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Photo-activatable antipathogenic carbon dots (CDs) were prepared by carbonization of citric acid and arginine (Arg) via 3 min microwave treatment for use in the eradication of common microorganisms. Nitrogen-doped Arg CDs were spherical in shape with a size range of 0.5 to 5 nm. The Arg CDs were modified with fluorescent dyes, such as fluorescein sodium salt (FSS, as Arg-FSS) and riboflavin (RBF, as Arg-RBF), to improve antimicrobial potency by enhancing their application in photodynamic therapy. The modified Arg CDs afforded fluorescence emission properties at 520 nm in the green region in addition to excellent blue fluorescence intensity at 420 nm under 345 nm excitation upon their FSS and RBF conjugation, respectively. Although the cytotoxicity of Arg CDs was decreased for Arg-RBF CDs to 91.2 ± 0.7% cell viability for fibroblasts, the Arg-based CDs could be safely used for intravenous applications at 1000 µg/mL concentration. The Arg CDs showed broad-spectrum antimicrobial activity against common pathogens and the minimum inhibitory concentration of Arg CDs was almost two-fold decreased for the modified forms without UV light. However, faster and more effective antibacterial activity was determined for photosensitive Arg-RBF CDs, with total bacterial eradication upon UV-A light exposure for 30 min.

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Glutathione (GSH) is present in almost every cell in the body and plays various integral roles in many biological processes. The Golgi apparatus is a eukaryotic organelle for the biosynthesis, intracellular distribution, and secretion of various macromolecules; however, the mechanism of GSH in the Golgi apparatus has not been fully elucidated. Here, specific and sensitive sulfur-nitrogen co-doped carbon dots (SNCDs) with orange-red fluorescence was synthesized for the detection of GSH in the Golgi apparatus. The SNCDs have a Stokes shift of 147 nm and excellent fluorescence stability, and they exhibited excellent selectivity and high sensitivity to GSH. The linear response of the SNCDs to GSH was in the range of 10-460 µM (LOD = 0.25 µΜ). More importantly, we used SNCDs with excellent optical properties and low cytotoxicity as probes, and successfully realized golgi imaging in HeLa cells and GSH detection at the same time.

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Indigo Carmine is a hazardous dye and produces an allergic action for humans despite the excessive use of the dye in several industrial fields. A sensitive and simple fluorescent assay for determining Indigo Carmine relying on quenching of the fluorescent europium-doped carbon dots by the action of inner filter effect was developed. This sensing platform involved the preparation of europium-doped carbon dots from the hydrothermal carbonization of tannic acid and europium chloride, which was used as fluorescent reagent with a distinctive excitation/emission wavelength at 307/340 nm. Both excitation and emission fluorescence of prepared carbon dots can be successfully quenched by adding Indigo Carmine dye. The developed spectrofluorimetric method exhibits good linearity with the concentration of Indigo Carmine dye in the range of 1.5 to 10.0 µg/ml and provided a limit of detection (LOD) value of 0.40 µg/ml. Furthermore, the prepared carbon nanoparticles were identified and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR), and ultraviolet (UV)-spectrophotometer techniques. In addition, the developed detecting approach was applied to determine Indigo Carmine in juice samples with acceptable recovery.

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Excessive use of antibiotics in aquaculture severely endangers human health and ecosystems, which has raised significant concerns in recent years. However, conventional laboratory-based approaches regularly required time or skilled manpower. Herein, we propose a point-of-care-testing (POCT) biosensor detection device for the simultaneous determination of multiantibiotics without complex equipment or professional operators. A laser-printed paper-based microfluidic chip loaded with multicolor fluorescence nanoprobes (mCD-µPAD) was developed to rapidly detect sulfamethazine (SMZ), oxytetracycline (OTC), and chloramphenicol (CAP) on-site. These "fluorescence off" detection probes composed of carbon dots (CDs) conjugated with aptamers (donor) and MoS2 nanosheets (acceptor) (CD-apt-MoS2) were based on Förster resonance energy transfer. Upon the addition of target antibiotics, the significantly recovered fluorescence signal on the µPAD can be sensitively perceived by employing a 3D-printed portable detection box through a smartphone. Under optimal conditions, this µPAD allowed for a rapid response of 15 min toward SMZ, OTC, and CAP with considerable sensitivities of 0.47, 0.48, and 0.34 ng/mL, respectively. In shrimp samples, the recoveries were 95.2-101.2, 96.4-105, and 96.7-106.1% with RSD below 6%. This paper-based sensor opens an avenue for on-site, high-throughput, and rapid detection methods and can be widely used in POCT in food safety.

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A novel bifunctional carbon dot (CD)-based sensing platform was constructed for detection of tetracyclines (TCs) and Al3+. The fluorescence CDs were fabricated by hydrothermal method using phenylenediamine (p-PD) and ethylenebis(oxyethylenenitrilo) tetraacetic acid (EGTA) as precursors. The obtained prepared CDs show bright yellow fluorescence (y-CDs, EX = 400 nm and Em = 556 nm), high fluorescence quantum yield (QY = 21.55 ± 0.06%), and preferable optical stability. TCs can directly quench the fluorescence of y-CDs based on static quenching characteristics and a small internal filtration effect (IEF). By adding Al3+ to the y-CDs + TCs system, the fluorescence is partly recovered because TCs escape from the surface of the y-CDs and form a more stable chelate with Al3+. The sensing platform displays good selectivity and high sensitivity to TCs and Al3+ with low detection limits of 0.057-0.23 µM and 0.091 µM, respectively. Importantly, this sensing platform has enabled the detection of TCs and Al3+ in milk samples with satisfactory recoveries and RSDs, confirming the reliability and feasibility of this method. Combining with low toxicity and preferable biocompatibility, the y-CDs are extended to cellular imaging and detection of CTC and Al3+ in A549 cells.

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Nitrogen (N-) and sulfur (S-) doped carbon dots (CDs) were synthesized in a single step in a few min, 1-4 min via microwave technique from five different types of amino acids viz. Arginine (A), Lysine (L), Histidine (H), Cysteine (C), and Methionine (M). These amino acid derived N- and/or S- doped CDs were found to be in spherical shapes with 5-20 nm particle size range determined by Transition Electron Microscope (TEM) images and Dynamic Light Scattering (DLS) measurements. Thermal degradation, functional groups, and surface potential of the CDs were determined by Thermogravimetric Analysis (TGA), FT-IR spectroscopy, and zeta potential measurements, respectively. Although the zeta potential value of Cysteine derived CD (C-CD) was measured as -7.45±1.32 mV, the zeta potential values of A-CD, L-CD, H-CD, and M-CD particles were measured as +2.84±0.67, +2.61±1.0, +4.10±1.50 and+2.20±0.60 mV, respectively. Amongst the CDs, C- CDs was found to possess the highest quantum yield, 89%. Moreover, the blood compatibility test of CDs, determined with hemolysis and blood clotting tests was shown that CDs at 0.25 mg/mL concentration, CDs has less than 5% hemolysis ratio and higher than 50% blood clotting indexes. Furthermore, A-CD was modified with polyethyleneimine (PEI) and was found that the zeta potential values was increased to +34.41±4.17 mV (from +2.84±0.67 mV) inducing antimicrobial capability to these materials. Minimum Inhibition Concentration (MIC) of A-CD dots was found as 2.5 mg/mL whereas the PEI modified A-CDs, A-CD-PEI was found as 1 mg/mL against Escherichia coli ATCC 8739 (gram -) and Staphylococcus aureus ATCC 6538 (gram +) bacteria strains signifying the tunability of CDs.

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Most carbon dots (CDs) conventional fabrication approaches produce single colored fluorescent materials, different methods are required to synthesize distinct carbon dots for specific optical applications. Herein, using one-pot hydrothermal treatment of Syringa obtata Lindl, a facile, low-cost and green assay is achieved in the controllable synthesis of blue and green fluorescent carbon dots. The fluorescent emission of CDs can be well-tuned by adding sodium hydroxide in the precursor solution. Blue fluorescent CDs are applied to Fe3+ sensing with a low detection limit of 0.11 µM of linear range from 0.5 to 80 µM, and then further extended to analysis river water samples. Green fluorescent CDs can be applied to pH detection, which show a remarkable linear enhancement in the green fluorescence emission region when the pH is increased from 1.98 to 8.95. Eventually, the detection of Fe3+ and pH are applied for the living cells fluorescent images in MCF-7 cells are achieved successfully, indicating as-synthesized CDs potential toward diverse application as promising candidate.

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