Facile Synthesis of Green Fluorescent Carbon Dots and Their Application to Fe3+ Detection in Aqueous Solutions

Probe-assisted detection of Fe3+ ions in a multi-functionalized nanopore

Iron is an essential element that plays critical roles in many biological/metabolic processes, ranging from oxygen transport, mitochondrial respiration, to host defense and cell signaling. Maintaining an appropriate iron level in the body is vital to the human health. Iron deficiency or overload can cause life-threatening conditions. Thus, developing a new, rapid, cost-effective, and easy to use method for iron detection is significant not only for environmental monitoring but also for disease prevention. In this study, we report an innovative Fe3+ detection strategy by using both a ligand probe and an engineered nanopore with two binding sites. In our design, one binding site of the nanopore has a strong interaction with the ligand probe, while the other is more selective toward interfering species. Based on the difference in the number of ligand DTPMPA events in the absence and presence of ferric ions, micromolar concentrations of Fe3+ could be detected within minutes. Our method is selective: micromolar concentrations of Mg2+, Ca2+, Cd2+, Zn2+, Ni2+, Co2+, Mn2+, and Cu2+ would not interfere with the detection of ferric ions. Furthermore, Cu2+, Ni2+, Co2+, Zn2+, and Mn2+ produced current blockage events with quite different signatures from each other, enabling their simultaneous detection. In addition, simulated water and serum samples were successfully analyzed. The nanopore sensing strategy developed in this work should find useful application in the development of stochastic sensors for other substances, especially in situations where multi-analyte concurrent detection is desired.

One-Pot, Optimized Microwave-Assisted Synthesis of Difunctionalized and B-N Co-Doped Carbon Dots: Structural Characterization

In this work, we employed a novel microwave-assisted synthesis method to produce nitrogen and boron co-doped carbon dots (B-N co-doped CDs). To achieve optimal synthesis, we conducted a comprehensive parameter modulation approach, combining various synthesis temperatures, times, and precursor concentrations, while keeping the power constant at 150 W and pH 5. Using maximum fluorescence emission as our response variable, the best conditions were identified as 120 °C, 3 min, and a precursor concentration of 1 mg/mL. Characterization using field emission scanning electron microscopy revealed these CDs to have a spherical morphology with an average size of 10.9 ± 3.38 nm. Further high-resolution transmission electron microscopy showed an interplanar distance of 0.23 nm, which is in line with prior findings of CDs that present a 0.21 nm distance corresponding to the (100) plane of graphite. Optical properties were ascertained through UV-vis absorption, identifying distinct π-π* and n-π* transitions. Fluorescence spectroscopy highlighted an emission peak at 375 nm when excited at 295 nm, achieving a quantum yield of 56.7%. Fourier-transform infrared spectroscopy and Raman spectroscopy analyses confirmed the boronic acid and amine groups' presence, underscoring the graphitic nature of the core and the co-doping of boron and nitrogen. These empirical observations were compared with theoretical investigations through simulated Raman spectra, proposing a potential structure for the CDs. X-ray photoelectron spectroscopy further endorsed the co-doping of nitrogen and boron, along with the detection of the specified functional groups. All these characteristics could lend this nanomaterial to different types of applications such as fluorescent probes for a broad range of analytes and for fluorescent cell imaging.

The study of a novel high selectivity pyrenyl-based fluorescence probe with aggregation-induced emission characteristics for Fe3+ detection designed by a structure modulation strategy

The past decades have witnessed the feat of fluorescent probes for Fe³⁺ detection, where eliminating the interference by other metal ions plays a pivotal role in its detection by probes in complex environments. Herein, by taking advantage of the substituent effects, the electron-withdrawing group (EWG) -CF₃ and electron-donating group (EDG) -CH₃ were introduced to 2-(1-pyrenyl)pyridine (pypyr) to prepare two turn-off fluorescence probes, 5-trifluoromethyl-2-(1-pyrenyl)pyridine (pypyr-CF3) and 5-methyl-2-(1-pyrenyl)pyridine (pypyr-CH3). Intriguingly, both probes displayed novel aggregation-induced emission (AIE) characteristics in MeCN/H₂O mixtures and the size and morphology of the aggregated particles were studied via DLS and TEM. By the modulation strategy, pypyr-CF3 can detect Fe³⁺ in the presence of 29 different metal ions without interference. Comparatively, pypyr-CH3 experienced serious interference from other metal ions such as Hg²⁺ and Zr⁴⁺. Besides, pypyr-CF3 not only demonstrated a higher photoluminescence quantum yield (PLQY) of 65.25% and wider pH adaptability but is also capable of Fe³⁺ detection over a wide pH range of 2-11 with a short response time (5 seconds). A plausible quenching mechanism based on the inner filter effect has also been demonstrated. More importantly, the versatile applications of pypyr-CF3, such as the quantitative analysis of Fe³⁺ in actual water samples, anti-forgery ink, fingerprint identification, etc., further corroborate its superb capabilities. This study aims to lend concrete support to the design and selectivity modulation of probes.

One-Step Synthesis of Nitrogen/Fluorine Co-Doped Carbon Dots for Use in Ferric Ions and Ascorbic Acid Detection

Carbon dots (CDs) have caught enormous attention owing to their distinctive properties, such as their high water solubility, tunable optical properties, and easy surface modification, which can be generally used for the detection of heavy metals and organic pollutants. Herein, nitrogen and fluorine co-doped carbon dots (NFCDs) were designed via a rapid, low-cost, and one-step microwave-assisted technique using DL-malic acid and levofloxacin. The NFCDs emitted intense green fluorescence under UV lighting, and the optical emission peak at 490 nm was observed upon a 280 nm excitation, with a high quantum yield of 21.03%. Interestingly, the spectral measurements illustrated excitation-independent and concentration-independent single-color fluorescence owing to the presence of nitrogen and fluorine elements in the surface functional groups. Additionally, the NFCDs were applied for the selective detection of Fe³⁺ and ascorbic acid based on the “turn-off” mode. The detection limits were determined as 1.03 and 4.22 µM, respectively. The quenching mechanisms were explored using the static quenching mechanism and the inner filter effect. Therefore, a NFCDs fluorescent probe with single color emission was successfully developed for the convenient and rapid detection of Fe³⁺ and ascorbic acid in environments.