Encapsulation of Quantum Dots into Polymer Nanoparticles by Miniemulsion Polymerization

A protocol for the embedding of single quantum dots into PMMA particles by miniemulsion polymerization has been established by Gao et al.⁹⁴ and Negele et al.¹¹ and was further optimized and expanded to the encapsulation of quantum dot/conjugated polymer hybrid particles in this work.

PMMA is particularly suitable as protective material as it is transparent in the visible regime and features a high photostability, both important aspects considering the necessary excitation of the embedded nanocrystal for photoluminescence measurements. Furthermore, the glass transition temperature of PMMA is high (105 °C) and consequently the polymer particle should be a hard sphere. This allows for mechanical manipulation and prevents film formation.

The established encapsulation procedure is as follows:¹¹ 0.2 mL of the hydrophobically stabilized nanocrystals dispersed in toluene are mixed with 1 mL of methyl methacrylate (monomer), 0.1 mL of hexadecane (hydrophobe) and 10 mg of azobisisobutyronitrile (AIBN, radical initiator). This mixture is injected into 80 mL of water containing 20 mg of the surfactant sodium dodecyl sulfate (SDS), directly followed by two minutes of ultrasonication. Polymerization is performed at 76 °C for 5 hours. This results in polymer particles with an average diameter of around 60 nm, of which approx.

13% contain an inorganic nanocrystal (the value is calculated from analysis of a minimum of 150 particles in TEM images). The number of monomer droplets that can be formed from 1 mL of monomer slightly exceeds the number of nanocrystals (by a factor of approx. 5). This ensures the embedding of single nanoparticles when no agglomeration occurs.

5.2 Results and Discussion

Negele et al. encapsulated high-optical quality CdSe/CdS quantum dots while preserving the photoluminescence efficiency of the emitters. A decrease in quantum yield after polymerization was not observed. Additionally, the embedded emitters featured an extraordinary photostability, while having their colloidal nature preserved.¹¹ Manipulation of single particles into the center of nanoantennae was demonstrated successfully by using an atomic force microscope (AFM). The manipulation had no influence on the optical properties of the embedded emitter.

Figure 52. Manipulation of a PMMA particle containing a single CdSe/CdS quantum dot with an overall particle diameter of approx. 60 nm into the gap of a nanoantenna by an AFM tip.

However, the manipulation is difficult and very time consuming. The diameter of the particles exceeds or is similar to the gap-size between the antennae (< 65 nm), which makes it difficult to position the QD in the feed gap of the antenna. This, however, is important for an effective coupling of the emitter to the antenna. For this reason, the encapsulation by miniemulsion polymerization was further optimized, with special attention being paid to a smaller overall diameter of the polymer particle. A further problem that arose during manipulation is that the particles tend to stick to the AFM tip. This ‘pick-up’ ruins the resolution of the AFM and hampers further manipulation. For this reason, cross-linked and potentially more rigid polymer particles were synthesized and investigated.

The embedding protocol was additionally extended to the encapsulation of CdSe/CdS/polyfluorene hybrid particles. Furthermore, the embedding of quantum dots into the more hydrophobic poly(tert-butyl methacrylate) (P^tBuMA), which additionally features a higher glass transition temperature (118 °C), was studied. A selection of embedding experiments is listed in Table 6.

Table 6. Embedding of quantum dots by miniemulsion polymerization under varying conditions and the

a US: ultrasonication. ^b Measured in toluene. ^c Number average. ^d The mean particle diameters were calculated from the area of a minimum of 100 particles measured in TEM images. ^e Standard conditions: Aqueous phase: 80 mL of degassed water with 20 mg of SDS. Oil phase: 0.2 mL of a CdSe/CdS QD dispersion in toluene (approx. 5 × 10^-5 M), 1 mL of MMA, 0.1 mL of hexadecane, 10 mg of AIBN. Ultrasonication for 2 min. with 60% intensity. ^f Before dialysis

for 2 weeks. ^g Vol% vs. water. ^h Vol% of ethylene glycol dimethacrylate vs. MMA. ^j Irgacure 369.

The experiment in Entry 1 was performed according to the protocol developed by Negele et al.¹¹, resulting in a polymer dispersion with a mean particle diameter of 60 nm according to TEM and DLS.

The quantum yield slightly decreased, from 36% for the nanocrystals in toluene to 24% for the encapsulated QDs (λexc: 400 nm).

Concerning the generation of particles with a smaller overall diameter, the ultrasound intensity was varied between 40% and 80%. However, this did not have an effect on the mean size of the particles. For all further experiments, the intensity was set to 60%. In the experiments listed in Entry 2, 3 and 4 it was studied if an increase of the concentration of SDS in the aqueous phase results in smaller mean particle diameters. This is the case, as a five-fold increase leads to a decrease in mean particle size by 20 nm (from 73 nm to 53 nm, Entry 2 & 3). Further increasing the amount of SDS does not have an effect (Entry 4). In these experiments, no QDs were present in the polymerization mixture, which might explain the somewhat larger particle diameter that is obtained in the experiment listed in Entry 2 vs. Entry 1.

In the experiment in Entry 5, 50 vol% of the monomer was replaced by toluene. When the toluene is removed after polymerization by dialysis or by applying mild vacuum (40 mbar), this might

5.2 Results and Discussion

mean diameter of 53 nm actually decreases after dialysis for 2 weeks to 41 nm. Unfortunately, the quantum yield suffers significantly and is reduced from 30% to 3%. If the emitter is located at the periphery or surface of the polymer particle, one might expect a reduction in quantum yield after removing excess surfactant, as the latter possibly stabilizes the part of the QD surface, which is located outside of the polymer particle. However, the position of the nanocrystals in the polymer particles is unaltered after dialysis. For this reason, it is unclear why dialysis results in a severe drop in quantum yield.

The higher surfactant concentration used in the experiment listed in Entry 3 was combined with a longer ultrasonication time of 3 min (Entry 6). This results in very small particles (Figure 53) with a mean diameter of 32 nm, while the quantum yield of the emitter is decreased from 41% to 23% by the embedding procedure.

Figure 53. TEM images and particle size histogram (based on a minimum of 100 particles) of PMMA particles with QDs embedded, synthesized according to the conditions described in Table 6, Entry 6. The inset in the right bottom depicts the fluorescence spectra (λexc: 400 nm) of the embedded QDs (blue line) and of the QDs dispersed in toluene

prior to embedding (red line).

droplets. Small particles with a mean diameter of 43 nm were obtained (Entry 7). The mean particle size did not further decrease as compared to particles obtained under the same experimental conditions but without iso-propanol (Entry 6, 32 nm). The dispersion features a high quantum yield of 35%. The addition of higher volume fractions of alcohol resulted in a destabilization of the miniemulsion system and in the formation of bulk polymer.

With the overall particle size being in a reasonable range (particle size of 30 nm – 40 nm vs. an antenna gap of approx. 60 nm), the addition of a cross-linker was studied. The glass transition temperature (Tg) of bulk PMMA is high (105 °C), however, the particles incline to stick to the AFM tip when being manipulated. It has to be considered that the glass transition temperature is particle-size dependent in the nano regime¹⁷⁵ and consequently might be significantly lower for the small particles synthesized in this work. Due to this ‘pick up’ of particles, the resolution of the AFM is ruined and the tip has to be replaced for further imaging or manipulation. It was investigated if the particle rigidity can be increased by cross-linking. To this end, 1 vol% of MMA was replaced by ethylene glycol dimethacrylate (Entry 8). Successful cross-linking was verified by extraction experiments. The particles of a non-cross-linked sample and of the cross-linked sample were precipitated by addition of methanol and collected by centrifugation. After drying, THF was added and the precipitate was treated with ultrasound. In the case of non-cross-linked particles, a clear solution was obtained. The cross-linked particles on the other hand did not dissolve in THF and were collected by centrifugation after ultrasonication. After several treatments with THF, still 85 wt% of the cross-linked particles remained, underlining the formation of an insoluble polymer network. The treated and dried particles still showed a bright fluorescence (Figure 54).

Figure 54. TEM images of PMMA particles (left) and cross-linked PMMA particles (right, 1 vol% of monomer replaced by ethylene glycol dimethacrylate) containing CdSe/CdS QDs. The image (center) depicts the residue after treating the

5.2 Results and Discussion

The addition of cross-linker has no influence on the average particle diameter (57 nm vs. 64 nm), while the quantum yield of the obtained polymer dispersion increases and exceeds the quantum yield of the employed nanocrystals (56% vs. 41% for the QDs in toluene). An increase of the amount of cross-linker to 10 vol% vs. MMA results in a polymer dispersion with a quantum yield of 47% (vs.

41% for the QDs in toluene) and an average particle diameter of 63 nm (Entry 9).

It remains unclear why the cross-linking has a positive influence on the fluorescence quantum yield. An increase in fluorescence intensity of (polymer-embedded) quantum dots in the presence of air instead of vacuum or dry inert gas has been reported by several groups.^176-178 In previous work, the influence of environmental parameters on the fluorescence efficiency of single CdSe/CdS/PMMA particles¹¹ was examined by single particle micro-photoluminescence measurements.¹⁴¹ Evacuating the sample chamber resulted in a reduction in emission intensity by one order of magnitude. The initial intensity could be reestablished by flooding the sample chamber with ambient air.

The reason for this strong dependency of QD fluorescence on environmental parameters can be explained by the saturation of surface-trap states by water and oxygen molecules (Figure 55).^176-178

Figure 55. Schematic representation of the influence of trap states on the emission efficiency of CdSe/CdS quantum dots embedded into PMMA particles. In ambient air, oxygen and water in the PMMA shell saturate surface-trap states

and their efficiency in quenching exciton emission is decreased. In vacuum, water and oxygen is removed from the nanocrystal surface and the emission efficiency is reduced. This process is reversible, meaning that flooding the sample

chamber results in an increase in emission intensity.¹⁴¹

For CdSe/CdS quantum dots, this positive effect of water and oxygen on QD emission is

In conclusion, the use of a cross-linker in the embedding process has no significant influence on particle size, while it seems to be beneficial for the fluorescence efficiency of the embedded emitters.

Force spectroscopy measurements were performed to investigate if the cross-linked particles feature a higher rigidity, which is required for manipulation experiments. The results will be discussed in the subsequent Chapter 5.2.2.

The replacement of a large fraction of monomer by toluene resulted in a polymer dispersion with a very low quantum yield after dialysis (Entry 5). This experiment was repeated, but with the addition of cross-linker. 50 vol% of the initial monomer amount was replaced by toluene and 25 vol%

of cross-linker (Entry 10, 1 mL of MMA was replaced by 0.5 mL of toluene, 0.25 mL of cross-linker and 0.25 mL of MMA). The resulting polymer dispersion features a mean particle diameter of 48 nm and a high quantum yield of 40% (vs. 41% for the QDs in toluene). Besides the possibility of obtaining smaller particles, the replacement of a part of the monomer by toluene was motivated by a better dispersability of CdSe/CdS/polyfluorene hybrid particles. These are poorly dispersible in monomer droplets containing mainly MMA. The embedding of those hybrids will be discussed in Chapter 5.2.3.

The embedding of hydrophobically stabilized nanocrystals into the more hydrophobic polymer P^tBuMA, which additionally features a higher Tg (118°C), was investigated. Polymerization and embedding of QDs into P^tBuMA (Entry 11) by the original protocol for PMMA described in Entry 1 resulted in particles with a similar average diameter (61 nm for P^tBuMA vs. 64 nm for PMMA).

However, the quantum yield of the emitters embedded into P^tBuMA decreased significantly (from 43% to 16%). The embedding efficiency was determined to be similar with 7% of the polymer particles containing a quantum dot (vs. approx. 10% for PMMA). Additionally, an agglomeration of QDs onto the stirrer and vessel wall was observed.

AIBN is used as radical source and demands for elevated temperatures to decompose. For this reason, polymerization is carried out at 76 °C. By replacing AIBN by a photo initiator, heating becomes expandable, which might be beneficial for the quantum yield of the resulting dispersion. A polymerization of ^tBuMA was carried out and AIBN was replaced by the photo initiator Irgacure 369 (Entry 12). This initiator was chosen as it is soluble in hydrophobic solvents and thus in the monomer droplets. After preparing the miniemulsion, it was illuminated for 10 minutes with approx.

18 mW/cm² with a wavelength of 365 nm. Small particles with a mean diameter of 41 nm were obtained. However, the dispersion featured a low quantum yield of 5% (vs. 43% for the QDs in toluene). Because no improvement in quantum yield was observed, the use of photo initiators was not pursued further.

In conclusion, by increasing the surfactant concentration and the ultrasonication time, the mean particle diameter could successfully be reduced by a factor of two from 60 nm to 30 nm. As an

5.2 Results and Discussion

alternative, particles with a mean diameter of 40 nm are accessible by the addition of iso-propanol to the aqueous phase. The use of a cross-linker results in increased quantum yields of the polymer dispersions, while the mean particle diameter is unaffected.