Direct Coupling of High-Performance Thin-Layer Chromatography with UV Spectroscopy and IR-MALDI Orthogonal TOF MS for the Analysis of Cyanobacterial Toxins

Ultrasensitive label-free electrochemical biosensor for detecting linear microcystin-LR using degrading enzyme MlrB as recognition element

A label-free electrochemical biosensor was firstly constructed to detect linear microcystin-LR (L-MC-LR) with high sensitivity. Degradation enzyme MlrB was used as recognition element for specific recognition of L-MC-LR. The electrode was modified with -COOH functionalized multi-walled carbon nanotube to increase the specific surface area and improve the conductivity, which was then applied to immobilize MlrB. The electrochemical signal was changed with the reaction between MlrB and L-MC-LR, which was recorded by using square wave voltammetry. The electrochemical biosensor showed superior sensitivity, with a dynamic range of 1 pg/mL to 100 ng/mL and a detection limit of 0.127 pg/mL. Moreover, the fabricated electrochemical biosensor exhibited excellent specificity toward L-MC-LR in real water samples. The concentrations of spiked L-MC-LR were 0.100, 5.00, 50.0 ng/mL, and the recovery rates were 95.0-104% with relative standard deviation (RSD) of 0.900-2.30% and 74.0-93.0% with RSD of 2.30-3.50% in lake water and tap water, respectively. Furthermore, the selectivity, reproducibility, and stability demonstrated the potential of degradation enzymes as recognition element in detection of cyanotoxins.

Infrared MALDI Mass Spectrometry with Laser-Induced Postionization for Imaging of Bacterial Colonies

Ultraviolet matrix-assisted laser desorption ionization mass spectrometry imaging (UV-MALDI-MSI) is a powerful tool to visualize bacterial metabolites in microbial colonies and in biofilms. However, a challenge for the method is the efficient extraction of analytes from deeper within the bacterial colonies and from the cytoplasm of individual cells during the matrix coating step. Here, we used a pulsed infrared (IR) laser of 2.94 μm wavelength to disrupt and ablate bacterial cells without a prior coating with a MALDI matrix. Instead, tissue water or, in some experiments, in addition a small amount of glycerol was exploited for the deposition of the IR laser energy and for supporting the ionization of the analytes. Compared to water, glycerol exhibits a lower vapor pressure, which prolonged the available measurement time window within an MSI experiment. Mass spectra were acquired with a hybrid Synapt G2-S HDMS instrument at a pixel size of 120 μm. A frequency-quadrupled q-switched Nd:YAG laser with 266 nm wavelength served for laser-induced postionization (MALDI-2). In this way, the ion abundances of numerous small molecules such as nucleobases, 2-alkyl-quinolones, a prominent class of Pseudomonas aeruginosa signaling molecules involved in one of the three quorum-sensing pathways, and also the signals of various bacterial phospholipids were boosted, partially by orders of magnitude. We analyzed single and cocultured colonies of Gram-negative P. aeruginosa and of Gram-positive Bacillus subtilis and Staphylococcus aureus as exemplary bacterial systems. To enable a rapid (within 5 s) MSI-compatible steam inactivation in a custom-made autoclave filled with hot water steam, bacterial cultures were grown on porous polyamide membranes. Compared to a UV-MALDI-2-MS measurement of the same systems, mass spectra with a reduced low mass background were generally generated. This resulted in the unequivocal detection of numerous metabolites only with the IR laser. In a fundamental part of our study, and to optimize the IR-MALDI-2 approach for the highest analytical sensitivity, we characterized the expansion dynamics of the particle plume as generated by the IR laser. Here, we recorded the total ion count and the intensities of selected signals registered from P. aeruginosa samples as a function of the interlaser delay and buffer gas pressure in the ion source. The data revealed that the IR-MALDI-2 ion signals are primarily generated from slow particles having mean velocities of ∼10 m/s. Interestingly, two different pressure/delay time regimes of the optimized ionization efficiency for phospholipids and smaller metabolites, respectively, were revealed, a result pointing to yet-unknown convoluted reaction cascades. The described IR-MALDI-2 method could be a helpful new tool for a microbial mass spectrometry imaging of small molecules requiring little sample preparation.

Nanobiochar paper based electrochemical immunosensor for fast and ultrasensitive detection of microcystin-LR

A portable, cheap and sensitive paper type electrochemical immunosensor was developed with conductive nanobiochar paper as the conductive layer and utilized for sensitive detection of microcystin-LR (MCLR) toxin in water. The paper immunosensor was constructed by coating of highly conductive and dispersible nanobiochar particle (nBC) and anti-MCLR antibody on the filter paper via dipping-drying method. The presence of MCLR could be specifically quantified amperometrically by the nBC-paper immunosensor with the response time of less than 5 min, and the lowest detection limit of 17 pM (0.017 μg/L) was achieved. Moreover, the proposed immunosensor exhibited high selectivity, reproducibility and storage stability, and was also used for environmental water detection with satisfactory recovery. The successful fabrication of low cost and ubiquitous biochar based paper type electrochemical immunosensing system would have significant value for the development of highly cost-effective electrochemical device.

Ultrasensitive detection of ochratoxin A based on biomimetic nanochannel and catalytic hairpin assembly signal amplification

In this paper, an ultrasensitive nanochannel sensor has been proposed for label-free Ochratoxin A (OTA) assay in combination with graphene oxide (GO) and catalyzed hairpin assembly (CHA). The high-performance sensor is segmented into two parts. One is composed of graphene oxide (GO) and DNA probes. In the presence of target OTA, OTA works as a catalyst to trigger the self-assembly pathway of the two probes and initiate the cycling of CHA circuits, which results in numerous double-stranded DNAs (dsDNA) in solution. The excess ssDNA probes are removed by GO. The other part is composed of biomimetic nanochannel coated with polyethyleneimine (PEI) and Zr⁴⁺, which can quantify the concentration of OTA by detecting the dsDNA in solution. The nanofluidic device has a detection limit of as low as 6.2 pM with an excellent selectivity. The nanochannel based assay was used to analyse food samples (red wine) with satisfied results. Thus, the proposed analytical method will provide a new approach the detection of OTA and can be applied for quality control to ensure food safety.

Determination of microcystins in environmental water samples with ionic liquid magnetic graphene

Microcystins is a class of monocyclic of heptapeptides with many different isomerides. It has become potential hazardous material in water environment for its toxic, distribution and stability. This project worked on a method for determination of trace microcystin (MC-LR and MC-RR) in environmental waters. The ionic liquid magnetic graphene (IL@MG) was prepared and applied to the concentration and determination of microcystins, based on magnetic solid phase extraction (MSPE), and coupled with ultra high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). The ionic liquid magnetic graphene was prepared by coprecipitatial synthesis and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FT-IR), specific surface area (BET), pore size distribution (BJH) and magnetic hysteresis loop. The experimental parameters of magnetic solid phase extraction, including amount of IL@MG, pH, extraction time and elution solvent were investigated by a univariate method and orthogonal screening. The method showed good linearity in the range of 0.01-10.0 g/L and 0.005-10.0 μg/L for MC-LR and MC-RR, when the pH of water samples was 4.00 and 10.0 mg adsorbents were used to extract targets for 18 min. The lowest detection limit was 0.414 ng/L and 0.216 ng/L for MC-LR and MC-RR respectively. The recoveries of the microcystins were in the range of 83.6-100.9%, and the relative standard deviation was less than 7.59%. The trace amount of MC-LR (0.020 μg/L) and MC-RR (0.003 μg/L and 0.021 μg/L) was detected in actural water samples. Attributed to its simple operator, low detection limit and high sensitivity, this method could be used for the detection of trace microcystins in water samples.

An ultrasensitive signal-on electrochemical aptasensor for ochratoxin A determination based on DNA controlled layer-by-layer assembly of dual gold nanoparticle conjugates

In this paper, a novel ultrasensitive signal-on electrochemical aptasensor has been proposed for Ochratoxin A (OTA) assay based on DNA controlled layer-by-layer assembly of dual gold nanoparticle (AuNP) conjugates. To construct the aptasensor, the 1^st AuNP conjugate was prepared by simultaneous immobilization of the capture probe 2 (CP2) and bridge probe (BP) onto the AuNPs. Then, OTA aptamer was loaded onto 1^st AuNPs by hybridization with CP2. The 1^st AuNP conjugate can be further immobilized onto the electrode by hybridization between BP and capture probe 1 (CP1), which was pre-immobilized on Au electrode. The 2^nd AuNP conjugate was prepared by immobilization of ferrocene (Fc) tagged SH-signal probe (SSP). Due to the recognition between aptamer on 1^st AuNP conjugate and OTA, CP2 was reformed in the ssDNA state, which can be utilized as the anchor for immobilization of 2^nd AuNP conjugate for electrochemical signal reporting. Because of the high surface-to-volume ratio and good conductivity of AuNPs, this dual AuNPs assembled nanoarchitecture finally lead to greatly improved abilities to load a large number of Fc molecules and significantly amply the electrochemical response even at a low target concentration. Both qualitative and quantitative analysis of OTA were thus realized by differential pulse voltammetry (DPV) signals, resulting in an excellent detection limit of 0.001 ppb and a wide dynamic range from 0.001 to 500 ppb over 6 orders of magnitude. Moreover, the real sample analysis towards OTA spiked wine samples was favorable, implying a great potential for practical applications. We envision that this unique dual AuNP conjugate assembly strategy would pave a new avenue for the development of versatile signal amplified electrochemical aptasensors.

Electrospun polymer nanofibres as solid-phase extraction sorbents for extraction and quantification of microcystins

Electrospun polymer nanofibres were used as novel solid-phase extraction (SPE) sorbents to extract and quantify the microcystins (MCs) including microcystin-RR (MC-RR) and microcystin-LR (MC-LR) from in-suit water samples. The parameters that influenced the extraction efficiency were studied, including the amount of nanofibre, eluted solvent, eluted volume, pH, and the water sample volume. Under optimized conditions, a linear response for MC-RR and MC-LR over the range of 0.25-4 µg/L was achieved with r(2) values of 0.998 and 0.997, respectively. The extraction recovery of MC-RR and MC-LR was 97-102% and 98-100%, respectively, when the MC concentration was 0.25-4 µg/L. When their concentrations ranged from 0.05  to 0.25 µg/L, the MCs could be detected with high accuracy by the nanofibre SPE sorbent combined with nitrogen gas. Due to its simplicity, environment-friendliness, high efficiency, reusability, and sensitivity, the electrospun polymer nanofibre can be applied as a novel SPE sorbent to extract and detect the MCs from in-suit water samples.

Methods to detect cyanobacteria and their toxins in the environment

Cyanobacteria blooms are since early times a cause for environmental concern because of their negative impact through the release of odors, water discoloration, and more dangerously through the release of toxic compounds (i.e. the cyanotoxins) that can affect both human and animal welfare. Surveillance of the aquatic ecosystems is therefore obligatory, and methods to achieve such require a prompt answer not only regarding the species that are producing the blooms but also the cyanotoxins that are being produced and/or released. Moreover, besides this well-known source of possible intoxication, it has been demonstrated the existence of several other potential routes of exposure, either for humans or other biota such as through food additives and in terrestrial environments (in plants, lichens, biological soil crusts) and the recognition of their harmful impact on less studied ecosystems (e.g. coral reefs). Nowadays, the most frequent approaches to detect toxic cyanobacteria and/or their toxins are the chemical-, biochemical-, and molecular-based methods. Above their particular characteristics and possible applications, they all bring to the environmental monitoring several aspects that are needed to be discussed and scrutinized. The end outcome of this review will be to provide newer insights and recommendations regarding the methods needed to apply in an environmental risk assessment program. Therefore, a current state of the knowledge concerning the three methodological approaches will be presented, while highlighting positive and negative aspects of each of those methods within the purpose of monitoring or studying cyanobacteria and their toxins in the environment.

Quantitative analysis of major dibenzocyclooctane lignans in Schisandrae fructus by online TLC-DART-MS

INTRODUCTION

Direct analysis in real time (DART) ion source is a powerful ionising technique for the quick and easy detection of various organic molecules without any sample preparation steps, but the lack of quantitation capacity limits its extensive use in the field of phytochemical analysis.

OBJECTIVE

To improvise a new system which utilize DART-MS as a hyphenated detector for quantitation.

METHODOLOGY

A total extract of Schisandra chinensis fruit was analyzed on a TLC plate and three major lignan compounds were quantitated by three different methods of UV densitometry, TLC-DART-MS and HPLC-UV to compare the efficiency of each method. To introduce the TLC plate into the DART ion source at a constant velocity, a syringe pump was employed. The DART-MS total ion current chromatogram was recorded for the entire TLC plate. The concentration of each lignan compound was calculated from the calibration curve established with standard compound.

RESULTS

Gomisin A, gomisin N and schisandrin were well separated on a silica-coated TLC plate and the specific ion current chromatograms were successfully acquired from the TLC-DART-MS system. The TLC-DART-MS system for the quantitation of natural products showed better linearity and specificity than TLC densitometry, and consumed less time and solvent than conventional HPLC method.

CONCLUSION

A hyphenated system for the quantitation of phytochemicals from crude herbal drugs was successfully established. This system was shown to have a powerful analytical capacity for the prompt and efficient quantitation of natural products from crude drugs.

Lab-on-a-plate: extending the functionality of MALDI-MS and LDI-MS targets

We review the literature that describes how (matrix-assisted) laser desorption/ionization (MA)LDI target plates can be used not only as sample supports, but beyond that: as functional parts of analytical protocols that incorporate detection by MALDI-MS or matrix-free LDI-MS. Numerous steps of analytical procedures can be performed directly on the (MA)LDI target plates prior to the ionization of analytes in the ion source of a mass spectrometer. These include homogenization, preconcentration, amplification, purification, extraction, digestion, derivatization, synthesis, separation, detection with complementary techniques, data storage, or other steps. Therefore, we consider it helpful to define the "lab-on-a-plate" as a format for carrying out extensive sample treatment as well as bioassays directly on (MA)LDI target plates. This review introduces the lab-on-plate approach and illustrates it with the aid of relevant examples from the scientific and patent literature.

Desorption electrospray ionization mass spectrometry analysis of lipids after two-dimensional high-performance thin-layer chromatography partial separation

Molecular imaging of separate but still incompletely resolved spots on high-performance thin-layer chromatography (HPTLC) plates is used for the direct analysis of porcine brain lipids by desorption electrospray ionization mass spectrometry (DESI-MS). Eight class-specific spots were imaged in the negative ion mode and shown to contain more than fifty lipids. A low lateral resolution of 400 x 400 microm allowed simple, rapid, and incomplete separation to be combined with DESI imaging for the identification of many components of these extremely complex mixtures. In this work, tandem mass spectrometry (MS/MS) was also employed to confirm the identity of particular lipids directly on HPTLC plates.