A microfluidic paper-based analytical device for rapid quantification of particulate chromium

Occupational exposure to Cr is concerning because of its myriad of health effects. Assessing chromium exposure is also cost and resource intensive because the analysis typically uses sophisticated instrumental techniques like inductively coupled plasma-mass spectrometry (ICP-MS). Here, we report a novel, simple, inexpensive microfluidic paper-based analytical device (μPAD) for measuring total Cr in airborne particulate matter. In the μPAD, tetravalent cerium (Ce(IV)) was used in a pretreatment zone to oxidize all soluble Cr to Cr(VI). After elution to the detection zone, Cr(VI) reacts with 1,5-diphenylcarbazide (1,5-DPC) forming 1,5-diphenylcarbazone (DPCO) and Cr(III). The resulting Cr(III) forms a distinct purple colored complex with the DPCO. As proof-of-principle, particulate matter (PM) collected on a sample filter was analyzed with the μPAD to quantify the mass of total Cr. A log-linear working range (0.23–3.75 μg; r² = 0.998) between Cr and color intensity was obtained with a detection limit of 0.12 μg. For validation, a certified reference containing multiple competing metals was analyzed. Quantitative agreement was obtained between known Cr levels in the sample and the Cr measured using the μPAD.     

Highly selective and sensitive paper-based colorimetric sensor using thiosulfate catalytic etching of silver nanoplates for trace determination of copper ions

A novel, highly selective and sensitive paper-based colorimetric sensor for trace determination of copper (Cu²⁺) ions was developed. The measurement is based on the catalytic etching of silver nanoplates (AgNPls) by thiosulfate (S₂O₃²⁻). Upon the addition of Cu²⁺ to the ammonium buffer at pH 11, the absorption peak intensity of AuNPls/S₂O₃²⁻ at 522 nm decreased and the pinkish violet AuNPls became clear in color as visible to the naked eye. This assay provides highly sensitive and selective detection of Cu²⁺ over other metal ions (K⁺, Cr³⁺, Cd²⁺, Zn²⁺, As³⁺, Mn²⁺, Co²⁺, Pb²⁺, Al³⁺, Ni²⁺, Fe³⁺, Mg²⁺, Hg²⁺ and Bi³⁺). A paper-based colorimetric sensor was then developed for the simple and rapid determination of Cu²⁺ using the catalytic etching of AgNPls. Under optimized conditions, the modified AgNPls coated at the test zone of the devices immediately changes in color in the presence of Cu²⁺. The limit of detection (LOD) was found to be 1.0 ng mL⁻¹ by visual detection. For semi-quantitative measurement with image processing, the method detected Cu²⁺ in the range of 0.5–200 ng mL⁻¹(R² = 0.9974) with an LOD of 0.3 ng mL⁻¹. The proposed method was successfully applied to detect Cu²⁺ in the wide range of real samples including water, food, and blood. The results were in good agreement according to a paired t-test with results from inductively coupled plasma-optical emission spectrometry (ICP-OES).     

Selective determination of homocysteine levels in human plasma using a silver nanoparticle-based colorimetric assay

The first use of silver nanoparticles (AgNPs) for the rapid, simple, and selective determination of homocysteine (Hcy) levels in human plasma was studied. Hcy and five other amino acids, including cysteine (Cys), could be distinguished by their different aggregation kinetics, which caused a change in the visible color and a shift in the UV-vis absorption spectra. The difference in the cross-linking (aggregation) rate between Hcy and Cys was used as the basis for developing a selective probe for Hcy and allowed the detection of Hcy in the linear range of 2-12 μM (R² = 0.9936). The limits of detection and quantification were found to be 0.5 μM and 1.7 μM, respectively. To investigate its selectivity and potential applicability, this AgNP-based method was successfully applied for the determination of Hcy levels in actual biological (human plasma) samples, where the determined levels of Hcy were within the error range of the measured level using the traditional chemiluminescence microparticle immunoassay (CMIA). Thus, the use of AgNPs is a feasible and potentially reliable method for the determination of Hcy levels in biological samples.     

Electrochemical sensors for the simultaneous determination of zinc,cadmium and lead using a Nafion/ionic liquid/graphene composite modified screen-printed carbon electrode

A simple, low cost, and highly sensitive electrochemical sensor, based on a Nafion/ionic liquid/graphene composite modified screen-printed carbon electrode (N/IL/G/SPCE) was developed to determine zinc (Zn(II)), cadmium (Cd(II)), and lead (Pb(II)) simultaneously. This disposable electrode shows excellent conductivity and fast electron transfer kinetics. By in situ plating with a bismuth film (BiF), the developed electrode exhibited well-defined and separate peaks for Zn(II), Cd(II), and Pb(II) by square wave anodic stripping voltammetry (SWASV). Analytical characteristics of the BiF/N/IL/G/SPCE were explored with calibration curves which were found to be linear for Zn(II), Cd(II), and Pb(II) concentrations over the range from 0.1 to 100.0 ng L⁻¹. With an accumulation period of 120 s detection limits of 0.09 ng mL⁻¹, 0.06 ng L⁻¹ and 0.08 ng L⁻¹ were obtained for Zn(II), Cd(II) and Pb(II), respectively using the BiF/N/IL/G/SPCE sensor, calculated as 3σ value of the blank. In addition, the developed electrode displayed a good repeatability and reproducibility. The interference from other common ions associated with Zn(II), Cd(II) and Pb(II) detection could be effectively avoided. Finally, the proposed analytical procedure was applied to detect the trace metal ions in drinking water samples with satisfactory results which demonstrates the suitability of the BiF/N/IL/G/SPCE to detect heavy metals in water samples and the results agreed well with those obtained by inductively coupled plasma mass spectrometry.     

Development of a sensitive micro-magnetic chemiluminescence enzyme immunoassay for the determination of carcinoembryonic antigen

A micro-magnetic chemiluminescence (CL) enzyme immunoassay with high sensitivity, selectivity, and reproducibility was developed for the determination of the tumor marker, carcinoembryonic antigen (CEA) in human serum. A sandwich scheme assay has been utilized with fluorescein isothiocyanate antibody (FITC)-labeled anti-CEA antibody and alkaline phosphate (ALP)-labeled anti-CEA antibody being used in the CL detection. The CL signal produced by the emission of photons from 4-methoxy-4-(3-phosphate-phenyl)-spiro-(1,2-dioxetane-3,2′-adamantane) (AMPPD) was directly proportional to the amount of analyte present in a sample solution. The influences of the reaction time of antigen with antibody, the reaction time of substrate with label, the dilution ratio of ALP-labeled anti-CEA antibody, the concentration of FITC-labeled anti-CEA antibody, and other relevant variables upon the CL signal were examined and optimized. The CL responses depended linearly on the CEA concentration over the range from 2 to 162 ng mL⁻¹ in a logarithmic plot. Assay sensitivity as low as 0.69 ng mL⁻¹ was achieved. A coefficient of variance of less than 13% was obtained for intra- and inter-assay precision. This method has been successfully applied to the analysis of CEA in human serum. According to the procedure based on spiked standards, the recoveries obtained were 80–110%. Comparison experiments were carried out with the commercially available CEA chemiluminescence immunoassay. Satisfactory results were obtained according to a paired t-test method (t value < t _critical at the 95% confidence level).     

Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples

We report the development of an electrochemical paper-based analytical device (ePAD) for the selective determination of dopamine (DA) in model serum sample. The ePAD device consists of three layers. In the top layer, SU-8 photoresist defines a hydrophilic sample application spot on the filter paper. The middle layer was made from transparency film and contained two holes, one for sample preconcentration and the other for the surfactant to allow transfer to the third layer. A screen-printed carbon electrode formed the bottom layer and was used for electrochemical measurements. In the absence of the anionic surfactant, sodium dodecyl sulfate (SDS), the oxidation peaks of DA, ascorbic acid (AA) and uric acid (UA) overlapped. With the addition of SDS, the DA oxidation peak shifted to more negative values and was clearly distinguishable from AA and UA. The oxidation potential shift was presumably due to preferential electrostatic interactions between the cationic DA and the anionic SDS. Indeed, whilst the SDS-modified paper improved the DA current five-fold, the non-ionic Tween-20 and cationic tetradecyltrimethylammonium bromide surfactants had no effect or reduced the current, respectively. Furthermore, only the SDS-modified paper showed the selective shift in oxidation potential for DA. DA determination was carried out using square-wave voltammetry between −0.2 and 0.8 V vs. Ag/AgCl, and this ePAD was able to detect DA over a linear range of 1–100 μM with a detection limit (S/N = 3) of 0.37 μM. The ePAD seems suitable as a low cost, easy-to-use, portable device for the selective quantitation of DA in human serum samples.     

Continuous monitoring with microfabricated capillary electrophoresis chip devices

We report on the direct coupling of hydrodynamically flowing stream to a microchip capillary electrophoresis (CE) for continuous assays of liquid samples. The new interface relies on mounting the sample tubing onto a sharp inlet tip and allows rapid, convenient and reproducible electrokinetic loading from a continuously flowing stream directly into the narrow separation microchannel. The sharp inlet interface is characterized by its efficiency, stability and simplicity. The effect of the sample flow rate, applied voltages and other relevant variables, is described. It was found that the peak intensity is independent of the flow rate. The performance of the new interface is illustrated for on-line CE-electrochemical monitoring of phenolic and explosive compounds. Conditions simulating continuous long-term monitoring, led to a highly stable response for a 15 ppm 1,3,5-trinitrobenzene solution (RSD = 3.7%, n= 40). Such ability to continuously introduce flowing samples into micrometer channels makes 'lab-on-a-chip' devices highly compatible with real-life monitoring applications.     

Microchip capillary electrophoresis/electrochemical detection of hydrazine compounds at a cobalt phthalocyanine modified electrochemical detector

This article reports on the use of cobalt(II) phthalocyanine (CoPc)-modified carbon paste amperometric detector for monitoring hydrazine compounds following their microchip separation. The marked catalytic electrochemical properties of CoPc-modified electrode display enhanced sensitivity compared with unmodified carbon pastes at a relatively low detection potential (+0.5 V versus Ag/AgCl). Factors influencing the on-chip separation and detection processes have been optimized. Three hydrazines (hydrazine, 1,1 dimethylhydrazine, and phenylhydrazine) have been separated within 130 s at a separation voltage of 1 kV using a 10 mM phosphate run buffer (pH 6.5). The detection limits obtained from using the CoPc-modified carbon paste electrodes for hydrazine and phenylhydrazine are 0.5 and 0.7 μM, respectively, with linearity over the 20–200 μM range examined. Such miniaturization and speed advantages of microchip CE are coupled to the highly sensitivity and convenient preparation of CoPc-modified carbon paste electrode. The resulting microsystem should be attractive for field monitoring of toxic hydrazine compounds in environmental applications.     

Graphene-polyaniline modified electrochemical droplet-based microfluidic sensor for high-throughput determination of 4-aminophenol

We report herein the first development of graphene-polyaniline modified carbon paste electrode (G-PANI/CPE) coupled with droplet-based microfluidic sensor for high-throughput detection of 4-aminophenol (4-AP) in pharmaceutical paracetamol (PA) formulations. A simple T-junction microfluidic platform using an oil flow rate of 1.8 μL/min and an aqueous flow rate of 0.8 μL/min was used to produce aqueous testing microdroplets continuously. The microchannel was designed to extend the aqueous droplet to cover all 3 electrodes, allowing for electrochemical measurements in a single droplet. Parameters including flow rate, water fraction, and applied detection potential (E_det) were investigated to obtain optimal conditions. Using G-PANI/CPE significantly increased the current response for both cyclic voltammetric detections of ferri/ferrocyanide [Fe(CN)₆]^3−/4− (10 times) and 4-AP (2 times), compared to an unmodified electrode. Using the optimized conditions in the droplet system, 4-AP in the presence of PA was selectively determined. The linear range of 4-AP was 50–500 μM (R² = 0.99), limit of detection (LOD, S/N = 3) was 15.68 μM, and limit of quantification (LOQ, S/N = 10) was 52.28 μM. Finally, the system was used to determine 4-AP spiked in commercial PA liquid samples and the amounts of 4-AP were found in good agreement with those obtained from the conventional capillary zone electrophoresis/UV–Visible spectrophotometry (CZE/UV–Vis). The proposed microfluidic device could be employed for a high-throughput screening (at least 60 samples h⁻¹) of pharmaceutical purity requiring low sample and reagent consumption.     

A flow injection method for the analysis of tetracycline antibiotics in pharmaceutical formulations using electrochemical detection at anodized boron-doped diamond thin film electrode

A method using flow injection (FI) with amperometric detection at anodized boron-doped diamond (BDD) thin films has been developed and applied for the determination of tetracycline antibiotics (tetracycline, chlortetracycline, oxytetracycline and doxycycline). The electrochemical oxidation of the tetracycline antibiotics was studied at various carbon electrodes including glassy carbon (GC), as-deposited BDD and anodized BDD electrodes using cyclic voltammetry. The anodized BDD electrode exhibited well-defined irreversible cyclic voltammograms for the oxidation of tetracycline antibiotics with the highest current signals compared to the as-deposited BDD and glassy carbon electrodes. Low detection limit of 10 nM (signal-to-noise RATIO = 3) was achieved for each drug when using flow injection analysis with amperometric detection at anodized BDD electrodes. Linear calibrations were obtained from 0.1 to 50 mM for tetracycline and 0.5–50 mM for chlortetracycline, oxytetracycline and doxycycline. The proposed method has been successfully applied to determine the tetracycline antibiotics in some drug formulations. The results obtained in percent found (99.50–103.01%) were comparable to dose labeled.