vapor sensing with a natural photonic cell€¦ · vukusic, 6. and olivier deparis. 1,* 1....

Vapor sensing with a natural photonic cell Sébastien R. Mouchet,1,2 Tijani Tabarrant,1 Stéphane Lucas,1 Bao-Lian Su,3,4,5 Pete

Vukusic,6 and Olivier Deparis1,* 1Department of Physics, Physics of Matter and Radiation (PMR), University of Namur (UNamur), Rue de Bruxelles

61, B-5000 Namur, Belgium 2Currently with College of Engineering, Mathematics and Physical Sciences, University of Exeter, Stocker Road,

Exeter EX4 4QL, United Kingdom 3Laboratory of Inorganic Materials Chemistry (CMI), University of Namur (UNamur), Rue de Bruxelles 61, B-5000

Namur, Belgium 4Department of Chemistry and Clare Hall College, University of Cambridge, Herschel Road, Cambridge CB3 9AL,

United Kingdom 5State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of

Technology, Luoshi Road 1222, 430070 Wuhan, Hubei, China 6College of Engineering, Mathematics and Physical Sciences, University of Exeter, Stocker Road, Exeter EX4 4QL,

United Kingdom *[email protected]

Abstract: Photonic structures encased by a permeable envelope give rise to iridescent blue color in the scales covering the male Hoplia coerulea beetle. This structure comprises a periodic porous multilayer. The color of these scales is known for changing from blue to green upon contact with water despite the presence of the envelope. This optical system has been referred to as a photonic cell due to the role of the envelope that mediates fluid exchanges with the surrounding environment. Following from previously studied liquid-induced changes in the color appearance of H. coerulea, we measured vapor-induced color changes in its appearance. This response to vapor exposure was marked by reflectance redshift and an increase in peak reflectance intensity. Different physico-chemical processes were investigated to explain the increase in reflectance intensity, a property not usually associated with vapor-induced optical signature changes. These simulations indicated the optical response arose from physisorption of a liquid film on the beetle scales followed by liquid penetration through the envelope and the filling of micropores within the body of the photonic structure. ©2016 Optical Society of America OCIS codes: (160.5298) Photonic crystals; (160.5293) Photonic bandgap materials; (330.1690) Color; (160.1435) Biomaterials; (280.4788) Optical sensing and sensors; (330.7310) Vision; (300.6170) Spectra.

References and links 1. S. Berthier, Iridescences, les Couleurs Physiques des Insectes (Springer, 2003). 2. P. Vukusic and J. R. Sambles, “Photonic structures in biology,” Nature 424(6950), 852–855 (2003). 3. S. Kinoshita, Structural Colors in the Realm of Nature (World Scientific Publishing Co, 2008). 4. A. L. Ingram and A. R. Parker, “A review of the diversity and evolution of photonic structures in butterflies,

incorporating the work of John Huxley (The Natural History Museum, London from 1961 to 1990),” Philos.Trans. R. Soc. Lond. B Biol. Sci. 363(1502), 2465–2480 (2008).

5. A. E. Seago, P. Brady, J.-P. Vigneron, and T. D. Schultz, “Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera),” J. R. Soc. Interface 6(Suppl 2), S165–S184 (2009).

6. D. W. Lee and J. B. Lowry, “Physical basis and ecological significance of iridescence in blue plants,” Nature 254(5495), 50–51 (1975).

7. S. Vignolini, P. J. Rudall, A. V. Rowland, A. Reed, E. Moyroud, R. B. Faden, J. J. Baumberg, B. J. Glover, andU. Steiner, “Pointillist structural color in Pollia fruit,” Proc. Natl. Acad. Sci. U.S.A. 109(39), 15712–15715(2012).

#259642 Received 18 Feb 2016; revised 14 Apr 2016; accepted 17 Apr 2016; published 26 May 2016 © 2016 OSA 30 May 2016 | Vol. 24, No. 11 | DOI:10.1364/OE.24.012267 | OPTICS EXPRESS 12267

8. L. P. Biró and J.-P. Vigneron, “Photonic nanoarchitectures in butterflies and beetles: valuable sources forbioinspiration,” Laser Photonics Rev. 5(1), 27–51 (2011).

9. J. H. Kim, J. H. Moon, S.-Y. Lee, and J. Park, “Biologically inspired humidity sensor based on three-dimensional photonic crystals,” Appl. Phys. Lett. 97(10), 103701 (2010).

10. Z. Wang, J. Zhang, J. Xie, C. Li, Y. Li, S. Liang, Z. Tian, T. Wang, H. Zhang, H. Li, W. Xu, and B. Yang,“Bioinspired water-vapor-responsive organic/inorganic hybrid one-dimensional photonic crystals with tunable full-color stop band,” Adv. Funct. Mater. 20(21), 3784–3790 (2010).

11. S. Mouchet, J.-F. Colomer, C. Vandenbem, O. Deparis, and J.-P. Vigneron, “Method for modeling additive coloreffect in photonic polycrystals with form anisotropic elements: the case of Entimus imperialis weevil,” Opt.Express 21(11), 13228–13240 (2013).

12. A. Yoshida, “Antireflection of the butterfly and moth wings through microstructure,” Forma 17, 75–89 (2002). 13. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of

butterflies,” Proc. R. Soc. Lond., B 273(1587), 661–667 (2006).14. O. Deparis, N. Khuzayim, A. Parker, and J.-P. Vigneron, “Assessment of the antireflection property of moth

wings by three-dimensional transfer-matrix optical simulations,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys.79(4), 041910 (2009).

15. L. Dellieu, M. Sarrazin, P. Simonis, O. Deparis, and J.-P. Vigneron, “A two-in-one superhydrophobic and anti-reflective nanodevice in the grey cicada Cicada orni (Hemiptera),” J. Appl. Phys. 116(2), 024701 (2014).

16. O. Deparis, S. R. Mouchet, L. Dellieu, J.-F. Colomer, and M. Sarrazin, “Nanostructured surfaces: bioinspirationfor transparency, coloration and wettability,” Mater. Today Proc. 1S, 122–129 (2014).

17. S. A. Jewell, P. Vukusic, and N. W. Roberts, “Circularly polarized colour reflection from helicoidal structures inthe beetle Plusiotis boucardi,” New J. Phys. 9(4), 99 (2007).

18. V. Sharma, M. Crne, J. O. Park, and M. Srinivasarao, “Structural origin of circularly polarized iridescence injeweled beetles,” Science 325(5939), 449–451 (2009).

19. H. Arwin, L. Fernández del Río, and K. Järrendahl, “Comparison and analysis of Mueller-matrix spectra fromexoskeletons of blue, green and red Cetonia aurata,” Thin Solid Films 571(3), 739–743 (2014).

20. L. T. McDonald, E. D. Finlayson, and P. Vukusic, “Untwisting the polarization properties of light reflected by scarab beetles,” Proc. SPIE 9341, 93410K (2015).

21. K. Kumazawa, S. Tanaka, K. Negita, and H. Tabata, “Fluorescence from wing of Morpho sulkowskyi butterfly,”Jpn. J. Appl. Phys. 33(1), 2119–2122 (1994).

22. P. Vukusic and I. Hooper, “Directionally controlled fluorescence emission in butterflies,” Science 310(5751), 1151 (2005).

23. E. Van Hooijdonk, C. Barthou, J.-P. Vigneron, and S. Berthier, “Detailed experimental analysis of the structural fluorescence in the butterfly Morpho sulkowskyi (Nymphalidae),” J. Nanophotonics 5(1), 053525 (2011).

24. E. Van Hooijdonk, S. Berthier, and J.-P. Vigneron, “Bio-inspired approach of the fluorescence emissionproperties in the scarabaeid beetle Hoplia coerulea (Coleoptera): modeling by transfer-matrix opticalsimulations,” J. Appl. Phys. 112(11), 114702 (2012).

25. E. Van Hooijdonk, C. Vandenbem, S. Berthier, and J.-P. Vigneron, “Bi-functional photonic structure in the Papilio nireus (Papilionidae): modeling by scattering-matrix optical simulations,” Opt. Express 20(20), 22001–22011 (2012).

26. V. L. Welch, E. Van Hooijdonk, N. Intrater, and J.-P. Vigneron, “Fluorescence in insects,” Proc. SPIE 8480,848004 (2012).

27. J.-P. Vigneron, J. M. Pasteels, D. M. Windsor, Z. Vértesy, M. Rassart, T. Seldrum, J. Dumont, O. Deparis, V.Lousse, L. P. Biró, D. Ertz, and V. Welch, “Switchable reflector in the Panamanian tortoise beetle Charidotella egregia (Chrysomelidae: Cassidinae),” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 76(3), 031907 (2007).

28. M. Rassart, J.-F. Colomer, T. Tabarrant, and J.-P. Vigneron, “Diffractive hygrochromic effect in the cuticle ofthe hercules beetle Dynastes hercules,” New J. Phys. 10(3), 033014 (2008).

29. F. Liu, B. Q. Dong, X. H. Liu, Y. M. Zheng, and J. Zi, “Structural color change in longhorn beetles Tmesisternus isabellae,” Opt. Express 17(18), 16183–16191 (2009).

30. M. Rassart, P. Simonis, A. Bay, O. Deparis, and J.-P. Vigneron, “Scale coloration change following waterabsorption in the beetle Hoplia coerulea (Coleoptera),” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 80(3),031910 (2009).

31. R. A. Potyrailo, H. Ghiradella, A. Vertiatchikh, K. Dovidenko, J. R. Cournoyer, and E. Olson, “Morpho butterfly wing scales demonstrate highly selective vapour response,” Nat. Photonics 1(2), 123–128 (2007).

32. R. A. Potyrailo, T. A. Starkey, P. Vukusic, H. Ghiradella, M. Vasudev, T. Bunning, R. R. Naik, Z. Tang, M.Larsen, T. Deng, S. Zhong, M. Palacios, J. C. Grande, G. Zorn, G. Goddard, and S. Zalubovsky, “Discovery ofthe surface polarity gradient on iridescent Morpho butterfly scales reveals a mechanism of their selective vaporresponse,” Proc. Natl. Acad. Sci. U.S.A. 110(39), 15567–15572 (2013).

33. L. P. Biró, K. Kertész, Z. Vértesy, and Z. Bálint, “Photonic nanoarchitectures occurring in butterfly scales asselective gas/vapor sensors,” Proc. SPIE 7057, 705706 (2008).

34. M. D. Shawkey, L. D’Alba, J. Wozny, C. Eliason, J. A. H. Koop, and L. Jia, “Structural color change following hydration and dehydration of iridescent mourning dove (Zenaida macroura) feathers,” Zoology (Jena) 114(2),59–68 (2011).


35. W. Wang, W. Zhang, X. Fang, Y. Huang, Q. Liu, J. Gu, and D. Zhang, “Demonstration of higher colourresponse with ambient refractive index in Papilio blumei as compared to Morpho rhetenor,” Sci. Rep. 4, 5591 (2014).

36. C. W. Mason, “Structural Colors in Feathers. I,” J. Phys. Chem. 27(3), 201–251 (1922). 37. C. W. Mason, “Structural Colors in Insects. I,” J. Phys. Chem. 30(3), 383–395 (1925). 38. C. W. Mason, “Structural Colors in Insects. II,” J. Phys. Chem. 31(3), 321–354 (1926). 39. C. W. Mason, “Structural Colors in Insects. III,” J. Phys. Chem. 31(12), 1856–1872 (1926). 40. C. M. Eliason and M. D. Shawkey, “Rapid, reversible response of iridescent feather color to ambient humidity,”

Opt. Express 18(20), 21284–21292 (2010).41. Y. Gao, Q. Xia, G. Liao, and T. Shi, “Sensitivity analysis of a bioinspired refractive index based gas sensor,” J.

Bionics Eng. 8(3), 323–334 (2011).42. X. Yang, Z. Peng, H. Zuo, T. Shi, and G. Liao, “Using hierarchy architecture of Morpho butterfly scales for

chemical sensing: experiment and modeling,” Sensors Actuat. A 167(2), 367–373 (2011).43. S. R. Mouchet, O. Deparis, and J.-P. Vigneron, “Unexplained high sensitivity of the reflectance of porous natural

photonic structures to the presence of gases and vapours in the atmosphere,” Proc. SPIE 8424, 842425 (2012).44. K. Kertész, G. Piszter, E. Jakab, Zs. Bálint, Z. Vértesy, and L. P. Biró, “Color change of Blue butterfly wing

scales in an air – Vapor ambient,” Appl. Surf. Sci. 281, 49–53 (2013).45. S. R. Mouchet, B.-L. Su, T. Tabarrant, S. Lucas, and O. Deparis, “Hoplia coerulea, a porous natural photonic

structure as template of optical vapour sensor,” Proc. SPIE 9127, 91270U (2014).46. T. Jiang, Z. Peng, W. Wu, T. Shi, and G. Liao, “Gas sensing using hierarchical micro/nanostructures of Morpho

buttery scales,” Sensors Actuat. A 213, 63–69 (2014).47. Y. Y. Diao, X. Y. Liu, G. W. Toh, L. Shi, and J. Zi, “Multiple structural coloring of silk-fibroin photonic crystals

and humidity-responsive color sensing,” Adv. Funct. Mater. 23(43), 5373–5380 (2013).48. M. N. Ghazzal, O. Deparis, J. De Coninck, and E. M. Gaigneaux, “Tailored refractive index of inorganic

mesoporous mixed-oxide Bragg stacks with bio-inspired hygrochromic optical properties,” J. Mater. Chem. C Mater. Opt. Electron. Devices 1(39), 6202–6209 (2013).

49. L. Bai, Z. Xie, W. Wang, C. Yuan, Y. Zhao, Z. Mu, Q. Zhong, and Z. Gu, “Bio-inspired vapor-responsive colloidal photonic crystal patterns by inkjet printing,” ACS Nano 8(11), 11094–11100 (2014).

50. O. Deparis, M. N. Ghazzal, P. Simonis, S. R. Mouchet, H. Kebaili, J. De Coninck, E. M. Gaigneaux, and J.-P. Vigneron, “Theoretical condition for transparency in mesoporous layered optical media: application to switching of hygrochromic coatings,” Appl. Phys. Lett. 104(2), 023704 (2014).

51. K. Kertész, G. Piszter, Z. Baji, E. Jakab, Z. Bálint, Z. Vértesy, and L. P. Biró, “Vapor sensing on bare andmodified blue butterfly wing scales,” Chem. Senses 4, 17 (2014).

52. G. Piszter, K. Kertész, Z. Vértesy, Z. Bálint, and L. P. Biró, “Substance specific chemical sensing with pristine and modified photonic nanoarchitectures occurring in blue butterfly wing scales,” Opt. Express 22(19), 22649–22660 (2014).

53. R. A. Potyrailo, R. K. Bonam, J. G. Hartley, T. A. Starkey, P. Vukusic, M. Vasudev, T. Bunning, R. R. Naik, Z.Tang, M. A. Palacios, M. Larsen, L. A. Le Tarte, J. C. Grande, S. Zhong, and T. Deng, “Towards outperforming conventional sensor arrays with fabricated individual photonic vapour sensors inspired by Morpho butterflies,”Nat. Commun. 6, 7959 (2015).

54. S. R. Mouchet, E. Van Hooijdonk, V. L. Welch, P. Louette, J.-F. Colomer, B.-L. Su, and O. Deparis, “Liquid-induced colour change in a beetle: the concept of a photonic cell,” Sci. Rep. 6, 19322 (2016).

55. J.-P. Vigneron, J.-F. Colomer, N. Vigneron, and V. Lousse, “Natural layer-by-layer photonic structure in the squamae of Hoplia coerulea (Coleoptera),” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 72(6), 061904 (2005).

56. O. Deparis, M. Rassart, C. Vandenbem, V. L. Welch, J.-P. Vigneron, and S. Lucas, “Structurally tuned iridescent surfaces inspired by nature,” New J. Phys. 10(1), 013032 (2008).

57. I. Tamáska, K. Kértész, Z. Vértesy, Z. Bálint, A. Kun, S.-H. Yen, and L. P. Biró, “Color changes upon cooling ofLepidoptera scales containing photonic nanoarchitectures, and a method for identifying the changes,” J. InsectSci. 13(87), 87 (2013).

58. K. Kertész, G. Piszter, E. Jakab, Z. Bálint, Z. Vértesy, and L. P. Biró, “Temperature and saturation dependence in the vapor sensing of butterfly wing scales,” Mater. Sci. Eng. C 39, 221–226 (2014).

59. P. Yeh, Optical Waves in Layered Media (Wiley-Interscience, 2005). 60. B. Edlén, “The Refractive Index of Air,” Metrologia 2(2), 71–80 (1966). 61. K. P. Birch and M. J. Downs, “An updated Edlén equation for the refractive index of air,” Metrologia 30(3),

155–162 (1993). 62. K. P. Birch and M. J. Downs, “Correction to the updated Edlén equation for the refractive index of air,”

Metrologia 31(4), 315–316 (1994).63. G. M. Hale and M. R. Querry, “Optical constants of water in the 200-nm to 200-µm wavelength region,” Appl.

Opt. 12(3), 555–563 (1973).64. J. Rheims, J. Köser, and T. Wriedt, “Refractive-index measurements in the near-IR using an Abbe

refractometer,” Meas. Sci. Technol. 8(6), 601–605 (1997).65. I. B. J. Sollas, “On the identification of chitin by its physical constants,” Proc. R. Soc. Lond., B 79(534), 474–

481 (1907). 66. Y. Saito, T. Okano, F. Gaill, H. Chanzy, and J.-L. Putaux, “Structural data on the intra-crystalline swelling of β-

chitin,” Int. J. Biol. Macromol. 28(1), 81–88 (2000).


67. I. Agnarsson, A. Dhinojwala, V. Sahni, and T. A. Blackledge, “Spider silk as a novel high performance biomimetic muscle driven by humidity,” J. Exp. Biol. 212(13), 1990–1994 (2009).

68. R. St-Gelais, G. Mackey, J. Saunders, J. Zhou, A. Leblanc-Hotte, A. Poulin, J. A. Barnes, H.-P. Loock, R. S. Brown, and Y.-A. Peter, “Gas sensing using polymerfunctionalized deformable Fabry-Perot interferometers,”Sensors Actuat. B 182, 45–52 (2013).

69. Y. Saito, J.-L. Putaux, T. Okano, F. Gaill, and H. Chanzy, “Structural aspects of the swelling of β chitin in HCl and its conversion into α chitin,” Macromolecules 30(13), 3867–3873 (1997).

70. M. Peesan, R. Rujiravanit, and P. Supaphol, “Characterisation of beta-chitin/poly(vinyl alcohol) blend films,”Polym. Test. 22(4), 381–387 (2003).

71. O. Deparis, C. Vandenbem, M. Rassart, V. Welch, and J.-P. Vigneron, “Color-selecting reflectors inspired frombiological periodic multilayer structures,” Opt. Express 14(8), 3547–3555 (2006).

1. Introduction

Natural photonic structures found in animals, mainly insects [1–5], or plants [6,7] have been optimized by millions of years of evolution and, for this reason, have inspired scientists with novel concepts and devices based on biomimetic approaches [8–10]. The interaction of light with these often macroporous structures, comprising voids (air or liquids) and biopolymers (chitin, keratin or cellulose) which exhibit various levels of spatial ordering, is responsible for a broad range of optical properties. Structural colors, namely colors that arise from coherent scattering, give rise to fascinating visual effects such as iridescence or color mixing [1,7,11]. Other aspects such as antireflection [12–16], polarization effects [17–20] or controlled fluorescence [21–26] have also been widely studied.

Natural structural colors have also attracted significant interest around the subject of sensing [27–33]. For instance, some insect cuticles or bird feathers are known to change color reversibly when they are in contact with liquids [27-30,34,35], a property that was already investigated more than 90 years ago by Mason [36–39] and is most often explained by the filling of mesopores (i.e. pore diameter ranging from 2 nm to 50 nm) and/or macropores (i.e. pore diameter larger than 50 nm) by liquids. Some butterfly wings are also sensitive to modifications of the local vapor environment. These changes are transduced in gas/vapor selective color changes. In 2007, Potyrailo et al. observed selective color changes in the wings of Morpho sulkowskyi butterflies which were induced by the vapor of water, methanol, ethanol and isomers of dichloroethylene [31]. The selectivity was later explained by a polarity gradient along the open Christmas-tree-like photonic structure of this butterfly species [32]. Following this discovery, gas- and vapor-induced color changes were investigated in other insect species with different photonic structures [33,40–46]. By taking inspiration from such natural photonic porous structures, optical gas sensors and other smart glass coatings were recently developed [9,10,47–53]. However, the dynamics of color changes induced by vapor or gases in natural photonic structures is far from fully understood. Most often, the photonic structures are open to the surrounding environment (e.g. Morpho wings). This feature enables the photonic structure to interact directly with fluids. These can then easily penetrate into pores and thereby modify the material through swelling or physisorption of vapor on the inner pore surface. It can be followed by capillary condensation at higher partial vapor pressures [43]. These effects most likely act in parallel in order to modify the optical reflectance response of the structure, hence its color. It is noteworthy that any biological function that might be related to fluid-induced color changes is so far still unknown.

In this work, vapor sensing associated with a natural photonic cell is reported. The concept of such a natural photonic cell emerged from our recent analysis of the photonic structure found in the male Hoplia coerulea beetle from the family Scarabaeidae, also known as cerulean chafer beetle, more specifically of its intriguing optical response to exposure to various liquids [54]. The blue-violet iridescent color of the beetle originates from a porous multilayer structure enclosed by a 100 nm-thick encasing envelope within the scales covering its elytra and thorax [1,38,55] [Figs. 1(a)-1(d)]. This multilayer structure has previously inspired the design of periodic TiO2/SiO2 iridescent coatings, which were deposited on glass plates by magnetron sputtering [56]. Remarkably, in spite of the fact that this structure is not


open to the surrounding environment, its color turns to green on contact with liquids [16,30,45,54]. This liquid-induced color change was shown to be mediated by the encasing envelope through which the physico-chemical properties and chemical composition foster the penetration of liquids [54] [Fig. 1(e)]. This interesting photonic structure was termed a “photonic cell” due to similarities it exhibits with certain biological cells in which plasma membranes control exchanges with the surrounding environment.

In this article, changes of reflectance from the elytra of H. coerulea beetle were measured after exposure to water and ethanol vapor in a controlled atmosphere. Increase of the reflectance peak intensity was observed, which could not be attributed to a combination of swelling of the structure, physisorption on the macropore surface and capillary condensation. In order to explain this unusual response, different physico-chemical processes were investigated and modeled in optical simulations of the photonic structure reflectance.

Fig. 1. Photonic cell of the male H. coerulea beetle. (a) Its blue-violet iridescent color is due to a photonic structure located inside the scales (b) covering its elytra. Enclosed by a 100 nm-thick encasing envelope, the photonic structure (c, d) consists of a periodic multilayer comprising layers of cuticle material (mainly chitin) and macroporous cuticle material (chitin rods spaced by voids). Due to its chemical composition (i.e. the presence of salts) and its physico-chemical properties, the envelope enables the penetration of liquids such as water whereas it acts as a barrier for other liquids such as ethanol. (e) The photonic structure enclosed by a permeable envelope is regarded as a photonic cell. [Scale bars: (b) 100 µm; (c) 1 µm; (d) 1 µm].

2. Materials and methods

The color changes induced by the interaction of H. coerulea’s photonic structures with vapor were investigated using a custom-made experimental set-up which allowed measurement of reflected light intensity under a controlled atmosphere [Fig. 2]. The set-up consisted of a sealed measurement chamber with gas inlet and outlet controls in which introduction of gases


was monitored by mass-flow controllers (Bronkhorst High-Tech EL-FLOW select). Vapor was formed by passing dry air as a carrier gas (Air Liquide Alphagaz 1: [N2 + O2] ≥ 99.999%; [O2] 20 – 22%; [H2O] < 3 p.p.m.; [CO] ≤ 1 p.p.m.; [CO2] ≤ 1 p.p.m.) through a bubbler containing the liquid to be vaporized (about 300 ml of liquid in a 400 ml bubbler). The mixture of dry air and vapor from the bubbler was then diluted in dry air in various proportions, while the total flow (dilution dry air flux plus dry air flux passing through the bubbler) was kept constant (200 sccm). After passing through the bubbler, the relative vapor pressure in air cannot be assumed to be equal to 100%, since vaporization of the liquid did not take place at equilibrium. Hence, the vapor concentration in the chamber was quantified in terms of flux ratio, i.e. ratio between air flow introduced into the bubbler and total flux entering the chamber. The pressure in the chamber was kept at 1.030 bar by a pressure controller (Bronkhorst High-Tech EL-PRESS select). This value was slightly higher than ambient pressure in order to avoid air intake from outside.

In order to perform optical measurements in situ, the chamber was equipped with two feedthroughs for optical fibers. The sample (a dead beetle elytron) was illuminated with white light using optical fibers at an incidence angle θ equal to 15° fixed by an Avantes AFH-15 fiber holder [Fig. 2]. Other fibers were used to detect the light reflected from the sample in the specular direction. A pulsed xenon lamp Avantes AvaLight-XE was used to deliver relatively high intensity light while avoiding continuous illumination of the sample. This could otherwise have led to undesirable warming of the sample during typically four or five hours of measurements at ambient temperature. In fact, during a long period of time, temperature variations in the measurement chamber could influence the interaction of vapor with the photonic structure. Indeed, it was recently demonstrated that the color of insect wings was also modified by temperature [57,58]. The temperature in the chamber was measured by Chromel/Alumel thermocouple (type K) and was measured to be stable during the experiments (less than 5.2% variation). Temperature change of the insect elytra due to warming by the incident light was also measured with a thermal camera FLIR B335 equipped with an IR LENS 10 mm objective. When a single elytron was illuminated by the lamp for one day, its maximal temperature change (at the end of the day) was equal to 2.5%. Reflected light intensity spectra, recorded while water or ethanol vapor were present in the atmosphere of the chamber, were measured using an Avantes AvaSpec-ULS2048-2-USB2 spectrophotometer. These spectra were transposed into reflection factors ( )R λ , often termed

reflectance, which are defined as the ratio between the intensity reflected by the sample ( )S λ

and the intensity reflected by a standard white diffusor ( )W λ with corrections ( )B λ taking

optical and electronic noise into account, namely; ( ) ( ) ( ){ } ( ) ( ){ }R S B W Bλ λ λ λ λ= − − . Due to recognized issues related to the directionality of scattered light from the diffusor, this reflection factor may sometimes exceed 100%. ( )S λ , ( )W λ and ( )B λ were measured with the same experimental configuration. For each set of measurements at a given vapor flux ratio, ( )W λ and ( )B λ were first measured before the sample was placed and the chamber was sealed. Then, three cycles of evacuation (down to 0.001 bar), followed by introduction of dry air into the chamber were performed in order to remove gases and vapor from the sample area, the pipes and the chamber. Following this, the vapor at a selected flux ratio was introduced into the chamber and the reflection factor spectra were recorded until they ceased to exhibit variations. The introduction of vapor was then stopped and only dry air was introduced into the chamber. Finally, the reflection factor spectra were recorded upon drying until they returned to their initial states. Spectral changes in reflection factor were calculated


by substracting the spectrum ( )0R λ measured just prior to vapor introduction from the

spectrum ( ),R tλ measured after a given exposure time t , i.e. ( ) ( ) ( )0, ,R t R t Rλ λ λ∆ = − .

Fig. 2. Experimental set-up for measuring spectral variations of the samples’ reflection factors induced by vapor flow through the sealed measurement chamber.

Since measurements were performed over several hours, a dual-channel spectrophotometer was used: one channel measured the reflected light from the sample and the other one allowed monitoring of the light source. The rate at which the exposed samples changed color depended on; the nature of the sample, the introduced vapor (water and ethanol were used in this study), the flux ratio, the total flux, the size of the beam spot on the sample (approximately 2 mm diameter) and the measurement chamber volume (about 1300 cm3). Variation in light source intensity turned out to be negligible at the time-scale of measurements.

Observations of the scales were made with an Olympus BX61 optical microscope equipped with an Olympus XC50 camera and an Olympus BX-UCB light source as well as with a Fei Tecnai 10 transmission electron microscope (TEM). Prior to the TEM analysis, pieces of H. coerulea’s elytra were prepared using standard TEM sample preparation methods. 100 nm-thick cross sections were cut with an ultramicrotome and transferred onto copper grids. A 3D reconstruction of the photonic structure of one sample was performed using a Fei Nova Nanolab 200 Dual-Beam scanning electron microscope (SEM) combined with a focus ion beam (FIB).

Theoretical reflectance spectra were calculated using a thin-film modeling computer code which relies on 1D scattering matrix formalism [59] to solve Maxwell’s equations rigorously in the case of planar layered photonic structures. Simulations were performed for unpolarized light at 15°-incidence angle. The refractive indices of air, water, ethanol and cuticle material (assumed to be chitin) were chosen to be 1.00 [60–62], 1.335 [63], 1.365 [64] and 1.56 [65], respectively. The macroporous layers were approximated by homogenous layers with an effective refractive index combining both indices of the cuticle material and of the macropores (filled with air, water or ethanol) [30,54]. This approximation was valid since the average distance between macropores was too short in comparison to visible light wavelengths giving rise only to non-zero order diffraction [30,54,55].

The comparison between measurements and simulations is not quantitative. This is the case because the measured variations of the reflection factor spectrum depend on the vapor


flux ratio. In contrast, the simulated reflectance spectra depend on the parameters of the photonic structures (refractive indices or dimensions) which could change as a result of exposure to the vapor. Despite this difference in physical quantities, both measured and simulated results can be qualitatively compared. This is presented in the following sections.

3. Results

When water vapor was introduced into the chamber, a redshift (20 – 25 nm) was observed in the reflection factor spectrum at a flux ratio greater than 50% [Fig. 3]. With ethanol vapor, a similar redshift (25 – 30 nm) was also measured but at a flux ratio greater than 75% [Fig. 4]. At low flux ratios (i.e. less than 50% for water and 75% for ethanol) no significant shift was observed (less than 2 nm).

In both cases (i.e. with water and with ethanol), the reflection factor spectra returned to their initial states (solid blue curve in Figs. 3 and 4) upon drying and an increase in the reflection factor peak intensity was observed. This increase was less pronounced at lower flux ratios. Such an increase of reflectance intensity induced by the presence of a vapor has already been observed experimentally in the case of butterfly wings that comprise a range of photonic structures [33,46,51]. Conversely, contact of certain beetle cuticles with liquids is known to induce a decrease of reflectance intensity. The reason for this change is well understood, arising due to the decrease of the macropore-cuticle refractive index contrast [28–30,54]. The increases in reflectance intensity associated with the butterfly wing measurements, however, remain unexplained so far.

Fig. 3. a, d, g, j) Reflection factor spectra ( )R λ measured at 15°-incidence angle on a H.

coerulea’s elytron in contact with water vapor in different flux ratios: 25% (a-c), 50% (d-f), 75% (g-i) and 100% (j-l). b, e, h, k) Differential spectra 0R R R∆ = − where R are spectra

measured at a given flux ratio and different exposure times and 0R is the spectrum measured just before vapor introduction into the measurement chamber – solid green curve: after exposure to vapor; solid blue curve: after complete drying. c, f, i, l) Variations of the peak wavelength peakλ of the reflection factor as a function of exposure time.


H. coerulea reversible color changes appear more sensitive to the presence of water vapor than to ethanol vapor (Figs. 3 and 4): a significant color change is induced at a lower flux ratio and the increase of the reflection factor intensity is higher with water vapor than with ethanol vapor. Moreover, the shift of the reflection factor peak wavelength takes place in discrete steps (Figs. 3 and 4) because scales appear to change their color abruptly, section by section, rather than gradually as was shown in previous studies [30,54].

Fig. 4. a, d, g, j) Reflection factor spectra ( )R λ measured at 15°-incidence angle on a H. coerulea’s elytron in contact with ethanol vapor in different flux ratios: 25% (a-c), 50% (d-f), 75% (g-i) and 100% (j-l). b, e, h, k) Differential spectra 0R R R∆ = − where R are spectra

measured at a given flux ratio and different exposure times and 0R is the spectrum measured just before vapor introduction into the measurement chamber – solid green curve: after exposure to vapor; solid blue curve: after complete drying. c, f, i, l) Variations of the peak wavelength peakλ of the reflection as a function of exposure time.

4. Discussion

Unlike open structures, color changes observed in H. coerulea scales in which the porous photonic structure is enclosed by an encasing envelope cannot be explained by physisorption and capillary condensation inside the pores since there is no phase transition within these pores. Indeed, vapor in contact with the scales first adsorbs onto their external surfaces, i.e. their encasing envelope. The liquid of this adsorbed film then penetrates into the porous photonic structure in a discrete stepwise manner. This is similar to penetration of scales by liquid droplets and resulting in a color change [30,54]. Furthermore, the differences in color change dynamics between water and ethanol are controlled by the physicochemical properties (microporosity) as well as the chemistry (presence of salts) of the encasing envelope that enable the penetration of specific fluids according to molecular polarity [54]. With an atmosphere containing low vapor content (i.e. low flux ratio), only a small amount of liquid penetrates into the scales: this leads to negligible color change ( 2nm< shift in peak wavelength). Conversely, at high flux ratios, three phenomena are likely to contribute to the production of a color change: (1) the swelling of the structure, (2) the filling of the macropores of the structure and (3) the filling of the micropores (i.e. pore diameter smaller


than 2 nm), e.g. through capillarity. In this section, we will first examine the mechanism involving only the formation of a thin adsorbed liquid film on the surface, without pore filling and we will then investigate these three likely possible processes.

If a film is assumed to adsorb onto the surface of the H. coerulea’s photonic structure (i.e. with a film thickness filmd ranging from 0 nm to 15 nm), noticeable variations in color reflectance and intensity are predicted. These are similar to the observations made by experiment [Fig. 5]. The simulated reflectance spectra and their relative variations R∆ [Fig. 5(b)] are similar to measured reflection factor spectra (Figs. 3 and 4). According to this modeled process, the peak wavelength of the simulated reflectance spectra [Fig. 5(a)] progressively redshifts from 453 nm ( film 0nmd = ) to 492 nm ( film 15nmd = ). Furthermore, a slight increase in the peak reflectance intensity is also predicted (about 0.2%) with a film thickness of 15 nm. With a further increase of the film thickness, the peak reflectance intensity decreases (e.g. a 20 nm-thick absorbed film is associated with no change in the reflectance intensity). This small increase of 0.2% is very slight, especially in comparison to the experimental results (Figs. 3 and 4). We note again here, however, that the measured quantity is not the reflectance intensity but instead is a normalized reflection factor. Although such an increase in reflectance intensity was observed in the cases of a few butterflies’ wings with different photonic structures [33,46,51], its mechanisms has not been explained. The observed increase of reflection intensity with H. coerulea can be understood by the formation of a thin film of adsorbate on the surface of the scales. However, this process does not explain the long time-scale of the color changes observed in the experiment as well as the discrete nature of the increases in the reflectance peak wavelength. Ordinarily, we might expect any adsorption of a film and resulting color change to be continuous and relatively rapid.

In dry conditions the difference in peak reflectance wavelength between measurement ( peak 430-435 nmλ = ) and simulation ( peak 453 nmλ = ) is rather small ( 5%< ). It can be explained by several factors. Firstly, the incidence and detection angles are experimentally not very well defined: the scales might have not been perfectly horizontal when the measurements were performed. Furthermore, in contrast to our idealized model of H. coerulea’s photonic structure, its actual photonic structure is not so well ordered. The junctions between its adjacent scales are not perfect, there are defects within scales such as variations of refractive indices and dimensions (e.g. layer thicknesses, lateral distance between rods…) or irregular layer interfaces. This leads to poorer color saturation and a variation of the reflection factor spectrum according to the location of the light spot on the insect’s elytron. Since regular flat interfaces were assumed in the photonic model, side lobe peaks are present in the simulated reflectance spectra due to Fabry-Pérot resonances. They are not observed in the experimental spectra (Figs. 3 and 4).

Fig. 5. a) Simulated reflectance spectra (unpolarized light at 15°-incidence angle) of the modeled structure (insert) onto which a liquid film is adsorbed (thickness ranging from 0 nm to 15 nm). b) Differences

film 0 nmdR R R =∆ = − between the spectra shown in (a) and the

reflectance spectrum for a modeled structure without adsorbed film (i.e. the solid blue curve in (a)).


The vapor-induced color change as well as the resulting reflectance peak shift can also be explained by a swelling of the structure. Biopolymers like chitin are known to swell when they are in contact with water [29,34,40,66,67]. Disruption of hydrogen bonds between polymeric chains due to water infiltration can lead to an increase of the structure size. This phenomenon is reversible: upon drying, hydrogen bonds re-form and the structure shrinks back to its initial dimensions. Polymer swelling due to the absorption of a liquid adsorbed on its surface is used in some gas sensor devices based on the deformation of a Fabry-Pérot interferometer or a multilayer [10,68]. Modification of mechanical properties of insect structures, as a result of water impregnation is in fact not surprising. Indeed, the display of specimens in museum collections usually requires prior humidification. One kind of chitin (β-chitin) was furthermore shown to expand in contact with HCl [69] and distilled water [70].

Although swelling of H. coerulea’s photonic structure has not been observed experimentally, we examine the optical properties arising should it occur. Any swelling of the structure would lead to an increase of the multilayer period and, hence, to modification of the reflectance spectrum. Simulations show that the reflectance peak wavelength would be redshifted [Fig. 6]. The swelling factor α (i.e. structure dimensions multiplied by 1 α+ ) was chosen between 0 and 0.10 (Figs. 6(a) and 6(b)). The simulated spectra are also quite similar to those measured experimentally (Figs. 3 and 4), especially with α between 0 and 0.01 (Figs. 6(c) and 6(d)). However, no variation of peak intensity was predicted. At 15°-incidence angle, the reflectance peak of the modeled structure is equal to 89.09% regardless of the swelling factor. Here, the change in reflectance spectrum is defined by 0R R Rα =∆ = − where R is the simulated spectrum with a swelling factor α and 0Rα = is the simulated spectrum of the original unexpanded structure. The value 0.01α = , which qualitatively reproduces the observed spectral shift, is rather low. For instance, swelling factors of the photonic structures of tree swallow birds (Tachycineta bicolor) and longhorn beetles Tmesisternus isabellae in contact with water vapor were estimated to be equal to 0.07α = [40] and 0.09 [29], respectively. Note that no variation of the refractive index of chitin due to the swelling (decrease of chitin density) was taken into account in our simulations. At these low swelling factors, this is a reasonable assumption.

Fig. 6. a, c) Simulated reflectance spectra (unpolarized light at 15°-incidence angle) of the modeled structure (insert) with different swelling factors. All dimensions of the structure were multiplied by 1 α+ , with α ranging (a) from 0 to 0.1 in steps of 0.02 and (c) from 0 to 0.01 in steps of 0.002. b, d) Differences 0R R Rα =∆ = − between the spectra shown in (a, c) and the reflectance spectrum for the structure modeled without swelling (i.e. the blue curve in (a)).


In the case where vapor adsorbed at the scale surface progressively fills the scales’ macropores as liquid, the effective refractive index of the macroporous layers increases from 1.26 (filled with air) to 1.45 (filled with water) or 1.46 (filled with ethanol) [30,54]. According to this modeled process, the peak wavelength of the simulated reflectance spectra [Fig. 7(a)] progressively redshifts from 453 nm to 504 nm (with water) or 506 nm (with ethanol). Although simulated reflectance spectra and their relative variations R∆ [Fig. 7(b)] are similar to those of measured reflection factor spectra (Figs. 3 and 4), the predicted variations are much more significant than the experimental ones. This implies, therefore, that the macropores of the photonic structure are not completely filled. Furthermore, a decrease of the intensity is predicted in this process, which is not observed in the measurements.

In experiments comprising exposure to liquid instead of vapor [16,30,45,54], the filling of the macropores is a realistic scenario since it can explain the observed decreases of both the reflection factor intensity and spectral width of the peak. Both decreases are predicted by the reduction of the refractive index contrast within the photonic structure [71].

Fig. 7. a) Simulated reflectance spectra (unpolarized light at 15°-incidence angle) of the modeled structure (insert) as the refractive index of the macropores increases from 1.000 (dry air – solid blue curve) to 1.335 (water – solid yellow curve) and to 1.365 (ethanol – solid grey curve). b) Differences dryR R R∆ = − between the reflectance spectra shown in (a) and the

reflectance spectrum of the modeled structure in which the macropores are filled with dry air (i.e. the solid blue curve in (a)).

In reality, the cuticle material of H. coerulea scales is microporous [54]. So far, we assumed the refractive index of this material was equal to 1.56 (i.e. the refractive index of chitin [62]) in our simulations. However, the infiltration of liquid into the micropores (e.g. through capillarity) may induce a small increase of the refractive index of this material. If an increase of the cuticle material refractive index from 1.56 to 1.62 is assumed [Fig. 8], the reflectance spectrum redshifts from 458 nm to 471 nm, similarly to the measurements, and the intensity increases from 89.09% to 91.69%. This increase is more significant than in the case of the film adsorbed on the scale surface. We did not consider the filling of the macropores in these simulations because micropores are known to be filled more easily by a liquid. The filling of micropores with a negligible filling of macropores is therefore conceivable. Furthermore, both filling processes have different effects on the reflectance spectrum. For this reason it is preferable to investigate them separately.

Fig. 8. a) Simulated reflectance spectra (unpolarized light and a 15°-incidence angle) of the modeled structure (insert) when the refractive index of the cuticle material increases from 1.56


to 1.62 due to liquid infiltration within the micropores. b) Differences 0R R R∆ = − between the spectra shown in (a) and the reflectance spectrum for a modeled structure of which the micropores are filled with dry air (i.e., the blue curve in (a)).

In order to assess the optical effect of this variation of the cuticle material refractive index, we calculated the micropore filling fraction iδ thanks to a Maxwell-Garnett effective medium

approximation [Fig. 9]: cuticle, eff m i m

cuticle, eff m i m2 2i

e e e eδ

e e e e − −

= + +

where 2cuticle, eff cuticle, effne = is the

effective dielectric constant of the medium (i.e. the microporous cuticle), me is the dielectric constant of the host medium (i.e. the dense cuticle material, free of micropores), and ie is the dielectric constant of the inclusions (i.e. micropores). In our simulations, the effective dielectric constant of the microporous cuticle was supposed to be equal to 2

cuticle, eff 1.56e = in

the dry state (with 2i 1.00e = ) and to range from 2

cuticle, eff 1.57e = to 2cuticle, eff 1.62e = in the

wet state (with 2i 1.335e = ). The intersection of the curve in Fig. 9 in the dry state with a

curve in the wet state gives the refractive index mn of the dense cuticle material and the micropore filling fraction iδ . For instance, assuming an effective refractive index in the wet state equal to cuticle, eff 1.59n = , the refractive index of the dense cuticle material mn is equal to 1.616 and the micropore filling fraction, to 9%. We note that filling fractions larger than the latter value do not seem plausible.

Fig. 9. Refractive index mn of cuticle material free of micropores as a function of the

micropore filling fraction iδ calculated using a Maxwell-Garnett effective medium

approximation for an effective refractive index 2cuticle, eff 1.56e = in the dry state

2i( 1.00 )e = and different effective refractive indices ranging from 2

cuticle, eff 1.57e = to 2

cuticle, eff 1.62e = in the wet state ( 2i 1.335e = ).

The observed increase of reflection intensity with H. coerulea is explained to some extent by this increase of effective refractive index of the cuticle material. Furthermore, this


phenomenon explains the time-scale of the measured changes in reflectance spectra: the infiltration of physisorbed liquid through the photonic cell is not instantaneous [54]. Moreover, such changes were shown to be discrete changes rather than continuous [30,54].

5. Conclusion

The blue iridescent color of Hoplia coerulea changes upon contact with water and ethanol vapor. Unlike many photonic structures found in many insect species, Hoplia coerulea’s periodic porous multilayer structure is quite unique in the sense it is not open to the environment but enclosed by a 100 nm-thick chitin microporous envelope. For this reason, this optical system was called photonic cell. Although there is no direct contact of the photonic structure with the surrounding atmosphere, the coloration of the photonic cell changes reversibly upon contact with vapor: a redshift and an increase of the peak reflectance intensity are observed. Such an increase was observed for different butterfly wings but the mechanism responsible for it has not been explained so far. Similarly to liquid-induced changes, H. coerulea’s photonic structure is more sensitive to water vapor than to ethanol vapor: reflectance changes are noticeable at an introduced vapor flux ratio which is lower with water than with ethanol. This difference is due to the permeable microporous envelope of the photonic cell formed by the scale which, much like the membrane of a biological cell, controls fluid exchanges with the environment. Among the four mechanisms identified in this work as forming possible explanations for these color changes, the filling of the micropores of the cuticle material is the most plausible. Namely, when vapor interacts with the scale, it first adsorbs onto its surface, then penetrates (more or less easily, depending on the nature of the liquid) into the scale and fills the available micropores. Physisorption on the scale surface and filling of the micropores both induce a redshift of the reflectance spectrum as well as an increase of the reflectance peak intensity. Usually, the microporosity of natural photonic structures is neglected in fluid-induced color changes but these results show that this feature plays a significant role. Although the other mechanisms (swelling of the structure and partial filling of the macropores) might influence the color change and induce a further redshift, our simulations indicate their effects seem to be less significant.

Acknowledgments

The authors thank Yvon Morciaux and Jean-François Colomer (Department of Physics, UNamur) for precious help during the development of the measurement chamber, Benoit Van der Schueren (CMI, UNamur) for technical help during the measurements as well as Louis Dellieu (Department of Physics, UNamur) for technical support during the collection of samples. This research used resources of the “Plateforme Technologique de Calcul Intensif (PTCI)”, UNamur (http://www.ptci.unamur.be), which is supported by the Belgian National Fund for Scientific Research F.R.S.-FNRS under the convention No. 2.4520.11 as well as of the Electron Microscopy Service (SME), UNamur (http://www.unamur.be/en/sevmel). PTCI and SME are members of the “Consortium des Équipements de Calcul Intensif (CÉCI)” (http://www.ceci-hpc.be) and of the “Plateforme Technologique Morphologie – Imagerie” (UNamur), respectively. This research was supported by F.R.S.-FNRS (Research credit CDR J.0035.13). S.R.M. was supported by F.R.S.-FNRS as Research Fellow.


vapor sensing with a natural photonic cell€¦ · vukusic, 6. and olivier deparis. 1,* 1....

Documents