Enhanced Catalytic Activity of Gold@Polydopamine Nanoreactors with Multi-compartment Structure Under NIR Irradiation

Highlights Gold@polydopamine particles with multi-compartment structure were synthesized by using block copolymer of PS-b-P2VP as soft template and characterized by 3D electron tomography technique. The particles can be applied as catalytic nanoreactors for the full kinetic study of reduction reaction of 4-nitrophenol by NaBH4. The nanoreactors show remarkable enhancement of the catalytic activity under NIR irradiation. Electronic supplementary material The online version of this article (10.1007/s40820-019-0314-9) contains supplementary material, which is available to authorized users.


Kinetic Analysis of the Catalytic Reduction of 4-Nitrophenol
The Langmuir-Hinshelwood kinetics [S1-S4] has been used for the mechanistic analysis of the catalytic activity. As shown in Fig. S7, 4-nitrophenol (Nip) is first reduced to 4-nitrosophenol and then to 4-hydroxylaminophenol (Hx). In the final step, Hx is reduced to 4-aminophenol (Amp) [S2]. The kinetic study follows a system of two coupled differential equations which describe the two steps of the reduction [S2-S4]: Here KNip, KHx and KBH4 are the Langmuir adsorption constants of the respective compounds, and ka, kb represent the reaction rate constants of step A and B, respectively. Equation (S1) describes the decay rate of 4-nitrophenol and the generation of Hx. Equation (S2) Fig. S8 it can be seen that the experimental data can be well fitted even when the conversion reaches 70%. This is the first time that full kinetic study has been approved to be applied for catalytic nanoreactor with complex nanostructure.

Theory for Surface Versus Diffusion Controlled Reactions
The total catalytic reaction time in a pseudo-unimolecular reaction, −1 , is the sum of the time for the reactant 4-nitrophenol to diffuse to the gold nanoparticles in the Au@PDA nanoreactors, −1 , and the time to get reduced by sodium borohydride NaBH4 adjacent to a nanoparticle, −1 , Likewise, the diffusion time has two contributions: the diffusion from the bulk to the Au@PDA nanoreactor, 0 −1 , and the diffusion from the outer surface of the PDA shell to the surface of a gold nanoparticle, −1 , i.e. −1 = 0 −1 + −1 . Since the density of gold nanoparticles in the nanoreactor is very large (e.g., as shown in Fig.  3), according to theoretical study for nanoreactors [S5] the rate limiting step in the diffusion approach (i.e. the slowest time) is the mean time to reach the nanoreactor from the bulk. Thus, in the case of a fully diffusion-controlled reaction, the macroscopically observable rate is given by the Smoluchowski rate [S5] ≈ 4 0 0 (S4) where 0~1 nm 2 ns -1 is the diffusion coefficient of 4-nitrophenol in water, 0~ (100-200) nm is the outer radius of the nanoreactor, and = 0.025 mg mL -1 is the concentration of nanoreactors in solution. We estimate the molecular weight of our Au@PDA nanoreactors as: being = (4 /3) 0 3 the nanoreactor volume, the fraction of empty space in the porous hybrid nanoreactors, which we estimate to be 0.5 (based on analysis of tomography, Fig. 5 in the main manuscript), and =1.5 g cm -3 the mass density of the nanoreactor which we assume to be equal to the mass density of condensed PDA [S6]. Taking this into account, we find that the molar concentration of Au@PDA nanoreactors is ~4 • 10 −12 M.
If we make an order of magnitude analysis, we conclude that −1 ~ 0.1 − 1 s. From Fig. S9 and the other experimental data, we note that the apparent rate constant is, ~ 10 −3 − 10 −2 s -1 , which means that the total catalytic reaction time is S7/S9 −1 ~ 100 − 1000 s. Since the diffusion time is approximately at least 2-3 orders of magnitude smaller than the total catalytic reaction time, using Eq. (S3) we clearly conclude that the reaction is surface-controlled.
We express the surface reaction rate, , as Where , is the 4-nitrophenol concentration in the nanoreactor next to the gold nanoparticles, and = Δ , being the fraction per unit time of the 4nitrophenol molecules that are reduced by NaBH4, and Δ the volume of the shell next to the gold nanoparticles where effectively the chemical reaction is happening. As a consequence, the surface reaction is directly proportional to the 4-nitrophenol and BH4concentrations within the reactive volume. For large concentrations, a competition of both reactants for reactive sites on the metal surface would lead to a saturation and to a subsequent slowdown of the surface reaction rate.
We find such behavior in the total reaction rate, Fig. S9. The apparent rate, , increases linearly with increasing BH4concentration. The diminution of with increasing 4-nitrophenol concentration is due to the nearly full coverage of the nanoparticles surface by 4-nitrophenol, which as well slows down the injection of electrons to the metal surface. If the reaction would be diffusion-controlled, a modification of the BH4concentration would not affect the total rate (since the concentration of BH4is in large excess and the bimolecular reaction can be effectively considered as pseudo-unimolecular). This fact supports our conclusion about the surface-controlled nature of the catalytic reaction.

Fig. S9
Dependence of the apparent rate constant, , with the borohydrate concentration for different initial 4-nitrophenol concentrations. The solid lines refer to the fit of the experimental data obtained using, e.g., Eq. (3a) in Ref. [S7] Fig. S10 Control experiment with only reactants of 4-nitrophenol and BH4but without Au@PDA nanoreactors under NIR irradiation (808 nm, 3 W cm -2 , 500 S)