Quantitatively Switchable pH-Sensitive Photoluminescence of Carbon Nanodots

Nitrogen-doped carbon quantum dots (NCQDs) detected to mercury ions in food monitoring

Using 2,3-diaminopyridine and citric acid as precursors, blue fluorescent nitrogen-doped carbon quantum dots (NCQDs) with a narrow size distribution (∼7.2 nm) were prepared and applied in the following assay for mercury ion detection at a weight ratio of 2,3-diaminopyridine:citric acid = 1:1 (0.2 g: 0.2 g, 20 mL for H2O), 220 °C, and 10 h. NCQDs was characterized by TEM, FT-IR, XPS, UV-Vis and EDS, and the prepared NCQDs display excitation-independent behavior due to less surface defects and uniform size. The optimal excitation and emission wavelengths of the NCQDs were 380 nm and 430 nm, respectively. Interestingly, the fluorescence of the NCQDs could be rapidly and selectively quenched by Hg2+ within 9 min at room temperature without further modification. Under optimal conditions, the limit of detection (LOD) was measured to be at the nanomolar level (42.4 nmol/L) with a linear range of 0-5.0 μmol/L, and fluorescence analysis of NCQDs was successfully used for the qualitative and quantitative analysis of mercury ions in food samples. Furthermore, our results revealed that fluorescence quenching occurred under the common fluences of the inner filter effect, and the static quenching effect was authenticated in the process in which Hg2+ coordinates with the NCQDs to form nonfluorescent complexes.

Multi-step strategies for synergistic treatment of urinary tract infections based on D-xylose-decorated antimicrobial peptide carbon dots

Urinary tract infections (UTIs) caused by Uropathogenic Escherichia coli (UPEC), often reoccur due to the formation of intracellular bacterial colonies (IBCs) and antibiotic resistance. Given the significance of YadC for UPEC infection in our previous study, we developed D-xylose-decorated ɛ-poly-L-lysine (εPL)-based carbon dots (D-xyl@εPLCDs) that can be traced, and employed multi-step approaches to elucidate the functional roles of D-xyl@εPLCDs in UPEC infection. Compared to undecorated particles, D-xyl@εPLCDs demonstrate YadC-dependent bacterial targeting and exhibit enhanced bactericidal activities both intracellularly and extracellularly. Moreover, pre-treatment of D-xyl@εPLCDs before infection blocked the subsequent adhesion and invasion of UPEC to bladder epithelial cells 5637. Increase of ROS production and innate immune responses were observed in bladder epithelial cells 5637 treated with D-xyl@εPLCDs. In addition, treatment of D-xyl@εPLCDs post-infection facilitated clearance of UPEC in the bladders of the UTI mouse model, and reduced ultimate number of neutrophils, macrophages and inflammatory responses raised by invaded bacteria. Collectively, we presented a comprehensive evaluating system to show that D-xyl@εPLCDs exhibits superior bactericidal effects against UPEC, making them a promising candidate for drug development in clinical UTI therapeutics.

Investigation of the Application and Mechanism of Nitrogen and Phosphorus Co-doped Carbon Dots for Mercury Ion Detection

In this paper, we obtained nitrogen and phosphorus co-doped carbon dots through a hydrothermal method using o-phenylenediamine and citric acid in a 40% phosphoric acid environment. The carbon dots emitted fluorescence at 476 nm under excitation at 408 nm and exhibited good selectivity and high sensitivity towards mercury ions. These carbon dots showed excellent dispersibility in water and maintained stable fluorescence even in high concentration salt environments. The interaction between mercury ions and functional groups on the carbon dots surface through electrostatic interaction resulted in static quenching. Simultaneously, by detecting the lifetime and transient absorption spectra of the carbon dots, we observed that the coordination of mercury ions with the carbon dots broadened the band structure of the carbon dots, and the existing photoinduced electron transfer process increased the non-radiative transition channel. The combined effect of dynamic quenching and static quenching significantly reduced the fluorescence intensity of the carbon dots at 476 nm. The carbon dots exhibited linear detection of mercury ions in the range of 0.01-1 µM, with a detection limit as low as 0.0245 µM. In terms of practical water environmental detection applications, these carbon dots were able to effectively detect mercury ions in tap water and lake water, demonstrating their broad application prospects in the field of environmental metal analysis.

Natural biomass carbon Dots-Based fluorescence sensor for high precision detection of vitamin B12 in serum

Vitamin B12(Vit B12) is an essential micronutrient for body growth, and abnormal levels of Vit B12 in the human body are closely associated with the prediction of certain diseases. Hence, a rapid, sensitive, and environment-friendly approach for Vit B12 detection was set up. Herein, the Bird's nest carbon dots (B-CDs) are synthesized by using a bird's nest and distilled water as precursors. One-step hydrothermal synthesis has created B-CDs without toxic ingredients or surface chemical modifications. The prepared B-CDs exhibited outstanding characteristics including excellent water solubility, brilliant fluorescence performance great biocompatibility, and fine stability in a broad pH range of 3.0-11.0 and high ionic strength solution. The experiment revealed that the fluorescence of the reaction system showed a regular decrease after the interaction of B-CDs with Vit B12. Additionally, there was an excellent linear relationship between the F/F0 of B-CDs and the concentration of Vit B12. The linear range was 0 ∼ 100 µM, R2 was 0.9929, and the detection limit was 0.24 µM. Finally, the proposed method successfully detected Vit B12 in human serum samples with recoveries of 96.2 %-100.3 %, showing broad clinical prospects.

A Ratiometric Fluorescent Sensor Based on Chelation-Enhanced Fluorescence of Carbon Dots for Zinc Ion Detection

Zinc ion, one of the most important transition metal ions in living organisms, plays a crucial role in the homeostasis of the organism. The disorder of zinc is associated with many major diseases. It is highly desirable to develop selective and sensitive methods for the real-time detection of zinc ions. In this work, double-emitting fluorescent carbon dots (CDs) are prepared by a solvothermal method using glutathione, L-aspartic acid, and formamide as the raw materials. The carbon dots specifically recognize zine ions and produce a decrease in fluorescence intensity at 684 nm and an increase at 649 nm, leading to a ratiometric fluorescent sensor for zinc detection. Through surface modification and spectral analysis, the surface groups including carboxyl, carbonyl, hydroxyl, and amino groups, and C=N in heterocycles of CDs are revealed to synergistically coordinate Zn2+, inducing the structural changes in the emission site. The CDs can afford a low limit of detection of ~5 nM for Zn2+ detection with good linearity in the range of 0.02-5 μM, showing good selectivity as well. The results from real samples including fetal bovine serum, milk powder, and zinc gluconate oral solution indicated the good applicability of the CDs in the determination of Zn2+.

Multifunctional Carbon Nanodots for Antibacterial Enhancement, pH Change, and Poisonous Tin(IV) Specifical Detection

In recent years, antibiotic-based carbon nanodots have been extensively developed and studied, because of their excellent synergistic fluorescence and antibacterial properties. These antibacterial carbon nanodots have also been developed with various new applications, such as heavy iron detection, pH sensitivity, temperature response, and bacterial count detection in various environments. In this article, using vancomycin hydrochloride as the only precursor, vancomycin hydrochloride carbon nanodots were rapidly synthesized by a one-step microwave method. The diameter of the vancomycin hydrochloride carbon nanodots was concentrated at 0.899 ± 0.40 nm with a uniform size and excitation-dependent fluorescence. Vancomycin hydrochloride carbon nanodots showed better antibacterial activity than the original vancomycin hydrochloride with low biological toxicity and good stability. In the pH range of approximately 7-13, there was a good linear relationship between the fluorescence intensity of the carbon nanodots and the pH value (R2 = 0.98516). Moreover, vancomycin hydrochloride carbon nanodots could quickly and specifically detect poisonous Sn4+ through changes in their fluorescence intensity, with a detection limit of approximately 5.2 μM. Multifunctional vancomycin hydrochloride carbon nanodots have good application prospects in the fields of antibacterial, toxic Sn4+ detection, and pH-sensitive aspects.

Deciphering the catalytic mechanism of superoxide dismutase activity of carbon dot nanozyme

Nanozymes with superoxide dismutase (SOD)-like activity have attracted increasing interest due to their ability to scavenge superoxide anion, the origin of most reactive oxygen species in vivo. However, SOD nanozymes reported thus far have yet to approach the activity of natural enzymes. Here, we report a carbon dot (C-dot) SOD nanozyme with a catalytic activity of over 10,000 U/mg, comparable to that of natural enzymes. Through selected chemical modifications and theoretical calculations, we show that the SOD-like activity of C-dots relies on the hydroxyl and carboxyl groups for binding superoxide anions and the carbonyl groups conjugated with the π-system for electron transfer. Moreover, C-dot SOD nanozymes exhibit intrinsic targeting ability to oxidation-damaged cells and effectively protect neuron cells in the ischemic stroke male mice model. Together, our study sheds light on the structure-activity relationship of C-dot SOD nanozymes, and demonstrates their potential for treating of oxidation stress related diseases.

Enhancing the photoluminescence and cellular uptake of fluorescent carbon nanodots via cubosome lipid nanocarriers

Carbon nanodots (C-dots) have attracted much attention for their use in the fields of bioimaging, drug delivery, and sensing due to their excellent fluorescent and photoluminescent properties, photostability, biocompatibility, and amenability to surface modification. Herein, we report a nanocomposite formulation of C-dots (<5 nm) encapsulated in lipid-based lyotropic liquid crystalline nanoparticles (∼250 nm) via either passive diffusion or electrostatic mechanisms. The physicochemical properties of the nanocomposite formulation including particle size, surface charge, internal cubic nanostructures, and pH-dependent fluorescent properties were characterised. Upon loading of C-dots into lipid nanoparticles, the highly ordered inverse bicontinuous cubic mesophase existed in the internal phase of the nanoparticles, demonstrated by synchrotron small angle X-ray scattering, molecular dynamic simulation and cryogenic transmission electron microscopy. The pH-dependent fluorescent property of the C-dots was modified via electrostatic interaction between the C-dots and cationic lipid nanoparticles, which further enhanced the brightness of C-dots through self-quenching prevention. The cytotoxicity and cellular uptake efficiency of the developed nanocomposites were also examined in an epithelial gastric adenocarcinoma cell line (AGS) and a macrophage cell line (stimulated THP-1). Compared to free C-dots, the uptake and cell imaging potential of the C-dot nanocomposites was significantly improved, by several orders of magnitude as demonstrated by cytoplasmic fluorescent intensities using confocal microscopy. Loading C-dots into mesoporous lipid nanocarriers presents a new way of modifying C-dot physicochemical and fluorescent properties, alternative to direct chemical surface modification, and advances the bioimaging potential of C-dots by enhancing cellular uptake efficiency and converging C-dot light emission.

Self-assembly of carbon nanodots induced by liquid-liquid phase separation in a surface microdroplet

Evaporating a sessile drop of ternary solutions containing one hydrotrope (such as ethanol) and two immiscible fluids exhibiting fascinating phase separation behaviours, has opened up a new pathway for controlled nanomaterial assembly. In this work, we studied the influence of liquid-liquid phase separation (LLPS) on the assembly of carbon nanodots (C-dots), 2 nm fluorescent carbon-based nanomaterials with high water solubility. Through self-evaporation of a micro-sized droplet containing ethanol, C-dot-water solution and different oils on a hydrophobic surface, C-dots rearranged into film, porous and granular structures by controlling the properties of oil component in the tenary droplet. Vapour pressure, solubility, surface tension and compositions of the oil components were investigated systematically for their impacts on the evaporation process of C-dot-laden droplets. By using confocal microscopy, we clearly revealed that C-dot assembly was triggered by LLPS in these four oil-water-ethanol ternary systems. The corresponding evaporation and assembly processes were unravelled to be determined by how the ternary solutions pass through the liquid-liquid equilibrium curves in the phase diagrams during evaporation. Our findings deepen the understanding of phase-separation behaviours for nanomaterial assembly as well as provide a novel, simple, and well-controlled approach for depositing different C-dot based nanostructures onto surfaces, which will benefit a wide range of practical applications in the fields of energy, environment and health.

Tunable fluorescent amino-functionalized Ti3C2Tx MXene quantum dots for ultrasensitive Fe3+ ion sensing

The development of sensors with high sensitivity, good selectivity and reproducibility are of great importance for the detection of Fe³⁺ in contaminated water for environmental monitoring. In this work, a reflux approach has been adopted to synthesize Ti₃C₂T_x quantum dots (QDs) based on the cutting effect of tetramethylammonium hydroxide (TMAOH) on Ti₃C₂T_x at high temperature. The surface-functionalized Ti₃C₂T_x QDs contained abundant amino groups and exhibited tunable pH-dependent emission, which was attributed to the protonation and deprotonation of the surface terminations. The linearity of the radiometric fluorescence intensity versus pH indicates its great potential as a dual-emission ratiometric pH sensor. Additionally, the Ti₃C₂T_x QDs exhibited tunable excitation-dependent emission behavior, which was related to the degree of passivation by the amino groups on the surface. Furthermore, the fluorescence intensity of the Ti₃C₂T_x QDs shows a linear response toward Fe³⁺ in the nanomolar to micromolar range with a low detection limit of 2 nM, originating from the oxidation and reduction between Fe³⁺ and Ti₃C₂T_x. This ultra-sensitive and selective detection capability demonstrated the environmental application potential for Ti₃C₂T_x QDs as a nanoprobe to monitor Fe³⁺.

Origin of proton induced fluorescence quenching of colloidal carbon dots: reshaping of Schrödinger wavefunctions and huge red shift of transition energy

The fluorescence quenching by protons is a universal phenomenon but the mechanism remains unclear. Here, we take the fluorescent amide-terminated carbon dots as a prototype to study the proton fluorescence quenching mechanism by using both experiments and time-dependent density functional theory calculations. The study reveals that when an approached proton is captured by the weakly negatively charged fluorophore group of the colloidal carbon dot, it will substantially change the electron wavefunctions owing to the strong proton-electron interaction, and this leads to highly diminished energy gap and resultant fluorescence quenching in the visible spectral region. The protons generated by hydrolysis of various types of metal ions also exhibit fruitful fluorescence quenching and the quenching efficiency is roughly proportional to the hydrolysis constant of the metal ion. This fluorescence quenching mechanism is quite distinct from the conventional ones involving electron or energy transfer.

High sensitivity pH sensing by using a ring resonator laser integrated into a microfluidic chip

We present a chip-scale integrated pH sensor with high sensitivity by using an optofluidic ring resonator (OFRR) laser. An optical fiber with a high refractive index (RI) is employed both as an optical cavity and the sensing reactor along a microchannel, while disodium fluorescein (DSF) aqueous solution with a low RI is served as the cladding gain medium and fluorescent probes. The pump light is introduced along the fiber axis and guided by the total internal reflection at the fiber/cladding interface. The evanescent field of the pump light extends out of the fiber surface and efficiently excites the dye molecules residing in the evanescent field region of the Whispering Gallery Modes (WGMs) of the OFRRs to produce lasing emission. This pumping scheme provides a uniform excitation to the gain medium and significantly increases the signal-to-noise ratio, ensuring a low lasing threshold and highly sensitive sensing. The lasing threshold property under different pH conditions is experimentally and theoretically conducted to evaluate the sensing performance, which shows that the lasing threshold highly depends on the pH value of the cladding solution due to the increasing deprotonation process. We further verify that the intensity of the lasing emission and the pH value shows good linearity in the pH range 6.51-8.13, with a 2-order-of-magnitude sensitivity enhancement compared to fluorescence measurement. The proposed OFRR lasing platform shows excellent robustness and low sample consumption, providing a powerful sensing strategy in medicine, and hazardous/toxic/volatile sensing, which require label-free, real-time, and in situ detection.