(666b) Development of a Novel Nanosensor Platform By Noncovalent Surface Engineering of Two-Dimensional Graphene Quantum Dots

Fluorescent nanomaterials have formed the core of nanosensor technologies with optical readouts, for real-time study of spatiotemporal dynamics in complex biological systems. In particular, electrostatic pinning of molecular recognition elements and biomimetic polymers on the surface of single-walled carbon nanotubes (SWCNTs) enables selective recognition of target biomolecules by the polymer and signal transduction by the near-infrared-emissive SWCNT [1]. In this work, we expand our nanosensor signal transduction element from a one-dimensional carbon nanotube to a two-dimensional graphene quantum dot (GQD). The flat surface of the GQD, in comparison to the highly curved SWCNT surface, provides an orthogonal opportunity to adsorb planar, sheet-forming polymers and thus allows further versatility in nanosensor design.

Existing work has focused on creating graphene sheet-based nanosensors and/or relies on covalent surface modification [2]–[4]. Here, we aim to create a new basis of fluorescent nanosensor development by (i) synthesizing GQDs with nanoscale dimensions that are accordingly the same order of magnitude in size as the biological systems they are designed to probe and (ii) employing noncovalent surface attachment to preserve the pristine graphitic domains that are responsible for the fluorescence emission properties. Although theory has predicted the ability of DNA nucleobases to adsorb to graphene surfaces [5]–[7] and this has been shown experimentally [8], [9], there has not yet been experimental confirmation for this system of noncovalently attached biopolymer to GQDs. We present a GQD-based sensing platform with accompanying comparisons of various GQD synthesis methods and GQD-polymer conjugate characterization. We first compare GQD synthesis schemes to create GQD preparations of varying sizes and oxidation levels. The GQDs are subsequently functionalized with a library of different polymers, including single-stranded DNA (ssDNA), synthetic nanosheet-forming peptoids [10], and phospholipids. Polymer adsorption is confirmed by a modulation in the fluorescence spectrum as well as AFM imaging. The role of GQD oxidation level, as determined by XPS analysis, and solution ionic strength are shown to play critical roles in the ability of polymers to adsorb to the GQD surface. GQDs with a lower oxidation level contain a larger proportion of` graphitic, sp²-conjugated carbon domains, and thus enable stronger binding to polymer coatings via pi-pi stacking interactions between aromatic rings. Comparisons with prior theoretical work on polymer-GQD molecular interactions are discussed. In summary, the development and characterization of this two-dimensional GQD sensing platform will enable a broader range of adsorbed polymers and thus expand sensing capabilities.

References

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