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

Polarity-Targeted Carbon Dots for Mitochondria and Lysosomes Imaging

Normally, electrostatic-dependent mitochondria localization can cause a decrease/loss of mitochondrial membrane potential (MMP), leading to the corresponding abnormal behaviors. So, achieving subcellular organelle localization and imaging with as little interference on their physiological activity is of significance for understanding cell activity. Herein, we discover and demonstrate that "polarity" can independently act as a novel kind of target for labeling at the organelle level. On this basis, mitochondria and lysosomes are precisely fluorescently imaged by two kinds of polarity-targeted carbon dots (C-dots), respectively. The two C-dots, named C-dots-1 and C-dots-2, have almost identical size and morphology as well as surface chemistry. The subtle difference is their polarity property: both of them are amphiphilic, with 1.54 and 0.95 for the log P values. Different from commonly used cationic-based organelle probes, both of the two C-dots possess slightly negatively charged surfaces (ζ-potential values ∼ -2.5 to -7.5 mV) at physiological conditions. Interestingly, the C-dots-1 and C-dots-2 have the capacity for highly selectively labeling and imaging mitochondria and lysosomes, whether cancer cells or normal cells. Because the targeting processes do not rely on electrostatic attraction effects, the MMP is not changed during localization processes. So, the corresponding cell abnormal behaviors caused by MMP diminishing, for example, the autophagy phenomenon, can be effectively avoided.

Gold nanoclusters Au25AcCys18 normalize intracellular ROS without increasing cytoplasmic alarmin acHMGB1 abundance in human microglia and neurons

This study focuses on the modulatory effects of gold nanoclusters with 25 gold atoms and 18 acetyl cysteines (Au25AcCys18) in human microglia, human iPSC-derived neurons and SH-SY5Y differentiated human neuronal cells. The combination of chemical, biological, and computational methods shows the well-retained viability of these human cells treated with Au25AcCys18, interactions between Au25AcCys18 and transcription factor TFEB (computational approach), interactions between TFEB and HMGB1 (proximity ligation assay and molecular modeling using AlphaFold), modulation of the abundance and location of acHMGB1 by Au25AcCys18 (immunocytochemistry), and the reduction of ROS in cells treated with Au25AcCys18 (CellROX live imaging). These novel findings in human neural cells, particularly neurons, encourage further studies in experimental animal models of neurological disorders and/or human organoids to exploit the unique structural and photophysical properties of gold nanoclusters and to better understand their ability to modulate molecular mechanisms in human cells.

Assessing the overflowing pile-up effect on the photoluminescence lifetime of nanomaterials

The tunable complex emissive states with nanosecond to microsecond lifetimes in nanomaterials, arise due to their structural heterogeneity, enabling them with a wide range of advanced optoelectronic applications. However, understanding the complex photoluminescence lifetime in these nanomaterials is critically challenged by the overflowing pile-up effect, which occurs due to the high repetition rate of the light source in the time-correlated single photon counting (TCSPC) technique. Here, we provide a quantitative lifetime analysis, especially in metal nanoclusters, metal complexes, and semiconductor quantum dots, which suggests that the same experimental parameters can mislead the lifetime data interpretation for long-ranged luminescent nanomaterials. We demonstrate that the overflowing pile-up effect could be fatal while analyzing the excited state lifetime. Furthermore, we provide the optimized parameters that could be used to get the actual lifetime data of samples. We hope that our findings will be crucial in obtaining the error-free and accurate excited state dynamics of these long-range lifetime nanomaterials.

Multi-Sensitive Au NCs/5-FU@Carr-LA Composite Hydrogels for Targeted Multimodal Anti-Tumor Therapy

Multifunctional targeted drug delivery systems have been explored as a novel cancer treatment strategy to overcome limitations of traditional chemotherapy. The combination of photodynamic therapy and chemotherapy has been shown to enhance efficacy, but the phototoxicity of traditional photosensitizers is a challenge. In this study, we prepared a multi-sensitive composite hydrogel containing gold nanoclusters (Au NCs) and the temperature-sensitive antitumor drug 5-fluorourac il (5-FU) using carboxymethyl cellulose (Carr) as a dual-functional template. Au NCs were synthesized using sodium borohydride as a reducing agent and potassium as a promoter. The resulting Au NCs were embedded in the Carr hydrogel, which was then conjugated with lactobionic acid (LA) as a targeting ligand. The resulting Au NCs/5-FU@Carr-LA composite hydrogel was used for synergistic photodynamic therapy (PDT), photothermal therapy (PTT), and chemotherapy. Au NCs/5-FU@Carr-LA releases the drug faster at pH 5.0 due to the acid sensitivity of the Carr polymer chain. In addition, at 50 °C, the release rate of Au NCs/5-FU@Carr-LA is 78.2%, indicating that the higher temperature generated by the photothermal effect is conducive to the degradation of Carr polymer chains. The Carr hydrogel stabilized the Au NCs and acted as a matrix for drug loading, and the LA ligand facilitated targeted delivery to tumor cells. The composite hydrogel exhibited excellent biocompatibility and synergistic antitumor efficacy, as demonstrated by in vitro and in vivo experiments. In addition, the hydrogel had thermal imaging capabilities, making it a promising multifunctional platform for targeted cancer therapy.

Monodispersed and Monofunctionalized DNA-Caged Au Nano-Clusters with Enhanced Optical Properties for STED Imaging

The performance of Stimulated Emission Depletion (STED) microscopy depends critically on the fluorescent probe. Ultrasmall Au nanoclusters (Au NCs) exhibit large Stokes shift, and good stimulated emission response, which are potentially useful for STED imaging. However, Au NCs are polydispersed in size, sensitive to the surrounding environment, and difficult to control surface functional group stoichiometry, which results in reduced density and high heterogeneity in the labeling of biological structures. Here, this limitation is overcome by developing a method to encapsulate ultrasmall Au NCs with DNA cages, which yielded monodispersed, and monofunctionalized Au NCs that are long-term stable. Moreover, the DNA-caging also greatly improved the fluorescence quantum yield and photostability of Au NCs. In STED imaging, the DNA-caged Au NCs yielded ≈40 nm spatial resolution and are able to resolve microtubule line shapes with good labeling density and homogeneity. In contrast, without caging, the Au NCs-DNA conjugates only achieved ≈55 nm resolution and yielded spotted, poorly resolved microtubule structures, due to the presence of aggregates. Overall, a method is developed to achieve precise surface functionalization and greatly improve the monodispersity, stability, as well as optical properties of Au NCs, providing a promising class of fluorescent probes for STED imaging.

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.

Glutathione ligand self-assembly enables luminescence from Au15 nanoclusters for highly sensitive and selective monitoring of blood Pb(II) ions

Lead Pb(II) ions is a cumulative toxicant that impacts several biological systems and poses severe harm to young children. Accurate Pb(II) ions monitoring is thus of paramount importance. Here, we present the synthesis and application of glutathione-capped Au15 nanoclusters (Au15(SG)13) as a luminescence probe for the accurate and selective monitoring of blood Pb(II). The introduction of Pb(II) ions triggers orderly self-assembly of Au15 nanoclusters, resulting in the formation of rigid shell around Au nuclei. This limits the localized vibration of the glutathione ligands and their interaction with water molecules, greatly reducing non-radiative energy loss, and thereby enhancing the photoluminescence signal. Consequently, Au15(SG)13 nanoclusters exhibit high sensitivity for Pb(II) detection. The detection signal displays a linear relationship with Pb(II) over a wide detection range (0-800 μg/L), demonstrating a substantial sensitivity of 35.29 μg/L. Moreover, the developed nanoclusters show superior selectivity for Pb(II) ions, distinguishing them from other prevalent heavy metals. This work pave the way for the development of advanced Pb(II) sensors with high sensitivity and selectivity.

Ultrabright Xanthene Fluorescence Probe for Mitochondrial Super-Resolution Imaging

Mitochondria are important organelles that provide energy for cellular physiological activities. Changes in their structures may indicate the occurrence of diseases, and the super-resolution imaging of mitochondria is of great significance. However, developing fluorescent probes for mitochondrial super-resolution visualization still remains challenging due to insufficient fluorescence brightness and poor stability. Herein, we rationally synthesized an ultrabright xanthene fluorescence probe Me-hNR for mitochondria-specific super-resolution imaging using structured illumination microscopy (SIM). The rigid structure of Me-hNR provided its ultrahigh fluorescence quantum yield of up to 0.92 and ultrahigh brightness of up to 16,000. Occupying the para-position of the O atom in the xanthene skeleton by utilizing the smallest methyl group ensured its excellent stability. The study of the photophysical process indicated that Me-hNR mainly emitted fluorescence via radiative decay, and nonradiative decay and inter-system crossing were rare due to the slow nonradiative decay rate and large energy gap (ΔEst = 0.55 eV). Owing to these excellent merits, Me-hNR can specifically light up mitochondria at ultralow concentrations down to 5 nM. The unprecedented spatial resolution for mitochondria with an fwhm of 174 nm was also achieved. Therefore, this ultrabright xanthene fluorescence probe has great potential in visualizing the structural changes of mitochondria and revealing the pathogenesis of related diseases using SIM.

Development of gold nanocluster complex for the detection of tumor necrosis factor-alpha based on immunoassay

Tumor necrosis factor-alpha, TNF-α, a cytokine recognized as a key regulator of inflammatory responses, is primarily produced by activated monocytes and macrophages. Measuring TNF-α levels serves as a valuable indicator for tracking several diseases and pathological states. Gold nanotechnology has been identified as a highly effective catalyst with unique properties for measuring inflammatory cytokines. This study aimed to synthesize gold nanoclusters (AuNCs) and the AuNCs-streptavidin system, along with their characterizations and spherical morphology. The detection of TNF-α antigen with AuNCs was determined, and a new immunoassay-based AuNCs analytical platform was studied. In this study, it was demonstrated that the synthesized AuNCs and AuNCs-streptavidin showed a bright-yellow appearance with absorption peaks at A600 and A610 nm, respectively. The approximately spherical shape was observed by TEM analysis. The AuNCs demonstrated a sensitivity limit for the detection of the TNF-α antigen, with a linear dose-dependent detection range of less than 1.25 ng/mL. The products of the band sizes and band intensities were proportional to the amount of TNF-α in the range of ∼80 kDa, ∼55 kDa, and ∼ 25 kDa in western blot analysis. The TNF-α in cell lysate was successfully detected using an immunoassay after the activation of RAW264.7 cells with lipopolysaccharide (LPS). This assay may serve as a viable alternative for TNF-α detection with high speed, sensitivity, and qualities, ensuring its broad applications.

Bridging Smart Nanosystems with Clinically Relevant Models and Advanced Imaging for Precision Drug Delivery

Intracellular delivery of nano-drug-carriers (NDC) to specific cells, diseased regions, or solid tumors has entered the era of precision medicine that requires systematic knowledge of nano-biological interactions from multidisciplinary perspectives. To this end, this review first provides an overview of membrane-disruption methods such as electroporation, sonoporation, photoporation, microfluidic delivery, and microinjection with the merits of high-throughput and enhanced efficiency for in vitro NDC delivery. The impact of NDC characteristics including particle size, shape, charge, hydrophobicity, and elasticity on cellular uptake are elaborated and several types of NDC systems aiming for hierarchical targeting and delivery in vivo are reviewed. Emerging in vitro or ex vivo human/animal-derived pathophysiological models are further explored and highly recommended for use in NDC studies since they might mimic in vivo delivery features and fill the translational gaps from animals to humans. The exploration of modern microscopy techniques for precise nanoparticle (NP) tracking at the cellular, organ, and organismal levels informs the tailored development of NDCs for in vivo application and clinical translation. Overall, the review integrates the latest insights into smart nanosystem engineering, physiological models, imaging-based validation tools, all directed towards enhancing the precise and efficient intracellular delivery of NDCs.

Complementary imaging of nanoclusters interacting with mitochondria via stimulated emission depletion and scanning transmission electron microscopy

The emerging stress caused by nanomaterials in the environment is of great concern because they can have toxic effects on organisms. However, thorough study of the interactions between cells and diverse nanoparticles (NPs) using a unified approach is challenging. Here, we present a novel approach combining stimulated emission depletion (STED) microscopy and scanning transmission electron microscopy (STEM) for quantitative assessment, real-time tracking, and in situ imaging of the intracellular behavior of gold-silver nanoclusters (AuAgNCs), based on their fluorescence and electron properties. The results revealed an aggregated state of AuAgNCs within the mitochondria and an increase in sulfur content in AuAgNCs, presumably owing to their reaction with thiol-containing molecules inside the mitochondria. Moreover, AuAgNCs (100 μg/mL) induced a 75% decline in mitochondrial membrane potential and a 12-fold increase of mitochondrial reactive oxygen species in comparison to control. This mitochondrial damage may be triggered by the reaction of AuAgNCs with thiol, which provides direct imaging evidence for uncovering the action mechanism of AuAgNCs on the mitochondria. The proposed dual-imaging strategy using STED and STEM is a potential tool to offer valuable insights into cytotoxicity between subcellular structures and diverse NPs, and can serve as a key strategy for nanomaterial biosafety assessment.

Copper Nanosheet-Based Wash-Free Fluorescence Imaging of Cancer Cells

Near-infrared fluorescence (NIRF) probes greatly facilitate in vivo imaging of various biologically important species. However, there are several significant limitations such as consuming washing steps, photobleaching, and low signal intensity. Herein, we synthesized fluorescent copper nanosheets templated with DNA scaffolds (DNS/CuNSs). We employ them and Cy5.5 of the fluorescence resonance energy transfer (FRET) system, which have a larger Stokes shift (∼12-fold) than the traditional NIRF dye Cy5.5. Based on their excellent fluorescence properties, we employ DNS/CuNSs-Cy5.5 for fluorescence probes in cancer cell imaging. Compared with the free Cy5.5 fluorescence probe, the novel fluorescence imaging probe implements wash-free imaging and exhibits enhanced anti-photobleaching ability (∼5.5-fold). Moreover, the FRET system constructed by DNS/CuNSs has a higher signal amplification ability (∼4.17-fold), which is more similar to that of Cu nanoclusters prepared with DNA nanomonomers as a template. This work provides a new idea for cancer cell MCF-7 imaging and is expected to promote the development of cancer cell fluorescence imaging.

Exploring the future of regenerative medicine: Unveiling the potential of optical microscopy for structural and functional imaging of stem cells

Regenerative medicine, which utilizes stem cells for tissue and organ repair, holds immense promise in healthcare. A comprehensive understanding of stem cell characteristics is crucial to unlock their potential. This study explores the pivotal role of optical microscopy in advancing regenerative medicine as a potent tool for stem cell research. Advanced optical microscopy techniques enable an in-depth examination of stem cell behavior, morphology, and functionality. The review encompasses current optical microscopy, elucidating its capabilities and constraints in stem cell imaging, while also shedding light on emerging technologies for improved stem cell visualization. Optical microscopy, complemented by techniques like fluorescence and multiphoton imaging, enhances our comprehension of stem cell dynamics. The introduction of label-free imaging facilitates noninvasive, real-time stem cell monitoring without external dyes or markers. By pushing the boundaries of optical microscopy, researchers reveal the intricate cellular mechanisms underpinning regenerative processes, thereby advancing more effective therapeutic strategies. The current study not only outlines the future of regenerative medicine but also underscores the pivotal role of optical microscopy in both structural and functional stem cell imaging.

Strong fluorescence-detected two-photon circular dichroism of chiral gold nanoclusters

Progress in syntheses and understanding of the intriguing properties of chiral noble metal nanoclusters sparks interest to extend investigations of their chiroptical response to the nonlinear optics regime. We present a quantitative determination of two-photon circular dichroism of chiral gold nanoclusters with ATT and L- or D-Arg ligands (ATT = 6-aza-2-thiotymine and Arg = arginine). Introduction of arginine ligands enables the formation of two enantiomers of the nanoclusters, with strong chiroptical effects in both linear and nonlinear regime. We present two-photon absorption and luminescent properties measured in a wide range of wavelengths, with the two-photon absorption cross section reaching 1743 GM and two-photon brightness ∼1102 GM at 825 nm. We report strong, 245-fold enhancement of the two-photon circular dichroism of nanoclusters with respect to the one-photon absorption counterpart - the dissymmetry factor. The presence of multiple advantages of nanoclusters: high fluorescence quantum yield, strong nonlinear optical properties and well-controlled chirality is a powerful combination for applications of such clusters in multiphoton microscopy.

Deep learning in single-molecule imaging and analysis: recent advances and prospects

Single-molecule microscopy is advantageous in characterizing heterogeneous dynamics at the molecular level. However, there are several challenges that currently hinder the wide application of single molecule imaging in bio-chemical studies, including how to perform single-molecule measurements efficiently with minimal run-to-run variations, how to analyze weak single-molecule signals efficiently and accurately without the influence of human bias, and how to extract complete information about dynamics of interest from single-molecule data. As a new class of computer algorithms that simulate the human brain to extract data features, deep learning networks excel in task parallelism and model generalization, and are well-suited for handling nonlinear functions and extracting weak features, which provide a promising approach for single-molecule experiment automation and data processing. In this perspective, we will highlight recent advances in the application of deep learning to single-molecule studies, discuss how deep learning has been used to address the challenges in the field as well as the pitfalls of existing applications, and outline the directions for future development.

Fluorescence CLEM in biology: historic developments and current super-resolution applications

Correlative light and electron microscopy (CLEM) is a powerful imaging approach that allows the direct correlation of information obtained on a light and an electron microscope. There is a growing interest in the application of CLEM in biology, mainly attributable to technical advances in field of fluorescence microscopy in the past two decades. In this review, we summarize the important developments in CLEM for biological applications, focusing on the combination of fluorescence microscopy and electron microscopy. We first provide a brief overview of the early days of fluorescence CLEM usage starting with the initial rise in the late 1970s and the subsequent optimization of CLEM workflows during the following two decades. Next, we describe how the engineering of fluorescent proteins and the development of super-resolution fluorescence microscopy have significantly renewed the interest in CLEM resulting in the present application of fluorescence CLEM in many different areas of cellular and molecular biology. Lastly, we present the promises and challenges for the future of fluorescence CLEM discussing novel workflows, probe development and quantification possibilities.

Parallel gold enhancement of quantum dots 565/655 for double-labelling correlative light and electron microscopy on human autopsied samples

Cadmium selenide quantum dots (QDs) are fluorescent and electron-dense nanoparticles. When used as reporter of immunolabeling, this dual visibility is essential for direct comparison of its fluorescent signals on light microscopy (LM) and their ultrastructrual counterparts on electron microscopy (EM) as correlative light and electron microscopy (CLEM). To facilitate EM recognition, QDs on EM grid were gold enhanced, which increased their size and electron density. On histological sections as well, gold-enhanced QDs, used as a reporter of immunolabeling, were easily recognized on EM. Because target structures are visible on bright field microscopy, gold enhancement facilitated trimming the target structures into final EM sections. Furthermore, gold enhancement of rod-shaped QD655 on EM grid was accentuated on their tips while spherical QD565 was gold-enhanced as sphere in contrast. This EM distinction was evident on histological sections where QD565 (green fluorescence) and QD655 (red fluorescence) were used as a reporter pair for double immunolabeling. Double-labeled immuno-fluorescent images, initially captured before EM processing, are now compared with their respective immuno EM counterparts. Specific labeling of each epitope was corroborated by mutual comparison between LM and EM. Although fluoronanogold may be a candidate reporter partner with QDs for gold-enhanced, double-labeling CLEM, its limited penetration into fixed tissue hampers universal use for thick histological sections. Gold-enhancement of QD immunolabeling, now expanded to double-labeling CLEM for human brain samples, will pave the way to translate molecular events into ultrastructural morphopathogenesis in situ.