Accepted Manuscript

Title: Mesoporous SiO2-TiO2 nanocomposite for pH Sensing

Author: Shumaila Islam Noriah Bidin Saira Riaz Rosly A.Rahman Shahzad Naseem

PII: S0925-4005(15)00854-0DOI: http://dx.doi.org/doi:10.1016/j.snb.2015.06.095Reference: SNB 18672

To appear in: Sensors and Actuators B

Received date: 7-4-2015Revised date: 4-6-2015Accepted date: 14-6-2015

Please cite this article as: S. Islam, N. Bidin, S. Riaz, R.A. Rahman, S. Naseem,Mesoporous SiO2-TiO2 nanocomposite for pH Sensing, Sensors and Actuators B:Chemical (2015), http://dx.doi.org/10.1016/j.snb.2015.06.095

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Mesoporous SiO2-TiO2 nanocomposite for pH Sensing

Shumaila Islam1), Noriah Bidin1)*, Saira Riaz2), Rosly A. Rahman1), Shahzad Naseem2) (1)Laser Centre, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi

Malaysia, Skudai, Johor 81310 Malaysia

e-mail: shumaium@yahoo.com, noriah@utm.my, roslyaraes@gmail.com (2)Centre of Excellence in solid State Physics, University of the Punjab, Lahore, Pakistan.

e-mail: saira_cssp@yahoo.com, shahzad_naseem@yahoo.com

ABSTRACT

Thermally stable mesoporous silica-titania nanocomposite matrix was obtained via sol-gel

method. The samples prior and after heat treatment were characterized by analytical instrumental

techniques like Field Emission Scanning Electron Microscope (FE-SEM), Energy Dispersive X-

Ray Spectroscopy (EDX), Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), Thermal

Gravimetric Analysis (TGA), surface area, porosity measurements and UV-Vis Spectroscopy.

The FE-SEM and AFM micrographs suggest that the indicator molecules are homogeneously

diffused into matrix pores of the silica-titania network. The matrix exhibits regular morphology

with large pores without any cracks. Energy Dispersive X-Ray Spectroscopy (EDX) analysis

verified the stoichiometric formation of encapsulated silica-titania nanocomposite matrix

whereas calculated average crystallite size 3.82 nm was obtained by XRD. N2 adsorption

isotherms explain the mesoporous nature of the silica-titania nanocomposite matrix with high

surface area value of 401 m2/g with a pore size of 3.3 nm and pore volume of 0.33 cm3/g. UV-

Vis spectroscopic determinations revealed that the prepared nanomatrix shows good sensitivity

between pH range of 3-5 for acidic medium within 6.9 seconds and pH 12 for basic medium

within 1.03 seconds; it is indicative that pH sensor is more responsive towards strong basis. It

can be deduced that the sensing species are probably better encapsulated in the matrix and show

faster response time. The fabricated pH sensor was found to have a six months age stability and

high reproducibility in terms of absorption and colour stability with response time. A reversible

response over a range of pH values between 3 and 12 were obtained. The encapsulated pH

indicator silica-titania nanocomposite matrix after 2hr heating at 150°C shows a great potential

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to be used as pH sensor with good material stability, reproducibility/repeatability, reversibility,

fast response and long life time behavior.

Keywords: Sol-gel method; Mesoporous nanocomposite; Encapsulated indicators; pH sensing

1. Introduction:

Organic–inorganic encapsulated indicators fabricated from silica based porous materials are now

of interest for investigations in many fields, especially antireflection coatings, catalyst, and

sensing purposes [1, 2]. The thermally stable inorganic network combines with organic

components to possess the stable chemical behavior in the sensing media [3]. In the inorganic

matrix, the presence of the heteroatoms can broaden the potentials of interesting characteristics.

The chosen materials for inorganic matrix are silica and titania. Mostly, silica-titania composite

compared with pure titania has good mechanical strength, high thermal stability and improved

dispersibility in solvents [4]. A number of publications also report the potential applications of

silica-titania inorganic composite as a sensor, waveguide, catalyst and support for some chemical

reactions by different fabrication and measuring techniques [5-15]. The most advantageous

approach to synthesize the composite material is the sol-gel method [16, 17]. One of the benefits

of the sol–gel method is to allow formation of the porous network and leads to the incorporation

of organic species in the inorganic matrices without degradation [5]. These sol-gel based

nanocomposite matrices are porous in nature and are encapsulated with co-indicators that are

used for pH sensing [18, 19]. Thus, the pH dyes are integrated into the porous sol-gel structure

during the fabrication process, before deposition of the sol-gel film onto a solid substrate with

spin coating. The reactions occurring in the sol-gel matrix between encapsulated reagents and the

analyte in solution induce a local change in pH, which results in a color change of the dye-loaded

layer. The porosity (i.e., pore volume, pore size, and surface area) of nanocomposites depends on

the structure and size of particles formed by hydrolysis and condensation reactions. The colloidal

species in the sol lead to the aggregation of these particles and develop the porous gel

microstructures [20]. This study primarily deals with the synthesis and characterization of

encapsulated pH sensing indicators such as phenolphthalein (C20H14O4), phenol red (C19H14O5S),

bromophenol blue (C19H10Br4O5S) and cresol red (C21H17NaO5S) in silica-titania nanocomposite

by the sol-gel process. The additive surfactant Cetyl Trimethyl Ammonium Bromide (CTAB)

was used as structure-directing agent that leads to the textural modification and porosity of

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nanocomposite network after the heat treatment at 150 °C for 2 hrs. Furthermore, in this study,

the relationships between processing condition, microstructure of porous materials and sensing

properties were systematically investigated. Additionally, the sensing mechanism was discussed

based on their physical properties such as surface area, pore size, and mesopore volume. This

presented work proceeds in the development/modification of encapsulated indicators silica-

titania matrix for new and efficient applications for sensing at much lower temperature as

compared to other conventional melting techniques. It is worth mentioning here that this is the

lowest temperature ever reported for the synthesis of encapsulated co-indicators silica-titania

nanocomposite matrix without any chelating agent or organic binder, broadening the pH range

for long lasting and reliable sensing.

2. Experimental

2.1. Synthesis of the Sols

In this multi-step synthesis process, the sol A (SiO2) was prepared by optimizing 2.5 ml of

tetraethylorthosilicate, TEOS [98% Aldrich] that was mixed with 15ml ethanol (EtOH) as it

contained mixture of water (H2O) and nitric acid (HNO3) [65% Merck]. The whole mixture was

stirred for 2hr at 70 ºC. The molar ratios were TEOS: EtOH: H2O: HNO3~0. 5:3.0:3.0:0.3. The

pH of the solution was kept below 1.5 to maintain the viscosity. The sol B (TiO2) was obtained

in two steps. In the first step, 5ml of titanium (IV) isopropoxide [97% Aldrich] was mixed into

10ml isopropanol (Grade GC, 99.7%). In the second step, 10ml propanol was added to the

mixture of water and HNO3. The molar ratio for tetraisopropylorthotitanate: propanol: H2O:

HNO3 of final sol was 1.0:4.0:1.0:0.5 respectively. The solution was stirred for 2hr at 100 ºC to

get a stable sol. The final product was a mild yellow gelatinous. To prepare encapsulated

indicators stable matrix, the two sols A and B were mixed in 1:1 volume ratio of silica to titania

sol. 2ml of 0.5M concentration of co-indicators mixture namely phenolphthalein (C20H14O4),

phenol red (C19H14O5S), bromophenol blue (C19H10Br4O5S) and cresol red (C21H17NaO5S) and

1ml of 0.5M concentration of CTAB solution were mixed into a solution of two sols. This

mixture was stirred and heated at 80 ºC for 1 hr to ensure the proper combination of the

constituents. After mixing, the sol turned reddish in color. Then the mixed sol was left for

several days for appropriate aging cycles to increase the viscosity of the solution and removal of

volatile components at room temperature. For uniform and adhesive coatings, the glass slides

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were thoroughly cleaned with acetone for 15 minutes and propanol for 10 minutes in an

ultrasonic bath in separate runs to remove any adsorbed gases or contaminants and then dried at

room temperature [20]. The prepared sol was deposited on cleaned glass substrates by spin

coating at a rate 3,000 rpm for 30 s. The coating was done at room temperature. The process was

repeated three times to obtain a uniform film at the same coating conditions. Solvent

evaporation, accompanied by further condensation reactions, resulted in deposition of a solid thin

film on the substrate pieces. The coated films were then annealed at 150 °C for 2 hrs in order to

produce a tough, inert and highly adhesive coating. The overall outline of synthesis and coating

is shown in Fig.1.

Fig. 1. Overall outline of the synthesis and coating

3. Characterization:

A field emission scanning electron microscope (FEI Nova NanoSEM 450) was used to study the

morphology of silica-titania nanocomposite structures and the EDX spectrum (Oxford Silicon

Drift Detector SDD) was used to know the percentage of elements present in the sample. For

surface topography analysis were made using Atomic Force Microscopy (AFM) equipment SPI

3800 N. X-ray diffraction data were obtained from Shimadzu XD3A equipment, using CuKα

radiation source. The thermal stability was obtained by TGA Mettler Toledo-brand Tga/sdta 851.

The surface areas of the nanocomposite material were determined from the BET (Brunauer,

Emmett and Teller) multipoint method and the pore size distribution was obtained using the BJH

(Barret, Joyner and Halenda) method. For the calculation of the surface areas, the BET method

was used to elaborate N2 adsorption-desorption isotherms. Moreover, the calculations of surface

areas and micropore volumes were employed from the α-plot method. Pore volumes were

obtained from the adsorbed amounts of N2 at P/Po = 0.98 desorption curve, supposing the

presence of N2 (density = 0.807 gcm-3) in the pores under these conditions. Then, BJH (Barret,

Joyner and Halenda) method was used to determine the pore size distribution for silica-titania

hybrid matrix. For sensitivity and absorption analysis of sol, the calculations were taken using a

Perkin Elmer Lambda 25 spectrometer over 250-900 nm range.

4. Results and Discussion:

4.1. Morphology Analysis:

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The FE-SEM micrographs suggest that the indicator molecules are homogeneously diffused into

matrix pores of the silica-titania network as shown in Fig. 2(a, c). The matrix exhibits regular

morphology with large pores without any cracks. While, the porosity decreases with heat

treatment due to diffusion of indicator species. This suggests that the temperature leads to more

homogeneous surface with a smaller crystallite size and enlarged exposed surface of the

encapsulated indicators [22]. These micrographs also indicate that the crystallinity increases with

the heat treatment that would be expected as a result of phase change [23].

Fig. 2 FESEM micrographs of silica-titania encapsulated indicators matrix (a) as-synthesized (b)

after heat treatment 150 °C for 2 hrs (c, d) corresponding EDX spectra respectively. AFM

micrographs of silica-titania encapsulated indicators matrix (e) as-synthesized (f) after heat

treatment 150 °C for 2 hrs

Table. 1. EDX analysis of as-synthesized and after heat treatment of silica-titania encapsulated

indicators matrix

Energy dispersive X-ray analysis was used to determine the elemental composition and to check

that the indicators have been incorporated into the silica-titania lattices. Fig 2 (c, d) shows the

EDX spectra obtained for silica-titania encapsulated pH sensing indicator dyes as-synthesized

and with heat treatments respectively. EDX analysis reveals that indicators are successfully

incorporated into the host matrix and shows the presence of N, S, Br, Na, which are elements of

indicators and shows variations after heat treatment. The values of elements are summarized in

Table.1. The variation in Si and Ti values increases after heat treatment owing to the growth of

particles. The amount of carbon incorporated into the matrix lattice is 5.44 % after heat treatment

that is a significant constituent of indicators and surfactant. Thus, encapsulation of indicators

within the matrix is successfully carried out. In order to investigate the silica-titania

nanoparticles’ surface modifications with encapsulation of indicators and heating, the samples

were evaluated by Atomic Force Microscopy (AFM). The morphological features of the

encapsulated pH indicators within silica-titania matrix thin film on the glass substrate were

examined over 5x5µm2 area. The topographies of samples are represented in terms of the

average roughness and root mean square roughness. The average roughness represents the 3-D

uniformity of the samples. A uniform distribution of grains can be observed in the as-synthesized

sample [Fig. 2(e)] with Ra= 2.15 nm and RMS= 6.33nm which shows relatively smooth surface.

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The dark regions can be assigned to the immobilized indicator molecules entrapped in the porous

voids of silica-titania matrix. The organic components fill the pores between the inorganic oxide

chains when integrated into the inorganic network. Thus, the film shows the appearance of

clusters, which are enormously small and closely packed [24]. The cluster structures are

impermeable to moisture and at a low temperature homogeneous and dense nanofilm was

obtained [25]. Furthermore, the higher homogeneity with low roughness values indicates good

encapsulation and strong interaction of indicators within the silica-titania network. Fig. 2 (f)

shows more regular surface after heat treatment for 2hr and observed large cavities with values

of Ra = 3.52 nm and RMC = 5.83 nm. The surface becomes rougher, and the broadening of the

roughness increases due to nucleation of the particles at temperature 150 °C for 2hr. The

particles grew in size and finally agglomerated. This is also consistent with the results reported in

the documented literature [26]. The increase of the particle size was also observed in XRD

section 4.2, calculated by Scherer formula. So, it can be suggested that roughness level

throughout the sample plays a significant role in sensing application.

4.2. Structural Analysis:

An XRD spectra of encapsulated indicators silica-titania nanocomposite matrix shows several

peaks of titania and silica throughout the samples as-synthesized and after heat treatment as

shown in Fig.3. In as-synthesized sample, (110) and (021) planes corresponding to orthorhombic

titania (JCPDS 23-1446) appeared. Under these conditions, less intense planes (200), (211) and

(213) belong to metastable anatase phase of titania (JCPDS 21-1276). Riaz et al. (2014) reported

that the sol synthesis and annealing conditions have a strong impact on crystallinity and phase

stability of specimen [27, 28]. Upon annealing the peak intensities corresponding to titania and

its phase (anatase) increases. Annealing strengthens the phases that were already present in the

as-synthesized sample. A low intense rutile phase peak with (211) plane indicating the

improvement in crystallinity. Rutile is the most thermodynamically stable form at all

temperatures and therefore most interesting for many optical applications. However, interestingly

the anatase metastable phase grows preferentially at lower temperatures and is suitable for film

formation on thermally sensitive substrates. [29]. Therefore, from the findings, it can be said that

annealing plays a vital role in phase variation with strength and crystalline growth of titanium

phases. Furthermore, a low intensity plane (002) corresponding to the silica phase (JCPDS 15-

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26) was also observed. So, the study shows that silica network incorporates the titania particles.

Furthermore, weak and broad diffraction patterns are attributed to less porosity due to the dyes

moieties, which extends into the pores of composite network, indicative of a lower crystalline

nature of films with short range mesostructured order. Additionally, it is inferred from XRD

results that mesostructure is prevalent in dye immobilized silica-titania films [30]. However,

crystallite size of encapsulated indicators silica-titania matrix as synthesized and after 2hr

heating at 150°C was calculated using Scherer formula given in (1)

D =kλ/βcosθ (1)

where k is the shape factor taken as 0.9, λ is the wavelength, β is full width at half maximum and

θ is the diffraction angle. Average crystallite size in as synthesized sample was calculated to be

3.07 nm, whereas larger crystallite sizes 3.82 nm were observed in the case of heat treated

sample. The crystallite size of thin films is extremely sensitive to the concentration of sol.

According to LaMer theory [31] in a homogenous solution, the particle formation depends on

nucleation process which leads to the coalescence of the particles from dissolved species; this

results in growth of nanoparticles to form larger particles [28, 32]. Thus, relatively larger

crystallite size was observed after heat treatment.

Fig. 3. XRD pattern of silica-titania encapsulated indicators (a) as-synthesized (b) after 2hr

heating at 150°C

4.3. Thermal Gravimetric Analysis:

The TGA (Thermal Gravimetric Analysis) and DTA (Differential Thermal Analysis) curves for

encapsulated indicators silica-titania matrix as-synthesized and heat treated sample in the 50−

600 °C temperature range are shown in Fig. 4 (a, b). The thermographs of both samples exhibit

almost a similar behavior. The as-synthesized sample followed two steps of mass weight loss.

The first step undergoing thermal degradation process, which starts at 60 °C with a weight loss

of 11.64 % and extending up to 160 ºC, is related to desorption of adsorbed water. In the second

step, the decomposition and expulsion of adsorbed organic fraction takes place in the range of

200 ºC- 550 ºC. These results reveal that sample was not fully dried and some physically bonded

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water was present in micropores and at the surface walls of composite, which caused a

subsequent weight loss of the 7.67 % at this temperature range [33]. This behavior relates to the

pyrolysis process of organic dyes, decomposition of surfactant (CTAB) [33], and follows the

escape of the physically bound water molecules from the silica-titania intermediate. Moreover, it

also indicates that the OH groups on the silica network are condensing to form Si–O–Si bridges

resulting in the weight loss. It has been previously reported that the weight loss at temperatures

lower than 270 °C in composites is due to the removal of physically absorbed water molecules

[34-37]. While, in the case of heat treated sample, the first regime corresponds to removal of

solvent residues and adsorbed water. The mass weight loss is 12.96%, which starts at lower

temperatures around 50 °C and is complete around 180 °C. The second regime corresponds to

thermal decomposition of dye in consonance with the melting point of dye followed by water

removal from framework hydroxyls [30, 37] and weight loss is 9.52% in the temperature range

of 200-550°C. The melting points of indicator dyes are summarized in Table.3 (section 4.5). The

weight loss in annealed sample is 1.85% higher than as-synthesized sample, that is indicative of

calcination of residual organics and development of titania phases. In both cases, as-synthesized

and annealed, the weight loss is not high, presumably because of good degree of polymerization

for silica-titania network formation and encapsulation of dyes within the network. Furthermore,

to pin−out the temperature corresponding to weight loss, the first order mathematical derivative

operation DTA was applied on both curves. The peak of the first derivative indicates the point of

greatest rate of change on the weight loss curve, this is also known as the inflection point. In

DTA, the loss of solvents and the combustion of organic material correspond to the peaks that

appear around 231 °C and shows that the material is endothermic in nature. These results reveal

that the sample was not fully dried and some physically bonded water was present in micropores

and at the surface walls of silica-titania crystals, which caused a subsequent loss of the mass at

the lowest temperatures. Furthermore, this indicates that after weight loss the indicators dyes

interact with the silica-titania mixed molecules more efficiently and leads to enhancement of the

sensing process. There is no significant change in weight beyond 300 °C implying that the silica

structure is fully developed, and the titania is transformed to the anatase and rutile phase.

Moreover, the effect of crystallization of matrix with small particle size is also considered.

However, it is seen that the materials do not show a significant weight loss after 400°C which is

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indicative of the thermally stable encapsulated dyes silica-titania matrix at the mentioned

temperature.

Fig.4. Weight (%) versus temperature (°C) and [dW/dT] (a) Silica-titania encapsulated indicators

as-synthesized (b) Silica-titania encapsulated indicators after 2hrs heat treatment at 150 °C

4.4. Surface area and pore size distribution characterization:

Mesoporous as-synthesized encapsulated indicators nanocomposite matrix has specific surface

area, pore diameter, and pore volume. This confirms that the nanocomposite structures are

preserved by drying at room temperature. However, after heat treatment at 150 °C for 2 hrs, it

has larger pore diameters, high pore volume and high surface area values which is advantageous

for sensing and catalytic applications. Table. 2 shows the values corresponding to surface area,

pore diameter, and pore volume. Both samples show a pronounced peak in the mesoporous

region 0-20 nm, as shown in Fig. 5.

Table.2 Surface area, pore diameter and pore volume distribution of silica-titania matrix

encapsulated indicators as-synthesized and heat treated samples

Fig.5. pore size distribution curves obtained by BJH (a) As-synthesized (b) after heat treatment

Fig.6. N2 Adsorption isotherms of (a) As-synthesized (b) after heat treatment

The adsorption-desorption isotherms of as-synthesized and annealed one are provided in Fig. 6.

The presence of a hysteresis loop indicates type IV behavior, particularly of mesoporous nature

and the hysteresis H2 type associated with ink-bottle pores is due to capillary condensation in the

pores of the matrix [38]. The samples have maximum area under the hysteresis loop indicating

that it has the maximum uniform pore size distribution in the mesopore region. The adsorption

isotherms of the annealed sample lie below indicating a lower pore volume, but have a similar

pattern as that observed in as-synthesized sample.

4.5. Chemical Analysis:

Basically, pH indicators are weak acids or base which is used in titrations to control the amount

of a chemical reaction in analytical experiments. When pH indicators are added into the solution,

they could bind hydrogen ion (H+) or hydroxide ions (OH-) and from changes in the way of when

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hydrogen ions are attached or detached from the electrons that are confined in a molecule

resulting in variations in color changes [39]. Usually, the indicator causes changes in colour of

the solution depending on the pH which can be observed visually. In this study, for the sensing

purpose of the broadening range, mixture of co-immobilized dyes namely

phenolphthalein (C20H14O4), phenol red (C19H14O5S), bromophenol blue (C19H10Br4O5S) and

cresol red (C21H17NaO5S) were selected. This mixture of indicators has spectral similarity in

absorption, emission and effective behavior towards the ionic strength. These dyes have valuable

properties because of their wide range from near UV to the near-IR regions of the spectrum [18,

19]. The characteristics of indicator dyes are summarized in Table. 3. For color determination,

from visual observations it can be observed that the co-indicators show yellowish-green color in

acidic solution that changes into blue color by increasing the pH values; this refers to Fig. 7(a).

In contrast, the silica-titania mixed sol with entrapped co-indicators exhibits several colours i.e.

bright yellow color at lower pH values, but when the pH level is increased, the color

progressively changes into light green and light purple to dark purple as observed in Fig. 7(b).

The colour change of the sol was fast within seconds when immersed in pH solutions. This is a

significant improvement over previous work reported in the literature and the stated response

times were in the order of 20-100 minutes [40]. Moreover, the variations in color in different pH

solutions are due to hydroxyl ions that freely penetrate into the matrix and react with indicators.

Fig. 7. Colour variation corresponding to different pH ranges 3-12 (a) Mixture of indicators (b)

encapsulated indicators within silica-titania matrix

Table.3. Characteristics/Properties of Indicator dyes

Every indicator exists in two forms: acid form and base form which corresponds to the two

absorption bands respectively. These bands are due to π-π* and n–π* transitions [36]. The

absorption spectrum of the free indicators before exposing to pH effect was low at 425 nm as

shown in Fig. 8. However, after exposing with pHs, there are two distinct transitions that can be

seen in Fig. 8; one, absorption band was observed at 593 nm corresponding to higher pHs with

maximum absorption for the mixture of free indicators in ethanol solution. Whereas, an

absorption band was observed at 300 nm corresponding to lower pHs with minimum absorption.

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The absorption spectra of the free indicators at different pH values were shown in Fig. 8. These

transitions were due to the changes in colours with the presence of two isosbestic points at 395

nm and 480 nm. This provides evidence for the presence of two equilibriums, the first

equilibrium at pH 3–7 is between acid and neutral forms of indicators and the second equilibrium

at pH 7–12 is between neutral and base forms. The pKa (acid dissociation constant) value of free

co-indicators mixture was calculated from the relationship between pH and absorbance and its

calculated value is 3.75 at the wavelength of 593 nm by UV-Vis spectroscopic method, which

can be seen in Fig. 8 [Inset (b)].

Fig.8. Absorption spectra of co-indicators solution at different pH values 3-12. Inset (a) is

showing the color variations in different pH solutions. Inset (b) pKa value of co-indicator

solution at 593nm for pH 3-12.

Fig.9. Absorption spectra of encapsulated indicators silica-titania matrix sol at different pH

values 3-13. Inset (a) is the variations in colors in different pH values 3-12 and inset (b) pKa

value of corresponding to absorption at pH 3-12.

The silica-titania matrix in the presence of surfactant has maximum absorption at lower

wavelengths. While absorption values decreases after the addition of indicators as shown in Fig.

9. When the encapsulated indicators-CTAB were treated with pH 3-12 solutions, the absorption

bands around 280-350 nm and 500-650 nm can be clearly observed in Fig. 9. These bands

correspond to different pH values with two indicating transitions. The first transition at pH 2–7

was due to the change of yellow color (acid form) to green color (neutral form) and the second

transition at pH 8–12 was due to the change of green color (neutral form) to purple color (base

form). In the presence of CTAB, the hydrophobic non-charged indicator molecules are

transferred to anionic hydrophilic species and formation of ion pairs between the CTAB cationic

surfactant and the anionic species of indicators. There is change of pKa to more basic in the

presence of cationic surfactant CTAB. Furthermore, the maximum absorption observed at lower

pH values is also indicated in color observations. However, when the sol is treated with pH 9, the

absorption band occurs at 430 nm without significant changes of absorbance with the presence of

an isosbestic point at 490 nm. This provides evidence for the presence of equilibrium, Pka found

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to be 9.3 is probably that in basic medium, the anions are perhaps involved in weak Vander

Walls interactions with silica-titania species. The variation of absorbance with pH of solution is

shown in Fig. 9 [Inset (b)]. This corresponds to the pH value at half the maximum absorbance

and favored with the presence of indicators anions. A larger pKa value has a smaller degree of

dissociation at any given pH. So, the encapsulation process has resulted in a pKa dye shift and

broadening in the dye pH range. Furthermore, the shifting of absorption bands towards higher

and lower energy values probably correspond to a change of the acidic form to basic form of

indicators [41]. It can be observed that sensitivity is good between pH 3-5 for acids and for bases

pH 11-12. To investigate the time response of fabricated sensor, a graph is plotted between

response time and absorption of the encapsulated co-indicator silica-titania matrix as shown in

Fig. 10. Fast response was observed for the fabricated sensor sol in basic medium at minimum

absorption. However, at maximum absorption, the response time of the sensor sol was quite low

in acidic medium. The time response recorded for pH 12 is 1.03 seconds and for pH 3 is 6.9

seconds.

Fig. 10 Time versus absorption of encapsulated co-indicator silica-titania matrix within different

pH ranges 3-12

4.6. Stability, reproducibility and reversibility of fabricated Sensor:

To investigate the stability of encapsulated indicators silica-titania nanocomposite matrix, the sol

was examined after a period of six months. After six months, the absorbance spectrum of the

film was the same with fast response time. The colour of the sol also has not changed. The result

is shown in Fig. 11. This may prove that the encapsulation of indicators within the network of

silica-titania nanocomposite matrix is fully stable after a period of six months. In addition, for

repeatability and reproducibility, three cycle absorption measurements against pHs 3-12 were

conducted. The colour variations were also recorded for these measurements. The cycles were

repeated every 2 hrs from the previous one to check the intraweek reproducibility. Fig. 12 shows

that the absorbance after each measurement does not show any remarkable changes; it is also

evident from color bar strips. These results indicated that the fabricated sensor has long life and

can be reproducible/repeatable.

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Fig. 11. Absorbance spectra of encapsulated indicator silica-titania matrix sol at pH 3-12 verses

time. Inset down strip showing the colour response to each pH value 3-12

Fig. 12. Absorbance spectra of encapsulated indicator silica-titania matrix sol at pH 3-12 verses

pHs. Inset down strip showing the colour response to each pH value 3-12

Two pH values 3 and 12 were used to check the extent of reversibility of silica-titania

encapsulated indicators matrix in terms of color change. Fig. 13 shows the changes of colours,

forward and backward. The response time was around 2 seconds in both ways. So, the study

shows that the prepared encapsulated matrix acts as fast reversible sensors.

Fig. 13 Reversibility of sol sensor at pH ranges 3 and 12 in terms of colour changes

5. Conclusion:

Encapsulated indicator silica-titania nanocomposite matrix was obtained via the sol-gel method.

The microscopic analysis indicates after heat treatment at 150 °C for 2hrs, that the particles are

uniformly distributed without any cracks/defects. It can also be observed that the

interconnectivity of indicators with matrix has a significant role in the increased stability and

sensitivity of the present sensor. EDX analysis substantiated the stoichiometric formation of

encapsulated silica-titania nanocomposite matrix. XRD results showed average crystallite size

3.82 nm in case of heat treated sample. Thermally stable matrix presents mainly mesoporous

nature with high surface area 401 m2/g, a pore size of 3.3 nm and pore volume of 0.33 cm3/g.

The highly porous nature makes silica-titania network to excellent host for sensing molecules

and it shows anion exchange properties that allow the encapsulation of pH sensing dyes.

Therefore, the sensing species straightforwardly diffuse towards the sensing centres. The

experimental findings reveal that thermally stable nanosensor allowed good indicator’s

interaction with silica-titania host matrix and improved signal sensitivity due to the good

incorporation between surfactant, indicators and host matrix species. Furthermore, UV-Vis

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spectroscopy determination revealed that the prepared nanomatrices shows good sensitivity

between pH ranges of 3-5 for acidic medium within 6.9 seconds and pH 12 for basic medium

within 1.03 seconds. The calculated dynamic pH value is 9.3 and is indicative that pH sensor is

more responsive towards strong basis. So, it can be concluded that the fabricated mesoporous

silica-titania nanocomposite matrix is an excellent material for binding organic compounds for

optical pH sensing and has potential for usage in industrial and scientific research field due to its

stability, reproducibility/repeatability, reversibility, fast response and long life time behavior.

Acknowledgments:

The authors would like to acknowledge the Malaysian Ministry of Education and Universiti

Teknologi Malaysia through RMC for the financial support of this project.

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Prof. Dr. Noriah Bidin is currently working as director and full Professor in Laser Centre, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia. She obtained her PhD degree from Universiti Teknologi Malaysia in 1995. She has more than 100 publications with 50 impact factor. Her research interests include nanomaterials and their applications in chemical sensors, Bio-medical and lasers.

Prof. Dr. Roslay Abdul Rahman is full Professor in Laser Centre, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia. He obtained his PhD degree from Universiti Teknologi Malaysia in 1995. He has many publications in reputed scientific journals. His research interests include chemical sensors, and photonic devices.

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Prof. Dr. Shahzad Naseem is currently working as director and full Professor in Centre of Excellence in Solid State Physics, University of the Punjab, Lahore Pakistan. He obtained his PhD degree from University of Northumbria at Newcastle, UK in 1985 and Post-Doc from University of Cambridge, UK in 1996. He has got many awards like Gold Medal for Outstanding Achievements, awarded by Pakistan Academy of Sciences, in 2013. Post-Doctoral Award of Association of Common wealth Universities, in 1996. Quaid-e-Azam Scholarship, Ministry of Education, Govt. of Pakistan, 1983-85. Gold Medal for standing 1st on Merit in M.Sc. (Physics), B.Z. University Multan, in 1981. He has more than 450 publications including 259 Journal Papers with 228.9 Impact Factor and 247 Conference Proceedings. He worked on PV Devices for space use and also put forward theory of working of these devices which was debated world over in the scientific community for many years to come. He started M.Phil. in Microelectronic Engineering & Semiconductor Physics for the first time in Pakistan. He has also introduced MS Nanotechnology for the first time in Pakistan. So far, he has supervised 173 research theses in the fields of PV, LCD, Microelectronics, Spintronics etc.

Dr. Saira Riaz is currently working as associate professor in Centre of Excellence in Solid State Physics, University of the Punjab, Lahore Pakistan. She obtained her Ph.D.  from  Solid  State  Physics, University  of  the  Punjab  in  2008  and  Post‐doc  from  Department  of Electronics,  University  of  York,  Heslington,  York,  UK  in  2008.  She  got many  awards  like  1st  position  in  District  Okara  in  S.S.C.  Examination. President Scholarship holder in S.S.C. & F.Sc. 1st position in M.Phil. Solid State Physics. Training Program Scholarship  from HEC, Pakistan  in 2006. International Research Support  Initiative  (IRSI) Scholarship from HEC,  from Pakistan,  in 2006. Post‐doc Fellowship, NCNST, Pakistan,  in 2008.  She has more than 200 publications including 150 Journal Papers with 100 Impact Factor and Conference Proceedings. She worked on electronic devices. So far, she has supervised 100 research theses in the fields of PV, Spintronics, nano-materials etc.

Dr. Shumaila Islam received a PhD- from Laser Centre, Ibnu Sina Institute for Scientific and Industrial Research, Universiti Teknologi Malaysia in 2015. She has many publications. Her research interests include nanomaterials and their applications in opto-electronic devices.

 

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Fig. 1. Overall outline of the synthesis and coating

 

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(e) (f)  

 

 

 

 

 

 

 

 

(a)

(b)

(c)

(d)

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  Fig. 2 FESEM micrographs of silica-titania encapsulated indicators matrix (a) as-

synthesized (b) after heat treatment 150 °C for 2 hrs (c, d) corresponding EDX spectra

respectively. AFM micrographs of silica-titania encapsulated indicators matrix (e) as-synthesized

(f) after heat treatment 150 °C for 2 hrs

 

 

Fig. 3. XRD pattern of silica-titania encapsulated indicators (a) as-synthesized (b) after 2hr

heating at 150°C

 

 

 

 

 

 

(a) (b)

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Fig.4. Weight (%) versus temperature (°C) and [dW/dT] (a) Silica-titania encapsulated indicators

as-synthesized (b) Silica-titania encapsulated indicators after 2hrs heat treatment at 150 °C

 

 

 

 

 

 

 

 

 

 

 

  Fig.5. pore size distribution curves obtained by BJH (a) As-synthesized (b) after heat

treatment

 

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Fig.6. N2 Adsorption isotherms of (a) As-synthesized (b) after heat treatment  

 

 

 

 

 

 

 

 

 

Fig. 7. Colour variation corresponding to different pH ranges 3-12 (a) Mixture of indicators (b)

encapsulated indicators within silica-titania matrix

 

(a)

(b)

3 4 5 6 7 8 9 10 11 12

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Fig.8. Absorption spectra of co-indicators solution at different pH values 3-12. Inset (a) is

showing the color variations in different pH solutions. Inset (b) pKa value of co-indicator

solution at 593nm for pH 3-12.

 

 

 

 

 

 

 

 

 

 

(a)

(b)

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  Fig.9. Absorption spectra of encapsulated indicators silica-titania matrix sol at different

pH values 3-13. Inset (a) is the variations in colors in different pH values 3-12 and inset (b) pKa

value of corresponding to absorption at pH 3-12.

 

 

 

 

 

 

 

 

(a)

(b)

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  Fig. 10 Time versus absorption of encapsulated co-indicator silica-titania matrix within

different pH ranges 3-12

 

 

 

  Fig. 11. Absorbance spectra of encapsulated indicator silica-titania matrix sol at pH 3-12

verses time. Inset down strip showing the colour response to each pH value 3-12

 

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  Fig. 12. Absorbance spectra of encapsulated indicator silica-titania matrix sol at pH 3-12

verses pHs. Inset down strip showing the colour response to each pH value 3-12

 

 

Fig. 13 Reversibility of sol sensor at pH ranges 3 and 12 in terms of colour changes 

 

 

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Table. 1. EDX analysis of as-synthesized and after heat treatment of silica-titania encapsulated

indicators matrix

 

 

Table.2 Surface area, pore diameter and pore volume distribution of silica-titania matrix

encapsulated indicators as-synthesized and heat treated samples

Name/ Chemical Formula

Melting Point

Structure with Acidic Medium, color Structure in Basic Medium, color

PKa /Absorption

Elements wt. (%) As-synthesized After heat treatment

C 6.57 5.44

N 2.12 0.64

O 40.66 43.69

Na 5.79 4.27

Si 36.51 38.10

S 0.52 0.7

Ti 5.97 6.23

Br 1.86 0.93

Samples

Surface area

(m2/g)

Pore Diameter

(nm)

Pore Volume

(cm3/g)

BJH Pore Diameter

(nm)

As-synthesized 342 2.8 0.24 3.2

After heat

treatment

401 3.3 0.337 3.6

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PhenolpthaleinC20H14O4

260 °C

0−8.2, Colorless 8.2−12.0, Fuchsia

9.6/

λmax= 552 nm (1st) and

374 nm (2nd)

Phenol Red C19H14O5S

300 °C

6.8, Yellow 8.2, Fuchsia

8.0 /

λmax = 430 - 435 nm (1st)

and 557 -560 nm (2nd)

Bromophenol blue C19H10Br4O5S

279 °C

3.0, Yellow 4.6, Purple

4.1/

λmax = 430 nm in its yellow

acidic form to 600 nm in

its blue basic form

[128].

Cresol red C21H17NaO5S

250 °C

7.2, Yellow 8.8, Purple

8.2/ λmax = 570

nm (1st) and 367 nm (2nd)