Selestina Gorgieva
1,2
, Darinka Fakin
1
, Alenka Ojstršek
1,2
1
 University of Maribor, Faculty of Mechanical Engineering, Institute of Engineering Materials and Design, Maribor, 
Smetanova ulica 17, 2000 Maribor, Slovenia
2
 University of Maribor, Faculty of Electrical Engineering and Computer Science, Institute of Automation, Koroška 
cesta 46, 2000 Maribor, Slovenia
Confocal Fluorescence Microscopy as a Tool for 
Assessment of Photoluminescent Pigments Print on 
Polyester Fabric
Konfokalna fluorescenčna mikroskopija kot orodje za 
ovrednotenje tiska fotoluminiscenčnih pigmentov na  
poliestrni tkanini
Original scientific article/Izvirni znanstveni članek
Received/Prispelo 10-2020 • Accepted/Sprejeto 11-2020 
 
Corresponding author/Korespondenčna avtorica: 
Assist Prof dr. Selestina Gorgieva 
E-mail: selestina.gorgieva@um.si 
ORCID: 0000-0002-2180-1603 
Tel.: +386 2 220 7935
Abstract
The size and distribution of the photoluminescent pigment particles within the selected binder may affect the 
quality and appearance of the final print significantly. Yet, the techniques for precise evaluation of size distri-
bution of the pigment particles within a 3D fabric space are rather limited, based on their intrinsic fluorescent 
properties. The presented work demonstrates a simple screen-printing process for the sustainable application of 
three different types of commercial fluorescent pigments on polyester (PES) fabric, using polydimethylsiloxane 
(PDMS) as a binder. A comprehensive toolbox was used to compare and study different commercial photo-
luminescent pigments and their corresponding prints, by means of size distribution and concentration effect 
of emission intensity, including Confocal Fluorescence Microscopy (CFM) and Scanning Electron Microscopy 
(SEM) in combination with complementary spectroscopic techniques, i.e. Energy Dispersive X-ray Spectroscopy 
(EDX) and Ultraviolet-visible (UV-vis) spectroscopy. The focus is on CFM utilised as a non-destructive tool, used 
for the evaluation of photoluminescent pigments´ spatial distribution within printing pastes, as well as on/
within the PES fabrics.
Keywords: Photoluminescent pigments, polyester fabric, confocal fluorescent microscopy, spectroscopy
Izvleček
Velikost in porazdelitev fotoluminiscenčnih delcev pigmenta v izbranem vezivu lahko bistveno vpliva na kakovost in 
videz končnega tiska. Vendar pa so tehnike za natančno oceno porazdelitve velikosti delcev pigmenta v 3-D prostoru 
tkanine na podlagi njihovih notranjih fluorescenčnih lastnosti precej omejene. Predstavljeno delo opisuje preprost po-
stopek sitotiska za trajnostno nanašanje treh različnih vrst komercialnih fluorescenčnih pigmentov na poliestrno (PES) 
tkanino z uporabo polidimetilsiloksana (PDMS) kot veziva. Uporabili smo kombinacijo orodij, s katerimi smo primerjali 
in proučevali različne komercialne fotoluminiscenčne pigmente in njihove ustrezne tiske s porazdelitvijo velikosti in 
učinka koncentracije na intenzivnost emisij, vključno s konfokalno fluorescenčno mikroskopijo (CFM) in vrstično elek-
16
Tekstilec, 2021, Vol. 64(1), 16–24 | DOI: 10.14502/Tekstilec2021.64.16-24

tronsko mikroskopijo (SEM) v kombinaciji z disperzno rentgensko spektroskopijo (EDX) in UV-vis spektroskopijo. Glavni 
poudarek je bil na CFM, ki se uporablja kot nedestruktivno orodje za oceno prostorske porazdelitve fotoluminiscenčnih 
pigmentov v tiskarskih vezivih in na PES-tkanini ali v njej.
Ključne besede: fotoluminiscenčni pigmenti, poliestrna tkanina, konfokalna fluorescenčna mikroskopija, spektroskopija
1 Introduction
Textile materials with minimum utility are far off 
today`s people`s needs and expectations; to this end, 
functional textiles, among which the photolumines-
cent textiles belong, have attracted significant atten-
tion in the textiles consumers’ end-user segment [1]. 
Photoluminescent textiles are obtained either by 
fixing pigments in the fibre or onto the polymer fi-
brous surface using chemical binders, by coating or 
printing diverse patterns onto textile fabrics [2]. The 
photoluminescent pigments as ˝glow-in-the-dark˝ 
powder particles are used as textile pigments for cre-
ating high-resolution patterns [3], as an alternative to 
LEDs, electro-luminescent wires and optical fibres for 
designing light-emitting fabrics, decoration, military 
facilities, communication and transportation, fire 
emergency systems, etc. One of the interesting, and 
cost-acceptable applications of luminescent pigments 
is the protection of the original products` originality 
[4]. In general, photoluminescent pigments are im-
mobilised onto textile fabrics by means of printing as 
the most affordable and simple procedure, fostering 
its use by 80% of printed merchandise [5]. Recently, 
the aqueous pigment-binder spray-coating technique 
was used as a facile method for production of coated 
garments hosting luminescent pigments [6].
A photoluminescent pigment consists mainly of 
crystals of aggregated elements and photonic traps, 
where, upon light source exposure, the crystals stay 
excited and keep discharging light, being supported 
by photonic traps (rare elements), which extend their 
phosphorescence time until the entire exhaustion 
of the stored light photons. Besides their chemical 
composition, the dimensions of photoluminescent 
pigments and their distribution within binders, influ-
ence the print quality and final appearance of printed 
material. Techniques used for precise assessment of 
pigments` size and pattern distribution when printed 
on fabric (s.c. in situ evaluation) are rather limited. 
Instead, the segmental and micro destructive sam-
pling is mainly utilised, followed by extensive labo-
ratory analysis [7]. In art works and cultural heritage 
examination of photoluminescent pigments` distri-
bution, several in situ identification techniques are 
in use, limited mostly to single point analysis such 
as vibrational spectroscopy, laser-induced breakdown 
spectroscopy and x-ray spectroscopy for atomic–level 
analysis, as well as diffuse reflectance imaging, flu-
orescence and fluorescence lifetime imaging [7], etc. 
The latter are not yet reported as relevant for evalua-
tion of printed textiles. When only particles are con-
sidered (excluding the printed fabric), Dynamic Light 
Scattering (DLS) methodology is applicable for nano 
pigments, preferably for monodisperse and, ideally, 
circular particles, and, as such, it is not completely 
applicable for the fairly heterogeneous (photolumi-
nescent) pigment used in textile printing. The precise 
microscopic tools (such as SEM and TEM microsco-
py) give distribution of patterns easily; however, the 
obtained information is limited to relatively small 
surfaces being examined, as well as costs of equip-
ment and the examination itself. When commercial 
pigments with unknown composition and proper-
ties (particles` sizes and their distribution, excitation 
wavelength, emission time, etc.) are utilised, together 
with additives (binders, brighteners), the complexity 
of pigments` evaluation may increase exponentially.
In the presented work, a facile procedure for 
screen-printing of luminescent pigments on poly-
ester (PES) fabrics is presented, using three types of 
commercial luminescent pigments and polydimeth-
ylsiloxane (PDMS) as a binder. Commercial pigments 
and their respective prints on PES fabrics were exam-
ined by complementary microscopic (SEM and CFM) 
and spectroscopic (EDX and UV-vis) techniques. 
Special focus in this research work is the utilisation 
of CFM as a non-destructive tool for simultaneous 
examination of the morphological and optical prop-
erties of photoluminescent pigments, and respective 
fabric prints in a 3D spatial manner. CFM is already 
used for micro particles [8−10] and auto fluorescent 
pigments` assessment [11], and recently our group in-
troduced it for assessment of pigment prints on cotton 
fabric [12], wherein this work presents the translation 
of a similar procedure to PES fabric. Nonetheless, 
examination of the spatial distribution of photolu-
minescent pigments is of high relevance, since the 
same was found to affect the propagation and exter-
nal emission of light highly, through mechanisms 
Confocal Fluorescence Microscopy as a Tool for Assessment of Photoluminescent Pigments Print on Polyester Fabric
17

such as charge diffusion, re-absorption, or changes 
in the mean free path of the emitted photons [13].
2 Materials and methods
2.1 Materials
PES fabric in plain weave was used, with a mass of 
165 g/m
2
, thickness of 0.505 mm, warp density of 
19 threads/cm and weft density of 21 threads/cm. 
Polydimethylsiloxane (PDMS), a two-part liquid 
elastomer kit, Sylgard 184 (Dow Corning, USA), 
that consists of a pre-polymer base and crosslinking 
curing agent in the ratio 10:1, was used as a binder. 
Three (violet, blue and yellow-green) commercially 
available photoluminescent pigments Sirius (Samson, 
Slovenia) were utilised in the printing procedure.
2.2 Fabrication of luminescent samples by 
screen-printing
Before printing, the PES fabric was washed at 40 °C 
for 30 min, using a solution of 2 g/L of standard neu-
tral non-ionic washing agent, without optical bleach-
er, water rinsed, and dried at ambient temperature. 
Individual luminescent pigment in concentrations: 
1%, 5%, 10% and 30 wt.% (per weight of PDMS) was 
admixed in a PDMS binder for 5 min, using a high-
speed paddle-stirring apparatus, to acquire homoge-
neous pigments` distribution. The prepared disper-
sions were applied onto PES fabric according to the 
flat screen-printing procedure using a semiautomatic 
printing table (Johannes Zimmer, Austria). In order 
to achieve an even coverage over the entire sample, a 
PES 125 mesh was used, as well as the following opti-
mal parameters: A roll-rod diameter of 15 mm, speed 
of 1 m/min, max magnet pressure, and 2 application 
layers. To cross-link the PDMS, printed PES fabrics 
were dried for 48 hours at ambient temperature.
2.3 Scanning electron microscopy (SEM) and 
energy dispersive X-ray (EDX) spectroscopy
Morphological analysis and elemental composition of 
three commercial photoluminescent pigments were 
accomplished by the SEM-EDX system, using a Zeiss 
Gemini Supra 35 VP Microscope (Carl Zeiss NTS 
GmbH, Germany), equipped with an X-ray energy 
dispersion spectrometer (EDX, Oxford Instruments, 
model Inca 400). Pigments in powder form were po-
sitioned onto the holder by double carbon tapes, and 
sputter coated with Palladium (Pd). 500 X magnifi-
cation and 1.00 kV setting were applied.
2.4 Confocal fluorescent microscopy (CFM)
Square pieces of (non)printed PES fabric were posi-
tioned on a transparent glass holder above the 20 x 
(dry) objective of an inverted CFM Leica TCS SP5 
II, equipped with an LAS AF software program. 
The photoluminescent pigments were excited with 
an argon laser (λ
ex
 = 458 nm), while the obtained 
signal was detected by two hybrid detectors, with 
a pre-set emission range from 500 nm to 550 nm. 
High-resolution images (1024 pixels x 1024 pixels) 
were obtained by image adjusted light gain and 8 x 
line averaging. Each sample was depicted at several 
positions, in order to check the printing uniformity 
in the x-y direction, while in the z-direction (sam-
ple thickness), the confocal mode with 10 µm stack 
thickness was applied, and assessed additionally by 
the depth (colour) codding function. By moving the 
focal plane in the z direction, up to 30 optical slices 
were obtained, and combined further in a 3D image 
stack for digital processing. The bright field images 
were captured in parallel, to depict the pigment-lean 
areas of printed PES fabrics, using a Dodt detector. 
The images from both the fluorescence and bright 
field channels were used in split or overlaid mode 
for further assessment by the ImageJ program, an 
image processing program, designed for scientific 
multidimensional images, in particular, the z-project 
function (for tracking of pigments` distribution with-
in the PES fabric). In parallel, the pigments`-PDMS 
binder dispersion was also coated onto the thin glass. 
The particle analysis plug-in (Image J) was employed 
for the assessment of the size and distribution of pig-
ments within the PDMS binder on the glass. Pure 
PES fabric and PDMS were analysed as controls.
2.5 UV–Vis spectroscopy: luminescence  
decay assessment
A qualitative assessment of the luminescence decay 
of the PES samples, printed with a combination of 
PDMS and individual photoluminescence pigment in 
different concentrations, was performed by initial, 5 
min exposure of samples under UV light with a wave-
length of 366 nm, within a UV chamber. Afterward, 
samples were exposed immediately to deep dark 
within the chamber, and after 20 seconds and 1 min, 
the photos were taken. For relative quantification of 
time-dependent luminescence decay, the same sam-
ples were also evaluated on a Tecan Infinite M200 
Pro microplate reader in luminescence mode, with-
out using emission filters. Printed PES fabrics were 
again excited under the same conditions as for the 
Tekstilec, 2021, Vol. 64(1), 16–24
18

qualitative assessment, and loaded immediately into 
the instrument, to commence measurement. Each 
decay curve was normalised by its initial intensity. 
Before testing, all printed PES fabrics were exposed 
to darkness for a 2-day period, to attenuate any af-
terglow illumination completely.
3 Results and discussion
The SEM micrographs on Figure 1 (left) depict lumi-
nescent pigment particles with irregular shape and 
large heterogenicity in terms of size in all three types, 
which we anticipate to be a consequence of their con-
ventional solid-state bulk synthesis, sintering and 
powdering procedures [14]. The morphologically 
distinct regions, as well as large size scattering (i.e. 
the presence of fine powder particles up to merged 
aggregates) are present in all pigments. This is expect-
ed to affect the luminescent properties, influencing 
the amount of light absorbed, and, consequently, 
the emission intensity. In contrast to SEM, the CFM 
resolved a larger number of particles due to the low-
er magnification used. Moreover, CFM has the ad-
vantage of generating high contrast images through 
optical sectioning, resulting from the superposition 
of sharp features from the focal plane and blurry 
features coming out of focus [15]. The CFM images 
(Figure 1, middle) depicture particles embedded into 
the binder, thus, excluding the necessity of examina-
tion of pigment powders, the latter having a shortage 
of presenting real state spatial distribution within 
the applied printing binder. The CFM data (Figure 1, 
right) demonstrates wide size distribution within all 
pigments in a range of 3−80 µm, which is reported 
for commonly used pigments containing Eu-doped 
strontium aluminates [16]. In the yellow-green pig-
ment, the smallest fraction (d ~ 3 µm) is missing, 
the same being dominant in the other two (blue and 
violet) pigments, which can be one of the factors con-
tributing to the highest glowing intensity over the 
other two types at the same concentration. The size 
distribution of the violet pigment differs significantly 
when compared to the other two pigments in terms 
Figure 1: SEM micrographs (left), CFM micrographs (middle) and size distribution extracted (right) from 
CFM images of a) violet, b) blue and c) yellow-green luminescent pigments. The scale bar in the CFM images 
corresponds to 250 µm.
Confocal Fluorescence Microscopy as a Tool for Assessment of Photoluminescent Pigments Print on Polyester Fabric
19

of count, which is approximately three times higher 
for particles in the 3-30 µm range, yet the presence of 
larger, irregular particles in this pigment is expected 
to decrease the luminescence intensity [17].
The EDX data, gathered in Table 1, identify the pres-
ence of the rare earth element europium (Eu), in the 
yellow-green and violet pigments. Eu is known as 
a photoluminescent pigments’ dopant [18]. Besides 
Eu, the yellow-green pigment contains strontium 
(Sr), aluminium (Al) and oxygen (O), which suggest 
the presence of strontium aluminates, also having 
phosphorescent properties, long afterglow at room 
temperature, and broad spectrum distribution of 
luminescence from the blue to green region. In the 
case of the violet pigment, the calcium aluminate 
elements were identified, presumably as mixed com-
pounds rather than a particular phase. As relatively 
diverse compositional profiles were identified for 
each pigment, we can speculate on the combined 
effect of composition, and size/distribution profile 
of pigments, the latter seeking further investigation 
involving fractionation and comparison of effects of 
by size-resolved components.
As promising for identification of pigments` size and 
distribution within the PDMS binder, the CFM was 
used further for the investigation of the 3D spatial 
distribution of pigments when printed on PES fabric. 
Indeed, CFM permits depiction and localisation of 
the fluorescing compounds unambiguously at light 
microscopy resolution [8], offering a non-destructive 
method for in-depth imaging, without demanding 
preparation [19], sometimes even without the need 
of additional fluorescent labelling [20, 21] which is 
also the case for PES fabric. This is due to the point 
by point illumination of samples, and rejection of out 
of focus light, which enables acquisition of z- stack 
(thickness) images and construction of 3D micro-
graphs [22, 23]. The same imaging parameters were 
applied for both pigments and PES fabrics. Under a 
specified excitation wavelength (λ
ex
 = 458 nm), the 
particles were visualised clearly and distinguished 
from the PES fabric (Figure 2), which itself exhibit-
ed a fluorescent signal tracked by a parallel detector. 
Image collects the maximum intensity signal from 
the consecutive images. Interestingly, the increase 
in pigment concentration from 1 up to 10 wt.% does 
not supplement the fluorescence signal significant-
ly; the same can also be observed in the fluorescence 
emission photographs (Figure 4). Relatively homoge-
neous distribution of pigments was observed from the 
maximum intensity image, like within single pigment 
analysis, which points-out the suitability of the se-
lected pigment-binder-printing procedure, allowing 
homogeneous distribution. At the same concentration 
(5%), the yellow-green pigment provides the largest 
filler pores´ blocking effect, due to the domination of 
large particle fractions over the other pigments.
Printed pigments were observed within an inspect-
ed thickness (< 300 µm), and not through the whole 
fabric thickness, as the thickness value is in the func-
tion of an objective aperture. To visualise the distri-
bution of pigments in the z axis better (thickness), 
a colour-coded projection is presented in Figure 3, 
demonstrating the inclusion of printed particles in 
deeper regions of the PES fabric. Bonding of pigments 
on individual PES yarns is pronounced in the case of 
the blue pigment (Figure 3a), which is not the case 
with the yellow-green pigment (Figure 3b), where the 
presence of fibrous, fibre-like glowing impurities and 
significantly different pigment particles by means of 
structure and size are observed.
For the macroscopic investigation of printed PES 
fabrics, the same were illuminated with UV at 254 
nm and 366 nm, and further depictured (Figure 4) 
and evaluated by means of afterglow decay kinetic 
(Figure 5).
Table 1: EDX data of violet, blue and yellow-green photoluminescent pigments
Violet pigment Blue pigment Yellow-green pigment
Element
Weight 
percentage
Atomic 
percentage
Element
Weight 
percentage
Atomic 
percentage
Element
Weight 
percentage
Atomic 
percentage
O K 34.82 65.54 O K 31.68 73.21 O K 34.82 65.54
Al K 17.72 19.78 Mg K 3.69 5.62 Al K 17.72 19.78
Sr L 34.28 11.78 Cl K 1.84 1.92 Sr L 34.28 11.78
Pd L 5.10 1.44 Sr L 29.76 12.56 Pd L 5.10 1.44
Eu M 8.08 1.45 Ta M 17.67 3.61 Eu M 8.08 1.45
W M 15.36 3.09
Tekstilec, 2021, Vol. 64(1), 16–24
20

Figure 2: CFM micrographs of PES fabric a) without (reference) and with 5% of b) violet, c) blue and d) yellow-
green pigment; obtained by fabric excitation by argon laser at λ
ex
 = 458 nm and signal accumulation by two 
hybrid detectors, with a pre-set emission range from 500 nm to 550 nm range. The white scale bar corresponds 
to 250 µm in all CFM micrographs.
a) b)
Figure 3: CFM micrographs of PES fabric with 5% of a) blue and b) yellow-green pigment; obtained by fabric 
excitation by argon laser at λ
ex
 = 458 nm and signal accumulation by two hybrid detectors, with a pre-set 
emission range from 500 nm to 550 nm. The white scale bar corresponds to 250 µm in all CFM micrographs. The 
colour scale bar refers to the image colours presenting the relative z depth, where glowing particles are positioned.
Confocal Fluorescence Microscopy as a Tool for Assessment of Photoluminescent Pigments Print on Polyester Fabric
21

a) b) c)
Figure 4: Photographs of screen-printed PES with 1%, 5%, 10% and 30 % of a) yellow-green; b) violet and c) blue 
pigments in PDMS; excited within a UV chamber at 254 nm and 366 nm for 5 min
0,00E+00
1,00E+07
2,00E+07
3,00E+07
4,00E+07
5,00E+07
6,00E+07
7,00E+07
8,00E+07
9,00E+07
1 30 900
Intensity (A.U.)
Time (min)
PES
PES + 1% yellow-green pigment
PES + 5% yellow-green pigment
PES + 10% yellow-green pigment
0,00E+00
1,00E+07
2,00E+07
3,00E+07
4,00E+07
5,00E+07
6,00E+07
7,00E+07
8,00E+07
9,00E+07
1 30 900
Intensity (A.U.)
Time (min)
PES
PES + 1% yellow-green pigment
PES + 5% yellow-green pigment
PES + 10% yellow-green pigment
0,00E+00
1,00E+07
2,00E+07
3,00E+07
4,00E+07
5,00E+07
6,00E+07
7,00E+07
8,00E+07
9,00E+07
1 30 900
Intensity (A.U.)
Time (min)
PES + 1% violet pigment
PES + 5% violet pigment
PES + 10% violet pigment
0,00E+00
1,00E+07
2,00E+07
3,00E+07
4,00E+07
5,00E+07
6,00E+07
7,00E+07
8,00E+07
9,00E+07
1 30 900
Intensity (A.U.)
Time (min)
PES + 1% violet pigment
PES + 5% violet pigment
PES + 10% violet pigment
0,00E+00
1,00E+07
2,00E+07
3,00E+07
4,00E+07
5,00E+07
6,00E+07
7,00E+07
8,00E+07
9,00E+07
1 30 900
Intensity (A.U.)
Time (min)
PES + 1% blue pigment
PES + 5% blue pigment
PES + 10% blue pigment
0,00E+00
1,00E+07
2,00E+07
3,00E+07
4,00E+07
5,00E+07
6,00E+07
7,00E+07
8,00E+07
9,00E+07
1 30 900
Intensity (A.U.)
Time (min)
PES + 1% blue pigment
PES + 5% blue pigment
PES + 10% blue pigment
Figure 5: Luminescence decay curves of screen-printed PES with 1%, 5% and 10 % of a) yellow-green; b) violet 
and c) blue pigments in a 180 min period
a)
b)
c)
Tekstilec, 2021, Vol. 64(1), 16–24
22

From the photographs in Figure 4, an expected in-
crease in luminescence intensity could be observed, 
with the increase in pigment’s concentration (irre-
spective of pigment type), while, among different 
pigments, the intensity decreased in the order yel-
low-green > blue > violet at the same concentrations. 
Luminescence decay curves, presented in Figure 5 
were obtained from the relatively wide time span 
measurement, i.e. from 1 up to 180 min. The pres-
ence of rare earth aluminates activated by Eu brought 
the highest emission intensity in the yellow-green 
pigment, as well as the longest afterglow, compared 
to the other two pigments.
4 Conclusion
Three commercial photoluminescent pigments 
were evaluated by complementary microscopic and 
spectroscopic techniques, before and after being 
screen-printed onto PES fabric by use of a PDMS 
binder. Herein, we identify CFM as an advanced, 
non-destructive visualisation tool for pigments` as-
sessment by means of their size distribution, as well 
as 3D (spatial) distribution, within a binder and PES 
textile fabric. The proposed assessment procedure 
can be applied further in analysis related to the con-
servation of artefacts. In this preliminary work we 
speculate on the potential size and distribution ef-
fect of particles onto their glowing intensity, and it’s 
complementing with the effect of their composition. 
Yet, further investigation and analysis are needed 
(e.g., including pre-fractionation of pigments) before 
providing a solid statement for our findings.
Acknowledgements
The MATUROLIFE project leading to this research has 
received funding from the European Union’s Horizon 
2020 Research and Innovation Program under Grant 
Agreement No 760789. The authors acknowledge the 
Textile Chemistry Program (P2-0118(B)) under fi-
nancial support from the Slovenian Research Agency 
(ARRS), as well as Dr. Silvo Hribernik for the SEM-
EDX measurements.
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