Journal
ACS APPLIED NANO MATERIALS
Volume 1, Issue 3, Pages 1430-1437Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsanm.8b00268
Keywords
optical moisture sensor; angle-independent resonance; surface lattice resonance; soil moisture monitoring; water scarcity
Funding
- National Science Foundation (NSF) [CMMI-1562884]
- NSF/DMR/Polymer program [DMR-1410253]
- NSF MRSEC Program [DMR-1120901]
- NSF SUNFEST-Summer Undergraduate Research in Sensor Technologies-REU Award [1359107]
- Directorate For Engineering
- Div Of Engineering Education and Centers [1359107] Funding Source: National Science Foundation
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Plasmonic nanostructures provide excellent platforms for colorimetric sensors in chemical, biological, and environmental applications. In contrast to the existing library of plasmonic sensors, We report an angle-independent optical sensor that is designed for monitoring soil moisture and operating on rough surfaces. The optical moisture sensor is constructed by coating hydrogel on top of an ultrathin, plasmonic Au nanorod lattice array, where the refractive index changes of the hydrogel upon exposure to moisture are transduced into spectral shifts of the resonances of the array. A modified Langmuir adsorption isotherm model is used to capture the dynamics of water adsorption and desorption at the interface between the sensor and the ambient environment. The nanorod length and the nanorod array pitch are systematically tuned to decouple the localized surface plasmon resonance of the nanorods and the Rayleigh anomalies of the nanorod array, creating sensors with angle-independent resonances (similar to 0.2 nm/deg). As a proof of concept, we place the sensor on uneven soil surfaces and demonstrate the consistent sensor resonance shift that only depends on the soil wetness. Robust, eco-friendly optical moisture sensors with angle-independent resonances provide a promising sensing platform for smart soil moisture monitoring important to tackle the challenge of water scarcity in agriculture.
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