Impact of Congo red dye in nano-porous silicon as pH-sensor

Sensors and Actuators B 216 (2015) 279–285

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Impact of Congo red dye in nano-porous silicon as pH-sensor Abdel-Hady Kashyout a , Hesham Soliman a , Marwa Nabil a,b,∗ , Ahmed Bishara b a Advanced Technology and New Materials Research Institute, City for Scientific Research and Technology Applications, New Borg El-Arab City, Alexandria, Egypt b Physics Department, Faculty of Science, Alexandria University, P.O.B. 21934 Alexandria, Egypt

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Article history: Received 20 January 2015 Received in revised form 22 March 2015 Accepted 28 March 2015 Available online 17 April 2015 Keywords: Nano-porous silicon Anisotropic alkali etching Congo red pH sensor

a b s t r a c t The combination of nano-porous silicon (nPS) with Congo red (CR) represents a nano-porous matrix which offer broad avenue of new and interesting properties depending on the involved materials as well as their morphology. Chemical route was utilized as the host material to achieve pores filling. They were impregnated with Congo red, which gave good results for the porous silicon as a promising pH sensor. The behavior of (nPS/CR) pH sensor, transformation of color (red-blue), is reversible after exposure to NH3 vapors, (red-blue-red). The ON/OFF behavior of the composite material depends upon the chemical processes occurring inside the pores. The fabrication of (nPS/CR) pH sensor depends on several factors {CR concentration, solvent type, and the chemical mechanism of CR adsorption on PS}. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Several materials with large specific surface area, such as polymer membranes, carbon nanotubes, and metal oxide have been studied as prospective candidates for future gas sensors. Porous silicon has attracted the attention of many experimental and theoretical researches. PS is an especially interesting material with large specific surface area, which is easily produced by electrochemical process [1]. Extensive investigations of the properties of porous silicon (PS) have revealed good prospects for its numerous applications in various fields; including solar cells, biotechnology, and sensor technology [2]. Recently, many chemical sensors using PS have been reported to be operated at room temperature, which is not possible for other semiconductor gas sensors. Additionally, it is easy to fabricate with an advantage of controlling the surface morphology through the variation of the formation parameters [1]. However, the instability of the hydro-silicon bond, which can undergo spontaneous oxidation in ambient atmosphere and results

∗ Corresponding author at: Advanced Technology and New Materials Research Institute, City for Scientific Research and Technology Applications, New Borg El-Arab City, Alexandria, Egypt. Tel.: +20 3 4593416; fax: +20 3 4593423. E-mail addresses: [email protected] (A.-H. Kashyout), [email protected] (H. Soliman), [email protected] (M. Nabil), [email protected] (A. Bishara). http://dx.doi.org/10.1016/j.snb.2015.03.099 0925-4005/© 2015 Elsevier B.V. All rights reserved.

in the degradation of surface structure, remains a key issue for industrial production [3]. Great effort has been done to discover simple, reliable and general techniques for preparation of porous silicon. In this view we present an original synthetic route to impregnation dispersed dye inside a porous silicon thin film structure [4]. Methods developed for the chemical etching of silicon make possible effective control over the parameters of the porous layer with a view to optimization of the adsorption properties of PS [2]. Chemical sensors based on PS display some advantages comparing with other transducers [5]. The open porous structure and the very large specific surface area make mesoporous silica and silicon ideal candidates to host specific organic molecules (dyes, surfactants, polymers, etc.) [6]. A new technique for the production nPS has been applied to be used as an acidic pH indicator. The impregnation method is described and studied by means of Fourier Transform Infrared Spectrophotometer. 2. Experimental work Porous silicon have been produced depending on our previous study [7], using alkaline medium that is preferable with respect to the disadvantages of using HF regarding its toxicity, corrosivity and hazardous to water. The used samples were already oxidized due to the exposure to atmosphere. Congo red; is a diazo anionic dye known as pH indicator (color change in the 3–5.2 pH interval), purchased by Sigma–Aldrich. N,N-dimethylformamide (DMF) has been selected as solvent. For the impregnation, solutions with

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concentration range 7 × 10−3 –6 × 10−2 M, i.e. widely below the solubility limit, have been used [8]. Homogeneously covered oxidized porous silicon (OPS) surface was prepared by 10 ␮l/cm2 of dye solution, that is determined by SEM (Scanning electron microscopy, JEOL (JSM 5300)), which deposited by a micro-pipette. The drying process must be slow and carefully controlled to ensure a homogeneous penetration of the dye along the pores; consequently it is carried out under a reduced pressure for about 4 h. The impregnation method is determined by FTIR (Fourier Transform Infrared Spectrophotometer-Shimadzu FTIR – 8400 s, Japan), which investigate the distribution of the dye along the pores. The linear scans on the OPS layer have been performed. Before each measurement, the freshly cleaved sample has been exposed to HCl vapors for about 10 s. HCl and NH3 vapors were obtained directly from commercial solutions. The main factors affecting the production of (nPS-CR-pH) sensor {dye concentration, solvent type, and the chemical mechanism} have been determined and studied. 3. Results and discussion (OPS) layers were examined with (SEM) before and after (CR) deposition. SEM images show the (OPS) layer prepared on a Si substrate were seen clearly on the surface view in Fig. A1a and b, which show (OPS) sample surface before deposition of (CR) and after uniformly deposited with (CR), respectively. A uniform dispersion of (CR) molecules was appeared in the difference between the values of pores thickness in Fig. A1c and d, the pore thickness 44 nm and enlarged to 120 nm, respectively, that as a result of presence of (CR) molecules in pores. Azo dyes constitute an abundant class of compounds containing N N group. Presumably more than 2000 different azo dyes are currently used for the dyeing of various materials such as

textiles, leather, plastics, cosmetics, and food. A wide range of functional groups (amino-, carboxy-, halo-, hydroxyl-, nitro-, sulfo-, etc.) can be found in the structure of these dyes together with one or more azo groups. Variety of functional groups and the structural arrangement determine the differences in the chemical properties of the dyes [9]. For example, the azo dye molecules can be both neutral and ionic, while the ionic ones may exist either as cationic, anionic or ionic species. A typical representative of this class is Congo red (CR), the sodium salt of benzidinediazo-bis-1naphthylamine-4-sulfonic acid, a diazo dye [10]. Fig. A2 shows FTIR spectra of nPS without the dye which contains the porous silicon peak at wave-number 1015 cm−1 . It also contains the FTIR spectra of CR alone by sharp peaks and high intensity of each bond in CR compound. Finally, the FTIR spectra of PS, which is coated by CR (0.06 M) in DMF as a solvent, is shown in Fig. A1. It is obvious in FTIR spectra of (nPS-CR), that the intensity of nPS-peak by itself is decreased after joining with CR. As shown in Fig. A2, the peaks in the FTIR spectrum of CR are associated with the structure of this compound as well. The broad band at 3450 cm−1 represents NH stretching vibration. Bands at 1611 cm−1 ( N N stretching vibration), 1364 cm−1 and 1227 cm−1 ( C N stretching vibration adjacent to aromatic ring), 1047 cm−1 (S O stretching vibrations in sulfonate groups) are in consistent with the structure of CR [11,6]. 3.1. Effect of Congo red dye concentration Figs. A3 and A4 show the effect of changing the CR dye concentration on the FTIR peaks in acidic or basic vapors. The band range of each chemical group which forms CR dye material is clearly shown with changing the dye concentration from 0 007 M to 0.06 M. At 0.06 M CR, there is no change in the FTIR spectral peaks for other bonds than sulfonate group by changing the concentration of CR dye. In addition, the lowest transmittance percent at the same

Fig. A1. SEM images: (OPS) sample surface before deposition of (CR). (OPS) sample surface after deposition of (CR). (OPS) sample cross-section before deposition of (CR), pore thickness = 44 nm. (OPS) sample cross-section after deposition of (CR), pore thickness enlarged to 120 nm.

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Fig. A2. FTIR spectra of {nPS, Congo red (CR) and nPS-CR} at (0.06 M) Congo red dissolved in DMF.

concentration of CR, which explains the adsorption, process of CR molecules on surface pores of nPS. Noticeable, the best concentration of CR dye is (0.06 M), which has clear broad band at sulfonate groups range. This means that sulfonate group is the only factor responsible for attachment on porous silicon surface. Figs. A3 and A4 show FTIR spectra of {pH-nPS-sensor} at several CR concentrations using DMF as a solvent of CR by exposure to acidic and basic vapors, respectively. Noticeable, intensity values of ( SO3 ), ( N N ) and ( N H ) bands are directly proportional with the CR concentrations during exposure to acidic vapors. In case of exposure to basic vapors, intensity values of ( SO3 ) and ( C C ) bands are directly proportional with CR concentrations. On the other hand, the intensity values of ( N N ), ( C C Aromatic) and ( N H ) are inversely proportional with CR concentrations. Then, CR concentration 0 06 M is the optimal concentration at exposure to acidic and basic vapors. Sulfonate group is only responsible of attachment, as shown in the following chemical mechanism [6], complex of CR dye to OPS

surface. As decreasing concentration of CR dye produce decreasing of the number of sulfonate groups. And thus the number of attachment groups in CR dye on OPS surface is decreased too, that appears in Fig. A5, disappear of sulfonate peak at the lowest concentration of CR dye with respect to the presence of Si O vibration band. 3.2. Chemical adsorption mechanism of Congo red dye Fig. A6a shows the wavenumber shift of the sulfonate group as a result of exposure to acidic vapors (HCl). It is clear at 0.03 M Congo red, the wavenumber shift of sulfonate group is nearly equal to zero. This means that there is no change in the chemical mechanism at this concentration. While, it has the highest shift value at 0.015 M as well as 0.06 M of Congo red. Fig. A6b shows the wavenumber shifting as a result of exposure to basic vapors (NH4 OH). Clearly; at 0.03 M Congo red, the wavenumber shift of sulfonate group is nearly equal zero, i.e. no change in

Fig. A3. FTIR spectra of {pH-nPS-sensor} at several concentrations of Congo red dissolved in DMF and exposure to acidic vapor.

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Fig. A4. FTIR spectra of {pH-nPS-sensor} at several concentrations of Congo red dissolved in DMF and exposure to basic vapor.

Fig. A5. Structural changes in CR molecules.

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indeed when the sample is exposed to NH4 OH vapors, a backwards shift is observed. Also the color of the surface turns back to red. These results confirm the pH indicator properties of CR embedded in the OPS. In the presence of ammonia, deprotonation occurs with formation of ammonium ions. Therefore, CR/OPS can be obtained only on using CR/DMF solutions with concentration 10−4 to 10−5 M, (i.e. a low amount of dye in the pores). The optical behavior, in the presence of acidic or basic vapors has been studied. Despite the low amount of CR, a dependency of pH-sensor efficiency upon the kind of vapors has been observed. A good reproducibility has been obtained after several cycles of exposures to HCl and NH4 OH. After ten cycles, the performance of the material is reduced because of the formation of NH4 OH that probably hampers the diffusion of vapors in the pores.

3.3. Type of solvent

Fig. A6. Effect of exposure pH-sensor to: (a) acidic vapors at different dye concentrations, (b) basic vapors at different dye concentrations.

chemical mechanism at that concentration. Also, minimum change in wavenumber at 0.03 M Congo red as detected for the acidic media. The peaks in the FTIR for {azo, alkene, and amine} bonds do not change due to any change for the CR dye concentration. Accordingly, the sulfonate group is the only group which is responsible for complexation of CR on porous silicon surface to form pH-nPS sensor. The color of PH-nPS sensor is changed {red–blue–red} as a result of exposure to HCl and NH4 OH vapors. We notice that, {( N N ), ( C C ) and ( N H )} peaks changed at acidic vapor and basic vapor. That appears as a shift in wavenumber value for each case. Then, Sulfonate group ( SO3 ) is only responsible of attachment, as shown in Fig. A7, complex of Congo red dye to oxidized porous silicon surface. Decreasing the concentration of CR dye, the number of sulfonate groups will also decrease and accordingly the number of attachment groups in Congo red dye on porous silicon surface is also decreased. The function groups {( N N ) azo group, ( C C ) alkene group, ( N H ) amine group}, in CR dye complex, are responsible of changing in colors due to the exposure to different pH whether acidic (HCl) or basic vapor (NH4 OH). pH-nPS sensor color is changed with respect to the value of pH vapor. The formation of ammonium form, by exposure to acidic vapor, produces blue color. The formation of azonium form, by exposure to basic vapor produces red color, as shown in Fig. A7. The color of the prepared samples, as shown in Fig. A8, will be changed after protonation of CR, as shown in Fig. A7 due to the shift of the band of the azo groups. Comparison with the spectrum of CR in DMF solution shows that this transition is influenced by solvation; it is evident on transformation from the CR/DMF solution to the CR/OPS system. The spectrum recorded after exposure to HCl vapors shows that the band shifted due to the protonation of the azo groups. This shift is accompanied by change in color of the sample surface, from red to blue (color of the acidic form of CR). The process is reversible,

In free solution, the color of CR depends not only on the pH but also governed by the nature of the solvent environment. In stained porous silicon, the porosity of the substrate may influence their color. The effect of non-aqueous solvents (i.e. DMF) is probably modifying the ionization state of the dye-substrate complex, thus altering the color of the CR. Such solvents may also change the aggregation or solvation states of the dye. In aqueous solution, CR is an acid–base indicator with a characteristic color transition to blue below pH 5 [12]. The characteristic pH-dependent changes in the appearance of CR were established in aqueous solutions at a variety of pH values. It was found that not only does the color of the dye alter with change of pH, but also its solubility. This pattern was markedly altered in the presence of non-aqueous solvents at identical measured pH values. In OPS substrate, the observed acid-induced color changes may be affected by the binding type of dye with substrate. Dye-substrate ionization, a possible reason for the differential susceptibility of various blue-stained porous layer components to treatment in solvents relates to the pH of the dye-substrate complex. CR-stained substrates at higher pH may be less resistant to solvent effect than dye-substrate combinations at lower pH [12]. Fig. A9 shows the FTIR spectra of OPS/CR substrate using several solvents of CR {non-aqueous (DMF) and aqueous}. The influence of solvents on ionization and dye aggregation can be correlated with the appearances of OPS/CR substrate in various solvents. The results suggest that change of solvent affects not only the CR indicator system but also the acid–base systems with which the dye interacts. The pH measurements of the buffer solutions in nonaqueous solvents indicate that the effect of DMF is to shift the pH of the acidified CR substrate complex to alkaline. This, coupled with the high disaggregating power of the solvent, renders all OPS/CR components red. Similarly, in aqueous, the pH of the acid–base system is shifted in the alkaline direction, but in this case the more moderate disaggregating power of the solvent leads some OPS/CR components to remain blue providing the exposure time is short. At low pH, acidic and neutral substrates tend to assume the blue coloration induced by the external acid environment, whilst basic substrate probably accepts protons from the cationic dye and thereby appear red. However, substrate structure and porosity may modulate these color appearances. The color of stained and acidified substrates is modified on exposure to nonaqueous solvents. Their effect is probably to alter the ionization state of the dye and substrate, and the degree of dye aggregation.

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Fig. A7. Structural changes in CR molecules reacting with nPS.

Fig. A9. Comparison between two FTIR spectra of {pH-nPS-sensor} at CR concentration = 0.06 M (dissolved in DMF and distl.H2 O). Fig. A8. Photographs of prepared pH-nPS sensor: (a) exposure to vapor (pH = 1–3), (b) exposure to vapor (pH = 4–8), (c) exposure to vapor (pH = 9–11) and (d) Exposure to vapor (pH = 12–14).

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4. Conclusion The preparation of PS layers on crystalline silicon wafers has been studied. An impregnation procedure for the preparation of CR/OPS composite materials has been developed. This process yields a homogeneous distribution of Congo red along the pores, as shown in SEM images. CR pH indicator properties are retained in the CR/OPS single and double layers. With low CR concentration, the guiding properties are retained. Notwithstanding the low amount of dye, HCl vapors cause a remarkable transformation of color to blue. This behavior is reversible after exposure to NH3 vapors. The ON/OFF behavior of the composite material depends upon the chemical processes occurring inside the pores. A good reproducibility has been obtained ten cycles of exposures to HCl and NH4 OH. After these cycles the performance of the material are reduced because of the formation of NH4 OH that probably hampers the diffusion of vapors in the pores. The appearance of OPS/CR substrate in different solvents which having effects on ionization and dye aggregation has been studied using FTIR spectra. The results obtained show that the type of the solvent affects not only the CR indicator system but also the acid–base systems in which the dye interacts. The pH measurements of CR solution in non-aqueous solvents indicate that the effect of DMF is to shift the pH of the acidified CR substrate complex to alkaline. This, coupled with the high disaggregating power of the solvent, renders all OPS/CR components red. In aqueous solution, the pH of the acid–base system is shifted in the alkaline direction, but in this case the more moderate disaggregating power of the solvent leads some OPS/CR components to remain blue provided the exposure time is short. References [1] S. Peng, H. Ming, L.I. Ming-Da, M.A. Shuang-Yun, Microstructure, electrical and gas sensing properties of meso-porous silicon and macro-porous silicon, Acta Phys. Chim. Sin. 28 (2012) 489–493. [2] E.A. Tutov, M.N. Pavlenko, E.E. Tutov, I.V. Protasova, E.N. Bormontov, Equilibrium and nonequilibrium electrode processes on porous silicon, Tech. Phys. Lett. 32 (2006) 558–560. [3] M. Atyaoui, W. Dimassi, G. Monther, R. Chtourou, H. Ezzaouia, Electrochemical deposition of cerium on porous silicon to improve photoluminescence properties, J. Lum. 132 (2012) 277–281. [4] E.D. Gaspera, V. Bello, G. Mattei, A. Martucci, SiO2 mesoporous thin films containing Ag and NiO nanoparticles synthesized combining sol–gel and impregnation techniques, Mater. Chem. Phys. 131 (2011) 313–319. [5] A. Benilov, I. Gavrilchenko, I. Benilova, V. Skryshevsky, M. Cabrera, Influence of pH solution on photoluminescence of porous silicon, Sens. Actuators A 137 (2007) 345–349. [6] P. Rivolo, P. Pirasteh, A. Chaillou, P. Joubert, M. Kloul, J.F. Bardeau, F. Geobaldo, Oxidised porous silicon impregnated with Congo red for chemical sensoring applications, Sens. Actuators B 100 (2004) 99–102.

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Biographies Abdel Hady Kashyout He received his B.Sc. from Alexandria University, Egypt in 1989 in Electrical Engineering. Since 1991 he has been employed by City for Scientific Research and Technology Applications (SRTA-City). He received his M.Sc. from Cairo University and Ph.D. from Alexandria University, in 1998 and 2001, respectively. He joined National Research Center at Messina, Italy as a research associate in 1993–1996. He also worked as a visiting professor at the Royal Institute of Technology (KTH). He was the Dean of Advanced Technology and New Materials Research Institute, City for Scientific Research and Technology Applications SRTA-City) (2008–2011). Currently, vice manager of SRTA-City. Hisham Soliman He received his B.Sc. from Alexandria University, Egypt in 1985 in Chemistry. Since 1987 he has been employed by research assistant in the national research centre in Cairo, Egypt followed by assistant researcher in 1992. He received his M.Sc. from Alexandria University and Ph.D. from Manchester, UK and Alexandria University, in 1991 and 1999, respectively. In 2004 worked as assistant professor in Nanotechnology and New Composite Materials Department, City for Scientific Research and Technology Applications, Alexandria, Egypt (SRTA-City) (2008–2011). Currently, Dean of Advanced Technology and New Materials Research Institute, (SRTA-City). Marwa Nabil She received her B.Sc. from Alexandria University, Egypt in 1999 in Physics and Chemistry. She received her M.Sc. and Ph.D. from Alexandria University, in 2004 and 2014, respectively. She was Demonstrator in Faculty of Education, Alexandria University – Damanhor Branch (2000–2004), Assistant Lecturer, Alexandria University – Damanhor Branch (2004–2005), Physical specialist in Advanced Technology and New Materials Research Institute, SRTA-City (2007–2011), Associate researcher in Electronics Materials Department, SRTA-City 2012–2014. Currently, researcher in Electronics Materials Department, SRTA-City. The fields of interest, renewable energy, nanotechnology in many applications, Porous silicon and its applications. Ahmed Bishara He received his B.Sc. from Alexandria University, Egypt in 1960 in Physics and Mathematics. Since 1960 he has been employed by Phys. Department, Faculty of Science, Alexandria University, Egypt. He received his M.Sc. from Alexandria University and Ph.D. from Moscow state University, in 1966 and 1974, respectively. He also worked as a Vice-Dean for student affairs, Faculty of Science, Alexandria University (1988–1990). He was Head of Physics Department, Faculty of Science, Alexandria University (1995–1998). Currently, Emeritus Prof. of Nuclear Physics, Physics Department, Faculty of Science, Alexandria University.