New frontiers in gold labeling

Stability and SERS signal strength of laser-generated gold, silver, and bimetallic nanoparticles at different KCl concentrations

Noble metal nanoparticles, specifically gold and silver, are extensively utilized in sensors, catalysts, surface-enhanced Raman scattering (SERS), and optical-electronic components due to their unique localized surface plasmon resonance (LSPR) properties. The production of these nanoparticles involves various methods, but among the environmentally friendly approaches, laser ablation stands out as it eliminates the need for toxic chemicals during purification. However, nanoparticle aggregation poses a challenge in laser ablation, necessitating the addition of extra materials that contaminate the otherwise clean process. In this study, we investigate the effectiveness of a biocompatible material, potassium chloride (KCl), in preventing particle aggregation. Although salt is known to trigger aggregation, we observed that certain concentrations of KCl can slow down this process. Over an eight-week period, we examined the aggregation rate, extinction behavior, and stability of gold, silver, and hybrid nanoparticles generated in different KCl concentrations. Extinction spectra, SEM images, SERS signal strength, and zeta potential were analyzed. Our results demonstrate that laser ablation in water and salt solutions yields nanoparticles with a spherical shape and a negative zeta potential. Importantly, we identified the optimal concentration of potassium chloride salt that maintains solution stability and SERS signal strength. Adsorbed chloride ions on silver nanoparticles were evidenced by low-frequency SERS band near 242 cm-1. A better understanding of the effect of KCl concentration on the properties of noble metal nanoparticles can lead to improved generation protocols and the development of tailored nanoparticle systems with enhanced stability and SERS activity.

On-Chip Micro-Supercapacitor with High Areal Energy Density Based on Dielectrophoretic Assembly of Nanoporous Metal Microwire Electrodes

Advances in the Internet of Things (IoT) technology have driven the demand for miniaturized electronic devices, prompting research on small-scale energy-storage systems. Micro-supercapacitors (MSCs) stand out in this regard because of their compact size, high power density, high charge-discharge rate, and extended cycle life. However, their limited energy density impedes commercialization. To resolve this issue, a simple and innovative approach is reported herein for fabricating highly efficient on-chip MSCs integrated with nanoporous metal microwires formed by dielectrophoresis (DEP)-driven gold nanoparticle (AuNP) assembly. Placing a water-based AuNP suspension onto interdigitated electrodes and applying an alternating voltage induces in-plane porous microwire formation in the electrode gap. The DEP-induced AuNP assembly and the gold microwire (AuMW) growth rate can be adjusted by controlling the applied alternating voltage and frequency. The microwire-integrated MSC (AuMW-MSC) electrically outperforms its unmodified counterpart and exhibits a 30% larger electrode area, along with 72% and 78% higher specific and areal capacitances, respectively, than a microwire-free MSC. Additionally, AuMW-MSC achieves maximum energy and power densities of 3.33 µWh cm-2 and 2629 µW cm-2, respectively, with a gel electrolyte. These findings can help upgrade MSCs to function as potent energy-storage devices for small electronics.

A correlated light and electron microscopy approach to study the subcellular localization of phosphorylated vimentin in human lung tissue

Correlative microscopy is an important approach for bridging the resolution gap between fluorescence light and electron microscopy. Here, we describe a fast and simple method for correlative immunofluorescence and immunogold labeling on the same section to elucidate the localization of phosphorylated vimentin (P-Vim), a robust feature of pulmonary vascular remodeling in cells of human lung small arteries. The lung is a complex, soft and difficult tissue to prepare for transmission electron microscopy (TEM). Detailing the molecular composition of small pulmonary arteries (<500μm) would be of great significance for research and diagnostics. Using the classical methods of immunochemistry (either hydrophilic resin or thin cryosections), is difficult to locate small arteries for analysis by TEM. To address this problem and to observe the same structures by both light and electron microscopy, correlative microscopy is a reliable approach. Immunofluorescence enables us to know the distribution of P-Vim in cells but does not provide ultrastructural detail on its localization. Labeled structures selected by fluorescence microscope can be identified and further analyzed by TEM at high resolution. With our method, the morphology of the arteries is well preserved, enabling the localization of P-Vim inside pulmonary endothelial cells. By applying this approach, fluorescent signals can be directly correlated to the corresponding subcellular structures in areas of interest.

Bioconjugation of Au25 Nanocluster to Monoclonal Antibody at Tryptophan

We report the first bioconjugation of Au₂₅ nanocluster to a monoclonal antibody at scarcely exposed tryptophan (Trp) residues toward the development of high-resolution probes for cryogenic electron microscopy (cryo-EM) and tomography (cryo-ET). To achieve this, we improved the Trp-selective bioconjugation using hydroxylamine (ABNOH) reagents instead of previously developed N-oxyl radicals (ABNO). This new protocol allowed for the application of Trp-selective bioconjugation to acid-sensitive proteins such as antibodies. We found that a two-step procedure utilizing first Trp-selective bioconjugation for the introduction of azide groups to the protein and then strain-promoted azide-alkyne cycloaddition (SPAAC) to attach a bicyclononyne (BCN)-presenting redox-sensitive Au₂₅ nanocluster was essential for a scalable procedure. Covalent labeling of the antibody with gold nanoclusters was confirmed by various analytical methods, including cryo-EM analysis of the Au₂₅ nanocluster conjugates.

Uncovering the Conformational Distribution of a Small Protein with Nanoparticle-Aided Cryo-Electron Microscopy Sampling

Here, we introduce the nanoparticle-aided cryo-electron microscopy sampling (NACS) method to access the conformational distribution of a protein molecule. Two nanogold particles are labeled at two target sites, and the interparticle distance is measured as a structural parameter via cryo-electron microscopy (cryo-EM). The key aspect of NACS is that the projected distance information instead of the global conformational information is extracted from each protein molecule. This is possible because the contrast provided by the nanogold particles is strong enough to provide the projected distance, while the protein itself is invisible due to its low contrast. We successfully demonstrate that various protein conformations, even for small or disordered proteins, which generally cannot be accessed via cryo-EM, can be captured. The demonstrated method with the potential to directly observe the conformational distribution of such systems may open up new possibilities in studying their dynamics at a single-molecule level.

Sigma 1 Receptor Co-Localizes with NRF2 in Retinal Photoreceptor Cells

Sigma 1 receptor (Sig1R), a modulator of cell survival, has emerged as a novel target for retinal degenerative disease. Studies have shown that activation of Sig1R, using the high affinity ligand (+)-pentazocine ((+)-PTZ), improves cone function in a severe retinopathy model. The rescue is accompanied by normalization of levels of NRF2, a key transcription factor that regulates the antioxidant response. The interaction of Sig1R with a number of proteins has been investigated; whether it interacts with NRF2, however, is not known. We used co-immunoprecipitation (co-IP), proximity ligation assay (PLA), and electron microscopy (EM) immunodetection methods to investigate this question in the 661W cone photoreceptor cell line. For co-IP experiments, immune complexes were precipitated by protein A/G agarose beads and immunodetected using anti-NRF2 antibody. For PLA, cells were incubated with anti-Sig1R polyclonal and anti-NRF2 monoclonal antibodies, then subsequently with (−)-mouse and (+)-rabbit PLA probes. For EM analysis, immuno-EM gold labeling was performed using nanogold-enhanced labeling with anti-NRF2 and anti-Sig1R antibodies, and data were confirmed using colloidal gold labeling. The co-IP experiment suggested that NRF2 was bound in a complex with Sig1R. The PLA assays detected abundant orange fluorescence in cones, indicating that Sig1R and NRF2 were within 40 nm of each other. EM immunodetection confirmed co-localization of Sig1R with NRF2 in cells and in mouse retinal tissue. This study is the first to report co-localization of Sig1R-NRF2 and supports earlier studies implicating modulation of NRF2 as a mechanism by which Sig1R mediates retinal neuroprotection.

Optimization of protocols for pre-embedding immunogold electron microscopy of neurons in cell cultures and brains

Immunogold labeling allows localization of proteins at the electron microscopy (EM) level of resolution, and quantification of signals. The present paper summarizes methodological issues and experiences gained from studies on the distribution of synaptic and other neuron-specific proteins in cell cultures and brain tissues via a pre-embedding method. An optimal protocol includes careful determination of a fixation condition for any particular antibody, a well-planned tissue processing procedure, and a strict evaluation of the credibility of the labeling. Here, tips and caveats on different steps of the sample preparation protocol are illustrated with examples. A good starting condition for EM-compatible fixation and permeabilization is 4% paraformaldehyde in PBS for 30 min at room temperature, followed by 30 min incubation with 0.1% saponin. An optimal condition can then be readjusted for each particular antibody. Each lot of the secondary antibody (conjugated with a 1.4 nm small gold particle) needs to be evaluated against known standards for labeling efficiency. Silver enhancement is required to make the small gold visible, and quality of the silver-enhanced signals can be affected by subsequent steps of osmium tetroxide treatment, uranyl acetate en bloc staining, and by detergent or ethanol used to clean the diamond knife for cutting thin sections. Most importantly, verification of signals requires understanding of the protein of interest in order to validate for correct localization of antibodies at expected epitopes on particular organelles, and quantification of signals needs to take into consideration the penetration gradient of reagents and clumping of secondary antibodies.

Biomolecular interactions of ultrasmall metallic nanoparticles and nanoclusters

The use of nanoparticles (NPs) in biomedicine has made a gradual transition from proof-of-concept to clinical applications, with several NP types meeting regulatory approval or undergoing clinical trials. A new type of metallic nanostructures called ultrasmall nanoparticles (usNPs) and nanoclusters (NCs), while retaining essential properties of the larger (classical) NPs, have features common to bioactive proteins. This combination expands the potential use of usNPs and NCs to areas of diagnosis and therapy traditionally reserved for small-molecule medicine. Their distinctive physicochemical properties can lead to unique in vivo behaviors, including improved renal clearance and tumor distribution. Both the beneficial and potentially deleterious outcomes (cytotoxicity, inflammation) can, in principle, be controlled through a judicious choice of the nanocore shape and size, as well as the chemical ligands attached to the surface. At present, the ability to control the behavior of usNPs is limited, partly because advances are still needed in nanoengineering and chemical synthesis to manufacture and characterize ultrasmall nanostructures and partly because our understanding of their interactions in biological environments is incomplete. This review addresses the second limitation. We review experimental and computational methods currently available to understand molecular mechanisms, with particular attention to usNP-protein complexation, and highlight areas where further progress is needed. We discuss approaches that we find most promising to provide relevant molecular-level insight for designing usNPs with specific behaviors and pave the way to translational applications.

Membrane Dynamics and Organization of the Phagocyte NADPH Oxidase in PLB-985 Cells

Neutrophils are the first cells recruited at the site of infections, where they phagocytose the pathogens. Inside the phagosome, pathogens are killed by proteolytic enzymes that are delivered to the phagosome following granule fusion, and by reactive oxygen species (ROS) produced by the NADPH oxidase. The NADPH oxidase complex comprises membrane proteins (NOX2 and p22^phox), cytoplasmic subunits (p67^phox, p47^phox, and p40^phox) and the small GTPase Rac. These subunits assemble at the phagosomal membrane upon phagocytosis. In resting neutrophils the catalytic subunit NOX2 is mainly present at the plasma membrane and in the specific granules. We show here that NOX2 is also present in early and recycling endosomes in human neutrophils and in the neutrophil-like cell line PLB-985 expressing GFP-NOX2. In the latter cells, an increase in NOX2 at the phagosomal membrane was detected by live-imaging after phagosome closure, probably due to fusion of endosomes with the phagosome. Using super-resolution microscopy in PLB-985 WT cells, we observed that NOX2 forms discrete clusters in the plasma membrane. The number of clusters increased during frustrated phagocytosis. In PLB-985NCF1ΔGT cells that lack p47^phox and do not assemble a functional NADPH oxidase, the number of clusters remained stable during phagocytosis. Our data suggest a role for p47^phox and possibly ROS production in NOX2 recruitment at the phagosome.

Ultrastructure of light-activated axons following optogenetic stimulation to produce late-phase long-term potentiation

Analysis of neuronal compartments has revealed many state-dependent changes in geometry but establishing synapse-specific mechanisms at the nanoscale has proven elusive. We co-expressed channelrhodopsin2-GFP and mAPEX2 in a subset of hippocampal CA3 neurons and used trains of light to induce late-phase long-term potentiation (L-LTP) in area CA1. L-LTP was shown to be specific to the labeled axons by severing CA3 inputs, which prevented back-propagating recruitment of unlabeled axons. Membrane-associated mAPEX2 tolerated microwave-enhanced chemical fixation and drove tyramide signal amplification to deposit Alexa Fluor dyes in the light-activated axons. Subsequent post-embedding immunogold labeling resulted in outstanding ultrastructure and clear distinctions between labeled (activated), and unlabeled axons without obscuring subcellular organelles. The gold-labeled axons in potentiated slices were reconstructed through serial section electron microscopy; presynaptic vesicles and other constituents could be quantified unambiguously. The genetic specification, reliable physiology, and compatibility with established methods for ultrastructural preservation make this an ideal approach to link synapse ultrastructure and function in intact circuits.

Electron Microscopic Detection of Single Membrane Proteins by a Specific Chemical Labeling

Electron microscopy (EM) is a technology that enables visualization of single proteins at a nanometer resolution. However, current protein analysis by EM mainly relies on immunolabeling with gold-particle-conjugated antibodies, which is compromised by large size of antibody, precluding precise detection of protein location in biological samples. Here, we develop a specific chemical labeling method for EM detection of proteins at single-molecular level. Rational design of α-helical peptide tag and probe structure provided a complementary reaction pair that enabled specific cysteine conjugation of the tag. The developed chemical labeling with gold-nanoparticle-conjugated probe showed significantly higher labeling efficiency and detectability of high-density clusters of tag-fused G protein-coupled receptors in freeze-fracture replicas compared with immunogold labeling. Furthermore, in ultrathin sections, the spatial resolution of the chemical labeling was significantly higher than that of antibody-mediated labeling. These results demonstrate substantial advantages of the chemical labeling approach for single protein visualization by EM.

Impact of conjugation strategies for targeting of antibodies in gold nanoparticles for ultrasensitive detection of 17β-estradiol

Antibody-coated nanoparticles have recently attracted considerable attention, with the focus falling on diagnostics. Nevertheless, controlled antibody bioconjugation remains a challenge. Here, we present two strategies of bioconjugation with the aim of evaluating the best approach for the coupling of antibodies on the surface of nanomaterials in an oriented way. We employed electrostatic interaction (physical adsorption) and covalent conjugation in the orientation of antibodies on the metallic surface as coupling methods, and their influence on the detection of 17β-estradiol was addressed with localized surface plasmon resonance. The understanding of these mechanisms is fundamental for the development of reproducible inorganic bioconjugates with oriented surface as well sensibility of immunoassays.

Surface Engineering of Gold Nanorods for Cytochrome c Bioconjugation: An Effective Strategy To Preserve the Protein Structure

The surface of gold nanorods (Au NRs) has been appropriately engineered to achieve a suitable interface for bioconjugation with horse heart cytochrome c (HCc). HCc, an extensively studied and well-characterized protein, represents an ideal model for nanoparticle (NP)-protein conjugation studies because of its small size, high stability, and commercial availability. Here, the native state of the protein has been demonstrated for the first time, by means of Raman spectroscopy, to be retained upon conjugation with the anisotropic Au nanostructures, thus validating the proposed protocol as specifically suited to mostly preserve the plasmonic properties of the NRs and to retain the structure of the protein. The successful creation of such bioconjugates with the retention of the protein structure and function along with the preservation of the NP properties represents a challenging but essential task, as it provides the only way to access functional hybrid systems with potential applications in biotechnology, medicine, and catalysis. In this perspective, the organic capping surrounding the Au NRs plays a key role, as it represents the functional interface for the conjugation step. Cetyltrimethylammonium bromide-coated Au NRs, prepared by using a seed-mediated synthetic route, have been wrapped with polyacrylic acid (PAA) by means of electrostatic interactions following a layer-by-layer approach. The resulting water-dispersible negatively charged AuNRs@PAA NPs have then been electrostatically bound to the positively charged HCc. The bioconjugation procedure has been thoroughly monitored by the combined analysis of UV-vis absorption, resonance Raman and Fourier transform infrared spectroscopies, transmission electron microscopy microscopy, and ζ-potential, which verified the successful conjugation of the protein to the nanorods.

Locating Reactive Groups on Nanomaterials with Gold Nanoclusters: Toward a Surface Reactive Site Map

Nanoparticles (NPs) are often functionalized with reactive groups such as amines and thiols for the subsequent conjugation of further molecules, e.g., stabilizing polymers, drugs, and proteins for targeting cells or specific diseases. In addition to the quantitative estimation of the reactive conjugation sites, their molecular positioning and nanoscale arrangement on single nanoparticles become more and more important for the tailored engineering and design of functional nanomaterials. Here, we use maleimide or sulfo-succinimidyl ester-modified 1.4 nm gold nanoclusters (AuNCs) to specifically label reactive thiol and amine groups with sub-2-nm precision on metal oxide and polymeric nanostructures. We confirm the binding of AuNCs by measuring and modeling sedimentation properties using analytical centrifugation, imaging their surface distribution and surface distances by transmission electron microscopy (TEM), and comparing the results to ensemble measurements of numbers of reactive surface groups obtained by common photometric assays. We map thiol and amine groups introduced on silica NPs (SiNPs), titania stars (Ti), silica inverse opals (SiOps), and polystyrene NPs (PS NPs). We show that the method is suitable for mapping local, clustered inhomogeneities of the reactive sites on single SiNPs introduced by masking certain areas during surface functionalization. Mapping precise positions of reactive surface groups is essential to the design and tailored ligation of multifunctional nanomaterials.

An Analytical Quantum Model to Calculate Fluorescence Enhancement of a Molecule in Vicinity of a Sub-10 nm Metal Nanoparticle

An analytical quantum model is used to calculate electrical permittivity of a metal nanoparticle located in an adjacent molecule. Different parameters, such as radiative and non-radiative decay rates, quantum yield, electrical field enhancement factor, and fluorescence enhancement are calculated by such a model and they are compared with those obtained by using the classical Drude model. It is observed that using an analytical quantum model presents a higher enhancement factor, up to 30%, as compared to classical model for nanoparticles smaller than 10 nm. Furthermore, the results are in better agreement with those experimentally realized.

Structural model of the dimeric Parkinson's protein LRRK2 reveals a compact architecture involving distant interdomain contacts

Leucine-rich repeat kinase 2 (LRRK2) is a large, multidomain protein containing two catalytic domains: a Ras of complex proteins (Roc) G-domain and a kinase domain. Mutations associated with familial and sporadic Parkinson's disease (PD) have been identified in both catalytic domains, as well as in several of its multiple putative regulatory domains. Several of these mutations have been linked to increased kinase activity. Despite the role of LRRK2 in the pathogenesis of PD, little is known about its overall architecture and how PD-linked mutations alter its function and enzymatic activities. Here, we have modeled the 3D structure of dimeric, full-length LRRK2 by combining domain-based homology models with multiple experimental constraints provided by chemical cross-linking combined with mass spectrometry, negative-stain EM, and small-angle X-ray scattering. Our model reveals dimeric LRRK2 has a compact overall architecture with a tight, multidomain organization. Close contacts between the N-terminal ankyrin and C-terminal WD40 domains, and their proximity-together with the LRR domain-to the kinase domain suggest an intramolecular mechanism for LRRK2 kinase activity regulation. Overall, our studies provide, to our knowledge, the first structural framework for understanding the role of the different domains of full-length LRRK2 in the pathogenesis of PD.

NANOGOLD decorated by pHLIP peptide: comparative force field study

The potential of gold nanoparticles (AuNPs) in therapeutic and diagnostic cancer applications is becoming increasingly recognized, which focuses on their efficient and specific delivery from passive accumulation in tumour tissue to directly targeting tumor-specific biomarkers. AuNPs functionalized by pH low insertion peptide (pHLIP) have recently revealed the capability of targeting acidic tissues and inserting into cell membranes. However, the structure of AuNP-pHLIP conjugates and fundamental gold-peptide interactions still remain unknown. In this study, we have developed a series of molecular dynamics (MD) models reproducing a small gold nanoparticle coupled to pHLIP. We focus on Au135 nanoparticles that comprise a nearly spherical Au core (diameter ∼ 1.4 nm) functionalized with a monomaleimide moiety, mimicking a commercially available monomaleimido NANOGOLD® labelling agent. To probe the structure and folding of pHLIP, which is attached covalently to the maleimide NANOGOLD particle, we have benchmarked the performances of a series of popular, all-atom force fields (FF), including those of OPLS-AA, AMBER03, three variations of CHARMM FFs, as well as united-atom GROMOS G53A6 FF. We found that CHARMMs and OPLSAA FFs predict that in an aqueous salt solution at a neutral pH, pHLIP is partially bound onto the gold surface through some short hydrophobic peptide stretches, while at the same time, a large portion of peptide remains in solution. In contrast, AMBER03 and G53A6 FFs revealed the formation of compact, tightly bound peptide configurations adsorbed onto the nanoparticle core. To reproduce the experimental physical picture of the peptide adsorption onto gold in unfolded and unstructured conformations, our study suggests CHARMM36 and OPLS-AA FFs as a tool of choice for the computational studies of NANOGOLD decorated by pHLIP.

Driving Adsorbed Gold Nanoparticle Assembly by Merging Lipid Gel/Fluid Interfaces

Surface forces between inorganic nanoparticles and lipid bilayer is of great relevance to biophysics, medicine, and nanobiotechnology. Adsorbed nanoparticles may influence the fluidity of the underlying lipids, which may in turn influence nanoparticle assembly. Herein three types of lipids (DOPC, Tc = -20 °C; DMPC, Tc = 23 °C; and DPPC, Tc = 41 °C) are used, all with the same phosphocholine (PC) headgroup. Gold nanoparticle (AuNP) color change is monitored as a function of lipid phase transition temperature (Tc), surface ligands on AuNPs, and temperature. Liposomes with higher fluidity induce much faster aggregation of AuNPs. Aside from the kinetic aspect of faster diffusion on fluid bilayers, this faster color change is attributed to the local lipid gelation and merging of gelled regions to eliminate the interface between different lipid phases.

Advantages and disadvantages of technologies for HER2 testing in breast cancer specimens

OBJECTIVES

Human epidermal growth factor receptor 2 (HER2) plays a central role as a prognostic and predictive marker in breast cancer specimens. Reliable HER2 evaluation is central to determine the eligibility of patients with breast cancer to targeted anti-HER2 therapies such as trastuzumab and lapatinib. Presently, several methods exist for the determination of HER2 status at different levels (protein, RNA, and DNA level).

METHODS

In this review, we discuss the main advantages and disadvantages of the techniques developed so far for the evaluation of HER2 status in breast cancer specimens.

RESULTS

Each technique has its own advantages and disadvantages. It is therefore not surprising that no consensus has been reached so far on which technique is the best for the determination of HER2 status.

CONCLUSIONS

Currently, emphasis must be put on standardization of procedures, internal and external quality control assessment, and competency evaluation of already existing methods to ensure accurate, reliable, and clinically meaningful test results. Development of new robust and accurate diagnostic assays should also be encouraged. In addition, large clinical trials are warranted to identify the technique that most reliably predicts a positive response to anti-HER2 drugs.

Silver Enhancement for Transmission Electron Microscopy Imaging of Antibody Fragment-Gold Nanoparticles Conjugates Immobilized on Ordered Mesoporous Silica

Ordered mesoporous silica (OMS) materials are receiving great attention as possible carriers for valuable but unstable drugs as, for example, therapeutic proteins. A key issue is to prove that the therapeutic protein is effectively able to penetrate the pores of OMS during the adsorption step. Here, we immobilized an antibody fragment [F(ab')GAMIgG] conjugated with ultrasmall gold nanoparticles (GNPs) onto amino-functionalized SBA-15 (SBA-NH2) mesoporous silica. The aim of this work is the visualization of the location of the conjugates adsorbed onto SBA-NH2 with transmission electron microscopy (TEM). Because of the ultrasmall size of GNPs (<1 nm), we use the silver enhancement procedure to amplify their size. In this procedure, ultrathin sections of conjugate-loaded SBA-NH2 particles are prepared by a ultramicrotome sectioning technique. The ultrasmall GNPs located on the top side of the 70-90 nm thick slices act as microcrystallization nucleation sites for the deposition of reduced metallic silver. Consequently, the ultrasmall GNPs increase their size. This allows for the direct imaging of the conjugates adsorbed. We clearly localize the F(ab')GAMIgG-GNPs conjugates either on the external surface of the particles or inside the mesopores of SBA-NH2 through TEM.

Supported lipid bilayers as dynamic platforms for tethered particles

Nanoparticle tethering to lipid bilayers enables the observation of hundreds of diffusing particles that are confined within a single field of view. A wide variety of materials ranging from plasmonic metals to soft matter can be stably tethered to the surface of a fluid bilayer by covalent or non-covalent means. The controlled environment of this experimental platform allows direct control over surface compositions and accurate tracking of nanoparticle interactions. This minireview will cover studies that use bilayer-tethered nanoparticles to investigate physical properties related to lipid mobility, biomolecule sensing, and surface interactions, as well as experiments to reversibly manipulate tethered nanoparticles by electric fields.

Fundamentals and application of magnetic particles in cell isolation and enrichment: a review

Magnetic sorting using magnetic beads has become a routine methodology for the separation of key cell populations from biological suspensions. Due to the inherent ability of magnets to provide forces at a distance, magnetic cell manipulation is now a standardized process step in numerous processes in tissue engineering, medicine, and in fundamental biological research. Herein we review the current status of magnetic particles to enable isolation and separation of cells, with a strong focus on the fundamental governing physical phenomena, properties and syntheses of magnetic particles and on current applications of magnet-based cell separation in laboratory and clinical settings. We highlight the contribution of cell separation to biomedical research and medicine and detail modern cell-separation methods (both magnetic and non-magnetic). In addition to a review of the current state-of-the-art in magnet-based cell sorting, we discuss current challenges and available opportunities for further research, development and commercialization of magnetic particle-based cell-separation systems.

Facile synthesis of multifunctional La1−xSrxMnO3@Au core–shell nanoparticles for biomedical applications

Multifunctional high-performance La_1−xSrxMnO₃@Au core–shell nanoparticles were synthesized by nanoemulsion with polymers, showing sharp Curie transition, excellent amphiphilic dispersibility and optical properties as well as biocompatibility.

Nanogold labeling of the yeast endosomal system for ultrastructural analyses

Endosomes are one of the major membrane sorting checkpoints in eukaryotic cells and they regulate recycling or destruction of proteins mostly from the plasma membrane and the Golgi. As a result the endosomal system plays a central role in maintaining cell homeostasis, and mutations in genes belonging to this network of organelles interconnected by vesicular transport, cause severe pathologies including cancer and neurobiological disorders. It is therefore of prime relevance to understand the mechanisms underlying the biogenesis and organization of the endosomal system. The yeast Saccharomyces cerevisiae has been pivotal in this task. To specifically label and analyze at the ultrastructural level the endosomal system of this model organism, we present here a detailed protocol for the positively charged nanogold uptake by spheroplasts followed by the visualization of these particles through a silver enhancement reaction. This method is also a valuable tool for the morphological examination of mutants with defects in endosomal trafficking. Moreover, it is not only applicable for ultrastructural examinations but it can also be combined with immunogold labelings for protein localization investigations.

Liquid scanning transmission electron microscopy: imaging protein complexes in their native environment in whole eukaryotic cells

Scanning transmission electron microscopy (STEM) of specimens in liquid, so-called Liquid STEM, is capable of imaging the individual subunits of macromolecular complexes in whole eukaryotic cells in liquid. This paper discusses this new microscopy modality within the context of state-of-the-art microscopy of cells. The principle of operation and equations for the resolution are described. The obtained images are different from those acquired with standard transmission electron microscopy showing the cellular ultrastructure. Instead, contrast is obtained on specific labels. Images can be recorded in two ways, either via STEM at 200 keV electron beam energy using a microfluidic chamber enclosing the cells, or via environmental scanning electron microscopy at 30 keV of cells in a wet environment. The first series of experiments involved the epidermal growth factor receptor labeled with gold nanoparticles. The labels were imaged in whole fixed cells with nanometer resolution. Since the cells can be kept alive in the microfluidic chamber, it is also feasible to detect the labels in unfixed, live cells. The rapid sample preparation and imaging allows studies of multiple whole cells.

Numerical simulation of x-ray luminescence optical tomography for small-animal imaging

X-ray luminescence optical tomography (XLOT) is an emerging hybrid imaging modality in which x-ray excitable particles (phosphor particles) emit optical photons when stimulated with a collimated x-ray beam. XLOT can potentially combine the high sensitivity of optical imaging with the high spatial resolution of x-ray imaging. For reconstruction of XLOT data, we compared two reconstruction algorithms, conventional filtered backprojection (FBP) and a new algorithm, x-ray luminescence optical tomography with excitation priors (XLOT-EP), in which photon propagation is modeled with the diffusion equation and the x-ray beam positions are used as reconstruction priors. Numerical simulations based on dose calculations were used to validate the proposed XLOT imaging system and the reconstruction algorithms. Simulation results showed nanoparticle concentrations reconstructed with XLOT-EP are much less dependent on scan depth than those obtained with FBP. Measurements at just two orthogonal projections are sufficient for XLOT-EP to reconstruct an XLOT image for simple source distributions. The heterogeneity of x-ray energy deposition is included in the XLOT-EP reconstruction and improves the reconstruction accuracy, suggesting that there is a need to calculate the x-ray energy distribution for experimental XLOT imaging.

Gold nanoparticles as physiological markers of urine internalization into urothelial cells in vivo

BACKGROUND

Urothelial bladder is the reservoir of urine and the urothelium minimizes the exchange of urine constituents with this tissue. Our aim was to test 1.9 nm biocompatible gold nanoparticles as a novel marker of internalization into the urothelial cells under physiological conditions in vivo.

METHODS

We compared normal and neoplastic mice urothelium. Neoplastic lesions were induced by 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in drinking water for 10 weeks. Nanoparticles, intravenously injected into normal and BBN-treated mice, were filtered through the kidneys and became constituents of the urine within 90 minutes after injection.

RESULTS

Gold nanoparticles were densely accumulated in the urine, while their internalization into urothelial cells depended on the cell differentiation stage. In the terminally differentiated superficial urothelial cells of normal animals, nanoparticles were occasionally found in the endosomes, but not in the fusiform vesicles. Regions of exfoliated cells were occasionally found in the normal urothelium. Superficial urothelial cells located next to exfoliated regions contained gold nanoparticles in the endosomes and in the cytosol beneath the apical plasma membrane. The urothelium of BBN-treated animals developed fat hyperplasia with moderate dysplasia. The superficial cells of BBN-treated animals were partially differentiated as demonstrated by the lack of fusiform vesicles. These cells contained the gold nanoparticles distributed in the endosomes and throughout their cytosol.

CONCLUSION

Gold nanoparticles are a valuable marker to study urine internalization into urothelial cells in vivo. Moreover, they can be used as a sensitive marker of differentiation and functionality of urothelial cells.

Labeling primary amine groups in peptides and proteins with N-hydroxysuccinimidyl ester modified Fe3O4@SiO2 nanoparticles containing cleavable disulfide-bond linkers

The surface of superparamagnetic silica coated iron oxide (Fe3O4@SiO2) nanoparticles was functionalized with a disulfide bond linked N-hydroxysuccinimidyl (NHS) ester group in order to develop a method for labeling primary amines in peptides/proteins. The nanoparticle labeled proteins/peptides formed after NHS ester reaction with the primary amine groups were isolated using a magnet without any additional purification step. Nanoparticle moieties conjugated to peptides/proteins were then trimmed by cleavage at the disulfide linker with a reducing agent. The labeled peptides were analyzed by LC-MS/MS to determine their sequences and the sites of NHS ester labeling. This novel approach allowed characterization of lysine residues on the solvent accessible surface of native bovine serum albumin. Low cost, rapid magnetic separation, and specificity toward primary amine groups make NHS ester coated Fe3O4@SiO2 nanoparticles a potential labeling probe to study proteins on living cell surfaces.

Nanoparticles based stem cell tracking in regenerative medicine

Stem cell therapies offer great potentials in the treatment for a wide range of diseases and conditions. With so many stem cell replacement therapies going through clinical trials currently, there is a great need to understand the mechanisms behind a successful therapy, and one of the critical points of discovering them is to track stem cell migration, proliferation and differentiation in vivo. To be of most use tracking methods should ideally be non-invasive, high resolution and allow tracking in three dimensions. Magnetic resonance imaging (MRI) is one of the ideal methods, but requires a suitable contrast agent to be loaded to the cells to be tracked, and one of the most wide-spread in stem cell tracking is a group of agents known as magnetic nanoparticles. This review will explore the current use of magnetic nanoparticles in developing and performing stem cell therapies, and will investigate their potential limitations and the future directions magnetic nanoparticle tracking is heading in.

Insights into cell entry and intracellular trafficking of peptide and protein drugs provided by electron microscopy

For widening the arsenal of protein and peptide therapeutics that act within cells, their cell-entry mechanisms, intracellular trafficking and distribution need to be characterized in detail. Immunofluorescence microscopy has been a prevalent tool for these studies. However, due to the limited resolution, it is often complemented with other methods. This article focuses on the perspectives of electron microscopy in tracking the intracellular delivery and trafficking of proteins, peptides and their carriers. This review introduces the electron microscopy techniques and labeling methods currently used for studying the cellular whereabouts of peptides and proteins with a focus on their intracellular trafficking. Since cell-penetrating peptides have widely been harnessed as carriers for proteins and peptides, and their usage is rapidly expanding, a particular emphasis has been placed on their applications and cell-entry mechanisms.

Whole-cell analysis of low-density lipoprotein uptake by macrophages using STEM tomography

Nanoparticles of heavy materials such as gold can be used as markers in quantitative electron microscopic studies of protein distributions in cells with nanometer spatial resolution. Studying nanoparticles within the context of cells is also relevant for nanotoxicological research. Here, we report a method to quantify the locations and the number of nanoparticles, and of clusters of nanoparticles inside whole eukaryotic cells in three dimensions using scanning transmission electron microscopy (STEM) tomography. Whole-mount fixed cellular samples were prepared, avoiding sectioning or slicing. The level of membrane staining was kept much lower than is common practice in transmission electron microscopy (TEM), such that the nanoparticles could be detected throughout the entire cellular thickness. Tilt-series were recorded with a limited tilt-range of 80° thereby preventing excessive beam broadening occurring at higher tilt angles. The 3D locations of the nanoparticles were nevertheless determined with high precision using computation. The obtained information differed from that obtained with conventional TEM tomography data since the nanoparticles were highlighted while only faint contrast was obtained on the cellular material. Similar as in fluorescence microscopy, a particular set of labels can be studied. This method was applied to study the fate of sequentially up-taken low-density lipoprotein (LDL) conjugated to gold nanoparticles in macrophages. Analysis of a 3D reconstruction revealed that newly up-taken LDL-gold was delivered to lysosomes containing previously up-taken LDL-gold thereby forming onion-like clusters.

Biological Atomic Force Microscopy for Imaging Gold-Labeled Liposomes on Human Coronary Artery Endothelial Cells

Although atomic force microscopy (AFM) has been used extensively to characterize cell membrane structure and cellular processes such as endocytosis and exocytosis, the corrugated surface of the cell membrane hinders the visualization of extracellular entities, such as liposomes, that may interact with the cell. To overcome this barrier, we used 90 nm nanogold particles to label FITC liposomes and monitor their endocytosis on human coronary artery endothelial cells (HCAECs) in vitro. We were able to study the internalization process of gold-coupled liposomes on endothelial cells, by using AFM. We found that the gold-liposomes attached to the HCAEC cell membrane during the first 15–30 min of incubation, liposome cell internalization occurred from 30 to 60 min, and most of the gold-labeled liposomes had invaginated after 2 hr of incubation. Liposomal uptake took place most commonly at the periphery of the nuclear zone. Dynasore monohydrate, an inhibitor of endocytosis, obstructed the internalization of the gold-liposomes. This study showed the versatility of the AFM technique, combined with fluorescent microscopy, for investigating liposome uptake by endothelial cells. The 90 nm colloidal gold nanoparticles proved to be a noninvasive contrast agent that efficiently improves AFM imaging during the investigation of biological nanoprocesses.

Preparation of gold nanocluster bioconjugates for electron microscopy

In this chapter, we describe types of gold nanoparticle-biomolecule conjugates and their use in electron microscopy. Included are two detailed protocols for labeling an IgG antibody with gold monolayer protected clusters. The first approach is a direct bonding approach that utilizes the ligand place exchange reaction. The second approach describes NHS-EDC coupling of Au(144)(pMBA)(60) with IgG. Also included are various characterization techniques for determining labeling efficiency.

Development and application of STEM for the biological sciences

The design of the scanning transmission electron microscope (STEM), as conceived originally by Crewe and coworkers, enables the highly efficient and flexible collection of different elastic and inelastic signals resulting from the interaction of a focused probe of incident electrons with a specimen. In the present paper we provide a brief review for how the STEM today can be applied towards a range of different problems in the biological sciences, emphasizing four main areas of application. (1) For three decades, the most widely used STEM technique has been the mass determination of proteins and other macromolecular assemblies. Such measurements can be performed at low electron dose by collecting the high-angle dark-field signal using an annular detector. STEM mass mapping has proven valuable for characterizing large protein assemblies such as filamentous proteins with a well-defined mass per length. (2) The annular dark-field signal can also be used to image ultrasmall, functionalized nanoparticles of heavy atoms for labeling specific amino-acid sequences in protein assemblies. (3) By acquiring electron energy loss spectra (EELS) at each pixel in a hyperspectral image, it is possible to map the distributions of specific bound elements like phosphorus, calcium and iron in isolated macromolecular assemblies or in compartments within sectioned cells. Near single atom sensitivity is feasible provided that the specimen can tolerate a very high incident electron dose. (4) Electron tomography is a new application of STEM that enables three-dimensional reconstruction of micrometer-thick sections of cells. In this technique a probe of small convergence angle gives a large depth of field throughout the thickness of the specimen while maintaining a probe diameter of <2 nm; and the use of an on-axis bright-field detector reduces the effects of beam broadening and thus improves the spatial resolution compared to that attainable by STEM dark-field tomography.

Electron tomography of late stages of FcRn-mediated antibody transcytosis in neonatal rat small intestine

The neonatal Fc receptor (FcRn) transports maternal immunoglobulin (IgG) across epithelia to confer passive immunity to mammalian young. In newborn rodents, FcRn transcytoses IgG from ingested milk across the intestinal epithelium for release into the bloodstream. We used electron tomography to examine FcRn transport of Nanogold-labeled Fc (Au-Fc) in neonatal rat jejunum, focusing on later aspects of transport by chasing Au-Fc before fixation. We observed pools of Au-Fc in dilated regions of the lateral intercellular space (LIS), likely representing exit sites where Au-Fc accumulates en route to the blood. Before weaning, the jejunum functions primarily in IgG transport and exhibits unusual properties: clathrin-rich regions near/at the basolateral LIS and multivesicular bodies (MVBs) expressing early endosomal markers. To address whether these features are related to IgG transport, we examined LIS and endocytic/transcytotic structures from neonatal and weaned animals. Weaned samples showed less LIS-associated clathrin. MVBs labeled with late endosomal/lysosomal markers were smaller than their neonatal counterparts but contained 10 times more internal compartments. These results are consistent with hypotheses that clathrin-rich basolateral regions in neonatal jejunum are involved in IgG exocytosis and that MVBs function in IgG transport while FcRn is expressed but switch to degradative functions after weaning, when the jejunum does not express FcRn or transport IgG.

Controlled formation of gold nanoparticle dimers using multivalent thiol ligands

Approaches for the controlled formation of gold nanoparticle dimers are investigated. These are based on a locally confined surface modification of gold nanoparticles followed by bridging two particles with an organic linker. A key factor in these approaches is the use of multivalent ligands. Citrate-stabilized gold nanoparticles are immobilized on a glass surface and mono- and multivalent thiol ligands are investigated regarding their ability to inactivate the nanoparticles sites facing away from the glass. A successful locally confined functionalization is only possible if multivalent ligands are used in this step. The application of monovalent ligands results in less stable particles without a permanent regioselective functionalization. This result can be explained by the dynamic equilibrium between bound and free ligands. Subsequently, the sites of the nanoparticles previously bound to the glass surface are functionalized with thiol ligands additionally bearing a reactive group. Approaches using dithiol linkers, diamine linkers, and coupling complementary functionalized particles are investigated. The highest yield of stable dimers is obtained from conditions where nanoparticles which are regioselectively functionalized with an N-hydroxysuccinimide ester are reacted with complementary amino-functionalized particles. The application of nanoparticles with activated carboxyl groups is essential since standard carboxyl activation agents induce an aggregation of the nanoparticles due to a reaction with remaining citrate molecules on the nanoparticle surface which reduces significantly electrostatic stabilization. This versatile approach using complementary regioselective with multivalent ligands functionalized nanoparticles may be also used for the coupling of particles with different size, shape, or composition, as well as a control of the interparticle distance.

DNA discrete modified gold nanoparticles

Recently, controlling assembly process in a precise manner has attracted increasing attentions in nanofabrication. Gold nanoparticles (AuNPs) modified with countable number of DNA strands, i.e., DNA discrete modified AuNPs, which bring AuNPs much more controllable manipulating possibilities have been playing an important role in this field. In this feature article, we will summarize recent progress on their preparation strategies and application in positioning assembly, which could benefit to the improvement of preparation methods of DNA discrete modified AuNPs and even other nanoparticles.

Development of cellulosic secondary walls in flax fibers requires beta-galactosidase

Bast (phloem) fibers, tension wood fibers, and other cells with gelatinous-type secondary walls are rich in crystalline cellulose. In developing bast fibers of flax (Linum usitatissimum), a galactan-enriched matrix (Gn-layer) is gradually modified into a mature cellulosic gelatinous-layer (G-layer), which ultimately comprises most of the secondary cell wall. Previous studies have correlated this maturation process with expression of a putative β-galactosidase. Here, we demonstrate that β-galactosidase activity is in fact necessary for the dynamic remodeling of polysaccharides that occurs during normal secondary wall development in flax fibers. We found that developing stems of transgenic (LuBGAL-RNAi) flax with reduced β-galactosidase activity had lower concentrations of free Gal and had significant reductions in the thickness of mature cellulosic G-layers compared with controls. Conversely, Gn-layers, labeled intensively by the galactan-specific LM5 antibody, were greatly expanded in LuBGAL-RNAi transgenic plants. Gross morphology and stem anatomy, including the thickness of bast fiber walls, were otherwise unaffected by silencing of β-galactosidase transcripts. These results demonstrate a specific requirement for β-galactosidase in hydrolysis of galactans during formation of cellulosic G-layers. Transgenic lines with reduced β-galactosidase activity also had biochemical and spectroscopic properties consistent with a reduction in cellulose crystallinity. We further demonstrated that the tensile strength of normal flax stems is dependent on β-galactosidase-mediated development of the phloem fiber G-layer. Thus, the mechanical strength that typifies flax stems is dependent on a thick, cellulosic G-layer, which itself depends on β-galactosidase activity within the precursor Gn-layer. These observations demonstrate a novel role for matrix polysaccharides in cellulose deposition; the relevance of these observations to the development of cell walls in other species is also discussed.

Synthesis of gold and silver nanoparticles using purified URAK

This study aims at developing a new eco-friendly process for the synthesis of silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) using purified URAK. URAK is a fibrinolytic enzyme produced by Bacillus cereus NK1. The enzyme was purified and used for the synthesis of AuNPs and AgNPs. The enzyme produced AgNPs when incubated with 1 mM AgNO3 for 24 h and AuNPs when incubated with 1 mM HAuCl4 for 60 h. But when NaOH was added, the synthesis was rapid and occurred within 5 min for AgNPs and 12 h for AuNPs. The synthesized nanoparticles were characterized by a peak at 440 nm and 550 nm in the UV-visible spectrum. TEM analysis showed that AgNPs of the size 60 nm and AuNPs of size 20 nm were synthesized. XRD confirmed the crystalline nature of the nanoparticles and AFM showed the morphology of the nanoparticle to be spherical. FT-IR showed that protein was responsible for the synthesis of the nanoparticles. This process is highly simple, versatile and produces AgNPs and AuNPs in environmental friendly manner. Moreover, the synthesized nanoparticles were found to contain immobilized enzyme. Also, URAK was tested on RAW 264.7 macrophage cell line and was found to be non-cytotoxic until 100 μg/ml.

Direct control of the spatial arrangement of gold nanoparticles in organic-inorganic hybrid superstructures

The directed assembly of gold nanoparticles is essential for their use in many kinds of applications, such as electronic devices, biological labels, and sensors. Herein an atomic alteration in the molecular structure of ligand-stabilized gold nanoparticles that can shift the interparticle distance up to 1 nm upon covalent coupling to organic-inorganic superstructures is presented. Gold nanoparticles are stabilized by two octadentate thioether ligands and have a mean diameter of 1.1 nm. The ligands contain a central rigid rod varying in length and terminally functionalized with a protected acetylene. The two peripheral functional groups on each particle enable the directed assembly of nanoparticles to dimers, trimers, and tetramers by oxidative acetylene coupling. This is a wet chemical protocol resulting in covalently bound nanoparticles. These organic-inorganic hybrid superstructures are analyzed by transmission electron microscopy, small angle X-ray scattering, and UV/vis spectroscopy. The focus of the comparison here is the subunit, which is anchoring the bridgehead, either a pyridine or benzene moiety. The pyridine-based ligands reflect the calculated length of the rigid-rod spacer in their interparticle distances in the obtained hybrid structures. This suggests a perpendicular arrangement that results from the coordination of the pyridine's lone pair to the gold surface. An atomic variation in the ligand's center leads to smaller interparticle distances in the case of hybrid structures obtained from benzene ligands. This large difference in the spatial arrangement suggests a tangential arrangement of the interparticle bridging structure in the latter case. Consequently a rather flat arrangement parallel to the particle surface must be assumed for the central benzene unit of the benzene-based ligand.