Ion sensor

ION SENSOR

FIELD OF THE INVENTION

The invention relates to an ion sensor having its electrode covered with a water-repelling organic chemically adsorbed film, wherein at least one hole permitting the passage of ions and preventing the passage of biopolymers is formed in the organic chemically adsorbed film, thereby permitting accurate measurements of ion density over a long period.

BACKGROUND OF THE INVENTION

The invention provides an ion sensor, which can be used even in a solution containing numerous impurities. More specifically, the invention provides an ion sensor, implanted in a living body, capable of monitoring activities in the body.

When a particular ion in solution is oxidized or reduced by an electrode, an oxidation or reduction current flows to the electrode. The value of the electric current is generally correlated to ion concentration. Thus, this principle has been utilized for developing ion sensors in many fields. For example, since many neurotransmitters are oxidized or reduced by a platinum and carbon electrode, a particular nervous tract of the brain can be simply and easily examined by implanting these microelectrodes in the brain.

Ion sensors are excellent for observing ions in a solution immediately; however, they have the problem that they are incapable of providing reliable data when in a solution containing numerous impurities likely to stick to the surface of the electrode. Particularly, in measuring a neurotransmitter of a living body by using a conventional ion sensor, organic molecules in a body fluid such as proteins stick to its electrode, and the sensor becomes incapable of measuring the neurotransmitter. This has been a serious problem because a sensor which is implanted in the body cannot be frequently taken out and re-implanted into the body.

SUMMARY OF THE INVENTION

An objective of the invention is to provide an ion sensor which can be applied even in a solution containing numerous impurities.

In order to accomplish the above objective, the ion sensor of the invention comprises an electrode with its surface formed with a thin film by covalently bonding water-repelling chemical admolecules, wherein at least one hole permitting the passage of ions and preventing the passage of biopolymers is formed in the chemically adsorbed thin film.

It is preferable in this invention that the size of the hole preventing the passage of biopolymers is in the range of 0.5-50 nm2.

It is preferable in this invention that the chemical admolecule is fixed to the electrode surface via covalent bonding containing siloxane groups (—SiO—).

It is preferable in this invention that the chemically adsorbed thin film is a monomolecular film.

It is preferable in this invention that the chemically adsorbed thin film is formed on an inorganic siloxanebased inner layer.

It is preferable in this invention that the thickness of the chemically adsorbed thin film is in range of 1-10 nm.

2

It is preferable in this invention that the molecule of the chemically adsorbed thin film contains a hydrocarbon or fluorocarbon group. It is preferable in this invention that the electrode is at 5 least one material selected from the group consisting of platinum, glassycarbon, silicon, gold and aluminum.

It is preferable in this invention that the procedures for manufacturing an ion sellsor comprise: dipping and holding an electrode having hydrophilic 10 groups on its surface ill a chemical adsorbent prepared by dissolving or dispersing chemical admolecules, containing water-repellent and chlorosilyl groups, and mask molecules with a physical adsorption property on their surfaces in a nonaque15 ous solution:

physically adsorbing the mask molecules to the electrode surface, thereby forming a chemically adsorbed film by a condensation reaction between the chemical admolecules and the chlorosilyl groups; 20 and

washing away the unreacted chemical admolecules and the mask molecules with a nonaqueous solution.

It is preferable in this invention that the mask mole

25 cule is at least one chosen from the group consisting of a carbocyanin and pyridinium compounds having molecular weight in a range of 500-1000.

According to the invention, impurities are not likely to stick to an electrode with its surface covered with a

30 water-repelling organic chemically adsorbed thin film. Particularly, when using such a sensor in a living body, organic compound (organic polymers) such as proteins rarely stick to the electrode. Micromolecules such as ions can reach the electrode by passing through a hole

35 on the scale of dozens of angstroms in diameter formed in the organic thin film; however, macromolecules such as protein can not penetrate the hole. Therefore, compared with the conventional microscopic electrode, the electrode of this invention has a superior reliability in

40 the sense that it cannot be contaminated by impurities on a scale larger than dozens of angstroms in diameter.

In the above-noted composition, the size of the hole is in the range of 0.5-50 nm2, thereby preventing the passage of biopolymers such as proteins while permitting

45 the passage of micromolecules such as ions.

An organic chemically adsorbed thin film, which is bonded to an electrode surface via covalent bonds comprising siloxane groups (—SiO—), is not easily peeled off, thereby providing an ion sensor with superior bio

50 logical adaptability, safety and endurance.

Moreover, the chemically adsorbed thin film is a monomolecular film with a uniform thickness.

It is also possible to increase the density of chemical admolecules by forming a chemically adsorbed thin film

55 on an organic siloxane-based inner layer.

If the thickness of the chemically adsorbed thin film is in range of 1-10 nm, it is easy to measure ions in a living body.

A water-repelling property can be effectively demon60 strated if hydrocarbon or fluorocarbon groups are contained in the molecules of the chemically adsorbed thin film.

In case that an electrode comprises at least one selected from the group consisting of platinum, glassycar65 bon, silicon, gold and aluminum, it is easy to form a chemically adsorbed thin film.

An ion sensor of the invention can be efficiently provided by the following procedures:

BRIEF DESCRIPTION OF THE DRAWINGS

10

preparing a chemical adsorbent by dissolving or dispersing chemical admolecules containing water-repellent and chlorosilyl groups and mask molecules with a physical adsorption property in a nonaqueous solution;

dipping and holding an electrode having hydrophilic groups on its surface in the prepared chemical adsorbent, thereby physically adsorbing mask molecules to the electrode surface; forming a chemically adsorbed film by a condensation reaction between hydrophilic groups on the electrode surface and the chlorosilyl groups of chemical admolecules; washing away unreacted chemical admolecules and 15

mask molecules with a nonaqueous solution. The hole, permitting the passage of micromolecules such as ions and preventing the passage of biopolymers such as protein, is accurately formed when the mask molecule is at least one chosen from the group consist- 20 ing of a carbocyanin and pyridinium compounds having molecular weights in a range of 500-1000.

25

40

FIGS. l(a)-l(c) are flow diagram showing the formation of an organic chemically adsorbed film in an embodiment of the invention.

FIG. 2 is a view showing the process of manufacturing an electrode in an embodiment of the invention. 30

FIG. 3 is a view showing the process of manufacturing an electrode in an embodiment of the invention.

FIG. 4 is a view showing the process of manufacturing an electrode in an embodiment of the invention.

FIG. 5 is a view showing the process of manufactur- 35 ing an electrode in an embodiment of the invention.

FIG. 6 is a view showing the process of octadecyl trichlorosilane being covalently bonded to an electrode surface in an embodiment of the invention.

FIG. 7 is a view showing the process of OTS being bonded to each other via siloxane coupling in an embodiment of the invention.

FIG. 8 is a view showing absorption spectrums of an organic chemically adsorbed film formed on a platinum 45 electrode in an embodiment of the invention.

FIG. 9 is a view showing a cyclic voltammogram measured by an electrode in a test solution immediately after dipping and holding the electrode in the solution in an embodiment of the invention. 50

FIG. 10 is a view showing a cyclic voltammogram measured by an electrode in a test solution after dipping and holding the electrode in the test solution for twelve hours, in an embodiment of the invention.

55

DETAILED DESCRIPTION OF THE
INVENTION

As examples of mask molecules of the invention, carbocyanin and pyridinium compounds or the like are shown in the following Formulas 1 or 2.

60

Formula 1

65

where ni represents 0,1 and 2, n2 represents 9 to 20 and n3 represents 9 to 20. (ni = 1 and ... 18 are preferable.)

In the case wherein ni = l and ... the compound is N,N'-dioctadecyloxacarbocyanine for Formula [1] and N,N'-dioctadecylthiacarbocyanine for Formula 2. In the case wherein ni=0 and ... the compound N,N'-dioctadecylindocarbocyanine for Formula 1 and N,N'-dioctadecyl-4,4'bipyridinium for Formula 2.

The silane-based surface active agent comprising alkyl groups is a reagent which can be chemically adsorbed to an electrode substrate via siloxane coupling. Such an active agent includes a trichlorosilane-based chemical adsorbent, such as ... a dichlorosilane-based chemical adsorbent, such as ... and ... or a monochlorosilane-based chemical adsorbent, such as CH3(CH2)„SiCl(CH3)2 and ... (where n represents 0-25. but most preferably 10-20). Among these examples, the trichlorosilane-based chemical adsorbent is preferred in that a siloxane coupling is formed on an electrode substrate surface and between adjacent molecules, thereby permitting the formation of a more firmly adsorbed film.

The silane-based surface active agent comprising fluoroalkyl groups can be a reagent which is chemically adsorbed to an electrode substrate via siloxane coupling, and such an agent include trichlorosilane-, monochlorosilane- or dichlorosilane-based chemical adsorbents.

Trichlorosilane-based chemical adsorbents include the following examples: ... ... ... ... ... CF3COO(CH2)i5SICl3; ...

Monochlorosilane- or dichlorosilane-based chemical adsorbents with lower-alkyl groups substituted are shown in the following examples:

...

CF3(CF2)7(CH2)2SiCl „(C2H5)3.n;

...

...

...

...

...

...

... where n represents 1 or 2.

Among these examples, a trichlorosilane-based chemical adsorbent is preferred in that a siloxane coupling is formed on an electrode substrate surface and between adjacent molecules, thus permitting the formation of a more firmly adsorbed film.

In incorporating a vinyl group (C=C) or acethyl group (ethynyl group) in an alkyl or fluoroalkyl group portion, the formed chemically adsorbed film can be crosslinked by being irradiated with an electron beam of about 5X106 rads, thus further improving the firmness of the chemically adsorbed film. The chlorosilane-based surface active agents capable of use according to the 5 invention are not limited to those in the form of a straight chain as noted above. It is possible to use agents with branched fluoroalkyl or hydrocarbon groups or those with silicons at one end substituted by fluoroalkyl or hydrocarbon groups (i.e., R2SiCl3, RsSiCl, 10 RiR2SiCl2, ... or the like, where R, R1, R2 and R3 represent fluoroalkyl or hydrocarbon groups). To increase the adsorption density, however, the straight chain form is preferred.

The nonaqueous solvent used to form a chemically 15 adsorbed film comprising alkyl or fluoroalkyl groups on an electrode substrate via siloxane coupling may be any organic solvent so long as it does not have active hydrogen which can react with the chlorosilane-based surface active agent. Any of the solvents including fluorine-, 20 hydrocarbon-, ether- and ester-based solvents can be a preferable organic solvent.

Examples of fluorine-based solvents are as follows:

1,1 -dichloro, 1 -fluoroethane;

l,l-dichloro,2,2,2-trifluoroethane; 25 l,l-dichloro-2,2,3,3,3-pentafluoropropane; 1,3-dichloro, 1,1,2,2,3-heptafiuoropropane; trifluoroalkylamine;

perfluorofuran and its fluoroalkyl derivative. Hydrocarbon-based solvents include the following: 30 hexane; octane; hexadecane; cyclohexane; etc. Ether-based solvents include the following: dibutylether; dibenzylether; etc. Ester-based solvents include the following: methyl acetate; ethyl acetate; isopropyl acetate; amyl 35 acetate; etc.

A single layer of a monomolecular chemically adsorbed film can be formed on an electrode substrate simply by following the procedures mentioned below:

preparing a solution by dissolving a chlorosilane- 40 based surface active agent in a nonaqueous organic solvent mentioned above;

dipping and holding the electrode substrate in the solution, thereby promoting a dehydrochlorination reaction between the hydrophilic groups on the 45 substrate surface and the chlorosilyl groups of the agent;

washing away the unreacted agent with a nonaqueous solvent; and

reacting the substrate with water. 50

As an electrode substrate used for the invention, any substrate with hydrophilic groups such as hydroxyl groups projecting from its surface may be used. However, platinum, glassycarbon, silicon, gold and aluminum are particularly excellent as electrode materials. 55

The invention is further described below by referring to the following practical embodiment.

A platinum wire 100 several urn in diameter was inserted in a glass tube 101 of 1 mm inside diameter with one end closed. The open end of the tube was con- 60 nected to a vacuum pump, and the inside of the tube was evacuated to a vacuum (FIG. 2). Glass tube 101 was melted by hearing the outside with a gas burner, and platinum wire 100 was inserted in the vacuum tube. Then, the end of the tube was ground with sand paper 65 and diamond paste until platinum wire 100 emerged from the surface (FIG. 4). The end of the tube was again ground with emery paper (manufactured by Buehler

The unreacted material was washed away with a nonaqueous solution, in this case chloroform. The electrode was then washed and reacted with pure water.

By the above-noted treatments, OTS 2 was chemically bonded to platinum electrode 1 via siloxane coupling, and a compound 3 in Formula 3 was physically adsorbed to the electrode (FIG. l-a and b). Only physically adsorbed compound 3 (Formula 3) was washed away with chloroform. After washing the electrode with water, the bond between each monomolecular film changed to siloxane bonds and a hole 4 was made on the spot where compound 3 existed. The film chemically adsorbed on the electrode surface is stable, and has holes at the molecular level (FIG. 1-c). In other words, the film has holes permitting the passage of ions while preventing the passage of biopolymers. The size of the hole can be easily controlled by changing the size of the mask molecules.

The procedures of covalent bonding between OTS and an electrode are more specifically described below.

After contacting the electrode surface with OTS, silanol bonds were formed by a dehydrochlorination reaction between the chlorosilyl groups of OTS and the hydroxyl groups on the surface (FIG. 6). The unreacted material was washed away with nonaqueous chloroform solution, and the electrode was washed with water, thereby substituting the remaining chlorosilyl groups with silanol groups. The electrode was then dried; as a result, the silanol groups were dehydrated and became siloxane bonds (FIG. 7).

The density of the holes in the OTS film on the electrode surface was estimated in the following ways:

Platinum was deposited on a glass substrate in a 1000 angstrom layer by vacuum deposition, providing an electrode. The electrode was dipped and held in a solution prepared by dissolving 30 mM/L OTS and 5 (xM/L of the compound in Formula 3 (a mixed solution of n-hexadecane, chloroform and carbon tetrachloride at a volume ratio of 80:8:12) for two hours. After removing the substrate from the solution, the substrate, which gained a water-repelling property, repelled the solution, and only a monomolecular film covalently bonded or physically adsorbed to the substrate stayed on the substrate.

The absorption spectrum of the compound contained in the film was measured by a reflection method. The results of absorption spectrum are shown in FIG. 8. In FIG. 8, curve A indicates the results of OTS containing the compound while curve B shows the results of OTS without the compound. In considering molecule occupying area of the compound as 1.7 m2, the number of