br Results and discussion br Conclusion The quadruple

Results and discussion
Conclusion The quadruple dye labelled dual oxygen and pH-sensitive ratiometric nanosensors detailed in this article are capable of measuring dissolved oxygen concentrations, from 0 to 100%, and the full physiological sphingosine ranges from pH3.5 to 7.5. We envisage that dual oxygen and pH nanosensor will find potential utility in the characterisation of diverse microenvironments, especially where there is interplay between pH and dissolved oxygen concentrations, such as, developing embryos and cancer tumours.
Experimental Control and measurement of dissolved oxygen concentrations — dissolved oxygen concentrations were measured using an Ocean Optics NeoFox (NFB0181) phosphorescence-based optical sensor probe. The phosphorescence lifetime of the probe can be calibrated to 0% and 100% dissolved oxygen concentrations. The calibrated probe was immersed in solutions of metalloporphyrins (0.01mg/mL, 10mL) and suspensions of oxygen-sensitive nanoparticles (0.50mg/mL, 10mL) in deionised water. The dissolved oxygen concentration of these solutions/suspensions was varied by bubbling in argon and oxygen gas, representing 0% and dissolved 100% oxygen concentration, respectively. Oxygen-dependent emission intensities of samples (1mL) were recorded using a Varian Cary Eclipse fluorescence spectrophotometer. Ratiometric oxygen-sensitive nanosensors (0.5mg/mL), composed of cationic platinum metalloporphyrin and oxygen-insensitive fluorophore TAMRA were excited at a single wavelength (400nm) and emission was collected between 540 and 700nm (slit size 10nm). For dual oxygen and pH-sensitive nanosensors each fluorophore/phosphor was excited at their own excitation wavelengths and slit sizes, to determine their luminescence properties; for example, the fluorescein derivatives (FAM and OG), TAMRA and the Pt metalloporphyrins were excited at 488nm (2.5nm slit size), 540nm (5.0nm slit size) and 405nm (10.0nm slit size), whereas their emission spectra were collected between 500–540nm (5.0nm slit size), 550–620nm (5.0nm slit size), and 630–750nm (20.0nm slit size), respectively. Zeta potential — Pt and Pd oxygen-sensitive and blank unfunctionalised nanoparticles were suspended (0.5mg/mL) in filtered pH buffer solutions (0.02μm, Millipore). Buffer solutions were prepared to cover the full physiological pH range, from 3.0 to 8.0 using mixtures of sodium phosphate dibasic (0.2M) and citric acid monohydrate (0.1M). Nanoparticle suspensions were transferred to zetasizer cuvettes (DTS1061, Malvern), flushed with filtered deionised water. Zeta potential measurements were made in triplicate for each nanoparticle suspension at each pH (constants used to make measurements for polyacrylamide nanoparticles: refractive index: 1.452, parameters used for dispersant deionised water dispersant; refractive index: 1.330, viscosity: 0.8872cP, dielectric constant: 78.5εr, Model Smoluchowski F (K) 1.5). All samples were allowed to equilibrate to 25°C for 120s prior to measurement.
Author contributions
Acknowledgements
Introduction The original reports of two-dimensional (2D) and three dimensional (3D) PADs by the Whitesides group [1,2] in 2007 and 2008, respectively, stimulated research in the field of highly functional paper-based sensors. Due to their lower cost, minimal required infrastructure, ease of fabrication, speed of fabrication, ease of use, potential to be used remotely, and ability to provide semi-quantitative results in a point-of-care fashion, those devices provide an alternative to elastomer (Polydimethylsiloxane, PDMS) and rigid polymer based, open-channel microfluidic systems [3]. Despite their advantages, current paper microfluidic technologies share some common disadvantages [4–6]. For example flows of complex fluids, such as whole blood or colloidal suspensions that contain particulates, are generally incompatible with wicking flow. Due to sample retention in the porous cellulose matrix, the volume that reaches the detection zones is usually less than 50% of the total volume within the device [7]. The groups of Website and Richard M. Crooks have developed hollow-channel paper analytical devices to overcome these disadvantages [8–10]. Those channels will allow micrometer-sized objects, such as bacteria or microbeads, to flow freely. However, an external force is needed to force the liquid into the channel, such as pressure arising from pumping or hydraulic pressure. In this paper we describe a simple well defined millimeter-sized channels, in which multiphase fluidics are transported without external force.