Fluorescent ionic liquid micro reservoirs fabricated by dual-step E-beam patterning

Article abstract of DOI:10.1016/j.materresbull.2021.111434

In this work the fabrication of fluorescent microstructures through the e-beam induced solidification of two types of room temperature ionic liquids (RTILs) with fluorescent organic dyes is reported. It is shown that, by introducing dual-step e-beam patterning method with controlled accelerating voltage, solid micro-sized reservoirs are fabricated with liquid phase RTIL sealed inside. The presence of liquid inside the containers is confirmed by measuring their fluorescence spectra and comparison with results obtained for solid RTIL. The impact of the electron dose (ED) used in the exposure process on robustness and fluorescence characteristics of the fabricated structures is investigated. The temperature response of the micro reservoirs is also examined. The advantage of liquid – filled micro reservoirs is that they remain highly fluorescent, while solid RTIL structures exhibit significant fall in fluorescence ability associated with e-beam exposure damage.

Full text of DOI:10.1016/j.materresbull.2021.111434

Materials Research Bulletin 142 (2021) 111434
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Fluorescent ionic liquid micro reservoirs fabricated by dual-step
E-beam patterning
a
,
*
a
a
,
b
a
c
Dominik Kowal , Krzysztof Rola , Joanna Cybinska , Marcin Skorenski , Adrian Zajac ,
c
,
d
c
e
a
Andrea Szpecht , Marcin Smiglak , Slawomir Drobczynski , Karolina Ciesiolkiewicz ,
a,e,*
Katarzyna Komorowska
a
Lukasiewicz Research Network – PORT Polish Center for Technology Development, Wroclaw 54-066, Poland
Faculty of Chemistry, University of Wroclaw, Wroclaw 50-383, Poland
b
c
Poznan Science and Technology Park, Rubiez 46, 61-612 Poznan, Poland
d
e
Faculty of Chemistry, Adam Mickiewicz University 61-614 Poznan, Poland
Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wroclaw 50-370, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work the fabrication of fluorescent microstructures through the e-beam induced solidification of two types
of room temperature ionic liquids (RTILs) with fluorescent organic dyes is reported. It is shown that, by intro-
ducing dual-step e-beam patterning method with controlled accelerating voltage, solid micro-sized reservoirs are
fabricated with liquid phase RTIL sealed inside. The presence of liquid inside the containers is confirmed by
measuring their fluorescence spectra and comparison with results obtained for solid RTIL. The impact of the
electron dose (ED) used in the exposure process on robustness and fluorescence characteristics of the fabricated
structures is investigated. The temperature response of the micro reservoirs is also examined. The advantage of
liquid – filled micro reservoirs is that they remain highly fluorescent, while solid RTIL structures exhibit sig-
nificant fall in fluorescence ability associated with e-beam exposure damage.
A. thin films
A. organic compounds
B. microstructure
B. luminescence
C. electron microscopy
1
. Introduction
Room temperature ionic liquids (RTILs) are low melting point
particularly beneficial that RTILs do not require additional solvents
which simplifies the film deposition procedure, makes it safer and
environmental friendly. There exist other examples of solvent-free re-
sists for lithography but these require processing at cryogenic temper-
atures [13,14,15].
organic salts which have been studied extensively since 1970s. As they
typically exhibit negligible volatility and very good thermal stability,
RTILs are often regarded as promising candidates to replace common
organic solvents in many industrial applications [1,2]. In particular,
RTILs are widely used as reaction supporters or catalytic converters and
their use in electrochemistry is also a subject of study [3–6]. RTILs can
be made polymerizable by the attachment of functional groups such as
allyl or vinylbenzyl to their molecular structure [7]. Resulting poly(ionic
liquids) are often used as solid electrolytes [8,9]. However, recent
demonstrations of UV or electron beam (e-beam) based solidification of
the RTILs [10–12], likely associated with polymerization and cross-
linking effects, brought new attention towards these materials. Solubi-
lity of the RTILs in ethyl or isopropyl alcohol changes drastically in
UV/e-beam modified films, which allows for reproducing micro-
structured patterns in the RTILs by lithographic techniques. It is
Furthermore there has been growing interest in synthesising RTILs
with fluorescent features which can be then applied in photonics, op-
toelectronics and broad field of chemical or biological sensing [16-19].
Fluorescent polymeric film sensors are widely used for the detection of
specific chemicals. Examples include copper, iron and lead ions [20-22],
2
but also gases like CO or ammonia and even nitroaromatic explosives
[23-25]. Analysis of fluorescence changes could be also used for sensing
environmental parameters such as temperature, humidity or pH
[26-28]. Fluorescent structures with micro and nanometer sizes have
been gaining increasing attention due to their application in biological
measurements, such as intracellular thermometry and glucose sensing
[29,30]. Due to the variety of fields where the fluorescence probes may
be possibly applied, studying the behaviour of fluorescent RTILs
*
Corresponding authors.
E-mail addresses: dominik.kowal@port.lukasiewicz.gov.pl (D. Kowal), katarzyna.komorowska@pwr.edu.pl (K. Komorowska).
https://doi.org/10.1016/j.materresbull.2021.111434
Received 8 March 2021; Received in revised form 31 May 2021; Accepted 3 June 2021
Available online 5 June 2021
0
025-5408/© 2021 Elsevier Ltd. All rights reserved.

D. Kowal et al.
Materials Research Bulletin 142 (2021) 111434
becomes a big issue in material science. Particularly, in case of UV or
e-beam treatment of the RTILs, the relation between exposure condi-
tions and resulting fluorescence intensity is to be resolved as this
possibly allows for tuning the fluorescence of the material at the pro-
cessing stage.
lithographic techniques but also thanks to the use of a unique material
such as ionic liquid, it allows for extensive modification of the properties
of the liquid molecules without the need to change the patterning
method.
E-beam lithography is a powerful tool for shaping the RTILs with sub-
micron resolution and such a solidification of RTILs has been up to date
reported by several authors [11,12,18,31]. In one of our recent works we
showed polymerizable fluorescent RTILs, which were obtained by the
presence of either fluorescein dyes or europium ions [18]. Thin films of
these RTILs were solidified by using electron beam and their lumines-
cent behaviour was examined. The luminescence of RTILs containing
organic dye was significantly decreased due to a self-quenching in the
aggregated state [32], or – what even more inevitable – the high energy
electrons damage of the dye. In this paper we present further study of the
e-beam treated RTILs with organic luminescent groups, where we focus
on the relation between fluorescence characteristics of the fabricated
structures and the exposure conditions chosen for the fabrication pro-
cess. First, we show that the intensity of the fluorescent light emitted by
the solidified structures gradually decays with the electron dose (ED)
used in the process, which is in agreement with previously reported data
2. Materials and methods
2.1. Synthesis of RTILs
Two kinds of RTILs with allyl functional group were studied: 1-allyl-
3-methylimidazolium chloride ([Allmim][Cl]) with the addition of
fluorescein dianions (concentration 1: 500), which has been reported in
the past and 1-[4-[4-(1,2,2-triphenylethenyl)phenoxy]butyl]-3-(prop-2-
en-1-yl)-1H-imidazol-3-ium chloride ([TPEBuImAll][Cl]) synthesised
in-house [18]. The target compound was prepared according to the
modified known methods [36,37]. Application of microwave reactor
allowed to achieve high reactions yields in a relatively short time.
Synthetic route is presented in Fig. 1. Detailed synthesis data are given
in the Appendix at the end of this paper. Shortly, 4-(1,2,2-triphe-
nylvinyl)phenol (TPEOH) was synthesized by means of coupling 1,2,
2-triphenylvinyl bromide with (4-hydroxyphenyl)boronic acid. Reac-
tion was catalyzed by Pd(PPh3)4 giving desired TPEOH. Then, linker
moiety was introduced to the TPEOH structure. Reaction with 1,4-dibro-
mobutane was carried out in microwave reactor under the standard
Williamson ether synthesis conditions. In the next step 1-(4-bromobu-
toxy)-4-(1,2,2-triphenylethenyl)benzene (TPEBu) was reacted with
imidazole under the microwave irradiation. At last [TPEBuImAll][Cl]
was synthesized by simple reaction with allyl chloride.
[
18]. In practice this creates serious obstacle for obtaining highly fluo-
rescent microstructures in solidified RTILs, however possible solution to
this issue is proposed, which is based on sealing the liquid phase RTIL in
solid micro reservoirs. These structures are fabricated by dual-step
pattering of the RTIL film with electron beam. Our method takes
advantage of controlled-acceleration voltage electron beam lithography
(
CAV-EBL), which has been reported to be useful for suspended struc-
tures fabrication in negative-tone resists [33]. We prove the presence of
a liquid phase inside the containers with spectral measurements and we
analyse temperature stability of the fabricated structures.
2
.2. Fabrication of micro reservoirs
Sealing very small volumes of liquid in micro-scaled solid reservoirs
could be useful in biomedical applications, such as drug delivery or
enzyme activity monitoring [34,35]. However the use of task specific
ionic liquids that posses properly designed optical or electrical charac-
teristics could also make them beneficial for the development of sensors
or intelligent materials and surfaces. Our fabrication approach could be
compared with what was achieved in the past by using soft lithography
Silicon (100) substrates were cleaned in oxygen plasma for 10 min
(
Plasma System GIGAbatch 310M). Prior to thin film formation RTILs
◦
were baked in 120 C in nitrogen in order to remove water content. Then
the layer was prepared by means of spin coating and the resulting film
thickness was 2.5 µm. In general RTILs do not require any additional
solvents, however [TPEBuImAll][Cl] was very viscous in room tem-
perature and so it has been dissolved in isopropyl alcohol for the purpose
of spin coating. The substrate was transferred to the chamber of the
electron microscope. The electron beam was controlled by RAITH Elphy
Multibeam system. Pure solid microstructures were fabricated in the
[
34,35]. Although the use of molded PDMS stamps allows for fast
isolation of liquids from the surrounding, it carries certain limitations
due to stamp sizes and integrity, making it difficult to fabricate the
reservoirs individually or to separate them. More flexible is the approach
based on stacking the structures, however it requires a complex
multi-step molding method [35]. On the contrary the proposed
approach is fast and straightforward. Not only it allows for reaching the
spatial resolution superior to what can be achieved through other
RTIL by using the accelerating voltage 30 kV and various electron doses
2
(
EDs) in the range 10 – 80 mC/cm . To fabricate the micro reservoirs
with liquid interior two stages of e-beam patterning process were per-
formed. In the first stage the sidewalls of the containers were solidified
by applying the 30 kV accelerating voltage and exposing the RTIL film to
◦
Fig. 1. Synthetic route to [TPEBuImAll][Cl]. Reagents and conditions: a) Pd(PPh
3
)
4
, K
2
CO
3
, THF, reflux, 16h; b) 1,4-dibromobutane, Cs
2
CO
3
, DMF, MW, 100 C, 45
◦
◦
min; c) imidazole, K
2
CO
3
, acetone, MW, 100 C, 45 min; d) allyl chloride, 60 C, 5h.
2

D. Kowal et al.
Materials Research Bulletin 142 (2021) 111434
the pattern which consisted of square, rectangular or round cells. After
that the RTIL uniform film overlapping the previously fabricated cells
was patterned again (large area rectangular sheet over the sidewall
pattern) with accelerating voltage set to 5 kV and the ED in 1 – 15 mC/
(SEM), FEI Dual Beam Helios 660, with the accelerating voltage set to 2
kV. Thickness of the e-beam modified structures was measured on the
stylus profiler (Bruker Dektak XT). Further on, for qualitative exami-
nation of the fluorescence emitted from the solidified RTIL the samples
were imaged by using fluorescence microscope (Carl Zeiss) with 350 –
500 nm filter for the excitation and longpass filter above 500 nm for
emission. For spectral measurements the samples were first imaged on
the Olympus fluorescence microscope and then the emission was ana-
lysed with the spectrometer (Ocean Optics QE65PRO-FL) attached to the
microscope. The imaging system allowed to spatially limit the excited
area to the diameter of 100 µm. LEDs centered at 405 nm and 490 nm
(Thorlabs M405D2 and M490D3) were used for sample excitation and
the emission spectra above 450 nm or 550 nm were acquired.
2
cm range. This way the “covers” of the containers were created.
Exposed samples were then developed in isopropyl alcohol and dried.
The whole fabrication process is schematically shown in Fig. 2a. For the
purpose of spectral measurements the design of the sample patterning
consisted of two partially intersecting zones: sidewalls of containers in
the form of square cell matrix (first step) and the thin cover layer above
the containers and directly on the substrate (second step). Such a design
of the process facilitated collection of the emission spectra from three
different regions separately: sidewalls, covers and closed reservoirs. This
patterning design is schematically shown on Fig. 2b and Fig. 2c presents
exemplary image from fluorescence microscope. It is worth to empha-
size that the presented fabrication method is straightforward as it is
mainly based on dual-step e-beam exposure and single-step develop-
ment. The use of lithography technique with controlled beam parame-
ters, resulting in controlled penetration depth of electrons, allowed us to
pattern both the sidewalls and the covers without removing the sample
from the process chamber in between times, which is clearly advanta-
geous for shortening the time of the fabrication and keeping the cost
down. Also, the proposed solidification method is based on chemical
ability of organic moieties to polymerize or crosslink, and so it is not
limited to the presented fluorescent RTILs, but could be also applied to
RTILs designed for other purposes, for example with paramagnetic
features [38].
3
. Results and discussion
Fig. 3a shows the fluorescence microscope image of rectangular
fluorescein-containing [Allmim][Cl] based structure. The structures
were solidified with accelerating voltage 30 kV and various EDs in the
2
range 10 – 80 mC/cm . The intensity of emitted fluorescent light
significantly decreased with the increase of the electron dose. There are
several possible reasons for this change. First, inelastic electron scat-
tering often leads to radiolysis (covalent bond breakage upon energy
absorption) especially in case of organic compounds [39,40]. It is then
likely that high energy electrons affect molecular structure of fluores-
cein ions upon impact. Second, it is possible that density of the solidified
material rises with the ED, which could bring fluorescein ions closer
together and quench the fluorescence. The shrinkage of the RTILs due to
e-beam induced solidification has been observed before [31]. Also, it is a
known fact that the polymer density depends on its molecular weight
[41], however the relation of the latter with the ED is yet to be measured
in RTILs. Thus, fluorescence quenching remains a possible option, but in
this case the issue could be solved by choosing another kind of dye.
Moreover, our observations of solidified [Allmim][Cl] imply that the
optical characteristics of pure [Allmim][Cl] (purchased from Sigma
Aldritch), without the presence of fluorophore, also depends on the ED.
Fig. 3b shows microscope image in reflected light of the solidified
structures in [Allmim][Cl], which grow darker with the increase of ED.
The absorption characteristics of solidified [Allmim][Cl] might be then
partially responsible for the fluorescence decrease shown in Fig. 3a.
2
.3. Structures characterization
Photoluminescence (PL), photoluminescence excitation (PLE)
spectra of the liquid phase RTIL were recorded with a FSL980-sm
Fluorescence Spectrometer from Edinburgh Instruments Ltd. A 450 W
Xenon arc lamp (PL and PLE) as an excitation source was used. TMS302-
X single grating excitation and emission monochromators with a focal
length of 300 mm were used. The luminescent photons were recorded by
Hamamatsu R928P high-gain photomultiplier detector. Emission
spectra were corrected for the recording system efficiency and excitation
spectra were corrected for the incident light intensity. RTILs solidified
by e-beam were first imaged by the use of scanning electron microscope
Fig. 2. a) Schematic view of the fabrication process of microcontainers, b) the patterning design used for the purpose of spectral measurements and c) fluorescent
microscope image presenting some of the fabricated structures.
3

D. Kowal et al.
Materials Research Bulletin 142 (2021) 111434
Fig. 3. a) Fluorescence from the solidified structures of
fluorescein-containing [Allmim][Cl] fabricated with 30 kV
2
accelerating voltage and various EDs (mC/cm ) indicated by
numbers (Thorlabs M490D3, shortpass filter below 500 nm for
excitation and long pass filter above 550 nm for emission). b)
Image in reflected light of the pure [Allmim][Cl] solidified
2
with EDs (mC/cm ) indicated by numbers. c) Emission spectra
of the fabricated structures (source and filter set as in 3a). d)
The relation between the peak emission intensity and the ED
(
peak wavelength shifts from 585 nm to 605 nm). e) Com-
parison between fluorescence spectra acquired for [Allmim]
Cl] with addition of fluorescein and without it. f) Emission
[
spectrum of the fluorescein-containing [Allmim][Cl] in liquid
phase (FSL980-sm Fluorescence Spectrometer from Edinburgh
Instruments Ltd., with 450 W Xenon arc lamp as an excitation
source and TMS302-X single grating excitation and emission
monochromators with a focal length of 300 mm). g) The
thickness of the container “covers” fabricated in fluorescein
containing [Allmim][Cl] with 5 kV accelerating voltage as a
function of the ED used.
One should note that the ED also influences the spatial resolution of
fluorescence is non-negligible. That means that either some fluorescent
compound is formed in the RTIL in result of e-beam exposure or perhaps
the [Allmim][Cl] used was not free from contaminants.
the process as it was reported in earlier works concerning e-beam
modification of the RTILs [12]. In particular, structures produced by
high ED e-beam tend to have smooth edges with “tails” of the material
that reach outside of the exposed area. This ED dependent effect
significantly limits the achievable spatial resolution. As we found out,
for the 30 kV frame the smallest features possible to fabricate in 2.5 µm
Moreover, it is noticeable that the peak wavelength of the emission
spectrum of fluorescein containing [Allmim][Cl] experiences a slight
redshift with the increase of the ED. For comparison the emission
spectrum of the same RTIL in liquid phase is presented on the Fig. 3f.
One should note the intensity values on this Fig. should not be compared
with the intensities acquired from exposed samples (Fig. 3c) due to
different experimental conditions (quantity of the material and the
experimental setup). The spectrum of the liquid phase shows maximum
at around 550 nm, which is in agreement with previous data obtained
for fluorescein in ionic liquids [18,19]. The peak wavelengths from
Fig. 3c range from 585 to 605 nm. The redshift may result from the
aggregation of the fluorescein ions upon solidification but it could also
be associated with self filtering of the light emitting film [32, 42]. That is
because the thickness of the film is expected to grow slightly with the
applied ED. Some redshift of fluorescein absorption and emission bands
in IL media was previously reported. The study with using fluorescein
anion as an ionic liquid component was made by Rodrigues et al. [19] It
has been shown that the emission spectrum of fluorescein dissolved in
2
RTIL film were roughly 0.5 µm for ED=10 mC/cm and 2 µm for ED=80
2
mC/cm . Interestingly the fluorescence of the tails seems unaffected by
the e-beam exposure, which is contrasted by darkening of the structure
body. Most probably the „tails“ are created by the electrons back-
scattered at the silicon substrate which is known as proximity effect in
electron beam lithography. Energy of these electrons or their number
might not be high enough to destroy the fluorophore. Emission spectra
of the objects from Fig. 3a were measured and they are presented in
Fig. 3c. Analysis of the emission intensities points to monotonic
dependence between ED and peak intensity which is shown in Fig. 3d,
rd
together with a 3 order polynomial fit. It has also been observed that
some fluorescence is associated with the [Allmim][Cl] itself, without the
presence of fluorescein. Structures shown in Fig. 3b were examined.
Their emission spectra are shown in Fig. 3e and compared to analogical
results obtained in the presence of the dye. Although the measured in-
tensities are much lower in case of pure RTIL, this background
+
ꢀ
the parent IL (in this case Quat Cl ) can shift to longer wavelengths.
Fluorescein dye was also investigated in different ionic liquids, and it
4

D. Kowal et al.
Materials Research Bulletin 142 (2021) 111434
was confirmed that some shift in the band position strongly depended on
the ionic liquid structure and dye concentration [43].
highlight the difference between this value and the power emitted from
the sealed container. The excess of power measured at the “container”
zone is associated with liquid presence inside it. Although we could not
see any other explanation for the observed fluorescence spectra, it can
only be considered as indirect evidence. A natural way to examine the
interior of the reservoirs is to cut through it with focused ion beam (FIB).
However, the ion beam, due to its high energy, is not neutral to the
structure and may cause solidification of its liquid part. Thus our efforts
to identify liquid and solid part in the cross-section of the reservoir were
not conclusive.
2
ED equal to 40 mC/cm was chosen to fabricate the sidewalls of the
micro reservoirs. This value was chosen in order to provide enough
spatial resolution of the fabricated structures and in the same time to
limit the fluorescence intensity of the solid part of the structure. More-
over, the dose of 40 mC/cm2 was well above the minimum dose for
solidification of the [Allmim][Cl] liquid. In this case the thickness of the
solidified structures is practically independent on the ED variations and
it reflects the thickness of the liquid film formed by spin coating [12].
That is because the applied accelerating voltage of 30 kV was more than
enough for electrons to penetrate through the entire film and induce its
solidification from the surface to the bottom. On the other hand, this was
not the case when 5 kV accelerating voltage was applied. Measured
thickness of the “covers” fabricated by 5 kV accelerating voltage is
shown in Fig. 3g as a function of the ED used. It was preferable to
fabricate the cover part of the containers with possibly small EDs as to
keep them possibly thin. However, as we found out, the cover layers
In Fig. 6a-b the fluorescence microscope image of the disc–like
microcontainers is presented, together with quantitative data of the
fluorescent maximum intensities. The discs were sealed with cover
layers fabricated with various EDs. Fluorescent intensity increases with
2
the ED and reaches maximum for ED = 6 mC/cm . For greater EDs the
intensity decreases. This data imply that the amount of liquid inside the
fabricated containers depends on the beam parameters used for the
fabrication of the cover layers. Using small EDs for the fabrication of the
covers results in thinner layer and this maximizes the volume of the
containers. On the other hand, as it was already mentioned, thin cover
layers often collapse, somehow allowing the liquid to be washed away
during the development in alcohol. It seems the thinnest layers are in
fact leaky which point to porous character of the solidified material,
however we found no evidence of porosity during SEM examination.
Superimposing these two features, volume decrease and tendency to
2
fabricated with small EDs (1-2 mC/cm ) have the tendency to collapse
inside the cells during sample development in isopropyl alcohol. To
demonstrate this several container structures are shown in Fig. 4a.
Square and circular containers fabricated with ED of the cover layer
2
equal to 1-2 mC/cm look very much drained and collapsed. For com-
parison it seems that the containers fabricated with ED of the cover layer
2
9
-12 mC/cm are filled with RTIL to greater extent. This difference is
2
most noticeable in case of circular containers. One should note that even
the containers fabricated with ED of the cover layer equal to 12 mC/cm2
show some kind of recess. Then it seems that some amount of the RTIL
closed in the container during patterning is always lost in the develop-
ment process. Furthermore in Fig. 4b similar containers are shown
fabricated in the two studied RTILs. For the same choice of ED used for
substance solidification, physical nature of the resulting solids is
different. E-beam modified regions in [Allmim][Cl] appear much
smoother than the ones in [TPEBuImAll][Cl] which exhibit more surface
roughness. Despite this fact we managed to successfully manufacture the
micro reservoirs in both RTILs.
collapse, makes the ED = 6 mC/cm the most preferable for making
containers.
Temperature, likewise the ED, is a factor the might induce the so-
lidification of the RTIL and influence its fluorescent characteristics. As
the liquid interior of the micro reservoirs is not altered by the electron
beam it can potentially be used for monitoring the heating history of the
sample. Also, the RTILs can be applied in the future as photo-
polymerisable resists with pre- and post bake processing steps - thermal
treatment is commonly used in lithography in order to remove solvents
or increase the stability of the resist. Thus we believe it is important to
know the impact of elevated temperature on the fluorescent character-
istics of the RTIL. For this purpose some additional tests have been
conducted (Fig. 6c). Samples were annealed for 10 min in temperatures
As was mentioned in Section 2.2, to confirm the presence of the
liquid phase RTIL inside the reservoir some special patterning design has
been introduced and spectral measurements have been performed. The
designs comprised of three zones as was schematically shown in Fig. 2b:
the “cover”, the “sidewall” and the “container” (“container” zone was in
fact an oversection of the first two zones). The emission spectra have
been collected for the three zones separately. The assumption was that
the “container” spectrum not only constituted of the power emitted by
the “cover” and “sidewall” but also another factor associated with the
liquid sealed in the container cells. In Fig. 5 the measured spectra are
shown for the two examined RTILs. Additionally, summarized power of
the “sidewall” and “cover” emitted light is presented on the plots as to
◦
between 50 – 200 C subsequently. The intensity of fluorescence
increased after first annealing process which might have been connected
to drying of the substance at elevated temperature. With temperature
◦
increasing above 100 C the fluorescence intensity gradually decreased
and it fell below 25% of the initial value once the temperature reached
◦
◦
200 C. It is not surprising that the fall of intensity started after exceeding
100 C taking into account the baking of the RTILs prior to film depo-
sition. The observed change was irreversible. One might have expected
that the liquid phase RTIL inside the containers would partially harden
at elevated temperature causing an additional drop of the fluorescence
intensity measured at the covered sections of the sample. However, no
such effect was observed as the fluorescence intensity of micro reservoirs
remained distinctly higher than for sidewalls and a cover summed up,
◦
even after annealing up to 200 C, as shown in Fig. 7 (compare with
Fig. 5). This suggests that the temperature required for hardening of the
RTIL has not been reached and, consequently, the thermal degradation
threshold of the dyes is the main reason for the fluorescence drop.
◦
◦
Decomposition of pure fluorescein takes place at 290 C (554 F), as can
be found in online database [44]. Other literature data also show fluo-
◦
rescein to be only little affected by temperatures below 210 C [45].
Decomposition of TPEOH which is responsible for fluorescence of
◦
[
TPEBuIm][Cl] is expected at 215 C [46]. On the other hand the thermal
behavior of fluorophores in RTIL environment might be quite different.
The decrease of fluorescence could be also associated with two other
effects, namely increased concentration of fluorophore due to material
shrinking and change of material transparency. It is unlikely though that
these effects play significant role, as in the conducted tests we found no
Fig. 4. a) Microcontainers fabricated in fluorescein-containing [Allmim][Cl]
with square and round shapes, covered with layers solidified with various EDs
2
(
1, 2, 9, 12 mC/cm ) as indicated by numbers. b) Microcontainers fabricated in
2
◦
two fluorescent RTILs with the cover layer solidified with ED set to 12 mC/cm .
confirmation of significant material shrinking in 200 C and we observed
5

D. Kowal et al.
Materials Research Bulletin 142 (2021) 111434
Fig. 5. Measured spectra of the micro reservoirs and their components fabricated in fluorescein containing [Allmim][Cl] and [TPEBuImAll][Cl] ([Allmim][Cl]:
Thorlabs M490D3, shortpass filter below 500 nm for excitation and longpass filter above 550 nm for emission; [TPEBuImAll][Cl]: Thorlabs M405D2, shortpass filter
below 420 nm for excitation and longpass filter above 450 nm for emission).
the transparency of [Allmim][Cl] (without fluorophore) to be nearly
◦
constant up to 220 C (Fig. 8).
4
. Conclusions
In this work we presented a novel method for the fabrication of micro
reservoirs with fluorescent RTILs sealed inside. The proposed method
relies on the use of e-beam patterning tool with accelerating voltage
change for RTILs solidification. Synthesis of two RTILs with different
fluorescence characteristics was performed and structures with various
designs were fabricated. We showed that the fluorescence abilities of
solid RTILs decay with the increase of ED used in the process, however
the liquid sealed in the containers remains highly fluorescent. This was
confirmed by fluorescence microscope imaging and spectral measure-
ments of the emitted light. We observed that the ED used for the fabri-
cation of the container covers must be higher than a threshold value to
prevent leakage of the liquid during development in isopropyl alcohol.
Thus, farther experiments must be conducted in order to reveal the
nature of the e-beam modified RTIL in nanoscale. Finally, we examined
fluorescence spectra of the fabricated containers annealed at tempera-
◦
◦
tures from 50 C to 200 C and we conclude that the structures are
◦
thermally stable up to around 100 C.
Fig. 6. a) Maximum fluorescence intensity from disc-like micro reservoirs
maximum of fluorescence spectrum acquired from 100 µm circular zone with
Author statement
(
individual container in the center) in fluorescein containing [Allmim][Cl] with
cover layers fabricated with various doses and b) fluorescence microscope
image of these structures. c) Normalized peak intensity of the fluorescence
spectrum of the micro reservoirs fabricated in the two examined RTILs annealed
Author; Contribution
1
. Dominik Kowal: Methodology, Investigation, writing – Original
Draft, Validation
2
3
4
. Krzysztof Rola: Methodology, Investigation, validation
. Joanna Cybi n´ ska: Supervision, methodology, conceptualization
. Marcin Skore n´ ski: Resources, Investigation, methodology
◦
◦
at temperatures from 50 C to 200 C.
Fig. 7. Fluorescence spectra of micro reservoir and its components fabricated
◦
in fluorescein containing [Allmim][Cl] annealed at 200 C.
Fig. 8. Normalized transmittance of the [Allmim][Cl] film as a function of
temperature measured at two wavelengths.
6

D. Kowal et al.
Materials Research Bulletin 142 (2021) 111434
5
6
7
. Adrial Zając: Resources, Investigation, methodology
purified by column chromatography on silica gel using chloroform:
. Andrea Szpecht: Resources, Investigation
methanol (v/v = 10:1) as eluent. Final product was obtained as a white
´
1
.
Marcin Smiglak: Supervision, Project administration,
powder in 79% yield (0.50 g). H NMR (500 MHz, CDCl
3
): 7.51 (s, 1H),
methodology
7.22 – 6.55 (m, 18H), 6.40 (d, J = 7.5 Hz, 2H), 4.05 (t, J = 7.8 Hz, 2H),
3.88 (t, J = 7.7 Hz, 2H), 2.00 – 1.88 (m, 2H), 1.80 – 1.62 (m, 3H). EI-MS
8
9
1
. Sławomir Drobczy n´ ski: Investigation, Funding Acquisition
. Karolina Ciesiołkiewicz: Investigation
m/z calcd for [M+H]+ C33
30 2
H N O, 471.2431; found 471.2425
0. Katarzyna Komorowska: Conceptualization, Writing – Review
Editing, Supervision, Project administration, Funding Acquisition
[TPEBuImAll][Cl]: Allyl chloride (0.40 ml, 4.00 mmol) was added to
◦
&
the TPEBuIm (0.22 g, 0.47 mmol). The mixture was heated at 65 C for 8
hours. Then allyl chloride was evaporated and the crude product was
purified by column chromatography on silica gel using chloroform:
Declaration of Competing Interest
methanol (v/v = 10:1) as eluent. Product was obtained as pale yellow
1
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
powder in 68% yield (0.17 g). H NMR (500 MHz, CDCl
3
): 7.22 – 6.51
(m, 19H), 6.60 – 6.55 (m, 2H), 5.06 – 4.93 (m, 2H), 4.20 – 4.01 (m, 5H),
3.89 (t, J = 7.8 Hz, 2H), 2.04 – 1.75 (m, 3H), 1.82 – 1.66 (m, 2H). EI-MS
m/z calcd for [M+H]+ C36
35 2
H N O, 511.2822; found 511.2825
Acknowledgments
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Financed by the Polish Ministry of Science and Higher Education No.
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