Boosted anodic electrochemiluminescence from blue-emissive sulfur quantum dots and its bioanalysis of glutathione

Homogeneous electrochemiluminescence sensor based on novel Ru-MOF nanosheets for highly sensitive and efficient detection of glutathione without immobilization

Glutathione (GSH), a key antioxidant, plays a crucial role in maintaining redox balance and other essential physiological functions in the body. Electrochemiluminescence (ECL) technology holds great potential for GSH detection due to its excellent performance. However, most existing ECL probes rely on heterodyne detection, where the probe must be immobilized on the electrode surface. This approach not only limits detection to the solid-liquid interface, thereby affecting identification efficiency, but also risks probe detachment or reduced activity due to instability during the immobilization process, which in turn compromises sensitivity and accuracy. These limitations make heterodyne detection challenging in practical applications, highlighting the need for improvements and innovation. In this study, we synthesized a water-soluble ECL probe using tris(2,2'-bipyridine)ruthenium (Ru(dcbpy)₃2⁺)-functionalized metal-organic frameworks (RuMOFNSs), enabling efficient and reliable GSH detection under homogeneous conditions. The experimental results demonstrated that the probe exhibited excellent sensitivity and selectivity across a concentration range from 10 mM to 10 pM, with a detection limit as low as 0.57 pM, and was able to achieve a rapid response within 5 min. In addition, the sensor showed excellent performance in analysis of real human serum samples. In this study, an ultrasensitive ECL sensing system for GSH detection based on MOF materials is proposed for the first time and provides an effective method for the monitoring of diseases related to GSH levels in the clinic.

Silicon oxide microchips functionalized with fluorescent probes for quantitative real-time glutathione sensing in living cells

Glutathione (GSH) plays a vital role in the regulation of intracellular functions which alterations in physiological glutathione levels are associated to various diseases. Molecular bioimaging is a sensitive method for GSH detection, but challenges persist in the development of fluorescent probes, mainly concerning long-term tracking of intracellular GSH concentration because of aggregation of molecular probes and their washout in cells. Engineered nanomaterials have shown great promise for increasing the disease diagnosis accuracy. Microchips generated by advanced microfabrication techniques can be applied in designing biomedical devices due to control over size, shape, and bioactive coatings utilization. In the current work, the synthesis and characterization of two GSH probes, Bdpy1 and Bdpy2, is reported, each offering irreversible and reversible GSH reactions, respectively. These GSH probes are immobilized on silicon oxide microchips (SOμC), micro-fabricated using photolithographic techniques, to give SOμC-Bdpy1 and SOμC-Bdpy2. Both functionalized microchips exhibited sensitivity to GSH, and, notably, the reversible SOμC-Bdpy2 showed less time dependency, making it more suitable for long-term intracellular GSH sensing. In vitro experiments in HeLa cells reveal both SOμC-Bdpy1 and SOμC-Bdpy2 were internalized in living cells, showing SOμC-Bdpy2 more reliable results (due to its less time dependency) for quantifying intracellular GSH. Remarkably, the intracellular GSH measurement was monitored by SOμC-Bdpy2 for 48 h, indicating the functionalized microchips capability to detect GSH amount in different time intervals. This study introduces a promising approach for long term quantification of intracellular GSH, overcoming the limitation of fluorescent probes and offering valuable insights into microchip-based sensing methodologies.

Advances in the synthesis and properties of sulfur quantum dots for food safety detection and antibacterial applications

Food safety is closely related to human health and has become a worldwide, pressing concern. Food safety analysis is essential for ensuring food safety. Sulfur quantum dots (SQDs), a new type of zero-dimensional metal-free nanomaterials, have recently become the focus of scientific research due to their good luminescence properties, dispersibility, biocompatibility, and inherent antibacterial properties. This review focuses on recent advances in SQDs, with emphasis on their practical applications in the food field. First, commonly used methods for the synthesis of SQDs are presented, including traditional and emerging strategies. The properties of SQDs are then analyzed in detail, particularly their luminescence properties, catalytic activities, and reducing properties. Next, the use of SQDs in food safety detection and antibacterial fields are elaborated. Finally, this review discusses the challenges associated with the use of SQDs in food safety detection and antimicrobial applications.

Facile synthesis of sulfur quantum dots with red light emission: Implications for electrochemiluminescence analysis application

Sulfur quantum dots (SQDs) have been reported as a potential candidate due to their low toxicity and high luminescent performance. Here, SQDs with red light fluorescence (FL) emission were synthesized by a one-step hydrothermal method using Na2CO3 as an etching agent, using sublimed sulfur powder as a sulfur source, and using bovine serum albumin (BSA) as a stabilizer. The choice of etching agent (NaOH or Na2CO3) realized the tuning of SQDs' FL emission with blue and red light. The synthesized SQDs showed good FL stability and high FL efficiency, with a quantum yield of 1.03 % in an aqueous solution at 575 nm. In addition, stable and efficient electrochemiluminescence (ECL) emission was achieved by employing SQDs as ECL emitters with K2S2O8 as the co-reactant. The resorcinol (RS) can enhance the ECL intensity of the SQDs-K2S2O8 system, and the ECL intensity had a good linear relationship with the concentration of RS in a range from 2.5 nM to 25 nM with a detection limit of 0.61 nM. This work provides an emerging red-light luminescent SQDs, which would open up a way for the development of new types of luminophor in FL or ECL analysis. It also provides convenience for bio-labeling of live cells, in vivo imaging and provide new materials for photoelectric devices.

Recent advances on nanomaterial-based glutathione sensors

Glutathione (GSH) is one of the most common thiol-containing molecules discovered in biological systems, and it plays an important role in many cellular functions, where changes in physiological glutathione levels contribute to the progress of a variety of diseases. Molecular imaging employing fluorescent probes is thought to be a sensitive technique for online fluorescence detection of GSH. Although various molecular probes for (intracellular) GSH sensing have been reported, some aspects remain unanswered, such as quantitative intracellular analysis, dynamic monitoring, and compatibility with biological environment. Some of these drawbacks can be overcome by sensors based on nanostructured materials, that have attracted considerable attention owing to their exceptional properties, including a large surface area, heightened electro-catalytic activity, and robust mechanical resilience, for which they have become integral components in the development of highly sensitive chemo- and biosensors. Additionally, engineered nanomaterials have demonstrated significant promise in enhancing the precision of disease diagnosis and refining treatment specificity. The aim of this review is to investigate recent advancements in fabricated nanomaterials tailored for detecting GSH. Specifically, it examines various material categories, encompassing carbon, polymeric, quantum dots (QDs), covalent organic frameworks (COFs), metal-organic frameworks (MOFs), metal-based, and silicon-based nanomaterials, applied in the fabrication of chemo- and biosensors. The fabrication of nano-biosensors, mechanisms, and methodologies employed for GSH detection utilizing these fabricated nanomaterials will also be elucidated. Remarkably, there is a noticeable absence of existing reviews specifically dedicated to the nanomaterials for GSH detection since they are not comprehensive in the case of nano-fabrication, mechanisms and methodologies of detection, as well as applications in various biological environments. This research gap presents an opportune moment to thoroughly assess the potential of nanomaterial-based approaches in advancing GSH detection methodologies.

Fabrication of highly luminescent and thermally stable composites of sulfur nanodots through surface modification and assembly

Sulfur nanodots (S-dots) have emerged as a promising luminescent material to excel over traditional heavy metal-based quantum dots. However, their relatively low emission efficiency and poor thermal stability in the solid state have limited their wide applications in photoelectric devices. In this work, highly luminescent, with a photoluminescence quantum yield higher than 50%, and thermally stable composites of S-dots were produced through modulating their surface states and aggregation behaviors by introducing pyromellitic dianhydride (PMDA) and benzoyleneurea (BEU), respectively. PMDA eliminated the relatively short-lived surface states and defects on the surface of S-dots and BEU regulated the aggregation states and facilitated the energy transfer from BEU to S-dots. The as-obtained composites also showed significantly improved thermal stability compared to S-dots, aided by the hydrophobic chemical groups and dense matrix of PMDA and BEU, which extended their applications in fabricating light-emitting diodes. Our presented results provide a new approach to produce highly luminescent S-dots, which widen their applications in the fields of bioimaging, sensing, photoelectric devices, and environmental science.

Potential-Resolved Electrochemiluminescence Multiplex Immunoassay for Florfenicol and Chloramphenicol in a Single Sample

The simultaneous detection of multiple antibiotic residues in food is of great significance for food safety. In this work, a novel dual-potential electrochemiluminescence (ECL) immunoassay was designed for the simultaneous detection of chloramphenicol and fluorfenicol residues in food. Ru@MOF was used as an anodic probe, and SnS2 QDs-PEI-Au-MoS2 was used as a cathodic probe. Notably, the coreactant for both luminophores was K2S2O8, avoiding interactions caused by different kinds of coreactants. Au nanoparticles functionalized with a nitrogen- and sulfur-doped graphene oxide-modified glassy carbon electrode to improve the electron transfer efficiency and provide a larger surface area for immobilization of antigen. The linear range for the detection of florfenicol was determined to be 0.1-1000 ng mL-1 with a detection limit of 0.03 ng mL-1, and the linear range for the detection of chloramphenicol was 0.01-1000 ng mL-1 with a detection limit of 3.2 pg mL-1 by recording the ECL responses at two different excitation potentials. The proposed immunoassay achieved a more stable recovery in the detection of actual samples and provided a new analytical method for the simultaneous detection of florfenicol and chloramphenicol residues with high sensitivity and specificity.

Sulfur quantum dots as a fluorescent sensor for N-acetyl-beta-D-glucosaminidase detection

N-acetyl-beta-D-glucosaminidase (NAG) is an important biomarker for early clinical diagnosis of renal disease, suggesting the necessity to develop a fast and sensitive method for its detection. In this paper, we developed a fluorescent sensor based on polyethylene glycol (400) (PEG-400)-modified and H2O2-assisted etched sulfur quantum dots (SQDs). According to the fluorescence inner filter effect (IFE), the fluorescence of SQDs can be quenched by the p-nitrophenol (PNP) generated by NAG-catalyzed hydrolysis of p-Nitrophenyl-N-acetyl-β-D-glucosaminide (PNP-NAG). We successfully used the SQDs as a nano-fluorescent probe to detect the NAG activity from 0.4 to 7.5 U·L-1, with a detection limit of 0.1 U·L-1. Furthermore, the method is highly selective and was successfully used in the detection of NAG activity in bovine serum samples, suggesting its great application prospect in clinical detection.

Shaping Sulfur Precursors to Low Dimensional (0D, 1D and 2D) Sulfur Nanomaterials: Synthesis, Characterization, Mechanism, Functionalization, and Applications

Low-dimensional sulfur nanomaterials featuring with 0D sulfur nanoparticles (SNPs), sulfur nanodots (SNDs) and sulfur quantum dots (SQDs), 1D sulfur nanorods (SNRs), and 2D sulfur nanosheets (SNSs) have emerged as an environmentally friendly, biocompatible class of metal-free nanomaterials, sparking extensive interest in a wide range application. In this review, various synthetic methods, precise characterization, creative formation mechanism, delicate functionalization, and versatile applications of low dimensional sulfur nanomaterials over the last decades are systematically summarized. Initially, it is striven to summarize the progress of low dimensional sulfur nanomaterials from versatile precursors by using different synthetic approaches and various characterization. Then, a multi-faceted proposed formation mechanism with emphasis on how these different precursors produce corresponding SNPs, SNDs, SQDs, SNRs, and SNSs is highlighted. Besides, it is essential to fine-tune the surface functional groups of low dimensional sulfur nanomaterials to form new complex nanomaterials. Finally, these sulfur nanomaterials are being investigated in bio-sensing, bio-imaging, lithium-sulfur batteries, antibacterial activities, plant growth along with future perspective and challenges in emerging fields. The purpose of this review is to tailor low dimensional nanomaterials through accurately selecting precursors or synthetic approach and provide a foundation for the formation of versatile sulfur nanostructure.

A resonant energy transfer electrochemiluminescence immunosensor based on low trigger potential of Zn-metal organic framework and CoOOH nanosheets for 5-fluorouracil detection

The organic luminophores have inspired widespread interest in electrochemiluminescence (ECL). Herein, a novel rod-like metal-organic framework was formed by chelating Zn ion with 9,10-di(p-carboxyphenyl)-anthracene (DPA), defined as Zn-MOF for simplicity. In this proposal, the prepared Zn-MOF was first used as a powerful organic luminophore with low trigger potential, thus developing a competitive ECL immunoassay for ultrasensitive detection of 5-fluorouracil (5-FU) with 1,4-diazabicyclo[2.2.2]octane (D-H₂) as the coreactant. The absorption spectrum of cobalt oxyhydroxide (CoOOH) nanosheets and the ECL emission spectrum of Zn-MOF could be highly matched, which ensured the occurrence of resonance energy transfer (RET). For that, ECL-RET was applied in the assembly strategy of the ECL biosensor, and Zn-MOF was used as the energy donor and CoOOH nanosheets as the acceptor. Taking advantage of the luminophore and ECL-RET, the immunoassay can be used for ultra-sensitive quantitative detection of 5-fluorouracil. The proposed ECL-RET immunosensor showed satisfactory sensitivity and accuracy with a wider linear range from 0.001 to 1000 ng/mL, and a lower detection limit (0.52 pg/mL). Hence, it is worth believing that this strategy can pave a bright research direction for the detection of 5-FU or other biological small molecules.

Solvent-Type Passivation Strategy Controls Solid-State Self-Quenching-Resistant Behavior in Sulfur Dots

Treatment of sulfur dots with polyethylene glycol (PEG) has been an efficient way to achieve a high luminescence quantum yield, and such a PEG-related quantum dot (QD)-synthesis strategy has been well documented. However, the polymeric insulating capping layer acting as the "thick shell" will significantly slow down the electron-transfer efficiency and severely hamper its practical application in an optoelectric field. Especially, the employment of synthetic polymers with long alkyl chains or large molecular weights may lead to structural complexity or even unexpected changes of physical characteristics for QDs. Therefore, in sulfur dot preparation, it is a breakthrough to use short-chain molecular species to replace PEG for better control and reproducibility. In this article, a solvent-type passivation (STP) strategy has been reported, and no PEG or any other capping agent is required. The main role of the solvent, ethanol, is to directly react with NaOH, and the generated sodium ethoxide passivates the surface defects. The afforded STP-enhanced emission sulfur dots (STPEE-SDs) possess not only the self-quenching-resistant feature in the solid state but also the extension of fluorescence band toward the wavelength as long as 645 nm. The realization of sulfur dot emission in the deep-red region with a decent yield (8.7%) has never been reported. Moreover, a super large Stokes shift (300 nm, λ_ex = 345 nm, λ_em = 645 nm) and a much longer decay lifetime (109 μs) have been found, and such values can facilitate to suppress the negative influence from background signals. Density functional theory demonstrates that the surface passivation via sodium ethoxide is dynamically favorable, and the spectroscopic insights into emission behavior could be derived from the passivation effect of the sulfur vacancy as well as the charge-transfer process dominated by the highly electronegative ethoxide layer.

Inner filter effect-based red-shift and fluorescence dual-sensor platforms with sulfur quantum dots for detection and bioimaging of alkaline phosphatase

Alkaline phosphatase (ALP), one of the vital biomarkers in several diseases, plays a role in indicating disease presence or severity in early diagnosis. Here, a simple H2O2 assisted top-down method was used to synthesize sulfur quantum dots (SQDs) with excitation and emission at 355 nm and 440 nm. Adding ALP into p-nitrophenyl phosphate (p-NPP) and SQDs was found to exhibit a red shift in the emission wavelength and fluorescence intensity quenching of SQDs, respectively, allowing us to propose dual-sensor platforms of red shift of emission wavelength (RSEW) and fluorescence quenching of SQDs. These dual-sensor platforms were highly sensitive and selective in ALP detection, with a linear response to ALP in the concentration range of 0.25 to 100 U L-1 and detection limits of 0.08 and 0.10 U L-1, respectively. The absorption of p-NP at 400 nm showed a good overlap with the excitation and emission of SQDs, leading to inner filter effect-based RSEW and fluorescence quenching of SQDs. This sensor platform was successfully applied in ALP sensing of serum samples as well as monitoring of ALP in cells. More importantly, this platform can serve as an example of using RSEW to detect ALP.

Selective and sensitive detection of tartrazine in beverages by sulfur quantum dots with high fluorescence quantum yield

In this work, sulfur quantum dots (TPA-SQDs) protected by terephthalic acid as a stabilizer were synthesized using a one-pot method. When excited at 310 nm, the synthesized TPA-SQDs solution emitted strong blue fluorescence at 428 nm, and the absolute quantum yield was as high as 85.99%. The proposed SQDs can be used as a fluorescent probe to specifically quench tartrazine (TZ), showing a good linear relationship (R² = 0.996) at TZ concentrations of 0.1-20 μM, with a detection limit of 39 nM. By analysing the fluorescence lifetime, UV-Vis absorption spectrum and zeta potential of the assay system, it can be speculated that the fluorescence quenching mechanism of TZ on TPA-SQDs is the inner filter effect (IFE). The proposed method was applied to the detection of TZ in vitamin water and orange juice, and the results were consistent with the determination results by high-performance liquid chromatography. The recoveries and relative standard deviations were 93.2-102.6% and 1.34-2.88%, respectively, which provided an alternative method for the determination of TZ in beverages or other food samples.

Synthesis of Fluorescent Sulfur Quantum Dots for Bioimaging and Biosensing

The rapid industrialization has had a serious impact on the environment, leading to an increase in disease and healthcare problems. The development of simple and effective biosensors to achieve specific analyte detection and bioimaging can provide useful information for disease prevention and treatment. Sulfur quantum dots (SQDs), a new class of metal-free fluorescent nanomaterial, are being studied and applied in diagnostic fields such as bioimaging and biosensing due to their advantages of simple synthetic process, unique composition, ultrasmall size, adjustable fluorescence, and low toxicity. This minireview highlights the main synthetic methods to synthesize fluorescent SQDs and their recent progress in cell and tissue imaging, as well as detection of biomolecules, metal ions, and temperature. Finally, the future development and some critical challenges of SQDs as a fluorescent probe in the field of bioimaging and biosensing are also discussed.

Research Update of Emergent Sulfur Quantum Dots in Synthesis and Sensing/Bioimaging Applications

Due to their unique optical property, low toxicity, high hydrophilicity, and low cost, sulfur quantum dots (SQDs), an emerging luminescent nanomaterial, have shown great potential in various application fields, such as sensing, bioimaging, light emitting diode, catalysis, and anti-bacteria. This minireview updates the synthetic methods and sensing/bioimaging applications of SQDs in the last few years, followed by discussion of the potential challenges and prospects in their synthesis and sensing/bioimaging applications, with the purpose to provide some useful information for researchers in this field.

Copper nanocluster-Ru(dcbpy)32+ as a cathodic ECL-RET probe combined with 3D bipedal DNA walker amplification for bioanalysis

In this study, an intra-molecular ECL-RET probe combining the 3D bipedal DNA walker amplification strategy was exquisitely designed to monitor platelet-derived growth factor BB (PDGF-BB).