Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
pubs.acs.org/JPCC
Covalent Immobilization of Gold Nanoparticles on Graphene M. Zakir Hossain* and Natsuhiko Shimizu
J. Phys. Chem. C Downloaded from pubs.acs.org by IOWA STATE UNIV on 02/04/19. For personal use only.
International Research and Education Center for Element Science, Graduate School of Science and Technology, Gunma University, Kiryu City, Gunma 376-8515, Japan ABSTRACT: The combination of the extraordinary electrical and optoelectrical properties of graphene and the binding potential of gold nanoparticles (AuNPs) with various small molecules and biological targets can aid in the development of ultrasensitive chemical and biological sensors. In this study, we carried out the covalent immobilization of AuNPs on the surface of graphene and investigated their further reaction with two target molecules. In the first step, epitaxial graphene (EG) on SiC was functionalized with −SH groups through the in situ primary amine diazotization reaction. The −SH-functionalized EG/SiC was then treated with HAuCl4 during the synthesis of AuNPs via the NaBH4 reduction process at 80 °C. The HAuCl4 treatment resulted in the decoration of graphene with covalently bonded (through −S−Au bonding) AuNPs (25−70 nm). The immobilized AuNPs further reacted with other reactive molecules such as hexanedithiol (HSC6H12SH) and pentachlorobenzenethiol (Cl5C6SH). Hence, the results demonstrated that AuNPs can be effectively immobilized on the surface of SH-functionalized EG/SiC, and the resulting assembly can be used for capturing specific biomolecules.
■
INTRODUCTION Its extraordinary electronic and optoelectronic properties make graphene the most promising two-dimensional material for next-generation ultrasensitive sensor technology.1−8 However, because of the bonding nature of C atoms in the hexagonal closed-packed structure, graphene is chemically inert to many functional molecules and atoms.9 The chemical inertness of graphene toward various molecules limits its application in chemical and biosensors because graphene-based electrodes do not interact with desired target molecules.6,7 Hence, to make graphene suitable for sensor applications, its surface properties should be modified.9 To date, various studies have been carried out to bind organic molecules onto the basal plane of graphene through various chemical methods.10 However, the functionalization of graphene with active organic moieties that can capture analyst molecules has not been studied much. Owing to its excellent electrical conductivity, graphene can be used as a super sensitive electrode for effectively transducing electrical signals in a sensor.7 Various efforts have been made to decorate graphene with various nanoparticles (NPs) including gold NPs (AuNPs) in order to make it suitable for capturing biomolecules.11,12 Most of the graphene−metal nanoparticle (MNP) assemblies are synthesized by the simultaneous reduction of metal salts and highly oxidized graphene derivatives. These MNPs are usually covered with surface-adsorbed ligands.13−15 Recently, Mendieta et al. decorated graphene oxide with ligand-free AuNPs using the pulse laser ablation technique in the liquid phase.11 Indeed, the decoration of graphene (which shows extraordinary electrical and optoelectrical properties) with MNPs can provide a larger electrochemically active surface area for the adsorption of biomolecules and hence can effectively enhance the electron transfer between the graphene electrode and the © XXXX American Chemical Society
molecules to be detected. Hence, MNP-decorated graphene can be utilized for developing ultrasensitive sensors. Recently, we carried out the functionalization of epitaxial graphene (EG) on SiC (EG/SiC) with −SH (thiol) groups through the in situ diazotization reaction.16 Since the −SH group can bind with Au atoms through covalent bonding, −SH-functionalized graphene can be utilized as a platform for capturing AuNPs, as shown in Figure 1. AuNPs possess distinct physical, chemical, and optoelectric properties that make them an excellent material for the fabrication of novel chemical and biological sensors.17 In addition, AuNPs can offer a suitable platform for multifunctionalization with a variety of organic or biological ligands for the selective binding of target molecules. In this study, we carried out the covalent immobilization of AuNPs on graphene under different experimental conditions and investigated the reaction of the resulting assembly with two target molecules using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Raman spectroscopy. We observed that the AuNPs could be covalently immobilized on graphene by immersing the −SH-functionalized graphene into the HAuCl4 solution during the AuNP synthesis through NaBH4 reduction at 80 °C. The −SH-functionalized graphene was also immersed into a presynthesized AuNP solution to obtain AuNP-decorated graphene. However, no chemical bonding occurred between the AuNPs and the −SH groups. The immobilized AuNPs reacted with thiol molecules, demonstrating the potential of the resulting assembly for sensing applications. Received: October 2, 2018 Revised: January 23, 2019
A
DOI: 10.1021/acs.jpcc.8b09619 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Schematic of thiol-functionalized graphene (left) and immobilized gold nanoparticles (AuNPs) on it.
■
EXPERIMENTAL METHODS EG/SiC was prepared by directly heating SiC at 1350 °C for 5−7 cycles of 1 min each in an ultrahigh vacuum (UHV) (maximum pressure ∼5.0 × 10−9 Torr) chamber. The typical sample (cut from a 6H-SiC(0001) wafer) size was ∼8 mm × 20 mm. The quality of the freshly prepared EG/SiC was examined ex situ by scanning tunneling microscopy and spectroscopy (STM/STS) and Raman spectroscopy, as reported earlier.18,19 Functionalization of EG/SiC was carried out in a borosilicate glass test tube to avoid the contact of the graphene surface with the wall of the reaction vessel.16 A 15 mL test tube was loaded with 1 equiv of aminoethanethiol in 5 mL of water followed by the addition of 2 equiv of concentrated HCl. The resulting mixture was thoroughly shaken to dissolve the amine completely. Aqueous solutions of aminoethanethiol are colorless. The EG/SiC sample to be functionalized was immersed vertically into the tube containing the clear aminoethanethiol solution. This tube was then cooled to a temperature of
