Thermoresponsive and Photocrosslinkable Poly(2-alkyl-2-oxazoline) Toolbox - Customizable Ultralow-Fouling Hydrogel Coatings for Blood Plasma Environments.

This study focuses on developing surface coatings with excellent antifouling properties, crucial for applications in the medical, biological, and technical fields, for materials and devices in direct contact with living tissues and bodily fluids such as blood. This approach combines thermoresponsive poly(2-alkyl-2-oxazoline)s, known for their inherent protein-repellent characteristics, with established antifouling motifs based on betaines. The polymer framework is constructed from various monomer types, including a novel benzophenone-modified 2-oxazoline for photocrosslinking and an azide-functionalized 2-oxazoline, allowing subsequent modification with alkyne-substituted antifouling motifs through copper(I)-catalyzed azide-alkyne cycloaddition. From these polymers surface-attached networks are created on benzophenone-modified gold substrates via photocrosslinking, resulting in hydrogel coatings with several micrometers thickness when swollen with aqueous media. Given that poly(2-alkyl-2-oxazoline)s can exhibit a lower critical solution temperature in water, their temperature-dependent solubility is compared to the swelling behavior of the surface-attached hydrogels upon thermal stimulation. The antifouling performance of these hydrogel coatings in contact with human blood plasma is further evaluated by surface plasmon resonance and optical waveguide spectroscopy. All surfaces demonstrate extremely low retention of blood plasma components, even with undiluted plasma. Notably, hydrogel layers with sulfobetaine moieties allow efficient penetration by plasma components, which can then be easily removed by rinsing with buffer.

Plasmonic nanomaterials with responsive polymer hydrogels for sensing and actuation.

Plasmonic nanomaterials have become an integral part of numerous technologies, where they provide important functionalities spanning from extraction and harvesting of light in thin film optical devices to probing of molecular species and their interactions on biochip surfaces. More recently, we witness increasing research efforts devoted to a new class of plasmonic nanomaterials that allow for on-demand tuning of their properties by combining metallic nanostructures and responsive hydrogels. This review addresses this recently emerged vibrant field, which holds potential to expand the spectrum of possible applications and deliver functions that cannot be achieved by separate research in each of the respective fields. It aims at providing an overview of key principles, design rules, and current implementations of both responsive hydrogels and metallic nanostructures. We discuss important aspects that capitalize on the combination of responsive polymer networks with plasmonic nanostructures to perform rapid mechanical actuation and actively controlled nanoscale confinement of light associated with resonant amplification of its intensity. The latest advances towards the implementation of such responsive plasmonic nanomaterials are presented, particularly covering the field of plasmonic biosensing that utilizes refractometric measurements as well as plasmon-enhanced optical spectroscopy readout, optically driven miniature soft actuators, and light-fueled micromachines operating in an environment resembling biological systems.

Dual Monitoring of Surface Reactions in Real Time by Combined Surface-Plasmon Resonance and Field-Effect Transistor Interrogation.

By combining surface plasmon resonance (SPR) and electrolyte gated field-effect transistor (EG-FET) methods in a single analytical device we introduce a novel tool for surface investigations, enabling simultaneous measurements of the surface mass and charge density changes in real time. This is realized using a gold sensor surface that simultaneously serves as a gate electrode of the EG-FET and as the SPR active interface. This novel platform has the potential to provide new insights into (bio)adsorption processes on planar solid surfaces by directly relating complementary measurement principles based on (i) detuning of SPR as a result of the modification of the interfacial refractive index profile by surface adsorption processes and (ii) change of output current as a result of the emanating effective gate voltage modulations. Furthermore, combination of the two complementary sensing concepts allows for the comparison and respective validation of both analytical techniques. A theoretical model is derived describing the mass uptake and evolution of surface charge density during polyelectrolyte multilayer formation. We demonstrate the potential of this combined platform through the observation of layer-by-layer assembly of PDADMAC and PSS. These simultaneous label-free and real-time measurements allow new insights into complex processes at the solid-liquid interface (like non-Fickian ion diffusion), which are beyond the scope of each individual tool.

Multi-diffractive grating for surface plasmon biosensors with direct back-side excitation.

A multi-diffractive nanostructure is reported for the resonant excitation of surface plasmons that are cross-coupled through a thin metallic film. It consists of two superimposed periodic corrugations that allow diffraction excitation of surface plasmons on the inner side of a thin metal film and their subsequent phase matching with counterpropagating surface plasmons travelling to the opposite direction on its other side. This interaction leads to establishing of a set of cross-coupled Bragg-scattered surface plasmon modes that exhibit an electromagnetic field localized on both metal film interfaces. The reported structure is attractive for surface plasmon resonance biosensor applications, where direct optical probing can be done through the substrate without the need of optical matching to a high refractive index prism. In addition, it can be prepared by mass production - compatible means with UV-nanoimprint lithography and its biosensing performance characteristics are demonstrated by refractometric and biomolecular affinity binding studies.

Shedding Light on the Dark Corners of Metal-Organic Framework Thin Films: Growth and Structural Stability of ZIF-8 Layers Probed by Optical Waveguide Spectroscopy.

Metal-organic framework (MOF) thin films are promising materials for multiple technological applications, such as chemical sensing. However, one potential limitation for their widespread use in different settings is their stability in aqueous environments. In the case of ZIF-8 (zeolitic imidazolate framework) thin films, their stability in aqueous media is currently a matter of debate. Here, we show that optical waveguide spectroscopy (OWS), in combination with surface plasmon resonance (SPR) spectroscopy, offers a convenient way for answering intriguing questions related to the stability of MOF thin films in aqueous solutions and, eventually provide a tool for assessing changes in MOF layers under different environmental conditions. Our experiments relied on the use of ZIF-8 thin films grown on surface-modified gold substrates, as optical waveguides. We have found a linear thickness increase after each growing cycle and observed that the growing characteristics are strongly influenced by the nature of the primer layer. One of our findings is that substrate surface modification with a 3-mercapto-1-propanesulfonate (MPSA) primer layer is critical to achieve ZIF-8 layers that can effectively act as optical waveguides. We observed that ZIF-8 films are structurally stable upon exposure to pure water and 50 mM NaCl solutions but they exhibit a slight swelling and an increase in porosity probably due to the permeation of the solvent in the intergrain mesoporous cavities. However, OWS revealed that exposure of ZIF-8 thin films to phosphate-buffered saline solutions (pH 8) promotes significant film degradation. This poses an important question as to the prospective use of ZIF-8 materials in biologically relevant applications. In addition, it was demonstrated that postsynthetic polyelectrolyte modification of ZIF-8 films has no detrimental effects on the structural stability of the films.

Plasmonic Hepatitis B Biosensor for the Analysis of Clinical Saliva.

A biosensor for the detection of hepatitis B antibodies in clinical saliva was developed. Compared to conventional analysis of blood serum, it offers the advantage of noninvasive collection of samples. Detection of biomarkers in saliva imposes two major challenges associated with the low analyte concentration and increased surface fouling. The detection of minute amounts of hepatitis B antibodies was performed by plasmonically amplified fluorescence sandwich immunoassay. To have access to specific detection, we prevented the nonspecific adsorption of biomolecules present in saliva by brushes of poly[(N-(2-hydroxypropyl) methacrylamide)-co-(carboxybetaine methacrylamide)] grafted from the gold sensor surface and post modified with hepatitis B surface antigen. Obtained results were validated against the response measured with ELISA at a certified laboratory using serum from the same patients.

Magnetic nanoparticle-enhanced surface plasmon resonance biosensor for extracellular vesicle analysis.

The sensitive analysis of small lipid extracellular vesicles (EVs) by using a grating-coupled surface plasmon resonance (GC-SPR) biosensor has been reported. In order to enable the analysis of trace amounts of EVs present in complex liquid samples, the target analyte is pre-concentrated on the sensor surface by using magnetic nanoparticles and its affinity binding is probed by wavelength interrogation of SPR. The GC-SPR has been demonstrated to allow for the implementation of efficient pulling of EVs to the sensor surface by using magnetic nanoparticles and an external magnetic field gradient applied through the sensor chip. This approach overcomes slow diffusion-limited mass transfer and greatly enhances the measured sensor response. The specific detection of different EV populations secreted from mesenchymal stem cells is achieved with a SPR sensor chip modified with antibodies against the surface marker CD81 and magnetic nanoparticles binding the vesicles via annexin V and cholera toxin B chain.

Nanostructured as-deposited indium tin oxide thin films for broadband antireflection and light trapping.

Indium tin oxide (ITO) thin films were sputter-deposited at ambient temperature on a glass-like substrate that was periodically nanostructured by UV nanoimprint lithography. Cross gratings of the corrugated and conformal ITO, with different periods and modulation depths, were tailored to exhibit light trapping or antireflection properties at specific spectral windows by combined optical simulations and experiments. For dense gratings, the light transmission in the 450-850 nm range was enhanced by 8% (absolute) compared to flat ITO films, which is one of the largest performance improvements reported in the literature for nanostructured transparent electrodes. Increasing the grating period shifts the threshold for diffraction coupling to waveguide modes in the visible and near infrared part of the spectrum, resulting in broad light trapping behaviour at wavelengths below this threshold. This work demonstrates a simple processing route at ambient temperature for the fabrication of high-performance transparent electrodes in order to fulfil different device requirements.

Reversibly tunable plasmonic bandgap by responsive hydrogel grating.

Reversible actuating of surface plasmon propagation by responsive hydrogel grating is reported. Thermo-responsive poly(N-isopropylacrylamide)-based (pNIPAAm) hydrogel nanostructure was designed and tethered to a gold surface in order to switch on and off Bragg scattering of surface plasmons which is associated with an occurrence of a bandgap in their dispersion relation. pNIPAAm-based grating with a period around 280 nm was prepared by using photo-crosslinkable terpolymer and laser interference lithography and it was brought in contact with water. The temperature induced swelling and collapse of pNIPAAm hydrogel grating strongly modulates its refractive index (Δn~0.1) which leads to the reversible opening and closing of a plasmonic bandgap. The experiments demonstrate partial opening of a bandgap with the width of 12 nm at wavelength around 800 nm where SPR exhibited the spectral width of about 75 nm.

Plasmonic amplification for bioassays with epi-fluorescence readout.

Corrugated metallic surfaces offer means for efficient amplification of fluorescence bioassay signal based on the near field coupling between surface plasmons and fluorophore emitters that are used as labels. This paper discusses the design of such plasmonic structure to enhance the sensitivity of immunoassays with epi-fluorescence readout geometry. In particular, crossed gold grating is theoretically and experimentally investigated for combined increasing of the excitation rate at the fluorophore excitation wavelength and utilizing directional surface plasmon-coupled fluorescence emission. For Alexa Fluor 647 dye, the enhancement factor of around EF = 102 was simulated and experimentally measured. When applied to a sandwich interleukin-6 immunoassay, highly surface-selective enhancement reaching a similar value was observed. Besides increasing the measured fluorescence signal associated with the molecular binding events on a surface by two orders of magnitude, the presented approach enables measuring kinetics of the surface reaction that is otherwise masked by strong background signal originating from bulk solution.

Metal-Organic Frameworks in Surface Enhanced Raman Spectroscopy-Based Analysis of Volatile Organic Compounds.

Volatile Organic Compounds (VOC) are a major class of environmental pollutants hazardous to human health, but also highly relevant in other fields including early disease diagnostics and organoleptic perception of aliments. Therefore, accurate analysis of VOC is essential, and a need for new analytical methods is witnessed for rapid on-site detection without complex sample preparation. Surface-Enhanced Raman Spectroscopy (SERS) offers a rapidly developing versatile analytical platform for the portable detection of chemical species. Nonetheless, the need for efficient docking of target analytes at the metallic surface significantly narrows the applicability of SERS. This limitation can be circumvented by interfacing the sensor surface with Metal-Organic Frameworks (MOF). These materials featuring chemical and structural versatility can efficiently pre-concentrate low molecular weight species such as VOC through their ordered porous structure. This review presents recent trends in the development of MOF-based SERS substrates with a focus on elucidating respective design rules for maximizing analytical performance. An overview of the status of the detection of harmful VOC is discussed in the context of industrial and environmental monitoring. In addition, a survey of the analysis of VOC biomarkers for medical diagnosis and emerging applications in aroma and flavor profiling is included.

Sandwich Immuno-RCA Assay with Single Molecule Counting Readout: The Importance of Biointerface Design.

The analysis of low-abundance protein molecules in human serum is reported based on counting of the individual affinity-captured analyte on a solid sensor surface, yielding a readout format similar to digital assays. In this approach, a sandwich immunoassay with rolling circle amplification (RCA) is used for single molecule detection (SMD) through associating the target analyte with spatially distinct bright spots observed by fluorescence microscopy. The unspecific interaction of the target analyte and other immunoassay constituents with the sensor surface is of particular interest in this work, as it ultimately limits the performance of this assay. It is minimized by the design of the respective biointerface and thiol self-assembled monolayer with oligoethylene (OEG) head groups, and a poly[oligo(ethylene glycol) methacrylate] (pHOEGMA) antifouling polymer brush was used for the immobilization of the capture antibody (cAb) on the sensor surface. The assay relying on fluorescent postlabeling of long single-stranded DNA that are grafted from the detection antibody (dAb) by RCA was established with the help of combined surface plasmon resonance and surface plasmon-enhanced fluorescence monitoring of reaction kinetics. These techniques were employed for in situ measurements of conjugating of cAb to the sensor surface, tagging of short single-stranded DNA to dAb, affinity capture of the target analyte from the analyzed liquid sample, and the fluorescence readout of the RCA product. Through mitigation of adsorption of nontarget molecules on the sensor surface by tailoring of the antifouling biointerface, optimizing conjugation chemistry, and by implementing weak Coulombic repelling between dAb and the sensor surface, the limit of detection (LOD) of the assay was substantially improved. For the chosen interleukin-6 biomarker, SMD assay with LOD at a concentration of 4.3 fM was achieved for model (spiked) samples, and validation of the ability of detection of standard human serum samples is demonstrated.

Collective localized surface plasmons for high performance fluorescence biosensing.

Metallic nanostructures supporting collective localized surface plasmons (cLSPs) are investigated for the amplification of signal in fluorescence biosensors. cLSPs modes are supported by diffractive arrays of metallic nanoparticles that are embedded in a refractive index-symmetrical environment. They exhibit lower damping and thus their excitation is associated with higher field intensity enhancement and narrower resonance than that for regular localized surface plasmons. Through finite difference time domain (FDTD) simulations, we designed a novel cLSP structure that exhibit two resonances overlapping with absorption and emission wavelengths of assumed fluorophore (similar to Cy5 or Alexa Fluor 647). The simulations of surface plasmon-enhanced fluorescence (PEF) took into account the cLSP-driven excitation, directional emission, and mediated quantum yield in realistic sandwich immunoassays that utilize fluorophore-labeled detection antibodies. Achieved results indicate that cLSP-based structures holds potential for extraordinarily high fluorescence intensity enhancement that exceeds a value of 10(3).

Compact surface plasmon-enhanced fluorescence biochip.

A new concept of compact biochip for surface plasmon-enhanced fluorescence assays is reported. It takes advantage of the amplification of fluorescence signal through the coupling of fluorophore labels with confined and strongly enhanced field intensity of surface plasmons. In order to efficiently excite and collect the emitted fluorescence light via surface plasmons on a metallic sensor surface, (reverse) Kretschmann configuration is combined with diffractive optical elements embedded on the chip surface. These include a concentric relief grating for the imaging of highly directional surface plasmon-coupled emission to a detector. Additional linear grating is used for the generating of surface plasmons at the excitation wavelength on the sensor surface in order to increase the fluorescence excitation rate. The reported approach offers the increased intensity of fluorescence signal, reduced background, and compatibility with nanoimprint lithography for cost-effective preparation of sensor chip. The presented approach was implemented for biosensing in a model immunoassay experiment in which the limit of detection of 11 pM was achieved.

The impact of plasmonic electrodes on the photocarrier extraction of inverted organic bulk heterojunction solar cells.

Nano-patterning the semiconducting photoactive layer/back electrode interface of organic photovoltaic devices is a widely accepted approach to enhance the power conversion efficiency through the exploitation of numerous photonic and plasmonic effects. Yet, nano-patterning the semiconductor/metal interface leads to intertwined effects that impact the optical as well as the electrical characteristic of solar cells. In this work we aim to disentangle the optical and electrical effects of a nano-structured semiconductor/metal interface on the device performance. For this, we use an inverted bulk heterojunction P3HT:PCBM solar cell structure, where the nano-patterned photoactive layer/back electrode interface is realized by patterning the active layer with sinusoidal grating profiles bearing a periodicity of 300 nm or 400 nm through imprint lithography while varying the photoactive layer thickness (L PAL ) between 90 and 400 nm. The optical and electrical device characteristics of nano-patterned solar cells are compared to the characteristics of control devices, featuring a planar photoactive layer/back electrode interface. We find that patterned solar cells show for an enhanced photocurrent generation for a L PAL above 284 nm, which is not observed when using thinner active layer thicknesses. Simulating the optical characteristic of planar and patterned devices through a finite-difference time-domain approach proves for an increased light absorption in presence of a patterned electrode interface, originating from the excitation of propagating surface plasmon and dielectric waveguide modes. Evaluation of the external quantum efficiency characteristic and the voltage dependent charge extraction characteristics of fabricated planar and patterned solar cells reveals, however, that the increased photocurrents of patterned devices do not stem from an optical enhancement but from an improved charge carrier extraction efficiency in the space charge limited extraction regime. Presented findings clearly demonstrate that the improved charge extraction efficiency of patterned solar cells is linked to the periodic surface corrugation of the (back) electrode interface. Supplementary Information: The online version contains supplementary material available at 10.1007/s00339-023-06492-6.

Negligible risk of surface transmission of SARS-CoV-2 in public transportation.

BACKGROUND: Exposure to pathogens in public transport systems is a common means of spreading infection, mainly by inhaling aerosol or droplets from infected individuals. Such particles also contaminate surfaces, creating a potential surface-transmission pathway. METHODS: A fast acoustic biosensor with an antifouling nano-coating was introduced to detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on exposed surfaces in the Prague Public Transport System. Samples were measured directly without pre-treatment. Results with the sensor gave excellent agreement with parallel quantitative reverse-transcription polymerase chain reaction (qRT-PCR) measurements on 482 surface samples taken from actively used trams, buses, metro trains and platforms between 7 and 9 April 2021, in the middle of the lineage Alpha SARS-CoV-2 epidemic wave when 1 in 240 people were COVID-19 positive in Prague. RESULTS: Only ten of the 482 surface swabs produced positive results and none of them contained virus particles capable of replication, indicating that positive samples contained inactive virus particles and/or fragments. Measurements of the rate of decay of SARS-CoV-2 on frequently touched surface materials showed that the virus did not remain viable longer than 1-4 h. The rate of inactivation was the fastest on rubber handrails in metro escalators and the slowest on hard-plastic seats, window glasses and stainless-steel grab rails. As a result of this study, Prague Public Transport Systems revised their cleaning protocols and the lengths of parking times during the pandemic. CONCLUSIONS: Our findings suggest that surface transmission played no or negligible role in spreading SARS-CoV-2 in Prague. The results also demonstrate the potential of the new biosensor to serve as a complementary screening tool in epidemic monitoring and prognosis.

Bacterial pathogen surface plasmon resonance biosensor advanced by long range surface plasmons and magnetic nanoparticle assays.

A new approach to surface plasmon resonance (SPR) biosensors for rapid and highly sensitive detection of bacterial pathogens is reported. It is based on the spectroscopy of grating-coupled long-range surface plasmons (LRSPs) combined with magnetic nanoparticle (MNP) assay. The interrogation of LRSPs allows increasing the biosensor figure of merit (FOM), and the employment of MNPs further enhances the sensor response by a fast delivery of the analyte to the sensor surface and through the amplified refractive index changes associated with the capture of target analyte. This amplification strategy is particularly attractive for detection of large analytes that diffuse slowly from the analyzed sample to the sensor surface. The potential of the presented approach is demonstrated in a model experiment in which Escherichia coli O157:H7 was detected at concentrations as low as 50 cfu mL(-1), 4 orders of magnitude better than the limit of detection achieved by regular grating-coupled SPR with direct detection format.

Surface plasmon-coupled emission on plasmonic Bragg gratings.

Surface plasmon-coupled emission (SPCE) from emitters in a close proximity to a plasmonic Bragg grating is investigated. In this study, the directional fluorescence emission mediated by Bragg-scattered surface plasmons and surface plasmons diffraction cross-coupled through a thin metallic film is observed by using the reverse Kretschmann configuration. We show that controlling of dispersion relation of these surface plasmon modes by tuning the refractive index at upper and lower interfaces of a dense sub-wavelength metallic grating enables selective reducing or increasing the intensity of the light emitted to certain directions. These observations may provide important leads for design of advanced plasmonic structures in applications areas of plasmon-enhanced fluorescence spectroscopy and nanoscale optical sources.

Rapid Actuation of Thermo-Responsive Polymer Networks: Investigation of the Transition Kinetics.

The swelling and collapsing of thermo-responsive poly(N-isopropylacrylamide)-based polymer (pNIPAAm) networks are investigated in order to reveal the dependency on their kinetics and maximum possible actuation speed. The pNIPAAm-based network was attached as thin hydrogel film to lithographically prepared gold nanoparticle arrays to exploit their localized surface plasmon resonance (LSPR) for rapid local heating. The same substrate also served for LSPR-based monitoring of the reversible collapsing and swelling of the pNIPAAm network through its pronounced refractive index changes. The obtained data reveal signatures of multiple phases during the volume transition, which are driven by the diffusion of water molecules into and out of the network structure and by polymer chain re-arrangement. For the micrometer-thick hydrogel film in the swollen state, the layer can respond as fast as several milliseconds depending on the strength of the heating optical pulse and on the tuning of the ambient temperature with respect to the lower critical solution temperature of the polymer. Distinct differences in the time constants of swelling and collapse are observed and attributed to the dependence of the cooperative diffusion coefficient of polymer chains on polymer volume fraction. The reported results may provide guidelines for novel miniature actuator designs and micromachines that take advantages of the non-reciprocal temperature-induced volume transitions in thermo-responsive hydrogel materials.

Field-Effect Transistor with a Plasmonic Fiber Optic Gate Electrode as a Multivariable Biosensor Device.

A novel multivariable system, combining a transistor with fiber optic-based surface plasmon resonance spectroscopy with the gate electrode simultaneously acting as the fiber optic sensor surface, is reported. The dual-mode sensor allows for discrimination of mass and charge contributions for binding assays on the same sensor surface. Furthermore, we optimize the sensor geometry by investigating the influence of the fiber area to transistor channel area ratio and distance. We show that larger fiber optic tip diameters are favorable for electronic and optical signals and demonstrate the reversibility of plasmon resonance wavelength shifts after electric field application. As a proof of principle, a layer-by-layer assembly of polyelectrolytes is performed to benchmark the system against multivariable sensing platforms with planar surface plasmon resonance configurations. Furthermore, the biosensing performance is assessed using a thrombin binding assay with surface-immobilized aptamers as receptors, allowing for the detection of medically relevant thrombin concentrations.