Article pubs.acs.org/Langmuir
Gold Nanoparticles Functionalized with Deep-Cavity Cavitands: Synthesis, Characterization, and Photophysical Studies Shampa R. Samanta, Revathy Kulasekharan, Rajib Choudhury, Pradeepkumar Jagadesan, Nithyanandhan Jayaraj, and V. Ramamurthy* Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States S Supporting Information *
ABSTRACT: In this report, we present methods of functionalization of AuNP's with deep-cavity cavitands that can include organic molecules. Two types of deep-cavity cavitand-functionalized AuNP's have been synthesized and characterized, one soluble in organic solvents and the other in water. Functionalized AuNP soluble in organic solvents forms a 1:1 host−guest complex where the guest is exposed to the exterior solvents. The one soluble in water forms a 2:1 host−guest complex where the guest is protected from solvent water. Phosphorescence from thiones and benzil included within heterocapsules attached to AuNP was quenched by gold atoms present closer to the guests included within deep-cavity cavitands. During this investigation, we have synthesized four new deep-cavity cavitands. Of these, two thiol-functionalized hosts allowed us to make stable AuNP's. However, AuNP's protected with two amine-functionalized cavitands tended to aggregate within a day.
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INTRODUCTION During the last five decades, supramolecular chemistry (supra) and the chemistry of metal nanoparticles (nano) have attracted considerable attention from both basic and applied aspects of chemistry. Following the serendipitous discovery of crown ethers by Pederson, in addition to natural host systems a number of synthetic cavitands, cryptands, hemicarcerands, and carcerands have been investigated in the context of stabilizing reactive intermediates as models for enzymes and as drug careers.1−3 Independent of supramolecular chemistry, nanochemistry, especially that dealing with gold nanoparticles (AuNP's), developed following an early observation by Faraday in 1857.4 Explosive activity in this area that occurred during the last two decades had to wait for reports of the easy and dependable synthesis of AuNP's.5 Functionalized gold nanoparticles (AuNP) by thiolate ligands displaying excellent stability and less flocculation tendency have played a predominant role in the realm of colloids and interfaces.6,7 The chemistry of AuNP's functionalized with a variety of receptors (through thiolate linkages) has made inroads into medicinal and analytical diagnostic applications as well as in molecular biology and surface and material sciences.8−14 In these areas, the ligand of interest is appended to AuNP's via thiolate linkages, thus requiring an independent synthesis. The marriage of supramolecular chemistry and nanochemistry has been brought about by a number of researchers by functionalizing the nanoparticles with a host capable of complexing with a variety of guest molecules.9,15 In this context, thiolated cyclodextrins, calixarenes, and resorcinarenes have been employed to synthesize host-functionalized AuNP's.16−23 However, their cavity size and poor water solubility limit their © 2012 American Chemical Society
use. To our knowledge, AuNP's are yet to be functionalized with water-soluble deep-cavity cavitands composed of three rows of aromatic rings (in contrast to calixarenes and resorcinarenes that have only one row of aromatic rings) that can form capsular assemblies in the presence of a guest in water. Such a possibility would open up new opportunities in bringing larger molecules closer to the gold surface while keeping them protected from exterior solvents and avoiding aggregation. During the past decade, we have been exploring the use of large deep-cavity cavitands of the structure shown in Scheme 1 as reaction vessels in water. We had previously demonstrated that the excited-state chemistry of a guest included in an octa acid (OA, Scheme 1) capsule was significantly altered24−26 and provided evidence of communication in terms of the energy, electrons, and spin between a capsule-incarcerated molecule and another stationed outside in aqueous solution.27−33 This prompted us to question whether one could manipulate the chemistry of a guest present within the capsule by neighboring metal atoms and if any heavy-atom effect would be induced. We have initiated this study with gold as an external atom requiring us to design capsules that could be linked to AuNP's. We have been interested in enhancing the phosphorescence of molecules through an encapsulation technique and heavy-atom effect and have established that the phosphorescence of thiones and benzils could be recorded in aqueous solution at room temperature within an organic capsule.34,35 Even aromatics Received: June 18, 2012 Revised: July 15, 2012 Published: July 18, 2012 11920
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cavitands have the same basic skeleton as octa acid (Scheme 1, structure 1, OA), which is well established to form capsular assemblies in the presence of guest molecules in water.39 Because OA could not be adsorbed on gold surfaces, we have explored the use of four deep-cavity cavitands 2−5 (Scheme 1) with amine and thiol functional groups that can interact with gold surfaces. Using guests 7−13 (Scheme 1), we have probed the ability of the AuNP-linked cavitands to include guest molecules. By examining the photophysical properties of guests 10, 11, and 13 included within the deep-cavity cavitand attached to the surfaces of AuNP's, we established that the neighboring gold atoms influenced the phosphorescence of these guests. In this article, we represent a guest included in the cavitand by guest@cavitand, a cavitand linked to a AuNP as cavitand∩AuNP, and a guest included in a host linked to the surface of a AuNP as guest@cavitand∩AuNP. Cartoon representations of the latter two are illustrated in Figure 1. The two possible types of host/guest complexes, namely, the 1:1 cavitandplex and 2:1 capsuleplex, are illustrated in Figure 1ii,iii.
Scheme 1. Chemical 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's. Of the various deep-cavity cavitands listed in Scheme 1, 1 (OA) and 2 (octa amine, OAm) were synthesized by reported procedures.39,40 The other four hosts were newly synthesized, and their synthesis procedures and characterization by 1H NMR and mass spectrometry are provided in the Supporting Information (pp S4−S12) section. AuNP functionalized with cavitands 2−5 were prepared in tetrahydrofuran (THF) via the single-phase method41−43 by the dropwise addition of 10 equiv of NaBH4 (in ice-cold water) to a vigorously stirred THF solution containing 1:0.5 equiv of HAuCl4·3H2O/cavitand. Depending on the cavitand, the solution turned red, violet, or blue. Details of the synthesis and purification procedures of cavitand-functionalized AuNP's are provided in the Experimental Section. The nanoparticles functionalized with cavitands were characterized by TEM, UV− vis absorption, IR, 1H NMR, dynamic light scattering (DLS), and thermogravimetric analysis (TGA).44,45 Although AuNP's functionalized with cavitands 4 (tetrathiol, TT) and 5 (tetrathiol tetraacid, TTTA) (in this notation, the first two letters indicate the number and nature of the functional group at the bottom of the cavitand, namely, R1, and the last two initials indicate the number and nature of functional groups at the top of the cavitand, namely, R2) were stable for several
and olefins emit phosphorescence within zeolites at room temperature in the presence of a heavy cation such as Cs+.36 The inclusion of guests within a capsule attached to a AuNP presented us with an opportunity to examine the influence of gold atoms on the phosphorescence of organic molecules in solution at room temperature. With the heavy atom gold with a fairly high spin−orbit coupling parameter (5104 cm−1) similar to that of iodine (5069 cm−1),37 we expected an influence on the radiative rate constant and an enhancement of the phosphorescence intensity while being aware of the possibility of its quenching.38 The above goals required us to functionalize AuNP's with deep-cavity cavitands. In this report, we present our results on the synthesis, characterization, and photophysical properties of AuNP's with a well-defined size (diameter of 2−5 nm) functionalized with deep-cavity cavitands. Our deep-cavity
Figure 1. Cartoon representations of (i) cavitand∩AuNP, (ii) guest@cavitand∩AuNP, and (iii) heteroternary 2:1 complexes of TTTA∩AuNP and TATP with a guest. 11921
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Figure 2. (Top) TEM images and (bottom) histograms of particle size distributions of (i) TT∩AuNP (2.5 ± 0.5 nm), (ii) TTTA∩AuNP (4.6 ± 1.5 nm), (iii) TAm∩AuNP (4.1 ± 2.3 nm) immediately after preparation, and (iv) TAm∩AuNP after 1 day of purification.
provided. The 1H NMR signals of cavitand TT in CDCl3 were sharp whereas the signals of various protons of the TT∩AuNP samples in CDCl3 and TTTA∩AuNP in NaOD/D2O were broad. In all three cases, the signals due to the alkyl chain that carried the thiol or amine group were nearly absent. (Note the signals marked with * in Figure 3.) The line broadening in the 1 H NMR spectra is consistent with what has been reported for various organically coated AuNP's.44 A comparison of the IR spectra of TT and TTTA with that of TT∩AuNP and TTTA∩AuNP suggested that AuNP's are coated with the above cavitands (Figures S12 and S13 in SI). Additional confirmation for cavitands being associated with AuNP came from dynamic light scattering (DLS) and theromogravimetric analysis (TGA) experiments. The average particle sizes of TT∩AuNP and TTTA∩AuNP in CHCl3 and NaOD/D2O as measured by DLS that included the organic layer were 4.6 ± 2.1 and 8.2 ± 2.4 nm, respectively (Figures S14 and S15 in SI). This is consistent with a layer of cavitand TT (length ∼1 nm) surrounding the metallic gold core. TGA traces shown in Figures S16 and S17 in
months, those functionalized with amines tended to aggregate. The plasmon band characteristic of AuNP's in the 520−550 nm region was recorded in all cases (Figure S10 in the Supporting Information (SI)).41,44,46 An examination of the TEM images shown in Figure 2 suggested that cavitand TT yielded small AuNP's with a narrow size distribution (diameter 2.5 ± 0.5 nm). Both TTTA and TAm gave slightly larger AuNP's (diameter ∼5 nm). However, as can be seen in Figure 2iv, TAm∩AuNP aggregated after a day in solution. The aggregation of the OAm∩AuNP nanoparticles was suggested by the TEM images provided in the SI (Figure S11 in SI). In this case, aggregation occurred even in solution within a few hours. Because of the aggregation problems, TAm∩AuNP and OAm∩AuNP were not studied further. Although the formation of the AuNP was inferred from the TEM images and plasmon band in the UV−vis absorption spectra, 1H NMR and IR spectra confirmed the cavitands attached to the surfaces of AuNP. In Figure 3, the partial 1H NMR spectra of the cavitand alone and cavitand∩AuNP are 11922
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Figure 4. 1H NMR partial spectra (500 MHz, in DMSO-d6) of (i) 11, (ii) 11@TT (host/guest 1:1), and (iii) 11@TT∩AuNP (host/guest 1:1). Bound and free guest signals are indicated with * and ⧫, respectively. Residual water and DMSO present in the solvent are denoted by • and ▲, respectively. Figure 3. 1H NMR (500 MHz) partial spectra of (i) TT (in CDCl3), (ii) TT∩AuNP (in CDCl3), (iii) TTTA (in 20 mM NaOD/D2O), and (iv) TTTA∩AuNP (in 20 mM NaOD/D2O). Methylene protons of the aliphatic chains of various hosts are denoted by *. Residual water and chloroform present in the solvents are denoted by • and ⧫, respectively.
when included in TT and TT∩AuNP, respectively. The variation is consistent with the expected changes in the dimensions of 11 when alone and included within TT and TT∩AuNP. Also, the diffusion constant of 11 was the same as that of TT∩AuNP when included in the latter, thus confirming its inclusion. The complexation behavior was further probed by isothermal titration calorimetry measurements.48−50 Thermodynamic parameters for the complexation of 7, 8, 10, and 11 with TT and TT∩AuNP were determined by monitoring the heat changes during the titration of the guest into a host solution. (For representative ITC data, see Figures S26−S33 in the SI.) All ITC experiments were carried out in DMSO. The association constant (K), ΔG, ΔH, ΔS, and stoichiometry of the complex provided in Table 1 were obtained by fitting the experimental titration curve with the computed one on the basis of an independent binding model. The binding constants in the range of 103 to 104 M−1 suggested all complexes to be moderately stable in DMSO. A stoichiometry of close to 1:1 for host/guest as inferred from the ITC data (Table 1) was consistent with the 1H NMR titration studies. A comparison of the above data between TT and TT∩AuNP as hosts suggested that generally the binding constant was slightly smaller with TT∩AuNP, and the decrease mainly seemed to be due to the decrease in entropy upon binding to the latter. We tentatively attribute this to the smaller mobility of guest@TT∩AuNP compared to that of guest@TT. From the data presented above, it is clear that guest molecules could be included within a deep-cavity cavitand∩AuNP. We believe that the moderately low binding in DMSO could be enhanced in water. Consistent with the general assumption that the guest-induced capsular assembly of deep-cavity cavitands is favored by hydrophobic interactions, no capsule formation was detected in DMSO solution with the above hosts and guests. Inclusion of Guests within Water-Soluble TTTAFunctionalized AuNP's. All complexation studies with TTTA and TTTA∩AuNP were carried out in NaOD/D2O (pH 12.3) solution. In Figure 5, the 1H NMR spectra of guests 7 and 9 included in TTTA and TTTA∩AuNP are provided. The inclusion of guests within the host is evident from the expected upfield shift of the guest signals when included within TTTA and TTTA∩AuNP. Most importantly, broader signals for TTTA∩AuNP complexes suggested that both 7 and 9 were included within TTTA-functionalized AuNP. 1H NMR titration experiments (Figures S34 and S35 in SI) confirmed the TTTA complexes to be 1:1. Thus, the behavior of TTTA in water toward guests 7 and 9 is similar to that of TT in DMSO.
the SI confirmed the presence of both organic and metallic components in the nanoparticle prepared in the presence of cavitands TT and TTTA. The organic weight fractions in the cases of TT∩AuNP and TTTA∩AuNP were found to be 74 and 40%, respectively. On the basis of the average core sizes of TT∩AuNP and TTTA∩AuNP, the numbers of gold atoms present in each particle were estimated to be ∼480 and ∼3000, respectively. From the TGA data, the average numbers of cavitands per nanoparticle were calculated to be 174 and 230 for TT∩AuNP and TTTA∩AuNP, respectively.44 On the basis of the above measurements, we believe we have functionalized AuNP's with deep-cavity cavitands, and those with thiol linkages (TT and TTTA) that gave very stable AuNP's were used for studies on guest inclusion. Inclusion of Guests within the Hydrophobic Cavitand (TT)-Functionalized AuNP. All complexation studies with TT∩AuNP that was soluble only in organic solvents such as CHCl3, THF, and DMSO were carried out in DMSO. First, we established with the help of 1H NMR titration experiments that guests 7−11 (Scheme 1) formed 1:1 host/guest complexes with TT in DMSO-d6. In all cases, the expected upfield shift of the guest protons due to paramagnetic shielding by the aromatic rings that form the framework of the cavitand was observed (Figures S18−S21 in SI).47 Similar studies were carried out with these guests with TT∩AuNP as the host in DMSO-d6, and 1H NMR titration spectra are provided in the SI (Figures S22−S25). As a representative of guests 7−11, we discuss below the results with camphorthione 11. In Figure 4, 1 H NMR spectra of 11 alone, in the presence of TT, and in the presence of TT∩AuNP in DMSO-d6 are provided. It must be noted that the signals due to methyl groups of free 11 in DMSO-d6 were above 0.5 ppm (Figure 4i). As an indication of inclusion within TT, these signals were upfield shifted (
