Imaging Dynamic Processes in Multiple Dimensions and Length Scales.

Optical microscopy has become an invaluable tool for investigating complex samples. Over the years, many advances to optical microscopes have been made that have allowed us to uncover new insights into the samples studied. Dynamic changes in biological and chemical systems are of utmost importance to study. To probe these samples, multidimensional approaches have been developed to acquire a fuller understanding of the system of interest. These dimensions include the spatial information, such as the three-dimensional coordinates and orientation of the optical probes, and additional chemical and physical properties through combining microscopy with various spectroscopic techniques. In this review, we survey the field of multidimensional microscopy and provide an outlook on the field and challenges that may arise.

Single-molecule photocatalytic dynamics at individual defects in two-dimensional layered materials.

The insightful comprehension of in situ catalytic dynamics at individual structural defects of two-dimensional (2D) layered material, which is crucial for the design of high-performance catalysts via defect engineering, is still missing. Here, we resolved single-molecule trajectories resulted from photocatalytic activities at individual structural features (i.e., basal plane, edge, wrinkle, and vacancy) in 2D layered indium selenide (InSe) in situ to quantitatively reveal heterogeneous photocatalytic dynamics and surface diffusion behaviors. The highest catalytic activity was found at vacancy in a four-layer InSe, up to ~30× higher than that on the basal plane. Moreover, lower adsorption strength of reactant and slower dissociation/diffusion rates of product were found at more photocatalytic active defects. These distinct dynamic properties are determined by lattice structures/electronic energy levels of defects and layer thickness of supported InSe. Our findings shed light on the fundamental understanding of photocatalysis at defects and guide the rational defect engineering.

Dynamin-dependent vesicle twist at the final stage of clathrin-mediated endocytosis.

Dynamin has an important role in clathrin-mediated endocytosis by cutting the neck of nascent vesicles from the cell membrane. Here, using gold nanorods as cargos to image dynamin action during live clathrin-mediated endocytosis, we show that, near the peak of dynamin accumulation, the cargo-containing vesicles always exhibit abrupt, right-handed rotations that finish in a short time (~0.28 s). The large and quick twist, herein named the super twist, is the result of the coordinated dynamin helix action upon GTP hydrolysis. After the super twist, the rotational freedom of the vesicle increases substantially, accompanied by simultaneous or delayed translational movement, indicating that it detaches from the cell membrane. These observations suggest that dynamin-mediated scission involves a large torque generated by the coordinated actions of multiple dynamins in the helix, which is the main driving force for vesicle scission.

Multiscale Evolution of Bulk Heterojunction Solar Cell Active Layers under Thermal Stress.

A multimodality spectromicroscopy imaging system has been developed to offer the essential capability of in situ characterization of functional materials at multiple length scales during the morphology evolution and phase development under external stimuli. The photoactive layer of bulk heterojunction solar cell, whose performance is strongly correlated to the structural features over a wide range of length scales, was characterized under thermal stress. Three stages of thermotropic evolution were monitored continuously by the spectromicroscopy imaging system to reveal the critical information from the molecular level to meso- and microscale. The optimized thermal annealing temperature window and preferred temperature dropping operation were identified to promote the performance of the photoactive layer.

Resolving cargo-motor-track interactions with bifocal parallax single-particle tracking.

Resolving coordinated biomolecular interactions in living cellular environments is vital for understanding the mechanisms of molecular nanomachines. The conventional approach relies on localizing and tracking target biomolecules and/or subcellular organelles labeled with imaging probes. However, it is challenging to gain information on rotational dynamics, which can be more indicative of the work done by molecular motors and their dynamic binding status. Herein, a bifocal parallax single-particle tracking method using half-plane point spread functions has been developed to resolve the full-range azimuth angle (0-360°), polar angle, and three-dimensional (3D) displacement in real time under complex living cell conditions. Using this method, quantitative rotational and translational motion of the cargo in a 3D cell cytoskeleton was obtained. Not only were well-known active intracellular transport and free diffusion observed, but new interactions (tight attachment and tethered rotation) were also discovered for better interpretation of the dynamics of cargo-motor-track interactions at various types of microtubule intersections.

Single Molecule Investigation of Nanoconfinement Hydrophobicity in Heterogeneous Catalysis.

Nanoconfinement imposes physical constraints and chemical effects on reactivity in nanoporous catalyst systems. In the present study, we lay the groundwork for quantitative single-molecule measurements of the effects of chemical environment on heterogeneous catalysis in nanoconfinement. Choosing hydrophobicity as an exemplary chemical environmental factor, we compared a range of essential parameters for an oxidation reaction on platinum nanoparticles (NPs) confined in hydrophilic and hydrophobic nanopores. Single-molecule experimental measurements at the single particle level showed higher catalytic activity, stronger adsorption strength, and higher activation energy in hydrophobic nanopores than those in hydrophilic nanopores. Interestingly, different dissociation kinetic behaviors of the product molecules in the two types of nanopores were deduced from the single-molecule imaging data.

Chemical modification of antibodies enables the formation of stable antibody-gold nanoparticle conjugates for biosensing.

Antibody-modified gold nanoparticles (AuNPs) are central to many novel and emerging biosensing technologies due to the specificity provided by antibody-antigen interactions and the unique properties of nanoparticles. These AuNP-enabled assays have the potential to provide significant improvements in sensitivity and multiplexed analysis compared to conventional immunoassays. However, a major challenge for these AuNP platform technologies is the synthesis of stable antibody-AuNP conjugates that resist aggregation in high salt environments and biological matrices. Moreover, synthetic strategies to form stable conjugates often require different solution conditions, e.g., pH, for each unique antibody. Herein we describe our effort to develop an approach to chemically modify lysine residues on antibodies to facilitate the formation of stable antibody-AuNP conjugates over a wide pH range. In this work, we systematically investigated the immobilization of native and chemically modified antibodies to 60 nm citrate-capped AuNPs as a function of pH and evaluated the stability of the antibody-AuNP conjugate in a saline environment. We have developed a method to chemically modify the lysine residues on an antibody prior to conjugation to the AuNP that results in stable conjugates over a wide pH range (6.0-8.5). Amino acid analysis and zeta potential measurements of native and modified antibodies reveal that the requisite modification correlates with the number of lysine residues, and a reduction in positive charge contribution from protonated lysine is required to form stable, pH-independent conjugates. Furthermore, we demonstrate that the chemically modified antibodies maintain antigen-binding capabilities. We apply this novel conjugation strategy to develop a surface-enhanced Raman spectroscopy (SERS)-based assay for the accurate subtyping of avian influenza viruses.

Asymmetrically Functionalized Antibody-Gold Nanoparticle Conjugates to Form Stable Antigen-Assembled Dimers.

Biomolecular assays based on the aggregation of modified gold nanoparticles (AuNPs) have been developed to provide low detection limits and rapid results with a simple one-step, wash-free procedure. However, a relatively narrow dynamic range, low sensitivity, and poor precision due to time-sensitive readout limit the application of these assay platforms. In this work we synthesized asymmetrically functionalized antibody-AuNP conjugates that are rationally designed to overcome the limitations of aggregation-based immunoassays. Solid-phase synthesis was used to chemically passivate the majority of the AuNP surface and restrict antibody immobilization into a small area of the AuNP surface. These asymmetric conjugates assembled into dimers with the addition of antigen and were stable for over 24 h. In contrast, conventional antibody-AuNP conjugates which are symmetrically modified with antibody assembled into large aggregates that continuously increased in size with the addition of target antigen. These results suggest that asymmetric antibody-AuNP conjugates have the potential to significantly improve the analytical performance of aggregation-based immunoassays.

A fluorescence-based method to directly quantify antibodies immobilized on gold nanoparticles.

The ability to evaluate antibody immobilization onto gold nanoparticles is critical for assessing coupling chemistry and optimizing the sensitivity of nanoparticle-enabled biosensors. Herein, we developed a fluorescence-based method for directly quantifying antibodies bound onto gold nanoparticles. Antibody-modified gold nanoparticles were treated with KI/I2 etchant to dissolve the gold nanoparticles. A desalting spin column was used to recover the antibody released from the nanoparticles, and NanoOrange, a fluorescent dye, was used to quantify the antibody. We determined 309 ± 93 antibodies adsorb onto a 60 nm gold nanoparticles (2.6 × 10(10) NP mL(-1)), which is consistent with a fully adsorbed monolayer based on the footprint of an IgG molecule. Moreover, the increase in hydrodynamic diameter of the conjugated nanoparticle (76 nm) compared to that of the unconjugated nanoparticle (62 nm) confirmed that multilayers did not form. A more conventional method of indirectly quantifying the adsorbed antibody by analysis of the supernatant overestimated the antibody surface coverage (660 ± 87 antibodies per nanoparticle); thus we propose the method described herein as a more accurate alternative to the conventional approach.