Surface modification of inorganic nanoparticles by organic functional groups

41 SURFACE MODIFICATION OF INORGANIC NANOPARTICLES BY ORGANIC FUNCTIONAL GROUPS

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APPLICATION 41

41

SURFACE MODIFICATION OF INORGANIC NANOPARTICLES BY ORGANIC FUNCTIONAL GROUPS

Nanoparticles are considered as key materials in informational, environmental and medical technologies. Various methods are proposed to synthesize the nanoparticles of metal, metal oxide and organic materials. The merits to use nanoparticles in these fields are summarized as below: (a) We can design new materials with hybridized functions by dispersing nanoparticles in a liquid or solid. (b) We can reduce resources and costs required to enable various functions that emerge at surfaces. (c) We can bind nanoparticles with biomolecules to enable diagnosis and cure with the minimal invasion. (d) We can fabricate nanostructures with less effort by organizing different kinds of nanoparticles. (e) We can explore new properties that arise based on quantum size effects. These merits emerge from the two characters of nanoparticles: (1) the new phase of materials that can mix with solids or liquids to form virtually continuous phases, and (2) the minimum unit of materials that have defined structure and functions, which arise from the decreased size of nanoparticles itself. However the decreased size also results in the difficult handling. We cannot manipulate each nanoparticle and the nanoparticles tend to aggregate. This nature of nanoparticles prohibits the handling similar to that of micrometer-sized particles. These tendencies come from the effects of surface atoms, which plays dominant role in the dispersion, aggregation and hybridization in the nanometer-sized materials. However, we can exploit this tendency to control the behavior of nanoparticles. The surface properties of nanoparticles are possibly changed by functional groups on the surface nanoparticles. Based on this idea, we propose the chemical modification of the surface of nanoparticles to realize better handling of nanoparticles. In this chapter, we discuss the attachment of organic functional groups on the surface of inorganic nanoparticles. 1. Surface-modified noble metal nanoparticles Sulfur atom has large affinity with noble metals. This affinity is used to produce surface-modified noble metal nanoparticles. Thiol-capped noble metal

nanoparticles are synthesized by reducing noble metal ions in the presence of thiols [1]. While the reduced noble metal atoms aggregate to form nanoparticles, thiol molecules attach on the surface of the nanoparticle to passivate the growing surface and minimize the chance to aggregate. The surface modification by alkanethiol (CnH2n⫹1SH) produces hydrophobic nanoparticles because SH groups bind with noble metal atoms and the nanoparticles are covered with alkyl groups. The modification can be performed with other molecules that have sulfur atoms [2]. The binding energy between noble metals and sulfur atoms is not so strong as covalent bonds. Based on this nature, we can realize surface modification with two kinds of thiols by dispersing thiolmodified noble metal nanoaparticles in a solution with a different kind of thiol. The exchange reaction between the thiol on the nanoparticles and the thiol in solution occurs to produce binary modified noble metal nanoparticles. Most noble metal nanoparticles including gold, silver, copper, palladium, platinum and nickel can be modified with thiols. Due to its easy experimental procedure, many researches have studied the surface modification of noble metal nanoparticles by thiols. 2. Organic modification of metal oxide nanoparticles Metal oxides have various properties including electron transport, semiconducting property, ferromagnetism, giant magnetoresistance, luminescence, ferroelectric property and catalysis (Table 41.1). In addition, most oxides are stable even in air and water. Due to these practical merits, various metal oxides are used in functional devices. However, incorporation of these properties of metal oxides into nanoscale devices requires two techniques, that is, synthesis of metal oxide nanoparticles and their surface modification. So far, few studies have been performed to synthesize surface-modified metal oxide nanoparticles. We are studying the metal oxide nanoparticles whose surface is covered by organic functional groups. We have developed simultaneous synthesis and modification of metal oxide nanoparticles because nanoparticles easily aggregate together irreversibly. Fig. 41.1 shows our method to synthesize surface-modified metal oxide nanoparticles. During hydrothermal synthesis, the growth of metal oxide nanoparticles proceeds by

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41 SURFACE MODIFICATION OF INORGANIC NANOPARTICLES BY ORGANIC FUNCTIONAL GROUPS

Table 41.1 Functions of metal oxides arranged by inputs and outputs. Input\output

Photonic

Electronic

Magnet

Photonic

Luminescence Laser

Photoconductivity Photovoltaic effect

Photoinduced magnet

Electronic

Light-emitting devices Electro luminescence

Semiconductor

Magnetoelectric effect

Faraday effect

Magnetic induction Colossal magnetic resistance

Magnetic

Thermal

Chemical Photo catalysis

Dielectrics

Thermal

Piezoelectric materials

Chemical

Fuel cell Chemical sensor

Magnetic heating

Magnetic phase transition Catalysis

Hydrothermal synthesis M

n+

M(OH)n

Metal oxide HO– nanoparticle Organic phase

Organic molecules (R-COOH, R-NH2) Hybridization with organic molecules

Water phase Organic-inorganic hybridized nanoparticles

Figure 41.1 Schematic of the surface modification of metal oxide nanoparticles during their hydrothermal synthesis.

the dehydration reaction between surface hydroxyl groups and metal hydroxides. We proposed surface modification through chemical bonding between the surface hydroxyl groups on the surface of metal oxide nanoparticles and organic reagents during the synthesis. Fig. 41.2 shows the surface-modified metal oxide nanoparticles. The hydrothermally synthesized metal oxide nanoparticles were covered with hydroxyl groups and therefore hydrophilic. On the other hand, the surface-modified metal oxide nanoparticles were covered with alkyl chains, which 594

Iron oxide nanoparticles synthesized without surface modifier

Iron oxide nanoparticles synthesized with surface modifier

Figure 41.2 Change in dispersion of nanoparticles by surface modification.

made the surface of metal oxide nanoparticles hydrophobic. This result confirms that the synthesis of metal oxide nanoparticles in the presence of organic reagents produces the surface-modified metal oxide nanoparticles. We also succeeded in modifying the surface of metal oxide nanoparticles with ⫺COOH or ⫺NH2 groups using the similar method. These modifications allow us to hybridize metal oxide nanoparticles with biomolecules, polymers and solid surfaces through strong chemical bonds.

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41 SURFACE MODIFICATION OF INORGANIC NANOPARTICLES BY ORGANIC FUNCTIONAL GROUPS

3. Hybridization of inorganic nanoparticles with biomolecules The synthesis of surface-modified inorganic nanoparticles enables the use of inorganic nanoparticles in various medical applications. The surface-modified nanoparticles can bind with biomolecules as well as inorganic materials. The hybridized nanoparticles have both inorganic properties and biological specificity. In this section, we discuss the alignment of metal nanoparticles on a ladder structure of deoxyribonucleic acid (DNA) through hybridization between nanoparticles and DNA single strands. Recently, Mao et al. [3] succeeded to build up ⬃10 nm rhombic lattices from 6 or 8 DNA single strands with designed sequences. They used the nature of DNA single strands to form Holliday junction that appears during the exchange of genetic information (Fig. 41.3). They also succeeded in combining the lattices together to form lattice and ladder structures of DNA. Based on this method, we tried to hybridize metal nanoparticles with the DNA lattice structure to align the metal nanoparticles. We designed the sequences of DNA single strands to prepare the similar structure as Mao et al. [3]. The designed sequence prepares a DNA ladder structure that allows a guest DNA single strand to attach on both side of the DNA ladder. We also designed a DNA single strand that can hybridize with the DNA ladder on one end and with a gold nanoparticle with the other end. By mixing gold nanoparticles, the designed single strand and the DNA ladder structure, we succeeded in aligning gold

(a)

nanoparticles as shown in Fig. 41.4 [4]. In addition to this work, we are trying to hybridize inorganic nanoparticles with enzymes and antibodies to realize various inorganic-bimolecular complexes. In this chapter, we discussed the surface modification of inorganic nanoparticles by organic functional groups. Recent development in the preparative methods of inorganic nanoparticles has opened the way to the application of the nanoparticles. The organic modification of the inorganic nanoparticles is the most suitable method to modify the surface properties including dispersion and hybridization of the nanoparticles. We believe that the control of the surface properties of the nanoparticles widens the range of application.

(a)

~19 nm

(b)

~31 nm

Figure 41.3 (a) Eight DNA single strands compose the rhombic structure as proposed by Seeman et al. (b) The rhombic structures attach together to from a planar structure.

(b)

DNA single strand

Au nanoparticle

DNA single strand

100 nm

Figure 41.4 (a) Designed arrangement of Au nanoparticles using the DNA ladder structure, (b) transmission electron micrograph of prepared gold structures.

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42 FABRICATION TECHNIQUE OF ORGANIC NANOCRYSTALS

References [1] M. Burst, M. Walker, D. Bethel, D.J. Stiffing and R. Why man: J. Chem. Soc. Chem. Common., 801–802 (1994). [2] A.C. Templeton, W.P. Wuelfing and R.W. Murray: Acc. Chem. Res., 33, 27–36 (2000).

[3] C. Mao, W. Sun and N.C. Seeman: J. Am. Chem. Soc., 121, 5437–5443 (1999). [4] Y. Hatakeyama, M. Minami, S. Ohara, M. Umetsu, S. Takami and T. Adschiri: Kobunshi Ronbunshu, 61, 617–622 (2004).

APPLICATION 42

42

FABRICATION TECHNIQUE OF ORGANIC NANOCRYSTALS AND THEIR OPTICAL PROPERTIES AND MATERIALIZATION

Organic nanocrystals are, for the definition, crystals of nanometer order of an organic compound. Organic nanocrystals are the nanomaterials that have been attracting attention in recent years, because they show some interesting characteristics for basic science, and many applications using organic nanocrystals are expected as well. Regarding the fabrication techniques of organic nanocrystals until around 1990, an evaporation technique in inert gas was reported as the bottom-up method by Toyotama or Yase et al. [1, 2]. Evaporation techniques, however, were not applicable for preparing nanocrystals of different organic compounds, because ordinary organics are unstable at high temperatures. On the other hand, the fabrication of organic pigments nanocrystals has been performed using milling technique mainly by company researchers. The milling method is effective to obtain the nanocrystals with particle size around 50–100 nm. However it is not easy for the milling method to reduce the particle size to 50 nm below because of structural rearrangement in crystals induced by a mechano–thermal effect. In this review, the authors introduce “the reprecipitation method” [3] and the related technique to fabricate organic and polymer -conjugated nanocrystals. And their sizedependent optical properties and application are described here. 1. The organic compounds used for nanocrystallization The -conjugated compounds attempted so far for the preparation of nanocrystals are summarized in Fig. 42.1. Polydiacetylenes are -conjugated polymer materials that can be obtained by irradiation of UV light or -ray or by thermal treatment through solid-state polymerization of diacetylenes [4]. The diacetylene derivative used in this paper was 1,6-di(N-carbazolyl)2,4-hexadiyne (DCHD), which has the carbazolyl group at both ends. 4-Dimethylamino-N-Methyl-4Stilbazolium Tosylate (DAST) is an ionic compound with stilbazolium structure. As the electric dipole moment of each molecule almost falls in the same 596

direction in a DAST crystals, it is well known that the crystals have large dipole moments and show very large second-order nonlinear optical properties. TPB and perylene are organic dye compounds having strong fluorescence even in crystalline state, and attract attention as fluorescence and electroluminescence (EL) materials. Titanylphthalocyanine (TiOPc), quinaridone, C60 and so on were used as hardly soluble compounds. TiOPc is known to have a superior optical photoconductive property and has been really used in usual photocopiers. Quinaridone has been used very well as a high-quality red pigment. All of the compounds shown in Fig. 42.1 are of interest for their optical and electronic properties in the crystalline state. 2. Fabrication techniques of organic nanocrystals (1) Reprecipitation method The scheme of reprecipitation method [3], which we found for a fabrication technique of organic nanocrystals is shown in Fig. 42.2. The reprecipitation method is a pure chemical technique to prepare organic nanocrystals simply by injecting solution of the target compound into a poor solvent. As distilled water is usually used as poor solvent, water-soluble solvent such as alcohol, acetone or THF was selected as the injected solution. The characteristics of reprecipitation method are that it is simple, quick and inexpensive. In addition, nanocrystallizations using this method are applicable for many kinds of organic compounds. In case of water-soluble organic dyes, the abovedescribed reprecipitation method cannot be applied. In such a case, the reprecipitation method was improved like the following two ways: the reprecipitation of dissolved salts into saturated aqueous solution instead of pure water, i.e., salting-out method [5], and the reprecipitation into a hydrocarbon solvent such as hexane, decalin, and so on, i.e., inverse-reprecipitation method [6]. In the reprecipitation method, the size and morphology of nanocrystals could be controlled by changing experimental condition such as combination between poor and good solvents, concentration of