Dual-modal Light-harvesting Microuidic System via Simultaneous Up- and Down-Conversions for Enhanced Flow Photocatalysis

Microuidic systems with large surface-to-volume ratios enable photocatalytic reactions to occur, avoiding the limitations of light penetration and allow the ecient transfer/mixing of mass and energy. For enhanced photocatalysis, the utilization of broad-spectrum light, especially over the entire solar spectrum, is highly desirable, but has been less explored in microuidic systems. Herein, we report a novel microuidic system with dual-modal light-harvesting capability via simultaneous up- and down-conversions to signicantly improve the photocatalytic eciency of C(sp 3 )-H functionalization reactions using ultraviolet (UV) to near-infrared (NIR) light. A transparent composite incorporating down-converting (DC) coumarin dye and up-converting (UC) lanthanide-doped nanocrystals (β-NaYF 4 :Yb/Er/Gd) was coated onto the inner surface of the microchannels, which showed effective dual conversion of UV/NIR to visible light. An improved photocatalytic organic transformation using our single- or double-stacked microuidic system was achieved utilizing a photocatalytic aza-Henry reaction with rose bengal (RB), which displayed a two-fold increase in reaction conversion.


Introduction
Arti cial photocatalysis mimics natural photosynthesis to achieve sustainable and environmentally benign chemical synthesis using light energy. In particular, photoredox catalysis using various organic photocatalysts is inexpensive and exhibits low toxicity. [1][2][3] However, photochemical organic transformations often occur at limited penetration distances due to the utilization of light through absorbing media. Consequently, the transformation e ciency may be low in conventional bulk reactors. Therefore, micro uidic systems with large surface-area-to-volume ratios are promising platforms to overcome such restrictions toward e cient photocatalytic organic synthesis. [4][5][6][7][8] This approach affords various advantages, such as short light pathway and uniform distribution of light, which lead to a noticeable decrease in reaction time and increase in the product yield or selectivity compared to those obtained using batch reactors, even under mild conditions. [9][10][11][12] Recently, several micro uidic systems e ciently harvest light by exploiting the Stokes-shift phenomenon. 13,14 For example, a photomicroreactor used for e cient chemical oxidation reactions utilizes a down-converting luminescent solar concentrator that renders an optical guiding effect in a physical medium. However, the broad space between the ow channels causes light loss due to incomplete total internal re ection. To overcome such limitations of solid-state optical guiding systems, a three-dimensional (3D) structured dual-channel micro uidic system: one for the luminophore in solution and the other acting as a reaction channel, has been developed to transfer the light effectively from the down-converting luminophore uid channel to the reaction channel to improve the reaction e ciency by applying a controlled channel system to obtain a homogeneous luminescent eld. 15 Nevertheless, these reported works only use the down-converting phenomenon to harness ultraviolet (UV) light, which is further attenuated by the inevitable light loss in the bulky physical medium during transfer.
In these contexts, the anti-Stokes-shift luminescence phenomenon of upconverting nanocrystals (UCNs) has gained attention because it enables a dual-modal light-harvesting approach using both up-and down-converters to harness UV and near-infrared (NIR) light by expanding the wavelength range of electromagnetic radiation used to activate the photocatalyst. 16,17 In addition, NIR light generally exhibits deep light penetration in physical media. Therefore, using a matrix consisting of UCNs is a promising approach toward the construction of an effective light harvester. Moreover, for e cient light transfer to a photocatalyst with less light scattering in the medium, a compact matrix of luminescence donors can be arranged in an intimate-distance manner.
In this paper, we report a rationally devised micro uidic system whose inner channel surface was coated with a thin and transparent composite comprised of up-and down-converters for dual-modal lightharvesting. This arrangement signi cantly increases the photocatalytic e ciency of C(sp 3 )-H functionalization reactions. Coumarin 153 (C153) acting as the down-converting (DC) component and NaYF 4 :Yb/Er/Gd nanocrystals as the up-converting (UC) component were homogeneously embedded on the surface layer of an elaborate serpentine microchannel, exhibiting high transparency (Fig. 1). The dualmodal light-harvesting behavior was con rmed using photoluminescence (PL) and photoelectrochemical (PEC) analyses under various conditions. The photocatalytic reaction of rose bengal (RB) in a single or double-stacked micro uidic system was remarkably promoted by doubling the conversion of 2-phenyl-1,2,3,4-tetrahydroisoquinoline within a short retention time (28 min) at room temperature. This result, even with a short reaction time, is the best conversion rate obtained for aza-Henry reaction with RB or other organic photocatalysts reported to date (Supplementary Table 1). Our dual-modal light-harvesting micro uidic system is a pioneering robust methodology, which can effectively enhance photoredox catalytic performance.

Results And Discussion
For e cient harvesting of a broad spectrum of light, we chose two types of highly emissive materials ( Fig. 2a). Coumarin 153 (C153) is conventionally used in OLED applications, which showss superior absorption and emission properties. 18,19 C153 is completely soluble in a photocurable silane-based inorganic polymer, allylhydridopolycarbosilane (AHPCS), which exhibits high chemical resistance, 20,21 and triallyl(phenyl)silane (TAPS), which acts as a cross-linker, without phase separation. To realize dualmodal light-converting phenomena, we prepared visible light emissive lanthanide-doped UCNs (β-NaYF 4 :Yb/Er/Gd; 30/2/30 mol%) using a conventional hydrothermal method ( Fig. 2a; the detailed synthetic method is described in the Supplementary Information). The unique up-converting luminescence characteristics of UCNs have been studied in various elds, including photovoltaics, encoding, bioimaging, and drug delivery. [22][23][24][25][26][27][28][29] Upon exposure to 980-nm NIR light, the synthesized UCNs showed luminescence at 530 and 660 nm, which overlaps with the absorption spectra of RB in the wavelength range 520-550 nm. The UCNs were capped with oleic acid (OA), which has a long hydrocarbon chain, to form a well-dispersed mixture in the hydrophobic AHPCS/TAPS matrix. Notably, the emission ranges of C153 and the UCNs are complementary to the absorption range of RB ( Supplementary Fig. 1), which were elaborately used for dual-modal light-harvesting via simultaneous upand down-conversions.
A thin-lm sample was rst manufactured using a polymeric mixture dispersed with C153 and UCNs using free radical polymerization to con rm their homogeneous dispersion in the solid-state composite. The FT-IR spectra (Fig. 2b) of the consolidated composite lm showed that the photo-and thermal-curing process eliminates the C = C bonds (1620-1630 cm − 1 ) originating from the OA in the UCNs and the AHPCS/TAPS pre-polymer. Accordingly, the OA-capped UCNs were covalently cross-linked with the silanebased polymer matrix via C-C bonding and uniformly distributed in the solid-state composite. We also investigated the transmittance of the composite lm on the PDMS substrate in the absorption range of RB using UV-Vis spectroscopy (Fig. 2c). A thin composite lm with a thickness of ~ 40 µm was then manufactured using the optimized content of C153 (1 wt.%) and UCNs (8 wt.%). The high transparency of the lm indicates the uniform distribution of C153 and UCNs without phase separation. The bare PDMS substrate has ~ 90% transparency in the UV, Vis, and NIR light region. The composite lm on the substrate exhibits the remarkable absorption of C153 at < 450 nm, presenting a slightly yellow tint by absorbing UV light ( Fig. 2c; see inset photograph). In addition, the high transmittance of ~ 80% in the wavelength range 450-600 nm allows su cient transfer of visible light to enable RB photocatalysis in the micro uidic channels even after coating with the composite.
We fabricated a composite-coated micro uidic system (Fig. 3a) to perform enhanced photocatalytic reactions by harvesting a broader spectrum of light. To replicate the polydimethylsiloxane (PDMS) microchannels, two molds with different cross-sectional dimensions (~ 1000 µm × 1000 µm vs. ~500 µm × 500 µm) and total lengths (33 cm vs. 132 cm) were made, which were manufactured using a 3D printer ( Supplementary Fig. 2). The inner surface of the PDMS microchannels was coated with a thin layer of the composite resin after a serial procedure consisting of partial UV curing, removal of the lled resin with airblowing, and post-thermal curing steps. The micro uidic systems with luminescence donors were successfully prepared with inner volumes of ~ 280 and ~ 215 µL, respectively. The detailed morphology of the fabricated microchannels was investigated by cutting the microchannel in both transverse and horizontal cross-sections, followed by SEM analysis (Fig. 3). The cross-section of the microchannel was slightly different from the square shape, presumably due to the low resolution of the 3D printed molds. From the SEM images, we can see that the thicknesses of the composite on the channel surfaces roughly varied with lengths in the range from several micrometers to ~ 40 µm. In addition, energy dispersive X-ray spectroscopy (EDS) mapping was performed using the horizontally cut samples to further examine the distribution of the UCNs (Fig. 3e). The uniform presence of Na, Gd, Yb, Er, and Y in the microchannels indicates the homogeneous dispersion of the UCNs in the coating of the microchannels. These results consistently suggest the uniform dispersion of the UCNs on the microchannels was achieved after the fabrication process.
The luminescence properties of the fabricated composite lm and micro uidic systems were con rmed using uorescence microscopy and the naked eye under UV (405 nm) and NIR (980 nm) light.
Photographs of the optical luminescence indicate that the luminescence donors (C153 and UCNs) were well-distributed throughout the entire microchannel pattern (~ 1000 µm channel width) (Fig. 4a, left and  Fig. 4b, left). A similar aspect of luminescence was observed in a more complex serpentine microchannel pattern (~ 500 µm channel width) (Supplementary Fig. 3). Moreover, we con rmed that the emitted visible light was quenched by RB dissolved in a mixed solution of acetonitrile, nitromethane, and deionized water under both UV and NIR light (Fig. 4a, right and Fig. 4b, right). The photoluminescence (PL) spectra were measured to further verify the light-converting phenomena (Fig. 4c, dark grey line and Fig. 4d, dark grey line). The PL spectra of the consolidated composite matrix showed that C153, which exhibited a maximum absorption wavelength (λ max ) at 400 nm and a minimum absorption wavelength (λ edge ) at 470 nm (Fig. 2b, blue line), emitted a luminescence λ max at 470 nm and λ edge at 580 nm (Fig. 4c). Hence, the luminescence emitted from C153 in the wavelength range 480-590 nm disappeared in the presence of RB in the microchannels. Characteristic luminescence peaks of the UCNs were observed at 520 and 550 nm when excited using 980-nm NIR light (Fig. 4d). The luminescence intensity of the UCNs in the visible light region also decreased in the presence of RB in the microchannels. That is, the luminescence of both C153 and UCNs was considerably quenched by absorption in the RB solution ( Supplementary   Fig. 1), con rming the dual-modal light transfer from C153 and the UCNs to RB, respectively.
We further performed photoelectrochemical (PEC) analysis to con rm the harvesting of up-and downconverted light from the composite lm by RB. PEC analysis was conducted using RB-coated TiO 2 nanotubes (NTs) as the working electrode ( Supplementary Fig. 6a). White and/or NIR light was irradiated on the working electrode through a glass slide with and without the composite lm layer (Fig. 4e and Supplementary Fig. 6b). Signi cantly increased photocurrent densities were measured at 1.36 V only when the light was irradiated through the composite lm for both white and NIR light. On the contrary, no or a negligible enhancement was observed when the bare glass slide was employed (Fig. 4e). Considering that the photocurrent can be induced when the adsorbed RB generates and injects photoexcited electrons into the TiO 2 NTs, these results demonstrate (1) the successful conversion of white and NIR light (i.e., dual conversion) to visible light by the composite lm and (2) the harvesting of the converted light by RB for photocatalysis.
We calculated the light penetration depth of the RB solution using the Beer-Lambert law (A = εcl = -logT; where A = absorbance, ε = molar absorption coe cient, c = concentration, l = path length, and T = transmittance) 11 to determine how much light can pass through the solution. The calculated plot showed that light transmittance dropped exponentially with the light penetration depth (Supplementary Fig. 8). RB with an absorption coe cient of 26,819 M − 1 cm − 1 at λ max = 559 nm ( Supplementary Fig. 1), may allow very little light to penetrate (< 0.5% at l = 250 µm) to the inside of the RB solution in acetonitrile, nitromethane, and deionized water in the micro uidic channels. Therefore, this estimation suggests the use of a micro uidic system with a shorter channel width (~ 500 µm) to further improve the organic transformation performance. The conversion in the channel without the up-and down-converters was 67%, which was higher than the 42% conversion obtained in the longer microchannel (~ 1,000 µm) at the same retention time (Table 1; entries 1 and 7). It was con rmed that the presumed light pathlength of 250 µm was favorable for superior photoredox catalysis with a channel width of ~ 500 µm.
Eventually, the use of a micro uidic system with a smaller channel width (~ 500 µm) coated with a composite containing UC and DC components showed a signi cantly improved conversion of 97% under dual-modal light harvesting conditions (Table 1 and Supplementary Fig. 9). To the best of our knowledge, this is the highest conversion versus reaction time obtained for the aza-Henry reaction using RB or other organic photocatalysts (Supplementary Table 1). This result was achieved due to the synergistic advantages of simultaneous up-and down-converting phenomena combined with the micro uidic system with effective light penetration and high surface-area-to-volume ratio (Supplementary Fig. 3).
The increase in the production capacity using microreactors has been considered without loss of reaction e ciency. 30,31 The dual-modal light-harvesting methodology of micro uidic systems was has been attempted in a similar manner by linearly stacking the composite-coated micro uidic system (Fig. 4). The second layered channel plate was tilted 15° over the rst layered plate to minimize the overlapped channels for effective light delivery ( Supplementary Fig. 10). This double-stacked system increases the reaction volume (~ 430 µL) and ow rate as much as two-fold in terms of the productivity. The aza-Henry reaction in the double-stacked system showed an identical conversion of 97% for a retention time of 28 min when compared to the result in the single devices. Moreover, the C-C coupling reactions of nitromethane and diethyl malonate gave product b with 99% conversion and C-P bond formation of the diethyl phosphite gave the corresponding product (c) with 63% conversion at a retention time of 28 min. These results show not only the superior photocatalytic e ciency, but also the increased production capacity of our stacked-up system manufactured using the UC and DC components.

Conclusions
In summary, we successfully developed a novel dual-modal light-harvesting micro uidic system by coating a transparent composite resin incorporated with C153 dye as the DC component and lanthanidedoped nanocrystals (NaYF 4 :Yb/Er/Gd) as the UC component. The manufactured AHPCS/TAPS based composite lm encapsulating C153 and the UCNs maintained excellent luminescence properties when irradiated with both UV and NIR light with high transparency (~ 80%) in the visible light region. The emitted light from the composite matrix was quenched in the presence of RB. The improved electron excitation of RB was also con rmed by observing the higher photocurrent density when white and NIR light was used to irradiate the composite lm on a glass substrate when compared to the bare glass substrate. Moreover, the serpentine micro uidic systems fabricated with the chosen composite showed visible light emission from the DC and UC components, and considerable luminescence quenching in the presence of RB. Based on the Beer-Lambert law, we obtained the highest conversion (97%) in the microsystem containing the DC and UC components with a narrow channel width (~ 500 µm) when conducting the photocatalytic aza-Henry reaction. This result is superior to the 42% conversion obtained in the wider system (~ 1000 µm) without the DC and UC components under identical light sources and a retention time of 28 min. Moreover, the double-stacked microsystem retained its excellent conversion of 97% with increased productivity. It is believed that rationally designed transparent up-and downconverting matrices on the micro uidic channels is a promising approach for light-harvesting to attain effective photocatalysis for sustainable chemistry. The PDMS micro uidic channel was prepared by replicating the polymer molds manufactured using a multi-jet printing (MJP) 3D printer (ProJet-MJP 3600). The 3D printed molds with the designed microchannel patterns were commissioned to K-TECH Co. Ltd in Korea ( Supplementary Fig. 2). The mixed viscous precursor of PDMS at a ratio of 10:1 was poured onto the 3D-printed mold and then cured at 80 °C for 2 h. After soft lithography, the PDMS and patterned PDMS substrates were treated with O 2 plasma for 2 min at 70 W and then bonded to each other to form the microchannels. The inlet and outlet tubing were connected to the PTFE tubing. Both C153 and UCNs were premixed at various ratios of 1:4-8 wt.% in the AHPCS/TAPS polymer resin with a photo-curing agent (irgacure369, 2 wt.%) and thermal curing agent (dicumyl peroxide, 2 wt.%). The mixer resin was introduced into the micro uidic channel via PTFE tubing. The fully lled micro uidic devices were exposed to a Xe lamp (300 W) for 3 min to homogeneously coat the inner channel surface and the uncured resin in the channel was removed and washed with isopropanol. The partially cured composite resin on the micro uidic channel surface was completely consolidated upon post-thermal curing in an oven at 80 °C overnight.

Characterization Of The Composite Film And Composite Coated Microchannels
Fourier-transform infrared spectroscopy (FT-IR) spectra were obtained on a Jasco FT/IR-4600 spectrometer using KBr pellets for the raw materials or neat using ATR accessories for the consolidated composite lm on the PDMS substrate. Scanning electron microscopy (SEM) images were obtained using a eld-emission scanning electron microscope (FE-SEM, Merlin, Carl Zeiss or JSM-7800F Prime, JEOL Ltd.) operated at 0.02-30 kV. To con rm the uniform distribution of C153 and UCNs in the composite-coated microchannel, electron dispersion spectroscopy (EDS) mapping and elemental analysis were performed using an EDS detector (XFlash 6l60, Bruker: Take-off angle, 35°; detector area, 60 mm 3 or Quad Detector (LED,UED,USD,BED) and EDS analysis software (ESPRIT 2.0.0, Bruker or Aztec, Oxford Instruments).

Analysis Of Dual-modal Light-harvesting Performance
The absorption properties of the RB solution (3.5 µM) and the composite lm were measured using a UV-Vis spectrometer (NANODROP 2000c, Thermo Scienti c) in a 10-mm quartz cuvette for the solution-state and a double-beam Shimadzu UV-2550 spectrophotometer over 300-800 nm for the solid-state lm. The luminescence properties of the composite lms and the solutions were measured using a PL spectro uorometer (FP-6500, JASCO). In addition, the PL spectra of the micro uidic channels fabricated with the composite lm were measured using a self-designed spectrometer (QE Pro, Ocean Optics) Declarations