Analytical Methods (v.10, #15)

Front cover (1665-1666).

Contents list (1667-1672).

Fast determination of fipronil and its metabolites in seafood using PRiME pass-through cleanup followed by isotope dilution UHPLC-MS/MS by Yun Zhang; Yong-Gang Zhao; He-Li Cheng; Nadeem Muhammad; Wei-Sheng Chen; Xiu-Qiong Zeng; Yan Zhu (1673-1679).
A simple and effective method based on the PRiME (process, robustness, improvements, matrix effects, ease of use) pass-through cleanup procedure followed by ultra-high performance liquid chromatography-tandem quadrupole mass spectrometry (UHPLC-MS/MS) was developed for analyzing fipronil and its metabolites in seafoods. Matrix effects in seafoods were effectively improved. Validation results demonstrated that this method was suitable for monitoring fipronil residues in seafoods.

A Ni3N–Co3N hybrid nanowire array electrode for high-performance nonenzymatic glucose detection by Xinglu Dai; Wenqing Deng; Chao You; Zhen Shen; Xiaoli Xiong; Xuping Sun (1680-1684).
In this communication, we report a new type of nickel cobalt nitride hybrid nanowire array in situ grown on a Ti plate (Ni3N–Co3N NW/Ti) topotactically converted from Ni–Co hydroxide as a high activity catalyst electrode for glucose electrooxidation in alkaline media. As a non-enzymatic glucose sensor, such a Ni3N–Co3N NW/Ti electrode exhibits superior electrochemical sensing performance with a short response time of less than 5 s, a wide detection range of 0.1 μM to 4.0 mM, a low detection limit of 30 nM (S/N = 3), and a high sensitivity of 4418.7 μA mM−1 cm−2, as well as satisfactory selectivity and reproducibility. Furthermore, it also performs efficiently for glucose detection in human blood serum, indicating that this novel electrode is a promising candidate for nonenzymatic glucose sensing.

Quantification of cocaine and its adulterants by nuclear magnetic resonance spectroscopy without deuterated solvents (No-D qNMR) by Willy W. F. Rocha; Júlia de A. Leite; Radigya M. Correia; Flávia Tosato; Natã C. L. Madeira; Paulo R. Filgueiras; Valdemar Lacerda; Jair C. C. Freitas; Wanderson Romão; Álvaro C. Neto (1685-1694).
In this study, a new method was developed to quantify cocaine and some adulterants (lidocaine, caffeine, phenacetin, procaine and benzocaine) using nuclear magnetic resonance spectroscopy without the use of deuterated solvents (No-D qNMR). No-D qNMR presents as main advantages: low cost (in comparison to the use of deuterated solvents), non-destructive and rapid analysis, and being able to detect non-volatile compounds. For analytical validation achievement, figures of merit such as selectivity and specificity, linearity, quantification limit, detection limit, accuracy, precision and robustness were obtained. The built models presented excellent precision (<5%), linearity (>0.99), and LOD values which varied between 0.126 and 0.666 mg mL−1. With the purpose of verifying the predictive capacity of the models, 34 crack samples seized by the Civil Police of Espírito Santo State – Brazil were analyzed, and the average cocaine content found was around 17.5 wt%, which is in line with expectations (up to approximately 30 wt%). Moreover, 50 wt% of the samples contained phenacetin, 9 wt% caffeine, and 3 wt% procaine and lidocaine, while benzocaine wasn't identified.

Ultrasensitive and direct fluorescence detection of RDX explosive vapor via side-chain terminal functionalization of a polyfluorene probe by Xin Peng; Huan Liu; Ao Liu; Wei Xu; Yanyan Fu; Qingguo He; Huimin Cao; Jiangong Cheng (1695-1702).
3,5-Trinitroperhydro-1,3,5-triazine (RDX), even more devastating than TNT but with an extremely low saturated vapor pressure (4.85 ppt), is often used in military and even terrorist attacks, and so there is a huge demand for its field detection. Conventional detection methods can hardly detect RDX, and no direct vapor detection has been reported based on a fluorescence method because RDX has no aromatic rings and hence it does not have a strong enough interaction with the fluorescent probe. By the introduction of Boc-NH– into the side chain terminal of polyfluorene, a conjugated polymer probe, PF6-Boc, has been designed and synthesized. The probe showed 91% fluorescence quenching in saturated RDX vapor and retained a 66% quenching response even after 8 cycles. A respectable ultrasensitive detection limit of 0.041 ppt was obtained. From simulated landmine detection experiments (RDX buried under sand), the probe showed a significant black quenching spot on the fluorescent filter paper within 5 min and the quenching spots became darker over time, even though the explosives were isolated in the soil and the air flow had a negative effect. The ultra-sensitive sensing performance, reusability and excellent performance in simulation experiments all demonstrate the great potential of PF6-Boc for the direct and on-site detection of RDX. The concept of the functionalization of the side chains of conjugated polymers to facilitate the PET process provides a new route for the detection of very low vapor pressure explosive vapors.

In this paper, a simple stereoselective sensing platform for the recognition of ascorbic acid (AA) and isoascorbic acid (IAA) was fabricated via immobilizing HS-β-cyclodextrin (HS-β-CD) on the surface of nanocomposite modified glassy carbon electrodes (GCEs). The nanocomposites were prepared using Pt and Pd bimetal nanowires (Pt–PdNWs) and reduced graphene oxide (rGO). The preparation process was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). After the modified electrodes interacted with the analytes, obvious differences in the peak currents of AA and IAA were obtained, hinting at the ability of the proposed sensor to recognize AA and IAA. This sensor with simple preparation, a rapid response, and excellent stability and reproducibility provided a promising perspective to recognize and determine AA and IAA.

Salicylidene Schiff bases (SASBs) have attracted significant attention because they have unique properties and can be simply synthesized by the condensation reactions; however, their poor fluorescence in solution makes them unable to be used as a fluorescent turn-off probe and limits their application fields. Herein, we have endeavoured to design and synthesize salicylidene Schiff bases with strong fluorescence in solution by structural modification. We found that although 3,5-dibromosalicylidene 2,3,4,5,6-pentafluoroaniline (DBPFA) showed very poor luminescence properties in solution, when it was reacted with NaOH to afford DBPFA-I at the same concentration, the emission intensity of DBPFA-I was 3890 times that of DBPFA. DBPFA-I could be used for detection of the nerve agent sarin mimic diethyl chlorophosphate (DCP) via nucleophilic attack of phenolic anion to DCP. The detection limit of the probe could be as low as 1.94 nM, which was much lower than other detection results. In addition, a paper strip device of DBPFA-I became non emissive immediately (in <15 s) upon exposure to DCP vapour (132 ppm (v/v)) with excellent selectivity. The sensing mechanism was explored by analyzing the sample via mass spectrometry.

Assay staining is an essential step to produce visual signals for the development of colorimetric biochips; it is, therefore, of practical importance to evaluate the conventionally adopted staining methods in terms of their quantitative performance in screening immunoassay biochips. Particularly, we have performed a systematic comparison of the color development speed, sensitivity, response range, and aging effects of three common staining protocols, i.e., gold nanoparticle-catalyzed silver deposition (silver staining), horseradish peroxidase (HRP)-catalyzed oxidation of tetramethylbenzidine (TMB staining), and alkaline phosphatase (ALP)-catalyzed reduction of tetrazolium salt (TAS staining). Our results have revealed that silver staining can afford the best screening performance in terms of sensitivity, response range, and aging effect. In comparison, TMB staining is fast in producing quantitative results (5–10 min). TAS staining can provide easily saturated signals with a low background, which is suitable for semi-quantitative or qualitative on-site applications. More importantly, we have demonstrated that the three staining methods can be adapted to the same biochip to fabricate “multichannel” colorimetric biochips for screening multiple tumor markers simultaneously.

Measuring the size and density of nanoparticles by centrifugal sedimentation and flotation by Caterina Minelli; Aneta Sikora; Raul Garcia-Diez; Katia Sparnacci; Christian Gollwitzer; Michael Krumrey; Alex G. Shard (1725-1732).
The successful translation of nanoparticle-based systems into commercial products depends upon the ability to reliably measure important physical and chemical properties of these particles. The density of nanoparticles is one such property, because it provides important information about the composition of the material. In this work, an analytical centrifugation approach based on line-start centrifugal sedimentation and flotation measurements is described. The two independent measurements permit both the size and the density of these nanoparticles to be determined with excellent precision. A set of monodisperse polystyrene nanoparticles of different sizes is used to demonstrate this method. The density and size measurements are validated by comparison to accurate Small Angle X-ray Scattering (SAXS) analysis for particles within the size range of SAXS, i.e. less than ∼300 nm in diameter. Both sedimentation and flotation measurements produce consistent high resolution size distributions of the particles and the measured size and density values are identical, within experimental uncertainty, to the SAXS results. This approach has the potential to provide useful characterisation of a range of particles of interest, for example, for medical application, such as liposomes and polymeric drug carriers.

Online coupling of tandem liquid-phase extraction with HPLC-UV for the determination of trace N-nitrosamines in food products by Weixia Li; Ning Chen; Yonggang Zhao; Weiqiang Guo; Nadeem Muhammd; Yan Zhu; Zhongping Huang (1733-1739).
N-Nitrosamines (NAs) are potent carcinogens and have been found in food products. Herein we report a simple tandem liquid-phase extraction (TLPE) method coupled with high-performance liquid chromatography (HPLC) for the determination of trace N-nitrosamines (NAs) in food products. The TLPE method is based upon the tandem implementation of pressurized hot water extraction (PHWE) and dispersive liquid–liquid microextraction (DLLME). The target analytes were first extracted with pressurized hot water, and then concentrated into an organic phase (n-octanol) in the DLLME procedure. The TLPE-HPLC system allowed the analytes to be extracted, concentrated and analyzed in a single step with minimal manipulation of the sample preparation, with an overall analysis time of 25 min. The optimized method showed good linearity (r2≥ 0.981) and satisfactory recoveries (74–117%) with good repeatability of the extractions (RSDs below 12.8%, n = 5). The limits of detection (LODs) for all nitrosamines in salted duck eggs were in the range of 0.08–0.55 μg kg−1.

Developing biosensors for Ca2+ detection is an important yet challenging analytical task, since it is quite difficult to distinguish Ca2+ from Mg2+, Cd2+ and even lanthanides in some cases. In addition to small molecule and protein based Ca2+ probes, DNA has recently emerged as a powerful platform for metal sensing. DNAzymes are DNA-based catalysts, and DNAzyme-based sensors are highly attractive due to their high stability, low cost and excellent sensitivity. EtNa is a recently reported RNA-cleaving DNAzyme that has high selectivity for Ca2+ over Mg2+ and Na+. In this work, a systematic mutation study was performed on EtNa, aiming to further understand its biochemical features and improve sensor performance. Among the 45 tested mutants, EtNa-C5T showed higher activity and even better Ca2+ selectivity compared to the original EtNa. The selectivity for Ca2+ over Na+ improved by ∼20-fold. In the presence of 1 mM Ca2+, C5T has a cleavage rate of 0.07 min−1. A catalytic beacon sensor was designed based on the C5T mutant with a detection limit of 11 μM Ca2+. This sensor was used to measure Ca2+ in five commercial milk samples and the results were compared to those from ICP analysis.

Reported herein is a series of new structural–functional relationship equations which provide relevant structural information of unknown fatty acid methyl esters (double-bonds, chain-length, and omega-bond position) based upon temperature induced shifts in equivalent chain length's (ECLs). The induced ECL changes were accomplished upon two cyanoalkyl (Supelco SP-2560, fast GC; Varian Factor 4, VF-23) and one stabilised PEG column (Innowax). Regression on this data-set permitted the development of classical linear equations. To calculate the ECL values, bracketing saturated fatty acid methyl esters (FAMEs) were run in the same admixture as the unsaturated FAMEs. The equations developed for each of the three columns are demonstrated to have varying capacity to accurately identify the chain-length, double-bond number and omega-bond position.

Sensitive recognition of ethion in food samples using turn-on fluorescence N and S co-doped graphene quantum dots by Fatemeh Nemati; Morteza Hosseini; Rouholah Zare-Dorabei; Mohammad Reza Ganjali (1760-1766).
To achieve a fast, sensitive and inexpensive method for the detection of ethion in the presence of Hg2+ ions, a nitrogen and sulfur co-doped graphene quantum dot (N,S/GQD)-based fluorescence sensing system was developed. The fluorescence of N,S/GQDs is sensitively quenched as a result of electrostatic interactions with Hg2+ ions adsorbed onto the negatively charged N,S/GQD surface. Following the introduction of ethion, the fluorescence of the Hg2+-quenched N,S/GQDs is recovered due to the strong coordination between Hg2+ and ethion. Based on the intensities of the recovered fluorescence of the N,S/GQDs, the ethion concentration was easily quantified. The linear response range of this method for ethion was from 19.2 μg L−1 M to 961.2 μg L−1. In this sensing system, not only was the detection limit as low as 8 μg L−1, which was as good as or better than those of similar assays, but the sensing system also displayed good selectivity in detecting ethion in the presence of different physiological molecules and inorganic ions. The proposed sensing method was successfully applied to the detection of ethion in water and food samples and was found to produce encouraging results.

Combining the high affinity and specificity of an aptamer for a protein, a highly sensitive strategy for protein detection was developed based on rolling circle amplification (RCA) and graphene oxide based fluorescence resonance energy transfer (FRET). In this strategy, a protein was captured using an antibody-coated microplate, and then a circularized product containing an aptamer–primer and padlock probe was added to form a sandwich structure. Under the influence of DNA polymerase, the RCA reaction extended the primer sequence, and generated hundreds of copies of the circular DNA template. Detection of the RCA products via FRET using graphene oxide allowed the sensitivity enhancement in target protein detection. This method was used to detect the target protein of cardiac troponin I (cTnI) with a detection limit as low as 14.40 pg mL−1. The highly sensitive and specific assay can be used as a promising tool in clinical sample analysis.

A rapid colorimetric method for the detection of deltamethrin based on gold nanoparticles modified with 2-mercapto-6-nitrobenzothiazole by Zhuqing Wang; Yunlong Huang; Dejin Wang; Li Sun; Chen Dong; Lanyun Fang; Yujie Zhang; Aiguo Wu (1774-1780).
In this study, we developed a rapid colorimetric method for the detection of deltamethrin using gold nanoparticles modified with 2-mercapto-6-nitrobenzothiazole (Au NPs–MNBT): deltamethrin adhered onto the surface of Au NPs–MNBT and a core–shell structure formed with Au NPs–MNBT as the core and deltamethrin as the shell. The formation of the core–shell structure caused the change of solution color and UV-visible absorption spectra of Au NPs, which can be used to quantitatively detect deltamethrin by the naked eye and using a UV-Vis spectrophotometer. The effect of MNBT content, detection pH and reaction time on the detection of deltamethrin was studied in detail, and the selectivity and anti-interference ability of the probe were tested. The results showed that the probe can be used to detect deltamethrin rapidly and accurately from 13 kinds of other pesticides. The detection of deltamethrin was not affected when 5 times the concentration of other pesticides coexisted with deltamethrin. The detection limit of deltamethrin was observed to be 0.25 μM by the naked eye and 0.005 μM by UV-Vis spectroscopy. The extract solutions of small tomato and cherry were used to evaluate the feasibility of this detection method in actual samples and the results were consistent with that of gas chromatography-mass spectrometry, which proved that the developed method has actual sample application potential for deltamethrin detection.

Development and application of a portable fluorescence detector for shipboard analysis of ammonium in estuarine and coastal waters by Yong Zhu; Jianfang Chen; Xiaolai Shi; Dongxing Yuan; Sichao Feng; Tingjin Zhou; Yongming Huang (1781-1787).
Herein, a novel homemade portable fluorescence detector that is simple, smaller, and transportable for the shipboard analysis of ammonium in estuarine and coastal waters has been developed and applied in a flow system for ammonium analysis based on the reaction of ammonium, OPA, and sulfite. The detector incorporates a UV-LED, two band pass filters, a photomultiplier tube (PMT), a modified flow cell, and an electronic circuit with a constant voltage supply and a constant current supply. The modified flow cell of the detector used in this study can avoid the interference of air bubbles in the fluorescence signal. A 100 nmol L−1 ammonium standard solution was continuously measured for 90 min. An RSD of 0.8% (n = 38) was obtained, indicating the good stability of the detector. Compared with the commercial PMT-FL fluorescence detector, the homemade detector was smaller and lightweight and demonstrated better sensitivity (approximately 11.4% higher). Moreover, the parameters of the flow system were optimized. Under optimal conditions, a detection limit of 2.1 nmol L−1 within the working range of 0–300 nmol L−1 was obtained. Using the underway sampling system, the homemade portable fluorescence detector was applied in Jiulongjiang estuary, and the distribution of ammonium in the surface seawater was obtained.

Expression of concern for ‘Synthesis of a monodisperse well-defined core–shell magnetic molecularly-imprinted polymer prior to LC-MS/MS for fast and sensitive determination of mycotoxin residues in rice’ by Mi-Cong Jin et al., Anal. Methods, 2017, 9, 5281–5292.

Back cover (1789-1790).