Absorption and Rayleigh scattering and resonance Rayleigh scattering spectra of [HgX 2 ] n nanoparticles

Transcription

1 Vol. 45 No. 6 SCIENCE IN CHINA (Series B) December 2002 Absorption and Rayleigh scattering and resonance Rayleigh scattering spectra of [HgX 2 ] n nanoparticles LIU Shaopu ( ) 1, JIANG Zhiliang ( p ) 1,2,KONGLing( ) 1 &LIUQin( ) 1 1. Institute of Environmental Chemistry, Southwest China Normal University, Chongqing , China; 2. Department of Resource and Environmental Science, Guangxi Normal University, Guilin , China Correspondence should be addressed to Liu Shaopu ( liusp@swnu.edu.cn) Received July 1, 2002 Abstract The composition and existing species of the reaction production of Hg 2+ and X (X= Cl, Br and I) under different conditions, and their absorption, Rayleigh scattering (RS) and resonance Rayleigh scattering (RRS) spectra have been studied. The results show that the products exist in the form of nanoparticles as [HgX 2 ] n aggregates under suitable conditions, and their average diameters increase with the increase of X diameters. The diameters of [HgCl 2 ] n,[hgbr 2 ] n and [HgI 2 ] n are less than 4 nm, equal to 9 nm and 70 nm respectively. There are bathchromic shifts gradually with the increase of X diameters in their absorption spectra. The absorption bands of [HgCl 2 ] n and [HgBr 2 ] n locate at ultraviolet region. However, the absorption band of [HgI 2 ] n is obvious in visible light region. Among three particles, only [HgI 2 ] n exhibits a strong RRS and its scattering peak is at 580 nm. The main reasons leading to the enhancement of resonance scattering are the large size of nanoparticle, the formation of the interface and the location of light scattering in the absorption band. There are three important factors affecting resonance scattering spectrum: the quanta colored effect and molecular absorption spectrum of [HgI 2 ] n nanoparticle as well as instrumental factors which mainly include the light source s emission spectral characteristics and the signal response curve of detector. Keywords: [HgX 2] n nanoparticle, absorption spectra, Rayleigh scattering, resonance Rayleigh scattering. Nanoparticle has many novel physical and chemical properties and has become a study hotspot at present. Nano-technology will have an important effect on physics, chemistry, biology, information science, material science, machine and engineering. Liquid phase nanoparticle is an important object of investigation in nanochemistry. They often exhibit various absorption and scattering spectral properties. The phenomenon is closely related to the complicated structure of nanoparticle, the physical and chemical reactions on the surface of nanoparticle, the size and shape of nanoparticle, and the incident light source. It is of great significance to study the relationship between these changes and the spectral properties for developing nanomaterial, especially nano-optical material, and for setting up new techniques and methods to determine and study nanoparticle. Resonance Rayleigh scattering (RRS) as well as correlative resonance nonlinear scattering such as second order scattering and frequency doubling scattering are new spectral ana-

2 No. 6 SPECTRA OF ABSORPTION & RAYLEIGH SCATTERING 617 lytical techniques. Owing to its high sensitivity and simplicity, the technique is interesting to researchers [1 3] and applied widely not only to the study of nucleic acids [4 7], proteins [8 10] and heparin [11,12], but also to the determination of trace inorganic ions [13 16] and some physical and chemical parameters [17]. Recent researches show that the spectral technique is also a useful means to investigate nanoparticle and it has been applied to studying and detecting many kinds of nanoparticles [18,19]. However, the colored phenomena of nanoparticle systems and their affecting factors are rarely investigated and the study of its resonance scattering spectral characteristics and the affecting factors are also inadequate. Most studies concentrate on gold and silver nanoparticles at present. Therefore, it is of theoretical and practical value to further study some other types of nanoparticles and their resonance scattering spectral characteristics, the cause of the enhancement of light scattering and the affecting factors. Our study has shown that the [HgX 2 ] n nanoparticles form in aqueous solutions when the ratio of [Hg 2+ ] and halide anion concentration [X ] is 1 2. The average diameters of [HgCl 2 ] n, [HgBr 2 ] n and [HgI 2 ] n nanoparticles are less than 4, equal to 9 and 70 nm respectively observed by transmission electron microscope (TEM). The formation of [HgX 2 ] n nanoparticles results in some different absorptions and scattering spectral phenomena. Because [HgCl 2 ] n and [HgBr 2 ] n have no absorption in visible region and scattering light is far from the absorption band, there is only Rayleigh scattering (RS). The intensity of Rayleigh scattering mainly depends on the diameter and concentration of nanoparticle and its scattering peak mainly depends on instrumental factors, namely, the emission spectral characteristics of light source and the signal response curve of the detector. Under the condition chosen in the work, both [HgCl 2 ] n and [HgBr 2 ] n have scattering peak at 470 nm. However, [HgI 2 ] n has an absorption band in visible region and scattering light wavelength is located at the absorption band, so there is a stronger resonance Rayleigh scattering. Its scattering intensity is 60 times stronger than that of [HgCl 2 ] n and 24 times stronger than that of [HgBr 2 ] n when their concentrations are the same. Besides the larger diameter of [HgI 2 ] n, the process of absorption-rescattering produced by the resonance between the scattering and the absorption is the main reason that causes the scattering intensity to enhance many times even several orders of magnitudes. Apart from the influence of instrumental factor, the light absorption is a more important factor to affect the resonance Rayleigh scattering spectrum. Therefore, the [HgI 2 ] n has a special RRS spectrum with the strongest characteristic scattering peak at 580 nm besides the peak at 470 nm. As for the study of nanoparticle, not only is the sensitivity of RRS higher than the normal RS, but also the selectivity is better. In this work, the aggregation of HgX 2, the size of scattering particle, the formation of the interface, chemical reaction, the ratio of [Hg 2+ ]/[X ]and media on absorption and scattering spectra have been studied. The reasons why scattering spectra change have also been discussed.

3 618 SCIENCE IN CHINA (Series B) Vol Experimental 1.1 Apparatus and reagents A model RF-540 spectrofluorophotometer (Shimadzu, Japan), U-560R ultraviolet visible spectrophotometer (Shanghai Precision & Scientific Instrument Co., LTD., China) and H-500 transmission electron microscope (TEM) (Electronic Company of Japan) were used mol/l Hg(NO 3 ) 2, 0.04 mol/l KI, 0.04 mol/l KBr and 0.04 mol/l KCl solutions were prepared with doubly distilled water. All chemicals are of analytical grade. 1.2 Experimental procedure Place 0.50 ml of 0.04 mol/l KI solutions into a graduated tube, add water to about 3 ml and mix up. Add 0.25 ml of 0.04 mol/l Hg(NO 3 ) 2 solution, mix up and dilute it to 10 ml after 5 min. Record the resonance Rayleigh scattering spectrum on the spectrofluorophotometer with synchronous scanning at λ ex = λ em ( λ = 0) from 300 to 700 nm. The absorption spectrum was recorded on a model U-560R spectrophotometer. 2 Results and discussion 2.1 Formation of mercury halide nanoparticle According to the experimental procedure, the compound HgX 2 forms when the ratio of [Hg 2+ ]/[X ] is 1/2. The HgX 2 molecule has hydrophobicity. The repelling effect of water on HgX 2 molecule makes HgX 2 close to each other and in the case the van der Waals force between HgX 2 molecules leads to the aggregation of HgX 2.AfterHg 2+ combines with X to form neutral molecule, the molecular permanent or instant dipole moment will result in intermolecular van der Waals bonding [20] which leads to the phenomenon that all molecules exist in the form of aggregate [HgX 2 ] n instead of single molecule HgX 2. It is known that the crystal ion radii of Hg 2+,Cl,Br and I are 0.110, 0.181, and nm [21], respectively. When HgX 2 crystal forms (in crystal, cation and anion are thought as round balls touching each other), its diameter should be less than 1 nm. However, the results show that the diameters of [HgI 2 ] n and [HgBr 2 ] n are about 70 and 9 nm, respectively. Because the diameter of [HgCl 2 ] n is less than the differentiate ratio of the TEM (3 4 nm), [HgCl 2 ] cannot be observed clearly and it can only be proved that its diameter is less than 4 nm (shown in figs.1(a) and (b)). In short, the reactions are as follows: Hg 2+ +2X =HgX 2 nhgx 2 =[HgX 2 ] n Single molecule Molecular aggregate Owing to the fact that the sizes of the three [HgX 2 ] n are in the range of nm, they are nanoparticles. It is the formation of these nanoparticles that leads to some novel absorption and

4 No. 6 SPECTRA OF ABSORPTION & RAYLEIGH SCATTERING 619 Fig. 1. The photograph of transmission electron microscope for [HgX 2] n nanoparticle. (a) mol/l HgI 2 ( 40000); (b) mol/l HgBr 2 ( 80000). light scattering phenomena. 2.2 Absorption spectra of liquid phase [HgX 2 ] n nanoparticles Fig. 2 shows the absorption spectra of the three [HgX 2 ] n nanoparticles in liquid phase. It can be seen that [HgCl 2 ] n,[hgbr 2 ] n and [HgI 2 ] n have strong light absorption when the wavelengths are less than 250, 270 and 350 nm, respectively. However, for the [HgCl 2 ] n and [HgBr 2 ] n systems, there is no absorption in the visible region. On the contrary, the [HgI 2 ] n nanoparticle system has a wide absorption band in the region of nm. However, there is no obvious absorption peak. Obvious absorption peak cannot be observed when the absorp- [X ], mol/l; [Hg 2+ ], mol/l. tion spectra in this region are further magnified. Absorption spectra can reflect the color of nanoparticles to a certain extent, for example, the [HgCl 2 ] n and [HgBr 2 ] n without absorption in the visible region are both white and the [HgI 2 ] n is pink with absorption in the range of nm. The change of color of mercury halide (HgX 2 ) is closely related to polorization of halide. I is most easily polarized, therefore, the color of iodide is most deep, that of bromide is the next and that of chloride is always white [22]. Attention should be paid to the fact that the absorption of nanoparticle system is different from the molecular absorption in water solutions, for nanoparticle is insoluble. Because there is interface between nanoparticle and liquid phase, its color is related Fig. 2. Absorption spectra. 1,[HgCl 2] n;2,[hgbr 2] n;3,[hgi 2] n.

5 620 SCIENCE IN CHINA (Series B) Vol. 45 to their absorption spectra, and also related to scattering, refraction, size effect and shape of nanoparticle. The colored effect is caused by all the above factors. 2.3 Resonance Rayleigh scattering spectra of [HgX 2 ] n nanoparticle The scattering with the scattering wavelength being equal to the incident wavelength is elastic scattering. If the size of the scattering particle is much less than the incident wavelength, this elastic scattering is Rayleigh scattering (RS). The diameters of [HgCl 2 ] n and [HgBr 2 ] n are less than 4 nm and equal to 9 nm respectively, which are much less than the incident light wavelength and their scattering peak at 470 nm is far from the absorption band, so the elastic scattering is typical Rayleigh scattering. Although the diameter of [HgI 2 ] n is larger (D =70nm),itisalsolessthanthe incident wavelength (λ 580 nm ). So the scattering also belongs to Rayleigh scattering. Because its scattering band is located in the absorption band, it produces the stronger RRS resulting from the resonance between the scattering and absorption. The scattering spectra of [HgX 2 ] n nanoparticle are shown in figs. 3 (a) and (b). Both [HgCl 2 ] n and [HgBr 2 ] n systems have the maximum scattering wavelength at 470 nm shown in fig. 3. The scattering spectra are similar to the signal response curve of the RF-540 spectrofluorophotometer which is mainly rest with the response curve of detector and the emission spectrum of Xe lamp (shown in fig. 4). But the system of [HgI 2 ] n has a maximum scattering peak at 580 nm and a weak scattering peak at 470 nm. In short, the Rayleigh scattering spectrum mainly depends on the spectral response characteristics of the instrument, especially the emission spectral characteristics of light source. However, the resonance Rayleigh scattering is affected by the quanta colored effect to a large extent besides the instrumental factors. So, colored nanoparticle system produces some new and characteristic RRS peaks. In this paper, the RRS peak at 580 nm is the characteristic peak of [HgI 2 ] n. Fig. 3. RS and RSS spectra. (a) [HgI 2] n particle. 1, mol/l I, mol/l Hg 2+ ; 2, mol/l I, mol/l Hg 2+ ; 3, mol/l I mol/l Hg 2+.Ordinatescale1.(b)[HgCl 2] n and [HgBr 2] n.1,0.001 mol/l Cl ; 2, mol/l Cl,3,0.001mol/LBr ; 4, mol/l Br ;[Hg 2+ ] = mol/l. Ordinate scale 5.

6 No. 6 SPECTRA OF ABSORPTION & RAYLEIGH SCATTERING 621 From the experimental results, the relative scattering intensities, I RS, of [HgCl 2 ] n and [HgBr 2 ] n, 22.0 and 51.2, have linear relationship with their diameters of and 9 nm, respectively. But the I RRS of [HgI 2 ] n, 1181, does not obey this linear relation. Its relative intensity is tens times as much as that of [HgCl 2 ] n and [HgBr 2 ] n. This proves that the resonance between scattering and absorption contributes much to the enhancement of scattering intensity. Therefore, resonance Rayleigh scattering can not only produce characteristic scattering peak, but also enhance the detection sensitivity considerably. So the [HgI 2 ] n was taken as an example to study its RRS and relevant properties. Fig. 4. Emission signal distribution of the RF-540 spectrofluorophotometer. Fig. 5. Effect of KI concentration on A 580 nm and I 580 nm. 1, [KI]/[Hg 2+ ]-A 580 nm; 2, [KI]/[Hg 2+ ]-I 580 nm Effect of KI concentration on absorptivity (A 580 nm ) and RRS intensity at 580 nm (I 580 nm ) When [Hg 2+ ] is a constant, HgI 2 molecule comes into being gradually and further aggregate to form [HgI 2 ] n molecules with the addition of KIshowninfig.5.BothA 580 nm and I 580 nm increase until the [I ]/[Hg 2+ ]is2/1,namely,when all Hg 2+ ions transform into HgI 2 and [HgI 2 ] n,the A 580 nm and I 580 nm become the maximum. When I concentration increases further, the [HgI 2 ] combines excessive I to form water-soluble [HgI 3 ] and [HgI 4 ] 2 complex ions, which lead to gradual destruction of [HgI 2 ] n, until the [I ]/[Hg 2+ ] is 4/1, when all the nanoparticles change into [HgI 4 ] 2 and A 580 nm and I 580 nm become the minimum. In a word, the following reactions take place: [ ] nhg + 2 I = HgI 2 n [ HgI ] + 2 I = [ HgI ] 2 n n n Effect of Hg 2+ concentration on A 580 nm and I 580 nm Place 0.20 ml of 0.04 mol/l KI into a graduated tube, add water to about 3 ml and mix up. n

7 622 SCIENCE IN CHINA (Series B) Vol. 45 Add 0.10 ml of 0.04 mol/l Hg(NO 3 ) 2 solutions and mix up. After about 5 min, the (HgI 2 ) n form. The nanoparticle concentration, expressed by HgI 2,is4 0 4 mol/l. Add different volumes of 0.04 mol/l Hg(NO 3 ) 2 solution continuously, dilute to 10 ml with water and mix up. The following operation is done according to the procedure. The relation between [Hg 2+ ]/[HgI 2 ]and A 580 nm and I 580 nm isshowninfig.6.fig.6indicates that the A 580 nm and I 580 nm decrease with the addition of Hg(NO 3 ) 2 ( mol/l) and when the [Hg 2+ ]/[HgI 2 ] is more than 2, both A 580 nm and I 580 nm are very low. Fig. 6. Effect of Hg 2+ concentration on the A 580 nm and I 580 nm. As shown in fig. 6, the abscissa of the point and represent A 580 nm and I 580 nm, respectively. where two lines intersect is 1.98 and 2.0, respectively. And it can be thought that the cationic complex [HgI 2 2Hg 2+ ]or[hg 3 I 2 ] 4+ forms. The formation of the cationic complex and its dissolvability in water result in the destruction of [HgI 2 ] n nanoparticle, which causes the absorptivity and RRS intensity to decrease sharply. That is, [ HgI ] + 2n Hg 2+ = n[ Hg I ] 4 2 n Effect of alcohol In a liquid phase system, nanoparticle and the existence of interface are important reasons that result in the enhancement of scattering signal. In the aqueous solution, Hg 2+ reacts with I to form pink [HgI 2 ] n nanoparticles, and an interface between solid [(HgI 2 ) n ] and water comes into being. So the scattering signal of [HgI 2 ] n nanoparticle system is stronger greatly than that of water. The experimental results indicate that the [HgI 2 ] n nanoparticles easily dissolve in alcohol, methanol and acetone to form very small [HgI 2 ] n nanoparticles or even HgI 2 molecule. Therefore, the nanoscale effect and quanta colored effect disappear, which result in great decrease of the scattering signal and the change of color from pink to colorlessness and the disappearance of the characteristic peak at 580 nm. The experiment shows that with the addition of alcohol (12% 50%, volume ratio), the color changes from pink to colorlessness, and A 580 nm decreases. I 580 nm also decreases until the characteristic peak at 580 nm vanishes. 2.7 Relation between the concentration of [HgI 2 ] n nanoparticle and A 580 nm and I 580 nm Under the optimum conditions chosen, the relations between the concentration of [HgI 2 ] n nanoparticle and A 580 nm and I 580 nm were studied. The results show that the [HgI 2 ] n nanoparticle concentration is proportional to the A 580 nm in the range of mol/l. The linear re- +

8 No. 6 SPECTRA OF ABSORPTION & RAYLEIGH SCATTERING 623 gression equation is A 580 nm = C The correlation coefficient is According to the formula of Anglister et al. [23], ( ) ( ) IRRS = 4000π n c λ0 N A dn dc dk d c + I0, where I RRS represents the intensity of RRS (unit, cd), I 0 is the incident light intensity, n is the refraction coefficient, C is molar concentration of particles, N A is Avogadro constant and dn/dc and dk/dc represent the increment of the real and imaginary parts of polarization ratio. When other factors are constant, the particle concentration is proportional to I RRS. The [HgI 2 ] concentration in the range of mol/l is proportional to I RRS. The regression equation is I 580 nm = C The correlation coefficient is Therefore, the spectrophotometric and RRS methods both can be used to determine the concentration of HgI 2. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No ), Natural Science Foundation of Chongqing and Natural Science Foundation of Guangxi Zhuang Autonomous Region (Grant No ). References 1. Pasternack, R. F., Collings, P. J., Resonance light scattering: A new technique for studying chromophore aggregation, Science, 1955, 269: Liu, S. P., Liu, Z. F., Studies on the resonance luminescence spectra of rhodanmine dyes and their ion-association complexes, Spectrochim. Acta, 1995, 51A: Liu, S. P., Liu, Z. F., Li, M., Analytical application of the second order scattering spectra of ion associated complex: Se (IV)-iodide-rhodamine B system, Acta Chimica Sinica (in Chinese), 1995, 54(12): Pasternack, R. F., Bustamante, C., Collings, P. J. et al., Porphyrin assemblies on DNA as studied by a resonance light-scattering technique, J. Am. Chem. Soc., 1993, 115: Huang, C. Z., Li, K. A., Tong, S. Y., Determination of nucleic acids by a resonance light-scattering technique with α, β, γ, δ-tetrakis[4-(trimethylammoniumyl)phenyl]porphine, Anal. Chem., 1996, 68: Huang, C. Z., Li, Y. F., Liu, X. D., Determination of nucleic acids at nanogram levels with safranine T by a resonance light-scattering technique, Anal. Chim. Acta, 1998, 375: Liu, S. P., Hu, X. L., Luo, H. Q. et al., Resonance Rayleigh scattering spectral characteristics of interaction of nucleic acids with some cation surfactants and their analytical applications, Science in China, Series B, 2002, 45(2): Ma, C. Q., Li, K. A., Tong, S. Y., Enhancement of Rayleigh light scattering of acid chrome Blue K by proteins and protein assay by the scattering technique, Analyst, 1997, 122(4): Liu, S. P., Liu, Q., Resonance Rayleigh scattering method for the determination of proteins with some monoazo dyes of chromotropic acid, Anal. Sci., 2001, 17(2): Liu, S. P., Liu, Q., Resonance Rayleigh scattering method for the determination of proteins with orange G, Anal. Sci., 2001, 17(2): Liu, S. P., Luo, H. Q., Li, N. B. et al., Resonance Rayleigh scattering study of the interaction of heparin with some basic diphenyl naphthylmethane dyes, Anal. Chem., 2001, 73: Luo, H. Q., Liu, S. P., Liu, Z. F. et al., Resonance Rayleigh scattering spectra for studying the interaction of heparin with some basic phenothiazine dyes and their analytical application, Anal. Chim. Acta, 2001, 449: Liu, S. P., Zhou, G. M., Liu, Z. F., Resonance Rayleigh scattering for the determination of trace amounts of mercury (II) with thiocyanate and basic triphenylmethane dyes, Anal. Lett., 1998, 37: Liu, S. P., Liu, Z. F., Huang, C. Z., Resonance Rayleigh scattering for indirect determination of trace amounts of selenium

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