**5. Conclusion**

and m = 248.3 μg for*ec*MNP250) and therefore the same enzyme amount, the difference between the MNP250- and the MNP600-filled reactors can only be attributed to other factors, for exam‐

The major difference can also stem from the remarkably smaller average microchannel diameters between the particles within the *ec*MNP250-filled chamber as compared to the *ec*MNP600-filled one. This can result in shortened diffusion path and therefore better mass transport [23]. An additional 40% increment was achieved using the 1:1 particle mixture, which was obviously resulted as a synergy of the higher enzyme content (17%) due to the higher chamber capacity and enhanced transport phenomena due to the small average microchan‐

Substrate screening experiments were performed with a single *ec*MNP-loading in the chip passing the solutions of the different substrates (**Scheme 1**: L-**1a** and *rac*-**1b-f**) through the same chip according to a predefined sequence [22]. The intensive washing procedure between the individual tests with various substrates ensured complete removal of any substrate or product from the preceding cycle (reaction). In the first cycle, the ammonia elimination was meas‐ ured from L-**1a** (the natural substrate of PAL). This reaction was chosen as reference for comparison to the other elimination reactions of PAL from the further substrates (*rac*-**1b-f**). The difference between the initial and final (control) measurement with L-**1a** was found to be only 1.5%, while the *SC* score remained below 2000. Surprisingly, in the MagneChip device, higher biocatalytic activities (*U*B) were observed with four of the unnatural substrates

**Figure 10.** Comparison of the specific biocatalytic activity of *Pc*PAL immobilized on MNPs with substrates L-**1a** and

rac-**1b-f** in MagneChip system [(S) = 20 mM, flow rate: 48.6 μL min−1] [22]. \*Control measurement.

(*rac*-**1b,c,e,f**), than with the natural substrate L-phenylalanine L-**1a** (**Figure 10**).

ple, to the differences in total surface area [23].

174 Lab-on-a-Chip Fabrication and Application

**4.3. Testing multiple substrates in MagneChip**

nel diameter [25].

Our results proved that the MagneChip microfluidic device is a reliable, reproducible, and efficient tool which was capable of fast, reliable, and fully automated screening and kinetic characterization of *Pc*PAL substrates using minimal solvent (~500 μl) and biocatalyst (~1 mg MNP) amounts for a test compound. Compared with shake vial, the volumetric productivity of the MNP biocatalyst in the chip exceeded the one of the shake vial by more than three orders of magnitude. The platform was also capable of studying enzymatic reactions with undiscov‐ ered substrates of *Pc*PAL. The above results suggest that the MagneChip platform would be successfully utilized as a novel and flexible tool for enzyme-catalyzed biotransformations.
