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Microfluidic systems with large surface‐to‐volume ratios and superior light transmission are used to efficiently transfer mass and convert energy, and to enhance photocatalytic reactions. Utilizing the entire solar spectrum for promoting photocatalytic reactions is highly desirable and near‐infrared (NIR) radiation, in particular, has a high transmission efficiency through common polymers and materials used to construct microfluidic devices. Herein, a reliable microfluidic system using bimodal light‐harvesting technique is reported to improve the photocatalytic efficiency of C(sp3)‐H functionalization reactions using coumarin dye (C153) and lanthanide‐doped upconversion nanocrystals (UCNs). Using two light‐harvesting components (C153 and UCNs) in polycarbosilane polymer matrix, a bimodal light‐harvesting microfluidic reactor is realized in which the inner surface of the microfluidic channel is reliably coated with a transparent composite of C153/UCNs to simultaneously downshift visible light and upconvert NIR light. A double‐stacked microfluidic system that successfully enhanced the photocatalytic conversion efficiency of Rose Bengal‐based aza‐Henry photocatalytic reactions by twofold (≈93% conversion). The study provides a design principle of next‐generation microfluidic reactor for a robust photocatalytic organic synthesis. [ABSTRACT FROM AUTHOR]

In the field of surface‐enhanced Raman scattering (SERS), advances in nanotechnology and surface chemistry have contributed to fabricating the metal substrates with highly sophisticated architectures and strong binding affinity to target molecules which enhanced the sensitivity to target molecules. However, the elaborate yet complicated steps for the synthesis, patterning, and surface modification of metal substrates have often resulted in compromising the reliability, reproducibility, and reusability as SERS substrates. Here, a fully programmable and automated digital maskless flow microlithography process that spatiotemporally controls the fluid flow, UV irradiation, and the shape and location of SERS polymer matrix is provided to fabricate a reliable, reproducible, and reusable hydrogel‐based 3D SERS substrate. The SERS substrates are located inside the microfluidic device in the form of disk‐shaped hydrogels. By rationally designing the functional group chemistry of the hydrogel microposts, Ag nanoparticles are homogeneously synthesized in situ, a target molecule is amplified by 25‐fold inside the microposts, and an enhancement factor as high as 2.4 × 108 is observed. Furthermore, a highly reusable multitarget sensing capability is demonstrated by a sequential analysis of multiple analytes without the trace of former analytes via the intermittent washing step. [ABSTRACT FROM AUTHOR]