Evaluation of inhibitory effect of TiO2 nanocoatings against microalgal growth on clay brick façades under weak UV exposure conditions

Building and Environment 64 (2013) 38e45 Contents lists available at SciVerse ScienceDirect Building and Environment journal homepage: www.elsevier.com/locate/buildenv Evaluation of inhibitory effect of TiO2 nanocoatings against microalgal growth on clay brick façades under weak UV exposure conditions Lorenzo Graziani a, Enrico Quagliarini a, Andrea Osimani b, Lucia Aquilanti b, Francesca Clementi b, Claude Yéprémian c, Vincenzo Lariccia d, Salvatore Amoroso d, Marco D’Orazio a, * a Department of Construction, Civil Engineering and Architecture (DICEA), Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy Department of Agricultural, Food and Enviromental Sciences (D3A), Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy Muséum National d’Histoire Naturelle (MNHN), Département “Régulations, Développement, et Diversité Moléculaire” (RDDM), UMR 7245 CNRS “Molécules de Communication etAdaptation des Microorganismes” (MCAM), USM 505 “Cyanobactéries, Cyanotoxines et Environnement” (CCE), 57, rue Buffon, préfabriqué de Botanique, Case N 39, 75005 Paris, France d Department of Biomedical Science and Public Health, Università Politecnica delle Marche, via Tronto 10/A, 60121 Ancona, Italy b c a r t i c l e i n f o a b s t r a c t Article history: Received 24 January 2013 Received in revised form 6 March 2013 Accepted 8 March 2013 Microalgal growth largely affects the aesthetical properties of building façades worldwide. It causes biodeterioration of building materials and, in a later stage, it can compromise integrity of the elements and their durability. Recently, the use of nanotechnology to prevent the growth of microalgae is rising. One of the most widespread and promising material is titanium dioxide (TiO2). Photocatalytic properties of TiO2 inhibit biofouling of microalgae when this coating is stimulated by UV radiation coming from the sun or from artificial light. In this study, the biocide effect of TiO2 coatings applied on clay brick specimens under weak UV radiation was assessed. Results revealed that TiO2 nanocoating was not able to fully prevent microalgal biofouling, but under optimal UV exposure conditions for the growth of microalgae it efficaciously prevented the adhesion of these microorganisms on the treated substrates through the formation of a superficial water film. This property resulted in a good self-cleaning efficiency of TiO2.
2013 Elsevier Ltd. All rights reserved. Keywords: Nano-coating TiO2 Façade biodeterioration Algae Cyanobacteria Durability 1. Introduction The aesthetic quality of outdoor exposed building façades can be seriously compromised by the development of biological stains caused by the growth of microorganisms, as it emerges from the available literature [1e5]. Algae and cyanobacteria are considered as pioneering inhabitants of outdoor exposed surfaces, in Europe algae being prevalent [4,6]. Biodeterioration, caused by microalgae, affects not only new building constructions but also ancient buildings of Cultural Heritage [7e9]. These organisms can adapt to a large variety of substrata, including clay brick façades, and their growth causes the loss of original aesthetic quality on all areas where a suitable combination of dampness, warmth and light occurs [10e 14]. Not only the climatic conditions, but also the properties of the * Corresponding author. Tel.: þ39 0712204587. E-mail addresses: l.graziani@univpm.it (L. Graziani), e.quagliarini@univpm.it (E. Quagliarini), a.osimani@univpm.it (A. Osimani), l.aquilanti@univpm.it (L. Aquilanti), f.clementi@univpm.it (F. Clementi), yep@mnhn.fr (C. Yéprémian), v.lariccia@univpm.it (V. Lariccia), s.amoroso@univpm.it (S. Amoroso), m.dorazio@ univpm.it (M. D’Orazio). 0360-1323/$ e see front matter
2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.buildenv.2013.03.003 substrate influence the growth of microalgae. For building materials such as stone, concrete and clay brick, surface roughness, moisture content, chemical composition, porosity, structure, and texture play a key role [12,13,15e18], affecting water (e.g. rain water) retention on building façades; this allows the growth of algae and cyanobatecteria first, and subsequently of lichens and moss, thus leading to the occurrence of large amounts of biological matter [17]. The biodeterioration effect exerted by microorganisms is due to the production of metabolites (e.g. organic acids and pigments) that damage building materials and cause undesirable changes of their properties [10,19]. In a later stage, this phenomenon can even compromise integrity of the elements and their durability [20,21]. Today, different treatments are available on the market for the prevention of biological stains due to the growth of algae and cyanobacteria, but traditional treatments do not ensure long-term protection, since they need re-application over time [11,22]. Water repellents and biocides are commonly used for removal of microorganisms. The first reduce wetting time of the materials, limiting the microbial growth, whereas biocides allow the microbiological activity to be decreased. Furthermore, these two types of L. Graziani et al. / Building and Environment 64 (2013) 38e45 products can be applied in combination. Algae and cyanobacteria require daylight for photosynthesis and hence multiplication, thus the use of photocatalytic materials to control their growth takes sense [23e25]. Two are the main advantages of using TiO2 for the treatment of building façades against the growth of microalgae. First, bonds formed between microalgae and substrata are destroyed by photo-induced oxidation of biofouling contaminants [24]. Secondly, the photocatalytic properties of TiO2 are responsible for the production of a super-hydrophilic interface that allows water to form a thin film on the solid surface that causes a better wetting of the contaminants [26e28], rendering the removal of macro-organisms due to water action and evaporation easier [25]. The effect of TiO2 to contrast the growth of algae in water and to prevent water contamination is well known [29e32]. In the last decades, some studies have been also carried out in the field of building constructions. For example, three types of photoactive kaolin/TiO2 composites were added on concrete blocks to study their effects against the growth of Chlorella vulgaris [33]. In the concrete field, treatment with TiO2 lead to a 66% inhibition of the algal growth in the presence of UV irradiation and this effect could be increased to 87% by adding a noble metal such as Pt or Ir [34]. Different TiO2 application procedures on cementitious material were even evaluated and the results proved that samples with TiO2 were efficient to avoid algal growth, while cement paste specimens containing TiO2 in the mixture seemed not to be adequate to the scope [35]. Further studies were aimed at evaluating the efficacy of TiO2 in preventing algal fouling on mortars, by using accelerated laboratory tests under different UV exposure conditions. Fonseca and colleagues [36] comparatively evaluated anatase TiO2 and two conventional biocides, whereas Zhang and colleagues [37] investigated the biofouling resistance of TiO2 nanoparticulate silane/ siloxane treatments. Finally, Gladis and colleagues [38] studied the biofouling resistance of TiO2 under low intensity UV exposure (northeast direction). The latter condition is favourable for the growth of microalgae, but it is not the optimum for activation of TiO2. To the authors’ knowledge, no data are available on the biocidal activity of TiO2 against the growth of microalgae on building façades, especially made of clay brick, exposed to low UV radiance. Based on the above premises, this study was aimed at investigating the biodeterioration effect of the green alga Chlorella mirabilis and the cyanobacteria species Chroococcidiopsis fissurarum on clay brick façades treated with TiO2 in the form of nanocrystalline anatase. 2. Phases, materials and methods 2.1. Phases This study articulates in three phases: (i) specimens characterization and application of nanostructured TiO2 so as to enhance its photocatalytic activity; (ii) evaluation of the microalgal growth on TiO2-treated and control specimens; and (iii) assessment of selfcleaning ability of TiO2. In the first phase, specimens were analysed for total porosity, surface roughness, and colour, to evaluate the morphology and the original aesthetic properties of the materials. In the second phase, the growth of two test strains ascribed to Chlorella mirabilis and Chroococcidiopsis fissurarum on eight TiO2treated and four untreated (control) specimens was evaluated through an accelerated growth test and a combination of colorimetric analysis and Digital Image Analysis (DIA) [16,39]. At the end of the accelerated test, the self-cleaning ability of activated TiO2 due to the formation of a super hydrophilic film was evaluated though a simple washing test; the strength of the biofilm adhesion and its thickness before and after washing were evaluated 39 by both DIA of scanned images and Confocal Laser Scanning Microscopy (CLSM). 2.2. Material characterization and TiO2 application Twelve prismatic (80  80  30 mm3) clay brick specimens were cut from clay brick panels used in aerated building façades with an average density equal to 1798 kg/m3. The material characterization was carried out before and after the application of TiO2. Before treatment, total porosity was evaluated with a mercury intrusion porosimeter (Micromeritics Autopore III) on three samples after drying at 60  C for 24 h. An average total porosity of 21.05% was measured. Surface roughness was also measured with a perthometer (Mahr model M4P) with a stylus tip radius of 2.0 mm. Ten measurements were made with a cut-off length of 0.8 mm following the method described in UNI EN 623-4:2005 [40]. The average roughness coefficient Ra was 7.22 mm. Colour identification of clay brick specimens was obtained by a colorimeter (Konica Minolta CM 2600d) with a 3 mm aperture. Twelve specimens were analysed and, for each specimen, nine measurements were repeated as recommended in UNI EN 15886:2010 [41]. Measurements were carried out using a daylight illuminant (D65) and 10 observer angle. Results were expressed in CIELAB colour space. An 8  8 reference spatial grid was used to ensure precise repeated measurements on the same points in subsequent tests. The average value of L*, a* and b* coordinate of tested clay brick was 52.33, 22.19, and 25.95, respectively. Two different commercial TiO2 water solutions (in the form of nanocrystalline anatase) were deposited on clay brick specimens [26e28]. The first solution (S1) had a TiO2 concentration equal to 1% (wg/vol), while the second solution (S0.5) had a concentration of 0.5% (wg/vol). Solutions S1 and S0.5 were applied on four specimens each, whereas the remaining four specimens remained untreated and hence were used as a control. Before TiO2 application, specimens were dried at 80  C, until the difference between two subsequent weighing was less than 0.1% (wg/wg). An air spray gun with a nozzle of 0.8 mm diameter was used to apply TiO2. Specimens were sprayed manually from a distance of approximately 250 mm, to better simulate a real application on building facades, and subsequently weighed with an electronic balance (Gibertini model EU4000AR) to determine the amount of solution applied on the specimens surface. Results were expressed as g/m2. After TiO2 application, specimens were dried at 60  C for 1 h to accelerate the drying process, as previously reported [26,28]. List of specimens and corresponding treatments are shown in Table 1. To understand the effect of TiO2 once it is applied on building façades, data from specimens treated with the same solution were averaged, and standard deviations were calculated. Table 1 Specimens identification with the amount of TiO2 applied on the surface. Sample name Type of treatment TiO2 amount (g/m2) Average TiO2 amount (g/m2) Standard deviation L1a L2a L3a L4a Untreated 0.00 0.00 0.00 0.00 e e L1 L2 L3 L4 One layer S1 0.17 0.30 0.28 0.25 0.25 0.07 L25 L26 L27 L28 One layer S0.5 0.16 0.07 0.08 0.15 0.11 0.05 40 L. Graziani et al. / Building and Environment 64 (2013) 38e45 A coefficient of variation (ratio of the standard deviation to the mean) of 28.0% and 45.4% were found for S1 and S0.5, respectively, thus indicating a heterogeneous application of TiO2. 2.3. Microbial cultures Two test strains ascribed to Chlorella cf. mirabilis Andreeva (ALCP 221B) and to Chroococcidiopsis fissurarum (Ercegovic) Komárek & Anagnostidis (IPPAS B445), both deposited at the Algotheque du Laboratoire de Cryptogamie of the Museum National d’Histoire Naturelle (MNHN), were selected for use in the accelerated growth test, since they are common colonizers of building façades [15]; their optimal growth conditions in terms of light intensity, temperature and relative humidity have previously been described [15,19,39,42]. Each strain was grown in batch culture in Bold’s Basal Medium (BBM) prepared as indicated in ASTM D558909 [43] and incubated at 24  C in a glass chamber with a 14 h/10 h light/dark photoperiod and light intensity of 1.500 lux. 2.4. Water run-off test Accelerated growth of the test strains on the specimens was achieved by performing a water run-off test in the laboratorymade system depicted in Fig. 1. Previous studies were used as the basis for the test design [16,33,37,39,43,44]. The system consisted of a 100  40  53 cm3 glass chamber containing two aluminium-glass composed racks inclined at 45 to increase the contact time between specimens and broth culture. The racks were positioned front-to-front along the long dimension of the glass chamber to maximize the number of specimens that could be processed simultaneously. The glass chamber was filled with 40 L of BBM and inoculated with both the test strains, at a final concentration of approximately 4 mg/L (dry mass determination), maintaining a temperature of 24  C with a heater (Heather Bluclima 150 W) and continuously agitating the broth culture with two wave pumps (Hydro, model Koralia 1600/425) placed on the base of the chamber. Temperature and relative humidity (RH) inside the glass chamber were recorded every 5 min over the nineweek-test using a remote data logger (Lascar Electronics model EL-USB-2). Daylight was provided by two 39 W neon lamps (Sylvania, model TopLife) with a light temperature of 5000 K, which were placed on the lid of the chamber at the same distance from the two racks. The samples were irradiated with an average UV intensity of 165 mW/cm2, as resulted from measurements made with a digital light meter (General Tools UV513AB), and a day length of 14 h. The glass chamber was covered with a non-hermetic lid and placed in a dark room to avoid entrance of natural light. Fig. 1. a) View and schematic view of test apparatus b) side view, c) front view. L. Graziani et al. / Building and Environment 64 (2013) 38e45 The system was equipped with two sprinkling rails made of PVC tubes with three 2 mm-holes drilled every 20 mm, in correspondence of each specimen. The rails were mounted approximately 20 mm over the specimens and connected to a 500 L/h water pump (Blupower) submerged in the broth culture with plastic hoses. The principle of the accelerated growth test has previously been described [15]. Briefly, the broth culture circulating through the rails was sprinkled over the top of the specimens and let to run down the surface, thus allowing the microalgal strains to adhere and form a biofilm. The suspension was sprinkled with a run/off cycle of 15 min for a duration of 6 h (3 h run and 3 h off); considering a flow of 5 L/h, each sample was sprinkled with 15 L of broth culture every light/dark cycle. RH in the chamber was 80%, but it reached 90% during water sprinkling. 2.5. Biofouling evaluation Biofouling due to the growth of the microalgal strains on the specimens surface was weekly evaluated by the naked-eye. In order to avoid human’s eye subjectivity, it was also quantitatively determined through a combination of colorimetric analysis and Digital Image Analysis (DIA) of scanned images of the samples. Colorimetric analysis was carried out with a portable spectrophotometer (Konika Minolta CM-2600D) onto nine points randomly selected over the surface of each sample. The result for 41 each point was the averaged value of three measurements automatically collected by the instrument. The points analysed were the same selected for characterization of the materials, as described in Sect. 2.2. Once a week, scans of the specimens surfaces were made using a scanner (Canon Scan Lide 700F) with a 600 dpi resolution. Percentage of specimen surface covered by the microalgal film was determined by DIA of scanned images using the ImageJ software, version 1.46r [45]. Before analysis, images were subjected to a binary conversion by filtering all the components of the CIELAB colour space. Low values of a* (green) and high values of b* (yellow) were assumed to indicate high concentration of chlorophylls and carotenoids, respectively, as previously reported [39]. Variation in lightness L* was also taken under consideration since soiling matter darkened the colour of all the samples. Colour changes due to the occurrence of the microalgal film were determined using the following formula: DE0;t rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2 2  L*0 L*t ¼ þ a*0 a*t þ b*0 b*t (1) where L*0, a*0 and b*0 are the CIELAB coordinates of untreated samples (time zero) and L*t, a*t and b*t are the CIELAB coordinates of the sample at each measuring time (once a week). Fig. 2. a) 3D projections of a sample image stacks. b) top projection of the sample and c) gradient of thickness from COMSTAT. 42 L. Graziani et al. / Building and Environment 64 (2013) 38e45 2.6. Self-cleaning ability of TiO2 At the end of the nine-week accelerated growth test, the selfcleaning ability of activated TiO2 was evaluated by manually washing the specimens with 500 mL of distilled water, using a laboratory dispenser with a conical tip from which a thin water jet was released. Samples were observed before and after the washing process by using both scanner and CLSM. A Carl Zeiss Axioplan 200M Microscope equipped with a Zeiss LSM 510 module was used to carry out microscopic observations with 10 magnification. After the acquisition of the images, DIA was used in order to measure the extension of biofilm on the surfaces. Images from scanner were used to evaluate the washed area from the entire surface of the specimens, while CLSM was exploited to evaluate the self-cleaning ability of the coating material in restricted zones. In this way, the self-cleaning ability of TiO2 was evaluated from either a global or a local (microscopic) point of view. Thickness of the biofilm was assessed by COMSTAT, as previously elucidated [46,47] after a 3D model reconstruction of the acquired images by CLSM (Fig. 2). Portions (10  5  3 mm3) of the specimens surface were cut before and immediately after washing and observed with CLSM. Microscopic observations were conducted as described in a previous study [47]. A series of images at discrete focal planes (z-stack) was captured at six locations across the surface of the specimens. Each image was 1273  1273 mm2 in size and the vertical distance between two subsequent focal planes was 10 mm. The bottom image was determined by the first focal plane that contained no in-focus cells, and the top image was the last focal plane that contained cells. Chlorophyll within the cells was excited using an argon laser with a wavelength of 488 nm. The resulting fluorescence was observed using a 505 nm long pass filter and a 10 magnification objective lens. In order to evaluate the superficial area covered by the microalgal film, LSM510 software program was used [48], as previously described [47]. Each stack was overlapped and top projection was obtained for each observed area. Resulting images were converted in grey value scale and background was subtracted by applying a threshold value of 20. Area reduction (in percentage) was calculated following eq. (2): DA ¼ ABW AAW  100 ABW (2) where ABW and AAW are the area (in mm2) covered by microalgae before and after the washing process, respectively. The average thickness of the biofilm was determined by COMSTAT as previously described [46,47]. Thickness reduction (in percentage) was calculated following eq. (3): DT ¼ TBW TAW  100˛ TBW (3) where TBW and TAW are the average thickness (in mm) of the biofilm before and after the washing process respectively. In order to study significance of results referring to self-cleaning ability, a one-way analysis of variance (ANOVA) was carried out, along with the TukeyeKramer honestly significant difference (HSD) using the JMP software [49]. Significance of differences was defined at P-value p < 0.05. Fig. 3. Fouling on untreated specimens a) and on treated specimens b). Numbers indicate week progression. Images are representative of four specimens for each treatment. L. Graziani et al. / Building and Environment 64 (2013) 38e45 43 3. Results & discussion 3.1. Weekly evaluation of biofouling with the naked eye During the water run-off test, the appearance of biological stains due to the microalgal growth on the specimens surface was weekly evaluated with the naked eye. From Fig. 3 one can see that after nine weeks all the specimens were characterized by the occurrence of a superficial green patina. However, in specimens treated with TiO2, the area covered by the biofilm seemed to be less expanse than that colonizing the control specimens; a neat difference between treated and untreated specimens was evident as early as week 4 (Fig. 3). 3.2. Colorimetric analysis The results of colour variation (DE) resulting from the colorimetric analysis are shown in Fig. 4. Although in the early phase of the monitoring period (from week 1 to week 3) specimens treated with TiO2 seemed to be characterized by a reduced fouling respect to uncoated specimens, no significantly differences were found and an overlapping of standard deviations calculated for TiO2-treated and control specimens was seen. From week 4 a comparable trend was observed for specimens treated with solution S0.5 and untreated specimens, with a slight discrepancy at week 8. As far as specimens treated with S1 are concerned, the maximum DE calculated respect to untreated specimens is 1.36. This difference is not visible by the naked eye, since the just noticeable difference (JND) is commonly fixed to 2.3 [50]. These evidences clearly suggest the inefficacy of TiO2 coating in inhibiting the superficial colonization by algae and cyanobacteria, and hence the appearance of biological stains. This result is in line with other available from the literature, reporting that photocatalytic TiO2 coating was not effective against phototrophic growth on roof tiles in Germany [38]; these evidences could be feasibly ascribed to a weak UV radiation unable to activate the TiO2 biocide effect. 3.3. Microbial growth on clay brick Percentages of area covered by the biofilm are given in Fig. 5. In order to study the correlation between amounts of TiO2 and covered area, specimens treated with the same TiO2 solution were grouped, and the corresponding data averaged. Fig. 4. Mean values  standard deviation (n ¼ 4) of biofouling assessed by colorimetric analysis. Growth of microalgae on treated specimens is comparable to that occurring on untreated specimens. Bars indicate standard deviations. Fig. 5. Percentages of area covered by the biofilm during the water run-off test. Mean value (n ¼ 4). Fig. 5 shows that TiO2 did not completely inhibit the growth of microalgae on the treated specimens. However, from week 3 to week 7, the effect of TiO2 is more evident. This latter finding might be ascribed to an insufficient UV radiation for the successful activation of photocatalysis until week 3, when TiO2 was finally activated and it started to inhibit algal adhesion on the substrata. Such a TiO2 effect can be clearly evinced from the evaluation of the curves shown in Fig. 5. Indeed, between weeks 4 and 5, the two curves referring to specimens treated with TiO2 show an inflection point, thus indicating a slowing down of the microalgal colonization. However, the microbial growth had a shadow effect on the coating, thus limiting the activity of the nano-coating and hence leading to a progressive increase of the covered area. Indeed, at the end of the nine-week-test, the percentage of covered area was almost the same in all the specimens, being equal to about 50%. From Fig. 5, a linear correlation between the amount of TiO2 applied on the specimens and its efficiency can be evidenced, and the higher was the quantity of photocatalytic coating, the slower was the coverage area due to the growth of the test strains. These results are in agreement with those obtained in a previous study, which was aimed at assessing the inhibitory effect of this photocatalytic coating on a roof exposed to northeast in Germany [38]. Fig. 6. Reduction (in percentage) of area covered by the microalgal film (DA) after manual washing of TiO2-treated and control specimens, calculated on the basis of the results from Digital Image Analysis (DIA) of scanned images and Confocal Laser Scanner Microscopy (CLSM). Mean values  standard deviations (n ¼ 4). 44 L. Graziani et al. / Building and Environment 64 (2013) 38e45 4. Conclusion Table 2 ANOVA test of results about washed area from the surface. Technique ANOVA between P-value DIA Untreated e S0.5 Untreated e S1 S0.5 e S1 0.0076 0.0008 0.2609 CLSM Untreated e S0.5 Untreated e S1 S0.5 e S1 0.0012 0.0021 0.9031 3.4. Self-cleaning ability on specimens with TiO2 Self-cleaning ability of TiO2 was assessed by evaluating both the percentage of washed area on the specimens surface and the reduction in biofilm thickness as described in Sect. 2.4. Fig. 6 shows that the percentage of washed area is greater in the treated specimens than in the controls ones. In order to assess the occurrence of significance differences, ANOVA test was carried out as described in Sect. 2.6 (Table 2). A significant difference between treated and control specimens was found irrespective of the analytical technique used (DIA or CLSM, P value < 0.05), whereas no significant differences were found between specimens treated with S0.5 and S1 (P value > 0.05). Thus, no correlation was found between the amount of TiO2 and its self-cleaning ability. This finding is in line with those reported in other studies, where different amounts of TiO2 nano-particles were found to be characterized by a similar self-cleaning efficiency [27]. Scanned images revealed that washed area on treated specimens is two times bigger than on control specimens, while this ratio is equal to three in the case of CLSM observations. Thus, the self-cleaning ability was more evident in the treated specimens than in the controls. This latter finding might be explained by the higher wettability of specimens treated with TiO2. Indeed, when TiO2 is activated by UV light, it forms a super-hydrophilic film on the specimens surface, which renders the adhesion of algae and cyanobacteria to the microstructure of clay bricks more difficult. A further effect of the increased wettability of the TiO2 treated specimens seems to be a reduction of the biofilm thickness, as shown in Fig. 7. Although treated specimens seemed to be characterized by a higher thickness reduction, this trend was not confirmed by statistical analysis. Fig. 7. Reduction (in percentage) of microalgal film thickness (DT) after manual washing of TiO2-treated and control specimens. Mean values  standard deviations (n ¼ 4). In this research, the inhibitory effect of TiO2 nano-coating against the adhesion of microalgae on clay brick façades was assessed by using an accelerated growth test under optimal conditions for multiplication of Chlorella mirabilis and Chroococcidiopsis fissurarum. Antifouling ability of TiO2 was monitored by colour change measurement, as well as DIA and CLSM. Colorimetric analysis showed that area covered by the two test strains on TiO2-treated and control specimens was the same, whereas DIA showed that TiO2 nano-coating was able to slow the biofilm formation on the specimens, but it was unable to stop the microalgal growth. An increase in the amount of TiO2 applied seemed not to produce appreciable differences. The main problem that causes this trend consists in the shadow effect caused by the microalgae on the nanofilm. Thus, when TiO2 is applied on façades exposed to weak UV radiation (i.e. north façades), it cannot stop the biofouling due to the multiplication of these microorganisms. Nevertheless, during a washing process, for instance due to wind-driven rain, TiO2 proved to greatly enhance the self-cleaning ability of brick surfaces as it was suggested by the more effective removal of the microalgal film from TiO2-treated specimens rather than the control. Thus, self-cleaning ability of TiO2 could reduce biodeterioration of building brick façades even under weak UV exposure conditions. Therefore, the exploitation of nano-structured TiO2 in addition to either traditional or innovative treatments [38] might enhance the inhibitory effect against microalgal fouling on clay brick façades. Further researches are currently in progress to confirm such an hypothesis. 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