A Low-Cost Beam-Scanning Second Harmonic Generation Microscope with Application for Agrochemical Development and Testing

• Published in: Analytical chemistry (Washington) Vol. 91; no. 18; pp. 11723 - 11730
• Main Authors: Grubbs, Benjamin A, Etter, Nicholas P, Slaughter, Wesley E, Pittsford, Alexander M, Smith, Connor R, Schmitt, Paul D
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 17-09-2019
• Subjects: Agrochemicals; Agrochemicals - chemistry; Agrochemicals - pharmacology; Cereal crops; Chemistry; Construction costs; Corn; Crops; Crystallization; Dependence; Deposition; Diagnostic systems; Equipment Design; Fiber lasers; Galvanometers; Glass; Glycine max; Griseofulvin - chemistry; Griseofulvin - pharmacology; Laser beams; Lasers; Leaves; Limit of Detection; Low cost; Microscopy; Pesticides; Pesticides - chemistry; Pesticides - pharmacology; Plant Leaves - chemistry; Plant Leaves - drug effects; Rotenone - chemistry; Rotenone - pharmacology; Scanning; Second harmonic generation; Second Harmonic Generation Microscopy - economics; Second Harmonic Generation Microscopy - instrumentation; Soybeans; Spraying; Substrates; Triticum; Tunable lasers; World population; Zea mays
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/acs.analchem.9b02304

A low-cost second harmonic generation (SHG) microscope was constructed, and, for the first time, SHG microscopy was used for imaging agrochemical materials directly on the surface of common commercial crop leaves. The microscope uses a chromatically fixed (1560 nm) femtosecond fiber laser, a commercial 2D galvanometer mirror system, and a PCIe digital oscilloscope card, which together kept total instrument costs under $40 000 (USD), a significant decrease in cost and complexity from common systems (commercial and home-built) using tunable lasers and faster beam-scanning architectures. The figures of merit of the low-cost system still enabled a variety of measurements of agrochemical materials. Following confirmation of largely background-free SHG imaging of common crop leaves (soybean, maize, wheatgrass), SHG microscopy was used to image active ingredient crystallization after solution-phase deposition directly on the leaf surface, including at industrially relevant active ingredient concentrations (<0.05% w/w). Crystallization was also followed in real-time, with differences in crystallization time observed for different application procedures (spraying vs single droplet deposition). A strong dependency of active ingredient crystallization on the substrate was found, with an increased crystallization tendency observed on leaves vs on glass slides. Different crystal habits for the same active ingredient were also observed on different plant species. Finally, a model extended-release formulation was prepared, with a decrease in active ingredient crystallinity observed vs solution-phase deposition. These collective results demonstrate the need for making diagnostic measurements directly on the leaf surface and could help inform the next generation of pesticide products that ensure optimized agricultural output for a growing world population.