Surface Modification of Hydroxy Carbonate Apatite ... - CSJ Journals

doi:10.1246/cl.121294 Published on the web May 5, 2013

486

Surface Modification of Hydroxy Carbonate Apatite Nanoparticles with PDMAEMA via Surface-initiated ATRP Faqi Yu,1 Xinde Tang,2 and Meishan Pei*1 School of Chemistry and Chemical Engineering, University of Jinan, Jinan, Shandong 250022, P. R. China 2 School of Material Science and Engineering, Shandong Jiaotong University, Jinan, Shandong 250023, P. R. China 1

(Received January 12, 2013; CL-121294; E-mail: [email protected]) A facile strategy for grafting poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) on the surface of hydroxy carbonate apatite (HCA) nanoparticles via surface-initiated ATRP has been presented. SEM, FT-IR, TGA, N2 adsorptiondesorption analysis, and DLS were employed for their characterization. The results demonstrated that the stimuli-responsive polymer had been successfully grafted onto the surface of HCA. The stimuliresponsive brushes attached to HCA could act as a switch to control the opening and closing of the pore on the surface of HCA. In this paper, the pH-responsive property was mainly focused on.

Hydroxyapatite (HAP) nanoparticles have become one of the most important biomedical materials in bone replacement and drug carrier fields due to excellent biocompatibility, good bone-bonding ability, and ability to not cause any systemic toxicity in human internal environment.17 Much work has been focused on the construction of intelligent surfaces with novel properties for perspective applications in biofunctional materials, drug delivery systems, and hybrid materials and surfaces.811 Hydroxy carbonate apatite (HCA) is the major inorganic constituent of natural bones, and the synthetic material has been widely used for bone tissue engineering.12,13 HCA nanoparticles synthesized by biochemical processes possess large penetrating pores on the surface that endow them with much more capacity to store than conventional HAP. The mesoporous structure makes it possible to incorporate high dosages of drugs into the mesopores and release them at a controlled rate.1416 Stimuliresponsive polymers grafting onto the external surface of HCA nanoparticles can regulate the transport of encapsulated molecules. Poly(N-isopropylacrylamide) (PNIPAM), as an extensively studied thermal-responsive polymer, has been reported to graft onto the mesoporous silica nanoparticles by atom-transfer radical polymerization (ATRP),1719 reversible additionfragmentation chain-transfer polymerization,20,21 and chemical coupling reaction.22 PNIPAM also has been grafted to HAP by ATRP for bone tissue engineering.23 Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as a thermal- and pHresponsive polymer with biocompatibility and antibacterial activity has been used to fabricate functional materials for medical and biomedical applications.24 PDMAEMA is a weak electrolyte. At low pH, such as pH 1, PDMAEMA is entirely protonated to form polyelectrolyte with positive charge, and the polymer chains are extending fully due to the electrostatic repulsions between PDMAEMA chains. Nevertheless, PDMAEMA chains are deprotonated and then became hydrophobic at high pH, and the polymer chains collapse.25 On the other hand, the ATRP technique, a powerful tool, has been widely used for the preparation of polymers with predetermined Chem. Lett. 2013, 42, 486488

molecular weight and narrow molecular weight distribution. As for the surface modification of HCA, to the best of our knowledge, there have been few reports concerning PDMAEMA-coated HCA nanoparticles so far. The PDMAEMA-modified HCA was prepared according to the following two procedures. Dried HCA powder26 (0.60 g), anhydrous THF (10 mL), and triethylamine (TEA) (3 mL, 21.5 mmol) were added to a 50-mL flask, and then, the system was mixed by ultrasonic agitation for 10 min. 2-Bromoisobutyryl bromide (BIBB) (2.5 mL, 20.2 mmol) in 10 mL of anhydrous THF was added dropwise into the mixture under ice bath. The resulting system was allowed to proceed with stirring at room temperature for 24 h. The solid was separated by filtration and washed with THF three times. Then, the product was redispersed in THF and separated by centrifugation at 3000 rpm. After that, HCA-Br was washed with methylene chloride and dried under vacuum overnight at 60 °C. HCA-Br (100.0 mg), CuCl (10 mg, 0.1 mmol), tris[2(dimethylamino)ethyl]amine (Me6TREN) (28 ¯L, 0.1 mmol), DMAEMA (2.5 mL, 15 mmol), and anisole (0.5 mL) were added in a 20-mL dry flask, which was then sealed with a rubber plug. The solution was degassed by three freezepumpthaw cycles and then placed in a thermostated oil bath at 85 °C for 6 h. The mixture was dispersed in THF, separated by centrifugation several times, and dried under vacuum overnight at 60 °C. As shown in Figures 1a and 1b, the synthesized porous HCA clearly displayed a wrinkled, worm-like porous structure. In comparison, from the images of HCA and HCA PDMAEMA, the porous structures still existed, which proved that the structures had not been destroyed during the modification. The FT-IR spectroscopy was employed to provide direct identification of chemical groups in HCA, HCA-Br, and HCA PDMAEMA (Figure 2). As shown in Figure 2a, two weak peaks at 1403 and 2927 cm¹1 were the characteristic deformation vibrations of CH, and the absorption band at 1650 cm¹1 was

(a)

(b)

Figure 1. SEM images of HCA (a) and HCAPDMAEMA (b).

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487 Table 1. Mesopore parameters of HCA, HCA-Br, and HCA PDMAEMA

a 873

2927

b

1403

Transmittance

Sample

c 1650

HCA HCA-Br HCA PDMAEMA

1738

1083

4000

3500

3000

2500

2000

1500

1000

70.9 93.4

Average pore diameter by BJH/nm 24.3 12.0

46.5

8.84

BET surface area/m2 g¹1

1650

Pore volume /cm3 g¹1 0.43 0.27 0.14

500

-1

Wavenumber/cm

300

90

Weight/%

80

a 70

Pore Volume/cm3 g−1

100

Quantity Adsorbed/cm3 (g STP)−1

0.035

Figure 2. FT-IR spectra of HCA (a), HCA-Br (b), and HCA PDMAEMA (c).

250

200

150

0.030 0.025 0.020 0.015 0.010 0.005 0.000 -0.005

0

20

40

60

80

100 120 140

Pore Diameter/nm

100

50

b

0 0.0

0.2

0.4

0.6

0.8

1.0

o

60

Relative Pressure (P/P ) c

50

100

200

300

400

500

600

700

Figure 4. Nitrogen adsorptiondesorption isotherm of HCA. Insert is BJH pore size distribution plot of the HCA.

T/°c 200

ascribed to the stretching vibration of C=O, which is owing to the yeast template residues. The absorption peaks at 1083 and 873 cm¹1 were ascribed to the stretching vibrations of PO and CO, respectively. After polymerization of DMAEMA on the surface of HCA, a new absorption peak at 1738 cm¹1 could be observed in Figure 2c, which corresponded to the stretching vibration of ester carbonyl group in the PDMAEMA. The weight loss of unmodified and modified HCA samples was investigated by TGA analysis in inert argon atmosphere. The weight losses of pure HCA below 700 °C was less than 26.2%, which was attributed to the adsorbed water in HCA surface and the lost of yeast template. The weight losses of HCA-Br and HCAPDMAEMA are 66.9 and 51.4%, respectively, when the samples were heated to 700 °C. Estimated from the TGA curves (Figure 3), the weight losses of ATRP initiator (HCA-Br) was about 6.90% compared to that of HCA. After the graft polymerization, the weight loss of HCAPDMAEMA increased by 15.5% than that of HCA-Br, which can be attributed to the grafted PDMAEMA brushes. Nitrogen adsorptiondesorption method was employed to measure the pore structures of HCA, HCA-Br, and HCA PDMAEMA. The results were summarized in Table 1. The BET isotherm of HCA exhibited the characteristic Type IV adsorptiondesorption pattern in Figure 4. The BJH pore size distribution of HCA is shown in Figure 4 inset, and the pore size was about 24.3 nm, which demonstrated that HCA had large penetrating pores on the surface that endowed them with much more capacity to store than conventional HAP.

Chem. Lett. 2013, 42, 486488



0.025

Figure 3. TGA curves of HCA (a), HCA-Br (b), and HCA PDMAEMA (c).

150

0.020 0.015 0.010 0.005 0.000

100

0

20

40

60

80

100

Pore Diameter/nm

50

0 0.0

0.2

0.4

0.6

0.8

1.0

o

Relative Pressure (P/P )

Figure 5. Nitrogen adsorptiondesorption isotherm of HCABr. Insert is BJH pore size distribution plot of the HCA-Br. The BET isotherm of HCA-Br is shown in Figure 5, and the BJH pore size distribution of HCA-Br is shown in Figure 5 insert. Comparing the mesopore parameters of HCA-Br with those of HCA, it is found that the inertial surface area increased and that the pore size decreased because of the existence of initiating sites for ATRP, 2-bromoisobutyryl bromide group. From Figure 6, the specific surface area was 46.5 m2 g¹1, and the pore size of HCAPDMAEMA was about 8.84 nm, which demonstrated that ATRP of DMAEMA took place not only on the exterior surface of the HCA but also on the internal surface of the mesopores. Thus, it was reasonable to presume that the compact polymer layers on the surface of HCA prevented penetration of the nitrogen. To examine the pH-dependent volume phase transition of the PDMAEMA shell, 0.5 mg mL¹1 of HCAPDMAEMA suspensions at varied pH was measured by Zetasizer Nano s90. DLS was measured at 25 °C, and the equilibrium time was



488 We are grateful for the financial support from the Natural Science foundation of Shandong Province (No. ZR2010BM006).



120

100

80

60

0.015 0.010 0.005 0.000 0

20

40

60

80

100

Pore Diameter/nm

40

20

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/P0 )

Figure 6. Nitrogen adsorptiondesorption isotherm of HCA PDMAEMA. Insert is BJH pore size distribution plot of the HCAPDMAEMA. 800 700 600

Size/nm

500 400 300 200 100 0 3

4

6

5

7

8

pH

Figure 7. pH dependence of hydrodynamic diameter of HCA PDMAEMA (Temperature: 25 °C).

5 min. The pH dependence of hydrodynamic diameter (Dh) of HCAPDMAEMA is shown in Figure 7. At low temperatures, the chains’ geometries are dominated by pH. PDMAEMA chain was soluble as a weak cationic polyelectrolyte, and the protonated chains adopt an extended conformation in acidic solution; on the other hand, at neutral or alkaline pH, the chain was too hydrophobic to be water-soluble, and the polymer chains in their uncharged form were collapsed. In acidic solution, at low pH, the repulsion of chains is stronger than at high pH, which leads to a larger Dh. However, PDMAEMAgrafted HCA flocculated in neutral or alkaline media, which resulted in a larger Dh than that of weak acid. In summary, a novel pH-sensitive nanosystem based on hydroxy carbonate apatite has been successfully prepared by surface-initiated atom-transfer radical polymerization of DMAEMA. PDMAEMA grafted on HCA could act as a good gatekeeper to control access to the pores via pH-dependent openclose mechanism, which is confirmed by the wellcontrolled release of drug from the pores through adjusting the pH. This nanodevice can have potential applications in siteselected drug delivery and release.

Chem. Lett. 2013, 42, 486488

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