Ratiometric fluorescence assay for L-Cysteine based on Fe-doped carbon dot nanozymes

Construction of a photo-controlled fluorescent switching with diarylethene modified carbon dots

Photo-controlled fluorescent switching is of great utility in fluorescence sensors, reversible data storage, and logic circuit, based on their modifiable emission intensity and spectra. In this work, a novel photo-controlled reversible fluorescent switching system was constructed based on photochromic diarylethene (DT) molecular modified fluorescent carbon dots (CDs). The fluorescent CDs acted as fluorescent donors and the photochromic diarylethene molecular functioned as acceptors in this fluorescent switching system. The fluorescence modulation efficiency of the fluorescent switching was determined to be 97.1%. The result was attributable to Förster resonance energy transfer between the CDs and the diarylethene molecular. The fluorescent switching could undergo 20 cycles without significant decay.

A universal sensing platform based on iron and nitrogen co-doped carbon dots for detecting hydrogen peroxide and related metabolites in human fluid by ratiometric fluorometry and colorimetry

A universal ratiometric fluorescence and colorimetric dual-mode sensing platform for detecting hydrogen peroxide (H₂O₂) and related metabolites in human fluid was constructed based on iron and nitrogen co-doped carbon dots (Fe/N-CDs). As a fluorescent nanomaterial with peroxidase-like property, Fe/N-CDs emits fluorescence at 449 nm (F₄₄₉) under excitation of incident ultraviolet light, and can catalyze the oxidation of o-phenylenediamine (OPD) by H₂O₂ for generating 2,3-diaminophenazine (oxOPD) that exhibits obvious absorption at 420 nm (A₄₂₀) and fluorescence emission at 555 nm (F₅₅₅). The Förster resonance energy transfer (FRET) between Fe/N-CDs and oxOPD would result in the fluorescence quenching Fe/N-CDs and the fluorescence enhancement of oxOPD, which facilitates the quantitation of oxOPD by ratiometric fluorometry. Since the amount of generated oxOPD is determined by the amount of H₂O₂ consumed during the oxidation reaction, the detection of H₂O₂ and related metabolites can be realized by monitoring both ratiometric fluorescent (F₅₅₅/F₄₄₉) and colorimetric (absorption, A₄₂₀) signals of oxOPD. This dual-mode sensing platform exhibits excellent selectivity and sensitivity toward with H₂O₂, xanthine and uric acid in both human serum and urine samples, demonstrating its good potential for monitoring H₂O₂ and metabolites involved in H₂O₂ metabolism in human body. The detection limits (LODs) of H₂O₂, xanthine and uric acid obtained by this sensing platform were 0.07, 0.15, and 0.14 μM for ratiometric fluorescence mode, and 0.12, 0.52, and 0.47 μM for colorimetric mode, respectively. By utilizing appropriate oxidases in this universal sensing platform, the determination of other metabolites involved with producing H₂O₂ can also be realized facilely.

Unveiling the Actual Catalytic Sites in Nanozyme-Catalyzed Oxidation of o-Phenylenediamine

Nanozymes have offered remarkable advantages over natural enzymes and found widespread applications including biosensors, immunoassays, nanomedicines, and environmental remediation. Oxidation of o-phenylenediamine (OPD) by nanozymes has been listed as a standard protocol for determining nanozyme activities. Given the complexity of OPD oxidation processes, however, the mechanism of nanozyme-catalyzed oxidation of OPD remains elusive. In this report, mechanistic studies of nanozyme-catalyzed oxidation of OPD are performed and a distinguishably different mechanism from that of natural enzymes is found. A combination of Fourier transform infrared spectroscopy, nuclear magnetic resonance, electrospray ionization mass spectrometry, and electron microscopic studies provides compelling evidence that polymerization of OPD occurs on the surface of several different nanozymes. The unexpected polymerization causes a dense coating layer of poly(o-phenylenediamine) (POPD) on nanozymes renders the intrinsic properties of nanozymes. Therefore, this fundamental discovery raise serious concerns using OPD-based colorimetric method for determining nanozyme activities. Without examining the surface change of nanozymes after catalytic reactions, the use of OPD-based colorimetric method for determining nanozyme activities is strongly discouraged. Furthermore, POPD is discovered as a new oxidase mimic, and this new mechanism also provides a general and robust method to coat nanomaterials with POPD polymers of enzyme-mimicking properties.

Progress in the Application of Carbon Dots-Based Nanozymes

As functional nanomaterials with simulating enzyme-like properties, nanozymes can not only overcome the inherent limitations of natural enzymes in terms of stability and preparation cost but also possess design, versatility, maneuverability, and applicability of nanomaterials. Therefore, they can be combined with other materials to form composite nanomaterials with superior performance, which has garnered considerable attention. Carbon dots (CDs) are an ideal choice for these composite materials due to their unique physical and chemical properties, such as excellent water dispersion, stable chemical inertness, high photobleaching resistance, and superior surface engineering. With the continuous emergence of various CDs-based nanozymes, it is vital to thoroughly understand their working principle, performance evaluation, and application scope. This review comprehensively discusses the recent advantages and disadvantages of CDs-based nanozymes in biomedicine, catalysis, sensing, detection aspects. It is expected to provide valuable insights into developing novel CDs-based nanozymes.

Red Emission Carbon Dots Prepared by 1,4-Diaminonaphthalene for Light-Emitting Diode Application and Metal Ion Detection

Carbon dots (CDs), as the most important type of carbon materials, have been widely used in many fields because of their unique fluorescence characteristics and excellent properties of biocompatibility. In previous studies, the fluorescence of CDs was mainly concentrated in the blue and green, whereas the red fluorescence was relatively less. Herein, we prepared efficient red-emitting CDs from 1,4-diaminonaphthalene using solvothermal methods. We discussed the effects of different solvothermal solvents on CDs. The results show that CDs prepared with octane and acetone as reaction media have the best fluorescence properties. The CDs dispersed in different organic solvents exhibited tunable emission across a wide spectrum from 427 nm to 679 nm. We further demonstrated the application of red light-emitting diode (LED) optoelectronics and fluorescence detection of Fe3+ in aqueous solution.

Ratiometric fluorescence and smartphone dual-mode detection of glutathione using carbon dots coupled with Ag+-triggered oxidation of o-phenylenediamine

Developing ratiometric fluorescence and smartphone dual-mode bioanalysis methods is important but challenging. A ratiometric fluorescence method for determining glutathione (GSH) using carbon dots (CDs) and Ag⁺-triggered o-phenylenediamine (OPD) oxidation is described here. Ag⁺oxidizes OPD to give 2,3-diaminophenazine (oxOPD), which effectively quenches CD fluorescence at 436 nm through the inner filter effect and causes a new emission peak at 561 nm. GSH chelates with Ag⁺and prevents the Ag⁺oxidizing OPD and therefore effectively preserves CD emission at 436 nm (blue) and allows only weak oxOPD fluorescence at 561 nm (orange) to occur. The oxOPD to CD fluorescence intensity ratio decreased linearly as the GSH concentration increased in the range 0-150 nM, and the detection limit was 15 nM. The ratiometric fluorescence probe lit with an ultraviolet lamp clearly changed color from orange to blue as the GSH concentration increased. An image was acquired using a smartphone camera and converted into digital values. The blue and red channel ratio was calculated and used to quantify GSH. The method therefore allows dual-mode detection of GSH.