Abstract
Graphene quantum dots (GQDs) are an allotrope of carbon with a planar surface amenable to functionalization and nanoscale dimensions that confer photoluminescence. Collectively, these properties render GQDs an advantageous platform for nanobiotechnology applications, including optical biosensing and delivery. Towards this end, noncovalent functionalization offers a route to reversibly modify and preserve the pristine GQD substrate, however, a clear paradigm has yet to be realized. Herein, we demonstrate the feasibility of noncovalent polymer adsorption to GQD surfaces, with a specific focus on single-stranded DNA (ssDNA). We study how GQD oxidation level affects the propensity for polymer adsorption by synthesizing and characterizing four types of GQD substrates ranging ~60-fold in oxidation level, then investigating noncovalent polymer association to these substrates. Adsorption of ssDNA quenches intrinsic GQD fluorescence by 31.5% for low-oxidation GQDs and enables aqueous dispersion of otherwise insoluble no-oxidation GQDs. ssDNA-GQD complexation is confirmed by atomic force microscopy, by inducing ssDNA desorption, and with molecular dynamics simulations. ssDNA is determined to adsorb strongly to no-oxidation GQDs, weakly to low-oxidation GQDs, and not at all for heavily oxidized GQDs. Finally, we reveal the generality of the adsorption platform and assess how the GQD system is tunable by modifying polymer sequence and type.
Highlights
Graphene is a two-dimensional hexagonal carbon lattice that possesses a host of unique properties, including exceptional electronic conductivity, mechanical strength, and adsorptive capacity[1,2,3]
The fluorescence maximum near 400 nm is described in previous literature as the intrinsic emission wavelength of Graphene quantum dots (GQDs) with low oxidation, which is in close agreement with our own GQD samples[10]
The GQD excitation-emission profiles demonstrate that the optical characteristics of low, med, and high-ox-GQDs depend on the excitation wavelength, where the maximum fluorescence wavelength is red-shifted as the excitation is moved to longer wavelengths
Summary
Graphene is a two-dimensional hexagonal carbon lattice that possesses a host of unique properties, including exceptional electronic conductivity, mechanical strength, and adsorptive capacity[1,2,3]. The bandgap of GQDs is attributed to quantum confinement[6,7], edge effects[8], and localized electron-hole pairs[9] This gives rise to tunable fluorescence properties based upon GQD size, shape, and exogenous atomic composition. Noncovalent functionalization of GQDs with biopolymers offers the advantages of reversible binding and preserving the fluorescent substrate properties, while reducing graphene dimensions to the nanoscale enables two-dimensional carbon applications at the molecular scale, of relevance to study biological processes[33]. We explore the effects of GQD oxidative surface chemistry on the strength of binding interactions between surface-adsorbed polymers and GQDs, while preserving, or in some cases enabling, intrinsic GQD fluorescence These results can serve as the basis for the design and optimization of polymer-GQD conjugates in various nanobiotechnology applications
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