On-line monitoring of a microwave-assisted chemical reaction by nanolitre NMR-spectroscopyw M. Victoria Gomez, a Hein H. J. Verputten, b Angel Dı´az-Ortı´z, a Andres Moreno, a Antonio de la Hoz* a and Aldrik H. Velders* b Received 26th November 2009, Accepted 16th April 2010 First published as an Advance Article on the web 25th May 2010 DOI: 10.1039/b924936b We report the use of a nanolitre nuclear magnetic resonance (NMR) spectroscopy microfluidic chip hyphenated to a continuous- flow microlitre-microwave irradiation set-up, for on-line moni- toring and rapid optimization of reaction conditions. Microwave-assisted organic synthesis (MAOS) has gained wide attention as alternative heating mode in fields such as organic chemistry and bioanalytical applications, medicine and high-throughput chemistry. 1 Microwave-assisted continuous flow organic synthesis (MACOS) has been introduced to minimize elaborate sample-handling, and combines the unique heating mechanism of microwave irradiation with the safety, reproducibility, facile automation and process control conditions of continuous flow techniques. 2 Recently, the development of MACOS with microreactors has further boosted the performance of microwave-assisted capillary organic synthesis, with significant impact on reaction rates and scale up. 3 Raman spectroscopy has been used for ‘‘in situ’’ monitoring of micro- wave reactions. 4 UV/vis spectroscopy has been coupled with a microwave reactor for continuous-flow on-line detection, using a reactor coil of 159 mL for a linear alcohol derivatization. 5 However, a chemical selective method for on-line monitoring of MACOS has not been implemented yet and ‘‘ex situ’’ analysis is usually carried out after product collection, that slows the overall synthetic process. 2,3 We here report the design and implementation of a microlitre- microwave reactor hyphenated with a custom-made nanolitre- NMR set-up, comprising a less than 2 mL reaction volume for the microwave flow cell and a 6 nL detection volume microfluidic NMR chip (Fig. 1). The detection volume of the NMR chip was chosen to be much smaller with respect to the microwave cell to have a dynamic range for the monitoring of different reaction times whilst keeping the flow rate and correlated reaction conditions constant. In this way the reaction volumes submitted to microwave heating at different irradiation times, i.e. at different starting positions in the capillary, can be analysed within one and the same on-flow experiment. To prove the microwave–NMR concept for optimization of reaction conditions, the well-known Diels– Alder cycloaddition of 2,5-dimethylfuran and dimethyl- acetylene dicarboxylate was chosen as model reaction. (see ESIw and Scheme S1 for details on the reaction). The inherent low sensitivity of NMR spectroscopy has for long thought to be a limiting factor for volume and mass-limited sample analysis. However, as the signal-to-noise ratio (SNR) increases with decrease in coil diameter, the microcoil concept can overcome this limitation. 6 Although planar coils have limitations regarding filling factor when compared to other small-volume NMR-coil designs, e.g. solenoidal microcoils, 6 stripline, 7 or microslot, 8 the planar- coils have several advantages: they are manufactured by rather straightforward and standardized microfabrication techniques, they allow a well-controlled geometry of the coil, as well as a precise positioning of the RF-transceiver coil and sample, and allow the straightforward integration with micro- fluidic connections and devices. We have recently designed microfluidic chips for 1 H NMR 9 and 19 F NMR 10 spectro- scopic applications, to study supramolecular assemblies and interactions at picomole concentrations. We here present the design of a 6 nL NMR-chip with integrated microfluidic connections for flow-through implementation with a microlitre- volume microwave-irradiation set-up (see Fig. 1). For an effective integration of all components of the setup, the two major components were designed and optimized, separately. First, the microfluidic NMR-chip was fabricated and validated to work at 7.05 T (300 MHz 1 H Larmor frequency), to obtain the required high sensitivity and resolution for chemical identification of (sub)nanomol quantities Fig. 1 Scheme of the hyphenated syringe–microwave–NMR set-up (left), with zoom on the microfluidic NMR chip (middle), and further zoom on the integrated planar radiofrequency transceiver microcoil on top of the 200 micron wide fluidic channel. The bars with different units indicate the different dimensions of the set-up. a Instituto Regional de Investigacio ´n Cientı´fica Aplicada (IRICA), Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain. E-mail: Antonio.Hoz@uclm.es; Fax: 34 9022 04130; Tel: 34 9022 04100 b NMR & MS Department, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, Enschede, The Netherlands. E-mail: a.h.velders@utwente.nl; Fax: 31 53489 4645; Tel: 31 53489 2980 w Electronic supplementary information (ESI) available: Set-up and reaction scheme, stopped-flow and on-flow data, MRI image of the sample channel. See DOI: 10.1039/b924936b 4514 | Chem. Commun., 2010, 46, 4514–4516 This journal is  c The Royal Society of Chemistry 2010 COMMUNICATION www.rsc.org/chemcomm | ChemComm