Photonic modulation of surface properties: a novel concept in chemical sensing This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2007 J. Phys. D: Appl. Phys. 40 7238 (http://iopscience.iop.org/0022-3727/40/23/S06) Download details: IP Address: 138.73.1.36 The article was downloaded on 14/05/2013 at 10:14 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Photonic modulation of surface properties: a novel concept in chemical sensing
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2007 J. Phys. D: Appl. Phys. 40 7238
(http://iopscience.iop.org/0022-3727/40/23/S06)
Download details:
IP Address: 138.73.1.36
The article was downloaded on 14/05/2013 at 10:14
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 40 (2007) 7238–7244 doi:10.1088/0022-3727/40/23/S06
Photonic modulation of surfaceproperties: a novel concept in chemicalsensingAleksandar Radu, Silvia Scarmagnani, Robert Byrne,Conor Slater, King Tong Lau and Dermot Diamond1
Adaptive Sensors Group, National Centre for Sensor Research, School of Chemical Sciences,Dublin City University, Dublin 9, Ireland
Received 5 April 2007, in final form 5 June 2007Published 16 November 2007Online at stacks.iop.org/JPhysD/40/7238
AbstractIn this paper we discuss the challenges and opportunities afforded bysurface-based photoswitchable chemical sensors. We focus on spiropyransas it is a well-studied system that can be photonically switched between twostates, only one of which exhibits ion-binding behaviour. Surfaceimmobilization and protection within a polymer matrix is identified as aroute that can successfully address the need for a localized hydrophobicenvironment in which a user can maintain control over thespiropyran-merocyanine equilibrium and at the same time improvephoto-fatigue resistance. Furthermore, we discuss the excellent potential oflight emitting diodes as light sources and detectors for photoswitchingbetween the states of spiropyran and measurement of bound species. Asimple, low-cost, low-power experimental setup provides spatial andtemporal control of surface illumination and surface binding. This, coupledwith low irradiance, is shown to generate significant improvement in fatigueresistance of spiropyrans-modified films, and may prove to be an importantstep towards the realization of chemical sensors that can be deployed inlarge-scale wireless chemical sensor networks.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
In modern society, exchange and circulation of information isof utmost importance. Digitalized modes of communicationssuch as the internet, e-mail or mobile phones help toconnect billions of people, places and objects. However, theinformation revolution should not stop there. The next stageof development is to establish communication between thedigital and molecular worlds, provided appropriate platforms(sensors) can be provided to make this happen. Environmentalmonitoring, remote sensing and monitoring personal healthand wellness are application areas that will significantly benefitfrom such a development. However, the realization of this
1 Author to whom any correspondence should be addressed.
vision requires large numbers of chemo/bio-sensors to beassembled into sensor networks and sensor communities.
Research in the area of wireless sensor networks(WSNs) is currently very extensive and focused on hardware,communication protocols and power management. Recentprogress has been outlined in a very interesting review byQi et al [1]. Interestingly, despite the exciting prospectof merging chemo/bio worlds with the digital world, thescientific literature describing wireless chemical sensornetworks (WCSNs) is still scarce. At present, demonstratordeployments of WSNs entirely focus on physical transducerslike temperature [2, 3], pressure [4] or resistance/conductanceutilized for measuring humidity or the concentration ofgases such as CO2 in room environments [5, 6] as wellas hydrogen gas [7]. Some examples of wireless remote
environmental monitoring involve colorimetric monitoringof chemical plumes [8] and electrochemical detection ofcadmium [9] and pH [10]. However, initial results in the twolater reports were reported from a single RF wireless node only.The inherent difficulties associated with field deployment ofautonomous chemo/bio sensors are largely responsible for thecurrent lack of real examples of WCSN deployments.
Unfortunately, simple chemo/bio sensors that aresufficiently selective and sensitive, and also capableof operating autonomously for long periods withoutsignificant drift are not yet available for most chemicallet alone biological species. Processes like fouling,decomposition or leaching significantly influence chemo/bio-sensor operating characteristics (sensitivity, selectivity,baseline) and consequently devices must be calibrated,meaning that the sensing surface should periodically beremoved from the sample and re-characterized. Therefore,accurate in situ chemical monitoring typically involvesrelatively complex instrumentation that incorporates pumps,fluidic manifolds, detectors and reagent reservoirs [11].
Clearly, a completely different approach to chemicalsensing is necessary if inexpensive reliable devices are to berealized that can function autonomously for extended periodsof time. In our research group, we believe that developmentof adaptive materials, and in particular, control of bindingprocesses at surfaces, may provide a route to the ultimaterealization of these devices. Adaptive materials have theability to modify their behaviour or characteristics in responseto external stimuli, for example through photochemical orelectrochemical changes in the molecular structure. If onlyone form has binding abilities, and others are passive, inprinciple a material can be switched ‘on’ and ‘off’ usingexternal stimuli. In the ‘off’ state, binding sites are hidden,hence the active surface is deactivated whereas in the ‘on’ state,the surface binding sites are activated and available. Hence itis possible to envisage a surface that can be switched betweenpassive and active forms using light, thereby addressing the socalled ‘chemical sensing paradox’ of combining the absoluterequirement of an active surface to bind the target species andgenerate the signal, with passive behaviour that minimizeschanges in the surface binding characteristics over time, inorder to reduce the need for calibration [12].
The aim of this paper is to discuss the challenges andopportunities associated with adaptive materials targeted atmetal ion sensing. We describe the development of a surface-based sensor exhibiting light-modulated Cu2+-binding wherethe user has complete control over the binding event, as wellas the passivation and regeneration of the sensing surfacefor subsequent measurements. Furthermore, we address theproblem of the simplicity of sensor construction and powerconsumption by utilizing a recently developed photometerbased on LEDs as a simple, inexpensive light source anddetector [13].
methacrylic acid 99% (redistilled), omega,omega-dimethoxy-omegaphenylacetophenone (DMPA), ethanol 96%, werepurchased from Aldrich. 0.5 mm polymethyl methacry-late (PMMA) was purchased from Goodfellow. 1-ethyl-3-(3- dimethylamino propyl) carbodiimide hydrochloride(EDC) and metal nitrates (purum grade) were purchasedfrom Fluka Scientific. The requisite spiropyran han-dle, 1′–(3-carboxypropyl)-3′,3′-dimethyl-6-nitrospiro[2H-1]-benzopyran-2,2′-indoline was produced in a three-step se-quence described elsewhere [14]. Aqueous solutions weremade with deionized Nanopure water (18 M�cm). The UVirradiation source BONDwand UV-365 nm was obtained fromthe Electrolite Corporation. The white light source wasobtained from Chiu Technical Corporation. Spectra wererecorded on a UV–Vis-NIR Perkin–Elmer Lambda 900 spec-trometer. Reflectance measurements were obtained using anin-house developed LED array instrument that is describedelsewhere [15]. The same device was used to illuminate thesurface for preset exposure times as described below.
Absorption spectra measurements on spiropyran-modifiedPMMA-based polymer films. 1,8-diaminooctane was co-valently attached to the surface of PMMA film as a linkergroup of the requisite length for the carboxylated spiropyranderivative as described elsewhere [16]. Stock solutions ofmetal salts in water were prepared and stored in the dark.Prior to each measurement, a small piece of the film ofapproximately 1 × 0.5 cm was cut out, immersed in deionizedwater, exposed to visible light for 1 min and the spectrum ofthe closed form (SP) was recorded. The reference spectrumof the opened form (MC) was obtained after the same filmwas exposed to UV light for 1 minute. The active film (MC)was then exposed to metal ion solution and new spectra wererecorded. Measurements using the LED array were obtainedon a dry, unexposed film upon illumination with a green LEDfor 1 min followed by illumination with the UV LED for thesame amount of time.
3. Discussion
Spiropyran has the ability to switch from an uncharged,colourless, passive spiropyran (SP) form, to a zwitterionic,highly coloured active merocyanine (MC) form uponillumination with UV light [17–19]. The phenolate groupin the MC form has ion-binding characteristics, particularlyfor heavy metals [20]. More importantly, if this complexis illuminated with green light, under certain conditions, theguest metal ion is ejected and MC rearranges back to SP asdepicted in figure 1. The spectral characteristics of these threestates are depicted in figure 2. The passive form of SP inethanol solution is colourless with minimal absorption in thevisible region. On the other hand, the MC form has a deeppurple colour due to strong absorption in the visible regioncentred around 560 arising from the formation of an extensivedelocalized �-system. The case of Cu2+ binding is typicalof a number of heavy metals, in that the Cu2+-MC complexgives rise to a new absorbance band at about 430 nm followedby simultaneous decrease in absorbance at 560 nm as shownin figure 2 (i.e. the colour changes enabling the SP, free MCand MC-ion complex states to be easily distinguished visually
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Figure 1. Reversible switching of SP to MC to MC-ion complexregulated by UV and visible light.
Figure 2. Absorbance spectra of surface film in SP form, the MCform (after irradiation with UV light) and MC-Cu2+ complexobtained upon exposure of the film to 9.1 × 10−4 M Cu2+.
or spectroscopically). Clearly, the prospect of utilization ofthis molecule in a user-controlled, self-indicating, heavy metalsensor is very exciting. Quantitative ejection of bound metalion implies potential full recovery of sensor thereby potentiallyreducing the need for calibration and significantly simplifyingthe device and its associated experimental setup. Sensors withsuch characteristics represent interesting candidates for futurelow-cost yet sophisticated chemo-sensing devices.
However, there are a number of challenges that must beaddressed before truly functioning devices can be realized withproperties suitable for use in real deployments:
• Insufficiently strong binding of metal ions. The phenolategroup in MC is the sole binding centre and it doesnot exhibit overly strong binding of metal ions therebyexpressing little selectivity [21]. This issue is usuallyaddressed by synthesizing various derivatives [22, 23]which strengthen the ion-binding properties, for examplethrough placement of the metal ion coordinating group(e.g. methoxy group) in the ortho-position to thepyran oxygen, thus facilitating cooperative binding bythe oxygen atoms [24, 25]. Other functional groups
include CO2Me [26], SO3 [27], azacrown ethers [28] andcalixarenes [29].
• Polar environments favour spontaneous transformation ofSP into MC thereby influencing the degree of control theuser can exert. Solvents such as water and alcohols canthermally stabilize the MC-metal ion complexes, therebyinhibiting the reversibility of the system. In such cases,visible/green light may not be sufficient to eject the metaland return the metal-MC complex into the passive SPform [30].
• Photodegradation of MC is regarded as a major limitationby many researchers as it can lead to a gradual reductionin sensitivity and eventual device failure.
• The necessity for a very simple, low-power and costefficient experimental setup is another important issuewith respect to eventual scale-up in deployment to largenumbers of sensors in WCSNs.
The following sections will address the latter three issues inmore detail.
3.1. SP-based photoswitchable surfaces
In order to maintain user-controlled switching between theSP and MC forms it is beneficial to maintain non-polarlocalized conditions. Recent years have witnessed a steadyincrease in publications describing the potential use of SPand its derivatives for heavy metals sensing, conceivedthrough incorporation of SP into hydrophobic liquid phases.Attempts have ranged from the use of hydrophobic solventssuch as acetonitrile [23], plasticizers like dicapryl phthalate[31] or polymers such as polymethylmethacrylate (PMMA),styrene-buthadiene-styrene (SBS) [32,33] and plasticized PVCmembranes [22].
In contrast, we have focused our attention on surface im-mobilized SP to generate adaptive, switchable surfaces as thissolves very significant issues with liquid phase experimen-tal design arising from solvent compatibility for the ‘host’ SPderivatives (lipophilic environment preferred) and the guestions (water environment for almost all applications). Fab-rication of thin films with switchable molecules bound to asolid substrate is a very effective way to tune a wide range ofsurface properties [34]. For example, covalent immobiliza-tion of a spiropyran on an appropriate surface can maintain itwithin a localized hydrophobic environment while simultane-ously ensuring that the charged MC does not leach out polarsample environment. The immobilization strategy should al-low enough flexibility for the spiropyran molecules to photoi-somerize between the active and passive forms, and associatewith neighbouring SP molecules in order to form sandwichcomplexes. The schematic in figure 3 depicts SP covalentlyimmobilized on a polymer surface and its mode of interactionwith divalent metal transition metal provided that the expectedstoichiometry is 1 : 2 [16]. One strategy is to attach the SPto the receiving polymer surface through a long chain alkyllinker group to provide the required degree of flexibility forphotoisomerization and complexation. SP-functionalized sur-faces as shown in figure 3 are particularly advantageous dueto their simplicity of preparation and utilization. For example,COOH derivativatized SP can be easily attached to -COOH-functionalized PMMA via diaminoalkyl linkers in a simple
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Figure 3. Representation of photo-controlled ion binding at a SP-modified surface. Bottom left—colourless SP-immobilized surface.Top—upon illumination with UV light, the surface becomes active and purple coloured due to the transformation of SP to MC. Byillumination of this surface with green light MC is switched back to SP. Bottom right—exposure of activated MC surface to an aqueoussolution of divalent metal ions (Me2+) leads to formation of the MC-Me2+ complex and the colour changes to pink. Irradiation of this surfacewith green light leads to break up of the complex, expulsion of the guest ion and reversion to the original passive, colourless SP form.(Colour online.)
Figure 4. UV–Vis spectra of SP-2, SP-4, SP-6 and SP-8 derivativescovalently attached to the surface of PMMA film upon exposure to1 × 10−2 M CoCl2. Effective binding is not observed until the tetherlength is 8-methylene groups long (SP-8).
two-step process using very mild experimental conditions [16].The tether length is easily controlled through selection of theappropriate diamino linker group. Figure 4 shows UV–VISspectra of merocyanine forms using tether lengths of 2, 4, 6and 8 carbons (SP-2, SP-4, SP-6 and SP-8) after exposure to10−2 M CoCl2. The spectra are given relative to the pure MCform in order to illustrate more clearly the formation of thecomplex. If the complex is indeed formed, the absorbance at430 nm, identified as the wavelength of the absorption bandof Co2+-MC complex, increases while in the same time, theabsorbance at 560 nm (wavelength of maximum for free MCform) decreases. These two processes are depicted as positive
and negative absorbencies, respectively using spectral subtrac-tion of the reference MC spectrum. While there is very littlechange in absorbance in the cases of SP-2, SP-4 and SP-6, thecomplexation is much more effective with SP-8 indicating thattether length of at least 8 carbons is required to provide enoughflexibility for effective formation of the Co2+-MC complex.
3.2. Reversible metal ion complexation
Demonstration of photo-control of fully reversible ion bindingin multiple switching cycles is probably the most importantrequirement for photoswitchable sensors. However, it isknown that SP undergoes photo degradation over time. Whilethe mechanism of photodegradation is not yet fully understood,it is believed to involve triplet excited spiropyran and tripletexcited oxygen [35–37]. Photodegradation is probably themain limiting factor for the application of spiropyran inreversible sensing devices and its suppression has been thefocus of many studies, for example by functionalizing ofspiropyran with antioxidant group or phosphoryl group [38]or sealing within an airtight environment [39].
Figure 5 depicts the first example of photoreversiblecopper binding to an SP-modified polymer surface. A smallpiece of SP-modified film was immersed in water, irradiatedwith visible light (1 min) and its spectrum recorded (opencircles). The same film was then irradiated with UV light(1 min) and the spectrum recorded (full circles). Waterwas then replaced with 9.1 × 10−4 M Cu(NO3)2 and thespectrum recorded 5 min after the exposure to the coppersolution (semi-filled circles). The copper solution was thenreplaced with water and the cycle repeated six times. Perhapssurprisingly, almost no degradation of signal was observed.More importantly, excellent reproducibility of the signal over
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Figure 5. Absorbance change during reversible andphotoswitchable detection of Cu2+ at 431 nm (——) and 560 nm(----) using UV (BONDwand) and white light sources.
Figure 6. Left—picture of LED array. Right—scheme depictingdiffuse reflectance measurement.
all stages of the cycle (SP to MC to MC-complex to SP) wasobtained enabling each state to be clearly differentiated.
In an earlier study a very interesting effect was observedshowing significantly reduced photodegradation of spiropyranin a screen-printed sheet containing 1 wt% in PMMA of SPmodified by attachment of a C18-alkyl chain to the nitrogenatom of the indoline group compared with a spin-coatedPMMA film containing 10 wt% of the same SP-derivative[39]. The lower concentration of SP in the spin-coated filmsrather than irradiation times was identified as the dominantfactor affecting fatigue resistance due to reduction of MCaggregation [39]. Surface immobilization of SP allows controlover behaviour like aggregation of the MC form and it mightbe an important strategy for obtaining stable photoswitchableand reversible sensing platforms.
3.3. Low-cost, low-power detector system
Recent advances in the development of LEDs in terms ofincreased spectral coverage and power output has stimulatedresearch into their use in low-cost, low-power optical sensingdevices [40]. LEDs are used as light sources in a widevariety of application fields, but they are very rarely used aslight detectors. Recently, we reported miniature, low-cost,low-power sensing systems employing LEDs as both lightsources and detectors [13, 15, 41, 42]. Typically, a reverse-biased low band-gap IR LED is used as the detector. Rather
Figure 7. Multiple optical switching and monitoring of a singlelocation on a SP-modified PMMA film using LED cluster. Despitean overall reduction of ∼25% (431 nm) and ∼14% (560 nm) in thesignal, a remarkable number of cycles can be achieved.
than measuring very small currents generated when photonshit the detector LED, the time required for the dischargeof small parasitic capacitance is measured within a fixedintegration time-cycle. Figure 6 (left) shows the pictureof the device wherein a range of exciter LEDs of differentwavelengths are placed around the detector LED. figure 6(right) demonstrates the operating principle. An exciter LEDilluminates the target and the resulting diffuse reflectance fromthe surface is measured by the detector LED. For the SP–MC system, we use UV, and blue (425 nm), green (560 nm)and red (660 nm) LEDs to illuminate the surface. The UVand green LEDs are used to photoswitch between the SPand MC forms whereas the blue and green LEDs are usedto monitor complex formation (λmax ∼ 430 nm) and MCformation (λmax ∼ 560 nm), while the red LED provides ameans for reference measurements since no significant spectralchanges occur in this region. For switching between the SPand MC forms, illumination for 60 s with UV and green LEDsis used, while the colour measurements are performed using asequence of 10 pulses of 0.5 s duration with the blue, green andred LEDs in sequence, and calculating the mean signal. Theshort pulsed illumination is particularly important at 560 nmand is a reasonable compromise that allows reproduciblecolorimetric measurements with minimal disruption of the SP–MC equilibrium.
Figure 7 demonstrates the utilization of the LED array forswitching between SP and MC and measuring the intensity ofthe colour at potentially important wavelengths over multiplecycles. A piece of approximately 1 × 1 cm of SP-modifiedPMMA film was cut and placed approximately 3 cm below theLEDs and the measuring protocol described above engaged.Figure 7 shows the response of the detector LED when thesurface is illuminated with the blue LED (425 nm) or greenLED (560 nm) over a large number of repeat switching cycles.It is striking that even after more than 370 cycles no substantialphotodegradation is observed. Therefore, switching to low-power sources (approximate irradiance of LEDs is 1 mW cm−2
[43]) can clearly help with the realization of surfaces that canbe repeatedly switched between active (sensing) and passive(non-sensing) states.
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4. Implications
Photoswitchable molecules like SP have the potential to playa key role in the development of next generation chemicalsensors. Despite the limited selectivity exhibited by the parentSP it is possible to address this issue by, for example, usingchemometric techniques [31] or more importantly, through thesynthesis of custom designed SP molecules with enhancedbinding, similar to that available from currently existingionophores [44]. Alternatively, binding of a range of potentialguests is not always an issue, for example, in applicationsinvolving separations or in active filtering of samples.
Surface immobilization is another very important strategyfor the development of functioning photoswitchable sensingdevices. Effective immobilization on low cost materialsunder mild experimental conditions ensures simple sensorpreparation. Interestingly, despite the substantial numberof literature reports on ion binding with SP-derivativesincorporated within polymers, there are very few examplesof their use in polar solvents such as water. Recently,Suzuki et al [45] demonstrated complexation of Pb2+ bya copolymer consisting of spiropyran methacrylate andperfluorooctylhydroxy methacrylate (SPMA-FHMA) from awater-CH3OH mixture (8 : 2) [45]. However, they haveobserved the incomplete ejection of Pb2+ which was attributedto insufficient exposure (1 min) of the polymer to visible light.Despite demonstrated reversibility in other systems, significantphotodegradation impeded longer lifetime of such sensors.
On the other hand, our data presented in thispaper suggested that excellent reversibility and reducedphotodegradation could be achieved. We have shown that it ispossible to successfully covalently immobilize SP and utilizeobtained polymer film for reversible detection of metal ions.Hence the surface can be maintained in its passive form untilthe need for detection arises, whereupon it is converted to theactive form using UV light, and the presence of metal ionsdetected using simple reflectance measurements with LEDs.A green LED can then be used to expel the ions and return theMC to the passive SP form.
According to Matsushima et al [39] immobilization ofSP to a polymer backbone seems to reduce aggregation ofMC thereby reducing the photodegradation. We speculate thiscould be the reason why we observe reduced photodegradationin our work. Similar results have been reportedly achieved byimmobilization of spiropyran within hydrophobic nanoparti-cles [30] and polymerized crystalline colloidal arrays [46].
The use of surface immobilized SP also allows the user todefine the area that is exposed to a LED source (e.g. using asimple mask). An example is shown in figure 8. The SP film isirradiated with UV light through a mask (depicting the structureof merocyanine) activating only the parts of the film exposed(figure 8(a)). On the other hand, in figure 8(b), the entire filmis initially activated to MC (purple) and the surface is thenirradiated with white light through a mask, deactivating onlythe area in the image of spiropyran. Clearly, the user can easilyaddress multiple locations on the surface for confirmation ofparallel measurements, or to extend the useful lifetime of thedevice through movement to a new location after a number ofmeasurements have been performed at a particular spot.
Finally, the sensing platform as described here couldbe utilized in a variety of other application fields. For
Figure 8. Spatial control of SP-modified film. (a) Film in passivemode irradiated with UV through the mask. (b) Film in active modeirradiated with white light through the mask.
example, it is known that the zwitterionic MC can interactwith zwitterionic amino acid molecules [47–49]. For example,it has been demonstrated that MC exposed to a solution oftyrosine does not revert to the SP form even after 100 h ofstorage in the dark as opposed to only several hours in theabsence of tyrosine [12]. This implies that these systems mayprovide routes to the controlled preconcentration, transport andthe delivery of amino acids or more complex biomolecules atparticular locations.
Acknowledgments
This work was financially supported by the Science Foundationof Ireland (grant 03/IN.3/1361) and the EnvironmentalProtection Agency (Grant 2004-RS-AIC-M4).
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