Rz ISO 21066:2018


A very sensitive test available for most PC materials. Visual test which gives compares materials according to ha fast they can photoreduce a dye. Follows a method developed by INTEC project. Can be easily adopted for in-house QC. Please check out this test for a basic and quick PC activity assessment.


A defined amount of blue coloured smart ink is applied on the surface of a test-piece as a thin film.  If the sample is photocatalyticaly active (self-cleaning, air/water purification), under UVA irradiation, then the colour of the film will turn pink.  The time of the colour change is proportional to the sample’s photocatalytic activity.  It is a very versatile and most of all very sensitive way of detecting photocatalytic activity of self-cleaning glass, self-cleaning tiles and similar commercial products.  For a more simple and cheaper test click here. Typical test conditions are given below:

Sample size 10 samples 2.5 cm x 2.5 cm and typically 1-15 mm thick
Suitable sample type Construction materials in flat sheet, board or plate shape; samples of lower activity such as self cleaning glass and tiles.
Unsuitable sample type Powder or granular photocatalytic materials
Sample pretreatment Mechanical cleaning (absolute alcohol) to remove organic impurities.
Test conditions Samples are coated with RZ ink and irradiated with UVA light (2 mW/cm2); the speed of colour change is recorded using a scanner.
Analytical method RGB colour extraction
Information returned Ttb, time to bleach the colour which is proportional to the photocatalytic activity.

Further Information

A Simple, Inexpensive Method for the Rapid Testing of the Photocatalytic Activity of Self-cleaning Surfaces;

The Rz ink test


There exist many different methods to assess the photocatalytic activity of a film, but in almost all cases: expensive analytical equipment, technical support and a long analysis time are required1-4. In a recent paper5 this group has reported the use of a photocatalyst activity indicating ink, containing a dye, Resazurin (Rz), and a sacrificial electron donor (i.e. SED, e.g. glycerol). The ink functions via a photo-reductive mechanism which can be summarized as follows: upon photoexcitation of an underlying photocatalyst film photogenerated holes react irreversibly with the SED in the overlying ink film, whereas photogenerated electrons reduce the Rz dye (blue) to Resorufin (Rf, pink) in the same ink film; the overall rate of the colour change, or time taken to change colour, provides a measure of the photocatalytic activity of the underlying self-cleaning film. In almost all cases of commercial self-cleaning films, the active coating is the semiconductor TiO2, and so interaction between an underlying titania self-cleaning film and an overlying Rz ink coating can be summarised as follows:

TiO2  TiO2 (h+, e)                                                         (1)

SED + TiO2*(h+, e)   SEDox + TiO2 (e˗)                                           (2)

Rz + 2e + 2H+  Rf  +  H2O                                                              (3)

Where Ebg is the band gap energy of the titania (3.2 eV for anatase), SEDox is glyceric acid or glyceraldehyde. Previous work reveals that the rate of the above, ink-based, photocatalyst driven process is linearly correlated with the much slower destruction of a film of stearic acid (SA)2,5.


In the Rz ink test all samples coated with the Rz ink by hand using a wire wound rod, manufactured by RK Print (K-bar #3). The wire gauge on the rod is 0.31 mm in diameter and gives a wet ink film of ~24 µm, which typically dries to give a film of ca. 800 nm thick, as measured by SEM.  The data below are for samples of commercial self-cleaning glass.


All irradiations were conducted using a Blak-Ray® XX-15 lamp and exposure stand purchased from Cole-Parmer. The bulbs used in the lamp were 15 W Blacklight tubes (Eiko) with lmax emission of 352 nm. The sample tray was set so as to irradiate the samples under test with a UV-A irradiance of ~2 mWcm-2, as measured using UVX Digital Ultraviolet Intensity Meter, with a UVX-36 sensor head for UV-A light (both from Cole-Parmer). UV-Visible spectra of the ink films were recorded using a Cary 50 Bio Varian spectrophotometer at different irradiation time intervals and, simultaneously, digital images of the ink-coated glass samples were recorded using an Ion CopyCat handheld document scanner. Red, green and blue, i.e. RGB, values were extracted from the digital images of the ink films.  A free download of the software used in the extraction is available6 and a set of guide notes are given elsewhere7, although most commonly available photo-editing software, like Adobe photoshop for example, allow RGB values to be similarly extracted from digital images. Fig. 1. Illustrates the main components used here to carry out the Rz ink test of the photocatalytic activity of a sample piece of self-cleaning glass, namely: the Rz ink, a K-bar (#3) and a hand-held scanner.

RZ test props

Fig. 1. – The equipment and materials used to conduct the test; a handheld document scanner, the Rz ink, a wire wound rod (K-bar #3) and a sample piece of self-cleaning glass.


In a typical experiment a sample of the self-cleaning and plain glass were coated with the Rz ink, dried in an oven at 70 °C for 10 minutes and then irradiated with UV-A light   (2 mW cm-2). The UV/Vis. spectra and digital images of the ink on glass samples were then recorded as a function of irradiation time spanning the range 0 – 450 seconds. The latter are shown in Fig. 2a for the Rz ink on self-cleaning (top images) and plain (bottom images) glass. Other work shows that of the three different colour parameters making up the digital image of the ink, only the red component, RGB (red)t, varies significantly, as the ink changes from blue (Rz) to pink (Rf) with increasing UV irradiation time, t, and so it is only this parameter that was extracted in all subsequent digital image work described here.

(a) Rz to Rf pics

(b) Rz to Rf graph

Fig. 2. – (a) A series of images recorded at 30 second irradiation intervals for two glass samples, namely; self-cleaning glass (top) and, plain glass (bottom). (b) A plot of variation of the RGB (red) with irradiation time, extracted from the images shown in (a). RGB (red)t is the value of the RGB (red) component at irradiation time, t. The time taken to bleach the RGB (red) component, ttb, is determined graphically as illustrated.

A plot of variation of the RGB (red)t values extracted from the images in Fig. 2a, are illustrated in Fig. 2b for self-cleaning and plain glass samples. In the case of the self-cleaning glass sample, the point in time at which the red component in the digital image has been bleached is taken as a measure of the activity of the photocatalyst film under test and is referred to as the time to bleach (ttb). Note that no colour change occurs in the Rz ink if there is no photocatalyst coating present, c.f. plain glass data in Fig. 2a.

In order to highlight the reproducibility and repeatability of the above method for assessing the photocatalytic activity of self-cleaning films using an Rz ink, a series of round-robin tests were conducted on the self-cleaning glass between groups across Europe. The results of this work, in the form of the measured ttb values for 5 samples of self-cleaning glass, are reported in Table 1. From the results of this work the repeatability appears low but variable, i.e. from 5.6 to <0.1%. The reproducibility was, as might be expected, a little less good, yielding an average value of 246 ± 33 s, i.e. ± 13%. However, in terms of repeatability and reproducibility, these errors are very good when compared to most of the current ISO photocatalyst activity tests, such as the methylene blue test, ISO 10178:2010, where the results of the inter-laboratory tests revealed a repeatability varying from 4.9 to 16.6% and a reproducibility of 28%! Recent, additional work shows the Rz ink test method is also very effective for assessing the photocatalytic activity of self-cleaning tiles and paint.

Other work shows that, for self-cleaning glass at least, the change in RGB (red) at any time, t, during an irradiation, i.e. DRGB (red)t is related directly to the change in absorbance at 610 nm, DAbs610(t), which, itself, is a measure of the [Rz]t, the concentration of Rz in the ink film at irradiation time, t.  This is relevant in that the rate of change of DAbs610 has been shown to correlate with the rate of destruction of stearic acid (d[SA]/dt) as noted earlier, implying d(RGB (red)t)/dt will also correlate with d[SA]/dt.

Table 1. – The average time to bleach the red component of the digital images of the ink on the same photocatalytic surface of self-cleaning glass (ttb) in seconds.

  Group 1 Group 2 Group 3 Group 4 Group 5 Group 6
Sample 1 253 260 214 259 251 282
Sample 2 239 252 227 262 252 245
Sample 3 239 257 208 265 248 207
Sample 4 238 259 215 269 247 260
Sample 5 271 250 206 250 247 248
Average /s 248 256 214 261 249 248
Std. Dev. /s 14 4 8 7 2 27


The use of an inexpensive digital scanner provides all the advantages of using a UV/Vis. spectrophotometry in recording the change in colour of the ink, without needing to use expensive, bulky equipment or significant technical support. The inter-laboratory repeatability of the Rz ink test is high (ca. 13 %) and better than many of the current ISO photocatalyst tests. As a consequence, the above method appears particularly suitable for measuring the photocatalytic activity of self-cleaning glass (and, from other, recent work, also paints and tiles) both in the lab and in the field, at little cost and with little training, but with a reasonable degree of precision.


The round robin inter-laboratory tests were conducted as part of the work of INTEC, a Coordination and Support Action, funded by the European Union’s Seventh Framework Programme for Research (FP7-NMP-2012-CSA-6, GA No. 319210).  The aim of the action is to explore the utilisation of photocatalyst indicator inks in a CEN standard for the rapid testing of the activities of various different self-cleaning surfaces, including: glass, tiles, paints, fabrics and concrete. We thank the EU for funding this exercise.


1.         A. Mills, C. Hill and P. K. J. Robertson, J. Photochem. Photobiol. A: Chem., 2012, 237, 7-23.

2.         J. Krysa, P. Novotna, S. Kment and A. Mills, J Photochem. Photobiol. A: Chem., 2011, 236, 81– 86.

3.         A. J. Julson and D. F. Ollis, Applied Catalysis B: Environmental, 2006, 65, 315–325.

4.         A. Mills and M. McGrady, J. Photochem. Photobiol.A: Chem., 2008, 193, 228–236.

5.         A. Mills, J. Wang, S.-K. Lee and M. Simonsen, Chem. Commun., 2005, 2721-2723.

6.         http://rsb.info.nih.gov/ij/download.html; accessed June 2013

7.         http://www.inkintelligent.com/wp-content/uploads/2013/06/RGB_Extraction_Guide.pdf; accessed June 2013