This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 132.236.27.111

This content was downloaded on 11/11/2014 at 20:11

Please note that terms and conditions apply.

A Surface Plasmon Resonance Study of Ag Nanoparticles in an Aqueous Solution

View the table of contents for this issue, or go to the journal homepage for more

2004 Jpn. J. Appl. Phys. 43 L119

(http://iopscience.iop.org/1347-4065/43/2A/L119)

Home Search Collections Journals About Contact us My IOPscience

A Surface Plasmon Resonance Study of Ag Nanoparticles in an Aqueous Solution

Tzu-Chiang CHEN�, Wen-Kuan SU and Yao-Leng LIN

Department of Electronic Engineering, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan 33509, Taiwan, R.O.C.

(Received June 27, 2003; revised November 11, 2003; accepted November 14, 2003; published January 9, 2004)

A novel method of detecting the concentration of Ag nanoparticles in an aqueous solution using surface plasmon resonance(SPR) reflectivity is reported. The laser ablation method was used to produce nanoparticles which were characterized using atransmission electron microscope (TEM) and ultraviolet-visible spectrometer. The particles had an average diameter ofapproximately 5 nm with an absorption peak at about 405 nm. A ‘‘Kretschmann’’ configuration surface plasmon resonancesensor revealed that SPR angle decreases with the relative concentration of the Ag nanoparticles. The SPR angle was shiftedby about 0.14� from the theoretical simulation results. The dielectric constant of an aqueous solution with the maximumconcentration of Ag nanoparticles was about 1.8167 and the refractive index of the solution was 1.3478.[DOI: 10.1143/JJAP.43.L119]

KEYWORDS: nanoparticles, laser ablation, surfactant, absorption spectrum, surface plasmon resonance

In recent years, extensive research on nanoparticles hasbeen performed. The main reason for this is that nanoparticlematerials manifest some characteristics that are differentfrom those of bulk materials, significantly altering thematerial science. For example, a decrease in nanoparticlesize will enhance the abilities of the catalyst and affect thecharacteristics of electro-optic devices.1,2) Nanoparticlemanufacturing methods can be broadly classified intochemical and physical methods. In the former, the mainmethod is the chemical reduction method which reducesmetal salts in a micelle or a reversed micelle.3) The laserablation method is an important physical manufacturingmethod. A high power laser beam is used to ablate a targetmetal placed in a surfactant solution.4,5) In general, nano-particle concentration has been measured by calculating theparticle site area from the transmission electron microscope(TEM) images; the drawback of this method is that TEMrequires complex image processing and is highly time-consuming.

A surface plasmon resonance (SPR) chemical sensor is asimple, noncontact, and nondestructive sensor. It has highsensitivity and high resolution for the changes in the opticalcharacteristics of media. SPR sensors are widely used toanalyze organic solutions6–9) and antigen biochemis-tries.10–12) It is highly suitable for studying the opticalcharacteristics of a solution containing metal nanoparticles.In this study, the relationships of reflectivity and theincidence angles of silver nanoparticles in an aqueoussolution are measured using a ‘‘Kretschmann’’ configura-tion13) SPR sensor and the results concurred with those of thetheoretical analysis.

Figure 1 shows a schematic representation of theKretschmann configuration surface plasmon resonance sen-sor apparatus. The Kretschmann configuration sensing partcomprised a BK7 cylindrical lens, a glass slide coated with asilver film, and a sample container, assembled on acomputer-controlled rotating stage. Matching oil was usedto form an optical contact between the cylindrical lens andthe glass slide. In the simulation, we can ignore thedifference between the dielectric constants of the cylindricalprism, matching oil and glass slide, and consider them as onelayer. This composition (cylindrical prism/ matching oil/

glass slide) seems to be a complicated experimental setupand broadens the reflectivity dip, but it realizes moreefficient coating and replaces the metal film of theKretschmann configuration. The light source was a He-Nelaser. An approximately 56 nm thick silver film wasevaporated onto a glass slide by thermal deposition.

The aqueous silver nanoparticle solution to be measuredwas produced by a Nd-YAG pulse laser (wavelength1064 nm, 350mJs/pulse) which was irradiated onto a silverrod in a 0.07M sodium dodecyl sulfate (SDS,C12H25OSO3Na) aqueous solution. Different volume frac-tions of SDS:Ag solution and SDS solution were mixed toyield relative concentrations of 1.00, 0.75, 0.5, 0.25, and0.00, respectively represented by Ag100 (4:0), Ag075 (3:1),Ag050 (2:2), Ag025 (1:3), and Ag000 (0:4). The absorptionspectrum of the silver nanoparticle solutions was measuredbetween 350�650 nm using a Hitachi U4001 spectrometer.The average size of silver nanoparticles was characterizedusing a high-resolution transmission electron microscope(Philip MC-200).

Figure 2 shows the ultraviolet-visible absorption spectrafor different concentrations of silver nanoparticle solutions.The wavelength is represented on the abscissa; the ordinateindicates the absorbance; ‘‘ ’’, ‘‘ ’’ ‘‘ ’’, and ‘‘H’’represent the absorption spectra of Ag100, Ag075, Ag050,and Ag025, respectively. The spectra are fitted by theLorentz curve equation. An absorption peak was observed atabout 405 nm. Mafune et al. attributed this absorptionphenomenon to the surface plasma wave resonance of silvernanoparticles.12) These results have proven the existence of

Fig. 1. Schematic representation of a surface plasmon resonance chemical

sensor.

�Corresponding author. E-mail address: chiang@ccit.edu.tw

Japanese Journal of Applied Physics

Vol. 43, No. 2A, 2004, pp. L 119–L 122

#2004 The Japan Society of Applied Physics

L 119

silver nanoparticles with superior orientation in SDSaqueous solution. We have found that the intensity of theabsorbance peak decreases with the concentration of silvernanoparticles in the solution, but full-width half-maximum(FWHM) increases as silver nanoparticle concentrationdecreases. These results indicate that the concentration ofsilver nanoparticles has some relation to absorption strengthand FWHM.

Figure 3 shows the reflectivity spectra of the samples withvarious silver concentrations measured using a SPR chemi-cal sensor. The abscissa indicates the angle of incidence andthe ordinate indicates the reflectivity; the spectra arenormalized. The SPR spectra of five samples with differentsilver concentrations represented by Ag000, Ag025, Ag050,Ag075, and Ag100, are denoted by ‘‘ ’’, ‘‘ ’’, ‘‘ ’’, ‘‘I’’,and ‘‘ ’’, respectively. In Fig. 3, we can observe that all theSPR spectra have one sharp absorption peak, and theresonance angles are in the range of 71.02�–71.16�. The

resonance angle difference between the Ag000 (‘‘ ’’)sample without silver nanoparticles, and the Ag100 (‘‘ ’’)sample containing silver nanoparticles, is about 0.14degrees.

To interpret the above phenomena, we refer to Fig. 2, thesurface plasma wave (SPW) resonance theory and theconditions of total reflection. In Fig. 2, the absorbances of allthe sample solutions are close to zero and showing no peaksat a wavelength of 632.8 nm. There is no localized surfaceplasma wave on the silver nanoparticle surface.11) Thus wecan perhaps attribute the intensity change of the SPW dip tothe adsorption of the silver particles or SDS molecules onthe silver film located at the bottom of the cylindrical prism.However this is clearly not the main cause of the differencein resonance angle. The SPW resonance theory indicates thatthe dielectric constant of the material is the factor thatdetermines the intensity and position of SPW dip. Theimaginary part of the dielectric constant mainly contributesto the absorption intensity, and the real part clearlyinfluences the strength and position of the SPW dip. Theeffect of the adsorption of silver nanoparticles and surfactantmolecules on the metal film located at the bottom of thecylindrical prism has been discussed by Hutter et al.14) Xuand Kall developed a multilayer system model to discuss theoptical response of the nanoparticle layer.15) In their experi-ments, a self-assembled monolayer of hexanedithiol orhydrochloride, a sulfuric and ammonia base covalentlybonded with the metal nanoparticles was inserted. However,we used a bare metal film in contact with an aqueoussolution in which silver nanoparticles covered by SDSmolecules were uniformly distributed. All the samples hadthe same SDS concentration in the experiment, so the changein dielectric constant can be attributed to the nanoparticles inthe aqueous solution. In addition, since the SDS moleculehas a hydrophilic head and a hydrophobic tail, the frequencyof collision and cohesion of the silver atoms will bedecreased by SDS molecules. Under insufficient momentum,

350 400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

(arb

. uni

ts)

Wavelength (nm)

Ag100 Ag075 Ag050 Ag025 Lorentz Fit

Fig. 2. Schematic representation of the UV-VIS absorption spectra of the

silver nanoparticle SDS solutions with different concentrations.

1 2 3 4 5 6 7 8 9 10

Abu

ndan

ce

Diameter (nm)

(a) (b)

Fig. 3. Transmission electron micrograph and size distribution of silver nanoparticles produced by the 1064 nm laser in a 0.07M SDS

aqueous solution.

L 120 Jpn. J. Appl. Phys., Vol. 43, No. 2A (2004) T.-C. CHEN et al.

the self-assembly effect of the nanoparticle will be unlikelyfor a short experimental duration. Thus, we can neglect theself-assembly effect, and assume that the aqueous solutionsamples are uniform and have average dielectric constants.

To avoid the self-assembly effect, we reduced themeasurement duration in this experiment as much aspossible, and used as unsaturated nanoparticle solution. Thisexperiment result can be verified by three-layer systemsimulation. To sum up, the silver nanoparticles wereintroduced in SDS aqueous solution in small amounts,which affected the dielectric constants of the solutions whenthe relative silver particle concentration changed (Ag025‘‘ ’’, Ag050 ‘‘ ’’, AG075 ‘‘I’’). The resonance angledecreases as silver particle concentration increases, but thevariation in the strength of the intensity was not notable.According to the electromagnetic theory, the dielectricconstant of a medium consists of two parts: imaginary part "iand real part "r. The imaginary part is mainly derived fromthe free electrons in the medium, and the real part is derivedwith the bonding electrons in the medium. When weconsider the third layer dielectric constant in Kretschmanngeometry: the imaginary part of the dielectric constantmainly contributes to the absorption intensity, and the realpart clearly influences the position of the SPW dip.According to the Lorentz-Lorentz effective medium expres-sion the average dielectric function of the medium can besimply considered as the average of the two components,which is a good approximation of the homogeneousdistribution on nanoparticles within the matrix.16) Becausethe real part of the dielectric constant of silver is negativeand with a value much larger than the imaginary part,17) thedielectric constant change of a diluted silver nanoparticlesolution was predominantly due to the change in the real partof the silver dielectric constant, and the dielectric constant ofsilver nanoparticle solution decreased with an increase insilver nanoparticle concentration. From theoretical simula-tions, the dielectric constants of the Ag000, Ag025, Ag050,Ag075, and Ag100 samples are about 1.8195, 1.8188,1.8181, 1.8175, and 1.8167 respectively. The relationship ofthe relative concentration of silver nanoparticles anddielectric constant is shown in Fig. 4. The vertical coordinate

indicates the dielectric constant and the horizontal coordi-nate the incidence angle; ‘‘þ’’ represents the dielectricconstant for the SDS aqueous solutions. In Fig. 4 we see thatSPR angle and silver nanoparticle concentration have alinear relation (solid line, " ¼ 1:8195� 0:00296c). Thisrelation can be used to detect the relative concentration ofsilver nanoparticles in the solution. At the same time, we canuse a known nanoparticle solution concentration as astandard to measure what of an unknown solution. To dothis, we must acquire many relevant optical characteristicsof silver nanoparticles, using an SPR chemical sensor todetermine small changes in the concentration of the solution.

In conclusion, we used a Nd-YAG pulse laser with awavelength of 1064 nm (350mJs/pulse) to ablate a silverrod in a 0.07M surfactant SDS aqueous solution whichproduced nanoparticles with an average grain size of about5 nm. The UV-VIS absorption spectra show that the silvernanoparticles in the aqueous solution have an absorptionpeak at 405 nm. Absorption intensity increased with theconcentration of the silver nanoparticles. Through the use ofa ‘‘Kretschmann’’ configuration SPR sensor, we found thatthe SPR angle of the silver aqueous solution decreases withthe relative concentration from 71.16 deg to 71.02 deg, andthe dielectric constant of silver nanoparticles in the SDSaqueous solution shifted from 1.8195 to 1.8167. This resultcan be expanded to measure the concentrations of othertypes of nanoparticles, and can also be used to design aninstant nanoparticle monitoring system. In the future, wewill report on the application to the production of metalnanoparticles.

This work was supported by the National Science Councilof Republic of China under the contract nos. NSC-91-2215-E-014-003 and NSC 90-2112-M-145-004.

1) Y. H. Chen and C. S. Yeh: Col. & Surf. A 197 (2002) 133.

2) S. I. Dolgaev, A. V. Simakin, V. V. Voronov, G. A. Shafeev and F.

Bozon-Verduraz: Appl. Surf. Sci. 186 (2002) 546.

3) N. Pradhan, A. Pal and T. Pal: Col. & Surf. A 196 (2002) 247.

4) A. V. Simakin, V. V. Voronov, G. A Shafeev, R. Brayner and F.

Bozon-Verduraz: Chem. Phys. Lett. 348 (2001) 182.

5) L. A. Lyon, M. D. Musick, P. C. Smith, B. D. Reiss, D. J. Pena and M.

J. Natan: Sens. & Actuat. B 54 (1999) 118.

6) J. Homola, S. S. Yee and G. Gauglitz: Sens. & Actuat. B 54 (1999) 3.

70.8 71.1 71.4

0.05

0.06

0.07

0.08

0.09

Ref

lect

ivit

y

Incidence Angle (deg)

Exp. Data Fitting CurveAg000Ag025Ag050Ag075Ag100

Fig. 4. Simulated and experimental SPW spectra for various relative

concentrations of silver nanoparticles in SDS aqueous solutions.

0.0 0.2 0.4 0.6 0.8 1.0

1.8165

1.8170

1.8175

1.8180

1.8185

1.8190

1.8195

1.8200

Die

lect

ric

Co

nst

ant

Relative concentrations (%)

Experimental Data Linear Fit

(ε =1.8195-0.0028 c)

Fig. 5. Schematic diagrams of the relationship of the relative concen-

tration and dielectric constant of silver nanoparticles in SDS solution.

Jpn. J. Appl. Phys., Vol. 43, No. 2A (2004) T.-C. CHEN et al. L 121

7) W. K. Su, C. M. Lee, Y. C. Cheng, L. B. Chang, J. H. Liou and J. M.

Shen: J. CCIT 27 (1998) 91.

8) Y. C. Cheng, W. K. Su and J. H. Liou: Appl. Surf. Sci. 136 (1998) 260.

9) Y. C. Cheng, W. K. Su and J. H. Liou: Opt. Eng. 39 (2000) 311.

10) K. Matsubara, S. Kawata and S. Minami: Opt. Lett. 15 (1990) 75.

11) M. Suzuki, Y. Nakashima and Y. Mori: Sens. & Actuat. B 54 (1999)

176.

12) F. Mafune, J. Y. Kohno, Y. Takeda and T. Kondow: J. Phys. Chem.

104 (2000) 9111.

13) E. Kreschmann and H. Raether: Z. Naturforsch. 23 (1968) 2135.

14) E. Hutter, J. H. Fendler and D. Roy: J. Appl. Phys. 90 (2001) 1977.

15) H. Xu and M. Kall: Sens. & Actuat. B 87 (2002) 244.

16) D. E. Aspnes: Thin Solid Films 89 (1982) 249.

17) P. B. Johnson and R. W. Christy: Phys. Rev. B6 (1972) 4370.

L 122 Jpn. J. Appl. Phys., Vol. 43, No. 2A (2004) T.-C. CHEN et al.