Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Surface plasmon resonance (SPR) has been widely used to detect a variety of biomolecular systems, but only a small fraction of applications report on the analysis of patients' samples. A critical barrier to the full implementation of SPR technology in molecular diagnostics currently exists for its potential application to analyze blood plasma or serum samples. Such capability is mostly hindered by the non-specific adsorption of interfering species present in the biological sample at the functional interface of the biosensor, often referred to as fouling. Suitable polymeric layers having a thickness ranging from 15 and about 70 nm are usually deposited on the active surface of biosensors to introduce antifouling properties. A similar approach is not fully adequate for SPR detection where the exponential decay of the evanescent plasmonic field limits the thickness of the layer beyond the SPR metallic sensor surface for which a sensitive detection can be obtained. Here, a triethylene glycol (PEG (3))-pentrimer carboxybetaine system is proposed to fabricate a new surface coating bearing excellent antifouling properties with a thickness of less than 2 nm, thus compatible with sensitive SPR detection. The high variability of experimental conditions described in the literature for the quantitative assessment of the antifouling performances of surface layers moved us to compare the superior antifouling capacity of the new pentrimeric system with that of 4-aminophenylphosphorylcholine, PEG-carboxybetaine and sulfobetaine-modified surface layers, respectively, using undiluted and diluted pooled human plasma samples. The use of the new coating for the immunologic SPRI biosensing of human arginase 1 in plasma is also presented.