Phosphorus-Doped Carbon Quantum Dots as Fluorometric Probes for Iron Detection.

Carbon quantum dots (CQDs), a novel fluorescent nanomaterial, have been extensively employed/explored in various applications, that is, biosensors, bioimaging, nanomedicine, therapeutics, photocatalysis, electrocatalysis, energy storage system, and so forth. In this study, we report the synthesis, characterization, and the application of phosphorus-doped CQDs (PCQDs), synthesized using trisodium citrate and phosphoric acid by the hydrothermal method. The effect of phosphorus doping on optical features and the formation of PCQDs have been explored elaborately by controlling the concentrations of precursors, reaction time, and the temperature. The fluorescent quantum yield for PCQDs was determined to be 16.1% at an excitation/emission wavelength of 310/440 nm. Also, the optical and structural properties of PCQDs were determined by using various spectroscopic and microscopic techniques. Static quenching of fluorescence was determined upon the addition of Fe3+ to PCQDs because of the formation of the fluorescent inactive complex (PCQDs-Fe3+). Hence, this chemistry leads to the development of a new fluorometric assay for the detection of Fe3+. The lower limit of Fe3+ detection is determined to be 9.5 nM (3σ/slope), with the linear fit from 20 nM to 3.0 µM (R 2 = 0.99). We have validated this new assay in the raw, ejected, and purified water samples of the RO plant by the standard addition method. These results suggest the possibility of developing a new commercial assay for Fe3+ detection in blood, urine, and various industrial waste and sewage water samples. Furthermore, recycling the pollutant water into the freshwater using filters that consist of PCQDs offers a great deal.

Impact of aminated carbon quantum dots as a novel co-reactant for Ru(bpy)32+: resolving specific electrochemiluminescence for butein detection.

Development of novel nanomaterial-based co-reactant is highly desired for enhancing ECL intensity and widespread analytical applications. Herein, we report the distinct role of amine-functionalized carbon quantum dots (f-CQDs) as a co-reactant, for the first time, augmenting the ECL property of Ru(bpy)32+ and demonstrating for biopharmaceutical (butein) detection. Unlike conventional co-reactants like tripropylamine (TPrA), 2-(dibutylamino)ethanol (DBAE), and pristine CQDs, the f-CQDs as a co-reactant yield superior ECL of Ru(bpy)32+. More importantly, the ECL intensity is independent of types of noble metals, metal oxide surfaces, and dissolved oxygen. Notably, the ECL intensity of Ru(bpy)32+-f-CQDs is linearly quenched with an increased concentration of butein, whereas no changes were observed with conventional co-reactants. ECL functionality of Ru(bpy)32+-f-CQDs has no interference with other similar phytochemicals and antioxidants. Enhanced selectivity is observed due to the formation of polyaminoquinone-like structures, which is confirmed by in situ spectroelectrochemical (UV-vis) and FT-IR studies. The present result envisaged that f-CQDs could be an alternative co-reactant for TPrA/DBAE, raising the ECL of Ru(bpy)32+ suitable for analytical studies. Graphical abstract.

Fluorescence Turn-On, Specific Detection of Cystine in Human Blood Plasma and Urine Samples by Nitrogen-Doped Carbon Quantum Dots.

Determination of cystine in blood and urine is very important to monitor and maintain the bio metabolism, immune systems, and prevent the tissue/DNA damage from free radicals, diagnosis of cystinuria disease, cancer, and related autoimmune diseases. Among the various detection methods, fluorometric detection is simple, rapid, and sensitive to cystine using nontoxic, inexpensive, highly fluorescent, stable carbon quantum dots (CQDs). The CQDs are prepared from p-phenylenediamine by the hydrothermal method to get the inherent optical features of pH-dependent and excitation wavelength-independent fluorescence emission along with high aqueous stability due to pre-eminent nitrogen content. The red emission of CQDs originates from the intrinsic core that is associated with photoinduced electron transfer (PET). The turn-on fluorescence observed in presence of cystine is due to decrease in the PET by oxidation of CQDs. On the basis of this observation, we have developed an assay for the determination of cystine with a concentration range of 10 nM to 10 µM and the limit of detection is 0.4 nM. Additionally, our assay shows good recoveries (93-105%) for the spiked blood plasma and urine samples using the standard addition method.

Efficient dual-mode colorimetric/fluorometric sensor for the detection of copper ions and vitamin C based on pH-sensitive amino-terminated nitrogen-doped carbon quantum dots: effect of reactive oxygen species and antioxidants.

We present a facile strategy for highly sensitive and selective determination of copper(II) ions and vitamin C (ascorbic acid, AA) using new amino-terminated nitrogen-doped carbon quantum dots (CQDs) synthesized from melamine as the carbon and nitrogen source by the hydrothermal method. The CQDs have superior optical features, including a pH-sensitive photoinduced electron transfer process. The CQDs form a complex with Cu2+ ions, leading to the development of naked-eye, colorimetric, and fluorometric determination. AA reduces the Cu2+ ions to Cu+ ions, which cannot form the complex. Thus the absorbance and fluorescence of the CQDs are recovered by addition of AA because of dissociation of the complex into Cu+ and CQD. The in situ generation of reactive oxygen species when AA is added to Cu-CQD complexes in the presence of dissolved oxygen leads to the sensitive determination of AA, proposed on CQDs for the first time. The in situ generation of reactive oxygen species was confirmed by a fluorescence method using a hydroxyl radical indicator (i.e., coumarin). This novel "turn-off/turn-on" sensing approach using amine-functionalized CQDs is potentially applicable to determining the concentration of Cu2+ ions and AA in the areas of materials chemistry, nanobiomedicine, nanobiotechnology, and bioengineering because of its high sensitivity, high selectivity, low cost, simple naked-eye readout, and good linearity. Graphical abstract.

Amygdalin-Functionalized Carbon Quantum Dots for Probing ß-Glucosidase Activity for Cancer Diagnosis and Therapeutics.

A fluorescence active nanosystem capable of targeting specific receptors of cancer cells with or without a biorecognition element is advantageous for biosensor studies. Herein, a naturally occurring anticancer drug, amygdalin (synthetic form: Laetrile, a misnomer: vitamin B17), has been modified on the surface of carbon quantum dots, prepared by a hydrothermal method, to probe ß-glucosidase activity. Despite its cyanide toxicity, amygdalin is recently revived to be an anticancer molecule, and the risk factor can be optimized by understanding its binding efficiency with ß-glucosidase in the cancer cells. In this study, an in vitro biorecognition pattern of amygdalin-functionalized carbon quantum dots (Amy@CQDs) toward ß-glucosidase is typically evaluated by an aggregation-induced fluorescence emission mechanism. The optical functionality and structural integrity of CQDs before and after functionalization with amygdalin are comprehensively studied by spectroscopic and microscopic techniques. Our results demonstrate that Amy@CQDs is a stable hydrophilic graphitic carbon nanostructure exhibiting selective fluorescence quenching upon interaction with ß-glucosidase, enabling the lowest detection limit of 134 nM. Hydrolysis products of amygdalin mediated by ß-glucosidase were further confirmed by HPLC and colorimetric methods, indicating the selective binding of the prepared Amy@CQDs, which may find a useful application in cancer diagnosis and therapeutics.

Cholesterol derived carbon quantum dots as fluorescence probe for the specific detection of hemoglobin in diluted human blood samples.

In the current decade, carbon quantum dots (CQDs) are promising fluorescence probe in bio/analytical chemistry due to its unique properties. The functional groups of CQDs can be tuning the optical properties and selectively make a strong bond with target molecules. The interactions between Hb and cholesterol through hydrophobic batches are more favorable than the usual π-π interaction between CQDs and Hb. Hence we prepared highly stable CQDs with a fluorescence quantum yield of 45% from cholesterol by the hydrothermal method to make hydrophobic interactions with Hb. Concurrently the CQDs possess graphitic crystalline and amphiphilic structure in nature. The fluorescence at 440 nm arises from the intrinsic core of CQDs and it will not affect by solution pH and excitation wavelengths. This fluorescence intensity was selectively quenched by Hb owing to the formation of fluorescence inactive complex (CQDs-Hb) through strong hydrophobic interactions. The quenching mechanism complies with the Forster non-radiative energy transfer (FRET) theory. This method shows good linearity from 0.1 µM to 2.9 µM with a limit of detection of 24 nM (S/N = 3). This observation is used for the quenchometric determination of Hb in diluted human blood samples.