Synthesis, Characterization, Guest Inclusion, and Photophysical

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Synthesis, Characterization, Guest Inclusion and Photophysical Studies of Gold Nanoparticles Stabilized with Carboxylic Acid Groups of Organic Cavitands Barnali Mondal, Nareshbabu Kamatham, Shampa Samanta, Pradeepkumar Jagadesan, Jibao He, and Vaidhyanathan Ramamurthy Langmuir, Just Accepted Manuscript • DOI: 10.1021/la403310e • Publication Date (Web): 23 Sep 2013 Downloaded from http://pubs.acs.org on October 4, 2013

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Synthesis, Characterization, Guest Inclusion and Photophysical Studies of Gold Nanoparticles Stabilized with Carboxylic Acid Groups of Organic Cavitands

Barnali Mondal, Nareshbabu Kamatham, Shampa R. Samanta, Pradeepkumar Jagadesan, Jibao He and V. Ramamurthy* Department of Chemistry, University of Miami, Coral Gables, FL, 33124 Central Instrumentation Facilities Department, Tulane University, New Orleans, LA 70118 [email protected]

Abstract Water-soluble gold nanoparticles (AuNP) stabilized with cavitands having carboxylic acid groups have been synthesized and characterized by a variety of techniques. Apparently, the COOH groups similar to thiol are able to prevent aggregation of Au NP. These AuNP were stable either as solids or in aqueous solution. Most importantly, these cavitand functionalized AuNP were able to include organic guest molecules in their cavities in aqueous solution. Just like free cavitands (eg., octa acid), cavitand functionalized AuNP includes guests such as 4,4’-dimethylbenzil and coumarin1 through capsule formation. The exact structure of the capsular assembly is not known at this stage. Upon excitation there is communication between the excited guest present in the capsule and gold atoms and this results in quenching of phosphorescence from 4,4’-dimethylbenzil and fluorescence from coumarin-1.

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Introduction Owing to the easy synthesis and unusual optical- and electronic properties and potential biomedical and opto-electronic applications, gold nanoparticles have received considerable attention.1-3 We believed the required proximity of heavy atoms such as gold and an excited organic molecule in exploring the interaction between them could be easily achieved in water where the hydrophobic organic molecule would seek out a hydrophilic environment. This necessitated the synthesis of water-soluble gold nanoparticles decorated with organic ligands. With the background in supramolecular chemistry, we visualized that functionalization of gold nanoparticles with organic host molecules would facilitate placing of organic guest molecules closer to the gold atoms. Gold nanoparticles have in the past been functionalized with thiolated cavitands such as cyclodextrins, calixarenes and resorcinarenes based on the well-known fact that gold atoms strongly bind to thiol group (---SH).4-10 To our knowledge no molecule of photochemical or photophysical interest has been investigated in such systems. Along this line, we recently reported the synthesis of gold nanoparticles functionalized with a thiolated deep cavity cavitand tetrathiol tetraacid (TTTA, 1, Figure 1) and demonstrated that the adjacent gold atoms altered the excited state properties of organic molecules included in TTTA.11 One shortcoming of this system was that individual guest included TTTA∩AuNP (∩ represents the host TTTA is located on top of AuNP; NP represents nanoparticle) could be prepared only through heterocapsule formation; homocapsular assemblies led to aggregation. Continuing on our interest in exploring the use of water-soluble deep cavity cavitands with functional groups such as acids, amines etc. to control excited state properties of organic molecules we recently established that such functionalized cavitands could be adsorbed on surfaces of silica, clay and titanium oxide.12-14 The excited state chemistry of organic guests included within such surface bound cavitands was significantly different from that in solution. Considering the easier synthesis of cavitands functionalized with COOH groups (octa acid, OA, Figure 1)15 than TTTA and that guest inclusion within TTTA adsorbed on AuNP could be carried out only through heterocapsule formation we were interested in exploring the use of water-soluble OA functionalized gold nanoparticles to probe the interaction between excited organic


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molecules and gold atoms. We drew inspiration from the early classic report of Turkevich that utilized citric acid both as reducing and stabilizing agents for gold nanoparticle formation.16-19 This observation prompted us to explore OA (2) and resorcinol modified octa acid (ROA, 3), tetra acid tetra alcohol (TATA, 4 and i-TATA, 5) as reducing and stabilizing agents. All four cavitands containing COOH groups were ideal to test whether RCOOH (R=cavitand) could be used to make stable nanoparticles. The results of our studies that include synthesis, characterization of functionalized gold nanoparticles (AuNP), inclusion of guests and their excited state behavior are presented in this Article. Hosts O

O

O

O

O

O

O

R1

1 2

TTTA OA

O O

R1

R1

O

O

R2

R2

R2

O

O

R2 O

O

O

R1

R1

R2

CH 2SH

COOH

COOH

R1

3 ROA

OH

COOH

Guests

R2

OCH 2

H OH

O

O

4 TATA

CH 2OH

COOH

5 iTATA

COOH

OH

O N

O

O

Coumarin-1 (C-1)

O 4,4'-dimethyl benzil (DMB)

Figure 1 Structures of hosts and guests used in this study.


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Results and Discussion Synthesis and characterization of deep-cavity cavitand functionalized AuNP: AuNP functionalized with cavitands 2 – 5 were prepared in water by drop wise addition of 4 equiv of NaBH4 to a vigorously stirred aqueous solution containing 10:1 equiv of HAuCl4 and cavitand. The red colored solution, indicative of AuNP formation, was evaporated at reduced pressure and elevated temperature (~40° C) to remove water and the obtained dark red solid particles dispersed again in water to rule out aggregation. Upon stirring with fresh water the AuNP functionalized with cavitands 2-4 regained their color and were stable for several months with no visible precipitation or aggregation. Only a black powder settled at the bottom of the test tube on dispersion of iTATA∩AuNP solid particles and was not characterized further. The nanoparticles functionalized with cavitands 2, 3 and 4 were characterized by TEM, UV-Vis absorption, IR, 1H NMR, dynamic light scattering (DLS) and thermogravimetric analysis (TGA). Details of synthesis of cavitand functionalized AuNP are provided in the Supporting Information section (SI). The plasmon bands characteristic of AuNP in the region 510 – 580 nm were recorded for cavitands 2, 3 and 4 functionalized AuNP (Figures S1 to S4 in SI).20, 21 The transmission electron microscopic (TEM) images of the above prepared samples evaporated on copper grid coated with carbon shown in Figures 2 and 3 confirming the formation of AuNP also suggested lack of aggregation and the nanoparticles to be spherical and narrowly distributed. Consistent with the TEM images, the hydrodynamic diameters of cavitand functionalized AuNP dispersed in water as estimated by DLS were uniform and slightly larger than the ones measured by TEM indicating a coating of organic layer around AuNP (Figures S5 to 11 in SI). In Figure 2 the TEM images of ‘bare’ nanoparticles (AuNP prepared by reduction with NaBH4 in the absence of stabilizing agent), and functionalized with OA are provided. Also included are the UVvisible absorption spectra of the same samples immediately after preparation and after removal of water and dispersion in water. These data suggested that AuNP functionalized with OA were stable and did not aggregate upon removal of water. Based on absorption spectra and DLS recorded periodically these cavitand functionalized AuNP were found to be stable for at least three months.


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Figure 2. (Top) TEM photographs of (i) OA∩AuNP (8.4 ±1.6 nm); (ii) bare AuNP (see text for definition) and (bottom) Absorption spectra of (i) OA∩AuNP (a) immediately after preparation (b) after distillation of H2O and redispersing the precipitate in H2O; Note the two spectra overlap; (ii) bare AuNP (a) immediately after preparation (b) after distillation of H2O and redispersing the precipitate in H2O.


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Figure 3 TEM photographs with histograms of particle size distributions of (i) ROA∩AuNP (6.5 ± 3.5 nm) (ii) TATA∩AuNP (9.6 ± 1.8 nm) (iii) DMB@[OA∩AuNP] (7.5 ± 2.5 nm), (iv) DMB@[ROA∩AuNP] (6 ± 2 nm).


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Theromogravimetric analysis (TGA) data confirmed the presence of both organic and metallic components in the nanoparticles prepared in presence of cavitands 2-4. TGA results with OA functionalized AuNP are provided in Figure 4 and the others in SI (Figures S12 to S14). Expectedly, heating of only OA resulted in 100% combustion of the sample around 500° C. OA∩AuNP at nearly the same temperature resulted in loss of 39% weight. Similar observations made with ROA∩AuNP and TATA∩AuNP confirmed the presence of metallic and organic components in the prepared samples. 1H NMR spectra provided in Figures S15 to S17 (SI) and IR spectra (Figures S20 and S22 in SI) of the cavitand alone and cavitand∩AuNP further confirmed that the cavitands were attached to the surfaces of AuNP. The 1H NMR signals of cavitands in D2O were sharp while the signals of various protons of the cavitand∩AuNP samples in basic D2O were slightly broad. The line broadening in the 1H NMR spectra is consistent with what has been reported for various organic molecule coated AuNP.21 In addition, the diffusion constants measured by DOSY for OA and OA∩AuNP at 1.7 x10-10 m2 s-1 and 1.03 x10-10 m2 s-1 respectively (Figures S18 and S19 in SI) confirmed the different sizes of free and AuNP adsorbed OA. Comparison of the IR spectra of OA, ROA and TATA with the corresponding cavitand∩AuNP suggested the cavitands to coat the AuNPs (Figures S20 and S22 in SI). Based on these various characterization results we conclude that stable AuNP functionalized with organic cavitands carrying COOH group could be prepared by a simple procedure outlined above. In Figure S23 the absorption spectra of various bare AuNP’s prepared by reduction with NaBH4 in the absence of any stabilizer and in the presence of cavitands 2-5 and citrate are presented (see Figure S24 for a visual cartoon representation; SI). These spectra reveal AuNP stabilized with cavitands 2-4 are much more stable than the ones prepared in presence of cavitand 5 and citrate; bare AuNP were least stable.


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Figure 4 TGA traces showing weight loss with respect to temperature: green, 100% loss for OA; red 38.5% for OA∩AuNP, and blue 41% for DMB@[OA∩AuNP]. Inclusion of guests within water-soluble cavitand functionalized AuNP: All guest complexation with cavitand∩AuNP (cavitand = OA, ROA or TATA) were carried out in aqueous solution. Fluorescent coumarin-1 (C-1) and phosphorescent 4,4’-dimethybenzil (DMB) (Figure 1) were chosen as guests to probe the interaction between excited guests and gold atoms. The synthesized nanoparticles were filtered (dialyzed) through cellulose membrane (Spectra/Por 1, MWCO: 6-8,000) to remove the unbound cavitands before using the cavitand∩AuNP for guest inclusion.22 The effectiveness of the above dialysis procedure in removing the unbound OA is evident from the 1H NMR spectra of OA, OA∩AuNP as synthesized, dialyzed filtrate containing only OA and OA∩AuNP that is retained in the dialysis bag (Figure S25 in SI). Guest inclusion within the cavitanddecorated AuNP’s was achieved by two methods: 1. Guest stirred with independently prepared cavitand∩AuNP- represented as guest@[cavitand∩AuNP]; 2. Prepared guestcavitand complexed used to synthesize AuNP- represensted as [guest@cavitand]∩AuNP. As the TEM, DLS and UV-Vis absorption data were identical for samples from both the methods all further preparations used the former method. In Figures S15, S16, S17 and S26 (SI) the 1H NMR spectra of guests C-1 and DMB included in cavitand∩AuNP (cavitand = OA, ROA and TATA) are provided.


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Inclusion of guests within the host is evident from the expected upfield shift of the guest signals, especially that of methyl and methylene protons.23 1H NMR spectra suggested that these guests formed the expected 2:1 (host:guest) capsular assemblies. For example in the case of DMB, presence of a single signal for both the methyl groups suggested the formation of a symmetrical capsuleplex rather than an unsymmetrical cavitandplex. Had this been a 1:1 cavitandplex the two methyl groups would be in distinctly different magnetic environments (one within and the other outside the cavity) and resulted in two signals with different chemical shifts. Further confirmation for DMB inclusion came from TGA data presented in Figure 4, which revealed a distinct increase in organic content between OA∩AuNP and DMB@[OA∩AuNP]. The difference we believe represented the included guest. Further support for the existence of guest@[OA∩AuNP] came from UV-visible absorption spectra presented in Figures S1-S4 and S27 in SI. For example, samples in which DMB and C-1 were included within cavitand∩AuNP (cavitand = OA, ROA and TATA) showed the expected plasmon band with a maximum at ~520 nm as well as absorption due to the guests. The overlap between the bands due to cavitand∩AuNP and guest@[cavitand∩AuNP] seen in the figures suggested that the structure, shape and size of cavitand∩AuNP were not affected by the inclusion of guests. DLS measurements for representative samples (Figure S28-S30) supported the above conclusion. For example the hydrodynamic radii for OA∩AuNP and DMB@[OA∩AuNP] were 14±0.6 nm and 15.2±1.5 nm. It is not clear whether the slight increase in hydrodynamic radius is the result of guest inclusion. TEM images for DMB@[OA∩AuNP] and DMB@[ROA∩AuNP] samples presented in Figure 3 further confirmed that OA∩AuNP were not disrupted by inclusion of DMB within OA. Most importantly, TEM, DLS and UV-Vis data of OA∩AuNP samples did not suggest aggregation of Au nanoparticles. This is in contrast to the behavior of TTTA∩AuNP where addition of guests resulted in the aggregation of the nanoparticles. From the results presented above it is clear that guest could be included within cavitands appended with COOH groups that stabilize AuNP. Although we can’t be certain of the orientation of the capsular assembly on AuNP surface, we visualize two possibilities and these are presented in Figure 5. Based on the observed inability of iTATA (Figure 1) to stabilize


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the AuNP unlike TATA we speculate a significant role for the benzoic acid groups at the cavitand’s broader rim than for propionic acid situated at the bottom.

Figure 5 Cartoon representations of visualized orientations of host-guest capsules on gold nanoparticles. Photophysics of guests included in cavitand∩AuNP: With the goal to manipulate the excited state properties of guest molecules our studies of organic molecules in confined environments such as water-soluble micelles, dendrimers, cavitands (cyclodextrins, cucurbiturils, and calixarenes and octa acid capsules) and solid surfaces such as silica, clay and zeolites and crystals have shown them excepting zeolites to be inert.24, 25 We have now extended this examination to organic molecules localized on gold nanoparticles visualizing the cavitand adsorbed on AuNP surface to provide better confinement and examine the influence of gold atoms as well. Both enhancement and quenching of the fluorescence of guest adsorbed on AuNP due to gold atoms’ influencing the radiative as well as non-radiative rate constants of the excited chromophores have been reported.26-31 Although the classic photoreactions such as cis-trans isomerization of stilbenes32-34 and Norrish Type II reactions of phenyl alkyl ketones35 are known to occur on the surfaces of AuNP via their triplet state no unequivocal establishment of the influence of AuNP exists, neither has any phosphorescence quenching by AuNP been reported other than our recent one.11 Thus it is not clear to what extent the AuNP influences the guest triplet state. To examine the quenching of excited singlet and triplet


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states of organic molecules we recorded the fluorescence of C-1 and phosphorescence of DMB included within OA, ROA and TATA localized on AuNP.

Figure 6 (i) Phosphorescence spectra of (i) (a) DMB@OA2, (b) DMB@[OA∩AuNP]; (ii) (a) DMB@TATA2, (b) DMB@[TATA∩AuNP]; (iii) (a) DMB@ROA2, (b) DMB@[ROA∩AuNP] (λex: 318 nm; solvent: water); (iv) Fluorescence spectra of (a) C1@OA2, (b)C-1@[OA∩AuNP] (v) (a) C-1@ROA2, (b) C-1@[ROA∩AuNP] (λex: 350 nm; solvent: water). [OA]=[ROA]=[TATA]=4×10-5 M, [DMB]=2×10-5 M and [C-1]= 2×10-5 M (host: guest = 2:1). In Figure 6, the phosphorescence spectra of DMB included in cavitands (OA, ROA and TATA) and in corresponding cavitand∩AuNP in water at room temperature are provided. Contrary to the known phosphorescence of DMB in solution only in oxygen free conditions, we have reported it to occur upon encapsulation within OA capsule in the presence of oxygen. It is clear from Figure 6 that the strong phosphorescence of DMB included in above cavitands at room temperature in water was quenched when included in the corresponding cavitand∩AuNP, an effect similar to the one on the fluorescence from various organic molecules. The observed results are consistent with the conclusion


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that the triplet of DMB is quenched by non-radiative pathway involving either energy or electron transfer process. Our future studies will address the mechanism of phosphorescence quenching by AuNP. To make sure that AuNP quenching does not require direct covalent linkage between the gold atoms and the probe, we examined the fluorescence of C-1 included within cavitand∩AuNP (cavitand = OA, ROA and TATA). The fluorescence spectra of C-1 included in cavitand and cavitand∩AuNP in water at room temperature are provided in Figure 6. The weak emission of C-1 within OA∩AuNP and ROA∩AuNP in comparison to the one in the absence of AuNP is obvious. Given that C-1 is a good electron donor the quenching could be due to electron transfer from S1 of C-1 to AuNP. Further work is required to fully understand this phenomenon. Most importantly, the above observation has opened a new approach to investigate the interaction between excited organic molecules and AuNP. We plan to explore the general applicability of this approach. Conclusions In the past, examination of emission quenching by AuNP required synthetic modification of the probe with anchor groups that could be linked to AuNP. 26-31 We surmised the elimination of such process and quick screening of emission quenching by placing a guest within a cavitand functionalized gold nanoparticles. We have realized this goal and demonstrated that AuNP could be stabilized with cavitands carrying COOH groups. This finding expands the type of groups (in addition to thiol) that could be used to stabilize metal nanoparticles. In this context recent reports of stabilizing AuNP with cucurbiturils (carbonyl functional group) and pillararene acids are noteworthy.36-38 These observations make a new beginning in supramolecular chemistry of bringing a photoactive organic chromophore and metal atoms closer without any covalent bonds. These metal nanoparticles themselves could be visualized to be hosts and we propose to use these assemblies to manipulate excited state chemistry of guest molecules. Acknowledgment


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VR is grateful to the National Science Foundation, USA for generous financial support (CHE-0848017) Supporting Information Available. Procedures for synthesis and 1H NMR spectral data of cavitands and cavitand functionalized AuNPs; UV-Vis and IR spectra, 1H NMR spectra, DLS and TGA traces of cavitand functionalized AuNPs and guest included cavitand functionalized AuNPs. This material is available free of charge via the Internet at http://pubs.acs.org References 1.

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29. Dulkeith, E.; Ringler, M.; Klar, T. A.; Feldmann, J.; Muñoz Javier, A.; Parak, W. J., Gold Nanoparticles Quench Fluorescence by Phase Induced Radiative Rate Suppression. Nano Lett. 2005, 5, 585-589. 30. Nakamura, T.; Hayashi, S., Enhancement of Dye Fluorescence by Gold Nanoparticles: Analysis of Particle Size Dependence. Jpn. J. Appl. Phys. 2005, 44, 6833–6837. 31. Pacioni, N. L.; González-Béjar, M. a.; Alarcón, E.; McGilvray, K. L.; Scaiano, J. C., Surface Plasmons Control the Dynamics of Excited Triplet States in the Presence of Gold Nanoparticles. J. Am. Chem. Soc. 2010, 132, 6298-6299. 32. Zhang, J.; Whitesell, J. K.; Fox, M. A., Photoreactivity of Self-assembled Monolayers of Azobenzene or Stilbene Derivatives Capped on Colloidal Gold Clusters. Chem. Mater. 2001, 13, 2323-2331. 33. Zhang, J.; Whitesell, J. K.; Fox, M. A., Photophysical Behavior of Variously Sized Colloidal Gold Clusters Capped with Monolayers of an Alkylstilbenethiolate. J. Phys. Chem. B 2003, 107, 6051-6055. 34. Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A., Competitive Photochemical Reactivity in a Self-Assembled Monolayer on a Colloidal Gold Cluster. J. Am. Chem. Soc 2001, 123, 1464-1470. 35. Kell, A. J.; Workentin, M. S., Aryl Ketone Photochemistry on Monolayer Protectyed Clusters: Study of the Norrish Type II Reaction as a Probe of Conformational Mobility and for Selective Surface Modification. Langmuir 2001, 17, 7355-7363. 36. Lee, T.-C.; Scherman, O. A., A Facile synthesis of Dynamic Supramolecular Aggregates of Cucurbit[n]uril (n=5-8) capped with Gold Nanoparticles in Aqueous Media. Chem. Eur. J. 2012, 18, 1628-1633. 37. Premkumar, T.; Geckeler, K. E., Cucurbit[7]uril as a Tool in the Green Synthesis of Gold Nanoparticles. Chem. Asian J. 2010, 5, 2468-2476. 38. Li, H.; Chen, D.-X.; Sun, Y.-L.; Zheng, Y. B.; Tan, L.-L.; Weiss, P. S.; Yang, Y.W., Viologen-Mediated Assembly of and Sensing with Carboxylatopillar[5]areneModified Gold Nanoparticles. J. Am. Chem. Soc. 2013, 135, 1570-1576.


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Synthesis, Characterization, Guest Inclusion, and Photophysical

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