Analytica Chimica Acta 506 (2004) 9–15

Fluorescent response of sol–gel derived ormosils foroptical ammonia sensing film

Xi Chena,∗, Ling Lin a, Peiwei Lib, Yuanjin Daia, Xiaoru Wanga

a The Key Laboratory of Analytical Sciences of Ministry of Education and Department of Chemistry,Xiamen University, Xiamen 361005, PR China

b Department of Biology, Indiana University, Bloomingtion, IN 47401, USA

Received 12 December 2002; received in revised form 16 October 2003; accepted 28 October 2003

Abstract

Optical sensing films for ammonia have been investigated based on a fluorescent indicator aminofluorescein (AF). AF was immobilizedin diverse organically modified silicates (ORMOSILSs) obtained by copolymerizing various proportions of methyltrimethoxysilane (Me–TriMOS), phenyltrimethoxysilane (Ph–TriMOS), dimethyldimethoxysilane (DiMe–DiMOS), and diphenyldimethoxysilane (DiPh–DiMOS)with tetramethoxysilane (TMOS). The effective polarities of ormosils were probed by using the solvatochromic dyeET (30) [2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)]phenolate. Compared with the fluorescent responses of fluorescein and fluorescein isothiocyanate, the fluorescentintensity enhancement of AF for ammonia was caused by the reaction between ammonia and the NH2 group on AF. The reaction may causethe reduction of the intermolecular self-quenching of AF to make a fluorescence enhancement of sensing film in NH3 solution. The ammoniasensing range and rate of response were found to highly depend on the type and content of organosilicon precursors employed. Films preparedwith a 1:3, 1:1.5, 1:1.2, or 1:1 mole ratio (TMOS:Me–TriMOS, TMOS:Ph–TriMOS, TMOS:DiMe–DiMOS, or TMOS:DiPh–DiMOS) wasfound to be the best in terms of stability and response. Their detection limits for ammonia in water was 0.01, 0.5, 0.2, and 0.5�g ml−1,respectively.© 2003 Elsevier B.V. All rights reserved.

Keywords:Ormosils; Fluorescence; Sensing film; Ammonia; Aminofluorescein

1. Introduction

A widely used approach for the development of chemicalsensors involves the immobilization of a suitable matrix,followed by an interconnection of this solid to a suitedtransducer. Compared with many organic polymers, organ-ically modified silicates (ORMOSILs) have been attractinggreat interest presently in the area of chemical sensors.They have been proven to be a better solid matrix to accom-modate to transducers owing to their better stability, opticaltransparency, flexibility, permeability[1–4], and porosity.Recently, ormosils-based optical oxygen[5] or ammoniasensors[6,7] have been extensively studied, since a morehydrophobic ormosils material can be obtained from sev-eral types of precursors, such as organotrialkoxysilane or

∗ Corresponding author. Fax:+86-5922186401.E-mail address:xichen@jingxian.xmu.edu.cn (X. Chen).

organodialkoxysilane [R–Si(OR′)3 or R2(OR′)2]. Generally,the sol–gel process consists of hydrolysis and condensationreactions. The silicate glasses are typically prepared by mix-ing alkoxysilanes with a low molecular mass, such as tetram-ethoxysilane (TMOS) or tetraethoxysilane (TEOS), withwater in a mutual solvent like methanol or ethanol[1,2,8,9].The conventional sol–gel is usually rather hydrophilic andsuitable for sensing polar ions, for instant, the hydrogen ion(H+). Alternatively, several types of organosilicon precur-sors can be hydrolyzed and co-condensed with TMOS orTEOS to form an organic–inorganic hybrid[10–13]. In thisregard, ormosils processes are dependent on a large num-ber of processing parameters such as alkoxide structure,reaction medium pH, solvent type, water–sol–gel precursorratio, aging temperature and pressure, drying temperatureand dying time as well[14–16]. With the substitutes ofunreactive and non-hydrolyzing alkyl groups for reactivealkoxyl groups in organosilicon precursors, the former onesstructurally act as a network modifier that terminates the

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.aca.2003.10.089

10 X. Chen et al. / Analytica Chimica Acta 506 (2004) 9–15

silicate networks. The substitution results in decreased po-larity and hydrophilicity of the produced ormosils[17–19],which in turn, increases the response of sensing weakerpolar substances like oxygen or ammonia.

Continuous analysis of ammonia in the environmentalsamples is important due to the increasing environmentalawareness and stricter regulations for pollution control. Con-sequently, a variety of ammonia sensors have been devel-oped, which are mainly based on electrochemical or opticaltransduction. Difficulties may arise in electrochemical meth-ods in terms of short lifetime, inadequate sensitivity, highsampling volume, miniaturzation and the less stability of ref-erence electrodes. So far, a great many optical ammonia sen-sors and probes have been described in literatures[20–23].In general, they are based on the pH-dependent absorption orfluorescence characteristics of dye indicators immobilized inthin films. The interaction between ammonia and the immo-bilized indicator results in absorbance or emission changes,which can be monitored by an optical fiber or planer waveg-uider [24–27]. Compared with the optical ammonia sensorsbased on the principle of pH measurement, in which an in-ternal buffer and a gas-permeable polymer membrane wereused, immobilized indicator methods can provide a betterpermeability and permeation selectivity, less response timeand lower detection limit. Recently, great attention has beenpaid to the methods of employing aminofluorescein (AF) asa fluorescent indicator dye in the preparation of ammoniasensor since its excitation maximum, beyond 400 nm, allowsthe use of inexpensive light sources[6,7]. Initial attemptsby Wolfbeis have been applied to produce ammonia probesbased on AF fluorescent reaction, in which AF is immo-bilized in an ormosils matrix[6]. In our previous studies,ammonia-sensing films were prepared by varying the ratio ofTMOS and organosilicon precursor DiMe–DMOS[7]. Theexperiments revealed that the response of ammonia sensorgreatly relies on the properties of ormosils, because the po-larity of the ormosils cages changed, with the molar ratiovarying of organosilicon and inorganosilicon precursors.

The work reported here aims to outline the effect oformosils polarity on the responses of ammonia sensingfilms, which were prepared via acid catalyzed hydrol-ysis and condensation of TMOS with one of the fourorganosilicon precursors, Me–TriMOS, DiMe–DMOS, Ph–TriMOS, and DiPh–DiMOS in different molar ratios. We are

Fig. 1. Aminofluorescein and its derivatives.

interested in developing materials with a lower pH, butlarger ammonia fluorescent response for ammonia sensingapplication. A solvatochromic dye,Et (30) [2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)]phenolate was used as thepolarity indicator for the prepared ormosils sensing films.In this report, the fluorescent reaction mechanism be-tween the fluorescent behavior of AF and ammonia inormosils cages was investigated by comparing AF and itsderivates. Furthermore, other characteristics of the sensingfilms including response time and detection limits are alsostudied.

2. Experimental

2.1. Chemical and standard solution

Si(OCH3)4(TMOS), CH3Si(OCH3)3(Me–TriMOS),(CH3)2Si(OCH3)2(DiMe–DMOS), PhSi(OCH3)3(Ph–Tri-MOS), Ph2Si(OCH3)2(DiPh–DMOS), (glycidyloxypropyl)-trimethoxysilane (GLYMO), and 5-aminofluorescein (AF,97%) were obtained from Fluka AG (Buchs, Switzerland).Et(30) (2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridino)phenolate(Reichardt’s dye), fluorescein (FLU), and fluorescein isoth-iocyanate (FITA, 90%) were purchased from Aldrich (Mil-waukee, WI, USA). All other chemicals were of guaranteedgrade. Twice-distilled deionized water was used throughout.The chemical structures of AF, FLU, and FITA are shownin Fig. 1.

The stock standard ammonia solution was prepared bydissolving ammonium chloride in 0.1 mol l−1 sodium phos-phate buffer of pH 8.0. The concentration of free ammo-nia was calculated by the Henderson–Hasselbach equation,which at pH 8.0 and 25◦C is:

[NH4Cl] = 1 + 10(pH − pKa)

10(pH − pKa)[NH3]

2.2. Apparatus

Fluorescence spectra and measurements were performedwith a Hitachi F-4500 spectrofluorimeter. The ammoniasensing layer was placed in the middle of the detectioncell, with λexit = 485 nm passing through an excitation

X. Chen et al. / Analytica Chimica Acta 506 (2004) 9–15 11

monochromator at a slit of 10 nm, and directed onto thesensing layer at an angle of 45◦. The synchronous fluo-rimetry (�λ = 20 nm) was used for the measurement offluorescence intensity throughout and all experimental re-sults were processed by a PC. Spin coating was performedwith a WS-400A-6NPP-LITE (Laurell, Technologies Co.,USA) at 1000 rpm. The dipole values of the compoundswere calculated by software CS Chem3D(R), Version 4.0(Cambridge Soft Corporation).ET (30) absorption valueswere determined by a visible-light spectroscopy (Beckman,USA), and theET (30) value was calculated from theλmaxvalues according to[28]:

ET(30) (kcal mol−1) = 28591

λmax(nm)

2.3. Sensing film preparation

In a typical experiment, ormosils were made for am-monia sensing films by mixing TMOS of different molarratios with organosilicon. A mixture of 40�l 5 mM AF,400�l methanol, and 300�l ormosils in methanol solu-tion was prepared. The ormosils were composed of vari-ous monomers with molar ratios as TMOS:Me–TriMOS 1:3,TMOS:Ph–TriMOS 1:1.5, TMOS:DiMe–DMOS 1:1.1, andTMOS:DiPh–DMOS 1:1, respectively, in a 5 ml vial. First,they were sonicated for 3 min, then 100�l hydrochloric acid(1 mmol l−1) was added, followed by sonicated for another3 min. Glass slides (15 mm× 30 mm× 0.15 mm) were ac-tivated after the treatment with concentrated nitric acid for24 h, then washed orderly with distilled water and ethanol,and later dried at 80◦C for 2 h. At last, Glass slides werespin-coated with the sensing films by pipetting 200�l ofthe respective sol-AF solutions. In all preparations, coatedormosils solutions were dried at room temperature in theambient atmosphere for 24 h, and then thermally treated at70◦C for 8 h. Coatings prepared in this way have an esti-mated thickness of 5�m [29]. The pH inside of the preparedormosils film was about 3.8, according to the comparison ofAF absorption spectra in MeOH solvent and sensing films.

2.4. Doping ormosils film with ET (30)

A typical component molar ratio of all silane materialshas been used forET (30) measurement, in which a 280�lMeOH, 120�l 1% methanolic solution ofET (30), 100�lsilanes, and 500�l water were added orderly. The mixturewas sonicated for 15 min, and then allowed to gel at roomtemperature for 24 h. After dried at 70◦C for 8 h, the or-mosils films withET (30) were ready for use.

2.5. Confocal microscopy experiment for AF and ET (30)probes

In order to study pore distribution of ormosils and givean evidence that AF andET (30) are located in the same

domains and regions in ormosils, the ormosil films whichimpregnated with AF andET (30) probes were imaged witha Nikon, Eclipse, E600 (Japan), confocal microscope. A15 mW argon-ion laser and mercury run were selected asthe excitation sources. According to the maximal excitationwavelengths of AF andET (30), the excitation wavelengthof 488 nm for AF and 350 nm forET (30) were used.

2.6. Pore distributions of ormosils

Pore distributions of ormosils were detected by Tristar3000 (Micrometritics Instrument Co., USA ). A 5.00 g sam-ple was crashed, rubbed, and then heated at 120◦C for 24 hunder the pressure of 0.0267 Pa. The 99.9% nitrogen (LindeCo., China) was employed as adsorbent gas.

3. Results and discussion

3.1. ET (30) absorption spectral behavior in ormosils

Although the polarity studies on ormosils using a keto-cyanine dye as a fluorescence probe have been described byLobonik and Wolfbies[30], confocal microscopic imagingtechnique was applied to confirm that AF andET (30) local-ize into the same regions of ormosils.Fig. 2 shows typicalconfocal microscope images of TMOS–Me–TriMOS filmimpregnated with AF andET (30) probes, from which sameluminescence dots could obviously be found when the filmwas illuminated at the excitation wavelength of 488 nm forAF and 350 nm forET (30). This result shows that two kindsof probes with different excitation wavelength were loadedinto the same domains of ormosil under the preparation con-ditions.

The behavior of the absorption spectra ofET (30) in or-mosils prepared from different molar ratios of Me–TriMOSto TMOS is shown inFig. 3. As compared to sampleswith Me–TriMOS addition, the sample prepared from 100%TMOS showed a maximum absorption wavelength bandat 498 nm (57.4 kcal mol−1). This value was found to beclose to the reference value of pure silica sol–gel (a max-imum absorption wavelength at 494 nm, 57.9 kcal mol−1)[31,32]. With addition of Me–TriMOS to the ormosils, asingle band with larger absorption intensity and red shiftedabsorption spectra could be observed. The same phenom-ena could be noticed by adding other kinds of organosili-cons to the ormosils. Interestingly, the polar trend inFig. 4is obviously associated with the substituent group, R. Thepolarity increases along with the size of the substitutes forall ratios of Me–TriMOS, Ph–TriMe, DiMe–DMOS, andDiPh–DMOS. Another important result presented inFig. 4lies in the difference inET (30) values of the ormosilswith unlike substituent groups. It should result from polar-ity changes and the diphenyl substituent ormosils introducedthe most obvious polarity decrease. As also seen inFig. 4,theET (30) value decreased by 78.4, 74.6, 72.8, and 70.0%,

12 X. Chen et al. / Analytica Chimica Acta 506 (2004) 9–15

Fig. 2. Confocal microscopy images of: (a) AF and (b)ET (30) in TMOS/Me–TriMOS ormosil. Excitation wavelength, 488 nm for AF, 350 nm forET (30).

respectively, for Me–TriMOS, Ph–TriMOS, DiMe–DMOS,and DiPh–DMOS ormosils films by changing the molar ra-tio from pure TMOS to TMOS/organosilicon= 1:1.

Table 1 displays the pH sensing characteristics of AFin the ormosils film with DiPh–DMOS as precursor. It isfound that the pH response of AF andET (30) value de-crease along with the increase of DiPh–DMOS content inthe precursor. Therefore, the resultant polarity of the sens-ing film is directly related with the silanol-group populationof the micro-porous glass. It is obvious that increasing or-mosils content results in the reduced presence of water insol–gel film [33]. Water molecule is essential for the pHsensing reaction of AF since water provides a hydrophiliccondition for hydrogen cation. In addition, the increase oforganosilicon content was found to decrease with the silanolgroup population of the film. It seemed that the ormosilsmatrix can provide more suitable and less polarity sites forreaction between AF and ammonia. Further support for thisassumption came when considering the responses of or-mosils with different precursors to ammonia and pH. The

Fig. 3. Absorbance spectra ofET (30) in different molar ra-tio of TMOS/Me–TriMOS films. (1) TMOS:TriMOS ratio= 10:0,λmax = 498 nm; (2) ratio= 9:1, λmax = 509 nm; (3) ratio= 8:2,λmax = 531 nm; (4) ratio= 7:3, λmax = 571 nm; (5) ratio= 4:6,λmax = 601 nm; (6) ratio= 5:5, λmax = 635 nm; (7) ratio= 1:3,λmax = 713 nm.

experimental results summarized inTable 1show that in aless polar environment (S1–4), fluorescent response to am-monia was enhanced, whereas that to pH decreased evidently(S1–1–S1–4). As shown inTable 2, ormosils films with mo-lar ratios as TMOS:Me–TriMOS 1:3; TMOS:Ph–TriMOS1:1.5; TMOS:DiMe–DMOS 1:1.1; TMOS:DiPh–DMOS 1:1displayed the most sensitive fluorescent response to AF,where their polarities reached the same value [ET (30) =40.2 ± 0.3 kcal mol−1]. This fact suggested that a suitablepolarity of ormosils film is critical for AF–NH3 fluorescentresponse.

3.2. AF fluorescent response of NH3

Aminofluorescein has been applied wildly owing to its pHsensitivities and higher fluorescence quantum yield. In AFmolecule, a photo-induced electron transfer process fromamino groups to aromatic hydrocarbon, causes intramolec-ular fluorescence self-quenching of AF molecule[34]. Insome derivatives of AF, the electron withdrawal groups, such

Fig. 4. ET (30) value as a function of the various R-group ormosils andof the composition (n = 3).

X. Chen et al. / Analytica Chimica Acta 506 (2004) 9–15 13

Table 1Characteristics of AF-ormosils films with different content of DiPh–DMOS precursors

Ormosils DiPh–DMOS:TMOS(molar ratio)

ET (30) (kcal mol−1) �pHa LODb (NH3 (�g ml−1))

S1–1 1:9 54.9± 0.3 0.05± 0.02 100S1–2 3:7 45.5± 0.2 0.20± 0.05 15S1–3 4:6 43.0± 0.3 1.8± 0.1 2.8S1–4 5:5 40.2± 0.5 0 0.5S1–5 6:4 39.2± 0.3 0 1.2

a pH ranges from 6.5 to 9.0 in 20 mmol l−1 NaOH–NaHPO4 buffer solution;�pH is the minimum detectable pH change.b Limit of determination.

as SCN, make their amide nitrogens a much poorer electronsource than that of AF, because they decrease the intramolec-ular electron-transfer effect by decreasing the intramolecularfluorescence self-quenching and, therefore, enhance fluo-rescence intensity of the substituted compounds[35]. Thatis why, AF shows remarkably weaker fluorescence intensitythan that of FLU or FITA. However, in the protonated AFmolecule, when the amino group is protonated, such inter-molecular electron transfer will be hindered, resulting in theobservation of a very large enhancement of fluorescence. Inaccordance with the above theoretical analysis, in our ex-periments, the ormosils films with the same concentrationof FLU or FITA did display a fluorescence enhancementin excess of 45-fold than those with AF. In common, FLUand its derivatives are typically used for their pH sensitivitybeginning at pH 4.0, where the molecules begin to form thehighly fluorescent dianionic structure, the xanthen-3-onetautomer. AF, on the other hand, shows an increase of flu-orescence intensity beyond pH 4.6, due to protonation ofthe aromatic amine[34]. At this stage, one must considerwhether the response of AF to ammonia in the sensing filmwas caused by the pH change upon the ammonia hydrolyz-ing in the matrix. Interestingly, no fluorescent response toammonia was found in the cases that FLU or FITA areused as fluorescence probes inS1–4 sensing films, sincea less polar environment of films prevents the entrance ofH+. Moreover, we should note that the structural differencebetween AF and FITA exist in no hydrogen atoms on theFITA amide nitrogen, and consequently, there was no fluo-rescence response to ammonia. These observations indicatethat hydrogen on amide nitrogen is a key factor for the fluo-rescence response to ammonia. The further consideration ofthe effect of NH2 group in AF on the fluorescence response

Table 2The maximal responses on NH3 of different ormosils sensing films (n = 3)

Ormosils film Molar ratio (organosilicon:TMOS) ET (30) (kcal mol−1) LOD (�g ml−1) Linear range (�g ml−1)

Me–TriMOS Ph–TriMOS DiMe–DMOS DiPh–DMOS

S2–1 3:1 40.1± 0.5 0.01 0.1–10S2–2 3:2 40.5± 0.2 0.8 1.0–20S2–3 6:5 40.3± 0.2 0.5 0.5–80S2–4 1:1 40.2± 0.5 0.5 0.5–20

was based on the following fact: we noticed that the blueshifts appeared if the AF sensing film was applied to detectammonia in solution. In a 20�g ml−1 ammonia solution, theabsorption wavelengths of AF changed from 490 to 485 nm,while 517 to 505 nm for excitation wavelength changes. Thisresult demonstrated that the existence of hydrogen bondbetween –NH2 of AF and NH3 should be taken into consid-eration. After the generation of AF–NH2 · · · NH3 hydrogenbond, the electron transition between amine nitrogen andphenol group will be weakened, causing a poorer electronssupply than that of amine nitrogen without hydrogen bondeffect. It results in the increase of AF fluorescence intensitysince intramolecular self-quenching of AF was reduced.

The sensing processes during the exposure of sensingfilms to ammonia solution can be described as the follow-ing:

NH3·H2O (solution)↔ NH3 (vapor) (balance betweenammonia in solution and vapor);

NH3 (vapor)↔ NH3 (sol–gel) (diffusion of ammonia intosensing film);

NH3 (sol–gel)+ NH2–AF (sol–gel)↔ –H3N · · · NH2–AF(sol–gel) (H bond generation, fluorescence intensityenhancement).

3.3. Fluorescent response of ormosils films

The excitation and emission spectra of AF doped ormosilsfilms prepared during this study demonstrate that the peakscentered at 485 and 505 nm, respectively. The characteristicsof sensing films with the different organosilicon precursorsand molar ratios were evaluated by monitoring their emis-sion intensity changes in the ammonia solution under the

14 X. Chen et al. / Analytica Chimica Acta 506 (2004) 9–15

Fig. 5. The fluorescent responses to different concentration of ammoniain TMOS/Me–TriMOS ormosils sensing films ((1) 0, (2) 0.05, (3) 0.1,(4) 0.5, (5) 1.0, (6) 3.0, (7) 5.0, and (8) 10.0�g ml−1).

same concentration. To obtain a stable baseline, the sensingfilms were conditioned under storage in distilled water for24 h before measurements. A typical fluorescent response ofS2–1 sensing film to different ammonia concentrations is pre-sented inFig. 5. These films exhibit a perfect linear responseto dissolved ammonia at room temperature. The calibrationcurve forS2–1, S2–2, S2–3 andS2–4 sensing films was linearfor ammonia concentration from 0.1 to 20, 1.0 to 20, 0.5 to80, and 0.5 to 20�g ml−1. The S2–1 sensing film gave thelowest detection limit, but contributed the narrowest linearrange. The reproducibility of the sensing films was deter-mined by measurement cycles between ammonia concentra-tion from 0 to 10�g ml−1, which is found to be 3.5, 4.3, 3.2,and 3.8% forS2–1, S2–2, S2–3, andS2–4 films. From distilledwater to 5�g ml−1 ammonia solution, the response time ofS2–1, S2–2, S2–3, andS2–4 sensing films, required to achieve95% of the full fluorescent response, was about 2.0, 5.6, 4.0,and 10 min, respectively. The time needed for returning tothe sensing line was 10 min for all films. The average sizeof the porosity in the ormosils films is correlated with thepore size, the average chain length and the linearity degreeof the organosilicon precursors. Mackenzie reported that theelongation ranges of tetraethoxysilane TEOS–Me–TriMOS,and TEOS–DiPh–DMOS were reported to be 113 and 3–15,respectively[35]. According to the response time of thesensing films, it could be assumed that the chain length oforganosilicon precursors in their hydrolysis process wouldbe a key effect factor for the cage size of these ormosilsmaterials, although the phenyl group of DiPh–DMOS pre-cursors has a larger size than the methyl group of TriMOS.According to the pores distributions experimental resultsshown inTable 3, the percent of pore with the size from100 to 10 nm forS2–1 film is 30.24,S2–2 film 14.41,S2–3film 18.91 andS2–4 film 11.18, respectively. It indicates thatTMOS–Me–TriMOS precursor contains a longer chain and

Table 3Pore distributions of ormosils

Pore ranges (nm) Relative volume (%)

S2–1 S2–2 S2–3 S2–4

100.0–10.0 30.24 14.41 18.91 11.1810.0–5.0 11.14 5.90 6.59 12.165.0–2.0 32.24 34.90 31.42 31.782.0–1.5 13.55 16.60 18.42 23.441.5–1.0 11.55 24.33 22.71 17.201.0–0.9 0 3.38 1.90 2.650.9–0.8 0.55 0.47 0 1.600.8–0.0 0.75 0 0 0

more suitable size of silica cage for AF embedding. Fur-thermore, the leakage of AF immobilized in the ormosilsfilms was also investigated. There was a slight detectablefluorescence in water after these films were stored in waterfor one month. GLYMO was selected as an immobilizationsol–gel reagent to overcome such leakage, because it can becoupled rather easily to the amino group of AF[36]. Afterthe covalent binding of AF to GLYMO, a 5 nm blue shiftfor AF maximum excitation wavelength occurred and theinitial fluorescence intensity of sensing films increased by15–23% forS2–1–S2–4 compared with that of the film with-out GLYMO covalent binding. In addition, no leaching wasfound for these sensing films because covalent bound indi-cators were hardly washed out. The initial fluorescence in-tensities of the films kept under dry condition decreased by

Table 4Effect of coexisting amines and amino acids on sensing film fluorescentresponsea

Compound �I/I0 × 100(%)b Dipolec (D)

S2–1 S2–2 S2–3 S2–4

Benzidine 0.5 1.1 0.6 −0.1 0.011o-Phenylenediamine 2.2 3.0 2.5 2.7 0.220Diphenylamine 3.2 1.9 2.3 4.8 0.987Ethanolamine 3.1 2.8 3.0 3.7 0.999Triethylamine 2.6 3.3 4.1 2.9 1.059Aniline 3.2 2.8 3.5 3.1 1.2961-Naphthylamine 6.2 6.8 7.6 8.2 1.324Cyclohexane 6.8 9.9 8.7 9.3 1.386Methylamine 12.7 13.2 10.5 9.0 1.396l-Glutamic acid 1.1 2.8 3.9 3.6 1.827l-Leucine 1.2 2.2 −1.4 1.1 2.041l-�-Alanine −0.1 2.5 3.0 0. 5 2.056l-Aspartic acid 2.9 1.8 −1.4 0.6 2.273l-Isoleucine 3.5 3.2 0.1 2.3 2.387dl-Glutamine 2.0 −1.2 3.3 1.5 2.755l-Serine −0.8 1.0 2.9 −1.3 2.910l-Hydroxyproline −0.7 2.3 −0.9 1.9 3.316

a The concentration of each compound is 20�g ml−1, the concentrationof ammonia 1�g ml−1, in 20 mmol l−1 NaOH–NaHPO4 buffer solution.

b I0: initial fluorescent intensity;I: fluorescent intensity after coexistingsubstance added;�I = I − I0.

c Results from the calculations by the software of CS Chem3D(R),Version 4.0 (Cambridge Soft Corporation). The dipolar value of ammoniais 1.551 D.

X. Chen et al. / Analytica Chimica Acta 506 (2004) 9–15 15

8–15% compared with films stored in distilled water, but theresponse to ammonia maintained over a period of 6 months.The denser structure of the ormosil network as a result of theabsence of water might be responsible for this phenomenon,because of the incomplete hydrolysis on these layers[6].

Effect of coexisting compounds is another essential fac-tor for ammonia sensing film. Our results showed that asthe polarity of the coexisting compounds approached thatof ammonia, the fluorescent responses increased accord-ingly (Table 4). Many other coexisting amines and aminoacids did not give apparent contribution to the measure-ments because of the distinct polarities between ammo-nia and these compounds, which prevented them into thematrix.

Similar work was performed to test the effect of mostmetal ions (Co2+, Hg2+, Sb2+, Bi2+, Sn2+, Ni2+, Cd2+,Fe3+, Cu2+, K+, Ca2+, Mg2+, Al3+, Zn2+, Mn2+, andBa2+, each at a concentration of 10�g ml−1) on the fluo-rescent intensities. Experimental results showed that all pre-pared sensing films were independent to those ions owingto their high hydrophobicity surface.

4. Conclusions

Organically modified silicate materials provide a conve-nient host matrix for the immobilization of aminofluoresceinfor ammonia sensing application. A high hydrophobicity oformosils matrix [ET (30) = 40.2±0.3 kcal mol−1] preparedby different kinds of organosilicon precursors provides ahigh fluorescent response for ammonia sensing film. Thepresented work has described the behaviors of fluorescent re-sponse of the reaction between AF and ammonia in ormosilsfilms when compared with several fluorescence derivates.The hydrogen bond between AF and ammonia causes thereduction of intramolecular fluorescence self-quenching, re-sulting in the fluorescence enhancement when sensing filmsexposed to ammonia solution. In this work, optimal resultsin terms of low detection limit, response time and stabilitywere obtained for films prepared with 1:3, 1:1.5, 1:1.2, or 1:1mole ratios as TMOS:Me–TriMOS, TMOS:Ph–TriMOS,TMOS:DiMe–DiMOS, or TMOS:DiPh–DiMOS, respec-tively. These materials produced a good linear response forammonia concentrations from 0.1 to 20, 1.0 to 20, 0.5 to80, and 0.5 to 20�g ml−1, and their detection limits forammonia in water were 0.01, 0.5, 0.2, and 0.5�g ml−1,respectively.

Acknowledgements

This research work was financially supported by Natu-ral Science Foundation of Fujian, China (D111011), andthe Research Foundation of Key Laboratory of Photochem-istry, Center for Molecular Science, Institute of Chemistry,

Chinese Academy of Sciences, which is gratefully acknowl-edged. At the meantime, we would also like to appreciateProfessor Xin Lu State Key Laboratory for Physical Chem-istry of Solid Surfaces and Department of Chemistry, Xia-men University for stimulating discussions and Dr. GuozhouChen, Chemical Department of Indiana University, USA, forhis contribution in the experiment of confocal microscopy.

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