Highly Luminescent Thermoresponsive Green Emitting Gold Nanoclusters for Intracellular Nanothermometry and Cellular Imaging: A Dual Function Optical Probe

Microviscosity-Assisted Disaggregation of a Model Ophthalmic Drug and FRET-Controlled Singlet Oxygen Generation in Lyotropic Liquid Crystals

Different phases of lyotropic liquid crystals (LLCs), made up of mesogen-like sodium dodecyl sulfate (SDS), mainly bestow different bulk viscosities. Along with this, the role of microviscosities of the individual LLC phases is of immense interest because a minute change in it due to guest incorporation can cause significant alteration in their property as a potential energy transfer scaffold. Recently, LLCs have been identified as plausible drug delivery agents for ocular treatments. In this direction, the present work illustrates photophysical modulations of an important laser dye as well as an ophthalmic medicine, coumarin 6 (C6), inside different LLC phases in an aqueous medium. C6 molecules spontaneously accumulate in water, leading to aggregation-caused quenching (ACQ) of fluorescence. However, the different phases of the LLCs prepared from SDS and water helped in disintegrating the C6 colonies to various extents depending upon the microviscosity. The heterogeneity in the LLC phases, in turn, could modulate the Förster resonance energy transfer (FRET) between C6 and the LLC incorporated with N-doped carbon nanoparticles (N-CNPs). The N-CNPs act as potential photosensitizers and generate singlet oxygen (1O2), a reactive oxygen species (ROS), to different extents. Microviscosities of the prepared LLCs were calculated by using fluorescence correlation spectroscopy (FCS). The different phases of the LLCs, viz., lamellar and hexagonal, with different microviscosities controlled the extent of C6 disaggregation and hence the FRET and the ROS generation. The results are encouraging since ROS generation has a significant role in the vision mechanism and PDT-based applications. LLC-based drug administration with potential FRET to control ROS generation may become handy in ophthalmology. The LLC phases used in this experiment not only served the purpose of drug delivery but also the photophysical events therein are compatible with the ocular environment.

Visualizing the intracellular aggregation behavior of gold nanoclusters via structured illumination microscopy and scanning transmission electron microscopy

Given the growing concerns about nanotoxicity, numerous studies have focused on providing mechanistic insights into nanotoxicity by imaging the intracellular fate of nanoparticles. A suitable imaging strategy is necessary to uncover the intracellular behavior of nanoparticles. Although each conventional technique has its own limitations, scanning transmission electron microscopy (STEM) and three-dimensional structured illumination microscopy (3D-SIM) combine the advantages of chemical element mapping, ultrastructural analysis, and cell dynamic tracking. Gold nanoclusters (AuNCs), synthesized using 6-aza-2 thiothymine (ATT) and L-arginine (Arg) as reducing and protecting ligands, referred to as Arg@ATT-AuNCs, have been widely used in biological sensing and imaging, medicine, and catalyst yield. Based on their intrinsic fluorescence and high electron density, Arg@ATT-AuNCs were selected as a model. STEM imaging showed that both the single-particle and aggregated states of Arg@ATT-AuNCs were compartmentally distributed within a single cell. Real-time 3D-SIM imaging showed that the fluorescent Arg@ATT-AuNCs gradually aggregated after being located in the lysosomes of living cells, causing lysosomal damage. The aggregate formation of Arg@ATT-AuNCs was triggered by the low-pH medium, particularly in the lysosomal acidic environment. The proposed dual imaging strategy was verified using other types of AuNCs, which is valuable for studying nano-cell interactions and any associated cytotoxicity, and has the potential to be a useful approach for exploring the interaction of cells with various nanoparticles.

Accurate Visualization of Metabolic Aberrations in Cancer Cells by Temperature Mapping with Quantum Coherence Modulation Microscopy

Specific metabolic aberrations of cancer cells rapidly generate energy with a minuscule but detectable temperature variation, which is a typical characteristic providing insight into cancer pathogenesis. However, to date, intracellular temperature mapping of cancer cell metabolism with high temporal and spatial resolution has not been realized. In this study, we mapped and monitored in real-time the intracellular temperature variations of mitochondria and cytoplasm at a subcellular scale via a single-molecule coherent modulation microscopy coupling targeted molecule labeling technique. According to the variation of the decoherence processes of targeted molecules as a function of intracellular temperature, we achieved a high temperature resolution (<0.1 K) and proved that this technique could eliminate interference from fluorescence intensity disturbance and external pH change. Furthermore, we showed a positive correlation between the determined temperature and the adenosine triphosphate production rate of mitochondrial metabolism in combination with a cell energy metabolic analyzer. This technology enables accurate real-time temporal and spatial visualization of cancer metabolism and establishes diagnoses and therapies for cancer.

Insight into the Lysozyme-Induced Aggregation of Aromatic Amino Acid-Functionalized Gold Nanoparticles: Impact of the Protein Conjugation and Lipid Corona on the Aggregation Phenomena

The aggregation and subsequent precipitation of gold nanoparticles (Au NPs) in the presence of protein molecules restrict the usefulness of NPs in biomedical applications. Till now, the influence of different properties of Au NPs (size, surface charge, surface coatings) and proteins (surface charge, chemical modification, folded and unfolded states) and pH and ionic strength of the solution on the aggregation of both Au NPs and proteins has been thoroughly discussed in the literature. However, the underlying different mechanistic pathways of the protein concentration-dependent aggregation of both Au NPs and proteins are poorly understood. The impact of the lipid corona on the protein-induced Au NP aggregation has remained an unresolved issue. In this context, we investigate the interaction of the negatively charged aromatic amino acid (phenylalanine and tyrosine)-functionalized gold nanoparticles (Au-AA NPs) with the positively charged globular protein lysozyme at different protein concentrations and compare the results with those of conventional citrate-functionalized Au NPs (Au-Cit NPs). Next, we conjugate lipids and proteins to Au NPs to impede the aggregation of Au NPs induced by the lysozyme. Our results reveal that the aggregation mechanism of the Au-AA NPs is distinctly different at low and high protein concentrations with the uniqueness of the Au-AA NPs over the Au-Cit NPs. Furthermore, we find that human serum albumin (HSA) protein-conjugated Au-AA and Au-Cit NPs are more effective in preventing the lysozyme-induced Au NP aggregation than bovine serum albumin (BSA)-conjugated Au NPs. For the first time, we also report the significant role of "hard" and "soft" lipid coronas in the aggregation of amino acid (phenylalanine)-functionalized gold nanoparticles in the presence of lysozyme protein.

Thermal Probing Techniques for a Single Live Cell

Temperature is a significant factor in determining and characterizing cellular metabolism and other biochemical activities. In this study, we provide a brief overview of two important technologies used to monitor the local temperatures of individual living cells: fluorescence nano-thermometry and an array of micro-/nano-sized thin-film thermocouples. We explain some key technical issues that must be addressed and optimised for further practical applications, such as in cell biology, drug selection, and novel antitumor therapy. We also offer a method for combining them into a hybrid measuring system.

Facile construction of highly luminescent and biocompatible gold nanoclusters by shell rigidification for two-photon pH-edited cytoplasmic and in vivo imaging

Gold nanoclusters (AuNCs), as a novel fluorescent material, have been extensively explored and developed for bioimaging because of their attractive advantages such as ultrasmall size, low toxicity and exceptional two-photon excitation properties. However, it still remains a challenge to produce water-soluble, biocompatible and ultrabright AuNCs. Herein, we report on a novel one-pot synthesis of highly luminescent and biocompatible AuNCs by using polyvinyl pyrrolidone (PVP), a water-soluble polymer, to rigidify the primary stabilizing layer (shell) that is composed of 6-aza-2-thiothymine (ATT) ligands bound to the particle. Such shell-rigidification resulted in a significant enhancement of the fluorescence efficiency, reaching a quantum yield of 39% under the best conditions, about 35-fold increase from the intrinsically weak fluorescence of the AuNCs stabilized by only ATT. The fluorescence enhancement mechanism was systematically characterized, and the results indicate that PVP coating rigidifies the ATT ligand shell through steric hindrance and reduces the nonradiative relaxation of the excited states. The biocompatible PVP-AuNCs were further examined for two-photon cellular and sentinel lymph node (SLN) bioimaging, and we observed pH-dependent cytoplasmic images and intense green fluorescence in SLN and lymphatic vessels.

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.

Excellent Multiphoton Excitation Fluorescence with Large Multiphoton Absorption Cross Sections of Arginine-Modified Gold Nanoclusters for Bioimaging

Fluorescent gold nanoclusters (Au NCs) with excellent one-photon and multiphoton properties have been demonstrated as promising candidates in many application fields. However, small multiphoton absorption (MPA) cross sections and weak multiphoton excitation (MPE) fluorescence impede their practical applications under near-infrared (NIR) excitation for biological imaging. Here, we report the regulated one-photon and multiphoton properties and mechanisms of arginine-stabilized 6-aza-2-thiothymine Au NCs (Arg/ATT-Au NCs) and the applications for MPE fluorescence imaging. The introduction of arginine into the capping layer of ATT-Au NCs significantly modifies the electronic structure, the absorption cross sections, and the relaxation dynamics of the lowest excited state, drastically reducing the nonradiative relaxation, suppressing the blinking, and greatly enhancing the fluorescence. Besides the improved one-photon properties, Arg/ATT-Au NCs demonstrate remarkable MPE fluorescence with a large MPA cross section. The two-photon (λ_ex = 850 nm), three-photon (λ_ex = 1400 nm), and four-photon (λ_ex = 1700 nm) absorption cross sections have been determined to be 6.1 × 10^-47 cm⁴ s¹ photon^-1, 1.5 × 10^-78 cm⁶ s² photon^-2, and 5.5 × 10^-108 cm⁸ s³ photon^-3, respectively, much higher than those of conventional organic compounds and previously reported Au NCs. Moreover, Arg/ATT-Au NCs have been successfully applied in two-photon and three-photon excitation fluorescence imaging of living cells with NIR excitation. The manifold advantages of small size, high quantum yield, suppressed blinking, good photostability and cytocompatibility, large MPA cross sections, and excellent MPE fluorescence imaging performances make fluorescent Arg/ATT-Au NCs a great candidate of imaging probes with vis-NIR excitation.

Photoluminescent nanocluster-based probes for bioimaging applications

In the continuous search for versatile and better performing probes for optical bioimaging and biosensing applications, many research efforts have focused on the design and optimization of photoluminescent metal nanoclusters. They consist of a metal core composed by a small number of atoms (diameter < 2-3 nm), usually coated by a shell of stabilizing ligands of different nature, and are characterized by molecule-like quantization of electronic states, resulting in discrete and tunable optical transitions in the UV-Vis and NIR spectral regions. Recent advances in their size-selective synthesis and tailored surface functionalization have allowed the effective combination of nanoclusters and biologically relevant molecules into hybrid platforms, that hold a large potential for bioimaging purposes, as well as for the detection and tracking of specific markers of biological processes or diseases. Here, we will present an overview of the latest combined imaging or sensing nanocluster-based systems reported in the literature, classified according to the different families of coating ligands (namely, peptides, proteins, nucleic acids, and biocompatible polymers), highlighting for each of them the possible applications in the biomedical field.

State of the Art and Perspectives on the Biofunctionalization of Fluorescent Metal Nanoclusters and Carbon Quantum Dots for Targeted Imaging and Drug Delivery

The interface of nanobio science and cancer nanomedicine is one of the most important current frontiers in research, being full of opportunities and challenges. Ultrasmall fluorescent metal nanoclusters (MNCs) and carbon quantum dots (CQDs) have emerged as promising fluorescent nanomaterials due to their unique physicochemical and optical properties, facile surface functionalization, good photostability, biocompatibility, and aqueous dispersity. These characteristics make them advantageous over conventional fluorophores such as organic dye molecules and semiconductor quantum dots (QDs) for the detection, diagnosis, and treatment of various diseases including cancer. Recently, researchers have focused on the biofunctionalization strategy of the MNCs and CQDs which can tailor their physicochemical and biological properties and, in turn, can empower these biofunctionalized nanoprobes for diverse applications including imaging, drug delivery, theranostics, and other biomedical applications. In this invited feature article, we first discuss some fundamental structural and physicochemical characteristics of the fluorescent biocompatible quantum-sized nanomaterials which have some outstanding features for the development of multiplexed imaging probes, delivery vehicles, and cancer nanomedicine. We then demonstrate the diverse surface engineering of these fluorescent nanomaterials with reactive target specific functional groups which can help to construct multifunctional nanoprobes with improved targeting capabilities having minimal toxicity. The promising future of the biofunctionalized fluorescent quantum-sized nanomaterials in the field of bioanalytical and biomedical research is elaborately demonstrated, showing selected recent works with relevant applications. This invited feature article finally ends with a short discussion of the current challenges and future prospects of the development of these bioconjugated/biofunctionalized nanomaterials to provide insight into this burgeoning field of MNC- and CQD-based diagnostics and therapeutic applications.

Gold nanoclusters: An ultrasmall platform for multifaceted applications

Gold nanoclusters (Au NCs) with a core size below 2 nm form an exciting class of functional nano-materials with characteristic physical and chemical properties. The properties of Au NCs are more prominent and extremely different from their bulk counterparts. The synthesis of Au NCs is generally assisted by template or ligand, which impart excellent cluster stability and high quantum yield. The tunable and sensitive physicochemical properties of Au NCs open horizons for their advanced applications in various interdisciplinary fields. In this review, we briefly summarize the solution phase synthesis and origin of the characteristic properties of Au NCs. A vast review of recent research work introducing biosensors based on Au NCs has been presented along with their specifications and detection limits. This review also highlights recent progress in the use of Au NCs as bio-imaging probe, enzyme mimic, temperature sensing probe and catalysts. A speculation on present challenges and certain future prospects have also been provided to enlighten the path for advancement of multifaceted applications of Au NCs.

Discriminatory Interaction Behavior of Lipid Vesicles toward Diversely Emissive Carbon Dots Synthesized from Ortho, Meta, and Para Isomeric Carbon Precursors

Photoluminescent carbon dots (C-dots) are widely used for bioimaging techniques to study different cellular processes. However, biocompatibility of C-dots is crucial because the wrong selection of C-dots may lead to an adverse effect on a particular cellular process. Herein, we investigate the interaction of zwitterionic lipid vesicles with photoluminescent C-dots derived from different isomeric (ortho, meta, and para) precursors of phenylenediamine (PDA) by spectroscopic and microscopic imaging techniques as well as dynamic light scattering methods. The study reveals that interaction of lipid vesicles with C-dots is highly dependent on the properties of the isomeric precursors. We find that vesicles retain their morphology upon interaction with ortho C-dots (oCD). The microscopic images reveal that oCD are selectively embedded in the lipid vesicles and can effectively be used for imaging purpose. On the contrary, meta and para C-dots (mCD and pCD) being located on the interfacial region induce aggregation in the vesicles. We explain the observation in terms of the location of the C-dots on the lipid vesicles, their electrostatic attraction at the vesicle interface, possible cross-linking with other vesicles and different hydration features of the isomeric precursors of the C-dots. The study may be helpful in understanding the interactions and attachment processes of C-dots at the interface of biological membranes.

In Vivo Spectral Distortions of Infrared Luminescent Nanothermometers Compromise Their Reliability

Luminescence nanothermometry has emerged over the past decade as an exciting field of research due to its potential applications where conventional methods have demonstrated to be ineffective. Preclinical research has been one of the areas that have benefited the most from the innovations proposed in the field. Nevertheless, certain questions concerning the reliability of the technique under in vivo conditions have been continuously overlooked by most of the scientific community. In this proof-of-concept, hyperspectral in vivo imaging is used to explain how unverified assumptions about the thermal dependence of the optical transmittance of biological tissues in the so-called biological windows can lead to erroneous measurements of temperature. Furthermore, the natural steps that should be taken in the future for a reliable in vivo luminescence nanothermometry are discussed together with a perspective view of the field after the findings here reported.

Thermo-responsive light-emitting metal complexes and related materials

This review discusses the fundamentals and design strategies for the development of thermo-responsive metal–ligand coordination materials and the applications of these materials in temperature sensing, bioimaging, information security, etc.