Oxygen Sensors and Their Uses
Field of the Invention
The present invention relates to oxygen sensors and their uses,
and more particularly to oxygen sensors for use in product
packaging for storing an article in a packaging envelope under
modified atmosphere conditions.
Background of the Invention
Modified atmosphere packaging, and in particular oxygen-free
packaging, are growing in importance for extending the shelf-life
of perishable products, such as foodstuffs, or providing a
protective atmosphere for other types of sensitive products.
However, this growth has been limited by the absence of suitable
sensors that can confirm that the oxygen levels are kept at
suitably low levels.
This failure in the art is a result of the diverse range of
requirements for an ideal sensor. Generally, the sensors need to
be inexpensive as packaging cannot incur high costs, especially
for consumable products such as foodstuffs. They also need to be
safe and non-toxic as they may be in contact with food and
leaching may occur, or they may be at risk of accidental
ingestion, for example by children. Furthermore the reaction of
the sensor to a packaging failure preferably needs to be
irreversible under normal packaging environment. This is
important as an increase in oxygen level when a packaging failure
occurs can lead to spoilage due to growth of microorganisms and
it can happen that their metabolic activity restores low oxygen
levels. This means that a reversible sensor may show that no
packaging failure has occurred. In addition, it would be useful
if the sensitivity of the sensor was tailorable as different
applications of the packaging may tolerate different oxygen
levels. Finally, the sensors should ideally be easy to use,
providing an observable change that an unsophisticated user can
appreciate, and without recourse to additional equipment, and be
easy to incorporate in the packaging.
There are a few products commercially available in the market,
namely, Oxy2Dot® RedEye® and Ageless Eye™. However, they suffer
from a range of significant limitations.
The Oxy2Dot®, which is manufactured by OxySense®, is a
fluorescence-based oxygen sensor. The product is a small circle
that sticks to the inside of the package and the fluorescence
intensity, which is inversely proportional to oxygen level, is
measured by a photodetector . The two main problems with this
sensor are (a) the need for expensive sensing equipment, and (b)
temporary high oxygen levels may go unnoticed as the sensor
operates in a reversible manner. The RedEye® operates on similar
principles to the Oxy2Dot® and, therefore, suffers from similar
limitations .
The Ageless Eye™, which is manufactured by Mitsubishi Gas
Chemical (MGC) , changes visible colour upon oxidation. At low
oxygen concentrations (<0.1%) the sensor appears pink, but when
the oxygen concentration increases the colour changes gradually
to blue. The limitations of this sensor are that it is not
irreversible, it is expensive (60p each) and the dye is harmful
(methylene blue) .
A further product is under development. A UV activated oxygen
indicator is currently being developed by Mills. This sensor,
which uses the same dye as Ageless Eye™ (methylene blue) , is
coated onto the inner side of packaging film and is only
activated when exposed to UV light, which changes its colour from
blue to white. When the sensor is exposed to oxygen, it regains
the blue colour. The main issues with this sensor are safety
issues due to the use of methylene blue, aesthetic (tainting of
food) , and the bleaching effect (blue to white) due to prolonged
exposure to shelf light, which may lead to false negatives.
There is also a need to develop an effective oxygen sensor for
use in non-food related fields, such as packaging pharmaceuticals
and nutraceuticals, and for use in other fields, such as the
storage of rare books and manuscripts, and for use in the
packaging of high value products, such as electronic devices and
components .
Oxygen sensing has been developed in other technical fields. US
3,663,176 describes the use of metal salts of elements in Group
IVB and VB of the periodic table as a colorimetric oxygen
indicator in stream of alkene (olefin) in polymerization
processes. US 2008/0300133 discloses an oxygen scavenger and
indicator comprising three components: (a) an oxygen sorbent
which is a metal or metal compound that can transfer from one
oxidation state to another, (b) a redox indicator and/or
complexing agent for the metal or the oxidised form of metal, and
(c) at least one polymer or gel electrolyte. The oxygen
indicator of this application apparently works when the oxidation
of the metal causes a change in a physical property of the oxygen
sorbent through a change in the interaction with the redox
indicator or the complexing agent, such as a colour change.
GB 2,369,808 discloses oxygen or water sensors for food packaging
based on a colour change of soluble transition metal compound,
generally a soluble coordination complex.
From the discussion above, it will be apparent that the provision
of an effective sensor for modified atmosphere packaging, and in
particular for packaging for foodstuffs, remains an unsolved
problem in the art.
Summary of the Invention
Broadly, the present invention relates to oxygen sensors and
their uses, and more particularly to oxygen sensors for use in
product packaging for storing an article in a packaging envelope
under modified atmosphere conditions wherein the oxygen sensors
comprise solid poly oxo-hydroxy metal ion materials, optionally
modified with one or more ligands. In some embodiments, the
solid poly-oxo-hydroxy metal ion material is present in a
hydrated, oxygen permeable matrix, for example formed from a
material, such as gelatine. While the present invention is
applicable in many technical fields, it is particularly
applicable in the field of food packaging. The presence of
oxygen in packaged food leads to food spoilage through enzymaticcatalysed
reactions, oxidation of flavours and nutrients and/or
by allowing aerobic food-spoiling microorganisms to grow.
odified-atmosphere packaging, in which the food package is
flushed with an inert gas such as nitrogen or carbon dioxide,
reduces oxygen levels and extends the shelf-live of food
products. The market for this packaging is growing but is
currently limited by the absence of cheap oxygen monitoring
devices, i.e. oxygen sensors, which ensure that low levels of
oxygen have been maintained during the storage and handling of
the food packages. The present invention addresses this problem
through the development of mineral oxo-hydroxide-based sensors
that are safely disposable (e.g. are environmentally friendly),
cheap to manufacture, and provide detectable changes in the
presence of oxygen that are easy to read. Additionally or
alternatively, the oxygen sensors of the present invention may be
synthesised from food-grade GRAS reagents which makes them
inherently safe for food applications.
Thus, the present invention uses solid poly oxo-hydroxy metal ion
materials in contrast to the traditional coordination complexes
used in US 2008/0300133 that rely on an interaction with a redox
indicator to produce detectable changes in response to the
presence of oxygen.
Accordingly, in a first aspect, the present invention provides an
oxygen sensor comprising a solid poly oxo-hydroxy metal ion
material having a transition metal ion in a first oxidation state
that is capable of oxidation to a second oxidation state in
response to oxygen, wherein the solid poly oxo-hydroxy metal ion
material has a polymeric structure in which one or more ligands
are non-stoichiometrically substituted for the oxo and/or hydroxy
groups, so that exposure of the material to oxygen causes the
oxidation of the metal ion in the solid poly-oxo-hydroxy material
to produce a detectable change in the material . In some
instances in this aspect of the present invention, the solid poly
oxo-hydroxy metal ion material is present in a nanoparticulate or
nanostructured form.
In a further aspect, the present invention provides an oxygen
sensor comprising a solid poly oxo-hydroxy metal ion material
having a transition metal ion in a first oxidation state that is
capable of oxidation to a second oxidation state in response to
oxygen, wherein the solid poly-oxo-hydroxy metal ion material is
present in a hydrated, oxygen permeable matrix, so that oxygen
permeating the matrix causes the oxidation of the metal ion in
the solid poly-oxo-hydroxy material to produce a detectable
change in the material.
Alternatively or additionally, the present invention provides an
oxygen sensor comprising a solid oxo-hydroxy metal ion material
having a transition metal ion in a first oxidation state that is
capable of oxidation to a second oxidation state in response to
oxygen, wherein the solid oxo-hydroxy metal ion material is
present in nanoparticulate or nanostructured form so that
exposure of the material to oxygen causes the oxidation of the
metal ion in the solid oxo-hydroxy material to produce a
detectable change in the material.
Preferably, the metal ion material used in any aspect of the
present invention may be dispersed in the matrix in a
nanoparticulate or nanostructured form. This helps to increase
the surface area of the material that can come into contact with
oxygen permeating the matrix. Preferably, in the sensors of the
present invention, the metal ion material does not form a soluble
complex with materials forming the matrix, as is the case in the
system disclosed in US 2008/0300133.
In one embodiment, the oxygen sensor comprises a solid poly oxohydroxy
metal ion material having a structure in which one or
more ligands are non-stoichiometrically substituted for the oxo
and/or hydroxy groups. As will be further explained below, this
generally means that the ligands are integrated into the solid
phase material and have at least some demonstrable metal-ligand
bonding.
In a further aspect, the present invention provides a product
packaging for storing an article in a packaging envelope under
modified atmosphere conditions, wherein the product packaging
comprises an oxygen sensor of the present invention within the
envelope, so that oxygen leaking into the packaging envelope
causes oxidation of the metal ion to produce a detectable change
in the material.
In a further aspect, the present invention provides the use of an
oxygen sensor as disclosed herein for detecting the leakage of
oxygen into product packaging for storing an article in a
packaging envelope under modified atmosphere conditions, so that
oxygen leaking into the packaging envelope causes oxidation of
the metal ion in the oxo-hydroxy material to produce a detectable
change in the material.
In a further aspect, the present invention provides a method of
detecting oxygen leaking into product packaging for storing an
article in a packaging envelope under modified atmosphere
conditions, the method comprising:
(a) providing an oxygen sensor of the present invention
within the packaging envelope under modified atmosphere
conditions, so that oxygen leaking into the packaging envelope
causes oxidation of the metal ion in the oxo-hydroxy material to
produce a detectable change in the material; and
(b) optionally detecting the change in the material to
indicate the leakage of oxygen into the packaging envelope.
In a further aspect, the present invention provides a process for
producing an oxygen sensor of the present invention, the process
comprising :
(a) mixing the solution comprising a transition metal ion,
and optionally one or more ligands, in a reaction medium at a
first pH(A) at which the components are soluble;
(b) changing the pH (A) to a second pH(B) to cause a solid
precipitate or a colloid of the ligand-modif ied poly oxo-hydroxy
metal ion material to be formed;
(c) separating, and optionally drying and/or formulating,
the solid poly oxo-hydroxy metal ion material produced in step
(b) ; and
(d) optionally processing the solid poly oxo-hydroxy metal
ion material so that a nanoparticulate or nanostructured material
is produced and/or
(e) optionally carrying our one or more post production
treatments such as heating.
As explained in more detail below, the process may involve the
further step of formulating the solid poly oxo-hydroxy metal ion
materials in a matrix by mixing the material, or a precursor
thereof, with one or more matrix forming materials to form a
hydrated, oxygen permeable matrix capable of sensing oxygen.
Alternatively or additionally, the one or more matrix forming
materials may be introduced at the time of the reaction to
precipitate the solid poly oxo-hydroxy metal ion material so that
the process comprises the step of precipitating the solid poly
oxo-hydroxy metal ion material in the presence of a solubilized
matrix material and solidifying the resulting material to produce
a solid poly oxo-hydroxy metal ion material in a semi-solid
matrix .
The present invention is based on the recognition that materials
containing a redox metal (M r) can convert oxidation state when
exposed to a new environment of differing oxygen content or
oxidative/reductive potential and that in some cases this will
lead to a change in colour in M -containing materials. The
present invention therefore utilises M -containing materials as
sensors of a changing oxygen or oxidative environment. These
materials have proven to be highly specific containing
materials that are capable of fulfilling one or more of the
general requirements o f oxygen sensors. These requirements
include (1) the need to be inexpensive as usually these are "one
off" sensors, (2) environmentally and biologically compatible,
especially for uses involving foodstuffs, (3) tailorable to the
different sensing needs of different environments and (4) easily
read and interpreted as a sensor, preferably without expensive
equipment or user training. Generally, for sensors in a solid or
semi-solid format, the sensor will include a degree of hydration
to facilitate redox activity, while in solution, suspension or
gel phase, the sensor is preferably dispersed adequately to
provide a large enough surface area that enables sensitive
detection. The present inventors have found that some specific
manipulation of M poly oxo-hydroxides , equally referred to as
hydroxy-oxides in the art, fulfils all of the above criteria and
provides for sensitive and tailorable sensing of an oxygen
environment. Moreover, they have identified that crystalline
forms, namely Mr oxides or Mr hydroxides can also be manipulated
in such a fashion to allow for useful oxygen sensing. Thus, Mr
oxo-hydroxides in their early stages of self-assembly that is
polymeric or cross-linked polymeric (denoted "poly") or their
more crystalline forms (denoted oxide or hydroxide) can be
modified and/or manipulated to usefully sense oxygen levels.it
may be clear to those skilled in the art that modification of the
materials is likely to lead to a reduction in their crystallinity
or an increase in their amorphous nature as described in more
detail below.
Embodiments of the present invention will now be described by way
of example and not limitation with reference to the accompanying
figures and examples.
"and/or" where used herein is to be taken as specific disclosure
of each of the two specified features or components with or
without the other. For example "A and/or B" is to be taken as
specific disclosure of each of (i) A , (ii) B and (iii) A and B ,
just as if each is set out individually herein.
Unless context dictates otherwise, the descriptions and
definitions of the features set out above are not limited to any
particular aspect or embodiment of the invention and apply
equally to all aspects and embodiments which are described.
Brief Description of the Figures
Figure la. Phase distribution during the synthesis of Fe
under nitrogen, where the nomeclature used to describe these
materials is described below.
Figure lb. Particle size of Fe +
0 H prior to incorporation into a
gelatine showing that, without modification, it would form large
agglomerates (N=3) .
Figure lc. TEM image of 2-15 nm Fe2+ OH nanoparticles dispersed
in a gelatine matrix. The nanoparticles (shown as small dark
rods in the TEM image) were produced and incorporated in a
gelatine matrix as described in Example 8 and then allowed to
oxidise by exposure to air.
Figure 2a. Phase distribution during the synthesis of Fe + OH-
-
Figure 2b. Particle sizing for Fe2+
0 H-T2 o nanoparticles after
synthesis (N=3) .
Figure 3 . Phase distribution during the synthesis of Fe2+ OHT
ooSucc under nitrogen.
Figure 4 . Particle sizing for Cu +
20 OH-Cys nanoparticles after
synthesis (N=3) .
Detailed Description
Detailed Description
The Metal Ion (M)
The present invention may employ solid poly oxo-hydroxy metal ion
materials as the source of the element of the oxygen sensor that
responds to the presence of oxygen. These may or may not be
solid ligand-modif ied poly oxo-hydroxy metal ion materials. The
production and characterisation of solid ligand-modif ed poly
oxo-hydroxy metal ion materials is disclosed in our earlier
application WO 2008/096130, expressly incorporated by reference
in its entirety, and these approaches may also be employed for
the production of solid poly oxo-hydroxy metal ion materials,
i.e. materials in which ligands are not incorporated into the
material. In the materials where ligands are incorporated, the
solid ligand-modif ied poly oxo-hydroxy metal ion materials may be
represented by the non-stoichiometric formula (M L (OH) ), where
represents one or more metal ions, L represents one or more
ligands and OH represents oxo or hydroxy groups, depending on
whether the groups are bridging (oxo groups) or surface groups in
the solid oxo-hydroxide material. As is well known in the art,
non-stoichiometric compounds are chemical compounds with an
elemental composition that cannot be represented by a ratio of
well-defined natural numbers, i.e. the x , y and n subscripts in
the formula above will not necessarily all be natural numbers,
even though the materials can be made in a reproducible manner
and have consistently reproducible properties.
Conveniently, the solid poly oxo-hydroxides may be formed when
the metal ion, originally present in the form of a salt, is
dissolved and then induced to form poly oxo-hydroxy materials.
This may optionally take place in the presence of one or more
ligands (L)and lead to some of the ligand becoming integrated
into the solid phase through formal M-L bonding (termed "ligand
bonding"), i.e. not all of the ligand (L) is simply trapped in or
adsorbed onto the bulk material. The bonding of the metal ion in
the materials can be determined using physical analytical
techniques such as infrared spectroscopy where the spectra will
have peaks characteristic of the bonds between the metal ion and
the ligand (L) , as well as peaks characteristic of other bonds
present in the material such as -0 , O-H and bonds in the ligand
species (L) . Preferred metal ions (M) are biologically
compatible under the conditions for which the materials are used
and are readily precipitatable from aqueous solution by forming
oxo-hydroxides .
In the present invention, the materials used to form the sensors
need to have at least two oxidation states and to produce a
detectable change, preferably a visible colour change or a UVvisible
change, where the transition metal ion in the materials
is oxidised from a first lower oxidation state to a second higher
oxidation state, in response to exposure to oxygen or an
oxidising environment. It is also preferred that they are
biologically compatible materials, especially for applications
where they are used in proximity to foodstuffs. These factors
mean that the sensors of the present invention are preferably
based on transition metal oxo-hydroxides as they typically have
the properties required and have the added advantages of being
produced by a simple method, easy to incorporate in a semi-solid
matrix, safe and undergoing irreversible changes upon exposure to
oxygen (under normal packaging conditions) . Examples of metal
ions include copper, iron, chromium, vanadium, manganese,
titanium, cobalt, molybdenum and/or tungsten. Some preferred
examples of metal oxidation states that produce the colour
changes that may be used in the present invention include: iron
(Fe2+ to Fe + ) , copper (Cu+ to Cu2+) and cobalt (Co2+ to Co +), all
of which have the advantage of being non toxic in both oxidation
states. It is also preferred that the change in the oxidation
state of the metal ion that leads to the colour change is
irreversible, or at least is irreversible under the conditions in
which it is employed as an oxygen sensor. In some embodiments,
more than one type of metal ion (2, 3 , 4 or more) may be used.
Without modification, the primary particles of the materials used
herein have metal oxide cores and metal hydroxide surfaces and
within different disciplines may be referred to as metal oxides
or metal hydroxides. The use of the term 'oxo-hydroxy' or -
hydroxide' is intended to recognise these facts without any
reference to proportions of oxo or hydroxy groups. Hydroxy-oxide
could equally be used therefore. As described above, the
materials of the present invention that are ligand doped are
altered at the level of the primary particle of the metal oxohydroxide
with at least some of the ligand L being introduced
into the structure of the primary particle, i.e. leading to
doping of the primary particle by the ligand L .
The primary particles of the solid poly oxo-hydroxy metal ion
materials described herein are produced by a process referred to
as precipitation. The use of the term precipitation often refers
to the formation of aggregates of materials that do separate from
solution by sedimentation or centrif ugation . Here, the term
"precipitation" is intended to describe the formation of all
solid phase material, including aggregates as described above and
solid materials that do not aggregate but remain as non-soluble
moieties in suspension, whether or not they be particulate, e.g.
nanoparticulate or nanostructured . These forms of the materials
have the advantage that the materials can readily be dispersed in
a matrix or used in powder form, with high surface areas for
reaction with oxygen. The skilled person can readily determine
whether materials are nanoparticulate or nanostructured, for
example using techniques such as dynamic light scattering, or an
equivalent technique, if present as aqueous suspension, or as
determined using TE , or an equivalent technique, if as a powder
or in a matrix. Preferably, nanostructured materials include
materials whose structural elements - clusters, crystallites or
molecules - have dimensions in the 1 to 100 nm range.
In the present invention, reference may be made to the metal oxohydroxides
having polymeric structures that generally form above
the critical precipitation pH. As used herein, this should not
be taken as indicating that the structures of the materials are
polymeric in the strict sense of having a regular repeating
monomer unit because, as has been stated, ligand incorporation
(where applicable) is, except by coincidence, non-stoichiometric .
The ligand species is introduced into the solid phase structure
by substituting for oxo or hydroxy groups leading to a change in
solid phase order. The polymeric nature of oxo-hydroxide metal
ion materials is discussed in Flynn, Chem. Rev., 84: 31-41, 1984.
Alternatively or additionally, for the ligand modified material
used in accordance with the present invention, there may be a
decrease in the crystallinity of the structure of the material or
increase in the disorder that can be determined by high
resolution transmission electron microscopy. High resolution
transmission electron microscopy allows the crystalline pattern
of the material to be visually assessed. It can indicate the
primary particle size and structure (such as d-spacing) and give
some information on the distribution between amorphous and
crystalline material. Using this technique, it is apparent that
the chemistry described above increases the amorphous phase of
our described materials compared to corresponding materials
without the incorporated ligand. This may be especially apparent
using high angle annular dark field aberration-corrected scanning
transmission electron microscopy due to the high contrast
achieved while maintaining the resolution thus allowing the
surface as well as the bulk of the primary particles of the
material to be visualised.
The reproducible physico-chemical property or characteristic of
the materials of the present invention will be dependent on the
application for which the material is intended. Examples of the
properties that can be usefully modulated using the present
invention include: particle size, light absorbing/reflecting
properties, hardness-softness, colour, redox capability,
dissolution and encapsulation properties. In this context, a
property or characteristic may be reproducible if replicate
experiments are reproducible within a standard deviation of
preferably ± 20%, and more preferably ± 10%, and even more
preferably within a limit of ± 5%.
The Ligand (L)
In embodiments of the present invention in which the oxo-hydroxy
metal ion materials used to form the oxygen sensors are ligand
modified, the solid phase ligand-modif ied poly oxo-hydroxy metal
ion species are represented by the formula ( L (OH) ) , where L
represents one or more ligands or anions that can be incorporated
into the solid phase ligand-modif ied poly oxo-hydroxy metal ion
material. By way of example, the ligands may be used to modulate
one or more of the following properties: the colour of the
material before and after exposure to oxygen, the rate of
(colour) change in response to oxygen, the sensitivity of the
response to oxygen, i.e. the level of oxygen, for example less
than 0.1%, or less than 0.5%, or less than 1.0% or less than
5.0%, needed to induce the detectable change or the kinetics of
the response to oxygen or an oxidising environment.
In the materials described herein, preferred examples of the
ligands may include one or more of the following types of ligand.
(a) a classical anion ligand selected from phosphate,
sulphate, silicate, selenate and/or bicarbonate; and/or
(b) a food additive ligand, such as maltol and ethyl maltol;
and/or
(c) an amino acid ligand, such as tryptophan, glutamine or
histidine; and/or
(d) a nutrient-based ligand, such as folate, ascorbate or
niacin; and/or
(e) a carboxylic acid ligand, such as gluconic acid or
lactic acid; and/or
(f) pendant groups from a semi-solid matrix, e.g. amino acid
side chains of gelatin.
Carboxylic acid ligands may be employed, such as linear
dicarboxylic acid ligands. Examples of these include ligands
represented by the formula HOOC - -COOH, where Ri is an optionally
substituted alkyl, alkenyl or alkynyl group, or an ionised form
thereof, e.g. is a C - alkyl group, wherein R is optionally
substituted with one or more hydroxyl groups. Specific examples
of these ligands include succinic acid, malic acid, adipic acid,
glutaric acid, pimelic acid, citric acid, aspartic acid or
tartaric acid, or an ionised forms thereof, i.e. where the ligand
is succinate, malate, adipate, glutarate, pimelate, citrate,
aspartate or tartrate.
Whether the carboxylic acid ligand is present as the acid or is
partially or completely ionised and present in the form of a
carboxylate anion will depend on a range of factors such as the
pH at which the material is produced and/or recovered, the use of
post-production treatment or formulation steps and how the ligand
becomes incorporated into the poly oxo-hydroxy metal ion
material. In some embodiments, at least a proportion of the
ligand will be present in the carboxylate form as the materials
are typically recovered at pH>4 and because the interaction
between the ligand and the positively charged metal ion would be
greatly enhanced by the presence of the negatively charged
carboxylate ion. For the avoidance of doubt, the use of
carboxylic acid ligands in accordance with the present invention
covers all of these possibilities, i.e. the ligand present as a
carboxylic acid, in a non-ionised form, in a partially ionised
form (e.g., if the ligand is a dicarboxylic acid) or completely
ionised as a carboxylate ion, and mixtures thereof.
Without wishing to be bound by any particular theory, the present
inventors believe that the ligand may be provided as pendant
groups from matrix material in which the metal oxo-hydroxide is
immobilised and/or dispersed. For example, a semi-solid matrix
such as gelatine may be used to immobilise and disperse the
sensors such that some of side chains of the amino acids in
gelatin may be incorporated in surface of primary particle of the
poly oxo-hydroxy metal ion material, thus altering its
physicochemical properties.
Typically, ligands are incorporated in the solid phase poly oxohydroxy
metal ion materials to aid in the modification of a
physico-chemical property of the solid material, e.g. as compared
to a poly oxo-hydroxylated metal ion species in which the
ligand (s) are absent. In some embodiments of the present
invention, the ligand (s) L may also have some buffering capacity.
Examples of ligands that may be employed in the present invention
include, but are by no means limited to: carboxylic acids such as
adipic acid, glutaric acid, tartaric acid, malic acid, succinic
acid, aspartic acid, pimelic acid, citric acid, gluconic acid,
lactic acid or benzoic acid; food additives such as maltol, ethyl
maltol or vanillin; 'classical anions' with ligand properties
such as bicarbonate, sulphate, nitrite, nitrate and phosphate;
mineral ligands such as silicate, borate, molybdate and selenate;
amino acids such as tryptophan, glutamine, proline, valine, or
histidine; and nutrient-based ligands such as folate, ascorbate,
pyridoxine or niacin or nicotinamide. Typically ligands may be
well recognised in the art as having high affinity for a certain
metal ion in solution or as having only low affinity or not be
typically recognised as a ligand for a given metal ion at all.
However, we have found that in poly oxo-hydroxy metal ion
materials, ligands may have a role in spite of an apparent lack
of activity in solution. Typically, one ligand or two ligands of
differing affinities for the metal ion are used in the production
of these materials although zero, one, two, three, four or more
ligands may be useful in certain applications.
For many applications, ligands need to be biologically compatible
under the conditions used and generally have one or more atoms
with a lone pair of electrons at the point of reaction. The
ligands include anions, weak ligands and strong ligands. Ligands
may have some intrinsic buffering capacity during the reaction.
The ratio of the metal ion(s) to the ligand (s) (L) is also a
parameter of the solid phase ligand-modif ied poly oxo-hydroxy
metal ion materials that can be varied according to the methods
disclosed herein to vary the properties of the materials.
Generally, the useful ratios of M :L will be between 10:1, 5:1,
4:1, 3:1, 2:1 and 1:1 and 1:2, 1:3, 1:4, 1:5 or 1:10.
Throughout the examples, the M +
jOH-L nomenclature was adopted to
describe the preparation for ligand-modif ied poly oxo-hydroxy
transition metal materials; where 1 ) refers to the transition
metal, i+ to its valence, and j its concentration in the initial
solution prior to synthesis and 2 ) L refers to a ligand and k to
its concentration. There is no limit to the number of ligands
and where no ligand was present the nomenclature used was M + H .
for example, the nanoparticulate poly oxo-hydroxy material
defined as Fe + OH-Tart 2 was prepared from an initial solution
that contained 40 m ferrous iron and 200 m tartaric acid.
Additional to the modified or unmodified poly-oxo-hydroxy
materials produced for oxygen sensing, it was shown that more
crystalline analogues, namely hydroxides and oxides may also be
employed. These more crystalline structures may be initially
similarly prepared through aqueous precipitation in the presence
of ligand but reacting conditions are chosen to ensure
crystallinity is achieved, which typically is assessed through Xray
diffraction measurements. It will however be clear to those
in the art that additional methods exist for the conversion of
poly oxo-hydroxy phases to crystalline phases including the use
of heat.
Producing and processing the materials used to make oxygen
sensors
Generally, the materials of the present invention may be produced
by a process comprising:
(a) mixing the solution comprising transition metal ion and
optionally one or more ligands in a reaction medium at a first
pH(A) at which the components are soluble;
(b) changing the pH(A) to a second pH(B) to cause a solid
precipitate or a colloid of the optionally ligand-modif ied poly
oxo-hydroxy metal ion material to be formed;
(c) separating, and optionally drying and/or formulating,
the solid poly oxo-hydroxy metal ion material produced in step
(b) ; and
(d) optionally processing the solid poly oxo-hydroxy metal
ion material so that a nanoparticulate or nanostructured material
is produced; and/or
(e) optionally carrying our one or more post production
treatments such as heating.
n cases where the ligands are provided by the materials forming
a semi-solid matrix, the process may involve the step of
precipitating the solid poly oxo-hydroxy metal ion material in
the presence of one or more solubilized matrix materials and
solidifying the resulting material to produce a solid poly oxohydroxy
metal ion material in a semi-solid matrix. While not
wishing to be bound by any particular theory, the present
inventors believe that in such materials the pendant groups from
the semi-solid matrix are capable of acting as ligands in the
solid poly oxo-hydroxy metal ion materials.
It will be apparent that as the materials used to form the oxygen
sensors are oxidisable in the presence of atmospheric oxygen, it
is advisable to prepare them under an inert atmosphere or reduced
oxygen conditions. Examples of other conditions that may be
employed include the following using a first pH(A) which is less
than 2.0 and the second pH(B) which is between 3.0 and 12.0,
preferably between 3.5 and 8.0, and more preferably between 4.0
and 6.0, and carrying out the reaction at room temperature (20-
25°C) . In general, it is preferred that in step (a), the
solution contains 20 to lOOmM e + and 0 to 250mM of a suitable
carboxylic acid ligand, and more preferably about 40mM Fe + and <
lOOmM of the ligand. If a ligand is used then a preferred ligand
is tartaric acid.
The separation of a candidate material may then be followed by
one or more steps in which the material is characterised or
tested. Examples of further steps or post production treatments
include, but are not limited to: heating, washing,
centrif ugation, filtration, spray-drying, freeze-drying, vacuumdrying,
oven-drying, dialysis, milling, granulating,
encapsulating, embedding (e.g. in gelatine), tableting, mixing,
compressing, nanosizing and micronizing.
When these materials are used as oxygen sensors, the further
steps include forming the solid poly oxo-hydroxy metal ion
material into the format in which it is to be employed as an
oxygen sensor. By way of example, the solid poly oxo-hydroxy
metal ion material may be dispersed in matrix forming materials
such as gelatine to form a semi-solid matrix, mixed with water to
form an aqueous suspension, or coated or spray dried on a
substrate to provide a sensor in the form of a powder coating.
It should be noted that different sizes and shapes of sensors may
be used to control sensing time. For example, sensing materials
produced in shallower moulds, than those described herein, will
change colour faster, since oxygen will permeate shorter
distances through the semisolid matrix before reaching the
entirety of the sensing material.
Hydroxy and xo groups
The present invention may employ any way of forming hydroxide
ions at concentrations that can provide for hydroxy surface
groups and oxo bridging in the formation of these poly oxohydroxy
materials. Examples include but are not limited to,
alkali solutions such as sodium hydroxide, potassium hydroxide
and sodium bicarbonate, that would be added to increase [OH] in
an ML mixture or M i+) solution, or acid solutions such as mineral
acids or organic acids, that would be added to decrease [OH] in
an ML mixture or M + solution.
The conditions used to produce the compositions of the present
invention may be tailored to control the physico-chemical nature
of the precipitate, or otherwise assist in its collection,
recovery or formulation with one or more excipients. This may
involve purposeful inhibition of agglomeration, or the use of
drying or grinding steps to subsequently affect the material
properties. However, these are general variables to any such
system for solid extraction from a solution phase. After
separation of the precipitated material, it may optionally be
dried before use or further formulation. The dried product may,
however, retain some water and be in the form of a hydrated solid
phase ligand-modif ied poly oxo-hydroxy metal ion material. It
will be apparent to those skilled in the art that at any of the
stages described herein for recovery of the solid phase,
excipients may be added that mix with the solid poly oxo-hydroxy
metal ion material, but do not modify the primary particle and
are used with a view to optimising formulation for the intended
function of the material. Examples of these could be, but are
not limited to, glycolipids, phospholipids (e.g. phosphatidyl
choline), sugars and polysaccharides, sugar alcohols (e.g.
glycerol), polymers (e.g. polyethyleneglycol (PEG)) and
taurocholic acid.
Uses
The oxygen sensors of the present invention may be employed in a
range of applications, in particular in the area of packaging and
storage, especially where the storage is under modified
atmosphere conditions that generally use nitrogen and/or carbon
dioxide in a packaging envelope. Accordingly, the sensors may be
used for packaging and/or storing products as diverse as food
products, pharmaceutical products, nutraceutical products,
documents, books or manuscripts, or electronic devices or
components .
Materials and Methods
All chemicals were purchased from Sigma Aldrich (Dorset, UK) ,
except uncoloured beef gelatine, which was from a commercial
supplier (Oetker) .
Synthesis of metal -hydroxides for use as oxygen sensors
The metal oxo-hydroxides described herein are produced by raising
the pH of an initial solution containing, at least, a soluble
transition metal in a low oxidation state that can undergo
oxidation to a higher oxidation state, such as Fe + or Cu+. This
initial solution is typically acidic, at 1.0>pH>7.0, but higher
pHs can also be used, providing that the metal can remain soluble
under such initial conditions. This initial solution can
optionally contain one or one or more ligands of the types
described herein such as tartaric acid or succinic acid.
Furthermore, an electrolyte, such as NaCl or KC1, can also be
added to the initial solution. Subsequently, an oxo-hydroxide
material is formed through a process of colloid formation and/or
precipitation by gradually increasing the pH (e.g. NaOH) until a
suitable pH is achieved. Precipitation is typically carried out
at room temperature (20-25°C) , but higher temperatures can also be
used, if required. Depending on the intended application,
reducing compounds, including reducing sugars such as glucose or
fructose can be added at any point during the synthesis process
for the purpose of tailoring the sensitivity of the materials to
oxygen. Finally, the mineral formed can be recovered through a
range of strategies dependent on the nature of the mineral and
the intended application. Note that the synthetic process should
be carried out, preferably, under low oxygen conditions since
this prevents the oxidation of the initial soluble metal or the
subsequently produced metal oxo-hydroxide . Low oxygen conditions
can be achieved through a range of strategies, such as nitrogen
flow, that are not described herein. The synthesis of more
crystalline sensors, alternatively the conversion of previously
produced amorphous materials described elsewhere in the examples
to more crystalline phases can be achieved using similar
conditions to those described above. However, higher pH' s (pH >
11) and/or higher temperatures (>60°C) may be employed in their
synthesis .
Post-synthesis recovery and testing
The techniques used for the post-synthesis recovery of metal oxohydroxides
of the present invention was dependent on the
application and nature of the minerals synthesised. The metal
oxo-hydroxides can be recovered by a range of methods such as
filtration or centrif ugation . In examples below, the metal oxohydroxides
were tested as (i) aqueous suspensions, (ii) dry
powders, or (iii) as part of a semi-solid matrix.
Aqueous suspensions
Upon synthesis, nanoparticulate metal oxo-hydroxides remain
stable in suspension and can be used directly for oxygen sensing.
Upon exposure to atmospheric oxygen, or any other source of
oxygen, the nanoparticulate material undergoes a process of
oxidation with an associated change in colour or UV absorbance
change that can be monitored.
Dry powders
synthesis, metal oxo-hydroxides can be dried and used for
oxygen sensing as dry powders. The metal oxo-hydroxide powder can
be dried directly from a final suspension or from a pellet
obtained by centrif ugation, filtration, or ultrafiltration. Upon
exposure to atmospheric oxygen, or any other source of oxygen,
the dry material undergoes a process of oxidation with an
associated change in colour that can be used for oxygen sensing.
The powders may also be incorporated in paint or spray coating
compositions for ease of application.
Semi-solid matrix
Upon synthesis, the metal oxo-hydroxides can be incorporated into
a semi-solid matrix, such as a gelatine matrix, that immobilises
and disperses the material in a nanoparticulate form. It may
also disperse materials which, in the absence of semi-solid
matrix, would have remained as micron-sized agglomerates. By way
of example, the matrix can be produced by dissolving a gelatine
powder such as beef gelatine in an aqueous suspension of metal
oxo-hydroxides and subsequently cooling it down, resulting in the
formation of a semi-solid matrix. Alternatively, the matrix may
be produced as part of the reaction that is used to form the
metal oxo-hydroxide. Conveniently, the gelatine is used in an
amount between 10-20% w/w. Upon exposure to atmospheric oxygen,
or any other source of oxygen, the metal oxo-hydroxide, which is
within the semi-solid matrix undergoes a process of oxidation
with an associated change in colour that can be used for oxygen
sensing .
Examples of materials that may be used as semi-solid matrices for
retaining the oxygen sensing materials of the present invention
in a disperse and, preferably, nanoparticulated form include
hydrocolloids or hydrogels from biological sources such as, but
not limited to:
Gelatine (e.g. beef, pork), Pectins, Starches (e.g. maize, wheat,
tapioca, potato, etc), starch derivatives (e.g., Carboxy Methyl
Starch, Starch Phosphate), cellulose, cellulose derivatives (e.g.
Hydroxyethyl Cellulose, carboxymethylcellulose) , Plant Gums (e.g.
Arabic, Guar Karaya, Locust bean, Tragacanth, Psyllium seed,
Quince seed, xanthan, larch, gatti), algae-derived gums
(Alginate, Carrageen, agar, agarose, Furcellaran) , bacteriaderived
sugars (e.g. dextran) , or chitosans .
Hydrocolloids or hydrogels from synthetic organic polymers such
as, but not limited to, poly(vinyl alcohol) [PVA] , poly(acrylic
acid) [PAA], poly (ethylene glycol) [PEG] poly (acrylonitrile)
[PAN] .
Hydrocolloids or hydrogels from inorganic polymers such as, but
not limited to, silicate, silicon dioxide, magnesium aluminium
silicate, other silicon based gels, aluminium hydroxide,
bentonite, other aluminium based gels, or borate based gels.
One consequence of the inclusion of the materials in matrices is
that the present inventors have found that some variation in the
colour of the sensor may be engineered through the choice of
matrix material and the conditions (such as pH) used to make the
oxygen sensor. This provides a further means in addition to the
use of ligand modification discussed above to tailor different
oxygen sensors for particular applications. By way of example,
if gelatine were added to a suspension of Fe + oOH the semi-solid
matrix containing ferrous nanoparticles would appear intense
bright green whereas the suspension would appear darker (blacker)
green. Interestingly, once oxidised the gelatine-based matrix
would become bright red whereas the gelatine-free suspension
would become brown/orange.
Phase distribution during synthesis
A similar procedure to that described above in "Synthesis of
metal oxo-hydroxides for use as oxygen sensors" was carried out
except that several aliquots were collected at different pH' s
during synthesis. First, an initial aliquot was also collected
for analysis of the "starting metal" concentration. Next the pH
was slowly increased by drop-wise addition of a concentrated
solution of NaOH with constant agitation until the mixture
reached a basic pH (generally >8.0). At different points during
the titration, a homogeneous aliquot (lm ) of the mixture was
collected and transferred to an Eppendorf tube. Any
centrifugable phase formed was separated from the solution by
centrifugation (10 minutes at 13000 rpm) . The iron concentration
in the supernatant fraction was then determined by ICPOES. To
differentiate between soluble iron and particulated noncentrif
ugable iron (generally <15 n diameter nanoparticles ) in
the supernatant, at each time point, each aliquot was also
ultraf iltered (Vivaspin 3,000 Da molecular weight cut-off
'polyethersulf one membrane. Cat. VS0192, Sartorius Stedium Biotech
GmbH, Goettingen, Germany) and again analysed by ICPOES.
Inductively Coupled Plasma Optical Emission Spectrometry analysis
(ICPOES)
Metal contents of solutions were measured using a JY2000-2 ICPOES
(Horiba Jobin Yvon Ltd., Stanmore, U.K.). Solutions were diluted
in 1-1.5% nitric acid prior to analysis.
TEM Analysis
The normal fixation pellet was then dehydrated through the
alcohol/acetylnitrile gradient before being fixed in non-aqueous
Quettol resin for 7 days. The non-aqueous pellet was resuspended
straight into 100% ethanol, overnight, followed by resuspension
in acetylnitrile and finally resin. The resultant resin embedded
pellets were sectioned in 100 nm thick sections on 400 mesh
copper grids for TEM analysis.
Oxygen Analysis
Oxygen levels were monitored using an oxygen meter (Rapidox 1100,
Cambridge Sensotec, Cambridge, UK) .
Particle Size Analysis
The hydrodynamic particle size of nanoparticulate suspensions was
determined by dynamic light scattering (DLS) , using a Zetasizer
Nano-ZS (Malvern Instruments, UK) . The hydrodynamic particle
size of the larger agglomerates was determined by static light
diffraction (SLD) , using a Mastersizer 2000 (Malvern Instruments,
UK) .
Examples
Example 1 . Ferrous oxo-hydroxide, Fe + o H
All solutions/suspensions were bubbled with nitrogen before and
throughout the synthesis to achieve low oxygen conditions. A
ferrous solution was prepared by adding ferrous sulphate to water
that had been previously acidified with hydrochloric acid. The
final iron concentration was 40 mM and the pH was generally below
4.0 and usually about 2.0. Once all of the ferrous salt
dissolved, the pH was raised with a 5M NaOH solution to pH 7.0-
9.0, usually 8.0, during which a green precipitate consisting of
micron sized agglomerates, i.e. ferrous oxo-hydroxide, was
formed. Finally, this material was incorporated as a
nanoparticulate dispersion in a semi-solid matrix.
Example 2 . Nanoparticulate tartrate modified ferrous oxohydroxide,
+ 0 OH-T o
All solutions/suspensions were bubbled with nitrogen before and
throughout the synthesis to achieve low oxygen conditions. A
ferrous solution was prepared by adding ferrous sulphate to water
that had been previously acidified with hydrochloric acid and
agitated until all of the ferrous salt dissolved. This solution
was then added to another solution containing tartaric acid. The
solution obtained from mixing the two solutions contained 40 mM
ron and 200 mM tartaric acid, and its pH was generally below 4.0
and usually about 2.0. The pH was then raised with a 5 M NaOH
solution to pH 7.0-9.0, usually 8.0, during which a green
nanoparticulate suspension, i.e. ferrous oxo-hydroxide
nanoparticles , was formed. Finally, this suspension was used
directly as a sensor, recovered through filtration for other
oxygen sensing methods, or, preferably, incorporated as a
nanoparticulate dispersion in a semi-solid matrix.
Example 3 . Nanoparticulate tartrate modified ferrous oxo~
hydroxide , Fe OH-T12o
Example 2 was repeated using 120 mM tartaric acid as the ligand
instead.
Example 4 . Nanoparticulate tartrate- and succinate-modified
ferrous oxo-hydroxide , F +
0 OH-T Succ2 o
Example 2 was repeated using 200 mM succinic acid in addition to
200 mM tartaric acid.
Example 5 . Succinate-modified ferrous oxo-hydroxide , Fe2+ OHSucc
oo
All solutions/suspensions were bubbled with nitrogen before and
throughout the synthesis to achieve low oxygen conditions. A
ferrous solution was prepared by adding ferrous sulphate to water
that had been previously acidified with hydrochloric acid and
agitated until all of the ferrous salt dissolved. This solution
was then added to another solution containing succinic acid. The
solution obtained from mixing the two solutions contained 40 mM
iron and 200 mM succinic acid, and its pH was about 2.0. The pH
was then raised with a NaOH solution to pH 6.5-9.0, usually 8.0,
during which a green precipitate, i.e. succinate modified ferrous
oxo-hydroxide, was formed. Finally, this suspension was used
directly as a sensor, recovered through filtration for other
oxygen sensing methods, or, preferably, incorporated as a
nanoparticulate dispersion in a semi-solid matrix.
Example 6 . Ferrous oxo-hydroxide, Fe2+ OH produced with disodium
carbonate
A ferrous oxo-hydroxide material was prepared as in Example 1 and
except .5M Na C03 was used instead of 5M NaOH. Finally, this
material was incorporated as a nanoparticulate dispersion in a
semi-solid matrix.
Example 7 . Dry nanoparticulate tartrate modified ferrous oxohydroxide,
Fe 0 -T
A nanoparticulate suspension was prepared as in Example 2 . This
suspension was then evaporated in a rotavapor at 60°C under
vacuum. Once dry the powder was ground and could be used for
sensing oxygen.
Example 8 . Ferrous oxo-hydroxide dispersed in a gelatin semi¬
solid matrix
The following process was carried out under a nitrogen
atmosphere. A ferrous oxo-hydroxide material was prepared as
described in Example 1 or 5 . Next, beef gelatine (15% w/w) was
added to this suspension while stirring. Then, the mixture was
heated to 40°C to dissolve the gelatine. Once the gelatine was
fully dissolved, the pH was re-adjusted back to its original
level with a NaOH solution. Finally, a semi-solid matrix that
immobilised and dispersed the ferrous oxo-hydroxide into
nanoparticles was formed by cooling this suspension to room
temperature. Note that different sizes and shapes of sensors
could be achieved by transferring aliquots of the final
suspension to suitably shaped and sized moulds prior to cooling.
Example 9 . Nanoparticulate ferrous oxo-hydroxide immobilised in
a gelatin semi-solid matrix
The following process was carried out under a nitrogen
atmosphere. A nanoparticulate ferrous oxo-hydroxide was prepared
as described in the Examples 2 , 3 or . Next, beef gelatine (15%
w/w) was added to this suspension while stirring. Then, the
mixture was heated to 40°C to dissolve the gelatine. Once the
gelatine was fully dissolved, the pH was re-adjusted back to its
original level with a NaOH solution. Finally, a semi-solid
matrix that immobilised the ferrous oxo-hydroxide nanoparticles
was formed by cooling this suspension to room temperature.
It should be noted that different sizes and shapes of sensors
could be achieved by transferring aliquots of the final
suspension to suitably shaped and sized moulds prior to cooling.
A s shown in Figures 1 to lb, e OH tends to form agglomerates in
the absence of a matrix such as gelatin, but when incorporated in
a matrix material, the physicochemical effect of matrix results
in physically disperse particles at sizes below lOOnm.
Example 10. Nanoparticulate tartrate modified ferrous oxohydroxide
dispersed in a hydroxyethyl cellulose semi-solid matrix
The following process was carried out under a nitrogen
atmosphere. A nanoparticulate ferrous oxo-hydroxide material was
prepared as described in Example 2 . Next, the suspension was
heated to 40°C and then hydroxyethyl cellulose (10% w/w) was
added to this suspension while stirring. A semi-solid matrix,
that immobilised the ferrous oxo-hydroxide particles, formed
quite rapidly after the addition of hydroxyethyl cellulose.
Example 11. Cuprous oxo-hydroxide, Cu+ OH
All solutions/suspensions were bubbled with nitrogen before and
throughout the synthesis to achieve low oxygen conditions. A
cuprous solution was prepared by adding cuprous chloride to water
that had been previously acidified with hydrochloric acid. The
final copper concentration was 10 mM and the pH was generally
below 2.0 and usually about 1.0. Once all of the cuprous salt
dissolved, the pH was raised with a NaOH solution to pH 7.0-9.0,
usually 8.0, during which a precipitate, i.e. cuprous oxohydroxide,
was formed. The material was initially a faint yellow
colour and turned blue/green when tested in an atmosphere having
21% oxygen. Finally, this material was incorporated as a
nanoparticulate dispersion in a semi-solid matrix.
Example 12. Cuprous oxo-hydroxide, Cu+
Example 11 was repeated using 20 mM copper instead. An initial
pH of about 1.0 was required, to ensure full dissolution of the
cuprous salt, prior to commencement of the synthesis.
Example 13. Gluconate-modified cuprous oxo-hydroxide , Cu+
2
Gluc100
All solutions/suspensions were bubbled with nitrogen before and
throughout the synthesis to achieve low oxygen conditions. A
cuprous solution was prepared by adding cuprous chloride to water
that had been previously acidified with hydrochloric acid and
agitated until all of the cuprous salt dissolved. This solution
was then added to another solution containing sodium gluconate.
The solution obtained from mixing the two solutions contained 20
m copper and 100 mM gluconate, and its pH was about 1.0-2.0.
The pH was then raised with a NaOH solution to pH 7.0-9.0,
usually 8.0, during which a blue/green precipitate, i.e.
gluconate modified cuprous oxo-hydroxide, was formed. Finally,
this suspension was used directly as a sensor, recovered through
filtration for other oxygen sensing methods, or, in some
preferred embodiments, dispersed in a semi-solid matrix.
Example 14. Nanoparticulate cysteine-modified cuprous oxohydroxide,
C Cys o
All solutions/suspensions were bubbled with nitrogen before and
throughout the synthesis to achieve low oxygen conditions. A
cuprous solution was prepared by adding cuprous chloride to water
that had been previously acidified with hydrochloric acid and
agitated until all of the cuprous salt dissolved. This solution
was then added to another solution containing cysteine. The
solution obtained from mixing the two solutions contained 20 mM
copper and 20 mM cysteine, and its pH was about 1.0. The pH was
then raised with a 5 NaOH solution to pH 7.0-9.0, usually 8.0,
during which a brown nanoparticulate suspension, i.e. cuprous
oxo-hydroxide nanoparticles, was formed that turned balck upon
oxidation/sensing. Finally, this suspension was used directly as
a sensor, recovered through filtration for other oxygen sensing
methods, or, preferably, incorporated as a nanoparticulate
dispersion in a semi-solid matrix.
Example 15. Cuprous oxo-hydroxide dispersed in a gelatin se isolid
matrix
The following process was carried out under a nitrogen
atmosphere. First, cuprous oxo-hydroxide was prepared as
described in Example 11, 12 or 13. Next, beef gelatine (15% w/w)
was added to this suspension while stirring. Then, the mixture
was heated to 40°C to dissolve the gelatine. Once the gelatine
was fully dissolved, the pH was re-adjusted back to its original
level with a NaOH solution. Finally, a semi-solid matrix that
immobilised and dispersed the cuprous oxo-hydroxide into
nanoparticles, was formed by cooling this suspension to room
temperature .
Example 16. Nanoparticulate cuprous oxo-hydroxide immobilised in
a gelatin semi-solid matrix
The following process was carried out under a nitrogen
atmosphere. A nanoparticulate cuprous oxo-hydroxide was prepared
as described in Example 14. Next, this suspension was heated to
40°C. Then, beef gelatine (15% w/w) was added to this suspension
while stirring. Once the gelatine was fully dissolved, the pH
was re-adjusted back to its original level with a NaOH solution.
Finally, a semi-solid matrix that immobilised the cuprous oxohydroxide
nanoparticles was formed by cooling this suspension to
room temperature.
Example 17. Comparison of the sensitivity ferrous oxo-hydroxide
materials immobilised in a gelatin semi-solid matrix: Fe2+ OH and
Fe + OH-Succ o
Fe +
4 H and Fe + H-Succ o where produced at pH 8.0 and immobilised
in gelatin as described in Example 8 . Prior to cooling down, 500
microliters of each suspension was dispensed under a flow of
nitrogen into top-opened cylindrical moulds (1.35 cm inner
radius) . The sensing materials remained in the top-opened moulds
for the duration of the sensing experiment and initially where
kept in an oxygen free chamber (<100 ppm 02). Next concentration
of oxygen was raised to 0.5% to allow oxidation. After 2h 30 in
under 0.5% 02, the Fe + 40 -Succ o material changed from green to
orange (Fe + to Fe + oxidation) while Fe2+ OH remained green,
showing that the incorporation of succinate increased sensitivity
in relation to Fe + OH.
In general, it should be noted that different sizes and shapes of
sensors impact on sensing time. For example, sensing materials
produced in shallower moulds, than those described herein, will
change colour faster, since oxygen will permeate shorter
distances through the semi-solid matrix before reaching the
entirety of the sensing material.
Example 18. Comparison of the sensitivity ferrous oxo-hydroxide
materials immobilised in a gelatin semi-solid matrix: Fe + OH and
Fe2+ OH-T2
e2 + OH and e2 H-T where produced at pH 8.0 and immobilised
in gelatin as described in Examples 8 and 9 . Prior to cooling
down, 500 microliters of each suspension was dispensed under a
flow of nitrogen into top-opened cylindrical moulds (1.35 cm
inner radius) . The sensing materials remained in the top-opened
moulds for the duration of the sensing experiment and initially
where kept in an oxygen free chamber (<100 ppm 0 ). Next
concentration of oxygen was raised to atmospheric level (21%) to
allow oxidation. After 40 min under 21% 02, a change from green
to orange (Fe + to Fe + oxidation) was already visible in the
Fe + O H material while Fe + 4oOH -T remained green.
Nanosensors : proof-o£-concept
Testing of oxygen sensors
In the examples that follow, the oxygen sensors were tested
directly in suspension or trapped in a gelatine matrix. The
sensors were then either exposed to atmospheric oxygen or kept in
an oxygen-free environment (i.e. nitrogen). A gelatine matrix
was use to stabilise some of the sensors in a 'solid form' for
proof-of-principle purposes but in commercial embodiments, other
materials may be used instead or in addition to gelatine.
Sensors in Suspension
e + OH-T 2o is a nanoparticulate ferrous oxo-hydroxide modified
with tartaric acid, which is dark green, but upon exposure to
oxygen, the solution changes to orange/brown. This is due to the
oxidation of ferrous iron (Fe2+ ) as ferrous oxo-hydroxide to
ferric iron (Fe ). Similar results were obtained with a
different ratio of iron to tartaric acid (Fe + oOH-Ti ) but the
colour change was found to occur more rapidly.
Succinate and citric acid were also used as ligands and the
resulting ferrous oxo-hydroxides showed similar behaviour. The
colour of e +
i - S c c 2 changed from green to orange/brown
upon oxidation. The nanoparticulate suspension is lighter in
colour and more precipitate is formed upon oxidation than with
e2+
0 H- T2oo but the colour change was still clearly seen.
Sensors in a solid matrix
Unmodified (i.e. with no added ligands) and tartrate-doped
ferrous oxo-hydroxides were immobilised and nano-dispersed in a
gelatine matrix as proof of principle. Advantageously, the
change of the colour in the solid matrix was even more defined
than the colour change in solution. Without wishing to be bound
by any particular theory, the present inventors believe that the
alteration in colour observed in these experiments using a matrix
material such as gelatine may be due to the incorporation of
pendant groups from the matrix into the ferrous oxo-hydroxide
material, and indicates that these pendant groups present in the
matrix material may, given the right synthetic conditions, alter
the physicochemical properties of metal ion oxo-hydroxide
materials in a similar fashion to free ligands, such as tartrate.
Overall, these results show that we can apply these oxygen
sensors to packaging by incorporating them in a matrix such as
gelatine and suggest that other ingredients that can form a gel
or solid matrix, such as starch or cellulose, can be used. Also,
this work showed that different sensitivity can be obtained by
producing sensors at different pH's. The use of hydrated
matrices also permits other components to be added to the sensors
such as other redox sensitive materials, including M containing
materials, minerals, compounds or complexes.
Oxygen Indicator in Powder Form
We have investigated the use of dry powders of unmodified and
tartrate-doped ferrous oxo-hydroxides as sensors, as dry powders
may be preferred for some applications, such as surface coating.
The unmodified ferrous oxo-hydroxide powder was originally green,
but slight oxidation during the drying process altered it to
brownish green. Nevertheless, subsequent exposure to oxygen
resulted in further colour change showing that the dry powder, if
dried in an oxygen free environment, can be used as a sensor.
The tartaric-doped ferrous oxo-hydroxide powder showed a more
subtle, and slower, colour change compared to the unmodified
ferrous oxo-hydroxide indicating that the sensitivity of sensors
based in dry powders may be tailored by ligand modification.
eact v ion of oxidized indicators
The packaging process may result in a partial oxidation of the
sensor due to unintended exposure to oxygen. We have found
evidence that it is possible to reactivate the sensors of the
present invention by exposing them to high temperatures (>80°C) .
We have also found that this reactivation process is
significantly more efficient if in the presence of reducing
sugars, such as glucose or fructose.
Copper-based sensor
Copper is a transition metal which also forms oxo-hydroxides at
high pH and that changes colour when oxidised (cuprous [Cu+] to
cupric [Cu +]). The colour change of unmodified cuprous oxohydroxide
in a gelatine matrix was originally very light (Cu+
to dark yellow (Cu+
20OH) to blue greenish (Cu +) in the presence of
oxygen. Cu+ H appeared transparent but in fact was a very faint
yellow.
Tailoring of sensitivity via incorporation of ligands
The incorporation of ligands into the metal ion oxo-hydroxide
materials can be used to either increase sensitivity (e.g. Fe +
0
OH-Succ oo > OH sensitivity) or to decrease it (Fe2+ OH-T 2o
sensitivity < Fe + OH sensitivity) . Without wishing to be bound
by a particular theory, the present inventors believe that
increased sensitivity appears is achieved by incorporating
ligands with low affinity for the metal ion, while the
incorporation of higher affinity ligands seems to stabilise the
materials at low valence states, thereby decreasing their
sensitivity for oxygen.
All documents mentioned in this specification are incorporated
herein by reference in their entirety.
Claims :
1 . An oxygen sensor comprising a solid poly oxo-hydroxy
metal ion material having a transition metal ion in a first
oxidation state that is capable of oxidation to a second
oxidation state in response to oxygen, wherein the solid poly
oxo-hydroxy metal ion material has a polymeric structure in
which one or more ligands are non-stoichiometrically
substituted for the oxo and/or hydroxy groups, so that exposure
of the material to oxygen causes the oxidation of the metal ion
in the solid poly-oxo-hydroxy material to produce a detectable
change in the material.
2 . The oxygen sensor of claim 1 , wherein the solid poly-oxohydroxy
metal ion material is present in nanoparticulate or
nanostructured form.
3 . An oxygen sensor comprising a solid oxo-hydroxy metal ion
material having a transition metal ion in a first oxidation
state that is capable of oxidation to a second oxidation state
in response to oxygen, wherein the solid oxo-hydroxy metal ion
material is present in nanoparticulate or nanostructured form,
so that exposure of the material to oxygen causes the oxidation
o f the metal ion in the solid oxo-hydroxy material to produce a
detectable change in the material.
4 . The oxygen sensor of any one of the preceding claims,
wherein the metal ion material is present in a hydrated, oxygen
permeable matrix so that oxygen permeating the matrix causes
the oxidation of the metal ion to produce a detectable change
in the material.
5 . The oxygen sensor of claim 4 , wherein the metal ion
material does not form a soluble complex with materials forming
the matrix.
6 . The oxygen sensor of any one of claims 3 to 5 , wherein
the metal ion material has a polymeric structure in which one
or more ligands are non-stoichiometrically substituted for the
oxo and/or hydroxy groups.
7 . The oxygen sensor o f any one of the preceding claims,
wherein the oxo and/or hydroxy groups are nonstoichiometrically
substituted by one or more ligands selected
from phosphate, sulphate, silicate, nitrite, nitrate, selenate
and/or bicarbonate.
8 . The oxygen sensor of any one of the preceding claims,
wherein the oxo and/or hydroxy groups are nonstoichiometrically
substituted by phosphate and where phosphate
becomes the dominant mineral phase above the oxo-hydroxide
mineral phase.
9 . The oxygen sensor of any one of the preceding claims,
wherein the oxo and/or hydroxy groups are nonstoichiometrically
substituted by silicate and where silicate
becomes the dominant mineral phase above the oxo-hydroxide
mineral phase.
10. The oxygen sensor of any one of the preceding claims,
wherein the colour change is irreversible under normal use in
the modified atmosphere packaging.
11. The oxygen sensor of any one of the preceding claims,
wherein the detectable change is a visible colour change.
12. The oxygen sensor of any one of claims 1 to 10, wherein
the detectable change is detectable under UV light.
13. The oxygen sensor of any one of the preceding claims,
wherein the metal ion is copper, iron, chromium, vanadium,
manganese, titanium, cobalt, molybdenum and/or tungsten.
14. The oxygen sensor of any one of the preceding claims,
wherein the metal ion is copper and/or iron.
15. The oxygen sensor of wherein the transition metal ion is
(a) iron and the first oxidation state is Fe(II) and the second
oxidation state is Fe(III); or (b) copper and the first
oxidation state is Cu(I) and the second oxidation state is
Cu(II); or (c) cobalt and the first oxidation state is Co(II)
and the second oxidation state is Co(III) .
16. The oxygen sensor of any one of the preceding claims,
wherein the matrix is formed from gelatine, agar, agarose, a
cellulosic material, or a combination thereof.
17. The oxygen sensor of claim 16 wherein the matrix is
formed from animal gelatine or a vegetarian gelatine
equivalent .
18. The oxygen sensor of any one of the preceding claims,
wherein the matrix comprises at least 50% w/w water.
19. The oxygen sensor of any one of the preceding claims,
wherein the matrix comprises between 5% and 25% w/w of the
solidmetal ion material .
20. The oxygen sensor of any one of the preceding claims,
wherein the oxygen sensor produces the detectable change in
response to an oxygen concentration greater than 1% within the
envelope under modified atmosphere conditions.
21. The oxygen sensor of any one of the preceding claims,
wherein the sensor is environmentally and/or biologically
compatible .
22. The oxygen sensor of any one of the preceding claims,
wherein the oxygen sensor is non toxic for compatibility with
food, food products, nutraceutical products or pharmaceutical
products .
23. The oxygen sensor of any one of the preceding claims,
wherein the matrix is a semi-solid matrix, a solid format, or
an aqueous suspension.
24. The oxygen sensor of any one of the preceding claims,
wherein the oxygen sensor is a powder or is surface coated.
25. The oxygen sensor of any one of the preceding claims,
wherein the ligand is a carboxylic acid ligand, such as
gluconic acid or lactic acid; a food additive ligand, such as
maltol and ethyl maltol; an amino acid ligand, such as
tryptophan, glutamine or histidine; a nutrient-based ligand,
such as folate, ascorbate or niacin; and/or a classical anionic
ligand such as phosphate, sulphate, silicate, selenate and/or
bicarbonate .
26. The oxygen sensor of any one of the preceding claims,
wherein the carboxylic acid ligand is a linear dicarboxylic
acid ligand.
27. The oxygen sensor of claim 26, wherein the carboxylic
acid ligand is represented by the formula HOOC-Ri-COOH, where
i is an optionally substituted alkyl, alkenyl or alkynyl
group, or an ionised form thereof.
28. The oxygen sensor of claim 26 or claim 27, wherein the
carboxylic acid ligand is succinic acid, malic acid, adipic
acid, glutaric acid, aspartic acid, tartaric acid, pimelic
acid, citric acid or an ionised form thereof.
29. A product packaging for storing an article in a packaging
envelope under modified atmosphere conditions, wherein the
product packaging comprises an oxygen sensor according to any
one of claims 1 to 28 within the envelope so that oxygen
leaking into the packaging envelope causes oxidation of the
metal ion to produce a detectable change in the material.
30. Use of an oxygen sensor according to any one o f claims 1
to 28 for detecting the leakage of oxygen into product
packaging for storing an article in a packaging envelope under
modified atmosphere conditions, so that oxygen leaking into the
packaging envelope causes oxidation of the metal ion to produce
a detectable change in the material.
31. A method of detecting oxygen leaking into product
packaging for storing an article in a packaging envelope under
modified atmosphere conditions, the method comprising:
(a) providing an oxygen sensor according to any one of
claims 1 to 2 8 within the packaging envelope under modified
atmosphere conditions, so that oxygen leaking into the
packaging envelope causes oxidation of the metal ion to produce
a detectable change in the material; and
(b) optionally detecting the change in the material to
indicate the leakage of oxygen into the packaging envelope.
32. The method of claim 32, wherein the modified atmosphere
conditions have low oxygen levels compared to atmospheric
conditions .
33. The method of claim 32, wherein the modified atmosphere
conditions use nitrogen and/or carbon dioxide.
34. The use or method of any one of claims 32 to 34, wherein
the packaging is for a product selected from a food product, a
pharmaceutical product, a nutraceutical product, a document,
book or manuscript, or an electronic device or component.
35. A process for producing an oxygen sensor according to any
one of claims 1 to 28, the process comprising:
(a) mixing the solution comprising a transition metal
ion, and optionally one or more ligands, in a reaction medium
at a first pH (A) at which the components are soluble;
(b) changing the pH(A) to a second pH(B) to cause a solid
precipitate or a colloid of the ligand-modif ied poly oxohydroxy
metal ion material to be formed;
(c) separating, and optionally drying and/or formulating,
the solid poly oxo-hydroxy metal ion material produced in step
(b) ; and
(d) optionally processing the solid poly oxo-hydroxy
metal ion material so that a nanoparticulate or nanostructured
material is produced.
36. The process of claim 36, wherein the first pH(A) is less
than 2.0 and the second pH(B) is between 3.0 and 12.0,
preferably between 3.5 and 8.0, and more preferably between 4.0
and 6.0.
37. The process of claim 36 or claim 37 wherein the step (a)
of the process is carried out at room temperature (20-25°C) .
38. The process of any one of claims 36 to 38, further
comprising chemically or physically altering the final particle
size of the solid poly oxo-hydroxy metal ion material.
39. The process of any one of claims 36 to 39, further
comprising formulating the solid poly oxo-hydroxy metal ion
materials in a matrix by mixing the material with one or more
matrix forming materials to form a hydrated, oxygen permeable
matrix capable of sensing oxygen.
40. The process of any one of claims 36 to 39, wherein
process comprises the step of precipitating the solid poly oxohydroxy
metal ion material in the presence of one or more
solubilized matrix materials and solidifying the resulting
material to produce a solid poly oxo-hydroxy metal ion material
in a semi-solid matrix.