Glass pipette and method for manufacturing the same

Abstract

To provide a glass pipette which has a small outer diameter / inner diameter ratio and small inner diameter to an extent that is impossible to achieve using laser puller widely used to manufacture glass nanopipettes in a conventional way.SOLUTION: A glass pipette of the present invention has an inner diameter of 20 nm or less and an outer diameter / inner diameter ratio of 1.8 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a glass pipette, and particularly to a glass pipette suitable for use as a probe of a scanning ion conductance microscope (SICM: Scanning Ion Conductance Microscopy) and a method for manufacturing the same.

Background Art

[0002] A scanning probe microscope (SPM: Scanning Probe Microscopy) that measures the surface shape and characteristics of a sample by using the interaction between a probe and the sample surface is known. Among them, SICM is an SPM that uses a glass pipette (glass nanopipette) as a probe. In SICM, electrodes are provided inside and outside the nanopipette filled with an electrolyte, so that an ionic current flows through the tip opening of the nanopipette when a bias is applied. When the nanopipette is brought close to the sample surface while keeping the applied bias constant, the value of the ionic current flowing through the tip of the nanopipette changes. By setting a threshold value for this ionic current change and recording the position of the nanopipette or the sample when the threshold value is exceeded, the shape of the sample surface can be obtained. In addition, measurements of the charge and mechanical properties of the sample surface are also performed by analyzing the change in the ionic current with respect to the distance between the pipette and the sample. Generally, the spatial resolution of SICM can be improved as the tip of the nanopipette and the sample surface are brought closer. However, on the other hand, since SICM is characterized by the ability to perform surface measurement with low invasiveness without the nanopipette contacting the sample surface, it is an important factor in SICM measurement that the tip of the nanopipette can be brought as close as possible without contacting the sample surface. In particular, when scanning the nanopipette as a probe along the sample surface, it is necessary to avoid contact on a rough surface.

[0003] Non-patent document 1 reports simulation results showing that the smaller the OD / ID ratio of the probe tip (nanopipette tip), the less likely it is to come into contact with the sample surface. Specifically, when using probes with OD / ID ratios of 1.72 and 1.28 to approach a sample with a large incline at the same set point, there is a difference of approximately 20 degrees in the maximum incline of the sample that can be approached.

[0004] Furthermore, from the perspective of spatial resolution in SICM, not only the OD / ID ratio but also the small ID value of the probe (nanopipette) tip is extremely important. Non-patent document 2 reports that when the probe tip and the sample surface are at a distance of approximately ID, spatial resolution up to about 1.5 times the ID is possible. Therefore, the present invention was arrived at after investigating glass pipettes (nanopipettes) with a small ID and a small OD / ID ratio. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Del Linz,et al.Anal.Chem.86(5)2353(2014). [Non-Patent Document 2] Rheinlaender and TESchaffer, Anal. Chem. 89, 11875 (2017). [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The present invention aims to provide a glass PET with a small outer diameter / inner diameter ratio and a small inner diameter, which is impossible to achieve with conventional methods such as laser puller fabrication, which are widely used to produce glass nanopipettes. [Means for solving the problem]

[0007] The glass pipette according to the present invention is characterized by having an inner diameter of 20 nm or less and an outer diameter / inner diameter ratio of 1.8 or less. In conventionally known glass pipettes (glass nanopipettes), those with an inner diameter (ID) reduced to about 10 nm have an outer diameter (OD) exceeding 20 nm, resulting in an OD / ID ratio of 2 or more. Glass pipettes with a sufficiently small OD / ID ratio were not known. In contrast, the glass pipette according to the present invention has an inner diameter of 20 nm or less, and an outer diameter / inner diameter ratio, or OD / ID ratio, of 1.8 or less, resulting in a glass pipette of unprecedented nanoscale size.

[0008] The glass pipette according to the present invention is suitable, for example, as a probe for a scanning ion conduction microscope and is effective in improving spatial resolution. In this case, the glass pipette according to the present invention is more preferably characterized by an inner diameter (ID) of 15 nm or less and an OD / ID ratio of 1.6 or less.

[0009] The glass pipette according to the present invention is manufactured by applying predetermined heat and tensile force to a glass capillary, and then subjected to alkaline etching treatment with an acid solution filled inside the glass pipette. Here, this technique is referred to as asymmetric etching.

[0010] In symmetric etching, where a glass pipette is simply immersed in an etching solution, both the outside and inside of the glass pipette are etched simultaneously, making it difficult to keep the outer diameter / inner diameter ratio (OD / ID ratio) small. Therefore, the present invention is characterized in that, with the inside of the glass pipette filled with an acidic solution, the entire glass pipette or its tip is immersed in an alkaline solution, thereby neutralizing the inside of the glass pipette with the acidic solution while etching its outer shape, which reduces the OD / ID ratio.

[0011] The type of packing solution and alkaline etching solution used inside a glass pipette are selected according to the material of the capillary. However, for applications where nanoscale etching control is possible, using an HCl solution as the packing solution and a KOH solution as the etching solution is practical in terms of cost and ease of handling when the capillary is made of glass. [Effects of the Invention]

[0012] Using the glass pipette according to the present invention results in a small OD / ID ratio and a small ID, which allows for approaching samples with large surface gradients in SICM, and provides excellent spatial resolution. Generally, sharpening the tip of this type of nanoprobe can potentially be achieved through microfabrication using a focused ion beam (FIB) device. However, for glass (nano)pipettes with a tip diameter of about 50 nm or less, the tip shape itself is extremely difficult to observe with a FIB device. Furthermore, since the tip of each glass pipette must be processed individually with a FIB device, the processing process is time-consuming, resulting in an extremely low throughput and making it difficult to reduce costs. In contrast, the method according to the present invention uses wet etching, which allows for batch processing of the manufacturing process and reduces costs compared to the method described above. [Brief explanation of the drawing]

[0013] [Figure 1] This shows the relationship between the inner diameter (ID) (nm), outer diameter / inner diameter ratio, and OD / ID ratio in glass pipettes. Regression equation (a) shows the case where the entire glass pipette is immersed in a 6M KOH aqueous solution without filling the inside with HCl. Regression equation (b) shows the glass pipette material fabricated using a CO2 laser puller. [Figure 2](a) shows the etching time and the change in the tip shape when the glass pipette material is immersed in 6M KOH aqueous solution without filling anything inside. (b) shows the etching time and the change in the tip shape when 6M HCl is filled inside the glass pipette material and the entire glass pipette material is immersed in 6M KOH aqueous solution. (c) shows the etching time and the change in the tip shape when 6M HCl is filled inside the glass pipette material and the tip of the glass pipette material is immersed in 6M KOH aqueous solution. [Figure 3] Graphs of (a), (b), and (c) in Fig. 2 are shown respectively. [Figure 4] (a) shows a wide-area imaging image of the PDMS grating, and (b) shows a schematic diagram of the PDMS grating. [Figure 5] (a) shows an imaging image at the nanoscale in Fig. 4, and (b) shows its line profile. [Figure 6] Imaging images of (a) sp = 1% and (b) sp = 8%, and (c) shows the line profiles of (a) and (b). [Figure 7] Imaging images of (a) sp = 2%, (b) sp = 4%, (c) sp = 8%, and (d) sp = 12%, and (e) shows the line profiles of (a) to (d).

Embodiments for Carrying out the Invention

[0014] The glass pipette according to the present invention and its manufacturing example will be described below, but the present invention is not limited thereto.

[0015] <Manufacture of Glass Pipette Material> Using a non-crystalline quartz glass capillary with an outer diameter of 1.0 mm and an inner diameter of 0.3 mm, a glass pipette material with an outer diameter / inner diameter ratio (OD / ID ratio) of 2.0 level, where the inner diameter is at the 10 nm level and the outer diameter is at the 20 nm level, was obtained by applying a predetermined tensile force and heating with a CO2 laser puller device (P - 2000, Sutter Instrument).

[0016] <Etching Treatment> The tip of the glass pipette was filled with ultrapure water by injecting ultrapure water into the inside of the aforementioned glass pipette material and heating it at 80°C for 30 minutes. Next, the interior was replaced with a 6M HCl aqueous solution using the temperature odor control method, and then the entire glass pipette material or its tip was immersed in a 6M KOH aqueous solution for etching. For comparison, a glass pipette material was immersed in a 6M KOH aqueous solution without any internal filling. Figures 2(a) to 2(c) show typical TEM image examples, and Figures 3(a) to 3(c) show graphs of the changes in etching time, inner diameter (ID) nm, and outer diameter / inner diameter ratio (OD / ID ratio). Figures 2(a) and 3(a) show glass pipette materials that have not been filled with anything and have been immersed in a 6M KOH aqueous solution. Although the OD / ID ratio decreases with etching time, the inner diameter becomes very large. In contrast, glass pipettes in which 6M HCl is packed inside and the entire pipette (b) or tip (c) is alkali-etched have their inner diameter suppressed while the outer surface is etched, resulting in an OD / ID ratio of 1.5 to 1.3.

[0017] <SICMによるイメージング> Using a glass pipette prepared by filling the inside with a 6M HCl aqueous solution and etching the entire material with a 6M KOH aqueous solution for 2 hours, the PDMS grating was observed using SICM. Figure 4(a) shows the imaging results over a wide area, and Figure 4(b) shows a schematic diagram of the PDMS grating used for imaging. Furthermore, Figure 5(a) shows the imaging results at the nanoscale, and Figure 5(b) shows the line profile. This shows that the PDMS grating is being imaged.

[0018] In SICM, to achieve high resolution, it is necessary that the tip of the probe (glass pipette) does not easily come into contact with the sample surface when brought close to it. In typical SICM imaging, the set point (sp), the threshold at which the ion current decreases as the probe tip approaches the sample surface, is often set to less than 1%. This is to ensure that the approach velocity of the probe tip to the sample surface, which is related to the time resolution of SICM, is at an appropriate value, while avoiding contact with the sample surface. The set point, changes in the imaging image, and line profile were compared using the glass pipette according to the present invention. Figure 6 shows imaging images of the edge portion of PDMS at (a) sp=1% and (b) sp=8%, and (c) the line profiles of (a) and (b). This result shows that with sp at 1%, the image has blurred edges, while increasing it to 8% makes the boundaries clearer. Next, Figure 7 shows the imaging images and line profiles obtained by gradually increasing the sp value to 2%(a), 4%(b), 8%(c), and 12%(d). As a result, we can see that structures that were not visible at sp=2% in Figure 7(a) are clearly visible at sp=12% in Figure 7(d). From the above, it is clear that being able to set a high sp in SICM imaging is extremely important for observing nanoscale structures. Using a glass pipette with a high OD / ID ratio can cause the tip of the glass pipette to come into contact with the sample surface, potentially superimposing noise on the imaging or, in some cases, even damaging the sample or probe. In contrast, using a glass pipette with a small inner diameter and a small OD / ID ratio according to the present invention reduces the aforementioned risks and improves resolution.

[0019] Figure 1 summarizes the useful glass pipette sizes based on the experimental results described above. In the graph shown in Figure 1, regression equation (a) shows the outer diameter / inner diameter ratio (OD / ID ratio, y in the regression equation) relative to the inner diameter (ID) nm (x in the regression equation) of a symmetrically etched glass pipette with a 6M KOH aqueous solution without any packing inside. Regression equation (b) shows the relationship between the inner diameter and the OD / ID ratio when manufactured using a CO2 laser puller. The glass pipette according to the present invention is located in the range to the lower left of regression equation (a). In the diagram, Symmetric is a glass pipette that has been alkali-etched entirely without any internal filling, Asymmetric is a pipette that has been filled with 6M HCl and then alkali-etched entirely, and Asymmetric (tip) is a pipette that has been filled with 6M HCl and then alkali-etched at the tip. From Figure 1, it is preferable that the inner diameter is 20 nm or less and the outer diameter / inner diameter ratio is 1.8 or less.

Claims

1. A method for manufacturing a glass pipette having an inner diameter of 20 nm or less and an outer diameter / inner diameter ratio of 1.8 or less, A method for manufacturing a glass pipette, characterized by performing an alkaline etching treatment while the inside of the glass pipette is filled with an acid solution.

2. A method for manufacturing a glass pipette according to claim 1, characterized by etching the inside of the glass pipette with a KOH solution.

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