Activating One/Two-Photon Excited Red Fluorescence on Carbon Dots: Emerging n→π Photon Transition Induced by Amino Protonation

Intermediate aminophenol enables hectogram-scale synthesis of highly bright red carbon quantum dots under ambient conditions

Carbon quantum dots (C-dots) have developed into potential nanomaterials for lighting, catalysis and bioimaging because of their excellent optical properties and good biocompatibility. However, it is still a challenge to produce efficient red emitting carbon quantum dots (R-C-dots) due to their obscure formation mechanism. This work offered a method to reveal the formation process from the precursor o-phenylenediamine (o-PDA) to R-C-dots. Different from traditional hydrothermal reactions, R-C-dots were synthesized at relatively low temperature and ambient pressure. The pre-oxidation intermediate aminophenol played an important role in the synthesis of R-C-dots, which further cross-linked and polymerized with o-PDA in an acid environment to form R-C-dots. The obtained R-C-dots had a photoluminescence quantum yield of up to 33.26% and excellent two-photon fluorescence properties. A white light-emitting diode (WLED) based on R-C-dots as the red phosphor exhibited standard white light CIE color coordinates of (0.33, 0.33) with a correlated color temperature of 5342 K and a high color rendering index (CRI) of 94.5. The obtained rendering index is the highest value among WLEDs with color coordinates of (0.33, 0.33) based on C-dots. This work provides a new perspective for the controllable large-scale synthesis of red C-dots.

Construction of optical dual-mode sensing platform based on green emissive carbon quantum dots for effective detection of ClO- and cellular imaging

Hypochlorite (ClO-) is an important redox regulator in reactive oxygen species, which play a considerable role in oxidative stress and related diseases. Hence, accurate and sensitive monitoring of ClO- concentration was urgently needed in the fields of life sciences, food and environment. Bright green fluorescent carbon quantum dots (G-CQDs) were synthesized utilizing one-step hydrothermal method with citric acid and acriflavine precursors. Through TEM, FTIR, XPS and zeta potential characterization procedures, G-CQDs illustrated uniformly dispersed and significant number of -NH2 and -OH on the surface. Meanwhile, the fluorescence and colorimetric analysis exhibited wide linear range and low detection limit response to ClO-. The fluorescence changes of G-CQDs were identified via smartphone to realize mobile sensing of ClO-. Subsequently, G-CQDs was applied for visualization and quantitative detection of ClO- in drinking water samples with satisfactory recovery rate. More importantly, G-CQDs demonstrated good water solubility, optical stability and excellent biocompatibility, which offered a promising analysis approach in cell imaging and exogenous ClO- detection in living cells. G-CQDs illustrated bright prospect and great potential in practical application of ClO- associated disease prevention and early clinical diagnosis.

Carbon dots with selective fluorescence response to hydroxyl radical for sensitive detection of bleomycin

In this work, antioxidant carbon dots (ACDs) with high quantum yield (63.98%) were developed through a one-step strategy using citric acid, ethylenediamine, and L-arginine as sources. The ACDs demonstrated excellent in vitro antioxidant activity. Additionally, ACDs displayed a unique fluorescence response to hydroxyl radical (OH). Leveraging this distinctive feature, a selective and sensitive fluorescence-based sensor was developed for detecting bleomycin (BLM) in the presence of ferrous ions (Fe2+). The studies on the oxidizing properties and dynamic quenching mechanism of ACDs by OH suggested that OH reduced the contents of the amino and hydroxyl groups of ACDs, leading to a decreased electron cloud density of ACDs for dynamic quenching of ACDs. Under optimally refined conditions, the detection limit of this strategy for BLM was 0.58 μg/mL, with a linear range spanning from 1 to 24 μg/mL when employing ACDs as the probe. This fluorescence sensing strategy provides a promising method for the detection of BLM in clinical samples.

One-step hydrothermal method synthesized pH-dependent carbon dots for multistage anti-counterfeiting

Work to combat counterfeiting has always been crucial to defending the interests of the public. The usual anti-counterfeiting marks are now fundamental and easy to imitate. Therefore, it is more beneficial to anti-counterfeiting work to develop an anti-counterfeiting mark with more variations to make forgery more difficult. Due to its exceptional stability and fluorescence variability, carbon dots (CDs), a newly developed fluorescent material, offer a wide range of potential applications in anti-counterfeiting. However, there currently needs to be more CD applications in multi-level anti-counterfeiting, and additional issues include high cost and environmental contamination. Therefore, considering the problems of green environmental protection and cost, CDs with excellent green (530 nm) and blue (475 nm, 486 nm) luminescence properties were prepared by a one-step reaction of m-phenylenediamine and glucose. The average fluorescence lifespan is longer than 5 ns, and the optimal quantum yield can reach 37%. Due to the large number of protonated amino groups and surface carboxyl functional groups, the prepared carbon dots exhibit green and blue fluorescence emission modes under acidic and alkaline conditions, respectively. Based on this situation, we produced CD ink and successfully used it for multi-level anti-counterfeiting.

Self-Precipitation of Highly Purified Red Emitting Carbon Dots as Red Phosphors

Colloidal carbon dots (C-dots) have attracted a great deal of attention for their unique optical properties. However, it is still a challenge to obtain highly purified C-dots without using multiple-step purification or postsize selection. In this work, a self-precipitation hydrothermal reaction was used to synthesize red-emitting C-dots (R-C-dots) using o-phenylenediamine (o-PDA) as a precursor without using any catalyst. The R-C-dots are able to precipitate on the wall of the reactor, which enables us to obtain solid-state C-dots with high purity. The R-C-dots have a photoluminescence quantum yield (PLQY) of as high as 36.75%, which is among the highest PLQY values reported previously for R-C-dots without using catalysts. The transient PL and transient absorption spectra revealed that 5,14-dihydroquinoxalino[2,3-b]phenazine linked on the surface of the C-dots determined the red luminescence behavior. This work provides a new path for the controllable synthesis of high-purity R-C-dots, showing potential applications in optoelectronic devices.

Construction of composite films using carbon nanodots for blocking ultraviolet light from the Sun

Carbon nanodots (CNDs) which demonstrate concentration-dependent emission and have a photoluminescence quantum yield of 45% were designed. Transparent CND-containing composite films (CND-films), obtained by combining the CNDs with polyvinyl alcohol in different proportions, were shown to block the UV component of sunlight. Whereas the pure PVA film could not block UV light, the ability of CND-films to block UV light could be adjusted by altering the proportion of CNDs in the film. The larger the proportion of CNDs, the greater the extent of UV blocking. CND-film containing 32 wt% CNDs completely blocked UV light (≤400 nm) from sunlight, without affecting the transmission of visible light (>800 nm). The ability of the CND-films to block the UV component of sunlight was investigated using a commercially available UV-induced color change card, which confirmed that the capacity of the CND-films to block UV light could be adjusted by altering the proportion of CNDs in the film. This study shows that CNDs with concentration-dependent long wavelength emission characteristics can be used as optical barrier units for the preparation of materials to block high-energy short wavelength light.