Analytical Methods (v.10, #3)

Front cover (267-268).

Contents list (269-274).

A novel paper based colorimetric assay for the detection of TiO2 nanoparticles by Gonca Bulbul; Hamed Eskandarloo; Alireza Abbaspourrad (275-280).
In this communication, we propose a new paper-based platform functionalized with methylene blue (MB) for the detection and removal of titanium dioxide nanoparticles (TiO2 NPs). Our design took advantage of color changes of the functional paper due to the photocatalytic effect of TiO2 on MB. The paper-based colorimetric sensor displayed a good sensitivity for TiO2, showing great potential for practical applications.

Provenance classification of nephrite jades using multivariate LIBS: a comparative study by Jianlong Yu; Zongyu Hou; Sahar Sheta; Jian Dong; Wen Han; Taijin Lu; Zhe Wang (281-289).
Provenance classification of nephrite jades is important since the unit price of jade changes drastically with its geological origin. In the present work, a detailed comparison between commonly applied multivariate methods is conducted to classify nephrite samples from five different locations via their laser-induced breakdown spectroscopy (LIBS) spectra. Five multivariate methods including principal component analysis (PCA), one-step and pairwise partial least squares discriminant analysis (PLS-DA), linear discriminant analysis (LDA), and support vector machine (SVM) were applied to provide the classification information on the samples. PCA was used for rough classification while the classification performance of the other four methods was discussed in detail. The results show a training accuracy of 89.0%, 99.2%, 99.8% and 100%, and a testing accuracy of 76.8%, 97.8%, 92.8% and 99.3% for the PLS-DA (one-step), PLS-DA (pairwise), LDA and SVM algorithms respectively. The superior model nature and the selection of suitable characteristic lines for weight differences led to the high performance of SVM, showing an excellent applicability for the provenance classification of nephrite jades using LIBS spectra.

Probing specific gravity in real-time with graphene oxide plasmonics by Ainash Garifullina; Nikhil Bhalla; Amy Q. Shen (290-297).
Specific gravity (SG), the ratio of the density of a substance to the density of a reference material, is a standard indicator of the concentration of an analyte in a given solution. SG is routinely used for product quality assessment in the food industry. However, currently available commercial SG meters, such as hand-held refractometers and density meters, are highly sensitive to humidity and temperature, and do not allow real-time measurements. For these reasons, SG detection is often time-consuming which leads to unwanted interruptions in food manufacturing processes. Therefore, highly sensitive, label-free, and real-time sensors for the detection of SG are urgently needed for food quality control. In this context, we develop a graphene oxide (GO)-coated gold (Au) surface plasmon resonance (SPR) sensor, for the first time, to measure the SG of food samples in real-time. The SG values of sample solutions are correlated with the refractive indices (RI) of these solutions, which are captured by the SPR measurements, with a sensitivity of 105 SPR response units. Moreover, the use of a GO coating provides a strong enhancement of plasmonic resonances due to its optoelectronic properties, doubling the sensitivity of SPR response units per RI unit (2 × 105) when compared to conventional Au SPR chips (1 × 105). We also validate our sensor performance by measuring the SG in real food samples. Our results demonstrate a highly sensitive, efficient, high throughput, and reproducible approach for SG measurements in food industry settings, and open new opportunities to utilize improved SPR sensor technology for many other label-free analytical sensing applications.

Development of a novel parallel determination platform: a feasibility study tested on a chemiluminescence device by Chunjiao Yang; Zhongfeng Sun; Guojun Zhang; Lijuan Wang; Jie Zhang; Xin Zhang (298-307).
The potential of obtaining incremental diagnostic information using a parallel assay is attractive. Herein, a novel parallel determination platform was developed for the simultaneous determination of hormones, including human growth hormone (GH), prolactin (PRL), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Polystyrene was used as a support material and the device comprised an antibody capture area, a connecting channel, and an isolating bar. After immobilizing antibodies in the antibody capture area, 50 μL antigens and 1.5 mL of HRP-labelled antibodies were mixed and bound to the capture antibodies by flowing in the connecting channel continually. After adding chemiluminescence substrates, the isolating bar enabled the connecting channel to enter into the detection site in a 1 : 1 ratio. Fluorescein isothiocyanate-labelled bovine serum albumin (FITC-BSA) was used as an internal standard. The developed method was validated in terms of limits of detection (LOD), linearity, recovery and precision. The LOD were 0.01 mIU L−1, 0.01 IU L−1, 0.03 mIU L−1, 0.05 mIU L−1, and 0.01 IU L−1 for GH, FSH, TSH, PRL, and LH, respectively. The assay was linear up to 230 mIU L−1 for GH (r2 = 0.995), 300 IU L−1 for FSH (r2 = 0.991), 100 mIU L−1 for TSH (r2 = 0.997), 8000 mIU L−1 for PRL (r2 = 0.994), and 260 IU L−1 for LH (r2 = 0.992). The mean recoveries were calculated at two concentration levels and the values were found to be between 90.5% and 108.3%. The intra- and inter-assay coefficients of variation were <10% and <11%, respectively. The method was successfully applied to determine serum sample (n = 120) and the results were strongly correlated with the Siemens and Abbott immunoassay, showing a bias offset of 1.7 mIU L−1, 2.4 IU L−1, 0.1 mIU L−1, 7.6 mIU L−1 and 1.5 IU L−1 for GH, FSH, TSH, PRL, and LH. This sensitive and cost-efficient platform is expected to be a powerful tool to measure combinations of various biomarkers.

Chemistry of extracting high-contrast invisible fingerprints from transparent and colored substrates using a novel phosphorescent label by G. Swati; Swati Bishnoi; Paramjeet Singh; Naina Lohia; Vishnu V. Jaiswal; M. K. Dalai; D. Haranath (308-313).
Traditionally used fluorescent powders for developing invisible (latent) fingerprints involve complicated operation and show characteristics of auto-fluorescence interference and high toxicity. To overcome these serious drawbacks we report a novel application and facile methodology to extract high contrast fingerprints on non-porous and porous substrates using a chemically inert, visible light excitable, and nanosized SrAl2O4:Eu2+,Dy3+ phosphorescent label in the dark. The chemistry of non-covalent physisorption interaction between the long afterglow phosphor powder and sweat residue in fingerprints has been discussed in detail. Real-time fingerprint development on porous and non-porous substrates has also been performed.

A rationally designed polymer (RDP) capable of recognizing α-tocopherol and other minor components in sunflower oil has been produced. It is known that sunflower oil is a source of various physiologically active compounds. Unfortunately, they are present in very minor quantities which make their purification from the complex oil matrix problematic. An extraction method presented here was developed with particular attention to the selectivity, efficiency and precision of the extraction process. The methacrylic acid-based RDP in combination with the optimised purification method allowed the extraction of α-tocopherol with 94% recovery. The synthesised polymer was used successfully to extract α-tocopherol together with other essential minor components of sunflower oil without any pre-treatment step. According to GC/MS, the compounds ‘harvested’ from sunflower oil using the developed polymer included palmitic, oleic and linoleic acids, α-tocopherol, campesterol, stigmasterol, and β-sitosterol.

Layer-by-layer fabrication of g-C3N4 coating for headspace solid-phase microextraction of food additives followed by gas chromatography-flame ionization detection by Yixin Yang; Peige Qin; Xiaoting Zhang; Jiahua Niu; Shufang Tian; Minghua Lu; Jinhua Zhu; Zongwei Cai (322-329).
This work demonstrates a simple layer-by-layer approach for producing graphitic carbon nitride (g-C3N4) coated fibers for solid-phase microextraction (SPME) with sol-SiO2 as the binder. With the prepared g-C3N4 coated SPME fiber, a headspace SPME coupled with GC (HS-SPME-GC) method was developed for the analysis of food additives. Detection limits of 0.2–4.0 ng mL−1 were obtained for the analysis of five food additives including methyl cinnamate, ethyl cinnamate, benzyl cinnamate, isobutyl cinnamate, and butylated hydroxytoluene by the developed HS-SPME-GC method with flame ionization detection. The method exhibited good linearity in the range of 0.5–1000.0 ng mL−1 with the coefficient of determination (R2) not lower than 0.9930. The repeatability of the prepared coating was 2.2–5.4% (n = 3) for the same fiber, and the reproducibility was in the range of 5.3–12.4% for three different fibers. The recovery of the developed method was in the range of 73.3–109.8%. Based on the as-prepared g-C3N4 coating, the developed HS-SPME-GC method was successfully applied to the analysis of real samples including biscuits and milk tea beverages.

A simple and rapid pretreatment method based on effervescent tablet-assisted dispersive liquid–liquid microextraction was introduced for the determination of three fungicides (azoxystrobin, chlorothalonil, and cyprodinil) in fruit juice samples followed by liquid chromatography-ultraviolet detection. The effervescent tablet composed of sodium dihydrogen phosphate, sodium carbonate and 1-dodecanol was prepared in a simple way. The ingredients consisting of the above three components were manually blended in an empty medicine strip and cooled in a refrigerator. Hence, an effervescent tablet was obtained within 5 min without the need for a press machine thanks to the solidification of 1-dodecanol at room temperature. In situ generation of carbon dioxide during the microextraction process enabled the dispersion of the extractant within the aqueous phase and enhanced the contact area both between phases. Under the optimized conditions, the extraction recoveries ranged from 53 to 88%. Good linearity was observed by the square of correlation coefficients better than 0.9980. Relative standard deviations were less than 4.8% for intra-day and 7.6% for inter-day precision at a concentration of 3.0 μg L−1 of each analyte. Limits of detection ranged from 0.09 (chlorothalonil) to 0.19 μg L−1 (azoxystrobin). The proposed new method was successfully applied for the analysis of three fungicides in real samples of fruit juices and satisfactory recoveries (76–104%) were achieved.

A preconcentration method based on the use of graphene oxide (GO) functionalized with an ionic liquid (IL) was developed for trace Hg determination in water samples. The IL–GO hybrid nanomaterial was prepared by a simple procedure to functionalize GO with the IL 1-butyl-3-dodecylimidazolium bromide ([C4C12im]Br) and its performance as a sorption material for Hg was evaluated. A microcolumn filled with the IL–GO nanomaterial was used for preconcentration and determination of Hg followed by electrothermal atomic absorption spectrometry (ETAAS) detection. Mercury was retained at pH 5.0 and 20% (v/v) HNO3 was used for the elution of Hg from the microcolumn. The effects of different variables, including the sample volume, extraction time, sample flow rate, type and concentration of eluent and eluent flow rate were carefully studied. High retention efficiency (100%) was achieved with the proposed IL–GO sorption nanomaterial without the need for additional chelating reagents or derivatization reagents, which is an important advantage compared with traditional preconcentration methods. A sensitivity enhancement factor of 100 and a low detection limit of 14 ng L−1 were obtained under optimal experimental conditions. The proposed method can be considered as a simple, cost-effective and efficient alternative for Hg determination in water samples like river, rain, mineral and tap water.

In the present study, a ZnO functionalized graphene oxide (GO) modified glassy carbon electrode (GO–ZnO/GCE) was used for the electrochemical sensory detection of phenol. The prepared material was characterised using cyclic voltammetry (CV), square wave voltammetry (SWV), ultraviolet-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The parameters, such as the scan rate, pH, comparative study, stability, repeatability and interference, were optimised for the experimental study. The result of the study revealed that GO–ZnO exhibited favourable electrochemical behaviour for phenol oxidation, which was assigned to the joint efficiency of the ZnO and GO properties. An electrochemical sensor based on GO–ZnO exhibited excellent electrochemical performance towards the detection of phenol when compared to GO and bare GCE. The peak current demonstrated a linear relationship with phenol concentration in the range of 5–155 μM. Depending on the signal-to-noise characteristics (S/N = 3), the limit of detection for phenol was observed to be 2.2 nM. In addition, the electrochemical sensor showed excellent selectivity, stability and repeatability in the experimental studies. Based on our analysis, it can be considered that the GO–ZnO based phenol sensor has great potential in different applications requiring the detection of trace amounts of phenolic compounds.

An approach for Brilliant Blue (BB) detection was established based on fluorescence resonance energy transfer, where YVO4:Eu nanocrystals (NCs) are used as the energy donors and BB as the energy acceptor. BB could be electrostatically combined with the YVO4:Eu NCs and the fluorescence of the nanocrystals was quenched. Under optimal conditions, the logarithm of fluorescence intensity of the YVO4:Eu NCs was linearly proportional to the BB concentration in the ranges of 2.5 × 10−6–2.5 × 10−4 and 2.5 × 10−4–2.5 × 10−3 g L−1. The limit of detection was 1.0 × 10−6 g L−1. The method was successfully applied for BB detection in juices and candies. This method has a number of advantages, such as the facile synthesis of YVO4:Eu NCs, a simple and quick quantitative procedure and competitive analytical performance.

An aptamer-based electrochemical biosensor for simple and sensitive detection of staphylococcal enterotoxin B in milk by Xiaohui Xiong; Xinping Shi; Yuanjian Liu; Lixia Lu; Jingjing You (365-370).
Staphylococcal food poisoning is one of the most commonly encountered foodborne illnesses that results from the consumption of foods containing staphylococcal enterotoxins (SEs) produced by enterotoxigenic strains of Staphylococcus aureus. Among the SEs, staphylococcal enterotoxin B (SEB) is one of the most commonly presented serotypes in staphylococcal food-poisoning cases, and rapid, accurate, and reliable detection of SEB is necessary and significant to protect public health from SEB. Herein, a label-free and convenient aptamer-based electrochemical biosensor for SEB detection was explored. In the absence of SEB, only the aptamer and mercaptohexanol (MCH) assembled on electrode surfaces, resulting in an efficient electron transfer and a small faradaic impedance response of [Fe(CN)6]3−/4−. Upon the addition of SEB, SEB assembled on gold electrode surfaces owing to the high specific recognition and affinity constants of the aptamer and SEB. In this state, the electron transfer of [Fe(CN)6]3−/4− was greatly inhibited, due to SEB on gold electrode surfaces which prevents [Fe(CN)6]3−/4− from accessing the electrode surface for efficient electron transfer, and a large faradaic impedance response of [Fe(CN)6]3−/4− was obtained. [Fe(CN)6]3−/4− was used as the redox probe to evaluate the interfacial electron transfer resistance of the biosensor, and the change in Ret against the logarithm of SEB concentration was found to be linear over the range from 0.5 ng mL−1 to 500 ng mL−1, with a detection limit of 0.17 ng mL−1 (S/N = 3). In order to demonstrate the applicability and reliability of the proposed method in complex matrices such as milk samples, the results of this assay and the ELISA kit were compared. The relative percentage error between the two methods ranged from −8.38 to 8.33, which indicates that there is no significant difference between the results obtained by the two methods. It also indicates that the proposed method holds great promise for real sample detection.

The good therapeutic effect of herbal medicines depends on their abundant components and it's extremely necessary to find out the main bioactive ingredients. In this paper, the extract of Dysosma versipellis and Glycyrrhiza uralensis was studied for the first time by chemometrics. A HPLC-UV method was developed and validated to establish fingerprint spectra of 46 batches of different samples and a total of 45 common components of all samples were quantitatively and qualitatively analyzed using HPLC-UV and UPLC-Q-TOF-MS/MS, respectively. The anticancer effect of the extract was obtained by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay on HeLa cells. After that, a support vector regression (SVR) model optimized by particle swarm optimization (PSO) was constructed to depict the relationship between the chemical constituents and anticancer effect of the extract. Then the mean impact value (MIV) method was introduced to evaluate the bioactivity of the concerned components based on the optimal SVR–PSO model. The results showed that the developed model has an excellent fitting accuracy and generalization ability, and a ranking of the components for their anticancer activity was obtained. The employed strategy provides an efficient and convenient access to active anticancer constituents from the extract of D. versipellis and G. uralensis. The identified components provide explicit guidance for screening anticancer compounds and the developed model can be used for predicting the activity of new samples.

A non-enzymatic glucose sensor based on the CuS nanoflakes–reduced graphene oxide nanocomposite by Xiaoyi Yan; Yue Gu; Cong Li; Bo Zheng; Yaru Li; Tingting Zhang; Zhiquan Zhang; Ming Yang (381-388).
In this work, a copper sulfide nanoflakes–reduced graphene oxide (rGO/CuSNFs) nanocomposite has been successfully synthesized via one-pot hydrothermal treatment, where the reduction of GO and in situ generation of CuS nanoflakes occurred simultaneously. Subsequently, a nonenzymatic electrochemical sensor was fabricated based on the disposable rGO/CuSNFs nanocomposite for the highly sensitive and selective detection of glucose. The prepared rGO/CuSNFs nanocomposite was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, respectively. Under optimized conditions, the prepared enzymeless sensor exhibited remarkable catalytic activity toward the glucose oxidation with a fast response time of <6 s, a wide linear range from 1 to 2000 μM, a high sensitivity of 53.5 μM (cm2 mM)−1 and a low detection limit of 0.19 μM. Furthermore, the rGO/CuSNFs/GCE also showed the great reproducibility, the long-term storage stability and the excellent anti-interference ability for glucose sensing. In addition, the as-prepared sensor was applied to detect glucose in human urine and blood serum samples with satisfactory results, indicating that the rGO/CuSNFs nanocomposite is an extremely promising material for non-enzymatic glucose sensing in practical samples.

The increment of infections with multi-drug resistant gram-negative bacteria together with the high attrition rate of new antibacterial development programs has led to the renaissance of colistin as a new hope. However, the current administration of colistin to humans requires pharmacokinetic drug monitoring to individualize its posology, avoiding the development of resistant bacteria and the attainment of toxic concentrations. In this context, accurate, precise and selective methodologies are required to determine colistin plasma concentration. The present work is aimed at developing and fully validating a new high-performance liquid chromatography with fluorescence detection assay for the quantification of colistin in plasma samples of hospitalized patients. The chromatographic separation of colistin and an internal standard was achieved using a C18 column with a mobile phase comprised of acetonitrile and water. The detector was set at excitation/emission wavelengths of 343/500 nm and the retention time of the drug was shorter than those reported using other analytical techniques. The method was revealed to be linear in the concentration range of 0.09–9.00 μg mL−1 (which includes the therapeutic range of colistin), precise (coefficient of variance ≤ 6.4%), accurate (bias ≤ 14%) and selective. After full validation, the method successfully quantified the total colistin in plasma from patients treated with colistimethate sodium.

Back cover (397-398).