Three-Dimensional Printed Photoluminescent Polymeric Waveguides

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Applications of Polymer, Composite, and Coating Materials

3D Printed Photoluminescent Polymeric Waveguides Francesca Frascella, Gustavo Gonzalez, Paula Bosch, Angelo Angelini, Annalisa Chiappone, Marco Sangermano, Candido Fabrizio Pirri, and Ignazio Roppolo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16036 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Applied Materials & Interfaces

3D Printed Photoluminescent Polymeric Waveguides

Francesca Frascellaa, Gustavo Gonzáleza,b, Paula Boschc, Angelo Angelinia, Annalisa Chiapponea, Marco Sangermanoa, Candido Fabrizio Pirria,b, Ignazio Roppoloa*

aDepartment

of Applied Science and Technology, Politecnico di Torino, Torino, Corso Duca

degli Abruzzi 24, 10129, Italy bCenter

for Sustainable Future Technologies @Polito, Istituto Italiano di Tecnologia, Corso

Trento 21, Torino 10129, Italy cDepartamento

de Química Macromolecular Aplicada, Instituto de Ciencia y Tecnología de

Polímeros, Consejo Superior de Investigaciones Cientíﬁcas (CSIC), C/ Juan de la Cierva 3, Madrid, 28006, Spain

Keywords: 3D printing, waveguides, optical sensor, DLP, photoluminescent materials

ABSTRACT

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In this work, we propose an innovative strategy for obtaining functional objects employing a light activated 3D printing process without affecting the materials’ printability. In particular, a dye is a necessary ingredient in a formulation for a Digital Light Processing 3D printing method in order to obtain precise and complex structures. Here we use a photoluminescent dye specifically synthesized for this purpose that enables the production of 3D printed waveguides and splitters able to guide the luminescence. Moreover, copolymerizing the dye with the polymeric network during the printing process, we are able to maintain the solvatochromic properties of the dye towards different solvents in the printed structures, enabling the development of solvents’ polarity sensors.

Introduction 3D Printing (3DP) is gaining in the last years more and more attention both in industrial and in scientific field due to the unique properties and manifolds advantages that the technologies included under this umbrella terms involve.1 In particular, taking advantage of 3DP, it is possible to produce objects with shapes impossible to achieve by classical subtractive manufacturing techniques, saving at the same time raw materials and energy.2, 3 3DP is exploding in a myriad of applications, leaving the first rapid prototyping use to new fields, ranging from electronics to aerospace,4, 5 passing through biomedical,6-8 mechanics9-11 and many others.12-15 Polymeric 3DP is actually the most developed branch in additive manufacturing world, due to the low cost of 3D printers and the relative large range of processable materials when compared with technologies for metals and ceramics processing.16, 17 However, the development of new printable materials is mandatory in order to further expand the use of this technology towards new


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frontiers.18 In this frame, among the different 3DP methods, light-based technologies such as stereolithography (SLA) and digital light processing (DLP) are known for being the most flexible since it is possible to impart new functionalities not only defining the geometry of the printed object but also easily playing on the different components of the printable formulations.19, 20 Since these technologies are based on the spatially-controlled solidification of liquid formulations through fast photopolymerization reaction,21 the most common strategy for implementing materials properties consists in dispersing (nano)fillers or functional additives in the photocurable formulation in order to enhance mechanical22-24 or functional18, 25 properties. In literature usually a top-down approach was followed,

18, 25, 26

however, when fillers are added, they must be

homogeneously dispersed and their stability in the mixture should be compatible with printing time. Moreover, their presence limits light penetration and increases the viscosity.

27

In order to

avoid these limitations an alternative bottom-up approach was proposed, dispersing precursors of an inorganic or metallic phase in the photocurable formulation. In those cases, the 3D printing step was followed by an in situ generation of NPs.19, 28 Nevertheless, the development of nanocomposites is not the only option for obtaining printable functional materials but it is possible to operate with the fundamental ingredients of the formulation: monomers, photoinitiator and dye. In fact it is possible to use monomers with inner functionalities29-31 or taking advantage of the photoinitiator32 for producing object with peculiar properties. More recently also the “intelligent” use of dye was proposed. 33 The dye is commonly added at low concentrations to the printable formulations to control the light penetration and avoid over-polymerization, allowing to obtain well-defined structures.34, 35 However, after the printing process the dye is usually useless, but it remains in the object leaving it a colour that in some case could be even undesired.19



In this work, we pursued this latter approach, developing 3D printable optical waveguides adding a photoluminescent dye to the formulation. Nowadays optical waveguides cover a broad range of applications, such as high optical power,36 photonic integrated circuits,37 communications38 and even sensors.39 Until now, many microfabrication processes have been developed, but to explore the possibilities of truly custom optical element, they need complicated three-dimensional microstructures and the use of different materials. In this approach, 3D printing technology based on polymers and hybrid materials provide an ideal platform.40 Recently, Lewis et al.41 report on the fabrication of optical waveguides via photocurable liquid core–fugitive shell printing, while two years before Parker et al.42 patterned silk waveguides by printing a concentrated, viscoelastic ink composed of silk fibroin through a fine deposition nozzle. Moreover, 3DP was used for the fabrication of microwave-range waveguides using fused filament deposition43,

44

or stereolithography44. Differently, in this work we used DLP technology for

printing the waveguides, which enables more complex geometries and lower amount of materials’ dishomogeneities. The dye employed is an aminoderivative of 7-nitrobenz-2-oxa-1,3-diazole (NBD), which has been functionalized with a methacrylic double bond. NBD-amino derivatives have been widely used in different fields because their synthetic precursor, the chloride derivative, can easily react with primary amines, which lead to fluorescent compounds. We have chosen this fluorophore because it joins together two interesting properties for our purposes: 1) The fluorescence emission of 7-nitrobenz-2-oxa-1,3-diazole (NBD) amino derivatives is dependent on the medium viscosity through a TICT (Twisted Intramolecular Charge Transfer) mechanism, and its fluorescence quantum yield increases as rigidity increases;45 and 2) it has very adequate absorption and emission properties, with a charge transfer (CT) intense absorption band in the red region (λabs=440-475nm) and intense fluorescence emission in the yellow-green (λem=497-


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526nm).46 Also, as it bears a methacrylic double bond, it will remain covalently linked to the material after 3D printing process, then avoiding long-term undesirable extraction reactions and at the same time gathering the materials properties with shape-induced characteristics. Therefore, here we demonstrate that complex optical waveguides and splitter could be easily produced by 3D printing taking advantage of the functional dye, without affecting the printability of the formulation. Moreover, these objects maintain the sensitivity versus the polarity of solvents that are placed in contact with the printed waveguides, enabling the development of polarity sensors. Experimental Section Synthesis

of

4-(N-Methyl-N9-ethanol)amino-7-nitrobenz-2-oxa-1,3-diazole

(NBD-NEtOH):

250mg of 4-chloro-7-nitrobenz-2-oxa-1,3-diazole (1.2mmol) and 15mL of ethanol were placed in a 30mL microwave (MW) glass vial, and then 240 μL of 2-(N-methyl) aminoethanol (1.5mmol) were slowly added under stirring. The vial was sealed and subjected to MW irradiation at 90ºC during 2min. The vial was left at room temperature and NBD-NEtOH slowly crystalized as bright orange platelets to obtain 285mg of pure compound (75% yield). NBD-NEtOH: 1H NMR (CDCl3): d = 8.46 (d, Har , J = 9.02 Hz), 6.18 (d, Har , J = 9.02 Hz), 4.35 (t, CH21O, J = 5.29 Hz), 4.06 (t, CH2-N, J = 5.29 Hz), 3.65 (s, OH), 3.52 ppm (s, CH3-N). Synthesis of Methacrylic Derivative of NBD (NBD-MA): Conventional acylation of NBD-EtOH was conducted as described elsewhere47. NBD-MA: 1H NMR (CDCl3): d = 8.46 (d, Har , J = 9.02 Hz), 6.32 (d, CH2-C, J = 1.56 Hz), 6.18 (d, Har J = 9.02 Hz), 5.95 (d, CH2-C, J = 1.56 Hz), 4.53 (m, CH2), 3.49 ppm (s, CH3-N).



Materials: The photocurable monomers Bisphenol A Ethoxylate (2 EO/phenol) diacrylate (Mw 572, BEDA) was purchased from Merck and used as received. Omnirad 819 (BAPO) from IGM Resins was used as photoinitiator. NBD-MA was used as photoluminescent dye. Acetonitrile from Merck was used as solvent for NBD-MA and for polarity sensitivity measurements. At last, all the other sensors used for polarity sensitivity measurements (dichloromethane, ethanol, N,Ndimethylformamide (DMF), toluene) were purchased from Merck. Formulations for the printing of waveguides: three different concentrations of NBD-MA were tested in this work: 0.01 per hundreds resins (phr), 0.005 phr and 0.001 phr in BEDA. All the formulations were prepared with 2 phr of BAPO. For the preparation of the printable formulations, first NBD-MA and BAPO were dissolved in acetonitrile using an Ultrasonic bath for 20 minutes (130 mg/mL). Then the BEDA was added and mixed by sonication for others 10 minutes. In addition, neat formulation containing 2 phr of BAPO and BEDA was prepared only for characterization tests. Preparation of the 3D structures: as printing equipment was employed a 3DLPrinter-HD 2.0 (Robot Factory) using a projector with a resolution of 50 µm (1920× 480 1080 pixels). The build area is 100 x 56.25 x 150 mm3 and a Z resolution from 10 µm to 100 µm. The power density of the light source is 10 mW/cm2. The irradiation time for the first 6 layers of the sliced object was 8 seconds to guarantee a good adhesion on building platform. The structures were then printed slightly modifying the irradiation time from 1 s layer to 1.3 s/layer (according to the amount of dye present). After printing, a post curing process (3 min) was performed in air with a broad band medium−pressure mercury lamp, also provided by Robot Factory (UV power density 50 mW/cm2).


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Methods: An Anton Paar MonowaveTM 300 microwave synthesis reactor provided with an infrared sensor (IR pyrometer) was used for the synthesis. Reaction was performed in pressureresistant 30-mL glass tubes sealed with silicon septum and using a magnetic stirring bar. Real-time rheological measurements were performed using an Anton Paar rheometer (Physica MCR 302) in parallel plate mode with a Hamamatsu LC8 lamp with visible bulb and a cut-off filter below 400 nm equipped with 8 mm light guide, light intensity was set at 10 mW/cm2. The gap between the two plates was set to 0.1 mm, the samples were kept at a constant temperature (25 °C) and under constant shear frequency of 1 rad/s; light was turned on after 1 minute in order to stabilize the system. Concomitant changes in viscoelastic material moduli during polymerization were measured as a function of exposure time. The measurements were carried out in the linear viscoelastic region (LVE) with a strain amplitude of 1%. The LVE was previously evaluated performing a amplitude sweep measurement at 1 rad/freq (strain from 0.1 to 1000%). Viscosity measurements were performed in parallel plate mode with a gap of 0.1 mm between both plates. The shear rate range was set up from 0.1 s-1 to 100 s-1. Differential scanning calorimetry (DSC) measurements were performed with a DSC1 STARe System apparatus of TA Instruments equipped with a low temperature probe. The experiments were carried out between 20 and 100 °C with a heating rate of 10 °C/min. Nicolet iS50 FT-IR spectrometer (Thermo Scientific, Milano, IT) equipped with an Attenuated Total Reflection (ATR) accessory (Smart iTX) was used to calculate the final acrylate conversion of the 3D printed samples. The spectra were collected using ATR accessory, first collecting a spectrum of the liquid photocurable formulations and then collecting ATR spectra of the 3D printed samples after printing step and after UV-post curing. The spectra were collected with a resolution of 2 cm−1 averaging 32 scans for each spectrum in the wavenumber range between 650 cm-1 and 4000 cm−1.



The conversion of acrylate double bonds was monitored by following the decrease of the peak area of C=C group at 1640 cm-1. The area of the peak was normalized by a constant signal in the spectra corresponding to the stretch of aromatic ring centered at 1510 cm-1. DMA measurements were performed on 3D printed films (thickness 200 μm) using a Triton Technology TTDMA. The experiments were performed scanning the mechanical response of the samples in the temperature range between – 40°C and 70°C with a heating rate of 3 °C/min, at a frequency of 1Hz, with a strain of 20 μm. Crosslinking density was calculated from E’ rubbery plateau value according to literature.48 The insoluble fraction (gel content) of the printed samples was determined according to the standard test method ASTM D2765-84. The samples were held in a metal net, accurately weighted and subsequently, submitted to extraction with chloroform (CHCl3) for 24 hours at room temperature to dissolve the not-cross-linked monomers and/or dye. Then the samples were dried overnight at 80°C. The insoluble fraction percentage was determined as weight difference before and after solvent extraction. The UV-Vis measurements were conducted by means of a double beam Lambda 40 instrument (Perkin-Elmer Italia, Milano, Italy). The range between 200 and 600 nm was monitored with a scan rate of 480 nm/min. The experiments on solid films were performed on 50 µm films coated on a glass slide. For checking the photodegradation of the dye, a solution of NBD-MA in acetonitrile was prepared (concentration 0.015 mg/ml), the UV-Vis spectra was collected before and after 3 minutes of UV irradiation The same experiments was performed on the solutions coming from the evaluation of insoluble fraction in order to investigate the successful copolymerization between dye and the polymeric matrix.


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The characterization of the optical performances of the 3D printed waveguides has been performed using a setup specifically built for this purpose, its description is reported in Supporting Information file (Figure S1). Results and Discussion Although the synthesis of NBD-EtOH precursor was described years ago, here we describe a different method, which greatly improves the process. In fact, in this case NBD-EtOH was produced by reaction under microwave irradiation. By using microwave irradiation, we: a) extraordinarily shorten reaction time (2min vs 2h), b) avoid the use of inert atmosphere (argon), c) avoid the use of toxic solvent and reactants (ethanol vs toluene and triethylamine), d) avoid laborious purification steps (the product crystallizes pure in reaction medium), and e) obtain similar yield of pure product. After methacrylation step, the so obtained dye was added at different amounts in a photocurable formulation based on BEDA, which was already tested to be 3D printable29, 33 Prior to the 3D printing process, some preliminary studies were carried out to investigate the effect of the addition of the NBD-MA on the properties of the BEDA-based photocurable formulations. Initially, the viscosity values of the formulations were determined. Viscosity is one of most critical parameter when a photosensitive resin is being developed; it is known that a high viscosity value during 3D printing could compromise the final 3D-printed part. Nevertheless, in certain circumstances, the 3D printer is specifically developed for a particular viscous formulation.49 Here, instead it is desired to maintain low viscosity for operating with any commercial 3D printer. Viscosity measurements indicated that all the formulations present a Newtonian behavior: the viscosity of the samples is not influenced by the shear rate (small variations are observed al low



shear rate due to the sensibility of the instrument, see Supporting Information, Figure S2). Table 1 reports the average values of viscosity calculated between 1 and 100 s-1 for Neat BEDA formulation and BEDA-based formulations at concentration of 0.001phr (per hundreds of resin), 0.005 phr and 0.01 phr of NBD-MA, all of them containing 2 phr of BAPO. As shown in Table 1, the presence of NBD-MA has no relevant influence on the viscosity of printable formulations, which is considerably lower than usual acceptable values for commercial DLP 3D-printers.25 In Figure S3 the curve relative for storage modulus (G’) and Loss modulus (G’’) versus irradiation time for neat BEDA formulation compared to those containing different amounts of NBD-MA (0.001, 0.005 and 0.01 phr of NBD-MA) are reported. The plots show similar reaction times (G’ = G’’) for all the formulations, indicating that the addition of NBD-MA does not strongly modify the printability. For all formulations the transition of G’ and G’' occurred after 10 seconds of illumination. On these films, acrylic double bond conversions were also measured (see Table 1). The formulations containing higher concentration of NBD-MA reach slightly lower degree of acrylic conversion after visible irradiation stage, which obviously increases to some extent after UV irradiation (see Table 1 and Figure S4). This could be explained considering that NBD-MA, acting as a dye, competes with BAPO in the light absorption during the visible irradiation step, decreasing the efficiency of photoinitiation and thus the double bond conversion.50 Nevertheless, such percentage of acrylic conversion is adequate for 3D printing process: in fact for this step it is sufficient that the gelation point is reached, enabling the shaping of the object. The insoluble fraction content was also evaluated for BEDA-based cured samples after UV irradiation, the results are reported in Table 1. The amount of insoluble fraction is slightly lower for formulations with higher NBD-MA concentration. This is in good agreement with IR evaluations reported above. The presence of NBD-MA has a slight effect also on the glass


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transition temperature (Tg) of the materials, showing a decrease of Tg values by increasing the amount of dye. This can be explained taking into account a decrease of crosslinking density by increasing the dye content, resulting in a looser and slightly more flexible network (Table 1). This is can be again related to a lower conversion but also to the fact that NBD-MA copolymerizes with BEDA, consequently generating larger polymeric networks, as demonstrated by a lower value of E’ in the rubbery plateau (Figure S5).48

Table 1. Table 1. Viscosity values at 25 °C (average values) , acrylate conversion (%) after visible light irradiation for 10 seconds and after UV irradiation for 180 seconds, glass transition temperature Tg (°C) and gel fraction (%) for Neat BEDA formulation and BEDA-based formulations at concentration for 0.001phr, 0.005 phr and 0.01 phr of NBD-MA. The formulations contain 2 phr of BAPO. Cross-linking density calculated by DMA experiments. Viscosity

Conversion

Conversion

Insoluble

Tg

Cross-linking density

value [Pa*s]

[%]a)

[%]b)

fraction [%]c)

[°C]d)

[mmol/cm3]e)

0.08

97

98

99

49

12.5

0.10

97

98

99

47

12

0.09

94

98

98

46

11

0.11

85

92

97

40

10

Formulation

Neat BEDA BEDA

+

0.001

phr

NBD-MA BEDA

+

0.005

phr

NBD-MA BEDA + 0.01 phr NBDMA

a) after Visible irradiation; b) after UV irradiation; c) measured after 24h of extraction in chloroform; d) T g

obtained from DSC; e) calculated by rubbery plateau in DMA tests.



In order to check the possible photodegradation of the dye under irradiation, UV-Visible spectra were taken on liquid uncured films and then checked after visible and UV irradiation of the same formulation. In Figure 1a, the experiment performed on neat BEDA sample is reported, as it possible to observe the liquid formulation show a broad absorption band between 350 nm and 420 nm which belongs to BAPO. After visible irradiation this band slightly decrease (λ> 400 nm) while after UV irradiation completely disappears, indicating complete photocleavage of BAPO photoinitiator. In the sample containing NBD-MA, in addition to the above-described broad band, another clear peak centered at 480 nm is evident (Figure 1b) which could be attributed to the dye (black curve in Figure 1c). While the band relative to BAPO disappears after UV irradiation step, the NBD-MA absorption peak is not affected by the light irradiation process, indicating that no photodegradation occurred to the dye. This was confirmed by direct UV irradiation of the dye (Figure 1c). Therefore, the dye is stable during photocrosslinking of the 3D object and it should maintain its photoluminescence properties. At last, we checked the successful incorporation of NBD-MA in the polymeric network by copolymerization with the acrylic double bond. UV-Vis tests were performed on the extracting chloroform solvent used for the insoluble fractions tests (Figure 1d). While the extracted solution for pure BEDA sample did not show any absorption peak, the solutions with the extracted fraction from the samples containing NBD-MA showed only a shoulder in the 360 nm-420 nm range. As previously mentioned, this band could be attributed to unreacted BAPO, which increases by increasing the dye content in the printable formulations, due competitive light absorption. More important, no absorption band is present at longer wavelength (480 nm), indicating that NBD-MA


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was not abstracted by chloroform and, as desired, remained chemically cross-linked in the polymeric network.

Figure 1. UV-VIS spectra of 50 µm films a) for neat BEDA formulation and b) BEDA-based formulation with 0.01 phr NBD-MA. c) UV-Vis spectra of NBD-MA in acetonitrile before and after UV irradiation; d) UV-VIS spectrum of extracting solvents for BEDA formulation (continuous line) and BEDA-based formulations with different NBD-MA concentration.

All these data confirm that NBD-MA successfully operates as dye for 3D printable formulations, competing with BAPO in light absorption, but at the same time being a part of the final light-cured materials. On the other hand, too high amounts of NBD-MA limit the double bonds conversion



and final crosslinking density, producing weaker material with unreacted moieties. Based on these studies, in the next step 3D structures were printed in order to produce photoluminescent waveguides. The BEDA-based formulations with 2 phr BAPO and NBD-MA at concentrations of 0.001, 0.005 and 0.01 phr were used to 3D-print optical waveguides with different geometries. At first, we aimed to find a concentration of NBD-MA that could ensure a good fabrication quality together with a good optical transmission. To this aim, we fabricated straight waveguides with a circular section of 2.8 mm and a length of 30 mm. The experimental setup is reported in the Supporting Information, Figure S7. Briefly, we measured the fluorescence intensity of the 3D printed structures in an end fire coupling configuration. The excitation is provided by a laser beam orthogonal to the waveguide direction. Such configuration ensures no propagation of the excitation beam within the waveguide, thus allowing a better localization of the fluorescence source. When the excitation point is faced with the objective coupled to the CMOS camera (Figure 2a, direct collection), we observe that the fluorescence has a sublinear dependence with respect to the dye concentration, probably due to a saturation effect. On the other hand, when the excitation point is at the opposite side with respect to the collection arm (Figure 2b), we collect fluorescence that propagates through the waveguide (remote collection) The transmittivity here is defined as the ratio between the fluorescence intensity 𝐼1 measured in direct collection of the waveguide and the fluorescence intensity 𝐼2 measured in remote collection. Figure 2b shows that increasing the concentration of NBD-MA lowers the transmittivity of the waveguide, which suggests the re-absorption of the fluorescence by the dye itself. Such hypothesis is confirmed by the fluorescence spectra collected in direct and remote configuration and reported in Supporting Information (Figure S9). On the other hand, by looking at the shape of the fabricated


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waveguides (Figure 2c-e) and the errors in the printed objects (Table S1 in Supporting Information), it is clear that the lower concentration of NBD-MA does not allow to print the desired geometry, while the 3D printed waveguides with 0.005 and 0.01 phr of NBD-MA (Figure 2d and e respectively) allow to maintain the round section of the waveguide.

Figure 2. a) Measured fluorescence intensity of the printed polymer versus concentration of the NBD-MA. The blue circles represent the experimental data, the black dashed line links the average intensity values (black diamonds). b) Measured transmittance T versus concentration of the NBDMA. c-e) Fluorescence images of the end face of the 3D printed waveguide at 0.001, 0.005 and 0.01 phr of NBD-MA respectively.



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In order to evaluate the guiding performances of waveguides, we fabricated a set of Y-shaped waveguides with 0.005 phr of NBD-MA (Figure 3) with different aperture angles 𝛼 (namely 30°, 40° 50° and 60°). The waveguides were fabricated with a square section of 2x2 mm. The square section was chosen because of easiness of fabrication and characterization, and we do not expect significant variations in the transmittance performances because of the multimodal nature of the considered waveguides. Due to total internal reflection, we expect a high transmittance of the waveguide as long as 𝛼 is lower than 40.7°, that is the critical angle, being from datasheet the refractive index of the polymeric compound at room temperature 1.534 at a wavelength of 589 nm. Figure 3a shows that the transmittance is almost constant below 40° and falls down to zero above the critical angle as expected. The fluorescent pictures show that the waveguide with 𝛼 = 40° (Figure 3b) guides light until the end facet, which appears bright, while the end facet of the waveguide in Figure 3c (𝛼 = 60°) appears darker, meaning that light is not guided anymore and is lost at the intersection point. In order to evaluate the balance between the two outputs, the experimental data have been reported in Supporting Information (Table S2)

a)

b)

c) ACS Paragon Plus Environment

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Figure 3. a) Transmittance through a branch of the Y-shaped waveguide versus aperture angle 𝛼. In the inset, a schematic view of the fabricated samples. Blue circles are experimental data collected on different samples at the two outputs. In the inset, a sketch of the experimental setup. The transmittance is calculated as the ratio between fluorescence collected in remote collection and the one collected in direct collection. b - c) Fluorescence pictures of the waveguides illuminated at one end of the waveguide. The pictures show that for angles smaller than the critical angle, the light is guided even for a sharp bending, while above the critical angle light is scattered at the intersection point of the three branches.

In a second set of experiments, 3D printed 1x7 branching circular cross-section optical waveguides were fabricated, here we report the data for the formulation with 0.005 phr of NBD-MA, since this concentration presents the best compromise between printability, transmittance and photoluminescence. Figure 4a reports the CAD designs of such waveguides, while in Figure 4b the 3D printed parts are shown. The geometry of such objects in the CAD design, resides in a rectilinear input arm (diameter 2.8 mm), a central rectilinear output arm (diameter 1.6 mm), which is surrounded by 6 non-rectilinear output arms (diameter 1.6 mm). The external arms are separated equidistantly from the central one generating a circumference with different diameters and so different angles between central and lateral arms (Figure 4b). We printed relatively big splitter for the ease of handling the samples and performing the experiments, however smaller structures were also produced reaching an ultimate resolution of 600 μm. (Figure S6). The light transmission ratio between later arms (TL) and central arm (TC) is reported in Figure 4c. Light transmittance values in branched splitters are reported in Table S3. Obviously, the larger is the splitter, the lower is the percentage of light guided in lateral arms, however, there is not a sharp decrease of transmission



since these smoother geometries allow a certain light guiding. This demonstrates that 3D printing could be effectively used for fabricating complex optical devices.

Figure 4. a) CAD files circular cross-section optical waveguides with different radius and b) correspondent 3D printed waveguides. c) Light transmission ratio in 3D printed optical waveguides between later arms (TL) and central arm (TC) in function of the radius of the external circumference.

At last, we explored the application of these devices as polarity sensor. Taking into account the well-known solvatochromism of some dyes

51

and in particular of this NBD dye 45, we tried to

hand over this properties in a 3D printed solid device. The characterization of the 3D printed solid device as polarity sensor has been performed by swelling the sample in different solvents using a


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handmade fluorescence set up, which description is reported in Supporting Information file (Figure S7). The fluorescence spectrum of the dye inside the solid resin (Figure S8) is dramatically different than its emission in different solvents: the emission is red shifted (maximum emission wavelength about 535 nm, while the literature values when dispersed in solvents are 514 nm in toluene or 519 nm in chloroform)45-47 and a shoulder appears at about 565nm. This finding should be caused by association of the NBD moieties inside the highly cross-linked solid matrix, which produced a new fluorescent species due, for example, to dipolar interactions arising from electronic excitation. Either simple dimeric association (excimers) or multimeric association (J-aggregates) can cause a bathochromic shift in fluorescence emission spectra. Formation of J-aggregates of NBD molecules in confined media has been previously reported,52 and the new fluorescence emission found for planar aromatic molecules through the so-called Aggregated Induced Emission phenomenon (AIE) is well known.53 Taking advantage to the fact that the dye was copolymerized with the polymeric network and thus cannot be extracted, we decided to perform emission spectra on the sample swollen in solvents with increasing solvent polarity parameter (ET30),54 in particular toluene, dichloromethane, dimethylformamide, acetonitrile and ethanol. When the device is swollen for 5 minutes in different solvents, there are subtle changes in the emission band depending on the polarity of the microenvironment inside the optical waveguide (Supporting Information Figure S10). Multiple fit peak of the fluorescence spectra has been performed for all the solvents (Supporting Information Figure S11, the data are summarized in Table S4).



As the polarity increases, the spectra of swollen samples show a bathochromic shift of the emission band of individual NBD molecules, as shown in Figure S11 in Supporting Information, whereas the band corresponding to the aggregates does not change in its position. In addition, a substantial decrease in the relative intensity of the shoulder with respect to the intensity at maximum wavelength is observed (Figure 5), which is consistent with our assumption that the shoulder corresponds to emission from aggregates. As solvent penetrates into the network, aggregates are partially destroyed by solvation. In addition, a decrease of the width of the aggregates emission band is found, confirming the behavior described above (Figure 5). Although the number of solvents studied is relatively limited, they cover a large interval of polarity and the linearity found for both parameters is very good, so these findings are exciting because they support the possible application of these printed devices not only as waveguides but also as polarity sensors.

Figure 5. Variation of fluorescence emission band of the 3D printed structures by swelling in different solvents represented as the ratio of the intensities (I1/I2) and the full widths at half-


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maximum height (W2/W1). Subscripts 1 and 2 refer to peaks assigned to individual NBD molecules and aggregates, respectively Conclusion In this work we applied an innovative strategy for implementing the capabilities of DLP 3D printable materials, employing a functional dye as ingredient both for obtaining precise and well defined complex geometries and for imparting functional properties to the printed structures. In particular, adding the right amount of NBD-based dye, we were able to obtain 3D printed photoluminescent waveguides and splitters. Moreover, we demonstrated that the dye was not degraded during the printing process, maintaining its properties in the object. Those optical devices demonstrated to guide the luminescence of the dye up to angle of 40°. At last, taking advantage that the synthesized dye was copolymerized with the polymeric network during the printing step, we demonstrated that we transferred the solvatochromic properties of the dye to the printed structures, enabling the use of these waveguides as solvent polarity sensors. The optical properties shown by this new device pave the way to a plethora of potential applications, in particular in the field of mobile platform for multiplexed detection.

ASSOCIATED CONTENT Supporting Information. Scheme of the optical setup, photorheology experiments, ATR spectra of the printed structures, evaluation of the printing precision, optical characterization of the 3D



printed structures and emission spectra of the printed structures in different solvents are reported in Supporting Information File. AUTHOR INFORMATION Corresponding Author * [email protected]

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Table of Contents (TOC)


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Three-Dimensional Printed Photoluminescent Polymeric Waveguides

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