Multitechnique characterization of oligo(ethylene glycol) functionalized gold nanoparticles

Optimization of the immunorecognition layer towards Brucella sp. on gold surface for SPR platform

A successful immunosensor is characterized by a proper antibody immobilization and orientation in order to enhance the antigen recognition. In this work, a thorough characterization of the antibody functionalized gold surface is performed to set up the best conditions to implement in an optical platform for the detection of Brucella sp. Two different strategies are evaluated, based on a random immobilization and on an oriented one: a direct antibody immobilization on carboxylic mixed polyethylene (PEG) self-assembled monolayer (SAM) or only carboxylic PEG SAM interface is compared to an oriented immobilization on a layer of protein G on the same PEG SAM interfaces. X-ray Photoelectron Spectroscopy (XPS), Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and contact angle (CA) are used to chemically characterize the gold functionalized surface and ToF-SIMS is also used to confirm the right antibody orientation. Optical characterization is applied to monitor the functionalization steps and fluorescence measurements are used to set up the proper experimental conditions and also to detect Brucella bacteria on the surface. Best results are obtained with a 10 ng/μl incubation solution of antibody immobilized, in an oriented way, on a mixed PEG SAM interface.

Nanoprojectile Secondary Ion Mass Spectrometry for Nanometrology of Nanoparticles and Their Interfaces

Nanoscale molecular characterization plays a crucial role in enhancing our insights into fundamental and materials processes occurring at the nanoscale. However, for many traditional techniques, measurements on different ensembles are mixed and the analytical result reflects the average surface composition or arrangement. Advances in nanometrologies that allow for measurements to be differentiated based on the chemical environment examined are critical for accurate analysis. Here, we present a variant of secondary ion mass spectrometry, SIMS, termed nanoprojectile SIMS, NP-SIMS, capable of nanoscale molecular analysis. The technique examines the sample with a suite, 10⁶-10⁷, of individual gold nanoprojectiles (e.g., Au₄₀₀⁴⁺) which stochastically probe the surface. Analysis of coemitted ions from each impact allows for the inspection of colocalized moieties within the ejected volume of a single projectile impact (10-15 nm in diameter). If some of these 10⁶-10⁷ measurements arise from nanodomains of similar composition, data can be grouped based on the detected secondary ions. We applied the method to examine a mixture of three different-sized nanoparticles with identical metal cores (3-5 nm in diameter), differing in the length of the attached ligand (decanetiol, tetradecanethiol, and hexadecanethiol). Using NP-SIMS, we determined the relative abundance of the three particles on the surface and isolated measurements based on the impact parameter between the impacting nanoprojectile and the surface particle, demonstrating that measurements occurring near the center of the particle can be differentiated from those at the particle-particle and particle-substrate interfaces. The results suggest that the described methodology is well-suited for molecular analysis of nanoassemblies and may be applied for tracking defects. Here we demonstrate that, using NP-SIMS, ensemble averaging can be avoided and molecular analysis can be undertaken at a scale below 5 nm, allowing for nanoscale molecular analysis of nano-objects and their interfaces.

Fabrication and Impact of Fouling-Reducing Temperature-Responsive POEGMA Coatings with Embedded CaCO3 Nanoparticles on Different Cell Lines

In the present work, we have successfully prepared and characterized novel nanocomposite material exhibiting temperature-dependent surface wettability changes, based on grafted brush coatings of non-fouling poly(di(ethylene glycol)methyl ether methacrylate) (POEGMA) with the embedded CaCO3 nanoparticles. Grafted polymer brushes attached to the glass surface were prepared in a three-step process using atom transfer radical polymerization (ATRP). Subsequently, uniform CaCO3 nanoparticles (NPs) embedded in POEGMA-grafted brush coatings were synthesized using biomineralized precipitation from solutions of CaCl2 and Na2CO3. An impact of the low concentration of the embedded CaCO3 NPs on cell adhesion and growth depends strongly on the type of studied cell line: keratinocytes (HaCaT), melanoma (WM35) and osteoblastic (MC3T3-e1). Based on the temperature-responsive properties of grafted brush coatings and CaCO3 NPs acting as biologically active substrate, we hope that our research will lead to a new platform for tissue engineering with modified growth of the cells due to the release of biologically active substances from CaCO3 NPs and the ability to detach the cells in a controlled manner using temperature-induced changes of the brush.

Windowless detection geometry for sum frequency scattering spectroscopy in the C-D and amide I regions

Understanding the structure and chemistry of nanoscopic surfaces is an important challenge for biointerface sciences. Sum frequency scattering (SFS) spectroscopy can specifically probe the surfaces of nanoparticles, vesicles, liposomes, and other materials relevant to biomaterial research, and, as a vibrational spectroscopy method, it can provide molecular level information about the surface chemistry. SFS is particularly promising to probe the structure of proteins, and other biological molecules, at nanoparticle surfaces. Here, amide I spectra can provide information about protein folding and orientation, while spectra in the C-D and C-H stretching regions allow experiments to determine the mode of interaction between particle surfaces and proteins. Methods used currently employ a closed liquid cell or cuvette, which works extremely well for C-H and phosphate regions but is often impeded in the amide I and C-D regions by a strong background signal that originates from the window material of the sample cells. Here, we discuss a windowless geometry for collecting background-free and high-fidelity SFS spectra in the amide I and C-D regions. We demonstrate the improvement in spectra quality by comparing SFS spectra of unextruded, multilamellar vesicles in a sample cuvette with those recorded using the windowless geometry. The sample geometry we propose will enable new experiments using SFS as a probe for protein-particle interactions.

Characterizing protein G B1 orientation and its effect on immunoglobulin G antibody binding using XPS, ToF-SIMS, and quartz crystal microbalance with dissipation monitoring

Controlling how proteins are immobilized (e.g., controlling their orientation and conformation) is essential for developing and optimizing the performance of in vitro protein-binding devices, such as enzyme-linked immunosorbent assays. Characterizing the identity, orientation, etc., of proteins in complex mixtures of immobilized proteins requires a multitechnique approach. The focus of this work was to control and characterize the orientation of protein G B1, an immunoglobulin G (IgG) antibody-binding domain of protein G, on well-defined surfaces and to measure the effect of protein G B1 orientation on IgG antibody binding. The surface sensitivity of time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to distinguish between different proteins and their orientation on both flat and nanoparticle gold surfaces by monitoring intensity changes of characteristic amino acid mass fragments. Amino acids distributed asymmetrically were used to calculate peak intensity ratios from ToF-SIMS data to determine the orientation of protein G B1 cysteine mutants covalently attached to a maleimide surface. To study the effect of protein orientation on antibody binding, multilayer protein films on flat gold surfaces were formed by binding IgG to the immobilized protein G B1 films. Quartz crystal microbalance with dissipation monitoring and x-ray photoelectron spectroscopy analysis revealed that coverage and orientation affected the antibody-binding process. At high protein G B1 coverage, the cysteine mutant immobilized in an end-on orientation with the C-terminus exposed bound 443 ng/cm² of whole IgG (H + L) antibodies. In comparison, the high coverage cysteine mutant immobilized in an end-on orientation with the N-terminus exposed did not bind detectable amounts of whole IgG (H + L) antibodies.

Dynamic single-molecule counting for the quantification and optimization of nanoparticle functionalization protocols

Applications of colloidal particles in the fields of i.e. biosensors, molecular targeting, or drug-delivery require their functionalization with biologically active and specific molecular ligands. Functionalization protocols often result in a heterogeneous population of particles with a varying density, spatial distribution and orientation of the functional groups on the particle surface. A lack of methods to directly resolve these molecular properties of the particle's surface hampers optimization of functionalization protocols and applications. Here quantitative single-molecule interaction kinetics is used to count the number of ligands on the surface of hundreds of individual nanoparticles simultaneously. By analyzing the waiting-time between single-molecule binding events we quantify the particle functionalization both accurately and precisely for a large range of ligand densities. We observe significant particle-to-particle differences in functionalization which are dominated by the particle-size distribution for high molecular densities, but are substantially broadened for sparsely functionalized particles. From time-dependent studies we find that ligand reorganization on long timescales drastically reduces this heterogeneity, a process that has remained hidden up to now in ensemble-averaged studies. The quantitative single-molecule counting therefore provides a direct route to quantification and optimization of coupling protocols towards molecularly controlled colloidal interfaces.

D-shaped plastic optical fibre aptasensor for fast thrombin detection in nanomolar range

The development of optical biosensors for the rapid and costless determination of clinical biomarkers is of paramount importance in medicine. Here we report a fast and low-cost biosensor based on a plasmonic D-shaped plastic optical fibre (POF) sensor derivatized with an aptamer specific for the recognition of thrombin, the target marker of blood homeostasis and coagulation cascade. In particular, we designed a functional interface based on a Self Assembled Monolayer (SAM) composed of short Poly Ethylene Glycol (PEG) chains and biotin-modified PEG thiol in ratio 8:2 mol:mol, these latter serving as baits for the binding of the aptamer through streptavidin-chemistry. The SAM was studied by X-ray Photoelectron Spectroscopy (XPS) analysis, static contact angle (CA), Surface Plasmon Resonance (SPR) in POFs, and fluorescence microscopy on gold surface. The optimized SAM composition enabled the immobilization of about 112 ng/cm² of aptamer. The thrombin detection exploiting POF-Aptasensor occurred in short times (5–10 minutes), the reached Limit of Detection (LOD) was about 1 nM, and the detection range was 1.6–60 nM, indicating the POF-Aptasensor well addresses the needs for a low-cost, simple to use and to realize, rapid, small size and portable diagnostic platform.

Label Free Particle-by-Particle Quantification of DNA Loading on Sorted Gold Nanostars

This paper describes a label free technique for determining ligand loading on metal nanoparticles using a variant of secondary ion mass spectrometry. Au₄₀₀⁴⁺ clusters bombard DNA-functionalized anisotropic gold nanostars and isotropic nanospheres with similar surface areas to determine ligand density. For each projectile impact, co-localized molecules within the emission area of a single impact (diameter of 10-15 nm) were examined for each particle. Individual nanoparticle analysis allows for determination of the relationship between particle geometry and DNA loading. We found that branched particles exhibited increased ligand density versus nanospheres and determined that positive and neutral curvature could facilitate additional loading. This methodology can be applied to optimize loading for any ligand-core interaction independent of nanoparticle core, ligand, or attachment chemistry.

Comparisons of Analytical Approaches for Determining Shell Thicknesses of Core-Shell Nanoparticles by X-ray Photoelectron Spectroscopy

We assessed two approaches for determining shell thicknesses of core-shell nanoparticles (NPs) by X-ray photoelectron spectroscopy (XPS). These assessments were based on simulations of photoelectron peak intensities for Au-core/C-shell, C-core/Au-shell, Cu-core/Al-shell, and Al-core/Cu-shell NPs with a wide range of core diameters and shell thicknesses. First, we demonstrated the validity of an empirical equation developed by Shard for determinations of shell thicknesses. Values of shell thicknesses from the Shard equation typically agreed with actual shell thicknesses to better than 10 %. Second, we investigated the magnitudes of elastic-scattering effects on photoelectron peak intensities by performing a similar series of simulations with elastic scattering switched off in our simulation software. Our ratios of the C-shell 1s intensity to the Au-core 4f_7/2 intensity with elastic scattering switched off were qualitatively similar to those obtained by Torelli et al. from a model that neglected elastic scattering. With elastic scattering switched on, the C 1s/Au 4f_7/2 intensity ratios generally changed by less than 10 %, thereby justifying the neglect of elastic scattering in XPS models that are applied to organic ligands on Au-core NPs. Nevertheless, elastic-scattering effects on peak-intensity ratios were generally much stronger for C-core/Au-shell, Al-core/Cu-shell, and Cu-core/Al-shell NPs, and there were second-order dependences on core diameter and shell thickness.

Biomedical surface analysis: Evolution and future directions (Review)

This review describes some of the major advances made in biomedical surface analysis over the past 30-40 years. Starting from a single technique analysis of homogeneous surfaces, it has been developed into a complementary, multitechnique approach for obtaining detailed, comprehensive information about a wide range of surfaces and interfaces of interest to the biomedical community. Significant advances have been made in each surface analysis technique, as well as how the techniques are combined to provide detailed information about biological surfaces and interfaces. The driving force for these advances has been that the surface of a biomaterial is the interface between the biological environment and the biomaterial, and so, the state-of-the-art in instrumentation, experimental protocols, and data analysis methods need to be developed so that the detailed surface structure and composition of biomedical devices can be determined and related to their biological performance. Examples of these advances, as well as areas for future developments, are described for immobilized proteins, complex biomedical surfaces, nanoparticles, and 2D/3D imaging of biological materials.