Aminoalkanoic Acids as Alternatives to Mercaptoalkanoic Acids for the Linker-Assisted Attachment of Quantum Dots to TiO₂

Linear 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 (K_ad) were ∼10⁴ M^-1 saturation amounts of the ligands per projected surface area of TiO₂ (₀) were ∼10^-7 mol cm^-2. Both K_ad and ₀ 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 K_ad were (1.8 ± 0.4) × 10⁶ and (2.4 ± 0.4) × 10⁶ M^-1, values of ₀ 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 (k_et) 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.