Researchers based in the Bionanophotonic Systems Laboratory in EPFL’s School of Engineering have successfully created a biosensor that creates only a steady flow of electrons – in the form of an applied electrical voltage – to illuminate and detect molecules at the same time.
This represents a breakthrough in creating a light-based biosensor without an external light source. This is due to existing difficulties with bulky, extensive equipment needed for these nanophotonic biosensors to generate and detect light, subsequently limiting their application in rapid diagnostics or point-of-care settings.
“If you think of an electron as a wave, rather than a particle, that wave has a certain low probability of ‘tunnelling’ to the other side of an extremely thin insulating barrier while emitting a photon of light. What we have done is create a nanostructure that both forms part of this insulating barrier and increases the probability that light emission will take place,” explained Mikhail Masharin, researcher, Bionanophotonic Systems Lab.
The design of the team’s nanostructure creates the right conditions for an electron passing upward through it to cross a barrier of aluminium oxide and arrive at an ultra-thin layer of gold. In the process, the electron transfers some of the energy to a collective excitation known as a plasmon, which then emits a photon.
Their design ensures the intensity and spectrum of this light changes in response to contact with biomolecules. This results in a powerful method for extremely sensitive, real-time, label-free detection.
“Tests showed that our self-illuminating biosensor can detect amino acids and polymers at picogram concentrations – that’s one-trillionth of a gram – rivaling the most advanced sensors available today,” said Hatice Altug, head, Bionanophotonic Systems Laboratory.
Dual-purpose metasurface
At the heart of the team’s solution is its dual functionality: the nanostructure’s gold layer is a metasurface, meaning it exhibits special properties that create the conditions for quantum tunnelling, and control the resulting light emission.
This control is made possible due to the metasurface’s arrangement into a mesh of gold nanowires, which act as ‘nanoantennas’ to concentrate the light at the nanometer volumes required to detect biomolecules efficiently.
“Inelastic electron tunneling is a very low-probability process, but if you have a low-probability process occurring uniformly over a very large area, you can still collect enough photons. This is where we have focused our optimisation, and it turns out to be a very promising new strategy for biosensing,” said Jihye Lee, former Bionanophotonic Systems Lab researcher and first author, now an engineer at Samsung Electronics.
As well as being compact and sensitive, the team’s quantum platform, fabricated at EPFL’s Center of MicroNanoTechnology, is scalable and compatible with sensor manufacturing methods. Less than a square millimeter of active area is required for sensing, creating an exciting possibility for handheld biosensors, in contrast to current table-top setups.
“Our work delivers a fully integrated sensor that combines light generation and detection on a single chip. With potential applications ranging from point-of-care diagnostics to detecting environmental contaminants, this technology represents a new frontier in high-performance sensing systems,” concluded Ivan Sinev, researcher, Bionanophotonic Systems Lab.
