The self-assembly of monodisperse gold and silver colloid particles into monolayers on polymer-coated substrates yields macroscopic surfaces that are highly active for surface-enhanced Raman scattering (SERS). Particles are bound to the substrate through multiple bonds between the colloidal metal and functional groups on the polymer such as cyanide (CN), amine (NH(2)), and thiol (SH). Surface evolution, which can be followed in real time by ultraviolet-visible spectroscopy and SERS, can be controlled to yield high reproducibility on both the nanometer and the centimeter scales. On conducting substrates, colloid monolayers are electrochemically addressable and behave like a collection of closely spaced microelectrodes. These favorable properties and the ease of monolayer construction suggest a widespread use for metal colloid-based substrates.
This paper details the kinetic aspects of covalent self-assembly
of colloidal Au particles from solution
onto immobilized organosilane polymers. On glass substrates,
surface formation can be monitored using UV−vis
spectroscopy and field emission scanning electron microscopy (FE-SEM).
Correlation of these data allows the effect
of nanostructure on bulk optical properties to be evaluated. At
short derivatization times, particle coverage is
proportional to (time)1/2. The particle sticking
probability p, defined as the ratio of bound particles to
the number
of particles reaching the surface in a given time period, can be
determined from a knowledge of the particle radius,
solution concentration, temperature, and solution viscosity; for
surfaces derivatized with (3-mercaptopropyl)trimethoxysilane (MPTMS), p ≈ 1. At longer
derivatization times, interparticle repulsions result in a
“saturation”
coverage at ≈30% of a close-packed monolayer. Two approaches
for modulating the rate of surface formation are
described: electrochemical potential control on organosilane-modified
SnO2 electrodes and charge screening by
organic adsorbates. Self-assembly of colloidal Au particles onto
functionalized substrate surfaces is a reproducible
phenomenon, as evidenced by UV−vis and surface enhanced Raman
scattering (SERS) measurements on identically
prepared substrates.
Two approaches to preparation of Ag-clad Au colloid monolayers are described. Each begins with a preformed monolayer of colloidal Au particles on glass, Au-coated quartz, or In-doped SnO2. Chemical reduction of Ag + from a commercial Ag coating formulation (LI Silver) or electrochemical reduction of Ag + leads to surfaces for which the amount of Ag deposited can be controlled. On transparent substrates, Ag deposition can be followed in real time by UV-visible spectroscopy. When the substrate is an electrode, electrochemical deposition can be monitored by coulometry; for the chemical process, anodic stripping voltammetry yields accurate values of the amount of Ag present, as confirmed by quartz crystal microgravimetry (QCM). Chemical reduction yields surfaces that are extremely enhancing for surface enhanced Raman scattering (SERS), although the deposition process required must be precisely tuned. In contrast, electrochemical deposition, while affording more accurate control of the reduction rate, produces only mildly enhancing surfaces. Atomic force microscopy (AFM) images of electrochemically-produced surfaces show formation of very large nonuniform Ag particles that are distinct from the Au colloid monolayer, while those prepared by LI Silver show both growth of Ag on Au and formation of smaller colloidal Ag particles attached to Ag-coated Au particles.
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