colloid stability of thymine-functionalized gold nanoparticles

Colloid Stability of Thymine-Functionalized Gold Nanoparticles

Jingfang Zhou, David A. Beattie, John Ralston,* and Rossen Sedev

Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes Campus, Mawson Lakes,Adelaide, SA 5095, Australia

ReceiVed July 3, 2007. In Final Form: September 6, 2007

Gold nanoparticles surface-coated with thyminethiol derivatives containing long hydrocarbon chains have beenprepared. The diameter of the particles is 2.2 and 7.0 nm, respectively, with a relatively narrow size distribution.Thyminethiol derivatives are attached to the gold particle surfaces with thymine moieties as the end groups. The colloidstability of the gold nanoparticles as a function of the type and concentration of monovalent salt, pH, and particlesize was investigated in alkaline, aqueous solutions. The gold particles are stable in concentrated NaCl and KClsolutions, but are unstable in concentrated LiCl and CsCl solutions. The larger gold particles are more sensitive tosalt concentration and aggregate at lower salt concentrations. The reversible aggregation and dispersion of the goldparticles can be controlled by changing the solution pH. The larger gold particles can be dispersed at higher pH andaggregate faster than the smaller particles, due to stronger van der Waals forces between the larger particles. Hydrationforces play an important role in stabilizing the particles under conditions where electrostatic forces are negligible.The coagulation of the gold nanoparticles is attributed to van der Waals attraction and reduced hydration repulsionin the presence of LiCl and CsCl.

I. Introduction

The colloid stability of aqueous dispersions can often bedescribed by the Derjaguin-Landau-Verwey-Overbeek (DLVO)theory, in which electrostatic and van der Waals interactions areaccounted for.1Deviations from DLVO behavior are well-known,however, and include steric, hydrophobic, and hydration interac-tions, among others.2 For example, it has long been realized thatsilica particles are much more stable than DLVO theory predictsat high ionic strength.3 Healy et al.4 found that amphoteric latexparticles are stable even in 3.0 M KCl. Ortega-Vinuesa et al.5

observed that polystyrene particles covered by IgG proteinscoagulate with increasing ionic strength, but are restabilized atsufficiently large electrolyte concentrations. Repulsive hydrationforces have been widely accepted to explain the above behavior.

Hydration forces have been intensively investigated in thepast two decades on hydrophilic or charged surfaces, for example,biopolymers,6 amphoteric latex particles,7 particles covered byhydrophilic macromolecules,8,9 mica,10 and metal oxides, suchas SiO2.11,12 The surface force apparatus (SFA)10 and atomicforce microscope (AFM)13 are commonly used to investigate

these forces. Several theoretical models14 have been proposedto elucidate the origin of hydration forces at the molecular level,based on experimental data and computer simulations. Despitethe debate over these models, the repulsive hydration force isgenerally attributed to the organization and orientation of waterin the vicinity of a surface, functioning at a short distance witha decay length in the 0.2-1.1 nm range. However, the pre-exponential factor in the force law may vary by more than anorder of magnitude, depending on the nature of the surface inquestion.11,15Force measurements also reveal that the hydrationforce is influenced not only by the hydrophilicity of the surfacein question but also by the nature and concentration of the hydratedcounterions that surround the surface.16 Israelachvilli and co-workers15 found that the hydration force oscillated between twomica surfaces immersed in aqueous KNO3solutions, but smoothedout for soft surfaces as well as for rough surfaces, such as thosefound on many particles.

Although the molecular origin of specific ion effects on colloidstability is still not completely understood, the structure-modifyingapproach15 is generally applied to explain these phenomenaqualitatively. Ions are assumed to adsorb onto the particle surfaceand hence modify water structure in the vicinity of the surface.The hydration force is large for strongly hydrated or “structuremaking” ions with strongly ordered local water structure; thelatter can be reduced remarkably by adding weakly hydrated or“structure breaking” ions, which can cause dispersions tocoagulate. Ruckenstein and Manciu17-21 recently proposed asurface dipole theory in which double-layer and hydration

* Corresponding author. E-mail: [email protected].(1) Hiemenz, P. C.; Rajagopalan, R.Principles of Colloid and Surface

Chemistry, 3rd ed.; Marcel Dekker Inc.: New York, 1997.(2) Lyklema, J. H.Fundamentals of Interface and Colloid Science; Vol IV:

Particulate Colloids; Elsevier Academic Press: New York, 2005.(3) Yotsumoto, H.; Yoon, R. H.; Wakamatsu, T.; Ito, S.; Sakamoto, H.Shigen

Sozai1993, 109, 909-15 (in Japanese).(4) Healy, T. W.; Homola, A.; James, R. O.; Hunter, R. J.Polym. Colloids

2 1978, Proc. Symp. Phys. Chem. Colloidal Part, 527-36.(5) Lopez-Leon, T.; Gea-Jodar, P. M.; Bastos-Gonzalez, D.; Ortega-Vinuesa,

J. L. Langmuir2005, 21, 87-93.(6) Rowe, A. J.Biophys. Chem.2001, 93, 93-101.(7) Valle-Delgado, J. J.; Molina-Bolivar, J. A.; Galisteo-Gonzalez, F.; Galvez-

Ruiz, M. J.Prog. Colloid Polym. Sci.2004, 123, 255-9.(8) Molina-Bolivar, J. A.; Galisteo-Gonzalez, F.; Hidalgo-Alvarez, R.Colloids

Surf., B1999, 14, 3-17.(9) Valle-Delgado, J. J.; Molina-Bolivar, J. A.; Galisteo-Gonzalez, F.; Galvez-

Ruiz, M. J.; Feiler, A.; Rutland, M. W.Langmuir2005, 21, 9544-54.(10) Pashley, R. M.J. Colloid Interface Sci.1981, 80, 153-62.(11) Valle-Delgado, J. J.; Molina-Bolivar, J. A.; Galisteo-Gonzalez, F.; Galvez-

Ruiz, M. J.; Feiler, A.; Rutland, M. W.J. Chem. Phys.2005, 123, 034708/1-034708/12.

(12) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J.J. Colloid Interface Sci.1994, 165, 367-85.

(13) Veeramasuneni, S.; Yalamanchili, M. R.; Miller, J. D.Colloids Surf., A1998, 131, 77-87.

(14) Leikin, S.; Parsegian, V. A.; Rau, D. C.; Rand, R. P.Annu. ReV. Phys.Chem.1993, 44, 369-95.

(15) Israelachvili, J.Intermolecular & surface forces, 2nd ed.; Academic PressLimited: London, 1991.

(16) Pashley, R. M.Chem. Scr.1985, 25, 22-7.(17) Ruckenstein, E.; Manciu, M.AdV. Colloid Interface Sci.2003, 105, 177-

200.(18) Manciu, M.; Ruckenstein, E.Langmuir2001, 17, 7061-70.(19) Manciu, M.; Ruckenstein, E.AdV. Colloid Interface Sci.2004, 112, 109-

28.(20) Manciu, M.; Ruckenstein, E.Langmuir2001, 17, 7582-92.(21) Ruckenstein, E.; Manciu, M.Langmuir2002, 18, 7584-93.

12096 Langmuir2007,23, 12096-12103

10.1021/la7019878 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 10/24/2007

interactions couple together. They modified the Poisson-Boltzmann formalism by including additional ion-hydration andion-dispersion interactions which account for specific ion effectson double-layer interactions.17 They discovered that couplingbetween the double layer and hydration can increase the decaylength of the repulsive force, especially at high ionic strength.Surface dipoles can increase or decrease the total repulsiondependingon thespecific ions involved.AlthoughOrtega-Vinuesaand co-workers5 questioned the validity of this approach, whenapplied to the influence of some anions on the stability of IgG-latex particles, colloid stability at high ionic strength,18 resta-bilization mechanisms,22and specific ions effects23can generallybe well-explained by this theory, especially for cations.

The aggregation and dispersion properties of nanosized colloidparticles are essential for their optical, electronic, and catalyticapplications.24,25 In most cases, however, nanoparticles aresurrounded by an organic layer. For colloids whose surfaces arecoated by surfactants, polymers, or biomolecules, the nature andchemistry of the organic layer play an important role in stabilizingthe particles.2 Non-DLVO forces operate and have a stronginfluence on stability, especially for small particle sizes.2,15Forcoated nanoparticles with a diameter less than 10 nm, the particlesize is comparable in dimension to that of the adsorbed organiclayer. There are very few studies of the colloid stability ofdispersions of this type, prompting the present investigation.

Thymine molecules are ionizable and pH-sensitive. Previousstudies have shown that the surface pKa of thyminethiolderivatives with long hydrocarbon chains is 11.2.26If the solutionpH is above 11.2, deprotonation of the hydrogen attached tothymine at the N3 position will occur. On the other hand, stronginteractions exist between thymine molecules, for example,H-bonding, dipole-dipole interaction, andπ-stacking of thethymine rings.27Therefore, if thymine-containing molecules areattached to particle surfaces, they may potentially cause strongadhesive attractions2,28 when the particles approach and makecontact. The purpose of this work is to study the aggregation anddispersion behavior of gold nanoparticles coated by thyminethiolderivatives containing long hydrocarbon chains. We focus onthe influence of particle size, solution pH, monovalent salt type,and concentration and on the role of the thymine end group incontrolling colloid stability.

II. Experimental Methodology

1. Reagents.AR grade reagents were purchased from Aldrichand used without further purification. The thyminethiol derivative,1-(10-mercaptodecyl)-5-methylpyrimidine-2,4-dione (TSH), wassynthesized in our laboratory according to the method we havereported previously.29 AR grade solvents used were obtained fromBDH or Chem-Supply. Water was purified by a Millipore Ultrapurewater system and had a resistivity of 18.2 MΩ‚cm at 25°C.

2. Techniques.The samples for transmission electron microscopy(TEM) were prepared by placing one drop of particle dispersion inpH 12.5 water onto standard carbon-coated Formvar films mountedon copper grids (200 mesh). A piece of tissue was used to adsorbwater and permit the samples to dry faster. TEM images were obtainedwith a Philips CM100 electron microscope operating at 100 kV. The

size distributions of the gold cores were measured using UniversalTEM Imaging Platform software and were based on counting atleast 150 individual particle images. Transmittance infrared absor-bance spectra in KBr pellet were acquired using a Nicolet Magna-IR750 spectrometer at a resolution of 2 cm-1.

The gold particles were dispersed in pH 12.5 water at aconcentration of 0.1 mg/mL. At this concentration, the goldnanoparticles are Rayleigh scatters and the “absorbance” is thuslinear with respect to concentration. In addition, the thymineabsorbance is less than 1, and is of course proportional to theconcentration of thymine molecules in the dispersion. When saltwas added, the solution was vigorously shaken and then left for asufficient time for the gold particles to settle, after which UV-visible absorption measurements were carried out with a VarianCary 5 UV-vis-NIR spectrophotometer at room temperature. Inthis manner, the behavior of gold particles dispersed in the aqueoussolution could be examined. The extinction coefficient of goldparticles in the UV-vis region is caused by the light scattering ofgold particles, as well as a surface plasmon band absorption around525 nm. The thymine molecules have a characteristic absorptionpeak near 270 nm. UV-vis absorption studies of a series of golddispersions with different concentrations were performed and it wasfound that the absorbance was linear with concentration, as expectedfor Rayleigh scatters.1Therefore, UV-vis absorption could be appliedto monitor the colloid stability of the gold particles. In the presentstudy, the maximum absorption of thymine was chosen as a referencepoint when accounting for the stability of the gold particles. Thedata were determined in duplicate or triplicate with a reproducibilityof (5% or better.

pH measurements were conducted in a Class 100 clean room ata constant temperature of 22°C. The pH probe (Model IJ44) fromIonode is a combination Li glass bulb and Ag/AgCl electrode. ThepH-sensitive glass has a very low sodium ion error and can becorrected using the standard curve provided by the company at highNa+ concentration. The measuring pH range is 0-14 with an accuracyof (0.02 pH. Before each experiment or set of experiments, the pHprobe was calibrated with fresh pH 7.0 and 10.0 standard Merckbuffer solutions.

The zeta potentials of the gold particles in aqueous dispersionswere determined using the phase analysis light scattering (PALS)technique. The PALS system was designed and constructed by theLaser Light Scattering and Materials Science Group at the Universityof South Australia.31 PALS uses a cross-beam technique. Acousto-optic module and Bragg cells are used to offset one of the laserbeams relative to the other. The system is capable of reliablymeasuring electrophoretic mobility down to at least 10-11 m2 s-1

V-1. The measurements were performed three times for eachcondition and the average value was used in the final data sets.

3. Preparation of Thymine Surface-Coated Au Nanoparticles(AuTSH). Thymine-coated gold nanoparticles were prepared usinga modified two-phase transfer method.30 In situ and ex situ (ligandreplacement) methods were chosen to obtain different sized goldparticles. HAuCl4 (0.12 mmol) dissolved in 20 mL of water wasadded to 0.48 mmol of tetraoctylammonium bromide (TOAB)dissolved in 20 mL of toluene. The mixture was stirred for 30 minand the water layer was discarded. The organic layer was used asa stock solution. For the in situ method, 0.24 mmol of thyminethiolmolecules dissolved in 20 mL of toluene were then added to thestock solution. After the solution was stirred for 10 min, 1.2 mmolof NaBH4, freshly dissolved in 2 mL of water, was quickly added.The solution turned brown immediately and a black powder appearedwith time. The solution was stirred overnight under N2 and the blackpowder was collected. For the ex situ method, 1.2 mmol of NaBH4

dissolved in 5 mL of water was slowly added to the stock solution.A purplish-red solution was obtained and stirred for 3 h under N2.Then, 0.24 mmol of thyminethiol molecules dissolved in 20 mL oftoluene were added drop by drop and stirred overnight under N2.

(22) Davalos-Pantoja, L.; Ortega-Vinuesa, J. L.; Bastos-Gonzalez, D.; Hidalgo-Alvarez, R.Colloids Surf., B2001, 20, 165-75.

(23) Peschel, G.; Van, Brevern, O.Prog. Colloid Polym. Sci.1991, 84, 405-8.(24) Hu, Y.; Dai, J.Miner. Eng.2003, 16, 1167-72.(25) Snoswell, D. R. E.; Duan, J.; Fornasiero, D.; Ralston, J.J. Colloid Interface

Sci.2005, 286, 526-35.(26) Jang, Y. H.; Sowers, L. C.; Cagin, T.; Goddard, W. A.J. Phys. Chem.

A 2001, 105, 274-80.(27) Inaki, Y.; Mochizuki, E.; Yasui, N.; Miyata, M.; Kai, Y.J. Photopolym.

Sci. Technol.2000, 13, 177-82.(28) Liu, J.; Zhang, L.; Xu, Z.; Masliyah, J.Langmuir2006, 22, 1485-92.(29) Lake, N.; Ralston, J.; Reynolds, G.Lamgmuir2005, 21, 11922-31.

(30) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R.J. Chem.Soc., Chem. Commun.1994, 801-2.

(31) http://www.unisa.edu.au/laser/Research/PALS.asp.

Thymine-Functionalized Gold Nanoparticles Langmuir, Vol. 23, No. 24, 200712097

The black powder was precipitated during reaction. The final productswere purified by sonicating in CHCl3 or MeOH and then collectedby centrifugation. The process of sonicating and centrifuging wasrepeated several times to remove any unbound thyminethiol moleculesand TOAB in the final product.

The gold nanoparticles may be dispersed in highly polar organicsolvents such as DMF and DMSO, where they form a transparentdispersion, as well as in high pH (>12) aqueous solutions. In thelatter case, deprotonation of the hydrogen attached to thymine occursand confers stability.29 From the solubility behavior and comple-mentary spectroscopic evidence, thyminethiol molecules are attachedto the gold particle surface through the thiol group, leaving thethymine group pendant, although it may exhibit some affinity forgold.

4. Morphology and Structure of Thymine-Coated GoldNanoparticles.The thyminethiol-coated gold nanoparticles preparedby the in situ and ex situ methods were examined by TEM. The

images and particle size distributions are shown in Figure 1. Howthe gold particles appear depends on the magnification of the TEMimages. They are rather spherical in shape at low magnification, butirregular and highly faceted at high magnification. The average corediameters are 2.2( 0.3 and 7.0( 1.0 nm for the in situ and ex situgold particles respectively. During the drying process on the coppergrid from high pH water, the gold particles formed nanostructuresin some areas, where similar-sized particles tended to form moreordered structures with a hexagonal packing. The average edge-to-edge separation between the metal cores can be calculated fromthe well-ordered region, yielding a value of about 2.0 nm for boththe smaller and larger gold particles. For the thyminethiol derivativesused in this study, the fully extended chain length is about 1.1 nm.The edge-to-edge distance is nearly twice the thickness of thethyminethiol derivatives, which means that interdigitation of theadsorbed organic layer did not occur. This behavior is different fromthat discovered for alkanethiol-coated gold particles, where inter-

Figure 1. TEM images and size distributions of Au-TSH nanoparticles: (a) in situ gold; (b) ex situ gold.

12098 Langmuir, Vol. 23, No. 24, 2007 Zhou et al.

digitation prevails during the drying process and the edge-to-edgedistance is generally equal to a single-chain length of the alkanethi-ols.32

The transmittance FT-IR spectra of the thyminethiol derivativesas well as the composite gold particles in KBr pellet are illustratedin Figure 2. The peak positions, along with the assigned functionalgroups,33,34 are summarized in Table 1. All of the characteristicpeaks of free thyminethiol derivatives can be found in the spectraof the composite gold particles, which means that the thyminethiolderivatives are the essential components in the final product. However,when compared with free thyminethiol molecules, all the absorptionpeaks for thyminethiol molecules tethered on the gold particle surfacebecome broader. The aromatic C-H and N-H vibrations for thesetethered thymines on the particle surface shift to a higher wavenumber.A new peak appeared at 3480 and 3446 cm-1 for the smaller andlarger particles, respectively. It is attributed to the formation ofhydrogen bonding between thymine units.33,35 The FT-IR spectraconfirmed that thyminethiol molecules interact strongly with eachother on the surface of the gold particle.

The gold nanoparticles were dispersed in pH 12.5 aqueous solu-tion. The in situ gold particles formed a brown, transparent dispersiondue to the scattering of the smaller gold particles, whereas the exsitu particles formed a clear, red dispersion resulting from the surfaceplasmon band absorption of the larger gold particles. The corre-sponding UV-vis absorption spectra of the thyminethiol derivativesand thymine-coated gold particles are shown in Figure 3. A peakappeared at around 535 nm for the larger particles, which was causedby the gold surface plasmon band absorption and is clear evidenceof gold particle formation.36 However, this peak broadenedsignificantly for the smaller particles. A maximum absorption peakaround 270 nm occurs for all the samples and is the characteristicabsorption peak of thymine. This peak shifted, by about 8 nm, tolower wavelength for the large particles compared with the smallparticles. This blue shift is attributed to theπ-stacking of thyminerings tethered on the larger particles.37,38

III. Results and Discussion

1. Stability of Colloidal Gold Nanoparticles. The goldnanoparticles were dispersed in pH 12.5 aqueous solution. The

PALS technique was applied to measure the electrophoreticmobility, ue, of the gold particles at different particle concentra-tions. The results obtained are shown in Table 2. The mobilityvalues are very small, as is the corresponding zeta potential,ú,irrespective of which method of calculation was used.39,40 It isreadily seen that the electrostatic interactions are very weak.

The monovalent salt influence on the stability of the goldnanoparticles was investigated. The UV-vis absorption spectraof 2.2 nm gold aqueous dispersion (pH) 12.5) as a function ofsalt concentration for different monovalent salts are shown inFigure 4. The corresponding normalized maximum absorbanceof thymine for each condition is shown in Figure 5. The resultsshowed that NaCl and KCl exerted little influence on colloidstability. These gold particle dispersions were stable, even insaturated NaCl or KCl solutions. On the other hand, the goldparticles started to aggregate when LiCl or CsCl was added tothe dispersion. Eventually, the gold particles coagulated fullyand sedimented in 1.25 M LiCl or 1.5 M CsCl solutions. Thesame trends were observed for the 7.0 nm gold particles, asshown in Figure 6. These specific ion effects on the coagulationof gold nanoparticles follow the Hofmeister series.41 The resultsalso follow the so-called “lyotropic series”1as coagulants, Cs+

> K+ > Na+, except for Li+ ion. Similar behavior was observedby Healy et al.4 who studied the coagulation behavior ofamphoteric polystyrene latex suspensions in the presence ofdifferent monovalent salts.

When NaCl or KCl was added to dispersions containingdifferent-sized gold particles, both the small and large goldparticles were remarkably stable in concentrated NaCl or KClsolutions. The behavior changes markedly for the 2.2 and 7.0 nmgold particles as a function of LiCl and CsCl concentration, asis shown in Figure 7 (combination of Figures 5 and 6). The largergold particles were more sensitive to LiCl or CsCl concentrationand coagulated fully at 0.75 M LiCl or CsCl concentration, amuch lower concentration than for the small particles. Thisbehavior reflects the stronger attractive van der Waals forcesbetween the large gold particles, for the van der Waals forcesare proportional to particle size. Of course, for the larger compositeparticles, the gold core will exert a stronger influence on the vander Waals attraction in comparison with their smaller cousins,2

as we will see below. The morphologies of the coagulated small

(32) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R.W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish,G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W.Langmuir1998, 14, 17-30.

(33) Silverstein, R. M.; Webster, F. X.Spectrometric identification of organiccompounds, 6th Ed; John Wiley & Sons, Inc.: New York, 1998.

(34) Zhang, S. L.; Michaelian, K. H.; Loppnow, G. R.J. Phys. Chem. A1998,102, 461-70.

(35) Hamlin, R. M.; Lord, R. C.; Rich, A.Science1965, 148, 1734-7.(36) Kamat, P. V.Nanoparticles and nanostructured films; Fendler, J., Ed.;

Wiley-VCH: Weinheim, 1998.(37) Whittten, D. G.; Chen, L.; Geiger, H. C.; Perlsten, J.; Song, X.J.Phys.

Chem. B1998, 102, 10098-10111.(38) Zhang, J.; Whitesell, J. K.; Fox, M. A.Chem. Mater.2001, 13, 2323-31.

(39) O’Brien, R. W.; White, L. R.J. Chem. Soc., Faraday Trans.1978, 274,1607-18.

(40) Delgado, A. V.; Gonzalez-Caballero, F.; Hunter, R. J.; Koopal, L. K.;Lyklema, J.Pure Appl. Chem.2005, 77, 1753-805.

(41) Lopez-Leon, T.; Jodar-Reyes, A. B.; Bastos-Gonzalez, D.; Ortega-Vinuesa,J. L. J. Phys. Chem. B2003, 107, 5696-708.

Figure 2. Transmittance FT-IR spectra of free thyminethiolderivatives (1), 2.2 nm gold particles (2), and 7.0 nm gold particles(3).

Figure 3. UV-vis absorption spectra of free thyminethiol molecules(1), 7.0 nm gold particles (2), and 2.2 nm gold particles (3).


and large gold particles in CsCl solutions are shown in Figure8. The gold particles formed densely packed aggregates, butretained their individual character, without agglomerating intolarger units.

The influence of pH on the colloid stability of the gold particlesis shown in Figure 9. With increasing pH, the gold particles weredispersed. The further increased the pH, the more dispersed theparticles. When the pH value was above 12.3, all the particleswere dispersed and transparent dispersions were formed. Whenthe pH was reduced, the particles coagulated and sedimented.This process was reversible and was repeated many times for

both small and large gold particles. However, more 2.2 nm goldparticles were dispersed at the same pH than were 7.0 nm diameterparticles due to the smaller van der Waals attractions betweensmall gold particles. The gold particles can be dispersedcompletely at pH 12.3, approximately one pH unit higher thanthe pKa of the thyminethiol derivatives. This means that almostall of the thymine molecules tethered to the gold particle surfacewere deprotonated when these particles were fully dispersed.

2. Forces between Gold Nanoparticles.van der Waals forcesbetween two identical spheres immersed in a homogeneousmedium are always attractive42and, in most cases, this attractionis responsible for the aggregation of colloidal particles. Accordingto traditional DLVO theory, the double-layer repulsion forceswill keep the particles dispersed. The double-layer repulsion iscaused by the charge on the surface, which can be regulated byadjusting solution pH for thymine-coated gold nanoparticles.The surface properties of thymine-coated gold nanoparticles atdifferent pH values are illustrated in Scheme 1. The particlesurface is neutral at a dispersion pH less than the pKa of thetethered thymine molecules, where there are no double-layer

Table 1. FT-IR Vibrations of Free Thyminethiol Molecules and Thyminethiol Surface-Coated Gold Nanoparticles

band position in cm-1

assignment free T-C10-SH 2.2 nm Au 7.0 nm Au

CH2 antisymmetric and symmetric stretching 2918; 2847 2920; 2850 2918; 2848CH2 and CH3 bendin 1470; 1429; 1385 1466; 1430; 1385 1466; 1431; 1385secondary amide CdO stretching 1699; 1680 broad centered at 1678 broad centered at 1683CdC skeletal in-plane stretching 1652 cannot distinguish cannot distinguishN-H stretching and bending 3153; 1639 3167; cannot distinguish 3165; cannot distinguisharomatic C-H stretching 3010 3036 3034C-N stretching 1356 1358 1358

Table 2. Electrophoretic Mobility and Zeta Potential of theGold Nanoparticles at pH 12.5 Aqueous Solution

concentration(mg/mL)

particlesize (nm)

electrophoreticmobility

(m2/V‚s× 10-9)

zetapotential

(mV)

0.5 2.2 -1.7( 2.0 -1.3( 1.47.0 -6.6( 1.4 -5.1( 1.1

1.0 2.2 -2.7( 1.2 -1.8( 0.97.0 -5.6( 2.1 -4.5( 1.6

Figure 4. Influence of the concentration and type of monovalent salts on the colloidal stability of the 2.2 nm diameter gold particles in pH12.5 water.


repulsion forces existing in the system, so that the particlesaggregate. When the solution pH increases above the pKa,deprotonation of thymine molecules will occur. The further thepH is from the pKa, the greater the surface potential and thestronger are the double-layer repulsion forces,2 all of which,along with the increased hydration forces and reduced adhesiveforces between gold particles,28lead to enhanced colloid stability.

The pKa of thymine is 11.2, so the gold particles are dispersedat pH 12.5. Under these conditions, the thickness of the electricaldouble layer (Debye-Huckel reciprocal length) at pH 12.5 and

an ionic strength of 0.03 M is 1.7 nm. The zeta potentials forthis system are very small (Table 2), which indicates thatelectrostatic repulsion plays a minor role in controlling colloidstability. Although it is known43,44that adsorbed organic layerscan reduce the van der Waals attractions or cause stericstabilization, in this study we observed that the stability of goldparticles can be controlled by adding salt or adjusting pH. Thequestion arises as to what forces might stabilize the goldnanoparticles at high pH.

(42) Hamaker, H. C.Physica1937, 4, 1058-1062.(43) Bittnera, A. M.Surf. Sci. Rep. 2006, 61, 383-428.(44) Vold, M. J.J. Colloid Sci.1961, 16, 1-11.

Figure 5. Influence of monovalent salt concentration on the colloidalstability of the 2.2 nm diameter gold particles in pH 12.5 water.

Figure 6. Influence of monovalent salt concentration on the colloidalstability of the 7.0 nm diameter gold particles in pH 12.5 water.

Figure 7. Comparison of colloid stability between the 2.2 and 7.0nm diameter gold particles in pH 12.5 water in the presence of LiCland CsCl.

Figure 8. TEM images of the aggregates of 2.2 (a) and 7.0 (b) nmdiameter gold nanoparticles in CsCl solutions (pH) 12.5).

Figure 9. Influence of pH on the colloid stability of the 2.2 and7.0 nm diameter gold nanoparticles without adding salt.


The particle surfaces are negatively charged at a dispersionpH of 12.5. Repulsive hydration forces can of course operate athydrophilic or negatively charged surfaces,10and may be modifiedby certain ions,4,15,19,45,46,47influencing colloid stability, asobserved here. Manciu and Ruckenstein18 considered the role ofthe hydration force in influencing colloid stability at high ionicstrength. If the hydration repulsion is large, the colloids canremain stable at any electrolyte concentration, which reasonablyexplains the stability of gold particles in this study in saturatedNaCl or KCl solutions. Therefore, the forces underlying the colloidstability of gold nanoparticles in high pH water appears to bea weak double-layer repulsion plus a strong hydration repulsionin the absence of added salt. With an increase in ionic strengthupon adding salt to the dispersion, the electrostatic forces arescreened and are eliminated at about 1.5 M electrolyte concen-tration.2 Under these conditions, the gold particles are stabilizedsolely by hydration repulsion forces.

We observed that LiCl acted as a more effective coagulantthan CsCl for these gold nanoparticle dispersions. The large andsmall gold nanoparticle dispersions coagulated completely atabout 0.75 and 1.25 M LiCl with corresponding pH values of11.6 and 11.4, respectively. The pH value of the suspensiondecreased in the presence of LiCl. When the dispersion pH wasdecreased, protonation of thymine molecules increased, leadingto a decrease in surface charge. Israelachvili et al.15 noted thatrepulsive hydration forces are reduced or eliminated for proton-bound surfaces. We also observed that the dispersion becamestable when the solution pH was readjusted to above 12.3. CsCldoes not reduce the dispersion pH, but certainly does destabilizethe dispersion.

To help explain the observations, the van der Waals forcesbetweengoldnanoparticleswerecalculated.Hamaker’sequation42

for the calculation of van der Waals attraction is based oncontinuum theory. Nanoparticles fall in the region between atomsand macroparticles. Fichthorn and Qin48-50as well as Wang andand co-workers51calculated the van der Waals attractions between

nanoparticles with diameters of 1.67 and 6.01 nm, respectively,using molecular-dynamics simulations and found that Hamaker’sequation can be used to describe van der Waals forces betweenspherical nanoparticles quite accurately. The adsorbed organiclayer will modify the van der Waals forces between bare goldparticles. Vold44 and Ninham and Parsegian,52 among others,developed models to calculate van der Waals forces betweencoated colloidal particles. A system of two identical sphereswith a single identical adsorbed layer in medium M is illustratedin Scheme 2. Based on the generic approach of Usui andBarouch,53the total van der Waals energy of interaction involvescontributions from the organic layer-organic layer interaction,organic layer-particle interaction, and particle-particle interac-tion, which in this case is given by

whereD is the separation distance between two particles,R isthe radius of the particle,Sis the thickness of adsorbed layer onthe particle surface, andA is the Hamaker constant, where

and

A1, A2, andA3 are the Hamaker constants for the organic layer-organic layer interaction, organic layer-particle interaction, andparticle-particle interaction for particles P with adsorbed layerS in medium M.AM, AS, andAPrepresent the Hamaker constantswith respect to vacuum for the medium, adsorbed organic layer,and particle. The corresponding values for water, thyminethioladsorbed layer with long hydrocarbon chain, and gold core are4.0× 10-20, 5.0× 10-20, and 40× 10-20 J, respectively.15 Thethickness of the adsorbed thymine layer was estimated to be 1.1nm. The corresponding van der Waals interaction as a functionof separation distance for 2.2 and 7.0 nm diameter goldnanoparticles is plotted in Figure 10. The van der Waalsinteractionscontributed fromthe thyminethiol layer-thyminethiollayer interactions and gold core-thyminethiol layer interactionsare only a small component of the total van der Waals interactions.The gold core-gold core interactions predominate in the totalinteractions for both 2.2 and 7.0 nm gold particles. At a givendistance, theGvdw is greater for the larger particles, but it isattractive for both. Therefore, the forces governing the colloidstability of thymine-functionalized gold nanoparticles in highpH water at different conditions may be described as follows:the Au sols are stable in concentrated NaCl and KCl solutions,as illustrated in Figures 5 and 6.Gvdw is attractive and electrostaticforces are very weak, so the observed colloid stability is due to(45) Allen, L. H.; Matijevic, E.J. Colloid Interface Sci.1969, 31, 287-96.

(46) Colic, M.; Fisher, M. L.; Franks, G. V.Langmuir1998, 14, 6107-12.(47) Colic, M.; Franks, G.; Fisher, M.; Lange, F.Langmuir1997, 13, 3129-

35.(48) Qin, Y.; Fichthorn, K. A.J. Chem. Phys.2003, 119, 9745-54.(49) Qin, Y.; Fichthorn, K. A.Phys. ReV. E 2006, 73, 020401(1-4).(50) Fichthorn, K. A.; Qin, Y.Ind. Eng. Chem. Res.2006, 45, 5477-81.

(51) Wang, J. C.; Neogi, P.; Forciniti, D.J. Chem. Phys.2006,125, 194717(1-6).

(52) Ninham, B. W.; Parsegian, V. A.J. Chem. Phys.1970, 52, 4578-4587.(53) Usui, S.; Barouch, E.J. Colloid Interface Sci.1990, 137, 281-8.

Scheme 1. Surface Properties of Thymine-Coated GoldParticles at Different p Valuesa

a Note: The diagram is not drawn to scale.

Scheme 2. Two Identical Spheres P with a Single IdenticalAdsorbed Layer S in Medium M

Gtotal_vdw ) Gs-s_vdw + Gp-s_vdw + Gp-p_vdw )

- 16[R + S

2

A1

D1+

2R(R + S)2R + S

A2

D2+ R

2

A3

D3]

A1 ) (AS1/2 - AM

1/2)2

A2 ) (AS1/2 - AM

1/2)(AP1/2 - AS

1/2)

A3 ) (AP1/2 - AS

1/2)2

D1 ) D

D2 ) D + S

D3 ) D + 2S


hydration repulsion. In the presence of LiCl, the dispersions aredestabilized, although Li+ ion is generally a structure maker.However, LiCl also reduces the dispersion pH. Thus, the hydrationforce is weakened as the particle surface is protonated, leadingto coagulation of the gold particles. For CsCl, the dispersion pHis not altered, but Cs+ ions are structure breakers. The hydrationforce is therefore disrupted and weakened, permitting the attractivevan der Waals forces to coagulate the gold particles. The largerthe gold particles, the greater the attractiveGvdw (Figure 10) andthe more sensitive are the gold particles to salt concentration andpH.

However, the traditional double-layer repulsion is calculatedaccording to the Poisson-Boltzmann equation, which is inac-curate at concentrations higher than approximately 0.05 M.2,15

Ruckenstein and Manciu17 found that an attractive force can begenerated between two surfaces when ion-dispersion interactionsare considered in the modified Poisson-Boltzmann approach athigh ionic strength. Therefore, double-layer attraction, as observed

by Bostrom et al.54and Ise et al.55may exist at high ionic strength.AFM studies28,56revealed that adhesive forces operated betweenpH-sensitive surfaces in contact at low pH. Thymine end groupscan interact strongly on the gold particle surface, as confirmedby FT-IR and UV-vis spectra, which might induce strongadhesive forces when two particles come into contact, contributingpotentially to the aggregation of the gold particles at low pH inaddition to van der Waals attraction.

IV. Conclusions

Thymine-coated gold nanoparticle dispersions are stable atpH 12.3 and above due to a combination of weak electrostaticrepulsion and strongly repulsive hydration forces. The combina-tion of these outweighs the van der Waals attraction, which isdominated by the gold nanocores with the adsorbed organic layerplaying a minor role. In the presence of concentrated NaCl andKCl solutions, the dispersions remain stable due to the stronghydration repulsion. Colloid instability occurs in the presence ofLiCl and CsCl due to weakened hydration repulsion. The largergold particles are more sensitive to salt concentration due togreater van der Waals forces. The controlled aggregation anddispersion of nanoparticles are of great interest for applicationsin nanoscience and nanotechnology with this work providingsome new insights.

Acknowledgment. Financial support through The AustralianResearch Council Special Research Centre Scheme is gratefullyacknowledged. Discussions with Assoc. Prof. Daniel Fornasieroare warmly acknowledged.

LA7019878

(54) Bostrom, M.; Williams, D. M. R.; Ninham, B. W.Phys. ReV. Lett.2001,87, 168103(1-4).

(55) Ise, N.; Okubo, T.; Sugimura, M.; Ito, K.; Nolte, H. J.J. Chem. Phys.1983, 78, 536-40.

(56) Kane, V.; Mulvaney, P.Langmuir1998, 14, 3303-3311.

Figure 10. Gtotal_vdwinteractions as a function of separation distancefor 2.2 and 7.0 nm thyminethiol-coated gold nanoparticles.


colloid stability of thymine-functionalized gold nanoparticles

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