Rapid Self-Assembly of Metal/Polymer Nanocomposite Particles as Nanoreactors and Their Kinetic Characterization

Self-assembled metal nanoparticle-polymer nanocomposite particles as nanoreactors are a promising approach for performing liquid phase reactions using water as a bulk solvent. In this work, we demonstrate rapid, scalable self-assembly of metal nanoparticle catalyst-polymer nanocomposite particles via Flash NanoPrecipitation. The catalyst loading and size of the nanocomposite particles can be tuned independently. Using nanocomposite particles as nanoreactors and the reduction of 4-nitrophenol as a model reaction, we study the fundamental interplay of reaction and diffusion. The induction time is affected by the sequence of reagent addition, time between additions, and reagent concentration. Combined, our experiments indicate the induction time is most influenced by diffusion of sodium borohydride. Following the induction time, scaling analysis and effective diffusivity measured using NMR indicate that the observed reaction rate are reaction- rather than diffusion-limited. Furthermore, the intrinsic kinetics are comparable to ligand-free gold nanoparticles. This result indicates that the polymer microenvironment does not de-activate or block the catalyst active sites.

Initially, the spectra of the polymer nanoreactor dispersion was recorded as a reference to be 26 subtracted from subsequent spectra as background. The 4-nitrophenol was added resulting in an 27 increase in absorbance at 425 nm. The increase in absorbance due to the addition of 4-nitrophenol 28 occurs within 30 seconds. After 1 minute, the sodium borohydride is added, which results in a further 29 increase in absorbance at 425 nm due to formation of the 4-nitrophenolate ion. The initiation of the 30 reaction corresponding to the start of the induction period was defined as the time at which the 31 absorbance at 425 nm increased to at least 10% of the maximum absorbance of the preceding plateau.

32
The induction time is characterized by a slow decrease (0.002 Abs/s) in absorbance that is 33 followed by a sharp (> 0.01 Abs/s) decline in absorbance indicating beginning of the reduction 34 reaction, which signifies the end of the induction period. Changes in the slope of the absorbance vs.  For analysis of the reaction rate, the data was normalized to the absorbance value at the end of 51 the induction period. The natural log of the normalized absorbance over time was plotted and 52 regions of two distinct slopes were observed. The first region has been attributed to formation of 53 an intermediate [2]. In order to avoid analysis of the intermediate reaction, the apparent reaction 54 rate was calculated from the second region corresponded to when the normalized absorbance fell 55 below 0.67. Data corresponding to a 15% conversion was analyzed when determining apparent 56 reaction rate. The induction time and apparent reaction rate are reported as the average ± standard 57 deviation of three experimental trials.

58
Because the reaction was carried out with a large excess of sodium borohydride compared to 4-59 nitrophenol, the reaction kinetics can be described by pseudo-first-order kinetics. For 60 heterogeneous catalysts, the apparent rate constant is assumed to be proportional to the surface of  and without irradiating the sample by utilizing a very selective pulse, typically a Gaussian pulse with 72 a long pulse duration, at a frequency identical to the resonance frequency the host molecule that is in 73 close spatial proximity to the ligand molecule of interest. Moreover, due to spin diffusion, the 74 signals from the ligands associated with the host molecule will be attenuated as well. A peak 75 subtraction is pursed between the spectra obtained with and without the selective irradiation. The

76
interacting ligands are then probed from the difference spectrum.

77
To evaluate effective transport of the 4-nitrophenol, 1 H-NMR spectroscopy and pulsed field 78 gradient (PFG) NMR were performed using a Bruker Avance II 800 MHz NMR with a 5-mm coil 1H- where I0 is the signal amplitude after the PFG pulse sequence with minimal gradients applied, the 86 gyromagnetic ratio, g the gradient strength applied, is the gradient pulse duration (1 ms were obtained by irradiating at 0 ppm spectrum. A difference spectra between 0 ppm and 6.8 ppm 97 was obtained (0 ppm spectrum -6. 88 ppm spectrum) to analyze the solute molecules that are within 98 the reactor. Using the PFG spectra, the solute peak intensity as a function of gradient strength was 99 plotted and the diffusion coefficient was determined from the slope of the linear fit.

106
The Langmuir Hinshelwood Kinetic Analysis is influenced by the data available at the end of 107 the induction time and start of the reaction. Therefore, we used a more precise method to define the 108 induction time.
Prior to the addition of 4-nitrophenol or sodium borohydride, a background 109 spectra of the polymer nanoreactors was taken in order to adjust the reaction data. With the addition 110 of 4-nitrophenol, the absorbance at 425 nm could be seen to rise from the initial absorbance of zero.

111
A plateau in absorbance occurs within 30 seconds after the addition of 4-nitrophenol, which is

118
The slope of absorbance was calculated over a centered 11-point data range. Noise in the 119 calculated slope was determined to be 10% of the maximum peak absorbance change prior to the 120 beginning of the induction time. The first negative slope change that had a magnitude greater than 121 the defined noise and was consistent over the following two data points was considered the 122 beginning of the reduction reaction and the termination of the induction time. All of the data was 123 then normalized to the absorbance value at the time point, marking the end of the induction period.

124
The reaction data immediately following the induction period was fit to the Langmuir Hinshelwood 125 model.

126
The reduction of 4-nitrophenol catalyzed by metal nanoparticles model is fully described  Figure S3 with good 137 agreement. The full fit parameters are provided in Table S1.

141
The black circles correspond to experimental data points and the red line represents the fitted curve.

142
Since the data is well described by the model, the assumptions of first-order rate kinetics,

169
The decrease in k1 after three recycles suggests leaching from the gold nanoreactor does occur 170 with multiple reuses. Performing ICP on the filtrate, there was ~30% reduction in gold content with 171 each recycle step; thus, the loss in activity corresponds to loss of gold. Since 70% retention of 172 nanoparticles has been reported using centrifugal based separations [6], the loss of gold can be 173 attributed, in part, to the loss of nanoreactors.

185
In this system, the nanoreactor size could be tuned from 110 nm to 170 nm by increasing the 186 block copolymer concentration while holding the core concentration constant (Supporting 187 Information, Figure S3). As expected, the overall nanoreactor size decreased with increasing block 188 copolymer concentration. This trend has been attributed to the relative change in core volume

217
A complementary scaling analysis approach is to consider the bimolecular reaction between 4-218 nitrophenol and nanoparticle catalyst using the Smoluchowski diffusion limited reaction model [8,9].

219
In the limit of slow diffusion, the bimolecular rate constant, kbm, is 220 = (S3)

221
The value of the kbm can be compared to the experimentally determined 2 nd order constants as an

229
Therefore, the 2 nd order rate constant, k, can be determined from measuring the kapp as a function of 230 catalyst concentration [8,9]. An experimentally determined k value approaching kbm would suggest a 231 diffusion limitation.

235
When the nanoreactor concentration or the gold loading was increased, kapp increased; the 2 nd 236 order rate constant was on the order of 10 6 M -1 s -1 . These values are much lower than the kbm ~ 10 8 M -237 1 s -1 indicating that neither internal nor external diffusion from the bulk solution to the nanoreactor 238 limit the apparent reaction kinetics.