Plasmonic Optical Fiber Sensor Based on Double ...

sensors Article

Plasmonic Optical Fiber Sensor Based on Double Step Growth of Gold Nano-Islands José M. M. M. de Almeida 1,2, * 1

2

*

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, Helena Vasconcelos 1 , Pedro A. S. Jorge 1

ID

and Luis Coelho 1

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CAP/INESC TEC—Technology and Science and FCUP—Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal; [email protected] (H.V.); [email protected] (P.A.S.J.); [email protected] (L.C.) Department of Physics, School of Sciences and Technology, University of Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal Correspondence: [email protected]

Received: 18 March 2018; Accepted: 18 April 2018; Published: 20 April 2018

Abstract: It is presented the fabrication and characterization of optical fiber sensors for refractive index measurement based on localized surface plasmon resonance (LSPR) with gold nano-islands obtained by single and by repeated thermal dewetting of gold thin films. Thin films of gold deposited on silica (SiO2 ) substrates and produced by different experimental conditions were analyzed by Scanning Electron Microscope/Dispersive X-ray Spectroscopy (SEM/EDS) and optical means, allowing identifying and characterizing the formation of nano-islands. The wavelength shift sensitivity to the surrounding refractive index of sensors produced by single and by repeated dewetting is compared. While for the single step dewetting, a wavelength shift sensitivity of ~60 nm/RIU was calculated, for the repeated dewetting, a value of ~186 nm/RIU was obtained, an increase of more than three times. It is expected that through changing the fabrication parameters and using other fiber sensor geometries, higher sensitivities may be achieved, allowing, in addition, for the possibility of tuning the plasmonic frequency. Keywords: optical fiber sensor; localized surface plasmon resonance; gold nanoparticles; gold thin film dewetting

1. Introduction The measurement of the refractive index (RI) of liquid solutions brings information about its chemical and physical properties, as well as the composition and concentration of dissolved biochemical or biological substances [1]. There are several methods to measure the RI using either free space propagation or guide wave optics [2,3]. Optical fiber sensors (OFS) based on surface plasmon resonance (SPR) can be made with metallic thin layers on top of specific optical fiber surfaces, creating features highly sensitive to the surrounding medium [4–6]. In certain conditions, the fabrication of noble metal thin films with nanoscale patterns, usually gold (Au) and silver (Ag), leads to a buildup of the electromagnetic field at the nanostructures boundaries and the phenomenon of localized surface plasmon resonance (LSPR) may take place. This effect results in the appearance of absorption bands whose resonant frequency depends upon the size, shape, and composition of the nanoparticles (NP) and the surrounding RI [7–9], allowing label-free real-time RI measurements. Several protocols have been developed to obtain Au NP with the chosen size and shape, dispersion stability and surface functionality. These methods can be classified either as physical or chemical [10–14]. However, while the physical methods require expensive equipment, the chemical methods are laborious and often lead to non-uniform films. The thermal annealing of metal thin

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films such as Au [15], Ag [16] and palladium [17], results in the formation of nano-islands on the surface (dewetting), offering a low-cost technique, with potential large-scale fabrication capability to produce plasmonic sensors with LSPR ability. The dewetting of Au thin films has been the object of studies on the influence of the thin film deposition conditions, substrate preparation, and thermal annealing parameters on the morphology of the nano-islands and on the characteristics of the plasmonic resonance [18–20]. Recently it was reported by Kang, M. [21], that repeating the standard thermal dewetting process of Au thin films, nano gap rich Au islands are obtained. The method provides enlarged Au nano-islands with small gap spacing, which increases the number of electromagnetic hotspots and thus enhances the extinction intensity as well as the tunability for plasmon resonance wavelength. Optical fiber sensors based on LSPR produced by dewetting of Au thin films were produced in unclad fiber [22] and, recently, a 50/3/50 µm hetero-core structured fiber coated with Au nano-islands around the cladding was demonstrated with wavelength sensitivity of 517 nm/RIU [23]. The optimization of OFS coated with Au thin film and annealed was presented [24]. The multimode fiber was immersed in piranha solution and then subjected to a lengthy silanization process for improving the Au adhesion to the silica core. When an SPR zone of 6 mm in length was built, a sensitivity higher than 700 nm/RIU was measured. The objective of the study presented here is the comparison of the performance of OFS where the plasmonic region is made either by the single or by the repeated thermal dewetting of Au thin film deposited only at the end of multimode fibers. In this way, an extra degree of freedom is introduced in the design of OFS/LSPR based on Au dewetting. 2. Materials and Methods 2.1. Sample Fabrication The fiber sensors were prepared using multimode fiber (FT600UMT Thorlabs, Dortmund, Germany) with a diameter of 600 µm which was split into fragments with a length of ~4 cm. The plastic fiber jacket was mechanically detached from the fiber and the TECS Hard Fluoropolymer Cladding (not compatible with the annealing temperatures) removed by dissolving in acetone to expose the silica core. Both ends were polished using 8 and 3 µm polishing disks (Fibrmet, Buehler, Lake Bluff, IL, USA) until an optical surface was reached. In addition, a set of silica planar substrates was used to fabricate and characterize nano-structured Au thin films using a broad range of conditions. Prior to deposition of Au, silica planar substrates and optical fibers were placed in a Teflon sample holder and washed in a mixture of detergent (Decon 90) with deionized water in an ultrasonic bath at 30 ◦ C for 10 min. Further, the samples were placed in acetone, methanol, and water following the same procedure. Then the samples were dried under a N2 stream and dried in an oven at 100 ◦ C for 10 min. For the deposition of Au, two metal holders were designed to hold the silica substrates and the fibers (with the end faces facing down). The holders were fixed onto a circular metal plate with a diameter of ~200 mm. The Au thin films were deposited using an electron beam evaporator model Auto 306 (Edwards, Ltd., Bolton, UK). A thickness monitor model FTM5 (Edwards, Ltd., UK) provides measurements with a resolution of 0.1 nm. The thickness of the Au thin films ranged from 2 to 12 nm and the adhesion to the substrate was improved with a 2 nm buffer layer of chromium (Cr). For the thermal dewetting of the Au thin films, the samples were placed in the central region of a 2500 W tubular furnace (Carbolite, UK) for 1 h at 500, 550, 600 and 650 ◦ C, respectively. The temperature of the oven was raised at a rate of ~10 ◦ C per minute and the cooling process took place at the same rate. The indicated annealing time was counted after the sample reached the chosen temperature until it starts to cool down.

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2.2. Sample Characterization Gold nano-islands formation on silica planar substrates was characterized by Scanning Electron Sensors 2018, 18, x FOR PEER REVIEW 3 of 13 Microscope (SEM) and by Energy Dispersive X-ray Spectroscopy (EDS). The SEM illustrates the Gold nano-islands formation on silica planar was characterized by Scanning Electron may be topologic changes due to thermal annealing, whilesubstrates EDS, being a semi-quantitative technique, (SEM) and byofEnergy Dispersivethe X-ray (EDS). The SEM illustrates the used toMicroscope confirm the presence gaps between Au Spectroscopy nano-islands. topologic changes due to thermal annealing, while EDS, being a semi-quantitative technique, may be The absorption spectra of planar samples were measured in transmission mode, using the setup used to confirm the presence of gaps between the Au nano-islands. illustrated in Figure 1a, from the ultraviolet (~300 nm) to near infrared (~900 nm), and were acquired The absorption spectra of planar samples were measured in transmission mode, using the setup with a resolution 1 nm1a, and anthe integration 100 tungsten equipped illustrated inofFigure from ultraviolettime (~300of nm) to ms. nearA infrared (~900halogen nm), andlight weresource acquired with anwith SMA905 fiber connector was used (Avantes, AvaLight-Hal, Apeldoorn, The Netherlands). a resolution of 1 nm and an integration time of 100 ms. A tungsten halogen light source equipped with an SMA905 connector wasand used (Avantes,asAvaLight-Hal, Apeldoorn, The Netherlands). The normalized spectrafiber were collected recorded an absorbance (in decibels) using a diode-array The normalized spectra were collected and recorded as an absorbance decibels) a diodefiber optic spectrometer (Speed+, SarSpec, Porto, Portugal). The (in light from using the fiber cable was array fiber optic spectrometer (Speed+, SarSpec, Porto, Portugal). The light from the fiber cable was collimated by a lens and after it passes through the planar sample was collected by a second lens, collimated by a lens and after it passes through the planar sample was collected by a second lens, where awhere second fiber fiber cablecable connects totothe a second connects thespectrometer. spectrometer.

Figure 1. (a) Setup for the measurement of the absorption spectra in transmission mode of planar

Figure silica 1. (a)samples Setup for the measurement of the absorption in transmission modemode of planar silica coated with gold nano-islands, (b) Setup spectra for measurement in reflection of samples coatedindex withsensitivity gold nano-islands, (b)coated Setupwith for gold measurement mode of refractive refractive of optical fibers nano-islands,in(c)reflection Set-up for coating with index sensitivity optical withAu gold nano-islands, (c) Set-up with Au only the Au only theof end surfacefibers of thecoated fibers; after coating, annealing took place infor an coating oven. end surface of the fibers; after Au coating, annealing took place in an oven.

The optical characterization of the fiber samples was performed in reflection mode using the setup illustrated in Figure 1b which includes a bifurcated fiber cable (SPLIT400-VIS-NIR, Ocean The optical characterization of thethe fiber samples was performed in reflection using the setup Optics, Largo, FL, USA) to connect 4 cm long fiber sensors to the light source andmode spectrometer. illustrated Figure which a bifurcated fiberwith cable (SPLIT400-VIS-NIR, Ocean1c, Optics, The in fibers were1b placed in includes a holder that enables coating Au only the end surface, Figure and Largo, after the thin were fiber annealed. Theto wavelength sensitivity the fiber samples FL, USA) tothat, connect the 4films cm long sensors the light source andofspectrometer. The was fibers were varying thecoating surrounding (SRI) from 1.300 to 1.640 using a set of the thin placed determined in a holderby that enables with refractive Au only index the end surface, Figure 1c, and after that, calibrated oils (Cargille–Sacher Laboratories, Inc., Cedar Grove, NJ, USA). The oil sample to be tested films were annealed. The wavelength sensitivity of the fiber samples was determined by varying the was placed in a plane mirror and the fiber sensor lowered until its tip was immersed in the oil drop surrounding refractive indexstage. (SRI)After from 1.300 to 1.640 using a set calibrated oils (Cargille–Sacher by means of a translation each measurement, the fiber andofthe mirror were flushed with Laboratories, Inc., Cedar Grove, NJ, USA). oil sample to be tested was placed in a plane mirror acetone to avoid contamination to the next The oil sample. A one-tailed t-test was implemented to immersed determine ifin two samples have particle diameter stage. and the fiber sensor lowered until its tip was thesets oilofdrop by means of a translation significantly differentthe from each other. verifywere whether the mean different After each measurement, fiber and the To mirror flushed with particle acetonediameter to avoidofcontamination to groups are equal, the analysis of variance (ANOVA) was applied [25].

the next oil sample. A 3. one-tailed t-test was implemented Experimental Results and Discussion to determine if two sets of samples have particle diameter significantly different from each other. To verify whether the mean particle diameter of different Characterization Planar Samples groups3.1. areSEM/EDS equal, the analysis of of variance (ANOVA) was applied [25]. Silica substrates coated with Au were analyzed by SEM/EDS techniques before and after the

3. Experimental Results and 2Discussion annealing process. Figure presents a sample coated with 6 nm thin film, (a) before and both (b) and (c) after annealing at 600 °C for 1 h. The photographs confirm that the thin films were converted to a

3.1. SEM/EDS Characterization of Planarafter Samples discontinuous nano island structure annealing.

Silica substrates coated with Au were analyzed by SEM/EDS techniques before and after the annealing process. Figure 2 presents a sample coated with 6 nm thin film, (a) before and both (b) and

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(c) after annealing at 600 ◦ C for 1 h. The photographs confirm that the thin films were converted to a discontinuous nano island structure after annealing. Sensors 2018, 18, x FOR PEER REVIEW

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Figure 2. Scanning Electron Microscope (SEM) photograph of a sample coated with 6 nm thick gold

Figure 2. Scanning Electron Microscope (SEM) photograph of a sample coated with 6 nm thick gold thin film: (a) before and (b,c) after annealing at 600 °C for 1 h. thin film: (a) before and (b,c) after annealing at 600 ◦ C for 1 h. Figure 3 shows the SEM photographs (taken with the built-in backscattered electron detector) of samples annealed at 500 and 600 °C for 1 h and coated with 2 to 10 nm Au thin films: (a) 2 nm/600 °C, Figure 3 shows the SEM photographs (taken with the built-in backscattered electron detector) of (b) 4 nm/600 °C, (c) 4 nm/500◦ °C, (d) 6 nm/600 °C, (e) 8 nm/600 °C and (f) 10 nm/600 °C. Distinct Figureat2.500 Scanning (SEM) photograph of 2 a sample coated with 6 nmfilms: thick gold samplesregions annealed andElectron 600 CMicroscope for 13dhwere and coated with to 10 nm Au thin (a) 2 nm/600 ◦ C, Z1 and Z2 marked on Figure analyzed by EDS allowing to estimate the composition thin film: (a) before and ◦(b,c) after annealing at 600 °C for 1 h. ◦ ◦ ◦ ◦ (b) 4 nm/600 C, (c) inhomogeneity 4 nm/500 C,at(d) 6 nm/600 C, (e) nm/600 C andin(f)all10thenm/600 of the surface different locations. The8 same is observed samples. C. TheDistinct regions average Z1 andFigure Z2 marked on Figure 3d were analyzed by EDS allowing to estimate the composition of particle diameter after annealing was calculated using the image analysis software ImageJ 3 shows the SEM photographs (taken with the built-in backscattered electron detector) of (National Institutes of Health, Bethesda, MD, USA). The average diameter of the nano-islands for samples annealed at 500 and 600 °C for 1 h and coated with 2 to 10 nm Au thin films: (a) 2 nm/600 °C, the surface inhomogeneity at different locations. The same is observed in all the samples. The average with the starting thickness of 2,6 4, 6, 8 and is image presented in(f) Table 1. From°C. the ANOVA (b) 4 nm/600 °C, (c) 4 nm/500 °C, (d) nm/600 °C,10 (e)nm 8 nm/600 °C and 10 nm/600 Distinct particle samples diameter after annealing was calculated using the analysis software ImageJ (National Z2 marked Figure analyzed by EDS allowing to estimate the composition testregions it mayZ1 beand concluded thaton the 2 and3d4 were nm thin film samples are not significantly different, as well Institutes of Health, Bethesda, MD, USA). The average diameter of the nano-islands for samples with surface inhomogeneity different locations. The is observed all the samples. The as of thethe 8 and 10 nm samples. Itatcan be observed that thesame island diameterinincreases with the film the starting thickness ofbetween 2, 4, 6, the 8 and 10 nm is presented intoTable 1. From ANOVA test it may be average particle diameter after annealing was calculated using thesimilar image analysis software ImageJ thickness. The gap different nano-islands seems be to thethe island size. However, (National Institutes of Health, Bethesda, MD, USA). The average diameter of the nano-islands for concluded that the 2 and 4 nm thin film samples are not significantly different, as well as the 8 and no quantitative validation was performed. There is a steady decrease in the island surface density samples with the starting thickness of 2, 4, 6, 8 and 10 nm isofpresented in Table 1.the From the ANOVA with the increase of the film thickness, although, analysis Figure 3b,c led to conclusion that 10 nm samples. It can be observed that the island diameter increases with the film thickness. itThe gap test it may be concluded that the 2 and 4 nm thin film samples are not significantly different, as well not affected by nano-islands the annealing temperature. betweenis the different seems to be similar to the island size. However, no quantitative as the 8 and 10 nm samples. It can be observed that the island diameter increases with the film

validation thickness. was performed. Therethe is adifferent steadynano-islands decrease in thetoisland surface with the increase of The gap between seems be similar to the density island size. However, no quantitative validation was performed. There is a steady decrease in the island surface density the film thickness, although, analysis of Figure 3b,c led to the conclusion that it is not affected by the the increase of the film thickness, although, analysis of Figure 3b,c led to the conclusion that it annealing with temperature. is not affected by the annealing temperature.

Figure 3. Scanning Electron Microscope (SEM) photograph of samples annealed at 500 and 600 °C for 1 h for 2 to 10 nm tick Au thin films: (a) 2 nm/600 °C, (b) 4 nm/600 °C, (c) 4 nm/500 °C, (d) 6 nm/600 °C, (e) 8 nm/600 °C and (f) 10 nm/600 °C. The marked regions Z1 and Z2 on (d) were analyzed by Energy Dispersive Spectroscopy (EDS). (SEM) photograph of samples annealed at 500 and 600 °C for Figure 3.X-ray Scanning Electron Microscope

◦ Figure 3. Scanning Electron Microscope (SEM) photograph of samples annealed 500 and 1 h for 2 to 10 nm tick Au thin films: (a) 2 nm/600 °C, (b) 4 nm/600 °C, (c) 4 nm/500 °C, (d)at 6 nm/600 °C,600 C for ◦ ◦ ◦ nm/600 (f) films: 10 nm/600 The marked Z1 and Z2 on analyzedC, by(d) Energy 1 h for 2 to (e) 10 8nm tick °C Auand thin (a)°C. 2 nm/600 C,regions (b) 4 nm/600 C,(d) (c)were 4 nm/500 6 nm/600 ◦ C, ◦ C and Dispersive X-ray Spectroscopy (e) 8 nm/600 (f) 10 nm/600 ◦(EDS). C. The marked regions Z1 and Z2 on (d) were analyzed by Energy Dispersive X-ray Spectroscopy (EDS).

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Table 1. Average diameter of the nano-islands for samples with 2, 4, 6, 8 and 10 nm. Table 1. Average diameter of the nano-islands for samples with 2, 4, 6, 8 and 10 nm. Nano-islands

Nano-islands

Thickness of Gold Thin Film (nm)

Thickness of Gold Thin Film (nm) 2 2 4 6 8 8 1010 4 6 c c a a b a a ± b 61.8 Diameter ±±standard standard deviation ± 5.1± 5.1 30.5 ±30.5 5.3 ± 5.3 ± 12.3 ± 15.2 41.9 6.1 ± 6.161.8 Diameter deviation 24.124.1 41.9 ± 12.3 c 73.4 73.4 ± 15.2 c Averages have differentlowercase lowercase letters different (p < 0.05). Averages thatthat have different lettersare aresignificantly significantly different (p < 0.05).

The EDS sample afterafter annealing is shown in Figure 4. The4. surface of the sample EDSanalysis analysisofofa 6a nm 6 nm sample annealing is shown in Figure The surface of the does not possess a significant number of impurities. Both spectra show the well-defined band at sample does not possess a significant number of impurities. Both spectra show the well-defined band around 1740 eVeV corresponding to to thethe KαKα Kβ at around 1740 corresponding , Kα 2 and Kβ1 1lines linesof ofthe theSi SiKKband. band. The The adhesion adhesion layer 1 , 1Kα 2 and band Cr L is overlapped by the wings of the larger oxygen line (O Kα) and a band related to carbon contamination (band (band CCK) K)isispresent. present.The The band related is present in region Z1 (nano-island) band related to to AuAu is present in region Z1 (nano-island) but but notregion in region Z2 (substrate). Therefore, the spots whitecorrespond spots correspond to pure Au nano-islands, not in Z2 (substrate). Therefore, the white to pure Au nano-islands, which is which is notin present in remaining surface, meaning thatthin the film Au thin film undergoes nucleation after not present remaining surface, meaning that the Au undergoes nucleation after thermal thermal treatment. treatment.

Figure 4. 4. Energy Energy Dispersive Dispersive X-ray X-ray Spectroscopy Spectroscopy (EDS) nm sample sample after after annealing annealing at at Figure (EDS) analysis analysis of of aa 66 nm ◦ 600 °C 600 C for for 11 h. h.

3.2. Optical of Planar Planar Samples Samples 3.2. Optical Characterization Characterization of The optic/plasmonic noble metal nanostructures depends on the size The optic/plasmonicproperties propertiesofof noble metal nanostructures depends on nanoparticle the nanoparticle and shape and distance between particles [9,19]. The morphology of the nano-structured films can be size and shape and distance between particles [9,19]. The morphology of the nano-structured films modified by controlling the initial film thickness, the temperature and the time of annealing. In can be modified by controlling the initial film thickness, the temperature and the time of annealing. addition, thethe deposition rate ofofthe its In addition, deposition rate theinitial initialthin thinfilm, film,the the annealing annealing atmosphere atmosphere pressure pressure and and its composition play an important role [18]. The present work shows the influence of the initial Au film composition play an important role [18]. The present work shows the influence of the initial Au film thickness and and the the annealing annealing temperature temperature on on the the LSPR LSPR characteristics. characteristics. thickness The Au coated planar samples were photographed before and and after after the the thermal thermal treatment treatment with with The Au coated planar samples were photographed before a digital camera (7 megapixel, Sony Cyber-shot). Before the thermal treatment, the Au films exhibit a digital camera (7 megapixel, Sony Cyber-shot). Before the thermal treatment, the Au films exhibit different colors colors changing changing from from light light gray gray for for 22 nm nm to to grayish-blue grayish-blue for for 66 nm nm and and to to yellowish yellowish golden golden different color for 12 nm thick films, as illustrated in Figure 5. After annealing for 1 h, the colors of the planar color for 12 nm thick films, as illustrated in Figure 5. After annealing for 1 h, the colors of the planar samples have changed. For the 2 nm sample a change can be already observed at 500 °C, while samples have changed. For the 2 nm sample a change can be already observed at 500 ◦ C, while thethe 4 to4 8 nm samples turn violet 550 °C.Thicker Thickersamples sampleschange changecolor coloronly onlyatat650 650◦°C. As can can be be seen seen in in ◦ C. 8tonm samples turn violet at at 550 C. As Figure 5, higher annealing temperature leads to a dark pink/violet color. Figure 5, higher annealing temperature leads to a dark pink/violet color.

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Figure 5. Gold coated planar samples photographed before and after annealing at 500, 550, 600 and 650 ◦°C C for 1 h with initial thicknesses of 2, 4, 6, 8, 10 and 12 nm.

The absorption and 2, 2, 6 The absorption spectra spectra before before annealing annealingof ofsamples sampleswith with22nm nmofofCrCr(adhesion (adhesionlayer), layer), and and 12 nm thick Au film deposited are shown in Figure 6. The absorption spectrum of the sample 6 and 12 nm thick Au film deposited are shown in Figure 6. The absorption spectrum of the sample with only only the the Cr Cr layer layer is is characterized characterized by by aa flat flat band band and and very very small small absorbance absorbance compared compared to to the the with samples with be concluded that that the Cr layer layer has low the optical samples with Cr Crand andAu. Au.It can It can be concluded theadhesion Cr adhesion haseffect low on effect on the spectroscopy. The sample with 2 nm of Au, shows a plasmonic broad absorption band with a peak optical spectroscopy. The sample with 2 nm of Au, shows a plasmonic broad absorption band with at ~600 nm. For samples with higher Au film thickness, the absorption bands exhibit similar a peak at ~600 nm. For samples with higher Au film thickness, the absorption bands exhibit similar characteristics, with with aa minimum minimum below below 600 600 nm nm followed followed by by aa smooth smooth band band for for higher higher wavelengths. wavelengths. characteristics, The wide absorption band for 2 nm Au sample may be ascribed to the formation of nanoThe wide absorption band for 2 nm Au sample may be ascribed to the formation of nano-structures structures the well-known process of thin films growth. In thin goldfilm thin growth film growth considering during theduring well-known process of thin films growth. In gold considering the the Volmer-Weber mode [26], isolated nano-islands are initially formed that nucleate and coalesce Volmer-Weber mode [26], isolated nano-islands are initially formed that nucleate and coalesce each each other [21].spectral The spectral characteristics of nm 6 tosamples 12 nm are samples are characteristic of semiother [21]. The characteristics of 6 to 12 characteristic of semi-continuous continuousthin plasmonic thin films [18,27]. plasmonic films [18,27]. As shown in Figure 7, the the absorption absorption spectra spectra of of samples samples with with different different Au Au thickness thickness changed changed As shown in Figure 7, significantly after annealing at 500, 550, 600 and 650 °C for 1 h. Figure 8a,b summarizes the variation significantly after annealing at 500, 550, 600 and 650 ◦ C for 1 h. Figure 8a,b summarizes the variation of the the peak peak wavelength, wavelength, extinction extinction intensity width at at half half maximum maximum (FWHM) (FWHM) with with the the of intensity and and full full width annealing temperature and thin film thickness. At 500 °C the thinner samples (2 to 6 nm) already ◦ annealing temperature and thin film thickness. At 500 C the thinner samples (2 to 6 nm) already show show well-defined plasmonic while the extinction value increases for thicker thinAt films. well-defined plasmonic bands,bands, while the extinction value increases for thicker Au thinAu films. 550, At 550, 600 and 650 °C the plasmonic band is now present in the 8 nm thick samples. The thicker 10 ◦ 600 and 650 C the plasmonic band is now present in the 8 nm thick samples. The thicker 10 and 12 nm and 12 nm samples retain the initial features of a continuous thin film, not presenting plasmonic samples retain the initial features of a continuous thin film, not presenting plasmonic features at 550 features 600 ◦°C, at 650 °C is a broad band is now observed, formation of and 600 ◦atC,550 butand at 650 C abut broad band now observed, indicating the indicating formation the of nano-islands. nano-islands. For 2 nm of Au the resonant band has a peak at 582 nm, and increasing the Au For 2 nm of Au the resonant band has a peak at 582 nm, and increasing the Au thickness to 4 thickness and 6 nm to 4 and 6 nm led to a red shift, to 593 and 598 nm, respectively. The redshift and the increasing of led to a red shift, to 593 and 598 nm, respectively. The redshift and the increasing of the extinction the extinction value for thicker AuAsfilms is patent. As8a,b, shown in temperature Figure 8a,b,increases, as the temperature value for thicker Au films is patent. shown in Figure as the for a given increases, for a given film thickness, the absorption band moves to shorter wavelengths and becomes film thickness, the absorption band moves to shorter wavelengths and becomes narrower. Within the narrower. Within the experimental error, the extinction intensity is not affected by the annealing experimental error, the extinction intensity is not affected by the annealing temperature, but only by temperature, but only by the film thickness. the film thickness.

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Figure 6. 6. Absorption Absorption spectra before annealing of planar samples with 2, and 12 nm gold thin film Figure spectra before before annealing annealing of of planar planar samples samples with with 2, 2, 666 and and 12 12 nm nm gold gold thin thin film film Figure 6. Absorption spectra deposited on substrates with 2 nm of chromium (adhesion layer) and of a sample with 2 nm deposited on 2 nm of chromium (adhesion layer) layer) and of and a sample 2 nmwith chromium deposited onsubstrates substrateswith with 2 nm of chromium (adhesion of a with sample 2 nm chromium thin film. thin film. thin film. chromium

Figure 7. Absorption spectra after annealing at 500, 550, 600 and 650 ◦°C for 1 h of planar samples with Figure Figure 7. 7. Absorption Absorption spectra spectra after after annealing annealing at at 500, 500, 550, 550, 600 600 and and 650 650 °C C for for 11 hh of of planar planar samples samples with with 2, 4, 6, 8, 10 and 12 nm gold thin film deposited on substrates with 2 nm of chromium (adhesion layer) 2, 4, 4, 6, 6, 8, 8, 10 10 and and 12 12nm nmgold goldthin thinfilm filmdeposited depositedon onsubstrates substrates with with22nm nmof ofchromium chromium(adhesion (adhesion layer) layer) and of a sample with 2 nm chromium thin film. and of a sample with 2 nm chromium thin film.

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Figure 8. Evolution of the plasmonic band as a function of the annealing temperature for 2, 4, 6 and 8 nm Figure 8. Evolution of the plasmonic band as a function of the annealing temperature for 2, 4, 6 and gold thin film: (a) peak wavelength and (b) full width at half maximum and peak intensity. 8 nm gold thin film: (a) peak wavelength and (b) full width at half maximum and peak intensity.

3.3. Repeated Thermal Dewetting of Au Thin Film in Planar Samples 3.3. Repeated Thermal Dewetting of Au Thin Film in Planar Samples The optical properties of metallic thin films depend on the dielectric constant of the surrounding The optical properties of metallic thin films depend the dielectric constant of the medium (εm). For nano-islands structured thin films, theon effective dielectric constant is surrounding a function of medium (ε ). For nano-islands structured thin films, the effective dielectric constant is a function m the volume fraction, F, (number of nano-islands times the volume of each nano-island) and the of the volume fraction, F, (number of nano-islands times the volume of each nano-island) the polarization of the nano-islands which is proportional to the F value [28]. Based on the free and electron polarization of the nano-islands which is proportional toof the F value Based the free electron model for metallic structures, the maximum absorption the LSPR [28]. (λm) and theon wavelength of the model for metallic structures, the maximum absorption of the LSPR (λ ) and the wavelength the m plasmon peak of bulk metal, λp (for Au, λp = 131 nm) is related with F and with the refractive of index plasmon peak of bulk metal, λp (for Au, λp = 131 nm) is related with F and with the refractive index of of the surrounding medium(ns) by Equation (1) [29]: the surrounding medium(ns ) by Equation (1) [29]: 1/2

  2 F   (1) m   p 1  2 + F  n2s21/2 λm = λ p 1 +  1  F ns  (1) 1−F For a given ns value, the maximum absorption increases with F and the LSPR peak shifts to For a given ns value, the maximum absorption increases with F and the LSPR peak shifts to higher higher wavelengths when ns increases. wavelengths when ns increases. A method for obtaining Au nano-islands with high density of nano-gaps for highly sensitive A method for obtaining Au nano-islands with high density of nano-gaps for highly sensitive surface-enhanced Raman scattering (SERS) was reported [21]. The method consists of repeating the surface-enhanced Raman scattering (SERS) was reported [21]. The method consists of repeating thermal dewetting process of Au thin films on the same substrate, providing enlarged gold nanothe thermal dewetting process of Au thin films on the same substrate, providing enlarged gold islands with small gap spacing. The result is an increase of the electromagnetic hotspots leading to nano-islands with small gap spacing. The result is an increase of the electromagnetic hotspots leading an increase of the extinction intensity as well as the tunability for plasmon resonance wavelength. to an increase of the extinction intensity as well as the tunability for plasmon resonance wavelength. Repeated dewetting was accomplished in first place on a set of planar silica substrates by Repeated dewetting was accomplished in first place on a set of planar silica substrates by alternating Au thin film evaporation and thermal annealing, as reported in [21]. The first step consists alternating Au thin film evaporation and thermal annealing, as reported in [21]. The first step consists in the thermal evaporation of Au thin film on silica substrates coated with 2 nm Cr followed by in the thermal evaporation of Au thin film on silica substrates coated with 2 nm Cr followed by thermal thermal annealing to obtain Au nano-islands. In the second step, a second Au thin film was deposited annealing to obtain Au nano-islands. In the second step, a second Au thin film was deposited on on the substrate with Au nano-islands obtained in the previous step and then annealed, following the substrate with Au nano-islands obtained in the previous step and then annealed, following the the same conditions. same conditions. Gold thin films with 6 nm thickness were deposited and annealed at 600 °C and the process Gold thin films with 6 nm thickness were deposited and annealed at 600 ◦ C and the process (deposition and annealing) repeated. Simultaneously, for comparison, a set of samples was prepared (deposition and annealing) repeated. Simultaneously, for comparison, a set of samples was prepared by single dewetting, using 6 and 12 nm thick Au films. by single dewetting, using 6 and 12 nm thick Au films. Figure 9 shows SEM images of Au nano-islands under different dewetting conditions: (a) single Figure 9 shows SEM images of Au nano-islands under different dewetting conditions: (a) single step and (b) repeated dewetting. The images show apparently engorged Au nano-islands and smaller step and (b) repeated dewetting. The images show apparently engorged Au nano-islands and smaller spacing after the repeated dewetting, compared to the single-step sample. The distribution of particle spacing after the repeated dewetting, compared to the single-step sample. The distribution of particle size for single or repeated dewetting, calculated with the software ImageJ software, shows that using size for single or repeated dewetting, calculated with the software ImageJ software, shows that using the single dewetting process ~66% of the particles have sizes between 26 and 46 nm, while for the the single dewetting process ~66% of the particles have sizes between 26 and 46 nm, while for the repeated dewetting the same percentage is fitted between 46 and 93 nm as shown in Figure 9. repeated dewetting the same percentage is fitted between 46 and 93 nm as shown in Figure 9. The repeated dewetting process exhibits higher packing density of nano-islands with an Area Fraction of 18.5%, due to the enlarged effective diameter, compared to the single-step dewetting


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which possesses Sensors 2018, 18, 1267an Area Fraction of 11.3%. A one-tailed t-test was performed and a p-value of 0.036 9 of 13 was achieved, meaning that the difference is statistically significant.

Figure Figure 9. 9. Scanning Scanning Electron Electron Microscope Microscope photographs photographs of of gold gold nano-islands nano-islands under under different different dewetting dewetting conditions of 6 nm thick gold film: (a) single step and (b) repeated dewetting. Plots (c,d), conditions of 6 nm thick gold film: (a) single step and (b) repeated dewetting. Plots (c,d),are arecalculated calculated showing showing distribution distribution of of particle particle size, size, for for samples samples shown shown in in (a,b), (a,b), respectively. respectively.

The nano-structured thin film developed initially, by forming isolate nano-islands, nucleate and The repeated dewetting process exhibits higher packing density of nano-islands with an Area merge during the first step, and according to [21], during the second dewetting, the nano-islands Fraction of 18.5%, due to the enlarged effective diameter, compared to the single-step dewetting which suffer rapid coalescence, rather than nucleation. Consequently, the second dewetting gives rise to an possesses an Area Fraction of 11.3%. A one-tailed t-test was performed and a p-value of 0.036 was increase of the size of the nano-islands formed in the first step and a reduction of the gap between achieved, meaning that the difference is statistically significant. nano-islands takes place. The nano-structured thin film developed initially, by forming isolate nano-islands, nucleate and For an Au film 6 nm thick, the extinction spectra shown in Figure 10, for single step and repeated merge during the first step, and according to [21], during the second dewetting, the nano-islands suffer dewetting, reveal that larger nano-islands with small gap augments the absorption and led to a shift rapid coalescence, rather than nucleation. Consequently, the second dewetting gives rise to an increase of the band to higher wavelength (~12 nm). These results are compatible with theoretical and of the size of the nano-islands formed in the first step and a reduction of the gap between nano-islands experimental published studies on the optimization of refractive index sensitivity of plasmonic takes place. coupled Au nanoparticles [30]. An increase of the size of the nano-islands cause a red shift of the For an Au film 6 nm thick, the extinction spectra shown in Figure 10, for single step and extinction band. In addition, the peak of the plasmon resonance depends not only on the size but also repeated dewetting, reveal that larger nano-islands with small gap augments the absorption and on the distance between the nano-particles [27]. When nano-islands are brought closer to each other, led to a shift of the band to higher wavelength (~12 nm). These results are compatible with theoretical their oscillations interact resulting in optical coupling, which results in red-shifted resonance modes. and experimental published studies on the optimization of refractive index sensitivity of plasmonic The absorption spectrum of a sample prepared by single step deposition and annealing of an Au coupled Au nanoparticles [30]. An increase of the size of the nano-islands cause a red shift of the film 12 nm thick is also shown in Figure 10, where it can be observed that the spectral features are extinction band. In addition, the peak of the plasmon resonance depends not only on the size but also characteristic of semi-continuous plasmonic thin films. Therefore, the repeated dewetting method on the distance between the nano-particles [27]. When nano-islands are brought closer to each other, plays an effective role in enhancing the extinction band. their oscillations interact resulting in optical coupling, which results in red-shifted resonance modes. The absorption spectrum of a sample prepared by single step deposition and annealing of an Au film 12 nm thick is also shown in Figure 10, where it can be observed that the spectral features are

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characteristic of semi-continuous plasmonic thin films. Therefore, the repeated dewetting method 10 the extinction band. 10of of13 13

Sensors 2018, 18, PEER plays effective inREVIEW enhancing Sensorsan 2018, 18,xxFOR FORrole PEER REVIEW

Figure 10. Absorption spectra of in samples prepared by single step and Figure 10. 10.Absorption Absorptionspectra spectra of nanostructures nanostructures in planar planar samples prepared bystep single and Figure of nanostructures in planar samples prepared by single and step repeated repeated dewetting of 6 nm thick gold film and of a sample prepared by single step dewetting of an repeated dewetting of 6 nm thick gold film and of a sample prepared by single step dewetting of an dewetting of 6 nm thick gold film and of a sample prepared by single step dewetting of an Au film Au film 12 nm thick film. Au film 12 nm thick film. 12 nm thick film.

3.4. 3.4. Optical Optical Characterization Characterization of of Fiber Fiber Sensors Sensors 3.4. Optical Characterization of Fiber Sensors The The edge edge surface surface of of the the core core of of aaa set setof of multimode multimode silica silica optical optical fibers fibers was was coated coated with with 666 and and 888 nm nm The edge surface of the core of set of multimode silica optical fibers was coated with and nm thick of Au film and annealed at 600 °C for 1 h. A sub set of the 6 nm coated fibers was re-coated after ◦ thick of of Au Au film film and and annealed annealed at at 600 600 °C for 11 h. h. A was re-coated re-coated after after thick C for A sub sub set set of of the the 66 nm nm coated coated fibers fibers was annealing with another 66 nm thick Au film and annealed again at 600 °C for 11 h. ◦ annealing with another nm thick Au film and annealed again at 600 °C for h. annealing with another 6 nm thick Au film and annealed again at 600 C for 1 h. Using the set up illustrated Figure 1b, the absorption spectra of fibers were measured Using the theset setup upillustrated illustratedinin inFigure Figure1b, 1b, the absorption spectra of these these fibers were measured Using the absorption spectra of these fibers were measured for for refractive index values of the surrounding medium from 1.30 up to 1.64, as shown in Figure 11 a,b for for refractive index values of the surrounding medium from 1.30 up to 1.64, as shown in Figure 11 a,b for refractive index values of the surrounding medium from 1.30 up to 1.64, as shown in Figure 11a,b for the single step and for the repeated dewetting fibers, respectively. the single step and for the repeated dewetting fibers, respectively. the single step and for the repeated dewetting fibers, respectively.

Figure Figure11. Evolutionof absorptionspectra spectrafrom from400 400to to1000 1000nm functionof therefractive refractiveindex index Figure 11. Evolution of the the absorption absorption spectra from 400 to 1000 nm as as aafunction ofthe index of of the surrounding medium from 1.30 to 1.64 of multimode silica optical fibers coated with nm of of the thesurrounding surrounding medium medium from from 1.30 1.30 to to1.64 1.64 of of multimode multimodesilica silicaoptical optical fibers fibers coated coated with with aaa 666 nm nm of of ◦ gold and fabricated at 600 °C for 1 h by (a) single step and (b) repeated dewetting. gold C for gold and and fabricated fabricated at at 600 600 °C for 11 hh by by (a) (a) single single step step and and (b) (b) repeated repeated dewetting. dewetting.

The The calculated calculated normalized normalized wavelength wavelength shift shift as as aaa function function of of the the surrounding surrounding refractive refractive index, index, The calculated normalized wavelength shift as function of the surrounding refractive index, from 1.30 to 1.64 is presented in Figure 12. The sensors presented here are meant to be used to monitor from 1.30 to 1.64 is presented in Figure 12. The sensors presented here are meant to be used to monitor from 1.30 to 1.64 is presented in Figure 12. The sensors presented here are meant to be used to liquid liquid mixtures mixtures in in the the range range 1.33 1.33 to to 1.40; 1.40; therefore, therefore, normalization normalization of of the the wavelength wavelength shift shift was was performed using, for each fiber, the peak wavelength at 1.30. The sensitivity to the surrounding performed using, for each fiber, the peak wavelength at 1.30. The sensitivity to the surrounding refractive refractiveindex indexwas wascalculated calculatedfrom fromFigure Figure12 12and andfor forthe the66and and88nm nmsingle singlestep stepdewetting dewettingthe thevalues values of 59.8 and 81.2 nm/RIU was achieved, respectively. Using the repeated dewetting process with of 59.8 and 81.2 nm/RIU was achieved, respectively. Using the repeated dewetting process with66 nm nm

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monitor liquid mixtures in the range 1.33 to 1.40; therefore, normalization of the wavelength shift was performed using, for each fiber, the peak wavelength at 1.30. The sensitivity to the surrounding refractive index was calculated from Figure 12 and for the 6 and 8 nm single step dewetting the 11 values Sensors 2018, 18, x FOR PEER REVIEW of 13 of 59.8 and 81.2 nm/RIU was achieved, respectively. Using the repeated dewetting process with 6 nm thick Au Aufilm filmaahigher highersensitivity sensitivityof of185.7 185.7nm/RIU nm/RIU was reached the same refractive index range, thick was reached forfor the same refractive index range, a a 3-fold improvement when compared the single process. 3-fold improvement when compared toto the single process.

Figure Figure 12. 12. Calculated Calculated normalized normalized wavelength wavelengthshift shiftas as aa function function of of the the surrounding surrounding refractive refractive index index from 1.30 to 1.64 of multimode silica optical fibers coated with a 6 and 8 nm of gold and from 1.30 to 1.64 of multimode silica optical fibers coated with a 6 and 8 nm of gold and fabricated fabricated at at 600 ◦ Cfor 600 °C for11hhby bysingle single step step (6 (6 and and 88 nm) nm) and and repeated repeated dewetting dewetting (6 (6 nm). nm).

The refractive index sensitivity changes with the size and shape of the nano-structures, but also The refractive index sensitivity changes with the size and shape of the nano-structures, but also with their interaction. The higher sensitivity for the 8 nm thick Au film compared with 6 nm thick with their interaction. The higher sensitivity for the 8 nm thick Au film compared with 6 nm thick may be explained by the larger particle size of the former. Regarding the repeated dewetting samples, may be explained by the larger particle size of the former. Regarding the repeated dewetting samples, the increase in sensitivity may be attributed to the higher particle size but also to the decrease in gap the increase in sensitivity may be attributed to the higher particle size but also to the decrease in spacing. gap spacing. 4. Conclusions 4. Conclusions This work reports the application of a simple fabrication method for obtaining nano-gap rich This work reports the application of a simple fabrication method for obtaining nano-gap rich gold gold nano-islands with enlarged sizes based on repeated deposition and thermal annealing of gold nano-islands with enlarged sizes based on repeated deposition and thermal annealing of gold thin thin film. These nano structures are meant to be used in the development of highly sensitive refractive film. These nano structures are meant to be used in the development of highly sensitive refractive index optical fiber sensors. A large increase in sensitivity was obtained over the single step dewetting index optical fiber sensors. A large increase in sensitivity was obtained over the single step dewetting process, even though only the core end surface of the optical fiber was coated with gold (600 μm in process, even though only the core end surface of the optical fiber was coated with gold (600 µm in diameter). diameter). It is expected that the initial and additional gold thin film thickness, the size, gap spacing, It is expected that the initial and additional gold thin film thickness, the size, gap spacing, packing packing density, and optical properties may be manipulated in order to design and fabricate specific density, and optical properties may be manipulated in order to design and fabricate specific optical optical fiber sensors. fiber sensors. Acknowledgments: byby thethe ERDF—European Regional Development Fund through the Thiswork workisisfinanced financed ERDF—European Regional Development Fund through Acknowledgments: This Operational Program for Competitiveness and Internationalization—COMPETE 2020 Program within project the Operational Program for Competitiveness and Internationalization—COMPETE 2020 Program within project POCI-01-0145-FEDER-006961, project Coral—Sustainable Ocean Exploitation: Tools and Sensors/ POCI-01-0145-FEDER-006961, project Coral—Sustainable Ocean Exploitation: Tools and Sensors/NORTE-01NORTE-01-0145-FEDER-000036. And by National Funds through the FCT—Fundação a Ciência Tecnologia 0145-FEDER-000036. And by National Funds through the FCT—Fundação parapara a Ciência e ae aTecnologia (Portuguese Foundation for Science and Technology) as part of project UID/EEA/50014/2013. The authors also (Portuguese Foundation for Science and Technology) as part of project UID/EEA/50014/2013. The authors also acknowledge the assistance and resources made available by TEC4SEA in support of this work. This infrastructure acknowledge the assistance and resources made available by TEC4SEA in support of this work. This infrastructure is financed by FEDER funds through Programa Operacional Regional do Norte—NORTE 2020 and Programa Operacional Regional do Algarve—ALGARVE21 and by National funds through FCT— Fundação para a Ciência e a Tecnologia (NORTE-01-0145-FEDER-022097). Helena Vasconcelos acknowledges financial support from FCT: SFRH/BD/120064/2016 and to the POCH program (Programa operacional capital humano).

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is financed by FEDER funds through Programa Operacional Regional do Norte—NORTE 2020 and Programa Operacional Regional do Algarve—ALGARVE21 and by National funds through FCT—Fundação para a Ciência e a Tecnologia (NORTE-01-0145-FEDER-022097). Helena Vasconcelos acknowledges financial support from FCT: SFRH/BD/120064/2016 and to the POCH program (Programa operacional capital humano). Author Contributions: José Almeida, Helena Vasconcelos and Luis Coelho did the fabrication and characterization. José Almeida, Pedro Jorge and Luis Coelho wrote the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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