Charge transfer processes with semiconductor
quantum dots (QDs)
have generated much interest for potential utility in energy conversion.
Such configurations are generally nonbiological; however, recent studies
have shown that a redox-active ruthenium(II)–phenanthroline
complex (Ru<sup>2+</sup>-phen) is particularly efficient at quenching
the photoluminescence (PL) of QDs, and this mechanism demonstrates
good potential for application as a generalized biosensing detection
modality since it is aqueous compatible. Multiple possibilities for
charge transfer and/or energy transfer mechanisms exist within this
type of assembly, and there is currently a limited understanding of
the underlying photophysical processes in such biocomposite systems
where nanomaterials are directly interfaced with biomolecules such
as proteins. Here, we utilize
redox reactions, steady-state absorption, PL spectroscopy, time-resolved
PL spectroscopy, and femtosecond transient absorption spectroscopy
(FSTA) to investigate PL quenching in biological assemblies of CdSe/ZnS
QDs formed with peptide-linked Ru<sup>2+</sup>-phen. The results reveal
that QD quenching requires the Ru<sup>2+</sup> oxidation state and
is not consistent with Förster resonance energy transfer, strongly
supporting a charge transfer mechanism. Further, two colors of CdSe/ZnS
core/shell QDs with similar macroscopic optical properties were found
to have very different rates of charge transfer quenching, by Ru<sup>2+</sup>-phen with the key difference between them appearing to be
the thickness of their ZnS outer shell. The effect of shell thickness
was found to be larger than the effect of increasing distance between
the QD and Ru<sup>2+</sup>-phen when using peptides of increasing
persistence length. FSTA and time-resolved upconversion PL results
further show that exciton quenching is a rather slow process consistent
with other QD conjugate materials that undergo hole transfer. An improved
understanding of the QD–Ru<sup>2+</sup>-phen system can allow
for the design of more sophisticated charge-transfer-based biosensors
using QD platforms