Abstract
Hybrid graphene oxide/silver nanocubes (GO/AgNCs) arrays for surface-enhanced Raman spectroscopy (SERS) applications were prepared by means of two procedures differing for the method used in the assembly of the silver nanocubes onto the surface: Langmuir–Blodgett (LB) transfer and direct sequential physisorption of silver nanocubes (AgNCs). Adsorption of graphene oxide (GO) flakes on the AgNC assemblies obtained with both procedures was monitored by quartz crystal microbalance (QCM) technique as a function of GO bulk concentration. The experiment provided values of the adsorbed GO mass on the AgNC array and the GO saturation limit as well as the thickness and the viscoelastic properties of the GO film. Atomic force microscopy (AFM) measurements of the resulting samples revealed that a similar surface coverage was achieved with both procedures but with a different distribution of silver nanoparticles. In the GO covered LB film, the AgNC distribution is characterized by densely packed regions alternating with empty surface areas. On the other hand, AgNCs are more homogeneously dispersed over the entire sensor surface when the nanocubes spontaneously adsorb from solution. In this case, the assembly results in less-packed silver nanostructures with higher inter-cube distance. For the two assembled substrates, AFM of silver nanocubes layers fully covered with GO revealed the presence of a homogeneous, flexible and smooth GO sheet folding over the silver nanocubes and extending onto the bare surface. Preliminary SERS experiments on adenine showed a higher SERS enhancement factor for GO on Langmuir–Blodgett films of AgNCs with respect to bare AgNC systems. Conversely, poor SERS enhancement for adenine resulted for GO-covered AgNCs obtained by spontaneous adsorption. This indicated that the assembly and packing of AgNCs obtained in this way, although more homogeneous over the substrate surface, is not as effective for SERS analysis.
Highlights
Organized films composed of metal nanoparticles have been extensively studied in recent years owing to their enormous potential in fields as diverse as photoelectrochemistry [1,2], optoelectronics [3], energy-harvesting applications [4], cancer imaging and therapy [5], sensing and biosensing applications [6,7]
Sensors based on arrays of noble metal nanoparticles have become increasingly popular for the ultrasensitive detection of a variety of species ranging from small molecules to large proteins by means of surface-enhanced Raman spectroscopy (SERS) [8,9]
In a closely related paper by our group [22], we investigated the influence of thickness and structuring of the graphene oxide layer covering a Langmuir–Blodgett film of silver nanocubes on SERS detection, in the same paper [22] we compared the experimental results with theoretical simulations obtained by a finite element method (FEM)
Summary
Organized films composed of metal nanoparticles have been extensively studied in recent years owing to their enormous potential in fields as diverse as photoelectrochemistry [1,2], optoelectronics [3], energy-harvesting applications [4], cancer imaging and therapy [5], sensing and biosensing applications [6,7]. Sensors based on arrays of noble metal nanoparticles have become increasingly popular for the ultrasensitive detection of a variety of species ranging from small molecules to large proteins by means of surface-enhanced Raman spectroscopy (SERS) [8,9]. These arrays offer additional sensing capabilities based on the localized surface plasmon resonance (LSPR) sensitivity to subtle changes in the refractive index of the surrounding molecular environment [10,11]. Methods rely on the random aggregation of silver or gold nanoparticles induced by a salt [16] whereas more recently external magnetic field were employed to dynamically control the interparticle spacing of a nanoparticle monolayer at the hexane/water interface [17]; the fabrication of controllable hot spots still remains a remarkable challenge
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