JP4246656B2 - Reversible control method of wettability of object surface - Google Patents

Description

  The present invention relates to a method for greatly and reversibly changing the wettability (hydrophilicity / hydrophobicity) of an object surface.

  Substances usually show hydrophilicity (oleophobic) or hydrophobicity (lipophilicity) and their properties do not change naturally. However, it is possible to change the wettability of the object surface by coating the surface of the substance (object) with an organic film. In recent years, a method for reversibly changing the wettability of an object surface has also been proposed.

  One of the methods for reversibly changing the wettability of the object surface is a method using adsorption / desorption of organic molecules to the object surface. In this method, by applying a voltage to an object, organic molecules are adsorbed or desorbed on the object surface, thereby changing the wettability of the object surface (Non-Patent Document 1).

  As another method, there is a method using a shape change of an organic molecule composed of a hydrophilic portion and a hydrophobic portion bonded to an object surface. In this method, organic molecules are oriented at a low density and approximately perpendicularly, and the charge on the object surface is changed to reversibly change the molecular shape so that a hydrophilic or hydrophobic part appears on the object surface. Change wettability. (Non-patent document 2).

  In addition, there is a method in which a conductive polymer film is made hydrophilic by doping a predetermined amount of ions according to the film thickness, and the film is made hydrophobic by dedoping (non-patent document). Reference 3). In this method, the amount of ions doped is changed by controlling the current flowing through the object.

  Furthermore, there is a method of changing the contact angle of water or the like by changing the voltage applied between the electrode provided with the fluoropolymer layer and the liquid in contact with the polymer (Non-Patent Documents 4, 5, and 6). .

  As another method, the wettability of the object surface is reversibly changed by light irradiation using organic molecules that dimerize when irradiated with light at a certain wavelength and become monomerized when irradiated with light at another wavelength. There is a method (Non-Patent Document 7).

  If the wettability of the object surface can be reversibly changed, for example, by changing the wettability of the substrate, the image displayed on the screen can be changed by controlling the movement of the ink placed on the substrate. It is possible to adjust the liquid flow rate by changing the wettability of the liquid transport tube wall of the analytical instrument (Non-Patent Document 4, Non-Patent Document 6).

Nicholas L. Abbott et al., “Langmuir”, (USA), 1995, 11, pp.16-18 Joerg Lahann et al., "Science" (USA), 2003, 299, pp.371-374 M. Niemitz et al., “Die Angewandte Makromolekulare Chemie” (Germany), 1991, 185, pp.147-154 Anthony Quinn et al., "Science", (USA), 2003, 107, pp.1163-1169 M. W. J. Prins et al., “Journal of Physical Chemistry B” (USA), 2001, 291, pp.277-280 Robert A. Hayes et al., "Nature" (UK), 2003, 425, pp.383-385 Scott Abbott et al., “Langmuir” (USA), 1999, 15, pp. 8923-8928

In the method of Non-Patent Document 1, there is a problem that some molecules are lost by diffusion during the adsorption / desorption process.
The method of Non-Patent Document 2 cannot be used for the purpose of requiring a large change in wettability such as hydrophobicity and hydrophilicity, since the degree of change in wettability is small. In addition, since organic molecules are oriented at the object surface at a low density, there is a problem that the interaction between the organic molecules is weak and the film is unstable.
In the method of Non-Patent Document 3, it is necessary to flow a large current of 0.1 mC / cm ² to the object in order to dope the object with an amount of ions whose wettability changes. .
Further, in the methods of Non-Patent Documents 4 to 6, it is necessary to apply a high voltage of 10 to several hundreds V or to confine the droplets in the microelectrode, so that it cannot be easily performed.
In the method of Non-Patent Document 7, it takes several minutes or more for the molecule to completely change its structure. Further, since the change in wettability before and after the structural change is small, it cannot be used for purposes requiring a large change in wettability.

  As described above, the conventional method is not satisfactory as a method for controlling the wettability of the object surface, such as the wettability of the object surface cannot be changed greatly or cannot be easily implemented. There wasn't.

  The problem to be solved by the present invention is to provide a method that can greatly change the wettability of an object surface and can be easily implemented.

In order to solve the above problems, the present invention has a binding functional group on a conductive layer formed on an object surface, which has a binding functional group that easily reacts with the conductive layer at one end of a molecular chain to form a chemical bond. The organic thin film is formed by self-organizing organic molecules having oxidation-reduction ability at the other end to form an organic thin film, and a voltage applied to both ends of the conductive layer is adjusted to provide a gradient potential in the surface of the organic thin film. The surface property control method is characterized in that the hydrophobic region or the hydrophilic region on the surface of the object is controlled by the above.

  In the present application, the “binding functional group” refers to a functional group that easily reacts with a metal or other conductive layer to form a chemical bond. Further, the “functional group having redox ability” (hereinafter referred to as “redox functional group”) refers to a functional group that is easily oxidized and reduced and is positively or negatively charged.

  Organic molecules are arranged almost vertically on the conductive layer (self-organization) due to the reaction between the binding functional group and the conductive layer and the interaction between adjacent organic molecules (self-organization). Form. At this time, since organic molecules are bonded to the conductive layer on the surface of the object by the binding functional group, the redox functional group appears on the surface of the thin film.

  The redox functional group is oxidized or reduced by adjusting the voltage applied to the conductive layer. By oxidizing or reducing the redox functional group on the surface side, the surface changes from hydrophilic (oleophobic) to hydrophobic (lipophilic), or conversely, from hydrophobic (lipophilic) to hydrophilic (oleophobic). ). The direction of this change depends on the type of organic thin film. For example, when ferrocene is used as the redox functional group, the potential of the organic thin film becomes higher than the redox potential of ferrocene (-0.5 V when the redox potential of gold oxide AuOx is used as a reference). Is oxidized to a ferricinium cation. If the potential is lower than that, the ferricinium cation is reduced and converted to ferrocene. When the redox functional group changes to a ferricinium cation, the surface becomes hydrophilic (oleophobic), and when the ferricinium cation changes to ferrocene, it becomes hydrophobic (lipophilic). Thereby, the wettability of the surface of an object can be changed.

  If the potential of the entire organic thin film becomes higher (or lower) than the redox potential of the redox functional group, the entire surface of the object exhibits hydrophobic or hydrophilic properties. On the other hand, when the surface potential of the organic thin film is inclined so as to cross the redox potential of the redox functional group, the surface of the object is divided into a hydrophilic part and a hydrophobic part at the place where the redox potential is crossed. .

  As described above, the oxidation-reduction of the redox functional group can be performed reversibly by adjusting the voltage applied to the object. Therefore, according to the method of the present invention, the wettability of the object surface can be reversibly changed.

  As the conductive layer provided over the substrate, gold, platinum, silver, copper, an alloy thereof, ITO, or the like can be used. Alternatively, a semiconductor whose conductivity is increased by doping a high concentration of impurities may be used.

  Highly reactive functional groups such as thiol groups, carbonyl groups, amino groups, phosphoric acid groups, sulfonic acid groups, phosphonic acid groups, or selenium (-SeH) are used as the binding functional groups of the molecules constituting the organic thin film. be able to. When germanium or a silicon semiconductor is used for the conductive layer, a functional group having a double bond at the terminal can also be used.

  As a redox functional group of an organic thin film molecule, a functional group that is easily oxidized and reduced electrochemically, such as ferrocene, ruthenium complex, quinones, and positively or negatively charged in an oxidized or reduced state, should be used. Can do.

  The structure of the organic molecule is not particularly limited as long as it has a functional group having redox ability at one end of the molecular chain and a binding functional group at the other end, and can form a self-assembled monolayer. It is desirable to use an organic molecule in which a binding functional group and a redox functional group are bonded to both ends of an alkyl chain having 8 or more carbon atoms. In this case, the van der Waals force between the alkyl chains of each molecule tends to form a stable film with high density and highly aligned orientation.

According to the method of the present invention using molecular redox, the surface of an object can be changed from hydrophobic to hydrophilic (or vice versa) by applying a voltage of several volts to the object.
The method according to the present invention can be used in, for example, a chemical analysis system using a micro flow channel, and the flow rate of the liquid and the flow channel can be changed by changing the wettability of the flow channel.

  Further, in the method according to the present invention, the wettability is controlled using an ultrathin film (thickness of several nm) made of organic single molecules, so that there is no coloring due to the formation of the thin film. Therefore, it can be used for purposes requiring transparency. For example, it is also possible to control the movement of image display ink of an image display device such as electronic paper by controlling wettability in a narrow region.

In the following, an embodiment of the method according to the invention will be described.
A glass plate on which a gold layer (conductive layer) having a thickness of 100 nm is formed is immersed in a 1 mM methanol solution of 11-ferrocenyl-1-undecanthiol (manufactured by Dojin Chemical Laboratory Co., Ltd.). Raised. As a result, a self-assembled monolayer of ferrocenylundecanethiol was formed on the gold layer (FIG. 1).

The substrate on which the self-assembled monolayer of ferrocenylundecanthiol was thus formed was immersed in a 0.1 M HClO ₄ aqueous solution, a Pt electrode was placed opposite thereto, and a DC voltage was applied as shown in FIG. By changing this voltage, the amount of current flowing between them was measured. In addition, all the potentials below are potential values on the gold layer (organic thin film) side based on the oxidation-reduction potential of AuOx.

  FIG. 3 is a graph showing voltage-current response characteristics when the voltage change rate (sweep rate) is changed to 10, 25, 50, 100, and 200 mV / s. Regardless of the voltage sweep rate, the current density peak due to oxidation of ferrocene and reduction of ferricinium cation appears at -0.6 to -0.5V.

Next, nitrobenzene exhibiting hydrophobicity was dropped into an aqueous HClO ₄ solution. Since nitrobenzene does not dissolve in water, nitrobenzene droplets were formed on the surface of the organic thin film.

  When the voltage change of the droplet was observed, the droplet shrunk when the voltage was increased (contact angle about 70 degrees). Conversely, when the voltage was lowered, the droplet spread on the substrate surface (contact angle approximately 25 degrees) (photo of Fig. 3). From this, it was shown that when the voltage is increased, the oxidation of ferrocene proceeds and the substrate surface becomes hydrophilic, and when the voltage is decreased, the reduction of ferricinium cation proceeds and the substrate surface becomes hydrophobic.

  FIG. 4 shows a change in the contact area between the droplet and the substrate when a 3 μL nitrobenzene droplet is placed on the substrate and the voltage is swept at a speed of 10 to 200 mV / s. The contact area was measured by attaching a crystal resonator to the substrate and measuring the resonance frequency of the substrate using a 53131A universal counter (manufactured by Agilent).

At the reduction potential of −0.8 V, the contact angle of the droplet with respect to the substrate was about 25 °. As the potential is increased, the contact area between the droplet and the substrate decreases, and the contact angle increases. Regarding the voltage change rate, even when the sweep rate is 200 mV / s (approximately 2 seconds from end to end of the potential width), the shape of the droplet changes almost in real time, and the response to voltage application is relatively fast. all right. In addition, the drop shape change rate of the reduction process (ferricinium cation → ferrocene) was slightly delayed compared to the rate of the oxidation process (ferrocene → ferricinium cation). This seems to be because in the reduction process, the droplet shape must be changed against the gravity of the droplet.
As described above, it has been shown that the wettability of the substrate surface is changed by changing the ferrocene appearing on the substrate surface to a ferricinium cation.

  Next, it was confirmed using the apparatus of FIG. 5 that the wettability of the surface can be changed for each place in the same plane. For the measurement, a substrate in which a ferrocenylundecanthiol self-assembled monolayer was formed on a glass plate on which an extremely thin (100 nm) gold layer was laminated was used. The reason why the gold layer is made extremely thin is to increase the resistance of the conductive layer so that a potential gradient is formed in the conductive layer. Note that in the case where the resistivity of the conductive layer itself is high as in the case where a semiconductor conductive layer or the like is used, it is not always necessary to use a thin film.

  The apparatus shown in FIG. 5 has a power source connected to points A and B on the substrate, and a potentiostat connected to point A on the substrate and a counter electrode (Pt electrode). The potentiostat is also connected with an AuOx electrode as a reference electrode.

Hereinafter, the voltage applied between the substrates AB is referred to as a bias voltage, and the voltage applied between the point A of the substrate and the counter electrode is referred to as a reference voltage.
The point A in FIG. 5 is the origin, and the AB direction is the x axis. Further, the voltage was set so that the bias voltage became negative toward the point B, and the reference voltage was set to be on the positive side with respect to the oxidation-reduction potential of ferrocene.

  In this device, almost no current flows between the substrate and the counter electrode except the ferrocene electrochemical reaction (oxidation-reduction reaction) that occurs on the substrate surface. Almost no longer flows. For this reason, the voltage across the entire substrate is substantially equal to the bias voltage.

FIG. 6 shows a case where a steep potential gradient is formed in the substrate by keeping the reference voltage constant (reference voltage Eoffset = -300 mV) and gradually increasing the bias voltage (Vbias) to negative (final voltage -0.7 V). It is the photograph showing the mode of the shape change of the nitrobenzene droplet (hydrophobicity) in three different positions. As the potential gradient became steeper, the contact area of the nitrobenzene droplet closer to the point B (left side of the photograph) with the substrate increased. This is because the hydrophobic region spreads from the B point toward the A point by making the reference voltage constant and increasing the bias voltage (Vbias) negative (Vbias1 → Vbias2) (FIG. 7).
Thus, it was shown that the position where the wettability of the surface changes in the substrate plane can be changed by changing the bias voltage.

Figure 8 shows three different positions when the bias voltage is constant (Vbias = -0.6V), the reference voltage is increased negatively, and the potential in the substrate is changed (change from -300mV to -400mV). It is the photograph showing the mode of the shape change of the nitrobenzene droplet in. By increasing the reference voltage negatively, the contact area of the droplet closer to the origin with respect to the substrate is increased. This is because the hydrophobic region spreads from the B point toward the A point by making the bias voltage constant and increasing the reference voltage negative (Eoffset1 → Eoffset2) (FIG. 9).
Thus, it was shown that the position where the wettability of the surface changes in the substrate plane can also be changed by changing the reference voltage.

Nitrobenzene droplets (2 mm in diameter) were moved in the plane using the change in wettability in the same plane by forming a potential gradient (FIG. 10). At this time, the bias voltage was kept constant (Vbias = -0.5V), and the reference voltage was changed.
(a) First, a nitrobenzene droplet was placed on the hydrophilic region (Eoffset = −270 mV).
(b) Thereafter, the reference voltage was increased negatively to bring the boundary between the hydrophilic region and the hydrophobic region closer to the droplet.
(c) When the droplet contacted the hydrophobic region, the liquid in the portion in contact with the hydrophobic region expanded the contact area toward the hydrophobic region. At this time, the liquid located in the hydrophilic region did not move (Eoffset = −340 mV).
(d) The reference voltage was further increased negatively so that the droplet was almost completely located in the hydrophobic region.
(e) Thereafter, the reference voltage was changed in the positive direction so that a part of the droplet was positioned in the hydrophilic region. As the reference voltage was changed, the contact area of the droplet located in the hydrophilic region with respect to the substrate became smaller and became a contracted shape.
(f) The reference voltage was changed so that the droplet was completely located in the hydrophilic region (Eoffset = −270 mV). As a result, the droplets were completely shrunk to the original shape. However, the position of the droplet on the substrate at this time moved from the position (a). If the above operation is repeated, the droplet can be moved to the end of the substrate.

  As described above, the droplet can be moved on the substrate by changing the position of the boundary where the wettability changes in the same substrate. In the above description, the droplet is moved by changing the reference voltage. However, it is also possible to move the droplet by changing the magnitude of the bias voltage.

  From the above, according to the method of the present invention, it was shown that the wettability of the object surface can be reversibly changed by applying a voltage as small as several volts to the object.

In the above, an aqueous HClO ₄ solution was used as an electrolyte solution, but an electrolyte other than an aqueous HClO ₄ solution may of course be used.
In the above description, the hydrophobic nitrobenzene droplets are moved on the surface of the object. However, it is also possible to move hydrophilic droplets such as water using an organic electrolyte in the same manner.

In a chemical analysis system using a micro flow channel, by applying a voltage between the liquid transport tube wall and the liquid to be transported to change the wettability of the liquid transport tube wall, It is possible to change.
Further, in an image display device such as electronic paper, the substrate is finely divided, and the movement of the ink for image display is controlled by controlling the wettability of the substrate for each of the divided areas, thereby displaying an image. It is also possible to change the state.
Furthermore, it is also possible to produce an upholstery product in which droplets move or change shape in a container filled with water or the like.

The figure showing the mode at the time of the oxidation reduction of a ferrocenyl undecanethiol self-assembled monolayer. The schematic block diagram of the apparatus for a voltage-current response characteristic measurement. The graph which shows the voltage-current response characteristic of the board | substrate which formed the self-organization molecular film of the ferrocenyl undecane thiol on the surface. The graph showing the change of the contact area of the droplet with respect to the board | substrate at the time of a voltage sweep. The schematic block diagram of the apparatus for changing the wettability in the same plane. A photograph showing how the shape of a nitrobenzene droplet changes when the bias voltage is changed while keeping the reference voltage constant. (a) The figure showing the relationship between the electric potential on a board | substrate, and wettability, (b) The figure showing the change of the relationship at the time of changing bias voltage. A photograph showing how the shape of the nitrobenzene droplet changes when the reference voltage is changed with a constant bias voltage. (a) The figure showing the relationship between the electric potential on a board | substrate, and a wettability, (b) The figure showing the change of the relationship at the time of changing a reference voltage. A photograph showing the movement of a nitrobenzene droplet using the change in wettability in the same plane.

Claims (4)

  A self-organizing organic molecule having a binding functional group that easily reacts with the conductive layer at one end of the molecular chain to form a chemical bond on the conductive layer formed on the surface of the object and has redox ability at the other end An organic thin film is formed by orienting vertically in order to control the hydrophobic area or hydrophilic area of the object surface by adjusting the voltage applied to both ends of the conductive layer and providing a gradient potential in the organic thin film plane. A method for controlling surface physical properties.   The surface property control method according to claim 1, wherein the conductive layer is a metal thin film. A self-organizing organic molecule having a binding functional group that easily reacts with the conductive layer at one end of the molecular chain to form a chemical bond on the conductive layer formed on the surface of the object and has redox ability at the other end An organic thin film is formed by orienting in a substantially vertical direction, and the voltage applied to both ends of the conductive layer is alternately changed in both positive and negative directions so that droplets on the object surface are alternately positioned in the hydrophilic region and the hydrophobic region. method of moving the droplet on the object surface reversibly by changing the boundary position between the hydrophobic and hydrophilic regions of the object surface such that.   4. The method of reversibly moving droplets on an object surface according to claim 3, wherein the conductive layer is a metal thin film.