Spectroscopic study of biologically active glasses

M. Szumera, I. Wacławska, W. Mozgawa, M. Sitarz*

Faculty of Materials Science and Ceramics, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Poland

Received 30 November 2004; accepted 4 January 2005

Available online 3 March 2005

Abstract

It is known that the chemical activity phenomenon is characteristic for some inorganic glasses and they are able to participate in biological

processes of living organisms (plants, animals and human bodies). An example here is the selective removal of silicate–phosphate glass

components under the influence of biological solutions, which has been applied in designing glasses acting as ecological fertilizers of

controlled release rate of the nutrients for plants.

The structure of model silicate–phosphate glasses containing the different amounts of the glass network formers, i.e. Ca2C and Mg2C, as a

binding components were studied. These elements besides other are indispensable of the normal growth of plants.

In order to establish the function and position occupied by the particular components in the glass structure, the glasses were examined by

FTIR spectroscopy (with spectra decomposition) and XRD methods.

It has been found that the increasing amount of MgO in the structure of silicate–phosphate glasses causes the formation of domains the

structure of which changes systematically from a structure of the cristobalite type to a structure corresponding to forsterite type. Whilst the

increasing content of CaO in the structure of silicate–phosphate glasses causes the formation of domains the structure of which changes from

a structure typical for cristobalite through one similar to the structure of calcium orthophosphate, to a structure corresponding to calcium

silicates. The changing character of domains structure is the reason of different chemical activity of glasses.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Silicate–phosphate glasses; Glass structure; Infrared spectroscopy; Glassy fertilizers

1. Introduction

The last decades have brought a considerable progress in

the field of the synthesis of new glasses for various, often

unconventional applications. Introduction into the glass

structure of a group of chemical elements participating in

the structure of living organisms allows to obtain glasses

demonstrating the property defined as ‘bioactivity’, i.e. the

ability to participate in the biological processes of living

organisms. The biological activity of glasses, which contain

as part of their composition appropriately selected bioele-

ments, has brought about their applications materials for the

production of implants in surgery and dentistry [1–3]. The

biological activity of glasses of appropriate composition

enables their participation in the biological processes of the

0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2005.01.023

* Corresponding author. Tel.: C48 12 617 2232; fax: C48 12 633 1593.

E-mail addresses: mszumera@poczta.onet.pl (M. Szumera), iwac@

interia.pl (I. Wacławska), msitarz@uci.agh.edu.pl (M. Sitarz).

growth of plants. An example of the latter are glasses

with mixed silicate–phosphate framework, demonstrating

the ability to accept in their composition the presence of a

number of elements indispensable in the biological

processes of the growth of plants and ability of their

selective release in the soil environment in a form available

for the plants. Glasses of this type can be used in practice as

ecological fertilizers of controlled release rate of the

nutrients for plants [4–6].

From the earlier investigations it follows that the

chemical activity of glasses of this type is determined by

their chemical composition and it increases with increasing

content of phosphorus and potassium in the glass structure

[7,8]. A different tendency can be observed in the case of

the increasing content of calcium and magnesium, at the

constant content of phosphorus and potassium in the

structure of glassy fertilizers [7,9].

Considering these facts, an attempt has been made to

determine the relation between the chemical composition

and the structure of silicate–phosphate glasses modified by

Journal of Molecular Structure 744–747 (2005) 609–614

www.elsevier.com/locate/molstruc

Table 1

The chemical composition of silicate–phosphate glasses

No. P2O5 K2O CaO

(mol %)

MgO SiO2

1 2 6 – – 92

MgO/SiO2

2 2 6 – 16 76 0.21

3 2 6 – 23 69 0.33

4 2 6 – 29 63 0.46

5 2 6 – 35 57 0.61

6 2 6 – 41 51 0.80

CaO/SiO2

7 2 6 12 – 80 0.15

8 2 6 18 – 74 0.24

9 2 6 24 – 68 0.35

10 2 6 28 – 63 0.44

M. Szumera et al. / Journal of Molecular Structure 744–747 (2005) 609–614610

an addition of elements playing the role of macroelements

(Ca, Mg) as well as their chemical activity under conditions

simulating the biological soil environment.

It is known that the structure of silicate–phosphate

glasses is a spatial framework in which every atom of silicon

is connected with four silicon or phosphorus atoms by

means of oxygen bridges. On the other hand, every

phosphorus atom has only three oxygen bridges bonds

because the oxygen atom appearing in the fourth corner of

a phosphorus–oxygen tetrahedron is connected with the

central phosphorus atom by a double bond [10]. In the

structure of the silicate–phosphate framework, built of

silicon and phosphorus atoms, surrounded in a tetrahedral

mode by the oxygen atoms, the components defined in the

chemistry of glass as modifiers play on important role. Their

introduction into the structure of silicate–phosphate glasses

causes both the breaking of bonds of P]O type and

formation of the bridging bonds of M–O–PO3 type as well

as the breaking of some part of Si–O–Si, Si–O–P and P–O–P

bonds, the result of which is the depolimerization of the

silicate–phosphate structure [11–13].

One of the most appropriate method of glass structure

investigation is IR spectroscopy. It may be use to determine

the main structural units present in the glass structure and

also allows to establish structural relations between

amorphous materials and their devitrificates. IR spec-

troscopy method is often used for investigation of the

structure of silicate–phosphate glasses [14–17], but there are

not information about IR study of glasses from MgO–K2O–

P2O5–SiO2 and CaO–K2O–P2O5–SiO2 systems.

Investigations which are the subject of the present study

take in the model silicate–phosphate glasses of constant

phosphorus amount, containing constant content of potass-

ium and varying amounts of magnesium and calcium in

their composition as the elements connected with the glass

forming components.

2. Experimental

The composition of glasses from the systems SiO2–

P2O5–CaO–K2O and SiO2–P2O5–MgO–K2O were selected

so that the melted glasses contained: 2 mol% of P2O5,

6 mol% of K2O, 11–28 mol% of CaO, 15–41 mol% of MgO

and 62–79 mol% of SiO2.

The glasses were obtained by the traditional method of

melting a mixture of pure materials, i.e. SiO2, H3PO4,

K2CO3, MgO and CaCO3, in platinum crucibles, in the

temperature range 1470–1600 8C. The obtained amorphous

material was refined to the grain size 0.1–0.3 mm.

The crystalline compounds were obtained by devitrifica-

tion of glassy samples. Structural examinations of the

glasses and their devitrificates were based on FTIR method

(Spectrometer Bio-Rad FTS-60 MV). Spectra were col-

lected after 256 scans at 4 cmK1 resolution. Samples were

prepared by the standard KBr pellets method. Spectra

decomposition has been carried out according to the method

described at the work [18].

To identify the crystal phases X-ray diffraction method

(Diffractometer X’Pert PRO) was applied.

The chemical compositions of the model silicate–

phosphate glasses are listed in Table 1.

3. Results and discussion

FTIR spectra in the middle infrared range (MIR) of

silicate–phosphate glasses modified by different kind and

content of cation modifiers are presented in Fig. 1.

FTIR spectra interpretation has been carried out accord-

ing to the assumption that the glass structure consists of

silicon–oxide bonds existing in amorphous SiO2 and

phosphorus–oxide bonds existing in amorphous P2O5.

FTIR spectra of the obtained amorphous materials are

characterized by three main absorption bands at 900–1200,

780–810 and 460–480 cmK1. These bands were attributed

to vibrations of the following structural units: the most

intensive bands at 900–1200 cmK1 were attributed to the

stretching vibrations Si–O as well as P–O. It represents a

superposition of some bands situated close to each other. In

all cases the intensity of these bands diminishes with

increasing content of the modifier in their structure. Their

position becomes shifted towards lower wavenumbers and

the half-width of the bands increases.

Bands at about 780–810 cmK1 appear in both groups of

glasses. Their intensity becomes reduced and the position of

the bands shows the tendency to shift towards lower

wavenumbers. These bands have been attributed to a

combination of vibrations of Si–O–Si, Si–O–P and P–O–P

bridges.

The next bands in the range 570–650 cmK1 is more

visible in the group of glasses containing calcium ions as the

structure modifier. The intensity of these bands increases

with increasing amount of the modifier in the glass structure.

They have been connected with P–O vibrations or pseudo-

lattice vibrations of group of phosphate tetrahedra [19].

Fig. 1. FTIR spectra of silicate–phosphate glasses containing magnesium (a) and calcium (b) as a modifiers.

M. Szumera et al. / Journal of Molecular Structure 744–747 (2005) 609–614 611

The bands appearing in both groups of glasses at 460–

480 cmK1 has been ascribed to the combination of bending

vibrations of O–Si–O and O–P–O bonds. It has been found

that with increasing content of magnesium ions in the glass

structure these bands decrease in their intensity and show

the tendency for a shift towards higher wavenumbers

(Fig. 1a). In the case of glasses containing a modifier in

Fig. 2. The decomposition of MIR spectra of silicate–phosphate glasses containin

the form of increasing content of calcium ions the situation

seem to be opposite (Fig. 1b).

It should be noted that P]O bond at about 1300 cmK1

did not appear in any group of examined glasses.

From the obtained spectra it follows that the increasing

content of modifiers in the form of cations of calcium and

magnesium in the structure of silicate–phosphate glasses,

g (a) 16 mol% of MgO, (c) 41 mol% of MgO and their devitrificates (b,d).

M. Szumera et al. / Journal of Molecular Structure 744–747 (2005) 609–614612

causes the breaking the oxygen bridges, i.e. Si–O–Si,

Si–O–P and P–O–P. It is responsible for the increasing

degree of depolimerization of the structure formed of

silicate and phosphate tetrahedra.

Complex and broad character of absorption spectra of the

investigated glasses causes, that obtaining of more structural

information is possible after their decomposition into

separate bands. According to the fact that structure of the

amorphous materials is similar to that of the structure of

their devitrificates [19,20], detail interpretation of absorp-

tion bands of the glasses spectra was based on their

comparison with absorption bands of crystalline samples of

the same chemical composition. Decomposition was carried

out for crystalline and amorphous samples. The spectra of

Fig. 3. The decomposition of MIR spectra of silicate–phosphate glasses containin

devitrificates (b,d,f).

chosen silicate–phosphate glasses and their crystalline

analogues after decomposition are presented in Fig. 2 and 3.

In all cases in the spectra of glasses after decomposition

of the main band at 900–1200 cmK1 there appeared

additional absorption bands situated in the range: 900–970

and 1100–1200 cmK1. The first of these bands originate

from a combination of stretching vibrations of P–O groups

in P–O–P bridges and from terminal vibrations of Si–OK

groups formed as a result of breaking Si–O–Si bridges [21].

With increasing content of the modifiers in the structure of

glasses their position becomes distinctly shifted towards

lower wavenumbers (Fig. 2a and c). The bands lying within

the range of higher frequencies have been assigned to

stretching vibrations of double Si]O bonds [20]. The

g (a) 12 mol% of CaO, (c) 24 mol% of CaO, (e) 28 mol% of CaO and their

M. Szumera et al. / Journal of Molecular Structure 744–747 (2005) 609–614 613

process of decomposition of the absorption bands in

the range 570–650 cmK1, occurring in glass structure

containing 16 mol% MgO (MgO/SiO2Z0.21) indicates

the existence of three component bands at 620, 590 and

566 cmK1 (Fig. 2a). The two last bands are connected with,

the occurrence of the earlier mentioned bending vibrations

of O–P–O bonds and pseudo-lattice vibrations of the groups

of PO4 tetrahedra. On the other hand, the band at 620 cmK1

is characteristic for vibrations of Si–O bonds occurring in

cristobalite, and a band of this type, besides other bands

characteristic for cristobalite (at 794, 1204 cmK1) [22],

appears in the product of crystallization of the glass under

discussion (Fig. 2b). With increasing content of MgO in the

structure of silicate–phosphate glasses (Fig. 2c and 2d), at

simultaneous decreasing of the SiO2 content, the character

of the absorption bands after decomposition process in the

discussed range of wavenumbers is changed. The absorption

band indicating the cristobalite-like character of the

structure of glasses at about 620 cmK1 becomes shifted

towards lower wavenumbers; simultaneously there appear

bands at about 880 and 510 cmK1 (Fig. 2c), corresponding

to vibrations of Si–O bonds, characteristic of magnesium

orthosilicate (forsterite) [23]. The presence of silicate of this

type in the devitrificates has been confirmed by X-ray

method.

Analysis of the component absorption bands obtained as

a result of the decomposition of spectrum in the range of the

wavenumbers 570–650 cmK1, present in glass containing in

its structure 12 mol% of CaO, points to the occurrence of

three bands lying at 590, 566 and 530 cmK1. In the

absorption spectrum of recrystallized glass these bands are

considerably more distinct (Fig. 3b), and their position

corresponds to the vibrations of P–O bonds, occurring in

calcium phosphate of Ca3(PO4)2 type [23]. The presence of

calcium phosphate in the devitrificates has been confirmed

by X-ray method. It should be noted that in the absorption

spectrum of devitrificate of this type of glass there are also

present the absorption bands at 795 and 620 cmK1,

illustrating vibrations of Si–O bonds occurring in

cristobalite.

With increasing amount of CaO (Fig. 3c) in the structure

of silicate–phosphate glasses (CaO/SiO2Z0.35) there dis-

appear the absorption bands characteristic for bonds

occurring in cristobalite. The absorption bands at 607 and

568 cmK1 which were found also in the spectrum of

recrystallized glass (Fig. 3d) indicate the presence of a

mixture of calcium orthophosphate and calcium orthosili-

cate. Analysis of the component absorption bands present in

a glass containing in its structure 28 mol% of CaO (CaO/

SiO2Z0.44) (Fig. 3e) indicates the occurrence of absorption

bands at about 590 and 860 cmK1, which correspond to

vibrations of Si–O bonds in calcium silicate of g–Ca2SiO4

type [23].

The above results indicate that with the change of the

modifier in the structure of silicate–phosphate glasses there

are formed domains characterized by certain degree of

ordering of the units present in their composition, while

the structure of the newly formed domains is similar to

the structure of the crystal compounds formed from glasses

with the same chemical composition.

The increasing amount of MgO in the structure of

silicate–phosphate glasses causes the formation of group-

ings (domains) the structure of which changes gradually

from a structure of the cristobalite type to a structure

corresponding to forsterite type.

Depending on the change in the CaO content in the

structure of silicate–phosphate glasses the newly formed

groupings of silicon–oxygen and phosphorus–oxygen units

change their structure from a structure typical for cristoba-

lite through one similar to the calcium orthophosphate, to a

structure corresponding to calcium silicates.

Obtained results suggest that the changing character of

domains structure, which formed in the considered glasses,

is the reason of their different chemical activity. As it

follows from the earlier study [7,9] the chemical activity of

glasses of this type depends on the mutual proportions

between the components forming their structure. In the case

of silicate–phosphate glasses with 2 mol% P2O5, containing

increasing amount of MgO, their solubility in citric acid

solution, simulating natural soil environment increases.

Thus, glasses containing domains with structure corre-

sponding to silicates are characterized by higher solubility

in comparison with glasses containing domains character-

istic for cristobalite type. At the same time, the solubility of

glasses containing Ca2C, as the structure modifier is slightly

different [7,9]. These glasses are characterized by decrease

of solubility with increasing content of calcium in their

structure till the moment when the value CaO/SiO2!0.35.

Next, the solubility rapidly increases after the above value

has been exceeded. Thus increased content of CaO in the

structure of glasses under discussion, changing domains

structure from typical for cristobalite to one similar to

calcium orthophosphate reduces their chemical activity in

solutions. Subsequent increase of content of calcium leads

to formation of domains with structure similar to calcium

silicates, which are characterized by higher solubility in

comparison with solubility of calcium orthophosphate.

4. Conclusion

Biological activity of silicate–phosphate glasses acting

as glassy fertilizers may be connected with the changing

character of domains which are formed in the structure of

glasses depending on the mutual properties between the

components forming their structure. Formation of domains

with structure corresponding of silicates causes the increase

of chemical activity of glasses. However, formation of

domains with structure similar to orthophosphates, which

are characterized by lower solubility in comparison with

solubility of silicates reduces chemical activity of glasses in

biological solutions.

M. Szumera et al. / Journal of Molecular Structure 744–747 (2005) 609–614614

Acknowledgements

The work was supported by Polish Committee for

Scientific Research under grant no 4 T08D 022 25.

References

[1] T. Kokubo, Proc. XVI Int. Congr. Glass, Madrid 1 (1992) 31:199.

[2] L.L. Hench, J. Am. Ceram. Soc. 74 (7) (1991) 1487.

[3] T. Kokubo, J. Non-Cryst. Solids 120 (1990) 138.

[4] M.F. Barba, P. Callejas, J.O. Arzabe, D. Ajo, J. Eur. Ceram. Soc. 18

(1998) 1313.

[5] I. Wacławska, L. Stoch, Proc. XIX Int. Congr. Glass, Edinburgh, 1–6

July 2001, Glass Technol. 43C (2002) 237–241.

[6] L. Stoch, I. Wacławska, A. Lis-Krzyscin, Chem. Agric. 2 (2001)

74–80.

[7] M. Szumera, I. Wacławska, Proc. XX Int. Congr. Glass, Kyoto 2004

(P-13-011).

[8] I. Wacławska, L. Stoch, J. Therm. Anal. Cal. 65 (2001) 141.

[9] I. Wacławska, M. Szumera, J. Therm. Anal. Cal. 72 (2003) 1065.

[10] V.G. Plotnichenko, V.O. Sokolov, V.V. Koltasher, E.M. Dianov,

J. Non-Cryst. Solids 306 (2002) 209.

[11] G. Walter, U. Hoppe, T. Baade, R. Kranold, D. Stachel, J. Non-Cryst.

Solid 217 (1997) 299.

[12] U. Hoppe, J. Non-Cryst. Solids 195 (1996) 138.

[13] T. Miya, V. Terunuma, T. Hosaka, T. Miyashita, Rev. Electr.

Commun. Lab. 27 (1979) 497.

[14] I.N. Chakraborty, R.A. Condrate, Phys. Chem. Glasses 26 (3) (1985)

68–73.

[15] U. Hoppe, J. Non-Cryst. Solids 195 (1996) 138–147.

[16] P.Y. Shih, Mater. Chem. Phys. 80 (2003) 299–304.

[17] L. Stoch, M. Sroda, J. Mol. Struct. 511/512 (1999) 77–84.

[18] M. Handke, W. Mozgawa, M. Nocun, J. Mol. Struct. 325 (1994) 129.

[19] M. Sitarz, M. Rokita, M. Handke, E. Galuskin, J. Mol. Struct.

651&653 (2003) 489.

[20] M. Handke, M. Nocun, Pol. Ceram. Soc. 43 (1993) 213.

[21] M. Handke, W. Mozgawa, J. Mol. Struct. 348 (1995) 341.

[22] M. Handke, W. Mozgawa, M. Nocun, Pol. Ceram. Soc. 43 (1993) 33.

[23] V.C. Farmer, The Infrared Spectra of Minerals, monograph 4,

Minerological Society, London, 1974.