Development of Fluorescent Aptasensors Based on G-Quadruplex Quenching Ability for Ochratoxin A and Potassium Ions Detection.

G-quadruplexes have received significant attention in aptasensing due to their structural polymorphisms and unique binding properties. In this work, we exploited the fluorescence-quenching properties of G-quadruplex to develop a simple, fast, and sensitive platform for fluorescence detection of ochratoxin A (OTA) and potassium ions (K+) with a label-free fluorophore and quencher strategy. The quenching ability of G-quadruplex was confirmed during the recognition process after the formation of the G-quadruplex structure and the quenching of the labeled fluorescein fluorophore (FAM). The fluorescence-quenching mechanism was studied by introducing specific ligands of G-quadruplex to enhance the quenching effect, to show that this phenomenon is due to photo-induced electron transfer. The proposed fluorescence sensor based on G-quadruplex quenching showed excellent selectivity with a low detection limit of 0.19 nM and 0.24 µM for OTA and K+, respectively. Moreover, we demonstrated that our detection method enables accurate concentration determination of real samples with the prospect of practical application. Therefore, G-quadruplexes can be excellent candidates as quenchers, and the strategy implemented in the study can be extended to an aptasensor with G-quadruplex.

A dual-mode nanoprobe for the determination of parathion methyl based on graphene quantum dots modified silver nanoparticles.

We developed a highly sensitive and selective method for double-signal analysis (fluorescence and ultraviolet-visible spectrophotometry) of organophosphorus pesticides (OPs), based on reversible quenching of graphene quantum dots (GQDs; fluorophores) with silver nanoparticles (AgNPs; absorbers). We used acetylcholinesterase to catalytically convert acetylthiocholine into thiocholine. In turn, by competitive binding to the AgNPs, the produced thiocholine displaces AgNPs from the GQDs and thus induces fluorescence recovery. However, OP analytes inhibit the activity of acetylcholinesterase and, in so doing, retain the silver-graphene nanoparticle complex and fluorescence quenching. The degree of quenching is proportional to the concentration of OPs; the detection limit is as low as 0.017 µg/L. The ultraviolet-visible absorption of GQDs/AgNPs at 390 nm decreases-because of AgNP aggregation that occurs after desorption from the GQDs-and the absorbance is linearly proportional to the OP concentration. Our system has good selectivity to substances that are commonly present in water and vegetables. We successfully applied our method to OP analysis in water, apple, and carrot samples.

Ultrasensitive detection of uric acid in serum of patients with gout by a new assay based on Pt@Ag nanoflowers.

A ultrasensitive assay for the determination of uric acid (UA) based on Pt@Ag nanoflowers (Pt@Ag NFs) was constructed. H2O2 was formed by the reaction of uricase and UA and produced the hydroxyl radical (˙OH). The system was catalyzed by Pt@Ag NFs to change the color of 3,3',5,5'-tetramethylbenzidine (TMB) from colorless to blue, and the morphology and chemical properties of Pt@Ag NFs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Under the optimized conditions, a linear relationship between the absorbance and UA concentration was in the range of 0.5-150 µM (R 2 = 0.995) with a limit of detection of 0.3 µM (S/N = 3). The method can be applied to detection of UA in actual samples with satisfactory results. The proposed assay was successfully applied to the detection of UA in human serum with recoveries over 96.8%. Thus, these results imply that the UA assay provides an effective tool in fast clinical analysis of gout.

Synchronous fluorescence and UV-vis spectroscopic studies of interactions between the tetracycline antibiotic, aluminium ions and DNA with the aid of the Methylene Blue dye probe.

Synchronous fluorescence spectroscopy (SFS) was applied for the investigation of interactions of the antibiotic, tetracycline (TC), with DNA in the presence of aluminium ions Al3+. The study was facilitated by the use of the Methylene Blue (MB) dye probe, and the interpretation of the spectral data with the aid of the chemometrics method, parallel factor analysis (PARAFAC). Three-way synchronous fluorescence analysis extracted the important optimum constant wavelength differences, deltalambda, and showed that for the TC-Al3+-DNA, TC-Al3+ and MB dye systems, the associated deltalambda values were different (deltalambda=80, 75 and 30 nm, respectively). Subsequent PARAFAC analysis demonstrated the extraction of the equilibrium concentration profiles for the TC-Al3+, TC-Al3+-DNA and MB probe systems. This information is unobtainable by conventional means of data interpretation. The results indicated that the MB dye interacted with the TC-Al3+-DNA surface complex, presumably via a reaction intermediate, TC-Al3+-DNA-MB, leading to the displacement of the TC-Al3+ by the incoming MB dye probe.

Synchronous fluorescence, UV-visible spectrophotometric, and voltammetric studies of the competitive interaction of bis(1,10-phenanthroline)copper(II) complex and neutral red with DNA.

Constant wavelength synchronous fluorescence spectroscopy (CW-SFS), UV-visible absorption spectroscopy, and cyclic and differential pulse voltammetry were applied to investigate the competitive interaction of DNA with the bis(1,10-phenanthroline)copper(II) complex cation ([Cu(phen)(2)](2+)) and a fluorescence probe, neutral red dye (NR), in a tris-hydrogen chloride buffer (pH 7.4). The results show that both the [Cu(phen)(2)](2+)and the NR molecules can intercalate competitively into the DNA double-helix structure. The cyclic voltammetry method showed that both anodic and cathodic currents of [Cu(phen)(2)](2+) decreased on addition of the DNA and the intercalated [Cu(phen)(2)](2+)-DNA complex formed (beta = (4.14 +/- 0.24) x 10(3)). CW-SFS measurements were facilitated by the use of the three-way resolution of the CW-SFS for NR, [Cu(phen)(2)](2+), and NR-DNA. The important constant wavelength (CW) interval, Deltalambda, was shown to vary considerably when optimized (135, 58, and 98 nm for NR, NR-DNA, and [Cu(phen)(2)](2+), respectively). This approach clearly avoided the errors that otherwise would have arisen from the common assumption that Deltalambda is constant. Furthermore, a chemometrics approach, parallel factor analysis (PARAFAC), was applied to resolve the measured three-way CW-SFS data, and the results provided simultaneously the concentration information for the three reaction components, NR, [Cu(phen)(2)](2+), and NR-DNA, for the system at each equilibrium point. The PARAFAC analysis indicated that the intercalation of the [Cu(phen)(2)](2+) molecule into the DNA proceeds by exchanging with the NR probe and can be attributed to two parallel reactions. Comprehensive information was readily obtained; the replacement of the intercalated NR commenced immediately on introduction of [Cu(phen)(2)](2+), approximately 50% of NR was replaced by [Cu(phen)(2)](2+) at a concentration of 0.45 x 10(-5) mol L(-1), and nearly all of the NR was replaced at a [Cu(phen)(2)](2+) concentration of 2.50 x 10(-5) mol L(-1). This work has the potential to improve extraction of information from the fluorescence intercalator displacement (FID) assay.

Synchronous fluorescence and UV-vis spectrometric study of the competitive interaction of chlorpromazine hydrochloride and Neutral Red with DNA using chemometrics approaches.

In this study, we have shown with the use of UV-vis spectrophotometry and the constant wavelength synchronous fluorescence spectroscopy (CW-SFS) techniques that the pharmaceutical drug, chlorpromazine hydrochloride (CPZ), intercalates into the deoxyribonucleic acid (DNA) double helix by partial exchange with the Neutral Red (NR) molecular probe. We have also demonstrated that with the use of three-way data plots, it is clear that it is important to have well-defined methodology for the selection of the important CW-SFS method parameter, Deltalambda. Ad hoc selection of this parameter, or even that based on experience, can readily lead to serious errors, which subsequently can be transferred to the interpretation of results. The said three-way plots provide a straightforward diagrammatic method, which improves the selection process of a satisfactory value for Deltalambda. Finally, we used PARAFAC modeling to resolve the complex three-way CW-SFS data, which provided simultaneously the concentration information for the three reaction components, NR, CPZ and NR-DNA, for the system at equilibrium. This PARAFAC analysis indicated that the intercalation of the CPZ molecule into the DNA proceeds by exchanging with the NR probe, and can be attributed to two parallel reactions.