Fast and sensitive detection of ochratoxin A in red wine by nanoparticle-enhanced SPR

Herein, we present a fast and sensitive biosensor for detection of Ochratoxin A (OTA) in a red wine that utilizes gold nanoparticle-enhanced surface plasmon resonance (SPR). By combining an indirect competitive inhibition immunoassay and signal enhancement by secondary antibodies conjugated with gold nanoparticles (AuNPs), highly sensitive detection of low molecular weight compounds (such as OTA) was achieved. The reported biosensor allowed for OTA detection at concentrations as low as 0.75 ng mL⁻¹ and its limit of detection was improved by more than one order of magnitude to 0.068 ng mL⁻¹ by applying AuNPs as a signal enhancer. The study investigates the interplay of size of AuNPs and affinity of recognition elements affecting the efficiency of the signal amplification strategy based on AuNP. Furthermore, we observed that the presence of polyphenolic compounds in wine samples strongly interferes with the affinity binding on the surface. To overcome this limitation, a simple pre-treatment of the wine sample with the binding agent poly(vinylpyrrolidone) (PVP) was successfully applied.     

Determination of colloidal gold nanoparticle surface areas, concentrations, and sizes through quantitative ligand adsorption

Determination of the true surface areas, concentrations, and particle sizes of gold nanoparticles (AuNPs) is a challenging issue due to the nanoparticle morphological irregularity, surface roughness, and size distributions. A ligand adsorption-based technique for determining AuNP surface areas in solution is reported. Using a water-soluble, stable, and highly UV–vis active organothiol, 2-mercaptobenzimidazole (MBI), as the probe ligand, we demonstrated that the amount of ligand adsorbed is proportional to the AuNP surface area. The equivalent spherical AuNP sizes and concentrations were determined by combining the MBI adsorption measurement with Au³⁺ quantification of aqua regia-digested AuNPs. The experimental results from the MBI adsorption method for a series of commercial colloidal AuNPs with nominal diameters of 10, 30, 50, and 90 nm were compared with those determined using dynamic light scattering, transmission electron microscopy, and localized surface plasmonic resonance methods. The ligand adsorption-based technique is highly reproducible and simple to implement. It only requires a UV–vis spectrophotometer for characterization of in-house-prepared AuNPs.     

Influence of size and functionality of polymeric nanoparticles on the adsorption behavior of sodium dodecyl sulfate as detected by isothermal titration calorimetry

Isothermal titration calorimetry was used to monitor the adsorption of the surfactant sodium dodecylsulfate (SDS) on different sized pure and carboxy functionalized polystyrene nanoparticles prepared by the mini-emulsion process. The ITC experiment gives, additionally to the CMC values, information about the interaction of the surfactant molecules to the particle’s surface due to the particle surface properties. The adsorption heat depends on the chemical composition of the polymer as well on the particle size. It also provides information about the surface coverage with surfactant and the number of additional adsorbed molecules per particle until full coverage by surfactant is obtained. The surfactant adsorption increases from 0.3 molecules per nm² for 50 nm to 8.5 molecules per nm² for carboxy functionalized particles with diameters larger than 160 nm. The area A _Surf-dens after the adsorption process gives information about the packing density of surfactant molecules on the particles in dependence of carboxy groups: an increasing number of carboxylic groups decreases the area occupied per SDS molecule. The adsorption process was also monitored by zeta potential measurements, where an increasing potential during the adsorption was detected.     

Polyelectrolyte Multilayer‐Mediated Growth of Gold Nanoparticle Films with Tunable Loading Density and Nanoparticle Shape

A facile method for preparing gold nanoparticle (AuNP) films with a high loading density based on the seed‐mediated growth of AuNPs on a polyelectrolyte multilayer (PEM) is reported. Use of PEMs as a base layer for gold seed adsorption confers controllability on the loading density of the AuNP film and size of the resulting AuNPs. In addition, the shape of the final AuNPs could be varied by adapting various species of polyelectrolytes. The optical response of the AuNP films is stable, because of the relatively uniform distribution of the AuNPs over a large area. The AuNP film has been used as a substrate for surface‐enhanced Raman scattering (SERS), and it shows stable and reproducible enhancement in the range from 10⁵ to 10⁷ depending on the fabrication condition.

     

Determination of human urinary kanamycin in one step using urea-enhanced surface plasmon resonance light-scattering of gold nanoparticles

The purpose of this study was to establish a simple, sensitive analytical method for kanamycin (KANA) in human urine. Enhancement of the plasmon resonance light-scattering (PRLS) of gold nanoparticles (AuNPs) by KANA provided the basis for this analytical method. At pH 6.7, KANA induced AuNPs aggregation with enhanced PRLS. The PRLS of the AuNPs–KANA system was further enhanced by addition of urea. The linear range and detection limit for KANA were from 20–800 nmol L⁻¹ and 2 nmol L⁻¹, respectively. Potential interfering substances present in urine had a negligible effect on the determination, thus preliminary sample separations were not necessary. Recovery of KANA from spiked human urine was 94–104%. This simple, sensitive method, using urea to enhance the PRLS of the AuNPs–KANA system, may provide a new approach for determination of compounds rich in OH groups.     

Label-free amperometric immunobiosensor based on a gold colloid and Prussian blue nanocomposite film modified carbon ionic liquid electrode

A novel experimental methodology based on a Prussian blue (PB) and gold nanoparticles (AuNPs) modified carbon ionic liquid electrode (CILE) was developed for use in a label-free amperometric immunosensor for the sensitive detection of human immunoglobulin G (HIgG) as a model protein. The CILE was fabricated by using the ionic liquid 1-octyl-3-methylimidazolium hexafluorophosphate as binder. Controllable electrodeposition of PB on the surface of the CILE and coating with 3-aminopropyl triethylene silane (APS) formed a film with high electronic catalytic activity and large surface area for the assembly of AuNPs and further immobilization of HIgG antibody. The electrochemistry of the formed nanocomposite biofilm was investigated by electrochemical techniques including cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. The HIgG concentration was measured through the decrease of amperometric responses in the corresponding specific binding of antigen and antibody. The decreased differential pulse voltammetric values were proportional to the HIgG concentration in two ranges, 0.05–1.25 ng mL⁻¹ and 1.25–40 ng mL⁻¹, with a detection limit of 0.001 ng mL⁻¹ (S/N = 3). This electrochemical immunoassay combined the specificity of the immunological reaction with the sensitivity of the AuNPs, ionic liquid, and PB amplified electrochemical detection and would therefore be valuable for clinical immunoassays.     

An ultrasensitive hydrogen peroxide biosensor based on electrocatalytic synergy of graphene–gold nanocomposite, CdTe–CdS core–shell quantum dots and gold nanoparticles

We first reported an ultrasensitive hydrogen peroxide biosensor in this work. The biosensor was fabricated by coating graphene–gold nanocomposite (G–AuNP), CdTe–CdS core–shell quantum dots (CdTe–CdS), gold nanoparticles (AuNPs) and horseradish peroxidase (HRP) in sequence on the surface of gold electrode (GE). Cyclic voltammetry and differential pulse voltammetry were used to investigate electrochemical performances of the biosensor. Since promising electrocatalytic synergy of G–AuNP, CdTe–CdS and AuNPs towards hydrogen peroxide was achieved, the biosensor displayed a high sensitivity, low detection limit (S/N = 3) (3.2 × 10⁻¹¹ M), wide calibration range (from 1 × 10⁻¹⁰ M to 1.2 × 10⁻⁸ M) and good long-term stability (20 weeks). Moreover, the effects of omitting G–AuNP, CdTe–CdS and AuNP were also examined. It was found that sensitivity of the biosensor is more 11-fold better if G–AuNP, CdTe–CdS and AuNPs are used. This could be ascribed to improvement of the conductivity between graphene nanosheets in the G–AuNP due to introduction of the AuNPs, ultrafast charge transfer from CdTe–CdS to the graphene sheets and AuNP due to unique electrochemical properties of the CdTe–CdS, and good biocompatibility of the AuNPs for horseradish peroxidase. The biosensor is of best sensitivity in all hydrogen peroxide biosensors based on graphene and its composites up to now.     

Luminescent Gold Nanoparticles with Size‐Independent Emission

Size‐independent emission has been widely observed for ultrasmall thiolated gold nanoparticles (AuNPs) but our understanding of the photoluminescence mechanisms of noble metals on the nanoscale has remained limited. Herein, we report how the emission wavelength of a AuNP and the local binding geometry of a thiolate ligand (glutathione) on the AuNP are correlated, as these AuNPs emit at different wavelengths in spite of their identical size (ca. 2.5 nm). By using circular dichroism, X‐ray absorption, and fluorescence spectroscopy, we found that a high Au?S coordination number (CN) and a high surface coverage resulted in strong Au^I–ligand charge transfer, a chiral conformation, and 600 nm emission, whereas a low Au?S CN and a low surface coverage led to weak charge transfer, an achiral conformation, and 810 nm emission. These two size‐independent emissions can be integrated into one single 2.5 nm AuNP by fine‐tuning of the surface coverage; a ratiometric pH response was then observed owing to strong energy transfer between two emission centers, opening up new possibilities for the design of ultrasmall ratiometric pH nanoindicators.     

Reactive capture of gold nanoparticles by strongly physisorbed monolayers on graphite

Anthracene Diels Alder adducts (DAa) bearing two long side chains (H-(CH₂)₂₂O(CH₂)₆OCH₂-) at the 1- and 5-positions form self-assembled monolayers (SAMs) at the phenyloctane - highly oriented pyrolytic graphite (HOPG) interface. The long DAa side chains promote strong physisorption of the monolayer to HOPG and maintain the monolayer morphology upon rinsing or incubation in ethanol and air-drying of the substrate. Incorporating a carboxylic acid group on the DAa core enables capture of 1-4 nm diameter gold nanoparticles (AuNPs) provided (i) the monolayer containing DAa-carboxylic acids is treated with Cu²⁺ ions and (ii) the organic coating on the AuNP contains carboxylic acids (11-mercaptoundecanoic acid, MUA-AuNP). AuNP capture by the monolayer proceeds with formation of Cu²⁺ - carboxylate coordination complexes. The captured AuNP appear as mono- and multi-layered clusters at high coverage on HOPG. The surface density of the captured AuNPs can be adjusted from AuNP multi-layers to isolated AuNPs by varying incubation times, MUA-AuNP concentration, the number density of carboxylic acids in the monolayer, the number of MUA per AuNP, and post-incubation treatments.     

FT-IR study of the nature and stability of NOx surface species on ZrO2, VOx/ZrO2 and MoOx/ZrO2 catalysts

The adsorption of NO, NO/O₂ mixtures and NO₂ on pure ZrO₂ and on two series of catalysts supported on ZrO₂, one containing vanadia and the other molybdena (ZV and ZMo, respectively), has been investigated. The V and Mo surface contents of the latter were ≤3 atoms nm⁻² and ≤5 atoms nm⁻², respectively. All samples had been previously submitted to a standard oxidation treatment. On all samples, only extremely minor amounts of NO_x surface species are formed by NO interaction at room temperature (RT). NO_x surface species are formed in greater amounts on pure ZrO₂ when NO and O₂ are coadsorbed at RT; they are mainly nitrites, small amounts of nitrates, and small amounts of (O₂NO−H)^δ− species; when ZrO₂ is warmed to 623 K in the NO/O₂ mixture, nitrites decrease, nitrates and (O₂NO−H)^δ− species increase. The same NO_x species as on the ZrO₂ surface free from V (or Mo) are formed on ZV (or ZMo) samples with surface V (or Mo) density <1.5 atoms nm⁻²; however, they occur in decreased amount with increasing V (or Mo) coverage. On ZV samples with a surface V density of 1.5–3 atoms nm⁻² (or ZMo samples with a surface Mo density of 1.5–5 atoms nm⁻²) when NO and O₂ are coadsorbed at RT, there is formation of small amounts of nitrites, nitrates (both on ZrO₂ surface free from V (or Mo) and at the edges of V- or Mo-polyoxoanions) and NO₂ ^δ+ species, associated with V⁵⁺ (or Mo⁶⁺) of very strong Lewis acidity; when samples are warmed up 623 K in the NO/O₂ mixture, nitrites disappear, nitrates increase, NO₂ ^δ+ species remain constant or slightly decrease. When NO₂ is allowed into contact at RT with oxidized samples, surface situations almost identical to those obtained for each sample warmed to 623 K in NO/O₂ mixture is reached. The NO_x surface species stable at 623 K, the temperature at which catalysts show the best performance in the selective catalytic reduction (SCR) of NO by NH₃, are nitrates, both on ZrO₂ and on polyvanadates or polymolybdates at high nuclearity. On the contrary, nitrites and NO₂ ^δ+ species are unstable at 623 K.     

Density functional study of substituted (–SH, –S, –OH, –Cl) hydrated ions of Hg<Superscript>2+</Superscript>

Abstract  

The electronic structure of Hg(II) ions, [Hg(L) _n (H₂O) _m ] ^q (L = HO⁻, Cl⁻, HS⁻, S²⁻) has been studied. Geometries were fully optimized. The B3LYP and PBE functionals give structures in good agreement with available experimental data. Calculated stretching frequencies generally correlate well with bond lengths. The role of the water molecule(s) in the solvated Hg(II) complexes has been investigated. The solvent can act as nucleophile, as hydrogen bond acceptor or as a spectator. The trans-effect results in lengthening of the Hg–L bond length. It can be understood as a competition between ligands in trans positions for the ability to donate their electron density to the 6s AO of Hg(II). The effect of the presence of water molecules on the Hg–L bond length depends on whether or not the water molecules form a direct coordination bond with Hg(II); it will not guarantee an increase in the stability of the complexes. The interaction energy, which represents the interaction between Hg(II) and ligand L and excludes all other interactions, is nucleophilicity-dependent. The interaction energy and the strength of the ligand nucleophilicity follow the order: S²⁻ > HS⁻ > HO⁻ > Cl⁻ > H₂O. The charge transfer, ΔN, is an indication for the type and strength of the interaction between ligand and Hg(II). Increasing the positive and negative value of ΔN will decrease and increase the Hg(II) total NBO charge, respectively, while decreasing the electrophilicity of Hg(II) will decrease its charge and the charge transfer, ΔN.     

Colorimetric detection of mercury(II) in a high-salinity solution using gold nanoparticles capped with 3-mercaptopropionate acid and adenosine monophosphate

A new colorimetric sensor for sensing Hg2+ in a high-salinity solution has been developed using gold nanoparticles (AuNPs) decorated with 3-mercaptopropionate acid (MPA) and adenosine monophosphate (AMP). Because of the high negative charge density of AMP on each AuNP surface, MPA/AMP-capped AuNPs are well dispersed in a high-salt solution. In contrast, the aggregation of MPA-capped AuNPs was induced by sodium ions, which shield the negative charges of the carboxylic groups of MPA. Through the coordination between the carboxylic group of MPA and Hg2+, the selectivity of MPA/AMP-capped AuNPs for Hg2+ in a high-salt solution is remarkably high over that of the other metals without the addition of a masking agent or a change in the temperature. We have carefully investigated the effect of the AMP concentration on the stability and sensitivity of MPA/AMP-capped AuNPs. Under optimum conditions, the lowest detectable concentration of Hg2+ using this probe was 500 nM on the basis of the measurement of the ratio of absorption at 620 nm to that at 520 nm. The sensitivity to Hg2+ can be further improved by modifying the MPA/AMP-capped AuNPs with highly fluorescent rhodamine 6G (R6G). By monitoring the fluorescence enhancement, the lowest detectable concentration of Hg2+ using R6G/MPA/AMP-capped AuNPs was 50 nM.     

Relationship between chemical and icosahedral local orderings in Al-Ni-Fe melts

An X-ray diffraction study and simulation of the structure of ternary Al_81.6Ni_14.9Fe_3.5, Al_71.6Ni₂₃Fe_5.4, and Al_61.1Ni_31.1Fe_7.3 melts by the reverse Monte Carlo method are conducted. An analysis of the structural models of melts is performed by the Voronoi-Delaunay partition. It is shown that the prepeak on the structure factor curves in the diffraction vector range of 13 nm⁻¹ to 22 nm⁻¹ is due to two factors: chemical ordering of atoms and non-crystalline close packing. The origin of the icosahedral short-range order in the melts as one of the variants of ordering of atoms in non-crystalline close packed clusters is discussed.     

Visual and surface plasmon resonance sensor for zirconium based on zirconium-induced aggregation of adenosine triphosphate-stabilized gold nanoparticles

Owing to its high affinity with phosphate, Zr(IV) can induce the aggregation of adenosine 5′-triphosphate (ATP)-stabilized AuNPs, leading to the change of surface plasmon resonance (SPR) absorption spectra and color of ATP-stabilized AuNP solutions. Based on these phenomena, visual and SPR sensors for Zr(IV) have been developed for the first time. The A_660 nm/A_518 nm values of ATP-stabilized AuNPs in SPR absorption spectra increase linearly with the concentrations of Zr(IV) from 0.5 μM to 100 μM (r = 0.9971) with a detection limit of 95 nM. A visual Zr(IV) detection is achieved with a detection limit of 30 μM. The sensor shows excellent selectivity against other metal ions, such as Cu²⁺, Fe³⁺, Cd²⁺, and Pb²⁺. The recoveries for the detection of 5 μM, 10 μM, 25 μM and 75 μM Zr(IV) in lake water samples are 96.0%, 97.0%, 95.6% and 102.4%, respectively. The recoveries of the proposed SPR method are comparable with those of ICP-OES method.     

Chiral separation of ofloxacin enantiomers by ligand exchange chromatography

Abstract  

The enantioseparation conditions of ligand exchange chromatography were examined using ofloxacin enantiomers. A C₁₈ column was used with the mobile phase consisting of a methanol–water solution (containing different concentrations of l-isoleucine and copper sulfate) at flow rate of 0.5 cm³ min⁻¹. The effect of different kinds and concentrations of ligands, bivalent ligand ions, and organic modifier, and temperature on enantioseparation were evaluated; the results showed that enantioselectivity was strongly affected by the ligand concentration of the mobile phase. Under the optimum conditions (methanol/water 20:80 v/v, containing 2.5 mmol dm⁻³ l-isoleucine and 0.6 mmol dm⁻³ Cu²⁺, room temperature), baseline separation of the two enantiomers was obtained with resolution of 1.32 in less than 30 min. The separation method was used to analyze the ofloxacin enantiomers in different commercial medicines.     

A novel colorimetric aptasensor for detection of chloramphenicol based on lanthanum ion–assisted gold nanoparticle aggregation and smartphone imaging

A label-free, rapid response colorimetric aptasensor for sensitive detection of chloramphenicol (CAP) was proposed, which was based on the strategy of ssDNA-modified gold nanoparticle (AuNP) aggregation assisted by lanthanum (La³⁺) ions. The AuNPs generated a color change that could be monitored in the red, green, and blue and analyzed by the smartphone imaging app. La³⁺, as a trigger agent, strongly combined with the phosphate groups of the surface of ssDNA-AuNPs probe, which helps create AuNP aggregation and the color change of AuNPs from red to blue. On the contrary, when mixing with CAP, the aptamer (Apt) bound to CAP to form a rigid structure of the Apt-CAP complex, and La³⁺ attached to the phosphate groups of the complex, which prevented the aptamer from binding to the surface of the AuNPs. As a result, the color of the AuNPs changed to violet-red. Finally, UV-vis absorption spectroscopy and the smartphone imaging app were employed to determine CAP with a lower detection limit of 7.65 nM and 5.88 nM, respectively. The proposed strategy featuring high selectivity and strong anti-interference ability for detection of CAP in practical samples was achieved. It is worth mentioning that the simple and portable colorimetric aptasensor will be used for facilitating on-site detection of food samples.

     

Gd<Superscript>3+</Superscript> binding of carboxylated polyglycidyl methacrylate latices and their colloidal stability

The binding of Gd³⁺ to two carboxylated polyglycidyl methacrylate latices was investigated. The latices differed in size (60 and 140 nm for CL6 and CL3, respectively) and surface charge density. The Gd³⁺ concentration in aqueous suspension was determined using an arsenazo (III) assay. Using ¹⁵³Gd³⁺, the bound amount was determined directly. Because of the high binding affinity, ligand depletion became evident. The binding was pH dependent, investigated in buffer solutions not influencing the arsenazo (III) assay. Optimal binding occurs by formation of sodium salts of the carboxylic groups and replacement of Na⁺ and H⁺ by Gd³⁺. The dissociation constants of the particles were k _D ≈ 5 × 10⁻⁵ mol/L (CL3) and 10⁻⁴ mol/L (CL6), without cooperativity (Hill plot). Colloidal stability was investigated. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Thermodynamics of mixed-ligand complex formation of copper (II) ethylenediaminetetraacetate with hexamethylenediamine in an aqueous solution

The mixed-ligand complex formation in the system Cu²⁺−Edta⁴⁻−(CH₂)₆(NH₂)₂ (L), where L is hexamethylenediamine has been calorimetrically, pH-potentiometrically and spectrophotometrically studied in aqueous solution at 298.15 K and the ionic strength of I = 0.5 (KNO₃). The thermodynamic parameters of formation of the CuEdtaL²⁻, CuEdtaHL⁻ (CuEdta)₂L⁴⁻ and (CuEdta)₂En⁴⁻ complexes have been determined. The most probable coordination mode for the complexone and the ancillary ligand in the mixed-ligand complexes was discussed.     

Adsorption properties of SCN? on (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes: a density functional study

Abstract  

The behavior of the thiocyanate anion (SCN⁻) adsorbed on the external surface of H-capped (6,0), (7,0), (8,0), and Al-doped (6,0) zigzag single-walled carbon nanotubes was studied by using density functional calculations. Geometry optimizations were carried out at the B3LYP/6-31G* level of theory using the Gaussian 03 suite of programs. We present the nature of the SCN⁻ interaction in selected sites of the nanotubes. Our results show that the pristine carbon nanotubes cannot significantly detect SCN⁻. The calculated binding energy of the Al-doped (6,0) single-walled carbon nanotubes indicated that SCN⁻ can be adsorbed significantly on the C and Al sites and these nanotubes can therefore be used for SCN⁻ storage. Binding energies corresponding to adsorption of SCN⁻ on the Al site in the Al-doped (6,0) single-walled carbon nanotubes was calculated as −286.38 kJ mol⁻¹. The calculated binding energies for SCN⁻ in N-down orientation are higher than those in S-down orientation for all of the configurations. More efficient binding could not be achieved by increasing the nanotube diameter. We also report the effects of SCN⁻ adsorption on the electronic properties of the nanotubes.