Photoisomerisation and ligand-controlled reversible aggregation of azobenzene-functionalised gold nanoparticles

Article abstract of DOI:10.1039/c4cc02250e

The photochemical behaviour of functionalised gold nanoparticles (AuNPs) carrying azobenzenethiolate-alkylthiolate monolayers was investigated. Repeated trans-cis and cis-trans isomerisation cycles could be performed in all cases with high efficiency. Reversible photoinduced aggregation was observed when azothiolates with long alkyl spacers (≥C₇) were combined with short (C₅) alkylthiolate coligands. The choice of a coligand thus offers control over the aggregation properties of the nanoparticles. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02250e

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Photoisomerisation and ligand-controlled
reversible aggregation of azobenzene-
functionalised gold nanoparticles†
Cite this: Chem. Commun., 2014,
50, 10105
Received 26th March 2014,
Accepted 11th July 2014
Anja Kohntopp, Alexandra Dabrowski, Michal Malicki‡ and Friedrich Temps*
¨
DOI: 10.1039/c4cc02250e
www.rsc.org/chemcomm
The photochemical behaviour of functionalised gold nanoparticles the switches in the ligand shell leads to aggregation of the NPs.
(AuNPs) carrying azobenzenethiolate–alkylthiolate monolayers was This effect may impede further switching after only a few cycles.5
investigated. Repeated trans–cis and cis–trans isomerisation cycles In cases where aggregation is desired, it is of great importance to
could be performed in all cases with high efficiency. Reversible control the process and the structure of the aggregates.8–10 Klajn
photoinduced aggregation was observed when azothiolates with and coworkers for example studied the behaviour of azobenzene
long alkyl spacers (ZC7) were combined with short (C5) alkylthiolate (AB) functionalised AuNPs as a function of AB concentration and
coligands. The choice of a coligand thus offers control over the solvent polarity and found different types of aggregates under
aggregation properties of the nanoparticles.
varying conditions.11,12
Here, we report on AB-functionalised AuNPs with high surface
Functionalised nanoparticles have gained tremendous importance coverage in solution in toluene that can be repeatedly photoswitched
in fundamental and applied sciences because of their unique, size- back and forth between their trans and cis forms and additionally
dependent optical, electronic, magnetic and chemical properties show virtually fully reversible aggregation when they are in the cis
with potential application in medical diagnostics, drug delivery, state. We evaluated the surface concentration of AB molecules on
cancer therapy, nanoelectronics and information storage, imaging the AuNPs independently by UV/Vis spectroscopy and by NMR
and super resolution microscopy, sensors, solar energy conversion, spectroscopy and determined the cis/trans-AB compositions in the
(photo)catalysis, surface coatings or the development of smart photostationary states upon irradiation with UV and Vis light. We
materials.1–3 Currently, particular interest exists in nanoparticles found that the aggregation potential of the functionalised AuNPs
with surface-bound molecular switches that can be controlled by can be controlled through the choice of the alkylthiolate coligand.
ultraviolet or visible light.4 Unlike in solution, however, the photo- Evidently, the tendency for aggregation due to attractive dipole–
switching properties of molecules on surfaces are subject to several dipole forces between the NP-bound cis-ABs depends strongly on the
mechanisms affecting or hindering the outcome: first, dense chain length of the coligand and besides the AB surface coverage.
packing of the surface-bound molecules may leave too little free
For our investigation of the photoswitching properties of ABs on
space for the transformation to proceed, especially when a photo- the gold surface, we functionalised AuNPs with AB-derivatised
isomerisation is associated with a large-amplitude structural alkylthiols (azothiols) as ligands and normal alkylthiols as coligands
change.4–7 Second, energy transfer either between the attached in mixed monolayers (see Fig. 1A and B) by ligand exchange from
switches or between the switch and the nanoparticle substrate may amine-stabilised NPs in mixed solutions (molar fractions xAB = 0.3,
lead to a rapid electronic deactivation of the photoexcited mole- 0.5 or 0.7) of azothiol and alkylthiol as detailed in the ESI.† The alkyl
cules and even prevent the reaction altogether.7 A third major issue (spacer) lengths of the thiols were systematically varied (Fig. 1B). The
arises when a change in properties of NPs upon isomerisation of obtained NPs were characterised using transmission electron micro-
scopy (TEM), NMR and UV/Vis spectroscopy. A typical TEM image is
displayed in Fig. 1C. It shows that the synthesis yielded nearly
Institute of Physical Chemistry, Christian-Albrechts-University Kiel,
spherical products with diameters typically of B4.0 Æ 0.8 nm (single
Olshausenstr. 40, D-24098 Kiel, Germany. E-mail: temps@phc.uni-kiel.de
† Electronic supplementary information (ESI) available: Syntheses details; details standard deviation). Considering the macroscopic density (rAu
on experimental methods; NMR spectra; evaluation of AB surface concentrations
=
19.3 g cmÀ3), the NPs should consist of B2000 Au atoms. 1H NMR
by UV/Vis and NMR spectroscopy; aggregation at different AB surface coverages;
and analysis of cis/trans-AB compositions in the photostationary states. See DOI:
10.1039/c4cc02250e
spectra confirmed the purity of the functionalised NPs (see Fig. 1D).
1
For organic molecules on AuNPs, the H NMR signals are strongly
broadened inhomogeneously and due to much faster spin–spin
relaxation13 so that surface-bound and free ligand molecules in
‡ Current address: European Space Research and Technology Centre, Keplerlaan 1,
PO Box 299, NL-2200AG Noordwijk, The Netherlands.
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Fig. 1 (A) Schematic representation of the functionalised AuNPs with azothiolate ligands and alkylthiolate coligands (NPs and ligands not drawn to scale).
(B) Structural formulas and abbreviations of the different thiols used in this work. (C) Typical TEM image of the AB-functionalised AuNPs (scale bar: 10 nm).
(D) Comparison of the 1H NMR spectra of free AB-OC7SH in solution (blue) and AB-OC7S-/C5S-NPs (red). The lack of sharp 1H signals of the AB-function in
the NP spectrum confirms that the ligands are bound to the NPs (residual sharp lines in the NP spectrum belong to the solvent and a trace of H2O).
the sample solution can be easily distinguished. The absence of sharp
signals of free AB thiols in the measured NMR spectrum thus
demonstrates that practically all AB molecules in the sample are
bound to the AuNPs. An observed high-field shift (see Fig. S1, ESI†) of
the aromatic proton signals with increasing AB surface concentration
supports an irregular, statistical distribution of the azothiolate and
alkylthiolate ligands in the mixed monolayer.
The UV/Vis spectra of the AB-functionalised NPs displayed in
Fig. 2 show two pronounced absorptions, the broad local surface
plasmon resonance (LSPR) band at around l = 515 nm and the pp*
band of the AB ligands in their trans form at l = 350 nm. The weaker
np* AB band at around l = 455 nm is only barely visible. Sizeable
surface enhancement of the AB pp* absorption could be ruled out in
our case by a combination of 1H NMR and UV/Vis spectroscopy as
detailed in the ESI.† Typical AB surface concentrations by both
methods were found to be around B100 molecules per NP. For
photoswitching, the solutions were irradiated at either l = 365 nm
or at l = 455 nm. The results obtained with the six different
azothiolate–alkylthiolate combinations are given in Fig. 2. As can
be seen, UV-induced trans–cis isomerisation leads to a drastic
decrease of the intensity of the pp* band, which recovers upon
Fig. 2 Measured UV/Vis spectra of the AB-functionalised AuNPs before
back-isomerisation with visible light. The photostationary states
PSS365 and PSS455 reached at the two wavelengths (for temporal
evolutions see Section S3, ESI†) represent the switching of the ABs on
the AuNPs to mostly the cis and mostly the trans forms, respectively.
In contrast, with two exceptions (Fig. 2C and E, discussed below) little
change was observed in the region of the LSPR bands. In all cases,
between three and five switching cycles were performed without any
noticeable signs of degradation or reduction in switching efficiencies
of the samples, as evident by the near-perfect agreement between the
measured spectra belonging to the respective photostationary states
which are plotted on top of each other.
irradiation (all-trans, black) and in PSS365 (blue) and PSS455 (red). All samples
were prepared using equimolar azothiol–alkylthiol solutions and switched
back and forth between the two photostationary states reproducibly at least
3 to 5 times, the individual spectra belonging to those switching cycles are
plotted on top of each other. In panels A, B, D and F, only photoisomerisation
of the AB moiety is observed. Panels C and E additionally show the reversible
aggregation of the NPs in PSS365 demonstrated by the broadened LSPR
bands; in this case the spectra show the overall extinction (sum of absorption
plus possible light scattering).
While this general behaviour is the same for all investigated
samples, significant differences are revealed depending on the
10106 | Chem. Commun., 2014, 50, 10105--10107
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alkyl chain lengths of the ligands: in PSS365, the NPs with the on the absolute length difference between the ligands and
shortest azoligand (AB-OC3S) still show significant residual coligands. This suggests that – at least in the present case –
350 nm (pp*) absorption by trans-AB, indicating that not all the solubility of the NPs is not the main driving force for
AB units were isomerised (see Fig. 2A and B). Assuming that the the aggregation, whereas the dipole–dipole forces are. When
spectra of the ABs on the NPs are similar in shape to those in the cis-ABs are isomerised back to trans, the aggregates are no
solution, we found switching efficiencies from the pure trans longer favoured and disaggregate.
form at the start to 70% cis/30% trans in PSS365 and 10% cis/90%
In conclusion, we studied AuNPs with mixed azothiolate–
trans in PSS455 (for details see Section S3.4, ESI†). With longer alkylthiolate monolayers. The samples could be photoswitched
azoligands, the trans-to-cis switching efficiency increased, while the back and forth reproducibly between two photostationary states
cis-to-trans efficiency decreased. For the ligand–coligand combi- (mostly cis and mostly trans) without visible photodegradation
nation with the longest alkyl spacer (AB-OC11S/C10S, Fig. 2F), we after several cycles. Observed differences in switching efficiency
reached switching to 95% cis/5% trans in PSS365 and 25% cis/75% indicate steric or electronic hindrance of the isomerisation
trans in PSS455.
when the alkyl spacer in the azoligand becomes too short.
Much more striking are the differences after irradiation in the Furthermore, we observed a fully reversible aggregation of NPs
shape of the LSPR bands when the longer azoligands are combined switched to the cis state when a long azoligand was combined
with short alkylthiolates (Fig. 2C and E), whereas little change is with a coligand possessing a shorter alkyl chain. Together with the
observed in all systems with decanethiolate coligands (Fig. 2B, D finding that the tendency for aggregation depends on the cis-AB
and F), a strong broadening and decrease in intensity of the LSPR surface concentration, our results lead to the conclusion that dipole–
band together with a sizeable red-shift are found after isomerisa- dipole attraction is the main driving force for the observed aggrega-
tion of samples carrying ABs with long alkyl spacers (C7, C11) in tion. Another critical factor appears to be solubility, as we found that
combination with pentanethiolate coligands (Fig. 2C and E). This azothiolate–AuNPs without alkylthiolates were practically insoluble.
phenomenon indicates aggregation of the NPs after switching of Our approach using mixed monolayers offers control of the photo-
the attached ABs to their cis form. As shown, it is completely aggregation potential of functionalised AuNPs by optimising the
reversible. Fig. S5 (ESI†) demonstrates that aggregation occurs chain length of the coligand and the surface concentration of
whether the sample solutions are stirred or not.
the azoligand. The photoswitching dynamics of our functiona-
The aggregation is attributed to the attractive dipole–dipole lised AuNPs are now under investigation using femtosecond
forces between the cis-ABs in the ligand shells around the NPs, time-resolved spectroscopy.
when they come into close proximity by Brownian motion. The
We gratefully acknowledge the support of this work by the
dipole moment of cis-AB (3.2 Debye) causes weaker solvent Collaborative Research Centre 677 ‘‘Function by Switching’’.
stabilisation of the NPs in nonpolar toluene such that they
aggregate. Related mechanisms have been discussed for various
Notes and references
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AuNP samples increases with higher AB surface concentration.
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NPs. In addition, the coligand chain length seems to play a
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closer contact between the cis-switched NPs and thereby stronger
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after prolonged exposure to UV light, demonstrating that pentane-
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the NPs. Further, the strength of aggregation seems to depend
mainly on the surface concentration of AB on the NPs rather than
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