For efficient 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 dual-modal light-converting phenomena, we prepared visible light emissive lanthanide-doped UCNs (β-NaYF4: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 fields, including photovoltaics, encoding, bioimaging, and drug delivery.22–29 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 up- and down-conversions.
A thin-film sample was first manufactured using a polymeric mixture dispersed with C153 and UCNs using free radical polymerization to confirm their homogeneous dispersion in the solid-state composite. The FT-IR spectra (Fig. 2b) of the consolidated composite film 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 silane-based polymer matrix via C–C bonding and uniformly distributed in the solid-state composite. We also investigated the transmittance of the composite film on the PDMS substrate in the absorption range of RB using UV-Vis spectroscopy (Fig. 2c). A thin composite film 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 film 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 film 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 sufficient transfer of visible light to enable RB photocatalysis in the microfluidic channels even after coating with the composite.
We fabricated a composite-coated microfluidic 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 filled resin with air-blowing, and post-thermal curing steps. The microfluidic 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 film and microfluidic systems were confirmed using fluorescence 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 confirmed 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 (λmax) at 400 nm and a minimum absorption wavelength (λedge) at 470 nm (Fig. 2b, blue line), emitted a luminescence λmax at 470 nm and λedge 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), confirming the dual-modal light transfer from C153 and the UCNs to RB, respectively.
We further performed photoelectrochemical (PEC) analysis to confirm the harvesting of up- and down-converted light from the composite film by RB. PEC analysis was conducted using RB-coated TiO2 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 film layer (Fig. 4e and Supplementary Fig. 6b). Significantly increased photocurrent densities were measured at 1.36 V only when the light was irradiated through the composite film 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 TiO2 NTs, these results demonstrate (1) the successful conversion of white and NIR light (i.e., dual conversion) to visible light by the composite film 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 coefficient, 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 coefficient of 26,819 M− 1 cm− 1 at λmax = 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 microfluidic channels. Therefore, this estimation suggests the use of a microfluidic 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 confirmed 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 microfluidic system with a smaller channel width (~ 500 µm) coated with a composite containing UC and DC components showed a significantly 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 microfluidic 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 efficiency.30,31 The dual-modal light-harvesting methodology of microfluidic systems was has been attempted in a similar manner by linearly stacking the composite-coated microfluidic system (Fig. 4). The second layered channel plate was tilted 15° over the first 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 flow 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 efficiency, but also the increased production capacity of our stacked-up system manufactured using the UC and DC components.