1 Miniaturized Sensing Probes Based on Metallic Dielectric Crystals 2 Self-Assembled on Optical Fiber Tips 3 Marco Pisco, †,⊥ Francesco Galeotti, ‡,⊥ Giuseppe Quero, † Agostino Iadicicco, § Michele Giordano,* ,∥ 4 and Andrea Cusano* ,† 5 † Optoelectronic Division, Engineering Department, University of Sannio, Benevento, Italy 6 ‡ Institute for Macromolecular Studies, National Research Council, Milano, Italy 7 § Department of Engineering, University of Naples “Parthenope”, Naples, Italy 8 ∥ Institute of Composite Biomedical Materials, National Research Council, Naples, Italy 9 * S Supporting Information 10 ABSTRACT: We propose a novel fabrication process to 11 realize optical sensing probes based on metal−dielectric 12 crystals self-assembled on an optical fiber tip. The breath 13 figure methodology has been adapted to work directly on 14 nonconventional substrates, such as optical fibers, enabling the 15 formation of regular and ordered metallo-dielectric crystals on 16 optical fiber tips. Accurate morphological characterization was 17 carried out to qualify the fabrication process. The reported 18 results indicate that the proposed fabrication technique 19 provides a method for rapid and cost-effective prototyping of 20 photonic−plasmonic nanoprobes for sensing applications. To 21 achieve this goal, we develop a technological platform via the 22 addition of polymer−metal crystals onto the tip of a standard 23 single optical fiber, which is able to support surface plasmon 24 resonances in the near-infrared. A dedicated numerical tool was developed to study and analyze arbitrary subwavelength 25 structures integrated on the optical fiber tip by taking into account finite-size effects. The numerical results are in good agreement 26 with the observed experimental spectra and reveal that the fabricated sensing probes act as structured interferometers that are 27 assisted by surface plasmon excitations at the metallo-dielectric interfaces. To prove the sensing capability of the proposed 28 platform, refractive index measurements were carried out, revealing a sensitivity of up to 2300 nm/RIU, outperforming most 29 plasmonic probes synthesized on optical fiber tips. The achieved performances, obtained using very small active areas, 30 demonstrate the effectiveness of these self-assembled fiber-optic probes for label-free chemical and biological sensing 31 applications. 32 KEYWORDS: breath figures, self-assembly, optical fiber sensors, sensing probes, optical fiber tip 33 I n recent decades, the development of sensors based on 34 surface plasmon resonances (SPRs) for the detection of 35 chemical and biological species has received considerable 36 scientific attention. Several optical configurations have been 37 proposed to exploit the properties of surface plasmons in 38 different sensing applications and in more disparate sectors, 39 such as medical diagnostics, environmental monitoring, and 40 food safety. 1,2 41 Surface plasmons are extremely sensitive to any local 42 refractive index change occurring at the metal−dielectric 43 interface. The excitation of an SPR leads to an increase in 44 the light absorbance at the resonance condition, which in turn 45 depends on the refractive index of the dielectric medium in the 46 proximity of the metallic surface. 1,2 This dependence is 47 exploited in various detection schemes for chemical and 48 biological sensing applications, thus enabling the label-free 49 detection of target molecules in real time and at very low 50 concentrations. 51 To efficiently trigger the excitation of surface plasmons, 52 traditional coupling schemes involve the use of prism couplers 53 and diffraction gratings, 3 which are currently outperformed by 54 miniaturized and integrated lab chip architectures and optical 55 waveguides. 4 56 Among them, SPR sensors based on optical fibers are 57 particularly attractive by virtue of the intrinsic advantages 58 associated with the use of fiber-optic technology. The challenge 59 in exploiting fiber-optic SPR phenomena in optical fiber sensors 60 arises from the difficulty of implementing an efficient and 61 controlled coupling method for plasmonic excitation within the 62 optical fiber. Despite these complications, a large number of Received: November 15, 2013 Article pubs.acs.org/journal/apchd5 © XXXX American Chemical Society A dx.doi.org/10.1021/ph500126v | ACS Photonics XXXX, XXX, XXX−XXX deb00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i5 HF03:4230 | 2.0 alpha 39) 2014/07/15 09:23:00 | PROD-JCA1 | rq_2855382 | 9/25/2014 09:14:38 | 11 | JCA-DEFAULT