Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
www.acsami.org
One-Pot Fabrication of Antireflective/Antibacterial Dual-Function Ag NP-Containing Mesoporous Silica Thin Films Kaikai Wang†,‡ and Junhui He*,† †
Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancundonglu 29, Haidianqu, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Thin films that integrate antireflective and antibacterial dual functions are not only scientifically interesting but also highly desired in many practical applications. Unfortunately, very few studies have been devoted to the preparation of thin films with both antireflective and antibacterial properties. In this study, mesoporous silica (MSiO2) thin films with uniformly dispersed Ag nanoparticles (Ag NPs) were prepared through a one-pot process, which simultaneously shows high transmittance, excellent antibacterial activity, and mechanical robustness. The optimal thin-filmcoated glass substrate demonstrates a maximum transmittance of 98.8% and an average transmittance of 97.1%, respectively, in the spectral range of 400−800 nm. The growth and multiplication of typical bacteria, Escherichia coli (E. coli), were effectively inhibited on the coated glass. Pencil hardness test, tape adhesion test, and sponge washing test showed favorable mechanical robustness with 5H pencil hardness, 5A grade adhesion, and functional durability of the coating, which promises great potential for applications in various touch screens, windows for hygiene environments, and optical apparatuses for medical uses such as endoscope, and so on. KEYWORDS: sol−gel method, antireflection, antibacterial, Ag NPs, thin film
■
INTRODUCTION Reflection often exists at the interfaces of transparent substrates resulting in undesirable optical loss. Take electronic devices, for example, the reflected sunlight off the screen considerably reduces the visibility and causes poor contrast on display. To address such issues, antireflective coatings have attracted significant attention in the past decades and pose a wide range of applications in photonic sensors, architectural glasses, optical lenses, and solar cells.1−7 For conventional single-layer antireflective coatings, zero reflection can be realized at a specific wavelength when they satisfy the following criteria: the film has a quarter-wavelength optical thickness (d = λ/4nc), and the refractive index (RI) is nc = nans , where na and ns are the RIs of air and substrate, respectively.8 According to criteria, quarter-wavelength mesoporous films with a low RI of ca. 1.23 were usually designed and applied as antireflective coatings on glass with an RI of ca. 1.52.9 Moreover, the fabrication of such mesoporous films generally uses the sol−gel method in consideration with its simplicity, controllability, low-cost, and easy large-scale production.10,11 Meanwhile, a thin film with antibacterial property would be interesting and attractive because it could effectively protect humans from some infection by microorganisms. In particular, © XXXX American Chemical Society
the popularity of electronic devices such as mobile phones and tablet computers, which have been reported to be a reservoir of clinically significant pathogens, largely increases the associated risk of infection.12,13 Although such devices make health-care delivery more efficient, they may serve as an exogenous source of cross-contamination and nosocomial infections among patients in health-care institutions.14 More seriously, these devices are seldom cleaned and disinfected within hospitals and homes because they are easily damaged by disinfectant fluids. To address these issues, intensive efforts had been devoted to design and fabricate antibacterial thin films that incorporate antibacterial agents to prevent microbial growth, such as antibiotics,15 cationic peptides,16 quaternary ammonium salts,17 metal nanostructures,18,19 and metal oxides.20 Among them, Ag nanoparticles (Ag NPs) show excellent antimicrobial effects against a broad spectrum of bacteria. Additionally, the NPs also have far lower propensity to induce bacterial resistance.21 Moreover, Ag NPs incorporated in thin films can decrease toxicity to human cells and release antibacterial Ag ion (Ag+) sustainably.22,23 Nowadays, Ag NPs have been increasingly Received: January 4, 2018 Accepted: March 16, 2018
A
DOI: 10.1021/acsami.8b00192 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
the liquids used. After the solution was aged at 25 °C for 24 h, a transparent yellow silica sol containing Ag NPs was obtained. Fabrication of Ag NPs-Containing Mesoporous Silica Thin Films. Typically, a cleaned slide glass was immersed in the silica sol for 60 s and then withdrawn at a speed of 3.5 cm/min from the sol. The obtained thin films were eventually calcined in air at 550 °C for 30 min and then at 720 °C for 2 min. The as-prepared thin films were named according to the molar ratio of Ag/Si and the content of P123. For example, film Ag0.2Si(P5) represents that the molar ratio of Ag/Si is 0.2 and the content of P123 is 5 wt %. Similarly, the others were named as Ag0.1Si(P5), Ag0.3Si(P5), Ag0.2Si(P2.5), and Ag0.2Si(P7.5), respectively. In addition to the above thin film samples, powder samples were also prepared for antibacterial tests via the same procedure as the Ag0.2Si(P5) thin film. In a typical procedure, an Ag NP-containing silica sol with identical composition to the Ag0.2Si(P5) thin film was first dried at 100 °C in air to obtain a gel. The gel was then calcined in air at 550 °C for 3 h, followed by additional heat-treatment at 720 °C for 2 min. Finally, the dried gel was ground to give a powder. Antibacterial Tests. In the current study, we chose Escherichia coli (E. coli) as the test bacterial strain, which is a common infectioncausing agent in many health-care settings.30 Preliminarily, the antibacterial efficacy of the corresponding powder samples with identical composition to the Ag0.2Si(P5) thin film was evaluated according to a previously reported procedure.20 First, the powder was dispersed in PBS at varied concentrations of 50−250 μg/mL. Then, the overnight E. coli bacterial culture suspension was diluted to ∼106 cells/mL with PBS. Finally, 10 μL of the diluted culture and 90 μL of PBS with varied concentrations of powder were spread uniformly over sterile Luria−Bertani agar plates. Thus, the final cell culture was 105 cells/mL, and the final powder concentrations that the bacteria were exposed to were 45−225 μg/mL. After incubation at 37 °C for 12 h, the colony-forming units on each plates were counted and calculated to estimate the antibacterial efficacy of the specimens compared with that without powder. Each experiment was performed in triplicates. Thereafter, the antibacterial performance of Ag0.2Si(P5) thin-filmcoated glass was also studied. The coated glass (1 × 1 cm2) and bare glass (1 × 1 cm2) were placed in a 24-well plate and incubated with 1 mL of E. coli culture (∼109 cells/mL) at 37 °C for 4 h. Then, E. coli cultures in the wells were replaced with a nutrient broth medium (1 mL) for further incubation. After 24 h, unattached cells were removed by washing gently with PBS 3 times. For confocal laser scanning microscopy (CLSM, Nikon-C2-SIM) observations, the samples were stained using a live/dead reagent and observed at a λexcitation of 488 nm. The live bacteria with intact cell membranes and dead bacteria cells with damaged membranes were stained in green dye and red dye, respectively. All experiments were performed in triplicates. Characterization. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations were carried out on a Hitachi S-4800 field-emission SEM operated at 5 kV and a JEOL JEM-2100F TEM at an acceleration voltage of 200 kV, respectively. Specimens were prepared referring to our previous work.31 X-ray diffraction (XRD) measurements were performed on a Bruker D8 Focus X-ray diffractometer using Cu Kα radiation (λ = 0.154184 nm). N2 adsorption−desorption measurements were carried out on a Quantachrome NOVA 4200e surface area analyzer at −196 °C using the volumetric method. The BET specific surface areas were calculated using the adsorption data in P/P0 = 0.04−0.20. The pore size distribution of Ag0.2Si(P5) powder was derived from the desorption branch of isotherm through the Barrett−Joyner−Halenda method. Transmittance and reflectance spectra were recorded on a Varian Cary 5000 UV−vis−NIR spectrophotometer, with an integrating sphere attached. Haze data were measured by a WGT-S transmittance/haze determinator from Shanghai Shenguang Instrument Company. Pencil scratching test followed the ASTM D3363-05 standard and was carried out using an Elcometer 3086 Motorised Pencil Hardness Tester by applying a loading weight of 7.5 N. The pencil was held firmly against the film at 45° and pushed away by the tester in a 6.5 mm stroke at 0.5 mm/s. Washability was tested by an Elcometer 1720 abrasion and washability tester. Thin films were washed by sponge brush for 100 cycles in 2 min. Adhesion of film to
applied in daily life, industrial, and technological areas.24−26 Therefore, Ag NPs are ideal agents to fabricate antibacterial thin films, considering their high antibacterial activity, durability, and low toxicity. Given the above, if a thin film simultaneously integrates antireflective and antibacterial properties, it would be of great significance in practical applications. Unfortunately, few related studies have been reported. Kim et al.27 designed a nanostructured surface with a period of 300 nm and an aspect ratio of 3.0 on a flat poly(methyl methacrylate) film, which exhibited a low reflectance of less than 0.5% in the visible wavelength range and good antimicrobial characteristics because of low adhesion. Because the surface showed no antibiofouling effect and failed in killing off the bacteria, the adherent bacteria would grow and multiply to form biofilms over time. Moreover, the fabrication process is complicated. Therefore, there is a high demand for a simple preparation process of multifunctional films with both good antireflective property and significant bactericidal activity. Herein, we developed a facile and effective dip-coating approach for the preparation of thin films simultaneously with broadband antireflective and antibacterial dual functions. The thin films were fabricated from a silica sol composed of PEG− PPG−PEG triblock copolymers (P123) micelles and Ag NPs. The low RI is attributed to homogeneous mesopores formed in the thin films by the evaporation-induced self-assembly process of P123, giving excellent antireflective property. Additionally, Ag NPs with mean size of 2.0 nm were in situ synthesized from AgNO3 using P123 without any other reducing agents.28,29 The uniformly dispersed ultrasmall Ag NPs in the thin films brought them a significant antibacterial activity. Besides, the thin films demonstrated good mechanical robustness (withstanding scratches by 5H pencil, enduring washing test, excellent (5A grade) adhesion to substrate). To the best of our knowledge, the current work represents the first example that simultaneously demonstrates high-performance antireflection, outstanding bactericidal activity, and excellent mechanical robustness.
■
EXPERIMENTAL SECTION
Chemicals. Poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (P123, MW ≈ 5800) and tetraethyl orthosilicate (TEOS, 98%) were purchased from Alfa Aesar. Silver nitrate (AgNO3, AR) was obtained from Guangdong Guanghua Technology Company. Absolute ethanol (EtOH, 99.5%) and hydrochloric acid (38%) were purchased from Beihua Fine Chemicals. Glass substrates were obtained from Jiangsu Swift Boat Glass and Plastic Company. Ultrapure water was obtained from a three-stage Millipore Milli-Q Plus 185 purification system (Academic) and was used in all experiments. Escherichia coli (ATCC 25922) was purchased from the China General Microbiological Culture Collection Center. Mueller Hinton agar was purchased from Beijing Jiangchenyuanyuan Bio-Technology Co. Phosphate-buffered solution (PBS, 0.01 mol/L) was obtained from Beijing Solarbio Science Technology Company. Live/dead reagent was purchased from Shanghai Yisheng Biological Company. Preparation of Silica Sols Containing Ag NPs. Ag NPs containing silica sols were prepared via the following procedure. First, 1−6 g P123 was dissolved in 67 mL EtOH in a flask. Then, 0−3 g AgNO3 and 10.8 mL H2O were mixed, and the aqueous AgNO3 was slowly added to the P123 ethanol solution under stirring within 5 min. Next, 11.15 mL TEOS and 14 μL hydrochloric acid (38%) were added sequentially, followed by vigorous stirring at 70 °C for 6 h. The molar ratio of TEOS/HCl/H2O/EtOH/AgNO3 was 1:0.005:5:41:x (x = 0.1, 0.2, 0.3). The content of P123 was calculated as a percentage per all of B
DOI: 10.1021/acsami.8b00192 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
were observed in the Ag0Si(P5) thin film without Ag NPs (Figure S3a). It was previously reported by He and Kunitake that NO3− ions in thin films decompose to release gaseous nitrogen, oxygen, and nitrogen oxides upon thermal treatment, leading to the formation of nanocraters.32 In the current work, similar processes might have also occurred, eventually resulting in the formation of nanopores. Figure 1b indicates that the Ag0.2Si(P5) film has a uniform thickness of 112 ± 2 nm. Its TEM image (Figure 1c) exhibits a homogenexous pore distribution, and the pore size was estimated to be ca. 4.1 nm. Furthermore, the N2 adsorption−desorption isotherms of Ag0.2Si(P5) thin film demonstrate a typical type-IV hysteresis with a sharp capillary condensation step in the relative pressure range of 0.407−0.702, indicative of the presence of uniform mesopores (Figure 2a). The pore size distribution (Figure S2b) derived from the desorption branch of isotherms reveals a uniform mesopore size of ca. 3.8 nm, which agrees well with the above TEM observation (Figure 1c). The BET-specific surface area of Ag0.2Si(P5) thin film was estimated to be 429.1 m2 g−1, and the pore volume (Vpore) was calculated to be 0.46 cm3 g−1 by the DFT method. As shown in Figure 1d, Ag NPs (indicated by blue arrows) are nearly monodisperse and homogeneously distributed in the mesoporous thin film with an average diameter of 2.04 nm and a standard deviation of 0.12 nm (inset in Figure 1d). The obtained XRD patterns (Figure S1) correspond to the metallic structure of Ag (JCPDS 65-2871), demonstrating the presence of metallic Ag NPs in the thin film. In addition, the broad diffraction peak at around 25° corresponds to the amorphous SiO2 matrix. The above results confirm that we successfully fabricated mesoporous silica (MSiO2) thin films with uniform-sized mesopores and homogeneously distributed monodisperse Ag NPs. Optical Properties of Ag NP-Containing Thin Films. The optical properties of the obtained thin films were investigated using a spectrophotometer. Figure 2a shows the transmission and reflection spectra of bare glass and Ag0.2Si(P5) thin-film-coated glass, and the maximum and average transmittances in the visible light region (400−800 nm) are shown in Table 1. After coating the Ag0.2Si(P5) thin film, the transmittance improved significantly as compared with that of the bare glass. The maximum transmittance reached 98.8% at 587 nm, and the average transmittance of Ag0.2Si(P5) thin film increased significantly by 6.04% as compared with a bare glass. In fact, the thickness of Ag0.2Si(P5) thin film (112 ± 2 nm) (Figure 1b) is very close to the optimal thickness (119 nm) of an ideal antireflective thin film at the wavelength of 587 nm. It
substrate was assessed by tape tests carried out according to the ASTM D3359 standard (Test Method A). An X-cut was first made through a thin film to the substrate, and then a pressure-sensitive tape (3M no. 710) was applied over the cut and firmly pressed on the surface and was finally pulled off rapidly.
■
RESULTS AND DISCUSSION Morphology and Structure of Ag NP-Containing Thin Films. A series of thin films were fabricated by varying the amount of AgNO3 or P123 toward a thin film with the maximum content of Ag NPs but without significantly lowering its antireflective property. A typical thin film Ag0.2Si(P5) is chosen as an example to discuss their morphology and structure in detail. As shown in Figure 1a, the overall thin film does not
Figure 1. Top-view (a) and cross-section (b) SEM and TEM (c,d) images of Ag0.2Si(P5) mesoporous thin film. Inset in (d) is the histogram of Ag NPs in the film.
have any cracks as usually observed on other thin films but does have some nanopores of two sizes on its surface, the diameters of which were ca. 4 nm (indicated by a blue arrow) and ca. 600 nm (indicated by a white arrow), respectively. Both Figures S3 and S4 show that the number of nanopores increases with increasing the amount of AgNO3. However, no such nanopores
Figure 2. (a) Transmission and reflection spectra of Ag0.2Si(P5)-coated glass in contrast to bare glass at the wavelength range of 400−800 nm. (b) Digital photographs of (i) coated glass and (ii) bare glass. C
DOI: 10.1021/acsami.8b00192 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 1. Maximum/Average Transmittance and Haze of Varied Composite Thin Films samplesa Tmax (%) Ta (%) haze (%) a
bare glass 92.0 91.1 0.19
Ag0.1Si(P5) 99.1 97.5 0.00
Ag0.2Si(P5) 98.8 97.1 0.00
Ag0.3Si(P5) 98.0 96.2 0.24
Ag0.2Si(P2.5) 93.2 92.2 0.16
Ag0.2Si(P7.5) 98.8 96.6 0.11
The errors for Tmax and Ta are all ±0.3% and for haze (≤0.5%) are ±0.1%.
Figure 3. (a) Transmission spectra and (b) reflection spectra of varied thin films on slide glasses: (1) Ag0.1Si(P5), (2) Ag0.2Si(P5), and (3) Ag0.3Si(P5).
identical to the optimal theoretical porosity (50%). However, the maximum transmittance (98.8%) of the thin film coated glass is slightly lower than the optimal theoretical transmittance (100%), which may be attributed at least partially to the presence of Ag NPs in the framework. Influence of the Content of Silver. The ability of killing the bacteria directly depends on the content of Ag NPs in the film. The more Ag NPs load in the film, the more Ag+ exist on the surface and thus the film exhibits higher killing efficiency. Therefore, increasing the content of silver in the thin film, while guaranteeing a good antireflective performance, is our final goal. To investigate the optical property changes of the films when the content of silver increases, we prepared three specimens Ag0.1Si(P5), Ag0.2Si(P5), and Ag0.3Si(P5) with increasing Ag/Si molar ratios. The SEM images of Ag0.1Si(P5) and Ag0.3Si(P5) are shown in Figure S3b,c. Clearly, increasing the ratio of Ag/ SiO2 varies the surface morphology. The AFM images (Figure S4a−c) of the thin films are also collected to further unveil the surface morphology. The root-mean-square roughness of Ag0.1Si(P5), Ag0.2Si(P5), and Ag0.3Si(P5) were estimated as ∼2.02, ∼7.77, and ∼21.50 nm, respectively, from a scanning area of 5 μm × 5 μm. As shown in Figure 3a,b, all of the coated glasses show remarkable antireflective performance, and the concrete values of the maximum transmittance and average transmittance of the three samples are listed in Table 1. Compared with Ag0.1Si(P5), the average transmittance of Ag0.2Si(P5) over 400−800 nm has a tiny decrease by 0.4%, whereas Ag0.3Si(P5) decreases much more by 1.3%. The inset of digital photographs in Figure 3b shows the surface appearances of Ag0.1Si(P5)-, Ag0.2Si(P5)-, and Ag0.3Si(P5)-coated glasses in turn. The difference in maximum wavelength and photonic response implies variation in film thickness. Haze is also an important indicator to evaluate the optical property, which is caused by light scattering within the materials or on the surface and expressed as the percentage of scattering light (deviation greater than 2.5° from incident light) and transmitting light. Here, the haze test was also carried out to further characterize the optical properties of the samples. The hazes of Ag0.1Si(P5) and Ag0.2Si(P5) were nearly to 0%, and the haze of Ag0.3Si(P5) is 0.24%, even higher than bare glass (haze = 0.19%).
is known that Ag NPs have a surface plasma resonance absorption peak around 400 nm, but the incorporation of Ag NPs in the mesoporous thin film did not lead to a significant decrease in transmittance around 400 nm, which might be because the film was too thin, and Ag NPs in the film were not enough to affect its optical performance visually. Figure 2b shows the interface reflection toward a fluorescent lamp on the surface of the bare glass and Ag0.2Si(P5) thin-film-coated glass. The reflected light was very strong from the surface of bare glass, which was, however, dramatically reduced on the surface of Ag0.2Si(P5)-film-coated glass. The nearly even reflection suggests the presence of uniform coating. In addition, the surface appearances of coated glass and bare glass at an appropriate angle under natural light are also shown in Figure 2b. The words under the coated glass were seen clearly, whereas they were hazy under bare glass, which is consistent with the results of the haze measurements (Table 1). The theoretical analysis of the optical performance is also discussed. Because the glass substrate is homogeneously covered by the film, it is available for us to reckon the RI of the mesoporous film by the following equation:9,33 nfilm = pna + (1 − p)nframework, where p, na, and nframework are the fraction porosity of film, the RI of air, and the RI of the film framework, respectively. An ideal quarter-wavelength single-layer antireflective film should have an RI (nfilm) of nans = 1.23, where na and ns are 1 and 1.52, respectively. If neglecting the presence of Ag NPs, the framework of the mesoporous film could be roughly considered as amorphous silica and then the RI of the film framework (nframework) could be estimated to be 1.46.34 By substituting nfilm, na and nframework with these values in the above equation (nfilm = pna + (1 − p)nframework), the porosity of the optimized antireflective thin film was estimated to be 50%. Meanwhile, the actual porosity of the mesoporous film is calculable according to the equation: porosity (p) = Vpore/(Vpore + Vframework), where Vpore and Vframework are the pore volume and the framework volume, respectively.33 As mentioned before, the pore volume (Vpore) is 0.46 cm3 g−1. In addition, the Vframework was calculated to be 0.45 cm3 g−1 from the density (ca. 2.2 g cm−3) of amorphous silica. Hence, the actual porosity of Ag0.2Si(P5) thin film was found to be ca. 50.5%, which is nearly D
DOI: 10.1021/acsami.8b00192 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Transmission spectra and (b) reflection spectra of varied thin films on slide glasses: (1) Ag0.2Si(P2.5), (2) Ag0.2Si(P5), and (3) Ag0.2Si(P7.5).
Ag0.1Si(P5) and Ag0.2Si(P5) increase the transmittance and reduce the light-scattering effect, so that the haze is reduced. Although Ag0.3Si(P5) increases the transmittance compared to bare glass, the increasing surface roughness and Ag NPs content give rise to light scattering, which finally increase the haze. Therefore, increasing the ratio of Ag/SiO2 will decrease the transmittance and increase the haze, which may be contributed to the increase of surface roughness and the poor optical transmittance of Ag NPs. Influence of the Content of P123. Because the P123 is the pore-forming agent of the film,35 the content of P123 is an important factor influencing the optical property. Therefore, it is necessary to research the influence of the content of P123. As mentioned above, when the molar ratio of Ag/Si is 0.2, the film Ag0.2Si(P5) performs best from the perspective of comprehensive properties. Hence, we changed the content of P123 in the sol solution, respectively, from 5 to 2.5 wt % and 7.5 wt % while keeping others the same as that of Ag0.2Si(P5), and they were correspondingly named as Ag0.2Si(P2.5) and Ag0.2Si(P7.5). The SEM images of Ag0.2Si(P2.5) and Ag0.2Si(P7.5) are shown in Figure S5a,b. The inset of digital photographs in Figure 4b shows the surface appearances of Ag0.2Si(P2.5)-, Ag0.2Si(P5)-, and Ag0.2Si(P7.5)-coated glasses in turn. From Figure 4 and Table 1, Ag0.2Si(P2.5) only results in a slight increase of 1.1% in the average transmittance of the substrate, whereas the average transmittance of Ag0.2Si(P7.5) increases dramatically by 5.5%, but it is also lower than Ag0.2Si(P5). In addition, the haze of both Ag0.2Si(P2.5) (0.16%) and Ag0.2Si(P7.5) (0.11%) is higher than Ag0.2Si(P5) (0%), which may be related to the transmittance and light scattering because of the internal structure of the film. The decrease of transmittance is mainly because the change of film porosities contributed to different contents of P123, which resulted in deviations of the reflective index from the optimal value. Thus, increasing or decreasing the content of P123 will decrease the transmittance and increase the haze as compared with Ag0.2Si(P5), which may be mainly contributed to the changes of reflective indices. Antibacterial Test of Ag0.2Si(P5) Thin Film. On the basis of both antireflective and antibacterial properties, the Ag0.2Si(P5) film was considered as the optimal one because the content of Ag NPs is maximum while not greatly harming the antireflective property. Therefore, the antibacterial and mechanical properties of Ag0.2Si(P5) were further characterized in detail. Two methods were applied to assess the antibacterial efficacy, including antibacterial efficacy of the corresponding powder and of the film Ag0.2Si(P5), respectively. As shown in Figure 5, E. coli was used to evaluate the antibacterial efficacy of the powder. With an increase of the powder concentration from
Figure 5. Antibacterial efficacy of powder toward E. coli. Data correspond to mean ± standard deviations calculated from three biological replicates.
0 to 225 μg/mL, the powder exhibits an enhanced antibacterial efficacy. When the concentration is 225 μg/mL, it effectively inhibits the growth of 80% bacterial cells. In addition, the mass ratio of SiO2 and Ag NPs in the powder is calculated about 2.8:1, so the actual concentration of Ag NPs is about 59.2 μg/ mL. Compared to the previous report, the concentration of Ag NPs to reduce 80% E. coli bacterial cell is 31 μg/mL,36 which illustrates that the presence of SiO2 slowed the release of Ag+ and thus decrease the toxicity to human cell. We refer that the high antibacterial efficiency is attributed to the ultrasmall size (∼2 nm) of Ag NPs, which induces high Ag atom efficiency and a high local concentration of Ag+ ions assembled on the surface.36 Thereafter, the antibacterial efficiency of Ag0.2Si(P5)-coated glass was assessed. Figure 6 shows the CLSM fluorescence images of Ag0.2Si(P5) coating as compared to bare glass in the presence of live/dead stains, where green, red, and yellow colors refer to alive, dead, and dying bacterial cells, respectively. As illustrated in Figure 6a, almost no dead cells appeared on the uncoated glass after 1 day incubation; as shown in Figure 6d, only a few active cells (ca. 20%) were observed on the coated glass surface, nearly half (ca. 50%) of which were undergoing death. Thus, only 10% of the bacteria were truly active on the coated substrate surface. These observations doubtlessly point to the great antibacterial efficacy (at least 90%) of Ag0.2Si(P5) coating on glass. It must be noted that the dead bacteria on the surface may cause biofouling, and the long-term use of such thin films is yet a potential challenge and needs to be addressed in future works. Mechanical Properties of Ag 0.2Si(P5) Thin Film. Mechanical properties of thin films are a crucial issue for real applications and were studied by pencil hardness test, sponge washing test, and tape adhesion test. First, the pencil hardness E
DOI: 10.1021/acsami.8b00192 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 6. CLSM images of E. coli grown on (a−c) glass surface and (d−f) coated glass surface in the presence of live/dead stain. Green and red staining refers to live and dead bacterial cells, respectively. All of the images obtained by CLSM show the general trend observed in three independent experiments.
Figure 8. (a) SEM images of Ag0.2Si(P5) thin film after washing test and (b) enlarged view of dashed box in (a). (c) Transmission spectra of Ag0.2Si(P5) thin film before and after washing test. (d) SEM image of Ag0.2Si(P5) thin film after tape test.
of Ag0.2Si(P5) film was assessed according to the ASTM D336305 standard. The low-magnification SEM images (Figure 7a,c,e,g) after 3H, 4H, 5H, and 6H pencil scratch indicate the thin film was not cut through to the substrate even after the 6H hardness test. The high-magnification SEM images of scratched areas (Figure 7b,d,f,h) indicate that the deformation degree increases with pencil hardness from 3H to 6H. Only a tiny part of the scratched area was deformed when the hardness of test pencil is no more than 5H. Thus, the thin film could withstand scratches by a 5H pencil. After a 6H pencil scratch, however, the thin film was significantly damaged, demonstrating that the hardness of the film is lower than 6H. Therefore, the overall thin film has a 5H pencil hardness. In many practical applications, washing is a necessary process to remove dust and contaminants from surfaces. Thus, sponge washing was also used to evaluate the mechanical stability of film. We observed the surface morphology and transmittance changes of the coated glass before and after the test. After washing, the thin film remains attached firmly to the substrate, and only tiny regions (
