A versatile photonic crystal sensing motif based on a twodimensional (2D) inverse opal monolayer of stimuli-responsive polyelectrolyte gel with tunable optical properties is reported. The photonic membrane shows prompt response to pH and can be readily

read out from either its optical spectra or interference colours

Akahane

Ben-Moshe

Bettina V. Lotsch

Boniacio

Cheng Li

Chow

Daghestani

Duan

Griffete

Hayward

Holtz

Hong

Huang

Karaman

Leeand

Lotsch

López-García

Nakayama

Porter

Singamaneni

Song

Tian

Tokarev

Ueno

Wang

Zhang

Zhao

English

Crossref

Chemical Communications

Stimuli-responsive 2D polyelectrolyte photonic crystals for optically encoded pH sensing

Li, Cheng

Lotsch, Bettina V.

Open Access LMU ( Ludwig-Maximilians-Univ. München)

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 6169–6171 6169Cite this: Chem. Commun., 2012, 48, 6169–6171Stimuli-responsive 2D polyelectrolyte photonic crystals for opticallyencoded pH sensingwCheng Liab and Bettina V. Lotsch*abReceived 15th March 2012, Accepted 25th April 2012DOI: 10.1039/c2cc31916kA versatile photonic crystal sensing motif based on a two-dimensional (2D) inverse opal monolayer of stimuli-responsivepolyelectrolyte gel with tunable optical properties is reported. Thephotonic membrane shows prompt response to pH and can be readilyread out from either its optical spectra or interference colours.The development of sensing techniques is driven by the everincreasing demand for miniaturized sensing platforms with fastresponse in areas such as bioassays, environmental monitoringand disease diagnostics.1 Among the various kinds of sensingarchitectures, photonic crystal (PC) sensors that employ stimuli-responsive materials as signal transducers have been demon-strated to be promising in that they are able to perform sensitivedetection in a non-destructive and label-free way.2 In past years,sensors have been developed by exploiting various PC motifsincluding opal films,3 inverse opals in the form of films4 ormicroparticles,5 Bragg stacks,6 porous silicon PCs,7 PC fibers,8and microcavities.9 Among them, hydrogel photonic crystalsensors are of particular interest because they can be tailoredto respond to various stimuli and show substantial volumechanges upon recognition of analytes by swelling that couldenhance the optical readout.3,4a–g,6c,d Asher’s group developedhydrogel sensing materials by polymerizing 3D3a–d and mostrecently 2D colloidal crystal arrays,3e in which a volume changeof the hydrogel upon recognition of the analyte led to modifica-tion of the optical thickness and therefore a shift in the positionof the photonic stop band and, hence, a colour change. Based ona similar principle, 3D inverse opals of hydrogel have beenshown to be applicable for the sensing of pH,4a,e,g glucose,4btemperature,4c and humidity.4d In terms of one dimensional PCarchitectures, Bragg stacks with hydrogel as a constituent layerhave recently been demonstrated to be capable of humiditysensing.6c,d However, the above sensing motifs all suffer fromslow response times when it comes to the sensing of solutes, suchas pH, or molecular analytes. It often takes more than tens ofminutes or even hours to reach maximum response, whichhampers applications in real-time sensing. The main reason forthis is that the above sensing motifs are made from thickhydrogel structures (from several to tens of microns), whichresults in slow diffusion of solutes into the hydrogel andtherefore slow response of the sensor. Although several measureshave been taken to speed up the response, they lack feasibilityand universality.3e,4eHerein, we report a versatile PC sensing motif, which is astimuli-responsive 2D PC based on a monolayer inverse opalof polyelectrolyte gel (2DPC-PG). A remarkable feature of the2DPC-PG is that it holds tunable photonic properties despite asub-micron thickness. Therefore, the photonic sensing forsolutes does not suffer from slow diffusion and the responsecan be greatly sped up.To prepare 2DPC-PGs, a monolayer of a polystyrene (PS)colloidal array was first assembled on a glass or siliconsubstrate, onto which a solution of quaternized poly-(2-vinylpyridine) (qP2VP) was spin-coated at the desired spin speed(ESIw). The PS monolayer was then selectively removed bydissolving it in toluene. Finally, a mechanically stable 2DPC-PGwas obtained after thermal cross-linking at 120 1C. Fig. 1 showsthe scanning electron microscopy (SEM) images of a 2DPC-PGprepared by using a monolayer opal assembled from 470 nmdiameter PS spheres and a spin speed of 2000 rpm. From theSEM images, one can notice a thin layer covering the void arrays,which owing to its small thickness appears almost transparent tothe electron beam in top view. It can further be seen in Fig. 1athat the P2VP gel replicates well the hexagonal order of the opalmonolayer and the lattice parameter of the resulting 2DPC-PGcorresponds to the diameter of the template PS spheres. Thecross-section SEM image in Fig. 1b gives clear evidence of thetop layer but at the same time shows that the height ofthe voids is decreased as compared to the original PS spheres,Fig. 1 SEM images of the 2D PC based on a monolayer inverse opalof polyelectrolyte gel (2DPC-PG) on a silicon wafer: (a) top-view and(b) side-view.aMax Planck Institute for Solid State Research, Heisenbergstrasse 1,70569 Stuttgart, GermanybDepartment of Chemistry and Center for Nanoscience,Ludwig-Maximilians-Universita¨t Mu¨nchen, Butenandtstr. 5-13,Haus D, 81377 Mu¨nchen, Germany. E-mail: b.lotsch@fkf.mpg.dew Electronic supplementary information (ESI) available. See DOI:10.1039/c2cc31916kChemComm Dynamic Article Linkswww.rsc.org/chemcomm COMMUNICATIONPublished on 27 April 2012. Downloaded by Ludwig Maximilians Universitaet Muenchen on 11/07/2013 09:44:29. View Article Online / Journal Homepage / Table of Contents for this issue6170 Chem. Commun., 2012, 48, 6169–6171 This journal is c The Royal Society of Chemistry 2012suggesting a deformation of the voids from spherical to oblate,which can be attributed to the low stiffness of the gel. As a result,the overall thickness of the 2DPC-PG is less than the diameter ofthe PS spheres, which is further confirmed quantitatively bythickness measurement. The small holes in the walls of the voidsare formed due to the close packing of the original PS spheres,indicating that the voids are interconnected with each other.The structural parameters of the 2DPC-PG, i.e. its overallthickness and the parameters of the optical lattice can be readilytuned with the presented method. The mean thickness of the2DPC-PG, which was measured with a profilometer, decreasesalmost linearly from 447, 417, 393, 363 to 332 nm as the spin-coating speed gradually increases from 1000, 1500, 2000, 2500 to3000 rpm (Fig. S1, ESIw). On the other hand, the lattice para-meters of the 2DPC-PG can be conveniently varied by using amonolayer template with different sphere diameters. For example,2DPC-PGs prepared with the same spin-coating speed of2000 rpm but sphere diameters of 470 and 535 nm possess latticeparameters of about 450 and 500 nm respectively (Fig. S2, ESIw).However, they still have a similar mean thickness of around395 nm as measured by profilometry. These results suggest thatthe thickness and the lattice parameter can be tuned separately,which enables us to study experimentally the relationship betweenstructural parameters and optical properties of the 2DPC-PG.The 2DPC-PG demonstrates tunable optical propertiesassociated with its structural parameters. As shown in Fig. 2,while an unstructured/dense P2VP gel film prepared on a bareglass substrate is transparent (a), the 2DPC-PG samples (b–e) allshow a major, well-defined dip in their transmission spectrum.Such a transmission dip corresponds to an attenuation of thetransmitted intensity as the incoming waves couple to the photonicmodes10 provided by the periodic structure of the 2DPC-PG(Scheme S1, ESIw). The position of the transmission dip can beassigned to the lattice parameter of the periodic structure anddescribed by ld ¼ffiffiffi3pdneff2,10d,e where d is the lattice parameterand neff is the effective refractive index (ESIw). The dip position of2DPC-PG is experimentally shown to be both thickness- andlattice parameter-dependent. For samples b, c and d preparedwith 470 nm spheres, the transmission dip red-shifts from 471, via485 to 502 nm with a decrease in spin-coating speed from 2500,via 2000 to 1500 rpm (i.e. an increase in overall thickness) asa result of an increasing neff. With a fixed spin-coating speedof 2000 rpm and hence a constant thickness, the transmissiondip of the samples c and e red-shifts from 485 to 520 nm as thetemplate sphere diameter increases from 470 to 535 nm.The pronounced dependence of the PC feature (i.e. trans-mission dip) of the 2DPC-PG on its structural parameters asshown above makes it promising for optically encoded sensing,as the swelling of the polyelectrolyte gel upon external stimuliwill lead to changes in the structural parameters that can be readout directly from the shift of the transmission dip. As P2VP iswell known as a weak cationic polyelectrolyte that exhibits fastand substantial swelling under acidic conditions due to proto-nation of the pyridine group,11 a pH sensor based on 2DPC-PGis demonstrated in this report. Fig. 3a shows the transmissiondip shift of a 2DPC-PG in response to pH 5, 4, 3 and 2, which is6, 15, 29 and 26 nm as compared to its initial position,respectively. Therefore, the variation of pH conditions can bereadily read out from the wavelength shift of the 2DPC-PG’stransmission dip. Besides, the swollen 2DPC-PGs can be entirelyrecovered by a quick wash with a basic solution or pure water.Accordingly, the optically encoded response of the 2DPC-PGupon pH changes is totally reversible and can be cycled morethan 10 times without degradation (Fig. S3, ESIw).To explain the optical response, i.e. the transmission dipshift of the 2DPC-PG as a function of pH, its related structuraland morphological changes under different pH conditions wasstudied. Under acidic conditions the 2DPC-PG swells dramaticallyand its thickness increases by 5%, 21%, 37% and 34% aftersoaking in pH 5, pH 4, pH 3 and pH 2 solutions, respectively(Fig. S4 and S5, ESIw). Such a trend in thickness variation is incomplete agreement with the transmission dip shifts as shownabove and indicates once more that the PC properties ofFig. 2 Transmission spectra of an unstructured P2VP gel film and2DPC-PGs prepared with varying sphere template diameters and spin-coating speeds on glass substrates.Fig. 3 Transmission dip shift (a), morphological changes (b) andinterference colours (c) of 2DPC-PGs in response to differentpH conditions. The 2DPC-PGs were prepared with 470 nm opalmonolayers and a spin-coating speed of 2000 rpm on glass (a) andsilicon wafer (b, c), respectively.Published on 27 April 2012. Downloaded by Ludwig Maximilians Universitaet Muenchen on 11/07/2013 09:44:29. View Article OnlineThis journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 6169–6171 6171the 2DPC-PG are a faithful replication of its structural para-meters. Moreover, the top-view SEM images of the swollen2DPC-PGs under different pH conditions (Fig. 3b) revealsthat the lattice parameter of the 2DPC-PG stays almost thesame, although the inner voids gradually diminish from pH 5via pH 4 to pH 3. Notably, upon pH 2 the suspended part ofthe overlayer in the 2DPC-PG exhibits significant wrinkling.The observed blue-shift of the transmission dip from pH 3 topH 2 clearly correlates with a buckling and, hence, a decreasein the optical thickness of the membrane. The 2DPC-PG isanchored to a stiff substrate and is thus subject to a gradedconstraint in its swelling, but unlike a dense P2VP gel filmwhich folds upon pH 2 due to the constraint imposed by thesubstrate,11b its smaller contact area to the substrate endows itwith higher flexibility and buckling deformation is restricted tothe suspended top layer, thus avoiding a large range folding ofthe film. The structural stability upon exposure to externalstimuli enables the 2DPC-PG to perform as a durable, rever-sible and mechanically strong pH sensor.Apart from the transmission dip arising from the in-planewaveguide-like mode and hence, the inherent PC properties ofthe membrane, the 2DPC-PG exhibits vivid colour whenprepared on highly reflecting substrates as a result of opticalinterference, which can also be exploited as a visibly percep-tible indicator for sensing events. Fig. 3c shows in a row thephotographs of a 2DPC-PG that was prepared on a siliconsubstrate before and after soaking in solutions of pH 5, 4, 3and 2 and subsequent drying. The varying hues from purple,blue, green to yellow upon different pH conditions are notice-able to the naked eye. The Fabry–Pe´rot interference fringes ofthe 2DPC-PGs within the visible region exhibit shifts incomplete agreement with the observed optical and thicknesschanges (Fig. S6, ESIw).Remarkably, the 2DPC-PG pH sensor exhibits a promptoptical response. As shown in Fig. 4, within 6 seconds soakingin a solution of pH 3 the transmission dip shift already reaches96% of the full response after 1 minute, indicating thatequilibrium can be well established within seconds. Recently,Asher’s group reported a powerful 2D PC hydrogel pH sensorthat however needs 30 minutes to reach its full response.3eCompared with the PC pH sensors designed so far, the 2DPC-PGpresents the thinnest gel structure, resulting in the observed fastestresponse times.In summary, we demonstrate the successful fabrication of2D inverse opal monolayers out of a swellable P2VP poly-electrolyte gel by a facile spin-coating method. The stimuli-responsive membranes give rise to a versatile photonic crystalpH sensing motif based on their tunable optical propertiesand exhibit fast response times owing to their sub-micronthickness. Such a 2D inverse opal sensing motif can be easilyextended for various sensing events when coupled with tailor-made responsive materials.Dr C. Li acknowledges support from the Alexander vonHumbolt Foundation. The authors thank Dr I. Tokarev andProf. V. V. Tsukruk for useful discussions and Dr SteffenSchmid and Viola Duppel for SEM measurements. Financialsupport from the Nanosystems Initiative Munich (NIM),Center for Nanoscience (CeNS) and Fonds der ChemischenIndustrie (FCI) is gratefully acknowledged.Notes and references1 (a) M. D. Porter, R. J. Lipert, L. M. Siperko, G. Wang andR. Narayanana, Chem. Soc. Rev., 2008, 37, 1001; (b) H. N.Daghestani and B. W. Day, Sensors, 2010, 10, 9630.2 (a) Y. Zhao, X. Zhao and Z. Gu, Adv. Funct. Mater., 2010, 20, 2970;(b) Y. Song, W. Wei and X. Qu, Adv. 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Stimuli-responsive 2D polyelectrolyte photonic crystals for optically encoded pH sensing.

Universität München: Elektronischen Publikationen

https://epub.ub.uni-muenchen.de/15878/1/stimuli-responsive2D.pdf

Stimuli-responsive 2D polyelectrolyte photonic crystals for optically encoded pH sensing.

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Open Access LMU ( Ludwig-Maximilians-Univ. München)

Universität München: Elektronischen Publikationen