Green synthesis of N,S-doped carbon dots for tartrazine detection and their antibacterial activities

A simple, green and low-cost method was developed for the synthesis of highly fluorescent N,S-doped carbon dots (N,S-CDs) via the hydrothermal treatment of Gandha Prasarini (GP) leaves as a natural source of carbon, nitrogen and sulfur. The as-prepared N,S-CDs exhibited excitation-dependent green fluorescence emission (λex = 450 nm, λem = 525 nm) with excellent stability, and were used as a fluorescent probe for the selective detection of tartrazine with a limit of detection of 0.18 μM. The fluorescence quenching of N,S-CDs was due to the inner filter effect. The developed method has been employed for the determination of tartrazine in honey and soft drinks with satisfactory recovery ranging from 92 to 110.2%. In addition, the antibacterial activity of the N,S-CDs was explored against both Gram-negative bacteria, Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa), and Gram-positive bacteria, Staphylococcus aureus (S. aureus). The antibacterial mechanism of the N,S-CDs was investigated. The results indicated that the antibacterial activity was due to the membrane damage of the bacteria by the N,S-CDs. Besides, the N,S-CDs showed negligible lytic effects on human erythrocytes. These findings will inspire further exploitation of CD-based nano-bactericides in biomedical applications.

Combination of colorimetry, inner filter effect-induced fluorometry and smartphone‑based digital image analysis: A versatile and reliable strategy for multi-mode visualization of food dyes

Herein, a multi-mode visualization platform was initiated for in-situ detection of food dyes (FDs) by combining colorimetry, fluorometry and smartphone‑based digital image analysis, in which water-dispersible quantum dots (QDs) were served as nanoprobes. Colorimetry was achieved by color comparison, while both fluorometry and fluorescence quantification were performed through inner filter effect (IFE)-induced fluorescence quenching, then color information (RGB & gray-scale values) of colorimetry and fluorometry was picked by a smartphone to reconstruct digitized alignments. Since IFE mechanism was concentration-dependent but did not rely on the interaction between fluorophore and quencher, the whole process of fluorescence response could be finished within 10 s, and both color gradients and fluorescence changes showed fine mappings to FDs concentrations in the range of 1.0 × 10^-3∼0.035 mg/mL for brilliant blue, and 1.0 × 10^-4∼0.1 mg/mL for Allura red and sunset yellow. As a proof-of-concept, the in-situ multi-mode visualization of these FDs in real beverages was experimentally proved to be highly feasible and reliable as compared with instrumental techniques like UV-vis/fluorescence spectrometry, along with HPLC. Finally, this strategy was extended to the multi-mode visualization of non-food dyes in three simulated wastewater samples with high credibility by contrast with the true additive amounts of model dyes.

Colorimetry Combined with Inner Filter Effect-Based Fluorometry: A Versatile and Robust Strategy for Multimode Visualization of Food Dyes

Herein, a strategy combining colorimetry and inner filter effect (IFE)-based fluorometry was developed for multimode visualization of food dyes (FDs) using CdTe quantum-dots-doped fluorescent indicator papers as a sample-to-answer device. Colorimetry was straightforwardly achieved by FDs extraction through electrostatic interaction and hydrophobic effect while fluorometry was implemented by IFE-induced fluorescence quenching. RGB/gray-scale values of colorimetry and fluorometry were furtherly picked by a smartphone application and applied to reconstruct color information-based digital image analysis for both direct alignments and linear regression analysis. The apparent color and fluorescence of FDs-bound indicator papers, together with their digitized color information, showed a good mapping to FDs concentrations in the range of 0-0.5 mg/mL for Sunset Yellow, 0-0.2 mg/mL for Allura Red, and 0-0.08 mg/mL for Brilliant Blue. As a proof of concept, the dosages of these FDs in real beverages and simulated dye effluents were deduced and cross-validated by different visualization modes, and finally double-checked by instrumental techniques such as spectrometric methods, high-performance liquid chromatography (HPLC), and mass spectroscopy (MS). The above findings concluded that (i) IFE mechanism is generally applicable to build fluorometric systems and (ii) cross validation of different visualization modes can markedly improve detection accuracy, which may provide references for design and fabrication of novel "lab-on-paper" devices for visualization applications with high reliability.

A novel phenolphthalein-based fluorescent sensor for Al3+ sensing in drinking water and herbal tea samples

In this study, 3,3-bis(4-hydroxy-3-((E)-((4-hydroxyphenyl)imino)methyl) phenyl)isobenzofuran-1(3H)-one (HMBP) was designed as a ''turn-on″ fluorogenic chemosensor to detect Al³⁺. Studies were performed in C₂H₅OH-HEPES (v/v, 9/1, pH 7.0) media at λ_em = 475 nm. The LOD value was found to be 0.113 µM. The stoichiometric ratio of HMBP-Al³⁺ was determined as 1:2 by Job's plot and ESI-MS as well as ¹H NMR titration. The binding constant of chemosensor HMBP with Al³⁺ from the Benesi-Hildebrand equation was determined to be 1.21 × 10⁸ M^-1. The quantum (Φ) yields were obtained as 0.040 and 0.775 for the chemosensor HMBP and HMBP-Al³⁺, respectively. The response of the chemosensor HMBP towards Al³⁺ was attributed to the strategies of blocking the photo-induced electron transfer (PET) and CN isomerisation mechanisms. Finally, the sensing of the chemosensor HMBP for the determination of Al³⁺ in real food samples, drinking waters and herbal teas, were employed.

Dual-emission carbon dots achieved by luminescence center modulation within one-pot synthesis for a fluorescent ratiometric probe of pH, Hg2+, and glutathione

Dual-emission carbon dots were synthesized by one-pot hydrothermal pyrolysis of citric acid and polyethyleneimine in the presence of rhodamine B at 160 °C for 5 h. The carbon dots have an average diameter of 2.51 nm with rhodamine moiety on their surface. Two emission bands centered at 447 and 581 nm are exhibited in their fluorescence spectra excited at 360 nm, and the former is sensitive while the latter is insensitive to Hg²⁺ and pH. Glutathione (GSH) can recover the fluorescence quenched by Hg²⁺. Therefore, the dual-emission carbon dots were developed as a fluorescent ratiometric probe employing the ratio of the two intensities at 447 and 581 nm (R_I447/I581) as the signal for the determinations of pH, Hg²⁺, and GSH. In the range of 5.0-10.0, a good linear relationship between R_I447/I581 and pH was built with a regression equation of R_I447/I581 = 11.95-0.56 pH (R² = 0.998). In the range from 0.0 to 8.0 μM, an excellent linear relationship between R_I447/I581 and the concentration of Hg²⁺ was obtained with a calibration equation of R_I447/I581 = 6.2317-0.4458c (R² = 0.995) and a limit of detection (LOD) of 0.24 μM. In the range from 1.0 to 10.0 μM, a linear equation, R_I447/I581 = 1.9133-0.4157c (R² = 0.995), was calibrated between R_I447/I581 and the concentration of glutathione with a LOD of 0.27 μM. The recoveries for the determinations of Hg²⁺ and GSH in real samples were in the ranges of 94.6 to 103.8% and 94.3 to 104.2%, respectively. Graphical abstract Dual-emission carbon dots achieved by luminescence center modulation within one-pot synthesis for a fluorescent ratiometric probe of pH, Hg²⁺, and glutathione.

A sensitive “ON–OFF” fluorescent probe based on carbon dots for Fe2+ detection and cell imaging

A sensitive fluorescent probe based on carbon dots has been synthesized by a one-pot hydrothermal method for the rapid detection of intracellular Fe²⁺.

Small molecules derived carbon dots: synthesis and applications in sensing, catalysis, imaging, and biomedicine

Carbon dots (CDs) are the new fellow of carbon family having a size less than 10 nm and attracted much attention of researchers since the last decade because of their unique characteristics, such as inexpensive and facile synthesis methods, easy surface modification, excellent photoluminescence, outstanding water solubility, and low toxicity. Due to these unique characteristics, CDs have been extensively applied in different kind of scientific disciplines. For example in the photocatalytic reactions, drug-gene delivery system, in vitro and in vivo bioimaging, chemical and biological sensing as well as photodynamic and photothermal therapies. Mainly two types of methods are available in the literature to synthesize CDs: the top-down approach, which refers to breaking down a more massive carbon structure into nanoscale particles; the bottom-up approach, which refers to the synthesis of CDs from smaller carbon units (small organic molecules). Many review articles are available in the literature regarding the synthesis and applications of CDs. However, there is no such review article describing the synthesis and complete application of CDs derived from small organic molecules together. In this review, we have summarized the progress of research on CDs regarding its synthesis from small organic molecules (bottom-up approach) via hydrothermal/solvothermal treatment, microwave irradiation, ultrasonic treatment, and thermal decomposition techniques as well as applications in the field of bioimaging, drug/gene delivery system, fluorescence-based sensing, photocatalytic reactions, photo-dynamic therapy (PDT) and photo-thermal (PTT) therapy based on the available literature. Finally, the challenges and future direction of CDs are discussed.