Patent Application
20130210679
The present invention relates to method for preparing a metal nanoparticle organic composite film, preferably a metal nanoparticle organic composite film of a chemical sensing device, to a metal nanoparticle organic composite film obtained by said method, and to a chemical sensing device comprising a metal nanoparticle organic composite film or an array of different metal nanoparticle organic composite films obtained by said method.
Composites from metal nanoparticles and organic molecules can be used as sensitive layers in chemical sensors [1]. Two different kinds of metal nanoparticle/organic composites exist. First, metal nanoparticles encapsulated with organic ligands (type “A”) and, second, metal nanoparticles which are connected (interlinked) by organic molecules (type “B”). While in both types of composite material the nanoparticles in the film are most important for the conductivity of the material, the kind and amount of organic molecules determine the volatile organic compound (VOC) sorption properties of the materials [1,2]. Thus, a broad variety of materials with tuneable selectivity can be achieved by choosing appropriate organic ligand or linker molecules [3].
Due to the conductivity of the material, the coating is especially suited for chemiresistor devices. Here, the sensing of VOCs is based on two effects:
Films comprising metal nanoparticles encapsulated with organic ligands (i.e. organic materials with only one functional group, which is bound to the surface) can be prepared by drop coating, spin coating, spray coating or dip coating as shown in
As shown in
Films prepared by ligand/linker exchange (“B1”) have the same properties as the film “A”, except that the mechanical stability is slightly enhanced due to the chemical interlinkage and that, in the film, unbound headgroups of the linker molecules may be present. However the process is not well controlled, and diffusion of the linker into and of the ligand out of the bulk of the composite is a problem. Additionally, these processes are known to often be very slow, especially when the organic linker and ligands have the same functional groups.
Ligand-linker exchange by co-precipitation is performed by mixing solutions of nanoparticles and linker molecules and waiting for the deposition of the composite on the substrate by precipitation. It is the worst method in terms of gaining control over the film structure. It generates the roughest, most disordered and often very thick films (“B2”). In addition, the ratio of ligand to bound and unbound linker is hard to control, which leads to the presence of unbound headgroups in the materials. The size and structure of their pores as well as the degree of interlinkage are undefined, as well.
Nanoparticle films interlinked with organic molecules can be additionally prepared by layer-by-layer (1-b-1) self-assembly on the sensor transducer [1,2]. Here, the substrates are alternately immersed into nanoparticle solutions and solutions of organic molecules, resulting in an assembly of the material by chemical reaction (ligand/linker exchange). An advantage of this preparation method (compared to the others) is the precise film architecture and composition that is controlled by the chemistry of the used nanoparticles, the organic molecules and the involved ligand/linker exchange reaction. The structure of film “B3” is the most homogenous one concerning composition and thickness. The ligands are exchanged with linkers in the very controlled layer-by-layer procedure. However, the pore structure is also not tuneable, and the degree of interlinkage and composition is solely determined by the chemistry between nanoparticles and organics.
Often, templates are used in order to tune the pore size and structure of materials. To synthesize porous inorganic solids, template-assisted sol-gel procedures are employed. For organic polymers, the molecular imprinting technology is applied (see e.g. [4]). For molecular imprinting of polymers, the polymerisation reaction of the functional monomers takes place in the presence of a template (additive), which is not reactive in the polymerization process. After removal of the template, the polymer is “imprinted”. The resulting pore can then be used to sorb molecules that are similar (in size and structure) to the template. A scheme of the process is shown in
One of the most interesting features of materials preparation is the locally confined deposition of the material on relevant regions on a surface. Drop coating of materials allows this intrinsically. For other deposition techniques (spin- spray- or dip-coating and evaporation) patterning of the material by lithographic methods is necessary. For organic materials or organic/inorganic composites conventional lithographic techniques cannot be used due to the solubility of the material in organic solvents, which are needed for the lithographic process. In this case, the use of a water-soluble mask can be used for patterning the surface [6].
For advanced sensing applications in trace detection of gaseous species, e.g. in the fields of medical diagnosis, food quality and environmental control, there is a large need for highly sensitive and reproducible sensors for volatile organic compounds (VOCs). Metal nanoparticle organic composites are well suited as sensitive layers on chemiresistors.
An optimized chemisensitive nanocomposite layer for VOC detection should have the following
a) structural properties:
b) chemical properties:
c) substrate interface properties:
d) patterning properties:
In terms of these requirements, the films grown by layer-by-layer self-assembly are the most suitable up to now. They are thin and have a reproducible structure and composition. The organics can be selected according to the desired sorption properties. However, it would also be desirable to tune the pore size and structure in a process that is similar to molecular imprinting and to tune the degree of interlinkage and the composition of the film, as swelling and pore filling are known to be important for the transduction of the sorption of VOCs into a measurable sensor signal. Additionally, it would be desirable to avoid dipping the substrates into solutions, to allow the use of plastic substrates and to avoid lithographic techniques for patterning. The current methods of preparation of such material do not offer these possibilities.
Thus, there is a need for a preparation method which allows the tuning of the (pore) structure, the degree of interlinking and the composition, while avoiding the dipping into solutions and maintaining the good properties of layer-by-layer self-assembled films with respect to film quality and reproducibility of formation. This preparation method should further allow to pattern the material on the surface in a desired manner in order to save material (and thus costs) and to avoid contaminations of other parts of the device with a semi-conductive layer.
The above objects of the present invention are solved by a method for preparing a metal nanoparticle organic composite film, preferably a metal nanoparticle organic composite film of a chemical sensing device, said method comprising the steps:
In one embodiment, said depositing b) and d) is performed by drop coating.
In one embodiment, said substrate comprises a material selected from glass, quartz, ceramics, polyethylene, polycarbonate, flexible polymer materials, silicon, ITO, FTO, metal oxides and carbon.
In one embodiment, said substrate is a transducer.
In one embodiment, said substrate is not a transducer, but has another function, e.g. in a tubing or display.
In one embodiment, said substrate is a flexible substrate.
In one embodiment, said flexible substrate comprises or is made of a polymer.
In one embodiment, said flexible substrate comprises or is a gel.
In one embodiment, said flexible substrate is a biological substrate, e.g. skin or tissue.
In one embodiment, said flexible substrate is a piece of fabric.
In one embodiment, said substrate is patterned. For example, said substrate may expose wells.
In one embodiment, prior to performing steps b) to j) (wherein steps f) and j) are optional), said surface of said substrate is at least partially functionalized to modify (i.e. to increase or to decrease) the wettability of said surface and/or the adhesion of said film to said surface, and/or is at least partially coated with a protecting layer, which, preferably, is inert to the used solvent, such as a layer made of SiO2 or other oxides.
Preferably, said solutions of said ligand stabilized metal nanoparticles and said organic linker molecule are dilute solutions. The term “dilute” solution is meant to refer to any solution that allows the production of a monolayer or submonolayer of the solute(s).
In one embodiment, the concentration of said ligand stabilized metal nanoparticles in said solution is selected so as to ensure that a given area of said surface is covered with a monolayer or submonolayer of nanoparticles. Preferably, 10 to 100% of said surface are covered with a monolayer, more preferably 50 to 100% of said surface are covered with a monolayer, most preferably 80 to 100% of said surface are covered with a monolayer.
In one embodiment, in step d), said organic linker molecule is deposited in an amount of from 1 to 500 pmol/mm2, preferably of from 20 to 100 pmol/mm2.
In one embodiment, in step b), said ligand stabilized nanoparticles are deposited such that a monolayer or submonolayer of particles is formed.
The phrase “said ligand stabilized nanoparticles are deposited such that a monolayer or submonolayer of particles is formed”, as used herein, is meant to refer to a way of applying the nanoparticles in a manner so as to result in a monolayer or submonolayer of particles. It should be noted that, in one embodiment, once the monolayer or submonolayer of particles is formed, no further deposition of nanoparticles occurs. It should be noted that in one embodiment, it is only of minor importance what the ultimate concentration of the deposition solution in terms of nanoparticle concentration is. Rather in this embodiment, it is more important how much material of nanoparticle from the solution is finally deposited on the substrate, after evaporation of the solvent. There are various factors that affect the amount of finally deposited material, such as applied volume which defines the total amount of materials besides the solvent, the concentration of the solution, the spreading of the solution which defines the area on which the solution is coated, and the material itself. If one assumes that a given total amount of material from a linker solution is deposited, the area of the substrate on which the defined volume with a defined concentration is applied is important. Consequently, in one embodiment, the linker concentration is with respect to the coated area (pmol/mm2). With respect to the nanoparticle deposition, the material itself is of relevance, in that the size, the size distribution and the geometry of the particles may vary. For this reason, a molar concentration, i.e. a number of particles in the solution, can not be generally defined, without unduly limiting the scope. Consequently, instead, in one embodiment, step b) is defined in terms of substrate coverage by a monolayer or submonolayer. A person skilled in the art knows how to deposit a solution of nanoparticles so as to achieve a monolayer or submonolayer coverage.
In one embodiment, said solution of an organic linker molecule further comprises an additive having a size similar to a desired pore size, which additive is removed during step h), optionally during steps f) and h). Preferred additives include aromatic and aliphatic hydrocarbons, hydrocarbons containing heteroatoms or water-soluble nanoparticles.
In one embodiment, said drying c), e) and/or i) is performed under an atmosphere selected from an ambient, inert, oxidising and reducing atmosphere. In one embodiment, the entire process is performed under an atmosphere selected from an ambient, inert, oxidising and reducing atmosphere.
In one embodiment, said drying c), e) and/or i) is performed under a humidity controlled atmosphere. In one embodiment, the entire process is performed under a humidity controlled atmosphere.
In one embodiment, said drying c), e) and i) is performed by means of a stream of gas, preferably of an inert gas. In one embodiment, said drying is performed by means of a stream of nitrogen.
In one embodiment, in step g), steps b) to e), optionally steps b) to f), are repeated at least 5 times, preferably at least 10 times, more preferably at least 15 times.
In one embodiment, said film has a thickness in the range of 10 nm to 500 nm, preferably 15 to 300 nm, more preferably 20 to 200 nm.
The solvent or solution used for the washing steps will depend on the kind of substrate, nanoparticles and linker molecules used in the method. Preferably, the same solvent as used for said solution of the organic linker molecule is used. Particularly preferred solvents include organic solvents, such as aromatic hydrocarbons (e.g. toluene), aliphatic hydrocarbons (e.g. hexane) or hydrocarbons containing heteroatoms (e.g. acetone, methanol, propanol, ethanol) and water.
In one embodiment, said washing h) further comprises ultrasonic treatment.
In one embodiment, in steps b) and d), said solution is deposited only onto a confined area of said surface or in a defined pattern.
The objects of the present invention are also solved by a metal nanoparticle organic composite film obtained by the method as defined above.
In one embodiment, said film has a homogenous composition, a homogenous pore size and structure, and/or a homogenous, preferably low, degree of interlinkage between said metal nanoparticles. The degree of interlinkage is represented by the ratio of the functional groups bound to the nanoparticles relative to the total number of functional groups. A ratio between 5% to 80% is preferred, a ratio of 10% to 60% is more preferred, a ratio of 20% to 50% is most preferred.
The objects of the present invention are also solved by an array of different metal nanoparticle organic composite films as defined above, wherein, preferably said different metal nanoparticle organic composite films are formed on a single substrate.
The objects of the present invention are further solved by a chemical sensing device comprising a metal nanoparticle organic composite film as defined above or an array of different metal nanoparticle organic composite films as defined above.
The term “nanoparticle”, as used herein, is not limited to spherical nanoparticles, but is meant to refer to structures (including rods or fibers) where at least one dimension of the structure is in the order of nanometers, i.e. <1 μm, preferably ≦500 nm, more preferably ≦300 nm, most preferably ≦100 nm.
Preferably, the metal nanoparticles comprise a metal selected from gold, silver, platinum, palladium, copper and alloys thereof. In one embodiment, said metal nanoparticles are core-shell nanoparticles, being electrically conductive and having a shell from a metal selected from gold, silver, platinum, palladium, copper and alloys thereof.
The term “ligand stabilized metal nanoparticles”, as used herein, is meant to refer to metal nanoparticles surrounded/encapsulated by organic or metal-organic ligands having a single functional group, which single functional group binds to said metal nanoparticles.
The term “metal nanoparticle organic composite”, as used herein, is meant to refer to a composite consisting of metal nanoparticles and organic molecules, in particular organic linker molecules interlinking said metal nanoparticles.
The term “organic linker molecule”, as used herein, is meant to refer to flexible or rigid and linear or branched organic or metal-organic molecules comprising at least two functional groups that bind to said metal nanoparticles (“bi-functional” or “poly-functional” linkers).
The length of the organic linker is important for the sensitivity. A length of 5 to 30 methylene units (or equivalents) is preferred, a length of 10 to 30 methylene units (or equivalents) is more preferred, a length of 20 to 30 methylene units (or equivalents) is most preferred.
A functional group may be selected from a hydroxyl group, amino group, carboxyl group, carboxylic acid anhydride group, dithiol carboxylic acid group, mercapto/thiol group, disulfide group, thioether group, thioctic acid group, trithiocarbamate group, dithiocarbamate group, xanthate group, isothiocyanate group, isocyanide groups, tin, selen or mercury group.
Preferred organic ligands include molecules which contain a functional group that can be easily exchanged against another functional group when bound to a nanoparticle surface; for example, amines bound to gold nanoparticles can be exchanged with thiols.
Preferred organic linker molecules include C5-C30-alkane dithiols, such as nonanedithiol, decanedithiol, undecanedithiol, dodecanedithiol, etc. Other exemplary linkers, which can be used in accordance with the present invention, are disclosed in references [1] to [3].
The inventors have surprisingly found that the method of layer-by-layer drop/spin/spray coating as described herein offers the possibility to tune the relevant film parameters of composition, pore structure and degree of interlinkage, and thus allows to prepare a sensing material with the proposed optimal structure. In addition, the suggested drop-supported layer-by-layer self-assembly allows molecular imprinting of the composites as well as patterning of the material.
In the process according to the present invention, nanoparticles and organic linker molecules are alternately deposited by drop coating, spray coating, or spin coating on a substrate, preferably a transducer (see
The number of applied consecutive deposition cycles will determine the thickness of the film and is, thus, a method to tune the resistance of the chemiresistor sensor for a given interdigitated electrode structure. To finalize the process, after the selected number of deposition cycles a final wash (possibly with ultrasonic treatment) is suitable, to remove unbound organic molecules and to generate pores, which will possibly collapse when the film is drying. This collapsed structure may then swell, when VOCs are present in the environment.
To tune the pore size and structure, the molecules in the linker solution are important. The solvent as well as un- or weakly bound linker or exchanged ligand molecules may be entrapped during the preparation process and may be removed in the final washing step leaving voids in the material. To expand this concept, selected additional molecules (additives) can be used together with the linker in the organic solution during deposition. This would allow the possibility to shape the pore size and structure in a way that is suitable to host the additive. Due to the layered nature of the deposition process, non-volatile additives can be easily incorporated during the drop, spin or spray coating process. If a removal afterwards is possible by the correct washing treatment, the size and structure of the pores can be tuned. In the easiest case, solvents with low volatility or surplus of linker or ligand molecules can be imprinted. By deposition and removal of a carefully selected additive (e.g. the desired analyte) the desired pore can be generated.
Due to the fact that in the layer-by-layer drop, spin, or spray coating an immersion of the substrate into the solvent can be omitted, plastic and or flexible substrates can be coated. In case of solubility/swellability of the substrate in contact with the solvent, thin protecting layers e.g. SiO2 can be applied. This allows continuous or even roll-to-roll processes.
An advantage of the layer-by-layer drop coating/casting method compared to the proposed layer-by-layer spin or layer-by-layer spray coating is that only the required material necessary for film preparation is used for deposition in dilute solutions. This saves chemicals, and thus production costs, and is preferred due to environmental reasons. Also, the preparation of arrays is favoured by layer-by-layer drop coating/casting, as the deposition of different materials at different locations on the substrate, i.e. patterning, is possible (see Example 6 and structures shown in
During the proposed layer-by-layer drop coating method, the following parameters allow to influence the formation of the film, and thus the final performance of the sensitive coating:
In summary, the preparation method according to the present invention allows for:
The improved composite films for sensing obtained by this method exhibit
and allow for
Reference is now made to the Figures, wherein
The invention is now further described by means of the following examples, which are intended to illustrate the present invention and not to limit it.
All work has been performed under ambient conditions. If not otherwise stated, the linker concentration was 0.625 M in toluene. The Au nanoparticles were prepared according to a procedure from the literature [7] and their absorbance of the plasmon band was set to 1.0. Before coating, all samples were aminosilanized as described in the same literature as the nanoparticle synthesis and the layer-by-layer self-assembly procedure [7]. For the layer-by-layer drop coating, the commercially available device “NANOPLOTTER” (Gesim mbH, Groβerkmannsdorf, Germany) was used. For film formation, 40 nl/mm2 of the respective solutions were spotted in accordance with the method shown in
For a comparison of the different assembly methods layer-by-layer drop coating (present invention) and layer-by-layer dip coating, sensor composites from gold nanoparticles and dodecanedithiol (DT) were prepared and their sensing properties towards toluene 1-propanol, 4-methyl-2-penanone and water were investigated. A comparison of the sensitivities is shown in
The layer-by-layer drop-coated film showed for all analytes an at least 50% higher sensitivity than the conventional layer-by-layer dip coated material. This is due to the higher swelling ability thanks to a lower degree of interlinkage of the layer-by-layer drop coated film.
For a comparison of the different assembly methods, sensor composites from gold nanoparticles and dodecanedithiol (DT) were prepared under ambient conditions and their degrees of interlinkage and oxidation were investigated by X-ray photoelectron spectroscopy (XPS). The analysis is shown in
The XP spectra shown in
To show that the degree of interlinkage is variable during the layer-by-layer drop coating process, composites from gold nanoparticles (AuNP) and nonanedithiol (NT) were prepared. The concentration of NT in the linker solution relative to the nanoparticle concentration was varied over 3 orders of magnitude and the samples were studied by XPS. In
The substrate signal (Si 2s) is visible for films prepared with low linker concentration. All films are oxidized (SOx) due to ambient air as preparation atmosphere. The degree of interlinkage varies in the optimal preparation region, as seen by the ratio of S—H to S—Au (from 2:1 to 0.5:1). As expected, the lower the linker concentration, the higher the degree of interlinkage.
To show that the film composition is variable during the layer-by layer drop coating process, composites from gold nanoparticles (AuNP) and nonanedithiol (NT) were prepared. The concentration of NT in the linker solution relative to the nanoparticle concentration was varied over 3 orders of magnitude and the samples were studied with XPS. The variation in composition is shown in
Three different composition regions can be identified:
As the degree of interlinkage and composition is expected to influence the sensitivity of the material, composites from gold nanoparticles (AuNP) and nonanedithiol (NT) in the optimal region were prepared and their sensitivities towards 5000 ppm toluene, 1-propanol, 4-methyl-2-pentanone and water were investigated. The results are shown in
It was observed that the response increases with decreasing concentration of linker and decreasing degree of interlinkage. The reason is presumably that excess of linker are not bound chemically but are entrapped in the network. Thus swelling is less possible, as the entrapped unbound linker is not removed completely during the washing.
In summary, the optimal structure is not too interlinked to limit effective swelling (like in the layer-by-layer grown films) and the unbound excess material has to be washed out effectively to enhance sorption of the desired analyte. The degree of interlinkage is represented by the ratio of the functional groups bound to the nanoparticles relative to the total number of functional groups. A ratio between 5% to 80% is preferred, a ratio of 10% to 60% is more preferred, a ratio of 20% to 50% is most preferred.
Drop coating allows the deposition of the material on selected areas on a device. The same works for the proposed layer-by-layer drop-coating approach. Beside the savings of cost and time, this may additionally result in
| Number | Date | Country | Kind |
|---|---|---|---|
| 11007362.4 | Sep 2011 | EP | regional |
