SERS optical fiber probe with plasmonic end‐facet

Abstract

Research article Received: 20 April 2016 Revised: 18 August 2016 Accepted: 20 August 2016 Published online in Wiley Online Library: 14 September 2016 (wileyonlinelibrary.com) DOI 10.1002/jrs.5031 SERS optical fiber probe with plasmonic end- facet Ming Xia, a * Pei Zhang, a Claris Leung a and Ya-Hong Xie a,b Surface-enhanced Raman spectroscopy (SERS) is a surface-sensitive technique that can enhance the intensity of Raman signal by several orders of magnitude, enabling even the detection of single molecule. This work presents the experimental and theoretical studies of an optical fiber probe with nano-structured end-facet for bio-sensing applications via SERS. The factors affecting the intensity of Raman signal passing through the fiber probe are investigated. These factors include the numerical aperture of the objective lens, the slit width of the spectrometer, the fiber length, and the size of SERS nano-array. The Raman signal loss through fiber compared with optical microscope-based free-space Raman detection is estimated. To further enhance the SERS enhance- ment factor, a hybrid graphene/Au nano-triangle structure is transferred on the end-facet of the fiber probe to enable SERS. Superimposing graphene layer on Au nano-structure is found to be superior over bare Au nano-structure in terms of the detection sensitivity. Copyright © 2016 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: SERS; optical fiber; graphene Introduction J. Raman Spectrosc. 2017, 48, 211–216 Methods The SERS fiber probe is prepared by transferring Au nano-triangle array onto the fiber facet. The fabrication process is shown in Fig. S1. SERS measurement is conducted using Renishaw inVia confocal Raman microscope with 785 nm laser (12.5 mW power with 20 s * Correspondence to: Ming Xia, Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA. E-mail: xiaming@g.ucla.edu a Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA b Jonsson Comprehensive Cancer Center, University of California, Los Angeles, Los Angeles, CA, 90095, USA Copyright © 2016 John Wiley & Sons, Ltd. Surface-enhanced Raman spectroscopy (SERS) is an important ana- lytical technique that is able to provide single molecule detection and high-resolution spectral information. [1] It is capable of single molecule detection and allows for label-free detection with high degree of specificity. [2] Molecules absorbed at the metallic surface experience a large amplification of the electromagnetic field because of local surface plasmon resonance leading to orders of magnitude increase in Raman yield and greatly enhanced Raman signal. To achieve high SERS enhancement factors (EFs), many ef- forts have been devoted to develop various metallic (mainly Au and Ag) nano-structures to enhance the local electromagnetic field. [3–5] In addition to the traditional metallic nano-structures, graphene and other two-dimensional materials have also been ex- plored to enhance the Raman signal. [6–9] SERS has been explored for in vivo tumor detection with labeled Au nanoparticles, [10,11] in which the specificity of detection is enabled by labeling technique. Compared with labeled nanoparticles, SERS optical fiber probe with single-ended measurement geometry [12–19] has been gaining at- tention for in vivo label-free bio-sensing because of their flexibility and compatibility with remote sensing. A label-free detection scheme allows real-time detection and eliminates the time and cost-consuming labeling procedures. [20] Moreover, label-free SERS renders vibrational information akin to fingerprints of the bio- molecules that is in principle more specific than any labeling approaches. The most common method to couple SERS substrates with optical fiber is to modify the fiber end with SERS substrates such as Ag or Au nanoparticles. [14–16,21] Nanoparticle-coated optical fiber probe for in vitro SERS measurement [14,16] can achieve EF on the order of 10 4 –10 5 , lower than the common single molecule SERS EF of ~10 7 –10 8 . [2] SERS fiber probes prepared using this method have spatial resolution similar to the fiber core diameter (usually larger than 10 μm). However, higher spatial resolution is demanded in certain bio-sensing, like in vivo neurochemical monitoring. [22] For instance, measurement of neurodynamics within individual synap- tic clefts (tens of nanometers [23] ) represents the most extreme chal- lenge. To expand the SERS fiber probe for in vivo bio-sensing, optimization of SERS fiber probe is required to provide higher SERS EF and spatial resolution. The optimization of SERS fiber probe can be achieved only if the key factors affecting the sensitivity are well understood. This paper describes our effort in gaining such understanding. We prepare SERS fiber probes based on Au nano-triangle array and propose a method to enhance its spatial resolution. The key factors studied here include the numerical aperture (NA) of objective lens, the slit width of spectrometer, the fiber length, and the size of SERS nano-array. The Raman signal loss through the fiber is estimated compared with normal Raman detection. To further enhance the SERS EF, a hybrid graphene/Au nano-triangle structure is trans- ferred on fiber facet. SERS fiber probe with hybrid graphene/Au nano-triangle structure is found to have better performance than the fiber probe with bare Au nano-triangle structure.