Electrical and optical evaluation of polymer composites for chemical sensing applications

Microelectronic Engineering 86 (2009) 1289–1292

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Electrical and optical evaluation of polymer composites for chemical sensing applications G. Dendrinos a,b, L. Quercia c, I. Raptis a, K. Manoli a, S. Chatzandroulis a, D. Goustouridis a,d, K. Beltsios b,* a

Institute of Microelectronics NCSR Demokritos, Aghia Paraskevi 15310, Greece Materials Science and Engineering Dept., University of Ioannina, Dourouti Campus, 45110 Ioannina, Greece c ENEA, Centro Ricerche Portici, 80055 Portici (NA), Italy d Theta Metrisis, Polydefkous 14, 12242 Aegaleo, Greece b

a r t i c l e

i n f o

Article history: Received 28 September 2008 Received in revised form 29 November 2008 Accepted 1 December 2008 Available online 11 December 2008 Keywords: Polymer composites Chemical sensing Electrical sensing

a b s t r a c t In this work we focus on poly(hydroxy ethyl methacrylate)/carbon black composites for the electrical and optical sensing of humidity and alcohols. The sensitivity is a non-monotonous function of carbon load, while processing with and without ultrasonic agitation can lead to substantially different sensitivity levels. In the case of a carbon-loaded film, optical sensing is limited by the transparency of the carbonloaded film, which, nevertheless, can be manipulated within limits through the choice of casting solvent. Electrical sensitivity of the composites is compared to the optical (thickness-based) and capacitance sensitivities of the carbon-free poly(hydroxy ethyl methacrylate) version. Composites offer a clear sensitivity advantage when their optimized, on the basis or electrical response, version is employed. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction The diverse range of physical and chemical properties of available polymers leads to one or more polymeric materials appropriate for each gas sensing application [1]. During the last decade, polymers have gained increased recognition in the field of gas sensor arrays and improved selectivity has been achieved over classical sensor materials such as semiconductors, metal oxides etc. Polymeric materials employed in gas sensing elements fall into one of the following categories: (i) plain dielectric (non-conducting) polymers, (ii) conductive polymers, (iii) molecular imprinted polymers and (iv) polymer composites. For example, sensors based on non-conducting polymers have been applied to the detection of various organics. In the latter case, the sorption of the analytes causes a change, usually an increase, of the dielectric constant monitored as capacitance change, e.g. [2]. On the other hand, the resistance of conductive polymers changes in the presence of particular analytes, e.g. [3]. However, the range of conductive polymers is limited and, sensing elements based on them tend to deteriorate upon prolonged use. Sensors based on molecular imprint polymers have attracted considerable attention and some interesting results have been reported, e.g. [4]. The fourth family corresponds to polymer composites, characterized by polymer matrices loaded with conductive particles, including carbon black and carbon nanotubes, e.g. [5,6]. * Corresponding author. E-mail address: [email protected] (K. Beltsios). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.12.001

The application of polymer composites in gas sensing is based on the conductivity decrease of polymeric films during the sorption of analytes. In particular, the sorption of analytes causes matrix swelling which in turn enhances the distances between conductive particles and leads to a conductivity reduction. This type of polymerbased sensors is highly attractive in terms of material selection and all solid non-conductive polymers are potential matrix materials. The polymer choice is governed by the nature of the analyte and additional factors, including the glass transition temperature of the polymer [7]. For example, the commercial e-nose Cyranose, is based on 32 polymer composite sensors [8]. In the composites-based configuration employed most frequently, the gas sensors devices are fabricated by dispersion of conductive particles in polymer solutions and deposition and drying of the dispersion between two electrodes predefined on a corresponding substrate. For a given concentration of a specific analyte, factors affecting response include the nature, ratio and spatial arrangements of the two components and the electrode geometry. Also, for a given nature and ratio of the two components, the spatial arrangement of the latter materials in the composite depends on deposition details, including the nature of the casting solvent. The reproducibility of sensing performance is an additional issue having a practical significance. In this work the evaluation of poly(hydroxy ethyl methacrylate) (PHEMA)/carbon black (CB) composites by optical and conductivity measurements is presented. A comparison of sensing performance with that of sensors based on capacitance changes of the neat polymer is included.

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information is also included herein. Films are drop-cast from the polymer/CB dispersions over predefined electrode areas. In Fig. 1A and B the thickness profile of a PHEMA/CB areas for a 2.5 mm scan is plotted for ethyl-S-lactate and HFIP casting solvents; main peaks with a width of 50–500 lm at half maximum are seen in both cases and result from main carbon islands. Both size and spatial distribution of carbon islands are important and additional features can be evaluated from micrographs such as that in Fig. 1C. A substantial portion of carbon in clustered in sizable islands (large island dimension of a few hundred microns and a narrow dimension of a few tens of microns) but there are also numerous islands having a size lower by one or two orders of magnitude and allowing for a substantial reduction of distances between conductive domains.

2. Polymer composite formulation The first step towards the fabrication of a polymer/CB sensor is the selection of the matrix polymer; the selection is largely guided by the exact sensing application in consideration. For example, in the case of polymer/CB sensors for polar analytes, a hydrophilic polymer should be selected. In this work the target application is the sensing of humidity and alcohols and the polymer selected is the hydrophilic PHEMA [9]. The carbon black fillers used in this study were Black Pearls 2000 from Cabot. Solvent casting allows for the very effective preparation of the polymer/conductive particle suspensions in terms of processing time and product homogeneity. Several casting solvents (such as ethyl-S-lactate, hexafluoroisopropanol (HFIP), methyl isobutyl ketone, methanol etc.) were evaluated on the basis of the following criteria: (a) sensitivity and reproducibility and (b) transparency for a substantial load of carbon black; a reasonably transparent carbon-loaded film can provide additional information through in situ thickness measurements. Satisfaction of criterion (a) was found possible for a variety of casting solvents and ethyl-S-lactate was chosen for the preparation of the bulk of the sensing composites considered in this work. Satisfaction of criterion (b) is far from trivial; most of the casting solvents tested led to fully non-transparent films when carbon load reached levels allowing for substantial conductivity-based sensitivity. It was only in the case of casting from HFIP that criterion (b) was met with some success and, consequently, some pertinent

3. Results 3.1. Evaluation of the swelling response In Fig. 2A and B typical swelling measurements for PHEMA films loaded with CB are presented. The swelling was monitored in situ by white light reflectance spectroscopy [10], a non-destructive optical methodology for the dynamic monitoring of film thickness changes [11]. The swelling response is higher in the case of humidity while the response in the case of low concentrations of methanol is very close to the measurement’s noise. In addition, the response exhibits a limited only variation upon change of the cast-

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Fig. 1. (A) Profilometer scan for a ethyl-S-lactate cast composite, with 8% PHEMA w/w and 8% CB w/w (see text). (B) Profilometer scan for a HFIP-cast composite with 0.5% PHEMA w/w and 5% CB w/w (see text). (C) Optical image (100) for a 8% PHEMA w/w loaded with 8% CB w/w cast from ethyl-S-lactate.

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Fig. 2. Swelling response of films (A) cast from ethyl-S-lactate with 8% PHEMA w/w and 8% CB w/w (B) cast from HFIP with 0.5% PHEMA w/w and 5% CB w/w.

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ing solvent. For comparison purposes it is noted that the swelling response of plain PHEMA to the same analytes and concentrations is the same within a factor of two (not shown).

Table 1 Comparison of the resistance and capacitance response of PHEMA/CB (ultrasonicated) and PHEMA IDE sensors. Water conc. (ppm)

Ro (Ohm)

DR/Ro

3.2. Electrical evaluation of the PHEMA/CB composites

6% CB 6% CB 8% CB 8% CB 10% CB 10% CB 12% CB 12% CB 14% CB 14% CB 16% CB 16% CB 18% CB 18% CB 20% CB 20% CB

1000 5000 1000 5000 1000 5000 1000 5000 1000 5000 1000 5000 1000 5000 1000 5000

412739.0 412739.0 64084.0 63802.0 626.7 626.6 668.7 668.6 610.3 610.1 582.2 581.9 399.3 399.5 63.8 63.5

0.0210 0.0200 0.0800 0.2200 0.0010 0.0130 0.0002 0.0026 0.0002 0.0180 0.0007 0.0300 0.0025 0.0990 0.0008 0.0760

Neat PHEMA

1000 5000

Co (pF) 76.5763 77.0879

DC/Co 0.01041 0.08660

In Fig. 3 the resistance changes due to the sorption of two polar analytes, water and methanol, are plotted. In general the response to water is higher than the response to methanol vapors. This can be attributed to the hydroxyl group of HEMA monomer. The same trend has been reported for plain PHEMA film sensors based on capacitance measurements [12]. The effect of application of ultrasonic-assisted agitation of the polymer suspensions is illustrated through DR/Ro comparisons for a wide range of CB concentration in the PHEMA/CB suspension. Dispersion processing with and without ultrasonic agitation can lead to sensitivity levels differing by as much as an order of magnitude. It also noted that ultrasonicated samples tend to exhibit enhanced sensitivities in the case of water vapors and reduced sensitivities in the case of methanol vapors; the difference might be due to the substantially different levels of swelling for the two analytes. Another key observation is that the response is a non-monotonous function of the CB load and our data suggest that one can design satisfactory sensors either in the low load CB regime or in the high load CB regime. While details are case specific, the aforementioned response complexity is not surprising in view of the intricacies of conductivity level vs. conductive component load when combined with swelling [6]. Finally, repeated cycles of sorption/desorption (not shown) indicate no alteration of response. 3.3. Sensing response comparison As it was mentioned in the introduction, non-conducting polymers are employed successfully as gas sensors based on the change of the dielectric constant induced upon the absorption of analytes, especially polar ones. Thus it is of interest to compare the results of the approach employing non-conducting neat polymers monitored via capacitance measurements with those of the approach that uses carbon-loaded versions of the same polymers monitored via resistance measurements; to our knowledge such a study has not been published so far. For this comparison, PHEMA films were drop-cast on interdigitated electrodes (IDEs). In this case, IDEs consisted of dense fingers with 5 lm width at 5 lm distance on dielectric substrate. It should be noted that the transducers do not exhibit the same critical dimensions in the resistance and capaci-

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tance approaches. This is because for resistance sensors with closely-spaced fingers there will be a short-circuit possibility via carbon black bridging, while, conversely, the initial capacitance of the sensor will unacceptably low [12] in case the electrodes of capacitance sensors are spaced apart as much as in resistance sensors. In Table 1 the capacitance and resistance initial values and responses in the case of 1000 and 5000 ppm H2O are listed. The initial resistance drops continuously with increasing CB level as expected. For certain CB loads and vapor concentrations the resistance-based sensitivity can be substantially higher than the capacitance based sensitivity; the case of 8% CB load at both water vapor concentrations provides an example. It should be noted that this enhanced response necessitates an optimum, in terms of CB load, composite film. Even for a PHEMA/CB sensor with a moderately different CB load the response gain can be diminished. Thus, optimized composite design and reproducible fabrication are characteristics crucial for corresponding reliable and high sensing capacity units. 4. Conclusions In this work we study the electrical and optical sensing response of PHEMA/CB composites for humidity and alcohols. The electrical response of polymer/CB based sensors is a non-monotonous function of carbon load; successful sensors can be fabricated with either a low or a high CB load. Optical sensors based on the same composites encounter transparency problems that, nevertheless, can be manipulated within limits through the choice of casting solvent; hexafluoroisopropanol offers some promise. An optimized PHMEMA/CB sensor can exhibit electrical sensitivity exceeding that of sensors based on changes of the dielectric constant of the corresponding CB-free polymer. Acknowledgement

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% CB Fig. 3. Resistance based sensitivity vs. CB load for a wide concentration range of CB with and without ultrasonication for 5000 ppm of methanol and humidity.

This work was partially financially supported by a Greece–Italy bilateral co-operation project. References [1] B. Adhikari, S. Majumdar, Prog. Polym. Sci. 29 (2004) 699. [2] S.V. Patel, S.T. Hobson, S. Cemalovic, T.E. Mlsna, Talanta 76 (2008) 872. [3] F. Tanaka, T. Kawai, S. Kojima, K. Yoshino, Synth. Met. 102 (1999) 1358–1359.

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