Polyethyleneimine-capped silver nanoclusters as a fluorescence probe for sensitive detection of hydrogen peroxide and glucose

In this work, we utilized polyethyleneimine-capped silver nanoclusters (PEI-Ag nanoclusters) to develop a new fluorometric method for the determination of hydrogen peroxide and glucose with high sensitivity. The PEI-Ag nanoclusters have an average size of 2 nm and show a blue emission at 455 nm. The photostable properties of the PEI-Ag nanoclusters were examined. The fluorescence of the PEI-Ag nanoclusters could be particularly quenched by H₂O₂. The oxidization of glucose by glucose oxidase coupled with the fluorescence quenching of PEI-Ag nanoclusters by H₂O₂ can be used to detect glucose. Under optimum conditions, the fluorescence intensity quenched linearly in the range of 500 nM–100 μM with high sensitivity. The detection limit for H₂O₂ was 400 nM. And a linear correlation was established between fluorescence intensity (F₀ − F) and concentration of glucose in the range of 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵ M and 1.0 × 10⁻⁵ to 1.0 × 10⁻³ M with a detection limit of 8.0 × 10⁻⁷ M. The method was used for the detection of glucose in human serum samples with satisfactory results. Furthermore, the mechanism of sensitive fluorescence quenching response of Ag nanoclusters to glucose and H₂O₂ has been discussed.     

Effect of the polarity of silver nanoparticles induced by ionic liquids on facilitated transport for the separation of propylene/propane mixtures

The effect of ionic liquids on the formation of a partial positive charge on the surface of silver nanoparticle and its subsequent effect on facilitated olefin transport were investigated. Three different ionic liquids of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM⁺BF₄⁻), 1-butyl-3-methylimidazolium triflate (BMIM⁺Tf⁻), and 1-butyl-3-methylimidazolium nitrate (BMIM⁺NO₃⁻) were employed to control the positive charge density of the surface of silver nanoparticles. The positive charge density of the silver nanoparticles, as characterized by the binding energy of the silver atom, was in the following order: BMIM⁺BF₄⁻/Ag ? BMIM⁺Tf⁻/Ag > BMIM⁺NO₃⁻/Ag. This order was consistent with the tendency of ionic liquids to form free ions. The best separation performance for the propylene/propane mixtures was a mixed gas selectivity of 17 and a permeance of 7 GPU through a composite membrane consisting of BMIM⁺BF₄⁻/Ag. A better separation performance for olefin/paraffin mixtures was observed with a higher positive charge density of the silver nanoparticles. It was therefore concluded that facilitated olefin transport was a direct consequence of the surface positive charge of the silver nanoparticles induced by ionic liquids.     

Mesoporous aluminosilicate ropes with improved stability from protozeolitic nanoclusters

Mesoporous aluminosilicate ropes with improved hydrothermal stability have been prepared through S⁺X⁻I⁺ route via self-assembly of protozeolitic nanoclusters with cetyltrimethylammonium bromides (CTAB) template micelles in HNO₃ solution. SEM observation confirmed that high-yield aluminosilicate ropes could be produced under proper HNO₃ concentration. NO₃⁻ ions had strong binding strength to the CTA⁺ ions and tended to form more elongated surfactant micelles, thus fibrous products were fabricated under the direction of these long rod micelles in shearing flow. At the same time, the NO₃⁻ ions combining with CTA⁺ ions generated more active (CTA⁺NO₃⁻) assembly, which effectively catalysed the polymerization of protozeolitic nanoclusters with large volume into highly ordered mesostructures. Compared with normal MCM-41 silica synthesized through S⁺X⁻I⁺ route in acidic media, the hydrothermal stability was improved considerably. These protozeolitic nanoclusters survived strongly acidic media and entered into mesostructured framework, which contributed to the improvement of hydrothermal stability.     

Detection of antibiotics in food: Extraction of fluoroquinolones by DNA

The ability of DNA to extract fluoroquinolones from model solutions and real probes of food was demonstrated and investigated quantitatively. The interaction between fluoroquinolones and different types of DNA was studied by equilibrium dialysis. The first application of this direct approach allowed us to determine binding constants and binding stoichiometries in different conditions. The binding of enrofloxacin to heat-denatured DNA (d-DNA) from herring sperm is pH- and magnesium-dependent; the highest fraction of bound drugs was found at pH 7 and a magnesium concentration of 0.5–1 mM. Results for three types of DNA: d-DNA, double-stranded DNA and single-stranded DNA were compared. The unwound DNA showed almost doubled binding constants and stoichiometries, thus indicating preferable interaction of enrofloxacin with single-strand regions of DNA. The binding of other fluoroquinolones (lomefloxacin, ciprofloxacin, norfloxacin, danofloxacin and sarafloxacin) with d-DNA is very similar to that of enrofloxacin: the binding constants are in the range from 0.94 × 10⁵ to 2.40 × 10⁵ M⁻¹, and the stoichiometries range from 4.1 to 6.9 fluoroquinolone molecules per 100 DNA bases. The binding properties were quantitatively the same for extraction of fluoroquinolones from model aqueous solutions and from liquid food (milk). The results indicate the efficiency of DNA for selective extraction of fluoroquinolones from real samples for further analysis. This selective binding also allows us to consider DNA as a natural receptor for development of analytical techniques for fluoroquinolones.     

Quantitative study of stereospecific binding of monoclonal antibody to anti-benzo(a)pyrene diol epoxide-N-dG adducts by capillary electrophoresis immunoassay

The stereospecific binding of monoclonal antibody (mAb) 8E11 to anti-benzo(a)pyrene diol epoxide (BPDE)-dG adducts in single nucleoside, long oligonucleotide, and genomic DNA were quantitatively evaluated using noncompetitive and competitive capillary electrophoresis (CE) immunoassays. Two single-stranded TMR-BPDE-90mers containing a single anti-BPDE-dG adduct with defined stereochemistry and a fluorescent label at 5′-end were used as fluorescent probes for competitive CE immunoassay. To quantitatively evaluate the binding affinity through competitive CE immunoassays, a series of equations were derived according to the binding stoichiometry. The binding of mAb 8E11 to trans-(+)-anti-BPDE-dG displays strongest affinity (K_b: 3.57 × 10⁸ M⁻¹) among all four investigated anti-BPDE-dG mononucleoside adducts, and the cis-(−)-anti-BPDE-dG displays lowest affinity (K_b: 1.14 ×10⁷ M⁻¹). The binding of monoclonal antibody (mAb) 8E11 to BPDE-dG adducts in long DNA (90mer) preferentially forms the complex with a stoichiometry of 1:1, and that mAb 8E11 displays a slightly higher affinity with trans-(+)-anti-BPDE-90mers (K_b: 6.36 ± 0.54 × 10⁸ M⁻¹) than trans-(−)-anti-BPDE-90mers (K_b: 4.52 ± 0.52 × 10⁸ M⁻¹). The mAb 8E11 also displays high affinity with BPDE-dG adducts in genomic DNA (K_b: 3.74 × 10⁸ M⁻¹), indicating its promising applications for sensitive immuno-detection of BPDE-DNA adducts in genomic DNA.     

Novel high-sensitive fluorescent detection of deoxyribonuclease I based on DNA-templated gold/silver nanoclusters

Herein, fluorescent DNA-templated gold/silver nanoclusters (DNA-Au/Ag NCs) are presented as a novel probe for sensitive detection of deoxyribonuclease I (DNase I). The procedure is based on quenching fluorescence of DNA-Au/Ag NCs by DNase I digestion of the DNA (5′-CCCTTAATCCCC-3′) template. This decrease in fluorescence intensity permitted sensitive detection of DNase I in a linear range of 0.013–60 μg mL⁻¹, with a detection limit of 3 ng mL⁻¹ at a signal-to-noise ratio of 3. Furthermore, the practicality of this probe for detection of DNase I in human serum and saliva samples was validated, demonstrating its advantages of simplicity, selectivity, sensitivity and low cost. Importantly, satisfactory agreement between results obtained by the fluorescent method described here and high performance liquid chromatography (HPLC) further confirmed the reliability and accuracy of this approach.     

Formation and Structure of Fluorescent Silver Nanoclusters at Interfacial Binding Sites Facilitating Oligomerization of DNA Hairpins

Fluorescent, DNA‐stabilized silver nanoclusters (DNA‐AgNCs) are applied in a range of applications within nanoscience and nanotechnology. However, their diverse optical properties, mechanism of formation, and aspects of their composition remain unexplored, making the rational design of nanocluster probes challenging. Herein, a synthetic procedure is described for obtaining a high yield of emissive DNA‐AgNCs with a C‐loop hairpin DNA sequence, with subsequent purification by size‐exclusion chromatography (SEC). Through a combination of optical spectroscopy, gel electrophoresis, inductively coupled plasma mass spectrometry (ICP‐MS), and small‐angle X‐ray scattering (SAXS) in conjunction with the systematic study of various DNA sequences, the low‐resolution structure and mechanism of the formation of AgNCs were investigated. Data indicate that fluorescent DNA‐AgNCs self‐assemble by a head‐to‐head binding of two DNA hairpins, bridged by a silver nanocluster, resulting in the modelling of a dimeric structure harboring an Ag₁₂ cluster.     

Fluorescence detection of glutathione reductase activity based on deoxyribonucleic acid-templated silver nanoclusters

Fluorescent silver nanoclusters stabilized by DNA (DNA-AgNCs) exhibit distinct response rates to thiol and disulfide. Glutathione reductase can catalyze the reduction of the oxidized glutathione (GSSG) quickly to reduced glutathione (GSH) in the presence of β-nicotinamide adenine dinucleotide 2′-phosphate reduced tetrasodium salt hydrate (NADPH). Consequently, DNA-AgNCs can serve as a new fluorescent platform for assaying the glutathione reductase (GR) activity. This newly proposed assay has a high sensitivity and a good selectivity toward GR. The GR activity can be detected in the range of 0.2–2.0 mU mL⁻¹ with a minimum detectable concentration of 0.2 mU mL⁻¹. Pepsin, lysozyme, trypsin, avidin, thrombin, myoglobin, and BSA have little effect on the fluorescence intensity of DNA-AgNCs. The GR activity assay is successfully used to monitor the inhibition of GR activity by a typical inhibitor 1,3-bis(2-chloroethyl)-1-nitrosourea.     

Electrophoresis mobility shift assay and biosensor used in studying the interaction between bleomycin A5 and DNA

DNA is the potential target of numerous drugs utilized widely in clinical cancer therapy. Here we employed bleomycin A5, with its deactivated form as contrast, to investigate the interaction between small pharmaceutical and DNA. Electrophoresis mobility shift assay (EMSA) which is a common technique used in studying specific interactions between DNA and proteins is applied in visualizing the binding of bleomycin A5 with DNA intuitively. More accurate association equilibrium constant for native and deactivated bleomycin A5 to DNA achieved on biosensor IAsys AUTO⁺ is 1.25 × 10⁴ and 1.3 × 10³ M⁻¹, respectively. With combination of EMSA and biosensor, a qualitative and quantitative method is described, which can be extended to studying the binding of most small molecules with targeting DNA and serves as a powerful tool in designing and screening for novel drugs.     

Phosphate selective alkylenebisurea receptors: structure-binding relationship

New host molecules for anions, adamantane, and alkyl urea derivatives substituted by naphthalene chromophores, were synthesized. Their binding with F⁻, Cl⁻, Br⁻, OAc⁻, HSO₄⁻, NO₃⁻, and H₂PO₄⁻ was investigated by UV-vis, fluorescence and NMR spectroscopy. The anion binding ability of adamantyl bisurea derivatives was compared with the analogous host molecules, wherein the urea moieties are separated by flexible alkyl linkers of the same length, and adamantane monourea derivative. The host molecules show the highest selectivity toward F⁻ and H₂PO₄⁻. The binding stoichiometry and the values of the association constants depend on the basicity of anions, availability of H-bonding sites, preorganization, and rigidity of the hosts, as well as solvent polarity and H-bonding availability. Rigid adamantane receptors, compared to flexible analogues show increased selectivity for H₂PO₄⁻, whereas binding of OAc⁻ is better with flexible receptors. The binding of OAc⁻ and H₂PO₄⁻ was investigated by microcalorimetry. The stoichiometries and the stability constants of the corresponding complexes obtained by this method were in good agreement compared to those determined by UV-vis titrations. In both cases the enthalpic contribution to the overall complex stability was predominant.     

Surface plasmon resonance biosensor for enrofloxacin based on deoxyribonucleic acid

A DNA-based surface plasmon resonance biosensor for enrofloxacin was developed. Heating denatured DNA immobilized on the gold-coated glass surface was exploited. The immobilization was performed by a layer-by-layer co-deposition with a cationic polymer. The sensor performance was tested with real biological probes. Direct and simple determination of enrofloxacin in milk samples was demonstrated. The sensor response obeys Langmuir binding isotherm being almost linear until about 20 μg mL⁻¹. The detection limit in milk samples was estimated to be 3 μg mL⁻¹.     

One-step synthesis and applications of fluorescent Cu nanoclusters stabilized by l-cysteine in aqueous solution

Herein, an innovative and simple strategy for synthesizing high fluorescent Cu nanoclusters was successfully established while l-cysteine played a role as the stabilizer. Meaningfully, the current Cu nanoclusters together with a quantum yield of 14.3% were prepared in aqueous solution, indicating their extensive applications. Subsequently, the possible fluorescence mechanism was elucidated by fluorescence, UV–vis, HR-TEM, FTIR, XPS, and MS. Additionally, the CuNCs were employed for assaying Hg²⁺ on the basis of the interactions between Hg²⁺ and l-cysteine; thus facilitating the quenching of their fluorescence. The proposed analytical strategy permitted detections of Hg²⁺ in a linear range of 1.0 × 10⁻⁷ mol L⁻¹ × 10⁻³ mol L⁻¹, with a detection limit of 2.4 × 10⁻⁸ mol L⁻¹ at a signal-to-noise ratio of 3. Significantly, this CuNCs described here were further applied for coding and fluorescent staining, suggesting may broaden avenues toward diverse applications.     

A sensitive sensor for anthraquinone anticancer drugs and hsDNA based on CdTe/CdS quantum dots fluorescence reversible control

Based on CdTe/CdS quantum dots (CdTe/CdS QDs) fluorescence (FL) reversible control, a new and sensitive FL sensor for determination of anthraquinone (AQ) anticancer drugs (adriamycin and daunorubicin) and herring sperm DNA (hsDNA) was developed. Under the experimental conditions, FL of CdTe/CdS QDs can be effectively quenched by AQ anticancer drugs due to the binding of AQ anticancer drugs on the surface of CdTe/CdS QDs and photoinduced electron transfer (PET) process from CdTe/CdS QDs to AQ anticancer drugs. Addition of hsDNA afterwards brought the restoration of CdTe/CdS QDs FL intensity, as AQ anticancer drugs peeled off from the surface of CdTe/CdS QDs and embedded into hsDNA double helix structure. The liner ranges and the detection limits of FL quenching methods for two AQ anticancer drugs were 0.33-9 μg mL⁻¹ and 0.09 μg mL⁻¹ for ADM and 0.15-9 μg mL⁻¹ and 0.04 μg mL⁻¹ for DNR, respectively. The restored FL intensity was proportional to concentration of hsDNA in the range of 1.38-28 μg mL⁻¹and the detection limit for hsDNA was 0.41 μg mL⁻¹. It was applied to the determination of AQ anticancer drugs in human serum and urine samples with satisfactory results. The reaction mechanism of CdTe/CdS QDs FL reversible control was studied.     

Fabrication of core-shell structured nanoparticle layer substrate for excitation of localized surface plasmon resonance and its optical response for DNA in aqueous conditions

LSPR from nanostructured noble metals such as gold and silver offers great potential for biosensing applications. In this study, a core-shell structured nanoparticle layer substrate was fabricated and the localized surface plasmon resonance (LSPR) optical characteristics were investigated for DNA in aqueous conditions. Factors such as DNA length dependence, concentration dependence, and the monitoring of DNA aspects (ssDNA or dsDNA) were measured. Different lengths and concentrations of DNA solutions were introduced onto the surface of the substrate and the changes in the LSPR optical characteristics were measured. In addition, to monitor the changes in LSPR optical characteristics for different DNA aspects, a DNA solutions denatured by means of heat or alkali were introduced onto the surface, after which optical characterization of the core-shell structured nanoparticle substrate was carried out. With this core-shell structured nanoparticle layer for the excitation of LSPR, the dependence upon specific DNA conditions (length, concentration, and aspect) could be monitored. In particular, the core-shell structured nanoparticle layer substrate could detect DNA of length 100-5000 bp and 400-bp DNA at a concentration of 4.08 ng mL⁻¹ (1 × 10⁷ DNA molecules mL⁻¹). Furthermore, the changes in LSPR optical characteristics with DNA aspect could be monitored. Thus, LSPR-based optical detection using a core-shell structured nanoparticle layer substrate can be used to determine the kinetics of biomolecular interactions in a wide range of practical applications such as medicine, drug delivery, and food control.     

A flow injection analysis system for monitoring silver (I) ion and iodine residuals in recycled water from recovery systems used for spaceflight

A laboratory-built flow injection analyzer is reported for monitoring the drinking water disinfectants silver (I) ion and iodine in water produced from NASA's water recovery system. This analyzer uses spectrophotometric detection with a custom made 10 cm optical flow cell. Optimization and interference studies are discussed for the silver (I) ion configuration. Subsequent results using the silver (I) configuration with minor modifications and alternative reagents gave promising results for iodine determinations as well. The estimated MDL values for Ag⁺ and I₂ are 52 μg L⁻¹ Ag⁺ and 2 μg L⁻¹ I₂; the mean percent recoveries were 104% and 96.2% for Ag⁺ and I₂ respectfully; and percent relative standard deviations were estimated at 1.4% for Ag⁺ and 5.7% for I₂. The agreement of this potentially multifunctional analyzer to reference methods for each respective water disinfectant is measured using Bland–Altman analysis as well as more traditional estimates.     

Enzyme-catalyzed silver deposition on irregular-shaped gold nanoparticles for electrochemical immunoassay of alpha-fetoprotein

A new and disposable electrochemical immunosensor was designed for detection of alpha-fetoprotein (AFP), as a model analyte, with sensitivity enhancement based on enzyme-catalyzed silver deposition onto irregular-shaped gold nanoparticles (ISGNPs). The assay was carried out with a sandwich-type immunoassay protocol by using ISGNP-labeled anti-AFP antibodies conjugated with alkaline phosphatase (ALP–Ab₂) as detection antibodies. The enzymatically catalytic deposition of silver on the electrode could be measured by stripping analysis in KCl solution due to the Ag/AgCl solid-state voltammetric process. Several labeling protocols including spherical gold nanoparticle-labeled ALP–Ab₂ and ISGNP-labeled ALP–Ab₂ were investigated for determination of AFP, and improved analytical properties were achieved with the ISGNP labeling. With the ISGNP labeling method, the effects of incubation time and incubation temperature for antigen-antibody reaction, and deposition time of silver on the current responses of the electrochemical immunosensors were also monitored. Under optimal conditions, the electrochemical immunosensor exhibited a wide dynamic range from 0.01 ng mL⁻¹ to 200 ng mL⁻¹ with a detection limit of 5.0 pg mL⁻¹ AFP. The immunosensor displayed a good stability and acceptable reproducibility and accuracy. No significant differences at the 95% confidence level were encountered in the analysis of 10 clinical serum samples between the developed immunoassay and the commercially available electrochemiluminescent method for determination of AFP.     

Fabrication of highly catalytic silver nanoclusters/graphene oxide nanocomposite as nanotag for sensitive electrochemical immunoassay

Silver nanoclusters and graphene oxide nanocomposite (AgNCs/GRO) is synthesized and functionalized with detection antibody for highly sensitive electrochemical sensing of carcinoembryonic antigen (CEA), a model tumor marker involved in many cancers. AgNCs with large surface area and abundant amount of low-coordinated sites are synthesized with DNA as template and exhibit high catalytic activity towards the electrochemical reduction of H₂O₂. GRO is employed to assemble with AgNCs because it has large specific surface area, super electronic conductivity and strong π-π stacking interaction with the hydrophobic bases of DNA, which can further improve the catalytic ability of the AgNCs. Using AgNCs/GRO as signal amplification tag, an enzyme-free electrochemical immunosensing protocol is designed for the highly sensitive detection of CEA on the capture antibody functionalized immunosensing interface. Under optimal conditions, the designed immunosensor exhibits a wide linear range from 0.1 pg mL⁻¹ to 100 ng mL⁻¹ and a low limit of detection of 0.037 pg mL⁻¹. Practical sample analysis reveals the sensor has good accuracy and reproducibility, indicating the great application prospective of the AgNCs/GRO in fabricating highly sensitive immunosensors, which can be extended to the detection of various kinds of low abundance disease related proteins.     

Ferric nanoparticle-based resonance light scattering determination of DNA at nanogram levels

A novel assay of DNA has been proposed by using ferric nanoparticles as probes coupled with resonance light scattering (RLS) detection. At pH 7.40, the RLS intensity of ferric nanoparticles can be greatly enhanced by the aggregation of positively charged ferric nanoparticles through electrostatic interaction with negatively charged DNA. The enhanced intensity of RLS at 452 nm is proportional to the concentration of DNA in the range of 0.01-0.8 μg ml⁻¹ for calf thymus and salmon sperm DNA and in the range of 0.005-0.3 μg ml⁻¹ for E. coli K12 genomic DNA. Detection limits are 3.6 ng ml⁻¹ for calf thymus DNA, 4.4 ng ml⁻¹ for salmon sperm DNA, and 1.9 ng ml⁻¹ for E. coli K12 genomic DNA, respectively. Compared with the chromophores previously used in RLS assay, the ferric nanoparticles have offered several advantages in easy preparation, good photostability and high sensitivity without being modified or functionalized.     

Electrochemical studies of the interaction of Basic Brown G with DNA and determination of DNA

A new method of electrochemical probe has been proposed for the determination of Herring Sperm DNA (DNA) based on its interaction with Basic Brown G (BBG). The electrochemical behavior of interaction of BBG with DNA was investigated on Hg electrode. In 0.1 mol L⁻¹ NH₃-NH₄Cl buffer solution (pH 8.0), BBG can be reduced on Hg electrode with a well-defined voltammetric peak at −0.67 V (versus SCE). In the presence of DNA, the reduction peak current of BBG decreases and the peak potential shifts to a more positive potential without the appearance of new peak. The study shows that a new BBG-DNA complex is formed by linear sweep voltammetry (LSV) and spectrophotometry. The decrease of the second order derivative of reductive peak current (Δi^″p) of BBG is proportional to the concentration of DNA in the range of 0.10-36 μg mL⁻¹. Limit of detection of DNA is 0.04 μg mL⁻¹. DNA of Hepatitis B Virus in serum samples was determined satisfactorily. Additionally, the binding mechanism was preliminarily discussed. The mode of interaction between BBG and DNA was found to be intercalation binding.     

Application of a bisindocarbocyanine reagent for dispersive liquid–liquid microextraction of silver with subsequent spectrophotometric determination

A novel, simple and environmentally friendly procedure for silver determination has been developed. The method is based on ion associate formation of AgI₂⁻ and bisindocarbocyanine chloride (BDIC) reagent, followed by dispersive liquid–liquid microextraction (DLLME) of the ion associate formed and subsequent UV–Vis spectrophotometric detection. The structure of BDIC and the reaction mechanism were investigated by MS and NMR measurements and quantum chemical calculations.The optimum experimental conditions were found to be: pH 6; 0.1 mol L^{− 1} KI; 5 × 10^{− 5} mol L^{− 1} BDIC. The DLLME procedure was carried out using an auxiliary solvent for adjustment of solvent density. A 0.5 mL mixture of toluene as extraction solvent, carbon tetrachloride as auxiliary solvent and ethanol as disperser solvent was used. Ultrasonication of the organic phase was applied to decrease the value of the blank test. The absorbance of the coloured extracts obeys Beer's law in the range 0.07–2.1 mg L^{− 1} of Ag at 566 nm wavelength. The limit of detection, calculated from a blank test (n = 10) based on 3 s, is 0.03 mg L^{− 1} of Ag. The developed procedure was applied to the determination of silver in real samples such as Nano Silver and semi-conductor thin films.