Flexible microsphere‐coupled surface‐enhanced Raman spectroscopy (McSERS) by dielectric microsphere cavity array with random plasmonic nanoparticles

Abstract

Surface‐enhanced Raman spectroscopy (SERS) is a powerful tool for nondestructive and ultrasensitive optical trace‐detection. However, the sophisticated fabrication processes and performance degradation on flexible substrates block SERS for practical uses. Here, we report a facile flexible microsphere‐coupled SERS (McSERS) substrate composed of a dielectric microsphere cavity array (MCA) and random gold nanoparticles (AuNPs) capping on a polydimethylsiloxane (PDMS) film (MCA/AuNPs/PDMS) for giant Raman enhancement. The random distribution of AuNPs provides a hydrophilic surface against to the coffee‐ring effect for uniform localized surface plasmon resonance (LSPR) response. The MCA capped on the AuNPs boosts the Raman intensity via the multiple optical manipulation processes, in which the photonic nanojet (PNJ) confines the excitation intensity near the AuNPs, whispering‐gallery mode (WGM) facilitates the energy transfer from microsphere cavities to AuNP gaps for LSPR boosting, and directional antenna effect converts near‐field Raman signals into far‐field with a small divergence. Therefore, the Raman scattering is dramatically improved with the enhancement factor (EF) to 107 for the limit of detection (LoD) of 4‐nitrobenzenethiol (4‐NBT) molecules down to 0.1 nM, two orders of magnitude higher via MCA coupling. Moreover, the flexible McSERS substrate exhibits outstanding durability and compatibility as an ultrasensitive Raman test strip, by which the thiram concentration is detectable down to 2.42 ng/cm2 on apple peels. The present work provides a facile strategy to fabricate SERS substrates with high flexibility for optical trace‐detection in real‐world applications.

Optoplasmonic Effects in Highly Curved Surfaces for Catalysis, Photothermal Heating, and SERS

“…SERS enhancements ranging from 3 to 10 times were obtained on the different curved surfaces in comparison to the aggregated Au NPs or up to 5 times in relation to the flat substrate with Au NPs templated with PS-b-P4VP (Figure 2A). This is a similar performance gain to that previously reported for a Au NP substrate covered with glass microspheres, 46 showing that integrating the plasmonic substrate to the curved surface has a similar effect on the SERS performance. The results in Figure 2A also demonstrate that SERS is enhanced The polydispersity of the glass microspheres and microfibers (Figure S2) and the tapered cone of the pulled fibers (Figure 1C) allowed researchers to investigate the effect of diameter on the SERS response.…”

“…The results in Figure 2A also demonstrate that SERS is enhanced The polydispersity of the glass microspheres and microfibers (Figure S2) and the tapered cone of the pulled fibers (Figure 1C) allowed researchers to investigate the effect of diameter on the SERS response. In all cases, the integrated area of the 4-MBA peak at 1590 cm −1 was maximal for diameters of approximately 5 μm, consistent with prior observations that enhancement is maximal at that diameter, 46,47 whereas both lower and higher diameters led to lower SERS enhancements (Figures 2B−D). In the case of microfibers, the optimal diameter was slightly higher at 6−8 μm but nonetheless consistent with the other curved substrates.…”

“…43−45 Using polydisperse microspheres and microfibers, the influence of the diameter of the optoplasmonic structures on the plasmonically enhanced spectroscopic and catalytic properties is investigated. We confirm recent reports that suggested maximal Raman enhancement on microspheres of 5 μm, 46,47 and show that this size relationship holds true for other types of curved surfaces. Importantly, we propose a theoretical framework to explain why a maximal response is observed at a specific diameter.…”

“…Microlensing effects using uncoated glass beads have previously been shown to improve the SERS response of flat substrates coated with Au NP. , Here, glass beads were functionalized with Au NPs, integrating a microlens and plasmonic substrate amenable to homogeneous SERS assays or densely packed photocatalysts. In addition to spherical microstructures, similar experiments were performed on microfibers (cylindrical) and pulled fibers (conical) to evaluate the generality of the curvature effects on SERS enhancement.…”

THz hooks generated on the basis of hollow reflection mirror

“…Photonic nanojet (PNJ) is a highly focused beam generated by a meso-scale dielectric particle under the irradiation of plane wave [1]. Due to high intensity, sub-wavelength beam waist and wavelength-scale propagation distance over which the diffraction is weak [2], PNJ shows great application potential in many fields, such as enhancement of Raman scattering signal [3][4][5], high resolution imaging [6][7][8] and detection of nanoparticles [9,10]. The PNJs are always generated in either a transmission or a reflection mode [11].…”