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Flexible Microsphere-Embedded Film for Microsphere-Enhanced Raman Spectroscopy Cheng Xing,† Yinzhou Yan,*,†,∥ Chao Feng,† Jiayu Xu,† Peng Dong,‡ Wei Guan,† Yong Zeng,†,§ Yan Zhao,†,∥ and Yijian Jiang† †
Institute of Laser Engineering and §Beijing Engineering Research Center of 3D Printing for Digital Medical Health, Beijing University of Technology, Beijing 100124, China ‡ Capital Aerospace Machinery Company, Beijing 100076, China ABSTRACT: Dielectric microspheres with extraordinary microscale optical properties, such as photonic nanojets, optical whispering-gallery modes (WGMs), and directional antennas, have drawn interest in many research fields. Microsphereenhanced Raman spectroscopy (MERS) is an alternative approach for enhanced Raman detection by dielectric microstructures. Unfortunately, fabrication of microsphere monolayer arrays is the major challenge of MERS for practical applications on various specimen surfaces. Here we report a microsphereembedded film (MF) by immersing a highly refractive microsphere monolayer array in the poly(dimethylsiloxane) (PDMS) film as a flexible MERS sensing platform for one- to threedimensional (1D to 3D) specimen surfaces. The directional antennas and wave-guided whispering-gallery modes (WG-WGMs) contribute to the majority of Raman enhancement by the MFs. Moreover, the MF can be coupled with surface-enhanced Raman spectroscopy (SERS) to provide an extra >10-fold enhancement. The limit of detection is therefore improved for sensing of crystal violet (CV) and Sudan I molecules in aqueous solutions at concentrations down to 10−7 M. A hybrid dual-layer microsphere enhancer, constructed by depositing a MF onto a microsphere monolayer array, is also demonstrated, wherein the WG-WGMs become dominant and boost the enhancement ratio >50-fold. The present work opens up new opportunities for design of cost-effective and flexible MERS sensing platforms as individual or associated techniques toward practical applications in ultrasensitive Raman detection. KEYWORDS: dielectric microsphere, PDMS film, microsphere-enhanced Raman spectroscopy, surface-enhanced Raman spectroscopy, flexible Raman enhancer
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INTRODUCTION Raman spectroscopy has been widely acknowledged as a nondestructive technique to observe vibrational, rotational, and other low-frequency modes in molecules and provide the structural fingerprints of materials for identification.1,2 However, Raman scattering signals are extremely weak and difficult to detect, owing to the small interactive cross section. In order to enhance the signal intensity, a series of Raman enhancement techniques have been developed for several decades based on various physical/chemical mechanisms: for example, surfaceenhanced Raman scattering (SERS), tip-enhanced Raman scattering (TERS), interference-enhanced Raman scattering (IERS), resonance Raman scattering (RRS), coherent antiStokes Raman scattering (CARS), and stimulated Raman scattering (SRS), etc.3−8 Plasmon-enhanced Raman spectroscopies (SERS and TERS) are the most investigated techniques due to their giant enhancement ratios, reportedly >108, by electromagnetic confinement in nanoscale hotspots and charge transfer with noble metals.9−13 This boosts the sensitivity for Raman trace chemo/biosensing and makes single-molecule detection possible.14−18 Particularly, the stamping SERS technique provides a simple way to fabricate SERS substrates © XXXX American Chemical Society
compatible with microfluidic chips and achieves a detection limit in the range of nanomoles for real-world biological specimens.19,20 Dielectric microspheres are an alternative approach for Raman enhancement. A fundamental difference from the traditional field enhancement seen in plasmonic structures is the low-loss electromagnetic responses in the full spectrum.21 Dielectric microspheres possess several extraordinary optical properties, such as photonic nanojets,22 whispering-gallery modes (WGMs),23 and directional antennas,24 to manipulate light focusing, confinement, and scattering on the microscale. Previous studies have demonstrated numerous applications of dielectric microspheres in nanoparticle detection, fluorescence enhancement, super-resolution imaging, and nanopatterning by photonic nanojets,25−29 ultra-low-threshold lasing and single molecule/nanoparticle transducing by WGMs,30−32 as well as fluorescence redirection by optical antenna effects.24,33,34 The dielectric microsphere array was first proposed for Raman Received: July 7, 2017 Accepted: September 11, 2017
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DOI: 10.1021/acsami.7b09884 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Fabrication of microsphere-embedded film (MF) and schematic of microsphere-enhanced Raman spectroscopy (MERS). (a) Fabrication of MF by microsphere-suspension dropping and self-assembly, PDMS spin-coating, curing, and peeling off. (b) Top and cross-section views of a 65μm BTOG MF. (c) Wrinkled and (d) self-resumed MF. (e) Schematic and (f) real image of backscattering configuration for MERS. (g) Schematic of MERS with MF for Raman detection.
optical properties in the PDMS film, for example, focusing properties, WGMs, and directional antenna effects. The flexibility makes the film suitable for any materials and structural surfaces (e.g., 3D surfaces). In this work, MFs demonstrate the capability for enhanced Raman detection of a variety of specimens ranging from one- to three-dimensional (1D to 3D), including Si single-crystal substrate, carbon nanotubes, graphene, and TiOx on a 3D-printed Ti6Al4V alloy component. Furthermore, MFs are found to be compatible with SERS substrates for improving the detection limit of crystal violet (CV) and Sudan I molecules in aqueous solutions. Microsphere dual-layer structures with higher enhancement ratios are also demonstrated by depositing the MFs on traditional microsphere arrays. The MF enhancer proposed in this work promises MERS as an individual or associated technique with SERS for ultrasensitive detection in biomedical, environmental, and material applications as well as chemosensing on 3D structured surfaces.
enhancement in 2007 and then optimized for enhancement ratio up to 1 order of magnitude.35−39 However, the corresponding enhancement mechanisms were not completely revealed until 2015, when we showed that Raman enhancement by microspheres with diameter
