Biomimetic Synthesis and Antibacterial
Characteristics of Magnesium
Oxide–Germanium dioxide
Nanocomposite Powders
C. P. AVANZATO, J. M. FOLLIERI AND I. A. BANERJEE*
Department of Chemistry, Fordham University, 441East Fordham Road
Bronx, New York 10458, USA
K. R. FATH
Department of Biology, The City University of New York and The Graduate Center
Queens College, 65-30 Kissena Blvd, Flushing, New York 11367, USA
ABSTRACT: The efficiency of growth of nanocrystalline magnesium oxide–
germanium dioxide nanocomposite powders at room temperature was investigated
in the presence of the amino acids histidine, aspartic acid, and the biopolymer polyL-lysine under varying conditions. It was observed that of the three, poly-L-lysine
and histidine were more efficient and formed higher yields of the products.
The growth of the nanocomposties was found to be pH-sensitive as indicated by zeta
potential as well as TEM, and dynamic light scattering analyses. Furthermore, the
nanocomposite powders were found to have significant antibacterial activity against
Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus)
bacteria.
KEY WORDS: magnesium oxide, nanocomposites, germania, amino acids.
INTRODUCTION
VER THE PAST decade, metal oxide nanocomposites have been attracting significant
attention due to their various potential applications in catalysis, electronics,
photonics, and sensors [1–8]. In addition to alumina, silica, and titania, germania has
been gaining tremendous importance mainly due to its enhanced reactivity and optical
properties [9,10]. Germania–silica hybrid materials, Ndþ3-doped germaina glasses,
and erbium-doped germania have been prepared for optical device fabrications [11–13].
Most methods employed for the synthesis of germanium oxide-based materials, however,
O
*Author to whom correspondence should be addressed. E-mail: [email protected]
Figure 4 appear in color online: http://jcm.sagepub.com
Journal of COMPOSITE MATERIALS, Vol. 43, No. 8/2009
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DOI: 10.1177/0021998308103158
ß SAGE Publications 2009
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involve the use of high-temperature conditions [14,15], which sometimes may lead to
sintering. Thus, milder methods for the synthesis of such nanomaterials would be
beneficial. Recently, germanium oxide was synthesized under mild conditions using
bioinspired methods [16–18]. The methods involved the use of specific peptide sequences
derived from the combinatorial phage-library or amino acids that helped in the efficient
mineralization of the materials. Nanocomposite powders of germanium oxide integrating
gold, palladium, tin oxide nanoparticles have also been prepared using biomimetic
methods [19,20]. Although the catalytic and optical applications of germanium oxide
nanocomposite powders are well known, research related to the antimicrobial activities of
germanium oxide nanocomposite powders is relatively sparse.
In this work we have explored the development of a new material, by preparing
nanocomposite powders of magnesium oxide–germania using amino acid catalysts. In the
past, magnesium oxide nanoparticles and microparticles have been used as a reinforcing
reagent, as well as a component in super conductors [21]. In addition, their high surface
reactivity, chemical, and thermal stability is desirable for applications in sensors, catalysis,
and additives [22,23]. Recently, the antimicrobial properties of MgO nanoparticles were
studied and they were found to be highly effective against bacteria. They have also been
found to be potent in toxic waste remediation [24–28]. In order to prepare new materials
with enhanced properties, composites of MgO such as alumina–magnesia, silica–magnesia
mixed oxides have been prepared by sol–gel methods and have been found to function as
efficient supports for various catalytic applications [29,30]. It has also been suggested that
magnesia or mixtures of MgO with other refractory inorganic oxides may have improved
properties for various applications [31]. Since both magnesium oxide and germanium
compounds possess antimicrobial properties [32], it would be interesting to study those
properties by preparing nanocomposite powders of germanium oxide–magnesium oxide.
In this article, the efficacy of growth of the magnesium oxide–germanium oxide
nanocomposite powders using biomimetic methods was studied in the presence of aspartic
acid, histidine, and poly-L-lysine. The biological approach used in this work is a mild
method of preparation of such nanocomposite powders, which upon calcination lead
to highly crystalline materials. Further, we examined the antibacterial effects of
nanocomposites of magnesia–germania against both Gram-negative and Gram-positive
bacteria. To our knowledge, this is the first time that nanocomposite powders of magnesium
oxide–germanium oxide have been prepared by biomimetic methods and tested for
antimicrobial properties. Such new materials may not only be potentially useful for catalysis,
sensors, and optics, but can also be used as antimicrobials for environmental remediation.
EXPERIMENTAL
Materials
Germanium (IV)–methoxide, magnesium methoxide, histidine, aspartic acid, and
poly-L-lysine were purchased from Sigma-Aldrich and used as received. Buffer solutions
of various pH were purchased from Fisher Scientific. Bacterial strains Escherichia coli
(XL-1 blue strain) and Staphylococcus aureus were a gift from the laboratory stock of the
Microbiology Laboratory at Queens College, CUNY. Other materials for bacteria
cultivation such as agar, yeast extract, Luria broth, sodium chloride, tryptone, petri dishes
were bought from Gibco or Sigma Aldrich.
Characteristics of Magnesium Oxide–Germanium dioxide Nanocomposite Powders
899
Methods
PREPARATION OF MAGNESIA–GERMANIA
The nanocomposite powders were prepared by adding 20 mL (0.1 M) magnesium
methoxide and germanium methoxide (10 mL) in methanol (2:1 molar ratio) to the
solutions of the amino acids (0.1 M) in respective buffer solutions (pH 4–9) (500 mL) every
15 min for 2 h under constant stirring. The magnesium oxide–germania nanocomposite
powders were thus prepared by co-gelling the precursors in the presence of the amino acids
and the biopolymer poly-L-lysine. The samples were vortexed slowly for 3 h, centrifuged at
4500 rpm, and washed twice with nanopure water. Finally, the resultant products obtained
were calcined in an inert atmosphere at 5008C for 3 h. All experiments were done in triplicates.
ANTIBACTERIAL ACTIVITY STUDIES
A single colony of E. coli (XL-1 blue strain) or S. aureus was grown overnight in Luria
Broth (LB) at 378C with shaking until late log phase. Then 100 mL of bacteria was
transferred into 30 mL of LB and then 2 mL of that were placed in separate sterile tubes and
100, 200, or 300 mL of various nanocomposite powders that were 50 mg/mL in doubledistilled water were added. As diluent controls, to a series of LB tubes we added 100, 200, or
300 mL of double-distilled water. All tubes were then incubated at 378C with shaking for
approximately 16 h. We determined the relative number of bacteria in the cultures by
measuring the turbidity of three separate 100 mL aliquots in 96-well plates at 630 nm light in
a BioTek plate reader.
Because LB contains many components we also tested the effects of incubating
nanocomposite powders with bacteria in water. We grew overnight cultures of E. coli and
S. aureus as before, but 100 mL was pelleted and re-suspended in 30 mL. We added 100,
200, or 300 mL solutions of various nanocomposite powders that were 50 mg/mL in
double-distilled water and then incubated at 378C with shaking for approximately 16 h. To
assay for the viability of the bacteria, 50 mL was spread onto the surface of LB agar plates
and incubated at 238C and tested. The number of viable cells in the sample was determined
by choosing the appropriate dilution of the sample onto LB agar plates and counting
colonies that appeared on the plates under a microscope. The average number of viable
cells was obtained by averaging the numbers in three replicate plates.
CHARACTERIZATION
Transmission Electron Microscopy (TEM): The particle sizes and morphology of the
samples were analyzed by TEM (JEOL 120 EX operated at 100 kV). The samples were
vortexed and 5 mL of the samples were pipetted on to a carbon-coated copper grid, dried
over night, and analyzed at various magnifications.
X-ray Diffraction: Powder X-ray diffraction studies were carried out using a PANalytical
X’Pert Pro diffractometer with the CuK radiation 180 (l ¼ 1.55 A8) source and equipped
with a diffractometer beam monochromator at 258C.
EDS (Electron Dispersive X-ray Analysis): The composition of the nanocomposite powders
was analyzed by an SEM (Amray 1000), equipped with EDS. For sample preparation, for
SEM–EDS, a few drops of the sample solution (in ethanol) were pipetted out of each sample
onto MCE filter paper (5 mm) with Whatman filter paper underneath to aid in absorbing the
fluids. The powders quickly built up on top of the MCE filter paper, which was air-dried.
Portions of the stained MCE filter paper were then cut out and mounted on SEM sample
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stubs with double-sided tape and carbon coated. The samples were then examined under the
SEM by EDS. The SEM was set at 20 keV, at a 338 sample tilt.
Dynamic Light Scattering (DLS) and Zeta Potential Measurement: A NICOMP 380 ZLS
zeta potential/particle sizer system (Santa Barbara, California, USA) was used to confirm
the charge and particle sizes of the samples. The samples were diluted with nanopure water
at room temperature, before measurements for the DLS analysis. Measurements of
-potential of the nanocomposite powders were done at 258C and at different pH values.
The concentrations of the suspensions were adjusted within the operational limits of the
instrument and the suspension pH was adjusted by the addition of standard buffer
solutions. The reported -potential values are the average of at least five measurements
(spread of values 5% of the reported mean values).
Infrared Spectroscopy: Fourier transform IR (FT-IR) spectra of the nanocomposite
powders were recorded using Mattson Infinity FTIR infrared spectrophotometer in the
range of 4000–400 cm1. Samples were dried at room temperature and then KBr pellets
were prepared. The transmittance of the samples was measured.
RESULTS AND DISCUSSION
The preparation of magnesia–germania nanocomposite powders was carried out by
biomimetic methods in the presence of a basic amino acid (histidine), an acidic amino acid
(aspartic acid), and a biopolymer (poly-L-lysine). In addition to being pH-sensitive,
poly(L-lysine) can form -helix, -sheet, or random coil conformation, though it was
recently shown that the transformation of the conformations is relatively inhibited within
silica matrices [33,34]. Since the amino acids as well as poly-L-lysine are all pH-sensitive, the
growth of the nanomaterials was examined at different pH. The magnesium oxide–germanium dioxide nanocomposite powders were prepared by co-gelling germanium methoxide
and magnesium methoxide in the presence of the amino acids. The pH was varied from
4 to 9. In general, in order to prepare metal oxide nanoparticles, one of the common
methods to make the solution basic is by the addition of ammonium hydroxide, or other
reagents but this method, does not necessarily allow for significant control on the growth of
the nanoparticles [35]. Our method is a mild biological method, which allows for the efficient
formation of crystalline nanocomposite powders with controlled size without the addition
of external additives such as ammonium hydroxide or sodium hydroxide. It was observed
that highest yields of product (Figure 1), was obtained in the presence of poly-L-lysine after
2 h of reaction time. Overall, it was observed that higher yields were obtained under basic
pH conditions for poly-L-lysine and histidine, while aspartic acid gave relatively higher
yields under acidic conditions. Even though the mechanism of the amino acid catalysis may
not be clear yet, the amino acids appear be involved in a two-step bio-mineralization process
and most likely influence the hydrolysis and polycondensation that provide the initial nuclei
for precipitation. Therefore the relative rates of hydrolysis and condensation also influence
the size of nuclei, and consequently the final precipitate morphology [36]. The solution
became instantly turbid under basic conditions in the presence of both histidine and
poly-L-lysine and a white precipitate progressively formed. Thus, the amino acids may be
increasing condensation rates at higher pH, leading to the formation of polycondensed
species. The imidazole ring of histidine has two nitrogens. One of the nitrogens is bound
to hydrogen and donates its lone pair to the aromatic ring and as such is slightly acidic,
while the other donates only one electron to the ring, hence has a free lone pair and is basic.
Characteristics of Magnesium Oxide–Germanium dioxide Nanocomposite Powders
901
100
80
pH 4
pH 9
% Yield
60
40
20
0
With aspartic acid
With histidine
With poly-L lysine
Figure 1. Yields of magnesia–germania nanocomposite powders obtained in the presence of the amino
acids at pH 4 and 9.
In the case of poly-L-lysine, additional NHþ
3 groups are present instead of the imidazole
ring system. Although protonated amino groups are generally not considered highly
catalytic but as the pH is increased and deprotonation occurs, electrostatic interactions
increase due to the availability of the lone pairs of electrons from nitrogen, leading to more
polycondensation.
The mechanism for gelation in the presence of aspartic acid seems to be different due
to the presence of the extra carboxyl group. Under acidic conditions, it can donate a proton
to the alkoxide group thus, the electron density is withdrawn from the Mg or Ge atom,
making it more electrophilic. Previous studies with propionic acid and acetic acid, have also
revealed similar interactions of the organic acids with germanium oxide [37].
The mechanism may be similar, and it is likely that the species goes through a short-lived
transition state, and then releases the alcohol group. In all probability the next step involves
the formation of networks of MgO–GeO2. Under basic conditions, even though the amino
group is deprotonated, the presence of two negatively charged –COO groups most likely
repels the negatively charged MgO and GeO2 groups thus causing less interactions and
consequently less product. Detailed kinetic studies would need to be performed to further
elucidate the exact mechanism. For silicates, it has been reported that in the presence of
amines, Si expands its coordination sphere to a penta-coordinate intermediate (a structure
similar to that found in silatranes) [38,39] and the mechanism includes a penta-coordinate
intermediate and a hexa-coordinated transition state. However, it is yet to be determined
whether the process is similar for Ge-based materials. Further studies exploring the exact
mechanism of the formation are being conducted.
The size and morphology of the MgO–GeO2 nanocomposite powders formed were
analyzed via TEM and AFM. Figure 2 shows the TEM images obtained in the presence of
poly-L-lysine. It was observed that pH played a significant role in the shapes and sizes of
the nanocomposite powders obtained in the case of poly-L-lysine catalyzed products.
At low pH, the particles were larger in size, (in the range of 100–150 nm) lower in yield
and had varied shapes such as rhombic or cubic shapes. However, at high pH, smaller
(30–50 nm) spherical particulates, which formed networks were observed. The electron
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Figure 2. TEM image of magnesia-germania formed in the presence of poly-L-lysine at (a) pH 4; (b) at pH 9.
diffraction patterns of the nanocomposite powders indicate that they are polycrystalline.
The trend was similar in terms of yields of nanocomposite powders obtained in the
presence of hisitidine, except that we did not observe cubic-shaped particles, and most of
the nanocomposites were spherical and did not vary significantly in size or in shape
irrespective of pH (data not shown). In the presence of aspartic acid, yields were relatively
low under basic conditions, but slightly higher in the case of acidic conditions and all
nanocomposites were spherical in shape. While the exact mechanism for the shape control
of the nanocomposite powders at low pH in the case of poly-L-lysine is not known at this
point, it appears that poly-L-lysine at low pH, acts as a template and wraps around the
nuclei, allowing for the reaction to go slower and growth on the 111 face of the
nanocomposites, leading to the cubic shapes. Further, it is not likely that the poly-L-lysine
itself aggregates at low pH.
DLS and Zeta Potential Analysis
The size range of the nanocomposite particulates obtained was further confirmed by
DLS analyses. Figure 3(a) shows the size range of magnesia–germania nanocomposite
powders formed at pH 4 in the presence of histidine. In general, particulates whose mean
diameters ranged from 125 to 135 nm were observed at low pH and at high pH a size range
of 35–65 nm were observed in the presence of histidine and poly-L-Lysine. For aspartic
acid, the mean diameter of particulates obtained irrespective of pH was found to be
around 80 nm. The effect of pH on the amino acid catalyzed growth of the nanocomposite
powders was also examined by the results obtained from measurements of the -potential.
The electrostatic interactions exerted between the negative sites of the metal oxide surfaces
and the positively charged protonated amino groups of the amino acids were found to be
largely dependent upon the pKa values of the amino acid side chains. The variation of
Characteristics of Magnesium Oxide–Germanium dioxide Nanocomposite Powders
(a)
REL.
903
INTENS-WT NICOMP DISTRIBUTION
100
80
60
40
20
0
50
200
100
500
(b)
1K
2K
5K
Poly-L- lysine
30
Nanocomposite grown in poly-L- lysine
Histidine
23
Nanocomposite grown in histidine
Aspartic acid
Zeta potential (mV)
16
Nanocomposite grown in the presence of aspartic acid
9
2
5
6
7
pH 8
9
10
−5
−12
−19
−26
Figure 3. (a) Dynamic light scattering analysis of nanocomposite powders obtained in the presence of
histidine at pH 4; (b) comparison of zeta potentials of amino acids before and after formation of
nanocomposites at various pH.
-potential with pH (Figure 3(b)) indicates that by and large, the zeta potentials are
positive in acidic conditions but negative in basic conditions. The point of zero zeta
potential was obtained at about pH 6.3 for the histidine samples, which is close to the pKa
value of 6.1–6.3 for the imidazole group in histidine. Below pH 7, the zeta potentials of the
nanocomposites increased almost linearly with the decrease of solution pH values from
7 to 4 (further lower pH was not tested as it may decrease the stability). This can be
attributed to the protonation of the amino groups (i.e., from –NH2 to NHþ
3 ). From
pH 7 to 10, the negative zeta potentials of the nanocomposite powders, however, did not
appear to decrease significantly with the increase of pH and seemed to level off after pH 8.
This may indicate that the amino groups were not completely deprotonated under these
pH conditions (from –NH2 to –NH). After pH 7, in all cases the zeta potential values
were found to be negative.
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XRD Analysis
The overall crystallinity of the nanocomposite powders was studied by X-ray diffraction
analysis. Figure 4 shows the diffraction pattern of a nanocomposite sample, obtained
using poly-L-lysine at pH 9. In general, crystallization of germanium dioxide sets in at
higher temperatures [14] and this phenomenon was observed in the nanocomposite
samples as well. The peaks observed, combined with the intensity of the 101 peak at 90,000
counts/s, indicates that the particles are of high purity. The characteristic peaks for MgO
(2 ¼ 39.5); (2 ¼ 441.8) for (111) and (200), respectively and that of GeO2 at (2 ¼ 20.5);
(2 ¼ 23.4) for (100) and (101) are observed confirming the formation of the
nanocomposite powders [40,41]. Further, we utilized the Debye–Scherrer formula [42]
(a)
PA010220080027 001,CAF
Counts
Germania
(101)
90000
40000
Germania
(100)
Magnesia
Magnesia (200)
Magnesia
(111)
(110)
10000
Germania
(202)
Magnesia
(220)
0
10
20
30
50
40
60
2θ (°)
(b)
Mg
Ge
Ge
0
5
10
15
20
Figure 4. (a) XRD analysis of calcined nanocomposite powder obtained in the presence of poly-L-Lysine at
pH 9; (b) EDS analysis of calcined nanocomposite powder obtained in the presence of poly-L-Lysine at pH 9.
Characteristics of Magnesium Oxide–Germanium dioxide Nanocomposite Powders
905
for crystallite size determination of the nanocomposite crystallite particles obtained using
poly-L-Lysine as the catalyst (since the materials obtained in the presence of poly-L-Lysine
had maximum yield and a significant size and shape control). The Scherrer formula
is given by D ¼ (Kl)/! cos where D is the crystallite size, l is the wavelength of X-ray,
! is the width on a 2 scale, and K is a constant close to unity. Values for the coefficient
‘K’ depend on the geometry of the crystallites and may not always be consistent in the
literature, is the Bragg’s angle. Initially, Scherrer developed the formula for cubic
crystallites and obtained a value of 0.94 for K. Klug and co-workers [43] further simplified
the derivation of the Scherrer equation leading to a value of 0.89 for K. As shown by
Nanda et al. [44,45], a modified form of the equation, which utilizes a value of 0.9 for K,
can be used to estimate the diameter of spherical particles from the width of a given Bragg
reflection. For the purposes of our calculations, we have used ‘K ¼ 0.94’ for cubic
crystallites obtained at low pH and ‘K ¼ 0.9’ for spherical particles obtained at higher pH.
The (101) peak was used for crystallite size determination. Table 1 shows a comparison of
the sizes of the nanocomposite crystallites obtained using poly-L-lysine by various
methods. The results indicate that the particle sizes of the samples obtained by TEM and
DLS analyses are fairly consistent with the average crystallite size determined from the
XRD data.
The composition of the nanocomposite powders was confirmed by EDS analysis and is
shown in Figure 4(b). The elemental ratio for Mg/Ge/O was found to be 24.2/15.6/60.2
when grown in the presence of poly-L-lysine. In general, the ratios were fairly consistent
with the molar ratios of the precursors used.
Infra-red Spectroscopy
The formation of the magnesia–germania nanocomposite powders was also confirmed
by IR spectroscopy. Figure 5 shows a comparison of IR spectra of non-calcined and
calcined nanocomposite powders. The non-calcined sample (Figure 5(b)) shows broad
peaks in the region of 860–1100 and 520–550 cm1. The peaks at 520 and 550 cm1
correspond respectively to the bending vibrational mode and the stretching vibrational
modes for MgO [46]. Bands centered at 1639 and 1460 cm1 are assigned to water
and the amide II peak most likely from the left over amino acids, respectively. The broad
peaks in the range of 860–1150 cm1 are assigned to the bending mode of different
types of surface hydroxyl groups. There is a decrease in their intensity after calcination,
which overlaps with the Ge–O–Ge peak. The calcined material (Figure 5(a)) shows
a sharp peak around 960 cm1 and a short peak at 1100 cm1, which correspond to
Ge–O–Ge [47].
Table 1. Comparison of the Mean Particle Diameter of Magnesia-Germania
Obtained using poly-L-lysine.
pH
4
9
Mean diameter (nm)
calculated using
Scherrer formula
Mean diameter (nm)
using TEM analysis
Mean diameter (nm)
using dynamic light
scattering analysis
129.6
42.1
128.2
41.2
129.3
40.8
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Transmittance (%)
(a)
(b)
360
860
1360
1860
2360
Wavenumber
2860
3360
3860
cm−1
Figure 5. FTIR analysis of (a) calcined vs. (b) noncalcined nanocomposite powder.
Antibacterial Activity
The antibacterial activity of the nanocomposite powders was evaluated by examining
the inhibition of bacterial growth of E. coli (XL-1 blue strain) (Gram negative) and
S. aureus (Gram positive). In preliminary studies (data not shown), we did not observe
significant size effects of nanocomposite powders (between 50 and 100 nm) on the growth
of bacteria, hence all the samples used for antibacterial studies were those nanocomposite
powders obtained using poly-L-lysine as catalyst at pH 9. The samples were washed,
centrifuged, and calcined before testing for antibacterial activity. Various concentrations
of nanocomposite powders were examined for inhibition studies to determine the minimal
inhibitory concentration (MIC). The effect of varying concentrations of nanocomposite
powders on the percent of bacterial growth is shown in Figure 6(a). On comparing
S. aureus with E. coli, we observed that the nanocomposite powders were more effective
against S. aureus rather than E. coli at lower concentrations. At higher concentrations
(45 mg/mL), the growth of bacteria was almost completely inhibited (495%) in both
cases. The MIC for S. aureus was found to be 0.05 mg/mL where as that of E. coli was
found to be 0.25 mg/mL. This may be due to the fact that the Gram-positive bacteria are
encased in a plasma membrane covered with a thick wall of peptidoglycan, while Gramnegative bacteria are encased in a triple layer, the outermost layer being a
lipopolysaccharide. In the past it has been shown in some cases that Gram-negative
bacteria may be more resistant to chemical agents than Gram-positive bacteria [48] and it
appears to be the case for the MgO–GeO2 nanocomposite powders as well. Another set of
bactericidal tests were conducted after washing the cultures from their growth medium in
water. We observed that the bactericidal activity was slightly higher (10% higher) in
water rather than in the growth medium. This may due to the fact that the growth medium
contains proteins, which may inhibit the attachment of nanocomposite powders to the
907
Characteristics of Magnesium Oxide–Germanium dioxide Nanocomposite Powders
E. coli
S. aureus
(a)
90
% of Bacteria
75
60
45
30
15
0
0.05
0.1
0.125
0.25
0.5
2.5
5
7.5
Concentration of nanocomposites (mg/mL)
(b)
(i)
(iii)
(ii)
(iv)
Figure 6. (a) Relative percentage growth of bacteria with different concentrations of nanocomposite
powders, (b) Photographs showing bacterial colonies in plates containing (i) 0 mg/mL; (ii) 3 mg/mL; (iii) 5mg/
mL and (iv) 7 mg/mL nanocomposites.
bacterial cell walls. In general, all samples were incubated overnight (10 h) before
examining bactericidal effects. Figure 6(b) shows the growth/no growth of E-coli bacteria
that were treated with different concentrations of nanocomposite powders in water and
then spread on agar plates. The growth of bacteria is completely inhibited when higher
concentrations (45mg/mL) of nanocomposite powders were added as seen in (iii) and (iv),
while at 3 mg/mL (ii), 10 CFUs were observed. Therefore from the above studies
conducted, it can be inferred that the MgO–GeO2 nanocomposite powders have significant
bactericidal activity, and overall may be more effective against Gram-positive bacteria.
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CONCLUSIONS
In summary, we have developed a simple biological method for the preparation of
magnesia–germania nanocomposite powders at room temperature in aqueous medium by
biomimetic methods. Of the amino acids used, poly-L-lysine and histidine were found to be
the most efficient for the formation of the nanocomposite powders. Calcination of the
products led to crystalline materials. The prepared materials were analyzed by several
individual methods. The nanocomposite powders were found to be bactericidal toward both
Gram-negative as well as Gram- positive bacteria, though they were more efficient against
Gram-positive bacteria.
ACKNOWLEDGMENTS
The authors thank Dr Areti Tsiola at the Queens College Core facility for Biomolecular
Imaging for the use of the transmission electron microscope. Dr I. Banerjee thanks the
Fordham University Faculty Research Grant and Dr K. Fath thanks the Professional Staff
Congress of The City University of New York for financial support.
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