Demanding applications in sensing, metasurfaces, catalysis, and biotechnology require fabrication of plasmonically active substrates. Herein, we demonstrate a bottom-up, versatile, and scalable approach that relies on direct growth of silver nanostructures from seed particles that were immobilized on polymer brush-grafted substrates. Our approach is based on (i) the uniform and tunable assembly of citrate-stabilized gold nanoparticles on poly(ethylene glycol) brushes to serve as seeds and (ii) the use of hydroquinone as a reducing agent, which is extremely selective to the presence of seed particles, confining the growth of silver nanostructures on the surface of the substrate. The diameter of the seed particles, concentration, as well as ratio of reactants and duration of the growth process are investigated for large-area growth of silver nanostructures with high surface coverage and plasmonic activity. The resulting silver nanostructures exhibit high levels of surface-enhanced Raman scattering activity at two different laser lines and allow detection of molecules at concentrations as low as 10 pM. The plasmonic properties of the silver nanostructures are further studied using ultrafast pump-probe spectroscopy. Spatially defined silver nanostructures are fabricated through the seed particles that are patterned via soft lithography, showing the capabilities of the presented approach in device applications.
We report completely sustainable processes and materials for inexpensive and scalable fabrication of plasmonically active solid substrates, which are critical for emerging applications in sensing, catalysis, and metasurfaces. Our approach involves grafting of poly(ethylene glycol) (PEG) onto silicon oxide terminated solid substrates using all-water based processing leading to an ultrathin (12 nm) and smooth (roughness of ∼1 nm) functional layer. The resulting surfaces facilitate robust and effective immobilization of gold nanoparticles (NPs) with a density that is superior to the organic solvent based processing. This new process achieves size dependent assembly of the citrate-stabilized gold NPs resulting in high plasmonic activity in surface-enhanced Raman scattering (SERS). The use of leaf extracts derived from Quercus pubescens as a reducing and stabilizing agent allowed for green synthesis of gold NPs with an average diameter of 25.6 ± 11.1 nm. The assembly of the green synthesized gold NPs on all-water processed PEG grafted layers enabled a fully sustainable route for fabrication of plasmonically active solid substrates. The resulting substrates exhibited high SERS response over the entire (∼1 cm 2 ) substrate surface with an analytical enhancement factor of 9.48 × 10 4 for the probe molecule rhodamine 6G under 532 nm laser excitation. A microfluidic device was also constructed on the fabricated platform for SERS mediated simultaneous detection of two nonsteroidal anti-inflammatory drugs, dexketoprofen and ibuprofen, which are widely used in human medicine and present as contaminants in wastewater. The biocompatibility of PEG together with all-water based processing overcome the need for waste management and ventilation of the working place enabling cost and energy efficient, environmentally benign fabrication of plasmonic devices.
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