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We report a smartphone–paper-based sensor impregnated with cetyltrimethylammonium bromide modified silver nanoparticles (AgNPs/CTAB) for determination of Fe3+ in water and blood plasma samples. The methodology for determination of Fe3+ is based on the change in signal intensity of AgNPs/CTAB fabricated on a paper substrate after the deposition of analyte, using a smartphone followed by processing with ImageJ software. The mechanism of sensing for detection and determination of Fe3+ is based on the discoloration of AgNPs which impregnated the paper substrate. The discoloration is attributed to the electron transfer reaction taking place on the surface of NPs in the presence of CTAB. Fe3+ was determined when the paper was impregnated with 1 mM AgNPs for 5 min of reaction time and the substrate was kept under acidic conditions. The linear range for determination of total iron in terms of Fe3+ was 50–900 μg L−1 with a limit of determination (LOD) of 20 μg L−1 and coefficient of variation (CV) of 3.2%. The good relative recovery of 91.3–95.0% and interference studies showed the selectivity of the method for determination of total iron in water and blood plasma samples. Smartphone–paper-based sensors have advantages of simplicity, rapidity, user-friendliness, low cost, and miniaturization of the method for on-site determination of total iron compared to methods that require sophisticated analytical instruments.

Despite the green energy generation with low cost compared to conventional fuel cells, microbial fuel cells (MFCs) still suffer with anode related constraints including laborious pretreatment and modification process of conventional electrodes, limited bacterial loading capacity, and inferior extracellular electron transfer efficiency. Accordingly, this investigation explores the waste tissue paper derived three dimensional (3D) carbon aerogel (CA) integrated with cerium dioxide (CeO2) nanotubes decorated nitrogen-doped reduced graphene oxide nanosheets (NRGO) as a competent anode to address these technical complements. The direct growth of NRGO and CeO2 over CA in the form of freestanding and binder-free NRGO/CeO2(1:2)/CA alleviates the significant constrains of conventional anode fabrication. The 3D hierarchical architectures of CA with open porous structure provide easy access of bacteria, thus increases the bacterial colonies per unit volume. Furthermore, the hydrogen bonding between the interfacial oxygen atoms of CeO2 and lysine residues of the cytochrome c in bacteria yields excellent extracellular electron transfer efficiency. The electrostatic interaction between the NRGO and bacteria cells improves the bacterial adhesion and biofilm formation, leading to the compact biofilm formation for the improved direct electron transference. With the profits of above, the MFC with NRGO/CeO2(1:2)/CA demonstrates a maximum power output and good lifespan performances. The present exploration facts thus access advanced avenues to converting waste matters of tissue paper, human urine, and wastewater into profitable constituents for the development of efficient and durable power producing systems.

Fluorescent test papers are promising for the wide applications in the assays of diagnosis, environments and foods, but unlike classical dye-absorption-based pH test paper, they are usually limited in the qualitative yes/no type of detection by fluorescent brightness, and the colorimetry-based quantification remains a challenging task. Here, we report a single dual-emissive nanofluorophore probe to achieve the consecutive color variations from blue to red for the quantification of blood glucose on its as-prepared test papers. Red quantum dots were embedded into silica nanoparticles as a stable internal standard emission, and blue carbon dots (CDs) were further covalently linked onto the surface of silica, in which the ratiometric fluorescence intensity of blue to red is controlled at 5:1. While the oxidation of glucose induced the formation of Fe(3+) ions, the blue emission of CDs was thus quenched by the electron transfer from CDs to Fe(3+), displaying a serial of consecutive color variations from blue to red with the dosage of glucose. The high-quality test papers printed by the probe ink exhibited a dosage-sensitive allochromatic capability with the clear differentiations of ~5, 7, 9, 11mM glucose in human serum (normal: 3-8mM). The blood glucose determined by the test paper was almost in accordance with that measured by a standard glucometer. The method reported here opens a window to the wide applications of fluorescent test paper in biological assays.

The goal of this work is to pursue analytical approaches that elucidate electron and proton diffusion inside the Shewanella oneidensis biofilm and bulk liquid, which will inevitably promote the translation of Microbial Fuel Cell (MFC) technology for renewable, "green energy" solutions that are in demand to sustain the world's ever-increasing energy demands and to mitigate the depletion of current resources. This study provides a novel strategy for monitoring electron/proton fluxes in 3-D multi-laminate structures of paper as a scaffold to support S. oneidensis biofilms and bulk media liquid. Multiple layers of paper containing bacterial cells and/or media are stacked to form a layered 3-D model of the overall biofilm/bulk liquid construct. Mass transport of electrons and protons into this 3-D system can be quantified along with the exploration of microbial energy production. Assembly of a 3D paper stack can be modular and allows us to control the thickness of the overall biofilm/bulk liquid construct with the different diffusion distances of the electrons/protons through the stack. By measuring the current generated from the 3-D stack, the electron and proton diffusivity through biofilms were quantitatively investigated. We found that (i) the diffusion length of the electrons/protons in the S. oneidensis biofilm/bulk liquid is a determinant factor for the MFC performance, (ii) the electron transfer through the endogenous mediators of S. oneidensis can be a more critical factor to limit the current/power generation of the MFCs than the proton transfer in the MFC system and (iii) the thicker biofilm allows higher and longer current generation but requires more time to reach a peak current value and increases the total energy loss of the MFC system.

We introduce for the first time a paper-based analytical device (PAD) for the non-enzymatic detection of glucose by modifying a screen-printed carbon electrode with cobalt phthalocyanine, graphene and an ionic liquid (CoPc/G/IL/SPCE). The modifying composite was characterized by UV-visible spectroscopy, energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The disposable devices show excellent conductivity and fast electron transfer kinetics. The results demonstrated that the modified electrode on PADs had excellent electrocatalytic activity towards the oxidation of glucose with NaOH as supporting electrolyte (0.1M). The oxidation potential of glucose was negatively shifted to 0.64V vs. the screen-printed carbon pseudo-reference electrode. The paper-based sensor comprised a wide linear concentration range for glucose, from 0.01 to 1.3mM and 1.3-5.0mM for low and high concentration of glucose assay, respectively, with a detection limit of 0.67µM (S/N = 3). Additionally, the PADs were applied to quantify glucose in honey, white wine and human serum. The disposable, efficient, sensitive and low-cost non-enzymatic PAD has great potential for the development of point-of-care testing (POCT) devices that can be applied in healthcare monitoring.

Bacteria able to transfer electrons to conductive surfaces are of interest as catalysts in microbial fuel cells, as well as in bioprocessing, bioremediation, and corrosion. New procedures for immobilization of Geobacter sulfurreducens on graphite electrodes are described that allow routine, repeatable electrochemical analysis of cell-electrode interactions. Immediately after immobilizing G. sulfurreducens on electrodes, electrical current was obtained without addition of exogenous electron shuttles or electroactive polymers. Voltammetry and impedance analysis of pectin-immobilized bacteria transferring electrons to electrode surfaces could also be performed. Cyclic voltammetry of immobilized cells revealed voltage-dependent catalytic current similar to what is commonly observed with adsorbed enzymes, with catalytic waves centered at -0.15 V (vs. SHE). Electrodes maintained at +0.25 V (vs. SHE) initially produced 0.52 A/m(2) in the presence of acetate as the electron donor. Electrical Impedance Spectroscopy of coatings was also consistent with a catalytic mechanism, controlled by charge transfer rate. When electrodes were maintained at an oxidizing potential for 24 h, electron transfer to electrodes increased to 1.75 A/m(2). These observations of electron transfer by pectin-entrapped G. sulfurreducens appear to reflect native mechanisms used for respiration. The ability of washed G. sulfurreducens cells to immediately produce electrical current was consistent with the external surface of this bacterium possessing a pathway linking oxidative metabolism to extracellular electron transfer. This electrochemical activity of pectin-immobilized bacteria illustrates a strategy for preparation of catalytic electrodes and study of Geobacter under defined conditions.