1 Introduction

Bio-inspired synthesis of highly intricate hierarchical structures using biological materials as templates or precursors is one of the interesting areas of green technology. Materials synthesized by such techniques have found their application in different fields like optics, electronics, biomedical and environmental technology [1–3]. In the recent past, many researchers have attempted to develop materials through green synthesis to solve various environmental problems. Pollution due to growth of industries is one the major problems faced by developing countries. Dyes are complex synthetic molecules that are major constituents of the industrial effluent streams of paint, textile and leather industries. Even small amount of these dyes in water bodies affects the biotic and abiotic factors in the ecosystem [4–6]. Various physico-chemical methods like adsorption, coagulation, ion-exchange, biodegradation etc., are available for decolorization of textile waste water [4, 6–10]. Among all these processes, photocatalytic oxidation appears to be more attractive [11–13]. The major advantages of photocatalytic oxidation over other physico-chemical methods are (1) complete mineralization of dyes (2) no sludge generation and (3) mild reaction conditions. Thus, application of photocatalysts for the elimination of organic pollutants from industrial effluents has been one of the most interesting areas of research.

In a semiconductor photocatalyst, photo irradiation causes excitation of electrons from valance band to the conduction band. This results in the formation of electron–hole pairs which are responsible for degradation of the dyes. Reactive intermediates formed by the reaction of OH radical with hole and intermediates formed due to water splitting reactions leads to mineralization of dye molecules [14–16]. In particular, TiO2 has been widely used in wastewater treatment because of its inert nature, stability, low-cost and resistance to photo-corrosion [17, 18]. However the major weakness of TiO2, as a photocatalyst, is that it requires UV light source and it absorbs only a small fraction of solar light. When TiO2 absorbs photons with energy greater than or equal to its band gap energy, electrons in its valance band are excited and jump to conduction band thereby generating electron–hole pairs. The holes can either oxidize organic pollutants or produce reactive oxygen species like ·OH. When this process takes place in the presence of oxygen, super-oxide anions (O ·−2 ) are produced. Photocatalytic performance of TiO2 is influenced by its crystallite size, surface area and dopants [19, 20]. Doping TiO2 with transition metals, like Ag, Fe, Mo, Ru, Os, Re and V improves photocatalytic efficiency of TiO2 by improving its visible light absorbance [12, 18, 21–23]. Transition metal doping of TiO2 results in formation of band gap traps and prevents recombination of electrons from conduction band or holes from valance band.

Nanoporous TiO2 is synthesized by several methods including, chemical vapor deposition (CVD), sol–gel, and hydro-thermal method. In the recent past, template assisted synthesis of TiO2 had been studied extensively and it had been shown that template controlled syntheses produce excellent surface morphological characteristics and result in enhanced catalytic efficiency [17, 24]. However, conventional templating methods are generally expensive, energy intensive and not eco-friendly. Thus, several scientists across the globe are investigating development of bio-inspired catalysts through green synthesis technology. Major strength of utilizing biological materials as templates in green synthesis is that, they are environmentally benign and the synthesized particles have controlled structural design and morphology. Several biomaterials like wood, egg shell, butterfly wings, plant materials, etc. [25, 26] have been employed as biotemplates in nanomaterial synthesis.

In this paper, we investigate a novel single step method for the synthesis of TiO2 using E. coli as biotemplate with simultaneous in situ silver incorporation. Enzyme nitrate reductase reduces silver ions in silver nitrate solution to metallic silver [27, 28]. E. coli was choosen in this study as it is capable of producing nitrate reductase enzyme which is required for this study. To the best of our knowledge, this is the first report on enzyme mediated synthesis of Ag–TiO2 using E. coli as biotemplate.

2 Experimental section

2.1 Materials

Analytical grade isopropyl alcohol (Merck), titanium (IV) isopropoxide (97 %, Sigma–Aldrich), and silver nitrate (Himedia) were used in this study without further purification. Reactive black B dye (C.I. 20505) was purchased from Indian Dyes and Chemicals, Tirupur, Tamilnadu, India. E. coli (MTCC 448) culture was grown on nutrient agar (Beef Extract 1.0 g/l, Yeast Extract 2.0 g/l, Peptone 5.0 g/l, NaCl 5.0 g/l, Agar 15.0 g/l). The culture was cultivated at 37 °C in LB broth containing Tryptone 10 g/l, Yeast extract 5 g/l and NaCl 5 g/l at 200 rpm in a rotary shaker.

2.2 Methods

2.2.1 Synthesis of TiO2 (TiO2-A)

The TiO2 precursor, titanium (IV) isopropoxide (2 ml), was dissolved in isopropyl alcohol (100 ml) and kept under stirring for 2 h. The solution was centrifuged at 10,000 rpm for 10 min and TiO2 pellets were collected. The TiO2 pellets were then calcined in a muffle furnace at 550 °C for 1 h. Calcined TiO2 was ground finely and stored for further use. This as prepared TiO2 was labeled as TiO2-A.

2.2.2 Synthesis of biotemplated TiO2 (TiO2-B)

Escherichia coli cells grown in Luria–Bertani (LB) broth were isolated by centrifugation at 6,000 rpm for 10 min. The harvested cells were re-suspended in 100 ml of isopropyl alcohol and stirred well for 30 min. 2 ml of titanium (IV) isopropoxide was added to this mixture drop-wise and the mixture was stirred for 2 h. Biotemplated TiO2 particles were separated from the mixture by centrifugation and then calcined at 550 °C for 1 h. This product was labeled as TiO2-B.

2.2.3 Synthesis of biotemplated Ag–TiO2 (TiO2-C)

Silver nitrate (0.5 M) solution 1–5 mol% was added to 200 ml of 20 h E. coli culture grown in LB broth. The culture with silver nitrate was agitated at 200 rpm for 30 min in an orbital shaker and then centrifuged at 10,000 rpm for 10 min. The pellets, cells loaded with silver, were used as biotemplates for TiO2 synthesis. The pellet was then re-suspended in isopropyl alcohol (100 ml) and 2 ml of titanium (IV) isopropoxide was added drop-wise. The mixture was then stirred for 2 h. The precipitate was then calcined at 550 °C for 1 h and finely ground for further use. The Ag–TiO2 samples thus synthesized with 1, 2, 3, 4 and 5 percentage of AgNO3 solution were labeled as TiO2-C1, TiO2-C2, TiO2-C3, TiO2-C4 and TiO2-C5 respectively.

2.2.4 Analysis

X-ray diffraction (XRD) pattern was recorded by irradiating the samples with Cu-Kα, analyzed at an angle 2θ and scan rate of 0.01 °/s using Bruker D8 X-ray diffractometer, Germany. The samples were pelletized at a pressure of 25 ton and analyzed for their elemental composition using S8 Tiger X-ray fluorescence (XRF) spectrometer, Bruker AXS, Germany. The optical spectra for the samples were obtained using Perkin-Elmer Lamda75 UV–Vis–NIR spectrophotometer. The morphology and elemental composition were analyzed by a cold field emission scanning electron microscope (FE-SEM, JSM 6701F, JEOL, Japan). Surface area (BET) and pore size distribution of the catalysts were determined by nitrogen gas adsorption method at 77.3 K using accelerated surface area and porosimetry system (ASAP 2020 V3.04 H, Micromeritics, USA).

2.2.5 Photocatalytic studies

The photocatalytic activities of synthesized materials were investigated by studying the degradation of a commercial dye, Reactive Black, under solar irradiation. 0.1 g TiO2 sample was added to 100 ml of 100 ppm reactive black dye solution and stirred for 20 min in dark. The mixture was then placed under sunlight and samples were drawn from the mixture at regular time intervals. Percentage decolorization was calculated from the optical density of dye solution recorded using Systronics UV–visible spectrophotometer 2201. Optical density of the sample was measured at the characteristic wavelength of reactive black dye (580 nm) corresponding to its maximum absorbance. All experiments were done in duplicates and the photocatalytic efficiencies of the samples were investigated statistically using Minitab 16 software. One-way ANOVA and Tukey’s test were performed to test the statistical significance between different TiO2 catalysts.

3 Results and discussion

3.1 Mechanism of enzyme mediated synthesis of E. coli templated Ag–TiO2

E. coli cells grown in LB broth were harvested during the stationary phase for the synthesis of biotemplated Ag–TiO2. As mentioned in Sect. 2.2.3 the harvested culture was centrifuged and the collected cells were re-suspended in isopropyl alcohol. Silver nitrate was added to E. coli cells collected during stationary phase (20 h). E. coli cells secrete enzymes required for the reduction of silver nitrate. Sigma factors (protein) in E. coli transcribe rpoS genes at the end of exponential phase into RpoS, regulatory gene for stationary phase [27]. RpoS genes respond to environmental stresses like osmotic shock, acidity, low temperature, UV exposure, nutrient limitation, etc. In order to manage the shortage of growth nutrients and minerals at the beginning of stationary phase, RpoS activity increases and induces the production of nitrate reductase enzyme in E. coli [27]. These enzymes are responsible for the reduction of silver ions in silver nitrate solution and silver nanoparticles are synthesized. At this stage, TiO2 precursor was added to the solution under constant stirring. TiO2, along with silver, precipitated on the internal and external surface of the cells. The precipitate was then collected and calcined at 550 °C to get biotemplated Ag–TiO2.

3.2 Characterization

X-ray diffraction studies were performed in order to confirm the presence of TiO2 and silver in its crystalline phase (Fig. 1). Relative crystallinity and the phase structure of the samples were confirmed by comparing the patterns with joint committee on powder diffraction standards (JCPDS) files for TiO2 and silver. Presence of anatase phase in the synthesized materials was identified from diffraction peaks at 25.6°, 37.9°, 48.3° and 54.2°. The 2θ values from these peaks correspond to the reflections from (1 0 1), (0 0 4), (2 0 0) and (1 0 5) planes (JCPDS No. 21-1272) [29]. Diffraction peaks at 38.1° and 44.3° as a response of reflection from (1 1 1) and (2 0 0) planes (JCPDS No. 4-783) [30] were sharp and strong in TiO2-C. This observation confirmed the presence of silver in TiO2-C and suggested that the product obtained was possibly a nanocomposite. Broader and less intense peaks of TiO2-A confirmed that the particles were smaller in size and amorphous in nature. The crystallite size was calculated using Debye–Scherrer formula (Eq. 1) and listed in Table 1. Since the enzymatic reduction is quite faster, silver nanoparticles are formed on the surface of TiO2 resulting in nanocomposites.

$$ D = \frac{k\lambda }{\beta \cos \theta } $$
(1)

where k = 0.89 (constant), λ = wavelength of X-ray (1.5406 Å), β = corrected peak broadening (full width at half maximum) and θ = diffraction angle (degree).

Fig. 1
figure 1

X-ray diffraction patterns of TiO2-A, TiO2-B and TiO2-C. The peaks of pure TiO2 (black thin line) and Ag (blue thick line) were shown as vertical lines (Color figure online)

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Table 1 Particle size analysis
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Presence of silver (10.22 %) in TiO2-C samples was also confirmed from the XRF elemental analysis. Traces of sodium, chlorine and potassium (0.04–0.94 %) were also observed in the samples TiO2-B and TiO2-C. The absorbance spectra recorded between 250 and 800 nm using UV–Vis–NIR spectrophotometer for all TiO2 samples and are shown in Fig. 2. Light absorbance in the visible range was maximum for TiO2-C among the three samples tested and is in the following order TiO2-C > TiO2-B > TiO2-A. TiO2-C exhibited a wide absorption spectrum in the range of 400–800 nm with maximum absorbance at 600 nm. This phenomenon was ascribed to surface plasmon resonance of Ag present in TiO2-C [31]. This observation was consistent with the XRD results discussed above.

Fig. 2
figure 2

UV–Vis absorbance measurements for TiO2-A, TiO2-B and TiO2-C

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Scanning electron micrograph and its corresponding energy-dispersive X-ray analysis for the TiO2 samples are shown in Fig. 3. The morphologies of the samples are depicted in SEM images with average grain size of the TiO2 samples were in the range of 35–85 nm. The samples appeared as clusters of nano-sized structures formed by nucleation and subsequent crystal growth. Biotemplated materials (TiO2-B) and (TiO2-C) were found to be more porous as compared to as prepared sample (TiO2-A). This may be due to the fact that TiO2 and Ag precursors penetrated deep inside the E. coli cells and utilized cell and its organelles as template. Primary particle formation started in supersaturated solution after reaching the metastable zone and then increased in size with crystal growth i.e., nucleation started at room temperature. Presence of silver and nitrate ions, coarsing time and temperature are the major parameters that decide the size of primary particle. Higher temperature resulted in formation of secondary particles by self-assembly and epitaxial growth of primary particles [32]. Size of particles as determined from XRD and SEM analysis is given in Table 1. These sizes correspond to crystallite size and grain size respectively. A grain is formed by agglomeration of several crystallites. Presence of titanium, oxygen and silver was confirmed from the elemental analysis of energy-dispersive spectroscopy (EDS) shown as inset in SEM images.

Fig. 3
figure 3

SEM and EDX (inset) images of TiO2-Commercial, TiO2-A, TiO2-B, and TiO2-C

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The surface area and pore size measurements were measured with N2 gas at 77.3 K. From Table 1 it was observed that TiO2-B had maximum surface area among the three samples followed by TiO2-C. Surface area was lowest for TiO2-A. The pore size of TiO2-A, TiO2-B and TiO2-C were 19.73, 23.95 and 21.63 nm respectively. Increase in surface areas and pore width of TiO2-B and TiO2-C were mainly due to biotemplating. The deposition of silver on the pores of TiO2 was probably the cause for the difference in surface area between TiO2-B and TiO2-C [33].

3.3 Photocatalytic studies

The photocatalytic activities of synthesized materials were analyzed by studying the degradation of commercial reactive black dye under solar irradiation. Decrease in color during photocatalytic oxidation was determined from the optical densities of samples. Biotemplated Ag–TiO2 samples with 1–5 mol% of silver concentration were synthesized and compared for their photocatalytic activity under solar light irradiation. The comparison is shown in Fig. 4a. TiO2 samples prepared with higher silver concentration in the synthesis solution had shown better decolorization kinetics. One-way ANOVA was performed to study the influence of silver nitrate concentration used in the synthesis and the results are shown in Table 2. Results confirmed that the silver nitrate concentration had statistically significant (p < 0.05) effect on the photocatalytic activity of the samples. Further, Tukey test was performed to test the null hypothesis (H0: equal means) and it was found that the means for TiO2-C4 and TiO2-C5 were not statistically different (p > 0.05).

Fig. 4
figure 4

a Photodegradation using biotemplated Ag doped TiO2-C (where C1, C2, C3, C4 and C5 corresponds to 1–5 mol% Ag). Dye concentration = 100 ppm, catalyst concentration = 1 g/l. b Photodegradation of reactive black using TiO2-C4 in comparison with TiO2-A, TiO2-B and TiO2-Commercial and TiO2 prepared by sol–gel, photodegradation, deposition and TiO2-C4 blank (dark adsorption). Dye concentration = 100 ppm, catalyst concentration = 1 g/l

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Table 2 Statistical analysis for TiO2-C (with 1–5 mol% Ag)
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Subsequently, TiO2-C4 was further compared with TiO2-Commercial, TiO2-A, TiO2-B and TiO2 synthesized by other methods like solgel, deposition and photoreduction. The comparison shown in Fig. 4b. It was evident from the figure that incorporation of silver had improved the solar light activity of the sample significantly. However, to confirm this, one-way ANOVA was performed and it was confirmed that there was a statistical difference between the samples (p < 0.05) (Table 3). Further, Tukey test confirmed that the photocatalytic efficiency of TiO2-C4 was statistically different from other samples. Meanwhile, TiO2-A and TiO2-B had shown 30 % color removal and their photocatalytic efficiencies were comparable to that of the commercial TiO2 photocatalyst. The difference between other three samples, TiO2-A, TiO2-B and TiO2-Commercial, were not statistically significant (p > 0.05).

Table 3 Statistical analysis for different TiO2 catalysts
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Among all the samples tested, Ag–TiO2 biotemplate material showed superior degradation in solar light (92 % color removal). As discussed in Sect. 3.2 presence of silver in TiO2-C is confirmed by XRD. Transfer of interfacial electrons and excitation of surface electrons due to surface plasmon resonance of silver [34] are the possible reasons for the increase in solar light activity of the samples. Oxygen vacancy is produced in Ag incorporated TiO2 materials and silver ions present in TiO2 acts as electron scavengers and enhance photodegradation under solar light. In order to reveal advantage points of the preparation method using enzyme, Ag–TiO2 powders were also prepared by photoreduction, sol–gel and deposition methods as described by Sahoo et al. [35], Senthilkumaar et al. [36] and Rupa et al. [37] respectively. The photocatalytic efficiencies of all these samples were compared and the comparison is shown in Fig. 4b. From the figure it can be seen that Ag–TiO2 prepared by enzymatic method showed superior degradation for higher concentration of dye. It could also be seen that the slope the concentration decay curve significantly changed in the case of TiO2-C4 when reaction mixture was exposed to sun light. It suggested that the TiO2-C4 sample possessed good activity in the solar range. For other samples the slope of the curve did not change appreciably. Further to elucidate the difference between adsorption and photodegradation, a blank experiment (dark experiment) was included for the sample TiO2-C4. It can be seen that the adsorption equilibrium was attained with in 25 min approximately. At adsorption equilibrium the percentage color removal was about 10 % while the color removal up to 90 % was achieved by photodegradation. With initial dye concentration of 100 mg/l, equilibrium dye uptake was 9 mg/g of catalyst under dark condition. However, with same initial dye concentration (100 mg/l) upon exposure of solar light 89 mg of dye was removed per g of catalyst after 80 min. This observation confirmed that silver modified TiO2 had solar activity.

4 Conclusions

Three different types of TiO2 samples (1) TiO2 by conventional sol–gel method (TiO2-A,); (2) TiO2 using E. coli as biotemplate (TiO2-B); and (3) Ag–TiO2 using E. coli as biotemplate (TiO2-C) were synthesized using eco-friendly method with an objective to produce solar light active TiO2. The synthesized materials were characterized for their morphology and chemical composition. The XRD and XRF data revealed the presence of anatase phase TiO2 and silver in synthesized material. From SEM, the morphology was found as clusters of nano spherical grains. EDS confirmed the presence of titanium, oxygen and silver through the elemental analysis. Activity shift to visible region due to incorporation of silver ions on TiO2 was confirmed from UV–Vis absorption spectra. From the results, it was shown that the enzyme mediated synthesis reported in the study was effective and significantly improved the photocatalytic efficiencies of the sample. Thus, the proposed method of enzyme mediated in situ synthesis of Ag–TiO2 with E. Coli as biotemplate appears to be an attractive eco-friendly synthesis of solar light active TiO2 photocatalyst.