Engineering Fluorescent Gold Nanoclusters Using Xanthate-Functionalized Hydrophilic Polymers: Toward Enhanced Monodispersity and Stability

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.

NIR-Emitting Gold Nanoclusters Encapsulated in PS-b-PEG Polymer Micelles

Gold nanoclusters (AuNCs) are an emerging type of luminescent probe, featuring good biocompatibility, high photostability, and large Stoke shifts. Their lack of colloidal stability is, however, a drawback for many applications. Here, we report the stabilization of AuNCs emitting in the NIR by a thiol-terminated polystyrene chain (Mn = 5000 g mol-1). The optical properties of this nanocomposite remain invariant for 2 years in THF. To use the PS5k-AuNCs in an aqueous environment, these were encapsulated into polymer micelles using a polystyrene-b-poly(ethylene glycol) copolymer. The resulting hierarchical constructs, with diameters of ca. 125 to 215 nm, have promising properties for applications as luminescent probes such as contrast agents for biomedical imaging.

Hybrid chain reaction and selective recognition-based homogeneous dual-fluorescence analysis of circulating tumor cells in clinical ovarian cancer samples

BACKGROUND

Oncological analysis is important in tumor diagnosis. We constructed a dual-fluorescence and binary visual analysis system for circulating tumor cells (CTCs) using the folate receptor as a biomarker, combined with hybridization chain reaction and nanomaterial amplification. This strategy integrates terminal protection, selective recognition properties of N-methyl mesoporphyrin IX and CdTe quantum dots for Cu2+ and double-stranded templated copper nanoparticles, and inkjet printing technology.

RESULTS

In fluorescence mode, folate receptor and A2780 ovarian cancer cells were specifically detected with a limit of detection of 0.1 fg mL-1, and 10 cells mL-1 were observed. The detection limits of both the color and distance reading modes were comparable to those obtained in fluorescence mode. The applicability of the method for quantifying CTCs was validated using 27 (6 negative and 21 positive) clinical ovarian cancer samples; the results agreed with those of both the clinical folate receptor-polymerase chain reaction kit and radiological and pathological results.

SIGNIFICANCE

This dual-fluorescence and binary visual CTCs detection method provides multiple options for clinical tumor liquid biopsy.

Antifouling Properties of Poly(2-Oxazoline)s and Poly(2-Oxazine)s: Direct Comparison of Polymer-Coated Surfaces with the Same Coating Parameters

This study presents a systematic comparison of the antifouling properties of water-soluble poly(2-oxazoline) (PAOx) and poly(2-oxazine) (PAOzi) brushes grafted to gold surfaces. PAOx and PAOzi are emerging polymer classes in biomedical sciences and are being considered superior alternatives to widely used polyethylene glycol (PEG). Four different polymers, poly(2-methyl-2-oxazoline) (PMeOx), poly(2-ethyl-2-oxazoline) (PEtOx), poly(2-methyl-2-oxazine) (PMeOzi), and poly(2-ethyl-2-oxazine) (PEtOzi), each of them in three different chain lengths, are synthesized and characterized for their antifouling properties. Results show that all polymer-modified surfaces display better antifouling properties than bare gold surfaces as well as analogous PEG coatings. The antifouling properties increase in the following order: PEtOx < PMeOx ≈ PMeOzi < PEtOzi. The study suggests that the resistance to protein fouling derives from both surface hydrophilicity and the molecular structural flexibility of the polymer brushes. PEtOzi brushes with moderate hydrophilicity show the best antifouling performance, possibly due to their highest chain flexibility. Overall, the research contributes to the understanding of antifouling properties in PAOx and PAOzi polymers, with potential applications in various biomaterials.

ROS-Responsive Fluorescent Sensor Array for Precise Diagnosis of Cancer via pH-Controlled Multicolor Gold Nanoclusters

Intracellular reactive oxygen species (ROS) are closely associated with cancer cell types. Therefore, ROS-based pattern recognition is a promising strategy for precise diagnosis of cancer, but such a possibility has never been reported yet. Herein, we proposed an ROS-responsive fluorescent sensor array based on pH-controlled histidine-templated gold nanoclusters (AuNCs@His) to distinguish cancer cell types and their proliferation states. In this strategy, three types of AuNCs@His with diverse fluorescence profiles were first synthesized by only adjusting the pH value. Upon the addition of various ROS, fluorescence quenching of three types of AuNCs@His occurred with different degrees, thereby forming unique optical "fingerprints", which were well-clustered into several separated groups without overlap by principal component analysis (PCA). The sensing mechanism was attributable to the oxidation of AuNCs@His by ROS, as revealed by X-ray photoemission spectroscopy, Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, and electrospray ionization mass spectrometry. Based on the ROS-responsive sensing pattern, cancer cell types were successfully differentiated via PCA with 100% accuracy. Additionally, the proposed sensor array exhibited excellent performance in distinguishing the proliferation states of cancer cells, which was supported by the results of the Ki-67 immunohistochemistry assay. Overall, the ROS-responsive fluorescent sensor array can serve as a promising tool for precise diagnosis of cancer, indicating great potential for clinical application.

Homogeneous Dual Fluorescence Count of CD4 in Clinical HIV-Positive Samples via Parallel Catalytic Hairpin Assembly and Multiple Recognitions

Regularly measuring the level of CD4+ cells is necessary for monitoring progression and predicting prognosis in patients suffering from an infection with the human immunodeficiency virus (HIV). However, the current flow cytometry standard detection method is expensive and complicated. A parallel catalytic hairpin assembly (CHA)-assisted fluorescent aptasensor is reported for homogeneous CD4 count by targeting the CD4 protein expressed on the membrane of CD4+ cells. Detection was achieved using CdTe quantum dots (QDs) and methylene blue (MB) as signal reporters. CdTe QDs distinguished CHA-assisted release of Ag+ and C-Ag+-C and MB that has differentiated cytosine (C)-rich single-stranded DNA (ssDNA) and C-Ag+-C, generating changes in fluorescence intensity. With the assistance of the CHA strategy and luminescent nanomaterials, this method reached limits of detection of 0.03 fg/mL for the CD4 protein and 0.3 cells/mL for CD4+ cells with linear ranges of 0.1 to 100 fg/mL and 1 to 1000 cells/mL, respectively. The method was validated in 50 clinical whole blood samples consisting of 30 HIV-positive patients, 10 healthy volunteers, and 10 patients with cancer or other chronic infections. The findings from this method were in good agreement with the data from clinical flow cytometry. Due to its sensitivity, affordability, and ease of operation, the current method has demonstrated great potential for routine CD4 counts for the management of HIV, especially in communities and remote areas.

Combination of Chemotherapy and Mild Hyperthermia Using Targeted Nanoparticles: A Potential Treatment Modality for Breast Cancer

Despite the clinical benefits that chemotherapeutics has had on the treatment of breast cancer, drug resistance remains one of the main obstacles to curative cancer therapy. Nanomedicines allow therapeutics to be more targeted and effective, resulting in enhanced treatment success, reduced side effects, and the possibility of minimising drug resistance by the co-delivery of therapeutic agents. Porous silicon nanoparticles (pSiNPs) have been established as efficient vectors for drug delivery. Their high surface area makes them an ideal carrier for the administration of multiple therapeutics, providing the means to apply multiple attacks to the tumour. Moreover, immobilising targeting ligands on the pSiNP surface helps direct them selectively to cancer cells, thereby reducing harm to normal tissues. Here, we engineered breast cancer-targeted pSiNPs co-loaded with an anticancer drug and gold nanoclusters (AuNCs). AuNCs have the capacity to induce hyperthermia when exposed to a radiofrequency field. Using monolayer and 3D cell cultures, we demonstrate that the cell-killing efficacy of combined hyperthermia and chemotherapy via targeted pSiNPs is 1.5-fold higher than applying monotherapy and 3.5-fold higher compared to using a nontargeted system with combined therapeutics. The results not only demonstrate targeted pSiNPs as a successful nanocarrier for combination therapy but also confirm it as a versatile platform with the potential to be used for personalised medicine.

Stepwise-Enhanced Tumor Targeting of Near-Infrared Emissive Au Nanoclusters with High Quantum Yields and Long-Term Stability

We developed an in situ coordination-driven spatially confined strategy for preparing near-infrared emissive gold nanoclusters encapsulated by fluorinated polymers (AuNCs@PF, λ_max = 810 nm) with good stability and high quantum yields (27.7%), far higher than those previously reported for NIR AuNCs (>800 nm). Based on the stepwise enhancements including long blood circulation-induced passive tumor targeting, fluoro-enhanced tumor permeation, and tumor microenvironment (weak acid)-induced aggregation retention in cells, these AuNCs demonstrated bright and stable NIR fluorescence imaging ability in tumors. Additionally, the AuNCs@PF were capable of fluorine magnetic resonance imaging and computed tomographic imaging. The multimodal imaging of tumor-bearing mice clearly implied the potential of AuNCs@PF in biomedical fields.

Gold nanomaterials and their potential use as cryo-electron tomography labels

Rapid advances in cryo-electron tomography (cryo-ET) are driving a revolution in cellular structural biology. However, unambiguous identification of specific biomolecules within cellular tomograms remains challenging. Overcoming this obstacle and reliably identifying targets in the crowded cellular environment is of major importance for the understanding of cellular function and is a pre-requisite for high-resolution structural analysis. The use of highly-specific, readily visualised and adjustable labels would help mitigate this issue, improving both data quality and sample throughput. While progress has been made in cryo-CLEM and in the development of cloneable high-density tags, technical issues persist and a robust 'cryo-GFP' remains elusive. Readily-synthesized gold nanomaterials conjugated to small 'affinity modules' may represent a solution. The synthesis of materials including gold nanoclusters (AuNCs) and gold nanoparticles (AuNPs) is increasingly well understood and is now within the capabilities of non-specialist laboratories. The remarkable chemical and photophysical properties of <3nm diameter nanomaterials and their emergence as tools with widespread biomedical application presents significant opportunities to the cryo-microscopy community. In this review, we will outline developments in the synthesis, functionalisation and labelling uses of both AuNPs and AuNCs in cryo-ET, while discussing their potential as multi-modal probes for cryo-CLEM.

Ultrafast Ratiometric Detection of Aflatoxin B1 Based on Fluorescent β-CD@Cu Nanoparticles and Pt2+ Ions

Rapid detection of aflatoxin B1 (AFB1) is a very important task in food safety monitoring. However, it is still challenging to achieve highly sensitive detection without antibody or aptamer biomolecules. In this work, a rapid detection of aflatoxin B1 was achieved using a ratiometric fluorescence probe without antibody or aptamer for the first time. In the ratiometric fluorescence system, the fluorescence emission of AFB1 at 433 nm was significantly enhanced due to the β-cyclodextrin-AFB1 host-guest interaction and the complexation of AFB1 and Pt²⁺. Meanwhile, the inclusion of aflatoxin B1 also quenched the fluorescence emission of β-CD@Cu nanoparticles (NPs) at 650 nm based on inner filter effect mechanism. On the basis of the above effects, the ratiometric detection of aflatoxin B1 was achieved in the range of 0.03-10 ng/mL with a low detection limit of 0.012 ng/mL (3σ/s). In addition, the β-CD@Cu NPs based nanoprobe could achieve stable response within 1 min to AFB1. The above ratiometric detection also demonstrated excellent application potential in the rapid on-site detection of AFB1 in food due to the advantages of convenience, rapidness, and high accuracy.

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.

Cucurbit[n]uril Supramolecular Assemblies-Regulated Charge Transfer for Luminescence Switching of Gold Nanoclusters

Host-guest molecular assemblies are highly desirable for precisely controlling the luminescence properties of nanomaterials. Unfortunately, the design of high-quality luminescent nanoswitches is still very challenging due to the low affinity of traditional macrocyclic molecules (e.g., cyclodextrin) and inherently sophisticated electronic structures of nanoemitters. The current work represents the first to fabricate a luminescent nanoswitch using cucurbit[n]uril supramolecular assemblies-regulated luminescence of gold nanoclusters (AuNCs). It is found that, similar to a small-molecule fluorophore-based system, the luminescence of fabricated AuNC-cationic quencher nanohybrids can be reversibly manipulated by cucurbit[7]uril through altering the key parameters of the charge transfer process including the reorganization energy and electronic coupling between charge-transfer reactants. This study demonstrates the crucial role of cucurbit[n]uril host-guest assemblies in modulating the luminescence of AuNCs and their application in luminescence switching, thus offering new avenues for the fabrication and development of optical devices and smart materials.

Multifunctional Host Polymers Assist Au Nanoclusters Achieve High Quantum Yield and Mitochondrial Imaging

The high biocompatibility and excellent photostability of Au nanoclusters (AuNCs) make them stand out in the bioimaging of nanoparticles. However, the low quantum yield and inferior targeting ability of water-soluble AuNCs greatly limit their biological applications. In this study, we designed and synthesized multifunctional host polymers PolySC₄AP and FGGC@AuNCs to fabricate PolySC₄AP/FGGC@AuNC assemblies via a host-guest interaction based on SC₄ (sulfonatocalix[4]arene) and positively charged FGGC ligands (phenylalanine-glycine-glycine-cysteine). Owing to the host-guest assembly strategy and rigid polymer matrix, the quantum yield of FGGC@AuNCs was significantly promoted from 7.0 to 35.3%, accompanied by considerable morphological changes of FGGC@AuNCs. Moreover, PolySC₄AP/FGGC@AuNCs could monitor the location of mitochondria along with R (Pearson's correlation coefficients) value for the co-localization as high as 0.9605, which provided a novel strategy for targeted bioimaging with luminophore.

Bright Future of Gold Nanoclusters in Theranostics

Quantum-sized gold nanoclusters (AuNCs) are emerging as theranostic agents-those that combine diagnostics and therapeutic properties-given their ultrasmall size <3 nm, which makes them behave more like a molecule rather than a nanoparticle. This molecule-like behavior endows AuNCs with interesting properties including photoluminescence, catalytic activity, and paramagnetism-all without the presence of any toxic heavy metal. But despite these fundamental advances, scalable synthetic approaches to produce high-quality AuNCs with well-controlled and programmable properties for biological applications as well as methods to determine their structure-property relationships are not widely available. In this Perspective, we will discuss what is known so far about AuNCs as well as how to move forward to propel AuNCs as a theranostic agent of choice for many biomedical applications.

Charge-reversal biodegradable MSNs for tumor synergetic chemo/photothermal and visualized therapy

Given the difficulties of biodegradation of mesoporous silica nanoparticles (NPs), enrichment and penetration of tumor sites, and real-time monitoring of the treatment process, we developed a kind of mannose-doping doxorubicin-loading mesoporous silica nanoparticle (MSN-Man-DOX) and coated by polydopamine-Gd³⁺ (PDAGd) metal-phenolic networks, as well as modified by poly (2-Ethyl-2-Oxazoline) (PEOz), constructing a novel nanomedicine MSN-Man-DOX@PDA-Gd-PEOz. Its pH-responsive charge reversal, photothermal, biodegradation, drug release, and magnetic resonance imaging (MRI) properties were evaluated in vitro. Cellular uptake, tumor penetration, lysosomal escape properties, as well as cell safety and toxicity of the nanoplatform were investigated through cell experiments. Finally, the MRI, organ distribution, photothermal condition, and comprehensive anti-tumor therapy in vivo were evaluated comprehensively through animal experiments. Research results showed that MSN-Man-DOX@PDA-Gd-PEOz had outstanding tumor enrichment and penetration abilities, which can produce excellent treatment effects through the synergistic effect of chemotherapy and photothermal therapy (PTT) with the function of magnetic resonance imaging contrast agent for disease monitoring. Besides, after finishing the therapeutic effect MSN-Man-DOX@PDA-Gd-PEOz can be biodegraded, so it had a good prospect of clinical application.

Antifouling fluoropolymer-coated nanomaterials for 19F MRI

We developed a multifunctional polymer coating for nanoparticles (NPs) that enables simultaneous detection by 19F MRI and shielding from blood plasma fouling. The coating is based on a water-soluble fluorinated poly(N-(2-fluoroethyl)acrylamide) (PFEAM) that shows high 19F MRI sensitivity, cytocompatibility and excellent antifouling properties, significantly outperforming polyethylene glycol. A proof-of-concept experiment was performed by synthesizing polymer-coated gold NPs that were successfully visualized by 19F MRI at magnetic fields close to the fields used in clinical practice. This universal approach can be used for coating and tracing of various NPs upon suitable polymer chain-end modification.

pH and Redox Dual-Response Disulfide Bond-Functionalized Red-Emitting Gold Nanoclusters for Monitoring the Contamination of Organophosphorus Pesticides in Foods

Most of the fluorescence sensors require choline oxidase or quenchers to detect organophosphorus pesticides (OPs) based on a single hydrolysate and suffer from high cost, complex procedures, weak stability, and low sensitivity. Here, we proposed a brand-new fluorescence strategy for highly sensitive detection of OPs based on both hydrolysate-response disulfide bond-functionalized gold nanoclusters (S-S-AuNCs) without additional substances. S-S-AuNCs were synthesized via a facile one-step redox reaction and emitted bright red light with ultrasmall size and high water dispersion. Interestingly, S-S-AuNCs displayed a unique response to thiol compounds and low pH values and were thus pioneered as a high-efficiency sensor for OPs based on acetylcholinesterase (AChE)-catalyzed hydrolysis of acetylthiocholine into thiocholine and CH₃COOH and OP inhibition of AChE activity. Further, S-S-AuNCs were employed to monitor the residue, distribution, and metabolization of methidathion in pakchoi with acceptable results. We believe that this work supplies a simpler and more highly sensitive approach for OP assay than the known ones and opens a new avenue to development of multistimulus-responsive and high-performance fluorescence substances.

Interactions of core cross-linked poly(2-oxazoline) and poly(2-oxazine) micelles with immune cells in human blood

Water-soluble poly(cyclic imino ether)s (PCIEs) have emerged as promising biocompatible polymers for nanomedicine applications in recent years. Despite their generally accepted stealth properties, there has been no comprehensive evaluation of their interactions with primary immune cells in human blood. Here we present a library of core cross-linked micelles (CCMs) containing various PCIE shells. Well-defined high molar mass CCMs (M_n > 175 kDa, Р< 1.2) of similar diameter (~20 nm) were synthesised using a cationic ring-opening polymerisation (CROP) - surfactant-free reversible addition-fragmentation chain-transfer (RAFT) emulsion polymerisation strategy. The stealth properties of the different PCIE CCMs were assessed employing a whole human blood assay simulating the complex blood environment. Cell association studies revealed lower associations of poly(2-methyl-2-oxazoline) (PMeOx) and poly(2-ethyl-2-oxazoline) (PEtOx) CCMs with blood immune cells compared to the respective poly(2-oxazine) (POz) CCMs. Noteworthy, PMeOx CCMs outperformed all other reported CCMs, showing overall low associations and only negligible differences in the presence and absence of serum proteins. This study highlights the importance of investigating individual nanomaterials under physiologically relevant conditions and further strengthens the position of PMeOx as a highly promising stealth material for biomedical applications.