1.5 Experimental section
1.5.3 Nanoparticle functionalization, purification, control experiments, and reaction setup
Thiol solution was first prepared by mixing BipySH (C6-Bipy) (8.51 mg, 0.0281 mmol) and TMASH (C20-TMA) (12.72 mg, 0.0281 mmol) in chloroform (3 mL). In a typical procedure, the thiol solution was added into 4 mL toluene solution of as-synthesized oleylamine-coated nanoparticles to perform ligand exchange. Then, charged nanoparticles were precipitated under non-polar solvents, and were allowed to settle by gravity over 2 hours. The clear supernatant which contained desorbed oleylamine
ligands and unreacted excess thiols was then removed, whereas the precipitated, charged NPs were washed by fresh portion of chloroform 5 times to completely remove any remaining ligand molecules.
After washing and drying, NPs were re-dissolved in 8 mL of methanol/water mixture (v/v = 1:1).
Figure 1.14 HR-TEM images of Au nanoparticles after ligand exchange
Figure 1.15 Thermodynamic calculations illustrating strong preference towards random spatial distribution of the two types of ligands on the NP surface. (a) Free energy and its components for
different degrees of disorder on the surface of NP. Disorder and entropy are measured with respect to the state (shown in panel b) that minimizes electrostatic energy: state with disorder is obtained from state by choosing a random subset Ψ out of all surface ligands such that Ψ has = elements total (rounded to the nearest integer) and then randomly reordering the elements within subset Ψ . That is, if a number out of ligands in the subset Ψ were charged, then subset Ψ is replaced with a one of ( , ) = !/( ! ( − )!) equiprobable sequences of length containing charged ligands. Thus, the number of charged ligands within subset Ψ is not changed by this procedure. Influence of gold core, screening, and dielectric saturation are not included in electrostatic energy. Van der Waals interaction energy was assumed to be equal to 20 between any pair of charged ligands and equal to 10 between any other pair of ligands.
Error bars correspond to standard deviation in an ensemble with a given degree of disorder . Temperature is 293 K. (b-d), Both hemispheres for various surface states: state (b), one of the states sampled with = 0.5 (c), and one of the states sampled with = 1 (d). Neutral ligands are indicated by grey balls, charged ligands – by orange balls. Although it might not seem so to a human eye, (d) is the perfectly random distribution of ligands.
The purified nanoparticles in methanol/water mixture (4 mL of methanol, 4 mL of water) were added to a vial containing solid CuI and this heterogeneous mixture was left for 1 hour (we found that at this stage, the NPs tended to precipitate under vigorous stirring). The amount of CuI was 10-fold molar excess with respect to the number of Bipy moieties attached to the surface of the nanoparticles. Next, the solution was transferred to a conical tube and centrifuged at 5000 rpm for 5 minutes to collect the excess solid CuI at the tube’s bottom. Importantly, the centrifugation did not precipitate the NPs from the water/methanol mixture. This centrifugation was repeated 5 times (each time transferring the supernatant containing the NPs to a fresh vial) to ensure complete removal of CuI.
Figure 1.16 HR-TEM images of Au nanoparticles after CuI addition
Control experiments confirmed that CuI can be removed from the methanol/water mixture in this manner. In the first experiment, only methanol/water mixture (8 mL) was added to a vial containing same amount of solid CuI and this mixture was subjected to the same centrifugation procedure (5 times).
ICP-AES elemental analysis of the supernatant after the fifth centrifugation confirmed that Cu signal was below the ICP machine detection limit (0.055 mg/kg). In the second control experiment, we verified that such a methanol/water mixture cleaned of CuI was not catalytically active to affect click reactions:
for the four individual reactions (azide-alkyne combinations 0+, 0−, +0, −0 at temperatures 25 °C, 40 °C, 60 °C, 80 °C, as in Figures 1.7a, b), the total yields of all products forming in a given reaction did not exceed ~1% after 24 hours; for reactions from Figures 1.9, 1.11d, the total yields did not exceed ~2%
(at 24 hours, for temperatures ranging from 25 °C to 80 °C). Finally, we performed experiments in which the NPs were covered not with a mixture of TMA and Bipy-terminated thiols but only with TMA ones – here, we wished to exclude the possibility of copper salts adsorbing onto the TMA ligands (via the so-called salt condensation phenomenon). Reassuringly, the pure-TMA NPs exhibited poor catalytic activity: (1.06 ± 0.92) % yield for reactions from Figures 1.9, 1.11d (at 24 hours, for temperatures ranging from 25 °C to 80 °C) This result also indicates that Bipy ligand is necessary for efficient binding of Cu to NPs.
Simultaneously, the amount of Cu loaded onto the Au NPs was measured from the ratio of Au to Cu.
To obtain this ratio, the NPs after the “cleaning” procedure described above were collected by completely drying the solvent, and their gold cores were dissolved by acid (8:1 v/v aqua regia : perchloric acid, 200 oC for 4 hours). Subsequently, the concentrations of gold and copper in the pretreated sample were measured by ICP-AES. The results (Figure 1.17) showed that there were 2.61 ± 0.30 Cu atoms per 1 nm2of NP surface. Given the known density of 4.67 thiols per 1 nm2Au surface67, it follows from ICP data that Bipy-Cu(I) ligands comprise (56 ± 7)% of all on-particle ligands – that is, close to the 1:1 ratio of TMASH (C20-TMA) and BipySH (C6-Bipy) ligands in the solution during ligand-exchange procedure.
Figure 1.17Surface density of Cu atoms on 4.2 nm AuNP evaluated using ICP data. Blue circle is the mean over 16 measurements. Error bar is the error of the mean. Grey and red crosses are individual data
points (arrows indicate pairs of points which are so close to each other that the markers overlap). The relatively large standard deviation in the ICP data comes not from sample-to-sample variation, but from the acid pretreatment and/or ICP measurement itself: this variation can be seen even on repeated measurements performed on the same sample (red markers, 5 measurements). In total, there were 11 samples, 16 measurements.
Reaction setup:First, methanol – important during purification of the NPs – was removed from the NP water/methanol suspension (1:1 v/v, 8 mL in total). This removal was motivated by our desire to directly compare the results of NP catalysis with those of CuSO4/NaAs (performed in water). The removal was done via rotary evaporation, first at the vacuum level of 60 mbar at room temperature (for at least 10 min) and then at 30 mbar (rt, ~ 5 min). In the second phase, some water also evaporated and it was later replenished to the original, 4 mL volume. The mole fraction of remnant methanol in the solution after this procedure was below 0.03%, as evaluated by refractometry with Kruss Optronic DR6300-T refractometer: after applying same rotary evaporation procedure to a mixture of methanol and pure water (of same initial proportion), the refractive index of the product differed by less than 10-5from the refractive index of pure DI water. All subsequent operations were performed in the glovebox, under Ar atmosphere.
Nanocatalysis: To a 20 mL reaction vial, the azide(s), alkyne(s), and a stirring bar were first added.
Next, 0.62 mL of Cu-loaded NPs solution was injected. More water was added to keep the total volume of 2 mL for each reaction, and the reaction solution was mixed for 24 hours under Ar.
Control reaction: To a 20 mL reaction vial, the azide(s), alkyne(s), and a stirring bar were first added.
Next, corresponding CuSO4and NaAs were added. Then water was added to keep the total volume of 2 mL for each reaction, and the reaction solution was mixed at the same temperature as for the NP system, for 24 hours and under Ar.
The solvent was then completely evaporated and the crude product was re-dissolved in deuterated solvent (D2O or DMSO-d6). The product conversion ratios (yields) were determined by integration of the respective product and precursor peaks, finding the ratio of these two values. Independently synthesized and characterized triazole products such as (0+), (+0), (0−), (−0), 1C, 2C, 3C, and 4C served as authentic references for the characteristic chemical shifts for each triazole.
Figure 1.18Nanoparticle sizes during the reaction of dialkyne 1. (a-c) TEM images of NPs either before the reaction (a) (left image shows NPs after synthesis, right image is after copper addition) or extracted from the reaction solution after 3 hours (b) or 24 hours (c) of the reaction progress. (d) Distributions (boxplots) of NP diameters obtained by analysis of TEM images. Midline (orange) is median, box edges are 25% and 75% quartiles, and whisker ends are the maximum and minimum values from the dataset.
Numbers of data points (particle analyzed) are N = 99 (before the reaction), N = 75 (3 hours) and N = 139 (24 hours). Temperature during reaction was 80 °C.
Figure 1.19Zeta potential of NP catalysts before reaction (but after ligand exchange), after 3 hours, and after 24 hours of the reaction progress (between dialkyne 1and negatively charged azide, at 80 °C) showing that the values of NPs after reaction remained similar to the initial particles.