Vanadium-Doped Porous Cobalt Oxide for Its Superior Peroxidase-like Activity in Simultaneous Total Cholesterol and Glucose Determination in Whole Blood Based on a Simple Two-Dimensional Paper-Based Analytical Device.

Vanadium-doped porous Co3O4 (V-porous Co3O4) was synthesized via a simple soft-templating method and used as a superior peroxidase mimic for the simultaneous colorimetric determination of glucose and total cholesterol (TC) in whole blood samples on a two-dimensional microfluidic paper-based analytical device (2D-µPAD). The large surface area and the presence of two metals in V-porous Co3O4 contributed to its excellent catalytic activity toward 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) and 3,3',5,5'- tetramethylbenzidine (TMB) with Michaelis-Menten constants (KM) of 0.1301 and 0.0141 mM, respectively. The 2D-µPAD was fabricated using simple wax screen-printing and cutting techniques. The colorimetric reactions of both glucose and TC on 2D-µPAD were simultaneously performed by adding a single drop of a whole blood sample on the sample zone made of the LF1 membrane. After the enzymatic reactions, the generated hydrogen peroxide (H2O2) was oxidized by V-porous Co3O4 to produce hydroxy radicals (•OH), inducing ABTS and TMB to generate colored products. The generated H2O2 was proportional to the intensities of the green and blue products of the glucose and TC systems, respectively. The developed 2D-µPAD required a short analysis time (∼5 min) with small volumes of samples (15 µL of whole blood) whereby no sample preparation was needed. Owing to several advantages including simplicity, low cost, long-term stability, and simultaneous readout, the novel V-porous Co3O4 coupled with 2D-µPAD proved to be promising for practical uses as a pioneering portable device for the determination of glucose, TC, and other important biomarkers without the need of technical supports.

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 (KM) 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.

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).

Pd-loaded hierarchical titanosilicalite-1 catalysts on CO2 cycloaddition with epoxides: Experimental and DFT investigations.

This work presents the synthesis of Pd-loaded microporous titanosilicalite-1 (Pd/TS-1) and Pd-loaded hierarchical titanosilicalite-1 (Pd/HTS-1) with abundant mesopores (2-30 nm) inside the framework via hydrothermal method using polydiallydimethyl ammonium chloride as the non-surfactant mesopore template. XRD, N2 sorption, FT-IR, FESEM-EDX, TEM, XPS, and DR-UV techniques were used to characterize the morphological and physicochemical properties of the synthesized materials. These materials were tested as heterogeneous catalysts, along with tetrapropylammonium bromide as co-catalyst, for cycloaddition reactions of CO2 with epoxides to produce cyclic carbonates. It was found that the epoxide conversions were influenced by acidity and pore accessibility of the catalysts. Using Pd/HTS-1 facilitated bulky substrates to access active sites, resulting in higher conversions than Pd/TS-1. Over 85 % conversions were achieved for at least five consecutive cycles without significant loss in catalytic activity. The interaction between the Pd active surfaces and epichlorohydrin (ECH) was further studied by DFT calculations. The existence of Pd(200) was more influential on adsorbing epichlorohydrin (ECH) and subsequent formation of dissociated ECH (DECH) intermediate than Pd(111) surface. However, Pd(111) was dominant in enhancing the activity of DECH species for capturing CO2. Therefore, the co-existence of Pd(200) and Pd(111) surfaces was needed for cycloaddition of CO2 with ECH.

Old silver mirror in qualitative analysis with new shoots in quantification: Nitrogen-doped carbon dots (N-CDs) as fluorescent probes for "off-on" sensing of formalin in food samples.

A novel fluorometric assay for selective and sensitive determination of formalin (FA) was developed based on nitrogen-doped carbon dots (N-CDs) coupled with silver mirror reaction. N-CDs was synthesized using the hydrothermal method with the ethylene glycol and ammonia solution as carbon and nitrogen precursors, respectively. The detection principle was based on "off-on" fluorescence switching. Specifically, the fluorescence signal of N-CDs was first turned off after incorporating the Ag+ and Tollens' reagents. Then, in the presence of FA, the Ag+ species on the N-CDs surface were reduced to Ag0 species and the fluorescence signal of N-CDs was switched back on. The fluorescence intensity due to the N-CDs signal linearly increased with the increasing FA concentrations in the range of 5-100 mg L-1, with the detection limit of 1.5 mg L-1. The proposed approach provides rapid, simple, sensitive, and selective detection of FA in various food samples.

Sensitive detection of trace level Cd (II) triggered by chelation enhanced fluorescence (CHEF) "turn on": Nitrogen-doped graphene quantum dots (N-GQDs) as fluorometric paper-based sensor.

Cadmium ion (Cd (II)) is a highly toxic heavy metal usually found in natural water. Exposure to Cd (II) can produce serious effects in human organs such as Itai-Itai disease. Therefore, the maximum allowance levels of Cd (II) in drinking water and herbal medicines imposed by the World Health Organization (WHO) are 3 µg L-1 and 300 µg kg-1, respectively. In this work, nitrogen-doped graphene quantum dots (N-GQDs) as a fluorescent sensor for Cd (II) determination was developed in both solution-based and paper-based systems. N-GQDs were synthesized from citric acid (CA) and ethylenediamine (EDA) via the hydrothermal method. The synthesized N-GQDs emitted intense blue fluorescence with a quantum yield (QY) of up to 80%. The functional groups on the surface of N-GQDs measured by FTIR were carboxyl (COO-), hydroxyl (OH-), and amine (NH2) groups, suggesting that they could be bound to Cd (II) for complexation. The fluorescence intensity of N-GQDs was gradually enhanced with the increase of Cd (II) concentration. This phenomenon was proved to result from the fluorescence enhancement (turn-on) based on the chelation enhanced fluorescence (CHEF) mechanism. Under the optimum conditions in the solution-based and paper-based systems, the limits of detection (LODs) were found to be 1.09 and 0.59 µg L-1, respectively. Furthermore, the developed sensors showed relatively high selectivity toward Cd (II) over ten other metal cations and six other anions of different charges. The performance of the sensor in real water and herbal medicine samples exhibited no significant difference as compared to the results of the validation method (ICP-OES). Therefore, the developed sensors can be used as fluorescent sensors for Cd (II) determination with high sensitivity, high selectivity, short incubation time (5 min). As such, the paper-based strategy has excellent promising potential for practical analysis of Cd (II) in water and herbal medicine samples with a trace level of Cd (II) concentrations.

Use of nitrogen-doped amorphous carbon nanodots (N-CNDs) as a fluorometric paper-based sensor: a new approach for sensitive determination of lead(ii) at a trace level in highly ionic matrices.

This work reports a facile synthesis of nitrogen-doped amorphous carbon nanodots (N-CNDs) and their use as a fluorometric paper-based sensor for the determination of Pb2+ at a low concentration. Both solution-based and paper-based systems were developed. The results show that the linearity ranges for Pb2+ determination were 0.010-10 mg L-1 (LOD = 0.008 mg L-1) and 0.005-0.075 mg L-1 (LOD = 0.004 mg L-1) for the solution-based and the paper-based sensors, respectively. Furthermore, the developed sensors show relatively high selectivity toward Pb2+ over ten other metal cations of different charges including As3+, Hg2+, Cd2+, Mg2+, Ni2+, Zn2+, Fe3+, Cu2+, Ba2+, and Ag+. The mechanism of Pb2+ determination was also investigated. It was found that the sensors exploited the quenching of the fluorescence intensity of N-CNDs by Pb2+via the photo-induced electron transfer (PET) mechanism. When applied to real water and herbal medicine samples, the performance of the sensor exhibited no significant difference as compared to the results of the validation method (ICP-OES). Overall, the developed sensors, especially the paper-based one, are promising for the practical analysis of Pb2+ in pharmaceutical and environmental samples with a low Pb2+ concentration.

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.

Screen-printed microfluidic paper-based analytical device (µPAD) as a barcode sensor for magnesium detection using rubber latex waste as a novel hydrophobic reagent.

This work reports the first use of cis-1,4-polyisoprene obtained from rubber latex (RL) waste as the hydrophobic reagent for the fabrication of a microfluidic paper-based analytical device (µPAD), providing a user-friendly means for magnesium detection. The µPAD was fabricated using a screen printing technique and the barcode-like paper sensor was then used for the detection of Mg(II) ions in RL and water samples. Using different types of paper media (paper towel, Whatman No.1 and Whatman No.4), the results indicate that the key factors in optimizing the quality of the fabricated µPAD include the viscosity of cis-1,4-polyisoprene solution which could be adjusted using different solvents and heating temperatures, the mesh screen size, the pore size of the paper substrates, and the dimension of the sample zone. The fabricated µPAD, which showed high chemical resistance, durability and design flexibility, was tested for the detection of Mg(II) ions using the reaction based on complexometric titration with EDTA where Eriochrome Black T was used as an indicator. An Android application "UBU OMg Sensor" was also developed to provide a simple, fast, and accurate means for end-users to interpret results generated by our developed µPAD.