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We study how the size of spherical gold nanoparticles (AuNPs) influences their ability to enhance the response of optical biosensors based on surface plasmon resonance (SPR). We present a theoretical model that relates the enhancement generated by the AuNPs to their composition, size, and concentration, thus allowing for accurate predictions regarding the SPR sensor response to various AuNPs. The effect of the AuNP size is also investigated experimentally using an SPR biosensor for the detection of carcinoembryonic antigen (CEA) in which AuNPs covered with neutravidin (N-AuNPs) are used in the last step of a sandwich assay to enhance the sensor response to biotinylated secondary antibody against CEA. The experimental data are in excellent agreement with the results of the theoretical analysis. We demonstrate that the sensor response enhancement generated by the N-AuNPs is determined by (i) the sensor sensitivity to N-AuNP surface density (Sσ) and (ii) the ability of the N-AuNPs to bind to the functionalized surface of the sensor. Our results indicate that, while Sσ increases with the size of the N-AuNP, the ability of the functionalized surface of the sensor to bind the N-AuNPs is affected by steric effects and decreases with the size of N-AuNP.

A nanoplasmonic ruler method is presented in order to measure the deformation of adsorbed, nm-scale lipid vesicles on solid supports. It is demonstrated that single adsorbed vesicles undergo greater deformation on silicon oxide over titanium oxide, offering direct experimental evidence to support membrane tension-based theoretical models of supported lipid bilayer formation.

Label-free characterization of single biomolecules aims to complement fluorescence microscopy in situations where labeling compromises data interpretation, is technically challenging or even impossible. However, existing methods require the investigated species to bind to a surface to be visible, thereby leaving a large fraction of analytes undetected. Here, we present nanofluidic scattering microscopy (NSM), which overcomes these limitations by enabling label-free, real-time imaging of single biomolecules diffusing inside a nanofluidic channel. NSM facilitates accurate determination of molecular weight from the measured optical contrast and of the hydrodynamic radius from the measured diffusivity, from which information about the conformational state can be inferred. Furthermore, we demonstrate its applicability to the analysis of a complex biofluid, using conditioned cell culture medium containing extracellular vesicles as an example. We foresee the application of NSM to monitor conformational changes, aggregation and interactions of single biomolecules, and to analyze single-cell secretomes.

Optical biosensors based on plasmonic nanostructures present a promising alternative to conventional biosensing methods and provide unmatched possibilities for miniaturization and high-throughput analysis. Previous works on the topic, however, have been overwhelmingly directed toward elucidating the optical performance of such sensors, with little emphasis on the topic of mass transport. To date, there exists no examination, experimental nor theoretical, of the bioanalytical performance of such sensors (in terms of detection limits) that simultaneously addresses both optical and mass transport aspects in a quantitative manner. In this work we present a universal model that describes the smallest concentration that can be detected by a nanoplasmonic biosensor. Accounting for both optical and mass transport aspects, this model establishes a relationship between bioanalytical performance and the biosensor’s design parameters. We employ the model to optimize the performance of a nanoplasmonic DNA biosensor consisting of randomly distributed gold nanorods on a glass substrate. Through both experimental and theoretical results, we show that the proper design of a nanostructured sensing substrate is one that maximizes mass transport efficiency while preserving the quality of the optical readout. All results are compared with those obtained using a conventional SPR biosensor. We show that an optimized nanoplasmonic substrate allows for the detection of DNA at concentrations of an order of magnitude lower with respect to an SPR biosensor.

Nanopatterned 2-dimensional Au nanocluster arrays with controlled configuration are fabricated onto reconstructed nanoporous poly(styrene-block-vinylpyridine) inverse micelle monolayer films. Near-field coupling of localized surface plasmons is studied and compared for disordered and ordered core-centered Au NC arrays. Differences in evolution of the absorption band and field enhancement upon Au nanoparticle adsorption are shown. The experimental results are found to be in good agreement with theoretical studies based on the finite-difference time-domain method and rigorous coupled-wave analysis. The realized Au nanopatterns are exploited as substrates for surface-enhanced Raman scattering and integrated into Kretschmann-type SPR sensors, based on which unprecedented SPR-coupling-type sensors are demonstrated.

Theoretical study of sensing properties of lattice resonances supported by arrays of gold nanoparticles expressed in terms of the figure of merit (FOM) is reported. Analytical expressions for the FOM for surface and bulk refractive index changes are derived to establish the relationship between the sensing performance and design parameters and to allow for the design of nanoparticle arrays with optimal sensing performance. It is demonstrated that lattice resonances exhibit about two orders of magnitude higher bulk FOM than localized surface plasmon (LSP) resonance and that the surface FOM provided by lattice resonances and LSP resonances are comparable.