Molecular cage-bridged plasmonic structures with well-defined nanogaps as well as the capability of reversible and selective guest trapping

Molecular cage-bridged gold nanoclusters with well-defined hotspots were demonstrated as novel plasmon-assisted nanoreactors.


Characterization.
1 H NMR spectra was obtained using a JEOLJNM-ECA300 at 400 MHz. XPS data was collected with a PHI5300 ESCA instrument using an Al K X-ray source. The UV/vis absorption spectra were recorded using a Perkin-Elmer UV/vis Lambda35 spectrometer. Scanning electron microscopy (SEM) images were obtained with a field-emission scanning electron microscope (LEO-1503, Germany). TEM image was obtained with a using JEM 2010 high-resolution transmission electronic microscope at an acceleration voltage of 100 kV. Raman spectra were measured upon excitation with a 633 nm laser line. Inelastically scattered light was collected with a LabRam HR system (Horiba-Jobin Yvon), equipped with a confocal optical microscope, high resolution gratings (1800 g mm -1 ), a Peltier CCD detector and an x, y, z motorized stage. Spectra were collected by focusing the laser line onto the sample, using a 50× objective (N.A. 0.5), with accumulation times of 90 s and laser power at the sample of 1 mW.

(2,6-dibromopyridin-4-yl)methanol (3):
Sodium borohydride (15.87 g) was added to a suspension of the 2 (25.0 g, 8.0 mmol) in 250 mL of ethanol. The mixture was refluxed for 3 h and cooled to room temperature. The ethanol was removed under vacuum, and then 320 mL of 1M HCl aqueous solution was added to decompose the excess sodium borohydride. The pH of the reaction mixture was adjusted to 7 by addition of saturated aqueous Na 2 CO 3 . The resulting solution was extracted with ethyl acetate (3 × 200 mL) and dried over anhydrous sodium sulfate, and the solvent was removed under vacuum. The desired solid was obtained in 53% yield and was used without further purification. 1 H NMR (400 MHz, CDCl 3 ): δ 4.72 (s, 2H), 7.47 (s, 2H).

2,6-bis(ethynyl)-4-[(methylthio)methyl]pyridine (7):
Under nitrogen gas trimethylsilylacetylene (2.94 g, 30.0 mmol) was added to a solution of 5 (2.98 g, 10.0 mmol) in deoxygenated distilled triethylamine (35 mL) and THF (35mL) with tetrakis(triphenylphosphine)palladium (0) (590 mg, 0.50 mmol) and CuI (38.1 mg, 0.2 mmol) in a 100 mL Schlenk flask. After stirring for 1 h at room temperature, the reaction system was heated to 60℃, at which the reaction solution was further stirred for 12 hours. The reaction system was cooled to room temperature and the THF was removed under vacuum. The mixture was dissolved in H 2 O and extracted with CH 2 Cl 2 three times. The combined dichloromethane layer was washed with brine and dried over sodium sulfate. After evaporation of the CH 2 Cl 2 , the crude 6 was brown oil. A solution of crude 6 (1.6 g, 4.8 mmol) in THF (30 mL) was reacted with TBAF (2.0 g, 14.4 mmol) at room temperature for 1 h. After removing the solvent, water was added and then the system was extracted with CH 2 Cl 2 three times. The solution was dried over sodium sulfate. After the solvent evaporated, the crude residue was purified by column chromatography on silica gel with petroleum ether/ethyl acetate (5%) as eluent to give pure 7 (450 mg, 45 %). 1

Synthesis of Pd 2 L 4 cage:
To a stirring solution of [Pd(CH 3 CN) 4 ](BF 4 ) 2 (11.1 mg, 0.025 mmol, 1 eq) in acetonitrile (dry, 1.5 mL) was added L (17 mg, 0.05 mmol, 2 eq.). The reaction mixture was heated at 50 °C for 30 minutes. The product was precipitated as a pale yellow solid by vapor diffusion of diethyl ether into the cooled reaction mixture.

Single-Crystal X-ray Crystallography
Single-crystal X-ray diffraction data were obtained on a D/max-RB (Japan, Rigaku) X-ray diffractometer equipped with a low temperature device and a fine-focus sealed-tube X-ray source (graphite monochromated Mo-Kα radiation, λ = 0.71073 Å, ω-scans with a 0.5 o step). Suitable single crystals were directly picked up from the mother liquor for data collections.

Host-guest interaction
For the guest 4-NP and HQ, after adding guests into the CD 3 CN solution of the cage, the obtained solution were examined by 1 H NMR, ESI-MS and UV-Vis. For the guest CP, DCTP and PDA, after adding guests into the CD 3 CN solution of the cage, the obtained suspension was sonicated for 10 min, and then the solution of the mixtures were examined by 1 H NMR and ESI-MS. However, due to the limited solubility of CP, DCTP and PDA in CH 3 CN solvent, the 1 H NMR titration and UV-Vis titration with the cages in CH 3 CN can not be performed.

UV Binding constant determination.
Each titration study was conducted at least three times and the results show good reproducibility. Binding constants for 1:1 association were obtained according to the reported literatures. [1] Binding constants for 1:2 association were obtained according to the non-cooperative binding model using bindfit. [2] 1 H NMR Titrations.
For each titration, a solution of cage (1 mM) was titrated with a solution of guest (1 mM), maintaining a constant total concentration throughout. For each observable peak shift of cage in the 1 H NMR spectrum and the Job plot was applied to determine the binding.

Preparation of plasmonic clusters
The assembly of Au NPs with aid of the Pd 2 L 4 cage in bulk gold colloidal solution.

The assembly of Au NPs with aid of the Pd 2 L 4 cage using microfluidic device.
Taking advantage of a microfluidic device to prepare the plasmonic clusters, the final concentration of cage and Au NPs was adjusted to be 10 -7 M and 10 -8 M, respectively. After reaction for 6 h, the different populations of clusters were purified by density gradient centrifugation. The density gradient was made by mixing different ratios of DMSO and glycerol (0-20% of glycerol by volume) and separation is then induced by the specific sedimentation velocity of each different cluster under centrifugation at 7500 rpm for 20 min. We found that a relative molar ratio of cage to the Au NP should be maintained between 50 and 100 to form the plasmonic structures with higher-CNs. When the molar ratio of cage was increased, a much faster reaction rate was observed and aggregates of NPs were formed.

Near field and SERS Calculation
FDTD calculation. The FDTD method was employed to determine the electric field intensities and distributions at the surface of 15 nm Au NPs by FDTD Solution software. In the calculation, the incident electric field is defined as a plane wave with a wave vector that is normal to the injection surface. The overall simulation time was set to 1000 fs and calculated at the wavelength of 633 nm. For the UV-vis spectra, the refractive index is 1.4.

SERS EF calculation.
Experimental EF was calculated according to the method proposed by Van Duyne et al. [3] Briefly, the following expression was used: where I SERS is the surface-enhanced Raman intensity, N SERS is the number of molecules bound to the enhancing plasmonic substrate, I normal is the regular Raman intensity collected from solid molecules, and N normal is the number of molecules in the excitation area. Figure S25. SERS enhancement factor of plasmonic clusters with different CNs.

Preparation of Plasmonic Substrate
The plasmonic substrate was prepared by layer-by-layer method. First, the substrate was modified by APS, followed by modification of Au NPs through the interaction between Au NPs and amino group of APS. Then through sequent immersion in the solution of Au NPs for 4 h and solution of cage for 2 h. The concentration of cage is 10 -4 M Figure S26. Raman spectra of cage solid and cage on plasmonic substrate.