Xu Wang

Amplified fluorescent aptasensor through catalytic recycling for highly sensitive detection of ochratoxin A

This paper describes a novel approach utilizing nano-graphite-aptamer hybrid and DNase I for the amplified detection of ochratoxin A (OTA) for the first time. Nano-graphite can effectively quench the fluorescence of carboxyfluorescein (FAM) labeled OTA specific aptamer due to their strong π-π; stacking interactions; while upon OTA addition, it will bind with aptamer to fold into an OTA-aptamerG-quadruplex structure, which does not adsorb on the surface of nano-graphite and thus retains the dye fluorescence. Meanwhile, the G-quadruplex structure can be cleaved by DNase I, and in such case OTA is delivered from the complex. The released OTA then binds other FAM-labeled aptamers on the nano-graphite surface, and touches off another target recycling, resulting in the successive release of dye-labeled aptamers from the nano-graphite, which leads to significant amplification of the signal. Under the optimized conditions, the present amplified sensing system exhibits high sensitivity toward OTA with a limit of detection of 20nM (practical measurement), which is about 100-fold higher than that of traditional unamplified homogeneous assay. Our developed method also showed high selectivity against other interference molecules and can be applied for the detection of OTA in real red wine samples. The proposed assay is simple, cost-effective, and might open a door for the development of new assays for other biomolecules. This aptasensor is of great practical importance in food safety and could be widely extended to the detection of other toxins by replacing the sequence of the recognition aptamer.

Magnified fluorescence detection of silver(I) ion in aqueous solutions by using nano-graphite-DNA hybrid and DNase I

This paper describes a novel approach utilizing nano-graphite-DNA hybrid and DNase I for the amplified detection of silver(I) ion in aqueous solutions for the first time. Nano-graphite can effectively quench the fluorescence of dye-labeled cytosine-rich single-stranded DNA due to its strong π-π stacking interactions; however, in the presence of Ag(+), C-Ag(+)-C coordination induces the probe to fold into a hairpin structure, which does not adsorb on the surface of nano-graphite and thus retains the dye fluorescence. Meanwhile, the hairpin structure can be cleaved by DNase I, and in such case Ag(+) is delivered from the complex. The released Ag(+) then binds other dye-labeled single-stranded DNA on the nano-graphite surface, and touches off another target recycling, resulting in the successive release of dye-labeled single-stranded DNA from the nano-graphite, which leads to significant amplification of the signal. The present magnification sensing system exhibits high sensitivity toward Ag(+) with a limit of detection of 0.3nM (S/N=3), which is much lower than the standard for Ag(+) in drinking water recommended by the Environmental Protection Agency (EPA). The selectivity of the sensor for Ag(+) against other biologically and environmentally related metal ions is outstanding due to the high specificity of C-Ag(+)-C formation. Moreover, the sensing system is used for the determination of Ag(+) in river water samples with satisfying results. The proposed assay is simple, cost-effective, and might open the door for the development of new assays for other metal ions or biomolecules.

Simple, Fast Matrix Conversion and Membrane Separation Method for Ultrasensitive Metal Detection in Aqueous Samples by Laser-Induced Breakdown Spectroscopy

A fast, low cost, and sensitive sample pretreatment method was specially developed for laser-induced breakdown spectroscopy (LIBS) based on metal precipitation and membrane separation for simultaneous elemental analysis in liquid samples. The metal elements were reacted with the chelating reagent 2,4,6-trimercapto-1,3,5-triazine (TMT) in the first step and separated by mixed cellulose ester microfiltration membrane subsequently. A specific membrane supporter with smaller aperture than the commercially available needle filter was designed and assembled in the presented research to increase the sensitivity. As a result, the detection limits of Cu, Ag, Mn, and Cr obtained in this research were 2.59 ng·mL(-1), 0.957 ng·mL(-1), 0.958 ng·mL(-1), and 1.29 ng·mL(-1) respectively, which were greatly improved from direct liquid analysis by LIBS. Satisfactory linearity, reproducibility, and accuracy were also obtained in the concentration range of 10-120 ng·mL(-1) for all four elements tested at the optimized experimental conditions. The analytical figures of merits of the proposed method are at an advanced level and are comparable to those reported among other non-LIBS research. In addition, the separation mechanism in this research was initially explored and further varied to be based on metal precipitates adsorption by membrane fibers, rather than a regular size-dependent obstruction.

Simultaneous and sensitive analysis of Ag(I), Mn(II), and Cr(III) in aqueous solution by LIBS combined with dispersive solid phase micro-extraction using nano-graphite as an adsorbent

In the present research, dispersive solid phase micro-extraction was combined with laser induced breakdown spectroscopy for the first time to achieve matrix conversion and preconcentration of Ag+, Mn2+ and Cr3+ in liquid samples. Nano-graphite was selected as a novel adsorbent due to its superior adsorption ability, water solubility and lower cost compared with carbon nanotubes and graphene. Sodium diethyldithiocarbamate was used as a complexing reagent to enhance the interaction between nano-graphite and metal ions. After adsorption, the nano-graphite powder was immobilized by mixing with commercial epoxy adhesive before LIBS analysis instead of the traditional tableting method to solve the problem that some carbonaceous materials are difficult to be tableted. Under optimized experimental conditions, the calibration curves were built in the range of 0.1–1.2 mg L−1 and good linearity (R squared better than 0.980 for all of the three target elements) can be obtained. The detection limits of Ag+, Mn2+ and Cr3+ were 16.59 ng mL−1, 10.85 ng mL−1 and 9.51 ng mL−1 respectively. In real sample analyses, the recoveries of three elements at different concentration levels were all in the range of 92.66–105.80% and the relative standard deviations of parallel samples were less than 8.98%.

Novel Signal-Enhancing Immunoassay for Ultrasensitive Biomarker Detection Based on Laser-Induced Fluorescence

An innovative signal-enhancing immunoassay for ultrasensitive biomarker detection based on laser-induced fluorescence (LIF) has been developed. A novel LIF optical system with high collection efficiency was constructed by using a parabolic mirror. Carboxyl-functionalized magnetic beads were used to immobilize antibody for achieving a conventional sandwich assay. Fluorescence from Rhodamine 6G (R6G)-labeled antibody was collected by the newly designed optical system. To reduce photobleaching of R6G under laser irradiation, ethanol instead of commonly used aqueous solution was used as assay buffer in the last stage. The newly developed LIF immunoassay displayed excellent analytical performance for α-fetoprotein (AFP) detection in the concentration range from 0.005 to 1.0 ng/mL with a detection limit of 0.0016 ng/mL. The detection limit obtained in this work is about 3 orders of magnitude better than that of conventional enzyme-linked immunosorbent assay (ELISA). In addition, the proposed method exhibited excellent precision, acceptable stability, and good reproducibility. Furthermore, the proposed immunoassay was successfully applied to AFP determination in real serum specimens. Therefore, the present immunoassay was demonstrated to be a powerful tool for ultrasensitive biomarker detection.

Dependence of pretilt angle on orientation and conformation of side chain with different chemical structure in polyimide film surface

A series of polyimide films were synthesized based on various side-chained diamines, 4-phenylphenyl-3,5-diaminobenzoate (DABB), 6-(4-phenylphenoxy)hexyl-3,5-diaminobenzoate (C6BB), 6-(4-(4-butoxylphenyl)phenoxy)hexyl-3,5-diaminobenzoate (C6BBC4) and 4-(4-butoxylphenyl-phenyl-3,5-diaminobenzoate (BBC4), the side chains of which contain a rigid biphenyl linked with flexible alkyl. The pretilt angle of liquid crystal molecules on the side-chained polyimide films based on C6BBC4 or BBC4 (PI-C6BBC4 or PI-BBC4) increased obviously with increasing side chain content and even achieved 87–89°. In contrast, the polyimide film based on C6BB or BB (PI-C6BB or PI-BB) even with a high side chain content could only achieve the homogeneous alignment. The properties and microscopic structure of the functionalized polyimide films were analyzed by polarized ATR-FTIR, computer simulations and surface energy etc. It was found that the side chains with different chemical structure get the different orientation and conformation in the outmost layer of polymer film, affecting the pretilt angle of liquid crystal molecules on this film surface. For PI-BBC4 and PI-C6BBC4, the side chains with the non-polar group at the tail take the vertical conformation in the outer layer of the polymer surface, which benefited the liquid crystal to get a large pretilt angle, even vertical alignment on the film surface. The side chain of PI-C6BB with the flexible alkyl spacer is equipped with good mobility. It is easy for its polar mesogen groups at the surface to take the conformation parallel to the polymer film, which leads to the formation of homogeneous alignment of LC molecules.

Study of the orientation of liquid crystal molecules on polyimide alignment films by FTIR with polarisation mode

The orientation of liquid crystal molecules on an alignment film was studied using Fourier transform infrared spectrometer (FTIR) with polarisation mode. The standard laboratory instrument was used to simultaneously determine the alignment direction and pretilt angle of the liquid crystal molecules. The dipole of the infrared active moiety of the rigid biphenyl of liquid crystal molecules can be used to represent their orientation, and the polarised light electrical vector coinciding with the dipole of the infrared active moiety increases in absorption intensity. Thereby, the infrared light absorbance of liquid crystal molecules in liquid crystal cells as a function of the polarised angle of incident light reveals the orientation of molecules, and the size of pretilt angle can be calculated. In particular, this method has no restriction on the range of pretilt angle.

Competitive immunoassay combined with magnetic separation and pulsed LIF system for cefalexin detection

In this study, a facile, ultrasensitive and interference-free method to detect cefalexin (CEX) was developed for the first time. This assay was carried out by covalently immobilizing cefalexin–ovalbumin (CEX–OVA) on high specific surface area amorphous nanoparticles of superparamagnetic iron oxide (SPIO). Here, the SPIO–CEX–OVA structure was rich in antibody domains for competitive immunological recognition to anti-CEX antibody and to AlexaFluor 488 labeled goat anti-mouse IgG. Compared with traditional laser induced fluorescence detection, the introduction of SPIO can preconcentrate analytes to reduce the detection limit and greatly shorten the assay time. A pulsed laser with higher peak energy was chosen as the excitation source for generating strong fluorescence signals and to improve sensitivity. The detection limit was 0.34 ng mL−1 with linearity in the range of 0.5 ng mL−1 to 50 ng mL−1, and the IC50 was 1.7 ng mL−1. The accuracy and reproducibility were determined by using spiked milk samples with three different concentrations of CEX (5, 20 and 50 ng mL−1). The recoveries of 85.2–111.4% were obtained with relative standard deviations of 5.3–9.1%, respectively. These results indicate that the method provides a pragmatic platform for convenient detection of small molecular residues due to its high sensitivity, selectivity and short assay time.

Increasing pretilt angle by grafting hexafluorobutyl acrylate into the surface of polyimide alignment films via electron beam irradiation

In this study, the polyimide (PI) alignment film whose surface contained fluorinated side chain was prepared by a new method. The fluorine compound, hexafluorobutyl acrylate (HFBA), was grafted into the surface of main-chained PI film as side chain via electron beam irradiation. The pretilt angle of liquid crystal (LC) was significantly improved from 2° to 20° or above. It was indicated by X-ray photoelectron spectroscopy spectra and attenuated total reflection Fourier transform infrared spectroscopy spectra that HFBA was successfully grafted into the thin surface layer of PI alignment films in the form of covalent bond. Scanning electron microscopy images showed that the irradiation process had no obvious influence on the surface morphology of the PI film. The surface energy of the PI film decreased from 49.85 to 31.50 mN/m after surface modification. Besides, the reason for the increase of pretilt angles was discussed.

Crystallization of inorganic silica based on interaction between polyimide and silica by sol–gel method

Crystalline silica in cristobalite phase was successfully prepared at a relatively low temperature of 800xa0°C by calcinating polyimide/silica hybrid films under ambient air. X-ray photoelectron spectroscopy measurements show that the product is silica after calcination. It is found that the crystallinity is dependent on the removing rate and the strength of the interaction between polyimide and silica. The presence of polyimide plays an important role in the growth of silica. Calcination to remove polyimide with rapid heating results in lower crystallinity compared with calcination with slow heating. For samples with the same content of silica, the crystallinity changes with the strength of the interaction between polyimide and silica molecules.