7.1 Nanofluidic devices for separation and purification

Nanofluidic devices are revolutionizing separation and purification techniques. By exploiting unique behaviors of fluids and particles at the nanoscale, these devices offer unprecedented precision and efficiency in separating molecules and particles based on size, charge, and other properties.

From DNA sequencing to water purification, nanofluidic separators are transforming various fields. This section explores the principles, mechanisms, and applications of these devices, highlighting their potential to solve complex separation challenges in chemistry, biology, and environmental science.

Principles of Nanofluidic Separation

Fundamental Concepts

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• Nanofluidic separation techniques rely on unique behavior of fluids and particles at nanoscale where surface forces and molecular interactions dominate over bulk fluid properties
• Electric double layer (EDL) affects distribution and transport of charged species near solid surfaces in nanofluidic separation
• Size-based separation exploits physical confinement of channels with dimensions comparable to target molecules or particles (DNA, proteins)
• Charge-based separation utilizes differential migration of ions or charged molecules under applied electric fields, known as electrophoresis
• Entropic trapping occurs when molecules or particles experience different degrees of confinement in nanochannels with varying cross-sections

Separation Mechanisms

• Hydrodynamic chromatography separates particles based on size-dependent velocities in pressure-driven flow through nanochannels
• Electrophoresis separates charged species based on their mobility in an electric field
• Dielectrophoresis uses non-uniform electric fields to separate particles based on their polarizability
• Exclusion-enrichment effect separates ions based on their ability to enter nanochannels of different sizes
• Affinity-based separation utilizes specific interactions between analytes and functionalized nanochannel surfaces

Factors Affecting Separation Efficiency

Channel Properties

• Channel geometry (width, depth, length) impacts separation efficiency by affecting fluid flow patterns and particle-wall interactions
• Surface chemistry of nanochannels influences separation through electrostatic interactions, adsorption, and formation of electric double layer
  • Hydrophilic surfaces enhance water flow and reduce adsorption of biomolecules
  • Hydrophobic surfaces can be used for separation of non-polar molecules
• Ratio of channel dimensions to characteristic length scales of analytes (hydrodynamic radius, Debye length) determines separation efficiency
  • Channels with dimensions similar to analyte size enhance size-based separation
  • Debye length affects the extent of electrostatic interactions in charge-based separations

Operating Conditions

• Applied electric field strength and distribution affect migration of charged species and overall separation performance in electrokinetic-based nanofluidic separations
  • Higher field strengths increase separation speed but may lead to Joule heating
  • Non-uniform fields can be used for focusing or trapping analytes
• Flow rate and pressure gradients in pressure-driven nanofluidic separations impact residence time and hydrodynamic effects on particles or molecules
  • Lower flow rates generally improve resolution but decrease throughput
  • Pressure-driven flow can be combined with electrokinetic effects for enhanced separation
• Temperature affects separation efficiency by influencing diffusion rates, viscosity, and molecular interactions within nanofluidic channels
  • Higher temperatures increase diffusion and may reduce separation resolution
  • Temperature gradients can be used for thermophoretic separation

Sample Characteristics

• Sample concentration and composition impact separation performance through effects on inter-particle interactions and channel surface saturation
  • High concentrations may lead to clogging or overloading of separation channels
  • Complex sample matrices can interfere with separation mechanisms
• Analyte properties such as size, charge, and shape determine their behavior in nanofluidic systems
  • Flexible molecules (DNA) may exhibit different separation behavior compared to rigid particles
  • Surface charge of analytes affects their interaction with channel walls and electric fields

Applications of Nanofluidic Devices

Biomolecule Separation

• Nanofluidic devices enable high-resolution separation of biomolecules based on size, charge, and conformational differences
  • DNA fragment separation with single-base resolution
  • Protein separation based on isoelectric point or molecular weight
• Lab-on-a-chip systems integrate nanofluidic devices for sample preparation and purification in biomedical diagnostics
  • Isolation of circulating tumor cells from blood samples
  • Purification of nucleic acids for genomic analysis

Water Treatment and Purification

• Nanofluidic membranes with precisely controlled pore sizes used for ultrafiltration and nanofiltration in water purification and desalination processes
  • Removal of bacteria and viruses from drinking water
  • Selective ion removal for water softening
• Ion selective nanofluidic systems employed for removal of specific ionic contaminants from water or other solutions
  • Arsenic removal from groundwater
  • Recovery of valuable metals from industrial wastewater

Material Processing and Analysis

• Continuous-flow nanofluidic separators allow for high-throughput purification of nanoparticles, colloids, and other nanoscale materials
  • Size-based sorting of gold nanoparticles for biomedical applications
  • Purification of carbon nanotubes based on chirality
• Nanofluidic systems used for isotope separation in nuclear fuel processing and other specialized applications requiring high selectivity
  • Uranium enrichment for nuclear fuel production
  • Separation of rare earth elements for electronic components

Nanofluidic System Design for Purification

Separation Mechanism Selection

• Determine primary separation mechanism based on physicochemical properties of target analytes and contaminants
  • Size-based separation for mixtures of differently sized particles or molecules
  • Charge-based separation for ionized species with different electrophoretic mobilities
  • Affinity-based separation for specific molecular recognition (antibody-antigen interactions)
• Choose appropriate channel dimensions and geometries to optimize separation efficiency for chosen mechanism and target analytes
  • Nanochannels with varying cross-sections for entropic trapping of DNA
  • Long, narrow channels for high-resolution electrophoretic separations

Material and Surface Considerations

• Select suitable materials for nanochannel fabrication to enhance selectivity and minimize undesired interactions
  • Glass or fused silica for optical detection and good chemical resistance
  • PDMS for rapid prototyping and flexibility in design
• Implement surface modifications to tailor channel properties for specific separations
  • Polymer brushes for protein separations
  • Self-assembled monolayers for controlling surface charge and hydrophobicity

System Integration and Optimization

• Incorporate electrodes and design electric field distributions for electrokinetic separations
  • Consider factors such as Joule heating and electroosmotic flow
  • Implement pulsed field techniques for improved separation of large molecules
• Integrate multiple separation stages or mechanisms in a single nanofluidic device to achieve higher resolution or multi-component separations
  • Two-dimensional separations combining size and charge-based mechanisms
  • Serial arrangement of nanochannels with different surface properties
• Design inlet and outlet structures to ensure uniform sample introduction and efficient collection of separated fractions
  • Hydrodynamic focusing for precise sample injection
  • Fraction collectors with multiple outlets for continuous separation
• Implement on-chip detection methods for real-time monitoring of separation performance and product purity
  • Fluorescence detection for labeled biomolecules
  • Electrical impedance measurements for label-free detection of particles