Field of the invention

The invention relates to a method of preparing a functionalised print paper surface on a substrate, and a substrate functionalised with a print paper surface. Background of the invention

There are many reasons why it may be advantageous to modify the properties of the surface of a substrate to exhibit particular desired properties. However, the methods for doing so are limited and are often expensive which limits their commercial use.

For example, many diagnostic devices have different zones that are functionalized with different biochemical and diagnostic agents. These diagnostic devices find use in a variety of fields, including biomedical and environmental testing. These devices may be simple "spot testing" devices, which are usually a substrate that includes an indicator zone for detecting the presence of an analyte.

In biomedical applications, spot testing is a widely used methodology for many biochemical and diagnostic assays. These tests are often qualitative or semiquantitative. Examples include chemical analysis (e.g. nitrogen analysis and heavy metal ions) and clinical diagnostics (e.g. enzyme-linked immunosorbent assay, or ELISA). The simplicity of many spot assays makes them difficult to be phased out, even though more sophisticated technologies have become available. This is in part due to these spot assays being inexpensive.

Similarly, spot testing finds use in environmental fields for identifying and quantifying concentrations of heavy metal ions in water, or contaminants in the air.

Many of these diagnostic devices have different zones that are functionalized with different biochemical and diagnostic agents. However, it can be difficult to functionalise these different zones, or fabricate a device with multiple zones of different functionality.

The present invention is directed towards providing a method for modification of a substrate surface, as well as a modified substrate surface in view of these problems. Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary of the invention

In one aspect of the invention, there is provided a method of preparing a functionalised print paper surface on a substrate, the method including: applying a print paper ink to a substrate to form a print paper surface, the print paper surface including organic nanofibers, and the print paper ink including at least a carrier liquid and the organic nanofibers; and wherein at least a portion of the organic nanofibers in the print paper surface are functionalised with a functionalisation agent.

The term "print paper" is different from "print on paper"; the latter has the meaning of applying a traditional ink (i.e., lithographic offset ink, flexographic ink, etc.,) on a substrate to form a desired pattern mostly for reading purposes. The former has the meaning of printing micro- or nano- fibres and other materials as "inks" onto a substrate, such as to form a patterned functional print paper surface (which micro- or nano- fibres may be functionalised before or after printing) to be used, for example, in diagnostic and environmental monitoring devices. The term nanofiber is intended to encompass a fibre that has at least one dimension that is less than 1000nm. Preferably, the organic nanofiber has at least one dimension that is less than 500nm; more preferably, less than 300nm; and most preferably less than 200nm. Alternatively, or additionally, it is preferred that the nanofiber has one dimension that exceeds 800nm, preferably 1000nm. The surface coverage and density of the nanofibers in the print paper surface can be tuned as desired. Generally, the surface density of the nanofibers used in the print paper surface is in the range of 1 to 1 10 g/m ² .

In an embodiment, the surface concentration of the functionalisation agent in the paper surface is at least 10 ^~11 M/m ² , preferably at least 10 ^~10 M/m ² , more preferably at least 10 ^"9 M/m ² . In an alternative embodiment, the surface concentration of the functionalisation agent in the paper surface is at least 10 ^"6 g/m ² , preferably at least 10 ^"5 g/m ² , more preferably at least 10 ^"4 g/m ² .

In an embodiment, the print paper ink further includes the functionalisation agent. In an embodiment, each dimension of the nanofiber is less than l OOOnm. Preferably each dimension is less than 800nm, more preferably less than 600nm, and most preferably less than 500nm.

In an embodiment, the concentration of the organic nanofiber in the carrier liquid is less than 5w/v %., preferably less than 4w/v%, more preferably less than 3w/v%, even more preferably less than 2w/v%, and most preferably less than 1 .5w/v%.

The carrier liquid may be any liquid that the organic nanofibers can be suspended in. Preferred liquids are water or organic liquids typically used in print applications. Generally, it is preferred to use a carrier liquid that has a moderate vapour pressure to assist with drying the print paper ink after application to the surface of a substrate.

In an embodiment, the organic nanofibers are functionalised prior to the step of applying the print paper ink to the substrate. This may be advantageous where a reaction medium is required to carry out the functionalisation. For example, the functionalisation agent may need to be solubilised in order to functionalise the organic nanofiber. Alternatively, where the functionalisation is achieved through physisorption, the reaction medium may facilitate the adsorption process. In an alternative embodiment, the organic nanofibers are functionalised after the step of applying the print paper ink to the substrate. In some instances the functionalisation agent may be attached, grafted, or adhered to the organic nanofibers in the print paper surface after the print paper ink has been applied.

In an embodiment, the print paper ink further includes a binder. Preferably, the binder is a non-hemicellulose and non-cellulose material. The binder may be used to adjust the physical properties of the solution, such as viscosity, to improve the printability of the print paper ink in certain situations. Similarly, a binder may be useful to improve the adherence of the ink to the surface of the substrate. In an embodiment, the print paper ink is applied to the substrate as at least one micro zone, or a plurality of micro zones. Preferably, the print paper ink is applied to the substrate as a plurality of micro zones in an ordered or patterned arrangement. In various forms, the micro-zones may exhibit a different response to a stimulus, be responsive to different stimuli, or provide a corresponding print paper surface that has different functionality or exhibits different properties from another micro-zone. Preferably, the substrate includes at least two different micro zones.

The term micro zone is intended to designate a small print paper surface zone. These zones can be of any shape and size, but typically they are circular in shape and have a diameter of from about 1 mm to about 4 mm. This is advantageous as it allows the micro zones to be visually inspected by eye or with simple instrumentation. This is particularly useful where the micro zones exhibit a change in colour in response to a stimuli or as a result of detection of a particular analyte.

In another aspect of the invention, there is provided a substrate with a functionalised print paper surface, the print paper surface including organic nanofibers that are functionalised with a functionalisation agent.

The embodiments and features generally discussed below are broadly applicable to the aspects discussed above.

In an embodiment, the functionalisation agent is a nanoparticle. The organic nanofiber may physisorb to the nanoparticle surface, or chemically bond with the nanoparticle surface depending on the specific type of nanoparticle that is selected. A range of different nanoparticles are contemplated. However, preferred nanoparticles are those formed of metals or metal salts. Preferred metals are those that are generally stable, such as Au, Pt, Ir, Pd, OS, Ag, Rh, Ru, Cu, Bi, Tc, Re. In an alternative embodiment, the nanoparticles may be organic compounds such as nanoparticles formed from polymers such as PTFE (Teflon). In still another embodiment, the nanoparticles may be nanoclays, or formed from mineral compositions such as silica, alumina, etc.

It is preferred that the nanoparticles are less than 80nm, preferably less than 70nm, more preferably less than 60nm, and most preferably less than 50nm. In an embodiment, the functionalisation agent are compounds that are covalently bonded to organic nanofibers. There are a wide range of suitable compounds that may be employed, the most preferred compounds are HRP and ALP enzyme systems.

The term print paper ink is broadly intended to encompass a print medium that includes organic nanofibers. In an embodiment, the organic nanofibers are selected from the group consisting of plant fibres and/or animal fibres. Suitable plant fibres include paper fibres, hemicellulose, cellulose, cotton, and/or linen. Suitable animal nanofibers include protein nanofibers, wool, and/or silk. Protein nanofibers and cellulosic nanofibers both have good adhesion to the substrates and many functional groups on their surface. As such, these are both good candidates for forming the print paper ink. Combinations of various plant and/or animal fibres may also be used. Preferably the nanofibers are selected from animal derived protein nanofibers, hemicellulose, cellulose, or combination thereof. More preferably, the nanofibers are hemicellulose, cellulose, or combination thereof. It is thought that the combination of hemicellulose together with cellulose nanofibers will increase the binding strength of the printed paper to the substrate. Most preferably, the nanofibers are cellulose. While a broad range of different functionalisation agents are contemplated, in one embodiment, the functionalisation agent is at least one of: a chemically active agent, bioactive agent, an electroactive agent, and a magnetoactive agent. Broadly, a chemically active agent is an agent that is responsive to the presence of a chemical of interest. Similarly, a bioactive agent is an agent that is responsive to a biochemical, a virus, or bacterium of interest. Preferably, the chemical, biochemical, virus, or bacterium of interest binds to the chemically or biochemically active agent, through an adsorption process such as physisorption or chemisorption. More preferably, this binding induces a change in the properties of the print paper. This change in properties may for example be a change in colour, or other physical or chemical properties that can be detected with an appropriate analytical method (e.g. Raman spectroscopy). Where the functionalization agent is an electroactive agent or a magnetoactive agent, the agent preferably provides electric, magnetic, or electromagnetic properties and/or is responsive to electromagnetic radiation, an electric field, and/or a magnetic field.

Alternatively, or in addition, the functionalisation agent is selected to provide a print paper surface that has a property selected from the group consisting of: hydrophobic, hydrophilic, conductive, adhesive, non-adhesive, or porous. Preferably, this property is considered relative to the substrate surface. That is, the print paper surface is functionalised to exhibit different properties from the substrate surface. In a limited example, the substrate surface may be plastic and therefore exhibit some degree of hydrophobicity. In such case, where the property of the functionalisation agent is to impart hydrophobicity to the print paper surface, the print paper surface exhibits a greater degree of hydrophobicity than the substrate surface. The same applies to other desired properties imparted by the functionalisation agent to the print paper surface. In an embodiment the functionalised print paper surface is sensitive to a stimulus, and exhibits a response on exposure to the stimulus. A range of stimuli are contemplated. However, preferred stimuli are selected from the group consisting of: chemical, biological, electric, magnetic, electromagnetic, heat, light intensity, pressure, humidity, and/or pH stimulus. On exposure to the stimulus, the print paper surface exhibits a response to indicate the presence or application of the stimulus. Preferably, the response scales with a magnitude of the stimuli, for example where the stimulus is heat, the response is greater for a greater difference in heat between the stimulus and the print paper surface. In one example, the response is manifested as a colour change in the print paper. The greater the magnitude of the stimulus, the more intense the colour change.

In an embodiment, the functionalised print paper surface is at least one of: hydrophobic, hydrophilic, conductive, adhesive, non-adhesive, porous, chemically selective, and/or biologically selective.

In an embodiment, the functionalisation agent has a function other than providing a pigment to the print paper surface. That is, the print paper surface and print paper ink are not intended to be used for the sole purpose of providing a print coloured surface (e.g. to display an image or letter). While the print paper surface may be of a different colour from the substrate surface (and thus be visually distinct), the print paper surface includes further functionality, such as having different physical properties from the substrate surface as generally discussed above, or being responsive to a stimuli. In some embodiments, the print paper surface will be transparent or the same colour as the substrate surface, and thus visually indistinct from the substrate surface.

In an embodiment, the print paper surface is on the surface of the substrate in the form of at least one micro zone, or a plurality of micro zones. Preferably, the print paper surface is on the surface of the substrate in the form of a plurality of micro zones in an ordered or patterned arrangement.

More preferably, the plurality of micro zones includes at least a first micro zone and a second micro zone, and the first micro zone and the second micro zone exhibit different sensitivity to a stimulus, are responsive to different stimuli, or have a different functionality from each other.

Where the print paper surface is on the surface of the substrate in the form of a plurality of microzones in an ordered or patterned arrangement, or where the print paper ink is applied in the form of a plurality of microzones in an ordered or patterned arrangement. Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

Figure 1 : Print paper applied to a glass substrate. Figure 2: The left image shows and electronic file for making the printing mask. A computer driven plotter cutter was used to cut out the dark circles to form the printing mask. The right image shows print paper applied to a transparency film using the mask.

Figure 3: A scheme illustrating two embodiments for pre-functionalisation of print paper ink to form a functionalised print paper surface on a substrate. Figure 4: Print paper functionalised with gold nanoparticles on a plastic substrate.

Figure 5: HRP detection obtained from bioactive print paper with visual output.

Figure 6: Hydrophobic functionalised print paper on paper substrate. Figure 7: Hydrophobic print paper on film.

Figure 8: Photos of porous print paper on glass slides.

Figure 9: SEM images of the print paper: (a) the edge of print paper; (b) the surface of the print paper (channels and pores retained in print paper). Figure 10: Photo of HRP detecting obtained from printed paper with visual output.

Detailed description of the embodiments

The invention is directed to methods and the use of print paper ink to functionalise a substrate surface. The invention is also directed towards substrate surfaces that have had print paper applied to them.

The term "print paper ink" relates to a print paper solution that may be applied to the surface of a substrate to impart a variety of different functionalities to the substrate surface. The print paper ink can be applied to the substrate in the form of a pattern including an organic fibrous material (such as cellulose nanofibers), a functionalized organic fibrous material (such as functionalised cellulose nanofibers), or a mixture of cellulose nano fibres and other functional compounds. This differs from traditional inks that are used in printing, as these traditional inks (which generally include dyes or other pigment additives) are for the purpose of forming visible patterns for reading or displaying an image, and in addition to lacking functionality, these typical inks do not generally contain materials derived from paper as a constituent.

A major advantage of print paper ink is that it enables the printing of differently functionalized zones on a substrate, potentially allowing more difficult and complicated functionalization of materials for application to the substrate.

The print paper inks of the present invention include at least an organic fibrous material suspended in a liquid carrier. The organic fibrous materials may be a cellulosic fibrous material such as cellulose or hemicellulose. This organic fibrous material is typically in the form of a nanofiber that is suspended within the carrier phase. The carrier phase may be water or an organic liquid such as those organic liquids that are typically used in inks. Organic liquids having a moderate to high vapour pressure are preferred as this provides for an organic phase that readily evaporates after application of the print paper ink to a substrate.

The print paper inks may also include additional constituents such as to provide desired physical or chemical properties. For example, the print paper ink may include a binder or other additive to provide a desired ink viscosity. The skilled addressee will appreciate that a number of other additives may be added for various purposes such as to provide improved composition stability, preventing or mitigating aggregation and/or settling of the fibrous components, and/or to tailor the print paper ink composition for use on a particular substrate. The print paper ink may be applied by printing or depositing the print paper ink using any printing or application method known to those skilled in the art. As such, use of print paper inks provides a versatile method for functionalising the surface of a substrate by applying the print paper to the surface of the substrate. For example, cellulose nanofibers can be easily functionalised with a number of different functionalisation agents to exhibit a range of different properties. Thus, the use of print paper inks to functionalise a substrate surface is easy and cost-effective.

The print paper ink may be functionalised prior to application of the ink to a substrate or after application of the ink to a substrate. For example, in one embodiment, the organic fibrous material is mixed with a functionalising agent in solution to modify the organic fibrous material to include a desired functionality. In another embodiment, print paper ink is applied to a substrate to form a print paper layer on the surface of the substrate. The print paper layer is then treated with a functionalising agent to modify the organic fibrous material in the print paper layer to include a desired functionality.

A range of different functionalising means and functionalising agents may be used. For example, the functionalising means may be through physisorption or chemisorption depending on the functionalisation agent that is used. Where the functionalisation agent is a small particle, such as a nanoparticle, it is expected that the organic fibrous material will adsorb to the surface of that nanoparticle through typical physical adsorption mechanisms, such as through charge attraction or through thermodynamically favourable hydrophobic or hydrophilic interactions depending on the specific carrier liquid that is used. Alternatively, the organic fibrous material may chemically bond with a part of the nanoparticle. In another example, the organic fibrous material is functionalised through chemical modification, such as to include a desired chemical moiety. This may be achieved through a variety of different methods, such as chemically grafting a small molecule with the desired moiety on to the organic fibrous material, or otherwise forming a chemical bond between the organic fibrous material and another compound. The organic fibrous material can be functionalised prior to forming the paper print ink composition, can be modified in-situ by adding the functionalising agent to the paper print ink composition, or by treating the organic fibrous material in the paper print after application to a substrate. Where the functionalisation is achieved after application of the paper print ink to the substrate, the modification can be done either prior to evaporation of the carrier liquid or after evaporation of the carrier liquid depending on the specific functionalisation agent that is selected and the nature of the functionalisation reaction.

Generally, it is envisaged that the print paper ink will be applied to the surface of the substrate in a patterned manner. Contact and non-contact printing processes and equipment can be used to produce such patterns. This is advantageous as it allows for the large scale functionalisation of substrates at high speed, with on-demand pattern variation.

Another major advantage of print paper is that it enables the printing of a range of different paper print inks that have organic fibrous materials that have different functionalisation on the same substrate surface, allowing certain more difficult functionalization of materials to be performed before printing. For example, in one form of the invention a substrate is printed with a plurality of print paper patterns. Each of the plurality of print paper patterns is functionalised so that it is sensitive to a different chemical species. A sample may be applied to the surface of the substrate, and each of the plurality of print paper patterns undergoes a visual change if a certain chemical species is present.

An advantage that can be easily demonstrated is that print paper offers much more flexibility in forming micro-zones of sensing and analytical applications. Print paper offers an easy solution for more difficult micro-zone functionalization to be first conducted using high-efficiency ex situ methods and the functionalized fibres or pulp to be then printed to form the micro zones or patterns. An example shown below is to use the print paper method to form patterned hydrophobic and superhydrophobic surfaces on a substrate. Another example is to modify the cellulose ink with conducting nano particles and to print patterned analytical zones for Surface Enhanced Raman Spectrometry.

If these paper micro zones are printed on paper, they are invisible. These zones can be reaction sites, letters and words, patterns that have different properties than the surrounding areas of the patterns.

In one specific illustrative example, a substrate includes three different print paper patterns that are sensitive to particular contaminant chemical species found in water such as arsenic, benzene, and cyanide. The first pattern is sensitive to arsenic, the second pattern is sensitive to benzene, and the third pattern is sensitive to cyanide. A sample of water is applied to the surface of the substrate, and if any of these contaminants are present they will bind to their respective print paper pattern effecting a colour change. This provides a qualitative assessment as to whether these contaminants are present in the water sample. The size, density, degree of functionalisation of the print paper patterns can potentially be modified to alter the sensitivity of the analysis.

In another illustrative example, a substrate may be printed with a plurality of print paper patterns that are sensitive to different viruses. A blood sample may be applied to the surface of the substrate where a virus, if present, will bind to the surface of a corresponding print paper pattern. Again, the print paper pattern may exhibit a colour change, or other measurable change to indicate the presence of a virus in the blood sample. The above illustrative examples relate to the use of print paper to functionalise at least a part of the surface of a substrate for use as chemical and biological sensors. However, it will be appreciated that a variety of other types of functionalisation are possible, and that the invention is not limited to the use of print paper as a chemical or biological sensor. Print paper may be functionalised to impart a variety of properties to a substrate, such as conductivity, hydrophobicity, hydrophilicity, or various other electronic and/or magnetic properties. Examples

The following examples relate to the use of a print paper formed from cellulose nanofibers.

Example 1 : Print paper on glass Nanocellulose (NC) water suspension with a solid content of 0.05% (w/v) was used as the print paper solution. 1 .5 μΙ_ of this suspension was transferred onto the glass slides to form one paper zone. As shown in Figure 1 , paper zones were printed on glass slides.

Example 2: Print paper on plastic Nanocellulose (NC) water suspension with solid content of 1 .4% (w/v) was used as the print paper ink. Commercial overhead transparency (Xerox) polymer films were chosen as an example for plastic substrates. A designed pattern was cut into a mask for printing demonstration by cutting patterns using a computer-driven plotter cutting through a transparency film. 20 μΙ of NC suspension was then transferred on the treated transparency film with the masks. A squeegee was then used to sweep across the mask, removing excess print paper ink, leaving ink in the cut circles to form the pattern (the concept of screen printing). After drying, the mask was removed, and printed NC paper pattern was obtained (as shown in the right hand image of Figure 2). For comparison, the electronic file of the mask was printed using the conventional black ink on paper (as shown in the left hand image of Figure 2). A post treatment process of passing the transparency film through hot rollers was found to improve the adhesion of print paper onto the transparency film. Alternatively, or additionally plasma pre- treatment of the surface of the transparency film could also be used to improve the adhesion of the print paper ink. Example 3: Print paper functionalised with gold nanoparticles and printed on to a paper substrate.

Print paper ink can be pre-functionalized for different applications. Gold nanoparticles were used as functional material for paper-based diagnostic devices, as they can covalently bind bio-molecules on their surface. Print paper ink can be pre- functionalized by two methods before it is printed with patterns on other substrates as illustrated in Figure 3.

Figure 3 provides a general schematic of two methods for functionalising the NC in the print paper ink with gold nanoparticles. A print paper ink 300 is provided including a carrier liquid and NC 302. In the first reaction scheme 304, gold nanoparticles are produced in situ through the addition of HAuCI ₄ and Na ₂ C ₆ H ₅ O ₇ . The Au(lll) is reduced in the carrier liquid to Au(0) and nucleates and grows to form gold nanoparticles 306. These gold nanoparticles physically bind with the NC 302 to form functionalised solution 307. In the second reaction scheme 308, gold nanoparticles 310 are prepared ex situ and then added to the print paper ink 300. As above, these gold nanoparticles physically bind with the NC 302 to form functionalised solution 31 1 . Either of solutions 307 or 310 may then be used as the functionalised print paper ink to form a functionalised print paper surface 312 on substrate 314.

In both cases, the gold nanoparticles were around 40 nm in diameter. In the first reaction scheme 304, the solid content of gold nanoparticles 307 was around 6 χ 10 ^~5 mol/m ³ . In the second reaction scheme 308, the solid content of gold nanoparticle used was around 2x 10 ^~10 mol/m ³ .

Figure 4 shows the paper print functionalised with gold nanoparticles applied to a plastic substrate. In this case, 20 μΙ of the functionalized paper ink was printed on plasma treated plastic. In the image shown in Figure 4, the print paper exhibits a reddish/pinkish colour which is from the gold nanoparticles.

Example 4: Paper print functionalized bioactive ink on plastic (the paper print ink was functionalized with biomolecules before printing).

Paper ink can be pre-functionalized by biomolecules for its further biomedical applications, such as ELISA. Here, a bio-reaction system with horseradish peroxidase (HRP) and 3,3',5,5'-tetramethylbenzidine (TMB) was used as an example. HRP (Abeam) was diluted into the concentrations of 1 μg/ml solution. 1 μΙ_ and 2.5 μΙ_ of HRP solution was added into paper ink (with solid content of 0.05 % w/v) individually. The paper ink was thus functionalized and became HRP bioactive paper ink with two different concentrations of HRP. 1 .5 μΙ_ of different inks (without and with HRP) were printed on the glass slides. A liquid substrate for HRP, 3,3',5,5'-tetramethylbenzidine (TMB, SIGMA), was used to generate colorimetric signal for analysis (see Figure 5).

Example 5: Paper print functionalized hydrophobic ink on paper and plastic (Paper ink is functionalized before printing) Print paper ink was pre-functionalized by adding low surface energy substances to functionalise the NP, in this case Teflon particles. The functionalised (hydrophobic) paper ink can be patterned and printed on other substrates, such as paper (Figure 6) or transparency film (Figure 7).

Example 6: Paper print functionalised to be porous and applied to a glass substrate (paper print ink is treated to have a porous filtration function before printing)

Paper ink can also be functionalized to build up pores and channels through the printed paper. For example, 0.2 g cellulose powder was mixed with 1 ml cellulose paper ink with solid content of 1 .4 w/v%, forming a paste. 20 μΙ of such paste was deposited on glass slides with patterns of circulars with diameter of 3 mm. A porous paper disk was thus printed on glass surface (Figure 8). The morphology of this print paper characterized by scanning electron microscopy confirms the pores and channels built up by the pre-functionalized paper ink (Figure 9). This kind of print paper builds up a porous hydrophilic substrate which can be printed into different patterns and on different substrates for making low cost chemical or biochemical assays. It highlighted the nature of conventional paper with its pores and surface chemistry, as well as the definition of ink.

Example 7: Post-functionalization of print paper with biofunctionalitv

Bioactive print paper can also be functionalized after the paper being printed. For example, the HRP-TMB reaction system can also be used for demonstrating the post- functionalization of print paper. 2.5 μΙ of HRP solution (250 ng/ ml) was deposited onto the print paper on the glass slides; when it was dry, add TMB to give colour evaluation (Figure 10). The post-functionalized bioactive print paper also works.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.