Electrolyte Solution Topography Imaging of Various Organic Samples

In-situ characterization of a sample is sometimes necessary to identify and understand the root cause of a problem and then develop an ideal countermeasure. A number of cutting edge applications follow this necessity such as:

• The study of dynamic and static biomechanics at the cellular level
• The study of membrane transport of ions between cells
• The study of failure mechanisms in battery electrodes

Common to all of these applications is the need to carry out in-situ measurements of a sample experiencing a phenomenon while in an electrolyte solution. Methods such as atomic force microscopy (AFM) have been used for in-liquid imaging of samples before, but AFM cannot be regarded as an appropriate solution.

Organic samples often tend to swell and soften when immersed in liquid, increasing their chances of being damaged by an AFM tip. A wet sample sways and changes position during scanning when there is even the slightest movement of an AFM probe. Since the challenges of a wet sample cannot be easily met with a method requiring direct probe-sample contact, an alternative to AFM is preferred.

This requirement has been met with the development of scanning ion conductance microscopy (SICM) and a new instrument called the Park NX10 SICM system.

In SICM, sample topography is obtained by modifying the ionic current flowing via the opening of a glass pipette as it passes across the surface of a sample, all while in an electrolyte solution. This mechanism applies no pressing force onto a sample and is considered to be completely non-invasive. Doing so prevents the chances of accidentally swaying a sample in an in-liquid setup and also overcomes AFM's limitations with respect to soft and delicate wet samples.

Experiments

The Park NX10 SICM is based on the Park NX10 AFM platform. The hardware is virtually the same with the only exception of an SICM head replacing the conventional AFM head on the system. The SICM head uses either a glass pipette with an inner diameter ranging from 80 to 100 nm or one made of quartz with an inner diameter of 30 to 50 nm instead of using an AFM tip to sense the tip-sample interaction force and image the sample topography.

After filling the pipette with an electrolyte solution, it is linked to an AgCl/Ag electrode while another electrode is linked to the sample in liquid (Figure 1a). This forms a closed circuit, and with an applied bias between the two electrodes, the ionic current passes through the pipette and then reaches the sample (Figure 1b).

As the pipette comes closer to the sample, the current reduces and the relationship between current and displacement (Figure 1c) is tracked by the accurate and fast feedback loop of the system. The current drops to zero when the pipette truly touches the sample surface.

It is possible to use the degree of current decrease to back calculate the surface topography. The surface is scanned by the pipette at a given current set-point (normally 99%), highlighting that it always maintains a distance of a few hundred nm away from the surface.

Similar to scanning tunneling microscopy (STM), which leverages the tunneling current to characterize conductive materials' surfaces, SICM monitors the current change in order to offer both non-contact and no-force imaging.

SICM also eliminates cantilever tuning which can complicate non-contact in-liquid AFM imaging. This method provides extremely stable imaging and quantitative data, and also makes it possible to observe extra sensitive or soft biological materials at nanoscale, including live cells.

Figure 1. The snapshots showing the (a) SICM hardware setup, (b) circuit mechanism and (c) current-distance relationship between the pipette end and the sample surface.

SICM imaging has two types of modes. The first is approach-retract scan (ARS) mode, which is extensively applied to samples with features higher than 1 µm. Due to the high variation in height, the pipette can easily break if used during a continuous scan on such samples.

Instead, the pipette in ARS mode approaches the sample until it reaches the set-point and completely retracts to a pipette-safe height before moving on to scan for the next pixel in the image. The processes of approach and retract are repeated at each pixel of the image to be created until it completes the entire scan.

Direct current (DC) mode is the second mode, which is mainly for samples with features that are within a few hundred nm in height. At a given current set-point, the pipette continuously scans on the surface indicating that the distance between the pipette and the sample surface is fixed.

Results and Discussions

Three representative samples, collagen fibrils, polycarbonate membrane with 400 nm pores, and polydimethylsiloxane (PDMS), were used to illustrate the accuracy and ease of use of SICM. ARS or DC mode was selected based on the materials' properties.

Polydimethylsiloxane (PDMS): PDMS was selected to be imaged first as it is a material used to develop standards for SICM system calibrations. PDMS has a variety of other uses including being a key component of contact lenses.

In-situ analysis of PDMS provides insight into how it acts in environments for specific application environments. With contact lenses, one environment would be the surface of the human eye, which is constantly moistened by basal tears, a biological lubricant containing electrolytes.

Park Systems provided two PDMS standard samples with varied geometries and these were imaged. One is the 117.5 nm high bar shaped grid, and the other is the XY square shaped grid, pitch size 10 µm. The electrolyte solution used was the phosphate-buffered saline (PBS) standard solution from Thermo Fisher Scientific.

Figure 2. Topography image of the XY standard sample acquired by DC mode. Scan size 20µm × 20µm. Image size 215 px × 215 px.

Figure 3. Topography image of the XY standard sample acquired by (DC mode. Scan size 20 µm × 20 µm. Image size 256 px × 256 px.

The topography images shown in Figures 2 and 3 were obtained by DC mode of the XY and Z standards, respectively. These images effectively show high contrast, unambiguous surface features. Park Systems’ XEI image processing software was used perform all image post-processing including quantification analysis.

For the XY standard sample, the ~160 nm grids height and the 10 µm pitch-to-pitch distance are accurately revealed (Figure 2) and for the Z standard sample, the height information recovered from DC mode is relatively close to the preferred value of 117.5 nm.

Poretics polycarbonate membrane: Nanopores are holes that measure several nanometers across and exist in thin membranes. The membranes can be strong sensors of ions and molecules due to the penetration of such molecules through the pores, and this feature is used in a wide range of fields such as medicine, biology, chemistry, and engineering.

Current improvements in nanotechnology can precisely monitor the morphology and the chemical and physical properties of the pores in order to increase their attractiveness to regulate and sense transport at the molecular level ^[1]. Nanopore membranes can be used for high-throughput nanoparticle filtration or separation with certain chemical modifications ^{[2, 3]}.

Extensive research was also performed in the modeling, characterization, and fabrication of nanopore membranes. Traditional AFM generally depends on the interaction forces between the sample surface and a probe tip. However, it is not sufficient enough to test the sharp geometry of a sample membrane surface or the ionic transportation through its pores. SICM has been shown to be an efficient method to accomplish both ^{[4, 5]}.

The poretics polycarbonate membrane was imaged with SICM was supplied by GE Water & Process Technologies, laced on a PDMS substrate, and immersed in PBS solution with SICM. Only ARS mode was used on this sample to prevent the pipette from being damaged by the potential depth of the pores.

Figure 4 displays the pore sizes obtained by SICM. The data reveals that the pores have an average depth of about 600 nm and an average diameter of approximately 416 nm. More advanced work with this type of sample has been performed, such as detecting the transport activity of individual pores ^{[4, 5]}.

Figure 4. Topography images of 400 nm diameter pore polycarbonate membrane recorded in PBS solution with SICM ARS mode. The average pore depth is ~ 600 nm.

Collagen fibrils: Collagen fibrils are a commonly accepted standard sample for measuring soft material and biological properties. These fibrils soften and swell after being rehydrated and can sway if disturbed with an AFM probe, which is similar to many biological samples.

Collaborators at Niigata University in Japan provided the third sample, which helped to demonstrate SICM's potential to image a sample whose nanoscale topography would otherwise be difficult to obtain with other microscopy methods.

After cutting and spin-casting the collagen fibrils on a petri dish, they were imaged with a Park NX10 system in PBS solution with SICM. Again, only the ARS mode was used due to the height difference of the collagen fibrils. All of the tests were performed with the help of the XEP software.

Figure 5 shows how the protein bundles as well as each individual fibril can be accurately identified in the 10 x 10 µm-sized images at a pixel resolution of 256 x 256. The thinnest fibril that is distinguished by SICM (indicated by the black arrow) is just 90 nm in width. This resolution is quite impressive as there is no real contact or force between the sample surface and the end of the pipette.

Figure 5. (a) Topography and (b) contrast enhanced topography images of collagen sample imaged in PBS solution with SICM ARS mode. Individual fibril can be clearly identified and the thinnest one observed is about 90 nm in width, as pointed out by the black arrow. Scan size 10× 10µm. Image size 256  × 256 px.

Conclusion

The topography images of all three samples were accurately and efficiently obtained in buffered solution using the Park Systems NX10 SICM. This innovative method completely resolves the problems existing in standard in-liquid AFM investigations and offers a no-force solution appropriate for all materials, particularly sensitive and soft ones.

The high performance and easy use of SICM enable researchers to acquire high-quality in-liquid images that will improve their understanding of the performance of the samples in different solutions at the nanoscale.

References and Further Reading

1. Adiga, S.P., et al., Wiley Interdiscip Rev Nanomed Nanobiotechnol, 2009. 1(5): p. 568-581.
2. Anmiv, S.P., et al., J. Phys. Condens. Matter, 2010. 22(45): p. 454107.
3. Tokarev, I. and S. Minko, Adv. Mater, 2010. 22(31): p. 3446-3462.
4. Chen, C., Derylo, M.A. and Baker, L.A., Anal. Chem., 2009. 81(12): p.4742-4751
5. Zhou, Y., Chen, C. and Baker, L.A., Anal. Chem., 2012. 84: p.3003-3009

This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.

For more information on this source, please visit Park Systems Inc.