Design of silver nanoparticles with graphene coatings layers used for LSPR biosensor applications

Highlights

• We proposed a hetero-structured glass/Ag-nanoparticles/graphene as high sensitive biosensor applications.

• Performance enhancement of graphene coated LSPR biosensor.

• The sensitivity is optimized by selecting the suitable graphene layers.

• The sensitivity improved by 304.60% when the pure AgNPs conventional biosensor was coated with graphene layer of the 9 nm.

Abstract

A new hybrid graphene nanoplasmonic structure, using silver nanoparticles (AgNPs) coated with a graphene film and deposited on a substrate is proposed in this paper. The aim is to obtain a plasmonic response with high sensitivity for plasmonic biosensors. We show, through the calculation of absorption spectra, that the localized surface plasmon resonance (LSPR) obtained in the SiO_x/AgNPs/Graphene nanostructure is very sensitive to variations in the thickness of the graphene and those of the refractive index of the detection medium. The study reveals a shift in plasmon resonance peaks resulting from the coupling between the AgNPs networks and the covering graphene layer. We obtained the red-shift of LSPR modes from 412 nm to 548 nm when the thickness of the graphene layer increases from 0.34 nm to 9 nm. We have shown that the increase in graphene thickness significantly affects the sensitivity of the device under study. An optimal sensitivity value is obtained for a graphene thickness of 9 nm, with a 304.60% gain in sensitivity value compared to the structure without graphene. These properties should enhance these nano-device and make them a preferred choice for biosensors applications, over other current biosensors.

Introduction

Surface plasmon resonance is an electromagnetic mode formed in a region of metal/dielectric surface and results from the interaction between free electrons and the excitation light [1]. Optical properties of metal nanoparticles (Ag or Au) and their contributions in several fields thanks to their multiple advantages that have brought to the forefront broad fields of fundamental and applied research. We would like to mention in particular from wave guiding to biosensor designs based on the nanoparticles array [[2], [3], [4]]. Localized surface plasmonic resonance biosensors (LSPR) are the most advantageous biosensors by virtue of their ability to providing biomolecular detection with higher sensitivity, mainly by removing the time-consuming marking process that reduces molecular bonding perturbations [[5], [6], [7]]. The plasmonic response of metal nanoparticles has been intensively studied and has shown a strong dependence on their sizes, shapes and constituent material [[8], [9], [10]]. It is evident from previous available studied literature (on metallic nanoparticles) that a number of parameters definitely govern their performance [11,12].

Spectacular progress on the control of optical properties of the graphene material sheets stimulated the search for new types of two-dimensional materials. Graphene is actually a two-dimensional crystal, composed of carbon atoms arranged in a hexagonal pattern, and whose stack constitutes graphite. In the visible field, it is an absorbent and dispersive material. It has many potential applications thanks to its unique electrical, mechanical, chemical and optical properties [[13], [14], [15], [16]]. Graphene may outperform existing transparent conductive materials, and a graphene based flexible touch screen was demonstrated by Bae et al. [17]. Multilayer graphene is a graphene of thin film with weak van der Waals interaction between the layers, and its electronic and optical properties are sensitive to the number of the layers as well as the stacking sequence [14].

Graphene is being considered a very interesting new kind of material that can be applied in various fields (including bio-plasmonics). It's well proven that AgNPs extensively absorb within restricted frequency bands in the visible spectrum when their LSPR are excited. Besides, the influence of the dielectric medium has been extensively studied [[18], [19], [20], [21]]. Some of this work concentrates on the effect of the graphene layers (whose refractive index is n_g [22,23]) on AgNPs optical properties. Graphene has received significant interest in late years due to its outstanding properties [2,14,24], an extremely sensitive plasmonic biosensor that uses the graphene layers to amplify the sensitivity induced by light absorbed in the graphene [18,25].

This is performed in the case when the graphene layers are deposited on the nanoparticles array. More precisely, we will analyze the effect by gradually varying the thickness of the graphene multilayers from 0.34 nm to 9 nm. Different dielectric coatings of the index n₂ are embedded above AgNPs/graphene, which in turn rest in direct or indirect contact with a SiO_x substrate (n₁). We study how the graphene layers are deposited; those are SiO_x/AgNPs/graphene. The associated work geometry is illustrated in Fig. 1(a) and (b). The incident-light wave (along the Oy-axis) whose electric field is TM polarized along Ox-axis.

The LSPR structures used throughout this work consist of a random assembly of silver nanostructures (AgNPs) supported on glass. The SEM image in Fig. 2(a) shows the distribution size as well as the overall shape of the resulting Ag nanoparticles. To validate the numerical model, we studied the response of a sample manufactured by our collaborators at the Lille University [12], the SEM image in the surface characterization Fig. 2 shows the sample in question which consists of Ag nanoparticles aletorically deposited on a glass substrate. We made an image analysis and extracted the relevant geometrical parameters. The nonoparticles were modelled by nanoribbons, the gometrical parameters considered are: l, h and a Fig. 1(a) and (b).

Throughout the document, the structural geometrical parameters will be defined such that l = 25 nm is the nanoparticles length, a = 70 nm is the network parameter (along x-axis), and h = 20 nm is the nanoparticles height.

Due to the important properties of the graphene as an effective solution to the sulfidation problem of Ag nanoparticles and the binding of biological elements [2,26], the AgNPs nanostructure was coated with a graphene layer on top, as shown in Fig. 1(b). The numerical results show that, compared to graphene is an LSPR substrate based biosensor/AgNPs, graphene in an LSPR substrate based biosensor/AgNPs/graphene offers a 304.60% higher sensitivity for nine graphene layers.

This work is structured as follow: Section 2 briefly introduces the theoretical model of numerical calculation and the method used to calculate the biosensor sensitivity. Section 3, demonstrates the corresponding results when AgNPs are covered with graphene multilayers, and when the trainers are applied on to a SiO_x substrate. Finally, section 4 will summarize the sensitivity results.