Article pubs.acs.org/Langmuir
Forming End-to-End Oligomers of Gold Nanorods Using Porphyrins and Phthalocyanines Alexander F. Stewart, Brandon P. Gagnon, and Gilbert C. Walker* Department of Chemistry, University of Toronto, Toronto, Ontario M5S3H6, Canada S Supporting Information *
ABSTRACT: The illumination of aggregated metal nanospecies can create strong local electric fields to brighten Raman scattering. This study describes a procedure to self-assemble gold nanorods (NRs) through the use of porphyrin and phthalocyanine agents to create reproducibly stable and robust NR aggregates in the form of end-to-end oligomers. Narrow inter-rod gaps result, creating electric field “hot spots” between the NRs. The organic linker molecules themselves are potential Raman-based optical labels, and the result is significant numbers of Raman-active species located in the hot spots. NR polymerization was quenched by phospholipid encapsulation, which allows for control of the polydispersity of the aggregate solution, to optimize the surface-enhanced Raman scattering (SERS) enhancement and permitted the aqueous solubility of the aggregates. The increased presence of Raman-active species in the hot spots and the optimizing of solution polydispersity resulted in the observation of scattering enhancements by encapsulated porphyrins/phthalocyanines of up to 3500-fold over molecular chromophores lacking the NR oligomer host.
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INTRODUCTION Self-assembled structures of metal nanospecies, in particular, those of gold nanoparticles, are applicable to therapeutic sensors,1 catalysis,2 and electrophoresis.3 This applicability is the result of interactions between localized surface plasmon resonances (LSPRs) of the nanospecies, which significantly amplify localized electric fields.4 Nanoparticle assembly has been accomplished in many ways, ranging from coordination chemistry5 to conventional electrostatic interactions.6 Selfassembled structures can be used as SERS probes, and it has been shown that proper control of the aggregate structures is critical for optimizing the observed signal due to the signal being dependent on both the proximity of the structures and the shape of their assembly.7 Nanorods are suitable as SERS probes because they can undergo longitudinal, end-to-end assembly in which electric field “hot spots” are created in the inter-rod gaps.8,9 Once illuminated, species located in these hot spots can display high Raman scattering enhancements.10 Previously, we described a method for achieving control of the self-assembly of polystyrene-functionalized gold NRs using phospholipid encapsulation, which allowed for the isolation of such longitudinally assembled short oligomer species, with short inter-rod gaps, that were temporally stable.11 These short oligomer species, primarily dimers and trimers, avoided the reduction in optical brightness found in longer chains.8 However, the use of polymers, such as polystyrene and others,12 for facilitating assembly required partially nonaqueous conditions for assembly and led to some undesirable side-byside assembly due to the length of even low-molecular-weight polymers, with a resultant decrease in the effective Raman enhancement of the active species. The problem we aim to © 2015 American Chemical Society
solve, then, is how to reduce the inter-rod gap to create more intense hot spots while retaining oligomerization control to create increased proportions of dimers and trimers and how to do so in a simpler solvent system. A better solution is needed. In the method reported here, we present a complete approach to achieving this better solution. We detail the formation of longitudinally assembled NR oligomer solutions through the use of short molecular linkers in conjunction with charged and Raman-active porphyrin/phthalocyanine species, using phospholipid mixtures for assembly duration control. We characterize the reduced inter-rod gap lengths compared to the mentioned previous methods as well as the method of population control to ensure the preferential isolation of short oligomer species. We examine the relative enhancements gained using each different porphyrin/phthalocyanine species in addition to other properties such as the observed fluorescence in order to determine the best species for overall future use. Scheme 1 illustrates our approach.
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EXPERIMENTAL SECTION
Synthesis and Functionalization of Gold Nanorods. Nanorods were prepared via a method slightly modified from that reported by Hafner.13 Briefly, a 200 mM stock solution of cetyltrimethylammonium bromide (CTAB) was prepared by combining 7.3 g of CTAB, purchased from Sigma-Aldrich, with 100 mL of deionized (DI) water. Stock solutions of 4 mM AgNO3, 15 mM HAuCl4, 100 mM L-ascorbic acid, and 10 mM NaBH4 (solid forms also purchased from SigmaAldrich) were prepared in a similar fashion. To prepare the growth Received: April 11, 2015 Revised: June 5, 2015 Published: June 7, 2015 6902
DOI: 10.1021/acs.langmuir.5b01323 Langmuir 2015, 31, 6902−6908
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Langmuir Scheme 1. Experimental Diagrama
a
The generation of lipid-encapsulated NR oligomers using short linkers and porphyrin/phthalocyanine species. (A) Cetyltrimethylammonium bromide (CTAB)-stabilized gold NRs, emphasizing the regions of primary CTAB association (orange). (B) Thiolated short linker N,N,N-trimethyl(11-mercaptoundecyl) ammonium chloride (TMA) on NR ends was introduced, associating with the end regions of the NRs where CTAB coverage is sparse. (C) Inclusion of the Raman reporter, the porphyrin/phthalocyanine species, inducing end-to-end self-assembly. (D) Quenching of selfassembly via transfer to a solution of phospholipid vesicles, encapsulating the present NR aggregates and preventing further aggregation.
solution, in an Erlenmeyer flask 23.75 mL of the 200 mM CTAB stock solution was combined with an additional 23.75 mL of DI water. This was combined with 1.4 mL of the 15 mM HAuCl4 stock and 750 μL of the 4 mM AgNO3 stock, which caused the solution to turn a deep redorange color from the influence of the gold. Reduction of the gold was achieved through the addition of 320 μL of the 100 mM ascorbic acid stock, turning the solution clear. The seed solution was prepared through the combination of 3.75 mL of 200 mM CTAB stock, 3.75 mL of DI water, 165 μL of 15 mM HAuCl4 stock, and 600 μL of 10 mM NaBH4 stock, which was chilled prior to addition and added under magnetic stirring of the overall solution. Once mixed and allowed to sit for approximately 30 min, 150 μL of the seed solution was added to the growth, which was lightly shaken, capped, and placed in a water bath set at 27 °C overnight, turning a purplish-blue color, indicating successful NR formation. To attach the N,N,N-trimethyl(11-mercaptoundecyl) ammonium chloride (TMA, purchased from ProChimia Surfaces) short linker to facilitate assembly, 10 (1.5 mL) Eppendorf vials were filled with the raw, CTAB-stabilized NR growth solution described and centrifuged twice at 7675 rcf (with solvent removal and replacement between and after cycles) to remove excess CTAB. The 10 vials were concentrated together and rediluted to 1.5 mL (at a concentration of approximately 1.85 nM, as estimated using previously determined molar extinction coefficients14 for collected spectra). This solution was then used for all subsequent NR additions. A 4.5 mM stock solution of TMA was created in a similar fashion to those above, and in a scintillation vial 250 μL of the 1.85 nM centrifuged NR solution, 250 μL of the 4.5 mM TMA stock, and 1.5 mL of DI water were combined. This was allowed to sit for 30 min prior to use. Preparation of Phospholipid Solution. The mixture utilized was DEC221, previously described by Ip et al.,15 a 2:2:1 molar ratio mixture of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), egg sphingomyelin (ESM), and bovine cholesterol. The raw materials were all purchased from Avanti Polar Lipids, with ESM and cholesterol obtained as solids and DOPC obtained as a solution in chloroform of 25 mg/mL. To create the general lipid solution, 25 mg of ESM and 6.8 mg of cholesterol were added to 1.1 mL of the DOPC solution in chloroform. Methanol (365 μL) was then added to the solution, and the resultant mixture was subdivided into 1 dram vials to a final amount of approximately 3 mg of lipid mixture per vial. The vials were desiccated overnight and frozen for storage. When preparing a vial of the frozen mixture for use, the dried lipids were rehydrated with 1 mL of water and heated to 50 °C under sonication for a period of time sufficient to produce a solution largely composed of small unilamellar vesicles (ULVs). This solution was diluted to 2 mL and set aside for use. Self-Assembly Initiation/Termination and Characterization. Self-assembly was initiated via the addition of the porphyrin or phthalocyanine species being investigated. These species were meso-
tetra(4-sulfonatophenyl) porphine tetrasodium salt (TSPP) and phthalocyanine tetrasulfonic acid (PTA), purchased from Frontier Scientific, and copper(II) phthalocyanine tetrasulfonic acid tetrasodium salt (CuPTA), purchased from Sigma-Aldrich. Stock solutions (20 μM) of each of the above species were prepared, and upon the addition of 40 μL of any of the 20 μM stock solutions, self-assembly was observed to have commenced. Self-assembly was quenched via dropwise addition of the assembling NR solution to the solution of lipid ULVs. Self-assembly, both progression and quenching, was observed via UV−vis−NIR spectroscopy utilizing a Cary 5000 spectrophotometer. Raman spectra were collected using a Renishaw inVia Raman spectrometer with 532 nm (at ∼12 mW), 638 nm (at ∼1 mW), and 785 nm (at ∼24 mW) lasers. Each spectrum was the additive result of five acquisitions each with a 5 s collection time at the above power levels. Fluorescence measurements were performed on a Mandel RF-5301 spectrophotometer using a constant excitation wavelength (638 nm) and slit width. Nanorod aspect ratio data and gap size measurements were obtained in ImageJ using the program measurement function and the scale bar provided on collected TEM images. Population analysis was conducted on the basis of the same TEM images, in addition to those collected at various stages in the selfassembly process (immediately upon salt introduction, after durations of assembly with encapsulation, etc.). Approximately 600 NRs from each stage were examined and sorted by appearing species (monomer, dimer, etc.). All TEM images were collected on a Hitachi H-7000 microscope operating at 100 kV, and the solutions were subjected to staining with uranyl acetate (1 μL/10 μL of solution). All SEM images were collected on a Hitachi S-5200 microscope operating at 15 kV, and the solutions were subjected to staining with phosphotungstic acid (1 μL/50 μL of solution) to obtain clear images of the phospholipid layer. Enhancement Comparison Method. To obtain the enhancement factor, the collected Raman spectra were scaled relative to the concentrations of porphyrins/phthalocyanines present. The resultant derived Raman signal of the encapsulated NR aggregate solutions was subsequently scaled by a factor corresponding to the decrease in the concentration of the NRs/Raman-active species between the preassembly state and the final encapsulated state, as determined from collected UV−vis−NIR profiles of the self-assembling solution.
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RESULTS AND DISCUSSION Synthesis, Assembly, and Encapsulation. As mentioned, the NRs were synthesized using a variation of an established method,13 where TMA, a short, cationic, thiolated linker, was introduced (Scheme 1A/B). During the growth process of the NRs, the preference for CTAB to associate with the facets along the sides of the rod, as opposed to the ends,16 along with electrostatic repulsion between the positive 6903
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Figure 1. Structures of short linker and Raman-active porphyrin/phthalocyanine species utilized to facilitate self-assembly. (A) TSPP, (B) PTA, (C) CuPTA, and (D) TMA.
the basis of previous work11 to result in the maximum number of oligomer species in the final solution, known to result in a greater overall solution electric field intensity,8 important for maximizing any Raman enhancement of active species in the vicinity. The observed reaction dynamic was that of a significant LSPR red shift over the first 5 min of assembly, followed by a slow progression (monitored for 35 min) in the red shift as longer NR oligomers formed and present aggregates condensed, prior to eventual precipitation. This was attributed to the initial sudden and rapid association of electrostatically charged TMA molecules on the NR ends with the oppositely charged porphyrin/phthalocyanine species and the formation of short oligomers. Following the consumption of the free porphyrin/phthalocyanine species, the oligomers begin associating with each other to form longer oligomers and, eventually, NR chains over a longer time period. A profile of a longduration assembly of CuPTA (compared to that described) is provided in the Supporting Information. Self-assembly was followed for NR solutions assembled with TSPP, PTA, and CuPTA, shown in Figure 2. Evidence of the ability of each species to facilitate assembly was seen as a result of the observed longitudinal LSPR peak red shifts as presented in Figure 2. NR aggregate solutions assembled using TSPP were clearly observed through the appearance of an intense band near 400 nm, corresponding to its Soret/B band.19 PTA exhibits a series of weak bands between 600 and 700 nm when in the solution state (Supporting Information), corresponding to the species’ Q bands,20 but were not clearly detected when facilitating NR selfassembly, suggesting a negligible additive influence on the base NR absorption trace when diluted through addition to the experimental mixture. For those aggregate solutions assembled
headgroups of the CTAB and the TMA, resulted in the preferential association of TMA through metal−thiol bonds at the NR ends. In addition, TMA is known to associate in a far more condensed manner on the NR surface than CTAB, which is known to occupy approximately 0.55 nm2 per molecular ion.17 Quaternary amines such as TMA are known to occupy only approximately 0.25 nm2 per molecular ion,18 resulting in significantly increased concentrations of positive charge at the NR ends as opposed to along the sides. It was found via transmission electron microscopy (TEM) that the synthesized NRs had an average length and diameter of 38.2 ± 7.3 and 8.0 ± 1.8 nm, respectively. Following the trend identified in earlier work,11 it was thought that this reduced gap size would further concentrate charge density in the gap region and increase the effective enhancement. The self-assembly of the NRs was initialized by introducing the Raman-active porphyrin/phthalocyanine species possessing a negative charge (Scheme 1C), which associated electrostatically with the dense groupings of TMA linkers on the ends of adjacent NRs, inducing self-assembly. Self-assembly was terminated by transferring the assembling solution into the DEC221 phospholipid solution. The mixture immediately encapsulated the aggregates and halted aggregation (Scheme 1D). The structures of the porphyrin/phthalocyanine species used, as well as that of TMA, are shown in Figure 1. The self-assembly process was characterized by UV−vis− NIR spectroscopy. The formation of longitudinally assembled aggregates in solution was detected through the red shift of the longitudinal LSPR NR extinction peak near 780 nm.8 The duration between triggering and quenching the self-assembly process (approximately 5 min, corresponding to approximately 15 nm of observed longitudinal LSPR red shift) was chosen on 6904
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linkers. During self-assembly, the CuPTA Q band near 680 nm experienced a red shift, but the other band did not and remained near 620 nm. This was confirmed through the identification and fitting of the constituent peaks in the region in both a sample of the dye solution and that of the assembled, encapsulated NR aggregate solutions (Supporting Information). Two separate bands rather than one degenerate one were explained as the result of the asymmetric charge distribution around the CuPTA molecule, where three of the sulfurcontaining groups were meta substituted and one was ortho substituted on the outermost ring structures, leading to two distinct “axes” of charge. This charge asymmetry leads, we surmise, to the existence of a preferential mode of association, as one CuPTA molecule fits closely alongside its neighbor to a great extent during their association with the TMA linkers. The fact that the 620 nm band stays fixed and the 680 nm band red shifts could reflect the formation of J aggregates,18 but other dye aggregates could also give these signatures. Examination of Encapsulated Aggregates. Following the observation of the desired degree of absorption spectrum red shift, the phospholipids were introduced, quickly quenching the self-assembly process as in previous work.11 The encapsulated aggregates were inspected after 24 h to confirm minimal additional aggregation. Staining of the encapsulated solutions with phosphotungstic acid and uranyl acetate allowed for confirmation of the presence of the phospholipid layer when examined under scanning electron microscopy (SEM) or TEM, as shown in Figure 3. These images were used to estimate an average gap size, which was found to correspond to 1.8 ± 0.2 nm, regardless of the connecting species used, and was significantly less than the approximately 2.5 nm gap size previously reported.11 When considering the species present between the rods (two TMA linkers and a porphyrin/ phthalocyanine species), the gap was seen to be smaller than the sum of the connecting species’ bond lengths (which, for the above, was determined to be approximately 4.6 nm, assuming a linear orientation). This suggested significant nonlinearity in the structure connecting the NR aggregates. To confirm the presence of primarily short oligomer species and to ensure that the allowed assembly time (before phospholipid introduction) was correct, population analysis was conducted using images collected via TEM of NR solutions at various points in the self-assembly process, both before and after the introduction of phospholipids. It was found that, as expected, the largest proportion of short oligomer species was isolated when introducing phospholipids after the 5 min assembly time/15 nm red shift (Figure 3A). Continued assembly led to increased proportions of undesirable longer oligomer species and the precipitation of very large aggregates. Low-magnification micrographs of both the regular and longterm assembly are provided in the Supporting Information for reference. The shorter inter-rod gap measured, compared to previous work,11 was expected to cause an increase in the amount of red shift at similar stages of assembly. This was investigated following the collection of population data and the NR selfassembly (directly prior to the introduction of phospholipids, as described in the procedure). As expected, a more pronounced LSPR peak red shift (∼15 nm) was observed in the porphyrin/ phthalocyanine-assembled NR oligomer solution than was observed in the polymer-assembled NR oligomers (∼10 nm), despite there being increased proportions of higher-order oligomers in the polymer-assembled solution.
Figure 2. Spectra illustrating the self-assembly of NR aggregate solutions facilitated by TSPP (A), PTA (B), and CuPTA (C). Each panel shows a representative spectrum of the NR solution prior to assembly (indicating the longitudinal and transverse peaks around 780 and 530 nm, respectively), immediately after the introduction of the porphyrin/phthalocyanine species, immediately after the introduction of the encapsulating phospholipid solution, and following a period of 24 h postencapsulation, demonstrating the red shift characteristic of longitudinal NR assembly and the quenching of assembly upon lipid introduction. The lipid was introduced in its own solution, precipitating the observed decrease in observed spectral intensity. As evidenced by the lack of shift in the transverse LSPR band near 540 nm, negligible side-by-side assembly was observed.
using CuPTA, the copper complex is evidenced through the appearance of its own Soret/B band near 350 nm20 as well as two other bands near 680 and 620 nm, representing the Q bands.21 The behavior of the CuPTA Q bands revealed information about their environment as they acted to bridge the TMA 6905
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Figure 3. (A) Population distribution analysis of NR aggregate solutions after 5 min of self-assembly (left), after 5 min of self-assembly followed by encapsulation, after 30 min (center), and after 35 min without encapsulation (right), showing the variance in species proportions among monomers, dimers, trimers, and longer oligomer species. (B) Phosphotungstic acid-stained NR monomer observed via SEM, showing the phospholipid sheath visible as a corona around the NR perimeter. The scale bar corresponds to 100 nm. (C) Uranyl acetate-stained NR dimers observed via TEM, showing the end-to-end assembly and typical gap size. Scale bars correspond to 100 nm.
Fluorescence Activity of Aggregate Solutions. The effectiveness of a particular species as a Raman probe is often compromised by its fluorescence activity, which can obscure the Raman signal of interest.22 For this reason, the fluorescence responses of TSPP, PTA, and CuPTA were investigated, as, due to the known fluorescence-quenching properties of metalchelated porphyrins/phthalocyanines,23,24 it was thought that a significant amount of fluorescence quenching could be achieved. It was found that though the species without chelated metals (TSPP and PTA) were similarly fluorescent over much of the spectrum, CuPTA was significantly less so due to the chelated copper ion. This is shown in Figure 4. Enhancement Efficiency of Porphyrin/Phthalocyanine-Containing Aggregates. The encapsulated NR aggre-
gate solutions were examined via Raman spectroscopy to determine the enhancement. It was estimated that there were ∼400 molecules of the corresponding Raman-active species present at each rod end following assembly (Supporting Information). In terms of overall enhancement, CuPTA was superior to TSPP and PTA because it possessed both the greatest relative enhancement and the lowest relative contributions from fluorescence, owing to the presence of the chelated metal ion. While NR aggregate solutions assembled using TSPP and PTA had relative enhancements of only approximately 300-fold and 130-fold, respectively, over molecular chromophores (Supporting Information), those assembled using CuPTA exhibited a relative enhancement factor of approximately 3500-fold over molecular chromophores, with significantly less overall noise, at 638 nm. The Raman enhancements of CuPTA-assembled NR oligomer solutions at other excitation wavelengths of interest (532 and 785 nm) were collected (Supporting Information). The enhancement factors exhibited over molecular chromophores were considerably lower than those observed at 638 nm: 182fold at 532 nm and 281-fold at 785 nm. To confirm that 638 nm was the optimal excitation wavelength, Raman measurements of TSPP- and PTA-assembled NR oligomer solutions were also collected at 532 and 785 nm, and their enhancement factors were identified. The enhancements were one or more orders of magnitude less than that of CuPTA at 638 nm (Supporting Information). As 638 nm was significantly removed from the plasmon resonance of the NR oligomers themselves, we concluded that the observed enhancement of the CuPTA NR oligomers is primarily the result of the formation of dipole antennas in the inter-rod gaps (rather than from plasmonic enhancement), which was enhanced by the CuPTA itself possessing an absorption band near 638 nm. The
Figure 4. Fluorescence measurements of NR aggregate solutions (excitation at 638 nm) prepared using TSPP, PTA, and CuPTA, demonstrating the quenching of observed fluorescence by chelated metal ions. 6906
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Langmuir Raman spectrum of a CuPTA-assembled NR solution at 638 nm is shown in Figure 5A.
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CONCLUSIONS
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ASSOCIATED CONTENT
We demonstrated a method for the creation of gold NR aggregates that provide for strong Raman signals in the solution state and that could be useful for use as plasmonic biosensors. Using TSPP, PTA, and CuPTA in conjunction with short linking agents, we achieved the formation of aggregates with usefully small inter-rod gaps. Using phospholipid encapsulation to control the aggregation, we achieved the formation of short oligomer species that are favorable to maximum Raman signal enhancement. We exploited the fluorescence quenching properties of CuPTA to minimize background fluorescence and noise. As a result, we formed dyes with NR aggregates with enhancements in the Raman signal of up to approximately 3500-fold that of the corresponding molecular chromophores. We believe that the variety of metals that could be substituted for the copper in this study, such as nickel, cobalt, zinc, and lead, which possess similar but significantly shifted Raman peak positions (on the order of tens of wavenumbers),29 as well as the potential for isotopic substitution within the phthalocyanine molecule itself (such as 15N)30 will allow for the extensive customization needed to create a diverse set of optical signatures of these aggregates and could be used for optically labeling antibodies attached to the NR aggregate probes.
S Supporting Information *
UV−vis−NIR/Raman spectral characterizations of TSPP/ PTA/CuPTA, comparative Raman spectra of PTA/CuPTA, Raman spectrum of a lipid layer, peak shift characterization/ fitting of CuPTA NR aggregate solutions, low-resolution TEM micrographs, and estimates of Raman species amounts on NR ends. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01323.
Figure 5. (A) Concentration-adjusted Raman measurements of NR aggregate solutions with assembly facilitated by CuPTA. The inset depicts the concentration-adjusted measurement of CuPTA molecular chromophores, demonstrating the significant enhancement obtained using CuPTA to facilitate NR aggregation. (B) Uncorrected Raman spectrum of the same solution, displaying the contributions from fluorescence.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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From Figure 5, the most evident modes for CuPTA existed at 1336 and 1536 cm−1 (Figure 5A) and corresponded to the Cα−Cβ and Cβ−Cβ stretches, respectively, in the pyrrole groups of the molecules.25 The background florescence that remained and was observed in the CuPTA-assembled NR solution’s Raman spectrum, despite the presence of the chelated ion, was significant. In the case of the two most intense modes mentioned, the fluorescence amplitude at a given wavelength was 20% of the observed peak Raman amplitude (Figure 5B). This contribution was attributed to several factors, such as variability in the copper content in the introduced CuPTA chromophore solution and the behavior of fluorophores when in close proximity to metal surfaces. In addition, it has been established that single-molecule fluorophores (such as TSPP, PTA, and CuPTA) can experience a fluorescence enhancement, rather than quenching, when located in close proximity to metal surfaces26,27 and especially when positioned between metal particles,28 as a result of the differing contributions by the local field enhancement to the radiative and nonradiative decay rates of the molecules.27
ACKNOWLEDGMENTS We thank I. Gourevich of the Centre for Nanostructure Imaging for assistance with acquiring TEM images.
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DOI: 10.1021/acs.langmuir.5b01323 Langmuir 2015, 31, 6902−6908
