Reversible end-to-end assembly of selectively functionalized gold nanorods by light-responsive arylazopyrazole–cyclodextrin interaction

We propose a two-step ligand exchange for the selective end-functionalization of gold nanorods (AuNR) by thiolated cyclodextrin (CD) host molecules. As a result of the complete removal of the precursor capping agent cetyltrimethylammonium bromide (CTAB) by a tetraethylene glycol derivative, competitive binding to the host cavity was prevented, and reversible, light-responsive assembly and disassembly of the AuNR could be induced by host–guest interaction of CD on the nanorods and a photoswitchable arylazopyrazole cross-linker in aqueous solution. The end-to-end assembly of AuNR could be effectively controlled by irradiation with UV and visible light, respectively, over four cycles. By the introduction of AAP, previous disassembly limitations based on the photostationary states of azobenzenes could be solved. The combination photoresponsive interaction and selectively end-functionalized nanoparticles shows significant potential in the reversible self-assembly of inorganic–organic hybrid nanomaterials.


Instrumentation and materials
Bright-field transmission electron microscopy (BF TEM) was performed on a Titan Themis G3 300 (FEI) operating at an accelerating voltage of 300 kV and a Libra 200 FE electron microscope (Zeiss) operating at 200 kV. Sample preparation was performed by incubation of a glow-discharged carbon-coated copper grid (S162, Plano) with 5 µL of the sample for 10 min followed by gentle blotting with filter paper. For the grid preparation of by UV light dissolved assemblies the sample (5 µL) was incubated for 10 min under irradiation (3 W, 365 nm) followed by blotting with filter paper. TEM images were analysed with TIA version 4.5 (FEI) and ImageJ version 1.52h (National Institutes of Health, USA, Java 1.8.0_66).
Ultrapure water was obtained with a PureLab UHQ (ELGA LabWater) water purification system. 1 H NMR and 13 C NMR spectra were recorded on a DPX 300 (Bruker), Avance II 300 (Bruker) or an Avance II

Experimental setup UV-vis titration
All assembly experiments were performed in ultrapure water. The nanoparticle stock solution was diluted to an UV-vis absorbance of the longitudinal SPR band of 0.5-0.7. For the titration experiments the total sample volume was 1 mL. dAAP was dissolved in DMSO (5 mM) and was added to the sample in amounts from 1-10 µL giving a total concentration from 0-50 µM. Two min after each addition a spectrum was recorded. For irradiation experiments, the sample cuvette was placed for 5 min in front of a light source of the respective wavelength (365 nm/520 nm) with the power of 3 W. For the switching experiments the solution was further diluted to a LSPR absorbance of 0.15 for better transmission. The UV-vis measurement was conducted immediately after irradiation. To avoid back isomerization the measurement room is tempered at 20 °C and kept dark.

Synthesis
Per-6-iodo-β-cyclodextrin [1] β-Cyclodextrin (11.6 g. 10.2 mmol, 1.0 equiv) was dispersed in toluene and the solvent removed under reduced pressure. This procedure was repeated to remove residual water from the cavities. Next, the β-cyclodextrin was dried for 24 h at 50 °C under high vacuum. To a solution of triphenylphosphine (40.1 g, 153 mmol, 15 equiv) and iodine (40.5 g, 160 mmol, 15.7 equiv) in DMF (160 mL) the dried βcyclodextrin was added under inert gas atmosphere and the solution was stirred at 80 °C for 18 h. The mixture was then concentrated under reduced pressure to half of the volume and the pH was adjusted

Per-6-thio-β-cyclodextrin (tCD) [2]
Per-6-iodo-β-cyclodextrin (4.17 g, 2.19 mmol, 1.0 equiv) was dissolved in DMF (43 mL) and thiourea (1.30 g, 17.08 mmol, 7.8 equiv) was then added and the reaction mixture heated to 70 °C under an inert gas atmosphere. After 19 h, the DMF was removed under reduced pressure to give a yellow oil, which was dissolved in H2O (200 mL). Sodium hydroxide (1.12 g) was added and the reaction mixture was heated to reflux under an argon atmosphere. After 1 h, the resulting suspension was acidified with a saturated aqueous solution of KHSO4 and the precipitate was filtered and washed thoroughly with distilled water. After drying under high vacuum, the desired product was obtained as white powder.

S5
Scheme S1: Chemical structure of the monovalent AAP molecule. The synthetic procedure can be found in a previously reported study [3].