CHAPTER TWO. Synthesis and characterization of uncapped and capped (surface. functionalized) silica nanoparticle

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1 CHAPTER TWO Synthesis and characterization of uncapped and capped (surface functionalized) silica nanoparticle 2.1 INTRODUCTION Silica nanoparticle (SNP) has gained much attention in scientific research because of its easy preparation and its wide range of industrial as well as biological application (Xu et al. 23). The variety of chemical and physical modification possible with silica increases its versatility, and its biocompatibility makes it suitable for extensive biological application. For instance SNPs are being used as biosensors for assay of glucose (Zhang et al. 24), biomarkers for leukaemia cell identification using optical microscopy (Tsuji et al. 26), drug delivery (Slowing et al. 28; Climent et al. 21; Escoto et al. 21; Manzano et al. 21; He et al. 211a, b; Stark, 211; Wang et al. 212;), DNA delivery (Bharali et al. 25; Gemeinhart et al. 25), enzyme immobilization (Qhobosheane et al. 21) cancer imaging (Santra et al. 21), targeting (Benezra et al. 211) and therapy (Hirsch et al. 23). SNP has huge potential in agriculture and veterinary sector (Majumder et al. 27) as well. It can be used as an entomotoxic agent (Debnath et al. 21; Debnath et al. 211), can control grasserie disease of silk worm (a viral disease caused by Bombyx mori nuclear polyhedrosis virus) and can also be applied as an anti-malarial drug in poultry farm (Rahman et al. 29). Actually silica is the second most abundant compound in the earth s crust. Silicas can be amorphous or crystalline, porous or non-porous (dense), anhydrous or hydroxylated [Iler 1979], regardless of their natural or synthetic nature. In a silica compound, the silicon atom is in tetrahedral coordination with 4 oxygen atoms. In crystalline silica the oxygen-sharing silicate tetrahedra are connected in an ordered fashion, whereas in amorphous form the 22

2 tetrahedra are randomly connected. In nature, amorphous silica can have different origins. It can be condensed from vapors emitted in volcanic eruptions. Natural silica can also be deposited from supersaturated natural water or polymerized in living organisms (biogenic silica). These amorphous biogenic silicas can be found as isolated particles, skeletal structures or surface elements in different living organisms. Diatomaceous Earth (DE), which is siliceous deposits from skeletal remains of diatoms, is one of the major resources of amorphous natural silica. There are two major synthetic routes for production of SNPs. It can either be synthesized by sol-gel method from an aqueous solution, with dissociated monomeric silicic acid, Si(OH) 4, or from a vapor of a silicon compound such as silicon tetrachloride. Stober et al. (1968) demonstrated a pioneering method for the synthesis of spherical and monodisperse SNPs from aqueous alcohol solution of silicon alkoxide in presence of ammonia as catalyst, and different sizes of SNPs were synthesized ranging from 5 nm to 1 µm with a narrow size distribution. Lee and Choi (22) synthesized SNPs of controlled morphology by using a CO 2 laser to re-heat flame synthesized SNP agglomerates, thereby sintering them in situ. Mӓdler et al. (22) presented a detailed study on synthesis of SNPs by flame spray pyrolysis from hexamethyldisiloxane. Among other non conventional routes of SNP production, Guo et al. (1999) demonstrated synthesis of SNPs by using Escherichia coli as template. Amutha et al. (21) extracted SNPs from rice husk, an agricultural waste. General scheme for synthesis of various silica materials is depicted in Fig

3 Fig. 2.1 General scheme for synthesis of silica (Adapted from Brinker CF, Schrerer GW: Sol- Gel Science. The Physics and Chemistry of Sol-Gel Processing. 2 nd edition. London: Academic Press; 199). Among various modes of synthesis, SNPs synthesized from silicon alkoxide as the starting material via sol-gel process, is the most popular. It involves simultaneous hydrolysis and condensation of silicon alkoxide. Spherical and monodisperse SNP of tunable size can be prepared by this process. Bogush and Zukoski (1988) obtained monodisperse silica particles having sizes ranging from 4 nm to several µm by controlled hydrolysis of tetraethylorthosilicate (TEOS) in ethanol, followed by condensation of the dispersed phase material. SNPs were also prepared by Sung Kyoo Park (26) from TEOS dissolved in ethanol by using a 24

4 semi-batch process in order to control the particle properties. Janet Gallardo and Pablo G Galliano (21) produced porous silica gels with high surface areas from TEOS and polyacrylic acid of high molecular weight in acidic media by sol gel method. Surface functionalization of these SNPs in situ in aqueous solution with silane can be done by covalent bond formation between the surface silanol groups and silane coupling agents. Here our main objective was to synthesize monodisperse, spherical, uncapped, thiol and Hexamethyldisilazane (HMDS) capped SNP, which can be used in various biological applications, especially in agricultural sector. The uncapped silica particle in nano form was synthesized from TEOS or Tetramethyl orthosilicate (TMOS) by slightly modifying Stober process (Stober et al. 1968). Surface modification of these SNPs was done by two ways (i) by capping them with 3-Mercaptopropyl triethoxysilane (MPTS) in neutral aqueous medium and (ii) by capping SNPs with HMDS. 2.2 MATERIALS & METHODS Chemicals TEOS, TMOS, ethanol, methanol, hydrochloric acid (35%) were obtained from Merck, Germany; ammonium hydroxide, 3-Mercaptopropyltriethoxysilane (3-MPTS) and HMDS were purchased from Sigma Aldrich. All the chemicals were of analytical grade and were used without further purification. Water used throughout the experiment was of Milli-Q grade with conductivity less than.1 µ S cm

5 2.2.2 Synthesis of uncapped and surface functionalized silica nanoparticles Uncapped SNPs were prepared by hydrolysis of TEOS and TMOS in alcoholic medium (ethanol in case of TEOS and methanol in case of TMOS) in presence of ammonium hydroxide. First, aqueous ethanolic or methanolic solution was taken and kept under ultrasonicator (BioTek, USA). After 5 minutes a known volume of TEOS was added in ethanolic solution and TMOS was added in methanolic solution while sonicating. After 1 minutes 25% NH 4 OH was added as a catalyst to promote the condensation reaction. Sonication was continued for further 3 minutes to get a white turbid suspension. The suspension was then lyophilized and the resultant powder was washed with deionized water several times to remove traces of ammonium hydroxide used in the process of synthesis. The reaction was performed at room temperature. The effect of reactant concentration and chain length of alcohol on size of SNPs was studied by changing the molar ratio of all the reactants and using different alcohols (ethanol and methanol). The schematic presentation of synthesis of amorphous SNPs by sol-gel method is depicted in Fig. 2.2a. Aqueous solution of silicon alkoxide (a) (b) 1. Hydrolysis 2. Polymerization Addition of 25% NH 4 OH Ultra sonication White turbid suspension of SNP Fig. 2.2 (a) Schematic presentation for synthesis of uncapped SNP by sol-gel method, (b) Equipment set up for synthesis of surface functionalized SNPs. 26

6 Thiol capped and HMDS capped SNP were synthesized by using a mixture of ethanol and water taken in a 3-neck round bottom flask equipped with reflux condenser and isopiestic funnel (equipment set up was designed by our research group, Fig. 2.2b). The mixture was stirred for about 1 minutes. TEOS was added to this mixture in such a way that the final molar ratio of TEOS: ethanol: water was.12: 4: 3. The mixture was kept under reflux condition at a temperature of 75 C and stirred vigorously for about 3 minutes. A solution of 3-MPTS in absolute ethanol was added drop wise to this reaction mixture with the help of an isopiestic funnel and the reaction was continued for about 45 minutes. Finally the mixture was centrifuged at 12 rpm for 1 minutes and washed several times with ethanol to remove excess of MPTS. The resultant thiol capped SNP was dried completely at a temperature of 8 C. HMDS capped SNP was synthesized under acidic medium with the core SNP functionalized by HMDS. Here, mixture of ethanol and water was taken in a 3-neck round bottom flask equipped with a reflux condenser and two isopiestic funnels containing aqueous.75(m) HCl and alcoholic solution of HMDS. TEOS was added to the aqueous mixture of alcohol in such a way that the molar ratio of TEOS: ethanol: water was.12: 4: 3. Initially half of the HCl was added to the reaction mixture with constant stirring and refluxing for about 3 minutes and the remaining aqueous HCl was added along with the alcoholic solution of HMDS with constant stirring. The reflux was continued for about 1 hour. Finally the reaction mixture was centrifuged at 12 rpm for 1 minutes and washed with ethanol several times to remove HMDS. The SNP was dried at a temperature of 8 C Characterization Average particle size of all the samples was measured by Dynamic Light Scattering (DLS, Malvern). DLS measurement was taken after an interval of 7 days and 28 days to see whether 27

7 the samples become agglomerated over a period of time. Surface morphology of the three types of SNPs was determined by SUPRA 4 Field Emission Scanning Electron Microscope (FE-SEM, Carl Zeiss) operated at acceleration voltage of 5. kv. For FE-SEM study micro pipettes were used to transfer a droplet of each sample of suspension on 2 cm 2 glass slide. The samples were allowed to dry and then images were taken at a number of random locations. During FE-SEM study Energy-dispersive X-ray spectroscopy (EDS) analysis was performed to determine the major constituent of the nano particulate matter. Particle size was confirmed from high resolution transmission electron micrograph (HR-TEM, JEOL 21). For HR-TEM analysis, a drop of well dispersed alcoholic sample solution was put into copper coated TEM grid and was air dried before imaging. FTIR spectra were recorded in Perkin-Elmer Spectrum 1. For FTIR experiments the samples were ground with KBr (spectroscopic grade) and pressed to pellets. Powder sample XRD (Bruker AXS analytical Instruments) was carried out by applying a primary beam monochromator to select the Kα1 component of the employed copper radiation of wavelength Å. The surface morphology of both SNPs was obtained from AFM study (NTEGRA Ts 15) in tapping mode. Stability of the capping material was obtained from TG-DTA analysis (Perkin Elmer Diamond TG/DTA) with a heating rate of 1 C per minute under nitrogen atmosphere. 2.3 RESULTS & DISCUSSION Determination of particle size of uncapped and surface functionalized silica nanoparticles from FE-SEM and HR-TEM study During synthesis of uncapped SNPs two sets of reactions were involved: (a) first silanol groups were formed by hydrolysis and (ii) siloxane bridges were formed by condensation reaction. This type of SNP was hydrophilic in nature. 28

8 Hydrolysis: Si-(OR) 4 + H 2 O Si-(OH) R-OH Condensation: 2Si-(OH) 4 2(Si-O-Si) + 4 H 2 O The condensation rate depended on reaction conditions which might result in the formation of three dimensional network or in formation of single monodisperse particles. According to Bogush Zukoski (1988) concentration of silicon alkoxide, ammonium hydroxide, water and chain length of alcohol chain played an important role in size determination of SNPs. Huang and co-researchers (22) have reported that sonication during a reaction could significantly increase the yield of carbodiimide-mediated amidations. In view of this, the effect of different reagents on particle size under sonicating condition was studied. The average particle size and the reaction conditions used are presented in Table 2.1. Table 2.1 Average particle size (± S.E.) of uncapped SNPs under different reaction conditions Sample Experimental conditions Average particle size (nm) Silicon alkoxide Alcohol Water NH 4 OH [M] [M] [M] [M] a. TEOS [.12] Ethanol [8] ± 19 b. TEOS [.12] Ethanol [8] ± 14 c. TEOS [.12] Ethanol [4] ± 8 d. TEOS [.12] Ethanol [4] ±6 e. TEOS [.12] Ethanol [8] ± 6 f. TEOS [.12] Ethanol [8] ± 21 g. TEOS [.12] Ethanol [4] ± 9 h. TEOS [.45] Ethanol [4] ± 9 i. TMOS [.12] Methanol [8] ± 7 j. TMOS [.12] Methanol [8] ± 8 k. TMOS [.12] Methanol [4] ± 6 l. TMOS [.45] Methanol [4] ± 7 29

9 Stober et al. reported that there was no effect of TEOS concentration on final particle size. But in our study it was observed that larger particles were generally produced with higher TEOS concentration. Moreover, in comparison with TMOS larger particles were obtained with TEOS (smaller particles were synthesized with TEOS when alcohol was used at 4 M concentration, except sample d). In both cases, TEOS and TMOS, the smallest particles were obtained when they were used at.45 M concentration. Generally hydrolysis is a very slow reaction. NH 4 OH was used as a catalyst for hydrolysis and condensation of silicon alkoxide in alcohol. NH 4 OH resulted in faster kinetics of the reaction. In our several attempts, smallest particles were obtained when 14 M NH 4 OH was used. Increased water concentration leads to smaller particles. In our experiment, the molarity of water varied from 14 M to 3 M. Smaller particles were obtained when 14 M water was used in comparison with 3 M water (other parameters remained constant). Alcohol plays a major role in monodispersity and size of SNPs. The particle size decreased with decreasing alcohol concentration (except sample d). In methanolic environment, smaller particles were produced. The smallest particles having 29 nm average size were obtained when.45 M TEOS, 4 M methanol, 14 M water and 14 M ammonium hydroxide were used. FE SEM image showed that all these SNPs were spherical in nature. Surface morphology and size distribution of these hydrophilic SNPs are presented in Fig. 2.3 and Fig. 2.4 respectively (these are FE-SEM images and particle size distribution of the samples already mentioned in Table 2.1). 3

10 Fig. 2.3 FE SEM image of uncapped SNPs (interpretation of the sample names given in Table 2.1). 4 (a) 6 (b) 3 (c) 6 (d) (e) (f) (g) Nanoparticle size (size) 4 (h) (i) 5 (j) 7 (k) 4 (l) Fig. 2.4 Particle size distribution of uncapped SNPs (sample names are interpreted in Table 2.1). 31

11 In situ surface functionalization of SNPs in aqueous medium was done with two organosilicons. For synthesis of HMDS capped SNP, first SNP core was synthesized by hydrolysis and condensation of TEOS. These SNPs had many free OH groups. Addition of HMDS caused covalent bond formation between free silanol groups and organosilanes (here SiMe). As these SNPs were capped with methyl groups, they were hydrophobic in nature. The steps of the reaction are depicted in Fig Fig. 2.5 Steps for synthesis of HMDS capped hydrophobic SNP. 32

12 Again in case of synthesizing thiol capped SNP nanosilica core was first synthesized from ethanolic solution of TEOS followed by covalent bond formation between free -OH group of uncapped SNP and SH group of 3-MPTS. The steps of the reaction are summarized in Fig Fig. 2.6 Steps for synthesis of thiol capped lipophilic SNP. 33

13 FE SEM images showed that both the types of SNPs were spherical in nature (Fig. 2.7a, b). TEM image showed that HMDS functionalized SNP had average size of 19 nm (± 2 nm) (Fig. 2.8a), whereas Figure 2.8b showed that MPTS functionalized SNP had average size of 32 nm (± 6 nm). Fig. 2.7 FE SEM image of (a) HMDS caped and (b) thiol capped SNP. (a) 6 (b) Fig. 2.8 TEM image of (a) HMDS capped SNP and (b) thiol capped SNP. Inset showing particle size distribution. 34

14 TEM image also revealed that both the particles were spherical in shape and distinct uniformity was maintained throughout and yet they did not agglomerate into clusters EDS analysis of silica nanoparticles Figure 2.9 shows one representative EDS analysis of uncapped SNP. It was observed from the EDS data that major constituent of the synthesized nano particulate material was Silicon (38.99%) and Oxygen (6.17%). So it was confirmed that synthesized NPs were nothing but SNP. The presence of gold was due to use of gold sputtering during sample preparation. Element Wt % At % O K SiK AuM Fig. 2.9 EDS data of uncapped SNP showing the major elements present XRD pattern of synthesized silica nanoparticles Powder XRD pattern is obtained from Bragg s Law λ = 2dSinθ using CuKα radiation. All three SNPs i.e. uncapped, HMDS and thiol capped SNP showed a single broad peak indicating their amorphous nature (Fig. 2.1A). Therefore non crystalline nature of the uncapped silica was maintained even after functionalization with HMDS or MPTS. This made the SNPs suitable for their biological application. 35

15 Inhaled crystalline silica induces chronic obstructive pulmonary disease, silicosis or even lung cancer in human beings when they are exposed to crystalline silica dust during mining, foundry work, mineral processing and at construction sites. But amorphous silica is generally considered to be safe and its commercial use is approved by Food and Drug Administration (Stathers et al. 24). A B Fig. 2.1 [A] X-ray diffraction pattern of (a) uncapped SNP; (b) HMDS capped SNP and (c) thiol capped SNP. [B] IR spectra of (a) SNP without any surface functionalization, (b) SNP functionalized with HMDS and (c) SNP functionalized with MPTS FTIR analysis proved HMDS and thiol functionalization on silica nanoparticles The surface capping of the SNPs was confirmed by FTIR analysis. SNPs without any surface functionalization showed four characteristic peaks [Fig. 2.1B (a)]. Peak obtained at 3452 cm -1 appeared for O-H stretching of the surface silanol groups. Characteristic 1638 cm -1 peak was for the C=O of the carboxylic acid group, 199 cm -1 was attributed towards Si-O 36

16 stretching of the SNP and another characteristic peak for SNP appeared at 798 cm -1. A small but distinct peak at 1248 cm -1 signified C-O stretching. In case of HMDS capped SNP [Fig. 2.1B (b)] two peaks for C-H stretching were observed at 2963 cm -1 and 295 cm -1 due to asymmetric stretching and symmetric stretching arising from the methyl fragment. The other characteristic peak of Si-O stretching appeared at 189 cm -1 and characteristic peak of silica was observed at 799 cm -1. For thiol capped SNP [Fig. 2.1B (c)], 3447 cm -1 peak was for O- H stretching, C-H asymmetric stretching frequency was observed at 2929 cm -1 and C-H symmetric stretching frequency appeared at around 28 cm -1. At the same time very weak stretching at around 26 cm -1 was for S-H stretching frequency of and the characteristic peak of the Si-O stretching of silica was obtained at around 177 cm -1 and lower stretching frequency around 8 cm -1 was attributed towards characteristic Si-O stretching. In both SNPs the peak at cm -1 was for the C=O stretching of CO 2 H group and their corresponding C-O peak appears ~125 cm -1. The intensity of the O-H stretching frequency was decreased in both cases due to corresponding co-condensation reaction Study of surface topology of synthesized silica nanoparticles The surface topology, obtained from AFM image, showed that all SNPs were spherical (Fig. 2.11). Though there was some roughness in the surface of the functionalized SNPs due to presence of capping agents (Fig. 2.11d & 2.11f). HMDS and thiol capped SNPs had surface roughness of 1.18 nm and.58 nm respectively. 37

17 Fig Atomic Force Micrograph of (a) uncapped SNP and (b) its surface roughness; (c) HMDS capped SNP and (d) its surface roughness; (e) thiol capped SNP and (f) its surface roughness. 38

18 2.3.6 TG-DTA analysis of synthesized silica nanoparticles The stability of the surface capping agent was studied by TG-DTA analysis. (Fig. 2.12) showed the percentage of weight loss with the variation of temperature. Uncapped SNP was very stable throughout the span of experimental temperature except in the initial temperature range. Major weight loss started from 5 C and continued up to 13 C; 4.8% weight loss during this temperature range was attributed towards the loss of water molecules, adhered to the surface. HMDS functionalized SNP underwent slow decrease in weight throughout the temperature range. It was quite stable upto 35 C. Whereas MPTS functionalized SNP showed initial weight loss of ~.2% due to the loss of surface adhered water molecules. Above 23 C gradual reduction of weight was due to thermolysis of 3-MPTS capping, which had boiling point of 215 C. Fig TG-DTA analysis of SNP without any surface functionalization (black line); HMDS functionalized SNP (red line) and SNP functionalized with MPTS (green line). 39

19 2.4 CONCLUSION The sol-gel technique for synthesis of SNPs has many processing merits mainly owing to facility, versatility, purity, homogeneity and modifications of particle size by changing synthesis parameters. Moreover, in situ surface functionalization of these SNPs in aqueous solution with silane can easily be done by covalent bond formation between the surface silanol groups and silane coupling agents. In this present study a facile and economic method for preparing spherical, monodisperse SNPs by slightly modifying Stober method followed by surface functionalization with HMDS and thiol was illustrated. Both uncapped and HMDS and thiol capped SNPs were purely amorphous in nature. Crystalline silica causes numerous health hazards in human beings. But amorphous silica is considered to be safe (Stathers et al. 24) and its commercial use is approved by Food and Drug Administration (FDA). So these synthesized SNPs can be used in biological application. As the uncapped SNPs had many free OH groups they were hydrophilic in nature. HMDS capped SNPs had methyl groups on their surface and MPTS capped SNPs had thiol group as their cap making them hydrophobic and lipophilic respectively. 4