**1. Introduction**

Indoor air pollution is a serious public health concern and a major cause of morbidity and mortality worldwide. In Europe, the total disease burden due to indoor air is about two million DALY (disability-adjusted life year) a year [1]. In 2006, the World Health Organization (Regional Office for Europe) started to draw up guidelines for indoor air quality [2] and addressed the three causes of indoor pollution that were most relevant for public health [3]:


The presence of microbial populations in damp indoor environments is one of the main causes of the degradation of indoor air quality and contributes to Sick Building Syndrome [6,7]. In Northern Europe and North America, the prevalence of mold contamination in buildings is estimated at between 20% and 40% [8]. Among the hundreds of microbial species that can be found in indoor environments [9–11], some are listed as potentially pathogenic species by the French High Council

**25** 

for Public Health and the France Environment Health Association [8,12,13]. Various studies have reported associations of mold growth with respiratory diseases in buildings, especially damp and water-damaged buildings [14]. Microorganisms may produce contaminants, *i.e.*, aerial particles, such as spores, allergens, toxins and other metabolites, that can be serious health hazards to occupants [15–23]. Frequent exposure to these contaminants can lead to various health troubles, including irritations and toxic effects, superficial and systemic infections, allergies and other respiratory or skin diseases [13,23–26]. Sick Building Syndrome has extensive economic and social impact [27–29]. A number of researchers have already pointed out that indoor building materials can become major sites of microbial growth when promoting conditions, such as high humidity and nutrient content, are present [30]. These conditions are easily satisfied in water-damaged buildings, damp buildings and badly-insulated buildings. Results from earlier studies have revealed that various microorganisms, including potentially pathogenic species, are detected on building materials [30].

A substantial amount of literature has been published on the effect of photocatalytic TiO2 nanoparticles on microorganisms [31–34]. These studies show that the photocatalytic process in water is effective against a wide range of organisms, such as algae, viruses, fungi and bacteria. It should be noted that the different tests were carried out in aqueous slurry or with aqueous inoculum (sprayed or dropped), emphasizing the major role of water in the microorganism photo-killing process. In addition, TiO2 nanoparticles can be used as (I) powder, usually dispersed in aqueous slurry or (II) film/coating applied to various substrates. Several works have highlighted very high bactericidal efficiency on different microorganisms: around 3 log after 30 min [35] and 6 log after 90 min [36] on *E. coli*, approximately 8 log after 90 min on mutans streptococci [37], *etc.* However, studies reporting such efficiencies used relatively strong light intensity, close to 10 W/m2 , and sometimes even beyond intensities in everyday use, up to 500 W/m2 , with photon wavelengths usually between 300 and 400 nm [38–40]. To our knowledge, no study reports such inactivation values with weaker light intensity, closer to a passive photocatalytic device. The efficiency of photocatalytic disinfection is attributed to the oxidative damage mainly induced by reactive oxygen species (ROS), such as O2 <sup>y</sup><sup>í</sup>, H2O2 and HO<sup>y</sup> . These reactive oxygen species are produced by redox reactions between adsorbed species (such as water and oxygen) and electrons and holes photo-generated by the illumination of TiO2. On the basis of studies on *Escherichia coli*, OH radicals were assumed to be the major cause of the bactericidal effect [41,42], although direct oxidation by "holes" (h+) from the valence band on the TiO2 surface is also highlighted in some works [43,44]. Regarding the process of degradation, the authors agree that the outer membrane, if present (Gram-negative bacteria), is the first barrier and, once it is damaged, the cytoplasmic membrane is attacked. The loss of cytoplasmic membrane integrity, which is involved in the process of cellular respiration, leads to the death of the cell.

This work is a preliminary study on transparent coatings formulated using TiO2 nanoparticles to fight against microbial proliferation in indoor conditions. As such, the first step of our work was to explore the different parameters influencing the efficiency of TiO2 nanoparticles when used alone for disinfection, *i.e.*, before being included in coatings. The aim of the paper was to emphasize the different factors determining disinfection efficiency and to show that the various performances reported in the literature should be correlated with experimental parameters. Passive devices in the form of semi-transparent photocatalytic coatings, easy to apply to the building material surfaces, are also considered.

Our previous investigations have already shown the efficiency of semi-transparent coatings on the abatement of NO*x* and VOC in air under various environmental conditions (Relative Humidity—RH, concentration of polluting gas, *etc.*) [45,46]. Such coatings consisted of ultra-light varnishes formulated using nanoparticles of TiO2, acrylic resin and silicates as the inorganic binder. The results obtained in air purification point out the interest of testing these transparent coatings for the photocatalytic disinfection of microorganisms. However, the coatings were found to be inefficient against green algae colonization in accelerated tests [47]. Regarding TiO2 nanoparticles alone, very good antibacterial performance is sometimes reported for photocatalytic TiO2, but may be related to very specific experimental conditions that are not representative of the natural conditions to be considered for passive devices. Three sets of experiments were carried out to highlight different factors determining the extent to which *Escherichia coli*, a Gram-negative bacterium, was inactivated by TiO2 photocatalysis: (1) the activity of TiO2 in the dark allowed the photocatalytic effect to be dissociated from the physical effect; (2) the deposited drop experiment was carried out to evaluate the influence of forced conditions between bacteria and particles; and (3) the stirring experiment, which was easier to carry out for the kinetics evaluation, enabled the effect of the suspension to be estimated.

We also highlight some of the issues to be faced in the formulation of such a product, for example the inclusion of nanoparticles within a binder matrix (acrylic resin here), which can act as a mask against UV absorption and/or can react with photogenerated radicals.
