# UV-Activated Luminescence/Colourimetric

Authors: Andrew Mills; Cheryl Tommons; Raymond T.  Bailey; M. Catriona Tedford; Peter J. Crilly
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

An oxygen indicator is described, comprising nanoparticles of titania dispersed in hydroxyethyl cellulose (HEC) polymer film containing a sacrificial electron donor, glycerol, and the redox indicator, indigo-tetrasulfonate (ITS). The indicator is blue- coloured in the absence of UV light, however upon exposure to UV light it not only loses its colour but also luminesces, unless and until it is exposed to oxygen, whereupon its original colour is restored.  The initial photobleaching spectral (absorbance and luminescence) response characteristics in air and in vacuum are described and discussed in terms of a simple reaction scheme involving UV activation of the titania photocatalyst particles, which are used to reduce the redox dye, ITS, to itsleuco form, whilst simultaneously oxidising  the glycerol to glyceraldehye.  The response characteristics of the activated, that is, UV photobleached, form of the indicator to oxygen are also reported and the possible uses of such an indicator to measure ambient O2 levels are discussed.


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## Body

## 1. Introduction

The presence of oxygen in food packaging usually has a
detrimental effect on the food products contained therein. Oxygen does not only react chemically with
food to cause oxidative rancidity, but moulds, growths, and aerobic microorganisms—which discolour and decompose food and make it offensive to
smell and dangerous to eat—thrive in the presence of oxygen [1]. Therefore, it is no surprise that the removal of oxygen in the food
packaging industry is of immense importance. This is usually achieved via modified
atmosphere packaging (MAP), a process in which the atmosphere within the food
package is flushed and replaced with an inert gas, such as nitrogen or carbon
dioxide, often combined with an efficient oxygen scavenger, resulting in an
oxygen level of 0.1% or less within the food package [1, 2].
MAP increases the shelf life of many food products by a factor of 3-4
compared to that in air, making it a popular method of packaging food in the
wholesale and retail food packaging industry.There are many established methods for the detection of oxygen, which include the Clark electrode [3] and gas chromatography [4], however, such methods are too
expensive and time consuming to allow 100% quality assurance. Consequently, there is an increasing interest
in the development of cheap, easy-to-use oxygen indicators [5].
This area of research has been dominated by the quenching of a polymer-encapsulated lumophore, such as
ruthenium(II)-tris(4,7-diphenyl-1,10-phenanthroline), Ru(dpp)32+, by oxygen. One of the few commercially-available
products based on this approach is the OxySense system [6], whereby, Ru(dpp)32+ is encapsulated in silicone rubber dots, called O2xyDots™, which can
be attached to the inside of a package or bottle. The O2xyDot is illuminated and the
readily measured luminescence lifetime of the lumophore is equated to the oxygen level within the package. Unfortunately, the detection of oxygen using
the OxySense system, or any optical sensor based on luminescence, requires the
use of relatively expensive instrumentation for making the required lifetime or
intensity measurements.Unlike changes in luminescence
intensity or lifetime, a sensor that changes colour in the presence of oxygen
would be most desirable for MAP, given that the human eye can then act as the
detector. Such indicators can take
several forms, such as a tablet [7, 8] or a printed layer [9, 10]. This technology is typified by the
Ageless Eye oxygen indicator, manufactured by the Mitsubishi Gas Company in
Japan [7, 8, 11], that comprises a redox-indicator,
usually methylene blue, which, in the absence of oxygen, is maintained in its
colourless, chemically reduced, leuco form, by a reducing agent, such as glucose in an alkali medium. In the presence of oxygen leuco, methylene blue is oxidised to a
highly coloured form. All components are
mixed together, along with a nonredox dye, such as Acid Red 52, which provides
a pink background colour, and pressed together to form a pellet, which is
subsequently encapsulated in an oxygen-permeable, ion-impermeable plastic sachet
to avoid any contact with the food. In
the presence of oxygen, the Ageless Eye indicator changes from pink to purple
in 2-3 hours, in a quasireversible process. However, this indicator needs to be
stored and handled under anaerobic conditions.
Such oxygen indicators are used in the food packaging industry, but
mostly used as a research tool or fault diagnostic, since they have limited use
elsewhere, because of cost and storage issues.Lee et al. [12, 13] recently developed a new range of
colourimetric oxygen indicators that are irreversible, reusable, and UV-light
activated. Such “intelligent ink” oxygen sensors comprise a UV-absorbing
semiconductor, such as TiO2, a redox-indicator, such as methylene
blue, a sacrificial electron donor, such as triethanolamine, and an
encapsulating polymer such as
hydroxyethyl cellulose; the ingredients are mixed together, with water as the
solvent, to form an ink. The ink can be coated
or printed subsequently onto a variety of substrates to produce a blue oxygen
indicator film, which, when activated by UV light, becomes colourless. The
activated, that is, UV-photobleached, film remains colourless unless, or until,
exposed to oxygen, at which point the reduced methylene blue is reoxidised back
to its original blue form.The basic working principles, by which such an irreversible oxygen
indicator works, are illustrated in Figure1.
Thus, upon UVA irradiation, ultraband gap illumination (hν) of the TiO2 semiconductor particles
create electron-hole pairs, TiO2*(e−,h+). The photogenerated holes, h+,
oxidise the mild sacrificial electron donor (SED) present, glycerol in this
case, in the ink film and the remaining photogenerated electrons, that is, e− or TiO2−, as in Figure 1, reduce the redox-sensitive dye, DOX to a reduced
form, DRed, which has a different colour to DOX. In an ink film, the above key components are
encapsulated in a polymer, such as
hydroxyethyl cellulose (HEC) that is soluble in a common solvent (usually
water). Thus, UV irradiation causes an O2-sensitive
ink film to change colour (step 1, Figure 1). In the absence of oxygen, the
photobleached dye will stay in this reduced—usually colourless—state
indefinitely. However, upon exposure to
oxygen, it is reoxidised to its original, highly coloured form (step 2, Figure 1).Figure 1
Schematic illustration of the key processes involved in the UV activation and
subsequent response towards oxygen of a TO2/redox dye, DOX/glycerol/HEC oxygen indicator, such as that used in this work, where DOX = ITS.In this paper, we describe a very oxygen sensitive version of this type of UV-activated
indicator, in which the sacrificial electron donor is glycerol and the redox
dye is indigo-tetrasulfonate (ITS). The
latter is highly coloured and nonluminescent in its oxidised state but
virtually colourless, highly luminescent, and very oxygen-sensitive in its
reduced state,leuco indigo-tetrasulfonate (leuco-ITS).


## 2. Experimental

### 2.1. Materials

Unless stated otherwise, all chemicals were purchased from Aldrich Chemical Company (Gillingham, Dorset, UK). The semiconductor, titanium dioxide (TiO2),
was P25 provided by Degussa (Frankfurt, Germany) and comprised
particles ca. 30 nm in diameter, with
an 80 : 20 anatase : rutile crystal phase composition.


### 2.2. Preparation of Indigo-Tetrasulfonate (ITS) Based Films

A typical example of an UV-activated
luminescence/colourimetric oxygen-sensitive, ITS-based indicator ink, used to make the
indicator films reported in this work, was prepared by adding 200 mg of glycerol
to 2 g of a 5% wt hydroxyethyl cellulose (HEC) aqueous solution, to which 20 mg
of P25 TiO2 and 5 mg of the redox indicator indigotetrasulfonate
(ITS) had been added. The resulting mixture was stirred magnetically for 15 minutes, followed by 15 minutes sonication
to disperse the usually aggregated titania particles, followed by a further 15
minutes stirring. Typically, an oxygen
indicator film was prepared by placing 3-4 drops (ca. 0.4 ml) of this ink onto a cut-glass microscope slide (0.8×3.8 cm), which was subsequently spun at 2500 rpm for 15 seconds. The resulting blue, transparent film was allowed to dry in
the dark for 30 minutes before use.


### 2.3. Methods

All UV/V is spectra and absorbance versus time profiles
were recorded using a Cary 50 BioVarian spectrophotometer. Luminescence spectra and luminescence
intensity versus time profiles
were recorded using a PerkinElmer LS50 fluorimeter and lifetime measurements were
made with an IBH Fluorocube time-correlated single-photon counting system,
using a NanoLed-03 source, which has its excitation peak wavelength at 370 nm. All UV irradiations were conducted
using a2×4 W BLB handheld UVA light source (typical irradiance = 4 mW cm−2).


## 2.1. Materials

Unless stated otherwise, all chemicals were purchased from Aldrich Chemical Company (Gillingham, Dorset, UK). The semiconductor, titanium dioxide (TiO2),
was P25 provided by Degussa (Frankfurt, Germany) and comprised
particles ca. 30 nm in diameter, with
an 80 : 20 anatase : rutile crystal phase composition.


## 2.2. Preparation of Indigo-Tetrasulfonate (ITS) Based Films

A typical example of an UV-activated
luminescence/colourimetric oxygen-sensitive, ITS-based indicator ink, used to make the
indicator films reported in this work, was prepared by adding 200 mg of glycerol
to 2 g of a 5% wt hydroxyethyl cellulose (HEC) aqueous solution, to which 20 mg
of P25 TiO2 and 5 mg of the redox indicator indigotetrasulfonate
(ITS) had been added. The resulting mixture was stirred magnetically for 15 minutes, followed by 15 minutes sonication
to disperse the usually aggregated titania particles, followed by a further 15
minutes stirring. Typically, an oxygen
indicator film was prepared by placing 3-4 drops (ca. 0.4 ml) of this ink onto a cut-glass microscope slide (0.8×3.8 cm), which was subsequently spun at 2500 rpm for 15 seconds. The resulting blue, transparent film was allowed to dry in
the dark for 30 minutes before use.


## 2.3. Methods

All UV/V is spectra and absorbance versus time profiles
were recorded using a Cary 50 BioVarian spectrophotometer. Luminescence spectra and luminescence
intensity versus time profiles
were recorded using a PerkinElmer LS50 fluorimeter and lifetime measurements were
made with an IBH Fluorocube time-correlated single-photon counting system,
using a NanoLed-03 source, which has its excitation peak wavelength at 370 nm. All UV irradiations were conducted
using a2×4 W BLB handheld UVA light source (typical irradiance = 4 mW cm−2).


## 3. Results and Discussion

### 3.1. Chemical Reduction of an ITS Solution

An aqueous solution ofleuco-ITS can be readily prepared from
an ITS solution via the addition of a
reductant, such as zinc amalgam. However, leuco-ITS is very oxygen-sensitive
and readily oxidized back to ITS by ambient oxygen and so, in order to prevent
this re-oxidation step occurring whilst recording the spectral properties of the
reduced redox dye in solution, any oxygen needs to be removed. Thus, in a typical experiment, a dilute
solution of ITS (2.8×10−5), at pH4, was prepared and the
absorbance spectra recorded, as illustrated in Figure 2 (broken line). ITS absorbs in the blue region of the visible
spectrum, with a wavelength of maximum absorbance, λmax⁡, at 590 nm, which is typical for this dye [14]. This solution was then
deoxygenated via 5 cycles of a
freeze-thaw process. Addition of a
zinc-amalgam pellet to the deoxygenated ITS solution reduced the highly
coloured ITS solution to its very pale yellow leuco-ITS form, as shown by the decrease in absorbance at 590 nm
and the increase in absorbance at 385 nm in Figure 2 (spectra were recorded
every 30 minutes). Excitation of leuco-ITS at this wavelength (385 nm)
revealed a blue luminescence with an emission maximum at 485 nm. The uncorrected excitation (λem=485 nm) and emission spectrum (λexcit=390 nm) of the leuco-ITS aqueous solution are illustrated in the insert diagram of
Figure 2. Single-photon counting
revealed the lifetime of luminescence of leuco-ITS
in aqueous solution, at pH4, to be 0.24±0.02 microseconds.Figure 2
UV/Vis spectra recorded for an indigotetrasulfonate (ITS) deoxygenated aqueous
solution2.8×10−5 mol dm−3, pH4): broken line. Upon reaction with a zinc amalgam pellet, the
blue ITS solution is reduced to leuco-indigotetrasulfonate
(leuco-ITS), signalled by the
decrease in absorbance at 590 nm and the increase at 390 nm (spectra were
recorded every 30 minutes). The insert
diagram illustrates the excitation spectrum (solid line; λem=485 nm) and emission spectrum broken line; λex=390 nm) of a 2.8×10−5 mol dm−3leuco-ITS deoxygenated aqueous solution, at pH4.


### 3.2. Photobleaching of an ITS Oxygen Indicator Film

A parallel study, to that above, was carried out on a typical ITS oxygen
indicator film, with one set of experiments carried out under vacuum (10−3 mbar), that is, O2 free, and the other under ambient conditions, that is, 21% O2.
A typical absorbance spectrum of a blue-coloured indicator film under
vacuum before (broken line) and after activation with UVA light is illustrated
in Figure 3(a). From these results it can
be seen that the initially blue-coloured ITS film is activated, that is, converted from ITS to leuco-ITS via the photoreduction of ITS by the UV-excited titania
particles (see Figure 1) in under 3 minutes of UVA irradiation. This photoreduction process (step 1, Figure
1) produces a fall in absorbance of the film at 600 nm as a function of UVA
irradiation time as indicated by the data in Figure 3(a) insert diagram. In contrast, under otherwise the same conditions,
a typical oxygen indicator in an ambient environment takes twice as long to activate
using the same UVA light source (I = 4 mW cm−2). The lower rate of photobleaching in the latter
system is due to the presence of oxygen in the ambient environment which is
able to reoxidise the reduced form of the dye via a dark reaction, that is step 2 in Figure 1.(a) Recorded change in the UV/Vis spectrum of an ITS oxygen indicator film
(broken line), in the absence of oxygen, upon exposure to UVA light (intensity:
4 mW cm−2) as a function of time.
The spectra illustrated were irradiated (from top to bottom at 600 nm)
for 0 second up to 160 seconds, in 10-second intervals. The insert diagram illustrates the change in
absorbance of the indicator film at 600 nm as a function of irradiation time,
derived from the data in the main diagram. (b) Photographs of an oxygen indicator film on a coverslip. (i) TO2/ITS/glycerol/HEC film
before and (ii) after 30 seconds UVA irradiation (I = 7 mW cm−2),
and (iii) film in photograph (ii) allowed to recover after 15 minutes in
air. (iv) Photograph of film (i) under
very low UV-light illumination in the presence of oxygen. (v) Photograph of film (ii) also under very
low UV-light illumination.
(a)(b)Figure3(b) contains photographs of a typical TiO2/ITS/glycerol/HEC ink spun
coated onto a coverslip (i) before and (ii) after photobleaching by UV
irradiation (30 seconds, I = 7 mW cm−2) in air. Photograph (iii), in Figure 3(b), is after ca. 15 minutes in the dark under ambient
conditions, during which time the ITS film regains its original colour (and
loses its luminescence), due to the reoxidation of leuco-ITS to ITS by oxygen.
Photographs (i)–(v) confirm that the ITS indicator films can
be photoactivated under ambient conditions to produce a luminescent yellow, leuco-ITS film that is oxygen-sensitive.The UV activation of a typical ITS oxygen indicator film can also be monitored by
luminescence, given thatleuco-ITS luminesces at 470 nm in the film (see Figure 3(b), photographs (iv) and  (v)). The observed change in the emission spectrum
of an ITS oxygen indicator film, under vacuum, upon exposure to UVA light as a
function of time is illustrated in Figure 4.
The insert diagram illustrates a plot of the variation in a film’s luminescence
intensity as a function of UVA irradiation time, and, as in the absorption
spectral changes in Figure 2, shows that in the absence of oxygen the film is activated
in less than 3 minutes, in comparison to an oxygen indicator film in an ambient
environment, which takes ca. 6
minutes due to the dark back reaction, that
is, step 2 in Figure 1.Figure 4
Recorded change in the emission spectrum of an ITS oxygen indicator film
(broken line), under vacuum, upon exposure to UVA light as a function of
time. The spectra illustrated were
irradiated (from bottom to top) for 0 second up to 140 seconds, in 10-second
intervals. The insert diagram
illustrates the change in luminescence intensity of the indicator film at 470 nm as a function of irradiation time, derived from the data in the main diagram.


### 3.3. Dark Oxidation of an Activated ITS Film

Once activated, that is, photobleached, via step 1 in the reaction scheme in Figure1, the stability of
the photobleached oxygen indicator film depends on the level of oxygen present
in the environment in which it finds itself.
Thus, in the dark and in the absence of oxygen, the UV-activated oxygen
indicator remains bleached indefinitely, whereas in the presence of oxygen, its
colour is restored within seconds and the luminescence associated with the
reduced form of ITS is quenched. This is
nicely illustrated by the data in Figures 5 and 6 (broken lines), which depict
the recorded change in absorbance at 600 nm and luminescence intensity at 470 nm, respectively, as a function of time, of an ITS-based oxygen indicator film
that has been UV irradiated in a vacuum and subsequently exposed to an ambient atmosphere
by opening up the system to air. In
contrast to this data, the solid lines in Figures 5 and 6 correspond to the
recovery of an UV-activated oxygen indicator film, irradiated under ambient conditions
(i.e., 21% O2) and
maintained in this environment once the photobleaching process was
stopped. Not surprisingly, the oxygen
indicator film in an evacuated environment recovered its original blue colour
when exposed subsequently to oxygen in the dark. However, the ITS indicator film which was
initially photobleached in air recovered only ca. 50% of its original colour, possibly due to some photodegradation
of the redox dye, presumably caused by the longer exposure to UVA light in air needed to photobleach the film,
which probably promotes dye degradation via
singlet oxygen production.Figure 5
Change in the absorbance, at 600 nm, of a typical UV-activated ITS oxygen
indicator film as a function of time, under an ambient atmosphere (solid line)
and an ITS film UV-activated under vacuum and subsequently exposed to air (broken line). Each film was fully photobleached, using UVA light,
before being allowed to recover. The
insert diagram is a first-order plot (natural log of the change in the
absorbance, that is, ln (ΔAbs), at 600 nm versus time), over
one half life, derived from the data in the main diagram. The rate
constants for this recovery step are given in Table 1.Figure 6
Change in the luminescence intensity, at 470 nm, of a typical, UV-activated ITS
oxygen indicator film as a function of time, under an ambient atmosphere (solid
line) and when exposed to air (broken line), having been first UV-activated under
vacuum. Each film was fully
photobleached, using UVA irradiation, before being allowed to recover in
air. The insert diagram is a first-order
plot, over one half life, derived from the data in the main diagram.The insert diagrams in Figures5 and 6 show that the dark process (step 2, Figure 1) gives a good fit to first-order kinetics for a UV-activated oxygen indicator
film when photobleached and left to recover in an ambient atmosphere, or when
photobleached in a vacuum and subsequently exposed to air. Such first-order kinetics are typical for UV-activated
oxygen film indicators and provide support for the proposed scheme, that the
kinetics of step 2 in the reaction scheme in Figure 1 depend directly upon the
rate of diffusion of O2 through the film. Other work indicates that the rate of this
dark recovery process is first order with respect to the partial pressure of
oxygen in the ambient atmosphere as with similar UV-activated, oxygen-sensitive
film indicators. The first-order rate
constants for the recovery of a typical UV-activated oxygen indicator film in
an ambient environment and in a vacuum are given in Table 1. It remains unclear why the kinetics of step 2,
the dye recovery step, are slower for an ITS film irradiated in air, compared
to that for films irradiated in a vacuum, although an appreciable degree of dye
degradation is observed for the latter process (vide supra). The above
results stress the need for oxygen-free conditions when UV-activating these ITS-based indicators.Table 1
Photobleaching and recovery kinetics of a typical UV-activated oxygen indicator.
Photobleaching kineticsRecovery kineticsEnvironmentAbs·k1(s−1)r2Int·k1(s−1)r2Abs·k1(s−1)r2Int·k1(s−1)r2Ambient0.00630.990.00750.960.00620.990.00910.99Vacuum0.0130.990.0150.970.0330.990.0270.98


## 3.1. Chemical Reduction of an ITS Solution

An aqueous solution ofleuco-ITS can be readily prepared from
an ITS solution via the addition of a
reductant, such as zinc amalgam. However, leuco-ITS is very oxygen-sensitive
and readily oxidized back to ITS by ambient oxygen and so, in order to prevent
this re-oxidation step occurring whilst recording the spectral properties of the
reduced redox dye in solution, any oxygen needs to be removed. Thus, in a typical experiment, a dilute
solution of ITS (2.8×10−5), at pH4, was prepared and the
absorbance spectra recorded, as illustrated in Figure 2 (broken line). ITS absorbs in the blue region of the visible
spectrum, with a wavelength of maximum absorbance, λmax⁡, at 590 nm, which is typical for this dye [14]. This solution was then
deoxygenated via 5 cycles of a
freeze-thaw process. Addition of a
zinc-amalgam pellet to the deoxygenated ITS solution reduced the highly
coloured ITS solution to its very pale yellow leuco-ITS form, as shown by the decrease in absorbance at 590 nm
and the increase in absorbance at 385 nm in Figure 2 (spectra were recorded
every 30 minutes). Excitation of leuco-ITS at this wavelength (385 nm)
revealed a blue luminescence with an emission maximum at 485 nm. The uncorrected excitation (λem=485 nm) and emission spectrum (λexcit=390 nm) of the leuco-ITS aqueous solution are illustrated in the insert diagram of
Figure 2. Single-photon counting
revealed the lifetime of luminescence of leuco-ITS
in aqueous solution, at pH4, to be 0.24±0.02 microseconds.Figure 2
UV/Vis spectra recorded for an indigotetrasulfonate (ITS) deoxygenated aqueous
solution2.8×10−5 mol dm−3, pH4): broken line. Upon reaction with a zinc amalgam pellet, the
blue ITS solution is reduced to leuco-indigotetrasulfonate
(leuco-ITS), signalled by the
decrease in absorbance at 590 nm and the increase at 390 nm (spectra were
recorded every 30 minutes). The insert
diagram illustrates the excitation spectrum (solid line; λem=485 nm) and emission spectrum broken line; λex=390 nm) of a 2.8×10−5 mol dm−3leuco-ITS deoxygenated aqueous solution, at pH4.


## 3.2. Photobleaching of an ITS Oxygen Indicator Film

A parallel study, to that above, was carried out on a typical ITS oxygen
indicator film, with one set of experiments carried out under vacuum (10−3 mbar), that is, O2 free, and the other under ambient conditions, that is, 21% O2.
A typical absorbance spectrum of a blue-coloured indicator film under
vacuum before (broken line) and after activation with UVA light is illustrated
in Figure 3(a). From these results it can
be seen that the initially blue-coloured ITS film is activated, that is, converted from ITS to leuco-ITS via the photoreduction of ITS by the UV-excited titania
particles (see Figure 1) in under 3 minutes of UVA irradiation. This photoreduction process (step 1, Figure
1) produces a fall in absorbance of the film at 600 nm as a function of UVA
irradiation time as indicated by the data in Figure 3(a) insert diagram. In contrast, under otherwise the same conditions,
a typical oxygen indicator in an ambient environment takes twice as long to activate
using the same UVA light source (I = 4 mW cm−2). The lower rate of photobleaching in the latter
system is due to the presence of oxygen in the ambient environment which is
able to reoxidise the reduced form of the dye via a dark reaction, that is step 2 in Figure 1.(a) Recorded change in the UV/Vis spectrum of an ITS oxygen indicator film
(broken line), in the absence of oxygen, upon exposure to UVA light (intensity:
4 mW cm−2) as a function of time.
The spectra illustrated were irradiated (from top to bottom at 600 nm)
for 0 second up to 160 seconds, in 10-second intervals. The insert diagram illustrates the change in
absorbance of the indicator film at 600 nm as a function of irradiation time,
derived from the data in the main diagram. (b) Photographs of an oxygen indicator film on a coverslip. (i) TO2/ITS/glycerol/HEC film
before and (ii) after 30 seconds UVA irradiation (I = 7 mW cm−2),
and (iii) film in photograph (ii) allowed to recover after 15 minutes in
air. (iv) Photograph of film (i) under
very low UV-light illumination in the presence of oxygen. (v) Photograph of film (ii) also under very
low UV-light illumination.
(a)(b)Figure3(b) contains photographs of a typical TiO2/ITS/glycerol/HEC ink spun
coated onto a coverslip (i) before and (ii) after photobleaching by UV
irradiation (30 seconds, I = 7 mW cm−2) in air. Photograph (iii), in Figure 3(b), is after ca. 15 minutes in the dark under ambient
conditions, during which time the ITS film regains its original colour (and
loses its luminescence), due to the reoxidation of leuco-ITS to ITS by oxygen.
Photographs (i)–(v) confirm that the ITS indicator films can
be photoactivated under ambient conditions to produce a luminescent yellow, leuco-ITS film that is oxygen-sensitive.The UV activation of a typical ITS oxygen indicator film can also be monitored by
luminescence, given thatleuco-ITS luminesces at 470 nm in the film (see Figure 3(b), photographs (iv) and  (v)). The observed change in the emission spectrum
of an ITS oxygen indicator film, under vacuum, upon exposure to UVA light as a
function of time is illustrated in Figure 4.
The insert diagram illustrates a plot of the variation in a film’s luminescence
intensity as a function of UVA irradiation time, and, as in the absorption
spectral changes in Figure 2, shows that in the absence of oxygen the film is activated
in less than 3 minutes, in comparison to an oxygen indicator film in an ambient
environment, which takes ca. 6
minutes due to the dark back reaction, that
is, step 2 in Figure 1.Figure 4
Recorded change in the emission spectrum of an ITS oxygen indicator film
(broken line), under vacuum, upon exposure to UVA light as a function of
time. The spectra illustrated were
irradiated (from bottom to top) for 0 second up to 140 seconds, in 10-second
intervals. The insert diagram
illustrates the change in luminescence intensity of the indicator film at 470 nm as a function of irradiation time, derived from the data in the main diagram.


## 3.3. Dark Oxidation of an Activated ITS Film

Once activated, that is, photobleached, via step 1 in the reaction scheme in Figure1, the stability of
the photobleached oxygen indicator film depends on the level of oxygen present
in the environment in which it finds itself.
Thus, in the dark and in the absence of oxygen, the UV-activated oxygen
indicator remains bleached indefinitely, whereas in the presence of oxygen, its
colour is restored within seconds and the luminescence associated with the
reduced form of ITS is quenched. This is
nicely illustrated by the data in Figures 5 and 6 (broken lines), which depict
the recorded change in absorbance at 600 nm and luminescence intensity at 470 nm, respectively, as a function of time, of an ITS-based oxygen indicator film
that has been UV irradiated in a vacuum and subsequently exposed to an ambient atmosphere
by opening up the system to air. In
contrast to this data, the solid lines in Figures 5 and 6 correspond to the
recovery of an UV-activated oxygen indicator film, irradiated under ambient conditions
(i.e., 21% O2) and
maintained in this environment once the photobleaching process was
stopped. Not surprisingly, the oxygen
indicator film in an evacuated environment recovered its original blue colour
when exposed subsequently to oxygen in the dark. However, the ITS indicator film which was
initially photobleached in air recovered only ca. 50% of its original colour, possibly due to some photodegradation
of the redox dye, presumably caused by the longer exposure to UVA light in air needed to photobleach the film,
which probably promotes dye degradation via
singlet oxygen production.Figure 5
Change in the absorbance, at 600 nm, of a typical UV-activated ITS oxygen
indicator film as a function of time, under an ambient atmosphere (solid line)
and an ITS film UV-activated under vacuum and subsequently exposed to air (broken line). Each film was fully photobleached, using UVA light,
before being allowed to recover. The
insert diagram is a first-order plot (natural log of the change in the
absorbance, that is, ln (ΔAbs), at 600 nm versus time), over
one half life, derived from the data in the main diagram. The rate
constants for this recovery step are given in Table 1.Figure 6
Change in the luminescence intensity, at 470 nm, of a typical, UV-activated ITS
oxygen indicator film as a function of time, under an ambient atmosphere (solid
line) and when exposed to air (broken line), having been first UV-activated under
vacuum. Each film was fully
photobleached, using UVA irradiation, before being allowed to recover in
air. The insert diagram is a first-order
plot, over one half life, derived from the data in the main diagram.The insert diagrams in Figures5 and 6 show that the dark process (step 2, Figure 1) gives a good fit to first-order kinetics for a UV-activated oxygen indicator
film when photobleached and left to recover in an ambient atmosphere, or when
photobleached in a vacuum and subsequently exposed to air. Such first-order kinetics are typical for UV-activated
oxygen film indicators and provide support for the proposed scheme, that the
kinetics of step 2 in the reaction scheme in Figure 1 depend directly upon the
rate of diffusion of O2 through the film. Other work indicates that the rate of this
dark recovery process is first order with respect to the partial pressure of
oxygen in the ambient atmosphere as with similar UV-activated, oxygen-sensitive
film indicators. The first-order rate
constants for the recovery of a typical UV-activated oxygen indicator film in
an ambient environment and in a vacuum are given in Table 1. It remains unclear why the kinetics of step 2,
the dye recovery step, are slower for an ITS film irradiated in air, compared
to that for films irradiated in a vacuum, although an appreciable degree of dye
degradation is observed for the latter process (vide supra). The above
results stress the need for oxygen-free conditions when UV-activating these ITS-based indicators.Table 1
Photobleaching and recovery kinetics of a typical UV-activated oxygen indicator.
Photobleaching kineticsRecovery kineticsEnvironmentAbs·k1(s−1)r2Int·k1(s−1)r2Abs·k1(s−1)r2Int·k1(s−1)r2Ambient0.00630.990.00750.960.00620.990.00910.99Vacuum0.0130.990.0150.970.0330.990.0270.98


## 4. Conclusion

A novel, fast acting luminescent and colourimetric oxygen indicator ink is
described, containing a sacrificial electron donor (glycerol) and the redox
indicator (indigotetrasulfonate). The
indicator is blue coloured in the absence of UV light, however, upon exposure
to UV light it not only loses its colour but also luminesces, unless and until
it is exposed to oxygen, whereupon, under dark conditions, its colour is
restored. The initial photobleaching
spectral (absorbance and luminescence) response characteristics in air and in
vacuum are described and discussed in terms of a simple reaction scheme
involving UV activation of the titania photocatalyst particles, which are used
to reduce the redox dye, ITS, to itsleuco form. The response characteristics of
the activated, that is, UV-photobleached form of the indicator, to oxygen are
also reported. This indicator appears
more sensitive towards oxygen than previous UV-activated indicators, based on
methylene blue, probably due to the lower redox potential [14] of ITS (−0.046 V versus SHE) compared to that
of methylene blue (+0.028 V versus
SHE) at pH7. This indicator appears
susceptible to appreciable photodegradation when UV-activated under ambient (21% O2)
conditions. In the absence of oxygen,
however, UV activation is not only more rapid, but also does not produce any
significant photodegradation. Thus, the
ink appears particularly suited for use in systems that are usually oxygen
free, before UV activation, as a means of indicating any subsequent leak or
tampering.



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*Source: 547301-2008-05-11.xml*