Nanophotonic Sensors Monitor Processes in Living Cells

A large-scale biosensor developed using nanoplasmonic material is able to accommodate a large number of cells on a single substrate and monitor cell proliferation in real time. The nanoplasmonic material allows for long-term cell survival and unaltered cellular responses.

Nanomushroom biosensors can be used to count dividing cells and detect biomolecules. Courtesy of OIST.

“Usually, when you put live cells on a nanomaterial that material is toxic and it kills the cells. However, using our material, cells survived for over seven days,” said researcher Nikhil Bhalla from the Okinawa Institute of Science and Technology (OIST).

The nanoplasmonic sensor resembles a piece of glass; however, its surface is coated with tiny mushroom-like structures, called nanomushrooms. The nanomushrooms have “stems” of silicon dioxide and “caps” of gold.

Researchers from OIST, who developed the sensor, said that the properties of the nanomushroom form a biosensor capable of detecting interactions at the molecular level. Biological molecules increase the sensitivity of the nanoplasmonic material, allowing it to sense extremely low concentrations of substances. For example, it can detect an increase of 16 cells in a group of 1000.

Schematic illustration of cells (blue mountain-like shapes) on top of nanoscale mushroom-like structures with silicone dioxide stems and gold caps, which have the potential to detect cell proliferation in real time. Courtesy of OIST.

“Using our method, it is possible to create a highly sensitive biosensor that can detect even single molecules,” said Bhalla.

To achieve uniformity across the entire material surface, researchers developed a novel printing technique for creating the nanomushrooms.

“Our technique is like taking a stamp, covering it with ink made from biological molecules, and printing onto the nanoplasmonic slide,” said researcher Shivani Sathish.

Using this method, researchers were able to develop a material consisting of approximately one million mushroom-like structures on a 2.5- × 7.5-cm silicon dioxide substrate.

The sensor uses the localized surface plasmons on the nanomushroom caps as optical antennae to monitor proliferating fibroblast cells. Changes in nanoplasmonic resonances of the nanomushrooms are directly proportional to the number of cells that bind to them.

Using their innovative printing technique, the Micro/Bio/Nanofluidics Unit at Okinawa Institute of Science and Technology (OIST) has developed a nanoplasmonic material containing millions of mushroom-like structures covered with a uniform layer of biomolecules. Courtesy of OIST.

When white light passes through the nanoplasmonic slide, the nanomushrooms absorb and scatter it, changing its properties. The absorbance and scattering of light is determined by the size, shape and type of nanomaterial. It is also affected by any medium in close proximity to the nanomushroom, such as cells that have been placed on the slide. By measuring how the light has changed once it emerges through the other side of the slide, researchers can detect and monitor processes occurring on the sensor surface, such as cell division.

The nanomushroom substrates preserve cell viability and could serve as a label-free platform for long-term monitoring of cell proliferation.

“Normally, you have to add labels, such as dyes or molecules, to cells, to be able to count dividing cells,” said Bhalla. “However, with our method, the nanomushrooms can sense them directly.”

Plasmonic and nanoplasmonic sensors could be used in a number of fields, from electronics to food production to medicine. The researchers believe that, in the future, nanoplasmonic materials could even be integrated with emerging technologies, such as wireless systems in microfluidic devices, allowing users to take readings remotely and thereby minimizing the risk of contamination. These results also open new opportunities in developing standard cell assays without chemical labels to detect cellular responses at the nanoscale.

The research was published in Advanced Biosystems (doi:10.1002/adbi.201700258).