Measuring binding kinetics of surface-bound molecules using the surface plasmon resonance technique

Grafting hyaluronic acid onto gold surface to achieve low protein fouling in surface plasmon resonance biosensors

Antifouling surfaces capable of reducing nonspecific protein adsorption from natural complex media are highly desirable in surface plasmon resonance (SPR) biosensors. A new protein-resistant surface made through the chemical grafting of easily available hyaluronic acid (HA) onto gold (Au) substrate demonstrates excellent antifouling performance against protein adsorption. AFM images showed the uniform HA layer with a thickness of ∼10.5 nm on the Au surface. The water contact angles of Au surfaces decreased from 103° to 12° with the covalent attachment of a carboxylated HA matrix, indicating its high hydrophilicity mainly resulted from carboxyl and amide groups in the HA chains. Using SPR spectroscopy to investigate nonspecific adsorption from single protein solutions (bovine serum albumin (BSA), lysozyme) and complex media (soybean milk, cow milk, orange juice) to an HA matrix, it was found that ultralow or low protein adsorptions of 0.6-16.1 ng/cm(2) (e.g., soybean milk: 0.6 ng/cm(2)) were achieved on HA-Au surfaces. Moreover, anti-BSA was chosen as a model recognition molecule to characterize the immobilization capacity and the antifouling performance of anti-BSA/HA surfaces. The results showed that anti-BSA/HA sensor surfaces have a high anti-BSA loading of 780 ng/cm(2), together with achieving the ultralow (<3 ng/cm(2) for lysozyme and soybean milk) or low (<17 ng/cm(2) for cow milk and 10% blood serum) protein adsorptions. Additionally, the sensor chips also exhibited a high sensitivity to BSA over a wide range of concentrations from 15 to 700 nM. Our results demonstrate a promising antifouling surface using extremely hydrophilic HA as matrix to resist nonspecific adsorption from complex media in SPR biosensors.

Non-labeled monitoring of targeted liposome interactions with a model receptor surface: effect of flow rate and water content

In this study, we present a novel in vitro approach that utilizes two surface-sensitive and label-free techniques, i.e. surface plasmon resonance (SPR) and quartz crystal microbalance (QCM), to study the interfacial events during liposome-target surface interactions. The flow channels of SPR and QCM devices were first synchronized via hydrodynamic modeling. Biotin-streptavidin was used as a model pair and self-assembled monolayers (SAMs) were utilized as model surfaces for targeted liposome-surface interaction studies. The interactions between biotin-liposomes and the streptavidin-biotin-SAM surfaces were investigated under controlled shear flows using the synchronized SPR and QCM devices. The response of the liposome interaction was monitored as a function of the flow rate. The affinity and the amount of bound liposome indicated that the increased flow rate improved the binding of the targeted liposomes to the model membrane surfaces. The combined use of the synchronized SPR and QCM devices for nanoparticle interaction studies clearly demonstrates the effect of the flow rate (or the shear stress) on the liposome binding. Our results suggest that the binding of liposomes to the model membranes is flow rate and shear stress regulated. Thus, the flow rate (or the shear stress), which is usually neglected, should be taken into account during the development and optimization of targeted liposome formulations. In addition, the water content within the liposome layer (including the water inside the liposomes and the water between the liposomes) had a significant influence on the visco-elasticity and the binding kinetics to the SAM surfaces.

Modeling analyte transport and capture in porous bead sensors

Porous agarose microbeads, with high surface to volume ratios and high binding densities, are attracting attention as highly sensitive, affordable sensor elements for a variety of high performance bioassays. While such polymer microspheres have been extensively studied and reported on previously and are now moving into real-world clinical practice, very little work has been completed to date to model the convection, diffusion, and binding kinetics of soluble reagents captured within such fibrous networks. Here, we report the development of a three-dimensional computational model and provide the initial evidence for its agreement with experimental outcomes derived from the capture and detection of representative protein and genetic biomolecules in 290 μm porous beads. We compare this model to antibody-mediated capture of C-reactive protein and bovine serum albumin, along with hybridization of oligonucleotide sequences to DNA probes. These results suggest that, due to the porous interior of the agarose bead, internal analyte transport is both diffusion and convection based, and regardless of the nature of analyte, the bead interiors reveal an interesting trickle of convection-driven internal flow. On the basis of this model, the internal to external flow rate ratio is found to be in the range of 1:170 to 1:3100 for beads with agarose concentration ranging from 0.5% to 8% for the sensor ensembles here studied. Further, both model and experimental evidence suggest that binding kinetics strongly affect analyte distribution of captured reagents within the beads. These findings reveal that high association constants create a steep moving boundary in which unbound analytes are held back at the periphery of the bead sensor. Low association constants create a more shallow moving boundary in which unbound analytes diffuse further into the bead before binding. These models agree with experimental evidence and thus serve as a new tool set for the study of bioagent transport processes within a new class of medical microdevices.

Correlation between the composition of multivalent antibody conjugates with colloidal gold nanoparticles and their affinity

Interactions between multivalent preparations of antibodies (conjugated with colloidal gold nanoparticles (GNP) as a carrier system) and a multivalent ligand were investigated. The aim of the present study was to reveal the relationship between the affinity of the conjugate and its composition (i.e., the valency). Surface plasmon resonance was applied to study the affinity and the kinetics of the interaction of multivalent conjugates and multivalent virus (on the example of the plum pox virus (PPV)). Three monoclonal antibodies against PPV were prepared. Five GNP preparations with an average particle size in the range from 5 to 60nm (according to electron microscopy measurements) were obtained. The series of preparations allowed us to synthesize GNP-antibody conjugates with different surface areas for immobilization of antibodies, and, consequently, conjugates with different valencies. It was shown that the affinity of the conjugates changes with size of colloidal carriers (i.e. with the valency of the conjugates). The affinity of the virus-antibody interaction (antibodies with affinities of 1.46.10(-8)M and 1.73.10(-8)M) is one to three orders of magnitude lower (depending on the valency of the conjugate) compared to that of the interactions of the virus with GNP conjugates (conjugates with the affinity varying from 1.69.10(-9) to 7.02.10(-12)M and from 2.39.10(-9) to 2.62.10(-11)M, respectively). An increase in the conjugate size leads to an increase in its affinity. The similar trends were observed for the potato virus X.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Molecular modeling and affinity determination of scFv antibody: proper linker peptide enhances its activity

One of existing strategies to engineer active antibody is to link V(H) and V(L) domains via a linker peptide. How the composition, length, and conformation of the linker affect antibody activity, however, remains poorly understood. In this study, a dual approach that coordinates molecule modeling, biological measurements, and affinity evaluation was developed to quantify the binding activity of a novel stable miniaturized anti-CD20 antibody or single-chain fragment variable (scFv) with a linker peptide. Upon computer-guided homology modeling, distance geometry analysis, and molecular superimposition and optimization, three new linker peptides PT1, PT2, and PT3 with respective 7, 10, and 15 residues were proposed and three engineered antibodies were then constructed by linking the cloned V(H) and V(L) domains and fusing to a derivative of human IgG1. The binding stability and activity of scFv-Fc chimera to CD20 antigen was quantified using a micropipette adhesion frequency assay and a Scatchard analysis. Our data indicated that the binding affinity was similar for the chimera with PT2 or PT3 and approximately 24-fold higher than that for the chimera with PT1, supporting theoretical predictions in molecular modeling. These results further the understanding in the impact of linker peptide on antibody structure and activity.

Whole serum BSA antibody screening using a label-free biophotonic nanoparticle array

Bovine serum albumin antibodies (aBSA) have been screened from whole leporine anti serum on a biophotonic array. The array was initially printed with seed gold nanoparticles into a 96-spot configuration, and 130-nm gold nanoparticles were synthesised in situ on the surface of each spot. The gold nanoparticle surface was then functionalized with the proteins bovine serum albumin (BSA), fibrinogen, and immunoglobulin G (IgG) and with the amino acid glycine. The concentration of aBSA in the whole serum was determined using a kinetic analysis of the time-dependent light scattering from the nanoparticles. The aBSA-BSA kinetic parameters derived from the array are k(a)=(1.3 +/- 0.3) x 10(5) M(-1) s(-1), k(d)=(4 +/- 2) x 10(-4) s(-1), and K(D)=3 nM, which compare favorably with those from continuous gold surfaces. The ultimate sensitivity of the array reader to the bulk refractive index (RI) is 1 x 10(-4) refractive index units (RIU), corresponding to 1 microg ml(-1) for aBSA. The nanoparticles appear to be more sensitive than the continuous gold surface to the aBSA binding event from whole serum, and this is interpreted in terms of the difference in RI contrast in the plasmon fields.

Measurement of two-dimensional binding constants between cell-bound major histocompatibility complex and immobilized antibodies with an acoustic biosensor

Gaining insights into the dynamic processes of molecular interactions that mediate cell-substrate and cell-cell adhesion is of great significance in the understanding of numerous physiological processes driven by intercellular communication. Here, an acoustic-wave biosensor is used to study and characterize specific interactions between cell-bound membrane proteins and surface-immobilized ligands, using as a model system the binding of major histocompatibility complex class I HLA-A2 proteins to anti-HLA-A2 monoclonal antibodies. The energy of the acoustic signal, measured as amplitude change, was found to depend directly on the number of HLA-A2/antibody complexes formed on the device surface. Real-time acoustic data were used to monitor the surface binding of cell suspensions at a range of 6.0 x 10(4) to 6.0 x 10(5) cells mL(-1). Membrane interactions are governed by two-dimensional chemistry because of the molecules' confinement to the lipid bilayer. The two-dimensional kinetics and affinity constant of the HLA-A2/antibody interaction were calculated (k(a) = 1.15 x 10(-5) mum(2) s(-1) per molecule, k(d) = 2.07 x 10(-5) s(-1), and K(A) = 0.556 mum(2) per molecule, at 25 degrees C), based on a detailed acoustic data analysis. Results indicate that acoustic biosensors can emerge as a significant tool for probing and characterizing cell-membrane interactions in the immune system, and for fast and label-free screening of membrane molecules using whole cells.