Application of disposable chiral plasmonics for biosensing and Raman spectroscopy

This thesis explores the capabilities of disposable chiral plasmonic metafilm assays, termed Disposable Plasmonic Assays, as a promising platform for biosensing and surface-enhanced Raman spectroscopy. The sensing and Raman properties of these metafilms arise from the excitation of surface plasmons when exposed to incident light. These plasmonic properties strongly depend on the geometric characteristics of the constituent nanostructures found in the metafilms. Specifically, the primary nanostructure employed throughout this research is the chiral 'shuriken' star, which generates chiral electromagnetic fields exhibiting greater chiral asymmetry than circularly polarized light. Monitoring changes in the resonance positions of the characteristic optical rotatory dispersion spectra produced by the Disposable Plasmonic Assays allows for the observation of surface binding events. By measuring resonance shift data and through the utilisation of various gold film functionalisation techniques, these assays are demonstrated as versatile, label-free biosensing platforms capable of specifically detecting a wide range of target proteins and virus particles from complex solutions. Furthermore, the multiplexing performance of these assays is showcased, enabling the detection of multiple different antigens and virions in a single experiment. These results highlight the potential of plasmonic metafilms as rapid and disposable point-of-care immunoassays for diagnostic applications. In addition to biosensing, the chiral geometry of Disposable Plasmonic Assays is exploited for the chiral discrimination of metal nanoparticles and small molecules using Surface Enhanced Raman Spectroscopy (SERS). By linking helicoid shaped gold nanoparticles to the metafilm surface via a dithiol linker, the chiral properties of both nanoparticles and metafilms combine, resulting in the creation of differential electromagnetic 'hotspot' regions based on their symmetry combinations. The electromagnetic intensity in these regions corresponds to the SERS signal obtained from the achiral dithiol linker molecule, facilitating a deeper understanding of the chirally dependent SERS phenomenon. These findings serve to validate and explain the differential SERS data obtained enantiomers of biomolecules and drug molecules from silver modified Disposable Plasmonic Assays

Attomole enantiomeric discrimination of small molecules using an achiral SERS reporter and chiral plasmonics

Biologically important molecules span a size range from very large biomacromolecules, such as proteins to small metabolite molecules. Consequently, spectroscopic techniques which can detect and characterize the structure of inherently chiral biomolecules over this range of scale at the femtomole level are necessary to develop novel biosensing and diagnostic technologies. Nanophotonic platforms uniquely enable chirally sensitive structural characterisation of biomacromolecules at this ultrasensitive level. However, they are less successful at achieving the same level of sensitivity for small chiral molecules, with less than nanomole typical. This poorer performance can be attributed to the optical response of the platform being sensitive to a much larger volume of the near field than is occupied by the small molecule. Here we show that by combining chiral plasmonic metasurfaces with Raman reporters, which can detect changes in electromagnetic environment at molecular dimensions, chiral discrimination can be achieved for attomole quantities of a small molecule, the amino acid cysteine. The signal-to-noise, and hence ultimate sensitivity, of the measurement can be further improved by combining the metasurfaces with gold achiral nanoparticles. This indirect enantiomeric detection is 9 orders of magnitude more sensitive than strategies relying on monitoring the Raman response of target chiral molecules directly. Given the generic nature of the phenomenon,this study provides a framework for developing novel technologies for detecting a broad spectrum of small biomolecules, which would be useful tools in the field of metabolomics

Chiral Metafilms and Surface Enhanced Raman Scattering For Enantiomeric Discrimination of Helicoid Nanoparticles

Chiral nanophotonic platforms provide a means of creating near fields with both enhanced asymmetric properties and intensities. They can be exploited for optical measurements that allow enantiomeric discrimination at detection levels greater than 6 orders of magnitude than is achieved with conventional chirally sensitive spectroscopic methods based on circularly polarized light. The optimal approach for exploiting nanophotonic platforms for chiral detection would be to use spectroscopic methods that provide a local probe of changes in the near field environment induced by the presence of chiral species. Here we show that surface enhanced Raman spectroscopy (SERS) is such a local probe of the near field environment. We have used it to achieve enantiomeric discrimination of chiral helicoid nanoparticles deposited on left and right-handed enantiomorphs of a chiral metafilm. Hotter electromagnetic hotspots are created for matched combinations of helicoid and metafilms (left-left and right-right), while mismatched combinations leads to significantly cooler electromagnetic hotspots. This large enantiomeric dependency on hotspot intensity is readily detected using SERS with the aid of an achiral Raman reporter molecule. In effect we have used SERS to distinguish between the different EM environments of the plasmonic diastereomers produced by mixing chiral nanoparticles and metafilms. The work demonstrates that by combining chiral nanophotonic platforms with established SERS strategies new avenues in ultrasensitive chiral detection can be opened

Ultrasensitive Raman detection of biomolecular conformation at the attomole scale using chiral nanophotonics

Understanding the function of a biomolecule hinges on its 3D conformation or secondary structure. Chirally sensitive, optically active techniques based on the differential absorption of UV–vis circularly polarized light excel at rapid characterisation of secondary structures. However, Raman spectroscopy, a powerful method for determining the structure of simple molecules, has limited capacity for structural analysis of biomolecules because of intrinsically weak optical activity, necessitating millimolar (mM) sample quantities. A breakthrough is presented for utilising Raman spectroscopy in ultrasensitive biomolecular conformation detection, surpassing conventional Raman optical activity by 15 orders of magnitude. This strategy combines chiral plasmonic metasurfaces with achiral molecular Raman reporters and enables the detection of different conformations (α-helix and random coil) of a model peptide (poly-L/D-lysine) at the ≤attomole level (monolayer). This exceptional sensitivity stems from the ability to detect local, molecular-scale changes in the electromagnetic (EM) environment of a chiral nanocavity induced by the presence of biomolecules using molecular Raman reporters. Further signal enhancement is achieved by incorporating achiral Au nanoparticles. The introduction of the nanoparticles creates highly localized regions of extreme optical chirality. This approach, which exploits Raman, a generic phenomenon, paves the way for next-generation technologies for the ultrasensitive detection of diverse biomolecular structures

Multiplexed biosensing of proteins and virions with disposable plasmonic assays

Our growing ability to tailor healthcare to the needs of individuals has the potential to transform clinical treatment. However, the measurement of multiple biomarkers to inform clinical decisions requires rapid, effective, and affordable diagnostics. Chronic diseases and rapidly evolving pathogens in a larger population have also escalated the need for improved diagnostic capabilities. Current chemical diagnostics are often performed in centralized facilities and are still dependent on multiple steps, molecular labeling, and detailed analysis, causing the result turnaround time to be over hours and days. Rapid diagnostic kits based on lateral flow devices can return results quickly but are only capable of detecting a handful of pathogens or markers. Herein, we present the use of disposable plasmonics with chiroptical nanostructures as a platform for low-cost, label-free optical biosensing with multiplexing and without the need for flow systems often required in current optical biosensors. We showcase the detection of SARS-CoV-2 in complex media as well as an assay for the Norovirus and Zika virus as an early developmental milestone toward high-throughput, single-step diagnostic kits for differential diagnosis of multiple respiratory viruses and any other emerging diagnostic needs. Diagnostics based on this platform, which we term “disposable plasmonics assays,” would be suitable for low-cost screening of multiple pathogens or biomarkers in a near-point-of-care setting

A chiral quantum metamaterial for hypersensitive biomolecule detection

Chiral biological and pharmaceutical molecules are analyzed with phenomena that monitor their very weak differential interaction with circularly polarized light. This inherent weakness results in detection levels for chiral molecules that are inferior, by at least six orders of magnitude, to the single molecule level achieved by state-of-the-art chirally insensitive spectroscopic measurements. Here, we show a phenomenon based on chiral quantum metamaterials (CQMs) that overcomes these intrinsic limits. Specifically, the emission from a quantum emitter, a semiconductor quantum dot (QD), selectively placed in a chiral nanocavity is strongly perturbed when individual biomolecules (here, antibodies) are introduced into the cavity. The effect is extremely sensitive, with six molecules per nanocavity being easily detected. The phenomenon is attributed to the CQM being responsive to significant local changes in the optical density of states caused by the introduction of the biomolecule into the cavity. These local changes in the metamaterial electromagnetic environment, and hence the biomolecules, are invisible to “classical” light-scattering-based measurements. Given the extremely large effects reported, our work presages next generation technologies for rapid hypersensitive measurements with applications in nanometrology and biodetection