Stabilization of Metal Nanoparticle Films on Glass
Surfaces Using Ultrathin Silica Coating
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Abstract
Metal nanoparticle (NP) films, prepared
by adsorption of NPs from
a colloidal solution onto a preconditioned solid substrate, usually
form well-dispersed random NP monolayers on the surface. For certain
metals (e.g., Au, Ag, Cu), the NP films display a characteristic localized
surface plasmon resonance (LSPR) extinction band, conveniently measured
using transmission or reflection ultraviolet–visible light
(UV-vis) spectroscopy. The surface plasmon band wavelength, intensity,
and shape are affected by (among other parameters) the NP spatial
distribution on the surface and the effective refractive index (RI)
of the surrounding medium. A major concern in the formation of such
NP assemblies on surfaces is a commonly observed instability, i.e.,
a strong tendency of the NPs to undergo aggregation upon removal from
the solution and drying, expressed as a drastic change in the LSPR
band. Since various imaging modes and applications require dried NP
films, preservation of the film initial (wet) morphology and optical
properties upon drying are highly desirable. The latter is achieved
in the present work by introducing a convenient and generally applicable
method for preventing NP aggregation upon drying while preserving
the original film morphology and optical response. Stabilization of
Au and Ag NP monolayers toward drying is accomplished by coating the
immobilized NPs with an ultrathin (3.0–3.5 nm) silica layer,
deposited using a sol–gel reaction performed on an intermediate
self-assembled aminosilane layer. The thin silica coating prevents
NP aggregation and maintains the initial NP film morphology and LSPR
response during several cycles of drying and immersion in water. It
is shown that the silica-coated NP films retain their properties as
effective LSPR transducers