Chapter 3 Supramolecular Assembly of H-Bonded
3.2 Experimental Section
3.3.6 TEM Analyses
To further confirm the modulation of fluorescence quenching effects on copolymers P1 and P4 by acid-donor-modified gold nanoparticles (AuSCOOH), transmission electron microscopy (TEM) analysis was carried out on copolymer nanocomposites containing AuSCOOH nanoparticles. This provides a further insight into the morphology of the nanoparticle aggregation. Solutions of copolymer nanocomposites, consisting of P1 and P4 (2 mg/mL) blended with AuSCOOH (0.5 mg/mL) in THF solvent, were drop-cast onto TEM grids. The morphologies of the copolymer nanocomposites into structural ensembles were controlled by the supramolecular self-assembly. Both carboxylic acid units in surface-functionalized gold nanoparticles (AuSCOOH) and copolymer P1 of nanocomposite
P1-AuSCOOH will compete with each other to form H-bonds with the pyridyl
groups in copolymer P1. The addition of acid-modified gold nanoparticles (AuSCOOH) to copolymers P1 and P4 created two distinct aggregate structures. In Figure 3.7(a), AuSCOOH nanoparticles (incompletely H-bonded to P1) were only partially dispersed in copolymer P1. This was due to the self-H-bonded copolymeric structures of the pendent pyridyl H-acceptors self-assembled with its own pendent H-donors in P1. Therefore, the layered self-H-bonded copolymeric structures of P1 were clearly visible in this TEM micrograph. On the other hand, as AuSCOOH
nanoparticles were blended with the non-self-H-bonded copolymer P4, more carboxylic acid surfactants from AuSCOOH nanoparticles were directly H-bonded with the pyridyl H-acceptor groups of the acid-protected copolymer P4. AuSCOOH nanoparticles were well distributed among copolymer P4, as shown in Figure 3.7(b).
The self-assembled phenomena of H-bonding between H-donors (from both copolymers and nanoparticles) and polymeric H-acceptors are dependent on all existing carboxylic acid groups (H-donors) available for the supramolecular architectures. Hence, AuSCOOH nanoparticles are more homogeneously dispersed in the acid-protected copolymer P4 and less uniformly dispersed in self-H-bonded copolymer P1. Besides, a similar aggregation trend of AuSCOOH nanoaprticles was observed in the TEM images of nanocomposites containing various isomeric copolymers (self-H-bonded copolymers P2-P3 and non-self-H-bonded copolymers
P5-P6). In order to distinguish the contribution from acid and acid-free surfactants on
surface-modified nanoparticles (AuSCOOH and AuSC10, respectively), the self-H-bonded copolymer P1 was blended with non-acid-modified AuSC10 nanoparticles (without H-bonds between copolymer P1 and AuSC10 nanoparticles).
Thus, it is clearly observed in Figure 3.7(c) that non-acid-modified nanoparticles (AuSC10) aggregate more extensively. This suggests that no H-bonding interactions occurred between AuSC10 nanoparticles and copolymer P1. Overall, the TEM
morphologies of H-bonded architectures demonstrate the versatility of the self-assembly processes in supramolecular nanocomposites of H-acceptor polymers and H-donor nanoparticles.
(a)
(b)
(c)
Figure 3.7 TEM images of acid-functionalized gold nanoparticles (AuSCOOH) blended with (a) self-H-bonded copolymer P1, (b) acid-protected copolymer P4, and (c) alkyl-functionalized gold nanoparticles (AuSC10) blended with self-H-bonded copolymer P1.
3.4 Conclusions
In conclusion, the mesomorphic and photoluminescent properties of H-bonded copolymers and homopolymer complexes are affected by the isomeric H-donors with different acid-substituted (para-, meta-, and ortho-) positions. These H-bonded complexes can generate supramolecular architectures either by self-H-bonded copolymers or by π-conjugated H-acceptor homopolymers blended with H-donor homopolymers. The supramolecular structures have the nematic and smectic C phases that are related to their bent and linear H-bonded structures, respectively. The photoluminescent properties of self-H-bonded copolymers, as well as the H-bonded homopolymer complexes, can be tuned by the isomeric H-donor moieties, and red-shifted PL emissions are expected in the H-bonded structures. Supramolecular architectures that contain H-bonds in self-H-bonded copolymers P1-P3 and H-bonded homopolymer complexes PBT1/P7-P9 are further confirmed by FT-IR spectroscopy and XRD measurements. Furthermore, the H-bonding interactions between acid-modified gold nanoparticles (AuSCOOH) and acid-protected copolymer P4 affect the fluorescence quenching more effectively, when compared with fluorescence titrations of acid-free-modified gold nanoparticles (AuSC10). Moreover, in contrast to the self-H-bonded copolymer P1, the acid-protected copolymer P4 more readily captures acid-modified gold nanoparticles (AuSCOOH) in the suparmolecular
assembly of nanocomposites. The H-bonding interactions between the pyridyl H-acceptor (from the copolymers) and the acid H-donor units (from both nanoparticles and copolymers) can explain the similarities in fluorescence quenching effects on both copolymers P1 and P4. Various nanocomposites containing two kinds of fluorescent copolymer counterparts (self-H-bonded copolymer P1 and acid-protected copolymer P4) and surface-modified nanoparticles (acid-modified
AuSCOOH and acid-free-modified AuSC10) were developed to display distinct aggregation phenomena in TEM images. The pyridyl H-acceptor units of copolymer
P1 would not only bind with its own para-benzoic acid groups but also with the other
proton donor surfactants from AuSCOOH nanoparticles. Overall, this study is the first to explore the suparmolecular assembly behavior of nanocomposites between fluorescent copolymers and surface-functionalized gold nanoparticles via both PL quenching phenomena and TEM morphologies. Based on the fluorescence quenching and recovery of gold nanocomposites, further chemosensor and biosensor applications of this study can be developed in the near future.
Chapter 4
Supramolecular Fluorescence Quenching Effects of H-Donor Surface-Modified Gold Nanoparticles on Fluorescent H-Acceptor Polymers/Copolymers
Containing Lateral Methyl- and Methoxy-Substituted Groups
This approach is exploring hydrogen-bonded (H-bonded) suparmolecular
assembled behavior via both TEM and fluorescence quenching studies through
organic solvent dissolving and evaporating processes. Different lateral methyl- and
methoxy-substituted groups with pyridyl terminus of fluorescent side-chain polymers
PBT1-PBOT3, it performed that the H-bonded interactions affect the fluorescence
quenching effectively upon the addition of surface-modified gold nanoparticles
bearing acid and acid-free surfactants in the fluorescence titrations experiments. We
demonstrated that homopolymer PBOT1 has the highest Ksv constant in the
compared fluorescent side-chain polymers. In addition, we established the
exponential equation to predict Stern-Volmer constant in various pyridyl units of
polymers from the experimental information. TEM studies displayed that interesting
H-bonded suparmolecular behavior of addition into the carboxylic acid units of
surface-modified gold nanoparticles. It is clearly observed that homogeneously gold
nanoparticles distributions are on the fluorescent side-chain polymers. Thus, the TEM
morphologies of H-bonded architectures demonstrate the versatility of the
self-assembled processes in supramolecular nanocomposites of H-acceptor polymers
and surface-modified gold nanoparticles.
4.1 Introduction
Supramolecular chemistry is a new and exciting branch of chemistry encompassing systems held together by non-covalent bonds,and such complexes have extensive potentials in the rapidly developing fields.97 More lately, directed self-assembled processes of nano-scaled building blocks using non-covalent interactions (e.g., hydrogen bonding, acid/base proton transfer, and electrostatic forces) have been amplified into macroscopically observable phenomena.45,54,98 In recent years, there is considerable interest in the fluorescence of conjugated polymers due to their possible applications as highly sensitive chemosensors or biosensors,32,85 which are based on the different fluorescence quenching capabilities caused by the varying degrees of supramolecular interactions with particular chemical or biological species.86,99 Poly(p-phenylene)s, poly(p-phenylene ethynylene)s, poly(p-phenylene vinylene)s, polythiophenes, and polyfluorenes with supramolecular receptor groups,
e.g., crown ethers, pyridine derivatives, and ionic groups, in the side-chains or main-chains of the polymers have been successfully used for sensing ions and biological moieties.87 Design and syntheses of fluorescent side-chain conjugated polymers with supramolecular asembly, which are able to exhibit either chromogenic and/or fluorogenic responses due to non-covalent interactions, have particularly gained broad attentions recently.88,100
For fluorescence superquenching by gold nanoparticles, self-assembly of nanoparticles into nanocomposites provides a direct pathway for incorporating the particles’ unique physical properties into the functional materials.33,101 Due to the stability and biocompatibility of gold, gold nanoparticles protected by mixed monolayer provide highly attractive models for biological and fluorescent conjugated polymers.102 Many high performance methods of fluorescence assays have been developed by taking advantage of this superquenching ability of gold nanoparticles for optically sensing important biological ions and molecules. Gold nanoparticles can be functionalized in order to become soluble in water as well as in organic solvents with readily variable monolayer structures. Rotello and co-workers reported the syntheses and self-assembly of gold nanoparticles with inherent optical properties in the literature.45,54 Starting with thiol-passivated gold nanoparticles, they used thermal ripening and the Murray place exchange reaction to create nanoparticles featuring
highly monodisperse cores surface-modified with carboxylic acid-functionalized monolayers. Murray and co-workers have investigated the quenching of fluorophores that are attached to monolayer-protected gold nanoparticles, and also the electron transfer from exited fluorophores to gold nanoparticles.90 Direct binding between a fluorophore and a metal surface often results in the quenches of the fluorophore’s excited states.91 In this scenario, both energy transfer and electron transfer processes are considered to be major deactivation pathways for excited fluoroprobes on a metal surface. Furthermore, in the presence of other metal ions, such as Cu2+, Co2+, Fe3+, Ni2+, Zn2+, Pb2+ and Ag+, the gold nanoparticles of the quenched nanocomposites (containing fluorophores) can be replaced with the metal ions to different extents due to the stronger re-coordination or re-complexation of the metal ions with fluorophores, and therefore recover the fluorescence of the chromophores to behave as chemosensors.58a,b In addition, the gold nanoparticles of the quenched nanocomposites can also be reacted with the reduced glutathione in the presence of glutathione reductase enzyme, and then recover the fluorescence of the chromophores to behave as biosensors.58c The sensor applications of this work can be further developed to detect the metal ions and biomolecules based on the modulation of fluorescence quenching and recovery.
Subsequently, it is more interesting to develop a two-stage self-assembly process
in which the recognition (or sensing) of different surface-functionalized gold nanoparticles (with acid and acid-free surfactants) is proceeded by the light-emitting polymers. Hence, side-chain conjugated polymers bearing fluorescent pyridyl H-acceptor pendants not only behave as highly selective chemosensors for carboxylic acid H-donors, but also exhibit distinct fluorescent quenching effects upon the addition of surface-functionalized gold nanoparticles (i.e., AuSCOOH and AuSC10, which contain acid and acid-free surfactants, respectively). To our knowledge, this approach is the interesting research of exploring H-bonded supramolecular assembly of nanocomposites via fluorescence quenching and TEM morphological analyses, where the hydrogen bonds were generated from the carboxylic acid units (as proton donors) of the surface-functionalized gold nanoparticles and the pyridyl groups (as proton acceptors) of lateral methyl-substituted groups polymers PBT1-PBT3 and methoxy-substituted groups polymers PBTO1-PBOT3 in the Figure 4.1 via organic dissolving and evaporating processes.
Figure 4.1 Schematic illustration of two series of different lateral methyl- and methoxy-substituted groups of H-bonded polymers blended with AuSCOOH.
4.2 Experimental Section
4.2.1 Materials
Chemicals and solvents were reagent grades and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Dichloromathane and THF were distilled to keep anhydrous before use. The other chemicals were used without further purification. The synthetic routes of proton acceptor polymers PBT and PBOT (as shown in Figure 4.2) were reported in Chapter 2.92 The chemical structures for all products were confirmed by 1H NMR spectroscopy and elemental analyses.
4.2.2 Synthesis of Surface-Functionalized Gold Nanoparticles AuSC10 and AuSCOOH
The surface-functionalized gold nanoparticles (AuSC10 bearing acid-free surfactants) used in this study were prepared through standard Brust-Schiffrin
mixed with tetraoctylammonium bromide (TOAB) in toluene solution (80 mL, 50 mmol). The two-phase mixture was vigorously stirred until all tetrachloroaurate was transferred into the organic layer, and then dodecanethiol (20 µL) was added to the organic phase. A freshly prepared aqueous solution of sodium borohydride (25 ml, 0.4 mol) was slowly added with vigorous stirring. After further stirring for 3 h, the organic phase was separated, and the standard Brust reaction mixture was evaporated without removing TOAB and dried completely under reduced pressure. The black solid obtained was heat-treated at 165°C at a heating rate of 2°C/min and held at this temperature for 30 min.104 The thermally ‘ripened’ product was dissolved in toluene and washed with methanol to remove excess thiol ligands and TOAB, then AuSC10 nanoparticles with alkyl surfactants were obtained. In the subsequent Murray place exchange reaction,105 AuSC10 nanoparticles (60 mg) were combined with the proper amount of 11-mercaptoundecanoic acid in dichloromethane (3 mg of AuSC10/mL) and reacted for 48 h. After the exchange reaction was completed, the reaction mixture was concentrated using a rotary evaporator. After washing these products with a large amount of ethanol and acetone, no further purifications were conducted on these samples. Then, the acid-functionalized gold nanoparticles (AuSCOOH) with a diameter ca. 5 ~ 6 nm were obtained, and the monolayer compositions of AuSCOOH nanoparticles with both acid and alkyl surfactants were characterized by 1H NMR
featured 79% decanethiol and 21% carboxylic acid thiol functionalities.106 In our studies of nanocomposites, both THF-soluble gold nanoparticles, i.e., alkyl-functionalized gold nanoparticles (AuSC10 bearing acid-free surfactants) and acid-functionalized gold nanoparticles (AuSCOOH bearing acid surfactants), were used.
Figure 4.2 Molecular structures of different lateral methyl- and methoxy-substituted groups of fluorescent side-chain polymers PBT1-PBT3 and PBOT1-PBOT3.
4.2.3 Preparation of Nanocomposites Consisting of Gold Nanoparticles and Polymers
The surface-functionalized gold nanoparticles, AuSCOOH and AuSC10, which contain acid and acid-free surfactants, respectively. Nanocomposites were prepared by mixing solutions (0.5 mg/ml) of surface-functionalized gold nanoparticles
AuSCOOH and AuSC10 in THF. This was then mixed with solutions of two series
O
gold nanoparticles began to assemble into nanocomposites within 2 min. The mixed solutions became visibly turbid, indicating that an aggregation process had occurred.
After 24 h, the solid precipitates were collected and washed extensively with hexane.
The nanocomposites were then dried overnight before being subjected to Transmission Electron Microscopy (TEM) measurements.
4.2.4 Measurements and Characterization
1H NMR spectra were recorded on a Varian unity 300M Hz spectrometer using CDCl3 and DMSO-d6 as solvents. UV-visible absorption spectra were recorded in dilute THF solutions (10-6 M) on a HP G1103A spectrophotometer, and photoluminescence (PL) spectra were obtained on a Hitachi F-4500 spectrophotometer. Fourier transform infrared (FT-IR) spectra were performed on a Nicolet 360 FT-IR spectrometer. Transmission Electron Microscopy (TEM) analyses were performed using a JEOL 2011 electron microscope with an acceleration voltage of 200 keV. The samples were prepared from THF solutions with a concentration of 1 wt%, and the aggregates were precipitated on TEM sample grids (200 Cu mesh/carbon films).
4.3 Results and Discussion
4.3.1 FT-IR Spectroscopic Studies
The formation of hydrogen bonding in two series supramolecular side-chain
polymers PBT1-PBT3 and PBOT1-PBOT3 containing acid-modified gold nanoparticles AuSCOOH was confirmed by FT-IR spectroscopy. As shown in Figure 4.3, IR spectra of PBOT1, AuSCOOH, and H-bonded nanocomposite
PBOT1/AuSCOOH are compared to analyze the hydrogen bonds. In contrast to
acid-modified gold nanoparticles AuSCOOH, the weaker O-H band observed at 2525 and 1920 cm-1 in H-bonded nanocomposite PBOT1/AuSCOOH is indicative of hydrogen bonding between the pyridyl group of PBOT1 and acid-modified gold nanoparticles AuSCOOH in the H-bonded nanocomposite. On the other hand, a C=O stretching vibration appeared at 1695 cm-1 in H-bonded nanocomposite
PBOT1/AuSCOOH, which shows that the carbonyl group was in a less associated
state than that in acid-modified gold nanoparticles AuSCOOH with a weaker C=O stretching vibration appeared at 1660 cm-1. Both results suggest that hydrogen bonds were formed between H-acceptor PBOT1 and H-donor AuSCOOH in the solid state H-bonded nanocomposite PBOT1/AuSCOOH. Some other supramolecular polymers also have the similar consequences of H-bonding formation as the H-bonded polymer complex demonstrated here.17
Figure 4.3 Infrared spectra for AuSCOOH, PBOT1, and H-bonded nanocomposite PBOT1/AuSCOOH at room temperature.
4.3.2 Fluorescence Quenching Effects of Copolymers by Surface-Modified Gold Nanoparticles
In the fluorescence quenching studies, where acid-donor-modified AuSCOOH nanoparticles and non-acid-modified AuSC10 nanoparticles (diameter ca. 5 ~ 6 nm) were doped, the photoluminescence (PL) spectra of two polymers PBOT1 and PBT1 were monitored in the presence of different concentrations of surface-modified gold nanoparticles (Figure 4.4). In Chapter 2, the fluorescence spectra of polymer PBT1 emitted blue light at 440 nm in THF solutions. In comparison with luminescent polymer PBT1, polymer PBOT1 was more red-shifted PL emissions at 440 nm due to the stronger electron donating effect of lateral methoxyl groups, which induce smaller
3000 2500 2000 1500
PBOT1
PBOT/AuSCOOH AuSCOOH
Transmittance
Wavenumber (cm-1)
1695 1660
2525 1920
energy band gaps in chromophores. In Figure 4.4(a) and 4.4(b), H-acceptor polymers
PBOT1 and PBT1 demonstrated dramatic decreases in fluorescence intensities upon
the addition of AuSCOOH nanoparticles (which contain 21% carboxylic acid surfactants). Upon the addition of AuSCOOH, progressive fluorescence quenching effects on polymers PBOT1 and PBT1 (blended with AuSCOOH) were observed by increasing the concentration of AuSCOOH. It is conceivable that the polymer nanocomposites PBOT1-PBT1/AuSCOOH reduced the fluorescence emission intensities by H-bonding complexation of the fluorescent pyridyl units with the acid surfactant on AuSCOOH. In Figure 4.4(a), the polymer PBOT1 was easily H-bonded with the acid H-donor surfactants on AuSCOOH nanoparticles due to the stronger electron donating effect of lateral methoxyl groups, which induce higher affinity on
AuSCOOH. However, as shown in Figure 4.4(b), the titration of methyl-substituted
polymer PBT1 by AuSCOOH nanoparticles displays a similar PL quenching trend as Figure 4.4(a). This suggests that the complexation of methyl-substituted polymer
PBT1 and AuSCOOH nanoparticles has a similar H-bonding interaction as that in the
nanocomposite PBOT1-AuSCOOH. Unlike the methoxyl-substituted polymer
PBOT1, weaker electron donating effect of lateral methyl groups from the
methyl-substituted polymer PBT1 during the H-bonding complexation process in the nanocomposite PBT1-AuSCOOH. Hence, the excitons of methoxyl-substituted
polymer PBOT1 are more easily trapped by the charge-transfer quenchers of
AuSCOOH than those of the methyl-substituted polymer PBT1. As a comparison,
the solutions of polymers PBOT1 and PBT1 were titrated with another nanoparticle counterpart, alkyl-functionalized gold nanoparticles (AuSC10, which bears acid-free surfactants). PL titrations of polymers PBOT1 and PBT1 by AuSC10 nanoparticles (Figure 4.4(c) and 4.4(d)) resulted in similar PL reductions after increasing the concentration of AuSC10 nanoparticles. However, due to less H-bonding interactions of non-acid-modified AuSC10 nanoparticles (containing acid-free surfactants) with polymers PBOT1 and PBT1, AuSC10 nanoparticles have much weaker PL quenching effects on PBOT1 and PBT1 than acid-modified AuSCOOH nanoparticles (Figure 4.4(a) and 4.4(b), respectively). Comparing the insets of Figure 4.4(a)-(d), the fluorescence quenching curves for polymers PBOT1 and PBT1 titrated by AuSCOOH (Figure 4.4(a)-(b)) are totally different from those titrated by AuSC10 (Figure 4.4(c)-(d)). The lower quenching effects observed for PL titrations of
AuSC10 nanoparticles on polymers PBOT1 and PBT1 are owed to the absence of
H-bonding interactions between polymers (PBOT1 and PBT1) and the acid-free surfactants on AuSC10 nanoparticles. In addition, the similar fluorescence quenching curve of AuSCOOH nanoaprticles were observed in the PL spectra of nanocomposites containing various the content of pyridyl groups in the copolymers
PBOT2-PBOT3 and PBT2-PBT3.
The PL quenching behavior follows the Stern-Volmer relation I0/I = 1+KSV[Q],96 where I0 and I are the emission intensities of the fluorescent polymers (PBOT1-PBT3) in the absence and presence of the quencher Q (surface-modified gold nanoparticles), respectively, KSV is the Stern-Volmer quenching constant, and [Q] is the concentration of the quencher. Figure 4.5 demonstrates Stern-Volmer plots of polymers
PBOT1-PBT3 for various concentrations of acid-modified AuSCOOH nanoparticles
and non-acid-modified AuSC10 nanoparticles, which are replotted from the insets of Figure 4.4(a)-(d). The quenching constants (Ksv) of polymers PBOT1 and PBT1 titrated with different nanoparticle quenchers (AuSCOOH and AuSC10) in THF solutions are obtained from the slope of Figure 4.5 and listed in Table 4.1. In comparison with the fluorescence quenching effects of AuSCOOH nanoparticles on polymers polymers PBOT1 and PBT1, the quenching constant (Ksv = 2.50 x 105 M-1) of PBT1 is smaller than that (Ksv = 3.31 x 105 M-1) of PBOT1. This can be explained by that the stronger electron donating effect of lateral methoxyl groups in polymer
PBOT1 which induce higher affinity to complex with AuSCOOH. However,
different to the previous results of polymers PBOT1 and PBT1 titrated with
AuSCOOH, as polymers titrated with AuSC10 nanoparticles, the quenching constant
(Ksv = 2.15 x 104 M-1) of PBOT1 is slightly smaller than that (Ksv = 2.30 x 104 M-1)
of PBT1. It is elucidated that the lateral methoxyl groups in polymer PBOT1 will connect with AuSC10 and competed with the pyridyl terminus of polymer PBOT1.
Due to reduced supramolecular interactions between AuSC10 and polymers PBOT1
Due to reduced supramolecular interactions between AuSC10 and polymers PBOT1