From synthesis to applications of biomolecule-protected luminescent gold nanoclusters

Gold nanoclusters (AuNCs) are a class of novel luminescent nanomaterials that exhibit unique properties of ultra-small size, featuring strong anti-photo-bleaching ability, substantial Stokes shift, good biocompatibility, and low toxicity. Various biomolecules have been developed as templates or ligands to protect AuNCs with enhanced stability and luminescent properties for biomedical applications. In this review, the synthesis of AuNCs based on biomolecules including amino acids, peptides, proteins and DNA are summarized. Owing to the advantages of biomolecule-protected AuNCs, they have been employed extensively for diverse applications. The biological applications, particularly in bioimaging, biosensing, disease therapy and biocatalysis have been described in detail herein. Finally, current challenges and future potential prospects of bio-templated AuNCs in biological research are briefly discussed.

Gold nanocluster-based fluorescent sensors for in vitro and in vivo ratiometric imaging of biomolecules

Gold nanoclusters (AuNCs) are promising nanomaterials for ratiometric fluorescent probes due to their tunable fluorescence wavelengths dependent on size and structure, as well as their biocompatibility and resistance to photobleaching. By incorporating an additional fluorescence spectral peak, dual-emission AuNC-based fluorescent probes have been developed to enhance the signal output reproducibility. These probes can be fabricated by integrating various luminescent nanomaterials with AuNCs. This review focuses on the preparation methods and applications of ratiometric fluorescent probes derived from AuNCs and other fluorescent nanomaterials or fluorescent dyes for both in vitro and in vivo bioimaging of target analytes. Additionally, the review delves into the sensing mechanisms of AuNC-based ratiometric probes, their synthetic strategies, and the challenges encountered when using AuNCs for ratiometric bioimaging. Moreover, we explore the application of protein-stabilized AuNCs and thiolate-capped AuNC-based ratiometric fluorescent probes for biosensing and bioimaging. Two primary methods for assembling AuNCs and fluorophores into ratiometric fluorescent probes are discussed: triggered assembly and self-assembly. Finally, we address the challenges and issues associated with ratiometric bioimaging using AuNCs and propose future directions for further advancing AuNCs as ratiometric imaging agents.

Protein-Templated Metal Nanoclusters: Molecular-like Hybrids for Biosensing, Diagnostics and Pharmaceutics

The use of proteins as biomolecular templates to synthesize atomically precise metal nanoclusters has been gaining traction due to their appealing properties such as photoluminescence, good colloidal- and photostability and biocompatibility. The synergistic effect of using a protein scaffold and metal nanoclusters makes it especially attractive for biomedical applications. Unlike other reviews, we focus on proteins in general as the protective ligand for various metal nanoclusters and highlight their applications in the biomedical field. We first introduce the approaches and underlined principles in synthesizing protein-templated metal nanoclusters and summarize some of the typical proteins that have been used thus far. Afterwards, we highlight the key physicochemical properties and the characterization techniques commonly used for the size, structure and optical properties of protein-templated metal nanoclusters. We feature two case studies to illustrate the importance of combining these characterization techniques to elucidate the formation process of protein-templated metal nanoclusters. Lastly, we highlight the promising applications of protein-templated metal nanoclusters in three areas-biosensing, diagnostics and therapeutics.

Synthesis of gold nanoclusters-loaded lysozyme nanoparticles for ratiometric fluorescent detection of cyanide in tap water, cyanogenic glycoside-containing plants, and soils

The modification of protein-stabilized gold nanoclusters with fluorophores has been intensively applied for the ratiometric detection of biomolecules, metal ions, and anions. This study developed a straightforward strategy to prepare lysozyme nanoparticle-encapsulated gold nanoclusters (LysNP-AuNCs) as a dual-emission probe for the ratiometric sensing of cyanide through fluorescence resonance energy transfer (FRET) without the conjugation of additional fluorophores. The reduction of gold ion precursors with lysozyme generated lysozyme-stabilized AuNCs under an alkaline pH, which were demonstrated to self-assemble into nanoaggregates during the formation of AuNCs. The aggregated lysozyme molecules on the AuNCs were treated with glutaraldehyde, triggering the conversion of the aggregated lysozymes into blue-emitting lysozyme nanoparticles. As a result, the AuNCs were well distributed inside a single lysozyme nanoparticle, as demonstrated by transmission electron microscopy. The presence of cyanide triggered the etching of the AuNCs in the LysNP-AuNCs, leading to the suppression of FRET from lysozyme nanoparticle to AuNCs. The LysNP-AuNC probe was implemented for FRET detection of cyanide with a linear range of 3-100 μM. Additionally, the selectivity of the LysNP-AuNC probe for cyanide toward other anions was remarkably high. The practicality of the proposed probe was evaluated by quantifying cyanide in tap water and soils and monitoring the liberation of hydrogen cyanide from cyanogenic glycoside-containing foods.

Charge Neutralization Strategy to Construct Salt-Tolerant and Cell-Permeable Nanoprobes: Application in Ratiometric Sensing and Imaging of Intracellular pH

Intracellular pH homeostasis is essential for the survival and function of biological cells. Negatively charged molecular probes, such as pyranine (HPTS), tend to exhibit poor salt tolerance and unsatisfactory cell permeability, limiting their widespread use in intracellular assays. Herein, we explored a charge neutralization strategy using multicharged cationic nanocarriers for an efficient and stable assembly with the pH-sensitive HPTS. Through immobilization and neutralization with poly(allylamine hydrochloride)-stabilized red-emitting gold nanoclusters (PAH-AuNCs), the resulting nanoprobes (HPTS-PAH-AuNCs) offered improved salt tolerance, satisfactory cell permeability, and dual-emission properties. The fluorescence ratio exhibited a linear response over the pH range of 3.0-9.0. Moreover, the proposed HPTS-PAH-AuNCs were successfully applied to determine and visualize lysosomal pH variations in living cells, which indicated great potential for biosensing and bioimaging applications in living systems. Benefiting from the charge neutralization strategy, various types of probes can be expected to achieve broader analytical applications.

A self-healing carboxymethyl chitosan/oxidized carboxymethyl cellulose hydrogel with fluorescent bioprobes for glucose detection

Self-healing hydrogel as a soft material with high durability and life-time has been successfully applied in various fields, including electronic skins, wearable electronic devices, and soft sensors. However, it is still a challenge to design a hydrogel with rapid self-healing, biodegradable and biosensing properties. Here, a self-healing carboxymethyl chitosan (CMCS)/oxidized carboxymethyl cellulose (OCMC) hydrogel with fluorescent bioprobes was developed for glucose detection. In this biosensing system, gold nanoclusters (AuNCs) and glucose oxidase (GOx) were encapsulated into the CMCS/OCMC hydrogel matrix as the fluorescent bioprobes. The CMCS/OCMC hydrogel with fluorescent bioprobes exhibited high sensitivity for glucose sensing with a linearly detection range of 100 μM to 5 mM and a detection limit of 0.029 mM, which covered the level of glucose in clinical detection. Furthermore, this hydrogel exhibited good biocompatibility. Finally, In vitro blood fluorescence tests and in vivo fluorescence investigation of the AuNCs-CMCS/OCMC hydrogel in diabetic mice indicated that this biocompatible and self-healing hydrogel based on fluorescent sensing system had potential application in implantable biosensing area for glucose monitoring.

Influence of Fe(III) on the Fluorescence of Lysozyme: a Facile and Direct Method for Sensitive and Selective Sensing of Fe(III)

Lysozyme is widely used for the synthesis of nanomaterials (e.g., gold nanoparticle) to fluorescently sense metal ions. However, the effect of metal ions on the fluorescence of lysozyme is not studied yet. Herein, we have explored the interactions of lysozyme with different metal ions to develop a direct sensing platform for Fe(III). It has been observed that the fluorescence of lysozyme was slightly decreased in the presence of Cu(II), Hg(II), As(V), Co(II), Cd(II), Cr(II), Fe(II), Mn(II), Pb(II), and Zn(II), while a significant decrease in the lysozyme fluorescence was observed for Fe(III). The effect of thermal stability on the fluorescence quenching was also studied from 25 to 60 °C. In the present study, the lysozyme sensing probe was able to selectively and accurately detect 0.5-50 ppm of Fe(III) with a LOD of 0.1 ppm (1.8 µM) at 25 °C.

Effect of a Good buffer on the fate of metastable protein-rich droplets near physiological composition

Metastable protein-rich microdroplets are produced from liquid-liquid phase separation (LLPS) of protein aqueous solutions. These globules can be intermediates for the formation of other protein-rich phases. Lysozyme aqueous solutions undergo LLPS around 0 °C in the presence of NaCl near physiological conditions. Here, it is shown that insertion of small amounts of 4-(2-hydroxyethyl)-1-piperazineethanesulfonate (HEPES, 0.1 M) as a second additive to lysozyme-NaCl-water solutions near physiological ionic strength (0.2 M) is an essential step for triggering conversion of protein-rich droplets into another phase. Specifically, LLPS induced by cooling reproducibly leads to a rapid and high-yield formation of compact tetragonal crystalline microparticles only in the presence of HEPES. These microcrystals exhibit small size (1-3 μm), narrow size distribution and guest-binding properties. The temperature-concentration phase diagram shows a characteristic topology with LLPS boundary metastable with respect to tetragonal microcrystals, which in turn become less stable than rod-shaped orthorhombic crystals above 40 °C. Interestingly, dynamic light scattering, hydrogen-ion titrations and isothermal titration calorimetry reveal that lysozyme-HEPES interactions were found to be weakly attractive and exothermic. Our findings indicate that additives of salting-in type can represent an important factor controlling the fate of metastable protein-rich microdroplets relevant to drug formulations, femtosecond crystallography, and potential implications in protein-driven cytoplasmic compartmentalization.

Synthesis of Exosome-Based Fluorescent Gold Nanoclusters for Cellular Imaging Applications

In recent years, fluorescent metal nanoclusters have been used to develop bioimaging and sensing technology. Notably, protein-templated fluorescent gold nanoclusters (AuNCs) are attracting interest due to their excellent fluorescence properties and biocompatibility. Herein, we used an exosome template to synthesize AuNCs in an eco-friendly manner that required neither harsh conditions nor toxic chemicals. Specifically, we used a neutral (pH 7) and alkaline (pH 11.5) pH to synthesize two different exosome-based AuNCs (exo-AuNCs) with independent blue and red emission. Using field-emission scanning electron microscopy, energy dispersive X-ray microanalysis, nanoparticle tracking analysis, and X-ray photoelectron spectroscopy, we demonstrated that AuNCs were successfully formed in the exosomes. Red-emitting exo-AuNCs were found to have a larger Stokes shift and a stronger fluorescence intensity than the blue-emitting exo-AuNCs. Both exo-AuNCs were compatible with MCF-7 (human breast cancer), HeLa (human cervical cancer), and HT29 (human colon cancer) cells, although blue-emitting exo-AuNCs were cytotoxic at high concentrations (≥5 mg/mL). Red-emitting exo-AuNCs successfully stained the nucleus and were compatible with membrane-staining dyes. This is the first study to use exosomes to synthesize fluorescent nanomaterials for cellular imaging applications. As exosomes are naturally produced via secretion from almost all types of cell, the proposed method could serve as a strategy for low-cost production of versatile nanomaterials.

Facile preparation of FITC-modified silicon nanodots for ratiometric pH sensing and imaging

A ratiometric fluorescent pH sensor was facilely constructed by covalent modification of amino-terminated silicon nanodots (SiND) with pH-sensitive fluorescein isothiocyanate (FITC). After optimization, the SiND-FITC(40:1) material with a SiND:FITC initial mass ratio of 40:1 was selected for the sensing of hydrogen ions. It was observed that the material inherits the unique features of SiND and FITC, and there is significant improvement of SiND acid-base stability, which is a favorable factor in terms of providing fluorescence reference signal. The SiND-FITC(40:1) material displays not only high pH sensitivity, but also good stability and anti-interference ability, and the response process is highly reversible. Deploying the SiND-FITC(40:1) material, we have made available a simple, sensitive, and precise approach for pH sensing. In aqueous solutions, the I₅₁₇/I₄₆₆ fluorescence intensity ratio of SiND-FITC(40:1) increases linearly in the pH range of 5.40-7.76. This dual emission nanosensor was successfully applied for pH sensing and cellular fluorescence imaging.

Rational Design of "Three-in-One" Ratiometric Nanoprobes: Protein-Caged Dityrosine, CdS Quantum Dots, and Gold Nanoclusters

Recently, multiplexed ratiometric fluorescence sensors for detecting several analytes have received much interest because of their multifunctionality. Here, we fabricate a novel trinity fluorescent nanoprobe in which one small-molecule fluorophore, blue-emissive dityrosine (diTyr) residues, and two nanomaterial fluorophores, green-emissive CdS quantum dots (CdSQDs) and red-emissive gold nanoclusters (AuNCs), are cocaged in a bovine serum albumin (BSA) molecule. The large differences of Stokes shifts among diTyr residues, CdSQDs, and AuNCs ensure their emission at a single excitation wavelength. The nanoprobes can be facilely integrated using two-step synthetic reactions. DiTyr residues and AuNCs are formed and bound to the protein cage through the redox reaction between Au3+ and tyrosine residues of BSA, and the CdSQDs are followed to be conjugated to the modified BSA cage-templated CdS combination reaction. With established benign biocompatibility, the nanoprobes can ratiometrically detect intracellular glutathione by significantly enhancing the green emission of the conjugated CdSQDs. Likewise, the ratiometric sensing of solution alkalinity and tris(2-carboxyethyl)phosphine can be achieved using blue-emitted diTyr residues and red-emitted AuNCs as the responsive units, respectively, and the corresponding other two fluorophores as the reference signals. This study addresses a concept of trinity fluorescence ratiometric sensing system with multiple targets and optional references, which should be a promising pathway to meet the challenges from complexing biochemical environments and multivariate analysis.

Red fluorescent AuNDs with conjugation of cholera toxin subunit B (CTB) for extended-distance retro-nerve transporting and long-time neural tracing

A retrograde transportation nerve probe, Au nanodots-cholera toxin B subunit (AuNDs-CTB), are prepared and fully characterized, which emit bright red fluorescence and show high quantum yield (7.2%) and good stability. The fluorescence emitted by the AuNDs is constant across a wide pH range (4-10) and after prolonged UV irradiation (>4 h). Previously, CTB has shown targeting characteristic for nerve cells with high sensitivity and effectiveness. After linking CTB to AuNDs through amidation reactions, AuNDs-CTB are obtained with excellent fluorescence property, nerve target characteristic, and, particularly, neural retrograde transportation feature. The red emission of the AuNDs-CTB is well distinguished from the blue autofluorescence of normal tissues, which provides potential for detection by naked eyes. Further, the fluorescence emission intensity maintains for 10 days in vivo, suggesting great utility for long-time monitoring and sensing of the nerve tissue. Furthermore, the AuNDs-CTB with bright red fluorescence can travel through the peripheral nerve to the spinal cord rapidly by retrograde transportation. The transportation occurs for a long distance (>5 cm) within only 2 days after injection of the AuNDs-CTB into the sciatic nerve. The present study exhibits a novel method for nerve visualization and drug delivery. STATEMENT OF SIGNIFICANCE: Au nanodots (AuNDs) conjugated with cholera toxin subunit B (CTB) have been developed for nerve labeling and neural retro-transporting. The red fluorescence from AuNDs-CTB is stable in vitro (pH 4-10 and 4 h UV irradiation) and in vivo (for a long time, more than 10 days). When injecting AuNDs-CTB into the sciatic nerve located at the midpiece of the thigh, the targeted nerve emits bright red fluorescence under UV light. Furthermore, the nerve can retrograde transport the AuNDs-CTB to the spinal cord for a distance of more than 5 cm just in 2 days. This work exhibits a novel method for nerve visualization by naked eyes and demonstrates the potential for intraoperative navigation.