Smart dual imprinted Origami 3D-ePAD for selective and simultaneous analysis of vanillylmandelic acid and 5-hydroxyindole-3-acetic acid carcinoid cancer biomarkers using graphene quantum dots coated with dual molecularly imprinted polymers

Measuring the levels of the biomarkers vanillylmandelic acid (VMA) and 5-Hydroxyindole-3-acetic acid (5-HIAA) is a valuable tool for clinical diagnosis not only of neuroblastoma or carcinoid syndrome, but also of essential hypertension, depression, migraine, and Tourette's syndrome. Herein, we explore using graphene quantum dots (GQDs) coated with molecularly imprinted polymer (MIP) as novel dual-imprinted sensors for selective and simultaneous determination of VMA and 5-HIAA in urine and plasma samples. The dual-MIP was successfully coated on the GQDs core via co-polymerization of (3-aminopropyl) triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS), acting as functional and cross-linking monomers, respectively. In addition, we successfully created the dual imprinted VMA and 5-HIAA shell on the GQDs' core via a one-pot synthesis. We fabricated a facile and ready-to-use Origami three-dimensional electrochemical paper-based analytical device (Origami 3D-ePAD) for simultaneous determination of VMA and 5-HIAA using a GQDs@dual-MIP modified graphene electrode (GQDs@dual-MIP/SPGE). The Origami 3D-ePAD was designed to form a voltammetric cell on a three-layer foldable sheet with several advantages. For example, they were quickly assembled and enhanced the device's physical durability with the hydrophobic backup sheet. The developed dual imprinted Origami 3D-ePAD leads to substantially enhanced sensitivity and selectivity to electrochemical signal amplification generated from increasing the electrode-specific surface area, electrocatalytic activity, and the large numbers of dual imprinted sites for VMA and 5-HIAA detection. The synthetic recognition sites are highly selective for 5-HIAA and VMA molecules with an imprinting factor of 8.46 and 7.10, respectively. Quantitative analysis relying on square wave voltammetry reveals excellent linear dynamic ranges of around 0.001-25 μM, with detection limits of 0.023 nM for 5-HIAA and 0.047 nM for VMA (3Sb, n = 3). The Origami 3D-ePAD provides high accuracy and precision (i.e., recovery values of 5-HIAA ranged from 82.98 to 98.40 %, and VMA ranged from 83.28 to 104.39 %), and RSD less than 4.37 %) in urine and plasma samples without any evidence of interference. Hence, it is well suited as a facile and ready-to-use disposable device for point-of-care testing. It is straightforward, cost-effective, reproducible, and stable. Furthermore, it allows for rapid analysis (analysis time ∼20s) useful in medical diagnosis and other relevant fields.

Patulin-imprinted origami 3D-ePAD based on graphene screen-printed electrode modified with Mn-ZnS quantum dot coated with a molecularly imprinted polymer

We developed a novel, compact, three-dimensional electrochemical paper-based analytical device (3D-ePAD) for patulin (PT) determination. The selective and sensitive PT-imprinted Origami 3D-ePAD was constructed based on a graphene screen-printed electrode modified with manganese-zinc sulfide quantum dots coated with patulin imprinted polymer (Mn-ZnS QDs@PT-MIP/GSPE). The Mn-ZnS QDs@PT-MIP was synthesized using 2-oxindole as the template, methacrylic acid (MAA) as a monomer, N,N'-(1,2-dihydroxyethylene) bis (acrylamide) (DHEBA) as cross-linker and 2,2'-azobis (2-methylpropionitrile) (AIBN) as initiator, respectively. The Origami 3D-ePAD was designed with hydrophobic barrier layers formed on filter paper to provide three-dimensional circular reservoirs and assembled electrodes. The synthesized Mn-ZnS QDs@PT-MIP was quickly loaded on the electrode surface by mixing with graphene ink and then screen-printing on the paper. The PT-imprinted sensor provides the greatest enhancement in redox response and electrocatalytic activity, which we attributed to synergetic effects. This arose from an excellent electrocatalytic activity and good electrical conductivity of Mn-ZnS QDs@PT-MIP, which improved electron transfer between PT and the electrode surface. Under the optimized DPV conditions, a well-defined PT oxidation peak appears at +0.15 V (vs Ag/AgCl) using 0.1 M of phosphate buffer (pH 6.5) containing 5 mM K₃Fe(CN)₆ as the supporting electrolyte. Our developed PT imprinted Origami 3D-ePAD revealed excellent linear dynamic ranges of 0.001-25 μM, with a detection limit of 0.2 nM. Detection performance indicated that our Origami 3D-ePAD possesses outstanding detection performance from fruits and CRM in terms of high accuracy (%Error for inter-day is 1.11%) and precision (%RSD less than 4.1%). Therefore, the proposed method is well-suited as an alternative platform for ready-to-use sensors in food safety. The imprinted Origami 3D-ePAD is an excellent disposable device with a simple, cost-effective, and fast analysis, and it is ready to use for determining patulin in actual samples.

Origami 3D-microfluidic paper-based analytical device for detecting carbaryl using mesoporous silica-platinum nanoparticles with a molecularly imprinted polymer shell

Herein, we present a novel Origami 3D-μPAD for colorimetric carbaryl detection using a super-efficient catalyst, namely mesoporous silica-platinum nanoparticles coated with a molecularly imprinted polymer (MSN-PtNPs@MIP). Morphological and structural characterization reveals that coating MIP on the MSN-PtNPs surface significantly increases the selective area, leading to larger numbers of imprinting sites for improved sensitivity and selectivity in determining carbaryl. The as-prepared MSN-PtNPs@MIP was used for catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂. Carbaryl selectively binds to the cavities embedded on the MSN-PtNPs surface and subsequently inhibits TMB oxidation leading the color to change to light blue. The change of reaction color from dark blue to light blue depends on the concentration of carbaryl within the 3D-μPAD detection zone. This design integrates the advantages of highly efficient sample delivery through micro channels (top layer) and efficient partition/separation paths (bottom layer) of the cellulose substrate to achieve both improved detection sensitivity and selectivity. Assay on the Origami 3D-μPAD can determine carbaryl by ImageJ detection, over a dynamic range of 0.002-20.00 mg kg^-1, with a very low limit of detection at 1.5 ng g^-1. The developed 3D-μPAD exhibit high accuracy when applied to detect carbaryl in fruits, with satisfactory recoveries from 90.1% to 104.0% and relative differences from the reference HPLC values less than 5.0%. Furthermore, the fabricated Origami 3D-μPAD provides reliable durability and good reproducibility (3.19% RSD for fifteen devices).

Fast, sensitive and selective simultaneous determination of paraquat and glyphosate herbicides in water samples using a compact electrochemical sensor

We report a new ready-to-use sensor for simultaneous determination of paraquat (PQ) and glyphosate (GLY) based on a graphite screen-printed electrode modified with a dual-molecularly imprinted polymer coated on a mesoporous silica-platinum core. Amino-mesoporous silica nanoparticles (MSN-NH2) were first synthesized by a simple co-condensation method using tetraethyl orthosilicate and 3-aminopropyltrimethoxysilane. PtNPs were then decorated on the surface of MSN-NH2 by chemical reduction. Finally, the dual-MIP was successfully coated on the MSN-PtNP core. This 3D-surface-imprinting strategy enhances the conductivity and monodispersity of the MSN-PtNPs@d-MIP. Quantitative analysis was performed by differential pulse voltammetry with an oxidation current appearing at -0.95 V for PQ and +0.97 V for GLY. The dual-MIP sensor shows good linear calibration curves in the range of 0.025-500 μM for both analytes with detection limits of 3.1 nM and 4.0 nM for PQ and GLY, respectively. The dual-MIP sensor shows high selectivity and specificity, attributed to the increased affinity of the imprinted cavities formed on the polymer film for the target PQ and GLY molecules. The proposed dual-MIP sensor was successfully applied to detect PQ and GLY concentrations simultaneously in water samples. The ready-to-use dual-MIP sensor is well suited for water-quality control and on-site applications without sophisticated instrumentation.

Smart sensor for assessment of oxidative/nitrative stress biomarkers using a dual-imprinted electrochemical paper-based analytical device

We present a novel dual-imprinted electrochemical paper-based analytical device (Di-ePAD) to simultaneously determine 8-hydroxy-2'-deoxyguanosine (8-OHdG) and 3-nitrotyrosine (3-NT) and assess oxidative and nitrative biomarkers in urine and plasma samples. The Di-ePAD was designed with hydrophobic barrier layers formed on filter paper to provide three-dimensional circular reservoirs and assembled electrodes. The molecularly imprinted polymer (MIP) was synthesized using a silica nanosphere decorated with silver nanoparticles (SiO₂@AgNPs) as a core covered with dual-analyte imprinted sites on the polymer to recognize selectively and bind the target biomarkers. This strategy drives monodispersity and enhances the conductivity of the resulting MIP core-shell products. 3-NT-MIP and 8-OHdG-MIP were synthesized by successively coating the surface of SiO₂@AgNPs with l-Cysteine via the thiol group, then terminating with MIP shells. The dual imprinted core-shell composites possess attractive properties for the target biomarkers' sensing, including catalytic activity, selectivity, and good conductivity. The Di-ePAD revealed excellent linear dynamic ranges of 0.01-500 μM for 3-NT and 0.05-500 μM for 8-OHdG, with detection limits of 0.0027 μM for 3-NT and 0.0138 μM for 8-OHdG. This newly developed method based on the synergistic effects of SiO₂@AgNPs combined with promising properties of MIP offers outstanding selectivity, sensitivity, reproducibility, simplicity, and low cost for quantitative analysis of 3-NT and 8-OHdG. The proposed Di-ePAD showed good accuracy and precision when applied to actual samples, including urine and serum samples validated by a conventional HPLC method.

Ready-to-use paraquat sensor using a graphene-screen printed electrode modified with a molecularly imprinted polymer coating on a platinum core

We propose the fabrication of a novel ready-to-use electrochemical sensor based on a screen-printed graphene paste electrode (SPGrE) modified with platinum nanoparticles and coated with a molecularly imprinted polymer (PtNPs@MIP) for sensitive and cost-effective detection of paraquat (PQ) herbicide. Successive coating of the PtNPs surface with SiO₂ and vinyl end-groups formed the PtNPs@MIP. Next, we terminated the vinyl groups with a molecularly imprinted polymer (MIP) shell. MIP was attached to the PtNPs cores using PQ as the template, methacrylic acid (MAA) as the monomer, ethylene glycol dimethacrylate (EGDMA) as the cross-linker, and 2,2'-azobisisobutyronitrile (AIBN) as the initiator. Coating the SPGrE surface with PtNPs@MIP furnished the PQ sensor. We studied the electrochemical mechanism of PQ on the MIP sensor using cyclic voltammetry (CV) experiments. The PQ oxidation current signal appears at -1.08 V and -0.71 V vs. Ag/AgCl using 0.1 M potassium sulfate solution. Quantitative analysis was performed by anodic stripping voltammetry (ASV) using a deposition potential of -1.4 V for 60 s and linear sweep voltammetric stripping. The MIP sensor provides linearity from 0.05 to 1000 μM (r² = 0.999), with a lower detection limit of 0.02 μM (at -0.71 V). The compact imprinted sensor gave a highly sensitive and selective signal toward PQ. The ready-to-use MIP sensor can provide an alternative approach to the determination of paraquat residue on vegetables and fruits for food safety applications.

Mimicking Peroxidase-Like Activity of Nitrogen-Doped Carbon Dots (N-CDs) Coupled with a Laminated Three-Dimensional Microfluidic Paper-Based Analytical Device (Laminated 3D-μPAD) for Smart Sensing of Total Cholesterol from Whole Blood

This work presents a simple hydrothermal synthesis of nitrogen-doped carbon dots (N-CDs), fabrication of microfluidic paper-based analytical device (μPAD), and their joint application for colorimetric determination of total cholesterol (TC) in human blood. The N-CDs were characterized by various techniques including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD), and the optical and electronic properties of computational models were studied using the time-dependent density functional theory (TD-DFT). The characterization results confirmed the successful doping of nitrogen on the surface of carbon dots. The N-CDs exhibited high affinity toward 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)-diammonium salt (ABTS) with the Michaelis-Menten constant (K_M) of 0.018 mM in a test for their peroxidase-like activity. Particularly, since hydrogen peroxide (H₂O₂) is the oxidative product of cholesterol in the presence of cholesterol oxidase, a sensitive and selective method of cholesterol detection was developed. Overall, the obtained results from TD-DFT confirm the strong adsorption of H₂O₂ on the graphitic N positions of the N-CDs. The laminated three-dimensional (3D)-μPAD featuring a 6 mm circular detection zone was fabricated using a simple wax screen printing technique. Classification of TC according to the clinically relevant criteria (healthy, <5.2 mM; borderline, 5.2-6.2 mM; and high risk, >6.2 mM) could be determined by the naked eye within 10 min by simple comparison using a color chart. Overall, the proposed colorimetric device serves as a low-cost, rapid, simple, sensitive, and selective alternative for TC detection in whole blood samples that is friendly to unskilled end users.

Enhanced selectivity and sensitivity for colorimetric determination of glyphosate using Mn-ZnS quantum dot embedded molecularly imprinted polymers combined with a 3D-microfluidic paper-based analytical device

We report a novel three-dimensional microfluidic paper-based analytical device (3D-μPAD) with colorimetric detection, using Mn-ZnS quantum dot embedded molecularly imprinted polymer (Mn-ZnS QD-MIP), for selective glyphosate determination in whole grain samples. Detection is based on the catalytic activity of Mn-ZnS QD-MIP in the H₂O₂ oxidation of ABTS. Glyphosate imprinted polymer is successfully synthesized on the Mn-ZnS QD surface using a poly (N-isopropylacrylamide) (NIPAM) and N, N'-Methylenebisacrylamide (MBA) as the functional monomers. The catalytic activity depends on binding or non-binding of glyphosate molecules on the synthetic recognition sites of the Mn-ZnS QD-MIP. Glyphosate selectively binds to the cavities embedded on the Mn-ZnS QD surface, and subsequently turns-off or inhibits the ABTS oxidation and color change to light green. The change of reaction color from dark green to light green depends on the concentration of glyphosate. We report, for the first time, using the relatively new penguard enamel colour to create a hydrophobic barrier. The foldable 3D-μPAD comprises three layers (top/center/bottom), named as the detection zone, immobilized Mn-ZnS QD-MIP disc, and sample loading. Assay on the 3D-μPAD can determine glyphosate by ImageJ detection, over an operating range of 0.005-50 μg mL^-1 and with a detection limit of 0.002 μg mL^-1. Our 3D-μPAD exhibits high accuracy, with a 0.4% (intra-day) and 0.7% (inter-day) relative difference from the certified CRM value. Moreover, the fabricated 3D-μPAD provides good reproducibility (1.7% RSD for ten devices). The developed 3D-μPAD was successfully applied to determine the glyphosate concentration in whole grain samples and shows great promise as an alternative highly selective and sensitive colorimetric method. The 3D-μPAD is well suited to food-quality control and onsite environmental-monitoring applications, without sophisticated instrumentation.

Acid-base titration using a microfluidic thread-based analytical device (μTAD)

This work presents the development and application of a novel analytical approach for the determination of acid and base concentrations by titration using a microfluidic thread-based analytical device (μTAD). This approach proved to be a simple to fabricate and to use, high precision, and cost-efficient means of acid-base quantification. The μTAD was fabricated by immobilizing the untreated cotton threads onto a wood frame, followed by pre-coating with an indicator (20 μL) and a primary standard solution (3 μL), and was tested using real samples including drug, food, and household products where 3 μL of each sample was dropped onto the center of a thread. Afterward, the distance of color change on the thread, easily observed and measured using the naked eye and a ruler, was used for analysis. The analysis using the μTAD, completed within 2 minutes and validated by the conventional titration, showed high accuracy and precision (RSD < 12.9%), good linearity ranges and low limit of quantification. The fabricated μTAD also remained stable for an extended period of time (>2 weeks under various storage conditions).

Novel paper-based colorimetric immunoassay (PCI) for sensitive and specific detection of salbutamol residues in flesh of swine and urine using Ag3 PO4 /Ag nanocomposite as label

Salbutamol (SAL) can cause potential hazards to human health and its use as a growth promoter in meat-producing animals is illegal. This work reports a novel approach for competitive paper-based colorimetric immunoassay (PCI) using the Ag₃ PO₄ /Ag nanocomposite as label for sensitive and specific determination of SAL in flesh of swine and urine. The Ag₃ PO₄ /Ag nanocomposite was synthesized by a one-step chemical bath method, which could instantly oxidize a chromogenic substrate for the color development under acidic conditions without the participation of H₂ O₂ . This approach provides high affinity between the Ag₃ PO₄ /Ag nanocomposite and the substrate (with the Michaelis-Menten constant of 0.44 mM). In addition, the fabrication process of the PCI was simple and cost-effective. Particularly, the novel PCI also exhibits simplicity and cost-effectiveness of the fabrication process through a simple wax screen-printing, which requires inexpensive equipment and material including a screen, wax, a squeegee, and a hair dryer. Under optimal conditions, the competitive PCI exhibited a linearity range of 0.025 to 1.00 µg/L. The developed approach offers advantages over the conventional ELISA for the purpose of routine use because it requires a shorter incubation time (<1 hr), significantly small volumes of reagents and samples (<100 µL each), and an inexpensive consumer-grade digital camera coupled with a simple gray-scale transformation of the RGB (Red Green Blue) color image for the purpose of quantification of the detection. PRACTICAL APPLICATION: Salbutamol (SAL) can cause potential hazards to human health and the use of which as growth promoter in meat-producing animals is illegal. This work introduces a novel approach for competitive immunoassay on paper-based colorimetric immunoassay using the Ag₃ PO₄ /Ag nanocomposite as the label (instead of using natural enzyme) for low-cost, sensitive, and specific determination of SAL residues at low level in flesh of swine and urine samples. The proposed approach offers advantages over the conventional ELISA for the purpose of routine use.

A non-toxic approach to assess total antioxidant capacity (TAC) of exotic tropical fruits from Thailand

A simple flow injection analysis (FIA) integrating with a metal-free approach for total antioxidant capacity (TAC) was developed. The non-toxic reaction was based on generating a vibrant blue radical from imipramine to avoid the potential interferents arising from the colorful fruit extracts. The blue radical can be rapidly scavenged by antioxidant compounds present in the sample. TAC values of Thai tropical fruit extracts were assessed by monitoring the quenching in absorbance of the test mixture following the addition of the antioxidant compounds/fruit extracts. The FIA co-operated in order to increase the sample throughput. The results demonstrated that Antidesma thwaiteaianum Muell. Arg. has the highest capacity followed by Terminalia chebula Retz. and Phyllanthus Emblica Linn., respectively. An excellent correlation between the proposed method was found with the DPPH assay. The proposed method allowed the TAC determination of fruit extracts in a high-throughput and straightforward way in accordance with the principles of green analytical chemistry.

Novel three-Dimensional molecularly imprinted polymer-coated carbon nanotubes (3D-CNTs@MIP) for selective detection of profenofos in food

A new and facile method for selective measurement of profenofos (PFF) using a simple flow-injection system with a molecularly-imprinted-polymer-coated carbon nanotube (3D-CNTs@MIP) amperometric sensor is proposed. The 3D-CNTs@MIP was synthesized by successively coating the surface of carboxylated CNTs with SiO₂ and vinyl end groups, then terminating with molecularly imprinted polymer (MIP) shells. MIP was grafted to the CNT cores using methacrylic acid (MAA) monomer, ethylene glycol dimethacrylate (EGDMA) as cross linker, and 2,2'-azobisisobutyronitrile (AIBN) as initiator. We constructed the PFF sensor by coating the surface of a glassy carbon electrode (GCE) with 3D-CNTs@MIP and removed the imprinting template by solvent extraction. Morphological and structural characterization reveal that blending of the MIP on the CNT surface significantly increases the selective surface area, leading to greater numbers of imprinting sites for improved sensitivity and electron transfer. The 3D-CNTs@MIP sensor exhibits a fast response with good recognition when applied to PFF detection by cyclic voltammetry and amperometry. The PFF oxidation current signal appears at +0.7 V vs Ag/AgCl using 0.1 M phosphate buffer (pH 7.0) as the carrier solution. The designed 3D-imprinted sensor provides a linear response over the range 0.01-200 μM (r² = 0.995) with a low detection limit of 0.002 μM (3σ). The sensor was successfully applied to detection of PFF in vegetable samples.

Complexometric and argentometric titrations using thread-based analytical devices

This work describes analytical approaches based on simple complexometric and argentometric titrations leading to the color change of a novel microfluidic thread-based analytical device (µTAD). The device was fabricated from a cotton thread (15 cm) treated with indicator solution, providing an easy-to-use platform for rapid measurement of analyte concentration in aqueous solution. The thread was immobilized onto a support, being a polypropylene sheet or box platform, to facilitate loading of liquid samples. Interaction between the deposited reagents and analytes in the samples then occurred within a few minutes. This resulted in zones of color change with different lengths along the thread depending on the analyte concentration. The interaction zones can be analyzed by human eyes based on comparison of the zone lengths with the printed scales which are correlated with the analyte concentrations. Complexometric titration using µTADs was initially investigated for Mg(II) determination in water and rubber latex samples. These devices consisted of two threads which were pretreated with Eriochrome Black T (EBT) and then treated with ethylenediaminetetraacetic acid (EDTA) in N-cyclohexyl-3-aminopropanesulfonic acid (CAPS) buffer at pH 10. Both threads were tied together with a central knot before being attached to the box platform prior to the analysis. Load of sample solution (6 µL) resulted in the length of red-violet color product on the threads being proportional to the concentration of Mg(II) in waters and rubber latex samples with the working concentration range of 25-1000 mg L^-1. In addition, µTAD with a supporting polypropylene sheet consisting of several threads treated with AgNO₃ and K₂CrO₄ indicators was applied for argentometric titration of chloride ion in water and food seasoning samples. After sample loading (3 µL), the initially red-brown threads turned into white corresponding to formation of AgCl(s) on the threads with a working concentration range of 75-600 mg L^-1. Greater selectivity towards Mg(II) and chloride compared with potential interference ions was also observed. All the developed μTADs were applied for analysis of real samples which showed results being in agreement with those obtained by classical titrations.

Use of a Smartphone as a Colorimetric Analyzer in Paper-based Devices for Sensitive and Selective Determination of Mercury in Water Samples

A smartphone application, called CAnal, was developed as a colorimetric analyzer in paper-based devices for sensitive and selective determination of mercury(II) in water samples. Measurement on the double layer of a microfluidic paper-based analytical device (μPAD) fabricated by alkyl ketene dimer (AKD)-inkjet printing technique with special design doped with unmodified silver nanoparticles (AgNPs) onto the detection zones was performed by monitoring the gray intensity in the blue channel of AgNPs, which disintegrated when exposed to mercury(II) on μPAD. Under the optimized conditions, the developed approach showed high sensitivity, low limit of detection (0.003 mg L^-1, 3SD blank/slope of the calibration curve), small sample volume uptake (two times of 2 μL), and short analysis time. The linearity range of this technique ranged from 0.01 to 10 mg L^-1 (r² = 0.993). Furthermore, practical analysis of various water samples was also demonstrated to have acceptable performance that was in agreement with the data from cold vapor atomic absorption spectrophotometry (CV-AAS), a conventional method. The proposed technique allows for a rapid, simple (instant report of the final mercury(II) concentration in water samples via smartphone display), sensitive, selective, and on-site analysis with high sample throughput (48 samples h^-1, n = 3) of trace mercury(II) in water samples, which is suitable for end users who are unskilled in analyzing mercury(II) in water samples.

Chromatic analysis by monitoring unmodified silver nanoparticles reduction on double layer microfluidic paper-based analytical devices for selective and sensitive determination of mercury(II)

This study demonstrates chromatic analysis based on a simple red green blue (RGB) color model for sensitive and selective determination of mercury(II). The analysis was performed by monitoring the color change of a microfluidic Paper-based Analytical Device (µPAD). The device was fabricated by using alkyl ketene dimer (AKD)-inkjet printing and doped with unmodified silver nanoparticles (AgNPs) which were disintegrated when being exposed to mercury(II). The color intensity was detected by using an apparatus consisting of a digital camera and a homemade light box generating constant light intensity. A progressive increase in color intensity of the tested area on the µPAD (3.0mm) was observed with increasing mercury(II) concentration. The developed system enabled quantification of mercury(II) at low concentration with the detection limit of 0.001mgL(-1) (3 SD blank/slope of the calibration curve) and small sample volume uptake (2µL). The linearity range of the calibration curve in this technique was demonstrated from 0.05 to 7mgL(-1) (r(2)=0.998) with good precision (RSD less than 4.1%). Greater selectivity towards mercury(II) compared with potential interference ions was also observed. Furthermore, the percentage recoveries of spiked water samples were in an acceptable range which was in agreement with the values obtained from the conventional method utilizing cold vapor atomic absorption spectrometer (CVAAS). The proposed technique allows a rapid, simple, sensitive and selective analysis of trace mercury(II) in water samples.

Amperometric flow injection analysis of glucose using immobilized glucose oxidase on nano-composite carbon nanotubes-platinum nanoparticles carbon paste electrode

We report a novel amperometric glucose biosensor based on glucose oxidase (GOx) immobilized on a carbon nanotube (CNTs)-poly(diallyldimethyl-ammonium chloride) (PDDA)-platinum nanoparticle (PtNPs) modified carbon-paste electrode (CNTs-PDDA-PtNPs/CPE). The CNTs-PDDA-PtNPs composite materials were characterized by TEM and electrochemical techniques. Cyclic voltammetric results reveal direct electron transfer of the immobilized GOx, indicated by two quasi-reversible redox peaks at a potential of 0.37V (vs. Ag/AgCl) in phosphate buffered solution (PBS) (0.10M, pH 7). The biosensor provides good glucose oxidation activity and retention of GOx electrocatalytic activity due to CNTs-PDDA-PtNPs enhancement of the redox response. The carbon paste electrode was installed as working electrode in a flow through electrochemical cell of a flow injection (FI) system. Glucose was quantified using amperometric measurements at 0.5V vs. Ag/AgCl and PBS carrier (0.10M, pH 7.0) at a flow rate of 1.0mLmin^-1. The linear working ranges for glucose measurements were 0.1-3mM (r²=0.995) and 5-100mM (r²=0.997), with corresponding sensitivities of 0.127 and 0.060 (μAs) mM^-1, respectively. The system provides good precision of 2.8% R.S.D with a calculated detection limit (3S/N) of 15μM. The proposed method was successfully applied to determination of glucose in food and pharmaceutical samples with throughput of 200 samplesh^-1.

Selective colorimetric sensors based on the monitoring of an unmodified silver nanoparticles (AgNPs) reduction for a simple and rapid determination of mercury

In this work, selective colorimetric sensors for simple and rapid detection of Hg(II) ions based on the monitoring of an unmodified silver nanoparticles (AgNPs) reduction were developed. The average diameter of synthesized AgNPs was 8.3±1.4nm which was characterized by transmission electron microscopy (TEM). The abrupt change in absorbance of the unmodified AgNPs was observed which progressively decreased and slightly shifted to the blue wavelength as the concentration of Hg(II) increased, indicating the oxidation of Ag(0) to Ag(I) occurred. It appears that the AgNPs were oxidized by Hg(II), resulting in disintegration of the AgNPs into smaller particles as well as mediating the reduction of Hg(II) to Hg(0) adsorbed onto the surface of AgNPs. The adsorption of Hg(0) resulted in the lack of sufficient charges on AgNPs surfaces due to the decrease in the surface coverage of negatively charged citrate molecules, which then leaded to enlargement of AgNPs. The calibration curve of this technique was demonstrated from 0.5 to 7ppm (r(2)=0.995), the limit of detection (LOD) was 0.06ppm (SDblank/slope of calibration curve) with the precision (RSD, n=4) of 3.24-4.53. Interestingly, the results show a significant enhance in the Hg(II) analytical sensitivity when Cu(II) is doped onto the unmodified AgNPs, which improves the quantitative detection limit to 0.008ppm. In addition, greater selectivity toward Hg(II) compared with the other metal ions tested was observed. Furthermore, the percentage recoveries of spiked drinking water, tap water and SRM1641d (mercury in water) were in acceptable range with a good precision (RSD) which were in agreement with the values obtained from graphite furnace atomic absorption spectrometer (GFAAS). The technique proposed in this study provides a rapid, simple, sensitive and selective detection method for Hg(II) in water samples.

Simple flow injection for determination of sulfite by amperometric detection using glassy carbon electrode modified with carbon nanotubes-PDDA-gold nanoparticles

A new approach is presented for sensitive and selective measurement of sulfite (SO3(2-)) in beverages based on a simple flow injection system with amperometric detection. In this work, the sulfite sensor was a glassy carbon electrode modified with multiwall carbon nanotubes-poly(diallyldimethylammonium chloride)-gold nanoparticles composites (CNTs-PDDA-AuNPs/GC). Electrochemical oxidation of sulfite with this electrode was first studied in 0.1M phosphate buffer (pH 7.0) using cyclic voltammetry. The results indicated that the CNTs-PDDA-AuNPs/GC electrode possesses electrocatalytic activity for the oxidation of sulfite with high sensitivity and selectivity. Sulfite was quantified using amperometric measurement with the new sensor at +0.4V vs Ag/AgCl in conjunction with flow injection. The linear working range for the quantitation of sulfite was 2-200 mg L(-1) (r(2)=0.998) with a detection limit of 0.03 mg L(-1) (3σ of blank) and an estimated precision of 1.5%.The proposed method was successfully applied to the determination of sulfite in fruit juices and wines with a sample throughput of 23 samples per hour.

Electrocatalytic oxidation of ascorbic acid using a poly(aniline-co-m-ferrocenylaniline) modified glassy carbon electrode

A poly(aniline-co-m-ferrocenylaniline) was successfully synthesized on a glassy carbon electrode (GCE) by electrochemical copolymerization using a scan potential range from −0.3 to +0.9 V (vs. Ag/AgCl) in 0.5 M H₂SO₄ containing 30% acetonitrile (ACN), 0.1 M aniline (Ani) and 0.005 M m-ferrocenyaniline (m-FcAni). The field emission scanning electron microscope (FESEM) and electrochemical methods were used to characterize the poly(Ani-co-m-FcAni) modified electrode. The poly(Ani-co-m-FcAni)/GCE exhibited excellent electrocatalytic oxidation of ascorbic acid (AA) in citrate buffer solution (CBS, pH 5.0). The anodic peak potential of AA was shifted from +0.55 V at the bare GCE to +0.25 V at the poly(Ani-co-m-FcAni)/GCE with higher current responses than those seen on the bare GCE. The scan number at the 10th cycle was selected as the maximum scan cycle in electrochemical polymerization. The limit of detection (LOD) was estimated to be 2.0 μM based on the signal-to-noise ratio (S/N = 3). The amperometric responses demonstrated an excellent selectivity for AA determination over glucose (Glu) and dopamine (DA).

A microfluidic system for evaluation of antioxidant capacity based on a peroxyoxalate chemiluminescence assay

A microfluidic system incorporating chemiluminescence detection is reported as a new tool for measuring antioxidant capacity. The detection is based on a peroxyoxalate chemiluminescence (PO-CL) assay with 9,10-bis-(phenylethynyl)anthracene (BPEA) as the fluorescent probe and hydrogen peroxide as the oxidant. Antioxidant plugs injected into the hydrogen peroxide stream result in inhibition of the CL emission which can be quantified and correlated with antioxidant capacity. The PO-CL assay is performed in 800-microm-wide and 800-microm-deep microchannels on a poly(dimethylsiloxane) (PDMS) microchip. Controlled injection of the antioxidant plugs is performed through an injection valve. Of the plant-food based antioxidants tested, beta-carotene was found to be the most efficient hydrogen peroxide scavenger (SAHP of 3.27x10(-3) micromol-1 L), followed by alpha-tocopherol (SAHP of 2.36x10(-3) micromol-1 L) and quercetin (SAHP of 0.31x10(-3) micromol-1 L). Although the method is inherently simple and rapid, excellent analytical performance is afforded in terms of sensitivity, dynamic range, and precision, with RSD values typically below 1.5%. We expect our microfluidic devices to be used for in-the-field antioxidant capacity screening of plant-sourced food and pharmaceutical supplements.