Use of a portable time-resolved fluorometer to determine oxytetracycline residue in four fruit crops

Automated method for the solid phase extraction of tetracyclines in wastewater followed by fluorimetric determination

This work presents a multisyringe flow injection analysis (MSFIA) system for the automatic extraction and determination of tetracyclines in wastewater samples. The sample was adjusted with Na2EDTA buffer before solid-phase extraction with an Oasis HLB column used for the analyte preconcentration. The europium (Eu3+)-based and citrate-mediated method (using Tris-HCl buffer) was selected for the fluorimetric analysis (λexc/em = 400/612 nm). For fluorescence detection, a low-cost system consisting of an USB 2000 CCD detector and a 3D-printed support that holds a LED light source was used. Under optimized conditions, the proposed method provided low limits of detection (9.4 μg L-1) and quantification (31 μg L-1), and good values for intra-day (<4 %) and inter-day precisions (<6 %). Recoveries of spiked TCs in wastewater samples ranged from 87 to 106 %. The results of this work were in good agreement with the measurements obtained by liquid chromatography coupled to a fluorescence detector.

A portable colorimetric point-of-care testing platform for MicroRNA detection based on programmable entropy-driven dynamic DNA network modulated DNA-gold nanoparticle hybrid hydrogel film

Point-of-care testing (POCT) platforms for microRNA (miRNA) detection have attracted considerable attention in recent years, due to the increasingly important role of miRNA as biomarkers for the diagnosis of many diseases, such as cancers. However, several limitations such as the requirement of enzyme-related amplification system, expensive preservation cost, sophisticated analysis instruments and tedious operations of conventional miRNA biosensing devices severely hinder their widespread applications. In this work, a portable and smart colorimetric analysis platform was developed by employing the ultrathin DNA-gold nanoparticle (AuNP) hybrid hydrogel film as the signaling unit and the enzyme-free entropy-driven dynamic DNA network (EDN) as the signal converter and amplification unit. By programming the DNA sequences of the EDN, the EDN could respond to a specific miRNA, with miRNA-155 or miRNA-21 as the model target, and release a converter DNA with amplified concentration to further trigger the release of AuNPs from the hydrogel film as a colorimetric signal output. To avoid the use of sophisticated spectral instruments, digital analysis based on primary three-color channel (R/G/B) was further introduced by using user-friendly camera and image processing software, and a detection limit at pM level was achieved. Moreover, by introducing H₂O₂-mediated AuNPs enlargement procedure in the colorimetric analysis platform, the detection limit for miRNA target could further be enhanced to fM level. The POCT platform is also portable and storable with a good storage stability for at least 45 days, suggesting its great potential in practical diagnosis applications.

Flavor classification and year prediction of Chinese Baijiu by time-resolved fluorescence

Baijiu is a traditional and popular Chinese liquor with enormous sale potential, which is affected by factors such as flavor and storage time. Chinese Baijiu is a complex and transparent mixture that makes analyzing difficult. The utility of time-resolved fluorescence helped to develop a new method to analyze Baijiu. Forty-two Baijiu samples among six brands with three flavors were prepared, and their fluorescence spectra were analyzed with an excitation light of 374.2 nm. Hexanoic acid and ethyl butyrate were found to have an impact on Baijiu fluorescence. The properties of lifetimes in Baijiu were investigated, and its mechanism was studied by calculations through density function theory. Using parameters of fluorescence lifetimes, Baijiu samples were classified according to their flavors. Additionally, the correlations between fluorescence lifetimes and storage time of Baijiu in Luzhou flavor were obtained, leading to a reliable and efficient method to establish the year forecast model of Chinese Baijiu with a mean error of 2.79 months. It also provides an important reference of the utility of time-resolved fluorescence for quantitative research of multi-component systems.

The degradation of oxytetracycline with ferrous oxalate under different light irradiation

Degradation of OTC under different light with Fe²⁺/C2O42- was investigated, and the reaction mechanism was also discussed. Although Fe(II/III)-C2O42- complex could weaken the inhibitory effect of Fe(II/III)-OTC complex on the detection of OTC by HPLC, the acidification could make inhibition become minimal. Response surface methodology was used to optimize the Fe²⁺/C2O42- dosage at low concentrations of Fe²⁺ and H₂C₂O₄. When the OTC concentration was 0.04 mM, the optimal dosage was OTC:Fe²⁺:H₂C₂O₄= 1:1.25:2 to attain a removal rate of 80% after 60 min under simulated solar light, and HO^• for degradation of OTC with Fe²⁺/C2O42+ could be 53%, furthermore, removal rate of OTC just increased 3 percentage points at 60 min when the simulated solar light changed into UV-254 nm, however, OTC could also be degraded for 48.89% removal rate after 3 h illumination of simulated visible light. The influence of Cl^-, CO32-, HCO3-, NO3-, NO2-, or H2PO4- on the degradation of OTC with Fe²⁺/C2O42- under simulated solar light was studied. NO2- could inhibit degradation for NO2- could react with free radicals, but the reason of inhibition of degradation by HCO3-, or CO32- was its influence on pH, whereas Cl^-, NO3- and H2PO4- had no influence on degradation. In addition, when the concentration of CO32- was higher than 4 mM, CO32- could promote the degradation for the direct photolysis of OTC at alkaline (pH > 10). Four products were detected by LC-MS, and the OTC degradation pathway was discussed.

Portable ratiometric probe based on the use of europium(III) coordination polymers doped with carbon dots for visual fluorometric determination of oxytetracycline

A novel ratiometric fluorescent probe for the determination of oxytetracycline (OTC) was developed. The method is based on the use of adenosine monophosphate/Eu(III) nanoscale coordination polymers doped with carbon dots (CDs) (CD@AMP/Eu NCPs). These were fabricated by self-assembly of Eu³⁺ and AMP on the surface of CDs containing large amounts of hydroxyl and carbonyl groups. Under the excitation at 310 nm wavelength, the doped NCPs display strong pink emission of Eu³⁺ at 615 nm and blue emission of the CDs at 430 nm on exposure to OTC. The ratio of fluorescence intensity (F₆₁₅/F₄₃₀) of such NCPs displays excellent linear relationship with OTC concentration ranging from 0.2 to 60 μM and the limit of detection (LOD) is 25 nM (3σ). The doped NCPs were evenly immobilized on common filter paper to prepare a visual ratiometric probe for the determination of OTC. Assisted by a digital camera with an APP color detector, the paper-based test strip was applied for the quantitative determination of OTC with a LOD of 0.5 μM and a wide linear scope of 1-100 μM. The method was applied to the determination of OTC in milk samples. Graphical abstractSchematic representation of the principle for oxytetracycline (OTC) determination using carbon dots (CD)@adenosine monophosphate (AMP)/Europium (Eu³⁺) paper-based ratiometic probe.

A Cathodic "Signal-off" Photoelectrochemical Aptasensor for Ultrasensitive and Selective Detection of Oxytetracycline

A novel cathodic "signal-off" strategy was proposed for photoelectrochemical (PEC) aptasensing of oxytetracycline (OTC). The PEC sensor was constructed by employing a p-type semiconductor BiOI doped with graphene (G) as photoactive species and OTC-binding aptamer as a recognition element. The morphological structure and crystalline phases of obtained BiOI-G nanocomposites were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The UV-visible absorption spectroscopic analysis indicated that doping of BiOI with graphene improved the absorption of materials in the visible light region. Moreover, graphene could facilitate the electron transfer of BiOI modified electrode. As a result, the cathodic photocurrent response of BiOI under visible light irradiation was significantly promoted when a suitable amount of graphene was doped. When amine-functionalized OTC-binding aptamer was immobilized on the BiOI-G modified electrode, a cathodic PEC aptasensor was fabricated, which exhibited a declined photocurrent response to OTC. Under the optimized conditions, the photocurrent response of aptamer/BiOI-G/FTO was linearly proportional to the concentration of OTC ranging from 4.0 to 150 nM, with a detection limit (3S/N) of 0.9 nM. This novel PEC sensing strategy demonstrated an ultrasensitive method for OTC detection with high selectivity and good stability.