Grantee Research Project Results

Final Report: Infrared Hyperspectral Microscope for Rapid Characterization of Microplastics

EPA Contract Number: 68HERC20C0019
Title: Infrared Hyperspectral Microscope for Rapid Characterization of Microplastics
Investigators: Yeak, Jeremy
Small Business: Opticslah, LLC
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2020 through August 31, 2020
Project Amount: $100,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2020) RFA Text |  Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Clean and Safe Water

Description:

The EPA has identified a need for new methods and instrumentation to characterize size, shape, and composition of microplastics, especially in the size range of 1 µm – 1 mm. To meet the needs identified by EPA, we seek to develop a portable sensor for improved microplastic sampling and characterization useable at remote measurement sites or fixed installations. The sensor would be connected to water sampling lines placed in regions where microplastic characterization is desired. Particulates in the size range 10 µm – 1 mm filtered from the water stream will be probed using high-performance infrared laser hyperspectral imaging/microscopy, based on swept-external cavity quantum cascade lasers. This powerful spectroscopic imaging technique will measure size and shape of particles collected from the microscope images, while at the same time determining the chemical composition via infrared spectroscopic analysis. A wide range of pure and weathered polymer/plastic materials will be identifiable using this technique and distinguished from inorganic particles or biological materials. By automating sample collection, measurement, and analysis, while eliminating the costly and labor-intensive steps of sample purification and cleaning, the sensor will operate continuously and autonomously. This operation mode will allow large volumes of data to be collected for microplastic characterization at multiple sites, as is needed to improve the understanding of the full effects of microplastics on the environment and human health. The technology will be immediately usable by researchers at academic institutions or government research laboratories to improve quality and quantity of microplastic data. In the future, these sensors would be installed in fixed locations at industrial sites, municipal water supplies, or other areas at high risk for microplastic contamination, ultimately allowing online monitoring for safety or regulatory purposes. The technology developed may also be applied to new markets where particle characterization is required, such as environmental monitoring or detection of explosives.

Summary/Accomplishments (Outputs/Outcomes):

The goal of the Phase I effort was to show feasibility of the measurement concept for microplastics characterization. Initial measurements in Phase I involved setting up custom benchtop hyperspectral microscopy systems including a swept-ECQCL (external cavity quantum cascade laser) system, magnification optics, sample substrates, an infrared detector/camera, and a visible camera. Microplastic particles of different compositions, sizes, and shapes were characterized using the ECQCL system. Plastic films of known thicknesses and compositions were used for system characterization and for acquisition of polymer library/reference spectra.

Mesh screens of different metals, and with different wire/pore sizes were characterized for suitability as sample substrates. The laboratory results were used to develop initial hyperspectral imaging data acquisition and analysis software for measuring and determining chemical composition of measured microplastics. The results of the Phase I research were used to determine the feasibility of the measurements and inform the design requirements for construction of a full prototype system in Phase II.

For the Phase I work, we selected a range of common polymer plastic materials for study. The polymers were all obtained as thin plastic films or sheets. By using films/sheets we were able to measure reference absorption spectra for the polymers, for direct comparison with microplastics which were generated from the exact same materials. The plastics were used to measure reference absorption spectra using the swept-ECQCL system. The plastics were also used to generate micro-plastic particles by grinding/shredding the films using sandpaper.

We measured transmission/absorption spectra of the polymer films using the swept-ECQCL, to serve as a spectral library for identification of microplastics. We developed a new method to increase the dynamic range of the measurements for the polymer films, to allow better quantification of the strongest absorption features. It was confirmed that different polymers exhibit different absorption spectra in the wavelength range probed by the swept-ECQCL system, as expected. Detection of absorption from polymer microparticles with thickness < 1 µm will be possible for many polymers.

Opticslah researched different optical designs for the micro-plastic hyperspectral microscopy laboratory system and constructed various laboratory measurement experiments. The first design for a hybrid-mode transmission microscope used a visible CMOS camera to acquire high- magnification visible images of the microplastics. Infrared absorption spectra were acquired by focusing the ECQCL beam to a spot size ~ 20 µm and measuring the transmitted intensity with a single-element infrared detector. Infrared spectra were acquired point-by-point with 20 µm spatial resolution by scanning the sample underneath the focused beam, and hyperspectral images were acquired by moving the sample over a grid of points. Investigation and analysis of the hyperspectral images showed that the micro-plastic particles could be imaged to show spatial features with the visible camera while at the same time the infrared absorption spectrum could be measured and used for identification of polymer type.

Attempts to use an infrared array detector (FLIR camera) for hyperspectral imaging were mostly unsuccessful. The problem was identified as the reflective microscope objective, whose properties were found to be incompatible with the broad-area infrared laser illumination needed for imaging. However, the reflective objective was shown to form excellent images in the visible and was able to focus the ECQCL beam to spot sizes of 15-20 µm, providing near diffraction-limited performance.

As part of the research, we also observed noticeable photo-thermal excitation of polymer micro- particles. The observation of a detectable change in the visible image due to applied infrared light is extremely significant, because it means we may be able to eliminate the infrared detector or camera entirely, and instead simply use the visible CMOS array for detection. Furthermore, the lower cost, higher speed, and larger array formats of visible cameras would provide other advantages over infrared array detectors. Finally, the use of a photothermal effect for detection would remove the need to focus the visible and infrared light through the same objective lens, opening up more possibilities to optimize the visible imaging system for larger field-of-views.

Conclusions:

The Phase I research showed that hyperspectral images could be obtained from micro-plastic particles, with the high-resolution visible images providing size/shape information and the infrared absorption spectrum providing material identification. A hyperspectral microscope configuration using a single-element infrared detector was found to provide better infrared imaging performance than configurations using infrared array detectors, primarily due to limitations with the all-reflective microscope objective used in the research. Alternate designs for the hyperspectral microscope were identified as promising for further development.

The poor infrared imaging performance of the reflective objective lens, combined with the poor optical properties of the micro-mesh substrates, indicates that the original concept for the system needs to be revised. Based on the Phase I research, we have therefore identified alternative concepts for infrared hyperspectral imaging which do not rely on reflections from the micro- mesh materials.

Hyperspectral imaging can be performed using the reflective objective in a hybrid-mode transmission or reflection microscope. In this configuration, the reflective objective can be used to focus the infrared beam to a small spot, while at the same time producing a high-quality visible image. Galvanometer-based scan mirror arrangements will be used to improve acquisition speed for the hybrid configuration using a single-element detector. Alternatively, we will investigate design of a custom objective lens to provide optimal imaging performance at infrared wavelengths, while still allowing transmission of visible wavelengths.

A new approach which can be pursued in parallel will use a photothermal mechanism to transfer the infrared absorption information into the visible image for detection. Preliminary measurements showed that the photo-thermal effect was observed in visible images due to expansion of microplastic particles when absorbing resonant infrared light from the applied ECQCL. We are currently investigating novel approaches to photo-thermal microscopy of micro-plastics.

There is an important and identified need for new methods of microplastic characterization. The primary disadvantage of the existing techniques is that they are time-consuming and labor- intensive. The faster speed and autonomous operation of the swept-ECQCL hyperspectral microscopy instrument will provide high immediate value to researchers. Onsite measurement is also an important but unmet need for researchers, which the swept-ECQCL product will address. We are currently reaching out to researchers and potential commercialization partners to gather more information and better identify the immediate and long-term needs of the market.

The technology developed may also be applied to new markets where particle characterization is required, such as environmental monitoring, detection of explosives, and medical imaging. New instruments for infrared spectroscopic characterization of solid materials will have additional market potential, for example in high-speed infrared spectroscopic ellipsometry or bidirectional reflectance distribution functions measurements.