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Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO2
journal contribution
posted on 2016-08-19, 00:00 authored by Natalia Rivera-González, Saurabh Chauhan, David F. WatsonLinear
aminoalkanoic acids (AAAs) and mercaptoalkanoic acids (MAAs)
were characterized as bifunctional ligands to tether CdSe QDs to nanocrystalline
TiO2 thin films and to mediate excited-state electron transfer
(ET) from the QDs to TiO2 nanoparticles. The adsorption
of 12-aminododecanoic acid (ADA) and 12-mercaptododecanoic acid (ADA)
to TiO2 followed the Langmuir adsorption isotherm. Surface
adduct formation constants (Kad) were
∼104 M–1; saturation amounts of
the ligands per projected surface area of TiO2 (Γ0) were ∼10–7 mol cm–2. Both Kad and Γ0 differed
by 20% or less for the two linkers. CdSe QDs adhered to ADA- and MDA-functionalized
TiO2 films; data were well modeled by the Langmuir adsorption
isotherm and Langmuir kinetics. For ADA- and MDA-mediated assembly
values of Kad were (1.8 ± 0.4) ×
106 and (2.4 ± 0.4) × 106 M–1, values of Γ0 were (1.6 ± 0.3) × 10–9 and (1.2 ± 0.1) × 10–9 mol cm–2, and rate constants were (14 ± 5)
and (60 ± 20) M–1 s–1, respectively.
Thus, the thermodynamics and kinetics of linker-assisted assembly
were slightly more favorable for MDA than for ADA. Steady-state and
time-resolved emission spectroscopy revealed that electrons were transferred
from both band-edge and surface states of CdSe QDs to TiO2 with rate constants (ket) of ∼107 s–1. ET was approximately twice as fast
through thiol-bearing linker 4-mercaptobutyric acid (MBA) as through
amine-bearing linker 4-aminobutyric acid (ABA). Photoexcited QDs transferred
holes to adsorbed MBA. In contrast, ABA did not scavenge photogenerated
holes from CdSe QDs, which maximized the separation of charges following
ET. Additionally, ABA shifted electron-trapping surface states to
higher energies, minimizing the loss of potential energy of electrons
prior to ET. These trade-offs involving the kinetics and thermodynamics
of linker-assisted assembly; the driving force, rate constant, and
efficiency of ET; and the extent of photoinduced charge separation
can inform the selection bifunctional ligands to tether QDs to surfaces.
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Keywords
kineticrate constantstime-resolved emission spectroscopytether CdSe QDsMAACdSe QDsTiO 2 Linear aminoalkanoic acidsnanocrystalline TiO 2TiO 2 nanoparticlesMDA-mediated assembly valuesETselection bifunctional ligandselectron-trapping surface statesexcited-state electron transferlinker-assisted assemblythiol-bearing linker 4- mercaptobutyric acidSurface adduct formation constantsADAscavenge photogenerated holesphotoinduced charge separationLangmuir adsorption isothermAAAK adTiO 2MBA12- aminododecanoic acidMDA-functionalized TiO 2 filmsABAamine-bearing linker 4- aminobutyric acid12- mercaptododecanoic acidΓ 0
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