Conformational Change-Induced Fluorescence of Bovine Serum Albumin–Gold Complexes

Impact of BSA and Au3+ concentration on the formation and fluorescence properties of Au nanoclusters

Bovine serum albumin-stabilized Au nanoclusters (BSA-Au NCs) have emerged as promising contenders for imaging agents and highly sensitive fluorescence sensors due to their biocompatibility and strong photoluminescence. Optimizing the synthesis conditions of BSA-Au NCs is crucial for enhancing fluorescence imaging and other nanocluster applications. In this study, for the first time, we systematically investigated the effects of BSA concentration and Au3+ on both particle size and optical characteristics of BSA-Au NCs. When the two components achieved a suitable concentration ratio, it was beneficial to form BSA-Au NCs with a high quantum yield (QY = 74.30%) and good fluorescence stability. In contrast, an inappropriate concentration ratio would lead to the formation of gold nanoparticles (Au NPs), and their internal filtration effect (IFE) would attenuate the fluorescence emission of BSA-Au NCs. The BSA-Au NCs were then employed as efficient fluorescence sensors for detecting Hg2+. Furthermore, the growth mechanism of BSA-Au NCs was elucidated by monitoring fluorescence changes during different incubation times. The BSA-Au NCs with a high quantum yield introduce a novel synthetic concept for sensitive fluorescent probes and expanding versatile applications of BSA-Au NCs in catalysis, chemical sensing and biomedicine.

A ratiometric fluorescent "off-on" sensor for acrylamide detection in toast based on red-emitting copper nanoclusters stabilized by bovine serum albumin

Acrylamide, as a Class 2A carcinogen, poses serious threats to human health. To achieve rapid and accurate determination of acrylamide in food, a ratiometric fluorescent "off-on" sensor was designed by incorporating red-emitting copper nanoclusters and glutathione. Copper nanoclusters with bimodal emission at 395 nm and 650 nm (excited at 310 nm) were synthesized by using bovine serum albumin as the ligand and ascorbic acid as the reductant. With glutathione addition, the fluorescence intensity at 650 nm gradually decreased, while the case at 395 nm slightly increased. The quenched fluorescence at 650 nm was subsequently restored by acrylamide through thiol-ene Michael addition reaction between acrylamide and glutathione. The constructed sensor showed excellent performance towards acrylamide detection in the range of 5-300 μM with a detection limit of 1.48 μM, and was further applied to real-sample detection of acrylamide in toast and exhibited good recoveries (90.29-101.30 %), indicating potential applications of this sensor.

Synthesis of Au Nanoclusters by Reduction of Bovine Serum Albumin: The Role of Sodium Hydroxide

Using bovine serum albumin (BSA) as both a reductant and ligand had been developed as one of the most used approaches for synthesis of fluorescent Au nanoclusters (NCs), in which first HAuCl₄ and BSA were mixed together and then NaOH was added to the mixture after a certain time to obtain the Au NCs. In this work, the role of sodium hydroxide in the formation and emission properties of the Au NCs was investigated systematically. It was revealed, for the first time, that activity of the gold precursor and, thus, emission properties of the resulting Au NCs are dependent upon the addition time of sodium hydroxide. Meanwhile, the reducing ability of BSA is dependent upon the concentration of sodium hydroxide added to the reaction solution. By optimization of the addition time and concentration of sodium hydroxide used, Au NCs with improved emission properties were successfully synthesized under relatively low BSA concentrations, which showed improved performance toward the sensing of Cu²⁺ ions.

Protein-Directed Au(0)-Rich Gold Nanoclusters as Ratiometric Luminescence Sensors for Auric Ions via Comproportionation-Induced Emission Enhancement

Gold nanoclusters (Au NCs) are widely used as fluorescent probes in biomedical sensing and imaging due to their versatile optical properties and low cytotoxicity. Surface engineering of gold nanoclusters (Au NCs) aims to design a surface with versatile physicochemical performances, but previous investigations have primarily focused on the acquisition of the "brightest" species. This has resulted in other types of Au NC being neglected. In the present study, our group prepared a series of Au NCs that were rich in surface Au(0), using the "aged" form of bovine serum albumin (BSA) via controlling the pH during synthesis. We found that slight increases of alkalinity during synthesis over that which produced Au NCs with the most intensive photoluminescence generated the "darkest" Au NCs, which exhibited the strongest absorption. These Au NCs included more Au atoms and had a higher Au(0) content. Furthermore, the addition of Au³⁺ quenched the emission of the "brightest" Au NCs, but increased that of the "darkest" Au NCs. The increased Au(I) proportion observed in the Au³⁺-treated "darkest" Au NCs resulted in a novel comproportionation-induced emission enhancement effect, which we utilized to construct a "turn-on" ratiometric sensor for toxic Au³⁺. The addition of Au³⁺ generated simultaneous, opposite effects on blue-emissive diTyr BSA residues and red-emissive Au NCs. After optimization, we successfully constructed ratiometric sensors for Au³⁺ with high sensitivity, selectivity, and accuracy. This study will inspire a new pathway to redesign the protein-framed Au NCs and analytical methodology via comproportionation chemistry.

Correlation between interfacial layer properties and physical stability of food emulsions: current trends, challenges, strategies, and further perspectives

Emulsions are thermodynamically unstable systems that tend to separate into two immiscible phases over time. The interfacial layer formed by the emulsifiers adsorbed at the oil-water interface plays an important role in the emulsion stability. The interfacial layer properties of emulsion droplets have been considered the cutting-in points that influence emulsion stability, a traditional motif of physical chemistry and colloid chemistry of particular significance in relation to the food science and technology sector. Although many attempts have shown that high interfacial viscoelasticity may contribute to long-term emulsion stability, a universal relationship for all cases between the interfacial layer features at the microscopic scale and the bulk physical stability of the emulsion at the macroscopic scale remains to be established. Not only that, but integrating the cognition from different scales of emulsions and establishing a unified single model to fill the gap in awareness between scales also remain challenging. In this review, we present a comprehensive overview of recent progress in the general science of emulsion stability with a peculiar focus on interfacial layer characteristics in relation to the formation and stabilization of food emulsions, where the natural origin and edible safety of emulsifiers and stabilizers are highly requested. This review begins with a general overview of the construction and destruction of interfacial layers in emulsions to highlight the most important physicochemical characteristics of interfacial layers (formation kinetics, surface load, interactions among adsorbed emulsifiers, thickness and structure, and shear and dilatational rheology), and their roles in controlling emulsion stability. Subsequently, the structural effects of a series of typically dietary emulsifiers (small-molecule surfactants,proteins, polysaccharides, protein-polysaccharide complexes, and particles) on oil-water interfaces in food emulsions are emphasized. Finally, the main protocols developed for modifying the structural characteristics of adsorbed emulsifiers at multiple scales and improving the stability of emulsions are highlighted. Overall, this paper aims to comprehensively study the literature findings in the past decade and find out the commonality of multi-scale structures of emulsifiers, so as to deeply understand the common characteristics and emulsification stability behaviour of adsorption emulsifiers with different interfacial layer structures. It is difficult to say that there has been significant progress in the underlying principles and technologies in the general science of emulsion stability over the last decade or two. However, the correlation between interfacial layer properties and physical stability of food emulsions promotes revealing the role of interfacial rheological properties in emulsion stability, providing guidance on controlling the bulk properties by tuning the interfacial layer functionality.

Valence-State-Engineered Electrochemiluminescence from Au Nanoclusters

To determine the intrinsic effects of body elements on the electrochemiluminescence (ECL) of metal nanoclusters (NCs), herein, a valence-state engineering strategy is developed to adjust the NCs' ECL with bovine serum albumin (BSA)-stabilized AuNCs as a model, in which engineering the valence state of the Au body element, i.e., Au(0) and Au(I), is performed via successively reducing the precursor AuCl₄^- to Au(I) and Au(0) with BSA. The obtained BSA-AuNCs/N₂H₄ system leads to three anodic ECL processes at 0.37 (ECL-1), 0.85 (ECL-2), and 1.45 V (ECL-3). ECL-1 is generated from the BSA-Au(0) section of BSA-AuNCs in a surface-defect-involved route and is much stronger and red-shifted compared to ECL-2 and ECL-3, which are generated from the BSA-Au(I) section of BSA-AuNCs in the band-gap-engineered route. Each of the anodic ECL processes can be selectively generated and/or suppressed via adjusting the Au(I)/Au(0) ratio of BSA-AuNCs, tunable ECL generation route, and triggering potential, and the emission intensity and waveband of metal NCs are conveniently achieved in body-element-involved valence-state engineering.

Microwave-Assisted Synthesis of Red Emitting Copper Nanoclusters Using Trypsin as a Ligand for Sensing of Pb2+ And Hg2+ Ions in Water and Tobacco Samples

In this work, a microwave assisted method was developed for synthesis of red fluorescent copper nanoclusters (NCs) using trypsin as a template (trypsin-Cu). The as-synthesized trypsin-Cu NCs are stable and water soluble, exhibiting fluorescence emission at 657 nm when excited at 490 nm. The as-prepared red-emitting trypsin-Cu NCs were characterized by using several analytical techniques such as ultraviolet-visible (UV-Vis) and fluorescence, fluorescence lifetime, Fourier transform infrared, and X-ray photoelectron spectroscopic techniques. Red-emitting trypsin-Cu NCs acted as a nanosensor for sensing both Pb2+ and Hg2+ ions through fluorescence quenching. Using this approach, good linearities are observed in the range of 0.1-25 and of 0.001-1 μM with the lower limit of detection of 14.63 and 56.81 nM for Pb2+ and Hg2+ ions, respectively. Trypsin-Cu NCs-based fluorescence assay was successfully applied to detect both Hg2+ and Pb2+ ions in water and tobacco samples.

Fabrication of avidin-stabilized gold nanoclusters with dual emissions and their application in biosensing

Protein-stabilized gold nanoclusters (Prot-Au NCs) have been widely used in biosensing and cell imaging owing to their excellent optical properties and low biotoxicity. However, several Prot-Au NCs reported in the literature do not retain the biological role of the protein, which greatly limits their ability to directly detect biomarkers. This study demonstrated for the first time the successful synthesis of dual-function avidin-stabilized gold nanoclusters (Av–Au NCs) using a one-pot method. The resulting Av–Au NCs exhibited intense blue and red emissions under 374 nm excitation. Furthermore, the Av–Au NCs retained the native functionality of avidin to bind to biotin. When DNA strands modified with biotin at both ends (i.e., linker chains) were mixed with Av–Au NCs, large polymers were formed, indicating that Av–Au NCs could achieve fluorescence signal amplification by interacting with biotin. Taking advantage of the aforementioned properties, we constructed a novel enzyme-free fluorescent biosensor based on the Av–Au NCs-biotin system to detect DNA. The designed fluorescent biosensor could detect target DNA down to 0.043 nM, with a wide line range from 0.2 nM to 20 µM. Thus, these dual-functional Av–Au NCs were shown to be an excellent fluorescent material for biosensing.

Avidin-stabilized gold nanoclusters (Av–Au NCs) were synthesized for the first time by a water-bath method.

The synthesized Av–Au NCs not only exhibited intense blue and red emissions under 374 nm excitation, but also retained the native functionality of avidin to bind to biotin.

The fluorescent signal amplification system constructed by the interaction of Av–Au NCs with biotin was successfully applied to detect target DNA in vitro.

Evidence of Au(II) and Au(0) States in Bovine Serum Albumin-Au Nanoclusters Revealed by CW-EPR/LEPR and Peculiarities in HR-TEM/STEM Imaging

Bovine serum albumin-embedded Au nanoclusters (BSA-AuNCs) are thoroughly probed by continuous wave electron paramagnetic resonance (CW-EPR), light-induced EPR (LEPR), and sequences of microscopic investigations performed via high-resolution transmission electron microscopy (HR-TEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray analysis (EDS). To the best of our knowledge, this is the first report analyzing the BSA-AuNCs by CW-EPR/LEPR technique. Besides the presence of Au(0) and Au(I) oxidation states in BSA-AuNCs, the authors observe a significant amount of Au(II), which may result from a disproportionation event occurring within NCs: 2Au(I) → Au(II) + Au(0). Based on the LEPR experiments, and by comparing the behavior of BSA versus BSA-AuNCs under UV light irradiation (at 325 nm) during light off-on-off cycles, any energy and/or charge transfer event occurring between BSA and AuNCs during photoexcitation can be excluded. According to CW-EPR results, the Au nano assemblies within BSA-AuNCs are estimated to contain 6–8 Au units per fluorescent cluster. Direct observation of BSA-AuNCs by STEM and HR-TEM techniques confirms the presence of such diameters of gold nanoclusters in BSA-AuNCs. Moreover, in situ formation and migration of Au nanostructures are observed and evidenced after application of either a focused electron beam from HR-TEM, or an X-ray from EDS experiments.

Advancement of Fluorescent and Structural Properties of Bovine Serum Albumin-Gold Bioconjugates in Normal and Heavy Water with pH Conditioning and Ageing

The red-emitting fluorescent properties of bovine serum albumin (BSA)–gold conjugates are commonly attributed to gold nanoclusters formed by metallic and ionized gold atoms, stabilized by the protein. Others argue that red fluorescence originates from gold cation–protein complexes instead, not gold nanoclusters. Our fluorescence and infrared spectroscopy, neutron, and X-ray small-angle scattering measurements show that the fluorescence and structural behavior of BSA–Au conjugates are different in normal and heavy water, strengthening the argument for the existence of loose ionic gold–protein complexes. The quantum yield for red-emitting luminescence is higher in heavy water (3.5%) than normal water (2.4%), emphasizing the impact of hydration effects. Changes in red luminescence are associated with the perturbations of BSA conformations and alterations to interatomic gold–sulfur and gold–oxygen interactions. The relative alignment of domains I and II, II and III, III and IV of BSA, determined from small-angle scattering measurements, indicate a loose (“expanded-like”) structure at pH 12 (pD ~12); by contrast, at pH 7 (pD ~7), a more regular formation appears with an increased distance between the I and II domains, suggesting the localization of gold atoms in these regions.

Component-optimized chemo-dynamic nanoagent for enhanced tumour cell-selective chemo-dynamic therapy with minimal side effect in glioma mouse model

Although CuO-deposited bovine serum albumin (CuO-BSA) and glucose oxidase (Gox) were combined to achieve H2O2 self-supplied chemo-dynamic therapy (CDT) and glucose consumption-based starvation therapy, the uses of copper and Gox...

Photoluminescent Nanoparticles for Chemical and Biological Analysis and Imaging

Research related to the development and application of luminescent nanoparticles (LNPs) for chemical and biological analysis and imaging is flourishing. Novel materials and new applications continue to be reported after two decades of research. This review provides a comprehensive and heuristic overview of this field. It is targeted to both newcomers and experts who are interested in a critical assessment of LNP materials, their properties, strengths and weaknesses, and prospective applications. Numerous LNP materials are cataloged by fundamental descriptions of their chemical identities and physical morphology, quantitative photoluminescence (PL) properties, PL mechanisms, and surface chemistry. These materials include various semiconductor quantum dots, carbon nanotubes, graphene derivatives, carbon dots, nanodiamonds, luminescent metal nanoclusters, lanthanide-doped upconversion nanoparticles and downshifting nanoparticles, triplet-triplet annihilation nanoparticles, persistent-luminescence nanoparticles, conjugated polymer nanoparticles and semiconducting polymer dots, multi-nanoparticle assemblies, and doped and labeled nanoparticles, including but not limited to those based on polymers and silica. As an exercise in the critical assessment of LNP properties, these materials are ranked by several application-related functional criteria. Additional sections highlight recent examples of advances in chemical and biological analysis, point-of-care diagnostics, and cellular, tissue, and in vivo imaging and theranostics. These examples are drawn from the recent literature and organized by both LNP material and the particular properties that are leveraged to an advantage. Finally, a perspective on what comes next for the field is offered.

Versatile Gold-Coupled Te-Carbon Dots for Quantitative Monitoring and Efficient Scavenging of Superoxide Anions

The superoxide anion (O₂^•-) is a reactive oxygen species (ROS) that functions as an important regulator of signal transduction in living systems. However, excess O₂^•- can cause metabolic imbalances and oxidative damage inside cells. Quantitative detection and efficient scavenging of O₂^•- are therefore critical for maintaining intracellular redox balance and homeostasis. In this work, a nanomaterial (Au-TeCD) composed of BSA-modified gold nanoparticles (AuNPs) complexed with tellurium-containing carbon dots (TeCDs) was constructed. The introduction of Au-TeCDs to solutions containing superoxide resulted in enhanced elimination of the anion, indicating that Au-TeCDs are able to scavenge O₂^•- from the surrounding environment. Notably, the respective TeCD and AuNP components of the Au-TeCDs were found to emit fluorescence at 425 and 640 nm upon exposure to superoxide anions. This unique spectroscopic property of Au-TeCDs allowed levels of O₂^•- in solution to be quantified using dual-fluorescence detection. The Au-TeCDs developed herein also exhibited low-cytotoxicity, versatile capabilities for in situ fluorescence imaging, and effective scavenging of O₂^•- in living cells. Taken together, these results suggest that Au-TeCDs act as effective tools for monitoring superoxide concentrations in complex mixtures and may be developed as possible therapeutics designed to scavenge excess ROS from diseased cells.

Rapid synthesis of fluorescent bovine serum albumin-gold nanoclusters complex for glutathione determination

A facile one-pot method for synthesis of bovine serum albumin (BSA)-gold nanoclusters (AuNCs) has been developed. The formation of BSA-AuNCs took only 30 s under mild conditions. BSA-AuNCs exhibited strong orange-yellow fluorescence, and the excitation and emission peaks were at 370 nm and 564 nm, respectively. In the process of forming BSA-AuNCs, the molecular chain of BSA has not been destroyed. Moreover, there were a large number of Au cations on the surface of BSA-AuNCs, which had strong oxidizing abilities. The reason for the ultrabright fluorescence of BSA-AuNCs was attributed to the Au(0)@Au(I)@Au(III)-ligand structure on the surface of BSA. In order to evaluate the fluorescence performance of BSA-AuNCs, BSA-AuNCs was used as a probe, realizing the sensitive and selective determination of glutathione (GSH) in a wide linear range of 0.01-0.48 μM and a detection limit of 3.3 nM. The proposed method not only offers a brand-new scheme for synthesizing BSA-AuNCs, but also provides a platform for studying the interaction between metal core and proteins. A facile one-pot method to synthesize ultrabright fluorescent BSA-AuNCs in tens of seconds has been introduced by mixing BSA suspension, KSCN, and HAuCl₄. The as-prepared BSA-AuNCs showed intensive orange-yellow fluorescence under a UV lamp (365 nm)_, and BSA still keeps the integral molecular chains during the whole synthesis process. Moreover, the as-prepared BSA-AuNCs have realized the sensitive and selective detection of glutathione (GSH) in a wide linear range of 0.01-0.48 μM and a detection limit of 3.3 nM.

Common Motif at the Red Luminophore in Bovine Serum Albumin-, Ovalbumin-, Trypsin-, and Insulin-Gold Complexes

We examined the static and dynamic characters of the red luminescence in the protein-Au(III) compounds, directly comparing multiple proteins: BSA, OVA, trypsin, and insulin. These four protein-Au(III) complexes showed a nearly identical excitation-emission pattern, not only the wavelength of luminescence (λ_em ∼ 640 nm). Lifetimes of the red luminescence shared a common value of ∼300 ns. Kinetics of the luminophore formation was consistently described by a Langmuir-type chemisorption of Au(III) for these proteins, coinciding with the protein conformation change at pH ∼ 10. These observations and the protein structural analyses support that the red luminophore formation involves Au(III) coordination to a common motif within these proteins.

Limited Proteolysis and Gel Electrophoresis in the Presence of Metal Cations: Au(III)-binding Luminescent Domain in Serum Albumins

The purpose of the presented protocols is to determine the domain of Au(III) binding in BSA. The BSA-Au(III) compound exhibits ultraviolet (UV)-excitable red luminescence (λem = 640 nm), with unusual Stokes shifts compared to the innate UV/blue fluorescence arising from the aromatic residues. Red-luminescent complexes are formed in highly alkaline conditions above pH 10 and require a conformation change within the protein to occur. In addition, preservation of Cys-Cys disulfide bonds in BSA is necessary to obtain this red luminescence. In order to understand the mechanism of this luminescence, elucidation of the luminophore-forming Au(III) binding site is essential. A facile way to assess the luminophore-forming site would be to (1) predictably fragment the protein by enzymatic digestion, (2) react the obtained fragments with Au(III), then (3) perform gel electrophoresis to observe the well-separated fragment bands and analyze the in-gel red luminescence. However, due to the alkaline conditions and the reaction with metal cations, new limited proteolysis techniques and gel electrophoresis conditions must be applied. Particularly, the presence of metal cations in gel electrophoresis can make the band separations technically difficult. We describe this new protocol in steps to identify the red-luminophore-forming metal binding domain in BSA. This protocol can thus be applied for analyzing protein fragments that must remain in a non-denatured or a partially denatured state, in the presence of metal cations. Because the majority of proteins need metal cations to function, analyses of metal-bound proteins are often desired, which have relied on x-ray crystallography in the literature. This method, on the other hand, could be used in supplement to study the interactions of proteins with metal cations without requiring the protein crystallization and at a desired pH condition.

Confinement-Enhanced Luminescence in Protein-Gold Nanoclusters

Confinement has profound effects on protein functions. Nanoscale probes for confinement or excluded volume interactions could help us understand how these interactions influence protein functions. This work reports on the increased luminescence of BSA-gold nanoclusters when confined. Confinement of the BSA-gold nanoclusters occurred within reverse micelles (RMs), where the size of the RMs determined the degree of confinement. The confinement-enhanced luminescence is reversible, i.e., the emission returns to its original value following cyclic changes in RM size. Circular dichroism measurements show an increase in alpha-helical character of the BSA-stabilized nanoclusters with confinement, which could provide a mechanism for the increase in luminescence. The alpha-helical character of the native proteins also increases with confinement, suggesting that the protein-nanocluster might sense confinement in an analogous fashion as the proteins. When the RMs approach the size of the protein, the intensity becomes independent of alpha-helical character, suggesting a different mechanism for the luminescence increase.

On the Different Mode of Action of Au(I)/Ag(I)-NHC Bis-Anthracenyl Complexes Towards Selected Target Biomolecules

Gold and silver N-heterocyclic carbenes (NHCs) are emerging for therapeutic applications. Multiple techniques are here used to unveil the mechanistic details of the binding to different biosubstrates of bis(1-(anthracen-9-ylmethyl)-3-ethylimidazol-2-ylidene) silver chloride [Ag(EIA)₂]Cl and bis(1-(anthracen-9-ylmethyl)-3-ethylimidazol-2-ylidene) gold chloride [Au(EIA)₂]Cl. As the biosubstrates, we tested natural double-stranded DNA, synthetic RNA polynucleotides (single-poly(A), double-poly(A)poly(U) and triple-stranded poly(A)2poly(U)), DNA G-quadruplex structures (G4s), and bovine serum albumin (BSA) protein. Absorbance and fluorescence titrations, mass spectrometry together with melting and viscometry tests show significant differences in the binding features between silver and gold compounds. [Au(EIA)₂]Cl covalently binds BSA. It is here evidenced that the selectivity is high: low affinity and external binding for all polynucleotides and G4s are found. Conversely, in the case of [Ag(EIA)₂]Cl, the binding to BSA is weak and relies on electrostatic interactions. [Ag(EIA)₂]Cl strongly/selectively interacts only with double strands by a mechanism where intercalation plays the major role, but groove binding is also operative. The absence of an interaction with triplexes indicates the major role played by the geometrical constraints to drive the binding mode.

Natural protein-templated fluorescent gold nanoclusters: Syntheses and applications

For the past decades, the synthesis of metal nanoclusters has been a great interest for research, for their unique physicochemical properties and great contributions to the catalytic, electrical and biomedical applications. Protein-templated gold nanoclusters (AuNCs) is a kind of fluorescent nanomaterials with good solubility, excellent stability, biocompatibility, decent quantum yields and active groups (-COOH, -NH₂) for facilitating modifications. Natural proteins are easily available, commercially affordable, diverse and multitudinous in animals, plants and foods, which provide a template pool for the exploration of AuNCs. This is one of the few reviews of specifically focusing on the natural protein-templated fluorescent AuNCs. The syntheses, properties and applications of different AuNCs were enumerated. Prospects were given on utilizing structure-modified proteins, bioactive enzymes, antibodies which should endow the AuNCs more favourable fluorescence performances and functional characteristics. The applications of AuNCs in analytical, biomedical and food sciences would be further heightened.

Luminescence Onset and Mechanism of the Formation of Gold(I)-Thiolate Complexes as the Precursors to Nanoparticles

Gold(I) (Au(I))-thiolate complexes are widely believed as the precursors to Au nanoparticle formations. While the literature suggests that the Au(III)-to-thiol ligand stoichiometric ratio of 1:3 is required to reduce a Au(III) and yield a Au(I)-thiolate, other stoichiometric ratios are also known to produce Au nanoparticles upon reduction. Using the characteristic red luminescence of Au(I)-alkanethiolates, we examined the process of their formations and their implications on the Au nanoparticle synthesis in detail. The onset of the luminescence, correlated with the Au(I)-thiolate formation, as well as the kinetics of the luminophore formation were evaluated in terms of the Au(III)-to-alkanethiol ratios. The onset of the luminescence was affected significantly by the solvent polarity during reaction but not post reaction. We found that the kinetics of the luminophore formation can vary widely, requiring from minutes to 24 h for completion depending on the thiol ligands and molar ratios, as well as solvents. This information could help in designing Au nanoparticle syntheses with the logical choice of Au(III)-to-thiol ratio, solvent, and the timing of reduction.

Identifying the Red-Luminophore-Forming Domain in Serum Albumin-Gold Complexes

Serum albumin-gold complexes exhibit UV-excitable red luminescence (λ_em = 640 nm) with unusual Stokes shifts compared with the innate UV/blue fluorescence arising from the aromatic residues. In order to understand the mechanism of this luminescence, we employed limited proteolysis and molecular cloning techniques and assessed the domain containing the red luminophore in bovine serum albumin (BSA) and human serum albumin (HSA). We identified that the luminophore is localized in a domain of serum albumin, residing within the N-terminus half.

Capping 1,3-propanedithiol to boost the antibacterial activity of protein-templated copper nanoclusters

We have prepared copper nanoclusters (Cu NCs) in the presence of bovine serum albumin (BSA) and 1,3-propanedithiol (PDT). The PDT/BSA-Cu NCs possess great activities against different types of bacteria, including non-multidrug-resistant bacteria (Escherichia coli, Salmonella Enteritidis, Pseudomonas aeruginosa, and Staphylococcus aureus) and multidrug-resistant bacteria (methicillin-resistant S. aureus). Their minimal inhibitory concentration (MIC) values are at least 242-fold and 10-fold lower than that of the free PDT and BSA-Cu NCs, respectively. The PDT/BSA-Cu NCs are strongly bound to the bacterial membrane, in which they induce the generation of ascorbyl (Asc) and perhydroxyl (HOO) radicals that result in disruption of their membrane integrity. At a concentration of 100-fold higher than their MIC for Escherichia coli, the PDT/BSA-Cu NCs exhibit negligible cytotoxicity towards the tested mammalian cells and show insignificant hemolysis. We have further demonstrated that low-cost PDT/BSA-Cu NCs-coated carbon fiber fabrics (CFFs) are effective against antibacterial growth, showing their great potential for antifouling applications.

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.

Photonic crystal enhanced gold-silver nanoclusters fluorescent sensor for Hg2+ ion

Luminescent nanoclusters (NCs) have attracted much attention because of their good photostability and low toxicity, however, the low quantum yield is still a deficiency, and many increasing efforts are being devoted to enhance the luminescence intensity of NCs. In this paper, a method of enhancing the fluorescent signal of gold-silver nanoclusters (AuAgNCs) by photonic crystals (PhCs) was proposed. The fluorescent intensity of AuAgNCs on PhCs can be enhanced 8.0-fold in comparison to the control sample without PhCs. Furthermore, a novel fluorescence sensor of AuAgNCs based on PhCs is used for the sensitive and selective detection of Hg²⁺ ion in the aqueous solution, the detection limit is 0.35 nM due to the PhCs enhancement effect for the fluorescence. This proposed method may not only develop a highly sensitive method for determination of Hg²⁺ ion, but also expand the application of AuAgNCs in ultra-trace analysis.

Label-Free SERS Strategy for In Situ Monitoring and Real-Time Imaging of Aβ Aggregation Process in Live Neurons and Brain Tissues

The aggregation of Aβ has been reported to closely correlate with Alzheimer's disease (AD). However, clear monitoring of the entire aggregation process of Aβ from monomer to fibril has been scarcely reported until now. Herein, we developed a label-free ratiometric surface enhanced Raman spectroscopic (SERS) platform for real-time monitoring of the entire process of Aβ aggregation in neurons and brain tissues. Different gold nanoparticles, generated in situ with Aβ monomer and fibril as templates separately, were served as effective SERS substrates to achieve a high sensitivity with a limit of detection (LOD) down to 70 ± 4 pM and 3.0 ± 0.5 pM for Aβ₄₀ monomer and fibrils, respectively. Besides, the introduction of ratiometric determination of Aβ monomer and fibril (I₁₂₄₄/I₁₂₆₈) realized real-time monitoring of the entire aggregation process of Aβ monomer with high accuracy and selectivity against other proteins and amino acids. The significant analytical performance of the developed platform, together with good biocompatibility, long-term stability, and remarkable spatial resolution, enabled the present SERS platform imaging and real-time monitoring and imaging of Aβ aggregation influenced by different metal ions (Cu²⁺, Zn²⁺, and Fe³⁺) in neurons and brain tissues at the single cell level. Our results suggested that Cu²⁺ and Zn²⁺ ion of low concentration (10 μM) promoted fibril formation, while Fe³⁺ and Zn²⁺ of high concentration (100 μM) showed inhibition of fibrosis.

The Effect of Fatty Acids and BSA Purity on Synthesis and Properties of Fluorescent Gold Nanoclusters

Fluorescent gold nanoclusters (AuNCs) are envisaged as a novel type of fluorophores. This work reports on the first comparative study investigating the effect of presence/absence/abundance of fatty acids (namely palmitic acid, PA) or other substances (like glycoproteins and globulins) in the protein (bovine serum albumin, BSA) on synthesis and properties of the final AuNCs. The most popular template (BSA) and microwave (MW)-assisted synthesis of AuNCs have been intentionally chosen. Our results clearly demonstrate that the fluorescent characteristics (i.e., fluorescence lifetime and quantum yield) are affected by the fatty acids and/or other substances. Importantly, the as-prepared AuNCs are biocompatible, as determined by Alamar Blue assay performed on Hep G2 cell line.

Hard X-ray excited optical luminescence from protein-directed Au∼20 clusters

Hard X-ray excited optical luminescence is a unique property of materials, which makes them promising for biological imaging applications. However, the preparation of biocompatible contrast agents for hard X-ray excited optical luminescence remains a considerable challenge that has, to date, not been overcome. In this study, we investigated the luminescence properties of protein-directed Au_∼20 clusters upon hard X-ray irradiation, both in solution and when embedded in films.

We investigated the luminescence properties of protein-directed Au_∼20 clusters upon hard X-ray irradiation, demonstrating the further possible application toward nanomaterial based bio-imaging and diagnostics.

NIR-Emitting Gold Nanoclusters-Modified Gelatin Nanoparticles as a Bioimaging Agent in Tissue

Gold nanocluster (AuNC) synthesis using a well-distinguished polymer for nanoparticle-mediated drug delivery paves the way for developing efficient theranostics based on pharmaceutically accepted materials. Gelatin-stabilized AuNCs are synthesized and modified by glutathione for tuning the emission spectra. Addition of silver ions enhances the fluorescence, reaching also high quantum yield (26.7%). A simplified model can be proposed describing the nanoclusters' properties-structure relationship based on X-ray photoelectron spectroscopy data and synthesis sequence. Furthermore, these modifications improve fluorescence stability toward pH changes and enzymatic degradation, offering different AuNCs for various applications. The impact of nanocluster formation on gelatin structure integrity is investigated by Fourier transform infrared spectrometry and matrix-assisted laser desorption/ionization time of flight mass spectroscopy, being important to further formulate gelatin nanoparticles (GNPs). The 218 nm-sized NPs show no cytotoxicity up to 600 µg mL^-1 and are imaged in skin, as a challenging autofluorescent tissue, by confocal microscopy, when transcutaneously delivered using dissolving microneedles. Linear unmixing allows simultaneous imaging of AuNCs-GNPs and skin with accurate signal separation. This underlines the great potential for bioimaging of this system to better understand nanomaterials' behavior in tissue. Additionally, it is drug delivery system also potentially serving as a theranostic system.

Targeted Gold Nanocluster-Enhanced Radiotherapy of Prostate Cancer

For over a hundred years, X-rays have been a main component of the radiotherapeutic approaches to treat cancer. Yet, to date, no radiosensitizer has been developed to selectively target prostate cancer. Gold has excellent X-ray absorptivity and is used as a radiotherapy enhancing material. In this work, ultrasmall Au₂₅ nanoclusters (NCs) are developed for selective prostate cancer targeting, radiotherapy enhancement, and rapid clearance from the body. Targeted-Au₂₅ NCs are rapidly and selectively taken up by prostate cancer in vitro and in vivo and also have fast renal clearance. When combined with X-ray irradiation of the targeted cancer tissues, radiotherapy is significantly enhanced. The selective targeting and rapid clearance of the nanoclusters may allow reductions in radiation dose, decreasing exposure to healthy tissue and making them highly attractive for clinical translation.

Nanodot-Directed Formation of Plasmonic-Fluorescent Nanohybrids toward Dual Optical Detection of Glucose and Cholesterol via Hydrogen Peroxide Sensing

Hybrid nanoparticles (NPs) have emerged as an important class of nanomaterials owing to their integrated enhanced properties and functionality. In this study, we have developed an effective nanodot templating strategy for the in situ formation of surfactant-free nanohybrids with unique plasmonic-fluorescent properties. A bright photoluminescent biodot synthesized from serine and histamine biomolecular precursors (Ser-Hist dot) was first engineered to have rich functional groups on the nanosurface capable of anchoring Ag⁺ ions via electrostatic interaction. Upon UV irradiation, free electrons could transfer from the photoexcited Ser-Hist dot to the Ag⁺ ions, facilitating the in situ growth of AgNPs. The resulting nanohybrid system (Bio@AgNPs) exhibits distinct characteristic surface plasmon resonance absorbance and highly quenched PL intensity due to the inner filter effect. Furthermore, the Bio@AgNP nanohybrid retains its redox capability, enabling hydrogen peroxide sensing via AgNP etching, which in turn empowers a dual colorimetric and fluorescent detection of glucose and cholesterol in complex biological samples (i.e., synthetic urine and human plasma) with high selectivity and sensitivity. This finding reveals a new effective and facile method for the preparation of highly functional hybrid nanomaterials for dual-mode detection of hydrogen peroxide-producing species and/or reactions.

Revisiting the conformational state of albumin conjugated to gold nanoclusters: A self-assembly pathway to giant superstructures unraveled

Bovine serum albumin (BSA) is often employed as a proteinaceous component for synthesis of luminescent protein-stabilized gold nanoclusters (AuNC): intriguing systems with many potential applications. Typically, the formation of BSA-AuNC conjugate occurs under strongly alkaline conditions. Due to the sheer complexity of intertwined chemical and structural transitions taking place upon BSA-AuNC formation, the state of albumin enveloping AuNCs remains poorly characterized. Here, we study the conformational properties of BSA bound to AuNCs using an array of biophysical tools including vibrational spectroscopy, circular dichroism, fluorescence spectroscopy and trypsin digestion. The alkaline conditions of BSA-AuNC self-assembly appear to be primary responsible for the profound irreversible disruption of tertiary contacts, partial unfolding of native α-helices, hydrolysis of disulfide bonds and the protein becoming vulnerable to trypsin digestion. Further unfolding of BSA-AuNC by guanidinium hydrochloride (GdnHCl) is fully reversible equally in terms of albumin's secondary structure and conjugate's luminescent properties. This suggests that binding to AuNCs traps the albumin molecule in a state that is both partly disordered and refractory to irreversible misfolding. Indeed, when BSA-AuNC is subjected to conditions favoring self-association of BSA into amyloid-like fibrils, the buildup of non-native β-sheet conformation is less pronounced than in a control experiment with unmodified BSA. Unexpectedly, BSA-AuNC reveals a tendency to self-assemble into giant twisted superstructures of micrometer lengths detectable with transmission electron microscopy (TEM), a property absent in unmodified BSA. The process is accompanied by ordering of bound AuNCs into elongated streaks and simultaneous decrease in fluorescence intensity. The newly discovered self-association pathway appears to be specifically accessible to protein molecules with a certain restriction on structural dynamics which in the case of BSA-AuNC arises from binding to metal nanoclusters. Our results have been discussed in the context of mechanisms of protein misfolding and applications of BSA-AuNC.

Identifying Reducing and Capping Sites of Protein-Encapsulated Gold Nanoclusters

The reducing and capping sites along with their local structure impact photo properties of the red bovine serum albumin-capped Au nanocluster (BSA-AuNC), however, they are hard to identify. We developped a workflow and relevant techniques using mass spectrometry (MS) to identify the reducing and capping sites of BSA-AuNCs involved in their formation and fluorescence. Digestion without disulfide cleavages yielded an Au core fraction exhibiting red fluorescence and [Au_nS_m] ion signals and a non-core fraction exhibiting neither of them. The core fraction was identified to mainly be comprised of peptides containing cysteine residues. The fluorescence and [Au_nS_m] signals were quenched by tris(2-carboxyethyl)phosphine, confirming that disulfide groups were required for nanocluster stabilization and fluorescence. By MS sequencing, the disulfide pairs, C75–C91/C90–C101 in domain IA, C315–C360/C359–C368 in domain IIB, and C513–C558/C557–C566 in domain IIIB, were identified to be main capping sites of red AuNCs. Peptides containing oxidized cysteines (sulfinic or cysteic acid) were identified as reducing sites mainly in the non-core fraction, suggesting that disulfide cleavages by oxidization and conformational changes contributed to the subsequent growth of nanoclusters at nearby intact disulfide pairs. This is the first report on precise identification of the reducing and capping sites of BSA-AuNCs.

Media Dependent Switching of Selectivity and Continuous near Infrared Turn-on Fluorescence Response through Cascade Interactions from Noncovalent to Covalent Binding for Detection of Serum Albumin in Living Cells

Abnormal level of proteins is proved to be associated with diseases. Thus, protein sensing is helpful for clinical diagnosis and therapy. However, there is a great variety of protein species and relatively low concentration of each protein in complicated biological systems including other nonprotein biomolecules. Therefore, it remains challenging to develop an effective method for detecting protein with high selectivity and sensitivity. Herein, a new self-assembly method based on a robust dye SQSS of which two squaraine molecules were conjugated through disulfide bond was developed for highly selective and sensitive detection of serum albumin (SA) in aqueous solution and live cells. SQSS can self-assemble into "compact" aggregates, offering "inert" disulfide group and very low background fluorescence through the combination of aggregation quenching and homogeneous fluorescence resonance energy transfer (homoFRET) quenching. The response of SQSS to SA undergoes two cascade stages. At the first stage, SA drives the compact assemblies of SQSS to form loose ones with fast speed (30 s) through noncovalent interaction, resulting in the enhancement of fluorescence to some extent. In this loose assembly state, the disulfide bond in SQSS is reactive. At the second stage, the Cys34 in SA slowly induced further disassembly through covalent binding with reactive disulfide bond, resulting in fluorescence further increasing and SQSS labeling to SA that cannot be displaced by site binding ligands of SA. The self-assemblies of SQSS can selectively detect SA with continuous near-infrared (NIR) turn-on fluorescence response in 100% aqueous buffer solution. In addition, SQSS showed the potential application of imaging SA in living cells. On the other hand, the loose assembly state of SQSS was also achieved in aqueous solution with 20% CH₃CN. In this media, thiol-containing glutathione (GSH) caused the disassembly of SQSS with turn-on fluorescence response through interaction with disulfide bond. SQSS can selectively recognize GSH over other amino acids even in the presence of other sulfhydryl amino acids. As a proof-of-concept method, the molecular self-assembly through multisteps interactions would provide an ideal strategy for detection and live-cell imaging of biorelated molecules with high selectivity and signal-to-noise ratio.

Ultrasmall All-In-One Nanodots Formed via Carbon Dot-Mediated and Albumin-Based Synthesis: Multimodal Imaging-Guided and Mild Laser-Enhanced Cancer Therapy

Integration of multiple diagnostic/therapeutic modalities into a single system with ultrasmall size, excellent photothermal/photodynamic properties, high cellular uptake efficiency, nuclear delivery capacity, rapid renal clearance, and good biosafety is highly desirable for cancer theranostics, but still remains challenging. Here, a novel type of multifunctional nanodots (denoted as BCCGH) was synthesized by mixing bovine serum albumin, carbon dots, and metal ions (Cu²⁺ and Gd³⁺), followed by the conjugation with a photosensitizer (HPPH). The nanodots hold great promise for fluorescence/photoacoustic/magnetic resonance/photothermal imaging-guided synergistic photothermal/photodynamic therapy (PDT) because of their appealing properties such as high photothermal conversion efficiency (68.4%), high longitudinal relaxivity (11.84 mM^-1 s^-1, 7 T), and superior colloidal stability with negligible Gd³⁺ release. Benefiting from the massive cellular uptake, endoplasmic reticulum/mitochondrion-targeting ability, and mild near-infrared laser irradiation-promoted nuclear delivery of BCCGH, a high anticancer therapeutic efficiency is achieved in the subsequent in vitro PDT. Besides, as revealed by the in vivo/ex vivo results, the nanodots also exhibit excellent tumor accumulation, efficient renal clearance, complete tumor ablation, and exceptional biosafety. To summarize, this work develops a carbon dot-mediated and albumin-based synthetic approach for constructing ultrasmall and multifunctional nanodots, which may hold great potential for cancer theranostics and beyond.

Luminophore Formation in Various Conformations of Bovine Serum Albumin by Binding of Gold(III)

The purpose of the presented protocols is to study the process of Au(III) binding to BSA, yielding conformation change-induced red fluorescence (λem = 640 nm) of BSA-Au(III) complexes. The method adjusts the pH to show that the emergence of the red fluorescence is correlated with the pH-induced equilibrium transitions of the BSA conformations. Red fluorescent BSA-Au(III) complexes can only be formed with an adjustment of pH at or above 9.7, which corresponds to the "A-form" conformation of BSA. The protocol to adjust the BSA to Au molar ratio and to monitor the time-course of the process of Au(III) binding is described. The minimum number of Au(III) per BSA, to produce the red fluorescence, is less than seven. We describe the protocol in steps to illustrate the presence of multiple Au(III) binding sites in BSA. First, by adding copper (Cu(II)) or nickel (Ni(II)) cations followed by Au(III), this method reveals a binding site for Au(III) that is not the red fluorophore. Second, by modifying BSA by thiol capping agents, another nonfluorophore-forming Au(III) binding site is revealed. Third, changing the BSA conformation by cleaving and capping of the disulfide bonds, the possible Au(III) binding site(s) are illustrated. The protocol described, to control the BSA conformations and Au(III) binding, can be generally applied to study the interactions of other proteins and metal cations.

Decorating a single giant DNA with gold nanoparticles

We decorated a single giant DNA (1.66 × 10⁵ base pairs) with gold nanoparticles through the simple procedure of mild warming, without denaturation of the DNA molecule. Single-molecule observation with fluorescence microscopy revealed that individual decorated DNA molecules stay in the bulk solution by avoiding aggregation and precipitation, and exhibit translational and conformational fluctuation, i.e., Brownian motion. An analysis of the intra-chain fluctuation of single DNA molecules revealed that the apparent spring constant and damping coefficient of a DNA chain increased by ca. 13- and 5-fold, respectively, upon decoration with gold nanoparticles. Observation by transmission electron microscopy revealed that gold nanoparticles were stably attached to the DNA skeleton. UV-visible measurements revealed the absence of any detectable change in surface plasmon resonance, suggesting that the gold nanoparticles assemble without the formation of a densely packed aggregate. CD measurements showed that the secondary structure of decorated DNA is still essentially the B-form.

We decorated a single giant DNA (1.66 × 10⁵ base pairs) with gold nanoparticles through the simple procedure of mild warming, without denaturation of the DNA molecule.