Ring-Oven Based Preconcentration Technique for Microanalysis: Simultaneous Determination of Na, Fe, and Cu in Fuel Ethanol by Laser Induced Breakdown Spectroscopy

Ring-Oven-Assisted In Situ Synthesis of Metal-Organic Frameworks on the Lab-On-Paper Device for Chemiluminescence Detection of Nitrite in Whole Blood

In situ synthesis of metal-organic frameworks (MOFs) on flexible materials for the fabrication of functional platforms and micro-devices is challenging. The time-/precursor-consuming procedure and uncontrollable assembly are stumbling blocks for constructing this platform. Herein, a novel in situ MOF synthesis method on paper substrates by use of the ring-oven-assisted technique was reported. Utilizing the ring-oven's heating and washing function, MOFs can be synthesized in 30 min on the designated position of paper chips with extremely low-volume precursors. The principle of this method was explained by steam condensation deposition. The MOFs' growth procedure was theoretically calculated by crystal sizes and the results conformed to the Christian equation. As different MOFs (Cu-MOF-74, Cu-BTB, Cu-BTC) can be synthesized successfully on paper-based chips, the ring-oven-assisted in situ synthesis method has great generality. Then, the prepared Cu-MOF-74 loading paper-based chip was applied to the chemiluminescence (CL) detection of nitrite (NO₂^-), based on the catalysis effect of Cu-MOF-74 on the NO₂^--H₂O₂ CL system. Also, by the delicate design of the paper-based chip, NO₂^- can be detected with the detection limit (DL) of 0.5 nM in whole blood samples without sample pretreatment. This work establishes a distinctive method for the in situ synthesis of MOFs and the application of MOFs on paper-based CL chips.

Determining metal elements in liquid samples using laser-induced breakdown spectroscopy and phase conversion technology

A phase conversion technology, involving the loading of brine samples with anionic polyacrylamide (APAM) colloidal droplets, is proposed to detect metal elements rapidly and accurately in liquid samples using laser-induced breakdown spectroscopy. The experimental conditions were optimized by comparing the obtained emission intensities and the signal-to-noise ratios, including the concentration of APAM, volume ratio of APAM solution to sample, delay time, and lens-to-sample distance (LTSD). Three kinds of complex brine samples with slightly soluble salts were used to test the analytical performance of the proposed method. The results show that the discrepancies of the concentrations of Li, Sr and Ca were 0.74-3.59%, compared with those obtained using inductively coupled plasma-optical emission spectrometry. This suggests that the proposed method can successfully determine metal elements in liquid samples, featuring short sample preparation time (less than 20 min), small sample volume (10 μL), and simple operation (no adsorption).

Ultrasensitive and Simultaneous Detection of Multielements in Aqueous Samples Based on Biomimetic Array Combined with Laser-Induced Breakdown Spectroscopy

Ultrasensitive detection of metallic elements in liquids has attracted considerable attention in fields such as environmental pollution monitoring and drinking water quality control. Hence, it is of great significance to develop a sensitive and simultaneous detection strategy for multiple metal elements in liquid. Laser-induced breakdown spectroscopy (LIBS) technology shows unique advantages because of its simple, rapid, and real-time in situ detection, but the laser energy will be greatly attenuated in the liquids; thus, the sensitivity of LIBS for direct detection of metal elements in liquid samples will decrease sharply. In this study, inspired by the structure of Stenocara beetle's back, a superhydrophobic biomimetic interface with hydrophilic array was prepared for enriching low-concentration targets into detection regions, and the biomimetic array LIBS (BA-LIBS) was successfully established. The ultrasensitive and simultaneous detection of nine metal elements in drinking water was realized based on the effective enrichment method. The limits of detection of the nine metal elements in mixed solution ranged from 8.3 ppt to 13.49 ppb. With these excellent properties, this facile and ultrasensitive BA-LIBS strategy might provide a new idea for the prevention and control of metal hazards in the liquid environment.

A novel hybrid filter/wrapper method for feature selection in archaeological ceramics classification by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been appreciated as a valuable analytical tool in the cultural heritage field owing to its unique technological superiority, particularly in combination with chemometric methods. Feature selection (FS) as an indispensable pre-processing step in data optimization, for eliminating the redundant or irrelevant features from high-dimensional data to enhance the predictive capacity and result comprehensibility of multivariate classification based on LIBS technology. In this paper, a novel hybrid filter/wrapper method based on the MI-DBS algorithm was proposed to enhance the qualitative analysis performance of the LIBS technique. The proposed method combines the advantages of the mutual information (MI) algorithm based filter method and bi-directional selection (DBS) algorithm based wrapper method. The MI algorithm is the first to remove the redundant or uncorrelated features so that a simplified input subset can be established. Then, the DBS algorithm is used to further select the retained features and hence to seek an optimal feature subset with good predictive performance. To benefit the above feature selection process, the wavelet transform denoising (WTD) method was used to reduce the noise from LIBS spectra. LIBS experiments were performed using 35 archaeological ceramic samples. Besides, the proposed hybrid filter/wrapper method was implemented through a random forest (RF) based nonlinear multivariate classification method. Through a comparison between several other feature selection methods and the proposed method, it has been seen that the proposed method is the best regarding the predictive performance and number of the selected features. Finally, the MI-DBS algorithm is used to seek the optimal features from the full spectrum (220-720 nm); the corresponding sensitivity, specificity and accuracy acquired through the RF classifier for the test set were 0.9722, 0.9956 and 0.9850. It is shown from the general results that the MI-DBS algorithm is more effective in terms of improving the model performance and decreasing the redundant or uncorrelated features and computational time and serves as a good alternative for FS in multivariate classification.

Matte photographic paper as a low-cost material for metal ion retention and elemental measurements with laser-induced breakdown spectroscopy

Matte photographic paper was explored as a low-cost material for metal-ion retention. The extraction was promoted by ultrasound, the mechanical waves of which forced adhesion to the photographic paper surface of an ion pair formed by DTAB (dodecyltrimethylammonium bromide) with the anionic coordination complex formed by metal ions and SPADNS (1,8-dihydroxy-2-(4-sulfophenylazo)naphthalene-3,6-disulfonic acid trisodium salt). As a proof of concept, the migration of copper ions present in aqueous solution to the solid phase and their direct measurement by laser-induced breakdown spectroscopy (LIBS) was evaluated. The main parameters that affected the extraction (paper type and sonication time) and analyte detection in the solid phase (delay, spot size and accumulated number of pulses) were optimised in a univariate way. The conditions influencing the complexation reaction and formation of the ion pair in aqueous solution (pH and copper, SPADNS and DTAB concentrations) were set according to a previous study. Under the optimised conditions, it was possible to use this extraction technology as an alternative in determining copper content in aqueous solutions by LIBS, overcoming the intrinsic difficulties of this instrumental technique in the analysis of liquid samples. The calibration curves were obtained in a linear range of 0.50-7.50 mg L^-1 copper in solution, with detection and quantification limits estimated at 0.08 and 0.24 mg L^-1, respectively. The precision of the method was less than 4.3% and the accuracy was checked by spike and recovery tests on liquid samples with different chemical compositions and by comparison of LIBS results with graphite furnace atomic absorption spectrometry data. The potential of the proposed method for extraction of Al³⁺, Ag⁺, Fe³⁺, Mn²⁺, Ni²⁺ and Zn²⁺ was also evaluated and adequate extraction efficiencies were obtained. The proposed method stands out due to its low cost and ease of execution.

Exploring the metallochromic behavior of pentacyanidoferrates in visual, electronic and Raman spot tests

Pentacyanidoferrate(II) complexes of aromatic N-heterocycles, such as 4-cyanopyridine, exhibit characteristic colors and strong metallochromism associated with the donor-acceptor interactions of the metal ions with the cyanide ligands. In the presence of transition metal ions insoluble polymeric complexes are formed, displaying bright yellow, red, brown and green colors with zinc(II), nickel(II), copper(II) and iron(III) ions, respectively. Such metallochromic response is better observed on filter paper, allowing applications in analytical spot tests. The effects can be explored visually and probed by means of modern instrumental facilities, including spectrophotometric and resonance Raman techniques. In this way, by using the cyanopyridinepentacyanidoferrates, the Prussian Blue test for ferric ions can be extended to the entire row of transition metal elements, providing a new and modern insight of such classical Feigl's spot tests.

Determination of trace heavy metal elements in aqueous solution using surface-enhanced laser-induced breakdown spectroscopy

Heavy metal pollution is one of the main problems in water pollution, which is harmful to humans. Surface-enhanced laser-induced breakdown spectroscopy (SENLIBS) has been applied to detect trace amounts of heavy metal elements in aqueous solution; however, it is still a big challenge to explore the relationship between the LIBS detection sensitivity and the substrate's physical properties. In this work, four typical substrates, zinc (Zn), magnesium alloy (Mg), nickel (Ni), and silicon (Si), were compared; and the mechanism of spectral enhancement by different substrates in SENLIBS was investigated. The results indicated that the limit of detection (LoD) of heavy metal elements on different substrates is positively proportional to the boiling of the substrate. That is mainly because a higher plasma excitation temperature and electron density are obtained, leading to more intense collision between particles. The signal enhancement is associated with the lower boiling point of the substrate (corresponding to a lower ablation threshold and higher ablation quantity from the substrate). As a result, the best LoD was 0.0011 mg/L for chromium (Cr) and 0.004 mg/L for lead (Pb) on an optimal Zn substrate, respectively. The LoDs were sufficiently low to meet the drinking water sanitation standard. These results showed that the detection sensitivity of heavy metal elements in aqueous solution can be improved by choosing a substrate with a lower boiling point in SENLIBS.

Highly concentrated, ring-shaped phase conversion laser-induced breakdown spectroscopy technology for liquid sample analysis

A highly concentrated, ring-shaped phase conversion (RSPC) method was developed for liquid sample analysis using the laser-induced breakdown spectroscopy (LIBS) technique. In this work, test samples were prepared by mixing the metal particles with polyvinyl alcohol (PVA) supporter in liquid phase. With heat, the PVA solution solidified inside a modified glass petri dish, forming a metal-enriched polymer ring film. Distinguished from other traditional liquid-to-solid conversing methods, the proposed new method takes advantage of enhanced homogeneity for the target elements inside the ring film. The modified glass petri dish was used to control the ring-shaped concentration. Due to the specially designed circular groove at the bottom of this dish, where the PVA solution and liquid sample mixture accumulated, the target elements were concentrated in this small ring, which is beneficial for enhancing and stabilizing the plasma signals compared to the direct liquid sample analysis using LIBS. The limits of detection for Ag, Cu, Cr, and Ba obtained with the RSPC-LIBS technology were 0.098  μg·mL^-1, 0.18  μg·mL^-1, 0.83  μg·mL^-1, and 0.046  μg·mL^-1, respectively, which provided greater improvement than the direct bulk liquid analysis using LIBS.

Zinc oxide nanorods functionalized paper for protein preconcentration in biodiagnostics

Distinguishing a specific biomarker from a biofluid sample containing a large variety of proteins often requires the selective preconcentration of that particular biomarker to a detectable level for analysis. Low-cost, paper-based device is an emerging opportunity in diagnostics. In the present study, we report a novel Zinc oxide nanorods functionalized paper platform for the preconcentration of Myoglobin, a cardiac biomarker. Zinc oxide nanorods were grown on a Whatman filter paper no. 1 via the standard hydrothermal route. The growth of Zinc oxide nanorods on paper was confirmed by a combination of techniques consisting of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS,) scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDX) analysis. The Zinc oxide nanorods modified Whatman filter paper (ZnO-NRs/WFP) was further tested for use as a protein preconcentrator. Paper-based ELISA was performed for determination of pre-concentration of cardiac marker protein Myoglobin using the new ZnO-NRs/WFP platform. The ZnO-NRs/WFP could efficiently capture the biomarker even from a very dilute solution (Myoglobin < 50 nM). Our ELISA results show a threefold enhancement in protein capture with ZnO-NRs/WFP compared to unmodified Whatman filter paper, allowing accurate protein analysis and showing the diagnostic concept.

Melted Paraffin Wax as an Innovative Liquid and Solid Extractant for Elemental Analysis by Laser-Induced Breakdown Spectroscopy

This work proposes a new development in the use of melted paraffin wax as a new extractant in a procedure designed to aggregate the advantages of liquid phase extraction (extract homogeneity, fast, and efficient transfer, low cost and simplicity) to solid phase extraction. As proof of concept, copper(II) in aqueous samples was converted into a hydrophobic complex of copper(II) diethyldithiocarbamate and subsequently extracted into paraffin wax. Parameters which affect the complexation and extraction (pH, DDTC, and Triton X-100 concentration, vortex agitation time and complexation time) were optimized in a univariate way. The combination of the extraction proposed procedure with laser-induced breakdown spectroscopy allowed the precise copper determination (coefficient of variation = 3.1%, n = 10) and enhanced detectability because of the concentration factor of 18 times. A calibration curve was obtained with a linear range of 0.50-10.00 mg L^-1 (R² = 0.9990, n = 7), LOD = 0.12 mg L^-1, and LOQ = 0.38 mg L^-1 under optimized conditions. An extraction procedure efficiency of 94% was obtained. The accuracy of the method was confirmed through the analysis of a reference material of human blood serum, by the spike and recovery trials with seawater, tap water, mineral water, and alcoholic beverages and by comparing with those results obtained by graphite furnace atomic absorption spectrometry.

Laser-induced breakdown spectroscopy technique for quantitative analysis of aqueous solution using matrix conversion based on plant fiber spunlaced nonwovens

In the present work, laser-induced breakdown spectroscopy (LIBS) was applied to detect concentrations of chromium and nickel in aqueous solution in the form of matrix conversion using plant fiber spunlaced nonwovens as a solid-phase support, which can effectively avoid the inherent difficulties such as splashing, a quenching effect, and a shorter plasma lifetime during the liquid LIBS analysis. Drops of the sample solution were transferred to the plant fiber spunlaced nonwovens surface and uniformly diffused from the center to the whole area of the substrate. Owing to good hydrophilicity, the plant fiber spunlaced nonwovens can hold more of the liquid sample, and the surface of this material never wrinkles after being dried in a drying oven, which can effectively reduce the deviation during the LIBS analysis. In addition, the plant fiber spunlaced nonwovens used in the present work are relatively convenient and low cost. Also, the procedure of analysis was simple and fast, which are the unique features of LIBS technology. Therefore, this method has potential applications for practical and in situ analyses. To achieve sensitive elemental detection, the optimal delay time in this experiment was investigated. Under the optimized condition, the limits of detection for Cr and Ni are 0.7 and 5.7  μg·mL(-1), respectively. The results obtained in the present study show that the matrix conversion method is a feasible option for analyzing heavy metals in aqueous solutions by LIBS technology.

Ring-Oven Washing Technique Integrated Paper-based Immunodevice for Sensitive Detection of Cancer Biomarker

A paper-based microfluidic immunodevice has recently attracted considerable interest for point-of-care testing (POCT) and a washing procedure was used as a standard procedure in immunoassay to eliminate the nonspecific binding protein from a paper surface. However, the traditional washing method cannot get rid of the nonspecific binding protein more completely to get a lower background. In this work, a novel washing strategy with a ring-oven technique integrated on a paper-based immunodevice was presented, which can effectively wash a nonspecific binding protein and enable a low background for sensitive detection of the carcinoembryonic antigen (CEA). By immobilizing the antibody on the detection area and incorporating the temperature-controlled ring-oven under the paper-based device, the continuous washing solution can carry the nonspecific binding protein to the waste area freely by capillary force and then the waste area dried quickly by heating. The paper device, which is matched to the size of the ring-oven, is composed of eight microfluidic channels by the simple and rapid paper-cutting fabrication method. With the HRP-catalyzed 3,3',5,5'-tetramethylbenzidine (TMB)-H2O2 colorimetric detection method, a lower detection limit of 0.03 ng/mL CEA can be obtained by enzyme-linked immunosorbent assay (ELISA). The washing efficiency for the nonspecific binding protein was improved a lot compared to the traditional washing methods, and the established paper-based device can be used in the determination of CEA in human serum with high sensitivity. The paper-based device provides a new washing strategy for sensitive immunoassay and point-of-care diagnostics.

Selective extraction and release using (EDTA-Ni)-layered double hydroxide coupled with catalytic oxidation of 3,3',5,5'-tetramethylbenzidine for sensitive detection of copper ion

Copper is an important heavy metal in various biological processes. Many methods have been developed for detecting of copper ions (Cu(2+)) in aqueous samples. However, an easy, cheap, selective and sensitive method is still desired. In this study, a selective extraction-release-catalysis approach has been developed for sensitive detection of copper ion. Ethylenediaminetetraacetic acid (EDTA) chelated with nickel ion (Ni(2+)) were intercalated in a layered double hydroxide via a co-precipitation reaction. The product was subsequently applied as sorbent in dispersive solid-phase extraction for the enrichment of Cu(2+) at pH 6. Since Cu(2+) has a stronger complex formation constant with EDTA, Ni(2+) exchanged with Cu(2+) selectively. The resulting sorbent containing Cu(2+) was transferred to catalyze the 3,3',5,5'-tetramethylbenzidine oxidation reaction, since Cu(2+) could be released by the sorbent effectively and has high catalytic ability for the reaction. Blue light emitted from the oxidation product was measured by ultraviolet-visible spectrophotometry for the determination of Cu(2+). The extraction temperature, extraction time, and catalysis time were optimized. The results showed that this method provided a low limit of detection of 10nM, a wide linear range (0.05-100μM) and good linearity (r(2)=0.9977). The optimized conditions were applied to environmental water samples. Using Cu(2+) as an example, this work provided a new and interesting approach for the convenient and efficient detection of metal cations in aqueous samples.