Nanocavity-in-Multiple Nanogap Plasmonic Coupling Effects from

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

Article

Nanocavity-in-Multiple Nanogap Plasmonic Coupling Effects from Vertically Sandwich-like Au@Al2O3@Au Arrays for Surface–Enhanced Raman Scattering Chen Yang, Ying Chen, Dan Liu, Cheng Chen, Jiemin Wang, Ye Fan, Shaoming Huang, and Weiwei Lei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17228 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Nanocavity-in-Multiple

Nanogap

Plasmonic

Coupling Effects from Vertically Sandwich-like Au@Al2O3@Au

Arrays

for

Surface–Enhanced

Raman Scattering Chen Yang,a Ying Chen,*a Dan Liu,a Cheng Chen,a Jiemin Wang,a Ye Fan,a Shaoming Huang,b and Weiwei Lei*a

a

Institute for Frontier Materials, Deakin University, Locked Bag 2000, Geelong, Victoria 3220,

Australia. b

Nanomaterials & Chemistry Key Laboratory, Wenzhou University, Wenzhou, P. R. China.

KEYWORDS: hybrid nanosheets, Au nanoparticles, 3D flexible substrate, vertical sandwich-like nanojunction, SERS

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

ABSTRACT: The development of ideal three-dimensional (3D) tailorable surface-enhanced Raman scattering (SERS) substrates with the properties of timesaving, large area, highthroughput, single or few molecules detection, reproducibility, reusable ability and high density of “hot spots”, has been the mainstream challenge and robust task. Here, we construct perpendicularly sandwich-like Au@Al2O3@Au hybrid nanosheets (PSHNs) on the Al foil as a 3D flexible substrate for SERS. The design of 3D PSHNs incorporates several advantageous aspects for SERS to enhance the performance of plasmonic diamers via bi-functions of vertical Al2O3 nanosheets (NSs) including the nano-scaffold and nano-baffle plate effects. As a nanoscaffold, it increases the space utilization of Au-Au diamers, while as a nano-baffle, it forms a densely homogenous Au@Al2O3@Au nanojunctions by sub-4 nm thickness of Al2O3 NSs as the dielectric isolated layer for the double-sided exposure of slit-like surface plasmon resonance (SPR). The optimized PSHNs substrate exhibits a fascinating SERS sensitivity with an experimental enhancement factor of 1012, and is able to detect the Rhodamine B (RhB) at extremely low concentration up to the limit of single or few molecules (10-18 M), as well as can be recycled without loss of SERS enhancement via the simple impregnation process. These advantages will greatly facilitate the wider use of SERS in many fields.

INTRODUCTION: Since the vital discovery of SERS phenomenon in 1970s, it has been applied in numerous fields including single molecule detection, specific identity of chemical fingerprints and large area Raman mapping images in genetics, medicine and biochemistry due to fast signal response, super-high sensitivity, trace analysis and non-destructive test.1-6 Essentially, the performance of SERS is based on the strong local electromagnetic focusing field arisen from the effect of plasmon couplings in the inter-particle nanogaps of metallic nanoparticles (NPs), namely “hot


2

Page 3 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spots”, and the charge transfer behaviour between the probe molecules and the substrate.7-8 Recently, the fabrication of nanostructures as a SERS-active substrate for extremely large electromagnetic field enhancement has become increasingly important with the aim to detect a single or a few molecules located at “hot spots”. These substrates not only require clean Raman background, excellent spatial reproducibility, and fabrication on large area through a costeffective method, but also require a high enhancement factor (EF) with the order over 108 and “hot spots” with a sub-10 nm gaps, high density and homogeneity in 3D distribution. Various substrates with special nanostructures have been designed during the past years for applications in SERS to improve their performance. As 3D hierarchical configuration-based SERS substrates, it can provide a larger surface area of metal nanostructure which can create higher density of “hot spots” and capture more probe molecules. Therefore, the building of 3D substrates has been the recent topic of intense study in SERS-based analysis. The most promising strategies to develop 3D hierarchical SERS substrates are to assemble noble metal NPs onto various non-plasmonic framework. Most of them are chosen Si or glass as supporting substrates to fabricate 3D hierarchical structures using plasma etching,9-10 decal transfer lithography,11 chemical vapor deposition,12 orthogonal reactive ion etching,13 or electrochemical etching method.14 However, the main drawbacks of these rigid 3D SERS substrates concentrate on the high cost, frangibility, time consuming, low-throughput and complicacy of preparation process, which are insufficient to satisfy the large batch of products for scientific and commercial demands. In order to promote the development of SERS, the exploration of flexible substrates and hierarchically 3D nanopore-introduced configurations have been proposed. For example, the carbon nanotube (CNT) sheet and various polymer matrix have been acted as the foldable substrates to resolve the special size or position requirement during practical tests.15-19 However,


3


Page 4 of 24

the Raman background and spectral overlapping interference from the carbon- or polymersbased matrix significantly influence the identification of target analytes. To improve signal cleanliness against substrate-induced fluctuations, Pisarek et al. reported that the presence of a flexible Al substrate at bottom of a nanoporous aluminium oxide layer covered with Ag NPs can increase Raman detection owing to nanoporous-involved multiple nanogaps for the enhancement of SERS activity and no Raman signal from the substrate.20 Recently, two-dimensional (2D) nanosheets (such as graphene and Mxene) veiled noble nanoparticles as SERS substrates with sub-nanometer gaps have been attracted much attention owing to their unique structural features to boost SERS chemical enhancement effect.21-22 However, these 2D SERS substrates underutilize the active confocal volume even though large-area “hot spots” are present with the x-y plane when the 3D laser confocal volume is used for SERS measurements. With the considerable extension on the third dimension (z-direction) to further improve the versatility of a 2D SERS platform, therefore, it is of great significance to construct unique and novel 3D architectures SERS substrates assembled by 2D nanomaterials and noble metal NPs through an ingenious structural design strategy, which are expected to reveal a remarkable high SERS activity. Here, for the first time we innovatively develop the tailorable sandwiched Au@Al2O3@Au nanostructures on Al foil as 3D PSHNs substrate via a simply solvothermal synthesis and followup Au NPs graft. This approach not only endows 3D PSHNs substrates numerous properties including light weight, clean background, high throughput and facile preparation, but also remarkably creates nanocavities to capture more probing molecules and potentially increase the light interference. Therefore, the plasmonic coupling mode of hierarchical nanocavity-inmultiple nanogaps has been investigated and explained the excellent Raman scattering signal (EF


4

Page 5 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


up to 1012) and outstanding cycling performance for 5 cycles when the concentration of Rhodamine B as super-low as 10-18 M. Moreover, the finite difference time domain simulation has been further applied to reveal the distribution and intensity of local electromagnetic field in three various nanogaps. Apart from the plasmon resonance couplings of adjacent Au NPs stemmed from the nano-scaffold effect, 3D PSHNs arrays provide additional “the nano-baffle plate function” from the Al2O3 NSs (thickness < 4 nm) to form and expose more slit-like SPR nanogaps at the dielectric interfaces of Au NPs/Al2O3 NSs. Consequently, the PSHNs substrates open a practical and feasible route to satisfy the requirement of commercialized production. Significantly, to the best of our knowledge, 3D well-defined nanostructures with nanocavity-inmultiple nanogaps on non-noble metals as flexible SERS substrates have not been reported yet.

RESULTS AND DISCUSSIONS

Figure 1. (a) The preparation process of 3D PSHNs substrates by following two steps: (i) vertical growth of boehmite NSs on Al foils; (ii) Au NPs formation. (b) Photographs describing the flexibility and tailorability of the PSHNs substrate. (c) Top-view SEM images of Al foil and (d) vertical boehmite NSs arrays. (e) Top- and (f) side-view SEM images of PSHNs arrays.


5


Page 6 of 24

The successful fabrication of perpendicularly sandwich-like Au@Al2O3@Au hybrid nanosheets was schematically displayed in Figure 1a. Firstly, the boehmite NSs grew on Al foils with the specific vertical morphology by the typical solvothermal synthesis. The as-synthesized boehmite NSs had an average length of 200±20 nm corroborated by scanning electron microscopy (SEM) images (Figure 1c and d). Secondly, the 3-4 nm thickness of Au film was coated on the surface of boehmite NSs via the e-beam evaporation method. Subsequently, the boehmite NSs could be dehydrated and transformed to Al2O3 NSs during the rapid thermal annealing (RTA) process and more detailed component characterization and analysis were shown in Figure S1.23 Additionally, the Al2O3 NSs could keep the originally 2D nanostructure and vertical morphology of boehmite NSs with the average height around 140 nm, as shown in Figure S2a and b. Meanwhile, sphere-like Au NPs with dispersive and homogeneous size were formed and anchored onto the double sides of Al2O3 NSs to configure a 3D sandwiched hybrid nanostructure after RTA treatment (Figure 1e, and Figure S3). The side-view of SEM image showed that the uniform Au@Al2O3@Au hybrid arrays further confirmed the sandwich-like textures (Figure 1f). Therefore, Au-Au dimers were better dispersed than those covered under single-sided nanosheets by taking advantage of 3D vertical nanostructures. Compared to 2D planar nanosheets, such distinctively 3D sandwiched structures endowed them with a significantly improved SERS sensitivity due to their strong multi-dimensional plasmonic coupling effects including the Au-Au dimers coupling with a narrow nanogap on the surface of Al2O3 NSs, Au-Au interparticle coupling from neighbouring Au@Al2O3@Au hybrid nanosheets, and Au-Au NPs coupling within the sub-4 nm nanogap created by the perpendicular Al2O3 NSs as the nanospacer, as well as their higher surface-to-volume ratio with richly nanocavities (

Nanocavity-in-Multiple Nanogap Plasmonic Coupling Effects from

Recommend Documents