The selective functionalization of graphene edges is driven by the chemical reactivity of its carbon atoms. The chemical reactivity of an edge, as an interruption of the honeycomb lattice of graphene, differs from the relative inertness of the basal plane. In fact, the unsaturation of the pz orbitals and the break of the π conjugation on an edge increase the energy of the electrons at the edge sites, leading to specific chemical reactivity and electronic properties. Given the relevance of the chemistry at the edges in many aspects of graphene, the present Review investigates the processes and mechanisms that drive the chemical functionalization of graphene at the edges. Emphasis is given to the selective chemical functionalization of graphene edges from theoretical and experimental perspectives, with a particular focus on the characterization tools available to investigate the chemistry of graphene at the edge.
A thermo-responsive polymer/quantum dot platform based on poly(N-isopropyl acrylamide) (PNIPAM) brushes 'grafted from' a gold substrate and quantum dots (QDs) covalently attached to the PNIPAM layer is presented. The PNIPAM brushes are grafted from the gold surface using an iniferter-initiated controlled radical polymerization. The PNIPAM chain ends are functionalized with amine groups for coupling to water-dispersible COOH-functionalized QDs. Upon increasing the temperature above the lower critical solution temperature (LCST) of PNIPAM the QD luminescence is quenched. The luminescence was observed to recover upon decreasing the temperature below the LCTS. The data obtained are consistent with temperature-modulated thickness changes of the PNIPAM layer and quenching of the QDs by the gold surface via nonradiative energy transfer.
We present experiments to determine the quantum efficiency and emission oscillator strength of exclusively the emitting states of the widely used enhanced green fluorescent protein (EGFP). We positioned the emitters at precisely defined distances from a mirror to control the local density of optical states, resulting in characteristic changes in the fluorescence decay rate that we monitored by fluorescence lifetime microscopy. To the best of our knowledge, this is the first emission lifetime control of a biological emitter. From the oscillation of the observed emission lifetimes as a function of the emitter to mirror distance, we determined the radiative and nonradiative decay rates of the fluorophore. Since only the emitting species contribute to the change in emission lifetimes, the rates determined characterize specifically the quantum efficiency and oscillator strength of the on-states of the emitter, in contrast to other methods that do not differentiate between emitting and dark states. The method reported is especially interesting for photophysically complex systems like fluorescent proteins, where a range of emitting and dark forms has been observed. We have validated the analysis method using Rhodamine 6G dye, obtaining results in very good agreement with the literature. For EGFP we determine the quantum efficiency of the on-states to be 72%. As expected for this complex system, our value is higher than that determined by methods that average over on- and off-states.
Multimode microscopy exploits the measurement of multiple spectroscopic parameters to yield a wealth of spatially resolved spectroscopic detail about the sample under study. Here, we describe the realization of a multimode microscope capable of wide-field transmission, reflectivity and emission imaging. The instrument also incorporates confocal spectral and lifetime imaging enabling convenient high-content imaging of complex samples, allowing the direct correlation of the data obtained from the different modes. We demonstrate the versatility of this imaging platform by reviewing applications to the modulation of fluorescent protein emission by inverse opal photonic crystals, to the detection and visualization of J-aggregate coupling of small molecule dyes intercalated into nanochannels in zeolites and to the visualization of fluorescent proteins micropatterned onto surfaces. In all cases, the combination of different microspectroscopic modes is essential for the resolution of specific photophysical details of the complex systems in question.
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