Rapid and ultrasensitive colorimetric detection of mercury(II) by chemically initiated aggregation of gold nanoparticles

The article describes a method for rapid and visual determination of Hg(II) ion using unmodified gold nanoparticles (Au-NPs). It involves the addition of Au-NPs to a solution containing Hg(II) ions which, however, does not induce a color change. Next, a solution of lysine is added which induces the aggregation of the Au-NPs and causes the color of the solution to change from wine-red to purple. The whole on-site detection process can be executed in less than 15 min. Other amines (ethylenediamine, arginine, and melamine) were also investigated with respect to their capability to induce aggregation. Notably, only amines containing more than one amino group were found to be effective, but a 0.4 μM and pH 8 solution of lysine was found to give the best results. The detection limits for Hg (II) are 8.4 pM (for instrumental read-out) and 10 pM (for visual read-out). To the best of our knowledge, this LOD is better than those reported for any other existing rapid screening methods. The assay is not interfered by the presence of other common metal ions even if present in 1000-fold excess over Hg(II) concentration. It was successfully applied to the determination of Hg(II) in spiked tap water samples. We perceive that this method provides an excellent tool for rapid and ultrasensitive on-site determination of Hg(II) ions at low cost, with relative ease and minimal operation.

Rapid and ultrasensitive detection of mercury ions using gold nanoparticle based label-free colorimetric method with excellent sensitivity, easy operation and low cost.

     

Ultrasensitive non-enzymatic glucose sensing at near-neutral pH values via anodic stripping voltammetry using a glassy carbon electrode modified with Pt3Pd nanoparticles and reduced graphene oxide

We describe an anodic stripping voltammetric (ASV) method for glucose sensing that widely expands the typical amperometric i-t response of glucose sensors. The electrode is based on a working electrode consisting of a glassy carbon electrode modified with Pt-Pd nanoparticles (NPs; in an atomic ratio of 3:1) on a reduced graphene oxide (rGO) support. The material was prepared via the spontaneous redox reaction between rGO, PdCl₄ ²⁻ and PtCl₄ ²⁻ without any additional reductant or surfactant. Unlike known Pt-based sensors, the use of Pt₃Pd NPs results in an ultrasensitive ASV approach for sensing glucose even at near-neutral pH values. If operated at a working voltage as low as 0.06 V (vs. SCE), the modified electrode can detect glucose in the 2 nM to 300 μM concentration range. The lowest detectable concentration is 2 nM which is much lower than the LODs obtained with other amperometric i-t type sensing approaches, most of which have LODs at a μM level. The sensor is not interfered by the presence of 0.1 M of NaCl.

We describe an anodic stripping voltammetric method for glucose sensing that widely expands the typical amperometric i-t response of glucose sensors (2 nM to 300 μM). The electrode is based on a glassy carbon electrode modified with Pt-Pd nanoparticles on a reduced graphene oxide (rGO) support.

     

Determination of trypsin activity using a gold electrode modified with a nanocover composed of graphene oxide and thionine

We report on an electrochemical method for the determination of the activity of trypsin. A multi-functional substrate peptide (HHHAKSSATGGC-HS) is designed and immobilized on a gold electrode. The three His residues in the N-terminal are able to recruit thionine-loaded graphene oxide (GO/thionine), a nanocover adopted for signal amplification. Once the peptide is cleaved under enzymatic catalysis by trypsin (cleavage site: Lys residue), the His residues leave the electrode, and the GO/thionine cannot cover the peptide-modified electrode anymore. Thus, the changes of the electrochemical signal of thionine, typically acquired at a voltage of -0.35 V, can be used to determine the activity of trypsin. A detection range of 1 × 10⁻⁴ to 1 U, with a detection limit of 3.3 × 10⁻⁵ U, can be achieved, which is better than some currently available methods. In addition, the method is highly specific, facile, and has the potential for the detection of trypsin-like proteases.

Graphene oxide was adopted as a nanocover for the development of a sensitive electrochemical method to detect the activity of trypsin.

     

Aptamer-functionalized magnetic nanoparticles for simultaneous fluorometric determination of oxytetracycline and kanamycin

This work describes a method for the simultaneous detection of oxytetracycline (OTC) and kanamycin (KMY) using aptamers acting as both recognition and separation elements, and complementary oligonucleotides labeled with a green emitting fluorophore (carboxyfluorescein, FAM) and a yellow emitting fluorophore (carboxy-X-rhodamine, ROX), respectively, as signal labels. An OTC aptamer and a KMY aptamer were immobilized on the surface of magnetic nanoparticles (MNPs) via avidin-biotin chemistry. The aptamers preferentially bind their respective targets and thereby cause the upconcentration of analytes. However, in their absence they bind fluorescently-tagged complementary oligonucleotide later added to the reaction system. This cause the NPs to become fluorescent, with emission peaks located at 520 and 608 nm, respectively. The effects of the concentration of avidin, aptamer, complementary oligonucleotide, incubation temperature and incubation time were optimized. Under the optimal conditions, linear relationships were obtained in the range of 1–50 ng∙mL⁻¹ for OTC and KMY, with limits of detection of 0.85 ng∙mL⁻¹ and 0.92 ng∙mL⁻¹, respectively. The method was applied to the analysis of pork, milk, and honey samples spiked with OTC and MKY. Recoveries ranged from 76.5 to 94.7 % and 77.8 to 93.1 %, respectively, and the relative standard deviation was <10.0 %.

This work describes an assay for the simultaneous detection of oxytetracycline and kanamycin using aptamer-modified as both recognition and separation elements, and complementary oligonucleotide labeled with FAM and ROX, respectively, as signal labels. The developed method possesses high sensitivity and selectivity, and short analysis time.

     

Graphene quantum dots combined with copper(II) ions as a fluorescent probe for turn-on detection of sulfide ions

A fluorescent probe for the sensitive and selective determination of sulfide ions is presented. It is based on the use of graphene quantum dots (GQDs) which emit strong and stable blue fluorescence even at high ionic strength. Copper(II) ions cause aggregation of the GQDs and thereby quench fluorescence. The GQDs-Cu(II) aggregates can be dissociated by adding sulfide ions, and this results in fluorescence turn on. The change of fluorescence intensity is proportional to the concentration of sulfide ions. Under optimal conditions, the increase in fluorescence intensity on addition of sulfide ions is linearly related (r ² = 0.9943) to the concentration of sulfide ions in the range from 0.20 to 20 μM, and the limit of detection is 0.10 μM (at 3 σ/s). The fluorescent probe is highly selective for sulfide ions over some potentially interfering ions. The method was successfully applied to the determination of sulfide ions in real water samples and gave recoveries between 103.0 and 113.0 %.

Graphene quantum dots (GQDs) emit strong blue fluorescence which, however, is quenched by copper(II) ions due to the formation of GQDs-Cu(II) aggregates. Fluorescence is recovered by sulfide ions due to the dissociation of GQDs-Cu(II) aggregates.

     

Selective assembly of Au-Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticle hetero-dimers

Hetero-dimeric magnetic nanoparticles of the type Au-Fe₃O₄ have been synthesised from separately prepared, differently shaped (spheres and cubes), monodisperse nanoparticles. This synthesis was achieved by the following steps: (a) Mono-functionalising each type of nanoparticles with aldehyde functional groups through a solid support approach, where nanoparticle decorated silica nanoparticles were fabricated as an intermediate step; (b) Derivatising the functional faces with complementary functionalities (e.g. amines and carboxylic acids); (c) Dimerising the two types of particles via amide bond formation. The resulting hetero-dimers were characterised by high-resolution TEM, Fourier transform IR spectroscopy and other appropriate methods.

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Graphical Abstract Nano-LEGO: Assembling two types of separately prepared nanoparticles into a hetero-dimer is the first step towards complex nano-architectures. This study shows a solid support approach to combine a gold and a magnetite nanocrystal.

     

Three Ternary Rare Earth(III) Complexes Based on 3‐[(4,6‐Dimethyl‐2‐pyrimidinyl)thio]‐propanoic Acid and 1,10‐Phenanthroline: Synthesis,Crystal Structure and Antioxidant Activity

Three ternary rare earth [Nd^III ( 1 ), Sm^III ( 2 ) and Y^III ( 3 )] complexes based on 3‐[(4,6‐dimethyl‐2‐pyrimidinyl)thio]‐propanoic acid (HL) and 1,10‐phenanthroline (Phen) were synthesized and characterized by IR and UV/Vis spectroscopy, TGA, and single‐crystal X‐ray diffraction. The crystal structures showed that complexes 1 – 3 contain dinuclear rare earth units bridged by four propionate groups and are of general formula [REL₃(Phen)]₂ · nH₂O (for 1 and 2 : n = 2; for 3 : n = 0). All rare earth ions are nine‐coordinate with distorted mono‐capped square antiprismatic coordination polyhedra. Complex 1 crystallizes in the monoclinic system, space group P2₁/c with a = 16.241(7) Å, b = 16.095(7) Å, c = 19.169(6) Å, β = 121.48(2)°. Complex 2 crystallizes in the monoclinic system, space group P2₁/c with a = 16.187(5) Å, b = 16.045(4) Å, c = 19.001(4) Å, β = 120.956(18)°. Complex 3 crystallizes in the triclinic system, space group P1 with a = 11.390(6) Å, b = 13.636(6) Å, c = 15.958(7) Å, α = 72.310(17)°, β = 77.548(15)°, γ = 78.288(16)°. The antioxidant activity test shows that all complexes own higher antioxidant activity than free ligands.