We report the development of a three-dimensional electrochemical paper-based analytical device (3D-ePAD) for the individual and simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The device was fabricated by alkyl ketene dimer (AKD)-inkjet printing of a circularly hydrophobic zone on filter paper for application of aqueous samples coupled with screen-printing of the electrodes on the paper which was folded underneath hydrophobic zone. A self-assembled threeelectrode system, comprising a graphite paste modified with Fe 3 O 4 @Au-Cys/PANI was fabricated on the patterned paper by screen printed through the pre-designed transparent film slit. The three electrodes of Fe 3 O 4 @Au-Cys/PANI modified graphite electrodes (Fe 3 O 4 @Au-Cys/PANI/GFE) on the layout paper were served as the working electrode, the reference electrode, and the counter electrode, respectively. Cyclic voltammetry (CV) was used to study the electrochemical mechanism of AA, DA and UA. The results indicated that a high sensitivity towards AA, DA and UA was observed. Our results suggested that coating the working electrode with anionic surfactant, SDS (1 mM, pH 2), provides the distinguishable oxidation peak potential of AA and did not overlap with the oxidation peak of DA and UA. As a result, simultaneous determination of these three molecules in a mixture can be achieved. Examples of individual quantification of DA and UA in pharmaceutical and urine samples were demonstrated using differential pulse voltammetry (DPV). Under the optimum condition, the developed 3D-ePAD gave a linearity ranged from 20 to 1,000 µM for both DA and UA. The detection limits were 2.19 and 1.80 µM for DA and UA, respectively. There are no significant matrix interferences in the analyzed samples which can be concluded that the proposed method is suitable for the quantification of DA and UA with sufficient accuracy and precision.

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A sensitive and selective non-enzymatic glucose sensor was developed based on magnetite (Fe 3 O 4 ) and nickel nanoparticles decorated carbon nanotubes (Fe 3 O 4 -CNTs-NiNPs). Fe 3 O 4 nanoparticles were in situ loaded on the surface of carboxylated multi-walled carbon nanotubes (CNTs-COOH) by a chemical co-precipitation procedure. Nickel nanoparticles (NiNPs) were prepared through reducing nickel chloride by hydrazine hydrate and then decorated on Fe 3 O 4 -CNTs using ultra-sonication. The as-prepared Fe 3 O 4 -CNTs-NiNPs was characterized using transmission electron microscopy (TEM) and X-Ray Diffraction (XRD). Glucose sensor was fabricated using glassy carbon (GC) coated with Fe 3 O 4 -CNTs-NiNPs composites film. Electrochemical investigations indicate that the Fe 3 O 4 -CNTsNiNPs/GC electrode possesses excellent performance in the electrochemical oxidation of glucose at an applied potential of +0.55 V (vs. Ag/AgCl) in 0.1 M NaOH solution. The linear dynamic range for glucose amperometric detection (E app = +0.55 V) was observed from 10 µM to 1.8 mM (r 2 = 0.998) with the sensitivity of 335.25 µA mM -1 ; and a low detection limit of 6.7 µM (S/N = 3). In addition, the fabricated sensor was successfully applied to determine glucose in honey and energy drinks with good results.

Nanoscale imprinting significantly increases the specific surface area and recognition capabilities of a molecularly imprinted polymer by improving accessibility to analytes, binding kinetics, and template removal. Herein, we present a novel synthetic route for a dual molecularly imprinted polymer (dual-MIP) of the carcinogen oxidative stress biomarkers 3-nitrotyrosine (3-NT) and 4-nitroquinolin-N-oxide (4-NQO) as coatings on graphene quantum-dot capped gold nanoparticles (GQDs-AuNPs). The dual-MIP was successfully coated on the GQDs-AuNPs core via a (3-mercaptopropyl) trimethoxysilane (MPTMS) linkage and copolymerization with the 3-aminopropyltriethoxysilane (APTMS) functional monomer. In addition, we fabricated a facile and compact three-dimensional electrochemical paper-based analytical device (3D-ePAD) for the simultaneous determination of the dual biomarkers using a GQDs-AuNPs@dual-MIP-modified graphene electrode (GQDs-AuNPs@dual-MIP/SPGE). The developed dual-MIP device provides greatly enhanced electrochemical signal amplification due to the improved electrode-specific surface area, electrocatalytic activity, and the inclusion of large numbers of dual-imprinted sites for 3-NT and 4-NQO detection. Quantitative analysis used square wave voltammetry, with an oxidation current appearing at −0.10 V for 4-NQO and +0.78 V for 3-NT. The dual-MIP sensor revealed excellent linear dynamic ranges of 0.01 to 500 μM for 3-NT and 0.005 to 250 μM for 4-NQO, with detection limits in nanomolar levels for both biomarkers. Furthermore, the dual-MIP sensor for the simultaneous determination of 3-NT and 4-NQO provides high accuracy and precision, with no evidence of interference from urine, serum, or whole blood samples.

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