All-Optical Imaging of Gold Nanoparticle Geometry Using Super-Resolution Microscopy

Point-Spread Function Deformations Unlock 3D Localization Microscopy on Spherical Nanoparticles

Nanoparticles (NPs) have proven their applicability in biosensing, drug delivery, and photothermal therapy, but their performance depends critically on the distribution and number of functional groups on their surface. When studying surface functionalization using super-resolution microscopy, the NP modifies the fluorophore's point-spread function (PSF). This leads to systematic mislocalizations in conventional analyses employing Gaussian PSFs. Here, we address this shortcoming by deriving the analytical PSF model for a fluorophore near a spherical NP. Its calculation is four orders of magnitude faster than numerical approaches and thus feasible for direct use in localization algorithms. We fit this model to individual 2D images from DNA-PAINT experiments on DNA-coated gold NPs and demonstrate extraction of the 3D positions of functional groups with <5 nm precision, revealing inhomogeneous surface coverage. Our method is exact, fast, accessible, and poised to become the standard in super-resolution imaging of NPs for biosensing and drug delivery applications.

Nanotherapeutic Heterogeneity: Sources, Effects, and Solutions

Nanomaterials have revolutionized medicine by enabling control over drugs' pharmacokinetics, biodistribution, and biocompatibility. However, most nanotherapeutic batches are highly heterogeneous, meaning they comprise nanoparticles that vary in size, shape, charge, composition, and ligand functionalization. Similarly, individual nanotherapeutics often have heterogeneously distributed components, ligands, and charges. This review discusses nanotherapeutic heterogeneity's sources and effects on experimental readouts and therapeutic efficacy. Among other topics, it demonstrates that heterogeneity exists in nearly all nanotherapeutic types, examines how nanotherapeutic heterogeneity arises, and discusses how heterogeneity impacts nanomaterials' in vitro and in vivo behavior. How nanotherapeutic heterogeneity skews experimental readouts and complicates their optimization and clinical translation is also shown. Lastly, strategies for limiting nanotherapeutic heterogeneity are reviewed and recommendations for developing more reproducible and effective nanotherapeutics provided.

Spatial Distributions of Single-Molecule Reactivity in Plasmonic Catalysis

Plasmonic catalysts have the potential to accelerate and control chemical reactions with light by exploiting localized surface plasmon resonances. However, the mechanisms governing plasmonic catalysis are not simple to decouple. Several plasmon-derived phenomena, such as electromagnetic field enhancements, temperature, or the generation of charge carriers, can affect the reactivity of the system. These effects are convoluted with the inherent (nonplasmonic) catalytic properties of the metal surface. Disentangling these coexisting effects is challenging but is the key to rationally controlling reaction pathways and enhancing reaction rates. This study utilizes super-resolution fluorescence microscopy to examine the mechanisms of plasmonic catalysis at the single-particle level. The reduction reaction of resazurin to resorufin in the presence of Au nanorods coated with a porous silica shell is investigated in situ. This allows the determination of reaction rates with a single-molecule sensitivity and subparticle resolution. By variation of the irradiation wavelength, it is possible to examine two different regimes: photoexcitation of the reactant molecules and photoexcitation of the nanoparticle's plasmon resonance. In addition, the measured spatial distribution of reactivity allows differentiation between superficial and far-field effects. Our results indicate that the reduction of resazurin can occur through more than one reaction pathway, being most efficient when the reactant is photoexcited and is in contact with the Au surface. In addition, it was found that the spatial distribution of enhancements varies, depending on the underlying mechanism. These findings contribute to the fundamental understanding of plasmonic catalysis and the rational design of future plasmonic nanocatalysts.

Toward Quantitative Nanothermometry Using Single-Molecule Counting

Photothermal heating of nanoparticles has applications in nanomedicine, photocatalysis, photoelectrochemistry, and data storage, but accurate measurements of temperature at the nanoparticle surface are lacking. Here we demonstrate progress toward a super-resolution DNA nanothermometry technique capable of reporting the surface temperature on single plasmonic nanoparticles. Gold nanoparticles are functionalized with double-stranded DNA, and the extent of DNA denaturation under heating conditions serves as a reporter of temperature. Fluorescently labeled DNA oligomers are used to probe the denatured DNA through transient binding interactions. By counting the number of fluorescent binding events as a function of temperature, we reconstruct DNA melting curves that reproduce trends seen for solution-phase DNA. In addition, we demonstrate our ability to control the temperature of denaturation by changing the Na⁺ concentration and the base pair length of the double-stranded DNA on the nanoparticle surface. This degree of control allows us to select narrow temperature windows to probe, providing quantitative measurements of temperature at nanoscale surfaces.

Imaging and Localization of Single Emitters near Plasmonic Particles of Different Size, Shape, and Material

Colloidal plasmonic materials are increasingly used in biosensing and catalysis, which has sparked the use of super-resolution localization microscopy to visualize processes at the interface of the particles. We quantify the effect of particle-emitter coupling on super-resolution localization accuracy by simulating the point spread function (PSF) of single emitters near a plasmonic nanoparticle. Using a computationally inexpensive boundary element method, we investigate a broad range of conditions allowing us to compare the simulated localization accuracy to reported experimental results. We identify regimes where the PSF is not Gaussian anymore, resulting in large mislocalizations due to the appearance of multilobed PSFs. Such exotic PSFs occur when near-field excitation of quadrupole plasmons is efficient but unexpectedly also occur for large particle-emitter spacing where the coherent emission from the particle and emitter results in anisotropic emission patterns. We provide guidelines to enable faithful localization microscopy near colloidal plasmonic materials, which indicate that simply decreasing the coupling between particle and molecule is not sufficient for faithful super-resolution imaging.

Super-Resolution Mapping of a Chemical Reaction Driven by Plasmonic Near-Fields

Plasmonic nanoparticles have recently emerged as promising photocatalysts for light-driven chemical conversions. Their illumination results in the generation of highly energetic charge carriers, elevated surface temperatures, and enhanced electromagnetic fields. Distinguishing between these often-overlapping processes is of paramount importance for the rational design of future plasmonic photocatalysts. However, the study of plasmon-driven chemical reactions is typically performed at the ensemble level and, therefore, is limited by the intrinsic heterogeneity of the catalysts. Here, we report an in situ single-particle study of a fluorogenic chemical reaction driven solely by plasmonic near-fields. Using super-resolution fluorescence microscopy, we map the position of individual product molecules with an ∼30 nm spatial resolution and demonstrate a clear correlation between the electric field distribution around individual nanoparticles and their super-resolved catalytic activity maps. Our results can be extended to systems with more complex electric field distributions, thereby guiding the design of future advanced photocatalysts.

Mapping Fluorescence Enhancement of Plasmonic Nanorod Coupled Dye Molecules

Plasmonically enhanced fluorescence is a widely studied and applied phenomenon, however, only a comparative theoretical and experimental analysis of coupled fluorophores and plasmonic nanoresonators makes it possible to uncover how this phenomenon can be controlled. A numerical optimization method was applied to design configurations that are capable of resulting in an enhancement of excitation and emission, moreover, of both phenomena simultaneously in coupled Cy5 dye molecule and gold nanorod systems. Parametric sensitivity studies revealed how the fluorescence enhancement depends on the molecule’s location, distance and orientation. Coupled systems designed for simultaneous improvement exhibited the highest (intermediate directional) total fluorescence enhancement, which is accompanied by intermediate sensitivity to the molecule’s parameters, except the location and orientation sensitivity at the excitation wavelength. Gold nanorods with a geometry corresponding to the predicted optimal configurations were synthesized, and DNA strands were used to control the Cy5 dye molecule distance from the nanorod surface via hybridization of the Cy5-labelled oligonucleotide. State-of-the-art dSTORM microscopy was used to accomplish a proof-of-concept experimental demonstration of the theoretically predicted (directional) total fluorescence enhancement. The measured fluorescence enhancement was in good agreement with theoretical predictions, thus providing a complete kit to design and prepare coupled nanosystems exhibiting plasmonically enhanced fluorescence.

High-Throughput, High-Resolution Interferometric Light Microscopy of Biological Nanoparticles

Label-free, visible light microscopy is an indispensable tool for studying biological nanoparticles (BNPs). However, conventional imaging techniques have two major challenges: (i) weak contrast due to low-refractive-index difference with the surrounding medium and exceptionally small size and (ii) limited spatial resolution. Advances in interferometric microscopy have overcome the weak contrast limitation and enabled direct detection of BNPs, yet lateral resolution remains as a challenge in studying BNP morphology. Here, we introduce a wide-field interferometric microscopy technique augmented by computational imaging to demonstrate a 2-fold lateral resolution improvement over a large field-of-view (>100 × 100 μm²), enabling simultaneous imaging of more than 10⁴ BNPs at a resolution of ∼150 nm without any labels or sample preparation. We present a rigorous vectorial-optics-based forward model establishing the relationship between the intensity images captured under partially coherent asymmetric illumination and the complex permittivity distribution of nanoparticles. We demonstrate high-throughput morphological visualization of a diverse population of Ebola virus-like particles and a structurally distinct Ebola vaccine candidate. Our approach offers a low-cost and robust label-free imaging platform for high-throughput and high-resolution characterization of a broad size range of BNPs.

Isolating strong nanoantenna-molecule interactions by ensemble-level single-molecule detection

Traditionally, the nanoscale interaction between single photon emitters and plasmonic nanostructures is studied by relying on deterministic, near-perfect, nanoscale-control, either top-down or bottom-up. However, these approaches are ultra-low throughput thus rendering systematic studies difficult and time-consuming. Here, we show a highly parallelised far-field tactic, combining multiplexed super-resolution fluorescence localization microscopy and data-driven statistical analysis, to study near-field interactions between gold nanorods and single molecules, even at bulk concentrations. We demonstrate that ensemble-level single molecule detection allows separating individual emitters according to their coupling strength with tailored resonant structures, which ultimately permits the reconstruction of super-resolved 2D interaction maps around individual nanoantennas.

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

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

A Robust and General Approach to Quantitatively Conjugate Enzymes to Plasmonic Nanoparticles

Bioconjugates of plasmonic nanoparticles have received considerable attention due to their potential biomedical applications. Successful bioconjugation requires control over the number and activity of the conjugated proteins and the colloidal stability of the particles. In practice, this requires reoptimization of the conjugation protocol for each combination of protein and nanoparticle. Here, we report a robust and general protocol that allows for the conjugation of a range of proteins to different types of nanoparticles using very short polyethylene-glycol(PEG) linkers, while simultaneously preserving protein activity and colloidal stability. The use of short linkers ensures that the protein is located close to the particle surface, where the refractive index sensitivity and near-field enhancement are maximal. We demonstrate that the use of a Tween20 containing stabilizing buffer is critical in maintaining colloidal stability and protein function throughout the protocol. We obtain quantitative control over the average number of enzymes per particle by either varying the number of functional groups on the particle or the enzyme concentration during incubation. This new route of preparing quantitative protein-nanoparticle bioconjugates paves the way to develop rational and quantitative strategies to functionalize nanoparticles for applications in sensing, medical diagnostics, and drug delivery.

Density Gradient Selection of Colloidal Silver Nanotriangles for Assembling Dye-Particle Plasmophores

A simple method based on sucrose density gradient centrifugation is proposed here for the fractionation of colloidal silver nanotriangles. This method afforded particle fractions with surface plasmon resonances, spanning from red to infrared spectral ranges that could be used to tune optical properties for plasmonic applications. This feature was exemplified by selecting silver nanotriangle samples with spectral overlap with Atto-655 dye's absorption and emission in order to assemble dye-particle plasmophores. The emission brightness of an individual plasmophore, as characterized by fluorescence correlation spectroscopy, is at least 1000-fold more intense than that of a single Atto-655 dye label, which renders them as promising platforms for the development of fluorescence-based nanosensors.

Revealing the Activity Distribution of a Single Nanocatalyst by Locating Single Nanobubbles with Super-Resolution Microscopy

It is challenging to uncover the catalytic activity at different locations of a single nanocatalyst for gas-generating reactions in real time. This research uses super-resolution microscopy to localize the center of single nanobubbles and reveal the local activity distribution at several to tens of nanometers accuracy. The distances between the centers of the nanobubbles and the center of the nanoplate usually distribute in a certain range from 0 to 500 nm, with the maximum population exhibiting at ∼200 nm. This research also shows that more nanobubbles appear near the tips of the Pd-Ag nanoplate compared with the edges, which indicates higher activity at the tips. In addition, the relationship between the location, lifetime, and turnover rate of the nanobubbles was also carefully studied. This work presents an effective, high-resolution method to localize the activity distribution of nanocatalysts during gas-generating reactions, such as photocatalytic water splitting, dehydrogenation, and electro-oxidation.