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A dual-signal sensor array for highly sensitive identification of biothiols is reported based on different optical responses of MnO2/curcumin (CUR) system to different biothiols. The addition of MnO2 nanosheets (MnO2 NSs) quenches the fluorescence of CUR, and the color of the mixture changes from yellow to brown. In the presence of reductive biothiols, MnO2 NSs are etched and lose their fluorescence quenching ability, resulting in an increase in the fluorescence intensity of CUR at 540 nm and a decrease in the absorbance at 430 nm. The sensor array generates specific response modes based on the varying reduction abilities of different biothiols, which can be distinguished by linear discriminant analysis (LDA). The sensor array successfully distinguished five biothiols (glutathione (GSH), dithiothreitol (DTT), cysteine (Cys), mercaptoethanol (ME), and homocysteine (Hcy)) across a wide concentration range (1 μM-100 μM) and biothiol mixtures with varing molar ratios. [ABSTRACT FROM AUTHOR]

A colorimetric method is described for the determination of uric acid (UA). The assay is based on oxidative etching of silver nanoparticles (AgNPs) by chloroauric acid (HAuCl4). The presence of UA suppresses the redox reaction between AgNPs and HAuCl4 because a competitive reaction occurs between HAuCl4 and UA. This results in a color change of the solution from brown to yellow. In parallel, the absorbance is blue shifted from 477 to 428 nm. The method has a detection limit as low as 30 pM (at S/N = 3) and a linear response range that covers the 0.1 nM to 0.1 mM UA concentration range. The reliability of the method was successfully demonstrated by analyzing spiked serum samples. [ABSTRACT FROM AUTHOR]

We report on a method for highly sensitive and selective colorimetric determination of Hg(II) via a signal amplification strategy. Cu@Au nanoparticles (NPs) are found to exhibit intrinsic peroxidase-like activity and can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine by HO. This is accompanied by a solution color change from colorless to green (with an absorption peak at 655 nm). The catalytic capability of the Cu@Au NPs (pale green) is strongly enhanced by a Cu@Au-Hg trimetallic amalgam (bluish), and this effect can be applied directly to the determination of Hg(II). The limit of detection as observed with the unaided eye is 10 nM, which is at least one order of magnitude lower than that of the known AuNP-based colorimetric assay. Due to excellent specificity of the amalgamation process, the assay is highly selective for Hg(II) and is not interfered by other metal ions in up to 0.5 μM concentrations. This assay was successfully applied to the determination of Hg(II) in tap water. In view of these advantages, we expect this colorimetric method to become an attractive tool for the quantitation of Hg(II) in biological, environmental, and food samples. [ABSTRACT FROM AUTHOR]

We report on a probe for the colorimetric and bare-eye detection of dopamine (DA). The optical effect is based on the finding that DA inhibits the melamine-induced aggregation of red gold nanoparticles (AuNPs) to form blue AuNP clusters. We presume that the aggregation induced by melamine is due to its strong hydrogen-bonding interactions with DA. The color changes can be monitored by spectrophotometry or the bare eye. The probes works over the 33 nM to 3.33 mM DA concentration range, and levels down to 33 nM can be quantified. The nanoprobe was successfully applied to the determination of DA in spiked serum. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

We are presenting a colorimetric assay for mercury (II) ions. It is based on citosan-functionalized gold nanoparticles (AuNPs) that act as a signaling probe. Hg (II) induces the aggregation of the chitosan-AuNPs through a chelation reaction that occurs between chitosan and Hg (II). This results in a strong decrease of the absorbance of the modified AuNPs and a color change from red to blue. This sensing system displays excellent selectivity over other metal ions and a detection limit as low as 1.35 μM which is lower than the allowed level of Hg (II) in drinking water (30 μM) as defined by World Health Organization. The method is inexpensive, facile, sensitive, and does not require the addition of other reagents in order to improving sensitivity. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

A highly sensitive colorimetric assay is described for the determination of glutathione (GSH). It is based on the use of tungsten disulfide (WS2) which is a peroxidase mimic. It catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by hydrogen peroxide to form a blue-colored product (oxTMB) at near neutral pH values. In the presence of the analyte GSH, it is transformed to its oxidized form (GSSG), while the blue oxTMB is increasingly reduced and eventually is converted to colorless TMB. This can readily be detected with bare eyes. GSH can be determined spectroscopically (at 652 nm) by this method at concentrations down to 61 pM, and the response is linear in the 100 pM to 10 nM GSH concentration range. The assay is cost-effective and simple. Conceivably, it provides a promising tool for the determination of GSH in food and medical samples. Graphical abstract Schemaric of a sensitive method for colorimetric detection of glutathione (GSH) based on WS2-catalyzed oxidation of TMB by H2O2 and the reduction of blue oxTMB by GSH. [ABSTRACT FROM AUTHOR]

A colorimetric approach is presented for the determinaton of cadmium(II) using unmodified gold nanoparticles (AuNPs) as a colorimetric probe. The assay is based on AuNP aggregation that is induced by the complexes formed between Cd(II) and glutathione (GSH) in a concentrated solution of NaCl. Unmodified AuNPs are known to aggregate in high-salt medium, but GSH can prevent aggregation. In the presence of Cd(II), it will bind GSH, and this will cause the AuNPs to aggregate as indicated by yellow and red dots under dark-field microscopy observation and the formation of a blue coloration. By monitoring the intensity change of AuNPs (as a ratio of absorbances at 600 and 520 nm), Cd(II) can be quantified with a linear response in the 17 pM to 16.7 nM concentration range and a detection limit of 4.3 pM. The method was successfully applied to the determination of Cd(II) in spiked lake water by the standard addition mode, and the detection limit is 4.5 pM.A ultrasensitive colorimetric assay of cadmium ions using unmodified gold nanoparticles as colorimetric probes with dark-field microscopy [ABSTRACT FROM AUTHOR]