Langmuir

Adsorption Kinetics of Au and Ag Nanoparticleson Functionalized Glass Surfaces

Sung-Ho Park, Jung-Hyuk Im, Jun-Wan Im, Byung-Hoon Chun, and Jae-Ho Kim1

Department of Applied Chemistry, Ajou University, Suwon 442–749, Korea

Accepted June 7, 1999

Well-defined two- or three-dimensional arrangements of nanosize Au and Ag particles werefabricated for surface-enhanced Raman scattering substrate applications and their unique opticalproperties. Two-dimensional arrays of colloidal films of Au and Ag were prepared on glass slidesmodified with silane compounds having bifunctional moieties that have specific affinity for gold orsilver. During the formation of colloidal films of Au and Ag on glass slides, UV–vis absorptionspectroscopy was used to monitor the progress of the surface immobilization reaction of colloidalparticles on solid substrates. Adsorption characteristics could be accurately modeled by the well-known Michaelis–Menten kinetics. It was found that thek3/k1 values were proportional to particlesize. This indicates that the rate of formation of the first monolayer with relatively small particles isfaster than that of the multilayers, whereas for larger colloids the multilayer formation rate is muchfaster. SERS spectra ofp-aminothiophenol (PATP) on a silver colloidal film were obtained with514.5-nm excitation. SERS intensities of PATP increased significantly with morphological change ofthe colloidal film. This morphological alteration, parallel to water evaporation from the colloidal film,was presumably induced by the difference in dielectric constants of air and water.© 1999 Academic Press

Key Words:adsorption kinetics; Au colloid; Ag colloid; colloidal metal film; self-assembledmonolayer; surface-enhanced Raman scattering spectroscopy.

INTRODUCTION

Since 1928, when Sir. Chandrasekhra Venkata Raman discovered the Raman scatteringphenomenon that bears his name, Raman spectroscopy has steadily become a more usefultool for the characterization of chemical materials and contributed to the elucidation oftheir structures. Improvements in the various components of Raman spectroscopic instru-mentation including lasers, detectors, electronics, and computers have given Ramanspectroscopy wide applications and enhanced its analytical sensitivity. The most notableexamples are Fourier transform Raman, resonance Raman, and surface-enhanced Ramanspectroscopy. The discovery of surface-enhanced Raman scattering (SERS) has promptednumerous theoretical and experimental studies (1–4). SERS is a spectroscopic techniquethat allows one to obtain vibrational spectra of molecules adsorbed on metal surfaces.Determination of the nature of the adsorbed molecules at the solid surface is important forunderstanding the physiochemical properties of intermolecular or adsorbate–substrateinteractions at the interface. Numerous studies have reported that many molecules ad-sorbed on appropriately prepared metal surfaces display Raman cross sections severalorders of magnitude greater than the corresponding quantity for an isolated molecule orfrom a solution (5, 6). Because of the large enhancement, small sample volumes can beused; detection limits of SERS technique lie in the picomole-to-femtomole range (7).

1 To whom correspondence should be addressed.

Microchemical Journal63, 71–91 (1999)Article ID mchj.1999.1769, available online at http://www.idealibrary.com on

0026-265X/99 $30.00Copyright © 1999 by Academic PressAll rights of reproduction in any form reserved.

71

The understanding of the mechanism responsible for SERS was concentrated in theearly study. There are two mechanisms that are now considered as the most appropriateexplanations for surface-enhanced Raman scattering. One is related to relatively long-range electronic resonance effects of the optical fields with the surface plasmon of theSERS-active substrate, namely, the electromagnetic theory. In the electromagnetic model,a molecule near an active conducting SERS surface experiences a stronger electromag-netic field compared with a free molecule (6). The other explanation is the chemicalenhancement theory related to the formation of charge-transfer complexes betweenadsorbed molecules and the substrate, causing distortion in the polarizability of themolecules (7). In the chemical enhancement model, the Raman scattering cross section ofthe molecule is amplified by molecule/metal interaction. Ample experimental evidencehas been reported for each of these mechanisms to suggest that more than one mechanismis responsible for the overall enhancement (6, 8). Although several different mechanismsfor the SERS effect have been proposed, a general agreement has been reached that thesurfaces of metals need to be roughened properly to obtain large surface enhancement.

Probably the most common substrates used for SERS are colloidal suspensions andsilver or gold particles and electrochemically roughened silver or gold electrodes becausetheir preparation requires no specialized instrumentation (9–12). Among the many SERSsubstrates providing appropriate characteristics, colloid hydrosols are the most commonlyemployed for several reasons: (a) their easy preparation, handling, and storage; (b) the lowelectric potential existing at the surface (11); and (c) the minimum influence of thesesystems on the structures of the adsorbate on the surface, especially when biologicalmolecules are used (13). The preparation of colloidal metals appears simple, but great caremust be exercised to produce a reproducible and stable colloid (4). The purity of the waterand reagents, as well as the cleanliness of the glassware, is critical (4). Small metalparticles exhibit novel physical and chemical properties that arise from size and surfaceeffects (14–16). In the case of novel metal colloids, the surface plasmon band of particleschanges with variation in particle diameter (16–18). Metal particles are surrounded byanions which produce an interparticle repulsive force. When a molecule is exposed tometal hydrosol, it is adsorbed on the particle by replacing the adsorbed anion, whichreduces the charge of the colloid particle and increases the possibility of the flocculationof the colloidal systems (19). The characteristics of this process are strongly influenced bythe size and shape of the particle, the ionic strength, the pH, and the concentration of thehydrosol solution. These parameters have contributed to rather poor reproducibility ofSERS effects of species adsorbed on metal hydrosols. Therefore, to fulfill the potential ofthe SERS technique as an established analytical method, a well-characterized SERSsubstrate and a reliable and reproducible preparation procedure are required. Althoughmany approaches have been reported on this account, preparation of a well-defined, stableSERS substrate having uniform roughness has proven difficult (20).

Recently the fabrication of nanostructures or nanoparticles has increased interestthroughout the fields of pure and applied science and technology due to their uniquephysical, optical, and chemical properties (21). A new method of generating macroscopicmetal surfaces, based on self-assembly of nanometer-scale colloidal gold and silverparticles from solution onto various solid substrates, has been reported by many researchgroups in the past several years (22, 23). It has been found that a well-defined and uniform

72 PARK ET AL.

nanometer-scale architecture of silver or gold colloid, called colloidal metal films (CMFs),can be generated and many possible applications, including SERS substrates, have beendemonstrated. CMFs as SERS substrates show unique advantages over other substratessince they can be characterized at both the macroscopic and microscopic levels, are highlyreproducible, are electrochemically addressable, and are simple to prepare in largenumbers (22–24). Furthermore, the surface roughness of CMFs can be defined andcontrolled by the colloid diameter, which provides not only the condition to optimizeSERS effect but also an opportunity to understand surface enhancement theory. However,little information is available on the critical parameters that determine surface roughness.The rate of formation of CMFs, depending on particle diameter, is particularly importantin control of the final topological properties, interparticle distance, and surface coverageof colloidal metal particles for SERS application.

In this study, we report on the kinetics of colloidal particle chemisorption on organosi-lane-modified glass substrates. The adsorption rate constants were calculated by using fourdifferent sizes of colloidal Au and Ag particles. Using the Michaelis–Menten equation andUV–vis absorption spectroscopy, we demonstrated that it is possible to control the surfacedensity of colloidal particles by changing immersion time and particle size. This result isuseful for fabricating reproducible colloidal metal films with specified physiochemicalproperties, which is crucial in using nanoparticle-based devices including SERS sub-strates.

MATERIALS AND METHODS

Materials. The following chemicals were obtained from Aldrich: HAuCl4 z 3H2O,3-aminopropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, trisodium citrate di-hydrate, acetone, methanol, 2-propanol,p-aminothiophenol, and H2O2. Silver nitrate waspurchased from Kojima Chemicals. The following materials were obtained from J. T.Baker: HCl, H2SO4, and HNO3. All chemicals were used as received. H2O (18 MV) wasobtained from Milli-Q. Glass slides were obtained from Gebruder Rettberg.

Colloid preparation.Various sizes of Au colloids were prepared according to thepreviously described procedure (25). A brief description of the procedure is as follows. Toan initial solution of 50 ml 0.01% (w/v) HAuCl4 that is at a boil, 1.75 ml of 1% (w/v)trisodium citrate is added. The solution initially develops a gray color, which changes toa lavender and finally red in 1–3 min of continuous boiling. All processes were done underreflux. By controlling the amount of trisodium citrate initially added, various sizes ofcolloidal particles were formed. The maximum absorption peak in the UV–vis spectrashifts from 520 nm to a longer wavelength as the citric acid/Au mole ratio decreases.Citrate-reduced Ag colloids were prepared according to a modified Lee and Meiselmethod (26). All glassware used in the colloid preparation was rigorously cleaned in abath of freshly prepared 3:1 HCl:HNO3 (aqua regia) (caution! aqua regia is extraordi-narily hazardous; it must be handled with the utmost care) and thoroughly rinsed withultrapure water. An aqueous solution (500 ml) of silver nitrate (90 mg) was heated to45°C, and then the temperature was increased rapidly to the boiling point while rigorouslystirring. As soon as the boiling commenced, a 1.0% (w/w) solution of sodium citrate (10ml) was rapidly added, and heat was reduced, but the solution was kept boiling gently for90 min and continuously stirred.

73Au AND Ag NANOPARTICLES ON GLASS

Surface derivation with organosilane coupling agent on a glass slide.The surface of aglass slide was modified with 3-aminopropyltrimethoxysilane (APTMS) or 3-mercapto-propyltrimethoxysilane (MPTMS). All glass slides used in this report were washed withdetergent and rinsed with distilled water followed by sonication in acetone and methanolfor 2 min each subsequently and dried in an oven before cleaning with a piranha solution(H2O2:H2SO4 1:3, v/v) (Caution! Piranha solutions are extremely dangerous, reactingexplosively with organic materials.) After exposure to a piranha solution for 30 min, theplates were rinsed thoroughly with distilled water and stored in methanol (HPLC grade)until use. An APTMS (or MPTMS) solution was prepared by adding 2 ml APTMS (orMPTMS) to 25 ml of 2-propanol. The clean plates were placed in the prepared solutionsfor 1 day. The modified plates were rinsed with 2-propanol and annealed at 110°C for 10min and stored in an inert atmosphere (27).

Colloid immobilization.Each modified glass slide was immersed in colloid suspension.After they were constructed on the plates, the CMFs were rinsed with distilled water andthen stored in distilled water. Particle size was determined by analysis of SEM photo-graphs and atomic force microscopy (AFM) images. The morphology of the CMFs wasmeasured by AFM.

Instrumentation.A JASCO V-550 UV/VIS spectrophotometer, working in the range190–850 nm, with a resolution of 0.5 nm per element, was used to carry out UV–visspectroscopy. AFM images were obtained by normal noncontact mode measurement withV-shaped silicon nitride cantilevers [spring constant: 0.067 N/m, SPM-LS, Park ScientificInstruments (PSI), Sunnyvale, CA]. Surface-enhanced Raman spectra were collectedusing a triple monochromator coupled with a blue intensified CCD array detector (SpexIndustries, Edison, NJ). A Coherent Innova 70 argon ion laser was used as an excitationsource (514.5 nm). The laser light was passed through a premonochromator to removeplasma emission lines. The scattered light was collected in a 45° angle geometry. Spectraobtained with 514.5-nm excitation were recorded using a 1200 groove/mm grating.

RESULTS AND DISCUSSION

We report on the kinetic behavior of the adsorption of colloid particles to a surfacemodified with organosilanes containing either thiol or amino-terminal groups for self-assembling Au and Ag colloids. The schematic diagram of the adsorption process ofcolloid particles is shown in the Fig. 1. To our knowledge, there is one report regardingthe kinetics of immobilization of colloids onto solid surfaces (28). Natanet al.showed thatthe surface coverage is roughly proportional to the square root of immersion time within45 min, and colloidal particle coverage is limited by repulsive interparticle interactioninhibiting additional particle immobilization on the Au particle. They observed that theabsorbance of CMFs constructed by longer exposure of the substrate to colloidal Au wasnot significantly changed even for periods of 6 weeks. In our study, an apparent plateauof the increase in absorbance at longer reaction time was observed, and this feature wasconsistently observed from immobilization reactions of Au colloids of various particlesizes. To elucidate the rate constant of the surface immobilization reaction of the colloidalparticle, the well-known Michaelis–Menten model was used to describe the relationshipof maximal absorbance at the absorption peak to immersion time.

The Michaelis–Menten model, which is often adopted to describe the kinetics of an

74 PARK ET AL.

enzyme-catalyzed reaction, allowed us to accurately describe the relationship of peakabsorbance with immersion time and to interpret the adsorption mechanism of Au colloidsonto modified solid substrates. Contrary to the previous report, the multilayer formationof Au colloidal film was observed by AFM.

To understand the nature of multilayer formation, it is necessary to elucidate detailedreaction pathways of chemisorption of Au particles on modified surfaces. The adsorptionreaction can proceed either by ion dissociation of the thiol or by formation of Hz asfollows (29):

glass, SH1 Au 3 glass, SAu2 1 H1, (1)

glass, SH1 Au 3 glass, SAu1 H z . (2)

Reaction (2) is favored considerably over (1) according to the previously reportedelectrochemical evidence (30–32) as well as conductometric analysis (29). This wasaffirmed by spectroscopic measurement in this study. If the adsorption reaction proceedsas indicated in Eq. (1), terminal SAu2 will induce significant colloid aggregation asoccurred after addition of anions such as Cl2 and OH2. However, no colloid aggregationwas observed during the immobilization reaction.

As shown in the Fig. 2, the absorption peak in UV–vis spectra indicated that the numberof adsorbed colloids is limited. If adsorption proceeds by reaction pathway (2), the surfacecharge will not develop. Instead, a slight partial change may appear on the surface of animmobilized colloidal particle due to the difference in electronegativity of Au and Satoms. Since the electronegativity of S is slightly higher than that of Au, a partial positivecharge appears in Au particles which act as nuclei for additional particle adsorption. Thiswas verified by AFM studies. Monodispersed Au colloids were covalently adsorbed onsurface-confined MPTMS-coated glass as shown in Fig. 3. If the initial negative charge of

FIG. 1. Schematic diagram of colloidal metal film preparation.k1 andk2 are adsorption and desorption rateconstants for the first layer.k3 and k4 are adsorption and desorption rate constants for the multilayers,respectively. The bars represent the glass substrate, and the circles, the Au or Ag colloid.


Au colloidal particles was significantly weakened, or at least converted to positive, athree-dimensional multilayer would form rather than a monolayer of Au colloids. Auparticles in suspension approach two different regions of the substrate as depicted inreactions (3) and (4). An Au particle diffuses toward surface-confined thiol functionalgroups as in case (3) or to already adsorbed Au particles on the glass surface as in (4):

glass–SH1 Au -|0k1

k2

glass–S–Au1 H z , (3)

glass–S–Au1 Au -|0k3

k4

glass–S–Aun. (4)

During the initial adsorption period, reaction (3) is favored over (4) because more thiolfunctional groups are available than adsorbed Au particles on the substrate. Most of thehydrogen radicals generated by immobilization of Au particles were converted to H2

molecules and consequently stored as H2O2 since a trace amount of O2 is available in thereaction (28). It is reasonable to consider that the reverse rate constantk2 in reaction (3)is negligible due to a strong covalent bond between the sulfhydryl and the Au particle.Furthermore, it was found that CMFs stored in water for several weeks maintain a constantoptical density, indicating thatk4 is small enough to be neglected too. Since the result of

FIG. 2. Absorption spectra of Au colloidal films immobilized on MPTMS-coated glass slides: The immer-sion times in a suspension of 16-nm-diameter Au colloid particles were 5 min, 10 min, 30 min, 2 h, 18 h, and24 h (from bottom to top, respectively).

76 PARK ET AL.

reaction (4) is the formation of a multilayer of Au colloidal film, the rate equation of themultilayer formation reaction can be expressed as follows with the rate constant k3:

V 5 k3@glass, SAu#@Au#. (5)

Although it appears that the reaction rate depends on both [glass; SAu] and [Au] in Eq.(5), it was found that [glass; SAu] governs the reaction rate of multilayer formation. Theeffective concentration of the Au colloid is estimated to be in the range of a fewnanomoles, whereas Au surface coverage at a fully adsorbed state was around 20 fmol.Based on the experimental findings and the concentration ratio, it is reasonable to neglect[Au] and simplify Eq. (5) to the rate of formation of a multilayer,

V 5 k3@glass, SAu#, (6)

the rate of formation of the first monolayer,

@glass, SAu# 5 k1@glass, SH#@Au#, (7)

FIG. 3. AFM image of Au colloidal film prepared by immersing a MPTMS-coated glass slide in Au colloid(d 5 16 nm) suspension for 30 min.


and the rate of breakdown of

@glass, SAu# 5 k3@glass, SAu#. (8)

The following relationships can be established under steady-state conditions:

k1@Au#@glass, SH# 5 k3@glass, SAu#, (9)

@glass, SAu# 5@Au#@glass, SH#

k3/k1. (10)

If KM is defined as

KM 5 k3/k1 (11)

Eq. (10) can be expressed as

@glass, SAu# 5@Au#@glass, SH#

KM. (12)

It is reasonable to say that the Au colloidal concentration in the suspension is consideredconstant throughout the reaction since the decrease in concentration of the colloid byadsorption on the solid substrate is near zero. Therefore, the following relationship can beestablished:

@glass, SH# 5 @glass, SH# total 2 @glass, SAu#. (13)

Substitution of the expression in Eq. (12) then gives

@glass, SAu# 5~@glass, SH# total 2 @glass, SAu#!@Au#

KM. (14)

Solving for [glass; SAu],

@glass, SAu# 5@glass, SH# total~@Au#/KM!

1 1 @Au#/KM(15)

or

@glass, SAu# 5 @glass, SH# total

@Au#

@Au# 1 KM. (16)

78 PARK ET AL.

Therefore, the rate of formation of the Au colloidal multilayer can be expressed as

V 5 k3@glass, SH# total

@Au#

@Au# 1 KM. (17)

If [Au] is much larger thanKM, the maximal rate of the adsorption reaction can beexpressed as

Vmax 5 k3@glass, SH# total if @Au# @ KM. (18)

Substitution of the expression forVmax into Eq. (17) then gives

V 5 Vmax

@Au#

@Au# 1 KM(19)

or

1

V5

1

Vmax1

KM

Vmaxz

1

@Au#. (20)

From Beer’s law,

A 5 abc, dA 5 d~abc!,dA

dt5

d~abc!

dt.

If the value ofab is constant,

dA

dt5 ab

dc

dt,

dc

dt5 V,

dA

dt5 abV,

dA 5 abVdt,

A 5 abVt.

Therefore,

V 5 A1

abtand Vmax 5 Amax

1

abt. (21)


If the above expression is substituted into Eq. (20),

1

A/abt5

1

Amax/abt1

KM

Amax/abtz

1

@Au#. (22)

Since [Au] is directly related to the time of immersion of the substrate in the colloidalsuspension, Eq. (22) can be expressed as

1

A5

1

Amax1

KM

Amaxz

1

@t#. (23)

Figure 4 is a plot of the reciprocal of the absorbance maxima versus the reciprocal ofimmersion time. All of the data points define a nearly straight line in accordance with Eq.(23). From the data in Fig. 4, we obtained the best-fit value ofAmax 5 0.089, whichcomplied well with the experimental results. The relationshipk3/k1 5 0.319 demon-strates that the rate of formation of the first monolayer is kinetically three times morefacile than that of the multilayer.

Many studies have proven that the adsorption kinetics of Au on amine and thiolfunctionalized surfaces are almost identical (23). The adsorption characteristics of Aucolloid of various particle sizes were investigated. We studied colloid suspensions havingmaximum absorbance atlmax 519.5, 526.5, 535.0, and 540.5 nm, corresponding to particlediameters of 12, 41, 71, and 147 nm, respectively. We used APTMS as a coupling agentand induced Eq. (23) to analyze adsorption behavior. The difference in wavelength ofabsorption peaks indicates the energy level of surface plasmon of the particle accordingto size. For each particle size, the absorbance saturation point was observed at longer

FIG. 4. Plot of the reciprocal of the absorbance of Au colloidal films atlmax versus the reciprocal of theimmersion time (t). The diameter of Au particles is 16 nm and the absorbance at the saturation point is 0.089.KM was calculated to be 0.319. The immersion time for each film preparation was the same as in Fig. 2.

80 PARK ET AL.

immersion times as shown in Fig. 5. It was found thatk3/k1 values were proportional toparticle size. For instance,KM values varied from 0.13 to 3.15 to 5.41 as the diameter ofthe colloid particle increased to 12, 41, and 71 nm, respectively. TheKM value estimatedfrom the adsorption of 16-nm Au colloid on an MPTMS-modified surface is well-fittedwith those obtained from adsorption of different-sized Au colloid on an APTMS-modifiedsurface. Therefore, it is reasonable to say that the adsorption kinetic behavior of Aucolloid of different sizes on an MPTMS-modified surface will closely resemble that on anAPTMS surface.

This result indicates that the adsorption kinetics of Au colloid varies drastically withparticle size. For example, for smaller Au particles, the rate of formation of the firstmonolayer is relatively faster than that of the multilayer. On the other hand, for largercolloids first layer formation occurs at a slow rate compared with multilayer formation.Facilitated multilayer formation for larger colloids reflects the fact that surface-confinedlarger particles act as nucleation sites more effectively for adhesion of Au particles on thetop of the first colloidal layer. Large metal particles tend to develop high surface chargeas well as high polarizability compared with smaller particles. This observation isconsistent with the well-known characteristics of the Au colloid, in that smaller colloidstend to be more stable and resistant against aggregation compared with larger particles.This is presumably responsible for the observation of largerk3/k1 values from large-particle reactions compared smaller-particle reactions. That is to say, the stability differ-ence varied with the sizes of the colloid and is directly related to the characteristics of thesurface adsorption reaction. In the initial period of immobilization, a larger particle is lesseffectively immobilized on the surface by the functional groups of thiol or amine, whichreflects smallerk1 with relatively largek3. Therefore, for larger Au colloids the immo-bilization reaction shows a largek3/k1 value, indicating more facilitated multilayerformation.

The extent of aggregation of gold colloid on the substrate can be measured by UV–visspectroscopy, especially in the region 600 to 700 nm. Prolonged immersion time leads toan increase in the low-energy band around 640 nm as shown in Fig. 5. The new band at640 nm indicates a typical electronic transition of aggregated Au particles (33). However,this feature was not seen when MPTMS was used as a coupling agent between glasssurface and Au colloid. It may be due to the electronegativity difference between nitrogenand sulfur atoms. Nitrogen is more electronegative than sulfur. Therefore, the amine groupserves as a nucleus more effectively than the thiol group, leading to effective particleflocculation. By observation of the low-energy band near 640 nm, it was found thatparticle size was limited. Occurrence of the low-energy band is also related to colloidalsize. When small Au colloid particles with a particle diameter of 12 nm were immobilizedon the APTMS-modified surface, the low-energy band did not appear, even at longimmersion times. As evidenced in Fig. 6, the longer immersion time leads only to highersurface coverage on the substrate with nearly identical electronic transition. It is interest-ing to compare AFM images of CMFs composed of different sizes of Au nanoparticles.Figure 7 clearly shows that the adsorption rate of small particles in the first layer is largerthan that of large particles, which is consistent with the UV–vis studies. Even thoughCMFs were constructed with the same immersion times, the surface coverage with colloidparticles 41 nm in diameter was higher than that with 71-nm particles.


FIG. 5. Absorption spectra of Au colloidal films and their plots according to Eq. (23). The diameters of Auparticles are 12 nm (a), 41 nm (b), and 71 nm (c). The immersion times for Au CMF construction were controlledat 15 min, 20 min, 30 min, 1 h, 2 h, 3 h, 18 h, and 24 h (from bottom to top, respectively).

82

FIG. 6. AFM images of Au colloidal films prepared by immersing APTMS-coated glass in a suspension of41-nm-diameter Au colloid particles for (a) 15 min, (b) 20 min, (c) 1 h, (d) 3 h, (e) 6 h, and (f) 24 h, respectively.


FIG

.7.

AF

Mim

ages

ofA

uco

lloid

alfil

ms

prep

ared

byim

mer

sing

AP

TM

S-c

oate

dgl

ass

slid

esfo

r6

hin

vario

ussi

zes

ofgo

ldpa

rtic

les.d

5(a

)41

nm,

(b)

71nm

,(c

)14

7nm

.

84 PARK ET AL.

It is well known that the silver particle is nearly identical in adsorption behavior to goldtoward thiol or amide moieties. However, the adsorption kinetics of Ag nanoparticles onsuch terminal groups on the solid substrate has not been investigated. When a surface-modified glass slide with an amide group was exposed to Ag colloid suspension, the peakabsorbance of Ag colloidal film in UV–vis spectra was proportional to the immersion timeas indicated in Fig. 8. The kinetic behavior of Ag colloid immobilization on an APTMS-modified glass surface is illustrated in Fig. 9, which is a plot of 1/A versus 1/t. Thetendency of the plot is almost identical as in the case of the Au colloid. It indicates thatthe adsorption characteristics of Ag are quite similar to those of Au.

Figure 10 shows the evolution of absorption bands of Ag colloid film on a glass slidein air at different immersion times. It was interesting that a blue shift in the absorptionmaximum and a new energy band were observed in UV–vis spectra of Ag colloidal filmwhen the intermediate material was changed from water to air by evaporation of waterfrom the colloid films. Two broad bands near 500 and 600 nm appeared at longerimmersion times, reflecting the formation of an aggregate multilayer of Ag nanoparticleson the surface. This blue shift may stem from the change in the aggregate state and ininterparticle interaction as observed in a previous report (35). Interparticle conglomerationis also responsible for peak broadening and appearance of a new energy band. Althoughthe Ag colloidal films of Fig. 10 were prepared under conditions identical to those underwhich the films in Fig. 8 were prepared, the absorption spectra in Fig. 10 were acquiredin a nitrogen atmosphere, not in water. Therefore, differences in the absorption spectra inFigs. 8 and 10 stem from the degree of aggregation, which was induced by the substitutionof intermediate materials between the colloid particles. The dielectric constant of a solvent

FIG. 8. Absorption spectra of 50- to 60-nm-diameter Ag colloidal films on APTMS-coated glass. Theimmersion time in Ag colloidal suspension was varied: 15 min, 30 min, 1 h, 2 h, 3 h, 6 h, 24 h, and 48 h (frombottom to top, respectively).


is often used as a measure of its capacity to cause separation of charged particles. Sincewater has a larger dielectric constant compared with air, water will separate unstablecharged silver particles more effectively than air. Consequently, a change in intermediatematerial from water to air would cause colloid particle aggregation on the substrate. Toconfirm this interpretation silver colloidal film was observed by AFM. As shown in Fig.11, silver colloid particles generally have a relatively broader size distribution than goldcolloid particles. As immersion time increased from 30 min to 3 h and finally to 48 h, theparticle size in the films also appeared to increase. In the case of Ag colloidal filmsconstructed by 48 h immersion, it was difficult to obtain AFM images due to the irregularsurface morphology. This observation of an increase in particle size with longer immer-sion times is analogous to the aggregation of Ag particles induced by replacing water withnitrogen in the drying process of the films.

Surface-enhanced Raman scattering spectroscopy was used to investigate the effects ofmorphology and surface plasmon variation by replacing intermediate materials. SERSspectra ofp-aminothiophenol (PATP) were obtained only on the multilayered silvercolloidal metal film, not on the monolayer of Ag CMFs (Fig. 12). The prepared silvercolloidal film was immersed for 15 min in a 1023 M ethanol solution of PATP, and thenwashed subsequently with ethanol and water. To obtain SERS spectra, it was necessary toincrease surface particle density to the multilayer extent.

There was a significant difference in SERS spectra of PATP measured on a driedcolloidal film and those on a wet film. When a wet colloidal film was used as a SERSsubstrate, Raman bands at 1573, 1440, 1391, and 1142 cm21 were tremendously increasedas water evaporated from the surface. All those modes have been assigned asb2, in-planemodes (34). Since Raman bands associated with the in-plane mode selectively showstrong intensities, PATP is adsorbed with a higher tilted angle to the substrate comparedwith the other two cases. On the other hand, the SERS spectrum in Fig. 12a indicates that

FIG. 9. Plot of the reciprocal of the absorbance of Ag colloidal films atlmax versus the reciprocal of theimmersion time (t): 1/A vs 1/t. The absorbance at the saturation point is 2.782 andKM is 1.97.

86 PARK ET AL.

PATP is oriented flat when it is adsorbed on a dried Ag colloid film. The selectiveenhancement of theb2 modes can also be explained by the charge-transfer mechanism(7). The charge transfer between metal particle and adsorbate induces a so-called “reso-nance Raman-like” scattering process on the Ag CMF surface. In the charge-transfermechanism, the increase in SERS intensity of the adsorbate in the vicinity of the substrateis explained by the change in the energy gap of the constituents. When the energy gapbetween the energy state of the Fermi resonance (Ef) and the electronic transition energylevel of the adsorbate becomes equal to the excitation energy, SERS is operative. As aconsequence of the appearance of new energy bands and peak broadening in UV–visspectra of the dried films, the energy of the excitation source of 514.5 nm is matched bythe energy gap between theEf of silver colloidal film and the electronic level ofp-aminothiophenol on the Ag colloidal film. However, if this is the sole reason for theselective increase inb2 modes, the SERS spectrum of PATP on a silver colloidal film,which was dried previously to expose PATP, must show a spectrum similar to thatobtained from a silver CMF film on which PATP was adsorbed in the wetted state andthen dried. The three SERS spectra in three different states of Ag CMFs in Fig. 12 areapparently different. This difference in the spectra can be explained by the electromag-netic enhancement effect, which involves the enhancement of the optical fields near thesurface. Electromagnetic (EM) enhancement requires small surface roughness to permitsurface plasmon resonance, resulting in strong local enhancement of the electric field ofboth incoming and scattered radiation. SERS intensity decays exponentially as thedistance between the metal and the sample increases. It has been known that when the

FIG. 10. Absorption spectra of dried Ag films in a nitrogen atmosphere. Films were prepared by immersingAPTMS-coated glass slides in Ag colloid suspension for 15 min, 30 min, 1 h, 2 h, 3 h, 6 h, and 24 h (from bottomto top, respectively).


FIG

.11

.A

FM

imag

esof

Ag

collo

idal

film

spr

epar

edby

imm

ersi

ngA

PT

MS

-coa

ted

glas

ssl

ides

inA

gco

lloid

susp

ensi

onfo

r(a

)30

min

,(b

)3

h,an

d(c

)48

h.

88 PARK ET AL.

sample is “sandwiched” between metal particles, maximal SERS intensity can be ob-tained. If the sample adsorbs to an irregular metal surface, it will not be easy to be“sandwiched” between metal particles. However, if the sample adsorbs on a relativelysmooth colloid aggregate surface, and then the surface morphology changes to an irregularmorphology by the gradual evaporation of water on the surface, it will be “sandwiched”between metal particles. Therefore, distinctive adsorption patterns of PATP induced bydifferent morphologies of the SERS substrates are reflected in the SERS spectra of Fig. 12.

CONCLUSIONS

Au and Ag colloidal films with well-defined two- or three-dimensional structures weresuccessfully constructed on glass slides modified with self-assembled monolayers (SAMs)of bifunctional silanes containing either amine or thiol groups on the opposite end of themolecule. The specific affinity of Au and Ag colloids to the amine functional groups ofthe SAMs on the glass surface allowed the fabrication of well-ordered, stable, andreproducible colloidal films with careful control of experimental conditions. The relation-ship between surface coverage and adsorption time was precisely described by using thewell-known Michaelis–Menten model.

The adsorption characteristics were investigated with various sizes of colloidal parti-cles. It was found that thek3/k1 values were proportional to particle size. This implies thatthe rate of formation of the first monolayer of relatively small particles is faster than therate of multilayer formation. Contrary to smaller particles, for large colloid particles therate of formation of the first layer is slower than the rate of multilayer formation,suggesting large colloid particles aggregate more effectively.

FIG. 12. SERS ofp-aminothiophenol on Ag colloidal films: (a) on a dried Ag colloidal film, (b) on a wetAg colloidal film, (c) on a wet Ag colloidal film that was dried in a nitrogen atmosphere.


Prolonged immersion time led to an increase in the low-energy band in the absorptionspectra of Au colloid films between 600 and 700 nm. This was found only with APTMS,not with MPTMS, as a coupling agent. The amine group has a stronger effect on particleflocculation than the thiol group.

Surface-enhanced Raman scattering spectra ofp-aminothiophenol adsorbed on a Agcolloidal film were obtained with 514.5-nm excitation. The SERS intensity ofp-amino-thiophenol varied significantly with the topology of the substrates. The surface morphol-ogy was monitored by AFM. The morphological variation affects the Fermi level of thecolloidal film. The increase in SERS intensities was explained by both electromagneticand charge-transfer effects. As a consequence of H2O evaporation, the space between Agparticles is replaced by air and the morphology of the Ag colloidal film is changed. Thismorphological alteration parallel to H2O evaporation is presumably induced by thedifference in dielectric constants of air and of H2O.

ACKNOWLEDGMENTS

The authors are grateful for the financial support of the Korean Research Foundation made in the program yearof 1997. We also thank Mr. Kyung-Ho Lee and Ms. Jin- Ho Park for their excellent experimental contributionsto this work.

REFERENCES

1. Fleischmann, M.; Hendra, P. J.; McGuillan, A. J.Chem. Phys. Lett.,1974,26, 163.2. Jeannair, D. J.; Van Duyne, R. P.J. Electroanal.,1997,84, 1.3. Albrecht, M. G.; Creighton, J. A.J. Am. Chem. Soc.,1977,99, 5215.4. Cotton, T. M.; Kim, J.-H.; Uphaus, R. A. J.Microchem. J.,1990,42, 44.5. Prog, Gi. X.Polym. Sci.,1994,19, 317.6. Ritchie, G.; Chen, C. Y.Pure Appl. Chem.,1980,52, 361.7. Cotton, T. M.; Kim, J.-H.; Chumanov, G. D.J. Raman Spectrosc.,1991,22, 729.8. Otto, A.Top. Appl. Phys.,54.; Cardona, M.; Guntherodt, G. (Eds.)Light Scattering in Solids,Vol. 4, p. 289.

Springer, Berlin, 1984.9. Moskovits, M.Rev. Mod. Phys.,1985,57, 783.

10. Creighton, J. A.Spectroscopy of Surfaces.Wiley, Chichester, 1988.11. Garrell, R. L.Anal. Chem.,1989,61, 401A.12. Koglin, E.; Sequaris, J.-M.Topics in Current Chemistry,p. 8. Springer-Verlag, Berlin/Heidelberg, 1986.13. De Groot, J.; Hester, R. E.J. Phys. Chem.,1987,91, 1693.14. Halperin, W. P.Rev. Mod. Phys.,1986,58, 533.15. Henglein, A.J. Phys. Chem.,1993,97, 5457.16. Kreibig, U. In Contribution of Cluster Physics to Materials Science and Technology(J. Davenas and P. M.

Rabette, Eds.), NATO ASI Series E, No. 104, p. 373. Martinus Nijhoff, Dordrecht, 1986.17. Turkevich, J.; Garton, G.; Stevenson, P. C.J. Colloidal Sci. Suppl.,1954,1, 26.18. (a) Kreibig, U.J. Phys. Colloq.,1977,38, C2–29. (b) Doremus, R. H.J. Chem. Phys.,1964,40, 2389. (c)

Hampe, W.Z. Phys.,1958,152,476.19. Creighton, J. A.Surface Studies with Lasers(F. R. Ausseneg, A. Leitner, and M. E. Lippish, Eds.), p. 55.

Springer-Verlag, New York, 1983.20. (a) Pettinger, B.; Bao, X.; Wilcock, I. C.; Muhler, M.; Ertl, G.Phys. Rev. Lett.,1994,72, 1561. (b) Van

Duyne, R. P.; Hulteen, J. C.; Treichel, D. A.J. Chem. Phys.,1993,99, 2101. (c) Roark, S. E.; Rowlen,K. L. Chem. Phys. Lett.,1993,212,50. (d) Schueler, P. A.;et al. Anal. Chem.,1993,65,3177. (e) Mullen,K. I.; Carron, K. T.Anal. Chem.,1991,63, 2196. (f) Byahut, S.; Furtak, T. E.Langmuir,1991,7, 508.

21. (a) Schnur, J. M.Science,1993,262,1669. (b) Huber, C. A.;et al., Science,1994,263,800. (c) Whitesides,G. M.; Mathias, J. P.; Seto, C. T.Science,1991,254,1312.

22. Grabar, K. K.; Freeman, R. G.; Hommer, M. B.; Natan, M. J.Anal. Chem.,1955,67, 735–743.

90 PARK ET AL.

23. Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.;Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J.Science,1995,267,1629–1632.

24. Bright, R. M.; Walter, D. G.; Musick, M. D.; Jackon, M. A.; Allison, K. J.; Natan, M. J.Langmuir,1996,12, 810–817.

25. Hayat, M. A.Colloidal Gold.Academic Press, San Diego, 1989.26. Munro, C. H.; Smith, W. E.; Garner, M.; Clarkson, J.; White, P. C.Langmuir,1995,11, 3713.27. Goss, C. A.; Charych, D. H.; Majda, M.Anal. Chem.,1991,63, 85.28. Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.;

Natan, M. J.J. Am. Chem. Soc.,1996,11, 1148.29. Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J.J. Am. Chem. Soc.,1996,118,9645.30. Karpovich, D. S.; Blanchard, G. J.Langmuir,1994,10, 3315.31. Widrig, C. A.; Chung, C.-J.; Porter, M. D.J. Electroanal. Chem.,1991,310,335.32. Weishaar, D. E.; Lamp, B. D.; Porter, M. D.J. Am. Chem. Soc.,1992,114,5860.33. Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M.J. Phys. Chem.,1995,99, 9466.34. Osawam, M.; Matsuda, N.; Yoshii, K.; Uchida, I.J. Phys. Chem.,1994,98, 12702.35. Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M.J. Am. Chem. Soc.,1995, 9466.


Documents

Langmuir