Modifying carbon dots with L-phenylalanine for rapid discrimination of tryptophan enantiomers

New chiral carbon dots (CDs), L-PCDs, for discriminating tryptophan (Trp) enantiomers were prepared in this work. Firstly, original CDs were synthesized through a hydrothermal method using pyridine-2,6-dicarboxylic acid and o-phenylenediamine as raw materials. Then, the surface of original CDs was modified with L-phenylalanine to create chiral fluorescent carbon L-PCDs. In the presence of D-Trp, the fluorescence intensity of L-PCDs decreased significantly while it remained unchanged in the presence of L-Trp. The chiral sensing system used in this study has a rapid response time of 3 minutes and can identify enantiomers with an enantioselectivity (ID/IL) of up to 3.3. For D-Trp, a good linear relationship can be obtained in the range of 0.3-4.2 mM with a limit of detection of 0.06 mM. This sensor allows for both quantitative detection of D-Trp and determination of enantiomeric percentage in the racemate. The chiral recognition mechanism is attributed to the different interaction between D-/L-Trp and L-PCDs.

Silicon Nanoparticle-Based Ratiometric Fluorescence Probes for Highly Sensitive and Visual Detection of VB2

In this work, blue fluorescent silicon nanoparticles (SiNPs) were prepared by a simple one-step hydrothermal method using (3-aminopropyl) triethoxy silane (APTES) and eriochrome black T as raw materials. The SiNPs showed favorable water solubility, thermal stability, pH stability, salt tolerance, and photobleaching resistance. At an excitation wavelength of 376 nm, the SiNPs emitted bright blue fluorescence at 460 nm. In the presence of vitamin B₂ (VB₂), the fluorescence intensity (FL intensity) of the SiNPs at 460 nm decreased obviously, and a new peak appeared at 521 nm. Based on this, a novel ratiometric fluorescence method was established for VB₂ detection. There was a good linear relationship between the fluorescence intensity ratio (F ₅₂₁/F ₄₆₀) and VB₂ concentration from 0.5 to 60 μM with a detection limit of 135 nM. This method was successfully applied to detect VB₂ content in the samples of vitamin B₂ drugs and beverages. Additionally, a simple paper sensor based on the SiNPs was designed to visualize detection of VB₂. With the support of color recognition software on a smartphone, the visual quantitative analysis of VB₂ was realized, ranging from 40 to 800 μM.

Polymer-Induced Emission-Active Fluorine-Embedded Carbon Dots for the Preparation of Warm WLEDs with a High Color Rendering Index

Exploration of many strategies has continuously contributed to producing aggregation-induced red-emissive carbon dots (CDs). In this work, we designed fluorine-embedded (F-embedded) CDs from 1,2,4-triaminobenzene, thiourea, and ammonium fluoride (NH₄F) exhibiting polymer-induced emission (PIE). The PIE phenomenon of fluorescent CDs is obtained in poly(vinyl alcohol) (PVA), showing emissions at 611 and 617 nm in the dispersed and solid states, respectively. The CDs exhibited a red shift of 28 nm in the PVA solution because PVA hydroxyl groups formed a robust bridge-like H-bonding network between CDs. The fluorine embedded in CDs enhanced the H-bond affinity toward PVA. It showed that this H-bond restricted the coupling of CDs' surface states and inhibited the nonirradiation transfer. For the solid state, surface PVA chains eliminated the π-π interaction of the conjugated core and constructed a self-quenching resistance polymeric system around CDs. As a result, CDs showed an unexpected red shift of fluorescence emission in PVA. Furthermore, white light-emitting diodes (WLEDs) have a correlated color temperature (CCT) of 5232 K, and a high color rendering index of 95 has been fabricated by integrating the red- and green-emissive films over the UV LEDs. Interestingly, the as-synthesized CDs showed room temperature phosphorescence (RTP), which enabled us to employ the CDs in double-security protection. Simultaneously, CDs have been used in fingerprint detection.

Multifunctional Biomedical Applications of Nitrogen and Sulfur Co-Doped Carbon Dots

Multifunctional carbon dots have drawn considerable attention due to their potential biomedical application value. We report the preparation of blue-green fluorescence-emitting, multifunctional, nitrogen-and-sulfur co-doped carbon dots (N, S-CDs) synthesized via a one-step process using 1-thioglycerol as a sulfur source, glucose and citric acid as carbon sources, and polyethyleneimine as a nitrogen source. Because of abundant amino and sulfur content, the CDs exhibited high sensibility and selectivity for detecting Cu₂₊ (detection limit: 0.01 μM, linear range: 0.025 to 50 μM). Fast and sensitive detection of tiopronin was also achieved on the basis of the fluorescence "off-on" mode considering the strong affinity between tiopronin and Cu₂₊. The N, S-CDs exhibited good biocompatibility as determined by fluorescence imaging using onion epidermal cells and gram-positive bacteria. The CDs also exhibited excellent antimicrobial ability against the gram-positive bacteria. Our results indicate that these novel N, S-CDs could be ideal candidates for several biochemical applications such as antibacterial treatment and detection of small biomolecules.

Yellow emission carbon dots for highly selective and sensitive OFF-ON sensing of ferric and pyrophosphate ions in living cells

A simple "OFF-ON" fluorescent system was proposed for selective and sensitive detection of ferric ion (Fe³⁺) and pyrophosphate (PPi) in living cells. The method was constructed based on the bright yellow emission of carbon dots (y-CDs), which were prepared using o-phenylenediamine (OPD) as the precursor via a facile hydrothermal treatment. The as-obtained y-CDs, with an average size of 2.6 nm, exhibited an excitation-independent emission peak at 574 nm. The fluorescence of y-CDs can be remarkably quenched by Fe³⁺ with high selectivity and sensitivity. Interestingly, the quenched fluorescence can be recovered regularly upon addition of PPi, showing a promising detection for PPi. The linear ranges for Fe³⁺ and PPi detections were 0.05-80 and 0.5-120 μM, respectively, and the corresponding limit of detections (LODs) were 22.1 and 73.9 nM. As we proved the y-CDs have negligible cytotoxicity and excellent biocompatibility, further application to the fluorescence imaging of intracellular Fe³⁺ and PPi were conducted, suggesting the prepared y-CDs can be used to monitor Fe³⁺ and PPi variation in living cells. Overall, our developed y-CDs-based OFF-ON switch fluorescent probe has the advantages of simplicity, agility, high sensitivity and selectivity, which provides a promising platform for environmental and biology applications, and paves a new avenue for monitoring the hydrolysis process of adenosine triphosphate disodium salt (ATP) by detection of PPi in organisms.

A carbon dot based theranostic platform for dual-modal imaging and free radical scavenging

Magnetofluorescent carbon dots (Cdots) doped with both P³⁺ and Mn²⁺ (abbreviated as PMn@Cdots) have been synthesized in an aqueous solution via a microwave-assisted pyrolysis method. In this system, a P³⁺ dopant was introduced to enhance the emission efficiency of the Cdots, while the presence of a Mn²⁺ dopant granted magnetic resonance imaging (MRI) capability. To the best of our knowledge, the present work is the first attempt to regulate red-emission and free radical scavenging of PMn@Cdots to serve as a dual-modal imaging nanoprobe and an antioxidant agent. Unlike most red-emitting Cdots, the as-prepared PMn@Cdots can be readily purified from unreacted precursors through antisolvent precipitation instead of by time-consuming purification methods. The whole synthetic procedure is rapid, facile, efficiently reproducible, and scalable. More importantly, further conjugation of the PMn@Cdots with hyaluronic acid (termed PMn@Cdots/HA) gives them good in vivo and in vitro biocompatibility as well as the capability to selectively target CD44-overexpressing cancer cells, as investigated by flow cytometry, fluorescence, and MRI. Meanwhile, PMn@Cdots exhibit antioxidant activity against multiple DPPH, hydroxyl, and superoxide radicals, which is comparable to that for ascorbic acid. Favorably, PMn@Cdots/HA showed a dose-dependent cytoprotective capability against H₂O₂-induced oxidative stress in B16F1, HeLa, and HEL cells. Therefore, the Cdot based theranostic platform can simultaneously function as a potential therapeutic candidate and as a dual-modal probe for enabling accurate diagnosis in future clinical applications.

Nucleolus-Targeted Red Emissive Carbon Dots with Polarity-Sensitive and Excitation-Independent Fluorescence Emission: High-Resolution Cell Imaging and in Vivo Tracking

Red-emitting carbon dots (CDs) have attracted tremendous attention due to their wide applications in areas including imaging, sensing, drug delivery, and cancer therapy. However, it is still highly challenging for red-emitting CDs to simultaneously achieve high quantum yields (QYs), nucleus targeting, and super-resolution fluorescence imaging (especially the stimulated emission depletion (STED) imaging). Here, it is found that the addition of varied metal ions during the hydrothermal treatment of p-phenylenediamine (pPDA) leads to the formation of fluorescent CDs with emission wavelengths up to 700 nm. Strikingly, although metal ions play a crucial role in the synthesis of CDs with varied QYs, they are absent in the formed CDs, that is, the obtained CDs are metal-free, and the metal ions play a role similar to a "catalyst" during the CD formation. Besides, using pPDA and nickel ions (Ni²⁺) as raw materials, we prepare Ni-pPCDs which have the highest QY and exhibit various excellent fluorescence properties including excitation-independent emission (at ∼605 nm), good photostability, polarity sensitivity, and ribonucleic acid responsiveness. In vitro and in vivo experiments demonstrate that Ni-pPCDs are highly biocompatible and can realize real-time, wash-free, and high-resolution imaging of cell nuclei and high-contrast imaging of tumor-bearing mice and zebrafish. In summary, the present work may hold great promise in the synthesis and applications of red emissive CDs.

Synthesis of Fluorescent Carbon Dots as Selective and Sensitive Probes for Cupric Ions and Cell Imaging

A novel sensing system has been designed for the detection of cupric ions. It is based on the quenched fluorescence signal of carbon dots (CDs), which were carbonized from poly(vinylpyrrolidone) (PVP) and L-Cysteine (CYS). Cupric ions interact with the nitrogen and sulfur atoms on surface of the CDs to form an absorbed complex; this results in strong quenching of the fluorescence of the CDs via a fast metal-to-ligand binding affinity. The synthesized water-soluble CDs also exhibited a quantum yield of 7.6%, with favorable photoluminescent properties and good photostability. The fluorescence intensity of the CDs was very stable in high ionic strength (up to 1.0 M NaCl) and over a wide range of pH levels (2.0-12.0). This facile method can therefore develop a sensor that offers reliable, fast, and selective detection of cupric ions with a detection limit down to 0.15 μM and a linear range from 0.5 to 7.0 μM (R2 = 0.980). The CDs were used for cell imaging, observed that they were low toxicity to Tramp C1 cells and exhibited blue and green and red fluorescence under a fluorescence microscope. In summary, the CDs exhibited excellent fluorescence properties, and could be applied to the selective and sensitive detection of cupric ion and multicolor cell imaging.