Inner-filter effect of nitrogen-doped carbon quantum dots-MnO2 nanotubes for smartphone-integrated dual-mode sensing of glutathione and captopril.

Nitrogen-doped carbon quantum dots (N-CQDs) exhibit unique fluorescence properties and are considered one of the best candidates for the development of fluorescence-based sensors for the detection of many analytes. In this work, a smartphone-assisted fluorescent sensor has been developed using N-CQDs and MnO2 nanotubes (MnO2 NTs) for the detection of glutathione (GSH) and captopril (CAP). N-CQDs were facilely synthesized via the solvothermal method, where o-phenylenediamine (o-PD) and urea were used as nitrogen precursors. Likewise, MnO2 NTs were synthesized using the hydrothermal method. Relying on the excellent fluorescence quenching ability of MnO2 NTs, a nanocomposite of N-CQDs and MnO2 NTs is prepared, wherein the fluorescence intensity of N-CQDs was effectively quenched in the presence of MnO2 NTs via the inner-filter effect (IFE). The addition of thiolated compounds (GSH and CAP) helped in the recovery of the fluorescence of N-CQDs by triggering the redox reaction and decomposing the MnO2 NTs. An investigation of fluorescence along with smartphone-based studies by evaluating the gray measurement using Image J software showed a great response towards GSH and CAP providing LODs of 4.70 µM and 5.22 µM (fluorometrically) and 5.76 µM and 2.81 µM (smartphone-based), respectively. The practical applicability of the sensing system has been verified using human blood plasma samples.

Antibacterial Activity of CdTe/ZnS Quantum Dot-ß Lactum Antibiotic Conjugates.

ß-Lactum antibiotics are broad class of antibiotics which kills bacteria by inhibiting the formation of peptidoglycan that constitutes the bacterial cell wall. The resistance that develops in bacteria for antibiotics led the scientific world to think about the future aspects for modifying the way through which antibiotics are acted on the bacteria and become lethal for them. In this consequence, the potential of latest marketed antibiotics e.g. Amoxiciline (I), ceftazidim (II) have been evaluated after being conjugated with quantum dots. The surface of quantum dots has been conjugated with antibiotics by carbodiimide coupling with the help of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as conjugating agent between antibiotic and functionalized quantum dots. The antibacterial properties of QD-conjugated antibiotics have been determined by disc diffusion assay. The potency of QD-conjugated antibiotics has been estimated by determining their MIC50 for the selected strain of Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. Minimum inhibitory concentration study, minimum bactericidal concentration and growth pattern analysis revealed that QD-antibiotic conjugates showed slightly more prospective than pure native antibiotics against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria.

Interaction of Folic Acid with Mn2+ Doped CdTe/ZnS Quantum Dots: In Situ Detection of Folic Acid.

To utilize the nanomaterials as an effective carrier for the drug delivery applications, it is important to study the interaction between nanomaterials and drug or biomolecules. In this study GSH functionalized Mn2+-doped CdTe/ZnS QDs has been utilized as a model nanomaterial due to its high luminescence property. Folic acid (FA) gradually quenches the FL of GSH functionalized Mn2+ - doped CdTe/ZnS QDs. The Stern-Volmer quenching constant (Ksv), binding constant (Ks) and effective quenching constant (Ka) for the FA-QDs system is calculated to be 1.32 × 105 M-1, 1.92 × 105 and 0.27 × 105 M-1, respectively under optimized condition (Temp. 300 K, pH 8.0, incubation time 40 min.). The effects of temperature, pH, and incubation time on FA-QDs system have also been studied. Statistical analysis of the quenched FL intensity versus FA concentration revealed a linear range from 1 × 10-7 to 5.0 × 10-5 for FA detection. The LOD of the current nano-sensor for FA was calculated to be 0.2 µM. The effect of common interfering metal ions and other relevant biomolecules on the detection of FA (12.0 µM) have also been investigated. L-cysteine and glutathione displayed moderate effect on FA detection. Similarly, the common metal ions (Na+, K+, Ca2+ and Mg2+) produced minute interference while Zn2+ Cu2+ and Fe3+ exert moderate interference. Toxic metal ions (Hg2+ and Pb2+) produced severe interferences in FA detection.Graphical abstract GSH-Mn2+ CdTe/ZnS QDs based Fluorescence Nanosensor for Folic acid.

CdTe QD-based inhibition and reactivation assay of acetylcholinesterase for the detection of organophosphorus pesticides.

An enzyme immobilized glutathione (GSH)-capped CdTe quantum dot (QD)-based fluorescence assay has been developed for monitoring organophosphate pesticides. In principle, GSH-capped CdTe QDs exhibit higher sensitivity towards H2O2 produced from the active enzymatic reaction of acetylcholinesterase (AChE) and choline oxidase (CHOx), which results in the fluorescence (FL) "turn-off" of the GSH-capped CdTe QDs. A "turn-on" FL of the CdTe QDs at 520 nm was recovered in the presence of organophosphate (OP). The FL changes of the GSH-capped CdTe QD/AChE/CHOx biosensor reasonably correspond to the amount of OP pesticides. The detection limit of the CdTe/AChE/CHOx biosensor towards paraoxon, dichlorvos, malathion and triazophos was 1.62 × 10-15 M, 75.3 × 10-15 M, 0.23 × 10-9 M and 10.6 × 10-12 M, respectively. The GSH-capped CdTe QDs/AChE/CHOx biosensor was applied as a FL nanoprobe for assaying the enzymatic activity of AChE. The inhibited AChE was reactivated up to 94% using pyridine oximate (2-PyOx-), and functionalized pyridinium oximates (4-C12PyOx- and 4-C18PyOx-) of varying chain lengths. It was found that the reactivation potency of the tested oximes varied with the chain length of the oximes. This biosensing system offers the promising benefit for the determination of the OP pesticides in food, water and environmental samples.

A colorimetric nanoprobe based on enzyme-immobilized silver nanoparticles for the efficient detection of cholesterol.

A large number of cardiovascular diseases have recently become of serious concern throughout the world. Herein, we developed a colorimetric probe based on functionalized silver nanoparticles (AgNPs) for the efficient sensing of cholesterol, an important cardiovascular risk marker. A simple sodium borohydride reduction method was employed to synthesize the AgNPs. The cholesterol oxidase (ChOx)-immobilized AgNPs interact with free cholesterol to produce H2O2 in proportion to the concentration of cholesterol, resulting in decreased AgNP absorbance (turn-off) at 400 nm due to electron transfer between the AgNPs and H2O2. The response of the sensor can also be observed visually. The absorption intensity of the AgNPs is recovered (turn-on) upon the addition of sodium dodecyl sulfate due to the inhibition of ChOx. This on-off mechanism was effectively applied to detect cholesterol within the concentration range 10-250 nM with a low detection limit of approximately 0.014 nM. Moreover, the selectivity of the sensor toward cholesterol was analyzed in the presence of a range of interfering organic substances such as glucose, urea, and sucrose. Finally, the potential of the proposed sensor was evaluated using real samples.

Surface plasmon resonance based spectrophotometric determination of medicinally important thiol compounds using unmodified silver nanoparticles.

The determination of thiol based biological molecules and drugs, such as cysteine (Cys) (I), α-lipoic acid (II), and sodium 2-sulfanylethane sulphonate (Mesna (III)) in human plasma are becoming progressively more important due to the growing body of knowledge about their essential role in numerous biological pathways. Herein we demonstrate a sensitive colorimetric sensor for the determination of medicinally important thiol drugs based on aggregation of the citrate capped silver nanoparticles (Ag NPs). This approach exploited the high affinity of thiols towards the Ag NPs surface which could tempt replacement of the citrate shell by the thiolate shell of target molecules, resulting in aggregation of the NPs through intermolecular electrostatic interaction or hydrogen-bonding. Because of aggregation, the plasmon band at around 400nm decreases gradually, along with the appearance of a new band connoting a red shift. The calibration curves are derived from the intensity ratios of A530/A400, which display a linear relation in the range of 1µM-150µM, 5µM-200µM and 10µM-130µM, respectively. The obtained detection limits (3σ) were found to be 1.5µM, 5.6µM and 10.2µM for compound I-III, respectively. The proposed method has been successfully applied for the detection of thiol compounds in real samples.

Antibacterial properties of amino acid functionalized silver nanoparticles decorated on graphene oxide sheets.

Graphene oxide (GO) sheets decorated with amino acid L-cysteine (L-cys) functionalized silver nanoparticles (GO-L-cys-Ag) was synthesized by AgNO3, trisodium citrate, and NaBH4. GO-L-cys-Ag nanocomposite was characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectra, ultraviolet-visible (UV-vis) absorption spectra, which demonstrated that a diameter of L-cys-AgNPs compactly deposited on GO. Antibacterial activity tests of GO-L-cys-Ag nanocomposite were carried out using Escherichia coli MTCC 1687 and Staphylococcus aureus MTCC 3160 as model strains of Gram-negative and Gram-positive bacteria, respectively. The effect of bactericide dosage on antibacterial activity of GO-L-cys-Ag nanocomposite was examined by plate count, well diffusion and broth dilution methods. Morphological observation of bacterial cells by scanning electron microscope (SEM) showed that GO-L-cys-Ag nanocomposite was more destructive to cell membrane of Escherichia coli than that of Staphylococcus aureus. The above technique establish that the bactericidal property of GO-L-cys-Ag nanocomposite with wide range of applications in biomedical science.

Green Luminescent CdTe Quantum Dot Based Fluorescence Nano-Sensor for Sensitive Detection of Arsenic (III).

Arsenic (As3+) is a hazardous and ubiquitous element; hence the quantitative detection of arsenic in various kinds of environmental sample is an important issue. Herein, we reported L-cysteine capped CdTe Quantum dot based optical sensor for the fluorometric detection of arsenic (III) in real water sample. The method is based on the fluorescence quenching of QDs with the addition of arsenic solution that caused the reduction in fluorescence intensity due to strong interaction between As3+ and L-cysteine to form As(Cys)3. The calibration curve was linear over 2.0 nM-0.5 µM arsenic with limit of detection (LOD) of 2.0 nM, correlation coefficient (r2) of 0.9698, and relative standard deviation (RSD %) of 5.2%. The Stern-Volmer constant for the quenching of CdTe QDs with As3+ at optimized condition was evaluated to be 1.17 × 108 L mol-1 s-1. The feasibility of the sensor has been analyzed by checking the inference of common metal ions available in the water such as K+, Na+, Mg2+, Ca2+, Ba2+, Cu2+, Ni2+, Zn2+, Al3+, Co2+, Cr2+, Fe3+ and its higher oxidation state As5+. Graphical Abstract Schematic representation of As3+ detection by L-Cysteine capped CdTe QDs.

Adsorption Kinetics and Binding Studies of Protein Quantum Dots Interaction: A Spectroscopic Approach.

Protein Quantum dots interaction is crucial to investigate for better understanding of the biological interactions of QDs. Here in, the model protein Bovine serum albumin (BSA) was used to evaluate the process of protein QDs interaction and adsorption on QDs surface. The modified Stern-Volmer quenching constant (Ka), number of binding sites (n) at different temperatures (298 308 and 318 K ± 1) and corresponding thermodynamic parameters (ΔG < 0, ΔH < 0, and ΔS > 0) were calculated. The quenching constant (Ks) and number of binding sites (n) is found to be inversely proportional to temperature. It signified that static quenching mechanism is dominant over dynamic quenching. The standard free energy change (ΔG < 0) implies that the binding process is spontaneous, while the enthalpy change (ΔH < 0) suggest that the binding of QDs to BSA is an enthalpy-driven process. The standard entropy change (ΔS > 0) suggest that hydrophobic force played a pivotal role in the interaction process. The adsorption process were assessed and evaluated by pseudofirst-order, pseudosecond-order kinetic model, and intraparticle diffusion model.

Nucleophilic attack of salicylhydroxamate ion at C=O and P=O centers in cationic micellar media.

The reaction between the salicylhydroxamate anion (SHA(-)) and p-nitrophenyl benzoate (PNPB), tris(3-nitrophenyl)phosphate (TRIS), and bis(2,4-dinitrophenyl)phosphate (BDNPP) have been examined kinetically. The α-nucleophile, SHA(-), incorporated into cetyltrimethylammonium bromide (CTAB) micelles accelerates dephosphorylation of tris(3-nitrophenyl)phosphate (TRIS) over the pH range 6.7-11.4. With a 1.0 mM of SHA in CTAB, the nucleophilicity of SHA followed the order of reactivity, PNPB (C=O, carboxylate ester) > TRIS (P=O, triester) > BDNPP (P=O, diester), and monoanionic SHA(-) and dianionic SA(2-) are the reactive species. The critical micelle concentration, cmc, of cetyltrimethylammonium bromide (CTAB) decreases and the fractional ionization constant, α, increases with increasing the concentration of SHA(-). Addition of 1 and 10 mM SHA under the reaction conditions (pH 9.2, borate buffer) led to saturation of the micellar surface and provided qualitative information for the micellar incorporation of hydroxamate ion. Plots of the pseudo-first-order rate constant, k(obs), log k(obs), fraction of hydroxamic acid ionized, α(SHA(-)) and α(SA(2-)), vs pH showed bifunctional nucleophilicity of hydroxamic acid under micellar condition. Plotting k(obs) vs [SHA](T) gave a straight line with intercept k(0). This indicates that hydroxamate ions are very strong nucleophiles for nucleophilic attack at the C and P center. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase.