Tải bản đầy đủ - 0 (trang)
1 Destruction of Bacteria with Ti(IV) and Ag Co-substituted Hydroxyapatite Under Weak UVA Irradiation

1 Destruction of Bacteria with Ti(IV) and Ag Co-substituted Hydroxyapatite Under Weak UVA Irradiation

Tải bản đầy đủ - 0trang

6 Solar Photocatalytic Disinfection by Nano-Ag-Based Photocatalyst



131



showed any bactericidal effects on E. coli. Additionally, no significant inactivation

of E. coli was observed under UVA irradiation with no catalyst. E. coli inactivation

of 1.8 log occurred at 180 min of irradiation on P25-TiO2 films. In contrast, E. coli

was almost completely killed within 150 min on HAPTiAg films under weak UVA

irradiation and in the dark (Fig. 6.3). Complete E. coli inactivation of 6.7 log

occurred at 150 min with or without UVA irradiation. The maximum concentration

of Ag+ ions leached out into aqueous solution from the HAPTiAg film was about

0.6 mg/L, which did not show any bactericidal effects on E. coli. Compared with all

the above results, the bactericidal activity of HAPTiAg film in both UVA and

darkness was much higher than that of P25-TiO2 film with UVA irradiation.

Furthermore, S. aureus was also killed efficiently on HAPTiAg films, respectively,

under UVA irradiation or in the dark (Fig. 6.4).

The above results indicated that it is quite possible that the E. coli was killed by

the synergy of the decomposition role of ROS and the bacteriostatic action of these

antibacterial ions. The involvement of O2À• radicals in different reaction systems

Fig. 6.3 Temporal course

of the E. coli inactivation in

HAPTiAg film (Reprinted

from Ref. [4] Copyright

© 2007 Elsevier)



8

+

0.6 mg/L Ag



log C



6



4

HAPTiAg in dark



2

HAPTiAg + UVA



0



0



50



100



150



200



Irradiation time/min



Fig. 6.4 Temporal course

of the S. aureus inactivation

in HAPTiAg film

(Reprinted from Ref. [4]

Copyright © 2007 Elsevier)



8



log C



6



4

HAPTiAg in dark



2

HAPTiAg + UVA



0



0



50



100



150



Irradiation time/min



200



132



C. Hu



was examined in methanol with an Nd:YAG laser (355 nm) irradiation source or in

the dark.

In HAPTiAg and HAPTi systems with and without UVA irradiation, six characteristic peaks of the DMPO- O2À• adducts were observed (Fig. 6.5), while in the

control system, DMPO with methanol did not exhibit any signal. Furthermore,

another control experiment was carried out. Superoxide dismutase (SOD) was

added into HAPTiAg and HAPTi methanol suspensions in the dark under otherwise

identical conditions. Obviously, the signal of DMPO- O2À• adducts was not

observed in the control experiment since SOD scavenged O2À•. The results confirmed that O2À• radicals were generated from HAPTiAg and HAPTi systems in the

dark. For the P25-TiO2, HAP systems (Fig. 6.6), six characteristic peaks of the

DMPO- O2À• adducts were observed under UV irradiation, but no such signals were

detected in the dark. The results verified that HAPTiAg and HAPTi can generate O2

À•

both under UVA irradiation and in the dark, while other materials have to be

excited by UV light and then O2À• is generated. Therefore, in the dark, HAP and

P25-TiO2 films had no bactericidal activity, whereas HAPTi did not show any

obvious effect either due to the little intensity of the DMPO- O2À•. However, in

HAPTiAg film systems, E. coli not only was decomposed by O2À• but also was

inhibited by silver ions, causing faster inactivation in the dark.

Similarly, under UV irradiation, both HAPTiAg films had the same role as that

in the dark, and the more O2À• was formed due to UV irradiation, the more



A



·



·



·



·



·



·



irradiated

dark

SOD + irradiated



control



3440



3460



3480



3500



3520



Magnetic Field/G

B



·



·



·



·



·



·



irradiated

dark

SOD+irradiated

control



3440



3460



3480



3500



3520



Magnetic Field/G

Fig. 6.5 DMPO spin-trapping ESR spectra recorded at ambient temperature in methanol dispersion under UVA irradiation or in dark, (a) HAPTiAg and (b) HAPTi (Reprinted from Ref. [4]

Copyright © 2007 Elsevier)



6 Solar Photocatalytic Disinfection by Nano-Ag-Based Photocatalyst



·



A



·



·



·



·



·



133



irradiated

dark

control



3440



3460



3480



3500



3520



Magnetic Field/G

B



·



·



·

·



·



·



irradiated

dark

control



3440



3460



3480



3500



3520



Magnetic Field/G

Fig. 6.6 DMPO spin-trapping ESR spectra in methanol dispersion under UVA irradiation or in

dark, (a) P25-TiO2 and (b) HAP (Reprinted from Ref. [4] Copyright © 2007 Elsevier)



inactivation of E. coli occurred. In contrast, HAP, HAPTi, and P25-TiO2 only had

the function of decomposing bacteria under UVA irradiation. Since the tested UVA

intensity was relatively weak, the amount of O2À• formed from HAP and HAPTi

was too small to lead to any obvious E. coli inactivation. However, the intensity of

O2À• from P25-TiO2 was stronger than that from HAP and HAPTi, and some E. coli

inactivation was observed. Nevertheless, the bactericidal activity of P25-TiO2 was

still much lower than that of HAPTiAg. The results demonstrated that the synergistic effect of the oxidation reaction and antibacterial reaction was much greater

than that of their sum.

The main mechanism for radical O2À• formation on HAP has been proposed.

The radical O2À• could be formed on HAP by heat treatment or UV irradiation

[5, 6]. The UV irradiation or heat treatment causes the changes of the surface PO4

group, probably the formation of an oxygen vacancy, which traps the electrons,

leading to the formation of the O2À• species [5]. HAPTi and HAPTiAg could

generate O2À• species at room temperature without UV irradiation. Moreover, the

intensity of O2À• signals formed in HAPTiAg was stronger than that in HAPTi. It is

quite possible that the substitution of Ti(IV) caused the oxygen vacancy in the

crystal of HAP because the valency of Ti(IV) is higher than that of Ca(II). The

formation of the oxygen vacancy was attributed to the possible formation of O2À• at

ambient temperature. Furthermore, HAPTiAg was characterized by XPS. The

silver species mainly exists as Ag0 (BE, 367.82 eV) and Ag+ (BE, 367.55 eV).



134



C. Hu



Fig. 6.7 (a) E. coli not treated, (b) and (c) TEM micrographs of E. coli in UVA-illuminated

HAPTiAg suspension for 2 h, and (d) and (e) TEM micrographs in HAPTiAg suspension in the

dark for 5 h (Reprinted from Ref. [4] Copyright © 2007 Elsevier)



Thus, the redox couple Ag0/Ag+ was formed in the structure of HAPTiAg. In the

presence of oxygen, the O2À• could be generated by the electron transfer of the

redox couple [7].



6.1.2



Cell Damage Mechanism



Figure 6.7 shows the appearance of E. coli after treatment of HAPTiAg films with

or without UVA irradiation. Before the reaction, the E. coli are a well-defined cell



6 Solar Photocatalytic Disinfection by Nano-Ag-Based Photocatalyst



135



wall as well as the rendered interior of the cell, which corresponds to the presence of

proteins and DNA (Fig. 6.7a). Great changes had taken place to the morphology of

E. coli after 150 min illumination (Fig. 6.7b, c). The cell wall was decomposed and

the rendered interior of the cell became white, indicating that the outer membrane

of the cell was damaged leading to leakage of the interior component. Similarly, the

cell wall of E. coli was also destructed resulting in leakage of the interior component (Fig. 6.7d, e) in the dark with HAPTiAg film. Based on the TEM investigation,

the cell wall, the peptidoglycan layer, and the cell membrane of the bacteria were

decomposed by O2À•. K+ exists universally in bacteria [8] and plays a role in the

regulation of polysome content and protein synthesis. Therefore, K+ leakage from

the inactivated bacteria can examine the change in cell membrane permeability. As

shown in Fig. 6.8, under only UVA irradiation or in the presence of 0.6 mg/L silver

ions, the K+ leakage was very slow. After the addition of HAPTiAg film with or

without UVA irradiation, K+ immediately started to leak from the E. coli cells, and

the leakage gradually increased with reaction time, paralleling the loss of cell

viability. The resultant K+ concentrations were much higher than that of the control

experiments. Moreover, the amount of the K+ leakage was almost equal under the

two conditions, indicating that the cell membrane was damaged to the same extent

with or without UVA irradiation. These results demonstrate that the K+ leakage was

consistent with the disruption of the cell wall and the cell membrane by the

oxidation and antibacterial action. In the bactericidal process of HAPTiAg, Ti

(iV) and Ag+ ions assist each other. On the one hand, the outer membrane of the

cell is attacked by O2À• produced from HAPTiAg. Successively, Ag+ ions are

effectively taken into the cytoplasmic membrane by the partially decomposed

outer membrane. Finally, the bacteria are inactivated by the bacteriostatic action

of Ag+. On the other hand, the bacteriostatic action of these ions enhances the

efficiency of O2À• in killing bacteria. In a conclusion, the high bactericidal activity

of HAPTiAg was due to the synergy of the oxidation role of the O2À• and the

bacteriostatic action of antibacterial ions.



Fig. 6.8 Leakage of K+

from E. coli cells in

HAPTiAg film (Reprinted

from Ref. [4] Copyright

© 2007 Elsevier)



3000

HAPTiAg + UVA



2000



log C



HAPTiAg in dark



1000

+

0.6 mg/L Ag



UVA



0

0



50



100



150



Irradiation time/min



200



136



6.2



C. Hu



Visible-Light Photocatalytic Degradation of Pathogenic

Bacteria Over Supported Silver Halides



Silver halides are well known as photosensitive materials and are widely employed

as source materials in photographic films. AgI and AgBr were supported on

P25-TiO2 by the deposition-precipitation method in an aqueous solution of

AgNO3 and NH4OH containing KI or KBr [9]. Silver halides could act as a good

visible-light photocatalyst candidate for the removal of pollutants when suitable

environmental conditions could be chosen to prevent their photodecomposition

[10–12]. AgI/TiO2 and AgBr/TiO2 show high efficiency and photostability in the

degradation of nonbiodegradable azo dyes under visible-light irradiation

[9, 13]. These catalysts were also found to be highly effective in killing bacteria

[13, 14].



6.2.1



Bacterial Inactivation Under Visible-Light Irradiation



The bactericidal activities of the samples were evaluated by the inactivation of

E. coli and S. aureus in water under visible-light irradiation. E. coli is a Gramnegative bacteria and S. aureus is a Gram-positive bacteria. 6.8 log E. coli was

completely inactivated within 60 min in the AgBr/TiO2 suspension under visiblelight irradiation (Fig. 6.9), whereas complete inactivation of 6.8 log S. aureus

occurred at 40 min of irradiation. Neither pure TiO2 with visible light nor AgBr/

TiO2 in the dark showed any bactericidal effects for the two bacteria (curves a and

b). Similarly, 7.8 log E. coli and 7 log S. aureus were almost completely killed

within 60 min and 100 min in the AgI/TiO2 suspension under visible-light irradiation (Fig. 6.10). These results indicated that AgBr and AgI were the main active

component of the catalyst under visible-light irradiation. Different times were

8

7



a



6



b



5



log C



Fig. 6.9 Temporal course

of the bacteria inactivation

(2 Â 107 cfu mLÀ1, 30 mL)

in aqueous dispersions

containing 0.2 g LÀ1 of

catalysts. (a) S. aureus/

E. coli + AgBr/TiO2 in dark,

(b) S. aureus/E. coli + TiO2,

(c) E. coli, and (d) S. aureus

in visible-light-illuminated

AgBr/TiO2 suspension

(Reprinted from Ref. [13]

Copyright © 2007 Elsevier)



4

3

2

1



c



d



0

0



10



20



30



40



50



Irradiation time/min



60



6 Solar Photocatalytic Disinfection by Nano-Ag-Based Photocatalyst



8



a

b

c



6



log C



Fig. 6.10 Temporal course

of the E. coli inactivation

(~5 Â 107 cfu/mL, 30 mL,

pH ¼ 4.04) in aqueous

dispersions containing

0.2 g/L of catalysts under

visible-light irradiation. (a)

No catalyst, (b) TiO2, (c)

AgI/TiO2 in dark, and (d)

AgI/TiO2 (Reprinted from

Ref. [13] Copyright © 2007

Elsevier)



137



4

2

0



d



0



10



20



30



40



50



60



70



80



90



Irradiation time/min

7

6

5



log C



Fig. 6.11 Cycling runs in

the photocatalytic

inactivation of E. coli in the

presence of AgBr/TiO2

under visible-light

irradiation. AgBr/TiO2

(0.2 g LÀ1); addition of

E. coli cells (~2 Â 107 cfu

mLÀ1/run) (Reprinted from

Ref. [13] Copyright © 2007

Elsevier)



4

3

2

1

0

0



50



100



150



200



250



300



Irradiation time/min

required for total cell inactivation of E. coli and S. aureus due to their dissimilar cell

wall constituents. Gram-negative bacteria have a thin layer of peptidoglycan and a

complex cell wall with two cell membranes: an outer membrane and a plasma

membrane. Gram-positive bacteria have only one membrane with a relatively thick

wall composed of many layers of peptidoglycan polymer. The addition of the outer

membrane of Gram-negative bacteria influences the permeability of many molecules, and under certain conditions, Gram-negative bacteria are more resistant to

many chemical agents than Gram-positive cells [15]. As shown in Fig. 6.11, the

catalyst’s activity did not significantly decrease in the inactivation of E. coli after

five successive cycles under visible-light irradiation, confirming the stability of

AgBr/TiO2. These results indicated that the supported AgBr and AgI were highly

effective at the killing of bacteria under visible light.

To illustrate the visible-light-induced bactericidal mechanism, the ESR spin-trap

technique (with DMPO) was used to detect the nature of the reactive oxygen

species generated on the surface of the catalysts under visible-light irradiation. As

shown in Fig. 6.12, •OH and O2À• radicals were formed in visible-light-irradiated



138



C. Hu



A



ã



ã



ã



B



ã



ă



ă

ă



irradiated



ăă



ă

irradiated



dark



3440



3460



3480



3500



dark



3520



3440



3460



Magnetic Field/G



3480



3500



3520



Magnetic Field/G



C



ã



ã



ã

ã



irradiated



dark



3440



3460



3480

3500

Magnetic Field/G



3520



Fig. 6.12 DMPO spin-trapping ESR spectra recorded at ambient temperature with AgBr/TiO2 as

catalyst in aqueous (a) or methanol (b) dispersion and with AgI/TiO2 as catalyst in aqueous

dispersion (c) under visible-light irradiation (λ > 420 nm) (Reprinted from Ref. [13] Copyright

â 2007 Elsevier)



AgBr/TiO2 suspension. OH was also observed in the aqueous AgI/TiO2 dispersion

under visible-light irradiation. No O2À• radicals were detected in the AgI/TiO2

dispersion in methanolic media. Furthermore, the formation of H2O2 was detected

in the visible-light-irradiated AgBr/TiO2 and AgI/TiO2 system as shown in the

previous work [9, 13]. These results indicated that •OH, O2À•, and H2O2 reactive

active species were involved in the photocatalytic bactericidal reaction.



6.2.2



Interaction of Bacteria with Photocatalysts



To clarify the interaction of bacteria-photocatalysts, the effects of pH and inorganic

ions on bacterial photocatalytic inactivation were investigated. As shown in the

previous work [13], the bactericidal activity of AgBr/TiO2 decreased significantly

with the pH increasing from 4.0 to 7.5. At pH 4 and pH 6.5, 7 log E. coli inactivation

occurred at 60 min irradiation, while at pH 7.5, no significant inactivation of E. coli

was observed. In the range of pH 4–8, the overall charges of the E. coli cells were

negative, whereas the surface of the catalyst was positively charged at pH < 4.8,



6 Solar Photocatalytic Disinfection by Nano-Ag-Based Photocatalyst



139



Fig. 6.13 The inactivation

of E. coli (~5 Â 107 cfu/mL)

under different pH

conditions with visiblelight-illuminated AgI/TiO2

(Reprinted from Ref. [14]

Copyright © 2007

American Chemical

Society)



log C



and it was negatively charged at pH > 4.8. Electrostatic attraction existed between

E. coli and the catalyst at pH 4, leading to more E. coli adsorption onto the surface

of the catalyst. Thus, the catalyst exhibits more activity for the killing of E. coli. At

pH 6.5 the surface of the catalyst was partly negatively charged because the pH was

approaching the isoelectric point (pH 4.8). The repulsive electrostatic force

between E. coli and the catalyst was weaker, so the inactivation of E. coli was

not significantly depressed at pH 6.5. However, at pH 7.5, the surface charge of the

catalyst became more negative, and the repulsive electrostatic force was stronger.

E. coli was not easily adsorbed on the surface of the catalyst, and the E. coli

inactivation was nearly inhibited. Only less than 1 log E. coli were inactivated at

60 min of irradiation. Similarly, in the AgI/TiO2 system, the inactivation of E. coli

was the highest at pH ¼ 4.04, and 6.1 log E. coli inactivation occurred at 40 min

irradiation, while at pH ¼ 7.75, only 0.67 log E. coli inactivation was observed at

the same irradiation time (Fig. 6.13).

The charges of the bacteria and AgI/TiO2 under different pH conditions are

shown in Fig. 6.14. In the range of pH 2–9, the overall charges of E. coli were

negative, while the surface charge property of AgI/TiO2 changed with the change of

solution pH. The isoelectric point of AgI/TiO2 was about 5.1. At pH < 5.1, the

surface of the catalyst was positively charged, while it was negatively charged at

8

7

6

5

4

3

2

1

0



pH = 8.78

pH = 7.75

pH = 6.03

pH = 4.04



0



20 30 40 50

Irradiation time/min



60



70



30



Zeta-potential/mV



Fig. 6.14 Zeta potentials

for a suspension of

AgI/TiO2 1.0 g/l in the

presence of KNO3 (10À3 M)

(Reprinted from Ref. [14]

Copyright © 2007

American Chemical

Society)



10



20



AgI/TiO2



10

0

-10

-20

-30

-40



2



4

E. coli



6



8

pH



10



12



140



C. Hu



pH > 5.1. Therefore, at pH ¼ 4.04, electrostatic attraction existed between E. coli

and catalyst, leading to E. coli being tightly bound with the catalyst surface. Thus, a

higher inactivation rate was obtained. At pH > 5.1, the electrostatic repulsive force

between the E. coli and the catalyst increased with the pH increasing due to the

more negative zeta potential. This led to the reduction of the E. coli inactivation

rate. The zeta potential of E. coli tended to be less negative at pH ¼ 6.03. Thus, the

inactivation of E. coli was inhibited to some extent. However, a stronger electrostatic repulsive force resulted in lower bactericidal efficiency at pH ¼ 7.75. At

pH ¼ 8.78, the zeta potentials of the catalyst and E. coli did not change much as

compared with the condition of pH ¼ 7.75, so the inactivation rate of E. coli was

similar to that at pH ¼ 7.75. The results indicated that interaction of TiO2 supported

AgBr or AgI with bacteria played an important role in disinfection. Hamouda and

Baker [16] also showed that if the antimicrobial composition had the same charge

as the bacteria cells, this induced repulsion and prevented contact, while the

addition of EDTA/Tris buffer to the formulation changed the charge and considerably improved the activity of the formulation. To further study the effects of the

interaction between bacteria and catalyst, Ni2+ or Mg2+ was added into the reaction

system. The inactivation rate of E. coli was greatly increased with the addition of Ni

2+

and Mg2+ at pH 7.5 in AgBr/TiO2 suspension and at pH 7.75 in AgI/TiO2

suspension under visible-light irradiation (Fig. 6.15). The single inorganic ions

with visible light did not show any bactericidal activity (Fig. 6.15, curves a and b),

indicating that the tested concentration of inorganic ions did not inhibit the growth

of bacteria. Furthermore, the zeta potential measurement showed that the zeta

potential of AgBr/TiO2 or AgI/TiO2 (Fig. 6.16) was more positive in the presence

of Ni2+or Mg2+ than without the addition of ions. Thus, a weaker repulsive

electrostatic force occurred between catalyst and bacteria at pH ¼ 7.5, resulting in

higher bactericidal activity. This result further confirmed the role of interaction for

E. coli-AgX/TiO2 in disinfection, although the addition of Ni2+ or Mg2+ may

enhance the separation of photogenerated electrons and holes. All of the previous



8



a

b



7

6



log C



Fig. 6.15 Survival of

E. coli with visible-lightilluminated AgI/TiO2

dispersions (0.2 g/L,

pH ¼ 7) under otherwise

different conditions: (a) Ni

(NO3)2 with no catalyst, (b)

Mg(NO3)2 with no catalyst,

(c) without the addition of

cations, (d) Mg(NO3)2, and

(e) Ni(NO3)2. Cation

concentration, 30 μM

(Reprinted from Ref. [14]

Copyright © 2007

American Chemical

Society)



5

4

3

2

1

0



e



0



10



20



30



40



Irradiation time/min



50



c

d



60



6 Solar Photocatalytic Disinfection by Nano-Ag-Based Photocatalyst



40



Zeta potential/mV



Fig. 6.16 Zeta potentials

for different suspensions of

AgI/TiO2 1.0 g/L in the

presence of KNO3 (10À3

M): (a) 30 μM Ni (NO3)2,

(b) 30 μM Mg (NO3)2, and

(c) without the addition of

ions (Reprinted from Ref.

[14] Copyright © 2007

American Chemical

Society)



141



30

20

10

0

-10



2



4



6

b



a8



10



12



pH



-20

-30



c



-40



Fig. 6.17 TEM images of E. coli for different reaction times in visible-light-illuminated AgI/TiO2

suspension: (a) E. coli before reaction, (b) E. coli treated for 30 min, and (c) and (d) E. coli treated

for 120 min (Reprinted from Ref. [14] Copyright © 2007 American Chemical Society)



experimental results indicated that the electrostatic force interaction of the bacteria

and catalyst is crucial for high bactericidal efficiency.



6.2.3



Destruction of Cell Structure



In Fig. 6.17, the morphology of bacteria at different stages during bactericidal

experiments showed the bactericidal mechanism of various reactive species (e.g., •



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

1 Destruction of Bacteria with Ti(IV) and Ag Co-substituted Hydroxyapatite Under Weak UVA Irradiation

Tải bản đầy đủ ngay(0 tr)

×