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An electrochemical sensor for Pb(II) is described that exploits (a) the outstanding adsorption ability of chitosan modified with quaternary ammonium groups (CQAS; cationic) and of lignosulfonate (LSN; anionic), and (b) the good electrical conductivity of polypyrrole nanoparticles (PPy NPs). A glassy carbon electrode (GCE) was modified with PPy NPs and polydopamine, and CQAS and LSN are used as dispersants in PPy. The modified GCE exhibits high selectivity and sensitivity for Pb(II) in the 0.1 to 50 μM concentration range, with a 55 nM detection limit (3σ method). The redox potentials for Pb(II) is around -0.55 V, and the sensor is not interfered by the presence of Hg(II) and Cu(II). The time dependent stability test showed that this sensor can maintain good reproducibility for one month. This sensor was applied to the determination of Pb(II) in wastewater samples. An electrochemical sensor for lead(II) is described that exploits the outstanding adsorption ability of chitosan that carries quaternary ammonium groups (CQAS), of lignosulfonate (LSN), and the good electrical conductivity of polypyrrole nanoparticles. CQAS and LSN are used as dispersants in PPy. The sensor exhibits high selectivity and sensitivity for Pb(II) in the range of 0.1 to 50 μM with a 55 nM detection limit.

Lateral flow assays, as a low-cost, simple, portable and disposable product of vitro diagnostic, are being widely used for point-of-care testing. However, the poor sensitivity of LFAs is the main challenge for commercialization. In order to enhance the sensitivity of LFAs, cellulose nanofibers (CNFs) have been integrated into LFAs to enhance the sensitivity of protein LFAs. A simple method is also presented to modify the properties of paper substrate by incorporating CNFs into a nitrocellulose membrane to enhance the sensitivity of nucleic acid LFAs. This method changes the pore size, porosity, surface groups and surface area of paper substrate and then increases the adsorption ability of biomolecules on paper substrate. The results indicate that the sensitivity of nucleic acid LFAs in Staphylococcus aureus testing achieves a 20-fold enhancement. Hence, we anticipate that this simple method has the potential for other paper-based devices to improve the performance. Graphical abstractA simple method is used to modify the properties of paper substrate by incorporating cellulose nanofibers (CNFs) into nitrocellulose (NC) membrane to enhance the sensitivity of nucleic acid LFAs.

Paper is a widely used support for use in devices for point-of-care testing (POCT) in clinical diagnosis, food safety monitoring and environmental pollution. Paper is inexpensive, biocompatible, biodegradable and allows a sample fluid to flow by capillary force. Numerous method have been developed recently for chemical modification of papers in order to introduce different functionalities. This review (with 148 refs.) summarizes the recent progress in paper-based POCT devices. The introduction summarizes the state of the art of paper-based POCT devices and the physicochemical properties of existing unmodified materials (including cellulose, cellulose-based composites, cotton fibers, glass fibers, nitrocellulose, thread). Methods for paper modification for sample pretreatment are summarized next, with subsections on sample storage and collection, sample separation, nucleic acid extraction and sample preconcentration. Another main section covers approaches for paper modification for improving POCTs, with subsections on assays for proteins, nucleic acids, drugs, ion and organic molecules. The advantages and disadvantages of these approaches are compared. Several tables are presented that summarize the various modification techniques. A concluding section summarizes the current status, addresses challenges and gives an outlook on future perspectives of POCTs. Graphical abstract This review summarizes the progress that has been made in paper based point-of-care testing (POCT) and lateral flow assays (LFAs), quite often by using advanced nanomaterials for paper modification.