Nanoparticle-Enhanced Plasmonic Biosensor for Digital Biomarker Detection in a Microarray

Nanoplasmonic devices have become a paradigm for biomolecular detection enabled by enhanced light–matter interactions in the fields from biological and pharmaceutical research to medical diagnostics and global health. In this work, we present a bright-field imaging plasmonic biosensor that allows visualization of single subwavelength gold nanoparticles (NPs) on large-area gold nanohole arrays (Au-NHAs). The sensor generates image heatmaps that reveal the locations of single NPs as high-contrast spikes, enabling the detection of individual NP-labeled molecules. We implemented the proposed method in a sandwich immunoassay for the detection of biotinylated bovine serum albumin (bBSA) and human C-reactive protein (CRP), a clinical biomarker of acute inflammatory diseases. Our method can detect 10 pg/mL of bBSA and 27 pg/mL CRP in 2 h, which is at least 4 orders of magnitude lower than the clinically relevant concentrations. Our sensitive and rapid detection approach paired with the robust large-area plasmonic sensor chips, which are fabricated using scalable and low-cost manufacturing, provides a powerful platform for multiplexed biomarker detection in various settings.

Phase-sensitive plasmonic biosensor using a portable and large field-of-view interferometric microarray imager

Hatice Altug, Alexander Belushkin, Yasaman Jahani, Filiz Yesilköy

Nanophotonics, and more specifically plasmonics, provides a rich toolbox for biomolecular sensing, since the engineered metasurfaces can enhance light–matter interactions to unprecedented levels. So far, biosensing associated with high-quality factor plasmonic resonances has almost exclusively relied on detection of spectral shifts and their associated intensity changes. However, the phase response of the plasmonic resonances have rarely been exploited, mainly because this requires a more sophisticated optical arrangement. Here we present a new phase-sensitive platform for high-throughput and label-free biosensing enhanced by plasmonics. It employs specifically designed Au nanohole arrays and a large field-of-view interferometric lens-free imaging reader operating in a collinear optical path configuration. This unique combination allows the detection of atomically thin (angstrom-level) topographical features over large areas, enabling simultaneous reading of thousands of microarray elements. As the plasmonic chips are fabricated using scalable techniques and the imaging reader is built with low-cost off-the-shelf consumer electronic and optical components, the proposed platform is ideal for point-of-care ultrasensitive biomarker detection from small sample volumes. Our research opens new horizons for on-site disease diagnostics and remote health monitoring.

2018

Mid-Infrared plasmonic nanoantennas for ultrasensitive detection of proteins and their secondary structure

Dordaneh Etezadi

Engineered plasmonic nanostructures have emerged as powerful sensing platforms and are promoting novel applications in various fields. By providing extreme field confinement down to molecular dimensions and enhancing light-matter interaction at the nanoscale, nanoplasmonics demonstrate outstanding potential for biomolecular analysis. In addition to their extensive applications at visible and near-infrared range, such nanoplasmonic structures deliver unique possibilities for biomolecular studies with surface enhanced infrared absorption spectroscopy based on the distinct molecular vibrations in mid-infrared (mid-IR) spectral range. The mid-IR chemical fingerprints of proteins incorporate comprehensive insight into their secondary structures and thus can provide conformational information. The main goal of this thesis is to extend the applications of mid-IR nanoplasmonics in ultrasensitive detection and analysis of proteins towards enabling their in vitro characterization with innovative approaches. First, we present suspended mid-IR polarization-insensitive nanoantennas on nanopedestals for ultrasensitive vibrational spectroscopy. This system provides fully accessible near-field intensity enhancements that are realized with a new isotropic etching nanofabrication process. Our experimental results demonstrate improved sensitivity compared to the conventional antennas on substrates based on the optimized field overlap with analytes as predicted in numerical simulations. Such a mid-IR plasmonic sensor can be effectively employed in chip-based ultrasensitive detection. Next, we demonstrate for the first time the secondary structure identification of nanometer thin layers of proteins as small as 14 kDa using mid-IR plasmonic nanorods on IR transparent substrates for implementation of challenging in-solution measurements. We sensitively resolved the spectral content of plasmonically enhanced amide I fingerprint. Through its second derivative analysis, we successfully extracted major protein secondary conformations, random coil and cross Î²-sheet, related to aggregation of Î±-synuclein protein involved in Parkinsonâs disease. Using an additional model protein with a native Î²-sheet structure, we demonstrate the sensitivity of our approach in extracting minute conformational differences. The tolerance of this new method for extracting the secondary structure signatures of thin protein layers to the resonance matching to amide I range due to any minor nanofabrication variations is an important finding for demonstrating its robustness and reproducibility. Finally, we present our biosensor integrated with a fluidic device for dynamic in situ secondary structure analysis of a protein monolayer upon external stimuli. We monitored reversible conformational changes from random coil to Î²-sheet in immobilized Î±-synuclein in real-time. Additionally, we establish the correlation between the obtained vibrational components using nanoplasmonics and that of IR transmission with polymer thin films and therefore demonstrate the reliability of our approach for mid-IR protein conformational investigations. Our platform can facilitate highly sensitive protein analysis at various conditions or in interactions with biomolecules such as lipids. These results are of great interest for multidisciplinary engineering and applied aspects of photonics and can open up a broad application area for mid-IR nanoplasmonics and devices.

EPFL2018

Quantifying the Limits of Detection of Surface-Enhanced Infrared Spectroscopy with Grating Order-Coupled Nanogap Antennas

Hatice Altug, Aurélian Michel John-Herpin, Andreas Tittl

Infrared spectroscopy is widely used for biomolecular studies, but struggles when investigating minute quantities of analytes due to the mismatch between vibrational cross sections and IR wavelengths. It is therefore beneficial to enhance absorption signals by confining the infrared light to deeply subwavelength volumes comparable in size to the biomolecules of interest. This can be achieved with surface-enhanced infrared absorption spectroscopy, for which plasmonic nanorod antennas represent the predominant implementation. However, unifying design guidelines for such systems are still lacking. Here, we introduce an experimentally verified framework for designing antenna-based molecular IR spectroscopy sensors. Specifically, we find that in order to maximize the sensing performance, it is essential to combine the signal enhancement originating from nanoscale gaps between the antenna elements with the enhancement obtained from coupling to the grating order modes of the unit cell. Using an optimized grating order-coupled nanogap design, our experiments and numerical simulations show a hotspot limit of detection of two proteins per nanogap. Furthermore, we introduce and analyze additional limit of detection parameters, specifically for deposited surface mass, in-solution concentration, and secondary structure determination. These limits of detection provide valuable reference points for performance metrics of surface-enhanced infrared absorption spectroscopy in practical applications, such as the characterization of biological samples in aqueous solution.

2018

Mid-Infrared plasmonic nanoantennas for ultrasensitive detection of proteins and their secondary structure

Dordaneh Etezadi

EPFL2018