Cm

RAa

b

a

ARRAA

KBMSIP

1

wmtc(haBmvlomitl

(

0d

Biosensors and Bioelectronics 26 (2010) 1392–1398

Contents lists available at ScienceDirect

Biosensors and Bioelectronics

journa l homepage: www.e lsev ier .com/ locate /b ios

haracterisation and analytical potential of a photo-responsive polymericaterial based on spiropyran

obert Byrnea,∗, Claudia Venturab, Fernando Benito Lopeza, Adelheid Walthera,ndreas Heiseb, Dermot Diamonda,∗

CLARITY: Centre for Sensor Web Technologies, National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin 9, IrelandSchool of Chemical Sciences, Dublin City University, Dublin 9, Ireland

r t i c l e i n f o

rticle history:eceived 19 April 2010eceived in revised form 9 July 2010ccepted 15 July 2010vailable online 22 July 2010

eywords:

a b s t r a c t

In this paper we consider the critical issues inhibiting the widespread deployment of bio/chemo-sensorsin wireless sensor networks. Primary among these is the problem of performing calibration at remotelocations, and the consequent need for integrated fluidic systems for performing tasks like sampling, cali-bration and detection. Our conclusion is that low-cost, bio/chemo-sensing platforms that provide reliableinformation over long periods of use will only be realised through the use of microfluidic platforms thatare much more biomimetic in nature than technologies employed in current devices. Central to driving

iomimeticicrofluidics

piropyranonogelsolymer actuator

down costs will be the development of fluidic platforms with integrated soft polymer actuators that willreplace existing pumps and valves. A particularly attractive approach is to employ photo-controlled poly-mer actuators, wherein the status of the material can be effectively switched using light, as this allowsphysical separation of the control layer from the fluidic platform layer in a planar system. This, in prin-ciple, should greatly simplify manufacturing and therefore drive down costs. In this paper, we describe

ar poolec

a polymeric gel and a lineto utilize photochromic m

. Introduction

For some years, we have been investigating strategies to developays to provide analytical platforms capable of long-term deploy-ent in remote locations. This key objective has been driven by

he emergence of ubiquitous digital communications and the asso-iated potential for widely deployed wireless sensor networksWSNs). Understandably, in these early days of WSNs, deploymentsave been based on very reliable sensors, such as thermistors,ccelerometers, flow meters, power meters, and digital cameras.iosensors and chemical sensors (bio/chemo-sensors) are largelyissing from this rapidly developing field, despite the obvious

alue of being able to measure molecular targets at multipleocations in real-time. Interestingly, while this paper is focusedn the issues with respect to wide area sensing of the environ-ent, the core challenge is essentially the same for long-term

mplantable bio/chemo-sensors (Diamond, 2008) i.e., how to main-ain the integrity of the analytical method at a remote, inaccessibleocation?

∗ Corresponding authors.E-mail addresses: robert.byrne@dcu.ie (R. Byrne), dermot.diamond@dcu.ie

D. Diamond).

956-5663/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.bios.2010.07.059

lymer modified with a photochromic moiety and show that it is possibleules for performing sensing and actuating functions.

© 2010 Elsevier B.V. All rights reserved.

The analytical method can be defined as the sequence of eventsthat must take place in order to obtain a reliable measurement.Clearly there are many potentially limiting factors in any WSNdeployment, including power management, communications strat-egy, and incorporation of a degree of local self-diagnostics orintelligence at the so-called sensing node. But these are commonto all WSN deployments, and as scaled-up deployments based onphysical transducers are happening, they are not the reason whythere are still no examples of WSNs employing large numbers ofbio/chemo-sensing devices. The core challenge for these devicesas mentioned above, is the ability to provide reliable data overextended periods of deployments (ideally years). So why afterdecades of research, and huge investments, are we still confoundedby this challenge? The answer lies in failure of the integrity ofthe analytical method over time (Byrne and Diamond, 2006). Inenvironmental water quality monitoring, the active sensing sur-faces of bio/chemo-sensors that are directly exposed to the sample,change with time, due primarily to biofouling, which causes theresponse characteristics (sensitivity, baseline, selectivity, etc.) tochange unpredictably, leading rapidly to device failure (Diamondet al., 2008a).

In an effort to counter this, the sensing event is now commonlyseparated from the sample, and a fluidic system is used to acquirea sample and perform integrated operations such as reagent addi-tion, calibration, cleaning, and the analytical measurement. This

Bioele

raWldacde

1

s

1

2

aftTarft2abaetbtmhD

ofpt

Sc

R. Byrne et al. / Biosensors and

equires the incorporation of fluid handling devices such as pumpsnd valves, as well as the bio/chemo-sensor or sensing method.hile this does provide a protective environment in which the ana-

ytical method integrity can be more effectively maintained, it alsorives up the complexity and cost of the platform. Consequently,utonomous environmental chemical analysers are expensive, withosts typically in excess of D20,000 per unit, which renders theeployment of multiple systems prohibitively expensive (Diamondt al., 2008b).

.1. Strategies to meet the core challenge

Presently, we are using a two-pronged approach to improve thisituation:

. Evolutionary: Drive down the cost of analysers through cleverengineering.

. Revolutionary: Develop disruptive technologies based on break-through in fundamental materials science.

The value of the evolutionary approach should not be under-ppreciated. For example, we have reduced the component costor an autonomous chemical analyser from over D2000 to lesshan D200, while maintaining the functionality of the platform.he devices have been deployed successfully for multiple weekst a variety of locations, including waste-water treatment plants,ivers and estuaries (Cleary et al., 2008). However, in order tourther drive the unit cost down towards D20, fundamental break-hroughs in materials science are required (Benito-Lopez et al.,009). We are particularly interested in the potential of switch-ble polymers in microfluidic platforms, through which is maye possible to control fluid movement (e.g. using soft polymerctuators to provide pumping and valving functions (Benito-Lopezt al., 2010), and to modulate the sensing interface itself (e.g.hrough switching between active (binding) and passive (non-inding) modes (Byrne et al., 2006; Radu et al., 2007)). Hencehe bio/chemo-sensing system of the future is likely to be much

ore biomimetic in nature, indefinitely self-sustaining, and per-aps capable of some degree of self-repair (Ramirez-Garcia andiamond, 2007).

The ability to manipulate the physical and chemical propertiesf a material using an external stimulus forms the basis for theseuturistic analytical platforms. In this paper, we describe a novelhoto-switchable polymer and characterised, based on spiropyranhat could fulfil some of the roles outlined above. Benzospiropy-

cheme 1. Photo- and chemo-isomerisation of benzospiropyran (BSP) into its opticallyhelating properties of divalent metal ions such as Cu2+.

ctronics 26 (2010) 1392–1398 1393

ran (BSP) is a well-known photochromic molecule that undergoesa heterocyclic ring cleavage at the C–O spiro bond to form a pla-nar and highly conjugated chromophore that absorbs strongly inthe visible region, this being the merocyanine (MC) isomer (seeScheme 1) (Crano et al., 1996; Minkin, 2004). The MC isomer alsobinds divalent metal ions (Gorner and Chibisov, 1998), H+(Raduet al., 2009), amino acids (Ipe et al., 2003) and DNA (Anderssonet al., 2008) resulting in a shift in the absorbance spectrum, anda corresponding colour change. Under the right conditions, thisbinding is photo-reversible, as illumination with white or greenlight decouples the guest from the MC, which then reverts tothe BSP. Hence spatial and temporal photocontrol (and colori-metric reporting) of guest uptake and release, for example, usingspiropyran-functionalised materials is possible (Scarmagnani et al.,2010). Herein, we have synthetically modified a polymeric gel anda liner polymer with BSP and show that it is possible to utilize theBSP molecule in this matrix for performing sensing and actuatingfunctions.

2. Experimental

2.1. Synthesis and characterisation of photo-responsive ionogels

The ionogel consists of three monomeric units: N-isopropylacrylamide (NIPAAm), N,N-methylene-bis(acrylamide)(MBAAm) and acrylated benzospiropyran in the ratio of 100:5:1,respectively and a phosphonium based ionic liquid as seen inFig. 1. The acrylated benzospiropyran is synthesised as describedelsewhere (Szilagyi et al., 2007). Photo-responsive ionogelswere prepared as described in supplementary information.Photo-responsive ionogels were characterised by IR (Perkin-Elmer Spectrum GX FT-IR system using a horizontal attenuatedtotal reflectance [ATR] sample holder) and Raman spectroscopy(Perkin-Elmer RamanStation 400 F). UV–vis spectroscopy ofphoto-responsive ionogels was characterised using a Cary 50UV–vis spectrophotometer coupled to fibre optic probe accessory.Photo-induced contraction experiments carried out at 25 ◦C usingOlympus-Motorised Inverted Research Microscope X81 (OlympusEurope Holding GmbH, Germany); the light source used was anXcite series 120 EXFO (Canada) and the software is Cell Imag-

ine. Data analysis was carried using Image J. Photo-responsiveionogels were kept for 2 h in 0.1 mM HCl aqueous solution beforephoto-induced contraction experiments were carried out. Ionogelcontaining no BSP was used as control material for photo-inducedcontraction experiments. Experiment was carried out as follows:

actuated isomer merocyanine (MC), protonated isomer (MC−H+) and bidentate

1394 R. Byrne et al. / Biosensors and Bioelectronics 26 (2010) 1392–1398

F -meth1 ds trihd

ssSas(owo

2

ft(piTwut1twsssyapafsc

dd(et5smas

ig. 1. (Left) Photo-responsive gel consists of poly(N-isopropylacrlamide) [Y], N,N00:10:1, respectively. (Right) Cation and anions that make up the three ionic liquiodecylbenzenesulfonate [dbsa]− , and dicyanoamide [dca]− .

ample of photo-responsive ionogel was placed on glass slide,ample was then covered in deionised water to avoid dehydration.ample size measurements were recorded while ionogel wasllowed to equilibrate for 90 s under UV light (360 nm), sampleize remained constant throughout UV exposure. Green light500 nm) irradiated sample after UV exposure for further 180 s,bservable changes in sample sizes were recorded. This procedureas repeated three times on each photo-responsive ionogel and

n the control ionogel.

.2. Synthesis and characterisation of BSP-PMMA-1 and -2

Synthesis and polymerization of BSP initiator procedure can beound in supplementary information. Molecular weight analysis ofhe polymers was carried out by Size Exclusion ChromatographySEC) with THF as an eluent using an Agilent 1200 series isocraticump (flow rate 1 ml/min) and an Agilent 1200 series refractive

ndex detector at 35 ◦C calibrated with poly(methyl methacrylate).wo PLGel 5 �m Mixed-C (300 mm × 7.5 mm) columns at 40 ◦Cere used. Injections were done by an Agilent 1200 autosamplersing a 50 �l injection volume. BSP-PMMA-1 and -2 were charac-erised using Cary 50 UV–vis spectrophotometer. Approximately,× 10−4 M solutions of each polymer were prepared in acetoni-

rile. For the protonation experiments, solutions of the polymersere kept in the dark for 2 h before 100 �l of 1 × 10−2 M aqueous

olution of hydrochloric acid was added to the colourless polymerolution. After 10 s, a strong yellow coloured was observed. Theolution was then exposed to green light (500 nm), after 30 s theellow colour disappeared, leaving a colourless solution. For Co2+

nd Cu2+ complexation experiments, 1 × 10−4 M solutions of theolymer were kept in dark before addition of equal molar equiv-lents of the metal salt (nitrate in both cases). The kinetic rate oformation of the coloured complex was monitored at 25 ◦C. Theolution was then exposed to green light (500 nm), after 30 s theolour disappeared, leaving a colourless solution.

Contact angle measurements were carried out using a FTÅ200ynamic contact angle analyser. Polymeric films were prepared byip coating glass slides into an acetonitrile solution of the polymer5 mg/ml). For the BSP-PMMA-2–Cu2+ complex containing films,qual molar equivalents of the metal salt were added to the solu-ion before dip coating the glass slide. For the control experiment,

mg/ml PMMA was used instead of the BSP-PMMA-2. Polymer

olutions containing the Cu(NO3)2 were allowed to equilibrate forore than 500 s. Slides were then washed with deionised water

nd placed in vacuum oven for 30 min before contact angle mea-urement. This procedure was repeated three times.

ylene-bis(acrylamide) [Z] and protonated benzospiropyran [X] in the molar ratioexyltetradecylphosphonium [P6,6,6,14]+, bis(trifluoromethylsulfonyl)imide [NTf2]− ,

3. Results and discussion

3.1. Photo-responsive poly(N-isopropylacrylamide) (pNIPAAm)ionogels

The photo-responsive ionogel structure is based on the commonpolymer used in smart hydrogels, poly(N-isopropylacrylamide)(pNIPAAm). In water, pNIPAAm gels undergo a volume phase tran-sition at their lower critical solution temperature (LCST), around32 ◦C, which can be substantially affected by modifying the chem-ical structure of the polymer network by copolymerization. It hasalso been reported that the LCST can be influenced by other factorssuch as salt additives and ionic liquids (Ueki and Watanabe, 2008;Xia et al., 2005).

Ionic liquids (ILs) are a class of novel solvents that consist only ofions, making them very different in nature to conventional molec-ular solvents. For example, in some cases ILs possess propertiessuch as negligible vapor pressure, high thermal stabilities, tunableviscosities, high electrochemical windows, and both hydrophobicand hydrophilic character (Fraser and MacFarlane, 2009; Welton,1999). For these reasons, along with the fact that ILs can undergomultiple solvation interactions (Anderson et al., 2002), ILs areattracting the attention of a growing number of scientists and engi-neers for a range of applications (Daibin et al., 2008; Meng et al.,2009; Olah et al., 2005; Pringle et al., 2006; Singh et al., 2009;Vijayaraghavan and MacFarlane, 2004; Welton, 1999). For mostapplications, ILs must be supported in a solid medium, while stillretaining their attractive and unique properties. For further read-ing on recent developments in ILs, a particularly good review byWatanabe is recommended (Ueki and Watanabe, 2008). The phasechange behaviour of pNIPAAm has been studied in several ILs andit has been shown that the anionic structure of the ILs mainly gov-erns the phase behaviour of the polymer in the IL, in particularpNIPAAm in [C2mim][NTf2] exhibits an upper critical solution tem-perature type phase behaviour, which is in complete contrast to thebehaviour of pNIPAAm in aqueous solutions (Ueki and Watanabe,2006).

Sumaru showed that a spiropyran-functionalised pNIPAAmcould undergo photo-sensitive solubility switching (Sumaru et al.,2004b). This formed the basis of a cross-linked pNIPAAm gel func-tionalised with BSP, which showed an ability to optically control thepermeability of a porous membrane (Sumaru et al., 2006). These

smart hydrogel materials do have significant limitations due totheir dependence on the presence of bulk water for optimal perfor-mance, and consequently these materials cannot operate in an openatmosphere for a long duration, or over a wide range of tempera-tures, because of water loss from the polymer network. However,

Bioelectronics 26 (2010) 1392–1398 1395

ttmit

Brngpcpac[bo

taioBsnl1caswtlI

it

Fial

R. Byrne et al. / Biosensors and

he ability of IL-based polymer gels to perform such volume phaseransitions could usher in a new era of environmentally stable poly-

er gels, as the swelling/shrinking behaviour should be possiblen open atmospheres over much broader temperature ranges andimescales, due to the inherent involatility of many ILs.

Bearing this in mind, we have co-polymerised pNIPAAm andSP within several phosphonium based ILs to create hybrid mate-ials for advanced functions. Apart from the advantage of ILs beingon-volatile, another advantage of impregnating photo-responsiveels with ILs over aqueous media is the possibility to tailor theroperties of the gels using different combinations of anions andations (polarity, viscosity, etc.). Therefore, we can prepare manyhoto-responsive gels with specific properties by substituting theppropriate IL. Three photo-responsive ionogels were preparedontaining three different phosphonium based ILs: [P6,6,6,14][NTf2],P6,6,6,14][dbsa], and [P6,6,6,14][dca] (see Fig. 1). Due to the hydropho-icity of the phosphonium ILs, negligible leaching of the IL wasbserved after 1 week of soaking in deionised water.

The ionogels were first characterised by vibrational spec-roscopy to verify the presence of the ionic liquids within the gelfter 1 week in deionised water. Due to the low concentration of BSPn the ionogel, no vibrational bands were found in either the Ramanr IR spectrum. Fig. 2 shows the Raman spectrum of [P6,6,6,14][dca]-SP ionogel and its individual components. This spectrum clearlyhows the incorporation of the IL within the gel structure. The sig-ature nitrile (–C N) stretch from the [dca]− anion in the IL is

ocated at 2190 cm−1, this is significantly shifted down field by0–2200 cm−1 when incorporated into the gel matrix. It can belearly seen that polymer chain vibrations within the gel are clearlyffected by the presence of the [P6,6,6,14][dca] The �C O amide Itretch in the polymer (due to the self associative hydrogen bondingithin the gel) located at 1649 cm−1 is unaffected by the introduc-

ion of the IL. Similar observation regarding the �C–N amide stretch

ocated at 1452 cm−1, this is also unaffected by the presence of theL.

The infrared spectrum of the [P6,6,6,14][dbsa]-BSP ionogel and itsndividual components are shown in ESI Fig. 1. It can be clearly seenhat equal contributions form the polymer and IL are present, in fact

ig. 3. (a) First-order kinetic plot of [P6,6,6,14][dca]-BSP ionogel absorbance decay at 440onogel under 500 nm irradiation monitored against time. Both data sets are the average ofter exposure to visible light (t = 150 s). (d) UV–vis spectra of [P6,6,6,14][dca]-BSP ionogel inight for 150 s.

Fig. 2. Raman spectrum of [P6,6,6,14][dca]-BSP ionogel, the gel without ionic liquid(hydrogel)and the IL [P6,6,6,14][dca].

the polymer chain vibrations are clearly affected by the presence ofthe [P6,6,6,14][dbsa]. Amide I (�C O stretch) and II (�N–H stretch) areshifted up field in both instances when the IL is introduced, amide Ifrom 1654 to 1641 cm−1 and amide II from 1543 to 1538 cm−1. The[dbsa]− anion, which is a para-substituted benzenesulfonate has itsown signature vibrational bands including the sulfonate asymmet-ric and symmetric stretching located at �as 1193 and �s 1120 cm−1,respectively. Also, the S–O− stretch has two sharp bands at 1010and 1032 cm−1. The infrared and Raman spectra for all ionogelsand its individual components can be found in ESI Figs. 2–7.

We have characterised the photochromic properties of BSPin ionic liquids previously and shown that the MC isomer �max

can be tuned solvatochromically using ILs (Byrne et al., 2008).We have also reported that some phosphonium ILs have unusualphysico-chemical interactions with the BSP and MC isomers (Byrneet al., 2009, 2010b). The optical properties of the ionogels con-

taining BSP were characterised using UV–vis spectroscopy. ESIFig. 8 shows the corresponding spectra of the three isomers of[P6,6,6,14][dca]-BSP ionogel illustrated in Scheme 1. The BSP iso-mer does not show any significant absorption bands in the visible

nm under visible light irradiation. (b) Percentage contraction of [P6,6,6,14][dca]-BSPf three experiments at 25 ◦C. (c) Images of [P6,6,6,14][dca] ionogel before (t = 0 s) andthe presence of 0.1 mmol aqueous HCl and when irradiated with 500 nm of visible

1396 R. Byrne et al. / Biosensors and Bioelectronics 26 (2010) 1392–1398

Fig. 4. (a) UV–vis spectra of 1 × 10−4 M BSP-PMMA-1 after the addition of 1 × 10−4 M Cu(NO3)2. (Inset) Absorbance change at 510 nm and 434 nm versus time. (b) UV–visspectra of 1 × 10−4 M BSP-PMMA-2 after the addition of 1 × 10−4 M Cu(NO3)2. (Inset) Absorbance change at 510 nm and 434 nm versus time. Measurements taken every 25 si

rannswhiwdakF[tiefct[piamfahgiiWarofgp

3

mptp

n acetonitrile at 25 ◦C.

egion, whereas its photo-isomer MC has an absorption band with�max at 520 nm (irradiation of sample with 254 nm). Upon proto-ation of [P6,6,6,14][dca]-BSP ionogel with 0.1 mmol aqueous HCl, aew absorption band appears at 436 nm representing the MC−H+

pecies. The MC−H+ absorption band disappears when irradiatedith visible light (500 nm) indicating that the MC−H+ speciesas returned to the ring closed BSP state (see Fig. 3(d)). Photo-

somerisation of the MC−H+ to the non-polar BSP isomer is coupledith a contraction in size of the [P6,6,6,14][dca]-BSP ionogel. This isue to the loss of the acidic proton from MC−H+, thus generatingmuch more hydrophobic environment, which follows first-orderinetics, with a rate constant estimated to be 9.2 × 10−3 s−1 (seeig. 3(a)). As a consequence, bulk water is expelled from theP6,6,6,14][dca]-BSP ionogel which causes the physical contraction ofhe material. This photo-responsive contraction of the gel was mon-tored against time under visible irradiation (see Fig. 3(b)). Threexperiments yielded a total contraction average of 27.5% (±6.2%)or [P6,6,6,14][dca]-BSP ionogel. This dramatic photo-responsiveontraction can clearly be seen in Fig. 3(c). Photo-induced con-raction results were also observed for [P6,6,6,14][dbsa]-BSP andP6,6,6,14][NTf2]-BSP, 21.6% (±7.4%) and 19. 6% (±1.7%). We havereviously reported how the ring closing kinetics of the MC to BSP

n certain ILs can be altered by varying the anion, in some cases byn order of magnitude (Byrne et al., 2008, 2009). This ring closingechanism is the basis of the ionogel deformation, therefore dif-

erent anion composition should have an implication in the actualctuation response of the ionogels (Sumaru et al., 2004a, 2006). Itas also been reported that the anionic structure of the IL mainlyoverns the phase transition behaviour of polymers in ILs, in a sim-lar fashion to the chaotropic and kosmotropic behaviour observedn the Hofmeister series (Hofmeister, 1888; Hua, 2006; Ueki and

atanabe, 2008). We believe that a combination of both processesttributes to the variation in degree of contraction, and we are cur-ently investigating the underlying physico-chemical interactionsf the ions with pNIPAAm in more depth. We believe that withurther development these photo-responsive ionogels can be inte-rated as task-specific optical actuators in microfluidic devices forumping and valving (Benito-Lopez et al., 2010).

.2. Chemo-responsive polymers

As the MC isomer of BSP has a phenolate site to which transitionsetal cations can bind, such as Cu2+ and Co2+, and this reversible

rocess can be controlled optically. It allows one to think of con-rolling supramolecular self assemblies using chemo-responsiveolymers, such as BSP-PMMA-1 and -2, as shown in ESI Scheme

1, as the binding of a transition metal ion requires two units of theMC isomer to form the most thermodynamically stable MC2–metalion complex (Byrne et al., 2006; Zakharova et al., 2010). Fries etal. (2010) recently published a chemo-responsive co-polymer ofPMMA and spiropyran methyl methacrylate, they demonstratedthat different metal ions gives rise to unique colorimetric responsesthat are dependent on the amount of spiropyran co-monomer con-tained in the polymer backbone. We have synthesized two PMMApolymers of different molecular weights (9800 and 28,000 g/mol)containing a single BSP unit located at the end of the polymer chain(see ESI Scheme 2). This was achieved by Atom Transfer RadicalPolymerization (ATRP) from a BSP functional ATRP initiator. Eachpolymer chain thus contains only one unit of BSP, and consequently,if the metal ion complex is to form, two polymer chains must cometogether to chelate the metal ion (see ESI Scheme 2).

Fig. 4(a) shows the kinetic monitoring of the BSP-PMMA-1 andCu2+ complex formation in acetonitrile. Upon addition of the Cu2+

ions, we see the emergence of a new absorption band at 510 nm.After 80 s we observe another distinct absorption band located at434 nm. As time progresses to 500 s, the absorption band intensityat 510 nm steadily decreases, whereas the band at 434 nm steadilyrises to become the only absorption band in the visible region. Thisprocess contains an isobestic point at 480 nm. We know from theliterature that the absorption band at 434 nm corresponds to theMC2–Cu2+ complex (Fries et al., 2010; Radu et al., 2007). We havenot encountered the absorption band at 510 nm before, and do notsee this when we do similar Cu2+ experiments with the single BSPmolecule in acetonitrile, but rather it appears in the same regionof the spectrum as the ring-opened MC isomer. Zhou describesthe metal chelation mechanism formation and disassociation asfollows: BSP + MC + Cu2+ → MC2–Cu2+ (Zhou et al., 1995). The MCisomer reacts readily with the Cu2+ ions as it is formed thermallyfrom BSP and this formation process is the rate-determining stepin the reaction sequence, with the Cu2+ ion concentration havinglittle effect on the reaction rate. This proposed mechanism wouldappear to hold true in this instance as we can see from Fig. 4(a)(inset) – if the absorption species at 510 nm is responsible for theMC isomer then it behaves as the rate-determining step. The con-centration of the species at 510 nm builds up to a maximum pointat 80 s, thereafter decreasing as the MC2–Cu2+ species (434 nm)starts to increase more rapidly. As the BSP-PMMA-1 polymer has

a molecular weight of 9800 g/mol, it was decided to polymer-ize a larger polymer containing BSP for comparison. BSP-PMMA-2(28,000 g/mol) due to its larger polymer chain should have a slowerrate of complex formation due to the increased methacrylate chaindensity.

R. Byrne et al. / Biosensors and Bioelectronics 26 (2010) 1392–1398 1397

and t

Ceootgdthpwe

gaTdai2tcppWPoPTwainwu

Fig. 5. Contact angle measurement of water on PMMA and PMMA-BSP-2 films

Fig. 4(b) shows the kinetic monitoring of the BSP-PMMA-2 andu2+ complex formation. Upon addition of the Cu2+ ions, we see themergence of a new absorption band at 500 nm, and after 122 s, webserve another distinct absorption band located at 410 nm. Thisccurs at a significantly longer time (approximately 40%) comparedo the BSP-PMMA-1 and Cu2+ complex formation. As time pro-resses to 500 s, the absorption band intensity at 510 nm steadilyecreases, whereas the band at 434 nm steadily rises to becomehe only absorption band in the visible region. The spectra alsoave a clear isobestic point at 480 nm. This process is completelyhoto-reversible, as when [MC-PMMA-2]2–Cu2+ is irradiated withhite light, the MC photo-isomerizes back to the closed BSP isomer,

jecting the Cu2+ ion (see ESI Fig. 9).The ability to control the properties of a surface has become a

oal of many biomimetic researchers, especially the wettability ofsolid surface, due to its importance in many aspects of nature.

he modification of surfaces with photochromic molecules to pro-uces surfaces with switchable wettability holds much promisend there are some very interesting examples using spiropyrann the literature (Rosario et al., 2002, 2004). When BSP-PMMA-

and Cu(NO3)2 are mixed together we observe the formation ofhe corresponding complex, due to the size of the polymer chainonnected to the BSP fragment, significant rearrangement of theolymer chains must happen, affecting the physical and chemicalroperties of the system as the BSP fragments coordinate the Cu2+.e have investigated the wettability of the BSP-PMMA-2 and [BSP-

MMA-2]2–Cu2+ using the contact angle measurement of a dropletf water. A significant contact angle is observed between the BSP-MMA-2 (77.32◦) and [BSP-PMMA-2]2–Cu2+ (32.12◦) (see Fig. 5).he Cu2+ coordination induces a 45.2◦ change on BSP-PMMA-2,hereas the PMMA polymer with no BSP moiety, we only observe

4.04◦ change upon addition of Cu2+. It is believed that the Cu2+

ons are retained in the BSP-PMMA-2 polymer film due to coordi-ation whereas in the PMMA film, the Cu2+ ions are removed whenashing with deionised water. This change in surface wettabilitytilizing the chelating properties of BSP is an improvement of over

he effect Cu2+ ions have on the polymeric films. Measurements taken at 20 ◦C.

10◦ on the recent work reported (Samanta and Locklin, 2008).

4. Conclusion

Biomimetic switchable materials will play a critical role inthe development of futuristic analytical devices, controlling fluidmovement through integrated soft polymer actuators, and uptakeand release of molecular guests depending on whether the materialis in an active (binding) or inactive (non-binding) state. With cer-tain BSP-functionalised ionogels, actuation and binding behaviourcan be readily controlled using light. We have shown that contrac-tions in these polymers of up to 25% can be triggered using light.In another derivative, the photonic control of uptake and releaseof certain guest ions (Cu2+) have been demonstrated. Clearly, theseearly demonstrations of stimuli–responsive materials are the firststeps in the development of future disruptive technologies that willlead to low-cost reliable biomimetic platforms capable of advancedanalytical functions. All in all, it seems clear that there are excit-ing times ahead in sensor science aligned with stimuli–responsivematerials (Byrne et al., 2010a).

Acknowledgements

We wish to acknowledge support for this research fromCLARITY, funded by Science Foundation Ireland under Grant No.07/CE/I1147. Special thanks to Al Robertson from Cytec Industriesfor supplying ionic liquids. AH wishes to acknowledge finan-cial support from Science Foundation Ireland under Grant No.07/IN.1/B1792.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.bios.2010.07.059.

1 Bioele

R

A

A

B

B

BB

B

BB

B

CC

D

D

D

DF

F

G

H

HI

398 R. Byrne et al. / Biosensors and

eferences

nderson, J.L., Ding, J., Welton, T., Armstrong, D.W., 2002. Journal of the AmericanChemical Society 124 (47), 14247–14254.

ndersson, J., Li, S., Lincoln, P., Andreasson, J., 2008. Journal of the American ChemicalSociety 130, 11836–11837.

enito-Lopez, F., Byrne, R., Raduta, A.M., Vrana, N.E., McGuinness, G., Diamond, D.,2010. Lab on a Chip 10 (2), 195–201.

enito-Lopez, F., Byrne, R., Wu, Y., Nolan, L., Kim, J., Lau, K.T., Wallace, G.G., Diamond,D., 2009. ECS Transactions 19 (6), 199–210.

yrne, R., Benito-Lopez, F., Diamond, D., 2010a. Materials Today 13 (7–8), 16–23.yrne, R., Coleman, S., Fraser, K.J., Raduta, A., MacFarlane, D.R., Diamond, D., 2009.

Physical Chemistry Chemical Physics 11 (33), 7286–7291.yrne, R., Coleman, S., Gallagher, S., Diamond, D., 2010b. Physical Chemistry Chem-

ical Physics 12 (8), 1895–1904.yrne, R., Diamond, D., 2006. Nature Materials 5 (6), 421–424.yrne, R., Fraser, K.J., Izgorodina, E., MacFarlane, D.R., Forsyth, M., Diamond, D., 2008.

Physical Chemistry Chemical Physics 10 (38), 5919–5924.yrne, R.J., Stitzel, S.E., Diamond, D., 2006. Journal of Materials Chemistry 16 (14),

1332–1337.leary, J., Slater, C., McGraw, C., Diamond, D., 2008. IEEE Sensors Journal 8, 508–515.rano, J.C., Flood, T., Knowles, D., Kumar, A., Van Gemert, B., 1996. Pure and Applied

Chemistry 68 (7), 1395–1398.aibin, K., Satoshi, U., Robin, H.-B., Shaik, M.Z., Michael, G., 2008. Angewandte

Chemie International Edition 47 (10), 1923–1927.iamond, D., 2008. Proceedings of the International Conference ‘International Per-

spectives on Environmental Nanotechnology’ 1, October 7–9, 2008, Chicago, pp.211–221.

iamond, D., Coyle, S., Scarmagnani, S., Hayes, J., 2008a. Chemical Reviews 108 (2),652–679.

iamond, D., Lau, K.T., Brady, S., Cleary, J., 2008b. Talanta 75 (3), 606–612.raser, K.J., MacFarlane, D.R., 2009. Australian Journal of Chemistry 62 (4), 309–

321.ries, K.H., Driskell, J.D., Samanta, S., Locklin, J., 2010. Analytical Chemistry 82 (8),

3306–3314.orner, H., Chibisov, A.K., 1998. Journal of the Chemical Society: Faraday Transac-

tions 94 (17), 2557–2564.ofmeister, F., 1888. Archiv fur experimentelle Pathologie und Pharmakologie 24,

247–260.ua, Z., 2006. Journal of Chemical Technology and Biotechnology 81 (6), 877–891.

pe, B.I., Mahima, S., Thomas, K.G., 2003. Journal of the American Chemical Society125 (24), 7174–7175.

ctronics 26 (2010) 1392–1398

Meng, Y., Pino, V.N., Anderson, J.L., 2009. Analytical Chemistry 81 (16), 7107–7112.Minkin, V.I., 2004. Chemical Reviews (Washington, DC, United States) 104 (5),

2751–2776.Olah, G.A., Mathew, T., Goeppert, A., Toeroek, B., Bucsi, I., Li, X.-Y., Wang, Q., Marinez,

E.R., Batamack, P., Aniszfeld, R., Prakash, G.K.S., 2005. Journal of the AmericanChemical Society 127 (16), 5964–5969.

Pringle, J.M., Forsyth, M., Wallace, G.G., MacFarlane, D.R., 2006. Macromolecules 39(21), 7193–7195.

Radu, A., Byrne, R., Alhashimy, N., Fusaro, M., Scarmagnani, S., Diamond, D., 2009.Journal of Photochemistry and Photobiology A: Chemistry 206 (2–3), 109–115.

Radu, A., Scarmagnani, S., Byrne, R., Slater, C., Lau, K.T., Diamond, D., 2007. Journalof Physics D: Applied Physics 40 (23), 7238–7244.

Ramirez-Garcia, S., Diamond, D., 2007. Journal of Intelligent Material Systems andStructures 18 (2), 159–164.

Rosario, R., Gust, D., Garcia, A.A., Hayes, M., Taraci, J.L., Clement, T., Dailey, J.W.,Picraux, S.T., 2004. Journal of Physical Chemistry B 108 (34), 12640–12642.

Rosario, R., Gust, D., Hayes, M., Jahnke, F., Springer, J., Garcia, A.A., 2002. Langmuir18 (21), 8062–8069.

Samanta, S., Locklin, J., 2008. Langmuir 24 (17), 9558–9565.Scarmagnani, S., Slater, C., Benito-Lopez, F., Diamond, D., Walsh, Z., Paull, B., Macka,

M., 2010. International Journal of Nanomanufacturing 5, 38–52.Singh, P.K., Kim, K.-W., Rhee, H.-W., 2009. Synthetic Metals 159 (15–16), 1538–1541.Sumaru, K., Kameda, M., Kanamori, T., Shinbo, T., 2004a. Macromolecules 37 (13),

4949–4955.Sumaru, K., Kameda, M., Kanamori, T., Shinbo, T., 2004b. Macromolecules 37 (21),

7854–7856.Sumaru, K., Ohi, K., Takagi, T., Kanamori, T., Shinbo, T., 2006. Langmuir 22 (9),

4353–4356.Szilagyi, A., Sumaru, K., Sugiura, S., Takagi, T., Shinbo, T., Zrinyi, M., Kanamori, T.,

2007. Chemistry of Materials 19 (11), 2730–2732.Ueki, T., Watanabe, M., 2006. Chemistry Letters 35, 964.Ueki, T., Watanabe, M., 2008. Macromolecules 41 (11), 3739–3749.Vijayaraghavan, R., MacFarlane, D.R., 2004. Chemical Communications (Cambridge,

United Kingdom) 6, 700–701.Welton, T., 1999. Chemical Reviews (Washington, DC) 99 (8), 2071–2083.Xia, Y., Yin, X., Burke, N.A.D., Stover, H.D.H., 2005. Macromolecules 38 (14),

5937–5943.Zakharova, M.I., Coudret, C., Pimienta, V., Micheau, J.C., Delbaere, S., Vermeersch, G.,

Metelitsa, A.V., Voloshin, N., Minkin, V.I., 2010. Photochemical and Photobiolog-ical Sciences 9 (2), 199–207.

Zhou, J.-W., Li, Y.-T., Song, X.-Q., 1995. Journal of Photochemistry and PhotobiologyA: Chemistry 87 (1), 37–42.