Higher-order assembly of BSA gold nanoclusters using supramolecular host–guest chemistry: a 40% absolute fluorescence quantum yield

Herein, we report for the first time a highly emissive higher-order assembled structure of BSA-Au NCs in the presence of cucurbit[7]uril, which enhances the absolute fluorescence quantum yield of BSA-Au NCs up to 40%. Cucurbit[7]uril neutralizes the surface charge of BSA-Au NCs, and hence aggregation happens. This aggregation shows reversible disaggregation in the presence of adamantylamine.

Experimental methods 3-4 4 Figure S1: Optical and Microscopic images of BSA-Au NC 4 5 Figure  Electronic Supplementary Material (ESI) for Nanoscale Advances. This journal is © The Royal Society of Chemistry 2022 Au NC in presence of CB7 15 Figure S12: Time-dependent FL study of BSA-Au NC 10 16 Figure S13: Encapsulation study of BSA-Au NC with CB7 10 17 Figure S14: Scattering study BSA-Au NC in presence of CB7 11 18 Figure S15: UV-vis absorption spectra of BSA-Au NC@CB7 11 19 Figure S16:

Synthesis. Synthesis of BSA-Au NC.
BSA-Au NCs were synthesized according to the reported procedure. 2 In a typical synthesis process, 5 mL of 10 mM tetrachloroauric acid trihydrate (HAuCl 4 .3H 2 O) was added to 250 mg of BSA in 5 mL MQ-water under vigorous stirring for 5 min. Then 0.5 mL of 1 M NaOH was added to the above solution to make the pH ~12. The mixture was allowed to incubate at 37 °C for 12 h until the solution turned golden brown color. The mixture was stored at 4 °C for further use.

Synthesis of Lys-Au NC.
We have synthesized Lys-Au NC according to the literature procedure. 3 To a 5 mL of 10 mg/mL lysozyme aqueous solution, 5 mL of 4 mM HAuCl 4 was added and diluted to 15 mL. After 5 minutes of mixing, a 0.5 mL of 1 M NaOH solution was added to make the pH of the solution ~ 12. The reaction mixture was incubated at 37 °C overnight.

Synthesis of CB7.
In a typical synthesis process, glycoluril (10 gm, 70.4 mmol) was added to a 250 mL round-bottomed flask equipped with a magnetic stir. A 37 % HCl (14.2 mL) was added to the flask. Then paraformaldehyde (4.22 gm, 140.7 mmol) was added in a stirring condition. The viscous product was allowed to stir for 30 min until it was set as a gel. Then heated it at 100 °C and refluxed at the same temperature for 18 h. Then the mixture was allowed to cool for 24 h to get the crystalline products. The purification was carried out according to the previously reported literature. 4 The 1 H NMR and LC-MS have been shown in Figures S3 and S4

UV-visible and Turbidity measurement.
UV spectrophotometer (Perkin Elmer, Lambda 750) was employed for the turbidity measurement at 550 nm where the system doesn't have any absorption. Turbidity is defined as 100 -% T, where T is transmittance. Turbidity was measured at different ratios of CB7 to BSA-Au NC. A pH-dependent turbidity study of BSA-Au NC@CB7 and Lys-Au NC@CB7 have done at different pH at 550 nm.

Fluorescence Study of Au 25 @BSA with CB7.
To study the interaction of Au 25 @BSA with CB7, 40 µL of cluster solution was taken and diluted to 200 µL by adding a different volume of CB7 (10 mM in PB of 5 mM). Time-dependent fluorescence studies were carried out by mixing CB7 and incubating at 37 °C for 15 min, 6 h, 12 h, and 24 h. The absolute fluorescence quantum yield of BSA-Au NC was measured in an integrated sphere with different molar ratios of CB7 to BSAAu NC. The fluorescence lifetime was also measured in the same way.

DLS study and Zeta potential measurement.
To know the hydrodynamic size and surface charge of the system, DLS study and Zeta potential measurements were performed respectively at different molar ratios of CB7 to BSA-Au NC.                          Table 2.