Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour of Mg(OH)<sub>2</sub> and MgO Nanoparticles

doi:10.4236/jbnb.2013.44046

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Journal of Biomaterials and Nanobiotechnology, 2013, 4, 365-373

http://dx.doi.org/10.4236/jbnb.2013.44046 Published Online October 2013 (http://www.scirp.org/journal/jbnb)

365

Effects of Precipitation Temperature on Nanoparticle

Surface Area and Antibacterial Behaviour of Mg(OH)2

and MgO Nanoparticles

Banele Vatsha1,2, Phumlani Tetyana1, Poslet Morgan Shumbula1, Jane Catherine Ngila2,

Lucky Mashudu Sikhwivhilu1, Richard Motlhaletsi Moutloali1

1Advanced Materials Division, DST/Mintek Nanotechnology Innovation Centre, Mintek, Randburg, South Africa; 2Department of

Applied Chemistry, University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa.

Email: [email protected]

Received May 28th, 2013; revised June 29th, 2013; accepted July 15th, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

A series of MgO nanoparticles were prepared by first precipitating and isolating Mg(OH)2 nanoparticles from

Mg(NO3)2 at three different temperatures using NaOH followed by their thermal decomposition also at three tempera-

ture settings. The effects of temperature at which precipitation and thermal decomposition of the hydroxide occurred

were studied to assess their influence on nanoparticle size and surface area. The synthesised nanoparticles were charac-

terized using a suite of techniques including Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Transmission

Electron Microscopy (TEM) and Scanning Electron Microscope (SEM) analysis. The average diameter range of MgO

nanoparticles ranged between 15 and 35 nm, while for the precursor Mg(OH)2 it varied between 28 and 45 nm. The

nanoparticle surface area obtained from BET studies was found in all cases to increase from 77 to 106.4 m2/g with in-

creasing temperature of precipitation. Antibacterial activities of the prepared Mg(OH)2 and MgO nanoparticles were

evaluated against the Gram-negative bacteria, Escherichia coli, and the Gram-positive bacteria, Staphylococcus aureus,

using agar diffusion method. A correlation between surface area and antibacterial activity supported the mechanism of

bacterial inactivation as the generation of reactive species. The Mg(OH)2 and MgO nanoparticles both exhibited pro-

nounced bactericidal activity towards the Gram positive bacteria than Gram negative bacteria as indicated by the extend

of the zone of inhibition around the nanoparticle.

Keywords: MgO; Nanoparticles; Precipitation; Crystallinity; Antibacterial

1. Introduction

The increased proliferation of infectious diseases due to

microorganisms found in medical devices, food packag-

ing, domestic appliances and water treatment systems has

elicited increased attention [1-3]. In the recent years, ex-

tensive efforts have been made to develop new or im-

prove known materials with antimicrobial properties.

Specifically, increased resistance of microorganisms to-

wards current biocides is of great concern specifically in

people of compromised immune systems like the elderly,

children and the sick. This has led to increased efforts to

explore new types of antimicrobial agents. In particular,

inorganic oxide nanomaterials like CaO, ZnO and MgO

have shown potential as effective alternatives in ad-

dressing some of these challenges [4-6]. For example, in

the last decade literature exploring the use of metal ox-

ides and specifically MgO has become widespread.

These studies revealed that indeed MgO is able to deac-

tivate both gram negative and gram positive bacteria and

several mechanisms of deactivation postulated [4,7]. Re-

lationship between particle size and activity was also

reported. This has led to our interest in Mg based materi-

als for applications in health and water [8].

Current research interest in inorganic nanomaterials

synthesis has focused more on controlling and tailor-

making materials into nanoparticles, nanorods, nanotubes,

nanoplates, etc. with the aim of streamlining their appli-

cations and efficiency [6,9]. Moreover, investigations to

develop versatile, cost effective and up-scalable methods

in achieving predictable nanoparticle properties are para-

mount to the success of introducing these materials for

widespread usage. These methods include sol-gel [10],

Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour

of Mg(OH)2 and MgO Nanoparticles

366

hydrothermal [11], laser vaporization, chemical gas phase

deposition and combustion aerosol synthesis [12]. How-

ever, in our investigations, there are drawbacks encoun-

tered from these conventional methods that tend to give

lower yield, poor control of shape and size and use of

corrosive chemicals. Indeed, it is generally known that

MgO’s unique properties are related to the size of the

nanoparticles [13]. Ouraipryvan et al. studied the synthe-

sis of crystalline MgO nanoparticle with mesoporous-

assembled structure via a surfactant-modified sol-gel

process [14]. The obtained results showed that the parti-

cle exhibited mesoporous structure and the diameter

range was approximately 35 to 50 nm. Continued search

for control of nanoparticle synthesis using simple meth-

ods is still relevant.

In this study the precipitation method was adopted

mainly because of its simplicity and relatively good con-

trol of the experimental conditions. The present study

was carried out to investigate the effect of precipitation

temperature on the morphology of MgO nanoparticles,

specifically the surface area, and its influence on anti-

bacterial (E. coli and S. aureus) activity. A strong corre-

lation between nanoparticle surface area and antibacterial

effects was observed giving an indirect way of establish-

ing mechanism of bacterial inactivation.

2. Experimental Section

2.1. Materials

Magnesium nitrate (Mg(NO3)2), sodium hydroxide (NaOH)

were purchased from Sigma Aldrich, South Africa. All

solutions were prepared using freshly prepared high

purity water from a Millipore unit, Q-POD. Escherichia

coli (E. coli) and Staphylococcus aureus (S. aureus)

strain were donated by Biolabels Unit at Mintek.

2.2. Characterisation Techniques

The XRD diffractograms were recorded on a Bruker D8

Advance X-ray diffractometer using a Co-Kα (1.7902Å)

monochromatic radiation source and a Ni filter with the

operating voltage and current maintained at 40 kV and 40

mA respectively in the 2



range of 5˚ - 80˚. The obtained

diffraction patterns were processed using Eva software.

Differential scanning calorimetric (DSC) analysis were

performed using air as oxidant at the heating rate of

10˚C·min −1 in a crimped aluminium crucible heated up to

a temperature of 500˚C using a Shimadzu DSC60 instru-

ment with a TA60WS thermal analyser and FC60A con-

troller. Scanning electron microspcopy (SEM) analyses

were done on a Nova NanoSEM 200 from FEI operating

at 10.0 KV. The Transmission Electron Microscopy

(TEM) images were acquired on a Philips CM120 Biot-

win Transmission Electron Microscope. The samples

were prepared by placing a drop of a dilute sample on a

carbon-coated copper grid (400 mesh, agar). The samples

were allowed to dry completely at room temperature.

Nitrogen adsorption measurements were performed at 77

K using a Micromeritics ASAP2010 system utilizing

Brunauer Emmett-Teller (BET) calculations for surface

area and BJH calculations for pore size distribution for

the desorption branch of the isotherm.

2.3. Synthesis and Characterisation of

Magnesium Oxide Nanoparticles

Mg(NO3)2 (0.2 M) and NaOH (0.4 M) solutions were

prepared using deionized water from Millipore water

purification system. Precipitation was induced by drop-

wise addition of NaOH into the Mg(NO3)2 solution under

continuous stirring for 1 hour at different reaction tem-

peratures (i.e. 23˚C, 60˚C and 85˚C) at pH 12. The re-

action mixture was cooled down to room temperature,

centrifuged and washed with copious amount of high

purity water and ethanol for effective removal of im-

purities. The final product was dried at 80˚C for 24 h and

calcined at different temperatures (i.e. 500˚C, 600˚C and

700˚C). Table 1 summarises the materials used and

reaction conditions.

2.4. Antibacterial Study Using Agar Diffusion

Method

Antibacterial activity was performed using agar diffusion

method adopted from Sundrarajan et al., [13]. Bacterial

cultures were grown overnight at 37˚C by adding a single

colony in 100 ml Luria Bertani Broth. E. coli and S.

aureus cultures (0.1 ml each) were plated out onto indi-

vidual Nutrient Agar plates using the Aseptic technique.

Holes/Wells were made on the nutrient agar inoculated

with bacteria and (about 0.5 mg) MgO nanoparticle

suspensions were decanted into the wells and the plates

Table 1. Precipitation and calcination temperatures used

for MgO nanoparticle synthesis.

Type Sample Precipitation

temperature (˚C)

Calcination

temperature (˚C)

1 23 500

A 2 23 600

3 23 700

4 60 500

B 5 60 600

6 60 700

7 85 500

C 8 85 600

9 85 700

Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour

of Mg(OH)2 and MgO Nanoparticles 367

incubated overnight at 37˚C. Finally, the diameters of

zones of inhibition around the wells were measured

against the control strain and measured with callipers.

Zone of inhibition is the area in which the bacterial

growth is stopped due to bacteriostatic effect of the com-

pound and it measures the inhibitory effect of compound

towards a particular microorganism.

3. Results and Discussion

3.1. Preparation of MgO from Mg(OH)2

Nanoparticles

The synthesis protocol employed for MgO nanoparticles

was a precipitation-calcination method. This involved

dropwise addition of sodium hydroxide into aqueous

solution of magnesium nitrate at various temperatures

between 25˚C to 85˚C to induce precipitation to form of

magnesium hydroxide. In order to finally obtain MgO

nanoparticles, the Mg(OH)2 powder was calcined at

500˚C, 600˚C and 700˚C through the decomposition of

Mg(OH)2 to MgO nanoparticles (Scheme 1).

Temperature of precipitation is one method of ma-

nipulating nanoparticle size and morphology. For exam-

ple, Yildirim and Duncan observed that the size of

spherical ZnO nanoparticles could be manipulated by the

choice of precipitation temperature (13.0 ± 1.9 nm at

25˚C and 9.0 ± 1.3 nm at 80˚C), which they attributed to

changes in the nature of adsorption events between ZnO

crystals and organic molecules used as surfactants [15].

Even though, no surfactants were used in the current

method, good control and manipulation of nanoparticle

morphologies were obtained by varying the precipitation

temperature. This indicates that precipitation temperature

might be an important factor in controlling morphologies

in instances of ionic effects rather surfactants are being

used. This will assist in lowering the overall cost associ-

ated with controlled nanoparticle synthesis. It is however

important to note that many methods and their variants

have been used to produce MgO nanoparticles [16-24].

3.2. Thermal Analysis of the Mg(OH)2

Transformation to MgO

Figure 1 shows the DSC and TGA profiles of the de-

composition and transformation of Mg(OH)2 nanoparti-

cles into MgO nanoparticles. The thermograms are do-

Scheme 1. MgO deri ved from Mg(NO3)2 precursor reaction

mechanism through the Mg(OH)2 intermediate.

Figure 1. TGA and DSC profiles of the decomposition of the

prepared Mg(OH)2 to MgO nanoparticles.

minated by one major thermal event centred on 380˚C

(DSC) with an onset at 250˚C (TGA) or 280˚C (DSC).

This onset marks the start of the decomposition of

Mg(OH)2 to MgO nanoparticles which occurs typically

during calcination of the hydroxide material [11,18,

25-27]. Other thermal events observed were in the tem-

perature regions around 100˚C, 240˚C and 465˚C. The

first event at 100˚C is probably related to the loss of

physically adsorbed or unbound water molecules on the

hydroxide. The thermal event around 240˚C is generally

attributed to the loss of chemisorbed water molecules

[28]. The thermal event at 465˚C that occurs after the

transformation of the hydroxide to the oxide is associated

with the phase transformation of amorphous MgO into its

cubic ordered phase [27]. This event, however, is not

widely reported or observed in literature during thermal

transformation of Mg(OH)2 to MgO.

3.3. X-Ray Characterisation of Mg(OH)2 and

MgO Nanoparticles

Figure 2 depicts the X-ray diffractogram of Mg(OH)2

nanoparticles precipitated at different temperatures, viz.

A at 23˚C, B at 60˚C and C at 85˚C. The three diffracto-

grams are similar and exhibit the same diffraction pattern

associated with Mg(OH)2 materials. The multiple dif-

fraction peaks from Mg(OH)2 at 22˚, 44˚, 60˚, 69˚, 74˚,

82˚ and 86˚ arise from the 001, 100, 011, 012, 110, 200

and 021 planes respectively according to a report by Sun-

drarajan et al. [13] and the XRD PDF2010: 000021169.

This confirmed that the formation of pure Mg(OH)2

nanoparticles is indifferent to the precipitation tempera-

ture used.

Figure 3 is the X-ray diffractograms of MgO nanopar-

ticles produced by calcining Mg(OH)2 at different tem-

peratures. The nanoparticles produced by calcination at

500˚C (A), 600˚C (B) and 700˚C (C) all exhibited iden-

tical diffraction patterns. The samples all had diffraction

Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour

of Mg(OH)2 and MgO Nanoparticles

368

Figure 2. Comparison of X-ray diffract ograms of Mg(OH)2 nanoparticles precipitated at 23˚C (A), 60˚C (B) and 85˚C (C).

Figure 3. X-ray diffractograms of MgO particles prepared from Mg(OH)2 precipitated at 23˚C (A), 60˚C (B) and 85˚C (C).

peaks at 44˚, 50˚ and 74˚ are assigned to 111, 200 and

222 planes respectively (XRD PDF2010:000021207).

This clearly indicated that Mg(OH)2 was completely

transformed to crystalline MgO with no traces of impuri-

ties observed. The results also show that calcining at the

relatively low temperature of 500˚C is adequate to pro-

duce pure MgO using this method. The crystallite sizes

(Table 2) were also calculated on the samples using 111,

200 and 220 diffraction maxima from the half-width of

diffraction peaks using Scherrer’s equation. The calcu-

lated crystallite sizes are comparable (on the lower end)

with the sizes obtained from TEM analysis for MgO

nanoparticles. According to the XRD data, the mean

crystallite sizes of the MgO nanoparticles were calcu-

lated by using the Debye Scherrer formula.

0.9

cos









Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour

of Mg(OH)2 and MgO Nanoparticles 369

Table 2. BET results of MgO nanoparticles calcined at

700˚C.

XRD crystallite sizes (nm)

Sample name

(precipitation

temperature)

Surface area

(m2/g) (111) (200) (220)

Mg(OH)2 (23˚C) 77.6 18.5 18.8 17.5

Mg(OH)2 (60˚C) 88.1 10.7 15.3 15.3

Mg(OH)2 (85˚C) 106.4 13.7 16.3 15.0

where λ = cobalt wavelength (1.7902Å), θ is the bragg

diffraction angle of the XRD peak, β is the measured

broadening of the diffraction line peak at an angle 2θ, at

half its maximum intensity (FWHM) in radian.

3.4. BET Analysis of Mg(OH)2 and MgO

Nanoparticles

The BET method was used to obtain specific surface area

for the MgO nanoparticles. BJH analysis was used to

determine the pore size distribution. This indicated that

MgO nanoparticles exhibit a bimodal pore distribution

which contains mesopores of between 1.7 - 5.2 nm and

larger mesopores with pore size averaging at 34.0 nm.

Rezaei et al. observed that the specific surface area of the

nanoparticles increased with increasing refluxing time

and temperature [29]. This is in line with our results that

showed that nanoparticles produced at different pre-

cipitation temperature showed that the specific surface

area increased with an increase in precipitation tem-

perature (Table 2).

3.5. TEM Analysis of Mg(OH)2 and MgO

Nanoparticles

TEM was used to analyse Mg(OH)2 nanoparticles pre-

cipitated at different temperatures. The general observa-

tion was that the shapes appear to be different (Figure 4).

It would be interesting to interrogate this observation

further but due to no access to a TEM instrument with

higher resolution, this idea was not explored further. The

average size distribution of the nanoparticles is shown in

Figure 4 with a mean of 37 nm for Mg(OH)2 precipitated

at 25˚C (Figure 4(a) ), 28 nm for those produced at 60˚C

(Figure 4(b)) and 45 nm for those precipitated at 85˚C

(Figure 4(c)). Literature shows that precipitation tem-

perature has an influence on the shapes and not size, with

flakes being observed when lower temperatures are used

[13,30,31]. Others have recently reported that there is a

correlation between precipitation temperature and final

particle size as long as the reaction is allowed to progress

for longer periods at lower temperatures to allow com-

plete reaction and other processes to take place [32]. This

aspect however widely reported, gives conflicting con-

clusions in literature to result in one predictive approach.

In the current work indications are that the size variance

does not show an obvious relationship with the precipita-

tion temperature used an observation that was also made

by Lv et al. [33].

MgO nanoparticles prepared by calcination of Mg(OH)2

materials at different temperatures, viz. 500, 600 and

700˚C were also characterised using TEM. The overall

average size range determined by TEM analysis results

ranged from 14 to 32 nm (Table 3). The average size of

nanoparticles obtained at 500 and 600˚C were compara-

ble irespective of the precipitation temperature used to

produce Mg(OH)2 particles with a narrow distribution

between 14 and 20 nm. On the other hand, nanoparticles

obtained at 700˚C were relatively larger (between 27 and

32 nm). Even though the general observation was that all

calcination temperatures resulted in average sizes of less

than 35 nm, the general appearance of the nanoparticles

seemed different. Sundrarajan et al. also observed the

effects temperature had on the morphology of MgO

nanoparticles where the initial flakes were transformed as

the calcination temperature was increased [13]. In so far

as the precipitation temperature was concerned, it was

observed that nanoparticles precipitated at higher tem-

perature (85˚C) exhibited more defined shapes compared

to those obtained at 23˚C and 60˚C, indicating that nano-

particle shape and size might be influenced by the pre-

cipitation temperature of the precursor Mg(OH)2 powder

was produced. Mageshwari and Sathyamoorthy used a

precipitation temperature of 100˚C to produce MgO na-

noparticles with a 5 nm average, an observation that in-

dicates that other parameters are more important at con-

trolling particle size than precipitation temperature [34].

3.6. Antibacterial Studies

The antibacterial effects of Mg(OH)2 and MgO nano-

particles were investigated using gram negative and gram

positive bacterial strains, E. coli and S. aureus respec-

tively, by the well diffusion agar method. The zones of

inhibition, the clearing zones around the wells without

visible bacterial growth, were measured from three se-

parate plates and are captured in Figure 5. These zones

indicate the antibacterial activity of Mg(OH)2 and MgO

since no bacterial growth is observed in those areas. The

antibacterial activity, from both Mg(OH)2 and MgO, was

higher in S. aureus than that observed in E. coli tests,

This can be attributed in part to the general observation

that gram positive bacterial strains are more susceptible

to antibacterial materials as compared to gram negative

strains due to differences in their cell wall structure [35].

On the other hand, MgO exhibited consistently higher

activity against S. aureus bacteria than Mg(OH)2 (Figure

5). This is in line with literature observation by Panacek

et al. [36] and Pan et al. [37] that smaller nanoparticles

Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour

of Mg(OH)2 and MgO Nanoparticles

370

Figure 4. TEM micrographs of MgO nanoparticles calcined at 700˚C (C) from Mg(OH)2 precipitated 23˚C (A), 60˚C (B),

85˚C (C) and with their respective histograms generated from the TEM micrographs.

are more active than larger ones and thus MgO nano-

particles, which are smaller than Mg(OH)2 nanoparticles,

are more effective towards inhibition of bacterial growth.

The mechanism of bacterial inhibition for nanoparticle is

usually attributed to the generation of reactive radicals

[38]. This mechanism is also assumed in this study and

therefore the different antibacterial activities of the mate-

rials are related to their ability to generate such radicals,

Effects of Precipitation Temperature on Nanoparticle Surface Area and Antibacterial Behaviour

of Mg(OH)2 and MgO Nanoparticles 371

Figure 5. Antibacterial activities of Mg(OH)2 and MgO as measured by zone of inhibition method.

Table 3. Summary of preparation conditions and average

nanoparticle sizes (nm).

Precipitation

temperature

(˚C)

Calcination

temperature

(˚C)

500 600 700 Mg(OH)2

25 15 14 27 37

60 16 18 23 28

85 18 20 32 45

a property that is dependent on the surface area and

crystal edges exposed. It is known that the higher the

surface area per unit mass, the more reactive material

becomes. The density of radicals produced, and hence

the inhibition ability is directly related to this property

and hence the trends observed in this study (Table 4).

For example, MgO is known to be more reactive than its

hydroxide and hence the activity trend observed whereby

the oxide is more active than the analogous hydroxide in

all cases studied. Within the materials themselves, e.g.

oxides, the activity trend follows the surface area and

nanoparticle size trend established in prior sections.

4. Conclusion

Synthesis and antibacterial effects of Mg(OH)2 and MgO

nanoparticles were undertaken and factors contributing

to the formation of well-defined nanoparticles were

explored. Temperature at which calcination of Mg(OH)2

to MgO was performed was found to be more important

than the temperature at which Mg(OH)2 was synthesised

in determining the final MgO nanoparticle size. A simple

and cost effective method to obtain nanoparticles with

narrow size dispersion was established in this study. The

antibacterial activities of the prepared Mg(OH)2 and

MgO nanoparticles were studied using well-diffusion

method. Both these nanoparticle types exhibited greater

Table 4. Relationship of MgO (calcined at 700˚C) anti-

bacterial activity towards S. aureus and surface area.

Activity against

S. aureus

(

)

Precipitation

temperature

Mg(OH)2MgO

Surface

area

MgO

(nm)

700˚C

Mg(OH)2

(nm)

23 6.5 8.6 77.6 27 37

60 6.8 7.0 88.1 23 28

85 7.5 10.2 104.2 32 45

antibacterial effects towards Gram-positive bacteria than

Gram-negative ones. In all cases, the antibacterial acti-

vity of Mg(OH)2 was lower than that of MgO, i.e. against

both E. coli and S. aureus. The antibacterial activity was

closely related to the surface area than to the actual nano-

particle size observed. The relatively low cost and abun-

dance of Mg based nanoparticles (Mg(OH)2 and MgO)

versus other metal based antibacterial agents make this

material a viable alternative for this application, i.e. E.

coli and S. aureus inhibition.

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