Low-Volume Reaction Monitoring of Carbon Dot Light Absorbers in Optofluidic Microreactors.

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Gentleman, Alexander S 
Lage, Ava 

Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming, requires substantial amounts of samples, and does not allow the analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet-visible absorption spectroscopy, with the potential for high-throughput automation using extremely low sample volumes with high sensitivity for monitoring of the analyte. HC-PCFs use interference effects to guide light at the center of a microfluidic channel and use this to enhance detection sensitivity. They open the possibility of comprehensively studying photocatalysts to extract structure-activity relationships, which is unfeasible with similar reaction volume, time, and sensitivity in cuvettes. Here, we demonstrate the use of HC-PCF microreactors for the screening of the electron transfer properties of carbon dots (CDs), a nanometer-sized material that is emerging as a homogeneous light absorber in photocatalysis. The CD-driven photoreduction reaction of viologens (XV2+) to the corresponding radical monocation XV•+ is monitored in situ as a model reaction, using a sample volume of 1 μL per measurement and with a detectability of <1 μM. A range of different reaction conditions have been systematically studied, including different types of CDs (i.e., amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry, viologens, and electron donors. Furthermore, the excitation irradiance was varied to study its effect on the photoreduction rate. The findings are correlated with the electron transfer properties of CDs based on their electronic structure characterized by soft X-ray absorption spectroscopy. Optofluidic microreactors with real-time optical detection provide unique insight into the reaction dynamics of photocatalytic systems and could form the basis of future automated catalyst screening platforms, where samples are only available on small scales or at a high cost.

photocatalysis, microreactors, carbon dots, optofluidics, laser spectroscopy, hollow-corephotonic crystal fibers
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ACS Catal
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American Chemical Society (ACS)
Leverhulme Trust (RPG-2018-256)
Engineering and Physical Sciences Research Council (1948662)
EPSRC (1948662)
Engineering and Physical Sciences Research Council (EP/L015978/1)
Engineering and Physical Sciences Research Council (EP/S025308/1)
Engineering and Physical Sciences Research Council (EP/S022953/1)
European Commission Horizon 2020 (H2020) Marie Sk?odowska-Curie actions (890745)
T.G.E. acknowledges the support from the Winton Programme for the Physics of Sustainability and the Isaac Newton Trust. T.L. and E.R. acknowledge the Cambridge NanoDTC (EPSRC Grant EP/L015978/1 and EP/S022953/1). C.C. and E.R. acknowledge the UKRI Cambridge Creative Circular Plastics Centre (EP/S025308/1). C.C. acknowledges the European Commission’s H2020 program for an MSCA IF (SmArtC, No.890745). A.L. acknowledges the EPSRC for a DTA studentship. T.G.E, A.S.G., and E.R. acknowledge the Leverhulme Trust (Research Project Grant RPG-2018-256). The authors acknowledge the kind support from the staff of the BESSY II synchrotron facility, especially Ronny Golnak, and thank the Helmholtz-Zentrum Berlin for the allocation of beamtime at the U49/2 PGM-1 beamline.
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