A. Madison, D. Westly, B. Ilic, C. Copeland, A. Pintar, C. Camp, J. Liddle, S. Stavis
National Institute of Standards and Technology,
United States
Keywords: nanoplastic, standards, microscopy, microspectroscopy, electron beam lithography
Summary:
Interest and concern in plastic nanoparticles are growing rapidly. Nanoplastic products have commercial applications ranging from optical probes1 to drug delivery2, whereas the unintentional release of nanoplastic byproducts into the environment poses a potential threat3. Optical microspectroscopy measurements of single nanoplastic particles are essential to fulfilling their potential as commercial products and assessing their hazards as environmental contaminants4. However, a lack of standards that are fit for purpose limits the accuracy of such measurements. In particular, the default format of a colloidal suspension has disadvantages for microspectroscopy calibrations, requiring sample preparation which results in, at best, disordered arrays of nanoplastic particles on featureless imaging substrates, and, at worst, particle agglomeration that confounds measurements of single particles. Moreover, existing nanoplastic standards can have ultrabroad and asymmetric distributions of optical properties, confounding inference of nanoplastic structures and requiring additional study5. Optical microspectroscopy systems employ several contrast mechanisms, including Rayleigh scattering, fluorescence emission, and Raman scattering to detect, quantify, and identify nanoplastic particles. Numerous issues limit accuracy, however, including optical responses that vary with particle size, shape, and refractive index5-7 and imaging systems that present nonuniform magnification, chromatic aberration, and focal variation8. These issues require standards that provide reference values of nanoplastic particle sizes, shapes, optical properties, and reference positions. The latter issue is entirely unexplored in this context, motivating new standards with multifunctional capabilities and spatial order. In this study, we introduce the concept of the nanoplastic array, addressing all of these issues. This prototype standard enables calibration, correction, and correlation of image data from various microscopy and spectroscopy systems, improving the accuracy of dimensional and optical property measurements of nanoplastic samples. To prove the concept, we fabricate nanoplastic arrays in nanoscale films of phenolic resin, containing fluorescent dopants, by electron-beam lithography. Our nanoplastic arrays (Figure 1 a-c) feature three types of nanostructures, each of which enables multiple calibrations. The simplest structure is a uniform film that enables correction of non-uniform irradiance for the accurate analysis of fluorescence and Rayleigh scattering intensity and that provides reference spectra for Raman measurements (Figure 1d). Building in complexity, a uniform pillar array with diameters of less than 1000 nm provides a repeating reference dimension and ordered reference positions for microscopy modes of electron scattering for size validation (Figure 1 e), and Rayleigh scattering (Figure 1f), fluorescence emission (Figure 1g), and Raman scattering (Figure 1h) for calibration. Finally, and most complex, variable pillar arrays with diameters ranging from 150 nm to 1000 nm in a repeating raster pattern facilitate systematic measurements of nanoplastic properties as a function of size. The smallest features of our nanoplastic arrays resemble point sources for calibration of the instrument response through focus (Figure 1i-j). Finally, fine gradations of the diameter of nanoplastic pillars enable quantification of the limiting signal-to-noise ratio at which detection of nanoplastics is possible under variable imaging conditions (Figure 1k-l). We expect that nanoplastic arrays will enable new order and accuracy in optical microspectroscopy, advancing the quantitative study of nanoplastic products and byproducts.