- Email: [email protected]
Chemical sol–gel-based sensors for evaluation of environmental humidity
Sensors and Actuators B 126 (2007) 455–460
Chemical sol–gel-based sensors for evaluation of environmental humidity N. Carmona a,∗ , E. Herrero a , J. Llopis b , M.A. Villegas a a
Centro Nacional de Investigaciones Metalurgicas (CENIM), CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain b Facultad de Ciencias Fisicas, Universidad Complutense, Ciudad Universitaria s/n, 28040 Madrid, Spain Received 2 August 2006; received in revised form 12 March 2007; accepted 26 March 2007 Available online 30 March 2007
Abstract Crystal violet and chlorophenol red pH indicators were independently encapsulated into sol–gel matrices to be used as environmental humidity and acidity optical sensors, respectively. Absorption spectra of the prepared sensors were recorded before and after exposure under different controlled artificial atmospheres. Results showed the possibility to estimate the environmental relative humidity with the crystal violet-doped sensor in the range between 20 and 90%, since a colour change occurred after approximately 10 min exposure. On the other hand, the presence of environmental pollutants with acid properties, mainly SO2 , was also possible to be detected by means of an optical absorption change of the chlorophenol red-doped sensor. Its response time was about 1 min. Both sensors are found to be reversible and reusable without showing optical fatigue. Consequently, the sensors are adequate for an easy and fast monitoring in one step of relative humidity and environmental acidity of engraved historical, artistic and cultural heritage assets and can help to improve the preventive conservation of this valuable legacy. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel; Crystal violet; Chlorophenol red; Humidity sensor; Conservation; Cultural heritage
1. Introduction Architectural monuments as well as museums, art galleries, libraries and archive collections are susceptible to deterioration. The presence of some environmental agents and/or non-adequate exhibition and storage conditions can yield severe consequences for the correct conservation of historical, artistic and cultural heritage assets [1,2]. The main dangerous environmental parameters for proper conservation of historical objects are: temperature, light, relative humidity and pollution agents [3]. The simultaneous presence of high relative humidity and pollutants (both atmospheric and non-atmospheric) enhances the chemical degradation of almost all materials [4–6]. Climate and pollution effects on museum collections have been investigated from the understanding of the whole deterioration risk submitted by the historical objects when they are exhibited and stored [7–9]. Conservation difficulties also include a limited access to sophisticated measurement equipments, thus adding a relative high number of places to be monitored and measured (e.g. museum halls, interior of showcases and storage boxes). There-
∗
Corresponding author. E-mail address: [email protected] (N. Carmona).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.03.030
fore, the possibility to monitor environmental parameters in an easy and low-cost way would have profits for the preservation of cultural heritage assets. As it has been pointed out by Cassar et al. [10], the study of the air pollution effects is a research to be developed to improve the basic knowledge on the fundamental mechanisms regulating the response of materials against microclimatic changes. Likewise, the research leading to the development of sensors for detecting atmospheric pollutants should be enhanced. Taking into account the sensitivity of sol–gel thin films based sensors, these kind of devices seems to be the most promising. Moreover, the design and development of multifunctional sensors is necessary to monitor the synergic effect of several pollutants or several environmental parameters, which contribute to the materials damage and final degradation. Sol–gel technology shows important advantages concerned with the preparation of thin films with functional properties. Inorganic and hybrid organic–inorganic sol–gel matrices are formed by polysiloxane networks with interconnected pores that allow the immobilisation of several kinds of chromophores, preserving their properties as well [11]. Reactions between the encapsulated chromophore (dopant) and some external chemical species as solvents, ions or molecules, water from relative humidity or gaseous atmospheric pollutants are favoured in
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N. Carmona et al. / Sensors and Actuators B 126 (2007) 455–460
sol–gel host materials. Thus, it is possible to design and prepare versatile sensors, especially oriented for a particular application connected with preventive conservation aims [12–18]. In order to monitor the environmental humidity, different approaches have been proposed on the basis of an indicator entrapped into an organic or inorganic matrix. Most of these studies are based on conductimetric and/or electrochemical measurements [19,20] or on the recording of refractive index differences [21], which need complicated and voluminous equipments difficult to handle by conservators. Among the available humidity sensors, some of them are characterised by the recording or monitoring optical properties (absorption, luminescence), although they are not based on the sol–gel technology [22–24]. The objective of the present work was to design, prepare and characterise optical sensors obtained by the sol–gel method for a simultaneous monitoring of relative humidity and environmental acidity. The sensor response should be easily monitored by means of a colour change. In addition, sensors should be reversible and with low response time to be finally applied as a useful device for the preventive conservation of historical and cultural heritage assets. 2. Experimental Two sol–gel systems were prepared from silicon tetraethoxide (Si(OCH2 CH3 )4 , TEOS, Fluka Chemika 98%) to be employed as an inert host matrix for the independent incorporation of two organic dyes: crystal violet (hexamethyl-p-rosaniline
chloride, CV, Panreac for analysis) and chlorophenol red (3 ,3 dichlorophenolsulfonephthalein, CR, Panreac for analysis). Both dyes were added in the proportion of 3 wt.% concerning to the final SiO2 content in the corresponding densified gel. TEOS was previously hydrolysed by a hydro-alcoholic solution formed with absolute ethanol (EtOH, Merck 99.9%) and aqueous HCl (final sol pH 2). Molar ratio TEOS:EtOH:H2 O:HCl was 1:8:4:0.03. The CV-doped sol was intense violet coloured while the CR-doped sol was deep red coloured. Both sols were kept under stirring during 30 min. Sols were applied on common glass slides by dip-coating at a drawing rate of 1.35 mm s−1 . Coatings surface was checked by optical microscopy. Neither cracks nor significant surface defects were observed. Coatings were transparent showing optical quality. Coatings thickness was measured by interference fringe method from the corresponding reflection spectra. Results indicated a thickness about 300 nm (±10 nm). The thin films obtained were thermally densified at 60 ◦ C during 3 days. After the heat-treatment the coatings are able to change reversibly their colour when they were dipped into different pH buffered solutions and no leaching was observed. Fig. 1 shows the tautomeric forms of both dyes (CV and CR) in aqueous solutions. General characterisation of the sensitive behaviour of the doped thin coatings under aqueous media has been previously made [25,26]. The following experiment was performed to test their sensitivity when exposed to different relative humidity percentages (CV sensor) and environmental acidities (CR sensor) [26]. The experimental procedure was as follows: the CV sensor was immersed for 2 min in an aqueous buffered solution at pH 2
Fig. 1. Tautomeric forms of (a) crystal violet (CV) and (b) chlorophenol red (CR), under different pH conditions in aqueous solutions.
N. Carmona et al. / Sensors and Actuators B 126 (2007) 455–460
457
Fig. 2. Optical absorption spectra of doped coatings dipped into buffered acid pH solution (continuous line) and buffered basic pH solution (dotted line): (a) CV and (b) CR.
and CR sensor was immersed in a buffered solution at pH 10 for the same time (2 min). In this way, CV molecules inside the corresponding sensor adopt the tautomeric CV acid form. In turn, CR molecules inside the corresponding sensor adopt the basic tautomeric form of CR. Both sensors were submitted side by side under controlled artificial atmospheres containing 20, 40, 60, 70 and 90% of RH at a constant temperature of 25.5 ◦ C and total absence of SO2 during 1, 3, 5, 15, 25, 40, 60, 90 and 120 min. UV–vis absorption spectra of the sensors were recorded before and after each exposure with an UV–Vis spectrophotometer Shimadzu model 3100 (recording range 300–800 nm). Reversibility test for the CV sensor was carried out recording the visible spectrum after immersion for 2 min into a concentrated HCl solution (1 M) and after exposure for 10 min to a 60% RH atmosphere (each cycle). The CR sensor reversibility was evaluated by dipping it successively 2 min in an acid buffered solution (pH 2) and afterwards 2 min in a basic buffered solution (pH 10). The accuracy of experimental data showed in all figures is affected by less than 3% error. 3. Results and discussion Optical behaviour of CV and CR molecules entrapped in the sol–gel matrix was evaluated by recording the absorption spectra after dipping the sensors for 1 min in aqueous buffered commercial solutions. Fig. 2(a) shows the optical spectra of the CV sensor when submitted to pH 2 and 10 buffered solutions. The CV molecule shows two absorption bands at 425 and 625 nm, which provide green colour when the solution pH is 2. The pH increase causes a decreasing in the intensity of the 430 nm absorption band, whereas the intensity of the absorption band
at 625 nm increases and slightly shifts to 590 nm. Finally, when the CV sensor was submitted to pH 10 solution the spectrum exhibited a sole absorption band peaked at 590 nm. Then the sensor displayed a deep blue colour. CR encapsulated molecules show a main absorption band at 430 nm when the sensor was submitted to a pH 2 buffered solution (orange colour). This band disappears at pH 10 and the spectra appeared dominated by an intense band at 570 nm, giving to the sensor a violet colour (Fig. 2(b)). For the experiments carried out under simulated atmosphere with different RH percentages, one CV sensor and one CR sensor, both previously submitted for 24 h to a neutral buffered solution, were employed. The starting optical conditions for the CV sensor correspond always with the spectrum obtained after dipping it for 2 min into a concentrated HCl solution (green colour). The CR sensor starting optical conditions correspond with the spectrum obtained after its immersion into a buffered solution at pH 10 (violet colour). Fig. 3(a) shows, for the CV sensor, the relative absorption intensity at λ = 590 nm as a function of exposure time at different relative humidity percentages. Details about data of Fig. 3(a) recorded for exposure times shorter than 20 min are plotted in Fig. 3(b). Fig. 3(c) shows, for the CR sensor, the evolution of the absolute absorption intensity at λ = 570 nm with the exposure time at different relative humidity percentages. The increasing of the relative absorption corresponding to the CV sensor is faster when the relative humidity percentages are high. For a certain time (e.g. 10 min in Fig. 3(b)), the absorbance increase of the CV sensor at λ = 590 nm versus RH proceeded as is shown in Fig. 4. This calibration curve correlates the change of colour of the CV sensor (from green to deep blue) with the different RH percentages at a fixed time (10 min). Such a time was selected according to
Fig. 3. (a) Relative optical absorption increase of CV sensor at λ = 590 nm; (b) detail of (a) in the first 20 min exposure of the sensor; (c) absolute optical absorption of CR sensor at λ = 570 nm; as a function of the exposure time to different controlled RH atmospheres (20, 40, 60, 70 and 90%).
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Fig. 4. Calibration curve of the CV sensor (relative variation of the optical absorbance vs. the relative humidity percentage) at λ = 590 nm.
Fig. 5. Calibration curve of the CR sensor (relative variation of the optical absorbance vs. the environmental pH) at λ = 430 nm.
the requirements for the final technical application of the sensor (i.e. preventive conservation of historical and cultural assets in museums). Therefore, as Fig. 4 demonstrates, CV sensor is sensitive for a wide RH range (20–90%). The mechanism that drives the CV change of colour by the action of relative humidity is based on a mixed phenomenon of neutralisation–dilution. Water from relative humidity is able to produce the dilution of the acid-basic chemical species (H3 O+ and OH− ), which will react with CV molecules. As is known, dilution induces pH changes up to the limit of neutral conditions. CV-doped coatings are especially sensitive against dilution by relative humidity, since the pH at which the change of colour takes place is in the very acid extreme of the pH scale. Thus, small changes of humidity (i.e. small dilution) can produce noticeable colour changes. Since CV-doped coatings are sensitive both against relative humidity and acid-basic chemical species [25], it is necessary to check the join presence or absence of such chemical species and humidity. The simplest procedure is to check the environmental acidity with a pH-sensor non-sensitive against humidity. For this purpose the CR-doped coating was used. As Fig. 3(c) demonstrates, CR sensor showed neither reduction nor displacement of its characteristic absorption band at 570 nm when submitted to 40, 70 or 90% of RH for 120 min. Thus, the CR sensor is not sensitive to RH changes. Previous results from several tests carried out with this sensor submitted to different concentrations of SO2 (10–50 ppm; SO2 generates sulphuric acid under humid environment) in a climatic chamber [26], showed
a small diminishing of the absorption band intensity at 570 nm and a proportional increasing of the intensity of the 430 nmband. Therefore, CR sensor is found sensitive against pollutants (SO2 ) with acid properties able to change the environmental pH. Fig. 5 shows the evolution of the optical response at λ = 430 nm of CR-doped sensor as a function of the environmental acidity changes. On the basis of the former explanation, one CV sensor and one CR sensor placed together will allow the monitoring of relative humidity and environmental acidity simultaneously. After 10 min exposure the deterioration risk of cultural, artistic and historical assets endangered due to non-adequate exhibition or storage conditions (e.g. interior of closed showcases or storehouses of museums) can be determined. With the presence of acid pollutants, the colour of the CR sensor will change from violet to orange, and the colour of the CV sensor will change from green to blue. On the other hand, if there is deterioration risk only due to high humidity under absence of pollutants, the CV sensor will change its colour in few minutes and will indicate the percentage of environmental relative humidity, while the colour of the CR sensor will remain unchanged. This will be possible using the calibration curves of Figs. 4 and 5 in a semi-quantitative way (a spectrophotometer will be necessary) or qualitatively (visual colour change and comparison with a colour scale). During the former operation under humid atmospheres the CR sensor will maintain its optical properties unchanged, since it does not show sensitivity against relative humidity (Fig. 3(c)).
Fig. 6. Reversibility and optical fatigue of: (a) CR sensor (each cycle = 2 min dipped in pH 2 buffered solution, afterwards 2 min dipped in pH 10 buffered solution; λmax = 570 nm); (b) CV sensor (each cycle = 2 min dipped in concentrated HCl solution, afterwards 10 min exposure to 60% RH atmosphere; λmax = 590 nm).
N. Carmona et al. / Sensors and Actuators B 126 (2007) 455–460
No fading or leaching of the CV or CR molecules incorporated in the sol–gel matrices was observed during the reversibility tests. Both sensors behaved without optical fatigue even after 10 cycles (Fig. 6). The absorption band intensities of the CR sensor were maintained during the cycles from extreme acid to extreme basic buffered solutions. The quick response time recorded (less than 1 min) favoured the results showed in Fig. 6(a). In contrast, the response time of the CV sensor was longer (∼10 min) and the band intensities were not so reproducible (Fig. 6(b)). This is due to the delayed response of the CV molecules encapsulated into the sol–gel matrix depending on the analyte concentration, i.e. the water saturation in the environment to be evaluated. Thus, they take longer times when the sensor is submitted to a more or less humid environment (gaseous phase) in comparison when the sensor is immersed into aqueous solutions (liquid phase) [25]. 4. Conclusions Sol–gel matrices are quite appropriate for the independent encapsulation of two organic dyes, CV and CR, in such a way their chemical reactivity is maintained. The preparation of two optical sensors for the simultaneous monitoring of relative humidity and environmental acidity has been possible. After 10 min exposure of the CV sensor, the RH of the environment where the sensor was placed can be estimated through the corresponding sensor calibration curve. If the CR sensor response does not change, the CV sensor indicates that its response is exclusively due to the environmental RH. The diminishing of the CR sensor absorption band after 10 min exposure, if any, indicates the presence of acid pollutants in the monitored environment (e.g. SO2 ) and, thus, such acid species would disturb the RH evaluation performed by the CV sensor. The CV sensor is sensitive in the 20–90% RH range. On the other hand, the CR sensor is sensitive against the presence of acid pollutants in the 10–50 ppm SO2 range. Both sensors can alert about risks of damage in cultural heritage materials by means of an optical response, i.e. a colour change. These sensors have small size, they are easy to handle, free of batteries and wires, with low-cost processing. These advantages are in fact added values, since conservation routines in museums need frequently to place sensors in special conditions or limited spaces with difficult access, view, position, etc. Acknowledgements The authors wish to acknowledge the financial support of the projects EC-MERG-CT-2004-516436 and FEDER-CICYT Ref. MAT-2003-03231. NC acknowledges CSIC-ESF for a postdoctoral contract. References [1] M. Garc´ıa-Heras, N. Carmona, C. Gil, M.A. Villegas, New optical sensors for monitoring acid environments in preventive conservation, Coalition 7 (2004) 5–7.
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[2] H. R¨omich, T. B¨ohm, Deterioration of glass by atmospheric attack, in: Climatic and air pollution effects on materials and equipment, Gesellsch. Umweltsimulat. 2 (1999) 187. [3] G. Thomson, The Museum Environment, 2nd ed., Butterworths, London, 1986. [4] L. Dupont, J. T´etreault, Cellulose degradation in an acetic acid environment, Stud. Conserv. 45 (2000) 201–210. [5] J. Leissner, Assessment and monitoring the environment of cultural property, in: Proceedings of the International Congress on Environment/Climate, Rome, Italy, 1996. [6] M.F. Mecklenburg, C.S. Tumosa, Temperature and relative humidity effects on the mechanical and chemical stability of collections, Ashrae J. 4 (2000) 77–82. [7] D. Camuffo, R. van Grieken, H.J. Busse, G. Sturaro, A. Valentino, A. Bernardi, N. Blades, D. Shooter, K. Gysels, F. Deutsch, M. Wieser, O. Kim, U. Ulrych, Environmental monitoring in four European museums, Atmos. Environ. 35 (1) (2001) 127–140. [8] P. Brimblecombe, The composition of museum atmospheres, Atmos. Environ. 24B (1) (1990) 1–8. [9] J. Tetreault, Measuring acidity of volatile products, J. IIC-CG 17 (1992) 17–25. [10] M. Cassar, P. Briblecombe, T. Nixon, C. Price, C. Sabbioni, C. S´aizJim´enez, K. Van Balen, in: C. S´aiz-Jim´enez (Ed.), Air Pollution and Cultural Heritage, A.A. Balkema Publishers/Taylor & Francis Group plc, London, UK, 2004, p. 249. [11] K. Matsui, Entrapment of organic molecules, in: H. Kozuka (Ed.), Handbook of Sol–Gel Science and Technology: Process, Springer, The Netherlands, 2005, pp. 459–484. [12] M. Garc´ıa-Heras, C. Gil, N. Carmona, J. Faber, K. Kromka, M.A. Villegas, Optical behaviour of pH detectors based on sol–gel technology, Anal. Chim. Acta 540 (2005) 147–152. [13] C. Rottman, A. Turniansky, D. Avnir, Sol–gel physical and covalent entrapment of three methyl red indicators: a comparison study, J. Sol–Gel Sci. Technol. 13 (1998) 17–25. [14] T.M. Butler, B.D. MacCraith, C. McDonagh, Leaching in sol–gel-derived silica films for optical pH sensing, J. Non-Cryst. Solids 224 (1998) 249–258. [15] M.A. Villegas, M.A. Garc´ıa, S.E. Paje, J. Llopis, Incorporation and optical behaviour of 4-dimethylaminazobenzene in sol–gel silica thin coatings, J. Eur. Ceram. Soc. 22 (2002) 1475–1482. [16] F.B.M. Suah, M. Ahmad, M.N. Taib, Applications of artificial neutral network on signal processing of optical fibre pH sensor based on bromophenol blue doped with sol–gel films, Sens. Actuators B: Chem. 90 (2003) 182–188. [17] F.R. Zaggout, Entrapment of phenol red pH indicator into a sol–gel matrix, Mater. Lett. 60 (2006) 1026–1030. [18] K. Chakrabarti, C.M. Whang, Silver doped Ormosil—an investigation on structural and physical properties, Mater. Sci. Eng. B: Solid 88 (2002) 26–34. [19] I.C. Cosentino, E.N.S. Muccillo, R. Muccillo, Development of zirconiotitania porous ceramics for humidity sensors, Sens. Actuators B: Chem. 96 (2003) 677–683. [20] C.T. Wang, C.L. Wu, I.C. Chen, Y.H. Huang, Humidity sensors based on silica nanoparticle aerogel thin films, Sens. Actuators B: Chem. 107 (2005) 402–410. [21] F.J. Arregui, Z. Ciurriz, M. Oneca, I.R. Matias, An experimental study about hydrogels for the fabrication of optical fiber humidity sensors, Sens. Actuators B: Chem. 96 (2003) 165–172. [22] S.M. Drew, J.E. Mann, B.J. Marquardt, K.R. Mann, A humidity sensor based on vapoluminescent platinum(II) double salt materials, Sens. Actuators B: Chem. 97 (2004) 307–312. [23] M.A. Zanjanchi, S. Sohrabnezhad, Evaluation of methylene blue incorporated in zeolite for construction of an optical humidity sensor, Sens. Actuators B: Chem. 105 (2005) 502–507. [24] B.D.G. Ratnanjali, A novel probe for a fiber optic humidity sensor, Sens. Actuators B: Chem. 80 (2001) 132–135. [25] M.A. Villegas, L. Pascual, Chemical and optical properties of dye-doped sol–gel films, J. Mater. Sci. 35 (2000) 4615–4619.
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[26] N. Carmona, M.A. Villegas, J.M. Fern´andez-Navarro, Optical sensors for evaluating environmental acidity in the preventive conservation of historical objects, Sens. Actuators A: Phys. 16 (2004) 398–404.
Biographies Dr. Noem´ı Carmona obtained her Ph.D. degree in Chemical Sciences from the University of Valladolid (Spain) in 2002, in the study of the alteration processes of historical stained glass windows and the treatments for their restoration and protection. She spend two years as Postdoctoral Marie Curie fellow in Fraunhofer-Institut fuer Silicatforschung, Wuerzburg, Germany. Actually she is an I3P CSIC-Program contracted scientist at Centro Nacional de Investigaciones Metal´urgicas, CENIM-CSIC, Madrid, Spain. Her research interests are: the preventive conservation of Cultural Heritage materials, especially glasses; protective coatings, paint consolidants for stained glass windows and environmental sensing. Dr. Eliseo Herrero studied Chemistry and Chemical Engineering at the University of Salamanca, where he also received his Ph.D. degree on Chemistry in 2005. He spend one year in Centro Nacional de Investigaciones Metal´urgicas, CENIM-CSIC. Actually he works as a research associate at the Materials & Engineering Research Institute of the Sheffield Hallam University. His main
research interests lie in the development of analytical methods for determination of pesticides and their degradation products, development of MIPs for SPE and as sensors, sol–gel chemistry and microbial fuel cells. Prof. Jos´e Llopis was appointed Professor of Physics at the University Complutense of Madrid in 2004, he received his degree in physics in 1974 and Ph.D. in Physics in 1980 both from the University Complutense of Madrid on the study of defects in ionic and semiconductor crystals and their role in the cathodoluminescence processes. Since taking a position at the University Complutense of Madrid he has been responsible of several research projects. His current research interests include sol–gel photonics and spectroscopy of metallic nanoparticles in inorganic materials -glasses and crystals-. ´ Dr. Mar´ıa-Angeles Villegas received her Ph.D. In “Preparation and study of glasses obtained by the sol–gel procedure” form the Universidad Aut´onoma de Madrid (Spain) in 1987. She was a postdoctoral fellow at Glass Department (Ceramic & Glass Institute, CSIC, Madrid) (1988–1989). She is in a permanent position at CSIC, firstly at the Ceramic & Glass Institute (1990–2000) and up to now at the Centro Nacional de Investigaciones Metal´urgicas, CENIM. Her field of research has been sol–gel glasses and coatings, as well as melted glasses for special applications. Her main research interests are: preparation of coatings for sensors; protective systems for historical glasses, metals and other materials; hybrid sol–gels; optically active glassy coatings and sol–gel nanocomposites.
Chemical sol–gel-based sensors for evaluation of environmental humidity N. Carmona a,∗ , E. Herrero a , J. Llopis b , M.A. Villegas a a
Centro Nacional de Investigaciones Metalurgicas (CENIM), CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain b Facultad de Ciencias Fisicas, Universidad Complutense, Ciudad Universitaria s/n, 28040 Madrid, Spain Received 2 August 2006; received in revised form 12 March 2007; accepted 26 March 2007 Available online 30 March 2007
Abstract Crystal violet and chlorophenol red pH indicators were independently encapsulated into sol–gel matrices to be used as environmental humidity and acidity optical sensors, respectively. Absorption spectra of the prepared sensors were recorded before and after exposure under different controlled artificial atmospheres. Results showed the possibility to estimate the environmental relative humidity with the crystal violet-doped sensor in the range between 20 and 90%, since a colour change occurred after approximately 10 min exposure. On the other hand, the presence of environmental pollutants with acid properties, mainly SO2 , was also possible to be detected by means of an optical absorption change of the chlorophenol red-doped sensor. Its response time was about 1 min. Both sensors are found to be reversible and reusable without showing optical fatigue. Consequently, the sensors are adequate for an easy and fast monitoring in one step of relative humidity and environmental acidity of engraved historical, artistic and cultural heritage assets and can help to improve the preventive conservation of this valuable legacy. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel; Crystal violet; Chlorophenol red; Humidity sensor; Conservation; Cultural heritage
1. Introduction Architectural monuments as well as museums, art galleries, libraries and archive collections are susceptible to deterioration. The presence of some environmental agents and/or non-adequate exhibition and storage conditions can yield severe consequences for the correct conservation of historical, artistic and cultural heritage assets [1,2]. The main dangerous environmental parameters for proper conservation of historical objects are: temperature, light, relative humidity and pollution agents [3]. The simultaneous presence of high relative humidity and pollutants (both atmospheric and non-atmospheric) enhances the chemical degradation of almost all materials [4–6]. Climate and pollution effects on museum collections have been investigated from the understanding of the whole deterioration risk submitted by the historical objects when they are exhibited and stored [7–9]. Conservation difficulties also include a limited access to sophisticated measurement equipments, thus adding a relative high number of places to be monitored and measured (e.g. museum halls, interior of showcases and storage boxes). There-
∗
Corresponding author. E-mail address: [email protected] (N. Carmona).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.03.030
fore, the possibility to monitor environmental parameters in an easy and low-cost way would have profits for the preservation of cultural heritage assets. As it has been pointed out by Cassar et al. [10], the study of the air pollution effects is a research to be developed to improve the basic knowledge on the fundamental mechanisms regulating the response of materials against microclimatic changes. Likewise, the research leading to the development of sensors for detecting atmospheric pollutants should be enhanced. Taking into account the sensitivity of sol–gel thin films based sensors, these kind of devices seems to be the most promising. Moreover, the design and development of multifunctional sensors is necessary to monitor the synergic effect of several pollutants or several environmental parameters, which contribute to the materials damage and final degradation. Sol–gel technology shows important advantages concerned with the preparation of thin films with functional properties. Inorganic and hybrid organic–inorganic sol–gel matrices are formed by polysiloxane networks with interconnected pores that allow the immobilisation of several kinds of chromophores, preserving their properties as well [11]. Reactions between the encapsulated chromophore (dopant) and some external chemical species as solvents, ions or molecules, water from relative humidity or gaseous atmospheric pollutants are favoured in
456
N. Carmona et al. / Sensors and Actuators B 126 (2007) 455–460
sol–gel host materials. Thus, it is possible to design and prepare versatile sensors, especially oriented for a particular application connected with preventive conservation aims [12–18]. In order to monitor the environmental humidity, different approaches have been proposed on the basis of an indicator entrapped into an organic or inorganic matrix. Most of these studies are based on conductimetric and/or electrochemical measurements [19,20] or on the recording of refractive index differences [21], which need complicated and voluminous equipments difficult to handle by conservators. Among the available humidity sensors, some of them are characterised by the recording or monitoring optical properties (absorption, luminescence), although they are not based on the sol–gel technology [22–24]. The objective of the present work was to design, prepare and characterise optical sensors obtained by the sol–gel method for a simultaneous monitoring of relative humidity and environmental acidity. The sensor response should be easily monitored by means of a colour change. In addition, sensors should be reversible and with low response time to be finally applied as a useful device for the preventive conservation of historical and cultural heritage assets. 2. Experimental Two sol–gel systems were prepared from silicon tetraethoxide (Si(OCH2 CH3 )4 , TEOS, Fluka Chemika 98%) to be employed as an inert host matrix for the independent incorporation of two organic dyes: crystal violet (hexamethyl-p-rosaniline
chloride, CV, Panreac for analysis) and chlorophenol red (3 ,3 dichlorophenolsulfonephthalein, CR, Panreac for analysis). Both dyes were added in the proportion of 3 wt.% concerning to the final SiO2 content in the corresponding densified gel. TEOS was previously hydrolysed by a hydro-alcoholic solution formed with absolute ethanol (EtOH, Merck 99.9%) and aqueous HCl (final sol pH 2). Molar ratio TEOS:EtOH:H2 O:HCl was 1:8:4:0.03. The CV-doped sol was intense violet coloured while the CR-doped sol was deep red coloured. Both sols were kept under stirring during 30 min. Sols were applied on common glass slides by dip-coating at a drawing rate of 1.35 mm s−1 . Coatings surface was checked by optical microscopy. Neither cracks nor significant surface defects were observed. Coatings were transparent showing optical quality. Coatings thickness was measured by interference fringe method from the corresponding reflection spectra. Results indicated a thickness about 300 nm (±10 nm). The thin films obtained were thermally densified at 60 ◦ C during 3 days. After the heat-treatment the coatings are able to change reversibly their colour when they were dipped into different pH buffered solutions and no leaching was observed. Fig. 1 shows the tautomeric forms of both dyes (CV and CR) in aqueous solutions. General characterisation of the sensitive behaviour of the doped thin coatings under aqueous media has been previously made [25,26]. The following experiment was performed to test their sensitivity when exposed to different relative humidity percentages (CV sensor) and environmental acidities (CR sensor) [26]. The experimental procedure was as follows: the CV sensor was immersed for 2 min in an aqueous buffered solution at pH 2
Fig. 1. Tautomeric forms of (a) crystal violet (CV) and (b) chlorophenol red (CR), under different pH conditions in aqueous solutions.
N. Carmona et al. / Sensors and Actuators B 126 (2007) 455–460
457
Fig. 2. Optical absorption spectra of doped coatings dipped into buffered acid pH solution (continuous line) and buffered basic pH solution (dotted line): (a) CV and (b) CR.
and CR sensor was immersed in a buffered solution at pH 10 for the same time (2 min). In this way, CV molecules inside the corresponding sensor adopt the tautomeric CV acid form. In turn, CR molecules inside the corresponding sensor adopt the basic tautomeric form of CR. Both sensors were submitted side by side under controlled artificial atmospheres containing 20, 40, 60, 70 and 90% of RH at a constant temperature of 25.5 ◦ C and total absence of SO2 during 1, 3, 5, 15, 25, 40, 60, 90 and 120 min. UV–vis absorption spectra of the sensors were recorded before and after each exposure with an UV–Vis spectrophotometer Shimadzu model 3100 (recording range 300–800 nm). Reversibility test for the CV sensor was carried out recording the visible spectrum after immersion for 2 min into a concentrated HCl solution (1 M) and after exposure for 10 min to a 60% RH atmosphere (each cycle). The CR sensor reversibility was evaluated by dipping it successively 2 min in an acid buffered solution (pH 2) and afterwards 2 min in a basic buffered solution (pH 10). The accuracy of experimental data showed in all figures is affected by less than 3% error. 3. Results and discussion Optical behaviour of CV and CR molecules entrapped in the sol–gel matrix was evaluated by recording the absorption spectra after dipping the sensors for 1 min in aqueous buffered commercial solutions. Fig. 2(a) shows the optical spectra of the CV sensor when submitted to pH 2 and 10 buffered solutions. The CV molecule shows two absorption bands at 425 and 625 nm, which provide green colour when the solution pH is 2. The pH increase causes a decreasing in the intensity of the 430 nm absorption band, whereas the intensity of the absorption band
at 625 nm increases and slightly shifts to 590 nm. Finally, when the CV sensor was submitted to pH 10 solution the spectrum exhibited a sole absorption band peaked at 590 nm. Then the sensor displayed a deep blue colour. CR encapsulated molecules show a main absorption band at 430 nm when the sensor was submitted to a pH 2 buffered solution (orange colour). This band disappears at pH 10 and the spectra appeared dominated by an intense band at 570 nm, giving to the sensor a violet colour (Fig. 2(b)). For the experiments carried out under simulated atmosphere with different RH percentages, one CV sensor and one CR sensor, both previously submitted for 24 h to a neutral buffered solution, were employed. The starting optical conditions for the CV sensor correspond always with the spectrum obtained after dipping it for 2 min into a concentrated HCl solution (green colour). The CR sensor starting optical conditions correspond with the spectrum obtained after its immersion into a buffered solution at pH 10 (violet colour). Fig. 3(a) shows, for the CV sensor, the relative absorption intensity at λ = 590 nm as a function of exposure time at different relative humidity percentages. Details about data of Fig. 3(a) recorded for exposure times shorter than 20 min are plotted in Fig. 3(b). Fig. 3(c) shows, for the CR sensor, the evolution of the absolute absorption intensity at λ = 570 nm with the exposure time at different relative humidity percentages. The increasing of the relative absorption corresponding to the CV sensor is faster when the relative humidity percentages are high. For a certain time (e.g. 10 min in Fig. 3(b)), the absorbance increase of the CV sensor at λ = 590 nm versus RH proceeded as is shown in Fig. 4. This calibration curve correlates the change of colour of the CV sensor (from green to deep blue) with the different RH percentages at a fixed time (10 min). Such a time was selected according to
Fig. 3. (a) Relative optical absorption increase of CV sensor at λ = 590 nm; (b) detail of (a) in the first 20 min exposure of the sensor; (c) absolute optical absorption of CR sensor at λ = 570 nm; as a function of the exposure time to different controlled RH atmospheres (20, 40, 60, 70 and 90%).
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Fig. 4. Calibration curve of the CV sensor (relative variation of the optical absorbance vs. the relative humidity percentage) at λ = 590 nm.
Fig. 5. Calibration curve of the CR sensor (relative variation of the optical absorbance vs. the environmental pH) at λ = 430 nm.
the requirements for the final technical application of the sensor (i.e. preventive conservation of historical and cultural assets in museums). Therefore, as Fig. 4 demonstrates, CV sensor is sensitive for a wide RH range (20–90%). The mechanism that drives the CV change of colour by the action of relative humidity is based on a mixed phenomenon of neutralisation–dilution. Water from relative humidity is able to produce the dilution of the acid-basic chemical species (H3 O+ and OH− ), which will react with CV molecules. As is known, dilution induces pH changes up to the limit of neutral conditions. CV-doped coatings are especially sensitive against dilution by relative humidity, since the pH at which the change of colour takes place is in the very acid extreme of the pH scale. Thus, small changes of humidity (i.e. small dilution) can produce noticeable colour changes. Since CV-doped coatings are sensitive both against relative humidity and acid-basic chemical species [25], it is necessary to check the join presence or absence of such chemical species and humidity. The simplest procedure is to check the environmental acidity with a pH-sensor non-sensitive against humidity. For this purpose the CR-doped coating was used. As Fig. 3(c) demonstrates, CR sensor showed neither reduction nor displacement of its characteristic absorption band at 570 nm when submitted to 40, 70 or 90% of RH for 120 min. Thus, the CR sensor is not sensitive to RH changes. Previous results from several tests carried out with this sensor submitted to different concentrations of SO2 (10–50 ppm; SO2 generates sulphuric acid under humid environment) in a climatic chamber [26], showed
a small diminishing of the absorption band intensity at 570 nm and a proportional increasing of the intensity of the 430 nmband. Therefore, CR sensor is found sensitive against pollutants (SO2 ) with acid properties able to change the environmental pH. Fig. 5 shows the evolution of the optical response at λ = 430 nm of CR-doped sensor as a function of the environmental acidity changes. On the basis of the former explanation, one CV sensor and one CR sensor placed together will allow the monitoring of relative humidity and environmental acidity simultaneously. After 10 min exposure the deterioration risk of cultural, artistic and historical assets endangered due to non-adequate exhibition or storage conditions (e.g. interior of closed showcases or storehouses of museums) can be determined. With the presence of acid pollutants, the colour of the CR sensor will change from violet to orange, and the colour of the CV sensor will change from green to blue. On the other hand, if there is deterioration risk only due to high humidity under absence of pollutants, the CV sensor will change its colour in few minutes and will indicate the percentage of environmental relative humidity, while the colour of the CR sensor will remain unchanged. This will be possible using the calibration curves of Figs. 4 and 5 in a semi-quantitative way (a spectrophotometer will be necessary) or qualitatively (visual colour change and comparison with a colour scale). During the former operation under humid atmospheres the CR sensor will maintain its optical properties unchanged, since it does not show sensitivity against relative humidity (Fig. 3(c)).
Fig. 6. Reversibility and optical fatigue of: (a) CR sensor (each cycle = 2 min dipped in pH 2 buffered solution, afterwards 2 min dipped in pH 10 buffered solution; λmax = 570 nm); (b) CV sensor (each cycle = 2 min dipped in concentrated HCl solution, afterwards 10 min exposure to 60% RH atmosphere; λmax = 590 nm).
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No fading or leaching of the CV or CR molecules incorporated in the sol–gel matrices was observed during the reversibility tests. Both sensors behaved without optical fatigue even after 10 cycles (Fig. 6). The absorption band intensities of the CR sensor were maintained during the cycles from extreme acid to extreme basic buffered solutions. The quick response time recorded (less than 1 min) favoured the results showed in Fig. 6(a). In contrast, the response time of the CV sensor was longer (∼10 min) and the band intensities were not so reproducible (Fig. 6(b)). This is due to the delayed response of the CV molecules encapsulated into the sol–gel matrix depending on the analyte concentration, i.e. the water saturation in the environment to be evaluated. Thus, they take longer times when the sensor is submitted to a more or less humid environment (gaseous phase) in comparison when the sensor is immersed into aqueous solutions (liquid phase) [25]. 4. Conclusions Sol–gel matrices are quite appropriate for the independent encapsulation of two organic dyes, CV and CR, in such a way their chemical reactivity is maintained. The preparation of two optical sensors for the simultaneous monitoring of relative humidity and environmental acidity has been possible. After 10 min exposure of the CV sensor, the RH of the environment where the sensor was placed can be estimated through the corresponding sensor calibration curve. If the CR sensor response does not change, the CV sensor indicates that its response is exclusively due to the environmental RH. The diminishing of the CR sensor absorption band after 10 min exposure, if any, indicates the presence of acid pollutants in the monitored environment (e.g. SO2 ) and, thus, such acid species would disturb the RH evaluation performed by the CV sensor. The CV sensor is sensitive in the 20–90% RH range. On the other hand, the CR sensor is sensitive against the presence of acid pollutants in the 10–50 ppm SO2 range. Both sensors can alert about risks of damage in cultural heritage materials by means of an optical response, i.e. a colour change. These sensors have small size, they are easy to handle, free of batteries and wires, with low-cost processing. These advantages are in fact added values, since conservation routines in museums need frequently to place sensors in special conditions or limited spaces with difficult access, view, position, etc. Acknowledgements The authors wish to acknowledge the financial support of the projects EC-MERG-CT-2004-516436 and FEDER-CICYT Ref. MAT-2003-03231. NC acknowledges CSIC-ESF for a postdoctoral contract. References [1] M. Garc´ıa-Heras, N. Carmona, C. Gil, M.A. Villegas, New optical sensors for monitoring acid environments in preventive conservation, Coalition 7 (2004) 5–7.
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Biographies Dr. Noem´ı Carmona obtained her Ph.D. degree in Chemical Sciences from the University of Valladolid (Spain) in 2002, in the study of the alteration processes of historical stained glass windows and the treatments for their restoration and protection. She spend two years as Postdoctoral Marie Curie fellow in Fraunhofer-Institut fuer Silicatforschung, Wuerzburg, Germany. Actually she is an I3P CSIC-Program contracted scientist at Centro Nacional de Investigaciones Metal´urgicas, CENIM-CSIC, Madrid, Spain. Her research interests are: the preventive conservation of Cultural Heritage materials, especially glasses; protective coatings, paint consolidants for stained glass windows and environmental sensing. Dr. Eliseo Herrero studied Chemistry and Chemical Engineering at the University of Salamanca, where he also received his Ph.D. degree on Chemistry in 2005. He spend one year in Centro Nacional de Investigaciones Metal´urgicas, CENIM-CSIC. Actually he works as a research associate at the Materials & Engineering Research Institute of the Sheffield Hallam University. His main
research interests lie in the development of analytical methods for determination of pesticides and their degradation products, development of MIPs for SPE and as sensors, sol–gel chemistry and microbial fuel cells. Prof. Jos´e Llopis was appointed Professor of Physics at the University Complutense of Madrid in 2004, he received his degree in physics in 1974 and Ph.D. in Physics in 1980 both from the University Complutense of Madrid on the study of defects in ionic and semiconductor crystals and their role in the cathodoluminescence processes. Since taking a position at the University Complutense of Madrid he has been responsible of several research projects. His current research interests include sol–gel photonics and spectroscopy of metallic nanoparticles in inorganic materials -glasses and crystals-. ´ Dr. Mar´ıa-Angeles Villegas received her Ph.D. In “Preparation and study of glasses obtained by the sol–gel procedure” form the Universidad Aut´onoma de Madrid (Spain) in 1987. She was a postdoctoral fellow at Glass Department (Ceramic & Glass Institute, CSIC, Madrid) (1988–1989). She is in a permanent position at CSIC, firstly at the Ceramic & Glass Institute (1990–2000) and up to now at the Centro Nacional de Investigaciones Metal´urgicas, CENIM. Her field of research has been sol–gel glasses and coatings, as well as melted glasses for special applications. Her main research interests are: preparation of coatings for sensors; protective systems for historical glasses, metals and other materials; hybrid sol–gels; optically active glassy coatings and sol–gel nanocomposites.