Introduction

Nanotechnology is one of the most interesting branches of science which could provide materials at the nanoscale size. Nanomaterials have been utilized broadly in various fields such as tissue engineering, drug delivery, bioscience, agriculture and water due to its unique physical and chemical properties [1,2,3]. Recently, nanoparticles (NPs) demonstrated a high impact in the antibacterial filed [4, 5]. Among them, zinc oxide (ZnO) is considered for its photocatalytic-driven bacterial activity in which it has a broad direct band gap energy of 3.3 eV and high exciton binding energy of 60 meV [6]. Stability, good biocompatibility, low cost, and easy preparation are some of the advantages of the ZnO nanoparticle that makes it a promising antibacterial agent [4, 7]. Beside these benefits, ZnO nanoparticle also has some important drawbacks that limited its applications as an antibacterial agent such as easily losing the active sites and showing a tendency of aggregation. More importantly, ZnO with a wide band gap (Eg = 3.3 eV) could generate charge carriers only in the UV portion of sunlight. It must be highlighted that to overcome these shortcomings of the ZnO nanoparticle, some simple and efficient strategies have been developed [6]. However, as an alternative, designing and fabrication of new nanomaterials with low band gap energy which is important in antibacterial activities could be considered as another strategy.

Organic–inorganic hybrid perovskite nanostructures (MAPbX3; MA = CH3NH3, X = Br, Cl or I) with tunable optical properties have attracted much attention. Perovskites are one of the wonderful groups of materials which demonstrate some practical properties such as high temperature superconductivity, piezoelectricity, pyroelectricity, ferroelectricity, and catalytic property [8]. Recently, as an alternative, all-inorganic perovskites (CsPbX3, X = Cl, Br, I) have received tremendous consideration due to their enhanced stability and outstanding electronic properties compared to their organic–inorganic counterparts. To the best of our knowledge, there are no reports about antibacterial activity of all-inorganic perovskites. Therefore, for the first time, we investigated and compared the antibacterial activity of inorganic cesium lead bromide (CsPbBr3) perovskite powder with ZnO powder against Gram-negative, rod-shaped Escherichia coli O157:H7 (abbreviated as E. coli). Experimental results showed that CsPbBr3 perovskite agent was more efficient than ZnO for the destruction of bacteria.

Materials and methods

CsPbBr3 perovskite nanoparticles were fabricated through the free-surfactant process. All starting materials used for CsPbBr3 Perovskite NPs including lead bromide (PbBr2) and cesium bromide (CsBr) were obtained from Sigma-Aldrich and used without further purification. In a typical synthesis, in a 250 mL round bottom flask containing 100 mL DMF, 1 mg PbBr2 was dispersed via ultra-sonication. After stirring at 75 °C for 20 min, the appropriate amount of CsBr in ethanol was added dropwise to the mixture solution under vigorous stirring. Reaction temperature was decreased to 50 °C and stirred for 10 min. The obtained solution was centrifuged at 2500 rpm for 10 min and then washed several times with 1-propanol. Finally, collected powders were annealed at 350 °C and dried at 80 °C for 24 h [9].

In the case of ZnO NPs synthesis, zinc acetate dihydrate [Zn(O2CCH3)2·2H2O], isopropanol and monoethanolamine were purchased from Sigma-Aldrich. ZnO NPs were prepared using the sol–gel pyrolysis method [10]. Typically, 1.1 g zinc acetate dihydrate was added in a mixture of isopropanol (15 mL) and monoethanolamine (0.55 mL). To obtain a clear and homogeneous solution, the mixture was stirred at 75 °C for 2 h. To make gel, the solution was kept for 2 days (aging). The gel was then preheated at 80 °C to evaporate the solvent and remove organic residuals. Finally, the obtained solid powders were annealed at 350 °C for 2 h. Shimadzu UV-1800 model spectrophotometer, SEM/EDX (MIRA3 TESCAN microscope) and TEM (Philips CM120). The agar well diffusion method was utilized to test the antibacterial property of NPs against Escherichia coli O157:H7 bacteria cells [11].

Results and discussions

Figure 1a, c depicts the XRD patterns of ZnO NPs and CsPbBr3 perovskite NPs, respectively. From Fig. 1a, the characteristic peaks at 2θ values of 31.75° (100), 34.44° (002), 36.25° (101), 47.54° (102), 56.55° (110), 62.87° (103), 66.38° (200), 67.91° (112), 69.05° (201), and 72.61° (004) can be related to the polycrystalline with wurtzite structure of ZnO (JCPS card no. 5-0664) [12]. The sharp and narrow peaks with no characteristic peaks of any impurities indicated that high-quality ZnO NPs were fabricated. The XRD pattern of CsPbBr3 perovskite NPs is shown in Fig. 1c. The synthesized CsPbBr3 powders show a monoclinic structure (a = b = 0.5827 nm and c = 0.5891 nm) [JCPS No. 018-0364]. This pattern is in very good agreement with other reports [13]. In addition, according to the Debye–Scherrer formula [14], the average crystallite size of ZnO NPs and CsPbBr3 perovskite NPs was found to be ~ 52.46 nm and ~ 39.20 nm (Fig. 1b, d).

Fig. 1
figure 1

a XRD pattern of ZnO NPs, b average crystalline size of ZnO NPs calculated by Scherrer’s equation, c XRD pattern of CsPbBr3 perovskite NPs, d average crystalline size of CsPbBr3 perovskite NPs calculated by Scherrer’s equation

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The optical behaviors of synthesized NPs were measured by absorption spectra in the UV/Vis wavelength range of 300–700 nm at room temperature and are illustrated in Fig. 2a. Absorption bands at 376 and 510 nm could be seen for ZnO and CsPbBr3 perovskite NPs, respectively. Compared to ZnO NPs, the higher red shift in the adsorption band of CsPbBr3 perovskite NPs indicated that a higher percentage of solar light can be used for electron–hole pair generation [15] which is substantial in antibacterial activity. Moreover, the optical band gaps of ZnO and CsPbBr3 perovskite NPs were calculated using Tauc’s equation, by plotting (αhυ)2 versus hυ [16], as depicted in Fig. 2b.The calculated optical band gap energy of ZnO and CsPbBr3 perovskite NPs is 3.3 and 2.4 eV, respectively. The lower band gap energy of CsPbBr3 Perovskite NPs exhibited that the morphology of crystals may have various main active facets and response various excitation energy with different direct bandgaps, resulting from quantum size effect.

Fig. 2
figure 2

a UV/Vis adsorption spectra of ZnO and CsPbBr3 perovskite NPs and b Tauc plot of ZnO and CsPbBr3 perovskite NPs

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Figure 3a, b depict the TEM images of ZnO and CsPbBr3 perovskite NPs, respectively. These images show that the synthesized nanoparticles had almost spherical shapes with smooth surfaces. The primary particle size of the ZnO and CsPbBr3 powders was found to be approximately 59 nm and 40 in diameters, respectively. These diameters of particles were found to be in good agreement with XRD data (Fig. 1). Moreover, Fig. 3c, d shows the EDX spectrum of ZnO and CsPbBr3 nanoparticles prepared on glass (Si/SiO2) substrates, respectively. The elemental constitution of ZnO nanoparticles was found to have a weight percentage of 43.65 of silicon, 3.28 of zinc and 32.62 of oxygen. The EDX spectrum of CsPbBr3 nanoparticles on glass substrate shows the existence of Cs, Pb, and Br elements. A ratio of 1.2:1.00:4.35 was obtained for Cs:Pb:Br elements from quantitative analysis, which is consistent with the expected composition.

Fig. 3
figure 3

TEM images of the a ZnO and b CsPbBr3 nanopowders annealed at 350 °C

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Escherichia coli O157:H7 was used as a model to evaluate the antibacterial activities of ZnO and CsPbBr3 perovskite NPs. As could be seen from Fig. 4, CsPbBr3 perovskite NPs show better antibacterial activity than ZnO NPs. Generally, two main factors determine the antibacterial activity of agents; band gap and exciton binding energy. The exciton binding energy (between electrons and holes) can be calculated using the Bohr diameter of the nanoparticle [17]. CsPbBr3 nanoparticles with a band gap of 2.4 eV and exciton binding energy (Eb) of 40 meV show a stronger antibacterial effect on E. coli rather than ZnO NPs with a wider band gap of 3.3 eV and larger exciton binding energy of 60 meV. The band gap determines the wavelength at which the material could absorb the incident light and the wavelength at which antibacterial agents operate most efficiently. A wider band gap agent (ZnO nanoparticles) is useful only at shorter wavelengths than CsPbBr3 nanoparticles. The band gaps ZnO (3.3 eV) and CsPbBr3 correspond to wavelengths of approximately 375 nm and 510 nm, which is UV and visible regions, respectively. Exciton binding energy refers to the stability against thermal dissociation of excitons which have a great tendency to dissociate due to the thermal energy. In ZnO NPs, a significant number of excitons exist (higher exciton binding energy), but due to its wider band gap, only few electron–hole pairs could be created at room temperature. These two important effects determine the antibacterial activity of nanoparticles. As a result, more free created electrons and holes have sufficient lifetime in CsPbBr3 which could induce photo-generation of reactive oxygen species (ROS) on the surface of CsPbBr3 from adsorbed oxygen.

Fig. 4
figure 4

Antibacterial activity of ZnO and CsPbBr3 on E. coli treated through the well diffusion method at different periods of treatment (1, 12 and 48 h)

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Conclusion

Two different types of nanoparticles, ZnO and CsPbBr3 perovskite, were successfully synthesized and characterized. Free-surfactant fabrication of CsPbBr3 perovskite NPs offered a fast, easy and inexpensive approach for the synthesis of nanomaterials, which show potential for antibacterial activity at a large scale. The antibacterial activity of nanoparticles against Escherichia coli O157:H7 was evaluated. Experimental results showed a significant room temperature antibacterial operation of CsPbBr3 perovskite nanoparticles due to its narrower band gap.