Luminescent Au Nanoparticles with a pH-Responsive Nanoparticle

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J. Phys. Chem. C 2010, 114, 12459–12468

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Luminescent Au Nanoparticles with a pH-Responsive Nanoparticle-Supported Molecular Brush Chang-won Lee,†,‡ Curtis Takagi,† Thanh Truong,§ Yen-Chi Chen,† and Agnes Ostafin*,†,‡ Departments of Materials Science & Engineering, Bioengineering, and Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: April 14, 2010; ReVised Manuscript ReceiVed: June 14, 2010

The pH-dependent properties of Au nanoparticles ∼2.2 nm in diameter decorated with bifunctional ligands possessing thiol and carboxylic acid functional groups are described. The Au-S linkage creates a polaritysensitive photoluminescent charge transfer (CT) complex at the particle periphery, which acts as a built-in sensor for ligand configuration at the nanoparticle interface. When a pH change eliminates the charge on the exposed carboxylic acid group, van der Waals interactions between the carbon chain of the ligand and the Au nanoparticle lead to collapse of the ligands, and the photoluminescent Au-S CT species responds to the change in local polarity accordingly. Electron microscopy, electrophoresis, dynamic light scattering, and mass spectrometry were used to characterize the physical nature of the nanoparticles. Absorption, fluorescence emission and excitation, and zeta potential measurements were used to understand the reversible pH-dependent spectral response. The number of charged ligands on each particle and pKa for the brush collapse were estimated. The Lippert-Mataga equation was used to provide evidence of the change in local polarity of the CT complex coincident with observed spectral changes. Introduction Molecular brushes are a topic of great interest and are being used in the development of smart, switchable, or multifunctional surfaces for control of wetting, adhesion, friction, or the emission of light.1 Brush architecture design can range from simple to complex branched molecules depending on the desired response and sensitivity. In this study, a molecular brush concept was used to design a pH-responsive photoluminescent Au nanoparticle with reversible spectral response in the range of pH 5 to 8. The brush consisted of mercaptoalkanoic acid molecules chosen with varying carbon chain length from C-6 to C-12. The structure of mercaptoalkanoic acid is that of a simple chain with no branches. Similar molecules have been used to form selfassembled surface monolayers.2 On binding to small Au nanoparticles ∼2 nm in diameter, mercaptoalkanoic acid molecules generate a photoluminescent Au-S charge transfer (CT) complex3-8 at the nanoparticle metal/ligand interface. Interaction between the Au nanoparticle and the CT complex makes the spectral characteristics of the CT complex somewhat different from free CT complexes reported in the literature. The singlet excited state of the CT complex is pH-sensitive and exhibits a reversible shift in its excitation spectrum to higher energy and a decrease in its photoluminescence yield when the surrounding pH becomes acidic. The mechanism for this response is suggested to be related to a change in local polarity at the metal/ligand interface where the emitting species is located. The mechanism for this change is attributed to the deprotonation of the carboxylic acid group on the mercaptoalkanoic acid molecule, which leads to a collapse of the molecule onto the nanoparticle surface. This in turn obscures the emitting Au-S site from the aqueous * Corresponding author. E-mail: [email protected]. † Department of Materials Science & Engineering. ‡ Department of Bioengineering. § Department of Chemistry.

surroundings and changes its local polarity. As a result, a shift in the position of the excitation spectrum of the Au-S complex is seen along with a change in emission intensity. When the pH is raised again, these changes are reversed. Other types of brushes exhibiting pH responsiveness have been demonstrated.9,10 Usually, the analysis of brush conformational change often relies on the change in interaction between two dye molecules located on adjacent brush “hairs”. In the present example, the photoluminescent Au-S ligand complex is fixed at the Au-ligand layer interface. The described materials could be used in the making of chemical and biological nanosensors11,12 along with other pH-responsive nanoparticles13-19 and molecules.20-24 They also may be useful in the study of the molecular brush dynamics at nanoparticle surfaces, which is important in drug delivery and immune response, microfluidics, and the development of environmentally responsive nanomaterials. In the future, it may be possible to redesign the molecular brush to respond to other analytes, further expanding the usefulness of this material in sensing. Experimental Methods Materials. Mercaptooctanoic acid (C8H16O2S) (MOA), mercaptohexanoic acid (C6H12O2S) (MHA), mercaptododecanoic acid (C12H24O2S) (MDA), hydrogen tetrachloroaurate (HAuCl4), sodium borohydrate (NaBH4), and ethanol (EtOH) were purchased from Sigma (St. Louis, MO). Precast tris-HCl 4-15% gradient gel, tris-glycine SDS buffer, and Laemmli sample buffer were purchased from Bio-Rad (Hercules, CA). Formvar-coated carbon TEM grids were obtained from Ted Pella (Redding, CA). Synthesis. All reactions took place in low intensity, indirect light at 20 °C to reduce photodecomposition. A solution of mercaptooctanoic acid was prepared by mixing 12.8 µL of mercaptooctanoic acid with 0.45 mL of ethanol. This was added to 2.3 mL of an aqueous Au solution prepared at a concentration of 0.012 M. Magnetic stirring at 600 rpm was initiated. About 10 s later, 1.75 mL of an aqueous sodium borohydride at a

10.1021/jp1033678  2010 American Chemical Society Published on Web 07/06/2010

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concentration of 0.143 M was added, and the mixture was stirred overnight at room temperature at 600 rpm. Parafilm was used to seal the top of flask to prevent evaporation. The next day, the sample was separated into three 1.5 mL centrifuge tubes and centrifuged at 12 000 rpm for 10 min using a benchtop centrifuge (Micromax RF, Thermo Scientific) at 25 °C. The supernatant was triturated with 200-proof EtOH, leaving the product pellet to dry. The dried pellet was weighed, and the final yield of the synthesis was determined to be 37.1 ( 2.7%. The pellet was resuspended in E-pure water to obtain the concentration of 0.2 mg/mL. For some syntheses, 0.45 mL of ethanol solution containing MHA or MDA was added instead of MOA. Electrophoresis. SDS-PAGE was performed with a MiniProtean 3 cell (Bio-Rad, Hercules, CA) using precast tris-HCl 4-15% gradient gel, tris-glycine SDS buffer, and Laemmli sample buffer. The pelleted sample after trituration was redispersed in 1 mL in E-pure water and mixed with 1 mL of sample buffer. Sample (30 µL) was loaded in the gel, and 200 V was applied for 1 h until the smallest size marker reached the gel bottom. We observed photoluminescence by placing the gel on the surface of a UV transilluminator (white/UV transilluminator, Upland, CA) providing 254 nm excitation light. In the gel image recorded by digital camera, the MW size ladder was observed as dark lines, whereas the sample was photoluminescent. Transmission Electron Microscopy. Transmission electron microscopy (TEM) images were obtained with a JEOL JSM840a TEM at the Electron Microscopy facility at Brigham Young University, Provo, UT and a Tecnai T12 TEM (Philips, Andover, MA) at the Heath Science Core Facility in the University of Utah. The sample (5 µL) was allowed to air-dry on the center of the TEM grid at room temperature. Zeta Potential. Zeta potential measurements for a suspension prepared at a particle concentration of 20 nM were obtained at 20 °C using a Zetasizer Nano ZEN3600 (Malvern Instruments, Malvern, Worcestershire, UK). This instrument is also capable of particle size measurements in the range of 0.6 nm to 6 µm and utilizes a configuration in which the scattered light is detected from the front of the cuvette at an angle of 7°. This means that the concentration of the sample is less critical for obtaining accurate size measurements than would be the case for conventional light scattering instruments in which the signal is detected at 90°. The mean particle hydrodynamic diameter, polydispersity index (PDI), and % polydispersity, defined as (PDI) 1/2 × 100 of the product, were also recorded. Mass Spectrometry. ICP-MS analysis using an Agilent 7500ce mass spectrometer (Santa Clara, CA) was used to obtain the Au (m/z 197) content of aqua regia-digested samples. The samples were diluted (1 in 100) in 5% HNO3 and run together with a calibration curve prepared from a soluble Au standard (Inorganic Ventures, Madrid, Spain). Iridium (3.3% in HCl, Inorganic Ventures) was used as internal standard (m/z 192). A self-aspirating PTFE nebulizer (ESI Scientific), PTFE cyclonic spray chamber (PC3 Elemental Scientific), and platinum cones were used. For sulfur (m/z 32) content analyses, the samples were similarly digested with aqua regia, diluted (1 in 2) using 2.4% HNO3, and run along with a calibration curve prepared from a soluble sulfur standard (H2SO4, Inorganic Ventures). In this case, terbium (m/z 159) was used as internal standard. To discount any interference of Au in the solution on the detection of sulfur, a known amount of sulfur standard (10 ppm) was mixed with the different concentrations of Au standard (0, 5, 10, 15, and 20 ppm). Results showed that there was no interference effect.

Lee et al. Spectroscopy. A Shimadzu UV mini 1240 (Kyoto, Japan) absorption spectrophotometer was used to obtain absorption spectra between 200 and 800 nm for samples diluted to a particle concentration of 1 µM at various pH. Spectra were recorded in a 1 cm path length quartz cuvette at 20 °C with a resolution of (2 nm. Fluorescence emission spectra of the samples were obtained from 400 to 800 nm at 20 °C in 1 cm quartz Suprasil cuvettes using a Cary Eclipse spectrophotometer from Varian (Palo Alto, CA). An excitation wavelength of 290 nm and spectral bandpass of (1 nm were used to record the emission spectra. Excitation spectra between 200 and 600 nm monitoring 610 nm emission were taken using the same instrument. Simulation. Simulation of the electronic orbital distribution of mercaptooctanoic-acid-Au complexes was performed with density functional theory using the B3PW91 model and pseudopotential Lanl2dz basis in the Gaussian 03 program25 at the Center for High Performance Computing, University of Utah. Two separate model systems were used: mercaptooctanoic acid and complexes of the acid with one Au atom. The geometry of this model system was fully optimized in aqueous environment using a continuum PCM solvation model. Lifetime. Lifetime measurements were performed at the Dixon laser laboratory in the department of Physics at the University of Utah. Au nanoparticle suspensions were placed in a UV-transparent quartz cuvette and positioned 2 cm in front of a photomultiplier tube (R636P, Hamamatsu Photonics, Hamamatsu, Japan) with a 10 ns rise time. The excitation wavelength at 266 nm was generated as the fourth harmonic of a Nd/YAG regenerative amplifier (4400 series, Quantronix, East Satauket, NY). The average laser power was measured to be 0.2 mW. A repetition rate of 770 Hz was used. The photomultiplier signal was collected with an SR400 photon counter (Stanford Research Systems, Sunnyvale, CA) triggered by the excitation pulse using a moving gate. Photobleaching. Photobleaching tests were performed at constant temperature of 30 °C. Au nanoparticle suspensions (pH 8.5) were placed in a 1 cm path length Suprasil quartz cuvette and mounted in a water-cooled jacket set at 30 °C. The output from a 300 W xenon arc lamp placed 15 cm away was directed at the cuvette, and the fluorescence spectrum of the sample was measured every 5 min. The light intensity was 14,200 LUX to 6 × 10-4 m2 of cuvette area at the point of exposure, as determined by digital illumination meter DX-200 from Edmund Optics (Barrington, NJ). Results Physical Characterization. A TEM image of the product at pH 8.5 is shown in Figure 1A. The TEM analysis was performed for pH 8.5 and 3.0 and yielded similar results. After counting 50 particles for each specimen, we found the Au nanoparticles prepared at pH 8.5 to have an average particle diameter of 2.2 ( 0.6 nm, whereas the particles prepared at pH 3.0 had an average particle diameter of 2.3 ( 0.5 nm. Electrophoresis analyses in a gradient gel format (Figure 1B) showed that the material consisted of relatively uniform nanoparticles with electrophoretic migration properties similar to that of MW markers ∼120 kDa in size. Because gradient gels have a continually decreasing porosity as the percentage of polyacrylamide increases toward the bottom of the gel, although the materials move on the basis of their charge-tomass ratio initially, particulates and large molecules will eventually stop moving when the gel porosity is comparable to the particle size. At this point size, rather than charge, dominates the separation. It was reported that Au25 clusters migrated

Luminescent Au Nanoparticles

Figure 1. (A) Transmission electron microscope image of Au nanoparticles decorated with photoluminescent Au-S charge transfer complexes at pH 8.5 (average size: 2.2 ( 0.6 nm (n ) 50)). Samples were prepared using mercaptooctanoic acid as the ligand. (B) Image of the electrophoresis gel showing (right lane) mercaptooctanoic-acidstabilized Au particles and 25-250 kDa MW markers (left lane). The gel is illuminated using 254 nm UV excitation so the Au particles appear brightly fluorescent whereas the MW marker appears as dark black lines. (C) High-resolution TEM micrograph of a typical nanoparticle.

similarly to those of 10 kDa proteins.3 Using this published result as a guide, the 120 kDa band observed in this study would correspond to a particle with over 300 Au atoms. This value is also consistent with the particle size estimated from TEM analyses (Figure 1A,C). Mass spectrometry (ICP-MS) analyses of Au and S content in the final product showed that the Au-to-S molar ratio was 1:0.27. This ratio is similar to that expected for thiol-stabilized Au nanoparticles with stoichiometry between Au309(thiol)75 and Au225(thiol)68.26 HOMO-LUMO Energies. Mercaptoalkanoic acids like MOA (C-8) have no appreciable emission of their own, and the HOMO-LUMO gap energies obtained experimentally are in the UV (Table 2). Simulated orbital isosurfaces obtained for MOA using the Gaussian 03 program show that for unbound MOA, the LUMO orbital is located at the carboxylic acid group and the HOMO orbital is located at the thiol end region of the molecule (Figure 2). When bound to an Au atom, a significant change in orbital distribution takes place, and the isosurface plot shows that both molecular orbitals are now located at the sulfur end of the molecule. The dipole moment of this species is expected to be significantly different from that of free MOA. Evidence of this is reflected in a different dependence of HOMO-LUMO energies on solvent polarity for free MOA and Au-bound MOA. In water, the HOMO-LUMO energy of free MOA increases as the solvent becomes more polar, whereas that of MOA-Au decreases. For free MOA, the absolute magnitude of the change in HOMO-LUMO energy with increasing solvent polarity was 0.19 eV, whereas that for Aubound MOA was 0.49 eV.

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Figure 2. Gaussian 03 simulated orbital isosurfaces of mercaptooctanoic acid: (A) LUMO, (B) HOMO; and Au-mercaptooctanoic acid: (C) LUMO, (D) HOMO. Yellow spheres: sulfur; red spheres: oxygen; gray spheres: carbon; white spheres: hydrogen.

Effect of pH on Emission, Excitation, and Absorption Spectra. The emission, excitation, and absorption spectra as a function of pH were measured (Figure 3A-C). The peak intensity at 610 nm emission (Figure 3A) declined approximately three-fold as the pH was changed from pH 9 to 6. The change of intensity as a function of pH was not sigmoidal, and there was a small recovery in emission intensity between pH 6.0 and 4.0 (Figure 3B). The change in emission intensity as a function of pH was found to be reversible between pH 9 and 5 (Figure 4). Below pH 4, tendency toward irreversible aggregation was observed using light scattering.27 At this pH, there was significant loss of surface charge, as reflected by the reduction in zeta potential of the suspended particles shown in Figure 5. Once the charge was reduced to a critical level, aurophillic interactions between Au nanoparticles dominated. The excitation spectrum wavelength maximum (Figure 3C) shifted from the 280-290 nm range to the 250-260 nm range at pH ∼5 (Figure 3E). This change was also reversible between pH 9 and 4. Absorbance spectra measured as a function of pH showed a large change in absorbance around 290 nm at pH ∼5 (Figure 3F). Together, these results indicate that there is a reversible change to the spectral properties of the Au materials that is linked to the pH titration. Lifetime. Exponential decay fitting of the laser-initiated timedependent emission using the equation y ) A exp(-x/t) + B yielded a lifetime of 1.45 µs. This lifetime is in the range of the other long lifetimes reported for Au-S complexes which has been used to suggest that the emission arises from a triplet state of the Au-S complex.5 Photobleaching. The effect of prolonged UV excitation with a 300 W xenon arc lamp on the optical density at 598 nm and

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Figure 3. Au nanoparticle emission, excitation, and absorption spectra at different pH. Particles were prepared with mercaptooctanoic acid as the ligand. (A) Emission spectra of fluorescence Au nanoparticles with the peak excitation as a function of pH. (B) Excitation spectra as a function of pH. (C) Absorption spectra as a function of pH. (D) Emission peak at 610 nm as a function of pH. (E) Intensity of excitation spectra maxima as a function of pH. (F) Absorbance at 290 nm as a function of pH.

Figure 4. Reversibility of emission intensity from Au nanoparticles between pH 8 and 5.5. Particles were prepared with mercaptooctanoic acid as the ligand.

Figure 5. Zeta potential of Au nanoparticles as a function of pH. Particles prepared with mercaptooctanoic acid as the ligand.

on the emission at 610 nm is shown in Figure 6A,B. The value at 598 nm was chosen by Kunkely et al28 as a measure of light scattering due to material aggregation. Their light sources were an Osram HBO 100 W/2 and a Hanovia Xe/Hg 977 B-1 (1 kW) lamp, and monochromatic light was obtained by means of a Schoeffel GM 250/1 high-intensity monochromator (band-

width, 23 nm). For comparison, the results of a similar experiment with our materials maintained at a constant temperature are plotted (solid dots in Figure 6A). In this case, the lamp intensity was 300 W, and no wavelength discrimination was used. No evidence of particle aggregation or change in photoluminescence emission wavelength was seen over the first

Luminescent Au Nanoparticles

Figure 6. (A) Optical density of Au nanoparticle suspensions at 598 nm during 20 min of UV illumination (O) and from ref 28. (B) Fluorescence intensity at 610 nm during 20 min of UV illumination. Inset: Fluorescence intensity at 610 nm during UV illumination over a longer interval. Particles were prepared with mercaptooctanoic acid as the ligand.

20 min of illumination time. About 20% loss in photoluminescence intensity took place over the course of 120 min (inset Figure 6B). Absorption Normalized Emission. Absorption-normalized emission intensities as a function of pH were obtained using a sample prepared such that the absorbance at 290 nm was 1.5 or less over the entire pH range. The corresponding fluorescence peak maximum intensities were then measured as before. No correction for inner filter effects was performed because the absorbance and emission wavelengths are greatly separated. At high pH, where the absorbance was much higher, poor penetration of the excitation and light scattering of the fluorescence may have reduced the detected emission intensity, leading to an apparent reduction in absorption-normalized emission. To obtain absorption-normalized emission intensities, the intensity was divided by the corresponding measured absorbance for each sample. Error bars were obtained to reflect the uncertainty of estimating the absorbance at 290 nm due to the rapidly changing slope of the absorption spectra in the UV region (see for reference Figure 3C) and the decreased light penetration in the fluorescence measurements. The uncertainty in the data is generally greater at higher pH values where the slope of absorption change is steeper and the absorbance is greater than 1. Discussion Physical Characterization. The synthesis of photoluminescent Au nanoparticles involves four steps: ionization, reduction,29,30 complex formation, and nucleation.

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HAuCl4 + H2O f AuCl4- + H3O+

(1)

Au3+ + 3BH4- f Au + 3BH3 + 3H+

(2)

xAu + ySR f Aux(SR)y

(3)

nAux(SR)y f (Aux(SR)y)n

(4)

Without reductant, reactions 2-4 are not favored. With sufficient amount of added reductant, clustered CT complexes formed in reaction 3 may continue to react with the permeating reductant to nucleate particles via reaction 4. During this process, ligands may be free to reorganize, and existing Au-ligand CT complexes can eventually become a capping species. Even though an Au nanoparticle larger than ∼2 nm in size is not likely to be luminescent31,32 because the photoluminescent CT complexes remain attached to the nanoparticles and cannot be separated by washing or electrophoresis, the particle suspension exhibits an emission maximum at 2.03 eV (610 nm) (Figure 2A). Electrophoresis indicated a monodisperse distribution of nanoparticles with electrophoretic migration similar to that of MW markers ∼120 kDa in size (Figure 1B). Free Au-S ligand CT complexes are expected at lower sizes ∼10-30 kDa.5,33,34 For example, according to Negishi et al,5 the electrophoretic band position for an Au25 cluster is at ∼10 kDa, so the 120 kDa band observed here could correspond to a nanoparticle size of ∼300 Au atoms. HRTEM confirmed numerous particles ∼2.2 nm in diameter with visible atomic lattice (Figure 1C) and d spacing ∼0.22 to 0.24 nm consistent with Au (111).35 The number of Au atoms in a ∼2.2 nm diameter Au nanoparticle of volume 5.6 nm3 should be ∼333. Surface Characterization. To estimate the surface coverage of the ligands on the nanoparticles, the electric double layer (Debye layer) thickness was calculated following the procedure by Kimura et al.36 In this procedure, the Debye parameter, κ, is defined by

κ)

(∑ ) zi2e2ni εrε0kT

0.5

(5)

in which zi is the valence of ith ion, e is the elementary electric charge, ni is the bulk concentration of ith ion connected to the total bulk ion concentration by n ) ∑niεr, and εr and ε0 are the relative and vacuum dielectric constants, respectively. The Debye layer thickness usually in nanometers is defined as 1/κ. Assuming that the added electrolytes, NaOH and HCl, are dissociated completely over the whole pH region, the degree of electrolyte dissociation, R, is defined as, R ) [R-]/R0, where R is the ligand, such as mercaptooctanoic acid. The values used to calculate R were taken from the experimentally determined measurement of pH-dependent zeta potential, ζ, which should be proportional to charge at the particle surface (Figure 5). The surface charge density in units of C/m2, σ, was derived as a function of pH using the degree of electrolyte dissociation R and the maximum surface charge density, σ0, via σ ) σ0R. Because mercaptooctanoic acid has charge of one, the calculated charge density for a carboxylic acid with one charge is -1.6 × 10-19 C/0.214 nm2 or 0.75 C/m2, where the denominator denotes the maximum surface area occupied by S atoms on an Au surface, reported to be 0.214 nm.2,37

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TABLE 1: Parameters Used in the Calculation of Number of Surface Charges Per Particlea pH

[H+] (mol/L)

[OH-] (mol/L)

[Cl-] (mol/L)

[Na] (mol/L)

1/k (nm)

R

σ (C/m2)b

surface charges (per particle)

ζ (mV)

ψ0 (mV)

10.5 9.5 8.9 8.3 7.9 5.3 2.7

2.9 × 10-11 3.2 × 10-10 1.1 × 10-9 5.0 × 10-9 1.1 × 10-8 5.0 × 10-6 2.0 × 10-3

3.5 × 10-4 3.2 × 10-5 9.6 × 10-6 2.0 × 10-6 9.3 × 10-7 2.0 × 10-9 5.0 × 10-12

0 2.5 × 10-9 3.7 × 10-9 0 7.1 × 10-9 9.6 × 10-9 1.1 × 10-8

7.7 × 10-9 7.7 × 10-9 7.7 × 10-9 0 7.7 × 10-9 7.7 × 10-9 7.7 × 10-9

23.2 76.8 139.7 305.4 441.1 192.6 9.67

1 0.996 0.990 0.973 0.958 0.399 0.0814

-0.0748 -0.0744 -0.0740 -0.0728 -0.0716 -0.0297 0.0061

71 71 70 69 68 28 6

-34.7 -34.6 -34.4 -33.8 -33.3 -13.8 -2.8

-347.2 -408.2 -438.4 -477.4 -495.2 -391.4 -88.6

a

Radius of particle: 1.1 nm. b Maximum surface charge density (estimated): -0.748 C/m2.

Using the relationship

σ)

Ve εε0ψ0 ) 1 area ( /κ)

(6)

where Ve is the charge of an electron in Coulombs, ε is the dielectric constant, ε0 is the vacuum permittivity, and ψ0 is the surface potential, the surface area actually occupied by one charge can be estimated. At the lowest pH, the estimated “footprint” of each charge is much larger than that of the estimated minimal footprint. By dividing the surface area of a 2.2 nm diameter particle by the estimated “footprint” at each pH, we obtained the number of charges per particle at each pH. The maximum number of charges occurred at basic pH and was calculated to be 71 molecules per particle. This result is very well matched with ICP-MS Au-to-S ratio analysis and other reported Au-to-S ratios of thiol-coated Au nanoparticles in the stoichiometric range of Au225(thiol)68.34 Plots of excitation spectrum peak maximum or absorbance at 290 nm (Figure 3E,F) and ζ (Figure 5) as a function of suspension pH exhibit a sigmoidally shaped trend with a point of maximum change occurring at pH ∼5.5. This transition likely reflects the pKa of the titratable carboxylic acid group. Although the pKa values of small carboxylic acids tend be around 3 to 4 pH, they can be shifted higher for molecules with longer aliphatic chains, such as 4.9 for acetic acid and 4.89 for octanoic acid. Interaction of titratable groups with nanoparticle surfaces can lead to further pKa shift by up to 1.1 pH units.38 At extremely low pH, ζ approaches zero and suspension instability is a problem because of strong aurophillic interaction. However, between pH 5 and 9, the response is reversible (Figure 4). Surface potential, ψo, could be obtained using the relation

σ)

( )[

(

)

The value of the calculated surface potential at pH 8 is significantly more negative than that of the measured ζ potentials (Table 1). Whereas this is expected, the large discrepancy has been described as unusual by Kimura et al., who attributed the difference to either unknown impurities strongly bound to the nanoparticle surface or the model not being appropriate for nanoparticle interfaces. Nanoparticle interfaces can deviate considerably from the idealized Gouy-Chapman model. For example, the double charge layer may not be continuous around the particle perimeter, especially if the ligands do not completely

2εxε0κkT y 2 1 1+ + sinh e 2 κa cosh2(y/4)

(

1 8 ln[cosh(y/4)] (κa)2 sinh2(y/2)

)]

1/2

(7)

The previously estimated surface charge density, σ, was used to obtain the parameter y in eq 3 that was then solved for ψo in mV using the relation, y ) eψ0/kT. The dependence of ψo on pH thus obtained has a similar profile to that obtained by Kimura et al. (Table 1). This is largely because of the term κ in the numerator of the leading term, which has its maximum value near pH 8. This position corresponds to the starting point of the titration where there are no sodium or chlorine atoms present and the Debye layer is thickest. Performing these measurements at constant ionic strength would lead to a sigmoidal profile for ψo similar to that of ζ.

Figure 7. (A) Lippert-Mataga plot for mercaptooctanoic-acid-based Au nanoparticles as a function of solvent polarity (b), Au nanoparticles as a function of ligand chain length (O), and mercaptooctanoicacid-based Au nanoparticles as a function of pH (]). The Stokes shift, expressed as νa - νf (in cm-1), and ∆f, the orientation polarizability defined in terms of the dielectric constant and refractive index as ((ε - 1)/(2ε + 1)) -((n2 - 1)/(2n2 + 1)), are plotted. Data points in the plot are listed in Table 2. Data points for nanoparticles as a function of pH and as a function of ligand chain length are assigned a ∆f value by assuming that polarity is the major factor determining Stokes shift and that the relation between νa - νf (in inverse centimeters) and ∆f is universally true and follows the fitted line equation. (B) Plot of estimated ∆f versus pH. The point of maximum transition is pH ∼5.5.

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TABLE 2: Excitation and Emission Maximum, Stokes Shift (νabs - νfluor), Orientation Polarizability, ∆f, and Dipole Moment Change (µabs - µfluor) for Au Nanoparticles As Function of Solvent Polarity, Ligand Chain Length, And pH molecule

pH

MOA 7 Au-MOA 7 Au-MHA Au-MOA Au-MDA

Au-MOA

solvent/polarity index

excitation (eV)/(cm-1)

emission (eV)/(cm-1)

Stokes shift (eV)

νabs - νfluor (cm-1)

∆f

isopropanol/3.9 ethanol/5.2 water/9 isopropanol/3.9 ethanol/5.2 water/9 water/9

6.11/49224.6 6.14/49466.3 6.30/50755.3 4.77/38429.0 4.43/35689.8 4.28/34481.4 4.21/33917.4 4.28/34481.4 4.65//37461.2 4.19/33783.8 4.19/33783.8 4.19/33783.8 4.2/33841 4.19/33783.8 4.185/33715.4 4.186/33726.8 4.19/33783.8 4.32/34806.8 4.77/38461.5 4.83/38910.5 4.81/39759.7 4.83/38910.5

none none none 2.03/16354.5 2.03/16354.5 2.03/16354.5 2.01/16193.4 2.03/16354.5 2.05/16515.6 2.04/16447.4 2.04/16393.4 2.04/16420.4 2.03/16366.6 2.012/16233.8 2.012/16260.2 2.012/16260.2 2.02/16260.2 2.02/16233.8 2.02/16233.8 2.01/16207.5 2.01/16207.5 2.01/16233.8

2.74 2.40 2.25 2.20 2.25 2.60 2.15 2.16 2.15 2.17 2.18 2.17 2.17 2.17 2.31 2.76 2.82 2.80 2.82

22074.5 19335.3 18126.9 17724.0 18126.9 20945.6 17336.4 17390.4 17363.4 17474.4 17550 17455.2 17466.6 17523.6 18573 22227.7 22703 23552.2 22676.7

0.299 0.312 0.327 0.328a 0.325a 0.305a 0.331a 0.331a 0.331a 0.330a 0.330a 0.330a 0.330a 0.330a 0.322a 0.296a 0.292a 0.286a 0.292a

7 10.19 9.75 9.24 8.71 7.94 7.44 6.62 6.02 5.56 5.02 4.24 3.78 2.96

water/9

µabs - µfluor (debye)

24.2

a Values for ∆f were estimated from the slope of the linear fit equation obtained for the Au-MOA system as a function of solvent polarity. This assumes that the slope of the Lippert-Mataga plot is the same regardless of the mechanism by which the local polarity of the absorbing and emitting states is changed.

cover the surface. A more systematic study using different types of ligands will shed more light on this discrepancy. To estimate the number of titratable charges on each particle, the σ obtained from the method above was divided by 1.6 × 10-19 and multiplied by the calculated surface area of a 2.2 nm diameter spherical nanoparticle. At the highest pH, the calculated number of charges on the nanoparticles plateaued at ∼70. Assuming that the footprint of a vertically standing ligand molecule is only ∼2 × 10-19 m2 (a ∼0.5 nm diameter circle), the maximum number of ligands that can fit on the surface of a 2.2 nm diameter spherical nanoparticle is ∼72. Therefore, the value obtained experimentally is close to the maximum number of charges possible. At the lowest pH, the number of charges drops to ∼6. Effect of pH on Emission and Excitation Spectra. The interactions between the fluorophore and solvent affect the energy difference between the excited and ground states. The energy difference can be estimated by the Lippert-Mataga equation, where νjA and νjF are the wavenumbers (in inverse centimeters) of the absorption and emission, respectively, h is Planck’s constant (erg · s), c is the speed of light (cm/s), R is the radius of the cavity in which the fluorophore resides (centimeters), ε is the dielectric constant of the solvent, n is the refractive index of the solvent, and µE and µG are dipole moments (debye) at excited and ground states, respectively.

VjA - VjF )

(

)

2 1 ε-1 n2 - 1 (µE - µG) - 2 hc 2ε + 1 2n + 1 a3

(8)

This equation is an approximation neglecting the polarizability and higher-order terms. The term in the above equation, ((ε - 1)/ (2ε + 1)) - ((n2 - 1)/(2n2 + 1)) is called the orientation polarizability (∆f, here after). In simplistic terms, the first term involving the dielectric constant accounts for shifts due to reorientation of the solvent dipoles and to the redistribution of the

electrons in the solvent molecules. The second term containing refractive index accounts for only the redistribution of electrons. Therefore, the difference of these two terms represents the spectral shifts due to the reorientation of the solvent molecules. Because the contribution of the refractive index has only a minor effect due to the approximately equal stabilization of the ground and excited states by the process, the Stokes shift is considered to be mostly affected by the solvent reorientation. In the Lippert-Mataga plot (Figure 7A), the slope of the line fitted to the data points obtained for the nanoparticle-supported Au-S CT complexes in various solvents is therefore equal to (2/hca3)∆µ with the units of (erg · cm3)/((erg · s) · (cm · s-1) · (cm3)) ) cm-1. Because the dipole moment unit, debye, is defined as erg0.5cm1.5, the change in dipole moment, µE - µG, could be determined by the following equation

(µE - µG) )

(

slope · h · c · a3 2

)

0.5

(9)

Using the above equation and the maximum excitation and emission peak energies from Figure 3, the dipole moment change for the range of solvents tested was calculated to be -24 D. The fluorophore cavity radius, R, was assumed to be half the length of the Au-MOA molecule. This is a reasonable change in dipole moment compared with dipole moment changes of other molecules. For example, molecules such as KBr have a dipole moment change of 10.4 D.39 The true fluorophore cavity is difficult to estimate because the conformation of the ligand complex may vary slightly with different solvents. The calculation result details are listed in Table 2. The Lippert-Mataga plot’s negative slope arises because the excitation energy maximum shifts to higher energy without any change to the emission wavelength. From this and the constancy of the emission at 610 nm, it can be concluded that the singlet excited state of the CT complex is most susceptible to

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Figure 8. Ratio of fluorescence intensity at the emission maximum (610 nm) over the absorbance at the 290 nm of the Au nanoparticles as a function of solution pH. Illustrated results were obtained using particles prepared with mercaptooctanoic acid as the ligand. Samples were prepared so that all absorbances did not exceed 1.5. No correction for inner filter effects was performed. Error bars reflect the uncertainty of estimating the absorbance at the excitation maximum due to the rapidly changing slope of the absorption spectra in the UV region. (See for reference Figure 3C and the text.) The error in measured absorption is significantly greater at higher pH where the slope of absorption change is steeper and approaches 1.5.

environmental polarity change, rising in energy as the pH decreases. The ground state and emitting triplet states are not influenced strongly. This is a relatively unusual situation unique to the nature of the Au-S CT complex. The evidence of the collapse of the molecular brush at low pH values is suggested by the observation that the change in Stokes shift as a function of solvent pH fits well to the trend obtained as a function of polarity (Figure 7A). The deduced ∆f values also follow a sigmoidal dependence with pH yielding an apparent transition point at pH ∼5.5 (Figure 7B). At high pH, the ∆f values are larger, suggesting a more polar environment, and at lower pH, they are smaller, suggesting a less polar environment. To show this, the measured values of νjA - νjF obtained for mercaptooctanoic-acid-terminated Au nanoparticles in various solvent were used to establish a trend line for νjA νjF versus ∆f. The apparent value of ∆f for nanoparticle samples of different chain length as well as for mercaptooctanoic acid as a function of pH was deduced from their respective measured Stokes shift assuming that the trend established for polarity is universally true regardless of the mechanism by which polarity is changed. If the change in Stokes shift was caused by another mechanism, then there would not be a linear dependence between the Stokes shift and the pH, or the values of ∆f deduced

Lee et al. would not be reasonable. The deduced ∆f values as a function of ligand chain length and pH are within the range expected for short chain ligands at surfaces and for semipolar solvents like isopropanol. For reference, chloroform has a calculated ∆f of 0.15 and hexane has a calculated ∆f of 0.01. Because the particles precipitate strongly in these solvents, it was not possible to make direct measurements under those circumstances. A more direct way to prove that molecular collapse is taking place, such as size exclusion chromatography, is not practical in this instance because the size of the ligand is too short to generate an appreciable change in particle diameter after its collapse. Other spectroscopic methods such as FTIR or NMR may be of use, and they are currently being explored. Role of the Au Nanoparticles on the Photostability and Emission Yield of the CT Complexes. In the case of thiolbased ligand complexes, Tzeng et al.3 reported that complexes with an intrinsic absorbance maxima at ∼3.1 eV (400 nm) and very weak emission at ∼2.7 eV (477 nm) were LMCT complexes in which one Au atom interacts with one S atom, whereas those with an absorbance at ∼3.9 eV (320 nm) and a very intense emission ∼1.94 eV (640 nm) are LMMCT in which two Au atoms interact with one S atom. On the basis of the wavelength maximum of emission and the measured lifetime of 1.45 µs, it is tempting to assign the emission we observe to an LMMCT complex. However, the excitation spectrum energy maximum is blue-shifted compared with a typical absorbances at ∼3.9 eV (320 nm). A possible explanation for this discrepancy is mixing between the electronic orbitals of Au atoms in the nanoparticle40-42 with that of the ligand complexes. NIST Atomic Reference Data provides a list of energy levels in Au and Au(I) species which range from 3.98 (312 nm) to 5.49 eV (226 nm).40-42 The absorption strength varies, and the strongest transitions are found at 4.40 (282 nm) and 6.15 eV (202 nm). Excitation at 290 nm can therefore be absorbed by both the CT complex and the Au atoms, and energy sharing between the Au and the CT complexes can skew the apparent excitation spectrum maximum wavelength accordingly. Energy sharing might also explain the improved photostability of these materials over free CT complexes reported elsewhere.43 Because the emission intensity and the absorbance are changing simultaneously as pH is decreased, we also evaluated how the fluorescence intensity at 610 nm normalized by the absorbance at 290 nm changes as a function of pH (Figure 8). It should be noted that there are several potential problems utilizing absorbance values to normalize the emissions. Among

Figure 9. Schematic showing the proposed conformational change to the molecular brush at the surface of Au nanoparticles at low (left) and high (right) pH.

Luminescent Au Nanoparticles these are instrument error due to low light levels for absorbances higher than 1 and instrument error following the rapidly changing slope of the absorbance in the UV, which amplifies uncertainty. We assumed that the error in measurement of the absorption was related to the slope of change in absorbance at 290 nm as well as the magnitude. Basically, the uncertainty range for each absorbance was the absorbance 2-4 nm higher and lower than the central value at 290 nm. This range was selected because the resolution of the spectrometer used is (2 nm. In other words, at higher pH, the absorption values were less accurate than at lower pH since the absorption at higher pH is closer to 1.5 and is changing more rapidly with wavelength. The absorption normalized emission as a function of pH shown in Figure 9 suggests that the emission yield increases somewhat with decreasing pH. The reason for a yield increase is not known but could be related to differences in the overlap between higher energy electronic levels of the CT complex and that of Au as the pH is changed. The molecular orbital diagram for d10 metal complexes of various has been extensively reviewed by others and indicates the presence of several higher energy UV absorption bands.7

J. Phys. Chem. C, Vol. 114, No. 29, 2010 12467 wavelength or change in absorption as a function of pH (Figure 3E,F) and possibly Figure 7B for this purpose. The problem with using the excitation peak wavelength shift is that the excitation is located in the UV, where many other molecules absorb light, and so limits the conditions where it can be used. The problem using the absorbance change is that it quickly increases at high pH values, leading to problems with accurate estimation of absorbance. At low pH, because the zeta potential of the particles becomes nearly neutral, the suspension is unstable. Light scattering of the aggregates can prevent accurate measurement of the absorbance. So whereas this material could be potentially be used as a measure of pH, other materials may be preferred because they would not suffer from these complications. Acknowledgment. We thank S. Lipsky of University of Minnesota for helpful discussion. This work was partially supported by the Utah Research Foundation Synergy Program. C.-w.L. was partially supported by a Sigma Xi Grant in Aid of Research and the Materials Science & Engineering Department. References and Notes

Conclusions The physical characteristics of the product were consistent with that of Au nanoparticles coated with a single layer of Au-S CT complexes. Au-S CT complexes bound to Au nanoparticles create a stable photoluminescent nanomaterial that exhibits reversible variations in luminescence intensity and excitation spectrum as a function of pH. The nature of this change is similar to what takes place for ligand complexes in solvents of different polarity, suggesting that both the pH and polarity responses are related. To explain this, it is suggested that the mercaptoalkanoic acid ligand used to form the CT complex behaves as a pH-responsive collapsible molecular brush on the surface of the Au nanoparticles. When the presenting carboxylic acid group is protonated, the molecule is free to collapse onto the Au nanoparticle surface in response to van der Waals interactions between the saturated carbon chain and Au atoms. When the ligand is collapsed, it prevents access of the water to the CT chromophore site. As a result, the collapsed saturated hydrocarbon chains lower the local polarity at the chromophore site, and the CT complex excitation and absorption characteristics change accordingly. A schematic of this concept is presented in Figure 9. The Lippert-Mataga analysis demonstrated that the pHdependent Stokes shift followed a linear trend with the orientation polarizability, ∆f, and that ∆f itself followed a sigmoidal dependence on pH, with a clear transition at around pH 5.5. As the pH is lowered, the transition is to values that suggest an environment significantly less polar than the water solvent in which the particles are prepared. These results were taken as an indication of collapse of the ligand molecule brush. Because the emission intensity versus pH graph lacks the sigmoidal shape typical of a titratable fluorophore (i.e., there are no high and low pH intensity plateaus), conventional analyses using the Henderson-Hasselbalch equation following emission intensity and pH are not applicable. It may be possible to use these particles to calibrate relative pH changes over a limited range (e.g., from pH ∼9 to 5) because the response of emission intensity to pH is nearly linear in this range and more importantly reversible (Figure 4). At lower pH values, the emission intensity increases again, so the correlation between emission intensity and pH loses its uniqueness. However, it is possible to utilize the change in the excitation peak maximum

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