Photochromic-fluorescent-plasmonic nanomaterials: Towards integrated three-component photoactive hybrid nanosystems

Article abstract of DOI:10.1039/c4cc02179g

Silica-coated gold nanorods functionalized with grafted fluorescent and photochromic derivatives were synthesized and characterized. Spectroscopic investigations demonstrated that cross-coupled interactions between plasmonic, photochromic, and fluorescence properties play a major role in such nanosystems, depending on the thickness of the silica spacer, leading to multi-signal photoswitchability.

Full text of DOI:10.1039/c4cc02179g

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Photochromic–fluorescent–plasmonic
nanomaterials: towards integrated three-
component photoactive hybrid nanosystems†
Cite this: Chem. Commun., 2014,
50, 7299
Received 24th March 2014,
Accepted 14th May 2014
K. Ouhenia-Ouadahi,a R. Yasukuni,‡ab P. Yu,c G. Laurent,a C. Pavageau,a J. Grand,b
J. Guerin,c A. Leaustic,c N. Felidj,b J. Aubard,b K. Nakatania and R. Metivier*a
´
´
´
´
DOI: 10.1039/c4cc02179g
www.rsc.org/chemcomm
Silica-coated gold nanorods functionalized with grafted fluorescent and between the metallic nanoparticle and the photochromic
photochromic derivatives were synthesized and characterized. Spectro- molecule,3b,6a and the overlap between the SPR and the electronic
scopic investigations demonstrated that cross-coupled interactions transition of the photochromic unit have been shown to play a major
between plasmonic, photochromic, and fluorescence properties play a role in the coupling strength.7 Besides, combining photochromism
major role in such nanosystems, depending on the thickness of the silica and fluorescence in the same system is of great advantage for
spacer, leading to multi-signal photoswitchability.
some applications such as optical data storage8 or high-resolution
imaging.9 Molecular systems showing both photochromic and
Hybrid systems showing coupling between plasmonic and fluorescence properties can have several different structures:10 the
organic units have recently generated numerous studies, with photochromic molecule may be fluorescent itself;11 the photo-
various innovative applications such as advanced sensing and chromic and the fluorescent units may be covalently bonded;12
biomedicine,1 allowed by an improved understanding of the or both units may be non-covalently bonded.13
enhancement mechanisms arising from the surface plasmon
In a hybrid system, the plasmonic unit plays a central role since it
resonance (SPR) excitation of metallic nanoparticles. Such allows modifying the photochromic or fluorescence activity of the
studies of hybrid metallic/organic materials, lying at the inter- system. Many studies used spherical nanoparticles as the plasmonic
section of chemistry, physics and nanotechnology, have been unit because of their easy chemical synthesis or their commercial
coined ‘‘molecular plasmonics’’. Within this scope of research, availability. However, using nanorods instead of nanospheres
the interplay between photophysical properties, such as fluores- presents several advantages: (i) due to their anisotropic geometry,
cence or photochromism, and surface plasmons has recently nanorods show two distinct SPR, (ii) as the longer axis increases,
triggered a great deal of interest. Concerning the coupling multipolar resonances are observed, giving rise to a higher electro-
between plasmon and fluorescence, the control of the distance magnetic enhancement.14 Changing the aspect ratio of gold nano-
allows the fine tuning of the interaction, leading to quenched rods during the synthesis allows setting precisely the SPR spectral
or enhanced fluorescence.2 In spite of fewer published reports, position within the visible-near infrared range.15
coupling plasmonic structures with photochromic molecules
In this work, we describe a three-component hybrid system
shows also appealing advantages.3 Several consequences of this based on functionalized gold nanorods (GNRs) showing plasmonic,
coupling have been demonstrated: it can lead to a shift or photochromic and fluorescence properties. To the best of our
distorsion of the SPR under light irradiation, a decrease of the knowledge, it is the first time that an all-in-one system is designed,
photochromic activity,4 an irreversibility of the photoreaction5 or built and characterized to obtain up to five different signal outputs:
an improvement of the photoswitching efficiency.6 The distance two visible light scattering resonances from the GNR core, UV and
visible light absorption and blue fluorescence emission from the
surrounding molecular species.
a
´
Laboratoire PPSM, ENS-Cachan, CNRS, 61 av President Wilson,
Scheme 1 presents the final hybrid nanostructure which is
composed of three different parts. The GNR core is coated with
a silica shell. The role of the shell is double. On one hand, the
silica coating is used as a spacer between the molecules and the
metallic particle which is a crucial parameter for the plasmon
and fluorescence–photochromism coupling.2,3b,6a On the other
hand, the silica shell is a convenient platform allowing further
covalent bonding of functionalized molecules. The GNRs were
94235 Cachan cedex, France. E-mail: metivier@ppsm.ens-cachan.fr
b
c
´
¨
Laboratoire ITODYS, Univ Paris Diderot, Sorbonne Paris Cite, 15 rue J-A de Baıf,
75205 Paris Cedex 13, France
´
ˆ
ICMMO, Universite Paris-Sud, Bat 410, 15 rue Georges Clemenceau,
91405 Orsay cedex, France
† Electronic supplementary information (ESI) available: Synthetic protocols and
size distribution histograms of the GNR samples. See DOI: 10.1039/c4cc02179g
‡ Present address: Laboratoire CSPBAT, Univ. Paris 13, 74 rue Marcel Cachin,
93017 Bobigny cedex, France.
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Fig. 1 SEM images of (a) the GNR cores, and the silica-coated GNR
(b) GNR-SiO2(a) and (c) GNR-SiO2(b), corresponding to silica-layers of
8 Æ 1 nm and 16 Æ 1 nm. Insets: SEM images of the corresponding hybrid
nanosystems incorporating grafted P molecular derivatives. (d) Extinction
spectra of the hybrid nanosystems in acetonitrile solution.
Scheme 1 Synthesis of the silica-coated gold nanorods with surface-
grafted photochromic and fluorescent derivatives.a
prepared by the ‘‘seed mediated growth method’’ devised by
El-Sayed et al.,16 and further coated with a silica shell through a
¨
modified Stober protocol (Scheme 1, step i, see details in the
ESI†).17 The thickness of the silica layer can be efficiently varied
with the silica precursor reaction time. Two samples were
obtained: GNR-SiO2(a) and GNR-SiO2(b), corresponding to 15 and
30 min of reaction time, respectively. Representative scanning
electron microscopy (SEM) images of the GNR before coating,
GNR-SiO2(a) and GNR-SiO2(b) samples are displayed in Fig. 1(a–c).
In both cases, the average length and diameter of the GNR are,
Scheme 2 Synthesis of the photochromic compound P.a
respectively, 50 Æ 2 nm and 20 Æ 2 nm (see ESI†). When the GNRs Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction20 to
are dispersed in acetonitrile, the longitudinal (resp. transversal) SPR one equivalent of 3-azidopropyltriethoxysilane (Scheme 1, step ii).
is located at 650 nm (resp. 510 nm). The silica coating is almost After 12 hours, the mixture was added to the silica-coated gold
uniform around the GNRs, even if a more accurate analysis reveals nanorod suspension (GNR-SiO2(a) or GNR-SiO2(b)) without prior
slightly smaller thickness at their extremities. The average value of purification (Scheme 1, step iii) with 3 equivalents of acetic acid.
the silica shell thickness has been measured over hundreds of After 12 h at room temperature, the resulting hybrid nanostructures
particles to be 8 Æ 1 nm and 16 Æ 1 nm for GNR-SiO2(a) and were purified by several centrifugation steps with acetonitrile.
GNR-SiO2(b), respectively. After silica coating, the longitudinal The effect of the grafting of P is shown in Fig. 1d. Comparing the
SPR peaks are red-shifted by 15 nm for GNR-SiO2(a) and 20 nm extinction spectra of GNR-SiO2-P(a) (resp. GNR-SiO2-P(b)) with the
for GNR-SiO2(b) (see ESI†), which is attributed to the increasing of extinction spectra of GNR-SiO2(a) (resp. GNR-SiO2(b)), a red shift
the refractive index of the surrounding medium.18
along with a broadening of both longitudinal and transversal SPR
The photochromic and fluorescent diarylethene P (Scheme 1) was are observed. This effect can be attributed to a variation of the local
chosen as an organic derivative to be combined to silica-coated refractive index due to the grafting of P molecules at the surface of
GNRs. P was prepared by the synthetic route outlined in Scheme 2. the silica shell.
Boronic ester 2 was synthesized in 3 steps in high yield from
The absorption spectrum of the diarylethene molecule in its open
2-(4-methoxy-phenyl)thiazole. Suzuki coupling with 3 provided P form (P-OF) in acetonitrile is plotted in Fig. 2a, with absorption
after deprotection of the methoxy group and propargyl grafting maxima at 277 nm and 317 nm. When irradiated at 335 nm, the
(see details in the ESI†). The functionalization procedure of the solution turns from colorless to blue, indicating the open to closed
silica-coated GNR was modified from the one developed for coating form reaction (P-OF - P-CF) reaction. A well-defined absorption
mesostructured silica shells on CTAB-stabilized inorganic nano- band of P-CF appears in the visible range (lmax = 588 nm), together
crystals.19 In the present case, the incorporation of the diarylethene with changes in the UV range (lmax = 271 nm and 331 nm). The
into the silica shell was realized by one-pot covalent grafting reversibility of the photochromic reaction is fully achieved under
as illustrated in Scheme 1. P was linked by the well-known irradiation at 575 nm.21 Fig. 2b and c, respectively, show the
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extinction spectra of colloidal suspension of hybrid samples broadening and wavelength shift of the SPR bands. Moreover, this
GNR-SiO2-P(a) and GNR-SiO2-P(b) in acetonitrile, before and after experimental effect was completely reversible under irradiation at
irradiation at 335 nm. In both cases the total extinction of the hybrid 575 nm. As this behavior occurs reversibly upon light irradiation of
system increases at 350 and 600 nm, and decreases at around the hybrid system, which induces the P-OF - P-CF (UV) or P-CF -
300 nm. These spectral modifications are directly attributed to the P-OF (visible) photochromic reactions, it clearly demonstrates
absorption of the grafted P-CF.
a direct interaction between the plasmonic GNR core and the
Fig. 2d shows differential extinction spectra DExt, obtained surrounding photochromic molecules, leading to a very sensitive
by subtracting extinction spectra recorded after and before photo-control of the SPR properties.22 The DExt spectrum of the
irradiation at 335 nm (see ESI†). These DExt spectra are useful GNR-SiO2-P(b) sample plot in Fig. 2d (red curve) is exactly overlaying
to identify the photochromic contribution during the irradiation the one of P in solution (dashed blue curve). In this case, there is
process and discriminate the possible modification of the SPR. In no modification of the position, the shape nor the intensity of the
the DExt spectrum of the photochromic compound P in acetonitrile SPR bands upon irradiation. This behavior shows the absence of
solution (Fig. 2d, blue dashed curve), which is used as a reference, coupling between the plasmon of the core and the photochromic
positive bands at 350 and 600 nm (resp. a negative one at 300 nm) molecules. The difference of behavior between the two samples is
are observed, corresponding to spectral regions where the P-CF attributed to the thickness of the silica shell, which is larger in the
absorption is higher (resp. lower) than the P-OF absorption. Inter- case of GNR-SiO2-P(b) than GNR-SiO2-P(a) (16 Æ 1 nm vs. 8 Æ 1 nm,
estingly, the DExt spectrum of the GNR-SiO2-P(a) sample displayed in respectively, as mentioned above). Indeed, many previous
Fig. 2d (green curve) exhibits many differences from the one of P in studies have already emphasized that the distance between
solution. For wavelengths below 400 nm, both spectra look similar, plasmonic structures and photoactive molecules is a critical
meaning that there is only a photochromic contribution at these parameter to observe coupling between the two parts.3b,6a The
wavelengths. Between 400–550 nm and 650–900 nm, the DExt observed phenomena and the difference between samples
spectrum of the sample GNR-SiO2-P(a) is below the P curve. These GNR-SiO2-P(a) and GNR-SiO2-P(b) clearly confirm the existence
two different areas correspond to the transversal and longitudinal of a distance-dependent coupling between the GNR and the
SPR of the GNR core, and indicate a negative contribution of the photochromic organic derivatives.
SPR. Contrarily, in the 550–650 nm region, the DExt spectrum of
Experiments have been conducted to evaluate the fluorescence
GNR-SiO2-P(a) is higher than the P curve, and denotes a positive properties of our hybrid nanosystems. In acetonitrile solution,
contribution of the SPR. These observations can be analyzed as a photochromic compound P actually shows fluorescence emission
combination of several SPR changes: decrease of the SPR intensities, in its open form (P-OF), and not in its closed form (P-CF).21 As
expected, the hybrid nanosystems GNR-SiO2-P(a) and GNR-SiO2-P(b)
are also fluorescent, revealing the emissive properties of the grafted
P-OF molecules at the surface of the silica-coated GNR. The emission
spectra were recorded in acetonitrile colloidal suspension and show
a broad band peaking at 435 nm. Under irradiation at 335 nm, the
P-OF - P-CF reaction is induced, leading to a significant decrease of
the fluorescence emission (80–85% of the initial fluorescence level
in both cases) which is fully recovered under irradiation at 575 nm
(P-CF - P-OF reaction), as evidenced in Fig. 3. Several complete
cycles can be achieved without any significant fatigue effect (Fig. 3,
insets). This behavior is similar whatever the considered sample.
Based on these results, fluorescence quantum yields (FF) have been
measured to be 0.038 and 0.048, respectively, for samples GNR-SiO2-
P(a) and GNR-SiO2-P(b). These values clearly show that both samples
are slightly more fluorescent than the free P molecule (FF = 0.016 in
acetonitrile)21 with a more pronounced fluorescence in the case
of the sample GNR-SiO2-P(b), probably due to a local field
enhancement in the presence of GNRs. Again, this result high-
lights the important role of the silica shell in modulating
plasmonic–fluorescence interaction and, following, the fluores-
cence emission of the hybrid system.
In conclusion, we designed, synthesized and characterized multi-
component hybrid nanosystems composed of a plasmonic GNR
core functionalized by molecules displaying both photochromic and
fluorescence properties. Accurate control of the distance between the
Fig. 2 (a) Absorption spectra of P-OF and P-CF molecules in solution. (b, c)
Extinction spectra of GNR-SiO2-P(a) and GNR-SiO2-P(b), respectively, before
and after irradiation at 335 nm. (d) Extinction difference spectra of the samples
plasmonic core and the photoactive organic counterparts has been
achieved by means of a silica shell, yielding evidence of plasmon–
photochromism coupling on one side and plasmon–fluorescence
obtained after subtraction of the corresponding extinction spectra after and
before irradiation at 335 nm. All spectra were recorded in acetonitrile.
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7302 | Chem. Commun., 2014, 50, 7299--7302
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