Analytical Methods (v.10, #39)

Front cover (4725-4725).

Inside front cover (4726-4726).

Contents list (4727-4731).

A smartphone-based surface plasmon resonance platform by Chutiparn Lertvachirapaiboon; Akira Baba; Kazunari Shinbo; Keizo Kato (4732-4740).
Surface plasmon resonance (SPR) originates from collective oscillation of electron density and associated electromagnetic field at a metal/dielectric interface. Reflection SPR, which includes prism-coupling and grating-coupling methods, and transmission SPR (TSPR) have been developed for use in a smartphone sensing platform. The strength of this approach relies on the SPR phenomenon: the strong electromagnetic oscillation of electrons that can occur at a metal/dielectric interface and is highly sensitive to changes in the localized dielectric on the metal surface. The observable SPR signals are strong enough to detect with a simple camera. By combining the SPR phenomenon with the high performance of a smartphone camera, smartphone-based SPR platforms have been developed for use in on-site and remote detection of biomedical and environmental samples, point-of-care devices, and practical use in household devices. This review gives an overview of smartphone-based SPR platforms in both reflection SPR and TSPR configurations, the design of the devices, and the typical setup used to acquire spectroscopic information and SPR images, as well as providing some examples of their use in the analysis of chemical and biological samples.

Real-time assessment of food freshness in refrigerators based on a miniaturized electronic nose by Min Wang; Fan Gao; Qian Wu; Junyu Zhang; Yingying Xue; Hao Wan; Ping Wang (4741-4749).
Food freshness has been paid great attention due to its direct relationship with human health and safety, and approaches for food freshness evaluation have attracted much interest from researchers. In this paper, we developed a miniaturized electronic nose system for convenient, direct and real-time food freshness evaluation by analyzing gases in a refrigerator (4 °C). The proposed system consists of a gas sampling module and MOS gas sensor array. The gas sampling module was used to extract gases from the refrigerator and clean the gas path by controlling a pump and a three-way valve. The gas sensor array is composed of three MOS sensors to monitor odor changes in the refrigerator. Meanwhile, a food freshness assessment model was established based on the sensor array results and a comparison with human sensory evaluation results. In order to confirm the effectiveness of the system, we performed experiments on meat, vegetables and fruits with three freshness levels including fresh, semi-fresh and spoiled. The accuracy of the system to identify the three freshness levels is 84.8%, 68.0% and 96.2% respectively. The experimental results demonstrated that the developed electronic nose can effectively evaluate the food freshness level. Therefore, the proposed electronic nose provides a non-destructive, low cost and convenient platform for fast and real-time evaluation of food freshness in refrigerators.

A novel pyrene-switching aptasensor for the detection of bisphenol A by Huai-Yuan Niu; Meng-Ke Li; Cheng-Gang Niu; Jishan Li (4750-4755).
BisphenolA (BPA) is an environmental endocrine disrupter which is highly harmful to human health. Although a number of traditional analysis methods, such as high performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS), enzyme linked immunosorbent assay (ELISA) and other biochemical tests, have been used for the measurement of BPA, these analyses are limited by the availability of commercial antibodies, need for sophisticated equipment and tedious prior sample treatment. To address these limitations, we report herein the design, synthesis and application of a competition-mediated pyrene-switching aptasensor for the selective detection of BPA in buffer and actual water samples. The detection principle is based on the attachment of pyrene molecules to both ends of a hairpin DNA strand, which becomes the partially complementary competitor to an anti-BPA aptamer. The fact that two pyrenes in close proximity to each other can form an excimer plays an important role in this method. Triggered by BPA, the molecular probe can form a hairpin structure, resulting in a fluorescence emission shift from 400 nm (pyrene monomer) to 488 nm (pyrene excimer). This method exhibits a linear detection range from 0 to 2.00 ng mL−1 for BPA with a detection limit of 0.094 ng mL−1 (3σ). In addition, the method has high selectivity for BPA and can be effectively applied to actual environmental water samples.

Comparison of poly(styrene-divinylbenzene)-based monolithic and bead-based methodologies used in NANOFLOW LCMS for proteomic studies by Pei-Lun Tsai; Tsu-Yi Sung; Chinh-Yen Chong; Sheng-Yu Huang; Sung-Fang Chen (4756-4764).
Nanoflow liquid chromatography coupled with electrospray tandem mass spectrometry (nanoLC-ESI-MS/MS) is a powerful tool in proteomics analysis. The optimum conditions for preparing and the performance of a high efficiency polymeric column were compared with those of micro-particle-filled capillary columns, including a totally porous silica C18 column and a HALO® fused core C18 column. Tryptic peptides were used as model compounds for evaluating the performance of three in-house fabricated columns. After optimization, a monolithic capillary column was prepared by the in situ polymerization of styrene and divinylbenzene (PS–DVB) within a 50 μm i.d. fused silica capillary using 1-propanol as the porogen. These continuous unitary porous structures are more robust and efficient compared with bead-based columns. A meter level PS–DVB column could substantially reduce the co-elution of peptides and suppress ion suppression, thus permitting the total ion current signal to be significantly enhanced. These three columns were comparable for the routine identification of peptides. For glycopeptides, the monolithic PS–DVB column gave the highest separation efficiency and a total of 20 N-linked glycopeptides could be identified in a tryptic digest of fetuin and bevacizumab (Avastin). The results indicate that a PS–DVB column has great potential for separating hydrophilic peptides. The novel monolithic media described represent a promising addition to the stationary phase used in capillary columns for proteome research.

Nitrogen doped carbon nanocage modulated turn-on fluorescent probes for ATP detection in vitro and imaging in living cells by Xianjiu Liao; Jing Wu; Yan Du; Wei Peng; Yao Yao; Jeremiah Ong’achwa Machuki; Deqin Geng; Fenglei Gao (4765-4775).
In this work, nitrogen-doped carbon nanocage (N-CNC) modulated turn-on fluorescent probes were developed for selective detection of adenosine monophosphate (ATP), and their application for imaging of ATP in living cells was demonstrated. The as-synthesized N-CNC nanoprobes exhibited minimal background fluorescence due to the interaction between ssDNA and the N-CNCs. The aptamer was adsorbed onto the surface of N-CNCs forming an aptamer/N-CNC nanocomplex, and the fluorescence was quenched by the N-CNCs through Förster resonance energy transfer. In the in vitro assay, the introduction of ATP led to the dissociation of the aptamer from the N-CNCs’ surface. The limit of detection was as low as 5 μM in the range of 5 μM to 2 mM, and the nanocomplex was highly selective toward ATP. The in vivo studies suggested that the N-CNC nanoprobes showed excellent biocompatibility and successfully worked for specific, high-contrast imaging of ATP in living cells. Noncovalent binding between N-CNCs and oligonucleotides along with excellent solubility and biocompatibility have shown N-CNCs to be an efficient carrier and provide excellent protection for cellular delivery of genes. They may be an excellent candidate for many biological applications, such as gene and drug delivery, intracellular imaging, and in vivo molecular analysis.

The unique quality of Pu-erh tea is due to the process of pile fermentation that the tea leaves undergo, but this process can lead to the risk of producing aflatoxin B1 (AFB1), the most toxic of aflatoxins. The commonly used methods for the determination of AFB1 in foods are complex and time-consuming. In this paper, an accurate and rapid method for the determination of aflatoxin B1 in Pu-erh tea was developed by isotope dilution mass spectrometry. After extraction of AFB1 in Pu-erh tea using acidified acetonitrile (1% formic acid), a purification step was performed by the QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) method with primary secondary amine (PSA) and octadecylsilyl silica (C18) as the absorbents. The quantification of AFB1 was performed by liquid chromatography tandem mass spectrometry (LC-MS/MS) in positive mode (ESI+) and selective reaction monitoring (SRM) with 13C-labeled aflatoxin B1 (13C17-AFB1) as the internal standard in less than 5 min. A complete validation study was carried out after method optimization to ensure adequate performance of the developed method. The results showed good linear correlation in the range from 0.10 to 10.00 μg L−1 with a correlation coefficient (R2) > 0.99. The average recoveries of AFB1 from blank samples fortified at three levels (0.50, 5.00, and 20.00 μg kg−1) ranged from 81.4 to 100.8% with the relative standard deviations (RSDs, n = 6) between 1.7 and 4.8%. The limit of detection (LOD) was 0.50 μg kg−1. The validation method was applied to an analysis of AFB1 in 50 raw Pu-erh teas and 50 ripened teas. None of the investigated Pu-erh samples showed an AFB1 content above the LOD. The results indicate that people who are keen on drinking Pu-erh tea are not at risk from AFB1 exposure. The proposed method was demonstrably simple, rapid, highly sensitive and accurate, and was effective in the rapid qualitative and quantitative analysis of AFB1 in Pu-erh tea.

An electrochemical sensor based on Fe3O4@PANI nanocomposites for sensitive detection of Pb2+ and Cd2+ by Ying Kong; Tingting Wu; Di Wu; Yong Zhang; Yaoguang Wang; Bin Du; Qin Wei (4784-4792).
In this study, an easy-to-use electrochemical sensor was developed for the quantitative detection of lead ions (Pb2+) and cadmium ions (Cd2+). Core-shell ferroferric oxide@polyaniline (Fe3O4@PANI) nanoparticles were used as a sensitive platform for the detection of Pb2+ and Cd2+. Fe3O4 with large specific surface area could increase the sensitivity for detection. PANI as a conductive polymer with excellent electronic conductivity not only improved the electron transport speed in the detection process, but also produced good chelation with heavy ions. PANI contains a large number of amino and imino groups on its surface, thereby improving the detection sensitivity. Under optimal experimental conditions, the linear range for the detection of Pb2+ was from 0.1 to 104 nmol L−1; the detection limit was 0.03 nmol L−1, error limit of LOD was 1.1% and LOQ was 0.09 nmol L−1. The calculated LOD value of Cd2+ was 0.3 nmol L−1, the error limit of LOD was 1.5%, and LOQ was 0.9 nmol L−1. The prepared electrochemical sensor presented high sensitivity, good specificity and excellent stability; also, it is simple and easy to operate. This analysis strategy has a huge application prospect in the detection of other heavy metal ions.

Simultaneous ultrafast determination of six alkaloids in mainstream cigarette smoke by DART-MS/MS by Chao Li; E'xian Li; Yiqin Wu; Junheng You; Wei Liu; Zhuwen Cui; Lijun Li; Zhihua Liu; Yunhua QIN (4793-4800).
To determine nicotine, nornicotine, myosmine, anatabine, anabasine and nicotyrine in mainstream cigarette smoke simultaneously, a novel method based on Direct Analysis in Real Time (DART) Ionization technique combined with mass spectrometry was established by optimizing parameters such as injection rate, type of extraction solvent and extraction solvent dosage. Ten types of flue-cured tobacco samples were analyzed by the established method, and the results were compared with those obtained using a GC/MS method: (1) under optimized conditions (30 mL ultrapure water as the extraction solvent and with extraction rate of 0.6 mm s−1), the analysis could be completed within 20 s. (2) The linear range of the method was 0.002–1000 μg mL−1 with R2≥ 0.9957, the recovery ranged from 80.0% to 122.5%, and the limit of detection and the limit of quantitation were 0.28–5.77 ng mL−1 and 0.92–19.23 ng mL−1, respectively. (3) The relative error between DART-MS/MS and GC/MS methods was 0.24–9.62%. The established method is green, rapid, reliable and suitable for the determination of 6 alkaloids in mainstream cigarette smoke.

Comparison of the performances of several commonly used algorithms for second-order calibration by Yong Li; Hui-Wen Gu; Hai-Long Wu; Xiang-Yang Yu (4801-4812).
The present study compared six commonly used algorithms, namely, alternating trilinear decomposition (ATLD), self-weighted alternating trilinear decomposition (SWATLD), alternating coupled two unequal residual functions (ACTUF), parallel factor analysis (PARAFAC), damped Gauss-Newton (dGN) and algorithm combination methodology (ACM). Their performances with respect to the pre-estimated component number, convergence rate, collinearity, noise, small component and slight non-linearity (chromatographic peak drift) were considered. The results indicate that the difference in the performances of the algorithms arises from differences in the computational mode and the construction of loss functions. Among the six algorithms, ATLD, SWATLD, ACTUF and ACM were insensitive to excessive factors; the ability to overcome severe collinearity was in the order of ACM, ACTUF and dGN > SWATLD > ATLD and PARAFAC; PARAFAC, dGN and ACM showed a stronger ability to resist noise; PARAFAC was the most sensitive to a small component; and ATLD and SWATLD could overcome some effect of slight non-linearity (chromatographic peak drift). Additionally, ATLD was found to be the fastest algorithm, and dGN and ACM had faster convergence rates than PARAFAC. This study is helpful for understanding the essential difference in the performances of algorithms and can provide guidance for choosing suitable algorithms for the analysis of three-way data.

Inexpensive simple extraction of trace PAHs from water using PS-DVB monolithic beads by Visakha Chunhakorn; Phitchayapa Ratchathamma; Harry J. Whitlow; Orapin Chienthavorn (4813-4820).
A new simple and sensitive analytical technique based on polystyrene-divinyl benzene (PS-DVB) monolithic bead extraction followed by gas chromatography-mass spectroscopic detection (GC-MS) was developed for the determination of polycyclic aromatic hydrocarbons (PAHs) in water. The PS-DVB monolithic bead was fabricated and used as the adsorbent. The experimental parameters controlling the performance of the bead extraction were optimized. Calibration curves showed excellent linear fits, with r2 > 0.995 for all PAHs in the 2–30 000 ng mL−1 range. The limits of detection (LODs) ranged from 0.01 to 0.47 ng mL−1. Recoveries of the PAHs in environmental water samples spiked at a concentration of 500 ng mL−1 were 72–109%, while they were 69–114% for water spiked at 20 ng mL−1. Both intraday and interday repeatabilities were high, with RSDs for 16 PAHs spiked standard solutions being less than 10%. The present study indicated that the proposed method is well suited for the quantification of PAHs in environmental and drinking water samples.

An effective analytical system based on an ultraviolet atomizer for trace cadmium determination using atomic fluorescence spectrometry by Xin Yuan; Li Yang; Siyan Liu; Haiyan Yang; Yuyi Tang; Ke Huang; Mei Zhang (4821-4826).
A novel ultraviolet (UV) atomization atomic fluorescence spectrometry (UV-AFS) system was developed for the quantitative analysis of trace cadmium ions (Cd) without preconcentration. Cd2+ can transform into Cd0 and Cd hydride (CdH2) gas vapor after hydride generation (HG) with KBH4. With the help of a simple, low power and low temperature UV atomizer, CdH2 can be converted to Cd0 and thus the total Cd was detected. Due to the high atomization efficiency and low carrier gas dilution of the UV atomizer, the analytical system provides high sensitivity with a limit of detection (LOD) of 0.006 ng mL−1, which was comparable to those obtained by preconcentration methods. The relative standard deviation (RSD) (n = 11) was better than 2.1% at a concentration of 0.5 ng mL−1. Its application was demonstrated by analyzing Certified Reference Materials and food samples for ultra-trace detection of Cd2+.

Da-Huang-Xiao-Shi decoction (DHXSD) is a representative formula for treating jaundice and could have bright prospects owing to its liver protecting effects. DHXSD might synthetically result in a change in the metabolic profiles of cholestasis rats compared with that of normal rats after oral administration. Metabolic profiling analysis has gained significant attention ever since the field of metabolomics began to develop. In this study, a highly sensitive, rapid and high chromatographic resolution ultra-high performance liquid chromatography-hybrid quadrupole-Orbitrap mass spectrometry (UHPLC-Q-Orbitrap MS) combined with multivariate statistical analysis method was established to investigate the metabolic profiles of DHXSD in normal and alpha-naphthylisothiocyanate (ANIT)-induced cholestasis rats. In addition, serum biochemical indices and pathological observation analysis were utilized to evaluate the animal model. As a result, a total of 68 prototype compounds and metabolites in normal and cholestasis rats in vivo were screened and tentatively identified according to their exact mass and the relevant literature. Among these, 15 of the compounds were filtered out as potential chemical markers that may be responsible for the pharmacological effects of DHXSD, which offers a basic foundation for quality evaluation, pharmacokinetic research and the clinical safety of drug usage. In conclusion, this study provides an insight into the metabolism of DHXSD in vivo to enable understanding of the metabolic process and therapeutic mechanism of cholestasis.

A near-infrared carbon dioxide sensor system using a compact folded optical structure for deep-sea natural gas hydrate exploration by Zhiwei Liu; Chuantao Zheng; Chen Chen; Hongtao Xie; Qiang Ren; Weilin Ye; Yiding Wang; Frank K. Tittel (4838-4844).
The detection of dissolved gas (e.g. carbon dioxide (CO2) and methane (CH4)) in seawater is important for deep-sea natural gas hydrate exploration, which requires that the sensor especially the optical structure should be of compact size and capable of operation in a deep-sea environment. A compact optical structure with a simple beam alignment and tracing method was proposed for tunable laser absorption spectroscopy (TLAS) based gas measurements, in order to minimize the sensor size and ease the beam alignment/tracing procedure for deep-sea operation. A near-infrared CO2 sensor system was developed based on the compact optical structure. A distributed feedback (DFB) laser centered at 6361.3 cm−1 and a multi-pass gas cell (MPGC) with an effective optical path length of 29.8 m were employed. The sensor system was integrated as standalone equipment by customizing an aluminum baseplate for a stable field operation. A series of experiments were carried out to assess the performance of the sensor system. A limit of detection (LoD) of ∼7.1 parts-per-million in volume (ppmv) at a 0.4 s averaging time was obtained, and the LoD was reduced to ∼277 parts-per-billion in volume (ppbv) at an optimum averaging time of 153.6 s. Considering the gas mixing time, the rise and fall times were measured to be ∼290 s and ∼200 s, respectively. The proposed compact sensor structure provides the basis for the further development of a sensor system for dissolved CO2 detection with a LoD of ppbv via the use of a mid-infrared tunable laser.

Back cover (4845-4846).