This study investigated the antimicrobial activity of amine-functionalised mesoporous silica microparticles against Listeria monocytogenes. The microparticles were successfully functionalised with amine and showed positive zeta potential values. Tests found the functionalised particles had antimicrobial activity against L. monocytogenes at concentrations between 10-50 μg/mL in buffer solution. However, their antimicrobial efficiency decreased in complex food systems due to interactions with organic molecules in the food that disrupted the positive charge of the particles. Further studies are needed to improve the particle efficiency in foods.

1. Antimicrobial activity of amine-functionalised mesoporous
silica microparticles
Niamh Mohan - niamh.mohan@mydit.ie
Supervisors – Jesus Frias & Jose M Barat
Co Supervisor – Maria Ruiz-Rico
Dublin Inst. of Technology, Faculty of Food Science and Environmental Health, Cathal Brugha street, Dublin.
Universidad Politecnica de Valencia, Faculty of Biochemical and Microbiological Engineering, Valencia, Spain.
Introduction & Aim of Research
The overuse of antibiotics and antimicrobial agents has resulted in the emergence of
multi drug resistant bacteria (Hajipour et al., 2012). Nanotechnology has helped develop
different strategies than those used in conventional therapies to tackle these resilient
microbes.
The concept of using nanotechnology to synthesise antibacterial agents is relatively
new. Ongoing research is being conducted on various types of nano- and microparticles
such as metal particles, magnetic particles and loaded/functionalised particles each with
different mechanisms of action. Mesoporous silica particles loaded and/or functionalised
with organic molecules have demonstrated interesting applications as suitable systems
for controlled release (Slowing et al., 2008). Another possibility involves using these types
of particles as antimicrobial supports as a result of the bioactive compound loaded or
attached in the matrix.
This project aims to determine the antimicrobial activity of amine-functionalised
mesoporous silica microparticles against Listeria monocytogenes. Finally, the activity of
the functionalised microstructures will then be tested against L. monocytogenes in
contaminated liquid foods to investigate the effect of the particles in a complex food
system.
Discussion
The zeta potential values revealed that the correct functionalisation of the mesoporous silica microparticles with amine was achieved. Figure 2 highlights that the particles were
positively charged due to the amine on their surface. The degree of functionalisation of the solid was determined by thermogravimetric analyses and the amount of amine anchored to
the particles was 35.97±2.59 mg/g SiO2. In contrast, the particles suspended in nutrient broth lost their positive charge due to interactions with components of the dissolvent. The
results of the particles in Ringer buffer imply that the functionalised particles are effective as antimicrobial agents with a minimum bacterial concentration between 10 and 50 µg/mL in
Ringer buffer. If the results of amine-functionalised particles and free amine are compared, it can be observed that free amine has no antimicrobial properties or shows an lower
effect than the attached amine (Fig 3). The study on the influence of different pH and nutrient broths show that the activity of the particles decreased when in the presence of the
organic molecules in the media. In the same way, the activity of the particles in the food system were affected by the presence of organic molecules such as proteins, sugars, etc.
(Fig. 4 and 5).
In conclusion, amine-functionalised mesoporous silica particles have antimicrobial activity against L. monocytogenes in model solutions. The antimicrobial efficiency of these
particles in food systems appears to be affected by the matrix and hence, the activity is decreased. Further studies should be carried out to understand the lose of efficiency
of the particles as well as trying other types of functionalisation.
Methodology
Microparticle synthesis &
functionalisation
Materials characterization
Transmission electron microscopy (TEM)
Zeta potential and amine quantification
Food System Assays
Fruit Juice
Milk
Microbiological Assays
Minimum bactericidal concentration (MBC)
Influemce of pH and media on antimicrobial activity
Results
Figure 1. TEM image of MCM-41
microparticulate structure.
0
1
2
3
4
5
6
pH 3.5 pH 5 pH 7.5 pH 9 pH 10.5
logCFU/mL
Ringer buffer
0
1
2
3
4
5
6
7
PBS BPW TSB BHB NB
logCFU/mL
Ringer buffer
Figure 4. Influence of pH and media on antimicrobial activity of amine-functionalised
microparticles (750 µg/mL) against L. monocytogenes. Control in dark blue and particles in
purple.
Figure 2. Zeta potential values of amine-
functionalised MSPs dispersed in different
buffer and culture broth.
-40
-20
0
20
40
60
80
Ringer
pH 3.5
Ringer
pH 5
Ringer
pH 7.5
Ringer
pH 9
Ringer
pH 10.5
PBS BPW TSB BHB NB
Zetapotential(mV)
Figure 3. Reduction of microbial growth of L. monocytogenes with amine-functionalised
microparticles and free amine at 2 h of treatment.
0
20
40
60
80
100
120
5 10 50 100 150 200
Survival(%)
Amine-functionalised MCM-41 (µg/mL)
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10
20
30
40
50
60
70
80
90
100
1000 1250 1500 1750 2000
Survival(%)
N3 concentration (µg/mL)
Figure 5. Effect of amine-functionalised microparticles against L.
monocytogenes inoculated in orange juice. 2 hours incubation in dark blue and
24 hours in purple.
0
20
40
60
80
100
120
100 500 1000 1500 2000 2500 3000 3500
Survival(%)
Amine-functionalised particles (µg/mL)
References
Hajipour, M. J., Fromm, K. M., Ashkarran, A. A., Jimenez de Aberasturi, D., de Larramendi, I. R., Rojo, T., Mahmoudi, M. (2012). Antibacterial properties of nanoparticles. Trends in Biotechnology, 30(10), 499–511.
Slowing, I. I., Vivero-Escoto, J. L., Wu, C.-W., & Lin, V. S.-Y. (2008). Mesoporous silica nanoparticles as controlled release drug delivery and gene transfection carriers. Advanced Drug Delivery Reviews, 60(11), 1278–88.