Analytical Methods (v.10, #45)

Front cover (5351-5352).

Contents list (5353-5357).

Simple lateral flow assays for microbial detection in stool by Wendy A. Henderson; Lichen Xiang; Nicolaas H. Fourie; Sarah K. Abey; Eric G. Ferguson; Ana F. Diallo; Natnael D. Kenea; Chang Hee Kim (5358-5363).
Diarrheal diseases claim the lives of 1300 children daily, mostly in the developing world. We have developed a simple lateral flow assay capable of detecting E. coli and EPEC DNA and RNA rapidly (<15 minutes) at the point-of-need, directly from stool without nucleic acid extraction or molecular amplification. The limit of detection of the method is 1 nM using synthetic DNA target substrates spiked into stool. However, due to the endogenous amplification of the 23S rRNA targets, we were able to detect the endogenous EPEC in pea-sized (5 mg) stool without labor-intensive and time-consuming nucleic acid purification or target amplification using enzymes. The significance of this method is that it is rapid (<15 minutes) and simple (without nucleic acid purification or molecular amplification) and does not require instrumentation, or access to a laboratory, cold chain or electric power. Thus, it is well-suited for point-of-need use in remote and/or resource-limited settings in the developing world where the mortality due to diarrheal diseases is especially high. The rapid testing of stool pathogens in real time at the point-of-need will decrease the loss of patients to follow-up, and enable patients to be treated earlier with the appropriate therapeutics in both the developed and developing world settings.

A new and label-free potentiometric aptasensing platform was designed for the sensitive detection of carcinoembryonic antigen (CEA) in human serum on a graphene oxide (GO) nanosheet-modified glassy carbon electrode by coupling with target recycling-assisted signal amplification. To construct such a signal-amplification aptasensing system, the nanosheets were initially immobilized on the electrode through physical adsorption, and then CEA aptamers were coated on the nanosheets viaπ-stacking interaction. Upon target CEA introduction, the analyte reacted with the aptamer to form a complex, thus resulting in the dissociation of the aptamer from the nanosheets. The formed CEA–aptamer complexes were cleaved in the presence of DNase I to release target CEA, thereby triggering the target recycling and catalytic recycling of DNase I. Thanks to the negatively charged oligonucleotide skeleton, the dissociation of the aptamers from the nanosheets could cause a change in the local electrical potential of the modified electrode. Under optimum conditions, the shift in the potential increased with the increment of target CEA concentrations, and exhibited good potential responses within a linear range of 0.01–100 ng mL−1 at a low detection limit of 9.4 pg mL−1. The specificity, precision, reproducibility and stability of the potentiometric aptasensor were acceptable. The accuracy of this method was evaluated for the analysis of human serum specimens, giving well-matched results with those obtained from the commercial human CEA ELISA kit.

Electrochemical sensing for 1-chloro-4-nitrobenzene based on β-cyclodextrin/carbon nanohorn nanohybrids by Odoom Jibrael Kingsford; Junjuan Qian; Depeng Zhang; Yinhui Yi; Gangbing Zhu (5372-5379).
The occurrence of 1-chloro-4-nitrobenzene (CNB) in the environment due to increased agricultural and economic activities worldwide has been a serious concern for various environmental agencies; hence the qualitative and quantitative determination of CNB is of great significance. In this study, we took into consideration the features of carbon nanohorns (CNHs), such as high surface area and electron transfer ability, and β-cyclodextrin (β-CD), such as high host–guest recognition ability with a hydrophilic outer layer, to design a highly sensitive electrochemical sensor for detecting CNB by preparing CNHs and β-CD nanohybrids through a simple ultrasonication process. The outcome of our study revealed that the CNHs/β-CD nanohybrid modified electrode is an excellent electrochemical sensor for CNB. Under the optimized conditions, the experimental results show a considerable linear response range, unsubstantial interference from familiar organic and inorganic compounds and a detection limit of 9.0 nM.

Development of a novel fluorescence ratiometric glucose sensor based on carbon dots and a potential fluorophore m-dihydroxybenzene by Hong Zhai; Yunfeng Bai; Haiqing Wang; Jun Qin; Huijun Liu; Feng Feng (5380-5386).
m-Dihydroxybenzene (mDHB) was used as a potential fluorophore for the first time to develop a new fluorescence ratiometric sensor with carbon dots (CDs) for glucose detection. mDHB itself cannot fluoresce, but its oxide can produce fluorescence and also quench the fluorescence of CDs. Using this characteristic of mDHB, we introduced two tandem enzyme catalytic oxidation reactions (catalysed by glucose oxidase and horseradish peroxidase) to quantitatively form mDHB oxide. Glucose was oxidized by oxygen to form hydrogen peroxide under the catalysis of glucose oxidase, and then hydrogen peroxide was used to quantitatively oxidize mDHB by the catalysis of horseradish peroxidase, so mDHB oxide was quantitatively related to glucose. Furthermore, the resulting mDHB oxide can cause changes in the fluorescence ratios of CDs and mDHB oxides. Therefore, glucose can be detected by the fluorescence ratio. The results showed that glucose content had a good linear relationship with the fluorescence ratio of mDHB oxide and CDs, and the detection limit can reach 0.35 μM. The strategy was simple and cost-effective as no chemical synthesis, modification and separation procedures were involved, which can open up a new avenue for glucose detection.

Selectivity is very important for stationary phase applications. However, selectivity is usually quite difficult to adjust once the ligands are fixed. Departing from the reported methods, dodecyl methacrylate (DOMA) and hydroxyethyl methacrylate (HEMA) were grafted onto a silica surface via two-step surface initiated-atom transfer radical polymerization (SI-ATRP) to synthesize reverse-phase/hydrophilic interaction mixed-mode stationary phases. The grafted amounts of C12 and OH functional groups were controlled by varying the ratios of DOMA to HEMA in the polymerization system. The resulting stationary phases were characterized by transmission electron microscopy (TEM), solid-state 13C NMR spectroscopy, elemental analysis (EA), Fourier transform infrared (FT-IR) spectrometry, and thermogravimetric analysis (TGA). The chromatographic performance and separation selectivity of the packed columns were investigated in different chromatographic modes using a wide range of analytes, including a non-polar benzene series, moderately polar β-agonists as well as organic acids, and strongly polar nucleosides. Exceptionally, the retention of the tested analytes displayed an obvious dependence on the ratio of the two functional monomers in the polymerization. In conclusion, the proposed strategy for the development of a mixed-mode chromatographic stationary phase can provide flexible selectivity by simply tuning the ratios of functional monomers.

Laser desorption/ionization mass spectrometry method on gold nanoparticle enhanced target (AuNPET) was used for rapid detection and quantification of lysine. The amino acid was tested directly in the concentration range from 100 μg mL−1 to 1 ng mL−1, which corresponds to lysine mass of 50 ng to 500 fg per analyzed spot. Detection limit value of 0.2 ng mL−1 or 100 fg per spot is one of the best among the techniques used to determine lysine. In addition, mass spectrometry imaging quantification approach shown in this study canceled out sweet spot effect on the measurement result.

For the first time, a robust, rugged, and low-cost ion sensor based on potentiometric transduction is presented here for rapid determination of piperidine. A conventional filter paper is used as a substrate to establish the sensors after coating a carbon-ink layer on the surface of the filter paper to make it conductive. Poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) was used as an ion-to-electron transducer and deposited through drop-casting on the paper-based carbon electrode. The polymeric membranes were based on the incorporation of two types of electroactive materials, namely ion association complexes such as piperidinium phosphomolybdate (Pip/PMA) (sensor I), piperidinium phosphotungstate (Pip/PT) (sensor II), piperidinium tetraphenyl borate (Pip/TPB) (sensor III), and β-cyclodextrin (β-CD) ionophore (sensor IV) in a plasticized polyvinyl chloride (PVC) matrix. The sensors revealed Nernstian slopes of 60.2 ± 0.5, 57.1 ± 0.6, 56.2 ± 0.8 and 54.2 ± 0.6 mV per decade with linear concentration ranges begin from 5.1 × 10−6, 7.4 × 10−6, 3.1 × 10−5 and 5.5 × 10−6 M for sensors I, II, III and IV, respectively. The detection limits range from 0.32 to 0.66 μg mL−1 for all the proposed sensors with a response time <10 seconds. The sensors exhibited clear selectivity towards piperidinium ions over several common organic and inorganic cations. Repeatability, reproducibility and stability have been studied to evaluate the properties of the sensors. The sensors were successfully utilized for piperidine quantification in wastewater and human urine samples. The obtained results agreed well with the acceptable recovery percentage and were better than those obtained by other previously reported routine methods.

Preparation and evaluation of molecularly imprinted membrane of teicoplanin by Ru Yao; Zhen Yu; Manman Wu; Haining Yu (5416-5422).
A molecularly imprinted membrane (MIM) with teicoplanin (TE) as a template and other related material membranes (organic nylon microporous membranes, polyvinylidene fluoride (PVDF) membranes, and polypropylene membranes) as supports were synthesized for the selective absorption of TE. These membranes can be used for pretreatment of biological samples and rapid determination of TE. Our results showed that PVDF MIM exhibited the best selectivity among the other three synthesized membranes and therefore, it was used in this study. The performance of PVDF MIM was examined, including surface structure, adsorption capacity, desorption capacity, selective adsorption capacity, and recovery rate; the precision, stability, repeatability, linearity and recovery rate of this method were investigated at the same time. Clean-up and enrichment of samples by TE MIM were reliable.

The present work proposed a simple and convenient electroanalytical method for the selective and simultaneous determination of anti-hypertensive drugs viz. amlodipine (AML) and losartan (LOS). The electrochemical response of AML and LOS was investigated at a glassy carbon electrode (GCE) modified with an iron metal–organic framework/mesoporous carbon (FeMOF/MC) nanocomposite by employing differential pulse voltammetry (DPV). Under the optimized experimental conditions, electrochemical sensing of both the molecules was performed at the FeMOF/MC/GCE in 0.1 M phosphate buffer (PBS) of pH 6.0. The DPV experiments revealed that the voltammetric signals of AML and LOS were very well resolved and separated with a potential difference of 50 mV. Also, the FeMOF/MC composite exhibited unique properties in terms of porosity, surface area and high electrical conductivity. As compared to the plain GCE, the modified electrode showed a thirteen- and nineteen-fold enhancement in the oxidation peak current of AML and LOS which indicates that the synergy of electrode materials provides excellent electrocatalytic activity towards the simultaneous determination of AML and LOS. The analytical curve of AML and LOS showed a linear response in the concentration range of 0.009 μM to 500 μM with a detection limit of 1.27 nM and 2.03 nM, respectively. The practical analytical utility of the proposed sensor was evaluated by employing the present method for simultaneous determination of AML and LOS in pharmaceutical formulations, and human blood serum, saliva and urine samples.

Electronic cigarettes (EC) and other electronic nicotine delivery systems (ENDS) have recently become popular choices for nicotine consumption due to the lower perceived harm compared to conventional tobacco products. Currently, only nicotine levels in EC fluids are regulated by the FDA. Besides nicotine and solvents such as propylene glycol and glycerin, the heating of EC fluids may produce thermal degradants that could impact a user's health. We proposed to use direct sample introduction (DSI) GC-MS/MS as a fast and reliable instrumental method to analyze EC fluid components and their thermal degradants generated in a simulated ENDS-like environment within the DSI enclosure. DSI GC-MS/MS separates and detects the target analytes even in the presence of a complex, viscous, and often “dirty” sample matrix. DSI utilizes a programmable-temperature vaporization (PTV) injector in conjunction with a ChromatoProbe accessory, in which a solid or liquid sample in a disposable glass vial can be heated and vapors introduced into the GC column for separation. DSI was used to mimic the heating behavior of an EC atomizer and introduce the EC vapors and degradants into GC-MS/MS in almost real-time. Subsequently, through tandem mass spectrometry, signature ion fragments for thermal breakdown components (e.g. aldehydes) were detected and quantified. Relative peak ratios of those thermal breakdown products and an internal standard were employed to study the effects of temperature ramp rate, maximum heating temperature, and maximum temperature hold time on the outcomes of thermal degradation.

Fast, cheap and easy routine quantification method for atrazine and its transformation products in water matrixes using a DLLME-GC/MS method by Alexandre Della-Flora; Raquel W. Becker; Marco F. Ferrão; Aline T. Toci; Gilcélia A. Cordeiro; Marcela Boroski; Carla Sirtori (5447-5452).
In recent years, Brazil has been one of the largest consumers of pesticides in the world, with atrazine (ATZ) being an active principle often used in Brazilian agriculture. In this context, this study is aimed at developing a fast, simple and economical routine methodology for the identification and quantification of ATZ and its transformation products, deisopropylatrazine (DIA) and deethylatrazine (DEA), by using dispersive liquid–liquid microextraction (DLLME) as the extraction technique associated with gas chromatography-mass spectrometry (GC/MS) analysis in surface water (class 2 river). The best extraction conditions were set using the Doehlert experimental design. Validation of the developed DLLME-GC/MS methodology showed limits of detection and quantification of 0.050 and 0.150 μg L−1 for ATZ, 0.250 and 0.500 μg L−1 for DEA and 0.250 and 0.900 μg L−1 for DIA, respectively. Parameters such as the matrix effect, repeatability and recovery were evaluated. The proposed method proved to be simple, easy and cost-effective for the analysis of a large number of different types of aqueous samples.

Back cover (5453-5454).