The
COVID-19 pandemic has clearly shown the importance of developments
in fabrication of advanced protective equipment. This study investigates
the potential of using multifunctional electrospun poly(methyl methacrylate)
(PMMA) nanofibers decorated with ZnO nanorods and Ag nanoparticles
(PMMA/ZnO–Ag NFs) in protective mats. Herein, the PMMA/ZnO–Ag
NFs with an average diameter of 450 nm were simply prepared on a nonwoven
fabric by directly electrospinning from solutions containing PMMA,
ZnO nanorods, and Ag nanoparticles. The novel material showed high
performance with four functionalities (i) antibacterial agent for
killing of Gram-negative and Gram-positive bacteria, (ii) antiviral
agent for inhibition of corona and influenza viruses, (iii) photocatalyst
for degradation of organic pollutants, enabling a self-cleaning protective
mat, and (iv) reusable surface-enhanced Raman scattering substrate
for quantitative analysis of trace pollutants on the nanofiber. This
multi-functional material has high potential for use in protective
clothing applications by providing passive and active protection pathways
together with sensing capabilities.
This paper presents electrospin nanolithography
(ESPNL) for versatile
and low-cost fabrication of nanoscale patterns of polymer brushes
to serve as templates for assembly of metallic nanoparticles. Here
electrospun nanofibers placed on top of a substrate grafted with polymer
brushes serve as masks. The oxygen plasma etching of the substrate
followed by removal of the fibers leads to linear patterns of polymer
brushes. The line-widths as small as ∼50 nm can be achieved
by precise tuning of the diameter of fibers, etching condition, and
fiber–substrate interaction. Highly aligned and spatially defined
patterns can be fabricated by operating in the near-field electrospinning
regime. Patterns of polymer brushes with two different chemistries
effectively directed the assembly of gold nanoparticles and silver
nanocubes. Nanopatterned brushes imparted strong confinement effects
on the assembly of plasmonic nanoparticles and resulted in strong
localization of electromagnetic fields leading to intense signals
in surface-enhanced Raman spectroscopy. The scalability and simplicity
of ESPNL hold great promise in patterning of a broad range of polymer
thin films for different applications.
Spatially
defined assembly of colloidal metallic nanoparticles
is necessary for fabrication of plasmonic devices. In this study,
we demonstrate high-resolution additive jet printing of end-functional
polymers to serve as templates for directed self-assembly of nanoparticles
into architectures with substantial plasmonic activity. The intriguing
aspect of this work is the ability to form patterns of end-grafted
poly(ethylene glycol) through printing on a hydrophobic layer that
consists of fluoroalkylsilanes. The simultaneous dewetting of the
underlying hydrophobic layer together with grafting of the printed
polymer during thermal annealing enables fabrication of spatially
defined binding sites for assembly of nanoparticles. The employment
of electrohydrodynamic jet printing and aqueous inks together with
reduction of the feature size during thermal annealing are critically
important in achieving high chemical contrast patterns as small as
∼250 nm. Gold nanospheres of varying diameters selectively
bind and assemble into nanostructures with reduced interparticle distances
on the hydrophilic patterns of poly(ethylene glycol) surrounded with
a hydrophobic background. The resulting plasmonic arrays exhibit intense
and pattern-specific signals in surface-enhanced Raman scattering
(SERS) spectroscopy. The localized seed-mediated growth of metallic
nanostructures over the patterned gold nanospheres presents further
routes for expanding the composition of the plasmonic arrays. A representative
application in SERS-based surface encoding is demonstrated through
large-area patterning of plasmonic structures and multiplex deposition
of taggant molecules, all enabled by printing.
This work reports scalable, low-cost, and simple fabrication of plasmonic heterostructures consisting of gold nanoparticles (NPs) of different sizes to generate intense hot-pots over large areas to serve as substrates for molecular sensing in SERS applications. Our approach involves assembly of massively-available colloidal gold NPs on substrates functionalized with end-grafted poly(ethylene glycol) (PEG) brushes without need for any sophisticated tools and post-modification of the particles and substrates. From real-time monitoring of the adsorption process by using a quartz crystal microbalance, we identified that the cyclic deposition of citrate-stabilized gold NPs on PEG brushes is an effective approach to modulate the kinetics of particle adsorption and greatly improves the surface coverage leading to reduced inter-particle distances. Cyclic deposition of NPs differing in size leads to placement of the small particles in close proximity of the large ones, yielding hot-spots as a consequence of the unique type of interaction between PEG chains and gold NPs. Assembly of heterostructures (60 nm+40 nm and 60 nm+20 nm) at optimized conditions resulted in strong SERS effects with enhancement factors as high as ≈2.0×10 and enabled detection of rhodamine 6G molecules in concentrations as low as 1 nm. The cyclic deposition of NPs also results in increase of the water contact angle without need for any post-modification of the substrate, resulting in ≈30 fold increase in the Raman intensity of aqueous molecules. The insights gained on the adsorption of gold NPs together with the simplicity of the presented approach show great promise for surface assembly of colloidal NPs for a broad range of applications.
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.
Approaches are needed for the tailored assembly of plasmonic building blocks on the surface of substrates to synergistically enhance their properties. Here we demonstrate selective immobilization and assembly of gold nanorods (NRs) on substrates modified and patterned with end-grafted poly(ethylene glycol) (PEG) layers. The ligand exchange from the initial cetyltrimethylammonium bromide to sodium citrate was necessary for the immobilization of gold NRs onto PEG grafted substrates. Linear nanopatterns of PEG were fabricated using electrospun nanofibers as masks in oxygen plasma etching. The selective immobilization of citrate-stabilized gold NRs with a length of ∼50 nm and a width of 20 nm on the nanopatterned PEG layers led to linear and registered arrays of rods. The number of gold NRs per line depended on the width of the patterns and approached 1 when the width of the patterns was comparable to the length of the rods. The confinement of the binding regions led to a ∼3 fold increase in the number of gold NRs immobilized per unit area. The selective and dense immobilization of gold NRs on the nanoscale patterns of PEG resulted in spatially defined and strong surface-enhanced Raman scattering activity enabling detection of molecules at concentrations as low as 1 nM.
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