Functionalized ultra-low fouling carboxy- and hydroxy-functional surface platforms: functionalization capacity, biorecognition capability and resistance to fouling from undiluted biological media

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.

Design of a Facile Antifouling Sensor Based on the Synergy between an Antibody and Phase-Transited BSA

Nonspecific adsorption has always been a critical challenge for sensor detection; thus, an efficient and facile approach for fabricating antifouling sensors is highly desirable. Here, we developed an antifouling coating on sensor surfaces, conveniently made with a simple drip of phase-transited BSA (PTB) followed by a modification with a peanut allergen antibody, which unexpectedly provides synergistic antifouling properties in sensors. Atomic force microscopy and scanning electron microscopy were used to evaluate the surface evenness. Optimizations in terms of PTB modification time and concentrations were performed using surface plasmon resonance by measuring protein resistance capabilities. Compared to bare Au surfaces, the PTB-modified surfaces exhibited low adsorption against BSA (<10 ng/cm2) and good resistance against lysozyme (Lyz). After immobilizing antibodies, the antifouling performance of the sensor coatings had an obvious enhancement, with almost no BSA adsorption and low lysozyme adsorption. The target recognition was also analyzed to verify the good sensing performance of the antifouling sensor. This understanding of antibody synergy provides suggestions for the development of antifouling sensors.

Influence of bovine and human serum albumin on the binding kinetics of biomolecular interactions

Bovine serum albumin (BSA) containing buffers are the standard blocking buffer in biosensing, yet human serum is the intended application for most clinical sensors. However, the effect of human serum albumin (HSA) on binding assays remains underexplored. A simple and well-studied assay (human IgG/goat anti-human IgG) was investigated with a surface plasmon resonance (SPR) sensor to address this fundamental question in sensing. Calibrations were performed with buffers containing various concentrations of bovine or human serum albumin, as well as full and diluted bovine or IgG-depleted human serum. It was found that HSA or human serum, but not BSA or bovine serum, significantly affected the SPR shift and binding constants of the assay. Interestingly, large differences were also observed depending on whether the animal or human antibody was immobilized on the SPR chip for detection, highlighting that matrix protein/analyte/receptor interactions play a significant role in the response. We find that the interaction of soluble HSA with human IgG interferes with the recognition region, affecting the binding constant, and thus results obtained in BSA are not necessarily applicable to clinical samples or in vivo conditions. We also clearly demonstrate why a minimum dilution of 1 : 10 is often required in SPR assays to remove most background effects. Taken together, these results show that: (1) BSA does not affect the binding constant between antibodies and thus serves its purpose well when only surface blocking is intended, (2) HSA is an adequate surrogate for human serum in assay optimization, and (3) blocking buffers should be prepared with HSA in the optimization steps of assays to be translated to human blood or serum.

Ordered Porous Layer Interferometry for Dynamic Observation of Non-Specific Adsorption Induced by 1-Ethyl-3-(3-(dimethylamino)propyl) Carbodiimide

Nonspecific adsorption (NSA) seems to be an impregnable obstacle to the progress of the biomedical, diagnostic, microelectronic, and material fields. The reaction path of bioconjugation can alter the surface charge distribution on products and the interaction of bioconjugates, an ignored factor causing NSA. We monitored exacerbated NSA introduced by a 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC) addition reaction, which cannot be resistant to bovine serum albumin (BSA) or polyethylene glycol (PEG) antifouling coating and Tween-20. And the negative effects can be minimized by adding as low as 7.5 × 10-6 M N-hydroxysulfosuccinimide (sulfo-NHS). We applied ordered porous layer interferometry (OPLI) to sensitively evaluate the NSA that is difficult to measure on individual particles. Using the silica colloidal crystal (SCC) film with Fabry-Perot fringes as in situ and real-time monitoring for the NSA, we optimized the surface chemistry to yield a conjugate surface without variational charge distribution. In this work, we propose a novel approach from the perspective of the reaction pathway to minimize the NSA of solely EDC-induced chemistry.

Antiviral Polymer Brushes by Visible-Light-Induced, Oxygen-Tolerant Covalent Surface Coating

This work presents a novel route for creating metal-free antiviral coatings based on polymer brushes synthesized by surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain transfer (SI-PET-RAFT) polymerization, applying eosin Y as a photocatalyst, water as a solvent, and visible light as a driving force. The polymer brushes were synthesized using N-[3-(decyldimethyl)-aminopropyl] methacrylamide bromide and carboxybetaine methacrylamide monomers. The chemical composition, thickness, roughness, and wettability of the resulting polymer brush coatings were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), water contact angle measurements, and ellipsometry. The antiviral properties of coatings were investigated by exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and avian influenza viruses, with further measurement of residual viable viral particles. The best performance was obtained with Cu surfaces, with a ca. 20-fold reduction of SARS-Cov-2 and a 50-fold reduction in avian influenza. On the polymer brush-modified surfaces, the number of viable virus particles decreased by about 5-6 times faster for avian flu and about 2-3 times faster for SARS-CoV-2, all compared to unmodified silicon surfaces. Interestingly, no significant differences were obtained between quaternary ammonium brushes and zwitterionic brushes.

Surface Passivation with a Perfluoroalkane Brush Improves the Precision of Single-Molecule Measurements

Single-molecule imaging is invaluable for investigating the heterogeneous behavior and interactions of biological molecules. However, an impediment to precise sampling of single molecules is the irreversible adsorption of components onto the surfaces of cover glasses. This causes continuous changes in the concentrations of different molecules dissolved or suspended in the aqueous phase from the moment a sample is dispensed, which will shift, over time, the position of chemical equilibria between monomeric and multimeric components. Interferometric scattering microscopy (iSCAT) is a technique in the single-molecule toolkit that has the capability to detect unlabeled proteins and protein complexes both as they adsorb onto and desorb from a glass surface. Here, we examine the reversible and irreversible interactions between a number of different proteins and glass via analysis of the adsorption and desorption of protein at the single-molecule level. Furthermore, we present a method for surface passivation that virtually eliminates irreversible adsorption while still ensuring the residence time of molecules on surfaces is sufficient for detection of adsorption by iSCAT. By grafting high-density perfluoroalkane brushes on cover-glass surfaces, we observe approximately equal numbers of adsorption and desorption events for proteins at the measurement surface (±1%). The fluorous-aqueous interface also prevents the kinetic trapping of protein complexes and assists in establishing a thermodynamic equilibrium between monomeric and multimeric components. This surface passivation approach is valuable for in vitro single-molecule experiments using iSCAT microscopy because it allows for continuous monitoring of adsorption and desorption of protein without either a decline in detection events or a change in sample composition due to the irreversible binding of protein to surfaces.

Echem methods and electrode types of the current in vivo electrochemical sensing

For a long time, people have been eager to realize continuous real-time online monitoring of biological compounds. Fortunately, in vivo electrochemical biosensor technology has greatly promoted the development of biological compound detection. This article summarizes the existing in vivo electrochemical detection technologies into two categories: microdialysis (MD) and microelectrode (ME). Then we summarized and discussed the electrode surface time, pollution resistance, linearity and the number of instances of simultaneous detection and analysis, the composition and characteristics of the sensor, and finally, we also predicted and prospected the development of electrochemical technology and sensors in vivo.

Biorecognition antifouling coatings in complex biological fluids: a review of functionalization aspects

Recent progress in biointerface research has highlighted the role of antifouling functionalizable coatings in the development of advanced biosensors for point-of-care bioanalytical and biomedical applications dealing with real-world complex samples. The resistance to nonspecific adsorption promotes the biorecognition performance and overall increases the reliability and specificity of the analysis. However, the process of modification with biorecognition elements (so-called functionalization) may influence the resulting antifouling properties. The extent of these effects concerning both functionalization procedures potentially changing the surface architecture and properties, and the physicochemical properties of anchored biorecognition elements, remains unclear and has not been summarized in the literature yet. This critical review summarizes these key functionalization aspects with respect to diverse antifouling architectures showing low or ultra-low fouling quantitative characteristics in complex biological media such as bodily fluids or raw food samples. The subsequent discussion focuses on the impact of functionalization on fouling resistance. Furthermore, this review discusses some of the drawbacks of available surface sensitive characterization methods and highlights the importance of suitable assessment of the resistance to fouling.

Peptide-Modified Surfaces for Binding Carbamylated Proteins from Plasma

Carbamylation of blood proteins is a common post-translational modification that occurs upon kidney dysfunction that is strongly associated with deleterious outcomes for patients treated using hemodialysis. In this study, we focused on the removal of two representative carbamylated plasma proteins, carbamylated albumin (cHSA) and fibrinogen (cFgn), through adsorption onto a surface functionalized with a specific peptide (cH2p1). Surfaces modified with poly(hydroxyethyl methacrylate) (p(HEMA)) were prepared using surface-initiated atom transfer radical polymerization (SI-ATRP) techniques and functionalized with cH2p1. cH2p1-functionalized surfaces showed selective binding toward cHSA and cFgn, compared to their native protein form, with NH-cH2p1 of superior selectivity than CO-cH2p1. The adsorption capacity of carbamylated protein on NH-cH2p1 was maintained in diluted plasma, and ultralow adsorption of native Fgn was observed. Similar to unmodified p(HEMA) surfaces, NH-cH2p1 showed a low platelet adhesion and activation, suggesting that the designed surface does not adversely affect platelets.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

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.

Recent progress on developing of plasmon biosensing of tumor biomarkers: Efficient method towards early stage recognition of cancer

Cancer is the second most extended disease with an improved death rate over the past several time. Due to the restrictions of cancer analysis methods, the patient's real survival rate is unknown. Therefore, early stage diagnosis of cancer is crucial for its strong detection. Bio-analysis based on biomarkers may help to overcome the problem Biosensors with high sensitivity and specificity, low-cost, high analysis speed and minimum limit of detection are practical alternatives for laboratory tests. Surface plasmon resonance (SPR) is reaching a maturity level sufficient for their application in detection and determination cancer biomarkers in clinical samples. This review discusses main concepts and performance characteristics of SPR biosensor. Mainly, it focuses on newly emerged enhanced SPR biosensors towards high-throughput and ultrasensitive screening of cancer biomarkers such as PSA, α-fetoprotein, CEA, CA125, CA 15-3, HER2, ctDNA, ALCAM, hCG, VEGF, TNF, Interleukin, IFN-γ, CD24, CD44, Ferritin, COLIV using labeling processes with focusing on the future application in biomedical research and clinical diagnosis. This article reviews current status of the field, showcasing a series of early successes in the application of SPR for clinical bioanalysis of cancer related biomolecules and detailing a series of considerations regarding sensing schemes, exposing issues with analysis in biofluids, while providing an outlook of the challenges currently associated with plasmonic materials, bioreceptor selection, microfluidics, and validation of a clinical bioassay for applying SPR biosensors to clinical samples. Research opportunities are proposed to further advance the field and transition SPR biosensors from research proof-of-concept stage to actual clinical usage.

Surface Design of Antifouling Vascular Constructs Bearing Biofunctional Peptides for Tissue Regeneration Applications

Antifouling polymer layers containing extracellular matrix-derived peptide motifs offer promising new options for biomimetic surface engineering. In this contribution, we report the design of antifouling vascular grafts bearing biofunctional peptide motifs for tissue regeneration applications based on hierarchical polymer brushes. Hierarchical diblock poly(methyl ether oligo(ethylene glycol) methacrylate-block-glycidyl methacrylate) brushes bearing azide groups (poly(MeOEGMA-block-GMA-N₃)) were grown by surface-initiated atom transfer radical polymerization (SI-ATRP) and functionalized with biomimetic RGD peptide sequences. Varying the conditions of copper-catalyzed alkyne-azide “click” reaction allowed for the immobilization of RGD peptides in a wide surface concentration range. The synthesized hierarchical polymer brushes bearing peptide motifs were characterized in detail using various surface sensitive physicochemical methods. The hierarchical brushes presenting the RGD sequences provided excellent cell adhesion properties and at the same time remained resistant to fouling from blood plasma. The synthesis of anti-fouling hierarchical brushes bearing 1.2 × 10³ nmol/cm² RGD biomimetic sequences has been adapted for the surface modification of commercially available grafts of woven polyethylene terephthalate (PET) fibers. The fiber mesh was endowed with polymerization initiator groups via aminolysis and acylation reactions optimized for the material. The obtained bioactive antifouling vascular grafts promoted the specific adhesion and growth of endothelial cells, thus providing a potential avenue for endothelialization of artificial conduits.

Functionalizable Antifouling Coatings as Tunable Platforms for the Stress-Driven Manipulation of Living Cell Machinery

Cells are continuously sensing their microenvironment and subsequently respond to different physicochemical cues by the activation or inhibition of different signaling pathways. To study a very complex cellular response, it is necessary to diminish background environmental influences and highlight the particular event. However, surface-driven nonspecific interactions of the abundant biomolecules from the environment influence the targeted cell response significantly. Yes-associated protein (YAP) translocation may serve as a marker of human hepatocellular carcinoma (Huh7) cell responses to the extracellular matrix and surface-mediated stresses. Here, we propose a platform of tunable functionable antifouling poly(carboxybetain) (pCB)-based brushes to achieve a molecularly clean background for studying arginine, glycine, and aspartic acid (RGD)-induced YAP-connected mechanotransduction. Using two different sets of RGD-functionalized zwitterionic antifouling coatings with varying compositions of the antifouling layer, a clear correlation of YAP distribution with RGD functionalization concentrations was observed. On the other hand, commonly used surface passivation by the oligo(ethylene glycol)-based self-assembled monolayer (SAM) shows no potential to induce dependency of the YAP distribution on RGD concentrations. The results indicate that the antifouling background is a crucial component of surface-based cellular response studies, and pCB-based zwitterionic antifouling brush architectures may serve as a potential next-generation easily functionable surface platform for the monitoring and quantification of cellular processes.

Antifouling Polymer Brushes via Oxygen-Tolerant Surface-Initiated PET-RAFT

This work presents a new method for the synthesis of antifouling polymer brushes using surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain-transfer polymerization with eosin Y and triethanolamine as catalysts. This method proceeds in an aqueous environment under atmospheric conditions without any prior degassing and without the use of heavy metal catalysts. The versatility of the method is shown by using three chemically different monomers: oligo(ethylene glycol) methacrylate, N-(2-hydroxypropyl)methacrylamide, and carboxybetaine methacrylamide. In addition, the light-triggered nature of the polymerization allows the creation of complex three-dimensional structures. The composition and topological structuring of the brushes are confirmed by X-ray photoelectron spectroscopy and atomic force microscopy. The kinetics of the polymerizations are followed by measuring the layer thickness with ellipsometry. The polymer brushes demonstrate excellent antifouling properties when exposed to single-protein solutions and complex biological matrices such as diluted bovine serum. This method thus presents a new simple approach for the manufacturing of antifouling coatings for biomedical and biotechnological applications.

Advancements in SPR biosensing technology: An overview of recent trends in smart layers design, multiplexing concepts, continuous monitoring and in vivo sensing

Inspired by the rapid progress and existing limitations in surface plasmon resonance (SPR) biosensing technology, we have summarized the recent trends in the fields of both chip-SPR and fiber optic (FO)-SPR biosensors during the past five years, primarily regarding smart layers design, multiplexing, continuous monitoring and in vivo sensing. Versatile surface chemistries, biomaterials and nanomaterials have been utilized thus far to generate smart layers on SPR platforms and as such achieve oriented immobilization of bioreceptors, improved fouling resistance and sensitivity enhancement, collectively aiming to improve the biosensing performance. Furthermore, often driven by the desires for time- and cost-effective quantification of multiple targets in a single measurement, efforts have been made to implement multiplex bioassays on SPR platforms. While this aspect largely remains difficult to attain, numerous alternative strategies arose for obtaining parallel analysis of multiple analytes in one single device. Additionally, one of the upcoming challenges in this field will be to succeed in using SPR platforms for continuous measurements and in vivo sensing, and as such match up other biosensing platforms where these goals have been already conquered. Overall, this review will give insight into multiple possibilities that have become available over the years for boosting the performance of SPR biosensors. However, because combining them all into one optimal sensor is practically not feasible, the final application needs to be considered while designing an SPR biosensor, as this will determine the requirements of the bioassay and will thus help in selecting the essential elements from the recent progress made in SPR sensing.

Interfacial Forces at Layered Surfaces: Substrate Electrical Double-Layer Forces Acting through Ultrathin Polymer Coatings

Manipulating the surface properties of materials via the application of coatings is a widely used strategy to achieve desired interfacial interactions, implicitly assuming that the interfacial forces of coated samples are determined exclusively by the surface properties of the coatings. However, interfacial interactions between materials and their environments operate over finite length scales. Thus, the question addressed in this study is whether interactions associated with bulk substrate materials could act through thin coatings or, conversely, how thick a coating needs to be to completely screen subsurface forces contributed by underlying substrates. Plasma polymer layers were deposited on silicon wafer substrates from ethanol vapor, with identical chemical composition, ultrasmooth surfaces, and varying thicknesses. Using colloid-probe atomic force microscopy, electrical double-layer forces were determined in solutions of various ionic strengths and fitted using the Derjaguin-Landau-Verwey-Overbeek theory. For the thicker ethanol plasma polymers, the fitted surface potentials reflected the presence of surface carboxylate groups and were invariant with thickness. In contrast, for coatings <18 nm thick, the surface potentials increased steadily with decreasing film thickness; the measured electrical double-layer forces contained contributions from both the coating and the substrate. Theoretical calculations were in agreement with this model. Thus, our observations indicate that the higher surface potential of the underlying SiO₂ surface can influence the interactions between a colloid particle and the multilayer structure if coatings are sufficiently thin. Such superposition needs to be factored into the design of coatings aimed at the control of material interactions via surface forces.

P4VP Modified Zwitterionic Polymer for the Preparation of Antifouling Functionalized Surfaces

Zwitterionic polymers are suitable for replacing poly(ethylene glycol) (PEG) polymers because of their better antifouling properties, but zwitterionic polymers have poor mechanical properties, strong water absorption, and their homopolymers should not be used directly. To solve these problems, a reversible-addition fragmentation chain transfer (RAFT) polymerization process was used to prepare copolymers comprised of zwitterionic side chains that were attached to an ITO glass substrate using spin-casting. The presence of 4-vinylpyridine (4VP) and zwitterion chains on these polymer-coated ITO surfaces was confirmed using 1H NMR, FTIR, and GPC analyses, with successful surface functionalization confirmed using water contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) studies. Changes in water contact angles and C/O ratios (XPS) analysis demonstrated that the functionalization of these polymers with β-propiolactone resulted in hydrophilic mixed 4VP/zwitterionic polymers. Protein adsorption and cell attachment assays were used to optimize the ratio of the zwitterionic component to maximize the antifouling properties of the polymer brush surface. This work demonstrated that the antifouling surface coatings could be readily prepared using a "P4VP-modified" method, that is, the functionality of P4VP to modify the prepared zwitterionic polymer. We believe these materials are likely to be useful for the preparation of biomaterials for biosensing and diagnostic applications.

Conformational Dynamics and Responsiveness of Weak and Strong Polyelectrolyte Brushes: Atomistic Simulations of Poly(dimethyl aminoethyl methacrylate) and Poly(2-(methacryloyloxy)ethyl trimethylammonium chloride)

The complex solution behavior of polymer brushes is key to control their properties, including for biomedical applications and catalysis. The swelling behavior of poly(dimethyl aminoethyl methacrylate) (PDMAEMA) and poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (PMETAC) in response to changes in pH, solvent, and salt types has been investigated using atomistic molecular dynamics simulations. PDMAEMA and PMETAC have been selected as canonical models for weak and strong polyelectrolytes whose complex conformational behavior is particularly challenging for the development and validation of atomistic models. The GROMOS-derived atomic parameters reproduce the experimental swelling coefficients obtained from ellipsometry measurements for brushes of 5-15 nm thickness. The present atomistic models capture the protonated morphology of PDMAEMA, the swollen and collapsed conformations of PDMAEMA and PMETAC in good and bad solvents, and the salt-selective response of PMETAC. The modular nature of the molecular models allows for the simple extension of atomic parameters to a variety of polymers or copolymers.

Polymer brush interfaces for protein biosensing prepared by surface-initiated controlled radical polymerization

This article discusses protein-binding polymer brushes and the various strategies that can be used to immobilize proteins on these films.

ZnO Quantum Dots Modified by pH-Activated Charge-Reversal Polymer for Tumor Targeted Drug Delivery

In this paper, we reported a pH responsive nano drug delivery system (NDDS) based on ZnO quantum dots (QDs) for controlled release of drugs. Zwitterionic poly(carboxybetaine methacrylate) (PCBMA) and poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) were introduced to modify ZnO QDs, which can help enhance water stability, increase blood circulation time, and promote endocytosis. After tuning of PCBMA/PDMAEMA ratios, the ZnO@P(CBMA-co-DMAEMA) nanoplatform shows a sensitive switch from strong protein adsorption resistance (with negatively charged surface) at physiological pH to strong adhesion to tumor cell membranes (with positively charged surface) at the slightly acidic extracellular pH of tumors. Anti-cancer drug, Doxorubicin (DOX), molecules were demonstrated to be successfully loaded to ZnO@P(CBMA-co-DMAEMA) with a relatively large drug loading content (24.6%). In addition, ZnO@P(CBMA-co-DMAEMA) loaded with DOX can achieve lysosomal acid degradation and release of DOX after endocytosis by tumor cells, resulting in synergistic treatment of cancer, which is attributed to a combination of the anticancer effect of Zn2+ and DOX.

Low-fouling, mixed-charge poly-l-lysine polymers with anionic oligopeptide side-chains

Biosensors and biomedical devices require antifouling surfaces to prevent the non-specific adhesion of proteins or cells, for example, when aiming to detect circulating cancer biomarkers in complex natural media (e.g., in blood plasma or serum). A mixed-charge polymer was prepared by the coupling of a cationic polyelectrolyte and an anionic oligopeptide through a modified "grafting-to" method. The poly-l-lysine (PLL) backbone was modified with different percentages (y%) of maleimide-NHS ester chains (PLL-mal(y%), from 13% to 26%), to produce cationic polymers with specific grafting densities, obtaining a mixed-charge polymer. The anionic oligopeptide structure (CEEEEE) included one cysteine (C) and five glutamic acid (E) units, which were attached to the PLL-mal(y%) polymers, preadsorbed on gold substrates, through the thiol-maleimide Michael-type addition. Contact angle and PM-IRRAS data confirmed monolayer formation of the modified PLLs. Antifouling properties of peptide-PLL surfaces were assessed in adsorption studies using quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance imaging (SPRI) techniques. PLL-mal(26%)-CEEEEE showed the best antifouling performance in single-protein solutions, and the nonspecific adsorption of proteins was 46 ng cm^-2 using diluted human plasma samples. The new PLL-mal(26%)-CEEEEE polymer offers a prominent low-fouling activity in complex media, with rapid and simple procedures for the synthesis and functionalization of the surface compared to conventional non-fouling materials.

SPR Biosensor for Quantification of Fetuin-A as a Promising Multibiomarker

Early diagnosis of ongoing malignant disease is crucial to improve survival rate and life quality of the patients and requires sensitive detection of specific biomarkers e.g. prostate-specific antigen (PSA), carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), etc. In spite of current technological advances, malignant diseases are still identified in rather late stages, which have detrimental effect on the prognosis and treatment of the disease. Here, we present a biosensor able to detect fetuin-A, a potential multibiomarker. The biosensing platform is based on polymer brush combining antifouling monomer units of N-(2-hydroxypropyl)methacrylamide (HPMA) and carboxybetaine methacrylamide (CBMAA), statistically copolymerized by surface-initiated atom transfer radical polymerization. The copolymer poly(HPMA-co-CBMAA) exhibits excellent non-fouling properties in the most relevant biological media (i.e. blood plasma) as well as antithrombogenic surface properties by preventing the adhesion of blood components (i.e. leukocytes; platelets; and erythrocytes). Moreover, the polymer brush can be easily functionalized with biorecognition elements maintaining high resistance to blood fouling and the binding capacity can be regulated by tuning the ratio between CBMAA and HPMA units. The superior antifouling properties of the copolymer even after biofunctionalization were exploited to fabricate a new plasmonic biosensor for the analysis of fetuin-A in real clinical blood plasma samples. The assay used in this work can be explored as label-free affinity biosensor for diagnostics of different biomarkers in real clinical plasma samples and to shift the early biomarker detection toward novel biosensor technologies allowing point of care analysis.

Surface Plasmon Resonance Optical Sensor: A Review on Light Source Technology

The notion of surface plasmon resonance (SPR) sensor research emerged more than eight decades ago from the first observed phenomena in 1902 until the first introduced principles for gas sensing and biosensing in 1983. The sensing platform has been hand-in-hand with the plethora of sensing technology advancement including nanostructuring, optical technology, fluidic technology, and light source technology, which contribute to substantial progress in SPR sensor evolution. Nevertheless, the commercial products of SPR sensors in the market still require high-cost investment, component, and operation, leading to unaffordability for their implementation in a low-cost point of care (PoC) or laboratories. In this article, we present a comprehensive review of SPR sensor development including the state of the art from a perspective of light source technology trends. Based on our review, the trend of SPR sensor configurations, as well as its methodology and optical designs are strongly influenced by the development of light source technology as a critical component. These simultaneously offer new underlying principles of SPR sensor towards miniaturization, portability, and disposability features. The low-cost solid-state light source technology, such as laser diode, light-emitting diode (LED), organic light emitting diode (OLED) and smartphone display have been reported as proof of concept for the future of low-cost SPR sensor platforms. Finally, this review provides a comprehensive overview, particularly for SPR sensor designers, including emerging engineers or experts in this field.

Design and preparation of bi-functionalized short-chain modified zwitterionic nanoparticles

An ideal nanomaterial for use in the bio-medical field should have a distinctive surface capable of effectively preventing nonspecific protein adsorption and identifying target bio-molecules. Recently, the short-chain zwitterion strategy has been suggested as a simple and novel approach to create outstanding anti-fouling surfaces. In this paper, the carboxyl end group of short-chain zwitterion-coated silica nanoparticles (SiO₂-ZWS) was found to be difficult to functionalize via a conventional EDC/NHS strategy due to its rapid hydrolysis side-reactions. Hence, a series of bi-functionalized silica nanoparticles (SiO₂-ZWS/COOH) were designed and prepared by controlling the molar ratio of 3-aminopropyltriethoxysilane (APTES) to short-chain zwitterionic organosiloxane (ZWS) in order to achieve above goal. The synthesized SiO₂-ZWS/COOH had similar excellent anti-fouling properties compared with SiO₂-ZWS, even in 50% fetal bovine serum characterized by DLS and turbidimetric titration. Subsequently, SiO₂-ZWS/COOH_5/1 was chosen as a representative and then demonstrated higher detection signal intensity and more superior signal-to-noise ratios compare with the pure SiO₂-COOH when they were used as a bio-carrier for chemiluminescence enzyme immunoassay (CLEIA). These unique bi-functionalized silica nanoparticles have many potential applications in the diagnostic and therapeutic fields.

STATEMENT OF SIGNIFICANCE

Reducing nonspecific protein adsorption and enhancing the immobilized efficiency of specific bio-probes are two of the most important issues for bio-carriers, particularly for a nanoparticle based bio-carrier. Herein, we designed and prepared a bi-functional nanoparticle with anti-fouling property and bio conjugation capacity for further bioassay by improving the short-chain zwitterionic modification strategy we have proposed previously. The heterogeneous surface of this nanoparticle showed effective anti-fouling properties both in model protein solutions and fetal bovine serum (FBS). The modified nanoparticles can also be successfully functionalized with a specific antibody for CLEIA assay with a prominent bio-detection performance even in 50% FBS. In this paper, we also investigated an unexpectedly fast hydrolysis behavior of NHS-activated carboxylic groups within the pure short-chain zwitterionic molecule that led to no protein binding in the short-chain zwitterion modified nanoparticle. Our findings pave a new way for the designing of high performance bio-carriers, demonstrating their strong potential as a robust platform for diagnosis and therapy.

Polymer Brush-Functionalized Chitosan Hydrogels as Antifouling Implant Coatings

Implantable sensor devices require coatings that efficiently interface with the tissue environment to mediate biochemical analysis. In this regard, bioinspired polymer hydrogels offer an attractive and abundant source of coating materials. However, upon implantation these materials generally elicit inflammation and the foreign body reaction as a consequence of protein fouling on their surface and concomitant poor hemocompatibility. In this report we investigate a strategy to endow chitosan hydrogel coatings with antifouling properties by the grafting of polymer brushes in a "grafting-from" approach. Chitosan coatings were functionalized with polymer brushes of oligo(ethylene glycol) methyl ether methacrylate and 2-hydroxyethyl methacrylate using photoinduced single electron transfer living radical polymerization and the surfaces were thoroughly characterized by XPS, AFM, water contact angle goniometry, and in situ ellipsometry. The antifouling properties of these new bioinspired hydrogel-brush coatings were investigated by surface plasmon resonance. The influence of the modifications to the chitosan on hemocompatibility was assessed by contacting the surfaces with platelets and leukocytes. The coatings were hydrophilic and reached a thickness of up to 180 nm within 30 min of polymerization. The functionalization of the surface with polymer brushes significantly reduced the protein fouling and eliminated platelet activation and leukocyte adhesion. This methodology offers a facile route to functionalizing implantable sensor systems with antifouling coatings that improve hemocompatibility and pave the way for enhanced device integration in tissue.

Clickable Antifouling Polymer Brushes for Polymer Pen Lithography

Protein-repellent reactive surfaces that promote localized specific binding are highly desirable for applications in the biomedical field. Nonspecific adhesion will compromise the function of bioactive surfaces, leading to ambiguous results of binding assays and negating the binding specificity of patterned cell-adhesive motives. Localized specific binding is often achieved by attaching a linker to the surface, and the other side of the linker is used to bind specifically to a desired functional agent, as e.g. proteins, antibodies, and fluorophores, depending on the function required by the application. We present a protein-repellent polymer brush enabling highly specific covalent surface immobilization of biorecognition elements by strain-promoted alkyne-azide cycloaddition click chemistry for selective protein adhesion. The protein-repellent polymer brush is functionalized by highly localized molecular binding sites in the low micrometer range using polymer pen lithography (PPL). Because of the massive parallelization of writing pens, the tunable PPL printed patterns can span over square centimeter areas. The selective binding of the protein streptavidin to these surface sites is demonstrated while the remaining polymer brush surface is resisting nonspecific adsorption without any prior blocking by bovine serum albumin (BSA). In contrast to the widely used BSA blocking, the reactive polymer brushes are able to significantly reduce nonspecific protein adsorption, which is the cause of biofouling. This was achieved for solutions of single proteins as well as complex biological fluids. The remarkable fouling resistance of the polymer brushes has the potential to improve the multiplexing capabilities of protein probes and therefore impact biomedical research and applications.

Ultralow-Fouling Behavior of Biorecognition Coatings Based on Carboxy-Functional Brushes of Zwitterionic Homo- and Copolymers in Blood Plasma: Functionalization Matters

Fouling from complex biological fluids such as blood plasma to biorecognition element (BRE)-functionalized coatings hampers the use of affinity biosensor technologies in medical diagnostics. Here, we report the effects the molecular mechanisms involved in functionalization of low-fouling carboxy-functional coatings have on the BRE capacity and resistance to fouling from blood plasma. The specific mechanisms of EDC/NHS activation of carboxy groups, BRE attachment, and deactivation of residual activated groups on recently developed ultra-low-fouling carboxybetaine polymer and copolymer brushes (pCB) as well as conventional carboxy-terminated oligo(ethylene glycol)-based alkanethiolate self-assembled monolayers (OEG-SAMs) are studied using the polarization modulation infrared reflection/absorption spectroscopy, X-ray photoelectron spectroscopy, and surface plasmon resonance methods. It is shown that the fouling resistance of BRE-functionalized pCB coatings is strongly influenced by a deactivation method affecting the ultra-low-fouling molecular structure of the brush and surface charges. It is revealed that, in contrast to free carboxy-group-terminated OEG-SAMs, only a partial deactivation of EDC/NHS-activated zwitterionic carboxy groups by spontaneous hydrolysis is possible in the pCB brushes. The fouling resistance of activated/BRE-functionalized pCB is shown to be recovered only by covalent attachment of amino acid deactivation agents to residual activated carboxy groups of pCB. The developed deactivation procedure is further combined with ultra-low-fouling brushes of random copolymer carboxybetaine methacrylamide (CBMAA) and N-(2-hydroxypropyl) methacrylamide (HPMAA) with optimized CBMAA content (15%) providing a BRE-functionalized coating with superior fouling resistance over various carboxy-functional low-fouling coatings including homopolymer pCB brushes and OEG-SAMs. The biorecognition capabilities of pHPMAA-CBMAA(15%) are demonstrated via the sensitive label-free detection of a microRNA cancer biomarker (miR-16) in blood plasma.

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.

Ultra-low fouling methylimidazolium modified surfaces for the detection of HER2 in breast cancer cell lysates

We synthesized novel ultra-low fouling ionic liquids and demonstrated their use with surface plasmon resonance (SPR) sensing for the analysis of HER2 in breast cancer cell lysates.

Copolymer Brush-Based Ultralow-Fouling Biorecognition Surface Platform for Food Safety

Functional polymer coatings that combine the ability to resist nonspecific fouling from complex media with high biorecognition element (BRE) immobilization capacity represent an emerging class of new functional materials for a number of bioanalytical and biosensor technologies for medical diagnostics, security, and food safety. Here, we report on a random copolymer brush surface - poly(CBMAA-ran-HPMAA) - providing high BRE immobilization capacity while simultaneously exhibiting ultralow-fouling behavior in complex food media. We demonstrate that both the functionalization and fouling resistance capabilities of such copolymer brushes can be tuned by changing the surface contents of the two monomer units: nonionic N-(2-hydroxypropyl) methacrylamide (HPMAA) and carboxy-functional zwitterionic carboxybetaine methacrylamide (CBMAA). It is demonstrated that the resistance to fouling decreases with the surface content of CBMAA; poly(CBMAA-ran-HPMAA) brushes with CBMAA molar content up to 15 mol % maintain excellent resistance to fouling from a variety of homogenized foods (hamburger, cucumber, milk, and lettuce) even after covalent attachment of BREs to carboxy groups of CBMAA. The poly(CBMAA 15 mol %-ran-HPMAA) brushes functionalized with antibodies are demonstrated to exhibit fouling resistance from food samples by up to 3 orders of magnitude better when compared with the widely used low-fouling carboxy-functional oligo(ethylene glycol) (OEG)-based alkanethiolate self-assembled monolayers (AT SAMs) and, furthermore, by up to 2 orders of magnitude better when compared with the most successful ultralow-fouling biorecognition coatings - poly(carboxybetaine acrylamide), poly(CBAA). When model SPR detections of food-borne bacterial pathogens in homogenized foods are used, it is also demonstrated that the antibody-functionalized poly(CBMAA 15 mol %-ran-HPMAA) brush exhibits superior biorecognition properties over the poly(CBAA).

Design and mechanisms of antifouling materials for surface plasmon resonance sensors

Surface plasmon resonance (SPR) biosensors have many possible applications, but are limited by sensor chip surface fouling, which blocks immobilization and specific binding by the recognizer elements. Therefore, there is a pressing need for the development of antifouling surfaces. In this paper, the mechanisms of antifouling materials were firstly discussed, including both theories (hydration and steric hindrance) and factors influencing antifouling effects (molecular structures and self-assembled monolayer (SAM) architectures, surface charges, molecular hydrophilicity, and grafting thickness and density). Then, the most recent advances in antifouling materials applied on SPR biosensors were systematically reviewed, together with the grafting strategies, antifouling capacity, as well as their merits and demerits. These materials included, but not limited to, zwitterionic compounds, polyethylene glycol-based, and polysaccharide-based materials. Finally, the prospective research directions in the development of SPR antifouling materials were discussed.

STATEMENT OF SIGNIFICANCE

Surface plasmon resonance (SPR) is a powerful tool in monitoring biomolecular interactions. The principle of SPR biosensors is the conversion of refractive index change caused by molecular binding into resonant spectral shifts. However, the fouling on the surface of SPR gold chips is ubiquitous and troublesome. It limits the application of SPR biosensors by blocking recognition element immobilization and specific binding. Hence, we write this paper to review the antifouling mechanisms and the recent advances of the design of antifouling materials that can improve the accuracy and sensitivity of SPR biosensors. To our knowledge, this is the first review focusing on the antifouling materials that were applied or had potential to be applied on SPR biosensors.

Hepatitis B plasmonic biosensor for the analysis of clinical serum samples

A plasmonic biosensor for rapid detection of protein biomarkers in complex media is reported. Clinical serum samples were analyzed by using a novel biointerface architecture based on poly[(N-(2-hydroxypropyl) methacrylamide)-co-(carboxybetaine methacrylamide)] brushes functionalized with bioreceptors. This biointerface provided an excellent resistance to fouling even after the functionalization and allowed for the first time the direct detection of antibodies against hepatitis B surface antigen (anti-HBs) in clinical serum samples using surface plasmon resonance (SPR). The fabricated SPR biosensor allowed discrimination of anti-HBs positive and negative clinical samples in 10min. Results are validated by enzyme-linked immunoassays of the sera in a certified laboratory. The sensor could be regenerated by simple treatment with glycine buffer.

Clickable and Antifouling Platform of Poly[(propargyl methacrylate)-ran-(2-methacryloyloxyethyl phosphorylcholine)] for Biosensing Applications

A functional copolymer platform, namely, poly[(propargyl methacrylate)-ran-(2-methacryloyloxyethyl phosphorylcholine)] (PPgMAMPC), was synthesized by reversible addition-fragmentation chain-transfer polymerization. In principle, the alkyne moiety of propargyl methacrylate (PgMA) should serve as an active site for binding azide-containing molecules via a click reaction, i.e., Cu-catalyzed azide/alkyne cycloaddition (CuAAC), and 2-methacryloyloxyethyl phosphorylcholine (MPC), the hydrophilic monomeric unit, should enable the copolymer to suppress nonspecific adsorption. The copolymers were characterized using Fourier transform infrared (FTIR) and (1)H NMR spectroscopies. Thiol-terminated, PPgMAMPC-SH, obtained by aminolysis of PPgMAMPC, was immobilized on a gold-coated substrate using a "grafting to" approach via self-assembly. Azide-containing species, namely, biotin and peptide nucleic acid (PNA), were then immobilized on the alkyne-containing copolymeric platform via CuAAC. The potential use of surface-attached PPgMAMPC in biosensing applications was shown by detection of specific target molecules, i.e., streptavidin (SA) and DNA, by the developed sensing platform using a surface plasmon resonance technique. The copolymer composition strongly influenced the performance of the developed sensing platform in terms of signal-to-noise ratio in the case of the biotin-SA system and hybridization efficiency and mismatch discrimination for the PNA-DNA system.

Phototriggered functionalization of hierarchically structured polymer brushes

The precise design of bioactive surfaces, essential for the advancement of many biomedical applications, depends on achieving control of the surface architecture as well as on the ability to attach bioreceptors to antifouling surfaces. Herein, we report a facile avenue toward hierarchically structured antifouling polymer brushes of oligo(ethylene glycol) methacrylates via surface-initiated atom transfer radical polymerization (SI-ATRP) presenting photoactive tetrazole moieties, which permitted their functionalization via nitrile imine-mediated tetrazole-ene cyclocloaddition (NITEC). A maleimide-functional ATRP initiator was photoclicked to the side chains of a brush enabling a subsequent polymerization of carboxybetaine acrylamide to generate a micropatterned graft-on-graft polymer architecture as evidenced by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Furthermore, the spatially resolved biofunctionalization of the tetrazole-presenting brushes was accessed by the photoligation of biotin-maleimide and subsequent binding of streptavidin. The functionalized brushes bearing streptavidin were able to resist the fouling from blood plasma (90% reduction with respect to bare gold). Moreover, they were employed to demonstrate a model biosensor by immobilization of a biotinylated antibody and subsequent capture of an antigen as monitored in real time by surface plasmon resonance.