Principles of ATR-FTIR
Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) relies on total internal reflection of an infrared beam through a high-refractive-index crystal. The evanescent wave penetrates a short distance into the sample in contact with the crystal, typically 0.5 to 5 micrometers depending on wavelength and crystal material. This shallow penetration makes ATR-FTIR inherently surface-sensitive.
The penetration depth (dp) follows the Harrick equation:
dp = λ / [2π n1 (sin²θ – (n2/n1)²)^½]
where λ is wavelength, n1 is crystal refractive index, n2 is sample refractive index, and θ is incidence angle.
| Wavenumber (cm⁻¹) | Penetration Depth (μm) |
|---|---|
| 4000 | 1.6 |
| 1000 | 6.4 |
Values calculated for diamond crystal (n1=2.4), organic sample (n2≈1.5), 45° incidence.
Critical Measurement Parameters
- Pressure: 50–100 psi ensures intimate contact without damaging crystal or altering nanoparticle properties.
- Excessive pressure may cause particle deformation or surface chemistry changes.
- Insufficient pressure leads to poor contact and spectral artifacts.
- For liquid suspensions, controlled pressure prevents meniscus formation.
Advantages Over Transmission FTIR
- Direct measurement of nanoparticle powders without KBr pellet preparation.
- Eliminates matrix interactions that can obscure surface features.
- Enables analysis of liquid suspensions without solvent evaporation artifacts.
- Reduced scattering artifacts from nanoparticle aggregation during pellet preparation.
- Reproducible sampling geometry improves quantitative reliability.
Case Studies Demonstrating Superiority
Platinum nanoparticles on alumina: Transmission FTIR failed to detect surface-adsorbed CO due to strong bulk absorption from the support. ATR-FTIR clearly identified linear CO at 2080 cm⁻¹ and bridged CO at 1850 cm⁻¹.
Titanium dioxide nanoparticles: Transmission measurements showed broad water bands obscuring surface hydroxyl signatures. ATR-FTIR resolved distinct Ti-OH stretching modes at 3675 cm⁻¹ and 3715 cm⁻¹ corresponding to different crystallographic faces.
Quantitative Analysis and Reproducibility
Studies on amine-functionalized silica nanoparticles achieved relative standard deviations below 5% for NH stretching band intensities using ATR-FTIR, compared to 15–20% variability in transmission measurements. The consistent contact area provides reliable intensity data for surface coverage calculations.
Limitations and Considerations
- Penetration depth may probe multiple nanoparticle layers in densely packed samples; monolayer deposition can mitigate this.
- Spectral distortions from anomalous dispersion near strong absorption bands affect band shapes and penetration depth calculations.
- Nanoparticles with strong surface plasmon resonances require careful data interpretation.
Recent Advancements and Applications
- Micro-ATR with spot sizes below 100 μm enables localized analysis of nanoparticle deposits.
- Imaging ATR-FTIR provides chemical maps of surface heterogeneity.
- Flow cells allow real-time monitoring of surface reactions in liquid media.
- Temperature-controlled stages facilitate stability studies under controlled conditions.
The technique applies to metals, metal oxides, polymers, and carbon-based nanoparticles. Applications include environmental contaminant adsorption studies on iron oxide nanoparticles, pharmaceutical coating analysis on polymeric nanoparticles, and industrial quality control for surface modification verification.