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AuNPs would hardly be applicable). We found that when the molar 45 represents the first example of CO2- and light-responsive nanoparticles,
percentage of 3 on the NPs exceeded 25%, the particles were
insoluble in toluene due to their high surface polarity. These
aggregated NPs could not be redissolved using visible light,
sonication, or by purging with N2 (to remove any possibly
DOI: 10.1039/C4CC08541H
light by assembling into aggregates, which could aggregate
further upon the delivery of CO2 (SI, Figure S6C). In other
5
complexed CO2). Therefore, the amount of the diamine 3 was 50 words, only upon pre-exposure to UV light were these specific
fixed at 25% throughout the optimisation of the mSAM composition.
Having screened the ratio of the remaining two ligands (AzoSH
and C12SH) from 1:2 to 2:1, we found that NPs containing 33-35
NPs responsive to CO2.
Conclusions
10 mol% of AzoSH within the mSAM exhibited the behaviour
shown in Fig. 1c. According to DLS, these NPs had hydrodynamic
diameters of 7-10 nm in toluene, and did not aggregate with either
only UV irradiation or CO2 treatment (bubbling into the solution)
(red and blue bars, respectively, in Figure 3). Only when both stimuli
15 were applied at once (4 min of UV irradiation during CO2 bubbling),
however, did the NPs rapidly assemble into aggregates of 800-
1000 nm in size, as shown by DLS (green bars in Figure 3). The
presence of aggregated NPs was also evident from TEM and
spectroscopic analysis (Figure 5b, c). Interestingly, these aggregates
20 were stable in a closed vial placed in the dark for more than 30
minutes.
We prepared gold nanoparticles co-functionalised with azobenzene-
and diamine-based ligands, which rendered the particles responsive
55 to UV irradiation and CO2, respectively. Exposure to UV and CO2
each led to a stepwise increase in the nanoparticle surface polarity,
triggering aggregation in a non-polar medium (toluene).42 The
composition of the mixed monolayer of ligands could be tuned in
order to afford NPs capable of aggregating only when both types
60 of stimuli are applied at once. Both of these stimuli could easily
be turned off / removed without the generation of any by-
products / chemical waste, after which the assembly-disassembly
cycles could be repeated. The aggregates only exist when both light
and CO2 are delivered simultaneously. An important future direction
65 concerning these and other man-made dynamically aggregating
materials is to study / engineer their emergent behaviours under
non-equilibrium conditions.
Notes and references
a Department of Organic Chemistry Weizmann Institute of Science 76100
70 Rehovot, Israel; E-mail: rafal.klajn@weizmann.ac.il
† Electronic Supplementary Information (ESI) available: Synthesis and
charaterisation of ligand 3; synthesis of functionalisation of gold
nanoparticles; control experiments. See DOI: 10.1039/b000000x/
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Figure 5. (A) TEM images of AuNPs before aggregation. (B) The same
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provided by removing one of the two stimuli. For example, when
a solution of aggregated NPs was sonicated, facile disaggregation
30 was observed even under constant UV irradiation (yellow bars in
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