Cellulose-based yellow-green emitting carbon dots with large Stokes shift as effective "turn off-on" fluorescence platforms for Cr (VI) and AA dual efficacy detection.

BACKGROUND: Hexavalent chromium (Cr (VI)) is highly carcinogenic to humans. Ascorbic acid (AA) deficiency can be hazardous to health. And the dual-effect fluorescence detection of them is an important research topic. Carbon dots (CDs) based on cellulose are excellent candidates for the fluorescence probes due to their low cost and environmental friendliness. But most of them exhibit shortwave emission, small Stokes shift and poor fluorescence performance, all of which limit their use. Therefore, there is an urgent need for cellulose CDs with longer emission wavelengths and larger Stokes shifts in dual-effect fluorescence detection of Cr (VI) and AA. RESULTS: Under optimal conditions (180 °C, 12 h), we prepared cellulose-based nitrogen-doped carbon dots (N-CDs) by a simple one-step hydrothermal process, which display longer emission wavelengths (ex: 370 nm, em: 510 nm), larger Stokes shifts (140 nm) and high fluorescence quantum yield (QY: 19.27 %). The continuous "turn-off" and "turn-off-on" fluorescence detection platforms were constructed based on the internal filtering effect (IFE) between Cr6+ and N-CDs, and Cr6+ reduced to Cr3+ by AA at pH = 6. The platform has been successfully simultaneous detect Cr (VI) and AA with a wide range of 0.01-40 µM and 0.1-100 µM. And the lowest limits of detection (LOD) are 0.0303 µM and 0.072 µM, respectively. In the presence of some other metals, non-metal ions and water-soluble acids in the fruits, this fluorescent platform can demonstrate a high level of interference immunity. SIGNIFICANCE AND NOVELTY: This represents the first yellow-green cellulose-based N-CDs with large Stokes shift for dual-effect detection of Cr (VI) and AA in real water samples and fresh fruits. The fluorescence detection platform has the advantage of low volume detection. Less than 2 mL of sample is required for testing and results are available in <5 min. This method is rare and supply a novel idea for the quantitative monitoring of Cr (VI) and AA.

[Experimental teaching reform of polymer molar mass determination using gel permeation chromatography coupled with light scattering].

Gel permeation chromatography coupled with light scattering (GPC-LS) is among the most common methods for determining the molar masses of polymers. GPC-LS is widely used in polymer science research and has been adopted for many industrial applications owing to its high sensitivity, accuracy, and precision. The determination of polymer molar masses using GPC-LS is an important experimental component of the "Polymer Physics Experiments" course. However, the present GPC-LS experimental teaching content tends to be overly simplistic and lacking in depth. Herein, the original experimental content is expanded and multiple sets of experiments are redesigned: (1) Using commercial polystyrene as an experimental sample, the molar mass, molar mass distribution, radius of gyration, and other molecular structure parameters are determined using GPC-LS; (2) Using two polyacrylonitriles with similar molecular structure parameters, subtle differences in the molar mass distributions of the samples are explored using differential mass distribution curves; (3) By comparing the chromatograms of a series of polyethylene glycols with different molar masses, the effect of molar mass on chromatographic peaks is investigated; and (4) For three different polymers (polyacrylonitrile, poly(methyl methacrylate), and poly(ß-cyclodextrin)), the polymer chain conformations are analyzed using conformation plots (i.e., radius of gyration vs. molar mass). In addition, the experimental teaching method is modified to convert passive learning into active learning, thereby improving the students' self-directed learning ability. This experimental teaching reform will help students obtain a more comprehensive understanding of GPC-LS principles and applications, stimulate their enthusiasm for learning, and improve the teaching quality of the experimental course.

Highly Sensitive Detection of Melamine Based on the Fluorescence Resonance Energy Transfer between Conjugated Polymer Nanoparticles and Gold Nanoparticles.

Adding melamine as additives in food products will lead to many diseases and even death. However, the present techniques of melamine detection require time-consuming steps, complicated procedures and expensive analytical apparatus. The fluorescent assay method was facile and highly sensitive. In this work, a fluorescence resonance energy transfer (FRET) system for melamine detection was constructed based on conjugated polymer nanoparticles (CPNs) and gold nanoparticles (AuNPs). The energy transfer efficiency is up to 82.1%, and the system is highly selective and sensitive to melamine detection with a lower detection limit of 1.7 nmol/L. Moreover, the interaction mechanism was explored. The results showed that the fluorescence of CPNs were firstly quenched by AuNPs, and then restored after adding melamine because of reducing FRET between CPNs and AuNPs. Lastly, the proposed method was carried out for melamine detection in powdered infant formula with satisfactory results.