The realization of a lab-on-a-chip system in which chemical reactions are carried out in a continuous flow fashion and monitored on-line by a suitable analytical technique is the main topic of this thesis. Two types of a lab-on-a-chip were realized, both using mass spectrometry (MS) as the on-line detection technique, viz. electrospray ionization (ESI) and Matrix Assisted Laser Desorption Ionization (MALDI). Microreactors were fabricated in glass or a combination of glass and silicon. Flow-driven pumping, using a microdialysis pump, was chosen as a mechanism to drive reagent solutions and reaction mixtures within the microchannels in a controlled way. A novel passive pressure-driven pumping mechanism, using the vacuum of the MALDI instrument, was used to activate the microchip integrated with MALDI MS (Chapters 7 and 8).
The basic concepts of microfluidics as well as the application of microfluidics devices to the field of analytical chemistry are described in the first part of Chapter 2. Subsequently, a literature overview is given, covering the application of microreactors to organic chemistry. Particular attention is given to the effects of downscaling the reaction vessel on the physical parameters of a reaction, as well as to practical issues in lab-on-a-chip technology such as fluidics handling and reaction monitoring.
Chapter 3 deals with the acid-catalyzed esterification reaction of 9-pyrenebutyric acid with ethanol to yield the corresponding ethyl ester carried out in a flow-driven glass microreactor. A higher efficiency in terms of product yields and reaction times was observed on-chip as compared to the conventional laboratory procedure. In a systematic investigation of the influence of acidic silanol groups at the microchannel wall on the acid-catalyzed reaction, it was demonstrated that surface phenomena play a key role in the observed reaction enhancement.
Two interfacing approaches for coupling of glass micro-chips to a Nanoflow ESI (NESI) mass spectrometer are described in Chapter 4. The microfluidics-based interfaces are based on two different integration designs. The first is a monolithic approach, in which the sample is sprayed from an open channel at the edge of the chip. The second is a modular approach, using commercially available Picotipä emitters attached to the chip outlet by means of low dead volume connectors. Both interfaces showed good ionization stability properties, giving a deviation of the total ion current over a few minutes acquisition time of about 8% and 1%, respectively. Offering a high versatility, the modular design was chosen for further applications and its high-throughput potential was demonstrated in Chapters 5 and 6. A microreactor design based on a novel mixing concept by laminating the incoming flows was developed, and its mixing dynamics was investigated by computational and experimental methods. At flow rates in the order of a few tens to a few hundreds nL min-1, complete reagents mixing is achieved in the microchannel within a few tens of milliseconds.
Chapter 5 deals with an on-chip qualitative and quantitative study of the binding strength of supramolecular interactions of different nature. Metal-ligand interactions of Zn-porphyrin with pyridine, 4-ethylpyridine, 4-phenylpyridine, N-methylimidazole, and butylimidazole in acetonitrile as well as host-guest complexations of b-cyclodextrins with N-(1-adamantyl)acetamide and 4-tert-butylacetanilide in water, were studied by mass spectrometry using the NESI-chip interface. Ka values of (4.6 ± 0.4) ´ 103 M-1 and (6.5 ± 1.2) ´ 103 M-1 were determined for the complexation of Zn-porphyrin with pyridine and 4-phenylpyridine, respectively. These Ka values are about four times larger than those obtained with UV/vis spectrophotometry, probably due to a higher ionization efficiency of the complexed compared to the uncomplexed Zn-porphyrin. A Ka value of (3.6 ± 0.3) ´ 104 M-1 was calculated for the complexation of N-(1-adamantyl)acetamide with b-cyclodextrin, which is in good agreement with that independently determined by microcalorimetry.
In Chapter 6 the NESI-chip interface is used for a kinetic study of the derivatization reaction of propyl-, and benzyl isocyanates, and toluene 2,4-diisocyanate with 4-nitro-piperazino-2,1,3-benzoxadiazole (NBDPZ) to yield the corresponding urea derivatives. Rate constants of 1.5 ´ 104 M-1s-1, 5.2 ´ 104 M-1s-1, and 2.4 ´ 104 M-1s-1 were determined for propyl isocyanate, benzyl isocyanate, and toluene 2,4-diisocyanate, respectively. The on-chip rate constants are 3 to 4 times higher than those determined using batch macro-scale conditions, demonstrating the remarkably efficient mixing by diffusion, occurring in the new microreactor design.
In Chapter 7 a lab-on-a-chip is described consisting of a glass / silicon hybrid microreactor integrated with a MALDI mass spectrometer. The integrated system enables (bio)chemical reactions to be carried out on-chip, inside the MALDI ionization chamber, and the reaction products to be in-situ analyzed by mass spectrometry. The effectiveness of the MALDI-chip integrated system was demonstrated for a number of reactions ranging from simple organic synthesis to polymer separation as well as peptide and oligonucleotide enzymatic digestion.
Chapter 8 focuses on the optimization of the MALDI-chip system described in Chapter 7, aiming to realize a lab-on-a-chip for reaction kinetics studies. Monitoring windows, consisting on freestanding silicon nitrate membranes, were opened in the top wafer along the fluidics path allowing sampling from the reaction mixture flowing through the channel. As a proof of principle, the simple reaction of 4-tert-butylaniline with 4-tert-butylbenzaldehyde in ethanol to yield the corresponding Shiff base was carried out on-chip in the MALDI ionization chamber. The product was detected not only at the outlet, but also through a sampling window.
The results presented in this thesis demonstrate that lab-on-a-chip systems are valuable tools to study (bio)chemical reactions. Microreactors offer a unique environment where reactions are carried out in micro- to nanoliter reaction volumes, profiting from the large surface-to-volume ratio available and the fast diffusive mixing under laminar flow regimes. The continuous-flow operative mode and the on-line monitoring offer high-throughput capabilities, which are unique for lab-on-a-chip systems.