Improving the Instrumental Resolution of Sensors Based on Localized Surface Plasmon Resonance

Neural network enabled nanoplasmonic hydrogen sensors with 100 ppm limit of detection in humid air

Environmental humidity variations are ubiquitous and high humidity characterizes fuel cell and electrolyzer operation conditions. Since hydrogen-air mixtures are highly flammable, humidity tolerant H2 sensors are important from safety and process monitoring perspectives. Here, we report an optical nanoplasmonic hydrogen sensor operated at elevated temperature that combined with Deep Dense Neural Network or Transformer data treatment involving the entire spectral response of the sensor enables a 100 ppm H2 limit of detection in synthetic air at 80% relative humidity. This significantly exceeds the <1000 ppm US Department of Energy performance target. Furthermore, the sensors pass the ISO 26142:2010 stability requirement in 80% relative humidity in air down to 0.06% H2 and show no signs of performance loss after 140 h continuous operation. Our results thus demonstrate the potential of plasmonic hydrogen sensors for use in high humidity and how neural-network-based data treatment can significantly boost their performance.

LSPR-Based Biosensing Enables the Detection of Antimicrobial Resistance Genes

The development of rapid, simple, and accurate bioassays for the detection of nucleic acids has received increasing demand in recent years. Here, localized surface plasmon resonance (LSPR) spectroscopy for the detection of an antimicrobial resistance gene, sulfhydryl variable β-lactamase (blaSHV), which confers resistance against a broad spectrum of β-lactam antibiotics is used. By performing limit of detection experiments, a 23 nucleotide (nt) long deoxyribonucleic acid (DNA) sequence down to 25 nm was detected, whereby the signal intensity is inversely correlated with sequence length (23, 43, 63, and 100 nt). In addition to endpoint measurements of hybridization events, the setup also allowed to monitor the hybridization events in real-time, and consequently enabled to extract kinetic parameters of the studied binding reaction. Performing LSPR measurements using single nucleotide polymorphism (SNP) variants of blaSHV revealed that these sequences can be distinguished from the fully complementary sequence. The possibility to distinguish such sequences is of utmost importance in clinical environments, as it allows to identify mutations essential for enzyme function and thus, is crucial for the correct treatment with antibiotics. Taken together, this system provides a robust, label-free, and cost-efficient analytical tool for the detection of nucleic acids and will enable the surveillance of antimicrobial resistance determinants.

pH-Modulated Nanoarchitectonics for Enhancement of Multivalency-Induced Vesicle Shape Deformation at Receptor-Presenting Lipid Membrane Interfaces

Multivalent ligand-receptor interactions between receptor-presenting lipid membranes and ligand-modified biological and biomimetic nanoparticles influence cellular entry and fusion processes. Environmental pH changes can drive these membrane-related interactions by affecting membrane nanomechanical properties. Quantitatively, however, the corresponding effects on high-curvature, sub-100 nm lipid vesicles are scarcely understood, especially in the multivalent binding context. Herein, we employed the label-free localized surface plasmon resonance (LSPR) sensing technique to track the multivalent attachment kinetics, shape deformation, and surface coverage of biotin ligand-functionalized, zwitterionic lipid vesicles with different ligand densities on a streptavidin receptor-coated supported lipid bilayer under varying pH conditions (4.5, 6, 7.5). Our results demonstrate that more extensive multivalent interactions caused greater vesicle shape deformation across the tested pH conditions, which affected vesicle surface packing as well. Notably, there were also pH-specific differences, i.e., a higher degree of vesicle shape deformation was triggered at a lower multivalent binding energy in pH 4.5 than in pH 6 and 7.5 conditions. These findings support that the nanomechanical properties of high-curvature lipid membranes, especially the membrane bending energy and the corresponding responsiveness to multivalent binding interactions, are sensitive to solution pH, and indicate that multivalency-induced vesicle shape deformation occurs slightly more readily in acidic pH conditions relevant to biological environments.

Unraveling How Cholesterol Affects Multivalency-Induced Membrane Deformation of Sub-100 nm Lipid Vesicles

Cholesterol plays a critical role in modulating the lipid membrane properties of biological and biomimetic systems and recent attention has focused on its role in the functions of sub-100 nm lipid vesicles and lipid nanoparticles. These functions often rely on multivalent ligand-receptor interactions involving membrane attachment and dynamic shape transformations while the extent to which cholesterol can influence such interaction processes is largely unknown. To address this question, herein, we investigated the attachment of sub-100 nm lipid vesicles containing varying cholesterol fractions (0-45 mol %) to membrane-mimicking supported lipid bilayer (SLB) platforms. Biotinylated lipids and streptavidin proteins were used as model ligands and receptors, respectively, while the localized surface plasmon resonance sensing technique was employed to track vesicle attachment kinetics in combination with analytical modeling of vesicle shape changes. Across various conditions mimicking low and high multivalency, our findings revealed that cholesterol-containing vesicles could bind to receptor-functionalized membranes but underwent appreciably less multivalency-induced shape deformation than vesicles without cholesterol, which can be explained by a cholesterol-mediated increase in membrane bending rigidity. Interestingly, the extent of vesicle deformation that occurred in response to increasingly strong multivalent interactions was less pronounced for vesicles with greater cholesterol fraction. The latter trend was rationalized by taking into account the strong dependence of the membrane bending energy on the area of the vesicle-SLB contact region and such insights can aid the engineering of membrane-enveloped nanoparticles with tailored biophysical properties.

Magnetoplasmonics beyond Metals: Ultrahigh Sensing Performance in Transparent Conductive Oxide Nanocrystals

Active modulation of the plasmonic response is at the forefront of today's research in nano-optics. For a fast and reversible modulation, external magnetic fields are among the most promising approaches. However, fundamental limitations of metals hamper the applicability of magnetoplasmonics in real-life active devices. While improved magnetic modulation is achievable using ferromagnetic or ferromagnetic-noble metal hybrid nanostructures, these suffer from severely broadened plasmonic response, ultimately decreasing their performance. Here we propose a paradigm shift in the choice of materials, demonstrating for the first time the outstanding magnetoplasmonic performance of transparent conductive oxide nanocrystals with plasmon resonance in the near-infrared. We report the highest magneto-optical response for a nonmagnetic plasmonic material employing F- and In-codoped CdO nanocrystals, due to the low carrier effective mass and the reduced plasmon line width. The performance of state-of-the-art ferromagnetic nanostructures in magnetoplasmonic refractometric sensing experiments are exceeded, challenging current best-in-class localized plasmon-based approaches.

Dual Gold-Nanoslit Electrodes for Ultrasensitive Detection of Antigen-Antibody Reactions in Electrochemical Surface Plasmon Resonance

We present the use of surface charges in dual gold-nanoslit electrodes to improve the surface plasmon resonance (SPR) detection limit by several orders of magnitude. The SPR is directly generated by gold-nanoslit arrays in the two electrodes. The SPR shifts for both nanoslit arrays are measured simultaneously with a simple hyperspectral setup. When biomolecules are captured by specific antibodies on the dual electrodes, the surface charge is changed during the electrochemical process due to the increase in surface impedance. The push-pull-type electrodes generate opposite surface charges. Using the differences in both spectral shifts, the change in surface charge is detected sensitively. We demonstrate that using a [Fe(CN)₆]^3-/4- redox process after antigen-antibody interactions, the dual nanoslit electrodes show an enhancement of the detection limit from 1 μg/mL to 10 pg/mL.

Fundamentals of Biosensors and Detection Methods

Biosensors have a great impact on our society to enhance the life quality, playing an important role in the development of Point-of-Care (POC) technologies for rapid diagnostics, and monitoring of disease progression. COVID-19 rapid antigen tests, home pregnancy tests, and glucose monitoring sensors represent three examples of successful biosensor POC devices. Biosensors have extensively been used in applications related to the control of diseases, food quality and safety, and environment quality. They can provide great specificity and portability at significantly reduced costs. In this chapter are described the fundamentals of biosensors including the working principles, general configurations, performance factors, and their classifications according to the type of bioreceptors and transducers. It is also briefly illustrated the general strategies applied to immobilize biorecognition elements on the transducer surface for the construction of biosensors. Moreover, the principal detection methods used in biosensors are described, giving special emphasis on optical, electrochemical, and mass-based methods. Finally, the challenges for biosensing in real applications are addressed at the end of this chapter.

How to Use Localized Surface Plasmon for Monitoring the Adsorption of Thiol Molecules on Gold Nanoparticles?

The functionalization of spherical gold nanoparticles (AuNPs) in solution with thiol molecules is essential for further developing their applications. AuNPs exhibit a clear localized surface plasmon resonance (LSPR) at 520 nm in water for 20 nm size nanoparticles, which is extremely sensitive to the local surface chemistry. In this study, we revisit the use of UV-visible spectroscopy for monitoring the LSPR peak and investigate the progressive reaction of thiol molecules on 22 nm gold nanoparticles. FTIR spectroscopy and TEM are used for confirming the nature of ligands and the nanoparticle diameter. Two thiols are studied: 11-mercaptoundecanoic acid (MUDA) and 16-mercaptohexadecanoic acid (MHDA). Surface saturation is detected after adding 20 nmol of thiols into 1.3 × 10⁻³ nmol of AuNPs, corresponding approximately to 15,000 molecules per AuNPs (which is equivalent to 10.0 molecules per nm²). Saturation corresponds to an LSPR shift of 2.7 nm and 3.9 nm for MUDA and MHDA, respectively. This LSPR shift is analyzed with an easy-to-use analytical model that accurately predicts the wavelength shift. The case of dodecanehtiol (DDT) where the LSPR shift is 15.6 nm is also quickly commented. An insight into the kinetics of the functionalization is obtained by monitoring the reaction for a low thiol concentration, and the reaction appears to be completed in less than one hour.

Solvent-induced conformational tuning of lysozyme protein adlayers on silica surfaces: A QCM-D and LSPR study

There is broad interest in functionalizing solid surfaces with lysozyme, which is a widely studied antimicrobial protein. To date, most efforts have focused on developing more effective immobilization schemes to promote lysozyme attachment in fully aqueous conditions, while there remains an outstanding need to understand how tuning the solution-phase conformational stability of lysozyme proteins can modulate adsorption behavior and resulting adlayer properties. Inspired by the unique conformational behavior of lysozyme proteins in water-ethanol mixtures, we conducted quartz crystal microbalance-dissipation (QCM-D) and localized surface plasmon resonance (LSPR) measurements to systematically investigate the adsorption behavior of lysozyme proteins onto silica surfaces across a wide range of water-ethanol mixtures. Our findings revealed that lysozyme adsorption behavior strongly depended on the ethanol fraction in a non-monotonic fashion and this trend could be rationalized by taking into account how competing effects of water and ethanol solvation influence solution-phase protein size and conformational stability. Integrated analysis of the QCM-D and LSPR measurement trends enabled quantitative determination of the solvent mass within lysozyme adlayers, which tended to decrease at higher ethanol fractions and supported that the hydrodynamic properties of lysozyme adlayers are mainly influenced by the degree of protein conformational flexibility as opposed to solvation effects alone.

Comparing Protein Adsorption onto Alumina and Silica Nanomaterial Surfaces: Clues for Vaccine Adjuvant Development

Protein adsorption onto nanomaterial surfaces is important for various nanobiotechnology applications such as biosensors and drug delivery. Within this scope, there is growing interest to develop alumina- and silica-based nanomaterial vaccine adjuvants and an outstanding need to compare protein adsorption onto alumina- and silica-based nanomaterial surfaces. Herein, using alumina- and silica-coated arrays of silver nanodisks with plasmonic properties, we conducted localized surface plasmon resonance (LSPR) experiments to evaluate real-time adsorption of bovine serum albumin (BSA) protein onto alumina and silica surfaces. BSA monomers and oligomers were prepared in different water-ethanol mixtures and both adsorbing species consistently showed quicker adsorption kinetics and more extensive adsorption-related spreading on alumina surfaces as compared to on silica surfaces. We rationalized these experimental observations in terms of the electrostatic forces governing protein-surface interactions on the two nanomaterial surfaces and the results support that more rigidly attached BSA protein-based coatings can be formed on alumina-based nanomaterial surfaces. Collectively, the findings in this study provide fundamental insight into protein-surface interactions at nanomaterial interfaces and can help to guide the development of protein-based coatings for medical and biotechnology applications such as vaccines.

Plasmonic Temperature-Programmed Desorption

Temperature-programmed desorption (TPD) allows for the determination of the bonding strength and coverage of molecular mono- or multilayers on a surface and is widely used in surface science. In its traditional form using a mass spectrometric readout, this information is derived indirectly by analysis of resulting desorption peaks. This is problematic because the mass spectrometer signal not only originates from the sample surface but also potentially from other surfaces in the measurement chamber. As a complementary alternative, we introduce plasmonic TPD, which directly measures the surface coverage of molecular species adsorbed on metal nanoparticles at ultrahigh vacuum conditions. Using the examples of methanol and benzene on Au nanoparticle surfaces, the method can resolve all relevant features in the submonolayer and multilayer regimes. Furthermore, it enables the study of two types of nanoparticles simultaneously, which is challenging in a traditional TPD experiment, as we demonstrate specifically for Au and Ag.

Ultrasensitive Surface Refractive Index Imaging Based on Quasi-Bound States in the Continuum

Herein, we demonstrate a cavity-enhanced hyperspectral refractometric imaging using an all-dielectric photonic crystal slab (PhCS). Our approach takes advantage of the synergy between two mechanisms, surface-enhanced fluorescence (SEF) and refractometric sensing, both based on high-Q resonances in proximity of bound states in the continuum (BICs). The enhanced local optical field of the first resonance amplifies of 2 orders of magnitude the SEF emission of a probe dye. Simultaneously, hyperspectral refractometric sensing, based on Fano interference between second mode and fluorescence emission, is used for mapping the spatially variant refractive index produced by the specimen on the PhCS. The spectral matching between first resonance and input laser is modulated by the specimen local refractive index, and thanks to the calibrated dependence with the spectral shift of the Fano resonance, the cavity tuning is used to achieve an enhanced correlative refractometric map with a resolution of 10^-5 RIU within femtoliter-scale sampling volumes. This is experimentally applied also on live prostate cancer cells grown on the PhCS, reconstructing enhanced surface refractive index images at the single-cell level. This dual mechanism of quasi-BIC spatially variant gain tracked by quasi-BIC refractometric sensing provides a correlative imaging platform that can find application in many fields for monitoring physical and biochemical processes, such as molecular interactions, chemical reactions, or surface cell analysis.

Optical Carbon Dioxide Detection in the Visible Down to the Single Digit ppm Range Using Plasmonic Perfect Absorbers

To tackle climate change and reduce CO₂ emissions, it is important to measure CO₂ output precisely. Even though there are many different techniques, no simple and cheap optical method in the visible is available. This work studies plasmonically enhanced optical carbon dioxide sensors in the visible wavelength range. The sensor samples are based on an inert plasmonic perfect absorber, which can be easily and cheaply fabricated by colloidal etching lithography. A CO₂-sensitive polyethylenimine (PEI) layer is then spin-coated on top to complete the samples. The samples are examined continuously by microspectroscopy during different CO₂ exposures to track spectral changes, particularly the position of the resonance centroid wavelength. The samples exhibit a resonance shift of up to 7 nm, depending on the CO₂ concentration and the temperature. The temperature influences the rise time as well as the sensitive concentration range. The concentration dependence of the resonance shift overall follows the shape of a Langmuir isotherm, which includes a nearly linear relation at lower concentrations and elevated temperatures and a saturating behavior at higher concentrations and lower temperatures. The results indicate that a sensitivity in the full range from 100 vol % to below 1 ppm can be achieved. The samples degenerate in a dry inert atmosphere in a matter of days but are useable over multiple weeks when exposed to humidity and CO₂. The PEI reacts very selectively to CO₂, showing no response to CO, NH₃, NO₂, CH₄, H₂, and only a very small response to O₂. Overall, polyethylenimine is very promising as a CO₂-sensitive material for many practical sensing applications over a wide range of concentrations. An adjustment of the temperature is mandatory to control the sensitivity and response time.

Dark-field hyperspectral imaging for label free detection of nano-bio-materials

Nanomaterials are playing an increasingly important role in cancer diagnosis and treatment. Nanoparticle (NP)-based technologies have been utilized for targeted drug delivery during chemotherapies, photodynamic therapy, and immunotherapy. Another active area of research is the toxicity studies of these nanomaterials to understand the cellular uptake and transport of these materials in cells, tissues, and environment. Traditional techniques such as transmission electron microscopy, and mass spectrometry to analyze NP-based cellular transport or toxicity effect are expensive, require extensive sample preparation, and are low-throughput. Dark-field hyperspectral imaging (DF-HSI), an integration of spectroscopy and microscopy/imaging, provides the ability to investigate cellular transport of these NPs and to quantify the distribution of them within bio-materials. DF-HSI also offers versatility in non-invasively monitoring microorganisms, single cell, and proteins. DF-HSI is a low-cost, label-free technique that is minimally invasive and is a viable choice for obtaining high-throughput quantitative molecular analyses. Multimodal imaging modalities such as Fourier transform infrared and Raman spectroscopy are also being integrated with HSI systems to enable chemical imaging of the samples. HSI technology is being applied in surgeries to obtain molecular information about the tissues in real-time. This article provides brief overview of fundamental principles of DF-HSI and its application for nanomaterials, protein-detection, single-cell analysis, microbiology, surgical procedures along with technical challenges and future integrative approach with other imaging and measurement modalities. This article is categorized under: Diagnostic Tools > in vitro Nanoparticle-Based Sensing Diagnostic Tools > in vivo Nanodiagnostics and Imaging Implantable Materials and Surgical Technologies > Nanoscale Tools and Techniques in Surgery.

A miRNA biosensor based on localized surface plasmon resonance enhanced by surface-bound hybridization chain reaction

The dysregulation of the concentration of individual circulating microRNAs or small sets of them has been recognized as a marker of disease. For example, an increase of the concentration of circulating miR-17 has been linked to lung cancer and metastatic breast cancer, while its decrease has been found in multiple sclerosis and gastric cancer. Consequently, techniques for the fast, specific and simple quantitation of microRNAs are becoming crucial enablers of early diagnosis and therapeutic follow-up. DNA based biosensors can serve this purpose, overcoming some of the drawbacks of conventional lab-based techniques. Herein, we report a cost-effective, simple and robust biosensor based on localized surface plasmon resonance and hybridization chain reaction. Immobilized gold nanoparticles are used for the detection of miR-17. Specificity of the detection was achieved by the use of hairpin surface-tethered probes and the hybridization chain reaction was used to amplify the detection signal and thus extend the dynamic range of the quantitation. Less than 1 h is needed for the entire procedure that achieved a limit of detection of about 1 pM or 50 amol/measurement, well within the reported useful range for diagnostic applications. We suggest that this technology could be a promising substitute of traditional lab-based techniques for the detection and quantification of miRNAs after these are extracted from diagnostic specimens and their analysis is thus made possible.

DNA-Biofunctionalization of CTAC-Capped Gold Nanocubes

Clinical diagnostics and disease control are fields that strongly depend on technologies for rapid, sensitive, and selective detection of biological or chemical analytes. Nanoparticles have become an integral part in various biomedical detection devices and nanotherapeutics. An increasing focus is laid on gold nanoparticles as they express less cytotoxicity, high stability, and hold unique optical properties with the ability of signal transduction of biological recognition events with enhanced analytical performance. Strong electromagnetic field enhancements can be found in close proximity to the nanoparticle that can be exploited to enhance signals for e.g., metal-enhanced fluorescence or Raman spectroscopy. Even stronger field enhancements can be achieved with sharp-edged nanoparticles, which are synthesized with the help of facet blocking agents, such as cetyltrimethylammonium bromide/chloride (CTAB/CTAC). However, chemical modification of the nanoparticle surface is necessary to reduce the particle’s cytotoxicity, stabilize it against aggregation, and to bioconjugate it with biomolecules to increase its biocompatibility and/or specificity for analytical applications. Here, a reliable two-step protocol following a ligand exchange with bis (p-sulfonatophenyl) phenyl phosphine (BSPP) as the intermediate capping-agent is demonstrated, which results in the reliable biofunctionalization of CTAC-capped gold nanocubes with thiol-modified DNA. The functionalized nanocubes have been characterized regarding their electric potential, plasmonic properties, and stability against high concentrations of NaCl and MgCl₂.

Plasmonic biosensors relying on biomolecular conformational changes: Case of odorant binding proteins

Plasmonic nanostructures serve in a range of analytical techniques that were developed for the analysis of chemical and biological species. Among others, they have been pursued for the investigation of odorant binding proteins (OBP) and their interaction with odorant molecules. These compounds are low molecular weight agents, which makes their direct detection with conventional surface plasmon resonance (SPR) challenging. Therefore, other plasmonic sensor modalities need to be implemented for the detection and interaction analysis of OBPs. This chapter provides a guide for carrying out such experiments based on two techniques that take advantage of conformation changes of OBPs occurring upon specific interaction with their affinity partners. First, there is discussed SPR monitoring of conformation changes of biomolecules that are not accompanied by a strong increase in the surface mass density but rather with its re-distribution perpendicular to the surface. Second, the implementation of surface plasmon-enhanced fluorescence energy transfer is presented for the sensitive monitoring of conformational changes of biomolecules tagged with a fluorophore at its defined part. Examples from our and other laboratories illustrate the performance of these concepts and their applicability for the detection of low molecular weight odorant molecules by the use of OBPs attached to the sensor surface is discussed.

Optimization of Au:CuO Nanocomposite Thin Films for Gas Sensing with High-Resolution Localized Surface Plasmon Resonance Spectroscopy

Gas sensing based on bulk refractive index (RI) changes, has been a challenging task for localized surface plasmon resonance (LSPR) spectroscopy, presenting only a limited number of reports in this field. In this work, it is demonstrated that a plasmonic thin film composed of Au nanoparticles embedded in a CuO matrix can be used to detect small changes (as low as 6 × 10^-5 RIU) in bulk RI of gases at room temperature, using a high-resolution LSPR spectroscopy system. To optimize the film's surface, a simple Ar plasma treatment revealed to be enough to remove the top layers of the film and to partially expose the embedded nanoparticles, and thus enhance the film's gas sensing capabilities. The treated sample exhibits high sensitivity to inert gases (Ar, N₂), presenting a refractive index sensitivity (RIS) to bulk RI changes of 425 nm/RIU. Furthermore, a 2-fold signal increase is observed for O₂, showing that the film is clearly more sensitive to this gas due to its oxidizing nature. The results showed that the Au:CuO thin film system is a RI sensitive platform able to detect inert gases, which can be more sensitive to detect noninert gases as O₂ or even other reactive species.

Enhanced Localized Surface Plasmon Resonance of Fully Alloyed AgAuPdPt, AgAuPt, AuPt, AgPt, and Pt Nanocrystals: Systematical Investigation on the Morphological and LSPR Properties of Mono Bi-, Tri-, and Quad-Metallic Nanoparticles

Multi-metallic alloy nanoparticles (NPs) can offer tunable or modifiable localized surface plasmon resonance (LSPR) properties depending upon their configurational and elemental alterations, which can be utilized in various applications, that is, in photon energy harvesting, optical sensing, biomedical imaging, photocatalysis, and spectroscopy. In this work, a systematic investigation on the morphological and LSPR properties of multi-metallic alloy NPs incorporating Ag, Au, Pd, and Pt is presented on c-plane sapphire (0001). The resulting NPs exhibit much enhanced and tunable LSPR bands in the UV-VIS wavelength as compared to the previously reported mono-metallic NPs based on the considerable improvement in size and shape of nanostructures along with the electronic heterogeneity. Solid-state dewetting of sputtered bilayers (Ag/Pt), tri-layers (Ag/Au/Pt), and quad-layers (Ag/Au/Pd/Pt) is employed to demonstrate a wide variety of configurations, sizes, densities, and elemental compositions of Pt, AgPt, AuPt, AgAuPt, AgAuPt, and AgAuPdPt NPs by the systematic control of annealing temperature and deposition schemes. The distinct morphology and elemental composition of surface nanostructures are obtained by means of surface diffusion, intermixing, and surface/interface energy minimization along with the applied thermal energy. In addition, the sublimation of Ag atoms from the alloy nanostructure matrix significantly influences the structural, elemental, and thus optical properties of NPs by reducing the average size and Ag percentage in the alloy NPs. Based on the specific size, shape, and elemental composition of NPs, the excitation of LSPR is correlated to the dipolar, quadrupolar, multi-polar, and higher order (HO) modes along with the finite difference time domain simulation of local electric-field. The LSPR intensity is generally stronger with a higher percentage of Ag atoms in the alloy NPs and gradually diminished by the sublimation loss. However, even the mono-metallic and alloy NPs without Ag exhibited significantly improved and dynamic nature of plasmonic bands in the UV and VIS wavelength.

Resonant Plasmonic Nanoslits Enable in Vitro Observation of Single-Monolayer Collagen-Peptide Dynamics

Proteins perform a variety of essential functions in living cells and thus are of critical interest for drug delivery as well as disease biomarkers. The different functions are derived from a hugely diverse set of structures, fueling interest in their conformational states. Surface-enhanced infrared absorption spectroscopy has been utilized to detect and discriminate protein monomers. As an important step forward, we are investigating collagen peptides consisting of  a  triple helix. While they constitute the main structural building blocks in many complex proteins, they are also a perfect model system for the complex proteins relevant in biological systems. Their complex spectroscopic information as well as the overall small size present a significant challenge for their detection and discrimination. Using resonant plasmonic nanoslits, which are known to show larger specificity compared to nanoantennas, we overcome this challenge. We perform in vitro surface-enhanced absorption spectroscopy studies and track the conformational changes of these collagen peptides under two different external stimuli, which are temperature and chemical surroundings. Modeling the coupling between the amide I vibrational modes and the plasmonic resonance, we can extract the conformational state of the collages and thus monitor the folding and unfolding dynamics of even a single monolayer. This leads to new prospects in studies of single layers of proteins and their folding behavior in minute amounts in a living environment.

Influence of NaCl Concentration on Bicelle-Mediated SLB Formation

The deposition of two-dimensional bicellar disks on hydrophilic surfaces is an emerging approach to fabricate supported lipid bilayers (SLBs) that requires minimal sample preparation, works at low lipid concentrations, and yields high-quality SLBs. While basic operating steps in the fabrication protocol mimic aspects of the conventional vesicle fusion method, lipid bicelles and vesicles have distinct architectural properties, and understanding how experimental parameters affect the efficiency of bicelle-mediated SLB formation remains to be investigated. Herein, using the quartz crystal microbalance-dissipation and localized surface plasmon resonance techniques, we investigated the effect of bulk NaCl concentration on bicelle-mediated SLB formation on silicon dioxide surfaces. For comparison, similar experiments were conducted with vesicles as well. In both cases, SLB formation was observed to occur rapidly provided that the NaCl concentration was sufficiently high (>50 mM). Under such conditions, the effect of NaCl concentration on SLB formation was minor in the case of bicelles and significant in the case of vesicles where it is expected to be related primarily to osmotic pressure. At lower NaCl concentrations, bicelles also formed SLBs but slowly, whereas adsorbed vesicles remained intact. These findings were complemented by time-lapsed fluorescence microscopy imaging and fluorescence recovery after photobleaching measurements that corroborated bicelle-mediated SLB formation across the range of tested NaCl concentrations. The results are discussed by comparing the architectural properties of bicelles and vesicles along with theoretical analysis of the corresponding adsorption kinetics.

CMOS-compatible plasmonic hydrogen sensors with a detection limit of 40 ppm

Sensing of leakage at an early stage is crucial for the safe utilization of hydrogen. Optical hydrogen sensors eliminate the potential hazard of ignition caused by electrical sparks but achieve a detection limit far higher than their electrical counterparts so far. To essentially improve the performance of optical hydrogen sensors in terms of detection limit, we demonstrate in this work a plasmonic hydrogen sensor based on aluminum-palladium (Al-Pd) hybrid nanorods. Arranged into high-density regular arrays, the hybrid nanorods are capable of sensing hydrogen at a concentration down to 40 ppm, i.e., one thousandth of the lower flammability limit of hydrogen in air. Different sensing behaviors are found for two sensor configurations, where Pd-Al nanorods provide larger spectral shift and Al-Pd ones exhibit shorter response time. In addition, the plasmonic hydrogen sensors here utilize exclusively CMOS-compatible materials, holding the potential for real-world, large-scale applications.

Rationally Designed PdAuCu Ternary Alloy Nanoparticles for Intrinsically Deactivation-Resistant Ultrafast Plasmonic Hydrogen Sensing

Hydrogen sensors are a prerequisite for the implementation of a hydrogen economy due to the high flammability of hydrogen-air mixtures. They are to comply with the increasingly stringent requirements set by stakeholders, such as the automotive industry and manufacturers of hydrogen safety systems, where sensor deactivation is a severe but widely unaddressed problem. In response, we report intrinsically deactivation-resistant nanoplasmonic hydrogen sensors enabled by a rationally designed ternary PdAuCu alloy nanomaterial, which combines the identified best intrinsic attributes of the constituent binary Pd alloys. This way, we achieve extraordinary hydrogen sensing metrics in synthetic air and poisoning gas background, simulating real application conditions. Specifically, we find a detection limit in the low ppm range, hysteresis-free response over 5 orders of magnitude hydrogen pressure, subsecond response time at room temperature, long-term stability, and, as the key, excellent resistance to deactivating species like carbon monoxide, notably without application of any protective coatings. This constitutes an important step forward for optical hydrogen sensor technology, as it enables application under demanding conditions and provides a blueprint for further material and performance optimization by combining and concerting intrinsic material assets in multicomponent nanoparticles. In a wider context, our findings highlight the potential of rational materials design through alloying of multiple elements for gas sensor development, as well as the potential of engineered metal alloy nanoparticles in nanoplasmonics and catalysis.

LSPR Biosensing Approach for the Detection of Microtubule Nucleation

Microtubules are dynamic protein filaments that are involved in a number of cellular processes. Here, we report the development of a novel localized surface plasmon resonance (LSPR) biosensing approach for investigating one aspect of microtubule dynamics that is not well understood, namely, nucleation. Using a modified Mie theory with radially variable refractive index, we construct a theoretical model to describe the optical response of gold nanoparticles when microtubules form around them. The model predicts that the extinction maximum wavelength is sensitive to a change in the local refractive index induced by microtubule nucleation within a few tens of nanometers from the nanoparticle surface, but insensitive to a change in the refractive index outside this region caused by microtubule elongation. As a proof of concept to demonstrate that LSPR can be used for detecting microtubule nucleation experimentally, we induce spontaneous microtubule formation around gold nanoparticles by immobilizing tubulin subunits on the nanoparticles. We find that, consistent with the theoretical model, there is a redshift in the extinction maximum wavelength upon the formation of short microtubules around the nanoparticles, but no significant change in maximum wavelength when the microtubules are elongated. We also perform kinetic experiments and demonstrate that the maximum wavelength is sensitive to the microtubule nuclei assembly even when microtubules are too small to be detected from an optical density measurement.

Heterodimers for in Situ Plasmonic Spectroscopy: Cu Nanoparticle Oxidation Kinetics, Kirkendall Effect, and Compensation in the Arrhenius Parameters

The ability to study oxidation, reduction, and other chemical transformations of nanoparticles in real time and under realistic conditions is a nontrivial task due to their small dimensions and the often challenging environment in terms of temperature and pressure. For scrutinizing oxidation of metal nanoparticles, visible light optical spectroscopy based on the plasmonic properties of the metal has been established as a suitable method. However, directly relying on the plasmonic resonance of metal nanoparticles as a built-in probe to track oxidation has a number of drawbacks, including the loss of optical contrast in the late oxidation stages. To address these intrinsic limitations, we present a plasmonic heterodimer-based nanospectroscopy approach, which enables continuous self-referencing by using polarized light to eliminate parasitic signals and provides large optical contrast all the way to complete oxidation. Using Au-Cu heterodimers and combining experiments with finite-difference time-domain simulations, we quantitatively analyze the oxidation kinetics of ca. 30 nm sized Cu nanoparticles up to complete oxidation. Taking the Kirkendall effect into account, we extract the corresponding apparent Arrhenius parameters at various extents of oxidation and find that they exhibit a significant compensation effect, implying that changes in the oxidation mechanism occur as oxidation progresses and the structure of the formed oxide evolves. In a wider perspective, our work promotes the use of model-system-type in situ optical plasmonic spectroscopy experiments in combination with electrodynamics simulations to quantitatively analyze and mechanistically interpret oxidation of metal nanoparticles and the corresponding kinetics in demanding chemical environments, such as in heterogeneous catalysis.

Gas identification with graphene plasmons

Identification of gas molecules plays a key role a wide range of applications extending from healthcare to security. However, the most widely used gas nano-sensors are based on electrical approaches or refractive index sensing, which typically are unable to identify molecular species. Here, we report label-free identification of gas molecules SO₂, NO₂, N₂O, and NO by detecting their rotational-vibrational modes using graphene plasmon. The detected signal corresponds to a gas molecule layer adsorbed on the graphene surface with a concentration of 800 zeptomole per μm², which is made possible by the strong field confinement of graphene plasmons and high physisorption of gas molecules on the graphene nanoribbons. We further demonstrate a fast response time (<1 min) of our devices, which enables real-time monitoring of gaseous chemical reactions. The demonstration and understanding of gas molecule identification using graphene plasmonic nanostructures open the door to various emerging applications, including in-breath diagnostics and monitoring of volatile organic compounds.

Plasmonic Nanosensor Array for Multiplexed DNA-based Pathogen Detection

In this research we introduce a plasmonic nanoparticle based optical biosensor for monitoring of molecular binding events. The sensor utilizes spotted gold nanoparticle arrays as sensing platform. The nanoparticle spots are functionalized with capture DNA sequences complementary to the analyte (target) DNA. Upon incubation with the target sequence, it will bind on the respectively complementary functionalized particle spot. This binding changes the local refractive index, which is detected spectroscopically as the resulting changes of the localized surface plasmon resonance (LSPR) peak wavelength. In order to increase the signal, a small gold nanoparticle label is introduced. The binding can be reversed using chemical means (10 mM HCl). It is demonstrated that multiplexed detection and identification of several fungal pathogen DNA sequences subsequently on one sensor array are possible by this approach.

Empowering Electroless Plating to Produce Silver Nanoparticle Films for DNA Biosensing Using Localized Surface Plasmon Resonance Spectroscopy

To facilitate the implementation of biosensors based on the localized surface plasmon resonance (LSPR) of metal nanostructures, there is a great need for cost-efficient, flexible, and tunable methods for producing plasmonic coatings. Due to its simplicity and excellent conformity, electroless plating (EP) is well suited for this task. However, it is traditionally optimized to produce continuous metal films, which cannot be employed in LSPR sensors. Here, we outline the development of an EP strategy for depositing island-like silver nanoparticle (NP) films on glass with distinct LSPR bands. The fully wet-chemical process only employs standard chemicals and proceeds within minutes at room temperature. The key step for producing spread-out NP films is an accelerated ripening of the silver seed layer in diluted hydrochloric acid, which reduces the nucleation density during plating. The reaction kinetics and mechanisms are investigated with scanning (transmission) electron microscopy (SEM/STEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy, with the latter enabling a convenient live monitoring of the deposition, allowing its termination at a stage of desired optical properties. The sensing capabilities of chemically deposited NP films as LSPR transducers are exemplified in DNA biosensing. To this end, a sensing interface is prepared using layer-by-layer (LbL) buildup of polyelectrolytes (PE), followed by adsorption and covalent immobilization of ssDNA. The obtained LSPR transducers demonstrate robustness and selectivity in sensing experiments with binding complementary and unrelated DNA strands.

Fabrication of micro-patterned substrates for plasmonic sensing by piezo-dispensing of colloidal nanoparticles

In this work we describe a very fast and flexible method for fabrication of plasmon-supporting substrates with micro-patterning capability, which is optimized for plasmonic sensing. We combined a wet chemistry approach to synthesize metallic nanoparticles with a piezo-dispensing system enabling deposition of nanoparticles on the substrates with micrometer precision. In this way, an arbitrary pattern consisting of 200 μm small spots containing plasmonic nanostructures can be produced. Patterns with various nanoparticles exhibiting different plasmonic properties were combined, and the surface density of the particles could be easily varied via their solution concentrations. We showed that under controlled conditions the dispensing process caused no aggregation of the particles and it enabled full transfer of the colloidal solutions onto the substrate. This is an important condition, which enables these substrates to be used for reliable plasmonic sensing based on monitoring the spectral shift of the nanoparticles. We demonstrated the functionality of such substrates by detection of small protein adsorption on the spots based on plasmon label-free sensing method.

Nonreciprocal hybrid magnetoplasmonics

The Faraday effect describes the phenomenon that a magnetized material can alter the polarization state of transmitted light. Interestingly, unlike most light-matter interactions in nature, it breaks Lorentz reciprocity. This exceptional behavior is utilized for applications such as optical isolators, which are core elements in communication and laser systems. While there is high demand for sub-micron nonreciprocal photonic devices, the realization of such systems is extremely challenging as conventional magneto-optic materials only provide weak magneto-optic response within small volumes. Plasmonics could be a key to overcome this hurdle in the future: over the last years there have been several lines of work demonstrating that different types of metallic nanostrutures can be utilized to greatly enhance the magneto-optic response of conventional materials. In this review we give an overview over the state of the art in the field and highlight recent developments on hybrid plasmonic Faraday rotators. Our discussions are mainly focused on the visible and near-infrared wavelength regions and cover both experimental realizations as well as analytical descriptions. Special attention will be paid to recent developments on hybrid plasmonic thin film systems consisting of gold and europium chalcogenides.

Large-Area Silver-Stibnite Nanoporous Plasmonic Films for Label-Free Biosensing

The development of various plasmonic nanoporous materials has attracted much interest in different areas of research including bioengineering and biosensing because of their large surface area and versatile porous structure. Here, we introduce a novel technique for fabricating silver-stibnite nanoporous plasmonic films. Unlike conventional techniques that are usually used to fabricate nanoporous plasmonic films, we use a room-temperature growth method that is wet-chemistry free, which enables wafer-scale fabrication of nanoporous films on flexible substrates. We show the existence of propagating surface plasmon polaritons in nanoporous films and demonstrate the extreme bulk refractive index sensitivity of the films using the Goos-Hänchen shift interrogation scheme. In the proof-of-concept biosensing experiments, we functionalize the nanoporous films with biotin-thiol using a modified functionalization technique, to capture streptavidin. The fractal nature of the films increases the overlap between the local field and the immobilized biomolecules. The extreme sensitivity of the Goos-Hänchen shift allows femtomolar concentrations of streptavidin to be detected in real time, which is unprecedented using surface plasmons excited via the Kretschmann configuration.

Temperature-Induced Denaturation of BSA Protein Molecules for Improved Surface Passivation Coatings

Bovine serum albumin (BSA) is the most widely used protein for surface passivation applications, although it has relatively weak, nonsticky interactions with hydrophilic surfaces such as silica-based materials. Herein, we report a simple and versatile method to increase the stickiness of BSA protein molecules adsorbing onto silica surfaces, resulting in up to a 10-fold improvement in blocking efficiency against serum biofouling. Circular dichroism spectroscopy, dynamic light scattering, and nanoparticle tracking analysis showed that temperature-induced denaturation of BSA proteins in bulk solution resulted in irreversible unfolding and protein oligomerization, thereby converting weakly adhesive protein monomers into a more adhesive oligomeric form. The heat-treated, denatured BSA oligomers remained stable after cooling. Room-temperature quartz crystal microbalance-dissipation and localized surface plasmon resonance experiments revealed that denatured BSA oligomers adsorbed more quickly and in larger mass quantities onto silica surfaces than native BSA monomers. We also determined that the larger surface contact area of denatured BSA oligomers is an important factor contributing to their more adhesive character. Importantly, denatured BSA oligomers were a superior passivating agent to inhibit biofouling on silica surfaces and also improved Western blot application performance. Taken together, the findings demonstrate how temperature-induced denaturation of BSA protein molecules can lead to improved protein-based coatings for surface passivation applications.

Nanoplasmonic pillars engineered for single exosome detection

Exosomes are secreted nanovesicles which incorporate proteins and nucleic acids, thereby enabling multifunctional pathways for intercellular communication. There is an increasing appreciation of the critical role they play in fundamental processes such as development, wound healing and disease progression, yet because of their heterogeneous molecular content and low concentrations in vivo, their detection and characterization remains a challenge. In this work we combine nano- and microfabrication techniques for the creation of nanosensing arrays tailored toward single exosome detection. Elliptically–shaped nanoplasmonic sensors are fabricated to accommodate at most one exosome and individually imaged in real time, enabling the label-free recording of digital responses in a highly multiplexed geometry. This approach results in a three orders of magnitude sensitivity improvement over previously reported real-time, multiplexed platforms. Each nanosensor is elevated atop a quartz nanopillar, minimizing unwanted nonspecific substrate binding contributions. The approach is validated with the detection of exosomes secreted by MCF7 breast adenocarcinoma cells. We demonstrate the increasingly digital and stochastic nature of the response as the number of subsampled nanosensors is reduced from four hundred to one.

Enhancing Surface Sensing Sensitivity of Metallic Nanostructures using Blue-Shifted Surface Plasmon Mode and Fano Resonance

Improving surface sensitivities of nanostructure-based plasmonic sensors is an important issue to be addressed. Among the SPR measurements, the wavelength interrogation is commonly utilized. We proposed using blue-shifted surface plasmon mode and Fano resonance, caused by the coupling of a cavity mode (angle-independent) and the surface plasmon mode (angle-dependent) in a long-periodicity silver nanoslit array, to increase surface (wavelength) sensitivities of metallic nanostructures. It results in an improvement by at least a factor of 4 in the spectral shift as compared to sensors operated under normal incidence. The improved surface sensitivity was attributed to a high refractive index sensitivity and the decrease of plasmonic evanescent field caused by two effects, the Fano coupling and the blue-shifted resonance. These concepts can enhance the sensing capability and be applicable to various metallic nanostructures with periodicities.

Hybrid Metamaterial Absorber Platform for Sensing of CO2 Gas at Mid-IR

Application of two major classes of CO2 gas sensors, i.e., electrochemical and nondispersive infrared is predominantly impeded by the poor selectivity and large optical interaction length, respectively. Here, a novel "hybrid metamaterial" absorber platform is presented by integrating the state-of-the-art complementary metal-oxide-semiconductor compatible metamaterial with a smart, gas-selective-trapping polymer for highly selective and miniaturized optical sensing of CO2 gas in the 5-8 µm mid-IR spectral window. The sensor offers a minimum of 40 ppm detection limit at ambient temperature on a small footprint (20 µm by 20 µm), fast response time (≈2 min), and low hysteresis. As a proof-of-concept, net absorption enhancement of 0.0282%/ppm and wavelength shift of 0.5319 nm ppm-1 are reported. Furthermore, the gas- selective smart polymer is found to enable dual-mode multiplexed sensing for crosschecking and validation of gas concentration on a single platform. Additionally, unique sensing characteristics as determined by the operating wavelength and bandwidth are demonstrated. Also, large differential response of the metamaterial absorber platform for all-optical monitoring is explored. The results will pave the way for a physical understanding of metamaterial-based sensing when integrated with the mid-IR detector for readout and extending the mid-IR functionalities of selective polymers for the detection of technologically relevant gases.

Quantitative Comparison of Protein Adsorption and Conformational Changes on Dielectric-Coated Nanoplasmonic Sensing Arrays

Nanoplasmonic sensors are a popular, surface-sensitive measurement tool to investigate biomacromolecular interactions at solid-liquid interfaces, opening the door to a wide range of applications. In addition to high surface sensitivity, nanoplasmonic sensors have versatile surface chemistry options as plasmonic metal nanoparticles can be coated with thin dielectric layers. Within this scope, nanoplasmonic sensors have demonstrated promise for tracking protein adsorption and substrate-induced conformational changes on oxide film-coated arrays, although existing studies have been limited to single substrates. Herein, we investigated human serum albumin (HSA) adsorption onto silica- and titania-coated arrays of plasmonic gold nanodisks by localized surface plasmon resonance (LSPR) measurements and established an analytical framework to compare responses across multiple substrates with different sensitivities. While similar responses were recorded on the two substrates for HSA adsorption under physiologically-relevant ionic strength conditions, distinct substrate-specific behavior was observed at lower ionic strength conditions. With decreasing ionic strength, larger measurement responses occurred for HSA adsorption onto silica surfaces, whereas HSA adsorption onto titania surfaces occurred independently of ionic strength condition. Complementary quartz crystal microbalance-dissipation (QCM-D) measurements were also performed, and the trend in adsorption behavior was similar. Of note, the magnitudes of the ionic strength-dependent LSPR and QCM-D measurement responses varied, and are discussed with respect to the measurement principle and surface sensitivity of each technique. Taken together, our findings demonstrate how the high surface sensitivity of nanoplasmonic sensors can be applied to quantitatively characterize protein adsorption across multiple surfaces, and outline broadly-applicable measurement strategies for biointerfacial science applications.

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.

Probing of sub-picometer vertical differential resolutions using cavity plasmons

Plasmon rulers can be used for resolving ultrasmall environmental, dimensional, and material changes owing to their high sensitivity associated with a light-scattering spectral shift in response to changes in the separation between plasmonic nanostructures. Here, we show, in several experimental setups, how cavity plasmons in a metal nanowire-on-mirror setup can be used to probe vertical dimensional changes with sub-picometer differential resolutions using two carefully chosen material systems. Specifically, we monitor the dielectric layer thickness changes in response to growth using atomic-layer deposition and to thermal expansion, demonstrating a sensitivity of 14-nm spectral shift per Ångström thickness change and 0.58 pm of vertical differential resolution, respectively. The findings confirm theoretical predictions and highlight the potential use of cavity plasmons in some ultrasensitive sensing applications.

Investigating how vesicle size influences vesicle adsorption on titanium oxide: a competition between steric packing and shape deformation

Understanding the adsorption behavior of lipid vesicles at solid-liquid interfaces is important for obtaining fundamental insights into soft matter adsorbates as well as for practical applications such as supported lipid bilayer (SLB) fabrication. While the process of SLB formation has been highly scrutinized, less understood are the details of vesicle adsorption without rupture, especially at high surface coverages. Herein, we tackle this problem by employing simultaneous quartz crystal microbalance-dissipation (QCM-D) and localized surface plasmon resonance (LSPR) measurements in order to investigate the effect of vesicle size (84-211 nm diameter) on vesicle adsorption onto a titanium oxide surface. Owing to fundamental differences in the measurement principles of the two techniques as well as a mismatch in probing volumes, it was possible to determine both the lipid mass adsorbed near the sensor surface as well as the total mass of adsorbed lipid and hydrodynamically coupled solvent in the adsorbed vesicle layer as a whole. With increasing vesicle size, the QCM-D frequency signal exhibited monotonic behavior reaching an asymptotic value, whereas the QCM-D energy dissipation signal continued to increase according to the vesicle size. In marked contrast, the LSPR-tracked lipid mass near the sensor surface followed a parabolic trend, with the greatest corresponding measurement response occurring for intermediate-size vesicles. The findings reveal that the maximum extent of adsorbed vesicles contacting a solid surface occurs at an intermediate vesicle size due to the competing influences of vesicle deformation and steric packing. Looking forward, such information can be applied to control the molecular self-assembly of phospholipid assemblies as well as provide the basis for investigating deformable, soft matter adsorbates.

LSPR Detection of Nucleic Acids on Nanoparticle Monolayers

Noble metal nanoparticles are well known for their unique optical properties. Density oscillations of the nanoparticle conduction electrons are induced at a specific frequency by an external incident light beam. This phenomenon is known under the term localized surface plasmon resonance (LSPR). The spectral position of the resonance band is determined by shape, size, and material of the nanoparticle and influenced by changes of the local refractive index of the surrounding medium. The latter gives the opportunity to use noble metal nanoparticles as label-free bioanalytical sensors. Biomolecules can be bound directly on the nanoparticle surface, which leads to a change of the local refractive index, and a shift of the peak maximum is detected by absorbance spectroscopy. This method is used for bioanalytical diagnostics. Here, a DNA sensing protocol for real-time measurements in situ using this system will be presented. A dense layer of noble metal nanoparticles is immobilized on a glass substrate and implemented in a microfluidic chamber, where the spectroscopic measurements are conducted.

Superior LSPR substrates based on electromagnetic decoupling for on-a-chip high-throughput label-free biosensing

Localized surface plasmon resonance (LSPR) biosensing based on supported metal nanoparticles offers unparalleled possibilities for high-end miniaturization, multiplexing and high-throughput label-free molecular interaction analysis in real time when integrated within an opto-fluidic environment. However, such LSPR-sensing devices typically contain extremely large regions of dielectric materials that are open to molecular adsorption, which must be carefully blocked to avoid compromising the device readings. To address this issue, we made the support essentially invisible to the LSPR by carefully removing the dielectric material overlapping with the localized plasmonic fields through optimized wet-etching. The resulting LSPR substrate, which consists of gold nanodisks centered on narrow SiO₂ pillars, exhibits markedly reduced vulnerability to nonspecific substrate adsorption, thus allowing, in an ideal case, the implementation of thicker and more efficient passivation layers. We demonstrate that this approach is effective and fully compatible with state-of-the-art multiplexed real-time biosensing technology and thus represents the ideal substrate design for high-throughput label-free biosensing systems with minimal sample consumption.