Local refractive index sensitivity of plasmonic nanoparticles

Probing the Local Near-Field Intensity of Plasmonic Nanoparticles in the Mid-infrared Spectral Region

The enhanced local field of gold nanoparticles (AuNPs) in mid-infrared spectral regions is essential for improving the detection sensitivity of vibrational spectroscopy and mediating photochemical reactions. However, it is still challenging to measure its intensity at subnanometer scales. Here, using the NO2 symmetric stretching mode (νNO2) of self-assembled 4-nitrothiophenol (4-NTP) monolayers on AuNPs as a model, we demonstrated that the percentage of excited νNO2 mode, determined by femtosecond time-resolved sum-frequency generation vibrational spectroscopy, allows us to directly detect the local field intensity of the AuNP surface in subnanometer ranges. The local-field intensity is tuned by AuNP diameters. An approximate 17-fold enhancement was observed for the local field on 80 nm AuNPs compared to the Au film. Additionally, the local field can regulate the anharmonicity of the νNO2 mode by synergistic effect with molecular orientation. This work offers a promising approach to probe the local field intensity distribution around plasmonic NP surfaces at subnanometer scales.

Anomalous spectral shift of localized surface plasmon resonance

We report the first observation of spectral blue shift of plasmon resonance of synthesized silver nanoparticles (AgNPs) due to a negative optical nonlinearity of a local ambient medium, i.e., indigo carmine (IC) solution at around 420 nm wavelength. The blue shift occurred at a larger concentration of AgNPs or at a larger concentration of IC solution, being in obvious contrast to spectral red shift which was widely witnessed in plasmon spectral shift in a linear regime. Plasmon-enhanced local fields could excite the third-order optical nonlinearity for blue shift even under continuous (non-pulsed) light illumination. We also found that the plasmon-excited nonlinearity could allow for differential nonlinear response of the IC solution to be even greater than its differential linear response, though appearing to be somewhat inconsistent with what was generally known in light-matter interaction. The demonstrated properties of such anomalous shift of plasmon spectral peaks and its accompanying properties indicated that plasmon technologies could be exploited not only in linear but also in nonlinear aspects for critical optimization in plasmon-energy harvesting systems such as in surface enhanced spectroscopy/microscopy, biomedical imaging/sensing, laser frequency conversion, ultrashort pulse generation, and all-optical switching.

Detection of SARS-CoV-2 Based on Nucleic Acid Amplification Tests (NAATs) and Its Integration into Nanomedicine and Microfluidic Devices as Point-of-Care Testing (POCT)

The coronavirus SARS-CoV-2 has highlighted the criticality of an accurate and rapid diagnosis in order to contain the spread of the virus. Knowledge of the viral structure and its genome is essential for diagnosis development. The virus is still quickly evolving and the global scenario could easily change. Thus, a greater range of diagnostic options is essential to face this threat to public health. In response to the global demand, there has been a rapid advancement in the understanding of current diagnostic methods. In fact, innovative approaches have emerged, leveraging the benefits of nanomedicine and microfluidic technologies. Although this development has been incredibly fast, several key areas require further investigation and optimization, such as sample collection and preparation, assay optimization and sensitivity, cost effectiveness, scalability device miniaturization, and portability and integration with smartphones. Addressing these gaps in the knowledge and these technological challenges will contribute to the development of reliable, sensitive, and user-friendly NAAT-based POCTs for the diagnosis of SARS-CoV-2 and other infectious diseases, facilitating rapid and effective patient management. This review aims to provide an overview of current SARS-CoV-2 detection methods based on nucleic acid detection tests (NAATs). Additionally, it explores promising approaches that combine nanomedicine and microfluidic devices with high sensitivity and relatively fast 'time to answer' for integration into point-of-care testing (POCT).

Optimization of Colloidal Gold Nanoparticles on Porous Anodic Aluminum Oxide Substrates for Refractometric Sensing

A new composite metal-insulator-metal (MIM) system consisting of exceptionally dense non-close-packed (NCP) arrays of gold or silver nanoparticles, porous anodic aluminum oxide (PAAO), and bulk aluminum substrate interacts strongly with visible light and may become a very useful component for optical applications. The proposed MIM structure can be synthesized using accessible lithography-free chemical and physical processes (anodization and capillary force assisted colloidal particle deposition) that are suitable for the low-cost production of specialized devices. Here, we present a systematic study to determine the essential MIM structure parameters (nanoparticle size and PAAO layer thickness) for localized surface plasmon resonance (LSPR) refractometric sensing. A performance comparison was done by recording the spectra of scattered light upon angled illumination in media with different refractive indices. A clear advantage for maximizing the signal to background ratio was observed in the case of 60 and 80 nm Au nanoparticles with a PAAO thickness in a narrow range between 300 and 375 nm. Sensitivity exceeding a 200 nm peak wavelength shift per refractive index unit was found for 60 nm Au nanoparticles on approximately 500-nm-thick PAAO. The experimental observations were supported by finite-difference time-domain (FDTD) simulations.

Research on Dual-Technology Fusion Biosensor Chip Based on RNA Virus Medical Detection

In recent years, the emergence of COVID-19 and other epidemics caused by RNA(ribonucleic acid)-type genetic viruses has aroused the close attention of governments around the world on emergency response to public safety and health emergencies. In this paper, an electrodeless biosensing detection chip for RNA virus medical detection is designed using quartz crystal microbalance technology and local surface plasmon resonance technology. The plasmonic resonance characteristic in the nanostructures of gold nanorods-quartz substrates with different parameters and the surface potential distribution of the quartz crystal microbalance sensing chip were studied by COMSOL finite element simulation software. The results show that the arrangement structure and spacing of gold nanorod dimers greatly affect the local surface plasmon resonance of nanorods, which in turn affects the detection results of biomolecules. Moreover, high concentrations of "hot spots" are distributed between both ends and the gap of the gold nanorod dimer, which reflects the strong hybridization of the multiple resonance modes of the nanoparticles. In addition, by simulating and calculating the surface potential distribution of the electrode area and non-electrode area of the biosensor chip, it was found that the biosensor chip with these two areas can enhance the piezoelectric effect of the quartz chip. Under the same simulation conditions, the biochip with a completely electrodeless structure showed a better sensing performance. The sensor chip combining QCM and LSPR can reduce the influence of the metal electrode on the quartz wafer to improve the sensitivity and accuracy of detection. Considering the significant influence of the gold nanorod dimer plasma resonance mode and the significant advantages of the electrodeless biosensor chip, an electrodeless biosensor combining these two technologies is proposed for RNA virus detection and screening, which has potential applications in biomolecular measurement and other related fields.

Shape Modulation of Plasmonic Nanostructures by Unconventional Lithographic Technique

Conventional nano-sphere lithography techniques have been extended to the fabrication of highly periodic arrays of sub-wavelength nanoholes in a thin metal film. By combining the dry etching processes of self-assembled monolayers of polystyrene colloids with metal physical deposition, the complete transition from increasing size triangular nanoprism to hexagonally distributed nanoholes array onto thin metal film has been gradually explored. The investigated nano-structured materials exhibit interesting plasmonic properties which can be precisely modulated in a desired optical spectral region. An interesting approach based on optical absorbance measurements has been adopted for rapid and non-invasive inspections of the nano-sphere monolayer after the ion etching process. By enabling an indirect and accurate evaluation of colloid dimensions in a large area, this approach allows the low-cost and reproducible fabrication of plasmonic materials with specifically modulated optical properties suitable for many application in biosensing devices or Raman enhanced effects.

Localized surface plasmon resonance-based biosensor on gold nanoparticles for Taenia solium detection

This paper describes, for the first time to our knowledge, a fast-response and specific biosensor for detection of Taenia solium, a parasite responsible for neurocysticercosis disease that affects the central nervous system. The biosensor is based on the localized surface plasmon resonance (LSPR) technique on gold nanoparticles (AuNPs) in colloidal suspension that were functionalized and activated with antibodies to perform an immuno-capture effect. The AuNPs were synthetized by Turkevich and seed-mediated growth methods. A variety of concentrations of T. solium antigen were added to test the detection and the dose-response profile. Small antigen concentrations were detected indicating that the limit of detection is lower than 0.1 µg/mL of antigen. The results demonstrate the potential of the AuNPs LSPR biosensor as a clinical tool for neurocysticercosis diagnostic.

Recent Advances in Smart Biomaterials for the Detection and Treatment of Autoimmune Diseases

Autoimmune diseases are a group of debilitating illnesses that are often idiopathic in nature. The steady rise in the prevalence of these conditions warrants new approaches for diagnosis and treatment. Stimuli-responsive biomaterials also known as "smart", "intelligent" or "recognitive" biomaterials are widely studied for their applications in drug delivery, biosensing and tissue engineering due to their ability to produce thermal, optical, chemical, or structural changes upon interacting with the biological environment. This critical analysis highlights studies within the last decade that harness the recognitive capabilities of these biomaterials towards the development of novel detection and treatment options for autoimmune diseases.

UV-Laser Interference Lithography for Local Functionalization of Plasmonic Nanostructures with Responsive Hydrogel

A novel approach to local functionalization of plasmonic hotspots at gold nanoparticles with biofunctional moieties is reported. It relies on photocrosslinking and attachment of a responsive hydrogel binding matrix by the use of a UV interference field. A thermoresponsive poly(N-isopropylacrylamide)-based (pNIPAAm) hydrogel with photocrosslinkable benzophenone groups and carboxylic groups for its postmodification was employed. UV-laser interference lithography with a phase mask configuration allowed for the generation of a high-contrast interference field that was used for the recording of periodic arrays of pNIPAAm-based hydrogel features with the size as small as 170 nm. These hydrogel arrays were overlaid and attached on the top of periodic arrays of gold nanoparticles, exhibiting a diameter of 130 nm and employed as a three-dimensional binding matrix in a plasmonic biosensor. Such a hybrid material was postmodified with ligand biomolecules and utilized for plasmon-enhanced fluorescence readout of an immunoassay. Additional enhancement of the fluorescence sensor signal by the collapse of the responsive hydrogel binding matrix that compacts the target analyte at the plasmonic hotspot is demonstrated.

Light-controlled nanoswitches: from fabrication to photoelectric switching

Plasmon-assisted optical growth significantly improves the controllability of polymerization on the nanoscale, which greatly facilitates the fabrication of nanodevices and their integration. Traditional approaches for nanodevices strongly rely on nanofabrication techniques, which involve complicated multi-steps and expensive instrumentation. Here, we build photoelectric nanoswitches by precisely implanting photoelectric active polymers, polypyrrole, into metallic bowtie nanogaps via charge transfer induced oxidative polymerization, which show conductivity up to 4 μS m^-1 with an optical switching response time of ∼50 ms. We further demonstrate their photoelectric switchability with different laser powers, wavelengths and amounts of polypyrrole in the gaps. This plasmon-assisted optical growth strategy not only provides a facile and accurate means to control the polymerization process, but also greatly expedites the fabrication of nanodevices and their integration.

Metallic photonic crystal-based sensor for cryogenic environments

We investigate the design, characterization, and application of metallic photonic crystal (MPC) structures, consisting of plasmonic gold nanogratings on top of a photonic waveguide, as transducers for lab-on-chip biosensing in cryogenic environments. The compact design offers a promising approach to sensitive, in situ biosensing platforms for astrobiology applications (e.g., on the "icy moons" of the outer solar system). We fabricated and experimentally characterized three MPC sensor geometries, with variable nanograting width, at temperatures ranging from 300 K to 180 K. Sensors with wider nanogratings were more sensitive to changes in the local dielectric environment. Temperature-dependent experiments revealed an increase in plasmonic resonance intensity of around 13% at 180 K (compared with 300 K), while the coupled plasmonic-photonic resonance was less sensitive to temperature, varying by less than 5%. Simulation results confirm the relative temperature stability of the plasmonic-photonic mode and, combined with its high sensitivity, suggest a novel application of this mode as the sensing transduction mechanism over wide temperature ranges. To our knowledge, this is among the first reports of the design and characterization of a nanoplasmonic sensor specifically for low-temperature sensing operation.

Nano-colorimetrically determined refractive index variation with ultra-high chromatic resolution

We develop a front-to-end solution where the shift of chromaticity from scattering of plasmonic nanoparticles is used as the reporter for nano-environmental refractive index variation. By co-projecting possible power combinations of RGB LEDs and digitized color grid density of CCD with various luminance onto the CIE 1931 chromaticity diagram, optimum condition for nanoenvironment sensing can be achieved. The highest resolution for local refractive index change is 0.0021 per distinguishable color, which is higher than that of a typical handheld spectrometer by 4.8 times. This result shows great potential in simplifying nano-environment sensing instruments and is particularly useful for multi-point dynamical process.

Chelation-assisted assembly of multidentate colloidal nanoparticles into metal-organic nanoparticles

We propose a chelation-assisted assembly of multidentate CNs into metal-organic nanoparticles (MONs). Multidentate CNs functionalized with coordination sites participate equally as organic linkers in MON construction, which is driven by chelation between metal ions and coordination sites. MONs assembled from Au nanoparticles display particle number- and size-dependent optical properties. In addition, the resulting CN-assembled MONs give evidence that assembly was dictated by the multidentate surface ligand rather than the size, shape or material of CNs. With this chelation-assisted strategy, it is possible to control the number of assembled CNs and build the connections between them.

Nanoplasmonic sensors for biointerfacial science

In recent years, nanoplasmonic sensors have become widely used for the label-free detection of biomolecules across medical, biotechnology, and environmental science applications. To date, many nanoplasmonic sensing strategies have been developed with outstanding measurement capabilities, enabling detection down to the single-molecule level. One of the most promising directions has been surface-based nanoplasmonic sensors, and the potential of such technologies is still emerging. Going beyond detection, surface-based nanoplasmonic sensors open the door to enhanced, quantitative measurement capabilities across the biointerfacial sciences by taking advantage of high surface sensitivity that pairs well with the size of medically important biomacromolecules and biological particulates such as viruses and exosomes. The goal of this review is to introduce the latest advances in nanoplasmonic sensors for the biointerfacial sciences, including ongoing development of nanoparticle and nanohole arrays for exploring different classes of biomacromolecules interacting at solid-liquid interfaces. The measurement principles for nanoplasmonic sensors based on utilizing the localized surface plasmon resonance (LSPR) and extraordinary optical transmission (EOT) phenomena are first introduced. The following sections are then categorized around different themes within the biointerfacial sciences, specifically protein binding and conformational changes, lipid membrane fabrication, membrane-protein interactions, exosome and virus detection and analysis, and probing nucleic acid conformations and binding interactions. Across these themes, we discuss the growing trend to utilize nanoplasmonic sensors for advanced measurement capabilities, including positional sensing, biomacromolecular conformation analysis, and real-time kinetic monitoring of complex biological interactions. Altogether, these advances highlight the rich potential of nanoplasmonic sensors and the future growth prospects of the community as a whole. With ongoing development of commercial nanoplasmonic sensors and analytical models to interpret corresponding measurement data in the context of biologically relevant interactions, there is significant opportunity to utilize nanoplasmonic sensing strategies for not only fundamental biointerfacial science, but also translational science applications related to clinical medicine and pharmaceutical drug development among countless possibilities.

Spatially Resolved Sensitivity of Single-Particle Plasmon Sensors

The high sensitivity of localized surface plasmon resonance sensors to the local refractive index allows for the detection of single-molecule binding events. Though binding events of single objects can be detected by their induced plasmon shift, the broad distribution of observed shifts remains poorly understood. Here, we perform a single-particle study wherein single nanospheres bind to a gold nanorod, and relate the observed plasmon shift to the binding location using correlative microscopy. To achieve this we combine atomic force microscopy to determine the binding location, and single-particle spectroscopy to determine the corresponding plasmon shift. As expected, we find a larger plasmon shift for nanospheres binding at the tip of a rod compared to its sides, in good agreement with numerical calculations. However, we also find a broad distribution of shifts even for spheres that were bound at a similar location to the nanorod. Our correlative approach allows us to disentangle effects of nanoparticle dimensions and binding location, and by comparison to numerical calculations we find that the biggest contributor to this observed spread is the dispersion in nanosphere diameter. These experiments provide insight into the spatial sensitivity and signal-heterogeneity of single-particle plasmon sensors and provides a framework for signal interpretation in sensing applications.

Split-GFP: SERS Enhancers in Plasmonic Nanocluster Probes

The assembly of plasmonic metal nanoparticles into hot spot surface-enhanced Raman scattering (SERS) nanocluster probes is a powerful, yet challenging approach for ultrasensitive biosensing. Scaffolding strategies based on self-complementary peptides and proteins are of increasing interest for these assemblies, but the electronic and the photonic properties of such hybrid nanoclusters remain difficult to predict and optimize. Here, split-green fluorescence protein (sGFP) fragments are used as molecular glue and the GFP chromophore is used as a Raman reporter to assemble a variety of gold nanoparticle (AuNP) clusters and explore their plasmonic properties by numerical modeling. It is shown that GFP seeding of plasmonic nanogaps in AuNP/GFP hybrid nanoclusters increases near-field dipolar couplings between AuNPs and provides SERS enhancement factors above 10⁸ . Among the different nanoclusters studied, AuNP/GFP chains allow near-infrared SERS detection of the GFP chromophore imidazolinone/exocyclic CC vibrational mode with theoretical enhancement factors of 10⁸ -10⁹ . For larger AuNP/GFP assemblies, the presence of non-GFP seeded nanogaps between tightly packed nanoparticles reduces near-field enhancements at Raman active hot spots, indicating that excessive clustering can decrease SERS amplifications. This study provides rationales to optimize the controlled assembly of hot spot SERS nanoprobes for remote biosensing using Raman reporters that act as molecular glue between plasmonic nanoparticles.

Mapping the local particle plasmon sensitivity with a scanning probe

We probe the local sensitivity of an optically excited plasmonic nanoparticle by changing the local dielectric environment through a scanning glass fiber tip. Recording the particle plasmon scattering spectrum for each tip position allows us to observe spectral resonance shifts concurrent with changes in scattering intensity and plasmon damping. For the tip-induced spectral shifts we find the strongest sensitivity at the particle edges, in accordance with the spatial plasmonic field profile. In contrast, the strongest sensitivity occurs at the center of the particle if the scattering intensity is probed at the short wavelength slope of the plasmon resonance instead of the resonance position. This bears important implications for plasmonic sensing, in particular when done at a single light wavelength.

Three dimensional sensitivity characterization of plasmonic nanorods for refractometric biosensors

An experimental three dimensional characterization of the local refractive index sensitivity of plasmonic gold nanorods is performed by controlled apposition of lithographic nanostructures. We show up to seven times higher sensitivity values to local changes in the refractive index at the particle tip than center. In addition, successive deposition of defined nm-thin dielectric layers on nanorods covered with stripe masks allows us to study the sensitivity decrease normal to the particle surface separately for different particle sites. Clear trends to a stronger sensitivity decay at sites of higher local sensitivity are demonstrated experimentally and theoretically. Our sensitivity characterization provides an important tool to find the most suitable particle type and particle site for specific bio-sensing applications.

Self-aligned placement and detection of quantum dots on the tips of individual conical plasmonic nanostructures

Hybrid structures of few or single quantum dots (QDs) coupled to single optical antennas are of prime interest for nano-optical research. The photoluminescence (PL) signal from single nanoemitters, such as QDs, can be enhanced, and their emission characteristics modified, by coupling them to plasmonic nanostructures. Here, a self-aligned technique for placing nanoscale QDs with about 10 nm lateral accuracy and well-defined molecular distances to the tips of individual nanocones is reported. This way the QDs are positioned exactly in the high near-field region that can be created near the cone apex. The cones are excited in the focus of a radially polarized laser beam and the PL signal of few or single QDs on the cone tips is spectrally detected.

Local refractive index sensitivity of gold nanodisks

We experimentally investigate the local refractive index sensitivity of plasmonic gold nanodisks by applying small polymer dots to selected disk sites by means of two-step lithography. Measured sensitivity profiles obtained from tracking the polymer-induced spectral shift of the plasmon modes are in excellent agreement with numerical simulation of both spectral sensitivity and the electric near field of the nanodisks. Based on the nanodisk sensitivity profile we tailor a sensitive and spatially uniform plasmonic sensor by capping the disk with a dielectric layer, thus restricting analyte access to the disk rim.

Engineering plasmon dispersion relations: hybrid nanoparticle chain-substrate plasmon polaritons

We consider the dispersion relations of the optical excitations in a chain of silver nanoparticles situated above a metal substrate and show that they are hybrid plasmon polaritons, composed of localized surface plasmons and surface plasmon polaritons. We demonstrate a strong dependence of the system's optical properties on the plasma frequency of the substrate and that choosing the appropriate plasma frequency allows one to engineer the modes to have a very high, very low or even negative group velocity. For the latter, Poynting vector calculations reveal opposite phase and energy propagation. We expect that our results will contribute to the design of nano-optical devices with specific transport properties.

Optimal geometric parameters of ordered arrays of nanoprisms for enhanced sensitivity in localized plasmon based sensors

Plasmonic sensors based on ordered arrays of nanoprisms are optimized in terms of their geometric parameters like size, height, aspect ratio for Au, Ag or Au0.5-Ag0.5 alloy to be used in the visible or near IR spectral range. The two figures of merit used for the optimization are the bulk and the surface sensitivity: the first is important for optimizing the sensing to large volume analytes whereas the latter is more important when dealing with small bio-molecules immobilized in close proximity to the nanoparticle surface. A comparison is made between experimentally obtained nanoprisms arrays and simulated ones by using Finite Elements Methods (FEM) techniques.

Simulation and implementation of a morphology-tuned gold nano-islands integrated plasmonic sensor

This work presents simulation, analysis and implementation of morphology tuning of gold nano-island structures deposited by a novel convective assembly technique. The gold nano-islands were simulated using 3D Finite-Difference Time-Domain (FDTD) techniques to investigate the effect of morphological changes and adsorption of protein layers on the localized surface plasmon resonance (LSPR) properties. Gold nano-island structures were deposited on glass substrates by a novel and low-cost convective assembly process. The structure formed by an uncontrolled deposition method resulted in a nano-cluster morphology, which was annealed at various temperatures to tune the optical absorbance properties by transforming the nano-clusters to a nano-island morphology by modifying the structural shape and interparticle separation distances. The dependence of the size and the interparticle separation distance of the nano-islands on the LSPR properties were analyzed in the simulation. The effect of adsorption of protein layer on the nano-island structures was simulated and a relation between the thickness and the refractive index of the protein layer on the LSPR peak was presented. Further, the sensitivity of the gold nano-island integrated sensor against refractive index was computed and compared with the experimental results.

Vibrational near-field mapping of planar and buried three-dimensional plasmonic nanostructures

Nanoantennas confine electromagnetic fields at visible and infrared wavelengths to volumes of only a few cubic nanometres. Assessing their near-field distribution offers fundamental insight into light–matter coupling and is of special interest for applications such as radiation engineering, attomolar sensing and nonlinear optics. Most experimental approaches to measure near-fields employ either diffraction-limited far-field methods or intricate near-field scanning techniques. Here, using diffraction-unlimited far-field spectroscopy in the infrared, we directly map the intensity of the electric field close to plasmonic nanoantennas. We place a patch of probe molecules with 10 nm accuracy at different locations in the near-field of a resonant antenna and extract the molecular vibrational excitation. We map the field intensity along a dipole antenna and gap-type antennas. Moreover, this method is able to assess the near-field intensity of complex buried plasmonic structures. We demonstrate this by measuring for the first time the near-field intensity of a three-dimensional plasmonic electromagnetically induced transparency structure.

Near-field effects generated by nanoantennas provide insights into light–matter interactions and new ways of sensing at low concentrations. By measuring characteristic vibrations from molecular patches, Dregely et al. are able to characterize the field intensity of buried three-dimensional nanostructures.

Plasmon-Enhanced Fluorescence Biosensors: a Review

Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

Trends and challenges of refractometric nanoplasmonic biosensors: a review

Motivated by potential benefits such as sensor miniaturization, multiplexing opportunities and higher sensitivities, refractometric nanoplasmonic biosensing has profiled itself in a short time span as an interesting alternative to conventional Surface Plasmon Resonance (SPR) biosensors. This latter conventional sensing concept has been subjected during the last decades to strong commercialization, thereby strongly leaning on well-developed thin-film surface chemistry protocols. Not surprisingly, the examples found in literature based on this sensing concept are generally characterized by extensive analytical studies of relevant clinical and diagnostic problems. In contrast, the more novel Localized Surface Plasmon Resonance (LSPR) alternative finds itself in a much earlier, and especially, more fundamental stage of development. Driven by new fabrication methodologies to create nanostructured substrates, published work typically focuses on the novelty of the presented material, its optical properties and its use - generally limited to a proof-of-concept - as a label-free biosensing scheme. Given the different stages of development both SPR and LSPR sensors find themselves in, it becomes apparent that providing a comparative analysis of both concepts is not a trivial task. Nevertheless, in this review we make an effort to provide an overview that illustrates the progress booked in both fields during the last five years. First, we discuss the most relevant advances in SPR biosensing, including interesting analytical applications, together with different strategies that assure improvements in performance, throughput and/or integration. Subsequently, the remaining part of this work focuses on the use of nanoplasmonic sensors for real label-free biosensing applications. First, we discuss the motivation that serves as a driving force behind this research topic, together with a brief summary that comprises the main fabrication methodologies used in this field. Next, the sensing performance of LSPR sensors is examined by analyzing different parameters that can be invoked in order to quantitatively assess their overall sensing performance. Two aspects are highlighted that turn out to be especially important when trying to maximize their sensing performance, being (1) the targeted functionalization of the electromagnetic hotspots of the nanostructures, and (2) overcoming inherent negative influence that stem from the presence of a high refractive index substrate that supports the nanostructures. Next, although few in numbers, an overview is given of the most exhaustive and diagnostically relevant LSPR sensing assays that have been recently reported in literature, followed by examples that exploit inherent LSPR characteristics in order to create highly integrated and high-throughput optical biosensors. Finally, we discuss a series of considerations that, in our opinion, should be addressed in order to bring the realization of a stand-alone LSPR biosensor with competitive levels of sensitivity, robustness and integration (when compared to a conventional SPR sensor) much closer to reality.

Refractive index sensing with subradiant modes: a framework to reduce losses in plasmonic nanostructures

Plasmonic modes with long radiative lifetimes, subradiant modes, combine strong confinement of the electromagnetic energy at the nanoscale with a steep spectral dispersion, which makes them promising for biochemical sensors or immunoassays. Subradiant modes have three decay channels: Ohmic losses, their extrinsic coupling to radiation, and possibly their intrinsic dipole moment. In this work, the performance of subradiant modes for refractive index sensing is studied with a general analytical and numerical approach. We introduce a model for the impact that has different decay channels of subradiant modes on the spectral resolution and contrast. It is shown analytically and verified numerically that there exists an optimal value of the mode coupling for which the spectral dispersion of the resonance line shape is maximal. The intrinsic width of subradiant modes determines the value of the dispersion maximum and depends on the penetration of the electric field in the metallic nanostructure. A figure of merit, given by the ratio of the sensitivity to the intrinsic width, which are both intrinsic properties of subradiant modes, is introduced. This figure of merit can be directly calculated from the line shape in the far-field optical spectrum and accounts for the fact that both the spectral resolution and contrast determine the limit of detection. An expression for the intrinsic width of a plasmonic mode is derived and calculated from the line shape parameters and using perturbation theory. The method of analysis introduced in this work is illustrated for dolmen and heptamer nanostructures. Fano-resonant systems have the potential to act as very efficient refractive index sensing platforms compared to Lorentz-resonant systems, due to control of their radiative losses. This study paves the way toward sensitive nanoscale biochemical sensors and immunoassays with a low limit of detection and, in general, any nano-optical device where Ohmic losses limit the performance.

Schottky-contact plasmonic dipole rectenna concept for biosensing

Nanoantennas are key optical components for several applications including photodetection and biosensing. Here we present an array of metal nano-dipoles supporting surface plasmon polaritons (SPPs) integrated into a silicon-based Schottky-contact photodetector. Incident photons coupled to the array excite SPPs on the Au nanowires of the antennas which decay by creating "hot" carriers in the metal. The hot carriers may then be injected over the potential barrier at the Au-Si interface resulting in a photocurrent. High responsivities of 100 mA/W and practical minimum detectable powers of -12 dBm should be achievable in the infra-red (1310 nm). The device was then investigated for use as a biosensor by computing its bulk and surface sensitivities. Sensitivities of ∼ 250 nm/RIU (bulk) and ∼ 8 nm/nm (surface) in water are predicted. We identify the mode propagating and resonating along the nanowires of the antennas, we apply a transmission line model to describe the performance of the antennas, and we extract two useful formulas to predict their bulk and surface sensitivities. We prove that the sensitivities of dipoles are much greater than those of similar monopoles and we show that this difference comes from the gap in dipole antennas where electric fields are strongly enhanced.

Universal scaling of plasmonic refractive index sensors

We establish experimental and numerical evidence that the refractive index sensitivities of various subwavelength plasmonic sensors obey a simple universal scaling relation that the sensitivities linearly increase with λm/neff (where λm is the resonant wavelengths and neff is the effective refractive index of the environment) and exhibit a slope equal to 1 instead of 2 predicted theoretically. The universal scaling relation is independent of the geometrical structures or contributions of multipolar resonances of individual metal structures (i.e. plasmonic atoms). It is also independent of spatial distributions or field-enhancements of electromagnetic hot spots in coupled metal structures (i.e. plasmonic molecules). The universal scaling relation reveals the fundamental standing wave resonances for all plasmonic atoms and the predominant near-field electric couplings for most plasmonic molecules. The established universal relation also helps to exclude some magnetically coupled plasmonic molecules for practical applications due to their reduced sensitivities.

Multimode resonances in silver nanocuboids

A rich variety of dipolar and higher order plasmon resonances have been predicted for nanoscale cubes and parallopipeds of silver, in contrast to the simple dipolar modes found on silver nanospheres or nanorods. However, in general, these multimode resonances are not readily detected in experimental colloidal ensembles, due primarily to the usual variation of size and shape of the particles obscuring or blending the individual extinction peaks. Recently, methods have been found to prepare silver parallopipeds with unprecedented shape control by nucleating the silver onto a tightly controlled suspension of gold nanorods (Okuno, Y.; Nishioka, K.; Kiya, A.; Nakashima, N.; Ishibashi, A.; Niidome, Y. Uniform and Controllable Preparation of Au-Ag Core-Shell Nanorods Using Anisotropic Silver Shell Formation on Gold Nanorods. Nanoscale 2010, 2, 1489-1493). The optical extinction spectra of suspensions of such monodisperse particles are found to contain multiple extinction peaks, which we show here to be due to the multimode resonances predicted by theoretical studies. Control of the radius of the nanoparticle edges is found to be an effective way to turn some of these modes on or off. These nanoparticles provide a flexible platform for the excitation, manipulation, and exploration of higher order plasmon resonances.

Teardrop-shaped surface-plasmon resonators

A new surface plasmon resonator, in the form of a metal cylinder shaped like a teardrop, is proposed and modeled using hypersingular integral equations. Illuminating the apex of the teardrop along its axis of symmetry with TM-polarised light excites equal-magnitude surface plasmons counter-propagating around its periphery. Interference of these counter-propagating surface plasmons leads to resonant modes having very sharp line-widths (~0.6 nm) and large quality factors (~3400). In water, a Au teardrop supports resonant modes having large bulk sensitivities (~1900 nm/RIU) and remarkably large figures-of-merit (~1700), about 1000 × larger than the figure-of-merit of, e.g., isolated Au nano-spheres. The scattering cross-section of the structure near resonant modes exhibits a Fano-like line-shape. The teardrop should find applications as a high-quality surface plasmon resonator or as a biosensing element.

Enhanced optical trapping and arrangement of nano-objects in a plasmonic nanocavity

Gentle manipulation of micrometer-sized dielectric objects with optical forces has found many applications in both life and physical sciences. To further extend optical trapping toward the true nanometer scale, we present an original approach combining self-induced back action (SIBA) trapping with the latest advances in nanoscale plasmon engineering. The designed resonant trap, formed by a rectangular plasmonic nanopore, is successfully tested on 22 nm polystyrene beads, showing both single- and double-bead trapping events. The mechanism responsible for the higher stability of the double-bead trapping is discussed, in light of the statistical analysis of the experimental data and numerical calculations. Furthermore, we propose a figure of merit that we use to quantify the achieved trapping efficiency and compare it to prior optical nanotweezers. Our approach may open new routes toward ultra-accurate immobilization and arrangement of nanoscale objects, such as biomolecules.

High-resolution biosensor based on localized surface plasmons

We report on a new biosensor with localized surface plasmons (LSP) based on an array of gold nanorods and the total internal reflection imaging in polarization contrast. The sensitivity of the new biosensor is characterized and a model detection of DNA hybridization is carried out. The results are compared with a reference experiment using a conventional high-resolution surface plasmon resonance (SPR) biosensor. We show that the LSP-based biosensor delivers the same performance as the SPR system while involving significantly lower surface densities of interacting molecules. We demonstrate a limit of detection of 100 pM and a surface density resolution of only 35 fg×mm-2 that corresponds to less than one DNA molecule per nanoparticle on average.