Full Length ArticleOptimization and analysis of pyrene-maltose functionalized graphene surfaces for Con A detection
Introduction
Graphene has attracted considerable interests in biosensors for the detection of bioactive molecules, including glucose, protein, nucleic acid (DNA and RNA), and bioactive particles including viruses and bacteria [1], [2], [3], [4], [5], [6]. Despite of the high sensitivity and great potentialities in such area, the inert surface of graphene results in the challenge for selective biomolecular detection [7], [8]. Various functionalization approaches have been developed to conjugate selective organic ligands on the surface of graphene, including both covalent and non-covalent modification [9]. Non-covalent modifications based on π-π stacking is advantageous as it does not require chemical modification and therefor maintains the structure and unique physical properties of graphene [10]. Among these non-covalent functional molecules, amphiphilic pyrene based molecules have been demonstrated useful for functionalization by π-π stacking [11], [12], [13]. Because of the aromatic structure, these amphiphiles can strongly absorb onto the surface of graphene, improving the dispersion of graphene in solvent [14], and at the same time exposing a hydrophilic group to act as linker for further functionalization, or to recognize the target biomolecules directly. For instance, Lin, et al. and Park, et al. reported the pyrene-polyglucose based graphene biosensors [15], [16], which showed good sensitivity and selectivity for Concanavalin A (Con A) molecules. However, these achievements rises several fundamental questions in the π-π stacking functionalized graphene, which have been rarely addressed, e.g. how do the functional assemblies on the graphene surface, what is the surface morphology, and how can the surface structure be optimized.
As compared to electrical and optical sensing mechanism, using mechanical signals in biosensing is an emerging field that recently has raised significant interest [17]. As an efficient tool in the surfaces analysis, atomic force microscope (AFM) has been used in the biosensors, where the surface morphology and properties will be distinguished after the surface absorption of biomolecules [18], [19]. Compared with other techniques, the AFM provides the direct visualization and micromechanical characterization of various biomolecules without complex device fabrication and special labels for detection. The mechanical properties, such as Young’s modulus, stiffness and viscosity, can be characterized by using AFM in the purpose to sense target biomolecules [20], [21], [22], [23]. For instance, by measuring the stiffness of cell membrane, cancer cells can be distinguished from normal cells. In addition, by functionalizing AFM tips with biomolecules, e.g. single strand DNAs and antibodies, the AFM tip can be directly employed to detect the target molecules by approaching the sample surfaces and measuring the interaction forces [24], [25]. Therefore, the AFM based mechanical sensing has the potential to be widely employed in the detection of bio-objects.
Here, we report the systematical investigations of the graphene surface functionalized by π-π non-covalent stacking mechanisms in terms of the morphology modifications, molecular self-assembly, protein detection. As a result, the graphene functionalization process is optimized. The pyrene-maltose molecules are used for the non-covalent functionalization of graphene through strong π-π stacking interactions and systematical analysis is performed to understand the effect of solvents on the agglomeration by using various spectroscopic techniques. AFM and scanning tunneling microscopy (STM) imaging confirms the successful formation of the self-assembled and the densely packed pyrene-maltose layer on the graphene surface, which gives distinguished atomic lattice structure as compared to pristine graphene. Finally, we demonstrated that the AFM could be used to selectively detect the target protein molecules by measuring the interfacial adhesion forces between the AFM probe and surfaces as the response signal.
Section snippets
Materials
Chemical vapor deposition (CVD) monolayer graphene was mechanically transferred onto the SiO2/Si wafer (Graphenea). Pyrene-maltose was synthesized by following the previous reported method [15]. Briefly, 80 mg (0.30 mmol) pyrenemethylamine hydrochloride (95%), 108 mg (0.30 mmol) D-(+)-maltose and excess amount of sodium cyanoborohydride (37 mg, 0.6 mmol) was added in 20 mL methanol/water (1:1, v/v) mixture, and then heated to 80oC under stirring for 48 h. The product was purified by silica gel
Results and discussion
Fig. 2 shows a schematic representation of the overall process. The starting material is a monolayer graphene with ultralow defect levels as evidenced by the Raman spectrum (Fig. 2b) that shows only a negligible D peak. After functionalizing the graphene surface non-covalently with the pyrene-maltose moieties bind through π-π stacking interactions, the maltose unit selectively binds the Con A allowing quantitative analysis using AFM.
To optimize the pyrene-maltose functionalization of graphene,
Conclusions
In this work, pyrene-maltose molecules are used to realize the non-covalent functionalization of graphene by the strong π-π stacking interactions. Different solvents are investigated to understand the agglomeration effect of the molecules and achieve a uniform functionalization layer, which is further confirmed by the AFM and STM. The densely packed pyrene-maltose molecules on graphene surface exhibits an ordered diamond-shape lattice, which is considerably distinguished from pristine graphene.
Acknowledgments
This work was financially supported by the Swedish Research Council (grant number 2016-05259), Knut and Alice Wallenberg foundation (Swedish graphene initiative) and China Scholarship Council (grant number 201404910509). The authors also acknowledge the financial support from the Swedish Research Council Formas (grant number 2019-01538).
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These authors contribute equally.