Tunable Assembly of Photocatalytic Colloidal Coatings for Antibacterial Applications

In this study, evaporation-induced size segregation and interparticle interactions are harnessed to tune the microstructure of photocatalytic colloidal coatings containing TiO2 nanoparticles and polymer particles. This enabled the fabrication of a library of five distinct microstructures: TiO2-on-top stratification, a thin top layer of polymer or TiO2, homogeneous films of raspberry particles, and a sandwich structure. The photocatalytic and antibacterial activities of the coatings were evaluated by testing the viability of Methicillin-resistant Staphylococcus aureus (MRSA) bacteria using the ISO-27447 protocol, showing a strong correlation with the microstructure. UVA irradiation for 4 h induces a reduction in MRSA viability in all coating systems, ranging from 0.6 to 1.1 log. Films with TiO2-enriched top surfaces exhibit better resistance to prolonged exposure to disinfection and bacterial testing. The remaining systems, nonetheless, present higher antibacterial activity because of a larger number of pores and coating defects that enhance light and water accessibility for the generation and transport of reactive oxygen species. This work establishes design rules for photocatalytic coatings based on the interplay between performance and film architecture, offering valuable insights for several applications, including antibacterial surfaces, self-cleaning/antifogging applications, and water purification.

deionised water were added to a 500 mL double-jacket glass reactor equipped with a condenser.CTPPA (1.7 g, 6.1 mmol), SSNa (19 g, 92 mmol) and sodium bicarbonate (NaHCO3) (0.15 g, 1.8 mmol) were then added to water.The mixture was deoxygenated with nitrogen for 30 min and then heated to 80 °C, and the stirring was set at 250 rpm.To start the experiment, ACPA (0.15 g, 0.6 mmol) was added together with 1,3,5-trioxane (3.3 g, added as an internal reference to follow the kinetics via NMR) in 1 mL of deionised and deoxygenated water.Full conversion (determined by 1 H NMR) was achieved after 3 h.The recovered solution had a solids content of 8.5 %.A sample was dried and the polymer recovered was analysed by MALDI-ToF mass spectrometry to determine the molar mass of the macroRAFT agent: Mn(MALDI-TOF) = 3021 g mol -1 .
PDMAPMA macroRAFT agent.The synthesis of the PDMAPMA macroRAFT was performed in water following the protocol described by Engström et al. 2 200 g of deionised water were added to a 500 mL double-jacket glass reactor equipped with a condenser.CTPPA (3.6 g, 13 mmol) and DMAPMA (40 g, 235 mmol) were added to the reactor allowing to dissolve the RAFT agent and obtained a homogeneous solution.The addition of HCl was necessary to set the pH at 6.The mixture was then deoxygenated with nitrogen for 30 min and after that heated to 70 °C.The stirring was set at 250 rpm.AIBA (0.71 g, 2.6 mmol) was added together with 1,3,5-trioxane (3.6 g) in 1 mL of deionised and deoxygenated water to start the experiment.After 2 h of reaction, 96% conversion was achieved.The temperature of the reactor was decreased to 45 °C to recover the product.The polymer was precipitated three times with cold acetone, and finally dried in vacuum for 24 h at room temperature.The precipitated polymer was analysed by MALDI-ToF mass spectrometry to determine the molar mass of the macroRAFT agent: Mn(MALDI-TOF) = 2850 g mol -1 .
Emulsion copolymerization of n-butyl acrylate and methyl methacrylate in the presence of PSSNa and PDMAPMA macroRAFT agents.Emulsion copolymerizations of BA and MMA (50/50 % in wt.) were performed at 70 °C in a 500 mL double-jacket glass reactor equipped with a condenser using either PSSNa or PDMAPMA macroRAFT agent (ensuring particle stabilization) and targeting a final solids content close to 20 wt%.
PSSNa-mediated emulsion copolymerization.150 g of deionised water were poured into the reactor and deoxygenated for 30 min.BA (20 g, 0.15 mol) and MMA (20 g, 0.2 mol) were then added to the reactor, followed by 3.5 mL of the macroRAFT solution (0.35 g, 0.12 mmol).The mixture was stirred at 250 rpm and heated to 70 °C.0.09 g of APS (0.39 mmol) dissolved in 1 mL of deionised and deoxygenated water was added to start the polymerization.The reaction was stopped after 4 h when full conversion was achieved, as determined by gravimetric analysis.
PDMAPMA-mediated emulsion copolymerization.200 g of deionised water were poured into the reactor and deoxygenated for 30 min.BA (25 g, 0.195 mol) and MMA (25 g, 0.25 mol) were then added to the reactor, followed by 0.16 g of dry macroRAFT (0.053 mmol).The mixture was stirred at 250 rpm and heated to 70 °C.0.11 g of APS (0.4 mmol) dissolved in 1 mL of deionised and deoxygenated water was added to start the polymerization.The reaction was stopped after 4 h when full conversion was achieved, as determined by gravimetric analysis.

Péclet Number Calculations
Péclet numbers for TiO2 and positive latex particles have been calculated as follows: Where μ is the viscosity of the solvent, R is the particle radius, H is the initial thickness of the wet film,  ̇ is the evaporation rate, k is the Boltzmann's constant, and T is the temperature in Kelvin.The particle radius was calculated as DH / 2 from the values obtained from DLS (45 ± 1 nm and 297 ± 2 nm for TiO2 and positively charged latex respectively).The initial film thickness was calculated to be 1.23 mm dividing the glass slide surface area (324 mm 2 ) by the casting volume (400 μl).The films were dried at slow and fast evaporation rates (see Table 1) and the value used for water viscosity was 1 × 10 -3 (294 K) and 0.5 × 10 -3 Pa.s (333 K) respectively.The value used for the slow evaporation rate was taken from Utgenannt et al. 3 (1.1 × 10 -7 ms -1 ).For the fast evaporation rate, the value was calculated to be 4.5 × 10 -7 ms -1 using the change in film weight (arising from water evaporation) over time (Δm/Δt) while the film was drying in the oven (see Figure S4).The evaporation rate can be calculated as: Where A is the film surface area and ρ is the density of water (1.0 g cm -3 ).

Figure S4
. Mass loss during film formation at fast evaporation conditions (see Table 1 for conditions).The area between the two blue lines, or above the orange line corresponds to where small-on-top stratification is predicted to occur according to each of the models.The points represent fast (red) or slow (green) evaporation rates, and the two TiO2 volume fractions used for the same-charge systems as mentioned in Table 1.Experimental observations for stratification are indicated by symbols: (+) for small-on-top, (×) for no stratification, or (•) for an intermediate situation, e.g., sandwich structure.serial dilutions (log dilutions) of initial recovery solution; b) number of repetition; c) stands for too numerous to count, where bacterial colonies were more than 350; d) average between Rep 1,2,3; e) standard deviation; f) Rep Ave./ Std.Er; g) number of bacterial colonies on the film.Calculated by Rep Aver.× dil.Number × 0.1 × 100 μl; h) log reduction of MRSA colonies, obtained after subtracting the number of viable colonies on control (non-treated coverslip) just after inoculation; i) log reduction of MRSA colonies after subtracting the number of viable colonies on control (nontreated coverslip) after UVA irradiation for 4 h, or in the dark for 4 h respectively; j) photocatalytic activity, calculated by LogRed UV -LogRed Dark.

Figure S3 .
Figure S3.Intensity vs hydrodynamic diameter of (a) the TiO2/positive latex blend, (b) the negative latex dispersion upon consecutive additions of TiO2 nanoparticles, and (c) the evolution of hydrodynamic diameter and zeta potential (ζ) of the same blends.

Figure S5 .
Figure S5.Péclet number (PeS) and volume fraction of small particles (ϕs) state diagram which illustrates the stratification of binary colloidal mixtures obtained from the Schulz-Sear model (blue line) or ZJD model (orange line).The area between the two blue lines, or above the orange line corresponds to where small-on-top stratification is predicted to occur according to each of the models.The points represent fast (red) or slow (green) evaporation rates, and the two TiO2 volume fractions used for the same-charge systems as mentioned in Table1.Experimental observations for stratification are indicated by symbols: (+) for small-on-top, (×) for no stratification, or (•) for an intermediate situation, e.g., sandwich structure.

Figure S6 .
Figure S6.Brownian dynamics simulations to model the L+Ti50,fast system.Snapshots of the films' (a) cross-section and (b) top surface taken at the end of the drying process, along with (c) particle probability distributions perpendicular to the film surface.Small (TiO2) and large (latex) particles are depicted in yellow and red, respectively.
Figure S9.UV-vis absorption spectrum of 0.1 wt.% TiO2 nanoparticle dispersion in water (pH = 3-3.5),prepared as described in the experimental section.Dashed line corresponds to the wavelength used in the antibacterial tests (368 nm).

Figure S10 .
Figure S10.Particle size distribution measured by dynamic light scattering (DLS) of water retrieved from the surface of L+Ti50,fast, L-Ti30,fast, and L-Ti30,slow films after 4h of soaking.

Table S1 .
Raw data spreadsheet obtained from testing the photocatalytic activity of films against MRSA bacteria (following ISO 27447:2009).

Table S2 .
Water contact angle data.