Microfluidic systems enhance analytical efficiency by reducing sample and reagent volumes, enabling rapid, miniaturized bioanalytical workflows without complex fabrication techniques. Microfluidic systems manipulate fluids in channels with dimensions typically ranging from tens to hundreds of micrometers. Importantly, microchannel fabrication can be achieved without traditional semiconductor lithography. A cost-effective alternative involves using double-sided adhesive tapes, which are cut and layered to form fluidic paths. This method enables rapid prototyping and customization of channel geometries, making microfluidics accessible for diverse laboratory applications.

Microfluidic platforms offer advantages in analytical science, including:

• Reduced reagent and significant advantages sample consumption, minimizing cost and waste.
   
• Accelerated reaction kinetics and analysis times due to enhanced surface-to-volume ratios.

Simplified instrumentation, allowing integration of multiple analytical steps into compact devices. Following bioanalytical techniques and instrumentation in our laboratory have implemented microfludic systems:

• Flow-based chemiluminescence microarray analysis on the Microarray Chip Reader (MCR): 

  This system utilizes microfluidic flow to deliver automatically reagents across immobilized probes on a microarray chip surface. Highly sensitive chemiluminescent signals are generated and read in real-time, enabling rapid multianalyte detection by biomolecular interactions with minimal sample input.

• Cartridge-based flow cytometry from rqmicro: 

  This platform combines immunomagnetic separation with microfluidic flow cytometry for the quantitative detection of Legionella pneumophila. Magnetic particles functionalized with antibodies selectively bind the target bacteria, which are then separated and quantified using optical detection in a microfluidic cartridge.

• Microfluidic synthesis of magnetic nanoparticles coupled with miniaturized NMR:

  A continuous-flow microreactor enables the controlled synthesis of magnetic nanoparticles. The integration with miniaturized nuclear magnetic resonance (NMR) allows online monitoring of relaxation properties, providing insights into particle size, surface chemistry, and magnetic behavior during synthesis.

Microfluidic systems are increasingly recognized for their role in:

• Point-of-care diagnostics: Compact and rapid testing platforms for diagnostics but also for rapid food and water analysis.
• Nanomaterial synthesis: Precise control over reaction conditions and product uniformity.
• Environmental monitoring: On-site detection of contaminants in water and air.

Literature:

Bemetz, J.; Wegemann, A.; Saatchi, K.; Haase, A.; Häfeli, U.O.; Niessner, R.; Gleich, B.; Seidel, M. Microfluidic-Based Synthesis of Magnetic Nanoparticles Coupled with Miniaturized NMR for Online Relaxation Studies. Analytical Chemistry, 2018, 90, 16, 9975-9982.  
DOI:10.1021/acs.analchem.8b02374.

Seidel, M. and Niessner, R., Chemiluminescence microarrays: a critical review. Analytical and Bioanalytical Chemistry,  2014,406, 5589–5612. http://link.springer.com/article/10.1007/s00216-014-7968-4. 

Streich, P.; Redwitz, J.; Walser-Reichenbach, S.; Herr, C.E.W.; Elsner, M.; Seidel, M. Culture-independent quantification of Legionella pneumophila in evaporative cooling systems using immunomagnetic separation coupled with flow cytometry. Applied Microbiology 2024, 4, 284-296. DOI: 10.3390/applmicrobiol4010019.

Wang, Y.; Seidel, M. Integration of 3D hydrodynamic focused microreactor with microfluidic chemiluminescence sensing for online synthesis and catalytical characterization of gold nanoparticles. Sensors, 2021, 21 (7), 2290.
https://www.mdpi.com/1424-8220/21/7/2290. 

Wang, Y.; Rink, S.; Baeumner, A.J.; Seidel, M.; Microfluidic flow-injection aptamer-based chemiluminescence platform for sulfadimethoxine detection. Microchimica Acta, 2022, 189, 117. https://link.springer.com/article/10.1007/s00604-022-05216-6. 

Surface modification is central to the performance of microarray chips and microfluidic systems, as it governs biomolecule immobilisation, signal quality, and assay reproducibility. Recent advances highlight the importance of tailoring surface chemistry to achieve 

• controlled orientation

• density

• and stability of immobilised ligands

For microarrays, functional coatings such as epoxides, aldehydes, and NHS-esters enable covalent attachment of proteins and nucleic acids, while polymer brushes and nanostructured films improve binding capacity and reduce nonspecific adsorption. 

In microfluidic devices, surface modification strategies must balance biocompatibility with fluid dynamics; silanisation, PEGylation, and plasma treatments are widely applied to tune wettability and prevent fouling.

Antigen Immobilisation

• Antigens must keep their natural shape for epitope recognition.

• Methods: protein A/G for oriented binding, site-specific click chemistry, His-tag/Ni-NTA affinity.

• Improves sensitivity in immunoassays.

Hapten Immobilisation

• Haptens are small molecules, harder to immobilise directly.

• Often linked to carrier proteins or attached via activated esters.

• Linker chemistry and surface activation help maintain recognition while ensuring stability.

Bioconjugation Methods underpin these immobilisation strategies.

• Classical methods:

  • EDC/NHS coupling (amide bonds)

  • Thiol–maleimide reactions

  • Diazonium chemistry

• Modern bioorthogonal methods:

  • Azide–alkyne click chemistry

  • Strain-promoted cycloaddition

• Advantages: high specificity, mild conditions, ideal for microfluidic biosensors.

• Polymer coatings and supramolecular assemblies allow multiplexing of antigens, haptens, and nucleic acids.

Anti-Fouling: The Jeffamine ED 2003 solution

To achieve high sensitivity and accurate results by eliminating non-target molecule adsorption, which causes high background noise, it’s a critical goal to prevent unspecific binding. Jeffamine ED-2003 is a type of polyetheramine, structurally related to polyethylene glycol (PEG) and has a broad application in surface functionalization, e.g. of:

• Microarray Chips: Both Glass and Polycarbonate (PC) substrates.

• Magnetic Nanoparticles: Used in separation and detection.

• MAF (Monolithic adsorption filtration)

Jeffamine's effectiveness stems from two primary characteristics:

• Hydrophilic Shielding: It forms a highly hydrophilic layer due to its PEG backbone. This environment disfavors interactions with hydrophobic regions of proteins. 

• Steric Hindrance: The long, flexible chains create a dynamic "brush" that physically excludes (sterically hinders) large biomolecules from approaching and permanently sticking to the surface.

Together, these innovations in surface modification and bioconjugation expand the analytical capabilities of microarrays and microfluidic systems, enabling sensitive detection of diverse biomolecules from proteins to small haptens. The cited studies demonstrate that careful design of immobilisation chemistry is essential for advancing diagnostic and bioanalytical technologies.

Literature:

Klüpfel, J., Koros, R.C., Dehne, K. et al. Automated, flow-based chemiluminescence microarray immunoassay for the rapid multiplex detection of IgG antibodies to SARS-CoV-2 in human serum and plasma (CoVRapid CL-MIA). Anal Bioanal Chem 413, 5619–5632 (2021).
https://doi.org/10.1007/s00216-021-03315-6

Klüpfel J.; Paßreiter S.; Weidlein N.; Knopp M.; Ungerer M.; Protzer U.; Knolle P.; Hayden O.; Elsner M.; Seidel M.; Fully automated chemiluminescence microarray analysis platform for rapid and multiplexed SARS-CoV-2 serodiagnostics. Analytical Chemistry, 2022, 94,2855–2864. doi.org/10.1021/acs.analchem.1c04672

Kloth, K., Rye-Johnsen, M., Didier, A., Dietrich, R., Märtlbauer, E., Niessner, R., Seidel, M., A Regenerable Immunochip for the Rapid Determination of 13 Different Antibiotics in Raw Milk, Analyst, 2009,134, 1433-1439.
https://pubs.rsc.org/en/content/articlelanding/2009/an/b817836d 

Wolter A, Niessner R, Seidel M. Preparation and characterization of functional poly(ethylene glycol) surfaces for the use of antibody microarrays. Anal Chem. 2007;79(12):4529–37. https://pubs.acs.org/doi/full/10.1021/ac070243a 

Bemetz, J.; Kober, C.; Meyer, V. K.; Niessner, R.; Seidel, M., Succinylated Jeffamine ED-2003 coated polycarbonate chips for low-cost analytical microarrays. Analytical and Bioanalytical Chemistry 2019,411, (10), 1943-1955.
https://doi.org/10.1007/s00216-019-01594-8