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Electronic Supplementary Information (ESI)Gold Coated Cicada Wings: Anti-Reflective micro-environment for Plasmonic Enhancement of Fluorescence from Upconversion NanoparticlesAkash Gupta,a Hao-Yu Cheng,b Kung-Hsuan Lin,b Chien Ting Wu,c Pradip Kumar Roy,a Sandip Ghosh,a and Surojit Chattopadhyay *aaInstitute of Biophotonics, National Yang-Ming University, 155, sec-2 Li Nong Street, Taipei 112, Taiwan.bInstitute of Physics, Academia Sinica, No. 128, Sec. 2, Academia Rd., Nangang Dist., Taipei 11529, ano Device Materials Characterization Division, National Nano Device Laboratories, Hsinchu-300, Taiwan.Experimental details of fluorescence lifetime measurements.The lifetimes of fluorescence were measured with the experimental setup shown in Fig. S1. We operated the cavity for Ti-sapphire pulsed laser (Tsunami HP, Spectra Physics, USA) in continuous-wave mode at wavelength of 980 nm. The power was controlled with the set of half-wave-plate and polarizing beamsplitter cube. The excitation laser was focused with a focusing lens (f = 10 cm), and the fluorescence was collected with an objective lens (20x/NA0.10, Olympus). A long-wave-pass dichroic mirror was used to reflect the fluorescence to the photomultiplier tube (PMT). Bandpass filters were used to measure the red (FL-647-657 Thorlabs) and green (MF525-40 Thorlabs) fluorescence. Fig. S1 Schematic of the experimental set up for frequency-domain fluorescence lifetime measurements. M: Mirror; PMT: Photomultiplier tube. BP: Bandpass filter. The fluorescence lifetimes were obtained with frequency-domain method [1,2]. We modulated laser at 500 Hz with a mechanical chopper and recorded the phase delay () of the emitted fluorescence to the excitation source with a lock-in amplifier (SR 830 DSP, Stanford Research System). Based on the assumption that the time-resolved fluorescence follows e-t/τ upon instantaneous photoexcitation, the fluorescence lifetime τ can be recovered with the equation τ=tanω, where ω =2f and f is modulation frequency [1,2]. Fig. S2 Energy dispersive X-ray spectroscopy (left), and transmission electron diffraction (right) for crystal composition and structure determination of the (a, b) core, and (c, d) core-shell UCNPs. Elementary chemical compositions of material in (a, c), and crystal planes are marked in (b, d) for the hexagonal phase UCNPs crystal. Fig. S3 (a) Optical image of the green luminescence from core (in hexane) (L), and core-shell (in water) (R) UCNPs under 2 W, cw 980 nm excitation. (b) Up conversion emission spectra of the core (in cyclohexane), and core-shell (in water) nanoparticles with peaks at 520, 540, and 655 nm, respectively under 0.3 W, cw 980 nm laser excitation. (c) CIE colour space image indicating calculated CIE chromaticity coordinates for core (0.254, 0.592), and core-shell (0.254, 0.593) from from the UCNP fluorescence spectrum (Fig. S3b). (d) UV-vis absorption spectra of core (top), and core-shell (bottom) UCNPs showing a ~980 nm peak (dashed line). (e) Photo stability, for 1 h, of the 540 nm emission of (black, top) core, and (red, bottom) core-shell UCNPs at 0.3 W, cw 980 nm excitation. Both the core, and the core-shell UCNPs (Fig. S3a) have three characteristic emission peaks at 520, 540, and 655 nm, respectively (Fig. S3b). Compared to the core UCNPs, the luminous intensity in the core-shell UCNPs is lower. The emission peaks originate from radiative transitions from 2H11/2 to 4I15/2, for the 520 nm, from 4S3/2 to 4I15/2, for the 540 nm, and from 4F9/2 to 4I15/2, for the 655 nm peaks, respectively. The resultant emission colour of the colloidal solution could be predicted by estimating the chromaticity indices, from the fluorescence plots, following CIE 1931 (Fig. S3c). Here, we have calculated chromaticity coordinates (x, y) as 0.254, 0.592, and 0.254, 0.593 for core, and core-shell, respectively. These coordinates values are marked on the CIE colour space diagram (Fig. S3c) and indicates strong fluorescence emission in green. Fig. S3d shows the UV-vis absorption spectra of both core (in hexane, refractive index 1.446), and core-shell UCNPs (in water, refractive index 1.33), which reveal photoexcitation at 980 nm is required for the upconversion process. The absorption in the core-shell UCNPs, compared to the core UCNPs, was lower because of the silica shell, and the use of water as the dispersing medium as reported previously. This is the reason for a lower fluorescence intensity in the core-shell UCNPs. Fig. S3e shows the photo stability of the emission @540 nm of both core and core-shell UCNPs. The UCNPs, irrespective of the shell, are absolutely photostable at least up to 1 hour, under 0.3 W of cw 980 nm laser irradiation.Fig. S4 Top view SEM images indicating Au morphology on flat substrates silicon, and quartz with different Au sputtering time for silicon (a) 0 (without Au), (b) 1, (c) 3, and (d) 8 min., and for quartz (e) 0, (f) 1, (g) 3, and (h) 8 min., respectively. Top view SEM images of nanostructured Cicada wing indicating morphology, (i) before (0 min. Au), after (j) 1, (k) 3, and (l) 8 min. of Au sputtering, respectively.Fig. S5 Power dependent fluorescence spectra of the (a-c) NaYF4: Yb3+, Er3+ (core), and (d-f) NaYF4: Yb3+, Er3+@SiO2 (core-shell) UCNPs dispersed on bare (a,d) silicon, (b,e) quartz, and (c,f) Cicada wing, respectively. Up arrows represents increasing powers of 0.3, 0.4, and 0.5 W of the cw 980 nm laser excitation.Fig. S6 (a-e) TEM images of NaYF4: Yb, Er@SiO2 (core-shell) UCNPs having different silica shell thicknesses of (a) 4, (b) 7, (c) 9, (d) 14, and (e) 18 nm, respectively. (f) Up conversion emission spectra of the core-shell UCNPs on gold coated Cicada wing using 0.3 W of cw 980 nm laser excitation. Fig. S7 Power dependent fluorescence spectra of the NaYF4: Yb, Er@SiO2 (core-shell) UCNPs dispersed on (a-c) 1 min., (d-f) 3 min., and (g-i) 8 min. Au coated (a,d,g) silicon, (b,e,h) quartz, and (c,f,i) Cicada wing substrates, respectively. Up arrows represents increasing powers of 0.3, 0.4, and 0.5 W, of the cw 980 nm laser excitation. Table S1. Comparison of ‘enhancement factors (EF)’* for green (540 nm), and red (655 nm) emission in core, and core-shell type UCNPs in presence of plasmonic nanostructures.Geometry of UCNPsSize of UCNPs (nm)Plasmonic structurePlasmonic materialEF@540 nmEF@655 nmReferenceCore 65PillarsAu2.22.73Core 30WireAg2.33.74Core 30SphereAu4.82.75Core 270IslandAu556Core 32GratingAg16397Core 39Rods and spacerAu22.6~98Core 6Rods and spacerAu35149Core CoreCore701630Nano hole SandwichSandwichAuAgAu351001198341000--101112Core-shell 45ParticleAg14.412.213Core-shell 40ParticleAu9.59~114Core-shell 50SphereAg43.2615Core-shell 30ShellAu2.62.116Core-shell 90ShellAu21117Core-shell 18SandwichAu171918Core-shell35Film on AR (Cicada wing)Au30*50*This work* EFs are calculated from the ratio of the emission (green or red) band intensity of the UCNPs in presence and absence of the plasmonic/AR nanostructure. The current work involves plasmons in gold film and anti-reflection property of the Cicada wing.Fig. S8 Power dependent variation of I520, I540, and I655 of the core-shell up conversion nanoparticles from different substrates including Cicada wing, quartz, and silicon, (a-c) without Au (0 min.), and with (d-f) 1 min, (g-i) 3 min., and (j-l) 8 min Au coating. The line joining the all the data points indicates linear fit to the points, and corresponding slopes (n) for all substrates are mentioned in each spectra. Upconversion emission intensity when plotted in log scale as a function of incident power yields a straight line (IUCL= KPn, where IUCL is the intensity of upconversion luminescence, P is the incident power density, ‘n’ is the number of photons involved in the absorption process, and K is a proportionality constant) whose slope gives an estimate of the number of photons involved in the fluorescence process. For multiphoton absorption, as in UCNPs, the value of n ≥ 2. Fig. S8 shows power dependence of characteristics emission bands at 520, 540, and 655 nm (from left to right columns), respectively, for substrates without Au (0 min.) (Fig. S8a-c), and with 1 min. (Fig. S8d-f), 3 min. (Fig. S8g-i), and 8 min. (Fig. S8j-l) Au coating. The line joining the all the data points indicates a linear fit to the data points, and corresponding slopes (n values) are mentioned. The slope (n) for Au coated (1, 3, and 8 min.) substrates are always higher as compared to the ones without Au. This enhancement in slope for Au coated substrates is consistent with emission enhancement in core-shell UCNPs on Au coated substrates in Fig. 4b-d. Such increased slopes were also observed before in presence of plasmonic materials [6,18]. Fig. S9 Confocal fluorescence microscopic images of 30 ?L core-shell UCNPs@SiO2 dispersed on bare (Au = 0 min.) (a-c), and (d-f) Au sputtered (Au = 8 min.) silicon, quartz, and Cicada wing substrate, respectively. The mean intensity (Imean) of fluorescence has been depicted in each of the images. All measurements were done under 10 mW, 980 nm laser excitation. Fig. S10 Confocal fluorescence microscopic images of 30 ?L core-shell UCNPs@SiO2 dispersed on bare (Au = 0 min.) (a-c), and (d-f) Au sputtered (Au = 8 min.) silicon, quartz, and Cicada wing substrate, respectively. The mean intensity (Imean) of fluorescence has been depicted in each of the images. All measurements were done under 20 mW, 980 nm laser excitation. Fig. S11 Confocal fluorescence microscopic images of 30 ?L core-shell UCNPs@SiO2 dispersed on bare (Au = 0 min.) (a-c), and (d-f) Au sputtered (Au = 8 min.) silicon, quartz, and Cicada wing substrate, respectively. The mean intensity (Imean) of fluorescence has been depicted in each of the images. All measurements were done under 30 mW, 980 nm laser excitation. Fig. S12 Confocal fluorescence microscopic images of 30 ?L core-shell UCNPs@SiO2 dispersed on bare (Au = 0 min.) (a-c), and (d-f) Au sputtered (Au = 8 min.) silicon, quartz, and Cicada wing substrate, respectively. The mean intensity (Imean) of fluorescence has been depicted in each of the images. All measurements were done under 40 mW, 980 nm laser excitation. Fig. S13 Confocal fluorescence microscopic images of 30 ?L core-shell UCNPs@SiO2 dispersed on bare (Au = 0 min.) (a-c), and (d-f) Au sputtered (Au = 8 min.) silicon, quartz, and Cicada wing substrate, respectively. The mean intensity (Imean) of fluorescence has been depicted in each of the images. All measurements were done under 50 mW, 980 nm laser excitation. Table S2 Analysis of measured phase delay (=?2- 1) of major emission peaks (540, and 655 nm) of core and core-shell UCNPs dispersed on bare, and Au coated silicon, quartz, and Cicada wing.Photonic SubstratesCore UCNPsCore-shell UCNPs for I540 (Degree) for I655 (Degree) for I540 (Degree ) for I655 (Degree)Bare Silicon1min. Au on Silicon3 min. Au on Silicon8 min Au on Silicon8191529845-40.45±1-30.20±1-33.42±2-32.50±2-40.45±1-30.20±1-33.42±2-32.50±2-46.85±1-36.34±2-36.87±1-36.44±3-40.78±1-37.27±1-36.83±1-35.7±4-48.25±4-44.36±1-43.92±1-40.86±5Bare Quartz1 min. Au on Quartz3 min. Au on Quartz8 min. Au on Quartz-41.18±5-35.72±2-27.76±1-27.66±2-44.73±3-42.35±3-33.1±1-31.15±2-39.34±2-37.42±3-32.98±1-31.77±4-46.52±3-45.76±1-45.06±2-41.85±5Bare Cicada wing1 min. Au on Cicada wing3 min. Au on Cicada wing8 min. Au on Cicada wing-43.73±4-31.2±1-29.07±3-30.52±4-50.4±4-38.95±1-35.45±4-16.33±6-39.76±1-38.11±4-31.15±1-32.92±1-53.42±4-50.72±5-42.38±4-43.1±2 ReferencesG. I. Redford and R. M. Clegg, Polar Plot Representation for Frequency-Domain Analysis of Fluorescence Lifetimes, Journal of Fluorescence 15 (2005) 805-815R.D. Spencer, G. Weber, Measurements of Subnanosecond Fluorescence Lifetimes with A Cross-Correlation Phase Fluorometer, Annals New York Academy of Sciences 158 (1969) 361-369H.P. Paudel, L. Zhong, K. Bayat, M.F. 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