Tracking single-particle rotation during macrophage uptake

Shedding Light on Luminescent Janus Nanoparticles: From Synthesis to Photoluminescence and Applications

Luminescent Janus nanoparticles refer to a special category of Janus-based nanomaterials that not only exhibit dual-asymmetric surface nature but also attractive optical properties. The introduction of luminescence has endowed conventional Janus nanoparticles with many alluring light-responsive functionalities and broadens their applications in imaging, sensing, nanomotors, photo-based therapy, etc. The past few decades have witnessed significant achievements in this field. This review first summarizes well-established strategies to design and prepare luminescent Janus nanoparticles and then discusses optical properties of luminescent Janus nanoparticles based on downconversion and upconversion photoluminescence mechanisms. Various emerging applications of luminescent Janus nanoparticles are also introduced. Finally, opportunities and future challenges are highlighted with respect to the development of next-generation luminescent Janus nanoparticles with diverse applications.

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. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Toward Quantitative Bio-sensing with Nitrogen-Vacancy Center in Diamond

The long-dreamed-of capability of monitoring the molecular machinery in living systems has not been realized yet, mainly due to the technical limitations of current sensing technologies. However, recently emerging quantum sensors are showing great promise for molecular detection and imaging. One of such sensing qubits is the nitrogen-vacancy (NV) center, a photoluminescent impurity in a diamond lattice with unique room-temperature optical and spin properties. This atomic-sized quantum emitter has the ability to quantitatively measure nanoscale electromagnetic fields via optical means at ambient conditions. Moreover, the unlimited photostability of NV centers, combined with the excellent diamond biocompatibility and the possibility of diamond nanoparticles internalization into the living cells, makes NV-based sensors one of the most promising and versatile platforms for various life-science applications. In this review, we will summarize the latest developments of NV-based quantum sensing with a focus on biomedical applications, including measurements of magnetic biomaterials, intracellular temperature, localized physiological species, action potentials, and electronic and nuclear spins. We will also outline the main unresolved challenges and provide future perspectives of many promising aspects of NV-based bio-sensing.

Modulation of Immune Responses by Particle Size and Shape

The immune system has to cope with a wide range of irregularly shaped pathogens that can actively move (e.g., by flagella) and also dynamically remodel their shape (e.g., transition from yeast-shaped to hyphal fungi). The goal of this review is to draw general conclusions of how the size and geometry of a pathogen affect its uptake and processing by phagocytes of the immune system. We compared both theoretical and experimental studies with different cells, model particles, and pathogenic microbes (particularly fungi) showing that particle size, shape, rigidity, and surface roughness are important parameters for cellular uptake and subsequent immune responses, particularly inflammasome activation and T cell activation. Understanding how the physical properties of particles affect immune responses can aid the design of better vaccines.

The Brownian and Flow-Driven Rotational Dynamics of a Multicomponent DNA Origami-Based Rotor

Nanomechanical devices are becoming increasingly popular due to the very diverse field of potential applications, including nanocomputing, robotics, and drug delivery. DNA is one of the most promising building materials to realize complex 3D structures at the nanoscale level. Several mechanical DNA origami structures have already been designed capable of simple operations such as a DNA box with a controllable lid, bipedal walkers, and cargo sorting robots. However, the nanomechanical properties of mechanically interlinked DNA nanostructures that are in general highly deformable have yet to be extensively experimentally evaluated. In this work, a multicomponent DNA origami-based rotor is created and fully characterized by electron microscopy under negative stain and cryo preparations. The nanodevice is further immobilized on a microfluidic chamber and its Brownian and flow-driven rotational behaviors are analyzed in real time by single-molecule fluorescence microscopy. The rotation in previous DNA rotors based either on strand displacement, electric field or Brownian motion. This study is the first to attempt to manipulate the dynamics of an artificial nanodevice with fluidic flow as a natural force.

Self-Assembly Behavior of Oppositely Charged Inverse Bipatchy Microcolloids

A directed attractive interaction between predefined "patchy" sites on the surfaces of anisotropic microcolloids can provide them with the ability to self-assemble in a controlled manner to build target structures of increased complexity. An important step toward the controlled formation of a desired superstructure is to identify reversible electrostatic interactions between patches which allow them to align with one another. The formation of bipatchy particles with two oppositely charged patches fabricated using sandwich microcontact printing is reported. These particles spontaneously self-aggregate in solution, where a diversity of short and long chains of bipatchy particles with different shapes, such as branched, bent, and linear, are formed. Calculations show that chain formation is driven by a combination of attractive electrostatic interactions between oppositely charged patches and the charge-induced polarization of interacting particles.

Mono-patchy zwitterionic microcolloids as building blocks for pH-controlled self-assembly

A directional molecular interaction between microcolloids can be achieved through pre-defined sites on their surface, "patches", which might make them follow each other in a controlled way and assemble into target structures of more complexity. In this article, we report the successful generation and characterization of mono-patchy melamine-formaldehyde microparticles with oppositely charged patches made of poly(methyl vinyl ether-alt-maleic acid) or polyethyleneimine via microcontact printing. The study of their self-aggregation behavior in solution shows that by change of pH, particle dimers are formed via attractive electrostatic force between the patchy and non-patchy surface of the particles, which reaches its optimum at a specific pH.

Generation of 3-dimensional multi-patches on silica particles via printing with wrinkled stamps

A simple route towards patchy particles with anisotropic patches with respect to a different functionality and directionality is presented. This method is based on microcontact printing of positively charged polyethylenimine (PEI) on silica particles using wrinkled stamps. Due to the wrinkled surface, the number of patches on the particles as well as the distance between two patches can be controlled.

Single-Janus Rod Tracking Reveals the "Rock-and-Roll" of Endosomes in Living Cells

Endosomes in cells are known to move directionally along microtubules, but their rotational dynamics have rarely been investigated. Even less is known, specifically, about the rotation of nonspherical endosomes. Here we report a single-Janus rod rotational tracking study to reveal the rich rotational dynamics of rod-shaped endosomes in living cells. The rotational reporters were Janus rods that display patches of different fluorescent colors on opposite sides along their long axes. When the Janus rods are wrapped tightly inside endosomes, their shape and optical anisotropy allow the simultaneous measurements of all three rotational angles (in-plane, out-of-plane, and longitudinal) and the translational motion of single endosomes with high spatiotemporal resolutions. We demonstrate that endosomes undergo in-plane rotation and rolling during intracellular transport and that such rotational dynamics are driven by rapid microtubule fluctuations. We reveal for the first time the "rock-and-roll" of endosomes in living cells and how the intracellular environment modifies such rotational dynamics. This study demonstrates a unique application of Janus particles as imaging probes in the elucidation of fundamental biological questions.

Receptor-Mediated Endocytosis of Nanoparticles: Roles of Shapes, Orientations, and Rotations of Nanoparticles

A complete understanding of the interactions between nanoparticles (NPs) and the cell membrane is essential for the potential biomedical applications of NPs. The rotation of the NP during the cellular wrapping process is of great biological significance and has been widely observed in experiments and simulations. However, the underlying mechanisms of the rotation and their potential influences on the wrapping behavior are far from being fully understood. Here, by coupling the rotation of the NP with the diffusion of the receptors, we set up a model to theoretically investigate the wrapping pathway and the internalization rate of the rotatable NP in the receptor-mediated endocytosis. Based on this model, it is found that the endocytosis proceeds through the symmetric-asymmetric or asymmetric-symmetric-asymmetric wrapping pathway due to the bending and membrane tension competition induced rotation of NP. In addition, we show that the wrapping rate in the direction that the wrapping proceeds can be largely accelerated by the rotation. Moreover, the time to fully wrap the NP depends not only on the size and shape of the NP but also on its rotation and initial orientation. These results reveal the roles of the shape, rotation, and initial orientation of the NP on the receptor-mediated endocytosis and may provide guidelines for the design of NP-based drug delivery systems.

Seeing the unseen: Imaging rotation in cells with designer anisotropic particles

Cellular functions are enabled by cascades of transient biological events. Imaging and tracking the dynamics of these events have proven to be a powerful means of understanding the principles of cellular processes. These studies have typically focused on translational dynamics. By contrast, investigations of rotational dynamics have been scarce, despite emerging evidence that rotational dynamics are an inherent feature of many cellular processes and may also provide valuable clues to understanding those cell functions. Such studies have been impeded by the limited availability of suitable rotational imaging probes. This has recently changed thanks to the advances in the development of anisotropic particles for rotational imaging. In this review, we will summarize current techniques for imaging rotation using particle probes that are anisotropic in shape or optical properties. We will highlight two studies that demonstrate how these techniques can be applied to explore important facets of cellular functions.

Interrogating Cellular Functions with Designer Janus Particles

Janus particles have two distinct surfaces or compartments. This enables novel applications that are impossible with homogeneous particles, ranging from the engineering of active colloidal metastructures to creating multimodal therapeutic materials. Recent years have witnessed a rapid development of novel Janus structures and exploration of their applications, particularly in the biomedical arena. It, therefore, becomes crucial to understand how Janus particles with surface or structural anisotropy might interact with biological systems and how such interactions may be exploited to manipulate biological responses. This perspective highlights recent studies that have employed Janus particles as novel toolsets to manipulate, measure, and understand cellular functions. Janus particles have been shown to have biological interactions different from uniform particles. Their surface anisotropy has been used to control the cell entry of synthetic particles, to spatially organize stimuli for the activation of immune cells, and to enable direct visualization and measurement of rotational dynamics of particles in living systems. The work included in this perspective showcases the significance of understanding the biological interactions of Janus particles and the tremendous potential of harnessing such interactions to advance the development of Janus structure-based biomaterials.

Effect of partial PEGylation on particle uptake by macrophages

Controlling the internalization of synthetic particles by immune cells remains a grand challenge for developing successful drug carrier systems. Polyethylene glycol (PEG) is frequently used as a protective coating on particles to evade immune clearance, but it also hinders the interactions of particles with their intended target cells. In this study, we investigate a spatial decoupling strategy, in which PEGs are coated on only one hemisphere of particles, so that the other hemisphere is available for functionalization of cell-targeting ligands without the hindrance effect from the PEGs. The partial coating of PEGs is realized by creating two-faced Janus particles with different surface chemistries on opposite sides. We show that a half-coating of PEGs reduces the macrophage uptake of particles as effectively as a complete coating. Owing to the surface asymmetry, Janus particles that are internalized enter macrophage cells via a combination of ligand-guided phagocytosis and macropinocytosis. By spatially segregating PEGs and ligands for targeting T cells on Janus particles, we demonstrate that the Janus particles bind T cells uni-directionally from the ligand-coated side, bypassing the hindrance from the PEGs on the other hemisphere. The results reveal a new mechanistic understanding on how a spatial coating of PEGs on particles changes the phagocytosis of particles. This study also suggests a new design principle for therapeutic particles - the spatial decoupling of PEGs and cell-targeting moieties reduces the interference between the two functions while attaining the protective effect of PEGs for macrophage evasion.

Measuring rotational diffusion of colloidal spheres with confocal microscopy

We report an experimental method to measure the translational and rotational dynamics of colloidal spheres in three dimensions with confocal microscopy and show that the experimental values reasonably agree with the theoretical values. This method can be extended to study rotational dynamics in concentrated colloidal systems and complex bio-systems.

Janus particles for biological imaging and sensing

Janus particles, named after the two-faced Roman god Janus, have different surface makeups, structures or compartments on two sides. This review highlights recent advances in employing Janus particles as novel analytical tools for live cell imaging and biosensing. Unlike conventional particles used in analytical science, two-faced Janus particles provide asymmetry and directionality, and can combine different or even incompatible properties within a single particle. The broken symmetry enables imaging and quantification of rotational dynamics, revealing information beyond what traditional measurements offer. The spatial segregation of molecules on the surface of a single particle also allows analytical functions that would otherwise interfere with each other to be decoupled, opening up opportunities for novel multimodal analytical methods. We summarize here the development of Janus particles, a few general methods for their fabrication and, more importantly, the emerging and novel applications of Janus particles as multi-functional imaging probes and sensors.

Controlling the Nanoscale Rotational Behaviors of Nanoparticles on the Cell Membranes: A Computational Model

Nanoparticles prefer to bind to a membrane with a surface coated by short or rigid ligands, as shown by computer simulations. To realize such a preferred configuration, the nanoparticle can spontaneously spin itself on the membrane surface, no matter what its initial orientation is.