Antifouling Lipid Membranes over Protein A for Orientation-Controlled Immunosensing in Undiluted Serum and Plasma

Curved Membrane Mimics for Quantitative Probing of Protein-Membrane Interactions by Surface Plasmon Resonance

A majority of biomimetic membranes used for current biophysical studies rely on planar structures such as supported lipid bilayer (SLB) and self-assembled monolayers (SAMs). While they have facilitated key information collection, the lack of curvature makes these models less effective for the investigation of curvature-dependent protein binding. Here, we report the development and characterization of curved membrane mimics on a solid substrate with tunable curvature and ease in incorporation of cellular membrane components for the study of protein-membrane interactions. The curved membranes were generated with an underlayer lipid membrane composed of DGS-Ni-NTA and POPC lipids on the substrate, followed by the attachment of histidine-tagged cholera toxin (his-CT) as a capture layer. Lipid vesicles containing different compositions of gangliosides, including GA1, GM1, GT1b, and GQ1b, were anchored to the capture layer, providing fixation of the curved membranes with intact structures. Characterization of the curved membrane was accomplished with surface plasmon resonance (SPR), fluorescence recovery after photobleaching (FRAP), and nano-tracking analysis (NTA). Further optimization of the interface was achieved through principal component analysis (PCA) to understand the effect of ganglioside type, percentage, and vesicle dimensions on their interactions with proteins. In addition, Monte Carlo simulations were employed to predict the distribution of the gangliosides and interaction patterns with single point and multipoint binding models. This work provides a reliable approach to generate robust, component-tuning, and curved membranes for investigating protein interactions more pertinently than what a traditional planar membrane offers.

Exploiting the design of surface plasmon resonance interfaces for better diagnostics: A perspective review

Surface Plasmon Resonance based-sensors are promising tools for precision diagnostics as they can provide tests useful for early and, whenever possible, non-invasive disease detection and monitoring. The design of novel, robust and effective interfaces enabling the sensing of a variety of molecular interactions in a highly selective and sensitive manner is a necessary step to obtain both accurate and reliable detection by SPR. This review covers the recent research efforts in this area, specifically emphasizing well-designed interfaces and applications in real-life samples. In particular, after a short introduction which identifies some of the critical challenges, the emerging strategies for the integration of the linker, the metal substrate and the recognition element on the sensing interface will be explored and discussed in three sections, as well as the opportunities for building SPR biosensors, easy to use, and with excellent sensitivities. Finally, a summary of some of the more promising and latest diagnostic applications will be provided, presenting a new window into the near-future perspectives.

Using the Water Absorption Ability of Dried Hydrogels to Form Hydrogel-Supported Lipid Bilayers

The formation of supported lipid bilayers (SLBs) on hydrogels can act as a biocompatible anti-fouling interface. However, generating continuous and mobile SLBs on materials other than conventional glass or mica remains a significant challenge. The interaction between lipid membrane vesicles and a typical hydrogel is usually insufficient to induce membrane vesicle rupture and form a planar lipid membrane. In this study, we demonstrate that the water absorption ability of a dried polyacrylamide (PAAm) hydrogel could serve as a driving force to facilitate the formation of the hydrogel-SLBs. The absorption driving force vanishes after the hydrogels are fully hydrated, leaving no extra interaction hindering lipid lateral mobility in the formed SLBs. Our fluorescence recovery after photobleaching (FRAP) results show that SLBs only form on hydrogels with adequate absorption abilities. Moreover, we discovered that exposure to oxygen during drying could lead to the formation of an oxidized crust on the PAAm hydrogel surface, impeding SLB formation. Therefore, minimizing oxygen exposure during drying is crucial to achieving high-quality hydrogel surfaces for SLB formation. This water absorption method enables the straightforward fabrication of hydrogel-SLBs without the need for additional substrates or charges, thereby expanding their potential applications.

Repurposing Fc gamma receptor I (FcγRI, CD64) for site-oriented monoclonal antibody capture: A proof-of-concept study for real-time detection of tumor necrosis factor-alpha (TNF -α)

The controlled orientation of biomolecules on the sensor surface is crucial for achieving high sensitivity and accurate detection of target molecules in biosensing. FcγRI is an immune cell surface receptor for recognizing IgG-coated targets, such as opsonized pathogens or immune complexes. It plays a crucial role in T cell activation and internalization of the cargos, leading downstream signaling cascades. In this study, we repurposed the FcγRI as an analytical ligand molecule for site-oriented ADA capture, a monoclonal antibody-based biosimilar drug, on a plasmonic sensor surface and demonstrated the real-time detection of the corresponding analyte molecule, TNF-α. The study encompasses the analysis of comparative ligand behaviors on the surface, biosensor kinetics, concentration-dependent studies, and sensor specificity assays. The findings of this study suggest that FcγRI has a significant potential to serve as a universal ligand molecule for site-specific monoclonal antibody capture, and it can be used for biosensing studies, as it represents low nanomolar range affinity and excellent selectivity towards the target. However, there is still room for improvement in the surface stability and sensing response, and further studies are needed to reveal its performance on the monoclonal antibodies with various antigen binding sites and glycoforms.

Peptide-anchored biomimetic interface for electrochemical detection of cardiomyocyte-derived extracellular vesicles

Cardiomyocyte-derived extracellular vesicles (EVs) are a promising class of biomarkers that can advance the diagnosis of many kinds of cardiovascular diseases. Herein, we develop a new electrochemical method for the feasible detection of cardiomyocyte-derived EVs in biological fluids. The core design of the method is the fabrication of a peptide-anchored biomimetic interface consisting of a lipid bilayer and peptide probes. On the one hand, the lipid bilayer provides excellent antifouling ability to the electrode interface and facilitates the anchoring of peptide probes. On the other hand, the peptide probes equip the electrode interface with excellent binding capability and affinity to CD172a, a specific marker of cardiomyocyte-derived EVs, thus enabling the efficient and selective detection of target EVs. Taking EVs derived from the heart myoblast cells H9C2 as the model target, the method displays a wide linear detection range from 1 × 10³ to 1 × 10⁸ particles/mL with a desirable detection limit of 132 particles/mL. Furthermore, the method shows good performance in biological fluids such as serum, and thus may have great potential for practical use in the diagnosis of cardiovascular diseases.

Fluorescent paper strip immunoassay with carbon nanodots@silica for determination of human serum amyloid A1

A fluorescent paper strip immunoassay in conjunction with carbon nanodots@silica (CND@SiO₂) as a label was developed for the quantitative measurements of human serum amyloid A1 (hSAA1) in serum at clinically significant concentrations for lung cancer diagnosis. Monodispersed CND@SiO₂ was prepared by cohydrolysis between silane-crosslinked carbon nanodots and silica precursors via the Ströber method and further attached covalently to anti-hSAA1 (14F8) monoclonal antibody [anti-hSAA1(14F8)] specific to the hSAA1 target. The hSAA1 concentrations were then determined by quantifying the blue fluorescence intensity upon 365 nm excitation of the captured hSAA1 with anti-hSAA1(14F8)-CND@SiO₂ conjugates in the test line on a paper strip where anti-hSAA1 (10G1) monoclonal antibody was physisorbed. The developed fluorescent paper strip with CND@SiO₂ can detect hSAA1 at concentrations ranging from 0.1 to 5 nM (R² = 0.995), with a limit of detection of  0.258 nM in 10 mM phosphate buffer pH 7.4 containing human serum albumin. The performance of  recovery (90.98-109.17%) and repeatability (coefficients of variation < 8.46%) obtained was also acceptable for quantitative determinations. The platform was employed for direct determination of hSAA1 concentrations in undiluted serum samples from lung cancer patients (relative standard deviation (RSD) < 7.46%) and healthy humans (RSD < 3.96%). The results were compared with those obtained using a commercially available enzyme-linked immunosorbent assay alongside liquid chromatography with tandem mass spectrometry measurements.

Antifouling lipid membrane coupled with silver nanoparticles for electrochemical detection of nucleic acids in biological fluids

Electrochemical method capable of detecting specific nucleic acids in complex fluid will undoubtedly advance the diagnosis of many kinds of diseases. Herein, by coupling lipid membrane with silver nanoparticles (AgNPs), we develop a new electrochemical method for sensitive and reliable detection of nucleic acids in biological fluids. The advantages of lipid membrane especially its excellent antifouling ability is employed to enhance the applicability of the method in complex environment; while the significant solid-state Ag/AgCl response of AgNPs is used to ensure the detection sensitivity of the method. The core of this method's workflow is the target-induced Y-shape structure formation, which results in the recruitment of AgNPs to the electrode surface, producing considerable electrochemical responses used for target nucleic acid detection. Taking highly upregulated in liver cancer (HULC), a liver cancer-related long non-coding RNA as a model target, the method exhibits high sensitivity, specificity, and reproducibility with a detection limit of 0.42 fM. Moreover, the method displays desirable usability in biological fluids such as serum, which will be of great potential in clinical diagnosis.

Rapid and regenerable surface plasmon resonance determinations of biomarker concentration and biomolecular interaction based on tris-nitrilotriacetic acid chips

The tris-nitrilotriacetic acid (tris-NTA) chip has been used for surface plasmon resonance (SPR) kinetic studies involving histidine (His)-tagged proteins. However, its full potential, especially for analyte quantification in complex biological media, has not been realized due to a lack of systematic studies on the factors governing ligand immobilization, surface regeneration, and data analysis. We demonstrate that the tris-NTA chip not only retains His-tagged proteins more strongly than its mono-NTA counterpart, but also orients them more uniformly than protein molecules coupled to carboxymethylated dextran films. We accurately and rapidly quantified immunoglobulin (IgG) molecules in sera by using the initial association phase of their conjugation with His-tagged protein G densely immobilized onto the tris-NTA chip, and established criteria for selecting the optimal time for constructing the calibration curve. The method is highly reproducible (less than 2% RSD) and three orders of magnitude more sensitive than immunoturbidimetry. In addition, we found that the amount of His-protein immobilized is highly dependent on the protein isoelectric point (pI). Reliable kinetic data in a multi-channel SPR instrument can also be rapidly obtained by using a low density of immobilized His-tagged protein. The experimental parameters and procedures outlined in this study help expand the range of SPR applications involving His-tagged proteins.

Detection of Tumor DNA in Human Plasma with a Functional PLL-Based Surface Layer and Plasmonic Biosensing

Standard protocols for the analysis of circulating tumor DNA (ctDNA) include the isolation of DNA from the patient's plasma and its amplification and analysis in buffered solutions. The application of such protocols is hampered by several factors, including the complexity and time-constrained preanalytical procedures, risks for sample contamination, extended analysis time, and assay costs. A recently introduced nanoparticle-enhanced surface plasmon resonance imaging-based assay has been shown to simplify procedures for the direct detection of tumor DNA in the patient's plasma, greatly simplifying the cumbersome preanalytical phase. To further simplify the protocol, a new dual-functional low-fouling poly-l-lysine (PLL)-based surface layer has been introduced that is described herein. The new PLL-based layer includes a densely immobilized CEEEEE oligopeptide to create a charge-balanced system preventing the nonspecific adsorption of plasma components on the sensor surface. The layer also comprises sparsely attached peptide nucleic acid probes complementary to the sequence of circulating DNA, e.g., the analyte that has to be captured in the plasma from cancer patients. We thoroughly investigated the contribution of each component of the dual-functional polymer to the antifouling properties of the surface layer. The low-fouling property of the new surface layer allowed us to detect wild-type and KRAS p.G12D-mutated DNA in human plasma at the attomolar level (∼2.5 aM) and KRAS p.G13D-mutated tumor DNA in liquid biopsy from a cancer patient with almost no preanalytical treatment of the patient's plasma, no need to isolate DNA from plasma, and without PCR amplification of the target sequence.

Recent Advances in Antifouling Materials for Surface Plasmon Resonance Biosensing in Clinical Diagnostics and Food Safety

Strategies to develop antifouling surface coatings are crucial for surface plasmon resonance (SPR) sensing in many analytical application fields, such as detecting human disease biomarkers for clinical diagnostics and monitoring foodborne pathogens and toxins involved in food quality control. In this review, firstly, we provide a brief discussion with considerations about the importance of adopting appropriate antifouling materials for achieving excellent performances in biosensing for food safety and clinical diagnosis. Secondly, a non-exhaustive landscape of polymeric layers is given in the context of surface modification and the mechanism of fouling resistance. Finally, we present an overview of some selected developments in SPR sensing, emphasizing applications of antifouling materials and progress to overcome the challenges related to the detection of targets in complex matrices relevant for diagnosis and food biosensing.

Recent Advancements in Receptor Layer Engineering for Applications in SPR-Based Immunodiagnostics

The rapid progress in the development of surface plasmon resonance-based immunosensing platforms offers wide application possibilities in medical diagnostics as a label-free alternative to enzyme immunoassays. The early diagnosis of diseases or metabolic changes through the detection of biomarkers in body fluids requires methods characterized by a very good sensitivity and selectivity. In the case of the SPR technique, as well as other surface-sensitive detection strategies, the quality of the transducer-immunoreceptor interphase is crucial for maintaining the analytical reliability of an assay. In this work, an overview of general approaches to the design of functional SPR-immunoassays is presented. It covers both immunosensors, the design of which utilizes well-known and often commercially available substrates, as well as the latest solutions developed in-house. Various approaches employing chemical and passive binding, affinity-based antibody immobilization, and the introduction of nanomaterial-based surfaces are discussed. The essence of their influence on the improvement of the main analytical parameters of a given immunosensor is explained. Particular attention is paid to solutions compatible with the latest trends in the development of label-free immunosensors, such as platforms dedicated to real-time monitoring in a quasi-continuous mode, the use of in situ-generated receptor layers (elimination of the regeneration step), and biosensors using recombinant and labelled protein receptors.

Diverse Dyes-Embedded Staphylococcus aureus as Potential Biocarriers for Enhancing Sensitivity in Biosensing

Nanomaterials-based immunochromatographic assays (ICAs) have gained great commercial success in real-life point-of-care testing (POCT). Exploring novel carriers of ICAs with improved signaling and sustained activity favors the development of sensitive POCT. Herein a potent signal biotag, colored Staphylococcus aureus (SA), was created for ICA carriers through a mild self-assembly strategy, providing high luminance and abundant specific binding sites for immobilization of monoclonal antibodies (mAbs). The biocompatible SA-dyes (SADs) retained both an intact surface structure for mAbs labeling (Fc portion) and the superior bioactivity of immobilized mAbs (affinity constant was about 10⁹ M^-1), thus waiving the intrinsic limitations of traditional nanomaterials and endowing high sensitivity. Proof-of-concept was demonstrated by employing Congo red- or/and fluorescein isothiocyanate-embedded SA (SACR, SAFITC, and SACR-SAFITC) as ICA carriers to detect zearalenone (ZEN) through colorimetric or/and fluorimetric signals. Furthermore, the ICAs satisfied the clinical requirement perfectly, including limit of detection (0.013 ng/mL, which was at least an 85-fold improvement over that of traditional gold nanoparticles-based ICA), linearity (R² > 0.98), reproducibility (RSD < 8%), selectivity, and stability. Importantly, the proposed biosensors could be well-applied in four real samples for ZEN monitoring with satisfactory recoveries, correlating well with the results from liquid chromatography-tandem mass spectrometry (LC-MS). This work also proved a universal design for tailoring coloration bands for SAD-ICA detection of multiple analytes.

Plasmonic Biosensing with Aluminum Thin Films under the Kretschmann Configuration

Aluminum has recently attracted considerable interest as a plasmonic material due to its unique optical properties, but most work has been limited to nanostructures. We report here SPR biosensing with aluminum thin-films using the standard Kretschmann configuration that has previously been dominated by gold films. Electron-beam physical vapor deposition (EBPVD)-prepared Al films oxidize in air to form a nanofilm of Al₂O_3, yielding robust stability for sensing applications in buffered solutions. FDTD simulations revealed a sharp plasmonic dip in the visible range that enables measurement of both angular shift and reflection intensity change at a fixed angle. Bulk and surface tests indicated that Al films exhibited superb sensitivity performance in both categories. Compared to Au, the Al/Al₂O₃ layer showed a marked effect of suppressing nonspecific binding from proteins in human serum. Further characterization indicated that Al film demonstrated a higher sensitivity and a wider working range than Au films when used for SPR imaging analysis. Combined with its economic and manufacturing benefits, the Al thin-film has the potential to become a highly advantageous plasmonic substrate to meet a wide range of biosensing needs in SPR configurations.