Photonic Crystal Microscope Quickly Detects Biomarkers for P-O-C Diagnostics

A compact photonic resonator absorption microscopy (PRAM) instrument, developed by a team at the University of Illinois Urbana-Champaign, provides fast, selective detection of cancer biomarkers for diagnostics at the point of care. The instrument takes advantage of the optical coupling between plasmonic gold nanoparticle tags and a photonic crystal surface to provide the contrast users need to observe and count gold nanoparticles that are linked to proteins or other biomarkers.

By matching the resonant wavelength of the photonic crystal to the wavelength of the gold nanoparticle’s surface plasmon, the microscope generates strong, localized quenching of the photonic crystal’s resonant reflection intensity. This allows users to clearly detect and count individual gold nanoparticles that are captured on the photonic crystal surface.

The researchers were able to see the nanoparticles captured on the surface by illuminating the photonic crystal with polarized light from a conventional red LED. they then recorded the reflected intensity of the crystal on a CMOS image sensor. When the red LED light was reflected off the photonic crystal, the image was captured by a webcam.

Conventional image processing algorithms discriminate darker regions of the reflected image from the brighter surrounding background to separate “in particle” from “not a particle” and count the number of captured nanoparticles, which show up as dark regions.

“The photonic crystals act like a mirror, but only for the color red. The gold nanoparticles are nonreflective and show up as dark spots,” professor Brian Cunningham said. Each gold nanoparticle tag provides a sufficiently high signal-to-noise ratio to enable digital resolution counting of tagged biomolecular targets using a compact, inexpensive instrument.

Artistic representation of gold nanoparticles on a photonic crystal surface. The optical coupling between the nanoparticle tags and the surface enable a photonic resonator absorption microscope, developed by University of Illinois Urbana-Champaign researchers, to provide contrast that is sufficient for the observation and counting of gold nanoparticles that are linked to proteins or other biomarkers. Courtesy of Janet Kirker.

The compact microscope captures images from a large field of view in a single image acquisition, rather than through a line-scanning process. By eliminating the need for some components of a line-scanning PRAM instrument, it achieves a small form factor suitable for desktop environments, with a cost for components that makes it feasible for clinics and diagnostic laboratories.

“Although our original photonic crystal microscope is very versatile, it is the size of a pingpong table,” Cunningham said. “We wanted to build a portable instrument that had the same detection capabilities. The new one we built can easily fit on a desk and costs around $7000, compared to the nonportable microscope, which costs $200,000.”

A single image acquisition requires only 6.1 msec of integration time to measure the reflected intensity of a 339 × 212 µm² area. This allowed researchers to acquire multiple images of the same region at different time points, to measure the kinetic properties of detection processes.

Because the PRAM instrument does not use fluorescent labels, it is not subject to photobleaching, which enables long-term kinetic data to be gathered without loss of signal.

The main components of the compact microscope are a low-intensity LED, a linear polarizing filter, a microscope objective, and a webcam-quality silicon CMOS image sensor.

The system itself can be put together in a day, researcher Nantao Li said.

Using the microscope, the researchers successfully detected specific microRNAs associated with prostate cancer. Each gold nanoparticle was attached to a piece of DNA that served as a probe; the probe DNA was then attached to piece of protector DNA. If the target microRNA sequence was present in a sample, the protector DNA was displaced, causing the gold nanoparticle to bind to the photonic crystal.

Since almost every cancer has micoRNAs associated with it, the microscope can, in theory, be used to detect different cancer types. Cunningham said the research team is collaborating with medical centers and hospitals to diagnose lung cancer, to measure the effects of chemotherapy in prostate cancer, and to undertake other initiatives. The researchers are also working to lower the cost of the microscope.

“We are trying to use smartphone cameras to capture the images. Our aim is to reduce the cost to less than $100,” Li said.

The team plans to build hand-held versions of the instrument, which would consist of a box with an image sensor that could communicate wirelessly with a smartphone.

The research was published in Biomedical Optics Express (www.doi.org/10.1364/BOE.427475).