Highly sensitive plasmonic metasensors based on atomically thin perovskite
(Nanowerk Spotlight) The convergence of nanotechnology, biology, and photonics opens the possibility of detecting and manipulating atoms and molecules that promises to revolutionize diagnostics and therapy at the cellular and even molecular level.
As a result, a large variety of biosensors, which incorporate biological probes coupled to a transducer, have been developed during the last two decades for environmental, industrial, and biomedical diagnostics.
"Biosensors are usually based on systems that can detect electronic or optical signals in terms of the concentrations of biological molecules, where molecular interactions can be monitoring by the signal change," Dr. Shuwen Zeng from the XLIM Research Institute at Université de Limoges, tells Nanowerk. "Useful applications include DNA analysis, glucose concentration test in human blood, and sensing of toxins in the water, food, and atmosphere."
Current challenges for biosensors, especially for handheld devices that can be deployed at the point of care, are to improve their detection sensitivity, and reduce their size and operating cost.
"The rapid development of the fabrication techniques for different types of nanomaterials has allowed many breakthroughs on the optical and electronics properties, e.g., high charge carrier mobility, negative refraction, hyperbolic dispersion," Zeng explains. "Typical examples are the discovery of monolayer graphene and metamaterial/metasurfaces, which showed to us how the properties and performances for optoelectronic sensing and imaging devices can be improved by engineering the materials at the nanoscale and even atomic scale."
More recently, perovskite nanosheets have been explored as novel optical materials with high optical absorption efficiency. They are promising materials for the plasmonic field as the sensing substrate to realize the zero-reflection for the phase singularity.
"More importantly, we have improved the phase singularity detection with the Goos–Hänchen (GH) effect," Zeng points out. "The GH shift is known to be closely related to the optical phase signal changes. And it is much more sensitive and more sharp than the phase signal at the plasmonic condition while the experimental measurement setup is much more compact than that of the common used interferometer scheme to exact the phase signals."
Schematic figure of a 2D perovskite-based Goos–Hänchen enhanced surface plasmon resonance biosensor, which is integrated with metasurface patterns. (Reprinted with permission by MDPI under CC BY license)
Plasmonic sensors or surface plasmon resonance (SPR) sensors are some of the most commonly-used optical sensing devices for real-time monitoring of chemical and biomolecular interactions. The resonance occurs at the interface between a metal and a dielectric, and is a result of collective electron charge oscillations in the metal coupled to an interfacial electromagnetic wave.
SPR is very sensitive to the surrounding environment, a property that is utilized in real-time and label-free detections. SPR sensors have been commercialized for more two decades, and they represent the current 'gold standard' for label-free biosensing. These sensors have been applied in various areas including food quality control, environmental monitoring, drug screening, and early-stage disease diagnosis.
However, as Zeng notes, SPR-based sensing is not sufficiently sensitive for the most demanding tasks: 1) Sensing small target analytes with a molecular weight less than 400 Dalton especially for cancer biomarkers, antibiotics, thyroid hormones, peptides, steroids, and bacterial pathogens in infectious diseases; and 2) detecting biological and chemical molecules with low concentration levels (i.e., < 10-15 mol/L) in complex matrices such as urine, saliva, and blood serum.
The use of optical phase singularity could be a powerful solution to address these two challenges.
Plasmonic resonances typically take the form of broadened Lorentz curves due to optical losses of metallic substrates or nanoparticles.
"However, plasmonic detection based on phase singularity is not dependent upon angular scanning and not affected by the broad resonance curves," Zeng explains. "The key factor that influences the phase-related plasmonic sensing is the minimum reflectivity at the resonance angle, which corresponds to a complete optical energy transfer from the incident light to SPR at the sensing interface."
As the team demonstrates in their work, the challenge of reaching an ultra-high plasmonic sensitivity for detecting small-molecule, low-concentration analytes can be overcome by engineering the sensing substrate to realize the zero-reflection condition.
For that purpose, they used an atomically thin perovskite nanomaterial with high absorption rate, sandwiched between hexagonal boron nitride and graphene layers, which enables the precise tuning of the depth of the plasmonic resonance dip.
Graphene/2D perovskite/hBN engineered on metasurface patterns. (Reprinted with permission by MDPI under CC BY license)
"As such, one can optimize the structure to reach near zero-reflection at the resonance angle and the associated sharp phase singularity, which leads to a strongly enhanced GH lateral shift at the sensor interface," Zeng says. "By integrating a two-dimensional perovskite nanolayer into a metasurface structure, we realized a strong localized electric field enhancement and further improved the the GH sensitivity to 1.5458 ×109 µm/RIU."
The researchers believe that this enhanced electric field together with the significantly improved GH shift will enable single molecular or even submolecular detection for hard-to-identify chemical and biological markers, including single nucleotide mismatch in the DNA sequence, toxic heavy metal ions, and tumor necrosis factor-α (TNFα).
In the next stage, the team will focus on the development of plasmonic sensors that can achieve multiplexed detection for different cancer biomarkers on a single chip. Also, they plan to study the nonlinear optical response with the 2D hybrid plasmonic metasurfaces to obtain a more significant sensing signal.
"The research topic of 2D hybrid metasurfaces is still in its infant stage," Zeng concludes. "One challenge is that a higher absorption rate of 2D nanomaterials is required for the visible and near-infrared regions and we will search for new 2D materials to form the van der Waals hybrid heterostructures to overcome this limit."