Optical waveguide spectroscopy for the investigation of protein-functionalized hydrogel films

FO-SPR Model for Full-Spectrum Signal Analysis of Back-reflecting FO-SPR Sensors to Monitor MOF Deposition

In this study, we explore the full-spectrum capabilities of fiber-optic surface plasmon resonance (FO-SPR) for analyzing heterogeneous samples with increased comprehensiveness. Our approach involves refining a literature-derived FO-SPR model to more precisely reflect experimental data obtained using a back-reflecting sensor configuration. Key enhancements in our model include adjustments to the thickness and permittivity of the gold SPR-active layer on the FO-SPR sensor as well as improvements to the angular distribution of light within the system. We apply this optimized model to the investigation of the deposition process of a metal-organic framework (MOF), specifically ZIF-8, using FO-SPR. By closely examining the temporal variations in the FO-SPR signal during MOF layer formation, we simultaneously determine the evolving thickness and refractive index (RI) of the MOF layer, offering a dual-parameter analysis. Our results demonstrate that a full-spectrum analysis of the FO-SPR signal can extract critical information from samples exhibiting radial heterogeneity. This advancement significantly enhances the quantitative assessment of various phenomena that alter the refractive index in the sensor's domain, such as adsorption and binding processes. This work thus represents a significant step forward in the field of FO-SPR sensor technology, promising broad applications in areas requiring the precise detection and analysis of complex samples.

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.

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.

Responsive Hydrogel Binding Matrix for Dual Signal Amplification in Fluorescence Affinity Biosensors and Peptide Microarrays

A combined approach to signal enhancement in fluorescence affinity biosensors and assays is reported. It is based on the compaction of specifically captured target molecules at the sensor surface followed by optical probing with a tightly confined surface plasmon (SP) field. This concept is utilized by using a thermoresponsive hydrogel (HG) binding matrix that is prepared from a terpolymer derived from poly(N-isopropylacrylamide) (pNIPAAm) and attached to a metallic sensor surface. Epi-illumination fluorescence and SP-enhanced total internal reflection fluorescence readouts of affinity binding events are performed to spatially interrogate the fluorescent signal in the direction parallel and perpendicular to the sensor surface. The pNIPAAm-based HG binding matrix is arranged in arrays of sensing spots and employed for the specific detection of human IgG antibodies against the Epstein-Barr virus (EBV). The detection is performed in diluted human plasma or with isolated human IgG by using a set of peptide ligands mapping the epitope of the EBV nuclear antigen. Alkyne-terminated peptides were covalently coupled to the pNIPAAm-based HG carrying azide moieties. Importantly, using such low-molecular-weight ligands allowed preserving the thermoresponsive properties of the pNIPAAm-based architecture, which was not possible for amine coupling of regular antibodies that have a higher molecular weight.

Actuated plasmonic nanohole arrays for sensing and optical spectroscopy applications

Herein, we report a new approach to rapidly actuate the plasmonic characteristics of thin gold films perforated with nanohole arrays that are coupled with arrays of gold nanoparticles. The near-field interaction between the localized and propagating surface plasmon modes supported by the structure was actively modulated by changing the distance between the nanoholes and nanoparticles and varying the refractive index symmetry of the structure. This approach was applied by using a thin responsive hydrogel cushion, which swelled and collapsed by a temperature stimulus. The detailed experimental study of the changes and interplay of localized and propagating surface plasmons was complemented by numerical simulations. We demonstrate that the interrogation and excitation of the optical resonance to these modes allow the label-free SPR observation of the binding of biomolecules, and is applicable for in situ SERS studies of low molecular weight molecules attached in the gap between the nanoholes and nanoparticles.

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

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

Light-microgel interaction in resonant nanostructures

Combination of responsive microgels and photonic resonant nanostructures represents an intriguing technological tool for realizing tunable and reconfigurable platforms, especially useful for biochemical sensing applications. Interaction of light with microgel particles during their swelling/shrinking dynamics is not trivial because of the inverse relationships between their size and refractive index. In this work, we propose a reliable analytical model describing the optical properties of closed-packed assembly of surface-attached microgels, as a function of the external stimulus applied. The relationships between the refractive index and thickness of the equivalent microgel slab are derived from experimental observations based on conventional morphological analysis. The model is first validated in the case of temperature responsive microgels integrated on a plasmonic lab-on-fiber optrode, and also implemented in the same case study for an optical responsivity optimization problem. Overall, our model can be extended to other photonic platforms and different kind of microgels, independently from the nature of the stimulus inducing their swelling.

Free-standing hydrogel-particle composite membrane with dynamically controlled permeability

The preparation and investigation of a free-standing membrane made from a composite of thermoresponsive poly(N-isopropylacrylamide) (pNIPAAm) and polystyrene nanoparticles (PS NP) with temperature-controlled permeability is reported. The method exploits the light-induced crosslinking of the photo-reactive pNIPAAm-based polymer and mechanical reinforcement of the membrane structure by the polystyrene nanoparticles. About micrometer thick layers were either directly attached to a gold surface or prepared as free-standing layers spanning over arrays of microfluidic channels with a width of about hundred microns by using template stripping. Diffusion of liquid medium, low molecular weight molecules, and large molecular weight proteins contained in blood through the composite membrane was observed with combined surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The swelling ratio, permeability, and nonspecific sorption to these composite membranes were investigated by SPR and OWS as a function of molecular weight of analyte, loading of PS NP in the composite film, and temperature. The authors show successful preparation of a defect-free membrane structure that acts as a thermoresponsive filter with nanoscale pores spanning over an area of several square millimeters. This membrane can be reversibly switched to block or allow the diffusion of low mass molecules to the sensor surface by temperature-triggered swelling and collapsing of the hydrogel component. Blocking of diffusion and low unspecific sorption of proteins contained in blood serum is observed. These features make this platform interesting for potential future applications in continuous monitoring biosensors for the analysis of low molecular weight drug analytes or for advanced cell-on-chip microfluidic studies.

Characterizing ultrathin and thick organic layers by surface plasmon resonance three-wavelength and waveguide mode analysis

A three-wavelength angular-scanning surface plasmon resonance based analysis has been utilized for characterizing optical properties of organic nanometer-thick layers with a wide range of thicknesses. The thickness and refractive index were determined for sample layers with thicknesses ranging from subnanometer to hundreds of nanometers. The analysis approach allows for simultaneous determination of both the refractive index and thickness without prior knowledge of either the refractive index or the thickness of the sample layers and without the help of other instruments, as opposed to current methods and approaches for characterizing optical properties of organic nanometer-thick layers. The applicability of the three-wavelength angular-scanning surface plasmon resonance approach for characterizing thin and thick organic layers was demonstrated by ex situ deposited mono- and multilayers of stearic acid and hydrogenated soy phosphatidylcholine and in situ layer-by-layer deposition of two different polyelectrolyte multilayer systems. In addition to the three-wavelength angular-scanning surface plasmon resonance approach, another surface plasmon resonance optical phenomenon, i.e., the surface plasmon resonance waveguide mode, was utilized to characterize organic sample layers whose thicknesses border the micrometer scale. This was demonstrated by characterizing both in situ layer-by-layer deposited polyelectrolyte multilayer systems and an ex situ deposited spin-coated polymer layer.

Active Control of SPR by Thermoresponsive Hydrogels for Biosensor Applications

The use of thermoresponsive poly(N-isopropylacrylamide)-based hydrogel (pNIPAAm) for rapid tuning of surface plasmon resonance (SPR) is reported. This approach is implemented by using an SPR layer architecture with an embedded indium tin oxide microheater and pNIPAAm film on its top. It takes advantage of rapid thermally induced swelling and collapse of pNIPAAm that is accompanied by large refractive index changes and leads to high thermo-optical coefficient of dn/dT = 2 × 10-2 RIU/K. We show that this material is excellently suited for efficient control of refractive index-sensitive SPR and that it can serve simultaneously as a 3D binding matrix in biosensor applications (if modified with biomolecular recognition elements for a specific capture of target analyte). We demonstrate that this approach enables modulating of the output signal in surface plasmon-enhanced fluorescence spectroscopy biosensors and holds potential for simple time-multiplexing of sensing channels for parallelized readout of fluorescence assays.

Bloch surface wave-enhanced fluorescence biosensor

A new approach to signal amplification in fluorescence-based assays for sensitive detection of molecular analytes is reported. It relies on a sensor chip carrying a one-dimensional photonic crystal (1DPC) composed of two piled up segments which are designed to increase simultaneously the excitation rate and the collection efficiency of fluorescence light. The top segment supports Bloch surface waves (BSWs) at the excitation wavelength and the bottom segment serves as a Bragg mirror for the emission wavelength of used fluorophore labels. The enhancement of the excitation rate on the sensor surface is achieved through the resonant coupling to BSWs that is associated with strong increase of the field intensity. The increasing of collection efficiency of fluorescence light emitted from the sensor surface is pursued by using the Bragg mirror that minimizes its leakage into a substrate and provides its beaming toward a detector. In order to exploit the whole evanescent field of BSW, extended three-dimensional hydrogel-based binding matrix that is functionalized with catcher molecules is attached to 1DPC for capturing of target analyte from a sample. Simulations supported by experiments are presented to illustrate the design and determined the performance characteristics of BSW-enhanced fluorescence spectroscopy. A model immunoassay experiment demonstrates that the reported approach enables increasing signal to noise ratio, resulting in about one order of magnitude improved limit of detection (LOD) with respect to regular total internal reflection fluorescence (TIRF) configuration.

Thin hydrogel films for optical biosensor applications

Hydrogel materials consisting of water-swollen polymer networks exhibit a large number of specific properties highly attractive for a variety of optical biosensor applications. This properties profile embraces the aqueous swelling medium as the basis of biocompatibility, non-fouling behavior, and being not cell toxic, while providing high optical quality and transparency. The present review focuses on some of the most interesting aspects of surface-attached hydrogel films as active binding matrices in optical biosensors based on surface plasmon resonance and optical waveguide mode spectroscopy. In particular, the chemical nature, specific properties, and applications of such hydrogel surface architectures for highly sensitive affinity biosensors based on evanescent wave optics are discussed. The specific class of responsive hydrogel systems, which can change their physical state in response to externally applied stimuli, have found large interest as sophisticated materials that provide a complex behavior to hydrogel-based sensing devices.

Long range surface plasmon-coupled fluorescence emission for biosensor applications

A biosensor scheme that employs long range surface plasmons (LRSPs) for the efficient excitation and collection of fluorescence light from fluorophore-labeled biomolecules captured in a three-dimensional hydrogel matrix is discussed. This new approach to plasmon-enhanced fluorescence (PEF) is experimentally and theoretically investigated by using the Kretschmann configuration of attenuated total reflection (ATR) method. A layer structure supporting LRSPs that consists of a low refractive index fluoropolymer layer, a thin gold film and a large binding capacity N-isopropylacrylamide (NIPAAm)-based hydrogel matrix swollen in an aqueous sample is employed. By using this layer architecture, the extended field of LRSPs probes the binding of biomolecules in the binding matrix at up to micrometer distances from the gold surface. With respect to regular surface plasmon-enhanced fluorescence spectroscopy (SPFS) and surface plasmon-coupled emission (SPCE), a narrower angular distribution of the fluorescence light intensity, a larger peak intensity and the excitation and emission at lower angles were observed.

Long range surface plasmon and hydrogel optical waveguide field-enhanced fluorescence biosensor with 3D hydrogel binding matrix: on the role of diffusion mass transfer

An implementation of evanescent wave affinity biosensor with a large-capacity three-dimensional binding matrix for ultra-sensitive detection of molecular analytes is investigated. In the experimental part of the work, highly swollen carboxylated poly(N-isopropylacryamide) (NIPAAm) hydrogel with up to micrometer thickness was grafted to a sensor surface, functionalized with antibody recognition elements and employed for immunoassay-based detection of target molecules contained in a liquid sample. Molecular binding events were detected by long range surface plasmon (LRSP) and hydrogel optical waveguide (HOW) field-enhanced fluorescence spectroscopy. These novel methods allowed probing an extended three-dimensional biointerface with an evanescent field reaching up to several micrometers from the sensor surface. The resonant excitation of LRSP and HOW modes provided strong enhancement of intensity of electromagnetic field that is directly translated into an increased fluorescence signal associated with the binding of fluorophore-labeled molecules. Experimental observations were supported by numerical simulations of mass transfer and affinity binding of target molecules in the hydrogel. Through the optimization of the hydrogel thickness and profile of the probing evanescent wave, low femtomolar limit of detection was achieved.

Electrochemical surface plasmon resonance and waveguide-enhanced glucose biosensing with N-alkylaminated polypyrrole/glucose oxidase multilayers

In this work, we report an electrochemical surface plasmon resonance/waveguide (EC-SPR/waveguide) glucose biosensor that could detect enzymatic reactions in a conducting polymer/glucose oxidase (GO(x)) multilayer thin film. In order to achieve a controlled enzyme electrode and waveguide mode, GO(x) (negatively charged) was immobilized with a water-soluble, conducting N-alkylaminated polypyrrole (positively charged) using the layer-by-layer (LbL) electrostatic self-assembly technique. The electrochemical and optical signals were simultaneously obtained from the composite LbL enzyme electrode upon the addition of glucose as mediated by the electroactivity and electrochromic property of the polypyrrole layers. Signal enhancement in EC-SPR detection is obtained by monitoring the doping-dedoping events on the polypyrrole. The real-time optical signal could be distinguished between the change in the dielectric constant of the enzyme layer and other nonenzymatic reaction events such as adsorption of glucose and the change of the refractive index of the solution. This was possible by correlation of both the SPR mode and the m = 0 and 1 modes of the waveguide in an SPR/waveguide spectroscopy experiment.

Responsive hydrogels for label-free signal transduction within biosensors

Hydrogels have found wide application in biosensors due to their versatile nature. This family of materials is applied in biosensing either to increase the loading capacity compared to two-dimensional surfaces, or to support biospecific hydrogel swelling occurring subsequent to specific recognition of an analyte. This review focuses on various principles underpinning the design of biospecific hydrogels acting through various molecular mechanisms in transducing the recognition event of label-free analytes. Towards this end, we describe several promising hydrogel systems that when combined with the appropriate readout platform and quantitative approach could lead to future real-life applications.