Detection of melamine in infant formula and grain powder by surface‐assisted laser desorption/ionization mass spectrometry

We have developed a method for the determination of melamine (MEL), ammeline (AMN), and ammelide (AMD) by surface‐assisted laser desorption/ionization mass spectrometry (SALDI‐MS) using gold nanoparticles (Au NPs). The major peaks for MEL, AMN, and AMD at m/z 127.07, 128.05, and 129.04 are assigned to the [MEL + H]⁺, [AMN + H]⁺, and [AMD + H]⁺ ions. Because the three tested compounds adsorb weakly onto the surfaces of the Au NPs through Au–N bonding, they can be easily concentrated from complex samples by applying a simple trapping/centrifugation process. The SALDI‐MS method provides limits of detection of 5, 10, and 300 nM for MEL, AMN, and AMD, respectively, at a signal‐to‐noise ratio of 3. The signal variation for 150‐shot average spectra of the three analytes within the same spot was 15%, and the batch‐to‐batch variation was 20%. We have validated the practicality of this approach by the analysis of these three analytes in infant formula and grain powder. This simple and rapid SALDI‐MS approach holds great potential for screening of MEL in foods. Copyright © 2012 John Wiley & Sons, Ltd.     

Catalytic gold nanoparticles for fluorescent detection of mercury(II) and lead(II) ions

In this study, we developed a fluorescence assay for the highly sensitive and selective detection of Hg²⁺ and Pb²⁺ ions using a gold nanoparticle (Au NP)-based probe. The Hg–Au and Pb–Au alloys that formed on the Au NP surfaces allowed the Au NPs to exhibit peroxidase-mimicking catalytic activity in the H₂O₂-mediated oxidation of Amplex UltraRed (AUR). The fluorescence of the AUR oxidation product increased upon increasing the concentration of either Hg²⁺ or Pb²⁺ ions. By controlling the pH values of 5 mM tris–acetate buffers at 7.0 and 9.0, this H₂O₂–AUR–Au NP probe detected Hg²⁺ and Pb²⁺ ions, respectively, both with limits of detection (signal-to-noise ratio: 3) of 4.0 nM. The fluorescence intensity of the AUR oxidation product was proportional to the concentrations of Hg²⁺ and Pb²⁺ ions over ranges 0.05–1 μM (R² = 0.993) and 0.05–5 μM (R² = 0.996), respectively. The H₂O₂–AUR–Au NP probe was highly selective for Hg²⁺ (>100-fold) and Pb²⁺ (>300-fold) ions in the presence of other tested metal ions. We validated the practicality of this simple, selective, and sensitive H₂O₂–AUR–Au NP probe through determination of the concentrations of Hg²⁺ and Pb²⁺ ions in a lake water sample and of Pb²⁺ ions in a blood sample. To the best of our knowledge, this system is the first example of Au NPs being used as enzyme-mimics for the fluorescence detection of Hg²⁺ and Pb²⁺ ions.     

Pt NPs catalyzed chemiluminescence method for Hg2+ detection based on a flow injection system

Establishing a simple and accurate method for Hg²⁺ detection is of great importance for the environment and human health. In this work, platinum nanoparticles (Pt NPs) with different capped agents and morphologies were synthesized. It was found that Pt NPs exhibited peroxidase‐like activity that can catalyze the chemiluminescence (CL) of the luminol system without H₂O₂. The most intensive CL signals were obtained by using PVP‐capped Pt NPs as catalysis. Based on the fact that Hg²⁺ could further enhance the CL intensity in the Pt NPs‐luminol CL system, a Pt NPs‐catalyzed CL method based on a flow injection system is developed for the sensitive analysis of Hg²⁺. When the concentration of Hg²⁺ in the system increases, the CL intensity would together increase, thereby achieving sensitive Hg²⁺ detection. The limit of detection (LOD) was calculated to be 8.6 nM. This developed method provides a simple and rapid approach for the sensitive detection of Hg²⁺ and shows great promise for applications in other complex systems.     

A Fluorescent Probe for Hg2+ Based on Gold(I) Complex with An Aggregation‐Induced Emission Feature

A gold(I) complex that exhibited aggregation‐induced emission in acetonitrile‐water mixtures was designed. It showed high selectivity and sensitivity for Hg²⁺ in acetonitrile‐H₂O (1:1, V:V) solution. Dynamic light scattering measurements were conducted to verify that the addition of Hg²⁺ changed the particle size and induced fluorescence quenching.     

A label-free colorimetric detection of lead ions by controlling the ligand shells of gold nanoparticles

We have developed a simple, colorimetric and label-free gold nanoparticle (Au NP)-based probe for the detection of Pb²⁺ ions in aqueous solution, operating on the principle that Pb²⁺ ions change the ligand shell of thiosulfate (S₂O₃²⁻)-passivated Au NPs. Au NPs reacted with S₂O₃²⁻ ions in solution to form Au⁺·S₂O₃²⁻ ligand shells on the Au NP surfaces, thereby inhibiting the access of 4-mercaptobutanol (4-MB). Surface-assisted laser desorption/ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) and inductively coupled plasma mass spectrometry (ICP-MS) measurements revealed that PbAu alloys formed on the surfaces of the Au NPs in the presence of Pb²⁺ ions; these alloys weakened the stability of the Au⁺·S₂O₃²⁻ ligand shells, enhancing the access of 4-MB to the Au NP surfaces and, therefore, inducing their aggregation. As a result, the surface plasmon resonance (SPR) absorption of the Au NPs red-shifted and broadened, allowing quantitation of the Pb²⁺ ions in the aqueous solution. This 4-MB/S₂O₃²⁻-Au NP probe is highly sensitive (linear detection range: 0.5-10 nM) and selective (by at least 100-fold over other metal ions) toward Pb²⁺ ions. This cost-effective sensing system allows the rapid and simple determination of the concentrations of Pb²⁺ ions in real samples (in this case, river water, Montana soil and urine samples).     

A novel Hg2+ fluorescence sensor TPE-TSC based on aggregation-induced emission

Abstract

A fluorescent sensor TPE-TSC with aggregation induced emission (AIE) characteristic is synthesized for detecting Hg²⁺ by attaching thiosemicarbazide (TSC) unit into tetraphenylethylene (TPE) group. TPE-TSC exhibits intense green emission in DMSO/H₂O (V:V?=?1:9) solution with the formation of the aggregation. TPE-TSC shows outstanding fluorescence quenching toward Hg²⁺ over other metal ions due to the formation of complex TPE-TSC/Hg²⁺ with a 2:1 binding ratio. The detection limit of TPE-TSC for Hg²⁺ is 1?×?10^?5 mol·L^?1.     

pH‐Switchable Redox Reactions and Bioelectrocatalytic Processes Using Au Nanoparticles‐Modified Electrodes

Boronate ester complexes generated between methylene blue (MB⁺)‐functionalized Au nanoparticles (NPs) and electrode surfaces are implemented to stimulate the bioelectrocatalyzed reduction of H₂O₂ in the presence of horseradish peroxidase (HRP). Two kinds of Au NPs are prepared: Class I includes MB⁺/phenylboronic acid as a modifying layer, whereas Class II includes MB⁺/dithiothreitol as a mixed capping layer. The Class I or II NPs form boronate ester complexes with a dithiothreitol‐ or phenylboronic acid‐functionalized Au electrodes, respectively. By the cyclic loading of the NPs on the electrodes (pH 8.1), and the removal of the NPs (pH 1.5), switchable bioelectrocatalyzed reduction of H₂O₂ is demonstrated.     

Detection of Hg2+ and F− Ions by Using Fluorescence Switching of Quantum Dots in an Au‐Cluster–CdTe QD Nanocomposite

A single probe of an Au nanocluster–CdTe quantum dots nanocomposite has been developed by using tripeptide‐capped CdTe quantum dots (QD) and bovine serum albumin (BSA) protein‐conjugated Au₂₅ nanocluster (NC) for detection of both Hg²⁺ ion and F^? ion. The formation of Au‐NC–CdTe QD nanocomposite has been confirmed by TEM, steady state and time resolved spectroscopy, CD and FTIR studies. A significant signal off (74 % PL quenching at 553 nm) phenomenon of this nanocomposite is observed in presence of 6.56×10^?7 M Hg²⁺ ion, due to salt‐induced aggregation. However, a dramatic PL enhancement (128 %) of the Au‐NC–CdTe QD nanocomposite is observed in presence of 8.47×10^?7 M F^? anion. The calculated limit of detections (LOD) of Hg²⁺ ion concentration and F^? ion concentration are found to be 9 and 117 nM , respectively, which are within the safety range set by the United States Environment Protection Agency. Thus, the simple Au‐NC–CdTe QD optical‐based sensor is very useful to detect both toxic cations and anions.     

Ultrasensitive detection of mercury(II) ion using CdTe quantum dots in sol-gel-derived silica spheres coated with calix[6]arene as fluorescent probes

We have developed a simple method for the preparation of highly fluorescent and stable, water-soluble CdTe quantum dots in sol-gel-derived composite silica spheres that were coated with calix[6]arene. The resulting nanoparticles (NP) were characterized in terms of UV, fluorescence and FT-IR spectroscopy and by TEM. The results show that the new NPs display more intense fluorescence intensity and are more stable than its precursors of the type SiO₂/CdTe. In addition, the new NPs exhibit a higher selectivity for the determination of Hg²⁺ than for other metal ions. Under the optimum conditions, the relative fluorescence intensity decreases with the concentration of Hg²⁺ in the range from 2.0 to 14.0?nmol?L^?1 and the detection limit is 1.55?nmol?L^?1. The method is based on the quenching of fluorescence by Hg²⁺ and expected to serve as a practical fluorescence test for rapid detection of Hg²⁺. A mechanism is suggested to explain the inclusion process by a Langmuir binding isotherm, and fluorescence quenching is best described by the Stern-Volmer equation.

A Reversible and Highly Selective Fluorescent Sensor for Mercury(II) Using Poly(thiophene)s that Contain Thymine Moieties

Summary: A reversible and highly selective assay method has been developed to detect mercury(II ) (Hg²⁺) ions using a conjugated polymer (CP). The transduction mechanism is based on Hg²⁺‐mediated interpolymer π‐stacking aggregation, which results in the fluorescence self‐quenching of the CP. CPs that contain thymine moieties, poly[3‐(N‐thymin‐1‐ylacetyl)ethylamine‐thiophene] (PTT), have been synthesized and characterized. In the absence of Hg²⁺ ions, the PTT chains remain separated from each other and the CP exhibits strong fluorescence emission. Upon adding Hg²⁺ ions, the formation of interpolymer π‐stacking aggregation induced by specific thymine–Hg–thymine interactions results in the fluorescence quenching of PTT. Distinguishing aspects of this assay include the signal amplification of CPs and the specific binding of Hg²⁺ ions to thymine‐thymine (T–T) base pairs.

The binding of Hg²⁺ ions causes the separate conducting polymer chains to aggregate with subsequent fluorescence self‐quenching.     

4-Mercaptophenylacetic acid functionalized Mn2+-doped ZnS nanoparticles fluorescence quenching caused by the addition of Cu2+

This paper reports that 4-mercaptophenylacetic acid functionalized Mn²⁺-doped ZnS nanoparticles (4-MPAA-ZnS-Mn²⁺ NPs) as fluorescent probes for the detection of copper ions in solution. The fluorescence quenching was due to the aggregation of copper ions with 4-MPAA-ZnS-Mn²⁺ NPs. These aggregations were confirmed by using UV lamp, UV–visible spectroscopy and dynamic light scattering (DLS). These 4-MPAA-ZnS-Mn²⁺ NPs were applied as fluorescent probes to detect copper ions in aqueous solution.     

Detection of Metal Ions (Cu2+, Hg2+) and Cocaine by Using Ligation DNAzyme Machinery

The Cu²⁺‐dependent ligation DNAzyme is implemented as a biocatalyst for the colorimetric or chemiluminescence detection of Cu²⁺ ions, Hg²⁺ ions, or cocaine. These sensing platforms are based on the structural tailoring of the sequence of the Cu²⁺‐dependent ligation DNAzyme for specific analytes. The tethering of a subunit of the hemin/G‐quadruplex DNAzyme to the ligation DNAzyme sequence, and the incorporation of an imidazole‐functionalized nucleic‐acid sequence, which acts as a co‐substrate for the ligation DNAzyme that is tethered to the complementary hemin/G‐quadruplex subunit. In the presence of different analytes, Cu²⁺ ions, Hg²⁺ ions, or cocaine, the pretailored Cu²⁺‐dependent ligation DNAzyme sequence stimulates the respective ligation process by combining the imidazole‐functionalized co‐substrate with the ligation DNAzyme sequence. These reactions lead to the self‐assembly of stable hemin/G‐quadruplex DNAzyme nanostructures that enable the colorimetric analysis of the substrate through the DNAzyme‐catalyzed oxidation of 2,2′‐azinobis(3‐ethylbenzothiazoline‐6‐sulfonic acid), ABTS^2?, by H₂O₂ into the colored product ABTS^.?, or the chemiluminescence detection of the substrate through the DNAzyme‐catalyzed oxidation of luminol by H₂O₂. The detection limits for the sensing of Cu²⁺ ions, Hg²⁺ ions, and cocaine correspond to 1 nM , 10 nM and 2.5 μM , respectively. These different sensing platforms also reveal impressive selectivities.     

Probing the Catalytic Activity of Reduced Graphene Oxide Decorated with Au Nanoparticles Triggered by Visible Light

Hybrid materials in which reduced graphene oxide (rGO) is decorated with Au nanoparticles (rGO–Au NPs) were obtained by the in situ reduction of GO and AuCl₄^?_(aq) by ascorbic acid. On laser excitation, rGO could be oxidized as a result of the surface plasmon resonance (SPR) excitation in the Au NPs, which generates activated O₂ through the transfer of SPR‐excited hot electrons to O₂ molecules adsorbed from air. The SPR‐mediated catalytic oxidation of p‐aminothiophenol (PATP) to p,p′‐dimercaptoazobenzene (DMAB) was then employed as a model reaction to probe the effect of rGO as a support for Au NPs on their SPR‐mediated catalytic activities. The increased conversion of PATP to DMAB relative to individual Au NPs indicated that charge‐transfer processes from rGO to Au took place and contributed to improved SPR‐mediated activity. Since the transfer of electrons from Au to adsorbed O₂ molecules is the crucial step for PATP oxidation, in addition to the SPR‐excited hot electrons of Au NPs, the transfer of electrons from rGO to Au contributed to increasing the electron density of Au above the Fermi level and thus the Au‐to‐O₂ charge‐transfer process.     

Selective detection of phosphaphenanthrenecontaining luminophors with aggregation-induced emission enhancement to transition metal ions

Transition metal ions (Pb²⁺, Zn²⁺, Cd²⁺, Co²⁺, Mn²⁺, Cu²⁺, Ni²⁺, Hg²⁺, Ag⁺, Fe³⁺) in water are used to quench emission of 2-(6-oxido-6H-dibenz 〈c,e〉 〈1,2〉 oxaphosphorin-6-yl)-1,4-phenylene-bis(p-pentyloxylbenzoate)s (MD5) with aggregation-induced emission enhancement (AIEE) in water-acetonitrile (AN) mixture (80:20 by volume). Among all metal ions, Fe³⁺ exhibits the highest quenching efficiency on AIEE of MD5 even when the concentration of Fe³⁺ is lower than 1×10⁻⁶ mol/L. The quenching efficiency of Hg²⁺ is lower than that of Fe³⁺ at the same concentration, though MD5 is used to detect Hg²⁺ efficiently, too. To other metal ions, low quenching efficiency has few relations with a wider concentration range. The UV absorbance spectra show only red shift of absorbance wavelength in the presence of Hg²⁺ and Fe³⁺, which indicates a salt-induced Jaggregation. SEM photos reveal larger aggregation and morphological change of nanoparticles of MD5 in water containing Hg²⁺ and Fe³⁺, which reduce the surface area of MD5 emission for further aggregation. The selective quenching effect of transition metal ions to emission of MD5 has a potential application in chemical sensors of some metal ions.     

Au‐TiO2/Graphene Nanocomposite Film for Electrochemical Sensing of Hydrogen Peroxide and NADH

We describe a simple method for preparing Au‐TiO₂/graphene (GR) nanocomposite by deposition of Au nanoparticles (NPs) on TiO₂/GR substrates. The as‐prepared Au‐TiO₂/GR was characterized by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The presence of Au NPs on TiO₂/GR surface remarkably improves the electrocatalytic activity towards the oxidation of hydrogen peroxide (H₂O₂) and β‐nicotinamide adenine dinucleotide (NADH). The Au‐TiO₂/GR modified glassy carbon (GC) electrode exhibits good amperometric response to H₂O₂ and NADH, with linear range from 10 to 200 µM and 10 to 240 µM, and detection limit of 0.7 and 0.2 µM, respectively.     

Detection of aminothiols through surface‐assisted laser desorption/ionization mass spectrometry using mixed gold nanoparticles

We have employed mixtures of two differently sized (average diameters: 3.5 and 14 nm) gold nanoparticles (Au NPs) as selective probes and matrices for the determination of aminothiols using surface‐assisted laser desorption/ionization mass spectrometry (SALDI‐MS). When using 38 and 150 pM solutions of the 3.5‐ and 14‐nm Au NPs, respectively, as the probe and matrix, SALDI‐MS provided limits of detection (signal‐to‐noise ratio = 3) of 2, 20, and 44 nM for 1.0 mL solutions of glutathione (GSH), cysteine (Cys), and homocysteine, respectively. The signal intensities of these analytes varied by less than 20% for SALDI‐MS analyses recorded over 50 sample spots; in contrast, they varied by as much as 60% when using a conventional matrix (2,5‐dihydroxybenzoic acid). We validated the practicality of this approach – with its advantages of sensitivity, reproducibility, rapidity, and simplicity – through the analysis of GSH in MCF‐7 cell lysates and Cys in plasma. Copyright © 2009 John Wiley & Sons, Ltd.     

Enhancement of Fe3O4/Au Composite Nanoparticles Catalyst in Oxidative Degradation of Methyl Orange Based on Synergistic Effect

Enhancement of Fe₃O₄ /Au nanoparticles (Fe₃O₄ /Au NPs ) catalyst was observed in the oxidative degradation of methyl orange by employing H₂O₂ as oxidant. To evaluate the catalytic activity of Fe₃O₄ /Au nanoparticles, different degradation conditions were investigated such as the amounts of catalyst, H₂O₂ concentration and pH value. Based on our data, methyl orange was degraded completely in a short time. The enhanced catalytic activity and increased oxidation rate constant may be ascribed to synergistic catalyst‐activated decomposition of H₂O₂ to •OH radical, which was one of the strong oxidizing species. Besides, Fe₃O₄ /Au nanoparticles have exhibited satisfying recycle performance for potential industrial application.     

Effect of the Counterion on Light Emission: A Displacement Strategy to Change the Emission Behaviour from Aggregation‐Caused Quenching to Aggregation‐Induced Emission and to Construct Sensitive Fluorescent Sensors for Hg2+ Detection

In this paper, a simple strategy to change the emission behaviour of luminogenic materials was developed. Tetraphenylethene (TPE)‐functionalised benzothiazolium salts with different counteranions (TPEBe?X; X=I^?, ClO₄^? and PF₆^?) were designed and synthesised. All the luminogens show weak red emission in the solution state that originates from intramolecular charge transfer from TPE to the benzothiazolium unit. Whereas aggregate formation enhances the light emission of TPEBe?ClO₄ and TPEBe?PF₆, that of TPEBe?I is quenched, thus demonstrating the phenomena of aggregation‐induced emission and aggregation‐caused quenching. TPEBe?I works as a light‐up fluorescent sensor for Hg²⁺ in aqueous solution with high sensitivity and specificity owing to the elimination of the emission quenching effect of the iodide ion by the formation of HgI₂ as well as the induction in aggregate formation by the complexation of Hg²⁺ with the S atom of the benzothiazolium unit of TPEBe?I. A solid film of TPEBe?I was prepared that can monitor the level of Hg²⁺ in aqueous solution with a detection limit of 1 μM .     

Highly Sensitive Electrochemiluminescence Detection of Mercury(II) Ions Based on DNA‐Linked Luminol‐Au NPs Superstructure

A novel highly sensitive electrochemiluminescence (ECL) detection protocol for mercury(II) ions was developed. Based on the strong and stable thymine? thymine mismatches complexes coordination chemistry, mercury(II) ions can specifically bind to a designed DNA strand, leading to the release of the complimentary DNA strand. The released DNA strand was then captured by magnetic beads modified with specific DNA, and then through the formation of DNA‐linked luminol‐Au nanoparticles (NPs) superstructure, a specific ECL system for mercury(II) ions was developed. Using 3‐aminopropyl‐triethoxysilane as an effective enhancer, the ECL system can detect Hg²⁺ ion within a linear range from 2.0×10^?10 mol L^?1 to 2.0×10^?8 M, with a detection limit as low as 1.05×10^?10 M (3σ). Moreover, this ECL system is highly specific for Hg²⁺, without interference from other commonly coexisted metal ions, and it can be used for the analysis of real samples.     

A Novel Hydrogen Peroxide Biosensor Based on the Synergistic Effect of Gold‐Platinum Alloy Nanoparticles/Polyaniline Nanotube/Chitosan Nanocomposite Membrane

Based on the immobilization of horseradish peroxidase (HRP) in chitosan(CS) on a glassy carbon electrode (GCE) modified with the Au‐Pt alloy nanoparticles (NPs) / polyaniline nanotube (nanoPAN) nanocomposite film, a novel hydrogen peroxide biosensor was constructed. The modified processes of GCE were monitored by cyclic voltammetry and electrochemical impedance spectroscopy. Au‐PtNPs/nanoPAN/CS had a better synergistic electrochemical effect than did AuNPs/nanoPAN/CS or PtNPs/nanoPAN/CS. The amperometric response of the biosensor towards H₂O₂ was investigated by successively adding aliquots of H₂O₂ to a continuous stirring phosphate buffer solution under the optimized conditions. Because Au‐PtNPs have unique catalytic properties and good biocompatibility, and especially Au‐PtNPs and nanoPAN have synergistic augmentation for facilitating electron‐transfer, the biosensor displayed a fast response time (<2 s) and broad linear response to H₂O₂ in the range from 1.0 to 2200 μmol L^?1 with a relatively low detection limit of 0.5 μmol L^?1 at 3 times the background noise. Moreover, the biosensor can be applied in practical analysis and exhibited high sensitivity, good reproducibility, and long‐term stability.