T

LMa

b

c

a

ARRAA

KSAESS

1

ammrbtp

tmcrssbt

t

0d

Sensors and Actuators B 151 (2010) 169–176

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

he effect of the sol–gel route on the characteristics of acid–base sensors

arissa Brentano Capeletti a, Francielle Leonardelli Bertottoa, João Henrique Zimnoch Dos Santosa,∗, Edwinoncadab, Mateus Borba Cardosoc

Instituto de Química, UFRGS, Av. Bento Goncalves, 9500, Porto Alegre 91501-970, BrazilBraskem S.A., III Pólo Petroquímico, Via Oeste, Lote 05, Triunfo 95853-000, BrazilLaboratório Nacional de Luz Síncrotron, CEP 13083-970, Caixa Postal 6192, Campinas, SP, Brazil

r t i c l e i n f o

rticle history:eceived 23 June 2010eceived in revised form 8 September 2010ccepted 9 September 2010vailable online 1 October 2010

eywords:

a b s t r a c t

The acid–base indicators alizarin red, brilliant yellow and acridine were encapsulated within a silicamatrix by three different sol–gel methods: namely, (1) the non-hydrolytic and the (2) acidic and (3) basichydrolytic routes. The sensors were characterized by several complementary techniques to investigatethe effect of the nature of the sensor and of the sol–gel route on the structural and textural characteristicsof the resulting sensor. The resulting acid–base sensors showed encapsulated contents ranging from0.1 to 15.2 �mol g−1. The encapsulation yield was dependent on the nature of the sensor and on the

ol–gelcid–base indicatorncapsulationilicaAXS

sol–gel route. The highest content was achieved in the encapsulation of acridine by the acidic route.The morphological and textural characteristics were also shown to be dependent on the sol–gel route.The highest surface area (867 m2 g−1) and a more opened fractal structure were obtained by the acidichydrolytic route. The sensors were evaluated in terms of the response time to ammonia gas by colorchanges. The response time was dependent on the nature of the indicator and on the sol–gel route. Theshortest response time was observed for the system obtained by the acidic route employing the alizarin

%).

red indicator (59 s ± 11.5

. Introduction

Sol–gel derived materials have received a peculiar interests chemical receptor matrices due to their optical transparency,echanical stability, chemical resistance and flexibility of sensororphological configurations. Furthermore, the control of textu-

al properties, in the case of pH sensors, enables a rapid responseecause the sol–gel matrix allows fast proton diffusion [1], andheir ability to respond rapidly makes them desirable for use inharmaceuticals, foods and chemicals industries [2].

Among the sol–gel derived materials, silica represents one ofhe most studied matrixes for entrapping sensor and/or receptor

olecules. The purpose of the encapsulation is to improve someharacteristics and properties, such as selectivity, sensibility, accu-acy, response time and useful life, which broaden the scope of theensor’s application [3]. Such applications may include pH mea-urement [4–6], and detection of gases [7,8], ionic species [9],

iomolecules like sugars, and others in food industry [10,11], justo mention a few.

One factor that may affect the characteristics and properties ofhe sensor is the synthetic route employed for its preparation. There

∗ Corresponding author. Tel.: +55 51 3316 7238; fax: +55 51 3316 7304.E-mail address: jhzds@iq.ufrgs.br (J.H.Z. Dos Santos).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.09.026

© 2010 Elsevier B.V. All rights reserved.

are many works reporting the effect of several variables to improvethe sensors properties, such as response time, reduction in leach-ability and enhancement of useful life (see for instance, [13–15]).Usually, papers report hydrolytic routes using acidic catalysis, nocatalysts or surfactants, in which different parameters are evalu-ated, such as pH, nature of silanes, solvents and surfactants andreagent concentration. The former alters the reaction rate [1], whilesilanes affect the matrix networks [15]. Furthermore, the entrappedmolecule (receptors) have been investigated in terms of the stericeffect, nature of the ligands [12,13], and concentration [14].

To our knowledge, comparative studies of the use of non-hydrolytic and hydrolytic routes catalyzed by basic and acidiccatalysts have not been described in the literature. These routeshave different characteristics because pH synthesis, the nature ofthe catalysts and rate of hydrolysis and condensation reactions canall influence the sensors properties. Furthermore, the nature ofthe synthetic route and the characteristics of the indicator itselfmay also affect the final properties, like the steric effect playedby the indicator molecule. The nature of functional group and thepH response range may also influence the structure and behavior

of the resulting systems. Therefore, in the present study, we havecompared three different commercial indicators: namely, acridine,alizarin red and brilliant yellow, as shown in Scheme 1. Acridinehas the smallest steric effect and possesses a nitrogen in an hetero-genic cycle, forming a planar molecule. Alizarin red has a similar

170 L.B. Capeletti et al. / Sensors and Act

O

O

OH

OH

Alizarin red

N

Acridine

N N C CH

NN

HO

HO

SO3Na

SO3Na

H

siab(

oofiTfnt

2

2

irlShe

2

tcaaar(hrow

Brilliant yellow

Scheme 1.

tructure, with carbonyl and hydroxyl groups on the ring, and isntermediate in terms of molecular volume. Brilliant yellow is anzoic compound and has a very different structure, giving it theiggest steric effect containing three different functional groups–SO3Na, –OH, –N N–) in its structure.

Therefore, in this study, we investigated the effect of the naturef the indicator molecule and the influence of the sol–gel routen the structural, textural and morphological characteristics of thenal sensor and its influence on the response to ammonia gas.hree different pH indicators were encapsulated by the three dif-erent sol–gel routes, the hydrolytic acidic, hydrolytic basic andon-hydrolytic routes, and the resulting system performance wasested as acid–base sensors.

. Material and methods

.1. Materials

Tetraethoxisilane (Si(OCH2CH3)4, TEOS, Merck, >98%) and sil-con tetrachloride (SiCl4, Sigma–Aldrich, 99%) were used aseceived. The acid–base indicators alizarin red (AL, 97%), bril-iant yellow (BY, 70%) and acridine (AC, 97%) were provided byigma–Aldrich. Hydrocloric acid (HCl, Nuclear, 38%), ammoniumydroxide (NH4OH, Nuclear, 29%) and FeCl3 (98%, Merck) weremployed as catalysts.

.2. Synthesis of sensors by sol–gel process

Three different routes were employed to prepare the sensors byhe sol–gel process from TEOS as raw material, according to pro-edures already described in the literature [16–18]. The employedcid–base indicators were alizarin red (AL), brilliant yellow (BY)nd acridine (AC), and the three employed routes were labeleds the acidic (AR), basic (BR) hydrolytic and non-hydrolytic (NHR)outes. The acidic route was catalyzed by chloridric acid in a 1:20

HCl:TEOS) ratio [16]. The basic route was catalyzed by ammoniumydroxide in a 1:1 ratio (NH4OH:TEOS) [17]. The non-hydrolyticoute employed TEOS and SiCl4 and was catalyzed by FeCl3 (0.5 wt.%f final product weight) [18]. The amount of acid–base indicatorsas 0.1 mol.% corresponding to the total amount of alkoxides.

uators B 151 (2010) 169–176

The sensors (powder) obtained by the acidic and basic routeswere prepared by aqueous and ethanolic solutions of acid–baseindicators, respectively, to which the catalyst and TEOS were addedwhile stirring at room temperature. The resulting solution was fur-ther stirred until gelation (about 4 h for the acidic route) or untilprecipitation, followed by one additional hour for the basic route,amounting to 1 h and 15 min. For the non-hydrolytic route, all theprocedures were executed under inert atmosphere. First, the cata-lyst and the indicator were added, followed by the addition of TEOSand SiCl4. The resulting solution was stirred at 80 ◦C until gelation(about 2 h). All solids were milled, washed with water and ethanol,then dried at 110 ◦C for 12 h (Scheme 2).

In the following discussion, the resulting sensors are labeledaccording to their nature (code of the indicator) and the prepa-ration route. For instance, ALNHR denotes the sensor prepared bythe non-hydrolytic route, containing alizarin red as the indicator.

2.3. Diffuse reflectance spectroscopy (DRS)

The UV–vis spectra of the sensors in powder form wereobtained on a Cary 100 UV–vis spectrophotometer using the diffusereflectance accessory. The pH indicator content was investigatedby UV-DRS employing the standard-addition analytical method,and all the calibration curves showed a linear correlation (R)greater than 0.99. Measurements were carried on Kubelka–Munkunits, using the silica (without indicator) synthesized by the sameroute as the investigated system. It is worth mentioning thatKubelka–Munk is a function employed in spectroscopy to converta given reflectance spectrum in its similar absorbance one. Usuallythis function is given by the equation:

F(R) = (1 − R)2

(2R)(1)

where R corresponds to the measured reflectance and F is the signalconverted in Kubelka–Munk units, which presents a linear correla-tion with concentration [19,20]. The indicator concentration withinthe silica matrix was calculated by the extrapolation of the calibra-tion curve to y = 0.

2.4. Scanning electron microscopy

The obtained sensors were analyzed by scanning electronmicroscopy (SEM) in a JEOL model JSM 6060, with a 20 kV accel-erating voltage and magnifications of 400 and 30,000. The sampleswere measured in their powder form on an aluminum stub coatedwith a thin gold layer.

2.5. Nitrogen porosimetry

A Micromeritics Gemini surface area analyzer was usedfor the nitrogen sorption studies. The samples were pre-heated at 150 ◦C for 8 h under vacuum before each mea-surement. The specific surface area was determined by theBrunauer–Emmett–Teller (BET) method at −196 ◦C, in the partialpressure range of 0.01 < P/P0 < 0.35. The pore sizes were calculatedby the Bartt–Joyner–Halenda equation (BJH method) using Halseystandards.

2.6. Small-angle X-ray scattering (SAXS)

SAXS experiments were carried out on the D2A and D11A beam

lines at the Brazilian Synchrotron Light Laboratory (LNLS, Camp-inas, Brazil) using a wavelength � = 1.488 nm. The X-ray beam wasmonochromatized by a silicon monochromator and collimated bya set of slits defining a pin–hole geometry. The incident beam wasdetected at two different sample-to-detector distances (1549.8 and

L.B. Capeletti et al. / Sensors and Actuators B 151 (2010) 169–176 171

- Washing with H2O/ethanol - Drying at 110°C during 12h

- Washing with H2O/ethanol - Drying at 110°C during 12h

- Aqueous solution HCl 0,1M - pH indicator - T = 25°C

- Washing with H2O/ethanol - Drying at 110°C during 12h

- t = 3h

TEOS

Sol

Gel

Sensor in powder form

rou

- t = 5min

- Aqueous Solution NH4OH conc. - pH indicator - T = 25°C

TEOS

Sol

Precipitate

Sensor in powder form

- t = 1h

- SiCl4

- FeCl3

- pH indicator - T = 25°C

TEOS

Sol

Gel

Sensor in powder form

eme 2

22bphSspwT

rPotsta

I

wpte

2

csiggs

as follows: AR < NHR < BR. Therefore, the non-hydrolytic route pro-vides systems with intermediate fixed amounts for VA and AB of3.74 and 7.99 �mol g−1, respectively, and the highest value to AC(15.2 �mol g−1). It is worth mentioning that, by the basic route,AL and BY were not detected in the final solid, and only a small

Table 1pH indicator content resulting for the three different sol–gel processes.

Indicator pH indicator content (�mol g−1)

AR (acid route) BR (basic

Sch

245.7 mm) to increase the scattering vector q (q = (4�/�) sin�;� = scattering angle) range. The dried samples were sandwichedetween two Kapton® foils, and the collimated X-ray beam wasassed through a chamber containing the stainless steel sampleolder. All measurements were performed at room temperature.ilver behenate powder was used as standard to calibrate theample-to-detector distance, the detector tilt and the direct beamosition. Transmission, dark current and Kapton® foil correctionsere performed on the 2D image before further data processing.

he isotropic scattering patterns were radially averaged.SAXS data analysis was performed using the Irena evaluation

outine [21] implemented in the Igor Pro Software (WaveMetrics,ortland, USA) [22]. A multi-level unified fit was used to describene or two levels of structural organization evident in the scat-ering data [23,24]. In this method, the scattering provided by eachtructural level is the sum of a Guinier exponential form and a struc-urally limited power-law tail. A generalized equation, representingny number of levels, can be written as [23,24]:

(q) =n∑

i=1

Gi exp

(−q2R2

gi

3

)+ Bi exp

(−q2R2

g(i+1)

3

)

×[

(erf(qRgi/√

6))3

q

]Pi

(2)

here n is the number of structural levels observed, G is the Guinierrefactor, Rg is the radius of gyration and B is a prefactor specifico the power-law scattering which is specified as the decay of thexponent P.

.7. Response time

Sensors were tested in an NH3 gas atmosphere. This test wasarried on in a closed glass system (total volume of 23.5 cm3) con-

isted of two vessels connected by a Rotaflow® valve. The sensors,n powder form, were set into the upper vessel. In a typical test, aiven mass of sensor (about 30 mg) was placed in contact with theas generated by a standard volume (1 mL) of NH4OH concentratedolution (29% w/v), corresponding to 5.1 × 10−4 g cm−3, at room

te) NHR (non-hydrolytic route)

.

temperature (25 ◦C). The response time was measured between thetime of opening the valve to the time when the color change couldbe observed with the eye. The measurements were carried on intriplicate.

3. Results and discussion

3.1. Effect of the sol–gel route on the encapsulated pH indicatorcontent

In a systematic approach, the three pH indicators under inves-tigation were encapsulated within the silica matrix by threedifferent sol–gel processes, namely acidic (AR), basic (BR) and non-hydrolytic routes (NHR), resulting in a total of nine systems, labeledas described in Section 2.2. Table 1 shows the pH indicator contentexpressed in units of �mol g−1.

According to Table 1, the values seem to be affected by thesol–gel route and the nature of the encapsulated indicator. TheAR sample possesses the highest encapsulated content for theAL and BY indicators, 7.32 and 9.95 �mol g−1, respectively. ACalso presented a large content (15 �mol g−1), while NHR has pre-sented a slightly higher value (15.2 �mol g−1). Such behavior canbe attributed to the low rate of the condensation reaction, whichprobably enhances the interaction between indicator moleculesand the silica network under formation, resulting in an increase inthe overall trapped indicator amount. This reaction rate increases

AR BR NHR

AL 9.95 – 7.99BY 7.32 – 3.74AC 15.0 0.13 15.2

172 L.B. Capeletti et al. / Sensors and Act

Table 2Wavelength absorption maxima of the different pH indicators: neat (in solid state)and after immobilization within the silica network by the three sol–gel routes.

Indicator Wavelength (nm)

Neat AR BR NHR

aaw

ieeidsirioiostcouilbhetat

3

otrsp

se[oimgsteto

BFfa

AL 452 414 – 484BY 439 400 – 399AC 358 408 356 328

mount of AC (0.13 �mol g−1) was observed. In contrast with thecidic route, the basic route presents a higher condensation rate,hich may hinder the indicator encapsulation.

The trapped amount of the three investigated indicators variesn the following order: BY < AL < AC. This behavior can be partiallyxplained by the volume impinged by each molecule and its stericffect on the silica network. AC presents the smallest volume ands a planar molecule. Therefore, it likely favors its incorporationuring the sol-gel condensation reactions, which explains encap-ulation yields as high as 15 �mol g−1. Another important factorn the case of AC is the presence of a nitrogen molecule in theing that can be easily protonated, increasing the possibility ofnteraction with the silica network by hydrogen bonding. On thether hand, due to the high molecular volume of AL, a decreasen the encapsulated amount is observed (9.9 �mol g−1). Studiesn alizarin red S (alizarin red with a SO3Na group attached) havehown that the encapsulated content slightly increases with an ini-ial added content, reaching values of the order of 60% [14], i.e.,orresponding to 11.5 �mol g−1, a slightly higher content than thatbtained in the present study. With an even higher molecular vol-me, BY affords even lower encapsulated content (7.3 �mol g−1),

.e., roughly 50% lower than that observed in the case of AC. In theiterature, higher encapsulated contents (about 18 �mol g−1) haveeen reported for BY. Nevertheless, in such cases, bridged ligandsave been employed to facilitate the encapsulation [26]. The stericffect played by the indicator molecule seems to be more crucial inhe case of the basic route than for the two other routes, because ALnd BY were not detected within the detection limits of the UV-DRSechnique.

.2. Effect of the sol–gel route on the structure of the sensors

The effect of the sol–gel route was further investigated in termsf the structure of the resulting systems. Bare compounds andhe resulting encapsulated sensors were characterized by diffuseeflectance spectroscopy in the UV–vis region (UV-DRS). Table 2hows the maximum absorbance wavelength of the three studiedH indicators and that of the resulting encapsulated systems.

Generally, the maximum band position of a molecule on theurface of a solid can be affected by several factors, such as stericffects, medium polarity, hydrogen bonding and surface acidity25]. In the acidic route, a hypsochromic shift (blue shift) wasbserved for AL and BY, while a bathocromic effect was detectedn the case of AC. This behavior can be explained by the increase of

edium polarity, because the indicator may interact with silanolroups of silica. In this situation, n–�* type transitions tend to hyp-ochromic shifts [25], which is the case of AL and BY [14,26]. Onhe other hand, �–�* transitions tend to result in bathochromicffects (red shift) [25], such as those observed in the case of the ACransition band, which strongly suggests that nitrogen protonationccurs in acidic media [27].

For the BR sensors, no bands could be detected for AL andY, probably due to the low amount of encapsulated indicators.or the ACBR sensor, one low intensity transition band could beound, indicating the presence of AC within the silica network (seelso Table 1). Nevertheless, practically no wavelength shift was

uators B 151 (2010) 169–176

observed, suggesting little to no interaction between the indica-tor and the silica framework. Finally, in the NHR systems, differenttrends were observed compared to those observed in the case ofAR. The differences probably occur due to the presence of catalystresidue (FeCl3), which was employed in this sol–gel synthesis andwhich might be interfering in the final sensor UV–vis reflectance. Insome cases, this residue even reflects more than the indicator itselfaffecting not only the spectrum, but also the final sensor colorationbecause the catalyst has a strong reddish tone. Attempts to employother Lewis acid catalysts (AlCl3), which would not absorb in thevisible region, were not successful because the gelation process didnot occur.

3.3. Effect of the sol–gel route on the morphological and texturalcharacteristics of the sensors

The morphological and textural characteristics of the sensorspresented a strong dependence on the synthesis route. Three tech-niques were used to investigate these characteristics: scanningelectron microscopy (SEM), small-angle X-ray scattering (SAXS)and nitrogen adsorption (BET and BJH methods). SEM was used forobserving the morphology of the particles obtained by each tech-nique. NHR impinges the generation of a monolith. This behavioris recurrent in routes under gelation processes [28]. In the caseof the AR, monolith formation can also be observed because thisroute also undergoes a gelation process. The most significant differ-ence among the three synthesis routes is observed for BR samples,which in contrast to the other two samples are formed through aprecipitation process. In the precipitation process, roughly spher-ical particles with a well-defined size can be obtained. A similarbehavior was observed to the other studied systems.

The solids obtained by the BR were further investigated to evalu-ate the effect of the nature of indicator on the resulting morphology.SEM micrographics are shown in Fig. 1, where high-magnification(30.000 × ) enables one to observe the morphology of the particlesmore clearly.

According to Fig. 1, the inclusion of a pH indicator seems toperturb the sol–gel reactions, because, if compared to bare sil-ica produced under the same conditions (Fig. 1d), a clear loss ofsphericity is observed (Fig. 1a–c). It can be observed that this effectis much more pronounced in the case of BY and AL (Fig. 1a and b) inwhich the size of the particles is approximately 0.5 �m. Conversely,bare silica prepared by BR exactly follows the literature parameters,i.e., spherical morphology and particle size in a range from 0.2 to2 �m [17].

Nitrogen adsorption was employed for analyzing the texturalproperties of the solid sensors. Table 3 provides data for specificsurface areas (SBET) and pore diameters (Dp).

The specific surface areas of the synthesized sensors by the threesol–gel routes varied widely within a range from 8 to 867 m2 g−1.The sensors produced by the acidic route present the highest SBETvalues, which are in agreement with data reported in the litera-ture [16]. Another relevant fact is the significant SBET increase withthe presence of the indicator in the medium when compared tobare silica prepared through the same experimental conditions.This fact indicates that the presence of these indicators perturbsthe system, causing an increase in the surface area. Another inter-esting result was an intermediate area for the NHR and also a strongdependence of SBET on the steric effect played by the encapsulatedmolecules, as already shown in the literature [29–31]. When the AR

and NHR are compared, no significant variation is observed in termsof Dp, while the BR affords the lowest SBET among the three routes.As a general trend, we have observed that as Dp increases, SBETdecreases independent of the presence or absence of the indicatorin the medium.

L.B. Capeletti et al. / Sensors and Actuators B 151 (2010) 169–176 173

Fig. 1. SEM micrographs of sensors prepared by BR: (a) BRBY; (b) BRAC; (c) BRAL; (d) bare silica.

Table 3Textural properties of sensors prepared by different synthetic routes determined by nitrogen porosimetry.

Indicator AL BR NHR

SBET(m2 g−1) DpBJH (Å) SBET(m2 g−1) DpBJH (Å) SBET(m2 g−1) DpBJH (Å)

AR 698 24 225 22 262 29

awEsoIepodtt

TU

AC 867 32 8BY 807 38 12Silica 301 26 11

To enable the elucidation of correlations between the texturalnd morphological properties of the prepared sensors, the samplesere further analyzed by small-angle X-ray scattering (SAXS) [26].

xamples of curves resulting from the three different routes arehown in Fig. 2. The unified fit [23,24] was used to interpret therganization in different length scales of the hierarchical structure.n this fitting procedure, the scattering curve can be fit by usingither one or several consecutive levels where each level is com-

osed by a shoulder-type Guinier region (allowing an estimationf radius of gyration, Rg) and/or a linear Power-law decay (givingetails about the organization of the material, ˛). At low-q, none ofhe samples possessed a Guinier region. This result indicates thathe material is formed with a very large length scale by particles or

able 4nified fit results to SAXS plots of prepared sensors by the three different routes.

AR BR

˛ Rg (nm) ˛

AL 3.7 1.2 ± 0.2 3.7BY 3.9 1.4 ± 0.2 3.9AC 3.9 1.0 ± 0.1 3.8Bare silica 3.9 1.1–0.1/+0.2 4.0

53 383 2550 332 2554 11 31

clusters with sizes larger than 100 nm, because smaller sizes implya reduction of the fit quality. It clearly corroborates with the SEMimages, because, in the basic route, particles of 0.5 �m are seen,while a gel-like structure is observed for the non-hydrolitic andacidic routes. However, in this length scale, a power-law decay,indicated by green lines in Fig. 3, is clearly observed. It is indica-tive of the fractal dimension of these particles or clusters, which, inthis specific case, provides information about their surfaces. The ˛

coefficients (power-law slopes) are as illustrated in Table 4.

According to Table 4, all solids have ˛ varying from 3.7 to 4.0(q < 0.02 nm−1), which indicates the degree of smoothness of thematerial’s surface under the length scale observed. Surface fractalshave ˛ exponents varying from 3 (completely rough surface) to 4

NHR

Rg (nm) ˛ Rg (nm)

1.4 ± 0.1 3.8 –– 3.8 –2.0–0.1/+0.2 3.8 –– 3.8 –

174 L.B. Capeletti et al. / Sensors and Actuators B 151 (2010) 169–176

Ft

(avttmIcciegt2wris(lm

Fg

Table 5Response time to NH3 gas of the sensors prepared by the three routes. (SD = 11).

Indicator t (s)AR BR NHR

ig. 2. SAXS plots of obtained sensors with AL by AR, BR and NHR routes and respec-ive unified fittings.

smooth surface). It was observed that the BR seems to be moreffected by the presence of indicators, as shown by the greater ˛ariation. This indicates that the presence of the indicator increaseshe roughness of the material; this behavior is in agreement withhe results obtained by other techniques, such the morphology

onitoring by SEM and indicator content measurement by DRS.n addition, it reinforces the presence of the influence of such indi-ators on the condensation reactions during the sol–gel process,hanging the aggregation mechanism of primary silica particlesn the basic route. On the other hand, an atypical surface fractalxponent is observed for acidic and non-hydrolytic routes, becauseel-like structures are obtained as a result of the chemical reac-ion. For a wet gel-like structure, ˛ coefficients between 1.8 and.5 were expected. Therefore, due to the drying of the material,e suggest that interstitial water was removed from the material

esulting in the appearance of surfaces that dominate the scatter-ng curve, as clearly indicated by the ˛ coefficients in Table 4. Themall degree of change of ˛ values within a same type of route

acidic and non-hydrolitic routes) is attributed to the surface simi-arities of the dried-gels rather than to the internal structure of the

aterial.

ig. 3. UV–vis DRS spectrum of ARAL sensor before and after contact with ammoniaas.

AL 59 – 149BY 98 – 133AC 135 – 164

At high-q (blue line) (q > 0.5 nm−1), a well-defined Guinierregion allows us to obtain the Rg, which can be attributed to thepresence of elementary silica particles. In addition, the Guinierregion followed a power-law decay (˛ = 4), indicating that all sys-tems present a smooth surface for this structural level. In the caseof the acidic route, a subtle increase of Rg is observed when thebare silica is compared with the material synthesized in the pres-ence of the indicators. We further speculate that the molecularvolume of the indicator, as well as its ability to form hydrogenbonds, can be directly correlated with the final size of the elemen-tary silica particles, i.e., AC < AL < BY. This trend can be observedin the case of Rg values for the acidic route. Although we do nothave a complete set of Rg values for the basic route, an oppositetrend is observed, because AC produced larger elementary parti-cles than AL. In addition, the basic route generates larger particlesthan the acidic route when the same indicators are compared. Thisis likely related to the physical properties of the indicators thathave distinct behaviors at different pH levels, changing the mech-anism of silica formation. However, an in depth investigation ofthe particle’s formation mechanisms is out of the scope of thismanuscript. On the other hand, for all solids obtained by the non-hydrolytic route and for two composites synthesized through thebasic procedure, the Rg calculation was not possible because onlyone structural organization level could be observed. For these cases,we suggest that the high scattering power of the composite sur-face has hindered the proper Rg calculation of the elementary silicaparticles.

3.4. Response time analysis

To evaluate the potential of the prepared sensors, tests wereperformed to determine the response time to NH3 gas. Fig. 3 illus-trates the UV–vis DRS spectrum of one obtained sensors before andafter reaction with gaseous NH3.

According to Fig. 3, a clear band shift from 414 to 503 nm isobserved after contact with the NH3 gas, and the solid color changedvisibly from yellow to pink. The response times for all the investi-gated systems are shown in Table 5.

According to Table 5, response times vary in the range from 59to 164 s. The response time for each analyzed indicator seems to bedependent on the experimental synthetic route. The measurementtime primarily fluctuated as function of the encapsulated indicator.The sensors that showed the sharpest (visual) and fastest changes(runs of 59 s and 97 s) were those prepared by the AR using AL andBY as indicators. Subsequently, the ARAC sensor presented the thirdshortest response time, probably due to the stronger interaction ofAC with the silica network. It is worth mentioning that becauseAC has a nitrogen atom (active site) in the ring which can be eas-ily protonated, its interaction with silica network is enhanced. Thisphenomenon may render the reaction with the analyte (NH3) moredifficult, because the indicator’s active site is already interactingwith silica. It is worth mentioning that the samples synthesized by

the NHR have shown longer response times compared with thosesynthesized by the AR with the same indicator. This behavior isexplained by the difficulty in identifying color change due to FeCl3residues. On the other hand, for the BR sensors, it was impossi-ble to observe visual color changes due to the low amount of fixed

nd Act

iist8

tamc

4

atncsotslsalncIsti

A

BFfi

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

L.B. Capeletti et al. / Sensors a

ndicator. In the literature, the response times to ammonia gas var-ed from 8 s to 8 min, depending on the sensor [32–34]. Optical fiberensors have shown the shortest response times (8–10 s) [34], andhe sol–gel based sensors have shown response times from 2 tomin [32].

The indicators entrapped within the sol–gel silicas were showno be stable even after period of one year. The reversibility wasttained after heating the system at 60 ◦C. All the sensors were sub-itted to reuse test and worked perfectly up to five investigated

ycles.

. Conclusion

The present study has shown that the structure of the sensor isffected by the method of sol–gel synthesis, which directly impactshe final characteristics of the material. The combination of theature of the indicator and the sol–gel route may provide clues forontrolling the response times to ammonia gas. The acidic routeeems to be the best route for preparing the solid sensors in termsf the amount of indicator entrapment without interfering withhe optical properties of the final material. Moreover, the greaterurface areas may also contribute to an easier access of the ana-yte to the sensor molecule within the silica framework despite themaller pore diameter. Among the three investigated indicators,lizarin red has shown the shortest response time when encapsu-ated in a silica prepared by the acidic sol–gel route. The use of theon-hydrolytic sol–gel route is limited by the presence of residualatalyst (FeCl3), which affects the optical behavior of the indicators.n summary, the present work contributes to the development ofolid sensors because we have pointed out that the synthesis condi-ion (sol–gel encapsulation route) may affect the indicator loading,ts nature and its behavior toward the analyte.

cknowledgements

This project was partially financed by the CNPq and FAURGS-raskem. L.B. Capeletti and F.L Bertotto thank the CNPq andAPERGS, respectively, for the grant, and the authors are thank-ul to the LNLS (Project D11A-SAXS1-8691) for the measurementsn the SAXS beamline.

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Biographies

Larissa Brentano Capeletti received her B.Sc. degree in industrial chemistry atUFRGS (2007). She is presently a master student at the same university. Her researchinterests cover the area of the sol–gel process in the development of hybrid materialsto applications as sensors, catalysts and biomaterials.

Francielle Leonardelli Bertotto is an undergraduate student in Chemical Engineer-ing. She has participated in research as part of her education in themes devoted toenvironmental monitoring.

João Henrique Zimnoch Dos Santos got his Bachelor of Science in chemistry (1985),Universidade Federal do Rio Grande do Sul (UFRGS, Porto Alegre, Brazil), his Masterof Science in Spectroscopy (1989) at the same University and Doctor in Molecular

Chemistry and Catalysis (1993) at Université Paul Sabatier (Toulouse, France). He didpost-doctoral research (1999/2000) at the Japan Institute of Science and Technology(JAIST, Ishikawa, Japan). He is presently Associate Professor in Analytical Chemistry(UFRGS). His research dealt with synthesis, characterization and evaluation of silica-based materials in different applications such as pre-concentration phases, catalystsupports, and sorbents. He is author of ca. 110 papers.

1 nd Act

E(UePne

ML

mer chemistry from Joseph Fourier University (France). In 2008 and 2009, hewas a post-doctoral fellow in the Chemical Sciences Division of the Oak Ridge

76 L.B. Capeletti et al. / Sensors a

dwin Moncada got his Chemistry Technology (1985) at Universidad De AntioquiaUdeA, Medellin, Colombia) and Food Engineering (2001) degree at Corporaciónniversitaria la Salle (U la Salle, Medellin, Colombia). He is PhD in Materials Sci-nce by the Universidad de Chile (2006). Presently, he is researcher at Braskem

etrochemical Co. (Triunfo, Brazil). His research dealt with polymeric, hybrid andanomaterials: their synthesis, characterization and evaluation properties in differ-nt commercial applications. He is author of ca. 10 papers and 6 patents.

ateus Borba Cardoso is an associate researcher at the Brazilian Synchrotronight Laboratory (LNLS-Brazil). He received his BSc and MSc degrees in the Federal

uators B 151 (2010) 169–176

University of Pelotas (Brazil). In 2007, he received his PhD in physical chemistryof polymers from Federal University of Rio Grande do Sul (Brazil) and in poly-

National Laboratory (ORNL-USA) where he worked on the development of hybridmaterials for hydrogen production. His present research interests concern the devel-opment of smart composite materials as well as their characterization through X-raytechniques.