Nanocomposites of Ag with organic montmorillonite (Ag-OMMT), Ag with montmorillonite (Ag-MMT), and organic montmorillonite (OMMT) were successfully prepared via a one-step solution-intercalated method. Sodium MMT, silver nitrate, and dimethyl octadecyl hydroxy ethyl ammonium nitrate were used as precursors. X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and energy dispersive spectroscopy analyses confirmed that the MMT layers were intercalated, and Ag+ was partly reduced to silver nanoparticles with diameters within 10–20 nm in Ag-OMMT. The decomposition temperature of the organic cations in OMMT and Ag-OMMT increased to 220°C, as revealed by differential scanning calorimetry-thermogravimetric analysis. The antimicrobial activity of the nanocomposites was tested by measuring the minimum inhibitory concentration (MIC) and killing rate. The MICs of Ag-OMMT against
Montmorillonite (MMT), an all-purpose clay, is widely used in a range of applications because of its high cation exchange capacity, swelling capacity, high surface areas, and strong adsorption and absorption capacities [
Meanwhile, polymers used in several industries such as food processing, biomedical devices, and filtering are required to have antiseptic ability to minimize the transmission of bacterial infections [
The aim of this work was to prepare organic antiseptic MMT with good compatibility and dispersibility for use as a nanoadditive in polymers. For this purpose, MMT was modified with Ag+ and quaternary ammonium nitrate via a one-step solution-intercalation technique. The structures of different antimicrobial organic MMTs were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) techniques, and the thermal stability was confirmed by differential scanning calorimetry-thermogravimetric (DSC-TG) analysis. The antimicrobial activity of the nanocomposites was evaluated by examining the minimum inhibitory concentration (MIC) and killing rate.
Sodium MMT used in this study was supplied by Zhejiang Fenghong Clay Chemicals Co., Ltd (China). The cation exchange capacity (CEC) of MMT was 90 meq (100 g)−1. Silver nitrate (AgNO3) with a purity of 99.8% was provided by Hunan Hipure Chemical Reagent Factory (China). Dimethyl octadecyl hydroxy ethyl ammonium nitrate (DOHEAN) at 50% (w/w) in butanol was provided by Jiangsu Hai’an Petrochemical Plant (China). Other reagents used in this study were of analytical grade.
10 g sodium MMT was dispersed in 200 mL deionized water and stirred at 80°C. AgNO3 (equimolar with the CEC) was dissolved in deionized water and then slowly dropped into the MMT sol, which was then kept at 80°C for 1 h with stirring. An equimolar quantity of DOHEAN was added to the Ag+ and MMT sol and kept at 80°C for 2 h with stirring. The intercalated montmorillonite (Ag-OMMT) was repeatedly washed with deionized water to remove residual AgNO3 and DOHEAN. This composite was then dried at 100°C for 24 h and then ground to a size less than 300 mesh.
The preparation of Ag-MMT and OMMT were consistent with the above methods but absented the process of addition AgNO3 and DOHEAN, respectively.
XRD measurements were performed using a D/Max 2550 diffractometer (Rigaku Electrical Co., Ltd.) with a Cu target and
Gram-positive bacteria
MIC tests were performed in MHA for the bacteria and fungi. A serial twofold dilution of Ag-OMMT was added to an equal volume of medium to obtain a concentration of 5000
The microorganism suspension was diluted using 0.9% (w/v) sterile saline water to 104 CFU/mL. 1 mL of cell suspension was added to 95 mL of 0.05, 0.025, and 0.0125 mg/mL nanocomposite (Ag-MMT and Ag-OMMT) solutions that had been autoclaved at 121°C for 20 min. Nanoscale SiO2 was used as a control. The samples were removed after 2 h shake cultivation. 50
The XRD patterns of unmodified MMT, Ag-MMT, OMMT, and Ag-OMMTs (modified with different amounts of Ag+) are presented in Figure
Sample | MMT | Ag-MMT | OMMT | 2.25 mmol |
4.5 mmol |
6.75 mmol |
9.0 mmol |
11.25 mmol |
---|---|---|---|---|---|---|---|---|
2 |
7.019 | 6.396 | 4.460 | 4.498 | 4.480 | 4.360 | 4.419 | 4.418 |
|
1.258 | 1.381 | 1.980 | 1.963 | 1.971 | 2.025 | 1.998 | 1.998 |
XRD patterns of MMT, Ag-MMT, OMMT, and Ag-OMMTs with different amounts of Ag+. MMT: unmodified MMT; Ag-MMT: MMT modified with Ag; OMMT: MMT modified with DOHEAN; Ag-OMMT: MMT modified with Ag and DOHEAN.
As shown in Figure
XRD patterns of Ag-MMT and Ag-OMMTs (different amounts of Ag+) after calcination at 750°C for 2 h. MMT: unmodified MMT; Ag-MMT: MMT modified with Ag; OMMT: MMT modified with DOHEAN; Ag-OMMT: MMT modified with Ag and DOHEAN.
EDS pattern of Ag-OMMT and TEM micrographs of Ag-MMT and Ag-OMMT.
TEM of Ag-MMT
TEM and EDS of Ag-OMMT
Figure
SEM micrographs of MMT and Ag-OMMT.
MMT
Ag-OMMT
DSC-TG curves of DOHEAN, MMT, Ag-MMT, OMMT, and Ag-OMMT are shown in Figure
DSC-TG profiles of DOHEAN, MMT, Ag-MMT, OMMT, and Ag-OMMT.
DOHEAN
MMT
Ag-MMT
OMMT
Ag-OMMT
The sharp exothermal peak on the DSC curve of DOHEAN represents the evaporation or decomposition of DOHEAN. However, there are two exothermal peaks on the DSC curves of OMMT and Ag-OMMT. The low-temperature exothermal peak corresponds to the evaporation or decomposition of DOHEAN on the silicate plate surfaces, and the other peak represents the evaporation or decomposition of DOHEAN between the silicate plates. The TG curves of OMMT and Ag-OMMT reveal that the evaporation or decomposition of DOHEAN occurs at approximately 220°C, which is higher than that of pure DOHEAN (160°C). This indicates that the organic cation has intercalated into the MMT layers, similar to the initial state, and that the silicate platelets have the ability to protect organic molecules from decomposition. [
Figure
FTIR spectra of MMT, DOHEAN, Ag-MMT, OMMT, and Ag-OMMT.
As shown in Table
MIC of the samples.
Samples | MIC (mg/mL) | ||
---|---|---|---|
|
|
|
|
Ag-MMT | 1.25 | 2.5 | 0.625 |
OMMT | 2.5 | >40 | >40 |
Ag-OMMT | 0.313 | 2.5 | 0.625 |
Killing rate of the samples.
|
Killing rate/% | |||||
---|---|---|---|---|---|---|
|
|
|
||||
Ag-MMT | Ag-OMMT | Ag-MMT | Ag-OMMT | Ag-MMT | Ag-OMMT | |
0.05 | 100 | 100 | 100 | 100 | 100 | 100 |
0.025 | 100 | 99.996 | 100 | 99.99 | 100 | 99.996 |
0.0125 | 100 | 99.995 | 100 | 90.15 | 100 | 93.68 |
OMMT can inhibit the growth of
Novel antimicrobial nanocomposites featuring sodium MMT, Ag+, and dimethyl octadecyl hydroxy ethyl ammonium nitrate were synthesized via a one-step solution-intercalated method. XRD, DSC-TG, FTIR, SEM, TEM, and EDS characterization indicated that Ag+ and DOHEAN were intercalated into the MMT layers. Ag formed both metallic species and Ag+ in the clay layer, while DOHEAN was chemically bonded with the MMT layers. The thermal stability of DOHEAN was improved by the protection from the MMT layers. The nanocomposite surface became crinkled and rough after modification, making it suitable for combining with polymers. Further, the nanocomposites showed a wide range of highly efficient antimicrobial activity. The results of this study may be used as a foundation for the future development of new types of nanocomposites of antimicrobial polymers in many industries, such as in wood adhesives, plastics, paints, and rubbers.
The authors declare that they have no conflicts of interest.
This work was supported by the National Special Program for International Science and Technology Cooperation (no. 2015DFA01120), the Hunan Province Major Program of Science and Technology (2017NK1010), the Key Projects in the National Science & Technology Pillar Program during the Eleventh Five-year Plan Period of China (2006BAD07A07-08), and the Hunan Province Natural Science Foundation (2015JJ5007).