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Glass and bioglass nanopowders by flame synthesis{

Tobias J. Brunner, Robert N. Grass and Wendelin J. Stark*
Received (in Cambridge, UK) 12th December 2005, Accepted 6th February 2006
First published as an Advance Article on the web 21st February 2006
DOI: 10.1039/b517501a

The preparation of amorphous nanopowders by flame synth- Table 1 Composition of glasses synthesised by flame spray synthesis
esis opens access to common soda-lime, metal-doped glasses or (in wt%) as measured by laser ablation-ICP-MS
bioglasses in the range of 20–80 nm and offers an alternative to SiO2 CaO Na2O P2O5 B2O3 Fa
conventional wet-phase preparation, solid state reactions or
Soda-lime glass 74.3 11.8 14
melting. Borosilicate glass 58.7 20.4 12.8 8.1
Bioglass 45S5 47.8 25.1 22.6 4.6
Glasses and glass ceramics consist of undercooled, partially or fully Bioglass 45S5F 45.4 26.6 21.4 4.3 2.4
amorphous materials. For thousands of years they have been Bioglass 77S 79.9 16.8 3.2
a
broadly used for their intriguing properties1,2 including elasticity,3 Nominal composition by introducing fluorobenzene as fluorine
source.19,20
transparency,4 and chemical resistance,5 and today they are
workhorses in data transmission.6–8 In medicine, certain glass
compositions (SiO2–CaO–P2O5–Na2O) were even found to form areas from 70 to 90 m2 g21 (soda-lime, borosilicate glass and 45S5)
tight bonds to living human bone and are applied in a variety of and 200 m2 g21 (77S). Soda-lime glass doped with cobalt (1.8 wt%)
bone grafts for bone repair and regeneration of defects arising and gold (0.6 wt%) resulted in an intensive colouring of the glass
from trauma, tumour and osteoporosis.9–12 Traditionally, oxide- best visible in pressed tablets as shown in Fig. 1. The blue colour of
based glasses have been derived by melting of the corresponding the cobalt doped glass is typical for cobalt(II) introduced into the
oxides at elevated temperatures13 and result in dense or low- glass matrix23,24 and the burgundy colour results from metallic
porosity texture materials. The application of complex glasses as gold doping.21,25 The chemical composition (Table 1) and
nanomaterials would stimulate the development of applications homogeneity of the glasses were verified by laser ablation
using their intriguing bulk properties at small scale applications. inductively coupled plasma mass spectrometry (LA-ICP-MS) by
Consequently, sol–gel processes have enabled production of some probing at least 8 times consecutively the same spot on a pressed
glasses14–18 and have resulted in porous structures with high tablet resulting in a composition depth profile (see ESI{). The
specific surface areas. Low temperature preparations, however, can formation of glasses was confirmed by the absence of reflections in
be limiting in terms of composition,14 high remaining water or X-ray powder diffraction (XRD) patterns of all as-synthesised
solvent content after synthesis and generally require the use of an powders (Fig. 2). The calcination of a representative glass (Bioglass
annealing or sintering step after preparation.17,18 The latter 45S5) was followed by thermal analysis coupled to a mass
inherently promotes the formation of hard agglomerates of the spectrometer (Fig. 3) and by measuring the X-ray diffraction
desired glass nanoparticles. We therefore would like to show that pattern after exposure to temperatures before (pattern 1) and after
the direct preparation of glass nanoparticles in a high temperature the corresponding exothermal event at 630 uC (patterns 2 and 3).
environment offers distinct advantages in terms of accessible The initial endothermic peak could be attributed to the desorption
compositions, homogeneity and particle size.
In order to illustrate the preparation of complex glasses by flame
synthesis we prepared commonly used soda-lime and borosilicate
glasses (Table 1) and cobalt or gold doped soda lime glass
(Fig. 1).21 In order to extend the range of accessible compositions
for high-surface bioglasses we included the preparation of several
silica-based bioglasses, optionally doped with fluoride.
Each particular glass was synthesized by combining and mixing
corresponding liquid metal and suitable anion precursors and
feeding the mixture into a flame reactor.22 The nanopowders were
collected on a filter mounted above the flame. Transmission
electron microscopy (TEM) of as-synthesised materials revealed
particles of about 20 to 80 nm size (Fig. 1) with specific surface

Institute for Chemical and Bioengineering, ETH Zurich, HCI E 107,

CH-8093 Zürich, Switzerland. E-mail: wendelin.stark@chem.ethz.ch;
Fig. 1 Transmission electron microscopy image of a flame-made,
Fax: +41 44 633 10 83; Tel: +41 44 632 09 80
{ Electronic supplementary information (ESI) available: additional XRD nanoparticulate Bioglass 45S5 sample showing a high degree of
pattern, electron micrographs, pore size distributions, UV-VIS spectra and agglomeration (left). Photographs of pressed soda-lime glass doped with
chemical compositions. See DOI: 10.1039/b517501a cobalt ions or gold displays bright-coloured glasses (right).

1384 | Chem. Commun., 2006, 1384–1386 This journal is ß The Royal Society of Chemistry 2006
Fig. 2 X-ray diffraction patterns of as-synthesised soda-lime glass and Fig. 4 Mercury intrusion porosimetry of calcined samples of Bioglass
45S5 Bioglass corroborating the formation of a glass (bottom). Bioglass 45S5. Inset: Scanning electron micrograph of Bioglass 45S5 after sintering
45S5 after calcination at different temperatures displays the formation of a at 600 uC.
glass ceramic phase. (&) Na2Ca2(SiO3)3, ($) Na2CaSi3O8.

pore size distribution with still unaffected pores around 30–60 nm

and large pores in the micrometer range. At 600 uC, the smaller
pores fully collapsed and the material mainly consists of a
microporous glass ceramic (insert, Fig. 4) as evidenced by the
XRD pattern. Controlled sintering of glass nanopowders therefore
gives access to materials with defined pore sizes.
A bioactive implant material forms an interfacial bond between
the implant and the host tissue. Bioactivity of bone implants can
be assessed in vitro by evaluation of surface reactions in
physiological fluids. Bioglass 45S5 was therefore tested by
immersion of pressed samples into simulated body fluid (SBF)
prepared according to Kokubo et al.30 at 37 uC. After exposure,
the surface of the material was investigated by Raman spectro-
scopy in backscattering mode as suggested by Notingher et al.31
Fig. 5 shows a 45S5 Bioglass sample after 7 days in SBF in
comparison to an as-prepared, untreated reference, pure hydro-
Fig. 3 The calcination of as-prepared bioglass to 800 uC was followed by xyapatite20 and calcium carbonate. The Raman signal at 960 and
differential thermal analysis (DTA) and thermogravimetry (TG) and 1080 cm21 indicated the formation of carbonated hydroxyapatite
resulted in the evolution of water (m/z 5 18) and CO2 (m/z 5 44). X-ray during immersion in SBF. The same sample investigated under the
diffraction pattern were taken separately at 600 to 800 uC (numbers 1–3).

of physisorbed water from the surface of the material (MS trace of

m/z 5 18). The evolution of carbon dioxide (MS trace m/z 5 44)
showed no prominent decarboxylations and corroborated the
stability of the glasses. The XRD pattern indicated that the
exothermal peak at 630 uC may be assigned to partial crystal-
lization of the bioglass and formation of a glass ceramic with
characteristic peaks for sodium calcium silicates26 (see Fig. 2).
The development of nano- or microporous glasses has been
suggested for nanofiltration,27 permeable membranes28 and
controlled degradation of biomaterials.29 The evolution of the
pore size distribution during sintering of a representative sample
(Bioglass 45S5) was therefore measured by mercury intrusion
porosimetry. A pronounced shift in the pore size distribution could
be observed with increasing sintering temperature (Fig. 4). As-
prepared glass nanopowders exhibited a relatively narrow pore size
distribution at 20–60 nm which can be mainly attributed to Fig. 5 Raman spectroscopy of bioglass after 7 days in simulated body
interparticle pores if compared to the transmission electron fluid in comparison to untreated bioglass, calcium carbonate (CaCO3) and
micrographs (Fig. 1). At around 500 uC, the material starts to hydroxyapatite (Ca10(PO4)6(OH)2) are shown. The formation of carbo-
sinter and pores partially collapsed at 550 uC resulting in a bimodal nated hydroxyapatite can be observed.

This journal is ß The Royal Society of Chemistry 2006 Chem. Commun., 2006, 1384–1386 | 1385
 Stoe STADI-P2 (Ge monochromator, CuKa1, PSD detector). Element
analysis was performed by laser ablation inductively coupled plasma mass
spectrometry (LA-ICP-MS).32 TEM images were recorded on a CM30 ST
(Philips, LaB6 cathode, operated at 300 kV, point resolution y4 Å).
Particles were deposited onto a carbon foil supported grid. SEM analysis
was performed on a LEO 1530 Gemini after sputtering the samples with
y4 nm of platinum. Simulated body fluid was prepared according to
Kokubo et al.30 and sterile filtered (Nalgene). Mercury intrusion
porosimetry was measured on a Micromeritics Autopore 9220. Raman
spectra were recorded on an EQUINOX 55 spectrometer equipped with a
FT-Raman accessory FRA 160/S (Bruker optics) in backscattering mode.
Differential thermal analysis was performed on a Linseis TG/STA-PT1600
thermoanalyser coupled to a mass spectrometer (Balzers, Quadstar).

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1386 | Chem. Commun., 2006, 1384–1386 This journal is ß The Royal Society of Chemistry 2006