Tunable fluorescence quenching near the graphene-aqueous interface.

With increased interest in graphene-based sensors for biomolecules and other targets, we investigated the impact of ionic strength on the steady-state emissions from fluorescein labels on proteins adsorbing on pristine CVD (chemical vapor deposited)-graphene on a silica support. Using the model system of fluorescein-tagged fibrinogen we demonstrated that, for fluorescein tags on adsorbed fibrinogen, emission intensity was very sensitive to the salt concentration. This behavior was not seen for fluorescein-tagged fibrinogen in solution. We demonstrated that fluorescein "quenching" in this system was a result of fluorescein's pH sensitivity: with changes in salt concentration, near-surface fluorescein experiences either the neutral bulk pH or, with a negatively charged surface, an acidic environment. The findings carry the important implication that the aqueous environment near silica-supported graphene is substantially acidic as a result of near-surface negative charge. This further implies, because of the purity of the graphene in this study and its lack of oxidation, that negative charge arises from ion adsorption and/or from the underlying silica support, which may be hydrated and present dissociated surface silanols. That is, the electrostatic potential from silica beneath the graphene may pass through the graphene, much as van der Waals interactions have been proven to do. Results were semi-quantitatively consistent with calculations that employed a Guoy Chapman model of the interface and the established pKa of the fluorescein. While these findings were obtained with adsorbed proteins, similar fluorescence quenching would be expected for any fluorescein-tagged species in the vicinity of silica-supported graphene. Thus, because of the negative charge at the aqueous graphene interface, ionic strength can be exploited as means of creating a molecular ruler of fluorescein emissions, and the emissions can be assessed within different distances, corresponding to the Debye length, from the graphene interface.

Evidence for negative charge near large area supported graphene in water: A study of silica microsphere interactions.

This study addresses the electrostatic and van der Waals interactions at the aqueous interface of large area CVD graphene, 1-3 layers thick on a silica support and assessed by Raman spectroscopy to have exclusive sp2 character. Ionic strength was found to substantially alter the interactions of silica microspheres with silica-supported graphene. Particles were nonadhesive at large Debye lengths but became irreversibly adherent at reduced Debye lengths about 2nm or less. This was demonstrated to be qualitatively parallel to the influence of ionic strength on silica-silica interactions. The observed ionic strength effects are best explained by negative charges in the vicinity, within a few nanometers, of the supported graphene. DLVO-based modeling of the silica-water-supported graphene interaction suggests that van der Waals interactions drive particle capture and that the surface potential at the supported graphene surface is at least -10 to -15mV (corresponding to a charge density of 0.02-0.06/nm2). This charge could result from ion adsorption or from charges on silica beneath the graphene. The conclusions are not substantially affected by inclusion of nanometer-scale interfacial roughness in the modeling.

A review of the mammalian unfolded protein response.

Proteins requiring post-translational modifications such as N-linked glycosylation are processed in the endoplasmic reticulum (ER). A diverse array of cellular stresses can lead to dysfunction of the ER and ultimately to an imbalance between protein-folding capacity and protein-folding load. Cells monitor protein folding by an inbuilt quality control system involving both the ER and the Golgi apparatus. Unfolded or misfolded proteins are tagged for degradation via ER-associated degradation (ERAD) or sent back through the folding cycle. Continued accumulation of incorrectly folded proteins can also trigger the unfolded protein response (UPR). In mammalian cells, UPR is a complex signaling program mediated by three ER transmembrane receptors: activating transcription factor 6 (ATF6), inositol requiring kinase 1 (IRE1) and double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). UPR performs three functions, adaptation, alarm, and apoptosis. During adaptation, the UPR tries to reestablish folding homeostasis by inducing the expression of chaperones that enhance protein folding. Simultaneously, global translation is attenuated to reduce the ER folding load while the degradation rate of unfolded proteins is increased. If these steps fail, the UPR induces a cellular alarm and mitochondrial mediated apoptosis program. UPR malfunctions have been associated with a wide range of disease states including tumor progression, diabetes, as well as immune and inflammatory disorders. This review describes recent advances in understanding the molecular structure of UPR in mammalian cells, its functional role in cellular stress, and its pathophysiology.