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ISO 27447:2019 Test Method for Antibacterial Activity of Semiconducting Photocatalytic Materials
Photocatalytic Antibacterial Testing by ISO 27447 — UV-A Activated Titanium Dioxide TiO2 Ceramics
1. Introduction to ISO 27447 and Photocatalytic Antibacterial Testing
ISO 27447:2019 specifies a test method for determining the antibacterial activity of semiconducting photocatalytic materials — primarily titanium dioxide (TiO₂) based coatings and composites. Under ultraviolet (UV) irradiation, photocatalysts generate reactive oxygen species such as hydroxyl radicals (•OH) and superoxide anions (O₂⁻), which attack bacterial cell membranes and inhibit colony formation. This standard provides two distinct protocols: the film cover method for flat sheet, board, and plate materials, and the glass cover method for textile-based photocatalytic surfaces.
The film cover method evaluates the log reduction of viable bacteria on photocatalytic surfaces after UV-A irradiation at 351 nm (peak wavelength), comparing treated specimens against non-treated controls. This is the preferred protocol for rigid construction materials such as ceramic tiles, glass panels, and metal cladding.
The core metric defined in ISO 27447 is the photocatalyst antibacterial activity value (R), calculated as the difference between the logarithmic counts of viable bacteria on non-treated and treated specimens after UV exposure. A higher R value indicates stronger antibacterial efficacy. The standard defines four distinct activity values to account for both the film cover and glass cover methods, and to distinguish between net photocatalytic killing and dark adsorption effects.
| Symbol | Definition | Significance |
|---|---|---|
| R (Film cover) | log(N_nonUV) − log(N_treated_UV) | Total antibacterial activity including dark effects |
| R_UV (Film cover) | log(N_dark) − log(N_treated_UV) | Net UV-activated photocatalytic killing only |
| R (Glass cover) | log(N_standard_UV) − log(N_treated_UV) | Antibacterial activity for textile specimens |
| R_UV (Glass cover) | log(N_treated_dark) − log(N_treated_UV) | Net UV activity for photocatalytic cloths |
2. Test Methodology and Critical Parameters
2.1 Bacterial Strains and Culture Preparation
The standard specifies the use of Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) as representative bacterial strains. The bacterial inoculum is prepared in 1/500 nutrient broth at a concentration of approximately 2.5 × 10⁵ colony-forming units per milliliter (CFU/mL). The 1/500 dilution is critical — it minimizes nutrient interference that could allow bacteria to multiply during the test, ensuring that any observed population reduction is attributable to photocatalytic action rather than starvation.
The 1/500 nutrient broth has a shelf life of only one week (revised from one month in the 2009 edition). Engineers establishing testing protocols must prepare fresh media weekly and verify sterility before each test batch to avoid false-negative results from contaminated broth.
2.2 UV Irradiation Conditions
ISO 27447 references ISO 10677 for the UV light source specification. The light source must be a black light blue (BLB) fluorescent lamp or an equivalent UV-A LED array with peak emission at 351 nm. The UV-A irradiance at the specimen surface is set to 2.0 mW/cm² ± 0.1 mW/cm² for the film cover method and 1.0 mW/cm² ± 0.1 mW/cm² for the glass cover method. Irradiation typically continues for 2 hours, though shorter durations may be adopted for comparative screening if validated.
One important design insight is that the UV radiometer must have good cosine response characteristics — a common source of measurement error in photocatalytic testing is the use of narrow-angle sensors that underestimate the total irradiance incident on the specimen, particularly when using diffuse UV sources.
3. Engineering Design Insights for Testing Apparatus
3.1 Moisture Control and Cover Film Selection
A frequently underestimated parameter in antibacterial photocatalysis testing is moisture retention. The bacteria must remain viable throughout the UV exposure period, which requires the cover film (for flat specimens) to maintain a relative humidity of ≥90% at the bacteria-film interface. Standard polyethylene films with a thickness of 40 μm to 50 μm typically achieve this, provided the film is laid without wrinkles or air pockets that could create localized drying.
For specimens with rough or textured surfaces — commonly encountered in architectural ceramic tiles and concrete panels — the glass cover method may not provide adequate bacteria-surface contact. The film cover method is always preferred for such materials, and the film must be gently pressed onto the specimen using a sterile roller to ensure intimate contact without damaging bacterial cells.
2.3 Enumeration of Viable Bacteria
After UV exposure, surviving bacteria are recovered by washing the specimen with SCDLP (soybean-casein digest broth with lecithin and polysorbate 80) medium. The addition of lecithin and polysorbate 80 (Tween 80) is essential — these surfactants neutralize any residual photocatalytic activity during the recovery step. The wash solution is then serially diluted (typically 10⁻¹ through 10⁻⁵) and plated on nutrient agar for colony counting after 48 ± 2 hours of incubation at 35 °C ± 2 °C.
The colony counting data are expressed in CFU per specimen, and the antibacterial activity value R is calculated as:
R = log₁₀(BL) − log₁₀(CL)
where BL is the average number of viable bacteria on non-treated control specimens after UV irradiation, and CL is the average on treated specimens after UV irradiation. A material with R ≥ 2.0 is conventionally considered to exhibit antibacterial activity.
4. Practical Applications and Material Qualification
ISO 27447 is widely referenced in product specifications for photocatalytic construction materials, including self-cleaning exterior tiles, antibacterial interior wall coatings, hospital-grade surface finishes, and HVAC system components. For manufacturers, compliance with this standard provides a defensible basis for marketing claims. When qualifying a new photocatalytic coating, engineers should run both dark control (no UV) and light exposure tests in triplicate on five separate production batches to establish statistical confidence in the R value.
Important limitation: ISO 27447 does not cover antifungal or antiviral testing, powder/granular materials, or visible-light-activated photocatalysts. Engineers evaluating visible-light photocatalysts (e.g., doped TiO₂, g-C₃N₄, BiVO₄) must consult supplementary standards or adapt the protocol with appropriate light sources. Using ISO 27447 with visible light invalidates the standard and produces non-compliant results.
FAQ
Q1: What is the minimum acceptable R value for antibacterial photocatalytic materials?
A: While ISO 27447 does not specify a pass/fail threshold, the photocatalytic industry conventionally regards R ≥ 2.0 (≥99% bacterial reduction) as indicative of meaningful antibacterial activity. Some certification programs require R ≥ 3.0 (≥99.9%).
A: While ISO 27447 does not specify a pass/fail threshold, the photocatalytic industry conventionally regards R ≥ 2.0 (≥99% bacterial reduction) as indicative of meaningful antibacterial activity. Some certification programs require R ≥ 3.0 (≥99.9%).
Q2: Can ISO 27447 be used to test antibacterial activity under indoor fluorescent lighting?
A: No — the standard explicitly requires UV-A irradiation at 351 nm peak wavelength and 1–2 mW/cm² irradiance. Indoor fluorescent lights emit negligible UV-A and will not activate TiO₂-based photocatalysts. For visible-light-active materials, the test protocol must be modified and declared as non-standard.
A: No — the standard explicitly requires UV-A irradiation at 351 nm peak wavelength and 1–2 mW/cm² irradiance. Indoor fluorescent lights emit negligible UV-A and will not activate TiO₂-based photocatalysts. For visible-light-active materials, the test protocol must be modified and declared as non-standard.
Q3: How does specimen roughness affect the test results?
A: Rough surfaces can shield bacteria from UV light and create micro-environments that retain nutrients, reducing the apparent antibacterial activity. The standard excludes permeable and rough-surface materials precisely for this reason. When testing rough materials, the film cover method with extended irradiation time may partially compensate.
A: Rough surfaces can shield bacteria from UV light and create micro-environments that retain nutrients, reducing the apparent antibacterial activity. The standard excludes permeable and rough-surface materials precisely for this reason. When testing rough materials, the film cover method with extended irradiation time may partially compensate.
Q4: What is the difference between ISO 27447 and JIS R 1702?
A: ISO 27447 is technically aligned with JIS R 1702 (Japanese Industrial Standard for photocatalytic antibacterial testing). The ISO version includes both film cover and glass cover methods, while JIS R 1702 primarily specifies the film cover method. Both use the same bacterial strains, UV conditions, and enumeration procedures.
A: ISO 27447 is technically aligned with JIS R 1702 (Japanese Industrial Standard for photocatalytic antibacterial testing). The ISO version includes both film cover and glass cover methods, while JIS R 1702 primarily specifies the film cover method. Both use the same bacterial strains, UV conditions, and enumeration procedures.
