PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Fluorescence enhancement and lifetime modification of single
nanodiamonds near a nanocrystalline silver surface
Tsong-Shin Lim,a Chi-Cheng Fu,b Kang-Chuang Lee,b Hsu-Yang Lee,b Kowa Chen,b
Wen-Feng Cheng,b Woei Wu Pai,c Huan-Cheng Chang*b and Wunshain Fannbd
Received 6th October 2008, Accepted 27th November 2008
First published as an Advance Article on the web 20th January 2009
DOI: 10.1039/b817471g
Fluorescent nanodiamond (FND) contains nitrogen-vacancy defect centers as fluorophores.
The intensity of its fluorescence can be significantly enhanced after deposition of the particle
(35 or 140 nm in size) on a nanocrystalline Ag film without a buffer layer. The excellent
photostability (i.e. neither photobleaching nor photoblinking) of the material is preserved even on
the Ag film. Concurrent decrease of excited state lifetimes and increase of fluorescence intensities
indicate that the enhancement results from surface plasmon resonance. Such a fluorescence
enhancement effect is diminished when the individual FND particle is wrapped around by DNA
molecules, as a result of an increase in the distance between the color-center emitters inside the
FND and the nearby Ag nanoparticles. A fluorescence intensity enhancement up to 10-fold is
observed for 35 nm FNDs, confirmed by fluorescence lifetime imaging microscopy.
Introduction
Fluorescent nanodiamond (FND) is emerging as a new type of
nanomaterial that holds great promise for biological applications.1,2 Containing a high concentration of nitrogen-vacancy
(N–V) defect centers as fluorophores, FND exhibits several
remarkable features, including emission of bright photoluminescence in the extended red region, no photobleaching and
photoblinking, and easiness of surface functionalization for
specific or nonspecific binding with nucleic acids and proteins,
etc.3,4 In particular, the capability of emitting light at B700 nm,
where cell autofluorescence signal is low,5 makes FND wellsuited for cellular imaging application. These excellent photophysical properties, together with the good biocompatibility of
the material,6 have enabled three-dimensional tracking of a single
FND particle with a size of 35 nm in a live mammalian cell over
a time period of more than 3 min using a wide-field fluorescence
microscope.7
Although the observation of a single 35-nm FND can be
readily made in solution and cells, improving the detection
sensitivity is desirable to widen its biological applications.8
A technology based on the use of metallic nanostructures that
interact with fluorophores to increase their emission intensity
has long been recognized since its discovery in 1980.9 Experiments with dye-doped polymer films10 and dye-labeled oligonucleotides11 on nanocrystalline Ag surfaces indicated that the
maximum fluorescence enhancement occurred at a separation
of B3 and B9 nm, respectively, between metal and fluorophore.
It means that covering the metallic film with a buffer layer of a
few nanometers in thickness is essential for achieving the largest
fluorescence enhancement effect. Typically, a 2- to 10-fold
enhancement in the fluorescence intensity is observed for
fluorophores with low quantum yields,12 whereas no significant
enhancement has been reported for high-quantum-yield
fluorophores.13 A number of biotechnological applications
taking advantage of this so-called metal-enhanced fluorescence
(MEF) have been developed, such as, to improve the sensitivity
of DNA hybridization assays.14 Aside from these applications,
which were all conducted for an ensemble of molecules, the
technique has also been applied to the detection of single
quantum dots (such as CdSe and CdTe) on silver island films
(SIFs), resulting in a 5-fold increase in fluorescence intensity.15,16
Herein, we show that the fluorescence intensity of (N–V)
centers, which have near unity photoluminescence quantum
efficiency in diamond,17 can also be significantly enhanced when
FNDs are in proximity to Ag nanoparticles on a SIF. Moreover,
for particles with sizes of 35 and 140 nm, no buffer layers are
required since the (N–V) centers are embedded in the diamond
lattice and are separated by several tens nm from the Ag surface.
Such a surface enhancement effect is particularly evident for
35-nm FNDs, where a near 10-fold increase in fluorescence
intensity was observed. Additionally, the enhancement effect is
diminished nearly completely when FNDs are coated with layers
of T4 DNA molecules, which act as a spacer that increases the
separation between FNDs and SIF. To the best of our knowledge, this is the first time that such MEF is observed for single
particles with sizes ranging from 35 to 140 nm.
a
Department of Physics, Tunghai University, Taichung, 407, Taiwan
Institute of Atomic and Molecular Sciences, Academia Sinica,
Taipei, 106, Taiwan. E-mail: [email protected];
Fax: (+886)2 23620200
c
Center for Condensed Matter Sciences, National Taiwan University,
Taipei, 106, Taiwan
d
Department of Physics, National Taiwan University, Taipei, 106,
Taiwan
b
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Materials and methods
Production and surface modification of FNDs
Synthetic type Ib diamond powders with a mean size of
140 nm (Micron+ MDA, Element Six, USA) and 35 nm
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(MSY, Microdiamant, Switzerland) were purified in concentrated H2SO4–HNO3 solution (3 : 1, v/v) at 90 1C for 30 min.
After washing extensively in deionized water, about 5 mg of
the diamond powders was deposited on a silicon wafer and
allowed to dry in air to form a thin film (area B0.5 cm2 and
thickness B50 mm). The dried diamond film was exposed
to a 3-MeV proton beam from a NEC tandem accelerator
(9SDH-2, National Electrostatics Corporation) at a dose
of B1 1016 ions cm2. Formation of (N–V) defect centers
was facilitated by annealing the proton-beam-treated nanodiamonds in vacuum at 800 1C for 2 h. The freshly prepared
FNDs were finally treated in concentrated H2SO4–HNO3
mixtures to remove graphitic surface structures and simultaneously functionalize the diamond surfaces with carboxyl and
other oxygen-containing groups.3
Poly-L-lysines (PLs) with a molecular weight of B30 000
were used to decorate FND surfaces with amino groups. This
was done by mixing 8 mg of N-(3-dimethylaminopropyl)-Nethyl-carbodiimide hydrochloride (EDC, Sigma) with 6 mg of
N-hydroxysuccinimide (Sigma) in a 5 mL solution containing
6 mg of acid-washed FNDs, followed by adding 3 mg of PLs
into the suspension. After incubation of the mixture at room
temperature for 24 h, the PL-coated FNDs were thoroughly
washed in deionized water. Approximately 1 mg of the amineterminated FND particles suspended in 100 mL of deionized
water were drop-cast on a glass plate with or without SIF
coating for fluorescence microscopy measurements.
Noncovalent conjugation of DNA to FND was made by
mixing 3 mg of the PL-coated FND particles suspended in
200 mL of 0.5 TBE buffer (Invitrogen) with a solution
containing 165.6-kb T4 phage DNA (Wako) at a molar ratio
of FND : DNA = 1 : 8. The conjugation was established by
pure electrostatic attraction. The mixture, after incubation at
room temperature for 10 min, was diluted with 0.5 TBE
buffer to a concentration suitable for single particle detection,
followed by drop-cast of the particles on SIFs for fluorescence
measurements.
Preparation of SIFs
SIFs were prepared by using the Tollens ‘‘mirror’’ reaction,
following the procedures described in ref. 10. In brief, glass
plates (25 25 1 mm) were first cleaned in H2SO4–H2O2
solution (3 : 1, v/v) and rinsed extensively in deionized water.
About 2–3 mL of concentrated NH4OH was added to 150 mL
of 0.1 M AgNO3 and after thorough stirring to dissolve any
precipitate that might have formed, 75 mL of 0.8 M KOH was
added to the mixed solution. To form SIFs, an equal amount
of this solution and 0.5 M dextrose were mixed together for
10 s at room temperature. Droplets of the mixed solution were
deposited on the acid-cleaned glass plate positioned on a hot
plate for 1 min at 35 1C prior to removal of excess reagents.
Wide-field fluorescence microscopy imaging
Fluorescence images of FND particles and SIFs were acquired
using a wide-field fluorescence microscope (IX70, Olympus)
equipped with a frequency-doubled Nd : YAG laser (DPSS
532, Coherent) operating at 532 nm and an electron multiplying charge-coupled device (CCD, DV887DCS-BV, Andor)
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set at an exposure time of 0.1 s. In imaging 140-nm FNDs, the
laser-excited fluorescence was collected with a 40, NA 0.75
objective (UPLFL 40, Olympus) and guided through a
565 nm long-pass filter (E565lp, Chroma Tech) before reaching the detector. As for the Ag nanoclusters and 35-nm FND
particles, the fluorescence was collected by using a 100, NA
1.35 oil objective (UPLFL 100, Olympus) and selected by a
680–730 nm band-pass filter (HQ655/150 m, Chroma Tech)
prior to detection.
Measurements of fluorescence lifetimes and spectra
Fluorescence lifetime measurements were performed with a
modified confocal optical microscope (E600, Nikon) equipped
with a 60, NA 0.7 objective (ELWD 60, Nikon) and a
frequency-doubled picosecond Nd : YAG laser (IC-532-30,
High Q Laser) operating at 532 nm and a repetition rate of
50 MHz.2,18 The picosecond-laser-excited fluorescence, after
passing through a 565 nm long-pass filter (E565lp, Chroma
Tech), was collected and detected by using either an avalanche
photodiode (SPCM-AQR-15, Perkin-Elmer) or a GaAsP
photon-counting photomultiplier tube (H7422P-40, Hamamatsu). The use of the latter allowed for fluorescence detection
with a time resolution of B300 ps, limited by the instrument
response function.
Confocal fluorescence images were acquired for particles
dispersed on either a glass plate or a SIF after a raster scan of
the specimen using a piezo-driven nanopositioning and scanning system (E-710.4CL & P-517.3CL, Physik Instrument). By
moving the particles of interest consecutively to the focal point
of the microscope objective, both the fluorescence time traces
and spectra of the individual particles were collected. The
former were recorded with a time-correlated single photon
counting module (SPC-600, Becker & Hickl) and the latter
were obtained with a monochromator (SP300i, Acton
Research) equipped with a liquid-nitrogen-cooled CCD camera (LN/CCD-1100-PB, Princeton Instruments). The intrinsic
lifetime of the fluorescence was analyzed by using a commercial program (FAST, Fluorescence Analysis Software
Technology), which deconvoluted the signal with the
instrument response function and fit each time trace with a
multi-exponential function as given by eqn (1).
Fluorescence lifetime gating and imaging
Fluorescence lifetime imaging microscopy (FLIM) was used to
obtain lifetime-resolved images of 35-nm FNDs with a timecorrelated single photon counting system (PicoHarp 300,
PicoQuant). Both the fluorescence decays of FNDs and Ag
nanoclusters were measured and analyzed. To obtain the
fluorescence images of 35-nm FNDs without the background
signals of SIFs, proper time-gating windows were chosen.
Time-gated counts were calculated by summing over all data
points constituting the fluorescence decay curve within this
selected window.
Results and discussion
Fig. 1a shows a typical atomic force microscopy image of an
as-grown SIF, composed of Ag nanoclusters with an average
size of B80 nm. Optical spectroscopic measurements indicated
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Fig. 1 (a) Typical atomic force microscopy image of SIF. (b) Comparison of the absorption spectrum of SIF (green) and the emission spectrum of
140-nm FNDs (red). (c) Wide-field epifluorescence image of SIF, obtained by laser excitation at 532 nm and emission collection at 680–730 nm.
(d) Typical fluorescence decay curve of an Ag nanocluster.
that this film absorbs strongly in the wavelength range of
350–600 nm with a maximum at B450 nm (Fig. 1b). The
band, originating from surface plasmon resonance (SPR),
overlaps well with the absorption band of FNDs at B560 nm,19
which ensures resonant interaction. The SPR band, on the
other hand, is fairly separated from the emission band (peaking
at 680 nm) of FNDs. A close examination of the Ag nanoclusters revealed that their fluorescence spectra are highly
heterogeneous. Each nanocluster shows a distinctly different
spectrum (Fig. 2). In particular, some nanoclusters (such as
particle 1 in Fig. 2) fluoresce in the spectral region overlapping
with that of FNDs.20 They appear as bright red spots as
undesirable backgrounds in the fluorescence image (Fig. 1c).
A preliminary experiment showed that the fluorescence brightness of these Ag nanoclusters is B6 times higher than that of
35-nm FNDs deposited on a glass plate, but is roughly one
order of magnitude lower than that of 140 nm FNDs. For the
Ag nanoclusters prepared in this work, they all have a
fluorescence decay lifetime of B300 ps, essentially limited by
the instrument response time (Fig. 1d). The result is in accord
with the findings of Dickson and coworkers,21 who reported
picosecond fluorescence lifetimes for Ag nanoclusters.
We started the MEF measurement with 140-nm FNDs
surface-coated with PLs. Fig. 3a shows a fluorescence image
along with its intensity histogram for single 140-nm FNDs
dispersed on a bare glass plate. The fluorescence intensity
distribution centers at the count number of B8000 with a
distribution width of 3900. A 2-fold increase in both mean
value and width of the distribution was observed when the
same particles were deposited on a SIF and excited under the
1510 | Phys. Chem. Chem. Phys., 2009, 11, 1508–1514
Fig. 2
Emission spectra of three single Ag nanoclusters in SIF.
same conditions (Fig. 3b). Such an increased distribution
width is attributed to the increase in heterogeneity of the
MEF effect as well as the size distribution of the Ag nanoclusters in the island film.
To further illustrate this MEF phenomenon, we increased
the FND-SIF distance by overcoating the PL-conjugated
FNDs with T4 DNA. As have been demonstrated elsewhere
for single FND/DNA detection,2 the DNA coating does not
result in any significant changes in the photoluminescence
property of the material, since the red emission of FND
originates from the (N–V) centers embedded in the diamond
lattice and their photophysical characteristics are insensitive to
surface functionalization. Furthermore, the individual 140-nm
FND particle is wrapped around by the T4 DNA molecule,
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Fig. 3 Wide-field epifluorescence images (left) and corresponding intensity histograms (right) of (a) PL-coated FNDs on a glass plate,
(b) PL-coated FNDs on SIF, and (c) PL-coated FNDs wrapped around by T4 DNA on SIF. Note the change of the contrast scale in (b).
which has an extended length of B60 mm and a radius of
gyration of B2 mm. Fig. 3c shows a fluorescence intensity
histogram of the FND/DNA complex. Indeed, the average
fluorescence intensity dropped to its original value, accompanied with a narrower distribution width. Assuming that only
one half of each DNA molecule is involved in the particle
wrapping process and the other half is extended freely into
solution, we estimate that the FND-SIF distance is increased
by B1 mm, which essentially eliminates all the MEF effect.
This distance-dependent measurement result supports our
suggestion that the observed fluorescence enhancement is
caused by SPR.
The origin of the MEF effect can be further elucidated by
fluorescence lifetime measurements. We compare in Fig. 4a the
fluorescence lifetime histograms of 140-nm FNDs on a bare
glass plate and on a SIF surface. Each fluorescence lifetime
was obtained by analyzing the fluorescence decay curve in
terms of a multi-exponential model after proper deconvolution
of the instrument response function as,22
I¼
X
ai expðt=ti Þ;
i
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ð1Þ
where ti is the lifetime of each component and ai is the
corresponding amplitude with Sai = 1. The amplitudeweighted lifetime is finally determined as
X
hti ¼
ai ti
ð2Þ
i
By summing the results over 30 single particles (Fig. 4b), we
found a shortening of the average fluorescence lifetime of
FNDs on the SIF to hti = 14.1 ns, compared with hti =
23.7 ns of the same FNDs deposited on the glass plate. The
near two-fold reduction in lifetime, together with the two-fold
increase in fluorescence intensity, again suggests that the
observed fluorescence enhancement derives from SPR.
Conventionally, the MEF effect can be observed more
readily for fluorophores with low quantum yields.23 The
fluorescence enhancement process can be understood as a
result of the quenching of a molecular excited state (exciton)
by non-radiative transfer of energy from the exciton to surface
plasmons supported by metallic structures in close proximity,
followed by the re-radiation of this energy from the
plasmons.24 Recently, there is a report on the fluorescence
enhancement of high-quantum-yield fluorophores.25 However,
Phys. Chem. Chem. Phys., 2009, 11, 1508–1514 | 1511
Fig. 4 (a) Histograms of fluorescence lifetimes and (b) averaged fluorescence decay curves of FNDs on a bare glass plate (blue) and on a SIF
surface (red).
Fig. 5 Comparison of photostability of 140-nm FNDs (blue) and Ag
nanoclusters (red) on SIF.
in that ensemble measurement, it is difficult to separate the
SPR-associated enhancement from other effects such as the
surface coverage effect. This work, combining single particle
imaging with fluorescence lifetime measurements, unambiguously demonstrates that the intensity of the photoluminescence from fluorophores with near unity quantum yields can
be metal-enhanced. To account for our observations, we
consider the spectral overlaps of the absorption and emission
bands of the fluorophores with those of the underneath
substrate. As shown in Fig. 1b, good spectral overlap was
achieved between FNDs and SIFs, whereas the emission band
of FND is well separated from the plasmon resonance. The
mechanism of the fluorescence enhancement process observed
herein is, therefore, most likely to involve excitation of SPR
by incident light, followed by transfer of the energy nonradiatively to nearby FNDs, and finally radiation from
the FND.26
It is an open question whether or not the excellent photostability of FND is preserved on the silver nanocrystalline film.
To answer this question, we monitored the fluorescence
intensities of the individual 140-nm FND particles over a time
period of 30 s with 100-ms time resolution. Similar to that
found previously for FNDs on a glass plate,2 excellent photostability was observed for the same particles on SIF. Under the
excitation with a cw 532-nm laser at a power density of
2 103 W cm2, the fluorescence intensity of the individual
FNDs stays essentially the same over 30 s and no sign of
intermittency was found. In contrast, the underneath Ag
nanoclusters exhibit distinct photoblinking behavior (Fig. 5).
In accord with previous observations for the same FNDs on
glass plates and in cells,2,7 no photobleaching was observed
over an excitation time period of more than 3 min (data not
Fig. 6 (a) Confocal fluorescence image of 35-nm FNDs on a glass plate and (b) comparison of confocal fluorescence image (left) and FLIM image
(right) of 35-nm FNDs on SIF. Six bright spots are marked and compared between these two images in (b). Note that the scale bars in the confocal
fluorescence image and the FLIM image are given in intensity (counts) and lifetime (ns), respectively.
1512 | Phys. Chem. Chem. Phys., 2009, 11, 1508–1514
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Fig. 7
FLIM images of 35-nm FNDs on SIF obtained at four different gating times.
shown). The exceptionally high photostability of FND is a key
property for the application of this nanomaterial in single
particle imaging and long-term tracking of a single particle
interacting with biomolecules on surface as well as in cells.
The success of enhancing the fluorescence intensity of
140-nm FNDs with the Ag nanostructures leads us to the
suggestion that smaller FND particles (such as the 35 nm
ones) should show more dramatic fluorescence enhancement.
However, in accord with their volume ratio, the fluorescence
intensity of 35-nm FNDs is B60-fold weaker (Fig. 6a).2,7
Detecting these particles thus becomes problematic owing to
the large fluorescence background from SIF.20 Fortunately,
time domain measurements provide a solution to overcome
this problem. As shown in Fig. 1d, the fluorescence decay
lifetimes of Ag nanoclusters are typically less than 300 ps,
which is much shorter than that of FNDs on SIF. It is
therefore possible to separate the signal of FND from that
of the Ag nanoclusters using a lifetime gating technique, or
FLIM.27 Fig. 6b (left) displays a fluorescence image of 35-nm
FNDs dispersed on a SIF surface, obtained by using a confocal fluorescence microscope. Several bright spots appear in
the image; however, the image alone does not allow us to tell
which is FND and which is Ag nanocluster. Only with the use
of the lifetime gating technique, they can be readily distinguished. Fig. 6b (right) shows the fluorescence lifetimeresolved image of the same specimen. Of the 6 particles circled
in the image, spots 1, 2, and 3 have significantly shorter
fluorescence lifetimes and they belong to the Ag nanoclusters,
whereas spots 4, 5, and 6 have longer fluorescence lifetimes
and they correspond to FND particles. Compared with
the same particles deposited on a coverglass slide and
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excited under the same experimental conditions (Fig. 6a), the
FNDs on SIF exhibit a B10-fold increase in fluorescence
intensity.
In Fig. 7, we also display the FLIM images of 35-nm FNDs
on SIF obtained at four different gating times of 0.0, 1.6, 3.2,
and 4.8 ns. The distinct changes of the images with the gating
time allow us to distinguish clearly FNDs from Ag nanoclusters in the specimen.
Summary and conclusion
We have observed significant fluorescence enhancements of
FNDs (35 and 140 nm in size) on SIF surfaces. The enhancement observed for the 140-nm FNDs is about 3-fold smaller
than that detected for single quantum dots of B5 nm in
diameter.15,16 The difference is easily comprehensible since
these FND particles are much larger and only B10% of the
fluorophores (i.e. N–V centers) inside the diamond lattices are
estimated to be located within the enhancement region
(B10 nm) above the metal surface. Although the fluorescence
background of the SIF makes it difficult to distinguish 35-nm
FNDs from nearby Ag nanoclusters, the former can still be
identified by using a time gating technique, thanks to the
excellent photostability and the long fluorescence lifetime of
the material. Further improvement of the detection sensitivity
is possible if FNDs containing a higher concentration of
(N–V) centers are available. Such a MEF effect, in combination with the easiness of surface modification, renders FND a
potentially useful tool for long-term observation of
DNA–protein and protein–protein interaction on surface at
the single particle level.
Phys. Chem. Chem. Phys., 2009, 11, 1508–1514 | 1513
Acknowledgements
This work was supported by Academia Sinica and the
National Science Council (Grant No. NSC 94-2120-M-002009- and NSC 96-2120-M-001-008-) of Taiwan. We thank one
of the referees for pointing out a possible mechanism for the
fluorescence enhancement observed in this experiment.
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