Self-Directed and Self-Oriented Immobilization of Antibody by Protein G−DNA Conjugate

Label free flexible electrochemical DNA biosensor for selective detection of Shigella flexneri in real food samples

An effective tool for early-stage selective detection of the foodborne bacterial pathogen Shigella flexneri (S. flexneri) is essential for diagnosing infectious diseases and controlling outbreaks. Here, a label-free electrochemical DNA biosensor for monitoring S. flexneri is developed. To fabricate the biosensor, detection probe (capture probe) is immobilized on the surface of poly melamine (P-Mel) and poly glutamic acid (PGA), and disuccinimidyl suberate (DSS) functionalized flexible indium tin oxide (ITO) electrode. Anthraquinone-2-sulfonic acid monohydrate sodium salt (AQMS) is used as a signal indicator for the detection of S. flexneri. The proposed DNA biosensor exhibits a wide dynamic range with concentration of the targets ranging from 1 × 10^-6 to 1 × 10^-21 molL^-1 with a limit of detection (LOD) of 7.4 × 10^-22 molL^-1 in the complementary linear target of S. flexneri, and a detection range of 8 × 10¹⁰-80 cells/ml with a LOD of 10 cells/ml in real S. flexneri sample. The proposed flexible biosensor provides high specificity for the detection of S. flexneri compared to other target signals such as discrete base mismatches and different bacterial species. The developed biosensor displayed excellent recoveries in detecting S. flexneri in spiked food samples. Therefore, the proposed biosensor can serve as a model methodology for the detection of other pathogens in a broad span of industries.

Precise and Prompt Analyte Detection via Ordered Orientation of Receptor in WSe2-Based Field Effect Transistor

Field-effect transistors (FET) composed of transition metal dichalcogenide (TMDC) materials have gained huge importance as biosensors due to their added advantage of high sensitivity and moderate bandgap. However, the true potential of these biosensors highly depends upon the quality of TMDC material, as well as the orientation of receptors on their surfaces. The uncontrolled orientation of receptors and screening issues due to crossing the Debye screening length while functionalizing TMDC materials is a big challenge in this field. To address these issues, we introduce a combination of high-quality monolayer WSe₂ with our designed Pyrene-based receptor moiety for its ordered orientation onto the WSe₂ FET biosensor. A monolayer WSe₂ sheet is utilized to fabricate an ideal FET for biosensing applications, which is characterized via Raman spectroscopy, atomic force microscopy, and electrical prob station. Our construct can sensitively detect our target protein (streptavidin) with 1 pM limit of detection within a short span of 2 min, through a one-step functionalizing process. In addition to having this ultra-fast response and high sensitivity, our biosensor can be a reliable platform for point-of-care-based diagnosis.

Detection and Quantification of Tp53 and p53-Anti-p53 Autoantibody Immune Complex: Promising Biomarkers in Early Stage Lung Cancer Diagnosis

Lung cancer is a leading cause of death worldwide, claiming nearly 1.80 million lives in 2020. Screening with low-dose computed tomography (LDCT) reduces lung cancer mortality by about 20% compared to standard chest X-rays among current or heavy smokers. However, several reports indicate that LDCT has a high false-positive rate. In this regard, methods based on biomarker detection offer excellent potential for developing noninvasive cancer diagnostic tests to complement LDCT for detecting stage 0∼IV lung cancers. Herein, we have developed a method for detecting and quantifying a p53-anti-p53 autoantibody complex and the total p53 antigen (wild and mutant). The LOD for detecting Tp53 and PIC were 7.41 pg/mL and 5.74 pg/mL, respectively. The detection ranges for both biomarkers were 0–7500 pg/mL. The known interfering agents in immunoassays such as biotin, bilirubin, intra-lipid, and hemoglobin did not detect Tp53 and PIC, even at levels that were several folds higher levels than their normal levels. Furthermore, the present study provides a unique report on this preliminary investigation using the PIC/Tp53 ratio to detect stage I–IV lung cancers. The presented method detects lung cancers with 81.6% sensitivity and 93.3% specificity. These results indicate that the presented method has high applicability for the identification of lung cancer patients from the healthy population.

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.

Site-Specific Linking of an Oligonucleotide to Mono- and Bivalent Recombinant Antibodies with SpyCatcher-SpyTag System for Immuno-PCR

Antibody-oligonucleotide conjugates (AOCs) are a versatile class of chimeric biomolecules for therapeutics and biotechnological applications. Most widely employed chemical labeling methods for proteins are based on targeting of Lys or Cys residues that leads to mixed stoichiometry in the degree of conjugation and may interfere with antigen binding, thus, compromising the function of the antibody. A site-specific oligonucleotide conjugation technology providing full control over valency in mild reaction conditions would be an advancement to the state-of-the-art in bioconjugation. Herein, we demonstrate the production of single-chain variable fragment antibodies with fused SpyCatcher (scFv-SpyCatcher, monovalent) and alkaline phosphatase-SpyCatcher (scFv-AP-SpyCatcher, bivalent) on C-terminus and their conjugation to SpyTag002-oligonucleotide in phosphate-buffered saline (PBS). The formation of a covalent isopeptide bond between the protein and SpyTag002-oligonucleotide was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis, and the functionality of the obtained AOCs was confirmed in immuno-polymerase chain reaction (PCR) assays for the detection of microcystin-LR and 17β-estradiol. Based on time-resolved fluorescence immunoassays with scFv-AP fusion constructs, we observed that the SpyCatcher and SpyCatcher-SpyTag002-oligonucleotide part lowered the absolute signal obtained from the assay by 27.6 and 48.4% at 2 nM and by 26.2 and 27.6% at 100 pM microcystin-LR and 17β-estradiol concentrations, respectively. Nevertheless, the overall sensitivity of the immuno-PCR assays was similar to the time-resolved fluorescence immunoassays performed with the same components. In this study, vectors for SpyCatcher-fusion construction were created for directional cloning with SfiI sites enabling the rapid generation of AOC constructs for site-specific SpyTag-oligonucleotide conjugation.

Antibody-Oligonucleotide Conjugates as Therapeutic, Imaging, and Detection Agents

Antibody-oligonucleotide conjugates (AOCs) are a novel class of synthetic chimeric biomolecules that has been continually gaining traction in different fields of modern biotechnology. This is mainly due to the unique combination of the properties of their two constituents, exceptional targeting abilities and antibody biodistribution profiles, in addition to an extensive scope of oligonucleotide functional and structural roles. Combining these two classes of biomolecules in one chimeric construct has therefore become an important milestone in the development of numerous biotechnological applications, including imaging (DNA-PAINT), detection (PLA, PEA), and therapeutics (targeted siRNA/antisense delivery). Numerous synthetic approaches have been developed to access AOCs ranging from stochastic chemical bioconjugation to site-specific conjugation with reactive handles, introduced into antibody sequences through protein engineering. This Review gives a general overview of the current status of AOC applications with a specific emphasis on the synthetic methods used for their preparation. The reported synthetic techniques are discussed in terms of their practical aspects and limitations. The importance of the development of novel methods for the facile generation of AOCs possessing a defined constitution is highlighted as a priority in AOC research to ensure the advance of their new applications.

Multifunctional Hybrid Magnetic Microgel Synthesis for Immune-Based Isolation and Post-Isolation Culture of Tumor Cells

Circulating tumor cells are of utmost importance among various biomarkers in liquid biopsies as a prognosis indicator of metastasis as well as in chemotherapeutic monitoring. This study introduces an efficient tool composed of soft nano/hybrid immune microgels for magnetic isolation of targeted tumor cells. The development process involves the in situ synthesis of magnetic nanoparticles within the three-dimensional matrix of thermoresponsive microgels. Surface modification and anti-EpCAM conjugation are adjusted by changing the temperature, and a conjugation efficiency of around 70% is achieved by using a protein G linker. Anti-EpCAM-conjugated nano/hybrid magnetic microgels are used to isolate EpCAM-expressing breast adenocarcinoma MCF-7 cells from culture media and whole blood with an efficiency of 75 and 70%, respectively. Furthermore, we demonstrate the ability of the hybrid microgels to isolate cancer cells with a purity of 65% and culture the cells post-isolation for further drug studies. The multifunctional hybrid microcarriers reported in this work can be potentially used for continuous monitoring of cancers and in personalized medicine.

Quantification of CYFRA 21-1 and a CYFRA 21-1–anti-CYFRA 21-1 autoantibody immune complex for detection of early stage lung cancer

Population-based screening of stage 0–I lung cancer is crucial for saving lives. The CIC/CYFRA 21-1 ratio allows the detection of stage I lung cancer with 76.0% sensitivity and 87.5% specificity.

A glass fibre membrane platform for ultra-sensitive detection of cardiac troponin T

A glass fibre membrane platform that allows quantification of circulating cTnT with a LoD of 0.87 pg mL^-1 is described. The proposed platform uses a glass fibre membrane, DNA-guided detection method, and antibody-conjugated fluorescent beads for the quantification of cTnT in the analytical detection range of 1-120 pg mL^-1 at room temperature in 30 min. Glass fibre membranes were chemically modified to immobilize the oligonucleotide probes that catch a biomolecular complex (FB-dAB-cTnT-cAB-DNA) containing complementary oligonucleotides. There were no interferences from human cTnI, cTnC, skTnT, biotin, and hemoglobin (each 1 μg mL^-1). The linearity in the serial dilution test of plasma samples indicates that this platform is highly applicable for regular health check-up to assess the risk of AMI and HF.

A critical insight into the development pipeline of microfluidic immunoassay devices for the sensitive quantitation of protein biomarkers at the point of care

The latest clinical procedures for the timely and cost-effective diagnosis of chronic and acute clinical conditions, such as cardiovascular diseases, cancer, chronic respiratory diseases, diabetes or sepsis (i.e. the biggest causes of death worldwide), involve the quantitation of specific protein biomarkers released into the blood stream or other physiological fluids (e.g. urine or saliva). The clinical thresholds are usually in the femtomolar to picolomar range, and consequently the measurement of these protein biomarkers heavily relies on highly sophisticated, bulky and automated equipment in centralised pathology laboratories. The first microfluidic devices capable of measuring protein biomarkers in miniaturised immunoassays were presented nearly two decades ago and promised to revolutionise point-of-care (POC) testing by offering unmatched sensitivity and automation in a compact POC format; however, the development and adoption of microfluidic protein biomarker tests has fallen behind expectations. This review presents a detailed critical overview into the pipeline of microfluidic devices developed in the period 2005-2016 capable of measuring protein biomarkers from the pM to fM range in formats compatible with POC testing, with a particular focus on the use of affordable microfluidic materials and compact low-cost signal interrogation. The integration of these two important features (essential unique selling points for the successful microfluidic diagnostic products) has been missed in previous review articles and explain the poor adoption of microfluidic technologies in this field. Most current miniaturised devices compromise either on the affordability, compactness and/or performance of the test, making current tests unsuitable for the POC measurement of protein biomarkers. Seven core technical areas, including (i) the selected strategy for antibody immobilisation, (ii) the surface area and surface-area-to-volume ratio, (iii) surface passivation, (iv) the biological matrix interference, (v) fluid control, (vi) the signal detection modes and (vii) the affordability of the manufacturing process and detection system, were identified as the key to the effective development of a sensitive and affordable microfluidic protein biomarker POC test.

Utilizing DNA for functionalization of biomaterial surfaces

DNA sequences are widely used for gene transfer into cells including a number of substrate surface-based supporting systems, but due to its singular structure property profile, DNA also offers multiple options for noncanonical applications. The special case of using DNA and oligodeoxyribonucleotide (ODN) structures for surface functionalization of biomedical implants is summarized here with the major focus on (a) immobilization or anchoring of nucleic acid structures on substrate surfaces, (b) incorporation of biologically active molecules (BAM) into such systems, and (c) biological characteristics of the resulting surfaces in vitro and in vivo. Sterilizations issues, important for potential clinical applications, are also considered.

From Protein Features to Sensing Surfaces

Proteins play a major role in biosensors in which they provide catalytic activity and specificity in molecular recognition. However, the immobilization process is far from straightforward as it often affects the protein functionality. Extensive interaction of the protein with the surface or significant surface crowding can lead to changes in the mobility and conformation of the protein structure. This review will provide insights as to how an analysis of the physico-chemical features of the protein surface before the immobilization process can help to identify the optimal immobilization approach. Such an analysis can help to preserve the functionality of the protein when on a biosensor surface.

Antigen detection with thermosensitive hydrophilicity of poly(N-isopropylacrylamide)-grafted poly(vinyl chloride) fibrous mats

The static water contact angle of stimuli-responsive fibrous mats is used as a convenient index for rapid antigen detection.

Human neonatal Fc receptor as a new potential antibody binding protein for antibody immobilization

A critical challenge in producing an antibody-based assay with the highest reproducibility and sensitivity is the strategy to immobilize antibodies to solid phase. To date, numerous methods of antibody immobilization were reported but each was subjected to its advantages and limitations. The current study proposes a new potential antibody binding protein, the human neonatal fragment crystallizable (Fc) receptor. This protein has shown its high affinity to the Fc of antibody either in vivo or in vitro. Human neonatal Fc receptor is a heterodimer constructed by p51 α-heavy chain and β2-microglobulin light chain; however, the binding sites toward the antibody are located in the p51 α-heavy chain. Hence, vector cloning and recombinant protein expression were carried out to express the p51 α-heavy chain of the human neonatal Fc receptor (hFcRn-α). The recombinant protein expressed, hFcRn-α, was adopted to pin rabbit IgG against hepatitis B virus surface antigen to a solid phase. A sandwich enzyme-linked immunosorbent assay was further developed to evaluate the efficiency of hFcRn-α-directed immobilization in antigen detection. The result was compared with the conventional physical adsorption method. The findings demonstrated that human neonatal Fc receptor was efficient in pinning antibodies and generating higher signals compared with the physical adsorption of antibody.

Strategies to develop endogenous stem cell-recruiting bioactive materials for tissue repair and regeneration

A leading strategy in tissue engineering is the design of biomimetic scaffolds that stimulate the body's repair mechanisms through the recruitment of endogenous stem cells to sites of injury. Approaches that employ the use of chemoattractant gradients to guide tissue regeneration without external cell sources are favored over traditional cell-based therapies that have limited potential for clinical translation. Following this concept, bioactive scaffolds can be engineered to provide a temporally and spatially controlled release of biological cues, with the possibility to mimic the complex signaling patterns of endogenous tissue regeneration. Another effective way to regulate stem cell activity is to leverage the inherent chemotactic properties of extracellular matrix (ECM)-based materials to build versatile cell-instructive platforms. This review introduces the concept of endogenous stem cell recruitment, and provides a comprehensive overview of the strategies available to achieve effective cardiovascular and bone tissue regeneration.

Ultra-Sensitive NT-proBNP Quantification for Early Detection of Risk Factors Leading to Heart Failure

Cardiovascular diseases such as acute myocardial infarction and heart failure accounted for the death of 17.5 million people (31% of all global deaths) in 2015. Monitoring the level of circulating N-terminal proBNP (NT-proBNP) is crucial for the detection of people at risk of heart failure. In this article, we describe a novel ultra-sensitive NT-proBNP test (us-NT-proBNP) that allows the quantification of circulating NT-proBNP in 30 min at 25 °C in the linear detection range of 7.0-600 pg/mL. It is a first report on the application of a fluorescence bead labeled detection antibody, DNA-guided detection method, and glass fiber membrane platform for the quantification of NT-proBNP in clinical samples. Limit of blank, limit of detection, and limit of quantification were 2.0 pg/mL, 3.7 pg/mL, and 7 pg/mL, respectively. The coefficient of variation was found to be less than 10% in the entire detection range of 7-600 pg/mL. The test demonstrated specificity for NT-proBNP without interferences from bilirubin, intra-lipid, biotin, and hemoglobin. The serial dilution test for plasma samples containing various NT-proBNP levels showed the linear decrement in concentration with the regression coefficient of 0.980-0.998. These results indicate that us-NT-proBNP test does not suffer from the interference of the plasma components for the measurement of NT-proBNP in clinical samples.

Orientation and density control of proteins on solid matters by outer membrane coating: Analytical and diagnostic applications

Autodisplay is an expression system for the display of recombinant proteins on the outer membrane (OM) of gram negative bacteria and has been developed for translocation studies, whole cell biocatalysis, bioremediation, inhibitor screening, and enzyme refolding. Recently, affinity proteins such as IgG-binding Z-domains and biotin-binding streptavidin have been autodisplayed on the OM of Escherichia coli for analytical and biomedical applications. The secretion mechanism of the autodisplay system was used and orientation and density control of these affinity proteins were determined. Affinity protein-autodisplaying E. coli cells have been used to coat solid supports in immunoassays. For this purpose, the OM of autodisplayed E. coli cells was separated and isolated by the aid of detergents. The structure of the resulting OM liposomes as well as their physico-chemical parameters, were analyzed. OM liposomes were used subsequently for coating various solid matters including microplates and biosensor transducer surfaces and the formation of OM layers were monitored. OM layer formation on solid matters was shown to increase the sensitivity of immunoassays and biosensors. In this review, analytical and diagnostic applications are described in particular concerning orientation and density control of autodisplayed affinity proteins.

Poly-protein G-expressing bacteria enhance the sensitivity of immunoassays

The sensitivities of solid-phase immunoassays are limited by the quantity of detection antibodies bound to their antigens on the solid phase. Here, we developed a poly-protein G-expressing bacterium as an antibody-trapping microparticle to enhance the signals of immunoassays by increasing the accumulation of detection antibodies on the given antigen. Eight tandemly repeated fragment crystallisable (Fc) binding domains of protein G were stably expressed on the surface of Escherichia coli BL21 cells (termed BL21/8G). BL21/8G cells showed a higher avidity for trapping antibodies on their surface than monomeric protein G-expressing BL21 (BL21/1G) cells did. In the sandwich enzyme-linked immunosorbent assay (ELISA), simply mixing the detection antibody with BL21/8G provided a detection limit of 6 pg/mL for human interferon-α (IFN-α) and a limit of 30 pg/mL for polyethylene glycol (PEG)-conjugated IFN-α (Pegasys), which are better than that of the traditional ELISA (30 pg/mL for IFN-α and 100 pg/mL for Pegasys). Moreover, the sensitivity of the Western blot for low-abundance Pegasys (0.4 ng/well) was increased by 25 folds upon mixing of an anti-PEG antibody with BL21/8G cells. By simply being mixed with a detection antibody, the poly-protein G-expressing bacteria can provide a new method to sensitively detect low-abundance target molecules in solid-phase immunoassays.

DNA Linkers and Diluents for Ultrastable Gold Nanoparticle Bioconjugates in Multiplexed Assay Development

A novel bioconjugation strategy leading to ultrastable gold nanoparticles (AuNPs), utilizing DNA linkers and diluents in place of traditional self-assembled monolayers, is reported. The protective capacity of DNA confers straightforward biomolecular attachment and multistep derivatization capabilities to these nanoparticles and, more significantly, substantially enhances their stability in demanding and complex sensing environments. The DNA/AuNPs were assembled through pH-assisted thiol-gold bonding of single stranded DNA and salt aging, with preconjugated biotin moieties facing outward from the gold surface. These nanoparticles remain a stable colloidal suspension under a wide range of buffers and ionic strengths and can endure multiple rounds of lyophilization while retaining high biological activity. Furthermore, the high stability of the DNA/AuNPs allows for multiple reactions and conjugations to be performed within the colloidal suspensions (i.e., Protein A and antibody binding) for tailored and specific recognition to take place. We have demonstrated the applications of the DNA/AuNPs for colorimetric assays and ELISA feasibility; additionally, SPR imaging analysis of a supported membrane microarray shows excellent results with DNA/AuNPs as the enhancing agent. Together, the properties imparted by this interface render the material suitable for clinical and point-of-care applications where stability, throughput, and extended shelf lives are needed.

Site-selective orientated immobilization of antibodies and conjugates for immunodiagnostics development

Immobilized antibody systems are the key to develop efficient diagnostics and separations tools. In the last decade, developments in the field of biomolecular engineering and crosslinker chemistry have greatly influenced the development of this field. With all these new approaches at our disposal, several new immobilization methods have been created to address the main challenges associated with immobilized antibodies. Few of these challenges that we have discussed in this review are mainly associated to the site-specific immobilization, appropriate orientation, and activity retention. We have discussed the effect of antibody immobilization approaches on the parameters on the performance of an immunoassay.

A self-directed and reconstructible immobilization strategy: DNA directed immobilization of alkaline phosphatase for enzyme inhibition assays

A DNA-directed immobilization technique is used to develop a common method for the reversible and self-directed immobilization of enzymes.

Multiplexed In-cell Immunoassay for Same-sample Protein Expression Profiling

In-cell immunoassays have become a valuable tool for protein expression analysis complementary to established assay formats. However, comprehensive molecular characterization of individual specimens has proven challenging and impractical due to, in part, a singleplex nature of reporter enzymes and technical complexity of alternative assay formats. Herein, we describe a simple and robust methodology for multiplexed protein expression profiling on the same intact specimen, employing a well-characterized enzyme alkaline phosphatase for accurate quantification of all targets of interest, while overcoming fundamental limitations of enzyme-based techniques by implementing the DNA-programmed release mechanism for segregation of sub-sets of target-bound reporters. In essence, this methodology converts same-sample multi-target labeling into a set of isolated singleplex measurements performed in a parallel self-consistent fashion. For a proof-of-principle, multiplexed detection of three model proteins was demonstrated on cultured HeLa cells, and two clinically-relevant markers of dementia, β-amyloid and PHF-tau, were profiled in formalin-fixed paraffin embedded brain tissue sections, uncovering correlated increase in abundance of both markers in the “Alzheimer’s disease” cohort. Featuring an analytically powerful yet technically simple and robust methodology, multiplexed in-cell immunoassay is expected to enable insightful same-sample protein profiling studies and become broadly adopted in biomedical research and clinical diagnostics.

Temperature-Responsive Poly(N-isopropylacrylamide) Modified Gold Nanoparticle-Protein Conjugates for Bioactivity Modulation

It is important to effectively maintain and modulate the bioactivity of protein-nanoparticle conjugates for their further and intensive applications. The strategies of controlling protein activity via "tailor-made surfaces" still have some limitations, such as the difficulties in further modulation of the bioactivity and the proteolysis by some proteases. Thus, it is essential to establish a responsive protein-nanoparticle conjugate system to realize not only controllable modulations of protein activity in the conjugates by incorporating sensitivity to environmental cues but also high resistance to proteases. In the work reported here, Escherichia coli (E. coli) inorganic pyrophosphatase (PPase) and poly(N-isopropylacrylamide) (pNIPAM) were both fabricated onto gold nanoparticles (AuNPs), forming AuNP-PPase-pNIPAM conjugates. The bioactivity-modulating capability of the conjugates with changes in temperature was systematically investigated by varying the molecular weight of pNIPAM, the PPase/pNIPAM molar ratio on AuNP, and the orientation of the proteins. Under proper conditions, the activity of the conjugate at 45 °C was approximately 270% of that at 25 °C. In the presence of trypsin digestion, much less conjugate activity than protein activity was lost. These findings indicate that the fabrication of AuNP-protein-pNIPAM conjugates can both modulate protein activity on a large scale and show much higher resistance to protease digestion, exhibiting great potential in targeted delivery, controllable biocatalysis, and molecular/cellular recognition.

Green fluorescent protein nanopolygons as monodisperse supramolecular assemblies of functional proteins with defined valency

Supramolecular protein assemblies offer novel nanoscale architectures with molecular precision and unparalleled functional diversity. A key challenge, however, is to create precise nano-assemblies of functional proteins with both defined structures and a controlled number of protein-building blocks. Here we report a series of supramolecular green fluorescent protein oligomers that are assembled in precise polygonal geometries and prepared in a monodisperse population. Green fluorescent protein is engineered to be self-assembled in cells into oligomeric assemblies that are natively separated in a single-protein resolution by surface charge manipulation, affording monodisperse protein (nano)polygons from dimer to decamer. Several functional proteins are multivalently displayed on the oligomers with controlled orientations. Spatial arrangements of protein oligomers and displayed functional proteins are directly visualized by a transmission electron microscope. By employing our functional protein assemblies, we provide experimental insight into multivalent protein–protein interactions and tools to manipulate receptor clustering on live cell surfaces.

Supramolecular protein assemblies can provide novel nano-architectures with diverse structures and functions. Here, the authors report a fabrication strategy for a series of monodisperse protein oligomers, which allows valency-controlled display of various functional proteins.

Surface plasmon resonance: a versatile technique for biosensor applications

Surface plasmon resonance (SPR) is a label-free detection method which has emerged during the last two decades as a suitable and reliable platform in clinical analysis for biomolecular interactions. The technique makes it possible to measure interactions in real-time with high sensitivity and without the need of labels. This review article discusses a wide range of applications in optical-based sensors using either surface plasmon resonance (SPR) or surface plasmon resonance imaging (SPRI). Here we summarize the principles, provide examples, and illustrate the utility of SPR and SPRI through example applications from the biomedical, proteomics, genomics and bioengineering fields. In addition, SPR signal amplification strategies and surface functionalization are covered in the review.

Modulating the activity of protein conjugated to gold nanoparticles by site-directed orientation and surface density of bound protein

The key property of protein-nanoparticle conjugates is the bioactivity of the protein. The ability to accurately modulate the activity of protein on the nanoparticles at the interfaces is important in many applications. In the work reported here, modulation of the activity of protein-gold nanoparticle (AuNP) conjugates by specifically orienting the protein and by varying the surface density of the protein was investigated. Different orientations were achieved by introducing cysteine (Cys) residues at specific sites for binding to gold. We chose Escherichia coli inorganic pyrophosphatase (PPase) as a model protein and used site-directed mutagenesis to generate two mutant types (MTs) with a single Cys residue on the surface: MT1 with Cys near the active center and MT2 with Cys far from the active center. The relative activities of AuNP conjugates with wild type (WT), MT1, and MT2 were found to be 44.8%, 68.8%, and 91.2% of native PPase in aqueous solution. Site-directed orientation with the binding site far from the active center thus allowed almost complete preservation of the protein activity. The relative activity of WT and MT2 conjugates did not change with the surface density of the protein, while that of MT1 increased significantly with increasing surface density. These results demonstrate that site-directed orientation and surface density can both modulate the activity of proteins conjugated to AuNP and that orientation has a greater effect than density. Furthermore, increasing the surface density of the specifically oriented protein MT2, while having no significant effect on the specific activity of the protein, still allowed increased protein loading on the AuNP and thus increased the total protein activity. This is of great importance in the study on the interface of protein and nanoparticle and the applications for enzyme immobilization, drug delivery, and biocatalysis.

Self-oriented nanoparticles for site-selective immunoglobulin G recognition via epitope imprinting approach

Molecular imprinting is a polymerization technique that provides synthetic analogs for template molecules. Molecularly imprinted polymers (MIPs) have gained much attention due to their unique properties such as selectivity and specificity for target molecules. In this study, we focused on the development of polymeric materials with molecular recognition ability, so molecular imprinting was combined with miniemulsion polymerization to synthesize self-orienting nanoparticles through the use of an epitope imprinting approach. Thus, L-lysine imprinted nanoparticles (LMIP) were synthesized via miniemulsion polymerization technique. Immunoglobulin G (IgG) was then bound to the cavities that specifically formed for L-lysine molecules that are typically found at the C-terminus of the Fc region of antibody molecules. The resulting nanoparticles makes it possible to minimize the nonspecific interaction between monomer and template molecules. In addition, the orientation of the entire IgG molecule was controlled, and random imprinting of the IgG was prevented. The optimum conditions were determined for IgG recognition using the imprinted nanoparticles. The selectivity of the nanoparticles against IgG molecules was also evaluated using albumin and hemoglobin as competitor molecules. In order to show the self-orientation capability of imprinted nanoparticles, human serum albumin (HSA) adsorption onto both the plain nanoparticles and immobilized nanoparticles by anti-human serum albumin antibody (anti-HSA antibody) was also carried out. Due to anti-HSA antibody immobilization on the imprinted nanoparticles, the adsorption capability of nanoparticles against HSA molecules vigorously enhanced. It is proved that the oriented immobilization of antibodies was appropriately succeeded.

Fabrication of antibody microarrays by light-induced covalent and oriented immobilization

Antibody microarrays have important applications for the sensitive detection of biologically important target molecules and as biosensors for clinical applications. Microarrays produced by oriented immobilization of antibodies generally have higher antigen-binding capacities than those in which antibodies are immobilized with random orientations. Here, we present a UV photo-cross-linking approach that utilizes boronic acid to achieve oriented immobilization of an antibody on a surface while retaining the antigen-binding activity of the immobilized antibody. A photoactive boronic acid probe was designed and synthesized in which boronic acid provided good affinity and specificity for the recognition of glycan chains on the Fc region of the antibody, enabling covalent tethering to the antibody upon exposure to UV light. Once irradiated with optimal UV exposure (16 mW/cm(2)), significant antibody immobilization on a boronic acid-presenting surface with maximal antigen detection sensitivity in a single step was achieved, thus obviating the necessity of prior antibody modifications. The developed approach is highly modular, as demonstrated by its implementation in sensitive sandwich immunoassays for the protein analytes Ricinus communis agglutinin 120, human prostate-specific antigen, and interleukin-6 with limits of detection of 7.4, 29, and 16 pM, respectively. Furthermore, the present system enabled the detection of multiple analytes in samples without any noticeable cross-reactivities. Antibody coupling via the use of boronic acid and UV light represents a practical, oriented immobilization method with significant implications for the construction of a large array of immunosensors for diagnostic applications.

Controlled protein embedment onto Au/Ag core-shell nanoparticles for immuno-labeling of nanosilver surface

Difficulties in stable conjugation of biomolecules to nanosilver surfaces have severely limited the use of silver nanostructures in biological applications. Here, we report a facile antibody conjugation onto gold/silver (Au/Ag) core-shell nanoparticles by stable and uniform embedment of an antibody binding protein, protein G, in silver nanoshells. A rigid helical peptide linker with a terminal cysteine residue was fused to protein G. A mixture of the peptide-fused protein G and space-filling free peptide was reacted with gold nanoparticles (AuNPs) to form a protein G-linked peptide layer on the particle surface. Uniform silver nanoshells were successfully formed on these protein G-AuNPs, while stably embedding protein G-linked peptide layers. Protein G specifically targets the Fc region of an antibody and thus affords properly orientated antibodies on the particle surface. Compared to Au nanoparticles of similar size with randomly adsorbed antibodies, the present immuno-labeled Au/Ag core-shell nanoparticles offered nearly 10-fold higher sensitivities for naked-eye detection of surface bound antigens. In addition, small dye molecules that were bonded to the peptide layer on Au nanoparticles exhibited highly enhanced surface-enhanced Raman scattering (SERS) signals upon Ag shell formation. The present strategy provides a simple but efficient way to conjugate antibodies to nanosilver surfaces, which will greatly facilitate wider use of the superior optical properties of silver nanostructures in biological applications.

A universal DNA-based protein detection system

Protein immune detection requires secondary antibodies which must be carefully selected in order to avoid interspecies cross-reactivity, and is therefore restricted by the limited availability of primary/secondary antibody pairs. Here we present a versatile DNA-based protein detection system using a universal adapter to interface between IgG antibodies and DNA-modified reporter molecules. As a demonstration of this capability, we successfully used DNA nano-barcodes, quantum dots, and horseradish peroxidase enzyme to detect multiple proteins using our DNA-based labeling system. Our system not only eliminates secondary antibodies but also serves as a novel method platform for protein detection with modularity, high capacity, and multiplexed capability.

Multiscale simulations of protein G B1 adsorbed on charged self-assembled monolayers

The orientation of an antibody plays an important role in the development of immunosensors. Protein G is an antibody binding protein, which specifically targets the Fc fragment of an antibody. In this work, the orientation of prototypical and mutated protein G B1 adsorbed on positively and negatively charged self-assembled monolayers was studied by parallel tempering Monte Carlo and all-atom molecular dynamics simulations. Both methods present generally similar orientation distributions of protein G B1 for each kind of surface. The root-mean-square deviation, DSSP, gyration radius, eccentricity, dipole moment, and superimposed structures of protein G B1 were analyzed. Moreover, the orientation of binding antibody was also predicted in this work. Simulation results show that with the same orientation trends, the mutant exhibits narrower orientation distributions than does the prototype, which was mainly caused by the stronger dipole of the mutant. Both kinds of proteins adsorbed on charged surfaces were induced by the competition of electrostatic interaction and vdW interaction; the electrostatic interaction energy dominated the adsorption behavior. The protein adsorption was also largely affected by the distribution of charged residues within the proteins. Thus, the prototype could adsorb on a negatively charged surface, although it keeps a net charge of -4 e. The mutant has imperfect opposite orientation when it adsorbed on oppositely charged surfaces. For the mutant on a carboxyl-functionalized self-assembled monolayer (COOH-SAM), the orientation was the same as that inferred by experiments. While for the mutant on amine-functionalized self-assembled monolayer (NH2-SAM), the orientation was induced by the competition between attractive interactions (led by ASP40 and GLU56) and repulsive interactions (led by LYS10); thus, the perfect opposite orientation could not be obtained. On both surfaces, the adsorbed protein could retain its native conformation. The desired orientation of protein G B1, which would increase the efficiency of binding antibodies, could be obtained on a negatively charged surface adsorbed with the prototype. Further, we deduced that with the packing density of 12,076 protein G B1 domain per μm(2), the efficiency of the binding IgG would be maximized. The simulation results could be applied to control the orientation of protein G B1 in experiments and to provide a better understanding to maximize the efficiency of antibody binding.

Site-directed antibody immobilization techniques for immunosensors

Immunosensor sensitivity, regenerability, and stability directly depend on the type of antibodies used for the immunosensor design, quantity of immobilized molecules, remaining activity upon immobilization, and proper orientation on the sensing interface. Although sensor surfaces prepared with antibodies immobilized in a random manner yield satisfactory results, site-directed immobilization of the sensing molecules significantly improves the immunosensor sensitivity, especially when planar supports are employed. This review focuses on the three most conventional site-directed antibody immobilization techniques used in immunosensor design. One strategy of immobilizing antibodies on the sensor surface is via affinity interactions with a pre-formed layer of the Fc binding proteins, e.g., protein A, protein G, Fc region specific antibodies or various recombinant proteins. Another immobilization strategy is based on the use of chemically or genetically engineered antibody fragments that can be attached to the sensor surface covered in gold or self-assembled monolayer via the sulfhydryl groups present in the hinge region. The third most common strategy is antibody immobilization via an oxidized oligosaccharide moiety present in the Fc region of the antibody. The principles, advantages, applications, and arising problems of these most often applied immobilization techniques are reviewed.

Regioselective covalent immobilization of recombinant antibody-binding proteins A, G, and L for construction of antibody arrays

Immobilized antibodies are useful for the detection of antigens in highly sensitive microarray diagnostic applications. Arrays with the antibodies attached regioselectively in a uniform orientation are typically more sensitive than those with random orientations. Direct regioselective immobilization of antibodies on a solid support typically requires a modified form of the protein. We now report a general approach for the regioselective attachment of antibodies to a surface using truncated forms of antibody-binding proteins A, G, and L that retain the structural motifs required for antibody binding. The recombinant proteins have a C-terminal CVIX protein farnesyltransferase recognition motif that allows us to append a bioorthogonal azide or alkyne moiety and use the Cu(I)-catalyzed Huisgen cycloaddition to attach the binding proteins to a suitably modified glass surface. This approach offers several advantages. The recombinant antibody-binding proteins are produced in Escherichia coli, chemoselectively modified posttranslationally in the cell-free homogenate, and directly attached to the glass surface without the need for purification at any stage of the process. Complexes between immobilized recombinant proteins A, G, and L and their respective strongly bound antibodies were stable to repeated washing with PBST buffer at pH 7.2. However, the antibodies could be stripped from the slides by treatment with 0.1 M glycine·HCl buffer, pH 2.6, for 30 min and regenerated by shaking with PBS buffer, pH 7.2, at 4 °C overnight. The recombinant forms of proteins A, G, and L can be used separately or in combination to give glass surfaces capable of binding a wide variety of antibodies.