A ratiometric nanoprobe based on silver nanoclusters and carbon dots for the fluorescent detection of biothiols

A review on the chemical and biological sensing applications of silver/carbon dots nanocomposites with their interaction mechanisms

The development of new nanocomposites has a significant impact on modern instrumentation and analytical methods for chemical analysis. Due to their unique properties, carbon dots (CDs) and silver nanoparticles (AgNPs), distinguished by their unique physical, electrochemical, and optical properties, have captivated significant attention. Thus, combining AgNPs and CDs may produce Ag/CDs nanocomposites with improved performances than the individual material. This comprehensive review offers an in-depth exploration of the synthesis, formation mechanism, properties, and the recent surge in chemical and biological sensing applications of Ag/CDs with their sensing mechanisms. Detailed insights into synthesis methods to produce Ag/CDs are unveiled, followed by information on their physicochemical and optical properties. The crux of this review lies in its spotlight on the diverse landscape of chemical and biological sensing applications of Ag/CDs, with a particular focus on fluorescence, electrochemical, colorimetric, surface-enhanced Raman spectroscopy, and surface plasmon resonance sensing techniques. The elucidation of sensing mechanisms of the nanocomposites with various target analytes adds depth to the discussion. Finally, this review culminates with a concise summary and a glimpse into future perspectives of Ag/CDs aiming to achieve highly efficient and enduring Ag/CDs for various applications.

A Competitive "On-Off-Enhanced On" AIE Fluorescence Switch for Detecting Biothiols Based on Hg2+ Ions and Gold Nanoclusters

In this study, a novel "on-off-enhanced on" approach to highly sensitive rapid sensing of biothiols was developed, based on competitive modulation of gold nanoclusters (AuNCs) and Hg2+ ions. In our approach, the AuNCs were encapsulated into a zeolite imidazole framework (ZIF) for predesigned competitive aggregation-induced luminescence (AIE) emission. To readily operate this approach, the Hg2+ ions were selected as mediators to quench the fluorescence of AuNCs. Then, due to the stronger affinities between the interactions of Hg2+ ions with -SH groups in comparison to the AuNCs with -SH groups, the quenched probe of AuNCs@ZIF-8/Hg2+ displayed enhanced fluorescence after the Hg2+ ions were competitively interacted with -SH groups. Based on enhanced fluorescence, the probe for AuNCs@ZIF-8/Hg2+ had a sensitive and specific response to trace amounts of biothiols. The developed fluorescence strategy had limit of quantification (LOQ) values of 1.0 μM and 1.5 μM for Cys and GSH molecules in serum, respectively. This competitive AIE strategy provided a new direction for developing biological probes and a promising method for quantifying trace amounts of biothiols in serum. It could promote progress in disease diagnosis.

Fluorescence probe for selectively monitoring biothiols within cells and mouse depression diagnosis

As a global mental disorder, depression is associated with oxidative stress in the brain. Cysteine, a reductive biothiols, regulates the oxidative situation in many biological events including the stress that occurs in the tissues. Exploring the pathology and physiology of depression is still a challenge and always in an urgent need. Thus, developing a new method that could track Cys level without the interferes from other competing substances is of great importance. Herein, we developed a fluorescence probe that could selectively sensing Cys over other biothiols. Besides, we have demonstrated its desirable performance in cellular applications and mouse brain. This work provides a new method for Cys imaging and understanding pathogenesis of depression. We hope the work described here could be used as a potential chemical approach for the diagnosis of Cys associated diseases in clinical applications.

A Novel Ratio Probe Based on Mixing of Thiocyanuric Acid- Enhanced Silver Nanoclusters with N, S Co-Doped Carbon Quantum Dots for Detecting Sodium 2,3-Dimercapto Propanesulfonic Acid

Background

The ratio fluorescent probe (RF-probe) has the characteristics of self-calibration and eliminating the influence of experimental factors (such as probe concentration, excitation intensity stability, and solution environment) in quantitative assays based on the linear relationship between the intensity ratio of two emission peaks and the concentration of analyte, especially in a complex biological matrix.

Objective

High-luminescent Ag nanoclusters (AgNCs) were prepared at room temperature due to aggregation-induced emission (AIE) through the incorporation of trithiocyanuric acid (TA). Subsequently, a new RF-probe based on mixing of AgNCs-TA with nitrogen and sulfur co-doped carbon quantum dots (N, S-CQDs) was prepared for sensitively and selectively assaying sodium 2, 3-dimercaptopropane-1-sulfonate (DMPS).

Methods

The fluorescence of N, S-CQDs was quenched in the presence of the AgNCs-TA mainly deriving from the inner filter effect (IFE), and its fluorescence intensities at 440 and 650 nm could recover and drop upon the addition of DMPS (λex = 370 nm).

Results

The RF-probe enables DMPS to be detected by fluorometry with a linear response in the 0.67–30.0 μmol/L concentration range and a 54.7 nmol/L detection limit (at 3σ/slope). At the same time, we also used the labeled recovery method to confirm the reliability of the method. The recoveries were 97.93–106.33%, and the corresponding standard deviations (RSD) were less than 1.87%.

Conclusion

The N, S-CQDs/AgNCs-TA RF-probe can also semi-quantitatively monitor DMPS by naked eyes.

Highlights

The mechanism of fluorescence enhancement of the AgNCs by TA also were investigated by the N, S-CQDs/AgNCs-TA-DMPS system. What’s more, the RF-probe of N, S-CQDs/AgNCs-TA was successfully utilized to monitor DMPS in real samples.

Combining metal nanoclusters and carbon nanomaterials: Opportunities and challenges in advanced nanohybrids

The development of functional materials with uniquely advanced properties lies at the core of nanoscience and nanotechnology. From the myriad possible combinations of organic and/or inorganic blocks, hybrids combining metal nanoclusters and carbon nanomaterials have emerged as highly attractive colloidal materials for imaging, sensing (optical and electrochemical) and catalysis, among other applications. While the metal nanoclusters provide extraordinary luminescent and electronic properties, the carbon nanomaterials (of zero, one or two dimensions) convey versatility, as well as unique interfacial, electronic, thermal, optical, and mechanical properties, which altogether can be put to use for the desired application. Herein, we present an overview of the field, for experts and non-experts, encompassing the basic properties of the building blocks, a systematic view of the chemical preparation routes and physicochemical properties of the hybrids, and a critical analysis of their ongoing and emerging applications. Challenges and opportunities, including directions towards green chemistry approaches, are also discussed.

Silver nanoparticles combined with amino-functionalized UiO-66 for sensitive detection of glutathione

Herein, a fluorescent nanosensor has been constructed for detection of glutathione (GSH) based on NH₂-UiO-66 and AgNPs. The NH₂-UiO-66 was synthesized where 2-amino-terephthalic acid as the organic ligand and Zr⁴⁺ as the center metal ions. The AgNPs can enhance the fluorescence of NH₂-UiO-66 based on metal enhanced fluorescence (MEF) effect. Moreover, in the present of GSH, the fluorescence of NH₂-UiO-66@AgNPs was quenched via electrostatic interaction and Ag-S reaction. The present sensing strategy shows good linear relation with the concentration of GSH in the range of 0.2-1.0 μM and 1.0-30 μM, and the limit of detection is 79 nM. Furthermore, our fluorescent nanosensor was utilized to detect GSH in human serum with a recovery of 96.8-102.5%. The results indicated that NH₂-UiO-66@AgNPs is successfully applied for high sensitive and selective detection of GSH in human serum.

Ratiometric determination of Cr(VI) based on a dual-emission fluorescent nanoprobe using carbon quantum dots and a smartphone app

A simple dual-colour fluorescent nanoprobe has been designed composed of blue and yellow emission carbon quantum dots (CQDs). This system is inexpensive and easy to operate and was successfully employed for on-site measurements based on a smartphone app. The designed nanoprobe exhibited increased selectivity for Cr(VI), leading to a double stable response of the two CQDs. The dual-emission nanoprobe showed blue-violet fluorescence upon UV irradiation, and the fluorescent emission peaks were located at 418 nm and 552 nm. The blue light emission of CQDs was quenched with increasing Cr(VI) concentration due to the inner filter effect, whereas the yellow light emission was enhanced due to the aggregation-induced emission effect. The different responses of the dual emissions to Cr(VI) resulted in a fluorescent colour variation, thus enabling facile macroscopic visualization. With a smartphone, the change in the fluorescence colour could be observed more apparently than that of a single fluorescence nanoprobe, and the response increased linearly so that the nanoprobe could be applied to instantaneous measurements. Furthermore, the dual-emission nanoprobe was successfully employed for analysing food and water samples. Accurate concentrations were obtained by constructing a calibration plot using a fluorescence spectrometer and a smartphone app; the recoveries were 81.6% to 107.7%, and the relative standard deviation was below 3.6%. Therefore, this smartphone-integrated dual-emission detection system is promising as a new portable method for the on-site measurement of Cr(VI) ions. * Y-CQDs: yellow emission carbon quantum dots. B-CQDs: blue emission carbon quantum dots. B/Y-CQDs: a mixture of B-CQDs and Y-CQDs.

Nanozyme based on CoFe2O4 modified with MoS2 for colorimetric determination of cysteine and glutathione

A nanozyme based on CoFe₂O₄ modified with MoS₂ was constructed for colorimetric determination of cysteine (Cys) and glutathione (GSH). Firstly, ferrite CoFe₂O₄ is synthesized, and it is then modified by MoS₂ to form a flower-like polymer (MoS₂@CoFe₂O₄). In the presence of H₂O₂, a redox interaction takes place, and the resulting hydroxyl promoted a colorimetric conversion from colorless to blue in the presence of 3,3',5,5'-tetramethylbenzidine (TMB). However, once Cys or GSH is added, they are capable to compete with the interaction of the hydroxyl with TMB, resulting in an inhibition of the colorimetric conversion. The colorimetric distinction is sensitive to the amount of target. The results obtained proved that the catalytic efficiency of MoS₂@CoFe₂O₄ is 4.4-fold and 1.8-fold to that of MoS₂ and CoFe₂O₄. Meanwhile, the K_m values to TMB and H₂O₂ are 0.067 and 0.048 mM, respectively, which are 6.5-fold and 77-fold, respectively smaller than those of natural peroxidase such as HPR. This indicates that the MoS₂@CoFe₂O₄ possesses a favorable interaction affinity. Additionally, the colorimetric distinction caused by the competition between TMB and cysteine or glutathione is obvious. The signal responses to cysteine and glutathione are linear in the range 0.5~15 μM and 0.5~35 μM, and the LODs are 0.10 and 0.21 μM, respectively. In practical assay of Cys in serum, the RSD of the sample tests is 4.6%, and the recoveries for the spiked assays are 95.3% and 96.0% with the RSD of 2.1% and 4.2%, respectively.

High-sensitivity Detection of Cysteine and Glutathione Using Au Nanoclusters Based on Aggregation-induced Emission

Gold nanoclusters (AuNCs) stabilized by glutathione (GSH) have been synthesized using a simple one-pot method, which were used as a fluorescence-enhanced probe for the detection of cysteine (Cys) and GSH. The detection is based on the finding that the weak yellow fluorescence of the AuNCs, with excitation/emission maxima of 430/600 nm, can be enhanced by Cys and GSH via NCs aggregation. This method is selective for Cys and GSH. According to the fluorescence enhancement, the detection ranges of AuNCs for Cys and GSH are 2.49 µM ~ 0.80 mM and 1.99 µM ~ 0.44 mM, with the detection limit of 0.42 µM and 0.27 µM, respectively. In addition, the probe has good anti-interference performance over other common biomolecules. Importantly, the probe is successfully used for the determination of Cys in human serum samples, displaying the potential application of the probe in the detection of biological sulfhydryl molecules in actual samples.

Carbon dots based ratiometric fluorescent sensing platform for food safety

Food safety has become a major global concern and the rapid detection of food nutritional ingredients and contaminants has aroused much more attention. Nanomaterials-based fluorescent sensing holds great potential in designing highly sensitive and selective detection strategies for food safety analysis. Carbon dots (CDs) possess tremendous prospects in fluorescent sensing food ingredients and contaminants due to their superior properties of chemical and photostability, highly fluorescence with tunability, and no/low-toxicity. Numerous endeavors are demanded to contribute to overcoming the challenge of lower sensitivity and selectivity of the sensors interfered by various components in intricate food matrices to ensure food safety and human health. Nanohybrid CDs based ratiometric fluorescent sensing with self-calibration is regarded as an efficient strategy for the CDs based sensors for the specific recognition of target analyte in the food matrices. This work is devoted to reviewing the development of nanohybrid CDs based ratiometric fluorescent sensing platform and the perspectives of the platform for food safety. The applications of nanohybrid CDs in sensing are summarized and the sensing mechanisms are briefly discussed.

A fluorescein-gold nanoparticles probe based on inner filter effect and aggregation for sensing of biothiols

Cysteine (Cys), homocysteine (HCys) and glutathione (GSH) are sulfhydryl-containing amino acids known as biothiols being able to bind to gold nanoparticles (AuNPs) via sulfhydryl group, resulting in the aggregation of AuNPs. Owning to their inner filter effect, AuNPs can weaken or even quench the fluorescence of fluorescein. However, the introduction of biothiols to fluorescein-AuNPs leads to the recovery of fluorescein fluorescence. Thus, a simple and reliable turn on fluorescence method was developed for monitoring biothiols with fluorescein-AuNPs as a probe. Several factors, including AuNPs concentration, pH value and incubation time, which might influence the fluorescence reclamation of fluorescein-AuNPs probe, were optimized by taking Cys as an example at room temperature. Under the optimal conditions, sensitive sensing of Cys, HCys and GSH was achieved. The detection limits for Cys, GSH, and HCys were 0.027, 0.023, and 0.030μΜ, respectively. This method was used to the determination of Cys in human serum samples with high precision and accuracy, indicating the potential of the method in practical applications with simple operation, good accuracy and high sensitivity.

Synthesis and characterization of maltol capped silver nanoparticles and their potential application as an antimicrobial agent and colorimetric sensor for cysteine

Maltol capped silver nanoparticles (McAgNPs) were synthesized using maltol (3-hydroxy-2-methyl-4-pyrone) as reducing and capping agent. McAgNPs were characterized by Visible and FTIR (Fourier transform infrared) spectroscopy, dynamic light scattering (DLS), and atomic force microscopy (AFM). Bright yellow color McAgNPs showed surface plasmon resonance (SPR) band at 436 nm, spherical shape and the average size between 35 to 50 nm. McAgNPs revealed higher stability against varying storage time, temperature, pH and salt concentrations. McAgNPs were successfully utilized for the selective and highly sensitive colorimetric detection of cysteine (Cys). Addition of Cys in a solution of McAgNPs, resulted a rapid change in color from yellow to orange because of the formation of nanoaggregates as confirmed by Visible/FTIR spectroscopy, DLS, and AFM studies. The estimated limit of detection (0.043 μM) was found to be more sensitive than previously reported other optical methods. The practical applicability of probe was also established by spiking the known concentrations of Cys in biological (blood plasma and urine) and environmental (tap and lake water) samples with significant recovery rates (92-104.6%). Despite being nontoxic to various tested cell lines, McAgNPs demonstrated potent antimicrobial, antibiofilm, and biofilm eradicating activities, thus potentially valuable in diagnostics and/or the synthesis of other nanocomposite material for broader applications.

Facile synthesis of highly efficient fluorescent carbon dots for tetracycline detection

Rampant use of tetracycline in animal feed is a threat to food security, the environment, and human health because of the risk of drug residues. Therefore, it is necessary to establish a sensitive, efficient, and reliable method for qualitative and quantitative detection of tetracycline. In this paper, we synthesized fluorescent carbon dots (FCDs) by thermal cracking of crab shell waste, and obtained a fluorescence quantum yield of 30%. Characterization of the FCDs by transmission electron microscopy, Fourier-transform infrared spectroscopy, ultraviolet visible absorption spectroscopy, and photoluminescence spectroscopy showed that they were fluorescent and evenly distributed with an average size of approximately 10 nm. We designed a sensitive probe for detecting tetracycline using the fluorescence intensity change of the FCDs. This method is sensitive, inexpensive, and environmentally friendly. The concentration of tetracycline was examined by comparing the fluorescence intensities of the FCDs before and after tetracycline addition. The limit of detection for tetracycline was 0.005 mg/L (signal-to-noise ratio = 3), which is promising for method development.

Biomolecule conjugated metal nanoclusters: bio-inspiration strategies, targeted therapeutics, and diagnostics

To help those suffering from viral infections and cancers, scientists are exploring enhanced therapeutic methods via metal nanoclusters (MNCs).

A ratiometric fluorescence and light scattering sensing platform based on Cu-doped carbon dots for tryptophan and Fe(III)

A new Cu-doped carbon dots (Cu-CDs) were synthesized rapidly and simply via one-step thermolysis of Na₂[Cu(EDTA)] and ascorbic acid (AA) at 250°C for 2h with a high quantum yield of 9.8%. The Cu-CDs exhibits two signals of fluorescence at 396nm and second-order scattering (SOS) at 617nm under a single excitation wavelength of 308nm, and can be obviously enhanced by tryptophan (Trp) or Fe(III) leading to the ratiometric fluorescence and SOS response with a good linear wider range of 0.5-250μM and 0.1-50μM, respectively. This sensing system exhibits good selectivity and sensitivity toward Trp and Fe(III) over other analytes with a low detection limit of 275nM and 46nM, respectively. Furthermore, the proposed sensing system displays a prospective application for quantitative assay of Trp and Fe(III) in practical sample.

Fluorescence lifetime-based pH sensing by platinum nanoclusters

In this work, we report for the first time the application of ultrasmall Pt nanoclusters (Pt NCs) as a fluorescence-lifetime-based pH sensor. The fluorescence properties of our prepared Pt NCs show strong pH dependence, due to the change of the surface states of Pt NCs under alkaline/acidic conditions. Interestingly, both the fluorescence intensity and the fluorescence lifetime of Pt NCs exhibit a good linear relationship in the physiologically relevant pH range of 6.02-7.54. Our Pt NC-based pH sensor possesses many important features as an attractive sensor, including high anti-jamming capability, excellent stability and good reversibility. Moreover, the use of fluorescence lifetime as the optical readout signal rather than the intensity makes the present pH sensor more reliable in practical applications because the fluorescence lifetime is invariant to the probe concentration or changes in excitation conditions. Application of the Pt NC-based system for pH detection in real water samples and simulated intracellular environment samples has also been successfully demonstrated.

Facile preparation of N,S-graphene oxide nanosheets as a fluorescence “off–on” sensing platform for sensitive detection of biothiols

Fluorescent N,S-graphene oxide was prepared via a one-step, solvent-free approach, and used as a probe for sensitive detection of biothiols.

Nitrogen and Sulfur Co-doped Fluorescent Carbon Dots for the Detection of Morin and Cell Imaging

Background:

Morin has many pharmacological functions including antioxidant, anticancer, anti-inflammatory, and antibacterial effects. It is commonly used in the treatment of antiviral infection, gastropathy, coronary heart disease and hepatitis B in clinic. However, researches have shown that morin is likely to show prooxidative effects on the cells when the amount of treatment is at high dose, leading to the decrease of intracellular ATP levels and the increase of necrosis process. Therefore, it is necessary to determine the concentration of morin in biologic samples.

Method:

Novel water-soluble and green nitrogen and sulfur co-doped carbon dots (NSCDs) were prepared by a microwave heating process with citric acid and L-cysteine. The fluorescence spectra were collected at an excitation wavelength of 350 nm when solutions of NSCDs were mixed with various concentrations of morin.

Results:

The as-prepared NSCDs were characterized by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The fluorescence intensity of NSCDs decreased significantly with the increase of morin concentration. The fluorescence intensity of NSCDs displayed a linear response to morin in the concentration 0.10-30 μM with a low detection limit of 56 nM. The proposed fluorescent probe was applied to analysis of morin in human body fluids with recoveries of 98.0-102%.

Conclusion:

NSCDs were prepared by a microwave heating process. The present analytical method is sensitive to morin. The quenching process between NSCDs and morin is attributed to the static quenching. In addition, the cellular toxicity on HeLa cells indicated that the as-prepared NSCDs fluorescent probe does not show obvious cytotoxicity in cell imaging. Our proposed method possibly opens up a rapid and nontoxic way for preparing heteroatom doped carbon dots with a broad application prospect.

Dual-channel fluorescence detection of mercuric (II) and glutathione by down- and up-conversion fluorescence carbon dots

The fluorescent carbon dots (CDs) with high fluorescent quantum yield (φ_f = 62%) and down- and up-conversion fluorescence properties were synthesized by one-pot hydrothermal treatment of citric acid and tris(hydroxymethyl)methyl aminomethane. The CDs displayed the capability to absorb excitation wavelength at 660 nm and 330 nm with fluorescence emission wavelength at 398 nm and 399 nm, respectively. The CDs showed high selectivity towards Hg²⁺ against various metal ions. Around 70% fluorescence was quenched by 40 μM Hg²⁺ through dynamic and static quenching mechanisms. Because of stronger affinity between the thiol and Hg²⁺, over 90% fluorescence was recovered by adding 40 μM glutathione to CDs-Hg²⁺ system. The calibration curves exhibited wide linear region for Hg²⁺ (0-4 μM) and glutathione (0-30 μM). The limits of detection with down- and up-conversion for Hg²⁺ were calculated to be 0.23 μM and 0.25 μM, and for glutathione were 0.28 μM and 0.29 μM, respectively. Inspired by the sensing results, logic gates with Hg²⁺ and glutathione as inputs were also established. Most importantly, this method was applied to detect Hg²⁺ and glutathione in tap water and lake water, and the recovery values were obtained to be 96.2%-110.4% and 93.4%-96.9%.