High-Density Super-Resolution Localization Imaging with Blinking Carbon Dots

Recent Advances in Fluorescent Nanoparticles for Stimulated Emission Depletion Imaging

Stimulated emission depletion (STED) microscopy, as a popular super-resolution imaging technique, has been widely used in bio-structure analysis and resolving the dynamics of biological processes beyond the diffraction limit. The performance of STED critically depends on the optical properties of the fluorescent probes. Ideally, the probe should process high brightness and good photostability, and exhibit a sensitive response to the depletion beam. Organic dyes and fluorescent proteins, as the most widely used STED probes, suffer from low brightness and exhibit rapid photobleaching under a high excitation power. Recently, luminescent nanoparticles (NPs) have emerged as promising fluorescent probes in biological imaging due to their high brightness and good photostability. STED imaging using various kinds of NPs, including quantum dots, polymer dots, carbon dots, aggregation-induced emission dots, etc., has been demonstrated. This review will comprehensively review recent advances in fluorescent NP-based STED probes, discuss their advantages and pitfalls, and outline the directions for future development.

Blinking Carbon Dots as a Super-resolution Imaging Probe

Single-molecule localization microscopy (SMLM) emerges as a powerful approach for super-resolution imaging that provides unprecedented resolution at the nanometer length scale. However, the development of appropriate probes with specific photophysical traits and characteristics is crucial for this approach. This study demonstrates two different fluorescent carbon dots (CDs) derived from the same molecular precursor─one emitting in red and the other in green─as a SMLM-based super-resolution imaging probe for different applications with an average localization precision of 20 nm and a resolution of 60 nm. Both the CDs exhibit spontaneous blinking with high photon count and low duty cycle but with different blinking cycles. The red emissive CDs with a lower blinking cycle are ideal for quantitative analysis, whereas green emissive CDs with a higher blinking cycle are ideal for high-resolution imaging. We show that the difference in blinking features is linked to their chemical compositions, and the presence of much denser trap states in red emitting CDs is responsible for the reduction of its blinking cycle. This study shows that CDs can be designed as a potential probe for SMLM-based super-resolution imaging for diverse bioimaging applications.

Intrinsic Burst-Blinking Nanographenes for Super-Resolution Bioimaging

Single-molecule localization microscopy (SMLM) is a powerful technique to achieve super-resolution imaging beyond the diffraction limit. Although various types of blinking fluorophores are currently considered for SMLM, intrinsic blinking fluorophores remain rare at the single-molecule level. Here, we report the synthesis of nanographene-based intrinsic burst-blinking fluorophores for highly versatile SMLM. We image amyloid fibrils in air and in various pH solutions without any additive and lysosome dynamics in live mammalian cells under physiological conditions. In addition, the single-molecule labeling of nascent proteins in primary sensory neurons was achieved with azide-functionalized nanographenes via click chemistry. SMLM imaging reveals higher local translation at axonal branching with unprecedented detail, while the size of translation foci remained similar throughout the entire network. These various results demonstrate the potential of nanographene-based fluorophores to drastically expand the applicability of super-resolution imaging.

Application of Multicolor Fluorescent Carbon Dots Based on Tea Polyphenols in a White Light-Emitting Diode and Room-Temperature Phosphorescence

Carbon dots (CDs) are new carbon nanomaterials, among which those prepared from biomass are popular due to their excellent optical properties and environmental friendliness. As representative natural phenolic compounds, tea polyphenols are ideal precursors with fluorescent aromatic rings and phenolic hydroxyl structures. Usually, polyphenolic precursors can only be used to produce blue or green fluorescent CDs, and fluorescence in long wavelength domains, such as orange or red, cannot be achieved. Herein, the high reactivity of the phenolic hydroxyl groups in tea polyphenols with o-phthalaldehyde was exploited to modulate the pH during the carbonation process, which led to redshifts of the fluorescence wavelengths. Different pH values during the reaction caused the precursors to take different reaction paths and form fluorescent groups exhibiting different conjugated structures, resulting in carbon dots providing different fluorescent colors. Finally, by utilizing the in situ hydrolysis of ethyl orthosilicate, the tea polyphenol-based carbon dots were embedded into a silica matrix, inducing phosphorescence of the carbon dots. This study provides a new approach for green preparation and application of natural polyphenolic CDs.

A photo-controlled fluorescent switching based on carbon dots and photochromic diarylethene for bioimaging

Carbon dots (CDs) as luminescent zero-dimensional carbon nanomaterials have good aqueous dissolution, photostability, high quantum yield, and tunability of emission color. It has great application potential in many fields, including bioimaging, labeling of biological species, drug delivery, and sensing in biomedical. However, controlling the fluorescence emission of carbon dots remains a formidable challenge. Herein, we designed and exploited a photo-controlled fluorescent switching based on photochromic diarylethene (DT) and CDs for bioimaging. It could be modulated reversibly between "ON" and "OFF" under UV/vis light exposure. The fluorescent modulation efficiency was as high as 95.3%. The fluorescent switching could be used to the bioimaging in HeLa cells with low cell toxicity. Therefore, this fluorescent switching could be a promising candidate in many potential application areas, especially in bioimaging.

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

A Fluorescent Vector of Carbon Dot to Deliver Rab13 and Rab14 Plasmids for Promoting Neurite Outgrowth

The complex processes of neuron differentiation and neuron repair are critical for treating nervous system injuries and neurodegenerative diseases. Neurite outgrowth plays a crucial role in these processes by enabling the formation of connections between neurons and the generation of neuroplasticity to restore the function of the nervous system. In this study, we fabricated functionalized carbon dots (CDs) with distinctive photoluminescence and low cytotoxicity for use as fluorescence imaging probes and nanocarriers to deliver plasmid DNAs to neurons effectively for inducing neurite outgrowth. CDs were prepared through a reflux process in nitric acid solution, and their surface was then modified using polyethylenimine (PEI) to obtain positively charged CDs for increasing the absorption of plasmid DNAs and the efficiency of cell uptake. Experimental results indicated that the fabricated CDs maintained a low cytotoxicity and exhibited a high neuron uptake of up to 97%. An improvement in the plasmid DNA ingestion of neurons resulted in enhanced expression of Rab13-Q67L and Rab14 proteins, which considerably promoted neurite sprouting and elongation. After the fabricated PEI-modified CDs were used to deliver the Rab13-Q67L and Rab14 plasmids, more than 56% of the neurons had a neurite length that was greater than twice the size of their soma. Thus, DNA delivery through functionalized CDs has a high potential for use in gene therapy for neuronal injuries and diseases.

Compact, Fast Blinking Cd-Free Quantum Dots for Super-Resolution Fluorescence Imaging

Quantum dots (QDs) can be used as fluorescent probes in single molecule localization microscopy to achieve subdiffraction limit resolution (super-resolution fluorescence imaging). However, the toxicity of Cd in the prototypical CdSe-based QDs can limit their use in biological applications. Furthermore, commercial CdSe QDs are usually modified with relatively thick shells of both inorganic and organic materials to render them in the 10-20 nm size range, which is relatively large for biological labels. In this report, we present compact (4-6 nm) CuInS2/ZnS (CIS/ZnS) and compare them to commercially sourced CdSe/ZnS QDs for their blinking behavior, localization precision and super-resolution imaging. Although commercial CdSe/ZnS QDs are brighter than the more compact Cd-free CIS/ZnS QD, both give comparable results of 4.5-5.0-fold improvement in imaging resolution over conventional TIRF imaging of actin filaments. This likely results from the fact that CIS/ZnS QDs show very short on-times and long off times which leads to less overlap in the point spread functions of emitting CIS/ZnS QD labels on the actin filaments at the same labeling density. These results demonstrate that CIS/ZnS QDs are an excellent candidate to complement and perhaps even replace the larger and more toxic CdSe-based QDs for robust single- molecule super-resolution imaging.

The power of super-resolution microscopy in modern biomedical science

Super-resolution microscopy (SRM) technology that breaks the diffraction limit has revolutionized the field of cell biology since its appearance, which enables researchers to visualize cellular structures with nanometric resolution, multiple colors and single-molecule sensitivity. With the flourishing development of hardware and the availability of novel fluorescent probes, the impact of SRM has already gone beyond cell biology and extended to nanomedicine, material science and nanotechnology, and remarkably boosted important breakthroughs in these fields. In this review, we will mainly highlight the power of SRM in modern biomedical science, discussing how these SRM techniques revolutionize the way we understand cell structures, biomaterials assembly and how assembled biomaterials interact with cellular organelles, and finally their promotion to the clinical pre-diagnosis. Moreover, we also provide an outlook on the current technical challenges and future improvement direction of SRM. We hope this review can provide useful information, inspire new ideas and propel the development both from the perspective of SRM techniques and from the perspective of SRM's applications.

Fluorescent Carbon Dots for Super-Resolution Microscopy

Conventional fluorescence microscopy is limited by the optical diffraction of light, which results in a spatial resolution of about half of the light's wavelength, approximately to 250-300 nm. The spatial resolution restricts the utilization of microscopes for studying subcellular structures. In order to improve the resolution and to shatter the diffraction limit, two general approaches were developed: a spatially patterned excitation method and a single-molecule localization strategy. The success of super-resolution imaging relies on bright and easily accessible fluorescent probes with special properties. Carbon dots, due to their unique properties, have been used for super-resolution imaging. Considering the importance and fast development of this field, this work focuses on the recent progress and applications of fluorescent carbon dots as probes for super-resolution imaging. The properties of carbon dots for super-resolution microscopy (SRM) are analyzed and discussed. The conclusions and outlook on this topic are also presented.

Near-Infrared Blinking Carbon Dots Designed for Quantitative Nanoscopy

Blinking carbon dots (CDs) have attracted attention as a probe for single molecule localization microscopy (SMLM), yet quantitative analysis is limited because of inept blinking and low signal-to-noise ratio (SNR). Here we report the design and synthesis of near-infrared (NIR) blinking CDs with a maximum emission of around 750 nm by weaving a nitrogen-doped aromatic backbone with surplus carboxyl groups on the surface. The NIR-CDs allow conjugation to monovalent antibody fragments for labeling and imaging of cellular receptors as well as afford increases of 52% in SNR and 33% in localization precision over visible CDs. Analysis of fluorescent bursts allows for accurate counting of cellular receptors at the nanoscale resolution. Using NIR-CDs-based SMLM, we demonstrate oligomerization and internalization of programmed cell death-ligand 1 by a small molecule inhibitor for checkpoint blockade. Our NIR-CDs can become a generally applicable probe for quantitative nanoscopy in chemistry and biology.

Nanoparticles for super-resolution microscopy: intracellular delivery and molecular targeting

Following an overview of the approaches and techniques used to acheive super-resolution microscopy, this review presents the advantages supplied by nanoparticle based probes for these applications. The various clases of nanoparticles that have been developed toward these goals are then critically described and these discussions are illustrated with a variety of examples from the recent literature.

Multimodal molecular imaging in the second near-infrared window

Near-infrared-II (NIR-II) fluorescence imaging has rapidly developed for the noninvasive investigation of physiological and pathological activities in living organisms with high spatiotemporal resolution. However, the penetration depth of fluorescence restricts its ability to provide deep anatomical information. Scientists integrate NIR-II fluorescence imaging with other imaging modes (such as photoacoustic and magnetic resonance imaging) to create multimodal imaging that can acquire detailed anatomical and quantitative information with deeper penetration by using multifunctional probes. This review offers a comprehensive picture of NIR-II-based dual/multimodal imaging probes and highlights advances in bioimaging and therapy. In addition, seminal studies and trends in multimodal imaging probes activated by NIR-II laser are summarized and several key points regarding future clinical translation are elucidated.

Fluorescent Nanoparticles for Super-Resolution Imaging

Super-resolution imaging techniques that overcome the diffraction limit of light have gained wide popularity for visualizing cellular structures with nanometric resolution. Following the pace of hardware developments, the availability of new fluorescent probes with superior properties is becoming ever more important. In this context, fluorescent nanoparticles (NPs) have attracted increasing attention as bright and photostable probes that address many shortcomings of traditional fluorescent probes. The use of NPs for super-resolution imaging is a recent development and this provides the focus for the current review. We give an overview of different super-resolution methods and discuss their demands on the properties of fluorescent NPs. We then review in detail the features, strengths, and weaknesses of each NP class to support these applications and provide examples from their utilization in various biological systems. Moreover, we provide an outlook on the future of the field and opportunities in material science for the development of probes for multiplexed subcellular imaging with nanometric resolution.

An insight into the potentials of carbon dots for in vitro live-cell imaging: recent progress, challenges, and prospects

Carbon dots (CDs) are a strong alternative to conventional fluorescent probes for cell imaging due to their brightness, photostability, tunable fluorescence emission, low toxicity, inexpensive preparation, and chemical diversity. Improving the targeting efficiency by modulation of the surface functional groups and understanding the mechanisms of targeted imaging are the most challenging issues in cell imaging by CDs. Firstly, we briefly discuss important features of fluorescent CDs for live-cell imaging application in this review. Then, the newest modulated CDs for targeted live-cell imaging of whole-cell, cell organelles, pH, ions, small molecules, and proteins are elaborately discussed, and their challenges in these fields are explained.

Carbon Dot Blinking Fingerprint Uncovers Native Membrane Receptor Organizations via Deep Learning

Oligomeric organization of G protein-coupled receptors is proposed to regulate receptor signaling and function, yet rapid and precise identification of the oligomeric status especially for native receptors on a cell membrane remains an outstanding challenge. By using blinking carbon dots (CDs), we now develop a deep learning (DL)-based blinking fingerprint recognition method, named deep-blinking fingerprint recognition (BFR), which allows automatic classification of CD-labeled receptor organizations on a cell membrane. This DL model integrates convolutional layers, long-short-term memory, and fully connected layers to extract time-dependent blinking features of CDs and is trained to a high accuracy (∼95%) for identifying receptor organizations. Using deep blinking fingerprint recognition, we found that CXCR4 mainly exists as 87.3% monomers, 12.4% dimers, and <1% higher-order oligomers on a HeLa cell membrane. We further demonstrate that the heterogeneous organizations can be regulated by various stimuli at different degrees. The receptor-binding ligands, agonist SDF-1α and antagonist AMD3100, can induce the dimerization of CXCR4 to 33.1 and 20.3%, respectively. In addition, cytochalasin D, which inhibits actin polymerization, similarly prompts significant dimerization of CXCR4 to 30.9%. The multi-pathway organization regulation will provide an insight for understanding the oligomerization mechanism of CXCR4 as well as for elucidating their physiological functions.

Surface-Engineered Gold Nanoclusters for Stimulated Emission Depletion and Correlated Light and Electron Microscopy Imaging

Stimulated emission depletion (STED) nanoscopy is an emerging super-resolution imaging platform for the study of the cellular structure. Developing suitable fluorescent probes of small size, good photostability, and easy functionalization is still in demand. Herein, we introduce a new type of surface-engineered gold nanoclusters (Au NCs) that are ultrasmall (1.7 nm) and ultrabright (QY = 60%) for STED bioimaging. A rigid shell formed by l-arginine (l-Arg) and 6-aza-2-thiothymine (ATT) on the Au NC surface enables not only its strong fluorescence in aqueous solution but also its easy chemical modification for specific biomolecule labeling. Au NCs show remarkable performance as STED nanoprobes, including high depletion efficiency, good photobleaching resistance, and low saturation intensity. Super-resolution imaging has been achieved with these Au NCs, and targeted nanoscopic imaging of cellular tubulin has been demonstrated. Moreover, the circular structure of lysosomes in live cells has been revealed. As a Au NC is also an ideal probe for electron microscopy, dual imaging of Aβ₄₂ aggregates with the single labeling probe of Au NCs has been realized in correlative light and electron microscopy (CLEM). This work reports, for the first time, the application of Au NCs as a novel probe in STED and CLEM imaging. With their excellent properties, Au NCs show promising potential for nanoscale bioimaging.

S, M. M, D. P. Improved citric acid-derived carbon dots synthesis through microwave-based heating in a hydrothermal pressure vessel

Carbon dots (CDs) synthesis from citric acid, ethylenediamine, and formamide by microwave-assisted hydrothermal carbonization in a pressurized vessel.

Fluorescent Probes for STED Optical Nanoscopy

Progress in developing fluorescent probes, such as fluorescent proteins, organic dyes, and fluorescent nanoparticles, is inseparable from the advancement in optical fluorescence microscopy. Super-resolution microscopy, or optical nanoscopy, overcame the far-field optical resolution limit, known as Abbe's diffraction limit, by taking advantage of the photophysical properties of fluorescent probes. Therefore, fluorescent probes for super-resolution microscopy should meet the new requirements in the probes' photophysical and photochemical properties. STED optical nanoscopy achieves super-resolution by depleting excited fluorophores at the periphery of an excitation laser beam using a depletion beam with a hollow core. An ideal fluorescent probe for STED nanoscopy must meet specific photophysical and photochemical properties, including high photostability, depletability at the depletion wavelength, low adverse excitability, and biocompatibility. This review introduces the requirements of fluorescent probes for STED nanoscopy and discusses the recent progress in the development of fluorescent probes, such as fluorescent proteins, organic dyes, and fluorescent nanoparticles, for the STED nanoscopy. The strengths and the limitations of the fluorescent probes are analyzed in detail.

Nanoscopic Imaging of Nucleolar Stress Enabled by Protein-Mimicking Carbon Dots

The nucleolus is a central hub for coordinating cellular stress responses during cancer development and treatment. Accurate identification of nucleolar stress response is crucially desired for nucleolus-based diagnostics and therapeutics but technically challenging due to the need to address the ultrastructural analysis. Here, we report a protein-like CD with the integration of fluorescent blinking domains and RNA-binding motifs, which offers the ability to perform enhanced super-resolution imaging of the nucleolar ultrastructure. This image allows extraction of multidimensional information from the nucleolus for accurate distinguishment of different cells from the same cell types. Furthermore, we demonstrate for the first time this CD-depicted nucleolar ultrastructure as a sensitive hallmark to identify and discriminate subtle responses to various stressors as well as to afford RNA-related information that has been inaccessible by conventional immunofluorescence methods. This protein-mimicking CD could become a broadly useful probe for nucleolar stress studies in cell diagnostics and therapeutics.

A Review of Fluorescent Carbon Dots, Their Synthesis, Physical and Chemical Characteristics, and Applications

Carbon dots (CDs) are a particularly useful type of fluorescent nanoparticle that demonstrate biocompatibility, resistance to photobleaching, as well as diversity in composition and characteristics amongst the different types available. There are two main morphologies of CDs: Disk-shaped with 1-3 stacked sheets of aromatic carbon rings and quasi-spherical with a core-shell arrangement having crystalline and amorphous properties. They can be synthesized from various potentially environmentally friendly methods including hydrothermal carbonization, microwaving, pyrolysis or combustion, and are then purified via one or more methods. CDs can have either excitation wavelength-dependent or -independent emission with each having their own benefits in microscopic fluorescent imaging. Some CDs have an affinity for a particular cell type, organelle or chemical. This property allows the CDs to be used as sensors in a biological environment and can even provide quantitative information if the quenching or intensity of their fluorescence is dependent on the concentration of the analyte. In addition to fluorescent imaging, CDs can also be used for other applications including drug delivery, quality control, photodynamic therapy, and photocatalysis.

Nonblinking carbon dots for imaging and tracking receptors on a live cell membrane

Blinking occurs with nearly all fluorophores including organic dyes, fluorescent proteins, semiconductor quantum dots and carbon dots (CDs). We developed non-blinking and photoresistant fluorescent CDs by introducing multiple aromatic domains onto a single carbon dot and demonstrated their great potential for imaging and tracking of receptors on a live cell membrane.

Protein-, polymer-, and silica-based luminescent nanomaterial probes for super resolution microscopy: a review

Super resolution microscopy was developed to overcome the Abbe diffraction limit, which effects conventional optical microscopy, in order to study the smaller components of biological systems. In recent years nanomaterials have been explored as luminescent probes for super resolution microscopy, as many have advantages over traditional fluorescent dye molecules. This review will summarize several different types of nanomaterial probes, covering quantum dots, carbon dots, and dye doped nanoparticles. For the purposes of this review the term "nanoparticle" will be limited to polymer-based, protein-based, and silica-based nanoparticles, including core-shell structured nanoparticles. Luminescent nanomaterials have shown promise as super-resolution probes, and continued research in this area will yield new advances in both materials science and biochemical microscopy at the nanometer scale.

Length-Dependent Distinct Cytotoxic Effect of Amyloid Fibrils beyond Optical Diffraction Limit Revealed by Nanoscopic Imaging

Fibrillar species have been proposed to play an essential role in the cytotoxicity of amyloid peptide and the pathogenesis of neurodegenerative diseases. Discrimination of Aβ aggregates in situ at high spatial resolution is therefore significant for the development of a therapeutic method. In this work, we adopt a rhodamine-like structure as luminescent centers to fabricate carbonized fluorescent nanoparticles (i.e., carbon dots, RhoCDs) with tunable emission wavelengths from green to red and burst-like photoblinking property for localization-based nanoscopic imaging. These RhoCDs contain lipophilic cationic and carboxyl groups which can specifically bind with Aβ_1-40 aggregates via electrostatic interaction and hydrogen bonding. According to the nanoscopic imaging in the Aβ_1-40 fibrillation and disaggregation process, different types of Aβ_1-40 aggregates beyond the optical diffraction limit have been disclosed. Additionally, length-dependent toxic effect of Aβ_1-40 aggregates beyond the optical diffraction limit is unveiled. Short amyloid assemblies with length of 187 ± 3.9 nm in the early stage are more toxic than the elongated amyloid fibrils. Second, disassembly of long fibrils into short species by Gramicidin S (GS-2) peptide might enhance the cytotoxicity. These results lay the foundation to develop functional fluorophore for nanoscopic imaging and also provide deep insight into morphology-dependent cytotoxicity from amyloid peptides.

Antifade Carbon Dots on a Plasmonic Substrate for Enhanced Protein Detection in Immunotherapy

Fluorescence microscopic analysis of checkpoint protein expression is capable of predicting clinical outcomes for checkpoint blockade immunotherapy. However, accurate detection of their expression levels is hindered by fluorophore photobleaching and cell autofluorescence. We now develop a sensitive and robust fluorescence microscopy method that uses antifade graphite-structured carbon dots (GCDs) on a plasmonic Ag substrate (named ACPAS) for the accurate detection of checkpoint proteins in immunotherapy. In ACPAS, a Ag substrate is used to enhance the fluorescence of GCDs while a continuous illumination is implemented to quench cell autofluorescence, thus enabling a dramatic improvement in the signal-to-background ratio by up to 33-fold. We use ACPAS to monitor programmed death ligand-1 (PD-L1) expression levels on various tumor cells and finely differentiate their microscopic changes in combination with chemokine receptor CXCR4-targeted treatments. ACPAS analysis reveals for the first time that CXCR4 agonist (SDF-1α) and antagonist (AMD3100) can potentiate PD-L1 expression by down-regulating CXCR4 expression on tumor cells, which provides valuable information on the development of anti-PD-L1 and anti-CXCR4 combination therapy. We envision that ACPAS will become a broadly useful tool for protein expression studies in biomedicine and life sciences.

Deep Red Blinking Fluorophore for Nanoscopic Imaging and Inhibition of β-Amyloid Peptide Fibrillation

Deposition and aggregation of β-amyloid (Aβ) peptides are demonstrated to be closely related to the pathogenesis of Alzheimer's disease (AD). Development of functional molecules capable of visualizing Aβ_1-40 aggregates with nanoscale resolution and even modulating Aβ assembly has attracted great attention recently. In this work, we use monocyanine fluorophore as the lead structure to develop a set of deep red carbazole-based cyanine molecules, which can specifically bind with Aβ_1-40 fibril via electrostatic and van der Waals interactions. Spectroscopic and microscopic characterizations demonstrate that one of these fluorophores, (E)-1-(2-(2-methoxyethoxy)ethyl)-4-(2-(9-methyl-9H-carbazol-3-yl)vinyl) quinolinium iodide (me-slg) can bind to Aβ_1-40 aggregates with strong fluorescence enhancement. The photophysical properties of me-slg at the single-molecule level, including low "on/off" duty cycle, high photon output, and sufficient switching cycles, enable real-time nanoscopic imaging of Aβ_1-40 aggregates. Morphology-dependent toxic effect of Aβ_1-40 aggregates toward PC12 cells is unveiled from in situ nanoscopic fluorescence imaging. In addition, me-slg displays a strong inhibitory effect on Aβ_1-40 fibrillation in a low inhibitor-protein ratio (e.g., I:P = 0.2). A noticeably reduced cytotoxic effect of Aβ_1-40 after the addition of me-slg is also confirmed. These results afford promising applications in the design of a nanoscopic imaging probe for amyloid fibril as well as the development of inhibitors to modulate the fibrillation process.

Core Size does not Affect Blinking Behavior of Dye-Doped Ag@SiO2 Core-Shell Nanoparticles for Super-Resolution Microscopy

Dye-doped nanoparticles have been investigated as bright, luminescent labels for super-resolution microscopy via localization methods. One key factor in super-resolution is the size of the luminescent label, which in some cases results in a frame shift between the label target and the label itself. Ag@SiO₂ core–shell nanoparticles, doped with organic fluorophores, have shown promise as super-resolution labels. One key aspect of these nanoparticles is that they blink under certain conditions, allowing super-resolution localization with a single excitation source in aqueous solution. In this work, we investigated the effects of both the Ag core and the silica (SiO₂) shell on the self-blinking properties of these nanoparticles. Both core size and shell thickness were manipulated by altering the reaction time to determine core and shell effects on photoblinking. Size and shell thickness were investigated individually under both dry and hydrated conditions and were then doped with a 1 mM solution of Rhodamine 110 for analysis. We observed that the cores themselves are weakly luminescent and are responsible for the blinking observed in the fully-synthesized metal-enhanced fluorescence nanoparticles. There was no statistically significant difference in photoblinking behavior—both intensity and duty cycle—with decreasing core size. This observation was used to synthesize smaller nanoparticles ranging from approximately 93 nm to 110 nm as measured using dynamic light scattering. The blinking particles were localized via super-resolution microscopy and show single particle self-blinking behavior. As the core size did not impact blinking performance or intensity, the nanoparticles can instead be tuned for optimal size without sacrificing luminescence properties.

Dye-doped nanoparticles have been investigated as bright, luminescent labels for super-resolution microscopy via localization methods.

Nanographenes: Ultrastable, Switchable, and Bright Probes for Super-Resolution Microscopy

Super‐resolution fluorescence microscopy has enabled important breakthroughs in biology and materials science. Implementations such as single‐molecule localization microscopy (SMLM) and minimal emission fluxes (MINFLUX) microscopy in the localization mode exploit fluorophores that blink, i.e., switch on and off, stochastically. Here, we introduce nanographenes, namely large polycyclic aromatic hydrocarbons that can also be regarded as atomically precise graphene quantum dots, as a new class of fluorophores for super‐resolution fluorescence microscopy. Nanographenes exhibit outstanding photophysical properties: intrinsic blinking even in air, excellent fluorescence recovery, and stability over several months. As a proof of concept for super‐resolution applications, we use nanographenes in SMLM to generate 3D super‐resolution images of silica nanocracks. Our findings open the door for the widespread application of nanographenes in super‐resolution fluorescence microscopy.

Modulation and study of photoblinking behavior in dye doped silver-silica core-shell nanoparticles for localization super-resolution microscopy

Blinking of fluorescent nanoparticles is a compelling phenomenon with widely debated mechanisms. The ability to inhibit or control blinking is important for applications in the field of optical, semiconductor and fluorescent imaging. Self-blinking nanomaterials are also attractive labels for localization-based super-resolution microscopy. In this work, we have synthesized silver core silica nanoparticles (Ag@SiO2) doped with Rhodamine 110 and studied the parameters that affect blinking. We found that under nitrogen rich conditions the nanoparticles shifted towards higher duty cycles. Also, it was found that hydrated nanoparticles showed a less drastic response to nitrogen rich conditions as compared to dried nanoparticles, indicating that surrounding matrix played a role in the response of nanoparticles to molecular oxygen. Further, the blinking is not a multi-body phenomena, super-resolution localization combined with intensity histogram analysis confirmed that single particles are emitting.

Efficient Modulation of β-Amyloid Peptide Fibrillation with Polymer Nanoparticles Revealed by Super-Resolution Optical Microscopy

β-Amyloid peptide (Aβ) aggregation is the essential hallmark of neurodegenerative disorders such as Alzheimer's disease. Efficient inhibitors are highly desired for the prevention of Aβ assembly that has been considered as the primary therapeutic strategy for neurodegenerative diseases. Apart from this, visualization of the aggregates and morphology at high spatial resolution is widely considered of crucial significance on biological treatment. In this work, we have developed small-sized (with diameter of ∼4.7 nm) and positively charged fluorescent conjugated polymer nanoparticles (CPNPs) with strong inhibition effect on Aβ_1-40 peptides fibrillation. Interestingly, the CPNPs also possess excellent photophysical properties, including high photon counts, robust blinking, and repetitive fluorescence switching, that are especially suitable for localization-based super-resolution imaging. Spatial resolution of ∼20 nm for these blinking CPNPs is readily achieved. According to the optical microscopic results, it was found that binding of CPNPs to the terminal of seed fibrils can effectively inhibit the fibrillation process. Owing to these attractive biological and unique photophysical properties, the small-sized CPNPs show high potential in a variety of super-resolution based biological applications.

Malic Acid Carbon Dots: From Super-resolution Live-Cell Imaging to Highly Efficient Separation

As-synthesized malic acid carbon dots are found to possess photoblinking properties that are outstanding and superior compared to those of conventional dyes. Considering their excellent biocompatibility, malic acid carbon dots are suitable for super-resolution fluorescence localization microscopy under a variety of conditions, as we demonstrate in fixed and live trout gill epithelial cells. In addition, during imaging experiments, the so-called "excitation wavelength-dependent" emission was not observed for individual as-made malic acid carbon dots, which motivated us to develop a time-saving and high-throughput separation technique to isolate malic acid carbon dots into fractions of different particle size distributions using C₁₈ reversed-phase silica gel column chromatography. This post-treatment allowed us to determine how particle size distribution influences the optical properties of malic acid carbon dot fractions, that is, optical band gap energies and photoluminescence behaviors.

Nanoparticles for super-resolution microscopy and single-molecule tracking

We review the use of luminescent nanoparticles in super-resolution imaging and single-molecule tracking, and showcase novel approaches to super-resolution imaging that leverage the brightness, stability, and unique optical-switching properties of these nanoparticles. We also discuss the challenges associated with their use in biological systems, including intracellular delivery and molecular targeting. In doing so, we hope to provide practical guidance for biologists and continue to bridge the fields of super-resolution imaging and nanoparticle engineering to support their mutual advancement.