Organized crystalline assemblies of cobalt salt and g-iron oxide have been produced by in situ matrixmediated biomimetic route. The process makes use of an organized supramolecular matrix and produces cobalt chloride crystals with characteristic morphology of coccolith of alga and nacreous structure of Pinctada martensii. Crystals of g-iron oxide have been produced with typical morphology of aragonite spherulites in regenerated shell of Pomaceae paludosa

Das, S K

Ramachandrarao, P

Rao, Venkatesh

Sinha, Arvind

English

eprints@NML

RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 79, NO. 5, 10 SEPTEMBER 2000 646Synthesis of organized inorganic  crystal assemblies Arvind Sinha*, Swapan Kumar Das, V. Rao and P. Ramachandrarao National Metallurgical Laboratory, Jamshedpur 831 007, India Organized crystalline assemblies of cobalt salt and g-iron oxide have been produced by in situ matrix-mediated biomimetic route. The process makes use of an organized supramolecular matrix and produces cobalt chloride crystals with characteristic morpho-logy of coccolith of alga nd nacreous structure of Pinctada martensii. Crystals of g-iron oxide have been produced with typical morphology of aragonite spher-ulites in regenerated shell of Pomaceae paludosa. NATURE has synthesized many inorganic crystals in living organisms with the help of biopolymers. The natural route consists of pre-organization of a supramolecular matrix, mineralization of it through interfacial molecular recogni-tion leading to the nucleation, growth and organization of the inorganic phase. A signature of the natural synthesis is the production of the same inorganic ph se with several distinctly different morphologies. For example, calcium carbonate crystals can grow in the form of plates, rods  or complex tubules1. Recent efforts in the laboratory, to duplicate this natural method of natural synthesis, have come to be known as biomimetic methods for synthesis2. An understanding of the mechanism involved in such  matrix-mediated synthesis has been recognized to be of great potential in the production of engineering materials. Thus, catalyst particles of controlled size and morpho-logy, magnetic materials with appropriate anisotropy, highly porous materials, composites and well-organized crystallite assemblies can be produced through bio-mimetic synthesis. A variety of technically important  inorganic materials such as CaCO3, CdS, TiO2, Fe2O3, Fe3O4, nano-sized copper and silver have been produced by the biomimetic route3–8. While these materials exhi-bited much of the sophistication expected in terms of shape, size and orientation, however, complex and un-usual morphologies found in living organisms could not be produced. The present communication reports the syn-thesis of cobalt chloride and iron oxide particles with morphologies that are characteristic of the coccolith of alga, nacreous structure of P. martensii and aragonite spherulites in the regenerated shell of P. paludosa9. In this respect, these results open up the exciting possibility of imitating nature more closely through the control of experimental conditions and producing several morpho- logies of the inorganic material in the same system.  In the present investigation polyvinyl alcohol (PVA) has been used as the supramolecular matrix. Being a water- soluble polymer with a monoclinic crystal structure, PVA having rigid carbon backbone and inter-chain hydrogen *For correspondence. (e-mail: arvind@csnml.ren.nic.in) Figure 1. a, Random distribution of cylindrical-shaped cobalt salt particles in a supernatant PVA film; b, Growth of cobalt salt precipitate with morphology typical of coccolith of algae; c, Cobalt salt precipitate with morphology similar to nacreous structure of P. martensii.   RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 79, NO. 5, 10 SEPTEMBER 2000 647bonds, has the ability to solvate. It can also undergo cross-linking and a series of gelation reactions. The gel can be crystalline and the degree of crystallinity is a func-tion of time. The presence of inorganic salts is also known to enhance gelation. Aqueous solutions containing differ-ent concentrations of PVA were produced by dissolving the polymer in deionized water at 90°C. The concentra-tions varied between 2.5 wt% and 10 wt% of PVA. Two different inorganic salts, viz. CoCl2· 6H2O and FeSO4· 7H2O were employed in the present study. The salts were dissol-ved in deionized water at room temperature to prepare 0.5 to 1.0 molar solutions and were filtered. Salt solutions were mixed with the aqueous solution of PVA in various proportions, stirred and allowed to react for a time period extending from 15 min to 15 days. Samples were drawn at regular intervals either by dipping a clean glass slide or by lifting the supernatant layer after the lapse of about a week or more. In the case of iron sulphate solutions, the polymer films with iron salt were reacted with a cold  solution of sodium borohydride and dried under con-trolled oxygen atmosphere. The borohydride reduces the iron salt to the metallic state through a combined process of hydrolysis and reduction10. The nano-sized metal parti-cles were then re-oxidized at room temperature to yield g-Fe2O3 in a single step. The films so obtained were exam-ined under scanning electron microscope (SEM). The The composition and structure of the film constituent w re confirmed by X-ray diffraction (XRD).  The pink-coloured supernatant films as well as the films coated on glass slides containing cobalt salt developed localized regions of blue colour under the polymer sur-face, when preserved in dry atmosphere at room tempera-ture for several days. The SEM study of the films containing cobalt salt showed randomly distributed particles with a hollow cylindrical morphology (Figure 1 a). Examination of the film at a higher magnification showed that these particles are the aggregates of crys als arranged in a circle around a rim of PVA (Figure 1 b). This mor-phology is identical to the coccolith of the alga, cocco-lithus huxleyi, made of radial growth of calcite in a biopolymer9. On the other hand, the films obtained on glass slide dipped into the PVA–cobalt chloride solution immediately after mixing, showed a distinctly different microstructure (Figure 1 c). In this case, a nacreous struc-ture of cobalt salt aligned in rows perpendicular to poly-mer crystals was obtained. This also resembl d the nacreous growth of P. martensii lamellae composed of aragonite crystals9. Both the above morphologies have been synthesized through the biomimetic route for the first time substituting CaCO3 by cobalt salt. Figure 2. a, EDAX analysis of the cobalt salt precipitate; b, XRD of the sample with ring-shaped aggregates of cobalt salt in polymer. b a Figure 3. a, g-iron oxide particles having growth arrangement similar to aragonite spherulites in regenerated shell of P. palud sa;b, XRD of the produced g-iron oxide particles.  b a RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 79, NO. 5, 10 SEPTEMBER 2000 648 In a recent publication, crystallization of a hexamer duplex [d (TGCGCG) and d(CGCGCA)] in similar ring-shaped crystals of hexadeoxyribonucleotide has been  reported in the presence of cobalt hexamine11. However, the EDAX analysis has indicated that the ring-shaped samples as well as rod-shaped samples produced by  the present investigators are rich in cobalt and chlorine (Figure 2 a). XRD of the ring-shaped aggregates encapsu-lated in polymer confirmed the presence of CoCl2 and CoCl2×2H2O phases embedded in PVA matrix (Figure 2 b). However, the XRD of the film coated on the glass slide showed a diffused broad peak characteristic of the glassy substrate. In the case of the films of the PVA–FeSO4 system, g-Fe2O3 formed with a range of morpho-logies extending from diffusion-limited aggregates to spherulite structures. Of these, the spherulite-like morphology of g-Fe2O3 in PVA resembled aragonite spherulites in the regenerated shell of P. paludosa9 (Figure 3 a). Formation of the phase was confirmed by XRD which showed the presence of g-Fe2O3 as a major phase along with a-Fe2O3 (Figure 3 b).  There are several reports available in the literature on the synthesis of agglomeration-free fine-sized transition metal powders using PVA and other hydrophilic poly-mers12,13. However, the present study highlights the possi-bility of producing inorganic materials with morphologies similar to those observed in living organism through biomimetic synthesis. This can be exploited to develop materials for magnetic and humidity sensors with high sensitivity.   1. Calvert, P. and Mann, S., J. Mater. Sci., 1988, 23, 3801–3815. 2. Addadi, L. and Weiner, S., Angew. Chem., Int. Ed. Engl., 1992, 31, 153–169. 3. Berman, A. and Charychi, D., Adv. Mater., 1999, 11, 296–300. 4. Mann, S., Heywood, Brig, R., Rajam, S., Derck, J. and Birchall, Nature, 1988, 334, 692–695. 5. Sobon, C. A., Bowen, H. K., Broad, A. and Calvert P., J. Mater. Sci. Lett., 1987, 6, 901–904. 6. Baskaran, S., Song, L., Liu, J., Chen, Yuan L. and Graff, Gordon L., J. Am. Ceram. Soc., 1998, 81, 401–408. 7. Mel, L., Schnablegger, H. and Mann, S., Nature, 1999, 402, 393–395. 8. Joshi, S. S., Patil, S. F., Iyer, V. and Mahamuni, S., Nanostruct. Mater., 1998, 10, 1135–1144. 9. Watabe, N., J. Crystal Growth, 1974, 24/25, 116–122. 10. Freitag, W. O., Sharp, T. A., Baltz, A. and Suchodolski, V., J. Appl. Phys., 1979, 50, 7801–7803. 11. Satheesh Kumar, P. and Gautham, N., Curr. Sci., 1999, 77, 1076–1078. 12. Savinova, E. R., Chuvilin, A. L. and Parmon, V. N., J. Mol.Catal., 1988, 48, 217–229. 13. Hirai, H., Wakabayashi, H. and Komiyama, M., Bull. Chem. Soc. Jpn., 1986, 59, 367–372.  Received 21 February 2000; revised accepted 5 July 2000   Detection of areal snow cover changes over Antarctica using SSM/I passive microwave data T. J. Majumdar* and K. K. Mohanty Marine and Water Resources Division, Remote Sensing Applications Area, Space Applications Centre (ISRO), Ahmedabad 380 015, India SSM/I (Special Sensor Microwave Imager) passive microwave data of two different months over the  Antarctic subcontinent have been used to (i) detect  the areal extent of snow cover, and (ii) to monitor the changes in the snow cover in this region. Minimum accumulation of snow over Antarctica occurs in Feb-ruary, and hence the detection of snow cover during January and February should indicate a depletion phase. An overall 14% decrease of snow has been  observed during this period. Corresponding data for accumulation/depletion of snow indicates a matching of general trend. SSM/I data, thus, can be used as a very promising tool for monitoring of snow over the Antarctic region since it is not affected by cloud cover/darkness. ANTARCTICA is the fifth largest continent on the earth (area approximately 14 million sq km), which surrounds the South Pole. It is almost totally covered by ice, devoid of any permanent human population. Ice-c vered region over Antarctica constitutes almost 4.5% of the earth’s surface area. It is also the coldest continent with ambient air temperature varying much below the freezing point of water (– 80°C to – 30°C) throughout the year. It is enriched with valuable minerals, coal and petroleum. Antarctica contains about 90 per cent of the world’s ice, and if melted, it would raise the ocean by around 60 m. Antarc-tic  is also the highest continent on the earth, because of the massive thickness of ice which overlays the land. The thickness of ice at places is beyond 4500 m (average thickness is approximately 2100 m). Snow and sea ice  are dynamic geophysical features on the earth’s surface. Together with land ice, they influence the local and global climate and oceanic regimes. Moreover, polar ice masses, and the ice sheets in particular, are sensitive indicators of climate change, which it is feared may result from  an increase in atmospheric ‘greenhouse gases’ such as methane and CO2,, the latter caused by burning of fossil fuels. Study on ozone hole over Antarctica has attracted global attention due to its multifaceted impact over global warming and related hazards due to increased penetration of UV rays on the earth through the ozone hole. The  seasonal sea ice covers of both polar regions may react to an increase in atmospheric temperature by reducing their areal extent1. Based on the study with Nimbus 7 (USA)  *For correspondence. (e-mail: rajesh_sathya@hotmail.com)  

Synthesis of organized inorganic crystal assemblies

Organized crystalline assemblies of cobalt salt and &#947;- iron oxide have been produced by in situ matrixmediated biomimetic route. The process makes use of an organized supramolecular matrix and produces cobalt chloride crystals with characteristic morphology of coccolith of alga and nacreous structure of Pinctada martensii. Crystals of &#947;-iron oxide have been produced with typical morphology of aragonite spherulites in regenerated shell of Pomaceae paludosa

Das, Swapan Kumar

Rao, V.

Ramachandrarao, P.

Publications of the IAS Fellows

RESEARCH COMMUNICATIONS 
CURRENT SCIENCE, VOL. 79, NO. 5, 10 SEPTEMBER 2000 646
Synthesis of organized inorganic  
crystal assemblies 
Arvind Sinha*, Swapan Kumar Das, V. Rao and 
P. Ramachandrarao 
National Metallurgical Laboratory, Jamshedpur 831 007, India 
Organized crystalline assemblies of cobalt salt and g-
iron oxide have been produced by in situ matrix-
mediated biomimetic route. The process makes use of 
an organized supramolecular matrix and produces 
cobalt chloride crystals with characteristic morpho-
logy of coccolith of alga nd nacreous structure of 
Pinctada martensii. Crystals of g-iron oxide have been 
produced with typical morphology of aragonite spher-
ulites in regenerated shell of Pomaceae paludosa. 
NATURE has synthesized many inorganic crystals in living 
organisms with the help of biopolymers. The natural route 
consists of pre-organization of a supramolecular matrix, 
mineralization of it through interfacial molecular recogni-
tion leading to the nucleation, growth and organization of 
the inorganic phase. A signature of the natural synthesis is 
the production of the same inorganic ph se with several 
distinctly different morphologies. For example, calcium 
carbonate crystals can grow in the form of plates, rods  
or complex tubules1. Recent efforts in the laboratory, to 
duplicate this natural method of natural synthesis, have 
come to be known as biomimetic methods for synthesis2. 
An understanding of the mechanism involved in such  
matrix-mediated synthesis has been recognized to be of 
great potential in the production of engineering materials. 
Thus, catalyst particles of controlled size and morpho-
logy, magnetic materials with appropriate anisotropy, 
highly porous materials, composites and well-organized 
crystallite assemblies can be produced through bio-
mimetic synthesis. A variety of technically important  
inorganic materials such as CaCO3, CdS, TiO2, Fe2O3, 
Fe3O4, nano-sized copper and silver have been produced 
by the biomimetic route3–8. While these materials exhi-
bited much of the sophistication expected in terms of 
shape, size and orientation, however, complex and un-
usual morphologies found in living organisms could not 
be produced. The present communication reports the syn-
thesis of cobalt chloride and iron oxide particles with 
morphologies that are characteristic of the coccolith of 
alga, nacreous structure of P. martensii and aragonite 
spherulites in the regenerated shell of P. paludosa9. In 
this respect, these results open up the exciting possibility 
of imitating nature more closely through the control of 
experimental conditions and producing several morpho- 
logies of the inorganic material in the same system. 
 In the present investigation polyvinyl alcohol (PVA) 
has been used as the supramolecular matrix. Being a water- 
soluble polymer with a monoclinic crystal structure, PVA 
having rigid carbon backbone and inter-chain hydrogen 
*For correspondence. (e-mail: arvind@csnml.ren.nic.in) 
Figure 1. a, Random distribution of cylindrical-shaped cobalt salt 
particles in a supernatant PVA film; b, Growth of cobalt salt precipitate 
with morphology typical of coccolith of algae; c, Cobalt salt precipitate 
with morphology similar to nacreous structure of P. martensii. 
 
 
RESEARCH COMMUNICATIONS 
CURRENT SCIENCE, VOL. 79, NO. 5, 10 SEPTEMBER 2000 647
bonds, has the ability to solvate. It can also undergo 
cross-linking and a series of gelation reactions. The gel 
can be crystalline and the degree of crystallinity is a func-
tion of time. The presence of inorganic salts is also known 
to enhance gelation. Aqueous solutions containing differ-
ent concentrations of PVA were produced by dissolving 
the polymer in deionized water at 90°C. The concentra-
tions varied between 2.5 wt% and 10 wt% of PVA. Two 
different inorganic salts, viz. CoCl2· 6H2O and FeSO4· 7H2O 
were employed in the present study. The salts were dissol-
ved in deionized water at room temperature to prepare 0.5 
to 1.0 molar solutions and were filtered. Salt solutions 
were mixed with the aqueous solution of PVA in various 
proportions, stirred and allowed to react for a time period 
extending from 15 min to 15 days. Samples were drawn at 
regular intervals either by dipping a clean glass slide or 
by lifting the supernatant layer after the lapse of about a 
week or more. In the case of iron sulphate solutions, the 
polymer films with iron salt were reacted with a cold  
solution of sodium borohydride and dried under con-
trolled oxygen atmosphere. The borohydride reduces the 
iron salt to the metallic state through a combined process 
of hydrolysis and reduction10. The nano-sized metal parti-
cles were then re-oxidized at room temperature to yield g-
Fe2O3 in a single step. The films so obtained were exam-
ined under scanning electron microscope (SEM). The 
The composition and structure of the film constituent 
w re confirmed by X-ray diffraction (XRD). 
 The pink-coloured supernatant films as well as the films 
coated on glass slides containing cobalt salt developed 
localized regions of blue colour under the polymer sur-
face, when preserved in dry atmosphere at room tempera-
ture for several days. The SEM study of the films 
containing cobalt salt showed randomly distributed 
particles with a hollow cylindrical morphology (Figure 1 a). 
Examination of the film at a higher magnification showed 
that these particles are the aggregates of crys als arranged 
in a circle around a rim of PVA (Figure 1 b). This mor-
phology is identical to the coccolith of the alga, cocco-
lithus huxleyi, made of radial growth of calcite in a 
biopolymer9. On the other hand, the films obtained on 
glass slide dipped into the PVA–cobalt chloride solution 
immediately after mixing, showed a distinctly different 
microstructure (Figure 1 c). In this case, a nacreous struc-
ture of cobalt salt aligned in rows perpendicular to poly-
mer crystals was obtained. This also resembl d the 
nacreous growth of P. martensii lamellae composed of 
aragonite crystals9. Both the above morphologies have 
been synthesized through the biomimetic route for the 
first time substituting CaCO3 by cobalt salt. 
Figure 2. a, EDAX analysis of the cobalt salt precipitate; b, XRD of 
the sample with ring-shaped aggregates of cobalt salt in polymer.
 
b 
a 
Figure 3. a, g-iron oxide particles having growth arrangement 
similar to aragonite spherulites in regenerated shell of P. palud sa;
b, XRD of the produced g-iron oxide particles. 
 
b 
a 
RESEARCH COMMUNICATIONS 
CURRENT SCIENCE, VOL. 79, NO. 5, 10 SEPTEMBER 2000 648
 In a recent publication, crystallization of a hexamer 
duplex [d (TGCGCG) and d(CGCGCA)] in similar ring-
shaped crystals of hexadeoxyribonucleotide has been  
reported in the presence of cobalt hexamine11. However, 
the EDAX analysis has indicated that the ring-shaped 
samples as well as rod-shaped samples produced by  
the present investigators are rich in cobalt and chlorine 
(Figure 2 a). XRD of the ring-shaped aggregates encapsu-
lated in polymer confirmed the presence of CoCl2 and 
CoCl2×2H2O phases embedded in PVA matrix (Figure 
2 b). However, the XRD of the film coated on the glass 
slide showed a diffused broad peak characteristic of the 
glassy substrate. In the case of the films of the PVA–
FeSO4 system, g-Fe2O3 formed with a range of morpho-
logies extending from diffusion-limited aggregates to 
spherulite structures. Of these, the spherulite-like morphology 
of g-Fe2O3 in PVA resembled aragonite spherulites in the 
regenerated shell of P. paludosa9 (Figure 3 a). Formation 
of the phase was confirmed by XRD which showed the 
presence of g-Fe2O3 as a major phase along with a-Fe2O3 
(Figure 3 b). 
 There are several reports available in the literature on 
the synthesis of agglomeration-free fine-sized transition 
metal powders using PVA and other hydrophilic poly-
mers12,13. However, the present study highlights the possi-
bility of producing inorganic materials with morphologies 
similar to those observed in living organism through 
biomimetic synthesis. This can be exploited to develop 
materials for magnetic and humidity sensors with high 
sensitivity. 
 
 
1. Calvert, P. and Mann, S., J. Mater. Sci., 1988, 23, 3801–3815. 
2. Addadi, L. and Weiner, S., Angew. Chem., Int. Ed. Engl., 1992, 
31, 153–169. 
3. Berman, A. and Charychi, D., Adv. Mater., 1999, 11, 296–300. 
4. Mann, S., Heywood, Brig, R., Rajam, S., Derck, J. and Birchall, 
Nature, 1988, 334, 692–695. 
5. Sobon, C. A., Bowen, H. K., Broad, A. and Calvert P., J. Mater. 
Sci. Lett., 1987, 6, 901–904. 
6. Baskaran, S., Song, L., Liu, J., Chen, Yuan L. and Graff, Gordon 
L., J. Am. Ceram. Soc., 1998, 81, 401–408. 
7. Mel, L., Schnablegger, H. and Mann, S., Nature, 1999, 402, 393–
395. 
8. Joshi, S. S., Patil, S. F., Iyer, V. and Mahamuni, S., Nanostruct. 
Mater., 1998, 10, 1135–1144. 
9. Watabe, N., J. Crystal Growth, 1974, 24/25, 116–122. 
10. Freitag, W. O., Sharp, T. A., Baltz, A. and Suchodolski, V., J. 
Appl. Phys., 1979, 50, 7801–7803. 
11. Satheesh Kumar, P. and Gautham, N., Curr. Sci., 1999, 77, 1076–
1078. 
12. Savinova, E. R., Chuvilin, A. L. and Parmon, V. N., J. Mol.
Catal., 1988, 48, 217–229. 
13. Hirai, H., Wakabayashi, H. and Komiyama, M., Bull. Chem. Soc. 
Jpn., 1986, 59, 367–372. 
 
Received 21 February 2000; revised accepted 5 July 2000 
 
 
Detection of areal snow cover changes 
over Antarctica using SSM/I passive 
microwave data 
T. J. Majumdar* and K. K. Mohanty 
Marine and Water Resources Division, Remote Sensing Applications 
Area, Space Applications Centre (ISRO), Ahmedabad 380 015, India 
SSM/I (Special Sensor Microwave Imager) passive 
microwave data of two different months over the  
Antarctic subcontinent have been used to (i) detect  
the areal extent of snow cover, and (ii) to monitor the 
changes in the snow cover in this region. Minimum 
accumulation of snow over Antarctica occurs in Feb-
ruary, and hence the detection of snow cover during 
January and February should indicate a depletion 
phase. An overall 14% decrease of snow has been  
observed during this period. Corresponding data for 
accumulation/depletion of snow indicates a matching 
of general trend. SSM/I data, thus, can be used as a 
very promising tool for monitoring of snow over the 
Antarctic region since it is not affected by cloud 
cover/darkness. 
ANTARCTICA is the fifth largest continent on the earth 
(area approximately 14 million sq km), which surrounds 
the South Pole. It is almost totally covered by ice, devoid 
of any permanent human population. Ice-c vered region 
over Antarctica constitutes almost 4.5% of the earth’s 
surface area. It is also the coldest continent with ambient 
air temperature varying much below the freezing point of 
water (– 80°C to – 30°C) throughout the year. It is enriched 
with valuable minerals, coal and petroleum. Antarctica 
contains about 90 per cent of the world’s ice, and if 
melted, it would raise the ocean by around 60 m. Antarc-
tic  is also the highest continent on the earth, because of 
the massive thickness of ice which overlays the land. The 
thickness of ice at places is beyond 4500 m (average 
thickness is approximately 2100 m). Snow and sea ice  
are dynamic geophysical features on the earth’s surface. 
Together with land ice, they influence the local and global 
climate and oceanic regimes. Moreover, polar ice masses, 
and the ice sheets in particular, are sensitive indicators of 
climate change, which it is feared may result from  
an increase in atmospheric ‘greenhouse gases’ such as 
methane and CO2,, the latter caused by burning of fossil 
fuels. Study on ozone hole over Antarctica has attracted 
global attention due to its multifaceted impact over global 
warming and related hazards due to increased penetration 
of UV rays on the earth through the ozone hole. The  
seasonal sea ice covers of both polar regions may react to 
an increase in atmospheric temperature by reducing their 
areal extent1. Based on the study with Nimbus 7 (USA)  
*For correspondence. (e-mail: rajesh_sathya@hotmail.com) 
 


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Synthesis of organized inorganic crystal assemblies

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