Nitrogen-doped carbon dots/Ni-MnFe-layered double hydroxides (N-CDs/Ni-MnFe-LDHs) hybrid nanomaterials as immunoassay label for low-density lipoprotein detection.

Nitrogen-doped carbon dots/Ni-MnFe-layered double hydroxides (N-CDs/Ni-MnFe-LDHs) are demonstrated as superior peroxidase mimic antibody labels alternative to horseradish peroxidase (HRP) in an immunoassay, potentially overcoming some of the inherent disadvantages of HRP and other enzyme mimicking nanomaterials. They revealed efficient peroxidase-like activity and catalyzed the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to form the intense blue product (at 620 nm) in the presence of hydrogen peroxide (H2O2). Using low-density lipoprotein (LDL) as a model target, an ultra-low limit of detection (0.0051 mg/dL) and a linear range of 0.0625-0.750 mg/dL were achieved, exhibiting higher sensitivity than the HRP-based immunoassay. Thus, the proposed N-CDs/Ni-MnFe-LDHs can be used as HRP mimicking analogs for developing highly sensitive colorimetric immunosensors for detection of biomarkers, as well as trace chemical analysis.

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.

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.