Photosensitized NO release from water-soluble nanoparticle assemblies

Article abstract of DOI:10.1021/ja070490w

The photoluminescence from CdSe/ZnS core/shell quantum dots, modified to make them water-soluble by exchanging the surface ligands with dihydrolipoic acid, is partially quenched by adding the chromium(III) complex trans-Cr(cylcam)(ONO)₂⁺

Full text of DOI:10.1021/ja070490w

Published on Web 03/17/2007
Photosensitized NO Release from Water-Soluble Nanoparticle Assemblies
Daniel Neuman,§ Alexis D. Ostrowski,§ Ryan O. Absalonson,§ Geoffrey F. Strouse,‡ and
Peter C. Ford*,§
Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106-9510, and
Department of Chemistry and Biochemistry, Florida State UniVersity, Tallahassee, Florida 32306-4390
Received January 22, 2007; E-mail: ford@chem.ucsb.edu
There is active biomedical interest in developing methodologies
for delivering the bioregulatory diatomic nitric oxide1 (NO) to
physiological targets for applications ranging from cardiovascular
control to radiation sensitization.2 Photochemical techniques provide
the opportunity to control the location and timing of the signal
leading to NO release. In this context, we and others have been
studying the photoreactions of transition metal nitrosyl and nitrito
complexes with the goal of developing thermally stable precursors
that release NO upon electronic excitation.3 One direction that we
have taken is the preparation of molecular constructs consisting of
the NO donor plus an antenna chromophore to collect the light via
single photon excitation (SPE)4 or two photon excitation (TPE).5
Figure 1. PL spectra (λex 366 nm) of water-soluble QDs (∼130 nM) in
Energy or electron transfer from the antenna to the NO precursor
would then lead to net NO release.
Nanocrystal quantum dots offer certain important advantages as
photosensitizing chromophores, including high optical cross-sections
for both SPE6 and TPE7 and the ability to tune the optical properties
phosphate buffer (15 mM, pH 8.2) with various concentrations of added 1
(0-630 µΜ). PL intensities were corrected for the absorbance by 1. The
PL λmax shifts slightly (∼3 nm) to the red at the highest value of [1].
Scheme 2
by varying the QD diameter.8 Multiple surface ligand sites provide
the opportunity to incorporate functionalities tethered to the QD
surface, such as solubility properties,9 biological specificity,10 and
the NO precursor or carrier.11 Furthermore, functionalized QDs have
been used as sensitizers for organic reactions12 for singlet oxygen
generation13 and for electron transfer.14 Here we demonstrate
enhanced NO photogeneration from electrostatic assemblies be-
tween water-soluble CdSe/ZnS core/shell QDs and the cationic
the first excitonic transition.18 A ZnS shell of ∼6 monolayers was
complex trans-Cr(cyclam)(ONO)2+ (1, cyclam ) 1,4,8,11-tetraaza-
grown, and surface ligands were exchanged with dihydrolipoic acid
cyclotetradecane), indicating that the QDs are serving as antennas
(DHLA)19 to give aqueous solubility in moderately basic solution.
to sensitize photoreactions of 1.
After purification, the result was a QD stock solution in aqueous
-
The dinitrito complex 1 (BF4 salt) is photoactive toward
phosphate buffer (PB) solution (15 mM, pH 8.2) that displayed a
cleavage of the CrO-NO bond to give NO plus a CrIVO intermedi-
570 nm photoluminescence (PL) maximum (fwhm ) 48 nm) with
ate (Scheme 1).4b,15 However, 1 is relatively weakly absorbing at
ΦPL ) ∼2% (Figure 1).
near-UV and visible wavelengths, thus rates of NO production from
When various concentrations of 1 were added to the QD solution
this photochemical precursor would be enhanced by an antenna
(∼130 nM),20 the PL intensity from the QDs decreased (Figure 1).
that increases the rate of light absorption. From this derives our
This quenching levels out at ∼60% for the highest value of [1]
interest in QDs as sensitizers.
(630 µM). Analysis of similar QD PL partial quenching by the
analogous trans-Cr(cyclam)Cl2+ complex led us to conclude21 that
Scheme 1
the quenching is due to electrostatic ion pairing between the cationic
Cr(III) complex and the anionic QD surface, as illustrated in Scheme
2. Although either energy or electron transfer from the excited QD
to Cr(III) cations at the surface would account for the quenching
seen with varied [1], the photostimulated release of NO (below)
parallels the excited-state chemistry of 1 (Scheme 1)
The QD PL quenching is also accompanied by enhanced
The water-soluble quantum dots were prepared by a three-step
photoreaction of the Cr(III) complexes. This was determined by
procedure involving synthesis of the CdSe cores,16 growth of the
using a NO-specific electrode immersed in a cuvette filled with
ZnS shell,17 and surface ligand exchange as described in the
Supporting Information. The average CdSe core diameter (3.8 nm)
was estimated from the 542 nm peak position (fwhm ) 33 nm) of
stirred PB solution irradiated by the continuous output of a high-
pressure Hg arc lamp (spectrally selected by a 320-390 nm band-
pass filter). When a small volume of 1 (also dissolved in PB
solution) was injected into the cuvette solution to give an initial
concentration of 200 µM, the electrode response due to photo-
§ University of California, Santa Barbara.
‡ Florida State University.
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J. AM. CHEM. SOC. 2007, 129, 4146-4147
10.1021/ja070490w CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
CARE grant from Los Alamos National Laboratory to P.C.F. and
G.F.S., and a National Institutes of Health grant (NBIB 7 R01
EB000832) to G.F.S. We thank Roberto da Silva for stimulating
discussions and technical help.
Supporting Information Available: Experimental procedures for
preparation of QDs and for evaluating NO photoproduction. Figure
comparing the solution spectra of 1 and the QDs. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
Figure 2. Detection of NO photochemically produced from 1 (200 µM)
in stirred buffer solutions (15 mM, pH 8.2) with (circles) and without
(squares) added QDs (100 nM). At the time designated, a 100 µL aliquot
from a stock solution of 1 was injected into the stirred solution. The
excitation source was a Hg arc lamp (320-390 nm band-pass filter), and
NO was detected with an amiNO-700 Innovative Instruments electrode.
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Acknowledgment. These studies were supported by a National
Science Foundation grant to P.C.F. (CHE-0352650), by a UC-
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