Design of a highly sensitive fluorescent probe for interfacial electron transfer on a TiO2 surface

Article abstract of DOI:10.1002/anie.201004976

Single-molecule fluorescent probe: The formation of the highly fluorescent 4-NHOH form of a 3,4-dinitrophenyl-substituted boron dipyrromethane dye can be used to study interfacial electron transfer on individual TiO₂ particles (see picture; B yellow, C gray, F green, H white, O red). Kinetic and image analyses of the resulting fluorescence bursts reveal temporal dynamics of molecular interactions and reactive-site distributions.

Full text of DOI:10.1002/anie.201004976

Angewandte
Chemie
DOI: 10.1002/anie.201004976
Fluorescent Probes
Design of a Highly Sensitive Fluorescent Probe for Interfacial Electron
Transfer on a TiO2 Surface**
Takashi Tachikawa,* Nan Wang, Soichiro Yamashita, Shi-Cong Cui, and Tetsuro Majima*
Electron transfer (ET) is a key process in various chemical
and biochemical reactions.[1] For example, interfacial ET in
nanoscale TiO2-based systems governs their photocatalytic
performance, and thus a proper understanding of the ET
mechanism can provide valuable information for designing
highly efficient photocatalysts.[2] Fluorescence at the single-
molecule or single-particle level has recently evolved as an
important tool for studying catalytic reactions on solid
surfaces, because of its high sensitivity, simplicity of data
collection, and high spatial resolution in microscopic imaging
techniques.[3] For instance, several organic dye probes have
been successfully employed to detect the generated reactive
oxygen species and identify the active sites on individual TiO2
nanoparticles by utilizing single-molecule fluorescence spec-
troscopy.[4] Nevertheless, there are very limited single-mole-
cule studies on dyes that respond to the reduction reactions
Figure 1. A) Photocatalytic generation of fluorescent HN-BODIPY from
involved in ET; hence, there is a tremendous need to develop
nonfluorescent DN-BODIPY. B) Normalized UV/Vis absorption (fine
new suitable fluorescent probes for exploring ET processes in
lines) and fluorescence (bold lines, excitation at 470 nm) spectra of
DN-BODIPY (solid lines), HN-BODIPY (dashed lines), and AN-
heterogeneous catalysis.[5]
We have now designed and synthesized a redox-respon-
sive boron dipyrromethane fluorescent probe, namely, 3,4-
dinitrophenyl-BODIPY (DN-BODIPY, Figure 1A) on the
basis of a photoinduced intramolecular ET mechanism. Both
ensemble-averaged and single-molecule fluorescence experi-
ments demonstrated that DN-BODIPY can act as a highly
sensitive nanosensor to monitor photoinduced ET process on
the TiO2 surface.
BODIPY (dotted lines). The inset shows the magnified absorption
peaks. C) Cyclic voltammograms of 1.0 mm DN-BODIPY, BODIPY, and
o-dinitrobenzene (DNB) in Ar-saturated anhydrous solutions of electro-
lytes in acetonitrile.
The dye DN-BODIPY is composed of a fluorescent
chromophore (BODIPY core) and a reactive site (dinitro-
phenyl group). The BODIPYs are of interest as fluorophores
due to their attractive properties, such as a high extinction
coefficient e, high fluorescence quantum yield Ffl , and good
chemical and photostability, which facilitate their use in
chemical and biosensor applications.[6] On the other hand,
reduction of aromatic nitro compounds to the corresponding
hydroxylamines or amines is one of the most important
transformations in synthetic organic chemistry and biochem-
istry, and thus has been used as a model system to investigate
(photo)catalytic reduction reactions with semiconductor and
metal nanoparticles.[7] However, in order to develop a
fluorogenic probe for monitoring ET process that exploits
the reduction of a nitro-substituted benzene moiety, a major
drawback must be overcome: nitrobenzene and its reduction
products (i.e., phenylhydroxylamine or aniline) are believed
to be strong quenchers of fluorescence dyes.[8] The nitro group
greatly lowers the LUMO energy level of the benzene moiety
at the meso position of the BODIPY core because of its
strongly electron withdrawing nature, and thus significant
quenching of BODIPY fluorescence occurs by intramolecular
ET from the excited fluorophore to the nitro-substituted
benzene moiety (donor-excited ET). Oppositely, because the
electron-donating hydroxylamino and amino groups greatly
[*] Dr. T. Tachikawa, N. Wang, S. Yamashita, S.-C. Cui,
Prof. Dr. T. Majima
The Institute of Scientific and Industrial Research (SANKEN)
Osaka University
Mihogaoka 8-1, Ibaraki, Osaka 567-0047 (Japan)
Fax: (+81)6-6879-8499
E-mail: tachi45@sanken.osaka-u.ac.jp
majima@sanken.osaka-u.ac.jp
N. Wang
College of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan 430074 (P.R. China)
[**] N.W. thanks Prof. Lihua Zhu at Huazhong University of Science and
Technology and the CSC (China Scholarship Council) program for
support. T.M. thanks the WCU (World Class University) program
through the National Research Foundation of Korea funded by the
Ministry of Education, Science and Technology (R31-10035) for
support. This work was partly supported by a Grant-in-Aid for
Scientific Research (Projects 22245022, 21750145, and others) from
the Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of the Japanese Government.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004976.
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increase the HOMO energy level of the benzene moiety, the
BODIPY fluorescence is quenched by an acceptor-excited
ET process (Section S4 of the Supporting Information).
Indeed, our preliminary experiments confirmed that a
mono-nitro-substituted BODIPY derivative (i.e., 4-nitro-
phenyl-BODIPY) was a failure as fluorescent probe, although
it underwent a similar reduction reaction to DN-BODIPY
(Figure S2 in the Supporting Information).
The redox properties of DN-BODIPY were determined
by cyclic voltammetry (CV) in acetonitrile. As shown in
Figure 1C, DN-BODIPY showed two reversible cathodic
waves in the window of 0 to ꢀ1.5 V versus normal hydrogen
electrode (NHE). Compared to the reference compounds
ortho-dinitrobenzene (DNB, reactive site) and BODIPY
without any substituent at the meso position (fluorophore
core; Invitrogen, D3921), the first cathodic wave at a half-
wave potential (E1/2) of ꢀ0.54 V versus NHE can be assigned
to the reduction of the para-nitro group, while the second
cathodic wave at an E1/2 of ꢀ0.98 V versus NHE is likely
related to reduction of the BODIPYunit to the corresponding
radical anion (BODIPYCꢀ).[9] This result suggests that reduc-
tion of the second nitro group in DN-BODIPY is difficult,
possibly because the produced para-hydroxylamino or amino
group would increase the electron density of the meta-nitro
group, which would be unfavorable for successive reduc-
tion.[10] It is thus anticipated that the above-mentioned
intramolecular ET process is suppressed when the produced
electron-donating group encounters the benzene moiety with
a nitro group, and a highly fluorescent compound is formed
(Scheme S4 and Table S1 in the Supporting Information).
The performance and applicability of our ET probe was
first examined by evaluating the photocatalytic reduction of
TiO2 nanoparticles (Ishihara Sangyo A-100, particle size 100–
200 nm) by ensemble-averaged spectroscopy. When TiO2
dispersions containing DN-BODIPY (50 mm, in Ar-saturated
methanol) were exposed to 365 nm UV light, a new fluores-
cence peak appeared at about 510 nm and its intensity
dramatically increased with increasing UV irradiation time
(Figure 2A). In control experiments, the increase in fluores-
cence intensity was negligible when the suspension was not
irradiated with UV light or no TiO2 photocatalyst was added
to the solution (Figure 2B). Since the photogenerated holes in
TiO2 are efficiently scavenged by methanol,[11] it can be
suggested that the photogenerated electrons in TiO2 are
mainly responsible for reduction of DN-BODIPY to generate
the fluorescent product.
Figure 2. A) Dependence on UV irradiation time of fluorescence spec-
tra of methanol solutions containing DN-BODIPY and TiO2 particles
(excitation at 470 nm). B) Fluorescence intensities measured before
&
and after UV irradiation of the DN-BODIPY solutions with ( ) and
~
*
without ( ) TiO2 or after being placed in the dark with TiO2 ( ).
Table 1: Photoabsorption and fluorescence properties of BODIPY deriv-
atives in methanol.
[a]
fl
Compound
lmax,abs [nm]
lmax,em [nm]
F
tfl [ns]
DN-BODIPY
HN-BODIPY
AN-BODIPY
506
499
501
525
510
512
0.00011
0.50
0.0025
<0.03
3.7
<0.05
[a] Calculated with fluorescein as fluorescence standard (Ffl =0.850 in
1n NaOH).
fluorescent product (Figure S3 in the Supporting Informa-
tion). Based on the above observations, we propose the
photocatalytic reduction route of DN-BODIPY over TiO2
shown in Figure 1A. It is thermodynamically possible that the
photogenerated electrons in TiO2 (conduction band edge
energy ECB = ꢀ0.88 V vs. NHE in methanol)[12] are trans-
ferred to DN-BODIPY molecules adsorbed on the TiO2
surface and lead to the generation of HN-BODIPY. Because
the reduction potential of phenylhydroxylamine to aniline is
more negative than that of nitrobenzene to phenylhydroxyl-
amine, successive reduction of HN-BODIPY would be
minor.[10,13]
We then focused on photocatalytic reduction of DN-
BODIPY molecules over single TiO2 particles by utilizing
total internal reflection fluorescence microscopy (TIRFM).
Figure 3A shows typical fluorescence images captured for a
single TiO2 particle in Ar-saturated methanol containing DN-
BODIPY (2 mm) under UV irradiation (middle and right
images). Individual single particles show a number of
We carefully separated the fluorescent product by prep-
arative layer chromatography and identified it as 4-hydroxy-
amino-3-nitrophenyl-BODIPY
(HN-BODIPY)
using
1H NMR, mass, UV/Vis absorption, and fluorescence (exci-
tation) spectroscopy (see Figure 1B and Section S6 of the
Supporting Information). Compared to DN-BODIPY and
other possible reduced species, that is, 4-amino-3-nitrophenyl-
BODIPY (AN-BODIPY), HN-BODIPY showed a high
fluorescence quantum yield and a greatly prolonged fluores-
cence lifetime (Table 1). The shapes of the fluorescence
excitation spectra recorded after UV irradiation were also
nearly identical to the absorption spectrum of the purified
fluorescent product, and this implies that there was only one
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toff is the characteristic time before
the formation of fluorescent products
on TiO2 , and ton the characteristic
time for which persistent emission is
exhibited, which should be related to
dissociation of products from the
surface rather than their photo-
bleaching or blinking (Figure S6 in
the Supporting Information). As
depicted in Figure 3C, the distribu-
tions of ton and toff are well fitted by a
single-exponential decay function
(R2 > 0.97). Plotting the reciprocals
of the average values of toff (htoffi) and
ton (htoni) against substrate concen-
tration [S] (Figure 3D) reveals that
the rate of fluorescent product for-
mation htoffiꢀ1 is dependent on [S], as
expected. At higher concentrations, a
faster adsorption equilibrium is
attained which assists reduction of
DN-BODIPY to the fluorescent
products and leads to an enhance-
ment in htoffiꢀ1.
Figure 3. A) Transmission (a) of a single TiO2 particle on the cover glass and fluorescence images
(b, c) of the same particle in Ar-saturated 2.0 mm DN-BODIPY solution under 488 nm laser and UV
irradiation (0.5 Wcmꢀ2 at the glass surface). The acquisition time of an image was 50 ms. The red
dots in the transmission image indicate the location of fluorescence bursts. The accuracy of
location was about 50 nm. B) A typical fluorescence intensity trajectory observed for a single TiO2
particle. The green dashed line indicates the threshold level separating the on and off states.
C) Off- (blue) and on-time (red) distributions constructed from over 100 events for 20 different
single TiO2 particles. D) Dependence on DN-BODIPY concentration of htoffiꢀ1 (blue) and htoniꢀ1
(red) obtained for TiO2. The solid blue and red lines were obtained from Equations (1) and (2),
respectively.
The dependence of the product
formation rate on [S] can be de-
scribed by Langmuir–Hinshelwood
equation (1)[15]
geffK1½Sꢂ
htoffiꢀ1
¼
ð1Þ
1 þ K1½Sꢂ
fluorescence bursts that have signals over the background
(also see Figure 3B). Control experiments also confirmed
that TiO2, DN-BODIPY, and UV excitation are essential for
generation of fluorescence bursts. The locations of the
fluorescence bursts, which were determined by fitting two-
dimensional Gaussian functions to the intensity distribution
of each fluorescence spot, are likely distributed over the
particle (see the red dots in the transmission image of
Figure 3A).[14] Moreover, the fluorescence lifetimes of the in
situ generated bursts over single TiO2 particles were mea-
sured by combining confocal microscopy with a time-corre-
lated single-photon counting (TCSPC) system. The fluores-
cence bursts exhibited a much longer lifetime than the
background signal from DN-BODIPY in solution, and thus
suggest that such a sudden intensity increase corresponds to
generation of the fluorescent product (i.e., HN-BODIPY;
Figure S4 in the Supporting Information). Compared to free
HN-BODIPY in bulk solution (tfl = 3.7 ns), these in situ
generated products on TiO2 showed much shorter lifetimes
(tfl ꢁ 1.3 ns), possibly due to ET from the excited BODIPY
chromophore to the TiO2 nanoparticles (Figure S5 and
Table S3 in the Supporting Information).
where K1 is the equilibrium adsorption constant for substrate
(K1 = k1[S]/kꢀ1), and geff the reactivity of all catalytic sites
(geff = kns, where k is the rate constant for one catalytic site,
and ns the total number of substrate binding sites on one
particle). The resulting value of K1 is (0.47 ꢃ 0.10) mmꢀ1 for
the adsorption of DN-BODIPY on TiO2 surface, which is of
the same order of magnitude as those for nitrobenzenes.[16]
In contrast, the DN-BODIPY concentration has no
significant effect on htoniꢀ1. According to the literature,[15]
the main feature of the kinetics of the substrate-assisted
product dissociation is described by Equation (2)
k2K2½Sꢂ þ k3
htoniꢀ1
¼
ð2Þ
1 þ K2½Sꢂ
where k2 is the rate constant of product dissociation in the
substrate-assisted pathway, k3 the rate constant of direct
product dissociation, and K2 = k1/(kꢀ1+k2). By fitting Equa-
tion (2) to the data, the k3 value was determined to be (1.7 ꢃ
0.1) sꢀ1, which is much higher than the k2 value of (0.33 ꢃ
0.43) sꢀ1 (Figure 3D) and thus suggests that the disappear-
ance of HN-BODIPY from the surface of the nanoparticles
does not involve the substrate-assisted steps.[17]
Finally, we demonstrate that single-molecule fluorescence
imaging with DN-BODIPY allows precise mapping of photo-
catalytic activity in individual TiO2 crystals at the nanometer
scale (Figure 4). Interestingly, most fluorescence spots were
Accordingly, we could directly evaluate the photocatalytic
reduction reactions on individual TiO2 particles using TIRFM
with DN-BODIPY as redox-responsive fluorescent probe.
Figure 3B shows a typical fluorescence intensity trajectory
obtained for a single TiO2 particle. In the single-particle
turnover trajectory, the actual photocatalytic events can be
separated into two characteristic durations toff and ton , where
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Experimental Section
BODIPY derivatives were synthesized according to literature proce-
dures with some modifications (see Supporting Information for
details)[20]
Microsized anatase TiO2 crystals with dominant (001) facets were
prepared by the hydrothermal method from titanium sulfate and
hydrofluoric acid according to literature procedures (see Supporting
Information for details).[21]
Steady-state UV/Vis absorption spectra were measured by a
Shimadzu UV-3100 UV/Vis/NIR spectrophotometer. Steady-state
fluorescence spectra were measured by a Hitachi 850 or HORIBA
FluoroMax-4 fluorescence spectrophotometer. Cyclic voltammetry
(CV) measurements were carried out at room temperature with an
electrochemical analyzer (ALS, 660A) with a standard three-elec-
trode configuration.
The experimental setup for single-particle experiments was based
on an Olympus IX71 inverted fluorescence microscope.[4] The details
of the experimental setup are described in the Supporting Informa-
tion. The position of the TiO2 particles immobilized on the cover glass
was determined from the transmission image obtained by illuminating
the sample from above with a halogen lamp (Olympus, U-LH100L-3).
Circularly polarized light emitted from a CW Ar ion laser (Melles
Griot, IMA101010BOS; 488 nm, 0.1 kWcmꢀ2 at the glass surface) was
reflected by a first dichroic mirror (Olympus, RDM450) toward a
second dichroic mirror (Olympus, DM505). The laser light passing
through an objective lens (Olympus, UPLSAPO 100XO; 1.40 NA,
100 ꢀ ) after the reflection at the second dichroic mirror was totally
reflected at the cover glass/methanol interface, which generated an
evanescent field, making it possible to detect a single fluorescence dye
molecule. For excitation of the TiO2 particles, the 365 nm light
emitted by an LED (OPTO-LINE, MS-LED-365) and passing
through a neutral density (ND) filter was passed through the
objective. The fluorescence emission from the fluorescent products
generated over a single TiO2 particle on the cover glass was collected
by using the same objective, magnified by a 1.6 ꢀ built-in magnifi-
cation changer, passed through a bandpass filter (Semrock, FF01-531/
40-25) to remove the undesired scattered light, and then imaged by an
electron-multiplying charge-coupled device (EM-CCD) camera
(Roper Scientific, Cascade II:512). The images were recorded at a
frame rate of 20 framessꢀ1. All experimental data were obtained at
room temperature.
Confocal fluorescence images were taken by using an objective-
scanning confocal microscope system (PicoQuant, MicroTime 200)
coupled to an Olympus IX71 inverted fluorescence microscope. The
samples were excited through an oil-immersion objective lens
(Olympus, UAPON 150XOTIRF; 1.45 NA, 150 ꢀ ) with a 485 nm
pulsed laser (PicoQuant, LDH-D-C-485; 2 kWcmꢀ2 at the glass
surface) controlled by a PDL-800B driver (PicoQuant). The emission
from the sample was collected by the same objective and detected by
a single-photon avalanche photodiode (Micro Photon Devices, PDM
50CT) through a dichroic beam splitter and bandpass filter (Semrock,
FF01-531/40-25).
Figure 4. A) Structure of anatase TiO2 crystal with preferential (001)
facets. Transmission (B) of a single TiO2 crystal on the cover glass and
fluorescence (C) images of the same crystal in Ar-saturated 2.0 mm
DN-BODIPY solution under 488 nm laser and UV irradiation
(30 mWcmꢀ2 at the glass surface). The red and blue dots in image B)
indicate the fluorescence bursts located on the (101) and (001)
surfaces, respectively, observed during 3 min irradiation. The precise
positions at which fluorescent products were generated were deter-
mined by centroid analysis. The arrow in image C) indicates the
fluorescence spot.
found to be preferentially located on the (101) surface of the
crystal (see red dots in image B). A similar tendency was
observed for more than five individual crystals examined. The
average counting rates of single dye molecules were roughly
estimated to be (53 ꢃ 10) and (17 ꢃ 4) molecules mmꢀ2 minꢀ1
for (101) and (001) surfaces, respectively; this strongly
indicates the effect of crystal facets on the photocatalytic
activity, which is not evident in the bulk measurement.[18] Our
finding is generally consistent with previous studies and may
be explained by differences in surface energy levels of the
conduction and valence bands, surface structures, and
adsorption energies of substrates on the exposed crystal
faces.[19] However, further work is needed to clarify the
detailed mechanism of the face-selective photocatalytic
reaction.
Received: August 10, 2010
In conclusion, we propose novel single-molecule fluores-
cence probe DN-BODIPY for studying the reduction process
involved in ET on the TiO2 surface. Single-molecule kinetic
and imaging analyses of the fluorescence bursts emitted from
the products revealed the temporal dynamics of molecular
interactions and interfacial ET, and heterogeneous distribu-
tions of reactive sites on individual catalyst particles. Our
methodology should provide a suitable approach to exploring
the electron-transport characteristics in various semiconduc-
tor (photo)catalysts as well as metal–semiconductor and
semiconductor–semiconductor nanocomposites.
Published online: October 4, 2010
Keywords: electron transfer · fluorescent probes · nanoparticles ·
.
photochemistry · single-molecule studies
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