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Simple Method for Tuning the Optical Properties of Thermoresponsive Plasmonic Nanogels Fei Han,† Alexander H. Soeriyadi,†,‡ S. R. C. Vivekchand,† and J. Justin Gooding*,†,‡,§ †
School of Chemistry, ‡Australian Centre for NanoMedicine, and §ARC Center of Excellence in Convergent Bio-Nano Science and Technology, The University of New South Wales, Sydney, New South Wales 2052, Australia S Supporting Information *
ABSTRACT: We report a straightforward way for forming and tuning the optical properties of thermally responsive plasmonic nanogels. Upon functionalization, a small red shift (2−3 nm) of the pNIPAM@AuNPs was observed due to changes in the refractive index surrounding the AuNP. By adding thermoresponsive poly-N-isopropylacrylamide (pNIPAM) into the pNIPAM@AuNP, its optical response was significantly increased. Heating the nanogel such that the pNIPAM collapsed and acted as a cross-link resulted in the aggregation of the AuNPs. The plasmonic response with red shifts of up to 20 nm was observed. The enlarged red shift was due to the increase in the dielectric constant around the particles and the interparticle interaction of the AuNPs. The interparticle interaction also leads to the broadening of the spectra. Experimental data and finite-difference time-domain (FDTD) calculation are in agreement with this observation. The temperature-dependent optical properties were reversible through multiple cycles of heating and cooling.
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section of the nanorods causes local heating of the pNIPAM and the conformational change. Other thermally responsive polymers can also be employed as shown by Liz-Marzan and co-workers7 who used oligo(ethylene oxide)methacrylate as the thermally responsive polymer that was grafted from gold nanoparticles modified with bovine serum albumin to which an atom transfer radical polymerization initiator was covalently attached. Another particularly elegant example of the use of thermoresponsive polymers in plasmonic materials is by Mirkin and co-workers8 who prepared gold nanoparticles modified with both single strands of DNA (ssDNA) and pNIPAM. Upon heating above 30 °C the pNIPAM collapses, and the ssDNA was made available to hybridize with complementary strands in solution causing the nanoparticles to aggregate. Thermally responsive polymer−plasmonic systems have been fabricated by attaching the polymers both to nanoparticles in solution5−7,9 as described in the examples above and to surfaces patterned with plasmonic nanoparticles.10,11 Both approaches share a challenge with optimizing these systems because the number of variables can be altered. This is particularly the case with the polymeric modification of nanoparticles in solution where a plasmonic gel or microgel is formed. Variables that can be changed include the size of the nanoparticles, mixture of nanoparticle size, shape, and amount
lasmonic material−polymer composite materials are attracting considerable interest as responsive plasmonic materials for a range of applications including sensing, photovoltaics, nanomotion, photothermal therapy, and drug delivery.1,2 Of particular interest are plasmonic particles coated in thermoresponsive polymers to give materials with plasmonic signatures that are temperature sensitive.3 Typically, these systems use the polymer poly-N-isopropylacrylamide (pNIPAM). With pNIPAM, the polymer chains are extended at low temperatures, and once the lower critical solution temperature (LCST) is reached, pNIPAM undergoes a hydrophilic-tohydrophobic transition and collapses in aqueous solution.4 Hence, in a gel containing pNIPAM and plasmonic nanoparticles the change in the plasmonic signature arises from either (1) a change in coupling between the nanoparticles as a function of their distance of separation or (2) a change in the refractive index of the polymer surrounding the nanoparticle due to the expulsion of water from the polymer and the replacement of the water by a higher refractive index organic polymer or (3) both mechanisms. An elegant example of how these thermally responsive materials can be used is demonstrated by Yang and co-workers5 where silver nanoparticles were grown in situ within a polymeric gel composed of pNIPAM. A change in polymer conformation with temperature resulted in temperature-sensitive surfaceenhanced Raman scattering (SERS) responses. Similarly, Feldman and co-workers 6 showed, using microgels of pNIPAM-coated gold nanorods, that optomechanical actuators could be produced. In this system, the large absorption cross© XXXX American Chemical Society
Received: March 19, 2016 Accepted: May 2, 2016
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DOI: 10.1021/acsmacrolett.6b00222 ACS Macro Lett. 2016, 5, 626−630
Letter
ACS Macro Letters
temperature (Tc) and is in good agreement with previously reported data.16 The pNIPAM chain undergoes a hydrophilicto-hydrophobic state change when the T > Tc. The pNIPAM polymer was then conjugated with gold nanoparticles (AuNPs), via the well-known Au−S bond formation, thereby resulting in the pNIPAM@AuNP in MilliQ water. All experiments were performed in Milli-Q water as Parak and co-workers17 have shown that electrolytes or proteins in the dispersion media can influence the reversibility of the thermoresponsive plasmonic materials. The AuNPs were 20, 40, and 80 nm in size (Figure S4). A high molar ratio of the pNIPAM:AuNP (107:1) was used in an attempt to have a high surface density of pNIPAM on the AuNP. For all three nanoparticle sizes, the coating of the AuNP with pNIPAM resulted in a reversible plasmonic shift as the dispersion was heated and cooled as illustrated in Figure 1a for pNIPAM@
of nanoparticles plus the molecular weight, composition, and amount of polymer. These existing methods of preparing plasmonic microgels typically require a new system to be synthesized each time a variable is altered. Herein we sought an alternative approach of preparing plasmonic microgels in a modular fashion where the amount and size of the nanoparticles and the amount of pNIPAM could easily be altered (Scheme 1) so that the thermal switching of the plasmonic Scheme 1. Schematic Illustrating the AuNP Functionalization Steps and the Generation of the Nanohybridsa
a
(a) The synthesis of the pNIPAM@AuNP, (b) the responsive behavior of pNIPAM@AuNP upon heating to its Tc and cooling, and (c) the responsive behavior of nanohybrids (with addition of the pNIPAM into the pNIPAM@AuNP) upon heating to its Tc and cooling.
response can easily be tuned. This is achieved using presynthesized gold nanoparticles of 20, 40, and 80 nm in size and pNIPAM of molecular weight 46 500 Da. The key aspect of the pNIPAM is that well-defined pNIPAM was prepared by the reversible addition−fragmentation transfer (RAFT) polymerization where 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT) was used as the transfer agents as the resultant thiol moieties can be used to graft the polymer onto the surface of gold nanoparticles.12−15 Using this system, pNIPAM-modified particles of any size and shape can be prepared, and microgel dispersions formed with different amounts of particles, combinations of particle sizes, and shapes and furthermore addition of pNIPAM not conjugated to particles can also be added to tune the response. Henceforth we will demonstrate how these variables impact the plasmonic switching of these microgels. In total nine different microgel systems were assessed as described in Table S1. The molecular weight of the pNIPAM was determined to be 46 kDa using NMR spectrometry (Figure S1). The monomer conversion (63%) was ascertained by comparing the vinyl proton signal (δ ∼ 5.4−6.3, 3H for acrylic monomers) to the total CH3 signal (δ ∼ 1.2−1.3, 6H). According to the Gel Permeation Chromatography measurement (Figure S2), the weight-average molecular weight (Mw) and polydispersity (Đ) of pNIPAM were in good agreement with the NMR data being 46 500 g/mol and 1.05, respectively. The cloud point temperature phase transition of pNIPAM was determined by monitoring the transmittance of a pNIPAM solution (5 mg/ mL) as a function of temperature (see Figure S3). A sharp decrease in the optical transmittance of the solution was observed at 32 °C, which is referred to as the cloud point
Figure 1. Representative UV−vis spectra characterizing the thermal behavior of the (a) pNIPAM@AuNP80 and (b) pNIPAM@AuNP80 with a simple addition of free pNIPAM (0.086 mM) dispersed in MilliQ water (Left panel: Location of the LSPR maxima at 25 and 40 °C; Right panel: thermal heating/cooling cycles). (c) Representative DLS of AuNP80 (black curve), pNIPAM@AuNP80 (yellow curve), and pNIPAM@AuNP80 with a simple addition of free pNIPAM (blue curve) at 25 and 40 °C (Left panel); thermal response measured for pNIPAM@AuNP80 (yellow curve) and pNIPAM@AuNP80 with a simple addition of free pNIPAM (blue curve).
AuNP80 (see Figure S5 for other particle sizes), although the shift was only 2−4 nm as determined using UV−visible absorbance spectra. This optical shift is attributed to the change in refractive index in the region of the nanoparticle because when T > Tc the pNIPAM polymer chains collapse around the AuNP surface which excludes water. As the collapsed pNIPAM has a higher refractive index (n) than water (swollen state),10 a red shift of the AuNP absorption is observed. The relationship between the state of the polymer and the optical response of the pNIPAM@AuNP was confirmed by a change in hydrodynamic radius of the nanogels as determined using dynamic light scattering (DLS) (Figure 1c). 627
DOI: 10.1021/acsmacrolett.6b00222 ACS Macro Lett. 2016, 5, 626−630
Letter
ACS Macro Letters In order to increase the magnitude of the optical response of these plasmonic nanogels, various architectures such as the core−satellite assembly,18 aggregation system,19 and networks20 have been studied. In this work, a simple addition of free pNIPAM was explored (nanogels 4−6 in Table S1). The hypothesis was that the addition of pNIPAM-free polymer would lead to two possible scenarios: (a) an increase in the concentration of pNIPAM in the surrounding of the AuNP which leads to larger change in refractive index and/or (b) the interaction of free pNIPAM with pNIPAM anchored AuNP would act as a cross-linker between the pNIPAM@AuNPs to form nanogels with multiple AuNPs in each entity. Such a possibility has recently been confirmed by Jones21 et al., who showed that free pNIPAM in solution can cause aggregation of pNIPAM-modified gold nanoparticles once the LCST was exceeded. With this latter possibility it was hypothesized that the reversibly aggregated and redispersed pNIPAM-modified gold nanoparticles would influence the interaction between AuNPs such that interparticle interactions would influence their plasmonic signatures. The temperature-dependent optical properties of nanogels (4−6) are displayed in Figure 1b for the pNIPAM@AuNP80 and Figure S6 for the other particle sizes. A pronounced red shift in pNIPAM@AuNP-based dispersion ranging between a maximum of 6 and 20 nm when T > Tc was observed with the magnitude increasing as the size of the AuNP increased from 20 to 80 nm. The majority of the red shift can be attributed to the increase in refractive index of the environment surroundings, as pNIPAM@AuNP became embedded with a denser pNIPAM medium as T > Tc. The slight amount of peak broadening of the resonances, especially for the pNIPAM@AuNP80-based hybrid system, is likely to arise from some internanoparticle interactions. As shown in Scheme 1, the agglomeration of the pNIPAM@AuNP shortens the distance between the gold nanoparticles at T > Tc. In cases where the AuNPs become sufficiently close enough, there will then be an increased red shift of LSPR.22 Therefore, controlling the dispersion or aggregation of AuNPs into polymer matrices is a possible route for increasing the optical response. Note the magnitude of the resonant shift is dependent on the amount of pNIPAM added with the shift approaching a maximum at 0.086 mM additional pNIPAM added. This observation suggests the graft-to approach we employed to form the pNIPAM@AuNP systems is either insufficiently dense or insufficiently thick to obtain the maximum refractive index change and hence maximum optical shift possible. Also note the optical response of different nanogels showed good reversibility (see Figure S6). To correlate the optical response with conformational changes with the pNIPAM@AuNP plus pNIPAM nanogel system, DLS and transmission electron microscopy were performed. As shown in Figure 1c, with the pNIPAM@ AuNP80 plus pNIPAM system, the increase in temperature above the LCST causes an increase in hydrodynamic diameter from 134 to 177 nm despite the polymer collapsing. This increase in size is a strong suggestion of the polymer collapse inducing aggregation via the free pNIPAM cross-linking between pNIPAM@AuNP80 entities. Such aggregation of pNIPAM@AuNP80 entities is also consistent with the observation of some peak broadenings; such aggregation can be rationalized by the pNIPAM becoming hydrophobic above the LCST which provides a driving force for the free pNIPAM to bind to the pNIPAM@AuNP80 units.23 The change in the nanohybrids was also visualized by TEM. Figure 2 shows that
Figure 2. Representative TEM images for the nanogel 5 (pNIPAM@ AuNP40 + pNIPAM@AuNP80 in the pNIPAM matrix (0.086 mM) solution): at 25 °C (left panel) and at 40 °C (right panel).
the nanohybrids are monodisperse at room temperature. Subsequently, heating to 40 °C leads to clustering of the AuNP and large highly anisotropic aggregates as in the as-cast films upon the nanohybrids. Note at 25 °C the particles are well separated with typically much larger than 50 nm separation between the particles, but upon collapse at 40 °C the interparticle distance appears to be less than 20 nm. To further investigate this system, finite difference timedomain (FDTD) simulation of an individual pNIPAM@ AuNP80 system was performed to calculate the position of the localized surface plasmon resonance peak for different refractive indices and to determine the impact of interparticle distance on the absorbance spectra. In the case of the refractive index, in the calculations it was varied from 1.33 (refractive index of water) to 1.52 (bulk refractive index of dry pNIPAM state). The calculated absorption cross-section of AuNP80 for the different refractive index is depicted in Figure 3. The red
Figure 3. FDTD simulations. (a) The spectra of the AuNP80 in different refractive index. (b) The corresponding location of the LSPR maxima at different refractive index.
shift in absorption cross-section increases in the range of 550− 575 nm when the refractive index increases from 1.33 to 1.52. For the pNIPAM@AuNP80, the red shift of 2−4 nm arises from a refractive index change of 0.015−0.03. With the expected variation in refractive index of the dielectric environment of the pNIPAM@AuNP-based nanohybrids reported between 1.33 and 1.5228, a red shift of
