated antibodies become much more likely to adhere to the gold electrodes than the hook ends.

Using this approach, the researchers more than doubled the sensitivity of a QCM sensor, creating new avenues for research using this type of device.