Controlled nanocrystallization of gold nanoclusters within surfactant envelopes: enhancing aggregation-induced emission in solution

The nanocrystallization of functional molecules has been a subject of recent interest in the current development of nanotechnology. Herein, we report the unprecedented synthesis of single nanocrystals of a molecular gold nanocluster in a homogeneous solution by using surfactant-based nano-envelopes. The co-assembling of a Au8 nanocluster carrying lipophilic phosphine ligands with sodium dodecyl sulfate (SDS) in an aqueous solution results in the formation of sphere-shaped amorphous nano-aggregates coated with the surfactant. Upon sonication, the spherical amorphous aggregates are smoothly shape-shifted into discrete rhombic nanocrystals, which can be tracked by TEM and solution XRD. The transformation into single nanocrystals occurs exclusively without further growth or agglomeration, implying that the crystal growth is restricted within the surfactant nano-envelopes. The robust but flexible nature of the wrapped surfactant is likely responsible for the controlled crystallization. We also demonstrate that the amorphous-to-nanocrystalline transition in solution remarkably enhances the photoluminescence emission from the nanocluster, providing a clear example of crystallization-induced emission enhancement. Notably, the obtained nanocrystals showed high stability in solution and retained their shape, size, and PL intensity even after several months, owing to the densely packed surfactant shell. The present surfactant-directed nanocrystallization method may be applicable to other molecular species to contribute to the development of nanocluster science as well as the designed synthesis of nanomaterials.


General
Tetrachloroauric(III) acid tetrahydrate (>47.5% for gold) was obtained from Tanaka Kikinzoku Kogyo.1,3-Bis(diphenylphosphino)propane (dppp) (Tokyo chemical industry), sodium dodecyl sulfate (SDS) (FUJIFILM Wako) and potassium cyanide (FUJIFILM Wako) were used as received.Other standard chemicals and solvents were purchased from Kanto chemical and used as received.Dynamic light scattering (DLS) and ζ-potential data were collected on a Malvern Zetasizer Advance-Pro (Red) light scattering system with a 633 nm He/Ne laser at 20 °C using a glass cuvette for DLS and a plastic cell for the ζ-potential measurement, respectively.Transmission electron microscopy (TEM) images were obtained by JEM-2000FX at Faculty of Engineering, Hokkaido University.Scanning transmission electron microscopy (STEM) images and energy dispersive X-ray spectroscopy images were obtained on a JEOL JEM-ARM200F at RIES, Hokkaido University.X-ray diffraction data in solution were obtained at BL05XU in SPring-8 (Hyogo, Japan).Diffraction data were collected using Dectris model Pilatus 3S 1M detectors.The incident X-ray beam (1.00 Å wavelength) was monochromated by a Si (111) double-crystal monochromator.The sample-to-detector distance was 249.8343 mm.The scattering vector q and the position of an incident X-ray beam on the detector were calibrated using several orders of layer reflections from silver behenate.A solution sample was held in a 1.5 mm-diameter glass capillary exposed to an Xray beam for 1.0 s at 25 °C.Powder X-ray diffraction (PXRD) profiles were obtained at room temperature on a Bruker D2 PHASER 2 nd Generation with CuKα radiation.The powder samples of 1-SDS composites (state II and III) were prepared by solvent evaporation using a freeze-drying machine, and were directly subjected to the measurement.Visible absorption and photoluminescence spectra were recorded at 25°C on a JASCO V-670 and FP-8600 spectrometers, respectively, using a quartz cell with a 1-cm path length.Photoluminescence spectra were corrected with rhodamine B and a JASCO ESC-333 standard light source unit, and the quantum yields were determined relative to rhodamine 6G in ethanol.Lifetime data were collected on a Hamamatsu photonics Quantaurus-Tau with the excitation wavelength at 280 nm.Electrospray ionization mass (ESI-MS) spectra were recorded on a Bruker microTOF-HS.Sonication treatments were performed on an SND US-102 ultrasound water bath.

Preparation of TEM and STEM samples
The sample solutions were diluted two-fold with the mixed solvent (MeCN/water = 10/90), and then the diluted solution was deposited on a TEM grid (Nisshin EM, Fine grid, Cat.653, 200 mesh, Cu).All grids were dried in vacuo for 24 hours and then subjected to the TEM and STEM measurements.

Fig. S3 .
Fig. S3.TEM image and DLS profile of the solution sample of state II after storage under ambient conditions for 5 days.

Fig. S5 .
Fig. S5.(a) Absorption (b) ESI-MS spectra of the Au cluster extracted from the solution sample of state III in MeCN/water with dichloromethane.For (a), the spectrum of the original cluster (1) is overlaid for reference.

Fig. S6 .
Fig. S6.PXRD profiles of solid samples of (a) SDS alone, 1-SDS composites (b state II; c state III, see Fig. 1c) prepared from the solutions of MeCN/water (10/90 v/v) by solvent evaporation, and (d) single crystals of the nitrate salt of 1 obtained by recrystallization from CH2Cl2/CH3OH/ether.Asterisks in (b) and (c) indicate the peaks due to the lamellar structure of SDS.

Fig. S7 .
Fig. S7.TEM images of the samples of 1 (0.03 mM) upon sonication for 1 h in MeCN/water (10/90 v/v) in the presence of SDS of varying initial concentrations (af).The initial concentration of SDS is given above each image in parenthesis.

Fig. S10 .
Fig. S10.a) TEM images of the sonicated 1-SDS composite ([1]0 = 0.03 mM; [SDS]0 = 0.20 mM; sonication time = 1 h) after being stored under ambient dark conditions for 3 months.b) Photoluminescence spectra of 1-SDS composite just after the sonication treatment (solid line) and the same sample after being stored under ambient dark conditions for 3 months (dashed line).