A fluorescent probe based on tryptophan-coated silver nanoclusters for copper (II) ions detection and bioimaging in cells

Developing a switch "OFF-ON" fluorescent probe for detection of melamine based on doubly-protected red emissive copper nanoclusters mediated by Hg2+ ions

Melamine, often used as an adulterant in infants' formula due to its high protein content, can be harmful when ingested in large amounts, leading to the formation of cyanurate-melamine co-crystals in infants and potentially causing kidney damage. In this study, we introduce a fluorescent method for the selective and reliable detection of melamine in milk and infants' formula. The fluorescent probe comprises copper nanoclusters (Cu NCs) functionalized with thiosalicylic acid (TSA) and polyvinylpyrrolidone (PVP) as double-protecting ligands. Upon the addition of Hg2+, the fluorescence emission of TSA-PVP@Cu NCs is diminished due to static quenching. Subsequently, the fluorescence emission of the TSA-PVP@Cu NCs + Hg2+ probe is restored upon the introduction of melamine, facilitated by the coordination interaction between melamine and Hg2+ and the formation of a stable chelate between them. Under optimized conditions, the fluorescence emission was recorded initially for the TSA-PVP@Cu NCs + Hg2+ probe (F°) and after melamine addition (F). The (F/F°) ratio increased with rising melamine concentrations within the range of 0.025-65 µM. The detection limit, calculated using a signal-to-noise ratio of 3, was determined to be 8.0 nM. The TSA-PVP@Cu NCs + Hg2+ probe was successfully employed to detect melamine in milk and infants' formula, yielding acceptable recovery percentages and relative standard deviations. These results underscore the reliability and efficacy of the proposed probe for the fluorometric detection of melamine in real-world samples.

A ratiometric fluorescence nanosensor for glutathione detection based on spatially confined dual-emission of α-lipoic acid-modified gold nanoclusters and silicon nanoparticles

The development of dual-emission ratiometric fluorescent probes with aggregation-induced emission enhancement (AIEE) overcomes the limitations of gold nanocluster (Au NC)-based probes, particularly their weak intrinsic fluorescence, in real-world applications. These AIEE probes also exhibit superior detection limits and enhanced sensitivity. A novel combination for the reliable fluorometric detection of glutathione (GSH) was proposed, utilizing aggregation-induced emission enhancement (AIEE) facilitated by electrostatic interaction and spatial confinement. The probe consists of a ratiometric combination of negatively charged α-lipoic acid-modified Au NCs (LA@Au NCs) and positively charged silicon nanoparticles (SiNPs). The addition of SiNPs causes aggregation of LA@Au NCs, enhancing the fluorescence of LA@Au NCs through the AIE effect under electrostatic interaction and spatial confinement. The addition of Cu2+ quenched the emission of LA@Au NCs as a result of charge transfer. The fluorescence emissions of LA@Au NCs were restored upon the addition of GSH due to the interaction between GSH and Cu2+. Simultaneously, the emission signal of SiNPs remains unchanged, serving as an internal reference signal during GSH measurement. It was found that the fluorescence ratio (F680/F465) is directly proportional to the concentration of GSH in the range of 0.05-100 μM, with a detection limit of 1.7 nM (S/N = 3). The proposed system was applied to detect GSH in real samples, including dietary supplements, human serum, and saliva samples. This work opens new avenues for constructing novel sensors based on AIEE for detecting biomolecules.

A facile synthesis of water-soluble copper nanoclusters as label-free fluorescent probes for rapid, selective and sensitive determination of alizarin red

Copper nanoclusters (FA@CuNCs) emitting blue fluorescence were successfully developed via a one-pot technique. In this method, the copper chloride, folic acid and hydrazine hydrate were applied as a precursor, protective agent and reducing agent, respectively. The absorption, fluorescence excitation and emission spectra of FA@CuNCs were carried out by using ultraviolet-visible and fluorescence spectrometry, respectively. The morphology, particle size, functional groups, oxidation states of elements of FA@CuNCs were discussed via using transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The stability of FA@CuNCs was studied under various conditions, such as storage time at 25 ℃, ultraviolet radiation time, sodium chloride solutione and pH. The FA@CuNCs displayed blue fluorescence under the excitation wavelength of 361 nm, and the fluorescence quantum yield was 7.45 %. As a result of the inner filter effect, the alizarin red could significantly weaken the blue fluorescence of FA@CuNCs. Thus, the as-prepared FA@CuNCs could be utilized as fluorescence nanosensors for the trace determination of alizarin red. This platform suggested an excellent linear range for alizarin red varying from 0.5 to 200 μM with a fitting coefficient of 0.9955. The detection limit was calculated to be 0.064 μM in the light of the 3b/k (b and k refer to the standard deviation and slope of fitted curve, respectively). Furthermore, the as-developed FA@CuNCs could be used to detect the alizarin red in real samples and for the sensing of temperature.

Synergism between copper and silver nanoclusters induces fascinating structural modifications, properties, and applications

Among the group 11 transition metal elements, Cu and Ag are widely studied due to their cost effectiveness and easy availability. However, the synergism between copper and silver is also very promising, exhibiting intriguing structures, properties, and applications. Nanoclusters, which are missing links between atoms and nanoparticles, are highly fluorescent due to their discrete energy levels. Their fluorescence can be efficiently tuned because of the synergistic behaviour of copper and silver. Furthermore, their fluorescence can be selectively altered in the presence of various analytes and sensing platforms, as reported by various groups. Moreover, copper clusters can be utilized for sensing silver while silver nanoclusters can be utilized for sensing ionic copper due to the strong interaction between copper and silver. Furthermore, DFT studies have been performed to understand the structural modification due to CuAg synergism. A concise summary of the synergism between copper and silver can open a new window of research for young scientists venturing into the field of environmental nanoscience.

Ultra-trace Ag doped carbon quantum dots with peroxidase-like activity for the colorimetric detection of glucose

Carbon quantum dots (CQDs) have significant applications in nanozymes. However, previous studies have not elucidated the structure-activity relationship and enzyme mechanism. In this study, we employed a one-step microwave method to synthesize ultra-trace Ag-doped carbon quantum dots (Ag-CQDs). In the presence of hydrogen peroxide (H2O2), we used the oxidative coupling reaction of 3,3',5,5'-tetramethylbenzidine (TMB) to evaluate the intrinsic peroxidase-like activity, kinetics, and mechanism of Ag-CQDs. The trace amount of doped Ag (1.64 %) facilitated electron transfer from the CQDs interior to the surface. The electron transfer triggered the peroxide activity of CQDs, producing hydroxyl radical (·OH), which oxidized the colorless TMB to blue-colored TMB (oxTMB). By coupling with glucose oxidase (GOx), the Ag-CQDs/H2O2/TMB system has been used for colorimetric glucose determination. The system demonstrated a low detection limit (0.17 µM), wide linear range (0.5-5.5 µM), and satisfactory results when fruit juice was analyzed. This study reports a feasible method for the colorimetric detection of glucose by synthesizing ultra-trace Ag-doped carbon quantum dots with peroxidase-mimicking activity.

Microwave-assisted preparation of yellow fluorescent graphitic carbon nitride quantum dots for trace tetracycline-specific detection

Tetracycline (TC) is extensively utilized in livestock breeding, aquaculture, and medical industry. TC residues seriously harm food security, the environment, and human health. There is an urgent need to exploit a highly efficient and sensitive testing method to monitor TC residue levels in aquatic environments. In this study, graphitic carbon nitride quantum dots (g-CNQDs) were successfully synthesized by a one-step microwave-assisted method using citric acid and urea as precursors. The as-prepared g-CNQDs with size of 1.25-3.75 nm exhibited bright yellow fluorescence at 523 nm when excited at 397 nm. Interestingly, this characteristic fluorescence emission of g-CNQDs could be selectively and efficiently quenched by TC. Based on this phenomenon, for TC detection was successfully explored and applied in real water samples. Wide linear scope of 7-100 μM, low detection limit (LOD) of 0.48 μM, satisfactory recovery of 97.77%-103.4%, and good relative standard deviation (RSD) of 1.05-5.87% were obtained. Mechanism investigations revealed that the static quenching and the inner filter effect (IFE) were responsible for this fluorescence quenching between g-CNQDs and TC. This work not only provided a facile approach for g-CNQDs synthesis but also constructed a g-CNQDs-based fluorescent sensor platform for the highly sensitive and selective detection of TC in aquatic environments.

Enhanced fluorometric detection of histamine using red emissive amino acid-functionalized bimetallic nanoclusters

Lysine-capped gold nanoclusters doped with silver (LYS@Ag/Au NCs) have been developed for the sensitive and selective "turn-off" fluorescence detection of histamine. This fluorescent probe demonstrates excellent stability and a high quantum yield of 9.45%. Upon addition of histamine, a positively charged biogenic amine, to the LYS@Ag/Au NCs fluorescent probe, its fluorescence emission is quenched due to electrostatic interaction, aggregation, and hydrogen bond formation. The probe exhibits good sensitivity for the determination of histamine within the range of 0.003-350 μM, with a detection limit of 0.001 μM based on a signal-to-noise ratio of 3. Furthermore, the probe has been applied to detect biogenic amines in complicated matrices, highlighting its potential for practical applications. However, interference from the analogue histidine was observed during analysis, which can be mitigated by using a Supelclean™ LC-SAX solid-phase extraction column for removal.

Tryptophanhydroxamic Acid-Stabilized Ultrasmall Gold Nanoclusters: Tuning the Selectivity for Metal Ion Sensing

Sub-nanometer-sized gold nanoclusters (Au NCs) were prepared via the spontaneous reduction of [AuCl4]-- ions with a hydroxamate derivative of L-tryptophan (Trp) natural amino acid (TrpHA). The prepared TrpHA-Au NCs possess intense blue emission (λem = 470 nm; λex = 380 nm) with a 2.13% absolute quantum yield and 1.47 ns average lifetime. The Trp-stabilized noble metal NCs are excellent metal ion sensors for Fe3+, but in this work, we highlighted that the incorporation of the hydroxamate functional group with an excellent metal ion binding capability can tune the selectivity and sensitivity of these NCs, which is a promising way to design novel strategies for the detection of other metal ions as well. Moreover, their simultaneous identification can also be realized. By decreasing the sensitivity of our nano-sensor for Fe3+ (limit of detection (LOD) ~11 µM), it was clearly demonstrated that the selectivity for Cu2+-ions can be significantly increased (LOD = 3.16 µM) in an acidic (pH = 3-4) condition. The surface-bounded TrpHA molecules can coordinate the Cu2+ confirmed by thermodynamic data, which strongly generates the linking of the NCs via the Cu2+ ions in acidic pH, and a parallel fluorescence quenching occurs. In the case of Fe3+, the degree of quenching strongly depends on the metal ion concentration, and it only occurs when the NCs are not able to bind more Fe3+ (~10 µM) on the surface, causing the NCs' aggregation.

A novel colorimetric/fluorescent dual-signal probe based on silver nanoparticles functionalized with L-cysteine and rhodamine 6G derivatives for copper ion detection and cell imaging

The dual-signal probe utilizing functionalized silver nanoparticles (AgNPs) is a promising sensing tool. Herein, a novel colorimetric/fluorescent dual-signal probe (AgNPs-L-Cys-Rh6G2) was fabricated for copper ion (Cu2+) detection and cell imaging by using L-cysteine as a "bridge" to connect AgNPs and rhodamine 6G derivatives. The AgNPs-L-Cys-Rh6G2 probe exhibits a dual-signal response to Cu2+ due to Rh6G2 hydrolysis, resulting in a high fluorescence response and a significant change in color from light yellow to pink under sunlight. The linear detection ranges of the AgNPs-L-Cys-Rh6G2 probe for Cu2+ were 100-450 μM and 150-650 μM using fluorescent and colorimetry methods, respectively. The detection limits were as low as 0.169 μM and 1.36 μM, respectively. Meanwhile, the proposed probe was applied to detect Cu2+ in the actual sediment with satisfactory recovery and low relative standard deviation. Furthermore, the probe was further employed for fluorescence imaging in HeLa cells. In brief, the developed AgNPs-L-Cys-Rh6G2 sensing platform can be used for simultaneous Cu2+ determination and cell imaging.

Highly photoluminescent tryptophan-coated copper nanoclusters based turn-off fluorescent probe for determination of tetracyclines

Employing cheap Cu nanoclusters to design a novel fluorescent probe have promising opportunities in the field of optical sensors. Here, we fabricated a highly photoluminescent D-tryptophan (D-Trp)-coated Cu nanoclusters (Trp-Cu NCs) by rapid microwave-assisted method to achieve precise quantification of tetracyclines (TC). Due to protecting groups of Trp, the synthesized Trp-Cu NCs have remarkable fluorescence stability with a quantum yield reached 12.5%. A distinct fluorescence quenching with the incremental addition of TC via the internal filtration effect (IFE). Based on turn-off fluorescence within 1 min, a detection method for detecting TC was constructed with a linear range of 0.3-120 μM and a limit of detection (LOD) of 0.12 μM. Besides, the proposed fluorescent probe has been employed for the determination of practical samples such as water samples, milk and honey, and exhibited satisfactory recoveries of 96.1%-108.2%, with relative standard deviations (RSD) lower than 5.0%. This is a sensitive, rapid and easily recognizable Trp-Cu NCs based sensing platform for the determination of TC, which could offer a powerful tool for ensuring food safety.

In situ microwave-assisted preparation of NS-codoped carbon dots stabilized silver nanoparticles as an off-on fluorescent probe for trace Hg2+ detection

An off-on fluorescent probe (NS-CDs-AgNPs) was synthesized based on a one-pot microwave process by utilizing N, S co-doping carbon dots (NS-CDs) and silver nitrate as precursors. The significant peak of NS-CDs-AgNPs at 393 nm in ultraviolet spectrum indicated silver nanoparticle (AgNPs) were successfully synthesized. A faint blue fluorescence emission (442 nm) was displayed when excited NS-CDs-AgNPs at 371 nm. A remarkable fluorescence recovery was observed upon adding of trance Hg2+, whereas the other heavy metal ions did not elicit this response. The reason for this phenomenon was revealed in this work that a spontaneous redox reaction occurred between NS-CDs-AgNPs and Hg2+, which leaded to the formation of NS-CDs-Agn-2NPsHg complexes. On the basis of this mechanism, a new off-on fluorescent analytical method was constructed for Hg2+ detection with linear range of 10-400 nM (R2 = 0.9941), and the detection limit (LOD) of 5.16 nM. Additionally, satisfactory recovery (90.28%-106.13%) and the relative standard deviation (RSD) (RSD＜5.21%) were obtained in water sample detection. More importantly, the NS-CDs-AgNPs exhibited lower cytotoxicity and better biocompatibility, indicating a huge potential in cell imaging and clinical medicine.

A Highly Sensitive Dual-Signal Strategy via Inner Filter Effect between Tween 20-Gold Nanoparticles and CdSe/ZnS Quantum Dots for Detecting Cu2

A highly sensitive and accurate dual-signal strategy is developed for trace Cu²⁺ detection based on the inner filter effect (IFE) between Tween 20-gold nanoparticles (AuNPs) and CdSe/ZnS quantum dots (QDs). Tween 20-AuNPs are utilized as colorimetric probes and excellent fluorescent absorbers. The fluorescence of CdSe/ZnS QDs can be quenched efficiently by Tween 20-AuNPs via IFE. In the presence of D-penicillamine, D-penicillamine induces the aggregation of Tween 20-AuNPs and the fluorescent recovery of CdSe/ZnS QDs at high ionic strength. Upon addition of Cu²⁺, D-penicillamine tends to selectively chelate with Cu²⁺ and then forms the mixed-valence complexes, which consequently inhibits the aggregation of Tween 20-AuNPs and the fluorescent recovery. The dual-signal method is used to quantitatively detect trace Cu²⁺, with low detection limits of 0.57 μg/L and 0.36 μg/L for colorimetry and fluorescence, respectively. In addition, the proposed method using a portable spectrometer is applied to the detection of Cu²⁺ in water. This sensitive, accurate and miniature sensing system has potential in environmental evaluations.

A copper ion-mediated on-off-on gold nanocluster for pyrophosphate sensing and bioimaging in cells

Herein, gold nanoclusters (AuNCs@EW@Lzm, AuEL) with the bright red fluorescence at 650 nm were prepared by egg white and lysozyme as double protein ligands, which exhibited good stability and high biocompatibility. The probe displayed highly selective detected pyrophosphate (PPi) based on Cu²⁺-mediated AuEL fluorescence quenching. Specifically, the fluorescence of AuEL was quenched once the Cu²⁺/Fe³⁺/Hg²⁺ is added to chelate with amino acids on the AuEL surface, respectively. Interestingly, the fluorescence of quenched AuEL-Cu²⁺ was significantly recovered by PPi, but not the other two. This phenomenon was attributed to the stronger bond between PPi and Cu²⁺ than that of Cu²⁺ with AuEL nanoclusters. The results demonstrated a good linear relationship between PPi concentration and the relative fluorescence intensity of AuEL-Cu²⁺ in the range of 131.00-685.40 μM with a detection limit of 2.56 μM. In addition, the quench AuEL-Cu²⁺ system can also be recovered in acidic environments (pH ≤ 5). And the as-synthesized AuEL showed excellent cell imaging and target the nucleus. Thus the fabrication of AuEL offers a facile strategy for efficient PPi assay and offers the potential for drug/gene delivery to the nucleus.

Sensitive detection and intracellular imaging of free copper ions based on DNA-templated silver nanoclusters aggregation-inducing fluorescence enhancement effect

Free copper ions (Cu⁺ and Cu²⁺) have critical toxicity to cells, although copper is an essential element for human body. Hence, sensitive monitoring is crucial to avoid over intake of Cu⁺/Cu²⁺. We herein designed a ssDNA sequence (A₃₁) for synthetizing A₃₁-templated silver nanoclusters (AgNCs), and demonstrated that Cu⁺/Cu²⁺ can induce the aggregation of A₃₁-templated AgNCs and thus greatly enhanced the fluorescence emission of A₃₁-templated AgNCs. Based on Cu⁺/Cu²⁺-induced fluorescence enhancement effect of A₃₁-templated AgNCs, a label-free and signal-on fluorescent sensing platform was developed for the specific and sensitive detection of Cu⁺/Cu²⁺ in biological samples and intracellular imaging of Cu⁺/Cu²⁺ in cells. The signal-on fluorescent sensing platform could be used to rapidly detect Cu⁺ and Cu²⁺ with a detection limit of 0.1 µM within 30 min., and to perform the intracellular imaging of Cu⁺ and Cu²⁺ in cells with good cell permeability and biocompatibility. By using the signal-on fluorescent sensing platform, we have successfully detected Cu⁺ and Cu²⁺ in cells fluids and human serum with a recovery of 90-104% and a RSD (n = 5) < 5%, and performed the imaging of Cu⁺/Cu²⁺ in Hela cells. The developed fluorescent sensing platform has obvious analytical and imaging advantages such as signal-on, simple operation, short analysis time, both Cu⁺ and Cu²⁺ detection, similar or higher sensitivity, good cell permeability and biocompatibility, which promising a reliable approach for the rapid and on-site detection or imaging of free copper ions in biological samples in clinical diagnosis.

Response surface methodology optimization for the synthesis of N, S-codoped carbon dots and its application for tetracyclines detection

In this study, using a convenient one-pot microwave-assisted method, we rapidly fabricated a N, S-codoped fluorescent carbon dots (NSCDs) through using citric acid (CA) and _d-penicillamine (DPA) for the detection of tetracyclines (TC). To rapidly and efficiently optimize the various synthesis parameters and significantly decrease the number of experimental runs, the effect of the synthesis factors on quantum yield (QY) by NSCDs was implemented through means of response surface methodology (RSM). The as-synthesized NSCDs presented superior photoluminescence stability with a strong quantum yield (QY) of 91.55% under optimal conditions, which was consistent with the predicted value using RSM. The fluorescence intensity of the NSCDs could be quenched effectively after adding TC by the inner filter effect (IEE) and photoinduced electron transfer (PET). Thus, the determination of TC by using NSCDs as a facile fluorescent probe was constructed in the range of 0.2-70 μM with a limit detection of 0.072 μM. Moreover, this detection approach has been utilized to detect TC in actual samples with satisfactory results.

Recent advances in the visual detection of ions and molecules based on gold and silver nanoclusters

Gold and silver nanoclusters (Au/AgNCs) exhibit excellent application potential in optical biosensors because of their low toxicity, excellent biocompatibility, and unique optical properties. Au/AgNCs-based visual analysis methods have emerged as powerful tools for detecting various targets with convenient readout. In this review, the applications of Au/AgNCs in the visual detection and bioimaging of metal ions, inorganic anions, small molecules, and biomacromolecules in various devices are summarized. Furthermore, this review also discusses the future perspectives of the field.