Molecular interaction analysis of lipid bilayer nanodomains by scanning probe nanospectroscopy in fluid environments

2019 Fiscal Year Research-status Report

• Project/Area Number: 19K15449
• Research Institution: Institute of Physical and Chemical Research
• Principal Investigator: BALOIS MariaVanessa 国立研究開発法人理化学研究所, 光量子工学研究センター, 基礎科学特別研究員 (00775083)
• Project Period (FY): 2019-04-01 – 2021-03-31
• Keywords: nanospectroscopy / Raman spectroscopy / surface science / optics

Outline of Annual Research Achievements

I have been able to finalize the designs of both the illumination and the detection optical systems. Through theoretical calculations done via FDTD simulations and electromagnetic field analysis using Mathematica, I determined the optimum field distribution needed to illuminate the sample and the specifications of the optics needed to generate this field. The necessary components were bought and assembled into the system. I have also conducted FDTD simulations to optimize the best shape and position of the Au probe that would give the highest amount of tip-enhancement and therefore, the highest signal from the sample. These simulations consider two kinds of environment for the sample: (1) the sample is dry and exposed to air and (2) the sample is in a liquid environment. To make the probe, a protocol was developed to electrochemically etch Au wires (diameter = 100 microns) to make a sharp Au probe (diameter ~ 32 nm).
The scanning probe head design has also been finalized and a home-made prototype scanning probe head has been successfully made. The head was able to approach and scan the surface of a chromium standard test sample (an array of square pits) using a tungsten tip as a test tip. A highly reproducible methodology was made to (1) make the probe mount, (2) mount the Au tip and (3) recycle the probe mount once the Au tip is not usable.
The design of the environment chamber that will house the entire system has been made and construction will be finished by July 2020. The sample-holding microfluidic device has been designed and the fabrication items have been gathered.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

Having a strong background in theoretical optics, I was able to conduct theoretical calculations related to the electromagnetic field generation based on the required experimental parameters of the system I am developing (numerical aperture of objective lens, wavelength of laser, thickness of sample, etc.) The necessary software to do the calculations (Mathematica and Lumerical - FDTD) were readily available in the laboratory. Therefore, the calculations could be done right away. Based on these calculations, I was able to determine the necessary optics I needed to procure and gather in order to build up the system. Fortunately, these items were procured early and avoided any delays caused by the current pandemic happening around the world.
Although the environment chamber is still under construction, I do not think that this will delay the project. For fabricating the Au probe, I had prior experience in making such kinds of probes therefore, it was not so difficult to learn how to electrochemically etch the Au wire to make the probe. To make the scanning probe head with Au tip, fortunately, there are equipment readily available in the laboratory that would help me manipulate the Au wire to mount it on the scanning probe head.
The research is progressing as scheduled even though there are times when we should do telework.

Strategy for Future Research Activity

From now, the fabrication of the lipid bilayer sample needs to be learned and done. Four kinds of samples need to be made: (1) self-assembled monolayers (SAM) which are homogeneous and made of benzenthiol, (2) mixed SAM layers: alkanethiol and biotynilated alkanethiol, (3) artificial lipid bilayers, and (4) real lipid bilayers in a liquid environment. Necessary items needed to create the SAMs and lipid bilayers will be purchased.
The microfluidic device that will house these samples needs to be fabricated. Training will be needed to operate electron-beam lithography machines that will make the microchannels on the microfluidic device. After making the microfluidic device, further optimization of the illumination, detection and scanning probe is needed after the introduction of the biological sample into the system. Testing the system on a liquid sample will done after conducting tests on the dry samples.
Since many Au tips will be used in the course of the experiments, learning how to efficiently fabricate batches of Au tips is necessary. The electrochemical etching system needed for making the Au tips will be designed and the necessary items to make this system will be purchased.
Preparation for upcoming conferences will also be made. The targeted conferences are: Japan Society of Applied Physics Autumn and Spring conferences (domestic), the International Conference on Advanced Vibrational Spectroscopy (international), and the SPP (Physics Society of the Philippines) (international).

Causes of Carryover

In the next fiscal year, I will be focusing on (1) nanospectroscopy experiments using electrochemically etched Au tips, (2) making the samples and microfluidic device and (3) reporting results in journals and conferences (both international and domestic).
The electrochemical system needed to etch the Au tips will be constructed in the start of the coming fiscal year and a considerable amount of Au wires will be needed for the experiments. This is especially important since the Au tips are non-recyclable and if any of the tips do not show plasmonic enhancement, these tips shall be discarded right away. Au will also be used to coat the sample chamber in the microfluidic device in order to (1) mount the sample and (2) create gap mode plasmons when the Au tip is present.
Samples will be made in the next fiscal year that will be used in the nanospectroscopy experiments. Self-assembled monolayers and artificial lipid bilayers are needed to create these samples. Since 4 kinds of samples will be used, 4 different microfluidic devices will be made since the samples should not be mixed with each other.