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), Xray 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′-azinobis(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 (H 2 O 2 ) 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 H 2 O 2 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.

The present work describes a simple hands-on experiment kit for colorimetric quantification of ferric (III) ion (Fe 3+ ) in an aqueous medium using anthocyanin extracted from Ruellia tuberosa L. as a green indicator. The extraction of a high amount of anthocyanin was easily accomplished by using only hot water instead of an organic solvent. The formation of the colored Fe 3+ −anthocyanin complex occurred on a homemade 24-well plate and the generated color was captured by a smartphone. The increase in color intensity was measured in the red, green, blue (RGB) system by the ImageJ software under the optimum conditions. The developed method enabled quantification of Fe 3+ at low concentrations with the detection limit of 0.03 mg L −1 and provided the linear range (0.05−2.0 mg L −1 ) with good linearity (R 2 = 0.9985) with Fe 3+ concentration. The concentrations of Fe 3+ in water samples determined by the developed method were not significantly different from those measured with UV−visible spectrophotometry at a 95% confidence level. In addition, the extracted anthocyanin stored at 4 °C was stable for two months. This hands-on experiment was implemented as a 2 h activity for 30 grade-12 students in which they were asked to determine the concentration of Fe 3+ in a water sample using the smartphone-assisted colorimetric method. The students' understanding of the related concepts of oxidation−reduction and determination of iron was collected by a diagnostic conceptual test. Having participated in the experiment, the students were found to have significantly improved understanding of both concepts.

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 (K M) 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.

This work presents a novel analytical approach for precise and quick (within 2 minutes) determination of acid and base concentrations (of very small amount) by titration using a microfluidic thread-based analytical device (μTAD).

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