Deposition of Tin Oxide Nanoparticles for Electrochemical Studies of Amyloid Peptides Paper

Deposition of Tin Oxide Nanoparticles for Electrochemical Studies of Amyloid Peptides

Alejandra M. De Jesús-Soto1, Kenny J. Colón-Colón2

1Department of Mathematics, University of Puerto Rico at Cayey, Puerto Rico2Department of Biology, University of Puerto Rico at Cayey, Puerto Rico

A B S T R A C T

Nanoparticles are microscopic materials that have physical dimensions ranging between 1-100 nm. Film depositions of Tin oxide nanoparticles were performed using DC magnetron sputtering, which is a physical process that vaporizes atoms from a solid target material in order to form a layer on a substrate. This part of the experiment was conducted at the University of Puerto Rico at Cayey. Then, sputtered Tin oxide nanoparticles will be tested with amyloid peptides in a cyclic voltammetry at University of Puerto Rico at Rio Piedras. Compounds obtained from the experiment showed decent deposition of the tin oxide nanoparticles. However, the Nanoparticles did not have a round appearance as wanted on Silicium substrate. Nanoparticles obtained after the deposition on Carbon glass substrate were closest to that required for the second part of the experiment. Adjustments of temperature and time exposure during film depositions will be performed in order to obtain an improved result.

Introduction

Nanoparticles are a group of microscopic materials that share physical dimensions ranging between 1 and 100 nanometers (nm). The use of nanoparticles in the field of medicine has been increasing due to the advantages they offer (Zhang et al. 2008). Nanoparticles are more accurate when they are needed to go directly to a target cell, cellular tissue, gland or groups of amino acids. Their size provides a greater surface area, which facilitates links to certain combinations of elements that would react when they reached the desired bio-compound. They could assure the development of enhanced and cost-effective tools for diagnosing a disease in a faster and more accurate process.

Some studies regarding the use of nanoparticles for treating diseases have developed certain interest in the neuroscience field, specifically in Alzheimer’s disease (AD). This disease is the most common chronic and progressive form of neurodegeneration of brains of patients that suffer from it (Brookmeyer et al. 2007). The disease goes in response of the deposition of β-amyloid (Aβ) peptides that contain nearly between 36-42 amino acids residues in the brain (Rauk. 2009; Rolinski et al. 2010).

The research of Lin Liu et al. (2013) suggest that the monomer form of the amyloid peptides (Aβ (1-16)) can serve as a

biomarker for diagnosing AD using gold nanoparticles and heme compound (iron) modified to them forming Aβ(1-16)-heme-AuNPs on a competitive assay. The product obtained was supposed get attached to a monoclonal antibody that was immobilized to the electrode of a cyclic voltammetry method which contained gold. These antibodies will be the ones attracting the N-terminus of the Aβ peptides and the Aβ (1-16)-heme-AuNPs. When the Aβ (1-16)-heme-AuNPs got attached to the monoclonal antibody on the electrode containing gold, readings from the cyclic voltammetry showed electrocatalytic O2 reduction. On another procedure of the same experiment, the electrode that contained gold was pre-incubated with Aβ, after adding the Aβ (1-16)-heme-AuNPs, the readings from the voltammetry responses showed a decrease of the reduction current of O2 to H2O2. This means that the competitive assay is sensitive and selective to Aβ peptides. The voltammetric responses varied with different concentrations of Aβ (0.02-1.5 nM) responding to a minimum limit of 10 pM (Liu et al. 2013).

The positive result of the experiment developed by Lin Liu et al. (2013) using gold nanoparticles has been of interest to Dr. Ana Guadalupe, a professor of the University of UPR at Río Piedras. She suggested the use of tin oxide nanoparticles sputtered in a carbon glass substrate. Then, Tin oxide nanoparticles will be tested with amyloid peptides in a cyclic voltammetry. She selected carbon glass as a substrate because of its inert characteristic. Tin oxide can be a reliable method for this research due to the fact that they work as

semiconductor with iron thin films, meaning that the heme compound would be added for reading the electrocatalytic signals from the voltammetry cycle when the monomer form of the Aβ peptide would be modified to it.

The new intended experiment starts with the sputtering protocol of the tin nanoparticles onto the carbon glass substrate. This part of the experiment would be held at the University of Puerto Rico at Cayey. First, the sputtering protocol would be tested using silicium as the substrate for assuring the correct standards of the vacuum chamber. The problem as set refers as to whether the sputtering method can work on a silicium substrate and on a carbon glass substrate using tin oxide nanoparticles. We hypothesized that sputtered Tin oxide nanoparticles on carbon glass substrate will be viable to continue studies with amyloid peptides. As the final product, it is required to have the substrate sputtered with the most rounded shaped and evenly dispersed nanoparticles as possible to assure that a larger surface area. This would help attach it to a ferrocene connector during the development of the other part of the experiment.

Materials and Methods

Film deposition was performed by DC (direct current) magnetron sputtering in a vacuum chamber, using a pure tin target in an Argon atmosphere. Basically, sputtering is a physical process in which atoms are ejected from a solid target material in order to form a layer on a substrate. In a vacuum environment, gas pressure is less than the ambient atmospheric pressure. Therefore it is also called low-pressure environment,

allowing a better flow of electrons and atoms.

Before starting the deposition, it is important to ensure that the chamber is clean. If necessary, Nitrogen was used to clean it. Then, the sample including the substrate was placed into the vacuum chamber in order to start the deposition process by sputtering. Also, a target was placed in the sputter source that has a magnetron (Figure 1). After closing the vacuum chamber, it was set to a base pressure of approximately 1 x 10-5 torr. Then, electrically neutral Argon atoms were introduced into it. A DC voltage placed between the target and substrate ionizes atoms and creates plasma. Plasma is a gaseous environment where there are enough ions and electrons for there to be appreciable electrical conductivity. Argon ions accelerate to the target. Their collision with the target ejects target atoms, which travel to the substrate and eventually settle, forming layers. Electrons released during Argon ionization are accelerated to the substrate, subsequently colliding with additional Argon atoms and creating more ions and free electrons in the process, continuing the cycle until the deposition time finishes. It is important to know that during this process, both temperature and time exposure can be adjusted. For better understanding of this procedure, see Figure 2. Before opening the chamber, it is important to remember to fill the chamber with nitrogen to balance the pressure from the outside. Sample was collected from the

chamber in order to see it in Scanning Electron Microscope (SEM)

SEM is a type of electron microscope that produces images of a sample by scanning it with a focused beam of electrons. Electrons interact with atoms in the sample, producing various signals that can be detected and that contain information about the sample's surface topography and composition.

Figure 1: Inside of a vacuum chamber and its parts.

Figure 2: What happens in a vacuum chamber during sputtering process? An atomic view.

Results

Figure 3: Results of the three depositions performed.

Compounds obtained from the experiment showed decent deposition of the tin oxide nanoparticles. On the other hand, the most expected part of the sputtering process, which was getting the nanoparticles in the roundish and most evenly dispersed way as possible, did not result as figured. However, it was recognized that the issue was due to maladjustments of temperature and time exposure of the substrates in the vacuum chamber. After using the silicium substrate, nanoparticles sputtered were too large in size and were not dispersed evenly because of the high temperature (160ºC) and the time exposure in the vacuum chamber (1 min). On the other hand, because of the room temperature into the vacuum chamber, nanoparticles were more evenly organized on the Silicium substrate. Nanoparticles did not have a round appearance as wanted. After reaching a certain balance of temperature and time exposure, carbon glass was used as a substrate. By exposing the nanoparticles to 150ºC during 10 seconds they reformed into a more spherical shape

and were almost no gaps between them. The carbon glass worked as similarly measured. However, it is still not the product wanted to be send for further research.

Discussion

The sputtering system is a versatile technique for depositing solid materials onto other substrates. In addition, this procedure assures the deposition of a film of tin nanoparticles over the exposed area of the silicium substrate and over the carbon glass substrate. Although the tested carbon glass substrate resulted nearly as figured, it is clear that the substrate has to have an even dispersion of the nanoparticles to assure the correct attachment of them to the monoclonal antibody and the ferrocine connector in the next part of the experiment. To make the nanoparticles more bound to the ferrocene connector it will require as much surface area to assure their attachment to it. Therefore, it is a concern that the product that would be developed for the study of amyloid peptides has the proper characteristics boundary for the most accurate results. There are still some adjustments that have to be done in order to

obtain an improved result. It is suggested that an adjustment of more time exposure would help cover those gaps that are still visible on the carbon glass substrate. The ferrocene will serve as a medium for connecting the Amyloid peptide with the electrode and the monoclonal antibody. It is expected that when the Aβ peptide gets attached to the monoclonal antibody, it would show response on the voltammetry reading, meaning that the compound functioned correctly, in a different way a tin oxide nanoparticle would respond when reaching to any other monoclonal antibody.

Acknowledgments

The authors would like to thank Dr. Wilfredo Otaño, Mr. Jose Cruz and the RISE Program at University of Puerto Rico at Cayey for their support during the entire project.

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