Dopamine assay based on an aggregation-induced reversed inner filter effect of gold nanoparticles on the fluorescence of graphene quantum dots

Synthesis of N-doped carbon dots for highly selective and sensitive detection of metronidazole in real samples and its cytotoxicity studies

The current investigation reports the synthesis of N-CDs using glucosamine, ascorbic acid, and ethylenediamine precursors by a simple hydrothermal technique. The formation of N-CDs was proved by various characterisation techniques such as X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Fourier-Transform Infrared Spectrophotometer (FT-IR). The optical properties were investigated by fluorescence and UV-vis spectrophotometer. Also, N-CDs showed high selectivity in detecting the MTZ compared to several other analytes. However, the metronidazole serves as an antibiotic against several microbial diseases but also a genotoxic, carcinogenic to the human when used in excessive dosage. The synthesised N-CDs showed high selectivity in detecting the MTZ compared to several other analytes. Besides, the cytotoxicity of the N-CDs was studied to evaluate its toxicity against the HeLa cancer cells. It showed 65.6% cell viability and 34.3% toxicity against the cancerous cells, and similarly 71% of cells viability against H9C2 cells. Thus, the current investigation explores the promising selective sensing of N-CDs against MTZ, along with that, it proved its cytotoxicity against HeLa cancerous cells and non-toxicity against H9C2 cells. The synthesised CDs can be better MTZ sensors and anti-cancer agents on further development at the industrial scale.

Post-synthetic modification of graphene quantum dots bestows enhanced biosensing and antibiofilm ability: efficiency facet

Graphene quantum dots (GQDs) are a luminescent class of carbon nanomaterials with a graphene-like core structure, possessing quantum confinement and edge effects. They have gained importance in the biological world due to their inherent biocompatibility, good water dispersibility, excellent fluorescence and photostability. The improved properties of GQDs require the logical enactment of functional groups, which can be easily attained through post-synthetic non-covalent routes of modification. In this regard, the present work has for the first time employed a simple one-pot post-modification method utilizing the salt of amino caproic acid, an FDA approved reagent. The adsorption of the modifier on GQDs with varying weight ratios is characterized through DLS, zeta potential, Raman, absorption and fluorescence spectroscopy. A decrease of 20% in the fluorescence intensity with an increase in the modifier ratio from 1 to 1000 and an increased DLS size as well as zeta potential demonstrate the efficient modification as well as higher stability of the modified GQDs. The modified GQDs with a high weight ratio (1 : 100) of the modifier showed superior ability to sense dopamine, a neurotransmitter, as well as competent biofilm degradation ability. The modified GQDs could sense more efficiently than pristine GQDs, with a sensitivity as low as 0.06 μM (limit of detection) and 90% selectivity in the presence of other neurotransmitters. The linear relationship showed a decrease in the fluorescence intensity with increasing dopamine concentration from 0.0625 μM to 50 μM. Furthermore, the efficiency of the modified GQDs was also assessed in terms of their antibiofilm effect against Staphylococcus aureus. The unmodified GQDs showed only 10% disruption of the adhered bacterial colonies, while the modified GQDs (1 : 100) showed significantly more than 60% disruption of the biofilm, presenting the competency of the modified GQDs. The unique modifications of GQDs have thus proven to be an effective method for the proficient utilization of zero-dimensional carbon nanomaterials for biosensing, bioimaging, antibacterial and anti-biofilm applications.

A flow injection fluorescence "turn-on" sensor for the determination of metformin hydrochloride based on the inner filter effect of nitrogen-doped carbon dots/gold nanoparticles double-probe

In this paper, an ultrasensitive and rapid "turn-on" fluorescence sensor, integrating flow-injection (FI) with nitrogen-doped carbon dots/gold nanoparticles (N-CDs/AuNPs) double-probe is established for the determination of metformin hydrochloride (MET) in biological fluids. The sensing strategy involves the weak inner filter effect between AuNPs and N-CDs due to aggregation products of MET with AuNPs. Unfortunately, the degree of AuNPs aggregation is difficult to control through manual assays, resulting in intolerable measurement error that limits further applications. However, the proposed method overcomes the above problem, and significantly lowers the consumption of expensive reagents (AuNPs: about 60 μL per test). Under optimal conditions, the fluorescence intensity at 400 nm excitation and 505 nm emission wavelengths display a linear correlation with MET concentration (5-100 μg L^-1) and the limit of detection is 2.32 μg L^-1 (3.3 S/k). The advantages of the presented method include high sensitivity, rapid speed (60 sample h^-1), good accuracy and precision (RSD ≤ 2.1%, n = 11) and low cost. Since MET is the first-line hypoglycemic agent in patients with type II diabetes, this method can preliminarily determine MET content in urine samples, giving satisfactory results.

A gold–carbon dots nanoprobe for dual mode detection of ketamine HCl in soda drinks

Fluorescent and color sensors for ketamine HCl determination. A dual-mode nanoprobe for the detection of a club drug in spiked beverage.

Carbon Dots Green Synthesis for Ultra-Trace Determination of Ceftriaxone Using Response Surface Methodology

The present study sought to develop a facile and green synthetic approach for producing fluorescent carbon dots (CDs) from a natural biomass using aqueous extraction of carbonized blue crab shell. Spherical carbon dots (6.00 ± 3.0 nm) exhibited an extended emission range with excellent quantum yield (14.5 ± 3.5%). In order to measure ceftriaxone, we offered a simple and sensitive method, based on fluorescence quenching of carbon dots in plasma and water with recovery values of 94.5-104.1%. Furthermore, with usage of central composite design (CCD) based response surface methodology (RSM); we optimized the effect of different factors. In addition, ANOVA evaluated the accuracy and suitability of quadratic model. Under optimal conditions, fluorescence quenching revealed a sensitive response in the concentration range of 20-1000 nM with the limit of detection 9.0 nM for ceftriaxone. Finally, carbon dots-based fluorescence quenching procedure was able to quantify ceftriaxone in plasma, as well as mineral and tap water. Spiked samples achieved satisfactory efficiencies.

Silver nanoparticles capped with carbon dots as a fluorescent probe for the highly sensitive "off-on" sensing of sulfide ions in water

The present study illustrates the synthesis of silver nanoparticles capped with carbon dots (AgNPs-CDs) and their application towards the sensitive and selective sensing of sulfide ions by colorimetry and spectrofluorimetry methods. The CDs were prepared from l-asparagine by pyrolysis at 234 °C. The as-synthesized CDs were then utilized as reducing and capping agents for the synthesis of AgNPs-CDs by the wet chemical method. The size of the AgNPs-CDs was found to be ~ 5.2 nm. They show a characteristic surface plasmon resonance band at 417 nm and emission maximum at 441 nm when excited at 348 nm. Since the AgNPs were formed on the surface of CDs, the emission intensity of AgNPs-CDs was drastically decreased in contrast to that of CDs. The as-synthesized AgNPs-CDs were then successfully used for the sensitive and selective determination of sulfide ions. The addition of 0.1 μM sulfide ions to AgNPs-CDs leads to a decrease in the absorbance intensity at 417 nm aside turning from yellow to colorless. In the contrary, the emission was "turned on" after the addition of sulfide ions. The decrease in the absorbance and increase in the emission were attributed to the rapid formation of Ag₂S. Finally, the practical application of the present method was demonstrated by determining dissolved H₂S in tap water samples.

Photoinduced holes transfer based visual determination of dopamine in human serum

A simple unexplored strategy was followed to construct ratiometric fluorescence-based sensing system for the detection of dopamine (DA) in human serum. Ratiometric fluorescence system was constructed through bonding energy transfer (TBET) by conjugating carboxyl functionalized CdTe quantum dots (QDs) and amine-capped Carbon quantum dots (CQDs). The photophysical properties of sensing system were characterized by standard methods. Photoluminescence (PL) of sensing system under excitonic wavelength (350 nm) depends on dual emission at 440 and 595 nm that corresponds to CQDs and CdTe QDs respectively. The developed sensing system was utilized for visual determination of DA, an unquenched blue fluorescence of CQDs in ratiometric system reveals the visual color differentiation for DA binding with CdTe QDs among the possible interferences (Alanine, Glycine, Glucose, Sucrose, Urea and Ascorbic acid). The limit of detection (LOD) and quantification (LOQ) was calculated as 8.1 and 27.2 nm respectively by using regression analysis. Photoinduced holes transfer (PHT) might have attributed the possible sensing mechanism for DA that quench the photoluminescence sequentially to enhance the sensing performance of QDs. The matrix interferences and reliability of the developed sensing platform were evaluated by testing DA spiked human serum and the sensing response was found to be field deployable.

Aequorin as a sensitive and selective reporter for detection of dopamine: A photoprotein inhibition assay approach

Dopamine is a metabolite that plays a key role in the human body and in biomedical and diagnostic applications. Thus, the concentration of this analyte has been considered in various diseases in therapeutic drug monitoring (TDM). In the present study, for the first time, a photoprotein inhibition assay strategy was developed by utilizing aequorin for the direct detection of dopamine as a receptor and reporter simultaneously. The results showed that bioluminescence emission of aequorin was effectively quenched by increasing concentration of dopamine at the range of 1 nM to 100 μM with a detection limit of 53 nM. The viability of this method for the monitoring of dopamine in spiked biological fluids was also established and it was successfully applied for the direct determination of dopamine in a blood serum and urine without preliminary treatment with satisfactory quantitative recovery 90-95% and 82-93%, respectively. The structural investigation using circular dichroism, fluorescence spectroscopy, and docking simulation indicated that, changes in the microenvironment of aromatic residues were significant, while minor conformational alterations of the protein were observed. It seems dopamine inhibits bioluminescence activity with specific binding to the residues involved in the light production.

MOF based fluorescent assay of xanthine oxidase for rapid inhibitor screening with real-time kinetics monitoring

The activity assay of xanthine oxidase (XO) is of great application value in clinical diagnosis because the abnormal level of this enzyme is related to a series of pathological states. In this work, a Zr based metal-organic framework (BTB-MOF) with stable photoluminescence in pure water and buffer solution was synthesized. The examination about the fluorescent responses of this material to xanthine and its oxidation product, uric acid, showed that, although both of them affected the emission of BTB-MOF in quenching form, the efficiencies presented much difference. Taking advantage of this feature, a fluorescent method was developed for the activity assay of XO, that is, BTB-MOF was added to the enzymatic oxidation system as a sensor to transduce the proceeding of the reaction real-timely to the signal of fluorescent intensity change. Our method can work under the interference of normal biologically related species, and precisely reflect XO activity in the range of 0.2-40 U L^-1 (detection limit = 0.004 U L^-1). With consecutive fluorescence intensity scan, this assay could be applied as a high speed screening method of XO inhibitors with the testing time of 1 min. This work shows the wide potential application of MOFs in enzyme analysis.

A fluorescent sensor for detecting dopamine and tyrosinase activity by dual-emission carbon dots and gold nanoparticles

In this work, we report a fluorescence strategy for detecting dopamine (DA) and sensing tyrosinase (TYR) activity on the basis of the dual-emission carbon dots (DECDs), which contain two emitters: the blue emitters (BE, maximum emission at 385nm) and yellow emitters (YE, maximum emission at 530nm). Gold nanoparticles (AuNPs) can effectively quench the two emissions of DECDs. The addition of DA aggregates AuNPs effectively, leading to the fluorescence recovery of dual emitters gradually. This strategy exhibits a high selectivity toward DA and shows good linear ranges, such as 0.5-3μM for BE and 0.1-3μM for YE. Additionally, the proposed method is successfully applied to the determination of DA in real samples with satisfactory recoveries. Subsequently, this DECDs-AuNPs platform is further taken advantage to assess TYR activity by the aid of TYR's capability for oxidation of DA into dopaquinone, which will not induce the agglomeration of AuNPs, so the fluorescence quenching of DECDs is associated with TYR activity. Finally, the mechanism of the reaction is discussed in detail, and the results suggest that both amine and phenolic hydroxyl groups of DA bring the aggregation of AuNPs.

Inner filter effect-based fluorescent sensing systems: A review

Inner filter effect (IFE) was previously considered as an error in fluorescence measurement. In recent years, it has been developed as an important non-irradiation energy conversion model of spectroscopic technique and found wide applications in the fields of chemical sensing and biosensing. In comparison with traditional techniques based on forster resonance energy transfer (FRET), the IFE-based fluorescent approach is more flexible and straightforward without the link of absorber with fluorescer. The present review for the first time introduces the state of the art in the progress of the IFE-based fluorescent sensing systems, including sensing strategy, essential conditions, materials option, and their applications for the detection of various target analytes, e.g., ionic species, small molecules, and macromolecules. In addition, the benefits and limitations of the IFE-based fluorescent sensing systems are also critically discussed and highlighted.