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A paper-based analytical device (PAD) with an integrated composite electrode has been designed and fabricated for non-enzymatic creatinine sensing. Reduced graphene oxide (rGO) was employed to modify the PAD so that it could function as a solid-contact transducer. A new macrocyclic pyrido-hexapeptide derivative was made and used as a special ionophore in the creatinine membrane sensor. The synthesized PAD showed a detection limit of 1.0 µM (S/N = 3) and a potentiometric response towards creatinine throughout a log-linear range of 2.0 µM–10 mM (R2 = 0.9998). The sensor shows significant selectivity for a few related substances, including ephedrine, codeine, ketamine, caffeine, urea, urate, carbinoxamine, and dextromethorphan. It has been established that the testing method is appropriate for the direct potentiometric detection of creatinine in a variety of human urine sample types. When an indicating electrode and a reference electrode are put on the same flexible disposable, this lets applications with a small sample volume be done. For point-of-care creatinine measurement, the developed paper-based analytical equipment is a good choice because it is affordable, easily accessible, and self-pumping (especially when combined with potentiometric detection). [ABSTRACT FROM AUTHOR]

A paper-based analytical device (PAD) is presented with colorimetric/electrochemical dual readouts for the simultaneous sensing of total chromium (Cr) and hexavalent chromium (Cr(VI)). This device consists of a homemade three-electrode system and a patterned paper chip, integrating multiple functions including electrochemical detection, fluid driving, online oxidation, and colorimetric detection. The fiberglass filter paper with a hydrophilic microchannel was used to achieve self-driving fluidics without external equipment. One end of the microchannel was integrated with a homemade three-electrode system to achieve sample loading and electrochemical detection. The middle region on the microchannel was modified with oxidizing reagents to perform online pretreatment, and the yield of Cr(III) oxidation can reach 97.9%, ensuring reliable colorimetric detection of total Cr at another end of the microchannel modified with chromogenic agents. With this device, the signals of Cr(VI) (the signal peak at 0.29 V vs. Ag/AgCl) and total Cr can be obtained in one single injection. After optimization, the limit of detection (LOD) of Cr(VI) and total Cr were 0.01 mg L−1 and 0.06 mg L−1 and the linear ranges were 0.05–3.0 mg L−1 and 0.2–3.0 mg L−1, respectively. The relative standard deviations (RSD) of the electrochemical testing of Cr(VI) results were in a range 1.3%–8.7% (n = 3), and the RSD values of the colorimetric testing of total Cr were between 0.7–9.2% (n = 3). The device's reliability was demonstrated by performing the practical speciation of Cr in tap water, river water, and electroplating wastewater while the recoveries obtained using the present method were in the range 93.5–106%. Overall, the proposed device provides high application prospect in the on-site rapid Cr speciation. [ABSTRACT FROM AUTHOR]

A microcapillary grooved paper-based analytical device capable of dual-mode sensing (colorimetric and electrochemical detection) was demonstrated for analysis of viscous samples (e.g., human saliva). Herein, a hollow capillary channel was constructed via laser engraved micropatterning functions as a micropump to facilitate viscous fluidic transport, which would otherwise impede analysis on paper devices. Using salivary thiocyanate as a model analyte, the proposed device was found to exhibit a promising sensing ability on paper devices without the need for sample pretreatment or bulky instrumentation, as normally required in conventional methods used for saliva analysis. An extensive linear dynamic range covering detection of salivary thiocyanate for both high and trace level regimes (5 orders of magnitude working range) was collectively achieved using the dual-sensing modes. Under optimal conditions, the limit of detection was 6 μmol L−1 with a RSD of less than 5%. An excellent stability for the μpumpPAD was also observed for over 30 days. Real sample analysis using the proposed device was found to be in line with the standard chromatographic method. Benefitting from simple fabrication and operation, portability, disposability, low sample volume (20 μL), and low cost (< 1 USD), the μpumpPAD is an exceptional alternative tool for the detection of various biomarkers in saliva specimens. [ABSTRACT FROM AUTHOR]

The authors describe a nitrocellulose membrane based paper matrix array for detection of mercury(II) with high throughput and repeatability. A thymine-rich signal reporting ssDNA probe labeled with gold nanoparticles (AuNPs) was designed, and detection is based on the strong T-Hg-T interaction which leads to a significant color change. In addition, catalytic signal amplification is accomplished by silver staining in order to enhance coloration. The method has a detection limit as low 0.5 ppt. This is at least 500-fold better than that of existing methods. By using an array, multiple sets of duplicate detections can be carried out simultaneously and in parallel. This improves the efficiency and accuracy of the assay. The method shows remarkable specificity since the signal of other competing metal ions is negligible compared to that of mercury(II) even at 100-fold higher concentration. The protocol was successfully applied to the determination of Hg(II) in spiked tap water and lake water samples. In our perception, this assay is a most promising tool for optical determination f ultra-low levels of Hg(II). [ABSTRACT FROM AUTHOR]

We describe a low-cost and disposable indium-tin-oxide (ITO) chip modified with multi-walled carbon nanotubes (MWNTs) and covered with filter paper. It is capable of sensitively detecting dihydronicotinamide adenine dinucleotide (NADH). The response to NADH occurs at a relatively low potential (0.42 V vs. Ag/AgCl) and is not interfered by ascorbic acid. The influence of electrode area and the amount of assembled MWNTs on the analytical signals were studied. Under optimized experimental conditions, the amperometric response to NADH is linear in the 0.25 μM to 0.1 mM concentration range, and the detection limit is 50 nM. Chronoamperometry was performed in order to study the electron transfer kinetics of NADH, and average diffusion coefficients and apparent rate constants were determined. In our perception, this approach provides an effective way to quantify NADH and therefore has a large potential in the field of NADH-related diagnostics and enzymology. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

An origami paper-based electrochemical immunoassay for C-reactive protein (CRP) detection is described. The assay integrates multiple steps of electrode modification into a single device. A graphene-modified screen-printed carbon electrode (G/SPCE) was employed to enhance sensitivity. Gold nanoparticles were first electrodeposited onto the G/SPCE, followed by a self-assembled monolayer of L-cysteine. The capture anti-CRP was then covalently immobilized on the modified electrode. CRP was quantified by measuring the changes in the charge-transfer resistance of the electrode by using hexacyanoferrate as the redox probe. Cyclic voltammetry and scanning electron microscopy were also applied to verify the successful modification of the electrode. Under optimal conditions, impedance increase in the 0.05-100 μg mL−1 CRP concentration range, and the limit of detection is 15 ng mL−1 (at S/N = 3). The immunoassay was successfully applied to the determination of CRP in a certified human serum sample. This method is simple, low-cost, portable and disposable.An origami paper-based analytical device (oPAD) is described that integrates the multistep of electrode modification, immobilization and detection into a single device. The direct conjugation between the capture antibody and target molecule was allowed to use in this system. The C-reactive protein (CRP) concentration in serum samples was determined using electrochemical impedance spectroscopy. [ABSTRACT FROM AUTHOR]