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Sensors and Actuators B 206 (2015) 74–83

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

mproving the selectivity of a free base tetraphenylporphyrin basedas sensor for NO2 and carboxylic acid vapors

. Evyapana,b,∗, A.D.F. Dunbarb

Department of Physics, University of Balikesir, Balikesir 10145, TurkeyChemical and Biological Engineering, University of Sheffield, Mappin Building S1 3JD, UK

r t i c l e i n f o

rticle history:eceived 8 April 2014eceived in revised form 15 August 2014ccepted 10 September 2014vailable online 18 September 2014

eywords:angmuir-SchaeferorphyrinMMAalixarenerganic sensor

a b s t r a c t

Langmuir-Schaefer (LS) films of a free base porphyrin 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine (EHO) are used in conjunction with a PMMA and carboxylic acid substitutedcalix[8]arene barrier layers deposited on top of the porphyrin films to enhance the selectivity of thisclass of vapor sensor. Initially, the monolayer properties of EHO without a barrier layer are investigatedby reporting �-A isotherms and the effect of transfer pressure on the gas sensing mechanism is stud-ied. Optimal surface pressures for transferring the EHO films onto glass substrates and the number oflayers to be transferred for gas sensing applications are presented. The prospect of tuning these vaporsensors in order to differentiate between large and small analyte molecules is investigated by applyingoverlying barrier layers which consists of a PMMA:calix[8]arene mixtures. EHO LS films with differentbarrier layers are exposed to NO2, acetic acid, butyric acid and hexanoic acid vapors. EHO films withoutany barrier layer are found to be highly sensitive to all four analytes studied; but the respective response

speed depends on the vapor being sensed. Upon addition of the PMMA:calix[8]arene barrier layers ontop of the EHO films this selectivity is further enhanced. The PMMA:calix[8]arene barrier layer acts as asize selective layer and the degree of selectivity is determined by the relative quantity of calix[8]arenemolecules in the selective layer. For all the sensors tested recovery is achieved by gently heating, and thereversibility of sensor material is shown to be very good.

. Introduction

Research on gas sensors has become important during the lastew decades because monitoring air quality has become an increas-ngly critical issue [1,2]. Organic materials are attractive as gasensor materials because of their low cost, easily tunable chem-cal structures, and ease of processing in solution to make thinolid state films for sensor device applications. There are a wideange of thin film preparation techniques which have been used torepare solid state films of organic materials from solution suchs Langmuir-Blodgett deposition [3], Langmuir-Schaefer deposi-ion [4], spin coating [5] and self-assembly [6]. Several differentrganic materials have been widely used as the active sensing mate-

ial: such as porphyrins [7,8], polymers [9], calixarenes [10] andhthalocyanines [11]. The porphyrins are of particular relevance

n relation to this work. They have strongly conjugated �-electrons

∗ Corresponding author at: University of Balikesir, Faculty of Art & Science, Depart-ent of Physics 10145, Cagis, Balikesir, Turkey. Tel.: +90 266 6121000;

ax: +90 266 6121215.E-mail address: mevyapan@gmail.com (M. Evyapan).

ttp://dx.doi.org/10.1016/j.snb.2014.09.023925-4005/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

[12] which result in interesting optical properties [13,14] includinghigh optical absorption coefficient in the UV–visible region [15]. Itis well know that exposure to certain gas molecules causes someremarkable electrical and optical changes to the porphyrins [16,17].Because of these specific properties, porphyrins are promisingsensing elements for optical gas sensing applications.

An ideal sensor device would simultaneously demonstrate highsensitivity, have a fast response, be recoverable and show selectiv-ity between analytes. Much of the research reported on porphyrinbased gas sensors is related to improvements in the sensitivity ofsensor devices. Improving chemical or structural characteristics ofthese materials has led to significant improvements in the sensi-tivity of this classification of gas sensors. Porphyrin based sensorshave been shown to be very sensitive to lots of vapors, they showfast response characteristics and also their recovery times are fast.However, also of considerable importance is the selectivity of thesensors. Porphyrins can be used either in the free base form [18,19]or with the two central hydrogen atoms replaced by a variety of

metal ions thereby significantly increasing the variety of analytesthat can be detected [5,20,21]. This approach allows the issue ofselectivity can be addressed to some extent when using porphyrinbased gas sensors. Different metallo-porphyrins can be used to

M. Evyapan, A.D.F. Dunbar / Sensors and Actuators B 206 (2015) 74–83 75

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ig. 1. Chemical structures of (a) 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)pholy(methyl methacrylate) (PMMA).

etect volatile organic gases with specific functional groups. How-ver, this does not help to differentiate between the wide varietiesf volatile organic gases which may all contain the same target func-ional group. The color change which is used to report the presencef the analyte gas in porphyrin sensors is sensitive to the presencef a specific functional group and therefore it is difficult to use thisechanism to differentiate between different molecules that con-

ain that group. In order to develop a sensor that is able to differenti-te between different analytes which contain the same functionalroups with a porphyrin based sensor system one approach is toncorporate a separate selection mechanism, which acts upon theas prior to its interaction with the porphyrin molecules.

In this work a simple size selection mechanism based onteric exclusion of larger molecules by incorporating a thinembrane on top of the optically active porphyrin film is

resented. Initially, the sensing characteristics of free base por-hyrin films upon exposure to four chemical analytes, NO2,cetic acid, butyric acid and hexanoic acid are presented. Theangmuir-Schaefer (LS) thin film preparation technique wassed to produce the solid-state porphyrin films which wereptimized for sensing applications. 5,10,15,20-tetrakis[3,4-bis(2-thylhexyloxy)phenyl]-21H,23H-porphine (EHO) was selected ashe sensor material to be used in these experiments because itsensor response to NO2 and carboxylic acids have previously beentudied extensively [13,14,19,20,22–25]. The main purpose of thistudy is to investigate the possibility of achieving molecular sizeelectivity using a barrier layer deposited on top of this type of opti-al sensing film. The barrier layer acts to control which moleculesan access the underlying porphyrin layer. PMMA was chosen toreate the barrier layer. Calix[8]arene molecules were incorpo-ated within the PMMA barrier layer to enhance the porosity ofhe barrier layer, thereby allowing some molecules to reach theensor molecules underneath. Gas sensing responses of uncoveredorphyrin, PMMA coated porphyrin films and PMMA/calix[8]areneoated porphyrin films were recorded using optical measurementystem and compared.

. Experimental details

.1. Materials and LS film fabrication

In this study, 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)

henyl]-21H,23H-porphine was used to prepare Langmuir-chaefer (LS) films. The synthesis and chemical structure of thisaterial has been described elsewhere[22]. In order to incorpo-

ate steric size selectivity layers of poly(methyl methacrylate)

21H,23H-porphine (EHO) and (b) carboxylic acid substituted calix[8]arene (c)

(PMMA) and carboxylic acid substituted calix[8]arene[23] weresubsequently applied on top of the EHO films. Chemical structuresof molecules are shown in Fig. 1. The EHO was dissolved in chlo-roform at a concentration ∼0.5 mg ml−1. PMMA and calix[8]arenesolutions in chloroform at concentration of ∼0.1 mg ml−1 and∼0.3 mg ml−1, respectively, were used to produce the barrier lay-ers. Investigations of the monolayer properties of EHO and LS filmpreparation were carried out using a NIMA Model 611 Langmuirtrough. All experiments were carried out at room temperature(∼18 ◦C) in a clean room. Ultra-pure water (ElgaPURELab Option>15 M� cm) was used as the sub-phase during the characterizationand deposition experiments. Surface pressure-area (�-A) isothermof the EHO were recorded under several different conditions.

For the deposition of the LS films, glass slides were used asthe substrate. Prior to deposition the glass slides were cleaned byrefluxing in IPA for several hours and then exposed to 1,1,1,3,3,3-hexamethyldisilazane vapors for min 12 h in order to render thesurface of the glass hydrophobic. LS film deposition was performedby contacting the substrate onto the floating monolayer on watersurface horizontally and subsequently lifting off a single LS layer.For multilayer LS film preparation, this process was repeated therequired number of times, at a different position on the floatingfilm in order to ensure that the area of floating film contacting thesubstrate was continuous for each deposition.

2.2. Optical measurements

An Ocean Optics USB2000 spectrometer and a MikropackMiniD2 UV–vis–IR light source were used for the optical characteri-zation experiments. Absorbance spectra were recorded in the range350–800 nm. Absorbance measurements necessitate the measure-ment of reference spectra, which were measured using either purechloroform in a quartz cuvette or a clean glass slide for the solutionand solid film absorbance measurements, respectively. Absorbancespectra of EHO solution were recorded before and after adding ofacetic acid in the ratio of 4:1 into chloroform solution in order todemonstrate the spectral shift upon exposure of EHO to acetic acid.Optical absorbance spectra of the solid state films were recorded ina custom built gas exposure cell before and after exposure to NO2,acetic acid (CH3COOH), butyric acid (C3H7COOH) and hexanoic acid(C5H11COOH) vapors.

2.3. Gas sensing setup

The experimental setup used to record the gas sensing charac-teristics of solid LS thin films has been described previously [24].

7 ors and Actuators B 206 (2015) 74–83

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6 M. Evyapan, A.D.F. Dunbar / Sens

his setup comprises a purpose built gas exposure chamber andwo Tylan FC-260 mass flow controllers in order to deliver ana-yte vapor and mix with clean dry nitrogen to obtain requirednalyte concentration. The gas exposure chamber consists of aetal cylinder sealed with O-rings. The volume of the chamber

s ∼16.5 cm3. For the NO2 sensing experiments a pre-mixed bottlef 5 ppm NO2 (the balance being clean dry nitrogen) was used asupplied by BOC Ltd. For the acetic acid, butyric acid and hexanoiccid sensing experiments one of the mass flow controllers was usedo deliver a controlled amount of clean nitrogen to a small ves-el containing the required analyte in its liquid phase. The analyteessel was maintained at a known temperature in a temperatureontrolled water bath. When the nitrogen gas passes through theeadspace in the vessel it mixes with the analyte vapor present inhe headspace. The partial pressure of the analyte in the headspaces determined by the temperature in the vessel. Control experi-

ents were conducted to ensure that the nitrogen flow rate wasufficiently low such that the vapor partial pressure remained inquilibrium (less than 500 cm3 s−1). The nitrogen stream contain-ng the analyte vapor could now be directed to the gas exposurehamber. The LS films were positioned on a Peltier device to enableeating and cooling. The exposures were performed at ∼20 ◦C andhe sensor recovery was conducted at ∼50 ◦C. There exact temper-tures may have been slightly lower than these values because thehermocouple used to measure temperature was attached to theeltier device rather than the sample. As a result the sensor filmsere separated from the thermocouple by the thickness the glass

ubstrates (1 mm thick). The recovery of EHO film can be acceler-ted by gently heating. Previously published studies on EHO basedensors showed that at temperatures significantly above room tem-erature the sensor response decreases [25]. Other studies showedhat over-heating an EHO sensor after first exposure can cause aesponse reduction in subsequent exposure cycles [22,23]. There-ore, in this study, recovery temperature was limited to ∼50 ◦C inrder to minimize these detrimental effects. The dynamic responsef EHO films to analyte exposure was carried out by measuring thebsorbance spectrum as a function of time during several exposurend recovery cycles. These spectra were subsequently analyzed andhe dynamic responses calculated by taking the difference betweenhe absorbance during exposure at 470 nm and subtracting the pre-xposure absorbance value also at 470 nm. 470 nm values weresed because this corresponds to the peak of the absorbance fea-ure that develops upon exposure of EHO to the analytes used in thisork. The exposure and recovery cycles were 10 min each. During

he first 5 min of the recovery cycle the sample temperature waslevated to ∼50 ◦C, for the remaining 5 min the temperature waseduced such that it had cooled down to ∼20 ◦C before the end ofhe recovery period. Additionally dry nitrogen was passed throughhe gas chamber during the recovery cycle to flush out any outassing analyte vapor. The dynamic measurements were carriedut for several cycles to observe reproducibility of the sensor mate-ial. In each measurement the first exposure and recovery cycle wasiscarded as the sensor response typically improved after the firstycle and then settled into a very repeatable pattern of cycles. Thepectral data was recorded using the same optical measurementystem described in the previous section and all processes wereontrolled by computer in order to ensure consistency.

. Results and discussion

.1. Monolayer behavior and LS film preparation

The behavior of EHO when deposited as a monolayer on a waterurface was investigated by measuring a �-A isotherm for eachaterial, as shown in Fig. 2. After the solution was spread on the

Fig. 2. Isotherm graph of EHO.

water surface, the surface pressure was recorded as a function ofsurface area available for the monolayer whilst it was compressedby the moving barrier. Monolayer stability was also determined byrecording a second compression isotherm. After the initial com-pression, the barrier was opened fully and the second isothermwas recorded by closing the barrier again. It has been reportedbefore that EHO does not form a uniform monolayer on watersurface, instead it forms small domains and when the barrier isclosed these domains coalesce [22]. This can be seen in Fig. 2 wherethe EHO isotherm shows a sudden increase in surface pressureat about ∼20 A2/molecule as the film is compressed. This unde-sirable aggregation behavior can be avoided if the EHO is firstexposed to NO2 vapor [22] or trifluoroacetic acid [19] or additivessuch as calix[8]arene added to the solution [26] before spread-ing onto the water surface. Richardson et al. [27] demonstratedthat exposure to trifluoroacetic acid causes the EHO monolayerto expand on water surface resulting in a more open film struc-ture and molecular packing upon transfer onto a solid substrate.This has important implications for the sensing characteristics [28].When untreated EHO molecules are transferred on a solid substrate,they tend to be J-aggregated and this can affect sensing properties[29]. The response of EHO sensors where J-aggregates are presentis decreased due to steric hindrance which reduces the availabilityof active binding sites within the EHO film.

In this work an EHO isotherm was recorded after spreadingfrom the chloroform solution of EHO and this was repeated for twocompression cycles in order to investigate the aggregation prop-erties of EHO molecules on water surface. Isotherms were alsorecorded for EHO after the addition of a small amount of aceticacid to the EHO solution (∼10 �l was added to 5 ml of the EHO inchloroform solution). The addition of acetic acid affects the EHOin a similar way to trifluoroacetic acid as described by Richard-son et al. [27]. Fig. 2 shows the �-A isotherms of EHO and EHOwith acetic acid for two compressions. Adding small amount ofacetic acid to the solution causes the monolayer to expand. Asseen in Fig. 2, there is a remarkable difference between the �-Aisotherms of the EHO monolayers with and without acetic acid.The �-A isotherm of the acid-free film shows a sudden surfacepressure increase and it does not exhibit any phase transitions, andits second compression shifts only very slightly to a lower surfacearea per molecule but the exhibits essentially the same monolayercharacteristics. Extrapolating of the isotherm curves for the acid-free EHO gives area per molecule of about 20 A2. This is attributedto the EHO molecules tending to J-aggregate and organize on water

surface edge on, facing each other which results with small area permolecule [19,22,27]. Upon being compressed a second time the filmbecomes very slightly more aggregated.

M. Evyapan, A.D.F. Dunbar / Sensors and Actuators B 206 (2015) 74–83 77

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ig. 3. (a) The absorption spectra of 7 layers EHO LS films using different transfer

inetic response of 7 layers EHO LS films using different surface pressures (5–15–30

When an EHO solution with acetic acid added is used, the initialsotherm is significantly different. The EHO becomes protonatednd therefore preferentially orients facing the water surface. Thiseans the monolayer adopts a more open structure with the por-

hyrin ring lying flat on the water surface [19,27]. This change inrientation of the acidified EHO molecules results in the significantncrease in area per molecule at which the acidified EHO monolayeregins to register some surface pressure. This can be noticed fromhe area per molecule value which is larger at around 90 A2 whenhe isotherm first starts to register a meaningful surface pressure.t can also be clearly seen in Fig. 2 that the acidified EHO mono-ayer exhibits phase transitions indicated by the inflection in thesotherm gradient as the monolayer is compressed. This is likely toe due to the orientation of the EHO molecules switching from lyingat on the water surface to an edge on orientation. The isotherm ofecond compression of monolayer exposed to acid shows a simi-ar behavior with the acid-free one indicating that once aggregatedhe EHO molecules remain aggregated. The flat open structure isot recovered presumably because of there is some attractive elec-rostatic interaction between EHO molecules. The surface area per

olecule values reported here are smaller than those reported pre-iously [27] indicating that some degree of aggregation may still beccurring in these films even when acidified. This kind of monolayerehavior can be seen in literature for different organic monolayers30].

In order to investigate the effect of aggregation behavior onensing characteristics of thin films deposited at three differenturface pressures (5 mN m−1, 15 mN m−1, 30 mN m−1) were cho-en for LS film preparation. All LS films were prepared during therst compression of an EHO solution with acetic acid added in ordero promote the production of a film with as much un-aggregatednd therefore responsive EHO as possible.

.2. Vapor sensing properties

Optical absorbance spectra for the 7 layer LS films of EHOeposited at 5 mN m−1, 15 mN m−1, 30 mN m−1 are shown inig. 3(a) both before exposure and after 10 min exposure to 5 ppmO2. All 3 films display a distinctive Soret band around 426 nm and

our weaker Q band peaks at longer wavelengths before exposureo NO2. The 30 mN m−1 sample also shows some other structure inhe Soret peak on the longer wavelength side. This is attributed to

he higher degree of aggregation in the EHO layers in this sampleaused by the high surface pressure used during this deposition.

Upon exposure to NO2 it is clear that all three filmsndergo significant spectral changes, similar to previously reported

res (5–15–30 mN m−1) before and after 10 min of exposure to 5 ppm NO2, (b) The−1) upon exposure of 5 ppm NO2.

observations [22,23]. The Soret band absorption peak decreasesin intensity and a new peak appears at approximately 470 nm.Also the Q bands become weaker and a new peak appears around710 nm. Similar results have been observed before in a study ofEHO thin films and these spectral changes were attributed to NO2binding to the EHO causing changes in the electronic energy lev-els, and hence the absorbance spectrum [22]. In all the exposedspectra there is still some evidence of absorbance at the Soretpeak energy indicating that not all the porphyrin molecules beingprobed undergo the spectral switch observed upon exposure. The426 nm Soret peak is associated with the EHO molecules which arenot exposed to NO2. There are significant differences in the extentof switching between the 30 mN m−1 sample and the other two.Fig. 3(b) shows the dynamic response of the new absorbance fea-ture which increases upon exposure to 5 ppm NO2 for 7 layers EHOLS film and provides further evidence of a less complete switch-ing for the 30 mN m−1sample. The absorbance is still increasing atthe end of the 10 min exposure time which is attributed to thevery low concentration on NO2 used and the increased aggrega-tion in this sample which hinders NO2 from accessing all the EHOmolecules. Steric effects due to aggregated EHO molecules may bepreventing exposure of the active site to NO2 molecules. It is wellknown that EHO molecules tend to aggregate and interact witheach other. When the deposition surface pressure is increased theEHO molecules are forced closer together and therefore they aremore likely to aggregate. Similar results have been observed inother studies which report that mixing EHO with other materi-als [23,31] or exposure to NO2 before preparing thin film [22] inorder to reduce the aggregation is known to increase the sensingresponse. In order to optimize the sensing properties of the filmsused in this study, a surface pressure of 15 mN m−1 for EHO solutionpre-treated with acetic acid was chosen as the deposition con-ditions for all further experiments. It is clear from our dynamicdata that not all the sensors reach dynamic equilibrium within the10 min exposure time applied, therefore a pragmatic approach wasadopted whereby 10 min exposures were used and the responsewas taken as the change in absorbance after 10 min, on the basisthat any longer is impractical for real world sensor applications.

As seen in Fig. 4(a), the response of the EHO film (7 layersdeposited at 15 mN m−1 using acetic acid treated EHO solution) isdirectly related to the concentration of NO2. Fig. 4(b) shows thatthere is a linear increase in the maximum response as a function of

increasing concentration of analyte. Fig. 4(b) also demonstrates thatat 5 ppm the sensor has not yet reached saturation. This reaffirmsthe conclusion that the low concentration of NO2 is insufficient tosaturate the sensor. The gradient of this straight line fit serves as a

78 M. Evyapan, A.D.F. Dunbar / Sensors and Actuators B 206 (2015) 74–83

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ig. 4. (a) The kinetic response of 7 layers EHO LS film upon exposure of different N

alibration constant for this sensors response to NO2 at concentra-ions below sensor saturation.

Fig. 5(a) shows the sensing response of four different samplesith one, three, five and seven LS layers of EHO deposited on the

lass substrate upon exposure to 5 ppm NO2. It is hardly surprisinghat the films with more layers have a higher optical density dueo the greater number of EHO molecules in the beam path. This isurther demonstrated by Fig. 5(b) which shows that there is a linearelationship between the maximum response and the number of LSayers applied, as would be expected from the Beer-Lambert law.

hat is more interesting is that the response occurs in two distincttages when multiple layers are present. This two stage processan be understood if one considers what happens when the NO2nteracts with a multi-layered sensor. First, however, consider theimple single layer sample. The first interactions occur betweenO2 vapor molecules and the easily accessible EHO moleculesxposed on the top surface of the thin film and is observed as the fastnitial sensor response. As time progresses the response increasesndicating a greater number of EHO molecules have interacted withhe vapor molecules. Subsequently, the vapor molecules will needo diffuse deeper into the film structure and access the less easilyccessible EHO molecules, such as those which may be aggregatedogether face-to-face. This can be seen as a very slow gradual rise onhe sensor response for the one layer sample after the initial surge.

urprisingly, this fast and then slowing response is not repeated forhe other multi-layered samples. For the multi-layered samples its clearly seen that the gradient of the response actually increasest some significant time after the initial exposure. The basic model

Fig. 5. (a) The kinetic response of different layers EHO LS films upon exposure o

ncentrations, (b) Maximum sensor response versus NO2 concentrations (ppm).

of fast surface and then slower diffusion limited exposure cannotexplain this. In order to explain these observations it is proposedthat diffusion of NO2 into the less accessible aggregated EHO nearthe top surface significantly restructures the film allowing access topreviously inaccessible un-aggregated EHO molecules in the under-lying layers. Therefore, as with the single layer sample, the firstgradient observed in the response curves is associated with the ini-tial fast surface response followed by diffusion limited response asthe NO2 penetrates any aggregated EHO. However, restructuring ofthese EHO aggregates may subsequently open up easy and there-fore fast diffusion pathways to the underlying layers with manyeasily accessible EHO molecules in the underlying layers. It is notedthat the inflection in the response gradient is most pronounced forthe films with the most layers which fits with this picture. Even-tually all the accessible EHO molecules that can switch will havedone so, and the rate of increase of the response slows and even-tually reaches saturation. This inflection in the response rate isnot observed in sensors upon exposure to carboxylic acids, so itis assumed that they cannot facilitate this process. It is also notedthat this inflection in the response rate is not observed in the sen-sors when exposed to NO2 (as discussed in the following section)where an overlying barrier layer is present. We tentatively proposethat the interaction with NO2 results in a dis-aggregation and/orswelling of the EHO film which is reversed upon recovery. Whereas

for EHO interactions with acids or NO2 exposures where there is abarrier layer present do not undergo this process. Presumably theaddition of a barrier layer prevents this process occurring for NO2exposures.

f 5 ppm NO2, (b) Maximum sensor response versus number of EHO layers.

M. Evyapan, A.D.F. Dunbar / Sensors and Actuators B 206 (2015) 74–83 79

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matrix.Fig. 8 shows the kinetic responses for EHO multilayers with var-

ious different PMMA-calix[8]arene mixtures forming the barrierlayer upon exposure to 5 ppm NO2. It is clear from the graph that

ig. 6. The kinetic response of 7 layers EHO LS films with 1 layer PMMA top layeroated with different coating pressures upon exposure of 5 ppm NO2.

.3. Selectivity measurements

In the previous sections, the sensing behavior of EHO films haseen discussed in detail. The aim of this study is to try to control theelectivity of organic thin film gas sensors. The EHO response alones not selective between different analytes. However, by monitoringhe EHO response in conjunction with a barrier layer which controlshe diffusion of analyte into the sensor film it is possible to differen-iate between similar analytes because the response rate changesccording to the analyte size, therefore enabling some degree ofelectivity to be achieved. When choosing a material to use as such

barrier layer it is important that it is transparent therefore doesot perturb the optical measurements used to determine the sensoresponse. It is well known that the interactions between thin filmsnd vapor start from the exposed surface of that thin film. In ordero control those interactions a thin PMMA layer was added as a bar-ier layer on top of the EHO LS film. This was done in order to limiturface interaction and control the diffusion of vapor moleculesnto film structure. PMMA is a long chain polymer, the polymerhains tend to stick to each other when they are close packed [30],herefore forming an effective barrier to gaseous diffusion.

Fig. 6 shows the effect of having a PMMA barrier layer depositedn top of a 7 layer EHO film as a function of PMMA deposi-ion surface pressure on the NO2 sensing properties of the film.

ith increasing deposition pressure the PMMA chains are forcedloser to each other and therefore the PMMA acts as a more effec-ive barrier to the NO2 at higher deposition surface pressures. Ashe deposition pressure in increased the PMMA layer significantlyeduces the number of EHO molecules that the NO2 can access.ince the sensor response is not quenched completely it is evidenthat there are still some pores through the PMMA film. However,t is clear that significant lateral diffusion within the EHO layerseneath the PMMA is not possible once the NO2 passes through theMMA layer because the final response intensity is reduced as theeposition surface pressure is increased. For the 2 mN m−1 PMMAarrier layer the sensor response is reduced by 47% (of the responsebserved for the uncovered sensor), whilst the 20 mN m−1 PMMAayer is reduced by 95% of the response to 5 ppm NO2.

Clearly in order to develop a highly selective sensor it is noteneficial to quench the sensor response completely, therefore forhe subsequent samples a PMMA deposition pressure of 10 mN m−1

as selected as a suitable intermediate case. In order to incorpo-

ate further control over the rate of analyte penetration throughhe PMMA barrier layer carboxylic acid substituted calix[8]areneas incorporated into the PMMA barrier layer. It is well known

hat calix[8]arene molecules consist of an open ring structure and

Fig. 7. Schematic illustration of the structure of barrier layer. The top layer onlyconsists of PMMA molecules in (a) and (b) shows the change in structure after calixadded.

have previously been used to produce porous films [23]. Thereforeby mixing PMMA with calix[8]arene and using it as a barrier layerresults in a more porous barrier layer. Fig. 7 shows a schematicrepresentation of how the incorporation calix[8]arene into thePMMA layer may influence the diffusion of analyte into the sen-sors. The presence of calix[8]arene permits more vapor moleculesto diffuse through to the underlying EHO LS film. Whether the ana-lytes diffuse through the centers of the calix[8]arene molecules ornot remains uncertain. However, because the carboxylic acid sub-stituted calix[8]arene molecules are amphiphilic in nature, whenblended with PMMA and spread on the Langmuir Blodgett troughthey should preferentially orient on the water surface such thattheir ring-like structure sits upright within the surrounding PMMA

Fig. 8. The kinetic response of 7 layers EHO LS films and a single top layer of PMMA-Calix mixture with different concentrations upon exposure of 5 ppm NO2.

80 M. Evyapan, A.D.F. Dunbar / Sensors and Actuators B 206 (2015) 74–83

Table 1Relative sensor response reduction for several top layer concentrations upon expo-sure of NO2, acetic acid and butyric acid.

Top layer concentration NO2 Acetic acid Butyric acid Hexanoic acid

100% PMMA 77% 79% 80% 87%75% PMMA-25%Calix 70% 70% 71% 83%

antsldePtrroistrlpro

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Fig. 9. The kinetic response of 7 layers EHO LS films and a single top layer of PMMA-Calix mixture with different concentrations upon exposure of 855 ppm acetic acid.

Fig. 10. The kinetic response of 7 layers EHO LS films and a single top layer of PMMA-Calix mixture with different concentrations upon exposure of 846 ppm butyric acid.

50%PMMA-50%Calix 57% 52% 56% 78%25%PMMA-75%Calix 44% 36% 38% 70%100%Calix 28% 19% 26% 57%

dding more calix[8]arene into the barrier layer increases the mag-itude of the sensor response achieved within the 10 min exposureime allowed when compared with 100% PMMA layer which is con-istent with the addition of calix[8]arene producing a more porousayer. After 10 min of exposure the sensors with barrier layers stillo not reach the same response level as the uncoated EHO film,ven for the sensor coated with highly porous 100% calix[8]arene.resumably, if we allow exposure times of more than 10 min fur-her EHO molecules will continue to interact, therefore, the sensoresponse is now clearly limited by diffusion through the porous bar-ier layer. The slope of the response graphs decreases as the amountf calix[8]arene in the barrier layer is decreased and the analyte gass forced to travel an increasingly tortuous path through the sen-or film, slowing the response down. In contrast, the response forhe 100% PMMA barrier layer reaches completion quickly but theesponse is considerably smaller in magnitude. Less of the under-ying porphyrins are accessible and those that are accessible areresumably only those near the surface which therefore respondapidly. This demonstrates that we can now control the diffusionf the analyte vapor entering our sensor films.

The decrease in the sensor responses as percentage of the uncov-red sensor response are presented in Table 1. These changes haveeen calculated using maximum response after 10 min of eachensor and given as a percentage decrease of that response. Thencreasing magnitude in the sensor response as the amount ofalix[8]arene increases implies that the calix[8]arene does indeedake the PMMA layer much more porous, helping the vaporolecules to diffuse through the barrier layer as speculated. In

ig. 8 saturation of the sensor response has only been observedor sensors with barrier layers with 100% and 75% PMMA. Thesewo films have relatively few pores and therefore a significantlyeduced number of EHO molecules are accessible. Therefore even atnly 5 ppm NO2 the fairly limited number of molecules present cannteract with all the accessible EHO sites and therefore the responseeaches saturation.

In order to further explore the influence of a porous barrierayer, three different sized carboxylic acids were chosen for com-arison. Vapors of acetic acid (CH3COOH) at 855 ppm, butyriccid (C3H7COOH) at 846 ppm and hexanoic acid (C5H11COOH)t 822 ppm were used to expose the sensors. Similar PMMA-alix[8]arene mix barrier layers were used. The kinetic responsef the sensors to acetic acid vapor is given in Fig. 9. If we comparehe 7 uncovered layers of EHO, exposure to acetic acid at 855 ppmesults in a higher response than the NO2 vapor at 5 ppm. This isresumably because of the difference in the ppm values used forhe acetic acid compared to the NO2 exposures. The response rate tocetic acid is also remarkably fast. It can be seen in Fig. 9 that incor-orating a barrier layer results in a smaller, slower response to thecetic acid exposure. Even with the high ppm value of acetic acid,he 100% PMMA layer can still limit exposure of the EHO moleculesemarkably.

Responses similar those for acetic acid were also measured

or butyric and hexanoic acids which are increasingly larger car-oxylic acid molecules. Therefore it was expected that they wouldlso induce a spectral response upon interaction with EHO, butecause they are larger molecules the diffusion restriction due to

Fig. 11. The kinetic response of 7 layers EHO LS films and a single top layer of PMMA-Calix mixture with different concentrations upon exposure of 822 ppm hexanoicacid.

the PMMA:calix[8]arene barrier layers should have a more dra-matic effect. Figs. 10 and 11 show the kinetic response of EHOfilms upon exposure to butyric acid at 846 ppm and hexanoic acid at

822 ppm, respectively. The responses to butyric and hexanoic acidsare similar but notably are significantly lower in magnitude thanthe responses to acetic acid vapor. This is despite the fact that theconcentration of each analyte was approximately the same. This

M. Evyapan, A.D.F. Dunbar / Sensors an

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diffuse through the pores which results in slower responses. How-

TT

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ig. 12. EHO solution spectra before and after adding acids (∼3 M acid added into ml and 1 mg ml−1 concentration of chloroform solution).

an be understood in terms of how easily the acid molecules canccess the EHO layers. The larger acid molecules find it more diffi-ult to diffuse into the EHO film structure. For both the butyric andexanoic acid responses they do not reach saturation within the0 min exposure time even with the uncovered 7 EHO layers. Thisould be because the interaction between EHO and the longer alkylhain acids is weaker than and therefore not as likely to occur as theHO and acetic acid interaction. Fig. 12 shows the spectral responseor equal moles of each of the analytes with EHO in solution. Thecetic acid curve shows a greater response than the butyric acidhich was greater than the hexanoic acid. The response to acetic

cid shows an almost complete switch whereas at the alkyl chainength increase the degree of switching reduces which suggestshat the interactions are indeed weaker. The increasingly long alkylhain reduces the likelihood of the acid binding to the EHO there-ore shifting the dynamic equilibrium between bound and unboundowards unbound. This could be a result of a slightly different dis-ribution of electrons within the carboxylic acid group as result ofhe longer alkyl chain attached.

Table 1 shows the relative changes in the response rate forhe three different carboxylic acids with various different bar-ier layers. The biggest reduction in response occurs for hexanoic

able 2he effect of top layer concentration on the speed of carboxylic acid response.

LS film NO2 Acetic acid

t50 (s) Max response t50 (s) Max r

7 layers EHO 266 0.130 33 0.160

+100%Calix 208 0.093 35 0.131

+25%PMMA-75%Calix 175 0.074 36 0.103

+50%PMMA-50%Calix 142 0.055 98 0.074

+75% PMMA-25%Calix 94 0.036 118 0.048

+100% PMMA 78 0.029 167 0.034

able 3ensitivity and LOD parameters of vapors.

LS film NO2 (5 ppm) Acetic acid (855 ppm)

Sensitivity (A/ppm) LOD (A) Sensitivity (A/ppm)

7 layers EHO 0.026 0.11 1.87 × 10−4

+100%Calix 0.018 0.16 1.52 × 10−4

+25%PMMA-75%Calix 0.014 0.21 1.16 × 10−4

+50%PMMA-50%Calix 0.010 0.30 0.84 × 10−4

+75% PMMA-25%Calix 0.007 0.42 0.52 × 10−4

+100% PMMA 0.005 0.60 0.37 × 10−4

d Actuators B 206 (2015) 74–83 81

acid with a 100% PMMA barrier layer. This combination almostcompletely quenches the response indicating that the hexanoicacid finds it very difficult to penetrate through the barrier layer.The hexanoic molecules are too large to fit through the poresbetween PMMA molecules. So only a very limited number of vapormolecules can diffuse into EHO film compared to other vapors.Increasing calix[8]arene content in the barrier layer allows morevapor through and therefore increases both the sensor responseand rate of response. Therefore by operating this type of porphyrinbased gas sensor in the diffusion limited regime by incorporating abarrier layer it is possible to differentiate between different sizedmolecules with the same active functional groups.

When a 100% calix[8]arene top layer is applied, there is stilla reduction in the maximum sensor response. Presumably this isbecause even though the ring like calix[8]arene molecules are veryporous some fraction of the under lying EHO molecules will be cov-ered by the calix[8]arene rings. ChemOffice software has been usedto estimate the footprint of the calix[8]arene molecules as a frac-tion of their cross-sectional area and it is found to be ∼19%. Thisis comparable to the response reductions measured and shown inTable 1; the decreases are 28%, 19% and 26% for NO2, acetic acid andbutyric acid, respectively.

Table 2 shows the time taken for the absorbance change toreach 50% of the total change achieved within the permitted expo-sure time of 10 min, denoted t50 (10 min). This measure of thesensor response rate was used because not all exposures hadreached equilibrium within 10 min, so the more typical t90 or t50parameters could not be determined, however the time taken forthe 50% of the maximum change observed in 10 min to occur(t50 (10 min)) still provides a meaningful indication of the sensorresponse rates. It is clear from the table that the sensor responsesof EHO films against acetic acid are remarkably faster than butyricacid response. It is assumed that the acetic acid molecules can usethe calix[8]arene pores to diffuse into film structure faster. Theacetic acid is not significantly hindered by the barrier layer until thePMMA:calix[8]arene ratio is 50:50 or higher. As a result the aceticacid responses reach saturation within the 10 min exposure time.It is more difficult for butyric acid and hexanoic acid molecules to

ever, the responses of 100% PMMA and 75% PMMA films exposedto butyric acid are reported as fast. This is because their totalabsorbance changes are very small and therefore they take short

Butyric acid Hexanoic Acid

esponse t50 (s) Max response t50 (s) Max response

180 0.055 159 0.023165 0.040 128 0.01160 0.033 79 0.007106 0.024 61 0.005

47 0.017 30 0.00438 0.012 28 0.003

Butyric acid (846 ppm) Hexanoic Acid (822 ppm)

LOD (A) Sensitivity (A/ppm) LOD (A) Sensitivity (A/ppm) LOD (A)

16 0.63 × 10−4 47 0.27 × 10−4 11019 0.47 × 10−4 63 0.10 × 10−4 30025 0.39 × 10−4 76 0.08 × 10−4 37535 0.28 × 10−4 107 0.06 × 10−4 50057 0.17 × 10−4 176 0.04 × 10−4 75081 0.11 × 10−4 272 0.02 × 10−4 1500

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imes to reach saturation; presumably only a few EHO moleculesear the surface are contributing to the response. From this data

t is clear that by taking into account the maximum response andhe t50 (10 min) times for each exposure it is possible to develop aensor which can selectivity differentiate between different sizedarboxylic acids.

The sensitivity and the limit of detection (LOD) parameters haveeen calculated and given in Table 3. The sensitivity value is defineds the change of absorbance at 470 nm per ppm (A/ppm) and theimit of detection was described by the following equation [32]:

OD = 3�

S(1)

here � is the noise level of measurement system, S is the sensi-ivity to specified vapor. The absorbance response is stable within0.001 so the noise level of the system is 1 × 10−3.

. Conclusion

EHO molecules can easily aggregate with each other while beingompressed to form a monolayer on a Langmuir-Blodgett troughnd their sensing characteristics are generally diminished by thisggregation. Adding acetic acid to the solution prior to spreadingas a similar effect to pre-bubbling the spreading solution with NO2r adding TFA, in that it reduces the aggregation of EHO films onhe Langmuir-Blodgett trough.

It is possible to improve the selectivity of EHO based gas sen-or films at the expense of sensitivity by applying a barrier layerver the EHO film to control the diffusion of analyte vapors intohe EHO layers. If a 100% PMMA barrier layer is applied on topf the EHO layers the sensor response is dramatically restricted.t a deposition pressure of only 20 mN m−1 100% PMMA can stop0% of vapor molecules entering the EHO layers. However, upon

ntroducing different quantities of calix[8]arene into the mixturesed for the barrier layer much of the sensitivity can be recovered.alix[8]arene molecules create some pores though the PMMA bar-ier because of their open ring-like molecular structure and theseores allow vapor molecules to diffuse into the underlying EHOlm. The results show that adding more calix[8]arene molecules

ncreases the sensing response for all vapors when compared with00% PMMA barrier layer. In these diffusion limited sensors aceticcid vapor generates a greater response than butyric or hexanoiccids. This is attributed partially to the stronger response associ-ted with acetic acid interacting with EHO molecules, but also dueo the smaller size of acetic acid molecules when compared withutyric or hexanoic acids. The pores introduced by the calix[8]areneolecules make the sensor device selectively more sensitive to

maller molecules than larger molecules. This is thought to beecause they limit the analyte diffusion and therefore change theensor response speed. Therefore by controlling the number ofores and considering the response rate for the sensors we cancquire information about the size of analyte vapor molecules.

cknowledgments

The authors would like to thank Professor Chris Hunter& Jor-an Hutchinson from the Chemistry Department at the Universityf Sheffield, for synthesizing the porphyrin used in this work. M.vyapan also thanks Turkish High Education Council (YOK) for theost. Doc. Scholarship.

ppendix A. Supplementary data

Supplementary material related to this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2014.09.023.

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d Actuators B 206 (2015) 74–83

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d Actuators B 206 (2015) 74–83 83

Biographies

Murat Evyapan is a research assistant at the Department of Physics at Balikesir Uni-versity. He received his M.Sc. and Ph.D. in physics from Balikesir University, Turkeyin 2006 and 2012. He had a Post. Doc. scholarship from Turkish High EducationCouncil in 2013 for Abroad Research in UK. His main interests are organic thin filmsand gas sensor for environment applications. He is currently working as a Post. Doc.Researcher in University of Sheffield, UK.

Alan Dunbar attained an M.Phys. in Physics with Electronics at UMIST (Manchester)in 1997, and also his Ph.D. in 2001. After his Ph.D., he moved to the University ofCanterbury, Christchurch for two years as a postdoctoral research fellow. He appliedpercolation theory to explain the onset of conduction in thin films of depositednano-particles in conjunction with morphological data obtained by atomic forcemicroscopy and electron microscopy. He then returned to the UK to begin work-ing in the University of Sheffield (Department of Physics and Astronomy) where hedeveloped gas sensors for volatile organic compounds. He then took up a researchpost focused on the optimization of polymer based solar cells through understand-

ing how the nano-scale morphology of the thin polymer films within the devicesinfluences the device efficiency. In 2009 he was appointed as lecturer in energy in theDepartment of Chemical and Biological Engineering at the University of Sheffield.His research interests include organic gas sensors, organic photovoltaic and nano-structured materials.