Analytica Chimica

Analytica Chimica Acta 414 (2000) 181–187 

Effect of the polymer matrix on the response of optical sensors for 

dissolved aliphatic amines based on the chromoreactand ETH T 4001 

Gerhard J. Mohr ∗ , Tomas Nezel, Ursula E. Spichiger-Keller 

Centre for chemical Sensors, ETH Technopark, Technoparkstrasse 1, CH-8005 Zurich, Switzerland 

Received 5 November 1999; accepted 28 January 2000 

Abstract 

The absorbance-based chromoreactand 4-(N, N-dioctylamino)-4 ′ -trifluoroacetylazobenzene (ETH T 4001) has been investigated 

in different polymer matrices for the optical sensing of dissolved aliphatic amines. Sensor layers composed of ETH T 

4001 and different polymer materials generally show a decrease in absorbance at around 500 nm and an increase in absorbance 

at around 420 nm wavelength upon exposure to dissolved aliphatic amines. This change in absorbance is caused by 

a conversion of the trifluoroacetyl group of the reactand into a hemiaminal or a zwitterion. 

Several polymer materials have been screened for their use in optical amine sensing, such as plasticized poly(vinyl chloride) 

(PVC), copolymers of acrylates, polybutadiene (PBD), and silicone. The choice of the polymer affects the sensitivity of the 

sensor layer, which is generally in the mM range for 1-butylamine, and the response time, which lies in the range of 10–30 min. 

Furthermore, the polarity of the polymer matrix has a strong influence on the diol formation caused by conditioning in water, 

and on the absorbance maximum of the solvatochromic reactand. However, the selectivity of the sensor layers for primary, 

secondary and tertiary amines is nearly unaffected by the polymer matrix. Thus, while it is possible to vary sensitivity towards 

amines and humidity by choosing the appropriate polymer matrix, it is not possible to modify the sensor’s selectivity within 

different amines. © 2000 Elsevier Science B.V. All rights reserved. 

Keywords: Aliphatic amines; Optical sensors; ETH T 4001; Chromoreactand; Polymers; Plasticizers; Humidity 

1. Introduction 

Recently, a new class of indicator dyes for optical 

sensing has been presented. These dyes respond to 

nucleophilic analytes by performing reversible chemical 

reactions. As a consequence of these chemical reactions, 

changes in both absorbance and fluorescence 

have been observed. Based on these new indicator 

dyes, called ‘chromoreactands’ or ‘fluororeactands’, 

∗ Corresponding author. Tel.: +41-1-445-1350; 

fax: +41-1-445-1233; www.chemsens.ethz.ch. 

E-mail address: gerhard@chemsens.pharma.ethz.ch (G.J. Mohr) 

optical sensors for alcohols, amines and humidity have 

been developed [1–6]. The optical sensors have mainly 

been used by combining a reactand and a catalyst 

(if necessary) with the plasticizer dioctylsebacate and 

poly(vinyl chloride) (PVC) as the polymer matrix. 

Polymers and plasticizers can have a tremendous 

effect on the response of both optical and electrochemical 

sensors. The lipophilicity and the polarity 

of a plasticizer can improve the selectivity of an ISE, 

e.g. for magnesium over calcium [7]. Polymers can 

exclude ions from permeating the material, while 

diffusion constants for gases remain high [8]. The 

polymer can enrich or exclude gaseous analytes from 

0003-2670/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. 

PII: S0003-2670(00)00794-7

182 G.J. Mohr et al. / Analytica Chimica Acta 414 (2000) 181–187 

the membrane. This happens, for instance, in polymer 

layers of electronic noses [9], or in the permselective 

membranes in gas separation techniques [10]. In order 

to be able to fine-tune the selectivity, it is important 

to investigate the response behaviour of ligands, 

indicators and reactands in different polymer layers. 

The present work discusses the effect of polymer 

materials on the response of optical sensors for dissolved 

aliphatic amines. Not only have plasticized 

polymers with significant differences in lipophilicity 

and polarity been used, but so too have polymer 

materials which are thought to function without any 

addition of plasticizers, e.g. silicones or methacrylates. 

The sensors have been characterized in terms 

of sensitivity, selectivity and response time. 

2. Experimental 

2.1. Reagents 

All the amines used were of analytic reagent grade. 

For membrane preparation, PVC (high molecular 

weight), tris(2-ethylhexyl)phosphate (TOP), tris(2-ethylhexyl)trimellitate 

(TOTM), 2-octyloxybenzonitrile 

(CPOE), hexakis-(2,3,6-tri-O-octyl)--cyclodextrine 

(-CD) and tetrahydrofuran (THF) were obtained 

from Fluka (Buchs, Switzerland). Poly(hexyl methacrylate) 

(PHMA), poly(vinyl chloride-co-vinyl acetate- 

co-2-hydroxypropyl acrylate) (PVC–OH) with a 

content of 81% vinyl chloride, 4% vinyl acetate 

and 15% 2-hydroxypropyl acrylate, and cis-transpolybutadiene 

(PBD) were obtained from Aldrich 

(Buchs, Switzerland). The PHMA was in solution in 

toluene and was evaporated to dryness before use. 

Ethylene vinyl acetate copolymers with a vinyl acetate 

content of 33% (EVA 33) and 45% (EVA 45) came 

from Scientific Polymer Products (Ont., NY). Acetic 

acid releasing silicone (E4) was from Wacker-Chemie 

GmbH (München, Germany). The synthesis and the 

optical properties of ETH T 4001 are described in 

detail elsewhere [1]. 

Amine solutions were prepared by dissolving the 

appropriate amount of each amine in 0.1 M sodium 

hydroxide solution. Due to the high pH of 13.0, the 

amines were mainly present in the electrically neutral 

form and not in the ammonium form. The correct 

amine concentrations were calculated by using the 

Henderson–Hasselbach equation and the pK a value of 

each amine [4,11]. 

2.2. Preparation of the sensor layers 

Sensor membranes M1–M6 were obtained by dissolving 

40 mg of PVC, 80 mg of the respective plasticizer 

and 1.0 mg of ETH T 4001 in 0.8 ml of THF (see 

Table 1). The sensor membrane M7 was obtained by 

dissolving 250 mg of silicone E4 and 1.0 mg of ETH T 

Table 1 

Composition and absorbance maxima of sensor layers M1–M11, prepared by dissolving the given components and 1 mg of ETH T 4001 

in 0.8 ml of THF 

Membrane Polymer Plasticizer Absorbance maximum 

TFA (nm) a 

M1 40 mg PVC 80 mg CPOE 510 434 

M2 40 mg PVC–OH 80 mg CPOE 509 432 

M3 40 mg PVC 80 mg -CD 485 420 

M4 40 mg PVC 80 mg DOS 497 424 

M5 40 mg PVC 80 mg TOP 501 423 

M6 40 mg PVC 80 mg TOTM 500 427 

M7 250 mg E4 – 462 417 

M8 65 mg EVA 33 – 494 423 

M9 65 mg EVA 45 – 494 423 

M10 30 mg PBD – 483 421 

M11 120 mg PHMA – 494 424 

a Absorbance maximum of the trifluoroacetyl form (TFA) of ETH T 4001 on exposure to dry air. 

b Absorbance maximum of the hemiaminal form (HA) of ETH T 4001 on exposure to 0.5 M aqueous 1-butylamine at pH 13.0. 

HA (nm) b

4001, M8 by dissolving 65 mg of EVA 33 and 1.0 mg 

of ETH T 4001, M9 by dissolving 65 mg of EVA 45 and 

1.0 mg of ETH T 4001, M10 by dissolving 30 mg of 

PBD and 1.0 mg of ETH T 4001, and M11 by dissolving 

120 mg of PHMA and 1.0 mg of ETH T 4001 each 

in 0.8 ml of THF. A dust-free glass plate was placed 

in a spin-coating device with a THF-saturated atmosphere 

[12]. 0.3 ml of each solution was transferred 

onto the rotating glass support with a syringe. The resulting 

membranes were placed in ambient air for drying. 

The membrane M6 was allowed to cross-link for 

48 h at ambient temperature and humidity. 

G.J. Mohr et al. / Analytica Chimica Acta 414 (2000) 181–187 183 

2.3. Apparatus 

The absorption spectra of the sensor layers were 

recorded on an Uvikon 942 spectrophotometer at 

22±1 ◦ C. The measurements were performed in a 

flow-through cell fixed inside the spectrophotometer 

by pumping the sample solutions at a flow rate 

of 1.5 ml min −1 (flux:5.4 cm min −1 ) using a Perpex 

peristaltic pump (Jubile, Switzerland) [13]. 

3. Results and discussion 

3.1. Sensor responses of polymers based on 

plasticized PVC (M1–M6) 

Two chemical reactions are encountered with all 

sensor layers, namely conversion of the trifluoroacetyl 

group of ETH T 4001 into a diol upon interaction with 

water (during conditioning), and further conversion of 

the diol into a hemiaminal upon interaction with aqueous 

amines (Fig. 1) [14]. These two chemical reactions 

cause the absorbance of ETH T 4001 to decrease at a 

wavelength of around 500 nm (representing the trifluoroacetyl 

form of the dye) and to increase at a wavelength 

of around 420 nm (corresponding to the diol 

and hemiaminal forms). The optical properties of the 

reactand are changed because the degree of electron 

delocalization of the reactand is affected by the chemical 

reaction, i.e. by the change in electron-acceptor 

strength of the trifluoroacetyl form after conversion 

into a diol or hemiaminal. 

The major advantage of plasticized PVC is that it 

allows the same polymer to be used with different 

Fig. 1. Reactions of water (1), primary and secondary amines (2), 

and tertiary amines (3) with ETH T 4001. 

plasticizers, and consequently, to tailor the characteristics 

of the sensor layer. In the present investigation, 

plasticizers of high polarity but low lipophilicity 

(CPOE), and of low polarity but high lipophilicity 

(TOTM) were used, and so were plasticizers with 

intermediate polarities and lipophilicities such as 

DOS and TOP (Table 2) [11,15,16]. Two parameters 

were investigated in more detail: the sensitivity of 

the plasticized PVC layers upon exposure to aqueous 

Table 2 

Relative permittivity ɛ r a,c and lipophilicities b,c (log P TLC values) 

of some polymers and plasticizers 

Plasticizer ɛ r log P TLC Polymer ɛ r 

CPOE 23.0 5.0 PVC 4.6 

DOS 3.9 10.5 silicone 3.2 

TOP 4.8 10.2 PBD ca. 2.3 

TOTM 4.6 16.9 

a Measured at 10 kHz [15]. 

b Lipophilicity determined by thin layer chromatography, which 

correlates with n-octan-1-ol-water partition coefficients [16]. 

c Data taken from [11,15,16].


Table 3 

Sensitivity, signal magnitude and response time of M1–M11, measured at 510 nm for M1 and M2, and at 500 nm for M3–M11 

Membrane RSC for diol RSC for hemiaminal Sensitivity to Forward response Reverse response 

formation a formation b 1-butyl-amine c time d (min) time e (min) 

M1 12 88 −1.90 10–15 5–10 

M2 19 81 −1.93 5–10 5–10 

M3 28 72 −1.51 10–15 6–12 

M4 24 76 −1.85 10–15 5–10 

M5 90 10 n.d. – – 

M6 9 91 −1.88 5–10 5–10 

M7 1 99 −1.04 5–15 5–15 

M8 31 69 −1.93 5–15 10–30 

M9 39 61 −1.95 6–15 10–25 

M10 1 99 −1.18 5–15 5–10 

M11 9 91 −2.11 6–12 6–12 

a Relative signal change (RSC) in % on being moved from dry air to 0.1 M sodium hydroxide solution. 

b Relative signal change (RSC) in % on being moved from 0.1 M sodium hydroxide solution to 0.5 M 1-butylamine in 0.1 M sodium 

hydroxide solution. 

c Logarithm of the concentration where the highest sensitivity for 1-butylamine was observed, i.e. at the point of inflection of the 

sigmoidal calibration graph. 

d Response time on being moved from lower to higher amine concentrations. Generally, a faster response at lower amine concentrations 

and a slower response at higher concentrations was observed. 

e Response time on being moved from higher to lower amine concentrations. 

1-butylamine was recorded in order to tailor the sensitivity 

range by choosing the appropriate plasticizer 

(Table 3), and the effect of different matrices on the 

selectivity was evaluated in order to identify the best 

matrix (Table 4). Table 4 shows that the selectivities 

of M1–M6 to 1-butylamine over interfering aliphatic 

amines are nearly identical. In all cases, the sensitivity 

towards primary aliphatic amines is governed 

by their increasing lipophilicity (see the logarithm 

of octanol–water partition coefficients, log K ow , in 

Table 4 

Selectivity coefficients (log K opt ) of M1–M11 for aliphatic amines and ethanol in comparison with 1-butylamine 

Analyte (log K ow [17]) a log K opt M1 log K opt M2 log K opt M3 log K opt M4 log K opt M6 

Ammonia −1.8 −1.8 −1.4 −1.5 −1.8 

Methylamine (−0.57) −1.3 −1.3 −1.3 −1.3 −1.5 

Ethylamine (−0.13) −1.1 −1.2 −1.2 −1.2 −1.3 

1-Propylamine (0.48) −0.5 −0.5 −0.6 −0.5 −0.7 

Diethylamine (0.58) −1.6 −1.9 −1.6 −1.6 −1.8 

Triethylamine (1.45) −1.1 −1.1 −1.1 −1.1 −1.1 

Ethanol (−0.30) −2.8 −2.9 −2.5 −2.7 −2.9 

log K opt M7 log K opt M8 log K opt M9 log K opt M10 log K opt M11 

Ammonia −1.6 −1.7 −1.6 −2.2 −1.7 

Methylamine −1.3 −1.3 −1.2 −1.5 −1.3 

Ethylamine −1.1 −1.2 −1.1 −1.3 −1.2 

1-Propylamine −0.6 −0.6 −0.5 −0.7 −0.4 

Diethylamine −1.4 −1.7 −1.7 −1.9 −1.7 

Triethylamine −0.7 −1.1 −1.1 −0.9 −1.4 

Ethanol −2.4 −2.8 −2.7 −2.7 −3.0 

a Logarithm of the partition coefficient of different amines and ethanol between n-octan-1-ol and water (1-butylamine has a log K ow of 

0.86).


Fig. 2. Absorbance spectra of M1 in contact with dry nitrogen and 

different concentrations of aqueous 1-propylamine. When changing 

from nitrogen to 0.1 M sodium hydroxide solution, the diol 

is formed, whereas when changing from plain buffer to aqueous 

1-propylamine, the hemiaminal form occurs. Both types of reactions 

are fully reversible. 

Table 4) [17], and is the highest for 1-butylamine. 

Although the lipophilicities of the secondary and tertiary 

amines lie within the same range, the response is 

significantly lower. We attribute this difference to the 

fact that secondary and tertiary amines are sterically 

hindered in approaching the trifluoroacetyl group by 

their bulky groups near the amino moiety [4]. 

The best selectivity to 1-butylamine over the investigated 

primary, secondary and tertiary amines is encountered 

with the membrane based on the lipophilic 

and non-polar plasticizer TOTM. All the plasticized 

membranes showed a significant formation of the 

diol during conditioning of the membranes in 0.1 M 

sodium hydroxide solution (Table 3). Considering 

that diol formation should be the highest in polar 

non-lipophilic plasticizers because of their water uptake, 

the limited interaction of ETH T 4001 with water 

in CPOE is remarkable (see also Fig. 2). Using the 

less polar and more lipophilic DOS and TOP leads 

to much larger diol formation. However, the extent of 

diol formation may also be affected by the interaction 

of the diol with specific functional groups of the 

polymer matrix. Thus, the diol might be stabilized 

by the formation of hydrogen bonds to the carbonyl 

or phosphoryl groups of the plasticizers [9]. When 

the water content of plasticized PVC membranes was 

investigated after conditioning in water, no direct correlation 

between relative permittivity or lipophilicity 

of the plasticizer and the water uptake was observed 

[18]. It was found that PVC plasticized with TOTM 

(plasticizer/PVC=2:1) exhibited a water content of 

0.32 wt.%, DOS, 0.48 wt.%, and TOP, 1.15 wt.% [18]. 

These data correlate much better with the encountered 

diol formation. 

The polymer membrane based on TOP exhibits a 

diol formation of 90% of the possible maximum signal 

change. Further signal changes with amines are small. 

Thus, this membrane is of no use for amine sensing. 

Nevertheless, the membrane could be of interest for 

sensing low relative humidity since its response to ambient 

air humidity of 24% RH already yields a signal 

change of around 65%. Due to the weak nucleophilicity 

of water, the response time of such a membrane is 

in the range of 2 h. This is too slow for fast reversible 

sensors, but the membrane could be used for test strips 

to detect exposure to humidity. 

In a preliminary investigation, calix[6]arene hexaester 

[19] and hexakis-(2,3,6-tri-O-octyl)--cyclodextrine 

[20] (in a ionophore/reactand ratio of 2:1) 

were added to M4 in order to enhance the sensitivity 

and the selectivity of the sensor layer. However, the 

addition of ionophores selective for alkylammonium 

ions did not affect any of the two parameters to a 

significant extent. Thus, the liquid -cyclodextrine 

itself was used as a plasticizer for PVC (M3). Again, 

the resulting data showed no positive effect on the selectivity 

and the sensitivity of the sensor membranes. 

The sensitivity to 1-butylamine was found to be smallest 

for the investigated PVC-based layers and the 

selectivity was also slightly lower than for the other 

plasticized membranes. It can therefore be concluded 

that the use of calixarene- and cyclodextrine-based 

ionophores for neutral analytes does not improve the 

sensor characteristics when using chromoreactands. 

3.2. Sensor responses of polymers based on 

plasticizer-free polymers (M7–M11) 

Plasticizer-free polymers have advantages over 

plasticized polymers in terms of operational and shelf 

lifetime. Plasticizers tend to evaporate when stored 

at temperatures higher than around 60 ◦ C. Depending 

on their lipophilicity, they can also be washed out. 

Plasticizers are required to soften polymers and enhance 

the diffusion and the solubility of the analyte.


If polymers without plasticizers are to be used, they 

will have to be smooth and flexible to allow fast analyte 

diffusion. Furthermore, they will have to form 

stable thin films on the mechanical support (glass 

plates, optical fibers etc.). Polymers that fulfill these 

requirements and have been investigated in this study 

are silicone (M7), ethylene vinyl acetate copolymers 

(M8, M9), PBD (M10) and PHMA (M11). All of 

these polymers are viscous, mechanically stable polymer 

materials that exhibit response times to amines 

that are comparable to the ones encountered with 

plasticized PVC (Table 3). Two of the polymers (silicone 

membrane M7 and PBD membrane M10) show 

remarkably low diol formation, which guarantees that 

the signal changes for amines are large. However, 

the sensitivity for 1-butylamine was found to be the 

lowest of all the membranes investigated (Table 3). 

This is probably due to the low polarity of the polymer 

material, which is not a good solvent for the 

relatively polar amines, and consequently, does not 

extract amines efficiently from the aqueous into the 

organic polymer phase. Nevertheless, the selectivity 

of M10 was among the best encountered in this study 

(Table 4). The ethylene vinyl acetate membranes, M8 

and M9, exhibited strong diol formation (between 

30 and 40%), but the sensitivity was slightly better 

than with plasticized PVC membranes. The strong 

response to both water and amines was due to the polarity 

of the membrane material caused by the acetate 

groups. The best membrane in terms of sensitivity 

combined with relatively low response to water (9%) 

was found to be the PHMA membrane M11. 

were slowed by a factor of 1.5–2. Membranes based 

on very non-polar materials, e.g. PBD and silicone, 

behaved significantly differently as their sensitivity 

was lower by a factor of 10, whereas their selectivities 

and response times were comparable to the 

more polar sensor membranes. The lower sensitivity 

was caused by the low solubility of polar amines in 

non-polar PBD and silicone. 

The absorbance maxima of ETH T 4001 in the different 

polymer materials give some insight into their 

polarity since the reactand exhibits a pronounced 

positive solvatochromism (Table 1) [21]. The absorbance 

maxima in different organic solvents can 

be used to establish a calibration graph between the 

polarity of the matrix (expressed by the wavelength 

of the absorbance maximum of Reichardt’s phenolbetaine 

in the respective solvent) and the ET 30 

value [21]. The resulting graph (Fig. 3) allows the 

polarity of the different polymer matrices to be estimated. 

This can throw some additional light on the 

overall effect of the polymer matrix on the sensor 

characteristics. These are a sum of single parameters 

(viscosity, lipophilicity and polarity of polymer and 

plasticizer) rather than just the dielectric constant or 

lipophilicity of the plasticizer alone. When calculating 

the ET 30 values from the absorbance maxima of 

ETH T 4001 in different organic solvents, PVC/CPOE 

had an ET 30 value of 44.8 kcal mol −1 , PVC/TOTM, 

41.7 kcal mol −1 , PVC/DOS, 40.7 kcal mol −1 ,EVA33 

and PHMA, 39.8 kcal mol −1 , PBD, 36.3 kcal mol −1 , 

3.3. Comparison of plasticized and plasticizer-free 

polymer membranes 

All the plasticized membranes studied respond to 

humidity and amines, and showed very similar selectivities 

and response times. This was due to the fact 

that the same basic polymer (PVC) was used. Furthermore, 

all plasticizers exhibited a significant polarity 

(Table 2) since they were either esters or ethers with 

carbonyl, phosphoryl or nitrile functions. For the 

plasticizer-free polymers with similar chemical functions 

(EVA 33, EVA 45, PHMA), a comparable sensitivity 

to membranes based on plasticized PVC was 

encountered (Table 3). However, the response times 

Fig. 3. Correlation between the ET 30 values of different organic 

solvents and the absorbance maxima of ETH T 4001 in these 

solvents.


and E4, 29.7 kcal mol −1 . In other words, we find 

that the absorbance maximum of the trifluoroacetyl 

form in cyclohexane (467 nm) correlates with the 

maximum in silicone, whereas the maximum in 

ethyl acetate (489 nm) correlates with that in the 

other plasticizer-free materials. The polar solvents 

(dichloromethane, 501 nm; acetone, 497 nm) correlate 

best with the plasticized PVC membranes, whereas 

none of the organic solvents gives as strong a red-shift 

as CPOE does. 

4. Conclusions 

Different polymer materials, such as plasticized 

PVC, vinyl acetate copolymers, PBD, and silicone, 

were investigated for their use in optical amine sensors 

based on a reversible chemical recognition. 

Generally, it was found that the sensitivity of sensor 

layers towards aliphatic amines is governed by 

their lipophilicity and chemical structure, and is the 

highest for lipophilic primary amines. The choice of 

the polymer matrix has little effect on the selectivity 

of the sensor layers to different aliphatic amines, 

whereas there is a strong effect on the diol formation 

of the reactand in the polymer. In contrast, the overall 

sensitivity is decreased by a factor of 10 when using 

highly non-polar materials. Since plasticizer-free 

membranes behave in a manner similar to plasticized 

polymers, albeit with slower response times, their use 

is preferred for practical applications. 

Acknowledgements 

This work was supported in part by Bosch Telecom 

GmbH, by the Swiss National Science Foundation 

within SNF-project 20-55588.98 and by the Swiss 

Federal Institute of Technology project TH-2/99-1 

(Reg. No. 03145). Their support is gratefully 

acknowledged. 

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