Colorimetric determination of staphylococcal enterotoxin B via DNAzyme-guided growth of gold nanoparticles

The authors describe a colorimetric method for the determination of the staphylococcal enterotoxin B (SEB) that also allows for visual readout. The assay is based on the growth of gold nanoparticles (AuNPs) mediated by a hemin/G-quadruplex DNAzyme which generates a color change from red to blue in the presence of SEB. The method is enzyme-free and does not require a label. The kinetics of the formation of the AuNPs is controlled by the hemin/G-quadruplex DNAzyme and this is key to the signal generation mechanism. In the presence of SEB, the reactions between aptamer and target modulated the amount of single probe G strands that form DNAzyme capable of consuming hydrogen peroxide. The growth process of AuNPs is influenced by the resulting concentration of H₂O₂ and leads to the color change. Under optimal conditions, a linear relationship exists between absorbance and SEB concentration in the range from 0.1 to 500 pg·mL￣¹ which covers the clinically relevant range. In case of visual detection, the lower limit of detection is 1 pg·mL^?1. The assay described here is sensitive, comparably inexpensive and can detect SEB rapidly without the need for sophisticated equipment. In our perception, the method has a wide scope in that it may be adapted to various nucleic acids, proteins and other biomolecules if respective aptamers are available.

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Graphical abstract Colorimetric determination of Staphylococcal enterotoxin B via DNAzyme-guided growth of gold nanoparticles

     

Colorimetric determination of Hg(II) via the gold amalgam induced deaggregation of gold nanoparticles

A method is described for the colorimetric determination of mercury(II). In the absence of Hg(II), aminopropyltriethoxysilane (APTES) which is positively charged at pH 7 is electrostatically absorbed on the surface of gold nanoparticles (AuNPs). This neutralizes the negative charges of the AuNPs and leads to NP aggregation and a color change from red to blue-purple. However, in the presence of Hg(II), reduced Hg (formed through the reaction between Hg(II) and citrate on the AuNP surface) will replace the APTES on the AuNPs. Hence, the formation of aggregates is suppressed and the color of the solution does not change. The assay is performed by measuring the ratio of absorbances at 650 and 520 nm and can detect Hg(II) at nanomolar levels with a 10 nM limit of detection. The specific affinity between mercury and gold warrants the excellent selectivity for Hg(II) over other environmentally relevant metal ions.

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Graphical Abstract Schematic of the method for determination of Hg²⁺ based on the gold amalgam-induced deaggregation of gold nanoparticles in the presence of APTES with the LOD of 10.1 nM.

     

Bovine serum albumin-stabilized gold nanoclusters as a fluorescent probe for determination of ferrous ion in cerebrospinal fluids via the Fenton reaction

Gold nanoclusters (AuNCs) stabilized with bovine serum albumin were utilized as a fluorescent probe for ferrous ion. The detection scheme is based on the quenching of the fluorescence of the modified AuNCs by hydroxyl radical (•OH) that is generated in the Fenton reaction between Fe(II) and H₂O₂. Fe(II) can be quantified in the 0.08 to 100 μM concentration range, and the limit of detection is as low as 24 nM. The method also displays good accuracy and high sensitivity when employed to the determination of Fe(II) in rat cerebrospinal fluids (CSFs). When applied to CSFs of a rat model of Alzheimer’s disease, it revealed enhanced levels of Fe(II) compared to a control, thereby showing the important physiological role of iron(II) in this disease.

BSA-stabilized gold nanoclusters (BSA-AuNCs) were utilized for the determination of ferrous ion in rat cerebrospinal fluids. The method is based on the quenching of the fluorescence by hydroxyl radical (•OH) which is generated in the Fenton reaction between Fe(II) and H₂O₂.

     

Colorimetric determination of copper ions based on the catalytic leaching of silver from the shell of silver-coated gold nanorods

We have developed a method for the colorimetric determination of copper ions (Cu²⁺) that is based on the use of silver-coated gold nanorods (Au@Ag NRs). Its outstanding selectivity and sensitivity result from the catalytic leaching process that occurs between Cu²⁺, thiosulfate (S₂O₃ ^2?), and the surface of the Au@Ag NRs. The intrinsic color of the Au@Ag NRs changes from bright red to bluish green with decreasing thickness of the silver coating. The addition of Cu²⁺ accelerates the leaching of silver from the shell caused in the presence of S₂O₃ ^2?. This result in a decrease in the thickness of the silver shell which is accompanied a change in color and absorption spectra of the colloidal solution. The shifts in the absorption maxima are linearly related to the concentrations of Cu²⁺ over the 3–1,000 nM concentration range (R?=?0.996). The method is cost effective and was applied to the determination of Cu²⁺ in real water samples.

Colorimetric determination of macrolide antibiotics using ferric ion

Macrolide antibiotics such as erythromycin, oleandomycin, spiromycin, and tylosin are found to react with Fe(3+) in the presence of an acetic acid-sulfuric acid mixture to form a colored product having a useful absorption band at 592 nm. Troleandomycin forms only a weakly colored product upon reaction. The molar absorptivity is about 2900 1 mol(-1) cm(-1) for erythromycin and the detection limit is 5 mug ml(-1). This colorimetric method permits the analysis of fermentation broths containing either erythromycin or tylosin without a separation step.     

Colorimetric determination of mercury(II) based on the inhibition of the aggregation of gold nanorods coated with 6-mercaptopurine

Microchimica Acta - The authors describe an optical assay for determination of mercury(II) ion by making use of gold nanorods (AuNRs) which are found to aggregate after addition of 6-mercaptopurine...     

Colorimetric determination of nitroalkanes via the janovsky reaction

Summary A determination of some lower nitroalkanes is suggested, via the colour reaction withm-dinitrobenzene in alkaline solution.

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Zusammenfassung Ein kolorimetrisches Verfahren zur Bestimmung niederer Nitroalkane mit Hilfe vonm-Dinitrobenzol in alkalischer Lösung wird vorgeschlagen.

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Résumé On propose un dosage colorimétrique de quelques nitroparaffines, au moyen de la réaction avec lem-dinitrobenzène en milieu alcalin.

     

Colorimetric determination of fluoride ion with a solid analytical reagent

A colorimetric procedure is proposed for the estimation of fluoride ions in μg range in aqueous samples. The method involves the use of a solid analytical reagent and exhibits an operational simplicity. The detection limit of 1 ppm in an aqueous sample is conveniently achieved in the direct application. The interfering ions are initially removed in an ion exchange step, which also serves as a concentration step for fluoride if present in low concentration.     

Colorimetric and visual determination of dicyandiamide using gallic acid-capped gold nanoparticles

Microchimica Acta - A new method is presented for the visual detection of dicyandiamide (DCD). Gold nanoparticles (AuNPs) capped with gallic acid (GA) were synthesized in a single step at room...     

Colorimetric and bare-eye determination of fluoride using gold nanoparticle agglomeration probes

We report on a sensitive detection scheme for fluoride that is based on a quinone-methide-type of rearrangement reaction to trigger a color change among cloaked gold nanoparticle agglomeration probes. Fluoride ions remove silyl moieties from phenol groups on the surface of the probes, and this causes a quinone-methide-type of rearrangement reaction to occur. This is accompanied by the release of a dithiol in a spontaneous and irreversible reaction at room temperature in aqueous medium. The released dithiol causes aggregation of the AuNPs which leads to a color change from pink-red to violet-blue that can easily be seen with bare eyes.. The generation of color by this cascade reactions is only caused by fluoride and not interfered by any other anions. The assay platform developed here offers a sensitive colorimetric assay for fluoride. The lower limit of detection is 120 μM, and the dynamic concentration range is from 120 μM to 1.5 mM.

Immobilization of gold nanorods onto acid-terminated self-assembled monolayers via electrostatic interactions

We report the immobilization of gold nanorods onto self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid (16-MHA). The simple two step protocol involves formation of a SAM of 16-MHA molecules onto gold-coated glass slides and subsequent immersion of these slides into the gold nanorod solution. The nanorods, formed by a seed-mediated, surfactant-assisted synthesis protocol, are stabilized in solution due to surface modification by the surfactant cetyltrimethylammonium bromide (CTAB). Attractive electrostatic interactions between the carboxylic acid group on the SAM and the positively charged CTAB molecules are likely responsible for the nanorod immobilization. UV-vis spectroscopy has been used to follow the kinetics of the nanorod immobilization. The nature of interaction between the gold nanorods and the 16-MHA SAM has been probed by Fourier transform infrared spectroscopy (FTIR). The surface morphology of the immobilized rods is studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. SEM was also used to determine the density of the immobilized nanorods as a function of the pH of immobilization. Control over the surface coverage of the immobilized gold nanorods has been demonstrated by simple pH variation. Such well-dispersed immobilized gold nanorods with control over the surface coverage could be interesting substrates for applications such as surface-enhanced Raman spectroscopy (SERS).     

End-to-end assembly of gold nanorods via oligopeptide linking and surfactant control

We report two novel approaches for fabricating self-assembled chains of end-to-end linked Au nanorods separated by a nanogap. In one approach, bi-functional cysteine end-capped oligopeptides of different lengths are used as the linking agent. The widths of the produced nanogaps scale with the length and tertiary structure of the peptide linker. Functionalized oligopeptides containing an acetylene group are also employed as a linker, and the functional group is uniquely identified using surface-enhanced Raman spectroscopy. The development of an oligopeptide-linking platform is motivated by the ease of synthesis and high modularity of peptides; these features enable the possibility to integrate diverse functionality into molecular nanogap junctions - synthesized in water. The stepwise nanochain formation is followed via the evolution of the longitudinal plasmon absorption band in combination with transmission electron microscopy. The reaction rate and extent is tuned by controlling the concentration of the stabilizing CTAB surfactant in the solution. At very low surfactant concentrations, spontaneous end-to-end linking of the Au nanorods is observed even in the absence of linking peptide. The assembled AuNRs may act as next-generation electrodes in a platform for molecular electronics and synthetic biology.     

Colorimetric determination of the fluoride ion based on interference with extractability

Summary The sensitivity of colorimetric determinations based on reduction of extinction by complex-forming anions can be increased by measuring the change of absorbancy in the organic phase after shaking with a suitable solvent, i. e. by determining the interference of the anion with extractability. This principle has been applied to the determination of the fluoride ion.

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Zusammenfassung Die Empfindlichkeit kolorimetrischer Bestimmungen, die auf Herabsetzung der Extinktion durch komplexbildende Anionen beruhen, kann durch Messung der Absorptionsänderung in der organischen Phase nach Schütteln mit einem geeigneten Lösungsmittel, das heißt durch Bestimmung der durch das Anion verminderten Extrahierbarkeit, erhöht werden. Dieses Prinzip wurde auf die Bestimmung von Fluoridion angewendet.

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Résumé La sensibilité des déterminations colorimétriques basée sur l'effet de diminution d'extinction dû à des anions donnant lieu à la formation de complexes peut être accrue par la variation de l'absorption dans la phase organique après agitation avec un solvant approprié, ce qui revient à déterminer l'effet d'empêchement d'extraction dû à l'anion. Ce principe a été appliqué au dosage de l'ion fluorure.

     

Colorimetric determination of diphenylmethane and phenylacetonitrile via the janovsky reaction

Summary A colorimetric method for determination of phenylacetonitrile in amounts down to 0.6g is described. It depends on the colour reaction withm-dinitrobenzene and potassium hydroxide in 50% methanol. Microgram amounts of diphenylmethane have been determined by nitration and subsequent treatment with diethylamine, yielding an intense blue colour.

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Zusammenfassung Eine kolorimetrische Methode zur Bestimmung von Phenylacetonitril in Mengen bis herab zu 0,6g wurde beschrieben. Sie beruht auf der Farbreaktion mit Kaliumhydroxid undm-Dinitrobenzol in 50%iger Methanol-lösung. Diphenylmethan konnte durch Nitrierung und anschließende Farbreaktion mit Diäthylamin bestimmt werden.

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Résumé On décrit une méthode colorimétrique de dosage de phénylacétonitrile pour les quantités descendant jusqu'à 0,6g. Elle répose sur la réaction avec lem-dinitrobenzène en milieu alcalin. Un procédé est également décrit pour le dosage de microquantités de diphénylméthane, au moyen de la nitration. Le dérivé nitré donne une coloration bleue en milieu basique.

     

铊(I)离子选择电极动力学分析法测定超痕量铁

Fe(II) induces the reaction between Tl3+ and H2O2. The rate of reaction is linearly proportional to the concentration of Fe2+ in the range 2.5 ?10-9-2.5 ?10-8 mol dm-3 (20? and 5 ?10-9-5 ?10-8 mol dm-3 (15?. The standard deviation is less than 0.071 ?10-8. A 1000-fold excess of Zn2+, Cd2+, Mg2+, Ni2+, Pb2+, Ba2+, Ca2+, Li+, Na+, Ag+, NO3-, SO42-, AcO-, HPO42-, 500-fold excess of Al3+, Fe3+, Co2+, Hg2+ and 100-fold excess of Ti4+, Cr3+, Cu2+, Br-, Cl- can be tolerated, but reducing agents such as (NH2)2SO4, NH2OH.HCl interfered. This kinetic method was applied to determine Fe(II) in standard zinc sample and fountain water, with satisfactory results.     

Colorimetric detection of fluoride ion in an aqueous solution using a thioglucose-capped gold nanoparticle

The thioglucose-capped gold nanoparticles have been prepared by the chemical reduction of HAuCl₄ using thioglucose as the reducing and capping agent, which displays selective colorimetric detection of fluoride ion in 10 mM HEPES buffer at physiological pH.