Total Internal Reflection-Based Extinction Spectroscopy of Single Nanoparticles

Interfacial Electron Modulation of AgxCuy@ZIF-8 for Photothermally Catalyzing CO2 Organic Transformations

Efficiently converting CO2 into valuable chemicals under mild conditions is extremely challenging due to its thermodynamic and kinetic stability. The carboxylation/cyclization of alkynes catalyzed by Cu or Ag nanoparticles (NP) is one of the green pathways for CO2 utilization. However, these reactions are often limited by harsh conditions, as well as the migration, aggregation, and leakage of metal NP during the reaction. Herein, the AgxCuy heterostructure alloy NP are surrounded by a porous metal-organic framework, forming core-shell AgxCuy@ZIF-8 catalysts. Thanks to the light-to-heat capability, these catalysts exhibited excellent catalytic activity in converting various alkynes and CO2 to alkynyl carboxylic acids and promoting the cyclization reactions of propargyl amines with CO2 under ambient conditions using blue LED irradiation. The remarkable catalytic activity of Ag1Cu1@ZIF-8 is attributed to the optimized electronic states of Ag and Cu NP, as well as the core-shell structure that enhances photothermal effects around the catalytic center. In addition, the ZIF-8 shell not only improves the substrate transport but also inhibits the aggregation, migration, and loss of alloy NP cores during the reaction, contributing to enhanced cycling performance compared to unencapsulated Ag1Cu1 NP. The catalytic reaction mechanisms were disclosed by a variety of spectral characterizations, control experiments, and DFT calculations.

Dielectric Surface-Based Biosensors for Enhanced Detection of Biomolecular Interactions: Advances and Applications

Surface plasmon resonance (SPR) biosensors are extensively utilized for analyzing molecular interactions due to their high sensitivity and label-free detection capabilities. Recent innovations in surface-sensitive biosensors with dielectric surfaces address the inherent limitations associated with traditional gold surfaces, such as thermal effects and biocompatibility issues, which can impede broader applications. This review examines state-of-the-art biosensor configurations, including total internal reflection, optical waveguide, photonic crystal resonators, Bloch surface wave biosensors, and surface electrochemical biosensors, which can enhance analyte signals and augment the molecular detection efficiency at the sensor interface. These technological advancements not only improve the resolution of binding kinetics analysis and single-molecule detection but also extend the analytical capabilities of these systems. Additionally, this review explores prospective advancements in augmenting field enhancement and incorporating multimodal sensing functionalities, emphasizing the significant potential of these sophisticated biosensing technologies to profoundly enhance our understanding of molecular interactions.

Multi-channel prismatic localized surface plasmon resonance biosensor for real-time competitive assay multiple COVID-19 characteristic miRNAs

A multi-channel prismatic localized surface plasmon resonance (LSPR) biosensor was developed for quantitative and real-time detection of multiple COVID-19 characteristic miRNAs. The well-dispersed and dense gold nanoparticles (AuNPs) arrays for LSPR biosensing were fabricated through a nano-thickness diblock copolymer template (BCPT). Both theoretical and experimental analyses were conducted to investigate the effects of particle size, interparticle spacing, and surface coverage on LSPR sensing spectrum and intensity sensitivity of varied AuNPs sizes. A competitive assay strategy was proposed and used for non-amplification miRNA detection with a low limit detection of 3.41 nM, while a four-channel prismatic LSPR system enables parallel detection of multiple miRNAs. Furthermore, this sensing strategy can effectively and specifically identify target miRNA, distinguish mismatched miRNA and interfering miRNA, and exhibit low non-specific adsorption. This BCPT-based LSPR biosensor demonstrates the practicality and potential of a multi-channel, adaptable, and integrated prismatic sensor in medical testing and diagnostic applications.

Determination of the Localized Surface Plasmon Resonance Alteration of AgNPs via Multiwavelength Evanescent Scattering Microscopy for Pb(II) Detection

We developed multiwavelength evanescent scattering microscopy (MWESM), which can acquire plasmonic nanoparticle images at the particle level using the evanescent field as the incident source and distinguish different LSPR (localized surface plasmon resonance) spectral peaks among four wavelengths. Our microscope could be easily and simply built by modifying a commercial total internal reflection fluorescence microscope (TIRFM) with the substitution of a beamsplitter and the addition of a semicircular stop. The ultrathin depth of illumination and rejection of the reflected incident source together contribute to the high sensitivity and contrast of single nanoparticle imaging. We first validated the capability of our imaging system in distinguishing plasmonic nanoparticles bearing different LSPR spectral peaks, and the results were consistent with the scattering spectra results of hyperspectral imaging. Moreover, we demonstrated high imaging quality from the aspects of the signal/noise ratio and point spread function of the single-particle images. Meaningfully, the system can be utilized in rapidly determining the concentration of toxic lead ions in environmental and biological samples with good linearity and sensitivity, based on single-particle evanescent scattering imaging through the detection of the alteration of the LSPR of silver nanoparticles. This system holds the potential to advance the field of nanoparticle imaging and foster the application of nanomaterials as sensors.

Ligand-Receptor Interaction Triggers Hopping and Sliding Motions on Living Cell Membranes

Exploring the surface-capturing and releasing processes of nanocargo on the living cell membrane is critical for understanding the membrane translocation process. In this work, we achieve total internal reflection scattering (TIRS) illumination on a commercial dark-field optical microscope without the introduction of any additional optical components. By gradually reducing the diaphragm size in the excitation light path, the angle of the incident beam can be well manipulated. Under optimal conditions, the excitation light can be totally reflected at the glass/water interface, resulting in a thin layer of evanescent field for TIRS illumination. Due to the exponential decay feature of the evanescent field, the displacement of the nanocargo along the vertical direction can be directly resolved in the intensity track. With this method, we selectively monitor the dynamics of the transferrin-modified nanocargo on the living cell membrane. Transition between confined diffusion and long-range searching is involved in the binding site recognition process, which exhibits non-Gaussian and nonergodic-like behavior. More interestingly, 2D fast sliding and 3D hopping motions are also distinguished on the fluidic cell membrane, which is essentially modulated by the strength of ligand-receptor interactions, as revealed by the free-energy profiles. These heterogeneous and dynamic interactions together control the diffusion mode of the nanocargo on the lipid membrane and, thus, determine the cellular translocation efficiency.

Optical mapping of the evolution of water content during the swelling of hydrophilic polymers

The water content of hydrophilic polymers is a fundamental property that regulates their performance. Herein, we report a new technique for optically imaging the water content of hydrophilic polymers both in static and dynamic evolution during swelling, based on mapping the interfacial refractive index of hydrophilic polymers with label-free total internal reflection microscopy.

Optical Microscopy Systems for the Detection of Unlabeled Nanoparticles

Label-free detection of nanoparticles is essential for a thorough evaluation of their cellular effects. In particular, nanoparticles intended for medical applications must be carefully analyzed in terms of their interactions with cells, tissues, and organs. Since the labeling causes a strong change in the physicochemical properties and thus also alters the interactions of the particles with the surrounding tissue, the use of fluorescently labeled particles is inadequate to characterize the effects of unlabeled particles. Further, labeling may affect cellular uptake and biocompatibility of nanoparticles. Thus, label-free techniques have been recently developed and implemented to ensure a reliable characterization of nanoparticles.

This review provides an overview of frequently used label-free visualization techniques and highlights recent studies on the development and usage of microscopy systems based on reflectance, darkfield, differential interference contrast, optical coherence, photothermal, holographic, photoacoustic, total internal reflection, surface plasmon resonance, Rayleigh light scattering, hyperspectral and reflectance structured illumination imaging. Using these imaging modalities, there is a strong enhancement in the reliability of experiments concerning cellular uptake and biocompatibility of nanoparticles, which is crucial for preclinical evaluations and future medical applications.

Evanescent scattering imaging of single protein binding kinetics and DNA conformation changes

Evanescent illumination has been widely used to detect single biological macromolecules because it can notably enhance light-analyte interaction. However, the current evanescent single-molecule detection system usually requires specially designed microspheres or nanomaterials. Here we show that single protein detection and imaging can be realized on a plain glass surface by imaging the interference between the evanescent lights scattered by the single proteins and by the natural roughness of the cover glass. This allows us to quantify the sizes of single proteins, characterize the protein-antibody interactions at the single-molecule level, and analyze the heterogeneity of single protein binding behaviors. In addition, owing to the exponential distribution of evanescent field intensity, the evanescent imaging system can track the analyte axial movement with high resolution, which can be used to analyze the DNA conformation changes, providing one solution for detecting small molecules, such as microRNA. This work demonstrates a label-free single protein imaging method with ordinary consumables and may pave a road for detecting small biological molecules.

Label-Free Imaging of Nanoscale Displacements and Free-Energy Profiles of Focal Adhesions with Plasmonic Scattering Microscopy

Cell adhesion plays a critical role in cell communication, cell migration, cell proliferation, and integration of medical implants with tissues. Focal adhesions physically link the cell cytoskeleton to the extracellular matrix, but it remains challenging to image single focal adhesions directly. Here, we show that plasmonic scattering microscopy (PSM) can directly image the single focal adhesions in a label-free, real-time, and non-invasive manner with sub-micrometer spatial resolution. PSM is developed based on surface plasmon resonance (SPR) microscopy, and the evanescent illumination makes it immune to the interference of intracellular structures. Unlike the conventional SPR microscopy, PSM can provide a high signal-to-noise ratio and sub-micrometer spatial resolution for imaging the analytes with size down to a single-molecule level, thus allowing both the super-resolution lateral localization for measuring the nanoscale displacement and precise tracking of vertical distances between the analyte centroid and the sensor surface for analysis of free-energy profiles. PSM imaging of the RBL-2H3 cell with temporal resolution down to microseconds shows that the focal adhesions have random diffusion behaviors in addition to their directional movements during the antibody-mediated activation process. The free-energy mapping also shows a similar movement tendency, indicating that the cell may change its morphology upon varying the binding conditions of adhesive structures. PSM provides insights into the individual focal adhesion activities and can also serve as a promising tool for investigating the cell/surface interactions, such as cell capture and detection and tissue adhesive materials screening.

Critical angle reflection imaging for quantification of molecular interactions on glass surface

Quantification of molecular interactions on a surface is typically achieved via label-free techniques such as surface plasmon resonance (SPR). The sensitivity of SPR originates from the characteristic that the SPR angle is sensitive to the surface refractive index change. Analogously, in another interfacial optical phenomenon, total internal reflection, the critical angle is also refractive index dependent. Therefore, surface refractive index change can also be quantified by measuring the reflectivity near the critical angle. Based on this concept, we develop a method called critical angle reflection (CAR) imaging to quantify molecular interactions on glass surface. CAR imaging can be performed on SPR imaging setups. Through a side-by-side comparison, we show that CAR is capable of most molecular interaction measurements that SPR performs, including proteins, nucleic acids and cell-based detections. In addition, we show that CAR can detect small molecule bindings and intracellular signals beyond SPR sensing range. CAR exhibits several distinct characteristics, including tunable sensitivity and dynamic range, deeper vertical sensing range, fluorescence compatibility, broader wavelength and polarization of light selection, and glass surface chemistry. We anticipate CAR can expand SPR′s capability in small molecule detection, whole cell-based detection, simultaneous fluorescence imaging, and broader conjugation chemistry.

Here, the authors present a method for quantifying molecular interactions on a glass surface, based on measuring surface refractive index changes via the reflectivity near the critical angle. They demonstrate tunable sensitivity and dynamic range, deep vertical sensing range, also for intracellular signals.

Single Out-of-Resonance Dielectric Nanoparticles as Molecular Sensors

Light scattering from single nanoparticles and nanostructures is a commonly used readout method for nanosensors. Increasing the spectral sensitivity of resonant nanosensors to changes in their local surrounding has been the focus of many studies. Switching from spectral to intensity monitoring allows one to investigate nonresonant or out-of-resonance dielectric nanoparticles. Here, we systematically compared such dielectric silica nanoparticles with plasmonic gold nanorods by deriving analytical expressions and by performing experiments. The experiments show a similar sensitivity for the detection of an adsorbate layer for both particle types, which is in good agreement with theory. The flat spectral response of dielectric silica nanoparticles simplifies the choice of illumination wavelength. Furthermore, such dielectric nanoparticles can be made from many oxides, polymers, and even biological assemblies, broadening the choice of materials for the nanosensor.

Probing Single-Molecule Binding Event by the Dynamic Counting and Mapping of Individual Nanoparticles

Measuring binding processes at the single-molecule level underpin significant functions in understanding biological events. Single-nanoparticle imaging techniques are providing a new concept for mapping the heterogeneous behaviors and characterizations of individual dynamics such as molecule-molecule interactions. Here, we develop the optical imaging techniques for directly counting and monitoring the binding and motion events of single nanoparticles linked to the substrate via the specific and reversible interactions between biomolecules. The one-step digital immunoassay realizes the biomolecular detection based on dynamic counting of the single nanoparticle binding event to substrate with the bright-field imaging. The detection limit achieves 8.4 pg/mL for procalcitonin with detection time of 14 min. Meanwhile, we map the accurate trajectory of single nanoparticle switching between different target molecules among the x-y plane with the total internal reflection imaging technique, which reveals the spatial coordinates of single target molecules on the substrate surface with high spatial and temporal resolutions.

Vertical Diffusion of Ions within Single Particles during Electrochemical Charging

Determining the trajectory of ionic transport and diffusion within single electroactive nanomaterials is critical for understanding the charging kinetics and capacity fading associated with ion batteries, with implications for rational design of excellent-performance electrode materials. While the horizontal pathway of mass transport has been feasibly investigated by optical superlocalization methods and electron microscopes, determination on the vertical trajectory has proven a more challenging task. Herein, we developed dual-angle total internal reflection microscopy by simultaneously introducing different angle-dependent illumination depths to trace the optical centroid shifts of nano-objects in the vertical dimension. We first demonstrated the proof of concept by resolving the vertical moving trails of a nanosphere doing Brownian motion and subsequently explored the picture of mass transport in the interior of single Prussian blue (PB) particles during electrochemical cycling. The results indicated that the vertical centroids of single PB particles remained unchanged when ions were inserted or extracted, suggesting an outside-in ionic transport pathway instead of bottom-up trajectory that one would intuitively expect.

Evanescent Wave-Guided Growth of an Organic Supramolecular Nanowire Array

The ordered assembly of molecules within a specific space of nanoscale, such as a surface, holds great promise in advanced micro-/nanostructure fabrication for various applications. Herein, we demonstrate the evanescent wave (EW)-guided organization of small molecules into a long-range ordered nanowire (NW) array. Experiment and simulation revealed that the orientation and periodicity of the NW array were feasibly regulated by altering the propagation direction and the wavelength of EW. The generality of this approach was demonstrated by using different molecule precursors. While existing studies on EW often took advantages of its near-field property for optical sensing, this work demonstrated the photochemical power of EW in the guided-assembly of small molecules for the first time. It also provides an enlightening avenue to periodic structure with fluorescence, promising for super-resolution microscopy and important devices applicable to optical and bio-related fields.

Single-particle spectroscopy and defocused imaging of anisotropic gold nanorods by total internal reflection scattering microscopy

Total internal reflection scattering (TIRS) microscopy is based on evanescent field illumination at the interface. Compared to conventional dark-field (DF) microscopy, TIRS microscopy has been rarely applied to the spectroscopic studies of plasmonic nanoparticles. Furthermore, there has been no detailed correlation study on the characteristic optical properties of single gold nanorods (AuNRs) obtained by DF and TIRS microscopy. Herein, through a single-particle correlation study, we compare the spectroscopic and defocusing properties of single AuNRs obtained by DF and TIRS microscopy, which have different illumination geometries. Compared to DF microscopy, TIRS microscopy yielded almost identical single-particle scattering spectra and localized surface plasmon resonance (LSPR) linewidth for the same in-focus AuNRs. However, TIRS microscopy, which is based on evanescent field illumination at the interface, provided a higher signal-to-noise ratio in the defocused image of the same AuNRs compared to DF microscopy. Furthermore, the heavily reduced background noise clarified the defocused scattering patterns of TIRS microscopy, which provided more detailed and accurate angular information than that obtained by conventional DF microscopy.

Single-particle spectroscopy for functional nanomaterials

Tremendous progress in nanotechnology has enabled advances in the use of luminescent nanomaterials in imaging, sensing and photonic devices. This translational process relies on controlling the photophysical properties of the building block, that is, single luminescent nanoparticles. In this Review, we highlight the importance of single-particle spectroscopy in revealing the diverse optical properties and functionalities of nanomaterials, and compare it with ensemble fluorescence spectroscopy. The information provided by this technique has guided materials science in tailoring the synthesis of nanomaterials to achieve optical uniformity and to develop novel applications. We discuss the opportunities and challenges that arise from pushing the resolution limit, integrating measurement and manipulation modalities, and establishing the relationship between the structure and functionality of single nanoparticles.

In situ mapping of activity distribution and oxygen evolution reaction in vanadium flow batteries

Understanding spatial distribution difference and reaction kinetics of the electrode is vital for enhancing the electrochemical reaction efficiency. Here, we report a total internal reflection imaging sensor without background current interference to map local current distribution of the electrode in a vanadium redox flow battery during cyclic voltammetry (CV), enabling mapping of the activity and reversibility distribution with the spatial resolution of a single fiber. Three graphite felts with different activity are compared to verify its feasibility. In long-term cyclic voltammetry, the oxygen evolution reaction is proved to enhance activity distribution, and homogeneity of the electrode and its bubble kinetics with periodic fluctuation is consistent with the cyclic voltammetry curve, enabling the onset oxygen evolution/reduction potential determination. Higher activity and irreversibility distribution of the electrode is found in favor of the oxygen evolution reaction. This sensor has potential to detect in situ, among other processes, electrochemical reactions in flow batteries, water splitting, electrocatalysis and electrochemical corrosion.

Identification of Nanoparticles via Plasmonic Scattering Interferometry

The development of optical imaging techniques has led to significant advancements in single-nanoparticle tracking and analysis, but these techniques are incapable of label-free selective nanoparticle recognition. A label-free plasmonic imaging technology that is able to identify different kinds of nanoparticles in water is now presented. It quantifies the plasmonic interferometric scattering patterns of nanoparticles and establishes relationships among the refractive index, particle size, and pattern both numerically and experimentally. Using this approach, metallic and metallic oxide particles with different radii were distinguished without any calibration. The ability to optically identify and size different kinds of nanoparticles can provide a promising platform for investigating nanoparticles in complex environments to facilitate nanoscience studies, such as single-nanoparticle catalysis and nanoparticle-based drug delivery.