BSA Capped gold Nanoclusters Modulated by Copper ion for Sensitive and Selective Detection of Histidine in Biological Fluid

This research proposed a sample and environmentally sustainable technique for the synthesis of bovine serum albumin capped gold nanoclusters (BSA-AuNCs) with outstanding fluorescence. The synthesized BSA-AuNCs were investigated using various ways before being combined with Cu2+ to produce a fluorescent switch probe (BSA-AuNCs-Cu2+) for histidine determination. After adding Cu2+, the fluorescence of the BSA-AuNCs was quenched, the fluorescence intensity was enhanced after adding histidine due to good coordination between Cu2+ and histidine. The significant chelation of histidine with Cu2+ demonstrated the viability of developing a selective "switch on" probe for histidine detecting over other amino acids. Unlike existing fluorescent nanomaterial-based approaches for detecting histidine, this study promises good selectivity, high efficiency, and the avoiding of chemical solvents. The designed BSA-AuNCs-Cu2+ fluorescent probe demonstrated an acceptable linear detection range of 0 to 240 µM under optimum circumstances, with a detection limit of 0.9 µM. The BSA-AuNCs-Cu2+ system was investigated in rat serum and human urine, with recoveries ranging from 97.2 to 108.2%, demonstrating its potential applicability for histidine detection with favorable results.

A simple, quantitative method for spectroscopic detection of metformin using gold nanoclusters

We synthesized bovine serum albumin (BSA)-stabilized gold nanoclusters (BSA-GNCs) and confirmed their ultra-small size using HRTEM (High-resolution Transmission Electron Microscope) and DLS (Dynamic Light Scattering). The fluorescence intensity of BSA-GNCs is "turned off" in the presence of Cu(II) metal ions. The resulting Cu(II)-mediated BSA-GNCs were utilized to detect metformin, a drug used to control diabetes. Metformin binds to and displaces Cu(II) ions from the BSA on the surface of the nanoclusters, which turns on the fluorescence of the nanoclusters. The interactions between the protein-stabilized nanoclusters were investigated in the absence and presence of Cu(II) using circular dichroism (CD) and Fourier-transform infrared spectroscopy (FTIR). Cu(II)-quenched BSA-GNCs had an extremely high sensitivity to detect metformin, with a low limit of detection (LOD) of 0.068 μM and a dynamic range of limit of quantification (LOQ = 10/3 LOD) of 0.22 to 11 μM. The ability of this novel "turn-on" nanosensor to detect metformin in human serum and urine samples was confirmed: the percentage recovery in fluorescence for spiked analyte ranged from 96.00-98.50% and 92.60-96.62% in human serum and urine samples, respectively. Thus, BSA-GNCs provide a valid, sensitive, specific fluorometric methodology for the detection of metformin in biomedical applications.

Selective determination of Ag+ in the presence of Cd2+, Hg2+ and Cu2+ based on their different interactions with gold nanoclusters

In this work, a fluorescence method was developed for selective detection of Ag⁺ in the presence of Cd²⁺, Hg²⁺, and Cu²⁺ based on gold nanoclusters (AuNCs). That is, bovine serum albumin (BSA) templated AuNCs with double emission peaks were synthesized using BSA as a protective agent. AuNCs with uniform distribution and average size between 2.0 and 2.2 nm were synthesized using a green and simple method, and showed bright orange-red fluorescence under ultraviolet light. AuNCs have two emission peaks at 450 nm and 630 nm with an excitation wavelength of 365 nm. Under alkaline conditions, Cd²⁺ can combine with the surface sulfhydryl groups of BSA–AuNCs to form Cd–S bonds, which cause AuNCs to aggregate, resulting in an increase in fluorescence intensity at 630 nm. Conversely, due to the d¹⁰–d¹⁰ metal affinity interaction, the addition of Hg²⁺ can reduce the fluorescence peak at 630 nm. Ag⁺ was reduced to Ag⁰ by gold nuclei in AuNCs, forming a stable hybrid Au@ AgNCs species with blue-shifted and enhanced fluorescence. Finally, the paramagnetic behavior of Cu²⁺ combined with BSA causes the excited electrons of the gold cluster to lose their energy via ISC, eventually leading to simultaneous quenching of the two emission peaks. The results show that the limit of detection (LOD) of Ag⁺, Hg²⁺, Cd²⁺ and Cu²⁺ is 1.19 μM, 3.39 μM, 1.83 μM and 5.95 μM, respectively.

A fluorescence method was developed for selective detection of Ag⁺ in the presence of Cd²⁺, Hg²⁺, and Cu²⁺ based on gold nanoclusters. The limit of detection for Ag⁺, Hg²⁺, Cd²⁺ and Cu²⁺ is 1.19 μM, 3.39 μM, 1.83 μM and 5.95 μM, respectively.

Detection and quantification of folic acid in serum via a dual-emission fluorescence nanoprobe

Folic acid (FA) is an essential vitamin in humans, and thus, rapid, accurate, and sensitive methods for its quantification in different biological samples are needed. This work describes a novel, simple, and effective dual-emission fluorescence nanoprobe for FA detection and quantification. The probe was covalently linked to amino-modified orange quantum dots (QDs) and carboxyl-modified blue graphene quantum dots (GQDs). The resulting material exhibited two emission peaks at 401 and 605 nm upon excitation at 310 nm. The probe had good selectivity and sensitivity toward FA with an exceptionally low detection limit (LOD = 0.09 nM). This probe was effectively used to quantify FA in animal serum samples. The method has potential utility for FA analysis in different types of biological samples. Graphical abstract.

Ratiometric fluorescent nanoprobes for visual detection: Design principles and recent advances - A review

Signal generation techniques for visual detection of analytes have received a great deal of attention in various sensing fields. These approaches are considered to be advantageous when instrumentation cannot be employed, such as for on-site assays, point-of-care tests, and he althcare diagnostics in resource-constrained areas. Amongst various visual detection approaches explored for non-invasive quantitative measurements, ratiometric fluorescence sensing has received particular attention as a potential method to overcome the limitations of intensity-based probes. This technique relies on changes in the intensity of two or more emission bands (induced by an analyte), resulting in an effective internal referencing which improves the sensitivity of the detection. The self-calibration, together with the unique optophysical properties of nanoparticles (NPs) have made the ratiometric fluorescent nanoprobes more sensitive and reliable, which in turn, can result in more precise visual detection of the analytes. Over the past few years, a vast number of ratiometric sensing probes using nanostructured fluorophores have been designed and reported for a wide variety of sensing, imaging, and biomedical applications. In this work, a review on the NP-based ratiometric fluorescent sensors has been presented to meticulously elucidate their development, advances and challenges. With a special emphasis on visual detection, the most important steps in the design of fluorescent ratiometric nanoprobes have been given and based on different classes of analytes, recent applications of fluorescent ratiometric nanoprobes have been summarized. The challenges for the future use of the technique investigated in this review have been also discussed.

Self-Referenced Ratiometric Detection of Sulfatase Activity with Dual-Emissive Urease-Encapsulated Gold Nanoclusters

In this study, on the basis of the biomineralization capability of urease, a facile, one-step, and green synthetic method has been proposed for the fabrication of gold nanoclusters (AuNCs). The prepared urease-encapsulated AuNCs (U-AuNCs) exhibited strong red fluorescence emission (λ_em = 630 nm) with a quantum yield as high as 17%. Interestingly, at a low concentration, the U-AuNC solution was found to be a dual-emissive system with the blue emission of the dityrosine (diTyr) residues of urease and the red emission of the embedded AuNCs. Further experiments demonstrated that p-nitrophenol (PNP) can selectively suppress the 410 nm emission of the diTyr residues of U-AuNCs without affecting the red emission of the U-AuNCs. The fluorescence quenching mechanism between U-AuNCs and PNP was systematically studied, and the leading role of the inner filter effect (IFE) was identified. Additionally, based on the sulfatase-catalyzed hydrolysis of p-nitrophenyl sulfate (PNPS) to release PNP, a self-referenced ratiometric detection method for sulfatase, which plays a crucial role in sulfur cycling, degradation of sulfated glycosaminoglycans and glycolipids, and extracellular remodeling of sulfated glycosaminoglycans, was developed by using dual-emissive U-AuNCs as the signal readout, in which the diTyr residues served as the probe and the AuNCs functioned as the internal reference. This IFE-based ratiometric sensing strategy showed a good linear relationship over the range of 0.01-1 U/mL ( R² = 0.997). The detection limit for sulfatase activity was 0.01 U/mL. The developed protocol was successfully used to detect sulfatase activity in human serum samples. The simplicity, rapidity, low cost, high credibility, good reproducibility, and excellent selectivity of the detection platform serve as an inspiration for further applications of fluorescent AuNCs in chemo/biosensing.

Time-dependent emission stokes shift in Au, Ag and Au/Ag fluorescent nanoclusters: evidence of multiple emissive states

Au, Ag and bimetallic Au/Ag nanoclusters were prepared under similar microwave-irradiated synthesis conditions in aqueous solutions, using BSA as a stabilizer. The nanoclusters exhibited strong fluorescence with characteristic emission spectral peak features that suggested the presence of multiple emissive states in each case. Time-resolved emission studies revealed the very rare incidence of the time-dependent emission Stokes shift in metal nanoclusters over a time-scale of ∼10 ns. The shifts were of varying degrees: ∼200 cm-1 for Ag, ∼750 cm-1 for Au and ∼1400 cm-1 for Au/Ag nanoclusters. Application of the Time-Resolved Area Normalized Emission Spectra (TRANES) method confirmed the existence of multiple emissive species in each of the nanoclusters, which also helped to explain the time-dependent emission Stokes shift.

Optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals

Optically-active nanocrystals (such as quantum dots and plasmonic noble metal nanoparticles) have received great attention due to their size-tunable optical properties. The indicator displacement assay (IDA) with optically-active nanocrystals has become a common practice for optical sensor development, since no sophisticated surface functionalization of nanoparticles is required. Among the IDA-based optical sensors, the use of metal ions as receptors seems to be attractive. Therefore, in this review, the research progress of optical sensing at the nanobiointerface of metal ion-optically-active nanocrystals has been summarized. In particular, metal ion-mediated selective recognition has been summarized here based on the classical Hard-Soft-Acid-Base (HSAB) principle, which has been seldom mentioned before. Most of the references were therefore categorized according to their located place based on the HSAB theory. Besides, several metal ion modulation strategies that were not related to the HSAB theory (e.g., redox modulation) were also included. Finally, due to the cross-talk of metal ions in selective recognition, we have also summarized sensor array development based on multiple metal ion receptors in IDA sensing with optically-active nanocrystals. Several interesting applications of the IDA sensing with metal ions as receptors and optically-active nanocrystals as indicators are presented, with specific emphasis on the design principles and photophysical mechanisms of these probes.

One-step eco-friendly approach for the fabrication of synergistically engineered fluorescent copper nanoclusters: sensing of Hg2+ ion and cellular uptake and bioimaging properties

Schematic illustration for one-step green synthetic approach for fabrication of synergistically engineered CuNCs.

A rainbow ratiometric fluorescent sensor array on bacterial nanocellulose for visual discrimination of biothiols

The crucial role of biothiols in many biological processes, which turns them into important biomarkers for the early diagnosis of various diseases, the development of an affordable, sensitive and portable probe for the detection and discrimination of these compounds is of great importance.

A biomimetic approach to conjugate vitamin B6 cofactor with the lysozyme cocooned fluorescent AuNCs and its application in turn-on sensing of zinc(II) in environmental and biological samples

This communication focusses on the synthesis of red fluorescent lysozyme cocooned gold nanoclusters (Lyso-AuNCs) that have been successfully applied for the selective and specific recognition of the vitamin B₆ cofactor pyridoxal-5'-phosphate (PLP). The red fluorescence of Lyso-AuNCs showed remarkable color change to yellow upon conjugation with PLP due to the formation of a Schiff base between the free -NH₂ present in the lysozyme and the -CHO group of PLP. The developed PLP conjugated Lyso-AuNCs (PLP_Lyso-AuNCs) was applied for the selective turn-on recognition of Zn²⁺ ions in aqueous medium. The yellow fluorescence of PLP_Lyso-AuNCs exhibited significant enhancement at 475 nm in the presence of Zn²⁺ producing bluish-green fluorescence attributed to the complexation-induced aggregation of nanoclusters. The nanoprobe exhibits nanomolar limit of detection for Zn²⁺ ions (39.2 nM) and the practicality of the nanoprobe was validated in various environmental water samples and biological plasma, urine, and beetroot extract, with fairly good recovery percent. Also, the system was successfully implemented for the intracellular detection and monitoring of Zn²⁺ in live HeLa cells. Graphical abstract Applications of red emitting lysozyme cocooned gold nanoclusters (Lyso-AuNCs) for the selective recognition of the vitamin B₆ cofactor pyridoxal-5'-phosphate (PLP) and the conjugated nano-assembly PLP_Lyso-AuNCs for turn-on detection of Zn²⁺ ions in various environmental and biological samples.

Hybridization of Metal Nanoparticles with Metal-Organic Frameworks Using Protein as Amphiphilic Stabilizer

Here, a facile strategy is reported to efficiently hybridize metal nanoparticles (MNPs) with typical metal-organic frameworks (MOFs) of ZIF-8 (zeolitic imidazolate framework-8), which employs bovine serum albumin (BSA, a serum albumin protein derived from cows) as the amphiphilic stabilizer to increase the affinity of MNP toward MOFs. For instance, the as-synthesized PdNPs/ZIF-8 composites with diameter from 100 to 200 nm always maintain well-defined crystalline structure, and the PdNPs with small size of ∼2 nm are well-dispersed in the crystal of MOFs without serious aggregations due to the BSA stabilizer. In Suzuki cross-coupling reactions of aryl halide, the PdNPs/ZIF-8 as catalysts have exhibited high activity and satisfied reusability owing to the use of BSA stabilizer as well as the fixing of MOFs matrixes. In addition, the strategy also can be extended to synthesize other kinds of MNPs/MOFs hybrid composites with tunable particle size, which brings more opportunity for functional MOFs hybrid materials.

Gold Nanocluster Containing Polymeric Microcapsules for Intracellular Ratiometric Fluorescence Biosensing

A new approach to sensing and imaging hydrogen peroxide (H₂O₂) was developed using microcapsule-based dual-emission ratiometric luminescent biosensors. Bovine serum albumin-capped gold nanoclusters (BSA-AuNCs) sensitive to H₂O₂ were coencapsulated with insensitive FluoSpheres (FSs) within polymeric capsules fabricated via the layer-by-layer method. Under single-wavelength excitation, the microcapsule-based biosensors exhibited emission bands at ∼516 and ∼682 nm resulting from the FSs and BSA-AuNCs, respectively. The polyelectrolyte multilayers lining the microcapsules were effective in protecting BSA-AuNCs from the degradation catalyzed by proteases (chymotrypsin, trypsin, papain, and proteinase K) and subsequent luminescent quenching, overcoming a key limitation of prior BSA-AuNC-based sensing systems. The luminescent response of the sensors was also found to be independent of local changes in pH (5-9). Quenching of the AuNCs in the presence of H₂O₂ enabled the spectroscopic quantification and imaging of changes in H₂O₂ concentration from 0 to 1 mM. The microcapsule sensors were easily phagocytized by murine macrophage cells (RAW 264.7), were effective as intracellular H₂O₂ imaging probes, and were successfully used to detect local release of H₂O₂ in response to an external chemical stimulus.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Microwave-assisted synthesis of BSA-stabilised gold nanoclusters for the sensitive and selective detection of lead(ii) and melamine in aqueous solution

MW-assisted synthesis of fluorescent BSA-AuNCs for the turn-off sensing of Pb(ii) and turn-on sensing of melamine.

Turn-on fluorescent sensing of glutathione S-transferase at near-infrared region based on FRET between gold nanoclusters and gold nanorods

A fluorescence resonance energy transfer (FRET) method based on gold nanoclusters capped glutathione (AuNCs@GSH) and amine-terminated gold nanorods (AuNRs) is designed for turn-on and near-infrared region (NIR) sensing of glutathione S-transferase (GST). The absorption band of AuNRs is tuned carefully to maximize the spectra overlap and enhance the efficiency of FRET. The FRET from multiple AuNCs to single AuNR quenches about 70% fluorescence emission of AuNCs. After GST is added, the strong specific interaction of GSH-GST can replace the AuNCs@GSH from AuNRs, FRET based on electrostatic interaction between AuNCs@GSH and AuNRs is switched off. Thus, emission enhancement of AuNCs@GSH is observed. The fluorescent enhancement is linearly with the increasing GST concentration over the range of 2-100 nM GST and the limit of detection for GST is about 1.5 nM.

Controlling the self-assembly and optical properties of gold nanoclusters and gold nanoparticles biomineralized with bovine serum albumin

While the size-dependent optical properties of BSA-stabilized gold nanoclusters are well known, the time-dependent growth mechanism remains to be described.

Transferrin-directed preparation of red-emitting copper nanoclusters for targeted imaging of transferrin receptor over-expressed cancer cells

Red-emitting fluorescent transferrin-functionalized copper nanoclusters were synthesized for the targeted bioimaging of HeLa cells.

Hydrogen peroxide sensitive hemoglobin-capped gold nanoclusters as a fluorescence enhancing sensor for the label-free detection of glucose

A novel label-free fluorescent probe based on blue-emitting gold nanoclusters capped by hemoglobin for the direct detection of glucose is presented.

Fluorescent nanohybrid of gold nanoclusters and quantum dots for visual determination of lead ions

Highly green emissive gold nanoclusters (Au NCs) are synthesized using glutathione as a stabilizing agent and mercaptopropionic acid as a ligand, and the intensity of fluorescence is specifically sensitive to lead ions. We then fabricated a ratiometric fluorescence nanohybrid by covalently linking the green Au NCs to the surface of silica nanoparticles embedded with red quantum dots (QDs) for on-site visual determination of lead ions. The green fluorescence can be selectively quenched by lead ions, whereas the red fluorescence is inert to lead ions as internal reference. The different response of the two emissions results in a continuous fluorescence color change from green to yellow that can be clearly observed by the naked eyes. The nanohybrid sensor exhibits high sensitivity to lead ions with a detection limit of 3.5 nM and has been demonstrated for determination of lead ions in real water samples including tap water, mineral water, groundwater, and seawater. For practical application, we dope the Au NCs in poly(vinyl alcohol) (PVA) film and fabricate fluorescence test strips to directly detect lead ions in water. The PVA-film method has a visual detection limit of 0.1 μM, showing its promising application for on-site identification of lead ions without the need for elaborate equipment.

Recent advances in the field of bionanotechnology: an insight into optoelectric bacteriorhodopsin, quantum dots, and noble metal nanoclusters

Molecular sensors and molecular electronics are a major component of a recent research area known as bionanotechnology, which merges biology with nanotechnology. This new class of biosensors and bioelectronics has been a subject of intense research over the past decade and has found application in a wide variety of fields. The unique characteristics of these biomolecular transduction systems has been utilized in applications ranging from solar cells and single-electron transistors (SETs) to fluorescent sensors capable of sensitive and selective detection of a wide variety of targets, both organic and inorganic. This review will discuss three major systems in the area of molecular sensors and electronics and their application in unique technological innovations. Firstly, the synthesis of optoelectric bacteriorhodopsin (bR) and its application in the field of molecular sensors and electronics will be discussed. Next, this article will discuss recent advances in the synthesis and application of semiconductor quantum dots (QDs). Finally, this article will conclude with a review of the new and exciting field of noble metal nanoclusters and their application in the creation of a new class of fluorescent sensors.

Protein-templated gold nanoclusters sequestered within sol-gel thin films for the selective and ratiometric luminescence recognition of Hg2+

Sequestration of bovine serum albumin (BSA)-stabilized gold nanoclusters (AuNCs@BSA) prepared using microwave assistance within sol-gel-derived mesoporous silica films permits the selective and highly sensitive quenchometric detection of aqueous Hg(2+) (limit of detection = 600 pM) with luminescence signal arising from oxidized BSA allowing for an analytically robust and reliable ratiometric detection. Overall, this work highlights a number of important advances, including the highest luminescence quantum yield reported to date for a protein-templated luminescent noble metal nanocluster (13%) made possible using a microwave-mediated synthesis followed by cold incubation. We also demonstrate the clear advantage of exploiting the luminescence signal arising from oxidized BSA as an internal reference to generate selectivity of response to Hg(2+). A careful Stern-Volmer quenching analysis reveals the persistence of two unique quenching sites for AuNCs@BSA entrapped within a sol-gel-derived glass, a minor population of which is unquenchable. Finally, based on these AuNCs@BSA nanosensors, we advise a path forward for paper-based indicator strip detection of heavy metals in aqueous streams, the implementation of which can be performed using the unaided eye, making it a meaningful approach for routine screening and in resource-limited situations.

Selective visual detection of trace trinitrotoluene residues based on dual-color fluorescence of graphene oxide–nanocrystals hybrid probe

The dual-color fluorescence nanohybrid probe comprising blue emissive fluorescent graphene oxide and red emissive nanocrystals has been developed for the visual detection of TNT residues in solution and on various surfaces.

Growth of in situ functionalized luminescent silver nanoclusters by direct reduction and size focusing

We have used one phase growth reaction to prepare a series of silver nanoparticles (NPs) and luminescent nanoclusters (NCs) using sodium borohydride (NaBH(4)) reduction of silver nitrate in the presence of molecular scale ligands made of polyethylene glycol (PEG) appended with lipoic acid (LA) groups at one end and reactive (-COOH/-NH(2)) or inert (-OCH(3)) functional groups at the other end. The PEG segment in the ligand promotes solubility in a variety of solvents including water, while LAs provide multidentate coordinating groups that promote Ag-ligand complex formation and strong anchoring onto the NP/NC surface. The particle size and properties were primarily controlled by varying the Ag-to-ligand (Ag:L) molar ratios and the molar amount of NaBH(4) used. We found that while higher Ag:L ratios produced NPs, luminescent NCs were formed at lower ratios. We also found that nonluminescent NPs can be converted into luminescent clusters, via a process referred to as "size focusing", in the presence of added excess ligands and reducing agent. The nanoclusters emit in the far red region of the optical spectrum with a quantum yield of ~12%. They can be redispersed in a number of solvents with varying polarity while maintaining their optical and spectroscopic properties. Our synthetic protocol also allowed control over the number and type of reactive functional groups per nanocluster.