A new plasmonic sensor developed by researchers at the University of Illinois at Urbana–Champaign will serve as a reliable early detection of biomarkers for many forms of cancer and eventually other diseases.
The sensor has been proven reliable to detect the presence of the cancer biomarker carcinoembryonic antigen (CEA) to the magnitude of 1 nanogram per milliliter. Most humans carry at least some amounts of CEA with an average range of 3–5 nanograms per milliliter. The researchers chose to focus on CEA because its presence in higher concentrations is an early indicator of many forms of cancer, including lung and prostate cancers.

The research team was led by Logan Liu, and Lynford Goddard, associate professors of electrical and computer engineering with students Abid Ameen and Lisa Hackett carrying out the project. The team published its results in the journal Advanced Optical Materials.

The plasmonic sensor is an improvement of the current state–of–the–art method for a few reasons. First, it was able to improve the limit of detection by at least two orders of magnitude. In fact, most methods aren’t able to accurately detect the presence of CEA until it reaches a higher concentration.

Secondly, because it works with much less instrumentation, it is less expensive and more portable and doesn’t require nearly the expertise to make a reading. It also means instead of needing a vile of blood for a test, a simple finger prick will do. This aspect will be especially important for those who don’t live close to an advanced medical facility, including those in developing nations. The device combines two sensing methods, which hadn’t until this time been able to be used together. First, it uses a 3D multi–layer nanocavity in a nanocup array, which allows for the light to be stored in the cavity comprised of two metal layers (in this case gold) surrounding one insulator layer. Secondly, it uses plasmonic sensing, which detects sensitive nanoscale light–matter interactions with biomolecules on the device surface. It produces an enhanced field confinement and an enhanced localized field. Because of the plasmonic structure, the light is out–coupled more efficiently as the surrounding refractive index changes.

“By combining plasmonic properties and the optical cavity properties together in one device we are able to detect lower concentration of biomarker by light confinement and transmission in the cavity layer and from the top of the device respectively, based on the thickness of the multilayers and the refractive index of the cavity layer,” Ameen explained.

“The nanocup array provides extraordinary optical transmission,” Hackett added. “If you take a thin metal film and try to shine light through it, there will be almost no light transmitted. However if you put a periodic array of nanoholes, or in our case a nanocup structure, then what you see is a resonance condition where at a certain wavelength, you will have a peak in the transmission through this device.”

Because the resonance is changing at a single wavelength and because the spectral features have reference locations, excitation and detection can be done reliably without any specialized equipment. With this device, a LED light source can be used instead of a laser and a photocell or camera image can be used instead of a high–end spectrometer. While this study demonstrated detection in a small human serum sample, the method could be used for the detection of other diseases down the road.

“In the future, if they are made very cost–effective and portable,” Hackett said, “it would be great to see people be able to take more control over their health and monitor something like this on their own.”