- The paper shows that multilayer PSS synthesized via ATRP delivers superior antifouling performance compared to MUA coatings for polymer nanoparticles.
- It details quantitative fluorescence and ISCAT imaging methods to evaluate trapping efficiency and fouling reduction under varied pH and ionic conditions.
- The research highlights methodological flexibility that enables customized passivation across diverse substrates, improving biosensor functionality and device reusability.
Surface Passivation Strategies for Optical Nanotweezers: Comparative Evaluation and Methodological Advances
Background and Motivation
Optical nanotweezers, particularly gold-based platforms, have advanced the manipulation, trapping, and analysis of nanoscale entities with unprecedented spatial precision. These devices leverage plasmonic and Mie resonances to confine light and drive AC electro-osmotic flows for massively parallel and scalable trapping. However, their operational robustness is fundamentally limited by surface fouling due to non-specific adsorption, which impedes reversible particle manipulation, particle recovery, and device reuse. Existing antifouling approaches, including surfactant addition and conventional thiol-terminated monolayers, offer incomplete mitigation, especially for diverse and challenging analytes such as polymer nanoparticles and extracellular vesicles (EVs).
This study systematically evaluates the antifouling efficacy of poly(sodium styrene sulphate) (PSS) grown via Atom Transfer Radical Polymerization (ATRP) compared to 11-mercaptoundecanoic acid (MUA) and zwitterionic poly(methacryloyloxyethyl phosphorylcholine) (PMPC) coatings, focusing on gold interferometric electrohydrodynamic tweezers (IET) and extending the relevance to broader nanotweezer substrate architectures.
Device Architecture and Surface Passivation Methods
IET devices consist of a gold film patterned with a microhole array, integrated with indium tin oxide electrodes and dielectric spacers in a microfluidic channel. Nanoparticle trapping is achieved by applying AC voltage, generating tangential fields that drive ACEO flows, with notable dependence on zeta potential, medium composition, and pH.
Surface passivation protocols were critically compared:
- MUA Coating: Forms a dense self-assembled monolayer via thiol-gold binding, with hydrophilic carboxyl terminal groups that deprotonate at neutral pH, enabling significant repulsion of negatively charged polystyrene nanoparticles.
- ATRP-Grown PSS: Utilizes a thiol initiator for robust attachment and enables multilayer growth, resulting in a highly charged, hydrophilic, and thick antifouling polymer film.
- ATRP-Grown PMPC: Provides a zwitterionic terminal group conducive to hydration layer formation and minimal fouling for EVs and other biological analytes.
Both ATRP strategies confer modularity, allowing precise tailoring of terminal group chemistry and surface charge for specific analyte classes.
Quantitative and qualitative imaging (fluorescence and ISCAT) established the superiority and limitations of each passivation strategy:
- Unpassivated Devices: Exhibit rapid, irreversible adsorption of polystyrene beads due to dominant Van der Waals interactions.
- MUA-Passivated Devices: Show a marked reduction in polystyrene fouling, with only a minimal number of adhered particles after extended operation at pH 7. However, effectiveness is restricted by pH sensitivity, single-layer deposition, and limited charge density.
- PSS-Coated Devices via ATRP: Demonstrate superior antifouling performance for polystyrene beads, with negligible particle adhesion (typically only at non-passivated regions), attributed to multilayer sulphate group repulsion and enhanced hydrophilicity. The contact angle measurements corroborate the transformation from hydrophobic to hydrophilic surface following passivation.
- PMPC-Coated Devices via ATRP and PSS-Coated Devices: Both show similar efficacy in reducing EV adsorption, indicating the functional equivalence in hydration layer formation and charge-based repulsion for EVs with negative zeta potential. PMPC is preferred in scenarios with heterogeneous particle charge distributions or unknown surface properties.
Materials with ethylene glycol terminal groups (OEG, TMOL) exhibited minimal fouling reduction for negatively charged polystyrene beads, highlighting the necessity of tailoring surface charge rather than relying solely on hydrophilicity.
Methodological Flexibility and Substrate Versatility
The ATRP methodology allows customization of initiator chemistry for substrate compatibility:
- Gold Substrates: Employ thiol-based initiators.
- Silicon/SiOâ‚‚/ITO Substrates: Compatible with silane and phosphate initiators, respectively.
ATRP confers precise control over polymer composition, layer thickness, and terminal group selection, offering a universal platform for device passivation across a diverse array of nanotweezer geometries and biosensing applications.
Implications and Future Directions
The findings substantiate several critical claims:
- Multilayer PSS synthesized via ATRP provides superior antifouling against polystyrene compared to MUA and equivalence to PMPC for EVs.
- ATRP-based passivation enables reversible particle trapping, device reusability, and broad compatibility with various particle types and substrates.
- Detailed knowledge of particle zeta potential and medium composition is imperative for optimal antifouling layer selection; for unknown or heterogeneous samples, zwitterionic passivation (e.g., PMPC) is recommended.
- Device operation in high ionic strength conditions may require further optimization, as hydration layer efficacy and charge repulsion can be compromised by electrical double-layer compression.
Future developments could leverage ATRP to integrate responsive or multifunctional antifouling layers, enabling dynamic modulation of trapping conditions, selective particle sorting, and improved biosensing specificity. The modularity of ATRP opens pathways for scaling high-throughput, label-free nanoscale analytics in medical diagnostics, environmental sensing, and materials science.
Conclusion
Comprehensive experimental evaluation demonstrates that multilayer PSS films grown via ATRP substantially enhance antifouling in gold-based optical nanotweezer platforms, surpassing MUA monolayers for polymer nanoparticles and matching PMPC for EVs. The ATRP-based approach provides versatile, substrate-independent, and application-specific surface passivation, enabling robust, reversible nanoparticle manipulation and advancing high-throughput nanoscale characterization. These results establish ATRP as a foundational methodology for next-generation optical nanotweezer technologies and biosensor devices (2606.20914).