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

Optical monitoring and screening of photocatalytic batchreactionsusing cuvettes ex situ is time-consuming, requiressubstantial amounts of samples, and does not allow the analysis ofspecies with low extinction coefficients. Hollow-core photonic crystalfibers (HC-PCFs) provide an innovative approach for in situ reaction detection using ultraviolet-visible absorption spectroscopy,with the potential for high-throughput automation using extremelylow sample volumes with high sensitivity for monitoring of the analyte.HC-PCFs use interference effects to guide light at the center of amicrofluidic channel and use this to enhance detection sensitivity.They open the possibility of comprehensively studying photocatalyststo extract structure-activity relationships, which is unfeasiblewith similar reaction volume, time, and sensitivity in cuvettes. Here,we demonstrate the use of HC-PCF microreactors for the screening ofthe electron transfer properties of carbon dots (CDs), a nanometer-sizedmaterial that is emerging as a homogeneous light absorber in photocatalysis.The CD-driven photoreduction reaction of viologens (XV2+) to the corresponding radical monocation XV & BULL;+ ismonitored in situ as a model reaction, using a samplevolume of 1 & mu;L per measurement and with a detectability of <1 & mu;M. A range of different reaction conditions have been systematicallystudied, including different types of CDs (i.e.,amorphous, graphitic, and graphitic nitrogen-doped CDs), surface chemistry,viologens, and electron donors. Furthermore, the excitation irradiancewas varied to study its effect on the photoreduction rate. The findingsare correlated with the electron transfer properties of CDs basedon their electronic structure characterized by soft X-ray absorptionspectroscopy. Optofluidic microreactors with real-time optical detectionprovide unique insight into the reaction dynamics of photocatalyticsystems and could form the basis of future automated catalyst screeningplatforms, where samples are only available on small scales or ata high cost.

Automated summary

Optical monitoring and screening of photocatalytic batch reactions using cuvettes ex situ is time-consuming and does not allow analysis of species with low extinction coefficients. Hollow-core photonic crystal fibers (HC-PCFs) provide an innovative approach for in situ reaction detection using absorption spectroscopy, with high sensitivity for monitoring the analyte. 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.