Artist's rendering of photonic PCR on a chip using light to rapidly heat and cool electrons at the surface of a thin film of gold. [Image courtesy of Luke Lee's BioPOETS lab]
Artist’s rendering of photonic PCR on a chip using light to rapidly heat and cool electrons at the surface of a thin film of gold. [Image courtesy of Luke Lee’s BioPOETS lab]

The amplification of minute amounts of genetic material is the cornerstone of every molecular biology laboratory and DNA sequencing facility. However, with respect to most other molecular techniques used, PCR is sluggish and represents a limiting factor for high-throughput analysis systems. For many years, scientists have been chasing the dream of developing ultrafast multiplex PCR systems that would be characterized by low power consumption, compact size, and simple operation—parameters that are critical for point-of-care diagnostics.

While various groups have proposed ultrafast PCR methods in recent years, none were able to address the challenges of creating a robust thermocycler unit that could handle heating and cooling speeds that were significantly greater than current systems. Now, bioengineers at the University of California, Berkeley have developed new technology they believe will dramatically increase heating and cooling speeds with the switch of a light. 

“PCR is powerful, and it is widely used in many fields, but existing PCR systems are relatively slow,” explained senior author Luke Lee Ph.D., professor of bioengineering at UC Berkeley. “It is usually done in a lab because the conventional heater used for this test requires a lot of power and is expensive. Because it takes an hour or longer to complete each test, it is not practical for use for point-of-care diagnostics. Our system can generate results within minutes.”

The findings from this study were released today in Light: Science & Application through an article entitled “Ultrafast photonic PCR.”

To increase the speed of the thermal cycling steps, Dr. Lee and his team took advantage of plasmonics, or the interaction between light and free electrons on a metal's surface. When exposed to light, the free electrons get excited and begin to oscillate, generating heat. Once the light is off, the oscillations and the heating stop.

As it turns out, gold is a popular metal for plasmonic photothermal heating since it is extremely efficient at absorbing light. In their study, the investigators used thin films of gold, about 120 nanometers (nm) thick that were deposited onto a plastic chip containing microfluidic wells in order to hold the PCR mixture and DNA sample.

Additionally, the Berkeley scientists used LED lights with a peak wavelength around 450 nm for most efficient light-to-heat conversion. The light was able to heat electrons at the interface of the gold films and DNA solution ramping temperatures up staggeringly fast at approximately 13°C per second. Cooling of the chip was equally impressive, dropping at rates around 6.6°C per second. In total, the scientists were able to cycle from 55°C to 95°C 30 times in less than 5 minutes—a cycle rate that would take current PCR machines about an hour to accomplish.   

In order to prove that their newly developed system has real-world applications, the researchers used the photonic PCR system to amplify a sample of DNA and found that the results compared well with conventional PCR systems.

“This photonic PCR system is fast, sensitive, and low-cost,” said Dr. Lee. “It can be integrated into an ultrafast genomic diagnostic chip, which we are developing for practical use in the field. Because this technology yields point-of-care results, we can use this in a wide range of settings, from rural Africa to a hospital ER.”