Quartz Crystal Microbalance Detection of Glutathione-Protected Nanoclusters Using Antibody Recognition

Protein Labeling Facilitates the Understanding of Protein Corona Formation via Fluorescence Resonance Energy Transfer and Fluorescence Correlation Spectroscopy

Once nanoparticles enter into the biological milieu, nanoparticle-biomacromolecule complexes, especially the protein corona, swiftly form, which cause obvious effects on the physicochemical properties of both nanoparticles and proteins. Here, the thermodynamic parameters of the interactions between water-soluble GSH-CdSe/ZnS core/shell quantum dots (GSH-QDs) and human serum albumin (HSA) were investigated with the aid of labeling fluorescence of HSA. It was proved that the labeling fluorescence originating from a fluorophore (BDP-CN for instance) could be used to investigate the interactions between QDs and HSA. Gel electrophoresis displayed that the binding ratio between HSA and QDs was ∼2:1 by direct visualization. Fluorescence resonance energy transfer (FRET) results indicated that the distance between the QDs and the fluorophore BDP-CN in HSA was 7.2 nm, which indicated that the distance from the fluorophore to the surface of the QDs was ∼4.8 nm. Fluorescence correlation spectroscopy (FCS) results showed that HSA formed a monolayer of a protein corona with a thickness of 5.5 nm. According to the spatial structure of HSA, we could speculate that the binding site of QDs was located at the side edge (not the triangular plane) of HSA with an equilateral triangular prism. The elaboration of the thermodynamic parameters, binding ratio, and interaction orientation will highly improve the fundamental understanding of the formation of protein corona. This work has guiding significance for the exploration of the interactions between proteins and nanomaterials.

Peptide conformational imprints enhanced the catalytic activity of papain for esterification

Peptide conformational imprints (PCIs) offer a promising perspective to directly generate binding sites for preserving enzymes with high catalytic activity and stability. In this study, we synthesized a new chiral cross-linker cost-effectively for controlling the matrix morphology of PCIs on magnetic particles (PCIMPs) to stabilize their recognition capability. Meanwhile, based on the flank part of the sequences on papain (PAP), three epitope peptides were selected and synthesized. Molecularly imprinted polymers (MIPs) were then fabricated in the presence of the epitope peptide using our new cross-linker on magnetic particles (MPs) to generate PCIMPs. PCIMPs were formed with helical cavities that complement the PAP structure to adsorb specifically at the targeted position of PAP. PCIMPs^65–79 were found to have the best binding parameters to the PAP with K_d = 0.087 μM and B_max = 4.56 μM. Upon esterification of N-Boc-His-OH, proton nuclear magnetic resonance (¹H-NMR) was used to monitor the yield of the reaction and evaluate the activity of PAP/PCIMPs. The kinetic parameters of PAP/PCIMPs^65–79 were calculated as V_max = 3.0 μM s⁻¹, K_m = 5 × 10⁻² M, k_cat = 1.1 × 10^–1 s⁻¹, and k_cat/K_m = 2.2 M⁻¹ s⁻¹. In addition, PAP is bound tightly to PCIMPs to sustain its activity after four consecutive cycles.

Deviation of Trypsin Activity Using Peptide Conformational Imprints

In this study, a methodology utilizing peptide conformational imprints (PCIs) as a tool to specifically immobilize porcine pancreatic alpha-trypsin (PPT) at a targeted position is demonstrated. Owing to the fabrication of segment-mediated PCIs on the magnetic particles (PCIMPs), elegant cavities complementary to the PPT structure are constructed. Based on the sequence on targeted PPT, the individual region of the enzyme is trapped with different template-derived PCIMPs to show certain types of inhibition. Upon hydrolysis, N-benzoyl-L-arginine ethyl ester (BAEE) is employed to assess the hydrolytic activity of PCIMPs bound to the trypsin using high-performance liquid chromatography (HPLC) analysis. Further, the kinetic data of four different PCIMPs are compared. As a result, the PCIMPs presented non-competitive inhibition toward trypsin, according to the Lineweaver-Burk plot. Further, the kinetic analysis confirmed that the best parameters of PPT/PCIMPs 233-245+G were Vmax = 1.47 × 10-3 mM s-1, Km = 0.42 mM, kcat = 1.16 s-1, and kcat/Km = 2.79 mM-1 s-1. As PPT is bound tightly to the correct position, its catalytic activities could be sustained. Additionally, our findings stated that the immobilized PPT could maintain stable activity even after four successive cycles.

Sensing HIV Protease and Its Inhibitor Using "Helical Epitope"-Imprinted Polymers

A helical epitope-peptide (lle⁸⁵-Gly⁹⁴) was selected from the α-helix structure of the HIV protease (PR) as the template, which represents an intricate interplay between structure conformation and dimerization. The peptide template was mixed with water, trifluoroethanol (TFE), and acetonitrile (ACN) at a certain ratio to enlarge the helical conformation in the solution for the fabrication of helical epitope-mediated molecularly imprinted polymers (HEMIPs) on a quartz crystal microbalance (QCM) chip. The template molecules were then removed under equilibrium batch rebinding conditions involving 5% acetic acid/water. The resulting HEMIPs chip exhibited a high affinity toward template peptide HIV PR_85-94, His-tagged HIV PR, and HIV PR, with dissociation constants (K_d) as 160, 43.3, and 78.5 pM, respectively. The detection limit of the developed HIV PR_85-94 QCM sensor is 0.1 ng/mL. The HEMIPs chip exhibited a high affinity and selectivity to bind HIV PR and subsequently to an inhibitor of HIV PR (nelfinavir). The HIV PR binding site was properly oriented on the HEMIPs-chip to develop a HIV PR/HEMIPs chip, which can effectively bind nelfinavir to establish a sandwich assay. The nelfinavir then attached to the HIV PR/HEMIPs chip, which can be easily removed involving 0.8% acetic acid/water. Therefore, HIV PR/HEMIPs chip can be useful to screen for other HIV PR inhibitors. This technique may improve drug targeting for HIV therapy and also strengthen investigations into other virus assays.

Selected DNA aptamers as hydroxyapatite affinity reagents

DNA aptamers were selected for their ability to bind specifically and quickly to crystalline hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂; HAP), the primary mineral component of enamel and bone. Aptamers were found to have an enhanced percent of G-nucleotides and a propensity for forming a G-quadruplex secondary structure. One aptamer was studied in comparison to control sequences and was found to bind with high affinity and at high loading capacity, with enhanced binding kinetics, and with specificity for crystalline HAP material over amorphous calcium phosphate (ACP) and β-tricalcium phosphate (TCP). The fluorescently-functionalized aptamer was demonstrated to specifically label HAP in a surface binding experiment and suggests the usefulness of this selected aptamer in biomedical or biotechnology fields where the labeling of specific calcium phosphate materials is required.

Qualitative and quantitative analysis of the biophysical interaction of inhaled nanoparticles with pulmonary surfactant by using quartz crystal microbalance with dissipation monitoring

Understanding the interaction between inhaled nanoparticles and pulmonary surfactant is a prerequisite for predicting the fate of inhaled nanoparticles. Here, we introduce a quartz crystal microbalance with dissipation monitoring (QCM-D)-based methodology to reveal the extent and nature of the biophysical interactions of polymer- and lipid-based nanoparticles with pulmonary surfactant. By fitting the QCM-D data to the Langmuir adsorption equation, we determined the kinetics and equilibrium parameters [i.e., maximal adsorption (Δm_max), equilibrium constant (K_a), adsorption rate constant (k_a) and desorption rate constant (k_d)] of polymeric nanoparticles adsorption onto the pulmonary surfactant (e.g., an artificial lipid mixture and an extract of porcine lung surfactant). Furthermore, our results revealed that the nature of the interactions between lipid-based nanoparticles (e.g., liposomes) and pulmonary surfactant was governed by the liposomal composition, i.e., incorporation of cholesterol and PEGylated phospholipid (DSPE-PEG₂₀₀₀) into DOPC-based liposomes led to the adsorption of intact liposomes onto the pulmonary surfactant layer and the mass exchange between the liposomes and pulmonary surfactant layer, respectively. In conclusion, we demonstrate the applicability of the QCM-D technique for qualitative and quantitative analysis of the biophysical interaction of inhaled nanoparticles with pulmonary surfactant, which is vital for rational design and optimization of inhalable nanomedicines.

Highly sensitive detection of antibodies in a soft bioactive three-dimensional bioorthogonal hydrogel

This three-dimensional detection method of antibodies offers a high sensitivity and good biomolecule stability for new biosensing devices.

Kinetic analysis of nanoparticle-protein interactions using a plasmon waveguide resonance

A plasmon waveguide resonance (PWR) sensor is proposed for studying the interaction between gold nanoparticles and proteins. The ability of the PWR sensor to operate in both TM and TE Polarizations, i.e. its polarization diversity, facilitates the simultaneous spectroscopy of the nanoparticles surface reactions using both polarizations. The response of each polarization to streptavidin-biotin binding at the surface of gold nanoparticles is investigated in real time. Finally, using the principles of multimode spectroscopy, the nanoparticle's surface reactions are decoupled from the bulk solution refractive index variations. Schematic diagram of the NP-modified PWR sensor.

Dynamic interactions of amelogenin with hydroxyapatite surfaces are dependent on protein phosphorylation and solution pH

Amelogenin, the predominant extracellular matrix protein secreted by ameloblasts, has been shown to be essential for proper tooth enamel formation. In this study, amelogenin adsorption to hydroxyapatite (HAP) surfaces, a prototype for enamel mineral, has been studied using a quartz crystal microbalance (QCM) to interrogate effects of protein phosphorylation and solution pH. Dynamic flow-based experiments were conducted at pH 7.4 and 8.0 using native phosphorylated porcine amelogenin (P173) and recombinant non-phosphorylated porcine amelogenin (rP172). Loading capacities (μmol/m²) on HAP surfaces were calculated under all conditions and adsorption affinities (K_ad) were calculated when Langmuir isotherm conditions appeared to be met. At pH 8.0, binding characteristics were remarkably similar for the two proteins. However, at pH 7.4 a higher affinity and lower surface loading for the phosphorylated P173 was found compared to any other set of conditions. This suggests that phosphorylated P173 adopts a more extended conformation than non-phosphorylated full-length amelogenin, occupying a larger footprint on the HAP surface. This surface-induced structural difference may help explain why P173 is a more effective inhibitor of spontaneous HAP formation in vitro than rP172. Differences in the viscoelastic properties of P173 and rP172 in the adsorbed state were also observed, consistent with noted differences in HAP binding. These collective findings provide new insight into the important role of amelogenin phosphorylation in the mechanism by which amelogenin regulates enamel crystal formation.

A highly sensitive quartz crystal microbalance immunosensor based on magnetic bead-supported bienzymes catalyzed mass enhancement strategy

A highly sensitive quartz crystal microbalance (QCM) immunosensor based on magnetic bead-supported bienzyme catalyzed mass enhanced strategy was developed for the detection of human immunoglobulin G (hIgG) protein. The high sensitive detection was achieved by increasing the deposited mass on the QCM crystal through the enhanced precipitation of 4-chloro-1-naphthol (CN) using higher amounts of horseradish peroxidase (HRP) and glucose oxidase (GOx) bienzymes attached on the magnetic beads (MB). The protein A (PA) and capture antibody (monoclonal anti-human IgG antibody produced in mouse, Ab1)-based QCM probe and the detection antibody (anti-human IgG antibody produced in goat, Ab2)-based MB/HRP/GOx bienzymatic bioconjugates were characterized using scanning electron microscope, transmission electron microscope, cyclic voltammetry, and electrochemical impedance spectroscopy techniques. Under the optimized experimental condition, the linear range and the detection limit of hIgG immunosensor were determined to be 5.0pg/mL-20.0ng/mL and 5.0±0.18pg/mL, respectively. The applicability of the present hIgG immunosensor was examined in hIgG spiked human serum samples and excellent recoveries of hIgG were obtained.

Real-time Recognition of Mycobacterium tuberculosis and Lipoarabinomannan using the Quartz Crystal Microbalance

A quartz crystal microbalance (QCM) immunosensor has been successfully employed to screen for both whole Mycobacteria tuberculosis (Mtb) bacilli and a Mtb surface antigen, lipoarabinomannan (LAM). One of the most abundant components of the Mtb cell surface, LAM, may be detected without the presence of the entire bacterium. Using available antibodies with proven utility in enzyme-linked immunoassays (ELISAs), a sensor was designed to measure Mtb bacilli and LAM. Equilibrium association constants (K(a)) were determined for the interaction of Mtb with immobilized α-LAM and anti-H37Rv antibodies, where avidity was seen to strengthen this interaction and provide for greater binding than might have otherwise been achieved. The binding of LAM to immobilized α-LAM had a high associate rate constant (k(a)) allowing for rapid detection. Evaluating these binding constants helped the compare the sensitivity of these immunosensors to conventional ELISAs. The use of these assays with the better antibodies may allow for immunosensor use in determining LAM as a point-of-care (POC) diagnostic for Mtb.

Quartz crystal microbalance analysis of DNA-templated calcium phosphate mineralization

A quartz crystal microbalance (QCM) sensor was developed for the quantitation of calcium phosphate mineralization and the assessment of DNA as a template molecule. Inherent advantages of QCM, such as nanogram sensitivity, temporal resolution, surface-based measurements, and flow capabilities, were leveraged in the design of this sensor, and in-line fluidic mixing was used to control precursor reaction. This research shows that DNA, a highly programmable anionic polymer, is able to template and control mineralization of calcium phosphate, with nucleation occurring in less than 15 min and initial rates ranging from 4 to 8 ng/min. FT-IR measurements show mineralized material to be calcium phosphate resembling hydroxyapatite (HAP) when a DNA template is used. DNA is a promising mineralization template, and the QCM proves to be a dynamic technique for a broad range of heterogeneous mineralization experiments in complement to classic, diffusion-limited, end-point analysis techniques.

Biomimetic monolayer-protected gold nanoparticles for immunorecognition

Gold nanoparticles (AuNPs) protected by self-assembled monolayers (SAMs) are capable of presenting precisely engineered surfaces at the nanoscale, allowing the mimicry of biomacromolecules on an artificial platform. Here we review the generation, characterization, and applications of monolayer-protected AuNPs that have been designed for immunorecognition by the integration of an oligopeptide epitope into the protecting monolayer. The resulting peptide-AuNP conjugate is an effective platform for biomimesis, as demonstrated by multiple studies. Recent work is presented and future directions for this field of research are discussed.

Short-chain PEG mixed monolayer protected gold clusters increase clearance and red blood cell counts

Monolayer-protected gold nanoparticles have great potential as novel building blocks for the design of new drugs and therapeutics based on the easy ability to multifunctionalize them for biological targeting and drug activity. In order to create nanoparticles that are biocompatible in vivo, polyethylene glycol functional groups have been added to many previous multifunctionalized particles to eliminate nonspecific binding. Recently, monolayer-protected gold nanoparticles with mercaptoglycine functionalities were shown to elicit deleterious effects on the kidney in vivo that were eliminated by incorporating a long-chain, mercapto-undecyl-tetraethylene glycol at very high loadings into a mixed monolayer. These long-chain PEGs induced an immune response to the particle presumably generating an anti-PEG antibody as seen in other long-chain PEG-ylated nanoparticles in vivo. In the present work, we explore the in vivo effects of high and low percent ratios of a shorter chain, mercapto-tetraethylene glycol within the monolayer using simple place-exchange reactions. The shorter chain PEG MPCs were expected to have better water solubility due to elimination of the alkyl chain, no toxicity, and long-term circulation in vivo. Shorter chain lengths at lower concentrations should not trigger the immune system to create an anti-PEG antibody. We found that a 10% molar exchange of this short-chain PEG within the monolayer met three of the desired goals: high water solubility, no toxicity, and no immune response as measured by white blood cell counts. However, none of the short-chain PEG mixed monolayer compositions enabled the nanoparticles to have a long circulation time within the blood as compared to mercapto-undecyl-ethylene glycol, which had a residence time of 4 weeks. We also compared the effects of a hydroxyl versus a carboxylic acid terminal functional group on the end of the PEG thiol on both clearance and immune response. The results indicate that short-chain-length PEGs, regardless of termini, increase clearance rates compared to the previous long-chain PEG studies, while carboxylated termini increase red blood cell counts at high loadings. Given these findings, short-chain, alcohol-terminated PEG, exchanged at 10%, was identified as a potential nanoparticle for further in vivo applications requiring short circulation lifetimes with desired features of no toxicity, no immune response, and high water solubility.

Effect of buried potential barrier in label-less electrochemical immunodetection of glutathione and glutathione-capped gold nanoparticles

The influence of potential barriers, introduced to the immunoglobulin-based sensory films, on voltammetric signals of a redox ion probe has been investigated. Films with positive and negative barriers have been examined by depositing charged self-assembled thiol monolayers as the basal layers of a sensory film. The studies performed with monoclonal anti-glutathione antibody-based sensors using ferricyanide ion probe have shown stronger sensor response to the layer components, as well as to the glutathione-capped gold nanoparticles acting as the antigen, for films with positive potential barrier buried deep in the film than for negative barrier films. The larger changes in differential resistance, peak separation and peak heights observed for films with positive barrier have been attributed to different depth and width of the charge distributions in these films. A buried positive barrier with narrow charge distribution width provides the best conditions for film stability and prevents fouling (less ion-exchanges with the medium). This conclusion has been confirmed by calculations of the electric field distribution and potential profiles in immunosensing films performed by numerical integration of Poisson equation for Gaussian distributions of fixed charges of covalently bound components. The proposed fixed-charge model can aid in rapid evaluation of sensory films in sensor development work. The implications of potential barriers in sensory film design are discussed.

Multifunctional nanoparticles as simulants for a gravimetric immunoassay

Immunoassays are important tools for the rapid detection and identification of pathogens, both clinically and in the research laboratory. An immunoassay with the potential for the detection of influenza was developed and tested using hemagglutinin (HA), a commonly studied glycoprotein found on the surface of influenza virions. Gold nanoparticles were synthesized, which present multiple peptide epitopes, including the HA epitope, in order to increase the gravimetric response achieved with the use of a QCM immunosensor for influenza. Specifically, epitopes associated with HA and FLAG peptides were affixed to gold nanoparticles by a six-mer PEG spacer between the epitope and the terminal cysteine. The PEG spacer was shown to enhance the probability for interaction with antibodies by increasing the distance the epitope extends from the gold surface. These nanoparticles were characterized using thermogravimetric analysis, transmission electron microscopy, matrix-assisted laser desorption/ionization-time of flight, and (1)H nuclear magnetic resonance analysis. Anti-FLAG and anti-HA antibodies were adhered to the surface of a QCM, and the response of each antibody upon exposure to HA, FLAG, and dual functionalized nanoparticles was compared with binding of Au-tiopronin nanoparticles and H5 HA proteins from influenza virus (H5N1). Results demonstrate that the immunoassay was capable of differentiating between nanoparticles presenting orthogonal epitopes in real-time with minimal nonspecific binding. The detection of H5 HA protein demonstrates the logical extension of using these nanoparticle mimics as a safe positive control in the detection of influenza, making this a vital step in improving influenza detection methodology.

Analytical strategies for detecting nanoparticle-protein interactions

A significant increase in biomedical applications of nanomaterials and their potential toxicity demands versatile analytical techniques to determine protein-nanoparticle (NP) interactions. These diverse analytical techniques are reviewed. Spectroscopic methods play a significant role in studying binding affinity, binding ratio, and binding mechanisms. To elucidate NP-proteome interactions, chromatography and electrophoresis techniques are applied to separate NP-bound proteins and matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) to identify these proteins. Since NP-protein binding is a dynamic event, surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) are methods of choice to study the kinetics of NP-protein binding.

Nanoparticle-based biologic mimetics

Centered on solid chemistry foundations, biology and materials science have reached a crossroad where bottom-up designs of new biologically important nanomaterials are a reality. The topics discussed here present the interdisciplinary field of creating biological mimics. Specifically, this discussion focuses on mimics that are developed using various types of metal nanoparticles (particularly gold) through facile synthetic methods. These methods conjugate biologically relevant molecules, e.g., small molecules, peptides, proteins, and carbohydrates, in conformationally favorable orientations on the particle surface. These new products provide stable, safe, and effective substitutes for working with potentially hazardous biologicals for applications such as drug targeting, immunological studies, biosensor development, and biocatalysis. Many standard bioanalytical techniques can be used to characterize and validate the efficacy of these new materials, including quartz crystal microbalance (QCM), surface plasmon resonance (SPR), and enzyme-linked immunosorbent assay (ELISA). Metal nanoparticle-based biomimetics continue to be developed as potential replacements for the native biomolecule in applications of immunoassays and catalysis.

Interparticle interactions in glutathione mediated assembly of gold nanoparticles

The understanding of the detailed molecular interactions between (GSH) glutathione molecules in the assembly of metal nanoparticles is important for the exploitation of the biological reactivity. We report herein results of an investigation of the assembly of gold nanoparticles mediated by glutathione and the disassembly under controlled conditions. The interparticle interactions and reactivities were characterized by monitoring the evolution of the surface plasmon resonance band using the spectrophotometric method and the hydrodynamic sizes of the nanoparticle assemblies using the dynamic light scattering technique. The interparticle reactivity of glutathiones adsorbed on gold nanoparticles depends on the particle sizes and the ionic strength of the solution. Larger-sized particles were found to exhibit a higher degree of interparticle assembly than smaller-sized particles. The assembly-disassembly reversibility is shown to be highly dependent on pH and additives in the solution. The interactions of the negatively charged citrates surrounding the GSH monolayer on the particle surface were believed to produce more effective interparticle spatial and electrostatic isolation than the case of OH (-) groups surrounding the GSH monolayer. The results have provided new insights into the hydrogen-bonding character of the interparticle molecular interaction of glutathiones bound on gold nanoparticles. The fact that the interparticle hydrogen-bonding interactions in the assembly and disassembly processes can be finely tuned by pH and chemical means has implications to the exploitation of the glutathione-nanoparticle system in biological detection and biosensors.

Self-contained, biomolecular motor-driven protein sorting and concentrating in an ultrasensitive microfluidic chip

We developed a molecular sorter that operates without external power or control by integrating the microtubule-based, biological motor kinesin into a microfluidic channel network to sort, transport, and concentrate molecules. In our devices, functionalized microtubules that capture analyte molecules are steered along kinesin-coated microchannel tracks toward a collector structure, concentrated, and trapped. Using fluorescent analyte molecules and nanoliter sample volumes, we demonstrated 14 fM sensitivity, even in the presence of high concentrations of other proteins.

Design and synthesis of an antigenic mimic of the Ebola glycoprotein

An antigenic mimic of the Ebola glycoprotein was synthesized and tested for its ability to be recognized by an anti-Ebola glycoprotein antibody. Epitope-mapping procedures yielded a suitable epitope that, when presented on the surface of a nanoparticle, forms a structure that is recognized by an antibody specific for the native protein. This mimic-antibody interaction has been quantitated through ELISA and QCM-based methods and yielded an affinity (K(d) = 12 × 10(-6) M) within two orders of magnitude of the reported affinity of the native Ebola glycoprotein for the same antibody. These results suggest that the rational design approach described herein is a suitable method for the further development of protein-based antigenic mimics with potential applications in vaccine development and sensor technology.

Enzyme-catalyzed signal amplification for electrochemical DNA detection with a PNA-modified electrode

The signal amplification technique of peptide nucleic acid (PNA)-based electrochemical DNA sensor was developed in a label-free and one-step method utilizing enzymatic catalysis. Electrochemical detection of DNA hybridization on a PNA-modified electrode is based on the change of surface charge caused by the hybridization of negatively charged DNA molecules. The negatively charged mediator, ferrocenedicarboxylic acid, cannot diffuse to the DNA hybridized electrode surface due to the charge repulsion with the hybridized DNA molecule while it can easily approach the neutral PNA-modified electrode surface without the hybridization. By employing glucose oxidase catalysis on this PNA-based electrochemical system, the oxidized mediator could be immediately reduced leading to greatly increased electrochemical signals. Using the enzymatic strategy, we successfully demonstrated its clinical utility by detecting one of the mutation sequences of the breast cancer susceptibility gene BRCA1 at a sample concentration lower than 10(-9) M. Furthermore, a single base-mismatched sample could be also discriminated from a perfectly matched sample.

Stable aqueous nanoparticle film assemblies with covalent and charged polymer linking networks

The construction of highly stable and efficiently assembled multilayer films of purely water soluble gold nanoparticles is reported. Citrate-stabilized nanoparticles (CS-NPs) of average core diameter of 10 nm are used as templates for stabilization-based exchange reactions with thioctic acid to form more robust aqueous NPs that can be assembled into multilayer films. The thioctic acid stabilized nanoparticles (TAS-NPs) are networked via covalent and electrostatic linking systems, employing dithiols and the cationic polymer poly(L-lysine), respectively. Multilayer films of up to 150 nm in thickness are successfully grown at biological pH with no observable degradation of the NPs within the film. The characteristic surface plasmon band, an optical feature of certain NP film assemblies that can be used to report the local environment and core spacing within the film, is preserved. Growth dynamics and film stability in solution and in the air are examined, with poly(L-lysine) linked films showing no evidence of aggregation for at least 50 days. We believe these films represent a pivotal step toward exploring the potential of aqueous NP film assemblies as a sensing apparatus.

Antigen–antibody interaction from quartz crystal microbalance immunosensors based on magnetic CoFe2O4/SiO2 composite nanoparticle-functionalized biomimetic interface

A new quartz crystal microbalance immunoassay for the detection of carcinoembryonic antigen (CEA) was developed by means of immobilizing anti-CEA onto magnetic CoFe2O4/SiO2 composite nanoparticles-functionalized biomimetic interface. Under optimal conditions, the frequency shift was proportional to the CEA concentration in the range of 2.5-55 ng/mL with a detection limit of 0.5 ng/mL at a signal-to-noise ratio of 3. Moreover, the immunosensor system showed an acceptable reproducibility and stability. Clinical serum specimens were assayed with this method, and the results were in acceptable agreement with those obtained from ELISA. Compared with the conventional ELISA assay, the proposed immunoassay system was simple and rapid without multiple labeling and separation steps. Importantly, the developed immunoassay protocol could be further extended for the determination of other antigens.

Electrospray Mass Spectrometry Study of Tiopronin Monolayer-Protected Gold Nanoclusters

Electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) and gel permeation chromatography (GPC) were used to study the synthesis of a series of tiopronin monolayer-protected gold nanoclusters (MPCs) and to monitor their postsynthesis peptide ligand place-exchange reactions. All mass spectra identified the presence of cyclic gold(I)-thiolates with a strong preference for tetrameric species. During the synthesis of pre-monolayer-protected nanoclusters (pre-MPCs), esterified gold(I)-thiolate tetramers were initially observed in minor abundance (with respect to disulfide bridged tiopronin species) before dramatically increasing in abundance and precipitating from solution. After conversion of pre-MPCs to MPCs, ESI-TOF mass spectra demonstrated an overall predominance of tetrameric species with conversion from ester-terminated end groups to carboxyl-terminated end groups. Further modifications were performed through postsynthesis ligand place-exchange reactions to validate the existence of the tetramers. This work suggests that monolayer protection is accomplished by cyclized gold(I)-thiolate tetramers on the gold core surface, and/or that gold(I)-thiolates are a basic building block within the nanoparticles.

A survey of the 2001 to 2005 quartz crystal microbalance biosensor literature: applications of acoustic physics to the analysis of biomolecular interactions

The widespread exploitation of biosensors in the analysis of molecular recognition has its origins in the mid-1990s following the release of commercial systems based on surface plasmon resonance (SPR). More recently, platforms based on piezoelectric acoustic sensors (principally 'bulk acoustic wave' (BAW), 'thickness shear mode' (TSM) sensors or 'quartz crystal microbalances' (QCM)), have been released that are driving the publication of a large number of papers analysing binding specificities, affinities, kinetics and conformational changes associated with a molecular recognition event. This article highlights salient theoretical and practical aspects of the technologies that underpin acoustic analysis, then reviews exemplary papers in key application areas involving small molecular weight ligands, carbohydrates, proteins, nucleic acids, viruses, bacteria, cells and lipidic and polymeric interfaces. Key differentiators between optical and acoustic sensing modalities are also reviewed.

Immobilization of Protein A on SAMs for the elaboration of immunosensors

Binary mixtures of 11-mercaptoundecanoic acid (MUA) and other thiols of various lengths and terminal functions were chemisorbed on gold-coated surfaces via S-Au bonds to form mixed self-assembled monolayers (SAMs). Several values of the mole fraction of MUA in the thiol mixtures were tested and the structure and composition of the resulted thin films were characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). The results made it clear that co-adsorption of MUA with thiols of similar chain length led to well-ordered monolayers whereas the co-adsorption of MUA with shorter thiols yielded less crystalline-like thin films, but with more reactive carboxylic acid terminal groups. This criterion appeared decisive for efficient covalent binding of Staphylococcus aureus Protein A (PrA), a protein that displays high affinity for the constant fragment (Fc) of antibodies of the IgG type from various mammal species. The ability of immobilized Protein A to recognize and bind a model IgG appeared to be optimal for the mixed SAM of MUA and the short-chain, omega-hydroxythiol 6-mercaptohexanol in the proportion 1-3.

Amplification of the antigen–antibody interaction from quartz crystal microbalance immunosensors via back-filling immobilization of nanogold on biorecognition surface

A new quartz crystal microbalance immunoassay method based on a novel transparent immunoaffinity reactor was developed for clinical immunoassay. To construct such an affinity reactor, resonators with a frequency of 10 MHz were fabricated by affinity binding of functionalized gold nanoparticles (nanogold) to quartz crystal with immobilized specific ligand for the label-free analysis of the affinity reaction between a ligand and its receptor. [Recombinant human tumor markers, carcinoembryonic antigen (CEA) was chosen as a model ligand.] The binding of target molecules onto the immobilized antibodies decreased the sensor's resonant frequency, and the frequency shift was proportional to the CEA concentration in the range of 3.0-50 ng/ml with a detection limit of 1.5 ng/ml at a signal/noise ration of 3. A glycine-HCl solution (pH 2.3) was used to release antigen-antibody complexes from the biorecognition surface. Good reusability was exhibited. Moreover, spiking various levels of CEA into normal human sera was diagnosed using the proposed immunoassay. Analytical results show the precision of the developed immunoassay is acceptable, implying a promising alternative approach for detecting CEA in clinical immunoassay. Compared with the conventional enzyme-linked immunosorbent assay, the proposed immunoassay system was simple and rapid without multiple labeling and separation steps. Importantly, the proposed immunoassay system could be further developed for the immobilization of other antigens or biocompounds.

Detection of Ebola virus envelope using monoclonal and polyclonal antibodies in ELISA, surface plasmon resonance and a quartz crystal microbalance immunosensor

Ebola virus (EBOV) Zaire, Sudan, as well as Ivory Coast are virulent human EBOV species. Both polyclonal and monoclonal antibodies (MAbs) were developed against soluble EBOV envelope glycoprotein (GP) for the study of EBOV envelope diversity and development of diagnostic reagents. Three EBOV Sudan-Gulu GP peptides, from the N-terminus, mid-GP, and C-terminus regions were used to immunize rabbits for the generation of anti-EBOV polyclonal antibodies. Polyclonal antisera raised against the C-terminus peptide could detect both Sudan-Gulu as well as Zaire GPs, while anti-N and mid-region peptide polyclonal sera recognized only EBOV Sudan-Gulu GP. Of the three anti-EBOV GP mouse MAbs produced, MAb 15H10 recognized all human EBOV GP species tested (Zaire, Sudan and Ivory Coast), and as well as reacted with the Reston non-human primate EBOV GPs. In addition, MAb 15H10 bound virion-associated GP of all known EBOV species. MAb 17A3 recognized GPs of both EBOV Sudan-Gulu and Zaire, while MAb 6D11 recognized only EBOV Sudan-Gulu GP. To detect EBOV GP, these antibody reagents were used in ELISA, surface plasmon resonance and in a quartz crystal microbalance immunosensor. Thus, polyclonal and monoclonal antibodies can be used in combination to identify and differentiate both human and non-human primate EBOV GPs.

Hemagglutinin linear epitope presentation on monolayer-protected clusters elicits strong antibody binding

Immunoreactive, multicomponent nanoclusters were assembled through the controlled presentation of a known, synthetic peptide epitope. The epitope comes from the hemagglutanin protein of influenza and is known to bind to a monoclonal anti-HA antibody. Antibody affinity for the immunoreactive MPC was compared to the affinity for traditionally used peptide arrays using the quartz crystal microbalance. The two systems had comparable affinities (Ka), ranging from 0.41 x 10(7) M(-1) to 1.8 x 10(7) M(-1), though the nanocluster used a much lower density of peptide relative to that of the peptide array. These results suggest that functionalized nanoclusters have potential in nanostructure assembly and medical applications. Water-soluble nanoparticles that present known neutralizing peptide epitopes of protein antigens might be used in antiviral influenza vaccines.