Electrothermal Vaporization for Universal Liquid Sample Introduction to Dielectric Barrier Discharge Microplasma for Portable Atomic Emission Spectrometry

Direct introduction of liquid sample into a microplasma for analytical atomic spectrometry can be a problem for its lowered atomization/excitation capability or can even extinguish it. The low power dielectric barrier discharge (DBD) microplasma has been widely used in optical spectrometry, but the number of detectable elements by atomic emission spectrometry (AES) is very limited, partially for the same reason. Here we use electrothermally vaporized analyte-containing species for sample introduction into a DBD microplasma, together with simple heating of the DBD, to enhance its atomization/excitation capability for AES. A compact tungsten coil electrothermal vaporizer (W-coil ETV) was used in this work, onto which a tiny volume of liquid sample was pipetted. Through administrating the heating program for the W-coil, sample solvent and matrix were removed first and subsequently atomized/vaporized analyte with extra energy provided by the W-coil was swept directly into the DBD microplasma for further atomization/excitation. These significantly contribute the stability of the DBD microplasma and save its power for reatomization/excitation of analyte thus improving the detectability. Under optimized experimental conditions, limits of detection of 0.8 μg L^–1 (0.008 ng) for cadmium and 24 μg L^–1 (0.24 ng) for zinc were obtained, with relative standard deviation (RSD) of 3.2% for 5 μg L^–1 Cd and 3.7% for 100 μg L^–1 Zn. Its potential application was also demonstrated by successfully analyzing several Certified Reference Materials. Its characteristics including compactness, low power consumption, cost effectiveness, tiny sample requirement, and easy operation make it very promising for field analytical chemistry

Dielectric Barrier Discharge Carbon Atomic Emission Spectrometer: Universal GC Detector for Volatile Carbon-Containing Compounds

It was found that carbon atomic emission can be excited in low temperature dielectric barrier discharge (DBD), and an atmospheric pressure, low power consumption, and compact microplasma carbon atomic emission spectrometer (AES) was constructed and used as a universal and sensitive gas chromatographic (GC) detector for detection of volatile carbon-containing compounds. A concentric DBD device was housed in a heating box to increase the plasma operation temperature to 300 °C to intensify carbon atomic emission at 193.0 nm. Carbon-containing compounds directly injected or eluted from GC can be decomposed, atomized, and excited in this heated DBD for carbon atomic emission. The performance of this new optical detector was first evaluated by determination of a series of volatile carbon-containing compounds including formaldehyde, ethyl acetate, methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol, and absolute limits of detection (LODs) were found at a range of 0.12–0.28 ng under the optimized conditions. Preliminary experimental results showed that it provided slightly higher LODs than those obtained by GC with a flame ionization detector (FID). Furthermore, it is a new universal GC detector for volatile carbon-containing compounds that even includes those compounds which are difficult to detect by FID, such as HCHO, CO, and CO₂. Meanwhile, hydrogen gas used in conventional techniques was eliminated; and molecular optical emission detection can also be performed with this GC detector for multichannel analysis to improve resolution of overlapped chromatographic peaks of complex mixtures

Miniaturized Dielectric Barrier Discharge Carbon Atomic Emission Spectrometry with Online Microwave-Assisted Oxidation for Determination of Total Organic Carbon

A simple, rapid, and portable system consisted of a laboratory-built miniaturized dielectric barrier discharge atomic emission spectrometer and a microwave-assisted persulfate oxidation reactor was developed for sensitive flow injection analysis or continuous monitoring of total organic carbon (TOC) in environmental water samples. The standard/sample solution together with persulfate was pumped to the reactor to convert organic compounds to CO₂, which was separated from liquid phase and transported to the spectrometer for detection of the elemental specific carbon atomic emission at 193.0 nm. The experimental parameters were systematically investigated. A limit of detection of 0.01 mg L^–1 (as C) was obtained based on a 10 mL sample injection volume, and the precision was better than 6.5% (relative standard deviation, RSD) at 0.1 mg L^–1. The system was successfully applied for TOC analysis of real environmental water samples. The obtained TOC value of 30 test samples agreed well with those by the standard high-temperature combustion coupled nondispersive infrared absorption method. Most importantly, the system showed good capability of in situ continuous monitoring of total organic carbon in environmental water

Protein Quantitation Using Ru-NHS Ester Tagging and Isotope Dilution High-Pressure Liquid Chromatography–Inductively Coupled Plasma Mass Spectrometry Determination

An accurate, simple, and sensitive method for the direct determination of proteins by nonspecies specific isotope dilution and external calibration high-performance liquid chromatography–inductively coupled plasma mass spectrometry (HPLC–ICPMS) is described. The labeling of myoglobin (17 kDa), transferrin (77 kDa), and thyroglobulin (670 kDa) proteins was accomplished in a single-step reaction with a commercially available bis­(2,2′-bipyridine)-4′-methyl-4-carboxybipyridine-ruthenium <i>N</i>-succinimidyl ester-bis­(hexafluorophosphate) (Ru-NHS ester). Using excess amounts of Ru-NHS ester compared to the protein concentration at optimized labeling conditions, constant ratios for Ru to proteins were obtained. Bioconjugate solutions containing both labeled and unlabeled proteins as well as excess Ru-NHS ester reagent were injected onto a size exclusion HPLC column for separation and ICPMS detection without any further treatment. A ⁹⁹Ru enriched spike was used for nonspecies specific ID calibration. The accuracy of the method was confirmed at various concentration levels. An average recovery of 100% ± 3% (1 standard deviation (SD), <i>n</i> = 9) was obtained with a typical precision of better than 5% RSD at 100 μg mL^–1 for nonspecies specific ID. Detection limits (3SD) of 1.6, 3.2, and 7.0 fmol estimated from three procedure blanks were obtained for myoglobin, transferrin, and thyroglobulin, respectively. These detection limits are suitable for the direct determination of intact proteins at trace levels. For simplicity, external calibration was also tested. Good linear correlation coefficients, 0.9901, 0.9921, and 0.9980 for myoglobin, transferrin, and thyroglobulin, respectively, were obtained. The measured concentrations of proteins in a solution were in good agreement with their volumetrically prepared values. To the best of our knowledge, this is the first application of nonspecies specific ID for the accurate and direct determination of proteins using a Ru-NHS ester labeling reagent

Analyte-Activable Probe for Protease Based on Cytochrome C‑Capped Mn: ZnS Quantum Dots

A new sensor format was proposed here by integrating conjugation of analyte-recognition sites and quenching the luminescence of quantum dots (QDs) in one pot during the synthesis of QDs, with protease as the model analyte. Inherently phosphorescence-attenuated Mn-doped ZnS QDs were prepared with electron transfer protein cytochrome C (Cyt C) as the ligand, which was capable of protease sensing in both label-free and activable format. This detection strategy eliminates the postsynthetic protein conjugation and responses to analyte in the turn-on mode, lowering the signal background. In the presence of protease, the initially “locked” phosphorescence of Mn-doped ZnS QDs could be activated, due to the enzymatic digestion of surface Cyt C ligand and removal of the electron-transfer quenching unit away from the close-proximity of QDs. The proposed probe exhibited good selectivity toward proteases over other proteins and enzymes. Besides, it was also capable of differentiating active and inactive serine proteases. Analytical performance of this probe was evaluated using trypsin as the model serine protease. Limits of detection (LOD) of 2 nM was obtained, which is well below the average urine trypsin level of patients. The analytical application of this probe was demonstrated in determination of trypsin in human pancreatic carcinoma (PANC-1 and 818.4) cells lysates, demonstrating the potential usefulness of this probe in future clinical diagnosis

Organic Solvent-Free Cloud Point Extraction-like Methodology Using Aggregation of Graphene Oxide

Because of its unique properties and capability of formation of well-dispersed aqueous colloids in aqueous phase, graphene oxide can be used for the efficient preconcentration of heavy metal ions prior to their determination. The complete collection of graphene oxide colloids from water has generally been considered to be insurmountable. Here, graphene oxide aggregation triggered by introducing NaCl was used to develop a novel organic solvent-free cloud point extraction-like method for the determination of trace toxic metals. The graphene oxide sheets were uniformly dispersed in aqueous samples or standard solutions for a fast and efficient adsorption of Pb­(II), Cd­(II), Bi­(III), and Sb­(III) owing to its hydrophilic character and the electrostatic repulsion among the graphene oxide sheets, and its aggregation immediately occurred when the electrostatic repulsion was eliminated via adding NaCl to neutralize the excessive negative charges on the surface of graphene oxide sheets. The aggregates of graphene oxide and analytes ions were separated and treated with hydrochloric acid to form a slurry solution. The slurry solution was pumped to mix with KBH₄ solution to generate hydrides, which were subsequently separated from the liquid phase and directed to an atomic fluorescence spectrometer or directly introduced to an inductively coupled plasma optical emission spectrometer for detection. On the basis of a 50 mL sample volume, the limits of detection of 0.01, 0.002, 0.01, and 0.006 ng mL^–1 were obtained for Pb, Cd, Bi, and Sb, respectively, when using atomic fluorescence spectrometry, providing 35-, 8-, 36-, and 37-fold improvements over the conventional method. Detection limits of 0.6, 0.15, 0.1, and 1.0 ng mL^–1 were obtained with the use of slurry sampling inductively coupled plasma optical emission spectrometry. The method was applied for analysis of two Certified Reference Materials and three water samples for these elements

Phosphorescent Differential Sensing of Physiological Phosphates with Lanthanide Ions-Modified Mn-Doped ZnCdS Quantum Dots

Phosphates, both inorganic and organic, play fundamental roles in numerous biological and chemical processes. The biological functions of phosphates connect with each other, analysis of single phosphate-containing biomolecule therefore cannot reveal the exact biological significance of phosphates. Sensor array is therefore the best choice for differentiation analysis of physiological phosphates. Lanthanide ions possess high affinity toward physiological phosphates, while lanthanide ions can also efficiently quench the luminescence of quantum dots (QDs). Taking lanthanide ions as cartridges, here we proposed a sensor array for sensing of physiological phosphates based on lanthanide ions-modified Mn-doped ZnCdS phosphorescent QDs in the manner of indicator-displacement assay. A series of lanthanide ions were selected as quencher for phosphorescent QDs. Physiological phosphates could subsequently displace the quencher and recover the phosphorescence. Depending on their varied phosphorescence restoration, a sensor array was thus developed. The photophysics of phosphorescence quenching and restoration were studied in detail for better understanding the mechanism of the sensor array. The exact contribution of each sensor element to the sensor array was evaluated. Those sensor elements with little contribution to the differentiation analysis were removed for narrowing the size of the array. The proposed sensor array was successfully explored for probing nucleotide phosphates-involved enzymatic processes and their metabolites, simulated energy charge changes, and analysis of physiological phosphates in biological samples

Biobar-Coded Gold Nanoparticles and DNAzyme-Based Dual Signal Amplification Strategy for Ultrasensitive Detection of Protein by Electrochemiluminescence

A dual signal amplification strategy for electrochemiluminescence (ECL) aptasensor was designed based on biobar-coded gold nanoparticles (Au NPs) and DNAzyme. CdSeTe@ZnS quantum dots (QDs) were chosen as the ECL signal probes. To verify the proposed ultrasensitive ECL aptasensor for biomolecules, we detected thrombin (Tb) as a proof-of-principle analyte. The hairpin DNA designed for the recognition of protein consists of two parts: the sequences of catalytical 8–17 DNAzyme and thrombin aptamer. Only in the presence of thrombin could the hairpin DNA be opened, followed by a recycling cleavage of excess substrates by catalytic core of the DNAzyme to induce the first-step amplification. One part of the fragments was captured to open the capture DNA modified on the Au electrode, which further connected with the prepared biobar-coded Au NPs-CdSeTe@ZnS QDs to get the final dual-amplified ECL signal. The limit of detection for Tb was 0.28 fM with excellent selectivity, and this proposed method possessed good performance in real sample analysis. This design introduces the new concept of dual-signal amplification by a biobar-coded system and DNAzyme recycling into ECL determination, and it is promising to be extended to provide a highly sensitive platform for various target biomolecules

Three Birds with One Fe₃O₄ Nanoparticle: Integration of Microwave Digestion, Solid Phase Extraction, and Magnetic Separation for Sensitive Determination of Arsenic and Antimony in Fish

An environmentally friendly and fast sample treatment approach that integrates accelerated microwave digestion (MWD), solid phase extraction, and magnetic separation into a single step was developed for the determination of arsenic and antimony in fish samples by using Fe₃O₄ magnetic nanoparticles (MNPs). Compared to conventional microwave digestion, the consumption of HNO₃ was reduced significantly to 12.5%, and the digestion time and temperature were substantially decreased to 6 min and 80 °C, respectively. This is largely attributed to Fe₃O₄ magnetic nanoparticles being a highly effective catalyst for rapid generation of oxidative radicals from H₂O₂, as well as an excellent absorber of microwave irradiation. Moreover, potential interferences from sample matrices were eliminated because the As and Sb species adsorbed on the nanoparticles were efficiently separated from the digests with a hand-held magnet prior to analysis. Limits of detection for arsenic and antimony were in the range of 0.01–0.06 μg g^–1 and 0.03–0.08 μg g^–1 by using hydride generation atomic fluorescence spectrometry, respectively, and further improved to 0.002–0.005 μg g^–1 and 0.005–0.01 μg g^–1 when inductively coupled plasma mass spectrometry was used as a detector. The precision of replicate measurements (<i>n</i> = 9) was better than 6% by analyzing 0.1 g test sample spiked with 1 μg g^–1 arsenic and antimony. The proposed method was validated by analysis of two certified reference materials (DORM-3 and DORM-4) with good recoveries (90%–106%)

A Target-Triggered DNAzyme Motor Enabling Homogeneous, Amplified Detection of Proteins

We report here the concept of a self-powered, target-triggered DNA motor constructed by engineering a DNAzyme to adapt into binding-induced DNA assembly. An affinity ligand was attached to the DNAzyme motor via a DNA spacer, and a second affinity ligand was conjugated to the gold nanoparticle (AuNP) that was also decorated with hundreds of substrate strands serving as a high-density, three-dimensional track for the DNAzyme motor. Binding of a target molecule to the two ligands induced hybridization between the DNAzyme and its substrate on the AuNP, which are otherwise unable to spontaneously hybridize. The hybridization of DNAzyme with the substrate initiates the cleavage of the substrate and the autonomous movement of the DNAzyme along the AuNP. Each moving step restores the fluorescence of a dye molecule, enabling monitoring of the operation of the DNAzyme motor in real time. A simple addition or depletion of the cofactor Mg²⁺ allows for fine control of the DNAzyme motor. The motor can translate a single binding event into cleavage of hundreds of substrates, enabling amplified detection of proteins at room temperature without the need for separation