Here we present a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of cadmium selenide quantum dots can be accurately visualized through cross-sectional fluorescence imaging.
The fabrication of polymer-nanoparticle composites is extremely important in the development of many functional materials. Identifying the precise composition of these materials is essential, especially in the design of surface catalysts, where the surface concentration of the active component determines the activity of the material. Antimicrobial materials which utilize nanoparticles are a particular focus of this technology. Recently swell encapsulation has emerged as a technique for inserting antimicrobial nanoparticles into a host polymer matrix. Swell encapsulation provides the advantage of localizing the incorporation to the external surfaces of materials, which act as the active sites of these materials. However, quantification of this nanoparticle uptake is challenging. Previous studies explore the link between antimicrobial activity and surface concentration of the active component, but this is not directly visualized. Here we show a reliable method to monitor the incorporation of nanoparticles into a polymer host matrix via swell encapsulation. We show that the surface concentration of CdSe/ZnS nanoparticles can be accurately visualized through cross-sectional fluorescence imaging. Using this method, we can quantify the uptake of nanoparticles via swell encapsulation and measure the surface concentration of encapsulated particles, which is key in optimizing the activity of functional materials.
The application of nanomaterials has long served as an area of increasing interest for novel technologies.1-3 This has included the growing use of nanoparticles in everyday items, including cosmetics, clothes, packaging and electronics.4-6 A major drive toward using nanoparticles in functional materials stems from their higher reactivity relative to the materials, in addition to the ability to tune properties by variation in particle size.7 One further advantage is the capability to easily form composite materials, introducing crucial properties to the host matrix, such as catalytic functionality, material strengthening and tuning of electrical properties.8-12
Nanoparticle-polymer composite materials can be achieved through a range of techniques, the simplest of which is direct integration of the desired nanoparticles during the fabrication of the host matrix.13,14 This results in a homogenous material with an even spacing of nanoparticulate material throughout. However, many applications only require the active material to be present at the external interfaces of the nanocomposites. As a result, direct incorporation does not result in efficient use of sometimes costly nanoparticle material as there is much nanoparticle waste through the bulk of the material.15,16 To achieve direct incorporation, the nanoparticles must also be compatible with host matrix formation. This may be challenging, especially in syntheses that require multifaceted reactions such as in the case of thermosetting polymers that are typically facilitated by metal complex catalysts mechanisms that may be affected by highly active nanoparticles.14
The considerable disadvantages associated with direct nanoparticle incorporation during the polymer synthesis, has led to the development of techniques aimed to limit nanoparticle incorporation to the surface layer.17-21 Swell encapsulation is one of the most successful strategies reported in the literature, to achieve high surface nanoparticle concentrations, with limited wastage in the polymer bulk.17-19 The technique utilizes the solvent driven swelling of polymer matrices, allowing for the incursion of molecular species and nanoparticles. Upon removal of the swelling solvent, the species within the matrix become fixed into place, with the highest concentration of species localized at the surface. To date, most of the reported uses of swell encapsulation are directed toward the fabrication of antimicrobial polymers, where it is key that the active agents are at the material surface. While many of these reports show enhanced antimicrobial activity, the precise surface nanoparticle composition is rarely probed in detail. Crick et al. recently demonstrated a method for the direct visualization of nanoparticle incursion, providing crucial insight into the kinetics and surface nanoparticle concentrations achieved by swell encapsulation.22
This work details the synthesis of cadmium selenide quantum dots (QD), their swell encapsulation into polydimethylsiloxane (PDMS) and the direct visualization of their incorporation using fluorescence imaging. The effect of varying swell encapsulation time and nanoparticle concentration in the swelling solution is explored. The fluorescence visualization technique allows for the direct imaging of nanoparticle incursion into the PDMS and demonstrates that the highest concentration of QDs is at the material surface.
1. Preparation of CdSe/ZnS Core/Shell Quantum Dots
2. Swelling Encapsulation of Nanoparticles into PDMS
3. Visualization of Nanoparticle Swell Encapsulation into PDMS
The quantum dots exhibited red fluorescence, with a lambda max of approximately 600 nm.22,28 The red emission was due to the confinement of the exciton by the quantum rod whose size dimensions are within the strong confinement regime. Li et al. showed that for quantum rods, the emission shifts to lower energy with an increase in either width or length of the rod. They further showed that the emission mainly determined by the lateral confinement, which plays an important role even when rods are very long, especially when the width is less than the Bohr radius of the material in question as it is in the strong confinement regime.29 Transmission electron microscope (TEM) imaging shows the elongated shape of the QDs (aspect ratio ~2.5). The average length of the QDs was shown to be 12.6 nm ± 2.1 nm (n = 200) (Figure 1). The QD solutions were stable under refrigeration for up to 3 months. Lower magnification images of the QDs are provided in the supplementary information (SI – 1).
During the encapsulation process, the silicone samples visually swelled, expanding to a maximum size of 15 mm × 15 mm × 2 mm after 1 hour in the swelling (original dimensions, 11 mm × 11 mm × 1 mm). The samples shrunk to their original size once the residual solvent evaporated (Figure 2). UV-Vis spectroscopy showed that nanoparticle encapsulation did not affect polymer coloration, whereby spectra remained unchanged for all of the encapsulated samples. Scanning electron microscope (SEM) imaging of the silicone subsequent to swell encapsulation showed that wrinkling at the surface, caused by the swell-shrink process. Energy dispersive X-ray spectroscopy (EDS) analysis showed evidence of the CdSe QDs, and indicated there was an increase in the presence of these elements (Cd/Se) with swelling time. The large detection volume of the EDS analysis did not allow for reliable quantitative analysis of surface coverage. SEM image and EDS data is provided in supplementary information (SI – 2/3).
The profile of the nanoparticle penetration through the polymer was shown through cross-sectional cutting of the silicone samples, in combination with laser excitation (microscope setup shown in supplementary information SI – 4). The fluorescent QD nanoparticles responded to the 488 nm incident laser scanning, emitting light in the red portion of the visible spectrum. The sample data indicated that CdSe QDs were concentrated at the external surfaces of the silicone, with a substantially reduced signal originating from the center of the sample. The incursion of the QDs into the silicone polymer was imaged using two-dimensional intensity weighted lifetime (τw) maps (photon count × lifetime). The exposure of the cross-sectional profile along the middle of silicone samples ensured that the full extent of nanoparticle movement through the polymer could be visualized (Figure 3). Longer swell encapsulation times (48 hours) provided samples with both the highest surface concentration of particles, and highest amount of particle permeation through to the bulk of the polymer, right through to the sample center. Shorter times encapsulation times (1, 4 and 24 hours) still show a higher number of particles at the surface, however the number of particles is reduced (Figure 3). Serial dilutions from the stock solution (100%) were used to investigate the effects of varying the nanoparticle concentration, on the subsequent uptake of nanoparticles into the polymer. The stock solution was diluted to achieve the following relative concentration swelling solutions to 66%, 50% and 33% v:v. No discernible differences in the fluorescence imaging was observed when the concentration was varied, when swell encapsulated for 48 hours, indicating that the swelling solution nanoparticle concentration does not impact the nanoparticle uptake into the polymer.
The highest nanoparticle surface concentration was observed for samples swell encapsulated for 48 hours. The fluorescence intensity of these samples is comparable to that in the swelling solution [~ 0.7 μM] (Supplementary information – SI – 5). The maximum penetration of particles is shown to be ~ 163 μm from the outer edge, with the concentration reaching half-maximum after 100 μm. The rate of maximum particle penetration is shown to slow as encapsulation time is increase, increasing from an average penetration rate of 3.4 μm/hour for 48 hours samples, to a rate of 28 μm/hour samples swell encapsulated for 4 hours (Supplementary information – SI – 5).
Figure 1. Quantum Dot Images. CdSe/ZnS QD TEM images showing rod like nanoparticles. Scale bar shows 10 nm. Outlines of individual particles are overlaid. Please click here to view a larger version of this figure.
Figure 2. Polymer Swelling. Photograph shows the silicone samples (a) before, (b) during and (c) after the solvent induced swelling. The size increase (from 11 mm to 15 mm), is reversed upon full drying of the silicone. Scale bar shows 10 mm. Please click here to view a larger version of this figure.
Figure 3. Fluorescence Lifetime Images. Images showing 2D intensity weighted lifetime maps (photon count lifetime). The images show the cross-sectional profiles of the center of the polymer portions after: (A) 0 hours, (B) 1 hour, (C) 4 hours, (D) 24 hours and (E) 48 hours of swell-encapsulation. (F) Encapsulation progress is shown by analyzing the normalized intensity weighed lifetime for each image. Scale bars show 100 μm. Error bars are show one standard deviation of the variation in the results obtained. This figure has been modified from [22], reproduced by permission of The Royal Society of Chemistry. Please click here to view a larger version of this figure.
Cross-sectional fluorescence imaging allows for direct visualization of nanoparticles during swell encapsulation. The kinetics of encapsulation has been shown, with the drive toward a high nanoparticle surface concentration demonstrated. The extent of nanoparticle incorporation is shown to vary with swell encapsulation time (described in section 2.3), with the total amount of incorporated nanoparticles increasing as this time is extended, with the particle concentration localized at the surface if the polymer samples are used. The variation in the concentration of the nanoparticle solution (up to a 3× dilution, described in section 2.2), indicates there is little change in the amount of encapsulated particles. The swell encapsulation behavior of the particles used (CdSe QDs, Ø – 12.6 ± 2.1 nm) have also been shown to be comparable to particles of similar size (Titanium dioxide, Ø – 13.1 ± 5.6 nm),22 which demonstrates the potential use of this technique as a valuable tool for further investigating swell encapsulation kinetics. The swell encapsulation rates observed in these experiments are imagined to be sensitive to nanoparticle size, and so any size variation may result in a modification of the encapsulation rate.
The accurate quantification of nanoparticles within nanocomposites is a challenge that faces many research groups.1-3 Fundamental methods exist which allow the identification of a materials overall composition, however an substantial increase in complexity arises when assessing localized nanoparticle concentration, or concentration gradients over micrometer (or even nanometer) distances. Existing standards rely upon bulk measurements, such as EDS and XPS, in addition to estimating the surface concentration through functional testing, however using these methods means the precise surface composition would remain unknown.17-21 The analysis technique detailed herein, provides direct imaging and facilitates quantitative analysis of these materials. Quantification of precise nanoparticle concentrations within the polymer is possible, when the fluorescence images are evaluated against known concentrations of the fluorescent material. It is hoped that the techniques described are able to contribute toward a developing a standard test for accurate surface concentration analysis of nanocomposites to further research in the development of more potent antimicrobial materials, as well as functional materials.
Cross-sectional fluorescence imaging has the potential to be applied to any fluorescent nanoparticulate species, it is possible that the fluorescence could originate from the nanoparticle itself or the surrounding ligands. Limitations of this technique will stem from the inability to incorporate nanoparticulate fluorescent species. Future developments of these experiments could aim to explore the nanoparticle size dependence of the swell encapsulation rate, with the composition of the swelling solvent also a potential focus. The development of a full understanding of the encapsulation process will be vital in the advance of swell encapsulation as a process for fabricating functional materials.
The authors have nothing to disclose.
C.R.C. would like to acknowledge the Ramsay Memorial Trust for funding.
Polydimethylsiloxane sheets | NuSil | – | Medical Grade |
Oleylamine | Sigma Aldrich | O7805 | Technical Grade |
Trioctylphosphine | Sigma Aldrich | 117854 | Technical Grade |
Trioctylphosphine oxide | Sigma Aldrich | 346187 | Technical Grade |
1-Octadecene | Sigma Aldrich | O806 | Technical Grade |
Zinc diethyldithiocarbamate | Sigma Aldrich | 329703 | – |
Oleic acid | Sigma Aldrich | 364525 | Technical Grade |
Triethylamine | Sigma Aldrich | 471283 | – |
Cadmium oxide | Alfa Aesar | 33235 | – |
Hexadecylamine | Alfa Aesar | B22459 | Technical Grade |
1-Dodecylphosphonic acid | Alfa Aesar | H26259 | – |
Selenium powder | Acros | 19807 | – |
Chloroform | Sigma Aldrich | 366919 | – |
n-Hexane | Sigma Aldrich | 208752 | – |
Microscope slides | VWR | 631-0137 | Thickness No. 1 |